Group 13 Radical and Lewis Acid Chemistry · (E = B, Al) led to a typical two electron pathway upon treatment with chloranil (p-C 6 Cl 4 O 2) and Ph 3 SnH. However, replacing the

Group 13 Radical and Lewis Acid Chemistry

by

Levy Liwei Cao

A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy

Department of Chemistry University of Toronto

© Copyright by Levy Liwei Cao 2019

ii

Group 13 Radical and Lewis Acid Chemistry

Levy Liwei Cao

Doctor of Philosophy

Department of Chemistry

University of Toronto

2019

Abstract

Since the discovery of frustrated Lewis pairs (FLPs), a pair of sterically encumbered Lewis acids

and bases that rapidly activate numerous small molecules, such advances have led to synthetic

applications in organic, polymer and radical chemistry. The objective for this thesis is to explore

the radical and Lewis acid chemistry of Group 13 compounds at a fundamental level, and their

applications in FLP chemistry.

Reduction of an N-heterocyclic carbene (NHC) stabilized planar borenium compound led to the

generation of a unique diborane compound which rapidly undergoes homolytic cleavage

reactions with various substrates. Furthermore, protection with steric bulk on the NHC of the

borenium with the combination of the FLP, B(C6F5)3 and tBu3P leads to generation of a boron

based dianion radical upon reduction.

Combination of tBu3P and E(C6F5)3 (E = B, Al) led to a typical two electron pathway upon

treatment with chloranil (p-C6Cl4O2) and Ph3SnH. However, replacing the phosphine with Mes3P

resulted in the reaction proceeding through single electron transfer (SET) to the Lewis acids,

with homolytic cleavage of the Sn-H bond.

Further studies on the SET chemistry between Lewis acids (E(C6F5)3, E = B, Al, Zn(C6F5)2), the

reductant Cp*2Fe and conventional weak Lewis bases (chalcogenide species) led to the synthesis

of various unique anions and radicals. The results of the stabilization of the weak Lewis acid-

base adducts from SET led to the attempts of one electron reduction of the reported unstable

(Ph2CN2)B(C6F5)3 adducts, resulting in the stabilization of the N2 fragment and activation of C-H

bonds.

iii

Reactions of hydride abstraction from (IBn)AlH3 resulted in unanticipated redistribution

reactions, which gave a rare dicationic aluminum-hydride stabilized by two NHCs (IBn). On the

other hand, attempts to prepare the analogous cyclic (alkyl)(amino) carbene (cAAC) alane

adducts gave rise to a monomeric neutral aluminum-hydride that rapidly undergoes a 1,2-hydride

shift between the Al and carbene carbon centers in solution.

Finally, attempts to prepare an ethylene-bridged intramolecular P/Al compounds have been

carried out. HAl(C6F5)2 and phosphinoalanes Mes2PAl(C6F5)2 have been successfully generated.

While HAl(C6F5)2 enabled the hydroalumination and dehydrocoupling reactions with terminal

alkynes, Mes2PAl(C6F5)2 demonstrated the addition across CO2 gas, and reacting with other

functional groups.

iv

Acknowledgments

Firstly, I would like to thank my supervisor, Prof. Doug Stephan. Thank you for allowing me to

work in the lab and always having the freedom to do what I wanted. Thank you for always

having the office door open when I had questions and sending me off for numerous conferences

to present my work and network with others in the field. You have made my PhD research an

extremely fun experience, and also made me a better person throughout the years. I would also

like to acknowledge the advice and feedback my committee members have given. Thank you,

Prof. Bob Morris and Prof. Ulrich Fekl.

Secondly, I would like to thank all the Stephan group members, past and present. Thank you to

Prof. Timothy Johnstone for teaching me how to refine countless badly disordered X-ray

structures. Thank you to Dr. Liu (Leo) Liu and Dr. Jiliang Zhou for being great and productive

collaborators and nice friends throughout the years. I would also like to thank for the discussions,

support and friendships from the Stephan group members: Alex Waked, Diya Zhu, James

LaFortune, Jolie Lam, Dr. Judy Tsao, Karlee Bamford, Ryan Andrews, especially those ones that

motivated me to go rock climbing and play basketball. Also, special thanks to my box mates: Dr.

Louie Fan, Chris Major, Erika Daley and Prof. Chris Caputo throughout the years to maintain a

great environment to work with my annoying chemistry. Thanks to the undergraduates and high

school volunteers, Sarah, Chris and Dan. You guys did a great job.

This work could not have been done without the excellent support staff in the department.

Thanks to Shanna Pritchard for keeping the lab organized. Thanks to Timothy Burrow for

advancing my skills with the EPR instruments. Also, I would like to all the NMR staff, Darcy

Burns, Sergiy Nokhrim, Dimity Pichugin, Jack Sheng, and Karl Demmans. They have put

tremendous effort into maintaining and accommodating all the requests we need. Thank you to

Alan Lough, Anna Liza Villavelez, Chung Fung, Jack O’Donnell, Ken Greaves, Patrick Wong,

Rose Balazs, and Violeta Gotcheva for the support throughout the years.

Lastly, I would like to thank my family for their support throughout my life. Thanks to my

parents for their support and love they give me and for raising me to who I am now. Finally, the

one person I would thank the most, my lifelong partner, Mingyue (Crystal) Wang. It would not

have been possible for me without your constant support, understanding, and inspiration.

v

Table of Contents

Abstract ........................................................................................................................................... ii

Acknowledgments.......................................................................................................................... iv

Table of Contents .............................................................................................................................v

List of Figures ..................................................................................................................................x

List of Schemes ........................................................................................................................... xvii

List of Tables ............................................................................................................................... xix

List of Abbreviations ................................................................................................................... xxi

Chapter 1 Introduction .....................................................................................................................1

1.1 Selected Chemical Process in the Industry ..........................................................................1

1.2 Group 13 Compounds ..........................................................................................................2

1.2.1 Boron in General ......................................................................................................2

1.2.2 Boron Chemistry ......................................................................................................2

1.2.3 Aluminum in General ..............................................................................................3

1.2.4 Aluminum Chemistry...............................................................................................3

1.3 A Brief Summary of Frustrated Lewis Pair Chemistry .......................................................4

1.3.1 Radical Chemistry in FLPs ......................................................................................5

1.3.2 Recent Advances in FLP Chemistry ........................................................................6

1.4 Carbenes ...............................................................................................................................7

1.4.1 A Short History of Carbenes ....................................................................................7

1.4.2 Carbene Stabilized Low Valent Main Group Compounds ......................................8

1.5 Scope of Thesis ....................................................................................................................9

1.6 References ..........................................................................................................................10

Chapter 2 Reduction of NHC-Stabilized Borenium: Generation of B-B Bonded Diboranes

and Boron-Based Radicals ........................................................................................................18

vi

2.1 Introduction ........................................................................................................................18

2.1.1 Diborane Compounds ............................................................................................18

2.1.2 Syntheses of B-B Bonded Diboranes(6) ................................................................18

2.1.3 Reactivity of B-B Bonded Diboranes(6)................................................................19

2.1.4 Boron-Based Radicals ............................................................................................20

2.2 Results and Discussion ......................................................................................................21

2.2.1 Generation of B-B Bonded Diboranes(6) ..............................................................21

2.2.2 Reactivity of Diborane ...........................................................................................27

2.2.3 Generating a Boron-Based Radical ........................................................................33

2.3 Conclusions ........................................................................................................................37

2.4 Experimental Details ..........................................................................................................37

2.4.1 General Considerations ..........................................................................................37

2.4.2 Syntheses and Characterizations ............................................................................39

2.4.3 X-ray Crystallography ...........................................................................................45

2.4.4 Electrochemistry ....................................................................................................49

2.4.5 Computational Chemistry ......................................................................................51

2.5 References ..........................................................................................................................52

Chapter 3 A Radical Mechanism for Frustrated Lewis Pair Reactivity .........................................59

3.1 Introduction ........................................................................................................................59

3.1.1 FLP Reaction Mechanism Chemistry ....................................................................59

3.1.2 Singlet Electron Transfer (SET) in Lewis Pairs ....................................................60

3.1.3 Work Carried Out After Our Publication...............................................................61


3.2.1 Two Electron Chemistry Between tBu3P/E(C6F5)3 (E = B, Al) ............................62

3.2.2 One Electron Chemistry Between Mes3P/E(C6F5)3 (E = B, Al) ............................64

3.3 Conclusion .........................................................................................................................68

vii

3.4 Experimental Details ..........................................................................................................69

3.4.1 General Considerations ..........................................................................................69

3.4.2 Syntheses and Characterizations ............................................................................70

3.4.3 X-ray Crystallography ...........................................................................................75

3.5 References ..........................................................................................................................78

Chapter 4 Single Electron Transfer to Lewis Acid-Base Adducts ................................................81

4.1 Introduction ........................................................................................................................81

4.1.1 SET Reactions with Lewis Acids ..........................................................................81

4.1.2 Zn Containing Radicals..........................................................................................82

4.1.3 Main Group N2 Activation .....................................................................................82


4.2.1 SET Reactions with E(C6F5)3 (E = B, Al)..............................................................83

4.2.2 SET Reactions with Zn(C6F5)2...............................................................................89

4.2.3 SET Reactions with Diazo Compounds .................................................................96

4.3 Conclusions ......................................................................................................................101

4.4 Experimental ....................................................................................................................102

4.4.1 General Considerations ........................................................................................102

4.4.2 Synthesis and Characterization ............................................................................103

4.4.3 Electrochemistry ..................................................................................................114

4.4.4 X-ray Crystallography .........................................................................................117

4.4.5 Computational Chemistry ....................................................................................123

4.5 References ........................................................................................................................125

Chapter 5 NHC-Stabilized Al Compounds ..................................................................................129

5.1 Introductions ....................................................................................................................129

5.1.1 Neutral Monomeric Aluminum Hydrides ............................................................129

5.1.2 Cationic Aluminum Hydrides ..............................................................................130

viii

5.1.3 NHC Stabilized Aluminum Compounds .............................................................131

5.1.4 Reversible Oxidative Addition at Carbon ............................................................131

5.2 Results and Discussion ....................................................................................................132

5.2.1 NHC-Stabilized Al hydride Cations ....................................................................132

5.2.2 Reversible 1,2-Hydride Migration .......................................................................137

5.2.3 Other NHC Based Al Compounds .......................................................................143

5.3 Conclusions ......................................................................................................................147

5.4 Experimental Details ........................................................................................................147

5.4.1 General Considerations ........................................................................................147

5.4.2 Synthesis and characterizations ...........................................................................148

5.4.3 Thermodynamic Calculations ..............................................................................156



5.5 References ........................................................................................................................163

Chapter 6 Preparation of HAl(C6F5)2 and Phospinoalanes ..........................................................167

6.1 Introduction ......................................................................................................................167

6.1.1 Aluminum Based FLPs ........................................................................................167

6.1.2 Phosphinoboranes and Phosphinoalanes..............................................................168

6.1.3 Possible Synthetic Route to P/A-based FLPs ......................................................169

6.2 Results and Discussion ....................................................................................................169

6.2.1 Synthesis of Bis(pentafluorophenyl) Alane (HAl(C6F5)2) ...................................169

6.2.2 Synthesis of Mes2P-Al(C6F5)2..............................................................................172

6.3 Conclusion .......................................................................................................................176

6.4 Experimental ....................................................................................................................177

6.4.1 General Consideration .........................................................................................177

6.4.2 Synthesis and Characterization ............................................................................177

ix



6.5 References ........................................................................................................................184

Chapter 7 Conclusions .................................................................................................................187

7.1 Thesis Summary...............................................................................................................187

7.2 Future Work .....................................................................................................................188

7.3 References ........................................................................................................................189

Copyright Acknowledgements.....................................................................................................190

x

List of Figures

Figure 1.1 – Selected examples of FLP reactions with small molecules. ....................................... 5

Figure 1.2 – Selected examples of isolable carbenes. ..................................................................... 8

Figure 1.3 – Selected carbene as in stabilized main group compounds. ......................................... 9

Figure 2.1 – Selected examples of B-B bonded diboranes(6) compounds. .................................. 19

Figure 2.2 – Selected examples of boron containing radicals. ..................................................... 20

Figure 2.3 - Energy plot vs. B-B bond distance of 2-3a (C: grey; N: blue; B: pink). .................. 23

Figure 2.4 - (a) Plot of frontier molecular orbitals for the 2-3a; (b) NBOs for B-B bonding. (c)

Plot of the gradient vector field of ρ(r) and the contour map of the Laplacian of the electron

density, ∇ρ(r), of 2-3a. .................................................................................................................. 25

Figure 2.5 – ORTEPs of 2-1b (left) and the cation of 2-2b (right) with thermal displacement

parameters drawn at 50 % probability. C: black; N: blue; B: yellow-green, Hydrogen atoms are

omitted for clarity except for the boron bound ones. .................................................................... 26

Figure 2.6 – 11B{1H} NMR spectra for the exchange reaction of generating compound 2-3a, 2-3b

and 2-3c. ........................................................................................................................................ 26

Figure 2.7 – ORTEPs of 2-4 (left) and 2-5 (right) with thermal displacement parameters drawn at

50 % probability. C: black; N: blue; B: yellow-green, O: red. Hydrogen atoms are omitted for

clarity. ........................................................................................................................................... 29

Figure 2.8 – (a) Computed spin density of 2-6, (b) Simulated and experimental EPR spectra for

2-6. ................................................................................................................................................ 30

Figure 2.9 - ORTEPs of 2-7 (left) and 2-8 (right) with thermal displacement parameters drawn at

50 % probability. C: black; N: blue; B: yellow-green, Cl: green. S: yellow Hydrogen atoms are

omitted for clarity. ........................................................................................................................ 32

Figure 2.10 – Computed spin density plot of the [(C2H2(NCH2C6H4)2CB)•] radical. .................. 33

xi

Figure 2.11 - ORTEPs of 2-9 (left) and 2-10 (right) with thermal displacement parameters drawn

at 50 % probability. H atoms and the cation [tBu3PH]+ of 2-10 are omitted for clarity. C: black,

F: pink, B: yellow-green, N: blue. ................................................................................................ 35

Figure 2.12 – Computed HOMO (left), LUMO (middle), and spin density (right) plots for 2-10.

....................................................................................................................................................... 36

Figure 2.13 – EPR spectrum of 2-11 in THF. ............................................................................... 37

Figure 2.14 - ORTEP of [Cp*2Co][B(C6F5)4] with thermal displacement parameters drawn at 50

% probability. C: black; F: Pink; B: yellow-green, Co: green. Hydrogen atoms are omitted for

clarity. ........................................................................................................................................... 41

Figure 2.15 - Cyclic voltammogram of 2-2a in THF (0.15 M of [nBu4N][B(C6F5)4] at room

temperature. Scan rate: 500 mV/s. ................................................................................................ 49

Figure 2.16 - Cyclic voltammogram of 2-9 in THF/0.1 M [nBu4N][B(C6F5)4] at room




Figure 2.18- Cyclic voltammogram of [tBu3PH][B(C6F5)4] in THF/0.1 M [nBu4N][B(C6F5)4] at

room temperature. Scan rate: 500 mV/s. ...................................................................................... 51

Figure 3.1 – Plausible mechanism of small-molecule activation by FLPs (a) mechanism of

activation of generic substrate (X2) by FLPs via an “encounter complex” (b) proposed

mechanism of activation of generic substrate (X2) by FLPs via the SET process. ....................... 60

Figure 3.2 – SET in Lewis pairs. .................................................................................................. 61


50 % probability. H atoms that are bonded to carbon atoms are omitted for clarity. C: black, F:

pink, B: yellow-green, P: orange, Al: cyan, Sn: silver, and H: light grey. ................................... 63

xii

Figure 3.4 – (a) Photo taken for the reaction mixture of Mes3P and Al(C6F5)3, (b) Simulated and

Experimental EPR spectra of the Mes3P and Al(C6F5)3 mixture. ................................................. 65

Figure 3.5 – UV spectra for (a) PMes3 and Al(C6F5)3 with p-O2C6Cl4 (b) PMes3 and B(C6F5)3

with p-O2C6Cl4 and (c) PMes3 with p-O2C6Cl4. ........................................................................... 66

Figure 3.6 – ORTEPs of 3-7 (a) and 3-8 (b) with thermal displacement parameters drawn at 50 %

probability. H atoms that are bonded to carbon atoms are omitted for clarity. C: black, F: pink, B:

yellow-green, P: orange, Al: cyan, H, light grey. ......................................................................... 67

Figure 4.1 - Interactions of main group systems with N2-fragments. ........................................... 82

Figure 4.2 - ORTEPs of the anions of 4-1a (a), 4-1b (b), 4-2a (c) and 4-2b (d) with thermal

displacement parameters drawn at 50 % probability. C: black; F: pink; B: yellow-green; O: red;

N: blue; Al: cyan; The [Cp*2Fe]+ cations and H atoms are omitted for clarity. ........................... 84

Figure 4.3 – Computed LUMOs for B(C6F5)3 and Al(C6F5)3 and TEMPO adducts. Hydrogen

atoms have been omitted for clarity. ............................................................................................. 84

Figure 4.4 - ORTEPs of the anions of 4-3 (a), 4-4 (b), 4-5 (c) and 4-6 (d) with thermal


Se: feldspar; Te: gold; Al: cyan; The [Cp*2Fe]+ cations are omitted for clarity. ......................... 88

Figure 4.5 - (a) Synthesis of 4-7; (b) ORTEP of the X-ray structure of the anion of 4-7 with

thermal displacement parameters drawn at 50 % probability. C: black; F: pink; S: yellow; Zn:

blue. All hydrogen atoms and [Cp*2Fe]+ cations have been omitted for clarity; (c) Picture of 4-7

crystals taken under a microscope. ............................................................................................... 90

Figure 4.6 - Energy (eV) of LUMO of (a) o-C14H8O2 and 4-8a and (b) o-C16H8O2 and 4-8b. .... 91

Figure 4.7 - (a) Synthesis of 4-8a/b and 4-9a/b; ORTEPs of the X-ray structure of (b) 4-8a (c) 4-

8b with thermal displacement parameters drawn at 50 % probability. C: black; F: pink; O: red;

Zn: blue. Hydrogen atoms have been omitted for clarity. (d) Picture of 4-8a crystals taken under

the microscope. (e) Picture of 4-8b crystals taken under the microscope, where the dark orange

block-shaped crystal is 4-8b and the orange needle-shaped crystal is o-C16H8O2. ...................... 91

xiii

Figure 4.8 - (a). Experimental and simulated X-band EPR spectrum of isolated 4-9a; (b)

Calculated spin density plot of the anion of 4-9a; (c) Experimental and simulated X-band EPR

spectrum of isolated 4-9b; (d) Calculated spin density plot of the anion of 4-9b. ....................... 92

Figure 4.9 – 1H NMR spectrum of the reaction between 4-9a with DMAP. ................................ 94

Figure 4.10 - 1H NMR spectrum of the reaction between 4-9b with DMAP. .............................. 95

Figure 4.11 – Summary reaction scheme for the reactions between 4-8a/b and 4-9a/b. ............. 95

Figure 4.12 – (a) Reactions of diazomethane with B(C6F5)3 and Cp*2Co. ORTEPs of the anion of

(b) 4-10 and (c) 4-11 with thermal displacement parameters drawn at 50 % probability. The

[Cp*2Co]+ cation and hydrogen atoms, except the NH, are omitted for clarity. C: black, N: blue;

B: yellow-green, H: white. ............................................................................................................ 97

Figure 4.13 – (a) Reactions of Ph2CN2, BPh3 and Cp*2Cr. ORTEPs of the anion of (b) 4-12 and

(c) 4-13 with thermal displacement parameters drawn at 50 % probability. The [Cp*2Cr]+ cation

and hydrogen atoms, except the NH are omitted for clarity. C: black, N: blue; B: yellow-green,

H: white. ........................................................................................................................................ 98

Figure 4.14 – (a) Reactions of C12H8CN2, BPh3 and Cp*2Cr. ORTEPs of the anion of (b) 4-15

and (c) 4-16 with thermal displacement parameters drawn at 50 % probability. The [Cp*2Cr]+

cation and hydrogen atoms, except the NH are omitted for clarity. C: black, N: blue; B: yellow-

green, H: white. ............................................................................................................................. 99

Figure 4.15 - ORTEP of the anion of 4-10a with thermal displacement parameters drawn at 50 %

probability. C: black; F: pink; S: yellow; Zn: blue; Fe: orange. All H atoms have been omitted

for clarity. .................................................................................................................................... 110

Figure 4.16 - ORTEP of the anion of 4-10b with thermal displacement parameters drawn at 50 %

probability. C: black; F: pink; S: yellow; Zn: blue. All H atoms have been omitted for clarity. 111

Figure 4.17 - Cyclic voltammogram of Cp*2Fe.......................................................................... 115

Figure 4.18 - Cyclic voltammogram of 4-8a. ............................................................................. 115

xiv

Figure 4.19 - Cyclic voltammogram of o-C14H8O2. ................................................................... 116

Figure 4.20 - Cyclic voltammogram of 4-8b. ............................................................................. 116

Figure 4.21 - Cyclic voltammogram of o-C16H8O2. ................................................................... 117

Figure 5.1 – Neutral monomeric aluminum mono hydrides that have been isolated in the solid

state. ............................................................................................................................................ 129

Figure 5.2 - Structurally characterized aluminum-hydride cations. ........................................... 130

Figure 5.3 - ORTEPs of the NHC-alane adduct 5-1 (left) and the dication of 5-2 (right) with

thermal displacement parameters drawn at 50 % probability. C: black, N: blue, Al: cyan, H: gray.

All ligand-based H atoms as well as the [B(C6F5)4]- anions are omitted for clarity. .................. 133

Figure 5.4 – (a) Computed structure for 5-22+ with WBI bond order in black given in a.u. and

NPA charges in red given in a.u.; (b) Lowest unoccupied molecular orbital (LUMO) plot for the

dication of 5-22+; (c–e) Selected NBOs for 5-22+ (c) 3pz for Al, (d) π(3c)1 for the phenyl group,

(e) π(3c)2 for the phenyl group.................................................................................................... 136



All ligand-based H atoms as well as the [B(C6F5)4]- anions are omitted for clarity. .................. 137

Figure 5.6 - ORTEPs of the molecular structure of 5-5 (left) and 5-6 (right) with thermal

displacement parameters drawn at 50 % probability; The hydrogen atoms are omitted except Al-

H and the C-H that the carbon is bonded to the Al center for clarity. C: black, N: blue, Al: cyan,

H: grey. ....................................................................................................................................... 139


50 % probability. The hydrogen atoms are omitted except Al-H for clarity. C: black, N: blue, Al:

cyan, H: grey. .............................................................................................................................. 139

Figure 5.8 - 1H-1H EXSY NMR spectrum for 5-7. ..................................................................... 141

Figure 5.9 – 1H NMR of the crude reaction of synthesis of 5-8. ................................................ 142

xv

Figure 5.10 - ORTEPs of 5-9’ (left) and 5-9 (right) with thermal displacement parameters drawn

at 50 % probability. C: black, N: blue, Al: cyan, Cl: green, K: purple, Si: pink. ....................... 144

Figure 5.11 – ORTEPs of the adduct 5-11 (left) and 5-12 (right) with thermal displacement

parameters drawn at 50 % probability. C: black, N: blue, Al: cyan, Cl: green, Si: pink. ........... 144

Figure 5.12 - Wireframe depiction of the molecular structure of 5-13; The hydrogen atoms are

omitted except Al-H and the CcAAC-H for clarity. C: black, N: blue, Al: cyan, H: grey. ........... 146

Figure 5.13 – ORTEPs of (cAACEt)Ru3(CO)11 with thermal displacement parameters drawn at 50

% probability. C: black, N: blue, Ru: pink, O: red. .................................................................... 153

Figure 5.14 – 31P NMR of the crude reduction reaction of 5-11 with P≡CAd. ........................ 154

Figure 5.15 - 1H VT-NMR spectra (range: 2.8 to 5.2 ppm) of 5-7 in tol-d8. .............................. 156

Figure 5.16 - Van’t Hoff plot for 5-7. ......................................................................................... 157

Figure 5.17 - Calculated frequency vs. experimental frequency for 5-22+. ................................. 162

Figure 6.1 – Selected examples of Al-based FLPs. .................................................................... 167

Figure 6.2 – Selected examples of phosphinoboranes and phosphinoalanes.............................. 168

Figure 6.3 – Possible synthetic routes to prepare intramolecular P/Al FLPs. ............................ 169


50 % probability. C: black, F: pink, Al: cyan, Br: brown, H: grey. All carbon bond H atoms were

excluded for clarity. .................................................................................................................... 171

Figure 6.5 – (a) Possible reaction mechanism for generating 6-4 and proposed reaction

mechanism. (b) ORTEP of 6-4 with thermal displacement parameters drawn at 50 % probability.

C: black, F: pink, Al: cyan, P: orange; Cl: green. All H atoms were excluded for clarity. ........ 172

Figure 6.6 – (a) and (b) electric structure of 6-5 and Mes2P-B(C6F5)2. (c) Computed structure of

6-5, with computed WBI bond order in black. C: grey; P: orange; Al: pink; F: green. (d) ORTEP

of Mes2P-B(C6F5)2 with thermal displacement parameters drawn at 50 % probability. C: black, F:

xvi

pink, B: yellow-green, P: orange. All H atoms were excluded for clarity. (e) Plot of frontier

molecular orbitals for the 6-5 and Mes2P-B(C6F5)2. (f) and (g) NBOs for the P lone pair electron

and Al empty p orbital. ............................................................................................................... 173

Figure 6.7 - ORTEP of 6-6 with thermal displacement parameters drawn at 50 % probability. C:

black, F: pink, Al: cyan, P: orange; O: red. All H atoms were excluded for clarity. .................. 174

Figure 6.8 – Multinuclear NMR spectra for 6-7 and 6-8. ........................................................... 176

Figure 6.9 – 19F{1H} NMR spectra of 6-1 in various solvents or concentrations. ..................... 178

Figure 6.10 - Molecular structures of [Al(C6F5)3·NaCl]8 and [Al(C6F5)2Cl·NaCl]n. Na: purple;

Cl: green; Al: pink; F: yellow. .................................................................................................... 178

xvii

List of Schemes

Scheme 1.1 – Dimethyl zirconocene activation with B(C6F5)3. ..................................................... 2

Scheme 1.2 – Possible reaction pathways with organoaluminium hydrides and alkynes. ............. 4

Scheme 1.3 – FLP-derived radicals and their reactivity. ................................................................ 6

Scheme 1.4 – Reaction schemes for the selected FLP reactions. ................................................... 7

Scheme 2.1 - Generation and reactivity of 2-3a and 2-3b. ........................................................... 22

Scheme 2.2 - Reactivity of 2-3a. .................................................................................................. 28

Scheme 2.3 – Synthesis of compounds 2-9, 2-10 and 2-11. ......................................................... 34

Scheme 3.1 – Reaction of tBu3P and E(C6F5)3 (E = B or Al) with p-O2C6H4 and Ph3SnH. ........ 63

Scheme 3.2 – Reaction scheme of Mes3P and E(C6F5)3 (E = B or Al) with p-O2C6Cl4 and

Ph3SnH. ......................................................................................................................................... 65

Scheme 4.1 – Selected SET examples with main group Lewis acids. .......................................... 81

Scheme 4.2 - Synthesis of 4-1a, 4-1b, 4-2a and 4-2b. ................................................................. 85

Scheme 4.3 - Reactions of O2, S8, Se and Te with E(C6F5)3 and Cp*2Fe. .................................... 86

Scheme 4.4 - DFT Calculation on free energies for the formation of the (Ph2CN2)BR3 and

[Ph2CN2(BR3)•]- (R = C6F5 and Ph). ............................................................................................. 96

Scheme 5.1 – Selected aluminum compounds. ........................................................................... 131

Scheme 5.2 - Selected examples of 1,2-hydride migration reaction examples on carbon centers.

..................................................................................................................................................... 132

Scheme 5.3 – Synthesis of compounds between 5-1 to 5-4. ....................................................... 134

Scheme 5.4 – Synthesis of compounds between 5-5 to 5-8. ....................................................... 138

xviii

Scheme 5.5 - Proposed reaction mechanism for the isomerization of 5-7. ................................. 142

Scheme 5.6 – Synthesis of 5-9 to 5-11. ...................................................................................... 143

Scheme 5.7 – Synthesis and reduction of 5-12. .......................................................................... 145

Scheme 6.1 – Reaction scheme of generation for 6-1 to 6-3. ..................................................... 170

Scheme 6.2 – Reaction of generation of 6-5 to 6-8. ................................................................... 175

xix

List of Tables

Table 2.1 - Computed B(sp3)-B(sp3) bond length for known diboranes(6) comparing with 2-3a.

....................................................................................................................................................... 24

Table 2.2 – Isotropic shift for 2-2a and 2-3a and 2-3b of the boron center. ................................ 24

Table 2.3 – Hyperfine coupling parameters for 2-6 and 2-6’. ...................................................... 31

Table 2.4 – Summary of crystallographic data for compounds 2-1b, 2-2b, 2-4, 2-5, 2-7. ........... 47

Table 2.5 - Summary of crystallographic data for compounds 2-8, [Cp*2Co][B(C6F5)4], 2-9, 2-10.

....................................................................................................................................................... 48

Table 2.6 - Reduction potential summary for 2-2a, 2-9, 2-10, [tBu3PH][B(C6F5)4] .................... 51

Table 3.1. Summary of crystallographic data for compounds 3-1, 3-4, 3-7, 3-8 and 3-9. ............ 77

Table 4.1 - Computed spin density for the proposed radicals [Ph2CN2B(C6F5)3•]- (left),

[C12H8CN2BPh3•]- (middle) and [Ph2CN2BPh3•]

- (right). ........................................................... 100

Table 4.2 - Summary of crystallographic data for compounds 4-1a, 4-1b, 4-2a and 4-2b. ....... 119

Table 4.3 – Summary of crystallographic data for compounds 4-3, 4-4, 4-5 and 4-6. ............... 120

Table 4.4 – Summary of crystallographic data for compounds 4-7, 4-8a, 4-8b and 4-9a. ......... 121

Table 4.5 – Summary of crystallographic data for compounds 4-9b, 4-10, 4-11 and 4-12. ....... 122

Table 4.6 – Summary of crystallographic data for compounds 4-13 and [Cp*2Fe][Al(C6F5)4]. 123

Table 4.7 - Hyperfine coupling parameters and spin density for 4-9a- (C: grey; F: green; Zn: deep

blue; O: red). ............................................................................................................................... 124

Table 4.8 - Hyperfine coupling parameters and spin density for 4-9b- (C: grey; F: green; Zn:

deep blue; O: red)........................................................................................................................ 125

Table 5.1- Average ratio between meso-5-7 and rac-5-7 at different temperature. ................... 156

xx

Table 5.2 - Summary of crystallographic data for compounds of 5-1 to 5-4. ............................. 159

Table 5.3 – Summary of crystallographic data for compounds of 5-5 to 5-9. ............................ 160

Table 5.4 - Summary of crystallographic data for compounds of 5-9’ to 5-13. ......................... 161

Table 5.5 - Calculated enthalpy for meso-5-7, (R, R)-5-7, and (S, S)-5-7. ................................ 162

Table 6.1 - Summary of crystallographic data for compounds 6-2, 6-3, 6-4 and 6-6. ................ 183

Table 6.2 - Isotropic shift for 6-5 and 6-6 of the phosphorus centers. ........................................ 184

xxi

List of Abbreviations

Å angstrom

° degrees

°C degrees Celsius

δ chemical shift

Δ heat

ΔG Gibbs free energy

ΔH enthalpy

ΔS entropy

∇ρ(r) Laplacian of the electron density

∇2ρ derivative of the Laplacian of the electron density

λ wavelength

π pi orbital

σ sigma orbital

ν wavenumber

µ bridging

Ar aryl

AIM atom-in-molecules

atm atmosphere

A(x) Hyperfine coupling of atom x

BIBn 1,3-dibenzyl-1H-benzo[d]imidazole-3-ium-2ide

Bn benzyl

cAAC cyclic (alkyl)(amino) carbene

cAACCyc 1-(2,6-iPr2C6H3)-3,3-diethyl-5,5-cyclohexylpyrrolidin-2-

ylidene

cAACEt 1-(2,6-iPr2C6H3)-3,3-diethyl-5,5-dimethylpyrrolidin-2-

ylidene

cAACMe 1-(2,6-iPr2C6H3)-3,3-dimethyl-5,5-dimethylpyrrolidin-2-

ylidene

cAACMenthyl 1-(2,6-iPr2C6H3)-3,3-dimethyl-5,5-menthylpyrrolidin-2-

xxii

ylidene

Cp Cyclopentadienide anion

Cp* 1,2,3,4,5-pentamethylcyclopentadienide anion

CV Cyclic voltammetry

Cy cyclohexyl

d doublet

DART Direct analysis in real time

DABCO 1,4-diazabicyclo[2.2.2]octane

DCB 1,2-dichlorobenzene

DCM dichloromethane

DFT Density functional theory

DFB difluorobenzene

Dipp 2,6-diisopropylphenyl

DMAP 4-dimethylaminopyridine

Dur 2,3,5,6-tetramethylphenyl

EPR electron paramagnetic resonance

Equiv. equivalents

ESI electrospray ionization

Et ethyl

FLP frustrated Lewis pair

g grams

GIAO gauge-independent atomic orbital

GOF Goodness of fit

h hours

HOMO Highest occupied molecular orbital

hpp 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate

HRMS High resolution mass spectrometry

Hz hertz

i ipso

IBn 1,3-Bis(benzyl)-1,3-dihydro-2H-2-ylidene

IDipp 1,3-Bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-2-ylidene

xxiii

IMes 1,3-Bismestiyl-1,3-dihydro-2H-2-ylidene

IPr 1,3-Bisisopropyl-1,3-dihydro-2H-2-ylidene

iPr isopropyl

IR infrared

nJxy n-bond scalar coupling constant between x and y atoms

K Kelvin

kcal kilocalories

kHz kilohertz

kJ kilojoule

LRMS Low resolution mass spectrometry

LUMO lowest unoccupied molecular orbital

M molarity

m meta

m multiplet

M06 a functional for DFT

M06-2X a functional for DFT

MALDI Matrix-Assisted Laser Desorption Ionization

Me methyl

MeCN acetonitrile

Mes mesityl, 2,4,6-trimethylphenyl

Mes* 2,4,6-tri-tert-butylphenyl group

mg milligram

MHz megahertz

min minutes

mL millilitre

mmol millimole

m/z mass to charge ratio

NBO Natural Bond Orbital

nBu Butyl

NHC N-heterocyclic carbene

NMR Nuclear Magnetic Resonance

xxiv

NPA Natural Population Analysis

o ortho

p para

Ph phenyl

ppm parts per million

q quartet

QTOF Quadrupole time-of-flight

rac racemic

rt room temperature

s singlet

sat. saturated

SET single electron transfer

SIMes 1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene

SMD standardized mean difference

t triplet

tBu tert-butyl

TEMPO (2,2,6,6-tetramethylpiperidin-1-yl)oxyl

THF tetrahydrofuran

Tmp 2,2,6,6-tertamethylpiperidine

tol toluene

tol-d8 deuterated toluene

trityl triphenylmethyl

UV/vis Ultraviolet/visible

VT variable temperature

WBI Wiberg bond index

1

Chapter 1

1 Introduction

1.1 Selected Chemical Process in the Industry

Development of human society is generally driven by advances in science and technology.

Discovery of the Haber-Bosch process for ammonia production led to the synthesis of nitrogen-

based fertilizers.1 While with Ziegler-Natta catalyis, commercial plastic products have

contributed to a huge part of our lives,2-4 recent research on synthesizing self-immolative5 or

recyclable polymers6 are alternative ways to counter the rising environmental concerns of plastic

waste. Moreover, improvement in medicine and pharmaceuticals manage to overcome several

previously deadly diseases, thus increasing human life-expectancy.7 Nowadays, there is still a

steady increase in the total revenue of the global chemical industry, with an overall total of over

$ 4,200 billion (US) in 2016.8 Billions of tons of hydrogenation products are produced and used

in food, agricultural and pharmaceutical industries. While most of the widely used hydrogenation

catalysts usually involved with application of precious metals, which are usually extreme costly

and toxic for the environment. With the recent emergence of green chemistry to reduce the

impact of such chemical processes,9 significant research is focused on replacing the heavy metal

with readily available transition metals, like Fe, Co and Ni, while another alternative is to

promote similar process with main group elements, which are relatively abundant in the Earth’s

crust and are environmental friendly. Recent research has suggested, in some instances, that main

group elements could behave similarly to transition metals.10 One significant example is metal-

free catalytic hydrogenation with frustrated Lewis pairs (FLPs).11, 12

The work outlined in this thesis focused on radical and Lewis acid chemistry for Group 13

compounds, and their application toward FLP chemistry on a fundamental level, which laid the

groundwork for developing main group systems that could replace transition metal catalysts on

an industry level in the future.

2

1.2 Group 13 Compounds

1.2.1 Boron in General

As the only non-metal in Group 13, boron is a unique and exciting element, which shows

similarities to its neighbour, carbon and its diagonal relative silicon in that it often forms

covalent compounds. However, boron also differs sharply from them for its electron deficiency,

due to its one less valance electron. This has a dominant effect on its chemistry. In the industry,

Borax is one of the dominant minerals sources for boron, and was known for the preparation of

glazes and hard glasses in the ancient world.13 The industrial production of amorphous boron

first came from 1892 by Moissan where high purity boron was obtained upon reduction of B2O3

with Mg.13, 14 Nowadays, about half of all boron consumption globally is as an additive in

fiberglass, about 900,000 metric tons in 2015, while other application include in agriculture,

ceramics, borosilicate glass, detergents and so on.14, 15

1.2.2 Boron Chemistry

With its high electron deficiency, boron compounds are commonly used as Lewis acids. While

boron trihalides, particularly BF3 and BCl3, are strong Lewis acids, the well-known Lewis acid

B(C6F5)3 was first reported by the groups of Chambers and Massey in early 1960s.16 However,

one of its most important applications was not revealed until 1990s, where B(C6F5)3 was applied

as an activator for Ziegler-Natta olefin polymerization catalyis.17-21 With its high Lewis acidity,

B(C6F5)3 was used for methyl group abstraction from the dialkyl-metallocene catalyst precursor

Cp2ZrMe2 to form an active alkyl-metallocene cation catalyst (Scheme 1.1).

Scheme 1.1 – Dimethyl zirconocene activation with B(C6F5)3.

Boron chemistry has had remarkable progress in the fields of inorganic and organic chemistry

over the past few decades.22-25 Three Nobel Prizes were awarded in the field of boron chemistry.

In 1976, Lipscomb was first awarded with the prize with his discovery for the bonding nature in

boranes, such as B2H6, diborane(6), and clusters,26 while Brown was awarded for his

3

contribution on the development of versatile organoboranes reagents in organic chemistry, one of

which is NaBH4.27 Finally, in 2010, a Nobel Prize was given to Professor Suzuki for his

outstanding work C-C bond forming cross-coupling reactions with organoboron compounds.28

Other application of boron compounds in FLP and radical chemistry will be discussed in more

details in the later chapters.

1.2.3 Aluminum in General

As the most abundant metal in the Earth’s crust, aluminum (8.3 %, by weight) is only exceeded

in abundance by O (45.5 %) and Si (25.7 %).29 The first impure metal form was isolated by the

Danish chemist Oersted in 1825 from the chemical reduction of AlCl3.30 A commercially process

was later independently developed in 1854 both by Deville and Bunsen from electrolysis of

fused NaAlCl4.29, 31 Despites the difficulty in isolating metallic aluminum at the time, the price of

aluminum metal has dropped dramatically from $1,200 per kg in 1852 to $ 0.16 per kg in 2016

since the discovery for the production process.29, 32 Current production process involves two

stages: (1) the extraction, purification and dehydration of bauxite and (2) the electrolysis of

Al2O3 dissolved in molten cryolite Na3AlF6. In 2018,29, 31 the semi-annual gross domestic

product (GDP) for the Canadian aluminum manufacturing was over $ 27 trillion (CAD).33

Among the aluminum products manufactured, aluminum alloys are widely used in

transportation, construction, electrical and consumer goods.34

1.2.4 Aluminum Chemistry

As with its widely explored application in the industry, aluminum compounds are also well

studied in academia. Herein, a brief summary of some most relevant aluminum chemistry from

the past half century is provided. The most common oxidation state for Al is 3+, and one of the

most famous aluminum compounds is LiAlH4, a versatile reducing and hydrogenating reagent

for both inorganic and organic compounds. Since the discovery of LiAlH4 in 1948, by 1951, the

number of functional groups that were known to react with lithium aluminum hydride was 23

and this rose to more than 60 by 1970s.29, 31 However, due to its highly reactive nature, it has

now generally been replaced by the more selective borohydrides or other organometallic

hydrides in organic transformations. More recently, LiAlH4 has been replaced with several

cheaper and safer organoaluminium hydrides, iBu2AlH, for example, is widely used in

hydroalumination reactions of unsaturated C-C bonds.35 While this reaction proceeds under mild

4

conditions with alkynes, hydroalumination of alkenes usually only proceeds at elevated

temperatures, and catalytic activation seems to be required in some cases. Due to the highly

polarized nature of Al-H bonds, and the electrophilic nature of the Al center, this addition

reaction is usually highly regioselective at carbon. However, when reacted with terminal alkynes,

competitive dehydrocoupling and formation of aluminum alkynides can also happen, due to the

Brønsted acidity of alkynes.36-41

Scheme 1.2 – Possible reaction pathways with organoaluminium hydrides and alkynes.

