Uranyl Peroxide Clusters Research Articles

Formation of Uranyl Peroxide Compounds via Dissolution of Studtite, [(UO2)(O2)(H2O)2](H2O)2, in Ionic Liquids.

Four uranyl peroxide compounds with novel structures were formed following the dissolution of studtite, [(UO2)(O2)(H2O)2](H2O)2, in imidazolium-based ionic liquids. The compounds were characterized using single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD), Raman and infrared (IR) spectroscopy, and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The ionic liquids used in the experiments were 1-ethyl-3-methylimidazolium (EMIm) diethyl phosphate, EMIm ethyl sulfate, and EMIm acetate. Each of the four uranyl peroxide compounds contain components from the ionic liquids as terminal ligands on uranyl peroxide molecular units, bridging ligands in uranyl peroxide sheet structures, or charge balancing cations located in the interstitial space. The studtite dissolved in and reacted with the ionic liquids, producing unique crystal structures depending on the anionic component of the ionic liquid, the temperature at which the synthesis was performed, and the introduction of additional ionic species into the solution. This is the first report of studtite dissolving in and reacting with ionic liquids to form uranyl peroxide compounds, which has the potential to vastly increase the number of synthetic routes for the formation of uranyl peroxide clusters and uranyl peroxide cage clusters.

Electrochemical/computational analysis and the derivation of Pourbaix diagrams of redox-active structural and adsorbed species in uranyl peroxide clusters (U60)

Electrochemical analysis of U60 nanoclusters (Li40K20[UO2(O2)(OH)]60(H2O)214) and their natural analog, the mineral studtite (UO2)O2(H2O)2·(H2O)2, was carried out using cyclic voltammetry with a powder microelectrode setup. Voltammetric analysis was supplemented by ab initio quantum mechanical modeling to understand the mechanism, thermodynamics, and structural changes during redox transition (with a semi-quantitative kinetics evaluation). This research aims to better understand the redox behavior and transport of dissolved and structural (incorporated) uranyl units in the environment. Further analysis is performed to determine if redox switching of actinyl ions in clusters can be performed while leaving the cluster intact or at what redox transition it disintegrates. Quantum-mechanical calculations were applied to approximate electrochemical peak potential shifts and add them to standard reduction potentials to more accurately assign peaks to specific redox transitions between different species. These peak potentials are calculated from oxidation state-dependent binding energies between different uranyl species either within or adsorbed to U60. This theoretical approach incorporates extensive error cancellation and serves to predict electrochemical redox potentials and identify the reaction taking place. Voltammograms of U60 in electrolyte solutions exhibit kinetically-inhibited coupled redox peaks assigned to the U(VI)/U(V) transition of cluster structural uranyl units at −0.34 V (vs. standard hydrogen electrode), indicative of U60 reduction from UO22+ to UO2+. The U(V)/U(IV) transition at −0.71 V is assigned to the subsequent U60 reduction of UO2+ to UO20. A method of observing these transitions in situ using synchrotron x-ray absorption spectroscopy was explored. Voltammetry, modeling, and peak analysis indicate a kinetically-inhibited two-step, one-e- (per step) reduction of U60 from U(VI) to U(V) to U(IV). Voltammetry and x-ray absorption spectroscopy measurements performed before and after exhaustive cycling and the highly stable nature of U60 provide evidence that clusters typically do not break apart upon reduction. Voltammograms of U60 collected in varied uranyl solutions are indicative of UO22+ adsorption to the cluster surface and its subsequent reduction at +0.24 V (U(VI)/U(V)) and −0.05 V (U(V)/U(IV)). For uranyl solutions in electrolyte, the two-step, one-e- (per step) reduction of U60 is superseded by uranyl adsorption from solution to the electrode surface with subsequent reduction via U(V) disproportionation. This study represents the sole electrochemical investigation of U60 in the literature and only the second of any uranyl peroxide nanocluster. It is also one of the first to use quantum–mechanical approaches in combination with existing solution-based standard reduction potentials to calculate Pourbaix (EH-pH) diagrams for adsorbed and structural species.

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 (Ouranyl) 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 (Vxc). These findings may help to promote the communication of confined water clusters and enrich the understating of confined hydrogen bonds.

Capture and Stabilization of the Hydroxyl Radical in a Uranyl Peroxide Cluster.

