Synthesis and Structure of U(VI), Np(VI), and Pu(VI) Phenylacetates

Radiochemistry - Phenylacetates [AnO2(C6H5CH2COO)2], where An = U (I), Np (II), or Pu (III), were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are...     

Synthesis and Structure of U(VI), Np(VI), and Pu(VI) 2-Fluorobenzoates

The compounds NH₄[AnO₂(C₆H₄FCOO₃], where An = U (I), Np (II), or Pu (III), CgH₄COO^? is the 2-fluorobenzoate anion, were synthesized and studied by single crystal X-ray diffraction. Compounds I–III are isostructural and crystallize in the cubic system, space group P2₁3, Z = 4. The main structural units of I–III are mononuclear complexes [AnO₂(C₆H₄COO)₃]^? belonging to crystal-chemical group AB₃¹ (A = AnO₂²⁺, B⁰¹ = C₆H₄FCOO^?). The actinide contraction in the structures of I–III is manifested in a regular decrease in the lengths of the An=0 bonds in the AnO₂²⁺ cations and in the volumes of the Voronoi-Dirichlet polyhedra (VDPs) of the An atoms in the series U-Np-Pu. The intermolecular interactions in crystal structures of I–III were analyzed by the method of molecular VDPs.

     

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic acid in aqueous solutions. Synthesis and structures of new crystalline compounds of U(VI), Np(VI), and Pu(VI)

Complexation of An(VI) (An = U, Np, Pu, Am) with 2,6-pyridinedicarboxylic (dipicolinic) acid in aqueous solutions was studied. All these actinides form with dipicolinic acid anion, PDC^2? 1: 1 and 1: 2 complexes. The PDC^2? ion coordinates to actinide(VI) ions in solutions in tridentate fashion. In 1: 2 complexes, the f-f transition bands in the electronic absorption spectra are very weak, which is associated with approximate central symmetry of the coordination polyhedron (CP) of the An atom. The apparent stability constants of Pu(VI) complexes were measured in a wide pH range, and the concentration stability constants of An(VI) (An = U, Np, Pu, Am) were determined. The crystalline complexes [Li₂AnO₂(PDC)₂]·2H₂O (An = U, Np, Pu) and [AnO₂(PDC)] _n (An = Np, Pu) were synthesized, and their structures were determined by single crystal X-ray diffraction. The X-ray data confirmed the conclusion that CP of An atoms in the complex ions AnO₂·(PDC) ₂ ^2? is centrosymmetrical. In the isostructural series of [Li₂AnO₂(PDC)₂]·2H₂O, the actinide contraction is manifested in shortening of the An-O distances in the “yl” groups in going from U to Pu.     

Behavior of Pu(VI) and Np(VI) in malonate solutions

Complexation of PuO ₂ ²⁺ in solutions containing malonate anions C₃H₂O ₄ ^2? (L^2?) is studied by spectrophotometry. Mono-and bimalonate complexes are formed. The monomalonate complex was isolated as PuO₂L · 3H₂O. It is isostructural to UO₂L · 3H₂O and forms rhombic crystals with the unit cell parameters a = 9.078(2), b = 7.526(2), and c = 6.2005(15) Å, space group Pmn2₁. The electronic absorption spectrum of the monomalonate complex is characterized by a strong band at 843 nm. In malonate solutions, Pu(VI) is slowly reduced to the pentavalent state even in the cold. The reduction of Np(VI) is considerably faster and more sensitive to increasing temperature. Some kinetic features of the reduction are discussed.     

Synthesis and single crystal X-ray diffraction study of U(VI), Np(VI), and Pu(VI) perchlorate hydrates

The compounds [AnO₂(H₂O)₅](ClO₄)₂ (An = Np, Pu) and [NpO₂(ClO₄)₂(H₂O)₃] were prepared as single crystals, which were studied by X-ray diffraction at 100 K. The structural type at room temperature was determined. The low-temperature modification of [UO₂(H₂O)₅](ClO₄)₂ was found and structurally studied. The coordination polyhedra in [AnO₂(H₂O)₅]²⁺ are weakly distorted pentagonal bipyramids with averaged interatomic distances An-O of 1.754, 1.744, and 1.732 Å in the “yl” groups and of 2.415, 2.416, and 2.409 Å in the equatorial planes for U, Np, and Pu, respectively. Hence, in the complex cations [AnO₂(H₂O)₅]²⁺ the actinide contraction is manifested only in regular shortening of the An-O interatomic distances in the “yl” groups. The compound [NpO₂(ClO₄)₂(H₂O)₃], isostructural to its known uranyl analog, appeared to be the first, proved by single crystal X-ray diffraction, example of a compound with coordination interaction between the perchlorate ion and the neptunyl(VI) cation.     

