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 tetraphenylporphyrin (TPP-Fe) and Fe tetra(pentafluorophenyl)porphyrin (TPFP-Fe), we showed their different electrochemical and chemical behaviors for ORR. Mechanism studies revealed that the Fe-superoxo species of TPP-Fe can undergo smooth protonation with trifluoroacetic acid (TFA) but the electron-deficient Fe-superoxo species of TPFP-Fe cannot be protonated with TFA. The Fe-superoxo reactivity difference between TPP-Fe and TPFP-Fe is the origin of their different catalytic ORR behaviors.