Cationic hafnium silyl complexes and their enhanced reactivity in σ-bond metathesis processes with Si-H and C-H bonds

Article abstract of DOI:10.1021/ja030024g

Reaction of the mixed-ring silyl methyl complex CpCp*Hf[Si(SiMe₃)₃]Me (4) with B(C₆F₅)₃ in bromobenzene-d₅ yielded the zwitterionic hafnium silyl complex [CpCpHfSi(SiMe₃)₃][MeB (C₆F₅)₃] (7), which is stable for at least 12 h in solution. Addition of PhSiH₃ to 7 rapidly produced HSi(SiMe₃)₃, CpCp*HfH(μ-H)B(C₆F₅)₃, and oligomeric silane products. Reactions of CpCp*Hf(SiR₃)Me (SiR₃ = Si^tBuPh₂, SiHMes₂) with B(C₆F₅)₃ rapidly produced HSiR₃ in quantitative yield along with unidentified hafnium-containing species. However, reactions of Cp₂Hf(SiR₃)Me (SiR₃ = Si(SiMe₃)₃ (8), Si^tBuPh₂ (9), SiPh₃ (10)) with B(C₆F₅)₃ quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp₂Hf(Si^tBuPh₂) (μ-Me)B(C₆F₅)₃ (13) was isolated by crystallization from toluene at -30 °C and fully characterized, and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The σ-bond metathesis reaction of 13 with Mes₂SiH₂ yielded HSi^tBuPh₂ and the reactive species Cp₂Hf(η²-SiHMes₂) (μ-Me)B(C₆F₅)₃ (16, benzene-d₆), which was also generated by reaction of Cp₂Hf(SiMes₂H)Me (11) with B(C₆F₅)₃. Spectroscopic data provide evidence for an unusual α-agostic Si-H interaction in 16. At room temperature, 16 reacts with benzene to form Cp₂Hf(Ph)(μ-Me)B(C₆F₅)₃ (17), and with toluene to give isomers of Cp₂Hf(C₆H₄Me) (μ-Me)B(C₆F₅)₃ (18-20) and Cp₂Hf(CH₂Ph)(μ-Me)B (C₆F₅)₃ (21). The reaction with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (ΔH? = 19(1) kcal/mol; ΔS? = -17(3) eu), a large primary isotope effect (k_H/k_D = 6.9(7)), and the experimentally determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond activation.

Full text of DOI:10.1021/ja030024g

Published on Web 07/12/2003
Cationic Hafnium Silyl Complexes and Their Enhanced
Reactivity in σ-Bond Metathesis Processes with Si-H and
C-H Bonds
Aaron D. Sadow and T. Don Tilley*
Contribution from the Department of Chemistry, UniVersity of California, Berkeley,
Berkeley, California 94720-1460
Received January 9, 2003; E-mail: tdtilley@socrates.berkeley.edu
Abstract: Reaction of the mixed-ring silyl methyl complex CpCp*Hf[Si(SiMe3)3]Me (4) with B(C6F5)3 in
bromobenzene-d5 yielded the zwitterionic hafnium silyl complex [CpCpHfSi(SiMe3)3][MeB(C6F5)3] (7), which
is stable for at least 12 h in solution. Addition of PhSiH3 to 7 rapidly produced HSi(SiMe3)3, CpCp*HfH(µ-
H)B(C6F5)3, and oligomeric silane products. Reactions of CpCp*Hf(SiR3)Me (SiR3 ) SitBuPh2, SiHMes2)
with B(C6F5)3 rapidly produced HSiR3 in quantitative yield along with unidentified hafnium-containing species.
However, reactions of Cp2Hf(SiR3)Me (SiR3 ) Si(SiMe3)3 (8), SitBuPh2 (9), SiPh3 (10)) with B(C6F5)3
quantitatively produced the corresponding cationic hafnium silyl complexes 12-14. The complex Cp2Hf-
(SitBuPh2)(µ-Me)B(C6F5)3 (13) was isolated by crystallization from toluene at -30 °C and fully characterized,
and its spectroscopic properties and crystal structure are compared to those of its neutral precursor 9. The
σ-bond metathesis reaction of 13 with Mes2SiH2 yielded HSitBuPh2 and the reactive species Cp2Hf(η2-
SiHMes2)(µ-Me)B(C6F5)3 (16, benzene-d6), which was also generated by reaction of Cp2Hf(SiMes2H)Me
(11) with B(C6F5)3. Spectroscopic data provide evidence for an unusual R-agostic Si-H interaction in 16.
At room temperature, 16 reacts with benzene to form Cp2Hf(Ph)(µ-Me)B(C6F5)3 (17), and with toluene to
give isomers of Cp2Hf(C6H4Me)(µ-Me)B(C6F5)3 (18-20) and Cp2Hf(CH2Ph)(µ-Me)B(C6F5)3 (21). The reaction
with benzene is first order in both 16 and benzene. Kinetic data including activation parameters (∆H‡ )
19(1) kcal/mol; ∆S‡ ) -17(3) eu), a large primary isotope effect (kH/kD ) 6.9(7)), and the experimentally
determined rate law are consistent with a mechanism involving a concerted transition state for C-H bond
activation.
Introduction
(e.g., Si-H) are generally more facile than C-H bond activa-
tions and may therefore be mediated by moderately electrophilic,
σ-Bond metathesis holds considerable promise for the de-
velopment of new catalytic transformations. This fundamental
bond activation mechanism, which involves concerted, four-
centered electrocyclic transitions states, was first described for
hydrogenations of d0 metal alkyl complexes.1 Such processes
are mediated by electrophilic, high valent transition metal or
f-element centers (d0 or fnd0) containing at least one open
coordination site adjacent to a reactive σ-bond.2 σ-Bond
metathesis steps have been implicated in catalytic reactions of
main group element-hydrogen bonds (E-H, E ) B, Si, P, Sn),
including dehydropolymerizations and the addition of E-H
bonds to olefins.3-8 Activations of the element-hydrogen bonds
16-electron group 4 complexes of the type Cp′2MR2 (M ) Ti,
Zr, Hf; Cp′2 ) Cp2, CpCp*, Cp*2, where Cp* ) η5-C5Me5; R
) H, alkyl, silyl).3,9 A more reactive class of compounds are
14-electron complexes of the type Cp*2MR (M ) Sc, Lu, Y; R
) hydride, alkyl), which can react with inert hydrocarbons,
including methane.10,11 However, although high valent com-
plexes capable of activating C-H bonds by σ-bond metathesis
(5) (a) Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1995, 117, 12645.
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9
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J. AM. CHEM. SOC. 2003, 125, 9462-9475
10.1021/ja030024g CCC: $25.00 © 2003 American Chemical Society

Cationic Hafnium Silyl Complexes
A R T I C L E S
Scheme 1. A Possible Catalytic Cycle for Hydrocarbon
Functionalization Involving σ-Bond Metathesis Steps
to silicon, and (3) the dehydrocoupling of Si-H and M-H
bonds to form a metal silyl derivative.18 The latter two steps of
this cycle have precedent in observed reactions of d0 complexes,
including those of hafnium.3,4,17,19 However, the activation of
C-H bonds via σ-bond metathesis with a metal-silicon bond
has not previously been reported. To develop more reactive d0
silyl complexes which might exhibit this chemistry, efforts were
directed toward the synthesis and study of cationic hafnium silyl
complexes.20 Herein, we report the first examples of zwitterionic
hafnium silyl complexes and their ability to activate Si-H and
C-H bonds.
Results
Synthesis of CpCp*Hf(SiR3)Me (SiR3 ) Si(SiMe3)3,
SitBuPh2, SiHMes2) Complexes. The neutral mixed-ring haf-
nium silyl methyl complexes CpCp*Hf(SiR3)Me (SiR3
)
have been known for almost 20 years, catalytic cycles incor-
porating these reactions have been limited to the alkylation of
pyridine.12 Recently, we reported the first examples of methane
conversions involving σ-bond metathesis steps, the hydro-
methylation of propene, and the dehydrosilation of methane.13,14
The study described here targets the development of new d0
complexes designed to expand the chemistry of element-
hydrogen bond activations and mediate new catalytic reactions.
The electrophilic complexes of interest are cation-like group 4
complexes that are expected to be highly reactive in σ-bond
metathesis reactions, based on analogies to cationic catalysts
in olefin polymerization.15,16 Previous studies have shown that
zwitterionic and cationic hafnium complexes containing methyl
and hydride ligands are highly reactive toward organosilanes
and more reactive than analogous neutral complexes.17 For
example, the zwitterionic hafnium hydride complex CpCp*HfH-
(µ-H)B(C6F5)3 rapidly catalyzes the conversion of PhSiH3 to
cross-linked oligosilanes and Ph2SiH2 by dehydrocoupling and
redistribution reactions involving Si-H and Si-C bond activa-
tions, respectively (eq 1). The corresponding neutral complex
CpCp*HfHCl more slowly (by > 1 order of magnitude)
catalyzes only the dehydropolymerization of PhSiH3.3,4
Si(SiMe3)3, SitBuPh2, SiHMes2) were prepared via silylation
of CpCp*HfCl2, followed by methylation. The hafnium silyl
chloride complexes 1-3 were prepared by the method of eq 2.
Treatment of compounds 1-3 with methyl Grignard in diethyl
ether at -78 °C afforded the desired hafnium silyl methyl
complexes 4-6 (eq 3). Compounds 1-6 are yellow, crystalline
materials, which are somewhat thermally and photochemically
sensitive and decompose in the solid state over 1 week at room
temperature.9,21 Thus, the complexes may be stored for extended
periods (∼3 months) in the dark at -30 °C.
Generation of [CpCp*HfSi(SiMe3)3][MeB(C6F5)3] (7). The
strong Lewis acid B(C6F5)3 has been reported to react with early
transition metal methyl complexes, such as Cp*2ZrMe2, via
methide abstraction.22 Similarly, in bromobenzene-d5, compound
4 reacted with B(C6F5)3 to produce [CpCp*HfSi(SiMe3)3][MeB-
(C6F5)3] (7), which is stable in solution at room temperature
1
for several hours. The H NMR spectrum of 7 contains two
Strategies for the development of catalytic hydrocarbon
conversions might be based on activation of a C-H bond by a
reactive metal silyl derivative, since metal-silicon bonds in d0
complexes are generally quite reactive (e.g., more reactive than
comparable metal-carbon bonds).3,9 A possible catalytic process
involving σ-bond metathesis is the dehydrosilation of hydro-
carbons, which would couple C-H and Si-H bonds to give an
Si-C bond and hydrogen. A three-step catalytic cycle for this
reaction (Scheme 1) involves (1) a C-H bond activation step,
(2) the well-known transfer of an organic group from the metal
sharp singlets corresponding to the Cp and Cp* ligands and
two broad singlets for the Si(SiMe3)3 (0.3 ppm, 14 Hz width at
half-height) and MeB(C6F5)3- (1.28 ppm, 18 Hz at half-height)
groups. The Si(SiMe3)3 resonance is further broadened at -20
°C (to 90 Hz at half-height) but begins to sharpen at the low
temperature limit for the bromobenzene-d5 solvent (mp ) -31
°C). At that temperature, the resonance splits into several new
SiMe3 peaks, suggesting hindered rotation about the Hf-Si
(18) King, W. A.; Marks, T. J. Inorg. Chim. Acta 1995, 229, 343.
(19) (a) Radu, N. S.; Hollander, F. J.; Tilley, T. D.; Rheingold, A. L. J. Chem.
Soc., Chem. Commun. 1996, 2459. (b) Castillo, I.; Tilley, T. D. Organo-
metallics 2000, 19, 4733. (c) Castillo, I.; Tilley, T. D. J. Am. Chem. Soc.
2001, 123, 10526.
(20) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 6814.
(21) (a) Tilley, T. D. Organometallics 1985, 4, 1452. (b) Arnold, J.; Roddick,
D. M.; Tilley, T. D.; Geib, S. J. Inorg. Chem. 1988, 27, 3510. (c) Roddick,
D. M.; Heyn, R. H.; Tilley, T. D. Organometallics 1989, 8, 324.
(22) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1991, 113,
3623. (b) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994,
116, 10015. (c) Beck, S.; Geyer, A.; Brintzinger, H.-H. J. Chem. Soc., Chem.
Commun. 1999, 2477.
(12) (a) Jordan, R. F.; Taylor, D. F. J. Am. Chem. Soc. 1989, 111, 778. (b)
Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990, 9,
1546. (c) Rodewald, S.; Jordan, R. F. J. Am. Chem. Soc. 1994, 116, 4491.
(13) Sadow, A. D.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 7971.
(14) Sadow, A. D.; Tilley, T. D. Angew. Chem., Int. Ed. 2003, 42, 803.
(15) (a) Imori, T.; Tilley, T. D. Polyhedron 1994, 13, 2231. (b) Dioumaev, V.
K.; Harrod, J. F. Organometallics 1994, 13, 1548.
(16) (a) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles
and Applications of Organometallic Chemistry; University Science
Books: Mill Valley, CA, 1987; p 577.
(17) Sadow, A. D.; Tilley, T. D. Organometallics 2001, 20, 4457.
9
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A R T I C L E S
Sadow and Tilley
bond. The latter methyl borate resonance is shifted significantly
downfield from that of the HfMe groups of 4 and CpCp*HfMe-
(µ-Me)B(C6F5)3 (-0.55 and 0 ppm, respectively)3c,17 and from
that of the bridging methyl borate group of CpCp*HfMe(µ-
Me)B(C6F5)3 (ca. 0 ppm, bromobenzene-d5).17 This downfield-
shifted resonance for the methyl group of MeB(C6F5)3- suggests
greater cation-anion separation than in CpCp*HfMe(µ-Me)B-
(C6F5)3 (in benzene-d6 and bromobenzene-d5). Compound 7 is
sparingly soluble in benzene-d6 and benzene-d6/bromobenzene-
d5 mixtures (the compound precipitates from solution as an oil
over 10 min). The solubility properties and the 1H NMR
spectrum of 7 imply that it contains significant charge separation.
