Hydrothermal Crystallization of Uranyl Coordination Polymers

Article pubs.acs.org/IC

Hydrothermal Crystallization of Uranyl Coordination Polymers Involving an Imidazolium Dicarboxylate Ligand: Effect of pH on the Nuclearity of Uranyl-Centered Subunits Nicolas P. Martin,† Clément Falaise,† Christophe Volkringer,†,‡ Natacha Henry,† Pierre Farger,§ Camille Falk,§ Emilie Delahaye,§ Pierre Rabu,§ and Thierry Loiseau*,† †

Unité de Catalyse et Chimie du Solide (UCCS)−UMR CNRS 8181, Université de Lille, ENSCL, Bat C7, BP 90108, 59652 Villeneuve d’Ascq, France ‡ Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France § Département de Chimie des Matériaux Inorganiques, IPCMS UMR7504 CNRS-UNISTRA, 23, rue du Loess, BP43, Strasbourg Cedex 2, France S Supporting Information *

ABSTRACT: Four uranyl-bearing coordination polymers (1−4) have been hydrothermally synthesized in the presence of the zwitterionic 1,3-bis(carboxymethyl)imidazolium (= imdc) anion as organic linkers after reaction at 150 °C. At low pH (0.8−3.1), the form 1 ((UO2)2(imdc)2(ox)·3H2O; ox stands for oxalate group) has been identified. Its crystal structure (XRD analysis) consists of the 8-fold-coordinated uranyl centers linked to each other through the imdc ligand together with oxalate species coming from the partial decomposition of the imdc molecule. The resulting structure is based on one-dimensional infinite ribbons intercalated by free water molecules. By adding NaOH solution, a second form 2 is observed for pH 1.9−3.9 but in a mixture with phase 1. The pure phase of 2 is obtained after a hydrothermal treatment at 120 °C. It corresponds to a double-layered network (UO2(imdc)2) composed of 7-fold-coordinated uranyl cations linked via the imdc ligands. In the same pH range, a third phase ((UO2)3O2(H2O)(imdc)·H2O, 3) is formed: it is composed of hexanuclear units of 7-fold- and 8-fold-coordinated uranyl cations, connected via the imdc molecules in a layered assembly. At higher pH, the chain-like solid (UO2)3O(OH)3(imdc)·2H2O (4) is observed and composed of the infinite edge-sharing uranyl-centered pentagonal bipyramidal polyhedra. As a function of pH, uranyl nuclearity increases from discrete 8- or 7-fold uranyl centers (1, 2) to hexanuclear bricks (3) and then infinite chains in 4 (built up from the hexameric fragments found in 3). This observation emphasized the influence of the hydrolysis reaction occurring between uranyl centers. The compounds have been further characterized by thermogravimetric analysis, infrared, and luminescence spectroscopy.

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INTRODUCTION Production of crystalline uranyl−organic framework (UOF) compounds has been intensively investigated in the last decades. Indeed, combination of the wide variety of organic linkers together with the diverse coordination modes of the uranyl cation (UO22+ in tetragonal, pentagonal, or hexagonal bipyramidal geometry) has led to the formation of a large number of organic−inorganic assemblies with different dimensionalities (0D−3D) and various nuclearities of uranylcentered building units.1−4 These crystalline assemblies are usually obtained from the use of carboxylic acids as O-donor molecules in association with the actinide elements. The organic part is usually related to aliphatic or aromatic polycarboxylate ligands, but use of imidazolium-containing molecules was not so often reported with hexavalent uranium. Indeed, numerous studies investigated the coordination to aromatic heterocycles carboxylates with N-donor function.5−18 The interest for imidazole-based molecule is also motivated by their use for the production of ionic liquids and their promising application of these salts in nuclear chemistry.19−22 Indeed, © XXXX American Chemical Society

ionic liquids exhibit very good results for uranium extraction and present good stability under irradiation.23 There exists one example of a complex based on the association of uranyl cation with a monocarboxylate bearing the positively charged imidazolium fragment. A mononuclear species was isolated from the reaction of uranium oxide UO3·2H2O with 1-carboxy3-methylmidazolium bis[(trifluoromethylsulfonyl)sulfonyl]imide.24 A second example describes the nitrate uranyl species interacting with the 1-(4-amidoximate)butyl-3-methyl-imidazolium ligand via its N-donor amidoxime function.25 Very recently, two novel uranium complexes were reported involving the tripodal flexible zwitterion 1,1′,1″-(2,4,6-trimethylbenzene1,3,5-triyl)-tris(methylene)-tris(pyridine-4-carboxylic acid) trichlorine (H3LCl3).26 This contribution is devoted to the reactivity of a ditopic carboxylate ligand bearing the imidazolium group (i.e., 1,3bis(carboxymethyl)imidazolium; noted imdc) with an aqueous Received: May 23, 2016

