Chemistry of Actinide Centers in Heterogeneous Catalytic Transformations of Small Molecules

The Curious Case of Pu+: Insight on 5f Orbital Activity from Inductively Coupled Plasma Tandem Mass Spectrometry (ICP-MS/MS) Reactions

A comprehensive understanding of when and how 5f orbitals participate in complex chemical bonding is important for a variety of applications. The actinides are unique in that they possess 5f orbitals and can access high oxidation states, which make them attractive for use in catalysis. Fundamental studies of actinide-ligand interactions offer a mechanism to examine the activation of the 5f orbitals so that the selectivity of 5f orbitals can be assessed. A previous study examined the reaction of Pu+ + CO2 and determined that the reaction efficiency is restricted by a barrier, namely, promotion from the Pu+ ground-state configuration, 5f67s, to a reactive-state configuration, 5f56d2. The present study illustrates the benefit of activation of Pu's 5f orbitals when studying the reaction of Pu+ + NO. In this reaction, PuO+ forms in an exothermic, barrierless process. The 5f orbitals can and do participate in forming a linear intermediate, [N-Pu-O]+, and this drives the exothermic reaction. Understanding the conditions under which 5f orbitals are active in chemical bonding is the key to exploiting the actinides' selective catalytic capabilities.

Molecular Models of Atomically Dispersed Uranium at MoS2 Surfaces Reveal Cooperative Mechanism of Water Reduction

Single atoms of uranium supported on molybdenum sulfide surfaces (U@MoS2) have been recently demonstrated to facilitate the hydrogen evolution reaction (HER) through electrocatalysis. Theoretical calculations have predicted uranium hydroxide moieties bound to edge-sulfur atoms of MoS2 as a proposed transition state involved in the HER process. However, the isolation of relevant intermediates involved in this process remains a challenge, rendering mechanistic hypotheses unverified. The present work describes the isolation and characterization of a uranium-hydroxide intermediate on molybdenum sulfide surfaces using [(Cp*3Mo3S4)UCp*], a molecular model of a reduced uranium center supported at MoS2. Mechanistic investigations highlight the metalloligand cooperativity with uranium involved in the water activation pathway. The corresponding uranium-oxo analogue, [(Cp*3Mo3S4)Cp*U(═O)], was also accessed from the hydroxide cluster via hydrogen atom transfer and from [(Cp*3Mo3S4)UCp*] through an alternative direct oxygen atom transfer. These results provide an atomistic perspective on the reactivity of low-valent uranium at molybdenum sulfide surfaces toward water, modeling key intermediates associated with the HER of U@MoS2 catalysts.

Leveraging a reduced polyoxomolybdate-alkoxide cluster for the formation of a stable U(v) sandwich complex

The synthesis and characterization of a series of (TBA)2[M{Mo5O13(OMe)4NO}2] (M = Zr, Hf, Th, and U) sandwich complexes is reported. A preformed lacunary, Lindqvist-type, polyoxomolybdate-alkoxide cluster provides access to first examples of actinide-polyoxomolybdate sandwich complexes isolated under non-aqueous conditions. Incorporation of metal(iv) cations into this framework was found to "switch on" reversible redox chemistry at the {Mo5} ligands, with the Zr and Hf containing complexes accepting up to two electrons, while the Th and U derivates accommodate as many as four additional electrons. The enhancement of the redox properties of the cluster upon actinide incorporation is an exciting observation, presenting actinide "doping" as a novel approach for accessing functional redox-active materials. Oxidation of the uranium containing sandwich complex (TBA)2[U{Mo5O13(OMe)4NO}2], chemically or electrochemically, allows access to the U(v) centered species, which was characterized both spectroscopically and by single crystal X-ray diffraction. This represents the first example of a U(v)-polyoxometalate sandwich complex to be isolated and structurally characterized.

Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene

The structure of and bonding in two base-free terminal actinide imido metallocenes, [η5-1,2,4-(Me3C)3C5H2]2An═N(p-tolyl) (An = U (1), Th (1')) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An═N(p-tolyl) moieties is significantly larger for 1 than for 1', which makes the bonds between the [η5-1,2,4-(Me3C)3C5H2]2U2+ and [(p-tolyl)N]2- fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(L) (L = OPMe3 (6), dmap (9), PhCN (14), and 2,6-Me2PhNC (17)) with Me3PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me2PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS2, isothiocyanates, thio-ketones, ketones, lactides, and acyl nitriles, forming the four- or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p-tolylCHO, the tetranuclear complex [η5-1,2,4-(Me3C)3C5H2]4[OCH(p-tolyl)CH(p-tolyl)O]2U4O4 (10) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N,N'-diisopropylcarbodiimide (DIC), 1, and the four-membered metallaheterocycle [η5-1,2,4-(Me3C)3C5H2]2U[N(p-tolyl)C(═NiPr)N(iPr)] (12). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph3CN3, CuI, Ph2S2, and Ph2Se2, yielding the uranium(V) imido complexes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(X) (X = N3 (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN3) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η5-1,2,4-(Me3C)3C5H2]2U=N(p-tolyl)[=NN=(9-C13H8)] (21), respectively.

Temperature Dependence of the Ring Opening of Cyclopropene Imines on Thorium Metallocenes

The reactions of two highly strained cyclopropenimine ligands L1H and L2H (L1H = N1,N1,N2,N2-tetraisopropyl-3-iminocycloprop-1-ene-1,2-diamine, L2H = N1,N1,N2,N2-tetracyclohexyl-3-iminocycloprop-1-ene-1,2-diamine) with three thorium precursors Cp*2ThCl2, Cp*2Th(Cl)(CH3), and Cp*2Th(CH3)2 were studied. At -20 °C, L1H and L2H react with Cp*2ThCl2 to form Th1 (Th1 = Cp*2ThCl2(L1H)) and Th2 (Th2 = Cp*2ThCl2(L2H)), respectively, where the neutral ligand coordinates to the thorium metal center. Coordination of the ligand to the thorium metal center introduces aromaticity at the cyclopropene ring of the ligand. Reaction at room temperature results in the ring opening of the ligand to form Th3 (Th3 = Cp*2ThCl2((Z)-2,3-bis(diisopropylamino)acrylonitrile) and Th4 (Th4 = Cp*2ThCl2((Z)-2,3-bis(dicyclohexylamino)acrylonitrile), where the cyclopropenimine converts into a nitrile and coordinates to the thorium metal center. Reaction of L1H and L2H with Cp*2Th(Cl)(CH3) and/or Cp*2Th(CH3)2 at -20 °C results in a rapid methanolysis reaction and forms Cp*2Th(L1/L2)(CH3/Cl)-type complexes Th5 (Th5 = Cp*2Th(L1)(CH3)), Th6 (Th6 = Cp*2Th(L2)(CH3), Th7 (Th7 = Cp*2Th(L1)(Cl), and Th8 (Th8 = Cp*2Th(L2)(Cl). On the other hand, at room temperature, these reactions result in a ring opening of the ligand. Room-temperature reaction of L1H and L2H with Cp*2Th(CH3)2 results in Th9 (Th9 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th10 (Th10 = Cp*2Th(CH3)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). Similarly, at room temperature, L1H and L2H react with Cp*2Th(Cl)(CH3) to form Th11 (Th11 = Cp*2Th(Cl)((Z)-3-imino- N1,N1,N2,N2-tetraisopropylbut-1-ene-1,2-diamine) and Th12 (Th12 = Cp*2Th(Cl)((Z)-3-imino-N1,N1,N2,N2-tetracyclohexylbut-1-ene-1,2-diamine). The ring-opening reaction is assisted by the nucleophilic attack of the thorium-coordinated methyl group to the highly strained cyclopropene imine carbon.

