Dioxygen Activation and Reduction by Pyridinophane-Copper Complexes

Homogeneous oxygen reduction reaction (ORR) studies rely on two predominant electron delivery modes, yet a critical solvent mismatch between electrochemically (aqueous) and chemically (nonaqueous) driven assays invalidates cross-system comparisons of catalytic activity and selectivity for Cu-based molecular catalysts. Herein, we compare ORR catalysis by two pyridinophane-copper(II) complexes, [Cu^tmpa-NH]²⁺ and [Cu^bmpa-NH]²⁺, under matched conditions to evaluate the intrinsic influence of electron delivery modes. Both follow an overall third-order rate law (first-order dependences on [catalyst], [O₂], [acid]) in acetonitrile. However, [Cu^tmpa-NH]²⁺ exhibits 40-fold higher activity and near-exclusive four-electron selectivity (96.42% H₂O), whereas [Cu^bmpa-NH]²⁺ displays mixed two-electron/four-electron behavior (58.22% H₂O₂). Critically, [Cu^tmpa-NH]²⁺ maintains consistent four-electron selectivity and activity across both electron delivery modes, confirming that its intrinsic ORR performance is independent of the electron delivery mode and governed solely by its coordination sphere. This work identifies an easily overlooked issue in chemical ORR assays: decamethylferrocene reacts with in situ formed H₂O₂ in acidic media, interfering with selectivity and kinetic analysis. Notably, a centrosymmetric bis(μ-OH)-bridged dinuclear Cu(II) intermediate generated from the reaction of [Cu^bmpa-NH]⁺ with O₂ has been successfully isolated and structurally characterized. Overall, this work establishes a structure-activity-selectivity relationship for pyridylamine-copper ORR catalysts and provides rational design guidance for their high-performance analogues.