A. M. Fedoseev

Crystal Structure of the Mixed-Valence Neptunium Compound Na6[(NpVO2)2(NpVIO2)(MoO4)5] · 13H2O

The crystal structure of the mixed-valence Np(V) and Np(VI) compound Na6[(NpVO2)2(NpVIO2)(MoO4)5] · 13H2O was determined. The structure is built of the anionic layers [(NpVO2)2(NpVIO2)(MoO4)5]6n-n with the Na+ cations and crystal water molecules between them. The Np(V) and Np(VI) atoms in the anionic layers are ordered. The motif of the anionic layer is close to that found in Mg2[(UO2)3(SeO4)5] · 16H2O. The isostructural mixed-valence Np(V) and U(VI) compound was also synthesized.

Interaction of neptunium(V) with picolinic, nicotinic and isonicotinic acids

Summary The interaction of Np(V) with all isomers of pyridinemonocarboxylic acid has been investigated, stability constants have been calculated using NIR spectroscopy. Single crystals of [NpO2(Pic)(H2O)2] (1), Cs[NpO2(Pic)2(HPic)]·3H2O (2), [NpO2(Nic)(H2O)] (3), and [NpO2(iso-Nic)(H2O)] (4) have been synthesized. Compound 1 contains infinite chains; compound 2 consists of isolated Cs+, [NpO2(Pic)2(HPic)]- groups and water molecules. In both 3 and 4 neptunoyl-ion NpO2 acts as monodentate ligand with one of the O atoms of NpO2 group entering in coordination polyhedron of the adjacent Np atom. Compounds 3 and 4 have layered and three-dimensional topologies, respectively. Crystallographic data: [NpO2(Pic)(H2O)2], space group P¯1, triclinic, a = 6.380(2), b = 7.733(2), c = 10.700(4) Å, α = 96.49(3), β = 106.78(3), γ = 103.83(3)°, and Z = 2 (1); Cs[NpO2(Pic)2(HPic)]·3H2O, space group P¯1, triclinic, a = 8.707(2), b = 9.777(2), c = 14.640(3) Å, α = 76.633(15), β = 78.75(2), γ = 86.55(2)°, and Z = 2 (2); [NpO2(Nic)(H2O)], space group P21/n, monoclinic, a = 11.901(4), b = 5.5628(15), c = 12.124(3) Å, β = 101.05(2)°, and Z = 4 (3); [NpO2(iso-Nic)(H2O)], space group P212121, orthorhombic, a = 5.742(3), b = 8.550(5), c = 18.512(10) Å, and Z = 4 (4). Electron and IR spectra of solid compounds have been measured.

Synthesis and X-ray diffraction study of complex neptunium(VI) potassium chromate K2[(NpO2)2(CrO4)3(H2O)] · 3H2O

The crystal structure of the K2[(NpO2)2(CrO4)3(H2O)] · 3H2O compound was established. The structure consists of anionic layers [(NpO2)2(CrO4)3(H2O)]n2n, between which K+ ions and crystallization water molecules are located. The coordination polyhedra of Np atoms are distorted pentagonal bipyramids. All chromate ions are bound in a tridentate-bridging fashion.

Neptunium(VI) Chromate Complex with 1,2-Diammoniopropane {H3NCH2CH(NH3)CH3}[(NpO2)2(CrO4)3(H2O)] · 3H2O: Synthesis, Crystal Structure and Properties

New neptunium(VI) complex {H3NCH2CH(NH3)CH3}[(NpO2)2(CrO4)3(H2O)] · 3H2O is synthesized; its crystal structure is determined and IR and near-IR absorption spectra are recorded. The crystallographic data are: a = 10.805(2) Å, b = 11.238(2) Å, c = 17.615(8) Å, space group P212121, Z = 4, V = 2139(1) Å3, R = 0.051, wR(F2) = 0.109. The crystal structure of the compound is built of the anionic layers of [(NpO2)2(CrO4)3(H2O)]2n–n. The {H3NCH2CH(NH3)CH3}2+ cations and crystallization water molecules are arranged between the layers. Coordination polyhedron of two crystallographically independent Np atoms has the shape of a pentagonal bipyramid. The equatorial plane in one Np polyhedron is formed by the oxygen atoms of four chromate ions and water molecule and by the oxygen atoms of five chromate ions in the other one.

Reaction of Plutonium(VI) with Orthosilicic Acid Si(OH)4

Reaction of Pu(VI) with Si(OH)4 (at concentration 0.004–0.025 mol l–1) in a 0.2 M NaClO4 solution at pH 3–8 is studied by spectrophotometric method. In the range of pH 4.5–5.5, PuO2(H2O)4OSi(OH)3+ complex is formed, while at pH > 6, PuO2(H2O)3O2Si(OH)2 or hydroxosilicate complex PuO2(H2O)3(OH)OSi(OH)3 is recorded. The equilibrium constants are calculated for the reactions of formation of PuO2(H2O)4OSi(OH)3+ and PuO2(H2O)3O2Si(OH)2 and their concentration stability constants: log K1 = –3.91 ± 0.17 and log K2≈ –10.5; log β1= 5.90 ± 0.17 and log β2≈ 12.6. The PuO2(H2O)4OSi(OH)3+ complex is significantly less stable than analogous complex of U(VI). Calculations of the forms of Pu(VI) occurrence at the Si(OH)4 concentration equal to 0.002 mol l–1 showed that the maximum fraction of the PuO2(H2O)4OSi(OH)3+ complex is ∼10% (pH 6.5), while the fraction of PuO2(H2O)3O2Si(OH)2 is almost 40% (pH 8).

