Synthesis and reactivity of 16-electron hafnocene silyl hydride complexes

Article abstract of DOI:10.1021/om9607560

The synthesis and reactivity of monomeric, 16-electron hafnocene silyl hydride complexes are described. The complex CpCp*Hf[Si(SiMe₃)₃]H (1; Cp* = η⁵-C₅Me₅), prepared by the reaction of CpCp*Hf(H)Cl with (THF)₃LiSi(SiMe₃),₃, reacts rapidly with both ethylene and diphenylacetylene with elimination of HSi(SiMe₃)₃ to afford the corresponding hafnacyclopentane and tetraphenylhafnacyclopentadiene complexes. Complex 1 reacts with acetone to give the insertion product CpCp*Hf(OCHMe₂)[Si(SiMe₃)₃] (4). Reaction of 1 with the secondary silane H₂Si(SiMe₃)₂ proceeds smoothly to the new silyl hydride complex CpCp*Hf[SiH(SiMe3)₂]H (5), which was characterized by X-ray crystallography. Deuterium labeling experiments indicate that the latter reaction occurs with significant scrambling of label between the SiH and HfH positions. The tertiary silane Ph₃SiH does not react under comparable conditions. The silane H₂Si(SiMe₃)₂ undergoes a similar σ-bond metathesis reaction with CpCp*Hf[Si(SiMe₃)₃]Cl, to give CpCp*Hf[SiH(SiMe₃)₂]Cl (7). For the latter reaction, there is a photochemical dependence on the rate and course of the reaction pathway. Exposure of the reaction mixture to room light is critical for complete conversion to 7.

Full text of DOI:10.1021/om9607560

8
Organometallics 1997, 16, 8-12
Articles
Syn th esis a n d Rea ctivity of 16-Electr on Ha fn ocen e Silyl
Hyd r id e Com p lexes
Gary L. Casty, Claus G. Lugmair, Nora S. Radu, T. Don Tilley,*
J ohn F. Walzer, and Davit Zargarian†
Department of Chemistry, University of California at Berkeley,
Berkeley, California 94720-1460
Received September 3, 1996X
The synthesis and reactivity of monomeric, 16-electron hafnocene silyl hydride complexes
are described. The complex CpCp*Hf[Si(SiMe3)3]H (1; Cp* ) η5-C5Me5), prepared by the
reaction of CpCp*Hf(H)Cl with (THF)3LiSi(SiMe3)3, reacts rapidly with both ethylene and
diphenylacetylene with elimination of HSi(SiMe3)3 to afford the corresponding hafnacyclo-
pentane and tetraphenylhafnacyclopentadiene complexes. Complex 1 reacts with acetone
to give the insertion product CpCp*Hf(OCHMe2)[Si(SiMe3)3] (4). Reaction of 1 with the
secondary silane H2Si(SiMe3)2 proceeds smoothly to the new silyl hydride complex CpCp*Hf-
[SiH(SiMe3)2]H (5), which was characterized by X-ray crystallography. Deuterium labeling
experiments indicate that the latter reaction occurs with significant scrambling of label
between the SiH and HfH positions. The tertiary silane Ph3SiH does not react under
comparable conditions. The silane H2Si(SiMe3)2 undergoes a similar σ-bond metathesis
reaction with CpCp*Hf[Si(SiMe3)3]Cl, to give CpCp*Hf[SiH(SiMe3)2]Cl (7). For the latter
reaction, there is a photochemical dependence on the rate and course of the reaction pathway.
Exposure of the reaction mixture to room light is critical for complete conversion to 7.
In tr od u ction
has focused on the possible role of group 4 silyl hydride
complexes, Cp′Cp′′MH(SiR3) (Cp′, Cp′′ ) cyclopentadi-
enyl derivatives), as intermediates in the polymeriza-
tion. Such complexes are thought to be highly reactive,
since they possess two different bonds which should
readily participate in σ-bond metathesis.1 The possibil-
ity that such species might react via the reductive
elimination of silane, as has been observed for related
alkyl hydride derivatives,6 has been suggested by Co-
rey.4 It is therefore important to more thoroughly
establish the chemical properties for d0 metallocene silyl
hydride complexes.
From our investigations of the chemistry of d0 metal-
silicon bonds, we have found that zirconocene and
hafnocene silyl complexes are active catalysts for the
dehydropolymerization of hydrosilanes.1 Other cata-
lysts which have been reported appear to convert to
metal silyl species during the polymerization.2 On the
basis of extensive mechanistic studies, we have sug-
gested that this dehydropolymerization occurs by a
purely σ-bond metathesis mechanism involving only two
steps.1 Other workers have suggested somewhat more
complicated mechanisms,3-5 but widespread attention
Postulates concerning the possible role of 16-electron
silyl hydride complexes in catalytic cycles have been
difficult to test, given the fact that none have been
isolated. However, closely related titanium(III) dimeric
complexes have been isolated by Harrod,3,7 and Lewis
base adducts of the type Cp2MH(SiR3)(PMe3) have been
prepared by Buchwald.8 Here we describe the synthesis
of monomeric, 16-electron hafnocene silyl hydride com-
plexes and their structural and chemical properties.
