Investigation of the Plutonium(IV) Interactions with Two Variants of the EF-Hand Ca-Binding Site I of Calmodulin.

Due to its presence in the nuclear industry and its strong radiotoxicity, plutonium is an actinide of major interest in the event of internal contamination. To improve the understanding of its mechanisms of transport and accumulation in the body, the complexation of Pu(IV) to the most common protein calcium-binding motif found in cells, the EF-hand motif of calmodulin, was investigated. Visible and X-ray absorption spectroscopies (XAS) in solution made it possible to investigate the speciation of plutonium at physiological pH (pH 7.4) and pH 6 in two variants of the calmodulin Ca-binding site I and using Pu(IV) in different media: carbonate, chloride, or nitrate solutions. Three different species of Pu were identified in the samples, with formation of 1:1 Pu(IV):calmodulin peptide complexes, Pu(IV) reduction, and formation of peptide-mediated Pu(IV) hexanuclear cluster.

Syntheses and evaluation of new hydrophilic azacryptands used as masking agents of technetium in solvent extraction processes.

The extraction of technetium, present in nitric acid medium as pertechnetate anion, is an issue in solvent extraction processes used to recover uranium and plutonium. In the present study, a complexing agent is added in the aqueous nitric acid solution to bind selectively the pertechnetate anion and prevent its extraction into the organic phase or to back extract it in the aqueous phase. Several azacryptands with the addition of hydrophilic groups were synthesized to improve the solubility of the previously studied cage molecule in nitric acid medium. Solvent extraction tests reveal that all the synthesized ligands have a similar complexation strength towards pertechnetate and are able to maintain this anion in the aqueous phase (0.5 M HNO3). These ligands are able to overcome the Hofmeister bias and selectively bind technetium in nitric acid solution. The azacryptand concentration can be increased by a factor of three in the liquid-liquid extraction conditions compared to our previous work. Coordination studies using microcalorimetry, Single Crystal X-Ray Diffraction (SC-XRD), infrared and Raman spectroscopies show the formation of an inclusion complex with hydrogen bonds stabilizing the oxo-anion within the cavity. This solubility improvement is promising for the introduction of this kind of macrocyclic azacryptands in a solvent extraction process.

Coordination Structures of Uranium(VI) and Plutonium(IV) in Organic Solutions with Amide Derivatives.

Carbamide and monoamide derivatives are very promising molecules to achieve U(VI) and Pu(IV) extraction and separation from spent nuclear fuels through solvent extraction. Herein, coordination structures of U(VI) and Pu(IV) complexes with carbamide derivatives were characterized using X-ray crystallography as well as infrared, UV-visible, and EXAFS spectroscopies. Coordination structures are compared to those obtained for monoamide derivatives in order to better understand the role of coordination chemistry in extraction properties. Single crystals were first synthesized with a short alkyl chain carbamide analog. Carbamide complexation in the solid state is found analogous to that in the monoamide. In organic solution, upon solvent extraction from nitric acid aqueous solution, it is shown that both amide derivatives can bind in the inner and outer coordination spheres of uranium(VI) and plutonium(IV). The amount of outer sphere coordination complexes increases with the amount of nitric acid. With uranium(VI), at a nitric acid concentration up to 5 mol·L-1, amide derivatives operate predominantly in the inner coordination sphere. In contrast, Pu(IV) coordination geometry is much more sensitive toward acid concentration or ligand structure than U(VI). Pu(IV) changes from inner sphere complexation at 0.5 mol·L-1 HNO3 to mostly outer sphere complexation at 4 mol·L-1. The proportion of outer-sphere complexes is strongly influenced by the ligand structure. Higher Pu(IV) extraction is found to be correlated with the amount of Pu(IV) outer sphere species. Secondary interactions in the outer sphere coordination shell appear to be of primary importance for plutonium extraction.

Perrhenate and pertechnetate complexation by an azacryptand in nitric acid medium.

