Synthesis and thermal stability study of a cobalt-organic framework with tetrahedral cages

    Synthesis and thermal stability study of a cobalt-organic framework with tetrahedral cages Xiao-Ying Lin, Qin-Qin Zheng, Xin-Zhong Liu, Xiao-Yu Jiang, Cheng Lin PII: DOI: Reference:

S1387-7003(16)30066-1 doi: 10.1016/j.inoche.2016.03.004 INOCHE 6254

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Inorganic Chemistry Communications

Received date: Revised date: Accepted date:

28 January 2016 1 March 2016 6 March 2016

Please cite this article as: Xiao-Ying Lin, Qin-Qin Zheng, Xin-Zhong Liu, XiaoYu Jiang, Cheng Lin, Synthesis and thermal stability study of a cobalt-organic framework with tetrahedral cages, Inorganic Chemistry Communications (2016), doi: 10.1016/j.inoche.2016.03.004

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Synthesis and thermal stability study of a cobalt-

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organic framework with tetrahedral cages

College of Ecological Environment and Urban Construction, Fujian University of Technology,

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Xiao-Ying Lina,*,Qin-Qin Zhenga ,Xin-Zhong Liua, Xiao-Yu Jianga, Cheng Linb

Fuzhou 350118, China b

School of Chemical Engineering, Fuzhou University, Fuzhou 350116, China

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E-mail: [email protected]

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Abstract: A new stable cobalt-organic framework, namely [Co2(oba)2(H2Me4bpz)]n (1) (H2oba = 4,4'-oxydibenzoic acid, H2Me4bpz = 3,3',5,5'-tetramethyl-4,4'-bipyrazole), has been synthesized by combining the mixed H2oba and H2Me4bpz with Co(CH3COO)2·4H2O under solvothermal conditions, which was characterized by infrared spectroscopy, single-crystal X-ray diffraction, powder X-ray diffraction, and thermogravimetric analyses. Single-crystal X-ray diffraction analysis reveals that 1 features a three-dimensional framework, which is built from interesting tetrahedral cages. The thermal stability of the complex have also been investigated.

Keywords: Mixed ligand, Crystal structure, MOFs, Thermal stability, Cobalt.

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ACCEPTED MANUSCRIPT Metal-organic frameworks (MOFs) are porous species featuring extended architectures, in which inorganic subunits and organic ligands are periodically linked via coordination bonds [1-

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4]. These promising crystalline materials as an exceptional class of gas capture and separation materials have been intensively explored because of their great advantages over other adsorbents

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with diverse structures, high surface areas, and modifiable pores [5-10]. However, rational design

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of MOFs with predicted structures and properties is still a challenge for chemists and material

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scientists, the current studies show that the employment of various polynuclear metallic units as secondary building units (SBUs) to link numerous organic linkers is a promising synthetic

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strategy for MOFs [11-14]. Among those reported MOFs, aromatic polycarboxylates ligands are always dominant research objects because they have rigid organic skeletons, excellent

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coordination capability, simple synthesis and easy functionalization with specific groups [15-17].

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Although the structural stability toward moisture in flue gas is one of crucial criteria for MOFs as

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adsorbents, most carboxylate based MOFs have to suffer hydrolysis in water or even humidity, yet due to weak metal-O bonds [18-22]. In contrast to undefined coordination fashions of carboxylate, azolates reveals simple and predictable coordination modes, and its MOF is more designable. Moreover, recent studies show that the employment of an azolate linker can be built chemically stable MOFs because of great robustness of M-N bonds arising from stronger basicity of azolates relative to carboxylates[2325]. Based on this finding, there is every reason to believe that the employment of aromatic carboxylates and azolate mixed ligands was a very effective approach to improve the stability of carboxylate-based MOFs. Herein, we have reported that this design concept is indeed viable. For the realization of this concept, we chose 4, 4'-oxydibenzoic acid (H2oba) and methylfunctionalized 3,3',5,5'-tetramethyl-4,4'-bipyrazole (H2Me4bpz) as ligands. H2oba has a ‘V’

