Oxothiomolybdenum Derivatives of the Superlacunary Crown Heteropolyanion {P8W48}: Structure of [K4{Mo4O4S4(H2O)3(OH)2}2(WO2)(P8W48O184)]30– and Studies in Solution

Reaction of the cyclic lacunary [H7P8W48O184]33– anion (noted P8W48) with the [Mo2S2O2(H2O)6]2+ oxothiocation led to two compounds, namely, [K4{Mo4O4S4(H2O)3(OH)2}2(WO2)­(P8W48O184)]30– (denoted 1) and [{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]36– (denoted 2), which were characterized in the solid state and solution. In the solid state, the structure of [K4{Mo4O4S4(H2O)3(OH)2}2(WO2)­(P8W48O184)]30– reveals the presence of two disordered {Mo4O4S4(H2O)3(OH)2}2+ “handles” connected on both sides of the P8W48 ring. Such a disorder is consistent with the presence of two geometrical isomers where the relative disposition of the two {Mo4O4S4(H2O)3(OH)2}2+ handles are arranged in a perpendicular or parallel mode. Such an interpretation is fully supported by 31P and 183W NMR solution studies. The relative stability of both geometrical isomers appears to be dependent upon the nature of the internal alkali cations, i.e., Na+ vs K+, and increased lability of the two {Mo4O4S4(H2O)3(OH)2}2+ handles, compared to the oxo analogous, was clearly identified by significant broadening of the 31P and 183W NMR lines. Solution studies carried out by UV–vis spectroscopy showed that formation of the adduct [{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]36– occurs in the 1.5–4.7 pH range and corresponds to a fast and quantitative condensation process. Furthermore, 31P NMR titrations in solution reveal formation of the “monohandle” derivative [{Mo4O4S4(H2O)3(OH)2}­(P8W48O184)]38– as an intermediate prior to formation of the “bishandle” derivatives. Furthermore, the electrochemical behavior of [{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]36– was studied in aqueous medium and compared with the parent anion P8W48.