Single-Crystal Synthesis and Structures of Highly Stable Ni8

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Single-Crystal Synthesis and Structures of Highly Stable Ni8-Pyrazolate Based Metal-Organic Frameworks Yong-Zheng Zhang, Tao He, Xiang-Jing Kong, Zhen-Xing Bian, Xue-Qian Wu, and Jian-Rong Li ACS Materials Lett., Just Accepted Manuscript • DOI: 10.1021/acsmaterialslett.9b00021 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Single-Crystal Synthesis and Structures of Highly Stable Ni8Pyrazolate Based Metal-Organic Frameworks Yong-Zheng Zhang,† Tao He,† Xiang-Jing Kong, Zhen-Xing Bian, Xue-Qian Wu, and Jian-Rong Li* Beijing Key Laboratory for Green Catalysis and Separation and Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, P. R. China

ABSTRACT: Pyrazolate-based (Pz-based) metal-organic frameworks (MOFs) acting as a class of highly stable porous materials have exhibited great application potential in various fields, and have been receiving a dense attention in developing new MOF members as well as expanding their practical applications. However, compared with carboxylate-based MOFs, the advancement in exploring the Pz-based MOFs is relatively slow, especially for the Ni8-pyrazolate (Ni8-Pz) based MOFs. One important reason is that it is quite difficult to obtain single crystals of this type of MOFs. Herein, single crystals suitable for single-crystal X-ray diffraction (SCXRD) of six Ni8-Pz based MOFs were synthesized through the fine control over the solvothermal reaction conditions, and structurally characterized. For all we know, this is the first time the structures of Ni8-Pz MOFs are definitely determined by SCXRD. Simultaneously, the first Ni8-Pz MOF with a 3-connected tri(pyrazoles) ligand was identified. The past more than two decades have witnessed rapid thriving of metal-organic frameworks (MOFs) in the design, synthesis, structural characterization, property, as well as application exploration in various fields including separation, sensing, catalysis, and so on.1-6 For the structural determination of MOFs, single-crystal X-ray diffraction (SCXRD) is undoubtedly the best method based on their crystalline feature. Therefore, the synthesis of single crystals suitable for SCXRD measurement is usually of prerequisite for developing new MOFs, but challenging in some cases, particularly for highly stable MOFs.7-9 However, many applications of MOFs involve in harsh conditions, requiring their good stability.10,11 The development of highly stable MOFs is indeed a sought-after goal to extend the scope of applications and promote use in practice, of this type of new materials.12-17 One strategy to overcome the vulnerability of MOFs is to enhance the strength of coordination bonds between metal ions and organic ligands.10,11,18 But, strong bonding interactions usually make difficulty for the reversible self-repairing of the coordination bonds to form singe crystals, even bigger ones. Carboxylate-based MOFs with high-valence metal ions (such as ZrIV, CrIII, and AlIII) and pyrazolate-based (pyrazolate is shorten as Pz thereafter) ones with divalent metal ions (such as NiII, CoII, and ZnII) are widely recognized to be chemically stable owning to associated strong coordination bonds, being in line with Pearson’s hard/soft acid/base principle.19 For the former of above mentioned two kinds of MOFs, in recent years, great success has been achieved in their single crystal synthesis, particularly for the ZrIV-carboxylate and CrIII-carboxylate based MOFs.20-22 The structures of them were definitely determined by SCXRD. These breakthroughs not only created/found plentiful new structures of MOFs, but also promoted the development of MOFs toward applications under harsh conditions. In contrast, the advancement in the Pz-

