Coordination chemistry and reactivity of early transition

metal complexes with the amine phenolate ligands

Thesis for the Degree of “Doctor of Philosophy”

By

Stanislav Groysman

Submitted to the Senate of Tel Aviv University

September 2005

Coordination chemistry and reactivity of early transition

metal complexes with the amine phenolate ligands

Thesis for the Degree of “Doctor of Philosophy”

By

Stanislav Groysman

Submitted to the Senate of Tel Aviv University

September 2005

This work was carried out under the supervision of Professor Moshe Kol and

Professor Israel Goldberg.

I am deeply grateful to my teacher and mentor, Professor Moshe Kol (Shiko),

for guidance, support, brilliant ideas, and never-ending inspiration.

I wish to thank Professor Israel Goldberg for his guidance in the beautiful field of X-

ray crystallography.

I wish to thank my group members, Dr. Edit Tshuva, Dr. Dalia Gut-Regev, Adi Yeori,

Shimrit Gendler, Sharon Segal, Ekaterina (Katy) Sergeeva, and Sheba D. Bergman,

for their support. Especially, I wish to express my deep appreciation to Mrs. Dvora

Reshef for the everyday help, and for the help in olefin polymerization project.

I wish to thank Professor Zeev Goldschmidt and Dr. Elisheva Genizi from Bar Ilan

university, and Dr. Michael Shuster from Carmel Olefins Ltd, for our fruitful

collaboration.

This work is dedicated to my parents, Alexander and Galina, and to my wife,

Marianna.

Table of contents

1 List of publications

3 Abstract

15 Introduction

15 I-1. Alkoxo (phenolate) ligand vs. Cp ligand

16 I-2. From a simple alkoxo to a bulky phenolate ligand

17

I-3. Multidentate ligands: Enhanced stabilization and easier design of

metal centers

19 I-4. Multidentate phenolate ligands

25

I-5. Synthetic pathways to early transition metal complexes bearing

phenolate ligands

26

I-6. Reactivity of early transition metal complexes supported by

multidentate phenolate ligands

27 I-6.1. Olefin polymerization by well-defined Group IV metal complexes

32

I-6.2. Stabilization of metal-carbon double bonds and olefin metathesis

reactivity

34 I-6.3. Phenolate ligands in bioinorganic chemistry

37 I-7. Amine phenolate ligands

38 I-7.1. The amine tris(phenolate) ligands

39 I-7.2. The amine bis(phenolate) ligands with the additional, sidearm, donor

41 I-7.3. The amine mono(phenolate) ligands with two sidearm donors

43 Discussion

43 D-1. Ligand synthesis

45 D-2. Complex synthesis

47 D-3. Coordination chemistry of the amine phenolate metal complexes

54

D-4. Reactivity of the early transition metal complexes with amine

phenolate ligands

54 D-4.1. Olefin polymerization

60 D-4.2. H-abstraction reactions

63 D-4.3. Modeling the active site of haloperoxidase

67 References

72 Appendix: Symbols and abbreviations

Publications

תקציר א

List of publications

1

List of publications presented in this work

1. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt Group IV complexes of an Amine Bis(phenolate) Ligand Featuring a THF Sidearm Donor: From Highly Active to Living Polymerization Catalysts of 1-Hexene Inorg. Chim. Acta 2003 345, 137.

2. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt From THF to Furan: Activity Tuning and Mechanistic Insight via Sidearm Donor Replacement in Group IV Amine Bis(phenolate) Polymerization Catalysts

Organometallics 2003, 22, 3013. 3. S. Groysman, E. Y. Tshuva, D. Reshef, S. Gendler, I. Goldberg,M. Kol, Z.

Goldschmidt, M. Shuster and G. Lidor High-Molecular Weight Atactic Polypropylene Prepared by Zirconium Complexes of an Amine Bis(phenolate) Ligand Isr. J. Chem. 2002, 42, 373.

4. S. Groysman, E. Tshuva, I. Goldberg, M. Kol, Z. Goldschmidt, and M. Shuster Diverse Structure-Activity Trends in the Amine Bis(phenolate) Titanium Polymerization Catalysts Organometallics 2004, 23, 5291.

5. S. Groysman, S. Segal, M. Shamis, I. Goldberg, M. Kol, Z.

Goldschmidt and E. Hayut-Salant Tantalum(V) Complexes of an Amine Triphenolate Ligand: a Dramatic Difference in Reactivity Between the Two Labile Positions J. Chem. Soc. Dalton Trans. 2002, 3425.

6. S. Groysman, S. Segal, I. Goldberg, M. Kol and Z. Goldschmidt Ta(V) Complexes of a Bulky Amine Tris(phenolate) Ligand: Steric Inhibition vs. Chelate Effect

Inorg. Chem. Commun. 2004, 7, 938. 7. S. Groysman, I. Goldberg, M. Kol and Z. Goldschmidt

Pentabenzyltantalum: Straightforward Synthesis, X-ray Structure and Application in the Synthesis of [O2N]TaBn3-Type and [O3N]TaBn2-Type complexes Organometallics 2003, 22, 3793.

8. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt

Tribenzyl Tantalum(V) Complexes of Amine Bis(phenolate) Ligands: Investigation of α-Abstraction vs. Ligand Backbone β-Abstraction Paths Organometallics 2004, 23, 1880.

9. S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldshmidt Exploring Routes to Tantalum(V) Alkylidene Complexes Supported by

List of publications

2

Amine Tris(Phenolate) Ligands Adv. Synth. Cat 2005, 347, 409.

10. S. Groysman, I. Goldberg, M. Kol and Z. Goldschmidt Vanadium(III) and Vanadium(V) Amine Tris(phenolate) Complexes Inorg. Chem. 2005, 44, 5073. 11. S. Groysman, E. Sergeeva, I. Goldberg and M. Kol Group IV Complexes of a Tetradentate Amine Mono(phenolate) Ligand: A Second Sidearm Donor Stabilizes Cationic Species Inorg. Chem. 2005, 44, 8188. 12. S. Groysman, E. Sergeeva, I. Goldberg and M. Kol Salophan complexes with Group IV Metals Eur. J. Inorg. Chem. 2005, 2480.

Abstract

3

Abstract

Living polymerization of 1-hexene by titanium amine bis(phenolate) complexes

This work is fully described in articles 1 and 2.

Ti complexes of the amine bis(phenolate) ligands having bulky t-Bu groups in the

ortho positions of the phenolate rings, and possessing a sidearm donor, have all shown a

tendency to catalyze a living polymerization, to a certain degree. Aiming at the most living

catalyst of this system, we postulated that a strong sidearm oxygen donor obtained via a THF

substituent, would induce a pronounced living polymerization activity of the derived catalyst.

TiO

R

t-Bu

t-Bu

O

R

Nt-Bu

Ot-Bu

R = CH2Ph, Me

TiO

R

t-Bu

t-Bu

O

R

Nt-Bu

Ot-Bu

R = CH2Ph

Figure A-1. Dialkyl Ti complexes bearing the amine bis(phenolate) THF and furan ligands.

Dialkyl Ti-THF complexes (Figure A-1) were activated toward 1-hexene by the

reaction with B(C6F5)3. This polymerization system showed an unprecedented behavior: The

polymerization was living for almost a week (at RT), and the resulting polymer had a very

high Mw (around 106), and a very narrow MWD (PDI = 1.09) (Figure A-2). In addition, this

catalytic system enabled the copolymerization of two α-olefins, 1-hexene, and 1-octene at

RT, forming a di-block copolymer of narrow MWD. To further support our hypothesis that

the pre-condition for the living polymerization in the amine bis(phenolate) catalysts was the

dative strength of the oxygen sidearm donor, we prepared the dibenzyl amine bis(phenolate)

Ti complex, having a weak oxygen donor: furan. The structure of the analogous Zr complex

indicated that the furan oxygen was bound to the metal, and that the bonding was indeed

Abstract

4

weak. As anticipated, the Ti-furan/B(C6F5)3 system showed a different character of the

polymerization catalysis, when compared with Ti-THF: the polymerization in this system was

ca. 10-fold faster, but was not “truly” living, as the MWD was narrow only in the first few

hours of the reaction. Further investigation of the THF-Ti and furan-Ti catalysts enabled a

mechanistic insight into the reactivity of the amine bis(phenolate) catalysts, supporting a

mononuclear nature of the latter, and displaying no polymeryl transfer.

Figure A-2. “Immortal” polymerization of 1-hexene by THF-Ti: dependence of molecular weight (Mw) on time.

The numbers indicate the polydispersity index (PDI) values of the polymer samples.

Highly active zirconium catalysts for 1-hexene and propylene polymerization

This work is fully described in articles 1, 2 and 3.

In analogy to the Ti species, the dibenzyl amine bis(phenolate) Zr complexes,

providing the most convenient form of the pre-catalyst, were prepared by a toluene-

elimination reaction of tetrabenzylzirconium with the ligand precursor. The activation of the

THF-Zr complex with a borane co-catalyst gave rise to a highly efficient 1-hexene

polymerization catalyst, whose activity (21,000 gpol mmolcat-1 h-1) is defined as “very high”.

The resulting polymer was of relatively high Mw, having a PDI of around 2, indicating a

“single-site” species. Similarly, the furan-Zr catalyst was highly active in 1-hexene

polymerization. In contrast to the THF-Zr, furan-Zr led to a low-Mw polymer.

1.12

1.061.06

1.051.07

1.061.11

1.09

0100000200000300000400000500000600000700000800000900000

0 2 4 6 8Polym. Time (days)

Mol

ecul

ar W

eigh

t

Abstract

5

Next, we investigated the reactivity of the Zr-based catalysts in the polymerization of

the industrially challenging monomer, propylene. Two types of the pre-catalyst were

employed: The first having two benzyl groups at the labile positions, and the second having

two chloro groups. Both pre-catalysts, upon activation with MAO, led to a very efficient

polymerization of propylene (Scheme A-1). The optimized activity of this system was as high

as 13,000 gpol mmolcat-1 h-1. The resulting polypropylene was uncommon, combining

complete atacticity and relatively high molecular weight, thereby displaying elastomeric

properties.

