Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium

Article pubs.acs.org/IC

Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium and Neptunium) with a Phthalate or Mellitate Ligand in an Aqueous Medium Nicolas P. Martin,† Juliane Mar̈ z,‡ Christophe Volkringer,†,§ Natacha Henry,† Christoph Hennig,‡ Atsushi Ikeda-Ohno,‡ and Thierry Loiseau*,† Unité de Catalyse et Chimie du Solide (UCCS), UMR, CNRS 8181, Université de Lille, École Nationale Supérieure de Chimie de Lille, Centrale Lille, Université Artois, 59000 Lille, France ‡ Institute of Resource Ecology, HDZR - Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany § Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France †

S Supporting Information *

ABSTRACT: Four metal−organic coordination polymers bearing uranium or neptunium have been hydrothermally synthesized from a tetravalent actinide chloride (AnCl4) and phthalic (1,2-H2bdc) or mellitic (H6mel) acid in aqueous media at 130 °C. With the phthalate ligand, two analogous assemblies ([AnO(H2O)(1,2-bdc)]2·H2O; An = U4+ (1) or Np4+ (2)) have been isolated, in which the square-antiprismatic polyhedra of AnO8 are linked to each other via μ3-oxo groups with an edgesharing mode to materialize infinite zigzag ribbons. The phthalate molecules play a role in connecting the adjacent zigzag chains to build a two-dimensional (2D) network. Water molecules are bonded to the actinide center or found intercalated between the layers. With the mellitate ligand, two distinct structures have been identified. The uranium-based compound [U2(OH)2(H2O)2(mel)] (3) exhibits a three-dimensional (3D) structure composed of the dinuclear units of UO8 polyhedra (square antiprism), which are further linked via the μ2-hydroxo groups. The mellitate linkers use their carboxylate groups to connect the dinuclear units, eventually building a 3D framework. The compound obtained for the neptunium mellitate ([(NpO2)10(H2O)14(Hmel)2]·12H2O (4)) reveals oxidation of the initial NpIV to NpV under the applied hydrothermal synthetic conditions, yielding the neptunyl(V) (NpO2+) unit with a pentagonal-bipyramidal NpO7 environment. This further leads to the formation of a layered assembly of the square-frame NpO7 sheets via the bridging oxygen atoms from the neptunyl oxo groups, which further coordinate to the pentagonal equatorial coordination plane of the adjacent neptunium unit (i.e., cation−cation interactions). In compound 4, the mellitate molecules act as bridging linkers between the NpO7 sheets by using four of their carboxylage groups, eventually building up a 3D structure.

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INTRODUCTION

(0D) to 3D and from mono- to polynuclear inorganic uranylcentered subunits.1−3 Compared with the extensive investigations on the UOFs, relevant studies on actinides with other oxidation states, such as tetravalent actinide (AnIV), are rather scarce even to date, and the precedent works focused mainly on tetravalent thorium (ThIV) and UIV complexes. Most of the precedent studies on these two AnIV complexes reported the syntheses and descriptions of networks with different nuclearities of AnIV building units. A typical illustration of such a polynuclear AnIV unit is the hexanuclear cationic core ([An6O4(OH)4]12+), which is stabilized by carboxylate ligands. The crystal structure of this type of complex has been reported for Th,4−9 U,4,10−14 and plutonium (Pu).15,16 Additionally, analogous hexameric units

Research on coordination polymers bearing actinides has grown intensively in the last few decades. This is largely due to the development of the metal−organic framework (MOF) class, which is mainly based on a combination of metallic centers with oxygen- or nitrogen-donor organic ligands, such as carboxylate, to materialize three-dimensional (3D) porous architectures. Among the series of actinides (An), uranium (U) is of major importance in general. It is a naturally occurring element and is stabilized as a hexavalent state as the uranyl cation (UO22+) with two trans-oxo UO bonds under oxic conditions, whereas the tetravalent uranium (UIV) could occur mainly under anoxic conditions as spherical U4+ ions. The uranyl(VI) moiety is able to react with a large number of organic ligands to yield a wide variety of mixed uranyl−organic frameworks (UOFs) with different dimensionalities from zero-dimensional © 2017 American Chemical Society

