Influence of Ligand Denticity and Flexibility on the Molecular Copper Mediated Oxygen Reduction Reaction.

To date, the copper complex with the tris(2-pyridylmethyl)amine () ligand (-) catalyzes the ORR with the highest reported turnover frequency (TOF) for any molecular copper catalyst. To gain insight into the importance of the tetradentate nature and high flexibility of the ligand for efficient four-electron ORR catalysis, the redox and electrocatalytic ORR behavior of the copper complexes of 2,2':6',2″-terpyridine () and bis(2-pyridylmethyl)amine () (- and -, respectively) were investigated in the present study. With a combination of cyclic voltammetry and rotating ring disk electrode measurements, we demonstrate that the presence of the and ligands results in a decrease in catalytic ORR activity and an increase in Faradaic efficiency for HO production. The lower catalytic activity is shown to be the result of a stabilization of the Cu state of the complex compared to the earlier reported - catalyst. This stabilization is most likely caused by the lower electron donating character of the tridentate and ligands compared to the tetradentate ligand. The Laviron plots of the redox behavior of - and - indicated that the formation of the ORR active catalyst involves relatively slow electron transfer kinetics which is caused by the inability of - and - to form the preferred tetrahedral coordination geometry for a Cu complex easily. Our study illustrates that both the tetradentate nature of the ligand and the ability of - to form the preferred tetrahedral coordination geometry for a Cu complex are of utmost importance for ORR catalysis with very high catalytic rates.