Unusual redox stability of neptunium in the ionic liquid [Hbet][Tf(2)N].

The behavior of neptunium in the ionic liquid betaine bistriflimide, [Hbet][Tf2N], has been studied spectroscopically at room temperature and 60 °C for the first time. An unprecedented complex redox chemistry is observed, with up to three oxidation states (iv, v and vi) and up to six Np species existing simultaneously. Both redox reactions and coordination of betaine are observed for Np(iv), (v) and (vi). Elevating the temperature accelerates the coordination of Np(v) with betaine and reduction reactions slow down.

Structure and dynamics of uranyl(VI) and plutonyl(VI) cations in ionic liquid/water mixtures via molecular dynamics simulations.

A fundamental understanding of the behavior of actinides in ionic liquids is required to develop advanced separation technologies. Spectroscopic measurements indicate a change in the coordination of uranyl in the hydrophobic ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]) as water is added to the system. Molecular dynamics simulations of dilute uranyl (UO2(2+)) and plutonyl (PuO2(2+)) ) solutions in [EMIM][Tf2N]/water mixtures have been performed in order to examine the molecular-level coordination and dynamics of the actinyl cation (AnO2(2+)) ); An = U, Pu) as the amount of water in the system changes. The simulations show that the actinyl cation has a strong preference for a first solvation shell with five oxygen atoms, although a higher coordination number is possible in mixtures with little or no water. Water is a much stronger ligand for the actinyl cation than Tf2N, with even very small amounts of water displacing Tf2N from the first solvation shell. When enough water is present, the inner coordination sphere of each actinyl cation contains five water molecules without any Tf2N. Water also populates the second solvation shell, although it does not completely displace the Tf2N. At high water concentrations, a significant fraction of the water is found in the bulk ionic liquid, where it primarily coordinates with the Tf2N anion. Potential of mean force simulations show that the progressive addition of up to five water molecules to uranyl is very favorable, with ΔG ranging from −52.3 kJ/mol for the addition of the first water molecule to −37.6 kJ/mol for the addition of the fifth. Uranyl and plutonyl dimers formed via bridging Tf2N ligands are found in [EMIM][Tf2N] and in mixtures with very small amounts of water. Potential of mean force calculations confirm that the dimeric complexes are stable, with relative free energies of up to −9 kJ/mol in pure [EMIM][Tf2N]. We find that the self-diffusion coefficients for all the components in the mixture increase as the water content increases, with the largest increase for water and the smallest increase for the ionic liquid cation and anion. The velocity autocorrelation functions also indicate changes in structure and dynamics as the water content changes.

Development and application of effective pairwise potentials for UO2(n+), NpO2(n+), PuO2(n+), and AmO2(n+) (n = 1, 2) ions with water.

Intra- and intermolecular force field parameters for the interaction of actinyl ions (AnO2(n+), where, An = U, Np, Pu, Am and n = 1, 2) with water have been developed using quantum mechanical calculations. Water was modeled with the extended simple point charge potential (SPC/E). The resulting force field consists of a simple form in which intermolecular interactions are modeled with pairwise Lennard-Jones functions plus partial charge terms. Intramolecular bond stretching and angle bending are treated with harmonic functions. The new potentials were used to carry out extensive molecular dynamics simulations for each hydrated ion. Computed bond lengths, bond angles and coordination numbers agree well with known values and previous simulations. Hydration free energies, computed from molecular dynamics simulations as well as from quantum simulations with a solvation model, were in reasonable agreement with estimated experimental values.

Hydrogen production in aromatic and aliphatic ionic liquids.

The radiolytic production of molecular hydrogen in the ionic liquids N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide ([N1114][Tf2N]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim][Tf2N]) has been examined with γ-rays, 2-10 MeV protons, and 5-20 MeV helium ions to determine the functional dependence of the yield on particle track structure. Molecular hydrogen is the dominant gaseous radiolysis product from these ionic liquids, and the yields with γ-rays are 0.73 and 0.098 molecules per 100 eV of energy absorbed for [N1114][Tf2N] and [emim][Tf2N], respectively. These low yields are consistent with the relative insensitivity of most aromatic compounds to radiation. However, the molecular hydrogen yields increase considerably on going from γ-rays to protons to helium ions with [emim][Tf2N] while they remain essentially constant for [N1114][Tf2N]. FTIR and UV-vis spectroscopic studies show slight degradation of the ionic liquids with radiation.

