Reactions of Alkyl Alkylidene Complexes with Silanes. Synthesis and Characterization of Novel Tantalum 1,1-Metallasilacyclobutadiene and Disilyl-Substituted Alkylidene Complexes

Full text of DOI:10.1021/ja972302f

J. Am. Chem. Soc. 1997, 119, 12657-12658
12657
Scheme 1
Reactions of Alkyl Alkylidene Complexes with
Silanes. Synthesis and Characterization of Novel
Tantalum 1,1-Metallasilacyclobutadiene and
Disilyl-Substituted Alkylidene Complexes
Jonathan B. Diminnie and Ziling Xue*
Department of Chemistry, The UniVersity of Tennessee
KnoxVille, Tennessee 37996-1600
ReceiVed July 11, 1997
We have been investigating cyclopentadienyl (Cp)-free silyl
alkylidene complexes of tantalum as models for possible reactive
intermediates in the reactions of alkyl alkylidene complexes
(RCH2)3TadCHR with SiH4.1,2 Recently, we reported the
synthesis and characterization of thermally unstable silyl alky-
lidene complexes (Me3ECH2)2Ta(dCHEMe3)SiPh2But (E ) C,
1; Si, 2), which react with PMe3 to form bis(phosphine)
singlets in the gated-decoupled 13C spectra. The molecular
structures of 5a,b have been determined by X-ray crystal-
lography and are found to be similar. The structure of 5a is
shown in Figure 1.9 Complex 5a exhibits distorted trigonal
bipyramidal geometry around the tantalum center, with the PMe3
ligands occupying axial positions. The TadC bond distances
of 1.947(12) and 1.962(12) Å are consistent with those observed
for other alkylidene complexes of tantalum (1.998(8) and
1.95(2) Å in 4,2 1.932(7) and 1.955(7) Å in Ta(dCHBut)2-
(mesityl)(PMe3)2,10 and 1.932(9) Å in [Ta(dCHBut)(CH2But)-
(PMe3)2]2(µ-N2)11). The metallasilacyclobutadiene ring in 5a
is planar (average deviation from least-squares plane ) 0.007
Å), which brings the silicon atom in close proximity to the
tantalum center (TasSi distance of 2.607(3) Å); however, the
fact that the metal center is formally d0 makes any metal-silicon
bonding interaction unlikely.12 The silicon atom of the metal-
lasilacyclobutadiene ring in 5a exhibits distorted tetrahedral
geometry, with bond angles ranging from 96.7(5)° to 115.9-
(6)°. The structure of 5a is novel, and to our knowledge
represents the first example of a 1,1-metallasilacyclobutadiene
complex. A number of metallasilacyclobutane complexes have
been synthesized and structurally characterized by Marks,
Girolami, Wilkinson, Petersen, and others,13-16 and a large
number of conjugated metallacyclobutadiene complexes are
known;17 however, to our knowledge, complexes 5a,b represent
the first cyclobutadiene complexes in which both double bonds
are localized exclusively on the metal atom.18
2
bis(alkylidene) complexes (Me3ECH2)Ta(PMe3)2[dCHEMe3]2
(E ) C, 3; Si, 4; 3 has been previously prepared by Schrock
and co-workers3). In order to further study the reaction of silane
with alkyl alkylidene complexes, we investigated the reactions
of tantalum alkylidene complexes containing phosphine sup-
porting ligands with silanes PhR′SiH2 (R′ ) Ph, Me), in the
hope that phosphine might help stabilize the resulting products.
When a solution of PhR′SiH2 (1 equiv) was added to a
solution of 4, we were surprised to find immediate H2 evolution
from the solution4 and formation of a novel metallasila-
cyclobutadiene complex (5, Scheme 1).5 Similarly, addition of
a solution of PhR′SiH2 to a solution of (Me3SiCH2)3Ta-
(PMe3)dCHSiMe3 (6)6 resulted in a nearly quantitative conver-
sion (by NMR) of 6 to (Me3SiCH2)3Ta[dC(SiMe3)SiPhR′H]
(7),5 again with evolution of H2. The reaction of the silane
with 4 and 6 occurred exclusiVely with the dCHSiMe3 ligands,
and the resulting products 5 and 7 were inert toward further
reaction with excess silane. In contrast, Berry and co-workers
t
have observed that Cp2Ta(dCH2)CH3 reacts with Bu2SiH2 to
give Cp2Ta(H)dCHSiHBut2 through a mechanism involving
oxidative addition of the silane to a d2 center, followed by CH4
elimination and alkylidene transfer and insertion steps.7a Bercaw
and co-workers have reported that the formation of d0 Cp*2-
Ta(H)(CH3)SiH3 (Cp* ) pentamethylcyclopentadienyl) from the
reaction of d0 Cp*2Ta(H)dCH2 with SiH4 is through the
oxidative addition of SiH4 to d2 Cp*2TasCH3, which is in
equilibrium with Cp*2Ta(H)dCH2.7b It is interesting to point
When the reaction of 6 with excess deuterated silane
PhMeSiD2 (2-5 equiv) was monitored by NMR, the predomi-
nant product was identified as (Me3SiCH2)3Tad[C(SiPhMeD)-
SiMe3] (7a-d1),5 along with a trace amount of 7a. PhMeSiHD
and PhMeSiH2 were also observed in the reaction mixture. The
possibility of H incorporation into PhMeSiD2 occurring by
exchange with the Me3SiCH2- ligands of 6 was investigated by
t
out that no reaction was observed between 4 and Bu2SiH2 in
benzene-d6, even at 65 °C.8
Spectroscopic properties of 5 and 7 are consistent with the
structure assignments.5 The 13C NMR alkylidene resonances
of 5a,b and 7a,b range from 238.2 to 255.4 ppm and appear as
(8) After a benzene-d6 solution of 4 and excess tBu2SiH2 was heated for
24 h at 65 °C, a 5% thermal decomposition of 4 to (Me3SiCH2)4Ta2(µ-
CSiMe3)219 had occurred; however, no reaction with tBu2SiH2 was observed.
