Influence of the distal guanidine group on the rate and selectivity of O₂ reduction by iron porphyrin

The O₂ reduction reaction (ORR) catalysed by iron porphyrins with covalently attached pendant guanidine groups is reported. The results show a clear enhancement in the rate and selectivity for the 4e^-/4H⁺ ORR. In situ resonance Raman investigations show that the rate determining step (rds) is O₂ binding to ferrous porphyrins in contrast to the case of mononuclear iron porphyrins and heme/Cu analogues where the O-O bond cleavage of a heme peroxide is the rds. The selectivity is further enhanced when an axial imidazole ligand is introduced. Thus, the combination of the axial imidazole ligand and pendant guanidine ligand, analogous to the active site of peroxidases, is determined to be very effective in enabling a facile and selective 4e^-/4H⁺ ORR.