Dative ligands with more than one donor

atom Metal complexes

Textbook H: Chapter 2.8

Textbook A: Chapter 8.7.4

Metal dihydrogen complexes Discovered in 1984 by Kubas: M(CO)3(PR3)2(H2) (M = Mo, W; R = Cy,

i-Pr)

Bonding

Reference: Kubas, G. J. et al. J. Am. Chem. Soc. 1984, 106, 451

dH-H = 0.84 Å (0.74 Å in free H2); dW-H = 1.75 ÅHH = 2690 cm-1; HD = 2360 cm-1

HH = -4.21 (24 Hz wide), HD = -4.21 (8 Hz wide, 1:1:1 triplet, JHD = 33 Hz, JHD (HD gas) = 43 Hz)

Good acceptors (CO, NO) are effective in stabilizing the H2 ligand.

2

M

H

H

M H2M H2

W

OC

COPR3

PR3

OCH

H

LnMH

HLnM

H

HLnM

H

H

hydridecomplex

dihydrogencomplex

Agostic interactions Agostic: Greek for “to hold on to oneself”; term coined by Brookhart. It refers to a C-H/Si-H bond, which interacts with a metal.

B: first agostic complex, Cotton, 1974

C is more stable than either the alkyl or the alkene-

hydride complexes.

Characterization1H NMR: peak shifted from that of a normal aryl or alkane towards that of a hydride ligand

JCH is typically around 70-100 Hz versus the 125 Hz of a normal sp3 C atomUsually, fluxional, exchange between agostic and terminal Hs

IR: sometimes, a reduced CH can be observedNeutron diffraction: dM-H in agostic complexes is from 1.85 to 2.4 Å

3B: Mo-H, 2.1 Ǻ; IR, 2704 and 2664 cm-1; 1H NMR, -3.8 ppm

CoTi

P

C

Cl

P

Cl

Cl

H

HH

A

N N

N N

B

MoCO

COC H

Me HEt

B

R3P CH2

HH

H

C

Alkane coordination

X-rayReed 1997

X-rayMeyer 2003

2-H,C

ReOC CO

n-heptane

TRIRGeorge 1997

ReOC CO

cylo-pentane

NMR -93 oCBall 19982-H,C

Possible coordination modes:

Published in: James A. Calladine; Simon B. Duckett; Michael W. George; Steven L. Matthews; Robin N. Perutz; Olga Torres; Khuong Q. Vuong; J. Am. Chem. Soc.  2011, 133 (7), pp 2303–2310DOI: 10.1021/ja110451kCopyright © 2011 American Chemical Society

Metal Hydrocarbyls and Related Ligands

Textbook H: Chapter 3.2.1, 3.8

Textbook A: Chapter 8

Stability and reactivity First metal alkyls: 1848, Frankland, EtZnI and ZnEt2. Alkyl compounds of main group elements (Al, Mg, Si, Sn, Pb) followed the

discovery of zinc alkyls. Similar alkyl compounds of TMs were considered unstable until 1960s.

7

Main group

alkyl

Hf

(kcal/mol)

BDE

(kcal/mol)

TM alkyl BDE

(kcal/mol)

CMe4

SiMe4

GeMe4

SnMe4

PbMe4

-40

-59

-17

-5

+32

86

75

60

52

36

Ti(CH2tBu)4

Zr(CH2tBu)4

Hf(CH2tBu)4

TaMe5

WMe6

47

60

64

62

38

LnM-X LnM + X

Binding modes

8

CM

M

M

M R

Terminal Bridging

MC

M

M M

M

R

M M

R

M

Metallacycles

M M M

M D

M NR2

M M

D

Bond strengths for classical -ligands Classical -bonding ligands (X = H, CH3, Cl) form strong M-X bonds. Bond dissociation energies (BDEs) of organometallic compounds are more difficult

to determine than for organic compounds. Useful in predicting reaction outcome.

There is a good correlation between M-X BDE and H-X BDE, except LnM-H is usually stronger than LnM-CH3 for middle to late TM. Explanation: destabilizing orbital interactions.

