UranylâPyromellitate Coordination Polymers: Toward Three

Article pubs.acs.org/crystal

Uranyl−Pyromellitate Coordination Polymers: Toward ThreeDimensional Open Frameworks with Large Channel Systems Ionut Mihalcea, Natacha Henry, Christophe Volkringer, and Thierry Loiseau* Unité de Catalyse et Chimie du Solide (UCCS)-UMR CNRS 8181, Université de Lille Nord de France, USTL-ENSCL, Bat C7, BP 90108, 59652 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: Five coordination polymers based on uranyl−pyromellitates have been hydrothermally synthesized, and their single-crystal XRD structures have been analyzed. These different compounds, obtained with different ammonia concentrations, exhibit either threedimensional (3D) or two-dimensional (2D) networks. Complex 1, (UO2)2(H2O)2(btec)·H2O, is a quite compact 3D structure containing isolated 7-fold coordinated uranyl cations linked through the pyromellitate (noted btec) and encapsulating free water species. Phase (NH4)[(UO2)2(OH)(H2O)(btec)]·1.75H2O (2) offers a second 3D architecture built up from dinuclear 7-fold coordinated uranyl units and mononuclear 8-fold coordinated uranyl units linked through the organic ligands. This framework is slightly more open because narrow one-dimensional (1D) channels trapping water species are visible. Phase 3, (NH4)2[(UO2)6O2(OH)4(btec)1.5]·11H2O, consists of large 1D lozenge-shaped channels (8.2 Å × 8.6 Å) delimited by infinite ribbons (composed of 7-fold coordinated uranyl polyhedra sharing edges) and pyromellitate ligands. Ammonium cations as well as water molecules are trapped within the channels. The fourth compound, (NH4)6[(UO2)3(btec)3]·12H2O (4), is lamellar with sheets containing 8-fold coordinated uranyl centers linked through the btec molecules, which have two nonbonded carboxylate functions interacting with the intercalated ammonium cations. Compound 5 also consists of a layered structure, (UO2)3(OH)2(H2O)2(btec), with uncommon trinuclear building blocks containing 7-fold (×2) and 8-fold (×1) coordinated uranyl centers, linked through the btec molecules. Fluorescence spectra of compounds 1, 3, and 4 are also measured.

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connect uranyl-centered moieties in multiple directions.5g,j,6a,7 In this situation, the structures exhibit quite narrow cavities or channels, in which are encapsulated solvent molecules such as water or amine species coming from decomposition of N,Ndimethylformamide, for instance. The use of flexible organic ligands from aliphatic molecules or aromatic molecules containing adjacent multidentate carboxylate arms seems to be one strategy for producing such 3D networks.2c,5j The presence of terminal aquo ligands attached to the uranyl cations is also another parameter, which may prevent the condensation reaction leading to the formation of 3D structures.5i In this work, we present a series of uranyl−organic frameworks obtained from the hydrothermal reaction with the 1,2,4,5-benzenetetracarboxylic acid (noted H4btec) under autogenous pressure. Previous work reported the crystallization of uranyl complexes with this polycarboxylate molecule from slow evaporation at room temperature, UO2(H2O)2(btec)·2H2O5b [one-dimensional (1D) chain], or in presence of the N-donor ligand 1,10-phenanthroline,

INTRODUCTION For the past decade, there has been growing interest in the reactivity of metallic cations with oxygen-donor or/and nitrogen-donor organic ligands for the generation of extended multidimensional architectures, so-called metal−organic frameworks or coordination polymers.1 In this class of solids, uranium was found to be a fascinating candidate because various organic−inorganic assemblies have been identified, especially for the hexavalent oxidation state. Its association with polytopic carboxylates successfully gave rise to the construction of polymeric complexes2 containing the different known coordination environments, defined by the double uranyl bondings and tetragonal, pentagonal, or hexagonal oxo planes,3 especially in case of the UO22+ cation (uranyl−organic framework or UOF). This chemical feature would then favor the formation of low-dimensional networks, because uranyl bonds are usually chemically inert and further condensation occurs mainly via the oxo groups in the equatorial plane. Indeed, this led to a large variety of molecular,4 chainlike,5 or layered structural motifs,5g,6 which is reflected by this specific preferential planar connection. Nevertheless, the formation of three-dimensional (3D) frameworks is also described and results from the conformation ability of the organic linkers to © 2011 American Chemical Society

