Mixed Formate-Dicarboxylate Coordination Polymers with Tetravalent

Article pubs.acs.org/crystal

Mixed Formate-Dicarboxylate Coordination Polymers with Tetravalent Uranium: Occurrence of Tetranuclear {U4O4} and Hexanuclear {U6O4(OH)4} Motifs Clément Falaise, 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: Two new three-dimensional coordination networks with tetravalent uranium have been solvothermally synthesized by associating UCl4 and a combination of two carboxylate-based ligands in N,N-dimethylformamide (DMF). The mixture of terephthalate (bdc) and formate (form) resulted in the formation MOF-type structure (Hdma)[U4O2(bdc)3(form)7] based on the tetranuclear uranium-centered motif {U4O4}. This inorganic building block is relatively rare in the actinides chemistry and generates here a negatively charged framework trapping dimethylammonium cations resulting from the in situ decomposition of DMF. The association of formate (form) and fumarate (fum) led to the isolation of the hexameric building block {U6O4(OH)4} in a three-dimensional framework (U6O4(OH)4(fum)5(form)2(H2O)2·3DMF) closely related to the topology observed with single ditopic ligand in the UiO-66 series. The porous solid contained two types of cages with estimated free pore diameters of 6.3 × 6.7 Å and 4.7 Å.

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carboxylates.7 This strategy was based on the use of solvothermal synthesis in organic solvent (typically, N,Ndimethylformamide noted DMF) under anaerobic conditions and with a controlled amount of water. These last points are crucial to control the hydrolysis rate and the condensation of U4+ cations without any oxidation reaction. Until now, we isolated a trinuclear building unit μ3-OU3 within a uranium trimesate7a and a hexanuclear motif U6O8 though an isoreticular family of solids constructed from dicarboxylic connectors (4,4′biphenyldicarboxylate, 2,6-naphthalenedicarboxylate, terephthalate, and fumarate).7b Herein, we continued our systematic study by combining tetravalent uranium and a mixture of two carboxylate-type ligands. The association of terephthalate (noted bdc) and formate (noted form) ligands gave rise to the formation of a solid (Hdma)[U4O2(bdc)3(form)7] (Hdma = dimethylammonium) (1) involving the tetranuclear building unit U4O4, whereas the mixture fumarate-formate (fum-form) led to the stabilization of the U6O4(OH)4 inorganic brick in U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2). Their synthesis, structural characterization, IR and UV/vis spectroscopies as well as thermal analysis are described in this paper.

INTRODUCTION For the past decade, the chemistry of uranium carboxylates has been intensively explored, leading to the production of fascinating architectures with new crystal chemistry.1 This enthusiasm could find its origin from the development of coordination polymers chemistry (including the subclass of metal−organic framework) initiated in the late 90’s, generating beautiful three-dimensional structures with many promising applications.2 However, the careful examination of the whole crystalline uranium-based carboxylate solids shows that some aspects are clearly underexploited. Indeed, the majority of crystal structures concerns the use of the uranyl ion (UO22+),3 and only a very small amount of complexes incorporates the lower oxidation states U3+, U4+, and U5+. Their extreme sensitivity to oxidation limits their stabilization under air atmosphere and the production on new carboxylate-based complexes. The propensity of U5+ to disproportionate into U4+ and U6+ explains the absence of any pure pentavalent uraniumbased carboxylate in the database. Besides a rich organometallic chemistry, we listed only one crystalline compound bearing trivalent uranium stabilized with a carboxylate ligand (formate).4 Among these low oxidation states, tetravalent uranium seems to be the most promising for a reaction with carboxylic acids in order to form coordination network solids. Actually, a handful of molecular complexes combining this class of ligands and U4+ have been already synthesized and presents a wide range of inorganic polyoxo clusters from monomeric unit to large motifs containing up to 38 uranium centers5 or infinite chainlike coordination polymers.6 Our group recently developed the production of threedimensional open-frameworks containing tetravalent uranium © 2013 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. Caution! Uranyl chloride (UCl4) is a radioactive and chemically toxic reactant, so precautions with suitable care and protection for handling such substances have been followed.