Reaction of ozone with Np(IV) and Pu(IV) oxalates in water

The reaction of the ozone–oxygen mixture with aqueous suspensions of Np(IV) and Pu(IV) oxalates was studied. Both metal cations and oxalate anions are oxidized in the process. The final products are Np(VI) and Pu(VI) hydroxides. The composition of Np(VI) hydroxide was confirmed by X-ray diffraction analysis. Oxidation of Np(IV) oxalate with oxygen leads to the accumulation of Np(V) oxalate and oxalic acid in the solution. At incomplete oxidation of Np(IV) oxalate with ozone in water, Np(V) is also accumulated. Heating considerably accelerates the ozonation. The possible reaction mechanism is briefly discussed. The Np(V) and Np(VI) ions participate in the catalytic cycle of the decomposition of oxalate ions with ozone.     

Behavior of Np(VII,VI, V) in silicate solutions

Properties of Np(VII, VI, V) in silicate solutions were studied spectrophotometrically. In noncomplexing media, the Np(VII) cation transforms into the anionic species at pH 5.5–7.5. In the presence of carbonate ions, this rearrangement occurs at pH 10–11.5, and in silicate solutions, at pH 10.5–12.0. These data show that Np(VII) cation forms complexes with carbonate and silicate ions, the latter being stronger. From the competitive reactions of Np(VI) complex formation with carbonate and silicate ions, the stability of NpO₂SiO₃ complex was estimated (log = 16.5) using the known stability constant of NpO₂(CO₃) ₃ ^4– . Complexation of Np(V) with SiO ₃ ^2– ions was not detected by the methods used.Translated from Radiokhimiya, Vol. 46, No. 6, 2004, pp. 527–530.Original Russian Text Copyright © 2004 by Shilov, Fedoseev, Yusov, Delegard.     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VII. Kinetics of Reduction of Pu(IV) and Np(VI) with Butanal Oxime in Undiluted TBP

The kinetics of reduction of Pu(IV) and Np(VI) with butanal oxime in undiluted TBP containing HNO₃ was studied spectrophotometrically. In the range [HNO₃] = 0.08-0.75 M the rate of Pu(IV) reduction is described by the equation -d[Pu(IV)]/dt = k[Pu(IV)]²[C₃H₇CHNOH]/{[Pu(III)][HNO₃]²} with the rate constant k = 0.068±0.017 mol l^-1 min^-1 at 20°C. The kinetic equation of the reduction of Np(VI) to Np(V) in the range [HNO₃] = 0.01-0.27 M is -d[Np(VI)]/dt = k[Np(VI)][C₃H₇CHNOH][H₂O]²/[HNO₃]0.5, where k = 0.058±0.007 l2.5 mol-2.5 min^-1 at 25°C, and the activation energy is 79±9 kJ mol^-1.     

Interaction of U(VI), Np(VI), and Pu(VI) ions with unsaturated heteropolytungstates of the 17th and 11th series in aqueous solutions

The stability constants of the complexes of U(VI), Np(VI), and Pu(VI) with the heteropolyanions (HPAs) P₂W₁₇O ₆₁ ^10? , SiW₁₁O ₃₉ ^8? , and PW₁₁O ₃₉ ^7? in solutions with pH from ?0.3 (2 M H⁺) to 5–5.5 in the presence of Na or K salts (up to 2 M) and without them were measured. All the complexes have exclusively the 1: 1 composition; their stability constants β^M(VI) in neutral solutions at a low ionic strength are close to 10⁸ 1 mol^?1. In 0.1–2.0 M acid solutions, log β^M(VI) for the complexes with P₂W₁₇O ₆₁ ^10? is within 1.4–3.9. The slope of the pH dependence of log β^M(VI) does not exceed 1.75; this fact suggests that no more than two protons are displaced from HPA upon complexation in acid solutions. In the presence of 1–2 M sodium salts, the β^M(VI) values reach a maximum at pH ～3 and drastically decrease with a further increase in pH. Actinides(VI) interact with HPAs appreciably more weakly than do actinides(III), which is apparently due to the fact that the denticity of HPAs in the complexes with An(VI), apparently, does not exceed 2.     