Received: November 16, 2011 Revised: November 22, 2011 Published: December 7, 2011 526

dx.doi.org/10.1021/cg201509v | Cryst. Growth Des. 2012, 12, 526−535

Crystal Growth & Design

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(NH4)[(UO2)2(OH)(H2O)(btec)]·1.75H2O (2). Only a few single crystals were isolated from a mixture of 502 mg (1 mmol) of UO2(NO3)2·6H2O, 127 mg (0.5 mmol) of pyromellitic acid, 1.2 mL (2.4 mmol) of NH4OH (2 M), and 3.7 mL (206 mmol) of H2O, which was placed in a Parr bomb and then heated statically at 200 °C for 24 h. The solution pH was 4.0 at the end of the reaction. The resulting yellow product was then filtered off, washed with water, and dried at room temperature. The major phase corresponds to compound 1, and phase 2 was found as an impurity. (NH4)2[(UO2)6O2(OH)4(btec)1.5]·11H2O (3). A mixture of 1005 mg (2 mmol) of UO2(NO3)2·6H2O, 64 mg (0.25 mmol) of pyromellitic acid, 1.0 mL (2 mmol) of NH3, and 4.0 mL (222 mmol) of H2O was placed in a Parr bomb and then heated statically at 110 °C for 24 h. The solution pH was 2.7 at the end of the reaction. The resulting yellow product was then filtered off, washed with water, and dried at room temperature. It gives crystallites with a specific block shape of 20−80 μm that can be observed by scanning electron microscopy (SEM) (Figure 1). Although phase 3 appeared over the pH range of 2.7−3.2 by varying the NH4OH concentration, it was difficult to obtain with high purity by following the conventional hydrothermal route because unidentified impurities (small amount) were always observed. A microwave-assisted procedure was attempted by using the MarsX CEM Corp. apparatus. A mixture of 1005 mg (2 mmol) of UO2(NO3)2·6H2O, 64 mg (0.25 mmol) of pyromellitic acid, 1.3 mL (2.6 mmol) of NH4OH (2 M), and 8.7 mL (483 mmol) of H2O was placed in a 100 mL Teflon autoclave and then heated statically at 110 °C for 2 h. After filtration with water, the resulting product was left at room temperature. SEM examination indicated a crystal morphology with a pseudohexagonal needlelike shape and 5−30 μm size (Figure 1). The difference in crystal shape and size was previously reported for the preparation of MOF-type materials and was attributed to the very fast kinetics of nucleation occurring under microwave irradiation,9 which prevented the growth of a large crystal in some cases. For 3, the XRD powder pattern clearly indicated the high purity of the final product (Supporting Information). We also observed the crystallization of the uranyl pyromellitate dihydrate UO2(H2O)2(btec)·2H2O (noted A hereafter),5b when heating such a starting mixture at lower temperatures (typically 2σ(I)] R indices (all data) largest difference peak and hole (e/Å3)

C10H10O15U2 844.22 293(2) yellow block 0.26 × 0.22 × 0.15 orthorhombic Pbcn 12.8489(2) 12.7051(2) 9.9736(1) 90 90 90 1628.16(4) 4, 3.444 19.951 2.25−31.5 −18 ≤ h ≤ 18 −18 ≤ k ≤ 18 −14 ≤ l ≤ 13 146460 2717 [R(int) = 0.0520] 123 1.087 R1 = 0.0158 wR2 = 0.0348 R1 = 0.0231 wR2 = 0.0377 1.307 and −1.205

2

3

C10H7.50NO15.75U2 869.73 293(2) yellow needle 0.27 × 0.06 × 0.06 monoclinic P21/n 9.1844(2) 21.6150(5) 11.5401(2) 90 112.701(1) 90 2113.47(8) 4, 2.733 15.378 1.88−24.94 −10 ≤ h ≤ 10 −25 ≤ k ≤ 25 −13 ≤ l ≤ 13 89921 3691 [R(int) = 0.0806] 256 1.150 R1 = 0.0296 wR2 = 0.0822 R1 = 0.0419 wR2 = 0.1028 2.779 and −1.133

the benzene ring were included in calculated positions and allowed to ride on their parent atoms. The crystal data are listed in Table 1. Supporting Information is available in CIF format. Thermogravimetric Analysis. The thermogravimetric experiments were conducted on a TGA 92 SETARAM thermoanalyzer under an air atmosphere with a heating rate of 1 °C/min from room temperature to 800 °C. X-ray thermodiffractometry was performed under a 5 L/h air flow in an Anton Paar HTK1200N instrument of a D8 Advance Bruker diffractometer (θ−θ mode, Cu Kα radiation) equipped with a Vantec1 linear position sensitive detector (PSD). Each powder pattern was recorded in the range of 5−60° (2θ) (at intervals of 20 °C between room temperature and 800 °C) with a 0.34 s/step scan, corresponding to an approximate duration of 35 min. The temperature ramps between two patterns were 5 °C/min. Infrared Spectroscopy. Infrared spectra of compounds 1, 3, and 4 (see the Supporting Information) were measured on a Perkin-Elmer Spectrum Two spectrometer between 4000 and 400 cm−1, equipped with a diamond attenuated total reflectance (ATR) accessory. Fluorescence. Fluorescence spectra of powdered compounds 1, 3, 4, and A were measured at room temperature on a SAFAS FLX-Xenius spectrometer between 400 and 800 nm, equipped with a xenon lamp. The fluorescence spectrum of uranyl dinitrate hexahydrate, UO2(NO3)2·6H2O, was also presented for comparison.

