Six-Fold Coordinated Uranyl Cations in Extended Coordination

Article pubs.acs.org/crystal

Six-Fold Coordinated Uranyl Cations in Extended Coordination Polymers Ionut Mihalcea, Natacha Henry, Till Bousquet, Christophe Volkringer, and Thierry Loiseau* Contribution from Unité de Catalyse et Chimie du Solide (UCCS), UMR CNRS 8181, Université de Lille Nord de France, ENSCL, Bat C7, BP 90108, 59652 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: Four uranyl-organic framework-type compounds have been hydrothermally synthesized with different ditopic aromatic dicarboxylates. The use of 1,4-benzenedicarboxylate (noted 1,4-bdc), 4,4′-biphenyldicarboxylate (noted 4,4′-bpdc), 4,4′-azobenzenedicarboxylate (noted 4,4′-adc), and 1,3-benzenedicarboxylate (noted 1,3-bdc) gave rise to the crystallization of a series of phases UO2(L) (L = 1,4-bdc (1), 4,4′-bpdc (2), 4,4′-adc (3), or 1,3-bdc (4)), which exhibited 6-fold coordinated uranyl centers as monomeric square bipyramidal unit. The crystal structures showed that the para position of the carboxylate groups favored the formation of layered extended assemblies in 1−3, whereas the meta isomer isophthalate induced the formation of a three-dimensional framework. In situ X-ray diffraction of the thermal behavior indicated the decomposition of the different networks in the range 360−420 °C, successively followed by the crystallization of the uranium oxide U3O8.

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INTRODUCTION For the past decade, the interest of metal−organic coordination polymers has grown exponentially, due to the possibility to generate three-dimensional architectures delimiting cavities and/or channel systems, with direct applications for molecular sorption, for instance.1 The strategy of the association of inorganic motifs with organic linkers was applied to almost all the metallic elements of the periodic table. In this course, the use of uranyl cations was successfully reported for the formation of the so-called uranyl−organic frameworks (UOFs), which exhibited fascinating and diverse assemblies with different network dimensionalities.2 Most of the UOF compounds involved the use of aliphatic3 or aromatic3f,4 polydentate carboxylates, which may also incorporate heterogroups such as nitrogen-donor4l,5 or sulfur-donor6 function. The coordination environment of the hexavalent uranium cation is commonly surrounded by two oxo groups in trans apical positions corresponding to the two short linear uranyl bonds (UO) and four, five, or six oxygen atoms, located in an equatorial plane, resulting in square, pentagonal, or hexagonal bipyramidal geometry, respectively.7 Indeed, these different coordination states are encountered in the UOF-type compounds, but the 7-fold or 8-fold coordination is preferentially observed in various oligomeric motifs (isolated multinuclear uranyl-centered unit or infinite chain-like unit). Surprisingly, only a few of these hybrid organic−inorganic assemblies are built up from the 6-fold coordinated unit for uranyl cations. In this contribution, we show different coordination polymers based on ditopic aromatic carboxylates, derived from the 1,4-benzenedicarboxylate ligand (noted 1,4-bdc). The © 2012 American Chemical Society

latter is a rigid molecule intensively used for the construction of many highly porous metal−organic framework (MOF)-type compounds (see, for instance, MOF-58 or MIL-1019) but also previously reported in an uranyl−organic network.4b Combined with the uranyl cation, four distinct compounds have been isolated by using the hydrothermal synthetic route, with 1,4benzenedicarboxylate, UO2(1,4-bdc) (1), 4,4′-biphenyldicarboxylate (noted 4,4′-bpdc), UO2(1,4-bdc) (2), 4,4′-azobenzenedicarboxylate (noted 4,4′-adc), UO2(4,4′-adc) (3), and 1,3benzenedicarboxylate (noted 1,3-bdc), UO2(1,3-bdc) (4). Contrarily to the other polycarboxylate molecules, the ligand bearing the aryl azo group is not so commonly reported in MOF-like compounds.10 This double nitrogen bonding is at the origin of conformation modifications under optical irradiation with the reversible conversion of the cis−trans isomers in some coordination polymers.11 The common structural feature in the series of compounds 1−4 is the occurrence of the 6-fold coordination for the uranyl centers, involved in layer-like or three-dimensional networks. This article deals with their synthesis, structural characterization, thermal behavior, and fluorescence signature.

