Reaction of [P(2)N(2)]Ta==CH(2)(Me) with ethylene: synthesis of [P(2)N(2)]Ta(C(2)H(4))Et, a neutral species with a beta-agostic ethyl group in equilibrium with an alpha-agostic ethyl group ([P(2)N(2)] = PhP(CH(2)SiMe(2)CH(2))(2)PPh).

Abstract

The photolysis of [P(2)N(2)]TaMe(3) ([P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) produces [P(2)N(2)]Ta=CH(2)(Me) as the major product. The thermally unstable methylidene complex decomposes in solution in the absence of trapping agents to unidentified products. However, in the presence of ethylene [P(2)N(2)]Ta=CH(2)(Me) is slowly converted to [P(2)N(2)]Ta(C(2)H(4))Et, with [P(2)N(2)]Ta(C(2)H(4))Me observed as a minor product. A mechanistic study suggests that the formation of [P(2)N(2)]Ta(C(2)H(4))Et results from the trapping of [P(2)N(2)]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum-methyl bond. The minor product, [P(2)N(2)]Ta(C(2)H(4))Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P(2)N(2)]Ta=CH(2)(Me) and is accompanied by the production of an equivalent of propylene. Pure [P(2)N(2)]Ta(C(2)H(4))Et can be synthesized by hydrogenation of [P(2)N(2)]TaMe(3) in the presence of PMe(3), followed by the reaction of ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a beta-agostic interaction between the ethyl group and tantalum center in [P(2)N(2)]Ta(C(2)H(4))Et. Partially deuterated analogues of [P(2)N(2)]Ta(C(2)H(4))Et show a large isotopic perturbation of resonance for both the beta-protons and the alpha-protons of the ethyl group, indicative of an equilibrium between a beta-agostic and an alpha-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the (1)H environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the beta-agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called beta-hydrogen transfer process.