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 CO 2 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 CO 2 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 CO 2 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 CO 2 to HCOOH using water as the proton source with 97% yield with no detectable H 2 or CO. The iron porphyrinoid can activate CO 2 in its Fe(I) state resulting in very low overpotential for CO 2 reduction in contrast to all reported iron porphyrins, which can reduce CO 2 in their Fe(0) state. Intermediates involved in CO 2 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 CO 2 binding to Fe(I), stabilizing the intermediates, and determining the fate of a rate-determining Fe(II)–COOH intermediate.