English

• 0.   Introduction

  The field of constructing coordination polymers (CPs) with controllable topology has now been studied extensively, and thus intriguing materials with poten-tial applications such as gas sorption and storage, pho-tocatalyst and luminescence, can be produced in differ-ent synthesized - methods^[1-6]. However, as we know, many multiple factors, such as temperature, solvent, central metal and co - ligands may affect the nature of CPs and the framework formation in the self-assembly process, which result in the complexity and uncertainty of them^[7]. Generally speaking, the fine tuning of CPs is difficult to realize, and even a subtle modulation of the synthesis conditions may lead to the formation of drasti-cally different structures. As is well known, the combi-nation of aromatic carboxylatic acids and flexible N - donor coligands, especially bis(imidazole) ligands, usu-ally have been used to construct CPs with interesting topologies. In such a system, the flexible bis(imidazole) ligands may bend and rotate freely when coordinating to the central metal atoms, which often result in struc-tural diversities. At present, significant work has been carried out by using different metal ions assembly with flexible bis(imidazole) ligands and a series of outstand-ing examples have been documented in the litera-ture^[8-11]. Furthermore, photoactive CPs have received much more attention as photocatalysts for dye effluent degradation due to large interfacial surface areas, short electron - hole diffusion lengths to internal interfaces, and multiple routes for band gap engineering through compositional and structural control^[12-15]. Recently, we synthesized 1, 5-bis-(2-methyl-imidazol)pentane (BMIP) as a flexible N- donor ligand, which has attracted atten-tion in the field of crystal engineering and has been not yet well developed^[16]. Taking inspiration from these views, herein, we report the syntheses and crystal struc-tures of two Zn(Ⅱ) metal-organic frameworks, namely [Zn(5-OHIP) (BMIP)]_n (1) and [Zn(5-BrIP) (BMIP)]_n (2) with BMIP and 5-hydroxyisophthalic acid (5-OHH₂IP) and 5-bromoisophthalic acid (5-BrH₂IP) ligands. More-over, the luminescent and photocatalyst properties of these complexes were investigated in the solid state at room temperature.

  1.   Experimental

  1.1   Materials and instruments

  The regents were used as commercial sources without further purification. The IR spectra were re-corded on Bruker Vector22 FT - IR spectrophotometer using KBr discs. Elemental analyses were performed on a Perkin - Elmer 240C elemental analyzer. Thermo-gravimetric analyses (TGA) were performed on a TGA V5.1A Dupont 2100 instrument heating from room tem-perature to 900 K under N₂ with a heating rate of 20 K· min^-1. The Rigaku D/max - 2500 X - ray powder diffrac-tometer with graphite monochromated Cu Kα radiation (λ=0.154 18 nm) was applied to analyze the powder X-ray diffraction (PXRD) data at 296 K and using ω - 2θ scan mode within 5°~50° at 40 kV and 40 mA. Fluores-cence measurements were performed using an F - 7000 fluorescence spectrophotometer at room temperature in the solid state. The photocatalytic activity studies were carried out in a UV-visible spectrometer using a Perki-nElmer Lambda 950 UV/Vis instrument.

  1.2   Syntheses of complexes 1 and 2

  A mixture of 5-OHH₂IP (0.018 g, 0.1 mmol), Zn(NO₃)₂·6H₂O (0.028 g, 0.1 mmol), BMIP (0.013 g, 0.05 mmol), H₂O (10 mL) and NaOH (0.008 g, 0.2 mmol) was placed in a Teflon-lined stainless-steel ves-sel, heated to 413 K for 3 d, and then cooled to room temperature within 24 h. Colorless block-shaped crys-tals of 1 were obtained with a yield of 29%. Anal. Calcd. for C₄₂H₄₇Zn₂N₈O₁₀(%): C, 52.80; H, 4.92; N, 11.73. Found(%): C, 52.76; H, 4.95; N, 11.78. IR (KBr pellet, cm^-1): 3 116 (w), 2 968 (w), 1 628 (w), 1 605 (s), 1 519 (m), 1 464 (m), 1 376 (s), 1 364 (m), 1 156 (s), 789 (w), 772 (m), 730 (m), 668 (w). The synthesis of complex 2 was similar to that of 1 but 5 - BrH₂IP was used (0.025 g, 0.1 mmol) instead of 5-OHH₂IP. Color-less block-shaped crystals were obtained (Yield: 25%). Anal. Calcd. for C₂₁H₂₃BrZnN₄O₄(%): C, 46.61; H, 4.25; N, 10.36. Found(%): C, 46.54; H, 4.32; N, 10.31. IR (KBr pellet, cm^-1): 3 120 (w), 2 960 (w), 1 626 (w), 1 608 (s), 1 521 (m), 1 468 (m), 1 379 (s), 1 366 (m), 1 150 (s), 793 (w), 775 (m), 732 (m), 670 (w).

