η1:η2-alkynyl-bridged W-Si complexes: Formation, structure, and reaction with acetone

Article abstract of DOI:10.1021/ja050441o

Reactions of (η⁵-C₅Me₄R)(CO)₂(MeCN)WMe (R = Me, Et) with HPh₂SiC≡C^tBu gave the novel alkynyl-bridged W-Si complexes, (η⁵-C₅Me₄R)(CO)₂W(μ-η^1<

Full text of DOI:10.1021/ja050441o

Published on Web 04/28/2005
η1:η2-Alkynyl-Bridged W-Si Complexes: Formation, Structure, and Reaction
with Acetone
Hiroyuki Sakaba,* Mako Yoshida, Chizuko Kabuto, and Kuninobu Kabuto
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
Received January 22, 2005; E-mail: sakaba@funorg.chem.tohoku.ac.jp
Activation of the Si-H bonds of hydrosilanes by transition-metal
complexes is a versatile synthetic route to transition-metal silyl
complexes.1 Recently, we showed the formation of an η3-silaallyl
complex by the reaction of the labile tungsten complex cis-
Cp*(CO)2(MeCN)WMe (1a; Cp* ) η5-C5Me5) with alkenyl-
hydrosilane HR2SiCHdCR′2 via Si-H bond activation.2 As a
further utilization of this methodology to synthesize a novel
tungsten-silicon-bonded complex, we have studied the reaction
of 1a with alkynylhydrosilane HR2SiCtCR′ in the expectation of
the formation of η3-silapropargyl/silaallenyl complex 2. Rosenthal
Figure 1. Molecular structure of 4.
Scheme 2
and co-workers have previously reported the formation of Cp2Ti(η2-
trans-HMe2SiCtCtBu), having an agostic Si-H-Ti interaction,
by the reaction of Cp2Ti(η2-Me3SiCtCSiMe3) with HMe2SiCt
CtBu (3).3 They made the interesting observation that a hydrido
η3-silaallenyl structure contributed considerably at low temperatures
and in the solid state. In our system using 1a and 3, a formal dimeric
complex of the expected η3-silapropargyl/silaallenyl complex was
isolated, and further studies using HPh2SiCtCtBu led to the
species on the silicon of another molecule. We then carried out the
reactions of 1a and its Cp′ derivative 1b (Cp′ ) η5-C5Me4Et) with
HPh2SiCtCtBu (5) having bulkier phenyl groups on the silicon
atom in order to prevent this dimerization. The reactions gave
monomeric species; however, their structures were revealed to be
the unprecedented alkynyl-bridged W-Si complexes 6a and 6b,
which were isolated as air-sensitive orange-brown solids in 63
and 64% yields, respectively (Scheme 2).
formation of a unique monomeric alkynyl-bridged W-Si complex.
In this communication, we describe the formation and structures
of these dimeric and monomeric complexes and the reaction of the
latter with acetone.
The reaction of 1a with 3 in toluene afforded the dinuclear
complex 4 (46%), a dimer of the expected Cp*(CO)2W-
(SiMe2CCtBu) species, as a yellow solid after removal of the
volatiles and recrystallization of the residue in pentane at -60 °C
(Scheme 1).4 The molecular structure of 4 was determined by X-ray
analysis and is depicted in Figure 1. The molecule has C2 symmetry,
and one carbonyl oxygen atom of each monomeric unit is bound
to each silicon atom. The W1-C9 distance (1.826(3) Å) is
considerably shorter than the W1-C10 distance (1.959(4) Å) of
the terminal carbonyl ligand and is comparable to W-C triple bond
distances (1.81-1.82 Å) in carbyne complexes of the type
Cp(CO)2WtCR.5 The Si1-O1 bond (1.707(3) Å) is at the upper
end of the range for Si-O single bonds (1.58-1.70 Å).6 These
structural features indicate that 4 has a siloxycarbyne structure.
