Pendent Amine Group at the Outer Coordination Sphere of a Copper(II) Complex Enhances the Rate of Oxygen Reduction Reaction

Abstract Enhancing the rate of an electrocatalytic reaction through the strategic incorporation of secondary coordination sphere (SCS) interactions—such as hydrogen bonding, proton relays, and electrostatic effects—is a critical factor influencing both turnover frequency (TOF) and overpotential. In this study, the oxygen reduction reaction (ORR) is investigated using two Cu(II) complexes (1 and 2) supported by a bis‐pyridine‐dioxime ligand framework. Complex 2 features an o–NMe₂–C₆H₄– substituent in the SCS. Comparative electrocatalytic ORR studies of 1 and 2 in phosphate buffer revealed that 2 exhibited ≈3 times higher TOF than 1. Similarly, ORR studies in acetonitrile in the presence of trifluoroacetic acid (TFAH) and decamethylferrocene (Fc^*) showed that 2 is about twice as active as 1. Based on the kinetic investigations, it is suggested that reduction of a Cu^II(O₂^•) intermediate occurs during the rate‐determining step (rds) for complex 1 in acetonitrile, whereas a different rds is noted for complex 2, which is the protonation of the Cu^II(O₂^•). Overall, this study underscores the pivotal role of SCS interactions in accelerating ORR and represents one of the few reports detailing the influence of the outer coordination sphere on ORR catalyzed by molecular Cu complexes.