Porous Uranium Diphosphonate Frameworks with Trinuclear Units

(1) The uranyl UO22+ dication dominates the chemistry of U(VI), and is present .... hydrogen atoms were placed geometrically and refined using a ridin...
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Article Cite This: Inorg. Chem. 2017, 56, 13249-13256

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Porous Uranium Diphosphonate Frameworks with Trinuclear Units Templated by Organic Ammonium Hydrolyzed from Amine Solvents Zhi-Hui Zhang,†,§ Ganna A. Senchyk,† Yi Liu,‡ Tyler Spano-Franco,† Jennifer E. S. Szymanowski,† and Peter C. Burns*,†,‡ †

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Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡ Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, United States § Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Changzhou University, Changzhou 213164, P. R. China S Supporting Information *

ABSTRACT: By varying solvent systems, the solvothermal treatment of uranyl nitrate and methylenediphosphonic acid (H4PCP) afforded three new porous uranyl-organic frameworks (UOFs). All were structurally characterized by single-crystal X-ray diffraction and formulated as (Et2NH2)2[(UO2)3(PCP)2](H2O)2.5 (1), (MeNH3)(H 3 O)[(UO 2 ) 3 (PCP)2 (H 2 O) 3 ] (2), and [Na(H 2 O) 4 ](H 3 O)[(UO2)3(PCP)2(H2O)2](H2O)5 (3). These compounds crystallize with three-dimensional anionic frameworks containing U(VI) and distinct cationic species due to in situ solvent hydrolysis. The solvent systems diethylformamide (DEF), N-methyl-2-pyrrolindone (NMP), and the additive sodium vanadate (Na3VO4) significantly impact the resultant structures, affording diethyl ammonium, methyl ammonium, and sodium cations captured in channels of the anionic frameworks of 1−3. In 1, a trinuclear U3O18 unit formed by three uranyl polyhedra that share edges is connected into a three-dimensional framework. Compound 2 has a three-dimensional framework formed from a uranyl-methylenediphosphonate layer that is pillared by UO7 pentagonal bipyramids. With the inclusion of sodium cations, 3 is a porous framework containing UO7 pentagonal bipyramids within a layer, with sodium cations and UO6 square bipyramids linking the adjacent layers. Compounds 1−3 feature the uranyl/ligand ratio of 3:2, but present diverse structural building units ranging from edge-shared trinuclear to heteronuclear assemblies. The compounds have been characterized by infrared (IR), Raman, and UV−vis spectroscopies, X-ray diffraction, and thermogravimetric analysis.

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INTRODUCTION A motivation for studies of uranium-based materials is their potential applications in advanced nuclear fuel cycles.1 The uranyl UO22+ dication dominates the chemistry of U(VI), and is present in Nature in a diverse family of uranyl minerals.2 The coordination environment about the uranyl ion is flexible, with four to six equatorial ligands, giving square, pentagonal, and hexagonal bipyramidal geometries in which the oxygen atoms of the uranyl ion forms the apexes. However, opportunities for controlling the dimensionality and porosity of uranyl-organic frameworks (UOFs) are largely undeveloped.3−10 Although the diphosphonate ligand was identified as a promising candidate for the construction of UOFs,10 the generally inert nature of the uranyl oxo atoms often results in one- or two-dimensional structural units, rather than three-dimensional extended frameworks. Development of UOFs with higher dimensionality is a potentially fruitful direction of effort.4 Methylenediphosphonate has been used to create diverse structures with uranyl that include cages,11 chains,12−14 layers,12,15−18 and three-dimensional frameworks.14,16,19,20 Among the five reported examples of three-dimensional frameworks, three contain UO7 mononuclear uranyl pentagonal © 2017 American Chemical Society

