Uranium Center Research Articles

Recent Advances in Low Valent Thorium and Uranium Chemistry

The synthesis, isolation, and characterisation of well-defined low-valent actinide complexes are reviewed with a main focus on compounds featuring uranium and thorium metal centres in formal oxidation states ≤ +3. The importance of the ligand environment in enabling access to these highly reactive species, as well as its influence on ground state electronic configurations and their reactivity, are emphasised. Furthermore, we highlight cyclic voltammetry (C.V.) studies as a more widely used method that can guide the synthesis of these highly reducing species.

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters.

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

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.

Oxidative Addition of E-H (E=C, N) Bonds to Transient Uranium(II) Centers.

Two-electron oxidative addition is one of the most important elementary reactions for d-block transition metals but it is uncommon for f-block elements. Here, we report the first examples of intermolecular oxidative addition of E-H (E=C, N) bonds to uranium(II) centers. The transient U(II) species was formed in-situ by reducing a heterometallic cluster featuring U(IV)-Pd(0) bonds with potassium-graphite (KC8). Oxidative addition of C-H or N-H bonds to the U(II) centers was observed when this transient U(II) species was treated with benzene, carbazole or 1-adamantylamine, respectively. The U(II) centers could also react with tetracene, biphenylene or N2O, leading to the formation of arene reduced U(IV) products and uranyl(VI) species via two- or four-electron processes. This study demonstrates that the intermolecular two-electron oxidative addition reactions are viable for actinide elements.

Oxidative Addition of E−H (E=C, N) Bonds to Transient Uranium(II) Centers

AbstractTwo‐electron oxidative addition is one of the most important elementary reactions for d‐block transition metals but it is uncommon for f‐block elements. Here, we report the first examples of intermolecular oxidative addition of E−H (E=C, N) bonds to uranium(II) centers. The transient U(II) species was formed in‐situ by reducing a heterometallic cluster featuring U(IV)‐Pd(0) bonds with potassium‐graphite (KC8). Oxidative addition of C−H or N−H bonds to the U(II) centers was observed when this transient U(II) species was treated with benzene, carbazole or 1‐adamantylamine, respectively. The U(II) centers could also react with tetracene, biphenylene or N2O, leading to the formation of arene reduced U(IV) products and uranyl(VI) species via two‐ or four‐electron processes. This study demonstrates that the intermolecular two‐electron oxidative addition reactions are viable for actinide elements.

Uranium Oxide Hydrate Frameworks with Dy(III) or Lu(III) Ions: Insights Into the Framework Structures With Lanthanide Ions.

Two uranium oxide hydrate frameworks (UOHFs) with either Dy3+ or Lu3+ ions, Dy1.36(H2O)6[(UO2)10UO13(OH)4] (UOHF-Dy) or Lu2(H2O)8[(UO2)10UO14(OH)3] (UOHF-Lu), were synthesized hydrothermally and characterized with a range of structural and spectroscopic techniques. Although SEM-EDS analysis confirmed the same atomic ratio of ~5.5 for U : Dy and U : Lu, they displayed different crystal morphologies, needles for UOHF-Dy in the orthorhombic C2221 space group and plates for UOHF-Lu in the triclinic P-1 space group. Both frameworks are composed of β-U3O8 type layers linked by pentagonal bipyramidal uranium polyhedra, with the Dy3+/Lu3+ ions inside the channels. However, the arrangements of Dy3+/Lu3+ ions are different, with disordered Dy3+ ions well aligned at the centers of the channels and single Lu3+ ions well-separated in a zigzag pattern in the channels. While the characteristic vibrational modes were revealed by Raman spectroscopy, the presence of a pentavalent uranium center in UOHF-Lu was confirmed with diffuse reflectance spectroscopy. The formation of two types of UOHFs with lanthanide ions, high or low symmetry, and the structure trend were discussed regards to synthesis conditions and lanthanide ionic radius. This work highlights the complex chemistry driving the formation of UOHFs with lanthanide ions and has implications to the spent nuclear fuel under geological disposal.

Laser-Induced Thermal Decomposition of Uranium Coordination Compounds with Non-oxidic Ligands to Produce Nitride and Carbide Materials.