Menn

Atactic polypropylene

Mw = 8,000 - 400,000250 - 1,100 equiv of MAO Me

Zr pre-catalyst

Scheme A-1. Propylene polymerization by the amine bis(phenolate) Zr catalyst.

Influence of the phenolate substituents on the reactivity of titanium catalysts

This work is fully described in article 4.

Aiming at developing of new activity modes, and discovering new structure activity

relationships, we prepared a series of Ti-based catalysts, possessing ortho (para) substituents

of varying steric and electronic nature (Scheme A-2). As the reactivity of these systems may

depend on the nature of the sidearm donor, two such series were prepared: One possessing a

NMe2 donor, and the other possessing an OMe donor. For the NMe2 series, the gradual

reduction of steric hindrance led to a considerable increase of the catalyst activity: The

catalyst having ortho Me groups had an activity of 1,400 vs. 30 for the catalyst having t-Bu

groups. An additional increase in activity (up to 8,000) was obtained using the chloro-

substituted ligands. Surprisingly, the reduction of the steric bulk, and use of electron-

withdrawing groups had no effect on the activity of catalysts featuring the OMe sidearm

Abstract

6

donor. The 13C NMR characterization of the polymer samples shed light on the origin of

these diverse trends for the two series: All catalysts of the NMe2 series produced a relatively

regioregular poly(1-hexene), whereas the reduction of the steric bulk in the OMe series led to

a substantial increase in the degree of regioerrors. The latter are known to decrease

polymerization rate significantly.

TiO

PhH2C

t-Bu

t-Bu

D

CH2Ph

Nt-Bu

Ot-Bu

t-Bu vs. Me

D = NMe2, OMe

TiO

PhH2C

Me

Me

D

CH2Ph

NMe

OMeTi

O

PhH2C

Me

D

CH2Ph

N

OMe

TiO

PhH2C

Cl

Cl

D

CH2Ph

NCl

OCl

Me

Me Me vs. Cl

D = NMe2, OMeD = NMe2, OMeD = NMe2

Me vs. H

Scheme A-2. Dibenzyl amine bis(phenolate) Ti(IV) complexes, displaying varying steric and electronic

influence at the metal center

The most efficient catalyst of these series led to an unusual polymer: Poly(1-hexene)

of an extremely high molecular weight (Mw = 4,300,000). In contrast to the “regular” poly(1-

hexene), which is a sticky oily substance, this polymer is in a rubbery state, showing the

properties of an elastomer.

Coordination chemistry of Ta(V) complexes with amine tris(phenolate) ligands

This work is fully described in articles 5 and 6

Wishing to extend the chemistry of the amine phenolate ligands beyond group IV

metals, we turned to investigate their chemistry with the group V metal, tantalum (V). The

Abstract

7

amine tris(phenolate) ligand, bearing Me groups at the ortho, para positions of the phenolate

rings, was chosen as a first ligand in this study. Two metal precursors, Ta(OEt)5 or

Ta(NMe2)5, reacted successfully with a ligand precursor, leading to a well-defined rigid

octahedral complexes of Cs-symmetry (Scheme A-3). Subsequently, these complexes were

treated with excess of Me3SiCl. Surprisingly, only one of the “labile” monodentate ligands

was exchanged. The solid-state structure determination revealed that the remaining OEt

(NMe2) group is trans to the central amine donor. The differentiation between the “labile”

positions was so strong, that the remaining OEt group did not undergo a metathesis reaction

at all, even when the reaction was carried out for weeks, or when stronger reagents (HCl, for

example) were employed. The desired dichloro complex could be obtained only via the

replacement of the more basic dimethylamido group, after prolonged reaction times.

TaO

X

Me

Me

O

X

NMe

OMe

Me

Me

HO

N

OHMe

Me

Me

Me

HO

Me

Me

TaX5

X = OEt, NMe2

TaO

Cl

Me

Me

O

X

NMe

OMe

Me

Me

xs. Me3SiCl

xs. Me3SiClTa

O

Cl

Me

Me

O

Cl

NMe

OMe

Me

Me

X = OEt, NMe2

4 days, X = NMe2

TaCl5

Scheme A-3. Coordination chemistry of Ta(V) in the amine tris(phenolate) environment

In contrast to the non-bulky ligand, the reaction of the sterically-crowded amine

tris(phenolate) ligand with Ta(NMe2)5 proceeded slowly, via a relatively stable dinuclear

intermediate, whose nature was established by means of X-ray diffraction. When formed, the

bis(dimethylamido) complex of the sterically-crowded ligand did not lead to a clean

Abstract

8

metathesis reaction with any chlorinating agent, while the analogous di(ethoxo) complex

allowed a clean substitution of a single group.

Synthesis, structure, and reactivity of pentabenzyltantalum

This work is fully described in article 7.

Pentabenzyltantalum was prepared by Schrock in 1976 by a two-step reaction

sequence: a reaction between tantalum pentachloride and dibenzyl zinc to produce

TaCl2(CH2Ph)3, which was further reacted with dibenzylmagnesium. We were able to

develop a more direct procedure, by preparation of pentabenzyltantalum via the single-step

reaction of TaCl5 with PhCH2MgCl. As the X-ray structure of this fundamental compound

was never reported, we solved its crystal structure as well (Figure A-3). Subsequently, we

evaluated the potential of Ta(CH2Ph)5 as a precursor to Ta(V) organometallic complexes with

the amine phenolate ligands. Two representative ligands of the amine bis(phenolate) and the

amine tris(phenolate) types were reacted with Ta(CH2Ph)5. Both ligands reacted smoothly,

leading to the expected tribenzyl, and dibenzyl complexes, respectively, in high yields.

Figure A-3. Molecular structure of pentabenzyltantalum.

Abstract

9

Investigation of α-abstraction vs. ligand backbone β-abstraction paths in the tribenzyl

tantalum(V) complexes of amine bis(phenolate) ligands

This work is fully described in article 8.

A variety of tribenzyl amine bis(phenolate) Ta(V) complexes could be prepared

smoothly using the pentabenzyltantalum route. All these complexes displayed a similar

propensity: They are hexacoordinate, and the fourth, sidearm donor is a “dormant” donor in

these species. We postulated that such a “dormant” donor may cause various organometallic

transformations in the parent tribenzyl species, and thus we carried out a thorough SAR study

of the H-abstraction reactions in this system as a function of various structural modifications.

TaO

PhH2C

R

R

D

CHPh

NR

ORTaO

PhH2C

R

R

D

CH2Ph

NR

OR

α-Hβ-H

C

Ta

N

PhH2C O

O CH2Ph

Ph HH

DHH

R

R

α

β

Scheme A-4. Possible H-abstraction routes for the tribenzyl complexes

When subjected to thermolysis, these species underwent two well-defined reactions:

Abstraction of an �-proton (from one of the benzyl groups), forming the metal-carbon double

bond (benzylidene), or abstraction of a β-proton (from the benzyl position in the amine

bis(phenolate) ligand), forming a new metal-carbon single bond (Scheme A-4). The choice of

the abstraction pathway was found to depend on two structural parameters: The presence of

the “dormant” donor, and the steric bulk at the ortho positions of the phenolate rings. The

presence of the sidearm donor, accompanied by small (H, Cl) ortho substituents led to

smooth �-H abstraction. When the sidearm donor was present, and the ortho substituents were

Abstract

10

bulkier, the main reaction was facile β−Η abstraction. In one particular case, i.e. t-Bu

phenolate substituents, the β−Η activation took place even at RT. In these cases, the sidearm

donor was bound to the metal following the H-abstraction and toluene elimination. When the

donor was absent (an [ONO]-type ligand), the reaction took the β−Η abstraction pathway

exclusively, and proceeded much slower. All the H-abstraction reactions were of first order in

the reactant, indicating an intramolecular mechanism.

Exploring routes to tantalum(V) alkylidene complexes supported by amine tris(phenolate)

ligands.

This work is fully described in article 9.

At the next step, an alkylidene functionality supported by amine tris(phenolate)

ligands was pursued. On the way to the desired alkylidene species we employed two

fundamental organometallic species, tris(neopentyl) mono(neopentylidene) tantalum(V)

(Ta(CH2t-Bu)3(CHt-Bu)), and pentabenzyltantalum, as starting materials. The reaction of the

amine tris(phenolate) ligand precursor with Ta(CH2t-Bu)3(CHt-Bu) proceeded via the O-H

addition to the metal-carbon double bond, forming a phenoxo-tetraneopentyl complex, one of

the most sterically congested organometallic species. Further reactivity of this complex was

less well-defined, as it decomposed into unidentified species, not containing an alkylidene

function.

The pentabenzyltantalum route led to the octahedral dibenzyl complexes with a

variety of amine tris(phenolate) ligand precursors. Unlike the tribenzyl amine bis(phenolate)

complexes, the dibenzyl complexes were found to be remarkably stable toward thermolysis,

decomposing only at 120 ºC after prolonged reaction times. Instead of forming the

mononuclear terminal alkylidene functionality, the reaction unexpectedly led to the rare

dinuclear µ-benzylidene complex, in which both Ta(V) centers were of octahedral geometry

(Figure A-4).

Abstract

11

Figure A-4. Molecular structure of µ-benzylidene di-Ta(V) complex.

Vanadium(III) and Vanadium(V) amine tris(phenolate) complexes

This work is fully described in article 10.

