Actinide Organometallic Complexes with π-Ligands

Recent developments and results from the organometallic chemistry of the actinides are reviewed. In the last one and a half years the structural data of about 15 organometallic complexes of transuranium actinides (Np or Pu) have been published, all involving π-ligands in the coordination sphere of the metal ion. On the basis of these data, a comparison of these molecules is presented. Depending on the steric demands of the ligands, effects like the actinide contraction seem to be stronger or weaker in the structural features. This indicates that the interplay between the actinide ion and the π-ligand is rather flexible, enabling the formation of stable bonds over a broad range of actinide ion oxidation states.     

Controlled Synthesis of Thorium and Uranium Oxide Nanocrystals

Very little is known about the size and shape effects on the properties of actinide compounds. As a consequence, the controlled synthesis of well‐defined actinide‐based nanocrystals constitutes a fundamental step before studying their corresponding properties. In this paper, we report on the non‐aqueous surfactant‐assisted synthesis of thorium and uranium oxide nanocrystals. The final characteristics of thorium and uranium oxide nanocrystals can be easily tuned by controlling a few experimental parameters such as the nature of the actinide precursor and the composition of the organic system (e.g., the chemical nature of the surfactants and their relative concentrations). Additionally, the influence of these parameters on the outcome of the synthesis is highly dependent on the nature of the actinide element (thorium versus uranium). By using optimised experimental conditions, monodisperse isotropic uranium oxide nanocrystals with different sizes (4.5 and 10.7 nm) as well as branched nanocrystals (overall size ca. 5 nm), nanodots (ca. 4 nm) and nanorods (with ultra‐small diameters of 1 nm) of thorium oxide were synthesised.     

Thermodynamics of Cm(III) in Concentrated Salt Solutions: Carbonate Complexation in NaCl Solution at 25°C

The carbonate complexation reactions of Cm(III) were studied by time-resolved laser fluorescence spectroscopy in 0–6 m NaCl at 25°C. The ionic strength dependence of the stepwise formation constants for the carbonato complexes Cm(CO₃) _n ^3–2n with n = 1, 2, 3, and 4 is described by modeling the activity coefficients of the Cm(III) species with Pitzer's ion-interaction approach. Based on the present results and literature data for Cm(III) and Am (III), the mean carbonate complexation constants at I = 0 are calculated to be: log ₁₀₁ ^o =8.1 ±0.3, log ₁₀₂ ^o =13.0 ± 0.6, log ₁₀₃ ^o =15.2 ± 0.4, and log ₁₀₄ ^o =13.0 ± 0.5. Combining these equilibrium constants at infinite dilution and the evaluated set of Pitzer parameters, a model is obtained, that reliably predicts the thermodynamics of bivalent actinide An(III) carbonate complexation in dilute to concentrated NaCl solution.     

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.     

Further insights in the ability of classical nonadditive potentials to model actinide ion–water interactions

Pursuing our efforts on the development of accurate classical models to simulate radionuclides in complex environments (Réal et al., J. Phys. Chem. A 2010, 114, 15913; Trumm et al. J. Chem. Phys. 2012, 136, 044509), this article places a large emphasis on the discussion of the influence of models/parameters uncertainties on the computed structural, dynamical, and temporal properties. Two actinide test cases, trivalent curium and tetravalent thorium, have been studied with three different potential energy functions, which allow us to account for the polarization and charge‐transfer effects occurring in hydrated actinide ion systems. The first type of models considers only an additive energy term for modeling ion/water charge‐transfer effects, whereas the other two treat cooperative charge‐transfer interactions with two different analytical expressions. Model parameters are assigned to reproduce high‐level ab initio data concerning only hydrated ion species in gas phase. For the two types of cooperative charge‐transfer models, we define two sets of parameters allowing or not to cancel out possible errors inherent to the force field used to model water/water interactions at the ion vicinity. We define thus five different models to characterize the solvation of each ion. For both ions, our cooperative charge‐transfer models lead to close results in terms of structure in solution: the coordination number is included within 8 and 9, and the mean ion/water oxygen distances are 2.45 and 2.49 Å, respectively, for Th(IV) and Cm(III). © 2012 Wiley Periodicals, Inc.     