Received: April 27, 2013 Revised: June 4, 2013 Published: June 19, 2013 3225

dx.doi.org/10.1021/cg400643g | Cryst. Growth Des. 2013, 13, 3225−3231

Crystal Growth & Design

Article

The compounds have been solvothermally synthesized under autogenous pressure using 23 mL Teflon-lined Parr type autoclaves (type 4746) by using the following chemical reactants: uranyl chloride (UCl4, obtained from the protocol using the reaction of hexachloropropene with uranium oxide, described in ref 7a), terephthalic acid (C8H6O4, 1,4-H2bdc, Aldrich, 98%), fumaric acid (C4H4O4, H2fum, Aldrich, 99%), formic acid (noted Hform, Aldrich, ≥95%), and anhydrous N,N-dimethylformamide (C3H7NO, DMF, Aldrich, 99.8%). The starting chemical reactants (except UCl4) were commercially available and have been used without any further purification. The reactant mixtures concerning the syntheses using UCl4 have been manipulated and weighted in a glovebox under argon atmosphere. They have been then placed in closed autoclaves, which are removed from the glovebox and then heated in an oven (under ambient atmosphere). (Hdma)[U4O2(bdc)3(form)7] (1): a mixture of 100 mg (0.26 mmol) of UCl4, 45 mg (0.27 mmol) of terephthalic acid, 1.5 mL (39 mmol) of formic acid, 30 μL (1.67 mmol) of water, and 4 mL (51 mmol) of N,N-dimethylformamide was placed in a Parr autoclave and then heated at 130 °C for 3 days. The resulting green powder was then filtered off, washed with DMF, and dried at room temperature in air atmosphere. The product is stable in air for several weeks. 1 was analyzed by scanning electron microscope (Hitachi S-3400N), and it shows agglomerates of platelike crystals (Figure S1). Yield: 57% (based on uranium). Chemical analysis: C: obs.: 21.08%, calc.: 20.52%; N: obs.: 0.74%, calc.: 0.77%; H: obs.: 1.44%, calc.: 1.49%). U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2): a mixture of 100 mg (0.26 mmol) of UCl4, 30 mg (0.27 mmol) of fumaric acid, 1.5 mL (39 mmol) of formic acid, 30 μL (1.67 mmol) of water and 4 mL (51 mmol) of N,N-dimethylformamide was placed in a Parr autoclave and then heated at 150 °C for 1 day. The resulting black powder was then filtered off, washed with DMF, and dried at room temperature in air atmosphere. The final product is stable in air for several weeks. 2 was analyzed by scanning electron microscope (Hitachi S-3400N), and it shows a mixture of spherical particles and small crystals with octahedral shape (Figure S2). Yield: 64% (based on uranium). Chemical analysis: C: obs.: 15.05%, calc.: 15.06%; N: obs.: 1.53%, calc.: 1.70%; H: obs.: 1.81%, calc.: 1.46%). Single-Crystal X-ray Diffraction. Crystals of compounds 1 and 2 were selected under 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 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.8 The substantial redundancy in data allowed a semiempirical absorption correction (SADABS V2.109) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least-squares on all F2 data using SHELX10 program suite through the WINGX11 interface. Hydrogen atoms of the benzene ring were included in calculated positions and allowed to ride on their parent atoms. The final refinements include anisotropic thermal parameters of all nonhydrogen atoms. The crystal data are given in Table 1. Supporting Information is available in CIF format. CCDC file numbers 929465 and 929466. Thermogravimetric Analysis. The thermogravimetric experiments have been carried out on a thermoanalyzer TGA 92 SETARAM under air atmosphere with a heating rate of 5 °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, Cu Kα radiation) equipped with a Vantec1 linear position sensitive detector (PSD). Each powder pattern was recorded in the range 6−60° (2θ) (at intervals of 20 °C between RT and 600 °C) with a 1 s/step scan, corresponding to an approximate duration of 27 min. The temperature ramps between two patterns were 5 °C·min−1. Infrared Spectroscopy. Infrared spectra of powdered compounds 1 and 2 (see Supporting Information) were measured on Perkin-Elmer