Following the investigation of the chemistry of the heavier group V member,

tantalum, we turned to the lighter group V member, vanadium. The chemistry of this metal

was studied especially in light of its bioinorganic relevance, as vanadium participates in the

active site of several enzymes. Three amine tris(phenolate) ligands featuring different steric

and electronic influence were employed in this study (Scheme A-5). V(III) complexes of

these ligands were obtained by reaction between the ligand precursors and VCl3(THF)3, in the

presence of Et3N. All the resulting complexes were of almost perfect TBP geometry, in which

the three phenolate oxygens lied in the equatorial positions, one axial positions was occupied

by the central nitrogen, and the other by an additional ligand: THF. The V(III) complexes of

the “electron-rich” ligands, having ortho, para Me and t-Bu phenolate substituents, were

found to undergo smooth and clean oxidation to V(V) oxo species, when exposed to air.

Alternatively, the V(V) complexes could be prepared directly, starting from VO(OPr)3. The

crystallographic and multinuclear NMR studies of these complexes supported their structure

Abstract

12

to be TBP, similar to the V(III) species, with the sole difference being replacement of the

axial neutral THF ligand by the dianionic oxo ligand. This structure is noteworthy: It mimics,

to a great extent, the structure of the (resting) active site in V-dependent haloperoxidase.

Scheme A-5. Coordination chemistry of V(III) and V(V) amine tris(phenolate) complexes

The reaction of the “electron-poor” ortho, para dichloro substituted ligand with

VO(OPr)3 resulted in the formation of V(V) species having a different structure. The X-ray

solution disclosed an octahedral amine tris(phenolate) complex, carrying an aqua ligand, in

addition to the oxo ligand. In addition to the octahedral species, the products mixture

contained a small fraction of a TBP complex. Thus, it appears that the amine tris(phenolate)

complexes are able to switch between the TBP and octahedral geometries. In addition, the

V(V) amine tris(phenolate) species have shown an oxygen-transfer reactivity, being able to

catalyze, albeit slowly, oxygen transfer from a peroxide to styrene and stilbene.

Group IV complexes of a tetradentate amine mono(phenolate) ligand: a second extra donor

stabilizes cationic species

This work is fully described in (submitted) article 11.

VO

OO

NR

R

R

RR

R

O

VO

L

Cl

Cl

ClO

O

NCl

O

Cl

Cl

N

OHHO

OH

R

R

R

R

R

RVCl3(THF)3

VO

OO

NR

R

R

RR

R

O

R =

R = Me, t-Bu, Cl

Me, t-BuairVO(OPr)3

R = Cl

VO(OPr)3R = Me, t-Bu

- L, ∆

+ L

Abstract

13

The amine mono(phenolate) ligand precursor was prepared by a single-step Mannich

condensation between a secondary amine, a substituted phenol, and formaldehyde. Upon

reaction with suitable Group IV metal precursors, this ligand precursor led to the highly

flexible tris(alkoxo) complexes with Ti(IV) and Zr(IV), and the tribenzyl complex with

Zr(IV) metal. At the next step, the removal of one of the alkoxo (alkyl) groups by means of

Lewis or Bronsted acid (B(C6F5)3 or [PhNMe2H][B(C6F5)4]) led to a fast and clean formation

of cationic complexes, in which the “extra” donor was bound to the metal (Scheme A-6). The

cationic alkoxo complexes display a rigid C1-symmetry, while the analogous alkyl species are

presumably of Cs-symmetry. The species demonstrate a remarkable stability: The cationic

alkoxo species are stable both in solution and in solid state for weeks, while the cationic alkyl

species are stable for days.

MO

O OR

OR

OR

Nt-Bu

t-BuO

Me

Me

OH

N

OMe

MeO

t-Bu

t-Bu

M(OR)4 MO

O O

OR

Nt-Bu

t-Bu

Me Me

OR

+

B(C6F5)4 -

[PhNMe2H]

[B(C6F5)4]

Scheme A-6. Synthesis and structure of the neutral tris(alkoxo) and cationic bis(alkoxo) Group IV metal

complexes with the amine mono( phenolate) ligand. M = Ti(IV), Zr(IV), R = Ot-Bu, Oi-Pr.

Salophan complexes of group IV metals

This work is fully described in article 12.

Sequential diamine bis(phenolate) Salophan ligands are similar to Salan ligands, with

the only difference between them being the nature of the link between the two nitrogen

donors: Flexible ethylene (diamine) bridge in Salan, and rigid ortho-phenylene (diamine)

bridge in Salophan. Early transition metal complexes of Salan ligands are in the focus of

Abstract

14

intensive investigation by several research groups; in contrast, no early transition metal

complexes of Salophan ligands have been reported thus far, with the exception of a single Mo

dioxo complex. We synthesized three new Salophan ligand precursors, featuring ortho Me,

ortho, para di-chloro, and ortho, para di-t-Bu phenolate substituents, in addition to the

previously reported prototypical Salophan ligand, bearing only hydrogen substituents at the

phenolate rings. The ligand pecursors were prepared by a simple sequence of condensation

and reduction. All the ligand precursors were reacted with Ti(Oi-Pr)4 and Zr(Ot-Bu)4

(Scheme A-8). We found that the unsubstituted Salophan led to a complex product mixture

for both metal precursors, whereas the ortho-Me substituted ligand led to a clean complex

only with zirconium. Other, namely ortho, para di-chloro, and ortho, para di-t-Bu substituted

ligands, gave a well-defined coordination chemistry with both metals. NMR analysis

indicated that all the Salophan ligands act as dianionic ligands, forming hexa-coordinate

complexes of C2-symmetry. X-ray structure analysis of these species revealed that the

wrapping mode of the ligands was fac-fac, and the orientation of the labile groups was cis

(Scheme A-7).

Scheme A-7. Wrapping mode of the Salophan ligands around Group IV metals

N N

OHR1

R2

HOR1

R2

H H

O

N

MN

OOR

R1

R1

R2

R2RO

H

H

M(OR)4M=Zr: R=Ot-Bu, R1=Me, R2=H

M=Zr: R=Ot-Bu, R1=R2=Cl

M=Ti: R=Oi-Pr, R1=R2=Cl

M=Ti: R=Oi-Pr, R1=R2=t-Bu

M=Ti: R=Oi-Pr, R1=R2=t-Bu

Introduction

15

Introduction

This thesis is devoted to the study of early transition metal complexes with amine

phenolate ligands, and presents their coordination chemistry and reactivity. In the

introduction I will describe various ligand systems with emphasis on oxygen-donor ligands,

their characteristic chemistry, and reactivity in the fields relevant to this work.

I-1. Alkoxo (phenolate) ligand vs. Cp ligand

In recent years we are witnessing a huge interest in the development of new ligand

systems that may be used as an alternative to the cyclopentadienyl (Cp) ligand in the

chemistry of early transition metals. Ligands based on the alkoxo- and amido donors have

attracted the most considerable attention, as these “hard” multi-electron donors successfully

stabilize high oxidation states of the early transition metals, that are defined as “hard acids”.1

The alkoxo (phenolate) ligands are especially attractive: The ligand precursors are

sufficiently acidic, bind the “oxophilic” early transition metals firmly, and may be viewed as

isoelectronic with the cyclopentadienyl ligand.2,3

Several important differences between cyclopentadienyl ligands and alkoxo ligands

should be taken into account. The alkoxo ligand is less electron-donating than the Cp ligand,

thus the alkoxo complexes are less electron-saturated.2-4 A simple alkoxo or phenoxo ligand

is less bulky than the cyclopentadienyl ligand. As a result, the use of a simple alkoxo

(phenoxo) ligand normally leads to complex product mixtures.2-4 An additional obstacle in

the chemistry of the alkoxo (phenoxo) ligands is their ability to bridge metal atoms.4 An

aggregation of metal complexes causes ligands redistribution, and provides low-energy

pathways for the decomposition of coordinatively unsaturated metal complexes.

Introduction

16

I-2. From a simple alkoxo to a bulky phenolate ligand.

Two general strategies have been developed to overcome the obstacles encountered in

the use of simple alkoxo or phenolate ligands: Employing bulky monodentate ligands, or

designing multidentate ligands. The use of bulky ligands allows accommodation of only a

limited number of ligands around the metal center, which could lead to the desired structure

of a metal complex, and to prevent aggregation. Throughout the last two decades, the motif of

bulky monodentate alkoxo, siloxo, and phenoxo ligands has been carefully investigated.2,3,5,6

The steric bulk of a ligand may be approximated using the “cone angle” concept.1,7 For the

Cp ligand, the cone angle was estimated to be 136º.7 Wolczanski and coworkers have

introduced a number of bulky alkoxo and siloxo ligands, with cone angles close to this value

(Figure I-1).2 The substantial steric bulk of the ligand allowed a clean metathesis reaction,

leading to the alkoxo/siloxo complexes of desired structures. As a result of the coordinative

unsaturation at the metal center, some of these complexes exhibited an unusual reactivity,

including carbon monoxide cleavage, oxidative addition of various substrates, and formation

of metal-metal bonded complexes.2

Figure I-1. Cone angles of the Cp, Ot-Bu, and “tritox” ligands

Bulky phenolates were subject of even more thorough investigation, especially by the

groups of Rothwell and Schrock.3,5,6 The phenolate may be viewed as a more "user-friendly"

ligand than simple alkoxide, as it is more acidic and its properties are more easily controlled

M

O

Ct-Bu

t-But-Bu

M

M

O

CMe MeMe

136 o < 90 o 125 o

Introduction

17

by the utilization of the appropriate substituents. As phenolate is a planar rather than a cone-

shaped ligand, the “cone angle” term is less appropriate for this ligand. The steric bulk of the

phenolate is normally transferred via its ortho substituents that point toward the metal; the

bulky ortho groups prevent aggregation of the metal centers and lead to the well-defined

constitution of a metal complex. For example, for 2,6 di-t-Bu phenolate ligands, complexes

containing only two such ligands (in axial positions), in addition to three alkyl ligands, were

typically formed for tantalum (V) (Figure I-2).5

CH3

Ta

CH3H3C

O O

t-Bu

t-Bu

t-Bu

t-Bu

Figure I-2. A well-defined mononuclear complex of Ta(V) carrying a 2,6 di-t-Bu phenolate ligand.