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EXPERIMENTAL SECTION

Synthesis. Caution! While uranyl nitrate UO2(NO3)2·6H2O is a radioactive and chemically toxic reactant, precautions with suitable care and protection for handling such substances have been followed. The compounds have been hydrothermally synthesized under autogenous pressure using 23 mL Teflon-lined Parr type autoclaves Received: June 25, 2012 Revised: July 24, 2012 Published: August 16, 2012 4641

dx.doi.org/10.1021/cg300853f | Cryst. Growth Des. 2012, 12, 4641−4648

Crystal Growth & Design

Article

Table 1. Crystal Data and Structure Refinement for Uranyl−Organic Frameworks formula formula weight temperature (K) crystal color crystal size (mm) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z, ρcalcd (g·cm−3) μ (mm−1) Θ range (deg) limiting indices

collected reflections 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

C8H4O6U 434.14 293(2) yellow 0.10 × 0.06 × 0.06 triclinic P1̅ 5.1044(3) 5.5593(3) 8.4574(3) 89.989(3) 85.615(3) 71.995(3) 227.50(2) 1, 3.169 17.841 2.42 - 33.34 −7 ≤ h ≤ 7 −8 ≤ k ≤ 8 −13 ≤ l ≤ 13 22967 1728 [R(int) = 0.0299] 70 1.129 R1 = 0.0160 wR2 = 0.0381 R1 = 0.0162 wR2 = 0.0383 0.923 and −1.910

C14H8O6U 510.23 293(2) yellow 0.25 × 0.19 × 0.06 monoclinic C2/m 9.0105(2) 14.4741(3) 5.3067(1) 90 103.364(1) 90 673.35(2) 2, 2.517 12.077 2.72 − 31.26 −13 ≤ h ≤ 13 −20 ≤ k ≤ 21 −7 ≤ l ≤ 7 11277 1143 [R(int) = 0.0476] 53 1.121 R1 = 0.0219 wR2 = 0.0448 R1 = 0.0231 wR2 = 0.0451 1.400 and −0.657

C14H8N2O6U 538.25 296(2) red 0.07 × 0.07 × 0.04 triclinic P1̅ 5.1366(10) 5.6106(11) 12.944(2) 95.950(10) 90.627(10) 107.037(11) 354.43(12) 1, 2.522 11.483 3.82 - 25.80 −6 ≤ h ≤ 6 −6 ≤ k ≤ 6 −14 ≤ l ≤ 15 8780 1341 [R(int) = 0.0438] 122 1.096 R1 = 0.0187 wR2 = 0.0445 R1 = 0.0187 wR2 = 0.0445 1.134 and −0.953

C8H4O6U 434.14 296(2) yellow 0.23 × 0.19 × 0.18 monoclinic C2/c 6.3566(2) 8.4626(2) 16.7062(4) 90 94.030(1) 90 896.46(4) 4, 3.217 18.111 2.44 - 36.38 −10 ≤ h ≤ 10 −13 ≤ k ≤ 12 −27 ≤ l ≤ 27 18917 2112 [R(int) = 0.0376] 72 1.113 R1 = 0.0145 wR2 = 0.0299 R1 = 0.0237 wR2 = 0.0332 0.631 and −0.857

