Reactions of tantalum alkylidene complexes with silanes. Synthesis and characterization of novel metallasilacyclodiene complexes and kinetic and mechanistic studies of their formation

Article abstract of DOI:10.1021/om000476r

The reactions of tantalum alkylidene complexes (RCH₂)₃Ta(L)=CHR (1) and RCH₂Ta(L)₂[=CHR]₂ (3) (R = SiMe₃, L = PMe₃) with phenylsilanes H₂SiR′Ph (R′ = Me, Ph) and (PhSiH₂)₂CH₂ were found to produce bis(silyl)-substituted alkylidene complexes (RCH₂)₃Ta=CR(SiHR′Ph) (4) and novel metallasilacyclobutadiene and metalladisilacyclohexadiene complexes. Reaction of the mixed-ligand trimethylsilylmethyl neopentylidene complex RCH₂Ta(L)₂[=CHBu^t]₂ (6) with H₂SiMePh also yielded a metallasilacyclobutadiene complex, but reaction of the neopenyl neopentylidene complex Bu^tCH₂Ta(L)₂[=CHBu^t]₂ (2) with H₂SiMePh yielded unidentified products. Deuterium-labeling and kinetic studies of the conversion of 1 → 4 were found to be consistent with a dissociative mechanism. The structures of the novel metallacyclic complexes 5a, 5b, 7, and 8 were determined by X-ray crystallography.

Full text of DOI:10.1021/om000476r

1504
Organometallics 2001, 20, 1504-1514
Articles
Rea ction s of Ta n ta lu m Alk ylid en e Com p lexes w ith
Sila n es. Syn th esis a n d Ch a r a cter iza tion of Novel
Meta lla sila cyclod ien e Com p lexes a n d Kin etic a n d
Mech a n istic Stu d ies of Th eir F or m a tion
J onathan B. Diminnie, J aime R. Blanton, Hu Cai, Keith T. Quisenberry, and
Ziling Xue*
Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600
Received J une 5, 2000
The reactions of tantalum alkylidene complexes (RCH2)3Ta(L)dCHR (1) and RCH2Ta(L)2-
[dCHR]2 (3) (R ) SiMe3, L ) PMe3) with phenylsilanes H2SiR′Ph (R′ ) Me, Ph) and (PhSiH2)2-
CH2 were found to produce bis(silyl)-substituted alkylidene complexes (RCH2)3TadCR-
(SiHR′Ph) (4) and novel metallasilacyclobutadiene and metalladisilacyclohexadiene complexes.
Reaction of the mixed-ligand trimethylsilylmethyl neopentylidene complex RCH2Ta(L)2[d
CHBut]2 (6) with H2SiMePh also yielded a metallasilacyclobutadiene complex, but reaction
of the neopenyl neopentylidene complex ButCH2Ta(L)2[dCHBut]2 (2) with H2SiMePh yielded
unidentified products. Deuterium-labeling and kinetic studies of the conversion of 1 f 4
were found to be consistent with a dissociative mechanism. The structures of the novel
metallacyclic complexes 5a , 5b, 7, and 8 were determined by X-ray crystallography.
Early transition metal alkylidene complexes have
silanes (Scheme 1).5 Mechanistic studies revealed that
these reactions involved direct insertion of the carbene
been the subject of enthusiastic study since the first
alkylidene complex, (ButCH2)3TadCHBut, was prepared
by Schrock in 1973.1 Alkylidene complexes have been
widely studied for their roles in effecting reactions such
as alkene and alkyne metathesis as well as olefin
polymerization.2 However, the reactivity of alkylidene
complexes toward silanes is a relatively unexplored
area. The complex Cp2Ta(dCH2)CH3 was found to react
with H2SiBut to give Cp2Ta(H)dCHSiHBut through
Sch em e 1
2
2
a mechanism involving oxidative addition of the silane
to a d2 Ta(III) center, followed by CH4 elimination and
alkylidene transfer and insertion steps.3 The formation
of d0 Cp*2Ta(H)(CH3)SiH3 (Cp* ) pentamethylcyclo-
pentadienyl) from the reaction of Cp*2Ta(H)dCH2 with
SiH4 was found to occur via oxidative addition of SiH4
to d2 Cp*2Ta-CH3, which is in equilibrium with Cp*2-
Ta(H)dCH2.4 In contrast, the reactions of some other
carbene complexes with silanes led to insertion reactions
of the carbene moiety into the Si-H bond to yield alkyl
unit into the Si-H bond5a and that the cleavage of the
Si-H bond and the formation of the new C-Si and C-H
bonds were concerted.5c To our knowledge, no such
direct reactions of silanes with the MdCHR moiety of
an early transition metal alkylidene complex have been
observed.6
We are interested in the reactions of alkylidene
complexes with silanes as part of our studies of the
(5) (a) Scharrer, E.; Brookhart, M. J . Organomet. Chem. 1995, 497,
61. (b) Mak, C. C.; Tse, M. K.; Chan, K. S. J . Org. Chem. 1994, 59,
3585. (c) Landais, Y.; Parra-Rapado, L.; Planchenault, D.; Weber, V.
Tetrahedron Lett. 1997, 38, 229. (d) Fischer, E. O.; Do¨tz, K. H. J .
Organomet. Chem. 1972, 36, C4.
(6) For reactions of metal complexes with silanes and silyl chemistry,
see, for example: (a) Corey, J . Y.; Braddock-Wilking, J . Chem. Rev.
1999, 99, 175. (b) Tilley, T. D. In The Chemistry of Organic Silicon
Compounds; Patai, S., Rappoport, Z.; Eds.; Wiley: New York, 1989;
Part 2, p 1415. (c) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai,
S., Rappoport, Z.; Eds.; Wiley: New York, 1991; p 245 and p 309. (d)
Eisen, M. S. In The Chemistry of Organic Silicon Compounds, Vol. 2;
Rappoport, Z.; Apeloig, Y.; Eds.; Wiley: New York, 1998; Part 3, p 2037.
(e) Gauvin, F.; Harrod, J . F.; Woo, H. G. Adv. Organomet. Chem. 1998,
42, 363.
(1) (a) Schrock, R. R. J . Am. Chem. Soc. 1974, 96, 6796. (b) Schrock,
R. R.; Fellmann, J . D. J . Am. Chem. Soc. 1978, 100, 3359. (c) Fellmann,
J . D.; Schrock, R. R.; Rupprecht, G. A. J . Am. Chem. Soc. 1981, 103,
5752. (d) Fellmann, J . D.; Rupprecht, G. A.; Wood, C. D.; Schrock, R.
R. J . Am. Chem. Soc. 1978, 100, 5964.
(2) (a) Ivin, K. J . Olefin Metathesis; Academic Press: London, 1983.
(b) Schrock, R. R. Acc. Chem. Res. 1990, 23, 158. (c) Breslow, D. S.
Prog. Polym. Sci. 1993, 18, 1141. (d) Sundararajan, G. J . Sci. Ind. Res.
1994, 53, 418.
(3) Berry, D. H.; Koloski, T. S.; Carroll, P. J . Organometallics 1990,
9, 2952.
(4) Parkin, G.; Bunel, E.; Burger, B. J .; Trimmer, M. S.; Van Asselt,
A.; Bercaw, J . E. J . Mol. Catal. 1987, 41, 21.
10.1021/om000476r CCC: $20.00 © 2001 American Chemical Society
Publication on Web 03/22/2001

Reactions of Tantalum Alkylidene Complexes
Organometallics, Vol. 20, No. 8, 2001 1505
reactions of silanes with coordinated ligands.7 We are
particularly interested in how H-Si bonds in silanes
react with the π-bonds in the MdCHR moiety of Cp-
free, high-oxidation-state alkylidene complexes. We
have chosen alkylidene complexes containing phosphine
ligands (RCH2)Ta(PMe3)2[dCHR′]2, in the hope that
such ligands would help stabilize the resulting reaction
products. In this paper, we report the synthesis and
characterization of novel bis(silyl)-substituted alky-
lidene, metallasilacyclobutadiene, and metalladisilacy-
clohexadiene complexes and kinetic and mechanistic
studies of the reactions of an alkylidene complex with
silanes. Preliminary results have been reported.8
The mixture turned green, then green-yellow, and finally
bright yellow as (Me3SiCH2)5Ta was produced. The reaction
was monitored carefully by 1H NMR until complete conversion
to (Me3SiCH2)5Ta14 was achieved, at which time the solvents
were removed in vacuo. The yellow residue was taken up in
hexanes at 0 °C and filtered to remove MgCl2. PMe3 (1.7 mL,
0.017 mol) was then added by syringe, and the solution was
heated to 50 °C for 1 h, during which time a bright orange
color developed. At this time NMR showed the conversion to
1 to be complete, and the solution was filtered to remove any
remaining MgCl2, concentrated, and cooled to -20 °C, produc-
ing 7.8 g (92%) of 1 as bright orange crystals. NMR: 1H NMR
(benzene-d6, 250.1 MHz, 23 °C) δ 6.17 (s, 1H, dCHSiMe3), 0.89
2
(d, 9H, J H-P ) 4.4 Hz, PMe3), 0.30 (s, 9H, dCHSiMe3), 0.27
(s, 27H, CH2SiMe3), 0.24 (s, 6H, CH2SiMe3); 13C{1H} NMR
(benzene-d6, 62,9 MHz, 23 °C) δ 251.8 (dCHSiMe3), 77.3 (CH2-
Exp er im en ta l Section
1
SiMe3), 15.9 (d, J C-P ) 8.6 Hz, PMe3), 3.6 (dCHSiMe3), 3.0
(CH2SiMe3).
