An observable silene/silylene rearrangement in a cationic iridium complex

Article abstract of DOI:10.1021/om010435a

The iridium complex Cp*(PMe₃)Ir(Me)(SiMe₂-OTf) (2) reacts with LiB(C₆F₅)₄(Et₂O)₂ to form a mixture of the silene complex [Cp*(PMe₃)Ir(η²-CH₂SiMe₂) (H)]- [B(C₆F₅)₄] (3) and the base-stabilized silylene complex [Cp*(PMe₃)Ir(Me)(SiMe₂(Et₂O))] [B(C₆F₅)₄] (4). The silene complex 3 has been crystallographically characterized. Addition of pyridine to this mixture affords Cp*(PMe₃)-Ir(Me)(SiMe₂(L)][B(C₆ F₅)₄] (5a, L = pyridine) in good yield, whereas addition of CO or ethylene results in clean formation of Cp*(PMe₃)Ir(L)(SiMe₃)][B(C₆F₅) ₄] (8a, L = CO; 8b, L = C₂H₄).

Full text of DOI:10.1021/om010435a

3220
Organometallics 2001, 20, 3220-3222
An Obser va ble Silen e/Silylen e Rea r r a n gem en t in a
Ca tion ic Ir id iu m Com p lex
Steven R. Klei, T. Don Tilley,* and Robert G. Bergman*
Department of Chemistry and Center for New Directions in Organic Synthesis (CNDOS),
University of California and Chemical Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received May 23, 2001
Summary: The iridium complex Cp*(PMe3)Ir(Me)(SiMe2-
OTf) (2) reacts with LiB(C6F5)4(Et2O)2 to form a mixture
of the silene complex [Cp*(PMe3)Ir(η2-CH2SiMe2)(H)]-
[B(C6F5)4] (3) and the base-stabilized silylene complex
[Cp*(PMe3)Ir(Me)(SiMe2(Et2O))][B(C6F5)4] (4). The silene
complex 3 has been crystallographically characterized.
Addition of pyridine to this mixture affords Cp*(PMe3)-
Ir(Me)(SiMe2(L)][B(C6F5)4] (5a , L ) pyridine) in good
yield, whereas addition of CO or ethylene results in clean
formation of Cp*(PMe3)Ir(L)(SiMe3)][B(C6F5)4] (8a , L )
CO; 8b, L ) C2H4).
Cp*(PMe3)Ir(Me)(SiMe2OTf) (2) as the final product.5,6
To determine whether this system can be used to gene-
rate an unsolvated silylene complex,7,8 we have inves-
tigated reactions designed to remove the triflate anion
from 2. Treatment of 2 with 1 equiv of LiB(C6F5)4(Et2O)2
in CH2Cl2 solvent at 25 °C produces an equilibrium
mixture of silene complex [Cp*(PMe3)Ir(η2-CH2SiMe2)-
(H)][B(C6F5)4] (3) and [Cp(PMe3)Ir(SiMe2(Et2O))(Me)]-
[B(C6F5)4] (4), established after 4 h at 25 °C, in a ratio
of 2.6:1 favoring silene complex 3. (eq 2). The salt
Numerous catalytic transformations are based on
conversions involving organosilanes, since Si-H activa-
tion via oxidative addition is a facile process.1 In
general, this process introduces a silyl ligand, which
may react further via metal-mediated rearrangements.
Thus, a thorough understanding of this rearrangement
chemistry is key to the development of new catalytic
processes. Despite much research focus, however, these
rearrangements are not well understood. Berry and co-
workers recently reported the first direct observation
of intramolecular activation (or â-hydrogen elimination)
of aliphatic C-H bonds in a 16-electron metal silyl to
generate a silene complex, showing that a silene ligand
can be derived from a silyl complex.2,3 Likewise, we have
shown that observable base-free silylene ligands may
be derived from silyl ligands by R-migration.4 Reactivity
studies in the iridium system described here provide
evidence for a third type of isomerization, involving
silene and silylene ligands (eq 1). Whereas the free
metathesis is complete within seconds, and monitoring
the reaction mixture after 5 min at 25 °C gave no
indication of observable reaction intermediates, al-
though the intermediacy of the Ir(III) silyl complex
[Cp*(PMe3)Ir(SiMe3)][B(C6F5)4] is suspected.
The room-temperature 1H NMR spectrum (CD2Cl2)
of complex 3 exhibits the expected resonances, including
those for diastereotopic methyl groups at 0.77 and 0.72
ppm, diastereotopic methylene protons at 0.46 and 0.05
ppm for the metalated carbon atom, and an Ir-H
resonance at -14.9 ppm. Several of the 1H NMR
resonances for 4 are broadened at room temperature,
indicating a fluxional process. However, at 190 K the
spectrum sharpens, and full spectroscopic characteriza-
tion of the diethyl ether complex is possible. A variable-
temperature 1H NMR spectroscopy experiment also
gave support for the 3:4 equilibrium, in that the ratio
of silene 3 to ether-stabilized silylene complex 4 is
approximately 1:1 at 220 K and returns to 2.6:1 upon
warming again to 298 K. For entropic reasons, a reduced
proportion of silene complex at lower temperature is
expected.
silene/silylene interconversion between SiMe2 and H2Cd
Si(H)Me has been the topic of considerable experimental
and theoretical investigation,1 this is the first report of
such a process within the coordination sphere of a
transition metal.
The Ir(III) complex Cp*(PMe3)Ir(Me)OTf (OTf )
OSO2CF3) (1) reacts rapidly with trimethylsilane by
Si-H activation followed by rearrangement, giving
(1) Brook, M. A. In Silicon in Organic, Organometallic, and Polymer
Chemistry; Wiley: New York, 2000.
(2) Dioumaev, V. K.; Plossl, K.; Carroll, P. J .; Berry, D. H. J . Am.
Chem. Soc. 1999, 121, 8391.
(3) Berry, D. H.; Procopio, L. J . J . Am. Chem. Soc. 1989, 111, 4099.
(4) Mitchell, G. P.; Tilley, T. D. Angew. Chem., Int. Ed. Engl. 1998,
37, 2524.
(5) Burger, P.; Bergman, R. G. J . Am. Chem. Soc. 1993, 115, 10462.
(6) Klei, S. R.; Tilley, T. D.; Bergman, R. G. J . Am. Chem. Soc. 2000,
122, 1816.
(7) Eisen, M. S. In The Chemistry of Organic Silicon Compounds;
Wiley: New York, 1998; Vol. 2, Chapter 35, p 2037.
(8) Tilley, T. D. In The Silicon-Heteroatom Bond; Wiley: New York,
1991; Chapters 9 and 10, pp 245 and 309.
10.1021/om010435a CCC: $20.00 © 2001 American Chemical Society
Publication on Web 06/23/2001

