Porous Uranyl Borophosphates with Unique Three-Dimensional Open

Jul 18, 2017 - EDS elemental analyses on selected single crystals of KUPB2 gave an average molar ratio of U:K:P = 12.15:2.06:11.97, which is in good a...
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Porous Uranyl Borophosphates with Unique Three-Dimensional Open-Framework Structures Yucheng Hao,† Gabriel L. Murphy,‡,§ Dirk Bosbach,† Giuseppe Modolo,† Thomas E. Albrecht-Schmitt,⊥ and Evgeny V. Alekseev*,†,∥ †

Institute of Energy and Climate Research (IEK-6), Forschungszentrum Jülich GmbH, 52428 Jülich, Germany School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia § Australian Nuclear Science and Technology Organization, Lucas Heights, New South Wales 2234, Australia ⊥ Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306-4390, United States ∥ Institute for Crystallography, RWTH Aachen University, 52066 Aachen, Germany ‡

S Supporting Information *

ABSTRACT: Two novel alkali-metal uranyl borophosphates have been prepared and characterized for the first time, namely, K 5 (UO 2 ) 2 [B 2 P 3 O 12 (OH)] 2 (OH)(H 2 O) 2 and K 2 (UO 2 ) 12 [B(H 2 PO 4 ) 4 ](PO 4 ) 8 (OH)(H 2 O) 6 denoted as KUPB1 and KUPB2, respectively. KUPB1 was obtained hydrothermally at 220 °C and crystallizes in a monoclinic structure in the chiral space group P21. The unit cell parameters of KUPB1 are a = 6.7623(2) Å, b = 19.5584(7) Å, c = 11.0110(4) Å, α = γ = 90°, β = 95.579(3)°, and V = 1449.42(8) Å3. It features a unique three-dimensional (3D) open-framework structure, composed of two corner-sharing linked one-dimensional (1D) anionic borophosphates (BP), [B2P3O13]5−, along the a axis and uranyl phosphate (UP), [(UO2)(PO4)3]7−, chains along the c axis, further bridged by PO4 tetrahedra. Multi-intersectional channels can be observed within the structure, in which the largest 11-ring (11-R) tunnel size is ∼7.0 Å × 8.8 Å. Its simplified framework can be described as a new 4-nodal net topological type with a point symbol of {4.84.10}{42.6}2{43.62.83.102}{82.10}. By modification of the synthetic conditions of KUPB1 through an increase in the amount of H3BO3 as flux 4-fold and a reduction of water as the reaction medium, the novel compound KUPB2 is generated. The unit cell parameters of KUPB2 are a = b = 21.8747(3) Å, c = 7.0652(2) Å, α = β = γ = 90°, and V = 3380.72(12) Å3. KUPB2 crystallizes in a tetragonal structure in the polar space group I4̅2m, and its structure is based on a highly complex 3D framework, {(UO2)12[B(PO4)4](PO4)8}9−, in which 1D 8-R UP [(UO2)(PO4)]− tubes can be observed along the c axis. The [(UO2)(PO4)]− tubes consist of three uranyl chains along the c axis, which are linked alternately by [PO4]3− tetrahedra. Those isolated 1D [(UO2)(PO4)]− tubes are further bridged through [(UO2)4B(PO4)4]− clusters, forming an exceptional 3D openframework structure. Its simplified cation network is a new 5-nodal net topological type such as {32.43.5.62.7.8}8{34.45.54.62}8{4.62.83}4{42.6}4{44.62}. Their facile hydrothermal synthetic routes, porous structure topology, thermal stability, and Raman spectroscopy properties are reported and discussed.

1. INTRODUCTION The design and synthesis of novel inorganic crystalline materials that possess advanced functional properties have been the subject of numerous studies for many decades.1−4 The archetype zeolite structure exemplifies this, with its prolific application in several areas including acid catalysis, luminescence, ionic exchange/sequestration, conductivity, gas adsorption, and separation.5−9 A plethora of structural derivatives have been found through modification of the zeolite Al−Si−O form through a variety of synthetic approaches, extending from traditional solid-state methods to the more recent successful application of organic templating agents.10−12 Further novel functional properties have been ascertained by advancing the © 2017 American Chemical Society

zeolite form toward the inclusion, or exchange, of further cations within the structural type such as in the case of the Al− Ge−O, Al−P−O, Ga−P−O, and Ge−Si−O systems.13−17 Interest in crystalline borate-derived materials is currently undergoing a resurgence because, despite the number of known structures being dwindled to that of the zeolite form, several of these show remarkable application toward areas traditionally targeted with zeolites.18 Crystalline borate materials typically consist of polymerized anionic [BO3]3− triangles and [BO4]5− tetrahedra, arranged in a plethora of structural permutations. Received: June 7, 2017 Published: July 18, 2017 9311