Technetium is present as the pertechnetate anion in spent nuclear fuel solutions, and its extraction by several extractant systems is a major problem for the liquid-liquid extraction processes used to separate uranium and plutonium. To prevent technetium extraction into the organic phase, a complexing agent may be added to the aqueous nitric acid phase to selectively bind the pertechnetate anion. In the present study, liquid-liquid extraction experiments reveal that technetium distribution ratios are considerably lowered with addition of an azacryptand, which is a good receptor for pertechnetate anion recognition. This ligand is able to overcome the Hofmeister bias and selectively bind techetium in nitric acid solution. Coordination studies using infrared and Raman spectoscopies and DFT calculations show the formation of an inclusion complex with hydrogen bonds stabilizing the oxo-anion within the cavity. For the first time, the cage molecules are studied for an extraction process.

Characterization of palladium species after γ-irradiation of a TBP-alkane-Pd(NO3)2 system.

The γ-irradiation of a biphasic system composed of tri-n-butylphosphate in tetrapropylene hydrogen (TPH) in contact with palladium(ii) nitrate in nitric acid aqueous solution led to the formation of two precipitates. A thorough characterization of these solids was performed by means of various analytical techniques including X-Ray Diffraction (XRD), Thermal Gravimetric Analysis coupled with a Differential Scanning Calorimeter (TGA-DSC), X-ray Photoelectron Spectroscopy (XPS), InfraRed (IR), RAMAN and Nuclear Magnetic Resonance (NMR) Spectroscopy, and ElectroSpray Ionization Mass Spectrometry (ESI-MS). Investigations showed that the two precipitates exhibit quite similar structures. They are composed at least of two compounds: palladium cyanide and palladium species containing ammonium, phosphorous or carbonyl groups. Several mechanisms are proposed to explain the formation of Pd(CN)2.

Complexation of Actinide(III) and Lanthanide(III) with H4TPAEN for a Separation of Americium from Curium and Lanthanides.

Previous studies have identified the TPAEN ligand as a potentially appropriate complexing agent in solvent extraction processes for the separation of americium (Am(III)) from the fission products including lanthanide (Ln(III)) and curium (Cm(III)) ions, a challenging issue for advanced nuclear fuel recycling. To get insight into the selectivity of this ligand, the complexation of selected trivalent Ln(III) and actinide (An(III)) cations with TPAEN was investigated in solution. First, the structure and stoichiometry of the TPAEN complex with Am(III) were characterized by extended X-ray absorption fine structure spectroscopy (EXAFS). Then complexation constants and thermodynamics data were acquired for the complexes using different methods: microcalorimetry for the Ln(III) cations, time-resolved laser fluorescence spectroscopy (TRLFS) for Eu(III) and Cm(III), and UV-visible spectroscopy for Nd(III) and Am(III).

Inner to outer-sphere coordination of plutonium(iv) with N,N-dialkyl amide: influence of nitric acid.

N,N-Dialkylamides are extensively studied as alternative organic ligands to achieve the extraction and separation of uranium(vi) and plutonium(iv). We report here the coordination structures of the plutonium(iv) ion with N,N-di(2-ethylhexyl)-n-butanamide as a function of nitric acid concentration in the aqueous phase. The coordination structure of Pu(iv) evolves gradually with increasing nitric acid concentration from an inner-sphere with two coordinated amide ligands toward an outer-sphere hexanitrate complex with only nitrate ions in the first coordination sphere and protonated amide ligands in the outer shell.

Structures of Plutonium(IV) and Uranium(VI) with N,N-Dialkyl Amides from Crystallography, X-ray Absorption Spectra, and Theoretical Calculations.