bipyramids, one has pairs of UO7 uranyl pentagonal bipyramids forming dimers by sharing edges as well as a UO7 uranyl pentagonal monomer, and the most remarkable compound is a mixed-valent uranium (IV/VI) diphosphonate with hexavalent uranium UO6 octahedra and eight-coordinated tetravalent uranium UO8 units.20 These compounds crystallize in both chiral and achiral space groups, influenced by the organic templates. The architectures of uranyl phosphonates are typically adapted by the degree of deprotonation of the phosphonate ligand caused by the amino templates such as aliphatic amines11,14,19−21 and aromatic amines.22−26 Although hydrolysis of dimethylformamide (DMF) or diethylformamide (DEF) in solvothermal synthesis of coordination systems has been documented,27 the extent to which solvent hydrolysis influences uranyl-organic framework formation is largely unexplored. Herein, we report a series of organic amine templated UOFs: (Et2NH2)2[(UO2)3(PCP)2](H2O)2.5 (1), (MeNH3)(H3O)[(UO2)3(PCP)2(H2O)3] (2), and [Na(H2O)4](H3O)[(UO2)3(PCP)2(H2O)2](H2O)5 (3). Received: August 6, 2017 Published: October 17, 2017 13249

DOI: 10.1021/acs.inorgchem.7b02019 Inorg. Chem. 2017, 56, 13249−13256

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Inorganic Chemistry Table 1. Crystallographic Data and Structure Refinement Results for Compounds 1−3 structure formula formula weight crystal system space group a (Å) b (Å) c (Å) α β (deg) γ V (Å3) Z crystal size (mm) ρcalcd (g/cm3) μ (mm−1) Rint a R1 [I > 2σ(I)] a wR2 (all data) b GOF on F2 Δρmax, Δρmin (e Å−3) absolute structure parameter a

1

2

3

C20H56N4O41P8U6 2684.62 orthorhombic Fdd2 21.5439 (15) 48.643 (3) 11.1769 (6) 90 90 90 11713.0 (12) 8 0.20 × 0.17 × 0.16 3.045 16.854 0.0293 0.0182 0.0411 1.031 1.51, −0.92 0.012 (3)

C6H23N2O43P8U6 2487.20 triclinic P1̅ 9.201 (3) 11.864 (3) 12.475 (3) 97.499 (3) 101.924 (3) 104.468 (3) 1266.1 (6) 1 0.20 × 0.18 × 0.17 3.262 19.477 0.0382 0.0367 0.0907 1.061 3.69, −3.67

C2H16NaO30P4U3 1381.11 monoclinic P21/c 7.5870 (5) 11.0699 (7) 16.1197 (11) 90 92.265 (1) 90 1352.79 (15) 2 0.20 × 0.10 × 0.07 3.391 18.283 0.0341 0.0222 0.0522 0.974 0.91, −1.15

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑w(Fo2)2]1/2. bGOF = {∑[w(Fo2 − Fc2)2]/(n − p)}1/2.

Table 2. Selected Bond Lengths (Å) for Compounds 1−3a compound 1 U1−O14 U1−O13 U1−O6i U1−O4 U1−O3i U1−O1 U1−O7

1.762 1.769 2.240 2.327 2.421 2.424 2.474

(5) (6) (5) (5) (5) (5) (5)

U2−O16 U2−O15 U2−O2 U2−O11ii U2−O10ii U2−O7 U2−O1 U2−O8

1.767 1.778 2.429 2.457 2.489 2.545 2.546 2.546

(5) (5) (5) (5) (5) (5) (5) (5)

U3−O17 U3−O18 U3−O5iii U3−O12 U3−O9ii U3−O10ii U3−O8

1.761 1.771 2.338 2.382 2.399 2.418 2.441

(5) (5) (5) (5) (5) (5) (5)

2.401 2.393 1.766 1.769 2.534 2.370 2.334

(7) (8) (9) (9) (9) (7) (8)

U3−O2 U3−O15iii U3−O17iv U3−O18iii U3−O19 U3−O20 U3−O21

2.392 2.349 2.322 2.295 1.781 1.798 2.524

(7) (7) (7) (7) (7) (8) (8)

2.376 2.514 1.771 2.305 2.309

(4) (4) (4) (4) (4)

Na1−O10 Na1−O11 Na1−O9

2.322 (6) 2.410 (5) 2.559 (4)

compound 2 U1−O1 U1−O4 U1−O5i U1−O7 U1−O8 U1−O9 U1−O14

2.382 2.335 2.340 2.535 1.773 1.785 2.325

(7) (7) (7) (7) (7) (8) (7)