Bicyclic and acyclic diamides: comparison of their aqueous phase binding constants with Nd(III), Am(III), Pu(IV), Np(V), Pu(VI), and U(VI)

This report describes affinity measurements for two, water-soluble, methyl-alkylated diamides incorporating the malonamide functionality, N,N,N',N' tetramethylmalonamide (TMMA) and a bicyclic diamide (1a), toward actinide metal cations (An) in acidic nitrate solutions. Ligand complexation to actinides possessing oxidation states ranging from +3 to +6 was monitored through optical absorbance spectroscopy, and formation constants were obtained from the refinement of the spectrophotometric titration data sets. Species analysis gives evidence for the formation of 1, 4, 1, and 2 spectrophotometrically observable complexes by TMMA to An(III, IV, V, and VI), respectively, while for 1a, the respective numbers are 3, 4, 2, and 2. Consistent with the preorganization of 1a toward actinide binding, a significant difference is found in the magnitudes of their respective formation constants at each complexation step. It has been found that the binding affinity for TMMA follows the well-established order An(V) < An(III) < An(VI) < An(IV). However, with 1a, Np(V) forms stronger complexes than Am(III). The complexation of 1a with Np(V) and Pu(VI) at an acidity of 1.0 M is followed by reduction to Np(IV) and Pu(IV), whereas TMMA does not perturb the initial oxidation state for these dioxocations. These measurements of diamide binding affinity mark the first time single-component optical absorbance spectra have been reported for a span of actinide-diamide complexes covering all common oxidation states in aqueous solution.     

Spontaneous Chelation-Driven Reduction of the Neptunyl Cation in Aqueous Solution

Octadentate hydroxypyridinone (HOPO) and catecholamide (CAM) siderophore analogues are known to be efficacious chelators of the actinide cations, and these ligands are also capable of facilitating both activation and reduction of actinyl species. Utilizing X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, as well as cyclic voltammetry measurements, herein, we elucidate chelation-based mechanisms for driving reactivity and initiating redox processes in a family of neptunyl–HOPO and CAM complexes. Based on the selected chelator, the ability to control the oxidation state of neptunium and the speed of reduction and concurrent oxo group activation was demonstrated. Most notably, reduction kinetics for the Np^VO₂^+//Np^IV redox couple upon chelation by the ligands 3,4,3-LI(1,2-HOPO) and 3,4,3-LI(CAM)₂(1,2-HOPO)₂ was observed to be faster than ever reported, and in fact quicker than we could measure using either X-ray absorption spectroscopy or electrochemical techniques.     

The Application of HEXS and HERFD XANES for Accurate Structural Characterisation of Actinide Nanomaterials: The Case of ThO2

The structural characterisation of actinide nanoparticles (NPs) is of primary importance and hard to achieve, especially for non-homogeneous samples with NPs less than 3 nm. By combining high-energy X-ray scattering (HEXS) and high-energy-resolution fluorescence-detected X-ray absorption near-edge structure (HERFD XANES) analysis, we have characterised for the first time both the short- and medium-range order of ThO₂ NPs obtained by chemical precipitation. By using this methodology, a novel insight into the structures of NPs at different stages of their formation has been achieved. The pair distribution function revealed a high concentration of ThO₂ small units similar to thorium hexamer clusters mixed with 1 nm ThO₂ NPs in the initial steps of formation. Drying the precipitates at around 150 °C promoted the recrystallisation of the smallest units into more thermodynamically stable ThO₂ NPs. HERFD XANES analysis at the thorium M₄ edge, a direct probe for f states, showed variations that we have correlated with the breakdown of the local symmetry around the thorium atoms, which most likely concerns surface atoms. Together, HEXS and HERFD XANES are a powerful methodology for investigating actinide NPs and their formation mechanism.     