Unraveling pH-Dependent Changes in Adsorption Structure of Uranyl on Alumina (012)

Mitigating uranium transport in groundwater is imperative for ensuring access to clean water across the globe. Here, in situ resonant anomalous X-ray reflectivity is used to investigate the adsorption of uranyl on alumina (012) in acidic aqueous solutions, representing typical UVI concentrations of contaminated water near mining sites. The analyses reveal that UVI adsorbs at two distinct heights of 2.4-3.2 and 5-5.3 Å from the surface terminal oxygens. The former is interpreted as the mixture of inner-sphere and outer-sphere complexes that adsorb closest to the surface. The latter is interpreted as an outer-sphere complex that shares one equatorial H2O with the terminal surface oxygen. With increasing pH, we observe an increasing prevalence of these outer-sphere complexes, indicating the enhanced role of the hydrogen bond that stabilizes adsorbed uranyl species. The presented work provides a molecular-scale understanding of sorption of uranyl on Al-based-oxide surfaces that has implications for environmental chemistry and materials science.

Reactivity of a Lewis base-supported uranium terminal imido metallocene towards small molecules

The Lewis base-supported uranium terminal imido metallocene [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(dmap) (1) readily reacts with various small molecules such as internal alkynes, isothiocyanates, thioketones, amidates, organic nitriles and imines, chlorosilanes, copper iodide, diphenyl disulfide, organic azides and diazoalkane derivatives. For example, treatment of 1 with PhCCCCPh and PhNCS forms metallaheterocycles originating from a [2 + 2] cycloaddition to yield [η5-1-(p-tolyl)NC(Ph)CHCC(Ph)CH2Si(Me)2-2,4-(Me3Si)2C5H2][η5-1,2,4-(Me3Si)3C5H2]U (2) and [η5-1,2,4-(Me3Si)3C5H2]2U[N(p-tolyl)C(NPh)S](dmap) (3), respectively. The reaction of 1 with the thioketone Ph2CS forms the known uranium sulfido complex [η5-1,2,4-(Me3Si)3C5H2]2US(dmap) (4), which reacts with a second molecule of Ph2CS to give the disulfido compound [η5-1,2,4-(Me3Si)3C5H2]2U(S2CPh2) (5). The imido moiety also promotes deprotonation reactions as illustrated in the reactions with the amide PhCONH(p-tolyl), the nitrile PhCH2CN and the imine (p-tolyl)2CNH to form the bis-amidate [η5-1,2,4-(Me3Si)3C5H2]2U[OC(Ph)N(p-tolyl)]2 (7), and the iminato complexes [η5-1,2,4-(Me3Si)3C5H2]2U[N(p-tolyl)C(CH2Ph)NH](NCCHPh) (8) and [η5-1,2,4-(Me3Si)3C5H2]2U[NH(p-tolyl)][NC(p-tolyl)2] (9), respectively. Addition of PhSiH2Cl to 1 yields [η5-1,2,4-(Me3Si)3C5H2]2U(Cl)[N(p-tolyl)SiH2Ph] (10). In contrast, the uranium(V) imido complexes [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(I) (11) and [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(SPh) (12), may be isolated upon addition of CuI or Ph2S2 to 1, respectively. Uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(NR) (R = p-tolyl (13), mesityl (14)) and [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)[NN(9-C13H8)] (15) are accessible from 1 on exposure to RN3 (R = p-tolyl, mesityl) and 9-diazofluorene, respectively. Complexes 2, 3, 5, and 7-15 were characterized by various spectroscopic techniques and, in addition, compounds 2, 3, 5, and 7-13 were structurally authenticated by single-crystal X-ray diffraction analyses.

Catalytic regeneration of metal-hydrides from their corresponding metal-alkoxides via the hydroboration of carbonates to obtain methanol and diols

Thorium complexes decorated with 5-, 6-, and 7-membered N-heterocyclic iminato ligands containing mesityl wingtip substitutions have been synthesized and fully characterized. These complexes were found to be efficient in the hydroboration of cyclic and linear organic carbonates with HBpin or 9-BBN promoting their decarbonylation and producing the corresponding boronated diols and methanol. In addition, the hydroboration of CO2 breaks the molecule into "CO" and "O" forming boronated methanol and pinBOBpin. Moreover, the demanding depolymerization of polycarbonates to the corresponding boronated diols and methanol opens the possibility of recycling polymers for energy sources. Increasing the core ring size of the ligands allows a better performance of the complexes. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading, and short reaction times, and shows a broad applicability scope. The reaction is achieved via the recycling of a high-energy Th-H moiety from a stable Th-OR motif. Experimental evidence and DFT calculations corroborate the formation of the thorium hydride species and the reduction of the carbonate with HBpin to the corresponding Bpin-protected alcohols and H3COBpin through the formate and acetal intermediates.