U2−O3 U2−O6 U2−O10 U2−O11 U2−O12 U2−O13ii U2−O16ii

U1−O9 U1−O8 U1−O3i U1−O5i U1−O1

1.776 1.782 2.336 2.343 2.366

(4) (4) (4) (4) (4)

U1−O4 U1−O7 U2−O12 U2−O2iii U2−O6

compound 3

Symmetry codes: 1, (i) −x + 2, −y + 3/2, z + 1/2; (ii) x − 1/4, −y + 5/4, z − 1/4; (iii) −x + 7/4, y − 1/4, z + 1/4; 2, (i) −x, −y + 2, −z; (ii) −x, −y + 1, −z; (iii) x + 1, y, z; (iv) −x, −y + 1, −z + 1; 3, (i) −x, y − 1/2, −z + 1/2; (iii) x + 1, y, z. a

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Each crystallizes with a three-dimensional framework and a uranyl/ligand ratio of 3:2 and contain diverse structural building units ranging from edge-shared trinuclear to heteronuclear assemblies. Diethylformamide (DEF) and Nmethyl-2-pyrrolindone (NMP) have significantly impacted the resultant structures of 1 and 2. For 3, sodium hydroxide produced by the hydrolysis of sodium orthovanadate influences the formation of a sodium templated porous framework.

EXPERIMENTAL SECTION

Synthesis. Caution! Although depleted uranium was used, precautions for handling radioactive materials should be followed that include proper training of workers and appropriate facilities. All chemicals were from commercial suppliers and used without further purification. Distilled and Millipore filtered water with a resistance of 18.2 MΩ cm was used in all reactions. All syntheses were conducted at 160 °C under solvothermal conditions in 23 mL Teflonlined stainless-steel reaction vessels for 3 days, followed by cooling to 13250