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C15H3N2O35U6 2199.3 293(2) yellow needle 0.20 × 0.08 × 0.03 orthorhombic Pnnm 13.198(2) 14.852(2) 23.407(4) 90 90 90 4588.1(12) 4, 3.183 21.198 1.62−33.21 −20 ≤ h ≤ 20 −22 ≤ k ≤ 22 −35 ≤ l ≤ 35 135027 8983 [R(int) = 0.0634] 146 2.94 R1 = 0.0444 wR2 = 0.0733 R1 = 0.0634 wR2 = 0.0747 4.81 and −5.51

C30H6N6O48U3 1836.5 293(2) yellow block 0.18 × 0.16 × 0.15 monoclinic C2/c 55.631(2) 7.1082(3) 13.1074(5) 90 95.831(1) 90 5156.4(4) 4, 2.365 9.521 1.47−27.63 −72 ≤ h ≤ 72 −9 ≤ k ≤ 5 −17 ≤ l ≤ 17 23709 5994 [R(int) = 0.0339] 366 1.31 R1 = 0.0277 wR2 = 0.0309 R1 = 0.0396 wR2 = 0.0324 1.89 and −1.10

5 C10H8O18U3 1130.25 293(2) yellow block 0.15 × 0.11 × 0.07 triclinic P1̅ 5.5266(3) 7.6559(3) 11.3208(4) 100.846(2) 100.440(2) 96.940(2) 456.66(3) 1, 4.110 26.629 1.87−30.03 −7 ≤ h ≤ 7 −10 ≤ k ≤ 10 −15 ≤ l ≤ 15 70902 2662 [R(int) = 0.0605] 142 1.142 R1 = 0.0181 wR2 = 0.0366 R1 = 0.0238 wR2 = 0.0457 0.886 and −1.285

O1U and 1.755(2) Å for U1O2U] and are located at the apical position of a pentagonal bipyramid. In the equatorial plane, the U−O distances range from 2.342(2) to 2.438(2) Å. A terminal uranium−water (U1−O1W) bond with a length of 2.415(2) Å exists, and that length agrees well with the presence of aquo species (confirmed by bond valence calculations;10 calcd value of 0.496). The uranyl-centered monomeric motifs are connected to each other through the four carboxylate arms of the pyromellitate linkers to generate a 3D framework. The organic ligand coordinates six neighboring uranyl cations with a syn-anti or anti-anti bidendate bridging mode for the carboxylate functions [μ6-η1:η1:η1:η1:η1:η1 (Figure 3)]. Two carboxyl oxygens of two neighboring carboxylate arms (in the 1,2 or 4,5 position) belong to the coordination sphere of one uranium atom, whereas the remaining carboxyl oxygen atoms bridge two distinct uranyl centers (Figure 2). It results in the formation of a relatively dense network, which encapsulates a disordered free water molecule (Figure 2). Its content was measured by thermogravimetric analysis (Supporting Information) and corresponds to 0.5H2O per UO2 unit (observed weight loss of 2.2%, calcd weight loss of 2.1%). Hydration water was removed when the sample was heated from room temperature to 110 °C. It exhibits strong hydrogen bond interactions with the terminal aquo species attached to the uranyl center. The TG curve (Figure S1c of the Supporting Information) also showed a second weight loss event between 160 and 230 °C, with a value of 4.2%. This step could be attributed to the removal of the water species bonded to the uranyl cations (calcd weight loss of 4.3%). Then a large weight

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RESULTS Structural Description. (UO2)2(H2O)2(btec)·H2O (1). Its structure is built up from one crystallographically independent uranium atom, which is 7-fold coordinated with two uranyl oxygen atoms, four carboxyl oxygen atoms, and one water molecule (Figure 2). As expected for hexavalent uranium, the two uranyl bonds have typical lengths [1.763(2) Å for U1 528



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Figure 3. Diversity of the connection modes of the pyromellitate linker in phases 1 (syn-anti and anti-anti bidendate, top left), 2 (chelate and syn-anti bidentate, top right), 3 (syn-syn bidentate, bottom left), 4 (chelate and monodendate, bottom right), and 5 (chelate and syn-anti bidentate): yellow circles for uranium, red circles for oxygen, black circles for carbon, and gray circles for hydrogen.

