Ligand Coordination Sphere Modulates the Kinetics of Oxygen Reduction Reaction Catalyzed by a Non-Heme Macrocyclic Cobalt(III) Complex

A molecular Co^III complex (<b>1</b>), supported by a 14-membered macrocyclic ligand, was developed. The oxygen reduction reaction (ORR) catalyzed by <b>1</b> was investigated under electrochemical and spectrochemical conditions in acetonitrile, using trifluoroacetic acid (TFAH) as the proton source, and revealed selective catalytic 4e^-/4H⁺ reduction in both cases. Kinetic analyses revealed a first-order dependence on the concentrations of both catalyst and O₂, but no dependence on TFAH or decamethylferrocene (under chemical conditions). The catalytic rate constant was determined to be 3.6 × 10³ M^-1 s^-1 under electrochemical and 90 M^-1 s^-1 under spectrochemical conditions. A reported Co(III) complex (<b>2</b>), featuring a bis-pyridine-dioxime ligand architecture, also catalyzed the 4e^-/4H⁺ reduction of O₂ but displayed first-order dependence on catalyst, TFAH, and O₂. These results suggest that variations in the coordination environment around the Co center lead to distinct ORR mechanisms, despite identical product selectivity. Complex <b>1</b> exhibited an effective overpotential of 0.78 V, which is 240 mV lower than that of <b>2</b> (η_eff = 1.02 V), underscoring the role of ligand architecture in tuning the catalytic overpotential. Overall, this study underscores the pivotal role of ligand design in shaping ORR kinetics, mechanism, and efficiency, offering valuable insights for the development of ORR catalysts.