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Inorganic Chemistry room temperature at a rate of 0.1 °C min−1. Thermogravimetric analysis (TGA) measurements were carried out using a TA-Instrument Q50 TG for crystal samples in Al crucibles under nitrogen atmospheres from room temperature to 800 °C at a heating rate of 10 °C/min. (Et2NH2)2[(UO2)3(PCP)2](H2O)2.5 (1). Uranyl nitrate aqueous solution (0.5 M, 0.5 mL), methylenediphosphonic acid solution (0.5 M, 0.5 mL), 0.5 mL of DEF, and 1.0 mL of water were added to the reaction vessel. The reaction vessel was then sealed and heated statically in an isothermal oven. Yellow needle-shaped crystals were collected by filtration of the product, followed by washing with distilled water three times. Yield: 90%, based on uranium. Phase purity was confirmed by comparing simulated and observed powder X-ray diffraction patterns (Figure S1). Crystals of 1 are insoluble in water, alcohol, acetone, acetonitrile, and DMF. (MeNH3)(H3O)[(UO2)3(PCP)2(H2O)3] (2). Uranyl nitrate aqueous solution (0.5 M, 0.5 mL), methylenediphosphonic acid solution (0.5 M, 0.5 mL), 0.5 mL of N-methyl-2-pyrrolindone (NMP), and 1.0 mL of water were combined in a reaction vessel. The reaction vessel was then sealed and heated statically in an isothermal oven. Yellow blocky crystals were collected by filtration of the mixture of products and reactants, followed by washing with distilled water. Yield: 85%, based on uranium. Phase purity was confirmed by comparing simulated and observed powder X-ray diffraction patterns (Figure S1). Crystals of 2 were found to be insoluble in water, alcohol, acetone, acetonitrile, and DMF. [Na(H2O)4](H3O)[(UO2)3(PCP)2(H2O)3](H2O)5 (3). Uranyl nitrate aqueous solution (0.5 M, 1.0 mL), methylenediphosphonic acid solution (0.5 M, 0.5 mL), and Na3VO4 (0.5 M, 0.5 mL) were combined in a reaction vessel. The vessel was sealed and heated statically in an isothermal oven. Yellow prismatic crystals formed mixed with light yellow unidentified precipitates. Due to the low and impure yield (ca. 5% on the basis of uranium), characterization beyond structure determination was not possible. X-ray Diffraction. Suitable single crystals of 1−3 were selected and mounted on glass fibers for single-crystal X-ray diffraction studies using a Bruker APEX II Quazar diffractometer equipped with graphite monochromated MoKα X-radiation provided by a microfocus source combined with Montel optics. A sphere of data was collected at 100 K for each compound using frame widths of 0.5° in ω. Data were integrated and corrected for background, Lorentz, and polarization effects using the APEX II software.28 Data were corrected for absorption using SADABS.29 Each structure was solved and refined using SHELXTL30 on the basis of F2. Generally, C-bound hydrogen atoms were placed geometrically and refined using a riding model. In the structure of 1, one water molecule is half occupied and its corresponding hydrogen atoms were not located. In the structure of 2, all non-H atoms were refined anisotopically. Initial analysis of the data indicated the presence of twinning that was addressed prior to the final refinement. Accounting for twinning lowered R1 by 0.025 and resolved several large peaks in the difference-Fourier map. Hydronium H atoms were not located due to disorder. In 3, the hydrogen atoms of the aqua ligands were located in difference-Fourier maps and refined using a rigid model. The hydrogen atoms of the interstitial water and hydronium were not located. Selected crystallographic parameters are provided in Table 1, and selected bond lengths and angles are listed in Table 2 and Table S1 (Supporting Information). Powder X-ray diffraction patterns of 1 and 2 were collected using a Bruker θ−θ diffractometer equipped with a Lynxeye one-dimensional solid-state detector and CuKα radiation from 5° to 50° (2θ) with a step width of 0.02° and a fixed counting time of 1 s/step. Patterns were calculated from the crystallographic parameters using Mercury.31 Spectroscopic Measurements. Absorption data were collected for crystals of 1−3 using a Craic Technologies UV−vis−NIR microspectrophotometer over the range of 200−1200 nm under ambient conditions. Infrared (IR) spectra were collected from powdered specimens using a SensIR Technology IlluminatIR FT-IR microspectrometer equipped with an attenuated total reflectance (ATR) objective. The spectra were taken from 650 to 4000 cm−1 with a beam aperture of 100 μm for samples stored in a desiccator for 24 h

prior to analysis. Raman spectra were collected under ambient conditions for single crystals using a Bruker Sentinel system linked by fiber optics to a Raman probe held in a microscope mount. The laser wavelength was 785 nm with a power of 400 mW.

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RESULTS Our objective was to synthesize uranyl diphosphonates that incorporated different organic amine cations through solvo-

Figure 1. (a) Depiction of the coordination environments of the trinuclear UVI sites in 1. (b) The three-dimensional coordination structure of 1 projected along [001]. (c) A perspective view of the three-dimensional network of 1 showing channels along [110]. Yellow and purple polyhedra correspond to uranyl and phosphonate, respectively.

thermal hydrolysis. Compounds 1−3 were obtained under solvothermal conditions subsequent to the straightforward addition of reactants appropriate to each stoichiometry. All diphosphonates in the resulting structures are fully deprotonated due to the alkalinity of the reaction systems. We also used DMF and CH3CN as potential organic templates, but unidentified fine-grained precipitates resulted that precluded 13251

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Figure 2. (a) Depiction of the coordination environments of the mononuclear UVI sites in 2. (b) The layered structure of 2 with nonbridging phosphonate oxygens. (c) The three-dimensional coordination framework of 2 showing the pillar-layered network. Yellow and purple polyhedra correspond to uranyl and phosphonate, respectively.

single-crystal structure analyses. Where a triethylamine (TEA) solvent was added, crystals formed that were isostructural with 1 and they included diethyl ammonium as the hydrolysis product. (Et2NH2)2[(UO2)3(PCP)2](H2O)2.5 (1). The structure analysis revealed that 1 consists of a polar three-dimensional open framework built from trinuclear clusters of uranyl polyhedra. The asymmetric unit contains three crystallographically independent uranyl cations, two deprotonated methylenediphosphonate ligands, two diethyl ammonium cations, as well as two and a half water molecules (Figure 1a). The three uranyl ions (Ur) have U−OUr bond lengths ranging from 1.761(5) to

Figure 3. (a) Depiction of the coordination environments of the UVI and NaI sites in 3. (b) The two-dimensional coordination structure of 3 projected along [001]. (c) A perspective view of the threedimensional network of 3 with sodium tetrahydrate inclusion. Yellow and purple polyhedra correspond to uranyl and phosphonate, respectively.

