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.