Reactions of secondary silanes with [{Pr2iPCH2CH2PPr2 i}Rh]2(μ-H)2. A series of dinuclear silyl-hydride complexes and their possible involvement in the catalytic hydro

Article abstract of DOI:10.1021/om950450d

Addition of a 1 equiv of a secondary silane (RR′SiH₂) to [(dippe)Rh]₂(μ-H)₂ (1) gives the complexes [(diPpe)Rh]₂(μ-H)(μ-η²-HSiRR′) (2a, R = R′ = Ph; 2b, R = R′ = Me; 2c, R = Ph, R′ = Me; dippe = 1,2-b

Full text of DOI:10.1021/om950450d

Organometallics 1996, 15, 2871-2880
2871
Rea ction s of Secon d a r y Sila n es w ith
[{P r i P CH2CH2P P r i }Rh ]2(µ-H)2. A Ser ies of Din u clea r
2
2
Silyl-Hyd r id e Com p lexes a n d Th eir P ossible
In volvem en t in th e Ca ta lytic Hyd r osilyla tion of Olefin s
Michael D. Fryzuk,*,1a Lisa Rosenberg, and Steven J . Rettig1b
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1
Received J une 13, 1995X
Addition of a 1 equiv of a secondary silane (RR′SiH2) to [(dippe)Rh]2(µ-H)2 (1) gives the
complexes [(dippe)Rh]2(µ-H)(µ-η2-HSiRR′) (2a , R ) R′ ) Ph; 2b, R ) R′ ) Me; 2c, R ) Ph,
R′ ) Me; dippe ) 1,2-bis(diisopropylphosphino)ethane). X-ray diffraction studies of 2a ,b
confirm the presence of a three-center, two-electron, Rh-H-Si bond. The observed
fluxionality of 2a -c in solution is due to exchange of the silicon and rhodium hydrides.
Complexes 2a -c lose hydrogen in the presence of 1 equiv of carbon monoxide to give
complexes 3a -c, [(dippe)Rh]2(µ-SiRR′)(µ-CO). A catalytic cycle is proposed for the hydro-
silylation of olefins by diphenylsilane to give Ph2SiR2 (R ) Et) or Ph2SiHR (R ) Bu), which
occurs in the presence of 1. One of the active species is thought to be [(dippe)Rh]2(µ-H)(µ-
η2-HSiPh2) (2a ).
In tr od u ction
accompanied by reductive elimination to regenerate a
Rh(I) silyl species; an example is shown in eq 3.8,9 This
Most studies of the reactivity of silanes toward
rhodium complexes have arisen from investigation of
their activity as hydrosilylation catalysts. In particular,
Wilkinson’s catalyst, RhCl(PPh3)3, has received much
attention as a catalyst precursor for the hydrosilylation
of various unsaturated organic molecules,2 and thus it
is not too surprising that the vast majority of known
rhodium silyl complexes are derived from this complex
and its analogues. As shown in eq 1, five-coordinate
-CH4
Rh(CH3)(PMe3)4 + HSiPh3
8 RhSiPh3(PMe3)3
-PMe3
(3)
is one of the few known Rh(I) silyl derivatives. Also,
reaction of silanes with some “half-sandwich” cyclo-
pentadienyl-rhodium complexes have produced silyl
complexes
-PR′3
RhCl(PR′3)3 + HSiR3
8 Rh(SiR3)H(Cl)(PR′3)2 (1)
silyl-hydride complexes of Rh(III) can be obtained by
oxidative addition of tertiary silanes to RhCl(PR3)3; a
wide variety of substituents on silicon can be tolerated,
and different phosphine ligands have been examined as
well.3-7 Octahedral Rh(III) silyl-hydride complexes
result upon oxidative addition of tertiary silanes to the
related chloro-carbonyl complex RhCl(CO)(PR3)2, as
shown in eq 2.3 When a Rh(I) complex contains a
with rhodium in the rare +5 oxidation state.10,11
There are not many examples in the literature of
dinuclear rhodium silyl complexes. In fact, addition of
silanes to dinuclear rhodium complexes such as Rh2-
(PF3)8 and [(C8H12)RhCl]2 (C8H12 ) 1,5-cyclooctadiene)
typically generates mononuclear products.12,13 Studies
of the dinuclear rhodium complex Rh2(µ-H)2(CO)2(µ-
dppm)2, however, have yielded dinuclear silyl complexes,
including that shown (dppm ) bis(diphenylphosphino)-
methane):14
RhCl(CO)(PR′3)2 + HSiR3 f
Rh(SiR3)H(Cl)CO(PR′3)2 (2)
hydrocarbyl ligand, then oxidative addition can be
X Abstract published in Advance ACS Abstracts, J une 1, 1996.
(1) (a) Tel: (604) 822-2897. FAX: (604) 822-2847. E-mail:
fryzuk@chem.ubc.ca. (b) Experimental Officer: UBC Crystallographic
Service.
(2) Ojima, I. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 1479.
(3) de Charenteney, F.; Osborn, J . A.; Wilkinson, G. J . Chem. Soc.
A 1968, 787.
(4) Haszeldine, R. N.; Parish, R. V.; Parry, D. J . J . Chem. Soc. A
1969, 683.
(5) Glockling, F.; Hill, G. C. J . Chem. Soc. A 1971, 2137.
(6) Ojima, I.; Nihonyanagi, M. J . Chem. Soc., Chem. Commun. 1972,
938.
(7) Haszeldine, R. N.; Parish, R. V.; Taylor, R. J . J . Chem. Soc.,
Dalton Trans. 1974, 2311.
(8) Thorn, D. L.; Harlow, R. L. Inorg. Chem. 1990, 29, 2017.
(9) Hofmann, P.; Meier, C.; Hiller, W.; Heckel, M.; Reide, J .; Schmidt,
M. U. J . Organomet. Chem. 1995, 490, 51.
S0276-7333(95)00450-X CCC: $12.00 © 1996 American Chemical Society
+
+

2872 Organometallics, Vol. 15, No. 13, 1996
Fryzuk et al.
In this work the reactions of a variety of secondary
silanes (RR′SiH2) with the dinuclear rhodium hydride
complex [(dippe)Rh]2(µ-H)2 (1; dippe ) 1,2-bis(diisopro-
F igu r e 1. Possible averaged structure in solution for
complexes 2a -c.
pylphosphino)ethane) are presented, and the charac-
terization and reactivity of the ensuing complexes are
discussed. The use of 1 as a catalyst precursor for the
hydrosilylation of olefins by diphenylsilane is also
described. A possible hydrosilylation catalytic cycle is
proposed.
Resu lts a n d Discu ssion
Syn th esis a n d P r op er ties of Din u clea r Silyl-
Hyd r id e Com p lexes. We have already reported15 that
the dinuclear rhodium hydride complex [(dippe)Rh]2(µ-
H)2 (1) reacts rapidly with 1 equiv of diphenylsilane
(Ph2SiH2) to afford a dinuclear product of empirical
formula [(dippe)Rh]2‚Ph2SiH2 (2a ) in good yield, with
concurrent evolution of 1 equiv of dihydrogen. This
reaction may be extended to dimethylsilane (Me2SiH2)
and methylphenylsilane (MePhSiH2), generating the
analogous complexes 2b,c. Isolated yields of these
F igu r e 2. Molecular structure and numbering scheme for
[(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ).
-H2
or doublet-of-multiplet patterns are observed in the
[(dippe)Rh]2(µ-H)2
1
31P{1H} NMR spectra (coupling to 103Rh, I ) /2, 100%
+ RR′SiH2
8
1
natural abundance). These results are consistent with
the proposed structure shown in Figure 1, with two
bridging hydrides and one bridging silylene ligand.
However, the broadness of the doublets observed in the
31P{1H} NMR spectra of 2a and 2c suggest that these
compounds are fluxional; variable-temperature 31P{1H}
[(dippe)Rh]2‚RR′SiH2
(4)
2a : R ) R′ ) Ph
2b: R ) R′ ) Me
2c: R ) Me, R′ ) Ph
complexes vary from 60 to 85% depending on the scale
used, due to their high solubility in organic solvents.
