Uranyl–Peroxide Capsule Self‐Assembly in Slow Motion

Abstract

Uranyl-peroxide capsules are the newestf amily of polyoxometalates. Although discovered 13 years previously with over 70 topologies reported, there is al ack in the fundamental understanding of assembly mechanisms, particularly the role of the alkali counterions.H erein, the reactionp athway anda ssembly of uranyl peroxide capsules is reported by tracking the conversion from K + uranyl triperoxide monomer to the K + uranyl-peroxide U 28 capsule by means of small-angle X-ray scattering and Raman spectroscopy.F or the first time, the K + uranyl-peroxide pentamer face is isolated and structurally characterized, giving credence to the long-held belief that these geometric faces serve as building blocks to the fully formed capsules. Once isolated and re-dissolved, the pentamer face undergoes rapid conversion to capsulef orms,u nderlining its high reactivity that challenges its isolation. Calorimetricm easurements of the studied speciesc onfirms the pentamer lies on the energy landscape between the monomer and capsule.Aqueousuranium speciation impactsall aspects of nuclear materials stewardship. These include safe storage, handling, and treatment of aqueous nuclear wastes;s torageo fs pent nuclear fuel;key steps of the cradle-to-grave nuclear fuel cycle;nuclear forensics;a nd environmental contamination. In the past 13 years, [1] uranyl-peroxide capsules have emerged as both a new molecule family akin to the transition-metalp olyoxometalates (POMs) [2] and as viable aqueous species in natural, [3] industrial, [4] and laboratory settings. Uranyl capsules have been borne out with now over 70 unique cluster topologies that have been structurally characterized, featuring variation in size (16 to 124 polyhedra), ligands (peroxide, hydroxide,o xalate, pyrophosphate) and heterometals in the capsule walls (Fe, V, W, Mo, Sm) and cavities (alkalis, alkaline earths, rare earths, Bi, Pb, Ta ,N b). [5] Peroxidei sf ormed by radiolysiso fw ater in the high radiation field of uranium ore deposits andf uel, and is stabilized by bondingt ou ranyl, forming the mineral studtite, [(UO 2 )(O 2 )(H 2 O) 2 ](H 2 O) 2 in both nature and as deposits on storednuclearf uel. [5a, 6] Peroxide is very effective for dissolving uranium in combination with am ild base or precipitating uranium in acidic to neutralc onditions.T he dissolution/precipitation behaviori so ne viable method to separateu raniumf rom other oxides (i.e. crude ore for yellowcake production).H owever,t he dichotomy of the stable uranyl-peroxideb ond deterring its controlled removal by heating,a nd the inherent tendencyo fp eroxide to disproportionatey ielding hazardouspressures of O 2 gas hasc reated dangerous scenarios with stored yellowcake [6] and other uranylp eroxide materials.With afocus on solid-state characterization and computation of uranyl peroxide capsules and related materials, we are gaining as tructural understanding of the uranyl-peroxide bond. [7] Less well-understood is its reactivity in conditions that promote the formation of soluble or precipitatedu rany...

Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

“…Although most POMs are produced via de-novo “one-pot” syntheses, some POMs have only been synthesized by derivation from other more readily accessible POMs via ligand or metal substitution, resulting in altered structures and properties . Great emphasis has been placed on understanding the solution behavior of POMs with special attention to formation mechanisms, − the role of counter cations, , crystallization, solubility, , and transformations . It has recently been suggested that counter cations can dramatically affect the conformation and reactivity of POMs in solution, in addition to balancing their charge and mediating aggregation and crystallization.…”

Supramolecular Assembly of Geometrically Unstable Hybrid Organic–Inorganic Uranyl Peroxide Cage Clusters and Their Transformations

“…Controlling the UO 2 2+ /Na ratio promotes multiple cluster topologies in the NaOH system, yielding U 20 , U 24 , U 28 , and U 32 . U 28 is a favored capsule topology when K is the only available countercation, perhaps because K + has an affinity with topological pentagons and hexagons formed by uranyl ions at room temperature during the self-assembly process. , A structural transformation methodology from a uranyl peroxide cluster precursor attracts our attention as a possible approach to achieve structural variety in the KOH system.…”

Calorimetric Study of Functionalized Uranyl Peroxide Nanoclusters and Their Monomeric Building Block

“…Uranyl triperoxide monomers can form in reaction solutions before uranyl ions assemble into UPCs and can be used as starting material to synthesize UPCs [2d,12,15,20] . The Δ H f ° (aq) obtained for Li−UT (aq) , Li−U 24(aq , ) [13a] and Li−U 28(aq) [13a] was used to calculate the enthalpy of formation of Li−U 24(aq) and Li−U 28(aq) from their monomeric building blocks (Δ H f‐mon ), see Reactions R1–2, Equation 2–3, and Table 5.…”

“…The affinity of counter cations to the topological windows (squares, pentagons, and hexagons) of UPCs impacts their aqueous solubility and crystallization [13–14] . Recent studies have provided insights into formation mechanisms with the isolation of an intermediate building block, the uranyl pentamer (U 5 ), while synthesizing K−U 28 from potassium uranyl triperoxo monomer (K−UT); [15] using precursor clusters to form clusters never synthesized before; [9c] and converting clusters from one to another [13a,16] …”