uranium, which adopts two distinct coordination environments (Figure 4). Uranium U1 is 8-fold coordinated to two uranyl oxygen atoms [1.766(6) Å for U1O11U and 1.762(7) Å for U1O12U] in apical position and six carboxyl oxygen atoms in the equatorial hexagonal plane, with the U−O distances in the range of 2.343(6)−2.510(6) Å. The second uranium, U2, is 7-fold coordinated to two uranyl oxygen atoms [1.770(7) Å for U2O21U and 1.766(8) Å for U2O22U], two carboxyl oxygen atoms [2.335(6) Å for U2O101 and 2.384(6) Å for U2O82], two hydroxyl groups [2.341(6) Å for U2O1H], and one water molecule in the terminal position [2.503(9) Å for U2−O1W]. Valence bond calculations10 gave values of 1.144 and 0.419 for the hydroxo (O1H) and aquo (O1W) groups, respectively. U1 surrounding defines a hexagonal bipyramid, whereas U2 surrounding is a pentagonal bipyramid. The hydroxo groups bridge two neighboring uranium-centered pentagonal bipyramids through a common edge, generating a dinuclear motif [the U2···U2 distance is 3.8657(2) Å]. This building block was not so commonly encountered in UOF-type polymeric complexes from the literature.5j,l,6g,k,11 The hexagonal biyramidal polyhedra are linked to each other through the carboxylate functions located in the 1,4 position of the aromatic ring along the c axis. The 1,4-carboxylate arms adopt a chelating bridging mode toward uranium U1. One of the carboxyl oxygens of the 2-carboxylate arm is also bonded to a distinct neighboring uranium U1, resulting in the formation of double infinite mixed chains of hexagonal bipyramids (chelated by 1,4carboxylate) and pyromellitates developing along the c axis (Figure 5). The remaining nonbonded carboxyl oxygen atoms

Figure 2. Structure of (UO2)2(H2O)2(btec)·H2O (1): (top) a 7-fold coordination and pentagonal bipyramidal polyhedron of the uranyl center, (middle panel) view in the a−c plane showing the connection of the U-centered pentagonal bipyramids with the pyromellitate linkers, and (bottom) view of the 3D framework showing the free water molecules encapsulated within the small channels along the a axis.

loss was observed from 320 °C and corresponds to the decomposition of the pyromellitate molecules. The remaining weight at 800 °C was 66.7% and agreed well with the expected value for U 3 O 8 (calcd value of 66.5%). The X-ray thermodiffraction analysis (Figure S1d of the Supporting Information) indicated that phase 1 was visible up to 140 °C and then is transformed into unidentified crystalline phases with relatively good crystallinity. From TG measurement, if one considers the successive removals of water, the chemical formula of the intermediate dehydrated phase should be “(UO2)2(btec)”. The latter persisted up to 300 °C and then was followed by a second phase transition up to 400 °C. Above this temperature, the form of α′-U3O8 appeared. (NH4)[(UO2)2(OH)(H2O)(btec)]·1.75H2O (2). The crystal structure contains two independent crystallographic sites for 529



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Figure 4. Structure of (NH4)[(UO2)2(OH)(H2O)(btec)]·1.75H2O (2): (top) 8-fold coordination and hexagonal bipyramidal polyhedron of uranyl center U1 and (bottom) dinuclear unit of 7-fold coordinated and pentagonal bipyramidal polyhedra of uranyl center U2.

Figure 5. Structure of (NH4)[(UO2)2(OH)(H2O)(btec)]·1.75H2O (2): (top) view of the double chain of hexagonal bipyramidal polyhedra linked via the chelating bridging mode of the 1,4-carboxylate arms of pyromellitates, along the c axis, and (bottom) view of the structure along the a axis showing the connection of chains of hexagonal bipyramids with the dinuclear blocks via the pyromellitates. Trapped water (red circles) and ammonium groups (blue circles) are encapsulated within channels running along the a axis.

