Article pubs.acs.org/IC
Sulfate-Centered Sodium-Icosahedron-Templated Uranyl Peroxide Phosphate Cages with Uranyl Bridged by μ−η1:η2 Peroxide Jie Qiu,† Tyler L. Spano,† Mateusz Dembowski,‡ Alex M. Kokot,‡ Jennifer E. S. Szymanowski,† and Peter C. Burns*,†,‡ †
Department of Civil and Environmental Engineering and Earth Sciences and ‡Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Two novel hybrid uranyl peroxide phosphate cage clusters, designated U20P6 and U20P12, contain peroxide bridges between uranyl in an unusual μ−η1:η2 configuration, as well as the common μ−η2:η2 configuration. These appear to be the only high-nuclearity metal peroxide complexes containing μ−η1:η2 peroxide bridges, and they are unique among uranyl peroxide cages. Both clusters contain 20 uranyl polyhedra, and U20P6 and U20P12 contain 6 and 12 phosphate tetrahedra, respectively. The 20 uranyl polyhedra in both cages are arranged on the vertices of distorted topological dodecahedrons (20 vertex fullerenes). Each cage is completed by phosphate tetrahedra and is templated by a sulfate-centered Na12 cluster with the Na cations defining a regular convex isocahedron. Whereas μ−η2:η2 peroxides are essential features of uranyl peroxide cages, where they form equatorial edges of uranyl hexagonal bipyramids, the μ−η1:η2 peroxide groups in U20P6 and U20P12 are associated with strong distortions of the uranyl polyhedra. Formation of U20P6 and U20P12 is a further demonstration of the pliable nature of uranyl polyhedra, which contributes to the tremendous topological variability of uranyl compounds. Despite the unusual structure and highly distorted polyhedral geometries of U20P6, small-angle X-ray scattering and Raman spectra suggest its stability in the aqueous solution and solid state. Encapsulation of oxyanions in the cages is rare,23,25,26 and the extent to which these may template formation of the cage is unclear. In the current contribution, we report two highly unusual uranyl peroxide phosphate cage clusters designated U20P6 and U20P12. A unique attribute of these clusters is the presence of μ−η1:η2 peroxide bridges, and they appear to be the only examples of high-nuclearity metal complexes containing peroxide in this configuration. A μ−η1:η2 peroxide bridge occurs in several homodinuclear complexes containing Rh(I), 30,31 Co(II), 32 Mo(VI), 33 W(VI), 33 or Pd(II),34 a heterodinuclear complex containing Fe(III) and Cu(II),6 and a trinuclear uranyl complex.35 Here we explore the topological and geometric aspects of incorporation of μ−η1:η2 peroxide bridges in complex cage clusters and the impact of this on their stability.
1. INTRODUCTION Metal peroxide complexes are important in catalytic oxidation as well as O2 transport in biological systems.1−5 X-ray crystallographic studies have identified structures of many such complexes and revealed that most of them are mononuclear or binuclear assemblies with peroxide groups in various binding modes.1,5,6 These include a peroxide group coordinated to a single metal cation as a nonbridging monodentate (η1) or bidentate (η2) ligand,4 or to two cations as a bridging ligand with μ−η1: η1, μ−η2: η2, or rarely μ−η1: η2 configurations.4 High-nuclearity metal peroxides often present complex structures with unusual peroxide binding modes, including a single peroxide group bridging four7−9 and six10 metal cations. Uranyl peroxide cage clusters are nanoscale materials with potential applications in nuclear fuel cycles.11,12 Since their discovery in 2005,13 more than 60 clusters have been described that contain from 16 to 124 uranyl ions and that have diameters extending to 4 nm. 11,14−17 Peroxide binds to uranyl strongly,18,19 and it is an essential component of this class of polyoxometalates.20 In each reported cage, peroxide bridges uranyl ions in a μ−η2:η2 configuration, and the U−(O2)−U dihedral angles are ∼130−150°, consistent with the curvature of the cage walls.11,20−22 These clusters are anionic and are charge-balanced by various countercations that are located both inside and outside the cages,23−26 and in some cases the countercations appear to stabilize topological squares, pentagons, or hexagons consisting of uranyl polyhedra.21,22,27−29 © XXXX American Chemical Society
