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  1   INTRODUCTION

  Metal-organic frameworks (MOFs) with variable structural motifs have attracted great considerable attention owing to their potential applications as functional materials in the fields of luminescence, catalysis, gas storage and magnetism^[1-5]. The basic method for the synthesis of MOFs is the self-assembly of metal ions and multifunctional organic ligands. Recent studies have proven that the use of mixed ligands is one of the most effective strategies for the building of MOFs with intriguing topological networks and interesting properties^[6-9]. In view of that, our current synthetic strategy is the mixedligand self-assembly strategy, which uses 1, 4-benzenedicarboxylic acid and ¹H-benzotriazole as the mixed ligands. To the best our knowledge, 1, 4-benzenedicarboxylic acid can display versatile coordination modes to bridge metal ions and the HBTA ligand can deprotonate into BTA^-, which can link metal ions into three kinds of coordination modes: μ₂-κN1:κN3, μ₂-κN1:κN2, μ₃-κN1:κN2:κN3^[10-12]. Via the hydrothermal reactions of Zn (OAc)₂·2H₂O, 1, 4-benzenedicarboxylic acid and HBTA, we successfully obtained a new 3D compound, namely [Zn (BTA)(BDC)_0.5]_n (1). Herein, we report the hydrothermal synthesis, crystal structure and luminescent property of the title Zn (II) compound.

  2   EXPERIMENTAL

  2.1   Materials and equipments

  All the 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. IR spectrum was recorded on a Nicolet Magna 750FT-IR spectrometer in the range of 400～ 4000 cm^-1. Powder X-ray diffraction data were collected using a PANalytical X'Pert Pro powder diffractometer with Cu-Kα radiation and 5≤2θ≤ 50°. Thermogravimetric analysis was 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 Bruker P4 diffractometer.

  2.2   Synthesis of {Zn (BTA)(pdc)_0.5}_n

  A mixture of Zn (OAc)₂·2H₂O (0.112 g, 0.5 mmol), H₂pdc (0.166 g, 1 mmol), HBTA (0.04 g, 0.34 mmol) and H₂O (10 mL) was sealed in a 23 mL Teflon-lined stainless steel vessel and heated at 160 oC for 3 d under autogenous pressure, and then the reactant mixture was cooled to room temperature at a rate of 10 oC/h. Colorless block crystals of 1 were collected in 52% yield based on HBTA. Elemental analysis calculated for C₁₀H₆N₃O₂Zn: C, 45.11; H, 2.18; N, 15.70%. Found: C, 45.19; H, 2.26; N, 15.82%. IR (KBr, ν, cm^-1): 3071(w), 1618(s), 1564(s), 1484(s), 1382(m), 1275(m), 1166(m), 1005(w), 928(w), 850(m), 729(s), 651(w), 586(w), 419(w), 541(w), 503(w).

  2.3   Structure determination

  A suitable single crystal with dimensions of 0.24mm × 0.22mm × 0.20mm was carefully selected and glued on a thin glass fiber. Structural determination was performed on a Bruker P4 diffractometer with a graphite-monochromatic Mo-Kα radiation (λ = 0.71070 Å) at 293 K. A total of 7138 reflections were collected with 2207 unique ones (R_int = 0.0143) in the range of 3.03＜θ＜27.48º by using an ω-2θ scan mode. Empirical absorption corrections were made using the SADABS program^[13]. The structure was solved by direct methods using SHELXS-97^[14] and refined on F₂ by full-matrix least-squares with SHELXL-97^[15]. All non-hydrogen atoms were refined anistropically, and all hydrogen atoms attached to carbon were placed at their ideal positions.

  The final R = 0.0221, wR = 0.0605 (w = 1/[σ²(F_o ²) + (0.0409P)₂ + 0.4314P], where P = (F_o ² + 2F_c ²)/3), S = 1.001 and (Δ/σ) _max = 0.000 for 2075 observed reflections (I > 2σ(I)). The maximum and minimum peaks on the final difference Fourier map are 0.342 and -0.405 e/ų, respectively. Selected bond lengths and bond angles are listed in Table 1.

