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Timestamp: 2019-04-19 02:18:55+00:00

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Single crystals of Na[(UO2)(i-C3H7COO)3]·0.7H2O (I), Cs[(UO2)(i-C3H7COO)3] (II) and (NH4)[(UO2)(i-C4H9COO)3] (III) were obtained via isothermal evaporation and their structures were solved using X-ray diffraction techniques. Even though the ligands are branched, bulky and spatial, many carbon and hydrogen atoms are still disordered in these crystal structures at low temperature. A new type of Na coordination is observed for the first time for this family of compounds, proposing high sensitivity of compound I to humidity. Depolymerization of the metal–oxygen frameworks for the new compounds is compared with the known ones. Coordination sequences of sodium/cesium and uranyl complexes with aliphatic monocarboxylate ions are calculated to show different crystal-chemical function of crystallographically independent atoms. As there are analogous compounds to the title ones with straight-chain ligands, such groups of similar compounds with single varying parameters are very advantageous for establishing correlations between composition and crystal structure.
Although all the mentioned compounds seem quite similar, it was shown earlier that their structures may feature different peculiarities, such as topological isomerism,10 depolymerization of the metal–oxygen frameworks,18 different packings with different systems of noncovalent interactions,19 and relation between the crystal system of compounds and the volumes of Voronoi–Dirichlet polyhedra of cations and ligands.16,20 Finally, such groups of similar compounds with single varying parameters (like the type of the ligand: branched or straight, or the cation) are very advantageous for establishing correlations between composition and crystal structure. Such correlations as well as structural peculiarities of the mentioned compounds are discussed below. Thus, besides having a possible practical application in the nuclear industry, the newly synthesized compounds serve in the development of theoretical crystal chemistry.
Caution! Compounds of U represent a potential health risk owing to radioactivity. Although the uranium precursors used contain depleted uranium, standard safety measures for handling radioactive substances must be followed.
Single crystals of I–III were obtained via isothermal evaporation of solutions in 20 or 100 ml glass vials at room temperature. Uranium oxide (UO3) was prepared according to the earlier reported procedure.21 Isobutyric acid (i-C3H7COOH, 99%), isovaleric acid (i-C4H9COOH, 99%), sodium hydroxide (NaOH, reagent grade, ≥98%), cesium hydroxide monohydrate (CsOH·H2O, technical grade) and ammonium hydroxide solution (ACS reagent, 28.0–30.0% NH3 in H2O) were obtained commercially from Sigma-Aldrich.
Na[(UO2)(i-C3H7COO)3]·0.7H2O (I). Uranium oxide (UO3) (0.7 mmol, 200 mg) was dissolved in a mixture of isobutyric acid i-C3H7COOH (3.5 mmol, 308 mg) and 10 ml of distilled water under moderate heating. Sodium hydroxide (NaOH) (0.7 mmol, 28 mg) was dissolved in the mixture. The final molar ratio of the reagents was 1 : 5 : 1, respectively. Evaporation of the transparent yellow solution in a few days resulted in yellow platy crystals that appeared to be the known phase, [UO2(i-C3H7COO)2(H2O)2].22 After a few more days of evaporation yellow prismatic crystals formed that appeared to be the new phase, Na[(UO2)(i-C3H7COO)3]·0.7H2O (I). The yield of the second phase was: ≈15%. Gravimetric analysis of uranium content in I resulted in a value of 41.9% (calculated 42.0%).
Cs[(UO2)(i-C3H7COO)3] (II). Uranium oxide UO3 (0.7 mmol, 200 mg) was dissolved in a mixture of isobutyric acid (i-C3H7COOH) (3.5 mmol, 308 mg) and 10 ml of distilled water under moderate heating. Cesium hydroxide (CsOH·H2O) (0.7 mmol, 118 mg) was dissolved in the mixture. The final molar ratio of the reagents was 1 : 5 : 1, respectively. Evaporation of the transparent yellow solution in a few days resulted in yellow prismatic crystals that appeared to be the new phase, Cs[(UO2)(i-C3H7COO)3] (II). The yield was: ≈65%. Gravimetric analysis of uranium content in II resulted in a value of 35.7% (calculated 35.8%).