Received: December 12, 2016 Published: February 16, 2017 2902

DOI: 10.1021/acs.inorgchem.6b02962 Inorg. Chem. 2017, 56, 2902−2913

Article

Inorganic Chemistry have also been observed for tetravalent cerium (CeIV),17−21 which is often used as a surrogate of radioactive actinides. Most of these works have applied conventional synthetic routes based on diffusion, slow evapolation, or solvothermal methods in organic solvents [e.g., N,N-dimethylformamide (DMF) or tetrahydrofuran (THF)]8,10,13,14,22−36 and have reported the association of aromatic polycarboxylate ligands, such as benzoates, terephathalates, and trimesates, in the complex assemblies or the formation of coordination polymers. There are, however, only a limited number of studies using pure aqueous reaction media, which are more relevant to the environmental conditions.37 In this case, the isolation of discrete polynuclear An(IV) entities could be facilitated by the use of multidentate oxygen- or nitrogen-donor organic ligands,4−7,9 the coordination of which can compete the strong tendency of AnIV toward hydrolysis and condensation reactions in aqueous solution.37,38 This also indicates that knowledge of the evolution mechanism of such aqueous AnIV polynuclear species would be of potential importance for an understanding of the migration behavior of An in the geosphere.39 In contrast to a large number of precedent studies reporting the syntheses of AnIV coordination polymers in organic solvents, there are only a few reports exploring the reactivity of AnIV with polydentate carboxylic acids in aqueous solutions. For instance, the complexes of AnIV with oxalic acids have been extensively investigated because of their importance in the reprocessing of spent nuclear fuels by PUREX.40 More recently, the reaction of ThIV with trimesic acid in an aqueous medium has been reported.26 The reaction of UIV with phthalic acid has also been investigated under biorelevant anaerobic conditions in the presence of bacteria, revealing the formation of a dinuclear complex.41 Such aromatic carboxylates are ubiquitous in the natural environment because of the natural decomposition of humic and fulvic acids. On the basis of these backgrounds, the aim of this study is to investigate the reaction of aromatic carboxylates with AnIV under aqueous conditions. The work focuses particularly on the use of phthalate, as well as mellitate (1,2,3,4,5,6-benzenehexacarboxylate), which can also be found in natural soils, such as in natural salts of aluminum minerals.42 Given the fact that similar issues of soil contamination would be expected for the geological disposal of radioactive wastes,43,44 this study employs not only UIV but also tetravalent neptunium (NpIV). In fact, the crystal chemistry of NpIV is poorly investigated even to date. To the best of our knowledge, only a small number of publications have reported the crystal structure of neptunium(IV) carboxylates, such as oxalates,45−48 formate,49 and tribromoacetate,50 while there are several studies reporting the carboxylate complexes with other oxidation states of Np, such as NpV 51−66 and NpVI.67−71 The present study reports the hydrothermal syntheses of UIV and NpIV with phthalate and mellitate. The crystal structures of the obtained compounds were determined by single-crystal X-ray diffraction (SC-XRD), and the results are compared between UIV and NpIV. We also report the thermal behavior and optical properties (IR and UV−visible) of uranium(IV) phthalate, which was obtained as a pure phase.

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Resource Ecology (Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany), which possesses the appropriate equipment for handling highly radioactive elements. Synthesis. The compounds were hydrothermally synthesized under an autogenous pressure using a 2 mL glass vial with a Teflon cap. The starting AnIV compound was uranium tetrachloride (UCl4), obtained from the reaction of hexachloropropene with uranium trioxide (UO3),25 or neptunium tetrachloride (NpCl4), obtained from a similar protocol based on the reaction of hexachloropropene with neptunium dioxide (NpO2). Actinide(IV) tetrachloride was mixed with either phthalic acid (C8H6O4, 1,2-H2bdc, Acros Organics, 99%) or mellitic acid (C12H6O12, H6mel, Aldrich, 99%) and deoxygenated deionized water in a glass vial. The chemicals (except UCl4 and NpCl4) are commercially available and were used without further purification. The preparation of the reactant mixtures was performed in an inert glovebox under an argon or a nitrogen atmosphere for U and Np compounds, respectively. The mixtures in the closed glass vials were then removed from the glovebox and heated in an oven under an ambient atmosphere. [UO(H2O)(1,2-bdc)]2·H2O (1). A mixture of 12 mg of UCl4 (0.03 mmol of U), 23 mg (0.14 mmol) of phthalic acid, and 0.5 mL (27.8 mmol) of water was placed in a closed glass vial and then heated statically at 130 °C for 24 h. The resulting product of 1 (green flat needlelike crystallites; Figure S1a) was then filtered off, washed with DMF, and dried at room temperature under an ambient atmosphere. Scanning electron microscopy (SEM) shows the formation of flat needle-shaped crystallites (Figure S1a). Compound 1 was obtained as a pure phase, which was proven by powder X-ray diffraction (XRD) pattern (Figure S2a). [NpO(H2O)(1,2-bdc)]2·H2O (2). A mixture of 12 mg of NpCl4 (0.03 mmol of Np), 23 mg (0.14 mmol) of phthalic acid, and 0.5 mL (27.8 mmol) of water was placed in a closed glass vial and then heated statically at 130 °C for 24 h. The resulting product of 2 (brown flat needlelike crystallites; Figure S1c) was dried at room temperature under an ambient atmosphere. Powder XRD analysis (Figure S2c) and optical microscopy observation show some unidentified impurities together with the phase of 2. U2(OH)2(H2O)2(mel) (3). A mixture of 12 mg of UCl4 (0.03 mmol of U), 40 mg (0.12 mmol) of mellitic acid, 50 μL of a 4 M NaOH solution, and 1 mL (55.5 mmol) of water was placed in a closed glass vial and then heated statically at 130 °C for 24 h. The resulting product of 3 (green platelike crystallites; Figure S1b) was then filtered off, washed with water, and dried at room temperature under an ambient atmosphere. The powder XRD pattern (Figure S2b) indicates that compound 3 was not observed as a pure phase, and unidentified phases were present, together with the phase of 3. [(NpO2)10(H2O)14(Hmel)2]·12H2O (4). A mixture of 10 mg of NpCl4 (0.026 mmol of Np), 40 mg (0.12 mmol) of mellitic acid, 50 μL of a 4 M NaOH solution, and 1 mL (55.5 mmol) of water was placed in a closed glass vial and then heated statically at 130 °C for 24 h. The resulting product of 4 (green platelike crystallites; Figure S1d) was dried at room temperature under an ambient atmosphere. Powder XRD analysis (Figure S2d) and optical microscopy observation show some unidentified impurities, together with the phase of 4. SC-XRD. Crystals of the U compounds (1 and 3) were analyzed on a Bruker DUO-APEX2 CCD area-detector diffractometer at 300 K using microfocused Mo Kα radiation (λ = 0.71073 Å) with an optical fiber as a collimator (UCCS, University of Lille). Crystals of the Np compounds (2 and 4) were analyzed on a Bruker D8 VENTURE diffractometer with a PHOTON 100 CMOS detector at 100 K using microfocused Mo Kα radiation (λ = 0.71073 Å; Helmholtz-Zentrum Dresden-Rossendorf). Crystals of 1−4 were selected under a polarizing optical microscope and glued on a glass fiber for SC-XRD experiments. Several sets of narrow data frames (20 s frame−1) were collected at different values of θ for two initial values of ϕ and ω, respectively, using 0.3° increments of ϕ or ω. Data reduction was performed using SAINT Plus, version 7.53a.72 The substantial redundancy in the data allowed a semiempirical absorption correction (SADABS, version 2.1073) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by