1.778(5) Å, which are typical.3−6,10,32 The U(2) uranyl ion is central to the trimer, where it is coordinated by three bidentate 13252

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Figure 4. Thermogravimetric curves of 1 and 2. * represents sampling points for PXRD and FT-IR. Red: 250 °C; purple: 725 °C; blue: 450 °C for 1 and 325 °C for 2.

phosphonate tetrahedra belonging to three different methylenediphosphonate ligands in an “end-on” arrangement. This creates a hexagonal bipyramidal coordination environment about U(2), in which three of the equatorial edges correspond to O−O edges of three different phosphonate tetrahedra. The U(2)−Oeq (eq: equatorial) bond lengths range from 2.429(5) to 2.546(5) Å, which are within normal ranges. The U(1) uranyl ion is coordinated by five phosphonate tetrahedra, with each tetrahedron providing a single oxygen atom to the coordination environment. Four of these phosphonate tetrahedra are provided by two methylenediphosphonate ligands in a “side-on” configuration relative to the uranyl ion, whereas the other tetrahedron is bidentate to the U(2) uranyl ion. U(3) is also in a pentagonal bipyramidal coordination environment produced by five phosphonate tetrahedra that share single O atoms with the uranyl ion. Four of these phosphonate tetrahedra belong to two methylenediphosphonate ligands in a “side-on” configuration relative to the uranyl ion, whereas the other methylenediphosphonate ligand is monodentate to the uranyl ion. The U−Oeq bond lengths of U(1) and U(3) range from 2.240(5) to 2.474(5) Å. The U(2) hexagonal bipyramid shares equatorial edges with each of the U(1) and U(3) pentagonal bipyramids, forming the trimers of uranyl polyhedra that are in turn linked into a framework through the coordinating methylenediphosphonate ligands with channels along [001], [101], and [110] (Figure 1b,c). Protonated Et2NH2+ cations and water molecules are located in the channels and stabilize the porous structure. Bond-valence sum calculations for U(2) yield a value of 5.47 valence units (vu), while U(1) and U(3) are 6.10 and 5.98 vu, respectively.33,34 (MeNH3)(H3O)[(UO2)3(PCP)2(H2O)3] (2). The asymmetric unit of 2 consists of three crystallographically unique uranyl cations, two methylenediphosphonate ligands, water that coordinates uranyl ions, as well as a hydronium counterion (Figure 2a). Each of the U(VI) cations is present as a uranyl ion, with U−OUr bond lengths ranging from 1.766(9) to 1.798(8) Å, which are within the typical range for uranyl ions in most structures.3−6,10,32 The calculated bond-valence sums for U(1), U(2), and U(3) are 5.97, 5.92, and 5.94 vu, respectively.33,34

Figure 5. Powder X-ray diffraction patterns of 1 (a) and 2 (b) as synthesized and subsequent to heating, compared to patterns simulated from the crystal structure parameters.

The structure of 2 is a framework that contains prominent layers that are parallel to (110) (Figure 2b). These contain the U(1) and U(2) uranyl ions, and each is present in a pentagonal bipyramidal coordination environment. Each is coordinated by a single bidentate methylenediphosphonate ligand in a “sideon” configuration that provides linkages with two phosphonate tetrahedra, as well as two other phosphonate tetrahedra that are part of methylenediphosphonate ligands that are bidentate to other uranyl ions. Each pentagonal bipyramid also contains a single H2O group at an equatorial position. Adjacent U(1) and U(3) pentagonal bipyramids are bridged through various phosphonate tetrahedra to form the overall sheet. The U(2) uranyl ions are located between the U(1)-U(3) layers and link them into a framework. The U(2) uranyl ion is coordinated by two bidentate “side-on” methylenediphosphonate ligands, as well as one H2O group, giving an overall pentagonal bipyramidal coordination environment about the U(VI) cation. The bond-valence sums for the U(1), U(2), and U(3) cations 13253

