Bimetallic Uranyl Organic Frameworks Supported by Transition-Metal

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

Bimetallic Uranyl Organic Frameworks Supported by TransitionMetal-Ion-Based Metalloligand Motifs: Synthesis, Structure Diversity, and Luminescence Properties Ran Zhao,† Lei Mei,† Kong-qiu Hu,† Ming Tian,† Zhi-fang Chai,†,‡ and Wei-qun Shi*,† †

Laboratory of Nuclear Energy Chemistry and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡ School of Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China S Supporting Information *

ABSTRACT: A bifunctional ligand, 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc), has been used in the investigation of constructing bimetallic uranyl organic frameworks (UOFs). Seven novel uranyl−transition metal bimetallic coordination polymers, [(UO2)Zn(bpdc)2]n (1), [Cd(UO2)(bpdc)2(H2O)2· 2H2O]n (2), [Cu(UO2)(bpdc)(SO4)(H2O)3·2H2O]n (3), [CuCl(UO 2 )(bpdc)(Hbpdc)(H 2 O) 2 ·H 2 O] n (4), [Cu(UO 2 )(bpdc) 2 (H 2 O)] n (5), [Co 2 (UO 2 ) 3 (bpdc) 6 ] n (6), and [Co3(UO2)4(bpdc)8(Hbpdc)(H2O)2]n (7), have been successfully constructed through the assembly of various transition-metal salts, uranyl ions, and H2bpdc ligands under hydrothermal conditions. UOFs 1, 5, 6, and 7 adopt three-dimensional (3D) frameworks with different architectures; UOFs 2 and 3 exhibit two-dimensional (2D) wavelike and stairlike layers, respectively, while UOF 4 is a one-dimensional (1D) chain assembly. These UOFs include a wide range of dimensionalities (1D−3D), interpenetrated frameworks, and cation−cation interaction species, suggesting that anion-dependent structure regulation based on the metalloligand [M(bpdc)m]n− motifs, the coordination modes of the metal centers and bpdc2− ligands, along with the reaction temperature, has a remarkable influence on the formation of bimetallic UOFs, which could be a representative system for the structural modulation of UOFs with various dimensionalities and structures. Furthermore, the thermal stability and luminescent properties of compounds 1, 3, and 6 are also investigated.

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rare,34−40 some simple and easily accessible systems, for example, 2,2′-bipyridine-based poly(carboxylic acid)s, which combine the coordination properties of diaryl carboxylic acids and 2,2′-bipyridine, are seemingly being neglected. We recently did a study on those ligands. For example, using 2,2′-bipyridine5,5′-dicarboxylic acid with CuII or ZnII atoms resulted in 3D bimetallic UOFs with polycatenated networks,41 while employing 2,2′-bipyridine-3,3′-dicarboxylic acid gave a uranyl-dimerbased helical motif in the presence of AgI.42 Given the significant differences between these isomeric 2,2′-bipyridinedicarboxylic acid species, another planar ligand in this series, 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc),43−49 which has two types of coordination sites (bipyridine and carboxylic groups) to bind both transition-metal ions and uranyl centers and is helpful in constructing high-dimensional coordination polymers with higher symmetry and rigidity, is introduced to construct new bimetallic UOFs with unique structures and

INTRODUCTION Uranyl organic frameworks (UOFs), which are the uranyl organic compounds with extended structure and can exploit the merit of the functionalities of uranyl and the organic ligands, have received widespread attention because of their fascinating topologies and various applications.1−13 However, compared to zero-dimensional (0D) molecular complexes, one-dimensional (1D) chains, and two-dimensional (2D) layers, three-dimensional (3D) structures represent a small number of UOFs.14−22 The main reason is the distinctive coordination geometry of linear uranyl ions (UO22+), which cause the incoming ligands to coordinate at only the equatorial sites. As a result, UOFs usually adopt 1D chain or 2D plate architectures. For the purpose of constructing 3D UOFs, one strategy that includes the incorporation of a second metal center has proven to be effective.23−33 In order to bind two different metal centers, bifunctional ligands that have coordination sites with different classifications based on Pearson’s hard−soft acid−base (HSAB) theory can meet this criteria. Although the examples of bimetallic UOFs that employ bifunctional ligands are not © XXXX American Chemical Society

Received: March 9, 2018

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DOI: 10.1021/acs.inorgchem.8b00634 Inorg. Chem. XXXX, XXX, XXX−XXX

formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z radiation type T (K) Dcalcd (g·cm−3) F(000) θmin/max range (deg) reflns collected/unique GOF on F2 Rint R1,wR2 [I ≥ 2σ(I)] R1, wR2 (all data)

2 C24H16CdN4O14U 934.84 orthorhombic Pnma 15.2365(19) 15.8147(18) 10.9952(13) 90 90 90 2649.4(5) 4 Mo Kα 289 2.344 760 2.968−27.485 32243, 3143 1.079 0.0431 0.0224, 0.0419 0.0279, 0.0432

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C24H12N4O10UZn 819.78 monoclinic P21/c 12.3897(11) 13.3628(11) 15.2821(14) 90 100.855(2) 90 2484.8(4) 4 Mo Kα 296 2.191 1544 1.674−27.559 16106, 5576 1.012 0.0404 0.0286, 0.0632 0.0449, 0.0688

C12H12CuN2O15SU 757.87 triclinic P1̅ 8.8579(15) 9.4869(17) 11.850(2) 87.011(5) 89.932(5) 85.394(5) 991.2(3) 2 Mo Kα 292 2.539 710 2.874−27.484 34247, 4531 1.100 0.0414 0.0213, 0.0545 0.0218, 0.0542

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Table 1. Crystal Data and Structure Refinements for All of the Compounds 4 C24H16ClCuN4O13U 905.43 triclinic P1̅ 7.3837(6) 12.0051(9) 16.1522(13) 70.316(3) 78.843(3) 85.920(3) 1322.59(18) 2 Mo Kα 299 2.274 860 2.812−27.484 50842, 6071 1.097 0.0278 0.0193, 0.0436 0.0260, 0.0457