X-ray quality crystals were obtained by recrystallization of 6b
from pentane, and the X-ray analysis showed a unique structure
with an alkynyl ligand bridged between the tungsten and silicon
atoms in an η1: η2-coordination mode (Figure 2). The W1-C1 bond
distance (2.050(7) Å) is comparable to W-C(alkynyl) bond
distances (2.05-2.09 Å) in di- and polynuclear complexes contain-
ing a Cp*(CO)2W(µ-η1: η2-CtCR) fragment.7 The Si1-C1 (1.937(7)
Å) and Si1-C2 (2.009(7) Å) distances in the three-membered ring
skeleton are significantly longer than the Si-C distances (1.80-
1.86 Å) of silacyclopropenes,8 whereas the C1-C2 distance
(1.270(9) Å) is shorter than their CdC distances (1.32-1.37 Å)
and is intermediate between those of standard CdC and CtC
bonds. The W1-Si1 bond distance (2.567(2) Å) is in the range of
the tungsten-silyl bonds (2.533-2.633 Å) in the structurally related
silyltungsten complexes Cp*(CO)2(L)WSiR3 (L ) PR′3, pyridine)
and is longer than the tungsten-silylene bond in a typical lone-
pair-donor-stabilized silylene complex Cp*(CO)2(H)WdSiPh2‚Py
(2.445 Å).9 These observations suggest that a dative WrSi
interaction between the 16e Cp*(CO)2W(CtCtBu) fragment and
the :SiPh2 silylene ligand, which is stabilized by π-complexation
of the alkynyl moiety to the silylene center, may be essential for
Scheme 1
The formation of 4 suggested that the dimerization was caused
by nucleophilic attack of the carbonyl oxygen of the monomeric
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10.1021/ja050441o CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
Figure 2. Molecular structures of 6b (left) and 7 (right).
the W-Si bond, leading to a novel formulation: an intramolecularly
π-bond-donor-stabilized silylene complex 6′ with a σ,π-bridging
alkynyl ligand.10 The importance of the interaction between a
silylene ligand and a C-C multiple bond has been shown in recent
studies. Tilley proposed a new mechanism for hydrosilylation
catalyzed by the ruthenium silylene complex [Cp*(iPr3P)(H)2Rud
Si(H)Ph‚Et2O][B(C6F5)4], in which addition of the Si-H bond of
the silylene ligand to alkene is a key step.11 Hall subsequently
reported a theoretical support for the mechanism and revealed that
the initial step is the formation of a π-complex between the silylene
center and alkene followed by insertion of the alkene into the Si-H
bond.12
of acetone into the silicon-alkynyl linkage. Future studies, includ-
ing theoretical studies, will be directed toward elucidating the
formation mechanism, isomerism, and bonding of this W-Si
complex and toward exploring its reactivity.
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Consideration of the resonance contribution of 6 leads to
additional interesting contributors 6′′ and 6′′′. The former is a
silacyclopropenyl complex with an R-C-Si agostic interaction
involving the strained C-Si bond, and the latter has a four-
membered cyclic tungstaallene structure.13 The 29Si resonances of
δ -48.1 for 6a and -47.8 for 6b at -70 °C, which are relatively
close to the very high field shifted 29Si resonances (around δ -100)
characteristic of silacyclopropenes,14 suggest a bonding interaction
between the silicon and two alkynyl carbon atoms; hence, the
contribution from 6′′′ is minor. We may view 6 as an intermediate
state between complexation of the triple bond to the silylene center
and coordination of the C-Si bond of the silacyclopropene ring to
the metal center. To obtain information on the best formulation
for 6, theoretical studies are necessary.
