Uranyl Complexes Research Articles

Theoretical unraveling the complexation and separation of uranyl–ligand complexes towards chiral R/S‐profenofos

AbstractThe identification and separation of chiral pesticides R/S‐profenofos are of great significance due to different biological toxicity of chiral isomers. In the study, we designed two novel ligands: (2,2′:6′,2″‐terpyridin)‐6‐yl(5‐methoxy‐1H‐pyrazol‐1‐yl) methanethione (TMMT) and (2,2′:6′,2″‐terpyridin)‐6‐yl(5‐methoxy‐1H‐pyrazol‐1‐yl) methanone (TMMO), and taking them as ligands coordinated with uranyl, two novel complexes [Uranyl‐TMMT]2+ and [Uranyl‐TMMO]2+ as receptors were successfully constructed. The complexation and enantioselectivity of two receptors towards R/S‐profenofos as guests were systematically unraveled based on the density functional theory (DFT) method. The results showed that the receptor [Uranyl‐TMMT]2+ containing thioamide group has stronger electron donating ability than the receptor [Uranyl‐TMMO]2+ containing amide group. All analyses including Mayer bond order (MBO), quantum theory of atoms in molecules (QTAIM), localized orbital locator (LOL) function, molecular orbital, and infrared spectrum illustrated that the UX (X = S, O) bonds play a larger role than the UN bonds in the selective complexation of receptors towards guests; the US bond in [Uranyl‐TMMT]2+ is more covalent than the UO bond in [Uranyl‐TMMO]2+. The thermodynamic calculations showed that the separation factors of [Uranyl‐TMMT]2+ towards R/S‐PFFs were between 8356 and 1219 in different solvents, and the separation factors of [Uranyl‐TMMO]2+ towards R/S‐PFFs were at range of 4951–710. The separation factors exhibited that two receptors have stronger coordination and complexation ability to S‐PFF than to R‐PFF, while [Uranyl‐TMMT]2+ exhibited significantly greater recognition ability and enantioselectivity for guests R/S‐PFFs molecules than the receptor [Uranyl‐TMMO]2+.

The stability and chemical bonding of a series tridentate ligand-actinyl complexes: [AnO2(L)2]2+ (An: U and Am)

In this paper, we studied the electronic structure and chemical bonding in a series tridentate ligand-actinyl complexes, [AnO2(L)2]2+ (An: U and Am; L: pyridine-2,6-dicarboxamide (PDA), pyridine-2,6-dicarboxthioamide (PDTA) and furan-2,6-dicarboxamide (FDA)) via using relativistic density functional theory. According to the calculation results, it is found that the actinyl is coordinated by two ligands via six donor atoms with the formation of a 1:2 complex. Among these thermodynamic achievable complexes, the [UO2(PDA)2]2+ species generally show the largest thermal stability, and differences in stability mainly arise from differences in the valence shell levels of the donor atoms. The main U − Ligand bonding character in uranyl complexes favors electrostatic interaction over orbital interaction. We subsequently assessed the bonding difference of uranium and americium in [AnO2(L)2]2+ (L: PDA and PDTA), finding out that binding ability of [AmO2]2+ to both ligands of PDA and PDTA is stronger than that of [UO2]2+, due to the larger orbital interactions in [AmO2(L)2]2+ complexes, which arises from significant participant of Am 5f in chemical bonding. This finding provides a fundamentally qualitative understanding of bonding nature in actinyl complexes and could provide some guidelines when selection of ligands for the best complexation behaviors toward actinyl.

Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes.