All manipulations were performed under a dry nitrogen
atmosphere with the use of either standard Schlenk techniques
or a glovebox. All solvents were purified by distillation from
potassium/benzophenone ketyl. Benzene-d6 and toluene-d8
were dried over activated molecular sieves and stored under
nitrogen. NMR spectra were recorded on a Bruker AC-250,
AMX-400, or Varian INOVA 600 Fourier transform spectrom-
eter and were referenced to solvents (residual protons in the
1H spectra). 29Si, 31P, and 2H chemical shifts were referenced
to SiMe4, external 85% H3PO4, and external toluene-d8,
respectively. Mass spectra were recorded on a VG ZAB-EQ
hybrid high-performance mass spectrometer at an ionization
voltage of 70 eV. TaCl5 (Strem) was sublimed before use. PMe3
(Aldrich) and 1.0 M anhydrous HCl in Et2O (Aldrich) were used
as received. H2SiMePh (Gelest) and H2SiPh2 (Aldrich) were
dried over activated molecular sieves and stored under nitro-
gen. Me3SiCH2MgCl,9 (ButCH2)3TadCHBut,1a,b (ButCH2)Ta-
(PMe3)2[dCHBut]2 (2),1c (Me3SiCH2)Ta(PMe3)2[dCHSiMe3]2
(3),10 and D2SiMePh11 were prepared by the literature proce-
dures. (PhSiH2)2CH2 was prepared by a procedure similar to
that in the literature.12 Elemental analyses were performed
by E + R Microanalytical, Parsippany, NJ .
P r ep a r a tion of (Me3SiCH2)Ta (P Me3)2[dCHBu t]2 (6). A
solution of 1.24 g of (tBuCH2)3TadCHBut (2.67 mmol) in 20
mL of toluene at -60 °C was treated dropwise with 12.6 mL
of HCl in Et2O (0.21 M, 2.6 mmol). The HCl/Et2O was prepared
by diluting 2.6 mL of 1.0 M HCl in Et2O with 10 mL of Et2O.
The resulting solution of (ButCH2)4TaCl1d was then treated
with 0.60 mL of PMe3 (5.8 mmol, excess) and warmed to room
temperature with stirring. After 4 h NMR spectra of the
resulting orange solution showed complete conversion to ClTa-
(PMe3)2[dCHBut]2.1c Me3SiCH2MgCl in Et2O (2.8 mL, 1.12 M,
3.1 mmol) was then added to the solution. After 1 h, the
volatiles were removed by vacuum, and the yellow-brown
residue was extracted with 30 mL of pentane. The pentane
solution was then filtered, concentrated, and cooled to -20 °C,
yielding three crops of orange crystals totaling 0.743 g (49.9%
yield based on (ButCH2)3TadCHBut). NMR: 1H NMR (benzene-
d6, 250.1 MHz, 23 °C) δ 7.27 (s, 1H, dCHBut), 1.44 (s, 1H,
2
dCHBut), 1.26 (t, 18H, J H-P ) 2.7 Hz, PMe3), 1.25 (s, 18H,
dCHCMe3), 0.27 (s, 9H, CH2SiMe3), -0.27 (t, 2H, 3J H-P ) 20.1
Hz, CH2SiMe3); 13C{1H} NMR (benzene-d6, 62,9 MHz, 23 °C)
δ 273.1, 242.2 (dCHBut), 47.4, 44.1 (dCHCMe3), 37.3 (CH2-
SiMe3), 35.3, 34.3 (dCHCMe3), 19.3 (t, 1J C-P ) 11.6 Hz, PMe3),
5.2 (CH2SiMe3). Anal. Calcd for C20H49SiP2Ta: C, 42.85; H,
8.81. Found: C, 42.84; H, 8.82.
P r ep a r a tion of (Me3SiCH2)3Ta (P Me3)[dCHSiMe3] (1).
The following is a modified procedure from the previously
reported synthesis.13 A slurry of 5.0 g (0.014 mol) of TaCl5 in
75 mL of hexanes at -20 °C was treated dropwise with 41.0
mL of a Me3SiCH2MgCl solution in Et2O (1.7 M, 0.070 mol).
P r ep a r a tion of (Me3SiCH2)3Ta [dC(SiMe3)SiHRP h ] (R
) Me, 4a ; P h , 4b). A solution of 0.90 g (1.5 mmol) of 1 in 30
mL of hexanes was treated dropwise with a solution of 0.20 g
(1.6 mmol) of H2SiMePh in 5 mL of hexanes. The reaction
mixture was then stirred for 18 h at room temperature, during
which time the color changed from orange to red-orange.
Removal of solvent yielded ca. 0.9 g of a red-orange oil of 4a ,
which also contained a small (<5% by 1H NMR) amount of
(Me3SiCH2)4Ta2(µ-CSiMe3)2. Attempts to crystallize the com-
pound were unsuccessful, and the instability of 4a in solution
precluded attempts to obtain analytically pure samples for
microanalysis. A similar result was obtained for 4b. Data for
4a : 1H NMR (benzene-d6, 250.1 MHz, 23 °C) δ 7.20-7.90 (m,
(7) (a) Liu, X.; Wu, Z.; Peng, Z.; Wu, Y.; Xue, Z. J . Am. Chem. Soc.
1999, 121, 5350. See also: (b) Xue, Z. Comments Inorg. Chem. 1996,
18, 223. (c) Chen, T.; Xue, Z. Chin. J . Inorg. Chem. 1999, 15, 413. (d)
Xue, Z.; Li, L.; Hoyt, L. K.; Diminnie, J . B.; Pollitte, J . L. J . Am. Chem.
Soc. 1994, 116, 2169. (e) Li, L.; Diminnie, J . B.; Liu, X.; Pollitte, J . L.;
Xue, Z. Organometallics 1996, 15, 5231. (f) McAlexander, L. H.; Hung,
M.; Li, L.; Diminnie, J . B.; Xue, Z.; Yap, G. P. A.; Rheingold, A. L.
Organometallics 1996, 15, 3520. (g) Wu, Z.; Diminnie, J . B.; Xue, Z.
Organometallics 1998, 17, 2917. (h) Wu, Z.; McAlexander, L. H.;
Diminnie, J . B.; Xue, Z. Organometallics 1998, 17, 4853. (i) Chen, T.;
Wu, Z.; Li, L.; Sorasaenee, K. R.; Diminnie, J . B.; Pan, H.; Guzei, I.
A.; Rheingold, A. L.; Xue, Z. J . Am. Chem. Soc. 1998, 120, 13519. (j)
Liu, X.; Li, L.; Diminnie, J . B.; Yap, G. P. A.; Rheingold, A. L.; Xue, Z.
Organometallics 1998, 17, 4597. (k) Wu, Z.; Diminnie, J . B.; Xue, Z.
Inorg. Chem. 1998, 37, 6366. (l) Wu, Z.; Diminnie, J . B.; Xue, Z.
Organometallics 1999, 18, 1002. (m) Choi, S.-H.; Lin, Z.; Xue, Z.
Organometallics 1999, 18, 5488. (n) Wu, Z.; Diminnie, J . B.; Xue, Z. J .
Am. Chem. Soc. 1999, 121, 4300.
(8) Diminnie, J . B.; Xue, Z. J . Am. Chem. Soc. 1997, 119, 12657.
(9) (a) Whitmore, F. C.; Sommer, L. H. J . Am. Chem. Soc. 1946, 68,
481. (b) Moorhouse, S.; Wilkinson, G. J . Organomet. Chem. 1973, 52,
C5. (c) Moorhouse, S.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1974,
2187.
(10) Diminnie, J . B.; Hall, H. D.; Xue, Z. Chem. Commun. 1996,
2383.
(11) Watanabe, H.; Ohsawa, N.; Sudo, T.; Hirakata, K.; Nagai, Y.
J . Organomet. Chem. 1977, 128, 27.
(12) Zech, J .; Schmidbaur, H. Chem. Ber. 1990, 123, 2087. In the
current procedure, CH2Cl2 was used in lieu of CH2Br2 to react with 2
equiv of H2SiClPh and Mg/Zn dust to give (PhSiH2)2CH2 in 43% yield.
(13) Rupprecht, G. A. Ph.D. Thesis, Massachusetts Institute of
Technology, 1979.
3
5H, SiHMePh), 4.90 (q, 1H, J H-H ) 3.6 Hz, SiHMePh), 1.20
2
(d, 3H, J H-H ) 12.1 Hz, CHaHbSiMe3), 0.92 (d, 3H, CHaHb-
SiMe3), 0.80 (d, 3H, SiHMePh), 0.30 (s, 9H, dCSiMe3), 0.19
(s, 27H, CH2SiMe3); 13C{1H} NMR (benzene-d6, 62.9 MHz, 23
°C) δ 240.6 (dCSiMe3), 137.6, 135.3, 129.5, 128.0 (SiHMePh),
89.6 (CH2SiMe3), 5.0 (dCSiMe3), 2.7 (CH2SiMe3), -1.9 (SiH-
MePh); 29Si{1H} NMR (benzene-d6, 79.5 MHz, 27 °C) δ 0.08
(CH2SiMe3), -13.2 (dCSiMe3), -49.9 (dCSiHMePh). Data for
4b: 1H NMR (benzene-d6, 250.1 MHz, 23 °C) δ 7.20-7.90 (m,
10H, SiHPh2), 5.07 (s, 1H, SiHPh2), 1.19 (s, 6H, CH2SiMe3),
0.31 (s, 9H, dCSiMe3), 0.19 (s, 27H, CH2SiMe3); 13C{1H} NMR
(benzene-d6, 62.9 MHz, 23 °C) δ 238.2 (dCSiMe3), 137.5, 136.5,
129.7, 128.1 (SiHPh2), 90.7 (CH2SiMe3), 5.19 (dCSiMe3), 2.72
(14) Li, L.; Hung, M.; Xue, Z. J . Am. Chem. Soc. 1995, 117, 12746.

1506 Organometallics, Vol. 20, No. 8, 2001
Diminnie et al.
(CH2SiMe3); 29Si{1H} NMR (benzene-d6, 79.5 MHz, 27 °C) δ
0.37 (CH2SiMe3), -13.1 (dCSiMe3), -48.3 (dCSiHPh2).
P r ep a r a tion of Meta lla sila cyclobu ta d ien e Com p lexes.
