Silane complexes of electrophilic metal centers

Article abstract of DOI:10.1021/ic052134p

Photolysis of solutions of M(CO)₆ (M = Cr, Mo, and W) in the presence of Et₃SiH affords the silane complexes Cr(CO) ₅(η²-HSiEt₃), Mo(CO)₅(η ²-HSiEt₃), and W(CO)₅(η²- HSiEt₃). Observed values of J_SiH in these complexes are consistent with modest elongation of the Si-H bond. With Ph₃SiH, complexes of Cr(CO)₅ and W(CO)₅ were obtained, but no complex with Mo was observed. When Ph₂SiH₂ was employed, only one Si-H bond interacts with the metal center. A dynamic exchange process observable on the magnetic resonance time scale exchanges the pendant and coordinated Si-H bonds of the coordinated diphenylsilane. Silanes bound to M(CO)₅ are activated with respect to reaction with nucleophiles. With methanol, catalytic methanolysis of HSiEt₃ has been observed in the presence of Cr(CO)₅(η²-HSiEt₃), affording Et₃SiOMe.

Full text of DOI:10.1021/ic052134p

Inorg. Chem. 2006, 45, 6453−6459
Silane Complexes of Electrophilic Metal Centers
Steven L. Matthews, Vincent Pons, and D. Michael Heinekey*
Department of Chemistry, UniVersity of Washington, Box 351700, Seattle, Washington 98195-1700
Received December 13, 2005
Photolysis of solutions of M(CO)6 (M
Cr(CO)5(
2-HSiEt3), Mo(CO)5( 2-HSiEt3), and W(CO)5(
consistent with modest elongation of the Si H bond. With Ph3SiH, complexes of Cr(CO)5 and W(CO)5 were obtained,
but no complex with Mo was observed. When Ph2SiH2 was employed, only one Si H bond interacts with the metal
center. A dynamic exchange process observable on the magnetic resonance time scale exchanges the pendant
and coordinated Si H bonds of the coordinated diphenylsilane. Silanes bound to M(CO)5 are activated with respect
to reaction with nucleophiles. With methanol, catalytic methanolysis of HSiEt3 has been observed in the presence
of Cr(CO)5(
2-HSiEt3), affording Et3SiOMe.
)
Cr, Mo, and W) in the presence of Et3SiH affords the silane complexes
η
η
η
2-HSiEt3). Observed values of JSiH in these complexes are
−
−
−
η
Introduction
electrophilic Re and Mn cations such as [(PPh3)Re(CO)4]+
and [mer-Mn(CO)3{P(OCH2)3CMe}2]+ bind silanes such as
Et3SiH more strongly than H2.8,9
Activation of Si-H bonds is a key step in hydrosilation,
a reaction widely employed for the derivatization of olefins.1
Silane complexes are thought to be intermediates on the
pathway to oxidative addition of Si-H bonds, analogous to
other σ-bond complexes such as alkane complexes and
dihydrogen complexes. Since the first observation of a
mononuclear silane complex by Hart-Davis and Graham,2
complexation and activation of Si-H bonds has become an
active area of research that has been thoroughly reviewed.3
Kubas has pointed out the many similarities between
complexation of Si-H bonds and the related chemistry of
C-H and H-H bonds.4 This aspect has also been empha-
sized in reviews by Crabtree5 and by Schneider.6
Kubas and co-workers have studied the interaction of
Si-H bonds with various group 6 carbonyl-containing
complexes. For example, Mo(CO)(Et2PCH2CH2PEt2)2 binds
silanes such as H2SiPh2. The closely related but less basic
Mo(CO)(Ph2PCH2CH2PPh2)2 only binds primary silanes and
does so more weakly.7 Sterically less congested but highly
Examples of highly electrophilic, carbonyl-rich metal
centers that bind silanes are rare, indeed Kubas has suggested
that the “electron-poor group 6 species such as M(silane)-
(CO)5 are not stable”.4 While such complexes may not be
isolable at room temperature, there is precedent for their
observation in solution. For example, Brown and co-workers
reported that photolysis of Cr(CO)6 in the presence of Et3-
SiH transiently formed Cr(CO)5(η2-HSiEt3).10 Burkey has
employed photoacoustic calorimetry (PAC) to estimate the
binding enthalpy for complexation of Et3SiH to M(CO)5 (M
) Cr, Mo, and W) to be 21, 22, and 28 kcal/mol,
respectively, and suggests that these complexes may be
reasonably stable with respect to silane dissociation.11
We have recently reported that moderately stable dihy-
drogen complexes (H2)Cr(CO)5 and (H2)W(CO)5 can be
prepared by photoextrusion of CO from the hexacarbonyls
in the presence of H2.12 Here we report our extensions of
these methods to the observation of silane coordination to
these highly electrophilic metal centers. We also report the
* To whom correspondence should be addressed. E-mail:
heinekey@chem.washington.edu.
(1) Marciniec, B. Coord. Chem. ReV. 2005, 249, 2374-2390.
(2) Hart-Davis, A. J.; Graham, W. A. G. J. Am. Chem. Soc. 1971, 93,
4388-4389.
(3) (a) Schubert, U. In AdVances in Organosilicon Chemistry; Marcinicec,
B., Chojnowski, J., Eds.; Gordon and Breach: Yverdon-lesBains,
Switzerland, 1994. (b) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV.