Core Excitations of Uranyl in Cs2UO2Cl4 from Relativistic Embedded Damped Response Time-Dependent Density Functional Theory Calculations

X-ray spectroscopies, by their high selectivity and sensitivity to the chemical environment around the atoms probed, provide significant insights into the electronic structures of molecules and materials. Interpreting experimental results requires reliable theoretical models, accounting for environmental, relativistic, electron correlation, and orbital relaxation effects in a balanced manner. In this work, we present a protocol for the simulation of core excited spectra with damped response time-dependent density functional theory based on the Dirac-Coulomb Hamiltonian (4c-DR-TD-DFT), in which environmental effects are accounted for through the frozen density embedding (FDE) method. We showcase this approach for the uranium M4- and L3-edges and oxygen K-edge of the uranyl tetrachloride (UO2Cl42-) unit as found in a host Cs2UO2Cl4 crystal. We have found that the 4c-DR-TD-DFT simulations yield excitation spectra that very closely match the experiment for the uranium M4-edge and the oxygen K-edge, with good agreement for the broad experimental spectra for the L3-edge. By decomposing the complex polarizability in terms of its components, we have been able to correlate our results with angle-resolved spectra. We have observed that for all edges, but in particular the uranium M4-edge, an embedded model in which the chloride ligands are replaced by an embedding potential reproduces rather well the spectral profile obtained for UO2Cl42-. Our results underscore the importance of the equatorial ligands to simulating core spectra at both uranium and oxygen edges.

A Comprehensive Study Concerning the Synthesis, Structure, and Reactivity of Terminal Uranium Oxido, Sulfido, and Selenido Metallocenes

Terminal uranium oxido, sulfido, and selenido metallocenes were synthesized, and their reactivity was comprehensively studied. Heating of an equimolar mixture of [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂UMe₂ (2) and [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U(NH-p-tolyl)₂ (3) in the presence of 4-dimethylaminopyridine (dmap) in refluxing toluene forms [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U═N(p-tolyl)(dmap) (4), which is a useful precursor for the preparation of the terminal uranium oxido, sulfido, and selenido metallocenes [η⁵-1,2,4-(Me₃Si)₃C₅H₂]₂U═E(dmap) (E = O (5), S (6), Se (7)) employing a cycloaddition-elimination methodology with Ph₂C═E (E = O, S) or (p-MeOPh)₂CSe, respectively. Metallocenes 5-7 are inert toward alkynes, but they act as nucleophiles in the presence of alkylsilyl halides. The oxido and sulfido metallocenes 5 and 6 undergo [2 + 2] cycloadditions with isothiocyanate PhNCS or CS₂, while the selenido derivative 7 does not. The experimental studies are complemented by density functional theory (DFT) computations.

N2-to-NH3 conversion by excess electrons trapped in point vacancies on 5f-element dioxide surfaces

Ammonia (NH3) is one of the basic chemicals in artificial fertilizers and a promising carbon-free energy storage carrier. Its industrial synthesis is typically realized via the Haber-Bosch process using traditional iron-based catalysts. Developing advanced catalysts that can reduce the N2 activation barrier and make NH3 synthesis more efficient is a long-term goal in the field. Most heterogeneous catalysts for N2-to-NH3 conversion are multicomponent systems with singly dispersed metal clusters on supporting materials to activate N2 and H2 molecules. Herein, we report single-component heterogeneous catalysts based on 5f actinide dioxide surfaces (ThO2 and UO2) with oxygen vacancies for N2-to-NH3 conversion. The reaction cycle we propose is enabled by a dual-site mechanism, where N2 and H2 can be activated at different vacancy sites on the same surface; NH3 is subsequently formed by H- migration on the surface via associative pathways. Oxygen vacancies recover to their initial states after the release of two molecules of NH3, making it possible for the catalytic cycle to continue. Our work demonstrates the catalytic activities of oxygen vacancies on 5f actinide dioxide surfaces for N2 activation, which may inspire the search for highly efficient, single-component catalysts that are easy to synthesize and control for NH3 conversion.

