The course of (R2R′SiO)3TaCl2 (R = tBu, R′ = H, Me, Ph, tBu (silox); R = iPr, R′ = tBu, iPr) reduction is dependent on siloxide size

Article abstract of DOI:10.1021/ic035407u

Various sized siloxides (Cy₃SiO > ^tBu ₃SiO > ^tBu₂PhSiO > ^tBu ₂MeSiO ～ ⁱPr₂^tBusiO > ⁱPr₃SiO > ^tBu₂HSiO) were used to make (R₂R′SiO)₃TaCl₂ (R = ^tBu, R′ = H (1-H), Me (1-Me), Ph (1-Ph), ^tBu (1); R = ⁱPr, R′ = ^tBu (1-ⁱPr₂); R = R′ = ⁱPr (1-ⁱPr₃); R = R′ = ^cHex (Cy)). Product analyses of sodium amalgam reductions of several dichlorides suggest that [(R₂R′SiO)₃Ta] ₂(μ-Cl)₂ may be a common intermediate. When the siloxide is large (1-^tBu), formation of the Ta(III) species ( ^tBu₃SiO)₃Ta (6) occurs via disproportionation. When the siloxide is small, the Ta(IV) intermediate is stable (e.g., [( ⁱPr₃SiO)₃Ta]₂(μ-Cl)₂ (2)), and when intermediate sized siloxides are used, solvent bond activation via unstable Ta(III) tris-siloxides is proposed to occur. Under hydrogen, reductions of 1-Me and 1-Ph provide Ta(IV) and Ta(V) hydrides [( ^tBu₂MeSiO)₃Ta]₂(μ-H)₂ (4-Me) and (^tBu₂PhSiO)₃TaH₂ (7-Ph), respectively.

Full text of DOI:10.1021/ic035407u

Inorg. Chem. 2004, 43, 3421−3432
t
The Course of (R R′SiO)3TaCl2 (R ) Bu, R′ ) H, Me, Ph, tBu (silox);
2
R ) Pr, R′ ) Bu, iPr) Reduction Is Dependent on Siloxide Size
i
t
Andrew R. Chadeayne, Peter T. Wolczanski,* and Emil B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853
Received December 8, 2003
Various sized siloxides (Cy3SiO > tBu3SiO > tBu2PhSiO > tBu2MeSiO ∼ Pr2tBuSiO > iPr3SiO > tBu2HSiO) were
i
i
used to make (R R′SiO)3TaCl2 (R ) tBu, R′ ) H (1-H), Me (1-Me), Ph (1-Ph), tBu (1); R ) iPr, R′ ) tBu (1-Pr2);
2
i
R ) R′ ) iPr (1-Pr3); R ) R′ ) cHex (Cy)). Product analyses of sodium amalgam reductions of several dichlorides
suggest that [(R R′SiO)3Ta]2(µ-Cl)2 may be a common intermediate. When the siloxide is large (1-Bu), formation
of the Ta(III) species (Bu SiO)3Ta (6) occurs via disproportionation. When the siloxide is small, the Ta(IV) intermediate
is stable (e.g., [(Pr3SiO)3Ta]2(µ-Cl)2 (2)), and when intermediate sized siloxides are used, solvent bond activation
t
2
t
3
i
via unstable Ta(III) tris-siloxides is proposed to occur. Under hydrogen, reductions of 1-Me and 1-Ph provide
t
t
Ta(IV) and Ta(V) hydrides [(Bu2MeSiO)3Ta]2(µ-H)2 (4-Me) and (Bu2PhSiO)3TaH (7-Ph), respectively.
2
Introduction
study of this unusual complex, including recent calculations
testifying to the relatively low energy of its singlet ground
state,7 an understanding of its synthesis and kinetic stability
is incomplete.
In a project originally designed to generate new metal-
metal bonded derivatives of early transition metals, different
siloxides were used to tune the tris-siloxide metal steric
parameter, while keeping a similar electronic environment.
By analysis of the reduction products of (R2R′SiO)3TaCl2
t
The Bu3SiO ligand (silox)1-3 has been used with great
success in the preparation of low coordinate, low valent, early
transition metal compounds.4-8 In particular, the generation
of (silox)3Ta8 has led to investigations of CO cleavage8-10
and a variety of other bond activations.4-15 Despite extensive
(1) Weidenbruch, M.; Pierrard, C.; Pesel, H. Z. Naturforsch. 1978, 33B,
1468-1471.
(2) (a) Dexheimer, E. M.; Spialter, L.; Smithson, L. D. J. Organomet.
Chem. 1975, 102, 21-27. (b) Weidenbruch, M.; Pesel, H.; Peter, W.;
Steichen, R. J. Organomet. Chem. 1977, 141, 9-21.
t
t
(R ) Bu, R′ ) H, 1-H; Me, 1-Me; Ph, 1-Ph; Bu, 1; R )
iPr, R′ ) Bu, 1-iPr2; R ) R′ ) Pr, 1-iPr3; R ) R′ ) Hex
(Cy)), a greater insight into the synthesis of (silox)3Ta (6)
was obtained. As a consequence, a uniform rationale for the
distribution of reduction products in all of the cases is
postulated, and the special nature of 6 was confirmed. Herein
are reported these findings along with the syntheses and
structures of two new Ta(IV) dimers, [(iPr3SiO)3Ta]2(µ-Cl)2
(2) and [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me).
t
i
c
(3) Marciniec, B.; Maciejewski, H. Coord. Chem. ReV. 2001, 223, 301-
335.
(4) (a) Wolczanski, P. T. Polyhedron 1995, 14, 3335-3362. (b) Miller,
R. L.; Lawler, K. A.; Bennett, J. L.; Wolczanski, P. T. Inorg. Chem.
1996, 35, 3242-3253. (c) Kleckley, T. S.; Bennett, J. L.; Wolczanski,
P. T.; Lobkovsky, E. B. J. Am. Chem. Soc. 1997, 119, 247-248. (d)
Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119, 10696-
10719. (e) Douthwaite, R. E.; Wolczanski, P. T.; Merschrod, E. J.
Chem. Soc., Chem. Commun. 1998, 2591-2592. (f) Veige, A. S.;
Wolczanski, P. T.; Lobkovsky, E. B. Angew. Chem., Int. Ed. 2001,
40, 3629-3632. (g) Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E.
B. J. Chem. Soc., Chem. Commun. 2001, 2734-2735.
(5) Covert, K. J.; Mayol, A.-R.; Wolczanski, P. T. Inorg. Chim. Acta 1997,
263, 263-278.
(6) Veige, A. S.; Kleckley, T. S.; Chamberlin, R. L. M.; Neithamer, D.
R.; Lee, C. E.; Wolczanski, P. T.; Lobkovsky, E. B.; Glassey, W. V.
J. Organomet. Chem. 1999, 591, 194-203.
(7) (a) Veige, A. S.; Slaughter, L. M.; Wolczanski, P. T.; Matsunaga, N.;
Decker, S. A.; Cundari, T. R. J. Am. Chem. Soc. 2001, 123, 6419-
6420. (b) Veige, A. S.; Slaughter, L. M.; Lobkovsky, E. B.;
Wolczanski, P. T.; Matsunaga, N.; Decker, S. A.; Cundari, T. R. Inorg.
Chem. 2003, 42, 6204-6224.
(8) (a) LaPointe, R. E.; Wolczanski, P. T.; Mitchell, J. F. J. Am. Chem.
Soc. 1986, 108, 6382-6384. (b) Neithamer, D. R.; LaPointe, R. E.;
Wheeler, R. A.; Richeson, D. S.; Van Duyne, G. D.; Wolczanski, P.
T. J. Am. Chem. Soc. 1989, 111, 9056-9072.
(9) Miller, R. L.; Toreki, R.; LaPointe, R. E.; Wolczanski, P. T.; Van
Duyne, G. D.; Roe, D. C. J. Am. Chem. Soc. 1993, 115, 5570-5588.
(10) Miller, R. L.; Wolczanski, P. T.; Rheingold, A. L. J. Am. Chem. Soc.
1993, 115, 10422-10423.
(11) Covert, K. J.; Neithamer, D. R.; Zonnevylle, M. C.; LaPointe, R. E.;
Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494-2508.
(12) (a) Bonanno, J. B.; Wolczanski, P. T.; Lobkovsky, E. B. J. Am. Chem.
Soc. 1994, 116, 11159-11160. (b) Strazisar, S. A.; Wolczanski, P. T.
J. Am. Chem. Soc. 2001, 123, 4728-4740.
(13) Bonanno, J. B.; Henry, T. P.; Neithamer, D. R.; Wolczanski, P. T.;
Lobkovsky, E. B. J. Am. Chem. Soc. 1996, 118, 5132-5133.
(14) Bonanno, J. B.; Veige, A. S.; Wolczanski, P. T.; Lobkovsky, E. B.
Inorg. Chim. Acta 2003, 345, 173-184.
(15) Chamberlin, R. L. M.; Rosenfeld, D. C.; Wolczanski, P. T.; Lobkovsky,
E. B. Organometallics 2002, 21, 2724-2735.
10.1021/ic035407u CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/28/2004
Inorganic Chemistry, Vol. 43, No. 11, 2004 3421

