Evolution of Actinyl Peroxide Clusters U₂₈ in Dilute Electrolyte Solution: Exploring the Transition from Simple Ions to Macroionic Assemblies

Abstract

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.

Selective Permeability of Uranyl Peroxide Nanocages to Different Alkali Ions: Influences from Surface Pores and Hydration Shells

“…An important feature of many macroions, including uranyl peroxide clusters, is that when carrying moderate charges, they strongly attract each other in dilute solutions although they carry the same type of charge and are highly soluble, leading to the reversible formation of hollow, spherical, single‐layered “blackberry”‐type structures 12. Those blackberry structures, driven by electrostatic interaction, will be sensitive to the environmental stimuli such as pH, solvent polarity, or extra electrolytes 11a. 13 Consequently, the change of blackberry size can serve as an accurate indicator to detect the ion‐transport process in aqueous solution.…”

Beyond Charge Balance: Counter‐Cations in Polyoxometalate Chemistry

“…Since their initial discovery, Blackberry formation is now accepted as a common phenomenon for inorganic POMs and for amphiphilic POMs that are functionalized by hydrophobic organic “tails” . Also noteworthy, the uranyl peroxide polyoxometalates of several distinct geometries and compositions, as well as the noble metal POMs, also assemble into blackberry structures, indicating the universality of the underlying structure formation processes. Beyond the scope of this Review, blackberry structures have also assembled from cationic coordination compound nanocages …”

“…Unlike tungstate and molybdate POMs, several uranyl peroxide capsules, self‐assemble in base rather than acid, and the capsules exhibit no acid–base chemistry, further solidifying the importance of the metal‐counter‐cations, usually alkali metals. Since the uranyl peroxide POMs possess a hollow‐capsule topology, alkali metal (and other) counter‐cations can exchange from the capsule inside into solution and vice versa .…”

Jenseits von Ladungsausgleich: Gegenkationen in der Polyoxometallat‐Chemie

“…Aufsätze gen [52,[96][97][98] sowie durch Edelmetall-POMs. [99,100] Dies lässt auf eine generelle Zugänglichkeit von POM-basierten Blackberrys schließen.…”