Chlorination of uranium metal in molten NaCl-CaCl2 via bubbling HCl

Journal of Radioanalytical and Nuclear Chemistry - Molten chloride salt fast reactors (MCFRs) will require UCl3 dissolved in molten salt mixtures as fuel for nuclear fission. For infusing the salt...     

Effects of precursor and support variation in the genesis of uranium oxide catalysts for CO oxidation and selective reduction of NO: Synthesis and characterization

A range of uranium oxide-based catalysts, derived from UO2(NO3)2.6H2O and UCl4 precursors, and supported on gamma-Al2O3, SiO2 and mesoporous H1SiO2, have been synthesized and then characterized using the following methods: isothermal nitrogen adsorption/desorption measurements, diffuse reflectance infrared spectroscopy (DRIFTS), gas titration of surface hydroxyl groups using Grignard reagents, U L(III) extended X-ray absorption fine structure (EXAFS), powder X-ray diffraction (PXRD), and thermogravimetric and differential thermal analysis. Brij76-templated H1SiO2 mesoporous silicas are found to be essentially stable under flowing oxygen after 16 h at 1073 K. At temperatures above this, however, extensive structural collapse, together with extensive dehydroxylation, ensues. Titration of the accessible hydroxyl group concentrations shows that in these materials the density of OH groups is considerably lower than in their amorphous counterparts. The adsorption of uranyl nitrate onto these dispersants results in a supported, and partially dehydrated, phase of the parent molecule with little obvious structural distortion; however, the adsorption of UCl4 leads to a complex adstructure which may best be described as U(O)2Cl2. The subsequent formation of the uranium oxide phase, nominally active for the oxidation of CO and selective reduction of NO (generally accepted to be U3O8), is found to be a considerable function of both the precursor and support system employed. Calcination of such systems to 1073 K results in extensive extrusion of the supported uranium phase from mesoporous supports, resulting in the formation of very large orthorhombic U3O8 domains. PXRD, however, shows that on amorphous SiO2 and gamma-Al2O3 similar treatment results in the formation of a hexagonal phase of U3O8. The formation of U3O8 is found to be promoted in mesoporous systems and by the presence of Cl in the catalyst make up. Some evidence is also found that suggests that a persistence of Cl limits the growth of U3O8 domains.     

The first urea azine molecule and its coordination to uranium in the first actinide guanidinate complexes

The urea azine molecule (CyNH)(2)C=N-N=C(HNCy)(2) (1) was easily prepared by reaction of the carbodiimide CyN=C=NCy and H(2)NNH(2) and this novel type of bis-guanidine proved useful in affording chelating and bridging ligands for the building of polynuclear compounds, as illustrated by the synthesis of the first uranium guanidinate complexes [(THF)(2)Li(mu-Cl)(2)UCl(mu-L)](2) (2) and [UCl(mu-L)(2)UCl(2)(micro-Cl)(2)UCl(mu-L)](2) (3) (L = - 2H); the X-ray crystal structures of compounds - were determined.     

Solvation of UCl6(2-) anionic complex by MeBu3N(+), BuMe2Im(+), and BuMeIm(+) cations

The complexes [MeBu 3N] 2[UCl 6] and [BuMe 2Im] 2[UCl 6] were characterized in the solid state and in solution of [MeBu 3N][Tf 2N], [BuMe 2Im][Tf 2N], and [BuMeIm][Tf 2N] room-temperature ionic liquids using single-crystal XRD, EXAFS, electrochemistry, UV-visible absorption spectroscopy, and NMR. In the solid state and in solution, the existence of hydrogen bonding between the UCl 6 (2-) complex and the ionic liquid cations was revealed by these techniques. The MeBu 3N (+) cation interacts with UCl 6 (2-) via the protons on the alpha-carbon atoms of nitrogen. The protons of the imidazolium ring account for the interaction between the BuMe 2Im (+) cation and the UCl 6 (2-) anion. For the BuMeIm (+) cation the major interaction was confirmed between the most acidic proton on C(2) and the chlorides of UCl 6 (2-). The experimental results also show that the intensity of the interaction between the UCl 6 (2-) anion and the cation varies with the ionic liquid cation in the following order: MeBu 3N (+) approximately BuMe 2Im (+) < BuMeIm (+).     