A possible mechanism for the formation of 6 is shown in Scheme
2. Complex 1 dissociates the coordinated acetonitrile to form the
coordinatively unsaturated species, which activates the Si-H bond
of 5 to give intermediate A. Reductive elimination of methane forms
unsaturated silyl complex B, from which two routes leading to 6
are conceivable: 1,2-alkynyl migration followed by the coordination
of the alkynyl ligand to the resulting silylene center or the direct
coordination of the alkynyl ligand to the metal center to form C
followed by isomerization to 6 via σ-π interchange of the
coordination mode. The σ-π interchange has often been proposed
in dinuclear complexes with a µ-η1: η2-alkynyl ligand.15
Supporting Information Available: Experimental details and
spectroscopic data (PDF). X-ray crystallographic data for 4, 6b, and 7
(PDF and CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
(2) Sakaba, H.; Watanabe, S.; Kabuto, C.; Kabuto, K. J. Am. Chem. Soc.
2003, 125, 2842.
(3) (a) Ohff, A.; Kosse, P.; Baumann, W.; Tillack, A.; Kempe, R.; Go¨rls, H.;
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(4) When the reaction of 1a with 3 was monitored by 1H NMR spectroscopy,
several species were observed prior to the formation of 4. One such species
may be ascribed to a monomeric species equilibrated with 4 on the basis
of the observation that a higher complex concentration favors 4. Because
of this phenomenon, attempts at its spectroscopic characterization and
isolation were unsuccessful.
(5) (a) Fischer, E. O.; Lindner, T. L.; Huttner, G.; Friedrich, P.; Kreissl, F.
R.; Besenhard, J. O. Chem. Ber. 1977, 110, 3397. (b) Fischer, E. O.;
Hollfelder, H.; Friedrich, P.; Kreissl, F. R.; Huttner, G. Angew. Chem.,
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(6) Sheldrick, W. S. In The Chemistry of Organic Silicon Compounds; Patai,
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G. K.; Chakraborty, S.; Tho¨ne, C. Organometallics 2002, 21, 5209.
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Chem. Soc. 2000, 122, 11511 and references therein.
(10) For the terminology of π-bond donor, see: Crabtree, R. H. The
Organometallic Chemistry of the Transition Metals, 3rd ed.; John Wiley
& Sons: New York, 2001; p 31.
(11) Glaser, P. B.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125, 13640.
(12) Beddie, C.; Hall, M. B. J. Am. Chem. Soc. 2004, 126, 13564.
(13) Complex 6 can be viewed as a tungstapropargyl/tungstaallenyl silicon
compound as suggested by a reviewer, showing an interesting structural
comparison with Cp2Ti(η2-trans-HMe2SiCtCtBu). We thank the reviewer
for insightful comments on its structure and the formation mechanism of
7.
When 6a was treated with acetone (1.5 equiv) in toluene-d8, the
sole formation of chelate-type alkyl-alkene complex 7 (98%) was
observed over the course of 2 days, with no intermediates detected
by 1H NMR spectroscopy (Scheme 3). The structure of 7 was
determined by X-ray analysis (Figure 2). A plausible mechanism
is shown in Scheme 3 and involves nucleophilic attack of the
carbonyl oxygen of acetone at the silicon center and C-C bond
formation between the carbonyl carbon and the â-carbon of the
alkynyl ligand to give cyclic silyl vinylidene intermediate D. 1,2-
Silyl migration in D to give E followed by C-H activation of the
tBu group would lead to the formation of 7. Alternatively, the direct
formation of E by acetone insertion into the silacyclopropene-like
framework of 6a is also possible. Acetone insertion into the Si-C
bond of a silacyclopropene has been demonstrated.16
(14) Seyferth, D.; Annarelli, D. C.; Vick, S. C. J. Organomet. Chem. 1984,
272, 123.
(15) For example, see: Cherkas, A. A.; Randall, L. H.; MacLaughlin, S. A.;
Mott, G. N.; Taylor, N. J.; Carty, A. J. Organometallics 1988, 7, 969.
(16) Seyferth, D.; Vick, S. C.; Shannon, M. L. Organometallics 1984, 3, 1897.
In summary, a novel η1: η2-alkynyl-bridged W-Si complex was
synthesized for the first time and was found to undergo the insertion
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