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) of Compound 1

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  BondDist.BondDist.Zn(1)-O(1)1.9470(12)Zn(1)-N(1)1.9690(14)Angle(º)Angle(º)O(1)-Zn(1)-N(1)112.58(6)O(1)-Zn(1)-N(3)a116.99(6)N(1)-Zn(1)-O(2)b98.67(5)N(3)a-Zn(1)-O(2)b103.48(6)

  Zn(1)-N(3)a	1.9786(14)	Zn(1)-O(2)b	2.0196(13)
  N(1)-Zn(1)-N(3)a	119.66(6)	O(1)-Zn(1)-O(2)b	100.87(5)
  Symmetry codes: (a) -x + 2, y - 1/2, -z + 1/2; (b) x, -y + 3/2, z - 1/2

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) of Compound 1

  3   RESULTS AND DISCUSSION

  3.1   IR spectroscopy

  The infrared spectrum of compound 1 has four characteristic absorption peaks at 1618, 1564, 1484, and 1382 cm^-1, which can be assigned to the asymmetric and symmetric vibration of the carboxylate groups of pdc^2- ligand. The absence of the absorption peak at 1700 cm^-1 indicates that the H₂pdc ligand is fully deprotonated, which is in accordance with the single-crystal X-ray analytical result.

  3.2   Description of structure 1

  Single-crystal X-ray diffraction analysis revealed that compound 1 crystallizes in the monoclinic P₂₁/c space group with the asymmetric unit containing one Zn (II) ion, one BTA^- ligand and half of a pdc^2- ligand. The pdc^2- ligand sits across a crystallographical inversion center, and the Zn (1) ion and BTAligand are at the general positions. As shown in Fig. 1, Zn (1) is tetrahedrally coordinated by two carboxylate oxygen atoms (O (1) and O (2b)) from two different pdc^2- ligands and two nitrogen atoms (N (1) and N (3a)) from two different BTA^- ligands. The Zn-O distances range from 1.9470(12) to 2.0196(13) Å and the Zn-N distances vary from 1.9690(14) to 1.9786(14) Å, which are comparable with that of the previously reported Zn (II) compounds^[16-17]. The O (N)-Zn-N (O) angles locate in the range of 98.67(5) ～ 119.66(6)° (Table 1). In 1, the BTA^- ligands adopted a μ₂-κN1:κN3 mode and connected the adjacent Zn (II) ions into a 1D chain extending along the c axis (Fig. 2a), with the Zn…Zn distance of 5.745 Å. The pdc^2- ligand is fully deprotonated and adopts a μ₄-κO: κO’:κO": κO" coordination mode linking four different Zn (II) ions. Each chain is interconnected into the adjacent chains through pdc^2- ligands, giving rise to the final 3D framework of 1 (Fig. 2b). Topologically speaking, if the Zn (II) ions and pdc^2- ligands linking four different Zn (II) ions are considered as 4-, 4-connected nodes, respectively, and the BTA^- ligands are treated as linkers, the overall structure of 1 can be simplified into a 4-connected topological network with the Schläfli symbol of {6⁴.8²}{6⁶}₂ (Fig. 2c), which was also identified as a bbf net according to the RSCR (Reticular Chemistry Structure Resource) symbol^[18-20]. To the best of our knowledge, only four Zn (II) coordination polymers assembled with H₂pdc and HBTA ligands have been reported and they all show 3D frameworks^[21-23]. The title compound 1 displays another new bbf-type topological framework, which further enriches the coordination chemistry of the mixed ligands of H₂pdc and HBTA.