DOI: 10.1021/acs.inorgchem.7b01443 Inorg. Chem. 2017, 56, 9311−9320

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Inorganic Chemistry

2. EXPERIMENTAL SECTION

With the addition of alkali-, alkaline-earth-, transition-, lanthanide-, or actinide-metal cations, a unique set of complex derived borate structures can be realized, ranging from zerodimensional (0D) clusters, one-dimensional (1D) chains, twodimensional (2D) sheets, and even condensed three-dimensional (3D) frameworks.18−22 The inclusion of additional anionic groups such as [PO4]3− into borate systems gives rise to compounds of borophosphates or borate−phosphates that often contain highly novel structural arrangements. However, in comparison to the alkali- and alkaline-earth metals, actinidebearing phases derived from the anionic B−P−O system have received considerably less attention.23−26 Presently, only two actinide borate−phosphate structures are known, An2 (BO4 )(PO 4) and Ba 5[(UO2 )(PO4 ) 3(B5 O9 )]· 0.125H2O (An = U, Th).27−29 U2(BO4)(PO4) is isostructural with Th2(BO4)(PO4), where the former can be prepared through high-temperature solid-state methods, whereas the latter requires more intricate conditions in the form of both high pressure and temperature (7.0 GPa and 700 °C). Their structures are based on 2D actinide-based layers connected by isolated [BO 4 ]5− and [PO 4 ] 3− tetrahedra. Ba 5 [(UO 2 )(PO4)3(B5O9)]·0.125H2O is composed of phosphate groups linked to uranyl polyhedra, which are further connected to nanotubular borate units. The first three actinide borophosphates isolated and reported, 30 Ag 2 (NH 4 ) 3 (UO 2 ) 2 B 3 O(PO 4 ) 4 (PO 4 H) 2 H 2 O, Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26, and Ag0.57(NH4)3(UO2)2B2P2.76As2.24O18.57(OH)1.43, show structural motifs similar to that of the aforementioned borate− phosphates in the case of Ag 2 (NH 4 ) 3 (UO 2 ) 2 B 3 O(PO4)4(PO4H)2H2O, forming a 2D-layered structure, whereas Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26 and Ag0.57(NH4)3(UO2)2B2P2.76As2.24O18.57(OH)1.43 show a more intricate 3D assemblage. Interestingly, both sets of described borate−phosphate and borophosphate structures possess porous inorganic 3D open frameworks. Such a property is well-known to be functionally utilized in the context of ionexchange and separation technologies. This is exemplified by the actinide borate [ThB5O6(OH)6][BO(OH)2]·2.5H2O, known as NDTB-1,31 which has been demonstrated to show exceptional ion-exchange activity toward the pertechnetate anion TcO4−.32 Such an exceptional and successful application was attributed to its highly porous open-framework structure and disordered anionic borate groups. However, the aforementioned borophosphates or borate−phosphates have porous 3D open frameworks comparable with that of NDTB-1, implying that they potentially possess a greater affinity toward enhanced ion-exchange properties and activity. Subsequently, it is salient that further novel structural derivatives of borophosphates and borate−phosphate are explored in the context of both the development of novel functional inorganic materials and further fundamental knowledge for nuclear waste disposal. Herein, we describe two novel alkali-metal uranyl borophosphates, KUPB1 and KUPB2. Both were found to be exceedingly porous materials, which are based on novel 3D open-framework structures. The highly unique topological structures have been analyzed through single-crystal X-ray diffraction (XRD), together with thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis in addition to Raman spectroscopic properties, which are discussed in relation to their chemistry.

Caution! The UO2(NO3)2·6H2O used in this work contained natural uranium; nevertheless, the standard precautions for handling radioactive materials must be followed. 2.1. Materials and Methods. Uranyl nitrate [UO2(NO3)2·6H2O; International BioanalyticalIndustries, Inc.], potassium hydroxide (KOH; Alfa-Aesar, 99.8%), phosphorous acid (H3PO3; Alfa-Aesar, 99.5%), boric axid (H3BO3; Alfa-Aesar, 99.5%), and ammonium dihydrogen phosphate (NH4H2PO4; Alfa-Aesar, 99.5%). 2.1.1. Synthesis of KUPB1. KUPB1 was obtained via a hydrothermal method. UO2(NO3)2·6H2O (0.0506 g, 0.10 mmol), KOH (0.0221 g, 0.40 mmol), H3PO3 (0.0332 g, 0.40 mmol), H3BO3 (0.0381 g, 0.60 mmol), and deionized water (0.5 mL), in a ratio of U:K:P:B = 1:4:4:6, were mixed thoroughly in an agate mortar, sealed in a Teflonlined stainless steel autoclave (23 mL with ∼0.65 mL of total reactant), and then transferred into a box furnace. The furnace was heated to 220 °C, held there for 36 h, and then cooled to 150 °C at a rate of 3 °C/h, before being further cooled to room temperature at a cooling rate of 5 °C/h. The resulting products were washed with hot water and filtered. Greenish block-shaped crystals of KUPB1 were obtained with a yield of 41% based on the uranium content, of which several were collected for analyses. A high-purity phase of KUPB1 was obtained by mechanically separating crystals of which the purity was confirmed by laboratory powder XRD (see Figure S1a). The separated highpurity phase sample was used for thermophysical property measurements. Energy-dispersive X-ray spectroscopy (EDS) elemental analysis on selected single crystals presented an average molar ratio of K:U:P = 5.02:2.05:6.11 for KUPB1, which is in good agreement with those obtained from single-crystal XRD studies (see Figure S1a and Table S3). 2.1.2. Synthesis of KUPB2. KUPB2 was prepared using a hydrothermal method. The initial compositions of UO2(NO3)2· 6H2O (0.0528 g, 0.10 mmol), KOH (0.0225 g, 0.40 mmol), H3PO3 (0.0332 g, 0.40 mmol), H3BO3 (0.0929 g, 1.50 mmol), and deionized water (0.2 mL) were used with a ratio of U:K:P:B = 1:4:4:15 for KUPB2. All of the chemicals were mixed thoroughly in an agate mortar and then sealed in a Teflon-lined stainless steel autoclave (23 mL). The autoclave was placed in a box furnace, heated to 220 °C for 24 h, and then cooled to 160 °C at a cooling rate of 3 °C/h, followed by further cooling to room temperature with a rate of 5 °C/h before the furnace was switched off. The resulting products were washed with hot water and filtered. Yellowish needle-shaped crystals of KUPB2 were obtained. The yield of KUPB2 wass greater than that of KUPB1 at 46% based on the uranium content. A high-purity phase of KUPB2 was obtained by mechanically separating crystals, of which the purity was confirmed by XRD analysis (see Figure S1b). The separated highpurity phase sample was used for thermophysical property measurements. EDS elemental analyses on selected single crystals of KUPB2 gave an average molar ratio of U:K:P = 12.15:2.06:11.97, which is in good agreement with the proposed chemical compositions of the structure (see Figure S2b and Table S3). 2.2. Crystallographic Studies and Powder XRD. Single-crystal XRD data for both compounds were collected on an Agilent Technologies SuperNova diffractometer with Mo Kα radiation (λ = 0.71073 Å) at room temperature. All data sets were corrected for Lorentz and polarization factors as well as for absorption by the multiscan method.33 The structures of both compounds were solved by direct methods and refined by a full-matrix least-squares fitting on F2 by SHELX-97.34 Their structures were checked for possible missing symmetry elements using PLATON with the ADDSYM algorithm, and no higher symmetry was found.35 Crystallographic data and structural refinements for both compounds are summarized in Table 1. More information on the important bond distances and angles of both compounds is listed in Tables S1 and S2. XRD measurements were made using a Bruker=AXS D8 Endeavor diffractometer (40 kV/40 mA) in Bragg−Brentano geometry. The diffractometer is equipped with a copper X-ray tube and a primary nickel filter, producing Cu Kα1,2 radiation (λ = 1.54187 Å). A linear silicon strip LynxEye detector (Bruker-AXS) was used. Data were 9312