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.

Actinide-Oxygen Multiple Bonds from Air: Synthesis and Characterization of a Thorium Oxo Supported by Redox-Active Ligands

The first non-uranyl, f-element oxo complex synthesized from dioxygen in dry air is presented in this work. The synthesis was accomplished by treating the redox-active thorium amidophenolate complex, [Th(^dippap)₃][K(15-c-5)₂]₂ (1-ap crown), with dioxygen in dry air, forming a rare terminal thorium oxo, [O═Th(^dippisq)₂(^dippap)][K(15-c-5)₂]₂ (2-oxo). Compound 1-ap crown was regenerated by treating 2-oxo with potassium graphite. X-ray crystallography of 2-oxo revealed a comparatively longer bond length for the thorium-oxygen double bond when compared to other thorium oxos. As such, several thorium-oxygen single bonds were synthesized for comparison, including Th(^dippisq)₂(OSiMe₃)₂(THF) (4-OSiMe₃), Th(OSiMe₃)₄(bipy)₂ (5-OSiMe₃), and [Th(OH)₂ (^dippHap)₄][K(15-c-5)₂]₂ (6-OH). Full spectroscopic and structural characterization of the complexes was performed via ¹H NMR spectroscopy, X-ray crystallography, EPR spectroscopy, and electronic absorption spectroscopy as well as SQUID magnetometry, which all confirmed the electronic structure of these complexes.

Synthesis of Uranium Single Atom from Radioactive Wastewater for Enhanced Water Dissociation and Hydrogen Evolution

As a common nuclide in radioactive wastewater, uranium (U) is generally treated by landfill, which induces the massive abandonment of uranium resources. In this work, a pulse voltammetry method for the synthesis of U single atoms on MoS₂ (U/MoS₂ ) nanosheets from radioactive wastewater for the electrocatalytic alkaline hydrogen evolution reaction (HER) is reported. The mass loading of U single atoms is facilely controlled with high selectivity for coexisting ions in radioactive wastewater. In the electrolyte of 1 m of KOH, U/MoS₂ nanosheets with 5.2% of U single atoms exhibit relatively low overpotentials of 72 mV at 10 mA cm^-2 . The mechanistic study reveals that the HER on U/MoS₂ includes the water dissociation on U single atoms to form OH* and H transfer from OH* to adjacent S-edge atoms. This procedure exhibits decreased activation energy for transition state in water dissociation and optimized Gibbs free energy for H* adsorption.

A Sacrificial Iminato Ligand in the Catalytic Cyanosilylation of Ketones Promoted by Organoactinide Complexes

Four new complexes containing the bis(pentamethylcyclopentadienyl)thorium(IV) moiety, Cp*₂Th(L1)(Me) (Th2), Cp*₂Th(L2)(Me) (Th3), Cp*₂Th(L1)Cl (Th5), and Cp*₂Th(L2)Cl (Th6), were synthesized in quantitative yields via the protonolysis reaction of the metallocene precursor complexes Cp*₂Th(Me)₂ (Th1) and Cp*₂Th(Me)Cl (Th4) and the respective six- and seven-membered N-heterocyclic neutral imine ligands L1H and L2H. The molecular structures of all the complexes were established by single-crystal X-ray structure analyses. The synthesized complexes along with the precursor complexes were employed as catalysts for the cyanosilylation reaction of ketones with trimethylsilyl cyanide (Me₃SiCN). The removal of the iminato ligand is necessary to trigger the reaction, allowing the formation of the active catalyst.

Base-free transfer hydrogenation of aldehydes and ketones catalyzed by imidazolin-2-iminato actinide complexes

A series of thorium and uranium complexes decorated with unsymmetrical imidazolin-2-iminato ligands were found to be effective as catalysts in the transfer hydrogenation of aldehydes and ketones with 2-propanol to form the corresponding alcohols.

Influence of additives on low-temperature hydrothermal synthesis of UO2+x and ThO2

UO2+x and ThO2 were synthesized through a hydrothermal reaction by adding aldehydes.