  1.3   X-ray crystallography

  The X - ray diffraction measurement for 1 and 2 was carried out on a Bruker Smart Apex Ⅱ CCD dif-fractometer equipped with a graphite-monochromatized Mo Kα radiation (λ=0.071 073 nm). The data were inte-grated by using SAINT program^[17], which also did the intensity corrections for Lorentz and polarization effect. An empirical absorption correction was applied using SADABS program^[18]. The structures were solved by direct methods using program SHELXS - 97 and all the non -hydrogen atoms were refined anisotropically on F ² by the full-matrix least-squares technique using SHELXL-97 crystallographic software package^[19-20]. Crystal data and structure refinement parameters are listed in Table 1. The selected bond lengths are given in Table 2.

  Table 1

  

  Table 1.  Crystal data and refinement parameters for complexes 1 and 2

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  Complex	1	2
  Empirical formula	C42H47Zn2N8O10	C21H23ZnBrN4O4
  Formula weight	954.62	540.71
  Crystal system	Monoclinic	Orthorhombic
  Space group	P21/n	Pna21
  a / nm	0.951 88(6)	0.887 40(6)
  b / nm	1.878 17(11)	1.518 59(10)
  c / nm	1.201 20(7)	1.710 17(11)
  β/(°)	101.363(10)
  V / nm3	2.105 4(2)	2.304 6(3)
  Z	2	4
  Crystal size / mm	0.26×0.22×0.20	0.20×0.14×0.10
  μ / mm-1	1.208	2.833
  F(000)	990	1 096
  No. of measured, independent, observed [I > 2σ(I)] reflections	11 405, 4 126, 3 208	11 998, 3 999, 3 634
  Data, restraint, parameter	4 126, 0, 282	3 999, 0, 283
  Final R indices [I > 2 σ (I)], S	R1=0.057 1, wR2=0.148 0, 1.048	R1=0.025 3, wR2=0.064 2, 1.034
  Largest diff. peak and hole / (e·nm-3)	540, -543	316, -497

  Table 2

  

  Table 2.  Selected bond lengths (nm) for complexes 1 and 2

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  1
  Zn1-N1	0.199 8(3)	Zn1-O1	0.196 7(3)	Zn1-N4A	0.201 6(3)
  Zn1-O3B	0.195 0(3)
  2
  Zn1-N1	0.203 2(2)	Zn1-O5	0.195 6(2)	Zn1-N4A	0.202 1(2)
  Zn1-O3B	0.192 5(2)
  Symmetry codes: A: -x, -y, 1-z; B: 1+x, 3/2-y, 1/2+z for 1; A: 5/2-x, -1/2+y, 1/2+z; B: 1-x, 1-y, 1/2+z for 2.

  CCDC: 1502156, 1; 1502157, 2.

