Cation-cation interactions: crystal structures of neptunyl(V) selenate hydrates, (NpO2)2(SeO4)(H2O)n (n=1, 2, and 4)

Green crystals of (NpO(2))(2)(SeO(4))(H(2)O)(4), (NpO(2))(2)(SeO(4))(H(2)O)(2), and (NpO(2))(2)(SeO(4))(H(2)O) have been prepared by hydrothermal methods. The structures of these compounds have been characterized by single-crystal X-ray diffraction. (NpO(2))(2)(SeO(4))(H(2)O)(4), isostructural with (NpO(2))(2)(SO(4))(H(2)O)(4), is constructed from layers comprised of corner-sharing neptunyl(V) pentagonal bipyramids and selenate tetrahedra that are further linked by hydrogen bonding with water molecules. Each NpO(2)(+) cation binds to four other NpO(2)(+) units through cation-cation interactions (CCIs) to form a distorted "cationic square net" decorated by SeO(4)(2-) tetrahedra above and below the layer. Each selenate anion is bound to two neptunyl(V) cations through monodentate linkages. (NpO(2))(2)(SeO(4))(H(2)O)(2) is isostructural with the corresponding sulfate analogue as well. It consists of puckered layers of neptunyl(V) pentagonal bipyramids that are further connected by selenate tetrahedra to form a three-dimensional framework. The CCI pattern in the neptunyl layers of dihydrate is very similar to that of tetrahydrate; however, each SeO(4)(2-) tetrahedron is bound to four NpO(2)(+) cations in a mondentate manner. (NpO(2))(2)(SeO(4))(H(2)O) crystallizes in the monoclinic space group P2(1)/c, which differs from the (NpO(2))(2)(SO(4))(H(2)O) orthorhombic structure due to the slightly different connectivities between NpO(2)(+) cations and anionic ligands. The structure of (NpO(2))(2)(SeO(4))(H(2)O) adopts a three-dimensional network of distort neptunyl(V) pentagonal bipyramids decorated by selenate tetrahedra. Each NpO(2)(+) cation connects to four other NpO(2)(+) units through CCIs and also shares an equatorial coordinating oxygen atom with one of the other units in addition to the CC bond to form a dimer. Each SeO(4)(2-) tetrahedron is bound to five NpO(2)(+) cations in a monodentate manner. Magnetic measurements obtained from the powdered tetrahydrate are consistent with a ferromagnetic ordering of the neptunyl(V) spins at 8(1) K, with an average low temperature saturation moment of 1.98(8) μ(B) per Np. Well above the ordering temperature, the susceptibility follows Curie-Weiss behavior, with an average effective moment of 3.4(2) μ(B) per Np and a Weiss constant of 14(4) K. Correlations between lattice dimensionality and magnetic behavior are discussed.     

Three new sodium neptunyl(V) selenate hydrates: structures, Raman spectroscopy, and magnetism