(NH4)[(UO2)(i-C4H9COO)3] (III). Uranium oxide (UO3) (0.7 mmol, 200 mg) was dissolved in a mixture of isovaleric acid (i-C4H9COOH) (4.2 mmol, 428 mg), 30 ml of distilled water and 30 ml of ethanol under moderate heating. Ammonium hydroxide solution was added to the reaction mixture (approximately 85 mg of solution, 1.4 mmol of NH3). The final molar ratio of the reagents was 1 : 6 : 2, respectively. Evaporation of the transparent yellow solution in a few days resulted in a mixture of yellow platy and prismatic crystals. Platy crystals appeared to be the amorphous uranyl isovalerate, whose crystalline state has not been reported yet. Crystalline prismatic crystals appeared to be the new phase, (NH4)[(UO2)(i-C4H9COO)3] (III). The yield of compound III was: ≈10%. Gravimetric analysis of uranium content in III resulted in a value of 43.2% (calculated 43.4%).
Single crystals of I–III were obtained from reaction mixtures. The intensities of reflections for I and II were measured with a Bruker Apex II DUO CCD diffractometer using Mo-Kα radiation (λ = 0.71073) at 120.0(2) K and were merged by SADABS.23 Intensity data for III were collected at 100.0(2) K at the BELOK beamline of the Kurchatov Synchrotron Radiation Source (Moscow, Russia) at a wavelength of 0.9699 Å using a Rayonix SX-165 CCD detector and merged using the SCALA24 package. All structures were solved by the direct method and refined by full-matrix least squares against F2. Non-hydrogen atoms were refined anisotropically except for some disordered atoms. The disordered carbon atoms (some methyl groups in I and the single unique alkyl group in III) were refined isotropically. A number of EADP, ISOR, SADI, RIGU and DFIX instructions were applied to refine some moieties. TWIN/BASF refinement was performed for III. Positions of hydrogen atoms were calculated and included in the refinement by the riding model with Uiso(H) = 1.5Ueq(X) for methyl groups and water molecules, and Uiso(H) = 1.2Ueq(X) for other atoms. All calculations were made using the SHELXL2014 25 and OLEX2 26 program packages.
Unfortunately, although atomic coordinates of all atoms could be obtained for a single crystal of II, including carbon atoms of a disordered anion, for all tested single crystals we obtained high convergence factors (Rint and Rf were higher than 0.3 and 0.12, respectively) and poor Flack parameter (0.2), anomalous bond distances and thermal parameters. The quality of single crystals decreased at low temperatures indicating the presence of twinning, but we failed to detwin our data. So, further refinement of the atomic coordinates and thermal parameters of II was carried out at room temperature using powder XRD.
The powder pattern of II was measured on a Bruker D8 Advance Vario diffractometer at RT with a LynxEye detector and Ge (111) monochromator, λ(CuKα1) = 1.54060 Å, θ/2θ scan from 5.6° to 89.5°, step size 0.00786°. The measurement was performed in the transmission mode, with the sample deposited between two Kapton films. All calculations were performed with the Bruker TOPAS package.27 The pattern was indexed with the SVD method28 in a cubic crystal system with crystal parameters a = 12.39075(4) Å and V = 1902.360(11) Å3. The space group P213 (similar to that obtained for a single crystal of II) was evaluated based on systematic absences. The atomic coordinates taken from single-crystal solution were included in the Rietveld refinement, with restraints applied to all covalent and metal–oxygen bonds, bond angles and metal atoms fixed on three-fold rotation axes. Hydrogen atoms were refined by the riding model. For each type of non-hydrogen atoms an isotropic thermal parameter was refined. The model was refined with the restraints proposed in ref. 29 and the average half-uncertainty window for the refinement as 0.10(2), indicating a consistent structural model. The preferred orientation was described with 4th order spherical harmonics. Line asymmetry was refined in a full axial model.30 At an average Δd of 0.01 Å the refinement converged to Rwp/R′wp/Rp/R′p/RBragg = 2.96/6.67/2.15/7.15/1.15%.
