CAO Zhuang WANG Ji-Jiang TANG Long WANG Xiao HOUXiang-Yang JU Ping REN Yi-Xia

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Two Zn(II)/Cd(II) Metal-organic Frameworks Constructed from 1,3,5-Benzenetricarboxylic Acid and 1,4-Bis(1-imidazolyl)benzene: Syntheses, Structures and Photoluminescent Properties①

CAO Zhuang WANG Ji-Jiang②TANG Long WANG Xiao HOUXiang-Yang JU Ping REN Yi-Xia

(716000)

Zn(II)/Cd(II), metal-organic frameworks, crystal structure, photoluminescent properties;

1 INTRODUCTION

Metal-organic frameworks (MOFs), also known as functional coordination polymers (CPs), the new type of organic-inorganic hybrid materials, are very appealing for their fascinating structures, promising properties and great potential applications in the fields of luminescence, gas adsorption, molecular recognition, catalysis and magnetism[1-13]. It is generally known that the design of a novel MOFrepresents a very interesting research. Several factors such as reaction solvents, reaction temperatures, molar ratio of organic ligands, metal ions, and so on are major challenges for the construction of desired complexes[14-19].Thus, the strategy of building functional MOFs inspires great interest.To our knowledge, the aromatic polycarboxylic 1,3,5-benzenetricarboxylic acid (H3btc) has been widely used as the ligand to form various com- plexes[21-23]. In our former work, the MOFs {[Zn (Hbtc)(bpyb)]?H2O}nand{[Cd(Hbtc)(bpyb)1.5]?0.5-bpyb}nhave been reported[24].To further extend our work, we continue to combine H3btc with Zn(II)/ Cd(II) metal oxides in the presence of bib ligand, and two new coordination polymers, namely {[Zn3- (btc)2(bib)2(H2O)2]?2H2O}n(1) and {[Cd3(btc)2(bib)1.5(H2O)6]?6H2O}n(2), have been successfully synthesized. In addition, the structures,thermal stability and luminescent properties of1 and 2 have been investigated.

2 EXPERIMENTAL

2.1 Materials and methods

All chemicals and solvents were commercially available and used directly. Elemental analyses for C, H, and N wereconducted on a Vario EL III elemental analyzer. IR spectra were performed on a Bruker EQUINOX-55 spectrometer with KBr pellets in the 4000～400 cm-1region. TGA was measured on a NETZSCH STA 449F3 analyzer in flowing nitrogenatmosphere.The fluorescence spectra were studied using a Hitachi F-4500 fluorescence spectrophoto- meter at room temperature.

2.2 Syntheses

2. 2. 1 {[Zn3(btc)2(bib)2(H2O)2]?2H2O}n(1)

ZnO(0.10 mmol, 8.20 mg), btc (0.10 mmol, 21.01 mg), bib (0.10 mmol, 21.00 mg), and 15 mL H2O were sealed in a 25 mL Teflon-lined reactor. After stirring for 30 min,the mixture was kept at 160℃ for 3 d, then cooled to room temperature. Colorless block crystals of complex 1 were obtained with42.5% yield (based on Zn). Anal. Calcd. for C42H34N8O16Zn3(%): C, 45.74; H, 3.11; N, 10.16. Found (%): C, 58.34; H, 3.52; N, 7.35. IR (KBr, cm-1): 3408 m, 3142 w, 2362 w, 2343 w, 1622 s, 1533 s, 1438 w, 1375 s, 1354 s, 1070 s, 962 m, 819 w, 758 s, 740 s, 725 s, 648 m, 536 w, 500 w, 463 w, 440 w.

2. 2. 2 {[Cd3(btc)2(bib)1.5(H2O)6]?6H2O}n(2)

An identical procedure with 1 was followed to prepare 2 except ZnO was replaced by CdO (0.10 mmol, 12.80 mg). Colorless block crystals complex 2 were obtained with46.7% yield (based on Cd). Anal. Calcd. for C36H45N6O24Cd3(%): C, 33.70; H, 3.54; N, 6.55. Found (%): C, 33.73; H, 3.52; N, 6.57. IR (KBr, cm-1): 3385 m, 3161 w, 2363 w, 2344 w, 1612 s, 1558 s, 1533 s, 1437 m, 1364 s, 1308 w, 1256 w, 1107 m, 1065 s, 961 m, 936 m, 835 m, 733 s, 648 w, 529 w, 507 w.

