Plutonium(IV) Cluster with a Hexanuclear [Pu6(OH)4O4]12+ Core

Mining for Metal-Organic Systems: Chemistry Frontiers of Th-, U-, and Zr-Materials

The conceptual framework presented in this Perspective overviews the design principles of innovative thorium-based materials that could address urgent needs of the medicinal, nuclear energy, and waste remediation sectors from the lens of zirconium and uranium analogs. We survey the intersections of Zr, Th, and U chemistry with a focus on how the intrinsic behavior of each metal translates to broader material properties, including, but not limited to, structural and topological diversity, preferential metal-ligand binding, and reactivity. On the example of several classes of materials, including organometallic complexes, polyoxometalates, and the primary focus of this Perspective, metal-organic frameworks (MOFs), the design principles that govern the preparation of Zr-, Th-, and U-compounds, including oxophilicity, variation in oxidation states, and stable coordination environments have been considered. Further, we highlight how the impact of the mentioned variables may shift throughout the progression from discrete molecular systems to extended structures. We discuss the common assumption that zirconium-organic materials are typically considered a close analog of thorium-based congeners in areas such as material design and preparation. Through consideration of fundamental chemistry principles, we shed light on the relationships between Zr-, Th-, and U-based materials and highlight how a critical analysis of their distinct properties can be used to target a desired material performance. As a result, we provide a detailed understanding of Th-based materials chemistry by anchoring their fundamental properties between two well-studied reference points, zirconium- and uranium-containing analogs.

Synthesis and Characterization of Cerium-Oxo Clusters Capped by Acetylacetonate

Cerium-oxo clusters have applications in fields ranging from catalysis to electronics and also hold the potential to inform on aspects of actinide chemistry. Toward this end, a cerium-acetylacetonate (acac1-) monomeric molecule, Ce(acac)4 (Ce-1), and two acac1--decorated cerium-oxo clusters, [Ce10O8(acac)14(CH3O)6(CH3OH)2]·10.5MeOH (Ce-10) and [Ce12O12(OH)4(acac)16(CH3COO)2]·6(CH3CN) (Ce-12), were prepared and structurally characterized. The Ce(acac)4 monomer contains CeIV. Crystallographic data and bond valence summation values for the Ce-10 and Ce-12 clusters are consistent with both clusters having a mixture of CeIII and CeIV cations. Ce L3-edge X-ray absorption spectroscopy, performed on Ce-10, showed contributions from both CeIII and CeIV. The Ce-10 cluster is built from a hexameric cluster, with six CeIV sites, that is capped by two dimeric CeIII units. By comparison, Ce-12, which formed upon dissolution of Ce-10 in acetonitrile, consists of a central decamer built from edge sharing CeIV hexameric units, and two monomeric CeIII sites that are bound on the outer corners of the inner Ce10 core. Electrospray ionization mass spectrometry data for solutions prepared by dissolving Ce-10 in acetonitrile showed that the major ions could be attributed to Ce10 clusters that differed primarily in the number of acac1-, OH1-, MeO1-, and O2- ligands. Small angle X-ray scattering measurements for Ce-10 dissolved in acetonitrile showed structural units slightly larger than either Ce10 or Ce12 in solution, likely due to aggregation. Taken together, these results suggest that the acetylacetonate supported clusters can support diverse solution-phase speciation in organic solutions that could lead to stabilization of higher order cerium containing clusters, such as cluster sizes that are greater than the Ce10 and Ce12 reported herein.

