A New Molybdenum Blue Structure Type: How Uranium Expands this Family of Polyoxometalates

Adding uranyl nitrate to a solution that would otherwise crystallize the MB wheel Mo₁₅₄ instead directed the formation and crystallization of Mo₇₂U_8, a new MB structure type. Uranium(IV) can be used as both heterometal and reductant in the formation of Mo₉₀U₁₀ type MB wheels. A dehydration induced phase change leads to formation of a sheet of these wheels connected by uranyl oxygen to molybdenum bonds, the first reported of their kind.

Abstract

The assembly of molybdenum polyoxometalates (POMs) has afforded large discrete nanoclusters with varied degrees of reduction such as the ~20 % reduced molybdenum blues. While many heterometals have been incorporated into these clusters to afford new properties, uranium has yet to be reported. Here we report the first uranium containing molybdenum blue clusters and the unique properties exhibited by this incorporation. The uranyl ion (UO₂ ²⁺) directs formation of Mo₇₂U₈, a square POM comprised of two faces connected by eight edge-sharing molybdenum dimers. Mo₇₂U₈, a chiral cluster, crystallizes as a racemic mixture and, in the solid state, has a ‘negative’ charge localized on one face of the cluster opposite the ‘positively’ charged face of another cluster. Using U(IV) as both heterometal and molybdenum reductant afforded crystals of Mo₉₇U₁₀, a wheel cluster with a heptamolybdate cap on one face. Mo₉₇U₁₀ dissociates in solution, losing the heptamolybdate, to form Mo₉₀U₁₀. Using more solvent during synthesis afforded crystals of Mo₉₀U₁₀S₄ which, instead of heptamolybdate, contains four sulfate ions. Crystals of Mo₉₀U₁₀S₄ undergo a dehydration induced phase change where clusters form a sheet through oxide bridges. Half of the bridges are cation-cation interactions between the uranyl oxygen atom and molybdenum, the first reported of this kind.

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