Structure and reactivity of chelating imido-amido complexes of tantalum. Mechanistic studies on the addition of silanes to Ta-N multiple bonds

The synthesis, structural characterization, and reactivity of tantalum complexes with chelating imido-amido ligands are reported. The highly bent imido complex Cp*Ta[=N(C6H3Me)(2)NSiMe3]Cl (4, Cp* = eta(5)-C-5-Me-5), with a Ta=N-C bond angle of 116.3(4)degrees, was synthesized from Cp*TaCl4 and the lithiated bis(silylamino)biphenyl (C6H3Me)(2)(NLiSiMe3)(2) (3). Compound 4 undergoes reactions with electrophiles at the nucleophilic imido nitrogen atom. The methyl derivative Cp*Ta[=N(C6H3Me)(2)NSiMe3]Me (5) reacts with xylyl isonitrile to give an insertion product, 6, which was structurally characterized. Addition of Mel to 5 gives a cationic diamide tantalum complex, {Cp*Ta[MeN(C6H3Me)(2)NSiMe3]Me}I-+(-) (7), the ionic structure of which was confirmed by X-ray crystallography. Reactions of 4 and 5 with unhindered silanes result in addition of the silane Si-H bond across the Ta=N double bond. Addition of PhSiH3 to 4 and 5 gave the hydrides Cp*Ta[PhSiH2N(C6H3Me)(2)NSiMe3](H)Cl (8) and Cp*Ta[PhSiH2N(C6H3Me)(2)NSiMe3](H)Me (9), respectively. The crystal structure of 9 was determined. Compounds 8 and 9 are unstable and decompose via elimination of HSiMe3. In the presence of CH2Cl2 and PhSiH3, 4 was slowly converted to another hydride complex, Cp*Ta[PhSiH2N(C6H3Me)(2)NSiPhHCl](H)Cl (12). A mechanism for this transformation, involving a sequence of silane addition/elimination reactions, is proposed. X-ray structural characterization of 12 revealed the presence of a nonclassical bonding interaction between the hydride ligand and a neighboring silyl group, leading to a short H-Si contact of 1.86(4) Angstrom and a distorted pentagonal bipyramidal geometry at silicon. Reactions of PhSiH3 and (CH2)(3)SiH2 (silacyclobutane) with 5 follow second-order kinetics, and an inverse deuterium isotope effect of k(H)/k(D) = 0.78(1) for the addition of PhSiH3 to 5 was observed. The elimination of HSiMe3 from 9 was found to follow a first-order rate law with approach to equilibrium (K-H = 0.025(2) mol/L) and exhibit an inverse isotope effect of k(H)/k(D) = 0.85(2). A study of the temperature dependence of the first-order rate constant for HSiMe3 elimination from 9 provided the activation parameters Delta H-double dagger = 25.5(3) kcal/mol and Delta S-double dagger = -0.3(1.0) cal/(mol.K). These findings are interpreted in terms of a mechanism involving slow, rate-determining formation of pentacoordinate silicon intermediates, coupled with a fast hydride shift between Ta and Si.