I-3. Multidentate ligands: Enhanced stabilization and easier design of metal centers.

Exploitation of the chelate effect provides a second possible strategy for the

stabilization of metal-alkoxide complexes. In general, multidentate ligands bind the metal

firmly due to the chelate effect,1 and, not less importantly, may enable precise control of the

structure and reactivity at the metal center. The following non-alkoxo ligand system, relevant

to the studies presented in this work, illustrates this trend. The triamidoamine [N3N] ligand

system, investigated initially by Verkade,8 was introduced to the early transition metals realm

by Schrock.9 These ligands were shown to bind to metals in a tetradentate, almost uniformly

C3v-symmetrical manner, creating a 3-fold-symmetric axial “pocket” (Figure I-3). In that

pocket, 3 orbitals are available for bonding with additional ligand(s): two π orbitals and one σ

orbital. In addition to the electronic consideration, this pocket was influenced sterically by

bulky substituents at the amido nitrogens. All this led to extremely rich chemistry of the

Introduction

18

metal-ligand multiple bonds. The formation of metal-ligand multiple bonds in this position

was frequently unavoidable: For instance, the reaction of various [N3N]TaCl2 complexes with

any Grignard reagent bigger than methyl led uniformly to the formation of the tantalum

alkylidene (M=CR2) function, and tungsten alkyl complexes [N3N]WR readily lost

dihydrogen at RT to form metal alkylidyne (M≡CR) complexes. In addition, the unique

ability of this ligand system to stabilize multiple bonds allowed to fulfill one of the “Holy

Grails” of modern inorganic chemistry: catalytic dinitrogen activation to ammonia via a 14-

step cycle including the states of imide and nitride, 10 of the intermediate species being

isolated and characterized.10

N

NN

R R

NR

z

dz2 dxz

N

N NTa

H R'R R

NR

N

NNW

R'R R

NR

N

NNMo

NR R

NR

N

NN

Mo

PR R

NR

Figure I-3. Orbitals available for the axial ligand(s) in a 3-fold symmetrical pocket (only one orbital of the

degenerate set (dxz, dyz) is shown); examples of multiply bonded ligands in the [N3N] metal complexes

Introduction

19

I-4. Multidenatate phenolate ligands.

Several multidentate phenolate-based ligand systems became widely known and

extensively used in recent years.4 Simple methylene- or ethylene-bridged bis(phenolate)

ligands have been investigated by several groups. These ligands, providing only two donors,

forming an 8-membered chelate ring (or a 9-membered chelate ring), and being relatively

flexible, do not impose a rigid geometry at the metal. Thus, the resulting early transition

metal complexes may be tetra-, penta-, or hexacoordinate, depending on the reaction

conditions (Scheme I-1).11 The use of a mononuclear metal precursor for these ligands

generally leads to a formation of a mononuclear product. In contrast, for dinuclear metal

precursors (Mo2(NMe2)6 and W2(NMe2)6), the ligand can bind either to a single metal or to

bridge between the metal centers (Scheme I-2).12

OHR1

R2

OHR1

R2

TiCl4

OR1

R2

OR1

R2

Ti

Cl Cl

+ THF- THF

OR1

R2

OR1

R2

TiCl

ClTHF

ZrCl4THF2

OR1

R2

OR1

R2

ZrCl Cl

THFTHF

Scheme I-1. Coordination behavior of methylene-bridged bis(phenolate) ligand with Group IV metals.

Introduction

20

OR1

R2

O R1

R2

Mo2(NMe2)6

OR1

R2

OR1

R2

Mo Mo

OR1

R2

OR1

R2

R2N

NR2 +Mo

MoO

R1

R2

OR1

R2

NR2

R2N

OH

R1

R2

OH

R1

R2

Scheme I-2. Coordination behavior of the bis(phenolate) ligand with Mo(VI) dinuclear metal centers.

Another example of a relatively simple chelate ligand system is the α, α’-bridged

bis(naphtholate) (BINOL) ligand.13 This ligand leads mostly to tetra- or penta-coordinate

complexes; in the case of bulky ortho substituents, the formation of bis(homoleptic)

complexes is diminished to a large extent (Scheme I-3). The most unique feature of this

ligand is its intrinsic chirality; thus, BINOL provides a chiral backbone in the resulting metal

species. For the Group IV metals possessing identical monodentate ligands (X), the resulting

tetrahedral complexes are C2-symmetrical, and the labile positions are homotopic.14

OHOH

OO

MXX

MX4

R

R

R

R

Scheme I-3. A BINOL ligand precursor, and the resulting tetrahedral M(IV) complex.

Sequential methylene-bridged tris(phenolate) ligands were investigated by Kawaguchi

and coworkers.15 As in the respective bis(phenolate) ligands, the phenolate rings in the

tris(phenolate) ligands are connected at the ortho positions through methylene linkers. These

Introduction

21

ligands demonstrated even more flexible behavior than the corresponding bis(phenolates):

Several conformations for the coordination of this type of ligands to the metal center were

reported. For the early transition metals (Ti(IV), Ta(V), Nb(V)), these ligands normally form

dinuclear complexes. The nearby metal centers in these complexes could be bridged by

additional monodentate (hydrido) or bidentate (DME) ligands, or by the phenolate oxygens

themselves (Figure I-4).

Figure I-4. A dinuclear di-Ti complex with the “open-chain” tris(phenolate) ligand.

The ligands presented in the previous paragraphs are all sequential (“open-chain”)

phenolate-only ligands. A very important family of multidentate phenolate ligands is the

family of cyclic poly(phenolate) ligands, termed calixarenes (“bowl-shaped”) after Gutsche.16

This general term includes all cyclic poly(phenolates), connected by methylene bridge; most

of the inorganic and organometallic chemistry was performed with the ligand containing four

phenolate units (calix[4]arene).17 This ligand may adopt various conformations, with the

“cone” conformation being the most common for the early transition metal complexes

(Scheme I-4). The basic calix[4]arene ligand is tetraanionic, which generally results in the

formation of dinuclear complexes for the Group IV metals, and may also lead to the dinuclear

species with Group V metals (Scheme I-4). For Group V metals, formation of a mononuclear

comlex may be achieved by using a Cp group as an additional, fifth ligand. In contrast, this

O

O

Ot-Bu

Ti

O

O

O

t-Bu

Ti

Cl

Cl

Introduction

22

tetraanionic tetradentate ligand normally forms a mononuclear well-defined species with

Group VI (M(VI)) metal centers. A variety of di- and trianionic calix[4]arene derivatives

could be synthesized by a selective Williamson alkylation at the phenolate oxygens. For the

corresponding dianionic ligands, mononuclear Group IV complexes are obtained.

Scheme I-4. Top: coordination chemistry of the tetraanionic calix[4]arene toward early transition metals.

Bottom: A mononuclear Zr(IV) complexes of a dianionic calix[4]arene.

The chemistry of chalcogen-bridged bis(phenolate) and bis(naphtolate) ligands

([OEO], E = S, Te) with Group IV metals has been studied mainly by Kakugo and coworkers,

and by Okuda and coworkers.18 These ligands are dianionic and tridentate, binding to the

OO O O

HH H H

t-Bu t-Bu t-Bu t-Bu

Ti(NMe2)4

OO O O

t-Bu t-Bu t-Bu t-Bu

OOOO

t-But-But-But-Bu

TiTi OOOO

t-But-But-But-Bu

WO

W(O)Cl4calix[4]arene

CpTaCl4

OOOO

t-But-But-But-Bu

Ta

OO O O

HMe H Me

t-Bu t-Bu t-Bu t-Bu

n-BuLi

ZrCl4O

O O OMe Me

t-Bu t-Bu t-Bu t-Bu

ZrCl Cl

OO O O

Me Me

t-Bu t-Bu t-Bu t-Bu

Zr

PhH2C CH2Ph

PhCH2MgCl

Introduction

23

metal through the phenolate oxygens and the central neutral donor. Due to the 5-membered

chelate rings, these ligands tend to adopt a more rigid coordination at the metal. The

coordination mode of these ligands is facial (fac) in octahedral geometry, which leads to the

cis disposition of the phenolate oxygens. For potentially bridging labile ligands (i.e., OR, Cl),

the resulting metal derivatives are dimeric, possessing nearly octahedral environment

(Scheme I-5). In contrast, the organometallic complexes of these ligands are monomeric and

pentacoordinate. Further studies on Mo(VI), W(VI), and Sm(III) metal complexes with this

type of ligand have all indicated a facial binding of the [OEO] ligand to the metal center.19

S

O Ti

O

R

RCl

Cl Cl

S

OTi

O

R

R

Cl

OH OH

RRTiCl4

MeLi

O O

RR

Ti

S

S

Me Me

S

O Ti

O

R

RTHF

Cl Cl

THF

Scheme I-5. Coordination chemistry of the [OSO] ligands at Ti(IV) metal centers.

Possibly, the most appreciated veteran in the field of multidentate phenolate ligands is

the “sequential” tetradentate di(imine) bis(phenolate) ligand, also known as SALEN ligand

(Scheme I-6).20 Complexes of nearly all early transition metals with this ligand have been

prepared and characterized. This ligand, being dianionic and tetradentate, leads to well-

defined penta- or hexacoordinate mononuclear complexes, generally avoiding the formation

Introduction

24

of the bis(homoleptic) or bridging species. In octahedral geometry, this rigid ligand normally

imposes a “mer-mer” planar disposition of its four donor atoms, leaving the two remaining

positions mutually trans (Scheme I-6).