Another important use of aluminum compounds is as aluminum-based Lewis acids. While

aluminum trihalides are used for Friedel-Crafts reaction, many research groups are also focused

on designing strong novel Lewis acids. While application of Al-based Lewis acids in FLP

chemistry will be discussed in more details in this later chapter, AlX3 (X = Cl, Br, I, C6F5) are

generally strong Lewis acids, some of which can be classified as Lewis superacids since their

Lewis acidity is stronger than the benched mark conversional Lewis acid SbF5 (for X = Br, I, and

C6F5).42, 43 However, these compounds could also suffer from aggregation, hydrolysis, or

explosive nature. As a result many research groups have developed new stable Al-based Lewis

superacids, including the group of Krossing42, 44 and Kӧgel.45 Other lower-valent Al(I) and Al(II)

species is also an interesting topics for many research groups, like Roesky,46-51 Uhl,52-56

Schnӧckel,57-62 Inoue,63, 64 Cowley65, 66 and Aldridge.67

1.3 A Brief Summary of Frustrated Lewis Pair Chemistry

In 1923, Gilbert Lewis classified of electron pair donors and acceptors as bases and acids,

respectively.68 Some anomalous Lewis acid-base combinations are known not to form adducts,69

but the reactive potential of such systems was uncovered in 2006 with the demonstration of

reversible, metal-free hydrogen activation by the species p-(Mes2P)C6F4(B(C6F5)2).12 In this

case, the Lewis base and acid centers are surrounded by bulky substituents that preclude dative

5

bond formation and allow the electron donor and acceptor to act cooperatively to split H2.

Shortly thereafter, this notion was generalized to include simple combinations of Lewis bases

and Lewis acids that are either non-interacting as a result of steric demands or form weak bonds

governed by dissociative equilibria. Such systems, known as “frustrated Lewis pairs” (FLPs),70

led to the development of metal-free hydrogenation catalysis11, 71 for a range of polar and non-

polar unsaturated compounds. Over the past decade, the chemistry of FLPs has been extended to

include the activation of a variety of small molecules including H2, CO2, SO2, N2O, NO, olefins

and alkynes, as well as C-H bonds (Figure 1.1).72-74 Such advances have led to synthetic

applications in organic, polymer and radical chemistry and the concept has also been applied to

uncover new transition-metal, enzymatic and surface chemistry.75

Figure 1.1 – Selected examples of FLP reactions with small molecules.

1.3.1 Radical Chemistry in FLPs

One of the major themes in this thesis focused on radical chemistry involving FLPs. Herein, the

related literature, reported before the discoveries in chapters 3 and 4 is summarized. More recent

developments will be discussed in more detail in the later chapters. In 2011, Erker and co-

workers trapped molecular NO with an ethylene-bridged P/B FLP to yield the persistent radical

Mes2P(CH2)2B(C6F5)2(NO)• (Scheme 1.3),76 intermolecular P/B FLP systems yield different

results.77, 78 This persistent radical generated reacts with toluene, to give

6

Mes2P(CH2)2B(C6F5)2(NOH) and Mes2P(CH2)2B(C6F5)2(NOCH2Ph). Also, related NO radical

FLP species behave as oxygen-centered radicals, which could be applied toward radical

polymerizations of styrene.79 In our own group, FLP N2O adducts derived from R3P (R = tBu,

Mes, napthyl) and two equivalents of Al(C6F5)3 provided a route to the transient frustrated

radical pair [R3P•][(m-O•)(Al(C6F5)3)2] (Scheme 1.3),80 which effects C–H bond activation of

either phosphine substituents or solvent to give salts including [tBu2PMe(C(Me)=CH2][(µ-

OH)(Al(C6F5)3)2] and [(napthyl)3PCH2Ph[(µ-OH)(Al(C6F5)3)2]. The analogous reaction of

Mes3P, Al(C6F5)3•toluene, and N2O in toluene gives the purple salt, [Mes3P•][(µ-

OH)(Al(C6F5)3)2], a rare example of a P(IV) radical cation salt.80 More recently, efforts to reduce

diones with H2 and B(C6F5)3 led to the isolation of boron-derived radicals (Scheme 1.3).81 This

methodology was later applied to o-iminoquinones and α-diimines.82

Scheme 1.3 – FLP-derived radicals and their reactivity.

1.3.2 Recent Advances in FLP Chemistry

While recent discoveries of single electron transfer (SET) mechanisms will be disclosed in later

chapters, other fundamental findings regarding FLP chemistry briefly discussed here. In 2018,

our group discovered that a stable classical Lewis acid-base adduct, (C6F5)3B-

7

P(MeNCH2CH3)3N, which does not exhibit any spectroscopic evidence of dissociation into free

Lewis acid and base, still generates FLP addition products upon mixing with PhNCO, PhCH2N3,

PhNSO and CO2.83 These results illustrated that FLP reactivity extends across the entire

continuum of equilibria governing Lewis acid-base adducts. Soon after that, Erker and co-

workers demonstrated that the Lewis acid-base pair, PCy3/B(C6F5)3, rapidly proceeded to

nucleophilic aromatic substitution reactions in solution, but maintained typical FLP reactivity in

the solid state with activation of various small molecules.84

Scheme 1.4 – Reaction schemes for the selected FLP reactions.

1.4 Carbenes

1.4.1 A Short History of Carbenes

Carbenes are defined as compounds bearing a neutral divalent carbon center with only six

valance electrons, are a fascinating molecular group in chemistry, with the first reported attempts

to synthesis such species appearing as early as 1835.85 Although diazo compounds and ketenes

could generate carbene species in situ,86 the first isolation of a free phosphino(silyl) carbene

came was disclosed in a seminal 1988 publication from Bertrand and co-workers.87 Three years

after, Arduengo et al. isolated the first crystalline N-Heterocyclic carbene (NHC), 1,3-

diadamantylimidazol-2-ylidene (IAd), drawing inspiration from insightful studies by Wanzlick88

and Öfele89 on metal-carbene complexes. While the bulky adamantly (Ad) groups prevented

8

carbene dimerization to form the corresponding olefin, also known as the Wanzlick

equilibrium,90 the N atoms adjacent to the carbene center also helped to exhibit a singlet ground

state electronic configuration with the σ-electron-withdrawing and π-electron-donating abilities.

The cyclic structure also helped to force the carbene carbon into a bent, more sp2 like

arrangement, which also contributes to its singlet ground state.85, 91 Since the pioneering work

from Bertrand and Arduengo, numerous studies detailing the synthesis of various NHCs as well

as new classes of carbenes over the past 30 years have emerged. One of the highly impactful

classes of carbene was again introduced from the Bertrand group in 2005, known as cyclic

(alkyl)(amino) carbenes (cAACs).92 Replacing one of the N atoms by a quaternary carbon

adjacent to the carbene carbon center resulted in an increase of the σ-donating ability as well as

the π-acidity for the carbene center comparing to their NHC analogues. This results in a decrease

in HOMO-LUMO gap of the carbene, which led to the discovery of facile oxidative addition

reactions between cAACs with H2 and NH3.93 Earlier last year, the Bertrand group have pushed

the boundary even further by replacing one of the N atoms with a H atom, leading to the first

crystalline monosubstituted carbene.94

Figure 1.2 – Selected examples of isolable carbenes.

1.4.2 Carbene Stabilized Low Valent Main Group Compounds

Despites the first transition metal-carbene complexes were reported by Chugaev in 1925,95 it was

not until the isolation of free carbenes led to important breakthroughs with NHC-stabilized

transition metal catalysts in organic transformations.85, 96 As this thesis is focused on the main

group compounds, a more detailed summary of transition metal-carbene complexes is not

presented here. It is noteworthy that the widely applied Grubbs’ second-generation catalyst was

9

synthesized from replacing a phosphine ligand with a SIMes carbene from the first-generation

catalyst, resulting a more robust reactivity. 97, 98

On the other hand, benefiting from its steric protection and electronic proprieties, application of

NHCs as ligands in main group chemistry allows for the isolation of many previously unstable

low valent main group compounds over the past few decades.99-104 Selected examples were

presented here (Figure 1.3),105-110 but most of the NHC-stabilized Group 13 compounds are

discussed in the later chapters.

Figure 1.3 – Selected carbene as in stabilized main group compounds.

1.5 Scope of Thesis

The objective for this thesis is to explore the radical and Lewis acid chemistry with Group 13

compounds in FLP chemistry. The projects presented here covers a wide range of synthetic

chemistry. Chapter 2 focus on the unique reactivity of a diborane compound generated from the

reduction of NHC stabilized borenium cation. While chapters 3 and 4 dives into exploring a new

reaction mode in FLP chemistry that involves single electron transfer. Chapter 5 presents the

preparation of some unique NHC-stabilized Al compounds. Lastly, chapter 6 explores the

synthesis of organoaluminium compounds for preparing a new class of intramolecular P/Al

FLPs.

10

The initial synthesis of compound 2-9 in chapter 2 was first accompolished by Dr. Jeffrey

Farrell, while all the other synthetic and computational work was performed by the author. While

the initial discovery of the formation of radicals with the combination of PMes3 and Al(C6F5)3

(in chapter 3) was from Dr. Gabriel Ménard, the synthetic reactions were designed by Dr. Liu

Liu and the author together for chapter 3 and 4. Specially, compounds 3-7, 3-8, 3-9, 4-1a, 4-2b,

4-3, 4-4, 4-5 and 4-6 were synthesized and characterized by Dr. Liu Liu together with the

associated computational work correlate with these compounds. Other isolated compounds

together with the X-ray, CV, UV-Vis, EPR, multinuclear NMR spectrum studies and DFT

calculations are all performed by the author. While everything included in chapter 5 and 6 were

solely performed by the author except that the HPMes2 used for the synthesis of 6-5 in chapter 6

was prepared in collaboration with Mr. James LaFortune. Molecular structure of Mes2P-B(C6F5)2

used in the discussion of chapter 6 were obtained from Mr. James LaFortune. All of the

elemental analyses and high resolution mass spectrometry were performed by ANALEST and

AIMS, respectively, at the University of Toronto.

Portions of each chapter have been published or drafted for publication at the time of writing:

Chapter 2: Cao, L. L.; Stephan, D. W. Organometallics 2017, 36, 3163-3170.

Chapter 3: Liu, L.; Cao, L. L.; Shao, Y.; Ménard G.; Stephan D. W. Chem, 2017, 3, 259-267.

Chapter 4: 1) Liu, L.; Cao, L. L.; Shao, Y.; Stephan, D. W. J. Am. Chem. Soc. 2017, 139,

10062-10071. 2) Cao, L. L.; Bamford, K. L.; Liu, L. L.; Stephan, D. W. Chem. Eur. J. 2018, 24,

3980-3983.

Chapter 5: 1) Cao, L. L.; Daley, E.; Johnstone, T. C.; Stephan, D. W. Chem. Commun. 2016, 52,

5305-5307. 2) Cao, L. L.; Stephan, D. W. Chem. Commun. 2018, 54, 8407-8410.

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18

Chapter 2

2 Reduction of NHC-Stabilized Borenium: Generation of B-B Bonded Diboranes and Boron-Based Radicals

2.1 Introduction

2.1.1 Diborane Compounds

Accessing and breaking C-C bonds has been well established in the literature.1-3 In comparison,

analogous B-B bonds are not as well documented. This can be attributable to the relatively low

bond dissociation energy of homonuclear sp3-sp3 boron-boron bonds (ca. 293 kJ/mol vs. 345

kJ/mol for an analogous C-C bond),4 making it difficult to stabilize such bonds. In recent years, a

variety of strategies have been developed to access such B-B bonded compounds including

haloborane reductions,5-7 dehydrocoupling,8-19 borylene coupling,20-23 hydroborations24 and

nucleophilic substitutions.25-30 The advent and reactivity of diborane species has had a significant

impact on organic31-33 and organometallic chemistry34, 35 and indeed such species have been

widely applied. Conventional diboranes(6) have B2H6 type structures, where the B centers were

bonded through bridging hydrides while no electron precise B-B bonded is included.36 Recent

advance in chemistry has led to the discovery of various electron precise B-B bonded

diborane(6) compounds. To clarify any possible misunderstanding, all diborane compounds

discussed in this chapter will be the electron precise B-B boned diborane compounds.

2.1.2 Syntheses of B-B Bonded Diboranes(6)

Several reports have described the synthesis of base stabilized diboranes and diboranes(6)

(Figure 2.1). The species [(IPr)BH2]2 (2-A) was reported by Robinson et al.37 and was derived

from the reductive coupling of the carbene adduct (IPr)BBr3 in reaction with KC8. Finze and co-

workers38 reported the structurally similar dianion [B2(CN)6]2− (2-B), prepared by reduction of

[FB(CN)3]− with tBuLi or KC8. Dehydrocoupling of a diboron precursor was used to generate

the species [HB(μ-hpp)]2 (2-C; hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidinate),

and a subsequent derivatization afforded the dicationic derivative [(Me2HN)HB(μ-hpp)]22+.39-44

More recently, Fontaine and co-workers45 have demonstrated spontaneous dehydrocoupling from

hydroborane to afford species 2-D. Grigsby and Power46 isolated the dimeric 9H-9-borafluorene

dianion 2-E (Ar = 2,4,6-triisopropylphenyl), while Wagner 47-50 described a radical coupling

19

affording the dianionic species 2-F and species 2-G. Tamao51 prepared the B−B bonded species

2-H via diborane reduction. Braunschweig52 afforded species 2-I from hydroboration of diborane

precursors, while most recently Kinjo and coworkers53 have reported the one-electron oxidation

of organoboron L2PhB: (L = oxazol-2-ylidene) to give the dicationic diborane

[L2PhB−BPhL2][X]2 2-J.

Figure 2.1 – Selected examples of B-B bonded diboranes(6) compounds.

2.1.3 Reactivity of B-B Bonded Diboranes(6)

While the above reports have focused on the synthesis of diboranes, fewer have probed the

subsequent reactivity of these species. Himmel and coworkers11 described the reaction of [HB(µ-

hpp)]2 with disulfides (RSSR) and elemental sulfur (S8) affording [(RS)B(µ-hpp)]2 and [(µ-

S)HB(µ-hpp)]2, respectively. The Braunschweig group has demonstrated the conversion of

carbene-stabilized boron-boron single bonded species to compounds containing double and triple

boron-boron bonds.31, 54 On the other hand, a 2015 report by Finze and co-workers38 showed the

hexacyanodiborane(6) dianion [B2(CN)6]2- to be remarkably water and air stable. In 2018, Kinjo

20

described the reactions of the dicationic diborane(6) 2-J with isonitriles and AuCl at elevated

temperatures to give the isocyano- and chloride-bound boronium cations [L2PhB-X]+ (X = CN or

Cl), respectively.53

2.1.4 Boron-Based Radicals

Figure 2.2 – Selected examples of boron containing radicals.

Given the isoelectronic relationship of triarylborane radical anion [Ar3B•]- and neutral

triarylmethyl radicals (Ar3C•), early studies of boron radicals originated from attempts to prepare

the triaryborane radical anions,55 but the first structurally characterized triaryborane radical anion

[Mes3B•]- was not obtained until 1986 by Power and coworkers.56 Since then, carbene-stabilized

boron-derived radicals have garnered attention, with the use of carbene donors to stabilize these

radicals being particularly attractive due to their π-acidity (Figure 2.2).57 In 2007, Gabbaï

reported the first isolable carbene-supported neutral boryl radical, while Lacôte, Curran and

Walton et al. have extensively studied the generation of transient species [(NHC)BH2•] and

[(NHC)BHR•] and their applications in various organic radical reactions.58-68 In 2014,

21

Braunschweig, Bertrand and Stephan independently prepared cAAC-stabilized boron radicals,

which were then used to prepare the corresponding borylene compounds.69-73 More recently, the

Tamm group has prepared the boron radical ((MeCN(tBu))2CBAr2•, Ar = 4-

trifluoromethylphenyl) stabilized with an electron rich NHC.74

Another strategy to stablize boron-based radicals is the applcation of electric repulsions. In 2013,

the Yamaguchi group has reported the synthesis of [(C6H3(C(CH3)2))3B•]- from the analogus

netural triarylborane.75, 76 Braunschweig then later isolated the first fully boron-centered radical-

cation radical-anion pair [((CHN(iPr))2CB)2•][C4Ph4B(Mes)•].77 Wagner has reported a direct B-

B one electron bonded radical anion [(C10H8B)2C12H8•]-,47 and they concluded the radical was

stablized by the electric repulsion from the negative charge, and with a similar startragy, they

have also isolated [(C10H4B)2CHCH2C(CH3)3•]- from 2-G (Figure 2.1).48 In 2018, the Xie group

in 2018 has prepared a nido-carborane-fused silylene stablized boron-centered radical anion [σ-

B10H10C2(PhC(NtBu)2Si)2BBr•]-.78

2.2 Results and Discussion

2.2.1 Generation of B-B Bonded Diboranes(6)

In recent work, the Stephan group have reported the reaction of (IBn)BH3 2-1a with trityl cation

prompting the subsequent borylations of the pendant benzyl groups and affording the borenium

cationic salt [C2H2(NCH2C6H4)2CB][B(C6F5)4] 2-2a (Scheme 2.1).79 The orientation of the at the

boron center 2-2a relative to the plane of this borenium cation suggested extended possible π-

conjugation and led us to investigate the one electron reduction of 2-2a. Compound 2-2a does

not react with THF as evidenced by NMR spectroscopy. A cyclic voltammogram (CV) of 2-2a

performed in THF using 0.1 M [nBu4N][B(C6F5)4] as an electrolyte, showed a quasi-reversible

one electron reduction wave at -1.55 V vs. the Cp2Fe/Cp2Fe+ couple (Figure 2.15), suggesting

the formation of a new species upon reduction. Based on this reduction potential, 2-2a was

combined with Cp*2Co at room temperature in C6D5Br. After five minutes, the reaction gives

rise to a sharp singlet resonance at -16.8 ppm in the 11B NMR spectrum with the complete

consumption of 2-2a, suggesting the formation of a new species 2-3a containing a four-

coordinate boron center (Scheme 2.1). The 1H NMR spectrum of 2-3a shows two doublets in the

benzylic region in contrast to 2-2a where only a single resonance was attributable to the benzylic

protons. These data are consistent with the formation of a tetrahedral geometry at boron.79

22

Similar results were reproduced in THF, C6H5Cl or 1,2-C6H4Cl2, however, efforts to separate 2-

3a from the reduction byproduct [Cp*2Co][B(C6F5)4] (Figure 2.14) were complicated by the

degradation of 2-3a after a few hours, even at low temperatures (-35 °C and -78 °C).

Scheme 2.1 - Generation and reactivity of 2-3a and 2-3b.

The structure of 2-3a was probed via computational studies using density functional theory

(DFT) methods at the M06-2X/Def2-SVP level of theory.80-82 The singlet state of 2-3a was

computed to be significantly lower in energy than two equivalents of the corresponding

monomeric B-based radical, and the bond dissociation enthalpy was calculated to be 42.9

kcal/mol (M06-2X/Def2-TZVP//M06-2X/Def2-SVP). Attempts to locate the transition state of

the dimerization process of 2-3a were performed from optimizing the structure of 2-3a with

various frozen B-B bond lengths (0.2 Å per step from 2.1 to 3.1 Å) at the (u)M06-2X/Def2-SVP

level (Figure 2.3). These results suggest the generation of 2-3a is a thermodynamically downhill

reaction. The computed structure predicts a B-B bond length of 1.901 Å, which is significantly

longer than most of the known B(sp3)-B(sp3) bond distances, which range from 1.753 to 1.849 Å

(Table 2.1)37, 38, 40, 52, 53

23

Figure 2.3 - Energy plot vs. B-B bond distance of 2-3a (C: grey; N: blue; B: pink).

The 11B NMR chemical shift of 2-3a was computed at the GIAO-B97-2/Def2-TZVP//M06-

2X/Def2-SVP level of theory82, 83 and the value (-14.5 ppm) correlates well with the

experimentally observed value -16.8 ppm in C6D5Br, see Table 2.2). The highest occupied

molecular orbital (HOMO) of 2-3a was primarily derived from the B-B bond, while the lowest

occupied molecular orbital (LUMO) was diffused across the whole molecule with a E(HOMO-

LUMO) = 6.31 eV (Figure 2.4a). The Wiberg bond index (WBI) of the central B-B bonding

orbital from the natural bond orbital (NBO) calculations84, 85 (M06-2X/Def2-TZVP//M06-

2X/Def2-SVP level) was found to be 0.67, and each boron center bares a natural atomic charge

of 0.006, and a bonding NBO between the boron centers was found with an occupancy of 1.65 e-.

Similarly, atom-in-molecules (AIM)86 studies reveal a bond critical point along the B-B bond

path with ρ = 0.098, and ∇2ρ = -0.126, indicating a covalent bonding interaction between the B

centers (Figure 2.4c). All of these results suggested that a diborane(6) species 2-3a that is

24

analogous to the species 2-G and 2-J (Figure 2.1) prepared by Wagner49, 50 and Kinjo,53

respectively.

Table 2.1 - Computed B(sp3)-B(sp3) bond length for known diboranes(6) comparing with 2-3a.

Optimized structures of 2-3b, 2-A, 2-B, 2-C, 2-Ia, & 2-J (Figure 2.1). C: grey. N: blue. B: pink.

O: red.

2-3a 2-3b 2-A 2-B 2-C 2-Ia 2-J

Computed

(Å)

1.90086 1.96675 1.83094 1.79261 1.75308 1.84878 1.85390

X-ray

structure

(Å)

- - 1.828(5) 1.782(4) 1.773(4) 1.838(2) 1.842(5)

Table 2.2 – Isotropic shift for 2-2a and 2-3a and 2-3b of the boron center.

δ (absolute)comp δ (ref)comp δ (C6D5Br)expt

2-2a -67.74 40.2 40.2

2-3a -122.46 -14.51 -16.8

2-3b -121.46 -13.51 -14.6

25

Figure 2.4 - (a) Plot of frontier molecular orbitals for the 2-3a; (b) NBOs for B-B bonding. (c)

Plot of the gradient vector field of ρ(r) and the contour map of the Laplacian of the electron

density, ∇ρ(r), of 2-3a.

The benzimidazolium analog of 2-2b was prepared following an analogous synthetic protocol

(Scheme 2.1). The precursor carbene adduct 2-1b was converted to the borenium cation

[C8H4(NCH2C6H4)2CB][B(C6F5)4] 2-2b. The formulations of these species were confirmed

spectroscopically and with X-ray crystallography data (Figure 2.5). Treatment of 2-2b with

Cp*2Co proceeded to give rise to a sharp singlet resonance at -14.9 ppm in the 11B NMR

spectrum with the complete consumption of 2-2b, consistent with the formation of 2-3b,

containing a four-coordinate boron center (Scheme 2.1).

26

Figure 2.5 – ORTEPs of 2-1b (left) and the cation of 2-2b (right) with thermal displacement

parameters drawn at 50 % probability. C: black; N: blue; B: yellow-green, Hydrogen atoms are

omitted for clarity except for the boron bound ones.

Figure 2.6 – 11B{1H} NMR spectra for the exchange reaction of generating compound 2-3a, 2-

3b and 2-3c.

27

While compound 2-3b exhibited similar sensitivity to 2-3a and was difficult to crystallized, its

synthesis permitted a cross-over experiment to be performed. To this end, combination of 2-3a

and 2-3b were monitored by 11B{1H} NMR spectroscopy. The mixture revealed resonances

attributable to both 2-3a and 2-3b as a well resonance consistent with the formation of the mixed

species 2-3c (Scheme 2.1). A similar spectrum was obtained from the reaction of a 1:1 mixture

of 2-2a and 2-2b with Cp*2Co (Figure 2.6). These observations with the support of the

computational results (Table 2.2) suggested the generation of 2-3a with a weak B(sp3)-B(sp3)

bond.

2.2.2 Reactivity of Diborane

Due to sensitive nature of 2-3a, reactivity studies were performed in situ by adding a substrate 15

minutes after addition of Cp*2Co to 2-2a. In this fashion, TEMPO was used initially (Scheme

2.2). The reaction mixture features a single new resonance at -6.5 ppm in the 11B NMR spectrum.

Following work-up, the product 2-4 was isolated as a white solid in 95 % yield. Recrystallization

afforded X-ray quality crystals allowing the connectivity of 2-4 to be confirmed as

[C2H2(NCH2C6H4)2CB(ONC5H6Me4)] (Figure 2.7), in which the B center is now four coordinate

via formation a new B-O bond (1.500(4) Å). The geometry about B is distorted in a pseudo-

tetrahedral, with B-CNHC and B-CPh distances of 1.609(4) Å, 1.651(4) Å, and 1.629(5) Å and C-

B-C angles of 118.9(3)°, 100.9(2)°, and 104.4(2).

28

Scheme 2.2 - Reactivity of 2-3a.

In a similar fashion, 2-3a generated in situ was allowed to react with one equivalent of benzoyl

peroxide, (PhC(O)O)2 (Scheme 2.2). The resulting species 2-5 affords a single resonance at -9.2

ppm in the 11B NMR spectrum. Following workup, this species was isolated as a white powder in

93 % yield. Crystals suitable for X-ray crystallography were grown from a saturated toluene

solution of 2-5 layered with pentane, and the collected data confirmed that 2-5 is

[C2H2(NCH2C6H4)2CB(O(O)CPh)] (Figure 2.7). The structure was analogous to that described

for 2-4 with a B-O bond length in 2-5 of 1.547(5) Å.

29

Figure 2.7 – ORTEPs of 2-4 (left) and 2-5 (right) with thermal displacement parameters drawn

at 50 % probability. C: black; N: blue; B: yellow-green, O: red. Hydrogen atoms are omitted for

clarity.

Compound 2-3a reacts with 2 equivalents of PhNO to yield the paramagnetic species 2-6

(Scheme 2.2). The product gave rise to an EPR spectrum and was completely NMR silent.

Simulation of the EPR spectrum afforded a g value of 2.0012 and hyperfine couplings of 3.6 G

to B, 10.0 and 1.0 G to N and 1.2 to 2.9 G to H atoms (Figure 2.8 and Table 2.3). Spin density

and Fermi contact coupling constants computed at the uM06-2X/Def2-TZVP//uM06-2X/Def2-

SVP level of theory were consistent with experimental data and confirmed thatthe unpaired

electron density is primarily located on the PhNO fragment. These data are thus consistent with

the formation of the nitrogen-centered radical [C2H2(NCH2C6H4)2CB(PhNO)•] 2-6 suggesting B-

N coordination although this could not be unambiguously confirmed despite repeated attempt to

crystalize 2-6 (Scheme 2.2).

30

Figure 2.8 – (a) Computed spin density of 2-6, (b) Simulated and experimental EPR spectra for

2-6.

Compound 2-3a also reacts with 2 equivalents of (tht)AuCl to generate a black solid in

suspension in C6H5Cl (Scheme 2.2). After filtering and subsequent work-up the product 2-7 was

ultimately isolated as a white solid in 81 % yield. The resulting product gives a sharp singlet at -

10.9 ppm in the 11B NMR spectrum. Following slow evaporation of the solvent, colorless

crystals of 2-7 were obtained. X-ray diffraction study confirmed the formulation of the product

to be [C2H2(NCH2C6H4)2CBCl] (Figure 2.9). The geometry about B is again pseudo-tetrahedral

with a B-Cl bond distance of 1.959(2) Å. These data infer chlorination of the diborane proceeds

with the concurrent reduction of Au(I), a strategy also explored by Kinjo and coworkers.53 In

their previous work, chlorination of the dication affords elemental Au and the boronium cation

[L2PhBCl]+.

31

Table 2.3 – Hyperfine coupling parameters for 2-6 and 2-6’.

Atoms Hyperfine

Coupling

(G)

Number of

atoms used of

simulation

Calculated Fermi

Contact Coupling (G)

2-6

Calculated Fermi

Contact Coupling (G)

2-6’

B (1) 3.58 1 -4.96 -2.08

N (4) 9.95 1 10.58 14.81

N (2 & 3) 0.95 2 -0.01 & -0.06 -0.02 & -0.01

H (5) 2.80 1 -2.60 -3.89

H (6) 1.21 1 0.81 1.28

H (7) 3.03 1 -2.62 -4.35

H (8) 1.21 1 0.80 1.27

H (9) 2.90 1 -2.60 -4.25

The corresponding reaction of 2-3a with excess S8 prompted the formation of a species 2-8

(Scheme 2.2) that was identified by a new and sharp signal in the 11B NMR spectrum at -13.4

ppm. In the 1H NMR spectrum, two multiplet resonances arise in the benzylic region, a spectral

pattern which differs from those seen for 2-4, 2-5, and 2-7. The formulation of 2-8 was

confirmed by X-ray diffraction. In the solid state structure, the molecule 2-8 sits on a

crystallographic two-fold axis with the asymmetric unit being comprised of a chelated-boron

center linked to an S2 unit. The overall molecular structure is comprised of four sulfur atoms in a

chain linking the two boron atoms and thus a molecular formulation for 2-8 of

[(C2H2(NCH2C6H4)2CB)2(S4)] (Figure 2.9). The B-S distance was determined to be 2.016(5) Å,

significantly longer than the B-S bonds (1.886(2) Å, 1.894(2) Å) reported in [(µ-S)HB(µ-

32

hpp)]211, presumably a reflection of the more electron-rich nature of the boron center in 2-8. The

S-S distances were found to be 2.077(2) Å and 2.078(3) Å with a B-S-S angle of 105.4(2)°.


50 % probability. C: black; N: blue; B: yellow-green, Cl: green. S: yellow Hydrogen atoms are

omitted for clarity.

The low computed B-B bond dissociation energy of 2-3a together with the reactivity described

herein, are consistent with a series of reactions that proceed via homolytic cleavage of the B-B

bond in 2-3a. Indeed, the reactivity of 2-3a with (PhC(O)O)2 is similar to that described for

boron radicals in reports by the groups of Braunschweig87 and Yamashita.88 In some sense, this

B-B reactivity is reminiscent of the dissociation of hexaphenylethane generating trityl radical as

first described in the seminal work of Gomberg.89 While the generation and characterization of

B-based radicals have prompted numerous studies,52, 87, 88, 90-97 the present report is, to the best of

our knowledge, the first to describe reactivity consistent with the generation of a transient radical

by homolytic cleavage of a B-B bond.

33

2.2.3 Generating a Boron-Based Radical

With the successful generation of 2-3a, and its radical-like reactivity, we started to focus on if we

could generate a stable boron radical based on the planar framework of 2-2a. DFT studies at

uM06-2X/Def2-TZVP//uM06-2X/6-31G of the proposed [(C2H2(NCH2C6H4)2CB)•] radical

reveals that most of the spin density is delocalized through the molecule. However, a significant

spin density was found at the benzylic protons (Figure 2.10), which could lead to the

decomposition of the radical compounds, As previous shown by Tamm’s report with NHC-

stabilized boron radical.74 Thus, we focused on the modification of 2-2a at the benzylic position

to probe possibility of stabilizing a planar boron-based radical precursor.

Figure 2.10 – Computed spin density plot of the [(C2H2(NCH2C6H4)2CB)•] radical.

The cation 2-2a was reacted with one equivalent of the FLP, tBu3P/B(C6F5)3. This leads to the

isolation of a new species 2-9 in 47 % yield. Monitoring the reaction by 31P NMR spectra

revealed the consumption of the phosphine and the formation of the corresponding phosphonium

cation, [tBu3PH]+. This infers the deprotonation of 2-2a. 11B NMR spectra of 2-9 shows two

resonances at -9.6 ppm and -14.6 ppm. These signals indicate the presence of two distinct four-

coordinate boron centers. The formulation of 2-9 was then confirmed as the neutral species, rac-

34

[trans-C2H2(NCH(C2H2(NCH2C6H4)2CB)C6H4)(NCH(B(C6F5)3)C6H4)CB], by X-ray

crystallography (Scheme 2.3, Figure 2.11).

Scheme 2.3 – Synthesis of compounds 2-9, 2-10 and 2-11.

The structure of 2-9 is comprised of a central borenium cation, in which the benzylic CH2 groups

are singly deprotonated. The BC3 plane is approximately planar with the sum of the angles about

B being 360 °. The geometry about the central B center in 2-9 is similar to that previously

reported for 2-2a and 2-2b. One of these CH centers is bonded to a second borenium fragment

forming a C-B bond, affording an NHC-borane adduct. The second CH fragment is bonded to

B(C6F5)3 yielding a borate anion. The resulting allyl borate in combination with the central

35

borenium cation result in an overall neutral species. The new C-B bonds adopt axial positions on

either side of the plane of the central borenium cation. The new C-B bond in the NHC-borane

adduct fragment was found to be 1.707(8) Å, while the other new C-B bond in the borate anion is

1.720(8) Å. The structure of 2-9 is reminiscent of the previously reported for cation [trans-

C2H2(NCH(C2H2(NCH2C6H4)2CB)C6H4)2CB][B(C6F5)4].79 This “triple-decker“ species was

generated by reaction with tBu3P which prompts deprotonation and oligomerization. In the case

of 2-9, it appears that deprotonation of a benzylic carbon of 2-3a also occurs but preferential

binding of B(C6F5)3 proceeds, presumably reflecting both steric and electronic factors.

Figure 2.11 - ORTEPs of 2-9 (left) and 2-10 (right) with thermal displacement parameters drawn

at 50 % probability. H atoms and the cation [tBu3PH]+ of 2-10 are omitted for clarity. C: black,

F: pink, B: yellow-green, N: blue.

Further exploring the reaction of 2-2a with the FLP, tBu3P/B(C6F5)3, the stoichiometry was

adjusted to a ratio of 1:2 of 2-2a and the FLP. In this case, the reaction proceeded in a similar

fashion to that affording 2-10 with all of the phosphine being converted to phosphonium cation

[tBu3PH]+ as evidenced by 31P NMR spectroscopy. The product 2-10 was isolated in 77 % yield

and the corresponding 11B NMR spectrum showed a resonance at -9.8 ppm. 1H NMR data

showed a single resonance attributable to the methine protons on the benzylic carbons. These

data suggested a formulation of 2-10 as [tBu3PH][trans-C2H2(NCH(B(C6F5)3)C6H4)2CB]

(Scheme 2.3). This was subsequently confirmed by an X-ray diffraction study (Figure 2.11),

which showed the anion to be pseudo two-fold symmetric, with anionic borate fragments

disposed on either side of the borenium cation fragment, In 2-10, the newly generated B-C bond

distances were found to be 1.714(2) Å and 1.716(3) Å. Similar to 2-9, the coordination sphere

about the central boron atom approaches a pseudo trigonal planar geometry.

36

Figure 2.12 – Computed HOMO (left), LUMO (middle), and spin density (right) plots for 2-10.

DFT studies at the (u)M06-2X/Def2-TZVP//(u)M06-2X/6-31G level reveals that the HOMO in

2-10 is delocalized on the aromatic plane, and the BBCF-C bonds, whereas the LUMO was

delocalized through the NHC and the boron center. Computational modeling of the

corresponding radical dianion [C2H2(NCH(B(C6F5)3)C6H4)2CB•]2- 2-11 (Scheme 2.3), suggesting

the spin density of 2-11 is delocalized on the plane with 60.8 % at the boron center (Figure 2.12).

Initial screening of the redox properties of the boron precursors were probed via cyclic

voltammetry (CV). While 2-9 reveals no evidence of reversible reductions (Figure 2.16), the CV

of 2-10 shows two features: an irreversible reduction at a potential of -1.61 V vs.

ferrocene/ferrocenium and a reversible wave centered at -2.23 V with a peak-to-peak separation

of 80 mV (Figure 2.17). The former feature was attributable to the irreversible reduction of the

phosphonium cation, producing H2 and phosphine. This was subsequently confirmed by an

independent electrochemical study of [tBu3PH][B(C6F5)4]. The reversible feature in the CV of 2-

10 was thus attributed to the reduction of the anion to the radical dianion 2-11. Given this

evidence, efforts to chemically reduce 2-10 with KC8 were undertaken. Monitoring of the

reaction mixture by 31P NMR spectroscopy confirmed the reduction of the phosphonium cation

to phosphine. Monitoring of the reaction mixture by EPR spectroscopy revealed the generation

of strong resonances centered at a g value of 2.0027 (Figure 2.13). However, attempts to obtain a

solid structure of 2-11 were unsuccessful.

37

Figure 2.13 – EPR spectrum of 2-11 in THF.

2.3 Conclusions

In conclusion, reduction of 2-2a and 2-2b generated diborane 2-3a and 2-3b. Weak B-B bonds

were generated which allowed exchange reactions to make the asymmetric diborane 2-3c. 2-3a

reacts with TEMPO and benzoyl peroxide to give the diamagnetic species 2-4 and 2-5,

respectively. Reaction with PhNO yields the radical 2-6, with the unpaired spin largely centered

at PhNO fragment. Reaction with (tht)AuCl affords the haloborane 2-7 and Au metal, while the

reaction with S8 gives 2-8, a species in which two boron centers are linked by an S4 unit. These

reactions provide rare illustrations of homolytic B-B bond cleavage. Efforts to generate a stable

planar boron-containing radical were carried out and two precursors 2-9 and 2-10 were produced.

The dianionic planar boron-containing radical 2-11 was generated based on CV and EPR studies.

2.4 Experimental Details

2.4.1 General Considerations

All manipulations were performed under an atmosphere of dry, oxygen-free N2 by means of

standard Schlenk or glovebox techniques (Innovative Technology glovebox equipped with a -35

°C freezer). Toluene, DCM, and pentane were collected from a Grubbs-type column system

manufactured by Innovative Technology. DCM, pentane, bromobenzene, o-dichlorobenzene

38

(DCB), and toluene were stored over 4 Å molecular sieves. Molecular sieves, type 4 Å (pellets,

3.2 mm diameter) purchased from Sigma Aldrich were activated prior to usage by iteratively

heating with a 1050 W Haier microwave for 5 minutes and cooling under vacuum. The process

was repeated until no further moisture was released upon heating. C6D6 and C6D5Br, purchased

from Cambridge Isotope Laboratories, were degassed and stored over 4 Å molecular sieves in

the glovebox for at least 8 h prior to use. THF was dried by sodium-benzophenone overnight

followed by distillation. CDCl3, purchased from Cambridge Isotope Laboratories, and C6H5Cl

were dried by stirring over CaH2 for several days followed by distillation. Unless otherwise

mentioned, chemicals were purchased from Sigma Aldrich, Strem Chemical or TCI, and used

without further purification. 1,3-Dibenzylimidazolium bromide and 1,3-dibenzyl-1H-

benzo[d]imidazole-3-ium bromide were prepared using literature methods.79 NMR Spectra were

recorded on a Bruker Avance III 400 MHz, Agilent DD2 500 MHz or an Agilent DD2 600 MHz

spectrometer and spectra were referenced to residual solvent in C6D5Br (1H = 7.28 ppm for meta

proton), C6D6 (1H = 7.16 ppm; 13C = 128.06 ppm), CDCl3 (

1H = 7.26 ppm; 13C = 77.16 ppm) or

externally (11B: BF3·Et2O, 19F: CFCl3). Chemical shifts (δ) are reported in ppm and the absolute

values of the coupling constants (J) are in Hz. In some instances, signal and/or coupling

assignment was derived from 2D NMR experiments. A Fisher Scientific Centrific Model 228

(Fixed speed: 3000 rpm) was used for centrifugation. Electron paramagnetic resonance (EPR)

measurements were performed at 298 K using a Bruker ECS-EMX X band EPR spectrometer

equipped with an EP4119HS cavity. Simulations were performed using PEST WinSIM

Software. Mass spectra were conducted using a Bruker Autoflex MALDI-TOF instrument while

high-resolution mass spectrometry was obtained using a JEOL Accu TOF modelJMS-T1000LC

instrument (with DART ion source). Compounds 2-1a and 2-2a were prepared as previously

reported.79 In some cases, elemental analysis was reproducibly low on carbon content, while

providing satisfactory H and N values. This phenomenon is attributed to the formation of boron

carbides during combustion and has been previously reported for other boron compounds. In the

case of compounds 2-8, the fragility of these compounds results in the observation of the B-

cation in the HR-MS data.

39

2.4.2 Syntheses and Characterizations

Synthesis of (BIBn)BH3 (2-1b)

Following the same procedure as 2-1a, with 1,3-dibenzyl-1H-benzo[d]imidazole-3-ium bromide

(1.503 g 3.96 mmol), 2-1b was obtained as a white solid in 35 % yield (430 mg) after

recrystallization from cold toluene/pentane (v:v = 1:1) at -35 °C, which is suitable for single

crystal X-ray diffraction study.

1H NMR (400 MHz, C6D6, 298K): δ 7.22 (m, 4H, Ph-H), 6.98 (m, 6H, Ph-H), 6.75 (m, 4H,

NCPh-H), 5.44 (s, 4H, Ph-CH2), 2.33 (q, 1JBH = 85 Hz, 3H, BH3). 13C{1H} NMR (126 MHz,

C6D6, 298K): δ 135.9, 132.6, 128.9, 128.6, 128.4, 128.2, 125.3, 123.2, 111.2, 49.2 (Ph-CH2). 11B

NMR (128 MHz, C6D6, 298K): δ -35.1 (q, 1JBH = 85 Hz). Anal. Calcd. For C21H21BN2: C 80.79

%, H 6.78 %, N 8.97 %. Found: C 80.78 %, H 6.86 %, N 8.64 %.

Synthesis of C6H4(NCH2C6H4)2CB][B(C6F5)4] (2-2b)

In an inert atmosphere glovebox, 2-1b (350.3 mg, 1.12 mmol) was weighed into a thick-walled

bomb charged with a magnetic stirrer. The compound was dissolved in 10 mL of C6H5Cl and a

solution of trityl tetrakis(pentafluorophenyl)borate (1.035 g, 1.12 mmol) in 10 mL of C6H5Cl was

added dropwise while stirring. The mixture was stirred for 30 minutes over which gas evolution

subsides. The bomb was sealed and stirred at 130 ºC for 26 hours. After cooling the reaction,

volatiles were removed in vacuo. The residue was dissolved in DCM, filtered through Celite.

