Highly Efficient Visible-Light-Driven CO₂-to-CO Conversion by Coordinatively Unsaturated Co-Salen Complexes in a...

The development of cost-effective catalysts for CO₂ reduction is highly desired but remains a significant challenge. The unsaturated coordination metal center in a catalyst is favorable for the process of catalytic CO₂ reduction. In this paper, two asymmetric salen ligands were used to synthesize two coordinatively unsaturated Co-salen complexes. The two Co-salen complexes exhibit an unsaturated coordination pattern and display high activity and CO selectivity for visible-light-driven CO₂ reduction in a water-containing system. The photocatalytic performance of 2 is higher than that of 1 because the reduction potential of the catalytic Co^II center and the energy barrier of the catalytic transition states of 2 are lower than those of 1, with turnover numbers (TON_CO), turnover frequencies (TOF), and CO selectivity values of 8640, 0.24 s^-1, and 97% for 2, respectively. The photocatalytic reduction of CO₂ to CO for 2 is well supported by control experiments and density functional theory (DFT) calculations.