Migrations in a polysilyl complex of rhodium. Trapping of an apparent silylene intermediate and observation of reversible Si-Si bond reductive elimination

Article abstract of DOI:10.1021/om960291d

Reaction of (Me₃P)₃RhCl with (THF)₃LiSi-(SiMe₃)₃ in the presence of 2-butyne or diphenylacetylene generates (Me₃P)₂(Me₃Si)RhSiMe ₂C(R)=C(R)SiMe(SiMe₃) (1, R = Me; 2, R= Ph). These complexes undergo a fluxional process which exchanges the SiMe₃ groups, apparently by Si-Si bond reductive elimination / oxidative addition.

Full text of DOI:10.1021/om960291d

Organometallics 1996, 15, 3477-3479
3477
Migr a tion s in a P olysilyl Com p lex of Rh od iu m .
Tr a p p in g of a n Ap p a r en t Silylen e In ter m ed ia te a n d
Obser va tion of Rever sible Si-Si Bon d Red u ctive
Elim in a tion
Gregory P. Mitchell and T. Don Tilley*
Department of Chemistry, University of California, Berkeley, California 94720-1460
Received April 16, 1996X
Summary: Reaction of (Me3P)3RhCl with (THF)3LiSi-
(SiMe3)3 in the presence of 2-butyne or diphenylacetylene
migrations in metal-silicon systems,2,3 this has yet to
be confirmed by direct observation. Of course, silylene
complexes formed by 1,2- and/or 1,3-migrations may be
difficult to observe simply because of their highly
reactive and transient nature. Thus, the current situ-
ation warrants serious consideration of alternative
mechanisms, such as ones involving concerted “double
migrations” or dyotropic shifts.5 Recently we reported
the observation of apparent 1,2- and 1,3-migrations in
the isomerization of the coordinatively unsaturated
rhodium and iridium polysilyl complexes (Me3P)3M-Si-
(SiMe3)3 (M ) Rh, Ir) to (Me3P)3M-SiMe2SiMe(SiMe3)2
derivatives. The latter complexes are thermally un-
stable, and the iridium species cleanly decomposes to
generates (Me3P)2(Me3Si)RhSiMe2C(R)dC(R)SiMe(SiMe3)
(1, R ) Me; 2, R ) Ph). These complexes undergo a
fluxional process which exchanges the SiMe3 groups,
apparently by Si-Si bond reductive elimination/ oxida-
tive addition.
Organosilicon1 and transition-metal silicon2,3 chem-
istry are replete with examples of intramolecular 1,2-
and 1,3-migrations. For transition-metal systems, such
migrations are often envisioned as proceeding via
discrete, intermediate silylene complexes.3 We have
recently provided some support for such scenarios, by
isolating and characterizing the first examples of
LnMdSiR2 species.4 However, despite compelling cir-
cumstantial evidence that silylene complexes mediate
(Me3P)3(H)IrSiMe2SiMe(SiMe3)SiMe2CH2 via cyclo-
metalation.6 Here we report further observations on the
migration chemistry associated with this system, which
have resulted from attempts to trap intermediates in
the above rearrangements.
X Abstract published in Advance ACS Abstracts, J uly 1, 1996.
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J . Am. Chem. Soc. 1994, 116, 10468.
Since alkynes are known to trap silylene groups which
have been extruded by transition-metal complexes,2a,b,7
we examined the reaction of (Me3P)3RhCl8 with (THF)3-
9
LiSi(SiMe3)3 in the presence of 1 equiv of 2-butyne. In
pentane this reaction produces a yellow-orange solution,
from which complex 1 was isolated in 72% yield (eq
1).10,11 The analogous reaction with diphenylacetylene
(2) (a) Tilley, T. D. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Chapter 24, p
1415. (b) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1991, Chapters 9 and 10, pp
245, 309. (c) Pannell, K. H.; Sharma, H. K. Chem. Rev. 1995, 95, 1351.
(d) Schubert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 419.
(3) (a) Pannell, K. H.; Cervantes, J .; Hernandez, C.; Cassias, J .;
Vincenti, S. Organometallics 1986, 5, 1056. (b) Pannell, K. H.; Rozell,
J . M., J r.; Hernandez, C. J . Am. Chem. Soc. 1989, 111, 4482. (c)
Pannell, K. H.; Wang, L.-J .; Rozell, J . M. Organometallics 1989, 8, 550.
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J ones, K. L.; Pannell, K. H. J . Am. Chem. Soc. 1993, 115, 11336. (f)
Hernandez, C.; Sharma, H. K.; Pannell, K. H. J . Organomet. Chem.