Kinetics of the Np(VI) + EDTA reaction in perchloric acid solutions

The stoichiometry of the Np(VI) + H2C2O4 and Np(VI) + H4Y reactions (Y is EDTA anion) in 0.2 M HClO4 solution was studied by spectrophotometry. With excess Np(VI), 1 mol of H2C2O4 or EDTA reduces, respectively, 2 or 4 mol of Np(VI) to Np(V). In 0.1–1.0 M HClO4 solution (the ionic strength of 1.0 was supported by adding LiClO4) containing 3–20 mM EDTA at 20–45°C, Np(VI) at a concentration of 1 mM and higher is consumed in accordance with the first-order rate law until less than 0.4 mM Np(VI) remains in the solution, after which the reaction decelerates. The reaction rate has the order of 1 with respect to EDTA and −1.5 with respect to H+ ions. The activated complex is formed with the loss of 1 and 2 H+ ions. The activation energy is 86.0 ± 3.5 kJ mol−1.

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, PDC2− 1: 1 and 1: 2 complexes. The PDC2− 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)22− 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.

Synthesis, crystal structure, and absorption spectra of 2,2′-bipyridine complexes of benzoates of hexavalent U, Np, and Pu

Complex benzoates [{UO2(C10N2H8)C6H5COO}2O2], [NpO2(C10N2H8)(C6H5COO)2], and [PuO2(C10N2H8)(C6H5COO)2] (C10N2H8 is 2,2′-bipyridine) were synthesized in the form of single crystals, and their structure was determined by X-ray diffraction analysis. The absorption spectra of the crystalline U(VI), Np(VI), and Pu(VI) complexes were measured and analyzed.

Crystal structure of complexes of pentavalent neptunium with 1,8-bis[2-(diphenylphosphinylmethyl)]phenoxy-3,6-dioxaoctane (L) and hexavalent actinides (U, Np, Pu) with 1,8-bis[2-(diphenylphosphinyl)]phenoxy-3,6-dioxaoctane (L1)

Complexes of pentaand hexavalent actinides with phosphoryl-containing podands incorporating a triethylene glycol fragment and 2-(diphenylphosphinyl)phenyl (L1) or 2-(diphenylphosphinylmethyl)phenyl (L) end groups were synthesized and structurally characterized. The complexes can be described by the following formulas: [NpO2L(C2H5OH)(NO3)] (I) for pentavalent neptunium and [AnO2(L1)2](OH)2 · nH2O (II), where An is U (IIa), Np (IIb), and Pu (IIc), for the isostructural compounds of hexavalent actinides. In I, L is a bidentate bridging ligand. The Np5+ coordination polyhedron is a pentagonal bipyramid. The bipyramid equatorial plane is formed by the oxygen atoms of two podands L, the bidentate nitrate ion, and the ethanol OH group. The oxygen atoms of the phosphoryl groups of the podand are involved in the coordination environment of two NpO2+ cations where they connect the electrically neutral neptunoyl nitrate fragments to infinite chains along the [100] direction, which are in turn connected into ribbons by strong hydrogen bonds. The crystal of II consists of the complex cations [AnO2(L1)2]2+, hydroxyl ions, and water molecules of crystallization. The environment of AnO22+ is formed by four ligands L1 whose oxygen atoms form a tetragonal-bipyramidal coordination environment. Each of the two crystallographically independent ligands L1 is connected to two AnO2+ cations. This gives positively charged layers of actinyl cations perpendicular to the [010] direction connected by molecular ligands. The layers contain channels accommodating hydroxyl ions and crystallization water molecules.

Crystal Structure and Absorption Spectra of Triguanidinium Bis(N-methyliminodiacetato)dioxoneptunate(V) {C(NH2)3}3[NpO2(mida)2]

This paper is a continuation of our on-going program for studying the structure and properties of pentavalent neptunium compounds with organic ligands containing several electron-donating atoms. Previously, we studied the neptunium(V) complexes with anions of pyridinecarboxylic acids, polypyridines, and found that the Np ion coordinates the oxygen and nitrogen atoms of heterocyclic ligands to form stable chelate rings [1‐3]. As is known, neptunium(V) in weak acid and basic aqueous solutions forms stable complexes with aliphatic polyaminoacetic acids (complexones). The compositions and stability constants were determined for the Np complexes with the anions of the following acids: iminodiacetic (H 2 ida), N -methyliminodiacetic (H 2 mida), ethylenediaminetetraacetic (H 4 edta), diethylenetetraminepentaacetic, and N -hydroxyethyliminodiacetic. It was noted that the complexes of pentavalent actinides in solutions are less stable than the complexes of hexavalent actinides with the same ligands [4‐6]. However, only neptunium(V) ethylenediaminetet