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DOI: 10.1021/acs.inorgchem.6b01232 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry uranyl nitrate solution under mild hydrothermal condition. Actually, this flexible zwitterionic dicarboxylate linker was mainly considered for the preparation of crystalline salts with alkaline-earth cations (Ca, Sr, Ba)27 or for the formation coordination polymers with transition metals (Cu, Mn, Co, Zn),28−30 lanthanides (La, Pr, Nd, Er),31,32 or lead33 metallic elements. With uranyl, four distinct phases have been obtained by using a relatively mild hydrothermal treatment after heating at 120−150 °C for 24 h in a reaction pH range (0.8−13). At lower pH (0.8−3.1), the compound 1 ((UO2)2(imdc)2(ox)· 3H2O; ox stands for oxalate group) forms as yellow block-like crystals, whereas two other compounds 2 (UO2(imdc)2) and 3 ((UO2)3O2(H2O)(imdc)·H2O) are observed when increasing the pH value (1.9−3.9) by adding a NaOH (4 M) solution. The compound (UO2)3O(OH)3(imdc)·2H2O (4) is obtained for higher pH range (3.1−13) and is synthesized as a pure phase for pH = 6.6. The synthesis and structural analysis of the four compounds are presented. The pure obtained phases (1, 2, and 4) have been further characterized by thermogravimetric analysis, infrared, and luminescence spectroscopy.

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EXPERIMENTAL SECTION

Synthesis. Caution! Uranium precursors are radioactive and chemically toxic reactants, so precautions with suitable care and protection for handling such substances have been followed. Compounds 1−4 have been hydrothermally synthesized under autogenous pressure using 23 mL Teflon-lined Parr-type autoclaves (type 4746) by using the following chemical reactants: uranyl nitrate hexahydrate (UO2(NO3)2· 6H2O, Merck, 99%), N,N-bis(carboxymethyl)-imidazolium chloride (C7N2O4H9Cl, noted [H2imdc][Cl]), sodium hydroxide (NaOH, Aldrich, 98%), and deionized water. The uranyl nitrate and sodium hydroxide reactants were commercially available and have been used without any further purification. N,N-Bis(carboxymethyl)-imidazolium chloride was obtained by following the synthesis procedure described in a previous work.30 Hereafter are given the synthetic conditions for the preparation of the compounds 1, 2, and 4 as pure phases (based on powder XRD pattern analysis, Figures S2). A synthetic batch of compound 3 was used for further XRD analysis by single-crystal technique. (UO2)2(imdc)2(ox)·3H2O (1). A mixture of 250 mg (0.5 mmol) of UO2(NO3)2·6H2O, 94 mg (0.5 mmol) of [H2imdc][Cl], and 5 mL (278 mmol) of H2O was placed in a Parr autoclave and then heated at 150 °C for 24 h. Initial pH was 0.8. The resulting yellow crystalline powder was then filtered off, washed with water, and dried at room temperature in air atmosphere. 1 was analyzed by a scanning electron microscope (Hitachi S-3400N) and showed yellow block-like crystals up to 150 μm size (Figure 1). Reaction yield (based on U): 15%. UO2(imdc)2 (2). From a mixture of 250 mg (0.5 mmol) of UO2(NO3)2·6H2O, 94 mg (0.5 mmol) of [H2imdc][Cl], 0.1 mL (0.4 mmol) of NaOH (4 M), and 5 mL (278 mmol) of H2O, heated at 150 °C, the phase 2 was obtained in mixture with compound 1 (Initial pH was 1.9). The pure phase 2 was obtained from the mixture of 250 mg (0.5 mmol) of UO2(NO3)2·6H2O, 94 mg (0.5 mmol) of [H2imdc][Cl], and 5 mL (278 mmol) of H2O, placed in a Parr autoclave, and then heated at 120 °C for 24 h. The resulting orange crystalline powder was then filtered off, washed with water, and dried at room temperature in air atmosphere. 2 was analyzed by a scanning electron microscope (Hitachi S-3400N) and showed orange needle-like crystals (Figure 1). Reaction yield (based on U): 19%. (UO2)3O2(H2O)(imdc)·H2O (3). A mixture of 250 mg (0.5 mmol) of UO2(NO3)2·6H2O, 94 mg (0.5 mmol) of [H2imdc][Cl]), 0.2 mL (0.8 mmol) of NaOH (4 M), and 5 mL (278 mmol) of H2O was placed in a Parr autoclave and then heated at 150 °C for 24 h. Initial pH was 3.1. The resulting mixture of yellow and orange crystals was then filtered off, washed with water, and dried at room temperature in air atmosphere. 3 was analyzed by a scanning electron microscope