Chadeayne et al.
Na0, toluene
Results
R2R′SiOH 111 °C, 4-24 h8
(5)
R2R′SiONa
R ) tBu, R′ ) H, Me, Ph;
Syntheses of R2R′SiH, R2R′SiOH, and R2R′SiONa (R
) tBu, R′ ) H, Me, Ph, tBu; R ) R′ ) iPr). The syntheses
R ) iPr, R′ ) tBu;
R ) R′ ) iPr, Cy
of the various silanols started from readily available precur-
t
sors: Bu2SiHCl, tBu2SiH2, iPr2SiHCl, iPr3SiH, and Cy3SiCl.
In a modification of a Doyle and West procedure,16 ther-
The sodium siloxides ranged from an amorphous, colorless
solid (iPr3SiONa, 87%)23 to white, crystalline materials
(iPr2 BuSiONa (59%), Cy3SiONa (>90%), and tBu2RSiONa
t
molysis of Bu2SiHCl and methyllithium in Et2O at 67 °C
in a bomb reactor afforded the colorless oil Bu2SiHMe in
t
t
i
94% yield (eq 1). Treatment of Pr2SiHCl with 1 equiv of
(R ) H (47%), Me (79%), Ph (69%)). Spectral data for the
silanes, silanols, and sodium siloxides are compiled in Table
1. Recent crystal structures of tBu3SiONa and tBu2PhSiONas
prepared via N2O treatment of the respective sodium
silicidessreveal these species to be tetramers, with Na and
O atoms occupying alternate corners of a cube.24 A related
tBuLi in a 3:4 mixture of pentane/heptane at 23 °C generated
t
iPr2 BuSiH in 43% yield (eq 2) upon distillation. A lower
boiling fraction accounted for 30% of the crude mass, and
spectroscopic analysis of this material failed to discern an
Si-H functionality. A pathway involving silylene formation
via deprotonation may be operable, and the generation of
silylenes via electron transfer is also well documented.17
related thermolysis of tBu2SiH2 and phenyllithium in heptane
structure of Bu2MeSiONa has also been determined.25
t
A
t
Syntheses of (R2R′SiO)3TaCl2 (R ) Bu, R′ ) H, Me,
t
i
t
i
Ph, Bu; R ) Pr2, R′ ) Bu; R ) R′ ) Pr). The tris-
siloxide tantalum dichlorides were synthesized via treatment
of TaCl5 with the sodium siloxides in benzene or toluene,
typically under thermolysis conditions (eq 6).
at 98 °C led to the elimination of LiH and the ultimate
isolation of Bu2SiHPh as a colorless oil (63%, eq 3).18
t
Et2O
tBu2SiHCl + MeLi
iPr2SiHCl + tBuLi
tBu2SiH2 + PhLi
8 tBu2MeSiH + LiCl (1)
67 °C, 20 h
TaCl5 + 3R2R′SiONa f (R2R′SiO)3TaCl2 + 3NaCl (6)
R ) tBu, R′ ) H (1-H), Me (1-Me),
Ph (1-Ph);
pentane/heptane
t
8 iPr2 BuSiH + LiCl (2)
23 °C, 16 h
heptane
R ) iPr, R′ ) tBu (1-iPr2)
8 tBu2PhSiH + LiH
(3)
R ) R′ ) iPr (1-iPr3), Cy (1-Cy3)
98 °C, 14 h
Exposure of the silanes to KOH/EtOH provided the
Although formation of (tBu2HSiO)3TaCl2 (1-H, 55%)
occurred in benzene at 23 °C after only 4 h of stirring,
(tBu2MeSiO)3TaCl2 (1-Me, 63%) required 16 h at 55 °C, and
t
i
silanols Bu2RSiOH (R ) Me (84%),19 Ph (87%)) and Pr2-
i
t
RSiOH (R ) Pr (98%), Bu (75%)) in excellent yields (eq
4).
(tBu2PhSiO)3TaCl2 (1-Ph, 69%), (iPr2 BuSiO)3TaCl2 (1-iPr2,
t
KOH, EtOH
R2R′SiH 78 °C, 20-36 h8
51%), (iPr3SiO)3TaCl2 (1-iPr3, 42%), and (Cy3SiO)3TaCl2 (1-
Cy3) were prepared by refluxing the reagents in toluene
(>110 °C) for 20, 16, 12, and 20 h, respectively. (silox)3TaCl2
(1) was also prepared in refluxing toluene during a 15 h
period.8 All of the dichlorides are white, crystalline com-
plexes, although 1-H is quite waxy.
Dichloride Reductions. 1. (iPr3SiO)3TaCl2 (1-iPr3).
Sodium amalgam reduction of (iPr3SiO)3TaCl2 (1-iPr3) in
THF or DME resulted in a blue-purple solution whose color
faded over the course of 1 h. Amidst several products,
(iPr3SiO)3TaO was tentatively identified on the basis of
spectral characteristics and comparison to related reactions,
but isolation was not attempted. Instead, Na/Hg reduction
of 1-iPr3 in Et2O afforded the Ta(IV) dimer [(iPr3SiO)3Ta]2-
(µ-Cl)2 (2, eq 7), and excess Na0 failed to further reduce the
complex.
(4)
R2R′SiOH
R ) tBu, R′ ) Me, Ph;
R ) iPr, R′ ) tBu;
R ) R′ ) iPr
t
tBu2PhSiOH, iPr2 BuSiOH, and iPr3SiOH are colorless oils,
but sublimed tBu2MeSiOH is a waxy, crystalline, white solid.
tBu2HSiOH was prepared according to a literature proce-
t
dure20 utilizing KOH/EtOH treatment of the chloride Bu2-
SiHCl. A related literature method was used to synthesize
t
Cy3SiOH from Cy3SiCl.21 Preparations1,2 of Bu3SiOH are
related to eqs 1-4, since 2 equiv of tBuLi reacts with tBu2-
t
t
SiF2, or 1 equiv of BuLi reacts with Bu2SiHF, to generate
tBu3SiH, and it is subsequently hydrolyzed with base.
A convenient preparation of Bu3SiONa involves ther-
molysis of the silanol with sodium metal,22 and this method
was extended to the new species (eq 5).
t
(iPr SiO) TaCl
Na/Hg, -2NaCl8
(7)
i
(16) Doyle, M. P.; West, C. T. J. Am. Chem. Soc. 1975, 97, 3777-3782.
(17) Driver, T. G.; Franz, A. I.; Woerpel, K. A. J. Am. Chem. Soc. 2002,
124, 6524-6525.
2
3
3
2
[( Pr3SiO)3Ta]2(µ-Cl)2
Et2O, 36-48 h
1-iPr3
2
(18) Lerner, H. W.; Scholz, S.; Bolte, M. Z. Anorg. Allg. Chem. 2001,
627, 1638-1642.
(19) Doyle, M. P.; West, C. T. J. Org. Chem. 1975, 40, 3835-3838.
(20) Lickiss, P. D.; Lucas, R. J. Organomet. Chem. 1995, 521, 229-234.
(21) Nebergall, W. H.; Johnson, O. H. J. Am. Chem. Soc. 1949, 71, 4022-
4024.
(23) Banditelli, G.; Pizzotti, M.; Bandini, A. L.; Zucchi, C. Inorg. Chim.
Acta 2002, 330, 72-81.
(24) Lerner, H.-W.; Scholz, S.; Bolte, M. Organometallics 2002, 21, 3827-
3830.
(22) Covert, K. J.; Wolczanski, P. T.; Hill, S. A.; Krusic, P. J. Inorg. Chem.
1992, 31, 66-78.
(25) Schutte, S.; Freire-Erdbruegger, C.; Kingebiel, U.; Sheldrick, G. M.
Phosphorus, Sulfur Silicon Relat. Elem. 1993, 78, 75-81.
3422 Inorganic Chemistry, Vol. 43, No. 11, 2004

The Course of (R2R′SiO)3TaCl2 Reduction
Table 1. 1H and 13C{1H} NMR Spectral Assignmentsa for R2R′SiX (X ) H, OH, or ONa), (R2R′SiO)3TaCl2, and Its Reduction Products
1H NMR (δ (J, Hz), assignt)
R/R′/other
13C{1H} NMR (δ, assignt)
compound
tBu
SiCMe3, SiC(CH3)3
R/R′/other
tBu2MeSiHb
tBu2PhSiHc
0.99
1.02
1.02
-0.01 (d, 4), Me
3.63 (q, 4), SiH
1.11 (d, 7.5), Me
1.13 (d, 7.5), Me
3.54, SiH
t
iPr2 BuSiHd
tBu2MeSiOHe
tBu2PhSiOH
0.97
1.02
-0.03, Me
7.21 (dd, 2, 5), p-Ph
7.44 (dd, 2, 8.5), m-Ph
7.64 (m, 2), o-Ph
1.07 (d, 5.6)
21.14, 30.09
128.31, Ph
133.94, Ph
144.28, Ph
t
iPr2 BuSiOHd
0.99
iPr3SiOHd
1.03 (d, 7.5), Me
3.59 (q, 2), OH
4.37 (SiH)
12.15, CH
18.28, Me
tBu2HSiONa
tBu2MeSiONa
tBu2PhSiONa
1.03
1.02
1.11
-0.06, Me
7.11 (m, 4.8), m-Ph
7.71 (dd, 1.6), o,p-Ph
1.14 (d, 7), Me
1.15 (d, 7), Me
1.08 (d, 7), Me
0.91 (sep, 7), CH
iPr2 BuSiONa
1.06
t
iPr3SiONa
(tBu2HSiO)3TaCl2 (1-H)
(tBu2MeSiO)3TaCl2 (1-Me)
(tBu2PhSiO)3TaCl2 (1-Ph)
1.15
1.31
0.31, Me
7.19 (m, 2, 8), m-Ph
8.03 (m, 7), o/p-Ph
21.54, 27.86
22.60, 28.70
-7.07, Me
127.87, Ph
129.85, Ph
133.80, Ph
135.30, Ph
14.20, CH
18.89, Me
(iPr2 BuSiO)3TaCl2 (1-iPr2)
1.22
1.26 (d, 8), Me
1.29 (d, 8), Me
1.40 (m, 8), CH
1.22 (d, 5.6)
22.00, 28.21
t
(iPr3SiO)3TaCl2 (1-iPr3)
(Cy3SiO)3TaCl2 (1-Cy3)
13.29, CH
7.95, Me
25.75
1.25 (dt, 2, 13)
1.36 (q, 8.5)
27.49
1.61 (q, 12.5)
1.76 (br s)
28.07
28.85
1.86 (d, 10.5)
2.07 (d, 13.5)
1.26 (br m, 7.2), CH
1.36 (br d, 6.8), Me
[(iPr3SiO)3Ta]2(µ-Cl)2 (2) (23 °C)
15.15, CH
15.23, CH
18.82, Me
19.19, Me
(70°C)
1.29 (br s), Me
1.42 (br m), CH
0.35
[(tBu2MeSiO)3Ta]2(µ-O)2 (3-Me)
1.22
1.26
[(iPr2 BuSiO)3Ta]2(µ-O)2 (3-iPr2)
1.31 (d, 7.2), Me
1.32 (d, 6.8), Me
1.38 (sep, 7.6), CH
0.47, Me
t
[(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me)
1.21
1.24
8.54, TaH
[(iPr2 BuSiO)3Ta]2(µ-H)2 (4-iPr2)
1.32 (d, 7.2), Me
1.33 (d, 7.2), Me
1.43 (sep, 7.6), CH
7.73, TaH
t
[(tBu2MeSiO)3TaH]2(µ-O) (5-Me)
1.22
1.26
1.20
0.36, Me
10.97, TaH
t
[(iPr2 BuSiO)3TaH]2(µ-O) (5-iPr2)d
1.14 (d, 11.2), Me
1.22 (d, 9.6), Me
7.11 (m, 5.7), o/p-Ph
7.90 (dd, 1.8,5.7), m-Ph
22.72, TaH
(tBu2PhSiO)3TaH2 (7-Ph)
(tBu2PhSiO)3Ta(η-C2H4) (8-Ph)
(tBu2MeSiO)3TaCH2(CH2)2CH2 (9-Me)
(iPr3SiO)3TaCH2(CH2)2CH2 (9-iPr3)d
1.15
1.11
2.61, C2H4
7.20 (m, 5.5), o/p-Ph
7.84 (m, 7), m-Ph
0.22, Me
2.13 (q, 3), (CH2)2
2.83 (q, 3), (CH2)2
1.17, (d, 3), Me
2.12, (m, 1.5), (CH2)2
2.84, (m, 2.5), (CH2)2
0.21, SiMe
(tBu2MeSiO)3Ta(η-C2Me2) (10-Me)
(tBu2PhSiO)3Ta(η-C2Me2) (10-Ph)
1.09
1.17
2.58, C2Me2
2.59, C2Me2
7.21 (m, 1.2, 4.8), o/p-Ph
7.87 (m, 2), m-Ph
1.17 (d, 3), Me
2.60, C2Me2
(iPr3SiO)3Ta(η-C2Me2) (10-iPr3)d
(tBu2PhSiO)3TaMe2 (11-Ph)
1.21
1.48, TaMe2
7.19 (m, 3), o/p-Ph
7.92 (m, 1.5), m-Ph
a Benzene-d6 unless otherwise noted. b Reference 16. c Reference 18. d Methine is obscured. e Reference 19.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3423

