The U28 Nanosphere: What's Inside?

Polyoxometalate-like behavior of actinyl-peroxide anions in aqueous alkaline media has been recently unveiled in the form of more than 20 reported crystal structures of clusters, each with 20-60 uranyl polyhedra composing capsule-like topologies. There is now opportunity to fully develop this new polyoxometalate (POM) family to include redox behavior, non-aqueous chemistry, complex materials from cluster building blocks, cluster-counterion interactions, etc. To pursue these opportunities, reliable syntheses rooted in an understanding of cluster assembly processes are imperative. To this end, using the U-28 nanosphere [UO2(O-2)(1.5)](28) as an example, we report high yield syntheses of a series of four U-28 salts that feature different templating cations (K, Rb, Cs) and anions [uranyl monomer, Nb(O-2)(4) and Ta(O-2)(4)]. The key to assembly and stability of U-28 is both 1) synthetic conditions that are not extreme or dynamic, and 2) templating cations and anions that ideally match each other and the topology of the capsule interior. U-28 salts are characterized in the solid-state by powder and single-crystal X-ray diffraction and infrared spectroscopy. Furthermore, Cs-templated U-28 is re-dissolved and characterized by Cs-133 NMR; providing information on solution stability, and revealing the interaction of the internal templating Cs+ with the central templating anion. While Cs+ internal to the cluster remains inside when U-28 is dissolved in a Na salt solution, the internal K+ will rapidly exchange with Na+, providing new routes to other cluster topologies and compositions.