Uranyl Cation Incorporation in the [P8W48O184]40â Macrocycle

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Uranyl Cation Incorporation in the [P8W48O184]40− Macrocycle Phosphopolytungstate Maxime Dufaye,† Sylvain Duval,*,† Greǵ ory Stoclet,‡ Xavier Trivelli,§ Marielle Huve,́ † Alain Moissette,∥ and Thierry Loiseau†

Inorg. Chem. Downloaded from pubs.acs.org by UNIV DE BARCELONA on 01/01/19. For personal use only.

†

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 ‡ Unité Matériaux Et Transformations (UMET) − UMR CNRS 8207, Bat 6, BP 90108, 59652 Villeneuve d’Ascq, France § Unité de Glycobiologie Structurale et Fonctionnelle (UGSF) − UMR CNRS 8576, Université de Lille Nord de France, Bat C9, BP 90108, 59652 Villeneuve d’Ascq, France ∥ Laboratoire de Spectroscopie Infrarouge et Raman (LASIR) − UMR CNRS 8516, Université de Lille Nord de France, USTL-ENSCL, Bat C7, BP 90108, 59652 Villeneuve d’Ascq, France S Supporting Information *

ABSTRACT: Association of uranyl nitrate with the macrocycle [P8W48O184]40− in formate buffered aqueous solution leads to the formation of a new compound (K11.3Li8.1Na22)[(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]· 89H2O (1). Its characterization by XRD reveals a high disorder of the uranyl cations and the formation of monodimensional chains of anionic [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]41.4− entities linked through formate ligands. The uranyl species are located either in the coordinating sites of the macrocycle [P8W48O184]40− or at its surface. Further studies of the molecule by SAXS and TEM show that the 1D chain collapses to give rise to the formation of polydisperse spherically aggregated species with an average radius of 129 Å.

surrogates elements.13−16 A class of actinide cations concerns the actinyl series where the metal is linked by two axial “yl” oxygen atoms through a triple iono-covalent bond to form an [AnO2]n+ group. This kind of cationic group appears very interesting due to its bipyramidal coordination environment with four to six labile oxo groups in the equatorial plane giving them a good flexibility for coordination chemistry. To our knowledge, several papers about polyvacant heteropolyoxometalates complexing actinyl cationic groups have been publish in the last two decades17,18 but, only a few concern the use of macrocyclic polyoxometalates. The most recent example was published in 2008 by Kortz et al., who reported interesting horseshoe [P8W36] compound (formed in situ starting from the [H6P4W24O94]18‑ unit) stabilizing an octanuclear uranyl-peroxo cluster.19 Another molecule, [(UO2)3(H2O)6As3W30O105]15‑ was obtained in 2002 by Pope et al., who observed the in situ formation of the [As3W30O105]21− macrocycle from decomposition of the [As4W40O184]28− polyanion.20 Lately, we have also successfully used the similar [As4W40O184]28− anion to bind up to four tetravalent uranium cations within the cavity and tested its

1. INTRODUCTION The chemistry of polyoxometalates (POM) has been widely studied during the past decades due to their structural diversities and abilities to bind a wide range of 3d, 4d, or 4f metallic cations. The combination of these elements gives rise to numerous properties and potential applications in medicine, magnetism, catalysis, photochemistry, material sciences, etc. The utilization of such POM moieties has also been investigated with actinides, in the nuclear area for waste storage or separation processes.1−8 One subclass of these molecules is the heteropolyoxometalates, which are built up from the polycondensation reactions of metallic cations, in aqueous acidic solution, around trigonal templating entities of XO3 type (X = AsIII, SbIII, BiIII, etc.) or tetrahedral templating entities of XO4 type (PV, SiIV, GeIV, etc.). For several years now, in our laboratory, we have been concerned with the complexation and stabilization of actinide cations or surrogate related elements (i.e., lanthanides) by O-donor organic molecules.9,10 For instance, with the tetravalent uranium cation, the largest polyoxometallic clusters to date were obtained and are almost exclusively stabilized by the complexation of O-donor organic ligands.11,12 More recently, we have continued our effort by developing an alternative approach using the coordination properties of lacunary heteropolyoxometalate species toward actinides or © XXXX American Chemical Society

Received: August 4, 2018

A

DOI: 10.1021/acs.inorgchem.8b02210 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