  Figure 1.  View of the coordination environments of Zn (II) ion in 1. Displacement ellipsoids are drawn at the 30% probability level. All H atoms have been omitted for clarity (Symmetry codes: (a) -x + 2, y - 1/2, -z + 1/2; (b) x, -y + 3/2, z - 1/2; (c) -x + 1, -y + 2, -z + 1)

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  Figure 2.  (a) One-dimensional chain structure constructed from BTA^- ligands and Zn (II) ions. (b) A view of the three-dimensional framework of compound 1. (c) Schematic representation of the 4-connected bbf topological network

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  3.3   Powder X-ray diffraction analysis and thermal analysis

  As shown in Fig. 3a, the peak positions of the experimental pattern match well with that of the simulated one based on the single-crystal X-ray diffraction data, indicating good purity of the bulk samples. In addition, thermal stability of compound 1 was also investigated under nitrogen atmosphere in the temperature range of 30～800 oC. From the TG curve of 1 (Fig. 3b), we found that there is almost no weight loss in the temperature range of 30～260 oC. Above 260 oC, a rapid weight loss was observed which can be attributed to the decomposition of organic ligands. Finally, the remnants of 30.83% may be ZnO (calcd.: 30.50%).

  Figure 3.  (a) Powder X-ray diffraction patterns for compound 1. (b) TGA curve of compound 1

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  3.4   Photoluminescent property

  The luminescent property of the title compound was investigated in the solid state at room temperature. As shown in Fig. 4, compound 1 displays the emission maxima at λ_em = 368 nm when excitated at λ_ex = 320 nm. According to the reported literatures, the free HBTA has a maximum emission at λ_em = 350 nm (λ_ex = 300 nm), and the free H₂pdc ligand has a maximum emission at λ_em = 447 nm (λ_ex = 349 nm)^{[24, 25]}. It is particularly noting that the luminescent emission spectrum of compound 1 is similar to that of the free HBTA ligand with a red shift of 18 nm. Therefore, the luminescence of 1 mostly derived from the HBTA intraligand charge transfer.

  Figure 4.  Emission spectrum and excitation spectrum (inset) of compound 1 in the solid state at room temperature

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  4   CONCLUSION

  In summary, a new 3D luminescent Zn (II) coordination polymer has been successfully synthesized via the hydrothermal reactions of Zn (II) ions and the mixed ligands of H₂pdc and HBTA. This compound features a 3D framework and can be topologically represented as a 4-connected bbf topological network.

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  Figure 1  View of the coordination environments of Zn (II) ion in 1. Displacement ellipsoids are drawn at the 30% probability level. All H atoms have been omitted for clarity (Symmetry codes: (a) -x + 2, y - 1/2, -z + 1/2; (b) x, -y + 3/2, z - 1/2; (c) -x + 1, -y + 2, -z + 1)

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  Figure 2  (a) One-dimensional chain structure constructed from BTA^- ligands and Zn (II) ions. (b) A view of the three-dimensional framework of compound 1. (c) Schematic representation of the 4-connected bbf topological network

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  Figure 3  (a) Powder X-ray diffraction patterns for compound 1. (b) TGA curve of compound 1

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  Figure 4  Emission spectrum and excitation spectrum (inset) of compound 1 in the solid state at room temperature

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (º) of Compound 1

  BondDist.BondDist.Zn(1)-O(1)1.9470(12)Zn(1)-N(1)1.9690(14)Angle(º)Angle(º)O(1)-Zn(1)-N(1)112.58(6)O(1)-Zn(1)-N(3)a116.99(6)N(1)-Zn(1)-O(2)b98.67(5)N(3)a-Zn(1)-O(2)b103.48(6)

  Zn(1)-N(3)a	1.9786(14)	Zn(1)-O(2)b	2.0196(13)
  N(1)-Zn(1)-N(3)a	119.66(6)	O(1)-Zn(1)-O(2)b	100.87(5)
  Symmetry codes: (a) -x + 2, y - 1/2, -z + 1/2; (b) x, -y + 3/2, z - 1/2

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