Neutral and anionic silyl hydride derivatives of the tantalum imido fragment Cp*(DippN=)Ta (Cp* = η5-C5Me5;: Dipp=2,6-iPr2C6H3).: Reactive σ-bonds and intramolecular C-H bond activations involving the silyl ligands

The syntheses and reactivities of alkyl, silyl, and hydride complexes of the type Cp*(DippN=)TaRR' (Cp* = eta(5)-C5Me5; Dipp = 2,6-(Pr2C6H3)-Pr-i; R = silyl, alkyl; R' = alkyl, hydride) are described. The compounds Cp*(DippN=)Ta(SiR3)Cl (SiR3 = Si(SiMe3)(3) (1), SiPh3 (2), SiHMes(2) (3); Mes = 2,4,6-Me3C6H2), prepared from Cp*(DippN=)TaCl2 and (THF)(n)LiSiR3, react with H-2 to yield the dimeric hydride [Cp*(DippN)TaCl(mu-H)](2) (4). The reaction of 4 with (THF)(3)LiSi(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 1 (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 eta(2)-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(mu-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)(2)SiMe2H]H (15). This complex exhibits a gamma-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[eta(2) -CH2(2-SiH(2)Mes-3,5-Me2C6H2)]H (20), which apparently results from decomposition of the expected silyl hydride. The alkyl hydride complex [Cp*(DippN=)TaMe(mu-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 alpha-agostic C-H interaction, and the corresponding deuteride 26-d slowly scrambles deuterium into the methylene positions.