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Inorganic Chemistry Table 3. Structural Comparison between the Uranyl Methylenediphosphonate Complexesa

a Uranium methylenediphosphonate complexes that contain metal cations beyond UO22+ are not included in this table. bU-CN means the coordination number of the uranium centers. cPCP-CM means the coordination mode of methylenediphosphonate (PCP). dThe authors addressed it a major product in a mixture. ePowder X-ray diffraction pattern was not available. fNd3+ as templating cation but is not incorporated into the final product. gYb3+ as templating cation but is not incorporated into the final product. hCrystallizing with concomitant U(IV)-PCP crystalline products.

are 6.25, 5.95, and 6.08 vu, respectively. The methylenediphosphonate ligands are deprotonated, and the framework contains solvent-accessible channels extending along [100] (Figure 2c). One methyl ammonium and one hydronium molecule reside within voids in the framework and provide for charge balance. [Na(H2O)4](H3O)[(UO2)3(PCP)2(H2O)2](H2O)5 (3). The structure of 3 contains two symmetrically distinct U(VI) cations (Figure 3a), each of which is present as a typical uranyl ion, with U−OUr bond lengths ranging from 1.776(4) to 1.782(4) Å. The U(1) uranyl ion is coordinated by two bidentate methylenediphosphonate ligands in “side-on” configurations, such that each of the phosphonate tetrahedra shares an oxygen atom with the uranyl polyhedron, as well as a single H2O group. U(2) is coordinated by four monodentate methylenediphosphonate ligands in “end-on” configurations with no additional coordinating ligands, and the overall coordination polyhedron is a square bipyramid with the uranyl ion oxygen atoms in apical positions. The U−Oeq bond lengths for both sites are in the range of 2.336(4)−2.514(4) Å, where the longest of these corresponds to the U(1)−H2O bond. The bond-valence sums for the U(1) and U(2) sites are 5.95 and 6.08 vu, respectively. The U(1) pentagonal bipyramids are linked through methylenediphosphonate ligands that bridge adjacent U(1) uranyl ions in “side-on” configurations, resulting in chains that extend along [010]. These are linked into a framework

structure through the U(2) square bipyramids, which are linked only to the methylenediphosphonate ligands that also coordinate U(1) uranyl ions (Figure 3b). The result is an open framework with channels that contain the Na cations that bond to two oxygen atoms of two different uranyl ions, as well as four H2O groups that bond to Na centers, giving an overall octahedral coordination environment about the Na cations (Figure 3c). It is interesting that incorporation of sodium ions in the structure did not lead to a strong cross-linking network with phosphonate oxygen atoms to construct a three-dimensional heterometal organic framework, as were the cases when K+ and Ba2+ were present.13,19 The channels also contain the O(13), O(14), and O(15) sites that exhibit partial occupancies and that may correspond to occupancy by Na or hydronium cations, as well as water. The IR spectra (see Figure S2) of 1−3 display strong bands at 810−925 cm−1 that arise from (UO 2)2+ stretching vibrations.35 Multiple bands in the range of 950−1120 cm−1 are assigned to the phosphonate groups. The carbonyl CO stretching vibration band observed around 1690 cm −1 corresponding to DEF is absent in the spectrum of compound 1, indicating its hydrolysis and the presence of diethyl amine cation templates in the lattice. The Raman spectra (Figure S3) contain bands at 775 and 845 cm−1 for 1 and 2 and 840 cm−1 for 3 that are assigned to uranyl symmetric stretching vibrations. The absorption spectra (Figure S4) of 1 and 2 show the typical vibrational-coupled electronic transitions at 13254

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complexes. Unique trinuclear units within 1 may be responsible for its outstanding thermal stability.