Expanding the Coordination of f-Block Metals with Tris[2-(2-methoxyethoxy)ethyl]amine: From Molecular Complexes to Cage-like Structures

Low-valent f-block metals have intrinsic luminescence, electrochemical, and magnetic properties that are modulated with ligands, causing the coordination chemistry of these metals to be imperative to generating critical insights needed to impact modern applications. To this end, we synthesized and characterized a series of twenty-seven complexes of f-metal ions including EuII, YbII, SmII, and UIII and hexanuclear clusters of LaIII and CeIII to study the impact of tris[2-(2-methoxyethoxy)ethyl]amine, a flexible acyclic analogue of the extensively studied 2.2.2-cryptand, on the coordination chemistry and photophysical properties of low-valent f-block metals. We demonstrate that the flexibility of the ligand enables luminescence tunability over a greater range than analogous cryptates of EuII in solution. Furthermore, the ligand also displays a variety of binding modes to f-block metals in the solid state that are inaccessible to cryptates of low-valent f-block metals. In addition to serving as a ligand for f-block metals of various sizes and oxidation states, tris[2-(2-methoxyethoxy)ethyl]amine also deprotonates water molecules coordinated to trivalent triflate salts of f-block metal ions, enabling the isolation of hexanuclear clusters containing either LaIII or CeIII. The ligand was also found to bind more tightly to YbII and UIII in the solid state compared to 2.2.2-cryptand, suggesting that it can play a role in the isolation of other low-valent f-block metals such CfII, NpIII, and PuIII. We expect that our findings will inspire applications of tris[2-(2-methoxyethoxy)ethyl]amine in the design of light-emitting diodes and the synthesis of extremely reducing divalent f-block metal complexes that are of interest for a wide range of applications.

Investigation of the Plutonium(IV) Interactions with Two Variants of the EF-Hand Ca-Binding Site I of Calmodulin

Due to its presence in the nuclear industry and its strong radiotoxicity, plutonium is an actinide of major interest in the event of internal contamination. To improve the understanding of its mechanisms of transport and accumulation in the body, the complexation of Pu(IV) to the most common protein calcium-binding motif found in cells, the EF-hand motif of calmodulin, was investigated. Visible and X-ray absorption spectroscopies (XAS) in solution made it possible to investigate the speciation of plutonium at physiological pH (pH 7.4) and pH 6 in two variants of the calmodulin Ca-binding site I and using Pu(IV) in different media: carbonate, chloride, or nitrate solutions. Three different species of Pu were identified in the samples, with formation of 1:1 Pu(IV):calmodulin peptide complexes, Pu(IV) reduction, and formation of peptide-mediated Pu(IV) hexanuclear cluster.

Metal-Oxo Cluster Formation Using Ammonium and Sulfate to Differentiate MIV (Th, U, Ce) Chemistries

Isolating isostructural compounds of tetravalent metals M^IV (Zr, Hf, Ce, Th, U, Pu, Np) improves our understanding of metal hydrolysis and coordination behavior across the periodic table. These metals form polynuclear clusters typified by the hexamer [M^IV₆O₄(OH)₄]¹²⁺. Exploiting the ammonium M^IV-sulfate (Ce^IV, Th^IV, and U^IV) phase space targeting rapid crystallization, we isolate the common hexamer [M^IV₆(OH)₄(O)₄]¹²⁺ but with different numbers of capping sulfates and water molecules for Ce^IV, Th^IV, and U^IV. These phases allowed a direct comparison of bonding trends across the series. Upon cocrystallization with the hexamers, higher complex structures can be identified. Thorium features assemblies with monomer-linked hexamer chains. Uranium features assemblies with sulfate-bridged hexamers and the supramolecular assembly of 14 hexamers into the U₈₄, [U₆(OH)₄(O)₄)₁₄(SO₄)₁₂₀(H₂O)₄₂]^72-. Last, cerium showcases the isolation from monomers to the Ce₆₂, [Ce₆₂(OH)₃₀(O)₅₈(SO₄)₇₁(H₂O)_33.25]^41-. Furthermore, small-angle X-ray scattering (room temperature) shows ammonium-induced cluster assembly for Ce^IV but minimal reactivity for U^IV and Th^IV. In this study, because the phases crystallized at elevated temperature demonstrates favorable cluster assembly, these solution phase results were surprising and suggest some other characteristics such as Ce's facile redox behavior, contributes to its solution-phase speciation.