Green crystals of Na(NpO(2))(SeO(4))(H(2)O) (1), Na(3)(NpO(2))(SeO(4))(2)(H(2)O) (2), and Na(3)(NpO(2))(SeO(4))(2)(H(2)O)(2) (3) have been prepared by a hydrothermal method for 1 or evaporation from aqueous solutions for 2 and 3. The structures of these compounds have been characterized by single-crystal X-ray diffraction. Compound 1 is isostructural with Na(NpO(2))(SO(4))(H(2)O) (4). The structure of 1 consists of ribbons of neptunyl(V) pentagonal bipyramids, which are decorated and further connected by selenate tetrahedra to form a three-dimensional framework. The resulting open channels are filled by Na(+) cations and H(2)O molecules. Within the ribbon, each neptunyl polyhedron shares corners with each other solely through cation-cation interactions (CCIs). The structure of 2 adopts one-dimensional [(NpO(2))(SeO(4))(2)(H(2)O)](3-) chains connected by Na(+) cations. Each NpO(2)(+) cation is coordinated by four monodentate SeO(4)(2-) anions and one H(2)O molecule to form a pentagonal bipyramid. The structure of 3 is constructed by one-dimensional [(NpO(2))(SeO(4))(2)](3-) chains separated by Na(+) cations and H(2)O molecules. These chains have two configurations resulting in two disordered orientations of the Se(2)O(4)(2-) tetrahedra. Each NpO(2)(+) cation is coordinated by one bidentate Se(1)O(4)(2-) and three monodentate Se(2)O(4)(2-) anions to form a pentagonal bipyramid. Raman spectra of 1, 2, and 4 were collected on powder samples. For 1 and 4, the neptunyl symmetric stretch modes (670, 676, 730, and 739 cm(-1)) shift significantly toward lower frequencies compared to that in 2 (773 cm(-1)), and there are several asymmetric neptunyl stretch bands in the region of 760-820 cm(-1). Magnetic measurements obtained from crushed crystals of 1 are consistent with a ferromagnetic ordering of the neptunyl(V) spins at 6.5(2) K, with an average low temperature saturation moment of 2.2(1) μ(B) per Np. Well above the ordering temperature, the susceptibility follows Curie-Weiss behavior, with an average effective moment of 3.65(10) μ(B) per Np and a Weiss constant of 14(1) K. Correlations between lattice dimensionality and magnetic behavior are discussed.     

Two new neptunyl(V) selenites: a novel cation-cation interaction framework in (NpO2)3(OH)(SeO3)(H2O)2 3H2O and a uranophane-type sheet in Na(NpO2)(SeO3)(H2O)

Dark green crystals of (NpO(2))(3)(OH)(SeO(3))(H(2)O)(2)·H(2)O (1) have been prepared by a hydrothermal reaction of neptunyl(V) and Na(2)SeO(4) in an aqueous solution at 150 °C, while green plates of Na(NpO(2))(SeO(3))(H(2)O) (2) have been synthesized by evaporation of a solution of neptunyl(V), H(2)SeO(4), and NaOH at room temperature. Both compounds have been characterized by single-crystal X-ray diffraction. The structure of compound contains three crystallographically unique Np atoms that are bonded to two O atoms to form a nearly linear O═Np═O NpO(2)(+) cation. Neighboring Np(5+) ions connect to each other through a bridging oxo ion from the neptunyl unit, a configuration known as cation-cation interactions (CCIs), to build a complex three-dimensional network. More specifically, each Np(1)O(2)(+), Np(2)O(2)(+), and Np(3)O(2)(+) cation is involved in three, five, and four CCIs with other units, respectively. The framework of neptunyl(V) pentagonal bipyramids is decorated by selenite trigonal pyramids with one-dimensional open channels where uncoordinated waters are trapped via hydrogen bonding interactions. Compound adopts uranophane-type [(NpO(2))(SeO(3))](-) layers, which are separated by Na(+) cations and water molecules. Within each layer, neptunyl(V) pentagonal bipyramids share equatorial edges with each other to form a single chain that is further connected by both monodentate and bidentate selenite trigonal pyramids. Crystallographic data: compound, monoclinic, P2(1)/c, Z = 4, a = 6.6363(8) ?, b = 15.440(2) ?, c = 11.583(1) ?, β = 103.549(1)°, V = 1153.8(2) ?(3), R(F) = 0.0387 for I > 2σ(I); compound (2), monoclinic, C2/m, Z = 4, a = 14.874(4) ?, b = 7.271(2) ?, c = 6.758(2) ?, β = 112.005(4)°, V = 677.7(3) ?(3), R(F) = 0.0477 for I > 2σ(I).     

