Electronic structure and bonding in actinyl ions and their analogs

Periodic Trends within Actinyl(VI) Nitrates and Their Structures, Vibrational Spectra, and Electronic Properties

A series of actinyl(VI) nitrate salts of the form MAnO₂(NO₃)₃, where M = NH₄⁺ K⁺, Rb⁺, Cs⁺, and Me₄N⁺ and AnO₂²⁺ = U, Np, Pu, and AnO₂(NO₃)₂(H₂O)₂·H₂O, and the uranyl tetranitrates M₂UO₂(NO₃)₄ have been synthesized from aqueous solution and their structures determined using single-crystal X-ray diffraction. Together, these complexes represent an isostructural series of actinide complexes among the salts crystallized with the same charge-compensating cation and have been studied using vibrational spectroscopy including Raman and Fourier-transform infrared. Periodic trends in both the structural properties of these complexes and their vibrational spectra are presented and discussed, in particular the invariant nature of the O≡An≡O asymmetric stretching frequencies observed across the actinyl series. Electronic structure calculations were performed at a variety of levels of theory to aid in the interpretation of the vibrational data and to correlate trends in the data with the underlying electronic properties of these molecules.

Progress in solid state and coordination chemistry of actinides in China

In the past decade, the area of solid state chemistry of actinides has witnessed a rapid development in China, based on the significantly increased proportion of the number of actinide containing crystal structures reported by Chinese researchers from only 2% in 2010 to 36% in 2021. In this review article, we comprehensively overview the synthesis, structure, and characterizations of representative actinide solid compounds including oxo-compounds, organometallic compounds, and endohedral metallofullerenes reported by Chinese researchers. In addition, Chinese researchers pioneered several potential applications of actinide solid compounds in terms of adsorption, separation, photoelectric materials, and photo-catalysis, which are also briefly discussed. It is our hope that this contribution not only calls for further development of this area in China, but also arouses new research directions and interests in actinide chemistry and material sciences.

Resolving the π-assisted U-N σf-bond formation using quantum information theory

We model the potential energy profiles of the UO₂ (NCO)Cl₂^- → NUOCl₂^- + CO₂ reaction pathway [Y. Gong, V. Vallet, M. del Carmen Michelini, D. Rios and J. K. Gibson, J. Phys. Chem. A, 2014, 118, 325-330] using different pair coupled-cluster doubles (pCCD) methods. Specifically, we focus on pCCD and pCCD-tailored coupled cluster models in predicting relative energies for the various intermediates and transition states along the reaction coordinate. Furthermore, we augment our study on energetics with an orbital-pair correlation analysis of the complete reaction pathway that features two distinct paths. Our analysis of orbital correlations sheds new light on the formation and breaking of respective bonds between the uranium, oxygen, and nitrogen atoms along the reaction coordinates where the "yl" bond is broken and a nitrido compound formed. Specifically, the strengthening of the U-N σ_f-bond is assisted by a π-type interaction that is delocalized over the C-N-U backbone of the UO₂ (NCO)Cl₂^- complex.

Uranyl Analogue Complexes—Current Progress and Synthetic Challenges

Uranyl ions, {UO2}n+ (n = 1, 2), display trans, strongly covalent, and chemically robust U-O multiple bonds, where 6d, 5f, and 6p orbitals play important roles. The synthesis of isoelectronic analogues of uranyl has been of interest for quite some time, mainly with the purpose of unveiling covalence and 5f-orbital participation in bonding. Significant advances have occurred in the last two decades, initially marked by the synthesis of uranium(VI) bis(imido) complexes, the first analogues with a {RNUNR}2+ core, later followed by the synthesis of unique trans-{EUO}2+ (E = S, Se) complexes, and recently highlighted by the synthesis of the first complexes featuring a linear {NUN} moiety. This review covers the synthesis, structure, bonding, and reactivity of uranium complexes containing a linear {EUE}n+ core (n = 0, 1, 2), isoelectronic to uranyl ions, {OUO}n+ (n = 1, 2), incorporating σ- and π-donating ligands that can engage in uranium–ligand multiple bonding, where oxygen may be replaced by heavier chalcogenido, imido, nitride, and carbene ligands, or by a transition metal. It focuses on synthetic methods of well-defined molecular uranium species in the condensed phase but also references gas-phase and low-temperature-matrix experiments, as well as computational studies that may lead to valuable insights.

Enhancing photosensitivity via the assembly of a uranyl coordination polymer

Synergistic assembly of uranyl centres and luminescent 2,6-bis(pyrazol-1-yl)pyridine-4-carboxylates (bppCOOH) gives rise to a uranyl coordination polymer, namely U-bppCOO, which exhibits a luminescence quenching response toward UV or X-ray irradiation doses. Notably, the photosensitivity of U-bppCOO has been significantly enhanced via metal-ligand assembly compared with that of the naked bppCOOH ligand.

Circularly Polarized Luminescence from Uranyl Improves Resolution of Electronic Transitions

The first reported example of circularly polarized luminescence from a chiral, molecular uranyl (UO₂²⁺) complex in solution is presented. This uranyl chiroptical activity is enabled by complexation with ibuprofen, an enantiopure chiral carboxylate ligand. Salt metathesis between [UO₂Cl₂(thf)₂]₂ and the sodium ibuprofenate salts results in the formation of the anionic tris complexes; these complexes are found to be luminescent in solution, both under visible excitation, directly targeting the metal, and through sensitization by UV absorption and energy transfer from the ligand. Each enantiomer displays both circular dichroism and circularly polarized luminescence (CPL) with |g_abs| ≤ 8.1 × 10^-2 and |g_lum| ≤ 8.0 × 10^-3 under UV excitation, comparable to chiral transition metal complexes or purely organic emitters. The strength of the CPL emission is found to be comparable following excitation of either the ligand or metal directly. Further, use of CPL allows for resolution of subcomponents of the emission spectrum not previously possible at room temperature using standard fluorescence techniques. Observation of CPL following direct uranyl excitation presents a new tool for probing speciation of uranyl complexes when chiral ligands are used, without the need for synthetic modification to incorporate a suitable chromophore, and could enable the design of improved ligands for uranyl extraction from wastewater.

