Origins of HCOOH Selectivity Over CO Mediated by an Unusual Fe(I)-Porphyrin Bearing a <i>β</i>-Substituted Cation

Molecular metalloporphyrins have been commonly reported to efficiently catalyze electrochemical CO₂-to-CO conversion. Unconventionally, Dey and coworkers reported that an iron-porphyrin analogue bearing a pendant amine binds with CO₂ at the Fe(I) state and reduces CO₂ into formic acid using water molecules as proton sources. However, the origins of HCOOH selectivity over the conventional CO product, as well as fundamental mechanistic details, are lacking. In the work, theoretical computations were employed to fundamentally investigate the reaction mechanisms. Our calculations reconfirmed that the formal Fe(I)-porphyrin would proceed with a direct CO₂-binding step, and this behavior could be ascribed to the significant hydrogen bonding and through-space electrostatic interactions between the cationic N-H and [CO₂^2-]-coordinated species. A two-electron transfer process in the key CO₂-binding step is found, which is estimated to proceed consecutively with protonation and 1e-reduction to give rise to an Fe(III)-COOH and Fe(II)-COOH intermediate, respectively. The cationic N-H plays vital roles in the stabilization of C-protonation species to yield HCOOH. Moreover, the cationic N-H terminal could hinder the dissociation of CO. Our computational results are consistent with experimental observations. The origins of HCOOH selectivity are elucidated, and an insightful mechanistic understanding of the cooperative roles of second-sphere hydrogen bonding and cationic effects is provided.