After removing the volatiles in vacuo, toluene (3 × 5 mL) and pentane (3 × 5 mL) were used to

wash the residue. The residue was dissolved in DCM and recrystallized from laying with pentane

to give crystals that were washed with toluene (3 × 5 mL) and pentane (3 × 5 mL) and dried in

vacuo to give a colorless crystal (836.7 mg, 76 %).

1H NMR (500 MHz, C6D5Br, 298 K, toluene omitted): δ 8.51, (d, 3JHH = 7.5 Hz, 2H), 7.57 (t,

3JHH = 7.5 Hz, 2H), 7.47 - 7.41 (m, 4H), 7.37, (dt , 3JHH = 3.6, 6.5 Hz, 2H), 7.15 (t, 3JHH = 7.5 Hz,

2H), 5.17 (s, 4H). 11B NMR (128 MHz, C6D5Br, 298 K): δ 45.2 (s, br), -16.1 (s). 13C{1H} NMR

(126 MHz, CDCl3, 298 K, partial): δ 142.6, 138.0, 137.6, 136.3, 132.4, 129.5, 129.1, 129.0,

128.2, 127.7, 125.3, 113.2, 48.7. 19F NMR (376 MHz, C6D5Br, 298 K): δ -131.6 (s, br, 8F, o-

40

C6F5), -162.0 (t, 3JFF = 20 Hz, 4F, p-C6F5), -165.9 (t, br, 3JFF = 19 Hz, 8F, m-C6F5). Anal. Calcd.

For C52H24B2F20N2: C 54.92 %, H 2.24 %, N 2.60 %. Found: C 58.13 %, H 2.55 %, N 2.16 %.

Generation of [C2H2(NCH2C6H4)2CB]2 (2-3a) and [C8H4(NCH2C6H4)2CB]2 (2-3b)

These compounds were prepared in a similar fashion. In an inert atmosphere, a solution of 2-2a

or 2-2b (10 mg, 0.010 mmol) in 0.5 mL of C6D5Br was added into a vial with Cp*2Co (3 mg,

0.01 mmol) at room temperature. The reaction mixture turned from green to brown/yellow color

at room temperature, and solids would crash out from the solution after 10-30 minutes. NMR

spectra of 2-3a and 2-3b was taken 10 minutes after the reaction has started, while crystal of the

[Cp*2Co][B(C6F5)4] were obtained from the crashed out solid redissolved in C6H5Cl and layered

with pentane at room temperature.

2-3a: 1H NMR (400 MHz, C6D5Br, 298 K, toluene and [Cp*2Co][B(C6F5)4] omitted): δ 7.62 (s,

br, 4H, PhH), 7.16-7.00 (m, 8H, PhH), 6.69 (s, br, 4H, PhH), 7.26 (s, 4H, NCH), 4.24 (d, 2JHH =

15.6 Hz, 4H, CHaHb) 3.11(d, 2JHH = 15.6 Hz, 4H, CHaHb). 11B NMR (128 MHz, C6D5Br, 298

K): -16.8 (s). HRMS(DART): m/z [M-H]+: calcd for [C34H27B2N4]+ 513.2422, measured

513.2433.

2-3b: 1H NMR (400 MHz, C6D5Br, 298 K, toluene and [Cp*2Co][B(C6F5)4] omitted): δ 7.62 (s,

br, 4H, PhH), 7.16-7.00 (m, 24H, PhH), 4.50 (d, 2JHH = 15.7 Hz, 4H, CHaHb) 3.34(d, 2JHH =

15.7 Hz, 4H, CHaHb ). 11B NMR (128 MHz, C6D5Br, 298 K): -14.9 (s). HRMS (ESI): m/z [M-

H]+: calcd for [C42H31N4B2]+ 613.2735, measured 613.2741.

41

Figure 2.14 - ORTEP of [Cp*2Co][B(C6F5)4] with thermal displacement parameters drawn at 50

% probability. C: black; F: Pink; B: yellow-green, Co: green. Hydrogen atoms are omitted for

clarity.

Synthesis of [C2H2(NCH2C6H4)2CB(ONC5H6Me4)] (2-4)

Following the procedure for generation of 2-3a (102 mg of 2-2a was used, reaction performed in

4 mL of THF), 10 minutes after the reaction started, TEMPO (16 mg, 0.10 mmol) was added to

the reaction mixture. The reaction mixture was allowed to stir for 1 h and the volatiles were

removed under vacuo. The desired product was extracted from toluene solution (3 × 2 mL), and

filtered through Celite. After dried under vacuo, the residue was washed with pentane (3 × 2 mL)

giving an off white solid (40 mg, 97 %). Single crystals for X-ray studies were obtained from

slow evaporation of a saturated benzene solution to give colorless crystals.

1H NMR (400 MHz, C6D6, 298 K toluene omitted): δ 8.54 (d, 3JHH = 7.5 Hz, 2H, PhH), 7.37 (t,

3JHH = 7.5 Hz, 2H, PhH), 7.19 (t, 3JHH = 7.5 Hz, 2H, PhH), 6.94 (d, 3JHH = 7.5 Hz, 2H, PhH),

5.79 (s, 2H, NCH), 5.34 (d, 2JHH = 15.6 Hz, 2H, NCH2Ph), 4.09 (d, 2JHH = 15.6 Hz, 2H,

NCH2Ph), 1.47-1.35 (m, 4H, CH2), 1.19 (s, 6H, CH3), 0.87 (d, 3JHH = 7.2 Hz, 2H, CH2), 0.72 (s,

CH3, 6H). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ 138.0, 135.7, 129.3, 128.6, 128.4, 128.3,

128.2, 128.1, 128.0, 126.9, 125.7, 125.2, 125.1, 59.6 (NCH), 52.3 (NC(CH3)2), 40.8, 34.7, 20.6,

18.2. 11B NMR (128 MHz, C6D6, 298 K): δ -9.3(s). HRMS (DART): m/z [M+H]+: calcd for

[C26H33BN3O]+ 414.2717, measured 414.2713.

42

Synthesis of [C2H2(NCH2C6H4)2CB(O(O)CPh)] (2-5)


4 mL of C6H5Cl), 10 minutes after the reaction started, (PhC(O)O)2 (12 mg, 0.05 mmol) was

added to the reaction mixture. The reaction mixture was al-lowed to stir for 1 h and volatiles

were removed under vacuo. The desired product was extracted with benzene (3 × 2 mL), and

filtered through glass fiber. After dried under vacuo, the residue was washed with pentane (3 × 2

mL) giving an off white solid (35 mg, 93 %). Single crystals for X-ray studies were obtained

from layering a saturated toluene solution with pentane to give colorless crystals.

1H NMR (400 MHz, C6D5Br, 298 K): δ 8.39 (d, 3JHH = 11.3 Hz, 2H, PhH), 7.95 (d, 3JHH = 11.3

Hz, 2H, PhH), 7.31 (m, 3H, PhH), 7.15 (m, 3H, PhH), 7.02 (m, 3H, PhH), 6.36 (s, 2H, NCH),

5.23 (d, 2JHH = 18.4 Hz, 2H, NCH2Ph), 4.48 (d, 2JHH = 18.4 Hz, 2H, NCH2Ph). 13C{1H} NMR

(126 MHz, C6D5Br, 298 K, partial): δ 138.1, 133.8, 127.6, 127.1, 118.3, 51.2 (NC(CH3)2), 40.9,

34.6, 20.7, 18.3. 11B NMR (128 MHz, C6D5Br, 298 K): δ -9.2 (s). LRMS (MALDI): m/z [M]+:

calcd for [C24H19BN2O2]+ 378.15, measured 378.38; HRMS (DART): m/z [2-2a]+: calcd for

[C17H14BN2]+ 257.1250, measured 257.1248.

Generation of [C2H2(NCH2C6H4)2CB(ONPh)] (2-6)

Following the procedure for generation of 2-3a (10 mg of 2-2a were used), after 10 min, the

solution was filtered through Celite, and nitrosobenzene (2 mg, 0.01 mmol) was added. The

reaction mixture turned yellow immediately, and the EPR spectrum of 2-6 was recorded. g =

2.0012, A(11B) = 3.6 G, A(14N) = 10.0, 1.0 G, A(1H) = 2.8. 1.2. 3.0, 1.2, 2.9 G. HRMS (DART):

m/z [2-2a]+: calcd for [C17H14BN2]+ 257.1250, measured 257.1248.

Synthesis of [C2H2(NCH2C6H4)2CBCl] (2-7)


4 mL of THF), 10 minutes after the reaction started, (tht)AuCl (32 mg, 0.10 mmol) was added to

the reaction mixture. The reaction mixture was allowed to stir for 1 h and volatiles was removed

under vacuo. The desired product was extracted with toluene (3 × 2 mL), and filtered through

glass fiber. After dried under vacuo, the residue was washed with pentane (3 × 2 mL) giving a

43

colorless solid (24 mg, 81 %). Single crystals for X-ray studies were obtained from layering a

saturated toluene solution with pentane to give colorless crystals.

1H NMR (400 MHz, CDCl3, 298 K): δ 8.25 (d, 3JHH = 8.4 Hz, 2H, PhH), 7.39 (dt, 3JHH = 7.8 Hz,

4JHH = 2.3 Hz, 2H, PhH), 7.24 (d, 3JHH = 1.4 Hz, 1H, PhH), 7.22 (d, 3JHH = 1.4 Hz, 1H, PhH),

7.21 (d, 3JHH = 2.4 Hz, 1H, PhH), 7.19 (br, 1H, PhH), 7.17 (s, 2H, NCH), 5.57 (d, 2JHH = 16.3

Hz, 2H, NCH2Ph), 5.11 (d, 2JHH = 16.3 Hz, 2H, NCH2Ph). 13C{1H} NMR (126 MHz, CDCl3,

298 K): δ 133.0, 127.6, 126.2, 125.8, 119.5, 51.4 (NC(CH3)2). 11B NMR (128 MHz, CDCl3, 298

K): δ -10.9 (s). HRMS (DART): m/z [2-2a]+: calcd for [C17H14BN2]+ 257.1250, measured

257.1249.

Synthesis of [(C2H2(NCH2C6H4)2CB)2(S4)] (2-8)


8 mL of C6H5Cl), 10 minutes after the reaction started, S8 (26 mg, 0.10 mmol) was added to the

reaction mixture. The reaction mixture was allowed to stir for 1 h and volatiles were removed

under vacuo. The desired product was extracted with benzene (3 × 2 mL), and filtered through

glass filter. After dried under vacuo, the residue was washed with pentane (3 × 2 mL) giving a

light yellow solid (40 mg, 63 %). Single crystals for X-ray studies were obtained from slow

evaporation of a pentane into a saturated DCM solution of 2-8 at -35 °C to give light yellow

crystals.

1H NMR (400 MHz, tol-d8, 298 K): δ 8.46 (m, 4H, PhH), 7.34 (m, 4H, PhH), 7.20 (m, 4H, PhH),

6.88 (m, 4H, PhH), 5.96 (m, 4H, -NCH-), 5.20 (m, 4H, -N-CH2-Ph), 4.20 (m, 4H, -N-CH2-Ph).

13C{1H} NMR (126 MHz, tol-d8, 298 K, partial): δ 133.8, 133.7, 118.7, 118.1, 50.9 (-

NC(CH3)2). 11B NMR (128 MHz, tol-d8, 298 K): δ -14.0 (s). LRMS (MALDI): m/z [M]+: calcd

for [C34H28BN4S4]+ 642.14, measured 642.33; HRMS (DART): m/z [2-2a]+: calcd for

[C17H14BN2]+ 257.1250, measured 257.1247.

Synthesis of [trans-C3H2N2(C7H5)2B(C3H2N2(C7H5)2B)(B(C6F5)3)] (2-9)

A solution of 2-2a·0.55(C7H8) (206 mg, 0.200 mmol) in 1 mL of C6H5Br was added to a mixture

of tBu3P (41 mg, 0.20 mmol) with B(C6F5)3 (51 mg, 0.10 mmol) in 2 mL of toluene. The

combined solution was left at room temperature for 16 h. Insoluble yellow crystals, combined

44

with colorless crystals of [tBu3PH][B(C6F5)4], were formed from the solution. The solvent was

decanted. The crystals were washed with cold C6H5Br (3 × 0.5 mL) and pentane (3 × 0.5 mL),

and were dried in vacuo. 2-9 was collected in 201 mg, but [tBu3PH][B(C6F5)4] crystallized with

2-9 could not be separated out (47 %, based on the 19F NMR the formula was determined as 2-

9·1.25[tBu3PH][B(C6F5)4]). Single crystals for X-ray studies were obtained by layering a

saturated solution in o-C6H4Cl2 with pentane to give yellow crystals.

1H NMR (600 MHz, CDCl3, 298K): δ 8.35 (d, 3JHH = 7.5 Hz, 2H, PhH), 8.29 (dd, 3JHH = 1.6, 7.6

Hz, 2H, PhH), 7.94 (d, 3JHH = 1.8 Hz, 2H, -NCH-), 7.87 (d, 3JHH = 8.0 Hz, 2H, PhH), 7.77 (td,

3JHH = 1.4, 7.6 Hz, 2H, PhH), 7.66 (t, 3JHH = 7.5 Hz, 2H, PhH), 7.54 – 7.48 (m, 5H, PhH & -CH-

), 7.40 (m, 5H, PhH & -CH-) 5.58 - 5.47 (m, 4H, -CH2-). 13C{1H} NMR (126 MHz, CDCl3,

298K): δ 141.3, 136.7, 135.8, 134.4, 133.5, 131.7, 130.2, 129.2, 128.8, 128.4, 127.1, 127.0,

126.4, 125.5, 122.7, 121.1, 50.8. 11B NMR (128 MHz, CDCl3, 298K): δ -9.6 (s), -14.6 (s). 19F

NMR (377 MHz, CDCl3, 298K, [B(C6F5)4]- omitted): δ -132.9 (dd, 3JFF = 24, 52 Hz, 1F, o-C6F5),

-133.1 (d, 3JFF =24 Hz, 1F, o-C6F5), -133.9 (dd, 3JFF = 24, 46 Hz, 1F, o-C6F5), -135.0 (m, 1F, o-

C6F5), -136.6 (dd, 3JFF = 24, 52 Hz, 1F, o-C6F5), -141.3 (d, 3JFF = 24 Hz, 1F, o-C6F5), -167.5 (t,

3JFF = 21 Hz, 1F, p-C6F5), -168.0 (t, 3JFF = 21 Hz, 1F, p-C6F5), -168.4 (t, 3JFF =21 Hz, 1F, p-

C6F5), -171.1 (t, 3JFF = 24 Hz, 1F, m-C6F5), -171.6 (t, 3JFF = 21 Hz, 1F, m-C6F5), -171.7 (t, 3JFF =

19 Hz, 1F, m-C6F5), -172.3 (t, 3JFF = 21 Hz, 1F, m-C6F5), -173.2 (t, 3JFF = 21 Hz, 1F, m-C6F5), -

175.0 (t, 3JFF = 21 Hz, 1F, m-C6F5). 31P{1H} NMR (162 MHz, CDCl3, 298K): δ 59.8 (s). Anal.

Calcd. For C52H26B3F15N4·1.25 C36H28BF40P: C 54.77 %, H 2.89 %, N 2.63 %. Found: C 49.72

%, H 3.13 %, N 0.89 %

Synthesis of [tBu3PH][trans-C3H2N2(C7H5)2B(B(C6F5)3)2] (2-10)

A solution of 2-2a·0.55(C7H8) (206 mg, 0.200 mmol) in 2 mL of C6H5Br was added to a mixture

of tBu3P (81 mg, 0.400 mmol) with B(C6F5)3 (205 mg 0.400 mmol) in 2 mL of C6H5Br. The

combined solution was left at room temperature for 16 h. The resulting green solution was mixed

with 4 mL of pentane and left at room temperature for 24 h. Insoluble yellow needle crystals

formed from the solution. The solvent was decanted. The crystals were washed with C6H5Br (3 ×

0.5 mL) and pentane (3 × 0.5 mL), and were dried in vacuo. 2-10 was collected in 457 mg. Based

on NMR, [tBu3PH][B(C6F5)4] also crystallized with 2-10 and could not be separated out (77 %,

based on the formula as 2-10·1.25[tBu3PH][B(C6F5)4] from 19F NMR data).

45

1H NMR (600 MHz, CDCl3, 298K): δ 7.86 (d, 3JHH = 7.0 Hz, 2H, PhH), 7.72 (d, 3JHH = 8.0 Hz,

2H, PhH), 7.51 (d, 4JHH = 2.1 Hz, 1H, -NCH-), 7.39 (d, 3JHH = 7.1 Hz, 2H, PhH), 7.33 (d, 3JHH =

7.2 Hz, 2H, PhH), 7.11 (d, 3JHH = 9.2 Hz, 2H, -N-CH-B(C6F5)3). 13C{1H} NMR (126 MHz,

CDCl3, 298K, pentane omitted): δ 151.7, 149.3, 147.4, 137.4, 134.6, 131.9, 131.7, 130.2, 127.0,

125.7, 122.7, 37.8 (P-C), 30.1 (-CH3). 11B NMR (128 MHz, C6D5Br, 298K, [B(C6F5)4]- omitted):

δ -9.8 (s). 19F NMR (564 MHz, C6D5Br, 298K, [B(C6F5)4]- omitted): δ -124.2 (dd, 3JFF = 25, 52

Hz, 2F, o-C6F5), -126.0 (dd, 3JFF = 46, 52 Hz, 2F, o-C6F5), -126.2 (d, 3JFF = 26 Hz, 2F, o-C6F5), -

127.1 (dd, 3JFF = 25, 52, 2F, o-C6F5), -128 (dd, 3JFF = 25, 52 Hz, 2F, o-C6F5), -133.0 (d, 3JFF = 25

Hz, 2F, o-C6F5), -159.1 (t, 3JFF = 21 Hz, 2F, p-C6F5), -159.9 (t, 3JFF = 21 Hz, 2F, p-C6F5), -160.4

(t, 3JFF = 21 Hz, 2F, p-C6F5), -163.0 (t, 3JFF = 23 Hz, 2F, m-C6F5), -163.4 (t, 3JFF = 24 Hz, 2F, m-

C6F5), -163.7 (t, 3JFF = 24 Hz, 2F, m-C6F5), -164.0 (t, 3JFF = 24 Hz, 2F, m-C6F5), -165.8 (t, 3JFF =

24 Hz, 2F, m-C6F5), -167.2 (t, 3JFF = 24 Hz, 2F, m-C6F5). 31P{1H} NMR (162 MHz, C6D5Br,

298K): δ 59.5 (s). HRMS (ESI-QTOF-): m/z [M]-: calcd for [C53H12B3F30N2]- 1279.080,

measured 1279.083.

Attempted generation of [K]2[trans-C3H2N2(C7H5)2B(B(C6F5)3)2] (2-11)

2-10·1.25[tBu3PH][B(C6F5)4] (40 mg, 0.014 mmol) was dissolved in 2 mL of THF and stored in

a -35 °C freezer for 30 min. After the solution was cooled, it was added dropwise to a vial

charged with KC8 (10 mg, 0.070 mmol) and a stir bar. The reaction mixture was allowed to warm

to room temperature, and was further stirred for 1 h. The obtained solution was filtered through a

pad of Celite to give an orange solution. The 31P NMR and the EPR spectra were recorded on the

obtained solution.

2.4.3 X-ray Crystallography

X-ray Data Collection and Reduction. Single crystals were coated with Paratone-N oil,

mounted using a glass fiber pin and frozen in the cold nitrogen stream of the goniometer. Data

sets were collected on a Bruker Apex II diffractometer. The data were collected at 150 (± 2) K

for all crystals. Data reduction was performed using the SAINT software package, and

absorption corrections were applied using SADABS. The structures were solved using XS and

refined by full-matrix leas squares on F2 using XL as implemented in the SHELXTL suite of

programs. Carbon-bound hydrogen atoms were placed in calculated positions using an

appropriate riding model and coupled isotropic temperature factors.

46

Structure Solution and Refinement. The structures were solved by the intrinsic phasing

methods using XT and subjected to full-matrix least-squares refinement on F2 using XL as

implemented in the SHELXTL suite of programs. All non-hydrogen atoms were refined with

anisotropically thermal parameters. Carbon bound hydrogen atoms were placed in geometrically

calculated positions and refined using an appropriate riding model and coupled isotropic thermal

parameters.

For 2-9, the structure was refined with 2 molecules of C6H5Cl and 0.5 molecules in total of n-

pentane, which were found from the electron density map. The n-pentane molecule was

modelled with disorder over 3 different positions, and the occupancy of each molecule were

freely refined with individual free variable (FVAR). The sum of the occupancies of the n-

pentane was determined to be 0.5 since they are disordered at a special position in the unit cell.

Thus, the 3 free variables used were restrained to sum up to be 0.5 by SUMP. The disordered n-

pentane was refined using the restraints SIMU and DELU.

On the other hand, the solvent disorder for 2-10 was refined from a model of bromobenzene and

a model of pentane occupied at the same position, and the occupancy of each molecule was

obtained from a full refinement from giving each solvent molecule an individual free variable

(FVAR). The resulting occupancies for the solvents were 0.53 for bromobenzene and 0.43 for

pentane.

47

Table 2.4 – Summary of crystallographic data for compounds 2-1b, 2-2b, 2-4, 2-5, 2-7.

2-1b 2-2b 2-4 2-5 2-7

empirical

formula

C21 H21 B N2 C47 H20 B2

Cl4 F20 N2

C29 H35 B N3

O

C26 H23 B Cl4

N2 O2

C17 H14 B Cl

N2

formula

weight

312.21 1156.07 452.41 548.07 292.56

crystal

system

Triclinic Triclinic Triclinic Monoclinic Monoclinic

space group P-1 P-1 P-1 P21/c P21/n

a (Å) 9.688(4) 10.3868(2) 10.954(2) 13.550(4) 10.634(2)

b (Å) 10.095(4) 15.389(2) 11.332(2) 13.708(3) 10.130(2)

c (Å) 10.469(4) 15.901(2) 11.352(2) 14.047(3) 12.866(2)

α (deg.) 69.37(3) 69.704(7) 83.03(1)

β (deg.) 64.56(2) 87.886(8) 85.59(1) 94.281(7) 90.170(4)

γ (deg.) 88.81(3) 72.038(7) 61.65(1)

vol (Å3) 854.9(7) 2260.6(6) 1230.7(4) 2601(1) 1385.9(3)

Z 2 2 2 4 4

ρ (calcd)

(Mg∙cm3)

1.213 1.698 1.221 1.399 1.402

μ (mm-1) 0.070 0.385 0.074 0.482 0.268

F(000) 332 1148 486 1128 608

Theta range

( )

2.182 to

27.747

1.370 to

27.724

1.808 to

27.560

1.507 to

25.843

2.481 to

27.696

T(K) 150(2) 150(2) 150(2) 150(2) 150(2)

reflections

collected

6930 33389 16354 36767 49492

unique

reflections

6930 9791 5380 4959 3214

Rint 0.0826 0.0584 0.0877 0.0645 0.0935

GOF (F2) 0.971 1.034 0.938 1.433 1.026

R1 indices

[I>2σ(I)]

0.0645 0.0961 0.0692 0.0643 0.0416

wR2 indices

(all data)

0.1610 0.2991 0.2099 0.2169 0.0875

Largest diff.

peak and

hole (e. Å-3)

0.353 & -

0.307

1.398& -

0.675

0.305 & -

0.369

0.561 & -

0.474

0.345 & -

0.266

CCDC No. 1558211 1558212 1501224 1541763 1541765

48

Table 2.5 - Summary of crystallographic data for compounds 2-8, [Cp*2Co][B(C6F5)4], 2-9, 2-

10.

2-8 [Cp*2Co]

[B(C6F5)4]

2-9 2-10

empirical

formula

C35 H30 B2 Cl2

N4 S4

C44 H30 B Co

F20

C66.49 H39.99 B3

Cl4 F15

C70.30 H47.78 B3

Br 0.52 F30 N2 P

formula weight 727.39 1008.42 1354.18 1595.84

crystal system Orthorhombic Orthorhombic Triclinic Monoclinic

space group Pbcn P212121

P C2/c

a (Å) 27.38(2) 16.573(1) 13.868(1) 33.417(2)

b (Å) 9.081(7) 19.785(2) 16.240(1) 15.8768(9)

c (Å) 13.834(11) 24.579(2) 16.248(1) 25.886(2)

α (deg.) 78.806(5)

β (deg.) 80.992(4) 108.751(3)

γ (deg.) 66.332(4)

vol (Å3) 3439(5) 8059(1) 3275.2(4) 13005(2)

Z 4 8 2 8

ρ (calcd)

(Mg∙cm3)

1.405 1.662 1.373 1.630

μ (mm-1) 0.465 0.553 0.267 0.498

F(000) 1504 4048 1370 6416

Theta range ( ) 1.488 to 26.371 1.321 to 27.573 1.384 to 25.000 2.033 to 28.283

T(K) 150(2) 150(2) 150(2) 150(2)

reflections

collected

39697 42135 49001 114745

unique

reflections

3523 18386 11550 16151

Rint 0.1453 0.0570 0.1134 0.0331

GOF (F2) 1.057 1.004 0.917 1.013

R1 indices

[I>2σ(I)]

0.0723 0.0558 0.0781 0.0434

wR2 indices (all

data)

0.2342 0.1403 0.2089 0.1164

Largest diff.

peak and hole

(e. Å-3)

1.194 & -0.484 0.393 & -0.403 0.686 & -0.496 0.511 & -0.465

CCDC No. 1541764 1501225 - -

49

2.4.4 Electrochemistry

Cyclic voltammetry experiments were performed using an BASi-Epsilon RDE-2 instrument. A

standard three-electrode cell configuration was employed using a glassy graphite working

electrode, a platinum wire counter electrode, and a silver wire serving as a reference electrode.

Formal redox potentials were referenced to the ferrocene/ferrocenium redox couple.

[nBu4N][B(C6F5)4] was used as supporting electrolyte. The scanning direction was from positive

to negative potential and then the reverse.

Figure 2.15 - Cyclic voltammogram of 2-2a in THF (0.15 M of [nBu4N][B(C6F5)4] at room

temperature. Scan rate: 500 mV/s.

50





51

Figure 2.18- Cyclic voltammogram of [tBu3PH][B(C6F5)4] in THF/0.1 M [nBu4N][B(C6F5)4] at

room temperature. Scan rate: 500 mV/s.

Table 2.6 - Reduction potential summary for 2-2a, 2-9, 2-10, [tBu3PH][B(C6F5)4]

2-2a 2-9 2-10 [tBu3PH][B(C6F5)4]

Ep (V) -1.55 -2.12 -2.23 -1.51

2.4.5 Computational Chemistry

All calculations were computed using the Gaussian 09 program.98 Geometry optimizations were

performed at (u)M06-2X functional.80 The Def2-SVP basis set was used for all the atoms. The

stationary nature of the converged upon geometry was confirmed by carrying out a frequency

calculation and ensuring the absence of imaginary frequencies. A stability calculation was also

performed on the geometry of 2-3a and 2-3b to confirm its singlet lowest energy state.

Attempting to locate the transition state of the dimerization process of 2-3a was performed by

optimized the structure of 2-3a with various freezing B-B bond length (0.2 Å per step from 2.1 -

3.1 Å) at M06-2X/Def2-SVP, and the triplet state of 2-3a was performed at uM06-2X/Def2-

SVP. Geometry optimizations of the literature known diboranes were performed at M06-

2X/Def2-SVP for all atoms with the solid-state structures as the starting point,37, 38, 40, 52, 53 and

checked with absence of imaginary frequencies. Frontier orbital energies and isotropic shifts for

2-3a and 2-3b were computed at the GIAO-B97-2/Def2-TZVP//M06-2X/Def2-SVP level of

52

theory using the parent compound 2-2a (δ(11B) = 40.2 ppm) as the reference point.99 Natural

bond orbital (NBO) analysis was carried out at M06-2X/Def2-TZVP//M06-2X/Def2-SVP level

with NBO 6.0 program.100 Atom in Molecule (AIM) analysis of 2-3a was performed with

MultiWin 3.3.8 program.101 For compound 2-6, initial estimates of the hyperfine coupling

constants were obtained by evaluating the Fermi contact coupling constants for each nucleus

using the expectation value of the spin operator and the spin density at that nucleus. To obtain

the requisite flexibility in the core region of the nuclei, the electronic structures of 2-6 & 2-6’

were obtained by deconstructing the Def2-TZVP basis set into its constituent primitive functions

for all atoms. The reported Fermi contact couplings were converted into guess using the

experimentally obtained g-factor of 2-6. Optimized structures were visualized using

ChemCraft102 or CYLview software.103

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59

Chapter 3

3 A Radical Mechanism for Frustrated Lewis Pair Reactivity

3.1 Introduction

3.1.1 FLP Reaction Mechanism Chemistry

Computational studies that have probed the mechanism of H2 activation have been critical to our

understanding of FLP chemistry.1-9 Such studies have been done for several systems and the

precise details differ slightly depending on the system and level of theory. In the case of tBu3P

and B(C6F5)3, a two-electron diamagnetic reaction mechanism is thought to proceed via an

“encounter complex”1, 8 (Figure 3.1a) that resides in a shallow thermodynamic well and features

an approach of the Lewis acid and base that is sterically limited to ca. 4 Å. Addition of H2 into

the polarized pocket of the encounter complex prompts interactions of the H-H σ-bond with the

Lewis acid and concurrent donation of the Lewis base to the σ* orbital of H2. The result is

protonation of the base and hydride delivery to the Lewis acid. More recently, detailed diffusion

and temperature-dependent studies10 provided experimental support for the association between

tBu3P or Mes3P and B(C6F5)3 in aromatic solvents and were consistent with a slightly endergonic

association via weak dispersive interactions. More recently, molecular dynamics simulations of

the tBu3P/B(C6F5)3 pair in toluene probed the degree of intermolecular association and suggested

that the concentration of reactive, pre-organized states (0.5 %) is comparable to levels typical of

reactive intermediates.11-14 This mechanism is thought to be operative for a range of main group

and transition metal FLPs. Moreover, it is directly analogous to that exhibited by bifunctional

Noyori-type transition-metal complexes.15 Conceptually, this is also related to the roles of an

acceptor and back-bonding orbitals that allow the activation of H2 at a single atom in transition

metal complexes16 and by singlet carbenes17, 18 and their analogs.19, 20

60

Figure 3.1 – Plausible mechanism of small-molecule activation by FLPs (a) mechanism of

activation of generic substrate (X2) by FLPs via an “encounter complex” (b) proposed

mechanism of activation of generic substrate (X2) by FLPs via the SET process.

3.1.2 Singlet Electron Transfer (SET) in Lewis Pairs

The notion of SET mechanisms in reactions of Lewis acids and bases is rare, and there are only a

few examples of SET from transition-metal complexes to strong Lewis acids (Figure 3.2).21, 22

For example, Wang and co-workers23 have recently reported the irreversible one-electron

oxidation of triarylamine by an equimolar portion of B(C6F5)3 generating the highly reactive

radical anion [•B(C6F5)3]-, which was quenched by subsequent reaction with solvent. These data

61

prompted questions of the possible role of SET reactions in the mechanism of FLP chemistry

(Figure 3.1b).

3.1.3 Work Carried Out After Our Publication

Figure 3.2 – SET in Lewis pairs.

62

Since the publication of our work in 2017,24 other researchers have also reported a few instances

where SET was observed in Lewis pairs. Through a calorimetric study of the H2 activation

reaction with B(C6F5)3 and PMes3,25 Autrey and co-workers were able to find that the reaction

would be well-modeled as a single, termolecular reaction step which is similar the metalloradical

reaction between H2 and [RhII(Tmp)•].26 More recently, Driess et al. revealed a relevant work

between silylium cations and various phosphines (Figure 3.2).27 Depending on the electron

affinity of the Lewis acid and the ionization potentials for the Lewis base, the different

combination of Lewis pairs could either proceed via with the radical pathway, or the traditional

two electron pathway. Later last year, Müller reported the observation of a SET reaction between

B(C6F5)3 and hafnocene-based germylene species (Figure 3.2).28


3.2.1 Two Electron Chemistry Between tBu3P/E(C6F5)3 (E = B, Al)

To begin to address the SET in FLP chemistry, the quintessential FLPs derived from the

combination of the tBu3P and one of E(C6F5)3 (E = B or Al) in toluene were examined. The

spectral data showed little reaction, with only traces of by-products. In the case of the borane

combination the minor by-product was identified as the para-substitution zwitterion

[tBu3PC6F4BF(C6F5)2].29 In a separate control experiment, combination of B(C6F5)3 with

tetrachloro-1,4-benzoquinone (p-O2C6Cl4) showed no reaction even on cooling to -78 ºC, while

Al(C6F5)3 forms a Lewis acid-base adduct with the quinone at room temperature. Regardless,

treatment of the FLP system consisting of tBu3P/E(C6F5)3 with an equimolar amount of p-

O2C6Cl4 in chlorobenzene immediately afforded light yellow solutions. Multinuclear NMR

spectroscopic data were consistent with the formation of new compounds derived from a 1:1:1

combination of the reagents, suggesting the formulation of these products as

[(tBu3POC6Cl4OE(C6F5)3] (E = B, 3-1; Al, 3-2) (Scheme 3.1). Crystallographic characterization

of 3-1 (Figure 3.3) confirmed its zwitterionic nature and revealed the P-O and B-O bond

distances to be 1.608(5) Å and 1.500(9) Å, respectively.

63

Scheme 3.1 – Reaction of tBu3P and E(C6F5)3 (E = B or Al) with p-O2C6H4 and Ph3SnH.

In an analogous fashion, E(C6F5)3 (E = B or Al) in reactions with Ph3SnH showed evidence of

only weak dative interactions; however, mixtures of tBu3P and E(C6F5)3 (E = B, Al) react readily

with Ph3SnH providing the (stannyl)phosphonium salts [tBu3PSnPh3][HB(C6F5)3] 3-3 and

[tBu3PSnPh3][(μ-H)(Al(C6F5)3)2] 3-4 (Scheme 3.1). The formation of the P-Sn bond was evident

from 119Sn satellites on the 31P NMR resonance at 66.5 ppm and the doublet structure of the 119Sn

NMR resonance at -128.5 ppm. Both of these signals exhibit average Sn-P coupling of 185 Hz.

In the case of 3-4, the solid-state structure was unambiguously confirmed by single crystal X-ray

diffraction analysis (Figure 3.3).


50 % probability. H atoms that are bonded to carbon atoms are omitted for clarity. C: black, F:

pink, B: yellow-green, P: orange, Al: cyan, Sn: silver, and H: light grey.

64

3.2.2 One Electron Chemistry Between Mes3P/E(C6F5)3 (E = B, Al)

The above observations are consistent with the accepted FLP mechanism involving concerted

action of the Lewis acid and base on the substrate, whereby the Lewis base donates an electron

pair to the substrate and the Lewis acid accepts an electron pair from the substrate. In these

cases, this reactivity results in FLP addition to the quinone or heterolytic cleavage of the Sn-H

bond.

In contrast, mixing Mes3P and E(C6F5)3 (E = B, Al) in an equimolar ratio in dry toluene or

chlorobenzene at room temperature gave rise to a very pale purple solution in the case of the

borane and a much more distinctly purple solution for the Al species. Although these

observations suggest that chemical reactions have occurred, both solutions exhibit NMR

spectroscopic features showing signals attributable to the initial reagents. In addition, evidence

for the formation of trace amounts of byproducts are evidenced by peaks in the 19F NMR spectra

after additional scans (e.g. for Mes3P/Al(C6F5)3 3300 scans). In the case of the borane reaction,

the EPR spectrum showed only a weak signal. However, the solution generated with

Mes3P/Al(C6F5)3 revealed a room-temperature EPR spectrum consisting of a doublet resonance

with a g-factor of 2.0089 and a hyperfine coupling constant of 238 G (Figure 3.4). This signal is

attributable to the coupling of an unpaired electron to a P center, and is consistent with the

formation of the known radical cation [Mes3P•]+ (Figure 3.4).27, 30, 31 This observation suggests

that this radical is formed via single electron transfer and generates the corresponding radical

anion [•Al(C6F5)3]-. This radical anion was not detected by EPR spectroscopy, which is

consistent with the short lifetimes of such species.32-34 Collectively, these observations of both

diamagnetic and paramagnetic species support the notion of SET equilibria between the FLP and

[Mes3P•]+ and [•Al(C6F5)3]-.

65

Figure 3.4 – (a) Photo taken for the reaction mixture of Mes3P and Al(C6F5)3, (b) Simulated and

Experimental EPR spectra of the Mes3P and Al(C6F5)3 mixture.

Scheme 3.2 – Reaction scheme of Mes3P and E(C6F5)3 (E = B or Al) with p-O2C6Cl4 and

Ph3SnH.

66

Figure 3.5 – UV spectra for (a) PMes3 and Al(C6F5)3 with p-O2C6Cl4 (b) PMes3 and B(C6F5)3

with p-O2C6Cl4 and (c) PMes3 with p-O2C6Cl4.

Quinones are well-known to be electron acceptors with the formation of charge transfer

complexes.35, 36 Control experiments revealed that no reaction was observed upon treatment of

Mes3P with p-O2C6Cl4 in C6H5Cl. Moreover, the UV-vis spectrum showed no absorption bands

from 450-900 nm (Figure 3.5c). However, slow addition of half an equivalent of p-O2C6Cl4 to

the combinations of Mes3P and E(C6F5)3 (E = B, Al) in toluene at -78 °C gave rise to deep purple

solutions that showed a visible absorption maximum at 573 nm, consistent with that of the

known radical [Mes3P•]+.30, 31 31P NMR spectra for these solutions showed no signals and 19F

NMR data documented the complete consumption of E(C6F5)3 within 30 min. The stoichiometry

and the spectral data are consistent with the formation of the radical cation salts

[Mes3P•]2[(C6F5)3EOC6Cl4OE(C6F5)3] (E = B 3-5, Al 3-6, Scheme 3.2). Attempts to crystallize

these salts were unsuccessful. However, exploiting a strategy previously used in the literature to

intercept radical salts,37, 38 Ph3SnH was added to these solutions, generating the phosphonium salt

[Mes3PH]2[(C6F5)3BOC6Cl4OB(C6F5)3] 3-7 (Scheme 3.2) as evidenced by the observation of the

expected resonance in the 31P NMR spectrum. Further confirmation of the formulation was

67

obtained by a crystallographic study that established the connectivity unambiguously (Figure

3.6a).

In a similar fashion, altering the stoichiometry to a 1:1:1 ratio of E(C6F5)3 (E = B, Al): Mes3P: p-

O2C6Cl4, gave solutions that immediately became deep purple in color with an absorption

maximum at 573 nm. Upon stirring at room temperature, this color faded to a light yellow over

the course of 3 min for B(C6F5)3 and 20 min for Al(C6F5)3 (Figure 3.5). The 31P NMR spectra of

the resulting solutions displayed singlets at 70.8 and 71.1 ppm for the B(C6F5)3 and Al(C6F5)3

reactions, respectively. The spectroscopically similar products of these reactions, isolated as

pale-yellow solids, were formulated as [(Mes3POC6Cl4OE(C6F5)3] (E = B 3-8, Al 3-9). This

formulation was confirmed via a single crystal X-ray diffraction study of 3-8 which revealed P-O

and O-B bond lengths of 1.606(3) Å and 1.500(5) Å, respectively (Figure 3.6).

Figure 3.6 – ORTEPs of 3-7 (a) and 3-8 (b) with thermal displacement parameters drawn at 50

% probability. H atoms that are bonded to carbon atoms are omitted for clarity. C: black, F: pink,

B: yellow-green, P: orange, Al: cyan, H, light grey.

The initially intense purple coloration of the solutions suggests the intermediate formation of 3-5

and 3-6 en route to 3-8 and 3-9. Addition of quinone to 3-5 or 3-6 is thought to generate the

transient radical anions [•OC6Cl4OE(C6F5)3]-, which are ultimately quenched by the radical

cation to give the zwitterionic products. This view is supported by independent reaction of

[Mes3P•][(μ-HO)(Al(C6F5)3)2]31 with p-O2C6Cl4 where no reaction was observed. The ultimate

products of these reactions employing Mes3P (3-8 and 3-9) are analogous to those obtained using

68

tBu3P (3-1 and 3-2) but the above data demonstrate that an alternative mechanism, initiated by

SET between the phosphine and the Lewis acid is operative.

To further support this hypothesis, addition of Ph3SnH to toluene solutions of Mes3P and one

equivalent of E(C6F5)3 (E = B, Al) resulted in an immediate color change. Multinuclear NMR

spectroscopic data confirmed the formation of the known salts [Mes3PH][HB(C6F5)3] 3-1039 and

[Mes3PH][(μ-H)(Al(C6F5)3)2] 3-11,40 respectively. Concurrent with this was the quantitative

formation of the distannane Ph3SnSnPh3 as evidenced by the observation of a singlet in the 119Sn

NMR spectra (-138.9 ppm (CH2Cl2) and -136.8 ppm (C6H5Br)) for reactions generating 3-10 and

3-11.41 The formation of these products is consistent with radical abstraction of H• from the Sn-

species. This evidence for homolytic cleavage of the Sn-H bond stands in marked contrast to the

corresponding reactions of tBu3P in which heterolytic cleavage of the Sn-H bond is observed.