based MOFs is relatively slow.23 One important reason is that it is difficult to obtain single crystals of Pz-based MOFs. Among Pz-based MOFs families, those containing strong Ni– N coordination bond Ni8(OH)4(OH2)2(Pz)12 (as Ni8) second building units (SBUs) are a group of prominent members because of their fascinating structures, ultrahigh stability, and great application potential, but developed very slowly.24-27 Up to now, all reported Ni8-Pz MOFs are only based on linear bis(pyrazoles) and porphyrinic tetra(pyrazoles) ligands with fcu and ftw-a topology, respectively. Structural determination of available Ni8-Pz MOFs is all based on the powder X-ray diffraction (PXRD) rather than SCXRD. Although PXRD structural analysis has advanced the field development, definitive structural solution by SCXRD has unique advantages, including determining linker distortion, framework interpenetration, guest arrangement, and finding new topology, which are vital for the application exploration of MOFs. Therefore, in order to facilitate the discovery of new members and explore applications in a broader range of Ni8-Pz MOFs, synthesizing single crystals of them suitable for SCXRD is in an urgent need. Herein, we report the precise control over the dosage of reactants and ratio of solvents to grow high-quality single crystals of Ni8-Pz MOFs and their structure solutions by SCXRD. Crystals of six Ni8-Pz MOFs, Ni8(OH)4(OH2)2(1,4BDP)6 (BUT-2, 1,4-BDP2– = 1,4-benzenedipyrozolate, BUT = Beijing University of Technology), Ni8(OH)4(OH2)2(1,4-BDPCN)6 (BUT-3, 1,4-BDP-CN2– = 2-cyan-1,4-benzenedipyrozolate), Ni8(OH)4(OH2)2(1,4-BDP-CHO)6 (BUT-4, 1,4BDP-CHO2– = 2-formyl-1,4-benzenedipyrozolate), Ni8(OH)4(OH2)2(1,3-BDP-CN)6 (BUT-48, 1,3-BDP-CN2– = 5-cyan-1,3benzenedipyrozolate), Ni8(OH)4(OH2)2(1,3-BDP-CF3)6 (BUT49, 1,3-BDP-CF32– = 5-trifluoro-methyl-1,3benzenedipyrozolate), and Ni8(OH)4(OH2)2(BTPP)4 (BUT123, BTPP3– = 1,3,5-tris((4-pyrozolate)phenyl)benzene) were synthesized and their structures were determined by SCXRD.

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MicroMax-007HF Rotating Anode X-ray Generator. Structure analyses show that the structures of BUT-2~4 are in agreement with the previously reported results from PXRD structure determination.25 While BUT-48, -49, and -123 have new structure types, and detailed discussions are given below. Crystal parameters and structure refinement of all MOFs are summarized in Table S1 and S2 of SI.

Figure 1. Optical microscopy images of six Ni8-Pz MOFs single crystals. The radius of crystal centering cycle is 0.1 mm. To the best of our knowledge, this is the first time the structures of Ni8-Pz based MOFs are definitely determined by the SCXRD. Among them, the MOFs constructed from the angular bis(pyrozolate) (BUT-48 and BUT-49) and the tri(pyrozolate) (BUT-123) ligands present new framework topologies in the Pz-based MOFs. Synthesis of single crystals and general characterizations. All single crystals of these new Ni8-Pz based MOFs were obtained from the solvothermal reaction between Ni(NO3)2·6H2O and respective free ligand acids in a mixed solvent of N,N-dimethylformamide (DMF) or N,Ndimethylacetamide (DMA) and deionized water (H2O). The synthesis details are given in the Supporting Information (SI). It was found that the dosage of reactants and composition of solvents have an important effect on the formation and size of single crystals, and the reaction time is also responsible for crystal quality and yield. After careful optimization of the synthetic conditions, high-quality single crystals suitable for SCXRD measurement were harvested in moderate or high yields up to 86%. These crystals present regular homogeneous morphologies under a microscope: green octahedron for BUT2~4, blue rhombohedron for BUT-48 and -49, and blue-green prism for BUT-123 (Figure 1). The sizes of BUT-2~4, BUT48, and -49 are in the range of 0.04~0.1 mm, and that of BUT2 can even reach up to 0.2 mm. For the prismatic BUT-123 crystals, the length and section diameter are about 0.1 and 0.02 mm, respectively. The newly synthesized single-crystal samples of BUT-48, -49, and -123 were also characterized by PXRD, infrared spectroscopy, and thermogravimetric analysis, demonstrating their phase purity, coordination between the ligands and metal ions, and thermal stability, respectively (Figure S5-S7). Crystal structure determination and description. The single-crystal diffraction data for these MOFs were collected with a Rigaku Supernova CCD diffractometer or Rigaku

