Electrochemical Reduction of CO to CHOH Catalyzed by an Iron Porphyrinoid.

Designing catalysts for the selective reduction of CO, resulting in products having commercial value, is an important area of contemporary research. Several molecular catalysts have been reported to facilitate the reduction of CO (both electrochemical and photochemical) to yield 2e/2H electron-reduced products, CO and HCOOH, and selective reduction of CO beyond 2e/2H is rare. This is partly because the factors that control the selectivity of CO reduction beyond 2e are not yet understood. An iron chlorin complex with a pendent amine functionality in its second sphere, known to selectively catalyze CORR to HCOOH with a very low overpotential from its formal Fe(I) state, can catalyze CORR from its formal Fe(0) state by 6e/6H, forming CHOH 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 d Fe-COOH intermediate species generated during CORR is crucial in determining the product selectivity of this reaction. In weakly acidic conditions, C-protonation of this Fe-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 CHOH, 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 CORR and enables 6e/6H reduction of CO to CHOH.