GUO Xing-Zhe LI Jia-Le SHI Shan-Shan ZHOU Hui HAN Shuai-Shuai CHEN Shui-Sheng

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Synthesis, Structure and Luminescent Property of a Zn(II) Complex with Mixed Multi-N Donor and 2,5-Dihydroxy-terephthalic Acid Ligands①

GUO Xing-Zhe LI Jia-Le SHI Shan-Shan ZHOU Hui HAN Shuai-Shuai CHEN Shui-Sheng②

(236041)(236041)

A new complex, [Zn2(HL)2(2,5-OH-pbda)]n(1, L = 1-(1-imidazol-4-yl)-4-(4- tetrazol-5-yl)benzene (H2L) and 2,5-OH-H2pbda = 2,5-dihydroxy-terephthalic acid), has been hydrothermally prepared and characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analysis and PXRD. Complex 1 crystallizes in monoclinic, space group21/with= 8.8101(10),= 17.105(2),= 9.4014(11) ?,= 105.704(2)o,= 1363.9(3) ?3,= 4, C14H9N6O3Zn,M= 374.64,D= 1.824 g/cm3,=mm-1,= 1.042,(000) = 756, the final= 0.0309 and= 0.1003 for 3130 observed reflections (> 2()). The H2L ligand was deprotonated to be an HL?anion, and coordinated with Zn2+to form two-dimensional (2D) Zn2(HL?)22+sheets, which were pillared by the 2,5-OH-pbda2?ligands to form athree-dimensional (3D) dmcnetwithPoint (Schl?fli) symbol of (4·82)(4·85). The large vacancy of the dmcnet is filledmutual interpenetration of another independent framework, leading to the formationof a 2-fold interpenetrating architecture. Solid state luminescent property of 1has been investigated.

metal-organic framework, crystal structure, luminescent property;

1 INTRODUCTION

The design and construction of metal-organic frameworks (MOFs) have attracted great interest in the fields of coordination chemistry and crystal engineering, because MOFs have intriguing variety of topologies, fascinating structures, interesting pro- perties and potential applications for instance, in gas absorption and separation, luminescence, catalysis, molecular magnetism, sensors,[1-7]. In order to obtain desirable frameworks with specific structures and functions, the primary selection is to design organic ligands. Generally, organic tectons mainly including polyazaheteroaromatic ligands and multi- carboxylic acids with N and O coordination atoms are the most widely used linkers for the assembly of various MOFs[8-12]. As an important type of multi- dentate N donor ligand, N donor ligands are usually nitrogen heterocyclic compounds, such as pyridine, imidazole, triazole, tetrazole, and so on[13-16]. The imidazole ligands are more effective ligands to build desirable MOFs as elaborated in our recent highlight[17],and we have deliberately designed mutil-N donor ligands containing the 4-imidazolyl groups such as 1,4-di(1H-imidazol-4-yl)benzene and 1,3,5-tri(1H- imidazol-4-yl)benzene, which can exhibit diverse coordination modes, employing as not only elec- trically neutral ligands, but also as anion ligands in case of their deprotonation to be imidazolate anions[18, 19]. On the other hand, polycarboxylate compounds have served as excellent candidates for building frameworks because of their strong binding ability and variable coordination modes. In this context, we have deliberately designed a multi-N donor ligand-1-(1H-imidazol-4-yl)-4-(4H-tetrazol- 5-yl)benzene (H2L), and six coordination complexes with diverse structures were successfully constructed from mixed ligands incurporating N-donor ligands and multi-carboxylates together with zinc(II) salts by hydrothermal methods under different reaction conditions[20]. In this paper, we focus our attention on the study on reactions of H2L ligand together with 2,5-dihydroxy-terephthalic acid (2,5-OH- H2pbda) and ZnSO4·7H2O for the assembly of MOFs as an extension of our work. Herein, we report the syntheses and crystal structure of a new coordination polymer [Zn2(HL)2(2,5-OH-pbda)]n(1) based on the mix ligand strategy with H2L and 2,5-OH-H2pbda with corresponding ZnSO4·7H2O under hydrothermal conditions.

2 EXPERIMENTAL

2. 1 Materials and measurements

All the commercially available chemicals and solvents were of reagent grade and used as received without further purification. Elemental analyses were performed on a Perkin-Elmer 240C Elemental Analyzer. IR spectra were recorded on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Power X-ray diffraction (PXRD) patterns were measured on a Shimadzu XRD-6000 X-ray diffractometer with Cu(= 1.5418 ?) radiation at room temperature. The luminescence spectra for the powdered solid samples at room temperature were measured on an Aminco Bowman Series 2 spectro- fluorometer with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5 nm, and all the measurements were carried out under the same experimental conditions.

