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  1   INTRODUCTION

  During the past two decades, the rational design and controlled synthesis of coordination polymers with interesting structures and/or peculiar properties, such as ion-exchange, catalysis, magnetism, adsorption, and luminescence, have been the focus of intense study^[1-7]. Generally, the preparation of such materials can be influenced by many factors, such as the nature of organic ligands, the coordination geometry of metal ions, solvent systems, metal-toligand ratios, etc^{[8, 9]}. A successful strategy in building such networks is to employ appropriate bridging ligands that can bind metal ions in different modes and provide a possible way to achieve more new materials with diverse architectures and excellent physical properties. Compared with those broadly used ligands aromatic benzene-based di- or multi-carboxylic acids^[10], related efforts on heterocyclic acids (such as pyridinecarboxylic acids, pyrazolecarboxylic acids, imidazolecarboxylic) and their derivatives are relatively less^[11-19]. The ligand bearing both carboxylate and N-heterocyclic groups, Hcptpy and H₂tzba can be used to construct metalorganic frameworks with interesting structures and properties. However, the investigations of coordination polymers synthesized by these mixed ligands of Hcptpy and H₂tzba have not been reported. Zinc, an essential trace element, plays a very important role in cell metabolism and physical growth and development. The applications of zinc compounds in optoelectronics research field have also aroused much attention. In this contribution, we would like to study the potential properties of zinc coordination polymers with these mixed polyfunctional ligands. It is found that the assembly of Hcptpy and H₂tzba with Zn (II) atoms results in a rare example of double-edge ladders, which is further extended to a 3D supramolecular network via O-H…O/N hydrogen bonds. Herein, we report the preparation and characterization of one new coordination polymer [Zn₂(cptpy)₂(tzba)(H₂O)]·H₂O (1). In addition, the thermal stability and photoluminescence properties of 1 have also been examined.

  2   EXPERIMENTAL

  2.1   General

  All chemicals and reagents were used as received from commercial sources without further purification. All reactions were carried out under hydrothermal conditions. Elemental analyses (C, H and N) were determined with an Elementar Vario EL (III) elemental analyzer. X-ray powder diffraction patterns (XRPD) of 1 were recorded at 293 K on a Bruker D₈ Advance diffractometer (CuKα, λ = 1.54056 Å) operated at 40 kV and 30 mA, using a Cu-target tube and a graphite-monochromator. Thermo gravimetric analysis (TGA) experiment was carried out on a Perkin-Elmer Diamond SII thermal analyzer from room temperature to 800 ℃ under a nitrogen atmosphere at a heating rate of 10 ℃/min. The emission/excitation spectra were recorded on an F-7000 (HITACHI) spectrophotometer at room temperature.

  2.2   Synthesis of the title compound

  A mixture of 4-(4-carboxyphenyl)-2, 2′:4′, 4′′- terpyridine (0.05 mmol, 0.0177 g), 4-(tetrazol-5-yl) benzeneca rboxylic acid (0.05 mmol, 0.0010 g), Zn (NO₃)₂·6H₂O (0.1 mmol, 0.0297 g) and NaOH (0.2 mmol, 0.0080 g) were added to water (8 mL) in a 25 mL Teflon-lined stainless steel vessel. The mixture was heated at 150 ℃ for 3 days. After the reactive mixture was slowly cooled to room temperature, colorless block crystals of 1 were obtained (yield: 45%, based on Zn). Elemental analysis calcd. (%) for C₅₂H₃₆O₈N₁₀Zn₂: C, 58.94; H, 3.42; N, 13.21%. Found: C, 58.79; H, 3.31; N, 13.15%. IR data (KBr , cm^-1): 3452(m), 1613(s), 1575(m), 1449(m), 1391(s), 1191(w), 763(m), 713 (m).

  2.3   X-ray structure determination

  A single crystal of the title complex (0.22mm × 0.19mm × 0.16mm) was mounted at 293 K on a Bruker SMART 1000 CCD diffractometer employing graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) by using a φ-ω scan mode at room temperature in the range of 2.90≤θ≤25.50° with index ranges of -15≤h≤18, -32≤k≤34 and -12 ≤l≤13. A semi-empirical absorption correction was applied using the SADABS program^[20]. The structure was solved by direct methods and refined by full-matrix least-squares on F₂ using the SHELXS-97 and SHELXL-97 programs, respectively^{[21, 22]}. Selected bond distances and bond angles are listed in Table 1.