5 C24H12CuN4O11U 833.95 triclinic P1̅ 8.6364(5) 12.2396(8) 13.8312(9) 89.038(2) 72.222(2) 71.460(2) 1314.74(14) 2 Mo Kα 101 2.107 786 3.106−27.484 46026, 6005 1.082 0.0417 0.0302, 0.0611 0.0395, 0.0650

6 C72H36Co2N12O30U3 2381.08 triclinic P1̅ 11.3944(3) 12.3861(3) 17.2365(5) 79.177(1) 81.632(1) 76.769(1) 2312.51(11) 1 Cu Kα 170 1.710 1122 3.714−66.595 58108, 8158 1.068 0.0601 0.0282, 0.0684 0.0318, 0.0699

7 C108H54Co3N18O46U4 3468.60 orthorhombic Pccn 48.5145(11) 14.6248(4) 19.1130(4) 90 90 90 13561.0(6) 4 Cu Kα 170 1.699 6580 3.913−68.528 81726, 12406 1.053 0.0540 0.0407, 0.0973 0.0460, 0.0999

Inorganic Chemistry Article

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Article

Inorganic Chemistry

[Co2(UO2)3(bpdc)6]n (6). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (13 mg, 0.05 mmol), Co(NO3)2·6H2O (14.5 mg, 0.05 mmol), and ultrapure water (1000 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 200 °C for 3 days, and then cooled to room temperature naturally. Orange block crystals were isolated. Yield: 16.3 mg (41.1% based on H2bpdc). [Co3(UO2)4(bpdc)8(Hbpdc)(H2O)2]n (7). A mixture of UO2(NO3)2· 6H2O (25.3 mg, 0.05 mmol), H2bpdc (13 mg, 0.05 mmol), Co(NO3)2·6H2O (14.5 mg, 0.05 mmol), and ultrapure water (1000 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 150 °C for 3 days, and then cooled to room temperature naturally. Orange needle crystals were isolated in low yield. The yield was not improved by varying the experimental conditions. Single-Crystal X-ray Structure Determination. Single-crystal X-ray diffraction data of compounds 1−4 were collected on an Agilent SuperNova X-ray CCD diffractometer with Mo Kα radiation (λ = 0.71073 Å). For 5−7, the crystallographic data were recorded with a Bruker D8 VENTURE X-ray CMOS diffractometer with Mo Kα radiation (λ = 0.71073 Å for 5) and Cu Kα radiation (λ = 1.54056 Å for 6 and 7). Empirical absorption corrections were performed with the SADABS program. The structures were solved by direct methods and refined by full-matrix least-squares techniques on F2 with anisotropic thermal parameters for all of the non-H atoms by using the SHELXTL and Olex2 software packages to convergence. A water solvent molecule in 2 (O4W) was given the occupancy parameter of 0.5 for disorder over two sites that are related by the mirror plane. A carboxylate O atom in 5 (O6) was given calculated occupancy parameters for disorder over two sites (O6A and O6B) to gain acceptable displacement parameters. All of the carbon-bound H atoms and the H atoms of the aqua ligands in 2 were located by geometric analysis and allowed to ride on their respective parent atoms, whereas the H atoms of the water solvent molecules for all compounds and from the uncoordinated carboxylate group of 7 were not included in the model. The H atom from the uncoordinated carboxylate group in 4 was retrieved from difference Fourier maps. The disordered solvent molecules in compounds 1 and 5−7 were removed by the SQUEEZE option of PLATON. The crystal data of all of the compounds mentioned above are given in Table 1, and selected bond lengths and angles are listed in Tables S1−S7. The CCDC numbers from compounds 1−7 are 1527543, 1814091, 1814092, 1814093, 1814094, 1814095, and 1814096, respectively. Crystal Structures. A single-crystal X-ray measurement shows that compound 1 belongs to the monoclinic space group P 21/c, and the asymmetric unit of 1 consists of one ZnII ion, one uranyl ion, and two bpdc2− anions. The ZnII center has a nearly octahedral coordination environment made up of four N atoms from two bpdc2− ligands and two carboxyl O atoms from two bpdc2− ligands (Figure 1a). The Zn− N and Zn−O bond lengths range from 2.124(4) to 2.180(4) Å and from 2.133(3) to 2.159(3) Å, respectively (Table S1). In 1, the uranyl ion is seven-coordinated with two UO bonds in its apical positions and five carboxyl O atoms from four different bpdc2− ligands in the pentagonal equatorial plane. The equatorial U−O bond distances range from 2.323(3) to 2.562(4) Å. As depicted in Scheme 1a,b, there are two types of bpdc2− ligands with different coordination modes in 1: modes a and b. For both modes, two N atoms of the bipyridyl rings coordinate to a ZnII center with a chelating bidentate mode. For mode a, every carboxylate group bridges one ZnII center and one uranyl center by employing two O atoms of a carboxylate group in a monodentate fashion, respectively. For mode b, two O atoms of a carboxylate coordinate to one uranyl center in a chelating bidentate mode, while one O atom of another carboxylate coordinates to another uranyl center in a monodentate fashion. All carboxylate groups of bpdc2− ligands in 1 are deprotonated, which could also be confirmed by the IR data. In the structure of 1, the ZnII center and its two adjacent bipyridyl-chelated bpdc2− ligands can be viewed as a [Zn(bpdc)2]2− motif. This [Zn(bpdc)2]2− motif and its adjacent uranyl center with a Zn−U distance of 5.26 Å, which are linked by two bridging carboxylate groups from the other two [Zn(bpdc)2]2− motifs, can be treated as a new [(Zn(bpdc)2)UO2] dinuclear unit (Figure 1b).

topologies. Through variation of the d-block transition-metal centers, seven different bimetallic UOFs associated with H2bpdc were synthesized under hydrothermal conditions and four of them have been tested for their thermal stability and emission spectrometry. This bimetallic UOF family includes a wide range of dimensionalities (1D−3D), interpenetrated frameworks, and cation−cation interaction species. This could be a representative system for the construction of transitionmetal-ion-centered bimetallic UOFs with various dimensionalities and structures.

