Ionothermal and Hydrothermal Flux Syntheses of Five New Uranyl

Dec 11, 2013 - Ralph A. Zehnder , James Jenkins , Matthias Zeller , Christian Dempsey , Stosh A. Kozimor , Gregory Jackson .... P. A. Smith , P. C. Bu...
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Ionothermal and Hydrothermal Flux Syntheses of Five New Uranyl Phosphonates T. Gannon Parker,† Justin N. Cross,‡ Matthew J. Polinski,† Jian Lin,§ and Thomas E. Albrecht-Schmitt*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftain Way, Tallahassee, Florida 32306, United States Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, Indiana 46556, United States § Department of Civil & Environmental Engineering & Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States ‡

S Supporting Information *

ABSTRACT: Four new uranyl phosphonate compounds have been synthesized via ionothermal flux in the ionic liquids 1butyl-3-methylimidazolium chloride ([Bmim][Cl]) and 1ethyl-3-methylimidazolium bromide ([Emim][Br]). [C 8 H 1 5 N 2 ][UO 2 (C 6 H 5 PO 3 H)(C 6 H 5 PO 3 )] ([Bmim][UPhPO]), [C8H15N2]2[(UO2)4(C6H5PO3)3Cl4] ([Bmim][UPhPOCl]), [C8H15N2][UO2(HO3P(CH2)3PO3)] (α-[Bmim][UC3DPO]), and [C6H11N2]2[(UO2)2(p-C6H4(PO3H)2)3]· 2H2O ([Emim][UPhDPO]) form one-dimensional chains, two-dimensional sheets, or three-dimensional frameworks. For comparison, analogous reactions were carried out hydrothermally, which lead to one new framework structure, [C8H15N2]2[(UO2)5(HO3P(CH2)3PO3)4] (β-[Bmim][UC3DPO]), and one previously characterized tubular uranyl phosphonate. It was found that the structure is equally dictated by the choice of flux method, the choice of ligand, and the choice of ionic liquid.

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pairs.5 This definition may be extended to include any ionic compound in the liquid phase under the reaction conditions.3 ILs exhibit exceptional tunability since the cation and anion can be systematically varied to achieve the properties desired from the solvent. Additionally, a ternary IL system consisting of a single cation and two distinct anions can be used to achieve intermediate properties.6 The current paradigm for these task-specific ionic liquids (TSILs) in the field of f-element chemistry in recent years has been focused primarily on liquid−liquid solvent extractions, wherein an ionic liquid could be substituted for dodecane as the organic phase in combination with an extractant to remove target elements from spent nuclear fuel. While the use of TSILs does present itself as a greener and safer alternative to more volatile and flammable organic solvents7 and have been shown in specific cases to be as effective or more effective than dodecane in the extraction of uranyl ions from acidic solutions,8,9 one of the major drawbacks to this system is the tendency of the ionic liquid to contaminate the aqueous phase through ion exchange processes.10,11 While numerous studies have been carried out to study these new ionic liquid separations technologies and the coordination chemistry of actinides in ionic liquids,12−15 the use of ILs as reactive solvents is still in its infancy and has only begun to truly develop over the past decade. The ionothermal flux synthesis has gained

INTRODUCTION The use of phosphorus-containing oxoanionic ligands in the synthesis of functional actinide framework materials has recently gained attention due in part to their potential for the safe disposal and remediation of nuclear waste. The use of tributylphosphate in the PUREX process for the separation of uranium and plutonium from nuclear waste has inspired the use of phosphonates as synthons for the production of actinide framework materials, capitalizing on the diversity of possible coordination geometries of U(VI).1 Hydrothermal syntheses of actinide frameworks, especially those incorporating Th and U, have dominated the literature to date, but an ionothermal flux presents itself as a unique medium through which new actinide framework materials may be accessed. Open-framework structures are typically synthesized by hydrothermal or solvothermal methods with water or an alcohol as the solvent.2 These reactions use an organic or alkali metal template that acts as a structure-directing agent which guides the reaction toward desired structure types.3 These template cations may be used to systematically tune specific properties of a microporous structure, especially pore size and shape.4 However, new methods of preparing hybrid porous materials are still significant because of the value of these materials for catalysis, ion exchange, and separations.2 Over the past decade, ionic liquids (ILs) have received considerable attention due to their ability to satisfy this demand via an ionothermal flux. ILs are defined as salts with melting points below 100 °C that are comprised entirely of ionic species and short-lived ion © 2013 American Chemical Society