around 420 nm, reflecting the chemical coordination environments about the U(VI) cations.18,36 No emission was observed in the fluorescence spectra from 250 to 800 nm. Thermogravimetric curves for 1 and 2 are in Figure 4. For 1, the first weight loss occurs from room temperature to ∼200 °C and corresponds to release of interstitial water (observed: 3.3%, calculated: 3.3%). Decomposition of diethyl ammonium begins at 325 °C with a sharp weight loss that ends at 450 °C (observed: 10.8%, calculated: 10.9%). The X-ray diffraction pattern collected for samples after heating indicates that the framework remains intact upon heating to 250 °C, but the material has become X-ray amorphous upon heating to 450 °C (Figure 5a). For 2, the first and second weight losses overlap over the temperature range of 200−440 °C and are attributed to the removal of methyl ammonium and water (found: 10.6%, calculated: 9.6%). Powder X-ray diffraction patterns for samples of 2 collected subsequent to heating (Figure 5b) indicate significant degradation of crystallinity by 200 °C and conversion to a mostly amorphous material by 325 °C. The superior thermal stability of 1 relative to 2 may be due to the synergistic effects of the trinuclear units and hydrogen bonding between host and guest moieties. To date, 12 uranyl methylenediphosphonate complexes (Table 3) have been reported, excluding those with U(IV) and/or heterometallic structures. UO7 pentagonal bipyramids are in all structures with at least one chelating PCP ligand. Other coordination environments about U(VI) are rare in this group of compounds. Besides a UO8 hexagonal bipyramid in 1, an example of UO6 square bipyramids was found in a neutral uranyl-PCP layer with a rare 2:1 stoichiometry.16 For the other compounds in this group, the 1:1 stoichiometry product is the most common. Comparing known U(VI) complexes of PCP, the deprotonation degrees of the PCP ligand and the coordination modes have little influence on the dimensionality of the overall structures. Aromatic diaminium templates often lead to layered coordination structures by serving as interlayered cations.12 Different amounts of ethylenediamine may have given rise to two distinct products with chains and an open channel structure.12,19 Notably, the uranyl-diphosphonate synthetic phase space is very labile. High reaction temperatures (over 180 °C), insufficient amine templates, and the addition of metal cations cause impurities of the final products.12,14,16,19 In this work, we successfully conducted a synthesis route for the facile synthesis of U(VI)-PCP products as pure phases through the inclusion of in situ hydrolysis of appropriate organic amines under mild solvothermal conditions. Additionally, the thermal stabilities of 1 and 2 were addressed and 1 has excellent thermal stability that is superior among all known uranyl-PCP complexes. This may be mostly attributed to its unique U3O18 trinuclear building blocks.

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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.7b02019. Powder X-ray diffraction patterns (PXRD), IR, Raman, and UV−vis spectra, and an additional table (PDF) Accession Codes

CCDC 1567065−1567067 contain 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.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhi-Hui Zhang: 0000-0002-8744-7897 Tyler Spano-Franco: 0000-0001-6572-9722 Peter C. Burns: 0000-0002-2319-9628 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, Grant No. DE-FG02-07ER15880. Z.-H.Z. acknowledges the Qing Lan Project of Jiangsu Province that provided an opportunity to conduct research at the University of Notre Dame. Sample characterization was done in the Materials Characterization Facility of the Center for Sustainable Energy at Notre Dame.

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REFERENCES

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CONCLUSIONS Three novel anionic UOFs based on methylenediphosphonate ligands have been solvothermally synthesized using different organic amines and sodium orthovanadate as the sources of cation templates. Each complex features the uranyl/PCP ratio of 3:2 and three-dimensional coordination frameworks adopting channels with the inclusion of diverse cations. The templating method of amines solvent hydrolysis provides for the formation of the novel porous frameworks of 1 and 2. Inclusion of different ammonium cations leads to distinct coordination units and network topologies. Excessive amine templates help to produce pure phases of uranyl-PCP 13255

DOI: 10.1021/acs.inorgchem.7b02019 Inorg. Chem. 2017, 56, 13249−13256

Article

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DOI: 10.1021/acs.inorgchem.7b02019 Inorg. Chem. 2017, 56, 13249−13256