However, in these relatively nonpolar, noncoordinating solvents,
tight ion pairing is expected.22
The upfield-shifted singlet at -15.04 ppm in the 11B NMR
spectrum of 7 indicates the presence of a four-coordinate borate
anion.23 Interestingly, the 29Si{1H} NMR resonances of 7 (-5.10
ppm, Si(SiMe3)3; -87.45 ppm, Si(SiMe3)3) are not significantly
shifted from those of the precursor 4, suggesting that the
electronic environment of the silyl ligand is not notably affected
by the change in formal charge at the hafnium center. Within
24 h at room temperature, 7 decomposed to HSi(SiMe3)3 and
several unidentified hafnium-containing products. The com-
pound was stable overnight in solution at -30 °C as a
bromobenzene-d5 solution but did not crystallize and could not
be isolated free of impurities.
synthesized. Unfortunately, the addition of B(C6F5)3 to these
compounds resulted in rapid elimination of the corresponding
silane, and no hafnium complexes could be isolated from the
reaction mixtures. Reactions of 5 or 6 with B(C6F5)3 in benzene-
d6 or bromobenzene-d5 produced a rapid color change to red
that was immediately followed by a bleaching to pale yellow.
The reactions of 5 and 6 with B(C6F5)3 in the presence of Lewis
bases (PMe3, pyridine, DMSO) also quantitatively produced the
corresponding free silane (1H NMR spectroscopy, benzene-d6,
5 min at room temperature). Methide abstraction from 5 in the
presence of diphenylacetylene (room temperature, benzene-d6)
quantitatively produced HSitBuPh2 along with a single hafnium-
1
containing product. The H NMR spectrum of this product
contains a singlet for the Cp ligand (4.91 ppm, 5 H) and a
complicated set of resonances corresponding to the Cp* ligand.
This species was not isolated, but based on its 1H NMR
spectrum, we postulate that the Cp* ligand is involved in a
decomposition of the initially formed silyl complex.
Synthesis of Neutral Precursor Complexes Cp2Hf(SiR3)-
Me (SiR3 ) Si(SiMe3)3 (8), SitBuPh2 (9), SiPh3 (10), SiHMes2
(11)). The possible involvement of the Cp* ligand in the rapid
decomposition of [CpCp*Hf(SiR3)]+ species suggested that
cationic hafnium silyl complexes might be isolable for ligand
sets possessing more inert C-H bonds (e.g., Cp2). The
complexes Cp2Hf(SiR3)Me (SiR3 ) Si(SiMe3)3 (8), SitBuPh2
(9)) were prepared by a two-step method analogous to the
synthesis of the mixed-ring hafnium silyl methyl derivatives,
involving reactions of Cp2HfCl2 with the appropriate silyl anion
reagent, followed by reaction of the resulting silyl complex with
methyl Grignard. The compounds Cp2Hf(SiPh3)Me (10) and
Cp2Hf(SiHMes2)Me (11) were prepared by the reaction of Cp2-
HfMeCl with the corresponding silyl anion reagent. Notably,
the reaction of Cp2HfCl2 with (THF)2.5LiSiHMes2 (in toluene
or Et2O, at -78 °C in the dark) did not yield Cp2Hf(SiHMes2)-
Cl.21c Like the mixed-ring complexes 1-6, the compounds 8-11
are somewhat thermally unstable and moderately sensitive to
room light. These compounds decomposed to HSiR3 and
unidentified hafnium-containing products over 1 week under
ambient lighting at room temperature but were stable for at least
4 months when stored in the dark at -30 °C. The X-ray crystal
structure of 9 was determined (Figure 1), and relevant structural
parameters for 9 are listed in Tables 1 and 2.
Other attempts to isolate the zwitterionic complex 7 were
unsuccessful. In benzene-d6, reaction of compound 4 with
B(C6F5)3 resulted in formation of a deep red solution; as noted
above, after ca. 10 min a red oil precipitated from solution. The
1H NMR spectrum of the reaction mixture contained several
broad resonances corresponding to Cp, Cp*, Si(SiMe3)3, and
-
MeB(C6F5)3 groups, and the quantitative formation of HSi-
(SiMe3)3 was observed after 24 h. Dissolution of 4 in pentane
followed by addition of B(C6F5)3 immediately produced a red
1
powdery precipitate, but the H NMR spectrum (benzene-d6)
of this product is the same as that of the mixture formed in
benzene-d6. Although 7 reacted with THF in benzene-d6 to form
Cp*CpHf[Si(SiMe3)3](THF)[MeB(C6F5)3] (7-THF; as indicated
1
by H NMR spectroscopy), the complex could not be isolated
free from HSi(SiMe3)3.
Reactions of 7 with primary and secondary silanes such as
PhSiH3, Ph2SiH2, and H2Si(SiMe3)2 at room temperature in
bromobenzene-d5 rapidly produced CpCp*HfH(µ-H)B(C6F5)3,
HSi(SiMe3)3, and unidentified silane products (t1/2 ≈ 5 min).
By comparison, reaction of the neutral hafnium silyl CpCp*Hf-
[Si(SiMe3)3]Cl with 3 equiv of PhSiH3 proceeded with a much
longer half-life (t1/2 ≈ 2 h) and yielded the previously reported
complex CpCp*Hf(SiH2Ph)Cl.3 Like the σ-bond metathesis
reactions of borane-activated complexes CpCp*HfMe(µ-Me)B-
(C6F5)3 and CpCp*HfH(µ-H)B(C6F5)3 with silanes,17 those of
the cationic complex 7 exhibit dramatically enhanced rates
relative to those of neutral analogues.
Attempted Syntheses of [CpCp*HfSiR3][MeB(C6F5)3] (SiR3
) SitBuPh2, SiHMes2). With the hope that other silyl ligands
might impart stability and/or crystallinity to a cationic hafnium
silyl complex, the precursor hafnium silyl methyl compounds
CpCp*Hf(SitBuPh2)Me (5) and CpCp*Hf(SiHMes2)Me (6) were
Synthesis of Cp2Hf(SiR3)(µ-Me)B(C6F5)3 (SiR3 ) Si-
(SiMe3)3 (12), SitBuPh2 (13), SiPh3 (14)). The bright yellow
solutions (benzene-d6 or toluene-d8) of 8-10 turned deep red
1
upon addition of B(C6F5)3 at room temperature, and H NMR
spectroscopy confirmed the quantitative formation of Cp2Hf-
(SiR3)(µ-Me)B(C6F5)3 (SiR3 ) Si(SiMe3)3 (12), SitBuPh2 (13),
SiPh3 (14); eq 4). Unlike the mixed ring system, compounds
12-14 were stable in benzene-d6 solution in the dark for
approximately 12 h. Complex 13 was isolated by crystallization
from toluene at -30 °C and characterized by X-ray crystal-
lography. Compounds 12 and 14 did not crystallize from toluene
(-78 °C) and were isolated as impure oils after the solvent was
removed under reduced pressure.
(23) Kidd, R. G. NMR of Newly Accessible Nuclei, Vol. 2; Academic Press:
1983.
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Cationic Hafnium Silyl Complexes
A R T I C L E S
Figure 2. ORTEP diagram of Cp2Hf(SitBuPh2)(µ-Me)B(C6F5)3 (13). The
hydrogen atoms on the Cp rings and on the substituents on the silyl ligand
were removed for clarity.
a singlet in the 11B NMR spectrum (-13.03 ppm) and by a
1
cross-peak between the methyl group and the boron in a H-
11B HMQC spectrum. As in mixed-ring zwitterionic 7, the
29Si{1H} NMR resonance of zwitterionic 13 is not shifted
significantly from that of the neutral precursor 9 (48.76 vs 52.66
ppm). Crystals of 14 were stable for at least 4 months at -30
°C but decomposed at room temperature over 2 days. Compound
14 decomposed slowly over the course of 4 days in benzene-
d6, at room temperature in the dark, with elimination of tBuPh2-
SiH.
Figure 1. ORTEP diagram of Cp2Hf(SitBuPh2)Me (9). The hydrogen atoms
on the Cp rings and on the substituents on the silyl ligand were removed
for clarity.
Table 1. Selected Bond Lengths (Å) for Cp2Hf(SitBuPh2)Me (9)
and Cp2Hf(SitBuPh2)(µ-Me)B(C6F5)3 (13)
9
13
Hf1-Si1
2.835(2)
2.287(6)
na
1.957(7)
1.899(7)
1.907(7)
2.200(2)
2.1677(5)
2.851(3)
2.48(1)
1.66(2)
1.92(1)
1.88(1)
1.90(1)
2.1605(5)
2.1694(5)
Hf1-C1
B1-C1
The spectroscopic features of zwitterionic hafnium silyl
species 12 and 14 are similar to those of 13. In compounds 12
and 14, the 11B NMR resonances (-13.10 and -13.11 ppm,
respectively, benzene-d6) establish the presence of a tetrahedral
borate center, and broad 1H NMR resonances (-0.33 and -0.15
ppm, respectively) corresponding to the abstracted methide
groups indicate the presence of bridging Hf‚‚‚Me-B interac-
tions. The 29Si{1H} NMR signal of 14 (39.41 ppm) does not
shift significantly from that of the neutral precursor 10 (37.18
ppm). In complex 12, however, the 29Si{1H} resonance of the
central Si (bonded to Hf) appears at -21.39 ppm, which is
substantially downfield of that of the neutral precursor (-84.12
Si-C12 (tBu)
Si1-C16 (Ph)
Si1-C22 (Ph)
Hf1-Cpcent
Hf1-Cpcent
Table 2. Selected Bond Angles for Cp2Hf(SitBuPh2)Me (9) and
Cp2Hf(SitBuPh2)(µ-Me)B(C6F5)3 (13)
9
13
Si1-Hf1-C1
93.6(2)
119.7(2)
105.9(2)
113.6(2)
106.7(3)
104.6(3)
105.4(3)
na
97.1(3)
117.2(4)
105.7(4)
116.2(4)
110.4(6)
101.7(6)
105.1(6)
162.0(9)
129.77(2)
113(1)
Hf1-Si1-C12 (tBu)
Hf1-Si1-C16 (Ph)
Hf1-Si1-C22 (Ph)
C12-Si1-C16
C12-Si1-C22
C16-Si1-C22
Hf1-C1-B1
1
ppm). In the H NMR spectrum of 12 at 190 K (toluene-d8),
the SiMe3 resonance splits into two broad singlets at 0.18 and
0.03 ppm, which integrate to 18 and 9 H, respectively, while
the Cp ligands remain equivalent. This low temperature 1H NMR
spectrum is consistent with a structure in which rotation of the
Si(SiMe3)3 group with respect to the Cp2Hf-framework is
hindered. Complexes 12 and 14 slowly decomposed at room
temperature over several days to produce the free silane and
mixtures of unidentified hafnium-containing products.
