QIN Ye-Yn ZHANG Xin SHEN Yi-Cheng YAO Yun-Gen②

a (Key Laboratory of Coal to Ethylene Glycol and Its Related Technology, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China)

b (University of Chinese Academy Sciences, Beijing 100039, China)

1 INTRODUCTION

In the past few decades, the construction of metalorganic frameworks (MOFs) with intriguing structures and topologies have attracted considerable interest from chemists because of their potential applications as functional materials in the fields of luminescence, magnetism, nonlinear optics, gas storage, catalysis and so on[1-4]. Until now, how to rationally design and synthesize MOFs at the molecular level remains a far-reaching challenge. In order to control the synthesis of desirable MOFs,great efforts have been made in this area, and several effective synthetic strategies, such as pillar-layer strategy, mixed-ligand method, post-synthetic modification, second building block approach and so on, have been well established[5-8]. Among these strategies to construct MOFs, the second building block method has been widely used to construct new crystalline materials with predictable structures owing to their intrinsic geometrics and directionality that can direct the network design[9-12]. According to literatures, 1,2,3-benzotriazole (HBTA) with three nitrogen atoms in the five-membered ring can deprotonate one proton into BTA-and multiple metal ions can be connected together by the BTA-ligands into polynuclear metal clusters or rod-shaped spiral chains[13-15]. In our previous work, we successfully exploited BTA-to build a plate-like tetranuclear Zn4(BTA)3building block[16]. As an extension and deepening of our research work, here we continue to use HBTA as an organic ligand to create polynuclear-based frameworks incorporated by the auxiliary HUCA ligand, which can link the benzotriazole controlled polynuclear clusters into extended frameworks. Herein, we report the hydrothermal synthesis, crystal structures and luminescent property of a new Zn(II) coordination polymer,namely [Zn(BTA)(UCA)]n(1, HBTA = 1,2,3-benzotriazole, HUCA = 4-imidazoleacrylic acid), which features a 2D layered structure based on benzotriazole controlled dinuclear [Zn2(BTA)2] subunits.Further packing of these 2D layers resulted in a 2D→ 3D polythreaded supramolecular framework directed by intermolecular hydrogen bonds and π···π interactions.

2 EXPERIMENTAL

2.1 Materials and equipments

All starting materials used in this work were commercially purchased and used without further purification. Elemental analyses of C, H and N were performed on an EA1110 CHNS-0 CE elemental analyzer. The IR spectrum was recorded on a Nicolet Magna 750FT-IR spectrometer in the range of 400～4000 cm–1. Thermogravimetric analyses were carried out on a NetzschSTA499C integration thermal analyzer under a nitrogen atmosphere from 30 to 800 ℃ at a heating rate of 10 ℃/min. The single-crystal data were collected on a Rigaku Mercury CCD diffractometer.

2.2 Synthesis of [Zn(BTA)(UCA)]n

A mixture of Zn(OAC)2·2H2O (0.112 g, 0.5 mmol), HUCA (0.138 g, 1 mmol), HBTA (0.04 g,0.34 mmol), H2O (10 mL) and CH3OH (5 mL) was sealed in a 23 mL Teflon-lined stainless-steel reactor under autogenous pressure at 160 ℃ for 72 h and then cooled to room temperature slowly. Colorless prism crystals yield in 35%. Anal. Calcd. (%) for 1 C12H9N5O2Zn: C, 43.69; H, 3.03; N, 21.24. Found(%): C, 60.37; H, 4.15; N, 6.70. IR (KBr pellet, cm–1)for 1: 3468(s), 1659(m), 1599(s), 1571(m), 1389(m),1371(s), 1345(s), 1271(m), 1231(m), 1171(m),1121(s), 979(s), 871(w), 793(w), 751(m), 701(w),651(w), 565(w).

