Selective 4e-/4H+ O2 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

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 H2O2 assay indicate that one out of the four electrons necessary to reduce O-2 to H2O 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.