Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry

Uranyl diacetate dihydrate is a useful reagent for the preparation of uranyl (UO22+) coordination complexes, as it is a well-defined stoichiometric compound featuring moderately basic acetates that can facilitate protonolysis reactivity, unlike other anions commonly used in synthetic actinide chemistry such as halides or nitrate. Despite these attractive features, analogous neptunium (Np) and plutonium (Pu) compounds are unknown to date. Here, a modular synthetic route is reported for accessing stoichiometric neptunyl(VI) and plutonyl(VI) diacetate compounds that can serve as starting materials for transuranic coordination chemistry. The new NpO22+ and PuO22+ complexes, as well as a corresponding molecular UO22+ complex, are isomorphous in the solid state, and in solution show similar solubility properties that facilitate their use in synthesis. In both solid and solution state, the +VI oxidation state (O.S.) is maintained, as demonstrated by vibrational and optical spectroscopy, confirming that acetate anions stabilize the oxidizing, high-valent +VI states of Np and Pu as they do for the more stable U(VI). All three acetate salts readily react with a model diprotic ligand, affording incorporation of U(VI), Np(VI), and Pu(VI) cores into molecular coordination compounds that occurs concomitantly with elimination of acetic acid; the new complexes are high-valent, yet overall charge neutral, facilitating entry into nonaqueous chemistry by rational synthesis. Computational studies reveal that the dianionic ligand framework assists in stabilizing the +VI O.S. via donation to the 5f shells of the actinides, highlighting the potential usefulness of protonolysis reactivity toward preparation of stabilized high-valent transuranic species.

Semiconductive Behavior and Photoconductivity of Uranyl Dithiophosphinate Single Crystal

Herein, we detail the synthesis, structure, and photoconductivity of the uranyl dithiophosphinate single crystal UO2[S2P(C6H5)2]2(CH3OH)·CH3OH (denoted as U-DPDPP). The formation of bonds between uranyl ions and sulfur-based ligands endows U-DPDPP with a distinct electronic absorption property with a broadband spectrum spanning from 250 to 550 nm, giving rise to a unique semiconductive property. Under X-ray illumination, U-DPDPP displays a distinctive photoconductivity response, with a charge carrier mobility lifetime (μτ) of 2.78 × 10-4 cm2·V-1 achieved, which contradicts the electronic-silence behavior of uranyl nitrate crystal.

Luminometric dosimetry of X-ray radiation by a zwitterionic uranium coordination polymer

A novel X-ray dosimeter based on a uranium coordination polymer has been developed by the judicious synergy between the luminescent uranyl centres and zwitterionic tritopic ligands.

Comparing coordination uranyl(vi) complexes with 2-(1H-imidazo[4,5-b]phenazin-2-yl)phenol and derivatives

Four derivatives of 2-(1H-imidazo[4,5-b]phenazin-2-yl)phenol have been synthesized and characterized structurally using X-ray crystallography. Coordination complexes with uranyl (UO₂²⁺) and copper (Cu²⁺) were prepared and absorption/emission spectra detailed. We observed increased fluorescence upon uranyl binding, in stark contrast to rapid quenching observed with the addition of copper. These phenomena have been further examined by DFT computational methods.

Dinuclear uranium(VI) salen coordination compound: an efficient visible-light-active catalyst for selective reduction of CO2 to methanol

A new dinuclear uranyl salen coordination compound, [(UO2)2(L)2]·2MeCN [L = 6,6'-((1E,1'E)-((2,2-dimethylpropane-1,3-diyl)bis(azaneylylidene))-bis(methaneylylidene))bis(2-methoxyphenol)], was synthesized using a multifunctional salen ligand to harvest visible light for the selective photocatalytic reduction of CO2 to MeOH. The assembling of the two U centers into one coordination moiety via a chelating-bridging doubly deprotonated tetradentate ligand allowed the formation of U centers with distorted pentagonal bipyramid geometry. Such construction of compounds leads to excellent activity for the photocatalytic reduction of CO2, permitting a production rate of 1.29 mmol g-1 h-1 of MeOH with an apparent quantum yield of 18%. Triethanolamine (TEOA) was used as a sacrificial electron donor to carry out the photocatalytic reduction of CO2. The selective methanol formation was purely a photocatalytic phenomenon and confirmed using isotopically labeled 13CO2 and product analysis by 13C-NMR spectroscopy. The spectroscopic studies also confirmed the interaction of CO2 with the molecule of the title complex. The results of these efforts made it possible to understand the reaction mechanism using ESI-mass spectrometry.