2.3 Crystal structure determination

Single-crystal data of complexes 1 and 2 were selected for diffraction datacollection at 296(2) K on a Bruker Smart APEX II CCD diffractometer equipped with a graphite-monochromatic Moα radiation (= 0.71073 ?). The absorption corrections were applied using semi-empirical methods of SADABS program[25]. Those structures were solved by direct methods with SHELXS-97 and refined by full-matrix least-squares methods on2using the SHELXL-97 programs[26, 27]. The positions of all non-hydrogen atoms were refined anisotropically. Hydrogen atoms were positioned in the geome- trically calculated positions. Detailed crystallo- graphic data and structural refinements of complexes 1 and 2 were gathered in Table 1. Selected bond distances and angles are listed in the Table 2.

Table 1. Crystal Data and Structural Refinement Parameters for 1 and 2

[a]=Σ||F|?|||/Σ|F|;[b]=[Σ[(2–2)2]/Σ[(2)2]]1/2

Table 2. Selected Bond Lengths (?) and Bond Angles (o) for 1 and2

3 RESULTS AND DISCUSSION

3.1 Crystal structure of {[Zn3(btc)2(bib)2(H2O)2] ?2H2O}n(1)

Single-crystal X-ray diffraction analysis reveals that complex 1is a three-dimensional framework. It crystallizes in the monoclinic2space group. In 1, an asymmetric unit contains one and a half of Zn(II) ions, one btc, one bib ligand, one coordination water molecule and onefree water molecule (Fig. 1). Zn(1) is four-coordinated by O(1), O(1A) from two btc ligands and N(1), N(1A) from two bib ligands, leaving a distorted {ZnO2N2} tetrahedral geometry. While Zn(2) is located in a distorted {ZnO5N} octahedral coordination geometry, linked by O(3), O(4), O(5B) and O(6B)from two btc ligands, N(4C)from one bib ligand, and O(7) from one coordination water molecule.The Zn–O bond lengths vary from 1.915(3) to 2.043(3) ?, and the Zn–N distances are within the range of 2.000(3)～2.014(3) ? (Table 2). All the bond lengths are consistent with those in the reported Zn(II)complexes[28, 29].

Fig. 1. Coordination environment of Zn(II) in 1. All hydrogen atoms are omitted for clarity

As shown in Fig. 2a, all the btc anions apply a3-1:1:1:1:0:1coordination mode to connect the Zn(II) ions to forman interesting infinite 1D [Zn4(btc)4]nloop-like chain along theaxis. Viewed along thedirection,the structure features a parallelogram-shaped helical tube with a diameter of 13.59 ?. Then, the bib ligands act as 2-connected nodes to connect the 1D loop-like chains, extending into a 3D framework (Fig. 2b). The potential large voids lead to the generation of a 3-fold inter- penetrating architecture(Fig. 2c). In addition, hydrogen bonding andstacking may play an important role in the assembly process to form a 3D entanglement framework, and further contribute to the stability of the 3-fold interpenetrating archi- tecture. The geometrical parameters of all hydrogen bonds andinteractions of complex 1 are listed in Table 3.

Fig. 2. (a) 1D loop-like chain structure. (b) 3D framework of 1 along thedirection. (c) View of the 3D framework of complex 1 space-filling

Table 3. Geometrical Parameters of Hydrogen Bonds and π···π Interactions for 1 and 2

3.2 Crystal structure of {[Cd3(btc)2(bib)1.5(H2O)6]?6H2O}n (2)

Fig. 3. Coordination environment of the Cd(II) in 2. All hydrogen atoms are omitted for clarity

The bridging3-1:1:1:1:1:1and3-1:1:1:1:0:1carboxyl groups of the btc ligands as well as the 2-connected bib ligandslink the Cd(II) ions to form an interesting 1D [Cd3(btc)2bib]nloop-like chain (Fig. 4a). By this means, the bib ligands connect adjacent 1D chains to create a 2D layerstructure (Fig. 4b). Interestingly, compared to the structure of complex 1, there also exist O–H···O hydrogen bonding and···stacking interactions in the 2D layer structure of complex 2. Furthermore, the adjacent 2D layers are expanded into a 3D supra- mulecular framework (Fig. 5) via hydrogen bonds and···interactions (Table 3).

