Versatile Flexible Azolate-based Metal-Organic Frameworks Possessing Different Bridging Ligands

Crystal 1 is a Zn-based two-dimensional MOF possessing one-dimensional channel with hcb topology, while Crystal 2 is a Cu-based three-dimensional MOF with Clas bridging ligand. In addition, the counterions of the metal salts also take part in the construction of the framework, resulting in versatile MOF structures. Metal-organic frameworks (MOFs) are gaining tremendous attention in recent years owing to their extremely high surface area, tunable porosity, facile synthesis and good stability [1]. With numerous combinations of metal ions or metal clusters as nodes and organic linkers as struts, MOFs can be designed into infinite number of structures with open channels [2], discrete cages and cagelike polyhedral [3], or functional groups throughout the whole framework [4], thus have been developed extensively for versatile applications in various fields, such as gas adsorption, catalysis, sensing, energy storage utilizations and biomedical applications [5-8]. But there are still many drawbacks to be resolved in the MOF structures: 1) many MOF structures, especially three-dimensional ones are usually rigid mainly due to the employment of rigid ligands such as terephthalic acid, making the robust framework difficult to sustain the integrity under volume change conditions or structure variations; 2) some polycarboxylato-based MOFs are unstable with poor water stability and chemical stability against acidic or basic media, making them easily to be broken in the harsh conditions of industrial applications [9].


INTRODUCTION
Crystal 1 is a Zn-based two-dimensional MOF possessing one-dimensional channel with hcb topology, while Crystal 2 is a Cu-based three-dimensional MOF with Cl -as bridging ligand.In addition, the counterions of the metal salts also take part in the construction of the framework, resulting in versatile MOF structures.Metal-organic frameworks (MOFs) are gaining tremendous attention in recent years owing to their extremely high surface area, tunable porosity, facile synthesis and good stability [1] .With numerous combinations of metal ions or metal clusters as nodes and organic linkers as struts, MOFs can be designed into infinite number of structures with open channels [2] , discrete cages and cagelike polyhedral [3] , or functional groups throughout the whole framework [4] , thus have been developed extensively for versatile applications in various fields, such as gas adsorption, catalysis, sensing, energy storage utilizations and biomedical applications [5][6][7][8] .But there are still many drawbacks to be resolved in the MOF structures: 1) many MOF structures, especially three-dimensional ones are usually rigid mainly due to the employment of rigid ligands such as terephthalic acid, making the robust framework difficult to sustain the integrity under volume change conditions or structure variations; 2) some polycarboxylato-based MOFs are unstable with poor water stability and chemical stability against acidic or basic media, making them easily to be broken in the harsh conditions of industrial applications [9] .

MOF STRUCTURES
In order to solve the problem as well as enhance the chemical and thermal inertness of MOFs, we have designed to synthesize MOF structures using semirigid ligand.The ditopic ligand 3,3',5,5'-tetramethyl-4,4'-bipyrazole (H 2 Me 4 bpz) is a promising candidate as the two pyrazolate (pz) rings can rotate around its C-C chemical bond so that can release some of the pressure caused by structure variations in the framework, and the metal-azolate bonds can also increase the stability of the framework [10] .As shown in Figure 1, H 2 Me 4 bpz has two coordination modes (mode I, mode II), as the two N atoms in diagonal positions coordinate with metals in the former usually resulting in three-dimensional structures, while four of the N atoms all coordinate with metal ions in the latter and result preferably in planar structures [11] .A variety of MOF structures have been synthesized through chemical reactions between H 2 Me 4 bpz with different metal ions such as Cu 2+ , Zn 2+ , Co 2+ , Cd 2+ and Ag + , in different modes mentioned above [12][13][14] .In addition, we have also successfully discovered a 2D wavy layered Ni-based MOF using H 2 Me 4 bpz as ligand (named Ni-Me 4 bpz) and investigated its electrochemical property as lithium ion battery anode material.The results showed that the flexibility e-ISSN:2321-6212 p-ISSN:2347-2278 of the ligand and the 2D layered construction greatly improved the chemical stability during the electrochemical test, after which the XRD pattern still exists [15] .