† Present address: Department of Chemistry, University of Mon-
treal, C.P. 6128, succursale A, Montreal, Quebec, Canada H3C 3J 7.
X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) (a) Tilley, T. D. Acc. Chem. Res. 1993, 26, 22. (b) Woo, H.-G.;
Tilley, T. D. J . Am. Chem. Soc. 1989, 111, 3757. (c) Woo, H.-G.; Heyn,
R. H.; Tilley, T. D. J . Am. Chem. Soc. 1992, 114, 5698. (d) Woo, H.-G.;
Walzer, J . F.; Tilley, T. D. J . Am. Chem. Soc. 1992, 114, 7047. (e) Imori,
T.; Tilley, T. D. Polyhedron 1992, 13, 2231.
(2) (a) Harrod, J . F.; Mu, Y.; Samuel, E. Polyhedron 1991, 10, 1239.
(b) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37. (c) Corey, J . In
Advances in Silicon Chemistry; Larson, G., Ed.; J AI Press: Greenwich,
CT, 1991; Vol. 1, p 327.
(3) (a) Aitken, C. T.; Harrod, J . F.; Samuel, E. J . Am. Chem. Soc.
1986, 108, 4059. (b) Mu, Y.; Aitken, C.; Cote, B.; Harrod, J . F.; Samuel,
E. Can. J . Chem. 1991, 69, 264. (c) Aitken, C. T.; Harrod, J . F.; Samuel,
E. Can. J . Chem. 1986, 64, 1677. (d) Harrod, J . F.; Mu, Y.; Samuel, E.
Can. J . Chem. 1992, 70, 2980. (e) Harrod, J . F. In Inorganic and
Organometallic Polymers; Zeldin, M., Wynne, K. J ., Allcock, H. R., Eds.;
ACS Symposium Series 360; American Chemical Society: Washington,
DC, 1988; Chapter 7.
(6) (a) Gell, K. I.; Schwartz, J . J . Chem. Soc., Chem. Commun. 1979,
244. (b) Gell, K. I.; Schwartz, J . J . Am. Chem. Soc. 1981, 103, 2687.
(c) Gell, K. I.; Posin, B.; Schwartz, J .; Williams, G. M. J . Am. Chem.
Soc. 1982, 104, 1846.
(7) (a) Samuel, E.; Mu, Y.; Harrod, J . F.; Dromzee, Y.; J eanin, Y. J .
Am. Chem. Soc. 1990, 112, 3435. (b) Woo, H. G.; Harrod, J . F.; Henique,
J .; Samuel, E. Organometallics 1993, 12, 2883.
(8) (a) Kreutzer, K. A.; Fisher, R. A.; Davis, W. M.; Spaltenstein,
E.; Buchwald, S. L. Organometallics 1991, 10, 4031. (b) Spaltenstein,
E.; Palma, P.; Kreutzer, K. A.; Willoughby, C. A.; Davis, W. M.;
Buchwald, S. L. J . Am. Chem. Soc. 1994, 116, 10308.
(4) (a) Corey, J . Y.; Zhu, X.-H.; Bedard, T. C.; Lange, L. D.
Organometallics 1991, 10, 924. (b) Shaltout, R. M.; Corey, J . Y. Main
Group Chem. 1995, 1, 115.
(5) (a) Hengge, E.; Weinberger, M. J . Organomet. Chem. 1992, 433,
21. (b) Hengge, E.; Weinberger, M. J . Organomet. Chem. 1993, 443,
167.
(9) Gutekunst, G.; Brook, A. G. J . Organomet. Chem. 1982, 225, 1.
S0276-7333(96)00756-X CCC: $14.00 © 1997 American Chemical Society

Hafnocene Silyl Hydride Complexes
Organometallics, Vol. 16, No. 1, 1997
9
(0.008 g, 0.0193 mmol) and (THF)3LiSi(SiMe3)3 (0.009 g, 0.0191
mmol) at room temperature, and the reaction was monitored
by 1H NMR spectroscopy. The initial orange color of the
solution turned dark blue and then colorless almost im-
mediately. Assignment of the structure was based purely on
1H NMR spectroscopy and comparison of the spectrum to
spectra for closely related derivatives.8a 1H NMR (benzene-
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All manipulations were per-
formed under an inert atmosphere of nitrogen or argon using
either standard Schlenk techniques or a Vacuum Atmospheres
glovebox. Dry, oxygen-free solvents were employed through-
out. To remove olefin impurities, pentane and benzene were
pretreated with concentrated H2SO4 and then 0.5 N KMnO4
in 3 M H2SO4, followed by NaHCO3 and finally MgSO4. All
solvents were distilled from sodium benzophenone ketyl.