Solvation of uranium hexachloro complexes in room-temperature ionic liquids. A molecular dynamics investigation in two liquids

We report a molecular dynamics study of the solvation of UCl(6)(-), UCl(6)(2-), and UCl(6)(3-) complexes in the [BMI][Tf(2)N] and [MeBu(3)N][Tf(2)N] ionic liquid cations based on the same anion (bis(trifluoromethylsulfonyl)imide (Tf(2)N-)) and the butyl-3-methyl-imidazolium+ (BMI+) or methyl-tri-n-butyl-ammonium (MeBu(3)N+) cation, respectively. The comparison of two electrostatic models of the complexes (ionic model with -1 charged halides versus quantum mechanically derived charges) yields similar solvation features of a given solute. In the two liquids, the first solvation shell of the complexes is positively charged and evolves from purely cationic in the case of UCl(6)(3-) to a mixture of cations and anions in the case of UCl(6)(-). UCl(6)(3-) is exclusively "coordinated" to BMI+ or MeBu(3)N+ solvent cations that mainly interact via their CH aromatic protons or their N-Me group, respectively. Around the less charged UCl(6)(-) complex, the cations interact via the less polar moieties (butyl chains of BMI+ or MeBu(3)N+) and the anions display nonspecific interactions. In no case does the uranium atom further coordinate solvent ions. According to an energy components analysis, UCl(6)(3-) interacts more attractively with the [BMI][Tf(2)N] liquid than with [MeBu(3)N][Tf(2)N], while UCl(6)(-) does not show any preference, suggesting a significant solvation effect of the redox properties of uranium, also supported by free energy perturbation simulations. The effect of ionic liquid (IL) humidity is investigated by simulating the three complexes in 1:8 water/IL mixtures. In contrast to the case of "naked" ions (e.g., lanthanide(3+), UO2(2+), alkali, or halides), water has little influence on the solvation of the UCl(6)(n-) complexes in the two simulated ILs, as indicated by structural and energy analysis. This is in full agreement with the experimental observations (Nikitenko, S. I.; et al. Inorg. Chem. 2005, 44, 9497).     

Low-temperature synthesis of uranium tetraboride by solid-state metathesis reactions

A novel synthesis of uranium tetraboride (UB(4)) by solid-state metathesis reaction is demonstrated. This approach significantly lowers the temperature required to synthesize this material to < or = 850 degrees C. When UCl(4) is reacted with 2 equiv of MgB(2) at 850 degrees C, crystalline UB(4) is formed. Powder X-ray diffraction and ICP-AES data support the reduction of UCl(4) to UCl(3) as the initial step in the reaction. The UB(4) product is purified by washing with water.     

Complexation of uranium(VI) with protocatechuic acid—application of iterative transformation factor analysis to EXAFS spectroscopy

The speciation of 1 mM uranium(VI) in carbonate-free aqueous solutions of 50 mM protocatechuic acid (PCA, 3,4-dihydroxybenzoic acid) was studied in the pH range of 4.0 to 6.8 using EXAFS spectroscopy. The uranium L_III-edge EXAFS spectra were analyzed using a newly developed computer algorithm for iterative transformation factor analysis (FA). Two structural different uranium(VI) complexes were observed. The speciation in the pH range of 4.0 to 4.8 is dominated by a 1:2 or 1:3 uranium(VI)/PCA complex with bidentate coordination of the carboxyl group to the uranium(VI) moiety. Already at pH 4.6 significant amounts of a second species are formed. This uranium(VI) species contains two PCA ligands that are bound to the uranium via their neighboring phenolic hydroxyl groups under formation of five-member rings.     