Synthesis and multi-scale properties of PuO2 nanoparticles: recent advances and open questions

Due to the increased attention given to actinide nanomaterials, the question of their structure-property relationship is on the spotlight of recent publications. Plutonium oxide (PuO₂) particularly plays a central role in nuclear energetics and a comprehensive knowledge about its properties when nanosizing is of paramount interest to understand its behaviour in environmental migration schemes but also for the development of advanced nuclear energy systems underway. The element plutonium further stimulates the curiosity of scientists due to the unique physical and chemical properties it exhibits around the periodic table. PuO₂ crystallizes in the fluorite structure of the face-centered cubic system for which the properties can be significantly affected when shrinking. Identifying the formation mechanism of PuO₂ nanoparticles, their related atomic, electronic and crystalline structures, and their reactivity in addition to their nanoscale properties, appears to be a fascinating and challenging ongoing topic, whose recent advances are discussed in this review.

Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands

Speciation of hexachloride tetravalent uranium, neptunium, and plutonium species in aqueous media has been investigated using density functional theory in the presence of inner sphere ligands such as chloride, nitrate, and solvent molecules. All possible structures with the formula [An^IV(Cl)_x(H₂O)_y(NO₃)_z]^4-x-z (An = U, Np, and Pu; x = 0-6, y = 0-8, and z = 0-6) were considered to explore the speciation chemical space of each actinide. The nature of the mixed-ligand complexes present in solution is controlled by the concentration of free ligands in solution. A low chloride concentration is suitable to drive the speciation away from the highly thermodynamically stable hexachloride species. Furthermore, the formation of dimeric species can proceed through both olation and oxolation mechanisms. Oxolation is preferred for monomers that contain fewer water ligands, while olation becomes favorable for complexes with more water ligands.

Leveraging Nitrogen Linkages in the Formation of a Porous Thorium-Organic Nanotube Suitable for Iodine Capture

We report the synthesis, characterization, and iodine capture application of a novel thorium-organic nanotube, TSN-626, [Th₆O₄(OH)₄(C₆H₄NO₂)₇(CHO₂)₅(H₂O)₃]·3H₂O. The classification as a metal-organic nanotube (MONT) distinguishes it as a rare and reduced dimensionality subset of metal-organic frameworks (MOFs); the structure is additionally hallmarked by low node connectivity. TSN-626 is composed of hexameric thorium secondary building units and mixed O/N-donor isonicotinate ligands that demonstrate selective ditopicity, yielding both terminating and bridging moieties. Because hard Lewis acid tetravalent metals have a propensity to bind with electron donors of rival hardness (e.g., carboxylate groups), such Th-N coordination in a MOF is uncommon. However, the formation of key structural Th-N bonds in TSN-626 cap some of the square antiprismatic metal centers, a position usually occupied by terminal water ligands. TSN-626 was characterized by using complementary analytical and computational techniques: X-ray diffraction, vibrational spectroscopy, N₂ physisorption isotherms, and density functional theory. TSN-626 satisfies design aspects for the chemisorption of iodine. The synergy between accessibility through pores, vacancies at the metal-oxo nodes, and pendent N-donor sites allowed a saturated iodine loading of 955 mg g^-1 by vapor methods. The crystallization of TSN-626 diversifies actinide-MOF linker selection to include soft electron donors, and these Th-N linkages can be leveraged for the investigation of metal-to-ligand bonding and unconventional topological expressions.

To form or not to form: PuO2 nanoparticles at acidic pH

The aim of this study is to synthesize PuO2 nanoparticles (NPs) at low pH values and characterize the materials using laboratory and synchrotron-based methods. Properties of the PuO2 NPs formed under acidic conditions (pH 1-4) are explored here at the atomic scale. High-resolution transmission electron microscopy (HRTEM) is applied to characterize the crystallinity, morphology and size of the particles. It is found that 2 nm crystalline NPs are formed with a PuO2 crystal structure. High energy resolution fluorescence detected (HERFD) X-ray absorption spectroscopy at the Pu M4 edge has been used to identify the Pu oxidation states and recorded data are analysed using the theory based on the Anderson impurity model (AIM). The experimental data obtained on NPs show that the Pu(iv) oxidation state dominates in all NPs formed at pH 1-4. However, the suspension at pH 1 demonstrates the presence of Pu(iii) and Pu(vi) in addition to the Pu(iv), which is associated with redox dissolution of PuO2 NPs under acidic conditions. We discuss in detail the mechanism that affects the PuO2 NPs synthesis under acidic conditions and compare it with one in neutral and alkaline conditions. Hence, the results shown here, together with the first Pu M4 HERFD data on PuF3 and PuF4 compounds, are significant for the colloid facilitated transport governing the migration of plutonium in a subsurface environment.