A spectroscopic, structural, and computational study of Ag-oxo interactions in Ag+/UO22+ complexes

Twelve novel Ag⁺/UO₂²⁺ heterometallic complexes have been prepared and characterized via structural, spectroscopic, and computational methods to probe the effects of Ag-oxo interactions on bonding and photophysical properties of the uranyl cation. Structural characterization reveals Ag-oxo interaction distances ranging from 2.475(3) Å to 4.287(4) Å. These interactions were probed using luminescence and Raman spectroscopy which displayed little effect on the luminescence intensity and the energy of the Raman active UO symmetric stretch peak as compared to previously reported Pb-oxo interactions. Computational efforts via density functional theory-based natural bond orbital analysis revealed that the highest stabilization energy associated with the Ag-oxo interaction had a value of only 11.03 kcal mol^-1 and that all other energy values fell at 7.05 kcal mol^-1 or below indicating weaker interactions relative to those previously reported for Pb²⁺/UO₂²⁺ heterometallic compounds. In contrast, quantum theory of atoms in molecules analysis of bond critical point electron density values indicated higher electron density in Ag-oxo interactions as compared to Pb-oxo interactions which suggests more covalent character with the Ag⁺. Overall, this data indicates that Ag⁺ has a less significant effect on UO₂²⁺ bonding and photophysical properties as compared to other Pb²⁺, likely due to the high polarizability of the cation.

Highlighting Recent Crystalline Engineering Aspects of Luminescent Coordination Polymers Based on F-Elements and Ditopic Aliphatic Ligands

Three principal factors may influence the final structure of coordination polymers (CPs): (i) the nature of the ligand, (ii) the type and coordination number of the metal center, and (iii) the reaction conditions. Further, flexible carboxylate aliphatic ligands have been widely employed as building blocks for designing and synthesizing CPs, resulting in a diverse array of materials with exciting architectures, porosities, dimensionalities, and topologies as well as an increasing number of properties and applications. These ligands show different structural features, such as torsion angles, carbon backbone number, and coordination modes, which affect the desired products and so enable the generation of polymorphs or crystalline phases. Additionally, due to their large coordination numbers, using 4f and 5f metals as coordination centers combined with aliphatic ligands increases the possibility of obtaining different crystal phases. Additionally, by varying the synthetic conditions, we may control the production of a specific solid phase by understanding the thermodynamic and kinetic factors that influence the self-assembly process. This revision highlights the relationship between the structural variety of CPs based on flexible carboxylate aliphatic ligands and f-elements (lanthanide and actinides) and their outstanding luminescent properties such as solid-state emissions, sensing, and photocatalysis. In this sense, we present a structural analysis of the CPs reported with the oxalate ligand, as the one rigid ligand of the family, and other flexible dicarboxylate linkers with –CH₂– spacers. Additionally, the nature of the luminescence properties of the 4f or 5f-CPs is analyzed, and finally, we present a novel set of CPs using a glutarate-derived ligand and samarium, with the formula [2,2′-bipyH][Sm(HFG)₂ (2,2′-bipy) (H₂O)₂]•(2,2′-bipy) (α-Sm) and [2,2′-bipyH][Sm(HFG)₂ (2,2′-bipy) (H₂O)₂] (β-Sm).

A Coupled EXAFS-Molecular Dynamics Study on PuO2+ and NpO2+ Hydration: The Importance of Electron Correlation in Force-Field Building

The physicochemical properties of the monovalent actinyl cations, PuO₂⁺ and NpO₂⁺, in water have been studied by means of classical molecular dynamic simulations. A specific set of cation-water intermolecular potentials based on ab initio potential energy surfaces has been built on the basis of the hydrated ion concept. The TIP4P water model was adopted. Given the paramagnetic character of these actinyls, the cation-water interaction energies were computed from highly correlated wave functions using the NEVPT2 method. It is shown that the multideterminantal character of the wave function has a relevant effect on the main distances of the hydrated molecular cations. Several structural, dynamical, and energetic properties of the aqueous solutions have been obtained and analyzed. Structural RDF analysis gives An-O_yl distances of 1.82 and 1.84 Å and An-O(water) distances of 2.51 and 2.53 Å for PuO₂⁺ and NpO₂⁺ in water, respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO₂⁺ and NpO₂⁺ are revisited and analyzed, assuming tetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MD simulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that both PuO₂⁺ and NpO₂⁺ cations in water are stable pentahydrated aqua ions.

Developing a Unique Hydrogen-Bond Network in a Uranyl Coordination Framework for Fuel Cell Applications

Crystalline materials with persistent high anhydrous proton conductivity that can be directly used as a practical electrolyte of the intermediate-temperature proton exchange membrane fuel cells for durable power generation remain a substantial challenge. The present work proposes a unique way of the axial uranyl oxo atoms as hydrogen-bond acceptors to form a dense hydrogen-bonded network within a stable uranyl-based coordination polymer, UO₂(H₂PO₃)₂(C₃N₂H₄)₂ (HUP-3). It exhibits stable and efficient anhydrous proton conductivity over a super-wide temperature range (-40-170 °C). It was also assembled into a H₂/O₂ fuel cell as the electrolyte and shows a high electrical power density of 11.8 mW·cm^-2 at 170 °C, which is among one of the highest values reported from crystalline solid electrolytes. The cell was tested for over 12 h without notable power loss.