2. EXPERIMENTAL SECTION Caution! Uranium-238 is radioactive and should only be handled by qualif ied personnel in appropriate facilities. 2.1. Materials. Uranyl nitrate hexahydrate (UO2(NO3)2·6H2O) was purchased from International Bio-Analytical Industries, Inc. Sodium sulfite (≥98 wt %), sodium sulfate (≥99 wt %, anhydrous), and phosphoric acid (H3PO4, ≥85 wt % in H2O) were purchased from Sigma-Aldrich Corporation. Lithium hydroxide monohydrate (LiOH· H2O) and hydrogen peroxide (H2O2, 30 wt % in H2O) were Received: October 10, 2016
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DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry purchased from VWR Company. Isotopically enriched 97% H218O was purchased from Cambridge Isotope Laboratories, Inc. All chemicals were reagent grade and used without purification to prepare solutions using ultrapure water. 2.2. Synthesis. Compound 1 with composition Li4Na12{(SO4)⊂Na12⊂[(UO2)20(O2)27(HPO4)6]}(H2O)42 was synthesized by combining aqueous solutions of UO2(NO3)2 (0.5 M, 0.1 mL), H2O2 (30%, 0.1 mL), LiOH (2.4 M, 0.12 mL), Na2SO3 (1 M, 0.05 mL), and H3PO4 (0.5 M, 0.1 mL) in a 5 mL glass vial, which resulted in a solution with a measured pH of 9.3. The vial was covered by parafilm with several small holes to allow for the slow evaporation of the solution at room temperature. Yellow blocky crystals of U2813 and orange blocky crystals of 1 with a yield lower than 5% based on uranium formed within three months. The quantities of sodium sulfite and phosphoric acid were adjusted to improve the yield of 1. Crystals of 1 formed following the mixing of aqueous solutions of UO2(NO3)2 (0.5 M, 1 mL), H2O2 (30%, 1 mL), LiOH (2.4 M, 1.2 mL), Na2SO3 (0.5 M, 2.5 mL), and H3PO4 (0.5 M, 1.5 mL) in a 20 mL glass vial. The resulting solution was cloudy and centrifuged to remove an unidentified light yellow precipitation, giving a clear solution with a pH of 7.4. The filtrate was placed in a new glass vial, and the vial was covered using parafilm with several small holes to allow for slow evaporation of the solution. Orange acicular crystals of 1 formed within 1 d with a yield of 20% based on uranium. The product contained no other precipitates, with the bulk of the uranium remaining in solution. Compound 2, with composition Li x Na 16−x {(SO 4 )⊂Na 12 ⊂[(UO2)20(O2)24(HPO4)6(H2PO4)6]}(H2O)n, formed after combination of aqueous solutions of UO2(NO3)2 (0.5 M, 0.1 mL), H2O2 (30%, 0.1 mL), LiOH (2.4 M, 0.12 mL), Na2SO4 (0.5 M, 0.15 mL), and H3PO4 (0.5 M, 0.125 mL) in a 5 mL glass vial, giving a pH of 9.2. The vial was covered by parafilm with several small holes to allow for slow evaporation at room temperature. Yellow blocky crystals of 2 with a yield less than 5% (based on uranium) formed with a fine precipitate within one month. Modified experiments did not improve the yield of 2 significantly. Crystals of 1 and 2 were also synthesized using 18O enriched uranyl nitrate. The preparation of U18O2(NO3)2 was conducted according to published procedures using 97% H218O.36 2.3. Single-Crystal X-ray Diffraction. Diffraction data for single crystals of 1 and 2 were collected using a Bruker APEX II Quazar diffractometer equipped with graphite monochromated Mo Kα X-ray radiation provided by a microsource sealed tube. A suitable crystal of each compound was harvested and mounted on a cryoloop using mineral oil. The crystal was cooled on the goniometer by flowing nitrogen gas at 100 K. A sphere of diffraction data was collected for each crystal using frame widths of 0.5° in ω and an exposure time of 30 s per frame. The Bruker APEX II software was used for data