Selective four-electron reduction of dioxygen by a mononuclear copper complex supported by a pentadentate polypyridylalkylamine ligand

Abstract

The selective four-electron reduction of O₂ to H₂O is central for efficient energy conversion. Copper-polypyridyl complexes are attractive Oxygen Reduction Reaction (ORR) catalysts, yet benchmark Cu-tris(2-pyridylmethyl)amine (Cu^tmpa) systems typically operate through a stepwise mechanism with H₂O₂ as an intermediate, which limits selectivity for water production. In this context, we report a mononuclear copper(II) complex, [Cu^tmpa-NH]²⁺, supported by the pentadentate polypyridylalkylamine ligand N-(2-methylpyridine)-2,11-diaza[3.3](2,6)pyridinophane (tmpa-NH), that enables selective four-electron reduction of O₂ to H₂O both in homogeneous electrochemical conditions in CH₃CN with 2,6-lutidinium tetrafluoroborate as proton source and in aqueous media. Single-crystal X-ray diffraction and low-temperature EPR spectroscopy establish a distorted octahedral Cu^II center with κ⁵ binding of tmpa-NH, while cyclic voltammetry shows a reversible Cu^II/I couple at E_1/2 ≈ −0.47 V vs. Fc⁺/Fc. Rotating ring-disk voltammetry and bulk electrolysis quantify near-exclusive H₂O formation (faradaic efficiency 91.7–97.8%) across −0.4 to −0.9 V, with an overpotential η ≈ 0.87 V referenced to O₂/4H⁺/2H₂O in CH₃CN. Kinetic analysis (foot-of-the-wave) reveals a third-order rate law, being first-order in [catalyst], [O₂], and [acid], with k_cat = (5.99 ± 0.14) × 10³ M⁻² s⁻¹. In a pH 7 phosphate buffer solution, [Cu^tmpa-NH]²⁺ retains robust ORR activity with high H₂O selectivity. The apparent rate constant (k_obs = 300 s⁻¹) in aqueous medium is significantly higher than in CH₃CN but lower than that of benchmark Cu^tmpa in water. By contrast, the complex with the tetradentate parent ligand, [Cu^bmpa-NH]²⁺, is inactive under identical conditions. The data are consistent with a mechanistic proposal for turnover control via proton-coupled electron transfer at an end-on Cu^II-superoxo intermediate, forming a Cu^II-hydroperoxo species. The added dimethyleneamine “cap” in tmpa-NH provides a fifth donor that preorganizes the first coordination sphere and directs the pathway toward the four-electron process, to enhance H₂O selectivity in copper-polypyridyl ORR catalysts across both organic and aqueous media.