Communications
Organometallics, Vol. 20, No. 15, 2001 3221
Sch em e 1
F igu r e 1. ORTEP diagram of the cationic portion of
1
complex 3 (iridium-bound hydrogen observed by IR and H
NMR spectroscopy not located; borate counterion not shown
for clarity). Selected bond lengths and angles: Ir-P 2.182-
(8) Å; Ir-C1 2.22(2) Å, Ir-Si 2.439(8) Å; Si-C1 1.84(2) Å;
Si-C2 1.84(2) Å; C1-Si-C2 119(1)°; C1-Si-C3 125(1)°;
C2-Si-C3 109(1)°.
Sch em e 2
The silene complex 3 can be isolated in 43% yield as
an analytically pure off-white solid by crystallization
from pentane/CH2Cl2. Use of the soluble salt LiB(C6F5)-
(Et2O)2 appears to be crucial in rapidly accessing the
silene complex before appreciable decomposition takes
place; salt metathesis with NaBPh4 or Na(B(3,5-CF3)-
C6H3)4 resulted in decomposition. The C-H coupling
constant for the methylene group of 3 was measured to
be 147.5 Hz by 13C NMR spectroscopy. This value lies
between the values assigned to sp3 (125.0 Hz for CH4)
and sp2 (156.2 Hz for C2H4) hybridization9 and indicates
partial contributions from two resonance forms involv-
ing coordinated silene and metallasilacyclopropane
structures.
Crystals of 3 suitable for an X-ray diffraction study
were grown from a CH2Cl2/pentane mixture. An ORTEP
diagram of the cationic portion of the complex can be
seen in Figure 1. The sum of the C-Si-C angles around
the silicon center is 353°, which again is between the
values assigned to sp3 (329°) and sp2 (360°) hybridiza-
tion, indicating partial double bond character between
silicon and the metalated carbon atom. The solid-state
structure of 3 exhibits Si-C bond lengths that are
within experimental error of each other, although
structural disorder mandates that the data obtained can
reliably be used only to establish atom connectivity (see
Supporting Information). Other silene complexes exhibit
modestly shortened Si-CH2 bonds.10-12
Treatment of a mixture of 3 and 4 with pyridine
produced [Cp*(PMe3)Ir(Me)(SiMe2(py))][B(C6F5)4] (5a ),
in 75% isolated yield as a bright yellow crystalline solid
(Scheme 1). We propose that equilibration of 3 and 4
and conversion of this mixture to 5a occur by the
pathway illustrated in Scheme 2. Carbon-hydrogen
reductive elimination (â-H transfer) results in formation
of cationic 16-electron silyl species 6, which then goes
on to form base-free metal-silylene complex 7 by
migration of a methyl group from silicon to iridium.5
Trapping of this species by pyridine leads to isolable
complex 5a ; the related complex, [Cp*(PMe3)Ir(Me)-
(SiMe2(py))][OTf] (5b), is prepared in 98% isolated yield
by treating the migrated triflato species Cp*(PMe3)-
Ir(Me)(SiMe2OTf) (2) with 1 equiv of pyridine.
In contrast to the reaction with pyridine, treatment
of 3 and 4 with carbon monoxide provides the nonmi-
grated silyl carbonyl complex [Cp*PMe3Ir(SiMe3)(CO)]-
[B(C6F5)4] (8a ) in 69% recrystallized yield. Again, we
postulate the intermediacy of the iridium silyl cation
6, which is trapped by CO (Scheme 2). We believe this
change in selectivity is due to the fact that, as a strong
π-acid, CO prefers coordination to the electron-rich Ir
center, while pyridine, a good σ-donor like Et2O, prefers
to bind to the silicon center. We find here that ethylene
reacts with a mixture of 3 and 4 like CO rather than
pyridine, leading to the silyl ethylene complex [Cp*PMe3-
Ir(SiMe3)(C2H4)][B(C6F5)4] (8b) in 70% isolated yield as
pale yellow crystals. To the best of our knowledge, this
is the first reported cationic silyl ethylene complex, and
it provides circumstantial evidence for the existence of
[Cp*P(OMe)3Co(SiEt3)(olefin)]+, a postulated intermedi-
(9) Breitmaier, E.; Voelter, W. 13C NMR Spectroscopy; Verlag
Chemie: New York, 1978; p 96.
(10) Campion, B. K.; Heyn, R. H.; Tilley, T. D. J . Am. Chem. Soc.
1990, 112, 4079.
(11) Campion, B. K.; Heyn, R. H.; Tilley, T. D.; Rheingold, A. L. J .
Am. Chem. Soc. 1993, 115, 5527. In contrast to the reactivity of the
ruthenium silene complex described in this paper, we have never
observed silicon-hydrogen reductive elimination behavior from silene
complex 3 to produce a species such as “Cp*(PMe3)Ir(CH2SiMe2H)”.
(12) Koloski, T. S.; Carroll, P. J .; Berry, D. H. J . Am. Chem. Soc.
1990, 112, 6405.
ate in catalytic hydrosilylation of olefins by [Cp*-
- 13
(P(OMe)3)CoCH2CH2-µ-H]+B[3,5-(CF3)2C6H3]4
.