Leveraging Actinide Hydrolysis Chemistry for Targeted Th and U Separations using Amidoxime-Functionalized Poly(HIPE)s

P olymerized h igh i nternal p hase e mulsions (poly(HIPE)s) are porous polymer monoliths whose synthesis can easily be tailored to allow incorporation of functional units. In this work, nitrile containing poly(HIPE)s have been prepared with either acrylonitrile (AN) or 4-cyanostyrene (4CS) comonomers. Post-synthetic modification of these nitrile-containing poly(HIPE)s yields their corresponding amidoximated analogues, which were studied for actinide uptake. These amidoxime-functionalized, porous polymers were shown to adsorb 95 % Th⁴⁺ species from aqueous solution within 30 minutes. In contrast to other amidoxime containing polymers the uptake of UO₂²⁺ in these poly(HIPE)s is lower under similar conditions. A critical analysis of actinide separations and high-energy X-ray scattering data provides insight into the polymers’ selectivity, enabled by the uptake of multinuclear Th clusters.     

Novel corresponding-states principle approach for the equation of state of isotopologues: H2(18)O as an example

The corresponding-states principle (CSP) has been considered for the development of the equations of state (EOS) of minor isotopologues that are usually unknown. We demonstrate that, for isotopologues of a given molecular fluid, a general extended multi-parameter corresponding-states EOS can be reduced to the three-parameter EOS, utilizing the critical parameters (temperature and density) and Pitzer's acentric factor as correlation parameters. Appropriate general CSP mathematical formalism and equations for constructing the EOS of minor isotopologues are described in detail. The formalism and equations were applied to isotopologues of water and demonstrated that the isotopic effect on the critical parameters and the acentric factor of H(2)(18)O can be successfully calculated from the EOS of H2O and experimental data on the isotope effects (liquid-vapor isotope fractionation factor and molar volume isotope effect). We have also shown that the experimental data on the vapor pressure isotope effect (VPIE) for 18O-substituted water are inconsistent within the framework of thermodynamics with the liquid-vapor oxygen isotope fractionation factor. The novel approach of CSP to isotopologues developed in this study creates a new opportunity for constructing the EOS of minor isotopologues for many other molecular fluids.     

Application of Multi-step Excitation Schemes for Detection of Actinides and Lanthanides in Solutions by Luminescence/Chemiluminescence Laser Spectroscopy

The use of laser radiation with tunable wavelength allows the selective excitation of actinide/lanthanide species with subsequent registration of luminescence/chemiluminescence for their detection. This work is devoted to applications of the time-resolved laser-induced luminescence spectroscopy and time-resolved laser-induced chemiluminescence spectroscopy for the detection of lanthanides and actinides. Results of the experiments on U, Eu, and Sm detection by TRLIF method in blood plasma and urine are presented. Data on luminol chemiluminescence in solutions containing Sm(III), U(IV), and Pu(IV) are analyzed. It is shown that appropriate selectivity of lanthanide/actinide detection can be reached when chemiluminescence is initiated by transitions within 4f- or 5f-electron shell of lanthanide/actinide ions corresponding to the visible spectral range. In this case chemiluminescence of chemiluminogen (luminol) arises when the ion of f element is excited by multi-quantum absorption of visible light. The multi-photon scheme of chemiluminescence excitation makes chemiluminescence not only a highly sensitive but also a highly selective tool for the detection of lanthanide/actinide species in solutions.     

Examples of Quantification in XPS on 5f Materials

 Various aspects of quantification in actinide research using photoelectron spectroscopy are discussed. In particular, the influence of instrumental factors, different types of the background, and shape of the line intensities are addressed. We give several examples of quantitative analysis of actinide bulk compounds and thin films.     