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EXPERIMENTAL SECTION

Materials, Syntheses, and Characterization. Caution! Uranium is a radioactive and chemically toxic element. Standard procedures for handling radioactive materials should be followed. All chemical reagents were purchased commercially and used without further purification. The ligand 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc) was prepared according to the previously documented procedure.50 Powder X-ray diffraction (PXRD) patterns were carried out on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5418 Å). Thermogravimetric analysis (TGA) was performed on a TA Instruments Q-500 thermogravimetric analyzer used in air with a heating rate of 5 °C·min−1. Solid-state fluorescence spectra were measured on a Hitachi F-4600 fluorescence spectrophotometer with an excitation wavelength of 345 nm in all cases. The Fourier transform infrared spectra were obtained from KBr pellets on a Bruker Tensor 27 spectrometer. [(UO2)Zn(bpdc)2]n (1). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (24 mg, 0.1 mmol), Zn(NO3)2·6H2O (14.9 mg, 0.05 mmol), 1 M NaOH (180 μL), and ultrapure water (820 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 200 °C for 3 days, and then cooled to room temperature naturally. Yellow block crystals of 1 were isolated from the whiteyellow precipitate. Yield: 20.9 mg (51.0% based on H2bpdc). [Cd(UO2)(bpdc)2(H2O)2·2H2O]n (2). A mixture of UO2(NO3)2· 6H2O (25.3 mg, 0.05 mmol), H2bpdc (12 mg, 0.05 mmol), Cd(NO3)2·4H2O (15.4 mg, 0.05 mmol), 1 M NaOH (60 μL), and ultrapure water (940 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 150 °C for 3 days, and then cooled to room temperature naturally. Yellow block crystals of 2 were isolated from the white-yellow precipitate in low yield. The yield was not improved by varying the experimental conditions. [Cu(UO2)(bpdc)(SO4)(H2O)3·2H2O]n (3). A mixture of UO2(NO3)2· 6H2O (25.3 mg, 0.05 mmol), H2bpdc (12 mg, 0.05 mmol), CuSO4· 5H2O (12.5 mg, 0.05 mmol), and ultrapure water (1000 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 150 °C for 3 days, and then cooled to room temperature naturally. Green block crystals of 3 were isolated. Yield: 8.6 mg (22.6% based on H2bpdc). [CuCl(UO 2 )(bpdc)(Hbpdc)(H 2 O) 2 ·H 2 O] n (4). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (12 mg, 0.05 mmol), CuCl2·2H2O (4.3 mg, 0.025 mmol), and ultrapure water (1000 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 200 °C for 3 days, and then cooled to room temperature naturally. Dark-green needlelike crystals of 4 were isolated from the green precipitate in low yield. The yield was not improved by varying the experimental conditions. [Cu(UO2)(bpdc)2(H2O)]n (5). A mixture of UO2(NO3)2·6H2O (25.3 mg, 0.05 mmol), H2bpdc (12 mg, 0.05 mmol), CuCl2·2H2O (6.8 mg, 0.04 mmol), and ultrapure water (1000 μL) was loaded into a 20 mL Teflon-lined autoclave. The autoclave was sealed, heated to 200 °C for 1 day, and then cooled to room temperature naturally. Greenish-blue block crystals were isolated from the blue precipitate in low yield. The yield was not improved by varying the experimental conditions. Besides, elongating the heating periods gave compound 4 with very low yields, and replacing CuCl2 with Cu(NO3)2 gave no crystalline products. C


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Inorganic Chemistry

Figure 1. (a) ORTEP drawing (at 50% probability) of the asymmetric unit of 1 with non-H atoms. (b) Coordination environment of the [(Zn(bpdc)2)UO2] dinuclear unit. (c and d) 3D framework of the UOF 1 viewed along the a and c directions, respectively. Polyhedra: U atoms, yellow; ZnII centers, turquiose.

Figure 2. (a) ORTEP drawing (at 50% probability) of the coordination environment of CdII and uranyl ions in 2 with non-H atoms. (b and c) 2D wavelike framework of the UOF 2 viewed along the b and c directions, respectively. Polyhedra: U atoms, yellow; CdII centers, turquiose.

Every dinuclear unit is connected to six other [(Zn(bpdc)2)UO2] dinuclear units through the linking of [Zn(bpdc)2]2− motifs, finally resulting in a 3D framework (Figure 1c,d). To better understand the crystal structure of compound 1, the simplification principle of a topological approach was adopted. Each [Zn(bpdc)2]2− motif and uranyl ion is connected to eight and five adjacent metal centers and thus can be viewed as 8- and 5-connected nodes, respectively. As a result, the whole network of 1 can be described as a novel 5,8connected (32·44·54)(34·45·513·66) topological framework, which is the first found among UOFs. Additionally, according to PLATON analysis, for the UOF 1, solvent-accessible voids exist in the crystal structure and possess about 28.5% of the crystal volume. Compound 2, which crystallizes in the orthorhombic space group Pnma, differs from 1 by the replacement of the d-block metal cation from ZnII to CdII, which usually leads to different structures because of the different coordination geometries of the d-block metal centers.51,52 As shown in Figure 2a, the asymmetric unit of 2 contains half of a uranyl ion, half of a CdII ion, two half bpdc2− anions, two half coordinated water molecules, and two half solvent water molecules. Among them, the uranyl ion (U1, O1, and O2), CdII ion (Cd1), two coordinated water molecules (O1W and O2W), and one solvent water

molecule (O3W) are located in the mirror plane, while the other solvent water molecule (O4W) is disordered over two sites related by the mirror plane. In 2, each bpdc2− ligand links two uranyl centers with one bidentate chelating group and a monodentate carboxylate group and one CdII ion with two N atoms from the bipyridine group (Scheme 1b). The U center shows a typical hexagonal-bipyramidal geometry with terminal UO bond lengths of 1.765(3) and 1.778(3) Å and a OUO bond angle of 177.92(15)° (Table S2). Each uranyl ion is bound to two bidentate chelating and two monodentate carboxylate groups from four different bpdc2− anions in its equatorial plane. The CdII ion is six-coordinated by two O and four N atoms derived from two coordinated water molecules and two bpdc2− ligands, respectively, which forms a single-ion [Cd(bpdc)2(H2O)2]2− motif. Each [Cd(bpdc)2(H2O)2]2− motif bridges four adjacent uranyl ions, and each uranyl ion is coordinated with four [Cd(bpdc)2(H2O)2]2− motifs, which finally yields a wavelike 2D sheet (Figure 2b,c). Both of the [Cd(bpdc)2(H2O)2]2− motifs and uranyl centers are acting as 4-connected nodes, and the resulting structure of

