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 Cp* = η5-C5Me5), with a TaN−C bond angle of 116.3(4)°, 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 reacts with xylyl isonitrile to give an insertion product, 6, which was structurally characterized. Addition of MeI to 5 gives a cationic diamide tantalum 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 TaN double bond. Addition of PhSiH3 to 4 and 5 gave the 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 hydrido 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) Å and a distorted pentagonal bipyramidal geometry at silicon. Reactions of PhSiH3 and (CH2)3SiH2 (silacyclobutane) with 5 follow second-order kinetics, and an inverse deuterium isotope effect of kH/kD = 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 (KH = 0.025(2) mol/L) and exhibit an inverse isotope effect of kH/kD = 0.85(2). A study of the temperature dependence of the first-order rate constant for HSiMe3 elimination from 9 provided the activation parameters ΔH⧧= 25.5(3) kcal/mol and ΔS⧧ = −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.