Insights into the Electronic Structure of a U(IV) Amido and U(V) Imido Complex

Reaction of the N-heterocylic carbene ligand i PrIm (L1 ) and lithium bis(trimethylsilyl)amide (TMSA) as a base with UCl4 resulted in U(IV) and U(V) complexes. Uranium's +V oxidation state in (HL1 )2 [U(V)(TMSI)Cl5 ] (TMSI=trimethylsilylimido) (2) was confirmed by HERFD-XANES measurements. Solid state characterization by SC-XRD and geometry optimisation of [U(IV)(L1 )2 (TMSA)Cl3 ] (1) indicated a silylamido ligand mediated inverse trans influence (ITI). The ITI was examined regarding different metal oxidation states and was compared to transition metal analogues by theoretical calculations.

Covalency in actinide(iv) hexachlorides in relation to the chlorine K-edge X-ray absorption structure

Chlorine K-edge X-ray absorption near edge structure (XANES) in actinideIV hexachlorides, [AnCl6]2- (An = Th-Pu), is calculated with relativistic multiconfiguration wavefunction theory (WFT). Of particular focus is a 3-peak feature emerging from U toward Pu, and its assignment in terms of donation bonding to the An 5f vs. 6d shells. With or without spin-orbit coupling, the calculated and previously measured XANES spectra are in excellent agreement with respect to relative peak positions, relative peak intensities, and peak assignments. Metal-ligand bonding analyses from WFT and Kohn-Sham theory (KST) predict comparable An 5f and 6d covalency from U to Np and Pu. Although some frontier molecular orbitals in the KST calculations display increasing An 5f-Cl 3p mixing from Th to Pu, because of energetic stabilization of 5f relative to the Cl 3p combinations of the matching symmetry, increasing hybridization is neither seen in the WFT natural orbitals, nor is it reflected in the calculated bond orders. The appearance of the pre-edge peaks from U to Pu and their relative intensities are rationalized simply by the energetic separation of transitions to 6d t2g versus transitions to weakly-bonded and strongly stabilized a2u, t2u and t1u orbitals with 5f character. The study highlights potential pitfalls when interpreting XANES spectra based on ground state Kohn-Sham molecular orbitals.

Relativistic Effects in Modeling the Ligand K-Edge X-ray Absorption Near-Edge Structure of Uranium Complexes

Accurate modeling of the complex electronic structure of actinide complexes requires full inclusion of relativistic effects. In this study, we examine the effect of explicit inclusion of spin-orbit coupling (SOC) versus scalar relativistic effects on the predicted spectra for heavy-element complexes. In this study, we employ a relativistic two-component Hamiltonian in the X2C form with all of the electrons in the system being considered explicitly to compare and contrast with previous studies that included the relativistic effects by means of relativistic effective core potentials (RECPs). A few uranium complexes are chosen as model systems. Comparison of the computed Cl K-edge X-ray absorption spectra with experimental data shows significantly improved agreement when a variational relativistic treatment of SOC is performed. In particular, we note the importance of SOC terms to obtain not only correct transition energies but also correct intensities for these heavy-element complexes because of the redistribution of ligand bonding character among the valence MOs. While RECPs generally agree well with all-electron scalar relativistic calculations, there are some differences in the predicted spectra of open-shell systems. These methods are still suitable for broad application to analyze the qualitative nature of transitions in X-ray absorption spectra, but caution is recommended for quantitative analysis, as SOC can be non-negligible for both open- and closed-shell heavy-element systems.

Stable Chelation of the Uranyl Ion by Acyclic Hexadentate Ligands: Potential Applications for 230U Targeted α-Therapy

Uranium-230 is an α-emitting radionuclide with favorable properties for use in targeted α-therapy (TAT), a type of nuclear medicine that harnesses α particles to eradicate cancer cells. To successfully implement this radionuclide for TAT, a bifunctional chelator that can stably bind uranium in vivo is required. To address this need, we investigated the acyclic ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox as uranium chelators. The stability constants of these ligands with UO22+ were measured via spectrophotometric titrations, revealing log βML values that are greater than 18 and 26 for the "pa" and "hox" chelators, respectively, signifying that the resulting complexes are exceedingly stable. In addition, the UO22+ complexes were structurally characterized by NMR spectroscopy and X-ray crystallography. Crystallographic studies reveal that all six donor atoms of the four ligands span the equatorial plane of the UO22+ ion, giving rise to coordinatively saturated complexes that exclude solvent molecules. To further understand the enhanced thermodynamic stabilities of the "hox" chelators over the "pa" chelators, density functional theory (DFT) calculations were employed. The use of the quantum theory of atoms in molecules revealed that the extent of covalency between all four ligands and UO22+ was similar. Analysis of the DFT-computed ligand strain energy suggested that this factor was the major driving force for the higher thermodynamic stability of the "hox" ligands. To assess the suitability of these ligands for use with 230U TAT in vivo, their kinetic stabilities were probed by challenging the UO22+ complexes with the bone model hydroxyapatite (HAP) and human plasma. All four complexes were >95% stable in human plasma for 14 days, whereas in the presence of HAP, only the complexes of H2CHXdedpa and H2hox remained >80% intact over the same period. As a final validation of the suitability of these ligands for radiotherapy applications, the in vivo biodistribution of their UO22+ complexes was determined in mice in comparison to unchelated [UO2(NO3)2]. In contrast to [UO2(NO3)2], which displays significant bone uptake, all four ligand complexes do not accumulate in the skeletal system, indicating that they remain stable in vivo. Collectively, these studies suggest that the equatorial-spanning ligands H2dedpa, H2CHXdedpa, H2hox, and H2CHXhox are highly promising candidates for use in 230U TAT.

A terminal neptunium(V)-mono(oxo) complex

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ_u > π_u energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium-mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal-ligand bond can be stabilized and studied in isolation.