Solid-state and solution-state coordination chemistry of lanthanide(III) complexes with (pyrazol-1-yl)acetic acid.

As a precursor of carboxyl-functionalized task-specific ionic liquids (TSILs) for f-element separations, (pyrazol-1-yl)acetic acid (L) can be deprotonated as a functionalized pyrazolate anion to coordinate with hard metal cations. However, the coordination chemistry of L with f-elements remains unexplored. We reacted L with lanthanides in aqueous solution at pH = 5 and synthesized four lanthanide complexes of general formula [Ln(L)3(H2O)2]·nH2O (1, Ln = La, n = 2; 2, Ln = Ce, n = 2; 3, Ln = Pr, n = 2; 4, Ln = Nd, n = 1). All complexes were characterized by single crystal X-ray diffraction analysis revealing one-dimensional chain formations. Two distinct crystallographic structures are governed by the different coordination modes of carboxylate groups in L: terminal bidentate and bridging tridentate (1-3); terminal bidentate, bridging bidentate, and tridentate coordination in 4. Comparison of the solid state UV-vis-NIR diffuse reflectance spectra with solution state UV-vis-NIR spectra suggests a different species in solution and solid state. The different coordination in solid state and solution was verified by distinctive (13)C NMR signals of the carboxylate groups in the solid state NMR.

Switchable phase behavior of [HBet][Tf2N]-H2O upon neodymium loading: implications for lanthanide separations.

Task-specific ionic liquids (TSILs) present an opportunity to replace traditional organic solvents for metal dissolution or separation. To date, a thorough investigation of the physical properties of biphasic TSIL-H(2)O systems and the effects of metal loading is lacking. In this work, the change in the liquid-liquid equilibrium of [HBet][Tf(2)N]-H(2)O upon the addition of Nd(III) is investigated by cloud-point measurements. The addition of Nd(III), drops the upper critical solution temperature by over 20 °C. Further investigation of the [HBet][Tf(2)N]-Nd(III) system led to the formation of single crystals of [Nd(2)(H(2)O)(8)(µ(2)-Bet)(2)(µ(3)-Bet)(2)][(Cl)(2)(Tf(2)N)(4)] from the TSIL phase.

Directed nucleation of monomeric and dimeric uranium(VI) complexes with a room temperature carboxyl-functionalized phosphonium ionic liquid.

The carboxyl-functionalized phosphonium ionic liquid (IL), [HCTMP][Tf(2)N], enabled the directed nucleation of monomeric or dimeric uranyl(vi) compounds. This new IL is the first carboxyl-functionalized IL which is liquid at room temperature and exhibits a wider electrochemical window and lower melting point than its ammonium analogue.

Solid-state and solution-state coordination chemistry of lanthanide(III) complexes with α-hydroxyisobutyric acid.

Despite the wide range of applications of α-hydroxyisobutyric acid (HIBA) in biochemical processes, pharmaceutical formulations, and group and elemental separations of lanthanides and actinides, the structures and geometries of lanthanide-HIBA complexes are still not well understood. We reacted HIBA with lanthanides in aqueous solution at pH = 5 and synthesized 14 lanthanide-HIBA complexes of the formula [Ln(HIBA)(2)(H(2)O)(2)](NO(3))·H(2)O (Ln = La (1), Ce (2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14)), isolating single crystals (1-7, 10, and 11) and powders (8, 9, and 12-14). Both single-crystal and powder X-ray diffraction studies reveal a two-dimensional extended structure across the entire lanthanide series. The environment around the eight-coordinated Ln(III) atom is best described as a distorted dodecahedron, where HIBA acts as a monoanionic tridentate ligand with one carboxylato oxygen atom and one hydroxyl oxygen atom chelating to one Ln(III) center. The carboxylato oxygen atom from a second HIBA ligand bridges to a neighboring Ln(III) atom to form a two-dimensional extended structure. While the coordination mode for HIBA is identical across the lanthanide series, three different structure types are found for La, Ce-Ho, and Er-Lu. Solution characterization using (13)C NMR further confirmed a single solution complex under the crystallization conditions. Raman and UV-vis-NIR absorbance and diffuse reflectance spectra of HIBA-Ln(III) complexes were also measured.

Exploring electrochemical windows of room-temperature ionic liquids: a computational study.