In situ spectroscopy and spectroelectrochemistry of uranium in high-temperature alkali chloride molten salts

Soluble uranium chloride species, in the oxidation states of III+, IV+, V+, and VI+, have been chemically generated in high-temperature alkali chloride melts. These reactions were monitored by in situ electronic absorption spectroscopy. In situ X-ray absorption spectroscopy of uranium(VI) in a molten LiCl-KCl eutectic was used to determine the immediate coordination environment about the uranium. The dominant species in the melt was [UO 2Cl 4] (2-). Further analysis of the extended X-ray absorption fine structure data and Raman spectroscopy of the melts quenched back to room temperature indicated the possibility of ordering beyond the first coordination sphere of [UO 2Cl 4] (2-). The electrolytic generation of uranium(III) in a molten LiCl-KCl eutectic was also investigated. Anodic dissolution of uranium metal was found to be more efficient at producing uranium(III) in high-temperature melts than the cathodic reduction of uranium(IV). These high-temperature electrolytic processes were studied by in situ electronic absorption spectroelectrochemistry, and we have also developed in situ X-ray absorption spectroelectrochemistry techniques to probe both the uranium oxidation state and the uranium coordination environment in these melts.     

An EXAFS and TRLFS investigation on uranium(VI) sorption to pristine and leached albite surfaces

Uranium(VI) was sorbed to freshly ground and leached albite in batch and flow-through systems in the pH range 5.0-6.4. The uranium(VI) surface complexes were studied by extended X-ray absorption fine structure (EXAFS) spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS). The EXAFS analysis of uranium(VI) sorbed on albite at pH 5.8 and 5 x 10(-6) M U(VI) showed one silicon atom at a USi distance of 3.09 A, which is indicative of the formation of an inner-sphere, mononuclear, bidentate uranium(VI) surface complex, Si(O)2UO2, on the silicate tetrahedra of albite. Two additional uranium(VI) sorption complexes were detected by TRLFS at higher initial aqueous U(VI) concentrations. However, the structure of these surface complexes could not be derived from EXAFS, since the measured EXAFS spectra represent the average of two surface complex structures. In order to simulate U(VI) sorption onto weathered feldspar surfaces, albite was leached with 0.01 M HClO4, resulting in surface material similar to amorphous silica gel. EXAFS showed that the equatorial oxygen shell of uranium(VI) sorbed on this material at pH 5.0 and 5.8 was split in two distances of 2.23 and 2.44 A. This indicates the formation of an inner-sphere surface complex.     

Cyclopentadienyluranium(IV)complexes with a chiral metallic center : I. Characterization of a prochiral structure by PMR spectroscopy

A range of compounds (C₅H₄R)₃UCl (R  alkyl) and the two tricyclopentadienyl compounds (C₅H₅)₂(C₅H₄R)UCl and C₅H₅(C₅H₄R)₂UCl, (R  CH(CH₃)₂) have been isolated and characterized by PMR spectroscopy. For the last compound, the PMR data indicate a magnetic non-equivalence for the cyclopentadienyl protons and for the methyl groups of R; both are due to the prochirality of the uranium atom.     

Density functional theory investigation of the redox properties of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes

The redox behavior of tricyclopentadienyl- and phospholyluranium(IV) chloride complexes L(3)UCl with L = C(5)H(5) (Cp), C(5)H(4)Me (MeCp), C(5)H(4)SiMe(3) (TMSCp), C(5)H(4)(t)Bu ((t)BuCp), C(5)Me(5) (Cp*), and C(4)Me(4)P (tmp), has been investigated using relativistic density functional theory calculations, with the solvent being taken into account using the conductor-like screening model. A very good linear correlation (r(2) = 0.99) has been obtained between the computed electron affinities of the L(3)UCl complexes and the experimental half-wave reduction potentials E(1/2) related to the U(IV)/U(III) redox systems. From a computational point of view, our study confirms the crucial importance of spin-orbit coupling and solvent corrections and the use of an extended basis set in order to achieve the best experiment-theory agreement. Considering oxidation of the uranium(IV) complexes, the instability of the uranium(V) derivatives [L(3)UCl](+) is revealed, in agreement with experimental electrochemical findings. The driving roles of both the electron-donating ability of the L ligand and the U 5f orbitals on the redox properties of the complexes are brought to light. Interestingly, we found and explained the excellent correlation between variations of the uranium Hirschfeld charges following U(IV)/U(III) electron capture and E(1/2). In addition, this work allowed one to estimate theoretically the half-wave reduction potential of [Cp*(3)UCl].     

