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

Abstract

High Resolution Image Download MS PowerPoint Slide Ruthenium hydride complexes supported by pincer ligands play a crucial role in the catalytic hydrogenation of CO 2 to reduced C 1 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 CO 2 hydrogenation catalyst ( acri PNP)RuH(CO)(PPh 3 ) ( 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 H 2 -bound species ( 5 ). Notably, the effective hydricity of complexes 1 and 2 in THF was determined by the H 2 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. CO 2 insertion into the Ru–H bond of the dihydride complex proceeds effectively under ambient conditions, suggesting that base-promoted H 2 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 CO 2 hydrogenation catalysts.

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