Thermodynamic Hydricity of a Ruthenium CO2 Hydrogenation Catalyst Supported by a Rigid PNP Pincer

Abstract

Ruthenium hydride complexes supported by pincer ligands play a crucial role in the catalytic hydrogenation of CO2 to reduced C1 chemicals such as formic acid and methanol. Toward a better understanding of their hydride transfer reactivity, knowledge of the underlying thermodynamic hydricity values is deemed critical, but relevant studies remain rare. Herein, we report the experimental thermodynamic hydricity of a new ruthenium CO2 hydrogenation catalyst (acriPNP)RuH(CO)(PPh3) (1) supported by a rigid, acridane-based PNP pincer ligand. We provide the synthesis, structure, and spectroscopic characterization of reaction intermediates involved in formate generation including the anionic dihydride (2), formate (3), five-coordinate purple species (4), and H2-bound species (5). Notably, the effective hydricity of complexes 1 and 2 in THF was determined by the H2 heterolysis method, revealing values of >52 and 32 kcal/mol, respectively. The corresponding hydricity values of 45-48 kcal/mol for related Ru dihydride complexes supported by neutral PNP pincer ligands highlight the effect of anionic complex charge in promoting stronger hydride donors. CO2 insertion into the Ru-H bond of the dihydride complex proceeds effectively under ambient conditions, suggesting that base-promoted H2 heterolysis is the rate-limiting step. Using 1 as a precatalyst, turnover frequencies in the order of 300 h-1 were obtained for formate generation. Broadly, our results provide valuable benchmark thermochemical data for the design of improved CO2 hydrogenation catalysts.