Surface Coverage- and Excitation Laser Wavelength-Dependent Luminescence Properties of U(VI) Species Adsorbed on Amorphous SiO2

Time-resolved luminescence spectroscopy is usefully used to identify U(VI) surface species adsorbed on SiO2. However, the cause of the inconsistent luminescence lifetimes and spectral shapes reported previously remains undetermined. In this study, the U(VI) surface coverage (Γ) and excitation laser wavelength (λex) were examined as the predominant factors governing the luminescence properties of U(VI) surface species. At neutral pH, the luminescence lifetimes of U(VI) surface species increased with decreasing Γ. In the low-Γ region, where a relatively large number of adjacent surface sites are involved in the formation of multidentate surface complexes, the displacement of more number of coordinated water molecules in the equatorial plane of U(VI) results in a longer lifetime. The pH-dependent luminescence lifetimes of U(VI) surface species at the same U(VI) to SiO2 concentration ratio in the pH range of 4.5–7.5 also explain the effect of the surface binding sites on the luminescence lifetime. The time-resolved luminescence properties of the U(VI) surface species were also investigated at different excitation wavelengths. Continued irradiation of the SiO2 surface with a UV laser beam at λex = 266 nm considerably reduced the luminescence intensities of the U(VI) surface species. The higher the laser pulse energy, the greater the decrease in luminescence intensity. Laser-induced thermal desorption (LITD) of U(VI) surface species is suggested to be the origin of the decrease in luminescence intensity. LITD effects were not observed at λex = 355 and 422 nm, even at high laser pulse energies.

Insights at the molecular level into the formation of oxo-bridged trinuclear uranyl complexes

Reaction of 1,3,4,6-tetra-O-acetyl-N-(2-hydroxy)-naphthylidene glucosamine (HL(Ac)) with uranyl acetate in ethanol leads to formation of dinuclear [(UO₂)₂(L)₂] (1). In a second step 1 is quantitatively transferred into the trinuclear oxo-bridged complex [(UO₂)₃(μ₃-O)(L)₃]^2- (2^2-) via deprotonation and coordination of a water molecule. This transformation was followed by NMR and UV/Vis spectroscopy and it proved possible to selectively introduce ¹⁸O into the μ₃-bridge.

Use of vibrational spectroscopy to identify the formation of neptunyl-neptunyl interactions: A paired Density Functional Theory and Raman spectroscopy study

Abstract: Actinyl-Actinyl interactions (AAIs) occur in pentavalent actinide systems, particularly for Np(V), and lead to complex vibrational signals that are challenging to analyze and interpret. Previous studies have focused on...

Simultaneous quantification of uranium(vi), samarium, nitric acid, and temperature with combined ensemble learning, laser fluorescence, and Raman scattering for real-time monitoring

Laser-induced fluorescence spectroscopy, Raman spectroscopy, and a stacked regression was developed for rapid quantification of uranium(vi) (1–100 μg mL−1), samarium (0–200 μg mL−1) and nitric acid (0.1–4 M) with varying temperature (20 °C–45 °C).

Covalency between the uranyl ion and dithiophosphinate by sulfur K-edge X-ray absorption spectroscopy and density functional theory

The dithiophosphinic acids (HS2PR2) have been used for the selective separation of trivalent actinides (AnIII) from lanthanides (LnIII) over the past decades. The substituents on the dithiophosphinic acids dramatically impact the separation performance, but the mechanism is still open for debate. In this work, two dithiophosphinic acids with significantly different AnIII/LnIII separation performance, i.e. diphenyl dithiophosphinic acid (HS2PPh2) and bis(ortho-trifluoromethylphenyl) dithiophosphinic acid [HS2P(o-CF3C6H4)2], are employed to understand the substituent effect on the bonding covalency between the S2PR2- anions (R = Ph and o-CF3C6H4) and the uranyl ion by sulfur K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory calculations. The two UO2(S2PR2)(EtOH) complexes display similar XAS spectra, in which the first pre-edge feature with an intensity of 0.16 is entirely attributed to the transitions from S 1s orbitals to the unoccupied molecular orbitals due to the mixing between U 5f and S 3p orbitals. The Mulliken population analysis indicates that the amount of \% S 3p character in these orbitals is essentially identical for the UO2(S2PPh2)2(EtOH) and UO2[S2P(o-CF3C6H4)2]2(EtOH) complexes, which is lower than that in the U 6d-based orbitals. The essentially identical covalency in U-S bonds for the two UO2(S2PR2)2(EtOH) complexes are contradictory to the significantly different AnIII/LnIII separation performance of the two dithiophosphinic acids, thus the covalency seems to be unable to account for substituent effects in the AnIII/LnIII separation by the dithiophosphinic acids. The results in this work provide valuable insight into the understanding of the mechanism in the AnIII/LnIII separation by the dithiophosphinic acids.

Luminescent Uranium-Thiophene Dicarboxylate Frameworks Supported by Transition-Metal-Organic Cations: Dimensionality and Luminescence Control in Uranyl Speciation

The study about the resourcfulization of uranium such as the usage as new materials can promote the harmless treatments of nuclear waste. In this work, with the direction of transition-metal...

Quantitative U=O bond activation in uranyl complexes via silyl radical transfer

Reductive silylation of the uranyl dication with 1,4-bis(trimethylsilyl)dihydropyrazine, or “Mashima’s Reagent”, is detailed. The substrate simultaneously delivers silylium ions and electrons to multiple uranyl complexes (e.g. pyridine dipyrrolide uranyl complex...