Received: September 25, 2013 Revised: December 10, 2013 Published: December 11, 2013 228

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received without further purification. 1-Butyl-3-methylimidazolium chloride ([Bmim][Cl]) and 1-ethyl-3-methylimidazolium bromide ([Emim][Br]) were prepared according to the literature.5 Reactions were run in PTFE-lined Parr 4749 autoclaves with 23 mL internal volumes. Distilled and Millipore-filtered water with a resistance of 18.2 M Ω·cm was used in all hydrothermal reactions. (Caution! While all of the uranium compounds in these studies contained depleted uranium salts, standard precautions were performed for handling radioactive materials, and all studies were conducted in a laboratory dedicated to studies on actinide elements.) Because the reactions resulted in the formation of an insoluble amorphous phase along with the crystalline phases of interest, calculating an accurate yield is exceedingly difficult and therefore omitted in this text. [Bmim][UPhPO] and [Bmim][UPhPOCl]. Uranyl nitrate hexahydrate (200 mg, 0.398 mmol), phenylphosphonic acid (126 mg, 0.797 mmol), and [Bmim][Cl] (348 mg, 1.99 mmol) were loaded into a 23 mL PTFE-lined stainless steel autoclave. After heating for 3 days at 150 °C, the autoclave was then cooled at an average rate of 5 °C/h to 25 °C. Yellow-green needles suitable for X-ray diffraction studies were isolated. α-[Bmim][UC3DPO]. Uranyl nitrate hexahydrate (20.4 mg, 0.0406 mmol), 1,3-propylenediphosphonic acid (24.4 mg, 0.119 mmol), and [Bmim][Cl] (110.3 mg, 0.630 mmol) were loaded into a 23 mL PTFE-lined stainless steel autoclave. After heating for 3 days at 150 °C, the autoclave was then cooled at an average rate of 5 °C/h to 25 °C. Yellow-green rods suitable for X-ray diffraction studies were isolated. [Emim][UPhDPO]. Uranyl nitrate hexahydrate (43.4 mg, 0.0864 mmol), 1,4-phenylenediphosphonic acid (22.1 mg, 0.0928 mmol), and [Emim][Br] (80.2 mg, 0.420 mmol) were loaded into a 23 mL PTFElined stainless steel autoclave. After heating for 3 days at 150 °C, the autoclave was then cooled at an average rate of 5 °C/h to 25 °C. Yellow-green blocks suitable for X-ray diffraction studies were isolated. β-[Bmim][UC3DPO]. Uranyl nitrate hexahydrate (50.2 mg, 0.100 mmol), 1,3-propylenediphosphonic acid (40.5 mg, 0.199 mmol), and [Bmim][Cl] (176 mg, 1.01 mmol) were loaded into a 23 mL PTFElined stainless steel autoclave with 2 mL of water. After heating for 3 days at 150 °C, the autoclave was then cooled at an average rate of 5 °C/h to 25 °C. The resulting yellow-green product was then washed with water, rinsed with ethanol, and allowed to air-dry at room temperature. Yellow-green blocks suitable for X-ray diffraction studies were isolated.

traction relative to hydrothermal and solvothermal syntheses in the formation of numerous compounds, including metal borophosphates,16 germanates,17,18 aluminophosphates,3,6 phosphate-borates,19 selenides,20 and organophosphonates.21,22 However, all of the above examples are based on transition metals, aluminum or tin, and to date, very few examples of ionothermal flux syntheses exist using f-elements, especially actinides. In this work, we expand on our previous syntheses of actinide materials with phosphonate linkers produced via hydrothermal flux.1,23−31 The IL flux (Figure 1) acts as both solvent and

Figure 1. 1-Butyl-3-methylimidazolium chloride (left) and 1-ethyl-3methylimidazolium bromide (right) were employed to act as both solvent and template in the ionothermal flux reactions.

template in these reactions and has been shown to play an important role in determining the structure of each compound. Herein, we report the synthesis and structural characterization of [C 8 H 15 N 2 ][UO 2 (C 6 H 5 PO 3 H)(C 6 H 5 PO 3 )] ([Bmim][UPhPO]), [C8H15N2]2[(UO2)4(C6H5PO3)3Cl4] ([Bmim][UPhPOCl]), [C 8 H 15 N 2 ][UO 2 (HO 3 P(CH 2 ) 3 PO 3 )] (α[Bmim][UC3DPO]), [C6H11N2]2[(UO2)2(pC 6 H 4 (PO 3 H) 2 ) 3 ]·2H 2 O ([Emim][UPhDPO]), and [C 8 H 15 N 2 ] 2 [(UO 2 ) 5 (HO 3 P(CH 2 ) 3 PO 3 ) 4 ] (β-[Bmim][UC3DPO]). See Table 1 for the crystallographic data.

