Electronic Effect Promoted Visible-Light-Driven CO 2 -to-CO Conversion in a Water-Containing System.

The design of unsaturated nonprecious metal complexes with high catalytic performance for photochemical CO 2 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 (-NO 2 ) on the aromatic ring of the salen ligand, exhibits the highest photochemical performance for visible-light-driven CO 2 -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 -CH 3 , -F, and -H groups, respectively), resulting in a lower reduction potential of active metal centers Co II and lower barriers for CO 2 coordination and C-O cleavage steps for 4 than those for catalysts 1 - 3 .