Reduction of Pu(IV) and Np(VI) with carbohydrazide in nitric acid solution

The reduction of Pu(IV) and Np(VI) with carbohydrazide (NH₂NH)₂CO in 1–6 M HNO₃ solutions was studied. The Pu(IV) reduction is described by a first-order rate equation with respect to Pu(IV). At [HNO₃] ≥ 3 M, the reaction becomes reversible. The rate constants of the forward and reverse reactions were determined, and their activation energies were estimated. Neptunium(VI) is reduced to Np(V) at a high rate, whereas the subsequent reduction of Np(V) to Np(IV) is considerably slower and is catalyzed by Fe and Tc ions. The possibility of using carbohydrazide for stabilizing desired combinations of Pu and Np valence states was examined.     

Synthesis and crystal structure of Th(IV), U(IV), and Np(IV) tribromoacetates

Actinide(IV) tribromoacetates of the composition [An(CBr₃COO)₄(H₂O)₂]₂ (An = Th, U, Np) were synthesized and studied. Their structural feature is the formation of electrically neutral dimeric complexes. The surrounding of the An(IV) atoms in the dimers is formed by the oxygen atoms of six CBr₃COO^? anions and two water molecules; the coordination number of An(IV) is 9, and the coordination polyhedron can be described as distorted base-monocapped tetragonal antiprism. Four independent CBr₃COO^? anions in the structure are coordinated to the An(IV) atoms in different fashions: monodentate, bidentate chelate, and bidentate bridging. Hydrogen bonding links the dimers in infinite chains along [010] direction. The hydrogen bonding noticeably influences the geometric characteristics of the coordination surrounding of the An(IV) atoms.     

Synthesis and properties of Np(VI) and Pu(VI) monophthalates

Neptunium(VI) and plutonium(VI) monophthalates were prepared and characterized. The complexes AnO₂ (COO)₂C₆H₄ 2H₂O were isolated from cold solutions, and AnO₂ (COO)₂C₆H₄ 1.33H₂O, from hot solutions. NpO₂ (COO)₂C₆H₄ b. 2H₂O and PuO₂ (COO)₂C₆H₄ 2H₂O crystalize in the triclinic and monoclinic systems, respectively. The complexes AnO₂(COO)₂C₆H₄ 1.33H₂O are isostructural and crystallize in the rhombohedral system. The thermal behavior of these complexes was studied. Their IR and electronic absorption spectra were recorded. The properties of these complexes were compared to those of known U(VI) monophthalates.Translated from Radiokhimiya, Vol. 46, No. 5, 2004, pp. 389–395.Original Russian Text Copyright © 2004 by Krot, Bessonov, Grigorev, Charushnikova, Makarenkov.     

Kinetics of Redox Reactions of U, Pu, and Np in TBP Solutions: VIII. Kinetics of Reduction of Pu(VI) and Np(VI) with N,N-Dibutylhydroxylamine in Undiluted TBP

The kinetics of Pu(VI) and Np(VI) reduction in TBP containing HNO₃ was studied spectrophotometrically. The rate of the reduction of Pu(VI) with N,N-dibutylhydroxylamine in undiluted TBP is independent of the Pu(VI) concentration and is described by the equation -d[Pu(VI)]/dt = k[(C₄H₉)₂NOH][H₂O]⁵, with k = (2.17±0.13) × 10^-5 l⁵ mol^-5 min^-1 at 12°C. The activation energy of the reaction, E = 85.2± 4.6 kJ mol^-1, was determined from the temperature dependence of k in the range 12.0-33.5°C. Reduction of Np(VI) in undiluted TBP is approximately described by the kinetic equation -d[Np(VI)]/dt = k[Np(VI)] × [(C₄H₉)₂NOH]/[HNO₃], with k 40 min^-1 at 25°C, and in a 30% solutio of TBP in n-dodecane, by the equation -d[Np(VI)]/dt = k[Np(VI)][(C₄H₉)₂NOH]/[HNO₃]0.7 with the rate constant k = 18.4±1.8 l0.3 mol-0.3 min^-1 at 25°C.     

