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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Convenient Syntheses of Trivalent Uranium Halide Starting Materials without Uranium Metal

Low-valent uranium coordination chemistry continues to rely heavily on access to trivalent starting materials, but these reagents are typically prepared from uranium turnings, which are becoming increasingly difficult to acquire. Here we report convenient syntheses of UI 3 (THF) 4 (THF = tetrahydrofuran) and UBr 3 (THF) 4 from UCl 4 , a more accessible uranium starting material that can be prepared from commercially available uranium oxides. UCl 3 (THF) 2 (1), UBr 3 (THF) 4 (2), and UI 3 (THF) 4 (3) were prepared by single-pot reductions from UCl 4 using KH and KC 8 and converted to 2 or 3 by halide exchange with the corresponding Me 3 SiX (where X = Br or I). Reduction of UI 4 (Et 2 O) 2 (4; Et 2 O = diethyl ether) and UI 4 (1,4-dioxane) 2 (5) was also shown to cleanly yield 3. Complex 1 was also synthesized separately by the addition of anhydrous HCl to U(BH 4 ) 3 (THF) 2 , which was prepared by thermal reduction of U(BH 4 ) 4 . All three trivalent uranium halide complexes were isolated in high crystalline yields (typically 85–99%) and their formulations were confirmed by single-crystal X-ray diffraction, elemental analysis, and 1 H NMR and IR spectroscopy. Elemental analysis conducted on triplicate samples of 1–3 exposed to vacuum for different time intervals revealed significant THF loss for all three complexes in as little as 15 min. Altogether, these results offer expedient entry into low-valent uranium chemistry for researchers lacking access to uranium turnings.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Mapping the Electronic Structure of the Uranium(VI) Dinitride Molecule, UN 2

A combined anion photoelectron spectroscopic and relativistic electronic-structure computational study of the electronic structure of the UN 2 molecule is reported. Since the photoelectron spectrum of uranium dinitride’s negative ion, UN 2 - , (UN 2 plus an additional electron) directly reflects the electronic structure of neutral UN 2 , we have measured and relied upon the photoelectron spectrum of the UN 2 – anion as a means of mapping the electronic structure of neutral UN 2 . In addition to the electron affinity of the UN 2 ground state (the energy released when attaching an electron to UN 2 molecule), energy levels of the UN 2 excited states (electronic states with higher energies than the ground state) were well characterized by the close interplay between experiment and high-level theory. We found that both electron attachment and electronic excitation significantly bend the UN 2 molecule and elongate its U≡N bond. Implications for the activation of UN 2 are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

The Exceptional Diversity of Homoleptic Uranium–Methyl Complexes

Homoleptic σ-bonded uranium–alkyl complexes have been a synthetic target since the Manhattan Project. The current study describes the synthesis and characterization of several unprecedented uranium–methyl complexes. Amongst these complexes, the first example of a homoleptic uranium–alkyl dimer, [Li(THF) 4 ] 2 [U 2 (CH 3 ) 10 ], as well as a seven-coordinate uranium–methyl monomer, {Li(OEt 2 )Li(OEt 2 ) 2 UMe 7 Li} n were both crystallographically identified. We report the diversity of complexes reported herein provides critical insight into the structural diversity, electronic structure and bonding in uranium–alkyl chemistry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Evidence of a Uranium‐Paddlewheel Node in a Catecholate‐Based Metal–Organic Framework

Abstract The interactions between uranium and non‐innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium‐based metal–organic frameworks (MOFs) offer a new angle to study these interactions, as these self‐assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non‐innocent linkers. We report the synthesis of the MOF NU‐1700 , assembled from U 4+ ‐paddlewheel nodes and catecholate‐based linkers. We propose this highly unusual structure, which contains two U 4+ ions in a paddlewheel built from four linkers—a first among uranium materials—as a result of extensive characterization via powder X‐ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Evidence of a Uranium‐Paddlewheel Node in a Catecholate‐Based Metal–Organic Framework

Abstract The interactions between uranium and non‐innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium‐based metal–organic frameworks (MOFs) offer a new angle to study these interactions, as these self‐assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non‐innocent linkers. We report the synthesis of the MOF NU‐1700 , assembled from U 4+ ‐paddlewheel nodes and catecholate‐based linkers. We propose this highly unusual structure, which contains two U 4+ ions in a paddlewheel built from four linkers—a first among uranium materials—as a result of extensive characterization via powder X‐ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations.