The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, (MesPDPPh)UO2(THF) and (Cl2PhPDPPh)UO2(THF), are reported as the UVI uranyl adducts of the bulky aryl substituted pincers (MesPDPPh)2– and (Cl2PhPDPPh)2– (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2MesPDPPh, Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H2Cl2PhPDPPh, Cl2Ph = 2,6-dichlorophenyl), respectively). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li2ArPDPPh, Ar = Mes of Cl2Ph), salt metathesis with [UO2Cl2(THF)2]2 afforded both compounds in moderate yields. The characterization of each species has been undertaken by a combination of solid- and solution-state methods, including combustion analysis, infrared, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal X-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional (ArPDPPh)2– pincer ligand. The electrochemical analysis of both compounds by cyclic voltammetry in tetrahydrofuran (THF) reveals rich redox profiles, including events assigned as UVI/UV redox couples. A time-dependent density functional theory study has been performed on (MesPDPPh)UO2(THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.

Synthesis and Structural Characterization of a Di-nuclear Uranyl Complex with Quinoline-6-carboxylate

Synthesis and Structural Characterization of a Di-nuclear Uranyl Complex with Quinoline-6-carboxylate

Benignly-fabricated supramolecular poly(amidoxime)-alginate-poly(acrylic acid) beads synergistically enhance uranyl capture from seawater

Design of amidoxime-based adsorbents in a cooperative and synergistic manner together with a concept of facile and green fabrication is becoming highly desirable to harvest uranium from seawater. Herein, taking benefit of supramolecular ionic-crosslinking and hydrogen bonding interactions, we have reported a simple and environmentally benign approach to construct novel bifunctional poly(amidoxime)-alginate-poly(acrylic acid) (PAO-A-PAA) composite beads. As a result of synergistic uranyl binding, the PAO-A-PAA composite beads reached a high adsorption capacity of 735.1 mg g−1 for uranium aqueous solution (36 ppm), which is 2.24 and 1.46 times that of monofunctional Alg-PAA (328.1 mg g−1) and Alg-PAO (502.2 mg g−1), respectively. More importantly, the PAO-A-PAA exhibits excellent selectivity towards U(VI) in the presence of natural organic matter and multiple coexisting metal ions in simulated seawater. The XPS analysis reveals the utilization and coordination of both amidoxime and carboxylate ligands of PAO-A-PAA for uranyl binding, which results in energetically more stable synergistic uranyl complexes (Ebinding = −9.45 to −10.34 eV) as proved by the density functional theory study. The adsorption efficiency of PAO-A-PAA is further supported by its ability to extract µg L−1-level uranium in real seawater (94.7–99.5%). The PAO-A-PAA also demonstrates good mechanical and chemical stability over a wide pH range, as well as good reusability. These findings provide insight into the significance of designing efficient sorbent material for enhanced uranyl capture via a supramolecular strategy.

Genesis of uranyl mineralization in the Arabian Nubian Shield: A review

This paper provides a review and synthesis of research literature on the geological, mineralogical and geochemical data to uranyl mineralization of the Arabian Nubian Shield (ANS). In general, the studied uranyl mineralization from the different locations of the ANS is similar from the mineralogical point of view. The mineral assemblages include, not conclusively, uranophane, kasolite, soddyite, boltwoodite, autunite, meta-autunite, liandratite, uranopyrochlore, ishikawaite and fergusonite. The compiled dataset supports a model of two main stages, namely hypogene or high-T alteration stage and supergene or low-T alteration stage. Both stages played an important role in the genesis of uranyl mineralization of the ANS. The late hydrothermal fluids leached uranium from the deep part of the host rocks and transported it in different uranous and uranyl complexes. While moving upward, the fluids gradually cooled to the temperature of meteoric water and probably have mixed with it. At these conditions, the pH of the fluids changes to be more alkaline and uranyl minerals start to develop in the fracture system mainly adsorbed on the clay minerals and probably coprecipitated with iron oxy-hydroxides. The requirement for supergene or low-T alteration stage would be saturation by oxic groundwater, a case implying the availability of meteoric water during the formation of uranyl mineralization in the ANS. Some of these minerals were dated by U-series method as 50–159 ka, and this could be attributed mainly to the Saharan II pluvial period.

Assessment of Various Density Functional Theory Methods for Finding Accurate Structures of Actinide Complexes.