VERSATILITY in MOFS
On the other hand, the counter ions of the metal salts as well as the solvent molecules can also actively take part in the configuration of MOF structures, leading to different type of versatility in MOFs [16] .Here in this respect, we choose CuCl 2 and ZnSO 4 as the metal source because not only can Cu 2+ and Zn 2+ coordinate with H 2 Me 4 bpz, but also the Cl -and SO 4 2-have been successfully served as the bridging ligands in many frame works [17] .As a result, two new MOFs have been synthesized using the above flexible azolate ligand and metal salts.Crystal 1 is a Zn-based two-dimensional MOF possessing one-dimensional channel with hcb topology, while Crystal 2 is a Cu-based three-dimensional MOF with Cl -as bridging ligand.The status of ligand H 2 Me 4 bpz in both crystals is in mode I.All the chemicals were commercial products of reagent grade, and were used without further purification.The synthesis of the H 2 Me 4 bpz ligand was according to our previous report [15] , and the two crystals were synthesized through a simple hydrothermal method.Synthesis of Crystal 1: 5.0 mg (2.6 × 10 -2 mmol) of H 2 Me 4 bpz and 6.9 mg (2.6 × 10 -2 mmol) of ZnSO 4 •7H 2 O were added to a 10 ml glass vial and dissolved in 1 ml of dimethylacetamide (DMAc).After sonicating for 20 min, the vial was capped tightly and placed in an oven at 90 0 C for 4 days.Colorless block crystals were collected.Synthesis of Crystal 2: 5.0 mg (2.6 × 10 -2 mmol) of H 2 Me 4 bpz and 4.4 mg (2.6 × 10 -2 mmol) of CuCl 2 •2H 2 O were added to a 10 ml glass vial and dissolved in 2 ml of a mixture of water, ethanol and dimethylformamide (1:1:1).After sonicating for 20 min, the vial was capped tightly and placed in an oven at 85 0 C for 4 days.Blue crystals were collected.The crystal was coated with Paratone oil on a Cryoloop pin and mounted on a Bruker SMART Apex(II) single X-ray diffractometer equipped with a CCD area detector, which was operated at 1500 W power (50 kV, 30 mA) to generate Mo Kα radiation (λ=0.071073nm) at room temperature.The crystal structure was analyzed and solved by the direct method and refined by full-matrix least-square techniques with the help of the SHELXL-97 program package.Further details of the crystal structure determination were deposited at the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication (Crystal 1, 1430112; Crystal 2, 1430113).
As shown in Figure 2, in the framework of Crystal 1, the Zn(II) ion is four coordinated with two N atoms from two deprotonated Me 4 bpz ligands, and two O atoms from two SO 4 2-.The bond lengths of Zn1-N1 and Zn1-O1 are 1.909 and 2.003 Å, respectively, indicating that the Zn(II) ion is in an distorted octahedron geometry.The ligand provides two N atoms coordinating with two Zn ions which are in the diagonal positions.In addition, in order to lower the energy level of the framework, the two pyrazolate (pz) rings rotate with a dihedral angle of 69.3 0 .Interestingly, the SO 4 2-also takes part in the constitution of the framework as bridging ligand, each coordinating with two Zn ions using two O atoms, which are in the same chemical situation.From Figure 2b, channels can be seen from the c-axis as a result of the stacking layers.rings, consisting of 40 atoms in total in each 2D layer, where four edges are made up by the ligand and two by SO 4 2-in each sixmembered ring.The lengths for the long edge and short edge are 9.438 Å and 4.412 Å, respectively (Zn-Zn).When the Zn(II) ion and the H 2 Me 4 bpz ligands are regarded as vertexes and edges, a hcb topology is obtained (Figure 2d).From the side direction of Crystal 1 as shown in Figure 3, each 2D layer composed of six-membered rings is packed by hydrogen bond forming a 3D super molecule configuration with 1D porous channels.The distance between each layer is 7.788 Å (Zn-Zn).Different from Crystal 1, Crystal 2 is a three-dimensional framework with H 2 Me 4 bpz and Cl -as the bridging ligand.As shown in Figure 4a, the Cu 2+ coordinates with two Cl -and two N atoms from two ligands, same as in Crystal 1 in mode I, the bond lengths of Cu1-Cl1 and Cu1-N1 are 2.557 and 1.936 Å, respectively.The dihedral angle of the two pyrazolate (pz) rings is 79.9 0 , which is larger than Crystal 1. Specially, apart from the H 2 Me 4 bpz ligand, each Cl -coordinates with two Cu ions from two different planes which is different from SO 4 2-in Crystal 1 (Figure 4b).With the participation of Cl -in the construction of the framework, the interconnected 2D planes bridge into 3D porous structures (Figure 4c).From the c-axis, it can be seen that there are two types of pores in Crystal 2. One is in the triangle shape while the other is in hexagon shape with the length of side about 9.497 Å.The detailed information about Crystal 1 and Crystal 2 is summarized in Table 1.

CONCLUSION
In summary, we have successfully synthesized two novel MOF structures using H 2 Me 4 bpz ligand and different salts as metal source, as well as the counterions (SO 4 2-, Cl -) as bridging ligands, through a facile solvothermal method.The as-obtained frameworks display unique structure morphologies as a result of the combination of metal ions, H 2 Me 4 bpz ligand and diverse counterions.The semirigid ligand H 2 Me 4 bpz delivered different dihedral angle of the two pyrazolate (pz) rings in order to adapt to the 2D or 3D framework under the lowest energy level as well as to resist the structure variations.