Structural and spectroscopic trends in actinyl iodates of uranium,neptunium, and plutonium

Two neptunyl(VI) iodates, NpO(2)(IO(3))(2)(H(2)O) (1) and NpO(2)(IO(3))(2).H(2)O (2), have been prepared from the aqueous reactions of Np(V) in HCl with KIO(4) or H(5)IO(6) at 180 degrees C and have been characterized by single crystal X-ray diffraction and Raman spectroscopy. Both compounds consist of two-dimensional arrangements of pentagonal bipyramidal [NpO(7)] polyhedra with axial neptunyl, NpO(2)(2+), dioxocations. In 1, the neptunium centers are bound in the equatorial plane by four bridging iodate anions and one terminal water molecule. The iodate anions link the [NpO(7)] units into corrugated sheets that interact with one another through intermolecular IO(3)(-)...IO(3)(-) interactions as also observed in UO(2)(IO(3))(2)(H(2)O). Compound 2 is isostructural with the recently reported PuO(2)(IO(3))(2).H(2)O, where oxygen atoms from bridging iodate anions occupy the five equatorial sites around the neptunyl moieties. The iodate anions occur as both mu(2)- and mu(3)-units and link the neptunyl polyhedra into sheets. Both types of iodate anions have their stereochemically active lone-pair of electrons aligned on one side of each layer creating a polar structure. Raman spectra of 1, UO(2)(IO(3))(2)(H(2)O), and PuO(2)(IO(3))(2).H(2)O show a sequential shift of the nu(1)(AnO(2)(2+)) stretch to lower wavenumber as the atomic number of the actinide is increased. Crystallographic data: 1, orthorhombic, space group Pcan, a = 7.684(2) A, b = 8.450(2) A, c = 12.493(3) A, Z = 4; 2, orthorhombic, space group Pna2(1), a = 7.314(1) A, b = 11.631(2) A, c = 9.449(2) A, Z = 4.     

Hydrothermal synthesis, structure, and magnetic properties of the mixed-valent Np(IV)/Np(V) Selenite Np(NpO2)2(SeO3)3

The reaction of NpO(2) with SeO(2) in the presence of CsCl at 180 degrees C results in the formation of Np(NpO(2))(2)(SeO(3))(3) (1). The structure of 1 consists of three crystallographically unique Np centers with three different coordination environments in two different oxidation states. Np(1) is found in a neptunyl(V), O[double bond]Np[double bond]O(+), unit that is further ligated in the equatorial plane by three chelating SeO(3)(2-) anions to create a hexagonal bipyramidal NpO(8) unit. A second neptunyl(V) cation also occurs for Np(2); it is bound by four bridging selenite anions and by the oxo atom from the Np(1) neptunyl cation to form a pentagonal bipyramidal, NpO(7), unit. The third neptunium center, Np(3), which contains Np(IV), is found in a distorted NpO(8) dodecahedron. Np(3) is bound by five bridging selenite anions and by three neptunyl units via cation-cation interactions. The NpO(7) pentagonal bipyramids and NpO(8) hexagonal bipyramids share both corners and edges. Both of these polyhedra share corners via cation-cation interactions with the NpO(8) dodecahedra creating a three-dimensional structure with small channels that house the stereochemically active lone pair of electrons on the selenite anions. Magnetic susceptibility data follow Curie-Weiss behavior over the entire temperature range measured (5 < or = T < or = 320 K). The effective moment, mu(eff) = 2.28 mu(B), which represents an average over the three crystallographically inequivalent Np atoms, is within the expected range of values. There is no evidence of long-range ordering of the Np moments at temperatures down to 5 K, consistent with the negligible Weiss constant determined from fitting the susceptibility data. Crystallographic data: 1, orthorhombic, space group Pbca, a = 10.6216(5), b = 11.9695(6), and c = 17.8084(8) A and Z = 8 (T = 193 K).     

Synthesis, structure, and infrared spectroscopy of the first Np5+ neptunyl silicates, Li6(NpO2)4(H2Si2O7)(HSiO4)2(H2O)4 and K3(NpO2)3(Si2O7)