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Therefore, a mixed flux of boric acid and phosphorous acid could be a unique reactive medium for the preparation of borophosphate compounds.38 The investigation into producing novel A−U−P−B−O (A = alkali metal) derived compounds yields two complex open-framework alkali-metal uranyl borophosphates, KUPB1 and KUPB2, through mild hydrothermal conditions. KUPB1 was achieved through the usage of UO2(NO3)2(H2O)6/H3BO3/H3PO3/KOH in a ratio of 1:6:4:4, together with 0.5 mL of deionized water. Interestingly, KUPB2 was obtained by increasing the amount of H3BO3 4-fold while holding all other reactants constant and using less water as a reaction medium; the other conditions lead to little difference. We postulate that H3BO3 has played the role of both flux and reaction medium for formation of the KUPB2 structure and crystals. Probably, an excess of H3BO3 leads to better solubility of uranium and its reaction with phosphorus. This, speculatively, can be a reason why KUPB2 has more uranium compared to KUPB1. In comparison, the first three actinide borophosphates, Ag2(NH4)3(UO2)2B3O(PO4)4(PO4H)2H2O, Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26, and Ag0.57(NH4)3(UO2)2B2P2.76As2.24O18.57(OH)1.43, prepared with a H3BO3/ NH4H2PO4 flux method at around 200 °C used considerably less water (50 μL).30 For comparison with the syntheses of the first series of uranyl borophosphates made by Wu et al., we attempt to synthesize KUPB1 and KUPB2 by changing H3PO3 to NH4H2PO4; however, all syntheses were unsuccessful. Substitution of H3PO3 with H3PO4 in the synthesis also does not lead to the formation of KUPB1 and KUPB2. We may suggest that H3PO3, a simultaneous reagent and low-temperature flux, has played a key role in the structure formation of uranium borophosphates. 3.1.1. Structure of KUPB1. The compound KUPB1 is the first alkali-metal uranyl borophosphate; it crystallizes in a monoclinic structure in the chiral space group P21 (No. 4). The unit cell parameters of KUPB1 are a = 6.7623(2) Å, b = 19.5584(7) Å, c = 11.0110(4) Å, α = γ = 90°, β = 95.579(3)°, and V = 1449.42(8) Å3. In an asymmetric unit, KUPB1 consists of five unique K, two U, four B, and six P atoms. It features a 3D open-framework structure, which is composed of corner-sharing 1D anionic borophosphate (BP) chains of [B2P3O13]5− and uranyl phosphate (UP) chains of [(UO2)(PO4)3]7−, as shown in Figure 1c,d. A pair of [BO4]5− tetrahedra share corners (O1/ O17), forming a [B2O7]8− dimer. The [B2O7]8− dimers and PO4 tetrahedra share corners, forming 1D anionic chains, [B2P3O13]5−, along the a axis, which can be viewed as the fundamental building block (FBB) written as (5□:⟨3□⟩=⟨3□⟩□) following Burns et al.’s designations39 (see Figure S3). The FBB for KUBP1, [B2P3O13]5−, is the first to be found within the actinide borophosphate structural family 26 but can be found in a few actinide free borophsophates, such as Na5[B2P3O13]11,40 Rb 3 [B 2 P 3 O 1 1 (OH) 2 ], 4 1 etc. Two chiral symmetric [B2P3O13]5− chains are contained in the structure of KUPB1, as shown in Figure S3d,e. The bond lengths in the BO4 tetrahedra are in the range of 1.384(11)−1.516(11) Å, and the bond lengths for P−O in PO4 tetrahedra are in the range of 1.483(8)−1.575(6) Å. The bond angle of B−O−B is 112.7(7)°, and that of B−O−P is in the range of 112.1(7)−133.8(6)° in the BP chains, as shown in Table S1. From BVS calculations, the valences for B cations are suggested to be 3+ with values for the B1, B2, B3, and B4 at ca. 3.11, 3.12, 3.08, and 2.95, respectively. The calculations of the BVS values for P1−P6 are

