The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type
Cp*(DippN)TaRR‘ (Cp* = η5-C5Me5; Dipp = 2,6-iPr2C6H3; R = silyl, alkyl; R‘ = alkyl,
hydride) are described. The compounds Cp*(DippN)Ta(SiR3)Cl (SiR3 = Si(SiMe3)3 (1), SiPh3
(2), SiHMes2 (3); Mes = 2,4,6-Me3C6H2), prepared from Cp*(DippN)TaCl2 and (THF)
n
LiSiR3,
react with H2 to yield the dimeric hydride [Cp*(DippN)TaCl(μ-H)]2 (4). The reaction
of 4 with (THF)3LiSi(SiMe3)3 (2 equiv) gave the 16-electron silyl hydride complex
Cp*(DippN)Ta[Si(SiMe3)3]H (6), which is Lewis acidic and reversibly binds halide ions to
afford anionic Ta(V) complexes of the type A+{Cp*(DippN)Ta(X)[Si(SiMe3)3]H}- (A = Li(THF)3, X = Cl (5); A = NBu4, X = Br (7) and I (8)). For complexes 5, 7, and 8, NMR and
X-ray data suggest the presence of weak three-center interactions involving Ta, Si,
and H. Silyl hydride 6 reacts with xylyl isocyanide (xylyl = 2,6-dimethylphenyl) and
acetonitrile to give the corresponding η2-silaimine and azomethine insertion products,
respectively, while treatment with acetone yields the stable isopropoxide product arising
from insertion into the Ta−H bond. When complex 6 is treated with ethylene or diphenylacetylene, elimination of HSi(SiMe3)3 occurs, along with formation of a five-membered
tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of
6 yields HSi(SiMe3)3 and a dimeric dihydride [Cp*(DippN)TaH(μ-H)]2 (14). At room
temperature, 6 rearranges (t
1/2 = 8.5 h) to an unusual alkyl hydride species resulting from
C−H bond activation, Cp*(DippN)Ta[CH2Si(SiMe3)2SiMe2H]H (15). This complex exhibits
a γ-agostic Si−H interaction with the metal center. An attempt to prepare Cp*(DippN)Ta(SiHMes2)H produced the alkyl hydride product Cp*(DippN)Ta[η2-CH2(2-SiH2Mes-3,5-Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride.
The alkyl hydride complex [Cp*(DippN)TaMe(μ-H)]2 (25) was prepared by the hydrogenolysis of Cp*(DippN)Ta[Si(SiMe3)3]Me (23), whereas Cp*(DippN)Ta(CH2CMe3)H (26)
was obtained by treatment of 4 with NpMgCl. Complex 26 possesses an α-agostic C−H
interaction, and the corresponding deuteride 26-
d
slowly scrambles deuterium into the
methylene positions.