Table 1. Crystal Data and Structure Refinements of Uranium Terephthalate-Formate (1) and Uranium Fumarate-Formate (2) 1 formula formula weight temperature/K crystal type crystal size/mm crystal system space group a/Å b/Å c/Å α/° β/° γ° volume/Å3 Z, ρcalculated/g·cm−3 μ/mm−1 Θ range/° 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

C33H27NO28U4 1837.68 293(2) green block 0.12 × 0.08 × 0.02 monoclinic C2/c 17.8701(5) 13.8872(4) 18.4391(5) 90 92.262(2) 90 4572.4(2) 4, 2.670 14.218 1.86−36.40 −29 ≤ h ≤ 29 −23 ≤ k ≤ 16 −30 ≤ l ≤ 30 65768 11125 [R(int) = 0.0470] 291 1.000 R1 = 0.0292 wR2 = 0.0507 R1 = 0.0499 wR2 = 0.0560 2.46 and −1.16

2 C13H5O17U3 1147.26 293(2) black block 0.13 × 0.162 × 0.08 orthorhombic Pnnm 10.6636(3) 13.3253(3) 20.0859(5) 90 90 90 2854.12(12) 4, 2.670 17.043 1.83−29.61 −14 ≤ h ≤ 13 −18 ≤ k ≤ 18 −27 ≤ l ≤ 27 34664 4129 [R(int) = 0.0389] 180 1.252 R1 = 0.0251 wR2 = 0.0956 R1 = 0.0342 wR2 = 0.1118 3.35 and −3.01

Spectrum Two spectrometer between 4000 and 400 cm−1, equipped with a diamond attenuated total reflectance (ATR) accessory. UV/Visible Spectroscopy. UV/vis spectra of powdered compounds 1 and 2 have been collected by using a Perkin-Elmer Lambda 650 spectrophotometer equipped with a powder sample holder set.

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RESULTS Structure Description. The structure of (Hdma)[U4O2(bdc)3(form)7] (1) is built up from a tetranuclear core U4O4, containing two independent crystallographic uranium cations, lying around an inversion symmetry center (Figure 1). Uranium atom U1 is 9-fold coordinated by one oxo group and eight carboxyl oxygen atoms (tricapped trigonal prism), whereas uranium atom U2 is 8-fold coordinated by two oxo groups and six carboxyl oxygen atoms (bicapped trigonal prism). The uranium centers are linked to each other through the oxo bridges OU1 and carboxyl bridge O11C (from formate group), which adopt a connection mode μ3. U1 is linked to two U2 via μ3-OU1 and μ3-O11C groups, the latter bridging two uranium centers and one carbon atom from formate species. U2 is linked to two U1 via μ3-OU1 groups (as previously mentioned) and to second neighboring U2 via two via μ3OU1 groups. The μ3-OU1−U distances are rather short for two of them (2.161(2), 2.188(2) Å) and is 2.403(2) Å for the third one. Bond valence calculations12 give the value of 2.08 for this μ3-O moiety, confirming its assignment as oxo group. The μ3O11C−U distances are 2.452(2) and 2.484(3) Å. These two 3226



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Figure 1. Coordination environments of the two independent crystallograhical sites of uranium U1 (9-fold coordinated, tricapped trigonal prismatic geometry) and U2 (8-fold coordinated, bicapped trigonal prismatic geometry) in (Hdma)[U4O2(bdc)3(form)7] (1). Dotted lines correspond to the trigonal prism.

types of μ3-O groups are located in a plane in order to form a planar tetranuclear motif (Figure 2). The U1−U2 and U2−U2