(9) Crystal data for 5a: monoclinic, P21/n (No. 14), a ) 10.647(3) Å,
b ) 17.757(6) Å, c ) 18.686(5) Å, â ) 96.91(3)°, V ) 3507(2) Å3, Z )
4, R(RwF2) ) 5.04 (12.40)% with 4617 unique reflections with F > 2.0σ-
(F), GOF ) 1.03, number of parameters refined ) 289.
(10) Churchill, M. R.; Youngs, W. J. Inorg. Chem. 1979, 18, 1930.
(11) Churchill, M. R.; Wasserman, H. J. Inorg. Chem. 1981, 20, 2899.
Turner, H. W.; Fellmann, J. D.; Rocklage, S. M.; Schrock, R. R.; Churchill,
M. R.; Wasserman, H. J. J. Am. Chem. Soc. 1980, 102, 7809.
(12) Our preliminary ab initio quantum mechanics calculations support
this conclusion. Wu, Y.-D.; Xue, Z. Unpublished results.
(13) Tikkanen, W. R.; Liu, J. Z.; Egan, J. W., Jr.; Petersen J. L.
Organometallics 1984, 3, 825.
(1) (a) Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J. B.; Pollitte, J. L. J. Am.
Chem. Soc. 1994, 116, 2169. (b) Li, L.; Diminnie, J. B.; Liu, X.; Pollitte,
J. L.; Xue, Z. Organometallics 1996, 15, 3520. (c) Xue, Z. Comments Inorg.
Chem. 1996, 18, 223. (d) Li, L.; Hung, M.; Xue, Z. J. Am. Chem. Soc.
1995, 117, 12746. (e) For a review of transition metal silyl complexes,
see: Tilley, T. D. In The Silicon Heteroatom Bond; Patai, S., Rappoport,
Z., Eds.; Wiley: New York, 1991; Chapters 9 and 10. Sharma, H. K.;
Pannell, K. H. Chem. ReV. 1995, 95, 1351.
(2) Diminnie, J. B.; Hall, H. D.; Xue, Z. J. Chem. Soc., Chem. Commun.
1996, 2383.
(3) Fellmann, J. D.; Schrock, R. R.; Rupprecht, G. A. J. Am. Chem. Soc.
1981, 103, 5752.
(4) A signal is observed at 4.47 ppm (relative to C6D5H at 7.15 ppm),
whose chemical shift is identical to that of H2 in benzene-d6 prepared
independently.
(5) See the Supporting Information for experimental and spectroscopic
details.
(6) Rupprecht, G. A. Ph.D. Thesis, Massachusetts Institute of Technology,
1979.
(14) Bruno, J. W.; Marks, T. J.; Day, V. W. J. Am. Chem. Soc. 1982,
104, 7357.
(15) Behling, T.; Girolami, G. S.; Wilkinson, G. J. Chem. Soc., Dalton
Trans. 1984, 877.
(16) Morse, P. M.; Spencer, M. D.; Wilson, S. R.; Girolami, G. S.
Organometallics 1994, 13, 1646.
(17) For a review of metallacyclobutadiene complexes, see: (a) Nugent,
W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley: New York,
1988; Chapter 7. (b) Engel, P. F.; Pfeffer, M. Chem. ReV. 1995, 95, 2281
and references therein.
(7) (a) Berry, D. H.; Koloski, T. S.; Carroll, P. J. Organometallics 1990,
9, 2952. (b) Parkin, G.; Bunel, E.; Burger, B. J.; Trimmer, M. S.; van Asselt,
A.; Bercaw, J. E. J. Mol. Catal. 1987, 41, 21.