Crystals of I belong to the orthorhombic crystal system (space group P212121, Z′ = 5). Five crystallographic sorts of uranium and five of sodium atoms occupy general positions. The U O distances and O U O angles of the uranyl ions fall in the ranges of 1.66(3)–1.82(2) Å and 177.6(10)–179.0(15)°. Compounds II and III crystallize in the space group P213 (Z′ = 1/3), which is rare among all compounds in CSD and ICSD,31,36,37 but is frequently observed for other uranyl carboxylates.16,20 In II and III, Cs, N and U atoms occupy special positions with C3 site-symmetry, as well as O atoms of uranyl ions in both crystal structures and one out of four H atoms of the ammonium ion in III. Thus, the uranyl ions in II and III are linear with U O distances in the range of 1.75(2)–1.76(3) Å.
Coordination polyhedra of all U atoms in I–III are hexagonal bipyramids (Fig. 1 and 2). The volumes of the Voronoi–Dirichlet polyhedra of U atoms in I–III are in the range of 9.08–9.50 Å3, which is in good agreement with the known value of 9.3(2) Å3 for U(VI) atoms in UOn polyhedra with n = 5, 6, 7, 8 or 9.38 In equatorial planes, the uranyl ions coordinate six oxygen atoms belonging to three isobutyrate or isovalerate ions with the B01 bidentate cyclic coordination mode (Fig. 1 and 2).14 All three crystals are constructed of typical anionic complex units [UO2L3]−, where L is an isobutyrate or isovalerate ion (Fig. 1 and 2). The crystallochemical formula14 of such complexes is AB013, where A = UO22+ and B01 = i-C3H7COO− or i-C4H9COO−. Many carbon and hydrogen atoms of the hydrocarbon chains in I–III are disordered over two positions. In I, one out of four independent water molecules has an occupancy of 0.5.
Fig. 1 Fragment of the crystal structure of Na[(UO2)(i-C3H7COO)3]·0.7H2O (I). Uranium atoms are shown as coordination polyhedra. Blue, light grey, dark grey and red spheres represent sodium, hydrogen, carbon and oxygen atoms, respectively. Hydrogen atoms of isobutyrate ions are not shown for clarity.
Fig. 2 Fragments of the crystal structures of Cs[(UO2)(i-C3H7COO)3] (II, left) and (NH4)[(UO2)(i-C4H9COO)3] (III, right). Uranium atoms are shown as coordination polyhedra. Blue, light grey, dark grey and red spheres represent cesium (left) or nitrogen (right), hydrogen, carbon and oxygen atoms, respectively.
Fig. 3 Hydrogen bonds (dashed lines) of the ammonium ion in the crystal structure of (NH4)[(UO2)(i-C4H9COO)3] (III). Uranium atoms are shown as coordination polyhedra. Blue, light grey, dark grey and red spheres represent nitrogen, hydrogen, carbon and oxygen atoms, respectively.
a Ω is the solid angle (in percent of 4π steradian), at which the shared face of the Voronoi–Dirichlet polyhedra of adjacent atoms is seen from the nucleus of any of them.
As in the case of several earlier studied uranyl complexes,10,18 the three-dimensional frameworks of crystal structures of I and II have the NaUO6 or CsUO6 composition, in which U atoms with coordination number equal to 8 are interconnected with Na or Cs atoms via the equatorial O atoms (Fig. 4a). All the remaining atoms and groups of atoms are terminal and ‘hang’ on the mentioned framework (the hydrocarbon chains, the O atoms of the uranyl cations and the water molecules).