Chadeayne et al.
Table 2. Crystallographic Data for [(iPr3SiO)3Ta]2(µ-Cl)2 (2) and
[(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me)
2
4-Me
formula
fw
C54H126Cl2 O6Si6Ta2
1472.89
C54H126O6 Si6Ta2
1401.99
space group
Z
C2/c
4
P1h
2
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
13.9922(8)
25.2621(14)
21.2191(11)
90
101.9680(10)
90
7337.3(7)
1.333
3.190
12.9177(16)
13.4760(17)
23.148(3)
82.096(3)
74.402(3
64.714(3)
3508.2(8)
1.327
F
calc, g‚cm-3
µ, mm-1
3.258
173(2)
temp, K
173(2)
λ, Å
0.71073
0.71073
R indices [I > 2σ(I)]a,b
R1 ) 0.0402
wR2 ) 0.0740
R1 ) 0.0680
wR2 ) 0.0810
1.020
R1 ) 0.0577
wR2 ) 0.1570
R1 ) 0.0829
wR2 ) 0.1689
1.146
R indices (all data)a,b
GOFc
2
a R1 ) S||Fo| - |Fc||/S|Fo|. b wR2 ) [∑w(|Fo| - |Fc|)2/∑wFo ]1/2
.
c GOF
Figure 1. Molecular view of [(iPr3SiO)3Ta]2(µ-Cl)2 (2).
(all data) ) [∑w(|Fo| - |Fc|)2/(n - p)]1/2, n ) number of independent
reflections, p ) number of parameters.
severe angle ( Ta-Cl1-Ta ) 70.47(2)°) to accommodate
the metal-metal interaction. The overall geometry is best
described as two edge-shared square pyramids, whose apical
siloxides are elongated (d(Ta-O2) ) 1.989(2) Å) relative
to the basal ligands (d(Ta-O1) ) 1.864(2) Å, d(Ta-O3) )
1.859(2) Å). The basal siloxides are splayed ( O1-Ta-
O3 ) 98.81(11)°, O1-Ta-Cl1 ) 89.46(8)°, O3-Ta-
Cl1A ) 88.17(8)°) relative to the µ-chlorides ( Cl1-Ta-
Cl1A ) 80.62(3)°, Cl-Ta-O3 ) 164.80(9)°, Cl1A-
Ta-O1 ) 163.18(9)°), but the sum of the basal angles is
357.1°; hence, there is only minor deviation from the plane,
and it is steric in origin. The apical-basal angles ( O1-
Ta-O2 ) 107.63(11)°, O3-Ta-O2 ) 105.79(11)°, Cl-
Ta-O2 ) 83.54(8)°, Cl1A-Ta-O2 ) 84.87(8)°) are a
testament to the regular nature of the square pyramids and
The dimer was obtained as purple crystals from pentane
in 40% yield. At 23 °C, the H NMR spectrum C6D6 of 2
1
consists of three broad asymmetric resonances in the methyl
region that overlap with a very broad feature whose integra-
tion is roughly consistent with the methine protons. At 70
°C, the spectrum simplifies immensely and the signals
coalesce to a broad singlet at δ 1.29 accompanied by a broad
multiplet at δ 1.42 corresponding to a single isopropyl group;
the original spectrum was reconstituted upon cooling. Further
heating (>70 °C) induced decomposition and a change in
color from purple to brown.
2. Structure of [(iPr3SiO)3Ta]2(µ-Cl)2 (2). A single crystal
X-ray diffraction study of [(iPr3SiO)3Ta]2(µ-Cl)2 (2) con-
firmed its dimeric nature and permitted rationalization of the
i
the interplay of the sterically encumbered Pr3SiO ligands.
1
aforementioned H NMR spectrum. The crystallographic
The C2 symmetry of the dimer is a consequence of a subtle
twist of the siloxide groups to minimize steric interactions
between the ligands on adjacent metals.
information is given in Table 2, pertinent geometric data are
presented in Table 3, and a molecular view is presented in
Figure 1. The tantalum atoms are bridged by two chlorides,
and the compound possesses C2 symmetry, which distin-
t
3. Reductions of (R2R′SiO)3TaCl2 (R ) Bu, R′ ) H
(1-H), Me, (1-Me); R ) iPr, R′ ) tBu (1-iPr2)). Reduction
of (tBu2HSiO)3TaCl2 (1-H) led to intractable material, and
evidence of C-O bond activation was obtained when (tBu2-
MeSiO)3TaCl2 (1-Me) was reduced with Na/Hg in ethereal
solvents. In THF, Na/Hg reductions led to numerous products
i
guishes each of the Pr3SiO ligands on one metal. A
somewhat long Ta-Ta single bond26-33 of 2.9773 Å is
present, presumably because the µ-Cl ligands must adopt a
(26) (a) Sattelberger, A. P.; Wilson, R. B., Jr.; Huffman, J. C. J. Am. Chem.
Soc. 1980, 102, 7113-7114. (b) Scioly, A. J.; Luetkens, M. L., Jr.;
Wilson, R. B., Jr.; Huffman, J. C.; Sattelberger, A. P. Polyhedron
1987, 741-757.
1
according to H NMR spectral analysis, including [(tBu2-
MeSiO)3Ta]2(µ-O)2 (3-Me) and species containing reso-
nances consistent with -O(CH2)CH2- fragments.5,9 In Et2O,
after ∼24 h, 1-Me was converted (Na/Hg or K/C8) to a
roughly 1:1 mixture of 3-Me and [(tBu2MeSiO)3Ta]2(µ-H)2
(4-Me), as indicated in Scheme 1. The compounds cocrys-
tallize as a dark red-brown material. A single crystal X-ray
(27) (a) Belmonte, P. A.; Schrock, R. R.; Day, C. S. J. Am. Chem. Soc.
1982, 104, 3082-3089. (b) Churchill, M. R.; Wasserman, H. J.;
Belmonte, P. A.; Schrock, R. R. Organometallics 1982, 1, 559-561.
(c) Belmonte, P. A.; Cloke, F. G. N.; Schrock, R. R. J. Am. Chem.
Soc. 1983, 105, 2643-2650. (d) Churchill, M. R.; Wasserman, H. J.
Inorg. Chem. 1982, 21, 226-230.
(28) Profilet, R. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1992, 11,
1559-1561.
(29) (a) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. Organometallics 2000,
19, 3931-3941. (b) Fryzuk, M. D.; McConville, D. H. Inorg. Chem.
1989, 28, 1613-1614.
(32) (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Wang, X. P. Inorg.
Chem. 1997, 36, 896-901. (b) Babaiankibala, E.; Cotton, F. A. Inorg.
Chim. Acta 1991, 182, 77-82. (c) Babaiankibala, E.; Cotton, F. A.;
Kibala, P. A. Inorg. Chem. 1990, 29, 4002-4005.
(33) Bott, S. G.; Sullivan, A. C. J. Chem. Soc., Chem. Commun. 1988,
1577-1578.
(30) Burckhardt, U.; Casty, G. L.; Tilley, T. D.; Woo, T. K.; Rothlisberger,
U. Organometallics 2000, 19, 3830-3841.
(31) Lee, T. Y.; Messerle, L. J. Organomet. Chem. 1998, 553, 397-403.
3424 Inorganic Chemistry, Vol. 43, No. 11, 2004