The production of ceramics from uranium coordination compounds can be achieved through thermal processing if an excess amount of the desired atoms (i.e., C or N), or reactive gaseous products (e.g., methane or nitrogen oxide) is made available to the reactive uranium metal core via decomposition/fragmentation of the surrounding ligand groups. Here, computational thermodynamic approaches were utilized to identify the temperatures necessary to produce uranium metal from some starting compounds─UI4(TMEDA)2, UCl4(TMEDA)2, UCl3(pyridine)x, and UI3(pyridine)4. Experimentally, precursors were irradiated by a laser under various gaseous environments (argon, nitrogen, and methane) creating extreme reaction conditions (i.e., fast heating, high temperature profile >2000 °C, and rapid cooling). Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligands yielding uranium ceramics. Residual gas analysis identified vaporized products from the laser irradiation, and the final ceramic products were characterized by powder X-ray diffraction. The composition of the uranium precursor as well as the gaseous environment had a direct impact on the production of the final phases.

Understanding solvent polarity effects on the separation of uranyl porphyrin-derivative complexes.

Solvation is a crucial task in developing efficient and selective extractants for actinide elements, but an understanding and perspective of solvent effects on the extraction of uranyl are still lacking. Herein, we present investigations into solvent effects on the geometry, stability and bonding properties of five uranyl porphyrin derivative complexes (UO2(L)) in four solvents via relativistic quantum chemical calculations, and reveal some trends in the influence of solvent polarity on uranyl compounds. All five [L]2- ligands equatorially coordinate [UO2]2+ in a hexa-dentate (κ6) fashion. Thus, the uranium center is bound to six N atoms by U-N bonds, and the properties of the U-N bond are affected by the ligands rather than by the solvent. The relative stability of these UO2(L) complexes is obviously affected by the solvent polarity; lower polar solvents stabilize more UO2(L) complexes from higher polar solvents, resulting in more extensive interaction of U-N. By computing the reaction energy, we track the conversion of pure [UO2]2+ to UO2(L) complexes in different solvents; the uranyl dipentafluorobenziamethyrin species exhibit the greatest selectivity to higher polarity solvents, with significant orbital interactions of U-N accounting for this stability and selectivity. This study provides a general procedure for theoretical screening of the binding ability and solvent selectivity of macrocyclic ligands towards uranyl, and searching for suitable ligands and solvents that will later be applied in nuclear science disciplines.

Temperature Dependence of Nuclear Quadrupole Resonance and the Observation of Metal-Ligand Covalency in Actinide Complexes: 35Cl in Cs2UO2Cl4.

We report a study of the temperature dependence of 35Cl nuclear quadrupole resonance (NQR) transition energies and spin-lattice relaxation times (T1) for 235U-depleted dicesium uranyl tetrachloride (Cs2UO2Cl4) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO22+ ion. The transition frequency decreases slowly with temperature below 75 K and with a more rapid linear dependence above this temperature. The spin-lattice relaxation time becomes shorter with temperature, and as temperatures increase, the T1 decrease becomes nearly quadratic. The observed trends are reproduced by a model that assumes phonon-induced fluctuations of the electric field gradient tensor and partial electron delocalization from Cl to U. The fit of the theoretical model to experimental data allows a Debye temperature of 96 K to be estimated. The generalization of this approach to investigations of covalency in actinide-ligand bonding is examined.

Near-Unity Energy Transfer from Uranyl to Europium in a Heterobimetallic Organic Framework with Record-Breaking Quantum Yield.

Lanthanide organic frameworks (Ln-MOFs) have attracted increasing research enthusiasm as photoluminescent materials. However, limited luminescence efficiency stemming from restricted energy transfer efficiency from the organic linker to the metal center hinders their applications. Herein, a uranyl sensitization approach was proposed to boost the luminescence efficiency of Ln-MOFs in a distinct heterobimetallic uranyl-europium organic framework. The record-breaking photoluminescence quantum yield (PLQY, 92.68%) among all reported Eu-MOFs was determined to benefit from nearly 100% energy transfer efficiency between UO22+ and Eu3+. Time-dependent density functional theory and ab initio wave-function theory calculations confirmed the overlap of excited state levels between UO22+ and Eu3+, which is responsible for the efficient energy transfer process. Coupled with intrinsically strong stopping power toward X-ray of the uranium center, SCU-UEu-2 features an ultralow detection limit of 1.243 μGyair/s, outperforming the commercial scintillator LYSO (13.257 μGyair/s) and satisfying the requirement of X-ray diagnosis (below 5.5 μGyair/s) in full.