Characterization of a Hexanuclear Plutonium(IV) Nanostructure in an Acetate Solution via Visible-Near Infrared Absorption Spectroscopy, Extended X-ray Absorption Fine Structure Spectroscopy, and Density Functional Theory

A new hexanuclear plutonium cluster has been stabilized in aqueous media with acetate ligands. To probe the formation of such a complex structure, visible-near infrared (vis-NIR) absorption spectroscopy, extended X-ray absorption fine structure (EXAFS) spectroscopy, and density functional theory (DFT) were combined. The presence of Pu₆O₄(OH)₄(CH₃COO)₁₂ species in solution was first detected by vis-NIR and EXAFS spectroscopy. To confirm unambiguously this structure, EXAFS spectra were simulated from ab initio calculations. Debye-Waller factors and structural parameters were derived from DFT calculations. A large number of 5f electrons were treated as valence or core electrons using small- and large-core relativistic effective pseudopotentials. It is possible to reproduce accurately the EXAFS spectrum of the octahedral hexamer cluster at both levels of calculations. Further DFT and EXAFS calculations were performed on clusters of lower or higher nuclearities and of different geometries using the 5f-core approximation. The result shows that trimer, tetramer, flat hexamer, and even 16-mer clusters exhibit different EXAFS patterns and confirm the very specific octahedral hexanuclear EXAFS signature.

Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis

The size and shape of a water-soluble hexanuclear plutonium cluster were probed by combining synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS). A specific setup coupling both techniques and dedicated to radioactive samples on the MARS beamline endstation at Synchrotron SOLEIL is described. The plutonium hexanuclear cores are well stabilized by the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid ligands and this allows a good evaluation of the setup to probe the very small plutonium core. The results show that, in spite of the constrained conditions required to avoid any risk of sample dispersion, the flux and the sample environment are optimized to obtain a very good signal-to-noise ratio, allowing the detection of small plutonium aggregates in an aqueous phase. The structure of the well defined hexanuclear cluster has been confirmed by EXAFS measurements in solution and correlated with SAXS data processing and modelling. An iterative comparison of classical fit models (Guinier or sphere form factor) with the experimental results allowed a better interpretation of the SAXS signal that will be relevant for future work under environmentally relevant conditions.

Beyond structural motifs: the frontier of actinide-containing metal-organic frameworks

In this perspective, we feature recent advances in the field of actinide-containing metal-organic frameworks (An-MOFs) with a main focus on their electronic, catalytic, photophysical, and sorption properties. This discussion deviates from a strictly crystallographic analysis of An-MOFs, reported in several reviews, or synthesis of novel structural motifs, and instead delves into the remarkable potential of An-MOFs for evolving the nuclear waste administration sector. Currently, the An-MOF field is dominated by thorium- and uranium-containing structures, with only a few reports on transuranic frameworks. However, some of the reported properties in the field of An-MOFs foreshadow potential implementation of these materials and are the main focus of this report. Thus, this perspective intends to provide a glimpse into the challenges, triumphs, and future directions of An-MOFs in sectors ranging from the traditional realm of gas sorption and separation to recently emerging areas such as electronics and photophysics.

