Inverse effect of a covalently attached electron-proton transfer mediator in the oxygen reduction reaction

In this work, the electron-proton transfer mediator (EPTM) BQ/H<inf>2</inf>Q was covalently attached to the salophen ligand (Sal-H<inf>2</inf>Q), and the ORR activity of its iron complex Fe(Sal-H<inf>2</inf>Q) was compared with that of the salophen complex of iron without the EPTM, using acetic acid as the external proton source in acetonitrile. The rate of the ORR was found to be much higher for Fe(Sal), giving a peak current of 222 μA, while for Fe(Sal-H<inf>2</inf>Q), it was 115 μA at a similar potential. Selectivity measurements revealed that both catalysts show selective water formation. This observation holds for other solvent systems as well. The findings in this study are contrary to the general observation that the covalent attachment of BQ/H<inf>2</inf>Q EPTM enhances the ORR reactivity.