Fig. 4 Schematic presentation of the surrounding of Na atoms in the NaUO6 fragments of crystal structures of sodium uranyl complexes with monocarboxylate ligands at CN of the Na atom equal to 6 (a, d) and 5 (b, c). The O atoms of the same monocarboxylate ligand are linked by arcs.
The CN's of Na atoms in compound I are also equal either to 5 (Na1, Na2, Na3, Na5) or to 6 (Na4), although their structural function appeared to be different. Atoms Na1, Na2 and Na5 are similar to Na atoms with CN 5 from the cited paper:18 they bind only two neighboring complex anions [UO2L3]− and one water molecule and they lead to the depolymerization of the framework (Fig. 4b). On the other hand, each of the Na3 and Na4 atoms is bonded with two neighboring complex anions [UO2L3]− through double Na–O–U bridges and with the third complex anion – through a single Na–O–U bridge (Fig. 4c and d). Moreover, the Na4 atom binds one water molecule (Fig. 4d). Such a coordination environment of Na3 and Na4 atoms saves the framework from depolymerization in those nodes, which is more like the situation with the ‘old’ potassium uranyl acetate, K[UO2(CH3COO)3]·0.5H2O,52 described in the cited paper.16 In the case of the Na3 atom the coordination polyhedron is very distorted and there is a void big enough for allocating one water molecule – similar to the case of the Na4 atom, which coordinates one water molecule with halved occupancy. That fact means that compound I should be very sensitive to humidity, as two out of five Na atoms can lose or gain one water molecule each without significant structural rearrangements, which is 0.4 water molecules per formula unit.
Such different coordination environments of Na atoms result in different coordination sequences for Na and for U atoms in crystal structures of the compounds under discussion.10,18,51Table 3 lists the coordination sequences CP for the five unique U atoms in the structure of compound I. While U1, U3 and U4 atoms have almost identical values of CP, the corresponding values for U2 and U5 are significantly lower. In fact, the U5 atom has the least number of neighboring atoms in the first three coordination spheres among all the known uranyl complexes with aliphatic monocarboxylate ions (Fig. 5). Thus, in spite of the similar coordination polyhedra of all five unique U atoms in the crystal structure of I, their structural function is very different in further coordination spheres. Although all the known sodium and uranyl complexes with aliphatic monocarboxylate ions feature the same composition of the metal–organic frameworks, NaUO6, their structures are different, which is an another example of topological isomerism.
Fig. 5 First three coordination spheres (CS) of the U5 atom in the crystal structure of Na[(UO2)(i-C3H7COO)3]·0.7H2O (I). The R–(O–R–)N chains elongate only in the upper direction, as there are only weak interactions from the down side. Only four neighboring R atoms compose the first three coordination spheres of the initial U5 atom. Uranium atoms are shown as coordination polyhedra. Blue, grey and red spheres represent sodium, carbon and oxygen atoms, respectively.
a The values of coordination sequences for the marked compounds are provided taking into account the uranyl-cation interactions.
Interestingly, analogous to I, compound Na[(UO2)(n-C3H7COO)3]·0.25H2O15 with unbranched butyrate ions features only one out of four crystallographic sorts of sodium atoms with CN 5, leading to depolymerization, which is 25% of all the Na atoms. The here-studied compound I has three out of five Na atoms leading to depolymerization, which is 60% of all the Na atoms. As a result, the compound with a higher number of Na atoms leading to depolymerization has lower values of coordination sequences, although it features the ligands with the same number of carbon atoms in the hydrocarbon chains.18 Probably, this effect is explained by the decreased flexibility of the branched hydrocarbon chain of the isobutyrate ion: it does not allow the initially ideal cubic NaUO6 framework10 to be preserved without depolymerization after the ligands are placed in the voids of this framework.