EXPERIMENTAL SECTION

Caution! Natural U and 237Np are radioactive and chemically toxic reactants. Therefore, precautions with suitable equipment and a facility for radiation protection are required for handling these elements. Experiments with 237Np were carried out in a controlled laboratory at the Institute of 2903


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Inorganic Chemistry Table 1. Crystal Data and Structure Refinements for the Obtained Uranium/Neptunium Carboxylates formula fw temperature/K cryst type cryst size/mm cryst syst space group a/Å b/Å c/Å α/deg β/deg γ/deg volume/Å3 Z, ρcalculated/g cm−3 μ/mm−1 θ range/deg limiting indices collected reflns unique reflns param GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff peak and hole/e Å−3

1

2

C16H12O13U2 888.32 300 green plate 0.14 × 0.06 × 0.06 triclinic P1̅ 7.8178(7) 9.5870(9) 13.2310(13) 77.492(6) 86.775(5) 83.487(5) 961.36(16) 2, 3.069 16.896 1.58−26.39 −9 ≤ h ≤ 9, −11 ≤ k ≤ 11, −16 ≤ l ≤ 16 24286 3929 [R(int) = 0.0581] 282 1.03 R1 = 0.0253, wR2 = 0.0481

C16H12O13Np2 886.26 100 brown plate 0.13 × 0.06 × 0.06 triclinic P1̅ 7.7880(8) 9.4753(10) 13.1220(14) 77.747(3) 86.824(3) 83.448(3) 939.60(17) 2, 3.062 10.821 2.63−26.40 −9 ≤ h ≤ 9, −11 ≤ k ≤ 11, −16 ≤ l ≤ 16 61364 3862 [R(int) = 0.0587] 257 1.02 R1 = 0.0217, wR2 = 0.0393

C12H4O16U2 880.21 300 green block 0.09 × 0.07 × 0.06 orthorhombic Fdd2 13.903(3) 19.813(5) 11.393(3) 90 90 90 3138.4(14) 8, 3.726 20.714 2.53−26.45 −17 ≤ h ≤ 17, −24 ≤ k ≤ 23, −14 ≤ l ≤ 14 13003 1589 [R(int) = 0.0907] 131 1.02 R1 = 0.0477, wR2 = 0.0995

3

C24O70Np10 3778.7 100 green plate 0.17 × 0.10 × 0.03 monoclinic Pn 12.9650(3) 12.9796(3) 20.1038(5) 90 95.1696(5) 90 3369.32(14) 2, 3.725 15.407 2.22−26.67 −16 ≤ h ≤ 14, −16 ≤ k ≤ 16, −25 ≤ l ≤ 25 78412 63506 [R(int) = 0.1871] 467 1.08 R1 = 0.0352, wR2 = 0.0372

4

R1 = 0.0395, wR2 = 0.0529 1.13 and −1.03

R1 = 0.0344, wR2 = 0.0418 1.02 and −0.98

R1 = 0.0707, wR2 = 0.1078 2.86 and −1.72

R1 = 0.0390, wR2 = 0.0486 1.50 and −1.84

IR Spectroscopy. IR spectra of compounds 1 and 3 (see the Supporting Information) were measured on a PerkinElmer Spectrum Two spectrometer equipped with a Diamond attenuated total reflectance (ATR) accessory. The spectra were recorded between 4000 and 400 cm−1. Phase transformation of compound 1 was characterized by in situ IR spectroscopy in air with a heating rate of 10 °C min−1 from room temperature up to 210 °C. During this heating period, 195 spectra were recorded on a PerkinElmer Spectrum Two spectrometer equipped with a Pike Special-IR Gladi ATR accessory in the range between 4000 and 400 cm−1 with a resolution of 4 cm−1. UV−Visible Spectroscopy. UV−visible absorption spectra were collected using a PerkinElmer Lambda 650 spectrophotometer equipped with a powder sample holder setup.

