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

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.