Scheme 1. Coordination Modes of the H2bpdc Ligand in UOFs 1−7a and Coordination Modes in Different UOFs: 1 (a and b), 2 (b), 3 (b), 4 (c and d), 5 (b and e), 6 (b and f), and 7 (b, f, and g)

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M: transition-metal ions. U: uranyl ions. D


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Inorganic Chemistry the 2D sheet can be simplified as a 4,4-connected binodal (44·62) sql network. Compounds 3−5 obtained with CuII centers show anion-dependent tunable structures that may be due to the flexible coordination modes of CuII and the combination of CuII with various anions.53−55 Singlecrystal X-ray diffraction measurement revealed that compound 3 crystallizes in a triclinic P1̅ space group and the asymmetric unit consists of one uranyl ion, one CuII ion, one bpdc2− unit, one sulfate anion, three coordinated water molecules, and two guest water molecules (Figure 3a). The U center adopts a seven-coordinated

binding to three CuII ions and one uranyl ion, while each CuII connecting three uranyl ions acts as a 3-connected node, and the resulting structure of this 2D layer can be viewed as a 3,4-connected binodal network with the Schlafli symbol (42·63·8)(42·6) with a V2O5like structure (Figure 3f). Compound 4 differs from 3 with the replacement of SO42− with Cl− and affords a totally different structure. Single-crystal X-ray diffraction analysis reveals that 4 crystallizes in the triclinic space group P1̅, with the asymmetric unit consisting of two half uranyl ions, one CuII ion, one bpdc2− unit, one Hbpdc− unit, one Cl− ion, two coordinated water molecules, and one solvent water molecule (Figure 4a). The

Figure 3. (a) ORTEP drawing (at 50% probability) of the asymmetric unit of 3 with non-H atoms. (b and c) Oblique and side views of the 1D [Cu(UO2)(bpdc)(SO4)(H2O)3]2n chain, respectively. (d) Detailed view of the heterometallic cation−cation interaction between uranyl and CuII ions in the UOF 3. (e) Side view of 2D stairlike layers of the UOF 3. (f) Topological feature of the UOF 3. Polyhedra and nodes: U atoms, yellow; CuII centers, turquiose; SO42− anions, orange. pentagon-bipyramidal environment and is surrounded by two uranyl axial O atoms, three O atoms from two bpdc2− ligands, and two O atoms from two SO42− ligands. All of the U−O bonds are in the normal ranges (Table S3).30 The CuII ion is in a six-coordinated distorted octahedral environment, with two N atoms from one bpdc2− ligand, three O atoms from three coordinated water molecules, and, more interestingly, one uranyl oxo atom (O1) to fulfill the coordination sphere. This bonding type, which includes the connection of a uranyl oxo atom and a second metal center, is often called a cation−cation interaction.25,56 The distorted octahedral environment of the CuII center is thus occupied by an N2O2 square in its equatorial plane, which involves one bpdc2− ligand and two water molecules with short bonds, while another water molecule and a uranyl oxo O atom are approaching from the axial positions of CuII. Each bpdc2− ligand links two uranyl centers by two different carboxylate groups adopting one monodentate fashion and one bidentate chelating mode, respectively, and one CuII center by two bipyridine N atoms (Scheme 1b), which forms a 1D [Cu(UO2)(bpdc)(H2O)3]n2n+ chain. Besides, pairs of sulfate ions with a distance of 4.381 Å bridge every two [Cu(UO2)(bpdc)(H2O)3]n2n+ chains faceto-face by linking the adjacent two uranyl centers, yielding a new 1D [Cu(UO2)(bpdc)(SO4)(H2O)3]2n chain (Figure 3b,c). These [Cu(UO2)(bpdc)(SO4)(H2O)3]2n chains are further stacked to each other through UOCu cation−cation interactions (Figure 3d), thereby creating compound 3 with stairlike 2D layers (Figure 3e). Topologically, each uranyl center acts as a 4-connected node by

Figure 4. (a) ORTEP drawing (at 50% probability) of the coordination environment of CuII and uranyl ions in 4. (b and c) 1D chain of the UOF 4 viewed along the a and c directions, respectively. Polyhedra: U atoms, yellow; CuII centers, turquiose. electroneutrality of 4 is supported by the uncoordinated protonated Hbpdc− anion. The U centers are located on the inversion center, with an eight-coordinated hexagonal-bipyramidal environment containing four O atoms from two carboxylate groups, two O atoms from two coordinated water molecules, and two uranyl axial O atoms. Each CuII ion is five-coordinated by four N atoms from one bpdc2− unit and one Hbpdc− unit and one Cl atom from Cl−, exhibiting a distorted trigonal-bipyramidal arrangement with N1, N3, and Cl1 in the equatorial plane and N2 and N4 occupying the axial positions. Adjacent bpdc2− and Hbpdc− ligands are binding to the same CuII ion by using its bipyridine N atoms in a bidentate chelating mode (Scheme 1c,d), forming the [CuCl(bpdc)(Hbpdc)]2− motif. Neighboring uranyl centers with a distance of 17.07 Å are further linked by these [CuCl(bpdc)(Hbpdc)]2− motifs with carboxylate groups in a bidentate chelating coordination mode to form a 1D chain structure of 4, which adopt a zigzag conformation in the ab plane (Figure 4b,c). Despite synthesis in the presence of CuCl2, in some cases, the final compound does not involve Cl−. This is true for compound 5 with shortened hydrothermal treatment time. Nevertheless, replacing CuCl2 with Cu(NO3)2 gave no crystalline products, indicating that the Cl− E