CeIV 70 Oxosulfate Rings, Frameworks, Supramolecular Assembly, and Redox Activity*

M^IV molecular oxo-clusters (M=Zr, Hf, Ce, Th, U, Np, Pu) are prolific in bottoms-up material design, catalysis, and elucidating reaction pathways in nature and in synthesis. Here we introduce Ce₇₀ , a wheel-shaped oxo-cluster, [Ce^IV ₇₀ (OH)₃₆ (O)₆₄ (SO₄ )₆₀ (H₂ O)₁₀ ]^4- . Ce₇₀ crystallizes into intricate high pore volume frameworks with divalent transition metals and Ce-monomer linkers. Eight crystal-structures feature four framework types in which the Ce₇₀ -rings are linked as propellers, in offset-stacks, in a tartan pattern, and as isolated rings. Small-angle X-ray scattering of Ce₇₀ dissolved in butylamine, with and without added cations (Ce^IV , alkaline earths, Mn^II ), shows the metals' differentiating roles in ring linking, leading to supramolecular assemblies. The large acidic pores and abundant terminal sulfates provide ion-exchange behavior, demonstrated with U^IV and Nd^III . Frameworks featuring Ce^III/IV -monomer linkers demonstrate both oxidation and reduction. This study opens the door to mixed-metal, highly porous framework catalysts, and new clusters for metal-organic framework design.

Decisive Role of 5f-Orbital Covalence in the Structure and Stability of Pentavalent Transuranic Oxo [M6O8] Clusters

Actinide metal oxo clusters are of vital importance in actinide chemistry, as well as in environmental and materials sciences. They are ubiquitous in both aqueous and nonaqueous phases and play key roles in nuclear materials (e.g., nuclear fuel) and nuclear waste management. Despite their importance, our structural understanding of the actinide metal oxo clusters, particularly the transuranic ones, is very limited because of experimental challenges such as high radioactivity. Herein we report a systematic theoretical study on the structures and stabilities of seven actinide metal oxo-hydroxo clusters [An^IV₆O₄(OH)₄L₁₂] (1-An; An = Th-Cm; L = O₂CH^-) along with their group 4 (Ti, Zr, Hf, Rf) and lanthanide (Ce) counterparts [M^IV₆O₄(OH)₄L₁₂] (1-M). The work shows the T_d-symmetric structures of all of the 1-An/M clusters and suggests the positions of the -OH functional groups, which are experimentally challenging to determine. Furthermore, by removing six electrons from 1-An, we found that oxidation could happen on the An^IV metal ions, producing [An^V₆O₄(OH)₄L₁₂]⁶⁺ (2-An; An = Pa, U, Np), or on the O^2- and OH^- ligands, producing [An^IV₆(O^•-)₄(OH^•)₂(OH)₂L₁₂]⁶⁺ (3-An; An = Pu, Am, Cm). On the basis of 2-An, we constructed a series of tetravalent and pentavalent actinide metal oxo clusters [An^IV₆O₁₄]^4- (4-An) and [An^V₆O₁₄]²⁺ (5-An), which proves the feasibility of the highly important pentavalent actinyl clusters, demonstrates the f orbital's structure-directing role in the formation of linear [O≡An^V═O]⁺ actinyl ions, and expands the concept of actinyl-actinyl interaction into pentavalent transuranic actinyl clusters.

Bridging the Transuranics with Uranium(IV) Sulfate Aqueous Species and Solid Phases

Isolating isomorphic compounds of tetravalent actinides (i.e., Th^IV, U^IV, Np^IV, and Pu^IV) improve our understanding of the bonding behavior across the series, in addition to their relationship with tetravalent transition metals (Zr and Hf) and lanthanides (Ce). Similarities between these tetravalent metals are particularly illuminated in their hydrolysis and condensation behavior in aqueous systems, leading to polynuclear clusters typified by the hexamer [M^IV₆O₄(OH)₄]¹²⁺ building block. Prior studies have shown the predominance and coexistence of smaller species for Th^IV (monomers, dimers, and hexamers) and larger species for U^IV, Np^IV, and Pu^IV (including 38-mers and 70-mers). We show here that aqueous uranium(IV) sulfate also displays behavior similar to that of Th^IV (and Zr^IV) in its isolated solid-phase and solution speciation. Two single-crystal X-ray structures are described: a dihydroxide-bridged dimer (U₂) formulated as U₂(OH)₂(SO₄)₃(H₂O)₄ and a monomer-linked hexamer framework (U-U₆) as (U(H₂O)_3.5)₂U₆O₄(OH)₄(SO₄)₁₀(H₂O)₉. These structures are similar to those previously described for Th^IV. Moreover, cocrystallization of monomer and dimer and of dimer and monomer-hexamer phases for both Th^IV (prior) and U^IV (current) indicates the coexistence of these species in solution. Because it was not possible to effectively study the sulfate-rich solutions via X-ray scattering from which U₂ and U-U₆ crystallized, we provide a parallel solution speciation study in low sulfate conditions, as a function of the pH. Raman spectroscopy, UV-vis spectroscopy, and small-angle X-ray scattering of these show decreasing sulfate binding, increased hydrolysis, increased species size, and increased complexity, with increasing pH. This study describes a bridge across the first half the actinide series, highlighting U^IV similarities to Th^IV, in addition to the previously known similarities to the transuranic elements.