Two Np(5+) silicates, Li(6)(NpO(2))(4)(H(2)Si(2)O(7))(HSiO(4))(2)(H(2)O)(4) (LiNpSi1) and K(3)(NpO(2))(3)(SiO(3)OH)(2) (KNpSi1), were synthesized by hydrothermal methods. The crystal structures were determined using direct methods and refined on the basis of F(2) for all unique data collected with Mo Kalpha radation and an APEX II CCD detector. LiNpSi1 crystallizes in orthorhombic space group Pnma with a =13.189(6) A, b = 7.917(3) A, c = 10.708(5) A, V = 1118.1(8) A3, and Z = 2. KNpSi1 is hexagonal, P62m, a = 9.734(1) A, c = 3.8817(7) A, V = 318.50(8) A3, and Z = 1. LiNpSi1 contains chains of edge-sharing neptunyl pentagonal bipyramids linked into two-dimensional sheets through direct linkages between the neptunyl polyhedra and the vertex sharing of the silicate tetrahedra. The structure contains both sorosilicate and nesosilicate units, resulting in a new complex neptunyl silicate sheet. KNpSi1 contains edge-sharing neptunyl square bipyramids linked into a framework structure through the sharing of vertices with the silicate tetrahedra. The neptunyl silicate framework contains channels approximately 6.0 A in diameter. These structures exhibit significant departures from other reported Np(5+) and U(6+) compounds and represent the first reported Np(5+) silicate structures.     

Oxoneptunium(v) as part of the framework of a polyoxometalate

(NH4)14Na4[(Np3W4O15)(H2O)3(BiW9O33)3].62H2O (1) and (NH4)14.5Na3.5[(Np3W4O15)(H2O)3(SbW9O33)3].40.5H2O (2) each contain three neptunyl(v) moieties encapsulated within heteropolyoxotungstate frameworks in which axial {NpO2}+ oxygens form one face of a WO6 octahedron.     

A comparison of neptunyl(V) and neptunyl(VI) solution coordination: the stability of cation-cation interactions

The solution coordination environments of pentavalent and hexavalent Np are studied by high-energy X-ray scattering. Np5+ and Np6+ both exist as the neptunyl moiety coordinated with five equatorial waters at Np-O distances of 2.46(2) and 2.37(2) A, respectively. NpO2(2+) also has a second coordination sphere of 6-10 waters at 4.37(3) A. The NpO2+ scattering is complicated by the presence of scattering at about 4.2 A that is attributed to Np-Np cation-cation interactions. The analysis of changing intensity of this peak as a function of Np concentration is used to determine a stability constant of Keq=0.74(9) M(-1) for the dimeric complex.     

Cation-cation interactions between neptunyl(VI) units

The boric acid flux reaction of NpO(2)(ClO(4))(2) with NaClO(4) affords Na[(NpO(2))(4)B(15)O(24)(OH)(5)(H(2)O)](ClO(4))·0.75H(2)O (NaNpBO-1). NaNpBO-1 possesses a layered structure consisting of double neptunyl(VI) borate sheets bridged by another Np(VI) site through cation-cation interactions. The sole presence of Np(VI) in NaNpBO-1 is supported by absorption and vibrational spectroscopy.     

A tetrameric neptunyl(v) cluster supported by a Schiff base ligand

The first tetrameric cation-cation neptunyl(v) cluster, [{NpO(2)(salen)}(4)(μ(8)-K)(2)][K(18C6)Py](2), has been synthesized in non-aqueous solution from the reaction of [(NpO(2)Py(5))(KI(2)Py(2))](n) with K(2)salen and its structure determined in the solid state and in solution where the complex retains its tetrameric form.     

Electronic transitions and vibronic coupling in neptunyl compounds

The low-lying electronic transitions of the neptunyl (NpO(2)(2+)) ion are characterized as either charge transfer (CT) or intra- 5f. Comparison of these classes of electronic transitions reveals significantly different photophysical properties, especially in vibronic coupling. An empirical model developed for analyses of uranyl CT vibronic transitions is used here to simulate the absorption (excitation) spectra of neptunyl in two compounds of different chemical compositions and structural symmetries. Analyses reveal that CT vibronic coupling in neptunyl has the same characteristics as that in typical uranyl analogues. The primary profile of the CT spectra is similar for neptunyl respectively with respect to chloride- and oxide-neptunium bonding interactions. On the other hand, vibronic coupling to the CT transitions is significantly different from that of f-f transitions, even within a given neptunyl compound. Electronic energy levels, vibronic coupling strength, and frequencies of various vibration modes were evaluated for transitions to the excited states of different origins in the region from 8000 cm(-1) to 21000 cm(-1) for two neptunyl compounds.     