Density functional theory (DFT) is a widely used computational method for predicting the physical and chemical properties of metals and organometals. As the number of electrons and orbitals in an atom increases, DFT calculations for actinide complexes become more demanding due to increased complexity. Moreover, reasonable levels of theory for calculating the structures of actinide complexes are not extensively studied. In this study, 38 calculations, based on various combinations, were performed on molecules containing two representative actinides to determine the optimal combination for predicting the geometries of actinide complexes. Among the 38 calculations, four optimal combinations were identified and compared with experimental data. The optimal combinations were applied to a more complicated and practical actinide compound, the uranyl complex (UO2(2,2′-(1E,1′E)-(2,2-dimethylpropane-1,3-dyl)bis(azanylylidene)(CH3OH)), for further confirmation. The corresponding optimal calculation combination provides a reasonable level of theory for accurately optimizing the structure of actinide complexes using DFT.

Influence of EDTA on the interaction between U(VI) and calcite

Uranium contamination is of great concern due to its long-term radioactivity and toxicity. The migration of uranium in environment is well known affected by its interaction with clay minerals and the presence of organic matters. The interaction of U(VI) with calcite in the presence of EDTA across weak to strong alkaline conditions was studied by batch sorption and coprecipitation experiments. Multiple spectroscopic methods and DFT calculation were applied to help analyze the experimental results, and the different roles EDTA plays in the interaction process between uranium and calcite were revealed. It is proposed that at hyperalkaline conditions (pH > 9.5), U(VI) favors to precipitate on calcite surface by forming uranium minerals, yet the presence of EDTA impedes the precipitation of uranium by increase the dissolution of calcite by complexing with Ca2+. In contrast, under weak alkaline conditions (pH 7.5–9.5), EDTA enhances the removal of uranium from solution by calcite, mainly through increasing the specific surface area and sorption sites of calcite due to the steps, vacancies and/or etch pits created by EDTA on calcite surface without modifying its lattice structure and surface functional groups. DFT calculations of Ca−UO2 −CO3 ternary complex interacted with calcite also support the theory that uranyl complexes prefer the sorption on the steps, vacancies and/or etch pits created by EDTA on calcite surface rather than on a flat calcite surface. These results unveil the details of interaction between uranium and calcite, including the presence of EDTA, will help to better understand the migration behavior of uranium in nature, especially in soil minerals.

Two-Dimensional Imprinting Strategy to Create Specific Nanotrap for Selective Uranium Adsorption with Ultrahigh Capacity

Uranium extraction is highly challenging because of low uranium concentration, high salinity, and a large number of competing ions in different environments. The template strategy is developed to address the defect of poor selectivity, but the adsorption capacity is limited by cavity blocking during the preparation of materials. Herein, a two-dimensional (2D) imprinting strategy is adopted to design 2D imprinted networks with specific nanotraps for effective uranium capture. The imprinted networks are established through the condensation polymerization of uranyl complexes, which are formed by aromatic building units coordinating with uranyl ions on the equatorial plane. Different from traditional imprinting materials that contain many invalid cavities (buried cavities or unreleased cavities), the as-prepared adsorbents possess tailored 2D nanotraps, which are open and specific to uranyl. Thus, the optimized networks not only show excellent selectivity for uranium (Kd = 964,500 mL/g in multi-ion solution) and slight disturbance of high salinity but also possess an ultrahigh adsorption capacity of 1365.7 mg/g. In addition, this adsorbent shows a high extraction efficiency for uranium under a wide range of pH conditions and exhibits good regeneration performance. This work proposes a pioneering strategy of 2D imprinting networks to capture uranium specifically with high capacity and can be applied to material design in many other fields.