direct methods, developed by successive difference Fourier syntheses, and refined by full-matrix least squares on all F2 data using the SHELX74 program suite and OLEX2 software.75 Hydrogen atoms of the benzene ring were placed at calculated positions and allowed to ride on their parent atoms. The final refinements include anisotropic thermal parameters of all non-hydrogen atoms, except the oxygen atoms of the water molecules. Crystals of compound 4 were systematically twinned. Therefore, the twinning treatment was performed by using JANA76 software. After the twinning treatment, the refinements indicate 57% of domain I and consequently 43% of domain II, with the orientation matrix (−0.0011, −1.0011, −0.1396), (−0.9989, 0.0011, 0.1396), and (0, 0, −1). The crystal data are given in Table 1. Supporting Information is available in CIF format (CCDC 1509604−1509607). Powder XRD. The powder XRD patterns for compounds 1 and 3 were collected at room temperature with a D8 Advance A25 Bruker apparatus with Bragg−Brentano geometry (θ−2θ mode). The D8 diffractometer is equipped with a LynxEye detector with Cu Kα radiation. For compounds 2 and 4, powder XRD patterns were collected on a Rigaku MiniFlex 600 with Bragg−Brentano geometry (θ−2θ mode), Cu Kα source (40 kV and 15 mA for X-ray generation), and a D/Tex Ultra Si strip detector. X-ray Thermodiffraction. X-ray thermodiffractometry was performed for compound 1 under 5 L h−1 of nitrogen flow in an Anton Paar HTK1200N on a D8 Advance Bruker diffractometer (θ−θ mode; Cu Kα radiation) equipped with a Vantec1 linear positionsensitive detector. Each powder XRD pattern was recorded in the range 5−50° (in 2θ) from room temperature to 800 °C at an interval of 20 °C with a 1 s step−1 scan, corresponding to the measurement duration of 37 min. The heating rate between two XRD patterns was 5 °C min−1. Thermogravimetric Analysis (TGA). The TGA experiment was carried out on a Setaram TGA92 thermoanalyzer under an argon atmosphere with a heating rate of 5 °C min−1 from room temperature up to 1100 °C.

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RESULTS Structure Description. The reaction of UIV or NpIV with the phthalate molecule resulted in the formation of an identical coordination polymer network [AnO(H2O)(1,2-bdc)]2·H2O [An = U4+ (1), Np4+ (2)], which possesses a two-dimensional (2D) organic−inorganic assembly. Their crystal structure is based on two crystallographically independent actinide centers (Figure 1), which exhibit the same coordination geometry: the metals An1 and An2 are 8-fold-coordinated by three oxo groups (O1 and O2), four carboxyl oxygen atoms (labeled OC, from phthalate molecule), and one aquo species (O1W and O2W). It defines a distorted square-antiprismatic geometry (AnO8), with typical U−O bond lengths of 2.178(4)−2.376(4) Å, U− OW bond distances of 2.616(5)−2.626(4) Å, and U−OC bond distances in the range 2.329(4)−2.483(4) Å. Because of actinide contraction, 77 the corresponding Np−O bond distances are slightly shorter than the corresponding U−O distances, with Np−O bond distances of 2.185(3)−2.362(3) Å, Np−OW bond distances of 2.617(3)−2.625(3) Å, and Np−OC 2904


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distances are slightly longer with the values of 3.866(1) and 3.842(1) Å in the fluorite-type structure of UO279 and NpO2,80 respectively. The occurrence of such an infinite one-dimensional (1D) network is rather uncommon in the AnIV coordination polymers, which usually exhibit discrete monomeric or polynuclear complexes instead. In fact, a similar chain structure has been previously reported for An IV with terephtalate (bdc) [U2O2(bdc)2(DMF)].14 Another type of infinite chain network occurs with acetate ligands.81 In this case, however, the connection of the bicapped square-antisprimatic polyhedra (UO10) via a trans edge-sharing mode generates straight ribbons instead of the zigzag ribbons found in compounds 1 and 2. A cis edge-sharing connection mode was encountered in the uranium sulfate U(OH)2(SO4),82 in which U is 8-fold-coordinated in square-antiprismatic geometry. Within one inorganic chain of compounds 1 and 2, which is aligned along with the a axis, there is one type of phthalate molecule bridging two pairs of An1−O−An2 subunits, where each carboxylate arm adopts a syn−syn configuration in a bidentate bridging mode between the An1 and An2 centers (Figure 2). A second type of phthalate molecule plays the role of linking the two adjacent chains with the same syn−syn bidentate connection fashion. This connection generates the formation of organic−inorganic layers of [AnO(H2O)(1,2bdc)]2 in the ab plane (Figure 3). Free water molecules (OW3)