Knapp, Julia G.↗
DOE OSTI2025

Four-electron oxidation and one-electron reduction of the bis(terphenylthiolate) U( II ) complex, U(SAr iPr6 ) 2 [Ar iPr6 = C 6 H 3 -2,6-(C 6 H 2 -2,4,6- i Pr 3 ) 2 ]

Here, the utility of the sterically bulky terphenylthiolate ligand, (SAr iPr6 ) 1− in expanding uranium reductive chemistry has been explored. Reduction of U(SAr iPr6 ) 2 I forms the U(II) complex, U(SAr iPr6 ) 2 , in which the metal is protected by the flanking arene rings of the ligand, but they move out of the way to accommodate the four electron reduction of PhN=NPh to form the U(VI) bis(imido) product U(SAr iPr6 ) 2 (=NPh) 2 (THF) 2 . Here, the KC 8 reduction of U(SAr iPr6 ) 2 generates a more reduced complex, KU(μ-SAr iPr6 ) 2 , initially identified by a −2.55 V vs. Fc + /Fc electrochemical reduction event in THF.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Effects of uranium metal carbon content on hydriding kinetics and corrosion blister number/area at sub-ambient pressures

Carbon is a common impurity in uranium metal, resulting in a number of uranium–carbon inclusion phases that contribute to an increase in metal defect density as carbon content increases. It is widely held that uranium hydride corrosion preferentially nucleates at these defect sites, and that an increase in carbon content will therefore represents an increase in uranium hydride corrosion sites on the metal surface. We hydrided six uranium sources with differing carbon contents to explore whether this assumption holds in a sub-ambient (~ 0.1 atm hydrogen), sealed environment, and report the resulting reaction kinetics and uranium hydride blister benchmarking data. We find that carbon content is not strongly correlated with reaction kinetics terms or the resulting hydride blister number and area, but that there is a tight relationship between corrosion blister number/area and kinetics as is expected. Further, we find that there is a strong trend of decreasing variance in the blister number, blister area, and induction time as carbon content increases (higher carbon content results in more reproducible blister populations). Additionally, we find a narrow band of uranium metal consumption at the end of the parabolic phase of reaction progress (beginning of linear growth phase) of 0.098 ± 0.011 w/w%, a fact that may be useful in assaying hydrogen corrosion of uranium metal within sealed environments generally.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2023

U–C Bond Insertion, Ring-Opening, and C–H Activation in a Uranium Bis(diisopropylamino)cyclopropenylidene (BAC) Adduct

Reaction of [U(NR 2 ) 3 ] (R = SiMe 3 ) with 1 equiv of bis(diisopropylamino)cyclopropenylidene (BAC) in Et 2 O results in the formation of [(NR 2 ) 3 U(BAC)] (1), which can be isolated in modest yields. Thermolysis of 1 in C 6 D 6 at 85 °C results in the formation of the ring-opened U(IV) product, [(NR 2 ) 2 U{N(R)(SiMe 2 CH=C(NiPr 2 )C(NiPr 2 )=CH)}] (2), which can be isolated in low yields. Mechanistic studies suggest that the formation of 2 proceeds via dissociation of BAC from 1 to regenerate [U(NR 2 ) 3 ], which converts into [U{N(R)(SiMe 2 CH 2 )}(NR 2 ) 2 ] at the elevated temperatures. BAC then inserts into the U–C bond of [U{N(R)(SiMe 2 CH 2 )}(NR 2 ) 2 ] to generate a cyclopropenyl intermediate, which undergoes ring opening and C–H activation to afford the final product, 2. We hypothesize that the ring-opening generates an unobserved carbene intermediate. Notably, thermolysis of a 1:1 mixture of independently prepared [U{N(R)(SiMe 2 CH 2 )}(NR 2 ) 2 ] and BAC results in clean formation of 2, providing strong support for the proposed mechanism. The formulations of both 1 and 2 were confirmed by X-ray crystallography. Finally, theoretical calculations indicate that the hypothesized uranium carbene intermediate features strong U–C bonding, potentially with some carbyne character in the electronic structure.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