1999, 99, 175-292.
(4) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure,
Theory and ReactiVity; Kluwer: New York, 2001; Chapter 11.
(5) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789-805.
(6) Schneider, J. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1068-1075.
(7) Luo, X.-L.; Kubas, G. J.; Bryan, J. C.; Burns, C. J.; Unkefer, C. J. J.
Am. Chem. Soc. 1994, 116, 10312-10313.
(8) Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. Inorg. Chim. Acta
1999, 294, 240-254.
(9) Fang, X.; Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. J.
Organomet. Chem. 2000, 609, 95-103.
(10) Zhang, S.; Dobson, G. R.; Brown, T. L. J. Am. Chem. Soc. 1991,
113, 6908-6916. Similar observations were also made by Harris and
co-workers: Kotz, K. T.; Yang, H.; Snee, P. T.; Payne, C. K.; Harris,
C. B. J. Organomet. Chem. 2000, 596, 183-192.
(11) Burkey, T. J. J. Am. Chem. Soc. 1990, 112, 8329-8333.
(12) Matthews, S. L.; Pons, V.; Heinekey, D. M. J. Am. Chem. Soc. 2005,
127, 850-851.
10.1021/ic052134p CCC: $33.50
Published on Web 03/03/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 16, 2006 6453

Heinekey et al.
addition of Et3SiH (ca. 1-2 mL; excess) via vacuum transfer, the
resulting yellow mixture was sonicated for 24 h. The volume of
Et3SiH was reduced under vacuum to leave a white solid in a small
quantity of HSiEt3. (CO)5ReCl (3 mg, 8.25 mmol) was added under
Ar, and after evacuation, C6H5F was vacuum-transferred into the
tube. The sample was then backfilled with Ar. H NMR (250 K,
C6H5F): δ -10.73 (Re-H-Si, JSiH ) 55 Hz).
chemical preparation of a silane complex of the very
electrophilic cation [Re(CO)5]+.
Experimental Section
General Procedures. Standard vacuum-line and drybox tech-
niques were employed in the manipulation of samples and in the
vacuum transfer of solvents. NMR spectra were acquired on a
Bruker Avance 500-MHz spectrometer running xwinnmr version
2.6. 1H and 13C NMR chemical shifts are referenced to the solvent
and are reported in ppm (δ) relative to tetramethylsilane. 31P NMR
chemical shifts are referenced to PMe3, which was located at δ
-61 relative to 85% H3PO4. Magnetization transfer experiments
were conducted using the double-pulsed field-gradient spin-echo
to achieve selective magnetization prior to standard pulse sequence
components to detect exchange.13 IR spectra were recorded on a
Bruker tensor 27 Fourier transform IR (FTIR) spectrometer.
Reagents. M(CO)6 (M ) Cr, Mo, and W; Aldrich or Strem)
were sublimed twice prior to use. CD2Cl2 (CIL) was distilled, placed
over activated silica, and then stored over CaH2 under vacuum.
HSiEt3 (Strem) and H2SiPh2 (Aldrich) were stored over LiAlH4
under Ar in a Teflon-stopcock-fitted glass bomb. HSiPh3, Cl2SiPh2,
LiAlH4, and LiAlD4 were used as received from Aldrich. Ph3-
B(C6F5)4 (Strem) was used as received. Cr(CO)5(PMe3) was made
by minor modifications to a published procedure14 in 50-mg batches
and purified by sublimation prior to use.
1
Photolysis Reactions. A typical sample preparation is as
follows: An NMR tube modified to fit a Teflon stopcock was
heated to 430 K overnight and pumped into an Ar-filled drybox
while still hot. The tube was charged with M(CO)6 (3-5 mg), and
the stopcock was fitted before being removed from the glovebox.
The tube was pump-cycled onto a vacuum line and evacuated
briefly. Approximately 0.5 mL of CD2Cl2 (or other solvent) was
vacuum-transferred into the tube at 77 K. Addition of the various
silanes was achieved either by a gastight syringe under a flow of
Ar or by vacuum transfer where possible. Addition of HSiPh3 was
performed in an Ar-filled glovebox. Photolysis (water-jacketed
450-W Hg-arc lamp) was conducted at 195 K in a quartz Dewar.
The sample was removed to a separate Dewar that was precooled
to 195 K using a dry ice slush for transportation. The sample was
then inserted into a precooled NMR probe for analysis. IR spectra
of the triethylsilane complexes were recorded on a Bruker tensor
27 FTIR spectrometer using 0.02 M solutions of M(CO)6 in
cyclopentane and an 8-fold excess of silane.