2. 2 Synthesis of complex[Zn2(HL)2(2,5-OH-pbda)]n (1)

A mixture of H2L (0.021 g, 0.1 mmol), 2,5-OH-H2pbda(0.020 g, 0.1 mmol), ZnSO4·7H2O (0.0287 g, 0.1 mmol) and NaOH (0.004 g, 0.1 mmol) in 10 mL H2O was sealed in a 25 mL Teflon-lined stainless-steel container and heated at 180 ℃ for 72 h. Colorless block crystals of 1 were collected with a yield of 63% by filtration and washed with water and ethanol for several times. Anal. Calcd. (%) for C14H9N6O3Zn: C, 44.88; H, 2.42; N, 22.43. Found (%): C, 44.66; H, 2.58; N, 22.73. IR(KBr): 3450～3060(m), 1615(m), 1525(m), 1462(s), 1388(s), 13558(m), 1306(s), 1256(m), 1142(s), 1086(m), 1012(m), 958(w), 858(s), 823(m), 752(m), 636(m), 582(m), 575(m).

2. 3 Crystal structure determination

The colorless crystals of complex 1 were selected for diffraction data collection at 296(2) K on a Bruker Smart Apex II CCD diffractometer equipped with a graphite-monochromatic Mo-radiation (= 0.71073 ?). A total of 9197 reflections were collected for 1,of which 3130 (int= 0.0221) were independent in the range of 2.38≤≤27.54o for 1 by using a-scan mode. Semi-empirical absorp- tion corrections were applied using the SADABS program[21]. The structure was solved by direct methods with SHELXS-97[22]program and refined by full-matrix least-squares techniques on2with SHELXL-97[23]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of 1 were generated geometrically. The final= 0.0309,= 0.1003 (= 1/[2(F2) + (0.0958)2+ 0.2985], where= (F2+ 2F2)/3),int= 0.0221, (Δ/)max= 0.000,= 1.042, (Δ)max= 0.907 and (Δ)min= –0.490 e/?3for 1. The selected bond distances and bond angles for complex 1 are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) of [Zn2(HL)2(2,5-OH-pbda)]n

Symmetry transformation: #1:–+3/2,–1/2, –+3/2;#2: –+2, –+1, –+2

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of 1

The result of X-ray diffraction analysis revealed that complex 1 crystallizes in monoclinic form with space group21/and the asymmetric unit consists of one deprotonated HL?ligand, a half of 2,5- OH-pbda2?and one Zn(II) atom. Fig.1 shows the coordinationenvironment of one independent Zn(II) atom with atomicnumbering scheme. It can be clearly seen that the Zn(1) atom isin a distorted tetrahedral coordination environment withthree nitrogen atoms (N(5), N(1A), N(4B)) from three different HL?ligands with the Zn(1)–N(5), Zn(1)–N(1A), and Zn(1)–N(4B) bond distances of 2.020(2), 2.016(2), and 1.992(2) ?, respectively,and one oxygen atom (O(1)) from 2,5-OH-pbda2?ligand with theZn(1)–O(1) bond in 1.958(2)?, and the coordination angles around Zn(1) are in the range of 102.94(9)～123.59(9)°(Table 1). If the coordination of 2,5-OH-pbda2?with Zn(II) is neglected, the deprotonated HL?employs a3-bridge to coordinate with three Zn(II) atoms, forming a two-dimensional (2D) Zn2(HL?)22+double-layer of fes network with 4·82topology in theplane (Fig. 2)[24], where both of the HL?linkers and Zn(II) atoms act as 3-con- nected nodes. And the Zn2(HL?)22+2D sheets are further pillared by the linear 2,5-OH-pbda2?to form a three-dimensional (3D) framework with the Zn···Zn separation of 10.935 ? between two adjacent 2D layers (Fig. 3). Topologically,each HL?links three Zn(II) atoms, which can be regarded as a 3-connected node. As for each Zn(II) atom, it in turn links three HL?and one2,5-OH-pbda2?. Hence, it can be treated as a 4-connector. Topology analysis[25]suggests that the resulting structure of complex 1 is a binodal (3, 4)-connecteddmc net, and the Point (Schl?fli) symbol is (4·82)(4·85) (Fig. 4)[26]. When viewed along theplane, the 3D structure contains1.452nm×15.56nm channels, which can facilitate the interpenetration. As for 1, the largecavities are completely filledmutual interpenetrationof another independent framework, leading to the formationof a 2-fold interpenetrating 3D architec- ture (Fig.5).