Figure 1. Optical microscope (left) and SEM (right) pictures of imidazolium-based dicarboxylates 1−4: (a) compound 1, (b) compound 2, (c) compound 3 in mixture with 1 and 2, and (d) compound 4. (Hitachi S-3400N) and showed orange block-like crystals (Figure 1). Attempts to get 3 as pure phase have failed. (UO2)3O(OH)3(imdc)·2H2O (4). A mixture of 250 mg (0.5 mmol) of UO2(NO3)2·6H2O, 94 mg (0.5 mmol) of [H2imdc][Cl]), 0.4 mL (1.6 mmol) of NaOH (4 M), and 5 mL (278 mmol) of H2O was placed in a Parr autoclave and then heated at 150 °C for 24 h. Initial pH was 6.6. The resulting yellow crystalline powder was then filtered off, washed with water, and dried at room temperature in air atmosphere. 4 was analyzed by scanning electron microscope (Hitachi S-3400N) and showed yellow needle-like particles up to 150 μm size (Figure 1). Reaction yield (based on U): 24%. Single-Crystal X-ray Diffraction. Crystals of compounds 1−4 were selected under a polarizing optical microscope and glued on a glass fiber for a single-crystal X-ray diffraction experiment. X-ray intensity data were collected on a Bruker DUO-APEX2 CCD area-detector diffractometer using Mo Kα radiation (λ = 0.71073 Å) with an optical fiber as collimator. Several sets of narrow data frames (20 s per frame) were collected with ω scans. Data reduction was accomplished using SAINT V7.53a.34 The substantial redundancy in data allowed a semiempirical absorption correction (SADABS V2.1035) to be applied, on the basis of multiple measurements of equivalent reflections. The structures (1, 3, and 4) were solved by direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix leastsquares on all data using the SHELX program suites.36 Hydrogen atoms of the imidazolium ring were included in calculated positions and allowed to ride on their parent atoms. The final refinements include anisotropic thermal parameters of all non-hydrogen atoms. Due to disordering of imidazolium ring and water molecules occurring in compound 1, the XRD intensities have been collected at 100 K. It was observed that the oxygen atoms related to free water molecules in compounds 1, 3, and 4 have large thermal parameters compared to the other atoms. This might be due to slight disordering situations for these water molecules, which are intercalated in voids of the crystal B


Article

Inorganic Chemistry Table 1. Crystal Data and Structure Refinement for Compounds 1−4 formula fw temp (K) cryst type cryst size (mm) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) vol. (Å3) Z, ρcalcd (g·cm−3) μ (mm−1) Θ range (deg) limiting indices

no. of collected reflns no. of unique reflections parameters Goodness-of-fit on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak and hole (e·Å−3)

1

2

3

4

C8H8N2O9.5U2 522.19 100 yellow block 0.16 × 0.16 × 0.12 orthorhombic Pnnm 5.7412(3) 14.4322(6) 15.7094(7) 90 90 90 1301.65(10) 4, 2.665 12.521 1.92−28.27 −6 ≤ h ≤ 7 −18 ≤ k ≤ 19 −20 ≤ l ≤ 19 6522 1679 [R(int) = 0.0324] 123 1.044 R1 = 0.0364 wR2 = 0.0911 R1 = 0.0551 wR2 = 0.1027 3.10 and −0.89

C14H14N4O10U 636.3 293 orange needle 0.13 × 0.07 × 0.06 monoclinic P21/c 8.2573(2) 14.3155(5) 14.6269(4) 90 97.680(2) 90 1713.54(9) 4, 2.467 9.541 2.00−27.52 −10 ≤ h ≤ 10 −18 ≤ k ≤ 18 −18 ≤ l ≤ 18 36 497 3094 [R(int) = 0.0486] 263 1.025 R1 = 0.0289 wR2 = 0.0683 R1 = 0.0402 wR2 = 0.0760 0.988 and −0.811

C14H16N4O18U3 1242.40 299 orange block 0.20 × 0.17 × 0.06 triclinic P-1 11.116(3) 11.411(2) 11.684(3) 116.620(9) 100.870(11) 97.906(10) 1259.0(5) 2, 3.277 19.339 1.928−27.482 −14 ≤ h ≤ 14 −14 ≤ k ≤ 14 −15 ≤ l ≤ 15 35 998 5752 [R(int) = 0.0354] 352 1.073 R1 = 0.0182 wR2 = 0.0440 R1 = 0.0214 wR2 = 0.0454 2.00 and −1.008

C7H7N2O16U3 1089.24 299 yellow needle 0.11 × 0.10 × 0.03 monoclinic C2/c 19.761(2) 12.5678(11) 15.5142(15) 90 101.723(5) 90 3772.7(6) 8, 3.835 25.773 1.932−26.620 −24 ≤ h ≤ 24 −15 ≤ k ≤ 15 −19 ≤ l ≤ 19 59 486 3903 [R(int) = 0.0835] 243 1.053 R1 = 0.0276 wR2 = 0.0567 R1 = 0.0435 wR2 = 0.0622 1.785 and −1.215

lamp. The fluorescence spectrum of uranyl dinitrate hexahydrate, UO2(NO3)2·6H2O, was also presented for comparison.

structure between the hybrid organic−inorganic components. In compound 4, this situation leads to very short interatomic distances of 2.46 Å (Alert A in checkcif procedure) between the two free water molecules, which are likely due to the disorder of these species. Crystals of compound 2 were systematically twinned (merohedral twinning). The orientation matrix for the two domains was determined using CellNow,37 and the intensities of each domain were extracted using SAINT V7.53a.34 Absorption correction was conducted by semiempirical method based on redundancy using Twinabs.38 The structure of 2 was solved using direct method with nonoverlapping reflections of domain I (hklf4 file generated by Twinabs). Then this structural model was refined using the hklf5 set of data (with all reflections). The final refinements include anisotropic thermal parameters of all non-hydrogen atoms. Afterward refinements indicate 68.2(1)% of domain I and consequently 31.8(1)% of domain II. The crystal data are given in Table 1. Supporting Information is available in CIF format. CCDC numbers: 1459393 for 1, 1456767 for 2, 1456770 for 3, and 1456769 for 4 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Infrared Spectroscopy. Infrared spectra of compounds 1, 2, and 4 (see Supporting Information) were measured on PerkinElmer Spectrum Two spectrometer between 4000 and 400 cm−1, equipped with a diamond attenuated total reflectance (ATR) accessory. The thermal behavior of 1, 2, and 4 was characterized by in situ IR spectroscopy under air atmosphere with a heating rate of 10 °C·min−1 from room temperature to 210 °C. During this temperature variation, 195 spectra were recorded in the range 4000−400 cm−1, with 4 cm−1 resolution, on a PerkinElmer Spectrum2 spectrometer equipped with a Pike Specialir GaldiATR accessory. Fluorescence. Fluorescence spectra of the powdered compounds 1, 2, and 4 were measured at room temperature on a SAFAS FLXXenius spectrometer between 400 and 650 nm, equipped with a xenon