least-squares on all F2 data using the SHELX program suite. The SQUEEZE procedure was applied (using PLATON v30715) to compound 1 to remove electronic densities on a large solvent accessible void where no electronic density was localized, probably due to very high disorder of cations and water solvent molecules. In this compound, uranyl centers are disordered on two crystallographically independent sites, with 0.65 and 0.25 occupancy factors for U1 and U2, respectively. These occupancy factors were determined on the basis of the electronic density trying to minimize the residual electronic diffraction peaks and holes around theses heavy metals, by decorrelation of the occupancy and the thermal parameters of these two atoms. The final refinements include anisotropic thermal parameters of all non-hydrogen atoms, except for the oxygen atoms of the water molecules or other atoms that would be nearly NPD or NPD if refined anisotropically. The crystal data are given in Table S1. Supplementary crystallographic data is available in CIF format (CCDC 1859464). SAXS Measurements. The measurements were performed on a SAXS Xeuss 2.0 apparatus (Xenocs) equipped with a micro source using a Cu Kα radiation (λ = 1.54 Å) and point collimation (beam size: 300 × 300 μm2). The sample to detector distance, around 35 cm, is calibrated using silver behenate as standard. The analyzed aqueous solutions are placed in 1.5 mm glass capillaries at a concentration of about 25 mM of [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]41.4− moiety. Before the analyses of each measurement, the contributions of the capillaries and the solvent are subtracted using a reference containing the solvent according to standard procedures. The experimental scattering data were compared to model calculated scattering data. Simulated scattering curves were computed by using the SolX software starting from the CIF files of the compounds.32−34 TEM Measurements. Images were obtained using a FEI Technai G2 20 TEM operated at 200 kV. The samples were prepared by dropping a small volume of solution containing compound 1 onto a carbon film on a metal grid. Electronic and Fluorescence Spectroscopies. Electronic spectroscopy was performed in solid state and in solution, on a PerkinElmer Lambda 650 photospectrometer in 1 cm length quartz vial for the solution measurements. Fluorescence measurements were performed on a SAFAS Xenius fluorimeter, using a xenon lamp. Thermogravimetric Analysis. The thermogravimetric experiment has 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. Infrared Spectroscopy. The infrared spectrum was measured on PerkinElmer Spectrum Two spectrometer between 4000 and 400 cm−1, equipped with a diamond attenuated total reflectance (ATR) accessory. No ATR correction was applied on the spectrum. EDX Analysis and Microscopic Photography. EDX measurements were performed on a crystal sample of compound 1 to estimate the elemental composition, with respect to the molecular composition. Images of crystals were collected by using a Keyence numerical microscope. Inductively Coupled Plasma Spectroscopy. Crystals of compound 1 were dissolved in 2% HNO3 solutions and analyzed using a Vista-Pro Varian ICP-OES to determine the Na/K/P/W/U/ Cl ratios of the compound. Based on the known number of W atoms on the molecular architecture, the percentages of each element can be determined.

chemical reactivity for the separation of U(IV)/Ln(III) mixtures.21 In this work, we have decided to use the readily formed macrocycle [H7P8W48O184]33− to bind the uranyl [UO2]2+ cationic group. This anionic moiety has been successfully used by other research teams to stabilize several polynuclear 2p, 3d, 4d, or 4f transition metal or semimetal clusters.22−29 Herein, we report on the synthesis and solid state and solution characterizations of the new uranyl containing polyanionic macrocycle [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]41.4−.

2. EXPERIMENTAL SECTION Synthesis. Caution! Uranyl nitrate UO2(NO3)2·6H2O is a radioactive (α, γ emitter) and chemically toxic reactant, so precautions with suitable care and protection for handling such substances have been followed. The Dawson anions K 6 [α-P 2 W 18 O 62 ]·14H 2 O and K 12 [αH2P2W12O48]·24H2O were synthesized by following published procedures.30 K28Li5[H7P8W48O184]·92H2O was synthesized according to the method reported by Constant and Tézé.30 In 150 mL of water were dissolved, successively, glacial acetic acid (8.5 g, 142 mmol), lithium hydroxide (3 g, 71 mmol), lithium chloride (3 g, 71 mmol) and K12[α-H2P2W12O48]·24H2O (4 g, 10 mmol). The solution was left in an open beaker. After 1 day, white needles appear and crystallization continues for several days under air atmosphere. One week later, the volume of the solution was evaporated to 100 mL and the crystals were collected by suction filtration on a coarse frit and washed with H2O (5 mL), ethanol (95%, 5 mL), and acetone (5 mL) and finally air-dried for 1 day. Yield: 1.84 g (46%). IR (cm−1): 1132 (m), 1081 (m), 1012 (w), 979 (w), 910 (m), 788 (s), 639 (s, br), 565 (w), 514 (s), 464 (s). Compound 1. The title compound was obtained by hydrothermal synthesis as follows: K28Li5H7[P8W48O184]·92H2O (0.050 g, 3.37 × 10−6 mol) and LiCl (0.2 g, 4.74 × 10−3 mol) were dissolved in 5 mL of a 1 M HCOOH/HCOONa buffer at pH = 3.7. The uranium salt UO2(NO3)2·6H2O (0.0133 g, 2.65 × 10−5 mol) was added, and the mixture was heated up to 160 °C during 30 h in a 23 mL Teflon-lined stainless autoclave. The large amount of LiCl is tentatively used to decrease the number of potassium cations encapsulated within the [P8W48O184]40− entity and also helps for the solubilization of the final product.31 The resulting solution was separated from an insoluble residue, and yellowish crystals were obtained after a few days by solvent evaporation in an opened beaker under air atmosphere. Yield: 0.023 g (39% based on W). Above 160 °C, only a fine unidentified powder was obtained. ICP/EDS: K (2.56%), Na (3.13%), U (9.41%), P (1.53%), W (47.3%), Cl (1.70%). Calculated for K11.3Na22Li8.1[(UO2)7.2(HCOO)7.8(P8W48O184)]·Cl8·89H2O (from XRD and TGA): K (2.59%), Na (2.97%), U (10.0%), P (1.46%), W (51.8%), Cl (1.67%). IR (cm−1): 1384 (w), 1351 (w), 1328 (sh), 1126 (m), 1083 (m), 1016 (w), 986 (w), 952 (sh), 913 (s), 848 (m), 760 (m, br) 634 (s, br), 565 (s), 512 (s), 461 (s). Single-Crystal X-ray Diffraction. Crystals of compound 1 were selected under polarizing optical microscope and glued on a mitegen loop 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 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 accomplished using SAINT V8.34a. The substantial redundancy in data allowed a semiempirical absorption correction (SADABS V2014/ 5) to be applied, on the basis of multiple measurements of equivalent reflections. The structure was solved by direct methods (I4/m, a = b = 32.2440(12) Å, c = 16.1510(16) Å, V = 16791.8(14) Å3), developed by successive difference Fourier syntheses, and refined by full-matrix

3. RESULTS Synthesis. The [P8W48O184]40− polyoxotungstate is a macrocycle constructed by the condensation of four hexavacant Dawson [P2W12O48]14‑ polyanionic moieties. This cyclic compound delimits a central cavity that is obviously not extremely reactive toward cationic species. Indeed, it has been shown to be able to hold relatively few 3d, 4d, or 4f oxo(thio)metallic clusters. (See refs 22−29.) An interesting feature lies B


Article

Inorganic Chemistry

Figure 1. Optical photographs of compound 1.