Table 1. Crystal Data and Structure Refinements for Compounds KUPB1 and KUPB2 K5(UO2)2[B2P3O12(OH)]2(OH) (H2O)2 (KUPB1) fw space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z λ (Å) F(000) Dc (g/cm3) GOF on F2 R1a wR2a

K2(UO2)12[B(H2PO4)4] (PO4)8(OH)(H2O)6 (KUPB1)

1428.62 P21

4546.05 I42̅ m

6.7623(2) 19.5584(7) 11.0110(4) 95.579(3) 1449.42(8) 2 0.71073 1306 3.273

21.8747(3)

3380.72(12) 2 0.71073 3864 4.446

1.021

1.164

0.0245 0.0645

0.0293 0.0659

7.0652(2)

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

a

recorded in the range of 2θ = 5−85° with 10 s/step and a step width of 0.02°. The aperture of the fixed divergence slit was set to 0.2 mm and the receiving slit to 8.0 mm. The discriminator of the detector was set to an interval of 0.16−0.25 V. 2.3. Scanning Electron Microscopy (SEM)/EDS Analysis. Elemental analysis and SEM/EDS were performed on a FEI Quanta 200F environment scanning electron microscope with a low-vacuum mode at 0.6 mbar. The SEM/EDS results are given in Figure S2. 2.4. Raman and IR Spectroscopy. The unpolarized Raman spectrum was recorded with a Horiba LabRAM HR spectrometer using a Peltier-cooled multichannel CCD detector. An objective lens with 50× magnification was linked to the spectrometer, allowing analysis of samples as small as 2 μm diameter. The samples were in the form of single crystals. The incident radiation was produced by a helium−neon laser line at a power of 17 mW (λ = 632.8 nm). The focal length of the spectrometer was 800 mm, and an 1800 grooves/ mm grating was used. The spectral resolution was approximately 1 cm−1 with a slit width of 100 μm. The spectra were recorded in the range of 100−4000 cm−1. 2.5. Thermal Analysis (TG−DSC Experiments). The thermal behavior of both compounds from room temperature to 1200 °C was studied by the method of DSC coupled with TG under a dry air atmosphere. A Netzsch STA 449C Jupiter apparatus was used for this propose with a heating rate of 10 °C/min. High-purity samples of KUBP1 or KUBP2, as shown by XRD measurements, were loaded into platinum crucibles, which were covered by platinum lids. During the measurements, a constant air flow of 20−30 mL/min was applied. 2.6. Bond-Valence-Sum (BVS) Analysis. As a semiempirical method for the approximate determination of valence states, the BVS values of all atoms in both phases were calculated. The BVS parameters for UVI−O, KI−O, PV−O, and BIII−O were used according to Brese and O’Keeffe.36,37

3. RESULTS AND DISCUSSION 3.1. Syntheses. Phosphorous acid (H3PO3) has a low melting point of 73.6 °C, and we suppose it has a potential reactive flux similar to the well-known flux of boric acid. However, as a reagent, phosphorous acid (phosphonic acid, IUPAC systematic name) is more active in terms of its redox behavior than boric acid. H3PO3 is a good reducing agent, and simultaneously it can be oxidized into H3PO4 around 200 °C. 9313

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handed) run along the 21 axis parallel to the a axis (see Figure 2). One 8-R channel 2(U−P−B−P) is along the b axis with a diameter of ca. 5.3 Å × 6.5 Å. The largest 11-R channel [2(B− P−U−P)−B−P−B)] has a diameter of ca. 7.0 Å × 8.8 Å along the c axis (see Figure 3d−g). These tunnels are occupied by the K+ cations as well as the water molecules. In order to reveal the complex zeolite-like topological network, we simplify the anionic uranyl borophosphate framework, [(UO 2 )(B2P3O13)]3−, of KUPB1 by removal of the O anions, while the B2O7 dimers were viewed as single nodes. As shown in Figure 3, the simplified anionic net of KUPB1 can be described as a new 4-nodal net topological type with a point symbol of {4.84.10}{42.6}2{43.62.83.102}{82.10}43−45 (see Figure 3a−c). On the basis of its cationic network, the framework density of porous KUPB1 is ∼16.4 M, where M is the framework-forming cation, which is B, P, and U here. This relatively lower framework density of KUPB1 is comparable with those of the zeolite materials reported previously.46,47 Natural tiling is an efficient approach to represent a network proposed by Blatov et al.,48 which can be used for illustrating the channel system and cavities by tracing the colors of the tiles clearly, as shown in Figure 5. The framework of KUPB1 is built from the novel composite building unit (CBU) [32·42·84·112], with the intersection of 8-, 8-, 8-, and 11-R tunnels (see Figure S5). Each [32·42·84·112] CBU connects to four other neighboring ones, via their 4- or 8-R channels defining a layer on the ab plane. Each layer is further connected to its adjacent ones through two additional linkers of [32·42] and [4· 112], which show that the structure of KUPB1 is constructed from [32·42·84·112] + [32·42] + [4·112] tiles. More details of its natural tiling structure are shown in Figure S4. K+ cations are 8-, 9-, and 11-fold-coordinated with O atoms, having K−O bond distances in the range of 2.650(11)− 3.412(9) Å. These K+ cation polyhedra corner or edge share with each other, forming 2D wave-shaped layers on the bc plane (see Figure S5a). When the K−O−K layer is simplified via the omission of oxygen anions, the 2D K sheet is observed to form with a new 3-nodal net topology, having a point symbol of {4.52}2{42.52.6.7}2{42.52.72} (see Figure S5b,d). The valence of the K+ cation is suggested to be 1+ as expected, according to the calculated BVS values for K1−K5 at 1.12, 1.18, 1.06, 0.96, and 1.08, respectively. It is interesting to compare the 3D open-framework structure of KUPB1 with the 3D uranyl borophosphate Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26, which crystallized in the centrosymmetric space group Pcmn (No. 62). It also has anionic borophosphate chains as in KUPB1; however, their anionic chains are comprised of different FBBs, which only contain [BO4]5− tetrahedra and no [BO3]3− triangle units (see Figure S6). The FBB of Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26, B2P5O20, is more complex, having a topology of (7□:□⟨4□⟩□|□); these polymerize along the b axis, forming BP chains with U-type gaps. UO7 polyhedra are filled in those U-type gaps along the [101] and [−101] directions, constructing the 3D uranyl borophosphate framework structure of Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26. In its 3D framework, two types of 1D elliptic-ring channels can be observed: one is a 6-R tunnel along the c axis and the other larger one is a 12-R channel along the b axis. The complex FBB is considered to be one of the factors that makes its 3D framework less open than that for KUPB1. Neighboring anionic BP chains are aligned with a more regular mode in the defining layer with a 1/2a shift