The Course of (R2R′SiO)3TaCl2 Reduction
Table 3. Bond Distances (Å) and Angles (deg) in [(iPr3SiO)3Ta]2(µ-Cl)2 (2)
Ta-Ta
Ta-O3
Si1-O1
Si-Cav
2.9773(3)
1.859(2)
1.670(3)
1.877(7)
Ta-O1
Ta-Cl1
Si2-O2
C-Cav
1.864(2)
2.5907(8)
1.657(3)
1.536(11)
Ta-O2
Ta-Cl1A
Si3-O3
1.989(2)
2.5702(9)
1.675 (3)
O1-Ta-O2
O1-Ta-Cl1
O2-Ta-Cl1A
Cl1-Ta-Cl1A
O2-Ta-Ta
Cl1A-Ta-Ta
Ta-O3-Si3
Si-C-Cav
107.63(11)
89.46(8)
84.87(8)
80.62(3)
123.21(8)
55.09(2)
O1-Ta-O3
O1-Ta-Cl1A
O3-Ta-Cl1
Ta-Cl1-Ta
O3-Ta-Ta
Ta-O1-Si1
O-Si-Cav
98.81(11)
O2-Ta-O3
O2-Ta-Cl1
O3-Ta-Cl1A
O1-Ta-Ta
Cl1-Ta-Ta
Ta-O2-Si2
C-Si-Cav
105.79(11)
83.54(8)
88.17(8)
108.09(9)
54.45(2)
171.35(18)
112.8(14)
163.18(9)
164.80(9)
70.47(2)
110.57(8)
169.74(16)
105.9(15)
110.4(8)
164.14(18)
114.0(19)
C-C-Cav
Scheme 1
structure determination of a representative crystal generated
geometric parameters and electron densities inconsistent with
a single compound. Spectroscopic investigations and frac-
tional crystallizations led to partial and then eventual
separation of the yellow dioxo (3-Me) and black dihydride
(4-Me) dimers. Assignment of 3-Me as a dimer is based on
the cocrystallization with 4-Me, its yellow color in com-
parison to monomeric, colorless (silox)3TaO,8 and solubility
properties.
signals. These gradually dissipated over 2 weeks, and
resonances attributable to [(tBu2MeSiO)3Ta]2(µ-O)2 (3-Me)
grew in concomitantly with the evolution of H2, which was
observed at δ 4.46. Thermolysis at 100 °C for 4 h completed
the conversion. Apparently, nitrous oxide oxidized Ta(IV)
to Ta(V), formulated as [(tBu2MeSiO)3TaH]2(µ-O) (5-Me),
and then formally oxidized hydride to dihydrogen as 3-Me
was generated.
t
The reduction chemistry of (iPr2 BuSiO)3TaCl2 (1-iPr2)
Dihydride 4-Me was more conveniently prepared via Na/
Hg reduction in Et2O under 1 atm of dihydrogen and isolated
from pentane as black crystals in 52% yield. A diagnostic
resonance at δ 8.54 that is absent in 4-Me-d2 is attributed to
the bridging hydrides, but IR spectra are less informative.
A shoulder present in 4-Me at ∼1350 cm-1 is lost upon
deuteration, but the expected position (∼960 cm-1) of the
corresponding Ta-D absorption in 4-Me-d2 is obscured by
ligand vibrations. Since the hydrides were not observed
crystallographically (vide infra), additional evidence for the
Ta(IV) formulation was obtained via treatment with excess
HCl. Toepler pump measurements of the gas evolved
indicated 2.8 equiv per dimer, consistent with a Ta(IV)
dihydride; two hydrides are protonated to give 2 equiv of
H2, and 2 equiv of HCl oxidizes two Ta(IV) centers to Ta-
(V), releasing 1 equiv of H2.
paralleled that of (tBu2MeSiO)3TaCl2 (1-Me), but it was not
pursued in detail because of the difficult isolations of the
silane and silanol. However, a small scale reduction of
t
1-iPr2 under dihydrogen in Et2O afforded [(iPr2 BuSiO)3Ta]2-
(µ-H)2 (4-iPr2) on the basis of its black appearance and a
hydride resonance at δ 7.73 that integrates properly versus
the siloxide ligands. Addition of 2 equiv of N2O to 4-iPr2
generated a new hydride resonance at δ 9.77, and thermolysis
at 160 °C (24 h) produced H2 and, presumably, dioxo
t
[(iPr2 BuSiO)3Ta]2(µ-O)2 (3-iPr2). The hydride resonance of
the intermediate suggests that it belongs to [(iPr2tBuSiO)3TaH]2-
(µ-O)2 (5-iPr2), given the related chemistry of 5-Me.
4. Structure of [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me). A
single crystal X-ray structure determination of [(tBu2-
MeSiO)3Ta]2(µ-H)2 (4-Me) was conducted, and the crystal-
lographic data (Table 2) and geometric features (Table 4)
are independently tabulated. Figure 2 illustrates the pseudo-
D3d symmetry that describes the dimer, whose hydrides were
not located. The d(Ta-Ta) value is 2.8713(4) Å, which is
When [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me) was exposed to
excess N2O in C6D6, a new set of hydride resonances were
observed to grow in at δ 10.97 along with new siloxide
Inorganic Chemistry, Vol. 43, No. 11, 2004 3425

Chadeayne et al.
Table 4. Bond Distances (Å) and Angles (deg) in [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me)
Ta-Ta
2.8710(5)
1.934(7)
1.929(8)
1.653(7)
1.657(8)
Ta1-O1
Ta2-O4
O1-Si1
O4-Si4
Si-Cav
1.894(8)
1.910(8)
1.663(8)
1.649(8)
1.894(17)
Ta1-O2
Ta2-O5
O2-Si2
O5-Si5
C-Cav
1.898(8)
1.917(8)
1.669(8)
1.648(8)
1.540(10)
Ta1-O3
Ta2-O6
O3-Si3
O6-Si6
O1-Ta1-O2
O4-Ta2-O5
O1-Ta1-Ta2
O4-Ta2-Ta1
Ta1-O1-Si1
Ta2-O4-Si4
O-Si-Cav
102.9(4)
105.0(4)
113.0(3)
113.5(3)
175.6(6)
175.4(6)
108.2(14)
108.2(8)
O1-Ta1-O3
O4-Ta2-O6
O2-Ta1-Ta2
O5-Ta2-Ta1
Ta1-O2-Si2
Ta2-O5-Si5
C-Si-Cav
106.5(3)
104.5(4)
112.5(3)
112.2(3)
166.6(6)
168.3(6)
110.7(30)
O2-Ta1-O3
O5-Ta2-O6
O3-Ta1-Ta2
O6-Ta2-Ta1
Ta1-O3-Si3
Ta2-O6-Si6
Si-C-Cav
104.5(4)
103.8(4)
116.1(3)
116.7(2)
173.8(6)
171.3(5)
110.7(23)
C-C-Cav
2.720(4) Å).9,26-34 Two tantalum oxygen distances are
marginally longer (d(Ta1-O3) ) 1.926(4) Å, d(Ta2-O6)
) 1.924(4) Å) than the remaining four (d(Ta-O)av ) 1.901-
(7) Å). The O-Ta-O angles are statistically the same ( O-
Ta-Oav ) 104.4(10)°), but Ta1-Ta2-O6 (116.55(12)°)
and Ta2-Ta1-O3 (116.18(13)°) were marginally larger
than the remaining Ta-Ta-O angles (113.1(6)° (av)).
Perhaps the hydrides are more oriented toward O3 and O6,
but the deviations about the ditantalum unit are so minimal
that it is difficult to assign their positions with any
confidence.
5. Reductions of (tBu2R′SiO)3TaCl2 (R′ ) Ph, 1-Ph; tBu,
1). The Na/Hg reduction of (silox)3TaCl2 (1) has previously
been reported to afford (silox)3Ta (6).8 Reduction in the
presence of H2 or simple exposure of 6 to dihydrogen
provides (silox)3TaH2 (7, δ(TaH) 21.99).9 By comparison,
reduction of (tBu2PhSiO)3TaCl2 (1-Ph) afforded a multitude
of products that constituted a tan oil; identification proved
unfeasible, but cyclometalation to various Ta(V) isomers is
likely. In the presence of dihydrogen, reduction provided the
dihydride (tBu2PhSiO)3TaH2 (7-Ph) in 80% yield as a
colorless oil (eq 8).
Na/Hg, -2NaCl, H2
(1 atm)8
(tBu2PhSiO)3TaCl2
(tBu2PhSiO)3TaH2
Et2O
1-Ph
7-Ph
(8)
1
The H NMR spectrum of 7-Ph, whose TaH2 resonance
is observed at δ 22.72, is diagnostic for a terminal dihy-
dride,9,27 and its IR spectrum contains a peak at a frequency
of ν(TaH(D)) ) 1751(1262) cm-1 and a corresponding wag
(δ(TaH2(D2)) at 783(533) cm-1.9 Cundari has calculated that
the lowest energy structure of (HO)3TaH2 (7′) is a trigonal
bipyramid with diequatorial hydrides; an isomer with axial
and equatorial hydrides is a few kilocalories per mole higher,
and the diaxial dihydride is not a minimum energy config-
uration.35,36 The infrared spectrum is also consistent with the
predicted diequatorial hydride structure.
Further Probes of Siloxide Steric Environment. It
appears that the size of the siloxide significantly affects the
Figure 2. Molecular views of [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me): (a) from
the side (hydrides not located) and (b) down the Ta-Ta axis.
(34) Hoskin, A. J.; Stephan, D. W. Coord. Chem. ReV. 2002, 233-234,
107-129.
(35) Cundari, T. R. University of North Texas. Unpublished results.
(36) (a) Ward, T. R.; Burgi, H. B.; Gilardoni, P.; Weber, J. J. Am. Chem.
Soc. 1997, 119, 11974-11985. (b) Bayse, C. A.; Hall, M. B.
Organometallics 1998, 17, 4861-4868. (c) Firman, T. K.; Landis, C.
R. J. Am. Chem. Soc. 2001, 123, 11728-11742.
shorter than that of [(iPr3SiO)3Ta]2(µ-Cl)2 (2) but longer than
the sum of covalent radii and longer than that of the
unbridged Ta(IV) dimers (e.g., [(silox)2TaH2]2, d(Ta-Ta) )
3426 Inorganic Chemistry, Vol. 43, No. 11, 2004