integration, including corrections for Lorentz, polarization, and background effects. SADABS37 was used to apply absorption corrections, and SHELXTL38 was used for structure solution and refinement. H atoms were not located in the structure. The initial model revealed that the structures of 1 and 2 contain void spaces, disordered solvent molecules, and disordered counterions. Therefore, the crystallographic data were further refined using the SQUEEZE tool of PLATON.39 Crystallographic information is summarized in Table S1. Bond-valence sums indicate that the phosphate groups in 1 are partially protonated as HPO42− (Table S2). In 2, phosphate groups are either HPO42− or H2PO4−, in equal proportions (Table S3). O atoms of μ−η1:η2 peroxide groups are not protonated in either 1 or 2 (Tables S4 and S5). 2.4. Elemental Analysis. X-ray fluorescence (XRF) analyses of individual crystals were done using an EDAX Orbis PC micro-XRF system equipped with a 50 kV, 50 W Rh tube, a 30 μm ultrahigh intensity polycapillary optic, and a 30 mm2 silicon drift detector (Apollo XRF ML-30). The position of the Cu Kα line from a standard wafer was used as a calibrant. X-rays were generated using a 35 kV voltage and 0.3 mA current. Samples were contained in an evacuated chamber for increased sensitivity. An amp time of 12.8 μs and 30 μm spot size resulted in ∼17 000 cps and a deadtime of ∼45% for all
analyses. The data (Figure S1) revealed that both 1 and 2 contain U, S, P, and Na. H and Li are not detectable using this method. Inductively coupled plasma optical emission spectra (ICP-OES) measurements were conducted using a PerkinElmer Optima 8000 ICP-OES spectrometer. Approximately 30 mg of 1 was dissolved in 5 mL of concentrated HNO3. This solution was diluted using 5 vol % HNO3 to achieve a final concentration of 1 to 20 ppm for each element of interest in a 10 mL total volume. Data were collected for matrix-matched standards to create a calibration curve for each element. The data gave the ratio of U/S/P/Na/Li as 20.00:1.04:6.24:24.15:3.93 for 1. 2.5. Thermogravimetric Analysis. TGA was conducted by using a Mettler Toledo thermal gravimetric analyzer. 15.6 mg of 1 was placed in a 70 μL alumina crucible and heated from 25 to 900 °C at a rate of 5 °C/min under flowing air. The data (Figure S2) indicate that 1 contains ∼42 H2O molecules per formula unit. 2.6. Spectroscopic Characterization. Infrared (IR) spectra for 1 were collected over the range from 600 to 4000 cm−1 using a SensIR Technologies IlluminatIR FTIR micro-spectrometer by placing a crystal on a glass slide and crushing it with a diamond attenuated total reflectance objective on the microscope. Raman spectra were collected using a Bruker Sentinel system linked via fiber optics to a Raman probe equipped with a 785 nm, 400 mW laser and a high-sensitivity, TE-cooled, 1024 × 255 CCD array. A crystal for each compound was selected and mounted on a glass slide that was placed on the stage of the microscope with a video-assisted fiber probe and measured. Many crystals of 1 were also ground and mounted on a glass slide, and spectra were repeatedly collected over time. Each spectrum was collected by using 15 s scans with three signal accumulations. 2.7. Small-Angle X-ray Scattering. A Bruker Nanostar equipped with a Cu microfocus source, Montel multilayer optics, and a HiSTAR multiwire detector was used for SAXS measurements. Scattering data were collected for water with and without a glassy carbon standard for the correction to absolute intensity and background subtraction. To monitor formation of clusters, aliquots of the reactant solution of 1 were collected and measured over time. Samples prepared by dissolving ∼200 mg of crystals of 1 in 20 mL of aqueous solution were also measured as a function of time. For each measurement, the solution was introduced to the sample chamber using a flow cell, and data were collected for 1 h with a detector distance of 26.3 cm.
