We have synthesized the mono-ruthenium substituted Keggin-type silicotungstate [SiW11O39RuIII(H2O)]5− (1a) by reaction of the mono-lacunary silicotungstate precursor [SiW11O39]8− with Ru(acac)3 under hydrothermal conditions and isolated as the caesium salt Cs5[SiW11O39RuIII(H2O)] (1). The DMSO-coordinated complex [SiW11O39RuIII(DMSO)]5− (2a) was prepared by reaction of 1a with DMSO in aqueous solution at 353 K and isolated as the caesium–potassium mixed salt Cs4.9K0.1[SiW11O39RuIII(DMSO)] (2). Both compounds 1 and 2 were characterized by single-crystal X-ray structure analysis, powder X-ray structure analysis, UV-Vis spectroscopy, cyclic voltammetry, IR-spectroscopy and elemental analysis. 1 crystallized in the tetragonal space group P42/ncm with a = 20.9299(4), c = 10.3603(4) Å, Z = 4. The ruthenium atom in the Keggin unit could not be distinguished from the tungsten due to disorder. The structural analysis of 2 (monoclinic, P21/c, a = 13.5850(4), b = 20.2764(7), c = 18.1326(4) Å, β = 90.8730(10)°, Z = 4) successfully revealed that the incorporated ruthenium atom is coordinated by DMSO through a Ru–S bond. Polyanion 2a represents the first mono-substituted Keggin ion in which the ruthenium center is not crystallographically disordered. UV-Vis spectroscopy combined with controlled potential electrolysis confirmed that the incorporated rutheniums in 1 and 2 have a valence state of +3. The IR spectra of both 1 and 2 were very similar. All these data indicate that 1 synthesized by reaction of the mono-lacunary silicotungstate K8[SiW11O39] with Ru(acac)3 under hydrothermal conditions is truly the mono-ruthenium substituted Keggin-type silicotungstate.
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