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

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  BondDist.BondDist.BondDist.Zn(1)-O(6)#11.932(4)Zn(2)-O(1)1.994(4)Zn(2)-O(7)2.129(4)Angle(°)Angle(°)Angle(°)O(6)#1-Zn(1)-O(4)107.79(19)O(1)-Zn(2)-N(8)107.2(2)O(1)-Zn(2)-O(2)57.03(17)O(1)-Zn(2)-N(1)#2142.8(2)O(7)-Zn(2)-O(3)171.15(17)N(8)-Zn(2)-O(7)86.99(18)

  Zn(1)-O(4)	1.947(4)	Zn(2)-O(1)	1.994(4)	Zn(2)-O(3)	2.201(4)
  Zn(1)-N(6)#2	2.047(5)	Zn(2)-N(1)#2	2.037(5)	Zn(2)-O(2)	2.490(5)
  Zn(1)-N(7)	2.057(5)	Zn(2)-N(8)	2.102(5)
  O(6)#1-Zn(1)-N(6)#2	95.31(19)	N(1)#2-Zn(2)-N(8)	109.9(2)	N(1)#2-Zn(2)-O(2)	85.80(17)
  O(4)-Zn(1)-N(6)#2	123.0(2)	O(1)-Zn(2)-O(7)	94.37(18)	N(8)-Zn(2)-O(2)	163.1(2)
  O(6)#1-Zn(1)-N(7)	114.49(19)	O(1)-Zn(2)-O(3)	93.78(17)	O(7)-Zn(2)-O(2)	99.79(17)
  O(4)-Zn(1)-N(7)	106.3(2)	N(1)#2-Zn(2)-O(3)	85.97(17)	O(3)-Zn(2)-O(2)	87.54(16)
  N(6)#2-Zn(1)-N(7)	109.89(19)	N(8)-Zn(2)-O(3)	87.32(18)	N(1)#2-Zn(2)-O(7)	89.61(17)
  Symmetry transformation: #1: -x+1, -y, -z+2; #2: x-1, y, z; #3: x+1, y, z

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  3   RESULTS AND DISCUSSION

  3.1   Crystal structure of

  Single-crystal X-ray diffraction analysis revealed that complex 1 crystallizes in the monoclinic system, P2₁/c space group, and features a 1D ladder-shaped polymer chain. The asymmetric unit of 1 contains two crystallographically independent Zn (II) atoms, one cptpy ligand, one tzba ligand, one aqua ligand as well as one guest water molecule. As shown in Fig. 1, the Zn (1) center is four-coordinated in the tetrahedral coordination geometry. The donor set is formed by two carboxylate oxygen atoms from two different tzba ligands, and two nitrogen atoms from one cptyp and one tzba ligands. While the Zn (2) center adopts a distorted octahedral coordination geometry surrounded by two nitrogen atoms from one cptyp and one tzba ligands, three carboxylate oxygen atoms from two different tzba ligands, and one oxygen atom from the water molecule.

  Figure 1.  Local coordination environment of Zn (II) atoms in the unit of 1

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  In the structure of 1, the cptpy ligand exhibits two different types of coordination modes. One cptpy acts as a bidentate ligand, while the other is a tridentate ligand. The carboxylate group of one cptpy exhibits a μ₁-η¹:η¹ coordination mode to connect one Zn (II) atom (Scheme 1a), whereas the -COO^- group of another cptpy adopts a μ₂-η¹:η² coordination mode to connect one Zn (II) atom (Scheme 1b). One pyridyl group of the cptpy ligand, which contains the N (6) atom, links one Zn (II) atom, whereas another two pyridyl groups of the cptpy ligand remain uncoordinated. As for the tridentate tzba ligand, the tetrazolyl group connects one Zn (1) atom and one Zn (2) atom through its 1- and 2-position N atoms. The carboxyl group adopts a monodentate coordination mode to bridge adjacent two Zn (1) atoms. The dihedral angles between the aromatic and tetrazolyl rings as well as carboxyl group and its corresponding phenyl ring are found to be about 48.22° and 7.98°, respectively. One tetrazolyl group of tzba and one carboxyl group of triden- tate cptyp ligand link adjacent two Zn (2) atoms to form a dimer with the Zn…Zn separation of 3.961 Å. Based on the coordination modes mentioned above, all Zn (2) atoms are interlinked by cptpy and tzba ligands to generate a 1D ribbon coordination po- lymer running along the [100] direction with rectan- gular windows of 12.915Å × 17.579Å (Fig. 2). It is noteworthy that 1 features a double-edge ladder with Zn (II) dimers as the nodes, pair of tzba and cptpy ligands as the inner rungs and cptpy as side rails, respectively, which is different from the previously observed simple ladder-shaped polymer chains^[23-25], and is unprecedented to the best of our knowledge. Moreover, uncoordinated outer pyridyl group of cptpy ligand acts as the side arm decorating on both sides of the ladder.

  Figure 1.  Various coordination modes of Hcptpy observed in complex 1

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  Figure 2.  View of the 1D ladder-shaped polymer chain of 1 running along the [100] direction

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  Observed from the crystal packing diagram of 1, these ladders are stacked in an offset fashion with an ABAB sequence and bond together by strong intermolecular hydrogen bonds, resulting in the formation of a 3D supramolecular network (Fig. 3). There are two kinds of hydrogen bonds in 1: O-H…O hydrogen bonding interactions between the aqua and carboxylate oxygen atoms (O (7)…O (2) ^a: 2.859 Å. Symmetry code: a = x, -y + 1/2, z + 1/2), and O-H…N hydrogen bonding interactions between the aqua and tetrazolyl ring of tzba ligand (O (7)…N (10) ^b: 2.811 Å. Symmetry code: b = x, -y + 1/2, z - 1/2).

