Proton-Coupled\nElectron Transfer between a Pendant\nThiol and a Ferrous Dioxygen Adduct

The mechanism of thiol oxidation by O<sub>2</sub>, as\ncatalyzed\nby ferrous porphyrins, is investigated by trapping intermediates of\nthe transformation at low temperatures and subsequently characterizing\nthem using continuous wave and pulsed electron paramagnetic resonance\nand resonance Raman spectroscopy. A Fe(III)-O<sub>2</sub><sup>•–</sup> species is initially formed in an iron porphyrin with a pendant\nthiol functional group, which undergoes proton-coupled electron transfer\n(PCET) (not HAT) to form an Fe(III)-OOH species. Following O–O\nbond homolysis, this forms a Fe(IV)O species with concomitant\noxidation of the thiol to an RSO<sub>3</sub>H group.