Benzene-d6 and toluene-d8 were purified by vacuum distillation
from Na/K alloy. 1H NMR yields were determined by relative
integration against Cp2Fe, using a long delay time and a short
pulse width. CpCp*Hf(H)Cl,1d (THF)3LiSi(SiMe3)3,9 CpCp*Hf-
d6, 300 MHz, 24 °C): δ 5.92 (s, 5 H, C5H5), 4.20 (sept, J HH
6.0 Hz, 1 H, OCHMe2), 1.89 (s, 15 H, C5Me5), 1.08 (d, J HH
)
)
6.0 Hz, 3 H, OCHMe2), 0.89 (d, J HH ) 6.0 Hz, 3 H, OCHMe2),
0.55 (s, 27 H, Si(SiMe3)3).
Cp Cp *Hf[SiH(SiMe3)2]H (5). CpCp*Hf(H)Cl (0.484 g, 1.17
mmol) and (THF)3LiSi(SiMe3)3 (0.544 g, 1.15 mmol) were
combined in an aluminum-foil-covered Schlenk flask. Diethyl
ether (30 mL) was added, and the solution was stirred for 5
min at room temperature. Subsequently, a diethyl ether
solution (5 mL) of H2Si(SiMe3)2 (0.265 g, 1.50 mmol) was added.
Immediately, the color of the reaction mixture changed from
light orange to dark orange. The reaction mixture was then
stirred at room temperature for 1 h. The reaction mixture was
filtered, and the volatile material was removed from the
filtrate by vacuum transfer to afford an orange residue. The
residue was dissolved in pentane (ca. 25 mL), and the
resulting filtered pentane solution was concentrated and cooled
to -35 °C to afford orange platelike crystals of complex 5 in
72% yield. IR (toluene, NaCl, cm-1): 2036 (Si-H), 1635 (Hf-
H). 1H NMR (benzene-d6, 300 MHz, 24 °C): δ 15.85 (s, 1 H,
Hf-H), 6.00 (s, 5 H, C5H5), 2.71 (s, 1 H, SiH(SiMe3)2), 1.95 (s,
15 H, C5Me5), 0.36 (s, 9 H, SiH(SiMe3)2), 0.32 (s, 9 H, SiH-
(SiMe3)2). 13C{1H} NMR (benzene-d6, 100 MHz, 24 °C): δ 118.4
(C5Me5), 108.9 (C5H5), 12.84 (C5Me5), 5.36 (SiH(SiMe3)2), 4.82
(SiH(SiMe3)2). 29Si{1H} NMR (benzene-d6, 99.3 MHz, 20 °C):
δ -4.85 (SiH(SiMe3)2), -4.89 (SiH(SiMe3)2), -29.7 (SiH-
(SiMe3)2). Anal. Calcd for C21H40HfSi3: C, 45.42; H, 7.26.
Found: C, 45.50; H, 7.20.
Cp Cp *Hf[SiH(SiMe3)2]Cl (7). To a benzene solution (ca.
20 mL) of CpCp*Hf[Si(SiMe3)3]Cl (0.200 g, 0.302 mmol) was
added H2Si(SiMe3)2 (0.160 g, 0.910 mmol) at room tempera-
ture, and the reaction mixture was stirred in the presence of
ambient room light for 12 h. The volatiles were removed in
vacuo, to afford a yellow oil which was extracted with pentane
(2 × 10 mL). The combined extracts were filtered, concen-
trated, and cooled to -40 °C to afford the product in 32% yield
(0.057 g). The reaction proceeds in 67% yield, as determined
by 1H NMR spectroscopy. IR (toluene, NaCl, cm-1): 2950 (m),
2900 (m), 2890 (m), 2860 (m), 2850 (m), 2052 (w), 2015 (w),
1493 (m), 1427 (w), 1244 (m), 841 (s sh), 810 (s sh). 1H NMR
(benzene-d6, 300 MHz, 22 °C): δ 5.85 (s, 5 H, C5H5), 2.81 (s, 1
H, SiH(SiMe3)2), 1.85 (s, 15 H, C5Me5), 0.46 (s, 9 H, SiH-
(SiMe3)2), 0.40 (s, 9 H, SiH(SiMe3)2). 13C{1H} NMR (benzene-
d6, 75.5 MHz, 22 °C): δ 119.75 (C5Me5), 111.01 (C5H5), 12.61
(C5Me5), 4.88 (SiCH3), 4.82 (SiCH3). 29Si{1H} NMR (benzene-
d6, 59.6 MHz, 22 °C): -4.69 (SiH(SiMe3)2), -6.18 (SiH-
10
[Si(SiMe3)3]Cl,1c and H2Si(SiMe3)2 were prepared according
to literature procedures. Elemental analyses were performed
by Mikroanalytisches Labor Pascher or the Microanalytical
Laboratory in the Department of Chemistry at the University
of California at Berkeley. NMR spectra were recorded on
AMX-300, Varian UN-500, and VBAMX-400 spectrometers,
and infrared spectra were recorded on a Perkin-Elmer 1330
instrument.
