Decomposition products of trinuclear ruthenium complexes as effective catalysts of water oxidation

Abstract

Ruthenium red, a di-µ-oxo-bridged ruthenium complex, and its oxidised form, ruthenium brown, have been studied as possible homogeneous redox catalysts for the oxidation of water to O₂ by Ce^IV ions in H₂SO₄ and HClO₄. In both media the Ce^IV ions oxidised the ruthenium red to brown and, with excess of Ce^IV, decomposed the ruthenium brown irreversibly to product(s) with three weak absorption bands at 390, 523 and 593 nm. Only in HClO₄ did the decomposition product(s) appear to act as a stable O₂ catalyst. Spectral evidence tentatively suggests that the active catalyst may be a hydrolysed Ru^IV polymeric species. The rate of catalysis was proportional to the initial concentration of ruthenium red/brown and the activation energy was determined as 36 ± 1 kJ mol^–1 over the temperature range ambient to ca. 50 °C. At temperatures greater than 50 °C the O₂ catalyst undergoes an irreversible thermal decomposition reaction.