X-ray absorption spectroscopy and actinide electrochemistry: a setup dedicated to radioactive samples applied to neptunium chemistry

A spectroelectrochemical setup has been developed to investigate radioactive elements in small volumes (0.7 to 2 ml) under oxidation-reduction (redox) controlled conditions by X-ray absorption spectroscopy (XAS). The cell design is presented together with in situ XAS measurements performed during neptunium redox reactions. Cycling experiments on the NpO22+/NpO2+ redox couple were applied to qualify the cell electrodynamics using XANES measurements and its ability to probe modifications in the neptunyl hydration shell in a 1 mol l-1 HNO3 solution. The XAS results are in agreement with previous structural studies and the NpO22+/NpO2+ standard potential, determined using Nernst methods, is consistent with measurements based on other techniques. Subsequently, the NpO2+, NpO22+ and Np4+ ion structures in solution were stabilized and measured using EXAFS. The resulting fit parameters are again compared with other results from the literature and with theoretical models in order to evaluate how this spectroelectrochemistry experiment succeeds or fails to stabilize the oxidation states of actinides. The experiment succeeded in: (i) implementing a robust and safe XAS device to investigate unstable radioactive species, (ii) evaluate in a reproducible manner the NpO22+/NpO2+ standard potential under dilute conditions and (iii) clarify mechanistic aspects of the actinyl hydration sphere in solution. In contrast, a detailed comparison of EXAFS fit parameters shows that this method is less appropriate than the majority of the previously reported chemical methods for the stabilization of the Np4+ ion.

Lanthanide-uranyl phosphonates constructed from diethyl ((phenylsulfonyl)methyl)phosphonate

Lanthanide-uranyl phosphonates possess intriguing crystal structures and huge application prospects, but the construction of these materials remains challenging. In this work, we demonstrate that new uranyl and lanthanide-uranyl sulfonylphosphonates with elegant crystal structures and photophysical properties can be assembled hydrothermally by employing the heterofunctional diethyl phosphonate (diethyl ((phenylsulfonyl)methyl)phosphonate, Et₂L) as a precursor ligand, which provides an effective strategy for the construction of lanthanide-uranyl phosphonates.

Heterobimetallic uranyl(VI) alkoxides of lanthanoids: formation through simple ligand exchange

Lanthanoid and actinoid silylamides are versatile starting materials. Herein we show how a simple ligand exchange with tert-butanol leads to the formation of the first trimeric heterobimetallic uranyl(VI)-lanthanoid(III) alkoxide complexes. The μ³ coordination of the endogenous uranyl oxo atom results in a significant elongation of the bond length and a significant deviation from the linear uranyl arrangement.

Experimental and Computational Study of a Tetraazamacrocycle Bis(aryloxide) Uranyl Complex and of the Analogues {E═U═NR}2+ (E = O and NR)

The reaction of [U(κ⁶-{(^t-Bu2ArO)₂Me₂-cyclam})I][I] (H₂{(^t-Bu2ArO)₂Me₂-cyclam} = 1,8-bis(2-hydroxy-3,5-di-tert-butyl)-4,11-dimethyl-1,4,8,11-tetraazacyclotetradecane) with 2 equiv of NaNO₂ in acetonitrile results in the isolation of the uranyl complex [UO₂{(^t-Bu2ArO)₂Me₂-cyclam}] (3) in 31% yield, which was fully characterized, including by single-crystal X-ray diffraction. Density functional theory (DFT) computations were performed to evaluate and compare the level of covalency within the U═E bonds in 3 and in the analogous trans-bis(imido) [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(NPh)₂] (1) and trans-oxido-imido [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(O)(NPh)] (2) complexes. Natural bond orbital (NBO) analysis allowed us to determine the mixing covalency parameter λ, showing that in 2, where both U-O_oxido and U-N_imido bonds are present, the U-N_imido bond registers more covalency with regard to 1, and the opposite is seen for U-O_oxido with respect to 3. However, the covalency driven by orbital overlap in the U-N_imido bond is slightly higher in 1 than in 2. The ¹⁵N-labeled complexes [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(¹⁵NPh)₂] (1-¹⁵N) and [U(κ⁴-{(^t-Bu2ArO)₂Me₂-cyclam})(O)(¹⁵NPh)] (2-¹⁵N) were prepared and analyzed by solution ¹⁵N NMR spectroscopy. The calculated and experimental ¹⁵N chemical shifts are in good agreement, displaying the same trend of δ_N (1-¹⁵N) > δ_N (2-¹⁵N) and reveal that the ¹⁵N chemical shift may serve as a probe for the covalency of the U═NR bond.

Creation of an unexpected plane of enhanced covalency in cerium(III) and berkelium(III) terpyridyl complexes

Controlling the properties of heavy element complexes, such as those containing berkelium, is challenging because relativistic effects, spin-orbit and ligand-field splitting, and complex metal-ligand bonding, all dictate the final electronic states of the molecules. While the first two of these are currently beyond experimental control, covalent M‒L interactions could theoretically be boosted through the employment of chelators with large polarizabilities that substantially shift the electron density in the molecules. This theory is tested by ligating BkIII with 4'-(4-nitrophenyl)-2,2':6',2"-terpyridine (terpy*), a ligand with a large dipole. The resultant complex, Bk(terpy*)(NO3)3(H2O)·THF, is benchmarked with its closest electrochemical analog, Ce(terpy*)(NO3)3(H2O)·THF. Here, we show that enhanced Bk‒N interactions with terpy* are observed as predicted. Unexpectedly, induced polarization by terpy* also creates a plane in the molecules wherein the M‒L bonds trans to terpy* are shorter than anticipated. Moreover, these molecules are highly anisotropic and rhombic EPR spectra for the CeIII complex are reported.