Figure 1. Views of the structure KUPB1 along the (a) c and (b) a axes. (c) 1D UP chain along the c axis. (d) 1D BP chain along the a axis. K atoms, UO7 polyhedra, and BO4 and PO4 tetrahedra are shown as blue, yellow, green, and pink, respectively.

ca. 5.17, 4.97, 4.95, 5.13, 5.07, and 5.03, which strongly suggest the expected phosphorus valence of 5+. U atoms are 7-fold-coordinated to O atoms, forming isolated UO7 pentagonal bipyramids. The collinear uranyl moieties, (OUO)2+, are 5-fold-coordinated to [PO4]3− tetrahedra through vertex sharing, forming 1D UP chains, [(UO2)(PO4)3]7−, along the c axis (see Figure 1c). This type of UP5 coordination environment is similar to that observed in Ag2(NH4)3[(UO2)2{B3O(PO4)4(PO4H)2}]H2O and the layered uranyl arsenates α,β-Rb[UO2(AsO3OH)(AsO2(OH)2)]42 with UAs5 moieties. The uranyl axial UO bond lengths are in the range of 1.779(5)−1.786(6) Å, whereas the equatorial bond lengths are 2.307(6)−2.400(6) Å. The two anionic chains of [B2P3O13]5− and [(UO2)(PO4)3]7− are bridged by the [PO4]3− tetrahedra, forming a 3D uranyl borophosphate open-framework [(UO2)(B2P3O13)]3− (see Figure 1a,b). The BVS calculations of U1 and U2 are ca. 6.08 and 5.72, which suggests that the valence of the U cations is 6+. Framework Topology Studies. KUPB1 has a new zeolitelike topology, with multi-intersecting channel systems, as shown in Figures 3 and 4. Two 8-R channels 2(U−P−B−P) exist along the a axis with diameters of ca. 6.2 Å × 6.4 Å and ca. 5.6 Å × 6.6 Å (distances are based on two opposite O atoms), respectively, in which two helical chains (left- and right-

Figure 2. (a) View of the framework of KUPB1 along the a axis. (b) Cross section and (d) profile of a left-handed helical chain running along the 21 axis parallel to the a axis formed by corner-sharing UO7, PO4, and BO4 polyhedra. (c) Cross section and (e) profile of a righthanded helical chain running along the 21 axis. 9314

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Figure 3. (a−c) Views of the framework of KUPB1 with their topology representations along the a, b, and c axes. (d and e) Two 8-R channels along the a axis. (f) One 8-R channel along the b axis. (g) One 11-R channel along the c axis.

Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26, it is presumed that this coerces the framework structure of KUPB1 to be exceedingly more open and contain much larger channels with a diameter of ca. 7.0 Å × 8.8 Å than those in the structure of Ag0.74(NH4)3(UO2)2B2P5O18.74(OH)1.26 with ca. 8.5 Å × 3.0 Å. Structural Relations with Actinide-Free Borophosphates. Na5[B2P3O13], Rb3[B2P3O11(OH)2], and a series of organic borophosphates49 possess the same FBB of [B2P3O13]5− with KUPB1; thus, it is worth comparing and contrasting their structural relations (see Figure 6a). All of them are crystallized in monoclinic space groups, whereas Na5[B2P3O13] and KUPB1 have the same chiral P2 1 space group and Rb3[B2P3O11(OH)2] crystallizes in the space group P21/c. As shown in Figure S7a, Na5[B2P3O13] has a chain structure along the a axis, the [B2P3O13]5− anionic chains are regularly arranged

Figure 4. 3D framework of KUPB1, with the orange sticks show the 3D interconnecting channel system.

Figure 5. (a) View of the channel system in KUPB1 using natural tiling. (b) New CBU cage [32·42·84·112]. (c and d) Two tiles of [32·42] and [4·112]. Figure 6. (a) View of the construction of structure KUPB2 along the c axis with (b) its cation framework topology representation. K and B atoms, UO7 polyhedra, and PO4 tetrahedra are shown as blue, green, yellow, and pink, respectively.

along the a axis, producing its centrosymmetric structure. Because of the templating K+ cations in KUPB1 having a larger ionic radii than the Ag+ cations in 9315

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Figure 7. (a) 8-R [(UO2)(PO4)] tube along the c axis built from parts b and c. (b) 1D uranyl chain along the c axis with a ball-and-stick presentation. (c) P1O4 tetrahedron with a ball-and-stick form. (d) FBB [B(PO4)4] group. (e) U2O7 pentagonal bipyramid. (f) One BP cluster [(UO2)4B(PO4)4]. The colors of the U, P, B, and O atoms are yellow, pink, green, and red, respectively.