Room-temperature ionic liquids (RTILs) are regarded as green solvents due to their low volatility, low flammability, and thermal stability. RTILs exhibit wide electrochemical windows, making them prime candidates as media for electrochemically driven reactions such as electro-catalysis and electro-plating for separations applications. Therefore, understanding the factors determining edges of the electrochemical window, the electrochemical stability of the RTILs, and the degradation products is crucial to improve the efficiency and applicability of these systems. We present here computational investigations of the electrochemical properties of a variety of RTILs covering a wide range of electrochemical windows. We proposed four different approaches with different degrees of approximation and computational cost from gas-phase calculations to full explicit solvation models. It was found that, whereas the simplest model has significant flaws in accuracy, implicit and explicit solvent models can be used to reliably predict experimental data. The general trend of electrochemical windows of the RTILs studied is well reproduced, showing that it increases in the order of imidazolium < ammonium < pyrrolidinium < phosphonium giving confidence to the methodology presented to use it in screening studies of ionic liquids.

Synthesis and structural characterization of molecular Dy(III) and Er(III) tetra-carbonates.

Single crystal structures of lanthanide carbonate and hydroxy-carbonate compounds have been previously reported in the literature, with the majority of these compounds being extended one- to three-dimensional compounds. Very few lanthanide compounds have been isolated that contain molecular moieties, and none have been reported for either erbium or dysprosium. Single crystals of the tetra-carbonate complexes, [C(NH(2))(3)](5)[Er(CO(3))(4)].11H(2)O (I) and [C(NH(2))(3)](4)[Dy(CO(3))(4)(H(2)O)](H(3)O).13H(2)O (II), were isolated from concentrated guanidinium carbonate solutions and characterized by single crystal X-ray diffraction studies. Compounds I and II are the first reported molecular carbonate structures for Er and Dy to be characterized via single crystal X-ray diffraction studies. Crystallographic data for I: monoclinic, space group P21/n, a = 8.8160(6) A, b = 21.0121(14) A, c = 19.6496(14) A, Z = 4. Data for II: tetragonal, space group P4/n, a = b = 15.3199(11) A, c = 7.5129(11) A, Z = 2.

Investigation of the structural properties of an extended series of lanthanide bis-hydroxychlorides Ln(OH)(2)Cl (Ln = Nd-Lu, except Pm and Sm).

The trivalent lanthanide bis-hydroxychloride compounds, Ln(OH)(2)Cl, (Ln = Nd through Lu, with the exception of Pm and Sm) have been prepared by hydrothermal synthesis starting with LnCl(3).nH(2)O. These compounds were synthesized at temperatures not exceeding the melting point of the Teflon liners in the Parr autoclaves ( approximately 220 degrees C). The compounds obtained were characterized by single crystal X-ray diffraction analysis, diffuse reflectance, FT-IR, and FT-Raman spectroscopies. Most of the lanthanide(III) bis-hydroxychlorides are isostructural and generally crystallize in the monoclinic space group P2(1)/m. The bis-hydroxychlorides of the heavier lanthanide(III) atoms with smaller ionic radii also crystallize in the orthorhombic crystal system. Apparently hydrogen bonds between the OH groups and the Cl atoms connect the layers in the "c" direction. These H-bonds seem to be the driving force for the angle beta of the monoclinic complexes to decrease with decreasing ionic radius of the Ln(III) ion and also for tying the layers together more strongly. As a result of this behavior, the structure of the heavier 4f analogues significantly resembles that of their orthorhombic counterparts. The heavier lanthanide bis-hydroxychlorides preferentially crystallize in the orthorhombic modification. The IR absorbance and Raman frequencies of the hydroxide ligands correlate as a function of the central lanthanide(III) ionic radius. This observation is corroborated by X-ray diffraction (XRD) structural data. These compounds are quite insoluble in near-neutral and basic aqueous solutions, but soluble in acidic solutions. It is expected that the analogue actinide bis-hydroxychlorides exhibit similar behavior and that this may have important implications in the immobilization and safe disposal of nuclear waste.

Directed synthesis of crystalline plutonium(III) and (IV) oxalates: accessing redox-controlled separations in acidic solutions.