The complexes are air- and moisture-sensitive both in
solution and in the solid state; they are thermally stable
in solution, as judged by the fact that heating to 80 °C
in d8-toluene in a sealed NMR tube for several days
caused no decomposition, as determined by 31P{1H}
NMR spectroscopy. Labeling studies using the corre-
sponding dideuteride [(dippe)Rh]2(µ-D)2 (d2-1) show that
the oxidative addition of the Si-H bond of Ph2SiH2 and
reductive elimination of H2 occurs nonselectively (with
complete scrambling of H/D) via a highly fluxional
intermediate.
1
and H NMR spectroscopic studies have confirmed that
some fluxional process is occurring in which the two
hydrides exchange between inequivalent sites. Indeed,
a fluxional process must also be occurring to exchange
the methyl and phenyl substituents on silicon for 2c,
to give an average structure in solution with four
equivalent phosphines. This is discussed in more detail
later.
Solid -Sta te Str u ctu r es of [(d ip p e)Rh ]2(µ-H)(µ-η2-
HSiR2) (R ) P h , 2a ; R ) Me, 2b). The solid-state
structure of the dinuclear complex [(dippe)Rh]2(µ-H)(µ-
η2-HSiPh2) (2a ) was evident from a single-crystal X-ray
diffraction analysis and has already been reported;15 the
analogous structure of [(dippe)Rh]2(µ-H)(µ-η2-H-SiMe2)
(2b) was also determined. Both solid-state structures
are different from the solution structures proposed in
the previous section. ORTEP diagrams of the dinuclear
complexes are shown in Figures 2 and 3, and selected
bond distances and bond angles for the two structures
are shown in Tables 2 and 3. In these structures the
two hydride ligands were both located and refined.
The most interesting feature of these two structures
is that while one hydride ligand bridges the two Rh
centers, the other is found bridging a Rh-Si bond. The
average lengthening of the bridged Rh-Si bond relative
to the unbridged bond in the two molecules is 0.17 Å,
which is consistent with the presence of the bridging
The room-temperature H and 31P{1H} NMR spectra
1
of complexes 2a -c suggest highly symmetric structures
in solution, where all four phosphorus centers are
chemically and magnetically equivalent, as are both
rhodium centers. A single rhodium hydride resonance,
a sharp multiplet with a relative intensity of 2, is
1
observed in each H NMR spectrum. Symmetric doublet
(10) Fernandez, M.; Bailey, P. M.; Bentz, P. O.; Ricci, J . S.; Koetzle,
T. F.; Maitlis, P. M. J . Am. Chem. Soc. 1984, 106, 5458.
(11) Duckett, S. B.; Perutz, R. N. J . Chem. Soc., Chem. Commun.
1991, 28.
(12) Bennett, M. A.; Patmore, D. J . Inorg. Chem. 1971, 10, 2387.
(13) Aylett, B. J . J . Organomet. Chem. 1980, 9, 327.
(14) Wang, W.; Eisenberg, R. J . Am. Chem. Soc. 1990, 112, 1833.
(15) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J . Organometallics
1991, 10, 2537.
+
+

Dinuclear Silyl-Hydride Complexes
Organometallics, Vol. 15, No. 13, 1996 2873
Ta ble 2. Selected Bon d Len gth s (Å) w ith
Estim a ted Sta n d a r d Devia tion s in P a r en th eses
[(dippe)Rh]2(µ-H)(µ-HSiPh2) (2a )
Rh(1)-Rh(2)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-Si
Rh(1)-H(1)
Rh(2)-P(3)
Rh(2)-P(4)
2.937(1)
2.213(2)
2.295(2)
2.298(2)
1.71(6)
Rh(2)-Si
Rh(2)-H(1)
Rh(2)-H(2)
Si-C(29)
Si-C(35)
Si-H(2)
2.487(2)
1.90(6)
1.61(6)
1.907(5)
1.902(5)
1.66(6)
2.221(2)
2.233(2)
[(dippe)Rh]2(µ-H)(µ-HSiMe2) (2b)
Rh(1)-Rh(2)
Rh(1)-P(1)
Rh(1)-P(2)
Rh(1)-Si(1)
Rh(1)-H(1)
Rh(2)-P(3)
Rh(2)-P(4)
2.8835(6)
2.2738(9)
2.2154(9)
2.308(1)
1.65(3)
Rh(2)-Si(1)
Rh(2)-H(1)
Rh(2)-H(2)
Si(1)-C(13)
Si(1)-C(14)
Si(1)-H(2)
2.463(1)
1.91(3)
1.51(4)
1.899(4)
1.899(4)
1.73(4)
2.246(1)
2.2165(9)
Ta ble 3. Selected Bon d An gles (d eg) w ith
Estim a ted Sta n d a r d Devia tion s in P a r en th eses
[(dippe)Rh]2(µ-H)(µ-HSiPh2) (2a )
P(1)-Rh(1)-P(2)
P(1)-Rh(1)-Si
87.02(6) P(4)-Rh(2)-H(2)
97.33(6) H(1)-Rh(2)-H(1)
172(2)
111(3)
124.1(2)
123.5(2)
101(2)
104.0(2)
90(2)
F igu r e 3. Molecular structure and numbering scheme for
[(dippe)Rh]2(µ-H)(µ-η2-HSiMe2) (2b).
P(1)-Rh(1)-H(1)
P(2)-Rh(1)-Si
170(2)
Rh(1)-Si-C(29)
164.73(5) Rh(1)-Si-C(35)
Ta ble 1. Cr ysta llogr a p h ic Da ta a
P(2)-Rh(1)-H(1)
Si-Rh(1)-H(1)
P(3)-Rh(2)-P(4)
P(3)-Rh(2)-H(1)
P(3)-Rh(2)-H(2)
P(4)-Rh(2)-H(1)
84(2)
92(2)
Rh(1)-Si-H(2)
C(29)-Si-C(35)
compd
[(dippe)Rh]2(µ-H)-
(µ-HSiPh2) (2a )
[(dippe)Rh]2(µ-H)-
(µ-HSiMe2) (2b)
C30H72P4Rh2Si
760.69
86.60(6) C(29)-Si-H(2)
157(2)
85(2)
77(2)
C(35)-Si-H(2)
108(2)
formula
fw
C
40H76P4Rh2Si
Rh(1)-H(1)-Rh(2) 109(3)
914.83
Rh(2)-H(2)-Si
99(3)
color, habit
cryst size, mm
cryst syst
space group
a, Å
red-orange, irregular orange, needle
[(dippe)Rh]2(µ-H)(µ-HSiMe2) (2b)
0.20 × 0.35 × 0.50
monoclinic
P21/n
0.08 × 0.15 × 0.45
monoclinic
P21/n
P(1)-Rh(1)-P(2)
87.45(3) Si(1)-Rh(2)-H(2)
44(1)
107(2)
74.29(3)
P(1)-Rh(1)-Si(1) 152.36(3) H(1)-Rh(2)-H(2)
P(1)-Rh(1)-H(1)
P(2)-Rh(1)-Si(1)
P(2)-Rh(1)-H(1)
Si(1)-Rh(1)-H(1)
P(3)-Rh(2)-P(4)
83(1)
Rh(1)-Si(1)-Rh(2)
19.338(8)
11.223(4)
22.934(9)
112.39(3)
4602(6)
4
13.386(3)
17.067(4)
17.862(4)
102.69(2)
3981(1)
4
98.12(4) Rh(1)-Si(1)-C(13) 132.4(1)
170(1)
92(1)
b, Å
c, Å
Rh(1)-Si(1)-C(14) 117.4(1)
Rh(1)-Si(1)-H(2) 102(1)
â, deg
86.71(3) Rh(2)-Si(1)-C(13) 108.4(1)
V, Å3
P(3)-Rh(2)-Si(1) 140.65(3) Rh(2)-Si(1)-C(14) 125.3(1)
Z
P(3)-Rh(2)-H(1)
P(3)-Rh(2)-H(2)
P(4)-Rh(2)-Si(1) 116.16(3) C(13)-Si(1)-H(2)
P(4)-Rh(2)-H(1)
P(4)-Rh(2)-H(2)
Si(1)-Rh(2)-H(1)
84(1)
168(1)
Rh(2)-Si(1)-H(2)
C(13)-Si(1)-C(14)
37(1)
99.8(2)
106(1)
91(1)
T, °C
21
21
Dc, g/cm3
F(000)
1.320
1.319
1920
1664
159(1)
82(1)
82(1)
C(14)-Si(1)-H(2)
Rh(1)-H(1)-Rh(2) 108(2)
Rh(2)-H(2)-Si(1) 99(2)
µ(Mo KR), cm-1
transmissn factors
(rel)
8.93
10.21
0.91-1.00
0.90-1.00
scan type
ω-2θ
ω-2θ
(1.48 ( 0.02 Å),16 and the Rh-H distances of 1.51(4)
and 1.61(6) Å are within the normal range for terminal
or bridging hydrides on rhodium. Three-center, two-
electron bonds such as these are no longer considered
rare for Si-H interactions with transition metals.16-19
They are commonly viewed as coordinated Si-H
scan range, deg in ω 1.31 + 0.35 tan θ
scan speed, deg/min 32 (up to 8 rescans) 16 (up to 8 rescans)
0.89 + 0.35 tan θ
data collected
2θmax, deg
cryst decay, %
total no. of rflns
no. of unique rflns
Rmerge
no. with I g 3σ(I)
no. of variables
R
+h,+k,(l
