Electronic Effect Promoted Visible-Light-Driven CO2-to-CO Conversion in a Water-Containing System

The design of unsaturated nonprecious metal complexes with high catalytic performance for photochemical CO2 reduction is still an important challenge. In this paper, four coordinatively unsaturated Co-salen complexes <bold>1</bold>-<bold>4</bold> were explored in situ using o-phenylenediamine derivatives and 5-methylsalicylaldehyde as precursors of the ligands in <bold>1</bold>-<bold>4</bold>. It was found that complex <bold>4</bold>, bearing a nitro substituent (-NO2) on the aromatic ring of the salen ligand, exhibits the highest photochemical performance for visible-light-driven CO2-to-CO conversion in a water-containing system, with TONCO and CO selectivity values of 5300 and 96%, respectively. DFT calculations and experimental results revealed that the promoted photocatalytic activity of <bold>4</bold> is ascribed to the electron-withdrawing effect of the nitro group in <bold>4</bold> compared to <bold>1</bold>-<bold>3</bold> (with -CH3, -F, and -H groups, respectively), resulting in a lower reduction potential of active metal centers Co-II and lower barriers for CO2 coordination and C-O cleavage steps for <bold>4</bold> than those for catalysts <bold>1</bold>-<bold>3</bold>.