Hyperstoichiometric Uranium Dioxides: Rapid Synthesis and Irradiation-Induced Structural Changes

Uranium dioxide (UO₂), the primary fuel for commercial nuclear reactors, incorporates excess oxygen forming a series of hyperstoichiometric oxides. Thin layers of these oxides, such as UO_2.12, form readily on the fuel surface and influence its properties, performance, and potentially geologic disposal. This work reports a rapid and straightforward combustion process in uranyl nitrate-glycine-water solutions to prepare UO_2.12 nanomaterials and thin films. We also report on the investigation of the structural changes induced in the material by irradiation. Despite the simple processing aspects, the combustion synthesis of UO_2.12 has a sophisticated chemical mechanism involving several exothermic steps. Raman spectroscopy and single-crystal X-ray diffraction (XRD) measurements reveal the formation of a complex compound containing the uranyl moiety, glycine, H₂O, and NO₃^- groups in reactive solutions and dried combustion precursors. Combustion diagnostic methods, gas-phase mass spectroscopy, differential scanning calorimetry (DSC), and extracted activation energies from DSC measurements show that the rate-limiting step of the process is the reaction of ammonia with nitrogen oxides formed from the decomposition of glycine and uranyl nitrate, respectively. However, the exothermic decomposition of the complex compound determines the maximum temperature of the process. In situ transmission electron microscopy (TEM) imaging and electron diffraction measurements show that the decomposition of the complex compound directly produces UO₂. The incorporation of oxygen at the cooling stage of the combustion process is responsible for the formation of UO_2.12. Spin coating of the solutions and brief annealing at 670 K allow the deposition of uniform films of UO_2.12 with thicknesses up to 300 nm on an aluminum substrate. Irradiation of films with Ar²⁺ ions (1.7 MeV energy, a fluence of up to 1 × 10¹⁷ ions/cm²) shows unusual defect-simulated grain growth and enhanced chemical mixing of UO_2.12 with the substrate due to the high uranium ion diffusion in films. The method described in this work allows the preparation of actinide oxide targets for fundamental nuclear science research and studies associated with stockpile stewardship.

5f Covalency Synergistically Boosting Oxygen Evolution of UCoO4 Catalyst

Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO₄ with alternative CoO₆ and 5f-related UO₆ octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. UCoO₄ exhibits a low overpotential of 250 mV at 10 mA cm^–2, surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co²⁺ ion in pristine UCoO₄ was converted to high-valence Co^3+/4+, while U⁶⁺ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co^3+/4+ was synergistically enhanced by the covalent bonding of U⁶⁺-5f in the U 5f–O 2p–Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

Irradiation-Driven Restructuring of UO2 Thin Films: Amorphization and Crystallization

Combustion synthesis in uranyl nitrate-acetylacetone-2-methoxyethanol solutions was used to deposit thin UO₂ films on aluminum substrates to investigate the irradiation-induced restructuring processes. Thermal analysis revealed that the combustion reactions in these solutions are initiated at ∼160 °C. The heat released during the process and the subsequent brief annealing at 400 °C allow the deposition of polycrystalline films with 5-10 nm UO₂ grains. The use of multiple deposition cycles enables tuning of the film thicknesses in the 35-260 nm range. Irradiation with Ar²⁺ ions (1.7 MeV energy and a fluence of up to 1 × 10¹⁷ ions/cm²) is utilized to generate a uniform distribution of atomic displacements within the films. X-ray fluorescence (XRF) and alpha-particle emission spectroscopy showed that the films were stable under irradiation and did not undergo sputtering degradation. X-ray photoelectron spectroscopy (XPS) showed that the stoichiometry and uranium ionic concentrations remain stable during irradiation. The high-resolution electron microscopy imaging and electron diffraction analysis demonstrated that at the early stages of irradiation (below 1 × 10¹⁶ ion/cm²) UO₂ films show complete amorphization and beam-induced densification (sintering), resulting in a pore-free disordered film. Prolonged irradiation (5 × 10¹⁶ ion/cm²) is shown to trigger a crystallization process at the surface of the films that moves toward the UO₂/Al interface, converting the entire amorphous material into a highly crystalline film. This work reports on an entirely different radiation-induced restructuring of the nanoscale UO₂ compared to the coarse-grained counterpart. The preparation of thin UO₂ films deposited on Al substrates fills an area of national need within the stockpile stewardship program of the National Nuclear Security Administration and fundamental research with actinides. The method reported in this work produces pure, robust, and uniform thin-film actinide targets for nuclear science measurements.