Disproportionation of Pu(VI) and reproportionation of Pu(V), Pu(VII) in aqueous alkali solutions

Disproportionation of Pu(VI) and reproportionation of Pu(V) and Pu(VII) in aqueous NaOH solutions was studied. With an increase in the NaOH concentration in solution over 7.5 M, the equilibrium of the reaction Pu(VII) + Pu(V)?2Pu(VI) is gradually shifted toward formation of Pu(V) and Pu(VII) as products of Pu(VI) disproportionation, and at [NaOH] + 13 M, Pu(VI) disproportionates virtually completely. At [NaOH] + 7.5 M, the equilibrium of the above reaction is shifted toward formation of Pu(VI). Based on the experimental data, the equilibrium constants of the reaction at various alkali concentrations in the solution and the formal potentials ?_{f[Pu(VII)/Pu(VI)]} were calculated. The data obtained showed that, with respect to reduction with water, Pu(VII) is stable in aqueous alkali solutions at NaOH concentrations exceeding 7.5 M.     

Synthesis and structural features of Np(VI) and Pu(VI) isophthalates

New An(VI) isophthalate complexes [PuO₂(C₈H₄O₄)] (I), Cs₂[(NpO₂)₂(C₈H₄O₄)₃]·4H₂O (II), [H₃O]₂[(NpO₂)₂(C₈H₄O₄)₃]·nH₂O (III), and [H₃O][NpO₂(C₈H₄O₄)(C₈H₅O₄)]·2H₂O (IV) with the An(VI): Lig ratios of 1: 1 (I), 1: 1.5 (II, III), and 1: 2 (IV) were synthesized and studied by single crystal X-ray diffraction. In complex I, the coordination polyhedron of the Pu(1) atom is a pentagonal bipyramid whose equatorial plane is formed by the oxygen atoms of four [C₈H₄O₄]^2– anions. The coordination capacity of the ligand in complex I is maximal among compounds I–IV and equal to 5, with each [C₈H₄O₄]^2– anion binding four PuO₂²⁺ cations into electrically neutral layers. In the structures of II and III, the coordination polyhedra of the Np(1) atoms are hexagonal bipyramids whose equatorial planes are formed by the oxygen atoms of three [C₈H₄O₄]^2– anions. Two crystallographically independent [C₈H₄O₄]^2– anions exhibit the coordination capacity equal to 4, each binding two NpO₂²⁺ cations in the chelate fashion. As a result, doubled anionic layers are formed in the crystals of II and III. Outer-sphere cations influence the packing of doubled layers in the crystals: Complex II crystallizes in the monoclinic system, and complex III, in the orthorhombic system. In the structure of IV, the coordination polyhedron of the Np(1) atom is a hexagonal bipyramid whose equatorial plane is formed by the oxygen atoms of two [C₈H₄O₄]^2– anions and one [C₈H₅O₄]^– anion. The crystallographically independent bridging anion [C₈H₄O₄]^2– exhibits the coordination capacity equal to 4 and binds in the chelate fashion two NpO₂²⁺ cations to form chains, and the independent hydrogen isophthalate anion [C₈H₅O₄]^– binds one neptunyl(VI) cation in the chain in the chelate fashion, exhibiting the coordination capacity equal to 2.     

Extraction of U(VI), Th(IV), Pu(IV), and Am(III) from nitric acid solutions with polyfunctional organophosphorus acids

Extraction of microamounts of U(VI), Th(IV), Pu(IV), and Am(III) nitrates from aqueous HNO₃ solutions with solutions of (diphenylphosphinylmethyl)phenylphosphinic, (di-p-tolylphosphinylmethyl)phenylphosphinic, and (dioctylphosphinylmethyl)phosphinic acids and of butyl hydrogen (diphenylphosphinylmethyl)phosphonate in organic diluents was studied. The metal: extractant stoichiometric ratio in the extractable complexes was determined, and the diluent effect on the extraction efficiency was examined. The possibility of using a macroporous polymeric sorbent impregnated with (dioctylphosphinylmethyl)phenylphosphinic acid for concentrating metal ions from HNO₃ solutions was demonstrated.     

