Electrochemical Reduction of CO2 to CH3OH Catalyzed by an Iron Porphyrinoid

Designing catalysts for the selective reduction of CO2, resulting in products having commercial value, is an important area of contemporary research. Several molecular catalysts have been reported to facilitate the reduction of CO2 (both electrochemical and photochemical) to yield 2e–/2H+ electron-reduced products, CO and HCOOH, and selective reduction of CO2 beyond 2e–/2H+ is rare. This is partly because the factors that control the selectivity of CO2 reduction beyond 2e– are not yet understood. An iron chlorin complex with a pendent amine functionality in its second sphere, known to selectively catalyze CO2RR to HCOOH with a very low overpotential from its formal Fe(I) state, can catalyze CO2RR from its formal Fe(0) state by 6e–/6H+, forming CH3OH as a major product with a Faradaic yield of ∼50%. Mechanistic investigations using in situ spectro-electrochemistry indicate that the reactivity of a low-spin d7 FeI–COOH intermediate species generated during CO2RR is crucial in determining the product selectivity of this reaction. In weakly acidic conditions, C-protonation of this FeI–COOH species, which is also chemically prepared and spectroscopically characterized, leads to HCOOH. The O-protonation, leading to C–OH bond cleavage and eventually to CH3OH, is ∼3 kcal/mol higher in energy and can be achieved in more acidic solutions. Hydrogen bonding to the pendent amine in the catalyst stabilizes reactive intermediates formed in the CO2RR and enables 6e–/6H+ reduction of CO2 to CH3OH.