Oxothiomolybdenum derivatives of the superlacunary crown heteropolyanion {P _8W _(48)}: Structure of [K _4{Mo _4O _4S _4(H _2O) _3(OH) _2} _2-(WO _2)(P _8W _(48)O _(184))] ~(30-) and studies in solution

摘要

Reaction of the cyclic lacunary [H _7P _8W _(48)O _(184)] ~(30-) anion (noted P _8W _(48))with the [Mo2S2O2(H _2O) _6] ~(2+) oxothiocation led to two compounds, namely, [K _4{Mo _4O _4S _4(H _2O) _3-(OH) _2} _2(WO _2)(P _8W _(48)O _(184))] ~(30-) (denoted 1)and [{Mo _4O _4S _4(H _2O) _3(OH) _2} _2(P _8W _(48)O _(184))]36-(denoted 2), which were characterized in the solid state and solution. In the solid state, the structure of [K _4{Mo _4O _4S _4(H _2O) _3(OH) _2} _2-(WO _2)(P _8W _(48)O _(184))] ~(30-) reveals the presence of two disordered {Mo _4O _4S _4(H _2O) _3(OH) _2} _2~+ "handles" connected on both sides of the P _8W _(48) ring. Such a disorder is consistent with the presence of two geometrical isomers where the relative disposition of the two {Mo _4O _4S _4(H _2O) _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 {Mo _4O _4S _4(H _2O) _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 [{Mo _4O _4S _4(H _2O) _3(OH) _2} _2(P _8W _(48)O _(184))] 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 [{Mo _4O _4S _4(H _2O) _3(OH) _2}(P _8W _(48)O _(184))]38-as an intermediate prior to formation of the "bishandle" derivatives. Furthermore, the electrochemical behavior of [{Mo _4O _4S _4(H _2O) _3(OH) _2} _2(P _8W _(48)O _(184))]36-was studied in aqueous medium and compared with the parent anion P _8W _(48).