55
+h,+k,(l
60
2.7
12 412
11 943
0.036
6473
342
negligible
11 434
11 112
0.036
5752
σ-bonds16,20-22sessentially
arrested
oxidative
additionssand are referred to as “agostic” Si-H bonds,
a term originally coined to describe the coordination of
a C-H σ-bond from an already coordinated ligand to
the metal center.23 Comparison of the relevant bond
lengths in the structures of 2a ,b with those for other
dinuclear complexes with agostic Si-H bridges suggests
that the oxidative addition of the Si-H bond to rhodium
432
0.037
0.043
1.46
0.029
0.027
1.35
Rw
GOF
max ∆/σ (final cycle) 0.03
0.19 (H coordinate)
residual density, e/Å3 -0.52, 0.78 (near Rh) -0.33, 0.37
a
Conditions and definitions: Rigaku AFC6S diffractometer,
takeoff angle 6.0°, aperture 6.0 × 6.0 mm at a distance of 285 mm
from the crystal, stationary background counts at each end of the
scan (scan/background time ratio 2:1), Mo KR radiation (λ )
0.710 69 Å), graphite monochromator, σ2(F2) ) [S2(C + 4B) +
(pF2)2]/Lp2 (S ) scan speed, C ) scan count, B ) normalized
background count, p ) 0.03 for 2a and 0.01 for 2b), function
minimized ∑w(|Fo| - |Fc|)2, where w ) 4Fo2/σ2(Fo2), R ) ∑||Fo| -
(16) Schubert, U. Adv. Organomet. Chem. 1990, 30, 151.
(17) Matarasso-Tchiroukhine, E. J . Chem. Soc., Chem. Commun.
1990, 681.
(18) Carren˜o, R.; Riera, V.; Ruiz, M. A.; J eannin, Y.; Philoche-
Levisalles, M. J . Chem. Soc., Chem. Commun. 1990, 15.
(19) Suzuki, H.; Takao, T.; Tanaka, M.; Moro-oka, Y. J . Chem. Soc.,
Chem. Commun. 1992, 476.
(20) Crabtree, R. H.; Hamilton, D. G. Adv. Organomet. Chem. 1988,
28, 299.
(21) Lichtenberger, D. L.; Rai-Chaudhuri, A. Inorg. Chem. 1990, 29,
975.
2
|Fc||/∑|Fo|, Rw ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and GOF ) [∑w(|Fo|
- |Fc|)2/(m - n)]1/2
. Values given for R, Rw, and GOF are based
on those reflections with I g 3σ(I).
(22) J essop, P. G.; Morris, R. H. Coord. Chem. Rev. 1992, 121, 155.
(23) Brookhart, M.; Green, M. L. H. J . Organomet. Chem. 1983, 250,
395.
hydride. The Si-H distances are only roughly 0.2 Å
longer than a typical Si-H bond length in free silane
+
+

2874 Organometallics, Vol. 15, No. 13, 1996
Fryzuk et al.
in 2a has been arrested at a relatively early stage, while
for 2b the addition has been arrested at an intermediate
stage.16
The strong trans influence of the silyl ligand (-SiR3)
manifests itself in the two structures as a lengthening
of the P-Rh distances for those phosphines trans to the
silicon, relative to the other P-Rh distances in the
structures.24 The most noticeable effects of the trans
influence are in the structure of 2a , where P(2)-Rh(1)
is ∼0.08 Å longer than P(1)-Rh(1) and P(3)-Rh(2). In
the structure of 2b P(1)-Rh(1) is 0.06 Å longer than
P(2)-Rh(1) and P(4)-Rh(2). It could be argued that
these effects are simply steric in origin, but in that case
one would expect the trend to be reversed. For example,
in 2a one would expect P(1) and P(3) to be pushed away
from the phenyl groups on silicon, resulting in longer
P(1)-Rh(1) and P(3)-Rh(2) bond lengths. Instead,
these are the shortest P-Rh bond lengths.
If the Si-H σ-interaction with rhodium is viewed as
a single ligand, the geometry at each of the rhodium
centers in complexes 2a ,b is roughly square planar,
though the two planes are twisted relative to each other.
The dihedral angles between the coordination planes
defined by P(1), Rh(1), P(2) and by P(3), Rh(2), P(4) in
the two structures are 116.6 and 133.8° for 2a and 2b,
respectively. This twisting of the chelate rings relative
to each other is probably due mainly to the size of the
bridging η2-HSiR2 ligand. The Rh-Rh separations
observed in these structures are similar to a range of
distances seen for the complexes [(P2)Rh]2(µ-H)(µ-X)
(where P2 ) dippe, (P(OMe)3)2; X ) η2-CRdCHR). For
example, in the vinyl hydride complex [(dippe)Rh]2(µ-
H)(µ-η2-CHdCH2)25 (5), the Rh-Rh distance is 2.8655(5)
Å, while for [{(MeO)3P}2Rh]2(µ-η2-C(Tolp)dCH(Tolp)(µ-
H)26 the separation is 2.936(1) Å.
F igu r e 4. Variable-temperature 121.4 MHz 31P{1H} NMR
spectra of [(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ) in C7D8 (a
small amount of 1 is indicated by an asterisk).
1
Var iable-Tem per atu r e 31P {1H} an d H NMR Spec-
tr oscop y of 2a -c. The variable-temperature 31P{1H}
1
and H (hydride region) NMR spectra of [(dippe)Rh]2-
(µ-H)(µ-η2-HSiPh2) (2a ) are shown in Figures 4 and 5,
respectively. While at room temperature the 31P{1H}
NMR spectrum of 2a is a slightly broad doublet, at lower
temperatures the spectrum becomes more complex,
illustrating two dynamic processes, one with a coales-
cence at about -40 °C and the other with a coalescence
at about -80 °C. At -66 °C the 31P{1H} NMR spectrum
indicates a species with two equivalent phosphines and
two other, inequivalent phosphines (three doublets of
integral ratio 2:1:1), whereas at -96 °C the spectrum
shows a broad set of signals attributable to the presence
of four inequivalent phosphines; thus, at very low
temperatures we observe a spectrum that corresponds
with the solid-state structure of 2a . In the intermedi-
ate-temperature regime, the 31P{1H} NMR spectrum is
consistent with structure B in Scheme 1. The presence
of a single hydride resonance for both the Si-H and the
1
F igu r e 5. Variable-temperature 300 MHz H NMR spec-
tra (hydride region) of [(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a )
in C7D8.
1
Rh-H-Rh moieties in the room-temperature H NMR
exchange is shown in Scheme 1; starting at the bottom
left, the complete oxidative addition of the Si-Ha bond
of 2 gives the species B, with a bridging silylene
fragment and a terminal hydride. The changes in
geometry at the rhodium centers required to accom-
modate this correspond to the lower temperature proc-
ess observed in the 31P{1H} NMR spectra. The terminal
hydride subsequently swings into a bridging position,
yielding a symmetric intermediate with the structure
A (proposed structure 2a in Figure 1). The reverse of
spectrum suggests that the fluxionality in 2a also
involves exchange of the bridging hydride Hb and the
silicon “hydride” Ha. A possible mechanism for this
(24) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987.
