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.将环状缺位型[H7P8W48O184]³³⁻阴离子（cyclic lacunary [H7P8W48O184]33– anion，下文简记为P8W48）与[Mo2S2O2(H2O)6]²⁺氧硫代阳离子（oxothiocation）反应，得到两种化合物，分别为[K4{Mo4O4S4(H2O)3(OH)2}2(WO2)(P8W48O184)]³⁰⁻（记为1）与[{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]³⁶⁻（记为2），二者均在固态与溶液状态下完成表征。固态结构分析显示，[K4{Mo4O4S4(H2O)3(OH)2}2(WO2)(P8W48O184)]³⁰⁻的结构中存在两个无序的{Mo4O4S4(H2O)3(OH)2}²⁺"柄状单元"，二者分别连接于P8W48环的两侧。该无序现象对应两种几何异构体：两种异构体中，两个{Mo4O4S4(H2O)3(OH)2}²⁺柄状单元的相对排布分别为垂直或平行模式。这一解释得到了31P与183W核磁共振波谱（NMR）溶液研究的充分支持。两种几何异构体的相对稳定性取决于内部碱金属阳离子的种类，即Na+与K+的差异；相较于氧代类似物，两个{Mo4O4S4(H2O)3(OH)2}²⁺柄状单元的不稳定性显著提升，该结论可通过31P与183W NMR谱线的明显宽化得到证实。紫外-可见（UV–vis）光谱溶液研究表明，加合物[{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]³⁶⁻的形成发生在pH 1.5~4.7范围内，对应一个快速且定量的缩合过程。此外，溶液中的31P NMR滴定实验显示，在形成"双柄"加合物之前，会先生成作为中间体的"单柄"衍生物[{Mo4O4S4(H2O)3(OH)2}(P8W48O184)]³⁸⁻。进一步地，本研究对[{Mo4O4S4(H2O)3(OH)2}2(P8W48O184)]³⁶⁻在水介质中的电化学行为进行了研究，并将其与母体阴离子P8W48进行了对比。