Origins of Iodine-Rich W6I12 Cluster Compounds and the Soluble Compound W6I22.

In search of iodine-rich compounds with an octahedral tungsten cluster, we explored the treatment of β-W6I12, the most stable tungsten iodide cluster compound, with liquid iodine. The most iodine-rich compound obtained from these reactions was W6I22, whose crystal structure adopts two closely related modifications. The remarkable connectivity of [W6I8]4+ clusters in the structure of W6I22 makes this compound the first example of a soluble binary octahedral tungsten iodide cluster, as demonstrated by dissolution experiments in several solvents. Differential scanning calorimetry showed that the thermolysis of triclinic α-W6I22 proceeds via a phase transformation into monoclinic β-W6I22, followed by the formation of W6I18 and W6I16 with release of iodine. A corresponding ambient-pressure study by combined differential thermal analysis and thermal gravimetry revealed the transformation of β-W6I22 into W6I14 and β-W6I12, which finally decomposes into the elements. On the basis of this simple example, we demonstrate how a complete reaction sequence, including preparation and subsequent phase transformations, can be monitored and analyzed by thermal scanning methods. Moreover, a reaction cycle is reported that relates a whole series of binary tungsten iodides. Syntheses of the new compounds α- and β-W6I22, and W6I14 are reported, and their crystal structures, as determined by X-ray diffraction techniques, are presented.