English

• 1. INTRODUCTION

  Particular attention has been recently devoted to coordination polymers (MCPs) not only due to their undisputed structural beauty but also to their promising applications in the fields of heterogeneous catalysis^[1], storage^[2], degradation of organic pollutants^[3-5], conductivity^[6], biological activity^[7], ferroelectricity^[8], magnetism^[9] and luminescence^[10]. It has been proved that the structures of metal coordination polymers (MCPs) have a close relationship with their functions. However, the prediction and design of target MCPs are still fraught with enormous challenges^[11]. Carboxylates, N-containing ligands or their co-ligand systems have received considerable attention^[12] because of their versatile coordination models for the design of multifunctional coordination polymers. Compared to well-known N-donor ligands, the 1, 3-BIP is a good N-donor ligand which allows the ligands to bend and rotate when they coordinate to metal centers^[13-15]. Linkers with electron withdrawing groups, such as 2, 3, 5, 6-tetrafluoro-1, 4-benzenedicarboxylylate (H₂TFBDC)^[16-18], have been used to build coordination polymers with transition metals. However, it has been reported that transition metal hybrids do not crystallize well in acidic environments, and the acidity of the parent acid requires a basic co-ligand such as a nitrogen base in order for the hybrid inorganic-organic materials to crystallize^[19-22].

  In this paper, we chose the flexible ligand 1, 3-bis(imidazole)propane as a N-donor ligand, and the rigid tetrafluoroterephthalic acid as an auxiliary linker for the construction of MCPs in order to satisfy and mediate the geometric requirements of metal centers. Herein we have prepared a new Cu(Ⅱ) coordination polymer, {[Cu(1, 3-BIP)(TFBDC)]· DMF}_n (1), and determined its structure by single-crystal X-ray diffraction analysis. In addition, its thermal stability and photocatalytic property have been investigated.

  2. EXPERIMENTAL

  2.1 Materials and physical measurements

  All of the chemical reagents were purchased from Ji'nan Heng Hua Science and Technology Company, and used without further purification. Elemental analyses for carbon, hydrogen, and nitrogen atoms were performed on a Vario EL Ⅲ elemental analyzer. The infrared spectra (4000~400 cm^–1) were recorded by using KBr pellet on an Avatar 360 E.S.P. IR spectrometer. Powder X-ray diffraction (PXRD) data were collected on a Rigaku Ultima ІV X-ray diffractometer with CuKα radiation (λ = 0.154056 nm) at room temperature in the 2θ range of 5~50º. UV-Vis spectroscopy was measured by F-4500 analytical instruments. Thermogravimetric analysis (TGA) was performed on a TA-SDT Q600 thermal analyzer under N₂ atmosphere at a heating rate of 10 ℃·min^–1 in the range of 30~1000 ℃.

  2.2 Synthesis of {[Cu(1, 3-BIP)(TFBDC)]·DMF}_n (1)

  A mixture of Cu(NO₃)₂·3H₂O (0.1 mmol, 24.2 mg), NaOH (0.15 mmol, 12 mg), 1, 3-BIP (0.1 mmol, 17.6 mg), H₂TFBDC (0.1 mmol, 23.8 mg), 2 mL DMF, and 4 mL H₂O was sealed in a 20 mL vial, which was heated to 90 ℃ for 3 days, and then cooled to room temperature over 24 hours. Blue block crystals of 1 were collected. Yield: 78% based on copper. Elemental analysis Calcd. (%) for C₂₀H₁₉CuF₄N₅O₅ (M_r = 548.94): C, 43.72; H, 3.16; N, 12.75. Found (%): C, 43.23; H, 3.42; N, 12.81. IR(cm^–1): 3432(bs), 3125(m), 2935(m), 2363(w), 1622(s), 1563(m), 1405(w), 1345(s), 1103(m), and 762(m).