The Course of (R2R′SiO)3TaCl2 Reduction
reduction chemistry of the various dichlorides. To help
provide further evidence of the size order in the siloxides,
small scale reactions were used to probe the formation of
olefin adducts versus metalacycles derived from ethylene and
2-butyne. 1H NMR spectroscopic assays of the crude reaction
mixture were used to ascertain the major product composi-
tion, but no purification or isolation was attempted. Examples
of metalacyclopentanes5,14,37 and ethylene and 2-butyne
adducts11 in these systems are documented. For the cases of
(tBu3SiO)3Ta (6) and the reduction of (tBu2PhSiO)3TaCl2 (1-
Ph), only the olefin adducts (tBu3SiO)3Ta(η-C2H4) (8-tBu)11
and (tBu2PhSiO)3Ta(η-C2H4) (8-Ph) were obtained (eq 9),
but for (tBu2MeSiO)3TaCl2 (1-Me) and (iPr3SiO)3TaCl2 (1-
iPr3), metalacycles (tBu2MeSiO)3Ta(-CH2(CH2)2CH2-) (9-
Me) and (iPr3SiO)3Ta(-CH2(CH2)2CH2-) (9-iPr3) were the
observed products (eq 10). Reductions in the presence of
2-butyne led to the monoadducts in all cases (eq 11), just as
exposure of 6 to 2-butyne afforded (silox)3Ta(η-C2Me2)
(10).11
fashion, treatment of 2 with 1 equiv of 2-butyne again gave
the dichloride 1-iPr3 concomitantly with the alkyne adduct
(iPr3SiO)3Ta(η-C2Me2) (10-iPr3, eq 13).
C6D6
[(iPr3SiO)3Ta]2(µ-Cl)2
+ 2C2H4
8
2
(iPr SiO) Ta(-CH (CH ) CH -) (iPr SiO) TaCl
+
(12)
3
3
2
2 2
2
3
3
2
9-iPr3
1-iPr3
C6D6
[(iPr3SiO)3Ta]2(µ-Cl)2
+ C2Me2
8
2
(iPr SiO) Ta(η-C Me ) (iPr SiO) TaCl
+
(13)
3
3
2
2
3
3
2
10-iPr3
1-iPr3
Addition of 2 equiv of MeMgBr to (tBu2PhSiO)3TaCl2 (1-
Ph) in Et2O provided the dimethyl derivative (tBu2-
PhSiO)3TaMe2 (11-Ph) as a colorless, crystalline solid (eq
14).
Na/Hg, -2NaCl, C2H4
(1 atm)8
(tBu2PhSiO)3TaCl2
Et2O
(tBu2PhSiO)3TaCl2
Et2O
+ MeMgBr -78 °C f 23 °C, 12 h8
1-Ph
1-Ph
(tBu2PhSiO)3Ta(η-C2H4)
(9)
(tBu2PhSiO)3TaMe2
+ MgBrCl (14)
8-Ph
11-Ph
(R2R′SiO)3TaCl2
R ) tBu, R′ ) Me (1-Me);
R ) R′ ) iPr (1-iPr3)
Na/Hg, -2NaCl, C2H4
(1 atm)8
No reaction was observed when derivatization of (silox)3-
TaCl2 (1) was attempted with a variety of methyl anion
equivalents. Dimethyl 11-Ph was thermolyzed for several
hours at 180 °C without noticeable decomposition.
(R2R′SiO)3Ta(-CH2(CH2)2CH2-) (10)
R ) tBu, R′ ) Me (9-Me);
R ) R′ ) iPr (9-iPr3)
Discussion
Background. Although the objective of creating metal-
metal bonded species without bridging ligands has yet to be
realized, this initial foray into attenuating the steric bulk of
(silox)3Ta (6) has provided valuable insights into the reduc-
tion chemistry of (R2R′SiO)3TaCl2. With the aid of calcula-
tions on the model (HO)3Ta (6′),7 the stability of 6, which
has pseudo-D3h symmetry, is now better understood. Its 1A1′
(R2R′SiO)3TaCl2
Na/Hg, -2NaCl, C2Me2
(1 atm)8
R ) tBu, R′ ) Ph (1-Ph);
R ) tBu, R′ ) Me (1-Me);
R ) R′ ) iPr (1-iPr3)
(R2R′SiO)3Ta(η-C2Me2) (11)
R ) tBu, R′ ) Ph (10-Ph);
R ) tBu, R′ ) Me (10-Me);
R ) R′ ) iPr (10-iPr3)
2
2
ground state reflects a (dz ) configuration that is ∼14 kcal/
3
mol below that of the E′′ state, which is the lowest lying
2
triplet state. The pair of electrons in the dz orbital serves as
a potent repulsive interaction to any approaching ligands;11
hence, the UV-vis spectrum of 6 is unchanged in nonpolar
media like hexane versus polar media containing moderately
strong donors for early metals such as THF. The pocket of
three (tBu3SiO) groups restricts the approach of any donor
to be along the z axis and greatly hampers attack at the empty
dxz and dyx (e′′) orbitals. It is the combination of sterics and
electronics that enables 6 to exist. Calculations reveal that
“(silox)3Nb” also possesses a singlet ground state, but its
lowest triplet state is only ∼2 kcal/mol away.7 The triplet
state is not repulsive to incoming ligands; hence, (silox)3Nb
has never been directly observed, and it binds L in cases
where 6 does not (e.g., PMe3). When generated in the absence
Additional NMR tube and small scale experiments were
also informative. Treatment of the Ta(IV) dichloride dimer
[(iPr3SiO)3Ta]2(µ-Cl)2 (2) with 1 equiv of ethylene swiftly
afforded the metalacycle (iPr3SiO)3Ta(-CH2(CH2)2CH2-)
(9-iPr3) and the Ta(V) dichloride (iPr3SiO)3TaCl2 (1-iPr3);
some starting material remained, and thus, the reaction can
be taken as a simple disproportionation (eq 12). In analogous
(37) (a) Lee, J.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2003, 22,
1546-1549. (b) Waratuke, S. A.; Thorn, M. G.; Fanwick, P. E.;
Rothwell, A. P.; Rothwell, I. P. J. Am. Chem. Soc. 1999, 121, 9111-
9119. (c) Thorn, M. G.; Hill, J. E.; Waratuke, S. A.; Johnson, E. S.;
Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1997, 119, 8630-
8641. (d) Johnson, E. S.; Balaich, G. J.; Rothwell, I. P. J. Am. Chem.
Soc. 1997, 119, 7685-7693. (e) Balaich, G. J.; Hill, J. E.; Waratuke,
S. A.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1995, 14, 656-
665. (f) Hill, J. E.; Balaich, G.; Fanwick, P. E.; Rothwell, I. P.
Organometallics 1993, 12, 2911-2924.
of L, cyclometalation to (silox)2HNbOSitBu2CMe2CH2 is
swift. The corresponding cyclometalation of 6 has a barrier
Inorganic Chemistry, Vol. 43, No. 11, 2004 3427

Chadeayne et al.
Scheme 2
of ∆Gq ) 24.5 (30) kcal/mol38 because the pair of electrons
in the dz orbital prevents binding of a C-H bond of a Bu
“arm” prior to activation.
etane to give (silox)3TaO(CH2)2CHdCH and (silox)3TaOCH2-
t
CMe2CH2, respectively.13
2
Under dihydrogen, reductions of (tBu2MeSiO)3TaCl2
t
(1-Me) and (iPr2 BuSiO)3TaCl2 (2-tBu) may be construed
Reduction Paradigm. Previously, the E°red at -1.90 V
(vs normal hydrogen electrode (NHE)) for (silox)3TaCl2 (1)
to [(silox)3TaCl2]- was determined to be reversible.6 Evi-
dence of further reduction at more negative potentials was
not observed, and it is plausible that, under Na/Hg conditions,
direct generation of (silox)3Ta (6) is not thermodynamically
feasible. Assuming that all tris-siloxide tantalum dichlorides
would behave similarly, the products derived from reduction
fit the pattern based on siloxide sterics illustrated in Scheme
2. Reduction of the Ta(V) dichlorides affords the bridging
dichloride Ta(IV) dimer, which is stable when the smallest
siloxide is utilized, that is, [(iPr3SiO)3Ta]2(µ-Cl)2 (2). For
all other cases, significant steric interactions of the adjacent
metal centers encourage disproportionation to the respective
Ta(V) dichlorides and tris-siloxide tantalum species. Note
that even 2 disproportionates when ethylene or 2-butyne is
present (eqs 11 and 12). Use of the most sterically encum-
bered siloxide, tBu3SiO, leads to the stable three coordinate
complex (silox)3Ta (6), but this species gradually cyclom-
as paralleling those of (silox)3TaCl2 (1-tBu) and (tBu2-
PhSiO)3TaCl2 (1-Ph) to afford the tris-siloxide dihydrides,
but these species are not stable. Either the Ta(V) dihydrides
undergo a dinuclear reductive elimination to produce [(tBu2-
t
MeSiO)3Ta]2(µ-H)2 (4-Me) and [(iPr2 BuSiO)3Ta]2(µ-H)2 (4-
iPr2) or the Ta(V) dihydrides scavenge the tris-siloxide Ta(III)
species faster than dihydrogen. In either instance, the
difference in products relative to (silox)3TaH2 (7) and (tBu2-
PhSiO)3TaH2 (7-Ph) stems from the smaller size of the
siloxides, which permits formation of the dinuclear Ta(IV)
complexes. It should be noted that no conditions have been
found to convert [(silox)2TaH2]2 to a mononuclear Ta(V)
hydride complex;9 hence, there is precedent for tantalum-
tantalum bond creation in preference to additional TaH bond
formation.
Reduction of (tBu2MeSiO)3TaCl2 (1-Me) in ethers provided
evidence of C-O bond scission, and in the case of diethyl
ether, the bis-µ-oxo [(tBu2MeSiO)3Ta]2(µ-O)2 (3-Me) and the
bis-µ-hydrido [(tBu2MeSiO)3Ta]2(µ-H)2 (4-Me) are the prod-
ucts. The fate of the ethyl groups has not been determined,
but no (tBu2MeSiO)3TaCH2(CH2)2CH2 (9-Me), which would
be the expected product from a tris-siloxide Ta(III) species
and ethylene, was detected. If disproportionation from the
bis-µ-chloride dimer affords (tBu2MeSiO)3Ta, it is proposed
that the smaller siloxide allows access to the dxz and dyz
orbitals by the nucleophilic oxygen of an ether molecule.
This juxtaposition of electrophilic (empty dxz and dyz) and
etalates to (silox)2HTaOSitBu2CMe2CH over a period of days
at room temperature. With a slightly less hindered siloxide,
tBu2PhSiO, the (tBu2PhSiO)3Ta species is not stable, and
while identification of the products was not possible,
cyclometalation to a number of isomers is a plausible result.
In the presence of H2, the Ta(III) intermediate is trapped to
afford (tBu2PhSiO)3TaH2 (7-Ph), just as the addition of H2
to 6 provides (silox)3TaH2 (7). In neither case is ether
cleavage noted, although, upon standing in THF, trace
amounts of (silox)3TaO are produced as 6 cyclometalates.
Whether the origin of the oxo-ligand is THF has not been
determined, but 6 has been shown to deoxygenate epoxides
and oxidatively add 2,3-dihydrofuran and 3,3-dimethylox-
2
nucleophilic (filled dz ) orbitals has previously been proposed
as the key element in C-O, N-H, C-N, and C-H bond
activations by (silox)3Ta (6),4-15 which can activate certain
ethers, as mentioned above.13 In the formation of 3-Me and
4-Me, C-O bond cleavage to yield (tBu2MeSiO)3EtTaOEt
(38) Veige, A. S. Ph.D. Thesis, Cornell University, 2002.
3428 Inorganic Chemistry, Vol. 43, No. 11, 2004