Cr(CO)5(η2-HSiEt3) (1). 1H NMR (CD2Cl2, 240 K): δ -13.58
(1H, Cr-H-Si, JHH ) 1.9 Hz, JSiH ) 95.2 Hz), 1.00 (9H, CH3,
JHH ) 7.5 Hz), 0.88 (6H, Si-CH2, JHH ) 1.9 and 7.5 Hz). 13C
NMR (CD2Cl2, 233 K): δ 223.3 (1C, trans-CO, JCH ) 2.3 Hz),
Preparation of HDSiPh2. A glass bomb fitted with a Teflon
stopcock was charged in the glovebox with a Teflon-coated stir
bar and equimolar amounts of LiAlH4 and LiAlD4 (250 mg, 276.5
mg, 6.59 mmol). The stopcock was fitted and the bomb removed
from the glovebox. The bomb was pump-cycled onto a vacuum
line. and under a flow of Ar, Cl2SiPh2 (4.0 mL, 17.6 mmol) was
added by syringe. The bomb was then evacuated by three freeze-
pump-thaw cycles and heated in an oil bath to 350 K for 3 days
with stirring. Upon cooling, hexane (10 mL) was added under a
flow of Ar to precipitate LiCl. The solution was filtered by a cannula
filter into a second Teflon-stopcock-fitted glass bomb, where hexane
215.1 (4C, cis-CO, JCH ) 2.9 Hz), 7.70 (CH2), 5.41 (CH3). IR (νCO
,
cyclopentane): 1951 cm-1
.
Mo(CO)5(η2-HSiEt3) (2). 1H NMR (CD2Cl2, 240 K): δ -8.36
(1H, Mo-H-Si, JHH ∼ 1.9 Hz, JSiH ) 96 Hz), 0.98 (9H, CH3, JHH
) 7.9 Hz), 0.85 (6H, Si-CH2, JHH ) ∼1.9 and 7.9 Hz). 13C NMR
(CD2Cl2, 233 K): δ 203.96 (cis-CO), 7.86 (CH2), 5.26 (CH3). IR
(νCO, cyclopentane): 1957 cm-1
.
W(CO)5(η2-HSiEt3) (3). H NMR (CD2Cl2, 240 K): δ -8.55
(1H, W-H-Si, JHH ∼ 1.5 Hz, JSiH ) 86 Hz), 0.99 (9H, CH3, JHH
) ∼7.5 and 1.5 Hz), 0.98 (6H, Si-CH2, JHH ) ∼7.5 and 1.5 Hz).
13C NMR (CD2Cl2, 233 K): δ 200.3 (1C, trans-CO, JCH unresolved),
1
1
was removed under vacuum. H NMR spectroscopy confirms the
identity of the product and shows that the signals due to Ph2SiH2
and Ph2SiHD are of approximately equal intensity, confirming that
the mole ratio of Ph2SiH2/Ph2SiHD is 1:2, as expected for ca. 50%
deuteration. A precise integration cannot be obtained because of
the small chemical shift separation. The level of deuteration was
also measured by mass spectroscopy, which indicated 55% deu-
teration.
13CO Enrichment of Cr(CO)6 and W(CO)6. A glass bomb
fitted with a Teflon stopcock was charged with M(CO)6 (M ) Cr
and W) in the glovebox. The bomb was pumped onto a vacuum
line, evacuated briefly, and tetrahydrofuran (THF) was vacuum-
transferred at 77 K. The solution was subjected to photolysis at
room temperature for 3 h and evacuated every 30 min by a freeze-
pump-thaw cycle. After the last freeze-pump-thaw cycle, the
bomb was backfilled with 13CO(g) (760 mmHg) and left to stand
overnight. The orange color of the solution of M(CO)5(THF) fades
to colorless, reforming M(13CO)(CO)5. This cycle was repeated to
form enriched M(CO)6, which was recrystallized from pentane prior
to use. Isotope incorporation was checked by IR and 13C NMR
spectroscopy.
196.8 (4C, cis-CO, JCH ) 1.3 Hz), 4.88 (CH2), 6.12 (CH3). IR (νCO
,
cyclopentane): 1951 cm-1
.
Cr(CO)5(η2-HSiPh3) (5). 1H NMR (CD2Cl2, 220 K): δ -11.65
(1H, Cr-H-Si, JSiH ) 111 Hz).
1
W(CO)5(η2-HSiPh3) (6). H NMR (CD2Cl2, 230 K): δ -6.38
(1H, W-H-Si, JSiH
) 101 Hz, JWH ) 36 Hz).
Cr(CO)5(η2-H2SiPh2) (7). 1H NMR (CD2Cl2, 220 K): δ -11.17
2
(1H, Cr-H-Si, JSiH ) 108 Hz, JHH ) 10.3 Hz), 6.02 (1H, Cr-
Si-H, JSiH ) 234 Hz, JHH ) 10.3 Hz). When complex 7 was
2
prepared with a 1:2 mixture of Ph2SiH2 and Ph2SiHD, integration
of the bound silane resonance at δ -11.2 versus the pendant silane
resonance at δ 6.0 showed that the bound silane resonance was
slightly more intense, with a ratio of 1.08:1.
Mo(CO)5(η2-H2SiPh2) (8). 1H NMR (CD2Cl2, 185 K): δ -6.49
(1H, Mo-H-Si, JSiH ) 111 Hz, 2JHH ) 11.8 Hz), 6.04 (1H, Mo-
2
Si-H, JSiH ) 232 Hz, JHH ) 11.8 Hz).