Figure 8. (a and b) Pore sizes of 4-R and 8-R windows of the [(UO2)(PO4)] tube. (c) Side view of one 8-R tube along the [010] direction with a ball-and-stick representation. (d) Unfolding the 8-R [(UO2)(PO4)] tube. (e) Hypothetical uranophane topology sheet.

parallel along the b axis, defining a layer C on the ab plane, the layers are stacked as a ···CCC··· mode within its structure (see Figure S7a). The closest distance between two [B2P3O13]5− chains is ca. 5.8 Å. Compared to Na5[B2P3O13], the defining layers of KUPB1 and Rb3[B2P3O11(OH)2] have the same stacking mode of ···CC′CC′···, as shown in Figure S7b,c, which have a 1/2 shift along the corresponding axis for C′ with a C layer. Owing to the larger ionic radius of Rb+ compared to Na+, the distance between the two neighboring anionic chains is larger with ca. 7.8 Å. Insertion of the (UO2)2+ uranyl groups between anionic chains in the chain structure of Na5[B2P3O13] and Rb3[B2P3O11(OH)2] configures the structure toward the KUPB1 type; the distance between the adjacent anionic chains further enlarges to ca. 9.9 Å. We presume that the cations absent in Rb3[B2P3O11(OH)2] but present in KUPB1 act as scaffolding templates, manipulating the spatial configuration of the anionic units within the structure of these [B2P3]-derived compounds. 3.1.2. Structure of KUPB2. KUPB2 crystallizes in a tetragonal structure in the noncentrosymmetric space group I42̅ m (No. 121), forming a tubular structure. The unit cell parameters of KUPB2 are a = b = 21.8747(3) Å, c = 7.0652(2) Å, α = β = γ = 90°, and V = 3380.72(12) Å3. The asymmetric unit of KUPB2 contains one K, two U, four B, and six P atoms. It features a 3D open-framework structure, composed of a 1D large UP tube, [(UO2)(PO4)]−, further bridged by a uranyl borophosphate cluster, [(UO2)4B(PO4)4]− (see Figure 6a). The 7-fold-coordinated U1O7 pentagonal bipyramids are edge (O2−O10) sharing along the c axis, forming a 1D uranyl chain

(in Figure 7b). P1O4 tetrahedra edge (O2−O10) and corner (O5) share with 1D uranyl chains alternatively. This structural configuration yields 1D 8-R [(UO2)(PO4)]− tubes along the c axis (in Figure 7a). B1O4 tetrahedra vertex (O11) share with the disordered P2O4 tetrahedra, forming [B(PO4)4]9− FBB units, as shown in Figure S6c. This BP group can be found in the actinide-free compounds of Pb6(PO4)[B(PO4)4]50 and Sr6[B(PO4)4](PO4),51 in which the [B(PO4)4]9− units form clusters.7 The P2 atoms are statistically disordered within the structure of KUBP2. They occupy a split symmetrical site within the unit cell with 50% occupation. As a result, two symmetrical O11 positons also appear as a split atomic site. Having disordered O11 atoms bonded to B1 atoms, each O11 atom has a 50% spatial chance to form a B−O bond. Because of the disorder in the oxygen environment, B1 atoms are also slightly moving along the c direction, and this is speculatively a reason for significant elongation of the B1 thermal ellipsoid. Four U2O7 pentagonal bipyramids corner (O8) share with the [B(PO4)4]9− units, forming uranyl borophosphate clusters [(UO2)4B(PO4)4]−, which are arranged parallel along the c axis (in Figure 7f). Uranyl borophosphate clusters [(UO2)4B(PO4)4]− further bridge the [(UO2)(PO4)]− tubes on the ab plane, constructing an unusual 3D uranyl borophosphate framework, {(UO2)12[B(PO4)4](PO4)8}9−; the K+ cations are located in the center of the [(UO2)(PO4)]− tubes for charging balance (see Figure 6a). The KUPB2 uranyl bond lengths were found to be within the range of 1.743(14)−1.764(7) Å, which is slightly shorter than that in KUPB1, whereas the equatorial U−O bond distances for KUPB2 are comparable with that in 9316

