Solution and Solid-State Structural Chemistry of Actinide Hydrates and Their Hydrolysis and Condensation Products

Structural Snapshots of Cluster Growth from {U6 } to {U38 } During the Hydrolysis of UCl4

Herein we report the assembly of large uranium(IV) clusters with novel nuclearities and/or shapes from the controlled hydrolysis of UCl₄ in organic solution and in the presence of the benzoate ligands. {U₆ }, {U₁₃ }, {U₁₆ }, {U₂₄ }, {U₃₈ } oxo and oxo/hydroxo clusters were isolated and crystallographically characterized. These structural snapshots indicate that larger clusters are slowly built from the condensation of octahedral {U₆ } building blocks. The uranium/benzoate ligand ratio, the reaction temperature and the presence of base play an important role in determining the structure of the final assembly. Moreover, the isolation of different size cluster {U₆ } (few hours), {U₁₆ } (3 days), {U₂₄ } (21 days) from the same solution in a chosen set of conditions shows that the assembly of uranium oxo clusters in hydrolytic conditions is time dependent.

Mixed-valent neptunium oligomer complexes based on cation–cation interactions

Mixing Np(iv) and Np(v) (as neptunyl(v)) results in the formation of tri- and tetranuclear oligomer complexes based on cation–cation interactions (CCIs), indicating the potential of CCIs to expand the oligomer/cluster chemistry of actinides.

Metal-Ion Displacement Approach for Optical Recognition of Thorium: Application of a Molybdenum(VI) Complex for Nanomolar Determination and Enrichment of Th(IV)

An azine-based molybdenum (Mo(VI)) complex (M1) is exploited for selective detection of thorium (Th(IV)) ions through a metal-ion displacement protocol. Th(IV) displaces Mo(VI) from M1 instantly leading to the formation of the Th(IV) complex, having orange-red emission. Consequently, a red shift of the emission wavelength along with 41-fold fluorescence enhancement is observed. This unique method allows detection of Th(IV) as low as 1.5 × 10^-9 M. The displacement of Mo(VI) from M1 by Th(IV) is established by spectroscopic studies and kinetically followed by the stopped-flow technique. The displacement binding constant for Th(IV) is notably strong, 4.59 × 10⁶ M^-1. Extraction of Th(IV) from aqueous solution to the ethyl acetate medium using M1 has been achieved. The silica-immobilized M1 efficiently enriches Th(IV) from its reservoir through solid-phase extraction. Computational studies (density functional theory) support experimental findings.

Detection and identification of solids, surfaces, and solutions of uranium using vibrational spectroscopy

The purpose of this review is to provide an overview of uranium speciation using vibrational spectroscopy methods including Raman and IR. Uranium is a naturally occurring, radioactive element that is utilized in the nuclear energy and national security sectors. Fundamental uranium chemistry is also an active area of investigation due to ongoing questions regarding the participation of 5f orbitals in bonding, variation in oxidation states and coordination environments, and unique chemical and physical properties. Importantly, uranium speciation affects fate and transportation in the environment, influences bioavailability and toxicity to human health, controls separation processes for nuclear waste, and impacts isotopic partitioning and geochronological dating. This review article provides a thorough discussion of the vibrational modes for U(IV), U(V), and U(VI) and applications of infrared absorption and Raman scattering spectroscopies in the identification and detection of both naturally occurring and synthetic uranium species in solid and solution states. The vibrational frequencies of the uranyl moiety, including both symmetric and asymmetric stretches are sensitive to the coordinating ligands and used to identify individual species in water, organic solvents, and ionic liquids or on the surface of materials. Additionally, vibrational spectroscopy allows for the in situ detection and real-time monitoring of chemical reactions involving uranium. Finally, techniques to enhance uranium species signals with vibrational modes are discussed to expand the application of vibrational spectroscopy to biological, environmental, inorganic, and materials scientists and engineers.

Electronic Structure and Chemical Bonding of [AmO2(H2O) n ]2+/1

Systematic americyl-hydration cations were investigated theoretically to understand the electronic structures and bonding in [(AmO2)(H2O) n ]2+/1+ (n = 1-6). We obtained the binding energy using density functional theory methods with scalar relativistic and spin-orbit coupling effects. The geometric structures of these species have been investigated in aqueous solution via an implicit solvation model. Computational results reveal that the complexes of five equatorial water molecules coordinated to americyl ions are the most stable due to the enhanced ionic interactions between the AmO2 2+/1+ cation and multiple oxygen atoms as electron donors. As expected, Am-Owater bonds in such series are electrostatic in nature and contain a generally decreasing covalent character when hydration number increases.