Cpcent-Hf1-Cpcent
C1-B1-C30
C1-B1-C36
C1-B1-C42
132.41(1)
na
na
104(1)
111(1)
na
Formation of zwitterionic 13 in benzene-d6 results in the
X-ray quality crystals of 13 were obtained from a concentrated
toluene solution cooled to -30 °C. The solid-state structure
confirms the formation of the zwitterionic species (Figure 2,
see Table 3 for data collection parameters). Selected bonds
lengths and angles are listed in Tables 1 and 2 along with
relevant structural parameters for 9. The Hf-C(Me) bond
distance (2.48(1) Å) is longer than that in 9 (2.287(6) Å); the
difference of hafnium-carbon(Me) distances (in 13 vs 9; 0.19
1
appearance of a broad signal at -0.25 ppm in the H NMR
spectrum, attributed to the bridging methyl group. For com-
parison, the 1H NMR resonance of free [MeB(C6F5)3]- (as the
counterion of [nBu3NCH2Ph]+) appears at 1.12 ppm in benzene-
d6.24 This indicates that in 13 there is a significant interaction
between the methide and the hafnium center. Also, cross-peaks
1
in the H NOESY spectrum of 13 confirm that the methide
remains in close proximity to the ligands on hafnium. Formation
of a tetrahedral methyl borate is confirmed by the presence of
Å) is smaller than those reported by Marks for CpR ZrMe(µ-
2
Me)B(C6F5)3 (CpR ) C5Me5, C5H3Me2, C5H3(SiMe3)2; Zr‚‚‚
Me ) 0.297-0.407 Å).22 The shorter M‚‚‚CH3B(C6F5)3 distance
in 13 is likely a consequence of weaker steric pressure exerted
(24) Radzewich, C E.; Guzei, I. A.; Jordan, R. F. J. Am. Chem. Soc. 1999, 121,
8673.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9465

A R T I C L E S
Sadow and Tilley
Table 3. Crystallographic Data for Compounds 9, 13, and 15
9
13
15
empirical formula
formula weight
crystal color, habit
crystal size (mm3)
crystal system
space group
a (Å)
HSiC27H32
563.13
yellow, blocks
0.27 × 0.12 × 0.08
triclinic
P1h(#2)
8.2187(2)
9.3202(2)
17.0472(4)
104.346(2)
91.339(2)
110.93(8)
1172.5(4)
HfSiC45BF15H32
1075.11
Hf2B2Br4C79F40H25
2432.22
yellow, plates
0.46 × 0.35 × 0.06
triclinic
red, blocks
0.17 × 0.13 × 0.10
orthorhombic
P21212 (#18)
20.1825(2)
21.1290(5)
10.0160(2)
90
P1h(#2)
14.6430(2)
15.23140(10)
17.3616(3)
95.234(1)
b (Å)
c (Å)
R (deg)
â (deg)
90
90
100.754(1)
104.326(1)
3647.31(9)
9636, 3.5-45.0
γ (deg)
V (Å3)
4271.2(3)
4849, 3.5-45.0
orientation reflection:
number, 2θ range (deg)
Z
3330, 3.5-45.0
2
4
2
D
calc (g/cm3)
1.595
1.672
2.214
F000
560.00
45.02
SMART
Mo KR
2112.00
25.64
SMART
Mo KR
2306.00
51.89
SMART
Mo KR
µ (Mo KR) (cm-1
diffractometer
radiation
)
(λ - 0.71069 Å)
(λ - 0.71069 Å)
(λ - 0.71069 Å)
Temperature (K)
scan type
scan rate
151
174
151
ω(0.3° per frame)
10.0 s per frame
52.3
ω(0.3° per frame)
10.0 s per frame
52.3
ω(0.3° per frame)
10.0 s per frame
49.4
data collected,
2θmax (deg)
reflections measured
total: 5673
unique: 3978
0.025
total: 207 40
unique: 7615
0.073
total: 18 385
unique: 11 518
0.039
Rint
transmission factors
Tmax ) 0.65
Tmin ) 0.46
SIR92
Tmax ) 0.81
Tmin ) 0.61
SIR92
Tmax ) 0.90
Tmin ) 0.41
SIR92
structure solution
no. of observed data
[I > 3σ(I)]
no. of parameters refined
data/parameter ratio
final residuals:
3194
3988
8562
262
12.19
0.033; 0.036; 0.048
594
6.71
1111
7.71
0.047; 0.059; 0.064
0.037; 0.036; 0.091
R; Rw; Rall
goodness of fit indicator
max shift/error
1.14
0.00
0.97
0.14
1.64
0.00
final cycle
max and min peaks,
final diff. map (e-/Å3)
0.89, -1.88
1.08, -0.79
2.07, -2.85
by its substantially smaller, unsubstituted Cp rings. The Si-
Hf-C(Me) angle increases from 93.6(2)° to 97.1(3)° upon
formation of the zwitterionic species. The B-C1 bond distance
of 1.66(2) Å is identical to the corresponding distance reported
for Cp*2ZrMe(µ-Me)B(C6F5)3.22 The locations of the hydrogen
atoms on the bridging methide were calculated based on a
tetrahedral geometry for the carbon, and their positions were
allowed to freely refine. In the resulting structure, the hydrogens
are directed toward the Hf center. The crystal structure also
confirms that partial methide abstraction does not significantly
perturb the overall coordination environment of the silicon atom.
The Hf-Si bond distance in 13 is lengthened by only 0.016 Å
relative to that in 9, and the only Si-C bond distance that
changes upon methide abstraction is that involving the tBu group
(lengthened by 0.04 Å).
Figure 3. ORTEP diagram of the cation in [Cp2Hf(µ-Br)]2[B(C6F5)4]2 (15).
Compounds 8-10 reacted with the methide abstraction agent
[Ph3C][B(C6F5)4] (benzene-d6, room temperature) with rapid
elimination of the corresponding free silane. However, in
bromobenzene-d5, the reaction of 9 with [Ph3C][B(C6F5)4]
produced Ph3CCH3 and one organometallic species, which was
crystallized by slow diffusion of pentane into the bromobenzene-
d5 solution at room temperature. X-ray crystallographic analysis
revealed the product to be the dimeric, dicationic complex [Cp2-
Hf(µ-Br)]2[B(C6F5)4]2 (15; Figure 3). Reaction of [Ph3C]-
[B(C6F5)4] with 9 in fluorobenzene produced a mixture of
unidentified products.
Synthesis and Characterization of Cp2Hf(η2-SiHMes2)-
(µ-Me)B(C6F5)3 (16) in Solution. As in the formation of 12-
14, the reaction of 11 with B(C6F5)3 yielded the partially
9
9466 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Cationic Hafnium Silyl Complexes
A R T I C L E S
Table 4. Spectroscopic Data for Mes2SiH2, 11, and 16
Mes2SiH
11
4.98
145
8.25
2077
16 (C D )
6 6
2
1H NMR (SiH)
1JSiH (Hz)
5.29
195
-61
2151
1.80
57
158
29Si{1H} NMR
ν(SiH)
1414
methide-abstracted hafnium silyl product, Cp2Hf(η2-SiHMes2)-
(µ-Me)B(C6F5)3 (16, quantitatively). At room temperature,
however, the resulting cationic hafnium silyl complex reacts
with benzene-d6 or toluene-d8, as described below. The reaction
of 11 with B(C6F5)3 in fluorobenzene, hexafluorobenzene,
chlorobenzene, and bromobenzene also produced 16 in good
1
yield (by H NMR spectroscopy), but in these solvents, the
complex decomposed to mixtures of products. In nonaromatic
hydrocarbon solvents such as pentane or cyclohexane, addition
of B(C6F5)3 to 11 rapidly induced precipitation of a powder
that contained 16 but was contaminated with Mes2SiH2. Thus,
complex 16 was generated and studied at -40 °C (in toluene-
d8), and under these conditions, it is stable for at least 8 h.
The most notable structural feature of 16 is an R-agostic SiH
interaction, which can be deduced from several distinctive
spectroscopic characteristics (see Table 4 for spectroscopic data
of Mes2SiH2, 11, and 16). In the 1H NMR spectrum of 16, the
SiH resonance at 1.80 ppm is far upfield of the typical range
for silicon hydrides (5-3.5 ppm).25 The 29Si NMR resonance
for the dimesitylsilyl ligand in 16 (158 ppm), detected via a
1H-29Si HMQC experiment, lies far downfield from the
resonance in 11 (8.3 ppm), indicating a substantial change in
Figure 4. Calculated gas-phase structure of [Cp2HfSiH3]+ (A).
Me)2YN(SiHMe2)2, with a â-agostic Si-H bond, exhibits a 1H
NMR shift of 2.97 ppm, a 1JSiH value of 142 Hz, and a ν(SiH)
frequency of 1804 cm-1 27
. Note that these comparisons suggest
strong donation of electron density from the Si-H bond to the
electrophilic hafnium center of 16. Harrod and co-workers have
isolated and spectroscopically characterized dimeric, dicationic
zirconocene hydrosilyl complexes formulated as [Cp′2Zr(µ-H)-
2+
(SiHR)]2 (R ) Ph, CH2Ph; Cp′ ) Cp, Cp*, C5H4Me), in
1
the electronic environment of the silicon atom. The JSiH
which very small SiH coupling constants (ca. 20 Hz) imply that
there is no direct interaction between the Zr-H and the Si
atom.28 R-Agostic interactions in alkyl complexes have been
extensively investigated due to their importance in olefin
polymerization.29 The R-agostic SiH interaction reported here
may be important in σ-bond metathesis reactions (Vide infra),
as complexes featuring such characteristics (i.e., 16) react with
the C-H bonds of arenes, while complexes lacking this agostic
interaction do not.
coupling constant of 57 Hz is dramatically decreased from the
corresponding value in 11 (150 Hz). This decrease is consistent
with substantial weakening of the Si-H bond resulting from
the R-agostic interaction with the cationic Hf center.26 The
strongly red-shifted Si-H stretching frequency at 1415 cm-1
(ν(SiD) ) 1015 cm-1) is also highly unusual and provides
additional support for the proposed R-agostic structure in 16.
Compound 16 reacts with DMSO to form the base-stabilized
complex Cp2Hf(SiHMes2)(DMSO)MeB(C6F5)3. Coordination of
DMSO to the cationic hafnium center disrupts the R-agostic
interaction, to produce a complex with spectroscopic data
corresponding to a typical hydrosilyl ligand (1H NMR: SiH )
Density Functional Theory (DFT) Calculations on the
+
Model Compounds Cp2HfSiH3 (A) and Cp2HfSiH(2,6-
Me2C6H3)2+ (B). To further explore the unusual bonding in 16,
density functional theory (B3LYP/LACVP**++) was used to
model the gas-phase structure of 16.30-32 The resulting structures
represent unsolvated, counterion-free models of compound 16.
The initial geometry of Cp2HfSiH3+ (A), modeled as a normal
hafnium silyl species with an Hf-Si σ-bond, a tetrahedral
geometry at silicon, and an overall charge of +1 (i.e., HfIV),
was minimized on the potential energy surface (PES). The final
geometry contains an R-agostic SiH moiety coordinated to the
Hf center and is consistent with the proposed structure of 16
(Figure 4, Table 5). The geometry of the silicon atom is severely
1
5.14 ppm, JSiH ) 145 Hz).21
Although this is the first example of a d0 R-agostic SiH
interaction, coordination of silicon-hydrogen bonds to transition
metal centers is common.25-28 For example, intramolecular
coordination of the â-Si-H bond (i.e., â-agostic) to the d0
centers in Cp2Zr[N(SiHMe2)tBu]X (X ) hydride, halide)
1
complexes results in similar upfield H NMR shifts for the
silicon hydride at 1.21-2.94 ppm, low 1JSiH coupling constants
of 113-135 Hz, and reduced values for the ν(SiH) stretching
frequency (1912-1998 cm-1).25 Similarly, Me2Si(η5-C9H5-2-
(29) (a) Grubbs, R. H.; Coates, G. W. Acc. Chem. Res. 1996, 29, 255. (b)
Clawson, L.; Soto, J. Buchwald, S. L.; Stiegerwald, M. L.; Grubbs, R. H.
J. Am. Chem. Soc. 1985, 107, 3377. (c) Piers, W. E.; Bercaw, J. E. J. Am.
Chem. Soc. 1990, 112, 9406. (d) Krauledat, H.; Britzinger, H.-H. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1412. (e) Cotter, D. W.; Bercaw, J. E. J.
Organomet. Chem. 1991, 417, C1. (f) Leclerc, M. K.; Brintzinger, H. H.
J. Am. Chem. Soc. 1995, 117, 1651.
(25) Procopio, L. J.; Carroll, P. J.; Berry, D. H. J. Am. Chem. Soc. 1994, 116,
177.
(26) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151.
(27) (a) Herrmann, W. A.; Eppinger, J.; Spiegler, M.; Runte, O.; Anwander, R.
Organometallics 1997, 16, 1813. (b) Hieringer, W.; Eppinger, J.; Anwander,
R.; Herrmann, W. A. J. Am. Chem. Soc. 2000, 122, 11983.
(28) (a) Dioumaev, V. K.; Harrod, J. F. J. Organomet. Chem. 1996, 521, 133.
(b) Dioumaev, V. K.; Harrod, J. F. Organometallics 1996, 15, 3859. (c)
Dioumaev, V. K.; Harrod, J. F. Organometallics 1997, 16, 2798. (d)
Dioumaev, V. K.; Rahimian, K.; Gauvin, F.; Harrod, J. F. Organometallics
1999, 18, 2249.
(30) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Volko, S. H.; Wilk,
L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (c) Lee, C.; Yang, W.; Parr,
R. G. Phys. ReV. B 1988, 37, 785.
(31) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. J. Chem. Phys. 1980, 72,
650.
(32) Jaguar 4.0, release 23; Schro¨dinger, Inc.: Portland, OR, 1998.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9467

A R T I C L E S
Sadow and Tilley
Table 5. Selected Bond Lengths (Å) and Angles (deg) for the
+
Calculated Structure Cp2HfSiH3 (A)
bond lengths
Hf-Si
Hf-Hbridge
Si-Hbridge
2.59
2.05
1.63
Si-Hterm
Hf-Cpcent
1.49
2.17
bond angles
Hf-Si-Hbridge
Hf-Hbridge-Si
Hf-Si-Hterm
52.3
88.6
124.6
Hterm-Si-Hterm
Hbridge-Si-Hterm
Cpcent-Hf-Cpcent
108.0
103.3
140.8
distorted to trigonal monopyrimidal, with the bridging hydride
occupying the pseudoaxial position. The Hf-Si-Hbridge and
Hterm-Si-Hbridge angles are 52.3° and 88.6°, respectively. The
Hf-Si-Hterm angles are identical (124.6°), and the Hterm-Si-
Hterm angle is 108.0°. The sum of these three angles is 357.3°,
indicating the near planarity of the silicon, the Hf, and the two
Hterm atoms (the equatorial substituents). The Si-Hbridge bond
distance of 1.63 Å is longer than that of Si-Hterm (1.49 Å) and
that of a 16-electron neutral complex Cp2Hf(SiH2Cy)Cl (1.51
Å). This neutral complex, which was isolated and fully
characterized previously,9 was modeled for comparison. The
Hf-Si bond distance in A is 2.60 Å, 0.17 Å shorter than the
corresponding distance calculated for Cp2Hf(SiH2Cy)Cl (2.76
Å). A calculation of the normal vibrational modes of structure
A confirms that it is indeed a minimum geometry on the
potential energy surface. The calculated Si-H stretching
frequency ν(SiH) of 1475 (after normalization to the ratio ν-
(SiH)calculated/ν(SiH)experimental, 2121 cm-1/2030 cm-1, for Cp2-
Hf(SiH2Cy)Cl) is consistent with the experimentally observed
value (νSiH ) 1415), indicating that the proposed R-agostic
structure and the calculated structure are similar. According to
the calculations, the R-SiH’s of the 16-electron complex Cp2-
Hf(SiH2Cy)Cl do not coordinate to the Hf center, as the Hf-
Si-H bond angles (110.0° and 112.4°) and Hf‚‚‚H through-
space distances (3.62 and 3.59 Å) preclude such interactions.