The Course of (R2R′SiO)3TaCl2 Reduction
phenyllithium (1.9 M, 48.3 mL, 91.8 mmol). The solution was
allowed to warm to 23 °C and brought to reflux. After 14 h,
precipitation of LiH from the red-purple solution was noted. It was
first cooled to 23 °C and subsequently cooled to 4 °C prior to
dropwise addition of 2-propanol, which led to vigorous bubbling
(H2) and the evolution of heat. After the solid had dissipated and
no further gas evolved, the amber solution was transferred to a
separatory funnel and washed (3 × 100 mL) with water. The
aqueous residues were then extracted once with 100 mL of hexanes.
The combined organic layers were dried with MgSO4, and the
solvent was removed via rotary evaporation to provide an amber
oil, from which was subsequently distilled (T ) 60-90 °C, 10-4
Torr) 16.7 g of a clear, colorless oil (85% yield).
is a logical start to numerous mechanisms; various binuclear
scenarios involving C-O bond activation are also reasonable.
While the reaction is clearly reproducible, the yields and the
corresponding amount of byproduct do not foment optimism
for further mechanistic evaluation.
Siloxide Steric Order. From the preceding discussion,
t
t
the rough order of siloxide size is Bu3SiO > Bu2PhSiO >
i
t
tBu2MeSiO ∼ Pr2 BuSiO > iPr3SiO. Not enough information
t
was obtained for Bu2HSiO to accurately order it, but
previous estimates of silicon steric factors indicate it as the
smallest of this grouping.39 Similarly, solubility difficulties
pertaining to the chemistry of Cy3SiO species precluded its
inclusion in this paper, and the same estimates show it as
the largest.39 The ability to alkylate (tBu2PhSiO)3TaCl2 (1-
Ph) under conditions that failed for (silox)3TaCl2 (1-tBu)
supports its ordering. Since 1-Ph is the only dichloride other
than 1-tBu to give an ethylene adduct when reduced in the
presence of C2H4, it certainly contains the next most bulky
siloxide investigated.
t
2. Bu2PhSiOH. Into a 100 mL three-neck flask charged with
an ethanolic solution of tBu2PhSiH (3.00 g, 13.1 mmol, 0.33 M in
ethanol) was transferred KOH (3.00 g, 53.6 mmol). The reaction
was brought to reflux for 36 h, cooled to 23 °C, and neutralized
with NH4Cl (satd). The resulting solution was then extracted (3 ×
40 mL) with hexanes, and the combined organic layers were washed
once with brine (40 mL), dried over MgSO4, and concentrated to
afford a yellow oil. From the crude yellow oil was distilled (60-
70 °C, 10-3 Torr) 2.8 g of a clear, colorless oil (87% yield).
Conclusions
t
t
3. Bu2PhSiONa. Into a 50 mL flask containing Bu2PhSiOH
(2.00 g, 8.18 mmol) and sodium metal (339 mg, 14.7 mmol) was
distilled 25 mL of toluene. The vessel was filled with an Ar
atmosphere and brought to reflux for 24 h. The toluene was removed
in vacuo to afford a yellow solid and residual sodium metal. The
resulting mixture was filtered through a glass frit and then extracted
five times with hexanes. The hexane filtrate was slowly concentrated
until precipitation began. This volume was then gently heated with
a warm water bath while stirring to resolubilize the material. The
solution was then allowed to stand at room temperature for 4 h to
initiate crystallization, subsequently cooled to -78 °C for 2 h, and
then filtered to collect 1.5 g of a white, crystalline solid (69% yield).
4. (tBu2PhSiO)3TaCl2 (1-Ph). To a 50 mL flask charged with
TaCl5 (1.50 g, 4.18 mmol) and tBu2PhSiONa (3.24 g, 12.5 mmol)
was distilled 20 mL of benzene. The resulting solution was brought
to reflux for 20 h. The amber slurry was cooled to 23 °C, and the
solvent was removed in vacuo to yield a tan solid. Pentane was
added, and the resulting suspension was filtered. The salt cake was
extracted (3 × 10 mL) with pentane, and the filtrate was then
concentrated to 3 mL. The concentrate was then cooled to -78 °C
for 4 h and filtered to yield 3.10 g of colorless crystals (78% yield).
Anal. Calcd for C42H69O3Si3Cl2Ta: C, 52.65; H, 7.26. Found: C,
52.25; H, 7.40.
5. (tBu2PhSiO)3TaH2 (7-Ph). To a 25 mL flask charged with
1-Ph (1.00 g, 1.04 mmol) and 2.2 equiv of Na/Hg (5.86 g, 2.29
mmol, 0.9% Na0) was distilled 15 mL of ether. While the ether
was kept frozen at 77 K, excess H2 was slowly admitted to the
reaction vessel until the pressure was 1 atm. The ether was allowed
to thaw, and the reaction was stirred for 16 h, resulting in a colorless
solution with a visible precipitate. The solvent was removed in
vacuo, and the resulting mixture was triturated (3 × 5 mL) with
pentane, suspended in 5 mL of pentane, and filtered through Celite.
The solvent was removed in vacuo to afford 800 mg of a clear,
colorless oil (87% yield).
The paradigm for the reduction of (siloxide)3TaCl2 satis-
factorily explains the experiments on this project to date and
aids in rationalizing 20 years of observations pertaining to
(silox)3Ta (6). Now that the sterics of the siloxides have been
assessed, it is hoped that the designed synthesis of unsup-
ported metal-metal bonded complexes will ultimately be
realized through the extension of this chemistry to other early
metals.
Experimental Section
General Considerations. All manipulations were performed
using either glovebox or high vacuum line techniques. Hydrocarbon
solvents, containing 1-2 mL of added tetraglyme, and ethereal
solvents were distilled under nitrogen from purple benzophenone
ketyl and vacuum transferred from the same. All glassware was
oven dried prior to use. NMR tubes and glassware that was used
in reductions were additionally flame dried under active vacuum
prior to use. Gaseous reagents (H2 (Matheson) and D2 (Cambridge
Isotope Laboratories) were slowly passed through a 77 K trap prior
to use; N2O and ethylene (Matheson) were passed through a -78
°C trap prior to use. TaCl5 (99.9%, Strem) was sublimed (115 °C,
10-4 Torr) prior to use. Unless otherwise specified, all reagents
were purchased from Aldrich. tBu2SiH2 was purchased from FMC
t
Lithium and used as received. (C6H11)3SiX (X ) Cl)21 and Bu2-
Si(H)OH20 were prepared via literature methods.
NMR spectra were obtained using Varian XL-400, INOVA-400,
and Unity-500 spectromenters. Chemical shifts are reported relative
to benzene-d6 (1H, s 7.15; 13C, t 128.0). Infrared spectra were
recorded on a Nicolet Impact 410 spectrophotometer interfaced to
a Gateway PC. Elemental analyses were performed by Oneida
Research Services, Whitesboro, NY, or Robertson Microlit Labo-
ratories, Madison, NJ.
6. tBu2MeSiH. Into a 100 mL bomb reactor charged with tBu2-
SiHCl (10.00 g, 46.9 mmol) was syringe-transferred (under Ar
purge) methyllithium (1.5 M in Et2O, 50 mL, 49.2 mmol). The
resulting solution was degassed, and the vessel was sealed and
heated to 67 °C for 20 h, resulting in a white precipitate. The
solution was cooled, and the excess methyllithium was quenched
by slow addition of 20 mL of 2-propanol. The contents were then
t
Procedures. 1. Bu2PhSiH. Into a 500 mL three-neck flask
charged with tBu2SiH2 (12.60 g, 87.4 mmol) and 200 mL of heptane
at -78 °C was syringe-transferred (under Ar purge) 1.05 equiv of
(39) Hwu, R. J.-R.; Tsay, S.-C.; Cheng, B.-L. In The Chemistry of
Organosilicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley
& Sons: New York, 1934; Vol. 2, pp 431-494.
Inorganic Chemistry, Vol. 43, No. 11, 2004 3429