Thiophene-functionalized heteronuclear uranium organic framework for selective detection and adsorption towards Mercury (II)

Mercury (II) metal ion is a headache problem due to its highly toxic and widely spread as an environmental pollutant. Herein, we design and synthesized a thiophene based heteronuclear uranyl organic framework {[Na(UO2)3(μ3-O)(μ2-OH)3(2,5-TDC)]•2H2O} (2,5-TDC ​= ​thiophene-2,5-dicarboxylic acid) (UOF1) for the selective sensing and adsorption towards Hg (II) in water. Among which, the regular 1D open channels and the accessible active soft base sulfur atoms arrayed on the pore walls play an important role for the Hg (II) grabbing and deposition, while the unique charge-transfer based “multi-peaks” fluorescent emission from the uranium centers act as the sensor towards the Hg (II). The detection limit and adsorption capacity are 2.488✕10−9 ​M and 244.32 ​mg ​g−1, respectively, which are among the top level performance for Hg (II) detection and remediation. In addition, density functional theory (DFT) calculations reveal that the Hg (II) affinity arising from the synergetic interactions of sulfur and oxygen supported by the UOF skeleton. This research demonstrates the dual functional utilization of a fluorescent MOF for both sensing and removal of metal ion, which highlights the facile construction of functionalized MOFs for heavy metals remediation.

Unconventional Pathways to Carbide Phase Synthesis via Thermal Decomposition of UI4(1,4-dioxane)2.

UI4(1,4-dioxane)2 was subjected to laser-based heating─a method that enables localized, fast heating (T > 2000 °C) and rapid cooling under controlled conditions (scan rate, power, atmosphere, etc.)─to understand its thermal decomposition. A predictive computational thermodynamic technique estimated the decomposition temperature of UI4(1,4-dioxane)2 to uranium (U) metal to be 2236 °C, a temperature achievable under laser irradiation. Dictated by the presence of reactive, gaseous byproducts, the thermal decomposition of UI4(1,4-dioxane)2 under furnace conditions up to 600 °C revealed the formation of UO2, UIx, and U(C1-xOx)y, while under laser irradiation, UI4(1,4-dioxane)2 decomposed to UO2, U(C1-xOx)y, UC2-zOz, and UC. Despite the fast dynamics associated with laser irradiation, the central uranium atom reacted with the thermal decomposition products of the ligand (1,4-dioxane = C4H8O2) instead of producing pure U metal. The results highlight the potential to co-develop uranium precursors with specific irradiation procedures to advance nuclear materials research by finding new pathways to produce uranium carbide.

Homoleptic Uranium–Bis(acyl)phosphide Complexes

The first uranium bis(acyl)phosphide (BAP) complexes were synthesized from the reaction between sodium bis(mesitoyl)phosphide (Na(mesBAP)) or sodium bis(2,4,6-triisopropylbenzoyl)phosphide (Na(trippBAP)) and UI3(1,4-dioxane)1.5. Thermally stable, homoleptic BAP complexes were characterized by single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy, when appropriate, for the elucidation of the electronic structure and bonding of these complexes. EPR spectroscopy revealed that the BAP ligands on the uranium center retain a significant amount of electron density. The EPR spectrum of the trivalent U(trippBAP)3 has a rhombic signal near g = 2 (g1 = 2.03; g2 = 2.01; and g3 = 1.98) that is consistent with the EPR-observed unpaired electron being located in a molecular orbital that appears ligand-derived. However, upon warming the complex to room temperature, no resonance was observed, indicating the presence of uranium character.

Dinitrogen cleavage and hydrogenation to ammonia with a uranium complex.

The Haber-Bosch process produces ammonia (NH3) from dinitrogen (N2) and dihydrogen (H2), but requires high temperature and pressure. Before iron-based catalysts were exploited in the current industrial Haber-Bosch process, uranium-based materials served as effective catalysts for production of NH3 from N2. Although some molecular uranium complexes are known to be capable of combining with N2, further hydrogenation with H2 forming NH3 has not been reported to date. Here, we describe the first example of N2 cleavage and hydrogenation with H2 to NH3 with a molecular uranium complex. The N2 cleavage product contains three uranium centers that are bridged by three imido μ 2-NH ligands and one nitrido μ 3-N ligand. Labeling experiments with 15Ndemonstrate that the nitrido ligand in the product originates from N2. Reaction of the N2-cleaved complex with H2 or H+ forms NH3 under mild conditions. A synthetic cycle has been established by the reaction of the N2-cleaved complex with trimethylsilyl chloride. The isolation of this trinuclear imido-nitrido product implies that a multi-metallic uranium assembly plays an important role in the activation of N2.