3. RESULTS 3.1. Structure of U20P6. Single-crystal X-ray diffraction studies showed that 1, Li4Na12{(SO4)⊂Na12⊂[(UO2)20(O2)27(HPO4)6]}(H2O)42, contains a novel anionic uranyl peroxide cage cluster that is designated U20P6 (Figure 1). The cage is built from 20 uranyl hexagonal bipyramids and six phosphate tetrahedra. Each uranyl ion is nearly linear, with UO bond lengths in the range from 1.78(1) to 1.84(1) Å, which is typical.40 The P−O bond lengths of the tetrahedra range from 1.50(1) to 1.61(1) Å and, together with the bond-valence sums at the O positions (Table S2), are consistent with assignment of these tetrahedra as HPO42−.14,41,42 It is convenient, from the perspective of describing the cluster structure, to define three building units consisting of multiple polyhedra that are in turn combined into the complete U20P6 cage (Figure 1). The largest of these is a tetramer of uranyl ions in which the central uranyl hexagonal bipyramid shares peroxide edges with each of three other uranyl hexagonal bipyramids (Figure 1b). Two such tetramers occur in U20P6, where they define the polar regions of the cluster in the orientation illustrated in Figure 1a. The peroxide bridges between uranyl ions in this unit are in the common μ−η2:η2 configuration. The second building unit consists of two distorted uranyl hexagonal bipyramids that are bridged by two phosphate tetrahedra, such that the tetrahedra are B
DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 1. Polyhedral representations of U20P6 in crystals of 1 (a) and three types of building units (b−d). The red arrow in (a) indicates the peroxide in a μ−η1:η2 configuration. Uranyl hexagonal bipyramids are shown in yellow, turquoise, and pink; phosphate tetrahedra are in purple, and sulfate tetrahedra are blue. Figure 2. Ball-and-stick representations of a fragment of U20P6. U atoms are shown in yellow, turquoise, and pink; P atoms are in purple, and O atoms are red. O atoms of μ−η1:η2 peroxide are shown as red hollow spheres.
monodentate to the uranyl ions (Figure 1c). Three such units occur in U20P6. The third unit, of which there are also three in U20P6, contains two uranyl hexagonal bipyramids in which the uranyl ions are bridged through normal μ−η2:η2 peroxide (Figure 1d). Aside from significant polyhedral distortions in some cases (see below), the three building units defined above (Figure 1bd) are common features of uranyl peroxide cages. However, the way in which these units are linked into the overall cage is highly unusual. Most notable are the two distinct roles of peroxide. Twenty-one peroxide in U20P6 are in the usual μ−η2:η2 configuration and these have O−O bond lengths in the range of 1.45(2) to 1.52(2) Å, which are typical.5,11 Six of the peroxide groups bridge uranyl ions in a μ−η1:η2 configuration (Figure 1a, shown by a red arrow), which is unique in uranyl peroxide cages and appears to be the first such configuration in a high nuclearity metal cluster. The O−O bond lengths in these peroxide groups range from 1.46(2) to 1.50(2) Å, which are also typical. The geometric distortions of several of the uranyl hexagonal bipyramids in U20P6 are noteworthy and are associated with the occurrence of peroxide in μ−η1:η2 configurations (Figure 2). In most structures containing uranyl ions that are coordinated by two or three peroxide groups bidentate to the uranyl ion, the peroxide groups are nearly coplanar with each other, and the O−O bond is roughly perpendicular to the uranyl ion. This is also the case for those uranyl hexagonal bipyramids in U20P6 that contain three peroxide groups that are bidentate to the uranyl ion, and it is notable that each of these peroxide groups bridge uranyl ions and are in a μ−η2:η2 configuration. For contrast, consider the U(10) hexagonal bipyramid in U20P6 (Figure 2c). The uranyl ion is coordinated by two peroxide groups, as well as two monodentate phosphate tetrahedra. The O4 and O3 atoms, which are part of the phosphate tetrahedra, are 0.09 and 0.04 Å below and above a plane drawn through the U cation that is perpendicular to the uranyl ion bonds, respectively. The O45 and O86 atoms belong to a μ−η1:η2 peroxide and are 0.29 and 0.10 Å above the same plane, respectively. For the μ−η2:η2 peroxide, the O53 atom is 0.31 Å below the same plane, and the O88 atom is on it. The departure of the O45−O86 and O53−O88 peroxide groups from the common coplanar configuration is consistent with