Here we describe the synthesis and characterization of a new uranyl peroxide cluster (UPC), U60 Ox30 *, which captures and stabilizes oxygen-based free radicals for more than one week. These radical species were first detected with a nitroblue tetrazolium colorimetric assay and U60 Ox30 * was characterized by single crystal X-ray diffraction as well as infrared (IR), Raman, UV-Vis-NIR, and electron paramagnetic resonance (EPR) spectroscopies. Identification of the free radicals present in U60 Ox30 * was done via room temperature solid and solution state X-band EPR studies using spin trapping methods. The spin trapping agent 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was definitive for identifying the free radicals in U60 Ox30 *, which are hydroxyl radicals (⋅OH) that are stable for up to ten days that also persist upon addition of the metalloenzymes catalase and superoxide dismutase. Addition of the spin trapping agent α-(4-pyridyl N-oxide)-N-tert-butylnitrone (POBN) further confirmed the radicals were oxygen based, and deuteration experiments showed that the origin of the free radicals was from the decomposition of H2 O2 in water. These results demonstrate that highly oxidizing species such as the ⋅OH radical can be stabilized in UPCs, which alters our understanding of the role of free radicals present in spent nuclear fuel.

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)1040- (U20Ppb10), (UO2)26(O2)33(C6H4P2O6)638- (U26Ppb6), and (UO2)20(O2)24(C6H4P2O6)632- (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.

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 (Oh) molecular symmetry. One-dimensional 31P NMR experiments provide direct evidence that both K+ and Rb+ ions trigger an Oh-to-D4h 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 D4h-to-Oh 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 D4h 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 Oh symmetry is conserved. Single-crystal X-ray diffraction measurements reveal that the mixed Li/Na/Cs salt adopts D2h 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.

Effects of H2O2 Concentration on Formation of Uranyl Peroxide Species Probed by Dissolution of Uranium Nitride and Uranium Dioxide.

Dissolution of uranium materials in alkaline aqueous conditions containing H2O2 results in uranyl peroxide species in solution, including anionic uranyl peroxide cage clusters. Uranyl peroxide cage clusters are generally highly soluble in water, where they persist as aqueous macroanions. Previous studies indicate that uranyl cluster speciation and dissolution of uranium materials is impacted by the concentration of alkali metal in solution, but in these studies, high concentrations of H2O2 were used. Herein, the role of hydrogen peroxide concentration is examined relative to the dissolution of powdered UN and UO2. Lower initial H2O2 concentrations reduce dissolution of UO2 and UN and tend to produce simple (small) uranyl peroxide species rather the highly soluble uranyl peroxide clusters. H2O2 availability will have implications for uranyl speciation and solubility where spent nuclear fuel is in contact with water and where alkaline peroxide conditions are used in dissolution of nuclear material.

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

Aqueous solutions containing the nanoscale uranyl peroxide cage clusters U60, [(UO2)(O2)(OH)]6060-, and U60Ox30, [{(UO2)(O2)}60(C2O4)30]60-, were monitored by in situ Raman spectroscopy during stepwise heating to 180 °C. In solutions containing U60, clusters persist to 120 °C, although conversion of U60 to U24, [(UO2)(O2)(OH)]2424-, occurs above 100 °C. U60Ox30 persisted in solutions heated to 150 °C, although partial conversion to smaller uranyl peroxide clusters species was observed beginning at 100 °C. Upon breakdown of the uranyl peroxide cage clusters, uranium precipitated as a compreignacite-like phase, K2[(UO2)3O2(OH)3]2(H2O)7, and metaschoepite, [(UO2)8O2(OH)12](H2O)10. The role of the countercations, oxalate bridge, and solution pH are examined in order to better understand the mobility of these species at elevated temperatures.

Energetic Trends in Monomer Building Blocks for Uranyl Peroxide Clusters.

The uranyl triperoxide anionic monomer is a fundamental building block for uranyl peroxide polyoxometalate capsules. The reaction pathway from the monomer to the capsule can be greatly altered by the counterion: both the reaction rate and the resulting capsule structure. We synthesized and characterized uranyl triperoxides Mg2UO2(O2)3·13H2O (MgUT), Ca2UO2(O2)3·9H2O (CaUT), Sr2UO2(O2)3·9H2O (SrUT), and K4UO2(O2)3·3H2O (KUT) and compared their thermodynamic stabilities. The enthalpies of formation from oxides and elements of these compounds were calculated by thermochemical cycles from measurements by high temperature oxide melt drop solution calorimetry. Their formation enthalpies from oxides become more negative linearly as a function of the increasing basicity of the respective oxides on the Smith scale. This relationship holds for previously Li and Na analogues. Further affirming the trend, Δ Hf,ox of MgUT departs from linearity, due to the distinct bonding environment of Mg2+, as compared to the other alkalis and alkaline earths in the series.