The possible, and previously unappreciated, role of SET processes in the reactivity of FLPs has

interesting implications. For example, the activation of H2 by the FLPs used herein are known to

give the salts [R3PH][HB(C6F5)3] and [R3PH][(μ-H)(Al(C6F5)3)2] (R = tBu, Mes). Although the

FLPs based on the two phosphines provide similar products, the data herein suggests differing

reaction mechanisms for the systems based on tBu3P and Mes3P. The former is thought to

operate via two-electron processes affording heterolytic cleavage of H2, however the present

study suggests the possibility of homolytic H2 cleavage via one-electron processes.

3.3 Conclusion

In conclusion, more than a decade after the uncovering of FLP reactivity, the divergent reactivity

observed for combinations of tBu3P/E(C6F5)3 or Mes3P/E(C6F5)3 (E = B, Al) with p-O2C6Cl4 and

Ph3SnH support differing reaction mechanisms. While the tBu3P systems react via the

conventional diamagnetic two-electron mechanism, the reactions of Mes3P proceed via

intermediate radical salts, generated by SET from the Lewis base to the Lewis acid. The

detection of this SET radical mechanism further broadens the potential reactivity of FLP

systems, which is currently the subject of on-going study.

69




standard Schlenk or glovebox techniques (MBraun LABmaster SP dry box and Innovation

Technology glovebox both equipped with a -35 °C freezer). Toluene, pentane, and

dichloromethane (CH2Cl2) were collected from a Grubbs-type column system manufactured by

Innovative Technology. These solvents, along with fluorobenzene (C6H5F), bromobenzene

(C6H5Br) and 1,2-dichlorobenzene (1,2-C6H4Cl2), were dried over 4 Å molecular sieves.

Molecular sieves, type 4 Å (pellets, 3.2 mm diameter) purchased from Sigma Aldrich were

activated prior to usage by iteratively heating with a 1050 W Haier microwave for 5 minutes and

cooling in vacuo. The process was repeated until no further moisture was released upon heating.

Dichloromethane-d2 (CD2Cl2), bromobenzene-d5 (C6D5Br) purchased from Cambridge Isotope

Laboratories, were degassed and stored over 4 Å molecular sieves in the glovebox for at least 8 h

prior to use. Chlorobenzene (C6H5Cl), benzene (C6H6) and chloroform-d (CDCl3) were degassed

and dried over calcium hydride. B(C6F5)3 was purified by sublimation at 100 °C under vacuum

followed by stirring with chlorodimethylsilane at room temperature for two hours, and dried

under vacuo. Al(C6F5)3•tol was prepared using literature methods.42 Combustion analyses were

performed in-house employing a Flash 2000 from Thermo Instruments CHN Analyzer. Spectra

were recorded on a Bruker Avance III 400 MHz, an Agilent DD2 500 MHz, and an Agilent DD2

600 MHz spectrometer and spectra were referenced to residual solvents of CD2Cl2 (1H = 5.32

ppm; 13C = 53.84 ppm), CDCl3 (1H = 7.26 ppm; 13C = 77.16 ppm), C6D5Br (1H = 7.28 ppm for

meta proton; 13C = 122.4 ppm for ipso carbon) or externally [11B: (Et2O)BF3 (δ 0.00), 19F: CFCl3

(δ 0.00), 31P: 85 % H3PO4 (δ 0.00)]. Chemical shifts (δ) are reported in ppm and the absolute

values of the coupling constants (J) are in Hz. In some instances, signal and/or coupling

assignment was derived from 2D NMR experiments. UV-Vis spectra were obtained on an

Agilent 8553 UV-is spectrophotometer using Quartz cells modified with a J-young NMR cap to

assure a near-perfect seal. Electron paramagnetic resonance (EPR) measurements were

performed at 298 K using a Bruker ECS-EMX X band EPR spectrometer equipped with an

EP4119HS cavity. Simulations were performed using PEST WinSIM Software.43

70

3.4.2 Syntheses and Characterizations

Synthesis of tBu3POC6Cl4OB(C6F5)3 (3-1)

A mixture of tBu3P (10.2 mg, 0.050 mmol), B(C6F5)3 (26.0 mg, 0.050 mmol) and p-O2C6Cl4

(12.0 mg, 0.050 mmol) was stirred in 2 mL of C6H5Cl at room temperature. After 5 min, white

solid product started to precipitate. The reaction mixture was allowed to further stir for 1 hour.

Pentane (5 mL) was added into the mixture with rapid stirring. The white solid was isolated by

filtration and washed with a C6H5Cl/pentane mixture (v:v = 1:3). All volatiles/solvents were

removed in vacuo to afford 3-1 as a white solid (44.7 mg, 93 %). Single crystals of 3-1 suitable

for X-ray diffraction were obtained from layering of pentane with a saturated C6H5Cl solution at

-35 °C.

1H NMR (400 MHz, C6D5Br, 298 K, C6H5Cl, pentane omitted): δ1.81 (bs, 27H, tBu3). 13C{1H}

NMR (126 MHz, CDCl3, 298 K, only partial peaks could be observed due to the since very

sparing solubility of compound 3-1, C6H5Cl, pentane omitted): δ 44.9 (d, 1JCP = 27 Hz, PC), 31.0

(d, 2JCP = 8 Hz, -CH3). 19F{1H} NMR (377 MHz, C6D5Br, 298 K): δ -132.3 (bs, 6F, o-C6F5), -

161.3 (bs, 3F, p-C6F5), -166.0 (bs, 6F, m-C6F5). 31P{1H} NMR (128 MHz, C6D5Br, 298 K): δ

125.4 (s). 11B NMR (128 MHz, C6D5Br, 298K): 1.36 (s).

Synthesis of tBu3POC6Cl4OAl(C6F5)3 (3-2)

A mixture of tBu3P (10.2 mg, 0.050 mmol), Al(C6F5)3•tol (31.0 mg, 0.050 mmol) and p-O2C6Cl4

(12.0 mg, 0.050 mmol) was stirred in 2 mL of C6H5Cl at room temperature. After 5 min, white

solid product started to precipitate. The reaction mixture was allowed to further stir for 1 hour.

Pentane (5 mL) was added into the mixture with rapid stirring. The white solid was isolated by

filtration and washed with a C6H5Cl/pentane mixture (v:v = 1:3). All volatiles/solvents were

removed in vacuo to afford 3-2 as a white solid (42.0 mg, 86 %).

1H NMR (400 MHz, C6D5Br, 298 K, C6H5Cl, pentane omitted): δ 1.81 (bs, 27H, tBu3). 13C{1H}

NMR (126 MHz, C6D5Br, 298 K, partial): δ 153.1 (s, C6Cl4), 149.8 (dm, 1JCF = 234 Hz, C6F5),

149.6 (dm, 1JCF = 233 Hz, C6F5), 140.5 (dm, 1JCF = 240 Hz, C6F5), 137.9 (s, C6Cl4), 137.8(s,

C6Cl4), 136.5 (dm, 1JCF = 256 Hz, C6F5), 130.4 (s, C6Cl4), 127.6 (s, C6Cl4), 40.4 (d, 1JCP = 27 Hz,

PC), 29.9 (bs, CH3). 19F NMR (377 MHz, C6D5Br, 298 K): δ -129.0 (br, 6F, o-C6F5), -163.3 (br,

71

3F, p-C6F5), -170.0 (br, 6F, m-C6F5). 31P{1H} NMR (128 MHz, C6D5Br, 298 K): δ 125.2 (s).

27Al NMR (104 MHz, C6D5Br, 298K): no signal detected.

Synthesis of [tBu3PSnPh3][HB(C6F5)3] (3-3)

A mixture of tBu3P (10.2 mg, 0.050 mmol), and B(C6F5)3 (26 mg, 0.050 mmol) was stirred in 2

mL of C6H5Cl, giving a light-yellow solution. Then Ph3SnH (17.8 mg, 0.050 mmol) was added

into the mixture. Immediately, the colour of the solution changed to colorless. The volatiles were

removed in vacuo. The resulting white solid was washed with pentane (3 x 3 mL), and dried in

vacuo to give 3-3 as a colorless oil (37.2 mg, 70 %).

1H NMR (400 MHz, CD2Cl2, 298 K): δ 7.77 (bs, 6H, m-PhH), 7.57 (bs, 9H, p-PhH & o-PhH),

3.61 (q, 1JBH = 91 Hz, 1H, BH), 1.55 (d, 3JPH= 15.5 Hz, 27H, tBu). 13C{1H} NMR (126 MHz,

CD2Cl2, 298 K): δ 148.3 (dm, 1JCF = 237 Hz, o-C6F5), 138.3 (dm, 1JCF = 238 Hz, p-C6F5), 136.8

(dm, 1JCF = 237 Hz, m-C6F5), 131.8 (s, 2JCSn = 12 Hz, o-PhC), 130.7 (s, 3JCSn = 58 Hz, m-PhC),

128.9 (s, 4JCSn = 7 Hz, p-PhC), 41.9 (d, 1JCP = 8 Hz, PC), 30.3 (bs, -CH3). 19F NMR (377 MHz,

CD2Cl2, 298 K): δ -132.1 (d, 3JCF = 18.6 Hz, 6F, o-C6F5), -163.3 (bs, 3F, p-C6F5), -166.1 (bs, 6F,

m-C6F5). 31P{1H} NMR (128 MHz, CD2Cl2, 298 K): δ 66.5 (s, 1JPSn = 185 Hz). 11B NMR (128

MHz, CD2Cl2, 298 K): δ -25.4 (d, 1JBH = 91 Hz). 119Sn{1H} NMR (186 MHz, CD2Cl2, 298 K): δ

-128.4 (d, 1JPSn = 185 Hz).

Synthesis of [tBu3PSnPh3][(µ-H)(Al(C6F5)3)2] (3-4)

A mixture of tBu3P (5.1 mg, 0.025 mmol), Al(C6F5)3•tol (31.0 mg, 0.050 mmol) was stirred in 4

mL of C6H5Cl, giving a light-yellow solution. Then Ph3SnH (9.0 mg, 0.025 mmol) was added

into the mixture. Immediately, the colour of the solution changed to colorless. The volatiles were

removed in vacuo. The resulting a white solid was washed with pentane (3 x 3 mL), and dried in

vacuo to give compound 3-4 as white powder (39 mg, 97 %). Single crystals of 3-4 suitable for

X-ray diffraction were obtained from layering of pentane with a saturated C6H5Cl solution at

room temperature.

1H NMR (400 MHz, C6D5Br, 298 K): δ 7.78 (bs, 6H, m-PhH), 7.60 (bs, 9H, p-PhH & o-PhH),

1.63 (d, 3JPH= 15.0 Hz, 27H, tBu). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K, only partial peaks

could be observed duo to the sparing solubility of compound 3-4): δ 139.4 (s, 1JCSn = 58 Hz,

72

PhC), 137.7 (s, 2JCSn = 40 Hz, PhC), 129.3 (s, 3JCSn = 3 Hz, PhC), 129.1 (s, 4JCSn = 3 Hz, PhC),

34.6 (d, 1JCP = 30 Hz, PC), 32.6 (d, 2JCP = 12 Hz, CH3). 19F{1H} NMR (377 MHz, C6D5Br, 298

K): δ -120.8 (bs, 12F, o-C6F5), -153.8 (bs, 6F, p-C6F5), -162.4 (bs, 12F, m-C6F5). 31P{1H} NMR

(128 MHz, C6D5Br, 298 K, 119Sn satellites could not be observed duo to the sparing solubility of

compound 3-4, however the structure was confirmed by X-ray studies): δ 66.2 (s). 27Al NMR

(128 MHz, C6D5Br, 298 K): no signal detected.

Synthesis of [Mes3PH]2[(C6F5)3BOC6Cl4OB(C6F5)3] (3-7)

A mixture of Mes3P (23 mg, 0.059 mmol), and B(C6F5)3 (30 mg, 0.059 mmol) was stirred in 2

mL of toluene at -78 °C under dry N2. p-O2C6Cl4 (7.25 mg, 0.030 mmol) was added very slowly

and a deep purple solution was observed upon rapid stirring at -78 °C. Ph3SnH (20.8 mg, 0.059

mmol) was added into the deep purple solution while stirring, leading to the immediate

formation of a colorless solution. The volatiles were then removed in vacuo and the resulting

white solid was washed with 3 pentane. Recrystallization from slow diffusion of pentane into a

saturated fluorobenzene solution at room temperature provided compound 3-7 as colorless

crystals (19.9 mg, 33 %).

1H NMR (400 MHz, CDCl3, 298 K): δ 8.62 (d, 1JPH = 496 Hz, 1H, PH), 7.14 (bs, 3H, m- C6H2),

7.02 (bs, 3H, m- C6H2), 2.38 (bs, 18H, o-CH3), 2.00 (bs, 9H, p-CH3). 13C{1H} NMR (126 MHz,

CDCl3, 298 K): δ 148.1 (dm, 1JCF = 235 Hz, o-C6F5), 147.3 (d, 4JCP = 3 Hz, p-C6H2), 144.0 (d,

2JCP = 11 Hz, o-C6H2), 142.6 (d, 2JCP = 10 Hz, o-C6H2), 138.3 (dm, 1JCF = 245 Hz, m-C6F5),

136.3 (dm, 1JCF = 246 Hz, p-C6F5), 133.3 (d, 3JCP = 11 Hz, m-C6H2), 131.9 (d, 3JCP = 11 Hz, m-

C6H2), 111.3 (d, 1JCP = 83 Hz, i-C6H2), 22.0 (d, 3JCP = 6 Hz, -CH3), 21.3 (d, 5JCP = 1 Hz, -CH3),

21.1 (d, 3JCP = 12 Hz, -CH3). 19F{1H} NMR (377 MHz, CDCl3, 298 K): δ -133.4 (d, 3JCF = 23

Hz, 6F, o-C6F5), -163.3 (t, 3JCF = 19 Hz, 3F, p-C6F5), -166.7 (m, 6F, m-C6F5). 31P{1H} NMR

(128 MHz, CDCl3, 298 K): δ -27.5 (s). 11B{1H} NMR (128 MHz, CDCl3, 298 K): δ -2.9 (s, br).

Synthesis of Mes3POC6Cl4OB(C6F5)3 (3-8)

A mixture of Mes3P (23 mg, 0.059 mmol), B(C6F5)3 (30 mg, 0.059 mmol) and p-O2C6Cl4 (14.5

mg, 0.059 mmol) was stirred in 2 mL of benzene at room temperature. Immediately, a deep

purple solution was observed, but the solution turned light yellow upon stirring at room

temperature. A white suspension appeared after stirring overnight. The solvent was then removed

73

in vacuo and the while solid was washed with 3 x 3 mL of pentane. All volatiles/solvents were

removed in vacuo to afford compound 3-8 as a white solid (54 mg, 80 %). Single crystals of 3-8

suitable for X-ray diffraction were obtained from slow diffusion of pentane into a saturated 1,2-

dichlorobenzene or dichloromethane solution. The reaction was also monitored by UV-Vis

spectroscopy based on the [•PMes3]+ radical absorption band at 571 nm as previously reported.31

1H NMR (400 MHz, CDCl3, 298 K): δ 6.96 (s, 6H, m-C6H2), 2.78 (bs, 3H, p-CH3), 2.34 (bs,

12H, o-CH3), 2.07 (bs, 9H, o-CH3/p-CH3 overlap), 1.79(bs, 3H, p-CH3). 13C{1H} NMR (126

MHz, CDCl3, 298 K): δ 154.2 (bs, p-C6H2), 148.1 (dm, 1JCF = 280 Hz), 147.1 (dm, 1JCF = 240

Hz), 145.6 (bs, o-C6H2), 144.8 (dm, 1JCF = 240 Hz), 144.6 (dm, 1JCF = 220 Hz), 142.4 (bs,

C6Cl4), 138.7 (dm, 1JCF = 246 Hz), 136.5 (s, m-C6H2), 136.4 (s, m-C6H2), 136.2 (dm, 1JCF = 245

Hz), 133.3 (dm, 1JCF = 280 Hz), 133.2 (dm, 1JCF = 257 Hz), 133.1 (bs, C6Cl4), 128.6 (d, 1JCP =

103 Hz, i-C6H2), 128.3 (dm, 1JCF = 200 Hz), 124.3 (dm, 1JCF = 270 Hz), 121.9 (dm, 1JCF = 230

Hz), 121.7 (dm, 1JCF = 173 Hz), 117.0 (bs, C6Cl4), 116.3 (bs, C6Cl4), 24.5 (bs, o-CH3), 23.6 (bs,

o-CH3), 23.3 (bs, o-CH3), 22.8 (bs, p-CH3), 22.7 (bs, p-CH3), 22.1 (bs, -CH3). 19F{1H} NMR

(377 MHz, CDCl3, 298 K): δ -130.7 (bs, 2F, o-C6F5), -133.2 (bs, 4F, o-C6F5), -159.8 (bs, 1F, p-

C6F5), -162.1 (bs, 2F, p-C6F5), -166.0 (bm, 6F, m-C6F5). 11B{1H} NMR (128 MHz, CDCl3, 298

K): δ -1.1 (bs). 31P{1H} NMR (128 MHz, CDCl3, 298 K): δ 70.8 (s).

Synthesis of Mes3POC6Cl4OAl(C6F5)3 (3-9)

A mixture of Mes3P (23 mg, 0.059 mmol), Al(C6F5)3•tol (37 mg, 0.059 mmol) and p-O2C6Cl4

(14.5 mg, 0.059 mmol) was stirred in 2 mL of toluene at room temperature. Immediately, a deep

purple solution was observed, but the solution turned light yellow upon stirring at room

temperature for 1 h. Pentane (5 mL) was added into the yellow solution with rapid stirring,

leading to the formation of a white precipitates. The mother liquor was then decanted away and

the while solid was washed with 3 x 3 mL pentane. All volatiles/solvents were removed in vacuo

to afford compound 3-9 as a white solid (51 mg, 74 %). Single crystals of 3-9 suitable for X-ray

diffraction were obtained from slow diffusion of pentane into a saturated dichloromethane

solution at -20 °C. The reaction was also monitored by UV-Vis spectroscopy based on the

[•PMes3]+ radical absorption band at 571 nm as previously reported.31

74

1H NMR (400 MHz, CDCl3, 298 K): δ 6.98 (bs, 6H, -C6H2), 2.84 (bs, 3H, p-CH3), 2.36 (bs,

12H, o-CH3), 2.16 (bs, 9H, o-CH3/p-CH3 overlap), 1.87 (bs, 3H, p-CH3). 13C{1H} NMR (126

MHz, CDCl3, 298 K): δ 153.1 (bs, p-C6H2), 149.8 (dm, 1JCF = 233 Hz), 149.6 (dm, 1JCF = 240

Hz), 147.1 (dm, 1JCF = 220 Hz), 145.8 (bs, C6Cl4), 145.6 (bs, o-C6H2), 143.0 (dm, 1JCF = 260

Hz), 140.3 (dm, 1JCF = 250 Hz), 136.2 (dm, 1JCF = 248 Hz), 135.4 (s, m-C6H2), 135.3 (s, m-

C6H2), 134.1 (dm, 1JCF = 250 Hz), 133.2 (dm, 1JCF = 263 Hz), 125.1 (d, 1JCP = 88 Hz, i-C6H2),

124.3 (dm, 1JCF = 280 Hz), 122.0 (dm, 1JCF = 278 Hz), 119.2 (dm, 1JCF = 230 Hz), 121.3 (dm,

1JCF = 288 Hz), 116.9 (bs, C6Cl4), 116.2 (bs, C6Cl4), 24.5 (bs, o-CH3), 23.7 (bs, o-CH3), 23.3 (bs,

o-CH3), 22.7 (bs, p-CH3), 22.0 (bs, p-CH3), 21.0 (bs, p-CH3). 19F{1H} NMR (377 MHz, CDCl3,

298 K): δ -122.5 (m, 6F, o-C6F5), -157.1 (t, 3JCF = 19 Hz, 3H, p-C6F5), -163.5 (m, 6F, m-C6F5).

31P{1H} NMR (128 MHz, CD2Cl2, 298 K): δ 71.1 (s). 27Al NMR (104 MHz, CDCl3, 298 K): no

signal detected.

Synthesis of [Mes3PH][HB(C6F5)3] (3-10)

A mixture of Mes3P (23 mg, 0.059 mmol), and B(C6F5)3 (30 mg, 0.059 mmol) was stirred in 2

mL of CH2Cl2, giving a light purple solution. Then Ph3SnH (41.4mg, 0.118 mmol) was added

into the mixture. Immediately, the color of the solution changed to light yellow. The volatiles

were removed in vacuo. The compound 3-1039 was extracted using diethyl ether (2 mL) to afford

a white powder (37.2 mg, 70 %). The generation of the sole tin-containing species Ph3SnSnPh3

was confirmed by 119Sn NMR spectroscopy (-138.9 ppm in CD2Cl2).

1H NMR (400 MHz, CDCl3, 298 K): δ 8.25 (d, 1JHP = 480.0 Hz, 1H, PH), 7.12 (bs, 3H, m-

C6H2), 7.04 (bs, 3H, m-C6H2), 3.62 (q, 1JHB = 98.0 Hz, 1H, BH), 2.37 (s, 9H, -CH3), 2.26 (s, 9H,

-CH3), 1.99 (s, 9H, -CH3). 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ 148.1 (dm, 1JCF = 234

Hz, o-C6F5), 147.1 (d, 4JCP = 3 Hz, p-C6H2), 144.0 (d, 2JCP = 10 Hz, o-C6H2), 142.6 (d, 2JCP = 10

Hz, o-C6H2), 137.5 (dm, 1JCF = 244 Hz, p-C6F5), 136.2 (dm, 1JCF = 245 Hz, m-C6F5), 133.2 (d,

3JCP = 12 Hz, m-C6H2), 131.8 (d, 3JCP = 11 Hz, m-C6H2), 111.2 (d, 1JCP = 82 Hz, i-C6H2), 21.8 (d,

3JCP = 6 Hz, -CH3), 21.2 (d, 5JCP = 1 Hz, -CH3), 20.9 (d, 3JCP = 12 Hz, -CH3). 19F{1H} NMR (377

MHz, CDCl3, 298 K): δ -133.2 (d, 3JCF = 21 Hz, 6F, o-C6F5), -164.3 (t, 3JCF = 20 Hz, 3F, p-

C6F5), -167.1 (m, 6F, m-C6F5). 31P NMR (128 MHz, CDCl3, 298 K): δ -27.5 (d, 1JPH = 480 Hz).

11B NMR (128 MHz, CDCl3, 298 K): δ -25.3 (d, 1JBH = 91 Hz).

75

Synthesis of [Mes3PH][(µ-H)(Al(C6F5)3)2] (3-11)

A mixture of Mes3P (23 mg, 0.059 mmol), and Al(C6F5)3•tol (74 mg, 0.118 mmol) was stirred in

3 mL of toluene, giving a light purple solution. Then Ph3SnH (41.4mg, 0.118 mmol) was added

into the mixture. Immediately, the colour of the solution changed to light yellow. The volatiles

were removed in vacuo. The compound 3-1140 was extracted using diethyl ether (2 mL) to afford

a white powder (63.1 mg, 74 %). The generation of the sole tin-containing species Ph3SnSnPh3

was confirmed by 119Sn NMR spectroscopy (-136.8 ppm in bromobenzene).

1H NMR (400 MHz, CDCl3, 298 K): δ 8.28 (d, 1JHP = 481 Hz, 1H, -PH), 7.13 (bs, 3H, -C6H2),

7.05 (bs, 3H, -C6H2), 4.41 (bs, 1H, Al-H-Al), 2.37 (s, 9H, -CH3), 2.26 (s, 9H, -CH3), 1.98 (s, 9H,

-CH3). 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ 149.7 (dm, 1JCF = 240 Hz, o-C6F5), 147.1 (d,

4JCP = 3 Hz, p-C6H2), 143.9 (d, 2JCP = 11 Hz, o-C6H2), 142.7 (d, 2JCP = 10 Hz, o-C6H2), 140.3

(dm, 1JCF = 248 Hz, p-C6F5), 136.3 (dm, 1JCF = 250 Hz, m-C6F5), 133.1 (d, 3JCP = 12 Hz, m-

C6H2), 131.8 (d, 3JCP = 10 Hz, m-C6H2), 111.3 (d, 1JCP = 82 Hz, i-C6H2), 21.8 (d, 3JCP = 6 Hz, -

CH3), 21.2 (d, 5JCP = 1 Hz, -CH3), 20.9 (d, 3JCP = 12 Hz, -CH3). 19F{1H} NMR (377 MHz,

CDCl3, 298 K): δ -122.3 (bs, 12F, o-C6F5), -151.3 (bs, 6F, p-C6F5), -160.6 (bs, 12F, m-C6F5). 31P

NMR (128 MHz, CDCl3, 298 K): δ -27.4 (d, 1JPH = 480 Hz). 27Al NMR (104 MHz, CDCl3, 298

K): no signal detected.


X-ray Data Collection and Reduction. Crystals were coated in Paratone-N oil in an N2 filled

glovebox, mounted on a MiTegen Micromount, and placed under a N2 stream, thus maintaining a

dry, O2-free environment for each crystal. The data were collected on a Bruker Apex II

diffractometer using a graphite monochromator with Mo Kα radiation (λ = 0.71073 Å). The data

were collected at 150(2) K for all crystals. The frames were integrated with the Bruker SAINT

software package using a narrow-frame algorithm. Data were corrected for absorption effects

using the empirical multiscan method (SADABS).

Structure Solution and Refinement. The structures were solved by the intrinsic phasing

methods using XT and subjected to full-matrix least-squares refinement on F2 using XL as



76


parameters. Hydrogens in compounds 3-4 and 3-7 were found through electron density

difference map and refined using an appropriate riding model and coupled isotropic temperature

factors. Repeated crystallization attempts of 3-9 yielded crystals of poor quality, nonetheless

preliminary X-ray studies confirmed its connectivity. Thus, only the unit cell parameters were

reported in the following table (Table 3.1).

77

Table 3.1. Summary of crystallographic data for compounds 3-1, 3-4, 3-7, 3-8 and 3-9.

3-1 3-4 3-7 3-8 3-9

empirical

formula

C48 H37 B Cl6

F15 O2 P

C66 H43 Al2

F30 P Sn

C96 H68 B2

Cl4 F30 O2 P2

C63 H44.55 B

Cl4.45 F15 O2 P

formula

weight

1185.25 1609.62 2048.86 1318.24

crystal

system

Monoclinic Monoclinic Monoclinic Monoclinic Monoclinic

space group P21/n P21 P21/n P21/c C2/c

a (Å) 10.8936(13) 10.146(6) 14.684(11) 16.705(3) 16.974(1)

b (Å) 15.122(2) 23.175(14) 19.939(15) 16.216(2) 19.578(2)

c (Å) 29.821(3) 14.045(8) 15.675(11) 21.744(3) 40.935(3)

α (deg.)

β (deg.) 91.844(7) 99.272(17) 102.996(15) 94.922(5) 101.834(5)

γ (deg.)

vol (Å3) 4909.8(10) 3259(3) 4472(6) 5868.3(15) 12214(2)

Z 4 2 2 4

ρ (calcd)

(Mg∙cm3)

1.603 1.640 1.522 1.492

μ (mm-1) 0.480 0.571 0.282 0.343

F(000) 2392 1600 2076 2677

Theta range 2.39 to 17.49 2.22 to 25.48 2.16 to 19.75 2.26 to 26.14

T(K) 150(2) 150(2) 150(2) 150(2) 150(2)

reflections

collected

32503 74699 95110 53837

unique

reflections

8657 14283 8494 12938

Rint 0.1788 0.0670 0.1283 0.0515

GOF(F2) 1.001 0.826 1.015 1.021

R1 indices

[I>2σ(I)]

0.0778 0.0402 0.0660 0.0741

wR2 indices

(all data)

0.1815 0.1243 0.1839 0.2336

Largest diff.

peak and hole

(e. Å-3)

0.792 &

-0.938

0.430 &

-0.427

0.493 &

-0.600

1.643 &

-0.664

CCDC No. 1539092 1539093 1539089 1539090 1539091

78

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25. Houghton, A. Y. and Autrey, T., Journal of Physical Chemistry A, 2017, 121, 8785-8790.

26. Wayland, B. B., Ba, S. and Sherry, A. E., Inorganic Chemistry, 1992, 31, 148-150.

27. Merk, A., Grossekappenberg, H., Schmidtmann, M., Luecke, M. P., Lorent, C., Driess,

M., Oestreich, M., Klare, H. F. T. and Müller, T., Angewandte Chemie International

Edition, 2018, 57, 15267-15271.

28. Dong, Z., Cramer, H. H., Schmidtmann, M., Paul, L. A., Siewert, I. and Müller, T.,


29. Marwitz, A. J. V., Dutton, J. L., Mercier, L. G. and Piers, W. E., Journal of the American


30. Pan, X., Chen, X., Li, T., Li, Y. and Wang, X., Journal of the American Chemical

Society, 2013, 135, 3414-3417.



32. Chen, J. and Chen, E. Y. X., Dalton Transactions, 2016, 45, 6105-6110.

33. Lawrence, E. J., Oganesyan, V. S., Wildgoose, G. G. and Ashley, A. E., Dalton

Transactions, 2013, 42, 782-789.

34. Ashley, A. E., Herrington, T. J., Wildgoose, G. G., Zaher, H., Thompson, A. L., Rees, N.

H., Kraemer, T. and O'Hare, D., Journal of the American Chemical Society, 2011, 133,

14727-14740.

35. Rosokha, S. V. and Kochi, J. K., Accounts of Chemical Research, 2008, 41, 641-653.

36. Patz, M. and Fukuzumi, S., Journal of Physical Organic Chemistry, 1997, 10, 129-137.

37. Li, T., Wei, H., Fang, Y., Wang, L., Chen, S., Zhang, Z., Zhao, Y., Tan, G. and Wang,

X., Angewandte Chemie International Edition, 2017, 56, 632-636.

38. Back, O., Celik, M. A., Frenking, G., Melaimi, M., Donnadieu, B. and Bertrand, G.,


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39. Welch, G. C. and Stephan, D. W., Journal of the American Chemical Society, 2007, 129,

1880-1881.

40. Ménard, G. and Stephan, D. W., Angewandte Chemie International Edition, 2012, 51,

8272-8275.

41. Khan, A., Gossage, R. A. and Foucher, D. A., Canadian Journal of Chemistry-Revue

Canadienne De Chimie, 2010, 88, 1046-1052.

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43. WINSIM; O'Brien, D. A., Duling, D. R. and Fann, Y. C., National Institute of

Enviromental Health Sciences, National Institutes of Health, USA, 2002.

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Chapter 4

4 Single Electron Transfer to Lewis Acid-Base Adducts

4.1 Introduction

In chapter 3, it was shown that simple Mes3P/E(C6F5)3 (E = B, Al) FLPs can, in some instances,

react via SET pathways, providing routes to homolytic cleavage of Sn-H bonds. While in the

case of reacting with p-O2C6Cl4, the generation of 3-5 and 3-6 suggested Mes3P was acted as a

one electron donor to the Lewis acids, E(C6F5)3. This leads us to explore reactions of the

combination of reductants with Lewis acids towards small molecule activation.

4.1.1 SET Reactions with Lewis Acids

Scheme 4.1 – Selected SET examples with main group Lewis acids.

The group of Agapie demonstrated the synthesis of a dianionic bis(borane) peroxide with the

reduction of O2 by 2 equivalents of ferrocene in the presence of B(C6F5)3.1 Since the publication

of our work in this chapter,2 Erker and coworkers found that the superoxide radical anion

analogues of Agapie’s compound could be generated by reaction of TEMPO or trityl radical with

B(C6F5)3 and dioxygen,3 and later on demonstrated SET between with B(C6F5)3 and quniones.3

Chen proposed the formation of the transient [•Al(C6F5)3]– followed by decomposition whilst

82

heating a C6D5Br solution of Al(C6F5)3 with ferrocene at 100 °C for 3 days.4 In 2019, Wildgoose

and co-workers have demonstrated homolytic cleavage of H2 with the boron radicals generated

from the reduction of the corresponding boranes.5

4.1.2 Zn Containing Radicals

In seemingly disparate chemistry, it was noted that reduction of Zn(II) has yielded a few

examples of Zn complexes formally containing Zn(I)-Zn(I) moieties.6 While previous studies

have described Zn-containing biradicals (S = 1) containing quinone,7-9 o-iminoquinone,10-14 and

carbenes15-19 ligands, radical Zn-containing anions have only been detected via photochemical

techniques20, 21 but are thought to play an essential role in molecular spin memory devices.22 The

use of redox-active ligands has been well studied in the literature and has been successfully

applied to the preparation of transition metal and main group containing radicals, and used in

catalytic reactions. One of the most widely used ligands groups is the oxolene family, which

have been used to prepare many quinonoid metal complexes.

4.1.3 Main Group N2 Activation

Figure 4.1 - Interactions of main group systems with N2-fragments.

Main group interactions with N2 have drawn much less attention than other small molecules. A

number of computational studies have addressed the interactions of N2 with Lewis acids, while

the species (N2)BF3 was spectroscopically characterized upon generation via supersonic

expansion at 600 torr and 170 K.23 The compound Ph3PNNPPh324 has described N2 stabilized by

two phosphine donors,25 although this species was not derived directly from N2. In a truly

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seminal findings, Braunschweig et al.26, 27 have described the first transition metal-free capture of

N2 via the generation of a cAAC-stabilized borylene.

In efforts by the Stephan group towards main group-N2 chemistry, studies involving

diazomethanes (which liberate N2) was initiated as such systems may provide insight for the

design of main group systems for N2 capture.28 In 2012, the Stephan group reported the insertion

of diazomethanes into B-C bonds of electrophilic boranes with the liberation of N2 (Figure 4.1).29

In more recent work,30 it was shown that the sterically-encumbered diazomethane, Ph2CN2 forms

a highly reactive borane-adduct, (Ph2CN2)B(C6F5)3, which raises the possibility of N2 capture

with metal-free Lewis pair system. This species was isolated at low temperature but was found

to liberate N2 at ambient temperature.


4.2.1 SET Reactions with E(C6F5)3 (E = B, Al)

Initial control experiments showed that the combination of Cp*2Fe with E(C6F5)3 (E = B or Al)

in a molar ratio of 1:1 in toluene led to the slow formation of the decamethylferrocenium cation

[Cp*2Fe]+, in less than 20 % conversion after 24 h for B(C6F5)3 and 1 h for Al(C6F5)3. This was

concurrent with the formation of unidentified by-products as evidenced by 19F NMR

spectroscopy. These results suggest SET from Cp*2Fe to strong Lewis acid E(C6F5)3 generating

[•E(C6F5)3]–. These species are known to be extremely reactive and are quenched via reaction

with solvent or itself at room temperature.31, 32 Additionally, as radical compounds are generally

reactive, attempts to observed the transient radical compounds with EPR studies were all

unsuccessful. Nonetheless, the possibility of the activation of a substrate molecule by transient

[•E(C6F5)3]– was examined.

4.2.1.1 Radical trap

The corresponding combination of benzoyl peroxide with E(C6F5)3 (E = B or Al) led to a mixture

of what are presumed to be weak Lewis adducts. However slow addition of benzoyl peroxide to

a mixture of E(C6F5)3 (E = B and Al) and Cp*2Fe selectively afford products 4-1a and 4-1b,

respectively (Scheme 4.2). The formulation of the salts [Cp*2Fe][PhC(O)OE(C6F5)3] (E = B 4-

1a, Al 4-1b) were confirmed by X-ray diffraction studies (Figures 4a and Figure 4.2b). These

reactions can be formally viewed as a homolytic cleavage of the peroxide O-O bond by a

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transient [•E(C6F5)3]– although the precise mechanistic picture may be more complicated. As

such, these observations are analogous to the capture of a free boryl radical with 0.5 equivalent

of benzoyl peroxide reported by Yamashita, Ohkoshi and Nozaki.33

Figure 4.2 - ORTEPs of the anions of 4-1a (a), 4-1b (b), 4-2a (c) and 4-2b (d) with thermal


N: blue; Al: cyan; The [Cp*2Fe]+ cations and H atoms are omitted for clarity.

Figure 4.3 – Computed LUMOs for B(C6F5)3 and Al(C6F5)3 and TEMPO adducts. Hydrogen

atoms have been omitted for clarity.

85

Scheme 4.2 - Synthesis of 4-1a, 4-1b, 4-2a and 4-2b.

While Cp*2Fe does not react with TEMPO, B(C6F5)3 is known to form a TEMPO adduct.34 In a

similar fashion, Al(C6F5)3 also forms a neutral radical adduct within seconds as supported by 19F

NMR spectroscopic data. DFT calculations reveal that the spin density of the radical adducts

TEMPO-E(C6F5)3 (E = B and Al) is predominantly distributed over the N [62.6 % and 61.6 %]

and O [31.5 % and 31.6 %] centers, respectively. The small WBIs of the E-O bond (0.47, E = B

and 0.29, E = Al) suggest a weakly dative description. The lowest unoccupied molecular orbitals

(LUMOs) are mainly localized at the E centers, as well as the π* orbitals of pentafluorophenyl

rings (Figure 4.3), indicating the Lewis acidity of both B and Al centers in the weakly combined

adducts. Nonetheless, these species could readily accept one electron from Cp*2Fe to prompt the

formation of the products 4-2a and 4-2b (Scheme 4.2). The nature of these products was

unambiguously confirmed by X-ray crystallography to be [Cp*2Fe][(C5H6Me4NOE(C6F5)3] (E =

B 4-2a, Al 4-2b, Figure 4.2a and b). The X-ray diffraction study of 4-2a revealed a drastic

lengthening of the O-N bond (1.461(4) Å) compared to that of the adduct of B(C6F5)3 with

TEMPO (1.311(2) Å). In addition, the B-O bond of 1.493(5) Å in anion of 4-2a is significantly

shorter than that reported for the neutral adduct (1.594(3) Å).35

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4.2.1.2 Reactions with Elemental O, S, Se, Te

Scheme 4.3 - Reactions of O2, S8, Se and Te with E(C6F5)3 and Cp*2Fe.

Alkylaluminum(III) compounds are known to react with O2 to ordinarily give aluminum

alkoxides or alkylperoxides via an insertion the Al-alkyl bonds.36-38 Exposure of a chlorobenzene

solution of Al(C6F5)3 to excess dry O2 at room temperature resulted in a light yellow solution.

However, the 19F NMR spectra showed no new species formed in 2 h. Upon standing in O2

atmosphere at room temperature for 36 h, Al(C6F5)3 decomposed slowly affording an

unidentified complex mixture.

In contrast to the Agapie synthesis of a dianionic bis(borane) peroxide by the direct reduction of

O2 by ferrocenes in the presence of B(C6F5)3,1 treatment of Al(C6F5)3/O2 in chlorobenzene with

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Cp*2Fe immediately gave a deep green solution. Although the 19F NMR spectra displayed a

complex mixture, after work-up salts 4-3 and [Cp*2Fe][Al(C6F5)4] were isolated (Scheme 4.3).

X-ray diffraction studies revealed the formulation of 4-3 as [Cp*2Fe][((C6F5)2Al(µ-

O)Al(C6F5)3)2] (Figure 4.4a). This confirms the cleavage of the O-O bond affording the unusual

dianionic species 4-3 with an Al2O2 core. The geometry of Al2O2 core is planar, featuring

average Al-O bond lengths of 1.828(5) Å, while the exocyclic O-Al(C6F5)3 distances averaged

1.813(5) Å. The Al-Al separation was found to be 2.656(3) Å, suggesting a very weak

interaction between two Al centers. NBO analysis revealed that the central Al atoms (1.95 and

1.95 a.u.) carried the most positive charges, while the O atoms (-1.41 and -1.42 a.u.) are

negatively charged.

Elemental selenium was also shown to react with Al(C6F5)3 and Cp*2Fe to form a salt 4-4

(Scheme 4.3) and [Cp*2Fe][Al(C6F5)4], as well as some unidentified by-products. The product,

[Cp*2Fe][((C6F5)2Al(µ-Se)Al(C6F5)3)2] 4-4 (Figure 4.4b) features a dianionic Al2Se2 core

analogous to that observed in 4-3. The central Al2Se2 ring is nearly square (Al−Se: 2.418 (2),

2.428(2) Å; Se−Al−Se: 92.70(2)°; Al−Se−Al: 87.29(8)°. The Al−Al distance was found to be

3.344(2) Å, which is much longer than that (2.656(3) Å) of 4-3. Two exocyclic Al(C6F5)3 units

are located in the trans-configuration of the planar Al2Se2 ring with the average Al−Se bond

length of 2.424(2) Å. The salt 4-4 is the first example of a dianionic Al2(μ-Se)2 species.

Treatment of elemental tellurium with Al(C6F5)3/Cp*2Fe in toluene at 0 °C afforded a green

suspension within 6 h. After workup, 19F NMR spectroscopy suggested two sets of independent

pentafluorophenyl resonances, indicating the generation of two major products. Indeed, the salt

4-5 with the formulation of [Cp*2Fe][(μ-Te)2(Al(C6F5)2)3] as well as [Cp*2Fe][Al(C6F5)4], were

isolated by fractional recrystallization. Single crystal X-ray structure analysis shows the anion of

4-5 to be a bicyclo[1,1,1]pentane-like core with two tricoordinated Te atoms and three

tetracoordinated Al atoms (Figure 4.4c). The average Te−Al bond length is 2.658(5) Å. The

pentafluorophenyl substituents exhibit π−π stacking surround the Al3Te2 core. Computations at

M06/TZVP//M06/SVP level predict the average charge of Al and Te atoms to be 1.14 a.u. and -

0.55 a.u., respectively.

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Figure 4.4 - ORTEPs of the anions of 4-3 (a), 4-4 (b), 4-5 (c) and 4-6 (d) with thermal


Se: feldspar; Te: gold; Al: cyan; The [Cp*2Fe]+ cations are omitted for clarity.