NN

HOOHRR

NN

OO

RR

M

X

X

MX4

Scheme I-6. SALEN ligand precursor, and the resulting metal (M(IV)) complex.

Reducing the C=N double bonds in the SALEN ligand leads to the di(amine)

bis(phenolate) [ONNO]-type SALAN ligand precursor (Scheme I-7). In analogy to the Group

IV SALEN metal complexes, SALAN metal complexes are also well-defined mononuclear

species of octahedral geometry. However, these saturated ligands demonstrate completely

different binding mode: The fac-fac coordination of the [ONN] fragments, resulting in cis

disposition of the monodentate ligands. Thus, the resulting metal species are C2-symmetrical

and chiral-at-metal.21 A similar binding mode has been reported for the di(thio)

bis(phenolate) [OSSO] ligands.22

NN

OHR

R

HO

R

RMX4

O

N

M

N

OX

R

R

R

RX

Scheme I-7. Wrapping mode of the SALAN ligands around Group IV metal centers

Introduction

25

“Breaking” the SALEN ligand into two parts leads to bidentate monoanionic

phenoxy-imine ligands. Group IV metal complexes bind two such ligands, in addition to two

monodentate ligands (chloro or alkyl groups). Similar to the di(amine) bis(phenolate) ligand

complexes, and in contrast to the SALEN metal complexes, the phenoxy-imine ligands are

oriented such that the overall symmetry is C2, the two phenolate oxygens mutually trans, and

the monodentate ligands are cis to one another (Scheme I-8).23

Scheme I-8. Phenoxy-imine ligands, and their Group IV complexes. The resulting catalysts are termed “FI”

catalysts after phenoxy-imine ligands.

The recent findings in the field of multidentate phenolate-based ligands were recently

reviewed.4 It should be noted, however, that this field remains relatively unexplored, in

comparison with the polydentate amido ligands,24 and major effort is invested today in the

design and exploration of new chelate phenolate-based ligands.

I-5. Synthetic pathways to early transition metal complexes bearing phenolate ligands

The salt metathesis route between the ligand salt (Li, Na, etc.) and the metal chloride

provides the most commonly used method for the preparation of early transition metal

complexes with various sorts of ligands (Scheme I-9). However, the chemistry of the

phenolate-based ligands with early transition metals is rather unique, as the phenol function

in the ligand precursor is substantially more acidic than most other functions (such as simple

MX4

R'N

OH2 MN

O

R'N

O

R'X X

R

R

R

Introduction

26

alkohol, amine, or alkane). This may be employed to help avoiding the rather notorious salt

metathesis route normally required for the amido or cyclopentadienyl ligands. Thus,

homoleptic metal alkoxides or amides may be employed as the metal precursors in alcohol or

amine elimination reaction, respectively (Scheme I-10). One of the goals of the current work

was to show that the homoleptic metal alkyl precursors, such as tetrabenzyltitanium or

pentabenzyltantalum, could be used as starting materials of choice in the chemistry of

phenolate ligands.

ArOH + BuLiMClx ArOMClx-1 + LiClArOLi

Scheme I-9. The salt metathesis route to early transition metal complexes with the phenolate ligands

ArOH + M(OR)x ArOM(OR)x-1 + ROH

ArOH + M(NR2)x ArOM(NR2)x-1 + R2NH

ArOH + MRx ArOMRx-1 + RH

Scheme I-10. The route of the alcohol, amine, and alkane elimination reactions, respectively, to early transition

complexes with phenolate ligands

I-6. Reactivity of early transition metal complexes supported by multidentate phenolate

ligands

In the last decade, phenolate-based chelating ligands became widely abundant in the

frontier fields of inorganic chemistry and catalysis. Among other fields, early transition metal

complexes of multidentate phenolates show an extraordinary reactivity in the fields of

enantioselective transformations,25 olefin polymerization, lactide and caprolactone

Introduction

27

polymerization,26 olefin metathesis, and bioinorganic chemistry. In the following sub-

paragraphs, a description of the fields relevant to this work is presented.

I-6.1. Olefin polymerization by well-defined Group IV metal complexes

One of the research fields most significantly influenced by the chelating phenolate-

based ligands is the field of α-olefin polymerization catalysts.27 The field of α-olefin

polymerization emerged in 1954, when Ziegler discovered that a combination of a simple

early transition metal complex (TiCl4), and a main group compound (AlEt3) catalyzes a

polymerization of ethylene to polyethylene under mild conditions.28 Soon after, Natta and

coworkers reported the stereoregular (isotactic) polymerization of propylene by the same

catalyst, producing a crystalline polymer of high melting point (Scheme I-11).29 This catalytic

system was heterogeneous, and therefore ill-defined. As a result, this multi-site system

produced polymers mixtures of broad molecular weight distribution and different properties,

and impeded a thorough understanding of the polymerization mechanism. Soon, it became

understood that the preparation of the well-defined polymers demands the preparation of

well-defined (“single-site”) catalysts.

Me

TiCl4

AlR3Me

n

x +

Mey

isotactic polypropylene: crystalline, high-melting

atactic polypropylene:amorphous, oily

Scheme I-11. Propylene polymerization by the heterogeneous system TiCl4/AlR3

The first homogeneous models were constructed from the Group IV bent

metallocenes (CpMX2) and alkyl aluminum compounds as activators (“co-catalysts”)

Introduction

28

(Scheme I-12). In contrast to the heterogeneous systems, these catalytic systems showed low

activity in the polymerization of ethylene, and negligible activity toward propylene and

higher alpha-olefins.30 Two major discoveries revolutionized this field. First, the reactivity of

the metallocene-based catalysts was significantly improved by the invention of a new co-

catalyst: Methylalumoxane (MAO), a partially hydrolyzed Me3Al having an oligomeric

form.31 Second, the invention of chiral (C2-symmetric) ansa-metallocenes has led the way to

the desired stereoregular (isotactic) polypropylene.32 Consequently, Cs-symmetric

metallocenes having enantiotopic sites led to the syndiotactic polypropylene,33 that was

inaccessible via the heterogeneous group IV Ziegler-Natta catalysts.

Scheme I-12. Top: Propylene polymerization by C2v-symmetric metallocenes. Middle: Propylene

polymerization by C2-symmetric ansa-metallocenes. Bottom: Propylene polymerization by Cs-symmetric ansa-

metallocenes.

ZrClCl

AlR3 Men

Me

n

Low activity, low Mw, atactic polypropylene

ZrCl Cl

ZrCl Cl

Me

n

Me

n

High activity, high Mw, isotactic polypropylene

High activity, high Mw, syndiotactic polypropylene

MAO

Al(Me)-O n(MAO)

Men

Me n/2Me

Introduction

29

One should not underestimate the contribution of metallocene catalysts to the field of

α-olefin polymerization, as these catalysts shed light on polymerization mechanism, and led

to novel polymer types.34 However, the versatility of a single ligand family in determining of

catalyst structure (metal geometry, its electron and steric saturation) is limited by definition.

In addition, the synthesis of various functionalized Cp ligands, and their metal complexes

may be quite cumbersome, and almost no metallocenes found their way to the polymer

industry for various reasons. Thus, in the middle of the 1990’s, a search for new ligand

systems for polymerization catalysts has begun.

At first, the amido-based ligands attracted most of the attention, as the derived

catalysts were shown to lead to “living polymerization”.24a Several criteria have been

proposed for living polymerization, among them are negligible termination (or chain transfer)

and fast initiation relative to propagation.35 Living polymerization can be of great importance,

as it leads to well-defined polymers of high-Mw and narrow molecular weights distribution. In

addition, living polymerization allows the preparation of block-copolymers. Prior to these

reports, no living polymerization catalysts, operating at ambient conditions, had been

reported in the field of Ziegler-Natta polymerization. In 1996, McConville and coworkers

have reported that a Ti-based catalyst, carrying a diamido ligand, polymerizes 1-hexene in a

living fashion (Scheme I-13).36 The living character of polymerization was indicated by a

linear rise in the molecular weight of the resulting polymer vs. time and vs. monomer

consumed, and by a narrow molecular weight distribution (MWD, or PDI). Since then, the

amido-based polymerization catalysts were thoroughly studied, especially in relation to living

polymerization.24a,37, 38

Introduction

30

Scheme I-13. Living polymerization of 1-hexene by diamido-based Ti catalyst

The great promise of the phenolate-based polymerization was demonstrated in the

work of Schaverien and coworkers in 1995 describing highly isotactic polymerization of 1-

hexene using BINOL Ti(IV) and Zr(IV) dibenzyl complexes (see Scheme I-3).14 It should be

noted, however, that the major interest in the phenolate-based catalysts has arisen only

recently, with the development of four families of polymerization catalysts.39 The first family

is the family of Ti catalysts based on the [OSO] ligands (Scheme I-5) that have led to the

active polymerization of ethylene and styrene.18 The second family is the family of “FI”

catalysts (Scheme I-6).23 All the complexes of this family are C2-symmetric, and the steric

and electronic parameters of this system may be easily controlled by the phenolate

substituents, and the imine substituents. As a result of fine structural modifications, some of

these catalysts exhibited unusual reactivities. For example, the FI-Zr catalysts, possessing a

bulky group at the ortho position of the phenolate rings were reported to be the most active

ethylene polymerization catalysts ever reported. The sterically-hindered and the electron-

deficient FI-Ti catalysts, possessing bulky groups at the ortho positions, and the meta-

difluorophenyl groups bound to the nitrogen donors, led to the living and syndiotactic

polymerization of propylene (Scheme I-14).