W(CO)5(η2-H2SiPh2) (9). 1H NMR (CD2Cl2, 220 K): δ -6.40
[Re(CO)5(η2-HSiEt3)][B(C6F5)4] (4). A screw-cap NMR tube
was charged with Ph3CB(C6F5)4 (10 mg, 10.8 mmol). After the
2
1
(1H, W-H-Si, JSiH ) 98 Hz, JHH ) 11.0 Hz, JWH ) 39 Hz),
2
6.50 (1H, W-Si-H, JSiH ) 236 Hz, JHH ) 11.0 Hz).
trans-Cr(CO)4(PMe3)(η2-HSiEt3) (10). 1H NMR (CD2Cl2, 210
(13) Braun, S.; Kalinowski, H. O.; Berger, S. 150 and More Basic NMR
Experiments: A Practical Course, 2nd ed.; Wiley: New York, 1999.
(14) Brown, R. A.; Dobson, G. R. Inorg. Chim. Acta 1972, 6, 65-71.
K): δ -12.47 (1H, Cr-H-Si, JSiH ) 102 Hz, JPH ) 16.8 Hz). 31
NMR (CD2Cl2, 210 K): δ 21.4.
P
6454 Inorganic Chemistry, Vol. 45, No. 16, 2006

Silane Complexes of Electrophilic Metal Centers
1
1
Figure 1. Partial (hydride region) H NMR spectra (CD2Cl2, 500 MHz,
Figure 2. Partial (hydride region) H NMR spectra (CD2Cl2, 500 MHz,
220 K) of 1. Inset: ×24 magnefication.
230 K) of 6. Inset: ×5 magnefication.
cis-Cr(CO)4(PMe3)(η2-HSiEt3) (11). 1H NMR (CD2Cl2, 210
K): δ -14.18 (1H, Cr-H-Si, JSiH ) 99 Hz, JPH ) 8.2 Hz). 31P
NMR (CD2Cl2, 210 K): δ 9.4.
carbonyl groups of 1 and 3. These are observed in an
approximate intensity ratio of 1:4 at δ 223.3 and 215.1 (1)
and δ 200.3 and 196.8 (3). These resonances are assigned
to the trans- and cis-carbonyl groups in these molecules,
Catalytic Methanolysis. In a 50-mL Schlenk tube connected to
a bubbler, 5.0 mg (23 µmol) of Cr(CO)6 was dissolved in 10.0 mL
(62 mmol) of Et3SiH. After photolysis for 30 min, 5.0 mL (123
mmol) of MeOH was added. Vigorous evolution of H2 was
observed, diminishing gradually over several hours. After 12 h,
analysis of the mixture by gas chromatography (GC)-mass
spectrometry (MS) revealed the formation of Et3SiOMe and
consumption of 50% of the starting Et3SiH. If the photochemical
preparation of Cr(CO)5(η2-HSiEt3) is 100% efficient, the turnover
number (TON) calculated from these data would be 1350. Separate
studies demonstrate that the photolytic preparation of Cr(CO)5(η2-
HSiEt3) has a yield of ca. 50%; thus, a better estimate of the TON
would be 2700. Similar methanolysis activity was observed when
preformed Cr(CO)5(THF) was added to a 1:1 mixture of MeOH
and Et3SiH.
2
respectively. In both 1 and 3, a two-bond coupling JCH
between the metal-bound 13CO and a 1H nucleus is detected
1
in the fully H-coupled 13C NMR spectra. The coupling of
2
the cis-13CO in 1 is JCH ) 2.9 Hz, larger than that of the
2
trans-13CO, where JCH is ∼2 Hz. This is also true for 3,
where the cis coupling 2JCH is ∼1.3 Hz and the trans coupling
is unresolved. The bound Si-H proton was confirmed as
the source of these couplings using a 1H-13C heteronuclear
multiple-quantum coherence (HMQC) experiment. Further
verification of the structures of 1-3 was provided by IR
spectroscopy in an alkane solution, where the CO stretching
vibrations can be readily observed. Consistent with previous
reports, only the strong E mode could be conclusively
assigned as a result of the overlap of other modes with
starting materials.11
Results
All photolysis reactions were carried out at 195 K. Initial
studies were carried out in toluene and alkane solvents. Low-
temperature solubility of the precursor complexes is quite
limited under these conditions, limiting the accessible
concentration of silane complexes. Attempts to isolate the
silane adducts were frustrated by incomplete photochemical
conversion to the desired complex. It was found that
methylene chloride provided much increased low-temperature
solubility. The stability of the complexes is somewhat
reduced in methylene chloride, but more satisfactory NMR
spectra were obtained.
Yields estimated for these reactions by NMR spectroscopy
are between 50% and 75%. Attempts to drive the reactions
to completion using longer irradiation times resulted in
decomposition. The complexes are persistent for hours in
toluene and alkanes at room temperature, with some gradual
regeneration of M(CO)6 observed along with decomposition
to unidentified products. More rapid decay was observed in
methylene chloride below room temperature.
The rhenium complex 4 can be generated by the reaction
of Re(CO)5Cl with [SiEt3][B(C6F5)4] in the presence of
HSiEt3 in a fluorobenzene solvent. A hydridic resonance is
observed at δ -10.73 as an unresolved heptet with satellites
Photolysis of methylene chloride solutions of the metal
hexacarbonyl complexes M(CO)6 in the presence of Et3SiH
(1 equiv) produces pale-yellow solutions of the σ silane
1
due to coupling to 29Si with JSiH ) 55 Hz.