Rigid NON- and NSN-ligand complexes of tetravalent and trivalent uranium: comparison of U-OAr2 and U-SAr2 bonding

A rigid NSN-donor proligand, 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylthioxanthene (H(2)[TXA(2)], 1) was prepared by palladium-catalyzed coupling of 2,6-diisopropylaniline with 4,5-dibromo-2,7-di-tert-butyl-9,9-dimethylthioxanthene. Deprotonation of 1 using (n)BuLi provided Li(2)(DME)(2)[TXA(2)] (2), and subsequent reaction with UCl(4) afforded [Li(DME)(3)][(TXA(2))UCl(3)] (4). The analogous NON-donor ligated complex [(XA(2))UCl(3)K(DME)(3)] [3; XA(2) = 4,5-bis(2,6-diisopropylanilino)-2,7-di-tert-butyl-9,9-dimethylxanthene] was prepared by the reaction of K(2)(DME)(x)[XA(2)] with UCl(4). A cyclic voltammogram (CV) of 3 in THF/[NBu(4)][B(C(6)F(5))(4)] at 200 mV s(-1) showed an irreversible reduction to uranium(III) at E(pc) = -2.46 V versus FeCp(2)(0/+1), followed by a product wave at E(1/2) = -1.83 V. Complex 4 also underwent irreversible reduction to uranium(iii) [E(pc) = -2.56 V], resulting in an irreversible product peak at E(pa) = -1.83 V. One-electron reduction of complexes 3 and 4 using K(naphthalenide) under an argon atmosphere in DME yielded 6-coordinate [(XA(2))UCl(DME)] (5) and the thermally unstable 7-coordinate [(TXA(2))U(DME)Cl(2)Li(DME)(2)] (6), respectively. The U-S distances in 4 and 6 are uncommonly short, the C-S-U angles are unusually acute, and the thioxanthene backbone of the TXA(2) ligand is significantly bent. By contrast, the xanthene backbone in XA(2) complexes 3 and 5 is planar. However, κ(3)-coordination and an approximately meridional arrangement of the ancillary ligand donor atoms is maintained in all complexes. DFT and Atoms in Molecules (AIM) calculations were carried out on 3, 4, 5, 6, [(XA(2))UCl(3)](-) (3B), [(TXA(2))UCl(2)(DME)](-) (6B) and [(TXA(2))UCl(DME)] (6C) to probe the extent of covalency in U-SAr(2) bonding relative to U-OAr(2) bonding.     

Facile syntheses of unsolvated UI3 and tetramethylcyclopentadienyl uranium halides

In the course of comparing the reaction chemistry of (C5Me5)3U, 1, and its slightly less crowded analogue (C5Me4H)3U, 2, new syntheses of UI3, (C5Me4H)3U, (C5Me4H)3UCl, 3, and (C5Me5)3UCl, 4, have been developed. Additionally, (C5Me4H)3UI, 5, and (C5Me4H)2UCl2, 6, have been identified for the first time. A facile synthesis of unsolvated UI3 is reported that proceeds in high yield with inexpensive equipment from iodine and hot uranium turnings. Both UI3 and UI3(THF)4 react with KC5Me4H in toluene to make unsolvated (C5Me4H)3U in higher yield than in previous reports that involve reduction of tetravalent (C5Me4H)3UCl, 3. A more atom-efficient synthesis of complex 3 is also reported that proceeds from reduction of t-BuCl, PhCl, or HgCl2 by 2. Similarly, (C5Me4H)3U reacts with PhI or HgI2 to generate (C5Me4H)3UI. These studies also provided a basis to improve the synthesis of (C5Me5)3UCl from 1 by employing t-BuCl or HgCl2 as the halide source. Like (C5Me5)3UCl, the (C5Me4H)3UCl complex reacts with HgCl2 to form (C5Me4H)2 and (C5Me4H)2UCl2, 6, but unlike (C5Me5)3UX (X = Cl or I), the less substituted (C5Me4H)3UX complexes do not reduce t-BuCl or PhX. The synthesis of 6 from (C5Me4H)MgCl x THF and UCl4 is also included.     

