CrVI→CrVTransition in Uranyl Chromium Compounds: Synthesis and High-temperature X-ray Diffraction Study of Cs2[(UO2)2(CrO4)3]

Secondary Role of Aliphatic and Heterocyclic Amines in the Formation of Low-Temperature Amine-Bearing U, Np, and Pu(VI) Chromates

Uranyl compounds with tetrahedral oxoanions demonstrate a significant structural and topological diversity. Complexes of transuranium elements with such anions are not equally well-represented in the literature. To answer the question about the structural similarity in a series of An⁶⁺ complexes with XO₄^2- anions, we synthesized and studied 10 new U, Np, and Pu chromates with outer-sphere organic cations. The structural analysis and comparison with the literature data shows that the Np and Pu complexes are generally based on the same structural blocks as the uranyl compounds. Moreover, the chromate anion does not show any unique structural role as compared to the sulfate and selenate ions. As a result, the neptunium and plutonium chromates contain 1D and 2D structural units similar to those found in the uranyl sulfates and selenates. The templating role of the outer-sphere cations in the actinyl complexes with tetrahedral oxoanions is also not evident, and there is no clear correlation between the nature of the outer-sphere cations and the topology of the structural units.

The Role of Acidity in the Synthesis of Novel Uranyl Selenate and Selenite Compounds and Their Structures

Herein, the novel uranyl selenate and selenite compounds Rb2[(UO2)2(SeO4)3], Rb2[(UO2)3(SeO3)2O2], Rb2[UO2(SeO4)2(H2O)]·2H2O, and (UO2)2(HSeO3)2(H2SeO3)2Se2O5 have been synthesized using either slow evaporation or hydrothermal methods under acidic conditions and their structures were refined using single crystal X-ray diffraction. Rb2[(UO2)2(SeO4)3] synthesized hydrothermally adopts a layered 2D tetragonal structure in space group P42/ncm with a = 9.8312(4) Å, c = 15.4924(9) Å, and V = 1497.38(15) Å, where it consists of UO7 polyhedra coordinated via SeO4 units to create units UO2(SeO4)58− moieties which interlink to create layers in which Rb+ cations reside in the interspace. Rb2[(UO2)3(SeO3)2O2] synthesized hydrothermally adopts a layered 2D triclinic structure in space group P1¯ with a = 7.0116(6) Å, b = 7.0646(6) Å, c = 8.1793(7) Å, α = 103.318(7)°, β = 105.968(7)°, γ = 100.642(7)° and V = 365.48(6) Å3, where it consists of edge sharing UO7, UO8 and SeO3 polyhedra that form [(UO2)3(SeO3)2O2] layers in which Rb+ cations are found in the interlayer space. Rb2[UO2(SeO4)2(H2O)]·2H2O synthesized hydrothermally adopts a chain 1D orthorhombic structure in space group Pmn21 with a = 13.041(3) Å, b = 8.579(2) Å, c = 11.583(2) Å, and V = 1295.9(5) Å3, consisting of UO7 polyhedra that corner share with one H2O and four SeO42− ligands, creating infinite chains. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 synthesized under slow evaporation conditions adopts a 0D orthorhombic structure in space group Cmc21 with a = 28.4752(12) Å, b = 6.3410(3) Å, c = 10.8575(6) Å, and V = 1960.45(16) Å3, consisting of discrete rings of [(UO2)2(HSeO3)2(H2SeO3)2Se2O5]2. (UO2)2(HSeO3)2(H2SeO3)2Se2O5 is apparently only the second example of a uranyl diselenite compound to be reported. A combination of single crystal X-ray diffraction and bond valance sums calculations are used to characterise all samples obtained in this investigation. The structures uncovered in this investigation are discussed together with the broader family of uranyl selenates and selenites, particularly in the context of the role acidity plays during synthesis in coercing specific structure, functional group, and topology formations.

