Revealing Significant Electronic Effects on the Oxygen Reduction Reaction with Iron Porphyrins

Understanding electronic effects on catalysis from a mechanism point of view is of fundamental significance but is also challenging. We herein report on electronic effects on the oxygen reduction reaction (ORR) with Fe porphyrins. By using Fe^III tetraphenylporphyrin (TPP-Fe) and Fe^III tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the Fe^III-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient Fe^III-superoxo species of TPFP-Fe cannot be protonated with TFA. The Fe^III-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.