Figure 3. View of the structure of (Hdma)[U4O2(bdc)3(form)7] (1) along the [110] (top) and [101] (bottom) directions, showing the connection of the tetrameric blocks via terephthalate ligands along the [1̅10] and [010] directions and the formate groups along the [001] direction. Blue circles indicate the nitrogen of the protonated dimethylamine.

connected to each other through the bidentate formate ligands, which ensure the three-dimensionality of the framework of 1. However, the network is negatively charged [U4O2(bdc)3(form)7]−, and its charge is compensated by the protonated dimethylamine located in small cavities. This monoamine molecules comes from the partial chemical decomposition of N,N-dimethylformamide solvent, which occurs under hydrothermal conditions. Previous contributions reported such a chemical behavior in the literature.13 The ammonium groups interact via hydrogen bond with some oxygens of the formate groups (N1G···O11E = 3.011(4) Å). Such a tetranuclear uranium-centered core has been previously reported in the literature as molecular assembly with other carboxylate molecules such as acetate5h or dimethylcarbamate.14 Similar configurations of the μ3-oxo groups bridging the uranium cations have been found in these tetrameric building units. The compound 2, (U6O4(OH)4(fum)5(form)2(H2O)2·3DMF), exhibits a structure based on the connection of hexanuclear building units with fumarate and formate ligands. The hexameric block is closely related to that observed in different tetravalent uranium complexes and corresponds to an octahedral configuration of uranium centers. Indeed, this motif is reported with formate,5m benzoate,5l or triflate15 ligands. More recently, we described a series of uranium(IV) dicarboxylates with the UiO-66/67 structural type,7b containing this hexamer as secondary building unit. In compound 2, three crystallographically independent uranium sites exist (Figure 4), which are located on the nodes of the octahedral polyhedron.

Figure 2. Polyhedral representation of the tetranuclear uraniumcentered motif {U4O4} in (Hdma)[U4O2(bdc)3(form)7] (1). Labels of uranium centers as well as hydrogen atoms of the formate groups are indicated. Other carboxylate arms belong to terephthalate molecules.

distances are 3.88678(17) and 3.7102(2) Å, respectively, preventing any U−U bonds. The other U−O distances from carboxylate arms are in the range 2.332(2)−2.724(3) Å and correspond to 6 terephthalate and 10 formate ligands, defining three types of connection fashions with the uranium centers in the tetrameric unit. The carboxylate groups of the terephthalate act as bidentate bridge (syn-syn mode) between two distinct adjacent uranium atoms U1 and U2. The two formate groups having the tricoordinate carboxyl oxygen are chelating the U1 atoms. Two other formate groups also adopt a bidentate bridging (syn-syn) mode with two distinct adjacent uranium atoms U1 and U2. The six remaining formate species have a bidentate bridging (anti-anti) mode between uranium atoms belonging to two distinct tetrameric blocks. These building units are linked to each other through the terephthalate ligands in order to generate layers in the (a,b) plane, with connections occurring along the [1̅10] and [010] directions, successively (Figure 3). These hybrid organic−inorganic sheets are 3227