  Figure 3.  Space-filling view of the extended 3D supramolecular architecture along [011] direction in 1

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  3.2   Powder X-ray diffraction patterns and thermal stability

  In order to check the phase purity of the complex, powder X-ray diffraction (PXRD) patterns of 1 was recorded at room temperature (Fig. 4). The peak positions of the simulated and experimental PXRD patterns are in agreement with each other, which confirms its phase purity of bulk sample. The difference in intensity of some diffraction peaks may be attributed to the preferred orientation of the crystalline powder samples. To examine the stability and molecular composition of 1, thermal gravimetric analysis (TGA) experiment was performed. As shown in Fig. 5, the TGA diagram reveals that 1 shows a two-step weight loss of 3.67% in the temperature range of 60～120 ℃, corresponding to the release of coordinated and guest water molecules (Calcd. 3.39%). The dehydrated phase remains stable up to 270 ℃ until the organic ligands start to release.

  Figure 4.  Comparing the simulated and experimental PXRD patterns of the title complex

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  Figure 5.  Thermal gravimetric analysis plot of 1

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  3.3   Luminescence property

  The luminescent properties of compounds containing d¹⁰ metal centers have been attracting more interest because of their potential applications in chemical sensors, photochemistry and electroluminescent displaying^[26]. To examine the luminescence properties of the d¹⁰ metal complex, the luminescence spectra of 1 (Fig. 6) have been measured. Complex 1 displays emission peak at 423 nm upon excitation at λ = 366 nm. In order to understand the nature of such emission bands, the fluorescent properties of the free Hcptpy and H₂tzba ligands were also investigated. Upon excitation at ca. 345 and 300 nm, the title compound shows emission maximum at 372 and 327 nm, respectively. Compared with the emission spectrum of Hcptpy and H₂tzba, red shift of emission bands for 1 has been observed. These emissions are neither metal-toligand charge transfer (MLCT) nor ligand-to-metal transfer (LMCT) in nature, since Zn (II) ions are difficult to oxidize or reduce due to their d¹⁰ configuration^[27]. These shifts of the emission bands may be attributed to both the deprotonated effect of Hcptpy and H₂tzba and the coordination interactions of the organic ligands to Zn (II) ions^[28].

  Figure 6.  Solid-state emission spectra of 1, free Hcptpy and H₂tzba ligands at room temperature

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• 

  Figure 1  Local coordination environment of Zn (II) atoms in the unit of 1

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  Figure 1  Various coordination modes of Hcptpy observed in complex 1

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  Figure 2  View of the 1D ladder-shaped polymer chain of 1 running along the [100] direction

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  Figure 3  Space-filling view of the extended 3D supramolecular architecture along [011] direction in 1

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  Figure 4  Comparing the simulated and experimental PXRD patterns of the title complex

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  Figure 5  Thermal gravimetric analysis plot of 1

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  Figure 6  Solid-state emission spectra of 1, free Hcptpy and H₂tzba ligands at room temperature

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  BondDist.BondDist.BondDist.Zn(1)-O(6)#11.932(4)Zn(2)-O(1)1.994(4)Zn(2)-O(7)2.129(4)Angle(°)Angle(°)Angle(°)O(6)#1-Zn(1)-O(4)107.79(19)O(1)-Zn(2)-N(8)107.2(2)O(1)-Zn(2)-O(2)57.03(17)O(1)-Zn(2)-N(1)#2142.8(2)O(7)-Zn(2)-O(3)171.15(17)N(8)-Zn(2)-O(7)86.99(18)

  Zn(1)-O(4)	1.947(4)	Zn(2)-O(1)	1.994(4)	Zn(2)-O(3)	2.201(4)
  Zn(1)-N(6)#2	2.047(5)	Zn(2)-N(1)#2	2.037(5)	Zn(2)-O(2)	2.490(5)
  Zn(1)-N(7)	2.057(5)	Zn(2)-N(8)	2.102(5)
  O(6)#1-Zn(1)-N(6)#2	95.31(19)	N(1)#2-Zn(2)-N(8)	109.9(2)	N(1)#2-Zn(2)-O(2)	85.80(17)
  O(4)-Zn(1)-N(6)#2	123.0(2)	O(1)-Zn(2)-O(7)	94.37(18)	N(8)-Zn(2)-O(2)	163.1(2)
  O(6)#1-Zn(1)-N(7)	114.49(19)	O(1)-Zn(2)-O(3)	93.78(17)	O(7)-Zn(2)-O(2)	99.79(17)
  O(4)-Zn(1)-N(7)	106.3(2)	N(1)#2-Zn(2)-O(3)	85.97(17)	O(3)-Zn(2)-O(2)	87.54(16)
  N(6)#2-Zn(1)-N(7)	109.89(19)	N(8)-Zn(2)-O(3)	87.32(18)	N(1)#2-Zn(2)-O(7)	89.61(17)
  Symmetry transformation: #1: -x+1, -y, -z+2; #2: x-1, y, z; #3: x+1, y, z

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