Figure 1. Coordination environments around the two types of actinide centers in 1 and 2. O1 and O2 are μ3-oxo groups shared between three neighboring actinide cations. OW1 and OW2 are aquo species in terminal positions. Color code: green circles, U or Np; red circles, O; gray circles, C; light-gray circles, H.

bond distances in the range 2.338(3)−2.484(3) Å. The oxo groups are shared between three adjacent actinide centers with a μ3-bridging configuration, whereas the aquo species are situated in a terminal position. The assignments of the μ3-O/ H2O groups are in good agreement with bond-valence-sum calculations,78 which give values of 2.03 and 2.11 for O1 and O2 in the U-based compound (expected value: 2.0). For the aquo species, bond-valence-sum calculations give 0.26 and 0.25 for O1W and O2W in the U-based compound (expected value: 0.4). The existence of the μ3-O bridging groups leads to the formation of an infinite ribbon, with AnO8 units sharing two edges with the adjacent one (Figure 2). Within this chain, the interatomic U···U distances are 3.6645(4) and 3.7094(5) Å for U1···U2 and U1···U1, respectively. In the Np analogue, the Np···Np distances are 3.6438(4) and 3.7031(5) Å for Np1··· Np2 and Np1···Np1, respectively. For comparison, An···An

Figure 3. Crystal structures of 1 and 2 in the bc plane. Isolated red circles indicate the free water molecules (O3W) intercalated between the inorganic chains.

are intercalated between the hybrid sheets stacking along the c axis. The O3W species reveal strong hydrogen-bonding interactions with the attached water molecule O2W [O3W··· H2WA−O2W = 2.088(10) Å in 1; O3W···H2WA−O2W = 2.016(6) Å in 2]. Cohesion of the structure is caused by the van der Waals interaction between the benzene rings. However, on the basis of TGA, the water content of the U compound was determined to be 0.5 H2O per U2O2 unit, instead of the 1 H2O revealed by XRD (see the Thermal Behavior of Compound 1 section). This difference might reflect disorder with a partial occupancy for the free water molecules of the O3W type. The solid-state UV−visible absorption spectrum (Figure S3d) collected for compound 1 shows a typical f−f transition for U4+,83 which consist of a broad band between 600 and 700 nm with some resolved components at 629, 654, and 678 nm (assigned to the transitions 3H4 → 1G6, 3H4 → 1D2, and 3H4 → 3 P0, respectively). Two other peaks at 500 and 550 nm are also observed and could be related to the transitions 3H4 → 1I6 and 3 H4 → 3P1, respectively.

Figure 2. Views of the infinite ribbons of square-antiprismatic AnO8 units developed along the a axis in 1 and 2. One type of connection mode of the phthalate ligand is shown within the chain. 2905


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Inorganic Chemistry

bidendate bridging mode, and the rest having two carboxylate groups adopt an anti monodentate connection mode. For the latter, the remaining nonbonded C−O distance is 1.20(4) Å (C6−O62), which is related to a CO bonding. When the crystal structure is viewed along the [001] direction (Figure 5),

The crystal structure of compound 3 consists of one crystallographically independent U4+ cation coordinated with five carboxyl oxygen (OC) atoms, and two hydroxo (OH) and aquo (O1W) groups (Figure 4). The U−O bond distances

Figure 4. Coordination modes of the 8-fold uranium center (U1) in compound 3 (top), the defined dinuclear unit {U2O10(OH)2(H2O)2} in which the uranium atoms are connected through two μ2-OH groups (middle), and the coordination mode of a mellitate ligand (bottom). Color code: green circles, U; red circles, O; gray circles, C; light gray circles, H.

Figure 5. (top) Crystal structure of compound 3 in the ab plane, showing the connection of the dinuclear units {U2O10(OH)2(H2O)2} with the mellitate ligands. (bottom) View of the structure of 3 in the bc plane, indicating the stacking of organic−inorganic sublayers in the ab plane along the [011] direction.

range from 2.288(18) to 2.430(18) Å for OC. The U1−OH bond lengths are 2.255(17) and 2.376(14) Å, whereas the U1− O1W length is slightly longer with bond distance of 2.52(2) Å. The assignments of OH/H2O are in good agreement with the results of bond-valence-sum calculations,78 with values of 1.18 for O1OH (expected value for the OH group: 1.2) and 0.33 for the O1W (expected value for H2O: 0.4). The 8-foldcoordinated U1 is defined by a square-antiprismatic polyhedron, which is linked via a common edge to an adjacent uranium center around 2 axis operation [U1···U1 distance = 3.775(2) Å]. The corresponding bridging group is the μ2-OH species. The aquo ligand is situated in the terminal position of the coordination sphere of U1 and is not further linked to another uranium atom. The resulting dinuclear unit {U2O10(OH)2(H2O)2} is connected to another by the mellitate molecules. The organic linker adopts an 8-fold coordination with the uranium centers (Figure 4). Two of six carboxylate arms of mellitate exhibit a syn−syn bidentate bridging fashion with the uranium centers, the other two have a syn−anti