Figure 2. Side and top view of compound 1.

order to circumvent this problem, we decided to use the hydrothermal route, adding a larger amount of lithium (See ref 30) and uranyl cations to (i) maximize the solubility of the polyanion and try to exchange some of the problematic potassium cations involved in the disorder by uranyl cations; (ii) use the autogenous pressure generated by the temperature to increase the solubility of the precursors and favor the uranyl incorporation into the macrocycle. This procedure allowed us to isolate a new set of yellow crystals of the compound 1 (Figure 1) in which up to 7.2 {UO2}2+ entities were incorporated within the [P8W48O184]40− cavity of the polyanion, in which less potassium/uranyl disorder is observed. Structure Description. Compound 1 crystallizes in the tetragonal I4/m space group (a = b = 32.2440(12) Å, c = 16.1510(6) Å, V = 16791.8(11) Å3). The asymmetric unit is constituted by one-half of a P2W12O48 unit, two crystallographically distinct hexavalent uranium cations, and two-half formate ligands. Applications of the C4 axis and a mirror plane perpendicular to this axis going through the phosphor atoms of the asymmetric unit give rise to the complete P8W48O184 macromolecule incorporating up to 7.2 {UO2}2+ uranyl cations (Figure 2). Surprisingly, the uranyl groups are not located in the center of the molecule as was generally reported for several 3d metal clusters (See ref 22−26 and 29). They appear to be slightly offcenter, forming a sort of crown on both sides of the cyclic molecule. Such a feature, where the metallic cluster is not located within the cavity of the phosphotungstic [P8W48O184]40− wheel, has previously been observed with 4d cations in [MoV4O10(H2O)3] and [MoV4O4S4(OH)2(H2O)3]2+ groups (See refs 27 and 28). In these two molecules, the

in the fact that bigger lanthanide cations have never been incorporated in a so great number compared to that reported for 3d−4d metallic cations. This is probably due to their larger ionic radius and more complex coordination sphere. From this point of view, direct encapsulation of uranyl [UO2]+ cationic groups within the [P8W48O184]40− cavity could represent a challenge. Indeed, as far as we know, no such study has been directly performed on this molecule regarding the complexation of the uranyl cations. One nice compound was previously obtained by Ulrich Kortz,19 starting from the [H6P4W24O94]18‑ derivative. This compound possesses horseshoes architecture and was constructed by three hexavacant Dawson moieties encapsulating an octamer of uranyl cations. Due to the synthetic conditions, this octanuclear subunit appears to be stabilized by peroxide bridges and several [PO4]3− tetrahedra directly coming from decomposition of the phosphotungstic precursor. To fulfill our goals regarding the direct incorporation of uranyl [UO2]2+ cationic groups in this macrocyclic molecule, we have first tried the use of the solution synthesis route using a formate buffer at pH 3.7 to prevent pH variation during the synthesis and to avoid the fast formation of uranyl carboxylate crystals when using an acetate buffer. At room temperature, this synthetic method leads to the formation of yellowish crystals which can be described in the I/4m space group (a = b = 32.3527(10) Å, c = 15.8831(5) Å, V = 16624.8(9) Å3). We attempted to solve the structure but a huge cationic disorder took place between potassium and uranyl species (either close to the uranium atoms or on some of the “yl” oxygen atoms), and this observation prevented us from getting an acceptable structural model. Nevertheless, we were able to estimate that a lower number of uranyl centers (ca. 5.2U/P8W48) was incorporated within the phosphotungstic macrocycle. In C


Article

Inorganic Chemistry

Figure 3. (a) Uranyl cluster disorder scheme of outer uranyl (U1, transparent) in compound 1, (b) highlight of the disorder between the potassium cation and the U1 “yl” oxygen atom, and (c) U−O bond distances of U1 and U2 centers. The equatorial red oxygen atoms belong to the [P8W48O184]40− polyanion while the pink ones belong to the bridging formate ligands.

Figure 4. Left: Inclusion of the potassium cation (pink sphere) in the uranyl crown cluster. Right: Ball and stick model of the connectivity of the potassium cation within the uranyl crown.