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RESULTS AND DISCUSSION Structure Description. Crystal structures of the compounds 1−4 have been characterized by single-crystal X-ray diffraction analysis. The structure of compound 1 ((UO2)2(imdc)2(ox)·3H2O) additionally shows the occurrence of oxalate species (ox), which have been revealed during X-ray diffraction data examination. There exists one inequivalent crystallographic site for the uranium (Figure 2), which is coordinated by eight oxygen atoms in a hexagonal bipyramidal geometry, with the two trans uranyl UO bondings of 1.739(5) Å. The distances of the equatorial U−O bonds are ranging from 2.458(6) to 2.503(5) Å. Two uranyl centers are linked to each other through the oxalate linker (ox), which adopts a tetradentate connection mode, since the two carboxylate arms are in bridging bidentate fashion, with two adjacent uranium atoms. This pair of uranyl centers (“U2ox”) is connected through two ditopic carboxylate ligands, which interact with the uranium atoms via a chelating mode. In fact, structure analysis indicates a statistically disordered situation for the imidazolium ring, which may adopt two positions, with an occupancy factor refined to 50% (Figure S1b). This connection results in the formation of an infinite ribbon developing along the c axis (Figure 3). The crystal structure of 1 consists of the stacking of such mixed organic−inorganic chains along the two [100] and [010] directions. Two distinct water molecules have been located between the ribbons nearby the disordered imdc ligand and correspond to a stoichiometry of 1.5 H2O for one U C


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Inorganic Chemistry

The in situ formation of oxalate moiety has been described in other uranyl carboxylates coordination polymers,40−48 as well as in recent works with other cations.49,50 The thermogravimetric curve of compound 1 (Figure S4a) shows a first weight loss up to 100 °C of 5.5%, which could correspond to the evacuation of the three free water molecules (calcd 5.30%) per 2 U centers, as revealed by XRD analysis. The second event appears upon heating between 325 and 365 °C and is assigned to the decomposition of the organic linkers. The final residue is U3O8 (obsd 54.6%; calcd 55.0%). The crystal structure of compound 2 (UO2(imdc)2) is built up from discrete uranyl units connected through the 1,3bis(carboxymethyl)imidazolium ligand. It is composed of one unique crystallographic site for uranium, which is 7-fold coordinated with a typical pentagonal bipyramidal geometry (Figure 2). The two trans uranyl bondings are characterized by UO distances of 1.764(4) and 1.770(4) Å. The U−O distances are ranging from 2.323(4) to 2.527(5) Å in the equatorial plane. The discrete UO7 units are linked to each other through two types of imidazolium dicarboxylate ligands. For one of them, the connection mode is chelating with the uranyl center for one carboxylate arm, whereas it adopts a monodentate fashion for the second arm (trans configuration). The remaining nonbonded C−O distance is 1.223(7) Å and corresponds to a nonprotonated state. The second type of ditopic organic linker has only a monodentate connection mode with the uranyl centers (cis configuration), resulting in two types of ligands that link only to two adjacent uranium atoms. The two remaining C−O bonds are not protonated, which is indicated by short distances of 1.213(7) and 1.225(7) Å. The connection of the uranyl centers with this two distinct imdc ligands generates infinite corrugated organic−inorganic two-dimensional network, developing along the (b,c) plane (Figure 3). The resulting neutral layers are stacked along the a axis via van der Waals interactions. They consist of a double layers of uranyl centers, and this arrangement is reminiscent of that described in the uranyl isophthalates39 intercalated by protonated 1,3-diaminopropane (UO2(1,3-bdc)1.5·0.5dap· 2H2O). Indeed, the angle of the carboxylate groups in the isophthalate species (1,3-position −120°) is closely related to that found in the imdc ligand (119.6°). Thermogravimetric analysis of the compound 2 (Figure S4b) indicates a one-step weight loss from 300 °C. The remaining weight value is 55.5% (obsd), in good agreement with the theoretical value of 55.3%, expected for U3O8 final residue obtained at 800 °C. The structure of the compound (UO2)3O2(H2O)(imdc)· H2O (3) contains hexanuclear units (Figure 4) composed of two 7-fold and one 8-fold uranyl centers (pentagonal bipyramid for U2 and U3; hexagonal bipyramid for U1), defined from three crystallographically independent actinide atoms (Figure S1e). For each uranyl cation, typical trans UO bond lengths are observed in the range 1.778(4)−1.797(4) Å. The U−O bond distances in the equatorial plane are ranging from 2.456(3) to 2.602(3) Å for the carboxyl oxygen atoms. Other types of oxygen atoms are found within the hexamer. Indeed, the uranyl centers are linked to each other through two distinct oxo groups (O1 and O2), which have a μ3-connection fashion. The uranyl-centered polyhedra are thus linked by sharing two (for U1 and U2) of three edges (for U3) in the equatorial plane. The U−O bond distances are rather shorter with lengths varying between 2.168(3) and 2.322(3) Å. The assignments of oxo-type group are in a good agreement with the bond valence considerations51 with values of 2.025 and 2.027 for O1 and O2,