the crystal structure shows a layer of dinuclear U units linked via the mellitate molecules with the anti monodentate and syn− anti bidendate carboxylate arms. The other carboxylate groups of the mellitate support the connection of the uranium atoms along the [011] direction, generating a dense 3D coordination polymer network (Figure 5). The formation of such a dinuclear entity with a double μ2-OH bridge is quite unique for UIV compounds.37 Up to now, only one structure has been reported for the dinucelar organometallic complex of UIV with the chelating ligand of 1,2,3,4-tetramethyl-5-(2-pyridyl)cyclopentadiene.84 In this complex, the two uranium centers are bridged by two μ2-oxo groups, but not by μ2-OH ones. The structure determination by SC-XRD on compound 4 reveals that the hydrothermal reaction of NpCl4 with mellitic acid results in the formation of an unexpected NpV-based network. This is in contrast to the result for compound 2 with phthalate, in which Np remains as NpIV. One neptunium(V) mellitate complex [Na4(NpO2)2(mel)·8H2O] has been previously reported to form by slow evaporation of an aqueous 2906


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Inorganic Chemistry solution at pH 6.5 in the presence of sodium (Na) as a countercation.62 Under our hydrothermal conditions applied in this study, a distinct structural assembly of 4 has been produced. It consists of 10 crystallographically independent neptunium atoms with a 7-fold coordination geometry (i.e., pentagonal bipyramids). Typical trans-dioxo neptunyl bonds are observed for all of the cations. The bond lengths of neptunyl (NpO) range from 1.824(8) to 1.872(7) Å. The Np−O bond lengths associated with carboxyl oxygen atoms on the equatorial pentagonal plane range from 2.342(7) to 2.653(7) Å. The formation of terminal Np−O bonds is also observed, where oxygen atoms are assigned to aquo species. The Np−OH2 bond lengths are in the range from 2.448(7) to 2.566(7) Å. These values are in agreement with those found in other NpV compounds with similar coordination environments.55,65,85−88 The coordination sphere of different neptunium centers can be distinguished by the presence or absence of terminal aquo species. That is, only Np7 contains three carboxyl oxygen atoms in the equatorial plane ({NpO7} unit). A single terminal aquo species is observed for Np2, Np4, Np8, and Np9 ({NpO6(H2O)} unit), whereas two terminal aquo ligands are present in the coordination sphere of Np1, Np3, Np5, Np6, and Np10 ({NpO5(H2O)2 unit}; Figure 6). All of

Figure 7. Polyhedral representation of the inorganic sheet in 4 showing the CCI linkage of the {NpO7} polyhedra in a square net.

isophthalate [(NpO2)2(1,3-bdc)(H2O)]·H2O55 obtained by the hydrothermal synthesis from a neptunyl(V) nitrate source [NpO2(NO3)]. Similar square Np network sheets have also been reported for inorganic compounds (e.g., sulfates, selenates, borates, iodates, etc.).85,86,90−93 The CCI-based Np network sheets are further linked to each other through the hexatopic mellitate molecules. Two types of crystallographically independent mellitate linkers (labeled A and B) are observed in the structure of 4 (Figure 8). The both mellitate molecules use

Figure 6. View of the asymmetric unit of 4 showing the connection mode of the 10 crystallographically independent neptunium centers linked to each other through “neptunyl” oxo groups (i.e., CCI). Code: green circles, Np; red circles, O; purple circles, terminal water molecules; blue bonds, neptunyl bond (NpO); black bonds, Np−O.

Figure 8. Coordination modes of the two distinct mellitate ligands (A and B) in compound 4.

the Np units are linked to each other via the bridging oxygen atoms of the “neptunyl” oxo groups, which further coordinate to the equatorial pentagonal plane of the adjacent Np unit. This bridging mode of the NpO−Np type has been reported for several Np-based compounds as cation−cation interactions (CCIs).89 The corresponding Np−O bond lengths in the equatorial plane range from 2.385(7) to 2.509(7) Å. The CCIbased bridging eventually materializes a 2D inorganic square network (Figure 7), which is developed along the pseudotetragonal ab plane. As a matter of fact, the cell parameters for the a and b axes [12.9650(3) and 12.9796(3) Å, respectively] are very close. The Np···Np distances between adjacent neptunium centers are in the range from 3.937(1) to 4.261(1) Å. The structural arrangement around the neptunium center in this CCI-based 2D Np network is comparable to those observed for the neptunium phthalate [(NpO2)2(1,2-bdc)]·4H2O65 and

seven oxygen atoms of the carboxylate arms to coordinate to the neptunium atoms. Two of the carboxylate arms exhibit a syn−syn bidentate bridging mode toward the neptunium centers (toward Np2 and Np3 and also Np2 and Np4 for A; toward Np1 and Np6 and also Np4 and Np5 for B; Figure 8). One other carboxylate arm adopts a syn−anti bidenate bridging fashion (toward Np8 and Np10 for A; toward Np8 and Np7 for B), and the other carboxylate arm is chelating to one Np center (Np9 for A; Np7 for B). The remaining two carboxylate groups are not bound to the neptunium centers: one exists in its protonated form with elongated C−OH bonds [1.289(12) and 1.318(12) Å for A and B, respectively] and short CO bonds [1.222(12) and 1.210(12) Å for A and B, respectively], and the other is not protonated, with the C−O bond distance ranging from 1.229(11) to 1.245(12) Å. The observed inorganic Np layer network is stacked along the c axis via the mellitate linkers 2907