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Inorganic Chemistry

can be represented as a 3,5-connected (63)(69·8) topological framework with the hms topological type (Figure 5e). Employing Co(NO3)2 in this uranyl−bpdc system leads to two different temperature-dependent 3D frameworks in terms of [CoIII(bpdc)3]3− and uranyl as building subunits. In these two cases, the oxidation of CoII to CoIII may be performed,45 which is also indicated by the bond-valence-sum (BVS) calculations in the PLATON software (Table S8). Single-crystal analysis reveals that 6 crystallizes in the triclinic space group P1̅. In Figure 6a, it is seen that

ion may still play an important role in the formation of 5. Compound 5 is crystallized in triclinic space group P1,̅ and its asymmetric unit is composed of one uranyl ion, one CuII ion, two bpdc2− units, and one coordinated water molecule (Figure 5a). The U atom is a 7-fold-

Figure 6. (a) ORTEP drawing (at 50% probability) of the asymmetric unit of 6 with non-H atoms. The 2D [(UO2)2Co2(bpdc)6]2− grids viewed along the (b) a and (c) [110] directions along with (d) its topological feature. Polyhedra and nodes: U atoms, yellow; CoIII centers, turquiose.

Figure 5. (a) ORTEP drawing (at 50% probability) of the asymmetric unit of 5 with non-H atoms. (b and c) 3D framework of the UOF 5 viewed from the a and c directions, respectively. Dashed line: undulated [(UO2)bpdc]n chain. (d) 2-fold 3D interpenetrating topological structure of the UOF 5. (e) Topological feature of the UOF 5. Polyhedra in parts b and c: U atoms, yellow; CuII centers, turquiose. The colors in parts d and e (red and green) represent different sets of interpenetrated frameworks.

the asymmetric unit of 6 contains one CoIII ion, one and a half uranyl ions, and three bpdc2− anions. The CoIII center is six-coordinated with all six N atoms from three bpdc2− anions and adopts an octahedral geometry. The Co−N bond lengths vary from 1.919(4) to 1.947(4) Å (Table S6), also falling in the range of those of CoIII-bpdc complexes.46 The chelating bpdc2− ligands with CoIII give a bite angle of N−Co−N in the range of 83.07(16)−83.82(16)°. There are two types of uranyl ions in 6. The U1 atom adopts a conventional 7fold pentagonal-bipyramidal geometry, surrounded by two axial O atoms with a U1O(axial) bond length of 1.764(4) Å and a O U1O bond angle of 179.06(18)° and five carboxylate O atoms from four different bpdc2− ligands. The U2 atom, which is located on the inversion center, is surrounded by two axial O atoms and four carboxylate O atoms belonging to four different bpdc2− ligands to furnish its square-bipyramidal coordination sphere. Three nearly planar chelating bpdc2− ligands in 6 adopt two distinct coordination modes, b and f (Scheme 1b,f). These ligands exhibit a bidentate N,Nchelating mode toward the CoIII centers, forming a [Co(bpdc)3]3− motif. The [Co(bpdc)3]3− motif in 6 is coordinated to six uranyl ions [four pentagonal-bipyramidal (U1) and two square-bipyramidal (U2)] to form a 3D porous framework. To better understand the whole structure of 6, the pentagonal-bipyramidal UO22+ centers (U1) and the [Co(bpdc)3]3− motifs were first connected to form a 4,4-connected 2D [(UO2)2Co2(bpdc)6]2− grid (Figure 6b−d), which is extended in the [11̅0] plane (Figure 7a). With the aid of the square-bipyramidal UO22+ center (U2), which was located between the adjacent layers, these 2D grids further linked to each other along the [110] direction, leading to a 3D UOF structure (Figure 7b−e). Besides, the simplification principle of a topological approach for the nature of the crystal structure of 6 was adopted. If each [Co(bpdc)3]3− motif is regarded as a 6-connected node and uranyl ions as a 4-connected node, the topological mode of 6 can be described as a 4,6-connected (44·62)(45·6)2(47·68)2 topological framework (Figure 7d), which is similar to NASTEH in the CCDC database.57 Interestingly, there exist

coordinated environment, adopting a pentagonal-bipyramidal coordination geometry with UO(axial) bond lengths of 1.755(4) and 1.782(4) Å and a OUO bond angle of 179.6(2)° (Table S5). The equatorial coordination plane of uranyl comprises five O atoms from three carboxylate groups and one coordinated water molecule. The CuII ion is five-coordinated by four N atoms from the bipyridine rings of two bpdc2− anions and one O atom from the carboxylate group of another bpdc2− anion to form a distorted trigonal-bipyramidal geometry. The Cu−N bond lengths are in the range of 1.994(3)− 2.154(4) Å, and the Cu−O bond lengths are 1.932(4) Å. In compound 5, the bpdc2− anion shows two types of coordination modes (Scheme 1e,b). In both cases, the bpdc2− ligand shows a N,Nchelating mode toward a CuII ion. In mode e, two carboxylate groups are coordinated with both the uranyl and CuII ions, while in mode b, only uranyl ions are bound to the carboxylic O atoms. In the structure of 5, two uranyl centers in the bc plane with a distance of 12.24 Å are linked by the mode b bpdc2− ligand to give an undulated [(UO2)bpdc]n chain along the b direction. These parallel chains are connected through the CuII ion and the mode e bpdc2− linkers, yielding a 2D layer in the bc plane (Figure 5b). One set of 3D frameworks is further constructed by the stacking of 2D layers through the connection of CuII ions and the mode e bpdc2− ligands along the a direction (Figure 5c). Finally, two sets of independent 3D frameworks interpenetrate each other to form the densely packed structure of 5 (Figure 5d). If each [Cu(bpdc)2]2− motif is regarded as a 5-connected node and uranyl ions as a 3-connected node, the whole structure of 5 F


