Low-valent Uranium Research Articles

On the origin of low-valent uranium oxidation state

The significant interest in actinide bonding has recently focused on novel compounds with exotic oxidation states. However, the difficulty in obtaining relevant high-quality experimental data, particularly for low-valent actinide compounds, prevents a deeper understanding of 5f systems. Here we show X-ray absorption near-edge structure (XANES) measurements in the high-energy resolution fluorescence detection (HERFD) mode at the uranium M4 edge for the UIII and UIV halides, namely UX3 and UX4 (X = F, Cl, Br, I). The spectral shapes of these two series exhibit clear differences, which we explain using electronic structure calculations of the 3d-4f resonant inelastic X-ray scattering (RIXS) process. To understand the changes observed, we implemented crystal field models with ab initio derived parameters and investigated the effect of reducing different contributions to the electron-electron interactions involved in the RIXS process. Our analysis shows that the electron-electron interactions weaken as the ligand changes from I to F, indicative of a decrease in ionicity both along and between the UX3 and UX4 halide series.

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.

Molybdenum sulphide clusters as redox-active supports for low-valent uranium.

The preparation of an actinide substituted cubane cluster, (Cp*3Mo3S4)Cp*UI2, and its reduced derivatives are reported. Structural and spectroscopic investigations provide insight into the unique interactions between the actinide and its redox-active molybdenum sulphide metalloligand, serving as a model to study atomically-dispersed, low-valent actinide ions on MoS2 surfaces. To probe the ability of the assembly to facilitate multielectron small molecule activation, the reactivity of the fully-reduced cluster, (Cp*3Mo3S4)Cp*U, with azobenzene was investigated. Addition of the substrate results in the formation of a cis-bis-imido cluster, (Cp*3Mo3S4)Cp*U(NPh)2. Cooperative reactivity between the actinide and redox-active support facilitates the 4e--reduction of substrate.

Multielectron Redox Chemistry of Uranium by Accessing the +II Oxidation State and Enabling Reduction to a U(I) Synthon.

The synthesis of molecular uranium complexes in oxidation states lower than +3 remains a challenge despite the interest for their multielectron transfer reactivity and electronic structures. Herein, we report the one- and two-electron reduction of a U(III) complex supported by an arene-tethered tris(siloxide) tripodal ligand leading to the mono-reduced complexes, [K(THF)U((OSi(OtBu)2Ar)3-arene)(THF)] (2) and [K(2.2.2-cryptand)][U((OSi(OtBu)2Ar)3-arene)(THF)] (2-crypt), and to the di-reduced U(I) synthons, [K2(THF)3U((OSi(OtBu)2Ar)3-arene)]∞ (3) and [(K(2.2.2-cryptand))]2[U((OSi(OtBu)2Ar)3-arene)] (3-crypt). EPR and UV/vis/NIR spectroscopies, magnetic, cyclic voltammetry, and computational studies provide strong evidence that complex 2-crypt is best described as a U(II), where the U(II) is stabilized by δ-bonding interactions between the arene anchor and the uranium frontier orbitals, whereas complexes 3 and 3-crypt are best described as having a U(III) ion supported by the di-reduced arene anchor. Three quasi-reversible redox waves at E1/2 = -3.27, -2.45, and -1.71 V were identified by cyclic voltammetry studies and were assigned to the U(IV)/U(III), U(III)/U(II), and U(II)/U(III)-(arene)2- redox couples. The ability of complexes 2 and 3 in transferring two- and three-electrons, respectively, to oxidizing substrates was confirmed by the reaction of 2 with azobenzene (PhNNPh), leading to the U(IV) complex, [K(Et2O)U((OSi(OtBu)2Ar)3-arene)(PhNNPh)(THF)] (4), and of complex 3 with cycloheptatriene, yielding the U(IV) complex, [(K(Et2O)2)U((OSi(OtBu)2Ar)3-arene)(η7-C7H7)]∞ (6). These results demonstrate that the arene-tethered tris(siloxide) tripodal ligand provides an excellent platform for accessing low-valent uranium chemistry while implementing multielectron transfer pathways as shown by the reactivity of complex 3, which provides the third example of a U(I) synthon.

Amplified Single-Atom U-O Interfacial Effect Originated from U 5f-O 2p Hybridization over UOx/GO for Enhanced Nitrogen Reduction Reaction.