2.3 Structure determination

A suitable single crystal with dimensions of 0.20mm × 0.18mm × 0.06mm was carefully selected and glued on a thin glass fiber. Structural determination was performed on a Rigaku Mercury CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ = 0.71073 ?) at 293(2) K. A total of 8069 reflections were collected with 2264 unique ones (Rint= 0.0229) in the range of 2.15<θ<24.99o by using an ω-2θ scan mode. The structure was solved by direct methods using SHELXS-97[17]and refined on F2by full-matrix least-squares with SHELXL-97[18]. All non-hydrogen atoms were refined anistropically, and all hydrogen atoms attached to carbon and nitrogen atoms were placed at their ideal positions.

The final R = 0.0255, wR = 0.0655 (w = 1/[σ2(Fo2)+ (0.0384P)2+ 0.5268P], where P = (Fo2+ 2Fc2)/3),S = 1.065 and (Δ/σ)max= 0.001 for 2104 observed reflections (I > 2σ(I)). The maximum and minimum peaks on the final difference Fourier map are 0.456 and –0.265 e/?3, respectively. Selected bond lengths and bond angles are listed in Table 1.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) of Compound 1

3 RESULTS AND DISCUSSION

3.1 Description of structure 1

Single-crystal X-ray structural analysis revealed that compound 1 features a 2D layered structure with the asymmetric unit containing one Zn(II) ion,one BTA-ligand and one UCA-ligand. As shown in Fig. 1, Zn(1) is tetrahedrally coordinated by one carboxylate oxygen atom (O(1a)) from one uacligand and three nitrogen atoms (N(1), N(1b) and N(4)) from two BTA-ligands and one UCA-ligand.The Zn–N bond distances range from 1.9858(17) to 2.0266(18) ? and the Zn–O bond distance is 1.9580(17) ?. The BTA-ligand links two Zn(II) ions in a μ2-N(1),N(2) mode, and two adjacent Zn(II)ions are bridged together by two BTA-ligands,giving rise to a dinuclear [Zn2(BTA)2] subunit with the Zn···Zn separation of 3.7594(16) ? (Fig. 2a). The UCA-ligand acts as a linear linker linking two Zn(II)ions in a μ2-N(4),O(1) mode. Each dinuclear[Zn2(BTA)2] subunit is coordinated by four UCA-ligands, and each UCA-ligand links two [Zn2(BTA)2]subunits. This connection mode further resulted in a 2D (4, 4) layer (Fig. 2b). Finally, the UCA-ligands connect the [Zn2(BTA)2] subunits together to form a 2D layer with large open windows (Fig. 2b). The most striking structure feature for 1 is the 2D → 3D polythreaded motif (Fig. 2c). In the packing framework of 1, the lateral BTA-ligands of each layer threaded into the opened windows of the adjacent layers (Fig. 2d). Notably, this polythreading was directed by two kinds of weak intermolecular interactions: intermolecular hydrogen bonds and π···π interactions. The N(5)–H(5A)···N(3) hydrogen bond distance between the BTA-and UCA-ligands from the adjacent layers is 2.835(3) ? and the angle of ∠NHN is 170o, and the centroid-to-centroid distance of π···π interactions between the phenyl rings of BTA-from the interval layers intermolecules is 3.7289(12) ?, which all fall in the normal ranges[19,20].

Fig. 1. An ORTEP drawing of the molecular structure of compound 1 (30% thermal ellipsoids).Hydrogen atoms associated with C atoms have been omitted for clarity.Symmetry codes: (a) x + 1, –y + 3/2, z – 1/2; (b) –x, –y + 1, –z

Fig. 2. (a) Benzotriazole controlled dinuclear [Zn2(BTA)2] subunits. (b) 2D layered structure of compound 1. (c) 2D → 3D polythreaded supramolecular framework of compound 1.(d) Detailed threaded motif directed by intermolecular hydrogen bonds and π···π interactions(Black dotted lines represent the hydrogen bonds and red dotted lines represent the π···π interactions)

3.2 . IR spectrum

In the IR spectrum of compound 1, the adsorption peak of 3468 cm-1can be assigned to the N–H stretching vibrations of imidazole ring, and the absence of absorption peaks in the range of 1730～169 cm-1indicates the deprotonation of urocanic acid. The characteristic peaks in the 1659, 1599,1571 and 1389, 1371 cm-1may be attributed to the ν(C=O) and ν(C–O) stretching vibrations, respectively[21].