Fig. 4. (a) 1D loop-like chains in 2; (b) 2D layers formed by bib ligands connecting adjacent 1D chains

Fig. 5. 2D layers of 1 connected into a 3D supramulecular framework through hydrogen bonds

3.3 IR spectra

The solid state IR spectra of the two complexes exhibit typical antisymmetric stretching bands of carboxyl groups (1622, 1533, 1438 cm-1in 1; 1613, 1559, 1533 cm-1in 2) and symmetric stretching bands of the same groups (1375, 1354, 1305 cm-1in 1; 1437, 1364, 1308 cm-1in 2). The maximum and minimum values Δ(as (COO-) –(COO-)) of 1 (247, 179 and 133 cm-1) and 2 (176, 195 and 225 cm-1), respectively indicate the coordination of btc ligands with Zn(II)/Cd(II) in not only monodentate but also bidentate-chelating modes[30].

3.4 Thermal analysis

To study the thermal stability of the complexes, thermal gravimetric analysis (TGA) was carried out from room temperature to 900 °C under nitrogen atmosphere (Fig. 6). In complex 1, the weight loss occurred from 94 to 146 ℃ (obsd. 6.29%, calcd. 6.53%), which corresponds to the decomposition of framework structure on two coordinated water mole- cules and two free water molecules. The decom- position of six coordinated water molecules and six free water molecules are observed from 55 to 190 ℃ (obsd. 16.7%, calcd. 16.84%) in complex 2. The frameworks of 1 and 2 decompose at 395and 334 ℃, and the final residues are ZnO (obsd. 23.01%, calcd. 22.14%) for 1 and CdO (obsd. 30.10 %, calcd. 30.03 %) for 2, respectively.

Fig. 6. Thermogravimetric analyses for 1 and 2

3.5 Fluorescence properties

The solid-state emission for free H3btc, 1 and 2were recorded at room temperature. As shown in Fig. 7, intense bands were observed at 322 nm for H3btc, 389 nm for 1, and 366 nm for 2 under the same excitation maximum at 301 nm. It is worth noting that the maximum emission peaks of 1 and 2 have a certain red shift (67 nm for 1, 44 nm for 2). 2 shows high fluorescent emission whereas 1 exhibits a weak fluorescent emission.These red shifts and emission intensities may result from the deprotonation of H3btc ligand and the coordination effects of theH3btc ligand to the Cd(II)/Zn (II) ions[31, 32].

Fig. 7. Emission spectra of free H3btc and complexes 1 and 2 in the solid state at room temperature

4 CONCLUSION

In summary, we have successfully synthesized two new Zn(II)/Cd(II) coordination polymers based on the aromatic polycarboxylic H3btc and the N-hetero- cyclic bib. The structural studies reveal that 1 shows a 3D framework, and the potential large voids in this network lead to a 3-fold interpenetrating architecture. 2 displays a 2D framework, and the adjacent 2D layers are expanded into a 3D supramulecular net- work via rich hydrogen bonds. Notably, the photo- luminescence investigations show that 2 shows high fluorescent emission, whereas 1 exhibits a weak fluorescent emission.

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29 April 2018;

3 July 2018 (CCDC 1835710 and 1835711)

the National Natural Science Foundation of China (No. 21373178, 21663031 and 21503183), the Scientific Research Foundation of Shaanxi Provincial Education Department (No. 16JK1857), and the Natural Scientific Research Foundation of Yan’an City Technology Division of China (No. 2016kg-01)

10.14102/j.cnki.0254-5861.2011-2060