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OU3) and in agreement with a μ3-oxo from bond valence calculations (2.09 and 2.236, respectively).16 The second one is in the range 2.390(11)−2.454(10) Å and in agreement with a μ3-hydroxo from bond valence calculations (1.24 and 1.22, respectively). The disordering of the μ3-anionic species is a recurrent situation for the {M6O4(OH)4} hexanuclear motif, due to the observation of relatively high crystal symmetry, which does not match to that of the local positions of the O/ OH species. The corresponding O/OH groups are differentiated by distinct U−O bond lengths. In the case of higher symmetry (cubic), an average distance of 2.32 Å is reported for such U-(μ3-O) distances,7b,17 but different groups of U-(μ3OH) (2.40−2.50 Å)5l,m and U-(μ3-O) (2.20−2.27 Å)5l,m,15 bond distances are described in other structural arrangements built up from the similar hexanuclear moiety. This type of hexameric core was previously described with other tetravalent actinides such as thorium5m,18 or plutonium19 in crystalline structures and in aqueous solution20 or neptunium in aqueous solution.21 Indeed, the occurrence of two distinct uranium environments (8- and 9-fold) within the hexameric unit is a new illustration of the flexibility of coordination for the actinide centers. It is also directly correlated to the connection mode of the {M6O4(OH)4} blocks to each other through either the fumarate or formate ligands. The ditopic fumarate molecules act as bidentate (syn-syn mode) bridging fashion between two adjacent uranium of one given hexamer and ensure the linkage between them through the second carboxylate arm. A total of 10 fumarate molecules surrounds the hexanuclear unit, which is linked to each to other through connection along the b and c axes (Figure 6). Along the third direction (a axis), the hexamers are linked via two groups of formate species, which act as bidentate bridge (anti-anti mode). The carbon atom of the latter organic linker is disordered on two equivalent positions. In fact, the connection mode of the hexamer with the two types of ligands is closely related to that occurring in the pure uranium fumarate (U6O4(OH)4(fum)6(H2O)6·5DMF),7b except that one of the fumarate linkers is replaced by two formate bridging groups (Figure 5). The presence of the formate groups induces a change of coordination since all the uranium atoms are 9-fold coordinated in the pure fumarate compound. The uranium atoms attached to the formate ligand, are 8-fold coordinated; the organic species is not bridging two uranium centers within a given hexamer but ensures the connection of adjacent hexamers to each other. The resulting structural arrangement of the hexameric blocks is similar to that observed in the pure uranium fumarate7b and corresponds to the distorted face-centered cubic lattice (considering the hexamers as nodes). This connection mode generates two types of cavities, in the same manner as described in the dense rock salt (NaCl) or blende (ZnS) phases with fcc close packing. The first void has a distorted octahedral geometry, whereas the second one has a tetrahedral geometry (Figure 7), with estimated free pore diameters of 6.3 × 6.7 Å and 4.7 Å, respectively. This three-dimensional framework was reported in the UiO-66 series incorporating zirconium as tetravalent cation with either terephthalate or fumarate molecule.22 Even if any molecules were localized within the pores by Xray diffraction, IR spectroscopy (Figure S10) shows the presence of free DMF molecules (v(C−O) at 1655 cm−1) that we supposed present in both cavities. The combination of thermogravimetric and elemental analyses indicates a number

Figure 4. Asymmetric unit in U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2) representing the three crystallographically inequivalent uranium centers (U1, U2, and U3), linked to each other via μ3-oxo (OU1, OU3) or μ3-hydroxo groups (OU2, OU4). The latter oxygen atoms are disordered on two positions (OU1/O U2 and OU3/OU4) as indicated by U−O bonding with dotted line. Cyan circle indicates the terminal water molecule (OW1).

Two of them (U2 and U3) are 8-fold coordinated with four tricoordinated oxygen atoms, three carboxyl oxygen atoms from fumarate molecules and one carboxyl oxygen atom from the formate group and defined by a square antiprismatic environment (Figure 5). The third uranium atom (U1) is 9-fold

Figure 5. Representation of the hexanuclear building unit {U6O4(OH)4} in U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2). The μ3-oxygen atoms (OUn, bridging the three uranium centers) disorder is not shown for clarity. U1 is 9-fold coordinated (capped square antiprismatic geometry) and have one terminal water molecule (cyan circle); U2 and U3 are 8-fold coordinated (square antiprismatic geometry), and one of the carboxylate linker corresponds to the formate ligand, for which the carbon atom is located on two equivalent positions (dotted line).

coordinated by four tricoordinated oxygen atoms and four carboxyl oxygen atoms from fumarate molecules. A ninth oxygen atom corresponding to terminal water molecule completes the coordination sphere (U1−OW1 = 2.591(10) Å) and is located above one face of the square anti prismatic polyhedron around the uranium U1, resulting in a monocapped square antiprism geometry (Figure 4). The oxygen atoms from the fumarate and formate linkers are well-defined and U−O bond distances are observed in the range 2.401(6)−2.442(5) Å and 2.391(9)−2.402(9) Å, respectively. However, the tricoordinated oxygen atoms (Figure 5) are disordered on two close equivalent positions (half occupancy) and assigned to μ3-oxo or μ3-hydroxy groups, which differ by two sets of the U−O bond distances. One is the range 2.193(8)−2.275(9) Å (for OU1 and 3228