Performing the reaction with S8, B(C6F5)3 and Cp*2Fe resulted in conversion to a single boron-

containing product as suggested by a singlet signal at -10 ppm in the 11B NMR spectrum and one

set of signals in 19F NMR spectrum. Single crystals of 4-6 (Figure 4.4d) suitable for a X-ray

diffraction studies were grown by slow vapor diffusion of pentane into a saturated

dichloromethane solution at -20 ° C. These data revealed the formulation of 4-6 as [Cp*2Fe][(µ-

S7)B(C6F5)3)2]. The dianion features a bent S7 chain with the average of S-S bond lengths of

2.036(3) Å. The B-S bond lengths are 1.965(9) and 1.988(8) Å, which is in the range of B-S

single bonds. The S7 chain carries a negative charge of -0.47 a.u., whereas boron atoms (0.42 and

0.42 a.u.) are positively charged. Repeated experiments using varying equivalents of S8 also

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afforded 4-6 suggesting that this species has both thermodynamic stability and appropriate

solubility properties to permit isolation.

4.2.2 SET Reactions with Zn(C6F5)2

In an initial attempt to demonstrate the SET reaction approach, a 1:1 combination of the electron

donor Cp*2Fe and the Lewis acid Zn(C6F5)2 in C6D5Br mixed at ambient temperature for 24 h

showed no reaction, as evidenced by 1H and 19F NMR spectroscopy. Nevertheless, treatment of

the mixture in C6H5Cl with 0.5 equivalents of [PhC(S)S)2] immediately gave rise to a deep green

solution. After work-up, the new species 4-7 was isolated as a green powder in 82 % yield. The

formulation of 4-7 as [Cp*2Fe][PhC(S)SZn(C6F5)2] was confirmed by single crystal X-ray

diffraction studies (Figure 4.5b). The zinc center is tetra-coordinated in a pseudo-tetrahedral

geometry with Zn-S bond lengths of 2.461(1) and 2.464(1) Å that are significantly longer than

the sum of the S and Zn covalent radii (2.27 Å)39 while the angle of S(1)-Zn-S(2) of 72.56(4)° is

much more acute than that the C(1)-Zn-C(7) angle of 128.2(2)°. DFT computations performed at

the M06/Def2-TZVP//M06/Def2-SVP level of theory and the subsequent NBO analysis of the

anion of 4-7 showed that two sulfur atoms each carry partial negative charges of -0.45 a.u., while

the zinc center is positively charged (1.38 a.u.). These results, coupled with the very small WBIs

found for the Zn-S bonds (0.15 each), lead to a description of the anion as a Lewis adduct of

thiocarboxylate with Zn(C6F5)2.

Independent combination of (PhC(S)S)2 with Zn(C6F5)2 gave rise to multinuclear NMR

spectroscopic data consistent with the formation of a weak adduct, while no reaction was

spectroscopically observed between (PhC(S)S)2 and Cp*2Fe in the absence of Zn(C6F5)2. This

observation, in addition to the absence of reaction between Cp*2Fe and Zn(C6F5)2, suggests that

the formation of 4-7 results from electron transfer from Cp*2Fe to the Zn-dithioperoxyanhydride

adduct effecting homolytic cleavage of the S-S bond.

90

Figure 4.5 - (a) Synthesis of 4-7; (b) ORTEP of the X-ray structure of the anion of 4-7 with

thermal displacement parameters drawn at 50 % probability. C: black; F: pink; S: yellow; Zn:

blue. All hydrogen atoms and [Cp*2Fe]+ cations have been omitted for clarity; (c) Picture of 4-7

crystals taken under a microscope.

This demonstration of a facile SET prompted us to probe the utility of this strategy to target zinc-

containing radical anions. To this end, the diones 9,10-phenanthrenedione (o-C14H8O2) and 4,5-

pyrenedione (o-C16H8O2) were individually combined with Zn(C6F5)2, each affording a dark red

solution. Multinuclear NMR spectroscopic data were consistent with the formation of Lewis

adducts 4-8a and 4-8b, respectively (Figure 4.7a),6, 40 similar to the observations for the FLP

system consisting of B(C6F5)3/dione.35 The formulations of 4-8a and 4-8b as

[(C14H8O2)Zn(C6F5)2] and [(C16H8O2)Zn(C6F5)2] were further confirmed by crystallographic

studies (Figure 4.7b and c). For 4-8a the Zn-O distances were found to be 2.178(2) and 2.159(2)

Å while the C(13)-C(14) and C-O distances are 1.527(4) Å, 1.234(4) and 1.225(4) Å). The

analogous Zn-O, C(13)-C(14) and C-O distances in 4-8b were found to be (2.173(2), 2.135(2) Å,

1.542(4) Å, 1.225(3) and 1.219(3) Å, respectively. It is also interesting to note that 4-8b co-

crystalized with a equivalent of o-C16H8O2.

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Figure 4.6 - Energy (eV) of LUMO of (a) o-C14H8O2 and 4-8a and (b) o-C16H8O2 and 4-8b.

Figure 4.7 - (a) Synthesis of 4-8a/b and 4-9a/b; ORTEPs of the X-ray structure of (b) 4-8a (c) 4-

8b with thermal displacement parameters drawn at 50 % probability. C: black; F: pink; O: red;

Zn: blue. Hydrogen atoms have been omitted for clarity. (d) Picture of 4-8a crystals taken under

92

the microscope. (e) Picture of 4-8b crystals taken under the microscope, where the dark orange

block-shaped crystal is 4-8b and the orange needle-shaped crystal is o-C16H8O2.

DFT calculations reveal that the LUMOs of 4-8a and 4-8b are primarily π*-antibonding orbitals

associated with the dione. The orbital energies of -4.16 eV and -4.26 eV, respectively, are

significantly lower than those of the free diones (-3.01 eV and -2.99 eV) (Figure 4.6), suggesting

the adduct should undergo facile reduction. Indeed, the cyclic voltammograms of 4-8a and 4-8b

show reversible reduction waves at -0.58 and -0.48 V versus Fc+/Fc, respectively. In each case, a

further redox process was also seen at -1.57 V attributable to the reduction of the free respective

diones. (Figure 4.18, Figure 4.19, Figure 4.20 and Figure 4.21). These observations are

consistent with the lability of the interaction between Zn(C6F5)2 and the diones in 4-8a and 4-8b.

Figure 4.8 - (a). Experimental and simulated X-band EPR spectrum of isolated 4-9a; (b)

Calculated spin density plot of the anion of 4-9a; (c) Experimental and simulated X-band EPR

spectrum of isolated 4-9b; (d) Calculated spin density plot of the anion of 4-9b.

93

Chemical reduction of 4-8a and 4-8b with one equivalent of Cp*2Fe afforded green solutions

from which species 4-9a and 4-9b were quantitatively isolated as black powders (Figure 5). The

19F NMR spectra of 4-9a and 4-9b are completely silent. Single crystals of 4-9a and 4-9b

suitable for an X-ray diffraction study were grown by slow vapor diffusion of pentane into a

saturated ortho-dichlorobenzene solution at room temperature. X-ray diffraction studies revealed

the formulations of 4-9a and 4-9b as [Cp*2Fe][((C14H8O2)Zn(C6F5)2)•] and [Cp*2Fe]

[((C16H8O2)Zn(C6F5)2)•], respectively (Figure 4.15 and Figure 4.16). The Zn-O and C(13)-C(14)

bond lengths in 4-9a (2.055(5) and 2.068 Å; 1.44(1) Å) and in 4-9b (2.047(2) and 2.069(2) Å;

1.447(3) Å) are significantly shorter than those in the corresponding neutral adducts. On the

other hand, the C-O bonds (4-9a: 1.274(9) and 1.276(9) Å and 4-9b: 1.281(3) and 1.283(3) Å)

are lengthened in comparison to the adducts. Similarly the O(1)-Zn-O(2) bond angles are

increased for the anions. Collectively, these observations suggest enhanced electron

delocalization in 4-9a and 4-9b.

The EPR spectra of 4-9a and 4-9b were recorded and simulations were performed to model the

hyperfine couplings observed (Figure 4.8). In both cases, isotropic g values of 2.0045 were seen.

In the case of 4-9a, hyperfine coupling to the hydrogen atoms were simulated as 1.7 G, 0.4 G,

1.9 G and 0.7 G; while for 4-9b: hyperfine constants of 2.0 G, 0.6 G, 2.3 G and 0.2 G were

obtained. The computed EPR parameters using DFT calculations at the uM06/Def2-

TZVP//uM06/Def2-SVP level of theory were in excellent agreement with these values (Table 4.7

andTable 4.8). Spin densities were computed to be primarily delocalized on the aromatic

skeleton with little relative population on the zinc center (4-9a: 0.4 %; 4-9b: 0.8 %).

The salts of radicals 4-9a and 4-9b are thermally stable at room temperature, both in the solid

state and in solution, but rapidly decompose when exposed to moisture. These species were not

found to react with tBu3P, in contrast to the closely related borocyclic radicals ((C6F5)2B(o-

C14H8O2)• and ((C6F5)2B(o-C16H8O2)•), for which nucleophilic aromatic C-H substitution was

seen.35, 41 However, treatment of a green C6D5Br solution of 4-9a with two equivalents of DMAP

immediately produced a light orange solution. The resulting solution was EPR silent, but gave

rise to 19F NMR signals (-115.1, -158.6, and -162.0 ppm) attributable to the known compound

Zn(C6F5)2(DMAP)2.6 1H NMR data were also consistent with the liberation of the dione o-

C14H8O2 and Cp*2Fe in a 1:1 ratio (Figure 4.9 and Figure 4.10). Similar results were obtained

upon treatment of 4-9b with DMAP. These observations suggest the Zn-O bonds are susceptible

94

to dissociation and replacement by sterically accessible Lewis bases such as DMAP. It also

appears that such displacement (partial or complete) of the Zn(C6F5)2 moiety from 4-9a and 4-9b

alters the redox potential of the transient dione-based radical allowing it to effect the one-

electron reduction of the decamethylferrocenium cation, regenerating decamethylferrocene and

liberating the dione with concurrent formation of the Zn-adduct, [Zn(C6F5)2(DMAP)2] (Figure

4.11).

Figure 4.9 – 1H NMR spectrum of the reaction between 4-9a with DMAP.

95

Figure 4.10 - 1H NMR spectrum of the reaction between 4-9b with DMAP.

Figure 4.11 – Summary reaction scheme for the reactions between 4-8a/b and 4-9a/b.

96

4.2.3 SET Reactions with Diazo Compounds

Scheme 4.4 - DFT Calculation on free energies for the formation of the (Ph2CN2)BR3 and

[Ph2CN2(BR3)•]- (R = C6F5 and Ph).

In previous work our group showed that weak Lewis acid-base adducts were stabilized upon

reduction. This prompted us to probe the reductive stabilization of (Ph2CN2)B(C6F5)3, precluding

the release of N2. DFT calculations at the SMD-M06-2X/Def2-TZVP//M06-2X/Def2-SVP level

of theory showed the free energies for adduct formation of (Ph2CN2)BR3 (R = C6F5 and Ph) are -

3.5 and 3.7 kcal/mol respectively. Interestingly, the corresponding free energies of the radicals

[Ph2CN2(BR3)]•-, were significantly lower at -13.5 and -23.1 kcal/mol, respectively. In the

present work, it is demonstrated that indeed single electron transfer to the diphenyldiazomethane

adducts stabilize the weak B-N interactions, providing reactive transient radicals which effect C-

H bond activation.

A 1:1 combination of diphenyldiazomethane (Ph2CN2) with B(C6F5)3 in chlorobenzene was

stirred at -35 °C. Treatment of the reaction mixture with an equal molar amount of Cp*2Co

immediately gives a yellow solution. The crude 19F NMR spectrum showed two sets of

resonances at -134.0, -163.9, and -167.3 ppm and -130.2, -161.9, -162.7 ppm attributable to

inequivalent C6F5 rings. The 11B NMR spectrum showed two resonances at -7.6 and -13.0 ppm

attributable to two tetra-coordinated boron species, 4-10 and 4-11, respectively. 1H NMR data

showed resonances at 6.61 and 2.43 ppm attributable to NH and CH2 fragments. Fractional

recrystallization permitted the formulations of the two products 4-10 and 4-11 via X-ray

crystallographic analysis. Compound 4-10 was found to be the salt

[Cp*2Co][Ph2CNNHB(C6F5)3] (Figure 4.12b). While the cation was unexceptional, the anion

was derived from the interaction of the hydrazide bound to borane, affording a diazoborate

derivative. The B-N(H) bond length was found to be 1.539(7) Å, while the N-N and N-C bonds

97

lengths were 1.342(5) and 1.303(7) Å, respectively. The B-N-N angle was determined to be

118.2(3)° while the C-N-N angle is 120.9(4)°. The second isolated product was confirmed to be

[Cp*2Co(C5Me4CH2B(C6F5)3)] 4-11 via a single crystal X-ray diffraction study (Figure 4.12c).

In this species, one of the hydrogen atoms in one of the Cp* methyl groups has been replaced by

borane, affording the zwitterionic Co(III)-borate species 4-11, with a methylene-boron B-C bond

length of 1.66(1) Å.

Figure 4.12 – (a) Reactions of diazomethane with B(C6F5)3 and Cp*2Co. ORTEPs of the anion

of (b) 4-10 and (c) 4-11 with thermal displacement parameters drawn at 50 % probability. The

[Cp*2Co]+ cation and hydrogen atoms, except the NH, are omitted for clarity. C: black, N: blue;

B: yellow-green, H: white.

Collectively, the identification of 4-10 and 4-11 are consistent with two possible reaction

mechanisms involving single electron transfer from a Co(II) center to either B(C6F5)342 or the

diazomethane adduct of the borane, Ph2CN2(B(C6F5)3).30 It is noteworthy that the anion

[HB(C6F5)3]-, which would be generated from C-H activation by the radical [B(C6F5)3)•]

-,43

98

showed no reaction with diazomethane when independently combined and studied by NMR

spectroscopy. This infers that compound 4-10 is formed via hydrogen atom abstraction from

Cp*2Co by the transient diazomethane-borane adduct radical [Ph2CN2(B(C6F5)3•]-. (Figure

4.12a). This accounts for the overall reaction ratio of diazomethane: Cp*2Co and B(C6F5)3 being

1: 2: 2. Efforts to observe the transient radical with EPR studies unsuccessful, consistent with

rapid hydrogen atom abstraction.

Figure 4.13 – (a) Reactions of Ph2CN2, BPh3 and Cp*2Cr. ORTEPs of the anion of (b) 4-12 and

(c) 4-13 with thermal displacement parameters drawn at 50 % probability. The [Cp*2Cr]+ cation

and hydrogen atoms, except the NH are omitted for clarity. C: black, N: blue; B: yellow-green,

H: white.

Diphenyldiazomethane (Ph2CN2) was combined with BPh3 in chlorobenzene at -35 °C.

Monitoring the reaction progress by multinuclear NMR spectroscopy revealed no evidence of

99

adduct formation. This is consistent with the poor Lewis basicity of the diazomethane and the

weaker Lewis acidity of the BPh3 in comparison with B(C6F5)3, in line with the computed free

energies in Scheme 4.4. Addition of Cp*2Cr to a mixture of BPh3 and diazomethane at -35 °C

generated an orange solution. The 11B NMR spectrum showed resonances at 23.0 and -3.5 ppm

consistent with the formation of two products, 4-12 and 4-13 which were isolated by fractional

crystallization. An X-ray diffraction study revealed that the species 4-12 to be

[Cp*2Cr][PhC(C6H4)NNBPh3] (Figure 4.13). While the cation was typical, the anion of 4-12 was

shown to be a borate with a substituent derived from the cyclization of the N2 fragment onto the

ortho position of one of the aryl rings on the diazomethane carbon. The resulting five membered

ring which is fused to the aryl ring is 1,3-disubstituted with the phenyl ring on carbon and BPh3

bound to nitrogen. The resulting N-B bond is 1.566(6) Å, while the N-N and new N-C bond

distances were determined to be 1.312(5) Å and 1.351(5) Å, respectively. The second product 4-

13 was also characterized crystallographically revealing its formulation as

[Cp*2Cr][Ph2CNNHBPh3] (Figure 4.13). The B-N and N-N distances in the anion of 4-13 were

determined to be 1.562(4) Å and 1.322(3) Å, respectively.

Figure 4.14 – (a) Reactions of C12H8CN2, BPh3 and Cp*2Cr. ORTEPs of the anion of (b) 4-15

and (c) 4-16 with thermal displacement parameters drawn at 50 % probability. The [Cp*2Cr]+

100

cation and hydrogen atoms, except the NH are omitted for clarity. C: black, N: blue; B: yellow-

green, H: white.

The formation of 4-12 and 4-13 demonstrates that electron transfer can provide an avenue to

stabilize a normally very weak diazomethane-Lewis acid interaction. Moreover, the transient

radical, [(Ph2CN2BPh3)•]- is very reactive, as it effects hydrogen atom abstraction in a

bimolecular fashion affording compounds 4-12 and 4-13.

These results encouraged us to attempt reactions of fluorenyldiazomethane (C12H8CN2) with

BPh3 and Cp*2Cr at -35 °C. Upon warming to room temperature and monitoring by 11B NMR

spectroscopy the generation of three products was evidenced by the presence of resonances at

26.2, 2.4 and -1.7 ppm. The peak at -1.7 ppm was isolated and identified as

[Cp*2Cr][C12H8CNNHBPh3] 4-15. The similarity of the chemical shift at 26.2 ppm to that

observed for 4-12 suggests the formulation of a second product as the analogous cyclized

product [Cp*2Cr][C6H4C(Ph)N2(BPh3)] 4-14. Finally, third species formed in the reaction 4-16

was isolated by crystallization and was unambiguously identified as [Cp*Cr(C5Me4CH2BPh3)]

via X-ray crystallography (Figure 4.14).

Table 4.1 - Computed spin density for the proposed radicals [Ph2CN2B(C6F5)3•]- (left),

[C12H8CN2BPh3•]- (middle) and [Ph2CN2BPh3•]- (right).

[Ph2CN2B(C6F5)3•]- [C12H8CN2BPh3•]- [Ph2CN2BPh3•]-

N1 (Boron bound) 68.9 % 64.8 % 63.2 %

N2 (N bound) 21.0 % 30.0 % 25.6 %

B 4.1 % 2.4 % 1.8 %

Cortho 1.5 % 1.1 % 1.8 %

101

To probe this reactivity difference, DFT calculations and NBO analyses were performed at the

uM06-2X/Def2-TZVP//uM06-2X/Def2-DefSVP level of theory to model the radicals

[Ph2CN2(BR3)•]- (R = C6F5, Ph) and [C12H8CN2(BPh3)•]

-. Spin density distributions were

computed (Table 4.1). As might be expected for the B(C6F5)3 derivative, the spin density is more

localized on the nitrogen atom adjacent boron, whereas for the BPh3 system the spin density is

dispersed onto the aryl rings of the diazomethane fragment. These observations are consistent

with the formation of 4-10 and 4-11 from the [B(C6F5)3•]- derived radical and the products 4-12

to 4-16 from the [BPh3•]- based radicals. Detailed computational works on the reaction

mechanism is currently under investigation

4.3 Conclusions

In general, the reactions described herein provide a novel route to unique salts featuring main-

group radical anions. These reactions can be viewed from two perspectives. One view involves

delivery of one electron (from Cp*2Fe, Cp*2Cr, and Cp*2Co) to the combination of a Lewis acid

(E(C6F5)3, E = B, Al, Zn(C6F5)2 and BPh3) and base (chalcogenide species and diazomethane

compounds). An alternatively perspective considers these reactions of a one-electron donor and

Lewis acid with a chalcogenide substrate molecule. In this latter view, the present results open a

new pathway to the generation of a transient frustrated radical pair (FRP) and a radical pathway

for FLP chemistry. Such an avenue has recently been uncovered and substantiated (Chapter 3).

Interestingly, the radical generated with the Zn(C6F5)2 and dione 4-9a and 4-9b undergo facile

ligand exchange with DMAP to regenerate [Cp*2Fe], and the dione along with

Zn(C6F5)2(DMAP)2. The isolated compounds 4-9a and 4-9b represents the first examples of Zn-

containing radical anions. Moreover, it was demonstrated that SET to highly unstable

diazomethane-borane adducts arrests the release of N2, which alternatively provides reactive

radical anions that undergo hydrogen atom abstraction from both sp3 and sp2 C-H bonds. The

further application of such SET reactions to simple donor-acceptor systems continues to be of

interest. In addition, the exploration of the chemistry of these unique radical anions is the subject

of ongoing study.

102

4.4 Experimental



standard Schlenk or glovebox techniques (MBraun LABmaster SP dry box and Innovation Tech-

nology glovebox both equipped with a -35 °C freezer). Toluene, pentane, and DCM were

collected from a Grubbs-type column system manufactured by Innovative Technology. These

solvents, along with fluorobenzene (C6H5F) and 1,2-dichlorobenzene (1,2-C6H4Cl2), were dried

over 4 Å molecular sieves. Molecular sieves, type 4 Å (pellets, 3.2 mm diameter) purchased

from Sigma Aldrich and were activated prior to usage by iteratively heating in a 1050 W Haier

microwave for 5 minutes and cooling under vacuum. The process was repeated until no further

moisture was released upon heating. CD2Cl2, and C6D5Br solvent purchased from Cambridge

Isotope Laboratories were degassed and stored over 4 Å molecular sieves in the glovebox for at

least 8 h prior to use. C6H5Cl, DFB, benzene (C6H6) and CDCl3 were degassed and dried over

calcium hydride. B(C6F5)3 was purified by sublimation at 100 °C followed by stirring with

chlorodimethylsilane at room temperature for two hours, and dried under vacuo. Unless

otherwise mentioned, chemicals were purchased from Sigma Aldrich, Strem Chemical or TCI.

Al(C6F5)3•tol,4 Zn(C6F5)2,44 o-C16H8O2,

45 Zn(C6F5)2DMAP2,6 Ph2CN2 and C12H8CN2

30 were

prepared using literature methods. Spectra were recorded on a Bruker Avance III 400 MHz, an

Agilent DD2 500 MHz, and an Agilent DD2 600 MHz spectrometer and spectra were referenced

to residual solvents of CD2Cl2 (1H = 5.32 ppm; 13C = 53.8 ppm), CDCl3 (

1H = 7.26 ppm; 13C =

77.0 ppm), C6D6 (1H = 7.16 ppm and 13C = 128.06 ppm) or externally [11B: (Et2O)BF3 (δ 0.00),

19F: CFCl3 (δ 0.00)]. Chemical shifts (δ) are reported in ppm and the absolute values of the

coupling constants (J) are in Hz. High-resolution mass spectra (HRMS) were obtained on a JMS-

T100LC JOEL DART. Electron paramagnetic resonance (EPR) measurements were performed

at 298 K using a Bruker ECS-EMX X-band EPR spectrometer equipped with an EP4119HS

cavity. The EPR spectra were collected with 2 mM solutions of compound 4-9a & 4-9b in

C6H5Cl. Simulations were performed using PEST WinSIM Software.46 Elemental analysis (C, H,

N) were also attempted for the compounds, but results repeatedly indicated low carbon content,

while providing satisfactory H and N values. This phenomenon is attributed to the formation of

boron carbides during combustion and has been previously reported for other boron compounds

in previous chapter as well.

103

4.4.2 Synthesis and Characterization

Synthesis of the salt [Cp*2Fe][PhC(O)OB(C6F5)3] (4-1a)

A mixture of Cp*2Fe (16.3 mg, 0.050 mmol) and B(C6F5)3 (26.0 mg, 0.050 mmol) was stirred in

3 mL of DCM. Benzoyl peroxide (6.1 mg, 0.025 mmol) was added slowly into solution. After 1

h, pentane (5 mL) was added into the solution with rapid stirring. The green solid was obtained

by filtration and washed with pentane (1 mL). All volatiles/solvents were removed in vacuo to

afford the salt 4-1a as a green solid (31.7 mg, 66 %). Single crystals of 4-1a suitable for X-ray

diffraction were obtained from layering of pentane with a saturated DCM solution at -15 °C.

1H NMR (400 MHz, CD2Cl2, 298 K): δ 8.12 (bs, 2H, o-C6H5), 7.52 (bs, 3H, p-C6H5 and m-

C6H5), -33.75 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K, B-bonded C could not be

observed): δ 167.8 (C=O), 148.3 (dm, 1JCF = 249 Hz, o-C6F5), 137.0 (dm, 1JCF = 255 Hz, p-

C6F5), 136.3 (dm, 1JCF = 246 Hz, m-C6F5), 131.4 (s, p-C6H5), 129.9 (s, CC(O)O), 128.6 (s, o-

C6H5), 128.2 (s, m- C6H5). 19F{1H} NMR (377 MHz, CD2Cl2, 298 K): δ -135.0 (d, 3JFF = 21 Hz,

6F, o-C6F5), -163.4 (t, 3JFF = 20 Hz, 3F, p- C6F5), -167.6 (m, 6F, m- C6F5). 11B{1H} NMR (128

MHz, CD2Cl2, 298 K): δ -4.7 (bs). HRMS (DART) calcd. for [C25H5BF15O2]- 633.0143, found

633.0137. Anal. Calcd. for C45H35BF15FeO2: calcd.: C 56.34 %, H 3.68 %, found: C 56.70 %, H

3.80 %.

Synthesis of the salt [Cp*2Fe][PhC(O)OAl(C6F5)3] (4-1b)

A mixture of Cp*2Fe (16.3 mg, 0.050 mmol) and Al(C6F5)3•tol (31.0 mg, 0.050 mmol) was

stirred in 3 mL of C6H5Cl. Benzoyl peroxide (6.1 mg, 0.025 mmol) was added slowly into

solution. After 2 min, pentane (5 mL) was added into the solution with rapid stirring. The green

solid was obtained by filtration and washed with pentane (1 mL). All volatiles/solvents were

removed in vacuo to afford the salt 4-1b as a green solid (37.1 mg, 76 %). Single crystals of 4-1b

suitable for X-ray diffraction were obtained from layering of pentane with a saturated C6H5Cl

solution at room temperature.

1H NMR (400 MHz, CD2Cl2, 298 K): δ 8.35 (bs, 2H, o-C6H5), 7.70 (bs, 3H, p-C6H5 and m-

C6H5), -36.75 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K, Al-bonded C could not be

observed): δ 169.6 (C=O), 150.4 (dm, 1JCF = 231 Hz, o-C6F5), 141.1 (dm, 1JCF = 245 Hz, p-

104

C6F5), 136.9 (dm, 1JCF = 252 Hz, m-C6F5), 135.2 (s, p-C6H5), 131.9 (s, o-C6H5), 130.6 (s,

CC(O)O), 128.5 (s, m-C6H5). 19F{1H} NMR (377 MHz, CD2Cl2, 298 K): δ -123.3 (bs, 6F, o-

C6F5), -158.3 (bs, 3F, p-C6F5), -164.6 (bs, 6F, m-C6F5). 27Al NMR (104 MHz, CD2Cl2, 298 K): δ

108.9 (bs). Anal. Calcd. for C45H35AlF15FeO2: calcd.: C 55.40 %, H 3.62 %, found: C 55.51 %,

H 3.78 %.

Synthesis of the salt [Cp*2Fe][(C5H6Me4NOB(C6F5)3] (4-2a)

A mixture of Cp*2Fe (16.3 mg, 0.050 mmol), TEMPO (7.8 mg, 0.050 mmol) and B(C6F5)3 (26.0

mg, 0.050 mmol) was stirred in 3 mL of toluene. After 1 min, pentane (5 mL) was added into the

solution with rapid stirring. The green solid was obtained by filtration and washed with pentane

(1 mL). All volatiles/solvents were removed in vacuo to afford the salt 4-2a as a green solid

(39.3 mg, 90 %). Single crystals of 10 suitable for X-ray diffraction were obtained from layering

of pentane with a saturated DCM solution at room temperature.

1H NMR (400 MHz, CD2Cl2, 298 K): δ 1.54 (bs, 4H, CH2), 0.98 (s, 12H, CH3), -36.74 (bs,

Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K): δ 147.9 (dm, 1JCF = 237 Hz, o-C6F5), 138.7

(dm, 1JCF = 247 Hz, p- C6F5), 136.7(dm, 1JCF = 248 Hz, m-C6F5), 124.6 (bs, BC), 68.3 (s, NC),

36.6 (s, CH2), 35.3 (s, CH2), 19.1 (bs, CH3). 19F{1H} NMR (377 MHz, CD2Cl2, 298 K): δ -136.0

(d, 1JFF = 19 Hz, 6F, o-C6F5), -163.2 (t, 1JFF = 18 Hz, 3F, p-C6F5), -167.1 (bs, 12F, m-C6F5). 11B

NMR (128 MHz, CD2Cl2, 298 K): δ -3.9 (s). HRMS (DART) calcd. for [C27H18BF15NO]-

668.1242, found 668.1244. Anal. Calcd. for C37H48BF15FeNO: calcd.: C 50.82 %, H 5.53 %, N

1.60 %, found: C 51.00 %, H 5.77 %, N 1.68 %.

Synthesis of the salt [Cp*2Fe][(C5H6Me4NOAl(C6F5)3] (4-2b)

A mixture of Cp*2Fe (16.3 mg, 0.050 mmol), TEMPO (7.8 mg, 0.050 mmol) and Al(C6F5)3•tol

(31.0 mg, 0.050 mmol) was stirred in 3 mL of DCM. After 1 min, pentane (5 mL) was added into

the solution with rapid stirring. The green solid was obtained by filtration and washed with

pentane (1 mL). All volatiles/solvents were removed in vacuo to afford the salt 4-2b as a green

solid (45.5 mg, 90 %). Single crystals of 4-2b suitable for X-ray diffraction were obtained from

layering of pentane with a saturated 1,2-C6H4Cl2 solution at room temperature.

105

1H NMR (400 MHz, CD2Cl2, 298 K): δ 1.72 (m, 2H, CH2), 1.50 (m, 4H, CH2), 1.31 (s, 6H,

CH3), 0.98 (s, 6H, CH3), -35.56 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2, 298 K, Al-

bonded C could not be observed): δ 150.0 (dm, 1JCF = 230 Hz, o-C6F5), 140.2 (dm, 1JCF = 245

Hz, p-C6F5), 136.6 (dm, 1JCF = 255 Hz, m-C6F5), 59.0 (s, N-C), 40.5 (s, CH2), 34.1 (s, CH2), 19.0

(s, CH3), 18.1 (s, CH3). 19F{1H} NMR (377 MHz, CDCl3, 298 K): δ -121.2 (bs, 12F, o-C6F5), -

160.1 (m, 6F, p-C6F5), -165.5 (bs, 12F, m-C6F5). 27Al NMR (104 MHz, CD2Cl2, 298 K): δ 106.8.

Anal. Calcd. for C47H48NAlF15FeO: calcd.: C 55.85 %, H 4.79 %, N 1.39 %, found: C 56.30 %,

H 4.91 %, N 1.40 %.

Synthesis of the salts [Cp*2Fe][((C6F5)2Al(µ-O)Al(C6F5)3)2] (4-3) and [Cp*2Fe][Al(C6F5)4]

Dry O2 (excess, 4 atm) was added into a chlorobenzene solution (1.5 mL) of Cp*2Fe (8.2 mg,

0.025 mmol) and Al(C6F5)3•tol (31.0 mg, 0.050 mmol). Immediately, the color of the solution

changed to be light green. Pentane (5 mL) was added into the solution with rapid stirring. The

green solid was obtained by filtration and washed with pentane (1 mL). All volatiles/solvents

were removed in vacuo to afford a mixture. Single crystals of 4-3 suitable for X-ray diffraction

were obtained from slow diffusion of pentane into a saturated C6H5Cl solution at room

temperature overnight (4 mg, 13 %). After collection of the crystals of 4-3, the resulting solution

was layered with pentane to give [Cp*2Fe][Al(C6F5)4] as green crystals (7.6 mg, 30 %). The

NMR data of the salt 4-3 have not been collected since the crystals of 4-3 were contaminated

with green oily byproducts that could not be completely removed.

The salt [Cp*2Fe][Al(C6F5)4]:

1H NMR (400 MHz, CD2Cl2, 298 K): δ -36.43 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2,

298 K, Al-bonded C could not be observed): δ 150.2 (dm, 1JCF = 232 Hz, o-C6F5), 140.8 (dm,

1JCF = 245 Hz, p-C6F5), 136.8 (dm, 1JCF = 252 Hz, m-C6F5). 19F{1H} NMR (377 MHz, CDCl3,

298 K): δ -122.9 (bs, 8F, o-C6F5), -158.5 (t, 3JFF = 18 Hz, 4F, p-C6F5), -164.8 (bs, 8F, m-C6F5).

27Al NMR (104 MHz, CD2Cl2, 298 K): δ 114.6 (s). HRMS (DART) calcd. for [C24AlF20]-

694.9496, found 694.9499. Anal. Calcd. for C34H30AlF20Fe: calcd.: C 45.30 %, H 3.35 %,

found: C 45.11 %, H 3.60 %.

106

Synthesis of the salt [Cp*2Fe][((C6F5)2Al(µ -Se)Al(C6F5)3)2] (4-4) and [Cp*2Fe][Al(C6F5)4]

A mixture of Cp*2Fe (8.2 mg, 0.025 mmol), Se (2 mg, 0.025 mmol) and Al(C6F5)3·tol (31.0 mg,

0.050 mmol) was stirred in 3 mL of C6H5Cl. After 12 h, pentane (5 mL) was added into the


(1 mL). All volatiles/solvents were removed in vacuo and the resulting green solid was collected

and fractional recrystallized to afford 4-4 (6.3 mg, 20 %) and [Cp*2Fe][Al(C6F5)4] (10.7 mg, 42

%). Single crystals of 4-4 suitable for X-ray diffraction were obtained from slow diffusion of

pentane into a saturated C6H5Cl solution at room temperature.

1H NMR (400 MHz, C6D5Br, 298 K) δ -33.50 (bs, Cp*H). 13C{1H} NMR (126 MHz, C6D5Br,

298 K) no signal could be observed after 10000 scans since the low solubility of the salt 4-4.

19F{1H} NMR (377 MHz, C6D5Br, 298 K) δ -120.7 (bs, 12F, o-C6F5, Al(C6F5)3), -123.4 (d, 3JFF

= 20 Hz, 8F, o-C6F5), -154.3 (t, 3JFF = 18 Hz, 4F, p-C6F5), -158.4 (bs, 8F, m-C6F5), -162.3 (bs,

6F, p-C6F5), -165.9 (bs, 12F, m-C6F5). 27Al NMR (104 MHz, C6D5Br, 298 K) δ no signal

detected. Trace unidentified impurities exist. Efforts were made to remove the trace unidentified

impurities but failed. Thus, the elemental analyses gave unsatisfying results.

Synthesis of [Cp*2Fe][(μ-Te)2(Al(C6F5)2)3] (4-5).

A mixture of Cp*2Fe (8.2 mg, 0.025 mmol), Te (3.2 mg, 0.025 mmol) and Al(C6F5)3·tol (31.0

mg, 0.050 mmol) was stirred in 3 mL of toluene at 0 °C. After 6 h, cold pentane (5 mL) was

added into the solution with rapid stirring. The green solid was obtained by filtration and washed

with cold pentane (1 mL). All volatiles/solvents were removed in vacuo and the resulting green

solid was collected and recrystallization at -15 °C to afford 4-5 (10.0 mg, 24 %) and

[Cp*2Fe][Al(C6F5)4] (12.8 mg, 50 %). Single crystals of 4-5 suitable for X-ray diffraction were

obtained from slow diffusion of pentane into a saturated 1,2-C6H4Cl2 solution at -15 °C.

1H NMR (400 MHz, CD2Cl2, 298 K) δ -33.35 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2,

298 K, Al-bonded C could not be observed) δ 149.7 (dm, 1JCF = 231 Hz, o-C6F5), 140.4 (dm, 1JCF

= 248 Hz, p-C6F5), 136.3 (dm, 1JCF = 256 Hz, m-C6F5). 19F{1H} NMR (377 MHz, CD2Cl2, 298

K) δ -120.2 (bs, 12F, o-C6F5), -158.8 (t, 3JFF = 18 Hz, 6F, p-C6F5), -162.1 (bs, 12F, m-C6F5). 27Al

NMR (104 MHz, CD2Cl2, 298 K) δ no signal detected. The salt 4-5 decomposed gradually in

107

both solution and solid state at room temperature with the formation of black powders,

preventing the acquisition of elemental analysis.

Synthesis of the salt [Cp*2Fe][(μ-S7)B(C6F5)3)2] (4-6)

A mixture of Cp*2Fe (16.3 mg, 0.050 mmol), S8 (12.8 mg, 0.050 mmol) and B(C6F5)3 (26.0 mg,

0.050 mmol) was stirred in 3 mL of DCM. After 12 h, pentane (5 mL) was added into the


(1 mL). All volatiles/solvents were removed in vacuo to afford the salt 4-6 as a green solid (36.6

mg, 77 %). Single crystals of 4-6 suitable for X-ray diffraction were obtained from layering of

pentane with a saturated DCM solution at -15 °C.

1H NMR (400 MHz, CD2Cl2, 298 K): δ -35.94 (bs, Cp*H). 13C{1H} NMR (126 MHz, CD2Cl2,

298 K): δ 148.1 (dm, 1JCF = 241 Hz, o-C6F5), 139.8 (dm, 1JCF = 241 Hz, p-C6F5), 137.2 (dm, 1JCF

= 246 Hz, m-C6F5), 120.3 (bs, BC), 19F{1H} NMR (377 MHz, CDCl3, 298 K): δ -131.1 (bs, 12F,

o-C6F5), -162.0 (bs, 6F, p-C6F5), -166.8 (bs, 12F, m-C6F5). 11B NMR (128 MHz, CD2Cl2, 298

K): δ -10.0. Anal. Calcd. for C76H60B2F30Fe2S7: calcd.: C 48.02 %, H 3.18 %, found: C 47.69 %,

H 3.10 %.

Synthesis of [Cp*2Fe][(PhC(S)S)Zn(C6F5)2] (4-7)

A mixture of Cp*2Fe (13.0 mg, 0.040 mmol), Zn(C6F5)2•tol (19.6 mg, 0.040 mmol), and

(PhC(S)S)2 (6.1 mg, 0.020 mmol) was stirred in 2 mL of chlorobenzene at room temperature.

After 5 min, 6 mL of pentane was added to the reaction mixture, and brown solid was

precipitated out from the reaction mixture. The solvent was decanted carefully with a pipette, and

the resulting solid was washed with 3 × 3 mL of pentane, and dried in vacuo to yield a brown

powder (28.7 mg, 82 %). Single crystals for X-ray studies were obtained from slow evaporation

of a pentane into a saturated chlorobenzene solution at -35 °C to give brown crystals.

1H NMR (400 MHz, C6D5Br, 298 K, pentane omitted): δ 8.37 (s, br, 1H, p-C6H5), 7.93 (s, br,

2H, m-C6H5), 7.19 (s, br, 2H, o-C6H5) -16.14 (s, br, 30H, -CH3). 13C{1H} NMR (128 MHz,

C6D5Br, 298 K, partial due to the low solubility of 4-7): δ 129.8 (-PhC) 129.5 (-PhC), 126.9 (-

PhC), 122.3 (-PhC). 19F NMR (376 MHz, C6D5Br, 298 K): δ -115.6 (s, br, 4F, o-C6F5), -159.2

108

(s, br, 2F, p-C6F5), -162.7 (s, br, 4F, m-C6F5). Anal. Calcd. For C39H36F10FeS2Zn: C 53.28 %, H

4.12 %. Found: C 52.65 %, H 3.95 %.

Synthesis of [(C14H8O2)Zn(C6F5)2] (4-9a)

Combining a solution of Zn(C6F5)2•tol (49.1 mg, 0.100 mmol) in 2 mL of chlorobenzene with o-

C14H8O2 (20.8 mg 0.100 mmol) leads to the formation of a red solution. After 2 minutes stirring

at room temperature, 8 mL of pentane was added to the reaction mixture resulting in the

precipitation of a red solid from solution. The solvent was quickly decanted with a pipette, and

the solid was washed with 3 × 3 mL of pentane, and dried under vacuum quickly to yield a red

powder (58.0 mg, 96 %). The compound is very sensitive to temperature, and will decompose

even store at -35 °C after a week. Single crystals suitable for X-ray studies were obtained from

layering a saturated chlorobenzene solution with pentane at -35 °C.

1H NMR (400 MHz, C6D5Br, 298 K, toluene omitted): δ 7.88 (d, br, 3JHH = 7.8 Hz, 2H, -C14H8,),

7.32 (m, 4H, -C14H8), 7.00 (m, 2H, -C14H8).13C{1H} NMR (128 MHz, C6D5Br, 298 K, toluene

omitted): δ 181.1 (C=O), 149.3 (dm, 1JCF = 223 Hz, o-C6F5,), 140.4 (dm, 1JCF = 250 Hz, p-C6F5),

139.5 (-ArC), 137.6 (-ArC), 136.7 (dm, 1JCF = 256 Hz, p-C6F5), 136.0 (-ArC), 132.6 (-ArC),

130.4 (-ArC), 124.9 (-ArC). 19F NMR (376 MHz, C6D5Br, 298 K): δ -117.6 (dd, 3JFF = 12, 30

Hz , 4F, o-C6F5,), -159.0 (t, 3JFF = 20 Hz, 2F, p-C6F5,), -161.4 (m, 4F, m-C6F5). Anal. Calcd. For

C26H8F10O2Zn: C 51.39 %, H 1.33 %. Found: C 52.33 %, H 1.53 %.

