Manganese–Porphyrin Catalysts with Second Coordination Sphere Hydrogen-Bonding Pendants for Enhanced Electrochemical CO₂ Reduction.

Abstract

Electrochemical CO₂ reduction (CO₂RR) using earth-abundant catalysts is hindered by high overpotentials, hydrogen evolution, and inefficient proton delivery. This dissertation employs spin-resolved density functional theory (UB3LYP-D3 with continuum solvation) to investigate the mechanism of a manganese(I) porphyrin bearing an ortho-acetamido pendant, [Mnᶦ(Por-o-acetamido)]⁻, with phenol as a proton donor.

Relaxed scans and transition-state optimisations across multiple spin manifolds identify a productive sextet pathway. The first proton-coupled electron transfer (PCET), CO₂ protonation to Mn–COOH, is essentially barrierless when assisted by the pendant’s hydrogen-bond relay, while the second PCET, C–O bond cleavage, proceeds with a modest barrier. Both steps are exergonic, yielding a fully downhill catalytic cycle with a theoretical onset potential of approximately −0.37 V vs SHE and effectively zero overpotential at 0 V.

Mechanistically, the pendant lowers entropic assembly costs, stabilises charge-developing transition states, and directs protons to substrate-derived intermediates, thereby suppressing hydrogen evolution. Benchmarking against Fe and Mn–TPP analogues shows that neutral, directional hydrogen-bond pendants shift the rate-determining step from initial protonation to C–O cleavage while preserving low driving-force operation. These results establish general design rules for secondary-sphere engineering and highlight Mn porphyrins as promising, earth-abundant catalysts for selective CO₂-to-CO conversion at the thermodynamic limit.