Detailed characterization of dioxouranium(vi) complexes with a symmetrical tetradentate N2O2-benzil bis(isonicotinoyl hydrazone) ligand

The reactions of UO2(OAc)2·2H2O with benzil bis(isonicotinoyl hydrazone) ligand (H2L) in varied solvent media resulted in the formation of a series of new dioxouranium(vi) complexes 1-3 of the type UO2(L)(X), [where 1, X = DMF; 2, X = DMSO; 3, X = H2O]. The complexes were systematically characterized by elemental analysis, UV-Visible spectroscopy, TGA, mass spectrometry, cyclic voltammetry, and powder X-ray diffraction study. Among all the complexes, 1 was confirmed by single-crystal X-ray diffraction study. It was found that 1 preferred a distorted pentagonal bipyramidal geometry, in which an equatorial coordination plane was formed by the ONNO-tetradentate cavity of the deprotonated hydrazone ligand along with an additional oxygen atom of the coordinated solvent molecule. Thermal analysis suggested that complexes 1 and 3 undergo weight loss in the temperature range 180-210 °C and 100-120 °C, respectively, due to the ready release of their coordinated solvent molecules. Complexes 1-3 exhibited analogous UV-Visible absorption bands and the intense band between 300-600 nm was assigned to the M ← L and n → π* transitions. Weakly resolved reduction waves assigned to {UO2}2+/{UO2}+ couple were observed for complexes 1 and 2 {1, -1.76 V; 2, -1.75 V; vs. ferrocenium/ferrocene (Fc+/Fc)} in DMSO solution, signifying the feeble electron-donating nature of the L2- ligand. Powder X-ray diffraction study suggested that the crystallite size of all the complexes was in the nanoscale range. Further analysis using density functional theory (DFT) calculations provided structural insights as well as information on the electronic properties of both complex 1 and the ligand.

Uranyl-Bound Tetra-Dentate Non-Innocent Ligands: Prediction of Structure and Redox Behaviour Using Density Functional Theory

Computational methods have been applied to understand the reduction potentials of [UO₂ -salmnt-L] complexes (L=pyridine, DMSO, DMF and TPPO), and their redox behavior is compared with previous experiments in dichloromethane solution. Since the experimental results were inconclusive regarding the influence of the uranyl-bound tetra-dentate 'salmnt' ligand, here we will show that salmnt acts as a redox-active ligand and exhibits non-innocent behavior to interfere with the otherwise expected one-electron metal (U) reduction. We have employed two approaches to determine the uranyl (VI/V) reduction potentials, using a direct study of one-electron reduction processes and an estimation of the overall reduction using isodesmic reactions. Hybrid density functional theory (DFT) methods were combined with the Conductor-like Polarizable Continuum Model (CPCM) to account for solvation effects. The computationally predicted one-electron reduction potentials for the range of [UO₂ -salmnt-L] complexes are in excellent agreement with shoulder peaks (∼1.4 eV) observed in the cyclic voltammetry experiments and clearly correlate with ligand reduction. Highly conjugated pi-bonds stabilize the ligand based delocalized orbital relative to the localized U f-orbitals, and as a consequence, the ligand traps the incoming electron. A second reduction step results in metal U(VI) to U(V) reduction, in good agreement with the experimentally assigned uranyl (VI/V) reduction potentials.

Chemistry, Alpha-glucosidase and Radical Scavenging Properties of Uranyl(VI) Hydrazide Complexes

BACKGROUND

Antioxidant, anti-inflammatory, antiviral and antitumoral activities among others are essential characteristics in the development of novel therapeutic compounds. Acid hydrazides can form complexation with certain metal ions that positively enhance these biological characteristics.