Formation of a new type of uranium(iv) poly-oxo cluster {U38} based on a controlled release of water via esterification reaction

A new strategy for the synthesis of large poly-oxo clusters bearing 38 tetravalent uranium atoms {U38} has been developed by controlling the water release from the esterification reaction between a carboxylic acid and an alcohol. The molecular entity [U38O56Cl40(H2O)2(ipa)20]·(ipa) x (ipa = isopropanol) was crystallized from the solvothermal reaction of a mixture of UCl4 and benzoic acid in isopropanol at temperature ranging from 70 to 130 °C. Its crystal structure reveals the molecular assembly of the UO2 fluorite-like inner core {U14} with oxo groups bridging the uranium centers. The {U14} core is further surrounded by six tetrameric sub-units of {U4} to form the {U38} cluster. Its surface is decorated by either bridging- and terminal chloride anions or terminal isopropanol molecules. Another synthesis using the same reactant mixture at room temperature resulted in the crystallization of a discrete dinuclear complex [U2Cl4(bz)4(ipa)4]·(ipa)0.5 (bz = benzoate), in which each uranium center is coordinated by two chlorine atoms, four oxygen atoms from carboxylate groups and two additional oxygen atoms from isopropanol. The slow production of water released from the esterification of isopropanol allows the formation of the giant cluster with oxo bridges linking the uranium atoms at a temperature above 70 °C, whereas no such oxo groups are present in the dinuclear complex formed at room temperature. The kinetics of {U38} crystallization as well as the ester formation are analyzed and discussed. SAXS experiments indicate that the {U38} species are not dominant in the supernatant, but hexanuclear entities which are closely related to the [U6O8] type are formed.

Employing an Unsaturated Th4+ Site in a Porous Thorium-Organic Framework for Kr/Xe Uptake and Separation

Actinide based metal-organic frameworks (MOFs) are unique not only because compared to the transition-metal and lanthanide systems they are substantially less explored, but also owing to the uniqueness of actinide ions in bonding and coordination. Now a 3D thorium-organic framework (SCU-11) contains a series of cages with an effective size of ca. 21×24 Å. Th⁴⁺ in SCU-11 is 10-coordinate with a bicapped square prism coordination geometry, which has never been documented for any metal cation complexes. The bicapped position is occupied by two coordinated water molecules that can be removed to afford a very unique open Th⁴⁺ site, confirmed by X-ray diffraction, color change, thermogravimetry, and spectroscopy. The degassed phase (SCU-11-A) exhibits a Brunauer-Emmett-Teller surface area of 1272 m²  g^-1 , one of the highest values among reported actinide materials, enabling it to sufficiently retain water vapor, Kr, and Xe with uptake capacities of 234 cm³  g^-1 , 0.77 mmol g^-1 , 3.17 mmol g^-1 , respectively, and a Xe/Kr selectivity of 5.7.

A neptunium(v)-mediated interwoven transuranium-rotaxane network incorporating a mechanically interlocked [c2]daisy chain unit

A combination of an Np^V center and a cucurbituril-based pseudorotaxane ligand generates the first transuranium-rotaxane complex, NRCP-1, which has a mechanically-interlocked [c2]daisy chain unit.

{Np38} clusters: the missing link in the largest poly-oxo cluster series of tetravalent actinides

The poly-oxo clusters of neptunium, {Np₃₈}, fill the gap in the largest poly-oxo cluster series of tetravalent actinides.

Actinide-based MOFs: a middle ground in solution and solid-state structural motifs

In this review, we highlight how recent advances in the field of actinide structural chemistry of metal–organic frameworks (MOFs) could be utilized towards investigations relative to efficient nuclear waste administration, driven by the interest towards development of novel actinide-containing architectures as well as concerns regarding environmental pollution and nuclear waste storage.

Redox-mediated formation of plutonium oxide nanoparticles

Precipitates formed by the neutralisation of Pu(iii), Pu(iv), Pu(v), and Pu(vi) solutions were characterised by HRTEM, SAXS, and XRD in the suspensions. PuO₂ nanoparticles uniform in size (typical diameter around 2.5 nm) and phase composition were observed in all cases under equilibrium conditions.

Structural and magnetic susceptibility characterization of Pu(v) aqua ion using sonochemistry as a facile synthesis method

The facile sonochemical preparation of pure, stable and concentrated Pu(v) aqueous solutions allowed to investigate its solvation environment and magnetic properties.

