A nonheme manganese(IV)–oxo complex, [MnIV(O)(BQCN)]2+, was generated in the photochemical and chemical oxidation of [MnII(BQCN)]2+ with water as an oxygen source, respectively. The photocatalytic oxidation of organic substrates, such as alcohol and sulfide, by [MnII(BQCN)]2+ has been demonstrated in both neutral and acidic media.

•Development of efficient homogeneous catalysts for water oxidation is important.•Recent progress on molecular complexes toward water oxidation is briefly reviewed.•How to distinguish homogeneous catalysts from heterogeneous catalysts is discussed.Artificial photosynthesis is considered a promising method to produce clean and renewable energy by sunlight. To accomplish this aim, development of efficient and robust catalysts for water oxidation and hydrogen production is extremely important. Owing to the advantages of easily modified structures and traceable catalytic processes, molecular water oxidation catalysts (WOCs) attract much attention during the past decade. However, the transformation of molecular WOCs to metal oxides/hydroxides or metal ions may occur under the harsh catalytic conditions, making the identification of true active species difficult. In this article, recent progress on molecular complexes acting as real catalysts or precursors toward water oxidation was briefly reviewed. We summarized the commonly used physical techniques and chemical methods that enable to distinguish homogeneous catalysts from heterogeneous catalysts. The factors that affect the nature of WOCs, such as reaction conditions, transition metal centers, and supporting ligands were discussed as well.

A nonheme manganese(IV)–oxo complex, [MnIV(O)(BQCN)]2+, was generated in the photochemical and chemical oxidation of [MnII(BQCN)]2+ with water as an oxygen source, respectively. The photocatalytic oxidation of organic substrates, such as alcohol and sulfide, by [MnII(BQCN)]2+ has been demonstrated in both neutral and acidic media.

A 1D chain-like Ag(I)-substituted Keggin polyoxotungstophosphate, K3[H3AgIPW11O39]·12H2O, has been synthesized in a high yield and characterized by single-crystal X-ray diffraction, XRD, IR, TG/DTA and elemental analysis. When the polyoxotungstophosphate is dissolved in aqueous solutions, 31P NMR, MS and conductivity analyses indicate that a Ag(I) anion-complex formulated as [H3AgI(H2O)PW11O39]3− is formed and is stable in a solution of pH 3.5–7.0. The oxidation of [H3AgI(H2O)PW11O39]3− by S2O82− has been studied by ESR, UV-Visible spectroscopy, 31P NMR and UV-Raman spectroscopy. It was found that [H3AgI(H2O)PW11O39]3− can be oxidized to dominantly generate a dark green Ag(II) anion-complex [H3AgII(H2O)PW11O39]2− and a small amount of Ag(III) complex [H3AgIIIOPW11O39]3−, simultaneously evolving O2. Compared with [AgI(2,2′-bpy)NO3] and AgNO3, [H3AgI(H2O)PW11O39]3− has the higher activity in chemical water oxidation. This illustrates that the [PW11O39]7− ligand plays important roles in both the transmission of electrons and protons, and in the improvement of the redox performance of silver ions. The rate of O2 evolution is a first-order law with respect to the concentrations of [H3AgI(H2O)PW11O39]3− and S2O82−, respectively. A possible catalytic water oxidation mechanism of [H3AgI(H2O)PW11O39]3− is proposed, in which the [H3AgII(H2O)PW11O39]2− and [H3AgIIIOPW11O39]3− intermediates are determined and the rate-determining step is [H3AgIIIOPW11O39]3− oxidizing water into H2O2.