from a mixture of uranyl nitrate hexahydrate (UO2(NO3)2·6H2O, Merck 98%), isophthalic acid (1,3-benzenedicarboxylic acid or 1,3bdcH2, Aldrich, 99%), terephthalic acid (1,4-benzenedicarboxylic acid or 1,4-bdcH2, Aldrich, 98%), 4,4′-biphenyldicarboxylic acid (4,4′bpdcH2, Aldrich 97%), 4,4′-azobenzenedicarboxylic acid (noted 4,4′adcH2), ammonium hydroxide solution (NH4OH, Prolabo, 28%), potassium hydroxide (Prolabo), and deionized water. The starting chemical reactants are commercially available (except 4,4′-adcH2) and have been used without any further purification. 4,4′-Azobenzenedicarboxylic acid was obtained following the synthetic procedure previously described.12 UO 2 (1,4-bdc) (1). A mixture of 503 mg (1 mmol) of UO2(NO3)2·6H2O, 83 mg (0.5 mmol) of terephthalic acid, 0.8 mL (1.6 mmol) of NH4OH (2 M), and 4.2 mL (233 mmol) of H2O was placed in a Parr bomb and then heated statically at 200 °C for 24 h. The solution pH was 2.6 at the end of the reaction. The resulting yellow product was then filtered off, washed with water, and dried at room temperature. Compound 1 was analyzed by a scanning electron microscope (Hitachi S-3400N) and showed agglomerates of parallelpiped-like crystallites of 10−20 μm size (Supporting Information). This phase was obtained over the pH range from 2.0 up to 3.4 by varying the NH4OH concentration. UO 2(4,4′-bpdc) (2). A mixture of 503 mg (1 mmol) of UO2(NO3)2·6H2O, 121 mg (0.5 mmol) of 4,4′-biphyneldicarboxylic acid, 0.6 mL (1.2 mmol) of NH4OH (2 M), and 4.4 mL (244 mmol) of H2O was placed in a Parr bomb and then heated statically at 200 °C for 24 h. The solution pH was 2.6 at the end of the reaction. The resulting yellow product was then filtered off, washed with water, and dried at room temperature. Compound 2 was analyzed by scanning electron microscope (Hitachi S-3400N) and showed quite large undefined crystals of 50−200 μm size (Supporting Information). This phase was obtained over the pH range from 2.6 up to 3.1 by varying the NH4OH concentration.

UO 2 (4,4′-adc) (3). A mixture of 503 mg (1 mmol) of UO2(NO3)2·6H2O, 139 mg (0.5 mmol) of 4,4′-azobenzenedicarboxylic acid, 0.6 mL (1.2 mmol) of NH4OH (2 M), and 4.4 mL (244 mmol) of H2O was placed in a Parr bomb and then heated statically at 200 °C for 24 h. The solution pH was 2.3 at the end of the reaction. The resulting red product was then filtered off, washed with water, and dried at room temperature. Compound 3 was analyzed by a scanning electron microscope (Hitachi S-3400N) and showed typical needlelike crystals of 5−10 μm size (Supporting Information). This phase was obtained over a wide pH range from 2.3 up to 3.2 by varying the NH4OH concentration. UO 2 (1,3-bdc) (4). A mixture of 503 mg (1 mmol) of UO2(NO3)2·6H2O, 83 mg (0.5 mmol) of isophthalic acid, 1.2 mL (2 mmol) of KOH (1 M), and 3.8 mL (211 mmol) of H2O was placed in a Parr bomb and then heated statically at 110 °C for 24 h. The solution pH was 2.8 at the end of the typical reaction. This phase appeared in the pH range 1.3−3.2 by changing the KOH content. The resulting yellow product was then filtered off, washed with water, and dried at room temperature. It gives crystallites with specific block shape of 200−500 μm as it can be observed by SEM (Supporting Information). For compounds 1−4, XRD powder patterns clearly indicated the high purity of the final product (Supporting Information). Single-Crystal X-ray Diffraction. Crystals of 1−4 were selected under apolarizing 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 X8-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.13 The substantial redundancy in data allowed a semiempirical absorption correction (SADABS V2.1014) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by direct methods, developed by successive difference Fourier 4642