  2.   Results and discussion

  2.1   Structure description

  The crystallographic data shows that complex 1 crystallizes in the monoclinic system with P2₁/n space group. There are one Zn(Ⅱ), one 5- OHIP^2- ligand, one BMIP ligand in the asymmetric unit of 1. Each Zn²⁺ ion is four - coordinated in a distorted tetrahedral geometry by two nitrogen atoms from two BMIP ligands and two oxygen atoms from two 5 - OHIP^2- ligands. The bond angles around the Zn²⁺ ion are in a range of 96.32(14)°~ 123.36(13)°. The bond lengths of Zn-O are 0.195 0(3)~ 0.196 7(3) nm and Zn-N are 0.199 8(3), 0.201 6(3) nm, respectively (Table 2). In 1, each 5-OHIP^2- anion serves as a μ₂ -bridge linking two adjacent Zn²⁺ ions in the bis-monodentate mode to give rise to a one-dimensional zigzag chain along c axis, while such one-dimensional chains are further double- bridged by BMIP ligands into a two-dimensional layer along the c axis (Fig. 1b). Interestingly, each flexible BMIP is ligat-ed to two Zn²⁺ ions with two terminal imidazole groups and a pair of oppositely arranged μ₂-BMIP ligands bind two Zn²⁺ ions from adjacent chains to form a [Zn₂ (BMIP)₂] 24-membered metallomacrocycle ring, and the Zn…Zn separation across the BMIP ligands is 1.167 4 nm. From the viewpoint of network topology^[21], if the Zn²⁺ ions are considered as 3 - connected nodes, 5 - OHIP^2-ligand and [Zn₂ (BMIP)₂] unit as linear connectors, the whole structure can be simplified to a 3 - connected 63 topological net (Fig. 1c). The 2D layers are further assembled by intermolecular hydrogen bonds with a H(5A) …O(2) distance of 0.170 nm and the angle of 160° (O(5)-H(5A)…O(2)), leading to formation of a 3D supermolecule structure (Fig. 1d).

  Figure 1

  

  Figure 1.  (a) Coordination environment of Zn (Ⅱ) in 1 showing 30% probability displacement ellipsoids; (b) 2D layer structure of 1; (c) 3-connected 63 topological network of 1; (d) 3D molecular structure of complex 1

  All H atoms are omitted for clarity; Symmetry codes: A:-x, -y, 1-z; B: 1+x, 3/2-y, 1/2+z

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  With 5-BrH₂IP ligand instead of 5-OHH₂IP ligand, new complex 2 with a three -fold interpenetrat-ing three - dimensional framework was obtained. X -ray structural analysis shows that 2 crystallizes in the orthorhombic system with Pna2₁ space group. Its asym-metric unit contains one Zn(Ⅱ), one BMIP ligand, and one 5 -BrIP^2- anion (Fig. 2a). The coordination environment around Zn(Ⅱ) is shown in Fig. 2a. Each Zn(Ⅱ) ion is in a distorted tetrahedron geometry defined by two nitrogen atoms (Zn1-N1 0.203 2(2) nm, Zn1-N4A 0.202 1(2) nm) of two BMIP ligands and two oxygen at-oms (Zn1 - O3B 0.192 5(2) nm, Zn1-O5 0.195 6(2) nm) from two separated 5 -BrIP^2- anions. In complex 2, Zn(Ⅱ) ions are bridged by 5-BrIP^2- ligands to give rise to one-dimensional helical structures, which coexist as two kinds of helical chains: a left-handed and a right-handed. The BMIP ligand adopts an anti-anti coordinat-ing fashion, while the completely deprotonated 5-BrIP^2-ligand adopts monodentately mode. The two carboxylic groups of 5-BrIP^2- take a uniform monodentate coordination mode. Zn(Ⅱ) ions are connected by 5-BrIP^2-anions to form a one-dimensional chain with a Zn…Zn distance of 1.047 5 nm (Fig. 2b top). Each BMIP ligand links two Zn(Ⅱ) ions to form a one - dimensional wave - like chain with an adjacent Zn…Zn distance of 1.354 6 nm. Firstly, two kinds of helical chains and BMIP ligands form the two - dimensional layer (Fig. 2b, bot-tom). Then the combination of two - dimensional layers is pillared by the other BMIP ligands to result in the formation of ultimate three-dimensional framework (Fig. 2c). Owing to the long distance of Zn…Zn, there is a large unoccupied void space existing in the single three-dimensional framework, which shows a possibili-ty that 2 may display interpenetrating structural charac-teristics. The potential voids are filled via mutual inter-penetration of two other independent equivalent frame-works, generating a three-fold interpenetrating 3D architecture. This feature can greatly enhance the sta-bility of the whole structure. To further demonstrate the overall 3D structure of 2, we consider each Zn(Ⅱ) ion as a four-connecting node, which is linked by two 5-BrIP^2-anions and two BMIP ligands. Moreover, the BMIP and 5-BrIP^2- anions are simplified as a linear linker. With further topological analysis by the TOPOS program^[21], the whole structure of 2 can be simplified to a three - fold three-dimensional framework 4-c dmp topology with the point symbols (6⁵.8) (Fig. 2d).