Figure 2. (Top) Coordination modes of uranyl-centered polyhedra with the imdc and oxalate ligands in (UO2)2(imdc)2(ox)·3H2O (1). (Bottom) Coordination modes of uranyl-centered polyhedra with imdc ligands in (UO2(imdc)2) (2). For 1, the disordering situation occurring for the imidazol ring and water molecules is not shown for clarity.

Figure 3. Views of (top) the infinite ribbons running along the c axis in (UO2)2(imdc)2(ox)·3H2O (1) and (bottom) the double layer in the (a,c) plane in (UO2(imdc)2 (2). For 1, the disordering situation occurring for the imidazol ring and water molecules as well as hydrogen atoms are not shown for clarity.

center. Chain-like arrangements have been also reported with the use of isophtalate ligand, but in the latter case, tetranuclear uranyl-centered building units were observed.39 The existence of the oxalate species, coming from partial in situ decomposition of organic molecule, is not new and has been previously reported for the synthesis of other compounds. It corresponds to the ring opening of the imidazolium group, followed by hydrolysis reaction under hydrothermal conditions. D


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Inorganic Chemistry

Figure 4. (Top) Coordination modes of uranyl-centered polyhedra with imdc ligands in the hexanuclear unit of (UO2)3O2(H2O)(imdc)· H2O (3). The two distinct imdc linkers are labeled A and B. (Bottom) Coordination modes of infinite chains of uranyl-centered polyhedra with the imdc ligand in (UO2)3O(OH)3(imdc)·2H2O (4).

respectively (expected value 2.0). One of the oxygen atoms attached to U2 is in the terminal position and attributed to an aquo species (bond valence 0.436; expected 0.4) with a U2− O1W distance of 2.482(3) Å. The three distinct uranyl centers are linked together around an inversion center to give a hexanuclear planar brick (Figure 4), in which the U1···U2, U1··· U3, and U2···U3 distances are 3.997(7), 3.9192(10), and 3.7068(9) Å, respectively. Such a hexameric block is very uncommon in the crystal chemistry of uranyl−organic frameworks. A closely related hexamer has been previously described in the uranyl 2,4-pyridinedicarboxylate,52 in which uranyl centers are connected through μ3-O or μ3-OH groups. However, all six uranium atoms are 7-fold coordinated in pentagonal bipyramidal geometries. Moreover, in the latter, a bending angle of 142.6° was reported between the two external uranium atoms, whereas it is close to 180° in compound 3. The connection of the hexameric units is ensured via the 1,3bis(carboxymethyl)imidazolium ligands. One type (labeled A) links through the two carboxylate arms (cis configuration), acting either as chelating mode (with U1) or as syn-syn bidendate bridging (with U2 and U3) mode in order to generate a chain-like subnetwork, developing along the [111] direction (Figures 4 and 5). The two-dimensional connection occurs through the second type of 1,3-bis(carboxymethyl)imidazolium linker (labeled B; cis configuration). One of its carboxylate groups adopts a monodentate connection fashion and is shared with U1 and U3 centers (μ3 configuration). The second oxygen atom is terminal, with a bond length of 1.214(6) Å, reflecting a CO bonding type. The second carboxylate arm is in both chelating mode with U1 and bidentate bridging mode with U1 and U2 (Figure S1e). The two perpendicular directions of coordinating 1,3-bis(carboxymethyl)imidazolium ligands result in the formation of hybrid organic−inorganic sheet, developing in the (b,c) plane (Figure 5). The layers are then stacked along the [−101] direction (Figure 5) with the intercalation of free water molecules (delocalized on two positions, with site occupancies of 50%). The crystal structure of compound 4 ((UO 2 ) 3 O(OH)3(imdc)·2H2O) consists of three crystallographically distinct uranyl centers (U1, U2, U3) which are bound to each other through oxo or hydroxo groups. The three uranium

Figure 5. Detailed polyhedral representations of the structure of (UO2)3O2(H2O)(imdc)·H2O (3). (a) Connection of the hexanuclear units through the imdc ligands type A, generating ribbon-like subnetwork (“hexamer-ligand A”), developing along the [111] direction. (b) View of the layer around the [−101] direction, formed from the infinite chain “hexamer-ligand A”, connected to each other through the ligand type B. (c) View of the structure of 3, showing the stacking of the sheet along the [−101] direction.