groups are disordered with an occupancy factor of 0.25 (Figure 3a), probably because of the presence of a potassium cation which was consequently set with a 0.75 occupancy factor. This potassium cation is located on the same crystallographic site as the center-directed “yl” oxygen of the U1 group, probably imposing this disorder and abnormally long UO bond distance, despite our synthetic conditions (Figure 3b). Its environment is composed of 11 oxygen atoms (K−O distances between 2.361(7) Å and 3.210(8) Å) and one chloride ion with K−Cl distances of 2.758(6) Å. On the eight remaining positions, the U2 uranyl groups were located with an occupancy factor at 0.65. The coordination sphere of these U2 cationic groups is also pentagonal bipyramidal (U−O bond length in the range 2.329(8) Å− 2.474(15) Å) with two oxygen atoms in the equatorial plane belonging to the polyanionic macrocycle. These two oxygen atoms are in common between U1 and U2 uranyl groups. The two “U−Oyl” distances on the U2 uranium cations (1.776(14) Å and 1.795(13) Å) are shorter than in the U1 group, but related to the classical “U−Oyl” distances. The U2 uranyl centers are bridged by several formate ligands that were set to have the same occupancy rate (0.65), with U2−Oformate distances ranging from 2.370(15) Å to 2.474(15) Å. A

clusters are off-centered as found for the uranyl cluster in our compound 1. Interestingly, the two molybdenum handles in refs 27 and 28 have been shown to be disordered over two positions, above and below the [P8W48O184]40− wheel, with a 50%/50% perpendicular or parallel relative disposition of the two molybdenum entities. For the molecular motif of compound 1, we observe a total of 16 uranyl positions, eight of them being located within the crown of the macrocycle (U1) and eight of them located upand downward from the crown of the macrocycle (U2). The eight U1 positions could be intuited as the oxygen atoms around this position, defining a pentagon that can actually match one of the classical geometries of [UO2]2+ uranyl groups (pentagonal bipyramid). The eight other positions (U2) are surprising as they were not expected at a first glance. The X-ray diffraction analyses reveal the uranyl cationic groups are all disordered on these two distinct groups of eight coordination positions. U1 uranyl groups are coordinated in their equatorial plane by the 5 oxygen atoms (U−O bond length in the range 2.088(8)−2.651(9) Å) coming for the polyanionic cycle discussed above completed by two “U−Oyl” bonds with distances of 1.89(3) Å and 1.94(5) Å to finally have a pentagonal bipyramidal coordination sphere. These U1 uranyl D


Article

Inorganic Chemistry

Figure 5. Packing of compound 1 along the c axis. {UO2} in yellow polyhedron, WO6 in gray octahedron, PO4 in pink tetrahedron, O in red sphere, and C in black sphere.

formate ensures the generation of the chains in the solid crystalline state. Along the a and b axes, the [U7.2P8W48O184] cyclic entities are interacting through the oxygen surface of the polyanion with several intercalated potassium and sodium cations. The charge balance of compound 1 is partially ensured by the Na+ and K+ cations located in the crystal lattice but, the remaining positively charged cations could not be accurately located from the X-ray data and may also be due to the presence of additional disordered Li+ cations. The complete number of Na + and K+ cations was determined using ICP measurements. Some water molecules have been located between the POM species, and the number of 89H2O/[P8W48O184] has been estimated from the TGA analysis (see below). Thermal Behavior. The TGA experiment was performed to determine the water content of compound 1 (Figure S1). Calculations were carried out based on the observation of the two first loss of weight at the first and second plateau (respectively until 250 and 400 °C) and compared with the calculated values obtained from the X-ray diffraction studies. The first weight loss corresponds to the departure of water molecules, and the second loss is related to the degradation of the organic part in the compounds i.e. the formate ligands. The TGA curve shows that compound 1 contains 89 (obs.: 9.4%; calc.: 5.4% from XRD) hydration water molecules and 9.5 (obs.: 2.5%; calc.: 2% from XRD) formate ligands. The difference between the observed and calculated water content for compound 1 is relatively important since all the water molecules could not be located in the lattice. This feature is relatively classical in POM chemistry because of the water disorder generally present, imposing a squeeze of the solvent molecules. The formate ligands located per cyclic [P8W48O184]40− entities in the structure were also checked by the TG analyses, and the obtained value corresponds relatively well to what was expected from the XRD. The last weight loss, starting around 600 °C, is due to the collapse of the molecular architecture. Indeed, the polyoxo-cluster part finally decomposes and forms the expected U3O8, P2O5, and WO3 oxides. Infrared and Raman Spectroscopy. The IR and Raman spectra were recorded and compared to the one of the uranium free [P8W48O184]40− precursor (Figure S2 and S3). We observe that the IR spectrum appears slightly affected by the uranyl complexation. The three P−O vibrations at 1132, 1081, and 1014 cm−1 are slightly shifted to 1126, 1083, and 1015 cm−1. A vibration band at 850 cm−1 also appears and could be related to the U−Oyl vibration in the [UO2]2+ groups. In the Raman spectrum, a band located at 797 cm−1 was also observed and classically related the [UO2]2+ cationic groups in agreement with previous work.17