Similar low-temperature irradiation experiments carried out
with HSiPh3 produces the complexes 5 and 6. Binding of
HSiPh3 to Mo(CO)5 has not been detected under these
conditions. The hydride signal for 5 is observed at δ -11.65
(JSiH ) 111 Hz), and the corresponding signal for 6 is
observed at δ -6.38 (JSiH ) 101 Hz). In the case of complex
1
complexes 1-3. The H NMR spectrum of each solution
(Figure 1) exhibits a new hydridic signal at δ -13.54 (Cr),
-8.36 (Mo), and -8.55 (W). The hydride resonances are
partially resolved heptets due to a small three-bond coupling
to the methylene protons of the silane. The value of 3JHH in
1 is observed to be 1.9 Hz, reduced from that in the free
silane (3.1 Hz). Coupling to 29Si (I ) -1/2, 6%) is clearly
visible in the hydride resonances for all three complexes and
allows the direct measurement of 1JSiH ) 95 (1), 96 (2), and
86 (3) Hz. In addition, 3 also exhibits coupling to 183W (I )
6, additional satellites due to coupling to 183W with JWH
)
36 Hz are also observed (Figure 2).
Complexes 7-9 are obtained similarly in the presence of
diphenylsilane. In this case, 1H NMR resonances due to both
coordinated (denoted HA) and pendant Si-H moieties
1
1/2, 14%) with JWH ) 36 Hz.
2
(denoted HB) are observed (with JHH ) 10-12 Hz), with
In the 13C NMR spectrum, the carbonyl signals for 1-3
are weak, but the use of 13CO-enriched Cr(CO)6 and W(CO)6
allows the observation of the 13C NMR signals for the
the former exhibiting diminished values of JSiH of 108 (7),
111 (8), and 98 (9) Hz. The hydridic resonance for complex
8 also exhibits coupling to 183W, JWH ) 39 Hz. For 7-9,
Inorganic Chemistry, Vol. 45, No. 16, 2006 6455

Heinekey et al.
1
Figure 3. Partial H NMR spectra (CD2Cl2, 500 MHz, 215 K) of 9.
Figure 5. Partial (hydride region) 1H NMR spectrum (CD2Cl2, 500 MHz,
210 K) of the products of photolysis of (PMe3)Cr(CO)5 with Et3SiH.
SiPh2 with 1:1 LiAlH4/LiAlD4. Irradiation of Cr(CO)6 with
1 equiv of this isotopically labeled silane affords a mixture
of complex 7, 7-d1, and 7-d2. Integration of the resonances
for protons of HA and HB in 7 and 7-d1 separately is not
1
possible because of overlap in the H NMR spectrum, but
because no isotope effect is expected in the H2SiPh2 complex,
both resonances can be integrated together. The ratio of
integrated intensity of the coordinated SiH resonance to the
pendant resonance is 1.08:1.
A phosphine-substituted derivative of Cr(CO)6 was in-
vestigated to provide a direct comparison to the [(PR3)Re-
(CO)4]+ system reported by Kubas and co-workers. Thus,
photolysis of (PMe3)Cr(CO)5 in the presence of Et3SiH was
found to afford both cis- and trans-silane complexes, along
with a small amount of complex 1 formed by phosphine loss
(Figure 5).
The major product exhibits a hydridic resonance at δ
-12.47 with JPH ) 16.8 Hz and JSiH ) 102 Hz. This
resonance is attributed to 10. The minor product gives a
resonance at δ -14.18 with JPH ) 9.4 Hz and JSiH ) 99 Hz.
This signal is assigned to 11.
Figure 4. 1H NMR spectra (CD2Cl2, 500 MHz) of 8 for pendant and bound
Si-H resonances at 180-225 K.
the pendant Si-H resonance is observed between δ 6 and
6.5, with JSiH significantly increased from that in the free
silane (Figure 3). Values of JSiH for the pendant SiH are 234
(7), 232 (8), and 236 (9) Hz, which are much greater than
JSiH ) 200 Hz observed in free diphenylsilane.
An interesting dynamic process is observed in complexes
7-9, which exchanges the coordinated and pendant Si-H
on the NMR time scale. At low temperatures, two sharp
resonances are observed. Upon an increase in the tempera-
ture, line broadening is observed as a result of the exchange
process (Figure 4). Coalesecence of the resonances could
not be observed because of decomposition at higher tem-
peratures. The resonance due to free Ph2SiH2 remains sharp
at all temperatures. The selective magnetization of the
protons in the HB environment allowed their exchange to be
monitored using spin saturation transfer. Exchange was
observed only between the HA and HB environments and not
with the free silane. This remained true even with the longest
mixing periods of up to 4 s.
Line-shape analysis over the temperature range 200-260
K allows the extraction of rate data for this exchange process
in complexes 7-9, which was used in an Eyring analysis.
Activation parameters are ∆Hq ) 9.9 ( 1.8 kcal/mol and
∆Sq ) -5 ( 7 eu (7); ∆Hq ) 8.0 ( 0.3 kcal/mol and ∆Sq
) -4 ( 2 eu (8); and ∆Hq ) 9.7 ( 0.5 kcal/mol and ∆Sq
) -5 ( 2 eu (9).