Uranium-based catalysts have been regarded as promising candidates for N2 fixation owing to the low-valent uranium metal active sites possessing the ability to enhance the electron back-donating to the π* antibonding orbitals of N2 for N≡N dissociation. Herein, we report a directional half-wave rectified alternating current electrochemical method to confine oxygen-rich uranium precursors over ultrathin 2D GO nanosheets. The as-prepared uranium catalysts exhibit a considerable Faradaic efficiency of 12.7% for NH3 and the NH3 yield rate of 18.7 μg h-1 mg-1 for N2 electroreduction. Operando XAS and isotope-labeling FTIR further unravel the preferred nitrogen adsorption reaction intermediate N-(2Oax-1 U-4Oeq) and confirm the key *N2Hy intermediate species derived from the fed N2 gas. Theoretical simulations demonstrate that the U-O atomic interface originated from U 5f-O 2p orbital hybridization can accumulate partial charge from GO, which can facilitate the N≡N dissociation and lower the thermodynamic energy barrier of the first hydrogenation step.

Conversion of Uranium(III) Anilido Complexes to Uranium(IV) Imido Complexes via Hydrogen Atom Transfer

A family of low-valent uranium(III) anilido complexes supported by the bulky hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) ligand was synthesized by combining Tp*2UBn (Bn = benzyl) (1-Bn) with anilines of varying steric bulk and electronic profiles, including 4-fluoroaniline, 3,5-difluoroaniline, 3,5-bis(trifluoromethyl)aniline, 3,5-dimethylaniline, 2,4,6-trimethylaniline, and 2,4,6-tri-tBu-aniline. The corresponding uranium(III) anilido species, Tp*2UNH(4-fluorophenyl) (2-pF), Tp*2UNH(3,5-difluorophenyl) (2-bisF), Tp*2UNH(3,5-bis(trifluoromethyl)phenyl) (2-bisCF3), Tp*2UNH(3,5-dimethylphenyl) (2-bisCH3), Tp*2UNH(2,4,6-trimethylphenyl) (2-Mes), and Tp*2UNH(2,4,6-tri-tBu-phenyl) (2-Mes*), were isolated, and reactivity was explored. Conversion to their respective uranium(IV) imido species (3-pF, 3-bisF, 3-bisCF3, 3-bisCH3, 3-Mes, and 3-Mes*) was achieved by hydrogen atom transfer using either Gomberg’s dimer or the 2,4,6-tri-tBu-phenoxy radical (·OMes*) which eliminates the need to use potentially explosive organic azides, a reagent that has been commonly used for the synthesis of uranium(IV) imido complexes. For comparison of yields and purity of all methods, the uranium imido complexes were also prepared using the corresponding organic azides. Where applicable, compounds were characterized by multinuclear NMR spectroscopy (1H, 11B, 19F), infrared spectroscopy, electronic absorption spectroscopy, and single crystal X-ray crystallography.

Isolation of uranium(III) primary phosphido complexes.

Low-valent uranium(III) primary phosphido complexes supported by hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) were synthesized with phosphines of varying steric and electronic profiles. Compounds were characterized by multinuclear NMR spectroscopy (1H, 11B, 31P NMR), infrared spectroscopy, electronic absorption spectroscopy, X-ray crystallography, and quantum chemical calculations.

Uranium Going the Soft Way: Low-Valent Uranium(III) Coordinated to an Arene-Anchored Tris-Thiophenolate Ligand.

The synthesis of a tripodal, S-based ligand, namely the mesitylene-anchored, tris-thiophenolate-functionalized (mes(Me,AdArS)3)3- (1)3-, and its coordination chemistry with low-valent uranium to form [UIII((SArAd,Me)3mes)] (1-U) are reported. Single-crystal X-ray diffraction analysis reveals a C3-symmetric molecular structure. Full characterization of 1-U was performed using nuclear magnetic resonance, UV-vis-NIR electronic absorption, and electron paramagnetic resonance spectroscopies as well as SQUID magnetometry, thus confirming the U(III) oxidation state. Alternating current magnetic studies show that 1-U exhibits single-molecule magnet behavior at low temperatures in a non-zero external field. Comparison of these results to those of the previously reported mesitylene-anchored complexes, [UIII((OArAd,Me)3mes)] and [UIII((OArtBu,tBu)3mes)], indicates a drastic change in the electronic structure when moving from phenolate-based ligands to thiophenolate-based 1, which is further discussed by means of computational analysis (NBO, DFT, and QTAIM). Despite the U-O bonds being stronger, a much higher covalency was found for the U-S analogue.

Reductive Coupling of Xylyl Isocyanide Mediated by Low-Valent Uranium

Reductive Coupling of Xylyl Isocyanide Mediated by Low-Valent Uranium

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 UI3(THF)4 (THF = tetrahydrofuran) and UBr3(THF)4 from UCl4, a more accessible uranium starting material that can be prepared from commercially available uranium oxides. UCl3(THF)2 (1), UBr3(THF)4 (2), and UI3(THF)4 (3) were prepared by single-pot reductions from UCl4 using KH and KC8 and converted to 2 or 3 by halide exchange with the corresponding Me3SiX (where X = Br or I). Reduction of UI4(Et2O)2 (4; Et2O = diethyl ether) and UI4(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(BH4)3(THF)2, which was prepared by thermal reduction of U(BH4)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 1H 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. Overall, these results offer expedient entry into low-valent uranium chemistry for researchers lacking access to uranium turnings.