(25) Fryzuk, M. D.; J ones, T.; Einstein, F. W. B. Organometallics
1984, 3, 185.
(26) Burch, R. R.; Shusterman, A. J .; Muetterties, E. L.; Teller, R.
G.; Williams, J . M. J . Am. Chem. Soc. 1983, 105, 3546.
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Dinuclear Silyl-Hydride Complexes
Organometallics, Vol. 15, No. 13, 1996 2875
Sch em e 1
doublet at 97.8 ppm, a doublet at 92.8 ppm, and a broad
singlet at 87.2 ppm, in a 2:1:1 ratio. A similar spectrum
(three doublets of integral ratio 2:1:1) is observed for
2a at -55 °C, with almost identical chemical shift
separations between the analogous peaks (Figure 4).
1
The H and 31P{1H} NMR spectra of [(dippe)Rh]2(µ-H)-
(µ-η2-HSiMe2) (2b) show even less change at low tem-
perature than those for the methylphenylsilyl analogue.
For 2b there is only a slight broadening of the doublet
observed in the 31P{1H} NMR spectrum and of the
1
hydride signal at -6.3 ppm in the H NMR spectrum
at -85 °C. Given that all three complexes 2a -c show
resonances in their room-temperature 31P{1H} NMR
spectra with practically identical chemical shifts, and
given the similar shifts observed in the low-temperature
31P{1H} NMR spectra of 2a and 2c, it is likely that the
low-temperature limit for the 31P{1H} spectra of all
three complexes would show peaks with similar shifts.
Thus, the differences in coalescence temperatures for
2a -c result mainly from differences in the activation
barrier to hydride exchange. There is a steady reduc-
tion of this activation barrier to exchange in complexes
2a -c which corresponds to the stepwise replacement
of phenyl groups with methyl groups; ∆Gq > ∆Gq
>
2a
2c
∆Gq for the process.
2b
One further point concerns the methylphenylsilyl-
hydride complex [(dippe)Rh]2(µ-H)(µ-η2-HSiMePh) (2c).
The above exchange mechanism in effect for these
molecules in solution (Scheme 1) does not explain why
the room-temperature 31P{1H} NMR spectrum for this
complex indicates the presence of four equivalent phos-
phines despite the unsymmetrically substituted silicon
in the averaged structure. This result requires the
substituents on silicon to be exchanging very rapidly,
which may be due to the coordinated Si-H bond rapidly
dissociating from and reassociating with the metal
center, as shown in Scheme 2. Rotation around the
remaining Rh-Si bond and inversion at the Si center
via a transition state labeled C would complete the
exchange. This process must be occurring for all three
silyl hydride complexes 2a -c.
these steps, with Hb swinging out to the terminal
position instead of Ha, accounts for the exchange the
two hydrides.15
1
The variable-temperature H NMR spectra shown in
Figure 5 are also consistent with the hydride fluxion-
ality described: at -91 to -96 °C the spectrum shows
two distinct hydride signals at -5.28 and -6.26 ppm.
This decoalescence corresponds to the higher tempera-
ture process observed in the 31P{1H} NMR spectra (i.e.,
the exchange involving structures A and B in Scheme
1). An approximate J H-P value of 40 Hz observed for
the lower field signal (-5.28 ppm) is larger tha the usual
cis H-P coupling but smaller than most trans coupling
constants measured in similar compounds. Because of
the broadening due to coupling to phosphorus and
rhodium no 29Si satellites are seen for either of these
hydride signals.27 It seems reasonable to conclude that
the three-center, two-electron Rh-H-Si bond observed
in the solid-state structure of 2a does exist in solution
but is labile with respect to complete oxidative addition
of the Si-H bond to the Rh center, rendering the silyl
hydride complex fluxional. A ∆Gq(193 K) value of 8.6
( 0.2 kcal/mol was calculated for 2a for the fluxional
process exchanging structures A and B in Scheme 1,
Disp la cem en t of Hyd r ogen by Ca r bon Mon oxid e
in th e Silyl Hyd r id e Com p lexes 2a -c. Dinuclear
rhodium complexes with bridging carbonyl and silylene
units (3a -c) are generated by the addition of ap-
proximately 1 equiv of CO to the silyl hydrides 2a -c
(eq 5). Complex 3a has been isolated in pure form, and
1
using the coalescence of the hydride signals in the H
NMR spectra.
For the methylphenylsilyl analogue 2c, [(dippe)Rh]2-
(µ-H)(µ-η2-HSiMePh), the activation barrier for ex-
change of the hydrides must be lower than for the
diphenyl derivative, since a decoalescence for the hy-
dride signals was not achieved even at -84 °C. As
shown in Figure 6, at that temperature the 31P{1H}
NMR spectrum of 2c shows three signals: a broad
(5)
complexes 3b,c have been observed in solution, though
they were not isolated pure in the solid state. Addition
of an excess of CO to the silyl hydride complexes gives
a mixture of products other than 3a -c which are
difficult to separate from each other and which have not
been identified. The approximately 1:1 reactions of the
silyl hydrides with CO are of interest because they
(27) 29Si NMR spectra for 2a and 2b show signals that are too broad
and poorly resolved to give any information on the connectivity at
silicon in these complexes, even at low temperature (for 2a ). This is
probably due to the extensive coupling of silicon to other spins,
combined with the fluxionality of the complexes.
+
+

2876 Organometallics, Vol. 15, No. 13, 1996
Fryzuk et al.
hydrosilylation of ethylene is the double-addition prod-
uct diethyldiphenylsilane, Ph2SiEt2. None of the single-
addition product, ethyldiphenylsilane, was observed in
reaction mixtures, even before completion (i.e., disap-
pearance of diphenylsilane). When this catalytic reac-
1
tion was monitored by H NMR, a turnover number of
960 h-1 was calculated. The hydrosilylation of 1-butene
results in the formation of the single-addition compound
n-butyldiphenylsilane, (n-Bu)SiHPh2, the expected anti-
Markovnikov addition product.29
A number of stoichiometric reactions relevant to the
hydrosilylation of ethylene catalyzed by 1 were carried
out in order to determine if the active species in the
catalytic cycle could be identified. For example, when
ethylene is added to the starting dihydride dimer
[(dippe)Rh]2(µ-H)2 (1), the dinuclear vinyl hydride com-
plex [(dippe)Rh]2(µ-H)(µ-η2-CHdCH2) (4) is formed.25
When ethylene is added to [(dippe)Rh]2(µ-H)(µ-η2-
HSiPh2) (2a ), the same vinyl hydride complex 4 is
isolated, along with diethyldiphenylsilane (eq 6). How-
F igu r e 6. Low-temperature 121.4 MHz 31P{1H} NMR
spectrum of [(dippe)Rh]2(µ-H)(µ-η2-HSiMePh2) (2c) in C7D8
at -84 °C.
Sch em e 2
2
+ C H (excess) f
[(dippe)Rh]2(µ-H)(µ-η -HSiPh2)
2
4
2a
2
+ Ph SiEt (6)
[(dippe)Rh]2(µ-H)(µ-η -CHdCH2)
2
2
4
ever, when diphenylsilane is added to the vinyl hydride
complex 4, the silyl hydride complex 2a is regenerated.
No diethyldiphenylsilane is observed in the 1H NMR
spectrum of this reaction, shown in eq 7; it is assumed
-C2H4
2
+ Ph SiH
8
[(dippe)Rh]2(µ-H)(µ-η -CHdCH2)
2
2
4
demonstrate the ease with which hydrogen is removed
from the Rh2Si core. The complete oxidative addition
of the second Si-H bond of diphenylsilane to Rh is
facile, as is the subsequent elimination of H2.
[(dippe)Rh]2(µ-H)(µ-η2-HSiPh2)
(7)
2a
that addition of the silane to the vinyl hydride causes
elimination of ethylene. The bis(µ-silylene) complex
[(dippe)Rh]2(µ-SiPh2)2 (5a ) is certainly present in any
mixture of 1 and excess silane.30 However, no reaction
is observed between 5a and ethylene, which rules out
the possible participation of the bis(µ-silylene) complex
in the hydrosilylation cycle.
