V. P. Perminov

Synthesis and structure of actinide(VII) compounds Rb3NpO4(OH)2·3H2O and Rb3PuO4(OH)2·3H2O

Actinide(VII) salts Rb3[NpO4(OH)2]·3H2O (I) and Rb3[PuO4(OH)2]·3H2O (II) were prepared as single crystals and examined by X-ray diffraction. The compounds are isostructural and crystallize in the monoclinic system, space group C2/c, Z = 4; unit cell parameters: a = 12.1544(3), b = 10.9942(2), c = 7.789(2) Å, β = 91.0930(11)° for I and a = 12.1254(3), b = 10.9506(2), c = 7.7699(2) Å, β = 90.8253(12)° for II. The main structural elements of I and II are centrosymmetrical anions [AnO4(OH)2]3− forming together with water molecules, owing to strong hydrogen bonding, chains oriented along [101]. In [AnO4(OH)2]3− anions, the central An(VII) atom has a tetragonal-bipyramidal oxygen surrounding. The An-O(OH) interatomic distances decrease in going from I to II owing to actinide contraction by a factor of ∼2 more strongly than the An-O bond lengths in the equatorial planes of the bipyramids. The previously studied structure of Cs3[NpO4(OH)2]·3H2O (III) was refined.

New data on sodium salts of Np(VII) and Pu(VII)

A new Pu(VII) compound, Na3[PuO4(OH)2]·2H2O, was prepared by X-ray diffraction analysis, and its structure was studied. To reveal the character of actinide contraction in going from Np(VII) to Pu(VII), the geometric parameters of the tetragonal-bipyramidal surrounding of Pu(VII) in the [PuO4(OH)2]3− anion were compared with those of Np(VII) in the previously studied isostructural compound Na3[NpO4(OH)2]·2H2O. To reveal the specific feature of hydrogen bonding in crystals of the general composition Na3[NpO4(OH)2]·nH2O (n = 0, 2, 4), the structure of the compound Na3[NpO4(OH)2]·4H2O studied previously by the photographic method was refined. The effect of hydrogen bonds on the geometric characteristics of the coordination polyhedra of the Np and Na atoms was considered.

Absorption Spectra of Some Crystalline Np(V) Compounds in the Visible Range

The elecotronic absorption spectra in the range 500-750 nm were measured for crystalline Np(V) compounds with the pentagonal-bipyramidal [NpO2OOCH·H2O, NpO2OOCCH2OH·H2O, (NpO2)2(DMSO)7·(ClO4)2·3H2O, NpO2ClO4·4H2O], hexagonal-bipyramidal [KNpO2CO3, (NpO2)2(NO3)2·5H2O], and tetragonal-bipyramidal [NpO2(TPPO)4ClO4, Cs3NpO2Cl4] ligand surrounding. The symmetry of the coordination polyhedra affects the symmetry of narrow absorption bands; however, the long-wave shift of the peak of hydrated NpO2+ ions at 617 nm, caused by cation-cation interaction, is considerably less pronounced than the shift of the band at 981 nm.

Synthesis and structure of complex nitrates of some Ln(III) and of Am(III) with 1,10-phenanthroline-2,9-dicarboxylic acid anions

The structures of Ln(III) and Am(III) complexes with anions of 1,10-phenanthroline-2,9-dicarboxylic acid (H2PDA) of the compositions [Pr(PDA)(NO3)(H2O)3]·H2O (I), [Ln(PDA)(NO3)(H2O)2]·H2O [where Ln = Nd (IIa), Sm (IIb), Gd (IIc)], [Am(PDA)(NO3)(H2O)2]·H2O (III), and [Nd(PDA)(NO3)· (H2O)] (IV), containing in the coordination sphere of the metal atom three different ligands, PDA2−, nitrate ions, and water molecules, were studied. The structures of compounds I–III prepared at room temperature differ essentially from that of compound IV isolated at ∼200°C. Compounds I–III have a chain structure, with compounds II and III being isostructural and differing from compound I. The structure of compounds I–III isolated at room temperature is based on zigzag cationic chains Ln3+,Am3+-PDA2− in which the anions have the coordination capacity of 5 and act as N,O-donor chelating ligands and as bridging monodentate ligands. Analysis of the hydrogen bonds shows that they influence the configuration of the cationic chains Ln3+,Am3+-PDA2− and are responsible for the difference between the cationic chain in I and those in II and III. The coordination polyhedron (CP) of the Pr atom in I is characterized by CN 10. The CP of Ln3+ and Am3+ in II and III can be described as a distorted tricapped trigonal prism (CN 9). Elimination of one water molecule in the high-temperature modification of IV is accompanied by incorporation of the oxygen atom of the PDA2− anion from the adjacent chain. As a result, the cationic chains become combined in cationic layers, and the coordination capacity of the ligand increases to 6. The CP of the Nd atom has nine vertices.