Chadeayne et al.
transferred to a separatory funnel and washed (3 × 10 mL) with
distilled water. The organic layer was separated, and the aqueous
residues were extracted three times with hexanes. The combined
organic layers were then dried over MgSO4 and concentrated to a
yellow oil, and distillation (90 °C, 100 Torr) afforded 7.00 g of a
clear, colorless oil (94% yield).
solvent was removed by rotary evaporation. The white solid was
dissolved in 100 mL of boiling hexanes and cooled to -78 °C to
afford 13.5 g of colorless crystals that were collected in three crops
(85% yield).
12. (C6H11)3SiONa. To a 250 mL flask charged with (C6H11)3-
SiOH (9.2 g, 31.0 mmol) and sodium metal (1.30 g, 56 mmol)
was distilled 100 mL of toluene. The resulting suspension was
brought to reflux and the silanol dissolved. Heating for 1 h resulted
in the formation of a white precipitate. The reflux was continued
for an additional 12 h, and the toluene was removed in vacuo. The
resulting white powder was washed (5 × 5 mL) with toluene and
used without further purification (8.41 g, 90% yield). NMR spectral
characterization was not possible due to low solubility.
13. ((C6H11)3SiO)3TaCl2 (1-Cy3). To a 100 mL flask charged
with (C6H11)3SiONa (4.00 g, 13.3 mmol) and TaCl5 (1.59 g, 4.44
mmol) was distilled 45 mL of toluene. The resulting suspension
was refluxed for 12 h, and the solvent was removed in vacuo. The
resulting white powder was suspended in toluene and filtered. The
insoluble solid was extracted (10 × 20 mL) with toluene. The
extracts were concentrated to 10 mL, cooled to -78 °C for 4 h,
and filtered to afford clear, colorless crystals (2.61 g, 52% yield).
Anal. Calcd for C54H99O3Si3Cl2Ta: C, 57.27; H, 8.81. Found: C,
57.02; H, 9.01.
t
7. Bu2MeSiOH. Into a 100 mL three-neck flask charged with
40 mL of an ethanolic solution of tBu2MeSiH (13.50 g, 85.3 mmol,
2.1 M) was transferred, under ambient conditions, KOH (19.1 g,
0.341 mol). The reaction was brought to reflux for 20 h, then cooled
to room temperature, and neutralized with a saturated solution of
NH4Cl. The resulting solution was then extracted (3 × 20 mL)
with hexanes, and the combined organic layers were washed once
with brine (30 mL), dried over MgSO4, and then concentrated to
afford a yellow, waxy solid. Sublimation (80 °C, 10-4 Torr) yielded
12.5 g of white crystals (84%).
8. tBu2MeSiONa. Into a 250 mL flask containing tBu2MeSiOH
(12.30 g, 70.6 mmol) and sodium metal (1.95 g, 84.7 mmol) was
distilled 125 mL of toluene. The vessel was placed under an Ar
atmosphere and brought to reflux for 24 h. The toluene was removed
in vacuo to afford a white solid amidst the residual sodium. A 50
mL portion of hexanes was added, the resulting mixture was filtered,
and the insoluble material was extracted (5 × 5 mL) with hexanes.
The hexane filtrate was concentrated until precipitation began,
cooled to -78 °C for 2 h, and filtered to collect 11.0 g of a white,
crystalline solid (79% yield).
i
14. Pr3SiOH. Into a 500 mL flask charged with 200 mL of an
ethanolic solution of iPr3SiH (50.0 g, 0.316 mol, 1.58 M) was added
KOH (71.0 g, 1.26 mol). The solution was brought to reflux for 24
h and cooled to 23 °C. To the solution was added aqueous HCl (1
M) until it was neutral by pH paper. The solution was extracted (3
× 50 mL) with hexanes. The combined extracts were concentrated
in vacuo to afford a yellow oil. Distillation (110 °C, 50 Torr)
afforded 54.0 g of a clear, colorless oil (98% yield).
9. (tBu2CH3SiO)3TaCl2 (1-Me). To a 50 mL flask charged with
tBu2MeSiONa (2.00 g, 10.2 mmol) and TaCl5 (1.22 g, 3.40 mmol)
was distilled 25 mL of benzene. The resulting solution was stirred
at 55 °C for 16 h, and the solvent was removed in vacuo. The
resulting solid was triturated (3 × 10 mL) with pentane, suspended
in pentane, and filtered. The salt cake was extracted with pentane
(3 × 5 mL), and the combined filtrate was concentrated to 3 mL.
The concentrate was then cooled to -78 °C for 4 h and filtered to
yield 1.60 g of off-white microcrystals (63% yield). Anal. Calcd
for C27H63O3Si3Cl2Ta: C, 42.01; H, 8.23. Found: C, 42.00; H, 8.39.
10. [(tBu2MeSiO)3TaH]2 (4-Me). a. Synthesis. Into a 50 mL
flask charged with 1-Me (1.00 g, 1.30 mmol) and 2.1 equiv of Na/
Hg (6.95 g, 0.9% Na0) was distilled 25 mL of diethyl ether. At 77
K, excess H2 (1 atm) was slowly added, after passing it first through
a liquid nitrogen trap. The ether was allowed to thaw, and the
solution was stirred under 1 atm of H2 for 20 h, resulting in an
black mixture with visible precipitate. The solvent was removed
in vacuo, and the resulting black solid was suspended in 10 mL of
pentane and filtered. The insoluble material was extracted with
pentane (3 × 10 mL), and the extracts were condensed to 5 mL
and cooled to -78 °C for 2 h, after which 0.92 g of black crystals
were collected by filtration (52% yield). Anal. Calcd for C27H64O3-
Si3Ta: C, 46.20; H, 9.19. Found: C, 46.27; H, 8.81. b. Toepler
Pump Analysis. Into a 25 mL flask containing 4-Me (118 mg,
0.0839 mmol) was added ∼10 mL of Et2O at 77 K. Excess HCl
(202 Torr, 3.02 mmol) was admitted via a gas bulb. Gas evolved
while the solution was allowed to warm to 24 °C (20 min), and the
black solution turned colorless. After the solution was stirred for 1
h, the gases were passed through a series of three liquid nitrogen
traps and collected via a Toepler pump (0.240 mmol). The collected
hydrogen was converted to water by cycling over CuO (300 °C),
and the remaining volatiles were re-collected (0.0199 mmol, 0.238
equiv). By difference, 0.220 mmol of H2 was produced (2.62 equiv
with respect to 4-Me, 87% of that expected).
15. iPr3SiONa. Into a 100 mL flask charged with iPr3SiOH (9.65
g, 55.4 mmol) and sodium metal (2.3 g, 99.6 mmol) was distilled
50 mL of toluene. The resulting solution was brought to reflux for
12 h, and the solvent was removed in vacuo. The resulting material
was triturated (3 × 10 mL) with hexanes, dissolved in 20 mL of
hexanes, and filtered. The solvent was removed to yield 9.5 g of a
clear, colorless, amorphous solid (87% yield).
16. (iPr3SiO)3TaCl2 (1-iPr3). Into a 100 mL flask charged with
iPr3SiONa (4.00 g, 20.4 mmol) and TaCl5 (2.23 g, 6.79 mmol) was
distilled 50 mL of toluene. The resulting solution was brought to
reflux for 12 h, after which time the solvent was removed in vacuo,
and the resulting material was triturated three times with 10 mL
portions of hexanes, dissolved in 10 mL of hexanes, and filtered.
The filtrate was concentrated to 5 mL, cooled to -78 °C for 2 h,
and filtered to collect 2.20 g of an off-white, microcrystalline solid
(42% yield). Anal. Calcd for C27H63O3Si3Cl2Ta: C, 42.01; H, 8.23.
Found: C, 40.72; H, 7.96.
17. [(iPr3SiO)3TaCl]2 (2). Into a 50 mL flask charged with
1-iPr3 (250 mg, 0.32 mmol) and Na/Hg (1.74 g of 0.9% Na/Hg,
0.68 mmol of Na0) was distilled 20 mL of ether. The solution took
on a faint pink color after stirring for 45 min and then became
purple over 12 h as a precipitate formed. The solution was stirred
for 38 h, and the solvent was removed. The residual amalgam was
decanted, and the remaining solid was suspended in pentane and
filtered. The insoluble material was washed with (3 × 10 mL)
pentane, and the filtrate was allowed to evaporate at -35 °C in a
nitrogen atmosphere. After 24 h, the soluble material was removed
by pipet and 200 mg of purple crystals were collected (40% yield).
Anal. Calcd for C27H63O3Si3ClTa: C, 44.04; H, 8.62. Found: C,
43.84; H, 8.84.
11. (C6H11)3SiOH. To a boiling ethanolic solution of (C6H11)3-
SiCl (3.0 g, 9.6 mmol, 0.19 M) and trace phenylphthalein was added
dropwise 10% aqueous KOH until the pink color persisted. The
t
18. iPr2 BuSiH. Into a 100 mL flask charged with iPr2SiHCl was
distilled 40 mL of heptane. The resulting solution was cooled to
3430 Inorganic Chemistry, Vol. 43, No. 11, 2004

