Synthesis and characterisation of novel ruthenium multi-substituted polyoxometalates: α,β-[SiW9O37Ru4(H2O)3Cl3]7−

Novel ruthenium multi-substituted polyoxometalate compounds of general formulae K6Na[SiW9O37RuIII4(H2O)3Cl3]·nH2O (α- and β-SiW9O34 isomers) were synthesised via the reactions of the trilacunary Keggin α- and β-[SiW9O34]10− anions with RuCl3 in aqueous solution. Characterisation was performed by analytical, spectroscopic, magnetic, and electrochemical techniques. The results are in accordance with the new polyoxometalates having the trivacant Keggin anion (α or β isomer) linked to a cubane-type Ru4O4 cluster. The co-ordination sphere of ruthenium is completed with water molecules and chloride anions. Magnetic measurements showed that the majority of the molecules have the Ru ions cluster in a compensated configuration constrained to an antiferromagnetic alignment at very low temperature and following a Curie-Weiss like behaviour. For both isomers, cyclic voltammetry revealed quasi-reversible redox processes at the ruthenium centres, namely RuIV/III and RuIII/II, which were found to be pH dependent. The novel polyoxotungstates incorporate the highest content of ruthenium (W:Ru = 2.25:1) ever reported for Keggin-, Dawson or sandwich-type transition metal-substituted polyoxometalates and may find promising applications as oxidative catalysts.

Graphical abstractTrivacant Keggin-type anions α- and β-[SiW9O34]10− react with RuCl3·nH2O to give novel polyoxoanions, formulated [SiW9O37Ru4(H2O)3Cl3]7−, isolated as potassium/sodium salts. The new compounds present comparatively high ruthenium content, are moderately soluble in water and are stable in the pH range 1–6, what may be interesting for several types of oxidative catalysis.Figure optionsDownload full-size imageDownload as PowerPoint slide