  Figure 2

  

  Figure 2.  (a) ORTEP drawing of 2 with 30% thermal ellipsoids; (b) 1D helical-chain constructed by Zn(Ⅱ) and 5-BrIP^2- ligands; (c) View of 3D framework of 2; (d) Topological representation of 3-fold 3D dmp structure of 2

  Hydrogen atoms are omitted for clarity; Symmetry codes: A: 5/2-x, -1/2+y, 1/2+z; B: 1-x, 1-y, 1/2+z

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  2.2   Thermal gravimetric analyses, PXRD and photoluminescence property

  Both complexes are stable under ambient condi-tions and insoluble in common solvents. To identify the thermal stabilities of the complexes, TGA measure-ments were carried out in a range of 293~950 K. Since there are not any solvent molecules in the two complex-es, they exhibit excellent thermal stability as no weight loss step occurred below 620 and 660 K for 1 and 2, respectively. When the temperature rose up, the whole framework began to collapse (Fig. 3a). PXRD was used to check the purity of complexes 1 and 2. The results show that all the peaks displayed in the measured pat-terns at room temperature closely match those in the simulated patterns generated from single - crystal dif-fraction data, indicating single phases of 1 and 2 were formed, as shown in Fig. 3b. On the other hand, CPs with d¹⁰ metal centers have been investigated for their photoluminescent properties and potential applications in chemical sensors and photochemistry. The solid - state luminescent properties of 1, 2 and BMIP were in-vestigated at room temperature and the solid - state emission spectra are shown in Fig. 4. BMIP exhibited an emission band at 475 nm when excited at 360 nm. Upon exciting at 330 nm, the maximum emission peaks at about 465 and 460 nm were observed for complexes 1 and 2, respectively. Compared with the emission peak of the free BMIP ligand, the luminescence emis-sion peaks of complexes 1 and 2 are blue shifted. Because Zn(Ⅱ) ions have fluorescent emissions that are tuned by the metal-ligand interactions and the deprot-onated effect of the dicarboxylic ligands, the results suggest that complexes 1 and 2 may be good candidate of potential blue - fluorescent materials, since they are highly thermally stable and insoluble in common sol-vents.

  Figure 3

  

  Figure 3.  (a) TG curves of complexes 1 and 2; (b) PXRD patterns of complexes 1 and 2

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  Figure 4

  

  Figure 4.  Emission spectra of 1, 2 and ligand BMIP in the solid state at room temperature

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  2.3   Photocatalytic activity

  As we know, organic dyes such as methyl orange, methylene blue (MB) and rhodamine B are extensively used in the textile industry, which are resistant to bio-degradation commonly. In recent years, much effort has been devoted to develop new photocatalytic materials based on CPs to reduce the environment pollution of such dye molecules^[22-23]. Hence, in this work, the photo-catalytic activities of complexes 1 and 2 were evaluat-ed by the degradation of MB under irradiation at room temperature. Typically, 15 mg of powder of the com-plexes and 0.5 mL of 30% H₂O ₂ were dispersed in a 50 mL MB aqueous solution, then magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium of the working solu-tion. The solution was then exposed to the irradiation from a 500 W high pressure mercury vapor lamp and kept under continuous stirring. And 5 mL samples were taken out and then analyzed by UV-Vis spectrom-eter. By contrast, the control photocatalysis experiment was also performed under the same conditions with on-ly H₂O₂ or H₂ O₂ and ZnSO ₄ without any catalysts. The photocatalytic properties of the crystalline products are illustrated in Fig. 5a~5c. Obviously, the irradiation caused a significant decrease of the absorption with in-creasing reaction time in the presence of two complex-es. The major absorption of MB at about 665 nm was selected to supervise the photocatalytic degradation process. Additionally, the ratios of concentration of MB (c/c₀) vs reaction time (t) for two complexes were plot-ted. It can be seen that approximately 15% of MB had been decomposed after reacting for 120 min with only H₂O ₂ without any catalyst in the solution, while 26% of MB had been decomposed in the same condition with H₂O ₂ and ZnSO₄. In comparison with that, the systems with photocatalysts have shown much better photocata-lytic activities during the degradation processes (Degra-dation rate: 88% for 1 and 77% for 2, respectively). These results indicate that the two complexes are good candidates for photocatalytic degradation of MB, and may have potential photocatalytic application in the reduction of some other organic dyes.