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Inorganic Chemistry

compound 7 with totally different structures. Single-crystal data reveal that 7 belongs to the orthorhombic space group Pccn. There are two uranyl ions, one and a half of CoIII ions, three and a half of bpdc2− ligands, and one coordinated water molecule in the asymmetric unit of 7 (Figure 8a). Considering the electroneutrality of 7, one of the uncoordinated bpdc2− units could be the protonated Hbpdc− species. The six-coordinated CoIII center in 7 is similar to the CoIII center in compound 6, with the Co−N bond lengths ranging from 1.926(5) to 1.951(9) Å and the N−Co−N bite angles between 82.5(3)° and 96.8(3)°. The U1 atom resides in a pentagonal-bipyramidal geometry provided by two axial O atoms and five equatorial O atoms from four carboxylate groups, while the U2 atom also adopts a pentagonalbipyramidal geometry filled by two axial O atoms, four equatorial O atoms from four carboxylate groups, and one equatorial O atom from a coordinated water molecule. Without consideration of CoIII, every two nearest uranyl ions with a U1−U2 distance of 5.91 Å are connected to each other by three mode f bpdc2− ligands (Scheme 1f) to form a [(UO2)2(bpdc)3]2− “M2L3”-typed coordination cage, which are extended to the [(UO2)2(bpdc)4]4− 1D chain along the c direction (Figure 8b,c) through the linking of mode b bpdc2− ligands (Scheme 1b). Finally, these [(UO2)2(bpdc)4]4− chains are further bridged to construct a 3D framework through the junction of two types of CoIII centers. Similar to compound 6, every CoIII center coordinated with three adjacent bpdc2− ligands with a bidentate N,N-chelating mode to form a [Co(bpdc)3]3− motif. The Co2-center-based [Co(bpdc)3]3− motif can coordinate with six uranyl centers and construct a dense wall along the bc plane (Figure 8d,e), while the [Co(bpdc)3]3− motif that sticks the adjacent walls together is centered by the Co1 atom, which is located on the 2-fold rotation axis (Figure 8e−g). This type of [Co1(bpdc)3]3− motif only links to four uranyl centers and two carboxylate groups are left free, leading to uncoordinated carboxylate groups with larger anisotropic displacement parameters (O22, O23, O22#2, and O23#2; Figure 8a). Moreover, parallel pores viewed along the c direction also exist, which should be attributed to the missing uranyl coordination for the free carboxylate groups around the

Figure 7. (a) 2D [(UO2)2Co2(bpdc)6]2− grids viewed along the [11̅0] direction. (b) 3D framework of the UOF 6 viewed along the [11̅0] direction. The topological feature of the UOF 6 viewed along the (c) [11̅0] and (d) b directions, along with (e) the space-filling model. Different colors in parts b−d (red, green, and blue) represent different sets of 2D [(UO2)2Co2(bpdc)6]2− grids. Polyhedra: U1 atoms, yellow; U2 atoms, purple; CoIII centers, turquiose. a set of large pores viewed along the a direction (Figure 7e). The total potential free void volume of 6 is 32.0%, which is calculated by PLATON. Compared to the UOF 6, the assembly of [Co(bpdc)3]3− motifs and water-coordinated pentagonal-bipyramidal uranyl ions leads to

Figure 8. (a) ORTEP drawing (at 50% probability) of the coordination environment of CoIII and uranyl ions in 7 with non-H atoms. (b and c) 1D [(UO2)2(bpdc)4]4− chain, which contains a [(UO2)2(bpdc)3]2− coordination cage. (d−f) Assemblies of 1D [(UO2)2(bpdc)4]4− chains by two types of CoIII centers. (g) 3D framework of the UOF 7 viewed along the c direction. (h) Topological feature of the UOF 7. Colors in parts d−f: Different sets of 1D [(UO2)2(bpdc)4]4− chains: red, green, blue, and light orange. Turquiose atoms: Co2 centers. Pink atoms: Co1 centers. Purple: free bpdc2− ligands that chelated to the Co1 centers. The turquiose and pink arrows for parts g and h represent the positions of the dense walls (constructed by the Co2 centers) and the Co1 centers, respectively. G


Article

Inorganic Chemistry [Co1(bpdc)3]3− motifs (Figure 8g). If each [Co(bpdc)3]3− motif is regarded as a 6-connected node (for Co2) and a 4-connected node (for Co1), respectively, and uranyl ions are acting as 4-connected nodes, the whole structure of 7 can be represented as a novel 4,6connected (411·64)2(43·83)4(86) topological framework (Figure 8h), which is also the first found among the UOFs. According to PLATON analysis, in 7, solvent-accessible voids exist in the crystal structure and possess about 9.4% of the crystal volume. Extension of the [M(bpdc)m]n− Motifs. As a bifunctional ligand, H2bpdc have both bipyridine N atoms and carboxylate O atoms. According to Pearson’s HSAB theory, the harder uranyl ions have a higher affinity toward O-donor groups, while d-block transition-metal ions show a strong tendency to bind with N-donor groups. In this study, compounds 1−7 were obtained via the treatment of various transition-metal salts with uranyl ions and H2bpdc ligands. As expected, all of the N atoms of the bpdc2− ligand are exclusively chelated to d-block transition-metal centers, in agreement with the specific binding preference of uranyl with carboxylate O atoms. The product of the conjunction of the d-block transition-metal center and bpdc2− ligands through the M−N bonds can be viewed as a whole [M(bpdc)m]n− “metalloligand” motif. Because the accessibility of the linear uranyl ions is limited to its equatorial positions, the dimensionality of bimetallic UOFs is mainly decided by the crosslinking ability of the Mn+ center. For compounds 1 and 5, the coordination spheres of ZnII in the [Zn(bpdc)2]2− motif and CuII in the [Cu(bpdc)2]2− motif are finished by another bpdc2− ligand with an oxygen-coordinated carboxylate group (Figure S1a,e), while for compounds 6 and 7, the [Co(bpdc)3]3− motif itself can extend in three different directions (Figure S1f,g). This makes 1 and 5−7 adopt a 3D framework. In compounds 2 and 3, the coordinated water molecules occupied the coordination sites of CdII and CuII, which prevents structural extension partially. The [Cd(bpdc)2(H2O)2]2− motif in 2 binds with uranyl ions through the carboxylate groups (Figure S1b), while [Cu(bpdc)(H2O)3] in 3 connected with uranyl ions through the carboxylate groups and the CuOU cation− cation interaction (Figure S1c). Both of them can extend in two different directions and the final dimensionality of the structure is 2. For compound 4, the coordination site of CuII in the [CuCl(bpdc)(Hbpdc)]2− motif is blocked by a Cl− anion, and the protonated carboxylate group also loses its bind ability (Figure S1d). For this reason, only the 1D chain structure of 4 is observed. These results are also in accordance with the anion-dependent structure regulation effect of bimetallic UOFs.41 Nevertheless, it is known that coordination modes (either the transition-metal center or the bpdc2− ligand) also make a great impact on the formation of coordination polymers. For example, the structural differences between compounds 1 and 2 could mainly be attributed to the different coordination spheres of the transition-metal centers. Although both the ZnII and CdII centers adopt a [N4O2] coordination, the two bpdc2− ligands around ZnII in 1 are chelating in the trans position, yielding a nonplanar [Zn(bpdc)2]2− motif, while a planar [Cd(bpdc)2(H2O)2]2− motif in 2 is constructed by the cis chelation of two bpdc2− ligands around CdII. For compounds 6 and 7, the exclusive affinity of the CoIII center with N atoms causes the cobalt(II) nitrate source to oxidize into a conventional 3D [Co(bpdc)3]3− motif. For the UOF 1 and the corresponding CuII-center-based UOF 5, although these two UOFs have similar molar ratios of M/U/bpdc with 1:1:2, they have totally different 3D structures. This is different from the 2,2′bipyridine-5,5′-dicarboxylic acid based bimetallic UOFs, in which both the CuII- and ZnII-based UOFs have the same structures.41 Compared to the different coordination modes of the bpdc2− ligands in the UOFs 5 (mode e) and 1 (mode a), the two carboxylic groups coordinated with two different metal centers (U and Cu) in a monodentate fashion in mode e, which is observed in a series of Ln−Cu-bpdc heterometallic 3D frameworks.43 while in mode a, each carboxylic group bridges two different metal centers (U and Zn) in a monodentate fashion, leading to the completely different structure of the UOF 1. To the best of our knowledge, the coordination mode of bpdc2− is first observed in the H2bpdc-based heterometallic coordination polymers.