It is also interesting to note that the minimized geometry for
the isoelectronic neutral compound Cp2ScSiH3 does not exhibit
an R-agostic structure.33 This is consistent with the spectroscopic
features of the isolated and crystallographically characterized
compound Cp*2ScSiH2SiPh3, which exhibits normal η1 bonding
of the silyl group to scandium.33
Figure 5. Calculated gas-phase structure of [Cp2HfSiH(2,6-Me2C6H3)2]+
(B).
Table 6. Selected Bond Lengths (Å) and Angles (deg) for the
+
Calculated Structure Cp2HfSiH(2,6-C6Me2H3)2 (B)
bond lengths
Hf-Si
2.63
2.02
1.67
2.18
Si-C1
1.90
1.91
2.18
Hf-Hbridge
Si-Hbridge
Hf-Cpcent
Si-C2
Hf-Cpcent
bond angles
Hf-Si-Hbridge
Hf-Hbridge-Si
Hf-Si-C
50.4
90.1
Hbridge-Si-C
103.2
108.8
137.8
Hbridge-Si-C
121.1
123.7
Cpcent-Hf-Cpcent
Hf-Si-C
reactions with silanes. Compound 13 rapidly reacted with 1
equiv of PhSiH3 to produce an immediate color change from
red to pale yellow, with formation of HSitBuPh2, PhMeSiH2,
and unidentified hafnium-containing species (by 1H NMR
spectroscopy). In contrast, the reaction of the neutral hafnium
silyl chloride complex Cp2Hf(SitBuPh2)Cl with PhSiH3 produced
The minimized geometry of Cp2Hf[SiH(2,6-Me2C6H4)2]+ (B)
(Figure 5, Table 6) also contains the R-agostic SiH structure
found in A and 16. The Hf-Si-Hbridge angle is severely
distorted to 50.4°, and the Si-Hbridge bond distance is elongated
to 1.67 Å. The Hf-Si bond distance of 2.62 Å is shorter than
the calculated distance for Cp2Hf(SiH2Cy)Cl (2.75 Å) or the
X-ray crystallographically determined distance of Cp2Hf-
(SitBuPh2)Me (2.835 Å). The Hf-Si-C bond angles are 121.1°
and 123.7°, and the C-Si-C angle is 114.3°. The sum of the
angles around silicon (ignoring the R-hydrogen substituent) is
359.1°, indicating the nearly trigonal planar geometry of the
silicon and its equatorial substituents. It is clear from these
calculations that the coordinatively unsaturated, electron-poor
metal centers of 16, A, and B are stabilized by the R-agostic
SiH interaction.
t
[Cp2HfHCl]x, BuPh2SiH, and a mixture of oligomeric silanes
over 3 days in benzene-d6. The reactions of 13 with primary
and secondary silanes, such as MesSiH3, Ph3SiSiH3, (Me3-
Si)3SiSiH3, (Me3Si)2SiH2, and Ph2SiH2, rapidly produced HSit-
BuPh2 and unidentified organometallic products at room
1
temperature (t1/2 < 5 min, H NMR spectroscopy).
The reaction of 13 with Mes2SiH2 at room temperature
produced Cp2Hf(SiHMes2)(µ-Me)B(C6F5)3 (16) in approxi-
1
mately 25% yield (by H NMR spectroscopy) in benzene-d6
t
after 2.5 h (eq 5). The byproduct BuPh2SiH was formed
quantitatively, and the low yield of compound 16 results from
its subsequent reaction with benzene to form Cp2HfPh(µ-Me)B-
(C6F5)3 (17, Vide infra). After 4 h, 17 was the primary hafnium-
containing product. Compounds 16 and 17 were identified by
comparisons of their 1H NMR spectra to the spectra of
independently synthesized samples (prepared via reactions of
11 and Cp2Hf(Ph)Me, respectively, with B(C6F5)3). In the
absence of Mes2SiH2, 17 was not formed from complex 13 in
benzene-d6 (1H NMR spectroscopy). Interestingly, the steric bulk
σ-Bond Metathesis Reactions of Cp2Hf(SitBuPh2)(µ-Me)B-
(C6F5)3 (13). With the zwitterionic hafnium silyl complex 13
in hand, we began to explore its reactivity in σ-bond metathesis
(33) Sadow, A. D.; Tilley, T. D. Manuscript in preparation.
9
9468 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Cationic Hafnium Silyl Complexes
A R T I C L E S
Scheme 2. Synthesis of Cp2Hf(η2-SiHMes2)(µ-Me)B(C6F5)3 (16)
and Its Reactions with Arenes via Mes2SiH2 Elimination
addition of small amounts of hexafluorobenzene (ca. 7-13%).
The results of these experiments indicate first-order dependence
of the rate on benzene and suggest a rate law of rate ) k[16]-
[C6D6] (k ) 5.0 × 10-6 M-1 s-1, 13.5 °C). The existence of a
large primary kinetic isotope effect (kH/kD ) 6.9(7)) confirmed
that benzene is involved in the rate-determining step. Further-
more, the magnitude of this isotope effect is consistent with
transfer of hydrogen from carbon to silicon in the â-position of
a concerted transition state.34 An Eyring plot provided activation
parameters of ∆H‡ ) 19(1) kcal/mol and ∆S‡ ) -17(3) eu,
which implicate an ordered transition state.35 These data are all
consistent with a σ-bond metathesis mechanism in which Hf-
Si bond cleavage and Hf-Ph bond formation occur simulta-
neously.
The cationic character of the reactive species and the role of
the borate anion were investigated by monitoring benzene
activations in the presence of excess [nBu3NCH2Ph][MeB-
(C6F5)3] (10 and 20 equiv). Since these rates were identical to
those observed in the absence of excess borate anion, it appears
that the dissociation of MeB(C6F5)3- is not involved in the rate-
determining step for C-H activation. Marks and co-workers
have studied the solution dynamics of cationic zirconium and
hafnium alkyl complexes with MeB(C6F5)3- counterions, based
on the coalescence of the diastereotopic methyl groups in the
C5H3Me2 ligand in fluxional cation-anion pairs, and have
determined the barriers for cation/anion separation.36 The values
for ∆G‡ and ∆H‡ (14.0(2) kcal/mol and 15(2) kcal/mol,
respectively; 20 °C in toluene) for the first-order cation-anion
reorganization process were reported for the compound (C5H3-
Me2)2Hf(CH2SiMe3)(µ-Me)B(C6F5)3.36 Comparisons of these
barriers with those of the second-order C-H bond activation
process for 16 (∆G‡ ) 24 kcal/mol at 20 °C; ∆H‡ ) 19(1)
kcal/mol, temperature range of 5-50 °C in benzene-d6) suggest
that the reaction of 16 with benzene is slower than cation-
anion separation. Furthermore, these activation barriers and the
fact that the neutral complexes 4, 6, and 11 do not react with
benzene imply that the reactive form of 16 contains considerable
cationic character.
Given the well-documented role of R-agostic assistance in
olefin polymerization,16,29 the R-agostic SiH interaction in 16
may be an important factor in the C-H bond activation step.
The negligible secondary isotope effect (kH/kD ) 1.1(1))
determined for the rate of benzene-d6 activation by Cp2Hf(η2-
SiDMes2)(µ-Me)B(C6F5)3 (16-d1) versus 16 indicates that the
Si-H‚‚‚Hf interaction is not disrupted in the transition state
for C-H activation. Furthermore, the cationic hafnium silyl
complexes 12-14 do not contain an R-SiH and do not react
with C-H bonds. These observations suggest that the R-agostic
Si-H interaction plays an important role in activating the Hf-
Si bond toward σ-bond metathesis.
of Mes2SiH2 and the SitBuPh2 substituent did not inhibit this
transformation; it is quite facile. Second-order plots of ln{[Mes2-
SiH2]/[13]} vs time were linear over three half-lives (k ) 8.3
× 10-4 s-1 M-1 at 26 °C). The first-order dependence of the
rate on both Mes2SiH2 and 13 supports the hypothesis that the
reaction proceeds via a concerted, σ-bond metathesis mecha-
nism.
C-H Bond Activation by Cp2Hf(η2-SiHMes2)(µ-Me)B-
(C6F5)3 (16). At room temperature, compound 16 rapidly reacted
with benzene-d6 (t1/2 ) 54 min) to form Cp2Hf(Ph-d5)(µ-Me)B-
(C6F5)3 (17-d5), with elimination of Mes2SiHD (Scheme 2). The
Mes2SiHD was identified by its 1:1:1 SiH triplet resonance in
1
the H NMR spectrum (2JHD ) 3.6 Hz) and by comparison to
an authentic sample of Mes2SiH2. R-Agostic 16 also reacted
with toluene-d8 (as solvent; over ca. 2.5 h at room temperature)
to give the four possible products of C-D bond activation, Cp2-
Hf(o-tolyl-d7)(µ-Me)B(C6F5)3 (18-d7), Cp2Hf(p-tolyl-d7)(µ-
Me)B(C6F5)3 (19-d7) (relative percentages of compounds 18 and
19 could not be resolved by 1H NMR spectroscopy but represent
45% of the total products), Cp2Hf(m-tolyl-d7)(µ-Me)B(C6F5)3
(20-d7, 32%), and Cp2Hf(benzyl-d7)(µ-Me)B(C6F5)3 (21-d7,
23%). The identity of each product was confirmed by its
independent synthesis from the appropriate Cp2Hf(C7H8)Me
derivative.
Kinetic investigations of the reaction of 16 with benzene
provide information regarding the mechanism of the C-H bond
activation step. In neat benzene-d6, linear plots of ln[16] versus
time established first-order rate dependence on the cationic
hafnium silyl complex. Attempts to determine the order in
benzene were complicated by the difficulty of finding an inert
solvent in which the zwitterionic complex was soluble; com-
pound 16 precipitated even from benzene solutions containing
low concentrations of nonaromatic hydrocarbon solvents. Polar
solvents such as diethyl ether and CH3CN interfered with the
C-H bond activation chemistry. However, it was possible to
vary the concentration of benzene over a small range by the
Discussion
The zwitterionic hafnium silyl complexes reported here are
significantly more reactive than corresponding neutral species
(34) Concerted four-centered transition states in which an H atom is transferred
in the â-position typically exhibit large primary isotope effects. For example,
see: Fendrick, C. M.; Marks, T. J. J. Am. Chem. Soc. 1986, 108, 425.
(35) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd ed.;
McGraw-Hill: New York, 1995; p 155.
(36) (a) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358. (b)
Beswick, C. L.; Marks, T. J. Organometallics 1999, 18, 2410. (c) Deck, P.
A.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 6128. (d) Deck, P. A.;
Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 1998, 120, 1772.
9
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A R T I C L E S
Sadow and Tilley
in σ-bond metathesis reactions. For example, the enhanced
reactivity of 13 is illustrated by its reaction with the sterically
hindered Mes2SiH2 to form 16, presumably through a very
hindered transition state. The high reactivity of 16 is demon-
strated by its ability to activate the C-H bonds of arenes. The
mechanism of the reactions of 16 with arene C-H bonds appears
to proceed via a concerted transition state, of the type normally
associated with σ-bond metathesis.1,2
The metalation of arenes by 16 is the first example of a direct
σ-bond metathesis reaction between a d0 metal-silyl and an
arene CsH bond. In previous studies, the scandium-silyl
complex Cp2Sc[Si(SiMe3)3](THF) was found to react with HCt
CPh to yield the products [Cp2ScCtCPh]2 and HSi(SiMe3)3,
via activation of the alkynyl CsH bond.37 While the TaIII
complex Cp2Ta(SiHtBu2)(PMe3) reacts with benzene in a
process which is similar to that of Scheme 2, the mechanism
of this metalation involves oxidative addition of a CsH bond
followed by reductive elimination of an SisH bond.38 Interest-
ingly, the microscopic reverse of the reaction of 16 with
benzene, the direct reaction of a d0 metal phenyl with a
hydrosilane to form a metal-silicon bond, with elimination of
arene, is also rare.33 Silanes typically react with d0 metal phenyl
(and alkyl) complexes to form Si-C bonds and metal hydride
species, as in lanthanide-catalyzed organosilane redistribution
and olefin hydrosilation.6,39
is maintained as the reaction passes through the transition
state. The latter interpretation is supported by the fact that
cationic hafnium silyl complexes lacking an R-SiH moiety,
such as 12-14, do not react with aromatic or aliphatic C-H
bonds.
The reactions of 16 with the C-H bonds of benzene and
toluene expand the scope of possible approaches for the catalytic
functionalization of arenes (e.g., Scheme 1). The three steps
which comprise this potential cycle (C-H bond activation by
a metal-silyl complex, aryl transfer from hafnium to the silicon
center of a hydrosilane, and silane metalation by a metal hydride
complex) have been observed as individual steps for cationic
hafnium complexes.17,20 The transfer of an aryl substituent from
a metal center to silicon has been reported for several transition
metal and f-block aryl complexes, including cationic hafnium
complexes.17,43 The dehydrocoupling of M-H and Si-H bonds
to form a metal-silyl complex and dihydrogen has also been
described in stoichiometric and catalytic reactions of primary
and secondary silanes with several d0 and fnd0 complexes.3,4,8,9
In addition, the metalation of Mes2SiH2 by Cp2Hf(SitBuPh2)-
(µ-Me)B(C6F5)3 indicates that 16 may be formed via σ-bond
metathesis (i.e., it is a viable participant in a catalytic cycle).
Unfortunately, the synthesis of 16 from Mes2SiH2 and Cp2HfH-
(µ-H)B(C6F5)3 is not possible because the latter hafnium hy-
dride complex appears to be unstable under the reaction
conditions.