Two uranyl-organic frameworks based on pyridine carboxylic acid and their electrochemistry properties study

In this paper, two novel 1D uranyl coordination polymers, [(UO2)(HPDTC)(H2O)]Cl (1) and [(UO2)(DPA)(H2O) (2) based on pyridine acid have been synthesized using hydrothermal method. The structure was obtained with the help of a Single Crystal X-ray diffraction meter and further characterized by IR, UV–vis, TG and others. The compounds show good luminescence properties and the uranium center exhibits the same pentagonal bipyramid coordination geometry among the compounds. Their electrochemical properties have been investigated through Cyclic Voltammetry (CV) and Galvanostatic Charge/Discharge (GCD). It reveals the counterion ions in compound 1 improve the electrical properties as the capacity is 56.94 ​F ​g−1.

Multi-electron Transfer by U(ɪɪ) and Masked U(ɪɪ) Complexes

Complexes of uranium in low oxidation state have shown the ability of activating non-reactive small molecules such as N2. However, the multi-electron transfer required for such activation, remains limited in uranium chemistry. Here, we review our recent research on the use of different strategies to overcome this issue, which has led to the isolation of a diuranium(ɪɪɪ) bridging oxide complex that reacts as a U(ɪɪ) synthon able to effect one-electron transfer per uranium center to N-heterocycles and multi-electron transfer to diphenylacetylene and azobenzene. We also showed that a closely related molecular U(ɪɪ) complex effects the same reactions providing the first unambiguous example of a monouranium four-electron transfer.

Multiple aspects of chirality in coordination polymers formed by the uranyl ion with (1R,3S)-(+)-camphorate ligands

Following previous work on complexes of the anions of (1R,3S)-(+)-camphoric acid (H2cam) with the uranyl cation, three novel species have been synthesized under solvo-hydrothermal conditions. [UO2(Hcam)2(4,4ʹ-bipy)] (1), where 4,4ʹ-bipy is 4,4ʹ-bipyridine, crystallizes as a monoperiodic polymer with bridging 4,4ʹ-bipy molecules and chelating, terminal Hcam– anions, hydrogen bonding between carboxylic and carboxylate groups in adjacent chains giving diperiodic networks. [UO2(cam)(2,2ʹ-bipy)] (2), where 2,2ʹ-bipy is 2,2ʹ-bipyridine, crystallizes also as a monoperiodic polymer, but bis-chelating cam2– anions are here bridging and the chelating 2,2ʹ-bipy ligands induce chirality at the uranium atoms; the chains are helical, left- or right-handed depending on the chirality of the associated uranium centres. [Zn(phen)3][(UO2)2(cam)3] (3), where phen is 1,10-phenanthroline, is an anionic diperiodic network with hcb topology displaying a grooved shape. In all three instances, the enantiomeric purity of the camphorate ligands does not produce significant enantioselectivity in regard to other aspects of the structural chirality.

Applications of Low-Valent Transition Metalates: Development of a Reactive Noncarbonyl Rhenium(I) Anion.