Syn th esis of 5a . A solution of 0.204 g (0.344 mmol) of (Me3-
SiCH2)Ta(PMe3)2[dCHSiMe3]2 (3) in 10 mL of hexanes was
treated with 0.044 g (0.35 mmol) of H2SiMePh in 5 mL of
hexanes. The solution was stirred at room temperature for 3
h, during which time the color changed from bright orange to
yellow. The solution was then concentrated and cooled to -20
°C, producing 0.190 g (78%) of 5a as yellow crystals. NMR:
1H NMR (benzene-d6, 250.1 MHz, 23 °C) δ 7.17-7.86 (m, 5H,
bright orange to yellow-orange. The solution was concentrated
and cooled to -20 °C, yielding 0.119 g of 8 as yellow crystals
(10.0%). Data for 8: 1H NMR (toluene-d8, 400.1 MHz, 27 °C)
δ 7.13-7.75 (m, 10H, SiHPh), 5.82 (m, 2H, SiHPh), 1.41 (m,
1H, Si-CHaHb-Si), 1.38 (2 overlapping d, 18H, PMe3), 0.88
(m, 1H, Si-CHaHb-Si), 0.22 (s, 9H, CH2SiMe3), 0.039 (s, 18H,
3
dCSiMe3), -0.58 (br t, 2H, J H-P ) 15.6 Hz, CH2SiMe3); 13C-
{1H} NMR (toluene-d8, 100.6 MHz, 27 °C) δ 262.7 (dCSiMe3),
145.4, 135.3, 128.5, 127.7 (Ph), 67.1 (CH2SiMe3), 18.6 (d, 1J C-P
) 21.1 Hz, PMe3), 16.7 (br d, 1J C-P ) 21.1 Hz, PMe3), 13.7 (Si-
CH2-Si), 6.22 (dCSiMe3), 5.24 (CH2SiMe3); 29Si{1H} NMR
(toluene-d8, 79.5 MHz, 27 °C) δ 10.5 (CH2SiMe3), -11.9 (Si-
CH2-Si), -19.3 (dCSiMe3); 31P{1H} NMR (toluene-d8, 162.0
2
SiMePh), 1.12 (d, 9H, J H-P ) 6.6 Hz, PMe3), 1.00 (s, 3H,
2
SiMePh), 0.88 (d, 9H, J H-P ) 6.7 Hz, PMe3), 0.35 (s, 18H, d
2
3
MHz, 27 °C) δ 3.07 (d, J P-P ) 155.8 Hz), -0.21 (d). The
CSiMe3), 0.25 (s, 9H, CH2SiMe3), -0.46 (t, 2H, J H-P ) 16.0
Hz, CH2SiMe3); 13C{1H} NMR (benzene-d6, 62.9 MHz, 23 °C)
δ 255.1 (dCSiMe3), 145.8, 136.3, 127.8, 127.2 (SiMePh), 50.4
assignment of two overlapping doublets for the PMe3 groups
in the 1H spectrum was confirmed by an HMQC (heteronuclear
multiple quantum coherence) experiment. Anal. Calcd for
1
1
(CH2SiMe3), 17.9 (d, J C-P ) 12.5 Hz, PMe3), 17.6 (d, J C-P
)
C
31H61P2Si5Ta: C, 45.57; H, 7.52. Found: C, 45.53; H, 7.53.
12.9 Hz, PMe3), 5.9 (dCSiMe3), 5.8 (SiMePh), 5.8 (CH2SiMe3);
29Si{1H} NMR (benzene-d6, 79.5 MHz, 27 °C) δ 0.40 (CH2SiMe3),
-15.4 (dCSiMe3), -76.8 (SiMePh); 31P{1H} NMR (benzene-
Rea ction of 1 w ith D2SiMeP h a n d An a lysis of th e
Hyd r ogen Ga s P r od u ced in Th is Rea ction by Ma ss
Sp ectr om etr y. To 1 (0.0303 g, 0.0501 mmol) dissolved in 0.4
mL of toluene-d8 in an NMR tube was added 20.0 µL (0.143
mmol) of D2SiMePh. The tube was then placed on an NMR
spectrometer which had been preheated to 50 °C, and the
reaction was monitored by NMR. After 20 min all of 1 had
been consumed, and the products were identified as 4a -d 1 and
PMe3. HDSiMePh, H2SiMePh, and a trace amount of 4a were
also identified. Data for 4a -d 1: 1H NMR (toluene-d8, 400.1
MHz, 50 °C) δ 7.20-7.81 (m, 5H, SiPhMeD), 1.10 (d, 3H, 2J H-H
) 12.1 Hz, CHaHbSiMe3), 0.91 (d, 3H, CHaHbSiMe3), 0.75 (s,
3H, SiPhMeD), 0.26 (s, 9H, dCSiMe3), 0.18 (s, 27H, CH2SiMe3);
13C{1H} NMR (toluene-d8, 100.6 MHz, 50 °C) δ 241.1 (d
CSiMe3), 137.8, 135.3, 129.7, 128.0 (SiDMePh), 89.7 (CH2-
SiMe3), 5.0 (dCSiMe3), 2.8 (CH2SiMe3), -1.8 (SiDMePh); 2H
NMR (toluene, 61.42 MHz, 27 °C) δ 4.92 (br s, SiDMePh). In
a separate experiment with 0.045 g of 1, 0.6 mL of toluene-d8,
and 47 µL (0.33 mmol) of D2SiMePh, 4,4′-dimethylbiphenyl (3.0
mg, 0.016 mmol, Aldrich) was added to the solution at the
beginning of the reaction as internal standard for NMR
2
d6, 162.0 MHz, 27 °C) δ -4.51 (d, J P-P ) 125 Hz), -7.03 (d).
The 13C resonance assignments were confirmed by the use of
13C-1H HETCOR NMR. Anal. Calcd for C25H55P2Si4Ta: C,
42.24; H, 7.80. Found: C, 42.11; H, 7.69.
Syn th esis of 5b. A solution of 0.199 g (0.336 mmol) of 3 in
10 mL of hexanes was treated dropwise with 0.070 g (0.38
mmol) of H2SiPh2 in 5 mL of hexanes. The reaction solution
was stirred for 4 h, during which time the color changed from
bright orange to yellow. The solution was then concentrated
and cooled to -20 °C, producing 0.114 g of 5b as yellow needles
(44.0% yield). NMR: 1H NMR (benzene-d6, 250.1 MHz, 23 °C)
2
δ 7.09-7.89 (m, 10H, SiPh2), 0.94 (t, 18H, J H-P ) 3.14 Hz,
PMe3), 0.41 (s, 18H, dCSiMe3), 0.26 (s, 9H, CH2SiMe3), -0.41
3
(t, 2H, J H-P ) 15.8 Hz, CH2SiMe3); 13C{1H} NMR (benzene-
d6, 62,9 MHz, 23 °C) δ 255.4 (dCSiMe3), 143.3, 137.4, 128.0,
127.1 (SiPh2), 52.5 (CH2SiMe3), 17.5 (t, 1J C-P ) 11.3 Hz, PMe3),
6.55 (dCSiMe3), 5.83 (CH2SiMe3); 29Si{1H} NMR (benzene-d6,
79.49 MHz, 27 °C) δ 0.70 (CH2SiMe3), -16.6 (dCSiMe3), -75.5
(SiPh2); 31P{1H} NMR (benzene-d6, 162.0 MHz, 27 °C) δ -6.72.
Anal. Calcd for C30H57P2Si4Ta: C, 46.61; H, 7.43. Found: C,
46.45; H, 7.49.
1
studies. After the reaction, H NMR at 600 MHz revealed the
presence of ca. 0.0065 mmol (0.088 equiv) of H2SiMePh and
0.024 mmol (0.33 equiv) of HDSiMePh in the product mixture.
The resonance of HDSiMePh is 0.010 ppm upfield-shifted from
that of H2SiMePh as a result of isotopic shift.
Syn th esis of 7. A solution of 0.548 g of (Me3SiCH2)Ta-
(PMe3)2[dCHBut]2 (6, 0.978 mmol) in 10 mL of pentane was
treated with 0.236 mL of H2SiMePh (1.93 mmol) in 2 mL of
pentane. The solution was stirred for 2 h, during which time
the color changed from yellow-orange to orange. Concentration
and cooling of the solution to -20 °C yielded 0.249 g of 7 as
yellow crystals (37.5%). NMR: 1H NMR (benzene-d6, 250.1
MHz, 23 °C) δ 7.10-8.00 (m, 5H, SiPhMe), 1.40 (s, 18H, d
Three samples were prepared for analysis of hydrogen gas.
In sample A, a J . Young valved NMR tube was charged with
a solution of 0.045 g (0.074 mmol) of 1 in 0.6 mL of toluene-d8
and 9.5 µL (0.068 mmol) of D2SiMePh. The solution was
immediately frozen in liquid nitrogen, and the tube was
evacuated to remove nitrogen. The Teflon valve on the tube
was then sealed, and the solution allowed to thaw and kept
at 23 °C for 25 min, at which time gas evolution from the
solution had ceased. The solution was frozen again in liquid
nitrogen, and the tube connected to the mass spectrometer.
The gaseous products were then pumped into the mass
spectrometer. In sample B, a J . Young valved NMR tube was
charged with a solution of 0.045 g (0.074 mmol) of 1 in 0.6 mL
of toluene-d8 and 47 µL (0.33 mmol) of D2SiMePh. The
procedure was repeated to obtain the mass spectrum of the
gaseous products.
