Photocatalytic CO2 Reduction Using a Robust Multifunctional Iridium Complex toward the Selective Formation of Formic Acid.

A highly efficient tetradentate PNNP-type Ir photocatalyst, Mes-IrPCY2, was developed for the reduction of carbon dioxide. The photocatalyst furnished formic acid (HCO2H) with 87% selectivity together with carbon monoxide to achieve a turnover number of 2560, which is the highest among CO2 reduction photocatalysts without an additional photosensitizer. Mes-IrPCY2 exhibited outstanding photocatalytic CO2 reduction activity in the presence of the sacrificial electron source 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in CO2-saturated N,N-dimethylacetamide under irradiation with visible light. The quantum yield was determined to be 49% for the generation of HCO2H and CO. Electron paramagnetic resonance and UV-vis spectroscopy studies of Mes-IrPCY2 with a sacrificial electron donor revealed that the one-electron-reduced species is the key intermediate for the selective formation of HCO2H.