atoms possess identical polyhedral surroundings, corresponding to the classical pentagonal bipyramidal geometry (7-fold coordination, see Figure S1f). Each of them has two uranyl bonds in trans positions, with typical UO bond distances varying from 1.767(6) to 1.785(6) Å. Within the equatorial pentagonal plane, the surrounding of each uranyl center is slightly different: U1 is coordinated to three hydroxo groups (U1−O = 2.367(5)−2.527(5) Å), one oxo group (U1−O1 = 2.230(5) Å), and one carboxyl oxygen atom (U1−O91 = 2.431(6) Å); U2 is bound to three hydroxo groups (U2−O = 2.341(5)−2.586(6) Å), one oxo group (U2−O1 = 2.248(5) Å), and one carboxyl oxygen atom (U2−O71 = 2.435(6) Å); U3 is coordinated to two hydroxo groups (U3−O = 2.464(5)− 2.508(5) Å), one oxo group (U3−O = 2.186(5) Å), and two carboxyl oxygen atoms (U3−O = 2348(6)−2.403(6) Å). The uranyl centers are linked to each other via central μ3-hydroxo groups (O2H, O3H) or μ3-oxo group (O1) as well as peripheral μ2-hydroxo groups (O1H). It results in the formation of an infinite chain containing edge-sharing pentagonal bipyramids, developing along the b axis (Figure 4). In a way, the hexanuclear brick observed in compound 3 can be viewed as a fragment of the inorganic chain in 4. The assignments of the OH/O groups are in good agreement with the bond valence calculations51 for the corresponding oxygen atoms, with values of 1.33 (O2H) and 1.26 (O3H) for the μ3hydroxo species, 2.16 (O1) for the μ3-oxo one, and 1.12 (O1H) for the μ2-hydroxo one (expected values for OH 1.2; O 2.0). Within the inorganic ribbon, the peripheral μ2-hydroxo groups alternate with two adjacent carboxylate bridges of the imdc E


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Inorganic Chemistry

pH Tuning of the Hydrolysis Reactions. The crystallization of the four compounds occurs for different pH ranges at 150 °C. The synthetic conditions of down and upper pH limit of occurrence of each phase are gathered in Table 2. Different concentrations of NaOH have been varied for exploring the pH range from 0.8 to 13 (Figure S3) in order to get a composition diagram as a function of pH. Figure 7

linker, the latter playing the role of bidentate groups (cis configuration), between the uranyl centers. The occurrence of such connection sharing 2 or 3 edges induces U···U distances, ranging from 3.7593(5) to 4.0026(5) Å. This topology of straight chain has been previously encountered in several uranyl−organic frameworks.1,52−61 The imdc ligand is linked to uranyl centers within one ribbon in a bidentate bridging mode, preventing any connection between the inorganic chains. This induces the generation of one-dimensional structure containing [(UO2)3O(OH)3(imdc)]∞ blocks, which are separated from each other through free water molecules (Figure 6). The crystal

Figure 7. Occurrence of the four distinct uranyl imidazolates compounds 1−4 as a function of reaction pH (range 0−7) after hydrothermal treatment at 150 °C.

indicates the occurrence of the four compounds 1−4 in the pH range 0−7. This diagram was obtained by taking some individual NaOH reactions, which correspond to down and upper pH limit (related to NaOH concentrations) of formation of a given compound. The phase 1 is obtained in pH range 0.8−3.1, whereas the phase 2 crystallized from pH = 1.9 to 3.9 (data are given for initial pH values). Indeed, for compound 1, the synthetic conditions at rather low pH led to crystallization of this mixed carboxylate uranyl complex, involving oxalate species, coming from partial decomposition of the imidazoliumbased ligand. At 150 °C, compound 1 can be obtained as pure phase at pH = 0.8 (without any addition of NaOH), whereas compound 2 is obtained as pure phase after a hydrothermal reaction at 120 °C. At higher NaOH concentration, compound 4 appears in a broad pH range 3.1−13 and can be obtained as pure phase at pH = 6.6. In fact, for lower pH range (1.9−3.9), compound 3 is always formed in a mixture with 2. Furthermore, for the lowest pH value in this range there is

Figure 6. View of stacking of infinite chains along the a axis in (UO2)3O(OH)3(imdc)·2H2O (4).

structure analysis revealed two distinct water molecules O1W and O2W (Figure S1g). Indeed, the cohesion of the crystal structure is ensured via the occurrence of hydrogen-bonded water with μ2-hydroxo and μ3-hydroxo groups, belonging to the inorganic chain. Along the [100] direction, there exist O1W··· O1W (2.59(2) Å) and O1W···O2H (2.74(2) Å) bondings. Along the [101] direction, the O2W interacts with O1W (2.46(3) Å) and the μ2-hydroxo group O1H (2.93(2) Å). The water molecule content has been confirmed by thermogravimetric analysis (Figure S4c). Up to 90 °C, the weight loss curve indicates a first step corresponding to the removal of free water molecules (obsd 3.3%; calcd 3.5%). The remaining residue is attributed to uranium oxide U3O8 (obsd 77.9%; calcd 76.8%).