  Figure 5

  

  Figure 5.  View of time-dependent absorption of MB solution in the presence of complex 1 (a) and complex 2 (b) during the decomposition reaction under UV light irradiation; (c) Photocatalytic performances of complexes 1 and 2

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  3.   Conclusions

  In summary, two new CPs with different frame-work structures have been successfully constructed based on the connectivity co-effect between the flexible BMIP ligand and 5- substitued carboxylates together with zinc salts under hydrothermal conditions. The re-sults exhibit that the structural diversification of CPs may result from different 5-substitued carboxylates. Furthermore, complexes 1 and 2 both show photolumi-nescence property, which appear to be potential hybrid inorganic - organic photoactive materials. And both of the complexes exhibit photocatalytic activity for the degradation of MB solution under ultraviolet light irra-diation.

  
  
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• 

  Figure 1  (a) Coordination environment of Zn (Ⅱ) in 1 showing 30% probability displacement ellipsoids; (b) 2D layer structure of 1; (c) 3-connected 63 topological network of 1; (d) 3D molecular structure of complex 1

  All H atoms are omitted for clarity; Symmetry codes: A:-x, -y, 1-z; B: 1+x, 3/2-y, 1/2+z

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  Figure 2  (a) ORTEP drawing of 2 with 30% thermal ellipsoids; (b) 1D helical-chain constructed by Zn(Ⅱ) and 5-BrIP^2- ligands; (c) View of 3D framework of 2; (d) Topological representation of 3-fold 3D dmp structure of 2

  Hydrogen atoms are omitted for clarity; Symmetry codes: A: 5/2-x, -1/2+y, 1/2+z; B: 1-x, 1-y, 1/2+z

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  Figure 3  (a) TG curves of complexes 1 and 2; (b) PXRD patterns of complexes 1 and 2

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  Figure 4  Emission spectra of 1, 2 and ligand BMIP in the solid state at room temperature

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  Figure 5  View of time-dependent absorption of MB solution in the presence of complex 1 (a) and complex 2 (b) during the decomposition reaction under UV light irradiation; (c) Photocatalytic performances of complexes 1 and 2

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  Table 1.  Crystal data and refinement parameters for complexes 1 and 2

  Complex	1	2
  Empirical formula	C42H47Zn2N8O10	C21H23ZnBrN4O4
  Formula weight	954.62	540.71
  Crystal system	Monoclinic	Orthorhombic
  Space group	P21/n	Pna21
  a / nm	0.951 88(6)	0.887 40(6)
  b / nm	1.878 17(11)	1.518 59(10)
  c / nm	1.201 20(7)	1.710 17(11)
  β/(°)	101.363(10)
  V / nm3	2.105 4(2)	2.304 6(3)
  Z	2	4
  Crystal size / mm	0.26×0.22×0.20	0.20×0.14×0.10
  μ / mm-1	1.208	2.833
  F(000)	990	1 096
  No. of measured, independent, observed [I > 2σ(I)] reflections	11 405, 4 126, 3 208	11 998, 3 999, 3 634
  Data, restraint, parameter	4 126, 0, 282	3 999, 0, 283
  Final R indices [I > 2 σ (I)], S	R1=0.057 1, wR2=0.148 0, 1.048	R1=0.025 3, wR2=0.064 2, 1.034
  Largest diff. peak and hole / (e·nm-3)	540, -543	316, -497

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  Table 2.  Selected bond lengths (nm) for complexes 1 and 2

  1
  Zn1-N1	0.199 8(3)	Zn1-O1	0.196 7(3)	Zn1-N4A	0.201 6(3)
  Zn1-O3B	0.195 0(3)
  2
  Zn1-N1	0.203 2(2)	Zn1-O5	0.195 6(2)	Zn1-N4A	0.202 1(2)
  Zn1-O3B	0.192 5(2)
  Symmetry codes: A: -x, -y, 1-z; B: 1+x, 3/2-y, 1/2+z for 1; A: 5/2-x, -1/2+y, 1/2+z; B: 1-x, 1-y, 1/2+z for 2.

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•