Catalytic Silane Alcoholysis. When solutions of 1 are
treated with methanol, the formation of MeOSiEt3 was
observed, identified by 1H and 13C NMR spectroscopy,15 and
confirmed by GC-MS. H2 gas was evolved, and the presence
of Cr(CO)5(MeOH) was noted. The methanol complex was
independently prepared by photolysis of toluene solutions
of Cr(CO)6 in the presence of methanol. Bound methanol in
this Cr complex has characteristic resonances in the 1H NMR
spectrum at δ 2.69 (3H) and 5.68 (1H). After generation of
complex 1 photochemically in neat Et3SiH, the addition of
MeOH to the pale-yellow solution turns it bright yellow and
H2 evolution was observed. The process is catalytic in Cr,
with ca. 2700 turnovers observed at room temperature over
12 h before catalyst deactivation occurs. Preformed Cr(CO)5-
(THF) was also an effective catalyst precursor. H2 evolution
and formation of MeOSiEt3 were observed upon the addition
of Cr(CO)5(THF) to a 1:1 Et3SiH/MeOH mixture.
Discussion
Complexation of Si-H bonds to a variety of transition-
metal fragments has been reported in the literature, usually
A small equilibrium isotope effect is seen in the coordina-
tion of HDSiPh2 to Cr(CO)5. A 1:2:1 mixture of H2SiPh2-
d0/H2SiPh2-d1/H2SiPh2-d2 was prepared by reaction of Cl2-
(15) Field, L. D.; Messerle, B. A.; Rehr, M.; Soler, L. P.; Hambley, T. W.
Organometallics 2003, 22, 2387-2395.
6456 Inorganic Chemistry, Vol. 45, No. 16, 2006

Silane Complexes of Electrophilic Metal Centers
with a mixture of Cp, arene, and phosphine coligands. Highly
electrophilic, carbonyl-rich metal centers are rarely observed
to bind silanes. Indeed, Kubas has suggested that such
molecules are not stable.4 While such complexes may not
be isolable at room temperature, there is precedent for their
observation in solution, as noted above.
values of JSiH, the interaction with Cr and Mo is seen to be
quite similar, while the interaction in the W complex 3 is
somewhat stronger. This is quite consistent with the PAC
data of Burkey, where the binding enthalpies of Et3SiH to
Cr(CO)5 and Mo(CO)5 were quite similar (21 and 22 kcal/
mol) but the binding to W(CO)5 was significantly stronger
(28 kcal/mol).
The rhenium complex 4 allows an opportunity to inves-
tigate the effect of charge on the activation of a coordinated
silane. We observe the Si-H bond to be lengthened
considerably (JSiH ) 55 Hz) with respect to the neutral
W(CO)5 analogue upon an increase in the charge of the
molecule. The activation of the bond is mostly due to σ
donation to a vacant metal orbital with a minimal contribution
from back-donation. Interestingly, complex 4 has a value
for JSiH very similar to that reported by Kubas in [cis-(PPh3)-
Re(CO)4(η2-HSiEt3)]+ (JSiH ) 61 Hz). If back-donation from
metal to bound silane were significant, the more electron-
rich metal center in the phosphine-substituted cation would
be expected to exhibit greater bond elongation and cor-
The preparation of M(CO)5(L) complexes by photoextru-
sion of CO in the presence of ligand L is well precedented
in the literature. We have previously demonstrated that this
procedure can be extended to the preparation of complexes
with L being a σ-bond donor such as H2.12 Similar to our
experience with H2, we find that silane complexes are formed
in sufficient yield by this procedure to allow for their
1
observation by H NMR spectroscopy. Because the silane
ligands are weakly bound, we have been unable to isolate
these complexes in a pure state, partly because of contamina-
tion with the starting material M(CO)6. Prolonged irradiation
leads to increased formation of unidentified decomposition
products.
Triethylsilane Complexes. Complexes 1-3 are indeed
observable in solution below room temperature. Formulation
as silane σ complexes is based upon the observation of large
values of JSiH ) 95 (1), 96 (2), and 86 (3) Hz. This
formulation as six-coordinate silane adducts is confirmed by
the C4V geometry of the carbonyl ligands indicated by the
13C NMR spectra.
respondingly lower JSiH
.
Triphenylsilane Complexes. In the case of Ph3SiH,
complexation affording 5 and 6 was observed, but no Mo
analogue could be obtained. Similar to the Et3SiH complexes,
the values of JSiH are diminished substantially from that of
the free silane (JSiH ) 200 Hz) upon coordination. Complex
5 has JSiH ) 111 Hz, and complex 6 exhibits JSiH ) 101 Hz.
For both HSiEt3 and HSiPh3, the Si-H bond is more
activated for W versus Cr. A small number of examples of
Ph3SiH coordination have been previously reported. In
comparison to these examples, the extent of activation of
the Si-H bond in 5 and 6 is quite modest. For example, in
(MeCp)Mn(CO)2(HSiPh3), JSiH ) 65 Hz.17 We conclude that
complexes 5 and 6 represent examples of relatively weak
silane complexation, presumably due to steric congestion in
Ph3SiH, which may also be responsible for our failure to
observe the Mo analogue. It is interesting to note that a prior
report of prolonged photolysis of W(CO)6 in the presence
of Ph3SiH led to a tricarbonyl species W(CO)3(η6-C6H5-
SiHPh2) with a coordinated phenyl group.18 It seems plausible
that complex 6 is an intermediate in the formation of this
tricarbonyl species, which we do not observe under our
conditions.