Synthesis of [(C16H8O2)Zn(C6F5)2] (4-9b)

Combining a solution of Zn(C6F5)2•tol (19.6 mg, 0.040 mmol) in 2 mL of chlorobenzene with o-

C16H8O2 (9.3 mg 0.040 mmol) leads to the formation of a deep red solution. After 2 min stirring

at room temperature, the reaction mixture was layered with 4 mL of pentane overnight at -35 °C,

yielding a mixture of dark crystals of 4-9b and orange needle crystals of 4,5-pyrenedione. 1H,

13C{1H} and 19F{1H} NMR spectra of 4-9b were collected for the crude reaction mixture, and

the product was not characterized by elemental analysis due to the fact that ratio of the product

4-9b to o-C16H8O2 is unknown.

1H NMR (600 MHz, C6D5Br, 298 K, toluene omitted): δ 8.19 (d, br, 3JHH = 7.4 Hz, 2H, -C16H8),

7.74 (d, br, 3JHH = 8.0 Hz, 2H, -C16H8,), 7.40 (s, br, 2H, -C16H8), 7.24 (t, br, 3JHH = 6.3 Hz, 2H, -

109

C16H8). 13C{1H} NMR (128 MHz, C6D5Br, 298 K, toluene omitted): δ 181.6 (C=O), 149.4 (dm,

1JCF = 224 Hz, o-C6F5), 140.6 (dm, 1JCF = 239 Hz, p-C6F5), 139.6 (-ArC), 137.6 (-ArC), 136.9

(dm, 1JCF = 256 Hz, p-C6F5), 133.4 (-ArC), 132.4 (-ArC), 128.6 (-ArC), 127.1 (-ArC), 125.9 (-

ArC). 19F NMR (376 MHz, C6D5Br, 298 K): δ -116.5 (dd, 3JFF = 11, 30 Hz, 4F, o-C6F5), -155.3

(t, 2F, 3JFF = 20 Hz, p-C6F5), -160.7 (m, 4F, m-C6F5).

Synthesis of [Cp*2Fe][((C14H8O2)Zn(C6F5)2)•] (4-10a)

A mixture of Cp*2Fe (26.1 mg, 0.080 mmol), Zn(C6F5)2•tol (39.3 mg, 0.080 mmol), and o-

C14H8O2 (16.7 mg 0.080 mmol) was stirred in 2 mL of chlorobenzene at room temperature. After

5 min, 6 mL of pentane was added to the reaction mixture, and black solid was precipitated from

the reaction mixture. The solvent was decanted carefully with a pipette, and the resulting solid

was washed with 3 × 3 mL of pentane, and dried under vacuum to yield a black powder (71.0

mg, 96 %). Single crystals for X-ray studies were obtained from slow evaporation of a pentane

into a saturated 1,2-C6H4Cl2 solution at room temperature.

1H NMR (400 MHz, C6D5Br, 298 K): δ 7.33 (br, -C14H8) 7.01 (br, -C14H8), 0.86 (br, 30H, -CH3).

13C{1H} NMR (128 MHz, C6D5Br, 298 K): no signal detected. 19F NMR (376 MHz. C6D5Br,

298 K): no signal detected. Anal. Calcd. For C46H38F10FeO2Zn: C 59.15 %, H 4.10 %. Found: C

58.64 %, H 4.06 %.

110

Figure 4.15 - ORTEP of the anion of 4-10a with thermal displacement parameters drawn at 50

% probability. C: black; F: pink; S: yellow; Zn: blue; Fe: orange. All H atoms have been omitted

for clarity.

Synthesis of [Cp*2Fe][((C16H8O2)Zn(C6F5)2)•] (4-10b)

A mixture of Cp*2Fe (26.1 mg, 0.080 mmol), Zn(C6F5)2•tol (39.3 mg, 0.080 mmol), and o-

C16H8O2 (18.6 mg 0.080 mmol) was stirred in 2 mL of chlorobenzene at room temperature. After

5 min, 6 mL of pentane was added to the reaction mixture, and black solid was precipitated from

the reaction mixture. The solvent was decanted carefully with a pipette, and the resulting solid

was washed with 3 × 3 mL of pentane, and dried in vacuo to yield a black powder (76.5 mg, 98

%). Single crystals for X-ray studies were obtained from slow evaporation of a pentane into a

saturated 1,2-C6H4Cl2 solution at room temperature.

1H NMR (400 MHz, C6D5Br, 298 K): δ 4.32 (br, -C16H8), -19.03 (br, 30H, -CH3). 13C{1H}

NMR (128 MHz, C6D5Br, 298 K): no signal detected. 19F NMR (376 MHz, C6D5Br, 298 K): no

signal detected. Anal. Calcd. For C48H38F10FeO2Zn: C 60.18 %, H 4.00 %. Found: C 59.98 %, H

3.97 %.

111

Figure 4.16 - ORTEP of the anion of 4-10b with thermal displacement parameters drawn at 50

% probability. C: black; F: pink; S: yellow; Zn: blue. All H atoms have been omitted for clarity.

Reaction of 4-10a and 4-10b with DMAP

A solution of 4-10a (9.3 mg, 0.010 mmol) or 4-10b (9.6 mg, 0.010 mmol) in 0.5 mL of C6D5Br

was mixed with DMAP (2.4 mg, 0.020 mol), and the resulting products were assigned by 1H and

19F NMR spectroscopic analysis.

Reaction with 4-10a:

1H NMR (500 MHz, C6D5Br, 298 K): δ 7.66 (d, 3JHH = 6.5 Hz, 4H, Zn(C6F5)2•2DMAP), 7.59 (d,

3JHH = 7.7 Hz, 2H, o-C14H8O2), 7.14 (d, 3JHH = 8.0 Hz, 2H, o-C14H8O2), 6.94 (t, 3JHH = 8.3 Hz,

2H, o-C14H8O2), 6.72 (t, 3JHH = 7.5 Hz, 2H, o-C14H8O2), 5.65 (s, br, 4H, Zn(C6F5)2•2DMAP),

2.06 (s, 12H, Zn(C6F5)2•2DMAP), 1.24 (s, 30H, Cp*2Fe). 19F NMR (376 MHz, C6D5Br, 298 K):

δ -115.1 (dm, 3JFF = 34 Hz, 4F, o-C6F5,), -158.6 (t, 3JFF = 20 Hz, 2F, p-C6F5), -162.0 (m, 4F, m-

C6F5).

112

Reaction with 4-10b:

1H NMR (500 MHz, C6D5Br, 298 K): δ 8.24 (d, 3JHH = 1.3, 7.7 Hz, 2H, o-C16H8O2,) 8.09 (d,

3JHH = 6.8 Hz, 4H, Zn(C6F5)2•2DMAP), 7.73 (dd, 3JHH = 1.3, 7.9 Hz, 2H, o-C16H8O2), 7.46 (s,

2H, o-C16H8O2), 7.36 (t, 3JHH = 7.7 Hz, 2H, o-C16H8O2), 6.05 (d, br, 3JHH = 6.8 Hz, 4H,

Zn(C6F5)2•2DMAP), 2.45 (s, 12H, Zn(C6F5)2•2DMAP), 1.54 (s, 30H, Cp*2Fe). 19F NMR (470

MHz, C6D5Br, 298 K): δ -114.7 (dm, 3JFF = 31 Hz, 4F, o-C6F5), -158.2 (t, 3JFF = 20 Hz, 2F, p-

C6F5), -161.7 (m, 4F, m-C6F5).

Synthesis of [Cp*2Co][(Ph2CNNHB(C6F5)3] (4-11) and [Cp*CoC5Me4CH2B(C6F5)3] (4-12)

A solution of Ph2CN2 (11.3 mg, 0.058 mmol) in 2 mL of pentane was stirred in a vial at -35 C,

while B(C6F5)3 (59.3 mg, 0.116 mmol) and Cp*2Co (38.2 mg, 0.116 mmol) were added

sequentially, resulting a yellow solid immediately precipitated out from the solution. The solvent

was decanted carefully with a pipette, and the resulting solid was washed with 3 × 2 mL of

pentane. Clean compound 4-12 was extracted from the crude product with 3 × 2 mL of benzene,

and dried as a yellow powder (16.5 mg, 34 %), while the residue was dried to give the clean

compound 4-11 as a yellow powder (34.0 mg, 57 %).

Single crystals for X-ray studies were obtained from slow evaporation of a pentane into a

saturated chlorobenzene solution for 4-11 and a saturated benzene solution for 4-12 at room

temperature to give yellow crystals.

For 4-11:

1H NMR (500 MHz, CD2Cl2, 298 K, pentane omitted): δ 7.52 (t, 3JHH = 7.5 Hz, 2H, -C6H5), 7.40

(t, 3JHH = 7.5 Hz, 1H, -C6H5), 7.35 - 7.28 (m, 2H, -C6H5), 7.14 - 7.01 (m, 4H, -C6H5), 6.94 (t,

3JHH = 6.7 Hz, 1H, -C6H5), 6.61 (s, br, 1H, -NH-), 1.70 (s, 30H, -CH3). 13C{1H} NMR (128

MHz, CD2Cl2, 298 K): δ 142.1, 135.9, 130.8, 130.2, 129.4, 129.3, 128.9, 128.6, 127.9, 127.7,

126.9, 124.7, 124.6, 94.4 (C-CH3), 8.2 (-CH3). 19F{1H} NMR (377 MHz, C6D5Br, 298 K): δ -

134.0 (d, 3JFF = 24 Hz, 6F, o-C6F5), -163.9 (t, 3JFF = 21 Hz, 3F, p-C6F5), -167.3 (t, 3JFF = 22 Hz,

6F, m-C6F5). 11B{1H} NMR (128 MHz, C6D5Br, 298 K): δ -7.6 (s).

113

For 4-12:

1H NMR (400 MHz, C6D6, 298 K, pentane omitted): δ 2.43 (s, br, 2H, CH2-B), 0.88 (s, 10H, -

CH3), 0.79 (s, 6H, -CH3) 0.67 (s, 6H, -CH3). 13C{1H} NMR (128 MHz, C6D6, 298 K): δ 93.1,

92.3, 91.9, 91.7, 7.1 (-CH3), 6.8 (-CH3), 6.7 (-CH3), 6.6 (-CH3). 19F{1H} NMR (377 MHz, C6D6,

298 K): δ -130.2 (s, br, 6F, o-C6F5), -161.9 (t, 3JFF = 26 Hz, 3F, p-C6F5), -162.7 (s, br, 6F, m-

C6F5). 11B{1H} NMR (128 MHz, C6D6, 298 K): δ -13.0 (s).

Synthesis of [Cp*2Cr][(PhC(C6H4)NNBPh3] (4-13) and [Cp*2Cr][Ph2CNNHBPh3] (4-14)

A solution of Ph2CN2 (55.4 mg, 0.285 mmol) in 2 mL of chlorobenzene was stirred in a vial at -

35 °C, while BPh3 (69.1 mg, 0.285 mmol) and Cp*2Cr (92.0 mg, 0.285 mmol) were added

sequentially, resulting in the precipitation of an orange solid. The reaction mixture was allowed

to further stir for another 2 h at room temperature to ensure the reaction is fully completed. The

solvent was removed under vacuum, and the resulting solid was washed with 6 mL of pentane

and extracted with 3 × 2 mL of benzene and dried in vacuo. The yield orange solid was further

washed with 3 × 2 mL of pentane and dried in vacuo to give clean compound 4-13 (45.6 mg, 42

%). The resulting leftover solid after extraction of compound 4-13 was further washed with 2 mL

of chlorobenzene and 3 × 2 mL of pentane, and dried in vacuo to give compound 4-14 (54.2 mg,

50 %)

Single crystals for X-ray studies were obtained from slow evaporation of a pentane into a

saturated chlorobenzene solution for 4-13 and a saturated DFB solution of 4-14 at room

temperature to give orange crystals.

For 4-13:

1H NMR (400 MHz, C6D6, 298 K, pentane and toluene are omitted): δ 21.35 (s, br, 30H, -CH3),

8.44 - 7.09(m, br, 24H), 5.88 (s, br, 2H), 5.07 (s, br, 2H). 13C{1H} NMR (128 MHz, C6D6, 298

K, partial due to the paramagnetic cation Cp*2Cr, pentane omitted): δ 143.1, 138.4, 117.6.

11B{1H} NMR (128 MHz, C6D5Br, 298 K): δ 23.1 (s).

114

For 4-14:

1H NMR (400 MHz, CDCl3, 298 K, pentane and toluene are omitted): δ 20.51 (s, br, 30H, -

CH3), 8.23 - 6.41 (m, br, 25H), 3.42 (m, br, 1H, -NH-). 13C{1H} NMR (128 MHz, CDCl3, 298

K): δ 135.0, 134.6, 132.8, 132.4, 130.8, 129.2, 128.8, 128.5, 128.4, 126.8, 126.6, 126.0, 125.5,

125.2, 123.3, 121.8. 11B{1H} NMR (128 MHz, CDCl3, 298 K): δ -3.7 (s).

Generation of [Cp*2Cr][C6H4C(Ph)N2(BPh3)] (4-15), [Cp*2Cr][C12H8CNNHBPh3] (4-16) and

[Cp*Cr(C5Me4CH2BPh3)] (4-17)

A solution of C12H8CN2 (54.7 mg, 0.285 mmol) in 2 mL of chlorobenzene was stirred in a vial at

-35 °C, while BPh3 (69.1 mg, 0.285 mmol) and Cp*2Cr (92.0 mg, 0.285 mmol) were added

sequentially, resulting in the precipitation of an orange solid. The reaction mixture was allowed

to further stir for another 2 h at room temperature to ensure the reaction is fully completed. The

solid was filtered and further washed with 3 × 2 mL of C6H5Cl and 3 × 2 mL pentane and dried

under vacuum to give crude compound of 4-16. The solvent used to wash 4-16 were collected

and removed under vacuum. The resulting solid was dissolved in C6H5Cl and layered with

pentane at room temperature to give a mixture of crystals for 4-15 and 4-17 suitable for

diffraction study. Single crystals suitable for X-ray studies for 4-16 were obtained from a

saturated DFB solution layered with pentane at room temperature.

4.4.3 Electrochemistry

Cyclic voltammetry experiments were performed using BASi-Epsilon RDE-2 model. A standard

three-electrode cell configuration was employed using a glassy graphite working electrode, a

platinum wire counter electrode, and a silver wire serving as a reference electrode. Formal redox

potentials were referenced to the ferrocenium/ferrocene redox couple. [nBu4N][BF4] (0.1 M) was

used as supporting electrolyte. The scanning direction was from positive potential to negative

potential first, with 50 mV/s scan rate. All CVs are measured with analytes/ferrocene as a 1 mM

solution in chlorobenzene (4-9a and 4-9b are made in situ).

115

Figure 4.17 - Cyclic voltammogram of Cp*2Fe.

Figure 4.18 - Cyclic voltammogram of 4-8a.

116

Figure 4.19 - Cyclic voltammogram of o-C14H8O2.

Figure 4.20 - Cyclic voltammogram of 4-8b.

117

Figure 4.21 - Cyclic voltammogram of o-C16H8O2.









Structure Solution and Refinement. The structures unless otherwise mentioned were solved by

intrinsic phasing using XS, whereas 4-9a was solved by the Patterson method. All the structures

were subjected to full-matrix least-squares refinement on F2 using XL as implemented in the

SHELXTL suite of programs. All non-hydrogen atoms were refined with anisotropically thermal

parameters. Carbon bound hydrogen atoms were placed in geometrically calculated positions and

118

refined using an appropriate riding model and coupled isotropic thermal parameters. For

compound 4-11 and 4-14, the N bound hydrogen atoms were located through electron density

difference map and modeled with appropriate riding model and coupled isotropic thermal

parameters.

119

Table 4.2 - Summary of crystallographic data for compounds 4-1a, 4-1b, 4-2a and 4-2b.

4-1a 4-1b 4-2a 4-2b

Moiety formula C22.50 H17.50

B0.50 F7.50 Fe0.50

O

C22.50 H17.50

Al0.50 F7.50 Fe0.50

O

C53 H53 B Cl

F15 Fe N O

C31.33 H32 Al0.67

F10 Fe0.67 N 0.67

O 0.67

Sum formula weight 479.69 487.78 1107.07 673.79

crystal system Triclinic Triclinic Triclinic Monoclinic

space group P-1 P-1 P-1 P21/n

a (Å) 10.592(2) 10.808(18) 9.8079(12) 9.8874(8)

b (Å) 13.454(3) 14.03(2) 21.726(3) 32.141(3)

c (Å) 14.360(3) 14.35(2) 23.941(3) 14.6181(12)

α (deg.) 83.346(6) 96.37(4) 92.850(4)

β (deg.) 89.680(6) 91.09(4) 91.521(3) 102.973(5)

γ (deg.) 81.366(6) 100.08(4) 102.432(3)

vol (Å3) 2009.4(7) 2128(6) 4972.1(11) 4526.9(6)

Z 4 4 4 6

ρ (calcd) (Mg∙cm3) 1.586 1.522 1.479 1.483

μ (mm-1) 0.485 0.479 0.454 0.452

F(000) 974 990 2276 2076

Theta range (°) 1.428 to 27.118 1.429 to 26.730 2.004 to 26.002 1.267 to 26.711

T(K) 150(2) 150(2) 150(2) 150(2)

reflections collected 51116 28754 84959 744878

unique reflections 8612 8996 19567 9444

Rint 0.0959 0.1300 0.0446 0.0938

GOF (F2) 1.009 0.965 1.041 1.064

R1 indices [I>2σ(I)] 0.0539 0.0706 0.0678 0.0528

wR2 indices (all data) 0.1444 0.1988 0.2034 0.1574

Largest diff. peak and

hole (e. Å-3)

0.466 &

-0.449

0.478 & -0.391 2.171 &

-1.299

0.549 & -0.611

CCDC No. 1546086 1546084 1546095 1546094

120

Table 4.3 – Summary of crystallographic data for compounds 4-3, 4-4, 4-5 and 4-6.

4-3 4-4 4-5 4-6


Al1.33 Cl0.33

F16.67 Fe0.67

O0.67

C33.33 H20 Al1.33

F16.67 Fe0.67

Se0.67

C37 H21 Al1.50

Cl3 F15 Fe0.50 Te

C38.50 H31 B Cl

F15 Fe S3.50


crystal system Monoclinic Monoclinic Triclinic Triclinic

space group P21/n P21/n P-1 P-1

a (Å) 14.168(3) 16.026(3) 13.251(7) 9.410(2)

b (Å) 39.796(8) 12.948(2) 14.356(6) 20.391(5)

c (Å) 18.693(4) 23.926(4) 22.384(11) 21.371(5)

α (deg.) 88.718(11) 95.423(6)

β (deg.) 99.024(14) 90.930(8) 77.885(12) 95.582(6)

γ (deg.) 69.221(10) 97.223(6)

vol (Å3) 10409(4) 4963.9(15) 3886(3) 4025.1(18)

Z 12 6 4 4

ρ (calcd) (Mg∙cm3) 1.644 1.732 1.800 1.639

μ (mm-1) 0.479 1.204 1.286 0.723

F(000) 5152 2564 2054 2004


T(K) 150(2) 150(2) 150(2) 150(2)



Rint 0.2122 0.1942 0.1924 0.1572

GOF (F2) 1.019 1.018 1.054 1.052

R1 indices [I>2σ(I)] 0.0974 0.0613 0.1077 0.0790



hole (e. Å-3)

2.053 & -1.176 0.597 &

-0.686

3.621 & -1.533 2.359 &

-1.192

CCDC No. 1546091 1546090 1546544 1546093

121

Table 4.4 – Summary of crystallographic data for compounds 4-7, 4-8a, 4-8b and 4-9a.

4-7 4-8a 4-8b 4-9a


Cl0.50 F10FeS2

Zn

C29H10.50 Cl0.50

F10O2Zn

C44H16F10O4Zn C46H38F10FeO2

Zn


crystal system Triclinic Monoclinic Triclinic Monoclinic

space group P-1 C2/c P-1 P21/c

a (Å) 10.8221(8) 26.836(4) 11.4423(5) 15.5673(13)

b (Å) 13.9215(8) 14.493(2) 11.9349(5) 10.3956(9)

c (Å) 15.0555(10) 14.364(2) 13.4211(6) 25.578(2)

α (deg.) 96.977(2) 102.780(2)

β (deg.) 100.662(2) 118.161(4) 101.712(2) 104.731(6)

γ (deg.) 100.342(2) 106.793(2)

vol (Å3) 2164.6(2) 4925.1(13) 1639.78(13) 4003.3(6)

Z 2 8 2 4

ρ (calcd) (Mg∙cm3) 1.490 1.791 1.750 1.550

μ (mm-1) 1.091 1.156 0.856 1.047

F(000) 994 2632 864 1904


T(K) 150(2) 150(2) 150(2) 150(2)



Rint 0.0352 0.0738 0.0513 0.0675

GOF (F2) 1.027 1.011 0.978 1.225

R1 indices [I>2σ(I)] 0.0468 0.0429 0.0406 0.0996



hole (e. Å-3)

1.393 & -0.467 0.482 & -0.497 0.379 & -0.505 1.067 & -1.000

CCDC No. 1588288 1588287 1588286 1588285

122

Table 4.5 – Summary of crystallographic data for compounds 4-9b, 4-10, 4-11 and 4-12.

4-9b 4-10 4-11 4-12

Moiety formula C48 H38 F10 Fe

O2 Zn

C54 H43.50 B Cl

0.50 Co F15 N2

C38 H29 B Co

F15

C51 H54 B Cr

N2


crystal system Monoclinic Triclinic Monoclinic Triclinic

space group P21/n P-1 P21/n P-1

a (Å) 11.9393(4) 11.9356(9) 11.506(2) 11.353(2)

b (Å) 13.5388(5) 11.961(2) 17.317(2) 11.402(2)

c (Å) 25.5220(10) 18.574(2) 17.589(2) 18.323(2)

α (deg.) 77.087(2) 99.080(4)

β (deg.) 95.737(2) 84.528(2) 96.728(4) 103.704(4)

γ (deg.) 72.178(2) 111.170(4)

vol (Å3) 4104.8(3) 2459.5(4) 3480.3(8) 2069.8(4)

Z 4 2 4 2

ρ (calcd) (Mg∙cm3) 1.550 1.476 1.604 1.216

μ (mm-1) 1.024 0.473 0.603 0.314

F(000) 1952 1114 1696 806


T(K) 150(2) 150(2) 150(2) 150(2)



Rint 0.0401 0.1065 0.1438 0.0679

GOF (F2) 1.029 1.058 0.940 1.050

R1 indices [I>2σ(I)] 0.0388 0.0658 0.0705 0.0665



hole (e. Å-3)

0.828 & -0.557 0.975 & -0.633 0.487 & -0.925 0.898 & -0.459

CCDC No. 1588289 1917670 1917669 1917668

123

Table 4.6 – Summary of crystallographic data for compounds 4-13 and [Cp*2Fe][Al(C6F5)4].

4-13 [Cp*2Fe][Al(C6F5)4]

Moiety formula C54 H58 B Cr F N2 C22 H15 Al0.50 F10

Fe0.50

Sum formula weight 816.83 510.75

crystal system Triclinic Triclinic

space group P-1 P-1

a (Å) 10.814(2) 10.719(5)

b (Å) 11.942(3) 13.037(6)

c (Å) 19.224(4) 15.005(6)

α (deg.) 85.793(7) 84.987(13)

β (deg.) 84.136(7) 88.873(12)

γ (deg.) 67.003(6) 82.307(14)

vol (Å3) 2271.7(8) 2070.0(16)

Z 2 4

ρ (calcd) (Mg∙cm3) 1.194 1.639

μ (mm-1) 0.294 0.510

F(000) 868 1026

Theta range (°) 2.104 to 25.682 1.998 to 27.103

T(K) 150(2) 150(2)

reflections collected 21869 44773

unique reflections 8517 9137

Rint 0.0447 0.0847

GOF (F2) 1.051 1.010

R1 indices [I>2σ(I)] 0.600 0.0530

wR2 indices (all data) 0.1788 0.1371


hole (e. Å-3)

0.379 & -0.518 0.452 &

-0.553

CCDC No. 1917667 1546092


All calculations were computed using the Gaussian 09 program.47 Geometry optimizations were

performed at (u)M06 or (u)M062x functional with the crystallographic coordinates used as

starting geometries when available. The Def2-SVP basis set was used for all atoms. The

stationary nature of the converged geometry was confirmed by carrying out a frequency

calculation and ensuring the absence of imaginary frequencies. Natural bond orbital (NBO)

analysis was carried out at (u)M06/Def2-TZVP//(u)M06/Def2-SVP of (u)M06-2X/Def2-

124

TZVP//(u)M06-2X/Def2-SVP level with NBO 6.0 program.48 Optimized structures were

visualized using ChemCraft49 or CYLview software.50

Table 4.7 - Hyperfine coupling parameters and spin density for 4-9a- (C: grey; F: green; Zn:

deep blue; O: red).

Atoms Hyperfine

Coupling (G)

Calculated Fermi

Contact Coupling

(G)

Calculated Spin density

Zn - -1.95 -0.453 %

H1 1.67 -2.05 & -2.07 -0.333 % & -0.333 %

H2 0.38 0.42 & 0.41 0.063 % & 0.062 %

H3 1.88 -2.58 & -2.56 -0.316 % & -0.312 %

H4 0.66 0.70 & 0.72 0.052 % & 0.053 %

125

Table 4.8 - Hyperfine coupling parameters and spin density for 4-9b- (C: grey; F: green; Zn:

deep blue; O: red).

4.5 References

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12893-12896.

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(G)

Calculated Spin density

Zn - -3.19 -0.829 %

H1 2.01 -2.41 & -2.57 -0.358 % & -0.374 %

H2 0.57 0.45 & 0.52 0.064 % & 0.073 %

H3 2.28 -2.67 & -2.84 -0.277 % & -0.297 %

H4 0.21 0.23 & 0.17 0.014 % & 0.006 %

126

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29. Neu, R. C. and Stephan, D. W., Organometallics, 2011, 31, 46-49.

30. Tang, C., Liang, Q., Jupp, A. R., Johnstone, T. C., Neu, R. C., Song, D., Grimme, S. and

Stephan, D. W., Angewandte Chemie International Edition, 2017, 56, 16588-16592.

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14727-14740.



33. Aramaki, Y., Omiya, H., Yamashita, M., Nakabayashi, K., Ohkoshi, S. and Nozaki, K.,


34. Fillion, E., Kavoosi, A., Nguyen, K. and Ieritano, C., Chemical Communications, 2016,

52, 12813-12816.

35. Longobardi, L. E., Liu, L., Grimme, S. and Stephan, D. W., Journal of the American


36. Lewiński, J., Zachara, J. and Grabska, E., Journal of the American Chemical Society,

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Prowotorow, I., Chemistry – A European Journal, 2000, 6, 3215-3227.

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Barragan, F. and Alvarez, S., Dalton Transactions, 2008, 2832-2838.

128

40. Mountford, A. J., Lancaster, S. J., Coles, S. J., Horton, P. N., Hughes, D. L., Hursthouse,

M. B. and Light, M. E., Organometallics, 2006, 25, 3837-3847.

41. Longobardi, L. E., Zatsepin, P., Korol, R., Liu, L., Grimme, S. and Stephan, D. W.,


42. Cummings, S. A., Iimura, M., Harlan, C. J., Kwaan, R. J., Trieu, I. V., Norton, J. R.,

Bridgewater, B. M., Jakle, F., Sundararaman, A. and Tilset, M., Organometallics, 2006,

25, 1565-1568.

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Society, 1999, 121, 7274-7275.

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2001, 20, 3772-3776.

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Peralta, J. E., Ogliaro, F., Bearpark, M., Heyd, J. J., Brothers, E., Kudin, K. N.,

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Canada 2009.

129

Chapter 5

5 NHC-Stabilized Al Compounds

5.1 Introductions

5.1.1 Neutral Monomeric Aluminum Hydrides

Aluminum hydrides are important reagents in organic, inorganic and materials chemistry. Such

species are used in a variety of roles including as reagents for reductions1-6 and in inorganic

synthesis.7, 8 Furthermore, applications as components in materials chemistry and in particular,

hydrogen storage materials9-11 have also garnered attention. From a structural perspective,

aluminum-hydride derivatives exhibit a range of geometries resulting from the low steric demand

of hydride and the ability of aluminum to accommodate coordination numbers ranging from

three to six. This led to only a few neutral monomeric aluminum monohydrides (R2AlH) to be

synthesized and isolated in the solid state (Figure 5.1).12 In 1995, Cowley reported the synthesis

of the first neutral monomeric aluminum monohydride utilizing the bulky Mes* (Mes* = 2,4,6-tri-

tert-butylphenyl) substituents.13 Later, Power and coworkers reported a one-pot synthesis of the

same Mes*2AlH compound and discovered it react with hexamethyldisilazane to give the

monomeric Mes*2Al(N(TMS)2).

14 Wehmschulte showed that hydride abstraction from R2AlH (R

= 2,6-dimestiyl-phenyl),15 led to the first divalent aluminum cation which was even stable in

dichloromethane due to the bulky R group.5, 16 Recently, Matsuo and co-workers have used the

Eind (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) ligand to prepare (EindAlH2)2 and

Eind2AlH.17

Figure 5.1 – Neutral monomeric aluminum mono hydrides that have been isolated in the solid

state.

130

5.1.2 Cationic Aluminum Hydrides

Figure 5.2 - Structurally characterized aluminum-hydride cations.

While the majority of reported aluminum hydride species are either neutral or anionic, cationic

aluminum-hydride species are less common. A matrix isolation study at 4 K described the

[AlH•]+ radical cation which was studied by EPR spectroscopy.18, 19 The first fully characterized

Al-hydride cations [H2Al((MeNCH2CH2N(Me)CH2CH2CH2)2)][AlH4] and

[H2Al(MeN(CH2CH2NMe2)2)][AIH4] (Figure 5.2) were prepared and reported by Atwood and

coworkers in 1991.20, 21 These species exhibited six- and five-coordinate aluminum centers,

respectively. In 1994, Soloveichik and coworkers described the structure of the salt

[AlH2(THF)4][(C5H5)3Yb(Na)Yb(C5H5)3] (Figure 5.2) which contained a six-coordinate

aluminum-dihydride cation.22 In 2004, our group employed an aluminum complex with a

phosphoranimine-amine ligand to generate salts of the aluminum-hydride cation

[(iPr2C6H3N)C(Me)CHPPh2(NC6H3iPr2)AlH][B(C6F5)4].23 Roesky and coworkers exploited a

bulky weakly coordinating anion to isolate the salts [H2Al(NMe3)2]2[(AlH)8(CCH2tBu)6] and

[H(nBu)Al(NMe3)2][(AlH)7(AlNMe3)(CCH2tBu)6] (Figure 5.2),24 which are examples of four-

coordinate aluminum hydride cations. Most recently, Wright and coworkers described the

structure of [(1,4-H-pyrid-1-yl)4Al][(pyridine)4AlH2], which was formed from the reaction of

(tBuO)AlH2 and pyridine.25 In 2016, we reported the isolation of the first dimeric aluminum-

hydride dication salt [(IDippAlH2)2][B(C6F5)4]2.26 Last year, the Inoue group prepared the

131

aluminum hydride cation stabilized by a bulky bisimino ligand [((IMes)NCH2)2AlH2][A] (A =

[AlB2H8]- or [BH4]

-).27

5.1.3 NHC Stabilized Aluminum Compounds

Over 25 years ago, Arduengo described the isolation of the first NHC-alane adduct.28 Since that

seminal work, numerous fundamental and intriguing NHC based Al species have been reported

(Scheme 5.1).26, 29-33 Surprisingly, few reports have probed the nature of cAAC analogs.31-34 It is

also noteworthy that Nikonov and co-workers have demonstrated reversible Al-H activation at an

Al(I) center.35

Scheme 5.1 – Selected aluminum compounds.

5.1.4 Reversible Oxidative Addition at Carbon

Bertrand’s synthesis of cyclic amino alkyl carbenes (cAACs) was an importance advance in

organometallic chemistry.36-38 Replacing one of the σ-withdrawing/π-donating amino groups of

N-heterocyclic carbenes (NHC) with a quaternary carbon gives cAACs enhanced π-accepting

character and a reduced HOMO-LUMO gap,37 resulting in previously unobtainable reactivity.

For example, in 2007 Bertrand showed that although NHCs are unreactive toward H2 and

132

ammonia, cAACs react readily to undergo oxidative addition.39 In 2010, Bertrand also described

the activation of the B-H bond of pinacol borane by cAACs leading to insertion of the carbene

center into the B–H bond (Scheme 5.2).40 In 2014, we collaborated with Bertrand to report a

transient cAAC-stabilized mono-amino-substituted borane which undergoes spontaneous hydride

migration to the cAAC carbon affording a boron monohydride (Scheme 5.2).41 One year later,

the Bielawski group reported a photoswitchable carbene system which allows for reversible N-H

bond activation depending on the wavelength of light.42 Recently, Braunschweig showed facile

reversible hydride migration to the cAAC carbon in a cAAC-borane adduct proceeded via a

nucleophilic addition at boron (Scheme 5.2).43 Related work by Marder and Radius demonstrated

reversible aryl migrations from the cAAC-carbon to boron (Scheme 5.2).44 Most recently,

Bertrand et al. have investigated that reversible oxidative addition reaction of E-H bonds (E = H,

B, N, Si, P) are influenced by the steric bulk around the cAAC carbon center.45

Scheme 5.2 - Selected examples of 1,2-hydride migration reaction examples on carbon centers.


5.2.1 NHC-Stabilized Al hydride Cations

Previously our group showed that the combination of (IDipp)AlH3 and [Ph3C][B(C6F5)4]

generates the dimeric aluminum-hydride dicationic salt [(IDippAlH2)2][B(C6F5)4]2 (Figure 5.2).26

133

To probe the impact of altering the carbene, 1,3-dibenzylimidazol-2-ylidene (IBn) was allowed

to react with AlH3·NEtMe2 in toluene, to readily afford (IBn)AlH3 (5-1) in 72 % yield after

recrystallization. This species gives rise to a broad singlet resonance in 1H NMR spectrum in tol-

d8 at 4.73 ppm corresponding to the aluminum-bound hydrides. The 27Al NMR spectrum of 5-1

shows a broad singlet resonance at 108.7 ppm. These data are comparable to those seen for

related NHC-alane adducts.26, 28 An X-ray crystallographic study of 5-1 (Figure 5.3) confirmed

the formulation, revealing an Al-C1 bond length of 2.059(2) Å, which falls within the range of

typical Al-C1 bonds (2.03-2.07 Å) involving carbene donors.



All ligand-based H atoms as well as the [B(C6F5)4]- anions are omitted for clarity.

Treatment of 5-1 with a stoichiometric amount of [Ph3C][B(C6F5)4] in C6H5Br at room

temperature results in the formation of Ph3CH and a new species 5-2 in 46 % yield. The 1H NMR

spectrum of 5-2 in C6D5Br shows resonances attributable to the IBn ligand but no signal was

observed in the 27Al NMR spectrum. The IR spectrum of 5-2 featured a signal at 1963 cm-1,

which is in the same region as observed for other cationic aluminum hydride species.21, 24, 25 The

solid state structure of 5-2 was unambiguously confirmed by X-ray crystallography. Compound

5-2 proved to be the dicationic borate salt [(IBn)2AlH][B(C6F5)4]2. The cation of this salt is a

planar monohydrido aluminum dication containing two IBn ligands (Figure 5.3). The planes of

the two carbenes are oriented approximately orthogonal to each other, allowing the benzyl

substituents to envelop the aluminum center. Two of the pendent arenes are positioned above and

134

below the pseudo-trigonal coordination plane of the aluminum center. The sum of the angles

about aluminum is 359° and the C(1)-Al-C(18) angle is 113.6(2)°. The Al-C bond distances in 5-

2 are 1.987(4) Å and 2.010(4) Å. The structural data also reveals that one of the benzyl

substituents on one of the IBn ligands is oriented such that a C(6)-C(7) π-bond is positioned

above the Al center with Al-C distances of 3.124(5) Å and 2.572(4) Å, respectively.

Scheme 5.3 – Synthesis of compounds between 5-1 to 5-4.

DFT calculations were performed at the M06-2X/Def2-TZVP//M06-2X/Def2-SVP level of

theory46, 47 to gain further insight into the electronic structure of dication 5-2. The optimized

geometry corresponded well with the crystallographically determined structure (non-hydrogen

RMSD = 0.30 Å), and a frequency calculation confirmed that that IR active band at 1963 cm-1

arises from a normal mode comprising almost exclusively the Al-H stretch. Based on natural

population analysis (NPA),48, 49 the Al atom of compound 5-2 bears a natural atomic charge of

0.75 a.u. (Figure 5.4). An NPA analysis reported for the similar dicationic hydrido boron

complex prepared by Ong and co-workers suggested that their boron complex was stabilized by

distribution of positive charge from the boron center onto the carbodicarbene supporting

ligands.50 The lack of such charge redistribution in 5-2 suggests that it is stabilized by other

effects. The LUMO of the dication (Figure 5.4) is mainly located on the Al atom and has

significant contribution from the proximal arene ring. An electronic interaction between the Al

135

center and the aromatic ring of the pendent benzyl group, inferred from the crystal structure of 5-

2 in addition to the delocalized nature of the LUMO, was corroborated by the results of a natural

bond orbital (NBO) analysis. Using second order perturbation theory, a donor acceptor

interaction was identified between a bonding NBO on the pendent arene ring and an empty lone

pair NBO on the Al center. The Al-based NBO has essentially pure 3pz character and the

bonding NBO, localized between the two carbon atoms of the arene ring that are closest to the

metal center, has π-symmetry and is oriented such that it overlaps with the Al-based NBO

(Figure 5.4) This interaction may impart stabilization on the complex, accounting for its

unexpected stability. For instance, NMR spectroscopy revealed no evidence of degradation of a

sample of 5-2 that had been stored in an inert atmosphere for over a month. The Al-arene

interaction in 5-2 is reminiscent of the olefin-Al interaction described for

CH2=C(CH3)CH2CH2CH2Al(C6F5)251, 52

and (C6H10)Al(C6F5)3 (C6H10 = cyclohexene),53 The

equivalence between the benzyl groups of 5-2 observed in the solution state NMR spectrum

suggest that this interaction is fluxional, at least on the NMR time scale. The transient nature of

the interaction, coupled with the fact that the distance separating the Al center and the nearest

carbon of the arene ring is greater than the sum of their van der Waals radii,54 led to the

formulation of this species as a 3-coordinate aluminum complex.

Although redistribution reactions are common in aluminum chemistry, the formation of 5-2 is a

rare example of such a redistribution involving a carbene ligand. One can speculate that the

reaction of 5-1 with [Ph3C][B(C6F5)4] proceeds to generate a dimeric analog of

[(IDippAlH2)2][B(C6F5)4]2 but that the lesser steric demands of IBn facilitate the redistribution

affording 5-2 and liberation of AlH3. These results lead us to attempt the hydride abstraction

reaction of 5-1 with [Ph3C][B(C6F5)4] in THF, so that THF could act as a Lewis base to stabilize

the generated [IBnAlH2]+ cation. With in the 1H NMR spectra, the formation of Ph3CH was

observed, along with the resonances attribute to IBn and THF (Scheme 5.3). Upon fractional

recrystallization of the products, the formulation of two new compounds were confirmed

unambiguously as [IBnAlH2(THF)2][B(C6F5)4] 5-3 and [IBnAlH2(THF)2][B(C6F5)4] 5-4 (Figure

5.5). For compound 5-3, the [IBnAlH2(THF)2]+ cation is sitting along a 2-fold axis along the Al-

C bond, resulting in the asymmetric unit of 5-3 to contain only half of the molecule. The two Al-

H bonds and the IBn carbene carbon are almost planar with a sum angle around the Al center

being 359°, while the THF molecules are orthogonal to the plane with the C-Al-O angle to be

136

90.7°. The Al-C bond length is 2.029(1) Å which is comparable with 5-2. While for 5-4, the

cationic structure of the [AlH2(THF)4]+ is similar to its analogue in the literature.22

Figure 5.4 – (a) Computed structure for 5-22+ with WBI bond order in black given in a.u. and

NPA charges in red given in a.u.; (b) Lowest unoccupied molecular orbital (LUMO) plot for the

dication of 5-22+; (c–e) Selected NBOs for 5-22+ (c) 3pz for Al, (d) π(3c)1 for the phenyl group,

(e) π(3c)2 for the phenyl group.

To investigate the mechanism of the generation of 5-4, we dissolved compound 5-2 in THF

resulting the in liberation of the IBn groups from the Al center and hydride abstraction

presumably from the solvent, resulting [AlH2(THF)4][B(C6F5)4] 5-4. These suggested that

compound 5-2 could be generated in the hydride abstraction reaction of 5-1 in THF, and further

reacted with THF to give compound 5-4, while 5-3 was made from THF trap of the hydride

abstraction product [IBnAlH2]+ cation (Scheme 5.3).

137



All ligand-based H atoms as well as the [B(C6F5)4]- anions are omitted for clarity.

5.2.2 Reversible 1,2-Hydride Migration

By analogy to the preparation of the NHC-alane adducts, treatment of the cAACEt (cAACEt = 1-

(2,6-iPr2C6H3)-3,3-diethyl-5,5-dimethylpyrrolidin-2-ylidene) with an equimolar portion of

AlH3·NEtMe2 in pentane at room temperature was undertaken. Multinuclear NMR spectroscopy

revealed the formation of a complex mixture, even on variation of the temperature (see section

5.4.2). Nonetheless, X-ray diffraction studies of crystals obtained from the mixture revealed the

co-crystallization of (cAACEtH)AlH2·NEtMe2 5-5 and (cAACEtH)2Al(µ-H)2AlH2·NEtMe2 5-6

(Scheme 5.4 and Figure 5.6). While 5-5 and 5-6 co-crystalized, there was no significant

interactions observed between the two molecules. In both cases, the CcAAC atoms adopt a

distorted tetrahedral geometry consistent with sp3-hybridisation. The bond length of CcAAC-Al in

5-5 is 2.027(3) Å, while those in 5-6 are 2.020(2) and 2.044(3) Å. The generation of 5-5 and 5-6

demonstrates that cAAC undergoes Al-H activation reaction presumably proceeding via initial

formation of a cAAC-alane adduct, analogous to previously characterized cAAC-borane adducts.