anion−anion repulsion, as the O45−O53 separation is only 2.66 Å even in the distorted configuration, with these two O atoms being displaced in opposite directions from the plane perpendicular to the uranyl ion. The O atom that is bonded to only one U(VI) cation (O45) is bonded to a Na cation inside the cage, and twisting away from the equatorial plane enhances the interaction with the Na cation. In this regard it is important to note that the bond-valence sums for the peroxide groups indicate that none are protonated (Table S4); thus, the O45 and analogous atoms form relatively strong bonds with Na cations inside the cage. The other uranyl hexagonal bipyramids in U20P6 that contain μ−η1:η2 peroxide groups are similarly distorted. The U20P6 cluster is a topological derivative of U20, which is a cage that consists of 20 uranyl ions arranged at the vertices of a fullerene topology consisting of 12 pentagons, also known as a dodecahedron.43,44 In U20 the uranyl ions are bridged only by peroxide groups, all of which are in a μ−η2:η2 configuration. Uranyl ions in U20P6 are arranged in the same topology, although topological pentagons are strongly distorted owing to the inclusion of phosphate tetrahedra and μ−η1:η2 peroxide in the cage (Figure 3). Three other cages with 20 uranyl ions based on the same fullerene topology contain from 6 to 10 pyrophosphate units that serve as bridges between uranyl ions.43 The U20P6 cage encapsulates 12 Na cations and a tetrahedral SO42− group formed through the presumed oxidation of sulfite by peroxide in the reaction (Figure 3). The S−O bond lengths range from 1.42(3) to 1.47(2) Å. Each of the sulfate oxygen atoms is bonded to three Na cations, and each Na cation is arranged at the vertex of a nearly regular convex icosahedron. As such, each Na cation is located inside a topological pentagon of the cage, where it coordinates to five O atoms of uranyl ions. Each of six Na cations is also bonded to the nonbridging O atom of the μ−η1:η2 peroxide group. 3.2. Structure of U20P12. Extending our synthetic efforts in search of additional sulfate-centered uranyl peroxide cage clusters produced crystals of 2, although they were in low yield C
DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
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groups in the U20P6 unit are replaced by six monodentate phosphate tetrahedra in the U20P12 unit. There are two of these tetramers in U20P12, and they are in the polar regions of the illustration in Figure 4. The other unit consists of two uranyl hexagonal bipyramids that are linked by a common peroxide group. A bond-valence analysis indicates that the phosphate tetrahedra correspond to both HPO42− and H2PO4−. The latter are each linked to two uranyl ions, whereas the former are three-connected in the cage. Distortions of uranyl polyhedra in U20P12 that contain μ−η1:η2 peroxide are similar to those observed in U20P6. 3.3. Spectroscopy. Raman spectra collected for crystals of 1 and 2 contain several modes in the range from 700 to 900 cm−1 that are attributed to uranyl and peroxide stretches (Figure 5) and are complicated by the existence of several structurally distinct uranyl and peroxide configurations. We also synthesized each with 18O uranyl nitrate, and the corresponding Raman spectra provide the expected blueshifts of the uranyl stretching frequencies, whereas those associated with peroxide stretches remain unchanged, because the peroxide were not isotopically labeled. Peroxide bands are centered at 833 and 860 cm−1 for 1 and at 830 and 853 cm−1 for 2. Peroxide stretches in uranyl clusters containing μ−η2:η2 peroxide normally fall in the range of 820−840 cm−1;14,45 thus, we tentatively assign the 860 and 853 cm−1 bands in 1 and 2 to the μ−η1:η2 peroxide. Additional bands in the spectra for the isotopically labeled material indicate 16O exchange from the solution for 