Cationic (d0) hafnium alkyl complexes containing R-agostic
interactions have been documented.40 Furthermore, R-agostic
assistance (i.e., the coordination of an R-hydrogen to the metal
center in the transition state for olefin insertion into an M-C
bond) has been proposed as an important effect in the polym-
erization of olefins. This assertion is based on the detection of
small secondary isotope effects (kH/kD ≈ 1.2) for the olefin
insertion step.29 The participation of R-agostic structures in
σ-bond metathesis has been suggested by theoretical studies of
gas-phase model molecules. For example, the ground-state, gas-
phase structure of 14-electron Cp2ZrMe+ contains an R-agostic
interaction, which is maintained upon coordination of H2 to the
cationic Zr center and in the transition state structure for Zr-C
bond hydrogenolysis.41a Also, an R-Si-H bond is coordinated
to the scandium center in the calculated transition state structure
for the σ-bond metathesis reaction of Cl2ScSiH3 with SiH4.41b
Additionally, in the isolated compound Cp*2Th(CH2CMe3)2, the
R-C-H bonds of one CH2CMe3 ligand are coordinated to the
Th center and this compound reacts via intramolecular C-H
bond activation and CMe4 elimination to form the cyclometa-
lated complex Cp*2Th(κ2-CH2CMe2CH2).42 In C-H bond
activation reactions with 16, participation of the R-agostic
SiH in the transition state appears to contribute to the high
reactivity of the Hf-Si bond. The negligible secondary isotope
effect (kH/kD ) 1.1(1)) suggests either that the agostic inter-
action is broken prior to the rate determining step or that it
Efforts to effect the type of catalysis depicted in Scheme 1
using Cp2Hf(SiHMes2)(µ-Me)B(C6F5)3 (16) were unsuccessful,
and the selective activation of benzene by cationic hafnium-
silyl complexes appears to be limited to the bis(cyclopentadi-
enyl) ligand set and the dimesitylsilyl substituent on 16.
Benzene-d6 solutions of Mes2SiH2 and 16 were heated to 60
°C for several days, but Mes2(C6D5)SiH was not formed,
probably because Mes2SiH2 is too hindered to occupy the
â-position of the four-centered transition state and participate
in a phenyl-transfer reaction. Given the possibility that less
hindered silanes might be more reactive toward phenyl deriva-
tives that form by C-H bond activation, PhSiH3 and Ph2SiH2
were investigated as components in 16/Mes2SiH2/benzene
reaction mixtures. Unfortunately, addition of PhSiH3 or Ph2-
SiH2 simply facilitated the decomposition of 16 to Mes2SiH2
and unidentified hafnium-containing products. Attempts to
extend the C-H bond activation reactions to CpCp*Hf-
derivatives were also unsuccessful; the reaction of CpCp*Hf-
(SiHMes2)Me with B(C6F5)3 (in benzene-d6) did not produce
identifiable quantities of either CpCp*Hf(SiHMes2)(µ-Me)B-
(C6F5)3 or CpCp*Hf(C6D5)(µ-Me)B(C6F5)3.17 Additionally, at-
tempts to initiate the catalytic dehydrosilation of benzene-d6 (as
solvent) with Mes2SiH2 were unsuccessful with Cp2HfMe(µ-
Me)B(C6F5)3 or CpCp*HfH(µ-H)B(C6F5)3 as potential catalyst
precursors (no Mes2(C6D5)SiH was detected after 2 d at 70 °C
by 1H NMR or GC-MS). Clearly, successful hydrocarbon
dehydrosilations by cationic group 4 complexes require species
capable of mediating facile and selective hydrocarbon metala-
tions via C-H bond activations. Despite the apparent lack of
catalytic activity in the systems described here, the observation
of C-H bond activations by 16 suggests that catalytic cycles
for hydrocarbon functionalization might be based on electro-
philic d0 metal-silyl species as the catalysts. This approach will
(37) Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics, 1993, 12,
2584.
(38) Berry, D. H.; Jiang, Q. J. Am. Chem. Soc. 1989, 111, 8049.
(39) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J. Organomet. Chem. 1996,
516, 41. (b) Voskoboynikov, A. Z.; Parshina, I. N.; Shestakova, A. K.;
Butin, K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K.
Organometallics 1997, 16, 4041.
(40) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics 1994, 13, 1424.
(41) (a) Ustynyuk, Y. A.; Ustynyuk, L. Y.; Laikov, D. N.; Lunin, V. V. J.
Organomet. Chem. 2000, 597, 182. (b) Ziegler, T.; Folga, E. J. Organomet.
Chem. 1994, 478, 57.
(42) Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.;
Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40.
(43) Castillo, I.; Tilley, T. D. Organometallics 2001, 20, 5598.
9
9470 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Cationic Hafnium Silyl Complexes
A R T I C L E S
approximately 10 mL, and cooled to -40 °C for crystallization. Yellow
require silyl complexes which are easily generated by the
dehydrocoupling of M-H and Si-H species and which readily
activate C-H bonds in a selective manner in the presence of
the Si-H and Si-C bonds of silanes.
crystals were isolated by filtration and dried under vacuum to yield
1
1.70 g of 2 (2.61 mmol, 79.5%). H NMR (300 MHz): δ 7.94 (m, 2
H, Ph), 7.48 (m, 2 H, Ph), 7.28 (m, 5 H, Ph), 6.11 (s, 5 H, C5H5), 1.64
t
(s, 15 H, C5Me5), 1.35 (s, 9 H, Bu). 13C{1H} NMR (100 MHz): δ
138.7 (Ph), 138.1 (Ph), 128.3 (Ph), 127.8 (Ph), 127.7 (Ph), 127.4 (Ph),
121.4 (C5Me5), 112.2 (C5H5), 32.1 (CMe3), 25.5 (CMe3), 12.9 (C5Me5).
29Si{1H} NMR (99 MHz): δ 51.48. IR (KBr, cm-1): 2113.6 (w), 1487
(m), 1454 (m), 1425 (s), 1379 (m), 1259 (m), 1174 (w), 1086 (s), 1026
(s), 816 (s). Anal. Calcd for C31ClH39HfSi: C, 56.96; H, 6.01. Found:
C, 56.60; H, 6.36. Mp: 100-102 °C (dec).
Conclusion
The results reported here demonstrate that cationic character
in a d0 silyl complex promotes σ-bond metathesis reactions of
M-Si bonds with Si-H and C-H bonds. The electrophilic,
coordinatively unsaturated, zwitterionic complex Cp2Hf(η2-
SiHMes2)(µ-Me)B(C6F5)3 (16) has been shown to exhibit
unusual spectroscopic features characteristic of an R-agostic
Si-H interaction with the hafnium center. The reactions
described here, including those of 13 with Mes2SiH2 and 16
with benzene, constitute further evidence that cationic d0 metal
complexes are highly reactive in σ-bond metathesis. Further-
more, we have observed three stoichiometric steps for a possible
catalytic cycle, involving cationic hafnium complexes, that
appear to represent a potential strategy for the catalytic
functionalization of hydrocarbons.
CpCp*Hf(SiHMes2)Cl (3). CpCp*HfCl2 (0.526 g, 1.17 mmol) and
(THF)2.5LiSiHMes2 (0.635 g, 1.52 mmol) were separately dissolved in
25 mL of ether. The solution of (THF)2.5LiSiHMes2 was slowly added
to the CpCp*HfCl2 solution. The reaction mixture was stirred in the
dark for 3 h. Solvent was removed under reduced pressure, and the
resulting solid was extracted with pentane (3 × 50 mL). The pentane
extracts were concentrated to ca. 15 mL and cooled to 0 °C. Small
yellow crystals were isolated by filtration and dried under vacuum to
1
yield 0.13 g of 3 (0.191 mmol, 16.3%). H NMR (400 MHz): δ 6.90
(s, 2 H, C6H2Me3), 6.86 (s, 2 H, C6H2Me3), 5.77 (s, 5 H, C5H5), 5.32
(s, 1 H, SiH), 2.69 (s, 6 H, C6H2Me3), 2.54 (s, 6 H, C6H2Me3), 2.21 (s,
3 H, C6H2Me3), 2.17 (s, 3 H, C6H2Me3), 1.84 (s, 15 H, C5Me5). 13C-
{1H} NMR (125 MHz): δ 141.6 (C6H2Me3), 139.7 (C6H2Me3), 136.3
(C6H2Me3), 136.4 (C6H2Me3), 128.6 (C6H2Me3), 127.8 (C6H2Me3), 127.5
(C6H2Me3), 119.9 (C5Me5), 112.2 (C5H5), 25.3 (p-C6H2Me3), 20.7 (o-
C6H2Me3), 20.6 (o-C6H2Me3), 11.8 (C5Me5). 29Si{1H} NMR (100
MHz): δ -0.423. IR (KBr, cm-1): 3589 (s), 3444 (br), 3226 (br),
3142 (br), 1421 (s), 1315 (s), 1265 (m), 1101 (m), 1051 (m), 976 (m),
831 (m). Anal. Calcd for C33ClH43HfSi: C, 58.14; H, 6.35. Found: C,
57.80; H, 6.42. Mp: 142-144 °C (dec).
Experimental Section
General. All manipulations were performed under an atmosphere
of nitrogen using Schlenk techniques and/or a glovebox. Dry, oxygen-
free solvents were employed throughout. Removal of thiophenes from
benzene and toluene was accomplished by washing each with H2SO4
and saturated NaHCO3 followed by drying over MgSO4. Olefin
impurities were removed from pentane by treatment with concentrated
H2SO4, 0.5 N KMnO4 in 3 M H2SO4, saturated NaHCO3, and then the
drying agent MgSO4. All solvents were distilled from sodium ben-
zophenone ketyl, with the exception of benzene-d6, which was purified
by vacuum distillation from Na/K alloy. Bromobenzene-d5 was placed
over 4-Å molecular sieves and degassed by repeated freeze-pump-
thaw cycles. The compounds CpCp*HfCl2,44 Cp2HfMeCl,45 CpCp*Hf-
[Si(SiMe3)3]Me (4),3c Cp2Hf[Si(SiMe3)3]Cl,46 B(C6F5)3,47 [Ph3C]-
[B(C6F5)4],47 (THF)3LiSi(SiMe3)3,48 (THF)3LiSitBuPh2,48 (THF)3LiSiPh3,48
and (THF)2LiSiMes2H21 were prepared according to literature proce-
dures. The materials MeLi, MeMgBr, (o-C6H4Me)MgCl, (m-C6H4Me)-
MgCl, and (p-C6H4Me)MgCl were purchased from Aldrich and used
as received. Elemental analyses were performed by the microanalytical
laboratory at the University of California, Berkeley. Infrared spectra
were recorded using a Mattson FTIR spectrometer at a resolution of 4
cm-1. All NMR spectra were recorded at room temperature in benzene-
d6 unless otherwise noted, using a Bruker AMX-300 spectrometer at
300 MHz (1H), 75.5 MHz (13C), a Bruker AM-400 spectrometer at
400 MHz (1H) 377 MHz (19F), or a Bruker DRX-500 at 500 MHz (1H),
125 MHz (13C), 160 MHz (11B), 100 MHz (29Si).
CpCp*Hf(SitBuPh2)Me (5). Compound 2 (0.329 g, 0.504 mmol)
was dissolved in Et2O (35 mL) and the solution was cooled to -78
°C. MeMgBr (0.20 mL, 3.0 M in diethyl ether, 0.60 mmol) was added
to the solution. The flask was allowed to slowly warm to room
temperature over 3 h. The volatile materials were removed from the
bright yellow solution under reduced pressure. The resulting solid was
extracted with pentane (3 × 25 mL), which was subsequently removed
under vacuum, affording a powdery solid. Analytically pure samples
of 5 were obtained by recrystallization from ether (3 mL) at -30 °C.
Yellow crystals were isolated by filtration, yielding 0.26 g (0.342 mmol,
1
67.8%) of 5. H NMR (500 MHz): δ 7.65 (d, 1 H, Ph), 7.39 (d, 1 H,
Ph), 7.27 (m, 4 H, Ph), 7.19 (m, 4 H, Ph), 6.02 (s, 5 H, C5H5), 1.59 (s,
15 H, C5Me5), 1.27 (s, 9 H, tBu), -0.44 (s, 3 H, HfMe). 13C{1H} NMR
(125 MHz): δ 147.3 (Ph), 147.1 (Ph), 128.7 (Ph), 128.6 (Ph), 128.0
(Ph) 127.8 (Ph), 127.5 (Ph), 119.2 (C5Me5), 111.1 (C5H5), 57.5 (HfMe),
32.0 (CMe3), 24.8 (CMe3), 12.5 (C5Me5). 29Si{1H} NMR (100 MHz):
δ 48.76. IR (KBr, cm-1): 2360 (w), 1427 (m), 1085 (m), 1016 (m),
814 (s), 739 (s), 706 (s). Anal. Calcd for C32H42HfSi: C, 60.69; H,
6.68. Found: C, 60.43; H 6.88. Mp: 112-113 °C (dec).
CpCp*Hf(SitBuPh2)Cl (2). A 250-mL Schlenk flask was charged
with CpCp*HfCl2 (1.47 g, 3.28 mmol) and (THF)3LiSitBuPh2 (1.68 g,
3.64 mmol). The flask was wrapped in aluminum foil, and 75 mL of
ether were added. The resulting solution was stirred at ambient
temperature in the dark for 5 h. Solvent was then removed under
reduced pressure, and the yellow solid was extracted with pentane (3
× 30 mL). The pentane extracts were combined, concentrated to
CpCp*Hf(SiHMes2)Me (6). Compound 3 (0.213 g, 0.312 mmol)
was dissolved in Et2O (ca. 40 mL) in a 100-mL Schlenk flask, which
was cooled to -78 °C. Via a syringe, 3.0 M MeMgBr in ether (0.150
mL, 0.45 mmol, 1.4 equiv) was added. The flask was allowed to slowly
warm to ambient temperature. Solvent was removed under reduced
pressure and the solid residue was extracted with hexane (2 × 30 mL).
