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.