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EXPERIMENTAL SECTION

Synthesis. Uranyl nitrate hexahydrate (98%, International BioAnalytical Industries), phenylphosphonic acid (≥98%, Aldrich), 1,3propylenediphosphonic acid (≥98%, Alfa Aesar), 1,4-phenylenediphosphonic acid (98%, Epsilon Chimie), chlorobutane (≥99%, Alfa Aesar), and 1-methylimidazole (99%, Alfa Aesar) were used as

Table 1. Crystallographic Data for [C8H15N2][UO2(C6H5PO3H)(C6H5PO3)] ([Bmim][UPhPO]), [C8H15N2]2[(UO2)4(C6H5PO3)3Cl4] ([Bmim][UPhPOCl]), [C8H15N2][UO2(HO3P(CH2)3PO3)] (α-[Bmim][UC3DPO]), [C6H11N2]2[(UO2)2(p-C6H4(POH)2)3]·2H2O ([Emim][UPhDPO]) and [C8H15N2]2[(UO2)5(HO3P(CH2)3PO3)4] (β[Bmim][UC3DPO])

a

compound

[Bmim][UPhPO]

[Bmim][UPhPOCl]

α-[Bmim][UC3DPO]

[Emim][UPhDPO]

β-[Bmim][UC3DPO]

formula mass color and habit space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z T (K) λ (Å) ρcalcd (g cm−3) μ (Mo Kα) (mm−1) R(F) for Fo2 > 2σ(Fo2)a Rw(Fo2)b

722.40 yellow-green, needle P21/c (no. 14) 10.225(2) 10.197(2) 23.917(4) 90 99.794(2) 90 4 296 0.71073 1.953 6.782 0.027 0.060

1177.35 yellow-green, needle P21/c (no. 14) 10.604(1) 16.431(1) 21.809(1) 90 90.493 90 4 100 0.71073 2.058 8.791 0.025 0.061

610.28 yellow-green, rod P1̅ (no. 2) 9.861(1) 10.042(1) 10.710(1) 71.712(1) 65.666(1) 68.082(1) 2 296 0.71073 2.303 9.442 0.020 0.044

751.27 yellow-green, block P1̅ (no. 2) 8.883(1) 11.770(1) 12.119(1) 118.082(1) 90.882(1) 91.923(1) 2 100 0.71073 2.191 7.522 0.023 0.058

2432.70 yellow-green, block P1̅ (no. 2) 9.461(2) 13.424(3) 13.453(3) 67.360(3) 70.649(4) 88.418(4) 1 100 0.71073 2.733 13.960 0.020 0.047