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Thermogravimetric curve of compound 1 (Figure S7) shows a continuous weight loss (exp. 1.7%) from room temperature up to 180 °C assigned to physisorbed molecules. Between 180 and 270 °C, a first weight loss is observed (obs: 11.2%) and followed by a second one from 350 up to 390 °C (exp. 26.6%). The total weight loss (exp: 37.8%; obs: 39.2%) is attributed to departure of the organic molecules (formate, terephtalate, and dimethylamine). The remaining plateau (exp. 60.2%) is assigned to the uranium oxide α-U3O8 (calc. 61.1%). Thermogravimetric curve of compound 2 (Figure S8) shows a first weight loss (obs: 2.2%) from room temperature up to 130 °C, which could be assigned to water molecules (calc: 1.4%). From 200 up to 630 °C a continuous weight loss is observed and attributed to a slow decomposition of carboxylate molecules (formate and fumarate) and DMF. The remaining weight (obs: 67.9%) corresponds to the uranium oxide α-U3O8 (calc. 68.2%). The sequence of solvent molecules (H2O and DMF) departures in 2 is confirmed by infrared spectroscopy as a function of temperature. From the IR spectrum collected at 170 °C (Figure S11), we still observed an intense peak at 1647 cm−1 corresponding to DMF (vCO). In parallel, we noted the disappearance of the broad peak centered at 3200 cm−1 characterizing the hydrogen bonding generated by water molecules. The evolution of the X-ray diffraction patterns as a function of temperature is well correlated with the TGA experiments and indicates that compound 1 persists up to 180 °C (Figure S5). The departure of organic molecules leads to the formation of an unidentified poorly crystallized phase which is transformed into α-U 3 O 8 at 360 °C. Unlike 1, after its decomposition at 180 °C, compound 2 generated an amorphous phase which is transformed into α-U3O8 at 420 °C (Figure S6). Since the bonded water is evacuated before 130 °C, the framework structure of the anhydrous form is retained up to 180 °C. Spectroscopy Characterization (IR, UV/vis). The IR spectrum of compound (Hdma)[U4O2(bdc)3(form)7] (1) (Figure S9), indicates the absence of any absorption peak around 1660 cm−1 (vCO), which confirms the absence of DMF molecule in the structure. The absorption peaks due to carboxylate arms are vasym(COO) around 1541 cm−1 and the vsym(COO) around 1372 cm−1. The weak peak at 1618 cm−1 is typical for the deformation δ(N−H) of the primary amine dimethylamine. One distinguishes three peaks at 3086, 2966, 2850 cm−1 that could correspond to the vibrations of C−H bonds associated to carboxylic acid and protonated dimethylamine. These vibrations are within in a broad band centered at 3000 cm−1 characteristic to the stretching vst and combination vibrations vcomb of ammonium group N−H+.23 For compound U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2) (Figure S10), the absorption peaks around 1655 cm−1 v(C−O) and 2928 cm−1 v(O=C−H) confirms the presence of DMF molecules inside the cavity. The absorption peaks due to carboxylate arms are vasym(COO) around 1541 cm−1 and the vsym(COO) around 1372 cm−1. The broad band centered at 3000 cm−1 reveals intermolecular hydrogen bonding between water and DMF molecules. The different absorption bands of the solid-state UV−visible spectrum (room temperature) of both compounds 1 and 2 are typical for tetravalent uranium (Figure 8).24 For 1, the signals at around 484 and 551 nm could be assigned to the distinct transitions 3H4 → 1I6 (484 nm) and 3H4 → 3P1 (551 nm),

