Exploring Mechanistic Details and Catalyst Resilience in Electrocatalytic Water Oxidation With a Cu(II) Complex Bearing a Redox-Active Ligand

Abstract

Herein, we report that a copper complex [Cu(dpaq)](ClO₄) (1) (H-dpaq = 2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamide) acts as a molecular water oxidation catalyst (WOC) under strong basic condition. Complex 1 oxidizes water to dioxygen in 0.1 M phosphate buffer solution at pH 12.0, exhibiting a turnover frequency of 3.1 × 10² s⁻¹ at a low overpotential (η) of ∼550 mV versus NHE at 1 mA cm⁻². A turnover number of 4.0 can be obtained during controlled potential electrolysis (CPE) using 0.25 mM complex 1 at a potential of 1.5 V at pH 12.0 for 3 h. Postelectrolysis analysis, rinse tests, and chelating assays collectively support the homogeneous nature of the electrocatalyst. Mechanistic investigations and quantum chemical calculations reveal a pathway wherein two successive ligand-centered oxidations transform the catalyst into a Cu(II)(dpaq•)O•. intermediate. Absence of any metal centered oxidation renders the oxidized intermediate less electrophilic, resulting in the survival of the methylene groups present on the ligand backbone against oxidation. The formation of the O─O bond is proposed to proceed via two consecutive single electron transfers (SET) from incoming hydroxide ions to the formal Cu^IV–oxo species.