The Course of (R2R′SiO)3TaCl2 Reduction
t
-78 °C, and BuLi (26 mL, 0.043 mol, 1.7 M in pentane) was
complete conversion to 3-Me and observation of H2 (δ 4.46 ppm).
t
syringe-transferred. A white precipitate formed while the solution
was allowed to warm while stirring. The solution was stirred for
12 h, cooled to 4 °C, and quenched by dropwise addition of 10
mL of 2-propanol. The resulting mixture was added to 40 mL of
water. The organic layer was separated, washed once with 20 mL
of distilled water, and dried over MgSO4. It was filtered and
concentrated to afford a yellow oil. Distillation (65-80 °C, ∼10-4
Torr) afforded 2.7 g of a clear, colorless oil (43% yield).
25. [(iPr2 BuSiO)3TaH]2 (4-iPr2) with N2O. Procedure 24 was used
with 29 mg (0.020 mmol) of 4-iPr2 and 0.041 mmol (∼2.1 equiv)
1
of N2O. After 12 h at 23 °C, the H NMR spectrum displayed
resonances tentatively assigned to [(iPr2 BuSiO)3TaH]2(µ-O)
t
t
(5-iPr2) and [(iPr2 BuSiO)3TaO]2 (3-iPr2). Thermolysis (160 °C, 24
h) resulted in complete conversion to 3-iPr2 and observation of H2.
Small Pot Reactions. Dichloride Reductions in the Presence
of Ethylene. 26. From (tBu2CH3SiO)3TaCl2 (1-Me). Into a 100
mL flask containing 1-Me (400 mg, 0.518 mmol) and 2.1 equiv of
Na/Hg (2.8 g, 0.9% Na0) was distilled 30 mL of 1,2-dimethoxy-
ethane (DME). Ethylene (1 atm), which was passed through a -78
°C bath, was admitted to the flask. After stirring for 5 h at 23 °C,
the solvent was removed and the residual solid was triturated three
times with 5 mL of hexanes. A 15 mL portion of hexanes was
added, and the solution was filtered and evaporated to dryness to
t
19. iPr2 BuSiOH. Into a 100 mL three-neck flask charged with
20 mL of an ethanolic solution of iPr2 BuSiH (2.70 g, 15.7 mmol,
t
0.78 M in ethanol) was transferred KOH (3.50 g, 62.6 mmol). The
reaction was brought to reflux for 22 h, cooled to 23 °C, and
neutralized with NH4Cl (satd). The resulting solution was then
extracted (3 × 40 mL) with hexanes. The resulting organic layer
was washed once with brine (40 mL), dried over MgSO4, and
concentrated to afford a yellow oil. From the crude yellow oil was
distilled (115-130 °C, 10-4 Torr) 2.2 g of a clear, colorless oil
(75% yield).
20. iPr2 BuSiONa. Into a 50 mL flask charged with iPr2 BuSiOH
(1.97 g, 10.5 mmol) and sodium metal (0.480 g, 99.6 mmol) was
distilled 30 mL of toluene. The resulting solution was refluxed for
21 h and cooled to 23 °C, and the solvent was removed in vacuo.
The resulting solid was triturated with pentane (3 × 5 mL),
dissolved in 10 mL of hexanes, and filtered. Residual insoluble
material was extracted (3 × 5 mL) with hexanes, and the combined
extracts were concentrated to 5 mL, cooled to -78 °C for 2 h, and
filtered to provide 1.30 g of white crystals (59% yield).
1
afford 411 mg of (tBu2MeSiO)3TaCH2(CH2)2CH2 (9-Me). A H
NMR spectral assay indicated 85% purity. 27. From (iPr3SiO)3TaCl2
(1-iPr3). Into a 50 mL flask charged with 200 mg (0.26 mmol) of
1-iPr3 and 1.4 g of Na/Hg (2.1 equiv, 0.9% Na0) was distilled 20
mL of THF. Procedure 26 was followed to afford a colorless
material assayed by 1H NMR spectroscopy to be 80% (iPr3-
SiO)3TaCH2(CH2)2CH2 (9-iPr3). 28. From (tBu2PhSiO)3TaCl2 (1-
Ph). Into a 50 mL flask charged with 1-Ph (250 mg, 0.26 mmol)
and 1.4 g of Na/Hg (2.1 equiv, 0.9% Na0) was distilled 20 mL of
THF. Procedure 26 was followed except that the reaction was stirred
for 2 h, during which time a color change from green to bright
t
t
1
orange was noted. The final orange material was assayed by H
t
21. (iPr2 BuSiO)3TaCl2 (1-iPr2). Into a 50 mL flask charged with
iPr2 BuSiONa (1.27 g, 6.04 mmol) and TaCl5 (0.720 g, 2.01 mmol)
NMR spectroscopy to be 75% (tBu2PhSiO)3Ta(η-C2H4) (8-Ph).
t
29. Reaction of [(iPr3SiO)3TaCl]2 (2) with C2H4. A 25 mL flask
equipped with a gas bulb was charged with 2 (100 mg, 0.068 mmol)
and 5 mL of diethyl ether. Into the 77 K reaction mixture was
condensed ethylene (0.075 mmol, 1.1 equiv) after passing through
a -78 °C trap. The purple color of the solution faded over 24 h
and became yellow after 48 h. The solvent was removed, and the
was distilled 25 mL of toluene. The solution was brought to reflux
for 16 h, and the solvent was removed in vacuo. The resulting solid
was triturated (3 × 10 mL) with pentane, suspended in 30 mL of
pentane, and filtered. The salt cake was extracted (3 × 5 mL) with
pentane, and the combined filtrates were concentrated to 3 mL.
Upon cooling to -78 °C for 4 h, 748 mg of off-white microcrystals
(51% yield) was isolated by filtration. Anal. Calcd for C30H69O3-
Si3Cl2Ta: C, 44.27; H, 8.54. Found: C, 43.82; H, 8.69.
22. Bu2HSiONa. Into a 50 mL flask charged with Bu2HSiOH
(2.00 g, 12.5 mmol) and sodium metal (516 mg, 22.5 mmol) was
distilled 30 mL of toluene. The resulting mixture was stirred for 4
h at 23 °C, and the solvent was removed in vacuo. The resulting
white solid was triturated (3 × 5 mL) with pentane, taken up in 25
mL of pentane, and filtered to remove residual sodium. The filtrate
was concentrated, cooled to -78 °C for 2 h, and filtered to collect
980 mg of white crystals (47% yield).
23. (tBu2HSiO)3TaCl2 (1-H). Into a 25 mL flask charged with
tBu2HSiONa (200 mg, 1.20 mmol) and TaCl5 (144 mg, 0.402 mmol)
was distilled 10 mL of benzene. The resulting solution was stirred
for 4 h at 23 °C, and the solvent was removed in vacuo. The
resulting material was triturated (3 × 5 mL) with pentane, dissolved
in 5 mL of pentane, and filtered. The solvent was removed in vacuo
to afford 150 mg of a waxy, white solid (55% yield).
NMR Tube Reactions. 24. [(tBu2MeSiO)3TaH]2 (4-Me) with
N2O. To a flame dried NMR tube equipped with a 180° joint was
added 4-Me (30 mg, 0.021 mmol) and 0.6 mL of C6D6. The solution
was then degassed, and N2O (0.044 mmol, ∼2.1 equiv) was added
via a gas bulb. The NMR tube was then flame sealed, and the
1
products were determined by H NMR spectroscopy to be a 1:1
mixture of 9-iPr3 and 1-iPr3.
Dichloride Reductions in the Presence of 2-Butyne. 30. From
(tBu2CH3SiO)3TaCl2 (1-Me). To a 25 mL flask charged with 275
mg of 1-Me (0.356 mmol) and 1.91 g of Na/Hg (2.1 equiv, 0.9%
Na0) was distilled 10 mL of THF at 77 K. To the frozen mixture
was distilled 2-butyne (0.56 mL, 0.39 g, 7.1 mmol, 20 equiv). The
reaction was allowed to stir for 2 h at 23 °C, and the solvent was
removed. Pentane (10 mL) was added, the mixture was filtered,
t
t
1
and the solvent was removed. A H NMR assay of the colorless
material indicated (tBu2MeSiO)3Ta(η-C2Me2) (10-Me) in 90%
purity. 31. From (tBu2PhSiO)3TaCl2 (1-Ph). To a 25 mL flask
charged with 250 mg of 1-Ph (0.261 mmol) and 1.4 g of Na/Hg
(2.1 equiv, 0.9% Na0) was distilled 6 mL of Et2O at 77 K. Procedure
30 was followed, and (tBu2PhSiO)3Ta(η-C2Me2) (10-Ph) was
1
assayed by H NMR spectroscopy as the product in 90% purity.
32. From (iPr3SiO)3TaCl2 (1-iPr3). Into a 25 mL flask charged
with 400 mg of 1-iPr3 (0.518 mmol) and 2.74 g of Na/Hg (0.68
mmol of 0.9% Na0, 2.1 equiv) was distilled 10 mL of Et2O at 77
K. Procedure 30 was followed except the reaction was allowed to
stir for 24 h at 23 °C. A 1H NMR assay confirmed the presence of
(iPr3SiO)3Ta(η-C2Me2) (10- iPr3) in the colorless oil in 80% purity.
33. Reaction of [(iPr3SiO)3TaCl]2 (2) with 2-Butyne. A 25 mL
flask was charged with 2 (100 mg, 0.068 mmol) and 5 mL of Et2O.
Into the 77 K reaction mixture was condensed 2-butyne (0.075
mmol, 1.1 equiv) via a gas bulb. The reaction was allowed to stir
for 24 h, culminating in a clear, yellow solution. The solvent was
1
reaction was monitored by H NMR spectroscopy. After 15 h at
23 °C, resonances tentatively assigned to [(tBu2MeSiO)3TaH]2(µ-
O) (5-Me) and those belonging to [(tBu2MeSiO)3Ta]2(µ-O)2 (3-
Me) were observed and continued to grow in over 48 h. Prolonged
reaction times (1 week) or thermolysis (160 °C, 2 h) resulted in
Inorganic Chemistry, Vol. 43, No. 11, 2004 3431

Chadeayne et al.
1
removed, and the products were determined by H NMR spectral
and refined anisotropically and hydrogen atoms were treated as
idealized contributions.
i
assay to be a 1:1 mixture of 1-iPr3 and 10- Pr3.
34. Attempted Reduction of (tBu2CH3SiO)3TaCl2 (1-Me) in
Et2O. To a 50 mL flask charged with 1-Me (1.00 g, 1.3 mmol)
and K/C8 (0.7 g, 5.2 mmol of K0, 4 equiv) was distilled Et2O. The
reaction immediately became olive green in color and then faded
to brown over the course of 1 h at 23 °C. After 2 h, the solvent
was removed and the crude brown solid was taken up and filtered
in hexanes. After concentration of the filtrate, 750 mg of brown
solid was collected. The 1H NMR spectrum for this material
revealed a 1:1 mixture of [(tBu2MeSiO)3TaH]2 (4-Me) and [(tBu2-
MeSiO)3Ta]2(µ-O)2 (3-Me).
36. [(tBu2MeSiO)3TaH]2 (4-Me). Dark brown-black crystals of
4-Me were grown by evaporating a concentrated hexane solution.
Upon isolation, a suitable crystal (0.40 × 0.20 × 0.15 mm3) was
immersed in polyisobutylene and placed under a 173 K N2 stream
on the goniometer head of a Siemens P4 SMART CCD area
detector system (graphite-monochromated Mo KR radiation). Mod-
est quality data was indicated by high mosaicity and somewhat
diffuse reflections. Absorption corrections were performed using
the SADABS program, and the structure was solved by direct
methods (SHELXS), completed by subsequent difference Fourier
syntheses, and refined by full-matrix least-squares procedures
(SHELXL).40 After an initial refinement, all non-hydrogen atoms
were treated and refined anisotropically and hydrogen atoms were
treated as idealized contributions. The tantalum hydrides were not
located but assigned bridging positions based on their chemical
shifts in the 1H NMR spectrum and the absence of terminal hydride
absorptions in the infrared spectrum.
Single-Crystal X-ray Diffraction Studies. 35. [(iPr3SiO)3TaCl]2
(2). Purple crystals of 2 were grown by evaporating a concentrated
pentane solution. Upon isolation, a suitable crystal (0.20 × 0.20 ×
0.20 mm3) was immersed in polyisobutylene and placed under a
173 K N2 stream on the goniometer head of a Siemens P4 SMART
CCD area detector system (graphite-monochromated Mo KR
radiation). Absorption corrections were performed using the SAD-
ABS program, and the structure was solved by direct methods
(SHELXS), completed by subsequent difference Fourier syntheses,
and refined by full-matrix least-squares procedures (SHELXL).40
After an initial refinement, all non-hydrogen atoms were treated
Acknowledgment. We thank the National Science Foun-
dation (CHE-0212147) and Cornell University for support
of this research.
(40) SMART and SAINT (software reference manuals, version 5.042,
Bruker Analytical X-ray Systems: Madison, WI, 1998) include
SADABS (software for empirical absorption correction, Sheldrick, G.
M. University of Gottingen, Germany, 2000), SHELXS, and SHELX-
TL (reference manuals, version 5.1, Sheldrick, G. M. University of
Gottingen, Germany, 1997).
Supporting Information Available: Crystallographic data in
CIF format. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC035407U
3432 Inorganic Chemistry, Vol. 43, No. 11, 2004