OBJECTIVE

Five new complexes of uranium with hydrazide ligands were synthesized at room temperature.

METHODS

The characterization was done by spectroscopic methods (ESI-Mass, IR, 1H-NMR, 13CNMR), CHN analysis and conductivity measurements. Metal complexes along with their respective ligands were further screened for their antioxidant (DPPH, superoxide and nitric oxide free radicals) properties and enzyme inhibition (α-glucosidase) activities.

RESULTS

Elemental and spectral data indicate octahedral geometry around uranyl (UO2 2+) species. Magnetic moments indicate the diamagnetic nature of uranyl(VI) ion in the complex in solid state. IC50 values showed potential antioxidant behavior of uranyl complexes demonstrating interesting structure-activity relationships. In general, hydrazide ligands were not active against superoxide and nitric oxide radicals while varying degree of results were observed against DPPH radical whereas all uranyl-complexes showed promising radical scavenging activities against all of them. Promising inhibitory potential was displayed by UO2 +2 hydrazide complexes against α- glucosidases whereas free hydrazide ligands were inactive.

CONCLUSION

Structure function relationship demonstrates that the nature of ligand, position of substituent, electronic and steric effects are significant factors affecting the radical scavenging and enzyme inhibition activities of the compounds.

Density functional theory (DFT) calculations of VI/V reduction potentials of uranyl coordination complexes in non-aqueous solutions

Of particular interest within the +6 uranium complexes is the linear uranyl(vi) cation and it forms numerous coordination complexes in solution and exhibits incongruent redox behavior depending on coordinating ligands. In this study, to determine the reduction potentials of uranyl complexes in non-aqueous solutions, a hybrid density functional theory (DFT) approach was used in which two different DFT functionals, B3LYP and M06, were applied. Bulk solvent effects were invoked through the conductor-like polarizable continuum model. The solute cavities were described with the united-atom Kohn-Sham (UAKS) cavity definition. Inside the cavity the dielectric constant matches the value of a vacuum and outside the cavity the dielectric constant value is the same as that of the solvent of interest, for example, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dichloromethane (DCM), acetonitrile and pyridine. With the help of the Nernst equation the calculated reduction potentials with respect to the ferrocene (Fc) reference electrode are converted into reduction free energies (RFEs). Uranyl complexes of organic ligands which range from mono- to hexa-dentate coordination modes were investigated in non-aqueous solutions of DMSO, DMF, DCM, acetonitrile and pyridine solutions. The effect of the spin-orbit correction and the reference electrode correction on the RFEs and various methods such as the direct method and the isodesmic reaction model were explored. Overall, our computational determination of RFEs of uranyl complexes in various non-aqueous solutions demonstrates that the RFEs can be obtained within ∼0.2 eV of experimental values.

Uranyl Functionalization Mediated by Redox-Active Ligands: Generation of O-C Bonds via Acylation

A series of uranyl compounds with the redox-active iminoquinone ligand have been synthesized, and their electronic structures elucidated using multinuclear NMR, EPR, electronic absorption spectroscopies, SQUID magnetometry, and X-ray crystallography. Characterization and analysis of the iminoquinone (iq⁰) complex, (^dippiq)UO₂(OTf)₂THF (1-iq), the iminosemiquinone (isq^1-) complex, (^dippisq)₂UO₂THF (2-isq), and the amidophenolate (ap^2-) complex, [(^dippap)₂UO₂THF][K(18-crown-6)(THF)₂]₂(3-ap crown) show that reduction events are ligand-based, with the uranium center remaining in the hexavalent state. Reactivity of 2-isq with B-chlorocatecholborane or pivaloyl chloride leads to U-O_uranyl bond scission and reduction of U(VI) to U(IV) concomitant with ligand oxidation along with organic byproducts. ¹⁸O isotopic labeling experiments along with IR spectroscopy, mass spectrometry, and multinuclear NMR spectroscopy confirm that the organic byproducts contain oxygen atoms which originate from U-O_uranyl bond activation.