Another important issue is the difference of reaction temperature. The UOFs 6 and 7 were obtained under the same conditions except for the different reaction temperatures of 200 °C for 6 and 150 °C for 7. Although the coordination modes of the bpdc2− and CoIII centers are similar in these two compounds, there is one coordinated water molecule on each uranyl center in 7, while no coordinated water molecule is bound to the metal centers in 6. This is in accordance with the previous reports, in which the coordinated water molecules decreased with increasing reaction temperature.58 The water coordinated with the uranyl center in 7 leads to the mismatching of the coordination sites of the bpdc2− ligands and uranyl, retaining several free carboxylate groups around the Co1 center and a totally different structure compared with 6. PXRD, IR Spectroscopy, TGA, and Luminescence Properties. Because of the very low yields of compounds 2, 4, 5, and 7, only compounds 1, 3, and 6 were subjected to further characterization. The PXRD patterns for 1, 3, and 6 matched with those of the simulated patterns, which clearly indicates the phase purity of these compounds (Figures S2−S4). In the IR spectra of 1, 3, and 6 (Figure S5), the strong absorption bands at ν = 1630−1340 cm−1 for these four compounds can be attributed to the asymmetric and symmetric stretching vibrations of carboxylic groups and the absorption of pyridine rings. The absence of characteristic absorption bands at ν = 1730−1690 cm−1 for compounds 1, 3, and 6 indicates that the carboxylate groups are coordinated completely to the metal centers. Besides, the strong absorption bands around 1130−1050 cm−1 in compound 3 disclose the existence of SO42− groups. These results are in agreement with the results of the single-crystal structural analyses. To test the thermal stability of these compounds, TGA for 1, 3, and 6 was carried out under an air atmosphere with a heating rate of 5 °C· min−1 from ambient temperature up to 800 °C (Figure S6). The framework of 1 is stable below 400 °C, and there is a a subsequent intense weight loss, which may correspond to the collapse of the structure.59 For 3, the thermal decomposition process can mainly be attributed to three steps. The first weight loss of 8.4% below 155 °C corresponds to the release of two guest water molecules (calcd 10.0%). The second weight loss begins at 320 °C, followed by the continuous decomposition of the framework until 550 °C. Then a weight loss occurring above 550 °C could be ascribed to decomposition of the SO42− groups.41 With similar chemical compositions, compound 6 shows a small weight loss before 200 °C for the loss of guest water molecules in their pores. Decomposition of the structure for 6 occurs at 300 °C. For these three compounds, the observed remaining weights are close to the expected values, which are calculated by treating Zn to ZnO, Cu to CuO, Co to 1/3Co3O4, and U to 1/3U3O8 (for 1, obsd 43.8%, calcd 44.2%; for 3, obsd 50.1%, calcd 47.5%; for 6, obsd 41.7%, calcd 42.1%). The photoluminescence spectra of 1, 3, and 6 were investigated at ambient conditions (Figure 9). When excited at λ = 345 nm, only 1 exhibits a strong yellow emission with six well-resolved peaks at λ = 479, 492, 513, 536, 561, and 589 nm, corresponding to the typical vibronic structure of the uranyl cation. These bands correspond to the electronic transitions S11−S00 and S10−S0v (v = 0−4), respectively,60 while nearly complete quenching of the uranyl emission is observed for compounds 3 and 6, which contain open-shell d-block metal ions such as CuII and CoIII, leading to d−d energy transfer and the following nonradiative decay.30

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CONCLUSION In summary, seven novel uranyl−transition metal bimetallic coordination polymers, 1−7, have been successfully constructed through the assembly of various transition-metal salts, uranyl ions, and H2bpdc ligands under hydrothermal conditions. Compound 1 exhibits a novel 5,8-connected (32·44· 54)(34·45·513·66) topological framework constructed by the linkage of [(Zn(bpdc)2)-UO2] dinuclear units. Compound 2 possess a wavelike 2D sql sheet based on [CdH