Exploring the Redox Properties of Bench-Stable Uranyl(VI) Diamido–Dipyrrin Complexes

The uranyl complexes UO2(OAc)(L) and UO2Cl(L) of the redox-active, acyclic diamido–dipyrrin anion L– are reported and their redox properties explored. Because of the inert nature of the complexes toward hydrolysis and oxidation, synthesis of both the ligands and complexes was conducted under ambient conditions. Voltammetric, electron paramagnetic resonance spectroscopy, and density functional theory studies show that one-electron chemical reduction by the reagent CoCp2 leads to the formation of a dipyrrin radical for both complexes [Cp2Co][UO2(OAc)(L•)] and [Cp2Co][UO2Cl(L•)].

Molecular dynamics study of uranyl adsorption from aqueous solution to smectite

Adsorption of aqueous uranyl species in pore spaces of clay minerals plays a key role in post-closure safety assessments for geological disposal of radioactive waste. Molecular Dynamics Simulations (MDS) were performed to study the adsorption of uranyl (UO22+) to the clay mineral smectite from its aqueous solution in the presence of carbonate (CO32−) and Na+ counter ions. The modelled system consisted of water-saturated clay layers and a 0.162 M aqueous uranyl carbonate solution. The large system size (up to 25,560 atoms) and long simulation times (over 200 ns) allowed investigating the effect of the electrical double layer on uranyl adsorption to smectite for different pore sizes (8.37, 25.1, and 33.5 nm). This study identified various polynuclear uranyl carbonate complexes on clay surfaces (e.g., [Na(UO2)3(CO3)3]+) and in aqueous solution (e.g., [Na2(UO2)5(CO3)6]0) which were not seen in other MDS studies due to statistical limitations caused by the small number of uranyl and carbonate ions in their super cell and/or insufficient simulation time. Uranyl complexes represented the majority of adsorbed species relative to uncomplexed uranyl, with different complexes for different pore sizes. The sorption parameter KD for uranyl ranged from 59 to 151 mL/g and was dependent on pore size (smaller KD for larger pore size). Electrostatic factors controlled the formation and locations of uranyl complexes. MDS-based KD values were in agreement with experimentally derived values for similar experimental conditions. MDS provides an efficient tool to derive sorption parameters for safety assessments, especially in the early stages of site selection and characterisation when access to cores from deep rocks may be limited.

Efficient UO22+ extraction by DAPhens with asymmetric terminal groups: The molecular design, spectral titration, liquid-liquid extraction and mechanism study

• DAPhens with asymmetric terminal groups show high selectivity to UO 2 2+ . • Methyl groups in amide moieties facilitate DAPhens’ extraction performance. • Less distortion of DAPhens in complex benefits its superior affinity to UO 2 2+ . Four asymmetric alkyl-substituted DAPhens (N,N’-Dimethyl-N,N’-Dialkyl-2,9-Diamide-1,10-Phenanthrolines) were synthesized and investigated as selective ligands for UO 2 2+ . The results confirm that the ligands have higher affinity for uranyl comparing to the symmetric analogues with the same 1,10-phenanthroline core moiety. The slope analysis indicated the formation of 1:1 (metal/ligand) extracting complex, which was identified in the organic phase as [UO 2 (DAPhen-4)(NO 3 )] + complex ion by electrospray ionization mass spectrometry (ESI-MS). The superior affinity of the ligands for UO 2 2+ over Th 4+ and Eu 3+ in solvent extraction was consistent with the stability constants of their complexes in acetonitrile obtained by UV–Vis spectroscopic titrations. The observed better affinity of the asymmetric alkyl-substituted DAPhens could originate from the less steric hindrance of methyl in the terminal groups, which was confirmed by the relatively mild distortion of the ligand structure in the uranyl complex according to the crystal and molecular structure of the solid-state complex [UO 2 (DAPhen-1)(NO 3 )](NO 3 )·(CH 3 OH) as characterized by single crystal X-ray diffraction.

Tetra-Substituted p-Tert-Butylcalix[4]Arene with Phosphoryl and Salicylamide Functional Groups: Synthesis, Complexation and Selective Extraction of f-Element Cations.

