Water-soluble multi-cage super tetrahedral uranyl peroxide phosphate clusters†
Combination of uranyl, peroxide, phosphate, and counter ions under ambient conditions in aqueous solution over the pH range 5.5–6.5 results in the self-assembly of a large super tetrahedral cluster containing 124 uranyl hexagonal bipyramids and 32 phosphate tetrahedra designated U124P32. With a diameter of 4 nm, U124P32 consists of four symmetrically identical cage clusters, each built from 23 uranyl hexagonal bipyramids and eight phosphate tetrahedra, as well as a central cage defined by 52 uranyl hexagonal bipyramids and 32 phosphate tetrahedra. It is the first uranyl-based cluster that contains multiple cages and it also has the highest number of uranyl ions. U124P32 contains 152 peroxide groups that are bidentate bridges between uranyl ions. Crystals of U124P32 provided structural details, and were dissolved for subsequent small-angle X-ray scattering and electrospray ionization mass spectrometry characterization that indicated U124P32 remains intact upon dissolution in water. Whereas uranyl phosphates are generally insoluble in water, prompting interest in their applications in remediating contaminated water, U124P32 is highly soluble, reflecting the impact of the nanoscale structure on solubility.