potassium cation (labeled K1) is also embedded within the small cavity defined by the [(UO2)4-(HCOO)4] crown (constructed from the U2-formate connections) with distances going from 2.764(14) Å to 2.935(14) Å (Figure 4). The coordination sphere of this potassium ion is constituted by 8 oxygen atoms with a distorted cuboidal geometry. This K+ cation might act as a templating agent of the crown shape cluster. It is noteworthy that using a larger quantity of Li+ cations in the synthesis to help in the solubilization of the materials did not remove this potassium atom, which highlights its potential role as a template. Several water molecules and four sodium cations (labeled Na4) with a distorted prismatic environment (Na−O distances between 2.24(3) Å and 2.54(1) Å) were located in the central cavity of the macrocycle. Eight chloride anions close to the U2 atom (dU−Cl = 2.425(3) Å) interact with the “yl” oxygen atoms of the two uranyl species. There are one very short distance of 1.690(13) Å with the “yl” oxo group of “U2−Oyl” bond and a longer one of 2.758(6) Å with the “yl” oxo group of “U1−Oyl” bond. These chloride anions come from the LiCl source used in the synthesis procedure. Its presence has been confirmed by the EDS analyses. Diffraction analyses suggest the presence of a fully occupied chloride atom, but the unrealistic short U2−Cl distance of 1.69 Å might account for some disorder between the uranyl and the chloride anion. The long-range organization of compound 1 reveals that the cyclic molecules are stacking along the c axis. This stacking is ensured through several formate ligands that are directly bound to the [(UO2)4-(HCOO)4] uranyl crown (with dU‑Oformate = 2.452(15) Å) (Figure 5) and related to the same 0.65 occupancy than the [(UO2)4-(HCOO)4] crown. Justification of this 0.65 occupancy for all the formate ligands linked to the U2 cation comes from the X-ray structural resolution and was confirmed by chemical analysis. From a chemical point of view, these formate ligands cannot be present without the U2 uranyl cations of the crown as they would not be bounded to any atoms. We can either assume that the four external U2 uranyl cations of the crown and the formate ligands coordinated to them form a single subunit that can be present (occupancy close to 2/3) or absent (occupancy close to 1/3) from the structure. In this configuration, the chain is broken when no U2-formate group is present. But, we think that a more realistic interpretation comes from a statistical point of view. Indeed, the [U7.2P8W48O184] macrocycles can be linked together by one to four formate bridges (with an average occupancy factor close to 2/3) to form the infinite chains observed along the c axis. In this case, the distribution of the four disordered external sites for U2E


Article

Inorganic Chemistry

Figure 6. Left: Experimental (dot) and simulated (strong line) X-ray scattering curves for aqueous solutions of compound 1. Right: scattering curve showing the aggregation process and the WAXS domain (orange dots).

d(U−O)/(pm) = 9141(ν3/cm−1)−2/3 + 80.4

(1)

d(U−O)/(pm) = 10650(ν1/cm−1)−2/3 + 57.5

(2)

spectra could be related to a convergence of the band due to the cluster aspect of the uranyl cations in compound 1. This phenomenon has already been observed for such condensed uranyl clusters.37 Measurements performed in aqueous solution (Figure S5 right) did not show the same trend. Apart from a red-shift in the emission bands of compound 1 compared to uranyl nitrate, the intensities of the bands are roughly similar. This could be due to the replacement of the formate ligands (in compound 1) and nitrate ligands (in uranyl nitrate) by water solvent molecule causing a radiative decay of the emission energy through U−OH2 bonds. Some discussion on the formate ligand solution behavior in compound 1 is further explained in the SAXS paragraph below. SAXS Measurements and Solution Study. SAXS (small angle X-ray scattering) was used to assess the solution stabilities of compound 1 using its crystallographic XRD data. With this method we are able to determine, in the aqueous solution, the size and the geometrical parameters of dissolved molecular species and eventually attest their long time stabilities by performing a quantitative analysis of the scattering data. Figure 6 depicts the comparison between the scattering curve measured for the title compound and the simulated scattering curve based on the X-ray diffraction analyses information. In the SAXS region, i.e. for q < 0.8 Å−1, the scattering curve observed is similar to the simulated scattering curve. Particularly, we observed a good agreement of positions of the intensity decrease, around q ≈ 0.2 Å−1, indicating that the structural size parameters of the compound are maintained in solution and are related to a spherically shaped compound with an Rg of 10.6 Å, closely related to the size of the molecular unit observed in the X-ray diffraction experiments (about 11.5 Å). Moreover, the SAXS data were collected 3 weeks later and no modification of the scattering curves was observed, revealing the stability of the molecules over this period. The WAXS curve was also measured and modeled. It showed a good correlation in the high q region considering a spherical model for compound 1. SAXS measurements in the low q region, starting at q < 0.1 Å−1 (Figure 6 right) showed an increase in the scattering signal indicating the presence of aggregated particles in solution. Based on the X-ray diffraction analyses, for which the formation of infinite chains was observed, we first thought that this aggregation process was driven by the linkage offered by the formate ligands bonding two or more molecular POM entities. A diffusion measurement experiment was tentatively

The use of the empirical equations (1) and (2) (respectively for IR and Raman spectroscopies) developed by Bartlett et al.,35 that correlate the IR and Raman vibrations energy to the “UOyl” distances, helps us to determine a theoretical “U− Oyl” distance of 1.78 Å−1.86 Å for IR and 1.78 Å−1.84 Å for Raman. These two calculated distances are in relatively good agreement with the observed “U2−Oyl” distances in the X-ray structure that were determined to be of about 1.776(14) Å and 1.795(13) Å. The “U1−Oyl” distances on the second uranyl atom appear to be abnormally longer, about 1.92(6) Å and 1.95(1) Å. This may be provoked by the disorder of this uranyl atom generating some uncertainties on these distances, but these long “U−Oyl” distances probably make them unavailable for this calculation process. Indeed, these correlated UO vibrations were not observed by IR and Raman spectroscopy, which indicates that these UOyl distances observed by XRD would surely come from the refinements artifacts due to the U/K disorder, for which the U occupancy is rather low (0.25). Finally, the presence of the formate ligands is also visible with the C−O bond vibration at 1585 cm−1 and by C−H vibrations at 1387, 1351, and 1329 cm−1 in the IR spectrum. Photoelectronic and Photoluminescence Properties. The 5f orbitals of the uranyl cation in the [UVIO2] group are free of electrons, indicating that no f-f electronic transitions can occur. The observed electronic transitions at 420 nm (Figures S4 and S5) are due to the presence of charge transfer between the orbital electrons of oxygen atoms and the 5f orbital of the uranium cation. The photoluminescence properties of compound 1 were measured and recorded in the solid state under excitation at a 365 nm wavelength. The classic vibronic structure of the UO22+ group was observed and corresponds to the S11 → S00 and S11 → S01−02−03−04 transitions.36 The vibronic structure of the title compound shows a drastic change in comparison with uranyl nitrate. On the solid-state emission spectrum (Figure S5 left), we mainly observed four emission bands at 455 nm, 500 nm, 525 nm, and 570 nm corresponding respectively to the uranyl cation. They all appear to be red-shifted compared to the uranyl nitrate emission bands. The most intense one is located at 500 nm but remains rather weak compared to uranyl nitrate. The observed differences between the two emission F


Article

Inorganic Chemistry

Figure 7. (a) TEM images showing the spherical aggregates and their polydispersity. (b) Comparison of the medialized SAXS (red graph) and experimental TEM (blue graph) particle distribution graphs.

