Benzoimidazole-Pyridylamido Zirconium and Hafnium Alkyl Complexes as Homogeneous Catalysts for Tandem Carbon Dioxide Hydrosilylation to Methane

Neutral Zr-IV and Hf-IV alkyl/amido complexes stabilized by a tridentate N ligand that contains a "rolling" heterodentate benzoimidazole fragment have been prepared and characterized. The ultimate nature of the ligand denticity, the electronic properties of the ligand binding pocket and the metal coordination environment are controlled by the protection/deprotection of the benzoimidazole NH group. The metal precursor used [M-IV(Bn)(4) or M-IV(NMe2)(4)] also has an influence on the final coordination sphere of the complex; indeed, a permanent central pyridine dearomatization occurs in the presence of dimethylamido ancillary groups. DFT calculations on the real system have been used to elucidate the mechanism. Selected alkyl species from this series have been scrutinized for the tandem hydrosilylation of CO2 to CH4 in combination with the strong Lewis acid B(C6F5)(3) using a variety of hydrosilanes. A positive effect of the hardness modification of the ligand donor atom set is observed in the catalytic outcomes. Indeed, kappa(3){N-,N,N-}Zr-IV(Bn)(2) catalyzes the process to methane selectively with a turnover frequency as high as 272h(-1) (at 96% substrate conversion) almost twice as much as that claimed for the benchmark kappa(3){O-,O,O-}Zr-IV(Bn)(2) complex under similar experimental conditions.