Insight into the Uranyl Oxyfluoride Topologies through the Synthesis, Crystal Structure, and Evidence of a New Oxyfluoride Layer in [(UO2)4F13][Sr3(H2O)8](NO3)·H2O.

A new strontium uranyl oxyfluoride, [(UO2)4F13][Sr3(H2O)8](NO3)·H2O, was synthesized under hydrothermal conditions. The single-crystal X-ray structure was determined. This compound crystallizes in the triclinic space group P1̅ (No. 2), with unit cell parameters a = 10.7925(16) Å, b = 10.9183(16) Å, c = 13.231(2) Å, α = 92.570(8)°, ß = 109.147(8)°, γ = 92.778(8)°, V = 1468.1(4) Å3, and Z = 2. The structure is built from uranyl-containing [Formula: see text] chains of tetrameric units of corner-sharing UO2F5 pentagonal bipyramids. These chains are linked through trimeric strontium units to form strontium-uranyl oxyfluoride layers further assembled by nitrate groups. The interlayer space is occupied by free water molecules. This compound was characterized by spectroscopic methods, especially 19F NMR highlighting the many different fluoride sites. Structural relationships with other uranyl oxyfluorides were investigated through the different F/O ratios, the structural building unit, and the structural arrangement.

Raman spectroscopic and ESI-MS characterization of uranyl peroxide cage clusters.

Strategies for interpreting mass spectrometric and Raman spectroscopic data have been developed to study the structure and reactivity of uranyl peroxide cage clusters in aqueous solution. We demonstrate the efficacy of these methods using the three best-characterized uranyl peroxide clusters, {U24}, {U28}, and {U60}. Specifically, we show a correlation between uranyl-peroxo-uranyl dihedral bond angles and the position of the Raman band of the symmetric stretching mode of the peroxo ligand, develop methods for the assignment of the ESI mass spectra of uranyl peroxide cage clusters, and show that these methods are generally applicable for detecting these clusters in the solid state and solution and for extracting information about their bonding and composition without crystallization.

Series of uranyl-4,4'-biphenyldicarboxylates and an occurrence of a cation-cation interaction: hydrothermal synthesis and in situ Raman studies.

Three uranium(VI)-bearing materials were synthesized hydrothermally using the organic ligand 4,4'-biphenyldicarboxylic acid: (UO2)(C14O4H8) (1); [(UO2)2(C14O4H8)2(OH)]·(NH4)(H2O) (2); (UO2)2(C14O4H8)(OH)2 (3). Compound 1 was formed after 1 day at 180 °C in an acidic environment (pH(i) = 4.03), and compounds 2 and 3 coformed after 3 days under basic conditions (pH(i) = 7.95). Coformation of all three compounds was observed at higher pH(i) (9.00). Ex situ Raman spectra of single crystals of 1-3 were collected and analyzed for signature peaks. In situ hydrothermal Raman data were also obtained and compared to the ex situ Raman spectra of the title compounds in an effort to acquire formation mechanism details. At pH(i) = 4.00, the formation of 1 was suggested by in situ Raman spectra. At an increased pH(i) (7.90), the in situ data implied the formation of compounds 1 and 3. The most basic conditions (pH(i) = 9.00) yielded a complex mixture of phases consistent with that of increased uranyl hydrolysis.

Correlations and differences between uranium(VI) arsonates and phosphonates.

Three new uranium arsonate compounds, UO(2)(C(6)H(5))(2)As(2)O(5)(H(2)O) (UPhAs-1), UO(2)(HO(3)AsC(6)H(4)AsO(3)H)(H(2)O)·H(2)O (UPhAs-2), and UO(2)(HO(3)AsC(6)H(4)NH(2))(2)·H(2)O (UPhAs-3) have been synthesized under mild hydrothermal conditions. UPhAs-1 is constructed from UO(7) pentagonal bipyramids that are chelated by the pyroarsonate moiety, [PhAs(O(2))OAs(O(2))Ph](2-), forming chains of layered uranyl polyhedra. Two of the phenylarsonic acids are condensed in situ to form the fused tetrahedra of the pyroarsonate moiety through a metal-mediated, thermally induced condensation process. The structure of UPhAs-2 consists of UO(7) pentagonal bipyramids that are chelated by phenylenediarsonate ligands, forming one-dimensional chains of uranyl polyhedra. UPhAs-3 consists of a rare UO(6) tetragonally distorted octahedron (D(4h)) that is on a center of symmetry and linked to two pairs of adjacent 4-aminophenylarsonate ligands. This linear chain structure is networked through hydrogen bonds between the lattice water molecules and the -NH(2) moiety. All three of these compounds fluoresce at room temperature, showing characteristic vibronically coupled charge-transfer based emission.

Synthesis of a uranyl persulfide complex and quantum chemical studies of formation and topologies of hypothetical uranyl persulfide cage clusters.

The compound Na(4)[(UO(2))(S(2))(3)](CH(3)OH)(8) was synthesized at room temperature in an oxygen-free environment. It contains a rare example of the [(UO(2))(S(2))(3)](4-) complex in which a uranyl ion is coordinated by three bidentate persulfide groups. We examined the possible linkage of these units to form nanoscale cage clusters analogous to those formed from uranyl peroxide polyhedra. Quantum chemical calculations at the density functional and multiconfigurational wave function levels show that the uranyl-persulfide-uranyl, U-(S(2))-U, dihedral angles of model clusters are bent due to partial covalent interactions. We propose that this bent interaction will favor assembly of uranyl ions through persulfide bridges into curved structures, potentially similar to the family of nanoscale cage clusters built from uranyl peroxide polyhedra. However, the U-(S(2))-U dihedral angles predicted for several model structures may be too tight for them to self-assemble into cage clusters with fullerene topologies in the absence of other uranyl-ion bridges that adopt a flatter configuration. Assembly of species such as [(UO(2))(S(2))(SH)(4)](4-) or [(UO(2))(S(2))(C(2)O(4))(4)](4-) into fullerene topologies with ~60 vertices may be favored by use of large counterions.

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.

Systematic evolution from uranyl(VI) phosphites to uranium(IV) phosphates.

Six new uranium phosphites, phosphates, and mixed phosphate-phosphite compounds were hydrothermally synthesized, with an additional uranyl phosphite synthesized at room temperature. These compounds can contain U(VI) or U(IV), and two are mixed-valent U(VI)/U(IV) compounds. There appears to be a strong correlation between the starting pH and reaction duration and the products that form. In general, phosphites are more likely to form at shorter reaction times, while phosphates form at extended reaction times. Additionally, reduction of uranium from U(VI) to U(IV) happens much more readily at lower pH and can be slowed with an increase in the initial pH of the reaction mixture. Here we explore the in situ hydrothermal redox reactions of uranyl nitrate with phosphorous acid and alkali-metal carbonates. The resulting products reveal the evolution of compounds formed as these hydrothermal redox reactions proceed forward with time.