Synthesis, spectral, DFT modeling, cytotoxicity and microbial studies of novel Zr(IV), Ce(IV) and U(VI) piroxicam complexes

The Zr(IV), Ce(IV) and U(VI) piroxicam anti-inflammatory drug complexes were prepared and characterized using elemental analyses, conductance, IR, UV-Vis, magnetic moment, ^IHNMR and thermal analysis. The ratio of metal: Pir is found to be 1:2 in all complexes estimated by using molar ratio method. The conductance data reveal that Zr(IV) and U(VI) chelates are non-electrolytes except Ce(IV) complex is electrolyte. Infrared spectroscopic confirm that the Pir behaves as a bidentate ligand co-ordinated to the metal ions via the oxygen and nitrogen atoms of ν(CO)_carbonyl and ν(CN)_pyridyl, respectively. The kinetic parameters of thermogravimetric and its differential, such as activation energy, entropy of activation, enthalpy of activation, and Gibbs free energy evaluated using Coats-Redfern and Horowitz-Metzger equations for Pir and complexes. The geometry of the piroxicam drug in the Free State differs significantly from that in the metal complex. In the time of metal ion-drug bond formation the drug switches-on from the closed structure (equilibrium geometry) to the open one. The antimicrobial tests were assessed towards some types of bacteria and fungi. The in vitro cell cytotoxicity of the complexes in comparison with Pir against colon carcinoma (HCT-116) cell line was measured.

Synthesis, Structural Characterization and Antimicrobial Activity of Cu(II) and Fe(III) Complexes Incorporating Azo-Azomethine Ligand

We are reporting a novel azo-azomethine ligand, HL and its complexes with Cu(II) and Fe(III) ions. The ligand and its complexes are characterized by various physico-chemical techniques using C,H,N analyses, FT-IR, ¹H-NMR, ESI-MS and UV-Vis studies. TGA analyses reveal complexes are sufficiently stable and undergo two-step degradation processes. The redox behavior of the complexes was evaluated by cyclic voltammetry. Furthermore, the ligand and its complexes were tested for antimicrobial activity against bacterial and fungal strains by determining inhibition zone, minimal inhibitory concentration (MIC) and minimal bactericidal concentration (MBC). The complexes showed moderate antimicrobial activity when tested against Gram +ve and Gram −ve bacterial strains. To obtain insights into the structure of ligand, DFT studies are recorded. The results obtained are quite close to the experimental results. In addition, the energy gap, chemical hardness, softness, electronegativity, electrophilic index and chemical potential were calculated using HOMO, LUMO energy value of ligand.

Six uranyl-organic frameworks with naphthalene-dicarboxylic acid and bipyridyl-based spacers: syntheses, structures, and properties

Six new uranyl complexes that exhibit promising photocatalytic activities for the degradation of TC under UV light based on 1,4-naphthalene dicarboxylic acid and nitrogen-based ligands were synthesized.

BR, Sundararajan M, Ghanty TK, Sivaraman N. Remarkable structural effects on the complexation of actinides with H-phosphonates: a combined experimental and quantum chemical study

Strikingly novel effects on pH-modulated actinide extraction by branched H-phosphonates are investigated by solvent extraction combined with DFT.

Oxocomplexes of U(vi) with 8-hydroxyquinoline-5-sulfonate in solution: structural studies and photophysical behaviour

Multinuclear (¹H and ¹³C) NMR, and Raman spectroscopy, combined with DFT calculations, provide detailed information on the complexation between U(vi) oxoions and 8-hydroxyquinoline-5-sulfonate (8-HQS) in aqueous solution. Over the concentration region studied, U(vi) oxoions (uranyl ions) form one dominant complex with 8-HQS in water in the pH range 3-6, a mononuclear 1 : 2 (metal : ligand) complex, with the metal centre (UO₂²⁺) coordinated to two 8-HQS ligands, together with one or more water molecules. An additional minor 1 : 1 complex has also been detected for solutions with a 1 : 1 metal : ligand molar ratio. The geometry of the dominant complex is proposed based on the combination of the NMR and Raman results with DFT calculations. Further information on the electronic structure of the complex has been obtained from UV/visible absorption and luminescence spectra. The complex of U(vi) and 8-HQS is non-luminescent, in contrast to what has been observed with this ligand and many other metal ions. We suggest that this is due to the presence of low-lying ligand-to-metal charge transfer (LMCT) states below the emitting ligand-based and uranyl-based levels which quench their emission. These studies have fundamental importance and are also relevant in the context of environmental studies, and the water soluble ligand 8-HQS has been chosen for application in uranium remediation of aqueous environments.

