Activating the Fe(I) State of Iron Porphyrinoid with Second-Sphere Proton Transfer Residues for Selective Reduction of CO2 to HCOOH via Fe(III/II)-COOH Intermediate(s).

The ability to tune the selectivity of CO2 reduction by first-row transition metal-based complexes via the inclusion of second-sphere effects heralds exciting and sought-after possibilities. On the basis of the mechanistic understanding of CO2 reduction by iron porphyrins developed by trapping and characterizing the intermediates involved ( J. Am. Chem. Soc. 2015, 137, 11214), a porphyrinoid ligand is envisaged to switch the selectivity of the iron porphyrins by reducing CO2 from CO to HCOOH as well as lower the overpotential to the process. The results show that the iron porphyrinoid designed can catalyze the reduction of CO2 to HCOOH using water as the proton source with 97% yield with no detectable H2 or CO. The iron porphyrinoid can activate CO2 in its Fe(I) state resulting in very low overpotential for CO2 reduction in contrast to all reported iron porphyrins, which can reduce CO2 in their Fe(0) state. Intermediates involved in CO2 reduction, Fe(III)-COOH and a Fe(II)-COOH, are identified with in situ FTIR-SEC and subsequently chemically generated and characterized using FTIR, resonance Raman, and Mössbauer spectroscopy. The mechanism of the reaction helps elucidate a key role played by a closely placed proton transfer residue in aiding CO2 binding to Fe(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe(II)-COOH intermediate.