Hydrogen-transfer reactions which generate new imine, imido, and trimethylenemethane complexes of tantalum

Abstract

The reactions of Cp*TaMe3C1 (Cp* = qs-CsMeS) with a variety of alkali-metal alkoxide, alkylamide, and alkyl reagents have been examined. Reaction with LiNMe2 produces Cp*Ta(NMez)Me3, which decomposes at 25 OC to an imine (or metallaazirane) complex, Cp*Ta(CH2NMe)Me2. The decomposition is a first-order, unimolecular process with a large kinetic isotope effect (kH/kD = 9.7). Monoalkylamides (LiNHR) react with Cp*TaMe3C1 to form imido complexes Cp*Ta(NR)Mez. Reaction of Cp*TaMe3C1 with lithium diisopropylamide forms a bridging methylene complex, Cp*Me2Ta(pCH2)(p-H)zTaMezCp*. The alkoxide compounds Cp*Ta(OR)Me3 (R = Me, CHMe2, CMe3) are very stable and decompose only over 100 OC. Alkyl complexes are stable only if the alkyl group does not have j3-hydrogens. Treatment of Cp*TaMe3C1 with (2-methylally1)magneium bromide affords an unstable tantalum 2-methylallyl compound, which decomposes cleanly to the trimethylenemethanec omplex Cp*TaMeZ(q4-C(CH2),)T. he rates of hydrogen abstraction or elimination processes in this system correlate with the nature of the atom bound to tantalum: for reactions involving a /3-hydrogen transfer the order is C > N > 0, while the facility of a-hydrogen abstraction reactions appear to decrease in the reverse order N > C. These reactivity patterns appear to reflect the variance in T a x , Ta-N, and Ta-O bond energies in this series. Hydrogenation of the imido compounds (Cp*Ta(NR)Me2) in the presence of phosphine ligands yields new examples of imido hydride complexes Cp*Ta(NR)H,(L) (L = PMe3, PMe2(C6Hs); R = CMe3, CHzCMe3). A moderately stable alkyl hydride complex, Cp*Ta(CH2NMe)Me(PMe3)H, has also been prepared.