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.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Detailed characterization of dioxouranium(vi) complexes with a symmetrical tetradentate N2O2-benzil bis(isonicotinoyl hydrazone) ligand

The reactions of UO2(OAc)2·2H2O with benzil bis(isonicotinoyl hydrazone) ligand (H2L) in varied solvent media resulted in the formation of a series of new dioxouranium(vi) complexes 1-3 of the type UO2(L)(X), [where 1, X = DMF; 2, X = DMSO; 3, X = H2O]. The complexes were systematically characterized by elemental analysis, UV-Visible spectroscopy, TGA, mass spectrometry, cyclic voltammetry, and powder X-ray diffraction study. Among all the complexes, 1 was confirmed by single-crystal X-ray diffraction study. It was found that 1 preferred a distorted pentagonal bipyramidal geometry, in which an equatorial coordination plane was formed by the ONNO-tetradentate cavity of the deprotonated hydrazone ligand along with an additional oxygen atom of the coordinated solvent molecule. Thermal analysis suggested that complexes 1 and 3 undergo weight loss in the temperature range 180-210 °C and 100-120 °C, respectively, due to the ready release of their coordinated solvent molecules. Complexes 1-3 exhibited analogous UV-Visible absorption bands and the intense band between 300-600 nm was assigned to the M ← L and n → π* transitions. Weakly resolved reduction waves assigned to {UO2}2+/{UO2}+ couple were observed for complexes 1 and 2 {1, -1.76 V; 2, -1.75 V; vs. ferrocenium/ferrocene (Fc+/Fc)} in DMSO solution, signifying the feeble electron-donating nature of the L2- ligand. Powder X-ray diffraction study suggested that the crystallite size of all the complexes was in the nanoscale range. Further analysis using density functional theory (DFT) calculations provided structural insights as well as information on the electronic properties of both complex 1 and the ligand.

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.

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.

Unraveling the structural stability and the electronic structure of ThO2 clusters

Unraveling the correlations between the geometry, the relative energy and the electronic structure of actinide oxide nanostructures is crucial for a better control of their size, shape and properties.

Polyoxometalate-based complexes as ligands for the study of actinide chemistry

The complexation of actinide cations by polyoxometalates (POMs) has been extensively studied over the past 50 years. In this perspective article, we present the rich structural diversity of actinide-POM complexes and their contribution to the extension of our knowledges of actinide chemistry, especially regarding aspect of their redox chemistry, as well as application for the capture and separation of these cations in the context of nuclear fuel remediation. These heterometallic assemblies have also proven highly valuable as model for heterogeneous systems based on actinides supported by metal oxide surfaces. In particular, activation of the An-O bond of actinyl fragments upon complexation with lacunary POMs has been reported, creating opportunities for future developments regarding the reactivity of these heterometallic assemblies.

Synthesis of 3-(2-Hydroxyphenyl)-5-(2-Pyridinyl)-1,2,4-triazoles as a potential chelate ligand for Uranyl ion

Two new uranyl complexes with the molecular formula [(UO2)(H2L1)](СH3OH) and [(UO2)(H3L2)](СH3OH) {H4L1 = 2-[5-[[5-[[5-(2-pyridyl)-1H-1,2,4-triazol-3‑yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]phenol and H5L2 = 2-[5-[[5-[[5-[[5-(2-pyridyl)-1H-1,2,4‑triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]phenol)} have been synthesized. All compounds have been characterized by NMR and IR spectroscopy. With H4L1 and H5L2 uranyl ion forms mononuclear complexes. In [(UO2)(H3L2)](СH3OH) pyridyl nitrogen was uncoordinated and bonding of H5L2 was realized only through phenol oxygen and N4-nitrogens of triazole cycles.