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 1-4 were explored in situ using o-phenylenediamine derivatives and 5-methylsalicylaldehyde as precursors of the ligands in 1-4. It was found that complex 4, 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 TON_CO and CO selectivity values of 5300 and 96%, respectively. DFT calculations and experimental results revealed that the promoted photocatalytic activity of 4 is ascribed to the electron-withdrawing effect of the nitro group in 4 compared to 1-3 (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 4 than those for catalysts 1-3.