In sample C, a solution of 0.045 g (0.074 mmol) of 1 in 0.65
mL of toluene-d8 and 47 µL (0.33 mmol) of D2SiMePh was
placed in a precalibrated J . Young valved NMR tube to give
2.51 mL of headspace. The tube was then immediately placed
in liquid N2 to freeze the solution. In this process, part of the
headspace was cooled as well. N2 in the tube was then removed
in vacuo. H2 at 1 atm was added to the tube to fill the
headspace. The solution was then allowed to thaw and kept
at 23 °C for 25 min, at which time gas evolution from the
solution had ceased. The solution was frozen again in liquid
N2, and the tube was connected to the mass spectrometer to
2
CCMe3), 1.18 (d, 9H, J H-P ) 6.2 Hz, PMe3), 1.12 (s, 3H,
2
SiPhMe), 1.09 (br d, 9H, J H-P ) 6.3 Hz, PMe3), 0.28 (s, 9H,
3
CH2SiMe3), -0.46 (br t, 2H, J H-P ) 15.2 Hz, CH2SiMe3); 13C-
{1H} NMR (benzene-d6, 62.9 MHz, 23 °C) δ 271.9 (dCBut),
145.4, 138.0, 127.1, 126.5 (SiMePh), 51.0 (CH2SiMe3), 47.9 (d
CCMe3), 37.3 (dCCMe3), 18.5 (overlapping d, PMe3), 5.95 (CH2-
SiMe3), 3.82 (SiMePh). Anal. Calcd for C27H55P2Si2Ta: C, 47.78;
H, 8.17. Found: C, 47.76; H, 8.18.
R ea ct ion of (Bu t CH 2)Ta (P Me3)2[dCH Bu t ]2 (2) w it h
H2SiMeP h . Complex 2 (0.029 g, 0.053 mmol) was dissolved
in benzene-d6 in an NMR tube. H2SiMePh (0.025 g, 0.20 mmol)
was added to the NMR tube. NMR spectra of the solution
showed the products to be H2, neopentane (CMe4), PMe3, and
other unidentified species. Another attempt in hexanes with
0.191 g of 2 and 0.050 g of H2SiMePh gave, after removal of
solvent, a red oil whose NMR spectra were similar to those
observed above.
Syn th esis of th e Meta lla d isila cycloh exa d ien e Com -
p lex 8. A solution of 0.860 g (1.45 mmol) of 3 in 15 mL of
pentane was treated dropwise with a solution of 0.378 g (1.65
mmol) of (PhSiH2)2CH2 in 2 mL of pentane. The solution was
stirred for 16 h, during which time the color changed from

Reactions of Tantalum Alkylidene Complexes
Organometallics, Vol. 20, No. 8, 2001 1507
Ta ble 1. Cr ysta llogr a p h ic In for m a tion for 5a , 5b, 7, a n d 8
5a
5b
7
8
formula
fw
C
25H55P2Si4Ta
C30H57P2Si4Ta
773.01
C27H55P2Si2Ta
678.78
C31H61P2Si5Ta
817.14
710.94
cryst size (mm)
temp (K)
cryst syst
space group
lattice params
0.30× 0.20 × 0.20
0.36 × 0.20 × 0.05
0.30 × 0.22 × 0.05
0.32 × 0.26 × 0.20
173(2)
173(2)
173(2)
173(2)
monoclinic
P21/n
monoclinic
P21/n
triclinic
P1h
orthorhombic
Pnma
a ) 10.647(3) Å
b ) 17.757(6) Å
c ) 18.686(5) Å
R ) 90°
a ) 12.199(5) Å
b ) 16.428(5) Å
c ) 19.526(6) Å
R ) 90°
a ) 10.154(3) Å
b ) 10.998(3) Å
c ) 16.492(5) Å
R ) 101.92(2)°
â ) 91.86(2)°
γ ) 111.98(2)°
1658.8(8)
a ) 10.667(3) Å
b ) 17.314(4) Å
c ) 25.591(5) Å
R ) 90°
â ) 96.91(3)°
γ ) 90°
â ) 93.96(3)°
γ ) 90°
â ) 90°
γ ) 90°
volume, Å3
3507(2)
3904(2)
4726(2)
Z
4
4
2
4
density(calc) (g/cm3)
1.346
1.315
1.359
1.148
µ (mm-1
F(000)
)
3.374
3.037
3.495
2.536
1456
1584
696
1680
scan type
ω-2θ
ω-2θ
ω-2θ
ω-2θ
θ range (deg)
index ranges
2.10-22.55
h, k, (l
1.62-22.54
h, k, (l
2.05-22.54
h, (k, (l
2.07-22.55
h, k, (l
no. of unique reflns
no. of params varied
R indices
4617 (Rint ) 0.0352)
289
5120 (Rint ) 0.0499)
4355 (Rint ) 0.0384)
3230 (Rint ) 0.0744)
334
289
208
0.0504 (wR2 ) 0.1240)
1.030
0.0606 (wR2 ) 0.1111)
1.077
0.0450 (wR2 ) 0.0901)
1.019
0.0583 (wR2 ) 0.1669)
1.035
goodness-of-fit on F2
pump the gaseous products into the spectrometer. After the
reaction, 4,4′-dimethylbiphenyl (4.1 mg, 0.022 mmol, Aldrich)
was added to the solution as internal standard for subsequent
NMR studies. 1H NMR at 600 MHz of the product mixture
showed the presence of ca. 0.0081 mmol (0.11 equiv) of H2-
SiMePh and 0.074 mmol (1.0 equiv) of HDSiMePh.
Kin etic Stu d ies of th e Con ver sion of 1f4a a n d 1f4a -
d 1. In a glovebox an NMR tube was charged with 1 (12.8-
45.0 mg) and 4,4′-dimethylbiphenyl (2.8-9.5 mg, an internal
standard), which were then dissolved in 400 µL of toluene-d8.
H2SiMePh or D2SiMePh was then added to the NMR tube via
calibrated micropipet so as to make [H2SiMePh]0/[1]0 between
5.34 and 34.8 and [D2SiMePh]0/[1]0 between 5.00 and 35.1. The
contents of the NMR tube were then mixed, and the tube was
cooled to -78 °C. The sample was then placed on an NMR
spectrometer which had been precooled to 10 °C, and NMR
spectra were recorded. The concentration of 1 was determined
by integration with respect to the internal standard 4,4′-
dimethylbiphenyl.
standard reflections were measured after every 97 reflections.
The intensity data were corrected for Lorentz and polarization
effects and for absorption using an empirical absorption
correction based upon ψ scans.
The structures were solved by direct methods using the
Siemens SHELXTL 93 (versions 5.0 and 5.10) software pack-
ages. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in calculated positions and
introduced into the refinement as fixed contributors with
isotropic U values of 0.08 Å2.
Resu lts a n d Discu ssion
Syn th esis of Disilyl-Su bstitu ted Alkyliden e Com -
p lexes. The addition of a solution of H2SiR′Ph (R′ )
Me, Ph) to a solution of the alkylidene complex (Me3-
SiCH2)3Ta(PMe3)dCHSiMe3 (1)13 resulted in the evolu-
tion of H2 and the nearly quantitative conversion (by
NMR) of 1 to the disilyl-substituted alkylidene complex
(Me3SiCH2)3Ta[dC(SiMe3)SiHR′Ph] (R′ ) Me, 4a ; Ph,
4b) (Scheme 2). The reaction of the silane occurred
exclusively with the alkylidene (dCHSiMe3) ligand, and
the resulting complexes 4a ,b were found to be unreac-
tive toward excess silane. No reaction was observed
between 1 and HSiPh3 at room temperature.
Spectroscopic properties of 4a ,b are consistent with
the structure assignments. The alkylidene resonances
of 4a and 4b occur at 240.6 and 238.2 ppm, respectively,
in the 13C{1H} NMR spectra, and appear as singlets in
the 1H-gated-decoupled 13C spectra. The R-H resonances
of the methylene groups of the (Me3SiCH2-) groups in
4a are diastereotopic and appear as two doublets in the
1H spectrum due to the stereogenic Si center in the
dC(SiMe3)Si*HMePh moiety.
Workup of the reaction mixture to produce 4a yielded
a red oil of reasonably pure 4a (>95% by 1H NMR).
However, all attempts to isolate analytically pure
samples of this compound were unsuccessful, as 4a was
found to slowly decompose in solution. When monitored
by NMR in benzene-d6, 4a slowly decomposed through
SiMe4 elimination; however a solution of 4a in benzene
could be frozen and stored at -20 °C for several weeks
without significant decomposition.
In kinetic studies involving excess PMe3, 1 (28.1 mg, 0.0465
mmol), 4,4′-dimethylbiphenyl (1.3 mg), and 420 µL (396 mg)
of toluene-d8 were placed in an NMR tube. PMe3 (97 µL, 71
mg, 0.94 mmol) and H2SiMePh (34 µL, 30 mg, 0.25 mmol) were
then added to this solution at 24 °C to make [H2SiMePh]0/[1]0
approximately 5.4. The reaction was conducted at 24 °C and
monitored by 1H and 13C NMR. In a control experiment, 1 (28.6
mg, 0.0473 mmol) and 4,4′-dimethylbiphenyl (5.1 mg) were
dissolved in 536 µL (504 mg) of toluene-d8. H2SiMePh (34 µL,
30 mg, 0.25 mmol) was then added at 24 °C to make
[H2SiMePh]0/[1]0 approximately 5.3. This reaction at 24 °C was
1
monitored by H NMR.
X-r a y Cr ysta l Str u ctu r e Deter m in a tion s of 5a , 5b, 7,
a n d 8. All crystal structures were determined on a Siemens
R3m/V diffractometer fitted with a Nicolet LT-2 low-temper-
ature device. Suitable crystals were coated with Paratone oil
(Exxon) and mounted under a stream of nitrogen at -100 °C.
The unit cell parameters and orientation matrix were deter-
mined from a least-squares fit of at least 25 reflections
obtained from a rotation photograph and an automatic peak
search routine. The refined lattice parameters and other
pertinent crystallographic information are given in Table 1.15
Intensity data were measured with graphite-monochro-
mated Mo KR radiation (λ ) 0.71073 Å). Background counts
were measured at the beginning and end of each scan with
the crystal and counter kept stationary. The intensities of three
Syn t h esis of Novel Met a lla sila cyclob u t a d ien e
Com p lexes. Addition of H2SiR′Ph (R′ ) Me, Ph) to
(15) See the Supporting Information for details.

1508 Organometallics, Vol. 20, No. 8, 2001
Diminnie et al.
