Uranyl Peroxide Cage Cluster Solubility in Water and the Role of the Electrical Double Layer

Enhanced Hydrogen Bonds of the (H2O)n (n = 4-8) Clusters Confined in Uranyl Peroxide Cluster Na20(UO2)20(O2)30

Water is a basic resource and an essential component of living organisms. It often exhibits some novel properties under confinement. The water clusters (H₂O)_n (n = 4-8) confined in the cavity of uranyl peroxide cluster Na₂₀(UO₂)₂₀(O₂)₃₀ (U₂₀) have been computationally investigated by using ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) calculations in this study. The results show that the confined water clusters can form hydrogen bonds with the internal oxygen atoms (O_uranyl) of U₂₀, and their conformations changed significantly. The average lengths (2.553-2.645 Å) of hydrogen bonds in confined (H₂O)_n are shorter than those (2.731-2.841 Å) in the corresponding free water clusters. Moreover, these confined hydrogen bonds show better hydrogen bond patterns according to the quantified indices. The natural bond orbital (NBO) calculations determine that there is electron transferring from the U₂₀ to its interior (H₂O)_n. It is the main reason for enhancing hydrogen bond interactions among the confined water molecules because their oxygen atoms are more negatively charged and their hydrogen atoms are more positively charged. The quantum theory of atoms in molecules (QTAIM) and interacting quantum atoms (IQA) analyses indicate that the confined hydrogen bonds are more covalent, based on the significant electron density ρ(r) and local energy density H(r) at the bond critical points (BCPs), and the stronger energies of interatomic exchange interactions (V_xc). These findings may help to promote the communication of confined water clusters and enrich the understating of confined hydrogen bonds.

Standalone 2-D Nanosheets and the Consequent Hydrogel and Coacervate Phases Formed by 2.5 nm Spherical U60 Molecular Clusters in Dilute Aqueous Solution

Unexpected hydrogel and coacervate are observed for dilute (1 mM) uranyl peroxide molecular cluster (Li₆₈K₁₂(OH)₂₀[UO₂(O₂)(OH)]₆₀, U₆₀) solution in the presence of di- or trivalent salts. We report the mechanism as the formation of anisotropic two-dimensional (2-D) single-layer nanosheets, driven by counterion-mediated attraction due to the size disparity between U₆₀ and small counterions. With weak monovalent cations, the nanosheets are bendable, resulting in hollow, spherical blackberry-type supramolecular assemblies in a homogeneous solution. With extra strong divalent or trivalent cations, the tough, free-standing sheets lead to gelation at ∼1 mM U₆₀. These stiff nanosheets are difficult to bend into spherical blackberry-type structures; instead, they stay in solution and form hydrogel based on their significant excluded volumes. At higher ionic strength, the large, thin filmlike nanosheet structures stack together more compactly and consequently lead to the transition from gel phase to a coacervate phase, another surprise since it was formed without the presence of bulky polycations.

Reactivity, Formation, and Solubility of Polyoxometalates Probed by Calorimetry

Room temperature calorimetry methods were developed to describe the energy landscapes of six polyoxometalates (POMs), Li-U24, Li-U28, K-U28, Li/K-U60, Mo132, and Mo154, in terms of three components: enthalpy of dissolution (ΔHdiss), enthalpy of formation of aqueous POMs (ΔHf,(aq)), and enthalpy of formation of POM crystals (ΔHf,(c)). ΔHdiss is controlled by a combination of cation solvation enthalpy and the favorability of cation interactions with binding sites on the POM. In the case of the four uranyl peroxide POMs studied, clusters with hydroxide bridges have lower ΔHf,(aq) and are more stable than those containing only peroxide bridges. In general for POMs, the combination of calorimetric results and synthetic observations suggest that spherical topologies may be more stable than wheel-like clusters, and ΔHf,(aq) can be accurately estimated using only ΔHf,(c) values owing to the dominance of the clusters in determining the energetics of POM crystals.