R(F) = Σ||Fo| − |Fc||/Σ|Fo|. bR(F2o) = [Σw(F2o − F2c )2/Σw(F4o )]1/2 229

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Figure 2. Structural views of [Bmim][UPhPO]. (a) Polyhedral view along the a axis where purple tetrahedra are C−PO32‑ or C−PO3H− moieties and yellow octahedra are U6+ metal centers. Weak interactions among the phenyl groups form channels in which the [Bmim] cations reside. (b) Same structure normal to the [ac] plane and clearly displays the chain structure which propagates parallel to the a axis. For clarity, all hydrogen atoms have been omitted, except for those participating in hydrogen bonds, and [Bmim] cations have been omitted from (b). Red spheres indicate hydrogen bond donor or acceptor oxygen atoms. Crystallographic Studies. Single crystals of [Bmim][UPhPO], [Bmim][UPhPOCl], α-[Bmim][UC3DPO], [Emim][UPhDPO], and β-[Bmim][UC3DPO] were mounted on cryoloops with viscous oil and optically aligned on a Bruker APEXII QUAZAR or D8 QUEST single crystal X-ray diffractometer using a digital camera. Initial intensity measurements were performed using a IμS X-ray source with a 30 W (QUAZAR) or 50 W (QUEST) microfocused sealed tube (Mo Kα, λ = 0.71073 Å) with high-brilliance and high-performance focusing multilayer optics. Standard APEXII software was used for the determination of unit cells and data collection control. The intensities of reflections of a sphere were collected by a combination of four sets of exposures (frames). Each set had a different φ angle for the crystal, and each exposure covered a range of 0.5° in ω. A total of 1464 frames was collected with an exposure time per frame of 10 or 40 s, depending on the size and quality of the crystal. SAINT was used for data integration, including Lorentz and polarization corrections. Semiempirical absorption corrections were applied using the program SCALE (SADABS).32 The program suite SHELX-97 was used for space group determination (XPREP), direct methods or Patterson methods structure solution (XS) and least-squares refinement (XL).33 All space group determinations were checked using the program PLATON.34 The final refinements included anisotropic displacement parameters for most atoms. The positions of all hydrogen atoms were calculated and their refinement parameters were constrained, except for those of water molecules which were not accounted for. Atomic coordinates, bond distances, and additional structural information are provided in the CIF files of the Supporting Information. UV−vis-NIR and Fluorescence Spectroscopy. UV−vis-NIR absorption data were acquired from single crystals of [Bmim][UPhPO], [Bmim][UPhPOCl], α-[Bmim][UC3DPO], [Emim][UPhDPO], and β-[Bmim][UC3DPO] using a Craic Technologies microspectrophotometer. Crystals were placed on quartz slides under Krytox oil, and the data were collected from 250 to 750 nm. The exposure time was automatically optimized by the Craic software. Fluorescence data were collected using 1 to 4 s exposures of 420 nm light from a mercury lamp for excitation. Absorption and emission data can be found in the Supporting Information. Raman Spectroscopy. Raman spectra were acquired from single crystals of [Bmim][UPhPO], [Bmim][UPhPOCl], α-[Bmim][UC3DPO], [Emim][UPhDPO], and β-[Bmim][UC3DPO] using a Craic Technologies Apollo 785 microspectrophotometer equipped with a 100 mW mercury lamp with a center wavelength of 785 nm over a range from 100 cm−1 to 2100 cm−1 with 5 s exposures. Raman spectra can be found in the Supporting Information.

phenylphosphonate ligands, which results in chains propagating normal to the [bc] plane (Figure 2). There appear to be weak interactions among the phenyl rings, which result in channels along which the imidazolium cations reside. The inclusion of the [Bmim] cations serves a role in both balancing the charge and controlling the topology of the structure. The structure shows no indication of coordination of either chloride (from the flux) or water to the uranium center. The uranium coordinates with two nearly linear “yl” oxo atoms at the axial positions, resulting in the conventional UO22+ moiety, with an O−U−O bond angle of 178.4(1)° and UO bond lengths of 1.781(3) Å and 1.783(3) Å. The uranyl unit is equatorially coordinated by four oxygen atoms from the phosphonate ligands with average bond lengths of 2.29(1) Å. Bond valence sum calculations based on these bond lengths gave a value of 6.104 for uranium, which agrees with the valence given by charge balance.35−37 One P−OH moiety was found in the structure. The P−OH was identified by its lack of coordination to uranium and its P−O bond length of 1.540(3) Å, which is somewhat longer than the average P−O bond length of 1.52(1) Å for the other oxygen atoms coordinated to uranium. This was corroborated by a bond valence sum of 1.189 for the P(2)−O(8) bond. There is likely a hydrogen bond between O(8) and O(7) (Figure 3) with an O−H···O distance of 2.459 Å. The P(1)−O(7) bond is exceptionally short with a bond length of 1.507(3) Å, presumably due to greater double bond character between P(1) and O(7). Structure of [C8H15N2]2[(UO2)4(C6H5PO3)3Cl4] ([Bmim][UPhPOCl]). [Bmim][UPhPOCl] crystallizes in the centrosymmetric monoclinic space group P21/c, and it is a onedimensional chain structure composed of one crystallographically unique UO6 tetragonal bipyramid and one crystallographically unique UO4Cl2 tetragonal bipyramid. The UO6 and UO4Cl2 moieties are isolated from one another but are bridged by corner-sharing interactions with the PO3 groups of the two unique phenylphosphonate ligands to form repeating [(UO2)2(C6H5PO3)2Cl2]2− units. These dinuclear units are further bridged through corner-sharing interactions by the phosphonate ligands to neighboring units, resulting in infinite one-dimensional chains propagating normal to the [bc] plane (Figure 4). The [Bmim] cations occupy the void space between parallel chains and serve a role in both balancing the charge and controlling the topology. [Bmim][UPhPOCl] shows no indication of coordination of water, but unlike [Bmim][UPhPO], the inner sphere coordination of chloride ions is observed for U(2) where two chloride ions coordinate cis in the equatorial plane. The two unique [Bmim] cations were found to have disordered alkyl substituents. All sp3 carbon atoms appear to be disordered over two sites. This disorder has been modeled for