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create files running along the a axis, and containing strict alternation of square bipyramids and bridging carboxylate groups. The latter adopt the syn−anti bidentate linkage mode between two adjacent uranyl centers. This connection generates infinite organic−inorganic layers perpendicular to the direction [011], which are parallel to the square plane of the uranyl-centered bipyramidal moieties (Figure 2). This com-

syntheses, and refined by full-matrix least-squares on all F2 data using SHELX15 program. Hydrogen atoms of the benzene ring were included in calculated positions and allowed to ride on their parent atoms. The crystal data are given in Table 1. Supporting Information is available in CIF format. Thermogravimetric Analysis. The thermogravimetric experiments have been carried out on a thermoanalyzer TGA 92 SETARAM under air atmosphere with a heating rate of 1 °C·min−1 from room temperature up to 800 °C. X-ray thermodiffractometry was performed under 5 L·h−1 air flow in an Anton Paar HTK1200N of a D8 Advance Bruker diffractometer (θ−θ mode, CuKα radiation) equipped with a Vantec1 linear position sensitive detector (PSD). Each powder pattern was recorded in the range 5−60° (2θ) (at intervals of 20 °C between RT 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−1. Infrared Spectroscopy. Infrared spectra of compounds 1−4 (see 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 the powdered compounds 1−4 were measured at room temperature on a SAFAS FLX-Xenius spectrometer between 400 and 800 nm, equipped with a xenon lamp. The fluorescence spectra of uranyl dinitrate hexahydrate, UO2(NO3)2·6H2O, and 4,4′-azobenzenedicarboxylic acid were also presented for comparison.

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RESULTS

Structural Description. UO2(1,4-bdc) (1). Its structure is built up from one crystallographic independent uranium atom (on special position 1b, 0 0 1/2), which is 6-fold coordinated with two uranyl oxygen atoms and four carboxyl oxygen atoms (Figure 1). There are two short UO distances of uranyl bond type with U1−O1u = 1.763(2) Å and four U−O distances of 2. 287(2) and 2.320(2) Å; the latter oxo groups are located in a perpendicular square plane relative to the double uranyl bond, resulting in a square bipyramidal surrounding. The monomeric uranyl centers are connected to each other through carboxylate groups from four distinct terephthalate anions, in order to

Figure 2. (top) View of one layer of the structure of 1 along the direction [111], showing the connection of the terephthalate ligands with the uranyl-centered square bipyramidal polyhedra. (bottom) View of the stacking of uranyl−organic layers the along the b axis.

pound is the second member of the uranyl−terephthalate series. The previous phase, (NH4)UO2(1,4-bdc)1.5·2.5H2O,4b was obtained from the slow hydrolysis of 1,4-cyanobenzene molecule, which led to the formation of the parent dicarboxylate linker. It gave rise to the formation of layered interpenetrated network, with monomeric 8-fold coordinated uranyl centers occupying the nodes of hexagonal rings. UO 2(4,4′-bpdc) (2). It contains one crystallographic independent uranyl center lying on the special position 2b (0 1/2 0). It is 6-fold coordinated to two oxygen atoms involved in the double uranyl bond with the typical short distances U1− O1U = 1.748(4) Å, forming an angle of 180° for O1UU1 O1U (Figure 1). Four other carboxyl oxygen atoms are located in a square equatorial plane, perpendicularly to the double bond uranyl axis. The U−O distances are equal to 2.287(3) Å. This resulting monomeric square bipyramidal unit is connected to two neighboring uranyl-centered units through carboxylate arms from four 4,4′-biphenyldicarboxylate ligands. Each carboxylate function has a bidentate mode bridging two uranyl cations to each other, with a syn−anti fashion, in order to generate files of uranyl polyhedra alternated by carboxylates groups along the c axis (Figure 3). A distance of 5.307(1) Å separates the uranium atoms to each other. A layered neutral network is then generated by the connection of the isolated uranyl centers via the two opposite carboxylate groups of the straight organic molecule, which develop an organic−inorganic sheet in the plane (b,c). The square planes of the uranyl