Low-valent transition metalates─anionic, electronic-rich organometallic complexes─comprise a class of highly reactive chemical reagents that find integral applications in organic synthesis, small-molecule activation, transient species stabilization, and M-E bond formation, among others. The inherent reactivity of such electron-rich metal centers has necessitated the widespread use of strong backbonding ligands, particularly carbonyls, to aid in the isolation and handling of metalate reagents, albeit sometimes at the expense of partially masking their full reactivity. However, recent synthetic explorations into transition-metalate complexes devoid of archetypic back-bonding ligands have led to the discovery of highly reactive metalates capable of performing a variety of novel chemical transformations.Building on our group's long-standing interest in reactive organometallic species, a series of rational progressions in early-to-middle transition-metal chemistry ultimately led to our isolation of a rhenium(I) β-diketiminate cyclopentadienide metalate that displays exceptional reactivity. We have found this Re(I) metalate to be capable of small-molecule activation; notably, the complex reversibly binds dinitrogen in solution and can be utilized to trap N2 for the synthesis of functionalized diazenido species. By employing isolobal analogues to N2 (CO and RNC), we were able to thoroughly monitor the mechanism of activation and conclude that the metalate's sodium counterion plays an integral role in promoting dinitrogen activation through a novel side-on interaction. The Re(I) metalate is also used in forming a variety of M-E bonds, including a series of uncommon rhenium-tetrylene (Si, Ge, and Sn) complexes that display varying degrees of multiple bonding. These metal tetrylenes act to highlight deviations in chemical properties within the group 14 elements. Our metalate's utility also applies to metal-metal bond formation, as demonstrated through the synthesis of a heterotetrametallic rhenium-zinc dimer. In this reaction, the Re(I) metalate performs a dual role as a reductant and metalloligand to stabilize a transient Zn22+ core fragment. Finally, the metalate displays unique reactivity with uranium(III) to yield the first transition metal-actinide inverse-sandwich bonds, in this case with three rhenium fragments bound through their Cp moieties surrounding the uranium center. Notably, throughout these endeavors we demonstrate that the metalate displays reactivity at multiple locations, including directly at the rhenium metal center, at a Cp carbon, through a Cp-sandwich mode, or through reversibly bound dinitrogen.Overall, the rhenium(I) metalate described herein demonstrates utility in diverse applications: small-molecule activation, the stabilization of reduced and/or unstable species, and the formation of unconventional M-E/M-M bonds or heterometallic complexes. Moving forward, we suggest that the continued discovery of noncarbonyl, electron-rich transition-metal anions featuring new or unconventional ligands should produce additional reactive organometallic species capable of stabilizing unique structural motifs and performing novel and unusual chemical transformations.

Assembling diuranium complexes in different states of charge with a bridging redox-active ligand.

Radical-bridged diuranium complexes are desirable for their potential high exchange coupling and single molecule magnet (SMM) behavior, but remain rare. Here we report for the first time radical-bridged diuranium(iv) and diuranium(iii) complexes. Reaction of [U{N(SiMe3)2}3] with 2,2'-bipyrimidine (bpym) resulted in the formation of the bpym-bridged diuranium(iv) complex [{((Me3Si)2N)3UIV}2(μ-bpym2-)], 1. Reduction with 1 equiv. KC8 reduces the complex, affording [K(2.2.2-cryptand)][{((Me3Si)2N)3U}2(μ-bpym)], 2, which is best described as a radical-bridged UIII-bpym˙--UIII complex. Further reduction of 1 with 2 equiv. KC8, affords [K(2.2.2-cryptand)]2[{((Me3Si)2N)3UIII}2(μ-bpym2-)], 3. Addition of AgBPh4 to complex 1 resulted in the oxidation of the ligand, yielding the radical-bridged complex [{((Me3Si)2N)3UIV}2(μ-bpym˙-)][BPh4], 4. X-ray crystallography, electrochemistry, susceptibility data, EPR and DFT/CASSCF calculations are in line with their assignments. In complexes 2 and 4 the presence of the radical-bridge leads to slow magnetic relaxation.

Uranium oxide hydrate frameworks with Er(III) or Y(III) ions: revealing structural insights leading to the low symmetry.

Two new mixed-valence uranium oxide hydrate frameworks (UOFs), incorporating either Er3+ or Y3+ ions, were successfully synthesised under hydrothermal conditions and characterised with single-crystal X-ray diffraction and a variety of other structural and spectroscopic techniques. Both frameworks are isostructural and crystallise in the triclinic P1̄ space group, consisting of β-U3O8 type layers pillared by additional uranyl centres, with the Er3+/Y3+ ions lying in the channels of the framework. SEM-EDS analysis found that both materials existed in plate-like morphologies, with a U:Er/Y ratio of 5.5. Bond valence sum analysis revealed the possible existence of pentavalent uranium centres, which was confirmed with diffuse reflectance spectroscopy. Being the first reported UOFs in this space group, this work highlights the complex and flexible nature of these materials, and the broader uranium oxide hydrate systems which exist in the surrounds of spent nuclear fuel disposal in the underground repository.