ensure the connection between the infinite chains and the discrete dinuclear unit of pentagonal bipyramids. The resulting three-dimensional framework reveals narrow tunnels with a larger aperture diameter (2.3 Å × 5.0 Å) along the a axis (Figure 5). As for 1, the pyromellitate molecule acts as a hexadendate ligand toward uranyl cations (Figure 3) but with a different connection mode because two of the carboxylate arms adopt chelating bridges and the two others have a syn-anti bidentate bridging mode (μ6-η1:η1:η1:η1:η1:η1). The framework of 2 is negatively charged, [(UO2)2(OH)(H2O)(btec)]−. Ammonium groups can be located within the channels and ensure the electroneutrality of the structure. Disordered water molecules also occupy the centers of the channels with strong hydrogen bonding networks between ammonium and oxygen belonging to the framework [2.79(3) Å for N1···O2W, 3.02(1) Å for N1···O1H, 2.94(3) Å for O3W···O1W, 2.84(2) Å for O4W···O1W, and 2.81(4) Å for O4W···O5W]. (NH4)2[(UO2)6O2(OH)4(btec)1.5]·≈11H2O (3). The structure of 3 is built up from the connection of 7-fold coordinated uranyl centers with the pyromellitate linker. There are three crystallographically independent sites for uranium (Figure 6). The corresponding pentagonal bipyramids consist of the typical short double uranyl bonds [1.748(6)−1.763(6) Å for U1O, 1.762(6)−1.778(6) Å for U2O, and 1.774(6)−1.776(6) Å for U3O] in apical positions, two carboxyl oxygen atoms [2.370(6)−2.414(6) Å for U−O], two hydroxo groups [2.418(6)−2.516(6) Å for U−O], and one oxo group [2.251(6)−2.276(6) Å for U−O] in the equatorial plane. The occurrence of the hydroxo and oxo groups is confirmed by bond valence calculations10 (1.395 and 1.375 Å for O1H and O2H, respectively and 1.972 Å for O1). Each hydroxo or oxo

Figure 6. Structure of (NH4)2[(UO2)6O2(OH)4(btec)1.5]·11H2O (3): (top) asymmetric unit showing the three independent uranium environments (U1−U3), where O1h and O2h are μ3-hydroxo groups and O1 is a μ3-oxo group, and (bottom) view of the infinite ribbons of 7-fold coordinated uranyl centers, running along the a axis.

ligand bridges three neighboring uranyl centers in the μ3connection fashion, to generate infinite ribbons running along 530



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the a axis. The U1···U2, U1···U3, and U2···U3 distances are 3.772(1), 3.965(1), and 3.782(1) Å, respectively. The topology of the chain is not new and was previously found in another uranyl compound12 with an organic carboxylate involving additional N-donor functionality. Related chains were also described with other mixed N- and/or O-donor ligands, but additional μ2-O bridging groups occurred between the uranyl centers.5h,8,13 The ribbons are then connected to each other through the four carboxylate arms of the pyromellitate molecules. This connection mode generates an unprecedented open framework (Figure 7) with one-dimensional channels

Figure 8. Thermogravimetric curve (top) of open framework 3 (under air, heating rate of 10 °C/min) and X-ray thermodiffraction patterns (bottom) as a function of the temperature of 3 (copper radiation).

Figure 7. Representation of the structure of (NH4)2[(UO2)6O2(OH)4(btec)1.5]·11H2O (3) along the a axis, showing the connection of infinite uranyl ribbons through the pyromellitate linkers, delimiting lozenge-shaped channels.

occurred and was attributed to the decomposition of the pyromellitate linker together with ammonium cations. The final remaining weight at 800 °C was 72.3%, and the XRD powder pattern of the residue showed the presence of α-U3O8 (calcd weight of 72.2%). The thermodiffractogram (Figure 8) of 3 indicated that Bragg peaks were clearly visible up to 240 °C, and their magnitudes decreased continuously up to 400 °C. The compound is then transformed in the crystalline form of α′-U3O8. The Brunauer−Emmett−Teller (BET) surface measurement (Micromeritics ASAP2010) was considered for dehydrated phase 3. Unfortunately, after the sample had been degassed at different temperatures under vacuum, very low BET surface values were found (≈6 m2/g). (NH4)6[(UO2)3(btec)3]·12H2O (4). The structure of 4 consists of 8-fold coordinated uranyl cations defining hexagonal bipyramids (Figure 9), lying on two independent crystallographic sites (Wickoff positions 8f and 4e). For both cations, short double uranyl bond are observed [1.764(4) Å for U1 O11u, 1.767(4) Å for U1O12u, and 1.768(4) Å for U2 O21u]. The equatorial hexagonal plane comprises carboxyl oxygen atoms only with U1−O distances in the range of 2.396(3)−2.534(3) Å and U2−O distances in the range of 2.434(3)−2.531(3) Å. Each uranyl center is isolated through the pyromellitate ligands. The latter interact with the metallic cations via two carboxylate arms (in the 1,4 position) with a chelating bridging mode and two other carboxylate arms with monodentate bridging modes. Nonbonded C−O linkages exhibit quite short distances of 1.237(7)−1.248(7) Å, which indicate a nonprotonated state for the monodentate carboxylate. Pyromellitate molecules have a tetradentate connection mode [μ4-η1:η1:η1:η0:η1:η1:η1:η0 (Figure 3)] with the uranyl centers. The positions of chelating or monodentate carboxylate

