Earth-Abundant Photocatalytic CO₂Reduction by Multielectron Chargeable Cobalt Porphyrin Catalysts: High CO/H₂Selectivity in Water Based on Phase Mismatch in Frontier MO Association

To stop global warming and climate changes, substantial efforts have been made to diminish CO2 emission. Photocatalytic and electrocatalytic CO2 reduction into fuels has thus become a highly important topic. Our recent interest has been to develop earth-abundant and environmentally friendly photocatalytic systems consisting of a non-precious-metal molecular CO2 reduction catalyst combined with subcomponents, especially using aqueous conditions without any organic solvents. However, CO2 reduction in water suffers from a drawback of decreasing its reaction yield due to concomitant H2 evolution, which can be driven by an overpotential less than that for CO2 reduction. Herein, we demonstrate a strategy to suppress H2 evolution using a cobalt porphyrin CO2 reduction catalyst possessing four N-methylpyridinium acceptors. The H2 evolution path is not favored by the active intermediate possessing a low-spin d7 CoII center because of its mismatch in forming an effective MO association with a 1s(H+) orbital. This is a rare example of catalysts avoiding the standard path relying on a filled (dz2)2 orbital in binding CO2. Instead, both π- and σ-type frontier MO associations are simultaneously formed by two degenerated π*(CO2) orbitals using a filled (dxz)2 and a half-filled (dz2)1 orbital. We also find that highly electron charged intermediates show switching in configuration from (dxz)2(dz2)1 to (dxz)1(dz2)2, leading us to allow the standard σ-type interaction with CO2. Correlation between the multielectron charging behaviors of cobalt porphyrins and the mechanism of photo- and electrocatalytic CO2 reduction is rationalized using our electrochemical and DFT results. This study sheds a light on strategies to rationally control the reaction rates and pathways based on the frontier MO engineering.