  2.3 Crystal structure determination

  Crystal data for MCP 1 were collected on a Bruker SMART APEX Ⅱ CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) in an ω-scan mode. A blue crystal (C₂₀H₁₉CuF₄N₅O₅) with dimensions of 0.22mm × 0.15mm × 0.05mm was selected for data collection which was performed on a Bruker P4 diffractometer equipped with a graphite-monochromatic MoKα radiation (λ = 1.54187 Å) by using a multi scan mode at 293(2) K. A total of 4293 reflections were collected in the range of 3.04≤θ≤25.01° (index ranges: –18 < h < 12, –16 < k < 16 –9 < l < 14) and 2111 were independent (Rint = 0.0221), of which 203 observed reflections with I > 2σ(I) were used in the structure determination and refinements. The structure was solved by direct methods using the program SHELXS-97^[23] and refined by full-matrix least-squares techniques against F² using the SHELXL-97^[24] crystallographic software package. All of the non-hydrogen atoms were easily found from difference Fourier map and refined anisotropically, whereas the hydrogen atoms of MCP 1 were placed by geometrical considerations and added to the structure factor calculation. The details of selected bond lengths and bond angles with their estimated standard deviations are listed in Table 1.

  Table 1

  

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

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  Bond	Dist.	Angle	(°)
  Cu(1)–N(2)#1	1.956(3)	N(2)#1–Cu(1)–N(2)	164.34(19)
  Cu(1)–N(2)	1.956(3)	N(2)#1–Cu(1)–O(1)#1	90.16(11)
  Cu(1)–O(1)#1	1.956(2)	N(2)–Cu(1)–O(1)#1	90.56(12)
  Cu(1)–O(1)	1.956(2)	N(2)#1–Cu(1)–O(1)	90.56(12)
  F(1)–C(3)	1.351(4)	N(2)–Cu(1)–O(1)	90.16(11)
  F(2)–C(4)	1.344(4)	O(1)#1–Cu(1)–O(1)	174.66(18)
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, y, –z; #2: –x+3/2, –y+1/2, –z+1

  3. RESULTS AND DISCUSSION

  3.1 Description of structure 1

  Single-crystal X-ray diffraction study revealed that MCP 1 belongs to triclinic, space group P$ \overline 1 $. The asymmetric unit of MCP 1 contains one Cu(Ⅱ) cation, one 1, 3-BIP ligand, one TFBDC^2- anion and one DMF molecule. As shown in Fig. 1, each Cu(Ⅱ) ion exhibits distorted tetrahedral geometry with two oxygen atoms from two TFBDC^2- anions (Cu–O 1.956 Å) and two nitrogen atoms from two 1, 3-BIP ligands (Cu–N 1.955 Å). The carboxyl groups of H₂TFBDC ligands are all deprotonated and two carboxylate oxygen atoms adopt µ₂-ŋ⁰: ŋ¹ fashions to connect adjacent Cu(Ⅱ) cations to form a one-dimensional chain (Fig. 2). Meanwhile, two gauche 1, 3-BIP ligands adopt cis-conformation with the dihedral angle between the two imidazole rings of 15.1°, which connect two adjacent Cu(Ⅱ) of the above-mentioned one-dimensional chains to form an interesting 2D sheet layer (Fig. 3). It seems like a bunch of Chinese red lanterns along the a-axis (Fig. 4). The lattice DMF molecule aggregates in 1 can be seen as an intercalation of guests into void spaces in the 2D sheet layers, which are further reinforced through strong intermolecular hydrogen bonding (C(12)–H(12B)···O(2) = 2.592 Å and C(7)–H(7)···O(2) = 2.427 Å) to form an overall 3D supramolecular framework (Fig. 5).

  Figure 1

  

  Figure 1.  Structure of MCP 1 (The H atoms have been omitted for clarity)

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  Figure 2

  

  Figure 2.  View of the infinite one-dimensional chain constructed by Cu(Ⅱ) cation and TFBDC ligand

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  Figure 3

  

  Figure 3.  View of the 2D sheet layer structure in MCP 1

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  Figure 4

  

  Figure 4.  View of Chinese lantern modelling in MCP 1

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  Figure 5

  

  Figure 5.  View of the 3D packing diagram of MCP 1

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  3.2 X-ray diffraction

  To confirm the purity of MCP 1, X-ray power dif-fraction analyses were carried out. As shown in Fig. 6, although the experimental patterns have a few un-indexed diffraction lines and some are slightly broadened in comparison with those simulated from the single-crystal models, it still can be considered that the bulk synthesized materials and the as-grown crystals for diffraction are homogeneous for MCP 1.