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Inorganic Chemistry KUPB1 in the range of 2.297(7)−2.55(2) Å. The average P−O bond lengths for [PO4]3− tetrahedra are ca. 1.536 Å, whereas for B−O bond lengths are ca. 1.445 Å. Full details can be found in Table S2. The tubular UP group [(UO2)(PO4)]− on the structural defining component of KUPB2 is composed of 1D U1O7 uranyl chains and P1O4 tetrahedra. As shown in Figure 6a, each 8-R [(UO2)(PO4)] tube is surrounded by four [(UO2)4B(PO4)4]− clusters, and four 8-R [(UO2)(PO4)]− tubes are also located around one [(UO2)4B(PO4)4]− cluster, with a [4,4] coordination mode. This 8-R [(UO2)(PO4)]− tube is built from four repeating −UO7−PO4−UO7−PO4− linkages with a size of ca. 7.6 Å × 7.6 Å (the distances are between two opposite O atoms), which is larger than the similar tunnels in Rb4(UO2)6(P2O7)4(H2O).52 There are 4-R quasi-square windows (ca. 3.6 Å × 3.8 Å) on the shaft of the [(UO2)(PO4)]− tubes, resulting in a 3D porous network structure, as shown in Figure 8a,b. If the 8-R [(UO2)(PO4)]− tube is unfolded, a hypothetical 2D uranophane topology sheet is obtained,53 which has a perimeter of ca. 26.6 Å (see Figure 8d,e). It is worth noting that, in the 3D anionic framework of Rb 4 (UO 2 ) 6 (P 2 O 7 ) 4 (H 2 O), the existing 9-R tunnel, [(UO2)3(P2O7)2]2−, is comparably smaller than that observed in KUPB2 with a size of ca. 5.2 Å × 6.1 Å. The tunnels have different connecting modes for Rb4(UO2)6(P2O7)4(H2O) and KUPB2; one 9-R tunnel of [(UO2)3(P2O7)2]2− is connected with two other neighboring ones through the sharing of common U2−U3 dimers along the b axis, defining a tunnel layer on the ab plane. Those parallel tunnel layers are further linked by two P2O7 dimers (P1−P2 and P3−P8) along the c axis, constructing its 3D framework. Note that the combination of [BO4]5− tetrahedra with [PO4]3− tetrahedra in KUPB2 has created a more complex topological structure than that in Rb4(UO2)6(P2O7)4(H2O). Framework Topology Studies. KUPB2 also possess a new zeolite-like topology with 1D large UP tubes. In order to present the novel zeolite-like topology network of KUPB2, we simplify the anionic framework [(UO2)12B(PO4)4](PO4)8]9− of KUPB2 by omitting the oxygen anions. As shown in Figure 6b, its simplified cation network is a new 5-nodal topological type with a point symbol of {32.43.5.62.7.8}8{34.45.54.62}8{4.62.83}4{42.6}4{44.62}. From its cationic network, we can observe that the 3D open framework of KUPB2 has a low framework density of ca. 14.7 M atoms per 1000 Å3, which is even lower than that in KUPB1 and comparable with the open zeolite faujasite54,55 with a framework density of ca. 13.5. The zeolitelike framework of KUPB2 can be considered as a OSI (framework-type code in zeolite database) zeolite type,56 which has a similar channel topology within their network structure (in Figure S6a). From the natural tiling point of view, the anionic uranyl borophosphate framework is the same as OSI with a 5-fold-coordinated net and each node occupied by a CBU. The CBU of KUPB2 is the quasi-d4r cage of [38·44·82], whereas for OSI, it is t-osi [64·122] (in Figure 9b). Four neighboring quasi-d4r cages of [38·44·82] are connected by another new cavity of [412·68·84]. The two adjacent CBUs are linked by two types of tiles, [34·82] and [83], defining a 2D layer on the ab plane. Those 2D layers are further connected along the c axis through tiles of [44·62], forming this unusual uranyl borophosphate framework. It is apparent to see that the 3D framework of KUPB2 was built from tiling signatures [38·44·82] + [44·62] + [34·82] + [83] + [412·68·84], in which quasi-d4r cages of [38·44·82] have occupied and traced the 8-R channel system

Figure 9. Views of the channel systems in KUPB2 using natural tiling (a), a new CBU cage [412·68·84] (f), and four other new building tiles (b−e).

of KUPB2 along the c axis, as shown in Figure S9. For comparison, the construction tiles of OSI are less as 2 × [42·64] + [63] + [64·122], which indicates that the 3D framework structure topology of KUPB2 is more complex. However, the channels along the [001] direction in [Al16P16O64]-OSI,57 having a 12-R with a size of ca. 5.2 Å × 6.0 Å, are smaller than the 8-R channels (ca. 7.6 Å × 7.6 Å) in KUPB2 along the [001] direction. More details of the tile description for KUPB2 are shown in Figure S8. The K+ cations in KUPB2 are 8-fold-coordinated, existing as square antiprisms, with K−O bond lengths in the range of 2.731(9)−2.806(8) Å. The K+ cations are face (O3−O3−O3− O3) sharing with each other, forming 1D K chains along the c axis, which are located in the center of 8-R tubes of [(UO2)(PO4)]− (see Figure S9). 3.2. Thermal Analyses. The TG and DSC measurements were performed from 50 to 1200 °C, as shown in Figure 10. TG analysis indicates that KUPB1 shows a weight loss in the range of 300−500 °C under a flowing air atmosphere, which is attributed to the removal of water molecules in the structure; an additional endothermic peak is exhibited at 342 °C from the DSC measurement. The mass loss observed from its TG curve is 5.68%, which matches well with the calculated one of 5.63%. The endothermic peak at 929 °C is attributed to the melting of the dehydrated product. Besides, we presumed that the broad DSC peak at ca. 1000 °C is caused by slow decomposition of the melted sample (see Figure 10a). TG analysis shows that KUPB2 has a weight loss from 300 to 600 °C, which corresponds to the elimination of 9.5 mol of water molecules per formula unit; this is further apparent in the DSC measurement, where an endothermic peak at 316 °C is observed. The mass loss observed from the TG curve of 3.65% is in agreement with the calculated one (3.73%). The endothermic peak at 907 °C corresponds to the melting of the dehydrated product (see Figure 10b). 3.3. Raman Spectroscopy Investigation. The Raman spectra were measured for both compounds, as shown in Figure 11. The Raman spectrum of KUPB1 was measured in a range of 100−4000 cm−1; for convenience, we have divided the spectra into two sections, a low frequency part 100−1300 cm−1 and a high-frequency region in 3000−4000 cm−1. More scattering peaks are in the range of 100−1000 cm−1, which is dominated by contributions from the uranyl, (UO2)2+, ion and [BO4]5− and [PO4]3− tetrahedral modes of KUPB1. The peaks in the range of 175−220 cm−1 could be assigned to the ν2 bending mode of the (UO2)2+ ion. The Raman spectrum shows a strong sharp peak at ∼822 cm−1 due to the symmetric vibration ν1 mode of the (UO2)2+ ion.58−60 Raman bands with week peaks around 300−500 cm−1 are attributed to the O−B− 9317