Successive Crystallization of Organically Templated Uranyl Sulfates: Synthesis and Crystal Structures of [pyH](H3O)[(UO2)3(SO4)4(H2O)2], [pyH]2[(UO2)6(SO4)7(H2O)], and [pyH]2[(UO2)2(SO4)3]

Three new uranyl sulfates, [pyH](H3O)[(UO2)3(SO4)4(H2O)2] (1), [pyH]2[(UO2)6(SO4)7(H2O)] (2), and [pyH]2[(UO2)2(SO4)3] (3), were produced upon hydrothermal treatment and successive isothermal evaporation. 1 is monoclinic, P21/c, a = 14.3640(13), b = 10.0910(9), c = 18.8690(17) Å, β = 107.795(2), V = 2604.2(4) Å3, R1 = 0.038; 2 is orthorhombic, C2221, a = 10.1992(8), b = 18.5215(14), c = 22.7187(17) Å, V = 4291.7(6) Å3, R1 = 0.030; 3 is orthorhombic, Pccn, a = 9.7998(8), b = 10.0768(8), c = 20.947(2) Å, V = 2068.5(3) Å3, R1 = 0.055. In the structures of 1 and 2, the uranium polyhedra and SO4 tetrahedra share vertices to form ∞3[(UO2)3(SO4)4(H2O)2]2− and ∞3[(UO2)6(SO4)7(H2O)]2− frameworks featuring channels (12.2 × 6.7 Å in 1 and 12.9 × 6.5 Å in 2), which are occupied by pyridinium cations. The structure of 3 is comprised of ∞2[(UO2)2(SO4)3]2− layers linked by hydrogen bonds donated by pyridinium cations. The compounds 1–3 are formed during recrystallization processes, in which the evaporation of mother liquor leads to a stepwise loss of hydration water.

Topological analysis of the layered uranyl compounds bearing slabs with UO2:TO4 ratio of 2:3

Nine new templated uranyl sulfates and selenates, [(H9O4)2(H2O)][(UO2)2(SO4)3(H2O)2] (H9US), [(C5H7 NO)2(H2O)][(UO2)2(SeO4)3(H2O)2](H2O) (OUSe), [C6H6N3][H5O2] [(UO2)2(SeO4)3(H2O)] (BH5USe), [C6H6N3][H7O3][(UO2)2(SO4)3 (H2O)](H2O) (BH7US), [C6H16N][H5O2][(UO2)2(SeO4)3(H2O)] (TeH5USe), [C6H18N2][(UO2)2(SO4)3(H2O)] (TmUS), [H5O2]2 [(UO2)2(SeO4)3(H2O)](H2O) (H5USe-1), [H5O2]2[(UO2)2(SeO4)3 (H2O)2](H2O)9 (H5USe-2), and [C4H14N2][(UO2)2(SeO4)3(H2O)](H2O) (DmUSe) have been prepared by isothermal evaporation of aqueous solutions containing extra sulfuric or selenic acid. Their crystal structures can be considered as organo-inorganic hybrids constructed of alternating [(UO2)2 (TO4)3(H2O) n ]2− slabs (T = Se6+, S6+, n = 1, 2) and layers containing templating organic moieties and/or hydronium ions and water molecules. The organic and inorganic parts of the structures are linked by multiple hydrogen bonds. Besides structure description, we offer topological analysis of the inorganic fragments with UO2:TO4 ratio of 2:3 as modular units resulting from self-assembly of fundamental chains formed by [(UO2)2(TO4)2] tetramers and TO4 tetrahedra.

Complex Uranyl Dichromates Templated by Aza-Crowns

Three new uranyl dichromate compounds templated by aza-crown templates were obtained at room temperature by evaporation from aqueous solutions: (H2diaza-18-crown-6)2[(UO2)2(Cr2O7)4(H2O)2](H2O)3 (1), (H4[15]aneN4)[(UO2)2(CrO4)2(Cr2O7)2(H2O)] (H2O)3.5 (2) and (H4Cyclam)(H4[15]aneN4)2[(UO2)6(CrO4)8(Cr2O7)4](H2O)4 (3). The use of aza-crown templates made it possible to isolate unprecedented and complex one-dimensional units in 2 and 3, whereas the structure of 1 is based on simple uranyl-dichromate chains. It is very likely that the presence of relatively large organic molecules of aza-crown ethers does not allow uranyl chromate chain complexes to condense into the units of higher dimensionality (layers or frameworks). In general, the formation of 1, 2, and 3 is in agreement with the general principles elaborated for organically templated uranyl compounds. The negative charge of the [(UO2)(Cr2O7)2(H2O)]2−, [(UO2)2(CrO4)2(Cr2O7)2(H2O)]4− and [(UO2)3(CrO4)4(Cr2O7)2]6− one-dimensional inorganic motifs is compensated by the protonation of all nitrogen atoms in the molecules of aza-crowns.