1993, 462, 259. (g) Pannell, K. H.; Brun, M.-C.; Sharma, H.; J ones,
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Tamao, K.; Kawachi, A. J . Am. Chem. Soc. 1995, 117, 8043.
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Soc. 1990, 112, 7801. (b) Grumbine, S. D.; Tilley, T. D.; Rheingold, A.
L. J . Am. Chem. Soc. 1993, 115, 358. (c) Grumbine, S. K.; Tilley, T.
D.; Arnold, F. P.; Rheingold, A. L. J . Am. Chem. Soc. 1993, 115, 7884.
(d) Grumbine, S. K.; Tilley, T. D.; Arnold, F. P.; Rheingold, A. L. J .
Am. Chem. Soc. 1994, 116, 5495. (e) Grumbine, S. K.; Tilley, T. D. J .
Am. Chem. Soc. 1994, 116, 6951. West has also isolated a three-
coordinate silicon LnMSiR2 species: Denk, M.; Hayashi, R. K.; West,
R. J . Chem. Soc., Chem. Commun. 1994, 33.
produced compound 2. Crystals of 1 contain enantio-
(5) (a) Heyn, R. H.; Tilley, T. D. Main Group Chem., in press. (b)
Reetz, M. T. Adv. Organomet. Chem. 1977, 16, 33.
(6) Mitchell, G. P.; Tilley, T. D.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1995, 14, 5472.
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Chem. 1971, 27, C31. (b) Seyferth, D.; Shannon, M. L.; Vick, S. C.;
Lim, T. F. O. Organometallics 1985, 4, 57. (c) Carlson, C. W.; West,
R. Organometallics 1983, 2, 1801. (d) Ishikawa, M.; Matsuzawa, S.;
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S0276-7333(96)00291-9 CCC: $12.00 © 1996 American Chemical Society

3478 Organometallics, Vol. 15, No. 16, 1996
Communications
Sch em e 1
F igu r e 1. Molecular structure of 1. Selected bond lengths
(Å) and angles (deg): Rh-Si(1) ) 2.309(2), Rh-Si(2) )
2.406(2), Rh-Si(3) ) 2.346(2), Rh-P(1) ) 2.382(2), Rh-
P(2) ) 2.410(2), Si(3)-Si(4) ) 1.877(7), C(12)-C(14) )
1.361(10); P(1)-Rh-P(2) ) 92.64(7), P(1)-Rh-Si(1) )
94.75(8), P(1)-Rh-Si(3) ) 97.87(7), P(2)-Rh-Si(1) )
112.98(7), P(2)-Rh-Si(2) ) 89.06(7), Si(1)-Rh-Si(2) )
87.97(7), Si(1)-Rh-Si(3) ) 85.15(7), Si(2)-Rh-Si(3) )
79.33(7), Si(2)-C(12)-C(14) ) 115.5(5), Si(2)-C(12)-C(13)
) 120.6(5), Si(3)-C(14)-C(12) ) 117.2(6), Si(3)-C(14)-
C(15) ) 121.7(7).
haps by 2 + 2 cycloaddition to give the transient
metallasilacyclobutene complex 5. Reductive elimina-
tion of a Si-C bond in 5 would give the rhodium silyl
6, which, via loss of a PMe3 ligand and oxidative
addition of a Si-Si bond, would be converted to the final
product 1. No intermediates were observed during the
1
formation of 1 at -70 °C, as determined by H and 31P
NMR spectroscopy. Interestingly, 1 is also formed by
the direct addition of 2-butyne to 4 (generated in
solution6), indicating that 4 is in equilibrium with the
silyl-silylene complex 3.
Metallacycles 1 and 2 exhibit fluxional behavior in
solution. Thus, at room temperature, the 1H NMR
spectrum of 1 displays only single resonances for the
SiMe3 and SiMe2 groups (benzene-d6 solution). Cooling
of a toluene-d8 solution of 1 allows observation of
decoalescence behavior (Tc ) -30 °C; ∆Gq ) 11(1) kcal
mol-1) and resolution of inequivalent SiMe3 and SiMe2
resonances.10 We propose that this fluxionality derives
from a Si-Si bond reductive elimination/oxidative ad-
dition cycle, as shown in eq 2. Rotation about a Si-C
meric pairs related by an inversion center, and the
molecular geometry (Figure 1)12 may be described as an
approximate square-based pyramid, with the trimeth-
ylsilyl ligand occupying the apical position. The five-
membered ring is only slightly puckered, such that the
angle between the RhSi2 plane and the C2Si2 least-
squares plane is 170°. Only the diastereomer shown
in eq 1 is observed in this reaction, presumably because
the other isomer would experience excessive nonbonded
repulsions between trimethylsilyl groups.