Separation and spectrophotometric determination of uranium(VI) by extraction with tri-n-octylphosphine oxide and benzophenone

Uranium(VI) is separated by extraction from nitric acid medium into a molten mixture of tri-n-octylphosphine oxide and benzophenone at about 50°. The organic phase solidifies on cooling and is separated and dissolved in ethanol. The uranium(VI) in this solution is then determined spectrophotometrically with 1-(2-pyridylazo)-2-naphthol.     

Structural characterization of U(VI) surface complexes on kaolinite in the presence of humic acid using EXAFS spectroscopy

To determine the influence of humic acid (HA), pH, and presence of atmospheric CO2 on the sorption of U(VI) onto kaolinite, the structure of the surface complexes was studied by U L III-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The best fits to the experimental EXAFS data were obtained by including two uranium coordination shells with two axial (O ax) and five equatorial (O eq) oxygen atoms at 1.77+/-0.02 and 2.34+/-0.02 A, respectively, and two coordination shells with one Al/Si atom each at 3.1 and 3.3 A. As in the case of the binary system U(VI)-kaolinite, uranium forms inner-sphere surface complexes by edge sharing with aluminum octahedra and/or silicon tetrahedra. HA and atmospheric CO2 as well as pH had no influence on the EXAFS structural parameters in the pH range of 5-8. Despite the presence of HA, U(VI) prefers to sorb directly onto kaolinite and not to HA that is bound to the clay surface. X-ray photoelectron spectroscopy (XPS) measurements of kaolinite particles that had been exposed to HA suspensions showed that significant parts of the kaolinite surface are not covered by HA.     

Density functional investigations of the properties and thermochemistry of UFn and UCln (n=1,...,6)

The structural properties and thermochemistry of the UF(n) and UCl(n) (n=1,...,6) molecules have been investigated using hybrid density functional theory and a small-core (60 electrons) relativistic effective core potential for the uranium atom. For the first time Bond dissociation energies for this whole series are computed and shown to be in good agreement with experiment. The geometry and electronic structure of each molecule was characterized.     

Spectroscopic and electrochemical studies of U(IV)-hexachloro complexes in hydrophobic room-temperature ionic liquids [BuMeIm][Tf2N] and [MeBu3N][Tf2N

The behavior of U(IV) octahedral complexes [cation]2[UCl6], where the [cation]+ is [BuMeIm]+ and [MeBu3N]+, is studied using UV/visible spectroscopy, cyclic staircase voltammetry, and rotating disk electrode voltammetry in hydrophobic room-temperature ionic liquids (RTILs) [BuMeIm][Tf2N] and [MeBu3N][Tf2N], where BuMeIm+ and MeBu3N+ are 1-butyl-3-methylimidazolium and tri-n-butylmethylammonium cations, respectively, and Tf2N- is the bis(trifluoromethylsulfonyl)imide anion. The absorption spectra of [cation]2[UCl6] complexes in the RTIL solutions are similar to the diffuse solid-state reflectance spectra of the corresponding solid species, indicating that the octahedral complex UCl6(2-) is the predominant chemical form of U(IV) in Tf2N--based hydrophobic ionic liquids. Hexachloro complexes of U(IV) are stable to hydrolysis in the studied RTILs. Voltammograms of UCl(6)2- at the glassy carbon electrode in both RTILs and at the potential range of -2.5 to +1.0 V versus Ag/Ag(I) reveal the following electrochemical couples: UCl6-/UCl6(2-) (quasi-reversible system), UCl(6)2-/UCl6(3-) (quasi-reversible system), and UCl(6)2-/UCl6(Tf2N)x-3+x (irreversible reduction). The voltammetric half-wave potential, Ep/2, of the U(V)/U(IV) couple in [BuMeIm][Tf2N] is positively shifted by 80 mV compared with that in [MeBu3N][Tf2N]. The positive shift in the Ep/2 value for the quasi-reversible U(IV)/U(III) couple is much greater (250 mV) in [BuMeIm][Tf2N]. Presumably, the potential shift is due to the specific interaction of BuMeIm+ with the uranium-hexachloro complex in ionic liquid. Scanning the negative potential to -3.5 V in [MeBu3N][Tf2N] solutions of UCl6(2-) reveals the presence of an irreversible cathodic process at the peak potential equal to -3.12 V (at 100 mV/s and 60 degrees C), which could be attributed to the reduction of U(III) to U(0).     