Sch em e 2
The spectroscopic properties of 5a ,b support the
structure assignments. The 13C{1H} NMR resonances
of the alkylidene ligands range from 250.3 to 255.4 ppm
and appear as singlets in the 1H-gated-decoupled 13C
spectra. The phosphine ligands in 5a are chemically
inequivalent, resulting in the appearance of two dou-
blets in the 1H, 13C{1H}, and 31P{1H} NMR spectra. This
is consistent with a trigonal bipyramidal structure with
axial phosphine ligands and a planar metallasilacyclob-
utadiene ring with the Ph and Me ligands above and
below the ring, thus making the phosphine ligands
chemically inequivalent. The two phosphine ligands in
5b are equivalent, leading to virtually coupled triplets
1
in the H and 13C{1H} spectra and a singlet in the 31P-
{1H} spectrum. The methylene resonances of the Me3-
SiCH2- ligands in 5a ,b appear as triplets in the 1H
NMR spectra. The structural assignments of 5a and 5b
were confirmed by X-ray crystallography, which is
discussed below.
The preferential reactions of the alkylidene ligands,
rather than the alkyl ligands, of 3 with H2SiPhR′ were
unexpected. To probe whether such a preference was
general, the reactions of the neopentyl analogue of 3,
(ButCH2)Ta(PMe3)2[dCHBut]2 (2),1c and the mixed-
ligand trimethylsilylmethyl neopentylidene complex
(Me3SiCH2)Ta(PMe3)2[dCHBut]2 (6) with H2SiMePh
were studied. The reaction of 6 with H2SiMePh also
generated H2 and a metallasilacyclobutadiene complex
7 (Scheme 2). However, the reaction of 2 with H2SiMePh
gave as products H2, CMe4, PMe3, and a mixture of
unidentified complexes. Spectroscopic properties of 7 are
similar to those of 5a . The alkylidene carbon resonance
occurs at 271.9 ppm in the 13C{1H} NMR spectrum and
appears as a singlet in the 1H-gated-decoupled 13C
spectrum. The phosphine ligands are chemically in-
(Me3SiCH2)Ta(PMe3)2[dCHSiMe3]2 (3) led to the evolu-
tion of H2 and the formation of novel 1,1′-metalla-3-
silacyclobutadiene complexes (Me3SiCH2)Ta(PMe3)2[d
C(SiMe3)SiPhR′C(SiMe3)] (R′ ) Me, 5a ; Ph, 5b) (Scheme
2). Again, preferential reactions with the alkylidene
ligands were observed, and the products were inert to
excess silane. In contrast to 4, no decomposition of
complexes 5a ,b in both solution and solid state was
observed over a long period of time, and 5a ,b are soluble
in a variety of aromatic, aliphatic, and ethereal solvents.
No reaction was observed between 3 and HSiPh3,
1
equivalent and appear as two doublets in both the H
and 13C{1H} spectra. However, one of the two phosphine
signals in the 1H spectrum is broad, indicating a
fluxional process involving one of the two phosphine
ligands. The structure of 7 was confirmed by X-ray
crystallography.
HSiButPh2, or H2SiBut .
2
The structures of 5a , 5b, 7, and 8 are novel and to
our knowledge represent the first examples of 1,1-
metallasilacyclobutadiene complexes. A number of met-
allasilacyclobutane complexes have been synthesized
and structurally characterized,16 and a large number
of conjugated metallacyclobutadiene complexes are
known;17 however, to our knowledge, 5a and 5b repre-
sent the first cyclobutadiene complexes in which both
double bonds are localized exclusively on the metal
atom.18
In contrast to complexes 5a ,b, which are stable in
solution, 7 was found to be unstable in solution,
decomposing by loss of PMe3 followed by loss of SiMe4
to give a mixture of decomposition products whose NMR
spectra were similar to those from the reaction of 2 with
H2SiMePh. One possibility for the lower stability of 7
in solution may be due to increased steric crowding
around the Ta center caused by the -But groups on the
alkylidene ligands; a C-C bond (ca. 1.54 Å) is shorter
than a C-Si bond (ca. 1.85 Å), resulting in the two
groups exo to the metallasilacyclobutadiene ring being
closer to the metal center. This may lead to the dis-
sociation of a phosphine ligand (as is supported by the
(16) (a) Tikkanen, W. R.; Liu, J . Z.; Egan, J . W.; Petersen, J . L.
Organometallics 1984, 3, 825. (b) Bruno, J . W.; Marks, T. J .; Day, V.
W. J . Am. Chem. Soc. 1982, 104, 7357. (c) Behling, T.; Girolami, G. S.;
Wilkinson, G.; Somerville, R. G.; Hursthouse, M. B. J . Chem. Soc.,
Dalton Trans. 1984, 877. (d) Morse, P. M.; Spencer, M. D.; Wilson, S.
R.; Girolami, G. S. Organometallics 1994, 13, 1646.
(17) For reviews of metallacyclobutadiene complexes, see: (a) Wig-
ley, D. E.; Gray, S. D. In Comprehensive Organometallic Chemistry II;
Abel, E. W.; Stone, F. G. A.; Wilkinson, G., Eds.; Pergamon: New York,
1995; Vol. 5, p 57. (b) Nugent, W. A.; Mayer, J . M. Metal-Ligand
Multiple Bonds; Wiley: New York, 1988; Chapter 7. (c) Engel, P. F.;
Pfeffer, M. Chem. Rev. 1995, 95, 2281, and references therein.
(18) A number of metallacyclopentadiene and -triene complexes have
been reported. For examples see: (a) Albers, M. O.; de Waal, D. J . A.;
Liles, D. C.; Robinson, D. J .; Singleton, E.; Wiege, M. B. J . Chem. Soc.,
Chem. Commun. 1986, 1680. (b) Hirpo, W.; Curtis, M. D. J . Am. Chem.
Soc. 1988, 110, 5218. (c) Hessen, B.; Teuben, J . H. J . Organomet. Chem.
1989, 367, C18.
1
broadened signal for one of the PMe3 ligands in the H
NMR spectrum of 7), resulting in a pseudo-tetrahedral
complex, which further decomposes by loss of SiMe4
(possibly via γ-H abstraction from a -But group) to give
unidentified products. It is therefore likely that the
reaction of 2 with H2SiMePh may also form a metal-
lasilacyclobutadiene complex, which then rapidly loses
PMe3 followed by CMe4 to give a product mixture
similar to that from the decomposition of 7.

Reactions of Tantalum Alkylidene Complexes
Organometallics, Vol. 20, No. 8, 2001 1509
Syn th esis of th e Meta lla d isila cycloh exa d ien e
Com p lex 8. The unexpected formation of metallasila-
cyclobutadiene complexes 5a ,b from the reactions of 3
with H2SiR′Ph prompted us to study whether such
chemistry could be extended to the reactions of alky-
lidene complexes with compounds containing more than
one silyl functionality. The reactions of such compounds,
such as a disilylmethane (H2PhSi)2CH2,12 which contain
two reactive groups, could possibly lead to the formation
of oligomeric, cyclic, or polymeric complexes in which
the disilylmethane moiety could bridge two metal
centers, as well as to the formation of a metalladisila-
cyclohexadiene complex. We therefore studied the re-
activity of (PhH2Si)2CH2 toward 3.
distorted tetrahedral geometry, with bond angles rang-
ing from 96.7(5)° to 115.9(6)°.
The structure of 5b is similar to 5a (Figure 2). As in
5a , the tantalum atom exhibits distorted trigonal bipy-
ramidal geometry with the PMe3 ligands occupying axial
positions. The TadC bond distances of 1.951(12) and
1.977(12) Å are similar to those listed above. The
metallasilacyclobutadiene ring is also planar, and the
silicon atom of the ring exhibits distorted tetrahedral
geometry, with bond angles ranging from 97.7(6)° to
115.7(6)°. Again, the planar nature of the ring brings
the Si atom in close contact to the Ta center (Ta-Si
distance of 2.599(4) Å).
The structure of 7 is similar to 5a , except that the
exo-SiMe3 groups of the metallasilacyclobutadiene ring
are replaced by -But groups (Figure 3). Again, the Ta
atom has a distorted trigonal bipyramidal geometry
with axial PMe3 ligands, and the TadC bond distances
of 1.952(9) and 1.953(8) Å are consistent with the
presence of two alkylidene moieties. Despite the insta-
bility of 7 in solution, in which it decomposes by loss of
a PMe3 ligand followed by SiMe4 elimination, there is
little in the structure of 7 to indicate increased steric
strain around the Ta center. The Ta-P distances of
2.621(3) and 2.628(3) Å are similar to those in 5a and
5b, which range from 2.596(4) to 2.664(3) Å. The Ta-
C-Si angle of the Me3SiCH2- ligand in 7 is 135.7(5)°,
which is wider than that (121.2(6)°) in 5a , but less than
that (137.8(7)°) in 5b. The Ta-Si distance of 2.621(3) Å
is slightly longer than those in 5a ,b. It is therefore
unclear from the solid-state structure of 7 why this
complex is unstable in solution, while complexes 5a ,b
are stable in solution. It is possible that the instability
of 7 is perhaps the result of a combination of steric and
electronic effects caused by the nature of the exo-R group
on the metallasilacyclobutadiene ring; the presence of
a SiMe3 group exo to the ring in 5a ,b may help to
stabilize these complexes toward decomposition. It is
well established that a silyl substituent helps to stabi-
lize adjacent metal-carbon bonds.20 The lack of silyl
substituents exo to the metallasilacyclobutadiene ring
in 7 may contribute to the lower stability of this complex
in solution.
