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

Neutral ZrIV and HfIV alkyl/amido complexes stabilized by a tridentate N-ligand containing a "rolling" heterodentate benzoimidazole fragment have been prepared and characterized. The ultimate nature of the ligand denticity, of the electronic properties of the ligand binding pocket and of the metal coordination environment are controlled by the protection/deprotection of the benzoimidazole NH group. The metal precursor choice [MIV(Bn)4 vs. MIV(NMe2)4] is also found to have an influence on the complex final coordination sphere; 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 underpinning mechanistic path. Selected alkyl species from this series have been successfully scrutinized for the tandem CO2 hydrosilylation to CH4 in combination with the strong Lewis acid B(C6F5)3 and using a variety of hydrosilanes. Catalytic outcomes have shown a positive effect of the hardness modification of the ligand donor atom set. Indeed, k3(N-,N,N-)ZrIV(Bn)2 from this work is found to catalyze the process selectively to methane with a TOF as high as 272 h-1 (at 96% substrate conv.) almost twice as much as that claimed for the benchmark k3(O-,O,O-)ZrIV(Bn)2 complex of the state-of-the-art under similar experimental conditions.