  Figure 6

  

  Figure 6.  PXRD pattern of MCP 1

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  3.3 Thermal behaviors

  To have insight into the changes occurring during heat treatment of the prepared MCP 1, thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) of the sample were carried out from 30 to 1000 ℃ at a heating rate of 10 ℃·min^-1. Based on the TGA curve depicted in Fig. 7, a gradual weight loss of 12.45% (calcd. 13.31%) is observed from 30 to approximately 150 ℃. It can be ascribed to the loss of one lattice DMF molecule per unit cell (calcd.: 13.31%), which is indicated by an exothermal peak at 150 ℃ in the DTA curve. The second obvious weight loss from 150 to 400 ℃ can be attributed to the decomposition of MCP 1, which is indicated by one endothermal peak at 220 ℃ in the DTA curve. After decomposition, the final residue is 14.28%, which can be attributed to CuO (calcd. 14.57%).

  Figure 7

  

  Figure 7.  TG-DTA curves for MCP 1

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  3.4 Photocatalytic property of MCP 1

  Photocatalysts have attracted much attention due to their potential applications in purifying water and air by thoroughly decomposing organic pollutants^{[25, 26]}. The organic dye methyl orange (MO) is the most common organic dye and difficult to decompose in waste water. Therefore, it was selected as the model dye contaminants to evaluate the photocatalytic effectiveness in the purification of waste water. Inspired by the previously reported studies about Cu(Ⅱ)/Co(Ⅱ) coordination polymers for photocatalytic degradation of organic dyes^{[27, 28]}, herein the photocatalytic activity of MCP 1 for the degradation of MO under xenon arc lamp irradiation was also explored. As can be seen in Fig. 8, the characteristic absorption peak of MO (465 nm) was gradually reduced with time increasing from 0 to 180 min. Besides, the changes in the C_t/C₀ plot of MO solutions versus irradiation time are shown in Fig. 9 to clarify the photocatalytic results, wherein C_t is the concentration of MO solutions at t time and C₀ is the concentration of MO solutions at the beginning of irradiation. As depicted in Fig. 9, the degradation ratio of MO reaches 16.5% without any photocatalyst, while it increases to 83.4% when MCP 1 was added to the mixture as catalyst within 180 minutes.

  Figure 8

  

  Figure 8.  UV-vis absorption spectra of MO solution during photocatalytic degradation of the MO solution using catalyst 1

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  Figure 9

  

  Figure 9.  Concentration changes of MO as a function of reaction time using MCP 1 catalyst and the blank experiment

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  4. CONCLUSION

  In summary, a new Cu(Ⅱ) coordination polymer has been solvothermally synthesized and structurally characterized. Single-crystal X-ray structural analysis revealed that MCP 1 shows a two-dimensional sheet layer structure. Photocatalytic property investigations show that MCP 1 can be used as a visiblelight photocatalyst for the degradation of MO.

  
  
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• 

  Figure 1  Structure of MCP 1 (The H atoms have been omitted for clarity)

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  Figure 2  View of the infinite one-dimensional chain constructed by Cu(Ⅱ) cation and TFBDC ligand

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  Figure 3  View of the 2D sheet layer structure in MCP 1

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  Figure 4  View of Chinese lantern modelling in MCP 1

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  Figure 5  View of the 3D packing diagram of MCP 1

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  Figure 6  PXRD pattern of MCP 1

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  Figure 7  TG-DTA curves for MCP 1

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  Figure 8  UV-vis absorption spectra of MO solution during photocatalytic degradation of the MO solution using catalyst 1

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  Figure 9  Concentration changes of MO as a function of reaction time using MCP 1 catalyst and the blank experiment

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

  Bond	Dist.	Angle	(°)
  Cu(1)–N(2)#1	1.956(3)	N(2)#1–Cu(1)–N(2)	164.34(19)
  Cu(1)–N(2)	1.956(3)	N(2)#1–Cu(1)–O(1)#1	90.16(11)
  Cu(1)–O(1)#1	1.956(2)	N(2)–Cu(1)–O(1)#1	90.56(12)
  Cu(1)–O(1)	1.956(2)	N(2)#1–Cu(1)–O(1)	90.56(12)
  F(1)–C(3)	1.351(4)	N(2)–Cu(1)–O(1)	90.16(11)
  F(2)–C(4)	1.344(4)	O(1)#1–Cu(1)–O(1)	174.66(18)
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, y, –z; #2: –x+3/2, –y+1/2, –z+1

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•