Understanding the reactivity landscape for the activation of water till the formation of the O-O bond and O 2 release in molecular chemistry is a decisive step in guiding the elaboration of costeffective catalysts for the oxygen-evolving reaction (OER). Copper (II) complexes have recently caught the attention of chemists as catalysts for the 4e -/4H + water oxidation process. While a copper (IV) intermediate has been proposed as the reactive intermediate species, yet no spectroscopic signature has been reported so far. Copper (III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper (II) o-phenylene bisoxamidate complexes are a function of the substitution pattern on the periphery of the aromatic ring. In-situ IR, EPR and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading towards the O 2 formation selectively at the copper center. A copper (II) superoxide species is identified as the reactive intermediate, the stability of which governs the selectivity of 4eoxidation of water to molecular oxygen.
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