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RESULTS AND DISCUSSION Structure of [C 8 H 15 N 2 ][UO 2 (C 6 H 5 PO 3 H)(C 6 H 5 PO 3 )] ([Bmim][UPhPO]). [Bmim][UPhPO] crystallizes in the centrosymmetric monoclinic space group P21/c, and it is a one-dimensional chain structure with a single crystallographically unique uranyl (UO22+) cation that adopts a tetragonal bipyramidal geometry. The UO6 tetragonal bipyramids are isolated from one another but are bridged by cornersharing interactions with the PO3 and PO3H moieties of the 230

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sum of 1.90(2), which is consistent with two-coordinate oxygen. Additionally, the average P−O bond length of 1.524(5) Å and the lack of an elongated P−O bond further points to a lack of protonated oxygen atoms. Structure of [C 8 H 15 N 2 ][UO 2 (HO 3 P(CH 2 ) 3 PO 3 )] (α[Bmim][UC3DPO]). α-[Bmim][UC3DPO] crystallizes in the centrosymmetric triclinic space group P1̅. It consists of UO6 tetragonal bipyramids bridged by PO3 and PO3H moieties to form chains propagating normal to the bc plane, which are linked by the propylene chains of the 1,3-propylenediphosphonate ligand resulting in the overall sheet topology (Figure 5).

Figure 5. Structural views of α-[Bmim][UC3DPO]. (a) Polyhedral view along the a axis where purple tetrahedra are C−PO32− or C− PO3H− moieties and yellow octahedra are U6+ metal centers. The propylenediphosphonate ligand acts as a linker between chains to form sheets, and the [Bmim] cations occupy the interlayer space. (b) Same structure normal to the [ac] plane and clearly displays the sheet topology. For clarity, all hydrogen atoms have been omitted, except for those participating in hydrogen bonds. Red spheres indicate hydrogen bond donor and acceptor oxygen atoms.

Figure 3. O(8) was determined to be protonated, and it forms a hydrogen bond with O(7) with an O−H···O distance of 2.459 Å.

all sp3 carbon atoms with the exceptions of C(19) and C(20), for which the disordered site(s) could not be satisfactorily resolved. C(19) and C(20) thus have somewhat larger thermal parameters of 0.108 and 0.127, respectively. All disordered carbon atoms were refined isotropically, and calculations for the positions of hydrogen atoms for the disordered atoms were omitted. N(3) was also found to be disordered over two sites but was nonetheless refined anisotropically. The two unique uranium centers both coordinate with two nearly linear “yl” oxo atoms at the axial positions with average bond angles of 178.3(5)° and average UO bond lengths of 1.781(1) Å. U(1) is coordinated by four oxygen atoms about its equatorial plane with an average U−O bond distance of 2.29(2) Å. U(2) is coordinated about its equatorial plane by two oxygen atoms and two chloride ions with average bond distances of 2.250(4) Å and 2.682(6) Å, respectively. Bond valence sum calculations for U(1) and U(2) yielded values of 6.139 and 6.188, respectively, which is consistent with U(VI). All of the phosphonate oxygen atoms are coordinated to a uranium center, which suggests the absence of any protonated oxygen atoms. This is corroborated by an average oxygen bond valence

The well-ordered [Bmim] cations occupy the interlayer space between the sheets and again serve the role of balancing the charge of the anionic network. Here again, as with [Bmim][UPhPO], the structure shows no indication of coordination of either water or chloride, and the uranium centers adopt the uncommon tetragonal bipyramidal geometry. There is one crystallographically unique uranyl center in the structure which coordinates with two nearly linear “yl” oxo atoms at the axial positions with an average bond angle of 177.9(1)° and UO bond lengths of 1.779(2) Å and 1.788(2) Å. The uranyl unit is equatorially coordinated by four nearly coplanar oxygen atoms from the phosphonate ligands with an average bond length of 2.291(9) Å. Bond valence sum calculations based on these bond lengths gave a value of 6.094 for uranium, which is consistent with U(VI). A protonated P−OH moiety was also found in the structure. O(7) was determined to be protonated because of the lack of

Figure 4. Structural views of [Bmim][UPhPOCl]. (a) Polyhedral view along the a axis where purple tetrahedra are C−PO32− moieties, green spheres are chlorine atoms, and yellow octahedra are U6+ metal centers. Disordered [Bmim] cations occupy the void space between parallel chains. (b) Same structure normal to the [ac] plane and clearly displays the chain structure which propagates parallel to the a axis. For clarity, all hydrogen atoms have been omitted and [Bmim] cations have been omitted from (b). 231