A relativistic DFT probe for small-molecule activation mediated by low-valent uranium metallocenes

DFT calculations rationalize the capability of uranium metallocenes in activating small molecules, and the experimentally inaccessible CO2adduct is addressed.

Theoretical design and exploration of low-valent uranium metallocenes via manipulating cyclopentadienyl substituent

Over a century of investigations on uranium complexes resulted in great progress in establishing their fundamental properties and developing applications. However, the in-depth understanding of uranium chemistry, particularly in record low + II valence, is still challenging due to rare crystallographically-identified U(II) complexes, extremely low stability and rigid synthetic conditions. Herein, to further expand experimental findings, various substituted-cyclopentadienyl uranium complexes, [U(CpX)3]n (X = Me, H, Cl, SiMe3 and 2SiMe3; n = -1 and 0), have been explored using relativistic density functional theory. It is shown that the reduction from [U(CpX)3] to [U(CpX)3]– slightly strengthens the U-Cp bonding, while the great enhancement is found in previously reported uranium arene analogues. The + III oxidation state is assigned to the metal of [U(CpX)3]; in contrast, it is approximately + II valence for [U(CpX)3]– because of non-negligible electron density residing on the CpX ligands, whose amount positively correlates with the electron-withdrawing ability of X substituent. Calculated reduction potentials range from −3.02 to −2.74 V while considering solvation and spin–orbit coupling effects. The largest value of the chloride complex couples is attributed to the better balance of electronic and steric effects.

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

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

The synthesis and versatile reducing power of low-valent uranium complexes.

This Perspective provides a detailed overview of the chemistry of low-valent (di- and trivalent) uranium. The reactivity of uranium(ii) and uranium(iii) complexes is discussed both to illustrate the general types of reactions that might be expected and to highlight the many unusual modes of reactivity observed with this element. A particular emphasis is given to redox reactions with uranium(iii) species, including reduction of small molecules, multi-electron reductions involving redox-active ligands, and formation of uranium-ligand multiple bonds. In addition, redox-neutral adduct formation with uranium(iii) complexes as well as the current state of the young field of uranium(ii) redox chemistry are also covered. Synthetic protocols to prepare a wide range of low-valent compounds are presented.

Ethene Activation and Catalytic Hydrogenation by a Low-Valent Uranium Pentalene Complex.

The reaction of the uranium(III) complex [U(η8-Pn††)(η5-Cp*)] (1) (Pn†† = C8H4(1,4-SiiPr3)2, Cp* = C5Me5) with ethene at atmospheric pressure produces the ethene-bridged diuranium complex [{(η8-Pn††)(η5-Cp*)U}2(μ-η2:η2-C2H4)] (2). A computational analysis of 2 revealed that coordination of ethene to uranium reduces the carbon-carbon bond order from 2 to a value consistent with a single bond, with a concomitant change in the formal uranium oxidation state from +3 in 1 to +4 in 2. Furthermore, the uranium-ethene bonding in 2 is of the δ type, with the dominant uranium contribution being from f-d hybrid orbitals. Complex 2 reacts with hydrogen to produce ethane and reform 1, leading to the discovery that complex 1 also catalyzes the hydrogenation of ethene under ambient conditions.

An Ultrastable Heterobimetallic Uranium(IV)/Vanadium(III) Solid Compound Protected by a Redox-Active Phosphite Ligand: Crystal Structure, Oxidative Dissolution, and First-Principles Simulation.

The first heterobimetallic uranium(IV)/vanadium(III) phosphite compound, Na2UV2(HPO3)6 (denoted as UVP), was synthesized via an in situ redox-active hydrothermal reaction. It exhibits superior hydrolytic and antioxidant stability compared to the majority of structures containing low-valent uranium or vanadium, further elucidated by first-principles simulations, and therefore shows potential applications in nuclear waste management.

Synthesis and characterization of hydrotris(3-phenylpyrazolyl)borate ligands on low-valent uranium

Treating both UI3(THF)4 and UI4(dioxane)2 with one equivalent of potassium hydrotris(3-phenylpyrazolyl)borate, KTpPh, resulted in the formation of new low-valent uranium derivatives, TpPhUI2(THF)2 (1) and TpPhUI3 (2), respectively. Spectroscopic characterization of these species shows that the TpPh ligand is 1,2-borotropically shifted, causing one phenyl group to point towards the B–H rather than the uranium center. Derivatization of these materials was possible via salt metathesis reactions, and in each case, the borotropically shifted phenyl group is maintained. Treating trivalent 1 with one equivalent of potassium phenoxide, KOPh, afforded trivalent TpPhU(OPh)I(THF)2 (1-OPh), whereas, a uranium(IV) derivative, TpPhUI2(Phpz)(THF) (2-Phpz), was synthesized by treating 2 with potassium 3-phenylpyrazolide, KPhpz. All compounds were characterized by 1H and 11B NMR, infrared, and electronic absorption spectroscopies, and where possible, X-ray crystallography.