The missing pieces of the PuO2 nanoparticle puzzle

The nanoscience field often produces results more mystifying than any other discipline. It has been argued that changes in the plutonium dioxide (PuO₂) particle size from bulk to nano can have a drastic effect on PuO₂ properties. Here we report a full characterization of PuO₂ nanoparticles (NPs) at the atomic level and probe their local and electronic structures by a variety of methods available at the synchrotron, including extended X-ray absorption fine structure (EXAFS) at the Pu L₃ edge, X-ray absorption near edge structure (XANES) in high energy resolution fluorescence detection (HERFD) mode at the Pu L₃ and M₄ edges, high energy X-ray scattering (HEXS) and X-ray diffraction (XRD). The particles were synthesized from precursors with different oxidation states of plutonium (III, IV, and V) under various environmentally and waste storage relevant conditions (pH 8 and pH > 10). Our experimental results analyzed with state-of-the-art theoretical approaches demonstrate that well dispersed, crystalline NPs with a size of ∼2.5 nm in diameter are always formed in spite of diverse chemical conditions. Identical crystal structures and the presence of only the Pu(iv) oxidation state in all NPs, reported here for the first time, indicate that the structure of PuO₂ NPs is very similar to that of the bulk PuO₂. All methods give complementary information and show that investigated fundamental properties of PuO₂ NPs, rather than being exotic, are very similar to those of the bulk PuO₂.

Experimental and Quantum Mechanical Characterization of an Oxygen-Bridged Plutonium(IV) Dimer

We report the synthesis and characterization of K₄ {[PuCl₂ (NO₃ )₃ ]₂ (μ₂ -O)}⋅H₂ O, which contains the first known μ₂ -oxo bridge between two Pu^IV metal centers. Adding to its uniqueness is the Pu-(μ₂ -O) bond length of 2.04 Å, which is the shortest of other analogous compounds. The Pu-(μ₂ -O)-Pu bridge is characterized by the mixing of s-, d-, and p-orbitals from Pu with the p-orbitals of O; the 5f-orbitals do not participate in bonding. Natural bond orbital analysis indicates that Pu and O interact through one 3c-2e σ_Pu-O-Pu and two 3c-2e π_Pu-O-Pu bonding orbitals and that the electron density is highly polarized on the μ₂ -O. Bond topology properties analysis indicates that the Pu-(μ₂ -O) bond shares both ionic and covalent character. Quantum mechanical calculations also show that the dimer has multiconfigurational ground states, where the nonet, septet, quintet, triplet, and singlet are close in energy. This work demonstrates the interplay between experimental and computational efforts that is required to understand the chemical bonding of Pu compounds.

Advancement of Actinide Metal-Organic Framework Chemistry via Synthesis of Pu-UiO-66

We report the synthesis and characterization of the first plutonium metal-organic framework (MOF). Pu-UiO-66 expands the established UiO-66 series, which includes transition metal, lanthanide, and early actinide elements in the hexanuclear nodes. The thermal stability and porosity of Pu-UiO-66 were experimentally determined, and multifaceted computational methods were used to corroborate experimental values, examine inherent defects in the framework, decipher spectroscopic signatures, and elucidate the electronic structure. The crystallization of a plutonium chain side product provides direct evidence of the competition that occurs between modulator and linker in MOF syntheses. Ultimately, the synthesis of Pu-UiO-66 demonstrates adept control of Pu(IV) coordination under hydrolysis-prone conditions, provides an opportunity to extend trends across isostructural UiO-66 frameworks, and serves as the foundation for future plutonium MOF chemistry.