along the a axis, delimited by four inorganic ribbons and four organic linkers. The inorganic chains occupy the edges of the lozenge window, whereas the benzene rings of the organic ligands are located at its corners. The pyromellitate molecules act as octadentate linkers, connecting eight uranyl cations with a syn-anti bidentate bridging mode [μ8-η1:η1:η1:η1:η1:η1:η1:η1 (Figure 3)]. Each carboxylate arm is bridging two uranyl centers of an inorganic ribbon, and consequently, pyromellitate is bonded to four distinct chains. The benzene ring plane is perpendicular to the chain axis. The aperture diameter of channels with a lozenge shape is ∼8.2 Å × 8.6 Å (based on the ionic radius of 1.35 Å for oxygen). The channels are filled with ammonium cations, acting as compensators of positive charge for the anionic framework [(UO2)6O2(OH)4(btec)1.5]2− and crystallization water molecules. This structural arrangement is particularly remarkable because most of the uranyl carboxylates consist of molecular packings of discrete or low-dimensional inorganic building blocks. Indeed, the number of 3D networks is rather small, and they were reported in some examples, exhibiting exclusively quite narrow pore systems.5g,j,6h,7b−d,14 The amount of water encapsulated within the channels has been estimated by thermogravimetric analysis (Figure 8). A first weight loss, assigned to the removal of trapped water, was observed up to 160 °C with the value of 8.9%, which corresponds to ∼11 H2O molecules per (UO2)6 unit (calcd value of 8.5%). From single-crystal X-ray diffraction analysis, only 5 H2O molecules per (UO2)6 unit were revealed by the Fourier examination, but it was quite difficult to locate these water molecules because of large statistical disorder within the 1D channels. Between 260 and 470 °C, a second weight loss 531



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Figure 9. Structure of (NH4)6[(UO2)3(btec)3]·12H2O (4): (top) 8-fold coordination environments of the two uranyl cations, U1 and U2, defining hexagonal bipyramidal polyhedra and (bottom) view of layer in the a−b plane showing the connection of discrete hexagonal UO8 bipyramids through the pyromellitate linkers.

(44.8%) was in very good agreement with the theoretical value (44.7%) expected for a U3O8 composition. However, the thermodiffraction experiment (Figure S3d of the Supporting Information) showed that phase 4 did not persist upon removal of water because Bragg peaks rapidly disappeared when the sample was heated from 60 °C. An amorphous product was then observed until its recrystallization occurred at 400 °C, with the formation of α′-U3O8, as the final compound up to 800 °C. (UO2)3(OH)2(H2O)2(btec) (5). The structure of 5 is also lamellar and composed of trinuclear uranyl blocks. There are two crystallographically independent sites for uranium (Figure 10). One, located at a special position, 1d, is 8-fold coordinated to two uranyl oxygen atoms [1.974(4) Å for U1−O1U], four carboxyl oxygen atoms [2.490(4)−2.586(3) Å for U1−O7n], and two hydroxo groups [2.319(3) Å for U1−O1H]. The second uranium center is located at general position 2i and is 7fold coordinated to two uranyl oxygen atoms [1.761(4)− 1.776(4) Å for U2−O2nU], three carboxyl oxygen atoms [2.308(4)−2.468(3) Å for U2−O], one hydroxo group [2.318(3) Å for U2−O1H], and one aquo group, in the terminal position [2.456(4) Å for U2−O1W]. The latter has a calculated bond valence of 0.458,10 in good agreement with the expected value for a water species (0.4). The U1-centered

groups differ within the equatorial plane for the U1 and U2 polyhedra. Chelating carboxylates are located in the trans position for U1, whereas they are located in the cis position for U2. This results in the formation of a mixed organic−inorganic layer (Figure 9) with a pavement of a strict alternation of hexagonal bipyramids and pyromellitate ligand in the plane (a− b). The remaining free C−O bonds are alternatively pointing up or down the layer. In the absence of protonated C−O bonds, the sheet is negatively charged, [(UO2)3(btec)3]6−, and compensated by positive ammonium groups, which are intercalated between the layers. Hydrogen bond interactions among three independent ammonium groups and the three terminal C−O bonds are observed [2.809(5) Å for N1···O71b, 2.792(5) Å for N2···O82a, and 2.801(6) Å for N3···O102a] as well as with water species located between the interlayer space. Thermogravimetric analysis (Figure S3c of the Supporting Information) indicated the departure of water molecules from room temperature to 110 °C (observed weight of 11.8%, calcd weight of 11.5%) under an air atmosphere. Then two weight loss events were observed from 200 to 320 °C (observed value of 10.5%) and from 320 to 430 °C (observed value of 31.7%), attributed to the removal of ammonium species and decomposition of the organic linker. The final weight loss 532