BUT-48 and BUT-49 are isostructural, here taking the former as the example for detailed structural description. BUT-48 crystallizes in the trigonal crystal system, space group of R–3. The crystallographic asymmetric unit contains one 1,3-BDP-CN2– ligand, 4/3 Ni atoms (1/3 Ni1 and one Ni2), and one O atom. All the Ni atoms are six-coordinated by linking to three N atoms of three pyrazolate groups from different ligands and three μ4-OH/OH2 entities from water molecules. Two Ni1 and six Ni2 atoms connect six μ4OH/OH2 entries to form a slightly distorted cubic Ni8(μ4OH)4(μ4-OH2)2 (Ni8O6) cluster, in which Ni1 atoms locate on two vertexes of body diagonal of the cube and other six Ni2 lie on the rest six vertexes (Ni1···Ni2 and Ni2···Ni2 distance is 2.96 and 2.99 Å, respectively (Figure 2a and S3a). In the structure, the bond distances of Ni−N are in the range of 2.01~2.03 Å, and those of Ni−O are in 2.14~2.20 Å. The angular 1,3-BDP-CN2– ligand has a 118.25o bridging angle (Figure 2b), deviating slightly from its ideal conformation (120o). The central phenyl ring and peripheral pyrazolyl rings are not coplanar with the dihedral angles of 31.98 and 24.43o, respectively (Figure S3b). As a result, the dihedral angle between two pyrazolate rings is 47.24o. Each 1,3-BDP-CN2– ligand, together with another crystallographically equivalent one bridges two Ni8O6 clusters, and each Ni8O6 cluster coordinates with twelve crystallographically equivalent 1,3BDP-CN2– ligands to form an overall three-dimensional (3D) framework with narrow channels (Figure 2c, top). From the topological point of view, each pair of 1,3-BDP-CN2– ligands can be regarded as an edge, the 12-coordinated Ni8 SBUs can be regarded as 6-connected vertices, and thus a distorted pcu net is formed by alternate connection of these two types of building units (Figure 2c, bottom). The void of BUT-48 accounts for 56.8% (54.6% for BUT-49) of its unit cell volume as estimated by PLATON.28 BUT-123 crystallizes in the tetragonal crystal system, space group of I4/m. In the asymmetric unit of BUT-123, there exist one crystallographically independent Ni atom and 1/2 BTPP3– ligands. The Ni atom is six-coordinated in an antidipyramid coordination geometry, and the coordinated atoms consist of three N atoms of three pyrazolate groups from different BTPP3– ligands and three μ4-OH/OH2 entities from water molecules. Eight adjacent Ni ions are bonded together by six μ4-OH/OH2 entities to form the classical octanuclear Ni8(μ4-OH)4(μ4-OH2)2 cluster. Each Ni8O6 cluster coordinates with twelve BTPP3– ligands, and each ligand bridges three Ni8O6 to form a 3D framework with onedimensional (1D) square channels (the side length is about 1.5 nm) along the (001) direction (Figure 2e, top). As shown in Figure S4a, the configuration of BTPP3– is obviously deviated from its ideal conformation of C3v symmetry, due to the rotation and distortion of peripheral phenyl and pyrazolyl rings away from the central phenyl plane. Interestingly, in the structure of BUT-123, there are unique spindle-shaped cages