Reductive activation of neptunyl and plutonyl oxo species with a hydroxypyridinone chelating ligand

Neptunyl(vi) and plutonyl(vi) oxo-activation with reduction to tetravalent hydroxides was investigated in gas and condensed phases, and by density functional theory.

Complexation of tetravalent uranium cations by the As4W40O140 cryptand

The polyanionic cryptand {As₄W₄₀O₁₄₀} was successfully used to bind up to four tetravalent uranium cations leading to the formation of three new cryptates. The obtained species appears to be stable in solution and the cryptand was used for U^IV/Nd^III separation studies.

Bottom-up synthesis of functionalized {Ce4(SiW9O34)2(l)2} polyoxometalates

Bottom-up synthesis allows the formation of four organic-functionalized Ce^IV containing polyoxometalates and one unprecedented polyanion stabilizing two different Ce^IV clusters.

A mesoporous cationic thorium-organic framework that rapidly traps anionic persistent organic pollutants

Many environmental pollutants inherently exist in their anionic forms and are therefore highly mobile in natural water systems. Cationic framework materials that can capture those pollutants are highly desirable but scarcely reported. Here we present a mesoporous cationic thorium-based MOF (SCU-8) containing channels with a large inner diameter of 2.2 nm and possessing a high surface area of 1360 m² g⁻¹. The anion-exchange properties of SCU-8 were explored with many anions including small oxo anions like ReO₄⁻ and Cr₂O₇²⁻ as well as anionic organic dyes like methyl blue and the persistent organic pollutant, perfluorooctane sulfonate (PFOS). Both fast uptake kinetics and great sorption selectivity toward PFOS are observed. The underlying sorption mechanism was probed using quantum mechanical and molecular dynamics simulations. These computational results reveal that PFOS anions are immobilized in SCU-8 by driving forces including electrostatic interactions, hydrogen bonds, hydrophobic interactions, and van der Waals interactions at different adsorption stages.

Cationic metal-organic frameworks provide promising opportunities to capture anionic pollutants, but stable frameworks with sufficiently large pores are lacking. Here the authors present a thorium-based mesoporous, cationic and hydrolytically-stable MOF that can rapidly trap inorganic and organic anionic pollutants.

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.

U(IV) Aqueous Speciation from the Monomer to UO2 Nanoparticles: Two Levels of Control from Zwitterionic Glycine Ligands

The fate of U(IV)O₂ 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 [U₆O₄(OH)₄(H₂O)₆(HGly)₁₂]·12Cl^-·12(H₂O) (U₆) were isolated at pH 2.15. The structure of U₆ is a hexanuclear oxo/hydroxo cluster U₆O₄(OH)₄ decorated by 12 glycine ligands and 6 water molecules. The effect of pH and temperature on U₆ conversion to UO₂ 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.

Coordination of Tetravalent Actinides (An=ThIV , UIV , NpIV , PuIV ) with DOTA: From Dimers to Hexamers

Three tetravalent actinide (An^IV ) hexanuclear clusters with the octahedral core [An₆ (OH)₄ O₄ ]¹²⁺ (An^IV =U^IV , Np^IV , Pu^IV ) were structurally characterized in the solid state and in aqueous solution by using single-crystal X-ray diffraction, X-ray absorption, IR, Raman, and UV/Vis spectroscopy. The observed structure, [An₆ (OH)₄ O₄ (H₂ O)₈ (HDOTA)₄ ]⋅HCl/HNO₃ ⋅n H₂ O (An=U(I), Np(II), Pu(III)), consists of a An^IV hexanuclear pseudo-octahedral cluster stabilized by DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) ligands. The six actinide atoms are connected through alternate μ₃ -O^2- and μ₃ -OH^- groups. Extended X-ray absorption fine structure (EXAFS) investigations combined with UV/Vis spectroscopy provide evidence for the same local structure in moderate acidic and neutral aqueous solutions. The synthesis mechanism was partially elucidated and the main physicochemical properties (pH range stability, solubility, and protonation constant) of the cluster were determined. The results underline the importance of: 1) considering such polynuclear species in thermodynamic models, and 2) competing reactions between hydrolysis and complexation. It is interesting to note that the same synthesis route with thorium(IV) led to the formation of a dimer, Th₂ (H₂ O)₁₀ (H₂ DOTA)₂ ⋅4 NO₃ ⋅x H₂ O (IV), which contrasts to the structure of the other An^IV hexamers.

Insights into the sonochemical synthesis and properties of salt-free intrinsic plutonium colloids

Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO₂ suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO₂ particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO₂ and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu L_III-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO₂ nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO₂ nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO₂ cores and hydrolyzed Pu(IV) moieties at the surface shell.