Analysis of the structure of the crystals of 8 revealed
them to be that of a meso-isomer (Figure 4), in which
the metalladisilacyclohexadiene ring is in a half-chair
conformation, and the Ph rings on the ring Si atoms
were found to occupy pseudoequatorial positions. The
molecule was found to exhibit crystallographically im-
posed mirror symmetry, with the Me3SiCH2- ligand
disordered over the mirror plane. This ligand was
therefore refined as two mirror images with site oc-
cupancy factors of 0.5. The Ta atom exhibits distorted
trigonal bipyramidal geometry, with the PMe3 ligands
occupying axial positions. The TadC distances of 1.985-
(15) and 1.985(14) Å are again consistent with other
alkylidene complexes of tantalum. The CdTadC bond
angle of 113.0(8)° is much wider than those in the
metallasilacyclobutadiene complexes 5a ,b and 7 (91.9-
(3)-93.3(5)°). This large angle causes the exo-SiMe3
groups [Si(1)Me3] to be in closer proximity to the Me3-
SiCH2- ligand, which in turn leads to a considerable
The reaction of PhSiH2CH2SiH2Ph with 3, when
conducted in an NMR tube in benzene-d6 and monitored
1
by H NMR, was found to immediately produce H2 and
other products. Increasing the scale of the reaction in
pentane and cooling the reaction mixture to -20 °C led
to the isolation of a 1,1′-metalla-3,5-disilacyclohexadiene
complex 8 in low yield (Scheme 2). No further products
could be isolated from the reaction mixture.
Crystals of 8 were found by X-ray crystallography to
be those of a meso-isomer, in which both phenyl rings
on the two stereogenic Si atoms are occupying pseu-
doequatorial positions on the metalladisilacyclohexadi-
ene ring. The NMR spectroscopic properties of crystals
of 8 are in agreement with the structure assignment of
the meso-isomer. The alkylidene carbons appear at
262.7 ppm in the 13C{1H} spectrum, and this signal
appears as a singlet in the 1H-gated-decoupled 13C
spectrum. The phosphine ligands are chemically in-
1
equivalent and lead to two doublets in the H, 13C{1H},
and 31P{1H} spectra. The silane protons appear as a
multiplet at 5.82 ppm, and the methylene protons as
multiplets at 0.88 and 1.41 ppm. 8 has an extremely
low solubility in benzene, but surprisingly is soluble in
toluene.
X-r a y Cr ysta l Str u ctu r es of 5a , 5b, 7, a n d 8. Slow
cooling of solutions of 5a ,b in hexanes and 7 and 8 in
pentane, respectively, produced crystals suitable for
analysis by X-ray crystallography. Crystal data, results
of analyses, and bond distances and angles are given
in Tables 1 and 2.15 The molecular structures of 5a , 5b,
7, and 8 are shown in Figures 1-4.
Complex 5a exhibits distorted trigonal bipyramidal
geometry around the tantalum center, with the PMe3
ligands occupying axial positions (Figure 1). The TadC
bond distances of 1.947(12) and 1.962(12) Å are consis-
tent with those observed for other alkylidene complexes
of tantalum (1.998(8) and 1.95(2) Å in 3, 1.932(7) and
1.955(7) Å in Ta(dCHBut)2(mesityl)(PMe3)2,19a and 1.932-
(9) Å in [Ta(dCHBut)(CH2But)(PMe3)2]2(µ-N2)19b,c). The
metallasilacyclobutadiene ring is planar (average devia-
tion from the least-squares plane ) 0.007 Å), which
brings the silicon atom in close proximity to the tanta-
lum center (Ta-Si distance of 2.607(3) Å); however, the
fact that the metal center is formally d0 makes any
metal-silicon bonding interaction unlikely. The silicon
atom of the metallasilacyclobutadiene ring exhibits
(19) (a) Churchill, M. R.; Youngs, W. J . Inorg. Chem. 1979, 18, 1930.
(b) Churchill, M. R.; Wasserman, H. J . Inorg. Chem. 1981, 20, 2899.
(c) 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.
(20) Fleming, I. In Comprehensive Organic Chemistry; J ones, D. N.,
Ed.; Pergamon: New York, 1979; Vol. 3, Part 13, p 541.

1510 Organometallics, Vol. 20, No. 8, 2001
Diminnie et al.
Ta ble 2. Selected In ter a tom ic Dista n ces a n d An gles for 5a , 5b, 7, a n d 8
Interatomic Distances (Å)
5a
7
Ta-P(1)
2.605(3)
Ta-P(2)
2.664(3)
Ta-P(1)
2.621(3)
1.953(8)
2.271(9)
1.796(9)
1.902(9)
1.879(9)
1.898(9)
1.542(12)
Ta-P(2)
2.605(3)
1.952(9)
2.628(3)
1.831(10)
1.885(8)
1.908(9)
1.885(11)
1.534(12)
Ta-C(1)
1.955(13)
2.289(12)
1.811(12)
1.904(12)
1.899(13)
Ta-C(2)
1.969(12)
1.824(13)
1.879(12)
1.905(12)
1.889(13)
Ta-C(1)
Ta-C(2)
Ta-C(3)
Si(1)-C(1)
Si(3)-C(3)
Si(4)-C(2)
Si(4)-C(20)
Ta-C(3)
Ta-Si(1)
Si(2)-C(2)
Si(4)-C(1)
Si(4)-C(19)
P(1)-C(22)
Si(1)-C(1)
Si(1)-C(12)
Si(2)-C(3)
C(1)-C(4)
P(2)-C(25)
Si(1)-C(2)
Si(1)-C(13)
Si(2)-C(19)
C(2)-C(8)
5b
8
Ta-P(1)
2.596(4)
Ta-P(2)
2.631(4)
1.951(12)
2.599(4)
1.81(2)
1.823(13)
1.865(13)
1.899(13)
Ta-P(1)
2.608(6)
1.985(15)
2.23(3)
1.87(3)
1.873(12)
Ta-P(2)
2.624(6)
Ta-C(1)
1.977(12)
2.282(12)
1.797(14)
1.816(13)
1.886(13)
1.901(13)
Ta-C(2)
Ta-C(1)
Ta-C(2)
Si(2)-C(2)
Si(3)-C(3)
Ta-C(1A)
Si(1)-C(1)
Si(3)-C(1)
Si(3)-C(14)
1.985(14)
1.847(15)
1.854(14)
1.880(18)
Ta-C(3)
Ta- -Si(4)
P(2)-C(7)
Si(2)-C(2)
Si(4)-C(2)
Si(4)-C(25)
P(1)-C(4)
Si(1)-C(1)
Si(4)-C(1)
Si(4)-C(19)
Bond Angles (deg)
5a
C(1)-Ta-C(2)
C(2)-Ta-C(3)
C(2)-Ta-P(1)
C(1)-Ta-P(2)
C(3)-Ta-P(2)
Si(2)-C(2)-Ta
Si(3)-C(3)-Ta
93.3(5)
132.4(5)
101.0(4)
91.0(4)
85.6(3)
146.8(7)
121.2(6)
C(1)-Ta-C(3)
C(1)-Ta-P(1)
C(3)-Ta-P(1)
C(2)-Ta-P(2)
Si(4)-C(1)-Ta
Si(4)-C(2)-Ta
Si(1)-C(1)-Ta
134.2(5)
90.1(4)
80.7(3)
96.9(4)
85.0(5)
84.6(5)
144.6(7)
C(3)-Si(3)-C(18)
C(1)-Si(4)-C(19)
C(2)-Si(4)-C(19)
Si(1)-C(1)-Si(4)
P(1)-Ta-P(2)
C(2)-Si(2)-C(13)
C(3)-Si(3)-C(17)
112.0(6)
115.0(6)
116.2(6)
130.4(7)
161.96(11)
113.0(6)
111.3(6)
C(1)-Si(4)-C(2)
C(1)-Si(4)-C(20)
C(2)-Si(4)-C(20)
Si(2)-C(2)-Si(4)
C(1)-Si(1)-C(12)
C(3)-Si(3)-C(16)
97.1(6)
114.0(6)
111.9(6)
127.9(7)
115.0(6)
114.4(6)
5b
C(1)-Ta-C(2)
C(2)-Ta-C(3)
C(4)-P(1)-Ta
91.9(5)
122.0(5)
117.1(5)
115.3(6)
114.7(7)
97.7(6)
C(1)-Ta-C(3)
146.0(5)
Si(3)-C(3)-Ta
C(2)-Ta-P(1)
C(1)-Ta-P(2)
C(3)-Ta-P(2)
Si(1)-C(1)-Si(4)
Si(4)-C(1)-Ta
Si(2)-C(2)-Ta
C(2)-Si(4)-C(25)
137.8(7)
98.8(4)
93.3(4)
82.3(3)
133.9(7)
84.5(5)
138.5(7)
113.0(6)
C(19)-Si(4)-C(25)
C(3)-Ta-P(1)
C(2)-Ta-P(2)
P(1)-Ta-P(2)
Si(1)-C(1)-Ta
Si(2)-C(2)-Si(4)
Si(4)-C(2)-Ta
103.5(5)
84.7(3)
99.6(4)
161.25(12)
141.4(7)
135.4(7)
85.8(5)
C(1)-Ta-P(1)
90.0(4)
102.4(9)
114.0(7)
113.0(7)
115.7(6)
113.6(6)
C(7)-P(2)-C(8)
C(1)-Si(1)-C(10)
C(3)-Si(3)-C(16)
C(1)-Si(4)-C(19)
C(2)-Si(4)-C(19)
C(7)-P(2)-Ta
C(2)-Si(2)-C(13)
C(1)-Si(4)-C(2)
C(1)-Si(4)-C(25)
113.9(6)
7
C(1)-Ta-C(2)
C(2)-Ta-C(3)
C(1)-Ta-P(2)
C(2)-Ta-P(2)
C(3)-Ta-P(2)
C(22)-P(1)-Ta
91.9(3)
121.7(3)
93.9(3)
96.6(3)
82.2(3)
119.2(4)
C(1)-Ta-C(3)
C(1)-Ta-P(1)
C(2)-Ta-P(1)
C(3)-Ta-P(1)
P(1)-Ta-P(2)
C(25)-P(2)-Ta
146.4(3)
94.0(3)
100.4(3)
81.7(3)
160.99(8)
113.0(4)
C(1)-Si(1)-C(2)
C(1)-Si(1)-C(13)
C(2)-Si(1)-C(13)
C(3)-Si(2)-C(19)
C(4)-C(1)-Ta
C(8)-C(2)-Si(1)
Si(1)-C(2)-Ta
95.7(4)
117.9(4)
112.3(4)
112.3(5)
147.6(6)
130.3(7)
86.4(3)
C(1)-Si(1)-C(12)
C(2)-Si(1)-C(12)
C(12)-Si(1)-C(13)
C(4)-C(1)-Si(1)
Si(1)-C(1)-Ta
113.2(4)
116.0(4)
102.6(4)
126.5(6)
85.9(3)
C(8)-C(2)-Ta
142.9(6)
135.7(5)
Si(2)-C(3)-Ta
8
C(1)-Ta-C(1A)
C(1A)-Ta-C(2)
C(1A)-Ta-P(1)
C(2A)-Ta-P(1)
C(1A)-Ta-P(2)
P(1)-Ta-P(2)
113.0(8)
115.3(11)
94.1(4)
82.6(7)
93.0(4)
C(1)-Ta-C(2)
C(1)-Ta-P(1)
C(2)-Ta-P(1)
C(1)-Ta-P(2)
C(2)-Ta-P(2)
C(10)-P(1)-Ta
131.7(10)
94.1(4)
82.6(7)
93.0(4)
84.8(7)
114.8(9)
C(12)-P(2)-Ta
C(1)-Si(3)-C(3)
C(3)-Si(3)-C(14)
Si(1)-C(1)-Ta
Si(3)-C(3)-Si(3A)
Si(2)-C(2)-Ta
112.9(8)
113.9(8)
101.9(9)
126.0(7)
113.0(11)
147(2)
C(1)-Si(1)-C(4)
C(1)-Si(3)-C(14)
Si(1)-C(1)-Si(3)
Si(3)-C(1)-Ta
C(15)-C(14)-Si(3)
C(2)-Si(2)-C(7)
108.0(8)
118.2(7)
112.8(8)
120.6(8)
121.8(19)
116.7(13)
167.25(18)
widening of the Ta-C-Si angle (147(2)°) in the Me3-
SiCH2- ligand.