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Figure 2. Construction and structures of two Ni8-Pz MOFs: (a) Ni8 cluster bridged with (b) 1,3-BDP-CN2– ligand to give (c) BUT48 with a disordered pcu topology and with the (d) BTPP3– ligand to give (e) BUT-123 with a (3,12)-connected topological network. Colour code: Ni, green; O, red; N, blue; C, black; hydrogen atoms are omitted for clarity. (internal pore size is about 1.1 × 1.4 nm), in which two aspectant branches of BTPP3– ligands cover the faces and two Ni8O6 clusters occupy the vertices (Figure S4b). These cages extend into a linear cage chain through sharing the vertex along the (001) direction, which are further linked by ligands to afford final 3D framework (Figure S4c). Topologically, the BTPP3– ligand and the Ni8 SBU can be seen as a 3-connected linker and a 12-connected node, respectively. BUT-123 thus exhibits a novel (3,12)-connected network with the point symbol of (420.628.818)(43)4 (Figure 2e, bottom), being the first example in the Pz-MOFs, as far as we know. PLATON calculation shows that the solvent-accessible volume in BUT123 was 77.6%.28 It should be pointed out that BUT-123 is the first example of Ni8-Pz MOF with a 3-connected tri(pyrazoles) ligand. Porosity and stability. N2 adsorption measurements at 77 K were carried out for BUT-48, -49, and -123 to evaluate their porosity. As shown in Figure 3a, saturated N2 uptakes of 266 and 238 cm3 g−1 (STP) are achieved for BUT-48 and -49, and evaluated Brunauer-Emmett-Teller (BET) surface area is 886 and 778 m2 g−1, respectively. The N2 uptake for BUT-123 is up to 810 cm 3 g −1 (STP) (Figure 3b), and the evaluated BET surface area is 2206 m2 g−1 with the pore volume of 1.25 cm3 g−1. Based on the N2 adsorption data, the pore size distribution of BUT-48, -49, and -123 are in the range of 1.2~1.6, 1.1~1.3, and 1.3~2.2 nm, respectively. These results are consistent with those from crystal structural analysis. In order to estimate the chemical stability of BUT-48, -49, and -123, their samples were treated in 1 M NaOH aqueous solution, pH = 5 HCl aqueous solution at room temperature, and in boiling water for 24 h. After treatments, checked PXRD patterns show unchanged diffraction peaks and good crystallinity, demonstrating their good chemical stability (Figure S5). Furthermore, the N2 adsorption isotherms of

Figure 3. N2 adsorption/desorption isotherms of (a) BUT-48 and -49 and (b) BUT-123 recorded at 77 K (Inset, pore size distribution).

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the treated samples were found to be almost the same as those of pristine ones as well, further confirming the robustness of these MOFs (Figure S8). It should be pointed out that, in pH < 5 HCl aqueous solution the frameworks of these MOFs are not so stable. The precise structures, as well as excellent porosity and stability of these Ni8-Pz MOFs would make accessible for the modification of their frameworks and diversified application exploration. In conclusion, single crystals of six Ni8-Pz MOFs suitable for SCXRD structure determination were synthesized under optimized solvothermal reaction conditions. This is the first time the Ni8-Pz MOFs can be structurally determined by SCXRD. Three of them possess new topologies in the PzMOFs, with permanent porosity and excellent stability. The present work can provide significant guidance on the design and synthesis of highly stable Ni8-Pz based MOFs, thereby accelerating their structure discovery, and simultaneously promoting application exploration of MOFs.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detail synthesis of ligands and MOFs, 1H NMR, additional figures of crystal structure, PXRD, FT-IR, TGA, N2 adsorption isotherms, crystal parameters and structure refinement are listed in the Supporting Information (PDF). Crystallographic data: BUT-2 (CIF) BUT-3 (CIF) BUT-4 (CIF) BUT-48 (CIF) BUT-49 (CIF) BUT-123 (CIF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions † Y.-Z. Zhang and T. He contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21771012, 21576006, 51621003).

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