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.