Both binary and ternary solid complexes of Pu(III) and Pu(IV) oxalates have been previously reported in the literature. However, uncertainties regarding the coordination chemistry and the extent of hydration of some compounds remain mainly because of the absence of any crystallographic characterization. Single crystals of hydrated oxalates of Pu(III), Pu(2)(C(2)O(4))(3)(H(2)O)(6).3H(2)O (I) and Pu(IV), KPu(C(2)O(4))(2)(OH).2.5H(2)O (II), were synthesized under moderate hydrothermal conditions and characterized by single crystal X-ray diffraction studies. Compounds I and II are the first plutonium(III) or (IV) oxalate compounds to be structurally characterized via single crystal X-ray diffraction studies. Crystallographic data for I: monoclinic, space group P2(1)/c, a = 11.246(3) A, b = 9.610(3) A, c = 10.315(3) A, Z = 4 and II: monoclinic, space group C2/c, a = 23.234(14) A, b = 7.502(4) A, c = 13.029(7) A, Z = 8.

First identification and thermodynamic characterization of the ternary U(VI) species, UO2(O2)(CO3)2(4-), in UO2-H2O2-K2CO3 solutions.

In alkaline carbonate solutions, hydrogen peroxide can selectively replace one of the carbonate ligands in UO2(CO3)3(4-) to form the ternary mixed U(VI) peroxo-carbonato species UO2(O2)(CO3)2(4-). Orange rectangular plates of K4[UO2(CO3)2(O2)].H2O were isolated and characterized by single crystal X-ray diffraction studies. Crystallographic data: monoclinic, space group P2(1)/ n, a = 6.9670(14) A, b = 9.2158(10) A, c = 18.052(4) A, Z = 4. Spectrophotometric titrations with H 2O 2 were performed in 0.5 M K 2CO 3, with UO2(O2)(CO3)2(4-) concentrations ranging from 0.1 to 0.55 mM. The molar absorptivities (M(-1) cm(-1)) for UO2(CO3)3(4-) and UO2(O2)(CO3)2(4-) were determined to be 23.3 +/- 0.3 at 448.5 nm and 1022.7 +/- 19.0 at 347.5 nm, respectively. Stoichiometric analyses coupled with spectroscopic comparisons between solution and solid state indicate that the stable solution species is UO2(O2)(CO3)2(4-), which has an apparent formation constant of log K' = 4.70 +/- 0.02 relative to the tris-carbonato complex.

Synthesis and structural characterization of a molecular plutonium(IV) compound constructed from dimeric building blocks.

Single crystals of Na(8)Pu(2)(O(2))(2)(CO(3))(6) x 12H(2)O, exhibiting bridging mu(2),eta(2)-O(2) ligands in unprecedented Pu(IV) dimeric units, were obtained at ambient temperature from an aqueous Pu(IV) peroxide carbonate solution.

Synthesis and characterization of f-element iodate architectures with variable dimensionality, alpha- and beta-Am(IO3)3.

Two americium(III) iodates, beta-Am(IO3)3 (I) and alpha-Am(IO3)3 (II), have been prepared from the aqueous reactions of Am(III) with KIO(4) at 180 degrees C and have been characterized by single-crystal X-ray diffraction, diffuse reflectance, and Raman spectroscopy. The alpha-form is consistent with the known structure type I of anhydrous lanthanide iodates. It consists of a three-dimensional network of pyramidal iodate groups bridging [AmO8] polyhedra where each of the americium ions are coordinated to eight iodate ligands. The beta-form reveals a novel architecture that is unknown within the f-element iodate series. beta-Am(IO3)3 exhibits a two-dimensional layered structure with nine-coordinate Am(III) atoms. Three crystallographically unique pyramidal iodate anions link the Am atoms into corrugated sheets that interact with one another through intermolecular IO3-...IO3- interactions forming dimeric I2O10 units. One of these anions utilizes all three O atoms to simultaneously bridge three Am atoms. The other two iodate ligands bridge only two Am atoms and have one terminal O atom. In contrast to alpha-Am(IO3)3, where the [IO3] ligands are solely corner-sharing with [AmO8] polyhedra, a complex arrangement of corner- and edge-sharing mu2- and mu3-[IO3] pyramids can be found in beta-Am(IO3)3. Crystallographic data: I, monoclinic, space group P2(1)/n, a = 8.871(3) A, b = 5.933(2) A, c = 15.315(4) A, beta = 96.948(4) degrees , V = 800.1(4) A(3), Z = 4; II, monoclinic, space group P2(1)/c, a = 7.243(2) A, b = 8.538(3) A, c = 13.513(5) A, beta = 100.123(6) degrees , V = 822.7(5) A(3), Z = 4.