Synthesis of a uranium(VI)-carbene: reductive formation of uranyl(V)-methanides, oxidative preparation of a [R2C═U═O]2+ analogue of the [O═U═O]2+ uranyl ion (R = Ph2PNSiMe3), and comparison of the nature of U(IV)═C, U(V)═C, and U(VI)═C double bonds

We report attempts to prepare uranyl(VI)- and uranium(VI) carbenes utilizing deprotonation and oxidation strategies. Treatment of the uranyl(VI)-methanide complex [(BIPMH)UO(2)Cl(THF)] [1, BIPMH = HC(PPh(2)NSiMe(3))(2)] with benzyl-sodium did not afford a uranyl(VI)-carbene via deprotonation. Instead, one-electron reduction and isolation of di- and trinuclear [UO(2)(BIPMH)(μ-Cl)UO(μ-O){BIPMH}] (2) and [UO(μ-O)(BIPMH)(μ(3)-Cl){UO(μ-O)(BIPMH)}(2)] (3), respectively, with concomitant elimination of dibenzyl, was observed. Complexes 2 and 3 represent the first examples of organometallic uranyl(V), and 3 is notable for exhibiting rare cation-cation interactions between uranyl(VI) and uranyl(V) groups. In contrast, two-electron oxidation of the uranium(IV)-carbene [(BIPM)UCl(3)Li(THF)(2)] (4) by 4-morpholine N-oxide afforded the first uranium(VI)-carbene [(BIPM)UOCl(2)] (6). Complex 6 exhibits a trans-CUO linkage that represents a [R(2)C═U═O](2+) analogue of the uranyl ion. Notably, treatment of 4 with other oxidants such as Me(3)NO, C(5)H(5)NO, and TEMPO afforded 1 as the only isolable product. Computational studies of 4, the uranium(V)-carbene [(BIPM)UCl(2)I] (5), and 6 reveal polarized covalent U═C double bonds in each case whose nature is significantly affected by the oxidation state of uranium. Natural Bond Order analyses indicate that upon oxidation from uranium(IV) to (V) to (VI) the uranium contribution to the U═C σ-bond can increase from ca. 18 to 32% and within this component the orbital composition is dominated by 5f character. For the corresponding U═C π-components, the uranium contribution increases from ca. 18 to 26% but then decreases to ca. 24% and is again dominated by 5f contributions. The calculations suggest that as a function of increasing oxidation state of uranium the radial contraction of the valence 5f and 6d orbitals of uranium may outweigh the increased polarizing power of uranium in 6 compared to 5.     

Chloroheptakis(dimethyl sulfoxide)­uranium(IV) trichloride

In the title complex, [UCl(C₂H₆OS)₇]Cl₃, the uranium metal center is coordinated in a distorted bicapped trigonal prism geometry by seven O atoms from di­methyl sulfoxide ligands and by a terminal chloride ligand. Charge balance is maintained by three outer‐sphere chloride ions per uranium(IV) metal center. Principle bond lengths include U—O 2.391 (2)–2.315 (2) Å, U—Cl 2.7207 (9) Å, and average S—O 1.540 (5) Å.     

<Superscript>99m</Superscript>Tc(CO)<Subscript>3</Subscript>-tosufloxacin dithiocarbamate complexation and radiobiological evaluation in male Wister rat model

The uranium ingot casting process is one of the steps which consolidate uranium deposits produced by electrorefiner in an ingot form in a pryprocessing technique. Since molten uranium metal reacts with a graphite crucible when the uranium is being dissolved, a graphite crucible cannot be used. Accordingly, a ceramic material must be selected which does not react with the dissolving uranium and this must be used as a coating material on the graphite crucible surface. As to this research, a reactivity experiments were performed between the coating layer and uranium by applying a thermal spray coating to the graphite material with alumina and YSZ ceramic material. As shown in the experimental result, the YSZ coating layer showed a stronger adhesive property on the side where there is no Ni–Al binding material. Moreover, no reaction was apparent between the YSZ coating layer and the uranium. Accordingly, the YSZ material and the process of thermal spray coating are considered to solve the reactive problem between uranium and a graphite crucible.