Cp Cp *Hf[Si(SiMe3)3]H (1). CpCp*Hf(H)Cl (0.484 g, 1.17
mmol) and (THF)3LiSi(SiMe3)3 (0.544 g, 1.15 mmol) were
combined in an aluminum-foil-covered Schlenk flask. Benzene
(30 mL) was added, and the solution was stirred for 15 min at
room temperature. The volatiles were removed in vacuo, and
the crude material was extracted with pentane (20 mL). The
pentane extract was filtered, concentrated (to ∼10 mL), and
cooled (to -40 °C) to afford complex 1 as bright orange needles
in 32% yield (0.228 g, 0.364 mmol). IR (toluene, NaCl, cm-1):
3028 (m), 2949 (m), 2895 (m), 2050 (w), 1638 (br m, Hf-H),
1495 (m), 1446 (w sh), 1439 (w), 1394 (w sh), 1387 (w), 1242
(s), 1017 (m), 837 (s sh), 833 (vs), 825 (s sh), 762 (m). 1H NMR
(benzene-d6, 500 MHz, 24 °C): δ 15.69 (s, 1 H, Hf-H), 6.02 (s,
5 H, C5H5), 1.94 (s, 15 H, C5Me5), 0.47 (s, 27 H, SiMe3). 13C-
{1H} NMR (toluene-d8, 125.7 MHz, -30 °C) δ 118.5 (C5Me5),
109.8 (C5H5), 13.15 (C5Me5), 6.93 (SiMe3). 29Si NMR (toluene-
d8, 99.3 MHz, -30 °C): δ -4.61 (Si(SiMe3)3), -67.1 (Si(SiMe3)3).
Anal. Calcd for C24H48HfSi4: C, 45.94; H, 7.71. Found: C,
45.85; H, 8.03.
Cp Cp *Hf[CH2(CH2)2CH2] (2). A pentane solution (15 mL)
of CpCp*Hf(H)Cl (0.215 g, 0.516 mmol) and (THF)3LiSi(SiMe3)3
(0.243 g, 0.516 mmol) was stirred under an atmosphere of
ethylene (1 atm) for 15 min. The initial orange color turned
yellow. Filtration and evaporation of the filtrate provided a
yellow powder. Crystallization from pentane at -35 °C gave
a pale yellow powder (0.140 g, 62%). 1H NMR (benzene-d6,
300 MHz, 24 °C): δ 5.82 (s, 5 H, C5H5), 2.39 (m, 2 H, Hâ), 2.06
(m, 2 H, Hâ), 1.74 (s, 15 H, C5Me5), 0.96 (m, 2 H, HR), 0.37 (m,
2 H, HR). Anal. Calcd for C19H28Hf: C, 52.47; H, 6.49.
Found: C, 51.03; H, 6.19. The low carbon value is presumably
due to loss of ethylene under vacuum and/or during the
combustion analysis.
(SiMe3)2), -48.64 (SiH(SiMe3)2). Anal. Calcd for C21H39
ClHfSi3: C, 42.77; H, 6.67. Found: C, 42.99; H, 6.79.
-
Cp Cp *Hf[C(P h )C(P h )C(P h )C(P h )] (3). A pentane solu-
tion (15 mL) of CpCp*Hf(H)Cl (0.305 g, 0.734 mmol) and
(THF)3LiSi(SiMe3)3 (0.347 g, 0.735 mmol) was added to diphen-
ylacetylene (0.262 g, 1.47 mmol), and the resulting reaction
mixture was stirred in the dark for 12 h. The reaction mixture
was filtered, and the volatile material was removed in vacuo
to afford a yellow-orange powder. Crystallization from hex-
amethyldisiloxane (∼5 mL) at -35 °C yielded complex 4 as a
yellow solid (0.290 g, 54%). 1H NMR (benzene-d6, 300 MHz,
24 °C): δ 7.00-6.71 (m, 20 H, C6H5), 6.01 (s, 5 H, C5H5), 1.80
(s, 15 H, C5Me5). Anal. Calcd for C43H40Hf: C, 70.24; H, 5.48.
