Cs(I) Cation Enhanced Cu(II) Catalysis of Water Oxidation

We report here a new catalytic water oxidation system based on Cu­(II) ions and a remarkable countercation effect on the catalysis. In a concentrated fluoride solution at neutral to weakly basic pHs, simple Cu­(II) salts are highly active and robust in catalyzing water oxidation homogeneously. F^– in solution acts as a proton acceptor and an oxidatively robust ligand. F^– coordination prevents precipitation of Cu­(II) as CuF₂/Cu­(OH)₂ and lowers potentials for accessing high-oxidation-state Cu by delocalizing the oxidative charge over F^– ligands. Significantly, the catalytic current is greatly enhanced in a solution of CsF compared to those of KF and NaF. Although countercations are not directly involved in the catalytic redox cycle, UV–vis and ¹⁹F nuclear magnetic resonance measurements reveal that coordination of F^– to Cu­(II) is dependent on countercations by Coulombic interaction. A less intense interaction between F^– and well-solvated Cs⁺ as compared with Na⁺ and K⁺ leads to a more intense coordination of F^– to Cu­(II), which accounts for the improved catalytic performance.