18O atoms of the labeled uranyl ions. Peaks in the region between 930 and 1170 cm−1 are related to stretches of both phosphate and sulfate groups.46,47 3.4. U20P6 in solution. Time-resolved SAXS profiles for the parent solution of 1 collected 1 h after combination of reactants demonstrate the presence of nanoscale objects (Figure 6). The data were successfully modeled as a sphere-shell with an outer radius of 8.5 Å, which is consistent with the size of U20P6. The scattering intensity increases modestly over several hours, and crystals containing U20P6 form after 24 h. Although there are several uranyl peroxide clusters with sizes that are similar to U20P6, it is likely that U20P6 formed rapidly in solution prior to crystallization. Given the unusual polyhedral connectivity observed in U20P6, the significant distortions of uranyl polyhedra, and the presence of very unusual μ−η1:η2 peroxide groups, we suspected that U20P6 might be unstable. Raman spectra were collected over time for a sample of ground crystals (Figure 7), and no significant spectral changes occurred over 45 d with the sample aged in an ambient atmosphere. SAXS profiles were collected for aqueous solutions into which crystals of U20P6 were dissolved, and these remained the same through 42 d (Figure 8) in contact with air. It is possible that U20P6 clusters in these time-resolved experiments transformed into other similarly sized clusters, but given that no excess peroxide was present in the solid state or the aqueous solution, it is more likely that U20P6 remained stable.
Figure 3. (a) Mixed ball-and-stick and polyhedral representation of U20P6 showing the coordination environment of a Na+ cation. The other 11 Na+ cations in the cage are omitted. (b) The sulfate-centered Na isocahedron contained in U20P6. U20P6 is shown as a graphical representation in which the vertices correspond to U6+ cations. U atoms and polyhedra are shown in yellow; phosphate tetrahedra are in purple, sulfate tetrahedra are in blue, and Na and O atoms are shown as green and red spheres, respectively.
and mixed with a fine precipitate, which precluded detailed chemical characterization. X-ray diffraction revealed that 2 contains a novel uranyl peroxide phosphate cage designated U20P12 with composition Li6Na6{(SO4)⊂Na12⊂[(UO2)20(O2)24(HPO4)6(H2PO4)6]}4− (Figure 4). XRF anal-
Figure 4. Polyhedral representations of U20P12 (2) (a) and two types of building units (b, c). The red arrow in (a) draws attention to peroxide in a μ−η1:η2 configuration. Uranyl hexagonal bipyramids are shown in yellow and pale green, phosphate tetrahedra in purple, and the disordered sulfate polyhedron is shown in blue.
ysis (Figure S1) confirmed 2 contains U, P, S, and Na. U20P12 is also a topological derivative of U20, but it differs from U20P6 in that it contains 12 phosphate tetrahedra, with P−O bond lengths ranging from 1.49(1) to 1.61(1) Å. The cluster also contains 18 μ−η2:η2 peroxide with O−O bond lengths ranging from 1.47(1) to 1.50(1) Å, and six μ−η1:η2 peroxide with O−O bond lengths averaging 1.47(1) Å. As in U20P6, U20P12 is built about a sulfate-centered isocahedral arrangement of Na cations, although the sulfate is positionally disordered in the X-ray structure (Figure S3). The U20P12 cage consisting of 20 uranyl polyhedra and 12 phosphate tetrahedra can be described in terms of two structural units (Figure 4). One of these contains a tetramer of uranyl hexagonal bipyramids (Figure 4b) that is similar to that found in U20P6 (Figure 1b), although three of the peroxide
4. DISCUSSION Reactions of solutions of UO2(NO3)2, H2O2, LiOH, H3PO4, and Na2SO3 (or Na2SO4) under ambient conditions gave clusters U20P6 and U20P12, which appear to be the only two high-nuclearity metal complexes with peroxide groups in a μ−η1:η2 configuration. Peroxide groups in these clusters link to uranyl polyhedra in two distinct fashions, the other being a μ−η2:η2 configuration. Despite the unusual connectivity of D
DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 5. Raman spectra of crystals of isotopically normal 1 (U20P6) and 2 (U20P12), as well as those crystallized using 18O enriched uranyl nitrate.
