Hydrogenation of Carbon Dioxide Catalyzed by PNP Pincer Iridium, Iron, and Cobalt Complexes: A Computational Design of Base Metal Catalysts

Abstract

The reaction mechanisms for hydrogenation of carbon dioxide catalyzed by PNP-ligated (PNP = 2,6-bis(di-iso-propylphosphinomethyl)pyridine) metal pincer complexes, (PNP)IrH3 (1-Ir), trans-(PNP)Fe(H)2CO (1-Fe) and (PNP)CoH3 (1-Co), were studied computationally by using the density functional theory (DFT). 1-Ir is a recently reported high efficiency catalyst for the formation of formic acid from H2 and CO2. 1-Fe and 1-Co are computationally designed low-cost base metal complexes for catalytic CO2 reduction. For the formation of formic acid from H2 and CO2 catalyzed by 1-Ir, 1-Fe, and 1-Co, the reaction pathways with direct H2 cleavage by OH– without the participation of the PNP ligand are about 20 kcal mol–1 more favorable than a previously postulated H2 cleavage mechanism that involves the aromatization and dearomatization of the pyridine ring in the PNP ligand. This finding reveals the essential role of the base, OH–, in the catalytic CO2 reduction cycle and suggests that the incorporation of strong bases ...