S0002-7863(97)02302-0 CCC: $14.00 © 1997 American Chemical Society

12658 J. Am. Chem. Soc., Vol. 119, No. 51, 1997
Communications to the Editor
Scheme 3
Figure 1. ORTEP of 5a, showing 50% thermal ellipsoids. Selected
bond distances (Å) and angles (deg): Ta-C(1) 1.947(12), Ta-C(2)
1.962(12), Ta-C(3) 2.285(11), Ta-P(1) 2.605(3), Ta-P(2) 2.665(3),
C(1)-Si(4) 1.901(11), C(2)-Si(4) 1.900(11), C(1)-Si(1) 1.832(12),
C(2)-Si(2) 1.819(11), P(1)-Ta-P(2) 161.97(10), P(1)-Ta-C(3) 80.7-
(3), P(2)-Ta-C(3) 85.6(3), C(1)-Ta-C(2) 93.1(4), C(1)-Ta-C(3)
134.4(4), C(2)-Ta-C(3) 132.5(4), C(1)-Si(4)-C(2) 96.7(5), Ta-
C(1)-Si(4) 85.3(5), Ta-C(2)-Si(4) 84.9(5).
Workup of the reaction mixture to produce 7 yields a red oil
of reasonably pure (>95% by 1H NMR) 7. However, all
attempts to isolate analytically pure samples of this compound
were unsuccessful, as 7 was found to be thermally unstable.
When monitored by NMR in benzene-d6, 7 slowly decomposed
through SiMe4 elimination; however, a solution of 7 in benzene
could be frozen and stored at -20 °C for several weeks without
significant decomposition. When the reaction to form 7 is
monitored by NMR, a partial conversion of 7 to 5, along with
formation of SiMe4, is observed after several days; 5 is not
observed during the formation of 7. A possible mechanism for
this conversion is shown in Scheme 3. Reactions similar to
those shown in Scheme 3 may be involved in the conversion
of 4 to 5: attack at one of the alkylidene ligands to produce H2
and 10, followed by intramolecular attack of the alkylidene
-SiPhR’H group on the second alkylidene as proposed above
to give 5, with a small excess of silane acting as a catalyst.
The preferential reaction of the alkylidene ligands, rather than
the alkyl ligands, of 4 and 6 with PhR′SiH2 was unexpected. In
order to probe whether such a preference is general, the reaction
of the neopentyl analog 3 with PhMeSiH2 was investigated.
Surprisingly, no H2 was observed; rather, a considerable amount
of CMe4 formed, along with a mixture of unidentified products.20
It therefore appears that the silicon atoms in the -CH2SiMe3
and dCHSiMe3 ligands of 4 may exert an important influence
over the course of the reactions of silanes with alkyl alkylidene
complexes.21 Kinetic studies are currently underway to further
probe the possible mechanistic pathways of these reactions.
Scheme 2
monitoring a solution containing PhMeSiD2 and (Me3SiCH2)4-
Ta2(µ-CSiMe3)2,19 which we had found to be inert toward
reaction with PhMeSiH2. No such H/D exchange was observed
1
by H NMR over 2.5 h at 50 °C, ruling out exchange with
residual protons in the solvent and suggesting that exchange
with -CH2SiMe3 ligands would be unlikely. In addition, analysis
of the gaseous products from the reaction with PhMeSiD2 by
mass spectrometry showed the predominant product to be D2,
with some HD also present.5 These observations lead us to
propose a possible mechanism for the formation of 7 (Scheme
2). The silane first reacts by addition across the TadC bond
to form a hydride (deuteride) intermediate 8, which then reacts
with a second equivalent of silane, producing H2 (D2) and a
silyl intermediate 9. Complex 9 then undergoes R-hydrogen
abstraction by the -SiPhR′H ligand to yield 7, reforming the
silane in the process. With PhR′SiD2, this final step would yield
PhR′SiHD, which could further react to form PhR′SiH2,
consistent with the results obtained from the reaction with
PhMeSiD2.
Acknowledgment is made to the NSF Young Investigator Award
program (CHE-9457368), the DuPont Young Professor Award, the
donors of the Petroleum Research Fund (28044-G3), administered by
the American Chemical Society, and the Camille Dreyfus Teacher-
Scholar Award for financial support of this research. We thank Professor
Gregory Girolami for helpful discussion regarding the crystal structures
of 5a,b and Dr. Al Tuinman for assistance in obtaining mass spectra.
Supporting Information Available: Experimental and spectro-
scopic data for 5 and 7; crystal structure of 5b along with crystal-
lographic and structural data for 5a,b (23 pages). See any current
masthead page for ordering and Internet access instructions.
JA972302F
(20) There was a report of the reactions of lanthanide alkyl complexes
with silanes to produce alkylsilanes and metal hydride complexes. Vosk-
oboynikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin, K. P.;
Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J. A. K. Organometallics 1997,
16, 4041.
(21) It is well-known that silicon stabilizes an adjacent carbon-metal
bond. Fleming, I. In ComprehensiVe Organic Chemistry; Jones, D. N., Ed.;
Pergamon: New York, 1979; Vol. 3, Chapter 13, p 541.
(18) Metallacyclopentadiene and -triene complexes have been reported;
see, e.g.: (a) Johnson, E. S.; Balaich, G. J.; Rothwell, I. P. J. Am. Chem.
Soc. 1997, 119, 7685. (b) Hessen, B.; Teuben, J. H. J. Organomet. Chem.
1989, 367, C18.
(19) Mowat, W.; Wilkinson, G. J. Chem. Soc. Dalton Trans., 1973, 1120.