Figure 7. Time-resolved Raman spectra of ground crystals of 1. These spectra suggest that U20P6 is stable in the solid state in contact with air at least for 45 d.
icosahedron in U20P6 is larger, because it contains a sulfate tetrahedron, and the larger size is incompatible with encapsulation by the U20 cluster. In other words, U20P6 and U20P12 appear to exhibit their unusual characteristics, including six μ−η1:η2 peroxide groups, because they are templated by the (SO4)⊂Na12 unit, which is too large for the more conventional U20 cage. However, it is not clear why U20P6 and U20P12 formed rather than a larger cluster consisting only of uranyl polyhedra, such as U24 or U28. The latter of these has been shown to encapsulate a range of species, including oxyanions. The pH values of the reaction solutions in the current case were 7.4− 9.3, which is within the range in which U60 and other cages of uranyl polyhedra form.49 Previous studies revealed significant distortions of uranyl polyhedra associated with inclusion of polycarboxylate ligands in a framework structure48 and phosphate groups in a uranyl peroxide cluster that only contains μ−η2:η2 peroxide groups.14 Powder X-ray diffraction studies showed that the framework structure possessed high resistance to both β and γ radiation in the solid state and that it persisted in aqueous solutions over a wide pH range.48 A combination of SAXS and electrospray ionization mass spectrometry measurements demonstrated that the uranylphosphate cluster was stable in aqueous solution.14 These studies, as well as our current study, indicate that uranyl
Figure 6. SAXS profiles collected for the parent solution of 1 (U20P6) at various times following combination of the reactants. For comparison, SAXS profiles collected for a solution that was aged for 5 d subsequent to dissolution of crystals of 1 in water are also given.
polyhedra in these clusters, evidence suggests that they form quickly in water and that they are stable in the solid state in air and in water. Similar to U20, both U20P6 and U20P12 also present dodecahedral topologies, although inclusion of μ−η 1 :η 2 peroxide and phosphate groups strongly distort their topological pentagons. Each of these three clusters encapsulate an icosahedron defined by Na cations. However, only the U20P6 and U20P12 clusters encapsulate a sulfate tetrahedron in addition to the Na cations. This may be significant in influencing the connectivity of the uranyl polyhedra and phosphate tetrahedra in the cage. The volume of the icosahedron defined by Na cations inside U20, as measured with Na at the vertices, is 91.06 Å3. In contrast, the analogous icosahedron in U20P6 is 19.2% larger with a volume of 108.54 Å3. We propose that the Na E
DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
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Characterization Facility of the Center for Sustainable Energy at the Univ. of Notre Dame.
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Figure 8. Selected SAXS profiles of aged aqueous solutions prepared by dissolving crystals of 1 (U20P6) in pure water. Crystals of 1 slowly dissolve in water, and U20P6 appears to be stable in the resulting solution over 42 d.
structures containing distorted uranyl polyhedra can be stable. Also, the bipyramidal coordination geometry of the uranyl ion is pliable and able to accommodate ligands with diverse geometries to form various complexes, which contributes to the structural diversity of uranyl compounds.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02429. Crystallographic parameters, infrared spectra, structure illustrations, X-ray fluorescence data, bond-valences (PDF) Crystallographic information (CIF)
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REFERENCES
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Tyler L. Spano: 0000-0001-6572-9722 Peter C. Burns: 0000-0002-2319-9628 Funding
This research is funded by the Office of Basic Energy Sciences of the U.S. Department of Energy as part of the Materials Science of Actinides Energy Frontier Research Center (DESC0001089). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Chemical analyses were conducted at the Center for Environmental Science and Technology at the Univ. of Notre Dame. Spectra and diffraction data were collected at the Materials F
DOI: 10.1021/acs.inorgchem.6b02429 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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