Time-resolved assembly of chiral uranyl peroxo cage clusters containing belts of polyhedra

Two chiral cage clusters built from uranyl polyhedra and (HPO ₃)^2- groups have been synthesized in pure yield and characterized structurally and spectroscopically in the solid state and aqueous solution. Synthesis reactions under ambient conditions in mildly acidic aqueous solutions gave clusters U₂₂PO₃ and U₂₈PO ₃ that contain belts of four uranyl peroxide pentagonal and hexagonal bipyramids, in contrast to earlier reported uranyl peroxide cage clusters that are built from four-, five-, and six-membered rings of uranyl hexagonal bipyramids. U₂₂PO₃ and U₂₈PO₃ are also the first chiral uranyl-based cage clusters, the first that contain uranyl pentagonal bipyramids that contain no peroxide ligands, and the first that incorporate (HPO₃)^2- bridges between uranyl ions. They are built from 22 uranyl polyhedra and 20 (HPO₃)^2- groups, or 28 uranyl polyhedra and 24 (HPO₃)^2- groups, with the outer and inner surfaces of the cages passivated by the O atoms of uranyl ions. Small-angle X-ray scattering (SAXS) profiles demonstrated that U ₂₂PO₃ clusters formed in solution within 1 h after mixing of reactants, and remained in solution for 2 weeks prior to crystallization. Time-resolved electrospray ionization mass spectrometry and SAXS demonstrated that U₂₈PO₃ clusters formed in solution within 1 h of mixing the reactants, and remained in solution 1 month before crystallization. Crystallization of U₂₂PO₃ and U₂₈PO₃ is accelerated by addition of KNO₃. Clusters of U ₂₂PO₃ with and without encapsulated cations exhibit markedly different aqueous solubility, reflecting the importance of cluster surface charge in fostering linkages through counterions to form a stable solid.