Unexpected Behavior of Np in Oxo-selenate/Oxo-selenite Systems

A study of neptunium (Np) chemistry in the complex oxo-selenium system has been performed. Hereby, two sets of precipitation experiments were conducted, investigating the influence of the initial oxidation state of selenium using Se^IVO₂ and H₂Se^VIO₄ with Np^V in alkali nitrate solution, keeping the ratio of Np/Se constant. Surprising results were observed. Five novel neptunium and selenium bearing compounds have been obtained by slow evaporation from aqueous solution. The novel Np^IV phase K_4-x[Np(SeO₃)_4-x(HSeO₃)_x]·(H₂O)_1.5 (1) crystallizes in green-colored, plate-shaped crystals and was obtained by adding SeO₂ and ANO₃ to a Np^V stock solution. Single-crystal X-ray diffraction reveals one-dimensional chain structures composed of square antiprismatic NpO₈ polyhedra linked via four trigonal pyramidal SeO₃ and HSeO₃ units. Raman spectral analysis supports the presence of both selenite and hydroselenite due to the presence of corresponding modes within the spectra. The addition of selenic acid to a Np^V stock solution resulted in the precipitation of elongated rose prisms of K₂[(NpO₂)₂(SeO₄)₃(H₂O)₂]·(H₂O)_1.5 (2), Rb₂[(NpO₂)₂(SeO₄)₃(H₂O)₂]·(H₂O)₂ (3) and K₉[(NpO₂)₉(SeO₄)_13.5(H₂O)₆]·(H₂O)₁₂ (4) as well as light red plates of Cs₂[(NpO₂)₂(SeO₄)₃] (5). To our knowledge, this is the first report of Np^VI selenates. All four structures show two-dimensional layered structures with alkali cations acting as charge balancing counter cations. Hereby the layers of compounds 2 and 3 are found to be orientational geometric isomers. Distinctly different phenomena are made responsible for the phase formation within these systems. The kinetically driven process of Np^V disproportionation led to the formation of the Np^IV selenites in the Se^IV-based system, whereas the oxidation of Np^V by reduction of nitrate in acidic conditions is responsible for the formation of the Np^VI selenates in the Se^VI system. The influence of air oxygen is also discussed for the latter reaction.

Microporous uranyl chromates successively formed by evaporation from acidic solution

The first microporous framework structures containing uranium and chromium have been synthesized and characterized. Rb2[(UO2)2(CrO4)3(H2O)2](H2O)3 (1) was crystallized from uranyl chromate solution by evaporation. Further evaporation led to increased viscosity of the solution and overgrowing of Rb2[(UO2)2(CrO4)3(H2O)](H2O) (2) on the crystals of 1. With respect to 1, the framework of 2 is partially dehydrated. Both frameworks differ compositionally by only one water molecule, but this seemingly small difference affects significantly the pore size and overall structural topology of the frameworks, which present very different flexibility of the U–O–Cr links. These are rigid in the pillared framework of 1, in contrast to 2 where the U–O–Cr angles range from 126.3 to 168.2°, reflecting the substantial flexibility of Cr–O–U connections which make them comparable to the corresponding Mo–O–U links in uranyl molybdates.

A2MnU3O11 (A = K, Rb) and Li3.2Mn1.8U6O22: Three New Alkali-Metal Manganese Uranium(VI) Oxides Related to Natrotantite

Single crystals of three new alkali-metal manganese uranium oxides, K2MnU3O11, Rb2MnU3O11, and Li3.2Mn1.8U6O22, have been grown from molten chloride fluxes and structurally characterized by single-crystal X-ray diffraction. The first two compounds crystallize in the trigonal space group, R3̅c, in the three-dimensional (3D), natrotantite structure composed of α-U3O8-topological layers connected via MnO6 octahedra. The Li-containing compound crystallizes in the monoclinic space group, Cc, with a related 3D structure, composed of β-U3O8-topological sheets connected via irregular MnO7 polyhedra. All three compounds exhibit typical uranyl, UO2(2+), coordination environments consisting of either UO7 pentagonal bipyramids or UO6 flattened octahedra. The lattice parameters of the new oxides are K2MnU3O11, a = 6.8280(2) Å, c = 36.8354(17) Å; Rb2MnU3O11, a = 6.8407(2) Å, c = 37.5520(17) Å; and Li3.2Mn1.8U6O22, a = 11.8958(8) Å, b = 10.9639(7) Å, c = 13.3269(8) Å, and β = 91.442(4)°. The magnetic susceptibilities of the K and Rb phases are discussed.