The structures of plutonium(IV) and uranium(VI) ions with a series of N,N-dialkyl amides ligands with linear and branched alkyl chains were elucidated from single-crystal X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), and theoretical calculations. In the field of nuclear fuel reprocessing, N,N-dialkyl amides are alternative organic ligands to achieve the separation of uranium(VI) and plutonium(IV) from highly concentrated nitric acid solution. EXAFS analysis combined with XRD shows that the coordination structure of U(VI) is identical in the solution and in the solid state and is independent of the alkyl chain: two amide ligands and four bidentate nitrate ions coordinate the uranyl ion. With linear alkyl chain amides, Pu(IV) also adopt identical structures in the solid state and in solution with two amides and four bidentate nitrate ions. With branched alkyl chain amides, the coordination structure of Pu(IV) was more difficult to establish unambiguously from EXAFS. Density functional theory (DFT) calculations were consequently performed on a series of structures with different coordination modes. Structural parameters and Debye-Waller factors derived from the DFT calculations were used to compute EXAFS spectra without using fitting parameters. By using this methodology, it was possible to show that the branched alkyl chain amides form partly outer-sphere complexes with protonated ligands hydrogen bonded to nitrate ions.

Hydrophilic Clicked 2,6-Bis-triazolyl-pyridines Endowed with High Actinide Selectivity and Radiochemical Stability: Toward a Closed Nuclear Fuel Cycle.

There is still an evident need for selective and stable ligands able to separate actinide(III) from lanthanide(III) metal ions in view of the treatment of the accumulated radioactive waste and of the recycling of minor actinides. We have herein demonstrated that hydrophilic 2,6-bis-triazolyl-pyridines are able to strip all actinides in all the different oxidation states from a diglycolamide-containing kerosene solution into an acidic aqueous phase. The ascertained high actinide selectivity, efficiency, extraction kinetics, and chemical/radiolytic stability spotlight this hydrophilic class of ligands as exceptional candidates for advanced separation processes fundamental for closing the nuclear fuel cycle and solving the environmental issues related to the management of existing nuclear waste.

1,10-Phenanthroline and non-symmetrical 1,3,5-triazine dipicolinamide-based ligands for group actinide extraction.

The synthesis and evaluation of new extractants for spent nuclear fuel reprocessing are described. New bitopic ligands constituted of phenanthroline and 1,3,5-triazine cores functionalized by picolinamide groups were designed. Synthetic routes were investigated and optimized to obtain twelve new polyaza-heterocyclic ligands. In particular, an efficient and versatile methodology was developed to access non-symmetric 2-substituted-4,6-di(6-picolin-2-yl)-1,3,5-triazines from the 1,3,5-triazapentadiene precursor in the presence of anhydride reagents. Extraction studies showed the ability of both ligand series to extract and separate actinides selectively at different oxidation states (U(VI), Np(V,VI), Am(III), Cm(III), and Pu(IV)) from an acidic solution (3 M HNO3). Phenanthroline-based ligands show the most promising efficiency for use in the group actinide extraction (GANEX) process due to a higher number of donor nitrogen atoms and a suitable pre-organization of the dipicolinamide-1,10-phenanthroline architecture.

On the structure of thorium and americium adenosine triphosphate complexes.

PURPOSE: The actinides are chemical poisons and radiological hazards. One challenge to better appraise their toxicity and develop countermeasures in case of exposure of living organisms is to better assess pathways of contamination. Because of the high chemical affinity of those actinide elements for phosphate groups and the ubiquity of such chemical functions in biochemistry, nucleotides and in particular adenosine triphosphate nucleotide (ATP) may be considered critical target building blocks for actinides. MATERIALS AND METHODS: Combinations of spectroscopic techniques (Fourier transformed Infra Red [FTIR], Electrospray Ionization Mass Spectrometry [ESI-MS], and Extended X-ray Absorption Fine Structure [EXAFS]) with quantum chemical calculations have been implemented in order to assess the actinides coordination arrangement with ATP. RESULTS: We describe and compare herein the interaction of ATP with thorium and americium; thorium(IV) as a representative of actinide(IV) like plutonium(IV) and americium(III) as a representative of all heavier actinides. In the case of thorium, an insoluble complex is readily formed. In the case of americium, a behavior identical to that described previously for lutetium has been observed with insoluble and soluble complexes. CONCLUSIONS: The comparative study of ATP complexation with Th(IV) and Am(III) shows their ability to form insoluble complexes for which a structural model has been proposed by analogy with previously described Lu(III) complexes.

The reaction chemistry of plutonyl(VI) chloride complexes with triphenyl phosphineoxide and triphenyl phosphinimine.