Table 2. Reactants Amounts and Molar Compositions for the Formation of Compounds 1-4 and Range of Occurrence Related to NaOH Concentration and pH Values for Hydrothermal Reactions Occurring at 150 °Ca phase 1 (down limit) phase 1 (up limit) phase 2 (down limit) phase 2 (up limit) phase 3 (down limit) phase 3 (up limit) phase 4 (down limit) phase 4 (up limit)

a

uranyl nitrate

imdc

H2O

0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol 0.25 g 0.5 mmol

0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol 0.094 g 0.5 mmol

5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol 5 mL 278 mmol

NaOH

[NaOH]

pHinitial

0 mL

0

0.8

0.2 mL 0.8 mmol 0.1 mL 0.4 mmol 0.3 mL 1.2 mmol 0.1 mL 0.4 mmol 0.3 mL 1.2 mmol 0.2 mL 0.8 mmol 0.5 mL 2 mmol

0.078 mol·L−1

3.1

0.154 mol·L−1

1.9

0.226 mol·L−1

3.9

0.154 mol·L−1

1.9

0.226 mol·L−1

3.9

0.078 mol·L−1

3.1

0.363 mol·L−1

13

The table indicates the down and up limit of the initial pH values for which the formation of a given compound is observed by XRD analysis. F


Article

Inorganic Chemistry

wavelength at 365 nm. For 1 and 2, they show a typical vibronic structure due to the double uranyl bonding and are characterized by five broad emission peaks (Figure 8, Table

also the formation of compound 1, whereas 4 is obtained for the highest value. It was therefore not possible to get the phase 3 alone. It was noticed that the increase of NaOH concentration induces the formation of the dense sodium uranyl compound (Na(UO2)O(OH), deriving from the Clarkeite mineral62) at the expense of compound 4. This study shows quite well the influence of the hydrolysis occurring between the uranyl cations in aqueous solution. As previously reported by Cahill46,63,64 in other carboxylate-based assemblies, the uranyl species undergo hydrolysis reaction upon pH increasing, which gives rise to the formation of oxo/hydroxo bridges between the metallic cations. From the observation of discrete uranyl centers in relatively acidic conditions, the oligomerization process thus led to the formation of a hexameric unit (nuclearity 6) and chain-like (one-dimensional infinite nuclearity) solids when increasing the pH reaction by adding NaOH base. Such a nuclearity increase from pH variation has also been explored with the phthalic acid (from discrete uranyl entity up to tetrameric units).65 The same trend is also observed for tetravalent actinides-based coordination polymers. However, the strong Lewis acidity of these cations promotes hydrolysis mechanism with the production of poly oxo cluster with high nuclearity (up to 38 An centers).66−70 Infrared Spectroscopy. Infrared spectra were collected for all solids (1, 2, 4) synthesized as pure phase (Figures S5a−c). In each solid, the presence of the imidazolium moiety in the different compounds is easily confirmed by vibrations ranging from 3200 to 2900 cm−1, assigned to methyl groups (νasym[CH2]) attached to the imidazolium ring. The resonances corresponding to carboxylate functions connected to uranyl cation covers the range 1660−1500 cm−1. The resonance corresponding to the asymmetrical vibration of the uranyl double bond νasym(UO) gives a peak at 923, 903, and 918 cm−1 for compounds 1, 2, and 4, respectively. These vibrations are in good agreement with the distances calculated71 from the crystal structure data (Table S1). For the hydrated compounds (1 and 4), IR spectra (Figures S5a−c) indicate resonances between 3700 and 3200 cm−1, assigned to vibrational modes of OH groups. To differentiate the bands coming from water and hydroxyl groups, we followed the dehydration of 1 and 4 by in situ infrared spectroscopy. In the case of 1 (Figure S5d), we observe a broad band centered at 3500 cm−1 and showing two shoulders at 3611 and 3542 cm−1. When heating above 60 °C, the intensity of the vibrations at 3542 cm−1 is decreasing and the band disappears at 100 °C. At this temperature, the peak initially situated at 3611 cm−1 is now shifted to 3642 cm−1 and starts to disappear. This resonance is no longer visible on the spectra at 130 °C. This thermal behavior highlighted by in situ infrared could correspond to a partial dehydration up to 100 °C, leading to the disappearance of hydrogen bonding (peak at 3542 cm−1). Therefore, the vibration located at 3611 cm−1 is characterized by O−H bonds involving water molecules that participate less in intermolecular bonding. The IR spectra of compound 4 collected at room temperature exhibit a peak at 3581 cm−1 with a sharpness characteristic of hydroxyl group (Figure S5c). The presence of water molecules is confirmed by the broad band centered at 3412 cm−1. As expected, the temperature increase induces the departure of a water molecule characterized by the attenuation of the intensity of this last band up to 200 °C (Figure S5e). Fluorescence. The solid-state luminescence spectra of compounds 1, 2, and 4 have been collected with an excitation

Figure 8. (Top) Solid-state fluorescence emission spectra of compounds (UO2)2(imdc)2(ox)·3H2O (1) (black line), (UO2(imdc)2 (2) (red line), uranyl nitrate hexahydrate (blue line) (given for comparison). (Bottom) Solid-state fluorescence emission spectra of compounds (UO2)3O(OH)3(imdc)·2H2O (4) (black line) and uranyl nitrate hexahydrate (blue line) (given for comparison). Excitation wavelength 365 nm at room temperature.