Pyrophosphate and Methylenediphosphonate Incorporated Uranyl Peroxide Cage Clusters

Six uranyl peroxide cage clusters containing 18–54 uranyl ions, as well as 6–27 pyrophosphate or methylenediphosphonate ligands, have been synthesized and characterized. These uranyl peroxide clusters self-assemble and crystallize from aqueous solutions containing uranyl nitrate, hydrogen peroxide, and pyrophosphate or methylenediphosphonate over a pH range from 5.3 to 8.1. In their structures, uranyl hexagonal bipyramids are linked by bridging peroxide, hydroxide, and pyrophosphate or methylenedisphosphonate ligands. The incorporation of pyrophosphate or methylenedisphosphonate results in several interesting features in the coordination of uranyl ions and the topology of the uranyl peroxide clusters.

Charge Density Influence on Enthalpy of Formation of Uranyl Peroxide Cage Cluster Salts.

More than 60 unique uranyl peroxide cage clusters have been reported that contain as many as 124 uranyl ions and that have overall diameters extending to 4 nm. They self-assemble in water under ambient conditions, are models for understanding structure-size-property relations as well as testing computational models for actinides, and have potential applications in nuclear fuel cycles. High-temperature drop solution calorimetry has been used to derive the enthalpies of formation of the salts of seven topologically diverse uranyl peroxide cage clusters containing from 22 to 28 uranyl ions that are bridged by various combinations of peroxide, pyrophosphate, and phosphite. The enthalpies of formation of these seven salts, as well as three salts of other uranyl peroxide clusters reported earlier, are dominated by the interactions of the alkali countercations with the clusters. There is an approximately linear relationship between the enthalpies of formation of the cluster salts and the charge density of the corresponding uranyl peroxide cluster, wherein salts containing clusters with higher charge densities have more negative enthalpies of formation.

Complexity of Uranyl Peroxide Cluster Speciation from Alkali-Directed Oxidative Dissolution of Uranium Dioxide.

Solid UO2 dissolution and uranium speciation in aqueous solutions that promote formation of uranyl peroxide macroanions was examined, with a focus on the role of alkali metals. UO2 powders were dissolved in solutions containing XOH (X = Li, Na, K) and 30% H2O2. Inductively coupled plasma optical emission spectrometry (ICP-OES) measurements of solutions revealed linear trends of uranium versus alkali concentration in solutions resulting from oxidative dissolution of UO2, with X:U molar ratios of 1.0, showing that alkali availability determines the U concentrations in solution. The maximum U concentration in solution was 4.20 × 105 parts per million (ppm), which is comparable to concentrations attained by dissolving UO2 in boiling nitric acid, and was achieved by lithium hydroxide promoted dissolution. Raman spectroscopy and electrospray ionization mass spectrometry (ESI-MS) of solutions indicate that dissolution is accompanied by the formation of various uranyl peroxide cluster species, the identity of which is alkali concentration dependent, revealing remarkably complex speciation at high concentrations of base.

Front Cover: Cation‐Directed Isomerization of the U28 Uranyl‐Peroxide Cluster (Eur. J. Inorg. Chem. 46/2017)

Front Cover: Cation‐Directed Isomerization of the U₂₈ Uranyl‐Peroxide Cluster (Eur. J. Inorg. Chem. 46/2017)

Uranyl-Peroxide Clusters Incorporating Iron Trimers and Bridging by Bisphosphonate- and Carboxylate-Containing Ligands.

A hybrid uranium-iron cage nanocluster, [(UO2)24(FeOH)24(O2)24(PO4)8(CH(COO)(PO3)2)24]96- (U24Fe24), was synthesized using bridging ligands containing bisphosphonate and carboxylate groups. U24Fe24 contains six tetramers of uranyl hexagonal bipyramids and eight iron trimers, each of which consists of three corner-sharing Fe3+ octahedra and is stabilized by in situ formed phosphate and 2,2-bis(phosphonato)acetate (C2P2) groups. Tetramers and trimers are bridged by 24 C2P2 groups into a cage cluster. Crystals of U24Fe24 present a paramagnetic-like behavior. X-ray scattering showed that U24Fe24 forms in the reactant solution prior to crystallization and is stable upon dissolution in water.

Benchmarking Uranyl Peroxide Capsule Chemistry in Organic Media

AbstractInvited for the cover of this issue is research led by May Nyman at Oregon State University in collaboration with scientists at the University of Notre Dame, Indiana, USA. The cover image depicts the transfer of uranyl peroxide clusters from an aqueous phase to an organic phase in a nuclear power plant cooling tower, showing how the fundamental science of actinides is related to the industrial nuclear fuel cycle.

