Concerted Proton-Electron Transfer in Electrocatalytic O-2 Reduction by Iron Porphyrin Complexes: Axial Ligands Tuning H/D Isotope Effect

The electrochemical O-2 reduction by thiolate- and imidazole-bound iron porphyrin complexes and H/D isotope effects on 4e(-) (determined by rotating disc electrochemistry) and 2e(-)(determined by rotating ring disc electrochemistry) O-2 reduction rates are investigated. The results indicate that a thiolate axial ligand shows an H/D isotope effect greater than 18 and 47 for the 4e(-) and 2e(-) O-2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e(-) and 2e(-) O-2 reduction, respectively. The catalytic O-2 reduction mechanism is investigated in situ with resonance Raman coupled with rotating disc electrochemistry. The data indicate that the rate-determining step changes from O-O bond heterolysis of Fe-III-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an Fe-II-OOH for an imidazole axial ligand.