References: Martinho Simöes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629 Ziegler, T. Pure & Appl. Chem. 1991, 63, 873 (DFT) 9

General trends

M-C bond enthalpy increases within a TM group. In main group alkyls, the M-C bond enthalpy decreases down in a group.

M-C: 35-70 kcal/mol Similar to main group alkyl bond strength. Comparable to the strength of a C-I bond.

The instability of TM alkyls is kinetic not thermodynamic. One strategy to isolate TM alkyls is to block available orbitals by

using coordinating ligands (bipy, phosphines).

10

Decomposition temp (°C)

Decomposition temp (°C)

TiMe4

TiEt4

~ -50 not observed

PbMe4

PbEt4

~ 200 (b.p. 110)~ 100

Kinetic instability: elimination

Modes of blocking -H elimination No -hydrogens The alkyl is oriented so that the beta position cannot access the metal

center (steric bulk or rigidity). The alkyl would give an unstable alkene as the product.

11

CMe3

M

neopentyl

CMe2Ph

M

neophyl

SiMe3

M

"silyl-neopentyl"Ph

M

benzyl

M R

M

alkynyl

norbornyl

LnMCH2 CH2

H -H elimination

LnM

H

H2C

CH2LnM H H2C CH2+

(or )-H elimination -hydride elimination

-hydride elimination

12

LnMHC

H

R

-H eliminationLnM

CHR

H

Li

tBu

Ta

Cl

CltBu

tBu

tBu

Zn

tBu

2Ta

tBu

tBu

tButBu

tBuHH

HH

Ta

tButBu

tBu

tBu

TaCl53 -H elimination

not isolated

Cp

ThCp

tBu

tBu

Cp

ThCp

tBu

Cp

ThCp

tBu

CH2

-agosticinteraction

H

Cp

ThCp

tBu

CH2

H

-bondmetathesis

starting

-bondmetathesis

ending

50 oC, 60 h-H elimination

Other decomposition modes

Reductive elimination: isolable alkyl hydrides are rare because the reaction is kinetically facile and thermodynamically favorable. The reverse reaction: C-H activation

Homolytic cleavage: rare

13

LnM

R

H

reductive

eliminationMLn + R-H LnM

R

X

reductive

eliminationMLn + R-Xbut

CoN

N N

N

Me

Me

+

- e-

Co(III)

CoN

N N

N

Me

Me

2+

Co(IV)unstable

CoN

N N

N

Me2+

Co(III)

+ CH3

Empirical relative stabilities 1-norbornyl > benzyl > trimethylsilyl > neopentyl > Ph ~ Me >> Et (1° R) > 2 °,

3 ° R Fluoroalkyl > alkyl (i.e. -CnF2n+1 > -CnH2n+1 for late TMs) CF bonds are very

strong (120-130 kcal/mol vs. 98-104 kcal/mol for alkyl C-H). Chelating (metallacycles) > nonchelating (acyclic)

3rd row > 2nd row > 1st row transition metals

14

PtL

LPt

L

L HPt

L

Lvs.

kdec = 1.0 s-1 (110 oC) kdec = 5.3 x 10-3 s-1 (110 oC)

2 MeI

RTno isolable alkyl

2 MeI

RT(OC)2Os

Me

Me

[Fe(CO)4]2-

[Os(CO)4]2-

Metal alkyls: bonding and characterization 1H and 13C NMR

for the C and H atoms to the metal are shifted to high field vs. those in the parent alkane.

Coupling of the 13C and 1H nuclei of the alkyl to the metal with spin I = ½: 103Rh (100% abundance); 195Pt (34%); 183W (14%); 199Hg (17%); 187Os (1.6%) or to phosphines (if present).

X-ray: M-C bonds are 1.9 – 2.2 Å

Reactivity M-R + Br2 gives M-Br M-R + HgCl2 gives R-HgCl

15

C

MR

RR

C

Al

CMe

MeAl

Me

Me

HH H

H H H

H C

M

HH

M

M C M

HH

H

Synthesis: nucleophilic attack on the metal

Very useful and rather general method Common reagents are RLi, RMgX (or R2Mg), ZnR2, AlR3, BR3, and PbR4.