Figure 1. Views of the 6-fold coordination of uranyl cation defining a square bipyramidal environment in UO2(1,4-bdc) (1, a), UO2(4,4′bpdc) (2, b), UO2(4,4′-adc) (3, c), and UO2(1,3-bdc) (4, d). Yellow circle, uranium; red circle, oxygen; yellow bond, uranyl bond, UO. 4643



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Figure 3. (top) View of one layer of the structure of 2 along the a axis, showing the connection of the biphenyldicarboxylate ligands with the uranyl-centered square bipyramidal polyhedra. (bottom) View of the stacking of uranyl−organic layers along the a axis.

bipyramids are strictly parallel to these sheets and induce the rotation of ∼32° of the aromatic rings of the organic linker. Each layer is then stacked to each other along the a axis, with a b/2 shift. UO2(4,4′-adc) (3). This phase consists of one crystallographically unique uranyl cation linked to each other through the azobenzenedicarboxylate ligand. The uranium center is located on the special position 1 h (1/2 1/2 1/2) and 6-fold coordinated to two oxo groups with short distances UO of 1.752(3) Å, as expected for the uranyl bond and four carboxyl oxygen atoms, engaged in a equatorial square plane, with longer distances U−O of 2.282(3) and 2.330(3) Å (Figure 1). The monomeric square bipyramidal polyhedra are isolated to each other by the organic molecules, which adopt a tetradentate connection mode with the uranyl cations. Each carboxylate arm has syn−anti bidentate fashion, bridging two adjacent uranyl centers. It generates files of square bypiramids along the a axis (Figure 4), linked to each other through the ditopic carboxylate molecule, in order to form a neutral layered network in the (a,c) plane. The square planes of the uranyl-centered polyhedral units are titled with an angle of ∼18.7° along the c axis, closely related to the situation occurring in UO2(1,4-bdc) (1) (angle of ∼11.8°), while they are strictly aligned in UO2(4,4′-bpdc) (2). The organic−inorganic layers are stacked along the b axis, with a perfect alignment of the square bipyramids, as for UO2(1,4-bdc) (1). UO2(1,3-bdc) (4). Its structure is built up from the connection of 6-fold coordinated uranyl cations with the isophthalate ligands (Figure 5). The unique independent crystallographical uranium atom lies on the inversion center (4a, 0 0 0) and is linked to two terminal oxygens with short

Figure 4. (top) View of one layer of the structure of 3 along the b axis, showing the connection of the azobenzenedicarboxylate ligands with the uranyl-centered square bipyramidal polyhedra. (bottom) View of the stacking of uranyl−organic layers along the b axis.

Figure 5. View of the three-dimensional structure of uranyl isophthalate UO2(1,3-bdc) (4) in the 110 plane.

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and crystallizing in the presence of hydrazine. In the first case, the uranyl is 8-fold coordinated as a monomeric unit,3f whereas it is found in 7-fold coordination in an octameric building block in the second uranyl isophthalate.4j

UO distances, which are related to the typical uranyl bonds (U1−O1U = 1.759(2) Å) forming a U1−O1−U1 angle of 180°. It is additionally bonded to four carboxyl oxygen atoms in the equatorial plane with U−O distances ranging from 2.290(2) to 2.330(1) Å (Figure 1), resulting in a square bipyramidal environment. The monomeric uranyl-centered polyhedral units are linked to each other through the carboxylate groups of the isophthalate molecules. One of the two carboxylate arms acts as a bidentate bridge between two uranyl cations with a syn−anti fashion. This generates infinite ribbons running along the [110] direction, with pairs of symmetrical carboxylate groups in the equatorial plane of the uranium atoms (Figure 5), which are separated from each other with a distance of 5.292(1) Å. The second carboxylate arm of the isophthalate ensures the connection with two other uranyl cations with a syn−syn bidentate bridging mode, which forms a second mixed carboxylate−uranium ribbon running along the [11̅ 0] direction. Dense layers of uranyl cations are developed along the (a,b) plane and intercalated by the isopththalate species. It results in an extended neutral three-dimensional network with flat rings delimited by six uranyl centers and four isophthalate linkers (Figure 6). These rings are densely packed along [1̅10],

