Ligand Topology and Secondary-Sphere Interactions Direct the Mechanistic Pathways of Chemical and Electrochemical Oxygen Reduction Reactions Catalyzed by Copper(II) Complexes

The oxygen reduction reaction (ORR) is crucial for renewable energy and peroxide production, but understanding the factors influencing catalytic efficiency and selectivity remains an important topic. Although a number of copper-based ORR catalysts using polydentate ligands of varying denticity and flexibility have been developed, the roles of ligand topology and secondary coordination sphere (SCS) interactions have been largely unexplored. To address these issues, we have developed four mononuclear copper(II) complexes supported by isomeric ethylenediamine-derived ligands and their amide-functionalized analogues. Structural, spectroscopic, electrochemical, and kinetic studies reveal that subtle changes in ligand architecture profoundly alter the electronic structure, redox properties, and catalytic behavior of the copper(II) complexes. Under chemical ORR conditions, all complexes selectively mediate the direct 4e–/4H+ reduction of O2 through a rate-determining proton-coupled electron transfer (PCET) step. The tripodal ligand framework enhances catalytic activity through proton-responsive modulation of Cu-oxygen intermediates, whereas amide incorporation accelerates ORR via SCS interaction that facilitates PCET. In contrast, electrochemical studies uncover a distinct mechanism in which O2 binding to Cu(I) becomes rate-limiting and the overall 4e– reduction proceeds via a sequential [2 + 2]e– pathway. These findings offer insights into design principles for molecular oxygen reduction catalysts with tunable activity and selectivity.