Such insertion of carbenes into Al-H bonds has been previously observed between cAAC and

NHC-alane compounds as reported by Radius and co-workers.34

138

Scheme 5.4 – Synthesis of compounds between 5-5 to 5-8.

Altering the stoichiometry of alane to cAACEt to a 1:2 ratio led to the formation of a monomeric

species (cAACEtH)2AlH 5-7 in 59 % yield (Scheme 5.4 and Figure 5.7). Single crystals of 5-7

suitable for X-ray diffraction studies were grown at -35 °C from a concentrated pentane solution.

The Al center features a trigonal planar geometry with the CcAAC-Al bond lengths of 1.963(1)

and 1.983(1) Å, which are much shorter than the those seen for NHC-alane adducts. Nonetheless

the Al-C distances in 5-7 are similar to the sum of the Al-C(sp3) covalent radii (1.97 Å),54 while

the cAAC-carbon center adopts a distorted tetrahedral geometry with CcAAC-N distances of

1.526(2) and 1.555(2) Å, and N-C-C angles of 107.3(1)° and 108.1(1)°. Compound 5-7

represents the first isolated example of a neutral monomeric dialkyl-aluminum hydride. It is

noteworthy that Cowley and co-workers have described Mes*2AlH where the bulky aryl groups

prevent oligomerization.13 The Al-N distances were found to be 2.286(1) Å and 2.434(1) Å,

which are shorter than the sum of Al-N van der Waals radii but longer than the sum of the

covalent radii.54 This infers that through space Al-N interactions in the solid state may help

stabilize this monomeric species.

139

Figure 5.6 - ORTEPs of the molecular structure of 5-5 (left) and 5-6 (right) with thermal

displacement parameters drawn at 50 % probability; The hydrogen atoms are omitted except Al-

H and the C-H that the carbon is bonded to the Al center for clarity. C: black, N: blue, Al: cyan,

H: grey.


50 % probability. The hydrogen atoms are omitted except Al-H for clarity. C: black, N: blue, Al:

cyan, H: grey.

140

The structure of 5-7 reveals the presence of two chiral carbons centers and thus in the solid state

was found to be the meso-isomer. The IR spectrum of the crystalline meso-5-7 reveals only one

characteristic band for the Al-H stretching at 1853 cm-1. Compound 5-7 is NMR silent in the 27Al

NMR spectrum, presumably due to its trigonal planar geometry around the quadrupolar Al

center. The 1H NMR spectrum of dissolved crystals of 5-7 in tol-d8 at room temperature shows

four inequivalent CDipp-H resonances together with two CcAAC-H signals at 3.61 and 3.10 ppm

with the corresponding 13C{1H} resonances at 73.7 and 70.2 ppm respectively. In addition, two

broad resonances at 4.89 and 3.81 ppm are attributable to the Al-H fragments. Integration of

these set of resonances were consistent with two closely related species of formulae

[(cAACEtH)2AlH]n. A VT-NMR study of the mixture over the temperature range -40 °C to 40 °C

showed the ratio of the intensity of the two sets of Al-H, CcAAC-H and CDipp-H resonances

coalesce with rising temperature (Figure 5.15). Also, returning the solution to room temperature

afforded the initial ratio of the two species indicating the interconversion is reversible. Analysis

of the NMR data suggests the thermodynamic parameters for the interconversion of the two

species with ∆H = 2.88 kcal/mol and ∆S = 0.01 kcal/(mol·K). The low value of ∆S is not

consistent with a dimerization or oligomerization process, rather it is consistent with a process

involving facile racemization of the chiral centers of meso-5-7 to generate rac-5-7 in solution. It

is noteworthy that the formation of related diastereomers have been described by single

activation reactions of E-H (E = Si, P, B) bonds with enantiopure cAACs.40 Further support for

this interpretation is provided by DFT calculations at the M06-2X/Def2-SVP level of theory.46, 47

These calculations reveal a computed ∆H of 3.4 kcal/mol for the conversion of meso-5-7 to rac-

5-7 (Figure 5.15), which is in good agreement with the VT-NMR data.

To gain more insight into the isomerization process between the diastereomers of 5-7, 1H-1H

EXSY NMR studies were performed (Figure 5.8) at room temperature revealing that both of the

two CDipp-H protons and the two CcAAC-H proton sites exchange. Moreover, cross peaks were

also found between the two CcAAC-H protons and the two Al-H signals. This suggests the

isomerization process proceeds by migration of a CcAAC-H proton to Al generating an

intermediate of the form (cAACEtH)AlH2(cAACEt) 5-7-Int. Redelivery of a hydride from the

AlH2 fragment results in a site-exchange process (Scheme 5.5). A similar reaction mechanism

was proposed for the exchange observed for (cAACMeH)BH2(cAACMe) by Braunschweig and

co-workers.43 In contrast to the present system, these authors isolated (cAACMeH)BH2(cAACMe),

141

which is directly analogous to the proposed intermediate 5-7-Int. It is also interesting to note that

the interconversion of the diastereomers of 5-7 via 5-7-Int amounts to reversible redox-

chemistry at the cAAC-carbon.

Figure 5.8 - 1H-1H EXSY NMR spectrum for 5-7.

142

Scheme 5.5 - Proposed reaction mechanism for the isomerization of 5-7.

Figure 5.9 – 1H NMR of the crude reaction of synthesis of 5-8.

Heating compound 5-7 in tol-d8 at 100 °C for 90 min. leads to complete conversion from the two

diastereomers of 5-7 to a new compound 5-8. 1H NMR spectroscopy revealed four inequivalent

CDipp-H protons and two inequivalent CcAAC-H protons each of equal intensity (Figure 5.9).

While the Al-H signal disappeared, a signal at 4.30 ppm corresponding to dihydrogen gas was

observed in the reaction mixture. These data suggested that the activation of CMe-H of a Dipp

group afforded 5-8 with loss of H2. The alkylation of Al was confirmed unambiguously by an X-

ray diffraction study. The formulation of 5-8 determined to be (cAACEtH)Al(CHC(Et)2CH2-

C(Me)2NC6H3(iPr)C(Me)CH2 in which Al is bound to two cAAC-carbons and the carbon of one

of the isopropyl substituents. The Al-C distances were determined to be 1.969(2) Å, 1.984(2) Å

and 1.999(2) Å, while the C-Al-C angles about Al were found to be 122.95(8)°, 114.01(8)° and

143

123.02(8)°, consistent with a pseudo-trigonal planar coordination sphere about Al. It is

noteworthy that the species 5-8 is chiral due to the two stereogenic cAAC-carbon centers,

although the centrosymmetric space group reveals the co-crystallization of the RR and SS

enantiomers.

The formation of 5-8 suggests that at elevated temperatures the interaction of 5-7 with the alkyl

substituent on the arene ring prompts σ-bond activation of one of the C-H bond of the methyl

group prompting loss of H2 and affording 5-8 in an irreversible process.

5.2.3 Other NHC Based Al Compounds

Adapting the procedure from Power and coworkers,55 an equimolar of KHMDS and AlCl3 in

toluene was stirred at room temperature affording a white solid upon workup, recrystallizing the

product results to formulate the compound as [K(tol)][AlCl3N(TMS)2] 5-9, and

[K][AlCl3N(TMS)2] 5-9’ was crystalized as a toluene free compound from benzene/pentane

layering at room temperature.

Scheme 5.6 – Synthesis of 5-9 to 5-11.

When 5-9 is mixed with various carbenes (SIMes and cAACEt) in toluene, 1H NMR

spectroscopy reveals a single set of resonances that can be assigned to the carbene and the TMS

groups, while the 13C{1H} NMR spectrum shows no signals corresponding to the free carbene

carbon, suggesting the carbene is bound to the Al center to form (SIMes)AlCl2N(TMS)2 5-10

and (cAACEt)AlCl2N(TMS)2 5-11. Although crystals for 5-10 were not obtained,

recrystallization of 5-11 from slow evaporation of a saturated toluene solution of 5-11 confirmed

the connectivity (Figure 5.11). The Al-CcAAC bond length is 2.102(6) Å, which is longer than

what was seen in 5-7 and 5-8, but still in the range of NHC-Al adducts.26, 31, 34 The CcAAC center

remains mostly planar with the sum of the angles about carbon being 359.8°. Comparing with 5-

9 and 5-9’, the bond length for the Al-N bond is longer as 1.839(4) Å for 5-9.

144

Figure 5.10 - ORTEPs of 5-9’ (left) and 5-9 (right) with thermal displacement parameters drawn

at 50 % probability. C: black, N: blue, Al: cyan, Cl: green, K: purple, Si: pink.

These results are comparable with the analogous (cAACCyc)BCl2(N(TMS)2), a precursor to the

corresponding boron radical and borylene compounds (cAACCycBCl(N(TMS)2• and

cAACCycBCl(N(TMS)2) prepared previously by our group previously.41 Similar, the preparation

of the Al analogues from reduction reactions of 5-11 was attempted. Reacting the compound

with excess amount of KC8 in THF resulted in a pink solution. However, all attempts to either

isolate the compound or to trap it were failed, and any observed decomposition products

suggested that the cAACEt was no longer bound to the Al center (Figure 5.13 and Figure 5.14).

Figure 5.11 – ORTEPs of the adduct 5-11 (left) and 5-12 (right) with thermal displacement

parameters drawn at 50 % probability. C: black, N: blue, Al: cyan, Cl: green, Si: pink.

145

To prepare a better precursor for nucleophilic Al compounds, we went back to investigate the

cAAC carbene and alane reaction. To prevent the second Al-H activation by the carbene, we

decided to use a very bulky cAACMenthyl carbene. Mixing an equimolar of enantiopure

cAACMenthyl with AlH3·NEtMe2 in toluene, readily afforded the colorless compound

((cAACMenthylH)AlH2)2 5-12

(

Scheme 5.7 and Figure 5.11) in 95 % yield upon recrystallization from a saturated pentane

solution at -35 °C. 1H NMR spectroscopy reveals a cAACMenthyl-H resonance at 3.50 ppm., and a

broad resonance attribute to the Al-H fragment at 3.92 ppm. Compound 5-12 crystalized in the

centrosymmetric space group P212121, while the asymmetric unit revealing a dimeric structure of

5-12. The Al-CcAAC bond lengths are 1.971(4) and 1.968(5) Å which are comparable to 5-7 and

5-8.

Scheme 5.7 – Synthesis and reduction of 5-12.

146

Figure 5.12 - Wireframe depiction of the molecular structure of 5-13; The hydrogen atoms are

omitted except Al-H and the CcAAC-H for clarity. C: black, N: blue, Al: cyan, H: grey.

Reduction of 5-12 were carried out with KC8 in THF at room temperature, resulting in a light-

yellow solution. After a workup and recrystallization from a saturated pentane solution at -35 °C,

micro crystal were obtained for 5-13

(

Scheme 5.7 and Figure 5.12). Due to poor diffraction of the crystal, only partial structure was

obtained. As shown in Figure 5.12, it reveals a tetrameric structure of 5-13 with two possible

formulation [K]4[(cAACMenthylH)AlH]4 or [K]4[(cAACMenthylH)AlH3]4 based on the connectivity

of the structure. The structure of 5-13 suggested the reduction reaction of 5-12 was successful,

however, as only limited amount of 5-13 were obtained, further studies is needed to further

confirm the structure of 5-13 and its reactivity.

147

5.3 Conclusions

Hydride abstraction reaction of NHC-alane adduct with trityl borate were shown to generate

monomeric three-coordinate aluminum-hydride salt 5-2, which represent the first dicationic

aluminum-hydride to be characterized. When the reaction is performed in THF, the THF trapped

intermediate 5-3 is generated. Furthermore, 5-2 decomposed in THF to give 5-4. Reactions of

alane and cAACEt allowed for the isolation of several Al-H species. Among them, compound 5-7

is to our knowledge, the first isolated example of a neutral monomeric dialkyl aluminum hydride.

VT and 1H-1H EXSY NMR spectroscopy demonstrate that 5-7 exists as a dynamic equilibrium

of diastereomers proceeding through a facile and reversible hydride migration between the Al

and cAAC-carbon center. The intermediate 5-7-int also provides thermal access to the C-H bond

activation of an isopropyl group of a carbene substituent providing the species 5-8. While other

NHC stabilized Al compounds were prepared, however, we found that the reduction reaction of

5-11 to prepare a nucleophilic Al compound was unsuccessful as cAACEt dissociates from the Al

center. Also, reduction of aluminum hydride compound 5-12 was successful but further studies

are needed to confirm the formulation of 5-13 and its reactivity.





°C freezer). Toluene and pentane were collected from a Grubbs-type column system

manufactured by Innovative Technology. Pentane, bromobenzene, and toluene were stored over

4 Å molecular sieves. Molecular sieves, type 4 Å (pellets, 3.2 mm diameter) purchased from

Sigma Aldrich were activated prior to usage by iteratively heating with 1050 W Haier


moisture was released upon heating. C6D6, C6D5Br and tol-d8 purchased from Cambridge Isotope


prior to use. Unless otherwise mentioned, chemicals were purchased from Sigma Aldrich or TCI.

1,3-Dibenzylimidazolium bromide,56 cAACEt and cAACMentyl,57 were prepared using literature

methods. NMR spectra were recorded on a Bruker Avance III 400 MHz, an Agilent DD2 500

MHz and Agilent DD2 600 MHz spectrometer and were referenced to residual solvents of

148

C6D5Br (1H = 7.28 ppm for meta proton; 13C = 122.4 ppm for ipso carbon), C6D6 (1H = 7.16

ppm; 13C = 128.06 ppm) or externally (11B: (Et2O)BF3, 19F: CFCl3,

27Al: Al(NO3)3). Chemical

shifts (δ) are reported in ppm and the absolute values of the coupling constants (J) in Hz. In some

instances, signal and/or coupling assignment was derived from 2D NMR experiments. IR spectra

were recorded on a Perkin-Elmer Spectrum One FT-IR instrument or a Bruker ALPHA

spectrometer equipped with an ATR sampling unit. Elemental analysis (C, H, N) were performed

in house.

5.4.2 Synthesis and characterizations

Synthesis of C3H2N2(CH2Ph)2) (IBn)

1,3-Dibenzylimidazolium bromide (2.013 g, 6.114 mmol) was suspended in 10 mL of toluene

and cooled to -35 °C. While stirring, solid KHMDS (1.219 g, 6.112 mmol) was then slowly

added. The reaction mixture was warmed to room temperature and stirred for 4 h. The resulting

mixture was passed through Celite, and volatiles were removed in vacuo. The obtained light

yellow free carbene solid (1.423 g, 93 %) was washed with pentane (3 x 5 mL) and used without

further purification.

1H NMR (400 MHz, C6D6, 298K): δ 7.15 (d, 3JHH = 6.9 Hz, 4H, p-PhH) 7.07 (t, 3JHH = 7.2 Hz,

4H, m-PhH), 7.02 (d, 3JHH = 6.9 Hz, 2H, o-PhH), 6.37 (m, 2H, NCH), 5.09 (s, 4H, PhCH2).

Synthesis of (IBn)AlH3 (5-1).

Free IBn carbene (758 mg, 3.03 mmol) was dissolved in 4 mL of toluene, and AlH3·NEtMe2 (6.0

mL of 0.5 M solution in toluene, 3.0 mmol) was added dropwise at room temperature. The

reaction was allowed to stir for another 2 h. The crude mixture was filtered through Celite and

volatiles were removed in vacuo. The resulting residue was washed with 3 x 2 mL pentane and

extracted with 3 x 5 mL toluene. The combined extracts were filtered through Celite and layered

with pentane to give yellow needles (597 mg, 72 %) that were suitable for X-ray crystallography.

1H NMR (500 MHz, C6D5Br, 298K): δ 7.16-7.06 (m, 10H, PhH), 6.35 (s, 2H, NCH), 5.24 (s,

4H, PhCH2), 4.38 (br s, 3H, AlH3). 13C{1H} NMR (126 MHz, C6D6, 298K): δ 136.1, 129.1,

128.6, 128.6, 128.4, 121.1 (NCH=CHN), 53.7 (PhCH2). 27Al{1H} NMR (130 MHz, C6D5Br,

149

298K): δ 108.7 (br s). Anal. Calcd. for C17H19AlN2: C 73.36 %, H 6.88 %, N 10.06 %. Found: C

73.04 %, H 6.73 %, N 9.95 %.

Synthesis of [(IBn)2AlH][B(C6F5)4]2 (5-2)·(C6H5Br).

Compound 5-1 (363 mg 1.303 mmol) was dissolved in 1.5 mL of C6H5Br in a vial, and trityl

tetrakis(pentafluorophenyl)borate (1.197 g 1.303 mmol) in 1.5 mL of C6H5Br was added

dropwise into the vial. The reaction mixture was allowed to sit at room temperature for 16 h.

White blocky crystals formed from the solution. The supernatant was decanted and the crystals

were washed with bromobenzene (3 x 1.5 mL), followed by pentane (3 x 2 mL) and

subsequently dried in vacuo to give (5-2)·(C6H5Br) (614 mg, 46 %, calculated based on IBn).

The crystals obtained were suitable for X-ray crystallography.

1H NMR (400 MHz, C6D5Br, 298K): δ 7.34 (s, 2H, PhH), 7.17-7.11 (m, 13H, PhH), 6.80 (dd,

3JHH=1.7, 7.6 Hz, 7H, PhH), 6.41 (d, 3JHH =1.6 Hz, 4H, NCH), 4.50 (s, 8H, PhCH2). 11B{1H}

NMR (160 MHz, C6D5Br, 298 K): δ -16.2 (s). 19F{1H} NMR (470 MHz, C6D5Br, 298 K): δ -

131.8 (br s, 16F, o-C6F5), -161.6 (t, 3JFF = 21 Hz, 8F, p-C6F5), -165.6 (br, t, 3JFF = 20 Hz, 8F, m-

C6F5). 13C{1H} NMR (125 MHz, C6D5Br, 298K): δ 131.3, 129.7, 128.2, 126.6, 122.1, 53.7. IR

(KBr pellet, cm-1): 1963 (s, νAl-H). Anal. Calcd. for C82H33AlB2F40N4·C6H5Br: C 51.82 %, H

1.88 %, N 2.75 %. Found: C 51.40 %, H 1.27 %, N 2.61 %.

Generation of [(IBn)AlH2(THF)2][B(C6F5)4] (5-3) and [AlH2(THF)4][B(C6F5)4] (5-4)

Compound 5-1 (693.2 mg, 2.49 mmol) was dissolved in 10 mL of THF, and [Ph3C][B(C6F5)4]

(2.287 g, 2.49 mmol) was slowly added to the solution. The reaction mixture was allowed to stir

at room temperature for 3 minutes, and the solvent was removed under vacuum. The crude

product was then washed with 3 x 4 mL of pentane, and dried in vacuo to give the crude product

mixture of 5-3 and 5-4. Crystals of 5-3 and 5-4 were obtained from a saturated solution of THF

layered with pentane at room temperature, but no clean product were isolated for NMR studies

due to the similar solubility of impurities.

Generation of [AlH2(THF)4][B(C6F5)4] (5-4) from 5-2

150

Compound 5-2·(C6H5Br) (255.4 mg, 0.125 mmol) was dissolved in 2 mL of THF, and the

reaction mixture were allowed to stir at room temperature for 10 minutes, and the solvent was

removed under vacuum. The crude product was then washed with 3 x 2 mL of pentane, and dried

in vacuo to give the crude product of 5-4. Crystals of 5-4 were obtained from a saturated solution

of THF layered with pentane, but no clean product were isolated for NMR studies due to the

similar solubility of the impurity.

Synthesis of (cAACEtH)AlH2·NEtMe2 (5-5) and (cAACEtH)2Al(µ-H)2AlH2·NEtMe2 (5-6)

A solution of AlH3·NEtMe2 in toluene (0.5 M, 0.57 mL, 0.29 mmol) was added dropwise to a

solution of cAACEt (90 mg, 0.29 mmol) in 2 mL of pentane while stirring at room temperature/-

35/-78 ºC. The reaction mixture was allowed to slowly warm up to room temperature and stirred

for another 2 h. The solvent was removed in vacuo to yield orange oil, which was dissolved in

minimum amount of pentane, and left in the freezer overnight to obtain crystalline products that

was suitable for single crystal X-ray studies for 5-5 and 5-6. Attempts to isolate the clean product

of 5-5 and 5-6 are unsuccessful due to the similar solubility as well as 5-5 and 5-6 release

NEtMe2 slowly at room temperature. Thus, NMR spectroscopy data was not obtained for clean

compounds.

Synthesis of (cAACEtH)2AlH (meso-5-7) and (rac-5-7)


solution of cAACEt (534 mg, 1.70 mmol) in 6 mL of pentane while stirring at room temperature.

The reaction mixture was stirred at room temperature for another 2 h. The solvent was removed

in vacuo to yield white powder, which was dissolved in minimum amount of pentane, and left in

the freezer overnight to afford crystalline products that was suitable for single crystal X-ray

studies. The resulting compound were washed with 3 × 2 mL of cold pentane and dried in vacuo

to yield white powder (328 mg, 59 % Yield).

1H NMR (600 MHz, tol-d8, 253K, dr(meso-5-7:rac-5-7) = 1: 0.6): δ 7.12-6.92 (m, 12H), 4.89 (s,

br, 1H, rac-5-7-AlH), 4.29 (p, 3JHH = 6.8 Hz, 2H, rac-5-7- HiPr), 4.20 (p, 3JHH = 6.7 Hz, 2H,

meso-5-7-HiPr), 3.91 (s, br, 1H, meso-5-7-AlH), 3.61 (s, 2H, meso-5-7-HCcAAC), 3.46 (p, 3JHH =

6.8 Hz, 2H, meso-5-7-HiPr), 3.22 (p, 3JHH = 6.7 Hz, 2H, rac-5-7-HiPr), 3.10 (d, 3JHH = 2.5 Hz,

2H, rac-5-7-HCcAAC), 1.98 (m, 3H), 1.78-1.12 (m, 54H), 1.06 (s, 4H), 0.92-0.85 (m, 12H), 0.78

151

(t, 3JHH = 7.4 Hz, 4H), 0.70 (d, 3JHH = 7.6 Hz, 6H), 0.46 (t, 3JHH = 7.3 Hz, 4H). 13C{1H} NMR

(101 MHz, C6D6, 298K): δ 152.0 (-PhC), 151.0 (-PhC), 149.7 (-PhC), 144.8 (-PhC), 148.0 (-

PhC), 143.7 (-PhC), 126.6 (-PhC), 125.7 (-PhC), 125.2 (-PhC), 124.7 (-PhC), 73.7 (-HCcAAC

(meso-5-7)), 70.2 (-HCcAAC (rac-5-7)), 66.2, 64.3, 55.6, 52.3, 49.5, 48.6, 34.3, 33.8, 33.5, 32.5,

32.1, 29.4, 29.0, 28.3, 27.7, 27.3, 26.4, 25.7, 25.5, 25.2, 24.7, 14.3, 10.7, 10.3, 10.0. 27Al{1H}

NMR (128 MHz, C6D6, 298K): no signal detected. IR (in benzene, cm-1): 1853 (s, br, νAl-H).

Anal. Calcd. For C44H73AlN2: C 80.43 %, H 11.20 %, N 4.26 %. Found: C 80.65 %, H 11.85 %,

N 3.86 %.

Synthesis of (cAACEtH)Al(CHC(Et)2CH2C(Me)2NC6H3(iPr)C(Me)CH2 (5-8)

Compound 5-8 (45 mg, 0.068 mmol) was dissolved in 0.5 mL of tol-d8 in a sealed 5 mm NMR

tube and heated at 100 °C for 90 minutes, the crude reaction 1H NMR spectroscopy were

collected after the reaction cooled down to room temperature. The NMR tube was then brought

into a glovebox, and the reaction mixture was transferred out with 2 mL of pentane, and in vacuo

to yield colorless crystals that is suitable for X-ray studies. The crystalline product was then

washed with 3 × 2 mL of cold pentane and dried to yield 5-8 (44 mg, >95 %).

1H NMR (400 MHz, C6D6, 298 K): δ 7.33-7.07 (m, 6H, ArH), 4.18 (m, br, 1H, -CH(CH2)

(CH3)), 4.07 (hept, 3JHH = 6.9 Hz, 1H, -HiPr), 3.64 (hept, 3JHH = 6.8 Hz, 1H, -HiPr), 3.37 (s, 1H,

HCcAAC), 3.26 (hept, 3JHH = 6.4 Hz, 1H, -HiPr), 3.13 (s, 1H, HCcAAC), 2.00-1.56 (m, 12H), 1.55-

1.03 (m, 35H), 0.99 (t, 3JHH = 7.4 Hz, 3H, -CH2CH3), 0.87 (t, 3JHH = 7.4 Hz, 3H, -CH2CH3), 0.66

(t, 3JHH = 7.0 Hz, 3H, -CH2CH3), 0.48 (t, 3JHH = 7.2 Hz, 3H, -CH2CH3). 13C{1H} NMR (101

MHz, C6D6, 298K): δ 153.8 (-PhC), 153.4 (-PhC), 151.2 (-PhC), 150.2 (-PhC), 145.0 (-PhC),

141.5 (-PhC), 127.0 (-PhC), 126.2 (-PhC), 125.7 (-PhC), 124.2 (-PhC), 124.1 (-PhC), 123.2 (-

PhC), 70.2 (-HCcAAC), 66.7 (-HCcAAC), 64.2, 63.5, 54.8, 54.5, 50.0, 49.7, 35.4, 33.1, 33.0, 33.0,

32.2, 32.0, 29.4, 29.0, 28.8, 28.7, 28.4, 28.3, 27.4, 27.3, 26.1, 26.0, 25.5, 25.3, 25.1, 25.0, 11.0,

10.9, 10.6, 10.5. 27Al{1H} NMR (128 MHz, C6D6, 298K): no signal detected. Anal. Calcd. For

C44H71AlN2: C 80.68 %, H 10.93 %, N 4.28 %. Found: C 81.04 %, H 11.47 %, N 3.92 %.

Synthesis of [K(tol)][AlCl3HMDS] (5-9) and [K][AlCl3N(TMS)2] (5-9’)

AlCl3 (0.976 mg, 7.32 mmol) and KHMDS (1.460 g, 7.32 mmol) were mixed in 10 mL of

toluene, and stirred at room temperature for overnight. The reaction mixture was then filtered,

152

and the solvent was removed under vacuum. The crude product was washed with 3 × 5 mL

pentane, and dried to yield compound 5-9 (1.742 g, 56 %). Crystals of 5-9 were grown from slow

evaporation of a saturated toluene solution, while crystals of 5-9’ were grown from a saturated

benzene solution layering with pentane at room temperature to give the solvent free compound.

Due to the low solubility of 5-9 in benzene for recrystallization of 5-9’, we decided to only use

5-9 for further studies.

1H NMR (400 MHz, C6D6, 298K): δ 0.58 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (126 MHz,

C6D6, 298K): δ 5.6 (s, N(Si(CH3)3)2). 27Al NMR (104 MHz, C6D6, 298K): no signal detected.

Synthesis of (SIMes)AlCl2N(TMS)2 (5-10)

Free carbene SIMes (67.0 mg, 0.219 mmol) were mixed with 5-9 (92.9 mg, 0.219 mmol) in 4

mL of benzene, and stirred under room temperature overnight. The solution was then filtered,

and the solvent was removed under vacuum. The resulting solid was then washed with 3 × 2 mL

of pentane, and dried in vacuo to give compound 5-10 (116 mg, 94 %).

1H NMR (500 MHz, C6D6, 298 K): δ 6.75 (dd, 4JHH = 0.7, 1.3 Hz, 4H, C6Me3H2), 2.96 (s, 4H,

CH2-CH2), 2.32 (d, 4JHH= 0.6 Hz, 12H, o-C6Me3H2), 2.08 (s, 6H, p-C6Me3H2), 0.33 (s, 18H,

N(Si(CH3)3)2). 13C{1H} NMR (400 MHz, C6D6, 298 K): δ 139.3 (ArC), 136.1 (ArC), 134.4

(ArC), 130.2 (ArC), 51.7 (CH2-CH2), 20.9 (p-C6Me3H2), 19.0 (o-C6Me3H2), 6.29 (N(Si(CH3)3)2).

27Al NMR (104 MHz, C6D6, 298K): no signal detected.

Synthesis of (cAACEt)AlCl2N(TMS)2 (5-11)

Free carbene cAACEt (228.8 mg, 0.730 mmol) were mixed with 5-9 (310.1 mg, 0.730 mmol) in 4

mL of benzene, and stirred under room temperature overnight. The solution was then filtered,

and the solvent was removed under vacuum. The resulting solid was then washed with 3 × 2 mL

of cold pentane, colorless crystal was obtained from a saturated toluene solution layered with

pentane at -35 °C for compound 5-11 (240 mg, 57 %).

1H NMR (400 MHz, C6D6, 298 K): δ 7.35-7.08 (m, 3H, ArH), 2.95 (p, 3JHH = 6.6 Hz, 2H, HiPr),

2.65 (dq, 3JHH = 7.4, 14.7 Hz, 2H, CH2CH3), 2.14 (dq, 3JHH = 7.1, 14.2 Hz, 2H, CH2CH3), 1.66

(d, 3JHH = 6.1 Hz, 6H, HiPr), 1.54 (s, 2H, CH2), 1.28 (d, 3JHH = 6.1 Hz, 6H, HiPr), 1.05 (t, 3JHH =

7.1 Hz, 6H, CH2CH3), 0.93 (s, 6H, C(CH3)2), 0.70 (s, 18H, N(Si(CH3)3)2). 13C{1H} NMR (101

153

MHz, C6D6, 298 K) 145.7 (ArC), 137.5 (ArC), 124.5 (ArC), 82.1 (C(CH3)2), 66.2 (CH2), 43.4,

30.7, 29.2, 29.1, 27.0, 22.2 (C(CH3)2), 9.9 (CH2CH3), 6.1 (N(Si(CH3)3)2). and 27Al{1H} NMR

104 MHz, C6D6, 298 K): no signal detected.

Reduction of 5-11

The general procedure for reduction of 5-11 (13.3 mg, 0.023 mmol) was performed in THF

solution with KC8 (13.6 mg, 0.101 mmol) at -35 °C for 10 minutes, and the solution was quickly

added to the substrates used to trap the product ((Ru(CO)4)3 (4.9 mg, 0.008 mmol), or P≡CAd

(4.1 mg, 0.023 mmol)), the decomposition product were either characterized by X-ray study or

NMR studies for literature known compound.

For (Ru(CO)4)3, the decomposition product was obtained by recrystallization of the crude

product from a toluene solution layered with pentane at -35 °C to give (cAACEt)Ru3(CO)11 based

on X-ray studies, which is a displacement of one of the CO molecule from the (Ru(CO)4)3 with a

free cAACEt indicating cAACEt dissociated from the Al center.

Figure 5.13 – ORTEPs of (cAACEt)Ru3(CO)11 with thermal displacement parameters drawn at

50 % probability. C: black, N: blue, Ru: pink, O: red.

154

Similar results were observed for the reaction with P≡CAd. The 31P NMR spectrum of the crude

reaction reveals the generation of ((cAACEt)C(Ad)P=PC(Ad)(cAACEt) at 522.2 ppm and its

monomer (cAACEt)C(Ad)P at 191.3 ppm, while excess of P≡CAd was shown at -74.9 ppm.58

Figure 5.14 – 31P NMR of the crude reduction reaction of 5-11 with P≡CAd.

Synthesis of ((cAACMenthylH)AlH2)2 (5-12)


solution of cAACMenthyl (167.3 mg, 0.438 mmol) in 4 mL of benzene, and stirred at room

temperature for 3 h. The solvent was then removed under vacuo, and the resulting crude solid

was dissolved in minimum amount of pentane, and stored in the freezer at -35 °C overnight to

give colorless crystalline 5-12 (171 mg, 95 %).

155

1H NMR (400 MHz, C6D6, 298 K): δ 7.27-6.93 (m, 6H, ArH), 4.06 (p, 3JHH = 6.7 Hz, 4H, HiPr),

3.92 (s, br, 4H, AlH2), 3.50 (s, 2H, CcAACH), 3.37 (p, 3JHH = 6.7 Hz, 1H, HiPr), 2.76 (d, 3JHH =

13.6 Hz, 6H, HiPr), 2.23 (m, 6H), 1.85 (d, 3JHH = 12.7 Hz, 6H), 1.65-0.83 (m, 45H). 13C{1H}

NMR (400 MHz, C6D6, 298 K) δ 152.4 (ArC), 140.3 (ArC), 125.6 (ArC), 125.3 (ArC), 61.9

(CcAACH), 58.6, 51.0, 50.1, 49.0, 35.7, 33.9, 32.3, 29.6, 28.9, 27.2, 27.1, 26.8, 26.5, 25.5, 25.4,

25.3, 24.7, 22.6, 20.0. 27Al{1H} NMR (104 MHz, C6D6, 298 K): no signal detected.

Generation of 5-13

A mixture of 5-12 (40.0 mg, 0.0485 mmol) and KC8 (30.0 mg, 0.222 mmol) were stirred in 3 mL

of THF at room temperature, the reaction mixture was then filtered and the solvent was removed

under vacuum, and the crude solid was then dissolved in minimum amount of pentane and stored

at -35 °C for days until colorless micro crystals of 5-13 were obtained. However, not enough

materials were obtained for mutlinuclear NMR spectroscopic analysis.

156

5.4.3 Thermodynamic Calculations

Figure 5.15 - 1H VT-NMR spectra (range: 2.8 to 5.2 ppm) of 5-7 in tol-d8.

Table 5.1- Average ratio between meso-5-7 and rac-5-7 at different temperature.

Temperature meso-5-7 rac-5-7

-40 °C 2.50 1.0

-20 °C 1.60 1.0

0 °C 1.05 1.0

20 °C 0.75 1.0

40 °C 0.50 1.0

157

Figure 5.16 - Van’t Hoff plot for 5-7.

ΔH = 12.042 kJ/mol = 2.88 kcal/mol

ΔS = 43.8 J/(mol·K) = 0.01 kcal/(mol·K)









158

Structure Solution and Refinement. The structures were solved by intrinsic phasing using XS.

All the structures were subjected to full-matrix least-squares refinement on F2 using XL as




parameters. The Al-bound hydrogen atoms for 5-1 to 5-7 were located in difference Fourier

maps and refined in an unrestrained manner.

For compound 5-13, as the dataset collected due to a breakdown of the diffractometer, and

repeated attempts for growing crystals of 5-13 were unsuccessful. Thus, only the unit cell

parameters of 5-13 is reported for further reference in this thesis.

159

Table 5.2 - Summary of crystallographic data for compounds of 5-1 to 5-4.

5-1 5-2·C6H5Br 5-3 5-4

Moiety

formula

C17 H19 Al N2 C88 H38 Al B Br

F40 N4

C49 H34 Al B

F20 N2 O2

C40 H34 Al B

F20 O4

Sum formula

weight

278.32 2039.73 1100.57 996.46

crystal system Monoclinic Triclinic Orthorhombic Orthorhombic

space group P21/n P-1 P21212 P21212

a (Å) 11.828(1) 12.470(1) 13.5725(6) 23.504(1)

b (Å) 11.514(1) 13.544(1) 21.402(1) 23.782(1)

c (Å) 12.649(1) 23.948(2) 8.0150(4) 7.4814(4)

α (deg.) 89.208(4)

β (deg.) 113.135(4) 88.213(4)

γ (deg.) 79.504(4)

vol (Å3) 1584.2(2) 3974.8(5) 2328.2(2) 4184.8(3)

Z 4 2 2 4

ρ (calcd)

(Mg∙cm3)

1.167 1.704 1.570 1.583

μ (mm-1) 0.120 0.675 0.168 0.180

F(000) 592 2024 1112 2016


T(K) 150(2) 150(2) 150(2) 150(2)

reflections

collected

14251 65355 21994 39366

unique

reflections

3614 18261 5364 9620

Rint 0.0564 0.0464 0.0437 0.0555

GOF (F2) 1.011 1.028 1.102 0.881

R1 indices

[I>2σ(I)]

0.0402 0.0740 0.0464 0.0486

wR2 indices

(all data)

0.1003 0.2421 0.1201 0.1495

Largest diff.

peak and hole

(e. Å-3)

0.242 & -0.200 2.774 & -1.866 0.646 & -0.510 0.362 and -

0.502

CCDC No. 1453309 1453310 - -

160

Table 5.3 – Summary of crystallographic data for compounds of 5-5 to 5-9.

5-5 & 5-6 meso-5-7 5-8 5-9

Moiety

formula

C74 H135 Al3 N5 C44 H73 Al N2 C44 H71 Al N2 C13 H26 Al Cl3 K

N Si2

Sum formula

weight

1175.80 657.02 655.00 424.96

crystal system Triclinic Triclinic Triclinic Triclinic

space group P-1 P-1 P-1 P-1

a (Å) 11.636(3) 9.4074(6) 11.9225(9) 13.071(6)

b (Å) 18.707(4) 11.4610(8) 13.304(1) 14.207(6)

c (Å) 19.303(5) 19.170(2) 13.382(1) 14.312(6)

α (deg.) 66.586(7) 92.871(2) 79.775(3) 98.708(9)

β (deg.) 76.109(8) 98.051(2) 80.203(2) 102.507(8)

γ (deg.) 86.462(8) 101.380(2) 79.872(3) 114.732(9)

vol (Å3) 3741(2) 1999.8(2) 2035.1(3) 2267(1)

Z 2 2 2 4

ρ (calcd)

(Mg∙cm3)

1.044 1.091 1.069 1.245

μ (mm-1) 0.092 0.082 0.080 0.727

F(000) 1306 728 724 888


T(K) 150(2) 150(2) 150(2) 150(2)

reflections

collected

44237 42000 45138 41674

unique

reflections

15956 9933 9692 8923

Rint 0.0593 0.0428 0.0696 0.1637

GOF (F2) 1.022 1.024 1.024 1.012

R1 indices

[I>2σ(I)]

0.0569 0.0450 0.0536 0.0788

wR2 indices

(all data)

0.1733 0.1295 0.1557 0.2098

Largest diff.

peak and hole

(e. Å-3)

0.544 & -0.477 0.310 & -0.320 0.271 & -0.298 1.487 & -0.595

CCDC No. 1850755 1850754 1850753 -

161

Table 5.4 - Summary of crystallographic data for compounds of 5-9’ to 5-13.

5-9’ 5-11 5-12 5-13

Moiety

formula

C6 H18 Al Cl3 K

N Si2

C28 H53 Al Cl2 N2

Si2

C54 H92 Al2 N2 -

Sum formula

weight

332.82 571.78 823.25 -

crystal system Orthorhombic Triclinic Orthorhombic Orthorhombic

space group Pbca P-1 P212121 -

a (Å) 9.015(3) 9.553(6) 9.7615(5) 35.03(4)

b (Å) 12.115(4) 9.783(4) 20.0288(9) 17.50(2)

c (Å) 29.44(1) 20.460(8) 26.862(2) 22.12(2)

α (deg.) 92.23(2)

β (deg.) 99.87(2)

γ (deg.) 116.06(2)

vol (Å3) 3215(2) 1678(2) 5251.7(4) 13566(26)

Z 8 2 4 -

ρ (calcd)

(Mg∙cm3)

1.375 1.132 1.041 -

μ (mm-1) 1.003 0.310 0.090 -

F(000) 1376 620 1824 -

Theta range ( ) 3.499 to 25.023 2.038 to 24.713 2.773 to 26.369 -

T(K) 150(2) 150(2) 150(2) -

reflections

collected

18540 22825 36325 -

unique

reflections

2838 5687 10710 -

Rint 0.1142 0.0721 0.1127 -

GOF (F2) 1.158 1.091 1.040 -

R1 indices

[I>2σ(I)]

0.0652 0.0585 0.0693 -

wR2 indices

(all data)

0.1449 0.2311 0.1352 -

Largest diff.

peak and hole

(e. Å-3)

0.463 & -0.520 0.610 & -0.886 0.290 & -0.412 -

CCDC No. - - - -


All calculations were computed using the Gaussian 09 program. Geometry optimizations were

performed at M06-2X functional46 with the crystallographic coordinates used as starting

geometries when available. The Def2-SVP basis set was used for all atoms.47 The stationary

162

nature of the converged geometry was confirmed by carrying out a frequency calculation and

ensuring the absence of imaginary frequencies. NBO calculations were performed using the

optimized geometry with NBO version 6.0.59 Optimized structures were visualized using

CYLview60 or Chemcraft software.61

Figure 5.17 - Calculated frequency vs. experimental frequency for 5-22+.

Table 5.5 - Calculated enthalpy for meso-5-7, (R, R)-5-7, and (S, S)-5-7.

Relative calculated enthalpy

(kcal/mol)

Experimental (kcal/mol)

meso-5-7 0 0

(R, R)-5-7 3.41717922

2.88 (S, S)-5-7 3.41592888

163

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167

Chapter 6

6 Preparation of HAl(C6F5)2 and Phosphinoalanes

6.1 Introduction

Earlier in chapter 3, we have discussed the radical pathway in FLP chemistry from the

combination of Mes3P and E(C6F5)3 (E = B, Al). Thus, this leads to a discussion if we could

prepare intramolecular P/Al-based FLPs, investigating if SET was feasible for P/Al-based FLPs

and their reactivity towards challenging substrates that are difficult for traditional FLPs.