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework

We report the structural properties of ultra-small ThO2 and UO2 nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO2 and UO2 NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO2 and UO2 materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th-O and U-O bond distances despite retaining average bond lengths that are characteristic of bulk ThO2 and UO2. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

Unexpected Roles of Alkali-Metal Cations in the Assembly of Low-Valent Uranium Sulfate Molecular Complexes

The directing effect of coordinating ligands in the formation of uranium molecular complexes has been well established, but the role of counterions in metal-ligand interactions remains ambiguous and requires further investigation. In this work, we describe the targeted isolation, through the choice of alkali-metal ions, of a family of tetravalent uranium sulfates, showing the influence of the overall topology and, unexpectedly, the U^IV nuclearity upon the inclusion of such countercations. Analyses of the structures of uranium(IV) oxo/hydroxosulfate oligomeric species isolated from consistent synthetic conditions reveal that the incorporation of Na⁺ and Rb⁺ promotes the crystallization of 0D discrete clusters with a hexanuclear [U₆O₄(OH)₄(H₂O)₄]¹²⁺ core, whereas the larger Cs⁺ ion allows for the isolation of a 2D-layered oligomer with a less condensed trinuclear [U₃(O)]¹⁰⁺ center. This finding expands the prevalent view that counterions play an innocent role in molecular complex synthesis, affecting only the overall packing but not the local oligomerization. Interestingly, trends in nuclearity appear to correlate with the hydration enthalpies of alkali-metal cations, such that the alkali-metal cations with larger hydration enthalpies correspond to more hydrated complexes and cluster cores. These findings afford new insights into the mechanism of nucleation of U^IV, and they also open a new path for the rational design and synthesis of targeted molecular complexes.

Unexpected structural complexity of thorium coordination polymers and polyoxo cluster built from simple formate ligands

A simple synthetic approach with [HCOOH]/[Th(iv)] and water controls the yield of six thorium formates with unexpected structural complexity.

Effect of solution acidity on the structure of amino acid-bearing uranyl compounds

A series of uranyl sulfates and selenates templated by protonated forms of amino acids (glycine, α- and β-alanine, threonine, nicotinic, and isonicotinic acid) has been prepared via isothermal evaporation of strongly acidic solutions. Their structures have been refined by the direct methods and can be classified as inorganic [(UO2)m(TO4)n (H2O)k] (T=S6+, Se6+) moieties combined with the protonated amino acid cations, water molecules and hydronium ions. Their overall motifs demonstrate common features with related structures templated by organic amines. The role of carboxylic acid groups depends on the nature of the corresponding amino acid. They can either link two protonated organic moieties into dimers, or contribute to hydrogen bonding between organic and inorganic parts of the structure. The ammonium ends of the amino acid cations form strong directional bonds to the oxygens of the uranyl and TO4 anions.

Time-controlled synthesis of the 3D coordination polymer U(1,2,3-Hbtc)2 followed by the formation of molecular poly-oxo cluster {U14} containing hemimellitate uranium(iv)