also observed to be slower than that of 1 with H2-
SiMePh; the latter was nearly complete after a few
hours at room temperature, while the former reaction
took 1 day to go to completion. This observation was
confirmed by kinetic studies of this reaction, which are
discussed below. The incorporation of hydrogen into the
excess deuterated silane D2SiMePh to give HDSiMePh
and H2SiMePh was unexpected, and the reaction was
Deu ter iu m La belin g Stu d ies of th e Con ver sion
of 1 to 4a . To further study the reaction of 1 with H2-
SiMePh to give the disilyl-substituted alkylidene com-
plex 4a , the reaction of 1 with the deuterated silane
D2SiMePh was investigated. Monitoring the reaction of
1 with D2SiMePh by NMR showed the product to be
(Me3SiCH2)3Ta[dC(SiMe3)SiDMePh] (4a -d 1) (Scheme
2). In addition, when 2.1 equiv of D2SiMePh was used
for the reaction, the formation of HDSiMePh and H2-
SiMePh was also observed; at larger excesses of silane
(>10 equiv), only HDSiMePh was found in the reaction
solution by NMR. The reaction of 1 with D2SiMePh was
2
thus monitored by H NMR to investigate whether the
hydrogen incorporation into D2SiMePh was due to
exchange of deuterium in D2SiMePh with protons in the
alkyl ligands (Me3SiCH2-) of 1. No incorporation of
deuterium into the alkyl ligands of the product 4a -d 1
was observed, and the only deuterium signals observed

Reactions of Tantalum Alkylidene Complexes
Organometallics, Vol. 20, No. 8, 2001 1511
F igu r e 1. ORTEP diagram of 5a showing 50% ellipsoids.
F igu r e 4. ORTEP diagram of 8 showing 30% ellipsoids.
(sample B), the mass spectrum showed the gaseous
products to be D2, HD, and H2 in an 89.6(1.2):9.1(1.1):
1.26(0.01) ratio.15 In such a reaction between 1 and 4.5
1
equiv of D2SiMePh, H NMR at 600 MHz showed the
presence of 0.088 equiv of H2SiMePh and 0.33 equiv of
HDSiMePh. In the third experiment, the gaseous prod-
ucts from the reaction of 1 with 4.5 equiv of D2SiMePh
under excess H2 (sample C) were analyzed as well by
mass spectrometry. The MS showed the products to be
D2, HD, and H2 in a 24.5(0.1):13.4(0.1):62.1(0.1) ratio.15
1H NMR at 600 MHz of this reaction mixture revealed
the presence of approximately 1.0 equiv of HDSiMePh
and 0.11 equiv of H2SiMePh. The results from samples
A and B showed that D2 is the major product in the
reaction, and the ratios of D2, HD, and H2 changed
slightly when D2SiMePh/1 ratios varied from ca. 1 to 5.
The result in sample C with added H2 in the reaction
mixture indicated that products and H2 added to the
system undergo hydrogen scrambling.
F igu r e 2. ORTEP diagram of 5b showing 50% ellipsoids.
Kin etic Stu d ies of th e Con ver sion of 1 to 4a a n d
Mech a n istic Con sid er a tion s of Th is Con ver sion .
To further investigate the reaction of phenyl-containing
silanes with alkylidene complexes, kinetic studies of the
reactions of 1 with H2SiMePh and D2SiMePh at 10 °C
were performed. During the NMR studies of this reac-
tion, no disilanes such as PhMeHSiSiHMePh or polysi-
lanes were observed. In addition, when the reaction of
1 with H2SiMePh was conducted in the presence of
HSiPh3 (which had been found to be unreactive toward
1), no crossover disilanes PhMeHSiSiPh3 were observed.
Thus it is unlikely that a mechanism involving silyl
radicals is involved in this reaction, as such a mecha-
nism is expected to involve disilane formation through
combination of silyl radicals as one of the chain termi-
nation steps. We therefore focused our attention on two
possible reaction pathways, which are shown in Scheme
3. The first pathway (path A) involves the loss of a PMe3
ligand to open a coordination site on the tantalum
center, followed by addition of the silane to the TadC
bond, leading to products. The second pathway (path
B) would involve an addition of the silane to the TadC
alkylidene bond, followed by loss of phosphine and con-
version to products. When monitored by NMR, no inter-
mediates were observed in the conversion of 1 to 4a .
If the dissociative mechanism in path A (Scheme 3)
were operative in this reaction, a steady-state ap-
F igu r e 3. ORTEP diagram of 7 showing 50% ellipsoids.
were those from D2SiMePh, HDSiMePh (overlapping
signals), and (Me3SiCH2)3Ta[dC(SiMe3)SiDMePh].15 This
result rules out the possibility that hydrogen incorpora-
tion into D2SiMePh is the result of exchange of deute-
rium in D2SiMePh with protons in the alkyl ligands of
1
1. In a separate experiment, H NMR spectra of a D2-
SiMePh solution in toluene-d8 showed no exchange of
the residual hydrogen in toluene-d8 with D2SiMePh
after 3 h at 50 °C.
The gaseous products from the reaction of 1 with ca.
1 equiv of D2SiMePh were analyzed by mass spectrom-
etry. The mass spectrum of sample A showed the
products to be D2, HD, and H2 in an 86.6(0.3):12.5(0.3):
0.86(0.03) ratio (by total ion current).15 In another
experiment when 1 reacted with 4.5 equiv of D2SiMePh

1512 Organometallics, Vol. 20, No. 8, 2001
Diminnie et al.
Sch em e 3
d[1]/dt ) k3[1][H2SiMePh]
(6)
For reactions conducted with an excess of H2SiMePh,
pseudo-first-order kinetics are expected (eq 7)
kobs ) k3[H2SiMePh]
(7)
Kinetic studies were performed with [H2SiMePh]0/[1]0
ranging from 5.34 to 34.8 and [D2SiMePh]0/[1]0 ranging
from 5.00 to 35.1. In these studies, [PMe3]av ranges from
0.0174 to 0.0575 M in the reactions with H2SiMePh and
0.0348 to 0.0785 M in the reactions with D2SiMePh.15
For each [Silane]av/[PMe3]av ratio, the experiment was
repeated with different [Silane]0 and [1]0. Under these
conditions the disappearance of 1 with time was found
to obey first-order kinetics,15 as is shown in the kinetic
plots. Values for the observed rate constants are given
in the Supporting Information. Plots of 1/kobs vs [PMe3]av/
[H2SiMePh]av and [PMe3]av/[D2SiMePh]av were found to
be linear (Figure 5), as would also be expected from the
dissociative mechanism rate law (eq 5). Such an obser-
vation is consistent with the mechanistic pathway path
A shown in Scheme 3, in which there is a dissociative
mechanism involving loss of PMe3 from 1 prior to the
reaction of 1 with H2SiMePh. From the plots of 1/kobs
vs [PMe3]av/[H2SiMePh]av, a value of k1 ) 5.45(10) ×
10-2 min-1 was obtained. In addition, from the slopes
of the plots shown in Figure 5, a kinetic isotope effect
value k2H/k2D was calculated to be 1.87. The magnitude
of this kinetic isotope effect is consistent with the
formation of or the breaking of a silicon-deuterium
bond in the rate-controlling step and is similar to that
(1.5) observed in a kinetic study of the reactions of an
iron carbene complex with silanes (Scheme 1).5c
proximation (d[1a ]/dt ) 0) gives21a
k1[1]
[1a ] )
(1)
(2)
(3)
(4)
k-1[PMe3] + k2[H2SiMePh]
and
k1k2[H2SiMePh][1]
ν ) -d[1]/dt )
k-1[PMe3] + k2[H2SiMePh]
or ν ) kobs[1] where
k1k2[H2SiMePh]
kobs
)
k-1[PMe3] + k2[H2SiMePh]
Path A (Scheme 3) involves initial dissociation of
PMe3. The presence of excess PMe3 is thus expected to
significantly slow the reaction. Kinetic studies of the
reaction of 1 with 5.4 equiv of H2SiMePh in the presence
of 20 equiv of PMe3 at 24 °C gave kobs ) 8.1 × 10-4
which may also be expressed in a linear form
k-1[PMe3]
1
kobs
1
k1
)
+
k1k2[H2SiMePh]
min-1 15
In comparison, the control experiment in the
.
absence of added PMe3 at 24 °C gave kobs′ ) 1.5 × 10-2
Thus under conditions of excess H2SiMePh, one would
expect to observe pseudo-first-order kinetics in [1].