Synthesis of a heterobimetallic actinide nitride and an analysis of its bonding

Reaction of [K(DME)][Th{N(R)(SiMe₂ CH₂)}₂(NR₂)] (R = SiMe₃) with 1 equiv. of [U(NR₂)₃(NH₂)] (1) in THF, in the presence of 18-crown-6, results in formation of a bridged uranium-thorium nitride complex, [K(18-crown-6)(THF)₂][(NR₂)₃U^IV(μ-N)Th^IV(NR₂)₃] (2), which can be isolated in 48% yield after work-up. Complex 2 is the first isolable molecular mixed-actinide nitride complex. Also formed in the reaction is the methylene-bridged mixed-actinide nitride, [K(18-crown-6)][K(18-crown-6)(Et₂O)₂][(NR₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NR)Th(NR₂)₂]₂ (3), which can be isolated in 34% yield after work-up. Complex 3 is likely generated by deprotonation of a methyl group in 2 by [NR₂]^-, yielding the new μ-CH₂ moiety and HNR₂. Reaction of 2 with 0.5 equiv. of I₂ results in formation of a U^V/Th^IV bridged nitride, [(NR₂)₃U^V(μ-N)Th^IV(NR₂)₃] (4), which can be isolated in 42% yield after work-up. The electronic structure of 4 was analyzed with EPR spectroscopy, SQUID magnetometry, and NIR-visible spectroscopy. This analysis demonstrated that the energies of 5f orbitals of 4 are largely determined by the strong ligand field exerted by the nitride ligand.

Secondary Role of Aliphatic and Heterocyclic Amines in the Formation of Low-Temperature Amine-Bearing U, Np, and Pu(VI) Chromates

Uranyl compounds with tetrahedral oxoanions demonstrate a significant structural and topological diversity. Complexes of transuranium elements with such anions are not equally well-represented in the literature. To answer the question about the structural similarity in a series of An⁶⁺ complexes with XO₄^2- anions, we synthesized and studied 10 new U, Np, and Pu chromates with outer-sphere organic cations. The structural analysis and comparison with the literature data shows that the Np and Pu complexes are generally based on the same structural blocks as the uranyl compounds. Moreover, the chromate anion does not show any unique structural role as compared to the sulfate and selenate ions. As a result, the neptunium and plutonium chromates contain 1D and 2D structural units similar to those found in the uranyl sulfates and selenates. The templating role of the outer-sphere cations in the actinyl complexes with tetrahedral oxoanions is also not evident, and there is no clear correlation between the nature of the outer-sphere cations and the topology of the structural units.

Involvement of 5f Orbitals in the Covalent Bonding between the Uranyl Ion and Trialkyl Phosphine Oxide: Unraveled by Oxygen K-Edge X-ray Absorption Spectroscopy and Density Functional Theory

Monodentate organophosphorus ligands have been used for the extraction of the uranyl ion (UO₂²⁺) for over half a century and have exhibited exceptional extractability and selectivity toward the uranyl ion due to the presence of the phosphoryl group (O═P). Tributyl phosphate (TBP) is the extractant of the world-renowned PUREX process, which selectively recovers uranium from spent nuclear fuel. Trialkyl phosphine oxide (TRPO) shows extractability toward the uranyl ion that far exceeds that for other metal ions, and it has been used in the TRPO process. To date, however, the mechanism of the high affinity of the phosphoryl group for UO₂²⁺ remains elusive. We herein investigate the bonding covalency in a series of complexes of UO₂²⁺ with TRPO by oxygen K-edge X-ray absorption spectroscopy (XAS) in combination with density functional theory (DFT) calculations. Four TRPO ligands with different R substituents are examined in this work, for which both the ligands and their uranyl complexes are crystallized and investigated. The study of the electronic structure of the TRPO ligands reveals that the two TRPO molecules, irrespective of their substituents, can engage in σ- and π-type interactions with U 5f and 6d orbitals in the UO₂Cl₂(TRPO)₂ complexes. Although both the axial (O_yl) and equatorial (O_eq) oxygen atoms in the UO₂Cl₂(TRPO)₂ complexes contribute to the X-ray absorption, the first pre-edge feature in the O K-edge XAS with a small intensity is exclusively contributed by O_eq and is assigned to the transition from O_eq 1s orbitals to the unoccupied molecular orbitals of 1b_1u + 1b_2u + 1b_3u symmetries resulting from the σ- and π-type mixing between U 5f and O_eq 2p orbitals. The small intensity in the experimental spectra is consistent with the small amount of O_eq 2p character in these orbitals for the four UO₂Cl₂(TRPO)₂ complexes as obtained by Mulliken population analysis. The DFT calculations demonstrate that the U 6d orbitals are also involved in the U-TRPO bonding interactions in the UO₂Cl₂(TRPO)₂ complexes. The covalent bonding interactions between TRPO and UO₂²⁺, especially the contributions from U 5f orbitals, while appearing to be small, are sufficiently responsible for the exceptional extractability and selectivity of monodentate organophosphorus ligands for the uranyl ion. Our results provide valuable insight into the fundamental actinide chemistry and are expected to directly guide actinide separation schemes needed for the development of advanced nuclear fuel cycle technologies.

Equatorial Electronic Structure in the Uranyl Ion: Cs2UO2Cl4 and Cs2UO2Br4

Electric field gradient (EFG) tensors in the equatorial plane of the linear UO₂²⁺ ion have been measured by nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments and computed by relativistic Kohn-Sham methods with and without environment embedding for Cs₂UO₂Cl₄ and Cs₂UO₂Br₄. This approach expands the possibilities for probing the electronic structure in uranyl complexes beyond the strongly covalent U-O bonds. The combined analyses find that one of the two largest principal EFG tensor components at the halogen sites points along the U-X bond (X = Cl, Br), and the second is parallel to the UO₂²⁺ ion; in Cs₂UO₂Cl₄, the components are nearly equal in magnitude, whereas in Cs₂UO₂Br₄, due to short-range bromide-cesium interactions, the equatorial component is dominant for one pair of Br sites and the axial component is larger for the second pair. The directions and relative magnitudes of the field gradient principal axes are found to be sensitive to the σ and π electron donation by the ligands and the model of the environment. Chlorine-35 NQR spectra of ²³⁵U-depleted and ²³⁵U-enriched Cs₂UO₂Cl₄ exhibited no uranium-isotope-dependent shift, but the resonance of the depleted sample displayed a 58% broader line width.

