The fate of U(IV)O in the environment in a colloidal form and its dissolution and growth in controlled environments is influenced by organic ligation and redox processes, where both affect solubility, speciation, and transport. Here we investigate U(IV) aqueous speciation from pH 0 to 3 with the glycine (Gly) ligand, the smallest amino acid. We document evolution of the monomeric to the hexameric form from pH 0 to 3 via UV-vis spectroscopy and small-angle X-ray scattering (SAXS). Crystals of the hexamer [UO(OH)(HO)(HGly)]·12Cl·12(HO) (U) were isolated at pH 2.15. The structure of U is a hexanuclear oxo/hydroxo cluster UO(OH) decorated by 12 glycine ligands and 6 water molecules. The effect of pH and temperature on U conversion to UO nanoparticles, or simply reversible aggregation, is detailed by transmission electron microscopy imaging, in addition to SAXS and UV-spectroscopy. Because of the zwitterion behavior of glycine, pH and temperature control over U(IV) speciation is complex. Unexpectedly, stability of the polynuclear cluster actually increases with increased pH. Speciation is sensitive to not only metal-oxo hydrolysis but also ligand lability and hydrophobic ligand-ligand interactions.

Uranyl peroxide capsules are a recent addition to polyoxometalate (POM) chemistry. Ten years of development has ensued only in water, while transition metal POMs are commonly exploited in aqueous and organic media, controlled by counterions or ligation to render the clusters hydrophilic or hydrophobic. Here, new uranyl POM behavior is recognized in organic media, including (1) stabilization and immobilization of encapsulated hydrophilic countercations, identified by Li nuclear magnetic resonance (NMR) spectroscopy, (2) formation of new cluster species upon phase transfer, (3) extraction of uranyl clusters from different starting materials including simulated spent nuclear fuel, (4) selective phase transfer of one cluster type from a mixture, and (5) phase transfer of clusters from both acidic and alkaline media. The capsule morphology of the uranyl POMs renders accurate characterization by X‐ray scattering, including the distinction of geometrically similar clusters. Compositional analysis of the aqueous phase post‐extraction provided a quantitative determination of the ion exchange process that enables transfer of the clusters into the organic phase. Preferential partitioning of uranyl POMs into organic media presents new frontiers in metal ion behavior and chemical reactions in the confined space of the cluster capsules in hydrophobic media, as well as the reactivity of clusters at the organic/aqueous interface.

Uranyl polyoxometalate clusters are both fundamentally fascinating and potentially relevant to nuclear energy applications. With only ten years of development, there is still much to be discovered about heterometal derivatives and aqueous speciation and behavior. Herein, we show that it is possible to encapsulate the polyoxocations [Bi O ] and [Pb O ] in [(UO )(O )(OH)] (denoted Bi@U and Pb@U ) in pure form and high yields despite the fact that under aqueous conditions, these compounds are stable on opposite ends of the pH scale. Moreover, [Pb O ] is a formerly unknown Pb polynuclear species, both in solution and in the solid state. Raman spectroscopic and mass spectrometric analysis of the reaction solutions revealed the very rapid assembly of the nested clusters, driven by bismuth- or lead-promoted decomposition of excess peroxide, which inhibits U formation. Experimental and simulated small-angle X-ray scattering data of Bi@U and Pb@U solutions revealed that this technique is very sensitive not only to the size and shape of the clusters, but also to the encapsulated species.

We obtained a kerosene-soluble form of the lithium salt [UO(O)(OH)] phase (Li-U), by adding cetyltrimethylammonium bromide surfactant to aqueous Li-U. Interestingly, its variable-temperature solution Li NMR spectroscopy resolves two narrowly spaced resonances down to -10 °C, which shift upfield with increasing temperature, and finally coalesce at temperatures> 85 °C. Comparison with solid-state NMR demonstrates that the Li dynamics in the Li-U-CTA phase involves only exchange between different local encapsulated environments. This behavior is distinct from the rapid Li exchange dynamics observed between encapsulated and external Li environments for Li-U in both the aqueous and the solid-state phases. Density functional theory calculations suggest that the two experimental Li NMR chemical shifts are due to Li cations coordinated within the square and hexagonal faces of the U cage, and they can undergo exchange within the confined environment, as the solution is heated. Very different than U in aqueous media, there is no evidence that the Li cations exit the cage, and therefore, this represents a truly confined space.

Supportinginformation and the ORCID identification number(s) for the author(s) of this article can be found under: http://dx.