Found: C, 70.14; H, 5.50.
X-r a y Cr ysta llogr a p h y. Orange platelike crystals were
obtained from a concentrated pentane solution at -35 °C. A
crystal of dimensions 0.24 × 0.15 × 0.06 mm was mounted on
a glass fiber using paratone N hydrocarbon oil. Data were
collected using a Siemens SMART diffractometer with a CCD
area detector. A preliminary orientation matrix and unit cell
parameters were determined by collecting 60 10 s frames,
followed by spot integration and least-squares refinement. A
hemisphere of data was collected using ω scans of 0.3°. Frame
data were integrated (XY spot spread 1.60°; Z spot spread
0.60°) using the program SAINT (SAX Area-Detector Integra-
tion Program, V4.024, Siemens Industrial Automation, Inc.,
Madison, WI, 1995). The data were corrected for Lorentz and
polarization effects. An absorption correction was performed
using XPREP (part of the SHELXTL Crystal Structure De-
termination Package, Siemens Industrial Automation, Inc.,
Madison, WI, 1995; µR ) 0.20, Tmax ) 0.79, Tmin ) 0.53). The
Cp Cp *Hf[Si(SiMe3)3](OCHMe2) (4). Acetone (0.002 mL)
was added to a benzene-d6 solution (1 mL) of CpCp*Hf(H)Cl
(10) Pae, D. H.; Xiao, M.; Chiang, M. Y.; Gaspar, P. P. J . Am. Chem.
Soc. 1991, 113, 1281.

10 Organometallics, Vol. 16, No. 1, 1997
Casty et al.
Ta ble 1. Su m m a r y of Cr ysta llogr a p h ic Da ta
Sch em e 1
empirical formula
fw
HfSi3C21H40
555.29
cryst color, habit
cryst dimens
crys syst
orange, plate
0.24 × 0.15 × 0.06 mm
monoclinic
cell determn (2θ range)
8192 (0.0-46.5°)
a ) 11.4044(2)Å
b ) 17.2453(1) Å
c ) 13.9402(2) Å
â ) 113.338(1)°
V ) 2517.34(6) Å3
P21/n (No. 14)
4
lattice params
space group
Z
Dcalcd
µ(Mo KR)
diffractometer
radiation
1.465 g/cm3
42.82 cm-1
Siemens SMART
Mo KR (λ ) 0.710 69 Å),
graphite monochromated
-149 °C
temp
scan type
no. of rflns measd
ω (0.3° per frame)
total, 10 616; unique, 3757
(Rint ) 0.078)
no. of observns (I > 3.00 σ(I))
structure soln
refinement
3108
direct methods (SIR92)
full-matrix least squares
0.032; 0.039
24 h). Therefore, for reactivity studies, it is most
convenient to generate complex 1 in situ.
residuals: R; Rw
R(all): R; Rw
max peak in final diff map
min peak in final diff map
0.036; 0.042
The reactivity of complex 1 toward selected small
molecules is summarized in Scheme 1. The reaction
with ethylene affords complex 2 in 62% yield. The
structure of 2 was assigned on the basis of analysis of
1.67 e/Å3
-2.55 e/Å3
1
both the H and 13C NMR spectra and comparisons to
10 616 integrated reflections were averaged in point group 2/m
to give 3757 unique reflections (Rint ) 0.078), but only 3108
reflections were considered observed (I > 3.00σ(I)). No decay
correction was necessary. Inspection of the systematic ab-
sences uniquely defined the space group as P21/n. The
structure was solved using direct methods (SIR92) and refined
by full-matrix least-squares methods using teXsan software.
The non-hydrogen atoms were refined anisotropically. Peaks
were found in the Fourier difference map at the expected
locations for the hydrogen atoms bonded to Hf(1) and Si(1)
(corresponding to Hf-H and Si-H distances of 1.76 and 1.25
Å, respectively). However, these H atoms could not be refined
well in these locations and were not included in the final
refinement. The number of variable parameters was 226,
giving a data/parameter ratio of 13.75. The maximum and
minimum peaks on the final difference Fourier map cor-
responded to 1.67 and -2.55 e/Å3, near the Hf atom. R )
0.032, Rw ) 0.039, and GOF ) 1.15. The crystallographic data
are summarized in Table 1.