S3). They correspond to the electronic transitions S10 → Sov (v = 0−4),72,73 since the S11 → S00 transitions are hardly visible. The average splitting of the S10 → Sov transitions is directly related to the symmetrical vibrations of the OUO bond and equal at 829 and 816 cm−1 for 1 and 2, respectively. In comparison with the spectrum of uranyl nitrate hexahydrate (maximum at 512 nm), the peaks located at 511 nm for 1 (Δλ = −1 nm) and 514.5 nm for 2 (Δλ = +2.5 nm) are blue and red shifted, respectively. Indeed, the discrete hexagonal bipyramidal environment observed in compound 1 is close to that of the uranyl nitrate. However, the coordination state differs in compound 2, for which discrete pentagonal bipyramidal geometry is found. This observation was previously reported in uranyl−pyromellitate frameworks,74−82 for which 7-fold environments were found for the uranyl centers, with such redshifted signals. The fluorescence spectrum of compound 4 (Figure 3) exhibits less resolution since a broad signal capped by four peaks located at 520, 535 (maximum intensity), 558, and 585 nm is observed. Such a spectrum has been reported in several uranyl−organic compounds based on such infinite inorganic chains.18,59−61,74 This specific broad feature could be due to the occurrence of direct oxo/hydroxo bondings between the uranyl centers, which would give rise to overlapping signals. G


Article

Inorganic Chemistry

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Accession Codes

CONCLUSION In summary, four new uranyl coordination polymers have been synthesized by using the 1,3-bis(carboxymethyl)imidazolium ligand under mild hydrothermal conditions (120−150 °C). At lower pH (0.8−3.1), compound 1 ((UO2)2(imdc)2(ox)·3H2O) was formed and its crystal structure revealed that the organic molecule is partially decomposed in generating oxalate molecule, which is associated in the final one-dimensional network. The oxidative ring-opening mechanism suggested for the formation of the oxalate from imidazolium moieties seems favored by acidic conditions. Actually, the mechanism was originally demonstrated for 2,3-pyrazinedicarboxylate ligands in hydrothermal condition at low pH of 1.0−1.5.49 When increasing the pH, the phases 2 (UO2(imdc)2) and 3 ((UO2)3O2(H2O)(imdc)·H2O) were obtained in the pH range 1.9−3.9. Their crystal structure contains either discrete 7-fold-coordinated uranyl centers (2) or hexanuclear uranylcentered brick (3), involved in layered assemblies. The last compound (UO2)3O(OH)3(imdc)·2H2O (4) crystallized for higher pH range (3.1−13) and is characterized by a chain-like structure with infinite ribbons of 7-fold-coordinated uranyl moieties linked to each other via edge-sharing polyhedra. This study is a nice illustration of the hydrolysis reactions occurring for uranyl species when increasing reaction pH in aqueous medium. Indeed, discrete uranyl units are observed in acidic conditions, and distinct building blocks are formed when adding NaOH solution, with higher nuclearity (hexamer for compound 3) or infinite oligomerization for compound 4, with the generation of 1D chain. In this system involving the 1,3bis(carboxymethyl)imidazolium ligand, we also observed that organic linker exists under its cationic form for the imidazole fragment and interacts only via the carboxylate groups with uranyl centers. This feature is due to the positions of the carboxylate groups (in 1,3), which prevent the connection of nitrogen atom with uranyl owing to steric hindrance of the ethylene-carboxylate group. This situation differs from other imidazole-based dicarboxylate molecules, for which one of the nitrogen atoms may belong to the coordination sphere of the uranyl center. This particular connection mode was observed in the uranyl imidazole-4,5-dicarboxylate (UO2(L); L = imidazole4,5-dicarboxylate) in which both carboxyl oxygen and nitrogen atoms are bound to uranyl centers.83 In the latter case, the nuclearity of the building unit is quite reduced; it is mononuclear with discrete units UO6N. If the carboxylate function is attached to the nitrogen atom of the imidazol cycle, the resulting ligand behaves as a classical polycarboxylate linker, with the possibility of playing with the nuclearity of uranylcentered building block. This leads to the possible production of polynuclear inorganic subnetworks, from mononuclear up to chain-like motifs.

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Crystallographic data for structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC nos. 1459393 for 1, 1456767 for 2, 1456770 for 3, and 1456769 for 4. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ (fax: +44-1223−336-033; e-mail: data_request@ ccdc.cam.ac.uk).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (33) 3 20 434 434. Fax: (33) 3 20 43 48 95. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mrs. Nora Djelal, Laurence Burylo, and Mr. Jason Delille, Philippe Devaux for their assistances with the synthesis, SEM images, and XRD powder patterns measurements (UCCS). The Chevreul Institute (FR 2638), “Fonds Européen de Développement Régional (FEDER)”, “CNRS”, “Infrastructure de Recherches de Résonance Magnétique Nucléaire (IR-RMN)”, “Région Nord Pas-de-Calais”, and “Ministère de l’Education Nationale de l’Enseignement Supérieur et de la Recherche” are acknowledged for the funding of Xray diffractometers and NMR spectrometers. The authors also acknowledge the Labex NIE (http://www.labex-nie.com/) and the Agence Nationale de la Recherche (ANR contracts no. ANR-15-CE08-0020-01) for financial support. The present work is part of the research activities supported by the icFRC (http://www.icfrc.fr) and the European COST action MP1202: HINT (http://www.cost-hint.cnrs.fr).

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01232. (CIF) (CIF) (CIF) (CIF) Powder XRD pattern, thermogravimetric curves, and IR spectra (PDF) H


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Hydrothermal Crystallization of Uranyl Coordination Polymers

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