Enhanced catalytic activity of solubilised species obtained by counter-cation exchange of K{CoII1.5[FeII(CN)6]} for water oxidation

Abstract

A cyano-bridged coordination polymer, K{Co^II_1.5[Fe^II(CN)₆]} {(K)Co–Fe}, reported as a highly active heterogeneous catalyst for water oxidation was solubilised by a conventional counter-cation exchange of K⁺ with Me₄N⁺ ions to provide the homogeneous catalyst of (Me₄N){Co^II_1.5[Fe^II(CN)₆]} {(Me₄N)Co–Fe}. (Me₄N)Co–Fe exhibited enhanced catalytic activity for photocatalytic water oxidation using [Ru(2,2′-bipyridine)₃]²⁺ and S₂O₈²⁻ as a photosensitiser and a sacrificial electron acceptor, respectively, in terms of the initial reaction rate (1.26 μmol min⁻¹), which is about twice that of (K)Co–Fe (0.61 μmol min⁻¹). Powder X-ray diffraction, pair distribution function and electrospray ionization mass spectrometry measurements of (Me₄N)Co–Fe manifested that small heptanuclear clusters of {Co₄[Fe(CN)₆]₃}⁴⁻ formed by depolymerisation are catalytically active species in solution.