Figure 6. (top) View of the connection of the hexanuclear units through the fumarate (along the b axis) and formate ligands (along the a axis) in U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2) in the (a, b) plane. (bottom) View of the hexanuclear units through the fumarate ligands in the (a, b) plane for the closely related structure of U6O4(OH)4(fum)6(H2O)6·5DMF.7b

Figure 7. Perspective view of the octahedral (left) and tetrahedral (right) cavities filled with DMF in U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2). Yellow sphere represents the center of the cavity and give an idea of its size.

of three DMF molecules per structural unit (U6O4(OH)4). Unfortunately, as discussed hereafter, the departure of solvent molecules trapped within the framework induces an amorphization that limits the accessible porosity of this material. Such a behavior has been previously reported in the uranium-based compound series deriving from the UiO-66 topology.7b Thermal Behavior. The two tetravalent uranium carboxylates have been characterized by thermogravimetric analysis and in situ thermodiffraction (up to 800 °C). 3229



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series called UiO-66 series and contained two types of cavities filled by DMF molecules. This two U4+-based solids were synthesized using similar quantities of reactants (UCl4, ligands, solvent), reaction time (24 h) and crystallized in the same range of temperature (130− 150 °C). Consequently, we could suppose that similar hydrolysis phenomenon occurs during the solvothermal reaction involving the same unidentified molecular species coexisting in solution. This point of view would involve that the formation of the inorganic secondary building appeared during the crystallization and was mainly under the control of the ligand combination. Moreover, the assembly of the organic species and the inorganic linkers has to be compatible with the final three-dimensional framework and the corresponding lattice energy.

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ASSOCIATED CONTENT

* Supporting Information S

SEM images, X-ray powder patterns, in situ X-ray diffractograms, thermogravimetric curves, FT-IR spectra, IR spectra. 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 20 434 973. Fax: (33) 3 20 43 48 95. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Pr. Marc Visseaux for his help in the synthesis of UCl4 and Pr. Francis Abraham for helpful discussions, and Mrs. Nora Djelal & Laurence Burylo for their technical assistance with the SEM images, TG measurements, and powder XRD (UCCS). The Fonds Européen de Développement Régional (FEDER), CNRS, Région Nord Pas-de-Calais and Ministère de l′Education Nationale de l′Enseignement Supérieur et de la Recherche are acknowledged for funding of X-ray diffractometers. C.F. acknowledges Université Lille 1 and Région Nord Pas-de-Calais for their Ph.D. grant supports.

Figure 8. Solid-state UV/vis spectra of (Hdma)[U4O2(bdc)3(form)7] (1, top) and U6O4(OH)4(fum)5(form)2(H2O)2·3DMF (2, bottom).

respectively. The intense and broad peak with some resolved components at 620 and 646 and 666 nm could be attributed to the terms 3H4 → 3P0, 3H4 → 1G4, and 3H4 → 1D2. The last broad and relatively flat band could correspond to the transition 3 H4 → 3H6. The solid state UV/vis spectrum of compound 2 is less resolved (Figure 8) and closely related to that observed in the series of uranium dicarboxylates bearing hexanuclear units.7b It consists of weak bands at 492 and 558 nm, assigned to the transitions 3H4 → 1I6 and 3H4 → 3P1, respectively. The most intense peak is in the range 610−710 nm and composed of weakly resolved peaks at 651, 671, and 678 nm, which could be assigned to the terms 3H4 → 3P0, 3H4 → 1G4, and 3H4 → 1 D2.

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REFERENCES

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CONCLUSION This contribution dealt with two rare examples of threedimensional MOF-type architectures obtained with tetravalent uranium and a mixture of organic ligands. The combination of terephthalate and formate led to the isolation of the tetranuclear unit U4O4 in 1, whereas the association of formate and fumarate generated the hexanuclear core U6O4(OH)4 in 2. Both solids exhibited an open framework, encapsulated organic species. In 1, dimethylammonium cations (from solvothermal decomposition of DMF) are localized in small cavities and compensate the negative charge of the network. The structural arrangement of 2 was quite similar to the zirconium-based 3230



Article

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