The 31P{1H} NMR spectra of complexes 3a -c show
simple doublets of multiplets, indicating that in solution
all four phosphines are chemically equivalent. A struc-
ture which is consistent with the NMR data contains
tetrahedral rhodium centers. For the unsymmetrically
substituted µ-methylphenylsilylene complex 3c, the
simple 31P{1H} NMR spectrum observed must be due
to fluxionality of the complexes in solution. One pos-
sibility is through the formation of fully planar, di-
nuclear structures which rapidly equilibrate with the
proposed tetrahedral ground-state geometry.
(8)
Ca t a lyt ic H yd r osilyla t ion of E t h ylen e a n d
1-Bu ten e. The complex [(dippe)Rh]2(µ-H)2 (1) has been
shown to be an efficient catalyst precursor for the
hydrogenation of olefins at ambient temperature and
pressure. For example, its use as a catalyst precursor
for the hydrogenation of 1-hexene (with a substrate/
catalyst ratio of approximately 1700) gives turnover
The above reactions suggest that the hydrosilylation
of ethylene catalyzed by 1 could involve the silyl-
hydride 2a as one of the active species in the catalytic
cycle. A possible catalytic cycle for the hydrosilylation
of ethylene based on this dinuclear active species is
shown in Figure 7. If, in the presence of excess silane,
the catalyst is converted to 5a , this would “shut down”
the hydrosilylation. However, with every oxidative
addition of Ph2SiH2 to 1 to produce 2a , or to 2a to
produce 5a , dihydrogen is evolved. In the presence of
H2, the bis(µ-silylene) complex 5a is converted back to
the silyl-hydride complex 2a .30 Thus, in the catalytic
numbers in the range 850-950 h-1 28
Given the reac-
.
tivity of this complex toward both hydrogen and olefins,
one of the initial goals of this study was to confirm that
1 is also a catalyst precursor for the hydrosilylation of
olefins.
The complex [(dippe)Rh]2(µ-H)2 (1) was found to be a
catalyst precursor for the hydrosilylation of both eth-
ylene and 1-butene by diphenylsilane. The product of
(29) Elschenbroich, C.; Salzer, A. Organometallics: A Concise In-
troduction, 2nd ed.; VCH: New York, 1992.
(30) Fryzuk, M. D.; Rosenburg, L.; Rettig, S. J ., unpublished results.
(28) Fryzuk, M. D.; Piers, W. E.; Einstein, F. W. B.; J ones, T. Can.
J . Chem. 1989, 67, 883.
+
+

Dinuclear Silyl-Hydride Complexes
Organometallics, Vol. 15, No. 13, 1996 2877
Ph2SiHBu unit directly from G to give D without the
intermediacy of H becomes dominant.
The steps in this catalytic cycle parallel those in the
cycle originally proposed for hydrosilylation catalyzed
by mononuclear platinum-group metal complexes in
that the olefin inserts into a metal-hydride bond and
the silicon-carbon bond is formed by reductive elimina-
tion of adjacent silyl and alkyl groups:38
(9)
There is evidence, however, that suggests that for some
catalysts the silicon-carbon bond-forming reaction is
an insertion of the olefin into the metal-silicon bond,
which is then followed by reductive elimination of
â-silylalkyl and hydride ligands:39,40
(10)
The evidence cited as support for this mechanism is the
observation of vinylsilanes (resulting from â-hydride
elimination from the â-silylalkyl intermediate) as side
products of the catalysis. In our system, vinylsilanes
were not detected and thus their absence supports the
catalytic cycle shown in Figure 7. To discover if the
silyl-hydride complex 2a undergoes fragmentation/
recombination reactions in solution (yielding mono-
nuclear species which could be responsible for the
observed catalytic activity), a mixture of 2a and [(dippp)-
Rh]2(µ-H)(µ-η2-HSiPh2) (2d ) in toluene was stirred and
heated to 100 °C. No signals due to the crossover
product [(dippe)Rh](µ-H)(µ-η2-HSiPh2)[Rh(dippp)] were
observed in the 31P{1H} NMR spectrum of the residues
of this reaction.41 Given the observed stoichiometric
chemistry of 2a , a catalytic cycle for hydrosilylation
based on the dinuclear complex seems reasonable.
F igu r e 7. Possible catalytic cycle for the hydrosilylation
of olefins based on the dinuclear active catalyst [(dippe)-
Rh]2(µ-H)(µ-η2-HSiPh2) (2a ) (the chelating diphosphine
ligands have been omitted to simplify the diagram).
reaction mixture where hydrogen is constantly being
evolved, there should always be local concentrations of
dihydrogen that are enough to regenerate 2a and
continue the catalytic cycle. Coordination of olefin to
2a generates the adduct E, which undergoes olefin
insertion to produce the alkyl-silyl derivative F ; sub-
sequent reductive elimination followed by oxidative
addition of the remaining Si-H bond produces G, which
undergoes coordination by olefin to produce H. This
species undergoes insertion and reductive elimination
to produce the hydrosilylation product and the un-
bridged Rh(0) dimer D. This species has never been
isolated or detected spectroscopically in reactions of 1
but has been proposed as an intermediate in some
reactions of the dimer.31,32 The formation of five-
coordinate silicon in structures G and H would be
facilitated by the unusually electron-rich Rh
centers;25,32-35 species containing three-center, two-
electron alkyl bridges btween rhodium centers, such as
is seen in F , are known.36,37 To rationalize the differ-
ence between ethylene and 1-butene, we suggest that
the formation of H by coordination of 1-butene is
sufficiently slow that reductive elimination of the
Exp er im en ta l Section
Gen er a l P r oced u r es a n d Rea gen t Syn th eses. All ma-
nipulations were performed under prepurified nitrogen in a
Vacuum Atmospheres HE-553-2 workstation equipped with
an MO-40-2H purification system or using Schlenk-type
glassware. The term “reactor bomb” refers to a cylindrical,
thick-walled Pyrex vessel equipped with a 5 mm (or 10 mm,
for larger bombs) Kontes Teflon needle valve and a ground-
glass joint for attachment to a vacuum line. NMR tubes used
for preparing sealed samples are 8 or 9 in. Wilmad 507 PP
NMR tubes with a female b14 joint attached by glass blowing.
Toluene and hexanes were predried over CaH2 and then
deoxygenated by distillation from molten sodium and sodium-
benzophenone ketyl, respectively, under argon. Pentane and
diethyl ether were dried and deoxygenated by distillation from
sodium-benzophenone ketyl under argon. Di-n-butyl ether
was dried by refluxing over CaH2. Deuterated benzene (C6D6,
(31) Fryzuk, M. D.; Piers, W. E. Polyhedron 1988, 7, 1001.
(32) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J .; Einstein, F. W. B.;
J ones, T.; Albright, T. A. J . Am. Chem. Soc. 1989, 111, 5709.
(33) Fryzuk, M. D.; J ang, M.; J ones, T.; Einstein, F. W. B. Can. J .
Chem. 1986, 64, 174.
(34) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J . Organometallics
1990, 9, 1359.
(35) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J . Organometallics
1993, 12, 2152.
(36) Schmidt, G. F.; Muetterties, E. L.; Beno, M. A.; Williams, J .
M. Proc. Natl. Acad. Sci. U.S.A. 1981, 78, 1318.
(37) Kulzick, M. A.; Price, R. T.; Andersen, R. A.; Muetterties, E. L.
J . Organomet. Chem. 1987, 333, 105.
(38) Chalk, A. J .; Harrod, J . F. J . Am. Chem. Soc. 1965, 87, 16.
(39) Harrod, J . F.; Chalk, A. J . In Organic Syntheses Via Metal
Carbonyls; Wender, I., Pino, P., Eds.; Wiley: New York, 1977; p 690.
(40) Tilley, T. D. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; p 1415.
(41) It should be noted that while this result appears to indicate
that no fragmentation is occurring, a similar result would be obtained
if only one of the two complexes underwent fragmentation. In the
absence of kinetic data it is impossible to prove the participation of a
dinuclear species in the catalytic cycle.
+
+

2878 Organometallics, Vol. 15, No. 13, 1996
Fryzuk et al.