spectroscopic data for the analogous Cp2Hf[CH2(CH2)2-
CH2] complex previously reported by Erker.12 Similarly,
the reaction of complex 1 with diphenylacetylene affords
the tetraphenylmetallacyclopentadiene complex 3 (in
54% yield), which was independently prepared by the
reaction of CpCp*HfCl2 with LiC(Ph)dC(Ph)C(Ph)dC-
(Ph)Li. The reactions with ethylene and diphenylacety-
lene produce HSi(SiMe3)3 (by 1H NMR spectroscopy),
and these reactions probably proceed by initial coordi-
nation of the incoming alkene or alkyne, followed by
reductive elimination of the silane.6 In contrast, com-
plex 1 reacts rapidly with 1 equiv of acetone in benzene-
d6 at room temperature via insertion, to produce the
isopropoxy complex 4. Therefore, silyl hydride 1 be-
haves analogously to the phosphine-stabilized complex
Cp2Zr(H)(SiPh3)(PMe3) described by Buchwald.8a
Previous studies have demonstrated that d0 M-Si
and M-H bonds are highly active toward σ-bond me-
tathesis processes.1 Due to their proposed role in
catalytic silane polymerizations, the reactivity of silyl
hydride complex 1 toward a series of silanes was
investigated. The reaction of 1 with 1 equiv of PhSiH3
in benzene-d6 at room temperature is very complex and
occurs rapidly to give numerous unidentified products,
including many which contain Si-H bonds. The reac-
tion of 1 with 1 equiv of Ph2SiH2 under analogous
conditions proceeds similarly, but at a slower rate. In
contrast, complex 1 does not react with Ph3SiH (benzene-
d6 solution). Given our previous success with the
synthesis and isolation of lanthanide-SiH(SiMe3)2 com-
Resu lts a n d Discu ssion
The silyl hydride complex CpCp*Hf[Si(SiMe3)3]H (1)
was prepared by the addition of (THF)3LiSi(SiMe3)3 to
CpCp*Hf(H)Cl in benzene at room temperature (Scheme
1). An analytically pure sample was obtained from
pentane at -35 °C as light-sensitive, thermally unstable
orange needles in 32% yield. Evidence supporting a
monomeric complex follows from 1H NMR and IR
1
spectroscopy. More specifically, the H NMR spectrum
of complex 1 reveals a characteristic downfield shift for
a terminal hydride ligand (15.68 ppm),1d,11 and the IR
spectrum contains a strong absorption at 1638 cm-1
which is consistent with the presence of a terminal
Hf-H bond.1d,11 Despite the low recovery of complex
1, the conversion is quantitative (by H NMR spectros-
copy). Complex 1 can be stored in the dark at -30 °C
(11) Diagnostic 1H NMR shifts and IR stretches for terminal Hf-
H: (a) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.;
Bercaw, J . E. Organometallics 1985, 4, 97. (b) Kablitz, H.-S.; Wilke,
G. J . Organomet. Chem. 1973, 51, 241. (c) Wolczanski, P. T.; Bercaw,
J . E. Organometallics 1982, 1, 793. (d) Highcock, W. J .; Mills, R. M.;
Spencer, H. L.; Woodward, P. J . Chem. Soc., Dalton Trans. 1986, 821.
(e) J ordan, R. F.; Bajgur, C. S.; Dasher, W. E.; Rheingold, A. L.
Organometallics 1989, 6, 1041.
1
for several days without significant decomposition, but
much shorter lifetimes are observed in solution (t1/2
(12) Erker, G.; Dorf, U.; Rheingold, A. L. Organometallics 1988, 7,
138.
≈

Hafnocene Silyl Hydride Complexes
Organometallics, Vol. 16, No. 1, 1997 11
Mechanistically, the formation of complex 5 from 1
and H2Si(SiMe3)2 could occur by several different path-
ways. One possibility is that the reaction proceeds via
initial reductive elimination of HSi(SiMe3)3 from com-
plex 1 to form the hafnocene CpCp*Hf, followed by
oxidative addition of the Si-H bond of H2Si(SiMe3)2.
This mechanism would therefore involve formation of
a Hf(II) species, which is expected to be a relatively
high-energy process.15 An alternative reaction pathway
might involve a four-centered transition state resulting
from interaction of Hf-Si and Si-H bonds. Other
mechanistic possibilities include a chain reaction cata-
lyzed by hydrogen16 and processes involving metalation
of the Cp* C-H bonds.17
To investigate the mechanism of this process, we
carried out deuterium-labeling experiments. The reac-
tion of 1 with D2Si(SiMe3)2 occurred rapidly (<5 min)
F igu r e 1. ORTEP view of the molecular structure of
Cp*CpHf[SiH(SiMe3)2]H (5).