TiN

N

Me

Me

i-Pri-Pr

i-Pr i-Pr

B(C6F5)3Bu

nBu

nPDI < 1.1

Living poly(1-hexene)

Introduction

31

Me

n

Living polymerization yielding

syndiotactic polypropylene

MAOMe n/2Me

TiN

O

t-Bu

RN

O

t-Bu

RCl Cl

FF

R =

Scheme I-14. Propylene polymerization by FI-Ti catalyst

The third family consists of the amine bis(phenolate) metal complexes discussed in

the present work. Finally, the fourth family is the family of the di(amine) bis(phenolate)

ligands.21,40 The labile positions, at which the polymerization reaction takes place, are

homotopic in this catalyst, due to the rigid C2-symmetric structure of the complex. Thus, in

analogy to the C2-symmetric metallocenes, an isotactic polymerization of various α-olefins

was observed.21a, 21b, 40 In addition, a living and isotactic polymerization of an α-olefin (1-

hexene) was achieved by the Zr catalyst of this family (Scheme I-15).21a The precondition for

the living polymerization in this system is the presence of bulky groups in ortho positions,

which are presumed to retard termination.

Scheme I-15. 1-hexene polymerization by di(amine) bis(phenolate) Group IV complex

nBu

nBu

Living and isotactic poly(1-hexene)

B(C6F5)3

O

N

ZrN

OCH2Ph

t-Bu

t-Bu

t-Bu

t-BuPhH2C

Me

Me

Introduction

32

Given these findings, the potential of the phenolate-based multidentate ligands has

been widely recognized by multiple research groups. Thus, at the present time, novel

phenolate-based olefin polymerization catalysts are designed, and their reactivity is

investigated by many research groups worldwide.

I-6.2. Stabilization of metal-carbon double bonds and olefin metathesis reactivity

The first metal-carbon double bond of the ”alkylidene” type was reported by Schrock

in 1974.41 In the course of investigation of Ta(V) homoleptic alkyl complexes, Schrock

discovered that the compound formed by the reaction between Ta(CH2t-Bu)3Cl2 and two

equivalents of t-BuCH2Li was actually Ta(=CHt-Bu)(CH2t-Bu)3, instead of Ta(CH2t-Bu)5

(Scheme I-16). Following the discovery that the alkylidene function mediates olefin

metathesis,42 metal-carbon multiple bonds became a subject of principal interest in

organometallic chemistry.43

Ta(CH2t-Bu)3Cl2 Tat-BuH2C

t-BuH2C

CH

CH2t-Bu

CH2t-Bu

t-Bu H

2 NpLi

- 2 LiCl

- NpHTa

CH2t-Bu

C(H)t-Bu

t-BuH2C

t-BuH2C

Scheme I-16. The α-hydrogen abstraction reaction, leading to the formation of an alkylidene complex

The major route that leads to the alkylidene function is the α-hydrogen abstraction

reaction, taking place in a dialkyl precursor (Scheme I-16).43 If the alkyl group carries β-

hydrogens, β-H elimination may compete with the α-hydrogen abstraction reaction. The α-

hydrogen abstraction reaction normally goes via a monometallic mechanism, being therefore

of first order in the reactant.44

Introduction

33

TaO

O

THF

R R

R

R

O

R

RCt-Bu

H

Mo

N

CHRO

O

i-Pr i-PrR

R RR

Figure I-5. Alkylidene complexes, supported by monodentate phenolate ligands.

Monodentate phenolate-based ligands were extensively used to support the alkylidene

functionality, and to study its structure and reactivity (Figure I-5).3,5 However, only recently

multidentate phenolate ligands have been introduced into this field. Following the highly

efficient Mo(VI) metathesis catalysts, supported by two monodentate alkoxo or phenolate

ligands, Schrock and coworkers prepared Mo(VI) alkylidene complexes, supported by a

single bis(phenolate) or BINOL ligands.45 The resulting species are mainly tetracoordinate,

containing an imido function in addition to the alkylidene and the phenolate oxygens

(Scheme I-17). Most significantly, these species are chiral, as a result of the ligand backbone,

and may be prepared in an enantiomerically pure form. These species have been found to be

highly effective stereoselective catalysts for various asymmetric olefin metathesis reactions.

For example, enantiomerically-pure small rings could be prepared from achiral precursors in

high yields, and with excellent enatiomeric excess (Scheme I-17).46

Introduction

34

Mo

N

CHROO

i-Pr i-Prt-Bu

t-Bu

Me

Me Me

MeO O

Me H

Me

Me2 mol %

99% ee, 93% yield

Scheme I-17. An asymmetric RCM reaction catalyzed by the Schrock-Hoveyda catalyst.

I-6.3. Phenolate ligands in bioinorganic chemistry

The field of bioinorganic chemistry is rapidly growing in the last years. In general,

this field is concerned with the active sites of metallo-enzymes, and with simple model

compounds, that mimic the structure and reactivity of the active site of the enzyme.47 The

metallo-enzymes that contain oxophilic early transition metals (mostly V, and Mo to a lesser

extent) may exhibit an oxygen-rich environment at the active site. For V, two enzyme

families are known today. The enzyme of the first family, V-dependent nitrogenase, reduces

dinitrogen into ammonia; the active site of this enzyme contains V ligated mainly by sulfur

donors.48,49 The second family of V-dependent enzymes is V-dependent haloperoxidases.48,49

These enzymes catalyze the oxidation of halide to hypohalous acid (using peroxide as the O

atom donor), which, in turn, halogenates organic substrates (Scheme I-18, top). In addition,

these enzymes catalyze the asymmetric O atom transfer to sulfides, forming optically active

sulfoxides (Scheme I-18, bottom).

Introduction

35

Scheme I-18. Reactivity of V-dependent haloperoxidase. Top: Halogenation of organic substrates (X = Cl, Br).

Bottom: Oxidation of sulfides to sulfoxides.

Considerable structural data has been gained about the V-dependent

haloperoxidases.50 The (resting) active site of all the enzymes of this family contains a V(V)

metal center in a trigonal bipyramidal geometry, coordinated to four oxygen atoms, and to

one nitrogen atom (Figure I-6). During the catalytic cycle, the metal retains its oxidation

state. The proposed catalytic cycle involves a peroxo (side-on) intermediate, in which the

activation of the O atom takes place. At the final step, before the release of the oxidant, the

metal center is of hexacoordinate octahedral geometry.51 In the hexacoordinate geometry, V

binds five oxygen ligands, and one nitrogen ligand (imine) in one of the axial positions.

X- + H2O2 + H+ "V" HOX + H2O

HOX + RH RX + H2O

RS

R+ H2O2 R

SR

+ H2O

O"V"

Introduction

36

Figure I-6. Proposed catalytic cycle for V-dependent haloperoxidase

Appropriate structural models for the active site of V-dependent haloperoxidase are

V(V) complexes, having NO4 donors set (where N is a neutral donor) preferably in a trigonal

bipyramidal geometry.48,49 It is worth noting that trigonal bipyramidal geometry is rare for

pentacoordinate V complexes; usually, a distorted square pyramidal geometry is observed.48c

In addition, the metal center should be able to switch between the penta- and hexacoordinate

geometries. The number of compounds strictly matching these conditions is very limited.

However, many complexes that come close to these conditions might be viewed as “good”

structural models. Most of these models are based on the bi- or tridentate imine-phenolate

ligands (Figure I-7).48,49 Some of these complexes have been found to be functional models

as well, being able to conduct oxidation of organic and inorganic substrates, such as olefins

(to epoxides) and sulfides (to sulfoxides).51 This field is of high interest to the inorganic

V

OHN(His404)

O

OOH

N(His496)

V

OHN(His404)

O

OOH

N(His496)

H -OOH

Cl-

-2 H2OVO

O

O

N(His496)O

Cl-

N(His404)

H+, H2O

HOCl

VO

O

O

N(His496)OH

N(His404)

H

ClO H

H2O2

Introduction

37

community nowadays, as it may open the way to efficient oxidation catalysts, operating

under ambient conditions, and using environment-friendly oxidants, such as hydrogen

peroxide or molecular oxygen.

Figure I-7. Selected structural models of the active site of V-dependent haloperoxidase

I-7. Amine phenolate ligands

The present work describes the structural chemistry and reactivity of early transition

metal complexes of the divergent tetradentate amine-phenolate ligands (with a minor

exception of sequential tetradentate Salophan ligands). This ligand family includes three sub-

groups: trianionic amine tris(phenolate) ligands; dianionic amine bis(phenolate) ligands,

having an additional donor on a sidearm; and monoanionic amine mono(phenolate) ligands,

carrying two such donors (Figure I-8). In contrast to many other ligand precursors, the

ligands of this family can be easily prepared from commercially available starting materials

in a single-step Mannich condensation. All these ligands may be viewed as “super-chelating”

ligands: they provide four donor atoms, and, as divergent ligands, do not require a specific

wrapping of the ligand precursor for the binding to the metal. In addition, these ligands may

enable a precise control of the geometry at the metal center, by restricting the number of

possible isomers in both penta- and hexacoordinate geometries.