Two coordination compounds bearing tetravalent uranium were synthesized in the presence of tritopic hemimellitic acid in acetonitrile with a controlled amount of water (H₂O/U ≈ 8) and structurally characterized. Compound 1, [U(1,2,3-Hbtc)₂]·0.5CH₃CN is constructed around an eight-fold coordinated uranium cationic unit [UO₈] linked by the poly-carboxylate ligands to form dimeric subunits, which are further connected to form infinite corrugated ribbons and a three-dimensional framework. Compound 2, [U₁₄O₈(OH)₄Cl₈(H₂O)₁₆(1,2,3-Hbtc)₈(ox)₄(ac)₄] ({U₁₄}) exhibits an unprecedented polynuclear {U₁₄} poly-oxo uranium cluster surrounded by O-donor and chloride ligands. It is based on a central core of [U₆O₈] type surrounded by four dinuclear uranium-subunits {U₂}. Compound 1 was synthesized by a direct reaction of hemimellitic acid with uranium tetrachloride in acetonitrile (+H₂O), while the molecular species ({U₁₄} (2)) crystallized from the supernatant solution after one month. The slow hydrolysis reaction together with the partial decomposition of the starting organic reactants into oxalate and acetate molecules induces the generation of such a large poly-oxo cluster with fourteen uranium centers. Structural comparisons with other closely related uranium-containing clusters, such as the {U₁₂} cluster based on the association of inner core [U₆O₈] with three dinuclear sub-units {U₂}, were performed.

The synthesis of a 3D coordination polymer [U(HL)₂] (L = hemimellitate) and a new poly-oxo cluster {U₁₄} afterwards is presented.

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.

Synthesis and structural characterization of the first neptunium based metal–organic frameworks incorporating {Np6O8} hexanuclear clusters

The synthesis of the first transuranium Metal–Organic Frameworks (TRU-MOFs) is reported here.

{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.

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.

Solution Species and Crystal Structure of Zr(IV) Acetate

Complex formation and the coordination of zirconium with acetic acid were investigated with Zr K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) and single-crystal diffraction. Zr K-edge EXAFS spectra show that a stepwise increase of acetic acid in aqueous solution with 0.1 M Zr(IV) leads to a structural rearrangement from initial tetranuclear hydrolysis species [Zr₄(OH)₈(OH₂)₁₆]⁸⁺ to a hexanuclear acetate species Zr₆(O)₄(OH)₄(CH₃COO)₁₂. The solution species Zr₆(O)₄(OH)₄(CH₃COO)₁₂ was preserved in crystals by slow evaporation of the aqueous solution. Single-crystal diffraction reveals an uncharged hexanuclear cluster in solid Zr₆(μ₃-O)₄(μ₃-OH)₄(CH₃COO)₁₂·8.5H₂O. EXAFS measurements show that the structures of the hexanuclear zirconium acetate cluster in solution and the solid state are identical.

Coordination polymers of uranium(IV) terephthalates

A series of tetravalent uranium terephthalates has been solvothermally synthesized in the solvent N,N-dimethylformamide (DMF) at temperature 100-150 °C with different water amounts. Composition diagrams have been determined for the U(4+) metallic cation in the presence of terephthalic acid, and their crystal structures revealed the occurrence of two- or three-dimensional coordination polymers. In the absence of water, a mixture of two polytypes T-U(2)Cl(2)(bdc)(3)(DMF)(4) (1) and M-U(2)Cl(2)(bdc)(3)(DMF)(4) (2) has been identified at low temperature (100-110 °C) for bdc/U = 1-4 (bdc = terephthalate linker). Their structures are built up from isolated uranium centers in nine-fold coordination, surrounded by 6 carboxyl oxygen atoms, 2 oxygen atoms coming from DMF molecules and one chlorine atom. The uranium cations are linked to each other through the bdc ligand in order to generate a 3D framework. By increasing the temperature (130-150 °C), a layered like compound has been isolated, U(2)(bdc)(4)(DMF)(4) (3). It is composed of discrete actinide centers in ten-fold coordination, with 8 carboxyl oxygen atoms and 2 oxygen atoms from DMF molecules. The connection of the UO10 units with the bdc linkers generates 2D sheets. When a controlled amount of water is added to the reaction medium, the crystallization of the UiO-66-like U(6)O(4)(OH)(4)(H(2)O)(6)(bdc)(6)·10DMF solid (containing a hexanuclear sub-unit) is observed for temperature 110-120 °C and the H(2)O/U molar ratio in the range of 2-10. At higher temperature (140-150 °C), a distinct phase appeared, U(2)O(2)(bdc)(2)(DMF) (4), which consists of infinite chains of uranium centers, linked to each other via the bdc ligands. Higher water contents led to the formation of urania UO(2).