18
at room temperature or -70 °C under ambient room
light to give significant (ca. 80%) incorporation of
deuterium into the HfH hydride position, as determined
by multiple integrations of 1H NMR spectra against an
internal ferrocene standard. The incomplete deutera-
tion of this position suggests competing processes and,
in particular, significant Hf-H/Si-H scrambling. In
fact, this type of scrambling is expected to be quite rapid
relative to other σ-bond metathesis processes.1d The
reaction of CpCp*Hf[Si(SiMe3)3]D with 1.05 equiv of H2-
Si(SiMe3)2 in benzene (or benzene-d6) results in signifi-
cant (but incomplete) transfer of deuterium to the Hf-H
and Si-H positions of 5 and production of both HSi-
Ta ble 2. Selected In ter a tom ic Dista n ces (Å) a n d
An gles (d eg) for Cp *Cp Hf[SiH(SiMe3)2]H (5)
(a) Bond Distances
Hf(1)-Si(1)
Si(1)-Si(3)
Si(2)-C(17)
Si(3)-C(19)
Si(3)-C(21)
Hf(1)-Cpa
2.744(1)
2.348(2)
1.880(5)
1.883(7)
1.896(6)
2.184
Si(1)-Si(2)
Si(2)-C(16)
Si(2)-C(18)
Si(3)-C(20)
C(1)-C(2)
2.352(2)
1.880(5)
1.876(5)
1.882(7)
1.437(7)
2.174
Hf(1)-Cp*a
(b) Bond Angles
109.00(7) Cp-Hf(1)-Cp*
Si(2)-Si(1)-Si(3)
135.94
105.48
Hf(1)-Si(1)-Si(3) 122.35(6) Si(1)-Hf(1)-Cp
Si(1)-Hf(1)-Cpa
109.90
Hf(1)-Si(1)-Si(2) 108.20(6)
2
(SiMe3)3 (ca. 75%) and DSi(SiMe3)3 (by 1H and H NMR
a
Cp and Cp* denote the centroids of the respective rings.
plexes obtained by σ-bond metathesis reactions of H2-
Si(SiMe3)2,13 we examined the interaction of complex 1
with this silane.
spectroscopy). No clear conclusion concerning the op-
erative mechanism can be drawn from the above label-
ing experiments, but we interpret the observation of
extensive deuterium scrambling as evidence for σ-bond
metathesis processes.1d,6c
In order to perhaps gain more information on this
σ-bond metathesis process, we examined the related and
much slower reaction of CpCp*Hf[Si(SiMe3)3]Cl1c (6)
with H2Si(SiMe3)2,which affords Cp*CpHf[SiH(SiMe3)2]-
Cl (7) in 67% yield, along with a quantitative amount
of HSi(SiMe3)3, as determined by 1H NMR spectroscopy
(eq 1).19 Initial efforts to follow the kinetics of this
Complex 1 reacts rapidly with H2Si(SiMe3)2 under
ambient room light in benzene-d6 to afford the new
hydrido silyl complex CpCp*Hf[SiH(SiMe3)2]H (5) in
1
86% yield by H NMR spectroscopy (Scheme 1). Our
observations indicate that complex 5 is much more
thermally and photochemically stable than 1, presum-
ably because of the less sterically demanding silyl
1
ligand. The H NMR spectrum contains a new Hf-H
signal (15.85 ppm), a singlet at δ 2.71 attributed to the
Si-H hydrogen, and two singlets for the diastereotopic
trimethylsilyl groups (at δ 0.36 and 0.32). Infrared
absorptions at 2036 and 1635 cm-1, assigned to the
Si-H and Hf-H stretches, respectively, are consistent
with a monomeric structure.1d,11 The molecular struc-
ture of complex 5 was confirmed by X-ray crystal-
lography. The structure is shown in Figure 1, and
selected bond distances and angles are summarized in
Table 2. Electron density peaks in the final difference
Fourier map were observed at the expected positions
for the hydrogen atoms bonded to Hf and Si. The Hf-
Si bond distance of 2.744(1) Å is comparable to analo-
gous distances in Cp*Cl2HfSi(SiMe3)3 (2.748(4) Å)14 and
Cp*CpHf(SiH2Ph)Cl (2.729(3) Å).1c Interestingly, these
distances are considerably shorter than those observed
for the independent Hf-Si bonds for Cp*CpHf[Si-
(SiMe3)3]Cl, 2.881(4) and 2.888(4) Å, which are probably
elongated by steric congestion in the molecule.1c
reaction revealed that products form only after a vari-
(15) Binger, P.; Podubrin, S. In Comprehensive Organometallic
Chemistry II: A Review of the Literature 1982-1994; Elsevier Science:
New York, 1995; Vol. 4, p 439.
(16) (a) Radu, N.; Tilley, T. D. J . Am. Chem. Soc. 1995, 117, 5863.
(b) Luinstra, G. A.; Teuben, J . H. J . Am. Chem. Soc. 1992, 114, 3361.