Inorganic Chemistry

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ACKNOWLEDGMENTS We acknowledge support of this work by the General Financial Grant from the China Postdoctoral Science Foundation (Grant 2017M610997), the National Natural Science Foundation of China (Grants 21577144, 21790373, and 21671191), the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of the National Natural Science Foundation of China (Grant 91426302), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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(1) Su, J.; Chen, J. In Lanthanide Metal−Organic Frameworks; Cheng, P., Ed.; Springer: Berlin, 2015; pp 265−295. (2) Wang, Y.; Liu, Z.; Li, Y.; Bai, Z.; Liu, W.; Wang, Y.; Xu, X.; Xiao, C.; Sheng, D.; Diwu, J.; Su, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. Umbellate Distortions of the Uranyl Coordination Environment Result in a Stable and Porous Polycatenated Framework That Can Effectively Remove Cesium from Aqueous Solutions. J. Am. Chem. Soc. 2015, 137, 6144−6147. (3) Liu, C.; Gao, C.-Y.; Yang, W.; Chen, F.-Y.; Pan, Q.-J.; Li, J.; Sun, Z.-M. Entangled Uranyl Organic Frameworks with (10,3)-b Topology and Polythreading Network: Structure, Luminescence, and Computational Investigation. Inorg. Chem. 2016, 55, 5540−5548. (4) Li, P.; Vermeulen, N. A.; Gong, X.; Malliakas, C. D.; Stoddart, J. F.; Hupp, J. T.; Farha, O. K. Design and Synthesis of a Water-Stable Anionic Uranium-Based Metal−Organic Framework (MOF) with Ultra Large Pores. Angew. Chem., Int. Ed. 2016, 55, 10358−10362. (5) Bai, Z.; Wang, Y.; Li, Y.; Liu, W.; Chen, L.; Sheng, D.; Diwu, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Wang, S. First Cationic Uranyl− Organic Framework with Anion-Exchange Capabilities. Inorg. Chem. 2016, 55, 6358−6360. (6) Hu, K.-Q.; Jiang, X.; Wang, C.-Z.; Mei, L.; Xie, Z.-N.; Tao, W.-Q.; Zhang, X.-L.; Chai, Z.-F.; Shi, W.-Q. Solvent-Dependent Synthesis of Porous Anionic Uranyl−Organic Frameworks Featuring a Highly Symmetrical (3,4)-Connected ctn or bor Topology for Selective Dye Adsorption. Chem. - Eur. J. 2017, 23, 529−532. (7) Hu, K.-Q.; Wu, Q.-Y.; Mei, L.; Zhang, X.-L.; Ma, L.; Song, G.; Chen, D.-Y.; Wang, Y.-T.; Chai, Z.-F.; Shi, W.-Q. Novel Viologen Derivative Based Uranyl Coordination Polymers Featuring Photochromic Behaviors. Chem. - Eur. J. 2017, 23, 18074−18083. (8) Liu, C.; Chen, F.-Y.; Tian, H.-R.; Ai, J.; Yang, W.; Pan, Q.-J.; Sun, Z.-M. Interpenetrated Uranyl−Organic Frameworks with bor and pts Topology: Structure, Spectroscopy, and Computation. Inorg. Chem. 2017, 56, 14147−14156. (9) Xie, J.; Wang, Y.; Liu, W.; Yin, X.; Chen, L.; Zou, Y.; Diwu, J.; Chai, Z.; Albrecht-Schmitt, T. E.; Liu, G.; Wang, S. Highly Sensitive Detection of Ionizing Radiations by a Photoluminescent Uranyl Organic Framework. Angew. Chem., Int. Ed. 2017, 56, 7500−7504. (10) Wang, X.; Wang, Y.; Dai, X.; Silver, M. A.; Liu, W.; Li, Y.; Bai, Z.; Gui, D.; Chen, L.; Diwu, J.; Zhou, R.; Chai, Z.; Wang, S. Phase transition triggered aggregation-induced emission in a photoluminescent uranyl-organic framework. Chem. Commun. 2018, 54, 627−630. (11) Xie, J.; Wang, Y.; Silver, M. A.; Liu, W.; Duan, T.; Yin, X.; Chen, L.; Diwu, J.; Chai, Z.; Wang, S. Tunable 4f/5f Bimodal Emission in Europium-Incorporated Uranyl Coordination Polymers. Inorg. Chem. 2018, 57, 575−582. (12) Hu, F.; Di, Z.; Lin, P.; Huang, P.; Wu, M.; Jiang, F.; Hong, M. An Anionic Uranium-Based Metal−Organic Framework with Ultralarge Nanocages for Selective Dye Adsorption. Cryst. Growth Des. 2018, 18, 576−580. (13) Liu, W.; Xie, J.; Zhang, L.; Silver, M. A.; Wang, S. A hydrolytically stable uranyl organic framework for highly sensitive and selective detection of Fe3+ in aqueous media. Dalton Trans. 2018, 47, 649−653.

Figure 9. Emission spectra of complexes 1, 3, and 6.

(bpdc)2(H2O)2]2− motifs. Compounds 3−5 based on the CuII center exhibit a V2O5-like 2D layer structure, a 1D chain structure, and a 2-fold-interpenetrated 3D hms-type framework, respectively, which also exhibit an anion-dependent structure regulation effect. Compounds 6 and 7 are built by the [Co(bpdc)3]3− motifs and adopt a 4,6-connected (44·62)(45· 6)2(47·68)2 topological framework and a 4,6-connected (411· 64)2(43·83)4(86) topological framework, respectively. The thermal stability and luminescent properties of compounds 1, 3, and 6 have been investigated. We expect that the successful synthesis of these seven bimetallic UOFs may be helpful for the design of other bimetallic UOFs with uncommon topology and architectures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00634. Tables of selected bond lengths and bond angles for UOFs 1−7, table of BVS data for Co atoms in compounds 6 and 7, structure for [M(bpdc)m]n− motifs and its adjacent metal centers in UOFs 1−7, and plots of PXRD, IR, and TGA for 1, 3, and 6 (PDF) Accession Codes

CCDC 1527543 and 1814091−1814096 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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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ran Zhao: 0000-0002-0418-499X Lei Mei: 0000-0002-2926-7265 Wei-qun Shi: 0000-0001-9929-9732 Notes

The authors declare no competing financial interest. I


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