Evolution of actinyl peroxide clusters U28 in dilute electrolyte solution: exploring the transition from simple ions to macroionic assemblies.

Actinyl peroxide clusters, a unique class of uranyl-containing nanoclusters discovered in recent years, are crucial intermediates between the(UO2)(2+) aqua-ion monomer and bulk uranyl minerals. Herein, two actinyl polyoxometalate nanoclusters of Cs15[(Ta(O2)4)Cs4K12(UO2(O2)1.5)28]⋅20 H2O (CsKU28) and Na6K9[(Ta(O2)4)Rb4Na12(UO2(O2)1.5)28]⋅20 H2O (RbNaU28) were synthesized by incorporating a central Ta(O2)4(3-) anion that templates a hollow shell of 28 uranyl peroxide polyhedra. When dissolved in aqueous solutions with additional electrolytes, those 1.8 nm-size macroanions self-assembled into spherical, hollow, blackberry-type supramolecular structures, as was characterized by laser-light scattering (LLS) and TEM techniques. These clusters are the smallest macroions reported to date that form blackberry structures in solution, therefore, can be treated as valuable models for investigating the transition from simple ions to macroions. Kinetic studies showed an unusually long lag phase in the initial self-assembly process, which is followed by a rapid formation of the blackberry structures in solution. The small cluster size and high surface-charge density are essential in regulating the supramolecular structure formation, as was shown from the high activation energy barrier of 51.2±2 kJ mol(-1). Different countercations were introduced into the system to investigate the effect of ion binding to the length of the lag phase. The current research provides yet another scale of self-assembly of uranyl peroxide complexes in aqueous media.