Interaction of U(VI), Np(VI), and Pu(VI) with picolinic (2-pyridinecarboxylic) acid: Complexation in aqueous solutions and synthesis, spectra, and thermal properties of complexes [CH6N3][AnO2(C6H4NO2)3] (An = U, Np, Pu)

Complexation of actinides(VI) (U, Np, Pu) with picolinic acid C₆H₅NO₂ (HPic) in solution was studied by spectrophotometry. In particular, data on the Np(VI) complexation were obtained for the first time. The complexes [AnO₂(HPic)]²⁺, [AnO₂(Pic)]⁺, [AnO₂(Pic)₂], and, presumably, [AnO₂(HPic)(Pic)]⁺, and also mixed picolinate-hydroxide complexes can exist in the solution under different conditions. The stability constants of the complexes were estimated. A series of crystalline actinide(VI) picolinate complexes Gu[AnO₂(Pic)₃] (An = Np, Pu; Pic is picolinate ion, Gu is guanidinium ion) were synthesized, and their structure was determined. The spectra and thermal behavior of the complexes are discussed.     

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of Fe(III) oxides and hydrous oxides and MnO<Subscript>2</Subscript>

Sorption of Np(V), Pu(V), and Pu(IV) on colloids of synthetic goethite (α-FeOOH), hematite (α-Fe₂O₃), maghemite (γ-Fe₂O₃), and amorphous MnO₂ was studied over wide ranges of solution pH and ionic strength by solvent extraction and X-ray photoelectron spectroscopy (XPS). Plutonium(V) is reduced upon sorption on α-FeOOH, but not on α-Fe₂O₃ and γ-Fe₂O₃. On the MnO₂ surface, Pu occurs as Pu(VI). From the pH dependences of the actinide sorption, the equilibrium constants of the reactions of Np(V)O ₂ ⁺ and Pu(V)O ₂ ⁺ with the surface hydroxy groups of the investigated colloid particles and a set of the equilibrium constants of the reactions of Pu(IV) hydroxo complexes with α-FeOOH were obtained. If no redox reactions occur on the surface of the colloid particles, these constants decrease in the order \(K_{MnO_2 } > K_{\alpha - FeOOH} > K_{\alpha - Fe_2 O_3 } \sim K_{\gamma - Fe_2 O_3 } \).     

Interaction of Np(V), (VI), and (VII) with Aluminosilicates in Alkaline Medium

The behavior of Np in higher oxidation states in alkaline solution containing silicate and aluminate ions was studied. In formation of a crystalline aluminosilicate in a solution, Np(V), (VI), and (VII) are not incorporated into its crystal structure but hamper formation of the solid phase. The possibility of sorption of Np on various aluminosilicates is primarily governed by its oxidation state. Np(V) and Np(VII) are not sorbed from strong alkali. Np(VI) is retained by aluminosilicate materials to various extents depending on the surface characteristics and surface area of these materials. On heating, the degree of Np(VI) sorption decreases, which suggests the physical nature of the process.     

Solubility of mixed-valence U(IV–VI) and Np(IV–V) hydroxides in simulated groundwater and 0.1 M NaClO4 solutions

The kinetics of U(VI) accumulation in the phase of U(IV) hydroxide and of Np(V) in the phase of neptunium(IV) hydroxide, and also the solubility of the formed mixed-valence U(IV)-U(IV) and Np(IV)-Np(V) hydroxides in simulated groundwater (SGW, pH 8.5) and 0.1 M NaClO₄ (pH 6.9) solutions was studied. It was found that the structure of the mixed U(IV–VI) hydroxide obtained by both oxidation of U(IV) hydroxide with atmospheric oxygen and alkaline precipitation from aqueous solution containing simultaneously U(IV) and U(VI) did not affect its solubility at the U(VI) content in the system exceeding 16%. The solubility of mixed-valence U(IV–VI) hydroxides in SGW and 0.1 M NaClO₄ is (3.6±1.9) × 10^?4 and (4.3 ± 1.7) × 10^?4 M, respectively. The mixed Np(IV–V) hydroxide containing from 8 to 90% Np(V) has a peculiar structure controlling its properties. The solubility of the mixed-valence Np(IV–V) hydroxide in SGW [(6.5 ± 1.5) × 10^?6 M] and 0.1 M NaClO₄ [(6.1±2.4) × 10^?6 M] is virtually equal. Its solubility is about three orders of magnitude as high as that of pure Np(OH)₄ (10^?9–10^?8 M), but considerably smaller than that of NpO₂(OH) (～7 × 10^?4 M). The solubility is independent of the preparation procedure [oxidation of Np(OH)₄ with atmospheric oxygen or precipitation from Np(IV) + Np(V) solutions]. The solubility of the mixed-valence Np hydroxide does not increase and even somewhat decreases [to (1.4±0.7) × 10^?6 M] in the course of prolonged storage (for more than a year).