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O1H···O72, 2.669(5) Å for O1H···O71, and 2.867(6) Å for O1H···O1W]. Photoluminescence Properties. The fluorescence measurements were performed under excitation at 365 nm at room temperature, on the purely obtained compounds 1, 3, and 4 and phase UO2(H2O)2(btec)·2H2O5b (or A). The latter exhibits a chainlike structure containing 8-fold coordinated uranium centers linked to four chelating carboxyl oxygen atoms, two terminal waters, and two terminal uranyl oxygen atoms. The fluorescence spectra (Figure 11) of 1, 4, and A5b

Figure 10. Structure of (UO2)3(OH)2(H2O)2(btec) (5): (top) trinuclear building block containing a central 8-fold coordinated uranyl cation linked via μ3-oxo (O72) and μ2-hydroxo (O1H) bridges to two 7-fold coordinated uranyl cations, (middle) polyhedral represenation of the trinuclear block with one central hexagonal bipyramid and two pentagonal bipyramids sharing edges, and (bottom) view of a layer in the (110) plane showing the connection of the trimeric units with the pyromellitate ligands.

hexagonal bipyamid is linked to two adjacent U2-centered pentagonal bipyramids via a trans edge-sharing connection mode with one hydroxo group (O1H) and one oxo group (O72, from the carboxylate group). The occurrence of the μ2hydroxo species agrees well with the bond valence calculations (1.195 for O1H).10 It results in the formation of a discrete trinuclear uranyl building block with a U1···U2 distance of 4.0312(2) Å. The existence of such a motif is new with carboxylate ligands because only trimeric bricks5g,6h,13c involving 7-fold coordinated uranyl cations were found in the literature. The trinuclear units are connected to each other through the pyromellitate ligands for generation of infinite neutral layers developed in the (110) plane (Figure 10). The organic molecule acts as a hexadentate linker [μ 6 η1:η1:η2:η1:η1:η1:η2:η1 (Figure 3)]: two of its carboxylate arms (1,4 position in the benzene ring) have a syn-anti bidentate bridging mode between two uranyl cations from pentagonal bipyramids, belonging to two distinct trimers. The two others (2,5 position in the benzene ring) have both the chelate mode with uranyl cation from hexagonal bipyramids and the bidentate bridging mode with two neighboring 7- and 8-fold coordinated uranyl cations. Carboxyl oxygen O72 is then μ3-coordinated between one carbon and two uranium atoms. The mixed organic−inorganic sheets are stacked in a close packing manner, along the [110] direction through hydrogen bond interactions between hydroxo or aquo groups and oxo groups from carboxylates [2.891(6) Å for O81···O1W, 2.572(6) Å for

Figure 11. Solid state emission spectra of compounds (top) 1 (black line), 3 (red line), (bottom) 4, and UO2(H2O)2(btec)·2H2O5b (A, red line) compared to that of the uranyl dinitrate hexahydrate [UO2(NO3)2·6H2O, blue line] at room temperature (excitation wavelength of 365 nm).

consist of the typical well-defined bands, corresponding to the S11 → S00 and S11 → S0v electronic transitions (v = 0−4).15 For 1, the most intense signal is located at 513.4 nm, whereas they are at 505.0 and 504.4 nm for 4 and A,5b respectively, reflecting the expected green fluororescence.16 The band spacings between S11 → S00 and S11 → S00 transitions are 744, 720, and 704 cm−1 for 1, 4, and A,5b respectively (Table S1 of the Supporting Information). The average energy splittings of the S11 → S0v transitions are 850, 818, and 830 cm−1 for 1, 4, and A,5b respectively, and are related to the symmetric vibrations of the OUO double uranyl bond. These values are in good agreement with those reported in the literature.17 It is interesting to note the band shift could be correlated to the different structural assemblies of uranyl for a given 533



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compound 3 with such a tetratopic ligand is quite unexpected because we think that the relatively high number of carboxylate groups together with its possible rotations around the benzene plane might allow metal linkages in many directions, which would induce the formation of rather complicated and compact three-dimensional networks as we observed in compounds 1 and 2. Moreover, the connection via the oxo groups of the equatorial plane of the bipyramidal polyhedra is also favored for uranyl because an oxo bridge, also called cation−cation interaction, from the UO group is rather rare. This connection feature is well illustrated with the isolation of lower-dimension networks such as compounds 4, 5, and UO2(H2O)2(btec)·2H2O,5b as well. Although it was suggested that the formation of a 3D framework could be favored by using flexible aliphatic linkers,2c,5j we observed in compound 3 the existence of a porous network via association of a rigid ligand (btec) together with straight infinite double chains of uranylcentered polyhedra. It results in a unique architecture for hexavalent uranium based on one-dimensional channel systems with a potential free diameter of 8.2 Å × 8.6 Å. Fluorescence spectra of compounds 1, 3, 4, and UO2(H2O)2(btec)·2H2O5b indicate red-shifted emission bands for structures containing 7fold coordinated uranyl centers and blue-shifted emission bands for structures containing 8-fold coordinated uranyl centers, compared to that of uranyl dinitrate hexahydrate.