A possible mechanism for the formation of 1 is given
in Scheme 1. The rhodium silyl complex which is un-
doubtedly formed initially, (Me3P)3RhSi(SiMe3)3, ap-
pears to undergo facile 1,2- and 1,3-migration processes
that in the absence of added alkyne give 4.6 Presum-
ably, 2-butyne traps the silyl-silylene complex 3, per-
(8) J ones, R. A.; Real, F. R.; Wilkinson, G.; Galas, A. M. R.; Hurst-
house, M. B.; Malik, K. M. A. J . Chem. Soc., Dalton Trans. 1980, 511.
(9) Gutekunst, G.; Brook, A. G. J . Organomet. Chem. 1982, 225, 1.
(10) Selected data for 1: 1H NMR (benzene-d6, 400 MHz, 25 °C) δ
3
0.23 (br s, 18 H, SiMe3), 0.58 (br s, 6 H, RhSiMe2), 0.85 (d, J HRh ) 4
Hz, 3 H, RhSiMeSiMe3), 0.97 (d, 2J HP ) 7 Hz, 9 H, PMe3), 1.08 (d, 2J HP
) 7 Hz, 9 H, PMe3), 2.05 (s, 3 H, CMe), 2.08 (s, 3 H, CMe); 1H NMR
(toluene-d8, 400 MHz, -70 °C) δ 0.22 (s, 9 H, SiMe3), 0.37 (s, 9H,
SiMe3), 0.53 (br s, 3 H, RhSiMe2), 0.83 (br s, 3 H, RhSiMe2), 0.88 (d,
3
2J HP ) 7 Hz, 9 H, PMe3), 0.94 (d, J HRh ) 4 Hz, 3 H, RhSiMeSiMe3),
2
1.00 (d, J HP ) 7 Hz, 9 H, PMe3), 2.17 (s, 3 H, CMe), 2.22 (s, 3 H,
bond in the intermediate vinylsilyl complex 7 would
CMe); 31P{1H} NMR (benzene-d6, 162.0 MHz, 25 °C) δ -28.50 (m).
Anal. Calcd for RhP2Si4C19H51: C, 40.98; H, 9.23. Found: C, 40.72;
H, 9.01. Selected data for 2: 1H NMR (benzene-d6, 400 MHz, 25 °C),
(11) Compound 1 was prepared by adding a solution of (THF)3LiSi-
(SiMe3)3 (0.385 g, 0.818 mmol) in 15 mL of pentane to a solution of
(Me3P)3RhCl (0.300 g, 0.818 mmol) and 2-butyne (100 µL, 1.3 mmol)
in 15 mL of pentane. After removal of solvent, the product was
extracted into pentane and then crystallized from that solvent in 72%
yield. For compound 8, a solution of (Me3P)3RhCl (0.300 g, 0.818 mmol)
in 20 mL of pentane was added to a solution of (THF)3LiGe(SiMe3)3
(0.421 g, 0.818 mmol) in 15 mL of pentane. The mixture immediately
became deep red, and it was stirred for 30 min before the volatile
materials were removed under pressure. The product was extracted
into pentane and then crystallized by concentration and cooling of this
solution (isolated yield 85%) to afford black, air-sensitive crystals.
(12) Crystal data for 1: RhP2Si4C19H51, fw 556.81, orange prisms,
triclinic, P1h, a ) 9.706(3) Å, b ) 10.011(3) Å, c ) 15.603(5) Å, R )
78.53(1)°, â ) 88.76(2)°, γ ) 78.38(1)°, V ) 1455.0(7) Å3, Z ) 2, µ(Mo
KR) ) 18.51 cm-1, T ) 155 K. Of 4115 empirically absorption corrected
data (2θmax ) 51.1°), 3797 were independent and observed. R(F) )
6.6%; R(wF) ) 7.7%.
3
δ 0.39 (br s, 18 H, SiMe3), 0.61 (br s, 6 H, RhSiMe2), 0.78 (d, J HRh
)
2
3 Hz, 3 H, RhSiMeSiMe3), 0.94 (d, J HP ) 7 Hz, 9 H, PMe3), 1.07 (d,
2J HP ) 7 Hz, 9 H, PMe3), 6.88 (m, Ph), 7.10 (m, Ph), 7.24 (m, Ph), 7.30
(m, Ph); 31P{1H} NMR (benzene-d6, 162.0 MHz, 25 °C) δ -29.55 (m).
