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* = η 5 -C 5 Me 5; Dipp = 2,6- i Pr 2 C 6 H 3; R = silyl, alkyl; R‘ = alkyl, hydride) are described. The compounds Cp*(DippN )Ta(SiR 3 )Cl (SiR 3 = Si(SiMe 3 ) 3 ( 1 ), SiPh 3 ( 2 ), SiHMes 2 ( 3 ); Mes = 2,4,6-Me 3 C 6 H 2 ), prepared from Cp*(DippN )TaCl 2 and (THF) n LiSiR 3, react with H 2 to yield the dimeric hydride [Cp*(DippN )TaCl(μ-H)] 2 ( 4 ). The reaction of 4 with (THF) 3 LiSi(SiMe 3 ) 3 (2 equiv) gave the 16-electron silyl hydride complex Cp*(DippN )Ta[Si(SiMe 3 ) 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(SiMe 3 ) 3 ]H} - (A = Li(THF) 3, X = Cl ( 5 ); A = NBu 4, 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 η 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(SiMe 3 ) 3 occurs, along with formation of a five-membered tantalacycle resulting from coupling of the unsaturated substrate at Ta. Hydrogenolysis of 6 yields HSi(SiMe 3 ) 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[CH 2 Si(SiMe 3 ) 2 SiMe 2 H]H ( 15 ). This complex exhibits a γ-agostic Si−H interaction with the metal center. An attempt to prepare Cp*(DippN )Ta(SiHMes 2 )H produced the alkyl hydride product Cp*(DippN )Ta[η 2 -CH 2 (2-SiH 2 Mes-3,5-Me 2 C 6 H 2 )]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(SiMe 3 ) 3 ]Me ( 23 ), whereas Cp*(DippN )Ta(CH 2 CMe 3 )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.