polycarboxylate ligand, here pyromellitate. The fluorescence spectrum of uranyl nitrate hexahydrate (powdered sample) has been measured for comparison (Figure 10, blue line). We observed that the shift seems to depend on the coordination state of the uranyl centers. The most intense peaks are located at 513.4 and 524.0 nm (red-shifted) for pentagonal bipyramidal environments (for 1 and 3, respectively) and 505.0 and 504.4 nm (blue-shifted) for hexagonal bipyramidal environments (for 4 and A, respectively). Compared to the spectrum of the uranyl dinitrate hexahydrate (maximum at 508.0 nm), the difference between those containing 8-fold coordinated uranyl centers (4 and A) is quite small (Δλ = −3−3.6 nm), whereas it is higher for those containing 7-fold coordinated uranyl centers (Δλ = 5.4 and 16.0 nm for 1 and 3, respectively). The surrounding around the uranyl center in this dinitrate is defined by a hexagonal bipyramid,18 and relatively slight differences in the band positions are observed with those of compounds 4 and A. These variations of positions could be due to the effect of the rigid benzene-containing tetracarboxylate ligand, which may shift the emission intensity of the whole spectrum. Such analyses of red-shifted or blue-shifted fluorescence spectra have been previously reported in a series of uranyl phases associated with aliphatic carboxylates,6i,j for instance, but no clear assignment depending on the uranyl coordination state had been made because of the different natures of ligands attached to the actinide cation. The fluorescence spectrum of phase 3 is composed of a broad signal in the range of 475−600 nm, but without resolution. Only some maximal peaks are visible at 505 and 524 nm together with shoulders at 545 and 570 nm. In comparison with those of 1, 4, or A, the resolution difference of compound 3 may be due to overlap contributions of the electronic transitions coming from the different uranyl centers, which are bonded to each other via oxo or hydroxo groups. The other phases contain only discrete uranyl center polyhedra isolated from each other by pyromellitate ligands. Such an observation has been previously described in a series of uranyl phthalates, for instance.5l

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ASSOCIATED CONTENT

S Supporting Information *

Powder XRD patterns of 1, 3, and 4, crystallographic data for 1−5 (CIF files), IR spectra of 1, 3, and 4, TG curves of 1 and 4, and X-ray thermodiffraction of 1 and 4. This material is available free of charge via the Internet at http://pubs.acs.org.

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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.

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ACKNOWLEDGMENTS We thank the GNR MATINEX of PACEN interdisciplinary program and French ANR Project n° ANR-08-BLAN-0216-01 for financial support. We also thank Pr. Francis Abraham and Dr. Pascal Roussel for helpful discussions and Mrs. Nora Djelal, Laurence Burylo, and Mr. Gérard Cambien for their technical assistance with the SEM images, TG measurements, and powder XRD and BET measurements (UCCS).

CONCLUSIONS This contribution displayed five different structural arrangements of the pyromellitate ligand with the uranyl cation, which are formed in aqueous medium after hydrothermal treatment. It revealed the wide diversity of connecting modes of this particular tetracarboxylate linker,19 offering the possibility of coordination via monodendate, bidendate, or chelating bridging types from four to eight uranyl centers (Figure 3). A previous work describing the room-temperature crystallization of the uranyl pyromellitate UO2(btec)·2H2O5b also reported the formation of a one-dimensional assembly composed of 8-fold coordinated uranyl cations linked through two carboxylate arms of the organic ligand (chelating mode). This example provided another illustration of the manner of connection of this tetracarboxylate. In the reaction pH range of 1−4, uranyl cation is known to undergo different steps of hydrolysis processes, which induce the formation of different oligomeric species.6g,h,20 The occurrence of different inorganic building motifs [monomer (1, 2, and 4), dimer (2), trimer (5), or infinite ribbon (3)] also contributed to the richness of the crystal chemistry for these uranyl−organic assemblies. In fact, a new trinuclear unit with 8-fold coordinated uranium trans-connected to 7-fold coordinated uranium has been observed in compound 5. Nevertheless, the formation of a porous open framework for

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REFERENCES

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UranylâPyromellitate Coordination Polymers: Toward Three

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