Concerted proton-electron transfer in electrocatalytic O2 reduction by iron porphyrin complexes: Axial ligands tuning H/D isotope effect

Abstract

The electrochemical O2 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) O2 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- O2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e- and 2e- O2 reduction, respectively. The catalytic O2 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 FeIII-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an FeII-OOH for an imidazole axial ligand.