3
99.6 atom % D) and deuterated toluene (C7D8, 99.6 atom % D)
were purchased from MSD Isotopes and dried over 4 Å
molecular sieves. The dried, deuterated solvents were then
degassed using three “freeze-pump-thaw” cycles and were
vacuum-transferred before use. Carbon monoxide, ethylene,
and 1-butene were purchased from Matheson Gas Products.
Deuterium gas was purchased from Matheson and passed
through a glass coil immersed in liquid nitrogen to remove
any traces of water and oxygen.
1H NMR spectra were recorded on Varian XL-300, Bruker
WP-200, Bruker WH-400, or Bruker AMX-500 spectrometers.
With d6-benzene as solvent the spectra were referenced to
C6D5H at 7.15 ppm, and with d8-toluene as solvent the spectra
were referenced to the CD2H residual proton at 2.09 ppm.
31P{1H} NMR spectra were recorded at 121.4 MHz on the
Varian XL-300 or at 202.3 MHz on the Bruker AMX-500 and
were referenced to external P(OMe)3 at 141.0 ppm relative to
85% H3PO4. 1H{31P} NMR spectra were recorded on the
Bruker AMX-500. 2H{1H} NMR spectra were run in C6H6 at
46.0 MHz on the Varian XL-300 and were referenced to
residual solvent deuterons at 7.15 ppm. 29Si{1H} and 29Si
NMR were run at 59.6 MHz on the Varian XL-300 and were
referenced to external TMS at 0.0 ppm.
Elemental analyses were carried out by Mr. P. Borda of this
department. Gas chromatography/mass spectrometry was
carried out by Ms. L. Madilao of this department.
Literature methods were used to prepare [(dippe)Rh]2(µ-H)2
(1)25,42 [(dippe)Rh]2(µ-D)2 (d2-1),25 and [(dippp)Rh]2(µ-H)2.42
Ph2SiH2 and MePhSiH2 were purchased from Aldrich Chemi-
cal Co., dried by refluxing over calcium hydride overnight, and
distilled. Me2SiH2 was prepared by reduction of Me2SiCl2,
purchased from Aldrich, using LiAlH4, by a slight modification
of the literature procedure.43
12.3 Hz, J H-H ) 6.9 Hz); Rh-H -6.25 (mult, 2H). Note: for
2b in d8-toluene the Si(CH3)2 resonance is seen at 1.02 ppm,
a solvent-related, upfield shift of 0.10 ppm. 31P{1H} NMR
(C6D6, ppm): 94.2 (d mult, J Rh-P ) 159 Hz). 29Si NMR (C7D8,
ppm): 163 (mult). Anal. Calcd for C30H72P4Rh2Si: C, 45.57;
H, 9.18. Found: C, 45.70; H, 9.23.
[(d ip p e)Rh ]2(µ-H)(µ-η2-HSiMeP h ) (2c). This complex
was prepared by the same method as for 2a (210 mg, 0.287
mmol of 1; 35 mg, 0.286 mmol of MePhSiH2). Dark, reddish
brown crystals were obtained in 62% yield (151 mg). 1H NMR
3
(C6D6, ppm): Hortho 8.00 (d, 2H, J H
) 6.6 Hz); Hmeta 7.29
-H
m
o
(t, 2H, 3J H
(av) ) 7.3 Hz); Hpara 7.15 (mult, 1H);
-H ,H -H
m
p
m
o
CH(CH3)2 1.97 (mult, 8H); PCH2CH2P, SiCH3 1.28-1.36
3
(overlapping d and s, 11H); CH(CH3)2 1.20 (dd, 12H, J P-H
)
3
14.8 Hz, J H-H ) 7.0 Hz); CH(CH3)2 1.12-0.94 (mult, 36H);
Rh-H -6.00 (pt, 2H, J P-H ) 19.5 Hz, J Rh-H ) 15.1 Hz).
31P{1H} NMR (C6D6, ppm): 94.3 (d mult, J Rh-P ) 160 Hz).
1
Anal. Calcd for C35H74P4Rh2Si: C, 49.30; H, 8.75. Found: C,
49.62; H, 8.86.
[(d ip p p )Rh ]2(µ-H)(µ-η2-HSiP h 2) (2d ). This complex was
prepared by the same method as for 2a (96 mg, 0.13 mmol of
[(dippp)Rh]2(µ-H)2; 22 mg, 0.12 mmol of Ph2SiH2). Dark,
reddish brown crystals were obtained in 64% yield (72 mg).
1H NMR (C7D8, ppm): Hortho 8.12 (d, 4H, 3J H
) 7.8 Hz);
o) 6.3 Hz);
-H
m
Hmeta 7.23 (t, 4H); Hpara 7.15 (t, 2H, 3J H
-H
m
p
PCH2CH2CH2P, CH(CH3)2 1.90-1.60 (overlapping mult, 12H);
PCH2CH2CH2P 1.18 (br mult, 8H); CH(CH3)2 1.09 (dd, 24H,
3J H-P ) 14.4 Hz, J H-H ) 7.2 Hz); CH(CH3)2 0.99 (dd, 24H,
3
3J H-P ) 11.8 Hz, J H-H ) 6.4 Hz); Rh-H -8.55 (pt, 2H, J P-H
3
) 20.9 Hz, J Rh-H ) 14.6 Hz). 31P{1H} NMR (C7D8, ppm): 37.2
(d, 1J Rh-P ) 155 Hz). Anal. Calcd for C42H80P4Rh2Si: C, 53.50;
H, 8.55. Found: C, 53.16; H, 8.71.
[(d ip p e)Rh ]2(µ-D)(µ-η2-DSiP h 2) (d 2-2a ). A solution of
[(dippe)Rh]2(µ-SiPh2)2 (5a ;30 75 mg, 0.068 mmol) in 10 mL of
toluene in a thick-walled reactor bomb was degassed by two
freeze-pump-thaw cycles and then cooled to -196 °C. Deu-
terium gas was introduced to 1 atm pressure, and the reactor
bomb was sealed. The mixture was warmed to room temper-
ature, giving a D2 pressure of 4 atm. The solution was stirred
at room temperature for 12-16 h, during which time the bright
orange changed to a deep red color. A workup procedure
identical with that described for 2a was followed, giving red
crystals in 64% yield (40 mg). In the 1H NMR spectrum of
the product the signal due to the bridging hydrides was absent.
2H NMR (C6H6, ppm): -5.92 (br s, w1/2 ) 14 Hz).
Rea ction of [(d ip p e)Rh ]2(µ-D)2 w ith P h 2SiH2. A proce-
dure identical with that described for the preparation of
[(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ) was followed using d2-1 (61
mg, 0.083 mmol). 1H and 2H NMR spectroscopy suggested a
product mixture of d0-, d1-, and d2-2a , by comparison with
spectra of authentic d0-2a and d2-2a complexes.
Syn t h eses of Com p lexes a n d R ea ct ivit y St u d ies.
[(d ip p e)Rh ]2(µ-H)(µ-η2-HSiP h 2) (2a ). To a stirred, dark
green solution of [(dippe)Rh]2(µ-H)2 (1; 150 mg, 0.205 mmol)
in toluene (3 mL) was added dropwise a solution of diphenyl-
silane (38 mg, 0.206 mmol) in toluene (2 mL) to give a dark
red solution. The toluene was removed under vacuum, and
the residue was dissolved in hexanes. After filtration of the
solution through a Celite pad, red crystals were obtained from
a minimum volume of hexanes (or pentane) by cooling to -40
°C. Yield: 77% (145 mg). 1H NMR (C6D6, ppm): Hortho 8.06
3
4
(dd, 4H, J H
o ) 7.9 Hz, J H o ) 1.4 Hz); Hmeta 7.27 (mult,
-H
-H
m
p
4H); Hpara 7.14 (tt, 2H, 3J H
) 6.6 Hz); CH(CH3)2 1.96 (mult,
H
p
m
8H, J H-H ) 6.8 Hz (from 1H{31P} NMR)); PCH2CH2P 1.25 (d,
3
2
3
8H, J P-H ) 13.1 Hz); CH(CH3)2 1.05 (dd, 24H, J P-H ) 15.1
3
3
Hz, J H-H ) 7.1 Hz); CH(CH3)2 0.95 (dd, 24H, J P-H ) 12.2
3
Hz, J H-H ) 6.9 Hz); Rh-H -6.17 (sept, 2H, J Rh-H ) 14 Hz )
1
J
P-H). 31P{1H} NMR (C6D6, ppm): 94.0 (br d, J Rh-P ) 155
Hz). 29Si NMR (C7D8, ppm): 137-141 (br mult, at room
temperature), 129-143 (br mult, at -89 °C). Anal. Calcd for
[(d ip p e)Rh ]2(µ-SiP h 2)(µ-CO) (4a ). To a red solution of
[(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ; 85 mg, 0.092 mmol) in
toluene (3 mL) in a thick-walled reactor bomb was added
approximately 2 equiv of carbon monoxide (0.184 mmol, 136
mmHg in a 25.2 mL bulb) by vacuum transfer. After the
solution was warmed to room temperature, the toluene was
removed under vacuum. The rust-brown residues were re-
precipitated from a minimum volume of hexanes at -40 °C; a
C
40H76P4Rh2Si: C, 52.52; H, 8.37. Found: C, 52.32; H, 8.45.