When [H2SiMePh] . [PMe3], the silane terms in the
rate expression (eq 3) would become dominant and the
observed rate would simplify to kobs ≈ k1. In other words,
one would observe saturation kinetics at large H2-
SiMePh excesses. Also, a plot of 1/kobs vs [PMe3]/[H2-
SiMePh] would be linear (eq 4), yielding values for k1
and k-1/k2.21b In the reactions shown in Scheme 2, 1
equiv of PMe3 is produced. If the average concentrations
of silane and PMe3 are [Silane]av ) [Silane]0 + [Si-
lane]end and [PMe3]av ) [1]/2, eq 4 then approximates
to
15
min-1, which is ca. 19 times larger than kobs
.
These
observations are consistent with path A.
The likelihood of path B was also investigated. Plots
of kobs vs [H2SiMePh]0 and kobs vs [H2SiMePh]av did not
reveal the linear relationship as defined in eq 7 for path
B,15 suggesting that path B is unlikely the pathway in
the reaction between 1 and H2SiMePh.
The observed kinetic isotope effect indicates that a
Si-H bond formation or bond cleavage occurs in the
rate-controlling step of the reaction. The deuterium-
labeling studies showed that D2 was the major gaseous
product in the reaction of 1 with excess D2SiMePh, and
the percentage of D2 in the D2-HD-H2 mixtures in-
creased slightly when D2SiMePh was raised from 1- to
5-fold. In addition, hydrogen incorporation into excess
D2SiMePh to yield HDSiMePh was also observed, and
hydrogen scrambling clearly occurs in the products.
While the mechanism of the conversion of 1 to 4a is
not clear, these observations offer some clues as to the
possible mechanistic pathways in this reaction. The
kinetic studies indicate that PMe3 initially dissociates
from 1 to give an intermediate complex 1a with an open
coordination site for the approach of silane. The silane
then reacts with the alkylidene ligand to yield a deu-
k-1[PMe3]av
1
kobs
1
k1
)
+
(5)
k1k2[H2SiMePh]av
If the reaction proceeded by the associative mecha-
nism shown in path B of Scheme 3, and if the first step
was rate controlling, a much simpler rate expression
would be obtained:
(21) (a) J ordan, R. B. Reaction Mechanisms of Inorganic and
Organometallic Systems; Oxford University Press: New York, 1991;
p 31. (b) For an example of the approaches here, see: Meyer, K. E.;
Walsh, P. J .; Bergman, R. G. J . Am. Chem. Soc. 1994, 116, 2669.

Reactions of Tantalum Alkylidene Complexes
Organometallics, Vol. 20, No. 8, 2001 1513
Sch em e 4
-1
F igu r e 5. Plot of kobs vs [PMe3]av/[Silane]av.
teride intermediate 9-d 2, possibly through nucleophilic
attack on the silane by the π-electron density of the
alkylidene bond. The alkylidene ligands of Schrock
alkylidene complexes have been shown to be nucleo-
philic in nature, and reactions of alkylidene complexes
with olefins and ketones are also believed to involve
some degree of nucleophilic attack on the olefin and
ketone by the alkylidene ligand.2 Similar reactions of
alkyl or amido complexes with silanes to give metal
hydride complexes and alkylsilanes or amidosilanes
have recently been reported.7a,22
At least the three processes in Scheme 4 may occur
in the next step. The ratio D2/HD ) 86.6(0.3)/12.5(0.3)
when D2SiMePh/1 ) ca. 1 suggested that 9-d 2 may then
react with a second silane molecule through a dehydro-
genative coupling reaction to yield D2 and a silyl
intermediate 10. Such reactions of hydride complexes
with silanes have been proposed in the dehydrogenative
polymerization of silanes.23 The silyl ligand in 10 then
undergoes R-hydrogen abstraction to give H2SiMePh or
HDSiMePh and 4a -d 1. The mechanism shown in path
A-1 is thus consistent with the observed dissociative-
mechanism from the kinetic studies and also accounts
for the preferential formation of D2 and the proton
incorporation into the unreacted D2SiMePh to give
HDSiMePh in the deuterium labeling studies. It should
be noted that this pathway (path A-1) first requires
H-SiHMePh addition to the TadC bond to give the
H-Ta-C′-Si moiety (C′ ) CHR) and at the last step
requires the elimination of H-SiDMePh from 10 with
the Si-Ta-C′′-H moiety (C′′ ) C(SiDMePh)R) of op-
posite regiochemistry.
The observations of hydrogen scrambling in products
when H2 was added indicate that path A-2 and path
A-3 may occur in the system. In path A-2, the deuteride
ligand in 9-d 2 undergoes σ-bond metathesis with H2 (or
HD) to give HD (or D2) and 9-d 1. This process may lead
to increased percentage of HD in sample C when the
reaction was conducted under H2. Path A-3 in Scheme
4, followed by the exchange HD + D2SiMePh h D2 +
HDSiMePh, may explain the observed hydrogen scram-
bling process. Another process that may occur is a
σ-bond metathesis reaction involving the R-C and H
atoms of the TadCHR ligand in 1a and the incoming
D2SiMePh, yielding HD and 4a -d 1 as products. Subse-
quent scrambling as shown in path A-3 may lead to the
observed product hydrogen isotope distributions.
It is interesting to note that the reaction of phenyl-
containing silanes with 1 may proceed through a step
involving some degree of nucleophilic attack on the
silicon center of the incoming silane molecule by the
π-electron density of the alkylidene ligand, leading to
the formation of a product 4 in which a new carbon-
silicon bond is formed. Similar chemistry was observed
in the reaction of zirconium amide complexes Zr(NMe2)4
and Zr(NMe2)3[SiR3]7k (R3 ) SiMe3) with phenyl-
containing silanes HxSiPh4-x (x ) 2, 3) to yield amidosi-
lanes and unstable zirconium hydride complexes (Scheme
5).7a These amide complexes possess some degree of
π-character (d-p π-bonds) in the N-Zr bonds through
interaction of the lone pair on the nitrogen atom of the
amide ligand with the unfilled d-orbitals on the zirco-
(22) Voskoboynikov, A. Z.; Parshina, I. N.; Shestakova, A. K.; Butin,
K. P.; Beletskaya, I. P.; Kuz’mina, L. G.; Howard, J . A. K. Organome-
tallics 1997, 16, 4041.
(23) (a) Woo, H.-G.; Walzer, J . F.; Tilley, T. D. J . Am. Chem. Soc.
1992, 114, 7047. (b) Xin, S. X.; Harrod, J . F. J . Organomet. Chem. 1995,
499, 181. (c) Procopio, L. J .; Carroll, P. J .; Berry, D. H. Polyhedron
1995, 14, 45. (d) Corey, J . Y.; Zhu, X. H.; Bedard, T. C.; Lange, L. D.
Organometallics 1991, 10, 924. (e) Hengge, E.; Gspaltl, P.; Pinter, E.
J . Organomet. Chem. 1996, 521, 145. (f) Banovetz, J . P.; Suzuki, H.;
Waymouth, R. M. Organometallics 1993, 12, 4700. (g) Spaltenstein,
E.; Palma, P.; Kreutzer, K. A.; Willoughby, C. A.; Davis, W. M.;
Buchwald, S. L. J . Am. Chem. Soc. 1994, 116, 10308. (h) Verdaguer,
X.; Lange, U. E. W.; Reding, M. T.; Buchwald, S. L. J . Am. Chem. Soc.
1996, 118, 6784. (i) Takahashi, T.; Hasegawa, M.; Suzuki, N.; Saburi,
M.; Rousset, C. J .; Fanwick, P. E.; Negishi, E. J . Am. Chem. Soc. 1991,
113, 8564.

1514 Organometallics, Vol. 20, No. 8, 2001
Diminnie et al.
Sch em e 5
philic attack of the π-electron density of the alkylidene
ligand in 1 on the incoming silane may explain the
observed preferential reaction of only the alkylidene
ligands, and not the alkyl ligands, of 1, 3, and 6 with
silane.
Ack n ow led gm en t is made to the National Science
Foundation (CHE-9904338, NSF Young Investigator
program CHE-9457368, and Research Experiences for
Undergraduates (REU) program), Camille Dreyfus
Teacher-Scholar program, and DuPont Young Professor
program for financial support of this research. The
authors also thank Dr. Albert A. Tuiman for help with
mass spectroscopy experiments, Karen T. Welch and Dr.
nium atom (Scheme 5). The d-p π-bonds may also be
involved in the nucleophilic attack on the silicon center
of an incoming silane molecule in the reactions of silanes
with the zirconium amide complexes, leading to the
formation of a silicon-nitrogen bond and a metal
hydride (Scheme 5).7a It should be also pointed out that
such reactivity is also in agreement with the formal
bond polarities of the reactants in both the alkylidene/
silane and amide/silane systems: the metal-carbon and
metal-nitrogen bonds are both polarized with a partial
negative charge on the carbon and nitrogen atoms,
respectively, while the Si-H bond of the silane is
polarized with a partial negative charge on the hydrogen
atom (the Pauling electronegativities of silicon and
hydrogen being 1.8 and 2.1, respectively). Such nucleo-
1
Hongjun Pan for assistance with H NMR at 600 MHz,
and referees for insightful suggestions.
Su p p or tin g In for m a tion Ava ila ble: 2H NMR spectrum
of the reaction mixture of 1 with D2SiMePh, mass spectra (MS)
of the hydrogen gas produced from the reaction of 1 with D2-
SiMePh, kinetic plots of the reactions of 1 with H2SiMePh and
D2SiMePh, a list of rate constants for the reactions of 1 with
H2SiMePh and D2SiMePh, plots of kobs vs [H2SiMePh]0 and
k
obs vs [H2SiMePh]av, and a complete list of the crystallographic
data for 5a , 5b, 7, and 8. This material is available free of
charge via the Internet at http://pubs.acs.org.
OM000476R