A study by transmission electron microscopy was performed on a solution sample of compound 1. The images show the presence of numerous aggregated spheres (Figure 7a). On the TEM images we can clearly observe polydispersity with particles size going from few nanometers up to 50 nm, but the greatest number of particles seems to stand around diameter of 15 to 35 nm. Based on this polydispersity, a particle size distribution graph could be obtained by measurements of the particles on the TEM images. It was compared with a polydispersity distribution graph obtained by SAXS measurements (Figure 7b). Comparison of the two graphs showed a good correlation between the two measurements indicating that several aggregated species could exist simultaneously in aqueous solution samples of compound 1, with average diameter of 25−26 nm.

performed by NMR spectroscopy to determine the diffusion coefficient of the formate associated with the POM molecule in solution and related it to its molecular weight. Unfortunately, the diffusion coefficient was measured (ca. 3.18 × 10−9 m2·s−1) to be much lower than expected for compound 1. In comparison, this diffusion coefficient is close to the one of the residual water in the solvent (ca. 5.08 × 10−9 m2·s−1) and is related to free formate molecules, which have been dissociated from the [U7.2P8W48O184] moieties. It is therefore not assigned to the large polyanionic [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8] 41.4− . The occurrence of such free formate ligands in the solution indicates the collapse of chains observed from X-ray diffraction analysis. Furthermore, simulation of the scattering data using a cylindrical model constructed with the linear aggregation of 6 to 10 [U7.2P8W48O184] monomers linked through formate ligands did not fit with the profile of the scattering curve in the area where the aggregation process is observed (See Figure S6). The slope of the curves showing an aggregation behavior was thus calculated at a value of 4.1, which seems to be in agreement with a sphere model. A gyration radius of 129 Å was extracted from this experimental curve. In this supposed model, the interactions between the negatively charged [(UO2)7.2(HCOO)7.8(P8W48O184)Cl8]41.4− monomers might come from the numerous alkaline cations present in the medium, stabilizing spherical aggregates despite the high polarity of the water solvent that should dissociate such aggregated species. Such a phenomenon has already been reported for instance with the related [Cu 2 0 Cl(OH)24(H2O)12P8W48O184]25− polyanion, and the alkaline cations were thought to maintain the link between the aggregated monomers by diminishing the electronic repulsion of the anionic entities.38 Interestingly, no aggregation was observed for the free [P8W48O184]40− wheel-like precursor (Figure S7a). Dissolution of compound 1 in tetramethylammonium (TMA)(NO3) (0.5 M) or LiNO3 (0.5 M) shows the conservation and the absence of an aggregation process, respectively (Figure 7b). This phenomenon might be explained by the fact that Li+ cations, in comparison with TMA ones (which are much bigger), did not provide enough attractive forces to compensate the repulsion of the highly negatively charged species in compound 1.

4. CONCLUSION We have successfully used the macrocyclic P 8 W 48 O 184 polyoxometalate to bind up to 7.2 uranyl cations that were found to be disordered on two different crystallographic sites above or below the polyanionic cavity. Chemical analyses and IR and Raman spectroscopies confirm the presence of the uranyl cation with some classical “U−O yl ” distances (1.776(14) Å−1.795(13) Å) although some abnormally longer ones (1.92(6) Å−1.95(1) Å) have been revealed from XRD analyses. The structural organization of the compound shows the formation of monodimensional chains constructed by [U7.2P8W48O184] units bridged by formate ligands. Aqueous solution studies of the compound by SAXS measurements confirm the stability of the molecular entity in compound 1 but suggest that the monodimensional chain aggregation constructed by coordination of the formate bridges is dispersed in solution and is replaced by stable polydisperse spherically aggregated species with an average gyration radius of 129 Å. We assume the stability of these spherical aggregates is ensured by the presence of numerous moderately large alkaline or ammonium cations (Na+, K+, and TMA+) since the presence of an excess of Li+ cation seems to prevent the occurrence of such an aggregation process. G