Uranyl peroxide clusters stabilized by dicarboxylate ligands: A pentagonal ring and a dimer with extensive uranyl–cation interactions

Great effort has been made to synthesize larger uranyl peroxide clusters whilst the understanding of the basic construction units for these clusters still needs to be further explored. Two small uranyl peroxide clusters stabilized by simple dicarboxylate ligands have been synthesized and characterized. Na10[(UO2)(O2)(C3H2O4)]5·20H2O (1) has a pentagonal ring stabilized by malonate and [Na4K2(UO2)2(O2)(C2O4)4]·6H2O (2) has a dimer stabilized by oxalate. Cation-cation interactions between uranyl and alkali metal ions exist in both 1 and 2, leading to the observation of elongated UO−yl bonds. Vibrational modes of uranyl ion, peroxide and malonate/oxalate ligands have been confirmed by Raman spectroscopy. The presence of malonate ligands prevents the pentagonal uranyl peroxide rings further assembling into larger clusters whilst the excess oxalate ligands inhibit the dimers further linking to nearby uranyl centers.

The Energy Landscape of Uranyl-Peroxide Species

Nanoscale uranyl peroxide clusters containing UO2(2+) groups bonded through peroxide bridges to form polynuclear molecular species (polyoxometalates) exist both in solution and in the solid state. There is an extensive family of clusters containing 28 uranium atoms (U28 clusters), with an encapsulated anion in the center, for example, [UO2(O2)(3-x)(OH)(x)(4-)], [Nb(O2)4(3-)], or [Ta(O2)4(3-)]. The negative charge of these clusters is balanced by alkali ions, both encapsulated, and located exterior to the cluster. The present study reports measurement of enthalpy of formation for two such U28 compounds, one of which is uranyl centered and the other is peroxotantalate centered. The [(Ta(O2)4]-centered U28 capsule is energetically more stable than the [(UO2)(O2)3]-centered capsule. These data, along with our prior studies on other uranyl-peroxide solids, are used to explore the energy landscape and define thermochemical trends in alkali-uranyl-peroxide systems. It was suggested that the energetic role of charge-balancing alkali ions and their electrostatic interactions with the negatively charged uranyl-peroxide species is the dominant factor in defining energetic stability. These experimental data were supported by DFT calculations, which agree that the [(Ta(O2)4]-centered U28 capsule is more stable than the uranyl-centered capsule. Moreover, the relative stability is controlled by the interactions of the encapsulated alkalis with the encapsulated anion. Thus, the role of alkali-anion interactions was shown to be important at all length scales of uranyl-peroxide species: in both comparing clusters to clusters; and clusters to monomers or extended solids.

Raman Spectroscopic and ESI-MS Characterization of Uranyl Peroxide Cage Clusters

Strategies for interpreting mass spectrometric and Raman spectroscopic data have been developed to study the structure and reactivity of uranyl peroxide cage clusters in aqueous solution. We demonstrate the efficacy of these methods using the three best-characterized uranyl peroxide clusters, {U24}, {U28}, and {U60}. Specifically, we show a correlation between uranyl-peroxo-uranyl dihedral bond angles and the position of the Raman band of the symmetric stretching mode of the peroxo ligand, develop methods for the assignment of the ESI mass spectra of uranyl peroxide cage clusters, and show that these methods are generally applicable for detecting these clusters in the solid state and solution and for extracting information about their bonding and composition without crystallization.

Ultrafiltration of Uranyl Peroxide Nanoclusters for the Separation of Uranium from Aqueous Solution

Uranyl peroxide cluster species were produced in aqueous solution by the treatment of uranyl nitrate with hydrogen peroxide, lithium hydroxide, and potassium chloride. Ultrafiltration of these cluster species using commercial sheet membranes with molecular mass cutoffs of 3, 8, and 20 kDa (based on polyethylene glycol) resulted in U rejection values of 95, 85, and 67% by mass, respectively. Ultrafiltration of untreated uranyl nitrate solutions using these membranes resulted in virtually no rejection of U. These results demonstrate the ability to use the filtration of cluster species as a means for separating U from solutions on the basis of size. Small-angle X-ray scattering, Raman spectroscopy, and electrospray ionization mass spectrometry confirmed the presence of uranyl peroxide cluster species in solution and were used to characterize their size, shape, and dispersity.

Dynamics of a Nanometer-Sized Uranyl Cluster in Solution

A class of uranyl peroxide clusters was discovered before as nanometer-sized ions that form spontaneously in aqueous solutions. The uranyl(VI) cluster investigated here is approximately 2 nm in diameter, contains 24 uranyl moieties, and 12 pyrophosphate units. NMR spectroscopy shows that the ion has two distinct forms that interconvert in milliseconds to seconds depending on the temperature and the size of the counterions. P blue, O red, U yellow.