Electronic spectra and excited states of neptunyl and its [NpO2Cl4]2- complex

Electronic states and spectra of NpO(2)(2+) and NpO(2)Cl(4)(2-) with a Np 5f(1) ground-state configuration, related to low-lying 5f-5f and ligand-to-metal charge-transfer (CT) transitions, are investigated, using restricted-active-space perturbation theory (RASPT2) with spin-orbit coupling. Restrictions on the antibonding orbital occupations have little influence on the 5f-5f transition energies, but an important impact on the CT states with an open bonding orbital shell. The present calculations provide significant improvement over previous literature results. The assignment of the experimental electronic spectra of Cs(2)NpO(2)Cl(4) is refined, based on our calculations of NpO(2)Cl(4)(2-). Assignments on the basis of bare NpO(2)(2+) are less reliable, since the equatorial Cl ligands perturb the excited-state energies considerably. Calculated changes of the Np-O bond lengths are in agreement with the observed short symmetric-stretching progressions in the f-f spectra and longer progressions in the CT spectra of neptunyl. A possible luminescence spectrum of the lowest quartet CT state is predicted.     

Boronic acid flux synthesis and crystal growth of uranium and neptunium boronates and borates: a low-temperature route to the first neptunium(V) borate

Molten methylboronic acid has been used as a reactive flux to prepare the first neptunium(V) borate, NpO(2)[B(3)O(4)(OH)(2)] (NpBO-1), and the first actinide boronate, UO(2)(CH(3)BO(2))(H(2)O) (UCBO-1). NpBO-1 contains cation-cation interactions between the neptunyl units. In contrast, the presence of the methyl groups in the uranyl boronate leads to a one-dimensional structure.     

Stabilising pentavalent actinides--visible-near infrared and X-ray absorption spectroscopic studies of the utility of the [(Np3W4O15)(H2O)3(MW9O33)3]18- (M = Sb, Bi) structural type

We report the interaction between B-type tri-lacunary heteropolyoxotungstate anions and actinyl(V) cations in aqueous solution, yielding a greater understanding of the stability of the O≡An≡O(1+) linear dioxo actinide moiety. Previously we reported that B-α-[BiW(9)O(33)](9-) and B-α-[SbW(9)O(33)](9-) will react with NpO(2)(1+) to yield [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb). Single crystal structural characterisation of salts of these complexes revealed a core in which three Np(V) atoms interact with a central W(VI) atom through bridging oxo groups. These bridging oxygen atoms come from one of the two axial oxygens in O≡Np≡O(1+) and represent a highly unusual interaction for a discrete molecular species. In this study visible/near infra-red spectroscopy indicates that [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) could be readily stabilized in solution at near neutral pH for several months, with (NH(4))(14)Na(4)[(Np(3)W(4)O(15))(H(2)O)(39)BiW(9)O(33))(3)]·62H(2)O crystallising from solution in high yield. At lower pH and [BiW(9)O(33)](9-) : NpO(2)(1+) ratios additional Np(V) species could be observed in solution. Stabilization of [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) in solution proved more challenging, with several distinctive Np(V) near infra-red transitions observed in solution. Slow complexation kinetics and reduction to Np(IV) was also observed. High [SbW(9)O(33)](9-) : NpO(2)(1+) molar ratios and careful control of solution pH was required to prepare solutions in which [(Np(3)W(4)O(15))(H(2)O)(3)(SbW(9)O(33))(3)](18-) was the only neptunium containing species. In stark contrast to the NpO(2)(1+) chemistry, [BiW(9)O(33)](9-) readily oxidizes PuO(2)(1+) to PuO(2)(2+) yielding further evidence of the decreased stability of Pu(V)vs. Np(V). Np L(II)-edge XAFS measurement revealed very good agreement with single crystal diffraction data for the Np structural environment for [(Np(3)W(4)O(15))(H(2)O)(3)(MW(9)O(33))(3)](18-) (M = Bi, or Sb) in the solid state. There was also good agreement between coordination shells for [(Np(3)W(4)O(15))(H(2)O)(3)(BiW(9)O(33))(3)](18-) in the solid state and in solution, yielding further confirmation of the high stability of this particular cluster.     