Organic Functionalization of Uranyl Peroxide Clusters to Impact Solubility

Benzene-1,2-diphosphonic acid (Ppb) was introduced into the uranyl peroxide cluster system, resulting in three Ppb-functionalized uranyl peroxide clusters, (UO₂)₂₀(O₂)₂₀(C₆H₄P₂O₆)₁₀^40- (U₂₀Ppb₁₀), (UO₂)₂₆(O₂)₃₃(C₆H₄P₂O₆)₆^38- (U₂₆Ppb₆), and (UO₂)₂₀(O₂)₂₄(C₆H₄P₂O₆)₆^32- (U₂₀Ppb₆). Dissolution experiments were performed for the potassium salts of U₂₀Ppb₁₀ and U₂₆Ppb₆, which revealed the capacity of U₂₀Ppb₁₀ to dissolve in the organic solvent dimethyl sulfoxide (DMSO). Unlike U₂₀Ppb₁₀, the K salt of U₂₆Ppb₆ did not dissolve in DMSO but was more soluble in water, perhaps due to the lower proportion of Ppb ligands in its structure. In this work, U₂₀Ppb₁₀ and U₂₀Ppb₆ formed as potassium salts and both adopt the fullerene topology of previously reported U₂₀. U₂₀ contains 20 uranyl peroxide units and encapsulates 12 Na cations. It is not possible for unfunctionalized U₂₀ to incorporate 12 K cations owing to space constraints, as is the case in the new clusters reported here. Transformation of U₂₀Ppb₁₀ in water over time to produce U₂₄ was observed, possibly owing to its ability to incorporate K cations, which have been associated with the formation of U₂₄.

Performance of group additivity methods for predicting the stability of uranyl complexes

Herein, we investigated the viability of two group additivity methods for predicting Gibbs energies of a set of uranyl complexes. In first place, we proved that both density functional theory (DFT)-based methods and Serezhkin's stereoatomic model provide equivalent answers in terms of stability. Moreover, we proposed a novel methodology based on Mayer's population analysis for estimating Serezhkin's empirical parameters theoretically. On the other hand, we showed that Cheong and Persson linear algebra methodology can be successfully applied to uranyl complexes, and analyzed its performance in connection with the chemical nature of the compounds employed in the model.

Dynamics of Cation-Induced Conformational Changes in Nanometer-Sized Uranyl Peroxide Clusters

Conformational changes of the pyrophosphate (Pp)-functionalized uranyl peroxide nanocluster [(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]^48- ({U₂₄Pp₁₂}), dissolved as a Li/Na salt, can be induced by the titration of alkali cations into solution. The most symmetric conformer of the molecule has idealized octahedral (O_h) molecular symmetry. One-dimensional ³¹P NMR experiments provide direct evidence that both K⁺ and Rb⁺ ions trigger an O_h-to-D_4h conformational change within {U₂₄Pp₁₂}. Variable-temperature ³¹P NMR experiments conducted on partially titrated {U₂₄Pp₁₂} systems show an effect on the rates; increased activation enthalpy and entropy for the D_4h-to-O_h transition is observed in the presence of Rb⁺ compared to K⁺. Two-dimensional, exchange spectroscopy ³¹P NMR revealed that magnetization transfer links chemically unique Pp bridges that are present in the D_4h conformation and that this magnetization transfer occurs via a conformational rearrangement mechanism as the bridges interconvert between two symmetries. The interconversion is triggered by the departure and reentry of K (or Rb) cations out of and into the cavity of the cluster. This rearrangement allows Pp bridges to interconvert without the need to break bonds. Cs ions exhibit unique interactions with {U₂₄Pp₁₂} clusters and cause only minor changes in the solution ³¹P NMR signatures, suggesting that O_h symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D_2h molecular symmetry, implying that while solvated, this cluster is in equilibrium with a more symmetric form. These results highlight the unusually flexible nature of the actinide-based {U₂₄Pp₁₂} and its sensitivity to countercations in solution.