Structural elucidation and physicochemical properties of mononuclear Uranyl(VI) complexes incorporating dianionic units

Two derivatives of organouranyl mononuclear complexes [UO2(L)THF] (1) and [UO2(L)Alc] (2), where L = (2,2'-(1E,1'E)-(2,2-dimethylpropane-1,3-dyl)bis(azanylylidene, THF = Tetrahydrofuran, Alc = Alcohol), have been prepared. These complexes have been determined by elemental analyses, single crystal X-ray crystallography and various spectroscopic studies. Moreover, the structure of these complexes have also been studied by DFT and time dependent DFT measurements showing that both the complexes have distorted pentagonal bipyramidal environment around uranyl ion. TD-DFT results indicate that the complex 1 displays an intense band at 458.7 nm which is mainly associated to the uranyl centered LMCT, where complex 2 shows a band at 461.8 nm that have significant LMCT character. The bonding has been further analyzed by EDA and NBO. The photocatalytic activity of complexes 1 and 2 for the degradation of rhodamine-B (RhB) and methylene blue (MB) under the irradiation of 500W Xe lamp has been explored, and found more efficient in presence of complex 1 than complex 2 for both dyes. In addition, dye adsorption and photoluminescence properties have also been discussed for both complexes.

Synthesis and electronic structure determination of uranium(vi) ligand radical complexes

Pentagonal bipyramidal uranyl (UO₂²⁺) complexes of salen ligands were prepared and the electronic structure of the one-electron oxidized species[1a–c]+were investigated in solution.

Dioxo-vanadium(v), oxo-rhenium(v) and dioxo-uranium(vi) complexes with a tridentate Schiff base ligand

The complexation of a tridentate Schiff base ligand with three oxo-metal ions lead to the formations of four new complexes.

Synthesis, characterization and molecular structure of a dinuclear uranyl complex supported by N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine

The preparation, characterization and the molecular structure of a dinuclear uranyl complex [(UO2)2L(OCMe2)2] supported by the bis-salophen ligand N,N′,N″,N′″-tetra-(3,5-di-tert-butylsalicylidene)-1,2,4,5-phenylenetetraamine (L4–) is described. [(UO2)2L(OCMe2)2] was prepared by reaction of uranyl nitrate with the neutral, protonated form of the ligand (H4L) in acetone. From a saturated acetone solution [(UO2)2L(OCMe2)2]·1.5(OCMe2) crystallizes triclinically, space group P1̅ with a = 1522.7(2), b = 1751.4(2), c = 1815.4(2) Å, α = 109.16(1), β = 99.29(1), γ = 105.29(1)° and Z = 2. Each uranium atom is surrounded in a distorted pentagonal bipyramidal fashion by two O and two N atoms of the salicylidene units, one O atom of an acetone ligand, and the two oxo groups. The cyclic voltammogram of [(UO2)2L(OCMe2)2] shows two quasi-reversible redox processes centered at +0.57 V and +0.82 V vs. Fc+/Fc attributed to the sequential oxidation of the coordinating phenolates to phenoxyl radicals. The crystal structure of an ethanol solvate of H4L was also determined by X-ray crystallography. H4L·5EtOH: triclinic, space group P1̅, a = 1003.4(3), b = 1187.7(3), c = 1905.1(5) Å, α = 75.75(2), β = 78.74(2), γ = 66.66(2)°, Z = 1.

Synthesis, spectral characterization, structural studies, molecular docking and antimicrobial evaluation of new dioxidouranium(vi) complexes incorporating tetradentate N2O2 Schiff base ligands

Two new uranyl(vi) Schiff base complexes were synthesized and characterized by physicochemical and spectroscopic methods. The antimicrobial activities of these complexes were also investigated against microorganisms.