Structural characterization of the first hydroxo-bridged plutonium compound, (PuO(2))(2)(IO(3))(mu(2)-OH)(3).

The hydrothermal reaction of a (239)Pu(IV) stock solution in the presence of iodic acid and 1 M KOH produces reddish-brown single crystals of (PuO(2))(2)(IO(3))(OH)(3). The structure consists of two-dimensional layers forming in the ac plane and is the first single-crystal structure of plutonium(VI) connected through hydroxide anions. The additional linkage of plutonium centers is completed through iodate ligands.

Tetrapotassium dicarbonatodioxoperoxouranium(VI) 2.5-hydrate, K4[U(CO3)2O2(O2)].2.5H2O.

The title compound was obtained by reacting UO2 powder in 2 M K2CO3 with hydrogen peroxide. The compound contains individual [U(CO3)2O2(O2)]4- ions, which are linked via an extended network of K atoms and hydrogen bonding. The U atom is coordinated to two trans-axial O atoms and six O atoms in the equatorial plane, forming distorted hexagonal bipyramids. The carbonate ligands are bound to the U center in a bidentate manner, with U-O bond distances ranging from 2.438 (5) to 2.488 (5) A. The peroxo group forms a three-membered ring with the U atom, with U-O bond distances of 2.256 (6) and 2.240 (6) A. The U=O bond distances of 1.806 (5) and 1.817 (5) A, and an O-U-O angle of 175.3 (3) degrees are characteristic of the linear uranyl(VI) unit.

Cs+ -selective membrane electrodes based on ethylene glycol-functionalized polymeric microspheres.

A new strategy for improving the robustness of membrane-based ion-selective electrodes (ISEs) is introduced based on the incorporation of microsphere-immobilized ionophores into plasticized polymer membranes. As a model system, a Cs(+)-selective electrode was developed by doping ethylene glycol-functionalized cross-linked polystyrene microspheres (P-EG) into a plasticized poly(vinyl chloride) (PVC) matrix containing sodium tetrakis-[3,5-bis(trifluoromethyl)phenyl] borate (TFPB) as the ion exchanger. Electrodes were evaluated with respect to Cs(+) in terms of sensitivity, selectivity, and dynamic response. ISEs containing P-EG and TFPB that were plasticized with 2-nitrophenyl octyl ether (NPOE) yielded a linear range from 10(-1) to 10(-5)M Cs(+), a slope of 55.4 mV/decade, and a lower detection limit (log a(Cs)) of -5.3. In addition, these membranes also demonstrated superior selectivity over Li(+), Na(+), and alkaline earth metal ion interferents when compared to analogous membranes plasticized with bis(2-ethylhexyl) sebacate (DOS) or membranes containing a lipophilic, mobile ethylene glycol derivative (ethylene glycol monooctadecyl ether (U-EG)) as ionophore.

Local and nanoscale structure and speciation in the PuO2+x-y(OH)2y.zH2O system.

Pu L(3) X-ray absorption fine structure spectra from 24 samples of PuO(2+x) (and two related Pu-substituted oxides), prepared by a variety of methods, demonstrate that (1) although the Pu sublattice remains the ordered part of the Pu distribution, the nearest-neighbor O atoms even at x = 0 are found in a multisite distribution with Pu-O distances consistent with the stable incorporation of OH(-) (and possibly H(2)O and H(+)) into the PuO(2) lattice; (2) the excess O from oxidation is found at Pu-O distances <1.9 A, consistent with the multiply bound "oxo"-type ligands found in molecular complexes of Pu(V) and Pu(VI); (3) the Pu associated with these oxo groups is most likely Pu(V), so that the excess O probably occurs as PuO(2)(+) moieties that are aperiodically distributed through the lattice; and (4) the collective interactions between these defect sites most likely cause them to cluster so as give nanoscale heterogeneity in the form of domains that may have unusual reactivity, observed as sequential oxidation by H(2)O at ambient conditions. The most accurate description of PuO(2) is therefore actually PuO(2+x-y)(OH)(2)(y).zH(2)O, with pure, ordered, homogeneous PuO(2) attained only when H(2)O is rigorously excluded and the O activity is relatively low.