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

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 (UO 2 2+ ) directs formation of Mo 72 U 8 , a square POM comprised of two faces connected by eight edge-sharing molybdenum dimers. Mo 72 U 8 , 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 Mo 97 U 10 , a wheel cluster with a heptamolybdate cap on one face. Mo 97 U 10 dissociates in solution, losing the heptamolybdate, to form Mo 90 U 10 . Using more solvent during synthesis afforded crystals of Mo 90 U 10 S 4 which, instead of heptamolybdate, contains four sulfate ions. Crystals of Mo 90 U 10 S 4 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.