Pb-Oxo Interactions in Uranyl Hybrid Materials: A Combined Experimental and Computational Analysis of Bonding and Spectroscopic Properties

Reported are the syntheses and characterization of six new heterometallic UO₂²⁺/Pb²⁺ compounds. These materials feature rare instances of M-oxo interactions, which influence the bonding properties of the uranyl cation. The spectroscopic effects of these interactions were measured using luminescence and Raman spectroscopy. Computational density functional theory-based natural bonding orbital and quantum theory of atoms in molecules methods indicate interactions arise predominantly through charge transfer between cationic units via the electron-donating uranyl O sp^x lone pair orbitals and electron-accepting Pb²⁺ p orbitals. The interaction strength varies as a function of Pb-oxo interaction distance and angle with energy values ranging from 0.47 kcal/mol in the longer contacts to 21.94 kcal/mol in the shorter contacts. Uranyl units with stronger interactions at the oxo display an asymmetric bond weakening and a loss of covalent character in the U═O bonds interacting closely with the Pb²⁺ ion. Luminescence quenching is observed in cases in which strong Pb-oxo interactions are present and is accompanied by red-shifting of the uranyl symmetric Raman stretch. Changes to inner sphere uranyl bonding manifest as a weakening of the U═O bond as a result of interaction with the Pb²⁺ ion. Comprehensive evaluation of the effects of metal ions on uranyl spectra supports modeling efforts probing uranyl bonding and may inform applications such as forensic signatures.

Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding

We report here a detailed experimental and theoretical investigation of hexavalent uranium in various local configurations with a high-energy-resolution fluorescence-detected X-ray absorption near-edge structure at the U M₄ edge. We show the pronounced sensitivity of the technique to the arrangement of atoms around the absorber and provide a detailed theoretical interpretation revealing the nature of spectral features. Calculations based on density functional theory and on crystal field multiplet theory indicate that for all local configurations analyzed, the main peak corresponds to nonbonding 5f orbitals, and the highest energy peak corresponds to antibonding 5f orbitals. Our findings agree with the accepted interpretation of uranyl spectral features and embed the latter in a broader field of view, which interprets the spectra of a large variety of U⁶⁺-containing samples on a common theoretical ground.

Synthesis, characterization, and theoretical analysis of a plutonyl phosphine oxide complex

The interplay of bond strength and covalency are examined in AnO₂Cl₂(OPcy₃)₂ (An = Pu, U) complexes. The synthesis of trans-PuO₂Cl₂(OPcy₃)₂, 1-Pu, has been carried out and confirmed by single crystal X-ray diffraction along with UV-vis-NIR, and ³¹P NMR spectroscopies. Theoretical analysis finds that despite a higher calculated covalency for the Pu-Cl interaction, the Pu-OPcy₃ interaction is stronger due to the accumulation of electron density in the interatomic region. The coordination of equatorial ligands slightly decreases the strength of the PuO_yl interactions relative to the free gas phase (PuO₂)²⁺ ion.

Electrochemical behaviour of uranium at a tripolyphosphate modified ITO electrode

UO₂²⁺ binds to the surface of a tripolyphosphate modified mesoporous indium tin-doped oxide electrode (nanoITO|P₃). Electrochemical studies reveal that nITO|P₃ electrodes catalyze the 2-electron interconversion between UO₂²⁺ and U⁴⁺ with the P₃-ligand assisting in the rate-limiting proton-coupled reduction of U(V) to U(IV), based on the kinetic isotope effect (1.8). Product composition between nITO|P₃(U⁴⁺) and surface adsorbed UO₂ can be controlled by adjusting the proton concentration and/or scan rate in voltammograms. These studies with uranium suggest that nITO|P₃ electrodes are good candidates for redox transformations with other actinides including neptunium, plutonium, and americium.

Unprecedented neptunyl(V) cation-directed structural variations in Np2Ox compounds

Studies on transuranic oxides provide a particularly valuable insight into chemical bonding in actinide compounds, in which subtle differences between metal ions and oxygen atoms are of fundamental importance for the stability of these compounds as well as their existence. In the case of neptunium, it is still mainly limited to specific Np oxide compounds without periodicity in the formation of stable structures or different oxidation states. Here, we report a systematic global minimum search of Np₂O_x (x = 1-7) clusters and the computational study of their electronic structures and chemical bonding. These studies suggest that Np(V) ion could play the structure-directing role, and thus the mixed-valent Np(III/V) in Np₂O₄ is predicted accessible. In comparison with lower oxidation state Np analogues, significant 5f-orbital covalent interactions with Np(V)O bonding are observed, which shows that these model neptunium oxides can provide new understandings into the behavior of 5f-electrons in chemical bonding and structural design.

Mass spectrometric and theoretical study on the formation of uranyl hydride from uranyl carboxylate

Uranyl hydride in the form of HUO₂Cl₂^- was prepared upon collision-induced dissociation of (RCO₂)UO₂Cl₂^- (R = H, CH₃CH₂, CH₃CH₂CH₂, CH₃CHCH, (CH₃)₂CH, C₅H₉, C₆H₁₁ and C₆H₅CH₂CH₂) in the gas phase. It was found that uranyl hydrides result from alkene and alkyne elimination with concomitant β-hydride transfer of uranyl alkylides RUO₂Cl₂^- following decarboxylation of the carboxylates with the exception of (HCO₂)UO₂Cl₂^-, and formation of HU^VIO₂Cl₂^- through alkene/alkyne loss is in competition with neutral ligand loss to give U^VO₂Cl₂^-. According to the calculations at the B3LYP level, loss of a neutral ligand is slightly less favorable in the cases of (CH₃CH₂)UO₂Cl₂^- and (CH₃CH₂CH₂)UO₂Cl₂^-, and the situations of (CH₃CHCH)UO₂Cl₂^-, ((CH₃)₂CH)UO₂Cl₂^-, (C₅H₉)UO₂Cl₂^-, (C₆H₁₁)UO₂Cl₂^- and (C₆H₅CH₂CH₂)UO₂Cl₂^- with β-hydrogen atoms should be similar despite the fact that the yield of uranyl hydride depends on the nature of the ligand. Although no uranyl hydride was observed when β-hydrogen is not available in the carboxylate precursor, there is no HUO₂Cl₂^- generated from (C₆H₅CO₂)UO₂Cl₂^-, (2-C₆H₄FCO₂)UO₂Cl₂^- and (CH₂CHCH₂CO₂)UO₂Cl₂^- with β-hydrogen either. This is attributed to the much more favorable formation of UO₂Cl₂^- over HUO₂Cl₂^- as revealed by the B3LYP calculations, which is similar to the absence of HUO₂Cl₂^- in the (CH₃CO₂)UO₂Cl₂^- case where highly reactive CH₂ would be formed.

