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.
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