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Figure 10. TG−DSC curves of phases KUPB1 (a) and KUPB2 (b).

lower frequencies in the range of 190−300 cm−1 could be attributed to the (UO2)2+ ion with a ν2 bending mode. Raman bands with a series of peaks around 476 cm−1 could be assigned to the O−B−O doubly degenerated symmetric bending ν2 mode in [BO4]5− tetrahedra; the Raman peak at ∼642 cm−1 is attributed to the bending character ν4 of [BO4]5− tetrahedra. Raman bands from 800 to 870 cm−1 should come from the symmetric vibration ν1 mode of the (UO2)2+ ions. The characteristic vibrations of the (UO2)2+ ion ν3 antisymmetrical stretching mode are at bands of 936 and 978 cm−1. The Raman bands at 1002 and 1018 cm−1 can be attributed to the ν1 [PO4]3− symmetric stretching and ν3 [PO4]3− antisymmetric stretching modes. The Raman peaks within the range of 1100− 1200 cm−1 can be attributed to the O−B−O triply degenerated asymmetric stretching ν3 mode in [BO4]5− groups. These assignments are according to the previously reported works.58−61

4. CONCLUSIONS We have shown by the use of a relatively facile hydrothermal synthetic route that two novel noncentrosymmetric alkali-metal uranyl borophosphates, KUBP1 and KUPB2, can be obtained with highly open-framework structures. Their structures were determined by X-ray crystallography and further confirmed by Raman spectroscopy. All bands observed in the Raman spectra could be assigned to the corresponding groups in both structures. Both compounds possess unique 3D open-framework structures. The 3D open framework of KUPB1 is based on two types of anionic chains: 1D BP chains, [B2P3O13]5−, and uranyl phosphate chains, [(UO2)(PO4)3]7−. The simplified network of KUPB1 is a new 4-nodal net topological type with a point symbol of {4.84.10}{42.6}2{43.62.83.102}{82.10}. KUPB2 possesses a BP framework, {(UO2)12[B(PO4)4](PO4)8}9−, in which 1D 8-R [(UO2)(PO4)]− tubes propagate along the c axis. The parallel-stacked [(UO2)4B(PO4)4]− clusters are further bridged by isolated [(UO2)(PO4)]− tubes, forming its 3D open framework. Its more complex cation network is a new 5-nodal net topological type with the point symbol {32.43.5.62.7.8}8{34.45.54.62}8{4.62.83}4{42.6}4{44.62}. Two different FBBs, which are seemingly the first identified within the actinide borophosphates structural family, were identified, namely, [B2P3O13]5− in KUPB1 and [B(PO4)4]9− in KUPB2. Generation of the first two noncentrosymetric actinide borophosphates gives greater insight into the structural complexity of uranyl borophosphate compounds. It is noteworthy to consider the templating role of both the uranyl and

Figure 11. Raman shifts of KUPB1 (a) and KUPB2 (b).

O doubly degenerated symmetric bending ν2 mode in [BO4]5− tetrahedra; the bending character ν4 of [BO4]5− tetrahedra are in the Raman spectral range of 506−750 cm−1. Raman bands with a very strong peak around 1007 cm−1 and a weak peak near 1035 cm−1 have been assigned to the ν1 [PO4]3− symmetric stretching and ν3 [PO4]3− antisymmetric stretching modes. The Raman spectrum show a few weak bands around the range of 1050−1250 cm−1 that have been assigned to the O−B−O triply degenerated asymmetric stretching ν3 mode in the [BO4]5− tetrahedra. The vibrational modes that come from the coordinated water molecules are observed at around 3500 and 3600 cm−1, respectively, with relatively low intensity. The Raman spectrum of KUPB2 was observed in the ranges of 100−1400 and 3000−4000 cm−1. Raman bands located in 9318

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alkali-metal cations in generating these unique porous structures, particularly in comparison to other actinide and nonactinide borophosphates, respectively. This study further demonstrates how subtle adjustments to synthetic conditions can reveal dramatic changes to the structural type and topology. Further investigations on the An−B−P−O system will be continued with similar synthetic conditions in order to obtain structures with higher porosity while simultaneously examining for ion-exchange properties among other associated functional properties.

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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.7b01443. Selected bond lengths and angles, SEM and EDS measurements, XRD patterns, and topological and tiling figures (PDF) Accession Codes

CCDC 1553927−1553928 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

Evgeny V. Alekseev: 0000-0002-4919-5211 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Helmholtz Association for funding within Project VH-NG-815. G.L.M. is thankful for the proceeds of a Joan R. Clarke scholarship for funding his stay with IEK-6. T.E.A.-S. was supported as part of the Center for Actinide Science and Technology, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0016568. The authors thank Dr. Schlenz for Raman data collection. Y.H. is thankful for financial support from the Chinese Scholarship Council.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b01443 Inorg. Chem. 2017, 56, 9311−9320

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DOI: 10.1021/acs.inorgchem.7b01443 Inorg. Chem. 2017, 56, 9311−9320