Fig. 3. TGA curve of compound 1

3.2 Thermal analysis

Thermogravimetric (TG) analysis was performed to investigate the thermal stability of compound 1(Fig. 3). From the TG curve of compound 1, it can be seen that there is no obvious weight loss in the temperature range of 30～395 ℃. Upon heating above 395 ℃, compound 1 begins to collapse owing to the decomposition of BTA-and UCA-ligands.The final residue weight of 25.14% can be assigned as ZnO (Calcd.: 25.26%).

Fig. 4. Solid-state fluorescence emission spectrum of compound 1

3.3 Fluorescence property

The fluorescence property of compound 1 was investigated at room temperature in the solid state.As shown in Fig. 4, upon excitation of 350 nm,compound 1 has a broad emission band with the maximum at 503 nm. According to the literature reports, we know that the free HBTA ligand displays fluorescence with the emission maximum at 350 nm(λex= 300 nm)[22]and free HUCA ligand displays fluorescence with the emission maximum at 453 nm(λex= 371 nm)[21]. Compared with the emission peaks of compound 1, free HBTA ligand and free HUCA ligand, the luminescence of compound 1 is distinctly different from the free ligands (HBTA and HUCA). Therefore, we speculate that the luminescent emission of 1 may be attributed to the ligandto-metal charge transfer (LMCT).

4 CONCLUSION

In summary, a novel luminescent Zn(II) compound has been successfully synthesized and structurally characterized. The compound features a 2D layer structure. Furthermore, these 2D layers are packed into a 3D supramolecular framework, displaying a 2D→3D polythreaded motif directing by the intermolecular hydrogen bonds.

(1) Zhang, X. J.; Wang, W. J.; Hu, Z. J.; Wang, G. N.; Uvdal, K. S. Coordination polymers for energy transfer: preparations, properties, sensing applications, and perspectives. Coord. Chem. Rev. 2015, 284, 206-235.

(2) Chen, W. X.; Tan, L.; Liu, Q. P.; Qiang, G. R.; Zhuang, G. L. The ionothermal synthesis, structure, and magnetism-structure relationship of two biphenyl tetracarboxylic acid-based metal-organic frameworks. Dalton. Trans. 2014, 16515–16521.

(3) Wang, C.; Liu, D. M.; Lin, W. B. Metal-organic frameworks as a tunable platform for designing functional molecular materials.J. Am. Chem. Soc.2013, 135, 13222–13234.

(4) Hu, P.; Morabito, J. V.; Tsung, C. K. Core-shell catalysts of metal nanoparticle core and metal-organic framework shell. ACS Catal. 2014,4409–4419.

(5) Li, B. Y.; Zhang, Y. M.; Ma, D. X.; Li, L.; Li, G. H.; Li, G. D.; Shi, Z.; Feng, S. H. A strategy toward constructing a bifunctionalized MOF catalyst:post-synthetic modification of MOFs on organic ligands and coordinatively unsaturated metal sites. Chem. Commun. 2012, 6151–6153.

(6) Chen, D. M.; Zhang, X. P.; Shi, W.; Cheng, P. Tuning two-dimensional layer to three-dimensional pillar-layered metal-organic frameworks:polycatenation and interpenetration behaviors. Cryst. Growth Des. 2014, 6261–6268.

(7) Chaemchuen, S.; Zhou, K.; Kabir, N. A.; Chen, Y.; Ke, X. X.; Van Tendeloo, G.; Verpoort, F. Tuning metal sites of DABCO MOF for gas purification at ambient conditions. Micropor. Mesopor. Mat. 2015, 277–285.

(8) Bloch, W. M.; Burgun, A.; Coghlan, C. J.; Lee, R.; Coote, M. L.; Doonan, C. J.; Sumby, C. J. Capturing snapshots of post-synthetic metallation chemistry in metal-organic frameworks. Nature Chem. 2014, 906–912.