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DISCUSSION Six-fold coordinated uranyl cation is rarely reported in UOFtype coordination complexes involving carboxylate ligands since pentagonal or hexagonal bipyramidal environments (implying two typical trans short axial UO bondings) are the most commonly encountered case. Nevertheless, this state of coordination is well-defined in the crystal chemistry of purely inorganic oxide-based solids bearing hexavalent uranium and reported several times in dense phases, over a wide range of U− O distances from 1.74 up to 2.34 Å.16 In fact, three distinct types of geometry around the uranium center have been established for the 6-fold coordination. For some of them, uranyl is trans-bonded to two oxo groups with short U−O distance of ∼1.79 Å; the four remaining oxo groups are located in a square equatorial plane and form a square bipyramidal polyhedron (U−O ≈ 2.28 Å). At the opposite, all the U−O bond distances are almost equal (2.0−2.1 Å), closely related to the ideal octahedral geometry. An intermediate state of coordination is also observed with two groups of U−O bond distances centered at around 1.88 and 2.20 Å, respectively. In our different coordination complexes, only the first case related to the square bipyramidal geometry is found. It is interesting to notice that such a surrounding was previously reported in very few complexes. A first illustration is found in the uranyl monocarboxylate UO2(bc)2 (bc = benzenecarboxylate) described by Cousson et al.17 in 1990 and exhibits a chain-like assembly. A second example is reported in a uranyl dicarboxylate6a involving the 3,3′-dithiobisbenzoate group (UO2(3,3′-dtba)4), with a 1D network. In this phase, the ligand is closely related to the 4,4′-azobenzenedicarboxylate used for the formation of 3, with the double nitrogen bonding NN replaced by the sulfur bonding S−S. The positions of the carboylate arms (4,4′- and 3,3′-) also differ and induce a Vshape conformation for the dithio-based ligand. The latter generates a monodimensional structure instead of a bidimensional one for 3. In other complexes, this type of square bipyramidal moiety is encountered in heterometallic assemblies with copper18 (UO2Cu(pdc)2(H2O)2) involving the 3,5pyrazoledicarboxylate (pdc) linker (2D net) or lanthanide ((UO2)2Ln(OH)(H2O)3(mel)·2.5H2O; Ln = Ce or Nd)19 with the mellitate (mel) linker (3D net). The last example is related to uranyl cations interacting with a dinitro derived 4,4′biphenyldicarboxylate (btnca) ligand in the complex (UO2)2U(OH)4(H2O)(btnca)3·5H2O.20 The positions of nitro groups attached to each benzene ring of the btnca molecule induce a torsion of the organic ligands, which resulted in the formation of a three-dimensional framework instead of a layered network for the phase 2 (involving the nonfunctionalized 4,4′biphenyldicarboxylate). This structure is quite interesting and unique since it consists of the intricate association of three distinct monomeric uranly centered building motifs. One uranium atom is 7-fold coordinated with the classical pentagonal bipyramidal environment; the second one is 6fold coordinated with a square bipyramidal environment, identical to that observed in the phases 1−4. The third uranyl cation exhibits an uncommon coordination state with four terminal hydroxyl oxygen atoms in a square plane but with unusual short values of U−O distances ranging from 1.92(3) up