NO

Vt-Bu

Me

Me

NO t-Bu

Me

Me

O

NN

OOMe

t-Bu t-Bu

MeV

O

O N

O O

V

OH2

OH2O

Br

Introduction

38

Figure I-8. The family of the amine phenolate ligands

I-7.1. The amine tris(phenolate) ligands

The amine tris(phenolate)s are old organic compounds but very young ligands.52 Even

though they are ideally suitable for stabilization of high-oxidation state early transition

metals, and despite their structural similarity with the well-known tripodal ligands, such as

triethanol amine,8 or the triamido amine,8,9 their potential has not been evaluated until the end

of 1990’s. The first transition metal (Fe(III)) complex of an amine tris(phenolate) ligand was

reported only in 1998.53a Soon after, the amine tris(phenolate) complexes of main group

elements (Ga(III), In(III), Si(IV), P(V)) were reported.53b,54 The amine tris(phenolate) ligands

were introduced into the chemistry of early transition metals by our group in 2001.55 A year

later, the amine tris(phenolate) Ti complexes were shown to lead to the active lactide

polymerization catalysts.56 Since then, these ligands draw an ever-increasing attention as a

rigid and versatile platform for the early transition metals.4

According to their coordination chemistry with the early transition (Ti(IV)), and the

main group metals, these ligands are indeed “tripodal”, or “atrane” ligands,8 as they lead to a

well-defined mononuclear TBP complexes, in which the phenolate oxygens occupy the

equatorial positions, and the amine occupies one of the axial positions, while the second axial

position is occupied by a fifth (monodentate) ligand (Figure I-9).55,56 In this respect, a

N

OH HOR R

R

HO

N

OH HO

D

R R

N

OHD

D

Amine tris(phenolate):Tetradentate trianionicno sidearm donors

Amine bis(phenolate):Tetradentate dianionicone sidearm donor

Amine mono(phenolate):Tetradentate monoanionictwo sidearm donors

R

Introduction

39

comparison between the reactivity of these tripodal oxygen-donor ligands and the reactivity

of tridodal nitrogen-donor ligands (triamidoamine) is revealing. Aiming at an easier

preparation of well-defined mononuclear complexes, the amine tris(phenolate) ligands should

be more useful than the triethanolamine ligands,8 according to the considerations presented

earlier, i.e., they are more acidic, and allow a precise steric control via their ortho

substituents. In addition, these ligands may possibly switch between hexacoordinate and

penta-coordinate metal geometries, thus enabling their participation in various catalytic

applications.

Figure I-9. A tripodal amine tris(phenolate) Ti(IV) complex

I-7.2. The amine bis(phenolate) ligands with the additional “sidearm” donor

The dianionic tetradentate amine bis(phenolate) ligands, carrying a sidearm donor,

were first reported in 1988 for Mo(VI).57 Our group has begun to explore the chemistry of

these ligands with Group (IV) metals in 1999, aiming at the preparation of α-olefin

polymerization catalysts. The complexes were prepared using per(alkoxo) (Ti(Oi-Pr)4) and

per(benzyl) (Zr(CH2Ph)4 and Hf(CH2Ph)4) metal precursors (Scheme I-19).58,59 For Ti, the

preparation of dialkyl complexes was accomplished by the reaction of the dialkoxo complex

with Me3SiCl, and further alkylation with the corresponding Grignard reagent.58b,58c In all

cases, the amine bis(phenolate) ligands have led to well-defined mononuclear complexes; in

most of the cases the sidearm donor was bound to the metal, accomplishing an octahedral

geometry at the metal center. The resulting complexes exhibited an almost uniform structure

TiO

OO

NR

R

R

RR

R

X

Introduction

40

at the metal center, in which the phenolate oxygens were mutually trans, and the labile

positions were cis (as required for an α-olefin polymerization pre-catalyst), being consistent

therefore with an overall Cs-symmetrical structure.

MO

X

t-Bu

t-Bu

D

X

Nt-Bu

Ot-Bu

N

OH HOt-But-Bu

t-Bu t-Bu

D

MX4

M = Ti, Zr, Hf

X = Oi-Pr, CH2Ph

D = NMe2, OMe, py, SMe, NEt2

- 2 HX

Scheme I-19. Synthesis and structure of the amine bis(phenolate) Group IV metal complexes

In combination with the co-catalyst (B(C6F5)3), the amine bis(phenolate) Group IV

metal complexes have led to active 1-hexene polymerization catalysts. Keeping in mind a

uniform structure of the pre-catalyst, a thorough structure-activity relationship study was

carried out. In general, the amine bis(phenolate) ligand precursor possesses several “degrees

of freedom”: The nature of the phenolate substituents, the type of the sidearm donor, and the

bridge between the central amine and the sidearm donor. At the first step of this study, most

of the ligands under investigation had bulky (t-Bu) substituents in the ortho positions, in

order to minimize the formation of the bis(homoleptic) complexes for the large metal ions (Zr

and Hf). The additional degree of freedom is the nature of the metal ion. Overall, the Zr

amine bis(phenolate) complexes exhibited the highest activity (standing among the highest

activities ever reported for 1-hexene polymerization),59a-c Hf complexes were somewhat less

active,59c and Ti complexes possessed much lower activity, displaying, however, a living

character of the polymerization in several cases.58b, 58d The presence of the sidearm donor was

found to be the most crucial parameter for high activity in Zr or for living polymerization in

the Ti series. As for the nature of sidearm donor, the non-bulky hard donors (NMe2 and OMe)

Introduction

41

led to the most active Zr catalysts, presenting activities of 21,000 and 50,000 gpol mmolcat-1 h-

1, respectively (Scheme I-20).59c For Ti, the OMe sidearm donor led to an unprecedented

living polymerization of 1-hexene for 31 h.58d Based on these results, the amine

bis(phenolate)-supported metal complexes seem to be one of the most promising non-

metallocene olefin polymerization catalysts today. Thus, further studies, aiming at the

discovery of highly active and living polymerization/block copolymerization catalysts, and

new activity modes, are being carried extensively.

Scheme I-20. Reactivity of Ti and Zr amine bis(phenolate) pre-catalysts in 1-hexene polymerization

I-7.3. The amine mono(phenolate) ligands with two sidearm donors

Viewing the tetradentate amine tris(phenolate) ligands and the amine bis(phenolate)

ligands as a single family, the amine mono(phenolate) ligands, carrying two sidearm donors

may be regarded as the “missing” relative in this family. These ligands possess an

“unnecessary”, at first sight, sidearm donor for Group IV M(IV) metals in octahedral

geometry. However, such a donor may probably be of high importance in cationic species, in

which one of the labile ligands is removed, and may possibly lead to dicationic

polymerization catalysts. This monoanionic ligand may successfully stabilize M(III)

complexes of octahedral geometry, possessing two additional labile groups. Furthermore, this

MO

PhH2C

t-Bu

t-Bu

D

CH2Ph

Nt-Bu

Ot-Bu

Bu

B(C6F5)3

Bun

for M = Zr: highly active polymerization catalysts;

for M = Ti: living polymerization catalysts

D = NMe2, OMen

Introduction

42

“dormant” donor may trigger the α-elimination reaction to form the metal-ligand multiple

bonds in the high-oxidation state early transition metals. Until now, no early transition metal

complexes of such ligands have been reported.60

Discussion

43

Discussion

This work encompasses coordination chemistry and reactivity of early transition metal

complexes with a variety of amine phenolate ligands. In this section various parameters that

bring this work together are discussed, starting from ligand and complex synthesis, and

concluding with the reactivity at the pre-designed metal sites. In addition, this section

highlights novelties brought to the realm of inorganic/organometallic chemistry and catalysis

by complexes of the amine phenolate ligands.

D-1. Ligand synthesis

The amine bis(phenolate) ligands and the amine tris(phenolate) ligands presented

herein have been prepared by the group of Prof. Z. Goldschmidt of Bar Ilan university. The

symmetrical amine bis(phenolate) ligands have been prepared by a single-step Mannich

condensation between the corresponding phenol, primary amine and formaldehyde (Scheme

D-1). The ligands were obtained as crystalline solids after recrystallization. The amine

tris(phenolate) ligands were synthesized by a straightforward reaction between

hexamethylene tetraamine and the corresponding phenol. Overall, this route is very practical,

leading to a large variety of ligand precursors in high yields. The amine mono(phenolate)

ligand was prepared in our laboratory following a similar preparation.

Discussion

44

R

R

OH+ CH2O D NH2+

MeOHR

RN

OH HOR

R

D

N N

N

N

R

R

OH+

R

RN

OH HOR

RMeOH

HO

R

R

R

R

OH+ CH2O D

NH+MeOH

R

RN

OHD

D

2

Scheme D-1. Synthesis of various amine phenolate ligands via Mannich condensation

Sequential (Salophan) ligands were synthesized by a different route, including a

condensation between the substituted 2-hydroxy benzaldehydes and ortho-phenylene

diamine, and subsequent reduction with sodium borohydride (Scheme D-2). For the majority

of the ligands, a useful one-pot synthesis was developed, followed by a simple work-up,

leading to the pure Salophan ligand precursors without a need for chromatography or

recrystallization.

R

R

OH

H

OH2N

H2N+

N

N

OH

OH

R

R

R

R

HN

HN

OH

OH

R

R

R

R

NaBH4MeOH

MeOH

Scheme D-2. Synthesis of Salophan ligands

Discussion

45

D-2. Complex synthesis

As described in the Introduction section, the phenolate ligand precursors are

sufficiently acidic, thus not requiring the notorious salt metathesis route for preparation of the

metal complexes. A variety of metal precursors, purely inorganic or organometallic, were

available for this purpose. The most convenient precursors are the commercially available

homoleptic alkoxide complexes, such as Ti(Oi-Pr)4, and Ta(OEt)5, or the related VO(OPr)3

(Scheme D-3). As the amine phenolate ligands are “super-chelate” ligands, and the alkoxide

function is more basic than the phenolate function, the reactions normally proceeded to full

conversion, and the yields were quantitative. The reactivity of the homoleptic amides, such as

Zr(NMe2)4, or Ta(NMe2)5, towards the amine phenolate ligand precursors resembled that of

the metal alkoxides to a large extent.

Ta(OEt)5 + LigH3 LigTa(OEt)2 + 3 HOEt

V(=O)(OPr)3 + LigH3 LigV(=O) + 3 HOPr

Scheme D-3. Samples for alcohol-elimination reactions between the amine tris(phenolate) ligand precursors and

metal-alkoxide precursors.