These values of JSiH are substantially reduced from the
value of 175 Hz observed in Et3SiH. In comparison, some
other reported values of JSiH in Et3SiH complexes are 62
Hz16 in cationic [CpFe(PEt3)(CO)(η2-HSiEt3)]+ and 61 Hz8
in cationic [cis-Re(CO)4(PPh3)(η2-HSiEt3)]+. Because the
reduction of JSiH is much less than that observed in other
Et3SiH complexes, we conclude that the interaction with the
metal center is relatively weak. In particular, because the
presence of five CO ligands renders the metal center highly
electron-deficient, back-donation from the metal d orbitals
into the Si-H σ* orbital is limited. This result is similar to
the reported observations in the case of dihydrogen com-
plexes such as Cr(CO)5(H2) and W(CO)5(H2), where the
properties of the molecules were found to be consistent with
limited back-donation from the metal center to the bound
H2.12
Diphenylsilane Complexes. Coordination of Ph2SiH2 was
observed for all of the group 6 metals, affording 7-9. As
expected, only one of the Si-H bonds coordinates, with JSiH
A
) 108 (7), 111 (8), and 98 (9) Hz. Again, the W complex
The observation of 1-3 provides a unique opportunity to
compare Si-H bond coordination in a homologous series
spanning the group 6 metals. If we assume that Si-H bond
elongation is inversely proportional to JSiH within a homolo-
gous series, we can correlate the observed couplings to bond
elongation and use this as a probe for the strength of the
interaction with the metal. On the basis of the observed
exhibits the greatest degree of Si-H bond elongation.
(17) Jetz, W.; Graham, W. A. G. Inorg. Chem. 1971, 10, 4-9.
(18) Ga¸dek, A.; Kochel, A.; Szyman˜ska-Buzar, J. Organomet. Chem. 2005,
690, 685-690.
(16) Scharrer, E.; Chang, S.; Brookhart, M. Organometallics 1995, 14,
5686-5694.
Inorganic Chemistry, Vol. 45, No. 16, 2006 6457

Heinekey et al.
Interestingly, JSiH increases slightly upon complexation to
precedent with agostic C-H bonds, it was expected that there
would be a nonstatistical occupancy of the bound and
pendant site by deuterium. We anticipated that H would
concentrate in the metal-bound environment because this
bond is weakened and elongated by coordination. Thus,
deuterium is expected to concentrate around the pendant site.
B
234 (7), 232 (8), and 236 (9) Hz. This large perturbation in
the coupling in the pendant SiH moiety has been previously
observed only in complexes of R2SiH2 with highly electro-
philic cationic metal centers, where the bound silane is also
activated toward reaction with nucleophiles. In the case of
[(PPh3)Re(CO)4(η2-HSiEt3)]+, liberation of [SiEt3]+ and
formation of products consistent with the formation of
(PPh3)Re(CO)4H has been observed.8 Thus, the increase in
1
Integration of the H NMR signals verifies this expectation
in that the intensity of the hydridic resonance is slightly
greater (1.08:1) than that for the pendant SiH. In a study of
alkane σ complexes of CpRe(CO)2, Ball and co-workers have
made similar observations and report a slight preference for
H to concentrate in the metal-bound environment.22
JSiH may indicate an increase in the s character of the Si-H
B
bond because of the increased polarization of the bound
Si-H bond and incipient formation of the silyl cation, which
would have sp2 hybridization.
In comparison to other reported complexes of Ph2SiH2,
complexes 7-9 have high values of JSiH, consistent with
relatively limited back-donation from the metal center to the
Si-H σ* orbital. Previously reported examples from the Cr
triad include JSiH ) 71 Hz in (η6-Me6C6)Cr(CO)2(η2-
HSiPh2H)19 and JSiH ) 50 Hz in (CO)Mo(dppe)2(η2-
HSiPh2H).7
Complexes 7-9 exhibit a very interesting intramolecular
dynamic process, observable by NMR line broadening and
magnetization transfer. The coordinated SiHA is observed
to exchange rapidly with the pendant SiHB, with enthalpies
of activation of 9-10 kcal/mol. Dissociation of the bound
silane is not involved because no line broadening is observed
in free silane. The entropies of activation are slightly
negative, consistent with an intramolecular process with an
ordered transition state. We suggest a mechanism for this
process proceeding via a species with both Si-H bonds
coordinated to the metal, as depicted in the following. Such
Cr(CO)4(PMe3)(η2-HSiEt3). Photolysis of (PMe3)Cr(CO)5
in the presence of Et3SiH gives two isomeric silane adducts
10 and 11 in a 20:1 ratio. The major product, complex 10,
is assigned to the trans structure based on the larger coupling
1
to 31P. This assignment was verified by a H-31P HMQC
experiment, which also confirmed the assignment of the 31
P
NMR resonances at δ 21.4 (10) and 9.4 (11). The starting
material (PMe3)Cr(CO)5 exhibits a 31P NMR signal at δ 9.0.