Article

Inorganic Chemistry

■

(8) Proust, A.; Matt, B.; Villanneau, R.; Guillemot, G.; Gouzerh, P.; Izzet, G. Functionalization and post-functionalization: a step towards polyoxometalate-based materials. Chem. Soc. Rev. 2012, 41 (22), 7605−7622. (9) Falaise, C.; Volkringer, C.; Loiseau, T. Mixed FormateDicarboxylate Coordination Polymers with Tetravalent Uranium: Occurrence of Tetranuclear {U4O4} and Hexanuclear {U6O4(OH)4} Motifs. Cryst. Growth Des. 2013, 13 (7), 3225−3231. (10) Falaise, C.; Volkringer, C.; Vigier, J. F.; Henry, N.; Beaurain, A.; Loiseau, T. Three-Dimensional MOF-Type Architectures with Tetravalent Uranium Hexanuclear Motifs (U6O8). Chem. - Eur. J. 2013, 19 (17), 5324−5331. (11) Falaise, C.; Volkringer, C.; Vigier, J. F.; Beaurain, A.; Roussel, P.; Rabu, P.; Loiseau, T. Isolation of the Large {Actinide}38 Poly-oxo Cluster with Uranium. J. Am. Chem. Soc. 2013, 135 (42), 15678− 15681. (12) Martin, N. P.; Volkringer, C.; Henry, N.; Trivelli, X.; Stoclet, G.; Ikeda-Ohno, A.; Loiseau, T. Formation of a new type of uranium(IV) poly-oxo cluster {U38} based on a controlled release of water via esterification reaction. Chem. Sci. 2018, 9, 5021−5032. (13) Duval, S.; Sobanska, S.; Roussel, P.; Loiseau, T. B-alphaAsW9O339‑ polyoxometalates incorporating hexanuclear uranium {U6O8}-like clusters bearing the UIV form or unprecedented mixed valence UIV/UVI involving direct UVI=O-UIV bonding. Dalton Trans 2015, 44 (46), 19772−19776. (14) Duval, S.; Beghin, S.; Falaise, C.; Trivelli, X.; Rabu, P.; Loiseau, T. Stabilization of Tetravalent 4f (Ce), 5d (Hf), or 5f (Th, U) Clusters by the alpha-SiW9O3410‑ Polyoxometalate. Inorg. Chem. 2015, 54 (17), 8271−8280. (15) Duval, S.; Roussel, P.; Loiseau, T. Synthesis of a large dodecameric cerium cluster stabilized by the SiW9O3410‑ polyoxometalate. Inorg. Chem. Commun. 2017, 83, 52−54. (16) Duval, S.; Trivelli, X.; Roussel, P.; Loiseau, T. Influence of the pH on the Condensation of Tetravalent Cerium Cations in Association with α-SiW9O3410‑ Leading to the Formation of a Ce6O4(OH)4 Core. Eur. J. Inorg. Chem. 2016, 34, 5373−5379. (17) Gaunt, A. J.; May, I.; Copping, R.; Bhatt, A. I.; Collison, D.; Fox, O. D.; Holman, K. T.; Pope, M. T. A new structural family of heteropolytungstate lacunary complexes with the uranyl, UO22+, cation. Dalton Trans 2003, 15, 3009−3014. (18) Gaunt, A. J.; May, I.; Helliwell, M.; Richardson, S. The first structural and spectroscopic characterization of a neptunyl polyoxometalate complex. J. Am. Chem. Soc. 2002, 124 (45), 13350− 13351. (19) Mal, S. S.; Dickman, M. H.; Kortz, U. Actinide Polyoxometalates: Incorporation of Uranyl-Peroxo in U-Shaped 36Tungsto-8-Phosphate. Chem. - Eur. J. 2008, 14 (32), 9851−9855. (20) Kim, K. C.; Gaunt, A.; Pope, M. T. New heteropolytungstates incorporating dioxouranium(VI). Derivatives of α-SiW9O3410‑, αAsW9O339‑, γ-SiW10O368‑, and As4W40O14028‑. J. Cluster Sci. 2002, 13 (3), 423−436. (21) Dufaye, M.; Duval, S.; Hirsou, B.; Stoclet, G.; Loiseau, T. Complexation of tetravalent uranium cations by the As4W40O140 cryptand. CrystEngComm 2018, 20, 5500−5509. (22) Pichon, C.; Mialane, P.; Dolbecq, A.; Marrot, J.; Riviere, E.; Keita, B.; Nadjo, L.; Secheresse, F. Characterization and electrochemical properties of molecular icosanuclear and bidimensional hexanuclear Cu(II) azido polyoxometalates. Inorg. Chem. 2007, 46 (13), 5292−5301. (23) Müller, A.; Pope, M. T.; Todea, A. M.; Boegge, H.; van Slageren, J.; Dressel, M.; Gouzerh, P.; Thouvenot, R.; Tsukerblat, B.; Bell, A. Metal-oxide-based nucleation process under confined conditions: Two mixed-valence V6-type aggregates closing the W48 wheel-type cluster cavities. Angew. Chem., Int. Ed. 2007, 46 (24), 4477−4480. (24) Mal, S. S.; Kortz, U. The wheel-shaped Cu20 tungstophosphate Cu20Cl(OH)24 (H2O)12(P8W48O184)25‑ ion. Angew. Chem., Int. Ed. 2005, 44 (24), 3777−3780.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02210. Crystal data and structure refinements parameters for compound 1 (Table S1), TGA curves for compound 1 (Figure S1), IR spectra in the region 1800−400 cm−1 for compound 1 (Figure S2), RAMAN spectra in the region 1200−100 cm−1 for compound 1 (Figure S3), Solid state electronic spectra for compound 1 (Figure S4), Solid state and solution emission spectra for compound 1 (Figure S5), and simulation of the SAXS curves considering the linear aggregation of 6 to 10 [U7.2P8W48O184] units (Figure S6) (PDF) Accession Codes

CCDC 1859464 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: (33) 3 20 434 973. Fax: (33) 3 20 43 48 95. ORCID

Maxime Dufaye: 0000-0002-5270-4718 Sylvain Duval: 0000-0002-3398-2501 Thierry Loiseau: 0000-0001-8175-3407 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors would like to thank Nora Djelal and 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. S.D. would like to thank the ANR for the funding of the POMAR project.