A new highly enriched 233 U reference material for improved simultaneous determination of uranium amount and isotope amount ratios in trace level samples

A highly-enriched 233 U reference material (>0.99987 n( 233 U)/n(U)) has been prepared and characterized for use as an isotope dilution mass spectrometry spike. An ion exchange separation was performed on 1 g of high purity 233 U to further reduce trace amounts of contaminant Pu in the material. The purified 233 U was then prepared as a master solution which was analyzed for molality of uranium by modified Davies and Gray titration. A portion of the master solution was quantitatively diluted and dispensed for reference material units. Selected units were analyzed for verification of uranium amount and to characterize uranium isotope amount ratios by multi-collector inductively couple plasma mass spectrometry. Furthermore, modelling of spike-corrected isotopic data show that the new spike will enable simultaneous measurements of uranium amount and isotope amount ratios with resulting uncertainties that are substantially less sensitive to over spiking than widely used 233 U certified reference materials.

233U↗
DOE OSTI2024

LLNL FY24 Aging and Lifetimes Exit Criteria: Milestone 9134, GC#2, EC#2

Surface chemistry of uranium from gases in the ambient atmosphere of the application environment presents significant challenges due to the high degree of reactivity of the substrate and the ubiquity of these gases in experimental setups. Relatively little is known about the chemical mechanism and reaction rates for the catalytic dissociation of atmospheric gases on solid plutonium or uranium as well as the degradation process to form the metal hydride. Experimental studies and age-aware modeling efforts would greatly benefit from detailed atomistic knowledge of the chemical reactivity in these materials.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Uranium-Mediated Peroxide Activation and a Precursor toward an Elusive Uranium cis -Dioxo Fleeting Intermediate

The activation of chalcogen–chalcogen bonds using organometallic uranium complexes has been well documented for S–S, Se–Se, and Te–Te bonds. In stark contrast, reports concerning the ability of a uranium complex to activate the O–O bond of an organic peroxide are exceedingly rare. Herein, we describe the peroxide O–O bond cleavage of 9,10-diphenylanthracene-9,10-endoperoxide in nonaqueous media, mediated by a uranium(III) precursor [(( Me,Ad ArO) 3 N)U III (dme)] to generate a stable uranium(V) bis-alkoxide complex, namely, [(( Me,Ad ArO) 3 N)U V (DPAP)]. This reaction proceeds via an isolable, alkoxide-bridged diuranium(IV/IV) species, implying that the oxidative addition occurs in two sequential, single-electron oxidations of the metal center, including rebound of a terminal oxygen radical. Furthermore, this uranium(V) bis-alkoxide can then be reduced with KC 8 to form a uranium(IV) complex, which upon exposure to UV light, in solution, releases 9,10-diphenylanthracene to generate a cyclic uranyl trimer through formal two-electron photooxidation. Analysis of the mechanism of this photochemical oxidation via density functional theory (DFT) calculations indicates that the formation of this uranyl trimer occurs through a fleeting uranium cis-dioxo intermediate. At room temperature, this cis-configured dioxo species rapidly isomerizes to a more stable trans configuration through the release of one of the alkoxide ligands from the complex, which then goes on to form the isolated uranyl trimer complex.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