The hexane solution was concentrated to ca. 5 mL and cooled to -30
°C. A red solid was isolated, yielding 0.128 g of 6 (0.193 mmol, 62.1%)
which was pure enough for reactivity studies. Analytically pure samples
of 6 were obtained by recrystallization from ether at -40 °C to yield
(44) Rogers, R. D.; Benning, M. M.; Kurihara, L. K.; Moriarty, K. J.; Rausch,
M. D. J. Organomet. Chem. 1985, 293, 51.
(45) Nishihara, Y.; Ishida, T.; Huo, S.; Takahashi, T. J. Organomet. Chem. 1997,
547, 209.
1
(46) Campion, B. K.; Falk, J.; Tilley, T. D. J. Am. Chem. Soc. 1987, 109, 9,
2049.
small red crystals. H NMR (300 MHz): δ 6.86 (s, 4 H, C6H2Me3),
5.73 (s, 5H, C5H5), 5.00 (s, 1 H, SiH), 2.48 (s, 6 H, o-C6H2Me3), 2.35
(s, 6 H, o-C6H2Me3), 2.21 (s, 3 H, p-C6H2Me3), 2.20 (s, 3 H,
p-C6H2Me3), 1.81 (s, 15 H, C5Me5), -0.52 (s, 3 H, HfMe). 13C{1H}
NMR (125 MHz): δ 144.65 (C6H2Me3), 142.48 (C6H2Me3), 142.42
(C6H2Me3), 136.82 (C6H2Me3), 136.72 (C6H2Me3), 129.34 (C6H2Me3),
(47) (a) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245. (b) Chien,
J. C. W.; Tsai, W.-M.; Rausch, M. D. J. Am. Chem. Soc. 1991, 113, 8570.
(48) (a) Gutekunst, G.; Brook, A. G. J. Organomet. Chem. 1980, 225, 1. (b)
Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993, 12,
2584. (c) Woo, H.-G.; Freeman, W. P.; Tilley, T. D. Organometallics 1992,
11, 2198.
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A R T I C L E S
Sadow and Tilley
129.12 (C6H2Me3), 128.58 (C6H2Me3), 118.64 (C5Me5), 111.61 (C5H5),
53.62 (HfMe), 26.38 (p-C6H2Me3), 25.73 (p-C6H2Me3), 21.46 (o-
C6H2Me3), 12.31 (C5Me5). 29Si{1H} NMR (100 MHz): δ 0.770. IR
(KBr, cm-1): 2364 (w), 2063 (m), 1601 (w), 1448 (m), 1376 (w), 1140
(w), 1023 (m), 813 (s). Anal. Calcd for C34H46HfSi: C, 61.75; H, 7.01.
Found: C, 61.42; H, 7.03. Mp: 157-159 °C (dec).
[CpCp*HfSi(SiMe3)3][MeB(C6F5)3] (7). A bromobenzene-d5 (∼0.3
mL) solution of B(C6F5)3 (0.015 g, 0.028 mmol) was added to a bright
yellow solution (∼0.3 mL) of 4 (0.018 g, 0.028 mmol). Upon addition,
the color instantaneously turned deep red. 1H NMR (500 MHz,
bromobenzene-d5): δ 5.71 (s, 5 H, C5H5), 1.81 (s, 15 H, C5Me5), 1.08
(br s, 3 H, MeB(C6F5)3), 0.151 (br s, 27 H, Si(SiMe3)3). 13C{1H} NMR
(125 MHz, bromobenzene-d5): δ 122.75 (C5Me5), 111.83 (C5H5), 12.91
(C5Me5), 6.50 (br, Si(SiMe3)3). 29Si{1H} NMR (100 MHz, bromoben-
zene-d5): δ -5.10 (Si(SiMe3)3), -87.45 (Si(SiMe3)3). 11B NMR (161
MHz, bromobenzene-d5): δ -15.07. 19F{1H} NMR (377 MHz,
bromobenzene-d5): δ -131.9 (m, 6 F), -163.9 (br, 3 F), -166.4 (br,
6 F).
9 H, CMe3), -0.57 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ
147.17 (Ph), 137.35 (Ph), 136.45 (Ph), 127.71 (Ph), 109.79 (C5H5),
55.59 (HfMe), 31.84 (CMe3), 23.83 (CMe3). 29Si{1H} NMR (99
MHz): δ 49.04. IR (KBr, cm-1): 3065 (w), 2971 (w), 2942 (w), 2851
(m), 1425 (m), 1089 (w), 1019 (m), 822 (s), 738 (m), 704 (s), 497 (s).
Anal. Calcd for C27H32HfSi: C, 57.59; H, 6.13. Found: C, 57.30; H,
5.90. Mp: 116-121 °C.
Cp2Hf(SiPh3)Me (10). Cp2HfMeCl (0.607 g, 1.69 mmol) and
(THF)3LiSiPh3 (0.860, 1.77 mmol) were separately dissolved in Et2O
(25 mL). The flask containing Cp2HfMeCl was cooled to -78 °C, and
the solution of LiSiPh3 was added in a dropwise fashion. The reaction
flask was wrapped in foil, and the reaction mixture was allowed to
warm to room temperature. The volatile materials were removed in
vacuo, and the resulting residue was extracted with pentane (3 × 50
mL). The combined extracts were subsequently concentrated to 45 mL
and cooled to -80 °C. A yellow powder was isolated by filtration at
1
-78 °C (0.611 g, 1.05 mmol, 62.1%). H NMR (500 MHz): δ 7.56
(d, 6 H, Ph), 7.27 (t, 6 H, Ph), 7.17 (t, 3 H, Ph), 5.68 (s, 10 H, C5H5),
-0.50 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ 146.21 (Ph),
137.07 (Ph), 128.34 (Ph), 128.11 (Ph), 109.87 (C5H5), 55.77 (HfMe).
29Si{1H} NMR (99 MHz): δ 37.18. IR (KBr, cm-1): 3104 (w), 3062
(w), 3007 (w), 2933 (w), 1479 (w), 1426 (m), 1132 (w), 1090 (m),
1015 (m), 819 (s), 736 (m), 703 (s), 509 (s). Anal. Calcd for C29H29-
HfSi: C, 59.73; H, 4.84. Found: C, 59.54; H, 4.73. Mp: 188-190
°C.
Cp2Hf[Si(SiMe3)3]Me (8). Cp2Hf[Si(SiMe3)3]Cl (0.473 g, 0.800
mmol) was dissolved in diethyl ether, and the solution was cooled to
-78 °C. A solution of MeMgBr (3.0 M, 0.29 mL) was added dropwise
with a syringe, and the resulting mixture was allowed to warm to
ambient temperature and stirred overnight in the dark. The solvent was
removed under vacuum, and the resulting solid was extracted with
pentane (2 × 50 mL). The solution was concentrated by evaporation
under reduced pressure, and compound 8 crystallized from ca. 45 mL
of pentane at -30 °C as small, needlelike, yellow crystals and was
Cp2Hf(SiMes2H)Me (11). Cp2HfMeCl (0.6073 g, 1.69 mmol) was
placed in a 100-mL Schlenk flask. A separate 100-mL Schlenk flask
was charged with (THF)2LiSiHMes2 (0.714 g, 1.71 mmol), and both
compounds were dissolved in Et2O (ca. 35 mL each). The solution of
Cp2HfMeCl was cooled to -78 °C in a dry ice/acetone bath. The
solution of (THF)2LiSiHMes2 was slowly added to the cooled Cp2-
HfMeCl/Et2O solution. The reaction flask was stoppered and wrapped
in aluminum foil. The reaction mixture was allowed to warm to room
temperature and stirred in the dark for 2 h. All volatile material was
removed under reduced pressure, and the remaining solids were
extracted with pentane (3 × 50 mL). The resulting yellow extracts were
combined, concentrated in vacuo to ca. 40 mL, and cooled to -78 °C.
A powdery yellow solid precipitated at low temperature yielding 0.681
1
isolated by filtration (0.232 g, 0.405 mmol, 50.7%). H NMR (500
MHz): δ 5.86 (s, 10 H, C5H5), 0.38 (s, 27 H, Si(SiMe3)3), -0.67 (s, 3
H, HfCH3). 13C{1H} (125 MHz): δ 109.47 (C5H5), 53.49 (HfMe), 5.81
(Si(SiMe3)3). 29Si{1H} NMR (100 MHz): δ -5.37 (Si(SiMe3)3), -84.12
(Si(SiMe3)3). IR (Nujol, cm-1): 2978 (w), 2845 (w), 1443 (s), 1144
(m), 1020 (s), 678 (s), 626 (s). Anal. Calcd for C20H40HfSi4: C, 42.04;
H, 7.05. Found: C, 41.93; H, 7.16. Mp: 140-142 °C.
Cp2Hf(SitBuPh2)Cl. Cp2HfCl2 (2.170 g, 5.72 mmol) and 1.05 equiv
of (THF)3LiSitBuPh2 (2.70 g, 5.84 mmol) were placed in separate 250-
mL Schlenk flasks, and ca. 75 mL of Et2O were added to each flask.
The flask containing the Cp2HfCl2/Et2O slurry was cooled to -78 °C.
Dropwise cannula addition of the (THF)3LiSitBuPh2/Et2O solution to
Cp2HfCl2 produced a yellow solution and a white precipitate. This
mixture was allowed to warm to room temperature and was then stirred
for 1 h. Solvent was removed under dynamic vacuum, and the yellow
residue was extracted with pentane (5 × 100 mL). The combined
extracts were concentrated and cooled to -80 °C. A yellow crystalline
solid was isolated by filtration (0.768 g, 1.32 mmol). The mother liquor
was concentrated and cooled to -78 °C to yield a second batch of
1
g (1.15 mmol, 68.2%) of 11. H NMR (500 MHz): δ 6.87 (s, 4 H,
m-C6H2Me3), 5.72 (s, 10 H, C5H5), 4.98 (s, 1 H, SiH), 2.36 (s, 12 H,
o-C6H2Me3), 2.21 (s, 6 H, p-C6H2Me3), -0.49 (s, 3 H, HfMe). 13C-
{1H} NMR (125 MHz): δ 144.69 (C6H2Me3), 141.57 (C6H2Me3),
137.26 (C6H2Me3), 129.35 (C6H2Me3), 109.77 (C5H5), 51.29 (HfMe),
25.64 (C6H2Me3), 21.45 (C6H2Me3). 29Si{1H} NMR (100 MHz): δ 8.25.
IR (KBr, cm-1): 2965 (s), 2916 (s), 2862 (m), 2077 (m), 1601 (m),
1442 (m), 1408 (m), 1017 (s), 819 (s), 804 (s), 749 (m). Anal. Calcd
for C29H36Hf1S1: C, 58.92; H, 6.14. Found: C, 58.63; H, 6.42. Mp:
100-102 °C (dec).
1
crystals of overall yield of 1.667 g (2.857 mmol, 49.9%). H NMR
(500 MHz): δ 7.65 (d, 4 H, Ph), 7.28 (t, 4 H, Ph), 7.21 (t, 2 H, Ph),
5.73 (s, 10 H, C5H5), 1.36 (s, 9 H, tBu). 13C{1H} NMR (125 MHz): δ
146.30 (Ph), 137.52 (Ph), 128.68 (Ph), 12.91 (Ph), 111.02 (C5H5), 31.93
(CMe3), 24.24 (CMe3). 29Si{1H} NMR (99 MHz): δ 49.41. IR (KBr,
cm-1): 2971 (w), 2943 (w), 2851 (m), 2364 (w), 2341 (w), 1425 (m),
1089 (m), 1021 (m), 822 (s), 738 (m), 705 (s), 497 (s). Anal. Calcd for
C26H29HfSiCl: C, 53.52; H 5.01. Found: C, 53.67; H 5.29. Mp: 134-
135 °C.
Cp2Hf[Si(SiMe3)3](µ-Me)B(C6F5)3 (12). A benzene-d6 solution (ca.
0.3 mL) of B(C6F5)3 (0.022 g, 0.043 mmol) was added to a solution
(ca. 0.3 mL) of 8 (0.024 g, 0.042 mmol) in an NMR tube. This
1
quantitatively produced 12. H NMR (500 MHz): δ 5.82 (s, 10 H,
C5H5), 0.12 (s, 27 H, Si(SiMe3)3), -0.33 (br s, 3 H, MeB(C6F5)3).
13C{1H} NMR (125 MHz): δ 149.94 (C6F5), 148.02 (C6F5), 142 (br,
C6F5), 138.97 (C6F5), 137.04 (C6F5), 124 (br, C6F5), 111.92 (C5H5),
33.7 (br, MeB(C6F5)3), 5.26 (Si(SiMe3)3). 11B NMR (161 MHz): δ
-13.10. 29Si{1H} NMR (99 MHz): δ -3.77 (Si(SiMe3)3), -21.39
(Si(SiMe3)3). 19F{1H} NMR (377 MHz): δ -132.5 (6 F), -158.9 (3
F), -164.0 (6 F).
