A Redox-Active Cobalt Platform for Selective Electro- and Photochemical O ₂ Reduction to H ₂ O ₂ in Near-Neutral Water

Hydrogen peroxide (H 2 O 2 ) constitutes a highly valuable and ecologically advantageous oxidizing agent that is pivotal to the domains of chemical production and water purification methodologies; however, its availability remains predominantly governed by centralized production processes that are resource-intensive. Here, we introduce a photoactive binuclear Co(II) complex (C1) supported by a redox-active hetero-azo donor–acceptor ligand ( N,N -dimethyl-4-((5-methylisoxazol-3-yl)diazenyl)aniline) that selectively channels dioxygen reduction through the 2e – /2H + pathway under both electrochemical bias and visible-light irradiation. In DMF, C1 promotes O 2 -to-H 2 O 2 conversion with a turnover frequency of 485 h –1 and a Faradaic efficiency of ∼84%, while retaining appreciable selectivity in near-neutral aqueous medium (68%). Under visible-light excitation (λ ex ∼ 440 nm) in the presence of a Ru-polypyridyl photosensitizer, the same molecular framework drives photochemical H 2 O 2 formation in near-neutral water with a turnover number of 814. Mechanistic analysis combining in-situ spectroelectrochemistry with electron paramagnetic resonance and Fourier transform infrared spectroscopy reveals a transient Co(III)-peroxo intermediate, consistent with a proton-coupled electron transfer manifold that suppresses O−O bond scission. Collectively, these results establish redox-active azo ligation as a modular strategy to integrate electro- and photochemical oxygen reduction reaction within a single molecular platform, providing a step toward decentralized, renewables-enabled H 2 O 2 generation under mild conditions.