Figure 6. Perspective views of the structure of UO2(1,3-bdc) (4) showing the connection mode of the isophthalate ligands with the isolated uranium-centered square bipyramids.

without any void, as indicated by the shortest distances C···C of 3.487(2) Å and terminal O1U···O1U of 3.074(2) Å. Two other UOF-type compounds were previously described with this organic ligand. They concerned the 2D uranyl isophthalate (UO2)1.5(1,3-bdc)2·DMF·H2O,3f obtained in N,N-dimethylformamide (DMF) solvent and the phase ((UO 2 ) 4 O(OH)2(H2O)2(1,3-bdc)2·2H2O4j), exhibiting a 3D framework 4645



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was 63.7% (calcd: 64.6%); for UO2(4,4′-bpdc) (2), the remaining weight value was 55.1% (calcd: 55.0%); for UO2(4,4′-adc) (3), the remaining weight value was 51.7% (calcd: 52.2%); for UO2(1,3-bdc) (4), the remaining weight value was 64.3%, (calcd: 64.5%). The in situ evolution of the XRD powder patterns as a function of temperature was also identical for the four compounds. The Bragg peaks of the different phases were visible up to 360 °C for 2, 380 °C for 1 and 3, and 420 °C for 4, indicating their relative good thermal stability. Above these temperatures, the structural collapse was then observed, and this phenomenon was immediately followed by the crystallization of the uranium oxide U3O8 (pdf file 74-562). This structural transformation differed from those of other uranyl polycarboxylates, which have been previously characterized by XRD thermodiffraction. For instance, the uranyl phthalates4i (see Supporting Information), pyromellitates,4k or mellitates19 were decomposed at lower temperatures or sometimes transformed into a transient phase, followed by the occurrence of amorphous phase, which then crystallized in uranium oxide U3O8 at temperature close to 400 °C. For the present study, it seems that the uranyl−organic networks based on 6-fold coordinated uranyl centers were quite thermally stable compared to the others, for which one of their structural features was the existence of 7- or 8-fold coordinated uranyl centers. This would be due to the nature of additional coordinating ligands around the uranyl (hydroxo or aquo groups), which may undergo dehydroxylation or dehydration transformations upon heating, which results in the formation of intermediate or structural decomposition before 400 °C. For the phases 1−4, the temperature of decomposition of the organic part matched well with that of the crystallization of U3O8. Photoluminescence Properties. The fluorescence measurements of compounds 1−4 were performed under irradiation at 365 nm at room temperature. The fluorescence spectra (Figure 8) of 1 and 4 consisted of the typical well-defined bands, corresponding to the electronic transitions S11 → S00 and S11 → S0v (v = 0−4), respectively. For 1, the most intense signal was located at 520.6 nm, whereas it was at 500.4 nm for 4, indicative of the expected green fluorescence. In comparison with the fluorescence spectrum of uranyl nitrate hexahydrate (max at 508.0 nm), the spectrum of 1 is red-shifted, whereas that of 4 is blue-shifted. The average energy splittings of the S11 → S0v transitions were 847 and 816 cm−1 for 1 and 4, respectively, and are related to the symmetric vibrations of the double uranyl bond OUO. These values are in good agreement with those reported in literature.5k,23 The fluorescence spectra of phases 3 and 4 were much more complex (Figure 8). For 3, it was composed of broad signals with maximum at 491.6 and 525.6 nm but without resolution. For 4, the expected signal coming from the vibronic character of the uranyl moiety was not observed and was hidden by the emission spectrum of the organic linker containing aza function in the region 480−540 nm.