Cp2Hf(SitBuPh2)Me (9). Cp2Hf(SitBuPh2)Cl (0.667 g, 1.15 mmol)
was placed in a 100-mL Schlenk flask, dissolved in diethyl ether (35
mL), and cooled to -78 °C. A solution of 3.0 M of MeMgBr (0.43
mL, 1.2 mmol) was added in a dropwise fashion from a syringe. The
reaction mixture was allowed to warm to room temperature, and the
resulting mixture was stirred for 1 h in the dark. The solvent was
removed under reduced pressure, and the resulting solid was extracted
with pentane (3 × 50 mL). The combined filtrates were concentrated
to ca. 50 mL and cooled to -78 °C, yielding small yellow crystals
Cp2Hf(SitBuPh2)(µ-Me)B(C6F5)3 (13). Compound 9 (0.100 g, 0.174
mmol) and B(C6F5)3 (0.092 g, 0.180 mmol) were dissolved in a minimal
amount of toluene (ca. 2 mL) in the glovebox. The resulting red solution
was cooled to -30 °C for several days until red blocklike crystals of
13 formed. The crystals were isolated by filtration from the mother
1
1
(0.157 g, 2.79 mmol, 40.9%). H NMR (500 MHz): δ 7.49 (d, 4 H,
liquor. H NMR (500 MHz): δ 7.26-7.11 (m, 10 H, Ph), 5.53 (s, 10
t
Ph), 7.26 (t, 4 H, Ph), 7.19 (t, 2 H, Ph), 5.70 (s, 10 H, C5H5), 1.25 (s,
H, C5H5), 0.90 (s, 9 H, Bu), -0.25 (br s, 3 H, MeB(C6F5)3). 13C{1H}
9
9472 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Cationic Hafnium Silyl Complexes
A R T I C L E S
NMR (125 MHz): δ 149.91 (C6F5), 148.02 (C6F5), 143.62 (C6H5),
139.03 (C6F5), 137.05 (C6F5), 136.61 (C6H5), 128.90 (C6H5), 128.48
(C6H5), 112.61 (C5H5), 40.05 (MeB(C6F5)3), 30.62 (CMe3), 25.60
(CMe3). 29Si{1H} NMR (99 MHz): δ 52.66. 11B NMR (161 MHz): δ
-13.03. 19F{1H} NMR (377 MHz): δ -132.2 (br, 6 F), -157.8 (br,
3 F), -163.2 (br, 6 F). IR (KBr, cm-1): 3121 (w), 3070 (w), 2943
(m), 2855 (m), 2690 (w), 2360 (w), 1645 (m), 1515 (s), 1462 (s), 1379
(m), 1282 (m), 1104 (s), 1015 (m), 974 (s), 902 (m), 838 (s), 803 (s),
735 (s), 705 (s), 498 (m). Anal. Calcd for C52H40BF15HfSi (including
1 C7H8): C, 53.51; H, 3.45. Found: C, 53.24; H, 3.07. Mp: 85-87
°C.
2 H, o-C6H5), 5.503 (s, 10 H, C5H5), 0.926 (br s, 3 H, MeB(C6F5)3).
13C{1H} NMR (125 MHz): δ 186.76 (C6H5), 136.71 (C6H5), 127.60
(C6H5), 127.33 (C6H5), 114.65 (C5H5), 24.20 (MeB(C6F5)3). 11B NMR
(160 MHz): δ -13.37. 19F NMR (377 MHz): δ -134 (6 F), -159 (3
F), -164 (6 F). IR (KBr, cm-1): 3045 (w), 2924 (w), 1649 (m), 1523
(m), 1468 (s), 1381 (m), 1321 (m), 1018 (m), 974 (s), 806 (s), 700
(w). Anal. Calcd for C35H18B1F15Hf1: C, 46.05; H, 1.99. Found: C,
46.00; H, 1.64. Mp: 135-136 °C.
Cp2Hf(CH2Ph)Me. Cp2HfMeCl (0.536 g, 1.49 mmol) and KCH2-
Ph (0.218 g, 1.68 mmol) were dissolved in THF (ca. 25 mL each) in
separate 100-mL Schlenk flasks. The flask containing the Cp2HfMeCl/
THF solution was cooled to -78 C with a dry ice/acetone bath. The
THF solution of KCH2Ph was added dropwise to the cold Cp2HfMeCl
solution. The resulting mixture was allowed to slowly warm to ambient
temperature and was then stirred under N2 for 1 h. THF was removed
under reduced pressure, and the salts were extracted with pentane (3
× 40 mL). The combined extracts were concentrated to ca. 25 mL and
cooled to -78 °C. Yellow crystals of Cp2Hf(CH2Ph)Me were isolated
by cold filtration in moderate yield (0.347 g, 0.836 mmol, 56.1%). 1H
NMR (500 MHz): δ 7.27 (t, 2 H, CH2C6H5), 6.90 (t, 1 H, CH2C6H5),
6.77 (d, 2 H, CH2C6H5), 5.50 (s, 10 H, C5H5), 1.33 (s, 2 H, CH2Ph),
-0.14 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ 152.81
(CH2C6H5),128.58 (CH2C6H5), 127.18 (CH2C6H5), 121.64 (CH2C6H5),
110.8 (C5H5), 64.51 (CH2C6H5), 38.44 (HfMe). IR (KBr, cm-1): 3091
(w), 3065 (w), 3014 (w), 2921 (w), 2864 (w), 1592 (m), 1484 (m),
1438 (w), 1207 (m), 1141 (w), 1016 (s), 811 (s), 751 (m), 704 (m).
Anal. Calcd for C18H20Hf1: C, 52.12; H, 4.81. Found: C, 52.20; H,
4.64. Mp: 61-61.5 °C.
Cp2Hf(SiPh3)(µ-Me)B(C6F5)3 (14). Compound 9 (0.012 g, 0.020
mmol) and B(C6F5)3 (0.011 g, 0.020 mmol) were dissolved separately
in benzene-d6 (∼0.3 mL each). The solution of B(C6F5)3 was added to
1
1
9, quantitatively producing 14 (by H NMR spectroscopy). H NMR
(500 MHz): δ 7.18 (m, 12 H, Ph), 7.12 (m, 3 H, Ph), 5.67 (s, 10 H,
C5H5), -0.15 (s, 3 H, MeB(C6F5)3). 13C{1H} NMR (125 MHz): δ
149.83 (br, C6F5), 147.90 (br, C6F5), 143.03 (Ph), 137.50 (br, C6F5),
136.80 (br, C6F5), 136.36 (Ph), 129.49 (Ph), 128.92 (Ph), 112.57 (C5H5),
41.0 (br, MeB(C6F5)3). 11B NMR (161 MHz): δ -13.11. 29Si{1H} NMR
(99 MHz): δ 39.41. 19F{1H} NMR (377 MHz): δ -133.3 (6 F), -159.1
(3 F), -164.3 (6 F).
Cp2Hf(η2-SiHMes2)(µ-Me)B(C6F5)3 (16). Compound 11 (0.014 g,
0.024 mmol) and B(C6F5)3 (0.013 g, 0.025 mmol) were dissolved in
toluene-d8 (∼0.7 mL) in an NMR tube. The solution was cooled to
-78 °C before being placed into the precooled spectrometer (-40 °C)
1
for analysis. The H NMR spectrum of the red solution at -40 °C
1
indicated that compound 16 had formed in quantitative yield. A H-
NOESY spectrum indicates through space interactions between the
MeB(C6F5)3 anion and the Cp ligands on hafnium and the mesityl
substituents on silicon. A 1H-11B HMQC reveals through bond coupling
between the methide group and the boron center. The reaction was
also performed in benzene, and the reaction mixture was immediately
frozen. The benzene was removed by sublimation under reduced
pressure in the dark at -30 °C, yielding a red powder (contaminated
Cp2Hf(m-C6H4Me)Me. To a 100-mL Schlenk flask containing Cp2-
HfMeCl (0.459 g, 1.28 mmol) was added Et2O (50 mL). The flask
was cooled to -78 °C, and a 1.0 M solution of (m-C6H4Me)MgCl in
Et20 (1.28 mL) was added via syringe. The reaction mixture was
allowed to warm to ambient temperature. After 0.5 h of stirring, the
volatile materials were removed under reduced pressure and the salts
were extracted with pentane (3 × 25 mL). The combined extracts were
concentrated to ca. 30 mL and cooled to -78 °C. An off-white powder
of Cp2Hf(m-C6H4Me)Me was isolated by cold filtration (0.400 g, 0.964
mmol, 75.4%). 1H NMR (500 MHz): δ 7.231 (t, 1 H, C6H4Me), 6.987
(s, 1 H, C6H4Me), 6.89 (m, 2 H, C6H4Me), 5.69 (s, 10 H, C5H5), 2.29
(s, 3 H, C6H4Me), 0.132 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz):
δ 194.22 (C6H4Me), 137.28 (C6H4Me), 136.34 (C6H4Me), 133.75 (C6H4-
Me), 128.20 (C6H4Me), 126.17 (C6H4Me), 111.24 (C5H5), 40.54
(HfMe), 22.75 (C6H4Me). IR (cm-1): 3086 (w), 2989 (w), 2914 (m),
2862 (w), 2787 (w), 1570 (w), 1439 (w), 1144 (w), 1097 (w), 1014
(m), 814 (s), 766 (s), 704 (m), 461 (m). Anal. Calcd for C18H20Hf1: C,
52.12; H, 4.81. Found: C, 52.06; H, 5.10. Mp: 111-112 °C.
Cp2Hf(o-C6H4Me)Me. The preparation of this compound was
similar to the synthesis described for Cp2Hf(m-tolyl)Me above, with
the modification that 1.05 equiv of 1.0 M (o-C6H4Me)MgCl in Et2O
were used instead of (m-C6H4Me)MgCl. Yield: 0.726 g of Cp2Hf(o-
C6H4Me)Me (1.75 mmol, 78.6%). 1H NMR (500 MHz): δ 7.12 (m, 3
H, C6H4Me), 7.08 (br, 1 H, C6H4Me), 5.665 (s, 10 H, C5H5), 1.983 (s,
3 H, C6H4Me), 0.185 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ
196.03 (C6H4Me), 145.98 (C6H4Me), 130.06 (C6H4Me), 129.04 (C6H4-
Me), 127.86 (C6H4Me), 126.23 (C6H4Me), 110.81 (C5H5), 57.13
(HfMe), 25.99 (C6H4Me). IR (KBr, cm-1): 3087 (m), 3044 (w), 2975
(w), 2912 (m), 2867 (m), 2793 (w), 1442 (s), 1145 (m), 1013 (s), 811
(s), 754 (s), 456 (s). Anal. Calcd for C18H20Hf1: C, 52.12; H, 4.81.
Found: C, 51.95; H, 4.64. Mp: 67-68 °C.
1
with Mes2SiH2) that was used for IR analysis. H NMR (toluene-d8,
-40 °C, 500 MHz): δ 6.60 (s, 4 H, C6Me3H2), 5.16 (s, 10 H, C5H5),
2.19 (s, 12 H, o-C6Me3H2), 2.04 (s, 6 H, p-C6Me3H2), 1.80 (s, 1 H,
SiH), -0.25 (br s, 3 H, MeB(C6F5)3). 13C{1H} NMR (toluene-d8, -40
°C, 125 MHz): δ 149.56 (C6F5), 147.63 (C6F5), 153.0 (br m, C6F5),
140.15 (Ar), 138.29 (C6F5), 137.60 (Ar), 137.15 (Ar), 136.38 (C6F5),
129.51 (Ar), 106.91 (C5H5), 24.60 (o-C6Me3H2), 20.86 (p-C6Me3H2),
-12.41 (MeB(C6F5)3). 29Si{1H} NMR (toluene-d8, -40 °C, 100 MHz,
detected via a 1H-29Si HMQC experiment): δ 158. 11B NMR (toluene-
d8, -40 °C, 160 MHz): δ -13.37. 19F{1H} NMR (toluene-d8, 377
MHz): δ -133.10 (d, 6 F), -160.35 (t, 3 F), -164.86 (t, 6 F). IR
(Nujol, cm-1): 1415 (m).
Cp2HfPhMe. A 100-mL Schlenk flask was charged with Cp2HfMeCl
(0.538 g, 1.50 mmol) and solid PhLi (0.128 g, 1.52 mmol). Benzene
(35 mL) was added, the flask was sealed, and the reaction mixture
was stirred under N2 for 3 h. The resulting solution was filtered and
the volatile materials were removed under reduced pressure, yielding
1
analytically pure Cp2HfPhMe (0.422 g, 1.05 mmol, 70.3%). H NMR
(500 MHz): δ 7.27 (t, 2 H, C6H5), 7.07 (m, 3 H, C6H5), 5.67 (s, 10 H,
C5H5), 0.108 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ 194.01
(C6H5), 136.79 (C6H5), 127.84 (C6H5), 125.33 (C6H5), 111.25 (C5H5),
40.59 (HfMe). IR (KBr, cm-1): 3044 (m), 2922 (m), 1439 (w), 1411
(w), 1015 (s), 808 (s), 700 (s). Anal. Calcd for C17H18Hf1: C, 50.94;
H, 4.53. Found: C, 50.54; H, 4.27. Mp: 78-80 °C (dec).
Cp2HfPh(µ-Me)B(C6F5)3 (17). Cp2HfPhMe (0.078 g, 0.194 mmol)
and B(C6F5)3 (0.109 g, 0.213 mmol) were placed in a 100-mL Schlenk
flask, 30 mL of pentane was added, and the resulting suspension was
stirred vigorously for ca. 2 h. The precipitate was allowed to settle,
and the desired complex was isolated by filtration from the reaction
mixture as a gray powder (0.103 g, 0.113 mmol, 58.4%). H NMR
(500 MHz): δ 7.42 (t, 2 H, m-C6H5), 6.930 (t, 1 H, p-C6H5), 6.516 (d,
Cp2Hf(p-C6H4Me)Me. The preparation of this compound was
similar to the synthesis described for Cp2Hf(m-C6H4Me)Me above, with
the modification that 1.05 equiv of 1.0 M (p-C6H4Me)MgCl in Et2O
were used instead of (m-C6H4Me)MgCl. Yield: 0.410 g (0.989 mmol,
67.3%). 1H NMR (500 MHz): δ 7.13 (d, 2 H, C6H4Me), 7.04 (d, 2 H,
C6H4Me), 5.69 (s, 10 H, C5H5), 2.25 (s, 3 H, C6H4Me), 0.12 (s, 3 H,
HfMe). 13C{1H} NMR (125 MHz): δ 190.01 (C6H4Me), 138.43 (C6H4-
1
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9473

A R T I C L E S
Sadow and Tilley
Me), 136.92 (C6H4Me), 134.23 (C6H4Me), 111.24 (C5H5), 40.38
(HfMe), 21.80 (C6H4Me). IR (KBr, cm-1): 3045 (w), 2197 (w), 1441
(w), 1144 (w), 1046 (w), 1015 (m), 812 (s), 792 (s), 487 (m), 465 (m).