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ACCEPTED MANUSCRIPT shape structure and nanosize length (11.3 Å), as well as flexibility due to the free rotation between the two phenyl rings, which are crucial factors for the assembly of polyhedral pore. In

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contrast to undefined coordination fashions of carboxylate, H2Me4bpz reveals simple and predictable coordination modes, and its MOF is more designable. Furthermore, the presence of

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methyl groups on the H2Me4bpz ligand may be beneficial to tune the size and facilitating the

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interactions with gas molecules. Therefore, using these two unique ligands can be expected to

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build unusual MOFs. Herein, a stable MOF, [Co2(oba)2(H2Me4bpz)]n (1), was constructed using

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these two ligands. Remarkably, it shows high thermal stability and water stability. Solvothermal reaction of Co(CH3COO)2 with H2oba and H2Me4bpz in DMF and MeCN

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mixed solvent at 160 °C for 3 days afforded purple block crystals of 1 [26]. The structure of 1 is

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characterized by single-crystal X-ray diffraction, elemental analysis, thermogravimetric analysis (Figure 2b), and IR spectrum. The purity of the crystalline materials generated has been

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investigated by X-ray diffraction on powder samples. The latter study reveals that almost only one phase has been observed for 1 because of a rather good fit between its simulated and observed patterns.

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Figure 1. (a) View of the coordination environment of the Co(II) ions in 1; symmetry codes: (a) x, 0.5-y, 0.25-z; (b) y, 1-x, 1-z; (c) y, -0.5+x, -0.75+z. (b) The tetrahedral cage substructure in 1. (c) View of the 3D framework of 1. (d) Schematic representation of six-connected topology, with the paddlewheel binuclear units as the six-connected node. Single crystal X-ray diffraction study revealed that 1 crystallized in the tetragonal I 42d space group [27]. There are one independent cobalt (II) ions, one oba2- ligands, and half of H2Me4bpz ligands in the asymmetric unit. In the structure of 1, each deprotonated oba2- ligand acts as a μ4linker and connects two paddle wheel [Co2(COO)4] units, while each paddle wheel [Co2(COO)4]

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ACCEPTED MANUSCRIPT unit is surrounded by four carboxylate groups from four oba2- ligands with the Co···Co distance of 2.786 Å (Figure 1a). Meanwhile, two apical sites of this [Co2(COO)4] unit are located by two

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non-deprotonated pyrazole group from two H2Me4bpz ligands. All Co-O and Co-N bond distances fall in the normal range of 2.000-2.092 Å [28-29]. The prominent structural feature of 1

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is the presence of tetrahedral cages in the framework (Figure 1b). Each tetrahedral cage with an

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inner diameter of 6 Å consists of four [Co2(COO)4] units, four oba2- ligands and two H2Me4bpz

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ligands. Each tetrahedral cage further links the same four cages by sharing [Co2(COO)4] units (Figure 1c). Finally, this cage-by-cage mode generates the 3D framework of 1. To the best of our

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knowledge, such framework based on tetrahedral cage is extremely scarce. In the 3D framework, the [Co2(COO)4] units and oba2-/H2Me4bpz ligands act as nodes and linkers, respectively. Thus,

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the whole framework of 1 can be topologically represented as a 6-connected net with a point

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(Schläfli) symbol of 36·66·73 (Figure 1d).

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The empty spaces of such 3D framework are filled by the other two identical frameworks, leading to a 3-fold interpenetrated network with 1D channels about 4.8×8.5 Å along the a axis (Figure 2). Significantly, some solvent molecules can be detected, but most of them are highly disordered and could not be well-located during the refining of the crystal structure, which is, indeed, supported by thermogravimetric analysis (Figure 3a). Accordingly, the contribution of all of the solvent molecules is subtracted from the data using SQUEEZE during the refinement [30]. After subtracting the solvent contribution, PLATON calculations indicate that the effective pore volume is 33.7%.

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Figure 2. Packing diagram of 1 shows 3-fold interpenetrated frameworks.