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the rigid phenylene unit of the 1,4-phenylenediphosphonate ligand occupies more space than the propylene chain of the 1,3propylenediphosphonate ligand in the above structure, which leaves an extra site available for coordination about the UO2 equatorial plane. A solvent water molecule is incorporated into the crystal structure which likely originates from the residual water in the hygroscopic ionic liquid. All atoms of the [Emim] cation, with the exception of C(10), were found to be disordered over two sites, and each site was assigned half occupancy. There is one crystallographically unique uranyl center in the structure which coordinates with two nearly linear “yl” oxo atoms at the axial positions with an average bond angle of 179.22(8)° and average UO bond lengths of 1.774(2) Å and 1.777(2) Å. The uranyl center is coordinated by five nearly coplanar equatorial oxygen atoms with an average bond length of 2.37(3) Å forming the common pentagonal bipyramid. Bond valence sum calculations based on these bond lengths gave a value of 6.029 for uranium, which is consistent with U(VI). All of the phosphonate groups were determined to have one protonated oxygen atom. The protonated oxygen atoms were identified by their average P−O bond length of 1.58(1) Å. This is somewhat longer than the P−O bonds for the oxygen atoms that coordinate the uranyl unit which have an average bond length of 1.500(5) Å. The assumption that O(5), O(8), and O(10) are protonated is corroborated by their bond valence sums of 1.105, 1.053, and 1.044, respectively. These hydroxyl groups likely participate in hydrogen bonding interactions (Figure 8) with O(4) and O(5); the O(5)−H···O(4) length is 2.456 Å, the O(8)−H···O(4) length is 2.638 Å, and the O(10)−H···O(5) length is 2.703 Å. An intermediate P−O bond length of 1.525(2) Å was observed for O(4), which does not appear to be protonated and does not coordinate the uranium atom; this intermediacy is likely due to enhanced double bond character in the P−O bond.

coordination to a uranium center, and this assumption was corroborated by a bond valence sum of 1.235. There is likely a hydrogen bond between O(7) and O(8) with an O−H···O distance of 2.430 Å (Figure 6).

Figure 6. O(7) was determined to be protonated, and it forms a hydrogen bond with O(8) with an O−H···O distance of 2.430 Å.

Structure of [C6H11N2]2[(UO2)2(p-C6H4(PO3H)2)3]·2H2O ([Emim][UPhDPO]). Like α-[Bmim][UC3DPO], [Emim][UPhDPO] crystallizes in the centrosymmetric triclinic space group P1̅. It consists of UO7 pentagonal bipyramids linked together in three dimensions by the PO3H moieties of the 1,4phenylenediphosphonate ligands. This bridging between the uranyl centers results in the formation of a porous threedimensional framework (Figure 7) with the pores having

Figure 7. Polyhedral view of [Emim][UPhDPO] along the a axis where purple tetrahedra are C−PO3H− moieties and yellow polyhedra are U6+ metal centers. The phenylenediphosphonate ligand acts as a linker in three-dimensions between U6+ metal centers, and disordered [Emim] cations and water molecules occupy the channels resulting from this connectivity.

dimensions of 5.717 Å [measured from O(8) to O(10)] by 9.758 Å [measured from O(4) to O(5)] . Disordered [Emim] cations and water molecules occupy the channels, which propagate normal to the [bc] plane. It is notable in this structure that the uranium centers do not adopt the tetragonal bipyramidal geometry as in α-[Bmim][UC3DPO]; instead, a phosphonate oxygen atom coordinates the uranyl unit and links parallel uranyl phosphonate sheets. This is presumably because

Figure 8. O(5), O(8), and O(10) were determined to be protonated. O(5) and O(8) form hydrogen bonds with O(4) with O−H···O distances of 2.456 Å and 2.638 Å, respectively. O(10) forms a hydrogen bond with O(5) as the acceptor with an O−H···O distance of 2.703 Å. 232