Mechanism of cerium(III) oxidation with ozone in sulfuric acid solutions

The kinetics of Ce(III) oxidation with ozone in 0.1–3.2 M H2SO4 solutions was studied by spectrophotometry. The reaction follows the first-order rate law with respect to each reactant. The rate constant k slightly increases with an increase in the acid concentration, which is associated with an increase in the O3/O3− oxidation potential. The activation energy in the range 17–35°C is 46 kJ mol−1. With excess Ce(III), the stoichiometric coefficient Δ[Ce(IV)]/Δ[O3] increases from 1.6 to 2 in going from 0.1 to 1–3.2 M H2SO4. The extent of the Ce(III) oxidation is 78% in 0.1 M H2SO4 and reaches 82% in 1 M H2SO4. The ozonation involves the reactions Ce(III) + O3 → Ce(IV) + O3−, O3− + H+ → HO3, HO3 → OH + O2, OH + HSO4− → H2O + SO4−, OH + Ce(III) → OH− + Ce(IV), and SO4− + Ce(III) → SO4/2− + Ce(IV). Low stoichiometric coefficient of the Ce(III) oxidation is associated with the hydrolysis of Ce(IV). The excited Ce(IV) ion arising from oxidation of Ce(III) with OH radical forms with the hydrolyzed Ce(IV) ion a dimer whose decomposition yields Ce(III) and H2O2. After the ozonation termination, Ce(IV) is relatively stable in sulfuric acid solution, with only 5–7% of Ce(IV) disappearing in 24 h.

Crystallization of salts M3[NpO4(OH)2]·nH2O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures. Synthesis and structure of a new Np(VII) compound, Na3[NpO4(OH)2]·6H2O

Precipitation of salts M3[NpO4(OH)2]·nH2O (M = Na, K, Rb, Cs) from concentrated alkali solutions at low temperatures (about −10°C) was studied. From solutions with [OH−] > 9.5 M, these compounds are isolated as coarse black crystals in high yield without impurity of other phases. The K, Rb, and Cs salts crystallize in the form of the previously studied compounds K3[NpO4(OH)2]·2H2O and M3[NpO4(OH)2]·3H2O (M = Rb, Cs). In the case of Na, a new hydrate Na3[NpO4(OH)2]·6H2O was obtained, and its crystal structure was determined. Crystals of the hexahydrate consist of centrosymmetrical tetragonal-bipyramidal anions [NpO4(OH)2]3−, crystallographically independent Na(1) and Na(2) cations, and water molecules. The coordination surrounding of the Np atom is characterized by noticeable difference (Δ = 0.0203 Å) in the Np-O bond lengths in the equatorial plane of the bipyramid. The [NpO4(OH)2]3− anions are combined with the Na(2) cations to form infinite chains [Na(2)NpO4(OH)2(H2O)2]2− in such a manner that the lateral edges of the anion are simultaneously the lateral edges of the Na(2) coordination polyhedron. Incorporation of one of the two crystallographically independent O atoms of the NpO4 group into the Na(2) coordination surrounding is responsible for a noticeable difference in the Np-O bond lengths in the equator of the [NpO4(OH)2]3− anion. The types of hydrogen bonding in the structures of Na3[NpO4(OH)2]·nH2O (n = 0, 2, 4, 6) are compared.

Synthesis and crystal structure of new Np(VII) compounds, M[NpO4(OH)]·nH2O (M = Ca, Sr, Ba)

Previously unknown Np(VII) compounds with alkaline earth metal cations of the composition M[NpO4OH]·nH2O (M = Ca, Sr, Ba) were prepared in the crystalline form and studied by single crystal X-ray diffraction analysis. The main structural motif in the compounds is formed by infinite anionic chains [NpO4· (OH)]n2n– in which the NpO4 groups are linked via common ОН groups. In the structures of Ca[NpO4(OH)]· 3H2O and Ca[NpO4(OH)]·4H2O, each Ca2+ cation links two anionic chains [NpO4(OH)]n2n–, so that electrically neutral layers are formed. In the structures of Sr[NpO4(OH)]·H2O and Ba[NpO4(OH)]·H2O, each of the Sr2+ and Ba2+ cations binds three anionic chains [NpO4(OH)]n2n–, so that a 3D framework is formed in the crystals. The outer-sphere cations influence the structure of the anionic chains.

Synthesis and Characteristics of Double Np(VI) Potassium and Pu(VI) Potassium Silicates K[(NpO2)(SiO3OH)]·H2O and K[(PuO2)(SiO3OH)]·H2O

K[(NpO2)(SiO3OH)]·H2O (I) and K[(PuO2)(SiO3OH)]·H2O (II) were prepared by hydrothermal synthesis at 120°C, and their IR and near-IR spectra were measured. The unit cell parameters of the compounds were determined from powder X-ray patterns [a = 7.068(1), b = 7.064(2), c = 6.640(1) Å. β = 106.68(1)°, V = 317.6 Å3 (I); a = 7.102(1), b = 7.100(2), c = 6.637(1) Å, β = 107.62(1)°, V = 318.9 Å3 (II)]. These compounds are isostructural with potassium boltwoodite K[(UO2)(SiO3OH)]·H2O.

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.

Reaction of ozone with uranium(IV) oxalate in water

Decomposition of aqueous suspensions of uranium(IV) oxalate under the action of an ozone–oxygen mixture was studied. The process occurs in two steps. In the first step, the U(IV) oxidation with the formation of oxalic acid uranyl solutions prevails. The second step involves decomposition of oxalate ions and hydrolysis of uranyl ions. An increase in temperature accelerates the transformation of uranium(IV) oxalate into uranium(VI) hydroxide compounds. In solutions containing KBr or UO2Br2, the following reaction occurs: O3 + Br– → O2 + BrO–. The arising hypobromite ions and hypobromous acid oxidize uranium(IV) oxalate extremely efficiently. The possible mechanism of ozonation of aqueous uranium(IV) oxalate suspensions is discussed.