The role of cations in uranyl nanocluster association: a molecular dynamics study

Actinyl ions can self-assemble in aqueous solution to form closed cage clusters ranging from 1.5 to 4.0 nm in diameter. The self-assembly, stability, and behavior of the nanoclusters depend on the nature of the aqueous environment, such as the pH and cations present. In this work, a classical force field for [(UO2)20(O2)30]20- (U20) peroxide nanoclusters in aqueous solution was developed from quantum-mechanical calculations. Using molecular dynamics simulations, the preferred binding sites of six cations (Li+, Na+, K+, Rb+, Cs+, and Ca2+) to the nanocluster were determined. Replica exchange molecular dynamics was used to equilibrate the structure and determine the equilibrium distribution of cations and water with respect to the nanocluster cage. In addition, the free energy barriers associated with cations entering the cluster were computed. Finally, the association of two cages was investigated by computing the free energy as a function of intercage distance. The free energy profiles reveal that the nanoclusters prefer to be associated when neutralized with divalent cations, but do not associate when neutralized with monovalent cations. This could explain the formation of tertiary structures observed experimentally.

Evidence for non-electrostatic interactions between a pyrophosphate-functionalized uranyl peroxide nanocluster and iron (hydr)oxide minerals

The terminal oxygen atoms of the pyrophosphate groups in the uranyl peroxide nanocluster U24Pp12 ([(UO2)24(O2)24(P2O7)12]48-) are not fully satisfied by bond valence considerations and can become protonated. This functionality could allow for specific interactions with mineral surfaces, as opposed to the electrostatically-driven interactions observed between non-functionalized uranyl peroxide nanoclusters and mineral surfaces. The sorption of U24Pp12 to goethite and hematite was studied using batch sorption experiments as a function of U24Pp12 concentration, mineral concentration, and pH. A suite of spectroscopic techniques, scanning electron microscopy, and electrophoretic mobility measurements were used to examine the minerals before and after reaction with U24Pp12, leading to a proposed conceptual model for U24Pp12 interactions with goethite. The governing rate laws were determined and compared to those previously determined for a non-functionalized uranyl peroxide nanocluster. The rate of uranyl peroxide nanocluster sorption depends on the charge density and functionalized component of the uranyl peroxide cage. Electrophoretic mobility and attenuated total reflectance Fourier transform infrared spectroscopy analyses show that an inner-sphere complex forms between the U24Pp12 cluster and the goethite surface through the terminal pyrophosphate groups, leading to a proposed conceptual model in which U24Pp12 interacts with the triply-coordinated reactive sites on the (110) plane of goethite. These results demonstrate that the behavior of U24Pp12 at the iron (hydr)oxide-water interface is unique relative to interactions of the uranyl ion and non-functionalized uranyl peroxide nanoclusters.

Calcium-Facilitated Aggregation and Precipitation of the Uranyl Peroxide Nanocluster U60 in the Presence of Na-Montmorillonite

The unique and diverse features of uranyl peroxide nanoclusters may contribute to the enhanced mobility of uranium in the environment. This study examines the sorption of the uranyl peroxide nanocluster [UO₂(O₂)(OH)]₆₀^60- (U₆₀) to Na-montmorillonite (SWy-2), plagioclase (anorthite), and quartz (SiO₂) as a function of time, U₆₀ concentration, and mineral concentration. SWy-2 was studied in both its untreated form as well as after two different pretreatments, denoted as partially treated SWy-2 and fully treated SWy-2. U₆₀ was removed (∼99%) from solution in the presence of untreated and partially treated SWy-2. However, U₆₀ was not removed from suspensions containing anorthite, quartz, or fully treated SWy-2, even after several months. The removal of U₆₀ from suspensions containing untreated SWy-2 is promoted in part by the exchange of Li⁺ counter-ions, normally weakly associated with U₆₀ in solution, for Ca²⁺ ions naturally present in the clay. In solution, Ca²⁺ ions induce the aggregation of nanoclusters, which precipitate on the surface of SWy-2. Ca-rich U₆₀ aggregates associated with SWy-2 were identified and characterized by scanning electron microscopy with energy dispersive spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. This research enhances our understanding of the molecular-scale processes controlling U₆₀ behavior at the mineral-water interface.

In situ Raman spectroscopy of uranyl peroxide nanoscale cage clusters under hydrothermal conditions

The behaviours of two uranyl peroxide nanoclusters in water heated to 180 °C were examined by in situ Raman spectroscopy.

Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U₆₀ concentration (0.5-2 g·L^-1), mineral concentration (100-500 m²·L^-1), and pH (8-10). The resulting rate law describing U₆₀ interactions with goethite at pH 9 was R = - k_rxn[U₆₀]^0.29±0.02[goethite]^1.2±0.1 where k_rxn = (6.7 ± 2.0) × 10^-4 (g·L^-1)^0.71±0.02(m²·L^-1)^-1.2±0.1(day^-1). The largest fraction of U₆₀ removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U₆₀ may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U₆₀ solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U₆₀ remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U₆₀ at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.

Uranyl dication mediated photoswitching of a calix[4]pyrrole-based metal coordination cage

A set of self-assembled tri- and tetrapodal metal coordination cage structures (cage-1 and cage-2, respectively) constructed from the uranyl dication (UO22+) and a dibenzoic acid functionalised cis-calix[4]pyrrole (1) are described. The inherent photochemical reactivity of the uranyl dication mediates the transformation of cage-1 to cage-2via the activation of molecular oxygen.

Uranyl Peroxide Nanocluster (U60) Persistence and Sorption in the Presence of Hematite

The presence of uranium-based nanomaterials in environmental systems may significantly impact our current understanding of the fate and transport of U(VI). Sorption of the uranyl peroxide nanocluster [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) to hematite (α-Fe₂O₃) was studied using batch sorption experiments with varying U₆₀, hematite, and alkali electrolyte (i.e., NaCl, KCl, and CsCl) concentrations. Data from electrospray ionization mass spectrometry and centrifugal microfiltration revealed that U₆₀ persisted in the presence of hematite and the background electrolyte for at least 120 days. K⁺ ions were removed from solution with uranium whereas Li⁺ ions remained in solution, indicating that the U₆₀ cluster behaved like an anion and that the Li⁺ ions did not play a significant role in the sorption mechanism. Analysis of the reacted mineral surface using X-ray photoelectron and Raman spectroscopies confirmed the presence of U(VI) and uranyl species with bridged peroxo groups associated with the mineral surface. These results indicate that uranyl peroxide nanoclusters may persist in the aqueous phase under environmentally relevant conditions for reasonably long periods of time, as compared to that of the uranyl cation.

Synthesis and structural characterization of a series of uranyl-betaine coordination complexes

Trimethylglycine and betainium class ionic liquids were employed in synthesizing six uranyl-betaine coordination complexes: [(μ2-(CH3)N2C3H3(CH2COO))2 (UO2(Cl)((CH3)N2C3H3(CH2COO)))2] 2[N(SO2CF3)2] (1), [K][UO2(Cl)3((CH3)3NCH2COO)] (2), [(CH3)3NCH2COOH][UO2(Cl)3((CH3)3NCH2COO)]⋅H2O (3), [LiUO2(μ2-(CH3)3NCH2COO)4] 3[N(SO2CF3)2] (4) {(μ2-(CH3)3NCH2COO)UO2(NO3)2((CH3)3NCH2COO)}2 UO2(NO3)2, UO2(NO3)2(H2O)2⋅H2O (5), and UO2(Cl)2(H2O)(μ2-(CH3)3NCH2COO) (6). These complexes expound upon the variability of monocarboxyl-functionalized uranyl coordination complexes, providing an enhanced framework for investigations into the structural chemistry of analogous actinyl systems.

Captivation with encapsulation: a dozen years of exploring uranyl peroxide capsules

Uranyl peroxide cages are an extensive family of topologically varied self-assembling nanoscale clusters with fascinating properties and applications.

Single-Crystal Time-of-Flight Neutron Diffraction and Magic-Angle-Spinning NMR Spectroscopy Resolve the Structure and 1H and 7Li Dynamics of the Uranyl Peroxide Nanocluster U60

Single-crystal time-of-flight neutron diffraction has provided atomic resolution of H atoms of H₂O molecules and hydroxyl groups, as well as Li cations in the uranyl peroxide nanocluster U₆₀. Solid-state magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy was used to confirm the dynamics of these constituents, revealing the transportation of Li atoms and H₂O through cluster walls. H atoms of hydroxyl units that are located on the cluster surface are involved in the transfer of H₂O and Li cations from inside to outside and vice versa. This exchange occurs as a concerted motion and happens rapidly even in the solid state. As a consequence of its large size and open hexagonal pores, U₆₀ exchanges Li cations more rapidly compared to other uranyl nanoclusters.