Selective 4e~-/4H~+ O_2 reduction by an iron(tetraferrocenyl)porphyrin complex: From proton transfer followed by electron transfer in organic solvent to proton coupled electron transfer in aqueous medium

摘要

An iron porphyrin catalyst bearing four ferrocenes and a hydrogen bonding distal pocket is found to catalyze 4e~-/4H~+ oxygen reduction reaction (ORR) in organic solvent under homogeneous conditions in the presence of 2-3 equiv of Trifluoromethanesulphonic acid. Absorption spectroscopy, electron paramagnetic resonance (EPR), and resonance Raman data along with H_2O_2 assay indicate that one out of the four electrons necessary to reduce O_2 to H_2O is donated by the ferrous porphyrin while three are donated by the distal ferrocene residues. The same catalyst shows 4e~-/4H~+ reduction of O_2 in an aqueous medium, under heterogeneous conditions, over a wide range of pH. Both the selectivity and the rate of ORR are found to be pH independent in an aqueous medium. The ORR proceeds via a proton transfer followed by electron transfer (PET) step in an organic medium and while a 2e~-/1H ~+ proton coupled electron transfer (PCET) step determines the electrochemical potential of ORR in an aqueous medium.