Preparation, Structure, and Properties of the Corner-Shared Double Cubes [Mo₆HgQ₈(H₂O)₁₈]⁸⁺ (Q = S, Se) and Tungsten Analogues

The purple corner-shared double cube [Mo6HgS8(H2O)18]8+ derivative of green [Mo3S4(H2O)9]4+, obtained under air-free conditions by the reaction with Hg0 (metal), is also formed with HgI2. The HgI2 reaction is accounted for by the disproportionation HgI2 ⇌ Hg0 + HgII, which is a source of Hg0. X-ray crystallographic information on the blue partially Cl- substituted cucurbituril supramolecular assemblies [Mo6HgQ8Cl4(H2O)14](C36H36N24O12)Cl4·14H2O (1) and of the Se analogue [Mo6HgSe8Cl4 (H2O)14](C36H36N24O12)Cl4·14H2O (2) have been determined. The product [W6HgSe8Cl4(H2O)14](C36H36N24 O12) Cl4·14H2O (3) has also been obtained, but there is no evidence for [W6HgS8(H2O)18]8+ and related forms. The formation of [Mo6HgS8(H2O)18]8+ by the reaction of [Mo3S4 (H2O)9]4+ with Hg0 under anaerobic conditions maximizes after ∼40 h in 2.0 M HCl, but requires longer reaction time (∼120 h) in 2.0 M Hpts (p-toluenesulfonic acid) and in 2 M HClO4 (∼6 days). In 2.0 M HCl there is little absorbance increase until [Mo3S4(H2O)9]4+ exceeds 1.2 × 10-3 M, which is explained by a dependence of the formation K (265 M-1) on [Mo3S4(H2O)94+]2. Furthermore, on dilution of column-purified [Mo6HgS8(H2O)18]8+, Beer's law is not obeyed and equilibria involving 2[Mo3S4(H2O)9]4+ are apparent. The kinetics of formation of [Mo6HgS8(H2O)18]8+ is first-order in [Mo3S4(H2O)9]4+, consistent with rate-determining formation of the single cube [Mo3HgS4(H2O)x]4+. The oxidations of [Mo6HgS8(H2O)18]8+ with [Fe(H2O)6]3+ and [Co(dipic)2]- are complicated by the release of [Hg(H2O)6]2+, which also functions as an oxidant. Similar results are obtained for [Mo6HgSe8(H2O)18]8+ and the less extensively studied [W6HgSe8(H2O)18]8+.