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tracarboxylate or pyromellitate)) demonstrate coordination assemblies different from that observed in 4, indicating the difference in the coordination behavior between mellitate and pyromellitate toward the neptunium center. With the mellitate linker, oxidation of NpIV to NpV is quite unanticipated, while we obtained single crystals of the uranium(IV) mellitate complex under the same synthetic conditions. These different results observed for the uranium and neptunium mellitate complexes probably stem from the different redox behavior between U and Np. It is obvious from the resultant neptunium(V) mellitate compound that the hydrothermal synthesis of the initial mixture of AnCl4 and mellitic acid results in the oxidation of AnIV to a higher oxidation state. The standard redox potential between UIV and UV (as UO2+) is lower than that between NpIV and NpV (as NpO2+),94 indicating that UIV could be oxidized more easily than NpIV. However, it is well-known that UV has a strong tendency to cause disproportionation (i.e., 2UO2+ + 4H+ → U4+ + UO22+ + 2H2O), while disproportionation of NpV is insignificant.94 This suggests that the UV species produced during the hydrothermal synthesis would be disproportionated instantly to eventually form UIV and UIV (as UO22+). As a matter of fact, the UV−visible absorption measurements on the uranium mellitate sample after the hydrothermal synthesis (t = 24 h in Figure 11) reveal the presence of UVI species (at 400− 450 nm) in the supernatant, supporting this hypothesis. Hence, it is reasonable to consider that the hydrothermal reaction of UCl4 and the mellitic acid mixture results in the formation of a uranium(IV) mellitate complex and additional UVI complexes as dissolved species and/or possible amorphous precipitates, which would account for the unidentified impurities found on the powder XRD measurement. In this case, the uranium(IV) mellitate compound would have a low solubility to be

(Figure 9). Free water molecules are intercalated between the layers, showing hydrogen-bonding interactions between the

Figure 9. Crystal structure of 4 in the bc plane showing the stacking of the square-layered Np network sheets (green) along the c axis. Isolated red circles represent the free water molecules intercalated between the inorganic sheets.

terminal aquo species bound to Np and the remaining nonbonded carboxylate groups. In total, 12 distinct water species have been identified from the SC-XRD analysis. In the 3D structure of compound 4, only four of the six carboxylate arms of the mellitate molecule interact with the neptunium centers, behaving like a pyromellitate ligand. However, the neptunyl(V) pyromellitate complexes reported thus far ([Na3NpO2(btec)]2·11H2O63 and [(NH4)3(NpO2)5(btec)2]·7H2O64 (btec = 1,2,4,5-benzenete-

Figure 10. (top) UV−visible absorption spectra of the solution containing UCl4 and phthalic acid in water (i.e., reaction mixture used for the synthesis of compound 1): black line; initial reaction mixture before the synthesis (t = 0); red line, supernatant after the hydrothermal treatment (t = 24 h). (bottom) Enlargement of the UV−visible absorption spectrum of the supernatant solution after the hydrothermal treatment (i.e., red line in the upper figure). 2908


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Figure 11. UV−visible absorption spectra of the solution containing UCl4 and mellitic acid in water (i.e., reaction mixture used for the synthesis of compound 3): black line; initial reaction mixture before the synthesis (t = 0); red line, supernatant after the hydrothermal treatment (t = 24 h).

Figure 12. In situ IR spectra of compound 1 at different temperatures between 20 and 200 °C. Only the selected ranges of 3800−3350 and 1600− 400 cm−1 are shown.

between the organic−inorganic sheets in compound 1. When one free water molecule (1 H2O) is assumed to be intercalated, the value is calculated to be 60.7%. The presence of UO2 in the final residue of thermal decomposition of 1 was confirmed by a X-ray thermodiffraction experiment under a nitrogen atmosphere (Figure S3b), indicating the growth of Bragg peaks corresponding to UO2 from 360 °C. It also confirmed that the structure of the initial compound 1 persists up to 140 °C. IR Spectroscopy of Compounds 1 and 3. The Fourier transform infrared (FT-IR) spectrum of compound 1 at room temperature (Figure S3c) shows various resonances in the range between 3700 and 3000 cm−1 (stretching vibrations), confirming the presence of water. Lanthanide-based coordination polymers containing only coordinated water molecules show the IR resonance of aquo groups at around 3300 cm−1 as a broad band.95 A similar broad band was also observed for compound 1 at around 3300 cm−1, which can therefore be assigned as the coordinated water molecules. The free water molecules show the resonance at 3607 cm−1. Bending vibrations of water molecules can also be identified by small peaks between 1650 and 1580 cm−1. The connection between the carboxylate arms and UIV leads to the specific vibrations νCO at 1535 and 1389 cm−1, which are assigned to the asymmetric and symmetric stretching of water molecules, respectively.

