A Monovacant Heteropolytungstate Thioderivative:  Synthesis and Characterization of [(PW11O39)2(H4Mo4S4O6)]10- and Related Isomers

[(PW11O39)2(Mo4S4O4(OH2)2)]10- anions were obtained through the stereospecific addition of the [Mo2S2O2]2+ oxothiocation to the monovacant α-[PW11O39]7- anion. K10[(PW11O39)2(Mo4S4O4(OH2)2)]·25H2O has been isolated as crystals and characterized by X-ray diffraction. The structure revealed a “sandwich-like” dimer of two α-[PW11O39]7- subunits assembled by the noteworthy central cluster {H4Mo4S4O6}. The crystallization of the crude product produces an isomerically pure compound, which was characterized by 31P and 183W NMR. IR data were also supplied. In solution, the compound isomerizes, giving a second diastereoisomer. A kinetic experiment, carried out by 31P NMR, allowed the conditions of the thermodynamic equilibrium to be determined. A structural relationship between the two isomers is proposed, fully consistent with NMR data. Cisoid and transoid isomers result in the relative disposition of each [PW11O39]7- subunit, either staggered or eclipsed. An investigation of the formation of the [Mo2O2S2]2+ unit from the polycondensed cyclic precursor [Mo10S10O10(OH)10(H2O)5] and the aggregation process resulting in the oxothio [(PW11O39)2(Mo4S4O4(OH2)2)]10- compound has been undertaken. The studies were monitored by 31P NMR and UV−vis spectroscopies. The reaction is quantitative in nearly stoichiometric conditions.