Biphasic kinetic investigations on the evaluation of non-salt forming reductants for Pu(IV) stripping from tributyl phosphate and N,N-dihexyl octanamide solutions in n-dodecane

The relative stability of different oxidation states of actinide elements is influenced by the nature of complexes formed and redox equilibria in aqueous/non-aqueous solutions. The reduction/stripping studies on Pu(IV) ions from loaded organic phases of 1.1 M tributyl phosphate and of 1.1 M N,N-dihexyl octanamide in n-dodecane were studied using organic soluble tert-butyl hydroquinone (TBH) and aqueous soluble reductants like acetaldoxime (AX) and hydroxyurea (HU). These studies were carried out as a function of reductant and nitric acid concentration (0.5–4 M HNO₃) and of time. The changes in Pu oxidation states were followed by spectrophotometry for TBH and by distribution ratio values for AX and HU as reductants. Spectrophotometric investigations using TBH as reductant showed that it was desirable to strip Pu(III) formed after reduction of Pu(IV) in the organic phase, which may otherwise be reconverted to extractable Pu(IV) by in situ generated HNO₂ from oxidative degradation of TBH to tert-butyl quinone. Similarly, the biphasic reduction/stripping of Pu(IV) using AX and HU as reductant rate was affected adversely with increased aqueous phase acidity. This data will help in the accurate simulation of Pu separation processes using these reductants in mixer-settlers/pulsed columns or centrifugal contactors.     

Thermodynamics of electrolyte solutions. The system HCl + CaCl2 + H2O at 298.15°K

Activity coefficients of hydrochloric acid have been determined from electromotive-force measurements of cells containing mixtures of hydrochloric acid and calcium chloride at constant total ionic strengthsI=0.1, 0.5, 1.0, 2.0, and 3.0 mole-kg^–1 at 298.15°K. Interpretations based on Scatchard's and Pitzer's equations indicate that Pitzer's equations probably provide a more convenient guide to the thermodynamic properties of the mixed-electrolyte solutions. Activity coefficients for calcium chloride were derived from these equations.     

Thermal conductivity equations for pure fluids in a heuristic extended corresponding states framework: I. Modeling techniques

The application of the extended corresponding states modeling technique to thermodynamic and transport properties has demonstrated that different conformality behaviors are followed by a same group of fluids inside each of these two categories of properties. The traditional extended corresponding states technique for transport properties requires the conventional thermodynamic shape factors, derived from accurate equations of state for both the target and the reference fluid, and an additional shape factor, derived from transport property data of the target fluid, in order to fit the transport properties themselves.A new extended corresponding states model is here proposed for thermal conductivity; the technique uses two shape factors, one for each independent variable, which are generated just from the available thermal conductivity data in the range of interest. As a consequence there is no need to import shape factors from thermodynamics.The shape factors are obtained as continuous functions through a neural network, because of its flexibility and high capability to fit the data. The accuracy of thermal conductivity data representation on the basis of this model is better than that achieved through conventional approach by summation of dilute gas, excess and critical enhancement contributions.     

锕酰冠醚配合物电子结构和化学成键理论研究进展

本文对锕系化合物的结构和性质的理论研究进行了规律性总结,并结合我们的研究成果,重点介绍了锕酰冠醚配合物的配位化学、电子结构和化学成键的基本特征。尽管近年来出现越来越多的光谱实验和晶体学数据报道,但是对锕系配合物的电子结构和化学成键的理论研究还不够系统。本文对锕酰冠醚配合物的配位结构、稳定能和光谱性质的计算结果进行了综述。大环配体(硫代)冠醚的腔体大小决定了配合物的结构特征。通过理论研究,在锕酰冠醚配合物中存在具有典型的An≡Oactinyl共价键和An-Oligand和An-Sligand离子键。对于离子键An-Oligand和An-Sligand,An和供电子配体之间通过价原子轨道的径向分布重叠形成微弱的共价相互作用。从U到Cm,配体向金属的电荷转移(LMCT)逐渐显著,导致Am和Cm的氧化态降低,金属离子与配体的作用变弱。这一成键规律和金属氧化态的变化规律,为实验上筛选合理且高效的镧锕分离配体提供重要理论指导。