crystallized, while the uranyl(VI) mellitates would be fairly soluble in our synthetic conditions. On the other hand, the NpV species could be stabilized without disproportionation, forming neptunium(V) mellitate complexes. For the phthalate system, the UV−visible spectrum of the sample after the hydrothermal reaction (t = 24 h in Figure 10) shows a very small amount of U species in the supernatant, indicating that the majority of the original UIV reacts with phthalic acid to form compound 1. Thermal Behavior of Compound 1. TGA was performed only for the uranium phthalate (1) compound, which could be obtained as a pure phase. The TGA curve recorded under an argon atmosphere (Figure S3a) indicates a two-step weight loss event. The first loss occurring up to 230 °C is attributed to the removal of attached and free water molecules. The chemical formula derived from SC-XRD analysis expects a value of 6.1 wt % for (2 + 1) H2O per U2O2 unit, while the observed weight loss is a lower value of 5.2%. This result indicates that the actual water content should be lower than that expected from SCXRD, which would agree with a stoichiometry of (2 + 0.5) H2O per U2O2 (calcd 5.10%). This hypothesis is also in good agreement with the weight value remaining at 1000 °C, which corresponds to the value after decomposition of the structure and degradation of the organic part. In fact, the final remaining weight value was found to be 61.5 wt %, which agrees well with the calculated value of 61.3 wt % for UO2, which is calculated by assuming 0.5 H2O as free water molecules intercalated 2909


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The thermal behavior of compound 1 was also followed by FT-IR spectroscopy from room temperature to 200 °C (Figure 12). The relatively low affinity of water molecules in the structure was confirmed by the disappearance of the bands between 3700 and 3000 cm−1, corresponding to the release of water molecules. At 120 °C, this phenomenon coincides with the appearance of a new band centered at 930 cm−1, which is further split into two independent vibrations at 945 and 913 cm−1 at higher temperature. These bands are characteristic of the uranyl(VI) vibrations, indicating the oxidation of UIV to UVI during the heating process as well as decomposition of the initial compound. Because of the presence of impurities in compound 3, we selected a few crystals of this phase for IR characterization. The acquired spectrum is shown in Figure S4. Similar to compound 1, coordinated water molecules in 3 produce a broad IR band centered at around 3200 cm−1, hiding the resonances coming from the hydroxyl group. The various connection modes between the carboxylate groups and U in 3 result in several resonances in the range between 1700 and 900 cm−1, which correspond to the stretching vibration νCO.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (33) 3 20 434 122. Fax: (33) 3 20 43 48 95. ORCID

Christoph Hennig: 0000-0001-6393-2778 Thierry Loiseau: 0000-0001-8175-3407 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Nora Djelal, Laurence Burylo, and Philippe Devaux for their technical assistance for SEM, TGA, and powder XRD measurements at UCCS. The Chevreul Institute (FR 2638), Fonds Européen de Développement Régional, CNRS, Région Hauts de France, and Ministère de l’Education Nationale de l’Enseignement Supérieur et de la Recherche are acknowledged for funding. The authors also thank TALISMAN for financial support (Project TALI_C05-18).

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CONCLUSION The reactivity of aromatic poly(carboxylic acid) of phthalic and mellitic acids, with UIV and NpIV, was investigated in an aqueous medium under moderate hydrothermal conditions. Three types of coordination polymers were revealed: two of them exhibit a similar 2D network with UIV or NpIV. This type of compound is composed of the connection of squareantiprismatic polyhedral units (AnO8) via edge-sharing modes with μ3-oxo bridges to materialize infinite zigzag chains. The chains are further linked to each other via carboxylate (phthalate) molecules. One UIV compound with mellitate has been isolated. Its crystal structure consists of dinuclear {U2O10(OH)2(H2O)2} units, with the μ2-hydroxo bridges connected by the hexadendate mellitate molecules, eventually forming a dense 3D framework. On the other hand, the reaction of mellitate with NpIV under the same synthetic conditions resulted in the formation of a layered neptunyl(V) network based on a square topology of NpO2+, being linked to each other via CCIs. This eventually resulted in the formation a 3D structure, with the mellitate acting as a linker between the inorganic NpO2+-based sheets. The compounds characterized in this study are rare examples of AnIV coordination polymers produced from aqueous media. Further works exploring the reaction of other polydendate carboxylates with AnIV in aqueous media are anticipated.

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02962. Crystallographic data for 1 in CIF format (CIF) Crystallographic data for 2 in CIF format (CIF) Crystallographic data for 3 in CIF format (CIF) Crystallographic data for 4 in CIF format (CIF) Optical and SEM photographs of 1 and 3, optical photographs of 2 and 4, powder XRD patterns of 1−4, thermogravimetric curve of 1, thermodiffraction diagram of 1, IR spectra of 1 and 3, and UV−visible spectrum of 1 (PDF) 2910


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Synthesis of Coordination Polymers of Tetravalent Actinides (Uranium

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