(17) (a) McDade, C.; Green, J . C.; Bercaw, J . E. Organometallics
1982, 1, 1629. (b) Miller, F. D.; Sanner, R. D. Organometallics 1988,
7, 818. (c) Cohen, S. A.; Auburn, P. R.; Bercaw, J . E. J . Am. Chem.
Soc. 1983, 105, 1136. (d) Schock, L. E.; Brock, C. P.; Marks, T. J .
Organometallics 1987, 6, 232. (e) J eske, G.; Lauke, H.; Mauermann,
H.; Swepston, P. N.; Schumann, H.; Marks, T. J . J . Am. Chem. Soc.
1985, 107, 8091.
(18) Deuterated silanes were prepared by reduction of the dichlo-
rosilane with LiAlD4: Boo, B. H.; Gaspar, P. P. Organometallics 1986,
5, 698.
(19) CpCp*Hf[SiH(SiMe3)2]Cl can be prepared by two routes: the
addition of methylene chloride to complex 5 at room temperature or
the addition of H2Si(SiMe3)2 to 6 in benzene at room temperature.
(13) (a) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J . Am. Chem.
Soc. 1992, 114, 8293. (b) Radu, N. S.; Tilley, T. D.; Rheingold, A. L. J .
Organomet. Chem. 1996, 516, 41.
(14) Arnold, J .; Roddick, D. M.; Tilley, T. D.; Rheingold, A. L.; Geib,
S. J . Inorg. Chem. 1988, 27, 3510.

12 Organometallics, Vol. 16, No. 1, 1997
Casty et al.
able induction time (ca. 0-3.3 h at 25 °C). Note that a
similar observation was made for the σ-bond metathesis
reaction of Cp*2SmCH(SiMe3)2 with H2Si(SiMe3)2, which
appears to proceed by a chain reaction catalyzed by H2.16
However, the reaction in eq 1 is not catalyzed by
hydrogen, and although CpCp*Hf(H)Cl does react with
H2Si(SiMe3)2 (very slowly over several days), the yield
of 7 is extremely low (trace amounts by 1H NMR
spectroscopy; benzene-d6 solution).
In addition, we have found that the reaction of
complex 6 with H2Si(SiMe3)2 is photochemically driven.
When a benzene-d6 solution of highly pure 6 (recrystal-
lized at least three times) was treated with H2Si(SiMe3)2
in the dark, complex 7 did not form and only the
decomposition of 6 to HSi(SiMe3)3 and unidentified
hafnium complexes was observed after several hours at
room temperature. After a benzene-d6 solution of 6 was
exposed to room light for 2 h, the color of the solution
changed from bright yellow to yellow-brown. By NMR
spectroscopy the purity of the compound appeared
unchanged; however, 10% of it had decomposed, as
determined by reference to an internal standard. Ad-
dition of H2Si(SiMe3)2 at this point, in the dark, resulted
in only a 10% conversion to 7. Exposure of this reaction
mixture to ambient room light, however, resulted in a
70% conversion to 7 after 12 h. Thus, 6 is photochemi-
cally converted to a species which appears to be para-
magnetic and may therefore contain Hf(III).20 This is
consistent with what we have previously observed for
photochemical reactions of 6 and its zirconium analogue.1c
Therefore, this reaction is photochemically driven and
an efficient radical chain process does not appear to be
operative.21 However, the reaction is clearly mediated
by a photochemically activated species. It may be that
photochemical Hf-Si bond homolysis is necessary for
this reaction to occur,1c but the nature and role of a
potential Hf(III) intermediate is not known at this time.
In summary, the preparation and reactivity of the
first monomeric, d0, 16-electron silyl hydride metal-
locenes are reported. For these systems, we have
observed reductive elimination, insertion, and σ-bond
metathesis processes. As observed previously,1c the
reaction of a hafnocene silyl complex (6) with a silane
is photochemically driven. Future investigations will
address the mechanism of this photochemical reaction.
Ack n ow led gm en t is made to the Divisions of Chem-
istry and Materials Research at the National Science
Foundation for support of this work. D.Z. is grateful to
the NSERC of Canada for a postdoctoral fellowship.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, collection, and refinement parameters, positional pa-
rameters, bond distances and angles, and anisotropic displace-
ment parameters (14 pages). Ordering information is given
on any current masthead page.
OM9607560
(21) (a) Espenson, J . H. Chemical Kinetics and Reaction Mecha-
nisms; McGraw-Hill: New York, 1981; p 134. (b) Lowry, T. H.;
Richardson, K. S. Mechanism and Theory in Organic Chemistry, 3rd
ed.; Harper and Row: New York, 1987; p 771.
(20) Extended photolysis of 6 with a UV lamp (10 °C, benzene-d6)
produces significant quantities of CpCp*HfCl2 and HSi(SiMe3)3, along
with a dark precipitate.