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The thermal stability of 1 has been investigated by thermogravimetric analysis (TGA) and

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powder X-ray diffraction (PXRD) pattern. The TGA curve suggests that 1 can stay stable at high temperatures up to 400 °C under a N2 atmosphere. The PXRD analysis show that 1 retains its

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crystallinity after being heated up to 350 °C in air or kept at 280 °C for 60 min in air. The TGA and PXRD analysis shows that 1 has excellent thermal stability in the class of bivalent metalbased MOFs. Our further tests reveal that 1 can also retain its crystallinity by suspending the crystal samples of 1 in water with hydrothermal treatment at 150°C for 12 hours (Figure S3).

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ACCEPTED MANUSCRIPT Figure 3. (a) TGA curve of 1. (b) PXRD patterns of the as-synthesized samples and samples after being heated at variable temperatures.

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By employment of mixed ligands 4,4'-oxydibenzoic acid and 3,3',5,5'-tetramethyl-4,4'-

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bipyrazole to assemble with the Co2+ ions, a microporous frameworks with tetrahedral cage

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substructures is presented here. Such an unusual structural assembly leads to a stable

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microporous framework and exhibits excellent thermal stability.

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Acknowledgments

This work was supported by the Fujian province Natural Science Foundation of China

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Fujian Province (2012H6002).

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(2015J01033), Industry-University Cooperation Key Project of Science and Technology of

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Appendix A. supplementary material

Materials and physical measurements, synthesis, X-ray crystallographic data in CIF format, PXRD and FT-IR, Tables S1 and S2. CCDC: 1447714 for 1 contain the supplementary crystallographic data for this paper. This data can be obtained free of charge from the Cambridge Crystallographic Data Center via http:// www.ccdc.cam.ac.uk/conts/retrieving.htm

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[26] Synthesis of 1. A mixture of Co(Ac)2∙4H2O (0.10 mmol, 24.9 mg), 4,4'-oxydibenzoic acid (0.10 mmol,

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25.8 mg), H2Me4bpz (0.05 mmol, 9.5 mg), N,N'-dimethylformamide (5 mL) and acetonitrile (4 mL) was sealed in a 25 mL Teflon-lined stainless steel vessel and heated at 160 °C for 72 hours. The reaction

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mixture was slowly cooled to room temperature at a rate of 3 °C/h, and the purple block crystals were obtained. Yield based on Co: 65.5%. Elemental analysis (%) for 1, Calcd. C, 55.58; H, 3.68; N, 6.82;

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found: C, 54.67; H, 4.24; N, 7.95. IR (KBr, cm−1): 3340 (w), 3071 (w), 2925 (w), 1679 (m), 1597 (s), 1493 (m), 1396(s), 1247(s), 1153 (s), 879 (m), 780 (m), 623(m).

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[27] X-ray diffractions were collected on a Bruker APEX II CCD diffractometer equipped with a graphite-

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monochromated Mo Κα radiation (λ = 0.71073 Å) at room temperature. The structure was solved by direct methods with SHELXS-97 and refined by the least-squares method with SHELXL-97 program.

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Crystal data for 1: C19H15N2O5Co, tetragonal, space group I-42d, Mr = 410.26, a = 25.041(4) Å, c = 15.542(3) Å, V = 9746(3) Å3, Z = 16, Dc = 1.118 g·cm−3, F(000) = 3360, μ = 0.729 mm-1, T = 293(2) K, GOF = 1.169. Data collection (3.08 < θ < 27.48) gave the final R1 = 0.0777, wR2 = 0.1897 and all data R1 = 0.0803, wR2 = 0.1914 for 5447 observed reflections with I > 2σ(I) out of 5587 unique reflections (Rint = 0.0886).

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Graphical abstract

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A new stable metal-organic framework, namely [Co2(oba)2(H2Me4bpz)]n (H2oba = 4,4'oxydibenzoic acid, H2Me4bpz = 3,3',5,5'-tetramethyl-4,4'-bipyrazole), has been synthesized and

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its thermal stability was explored.

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Highlights · Co-bipyrazole-dicarboxylates MOFs;

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· porous framework based Co ions

· three-dimensional structure with tetrahedral cages;

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· shows high thermal stability.

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