Uranium-mediated electrocatalytic dihydrogen production from water.

Depleted uranium is a mildly radioactive waste product that is stockpiled worldwide. The chemical reactivity of uranium complexes is well documented, including the stoichiometric activation of small molecules of biological and industrial interest such as H2O, CO2, CO, or N2 (refs 1 - 11), but catalytic transformations with actinides remain underexplored in comparison to transition-metal catalysis. For reduction of water to H2, complexes of low-valent uranium show the highest potential, but are known to react violently and uncontrollably forming stable bridging oxo or uranyl species. As a result, only a few oxidations of uranium with water have been reported so far; all stoichiometric. Catalytic H2 production, however, requires the reductive recovery of the catalyst via a challenging cleavage of the uranium-bound oxygen-containing ligand. Here we report the electrocatalytic water reduction observed with a trisaryloxide U(III) complex [(((Ad,Me)ArO)3mes)U] (refs 18 and 19)--the first homogeneous uranium catalyst for H2 production from H2O. The catalytic cycle involves rare terminal U(IV)-OH and U(V)=O complexes, which have been isolated, characterized, and proven to be integral parts of the catalytic mechanism. The recognition of uranium compounds as potentially useful catalysts suggests new applications for such light actinides. The development of uranium-based catalysts provides new perspectives on nuclear waste management strategies, by suggesting that mildly radioactive depleted uranium--an abundant waste product of the nuclear power industry--could be a valuable resource.

Reductive disproportionation of nitric oxide mediated by low-valent uranium.

The reductive disproportionation of nitric oxide (1 atm) is mediated by the bulky U(III) aryloxide [U(III)(OAr(Ad,Ad,Me))3] ((Ad,Ad,Me)ArO = O-C6H2-2,6-Ad-4-Me) (1) to form the U(V) terminal oxo species [((Ad,Ad,Me)ArO)3U(V)(O)] (2) and N2O, as confirmed by single crystal X-ray diffraction and GC-MS measurements. The reaction is quantitative in the solid state. Mechanistic and theoretical studies of the reaction suggest that the N-N bond is formed by the coupling of an η(1)-O bound nitric oxide ligand with gaseous NO to give an η(1)-(N2O2)(1-) intermediate prior to the spontaneous extrusion of N2O to yield the U(V) terminal oxo species 2.

Theoretical investigation of low-valent uranium and transuranium complexes of a flexible small-cavity macrocycle: structural, formation reaction and redox properties.

The flexible small-cavity macrocycle, trans-calix[2]benzene[2]pyrrolide (H2L), has been found experimentally to complexate low-valent UIII and UIV with binding pockets (BP) of bis(arene) (Ar) and bis(pyrrolide) (Pl), respectively. This switchable coordination of the uranium center has been explored using relativistic density functional theory (DFT) in this work. Systematic calculations of [(BP-L)Anm(η-H)nBH4-n)]z+ (BP = Ar and Pl; An = U, Np and Pu; m = III and IV; n = 2 and 3; and z = 0 and 1), labeled as BP-Anm-nH, were carried out. Energetics and geometrical/electronic-structure analyses reveal that the size matching between actinide ions and the binding pocket plays a significant role in determining the energetic ordering of isomers. The relatively large-size UIII and NpIII ions are selectively preferred by the large bis(arene) pocket, yielding the most stable isomer of Ar-An-2H; simultaneously formed δ(An-Ar2) bonding helps stabilizing the system. In contrast, the small-size PuIII and AnIV are held by the smaller bis(pyrrolide) to show the energetically favored Pl-An-3H isomer. This size argument is further supported by calculations on the related Th and Pa compounds. The formation reactions of BP-Anm-nH demonstrate an endothermic process when using the H2L ligand reactant. Applying a more basic alkali ligand (A2L; A = Li, Na and K) as the reactant significantly reduces the reaction energy and presents thermodynamic possibility to prepare the low-valent actinide complexes. This is in agreement with the experimental synthesis where K2L was utilized. The redox potentials (E0) from tri- to tetravalent actinides were calculated while including both solvation and spin-orbit coupling effects. The highly reductive nature of the UIII complex was manifested by the calculated E0 of over 1.1 V.