16

ClFe

Cp

OCOC

+ RMgX RFe

Cp

OCOC

+ MgX2

CrCl3AlMe3

THFMeCrCl2(THF)3

using RLi leadsto reduction

TiCl4 + 4 MeLiEt2O

TiMe4(Et2O)2

very unstable(decomp. at -60 oC)

dmpeTiMe4(dmpe)

very stable

TaCl5 + 3/2 ZnMe2C5

[TaMe2Cl3]x

2 MeLiTaMe5 yellow

melts ~ 25 oCdecomposes to "TaCxHy" by losing CH4 and H2

WCl6 + 6 AlMe3C5

WMe66 AlMe2Cl

Wilkinson, 1973red

decomposes ~ 35 oC explosively!

Metal hydrides: importance and characterization

1931, Hieber reports H2Fe(CO)4

1955 - 1964, Cp2ReH, PtHCl(PR3)2, K2[ReH9] M-H bonds can undergo insertion reactions with unsaturated substrates Characterization

1H NMR +25 to -60 ppm (usual: -5 to -15 ppm) Coupling

With the metal (if it has I = 1/2) With cis and trans phosphines: stereochemistry determination With each other (if inequivalent, J = 1 – 10 Hz)

IR: (M-H) 1500 – 2000 cm-1, not very useful (weak intensities) Neutron diffraction (vs. X-ray diffraction): large crystals are

needed (1 mm3 vs. 0.01 mm3)

17

LnM-H

Metal hydrides: synthesis from a ligand From alkyl ligands:

-hydride elimination: Ziegler-Natta polymerization, carbene formation

-hydride elimination: stability of metal-alkyl complexes

From other ligands

18

LnMHC

H

R

-H eliminationLnM

CHR

H

LnMCH2 CH2

H -H elimination

LnM

H

H2C

CH2LnM H H2C CH2+

RuCl2(PPh3)3 + 2 KOCHMe2 + PPh3 RuH2(PPh3)4 + Me2CO + 2 KCl

Cr(CO)6 + HO- [(OC)5Cr-H-Cr(CO)5]-

- CO2

Cr(CO)6

- CO[Cr(CO)5(COOH)]- [CrH(CO)5]-

Silyl complexes: M-SiR3 (R = alkyl, aryl, OR) First complex: CpFe(CO)2(SiMe3), Wilkinson 1956

Trimethylsilyl (TMS) complexes are more numerous than t-Bu complexes (rare) -elimination inhibited due to instability of the Si=C bonds Sterically less congested because M-Si is longer than M-C M-SiR3 bonds are stronger than M-C bonds due to -interaction between M and SiR3 fragment

Most common synthesis method: oxidative addition of Si-H bonds (in contrast to C-H activation)

Rich chemistry http://www.cchem.berkeley.edu/tdtgroup/organometallic.html

19

SiR3

-backdonation

-donation

* empty

d filled

LnMH

SiR3

+ LnMH

SiR3

LnMH

SiR3

LnMH

SiR3

Na[Fe(CO)2Cp] + ClSiMe3 Cp(CO)2Fe-SiMe3 + NaCl

Amide, oxides, and halide ligands Extra lone pairs present on the

heteroatom Late TM: when 18e, repulsion

between lone pairs and filled d orbitals; weakening of the M-heteroatom bond

Early TM: when d0, empty orbitals available for interaction: strengthening of the M-heteroatom bond

20

Ru

TiSi

NCl

Cl

Dowpropene polymerization

catalyst

H2N

NTs

Noyorihydrogenation catalyst

Mo Mo

OO

O

O

OO

OO

t-But-But-Bu

t-Bu120o

O

TaO C

O

(t-Bu)3Si

(t-Bu)3Si

(t-Bu)3Si

O

Ta OC

O

Si(t-Bu)3

Si(t-Bu)3

Si(t-Bu)3

180o