Hydrogenation of Alkylzirconium(IV) Complexes: Heterolytic Activation of Hydrogen by a “Homogeneous” Metal Alkyl

Bis(η²-cyclopentadienyl)zirconium(IV) alkyl chlorides and hydrides have been prepared and characterized. Hydrogenation of these species yields the corresponding alkane and zirconium hydride complexes, Cp₂ZrHCl and Cp₂ZrH₂, respectively. Deuterium labeling experiments suggest that these d⁰ complexes activate H₂ by heterolytic attack on that molecule. Qualitative rates for hydrogenation of a series of complexes were Cp₂Zr(R)H > Cp₂Zr(R)Cl ≃ Cp₂ZrR₂ > [(Cp₂ZrCl)₂-(μ-OCHR)] > Cp₂Zr(COR)Cl. This rate trend is the same as that for carbonylation and suggests a conceptual link between mechanisms for hydrogenation and carbonylation of these unsaturated complexes. A possible relationship is noted between heterolytic activation and oxidative addition of H₂ to transition-metal species.