The major achievement of this work from a synthetic point of view is the pursuit of

the “alkane elimination” reactions between the homoleptic organometallic metal precursors

and the amine phenolate ligand precursors. These routes are advantageous for the preparation

of an organometallic complex. Some homoleptic organometallic species e.g., Zr(CH2Ph)4,

and Hf(CH2Ph)4 are known for their stability, and thus have been widely used as metal

precursors in “toluene elimination” reactions with acidic ligand precursors. However, the

remaining member of the Group IV triad, Ti(CH2Ph)4, is much less stable, and therefore was

seldom used as a starting material. Previously, an alternative route was developed in our

Discussion

46

group, including the reaction of the ligand precursor with Ti(Oi-Pr)4, followed by

chlorination with TMSCl, and finally an alkylation with PhCH2MgCl (Scheme D-4).58 In the

present research, we demonstrated that Ti(CH2Ph)4 is stable enough, and highly useful for

preparation of titanium benzyl complexes with amine phenolate ligands.

Ti(Oi-Pr)4 + LigH2 LigTi(Oi-Pr)2

Ti(CH2Ph)4 + LigH2

Me3SiCl LigTiCl2PhCH2MgCl LigTi(CH2Ph)2

LigTi(CH2Ph)2

Scheme D-4. Top: A previous synthesis of amine bis(phenolate) Ti(IV) dibenzyl complexes, consisting of the

three steps; Bottom: A single-step synthesis of amine bis(phenolate) Ti(IV) dibenzyl complexes

Until recently, the chemistry of “toluene elimination” reactions had been confined to

the tetrabenzyl Group IV metal complexes only. In the course of our study of Ta(V)

chemistry with the amine phenolate ligands, we attempted the use of the Group V (Ta(V))

pentabenzyl complex as a starting material in that reaction. First, we developed a simplified

preparative route for pentabenzyltantalum, and solved its crystal structure, which had not

been reported previously. Thereafter, we reacted it with a variety of amine bis- and amine

tris(phenolate) ligands (Scheme D-5). This work proved that pentabenzyltantalum is the

“precursor of choice” in this chemistry, as it led to the desired organometallic species in high

yields in a single step.

Discussion

47

Scheme D-5. Structure, and toluene-elimination reactions of pentabenzyltantalum with the amine phenolate

ligand precursors

An additional synthetic route that we employed in this work relies on the reaction

between a metal chloride precursor and the ligand precursor in the presence of a mild base:

Et3N. This route was found to be particularly useful when the utilization of metal alkoxide or

metal alkyl precursors was not possible, as in the case of V(III) complexes. In this route,

triethylamine serves to absorb HCl, forming the EtN·HCl salt that is insoluble in common

organic solvents such as ether or THF. Thus, the desired products are obtained by a rather

simple filtration-recrystallization sequence.

D-3. Coordination chemistry of the amine phenolate early transition metal complexes

One of the most pronounced advantages of the amine phenolate ligands is their well-

defined coordination chemistry. Two isomers are feasible in octahedral geometry for the

amine bis(phenolate) complexes of the LigMX2 type (Figure D-1). In the first isomer, the

phenolate oxygens are trans to each other, leading to an approximate Cs-symmetry of the

metal complexes. In the second isomer, the phenolate oxygens are cis to each other, and the

resulting symmetry is C1. Throughout this research, we have synthesized dozens of such

complexes, bearing alkoxo, amido, benzyl or methyl monodentate ligands. All of these

LigH2LigTa(CH2Ph)3- 2 CH3Ph

LigH3LigTa(CH2Ph)2

- 3 CH3Ph

Discussion

48

complexes exhibited a trans disposition of the phenolate oxygens. However, Mountford and

coworkers have recently demonstrated the viability of the C1-symmetric zirconium

complexes as well, in which the oxygens are in a cis disposition.61

MO

R

R1

R1

D

R

NR1

OR1

M = Ti(IV), Zr(IV), Hf(IV)

R = Oi-Pr, NMe2, Me, CH2Ph, Cl

R1 = t-Bu, Me, Cl, Br

D = NMe2, OMe, OEt, THF, furan, py, SMe

MO

R

R1

O

R

NR1

D

R1

R1M = Zr(IV)

R = Cl

R1 = t-Bu

D = py

Cs-symmetry C1-symmetry

N

OH HOR1

R1

R1

R1

D

MO

R

R1

R1

R

R

NR1

OR1

M = Ta(V)

R = CH2Ph

R1 = t-Bu, Me, Cl, Br

D = NMe2, OMe, Me

Cs-symmetry

D

Figure D-1. Possible wrapping modes of the tetradentate amine bis(phenolate) ligands around a Group IV

(M(IV)), and Group V (Ta(V)) metals in octahedral geometry

The second structural parameter under investigation in the chemistry of the amine

bis(phenolate) ligands was binding of the fourth, sidearm, donor. Previously, it was shown

that, because of steric pressure, the sidearm donor might remain uncoordinated to the Group

Discussion

49

IV metal, forming a pentacoordinate complex.59b Thus, the binding of a sidearm donor should

not be taken for granted a priori. In this work, we employed several types of sidearm donors:

THF, open-chain ether (-OMe), non-bulky amine (-NMe2), and furan. All these, including the

weak aromatic donor (furan), were found to bind to the Group IV metal center, thus leading

to octahedral geometry at the metal.

For a Group V metal complex (Ta(V)) not bearing multiply bonded ligands, the

tetradentate dianionic amine bis(phenolate) ligand is expected to bind in a tridentate fashion

(i.e. the sidearm donor remaining unbound) yielding octahedral geometry at the metal center

(Figure D-1). This was supported by numerous X-ray structure determinations. However, this

“dormant” donor may coordinate, or even trigger the removal of one of the benzyl groups in a

LigTa(CH2Ph)3-type complex, which will be demonstrated in the following section.

The coordination chemistry of the amine tris(phenolate) complexes was found to be

well-defined as well. Octahedral LigMX2 complexes of M(V) metal centers may exist as a

single geometrical isomer (for identical X’s, see Figure D-2). However, these ligands may

probably lead to pentacoordinate metal centers, in the case of metal centers in lower than

M(V) oxidation state (V(III), Ti(IV)), or for the metal centers in M(V) oxidation state bearing

a multiply bound ligand (oxo (=O), alkylidene (=CHR)).

Discussion

50

MO

X

R1

R1

O

X

NR1

OR1

R1

R1

MO

OO

NR1

R1

R1

R1 R1

R1

L

M = Ta(V)

X = OEt, NMe2, Cl, CH2Ph, µ-CHPh

R1 = Me, t-Bu, Cl

M = V(III),

L = THF

R1 = Me, t-Bu, Cl

MO

OO

NR1

R1

R1

R1 R1

R1

X

MO

OO

NR1

R1

R1

R1 R1

R1

R

M = Ti(IV)

L = Oi-Pr

R1 = Me, t-Bu, Cl

M = V(V)R = OR1 = Me, t-Bu

N

OH HOR1

R1

R1

R1

HO

R1

R1

C3-symmetry C3-symmetry

Cs-symmetryC3-symmetry

Figure D-2. Coordination modes for early transition metals in the amine tris(phenolate) environment

All the Ta(V) complexes with the amine tris(phenolate) ligands were found to be

octahedral. As the amine tris(phenolate) is a tripodal ligand, possessing a central donor, and

three equivalent “arms”, its chemistry may be correlated with the chemistry of other tripodal

ligands, and in particular triamidoamine. The coordination chemistry of the amine

tris(phenolate) ligands with Ta(V) stands in sharp contrast to the behavior of the

triamidoamine ligands: The triamidoamine ligands did not lead to octahedral complexes

when the metal center was hexa-coordinate, giving instead a C3-symmetrical pocket.9 For one

specific monodentate ligand (benzyl), the amine tris(phenolate) Ta(V) complexes were C3v-

Discussion

51

symmetric on the NMR timescale down to 203 K; however, this may probably be explained

by a dynamic process recently found in the hexacoordinate amine tris(phenolate) Ti(IV)

complexes.62 Furthermore, the amine tris(phenolate) ligands do not show the tendency found

for the triamidoamine ligands, as they do not lead to TBP Ta(V) complexes with the axial

position occupied by a multiply-bonded ligand. Even when the remaining (“monodentate”)

ligand was nominally “double-bonded” (alkylidene), the complex was of octahedral

geometry, and the alkylidene ligand was found in a rare bridging (µ-alkylidene) mode. This

difference in coordination chemistries between two ligand systems may stem from the

presence of an additional π orbital on the phenolate oxygen, that destabilizes the orbitals

configuration in the “axial pocket” found for the triamidoamine ligands (Figure I-3).

In contrast, vanadium complexes of the amine tris(phenolate) ligands normally

featured a penta-coordinate TBP geometry. For V(III), the remaining axial position was

occupied by a neutral ligand (THF). For V(V), THF was replaced by a doubly-bonded ligand,

the oxo group. This structure provides the first evidence for viability of the “triamidoamine-

type” binding of our tripodal ligand to the metal: i.e., the structure in which the phenolate

oxygens occupy equivalent equatorial positions, and the multiply bonded ligand is found in

the axial position.

We found that an exchange between these two geometries could take place if an

electron-deficient amine tris(phenolate) ligand was bound to V(V) (Scheme D-6). This

complex was found to support both geometries at RT, coordinating an additional neutral

molecule at the sixth coordination site. As expected, the equilibrium between the

pentacoordinate and the hexacoordinate geometries was temperature-dependent, with the

TBP form prevailing at elevated temperatures, and the octahedral form prevailing at RT. The

preference for a hexacoordinate complex in this case is attributed to the overall electron-

deficiency at