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Structure of [C8H15N2]2[(UO2)5(HO3P(CH2)3PO3)4] (β[Bmim][UC3DPO]). β-[Bmim][UC3DPO] crystallizes in the centrosymmetric triclinic space group P1̅. The structure contains tetramers of edge-sharing UO7 pentagonal bipyramids, where two of the U atoms are crystallographically unique. The edge-sharing interactions are formed through O(4) and O(5), which are donated from PO3 groups. These tetramers are linked by individual bridging phosphonate groups to form onedimensional chains, which are further linked by diphosphonate ligands to form two-dimensional sheets. These sheets are then linked together by corner-sharing interactions between U(3) and two distinct phosphonate groups, ultimately resulting in a bridge between U(2) of one sheet to U(2) of the next. The result of this connectivity is a three-dimensional porous structure (Figure 9), with disordered [Bmim] cations

elongated P−O bonds for O(15) and O(16) which were 1.576(3) Å and 1.562(3) Å, respectively, compared to an average of 1.53(2) Å for the other P−O bonds coordinated to uranium. This assumption is also validated by bond valence sums of 1.079 and 1.117 for the P(4)−O(15) and P(3)−O(16) bonds, respectively. O(17) stands out for its particularly short P−O bond length of 1.505(3) Å and lack of coordination to a uranium center. This is probably due to greater P−O double bond character. There are likely hydrogen bond interactions (Figure 10) with O(17) for both O(15) and O(16); the O(15)−H···O(17) distance is 2.649 Å and the O(16)−H··· O(17) distance is 2.606 Å.

Figure 10. O(15) and O(16) were determined to be protonated. O(15) and O(16) form hydrogen bonds with O(17) with O−H···O distances of 2.649 Å and 2.606 Å, respectively. Figure 9. Structural views of β-[Bmim][UC3DPO]. (a) Polyhedral view along the a axis where purple tetrahedra are C−PO32− or C− PO3H− moieties and yellow polyhedra are U6+ metal centers. The propylenediphosphonate ligand acts as a linker between chains of tetramers to form sheets, which are further connected in a third dimension by corner-sharing interactions with U(3), which is shown in (b). The result is a porous three-dimensional structure with disordered [Bmim] cations occupying the pores. For clarity, all hydrogen atoms have been omitted.

Fluorescence Spectroscopy. The expected green emission centered between 520 and 530 nm was observed for all five of the title compounds and is the result of charge transfer between the uranyl centers and the ligands related to both symmetric and antisymmetric vibrational modes of the uranyl cation. The emissions of all of the compounds exhibit characteristic vibronic fine-structure of varying resolution, the features of which have previously been thoroughly studied and identified.38 The fine-structure could be resolved in all compounds, but it is especially pronounced in the spectra collected for [Emim][UPhDPO] and β-[Bmim][UC3DPO]. The minor differences in the spectra are likely due to the different coordination environments of the uranium centers.39 The differences in resolution of the spectra can also be attributed to the size and quality of the crystal being studied and disorder about the equatorial plane of the uranyl cation.1 UV−vis-NIR Spectroscopy. The UV−vis-NIR spectroscopy of U(VI) is dominated by a vibronically coupled charge transfer band centered at ∼420 nm due to the uranyl ion, and the spectra for these compounds are no exception. The peak is the result of an oxo-to-metal charge transfer between the HOMO and LUMO orbitals of the uranyl cation, which are formed from the hybridization of the 5f and 2p orbitals. The spectra for all five compounds also feature an intense and broad peak at ∼300 nm due to the ligand-to-metal charge transfer.40 Raman Spectroscopy. The spectra of [Bmim][UPhPO], [Bmim][UPhPOCl], α-[Bmim][UC 3 DPO], [Emim][UPhDPO], and β-[Bmim][UC3DPO] all exhibit sharp intense peaks at 818, 833, 840, 830, and 822 cm−1, respectively, which correspond to the symmetric “yl” U−O stretching mode.

occupying the pores, which measure 7.526 Å by 12.082 Å (measuring from O(15) and O(14), respectively, to their corresponding symmetry equivalents). It is remarkable that this structure, synthesized via hydrothermal methods, is so vastly different than its ionothermally synthesized counterpart, α[Bmim][UC3DPO]. The [Bmim] cation was found to have a disordered butyl chain. C(13) and C(14) were each found to be disordered over two sites, and all four sites were assigned half occupancy. C(13A) and C(14B) were refined anisotropically; however, it was necessary to refine C(13B) and C(14A) isotropically. The structure consists of three crystallographically unique uranium centers, two of which adopt pentagonal bipyramidal geometries and one which takes a tetragonal bipyramidal geometry. All three uranium centers each coordinate with two nearly linear “yl” oxo atoms at the axial positions with an average bond angle of 178.8(1)° for U(1) and U(2) and 180.0° for U(3), which lies on an inversion center. The average oxo bond length for all three uranium centers was 1.771(4) Å. U(1) and U(2) were coordinated about their equatorial plane by five nearly coplanar oxygen atoms with average bond distances of 2.39(1) Å and 2.40(1) Å, respectively. U(3) is coordinated about its equatorial plane by four nearly coplanar oxygen atoms with an average bond distance of 2.30(2) Å. Bond valence sum calculations for U(1), U(2), and U(3) yielded values of 5.993, 5.941, and 5.831, respectively, which fit nicely into the regime of U(VI). Both of the diphosphonate ligands presumably have a single protonated oxygen atom based on the relatively

