Solvent Effects in Electrocatalytic CO2-to-CO Conversion Mediated by Iron Tetraphenylporphyrins

Iron tetraphenylporphyrin (FeTPP) is an efficient molecular electrocatalyst for CO2-to-CO conversion. Masaoka and co-workers employed counteranion exchange to obtain a more soluble FeTPP-ClO4 complex and found that FeTPP exhibited significantly higher catalytic activity in acetonitrile (MeCN) than in N,N-dimethylformamide (DMF). They proposed that Fe(I) was the active species responsible for the enhanced activity in MeCN. However, Lassalle-Kaiser and co-workers found that the Fe(I) species only loosely bound CO2, whereas the Fe(0) species acted as the true active form. Despite these findings, the fundamental origin of the solvent effects in the CO2 reduction reaction (CO2RR) remains poorly understood. Herein, theoretical calculations were employed to systematically investigate the solvent effects in the reaction. Our calculations revealed that the formal Fe(I) species cannot effectively activate CO2, whereas the formal Fe(0) species directly binds to CO2. The superior catalytic activity in MeCN originates from the axial coordination of a MeCN molecule to the Fe(II)-COOH intermediate. Trans-axial MeCN coordination reduces the doming effect, resulting in a more negatively charged hydroxyl oxygen atom in the COOH moiety. This charge accumulation facilitates reprotonation and C-O bond cleavage. This work elucidates the mechanistic origin of solvent effects and highlights the critical role of axial coordination in electrocatalytic CO2 reduction.