Reaction of [P2N2]TaCH2(Me) with Ethylene: Synthesis of [P2N2]Ta(C2H4)Et, a Neutral Species with a β-Agostic Ethyl Group in Equilibrium with an α-Agostic Ethyl Group ([P2N2] = PhP(CH2SiMe2NSiMe2CH2)2PPh)

Abstract

The photolysis of [P₂N₂]TaMe₃ ([P₂N₂] = PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh) produces [P₂N₂]TaCH₂(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 [P₂N₂]TaCH₂(Me) is slowly converted to [P₂N₂]Ta(C₂H₄)Et, with [P₂N₂]Ta(C₂H₄)Me observed as a minor product. A mechanistic study suggests that the formation of [P₂N₂]Ta(C₂H₄)Et results from the trapping of [P₂N₂]TaEt, formed by the migratory insertion of the methylene moiety into the tantalum−methyl bond. The minor product, [P₂N₂]Ta(C₂H₄)Me, forms from the decomposition of a tantalacyclobutane resulting from the addition of ethylene to [P₂N₂]TaCH₂(Me) and is accompanied by the production of an equivalent of propylene. Pure [P₂N₂]Ta(C₂H₄)Et can be synthesized by hydrogenation of [P₂N₂]TaMe₃ in the presence of PMe₃, followed by the reaction of ethylene with the resulting trihydride. Crystallographic and NMR data indicate the presence of a β-agostic interaction between the ethyl group and tantalum center in [P₂N₂]Ta(C₂H₄)Et. Partially deuterated analogues of [P₂N₂]Ta(C₂H₄)Et show a large isotopic perturbation of resonance for both the β-protons and the α-protons of the ethyl group, indicative of an equilibrium between a β-agostic and an α-agostic interaction for the ethyl group in solution. An EXSY spectrum demonstrates that an additional fluxional process occurs that exchanges all of the ¹H environments of the ethyl and ethylene ligands. The mechanism of this exchange is believed to involve the direct transfer of the β-agostic hydrogen atom from the ethyl group to the ethylene ligand, via the so-called β-hydrogen transfer process.