[(d ip p e)Rh ]2(µ-H)(µ-η2-HSiMe2) (2b). One equivalent of
dimethylsilane (0.139 mmol, 103 mmHg in a 25.2 mL constant
volume bulb) was vacuum-transferred to a dark green solution
of [(dippe)Rh]2(µ-H)2 (1; 102 mg, 0.139 mmol) in frozen toluene
(8 mL) at -196 °C. The solution was warmed to room
temperature, by which time the solution had changed to a deep
red color. The toluene was removed under vacuum, and the
residue was dissolved in hexanes. After filtration of the
solution through a Celite pad, red crystals were obtained from
a minimum volume of hexanes (or pentane) by cooling to -40
°C. Yield: 85% (93 mg). 1H NMR (C6D6, ppm): CH(CH3)2 2.05
(overlapping d sept, 8H); PCH2CH2P 1.33 (d, 8H, 2J P-H ) 12.9
brown powder was obtained. Yield: 70% (61 mg). 1H NMR
(C6D6, ppm): Hortho 8.29 (d, 4H, J H
(t, 4H); Hpara 7.15 (mult, 2H); CH(CH3)2 2.11 (br s, 8H);
3
) 7.5 Hz); Hmeta 7.30
-H
m
o
2
PCH2CH2P 1.35 (d, 8H, J P-H ) 13.2 Hz); CH(CH3)2 1.07 (dd,
3
3
24H, J P-H ) 14.7 Hz, J H-H ) 6.9 Hz); CH(CH3)2 0.90 (dd,
24H, J P-H ) 11.5 Hz, J H-H ) 7.1 Hz). 31P{1H} NMR (C6D6,
3
3
3
3
1
Hz); CH(CH3)2 1.23 (dd, 24H, J P-H ) 15.0 Hz, J H-H ) 7.2
ppm): 79.8 (d mult, J Rh-P ) 161 Hz). IR (KBr pellet): νCO
3
1703.4 cm-1 (s). Anal. Calcd for C41H74OP4Rh2Si: C, 52.34;
H, 7.93. Found: C, 52.21; H, 8.07.
Hz); Si(CH3)2 1.12 (s, 6H); CH(CH3)2 1.03 (dd, 24H, J P-H
)
(42) Fryzuk, M. D.; McConville, D. H.; Rettig, S. J . J . Organomet.
Chem. 1993, 445, 245.
(43) Doyle, M. P.; DeBruyn, D. J .; Donnelly, S. J .; Kooistra, D. A.;
Odubela, A. A.; West, C. T.; Zonnebelt, S. M. J . Org. Chem. 1974, 39,
2740.
[(d ip p e)Rh ]2(µ-SiMe2)(µ-CO) (4b). This complex was
prepared using the same method as for 4a and has been
identified in solution using NMR spectroscopy, but it was not
isolated in a pure form. 1H NMR (C6D6, ppm): CH(CH3)2 2.14
+
+

Dinuclear Silyl-Hydride Complexes
Organometallics, Vol. 15, No. 13, 1996 2879
(mult, 8H); PCH2CH2P 1.40 (d, 8H, 2J P-H ) 13.2 Hz); Si(CH3)2
Mass spectrometric data for diethyldiphenylsilane are listed
above in the description of the reaction of ethylene with 2a ,
as are the 1H and 13C{1H} NMR spectroscopic data.
3
3
1.28 (s, 6H); CH(CH3)2 1.15 (dd, 24H, J P-H ) 15.1 Hz, J H-H
3
3
) 7.0 Hz); CH(CH3)2 1.04 (dd, 24H, J P-H ) 11.8 Hz, J H-H
)
7.1 Hz). 31P{1H} NMR (C6D6, ppm): 82.9 (br d, J Rh-P ) 172
1
P r ogr ess of th e Rea ction Mon itor ed by 1H NMR.
Diphenylsilane (126 mg, 0.700 mmol) was dissolved in 3.0 mL
of d6-benzene, and this solution was placed in a 250 mL reactor
bomb with a stirbar. The reactor bomb was attached to the
vacuum line, degassed by evacuation of the head space, and
then placed under 1 atm of ethylene. This substrate solution
was stirred for 15 min before the addition of the catalyst, to
ensure saturation of the solution with ethylene. The catalyst
(1; 0.014 mmol, 10 mg in 1.0 mL of d6-benzene) was added by
syringe to the substrate solution under a strong flow of
ethylene. The reactor bomb was left open to the manifold full
of ethylene, which was kept at 1 atm by periodic addition of
more gas from the cylinder. Samples of the mixture were
withdrawn periodically by syringe, under a strong flow of
ethylene. 1H NMR spectra were run within 10 min of removal
of the samples from the mixture, and in some cases 31P{1H}
NMR spectra were run afterwards.
The first sample was withdrawn from the catalytic mixture
3 min after the catalyst was added to the substrate. Its 1H
NMR spectrum showed that 100% conversion of Ph2SiH2 to
Ph2SiEt2 had already occurred. Three more samples removed
from the reaction mixture over the next 2 h showed the same
ratio of product to starting material. The assumption that the
hydrosilylation reaction was finished in 3 min leads to a
turnover number of 16 mol min-1 (mol of catalyst)-1 (or 16
min-1, which is 960 h-1). The 31P{1H} NMR spectra of the
samples all showed [(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ) to be
the major complex in solution.
Hz).
[(d ip p e)Rh ]2(µ-SiMeP h )(µ-CO) (4c). This complex was
prepared using the same method as for 4a and has been
identified in solution using NMR spectroscopy, but it was not
isolated in a pure form. 1H NMR (C6D6, ppm): Hortho 7.94 (d,
2H, 3J H
) 6.0 Hz); Hmeta 7.33 (t, 2H); Hpara 7.15 (t, 1H,
-H
m
o
3J H
) 7.5 Hz); CH(CH3)2 2.23 (mult, 8H); PCH2CH2P 1.70
-H
m
p
(s, 4H); PCH2CH2P, SiCH3, CH(CH3)2 1.58-1.17 (overlapping
mult, 19H); CH(CH3)2 1.03 (mult, 36H). 31P{1H} NMR (C6D6,
1
ppm): 81.6 (br d mult, J Rh-P ) 172 Hz).
Rea ction of [(d ip p e)Rh ]2(µ-H)(µ-η2-HSiP h 2) w ith C2H4.
A dark red solution of [(dippe)Rh]2(µ-H)(µ-η2-HSiPh2) (2a ; 25
mg, 0.027 mmol) in d8-toluene (0.6 mL) was placed in a
sealable NMR tube. The tube was attached to a vacuum line
by a needle valve adapter, and slightly less than 1 atm of
ethylene was introduced. The tube was cooled to -196 °C and
then sealed. After 1 week the solution color had changed to a
1
lighter orange, and H and 31P{1H} NMR spectra showed the
presence of [(dippe)Rh]2(µ-H)(µ-η2-CHdCH2) (4),25 dissolved
ethylene (1H NMR (ppm, C7D8) 5.4 (s)), and some compound
containing the SiPh2 fragment, as determined by the presence
of signals in the aromatic region of the 1H NMR spectrum.
