Mechanisms of dihydrogen activation by zirconocene alkyl and hydride derivatives : A molecular orbital analysis of a 'direct hydrogen transfer' reaction mode

Abstract

The formation of an adduct between a dihydrogen and a d⁰ species of the general type (C₅R₅)₂Zr(R′)₂ (R,R′ = H or CH₃) is suggested to involve a process analogous to that in CO adduct formation, and is studied by an extended Hückel molecular orbital analysis. Back donation into σ* H₂ orbitals arises from high-lying M-R′ bonding orbitals and determines the stability of alternative adduct geometries. Two isomeric H₂ adducts can interconvert by an intramolecular hydrogen shift. With D₂, this reaction sequence leads to hydrogen isotope exchange. The transition state of this reaction mode contains a three-centre (H⋯H⋯H)⁻ ligand configuration with bonding properties similar to those of an allyl ligand; it does not involve an oxidative change in the charge on the metal nor generation of a positively charged, protonic hydrogen species by heterolytic H₂ cleavage. The term ‘direct hydrogen transfer’ is proposed for this type of elementary reaction. For H₂-induced alkane elimination from group IV metallocene alkyl derivatives, an analogous reaction mechanism, involving formation of a H₂ adduct and a subsequent intramolecular hydrogen shift toward the alkyl ligand, is predicted to proceed with moderate activation energies.