■

REFERENCES

(1) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. (2) Pope, M. T.; Müller, A. Polyoxometalate Chemistry - an old field with new dimensions in several disciplines. Angew. Chem., Int. Ed. Engl. 1991, 30 (1), 34−48. (3) Pope, M. T., Müller, A., Eds. Polyoxometallates: from Platonic Solids to Anti- Retroviral Activity. Top. Mol. Organ. Eng. 1994.101 (4) Hill, C. L. Special Thematic Issue on Polyoxometallates. Chem. Rev. 1998, 98, 1−390. (5) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Hybrid Organic-Inorganic Polyoxometalate Compounds: From Structural Diversity to Applications. Chem. Rev. 2010, 110 (10), 6009−6048. (6) Pope, M. T.; Kortz, U. Encyclopedia of Inorganic and Bioinorganic Chemistry; John Wiley & Sons, Ltd.: New York, 2012. (7) Long, D. L.; Tsunashima, R.; Cronin, L. Polyoxometalates: Building Blocks for Functional Nanoscale Systems. Angew. Chem., Int. Ed. 2010, 49 (10), 1736−1758. H


Article

Inorganic Chemistry (25) Mal, S. S.; Nsouli, N. H.; Dickman, M. H.; Kortz, U. Organoruthenium derivative of the cyclic H7P8W48O18433‑ anion: {K(H2O)}3{Ru(p- cymene)(H2O)}4P8W49O186(H2O)227‑. Dalton Trans 2007, 25, 2627−2630. (26) Ismail, A. H.; Bassil, B. S.; Yassin, G. H.; Keita, B.; Kortz, U. {W48} Ring Opening: Fe16-Containing, Ln4-Stabilized 49-Tungsto-8Phosphate Open Wheel Fe 16 O 2 (OH) 23 (H 2 O) 9 (P 8 W 49 O 189 )Ln4(H2O)2011‑. Chem. - Eur. J. 2012, 18 (20), 6163−6166. (27) Korenev, V. S.; Floquet, S.; Marrot, J.; Haouas, M.; Mbomekalle, I.-M.; Taulelle, F.; Sokolov, M. N.; Fedin, V. P.; Cadot, E. Oxothiomolybdenum Derivatives of the Superlacunary Crown Heteropolyanion {P8W48}: Structure of K4{Mo4O4S4(H2O)3(OH)2}2-(WO2)(P8W48O184)30‑ and Studies in Solution. Inorg. Chem. 2012, 51 (4), 2349−2358. (28) Sousa, F. L.; Boegge, H.; Merca, A.; Gouzerh, P.; Thouvenot, R.; Müller, A. Vectorial growth/regulations in a {P8W48}-type polyoxotungstate compartment: trapped unusual molybdenum oxide acts as a handle. Chem. Commun. 2009, 48, 7491−7493. (29) Yang, P.; Alsufyani, M.; Emwas, A.-H.; Chen, C.; Khashab, N. M. Lewis Acid Guests in a {P8W48} Archetypal Polyoxotungstate Host: Enhanced Proton Conductivity via Metal-Oxo Cluster within Cluster Assemblies. Angew. Chem., Int. Ed. 2018, 57 (40), 13046− 13051. (30) Tézé, A.; Hervé, G. Inorganic syntheses; John Wiley and Sons, 1990; Vol. 27, p 85. (31) Boyd, T.; Mitchell, S. G.; Gabb, D.; Long, D.-L.; Cronin, L. Investigating Cation Binding in the Polyoxometalate-Super-Crown [P8W48O184]40−. Chem. - Eur. J. 2011, 17 (43), 12010−12014. (32) Forster, F.; Webb, B.; Krukenberg, K. A.; Tsuruta, H.; Agard, D. A.; Sali, A. Integration of small-angle X-ray scattering data into structural modeling of proteins and their assemblies. J. Mol. Biol. 2008, 382 (4), 1089−1106. (33) O’Donnell, J. L.; Zuo, X. B.; Goshe, A. J.; Sarkisov, L.; Snurr, R. Q.; Hupp, J. T.; Tiede, D. M. Solution-phase structural characterization of supramolecular assemblies by molecular diffraction. J. Am. Chem. Soc. 2007, 129 (6), 1578−1585. (34) Zuo, X. B.; Cui, G. L.; Merz, K. M.; Zhang, L. G.; Lewis, F. D.; Tiede, D. M. X- ray diffraction ″fingerprinting″ of DNA structure in solution for quantitative evaluation of molecular dynamics simulation. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (10), 3534−3539. (35) Bartlett, J. R.; Cooney, R. P. On The Determinaton of Uranium Oxygen Bond Lenghts in Dioxouranium(VI) Compounds by RAMAN Spectroscopy. J. Mol. Struct. 1989, 193, 295−300. (36) Rabinowitch, E.; Belford, R. L. Spectroscopy and Photochemistry of uranyl compounds; Macmillan: 1964. (37) Clark, D. L.; Conradson, S. D.; Donohoe, R. J.; Keogh, D. W.; Morris, D. E.; Palmer, P. D.; Rogers, R. D.; Tait, C. D. Chemical speciation of the uranyl ion under highly alkaline conditions. Synthesis, structures, and oxo ligand exchange dynamics. Inorg. Chem. 1999, 38 (7), 1456−1466. (38) Liu, G.; Mal, S. S.; Kortz, U. J. Am. Chem. Soc. 2006, 128, 10103−10110.

I


Uranyl Cation Incorporation in the [P8W48O184]40â Macrocycle

Recommend Documents

Uranyl Cation Incorporation in the [P8W48O184]40â Macrocycle