A short tutorial on uranium electropolishing

Electropolishing of uranium metal is best performed in a simple aqueous solution of phosphoric acid. With +3 to +10 volts of bias potential applied to the uranium (anode) relative to a Pt counter electrode (cathode). As summarized in a Pourbaix stability diagram, these conditions yield dissolution of the U surface as UOX containing solution species without the possibility of injecting hydrogen into the metal surface. However, if the uranium is biased to negative potentials relative to the counter electrode then water may be reduced at the U surface and hydrogen gas becomes available for reaction with the U metal. The stability diagram may be modified if there are other complexing species present in solution and the potential scale shifted if a different metal is use for the counter electrode. These effects can be understood in a known way as long as solution and electrochemical cell conditions are recognized.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2021

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA) 2 ] 2 (μ-N)(μ-κ 1 :κ 1 -BCMA) (7), [(U(BIMA) 2 ) 2 (μ-N)(μ-N i Pr)(K 2 (μ-η 3 :η 3 -CH 2 CHN i Pr)] 2 (8), and [U(BIMA) 2 ] 2 (μ-N)(μ-κ 1 :κ 1 -BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. 15 N isotopic labeling experiments confirmed the bridging nitride moieties in 7–9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1–, 2–, and 3– charges. Additionally, chemical oxidation of the U(IV) precursor U(N 3 )(BCMA) 3 yielded the cationic U(V) species [U(N 3 )(BCMA) 3 ][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA) 3 , U(N 3 )(BCMA) 3 , and [U(N 3 )(BCMA) 3 ][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2023

Temperature Dependence of Nuclear Quadrupole Resonance and the Observation of Metal–Ligand Covalency in Actinide Complexes: 35 Cl in Cs 2 UO 2 Cl 4

We report a study of the temperature dependence of 35 Cl nuclear quadrupole resonance (NQR) transition energies and spin–lattice relaxation times (T 1 ) for 235 U-depleted dicesium uranyl tetrachloride (Cs 2 UO 2 Cl 4 ) aimed at elucidating electronic interactions between the uranium center and atoms in the equatorial plane of the UO 2 2+ 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 T 1 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. Finally, the generalization of this approach to investigations of covalency in actinide–ligand bonding is examined.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Spectroscopic and Computational Evidence of Uranium Dihydrogen Complexes

Dihydrogen complexation, a phenomenon with robust precedent in the transition metal series, is spectroscopically detected for a uranium(III) complex and thereby extended for the first time to the 5f series. The vacant coordination site and low valence of (C 5 H 4 SiMe 3 ) 3 U prove to be key to the reversible formation of (C 5 H 4 SiMe 3 ) 3 U–H 2 (complex 1), and the paramagnetism of the f 3 center facilitates the detection of complex 1 by NMR spectroscopy. Density functional theory calculations reveal that the delocalization of the 5f electron density from (C 5 H 4 SiMe 3 ) 3 U onto the side-on dihydrogen ligand is crucial to complex formation, an unusual bonding situation for an actinide acid–base complex. Here, the spectroscopic and computational results are compared to those reported for lanthanide metallocenes to yield insight into the nature of–and future possibilities for–f-element dihydrogen complexation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Online monitoring the hydrolysis of uranium hexafluoride for intermediates by cryogenic layering and FTIR

Uranium hexafluoride (UF 6 ) is a commonly utilized material feedstock in uranium enrichment processes due to its high vapor pressure and ease of sublimation. When exposed to air, UF 6 undergoes spontaneous hydrolysis to form uranyl fluoride (UO 2 F 2 ) particulates which are utilized for the detection of undeclared nuclear activities by nuclear safeguards organizations. In this study, the kinetics of the hydrolysis reaction and how they relate to particle morphology of the product are still debated in the literature. Here, we report the direct, in situ observation of UF 6 reaction intermediates by cooling the reaction to cryogenic temperatures to significantly reduce the rate of hydrolysis. The reaction is then observable by Fourier transform infrared (FTIR) spectroscopy. The conversion of UF 6 to UOF 4 is observed as well as several other bands associated with possible long lived intermediate complexes. Chemometrics are used to further elucidate the reaction pathway from UF 6 to UO 2 F 2 .

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