(9) Guillerm, V.; Weselinski, L. J.; Belmabkhout, Y.; Cairns, A. J.; D'Elia, V.; Wojtas, L.; Adil, K.; Eddaoudi, M. Discovery and introduction of a(3,18)-connected net as an ideal blueprint for the design of metal-organic frameworks. Nature Chem. 2014, 673–680.

(10) Cho, W.; Lee, H. J.; Choi, G.; Choi, S.; Oh, M. Dual changes in conformation and optical properties of fluorophores within a metal-organic framework during framework construction and associated sensing event. J. Am. Chem. Soc. 2014, 12201–12204.

(11) Vlad, A.; Zaltariov, M. F.; Shova, S.; Novitchi, G.; Varganici, C. D.; Train, C.; Cazacu, M. Flexible linkers and dinuclear metallic nodes build up an original metal-organic framework. CrystEngComm. 2013, 5368–5375.

(12) Reger, D. L.; Leitner, A.; Smith, M. D.; Tran, T. T.; Halasyamani, P. S. Homochiral helical metal-organic frameworks of group 1 metals. Inorg. Chem.2013, 10041–10051.

(13) Zhou, W. L.; Liang, J.; Qin, C.; Shao, K. Z.; Wang, F. M.; Su, Z. M. Syntheses, crystal structures and properties of inorganic-organic hybrids constructed from Keggin-type polyoxometalates and silver coordination compounds. CrystEngComm. 2014, 7410–7418.

(14) Li, Y. W.; Liu, S. J.; Hu, T. L.; Li, D. C.; Dou, J. M.; Chang, Z. A new Co-based metal-organic framework constructed from infinite sinusoidal-like rod-shaped secondary building units. Inorg. Chem. Commun. 2014, 67–70.

(15) Jiang, Z. Q.; Jiang, G. Y.; Wang, F.; Zhao, Z.; Zhang, J. Ring-size controllable metallamacrocycles as building blocks for the construction of microporous metal-organic frameworks. Chem. Commun. 2012, 48, 3653–3655.

(16) Qin, Y. Y.; Zhang, J.; Li, Z. J.; Zhang, L.; Cao, X. Y.; Yao, Y. G. Organically templated metal-organic framework with 2-fold interpenetrated{33.59.63}-lcy net. Chem. Commun. 2008, 2532–2534.

(17) Sheldrick, G. M. SHELXS-97, Program for Refinement of Crystal Structures. Institute for Inorganic Chemistry, University of G?ttingen: G?ttingen,Germany 1997.

(18) Sheldrick, G. M. SHELXL-97, Program for Solution of Crystal Structures. Institute for Inorganic Chemistry, University of G?ttingen: G?ttingen,Germany 1997.

(19) Wang, S.; Ding, X. H.; Li, Y. H.; Huang, W. Supramolecular assemblies through host-guest interactions of 18-crown-6 with ammonium salts:geometric effects of amine groups on the hydrogen-bonding architectures. Supramol. Chem. 2015, 213–223.

(20) Guo, F.; Zhu, B. Y.; Xu, G. L.; Zhang, M. M.; Zhang, X. L.; Zhang, J. Tuning structural topologies of five photoluminescent Cd(II) coordination polymers through modifying the substitute group of organic ligand. J. Solid State Chem. 2013, 199, 42–48.

(21) Feng, X.; Wen, Y. H.; Lan, Y. Z.; Feng, Y. L.; Pan, C. Y.; Yao, Y. G. Multifunctional zinc(II) urocanate with rare fivefold interpenetrating diamondoid network. Inorg. Chem. Commun. 2009, 89–91.

(22) Lu, J.; Zhao, K.; Fang, Q. R.; Xu, J.; Yu, J. H.; Zhang, X.; Bie, H. Y.; Wang, T. G. Synthesis and characterization of four novel supramolecular compounds based on metal zinc and cadmium. Cryst. Growth Des. 2005, 1091–1098.