Critical role of water content in the formation and reactivity of uranium, neptunium, and plutonium iodates under hydrothermal conditions: implications for the oxidative dissolution of spent nuclear fuel

The reactions of 237NpO2 with excess iodate under acidic hydrothermal conditions result in the isolation of the neptunium(IV), neptunium(V), and neptunium(VI) iodates, Np(IO3)4, Np(IO3)4.nH2O.nHIO3, NpO2(IO3), NpO2(IO3)2(H2O), and NpO2(IO3)2.H2O, depending on both the pH and the amount of water present in the reactions. Reactions with less water and lower pH favor reduced products. Although the initial redox processes involved in the reactions between 237NpO2 or 242PuO2 and iodate are similar, the low solubility of Pu(IO3)4 dominates product formation in plutonium iodate reactions to a much greater extent than does Np(IO3)4 in the neptunium iodate system. UO2 reacts with iodate under these conditions to yield uranium(VI) iodates solely. The isotypic structures of the actinide(IV) iodates, An(IO3)4 (An=Np, Pu), are reported and consist of one-dimensional chains of dodecahedral An(IV) cations bridged by iodate anions. The structure of Np(IO3)4.nH2O.nHIO3 is constructed from NpO9 tricapped-trigonal prisms that are bridged by iodate into a polar three-dimensional framework structure. Second-harmonic-generation measurements on a polycrystalline sample of the Th analogue of Np(IO3)4.nH2O.nHIO3 reveal a response of approximately 12x that of alpha-SiO2. Single-crystal magnetic susceptibility measurements of Np(IO3)4 show magnetically isolated Np(IV) ions.     

Cation-cation interactions in neptunyl(V) compounds: hydrothermal preparation and structural characterization of NpO2(IO3) and alpha- and beta-AgNpO2(SeO3)

The hydrothermal reaction of NpO(2) with IO(3)(-) in the presence of nitrate results in the formation of NpO(2)(IO(3)) (1). Under similar conditions, NpO(2) reacts with AgNO(3) and SeO(2) to yield alpha-AgNpO(2)(SeO(3)) (2) and beta-AgNpO(2)(SeO(3)) (3). The structure of 1 consists of distorted pentagonal bipyramidal Np(V) centers that are bridged by iodate anions. In addition, the oxo atoms of the neptunyl(V) cations coordinate adjacent Np(V) centers creating layers that are linked into a three-dimensional network structure by the iodate anions. The structure is polar owing to the alignment of the stereochemically active lone pair of electrons on the iodate anions along the c-axis. alpha-AgNpO(2)(SeO(3)) (2) forms a layered structure consisting of hexagonal bipyramidal NpO(8) polyhedra that are bound by chelating and bridging selenite anions. The primary and secondary structures of 3 are similar to those of 1, and neptunyl-neptunyl interactions are partially responsible for the creation of a three-dimensional network structure. However, the selenite anions in 3 are rotated with respect to the iodate anions found in 1, and the structure is centrosymmetric. The network found in 3 consists of interconnecting, approximately square channels that house the Ag(+) cations. A bond-valance sum parameter of 2.036 A for Np(V) bound exclusively to oxygen has been developed with b = 0.37 A. Crystallographic data: 1, orthorhombic, space group Pna2(1), a = 13.816(2) A, b = 5.8949(8) A, c = 5.5852(8) A, Z = 4; 2, monoclinic, space group P2(1)/n, a = 4.3007(3) A, b = 9.5003(7) A, c = 11.5877(9) A, beta = 95.855(1) degrees, Z = 4; 3, triclinic, space group Ponemacr;, a = 7.1066(6) A, b = 8.3503(7) A, c = 8.3554(7) A, alpha = 89.349(1) degrees, beta = 77.034(1) degrees, gamma = 76.561(1) degrees, Z = 2.     