Fig. 1. Coordination environment of the Zn(II) atom in 1 with ellipsoids drawn at the 30% probability level. The hydrogen atoms are omitted for clarity

Fig. 2. (a) 2D layer framework built from Zn(II)-HL?and(b) Simplified 2D fes net

Fig. 3. 3D structure of 1 constructed from 2D networks (turquiose) pillared by 2,5-OH-pbda2?ligands (yellow)

Fig. 4. Topological view of the dmc topology of complex 1, where the turquiose and blue balls represent the Zn(II) atoms and the centers of benzene rings of HL?ligands, respectively

Fig. 5. Schematic representation of the 2-fold interpenetrated dmc net of 1

3. 2 Thermal stability and powder X-ray diffraction

Complex 1 was subjected to ascertain the stability of supramolecular architecture by thermogravimetric analysis (TGA), as shown in Fig. 6. No obvious weight loss was found for 1 before the decomposi- tion of the framework occurring at about 355 ℃, which is in good agreement with the result of crystal structure analysis. The TGA analysis showed that the framework has collapsed step by step around 400 and 535 °C. Apparently, complex 1 showed a weight loss of 25.8% in the temperature range of 355～420 °C, corresponding to the release of 2,5-OH- pbda2?ligand (calcd. 26.3%), followed by the decomposition of the residue of HL?ligand. Powder XRD experiment was performed to confirm the phase purity of bulk sample, and the experimental patterns of the as-synthesized sample are well consistent with the simulated ones, indicating the phase purity of the sample (Fig. 7).

Fig. 6. TG curve of 1

Fig. 7. Simulated and experimental XRPD patterns of complex 1

3. 3 Luminescent property

Compounds constructed by10metal centers and conjugated organic linkers can be promising candi- dates employed as chemical sensors and photoche- mistry[27,28].In this paper, the solid-state photolu-minescent property of complex 1 has been investigated together with free H2L ligands at room temperature(Fig. 8). The free H2L ligand shows blue photolu- minescence emission at 385 nm upon excitation at 338 nm, which is probably attributable to the* → n or* →transitions[29]. As previously reported[30], the emission bands of solid-state benzenecarboxylate ligands can be assigned to the* → n transition, but fluorescent emission of benzene-dicarboxylate ligands resulting from the* → n transition is very weak compared with that of the* →transition of the H2L ligand, so benzene-carboxylate ligands almost have no contribution to the fluorescent emission of the assynthesized coordination polymers[31]. On com- plexation of these ligands with Zn(II) atoms, excitation of the microcrystalline samples leads to the generation of strong blue fluorescent emissions, with the maximal peaks occurring at 410 nm (ex= 345 nm) for 1. It can be seen that compound 1 exhibits strong broad blue photoluminescence with emission maxima at 410 nm upon excitation at 345 nm. In contrast to the case for the free ligand, the emission band of 1 is 25 nm red-shifted. Such broad emission bands mainly originate from the ligand-based luminescence, with the corresponding shifts originating from the ligand-to-metal charge transfer[32-33]. In addition, it is noteworthy that the enhancement of luminescence for complex 1, compared with the free ligand under the same conditions, may mainly originate from the coordination interactions between the metal Zn(II) atom and the ligand, which enhanced its confor- mational rigidity and then decreased the nonradia- tive energy loss[34].

Fig. 8. Excitation (left) and emission (right) spectra of H2L and complex 1

4 CONCLUSION

A new complex was constructed from the mixed 1-(1H-imidazol-4-yl)-4-(4H-tetrazol-5-yl)benzene and 2,5-dihydroxy-terephthalic acid ligands together with ZnSO4·7H2O under hydrothermal method, and the structure was characterized by single-crystal X-ray diffraction, IR spectroscopy, elemental analy- sis and PXRD. The result of X-ray diffraction analysis reveals the complex is a 2-fold interpene- trating dmc net. The result confirms that the com- bination of multi-N-donor ligand and polycar- boxylate ligand is a good choice for the construction of MOFs with specific structures and properties.

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11 October 2017;

11 December 2017 (CCDC 1578692)

①This work was supported by the National Natural Science Foundation of China (No. 21171040), the Natural Science Foundation of Anhui Provincial Education Commission (No. KJ2017ZD29) and National Undergraduates Innovation Project (201710371018)

. Chen Shui-Sheng, doctor, majoring in coordination chemistry. E-mail: sscfync@163.com

10.14102/j.cnki.0254-5861.2011-1849