The coordination sequences for the known cesium and uranyl complexes with aliphatic monocarboxylate ions are provided in Table 3. Let us remind, that in ideal AUO6 frameworks (A is a secondary metal atom), schematically represented in Fig. 4a, every R atom (R is either A or U) is connected with three R atoms via the R–O–R bridges and, thus, the R–(O–R–)N chains consecutively bifurcate on R atoms (see Fig. 4 and 6).10,16 This fact gives the highest theoretically possible number of neighbors equal to 3, 6, 12, 24, 48, 96, 192 and so on in the coordination spheres of R atoms in such AUO6 frameworks.10 Among seven listed Cs-containing compounds in Table 3, three compounds do not fit in the earlier described representation with the three-dimensional CsUO6 frameworks. Two of these compounds have layered structures (see Table 3) and, thus, no three-dimensional CsUO6 frameworks. Moreover, in the cases of compounds with acetate53 and crotonate48 ligands (see Table 3) the R–(O–R–)N chains bifurcate also on O atoms, but not only on R atoms (see the black and grey disks in Fig. 6), making it possible to have four neighbors in the first coordination sphere. In the case of the Cs[UO2(n-C3H7COO)3][UO2(n-C3H7COO)(OH)(H2O)] compound17 one out of two U atoms has CN 7 in contrast to all the other discussed U atoms with CN 8. The acetate, crotonate and n-butyrate hydroxo containing compounds also feature uranyl-cation UO22+–Cs+ interactions,48–50 which were taken into account for the calculation of coordination sequences, which makes the values of coordination sequences so high for the acetate complex (see Table 3 and black circles in Fig. 6). However, the layered structures of crotonate and n-butyrate hydroxo compounds prevent the values of coordination sequences from being as high as in the acetate containing complex (see Table 3).
Fig. 6 Bifurcation of R–(O–R–)N chains (depicted as black lines), starting from an initial atom. In Cs[(UO2)(i-C3H7COO)3] (II, left) the chains bifurcate only on R atoms, while in Cs[UO2(C3H5COO)3] (right) the chains bifurcate on R (nodes shown with black disks) and oxygen (nodes shown with grey disks) atoms. Digits indicate the numbers of coordination spheres. Uranyl-cation interactions are shown with black circles. The crystal structures are shown in a stick representation. Blue, yellow and red colors correspond to cesium, uranium and oxygen atoms, respectively.
The remaining four cesium and uranyl complexes with acrylate,54 propionate,46n-butyrate16 and isobutyrate (compound II) ligands from Table 3 can be represented with the concept of three-dimensional CsUO6 frameworks. Interestingly, they realize the same coordination sequences even though the size of the ligand changes considerably (see Table 3). Along with Na[UO2(CH3COO)3],55 K[UO2(C2H5COO)3]47 and K[UO2(C3H5COO)3]48 these are the only compounds that maintain the highest theoretically possible number of neighbors up to the fourth coordination sphere.10 In correspondence with the results in ref. 16 and 20, these crystal structures represent the optimal ratios between the sizes of the ligands and the pores of metal–oxygen frameworks, determined by the sizes of the secondary metal atoms.
Finally, it is worth noting, that sodium and cesium containing compounds with unbranched n-butyrate ions, analogous to I and II, possess lower symmetry than the branched ones: monoclinic Na[(UO2)(n-C3H7COO)3]·0.25H2O15vs. orthorhombic Na[(UO2)(i-C3H7COO)3]·0.7H2O (I) and orthorhombic Cs[UO2(n-C3H7COO)3] 16vs. cubic Cs[(UO2)(i-C3H7COO)3] (II).16 Thus, in spite of promoting depolymerization, branched isobutyrate ions contribute to maintaining a higher crystal system.
This research was supported by the Russian Science Foundation (project no. 17-73-10117). Synchrotron measurements were performed at the unique scientific facility Kurchatov Synchrotron Radiation Source supported by the Ministry of Education and Science of the Russian Federation (project code RFMEFI61917X0007). The contribution of the center for molecular composition studies of INEOS RAS is acknowledged.
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V. 
 V. 
 V. 
 V.