A Single Iron Porphyrin Shows pH Dependent Switch between Push and Pull Effects in Electrochemical Oxygen Reduction

The push-pull effects associated with heme enzymes manifest themselves through highly evolved distal amino acid environments and axial ligands to the heme. These conserved residues enhance their reactivities by orders of magnitude relative to small molecules that mimic the primary coordination. An instance of a mononuclear iron porphyrin with covalently attached pendent phenanthroline groups is reported which exhibit reactivity indicating a pH dependent push to pull transition in the same molecule. The pendant phenanthroline residues provide proton transfer pathways into the iron site, ensuring selective 4e(-)/4H(+) reduction of O-2 to water. The protonation of these residues at lower pH mimics the pull effect of peroxidases, and a coordination of an axial hydroxide ligand at high pH emulates the push effect of P450 monooxygenases. Both effects enhance the rate of O-2 reduction by orders of magnitude over its value at neutral pH while maintaining exclusive selectivity for 4e(-)/4H(+) oxygen reduction reaction.