Anal. Calcd for C18H20Hf1: C, 52.12; H, 4.81. Found: C, 52.47; H,
4.72. Mp: 84-85 °C.
Me3), 141.02 (C6H2Me3), 130.20 (C6H2Me3), 128.74 (C6H2Me3), 23.58
(C6H2Me3), 21.43 (C6H2Me3). 29Si{1H} NMR (100 MHz): δ -18.79
1
(t, JSiD ) 35.67 Hz). IR (KBr, cm-1): 2956 (s), 2926 (s), 2856 (s),
1604 (s), 1452 (s), 1411 (m), 1076 (m), 877 (m), 657 (s), 633 (s), 569
(s). Anal. Calcd for C18H22ClDSi: C, 71.14; H, 7.30. Found: C, 71.23;
H, 7.44. Mp: 67-68 °C.
Cp2Hf(o-C6H4Me)(µ-Me)B(C6F5)3 (18). A pentane solution (15 mL)
of B(C6F5)3 (0.352 g, 0.688 mmol) was added to a pentane solution
(25 mL) of Cp2Hf(o-C6H4Me)Me (0.284 g, 0.686 mmol) at -78 °C.
Immediately a white precipitate formed. The mixture was gradually
warmed to room temperature, and the powdery solid was isolated by
filtration. Residual solvent was removed in vacuo, and analytically pure
18 was isolated in good yield (0.532 g, 0.573 mmol, 83.6%). 1H NMR
(toluene-d8, 500 MHz): δ 6.98 (m, 2 H, C6H4Me), 6.89 (t, 1 H, C6H4-
Me), 6.14 (br, 1 H, C6H4Me), 5.49 (s, 10 H, C5H5), 1.66 (s, 3 H,
C6H4Me), 1.04 (br s, 3 H, MeB(C6F5)3). 13C{1H} NMR (100 MHz): δ
192.2 (C6H4Me), 146.7 (C6H4Me), 132.0 (C6H4Me), 126.3 (C6H4Me),
122.4 (C6H4Me), 113.6 (C5H5), 25.2 (C6H4Me). 11B NMR (160 MHz):
δ -13.69. 19F NMR (377 MHz): δ -133 (6 F), -159 (3 F), -164 (6
F). IR (KBr, cm-1): 3126 (w), 2978 (w), 1645 (m), 1516 (s), 1462 (s),
1379 (w), 1279 (w), 1099 (s), 1018 (w), 972 (s), 829 (s), 737 (m).
Anal. Calcd for C36H20BF15Hf: C, 46.65; H, 2.17. Found: C, 46.58;
H, 2.10. Mp: 107-108 °C.
Cp2Hf(p-C6H4Me)(µ-Me)B(C6F5)3 (19). A similar procedure to that
for 18 was used, with Cp2Hf(p-C6H4Me)Me (0.270 g, 0.651 mmol).
Compound 19 was isolated in good yield (0.463 g, 0.499 mmol, 76.8%).
1H NMR (500 MHz): δ 6.931 (d, 2 H, C6H4Me), 6.524 (d, 2 H, C6H4-
Me), 5.482 (s, 10 H, C5H5), 2.127 (s, 3 H, C6H4Me), 0.895 (br s, 3 H,
MeB(C6F5)3). 13C{1H} NMR (125 HMz): δ 183.5 (C6H4Me), 137.5
(C6H4Me), 129.2 (C6H4Me), 114.5 (C5H5), 21.4 (C6H4Me). 11B NMR
(160 MHz): δ -13.36. 19F NMR (377 MHz): δ -134 (6 F), -159 (3
F), -164 (6 F). IR (KBr, cm-1): 3120 (w), 2922 (s), 2756 (s), 1645
(m), 1516 (s), 1462 (s), 1281 (w), 1093 (s), 1016 (w), 974 (s), 829
(m). Anal. Calcd for C36H20BF15Hf: C, 46.65; H, 2.17. Found: C, 46.37;
H, 2.14. Mp: 87-88 °C (dec).
Cp2Hf(m-C6H4Me)(µ-Me)B(C6F5)3 (20). A similar procedure to that
of 18 was used, with Cp2Hf(m-C6H4Me)Me (0.115 g, 0.277 mmol). 20
was isolated in modest yield (0.113 g, 0.122 mmol, 44.0%). 1H NMR
(toluene-d8, 500 MHz): δ 7.02 (m, 1 H, C6H4Me), 6.76 (d, 1 H, C6H4-
Me), 6.44 (s, 1 H, C6H4Me), 6.29 (d, 1 H, C6H4Me), 5.72 (s, 10 H,
C5H5), 2.11 (s, 3 H, C6H4Me), 0.94 (br s, 3 H, MeB(C6F5)3). 13C{1H}
NMR (toluene-d8, 125 MHz): δ 187.39 (C6H4Me), 149.96 (C6H4Me),
148.09 (C6H4Me), 138.14 (C6H4Me), 136.66 (C6H4Me), 133.88 (C6H4-
Me), 114.63 (C5H5), 21.94 (C6H4Me). 11B NMR (160 MHz): δ -13.34.
19F NMR (377 MHz): δ -134 (6 F), -159 (3 F), -164 (6 F). IR
(KBr, cm-1): 2992 (w), 2923 (w), 1644 (m), 1516 (s), 1460 (s), 1381
(w), 1280 (w), 1091 (s), 1016 (m), 973 (s), 829 (s), 803 (m), 769 (m).
Anal. Calcd for C36H20BF15Hf: C, 46.65; H, 2.17. Found: C, 46.53;
H, 2.38. Mp: 104-106 °C (dec).
(THF)1.8LiSiDMes2. (THF)2LiSiMes2D was synthesized by reduction
of Mes2SiDCl with 10 equiv of Li0 in THF at 0 °C, following the
original synthesis of (THF)2.5LiSiMes2H.21 Yield: 1.663 g, (4.11 mmol,
1
60.2%). H NMR (500 MHz): δ 6.91 (s, 4 H, C6H2Me3), 3.30 (t, 7.5
H, C4H8O), 2.67 (s, 12 H, o-C6H2Me3), 2.25 (s, 6 H, p-C6H2Me3), 1.13
(t, 7.5 H, C4H8O). 13C{1H} NMR (125 MHz): δ 145.36 (C6H2Me3),
144.28 (C6H2Me3), 134.65 (C6H2Me3), 129.66 (C6H2Me3), 68.80, 26.05,
1
25.61, 21.63. 29Si{1H} NMR (100 MHz): δ -72.42 (t, JSiD ) 18.92
Hz). IR (KBr, cm-1): 2962 (s), 2916 (s), 1603 (m), 1458 (m), 1414
(w), 1261 (m), 1045 (s), 847 (m), 818 (s), 658 (m). Anal. Calcd for
(OC4H8)1.8LiSiC18H22D: C, 74.70; H, 9.05. Found: C, 74.88; H, 9.22.
Mp: 94-98 °C (dec).
Cp2Hf(SiDMes2)Me. The synthesis of Cp2Hf(SiMes2D)Me followed
that of 11, using (THF)2LiSiMes2D instead of (THF)2LiSiMes2H. Yield
) 0.519 g (0.876 mmol, 80.0%). 1H NMR (500 MHz): δ 6.88 (s, 4 H,
C6H2Me3), 5.72 (s, 10 H, C5H5), 2.36 (s, 12 H, o-C6H2Me3), 2.21 (s, 6
H, p-C6H2Me3), -0.49 (s, 3 H, HfMe). 13C{1H} NMR (125 MHz): δ
144.71 (C6H2Me3), 141.55 (C6H2Me3), 137.26 (C6H2Me3), 129.35 (C6H2-
Me3), 109.78 (C5H5), 51.40 (HfMe), 25.63 (C6H2Me3), 21.45 (C6H2Me3).
1
29Si{1H} NMR (100 MHz): δ 6.97 (t, JSiD ) 50.72 Hz). IR (KBr,
cm-1): 2966 (m), 2916 (m), 1601 (w), 1512 (w), 1462 (s), 1261 (m),
1090 (m), 1016 (s), 850 (m), 822 (s), 621 (m), 579 (m). Anal. Calcd
for C29H35DHfSi: C, 58.82; H, 5.96. Found: C, 59.02; H, 6.26. Mp:
75-79 °C (dec).
Kinetics Measurements of the Reaction of 13 with Mes2SiH2. The
reaction was monitored by 1H NMR spectroscopy with a Bruker
DRX500 spectrometer, using a 5 mm Wilmad NMR tube equipped
with a J. Young Teflon screw cap. The sample was prepared by addition
of benzene solutions of 9 (ca. 0.02 M) and B(C6F5)3 (1:1 molar ratio)
to 5-10 equiv of Mes2SiH2 in benzene-d6. The solution was frozen in
liquid N2 immediately after preparation and thawed before being placed
in the probe which was preheated to 26 °C. The probe temperature
was calibrated using neat ethylene glycol and monitored with a
thermocouple. Single scan spectra were acquired automatically at preset
time intervals. The peak areas were integrated relative to cyclooctane
as an internal standard. Rate constants were obtained by nonweighted
linear least-squares fit of the second-order rate law, ln{[Mes2SiH2]t/
[13] ) ln{[Mes2SiH2]0/[13]0} + k∆0t.
Kinetic Measurements of Reactions of 16 and Benzene. Reactions
were monitored by 1H NMR spectroscopy with a Bruker DRX500
spectrometer, using 5 mm Wilmad NMR tubes equipped with J. Young
Teflon screw caps. The samples were prepared by addition of a benzene-
d6 solution of B(C6F5)3 to a solution of 11. The reaction solutions were
frozen in liquid N2 immediately after preparation and thawed just before
being placed in the probe which was precooled to the required
temperature. The probe temperature was calibrated using neat ethylene
glycol or methanol and monitored with a thermocouple. Single scan
spectra were acquired automatically at preset time intervals. The peak
areas were integrated relative to cyclooctane as an internal standard.
Rate constants were obtained by nonweighted linear least-squares fit
of the pseudo-first-order rate law, ln[C] ) ln[C0] + kobst. The benzene-
d6 concentration was varied by dilution with hexafluorobenzene.
Cp2Hf(CH2Ph)(µ-Me)B(C6F5)3 (21). A toluene-d8 solution (ca. 0.3
mL) of B(C6F5)3 (0.017 g, 0.033 mmol) was added to a toluene-d8
solution (0.3 mL) of Cp2Hf(CH2Ph)Me (0.014 g, 0.033 mmol) to
1
quantitatively generate 21. H NMR (toluene-d8, 500 MHz): δ 7.07
(s, 1 H, CH2C6H5), 6.77 (s, 2 H, CH2C6H5), 6.22 (s, 2 H, CH2C6H5),
5.40 (s, 10 H, C5H5), 1.71 (s, 2 H, CH2C6H5), 0.57 (s, 3 H, MeB-
(C6F5)3). 13C{1H} NMR (125 MHz): δ 129.1 (CH2C6H5), 126.9
(CH2C6H5), 124.3 (CH2C6H5), 114.0 (CH2C6H5), 113.3 (C5H), 67.3
(CH2C6H5). 11B NMR (160 MHz): δ -13.37. 19F NMR (377 MHz):
δ -134 (6 F), -159 (3 F), -164 (6 F).
Mes2SiDCl. Mes2SiDCl was synthesized by an adaptation of a Corey
and West synthesis for Ph2SiHCl.49 Mes2SiD2 (2.67 g, 9.95 mmol) and
Ph3CCl were dissolved in toluene in a Teflon-sealed flask and heated
to 115 °C for 2 days. Mes2SiDCl was isolated in reasonable yield (2.21
Computational Details. The B3LYP/LACVP**++30,31 level of
theory with “high” grid density as implemented in the Jaguar 4.0
quantum chemistry program package32 was used for these studies. Full
geometry optimization was performed for model compounds A and B.
Analytical vibrational frequency calculation was performed for model
compound A; this stationary point was characterized by exactly zero
imaginary vibrational modes. No frequency analysis was performed
for model compound B.
1
g, 7.27 mmol, 73.1%) by cooling the reaction mixture to -78 °C. H
NMR (500 MHz): δ 6.63 (s, 4 H, C6H2Me3), 2.62 (s, 12 H, o-C6H2Me3),
2.03 (s, 6 H, p-C6H2Me3). 13C{1H} NMR (125 MHz): δ 114.87 (C6H2-
(49) Corey, J. Y.; West, R. J. Am. Chem. Soc. 1963, 16, 2430.
9
9474 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Cationic Hafnium Silyl Complexes
A R T I C L E S
Acknowledgment. Acknowledgment is made to the National
Science Foundation for their generous support of this work, to
Dr. Robert Bergman and Dr. Benjamin King for valuable help
with DFT calculations, and to Dr. Fred Hollander, Dr. Dana
Caulder, and Dr. Alan Oliver for assistance with X-ray crystal-
lography.
Supporting Information Available: Supporting Information
available: representative kinetics plots and Eyring plots for the
reaction of 16 with benzene (PDF) and X-ray crystallographic
data for 9, 13, and 15 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA030024G
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J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9475