Anal. Calcd for RhP2Si4C29H55: C, 51.15; H, 8.14. Found: C, 50.75;
H, 8.40. Selected data for 8: 1H NMR (benzene-d6, 400 MHz, 25 °C)
δ 0.55 (s, 27 H, SiMe3), 1.14 (br s, 27 H, PMe3); 31P{1H} NMR (benzene-
d6, 162.0 MHz, 25 °C) δ -22.20 (m). Anal. Calcd for RhGeP3-
Si3C18H54: C, 39.52; H, 9.95. Found: C, 39.20; H, 10.31. Selected data
for 9: 1H NMR (benzene-d6, 400 MHz, 25 °C) δ -11.4 (dt, H, J HP
)
2
2
160 Hz, J HP ) 20 Hz, IrH), -0.64 (m, 1 H, IrCH2), -0.55 (m, 1 H,
IrCH2), 0.55 (s, 3 H, SiMe), 0.59 (s, 9 H, SiMe3), 0.63 (s, 9 H, SiMe3),
2
2
0.68 (s, 3 H, SiMe), 1.11 (d, J HP ) 8 Hz, PMe3), 1.27 (d, J HP ) 8 Hz,
PMe3), 1.30 (d, J HP ) 8 Hz, PMe3); 31P{1H} NMR (benzene-d6, 162.0
2
MHz, 25 °C) δ -63.55 (m, 2 overlapping resonances, 2 P), -54.90 (m,
1 P). Anal. Calcd for IrGeP3Si3C18H54: C, 30.33; H, 7.64. Found: C,
30.20; H, 7.90.

Communications
Organometallics, Vol. 15, No. 16, 1996 3479
allow addition of a second Si-Si bond and exchange
of the SiMe3 groups. The observed fluxionality there-
fore lends some support to the latter part of the
mechanism outlined in Scheme 1. Addition of excess
PMe3 to 1 did not result in trapping of the proposed
vinylsilyl complex 6 and had no effect on the rate of the
fluxional process.
With the hope that a germylene complex analogous
to (Me3P)3(Me3Si)RhdSi(SiMe3)2 might be more stable
and therefore observable, we attempted to generate such
The results described above provide further evidence
for the participation of 1,2- and 1,3-migrations in metal
silyl complexes. This migration chemistry results in the
dramatic rearrangement of a polysilyl complex to give
a novel tris(silyl)rhodium derivative. Currently it ap-
pears that silylene intermediates are involved, but it is
impossible at this time to definitively rule out other
mechanistic possibilities. Also of interest is the revers-
ible Si-Si bond oxidative addition/reductive elimination
equilibrium observed for 1 (eq 2). Given the rarity of
both steps,2 the facility of this process is quite remark-
able. As indicated in eq 2, this intramolecular process
may be assisted by formation of an intermediate η3-
silaallyl structure (7). Apparently, an alternative C-H
activation in this intermediate would produce an exces-
sively strained chelate ring. We are continuing to
explore this system in search of new information.
a
species by the reaction of (Me3P)3RhCl with
13
(THF)3LiGe(SiMe3)3 in pentane. Surprisingly, this
reaction gave the deep red complex (Me3P)3RhGe-
(SiMe3)3 (8), isolated in 85% yield.10,11 Complex 8 is
stable at room temperature but gradually decomposes
at 70 °C over 2-3 h to a mixture of products, including
several hydride complexes. It does not react with 1
equiv of 2-butyne at room temperature over 2.5 h. In a
similar manner, reaction of (THF)3LiGe(SiMe3)3 with
“(Me3P)3IrCl” (generated in situ by addition of PMe3 to
[(COE)2IrCl]2)14 produces the cyclometalated product
Ack n ow led gm en t is made to the National Science
Foundation for their generous support of this work. We
also thank J ohnson-Matthey Aesar, Inc., for a loan of
RhCl3‚xH2O.
fac-(Me3P)3Ir(H)Ge(SiMe3)2SiMe2CH2 (9), isolated as
colorless crystals in 29% yield.10 Thus, no convincing
evidence for migration chemistry was observed in either
germyl complex.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental procedures and characterization data for compounds
and tables of crystal, data collection, and refinement param-
eters, atomic coordinates, bond distances and angles, and
anisotropic displacement parameters (21 pages). Ordering
information is given on any current masthead page.
(13) Brook, A. G.; Abdesaken, F.; So¨llradl, H. J . Organomet. Chem.
1986, 299, 9.
(14) Aizenberg, M.; Milstein, D. Angew. Chem., Int. Ed. Engl. 1994,
33, 317.
OM960291D