The reaction between Ph(3)PO dissolved in acetone and "PuO(2)Cl(2)" in dilute HCl resulted in the formation of [PuO(2)Cl(2)(Ph(3)PO)(2)]. Crystallographic characterization of the acetone solvate revealed the expected axial trans plutonyl dioxo, with trans Cl and Ph(3)PO in the equatorial plane. Spectroscopic analyses ((31)P NMR, (1)H NMR, and vis/nIR) indicate the presence of both cis and trans isomers in solution, with the trans isomer being more stable. Confirmation of the higher stability of the trans versus cis isomers for [AnO(2)Cl(2)(Ph(3)PO)(2)] (An = U and Pu) was obtained through quantum chemical computational analysis, which also reveals the Pu-O(TPPO) bond to be more ionic than the U-O(TPPO) bond. Slight variation in reaction conditions led to the crystallization of two further minor products, [PuO(2)(Ph(3)PO)(4)][ClO(4)](2) and cis-[PuCl(2)(Ph(3)PO)(4)], the latter complex revealing the potential for reduction to Pu(IV). In addition, the reaction of Ph(3)PNH with [PuO(2)Cl(2)(thf)(2)](2) in anhydrous conditions gave evidence for the formation of both cis- and trans-[PuO(2)Cl(2)(Ph(3)PNH)(2)] in solution (by (31)P NMR). However, the major reaction pathway involved protonation of the ligand with the crystallographic characterization of [Ph(3)PNH(2)](2)[PuO(2)Cl(4)]. We believe that HCl/SiMe(3)Cl carried through from the small scale preparation of [PuO(2)Cl(2)(thf)(2)](2) was the source of both protons and chlorides. The fact that this chemistry was significantly different from previous uranium studies, where cis-/trans-[UO(2)Cl(2)L(2)] (L = Ph(3)PO or Ph(3)PNH) were the only products observed, provides further evidence of the unique challenges and opportunities associated with the chemistry of plutonium.

Comparison of two tetrapodal N,O ligands: impact of the softness of the heterocyclic N-donors pyridine and pyrazine on the selectivity for Am(III) over Eu(III).

To quantify the impact of the N-donor softness on the coordination of f elements in aqueous solution, and in particular on the selectivity for Am(III) over Eu(III), we have designed the two tetrapodal hexadentate ligands N,N-bis(2-pyridylmethyl)ethylenediamine-N',N'-diacetic acid (Lpy) and N,N-bis(2-pyrazylmethyl)ethylenediamine-N',N'-diacetic acid (Lpz). These ligands bear two hard acetate groups to provide stability to the An(III) and Ln(III) complexes and two N-heterocyclic soft groups to provide Am(III) versus Eu(III) selectivity. They only differ in their N-donor moieties, pyridine or pyrazine. The proton NMR and potentiometric analyses performed on the lanthanide complexes of the two ligands indicate that a unique metallic complex, LnL, is formed and that LnLpy+ and LnLpz+ have the same structure in water. Furthermore, the hydration numbers of the europium and terbium ions in these complexes, measured by luminescence decay, have the same value (q = 3), indicating that the two ligands act as hexadentate donors in both systems. As expected, the softer pyrazine-based ligand gives less stable complexes than the pyridine-based ligand with the hard Ln(III) cations. The fragment N(CH2pz)2 containing two pyrazine functions has a very low contribution to the stability of the lanthanide complexes, even though the pyrazine groups are coordinated to the cation in water. The stabilities of the americium(III) complexes were determined by potentiometry and are greater than those found for the isoelectronic europium complexes. The selectivity for Am(III) over Eu(III) increases from 60 to 500 when the pyridine-containing fragment N(CH2py)2 is substituted by the pyrazine-containing fragment N(CH2pz)2, which demonstrates that the selectivity for Am(III) over Eu(III) is significantly enhanced when the softness of the N-heterocycle increases from pyridine to pyrazine. These new hydrophilic ligands present attractive selectivities for Am(III) over Eu(III) that could make them good candidates for the selective back extraction of Am(III) from organic solutions containing 4f and 5f elements.