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CONCLUSIONS The five new materials presented herein demonstrate the ability of an ionothermal flux to produce new and structurally diverse compounds that differ significantly from similar compounds produced via hydrothermal methods. To demonstrate the difference the flux makes in structure determination, syntheses 233

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Chemistry and Biochemistry, for his contributions and thoughtful discussions on crystallography.

similar to those used for [Bmim][UPhPO], [Bmim][UPhPOCl], α-[Bmim][UC3DPO], and [Emim][UPhDPO] were executed, with the primary difference in the reactions being that 2 mL of water was added to the reaction mixture. In this way, a previously reported structure41 was obtained using the PhPO ligand, no crystalline product was obtained using the PhDPO ligand, and β-[Bmim][UC3DPO] was formed using the C3DPO ligand. In all of the cases reported above, the cation of the ionic liquid is incorporated into the structure and appears to satisfy the roles of both charge-balancing ion and structuredirecting agent. The primary goal of this study was to use the ionic liquid as both solvent and template for the synthesis of threedimensional framework structures which could be used for catalysis, ion exchange, or separations; however, it was found that the ligand, the flux method (hydrothermal or ionothermal), and ionic liquid used ([Bmim][Cl] or [Emim][Br]) all play an equally important role in structure determination. While it was not surprising that the reaction with phenylphosphonic acid did not yield high-dimensionality structures, it was surprising that β-[Bmim][UC3DPO], synthesized hydrothermally, was three-dimensional, yet its ionothermally synthesized counterpart α-[Bmim][UC3DPO] was two-dimensional. As mentioned above, it is also interesting that the ionothermal flux can succeed in forming a crystalline product when a hydrothermal flux may otherwise fail, as was the case for [Emim][UPhDPO]. Finally, the importance of the specific ionic liquid used was indicated by the formation of [Emim][UPhDPO] when [Emim][Br] was used as flux and the lack of any similar crystalline phase when [Bmim][Cl] was used as flux. Future studies with the ionothermal flux syntheses of actinide phosphonates will seek to systematically deduce the importance of the cation and anion in structure determination. We will also expand on our knowledge of transuranic phosphonate materials formed using an ionothermal flux synthesis in an effort to find new nuclear waste forms and separations technologies.

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ASSOCIATED CONTENT

S Supporting Information *

X-ray crystallographic files in CIF format for [C8H15N2][UO 2 (C 6 H 5 PO 3 H)(C 6 H 5 PO 3 )] ([Bmim][UPhPO]), [C8H15N2][UO2(HO3P(CH2)3PO3)] (α-[Bmim][UC3DPO]), [C 6 H 11 N 2 ] 2 [(UO 2 ) 2 (p-C 6 H 4 (POH) 2 ) 3 ]·2H 2 O ([Emim][UPhDPO]), and [C8H15N2]2[(UO2)5(HO3P(CH2)3PO3)4] (β-[Bmim][UC3DPO]). Selected bond angles, crystal images, absorption spectra, emission spectra, and Raman spectra are also available. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for support provided by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, Heavy Elements Chemistry Program, U.S. Department of Energy, under Grant DE-FG0213ER16414. The author would also like to thank Allen G. Oliver of the University of Notre Dame, Department of 234

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(39) Leung, A. F.; Hayashibara, L.; Spadaro, J. J. Phys. Chem. Solids 1999, 60, 299−304. (40) (a) Liu, G.; Beitz, J. V. Spectra and Electronic Structures of Free Actinide Atoms. In The Chemistry of the Actinide and Transactinide Elements; Morss, L. R., Edelstein, N. M., Fujer, J., Eds.; Springer: The Netherlands, 2006; Vol. 4, Chapter 16, pp 2013−2111; (b) Carnall, W. T.; Liu, G. K.; Williams, C. W.; Reid, M. F. J. Chem. Phys. 1991, 95, 7194−7203. (41) Aranda, M. A. G.; Cabeza, A.; Bruque, S.; Poojary, D. M.; Clearfield, A. Inorg. Chem. 1998, 37, 1827−1832.

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