A new series of lanthanide (1–5) and uranyl (6) complexes with a tetra‐substituted bifunctional calixarene ligand H2L is described. The coordination environment for the Ln3+ and UO2 2+ ions is provided by phosphoryl and salicylamide functional groups appended to the lower rim of the p‐tert‐butylcalix[4]arene scaffold. Ligand interactions with lanthanide cations (light: La3+, Pr3+; intermediate: Eu3+ and Gd3+; and heavy: Yb3+), as well as the uranyl cation (UO2 2+) is examined in the solution and solid state, respectively with spectrophotometric titration and single crystal X‐ray diffractometry. The ligand is fully deprotonated in the complexation of trivalent lanthanide ions forming di‐cationic complexes 2 : 2 M : L, [Ln2(L)2(H2O)]2+ (1–5), in solution, whereas uranyl formed a 1 : 1 M : L complex [UO2(L)(MeOH)]∞ (6) that demonstrated very limited solubility in 12 organic solvents. Solvent extraction behaviour is examined for cation selectivity and extraction efficiency. H2L was found to be an effective extracting agent for UO2 2+ over La3+ and Yb3+ cations. The separation factors at pH 6.0 are: βUO 22+ /La 3+ =121.0 and βUO 22+ /Yb 3+ =70.0.

Theoretical Insights on Improving Amidoxime Selectivity for Potential Uranium Extraction from Seawater.

Extraction of uranium from seawater is one of the important ways to solve the shortage of terrestrial uranium resources. Thereinto, the competition between uranyl and vanadium cations is a significant challenge in the commonly used amidoxime-based adsorbents for extracting uranium from seawater. An in-depth understanding of the extraction behaviors of modified amidoxime groups with uranyl and vanadium ions is one of the effective means to design and develop efficient adsorbents for selective uranium sequestration. In this work, we have designed and systematically investigated the alkyl and amino functionalized amidoxime, (Z)-2-amino-N'-hydroxy-N,N-dimethylbenzimidamide (L1), and its phenyl and methoxy derivatives ((Z)-3-amino-N'-hydroxy-N,N-dimethyl-2-naphthimidamide (L2) and (Z)-2-amino-N'-hydroxy-4-methoxy-N,N-dimethylbenzimidamide (L3)) by quantum chemistry calculations. In the uranyl complexes, the amidoxime groups prefer to act as η2-coordinated ligands as the amidoximes increase, and there exist substantial hydrogen bond interactions, which are different from the vanadium complexes. Various bonding analyses show that the L1 ligand possesses a stronger binding affinity to UO22+, and the -C6H5 and -CH3O substituent groups seem to have no effect on the improvement of extraction ability. Thermodynamic analysis confirms that the L1 ligand has a stronger extraction capability to uranyl ion compared to L2 and L3. According to the calculations of the vanadium (V) (VO2+ and VO3+) complexes with the L1 ligand, L1 is more likely to react with [H2VO4]- and [HVO4]2- to form VO2+ complexes. Expectantly, thermodynamic analysis displays a higher extraction capacity for uranyl ions than vanadium ions. Therefore, these alkyl and amino functionalized amidoxime ligands demonstrate high selectivity for uranyl over vanadium ions, which is mainly due to the coordination mode changes of these ligands toward vanadium in conjunction with the considerable hydrogen bonds in the uranyl complexes. These results are expected to afford useful clues for the design of efficient adsorbents for uranium extraction from seawater.

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 reagent simultaneously delivers silylium ions and electrons to multiple uranyl complexes, i.e. a pyridine dipyrrolide uranyl complex and a common uranyl-containing starting material, UO2Cl2(OPPh3)2. This reaction results in quantitative activation of thermodynamically robust UO bonds. The reductive functionalization of environmentally persistent actinyl species with Mashima's reagent is seen as a promising solution for nuclear waste remediation.

