Regulating the Electronic Structures of Co Sites in Salen Cobalt Complexes for Boosting CO₂ Photoreduction

The performance of photocatalytic CO₂ reduction is highly dependent on the electronic structures of active sites, while achieving delicate regulation of electronic states to enhance the catalytic performance remains challenging. Herein, we designed and synthesized four salen cobalt complexes with different functional groups in organic ligands (Co-X, X = -NO₂, -COOMe, -tBu, and -OMe) for photocatalytic CO₂ reduction. Impressively, by changing the -X groups of the organic ligands, the electronic structures of Co ions in Co-X can be regulated, which results in obviously different photocatalytic performance for the reduction of CO₂ to CO, following the -X group sequence of -NO₂ > -COOMe > -tBu > -OMe. The optimized Co-NO₂ with the strongest electron-withdrawing ability of the -NO₂ group achieves the largest turnover number (TON) and turnover frequency (TOF) values of 5727 and 0.32 s^-1, respectively, 4.5 times higher than those of the Co-OMe. Theoretical calculations reveal that the spin populations of Co in Co-NO₂ are the largest among the four cobalt complexes, which is the most beneficial for absorbing and activating the CO₂ molecule, thereby accounting for its highest photocatalytic activity.