3222 Organometallics, Vol. 20, No. 15, 2001
Communications
The chemistry of metal complexes containing reactive
silicon-based fragments appears to be highly relevant
to a number of catalytic processes. For example, a newly
discovered dehydropolymerization of silanes to carbosi-
lanes appears to involve silene complexes.14 Also, con-
siderable speculation has centered on the potential role
of silylene complexes in the dehydrocoupling of silanes
to polysilanes, and Berry has presented convincing
evidence for participation of a germylene complex
formed via R-migration in the demethanitive coupling
of germanes to polygermanes.15 Clearly, an understand-
ing of the factors controlling the pathway for elimination
reactions in metal silyl derivatives (e.g., R vs â) are key
to development of catalytic reactions in these systems.
In this context the observed interconversion of the silene
and silylene ligands is interesting in that it provides
information regarding factors that can be used to control
the course of reactions from this equilibrium manifold.
We are currently exploring the relevance of these
observations in attempts to develop new catalytic trans-
formations.
Ack n ow led gm en t. We are grateful for support of
this work (to R.G.B.) from the Director, Office of Energy
Research, Office of Basic Energy Sciences, Chemical
Sciences Division, of the U.S. Department of Energy
under Contract No. DE-AC03-7600098. The National
Science Foundation is thanked for financial support (to
T.D.T.) and for a Graduate Fellowship (to S.R.K.).
Determination of the solid-state structure of 3 was
carried out by Dr. F. J . Hollander and Dr. A. Oliver of
the U.C. Berkeley X-ray diffraction facility (CHEXRAY).
We are very grateful to the Boulder Scientific Company
for their generous gift of lithium tetrakis(pentafluo-
rophenyl)borate etherate complex. CNDOS is supported
by Bristol-Myers Squibb as founding member.
Su p p or tin g In for m a tion Ava ila ble: Characterization
data and crystallographic information (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(13) Brookhart, M.; Grant, B. E. J . Am. Chem. Soc. 1993, 115, 2151.
(14) Procopio, L. J .; Berry, D. H. J . Am. Chem. Soc. 1991, 113, 4039.
(15) Reichl, J . A.; Popoff, C. M.; Gallagher, L. A.; Remsen, E. E.;
Berry, D. H. J . Am. Chem. Soc. 1996, 118, 9430.
OM010435A