to 1.99(3) Å. Four additional carboxyl oxygen atoms are located upward and downward the equatorial plane and complete the coordination sphere of this uranyl, with longer U−O distances in the range 2.42(1)−2.54(1) Å. This specific type of geometrical configuration is referred to as the tetraoxido core motif,21 which was very rarely observed for uranium(VI) in inorganic compounds. In these different coordination complexes, it is noteworthy to observe the 6-fold coordinated uranyl cations occurring as monomeric building unit only. The second feature is the implication of aromatic ditopic carboxylate molecules as linker between the uranyl centers (except for the phase based on the hexadentate mellitate19 molecule). To our knowledge, this type of coordination was not found in complexes with aliphatic carboxylates. Nevertheless, 6-fold coordinated uranyl species was found in a larger oligomer containing six metallic centers, surrounded by other types of O-donor ligands such as the pbenzylcalix[7]arene (H 7 L) in (UO 2 ) 6 (L) 2O 2 (Hdabco)6 ·solvent.22 Thermal Behavior. The four uranyl dicarboxylates 1−4 have been studied by thermogravimetric and X-ray thermodiffraction analyses. They exhibited a similar thermal behavior upon heating and one illustration, related to the decomposition of UO2(1,4-bdc) (1), is given in Figure 7. All the thermogravimetric curves indicated a unique weight loss event, occurring between 350 and 450 °C and related to the removal of the organic linker. Expected remaining weight losses are observed and were in good agreement with the calculated one based on the stoichiometric chemical formula 1/ 3(U3O8). For UO2(1,4-bdc) (1), the remaining weight value

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CONCLUSIONS This contribution presented the hydrothermal synthesis and structural characterization of four uranyl−organic frameworktype compounds containing different ditopic aromatic dicarboxylates, such as 1,4-benzenedicarboxylate, 4,4′-biphenyldicarboxylate, 4,4′-azobenzenedicarboxylate, and 1,3-benzenedicarboxylate. These four distinct phases UO2(L) (L = 1,4-bdc

Figure 7. (top) Thermogravimetric curve of the compounds UO2(1,4bdc) (1) (under air, heating rate 1 °C/min). (bottom) X-ray thermodiffraction patterns as a function of temperature of 1 (copper radiation) under air atmosphere. 4646



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ASSOCIATED CONTENT

S Supporting Information *

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

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AUTHOR INFORMATION

Corresponding Author

*Phone: (33) 320 434 434. Fax: (33) 320 434 895. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the GNR MATINEX of PACEN interdisciplinary program and the French ANR project no. ANR-08-BLAN-0216-01 for financial support. We also would like to thank Pr. Francis Abraham for helpful discussions and Mrs. Nora Djelal and Laurence Burylo for their technical assistances with the SEM images, TG measurements, and powder XRD (UCCS). The ″Fonds Européen de Développement Régional (FEDER)″, ″CNRS″, ″Région Nord Pas-deCalais″, and ″Ministère de l’Education Nationale de l’Enseignement Supérieur et de la Recherche″ are acknowledged for funding of X-ray diffractometers.

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Figure 8. (top) Solid-state fluorescence emission spectra of compounds UO2(1,4-bdc) (1) and UO2(1,3-bdc) (4); (middle) UO2(4,4′-bpdc) (2) (uranyl nitrate hexahydrate (blue line) is given for comparison); and (bottom) UO2(4,4′-adc) (3) and the ligand 4,4′azobenzenedicarboxylic acid (4,4′-adcH2) under excitation wavelength 365 nm at room temperature.

(1), 4,4′-bpdc (2), 4,4′-adc (3), and 1,3-bdc (4)) exhibit 6-fold coordinated uranyl centers as a monomeric square bipyramidal unit, and this specific environment is not so common for hexavalent uranium cation in UOF-type solids. The crystal structures showed that the para position of the carboxylate groups favored the formation of the layered extended assemblies in 1−3, whereas the meta isomer isophthalate induced the formation of a three-dimensional framework. In situ X-ray diffraction of the thermal behavior indicated the decomposition of the different networks in the range 360−420 °C, successively followed by the crystallization of uranium oxide U3O8. 4647



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Six-Fold Coordinated Uranyl Cations in Extended Coordination

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