6.1.1 Aluminum Based FLPs

Since the discovery of FLPs in 2006, the combinations of steric inhibited Lewis acids and bases

have been shown to activate various small molecules. So far, various Lewis bases have been

applied, including phosphines, amines, thioethers, and carbenes, etc., in FLP chemistry.1-4 On the

other hand, as Lewis acids, there are much less examples with alanes even though these are

generally better Lewis acids.5

Figure 6.1 – Selected examples of Al-based FLPs.

Aside from our own research on AlX3 (X = C6F5, Cl, Br, I) combined with various phosphines,6-

14 The Chen group has reported various examples of using Al(C6F5)3 with amines in polymer

synthesis.15-18 Since 2011, Uhl and coworkers have prepared a family of geminal P/Al-based

168

FLPs prepared from hydroalumination of alkynylphosphines (Figure 6.1).5, 19-24 In the same year,

Fontaine and coworkers have illustrated the capability of CO2 capture with (Me2PCH2AlMe2)2.25

While the Harder group has demonstrated the FLP reactivity with Al in two instances: an

aluminum-based cation [nacnacDippAl]+ coupled with Ph3P26 and a phosphinoamide-AlMe2

complex, (Ph2P(Dipp)NAlMe2)2,27 where the FLP reactivity was masked with the weak Al-P

interaction (Figure 6.1). Recently, Breher has reported the synthesis of aluminium

diphosphamethanides (Figure 6.1),28 a hidden FLP that can activate H2 molecules to give the

corresponding alane and phosphine.

6.1.2 Phosphinoboranes and Phosphinoalanes

Figure 6.2 – Selected examples of phosphinoboranes and phosphinoalanes.

The study of stable compounds with multiple bonds between heavier main group elements is one

of the central themes of organometallic and inorganic chemistry for decades.29, 30 One of most

studied combinations is multiple bonding behavior Group 13 and Group 15 elements.29, 30 With

the discovery of FLP chemistry, previously synthesized phosphinoboranes have received more

attention in the recent years for their reactivity. In 2008, our group has reported the synthesis of

R’2PB(C6F5)2 (R’ = tBu, Cy) and demonstrated their ability for H2 activation.31, 32 More recently,

Westcott and coworkers have synthesized Ph2PBPin and illustrated the phosphinoboration of on

unsaturated bonds.33-35 However, there are only limited examples in the literature of

phosphinoalanes and their reactivities. In 1994, Power has isolated the first phosphinoalane that

showed a pyramidal geometry at the P center with a planar geometry at Al center, which

suggested a single bonded structure.36 This is true for most of phosphinoalanes synthesized in the

169

literature.29, 30 Until 2007, Nӧth and Paine have reported the preparation of phosphinoalanes with

Tmp (Tmp = 2,2,6,6-tetramethylpiperidine) group on the Al center.37 Although the mono-

substituted phosphinoalane remained a typical single bonded structure, the P atom center shifted

to a planar geometry surround by two planar Al centers in the solid state structure.

6.1.3 Possible Synthetic Route to P/A-based FLPs

Thus, to prepare an intramolecular P/Al-based FLPs, this chapter will focus on the synthesis of

HAl(C6F5)2 and phosphinoalane, which could further hydroaluminate or phospinoaluminate C≡C

triple bond to generate the target compounds.

Figure 6.3 – Possible synthetic routes to prepare intramolecular P/Al FLPs.


6.2.1 Synthesis of Bis(pentafluorophenyl) Alane (HAl(C6F5)2)

In 2006, Collins has reported the generation of HAl(C6F5)2 6-1 from ClAl(C6F5)2 and a

zirconocene hydride source.38 Following the literature procedure, ClAl(C6F5)2 was prepared from

redistribution reaction between Al(C6F5)3·tol and AlCl3 to give colorless crystals in 67 % yield.28

Following the analogous procedure of preparing of Piers’ borane HB(C6F5)2, from ClB(C6F5)2

and Et3SiH,39 the combination of ClAl(C6F5)2 with an equimolar ratio of Et3SiH at room or

elevated temperature in toluene revealed no reaction, while addition of catalytic amount of

[Ph3C][B(C6F5)4] results decomposition of the reaction mixture indicated by the 19F NMR

spectroscopy studies. However, stirring ClAl(C6F5)2 together with excess amount of NaH in

C6H5F at room temperature led to generation of a new product, which was isolated as a white

solid after workup. The 1H NMR spectrum of the crude product reveals a broad resonance at 4.38

ppm attributed to Al-H signal. Also, the 19F NMR spectrum showed the full consumption of

ClAl(C6F5)2 and a new set of signals at -122.6, -124.1, -149.8, -151.3, -159.9 and -161.0 ppm,

which are similar to those Collins has reported.38 Moreover, it was also found that the signals

170

vary depending on the concentration and the solvent that 6-1 is dissolved in (Figure 6.9),

suggesting dimer or trimer 6-1’ were formed in solution, similar as other aluminium hydrides.40

We were able to trap 6-1 with an external base, DMAP, to give the compound 6-2, where single

crystals suitable for X-ray study were obtained from a saturate solution of 6-1 in toluene layered

with a saturated pentane solution of DMAP at -35 °C. The formulation of 6-2 was confirmed as

HAl(C6F5)2(DMAP)2 (Figure 6.4). The Al center features a trigonal bipyramidal geometry, while

the two DMAP molecules located at the axial position, with Al-N bond length as 2.102(3) and

2.091(3) Å. The Al bound H atom was located through electron density difference map, and the

sum angle of the AlC2H plane is 359.9°. These results supported the generation of 6-1. However,

when we were trying to obtain the solid state structure of 6-1, a trace amount of decomposition

product was isolated as crystals that were suitable for diffraction study. X-ray crystallography of

the decomposition products suggested that the crude mixture of 6-1 contained NaCl, generated

from the metathesis reaction. As the impurity, NaCl, is NMR silent, and attempts to remove the

impurity have failed due to the similar solubility, and presumably, weak interaction with product

6-1 (Figure 6.10). We conclude that the metathesis reaction between ClAl(C6F5)2 with NaH

successfully generated the desire product 6-1, but the crude product isolated herein contained

NaCl impurities that cannot be removed completely. However, this did not stop us from

exploring hydroalumination reaction between 6-1 with alkynes.

Scheme 6.1 – Reaction scheme of generation for 6-1 to 6-3.

While no reaction was observed with internal alkynes (PhC≡CPh), 6-1 rapidly reacted with

terminal alkynes (p-BrC6H4C≡CH) at room temperature. The 1H NMR spectroscopy studies of

the reaction mixture suggested that the hydroalumination reaction of the C≡C triple bond did

171

occur, with two alkene proton signals each showed up as a doublet at 5.51 and 5.08 ppm, with

3JHH = 18 Hz, and 3JHH = 11 Hz, respectively, while the 19F NMR spectrum revealed two set of

C6F5 signals at -121.7, -158.1, -162.7 ppm and -124.9, -151.8, -160.7 ppm, which suggested

another product was generated. Recrystallization of a saturated toluene solution of the crude

mixture layered with pentane at -35 °C gives compound (p-(Br(C6H4)C≡C)Al(C6F5)2)2 6-3

(Figure 6.4). Instead of the hydroalumination product, the dehydrocoupled compound 6-3 was

obtained. The solid state structure of 6-3 reveals a dimeric structure with Al-Ctriple bonds lengths

of 2.000(3) and 2.126(3) Å, while these lengths are longer than the sum of the covalent radii.41

We are continuing the investigation of the hydroalumination of alkynylphosphines.


50 % probability. C: black, F: pink, Al: cyan, Br: brown, H: grey. All carbon bond H atoms were

excluded for clarity.

Another alternative synthetic route for preparing the target intramolecular P/Al-based FLPs lead

us to attempt haloalumination reactions between haloalane with phosphinylacetylenes. The

treatment of ClAl(C6F5)2 with an equimolar amount of Ph2PC≡CH was carried out at room

temperature in toluene. Multinuclear NMR spectra studies revealed two doublet peaks, coupling

with each other in the 31P NMR spectrum with 2JPP = 190 Hz, while 19F NMR spectroscopy

showed two set of C6F5 signals. The formulation of compound 6-4 was unambiguously

172

confirmed as (C6F5)2ClAl-CH=C(PPh2)2CH=C-AlCl(C6F5)2 from X-ray crystallography (Figure

6.5). The formation of 6-4 can be attributed to a typical sequence of 1,2-P/Al FLP addition

reactions to the C≡C triple bond of a pair of the starting material Ph2PC≡CH (Figure 6.5a).

Similar FLP addition reactions with Ph2PC≡CH were also found in the literature with strong B

based Lewis acids (RB(C6F6)2, R = C6F5 or Me).42

Figure 6.5 – (a) Possible reaction mechanism for generating 6-4 and proposed reaction

mechanism. (b) ORTEP of 6-4 with thermal displacement parameters drawn at 50 % probability.

C: black, F: pink, Al: cyan, P: orange; Cl: green. All H atoms were excluded for clarity.

6.2.2 Synthesis of Mes2P-Al(C6F5)2

Adapting the procedure from its boron analogue,31 an equimolar amount of LiPMes2 was mixed

with ClAl(C6F5)2 in toluene (Scheme 6.2). Multinuclear NMR studies revealed the consumption

of starting material with the growth of a new set of resonances at -122.9, -153.4 and -161.1 ppm

that attributes to the C6F5 ring in 19F NMR spectrum, together with a broad singlet at -91.4 ppm

in 31P{1H} NMR spectrum. Additionally, a set of signals can be assigned to the Mes group in the

1H NMR spectrum. However, all attempts to confirm the solid state structure of 6-5 have failed,

as compound is very reactive, and only decomposition compounds were obtained. The observed

173

31P chemical shift is well matched to that computed for the proposed Mes2PAl(C6F5)2 6-5 (-88.3

ppm) at the GIAO-B97-2/Def2-TZVP//M06-2X/Def2-SVP of theory.

Figure 6.6 – (a) and (b) electric structure of 6-5 and Mes2P-B(C6F5)2. (c) Computed structure of

6-5, with computed WBI bond order in black. C: grey; P: orange; Al: pink; F: green. (d) ORTEP

of Mes2P-B(C6F5)2 with thermal displacement parameters drawn at 50 % probability. C: black, F:

pink, B: yellow-green, P: orange. All H atoms were excluded for clarity. (e) Plot of frontier

molecular orbitals for the 6-5 and Mes2P-B(C6F5)2. (f) and (g) NBOs for the P lone pair electron

and Al empty p orbital.

DFT studies of 6-5 and its boron analogues Mes2PB(C6F5)2 at M06-2X/Def2-TZVP//M06-

2X/Def-SVP revealed that 6-5 is similar to other phosphinoalanes that were reported featuring a

single bond between Al and P centers, with a WBI bond order as 0.85 a.u. While 6-5 features a

pyramidal geometry at the P atom and a planar structure at the Al atom, which is significantly

different from that of the planar Mes2PB(C6F5)2. These structural differences also show in their

molecular orbitals (Figure 6.6). The HOMO of 6-5 shows as the electron lone pair at the P

174

center, while the LUMO of 6-5 was dominated by the empty p orbital at the Al center, these were

also found with NBO analysis. In contrast, the HOMO and LUMO represented π and π* for the

double bonded P-B center in Mes2PB(C6F5)2. This difference suggested 6-5 should be very

reactive.

Initial combinations of 6-5 with typical Lewis bases, DMAP and DABCO, seems to cleave the

P-Al bond, resulting to a complex mixture based on the 19F and 31P{1H} NMR spectroscopy

studies. Thus, we turned to explore the reactivity of 6-5 with small molecules.

Firstly, 13CO2 was charged in a J-Young tube with a solution of 6-5 in DFB at room temperature.

The 13C{1H} NMR spectrum shows a diagnostic doublet signal at 197.5 ppm coupling to the 31P

atom (1JPC = 7 Hz), which appeared at -13.8 as a broad singlet in the 31P{1H} NMR spectrum.

These results are comparable with a single CO2 addition product. Formulation of the addition

product (Mes2P(13C(O)O)Al(C6F5)2)2 6-6 was confirmed from diffraction study (Figure 6.7). A

dimeric structure was found upon growing the asymmetric unit of 6-6.

Figure 6.7 - ORTEP of 6-6 with thermal displacement parameters drawn at 50 % probability. C:

black, F: pink, Al: cyan, P: orange; O: red. All H atoms were excluded for clarity.

175

Scheme 6.2 – Reaction of generation of 6-5 to 6-8.

The isolation of 6-6, supported the generation of the target P-Al compound and suggested that 6-

5 is much more reactive than its boron analogue, as there was no reaction observed with Mes2P-

B(C6F5)2 towards CO2. This is also in agreement with the computational prediction that 6-5, with

its unquenched lone pair of electrons on the P atom and an empty orbital on the Al center, should

be much more reactive than its double bonded planar analogue Mes2PB(C6F5)2. Indeed, other

than CO2, compound 6-5 also rapidly reacts with other small molecules like O2, internal and

external C≡C triple bonds, diazomethane, and azobenzene, according to multinuclear NMR

spectroscopy studies. However, as no solid-state structure has been obtained for any of the

desired products yet, we will only limit our discussion of the addition reaction with 6-5 and

Ph2PC≡CPh.

Monitoring the crude reaction of 6-5 with Ph2PC≡CPh in toluene, we saw the consumption of the

phosphinyl alkyne at -33 ppm in 31P{1H} NMR spectra gave raise to a doublet and a doublet of

pentet with a 31P-31P coupling as 46 Hz (Figure 6.8), while the pentet could attribute to the

through-space coupling with 19F atoms on the Al fragment. The 19F{1H} NMR spectrum

revealed a set of signals for the C6F5 group at -117.7, -154.0, -162.1 ppm. These suggested that

6-5 added across the C≡C triple bond to give compound (Mes2P)(Ph2P)C=C(Ph)Al(C6F5)2 6-7.

While reacting 6-5 with other R2PC≡CPh (R = Mes, tBu) showed no reaction, possibly due to the

steric bulk around the P center. Freshly generated 6-7 was placed in an atmosphere of 13CO2 at

176

room temperature and the resulting solution gave rise to a new doublet at 167.4 ppm in the

13C{1H} with a 1JPC = 121 Hz (Figure 6.8). In addition, the 31P{1H} NMR spectrum revealed a

doublet resonance at 12.9 with a 2JPP = 138 Hz, while another doublet of doublet resonance at -

6.3 ppm with 1JPC = 121 Hz and 2JPP = 138 Hz. These data suggested that CO2 was trapped with

the previously generated P/Al FLP 6-7 to give compound (Mes2P)(Ph2)PC=C(Ph)13CO2Al(C6F5)2

6-8.

Figure 6.8 – Multinuclear NMR spectra for 6-7 and 6-8.

6.3 Conclusion

In conclusion, we have generated the desired 6-1 and 6-5. Initial testing revealed that 6-1 could

undergo hydroalumination and dehydrocoupling reactions with terminal alkynes, while 6-5 was

able to capture 13CO2 to form 6-6. Also, multinuclear NMR spectra studies suggested the

phospinoaluminate reaction transpired between 6-5 and phosphinyl alkyne to give the desired

intramolecular P/Al-based FLP 6-7.

177

6.4 Experimental

6.4.1 General Consideration



°C freezer). Toluene and pentane were collected from a Grubbs-type column system

manufactured by Innovative Technology. Pentane, bromobenzene, and toluene were stored over

4 Å molecular sieves. Molecular sieves, type 4 Å (pellets, 3.2 mm diameter) purchased from

Sigma Aldrich were activated prior to usage by iteratively heating with 1050 W Haier


moisture was released upon heating. C6D6, C6D5Br, tol-d8 purchased from Cambridge Isotope


prior to use. Unless otherwise mentioned, chemicals were purchased from Sigma Aldrich or TCI.

Cl-Al(C6F5)2,28 Ph2PC≡CH, Ph2PC≡CPh,42 LiPMes2

31 were prepared using literature methods.

NMR spectra were recorded on a Bruker Avance III 400 MHz, an Agilent DD2 500 MHz, and

Agilent DD2 600 MHz spectrometer and spectra were referenced to residual solvents of C6D5Br

(1H = 7.28 ppm for meta proton; 13C = 122.4 ppm for ipso carbon), C6D6 (1H = 7.16 ppm; 13C =

128.06 ppm), or externally (19F: CFCl3, 27Al: Al(NO3)3). Chemical shifts (δ) are reported in ppm

and the absolute values of the coupling constants (J) are in Hz. In some instances, signal and/or

coupling assignment was derived from 2D NMR experiments.

6.4.2 Synthesis and Characterization

Synthesis of HAl(C6F5)2 (6-1)

A mixture of ClAl(C6F5)2 (1.023 g, 2.58 mmol) and NaH (61.9 mg, 2.58 mmol) were stirred in

10 mL of C6H5F overnight. The reaction mixture was then filtered, and solvent was removed

under vacuum. The resulting crude solid was washed with 3 x 5 mL of pentane and dried in

vacuo to yield compound 6-1 (770 mg, 82 %).

1H NMR (400 MHz, C6D6, 298K): δ 4.28 (s, br, 1H, AlH). 13C{1H} NMR (101 MHz, C6D6,

298K): δ 150.3 (dd, 1JCF = 233 and 3JCF = 24 Hz, o-C6F5), 142.6 (d, 1JCF = 255 Hz, p-C6F5),

137.0 (d, 1JCF = 253 Hz, m-C6F5), 111.6 (s, br, i-C6F5). 19F{1H} NMR (377 MHz, C6D6, 298K, 6-

1 : 6-1’ = 1:3): δ -122.6 (s, br, 4F, 6-1-o-C6F5), -124.1 (s, br, 4F, 6-1’-o-C6F5), -149.8 (s, br, 4F,

178

6-1’-p-C6F5), -151.3 (s, br, 4F, 6-1-p-C6F5), -159.9 (s, br, 4F, 6-1’-m-C6F5), -161.0 (s, br, 4F, 6-

1-m-C6F5),. 27Al{1H} NMR (104 MHz, C6D6, 298K): no signal detected.

Figure 6.9 – 19F{1H} NMR spectra of 6-1 in various solvents or concentrations.

Figure 6.10 - Molecular structures of [Al(C6F5)3·NaCl]8 and [Al(C6F5)2Cl·NaCl]n. Na: purple;

Cl: green; Al: pink; F: yellow.

179

Synthesis of HAl(C6F5)2(DMAP)2 (6-2)

A mixture of 6-1 (25.9 mg, 0.0715 mmol) and DMAP (17.5 mg, 0.143 mmol) was stirred in 5

mL of toluene for 2 h. The solvent was removed under vacuum, and the resulting white solid was

washed with 3 x 2 mL of pentane and dried in vacuo to yield compound 6-2 (32 mg, 74 %).

Crystals suitable for X-ray crystallography were grow from a saturate 6-1 solution in C6H5Cl

layered with a saturated pentane solution of DMAP at -35 °C overnight.

1H NMR (400 MHz, C6D6, 298K): δ 8.37 (s, br, 4H, o-NC5H4), 5.83 (s, br, 4H, m-NC5H4), 2.08

(s, 12H, N(CH3)2). 13C{1H} NMR (101 MHz, C6D6, 298K): δ 154.5 (s, p-NC5H4), 148.0 (s, o-

NC5H4), 106.4 (s, p-NC5H4), 38.0 (s, N(CH3)2). 19F{1H} NMR (377 MHz, C6D6, 298K): δ -121.7

(m, 4F, o-C6F5), -158.1 (t, 3JFF = 20 Hz, 2F, p-C6F5), -162.7 (p, 3JFF = 13 Hz, 4F, m-C6F5).

27Al{1H} NMR (104 MHz, C6D6, 298K): δ 114.9 (s, br).

Synthesis of (p-(Br(C6H4)C≡C)Al(C6F5)2)2 (6-3)

A mixture of 6-1 (23.6 mg, 0.0652 mmol) and p-(Br(C6H4)C≡CH (11.8 mg, 0.0652 mmol) was

stirred in 5 mL of toluene for 2 h. The solvent was removed under vacuum, and the resulting

crude solid was washed with 3 x 2 mL of -35 °C pentane and dried in vacuo to yield compound

6-3 (13 mg, 37 %). Crystal suitable for X-ray crystallography were grow from a saturated

solution of 6-3 in toluene layered with pentane at - 35 °C overnight.

1H NMR (400 MHz, C6D6, 298K): δ 7.23 (d, 3JHH = 8.5 Hz, 2H, o-C6H4Br), 6.82 (d, 3JHH = 8.5

Hz, 2H, m-C6H4Br). 13C{1H} NMR (101 MHz, C6D6, 298K): δ 150.6 (s, o-C6H4), 150.5 (d, 1JCF

= 228 Hz, o-C6F5), 142.9 (d, 1JCF = 256 Hz, p-C6F5), 137.3 (d, 1JCF = 231 Hz, m-C6F5), 136.8 (s,

p-C6H4), 133.1 (s, m-C6H4), 115.2 (s, i-C6H4), 85.5 (s, p-Br-C6H4-C≡C). 19F{1H} NMR (377

MHz, C6D6, 298K): δ -122.5 (dd, 3JFF = 11, 27 Hz, 8F, o-C6F5), -149.2 (t, 3JFF = 20 Hz, 4F, p-

C6F5), -160.0 (m, 8F, m-C6F5). 27Al{1H} NMR (104 MHz, C6D6, 298K): no signal detected.

Synthesis of (C6F5)2ClAl-CH=C(PPh2)2CH=C-AlCl(C6F5)2 (6-4)

A mixture of ClAl(C6F5)2 (39.7 mg, 0.100 mmol) and Ph2PC≡CH (21.0 mg, 0.100 mmol) were

dissolved in 10 mL of toluene and stirred for 4 h at room temperature. The solvent was removed

under vacuum, and the resulting crude solid was washed with 3 x 5 mL of pentane, and the

180

compound was then dissolved in minimum amount of toluene and stored at -35 °C overnight to

yield colorless crystals of 6-4 (26.3 mg, 43 %) that was suitable for diffraction study.

1H NMR (400 MHz, C6D6, 298K): δ 9.69 (dd, 3JPH = 33, 56 Hz, 1H, =CH), 7.91 (dd, 3JPH = 34,

57 Hz, 1H, =CH), 7.52 (ddt, 3JPH = 7, 14 Hz, 3JHH = 2.3 Hz, 2H, o-C6H5), 7.31 (dd, 3JPH = 8, 14

Hz, 2H, o-C6H5), 6.92-6.74 (m, 12H, C6H5). 13C{1H} NMR (101 MHz, C6D6, 298K): δ 150.0 (d,

1JCF = 234 Hz, o-C6F5), 141.8 (d, 1JCF = 238 Hz, p-C6F5), 137.4 (d, 1JCF = 257 Hz, m-C6F5),

135.9 (d, 2JPC = 3 Hz, o-C6H5), 135.68 (d, 2JPC = 3 Hz, o-C6H5), 134.0 (d, 4JPC = 12 Hz, p-C6H5),

133.6 (d, 4JPC = 11 Hz, p-C6H5), 130.3 (d, 3JPC = 13 Hz, m-C6H5), 130.03 (d, 3JPC = 14 Hz, p-

C6H5), 121.3 (d, 1JPC = 52 Hz, =CH), 120.4 (d, 1JPC = 52 Hz, =CH), 117.8 (d, 1JPC = 87 Hz,

C=CH), 116.1 (d, 1JPC = 85 Hz, C=CH). 31P{1H} NMR (162 MHz, C6D6, 298K): δ 42.8 (d, 2JPP

= 190 Hz), 18.5 (d, 2JPP = 190 Hz). 19F{1H} NMR (377 MHz, C6D6, 298K): δ -121.1 (dd, 3JFF =

13, 27 Hz, 4F, o-C6F5), -122.0 (dd, 3JFF = 12, 28 Hz, 4F, o-C6F5), -153.3 (t, 3JFF = 20 Hz, 2F, p-

C6F5), -154.1 (t, 3JFF = 20 Hz, 2F, p-C6F5), -161.6 (m, 4F, m-C6F5), -161.9 (m, 4F, m-C6F5).

27Al{1H} NMR (104 MHz, C6D6, 298K): no signal detected.

Synthesis of Mes2P-Al(C6F5)2 (6-5)

A solution of LiPMes2 (246.0 mg, 0.890 mmol) was dissolved in 10 mL of toluene and Cl-

Al(C6F5)2 (353.1 mg, 0.890 mmol) was added slowly at room temperature. The reaction mixture

was allowed to stir for another 4 h and then filtered and dried in vacuo. The crude mixture was

then washed with 3 x 2 mL of pentane and dried to yield compound 6-5 (257.8 mg, 46 %).

1H NMR (400 MHz, C6D6, 298K): δ 6.59 (s, 4H, MesH), 2.24 (s, 12H, o-CH3), 1.97 (s, 6H, p-

CH3). 13C{1H} NMR (126 MHz, C6D6, 298K): δ 152.6, 149.6 (dd, 1JCF = 229 Hz, 3JCF = 21 Hz,

o-C6F5), 142.8 (d, 3JCP = 10 Hz, o-C6H2), 140.2 (d, 1JCF = 265 Hz, p-C6F5). 136.8 (s, br, p-C6H2)

136.7 (d, 1JCF = 247 Hz, m-C6F5), 132.8 (d, 3JCP = 25 Hz, m-C6H2), 24.4 (d, 3JCP = 10 Hz, o-CH3),

20.4 (s, p-CH3). 31P{1H} NMR (162 MHz, C6D6, 298K): δ -91.2 (br). 19F{1H} NMR (377 MHz,

C6D6, 298K): δ -122.9 (s, br, 4F, o-C6F5), -153.4 (s, br, 2F, p-C6F5), -161.1 (s, br, 4F, m-C6F5).

27Al{1H} NMR (104 MHz, DFB, 298K): no signal detected.

Synthesis of (Mes2P(13C(O)O)Al(C6F5)2)2 (6-6)

181

A 0.5 mL of solution of 6-5 (58.6 mg, 0.093 mmol) in DFB was subjected to three freeze-pump

thaw cycles and charged with 1 atmosphere of carbon dioxide in a J-Young tube. The mixture

was allowed to sit at room temperature overnight, and the solvent was removed under vacuum,

leaving of white solid (119.0 mg, 95 %). The crystals of the compound were grown from a

saturated pentane solution of 6-6 at -35 °C overnight.

1H NMR (400 MHz, C6D6, 298K): δ 6.55 (d, 4JHH = 3.9 Hz, 4H, MesH), 2.34 (s, 12H, o-CH3),

1.92 (s, 6H, p-CH3). 13C{1H} NMR (101 MHz, DFB, 298K): δ 197.5 (d, 1JPC = 7 Hz, P-13C),

150.4 (dd, 1JCF = 249 Hz, 3JCF = 14 Hz, o-C6F5), 143.9 (d, 3JCP = 17 Hz, o-C6H2), 141.4 (s, p-

C6H2, 129.6 (s, m-C6H2), 22.4 (d, 3JCP = 14 Hz) 19.7 (s, p-CH3). 31P{1H} NMR (162 MHz,

C6D6, 298K): δ -13.8 (br). 19F{1H} NMR (377 MHz, DFB, 298K): δ -122.4 (s, br, 4F, o-C6F5), -

151.5 (s, br, 2F, p-C6F5), -161.3 (s, br, 4F, m-C6F5). 27Al{1H} NMR (104 MHz, DFB, 298K): no

signal detected.

Synthesis of (Mes2P)(Ph2P)C=C(Ph)Al(C6F5)2 (6-7)

A solution of 6-5 (55.0 mg, 0.0872 mmol) in 3 mL of C6H5Cl was added slowly to a solution of

Ph2PC≡CPh (25.0 mg, 0.0873 mmol) in 2 mL of C6H5Cl. The reaction mixture was then stirred

at room temperature for 2h, and the solvent was removed under vacuum. The resulting solid was

then washed with 3 x 2 mL of pentane and dried in vacuo to give compound 6-7 (Only crude

product was isolated, further purification is needed to obtain clean 1H and 13C{1H} NMR spectra

for the compound).

31P{1H} NMR (162 MHz, C6D6, 298K): δ 5.1 (d, 2JPP = 46 Hz, PPh2), 4.2 (dp, 2JPP = 46 Hz, JPF

= 8 Hz, PMes2). 19F{1H} NMR (377 MHz, C6D6, 298K): δ -117.7 (d, 3JFF = 26 Hz, 4F, o-C6F5), -

154.0 (t, 3JFF = 20 Hz, 2F, p-C6F5), -162.1 (tt, 3JFF = 11, 19 Hz, 4F, m-C6F5). 27Al{1H} NMR

(104 MHz, C6D6, 298K): no signal detected.

Reaction of 6-7 with 13CO2 to generate 6-8

A 0.5 mL of DFB solution of 6-7 (50 mg, 0.055 mmol) was subjected to three freeze-pump thaw

cycles and charged with 1 atmosphere of carbon dioxide in a J-Young tube. The mixture was

allowed to sit at room temperature overnight, and the solvent was removed, washed with 3 x 2

182

mL of pentane and dried under vacuum, leaving white solid (38.0 mg, 73 %). 1H NMR spectrum

of 6-8 will be collected in the future once it is purified.

13C{1H} NMR (126 MHz, C6H5Cl, 298K): δ 167.4 (d 1JPC = 117 Hz, 13C-PMes2), 149.7 (d, 1JCF

= 233 Hz, o-C6F5). 31P{1H} NMR (162 MHz, C6D6, 298K): δ 12.9 (d, 2JPP = 138 Hz, PPh2), -6.3

(dd, 2JPP = 132 Hz, 1JPC = 121 Hz, 13C-PMes2). 19F{1H} NMR (377 MHz, C6H5Cl, 298K): δ -

120.5 (s, br, 4F, o-C6F5), -154.5 (s, br, 2F, p-C6F5), -162.0 (s, br, 4F, m-C6F5). 27Al{1H} NMR

(104 MHz, C6H5Cl, 298K): no signal detected.









Structure Solution and Refinement. The structures were solved by intrinsic phasing using XS.

All the structures were subjected to full-matrix least-squares refinement on F2 using XL as




parameters. The Al-bound hydrogen atom 6-2 was located from difference Fourier maps and

refined in an unrestrained manner.

183

Table 6.1 - Summary of crystallographic data for compounds 6-2, 6-3, 6-4 and 6-6.

6-2 6-3 6-4 6-6

Moiety

formula

C31 H33 Al F10 N4 C45 H20 Al2 Br2

F20

C73 H46 Al2 Cl2

F20 P2

C31 H22 Al F10 O2

P

Sum formula

weight

678.59 1154.39 1489.90 674.43

crystal system Monoclinic Triclinic Triclinic Monoclinic

space group P21/c P-1 P-1 C2/c

a (Å) 11.6102(5) 10.8873(4) 13.739(1) 22.09(2)

b (Å) 25.924(1) 11.0051(3) 14.628(1) 14.94(1)

c (Å) 11.8202(5) 11.6569(4) 17.264(1) 18.03(1)

α (deg.) 86.113(2) 83.560(2)

β (deg.) 113.369(2) 63.315(2) 88.164(2) 90.15(5)

γ (deg.) 61.639(2) 83.113(2)

vol (Å3) 3265.9(2) 1079.16(7) 3422.2(5) 5948(8)

Z 4 1 2 8

ρ (calcd)

(Mg∙cm3)

1.380 1.776 1.446 1.506

μ (mm-1) 0.147 2.040 0.265 0.214

F(000) 1400 566 1504 2736


T(K) 150(2) 150(2) 150(2) 150(2)

reflections

collected

73711 23978 49273 20367

unique

reflections

8095 7509 12526 5193

Rint 0.1038 0.0329 0.0636 0.0678

GOF (F2) 0.941 1.006 1.027 1.040

R1 indices

[I>2σ(I)]

0.0700 0.0424 0.0763 0.0580

wR2 indices

(all data)

0.2099 0.1263 0.2492 0.1679

Largest diff.

peak and hole

(e. Å-3)

1.100 & -0.620 0.925 & -1.043 0.848 & -0.801 0.529 & -0.487

CCDC No. - - - -


All calculations were computed using the Gaussian 09 program. Geometry optimizations were

performed at M06-2X functional43 with the crystallographic coordinates used as starting

geometries when available. The Def2-SVP basis set was used for all atoms.44 The stationary

184

nature of the converged geometry was confirmed by carrying out a frequency calculation and

ensuring the absence of imaginary frequencies. NBO calculations were performed using the

optimized geometry with NBO version 6.0.45 Optimized structures were visualized using

CYLview46 or Chemcraft software.47

Table 6.2 - Isotropic shift for 6-5 and 6-6 of the phosphorus centers.

δ (absolute)comp δ (ref)comp δ (C6D6)expt

6-5 -377.1387 -88.3 -91.4

6-6 -302.5934 -13.8 -13.8

6.5 References

1. Stephan, D. W., Science, 2016, 354, aaf7229.

2. Stephan, D. W., Accounts of Chemical Research, 2015, 48, 306-316.

3. Stephan, D. W., Journal of the American Chemical Society, 2015, 137, 10018-10032.

4. Stephan, D. W. and Erker, G., Angewandte Chemie International Edition, 2015, 54,

6400-6441.

5. Uhl, W. and Würthwein, E.-U., in Frustrated Lewis Pairs II: Expanding the Scope, eds.

Erker, G. and Stephan, D. W., Springer Berlin Heidelberg, Berlin, Heidelberg, 2013, pp.

101-119.

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Chapter 7

7 Conclusions

7.1 Thesis Summary

All the work discussed in this dissertation presented the exploration of the radical and Lewis acid

chemistry with Group 13 compounds in FLP chemistry. Reduction of the planar borenium

[C2H2(NCH2C6H4)2CB]+ 2-2a+ generated the diborane compound (C2H2(NCH2C6H4)2CB)2 2-3a.

This diborane persist a unique reactivity with homolytic B-B bond cleavage reactions towards

various substrates. This reactivity also resembles the dissociation of hexaphenylethane

generating trityl radical, as first proposed in the seminal work of Gomberg.1 Moreover,

substitution of one of the benzylic protons on 2-2a with B(C6F5)3 leads to the isolation of 2-10,

which generated the corresponding boron based radical that was observed by EPR.

Chapters 3 and 4 explored the radical chemistry in FLPs and Lewis acid-base adducts. We first

demonstrated with the same substrates, p-C6Cl4O2 and Ph3SnH, changing the Lewis base from

tBu3P to Mes3P with the same Lewis acids E(C6F5)3 (E = B, Al) alter the reacting from the

traditional two electron pathway to a single electron pathway, where the Lewis base Mes3P acted

as a single electron donor delivering agent to the Lewis acid that results in homoleptic cleavage

of the Sn-H bond. This discovery promoted us to attempt the SET reactions with replacing one

electron donor to more conventional reducing agents, metallocene compound like Cp*2M (M =

Fe, Cr, Co) in combination of various Lewis acids (E(C6F5)3, E = Al, B; or Zn(C6F5)2 and BPh3).

This combination leads to the SET reaction with conventional weak Lewis base (chalcogenide

species and diazomethane compounds). While one could think these reactions could result from

the generation of the corresponding transient radicals from SET to the Lewis acid, an alternative

perspective could also be the combination of the weak bases with the Lewis acids formed weak

or unstable Lewis acid-base adducts, which lower the reducing potential of the Lewis acids

raised the electron transfer from the reducing agent with Cp*2M (M = Fe, Cr, Co). The latter

notion seek the reduction reactions of the very unstable Lewis acid-base adducts, (Ph2CN2)BR3

(R = C6F5 or Ph), which would rapidly release N2 upon mixing. This results the stabilization of

the N2 fragment and generation of the C-H activation products.

188

The next portion of this thesis focused on NHC-stabilized Al compounds, where we focused on

prepare different family NHC used to stabilized active Lewis acidic Al center. We first

demonstrated hydride abstraction from the NHC-alane adduct 5-1 reveals generation of the first

bis-carbene stabilized aluminum monohydride dication [(IBn)2Al-H]2+ 5-2 from the

redistribution reaction of the Al cation generated. Changing the NHC carbenes from IBn to

cAACEt with alane leads to the oxidative addition of the Al-H to the carbene carbon center.

Moreover, with 1H-1H EXSY NMR studies of the double oxidative addition product 5-7, we

observed the reversible 1,2-hydride shift between the Al and the carbene carbon center.

Lastly, to attempt the synthesis of intramolecular P/Al-based FLP compounds for further

investigation of its radical pathway, we have generated the Al analogue of Piers’ borane 6-1, and

phosphinoalanes 6-5. Reaction of 6-1 with terminal alkyne led to both hydroalumination and

dehydrocoupling reactions. On the other hand, 6-5 has shown to react with CO2 and internal C≡C

triple bonds.

7.2 Future Work

Boron containing heterocyclic compounds are widely used in luminescent materials.2, 3 Thus,

exploration of synthesizing various planar NHC stabilized boron containing compounds, and the

corresponding radical species could reveal interesting luminescent properties for material

chemist as doublet emitters can reach 100 % maximum electroluminescene yield vs. only 25 %

for typical singlet-based fluorescent materials.4 This notion could also be applied to the heavier

Group 13 analogues.

As mentioned earlier in Chapter 3 and 4, since our discovery of the SET mechanism in FLP

chemistry, Dreiss,5 Erker,6, 7 Wildgoose8 and others have further broadened the field in the recent

year. Further application of such SET to simple donor-acceptor systems continues to be of

interest. For the reduction of diazo compounds with the combination of Lewis acids and reducing

agents, DFT studies on the reaction mechanism has currently been carried out with the

collaboration with Dr. Zheng-wang Qu at Universität Bonn. Moreover, the notion of weak Lewis

acid and base adducts could be stabilized from addition of reducing agent could also be applied

to FLP chemistry, which might lead to the activation of N2, CH4 and other challenging

substrates.

189

While chapter 5 focused on the preparation of NHC-stabilized Lewis acidic Al compounds,

recent studies show considerable interests in preparation of nucleophilic Al centers.9, 10

Preliminary studies on reduction of cAACEt-stabilized Al chloride compounds led to

fragmentation of the molecules due to the weak bonding between the cAACEt with the Al center.

However, reduction of Al-H compound 5-12 suggested the generation of anionic Al(III)

compound, confirming the molecular structure of 5-12 and exploring its reactivity towards Lewis

acids and small molecules could be interesting. Reactivity studies of 6-1 and 6-5 are currently

under investigation.

7.3 References

1. Gomberg, M., Journal of the American Chemical Society, 1900, 22, 757-771.

2. Mellerup, S. K. and Wang, S., Trends in Chemistry, 2019, 1, 77-89.

3. Hirai, M., Tanaka, N., Sakai, M. and Yamaguchi, S., Chemical Reviews, 2019, 119,

8291-8331.

4. Segal, M., Baldo, M. A., Holmes, R. J., Forrest, S. R. and Soos, Z. G., Physical Review B,

2003, 68, 075211.

5. Merk, A., Grossekappenberg, H., Schmidtmann, M., Luecke, M. P., Lorent, C., Driess,

M., Oestreich, M., Klare, H. F. T. and Müller, T., Angewandte Chemie International

Edition, 2018, 57, 15267-15271.

6. Tao, X., Daniliuc, C. G., Janka, O., Pottgen, R., Knitsch, R., Hansen, M. R., Eckert, H.,

Lubbesmeyer, M., Studer, A., Kehr, G. and Erker, G., Angewandte Chemie International

Edition, 2017, 56, 16641-16644.

7. Tao, X., Daniliuc, C. G., Knitsch, R., Hansen, M. R., Eckert, H., Lubbesmeyer, M.,

Studer, A., Kehr, G. and Erker, G., Chemical Science, 2018, 9, 8011-8018.

8. Bennett, E. L., Lawrence, E. J., Blagg, R. J., Mullen, A. S., MacMillan, F., Ehlers, A. W.,

Scott, D. J., Sapsford, J. S., Ashley, A. E., Wildgoose, G. G. and Slootweg, J. C.,


9. Hicks, J., Vasko, P., Goicoechea, J. M. and Aldridge, S., Nature, 2018, 557, 92-95.

10. Hofmann, A., Pranckevicius, C., Troster, T. and Braunschweig, H., Angewandte Chemie


190

Copyright Acknowledgements

Chapter 2: Reprinted (adapted) with permission from (Cao, L. L.; Stephan, D. W.

Organometallics 2017, 36, 3163-3170) Copyright (2017) American Chemical Society.

Chapter 3: Reprinted (adapted) with permission from (Liu, L.; Cao, L. L.; Shao, Y.; Ménard G.;

Stephan D. W. Chem, 2017, 3, 259-267) Copyright (2017) Elsevier Inc.

Chapter 4: Reprinted (adapted) with permission from (Liu, L.; Cao, L. L.; Shao, Y.; Stephan, D.

W. J. Am. Chem. Soc. 2017, 139, 10062-10071) Copyright (2017) American Chemical Society,

and from (Cao, L. L.; Bamford, K. L.; Liu, L. L.; Stephan, D. W. Chem. Eur. J. 2018, 24, 3980-

3983) Copyright (2018) Wiley-VCH Verlag GmbH & Co.

Chapter 5: Reprinted (adapted) with permission from (Cao, L. L.; Daley, E.; Johnstone, T. C.;

Stephan, D. W. Chem. Commun. 2016, 52, 5305-5307) Copyright (2016) The Royal Society of

Chemistry, and from (Cao, L. L.; Stephan, D. W. Chem. Commun. 2018, 54, 8407-8410)

Copyright (2018) The Royal Society of Chemistry.

Documents

Group 13 Radical and Lewis Acid Chemistry · (E = B, Al) led to a typical two electron pathway upon treatment with chloranil (p-C 6 Cl 4 O 2) and Ph 3 SnH. However, replacing the