Theoretical studies on structures of neptunyl carbonates: NpO2(CO3)m(H2O)n(q-) (m = 1-3, n = 0-3) in aqueous solution

Extensive ab inito computations have been carried out to study the equilibrium structure, infrared spectra, and bonding characteristics of a variety of hydrated NpO2(CO3)m(q-) complexes by considering the solvent as a polarizable dielectric continuum as well as the corresponding anhydrate complexes in the gas phase. The computed structural parameters and vibrational results at the MP2 level in aqueous solution are in good agreement with Clark et al.'s experiments and provide realistic pictures of the neptunyl complexes in an aqueous environment. Our computed hydration energies reveal that the complex with water molecules directly bound to it yields the best results. Our analysis of the nature of the bonding of neptunyl complexes provides insight into the nature of 6d and 5f bonding in actinide complexes.     

The structural and spectroscopic characterisation of three actinyl complexes with coordinated and uncoordinated perrhenate: [UO2(ReO4)2(TPPO)3], [[(UO2)(TPPO)3]2(mu2-O2)][ReO4]2 and [NpO2(TPPO)4][ReO4

The first structural characterization of an actinide complex with coordinated perrhenate is reported, [UO2(ReO4)2(TPPO)3] (1). In this [UO2]2+ complex two [ReO4]- anions and three TPPO (triphenylphosphine oxide) P=O donor ligands are coordinated in the equatorial plane in a cisoid arrangement. This bonding arrangement, and apparent strain observed in the equatorially bonded ligands, is attributed to the solid state packing in adjacent molecules in which hydrophobic TPPO ligands form an effective "shell" around a hydrophilic core of two UO2(ReO4)2 moieties. Solid state vibrational spectroscopy (infrared and Raman), 31P CP MAS NMR and elemental analysis are also consistent with the formula of 1. Solution state vibrational spectroscopy and 31P NMR measurements in EtOH indicate the lability of the TPPO and [ReO4]- groups. The photolytic generation of peroxide in EtOH solutions of 1 leads to the formation of trace quantities of [[(UO2)(TPPO)3]2(mu2-O2)][ReO4]2, 2, in which the coordinated [ReO4]- groups of 1 have been displaced by bridging O2(2-), derived from atmospheric O2. Finally, attempts to synthesise a [NpO2]+ analogue of have resulted only in the formation of [NpO2(TPPO)4][ReO4], 3, in which [ReO4]- acts solely as a counter anion. From these results it can be concluded that [ReO4]- will bond to [UO2]2+, but will be readily displaced by a more strongly coordinating ligand (e.g. peroxide) and will not coordinate to an actinyl cation with a lower charge, [NpO2]+, under the same reaction conditions.     

Cation-cation interactions between NpO2(+) and UO2(2+) at different temperatures and ionic strengths

Cation-cation interactions between NpO(2)(+) and UO(2)(2+) were studied at different temperatures (283.15 K to 358.15 K) and different ionic strengths (3-4.5 mol dm(-3)) by spectrophotometry and microcalorimetry. The cation-cation complex between NpO(2)(+) and UO(2)(2+) was weak and became stronger as the temperature was increased from 283.15 K to 358.15 K. The molar enthalpy of complexation was directly determined for the first time by microcalorimetry to be (4.2 ± 1.6) kJ mol(-1) at 298.15 K, in good agreement with the trend in the stability constant at different temperatures. The small and positive enthalpy and entropy of complexation support the argument that the cation-cation complex between NpO(2)(+) and UO(2)(2+) is of inner-sphere type. At each temperature, the stability constants of the cation-cation complex were found to increase as the ionic strength was increased. The specific ion interaction theory (SIT) was used to obtain the stability constants at infinite dilution and variable temperatures.     

A versatile precursor for non-aqueous neptunyl(V) chemistry

The polymeric complex [(NpO(2)Py(5))(KI(2)Py(2))](n) is prepared from dry "NpO(2)Cl" by anion exchange with potassium iodide in pyridine affording the first convenient starting material for the development of NpO(2)(+) coordination chemistry in anhydrous organic media.