Oxo-sulfido molybdenum and tungsten fluorides with M-O and M-S multiple bonds

Oxo-sulfido molybdenum/tungsten difluorides in the form of Mo(O)(S)F₂ and W(O)(S)F₂ were prepared in cryogenic matrices via the reactions of laser-ablated metal atoms and SOF₂. Both complexes were characterized to possess one oxo, one sulfido and two fluoro ligands terminally bound to the metal center according to the results of infrared spectroscopy combined with isotopic substitution, and non-planar C_s symmetries with closed shell singlet ground states were established on the basis of density functional calculations. The SMoO and SWO bond angles of Mo(O)(S)F₂ and W(O)(S)F₂ are around 107°, which are close to those of bent MoO₂²⁺ and WO₂²⁺ (∼101°). Natural bond orbital calculations indicate the presence of a Mo/W-O double bond in Mo(O)(S)F₂ and W(O)(S)F₂ while the Mo/W-S bond is better described as a triple bond upon F^- coordination to SMoO²⁺ and SWO²⁺. UV-Vis irradiation is required in order to form the oxo-sulfido molybdenum/tungsten difluorides when metal atoms react with SOF₂ in cryogenic matrices.

Photodissociation and Infrared Spectroscopy of Uranium-Nitrogen Cation Complexes

Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U⁺(N₂)_n (n = 1-8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U⁺(N₂)_n indicate that N₂ ligands are intact molecules and that there is no insertion chemistry resulting in UN⁺ or NUN⁺. Fixed frequency photodissociation at 532 and 355 nm indicate that the U⁺-N₂ bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U⁺-N₂ dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N-N stretch vibration via elimination of N₂ molecules. These resonances are observed to be shifted about 130 cm^-1 to the red from the free-N₂ frequency for complexes with n = 3-8. Density functional theory indicates that U⁺ is most stable in the sextet state in these complexes and that N₂ molecules bind in end-on configurations. The fully coordinated complex is predicted to be U⁺(N₂)₈, which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U⁺-N₂ dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.

Photoluminescence of Pentavalent Uranyl Amide Complexes

Pentavalent uranyl species are crucial intermediates in transformations that play a key role for the nuclear industry and have recently been demonstrated to persist in reducing biotic and abiotic aqueous environments. However, due to the inherent instability of pentavalent uranyl, little is known about its electronic structure. Herein, we report the synthesis and characterization of a series of monomeric and dimeric, pentavalent uranyl amide complexes. These synthetic efforts enable the acquisition of emission spectra of well-defined pentavalent uranyl complexes using photoluminescence techniques, which establish a unique signature to characterize its electronic structure and, potentially, its role in biological and engineered environments via emission spectroscopy.

The influence of piperazine diamine templates on the synthesis, structures and properties of uranyl oxalate complex

As an important nuclear material, uranium is one of the most concerned elements in the nuclear fuel cycle, which could interact with many inorganic and organic ligands. Amine templates have a significant structural-oriented effect on the construction of uranyl oxalate complex. In this work, the piperazine diamine templates were used to synthesize uranyl oxalate complex and their crystal structures were resolved by single crystal diffraction, and their spectra were studied by IR, Raman, UV–vis, fluorescence, and EPR techniques. The final results show that crystal structures, properties and applications of uranyl oxalate complex have a close correlation with polyamine templates. The single crystal structure results show that the structural-oriented effect of piperazine diamine template is greatly affected by the proportion and concentration of solute in the surrounding environment. And the alkyl substituents on N atoms of amine templates are related to the tight of structures. Interestingly, 5# has a potential application as the original material for multiple reuse of fluorescent sensor materials. At present, there is no clear and in-depth study on the internal mechanism of such phenomena in solid uranyl complexes, and the specific mechanism needs to be further explored.

Topology of the Electron Density and of Its Laplacian from Periodic LCAO Calculations on f-Electron Materials: The Case of Cesium Uranyl Chloride

The chemistry of f-electrons in lanthanide and actinide materials is yet to be fully rationalized. Quantum-mechanical simulations can provide useful complementary insight to that obtained from experiments. The quantum theory of atoms in molecules and crystals (QTAIMAC), through thorough topological analysis of the electron density (often complemented by that of its Laplacian) constitutes a general and robust theoretical framework to analyze chemical bonding features from a computed wave function. Here, we present the extension of the Topond module (previously limited to work in terms of s-, p- and d-type basis functions only) of the Crystal program to f- and g-type basis functions within the linear combination of atomic orbitals (LCAO) approach. This allows for an effective QTAIMAC analysis of chemical bonding of lanthanide and actinide materials. The new implemented algorithms are applied to the analysis of the spatial distribution of the electron density and its Laplacian of the cesium uranyl chloride, Cs2UO2Cl4, crystal. Discrepancies between the present theoretical description of chemical bonding and that obtained from a previously reconstructed electron density by experimental X-ray diffraction are illustrated and discussed.