Enhanced luminescence of tris(carboxylato)uranyl(VI) complexes and energy transfer to Eu(III): a combined spectroscopic and theoretical investigation.

Complex formation between uranyl and carboxylate ligands (benzoate, nicotinate and isonicotinate) has been studied extensively by absorption and luminescence spectroscopy in acetonitrile medium. Experimental data had indicated the existence of stable and enhanced luminescent tris(carboxylato) uranyl(VI) complexes i.e. [UO2(L)3]- with D3h symmetry. The high luminescence of these complexes was due to the sensitization of the Oyl → U ligand to metal charge transfer (LMCT) emission by extremely intense equatorial (carboxylate ligands) LMCT bands. The variation in the experimentally observed parameters such as intensity of equatorial LMCT bands, luminescence lifetimes, quantum yields and structural parameters among tris(carboxylato) uranyl(VI) complexes are affirmed by quantum chemical calculations using density functional theory and the computational results are found to be in good agreement with experimental findings. Interestingly, in a very dilute mixture of [UO2(L)3]- and Eu(III), energy transfer from uranyl to Eu(III) is observed and it leads to the detection of europium at trace levels. This is an intriguing observation as none of the previous studies have reported such a low level of detection limit of Eu(III) by means of energy transfer from any metal.

Synthesis and characterization of a uranyl(vi) complex with 2,6-pyridine-bis(methylaminophenolato) and its ligand-centred aerobic oxidation mechanism to a diimino derivative

Both aminomethylene groups of a uranyl(vi) complex bearing a 2,6-pyridine-bis(methylaminophenolato) ligand are oxidized to azomethine ones by an O2 molecule under an ambient atmosphere to provide a diimino derivative of the parent uranyl(vi) complex.

Complexation of uranyl(vi) with succinimidedioxime in comparison with glutarimidedioxime

Glutarimidedioxime favors 1 : 2 neutral uranyl complex species, whereas succinimidedioxime tends to form a cross-linked 2 : 2 binuclear complex with uranyl ions.

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- (22-) 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.

Effects of coordinating heteroatoms on molecular structure, thermodynamic stability and redox behavior of uranyl(vi) complexes with pentadentate Schiff-base ligands.

Uranyl(vi) complexes with pentadentate N3O2-, N2O3- and N2O2S1-donating Schiff base ligands, tBu,MeO–saldien–X2− (X = NH, O and S), were synthesized and thoroughly characterized by 1H NMR, IR, elemental analysis, and single crystal X-ray diffraction. The crystal structures of UO2(tBu,MeO–saldien–X) showed that the U–X bond strength follows U–O ≈ U–NH > U–S. Conditional stability constants (βX) of UO2(tBu,MeO–saldien–X) in ethanol were investigated to understand the effect of X on thermodynamic stability. The log βX decrease in the order of UO2(tBu,MeO–saldien–NH) (log βNH = 10) > UO2(tBu,MeO–saldien–O) (log βO = 7.24) > UO2(tBu,MeO–saldien–S) (log βS = 5.2). This trend cannot be explained only by Pearson's Hard and Soft Acids and Bases (HSAB) principle, but rather follows the order of basicity of X. Theoretical calculations of UO2(tBu,MeO–saldien–X) suggested that the ionic character of U–X bonds decreases in the order of U–NH > U–O > U–S, while the covalency increases in the order U–O < U–NH < U–S. Redox potentials of all UO2(tBu,MeO–saldien–X) in DMSO were similar to each other regardless of the difference in X. Spectroelectrochemical measurements and DFT calculations revealed that the center U6+ of each UO2(tBu,MeO–saldien–X) undergoes one-electron reduction to afford the corresponding uranyl(v) complex. Consequently, the difference in X of UO2(tBu,MeO–saldien–X) affects the coordination of tBu,MeO–saldien–X2− with UO22+. However, the HSAB principle is not always prominent, but the Lewis basicity and balance between ionic and covalent characters of the U–X interactions are more relevant to determine the bond strengths.