The U 28 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 [UO 2 (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 133 Cs 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.