The sealed NMR tube was cut open, and the solvent and
excess ethylene were removed from the sample in vacuo. The
residues were dissolved in hexanes and passed down a small
column of alumina to remove the vinyl hydride complex 4. A
1H NMR spectrum of the eluted compound showed it to be
diethyldiphenylsilane, by comparison with an authentic sample
prepared by the addition of excess EtMgBr to dichlorodi-
phenylsilane. 1H NMR (C6D6, ppm): Hortho 7.62-7.53 (mult,
4H); Hmeta, Hpara 7.38-7.16 (mult, 6H); CH2CH3 1.18-0.88
(mult, 10H). 13C{1H} NMR (CDCl3, ppm relative to CDCl3 at
77.0): Cipso 136.26; Cmeta 134.97; Cortho, Cpara 129.10, 127.80;
Hyd r osilyla tion of 1-Bu ten e by Dip h en ylsila n e Usin g
[(d ip p e)Rh ]2(µ-H)2 (1) a s Ca ta lyst. In a glovebox, diphenyl-
silane (248 mg, 1.35 mmol) was dissolved in 1.5 mL of
d6-benzene, and this solution was placed in a reactor bomb
with a stir bar. The catalyst [(dippe)Rh]2(µ-H)2 (1; 2 mg, 0.003
mmol) was added to the diphenylsilane solution. The dark
green solid immediately changed to orange and began to
dissolve, and bubbles were evolved. The bomb was attached
to a vacuum line, and the head space was evacuated before
the bomb and the vacuum manifold were filled with 1 atm of
1-butene. The mixture was stirred for 24 h; then the head
space was evacuated and a 1H NMR spectrum was run.
The initial spectrum showed the reaction had not gone to
completion. To remove the volatile starting material, the NMR
sample was placed under vacuum while being cooled in an ice
bath. 1H NMR spectroscopy showed the only silicon-containing
product to be n-butyldiphenylsilane and a very small amount
of triphenylsilane (a product of disproportionation of di-
phenylsilane, which probably formed before the 1-butene was
added to the reaction mixture).
1
Cmethyl 7.47; Cmethylene 3.96. A HETCOR (13C, H) experiment
carried out on the Bruker AMX-500 machine shows the alkyl
resonances in the 1H NMR spectrum to be as follows (ppm):
3
CH2CH3 1.004 (q, 4H, J H-H ) 8 Hz); CH2CH3 0.982 (t, 6H).
Mass spec (EI; m/e): 240 (M+), 211, 183, 159, 131, 105, 79.
Rea ction of [(d ip p e)Rh ]2(µ-η2-CHdCH2) (4) w ith
1
Equ iv of P h 2SiH2. To a solution of [(dippe)Rh]2(µ-H)(µ-η2-
CHdCH2) (4; 24 mg, 0.032 mmol) in toluene (5 mL) was added
a solution of 1 equiv of diphenylsilane (6 mg, 0.032 mmol) in
toluene (3 mL). The originally pale orange solution im-
mediately darkened to a deep red-orange color. The solvent
was removed in vacuo, and the residues were dissolved in d6-
benzene for analysis by NMR spectroscopy. The only product
observed in the 31P{1H} NMR spectrum was [(dippe)Rh]2(µ-
H)(µ-η2-HSiPh32) (2a ). In the 1H NMR spectrum the expected
resonances due to 2a were observed. No signals due to
Ph2SiEt2 were observed.
For n-butyldiphenylsilane: 1H NMR (C6D6, ppm) Hortho 7.64
(mult, 4H), Hmeta, Hpara 7.24 (mult, 6H), Si-H 5.19 (t, 1H, 3J H-H
1
) 3.2 Hz, J Si-H ) 192 Hz (measured from 29Si satellites)),
SiCH2CH2CH2CH3 1.54 (mult, 2H), SiCH2CH2CH2CH3 1.40
(mult, 2H), SiCH2CH2CH2CH3 1.17 (mult, 2H), SiCH2CH2-
Ca ta lytic Rea ction s. Typ ica l Con d ition s for th e Hy-
d r osilyla t ion of E t h ylen e b y Dip h en ylsila n e Usin g
[(d ip p e)Rh ]2(µ-H)2 (1) a s Ca ta lyst. A 253 mg (1.38 mmol)
amount of diphenylsilane was dissolved in 2.5 mL of toluene,
and this solution was placed in a 50 mL reactor bomb with a
stirbar. The reactor bomb was attached to the vacuum line,
degassed by evacuation of the head space, and then placed
under 1 atm of ethylene. The catalyst (1; 0.014 mmol, 0.25
mL of a 0.55 M solution in toluene) was added to the substrate
solution by syringe under a strong flow of ethylene. The
reactor bomb was left open to the manifold full of ethylene,
which was kept at 1 atm by periodic addition of more gas from
the cylinder. The mixture was stirred for 24 h, at which time
it was taken into the glovebox and the catalyst was removed
on a Florisil column. The product mixture was analyzed by
both GC-MS and 1H NMR spectroscopy, which showed that
all of the diphenylsilane had been consumed. The major
product was diethyldiphenylsilane (estimated at >90%).
3
CH2CH3 0.88 (t, 3H, J H-H ) 7.3 Hz); Mass spec (m/e; EI) 240
(M+), 183, 162, 134, 105, 84.
Ca lcu la tion of ∆Gq for th e Hyd r id e Exch a n ge in 2a .
∆Gq was calculated using the value for the rate constant kC
(where kC ) π∆νC/21/2) in the Eyring equation44
∆Gq ) -RTC ln[(kCh)/(kBTC)]
where R ) the gas constant, TC ) temperature of coalescence,
∆νC ) peak separation at the low-T limit, h ) Planck constant,
and kB ) Boltzmann constant. For the hydride resonances in
1
the variable-temperature H NMR spectra of 2a , TC ) 193 K
(-80 °C) and ∆νC ) 309 Hz. The coalescence temperature was
(44) Thomas, W. A. Annu. Rev. NMR Spectrosc. 1968, 1, 43.
+
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2880 Organometallics, Vol. 15, No. 13, 1996
Fryzuk et al.
estimated visually from the spectra and has an error of
approximately (5 K.
dispersion corrections for all atoms were taken from ref 46.
Final atomic coordinates and equivalent isotropic thermal
parameters are given in the Supporting Information, and
selected bond lengths and angles appear in Tables 2 and 3.
Measured and calculated structure factor amplitudes are
available from the authors.
X-r a y Cr ysta llogr a p h y An a lyses of [(d ip p e)Rh ]2(µ-H)-
(µ-H-SiR2) (R ) P h , 2a ; R ) Me, 2b). Crystallographic data
appear in Table 1. The final unit-cell parameters were
obtained by least squares on the setting angles for 25 reflec-
tions with 2θ ) 31.2-44.7° for 2a and 25.2-33.1° for 2b. The
intensities of 3 standard reflections, measured every 200
reflections throughout the data collections, decayed uniformly
by 2.7% for 2b and remained constant for 2a . The data were
processed45 and corrected for Lorentz and polarization effects,
decay (for 2a ), and absorption (empirical; based on azimuthal
scans for three reflections).
The structures were solved by conventional heavy-atom
methods. The non-hydrogen atoms of both structures were
refined with anisotropic thermal parameters. The metal and
silyl hydride atoms were refined with isotropic thermal
parameters. All other hydrogen atoms were fixed in idealized
Ack n ow led gm en t. Financial support for this work
was provided by the NSERC (operating grants to M.D.F.
and a postgraduate scholarship to L.R.). We also thank
J ohnson Matthey for the generous loan of RhCl3‚3H2O.
Su p p or tin g In for m a tion Ava ila ble: Complete tables of
non-hydrogen atom parameters, bond lengths and angles,
calculated hydrogen atom parameters, anisotropic thermal
parameters, torsion angles, intermolecular contacts, and least-
squares planes for 2a ,b (51 pages). Ordering information is
given on any current masthead page.
positions (staggered methyl groups, C-H ) 0.98 Å, BH
)
1.2Bbonded atom). Corrections for secondary extinction were not
OM950450D
necessary. Neutral atom scattering factors and anomalous
(46) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K. (present distributor Kluwer Academic
Publishers: Boston, MA), 1974; Vol. IV, pp 99-102, 149.
(45) TEXSAN: Structure Analysis Package (VMS Version 5.1);
Molecular Structure Corp., The Woodlands, TX, 1985.
+
+