Complexation of Et3SiH cis to PMe3 has very little effect
on the 31P NMR chemical shift, while replacement of a trans-
CO group with Et3SiH in 10 leads to a significant chemical
shift change to δ 21.4.
Surprisingly, both isomers exhibit nearly identical values
of JSiH of 102 (10) and 99 (11) Hz, which are also quite
similar to the value for 1 of 95 Hz. This outcome is
unexpected because it is generally believed that the identity
of the trans ligand controls the degree of bond activation in
σ complexes. In these very electrophilic metal centers, the
interaction with the Si-H bond is dominated by donation
from the Si-H bond to an acceptor orbital on Cr, with
minimal contributions from back-donation. Thus, the strong
π-acid CO trans to bound silane versus the phosphine ligand
leads to a very similar degree of Si-H bond elongation. A
similar outcome was recently reported for coordination of
H2 to these Cr centers.23 The increase in JSiH upon inclusion
of a phosphine ligand in the metal coordination sphere is
similar to that observed between [Re(CO)5(η2-HSiEt3)]+ and
[Re(CO)4(PPh3)(η2-SiEt3)]+. Because the electronic nature
of the Cr-HSi interaction is very similar in 10 and 11, the
observed product ratio of 20:1 versus the statistical ratio of
1:4 must arise from other factors. We suspect either that there
is a steric influence to the preferred structure or that the
products are themselves photoactive, and the observed
product ratio represents that from a photostationary state
attained during photosynthesis.
a rapid exchange of free and coordinated Si-H bonds in
metal complexes of SiH2R2 has not been previously observed.
Slow exchange of this type was detected using deuterium
labeling in the manganese complex CpMn(CO)2(η2-DSi-
HRR′).20 In the ruthenium silyl hydride Ru(PtBu2Me)(CO)-
HA(SiHBPh2), exchange of HA and HB was detectable by spin
saturation transfer.21 We suggest that the rapid exchange
observed in 7-9 is a further manifestation of the highly
electrophilic nature of these metal centers. The coordination
of a second Si-H bond to form the η3-H,Si,H structure is
lowered in energy with respect to the η2-Si,H structure in
these electrophilic complexes, which enables the exchange
to happen more rapidly than has been observed previously.
Equilibrium isotope effects were probed by preparing
complex 6 with partially deuterated diphenylsilane using a
mixture of H2SiPh2 and HDSiPh2. On the basis of extensive
(19) Schubert, U.; Mu¨ller, J.; Alt, H. G. Organometallics 1987, 6, 469-
472.
(20) Colomer, E.; Corriu, R. J. P.; Marzin, C.; Vioux, A. Inorg. Chem.
1982, 21, 368-373.
(21) Gusev, D. G.; Nadasdi, T. T.; Caulton, K. G. Inorg. Chem. 1996, 35,
6772-6774.
(22) Lawes, D. J.; Geftakis, S.; Ball, G. E. J. Am. Chem. Soc. 2005, 127,
4134-4135.
(23) Matthews, S. L.; Heinekey, D. J. Am. Chem. Soc. 2006, submitted
for publication.
6458 Inorganic Chemistry, Vol. 45, No. 16, 2006

Silane Complexes of Electrophilic Metal Centers
Catalytic Methanolyis. Binding of silanes to transition
metals is well-known to lead to enhanced reactivity with
nucleophiles. Particularly important in this respect is the use
of alcohols as the nucleophile, which affords silyl ethers.
This is a useful reaction in organic synthesis for reversible
protection of OH functionalities.
We find that Cr(CO)5(L) complexes (L ) THF and Et3-
SiH) are effective catalysts for methanolysis of Et3SiH. We
have not optimized the conditions but find that MeOSiEt3
can be efficiently formed from MeOH and Et3SiH, with 2600
turnovers prior to catalyst deactivation, which occurs after
about 12 h. These results are comparable to those reported
by Brookhart and co-workers24 using [CpFe(CO)2]+ but
slower than those of Luo and Crabtree’s catalyst [IrH2(PPh3)2-
(THF)2]+.25 We suggest the mechanism depicted below for
the catalytic reaction, which is similar to that suggested by
Brookhart and co-workers.24
ligand. Both cis and trans isomers are observed in a
nonstatistical ratio.
Conclusions. Silanes bind weakly to electrophilic M(CO)5
fragments, with modest activation of the Si-H bond. An
unusual rapid exchange between bound and pendant Si-H
bonds has been observed in Ph2SiH2 complexes. Replacement
of one CO ligand with PMe3 has surprisingly little effect on
the activation of Et3SiH on Cr. The electronic basis of the
SiH/metal interaction is relatively insensitive to the coligands,
suggesting a minor role for back-donation from metal to
Increasing the charge on the complex is seen to increase
the extent to which the coordinated bond is elongated on
coordination, consistent with dominant σ donation from
silane to the metal center and minor contributions to the
interaction from back-donation. Complexation activates
silanes toward reaction with nucleophiles, which has been
applied to catalytic methanolysis.
Acknowledgment. This research was supported by the
National Science Foundation.
(24) Chang, S.; Scharrer, E.; Brookhart, M. J. Mol. Catal. A 1998, 130,
107-119.
(25) Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc. 1989, 111, 2527-
2535.
IC052134P
Inorganic Chemistry, Vol. 45, No. 16, 2006 6459