A 3D 2-fold interpenetrating Cu(II) coordination polymer based on 4,4′-oxybis(benzoic acid) and 1,3-bis(2-methyl-imidazol-1-yl) benzene exhibiting photocatalytic properties

2.1 Chemicals and reagents

Reagents and solvents were purchased from the Jinan Camolai Trading Company and used directly without any further treatment. The synthetic method for the 1,3-BMIB ligand was based on literature [13], [ 14].

2.2 Physical measurements

The C, H and N elemental analysis were performed on a Vario EL III elemental analyzer. Thermal gravimetric analysis (TGA) was carried out under N2 atmosphere on a Perkin-Elmer Pyris 1 TGA analyzer in the temperature range 25–800 °C with a heating rate of 10 K min–1. FT-IR spectra ranging from 4000 to 400 cm⁻¹ were recorded on a VECTOR 22 spectrometer by using KBr pellets. The fluorescence spectrum was obtained from a Fluoro Max-P spectrophotometer at room temperature.

2.3 Synthesis of [Cu₂(1,3-BMIB)(OBA)₂]_n (1)

A mixture of H₂OBA (25.8 mg, 0.1 mmol), 1,3-BMIB (23.8 mg, 0.1 mmol), CuCl₂·2H₂O (17.0 mg, 0.1 mmol), H₂O (2 mL) and DMF (4 mL) was placed in a Teflon-lined stainless steel vessel, heated to 100 °C for two days, and then cooled to room temperature over 12 h. Blue crystals of 1 were obtained. Yield: 52% based on CuCl₂·2H₂O. Elemental analysis (%) calcd for C₄₂H₃₀Cu₂N₄O₁₀ (877.800): C 57.47, H 3.44, N 6.38; found C 57.65, H 3.45, N 6.40. IR (cm⁻¹): 3438 w, 1672 m, 1629 s, 1503 m, 1400 s, 1235 s, 1163 m, 1095 w, 1008 w, 878 m, 776 m, 741 w, 661 m, 460 w.

2.4 Photocatalytic measurement

Typical degradation experiments were done as follows: 30 mg of complex 1 and 10 μL of 30% H₂O₂ were mixed into 50 mL of the prepared aqueous solutions of MB and RhB (10 mg L⁻¹) with magnetic stirring under irradiation by a 300 W medium-pressure mercury vapor lamp. To guarantee the adsorption-desorption equilibrium, the solutions were magnetically stirred in the dark for 30 min before irradiation. Later, 1 mL of the reaction solution was taken and analyzed with a UV/Vis spectrophotometer at specific intervals. The MB and RhB concentrations were measured by using the UV/Vis spectrophotometer at their maximum absorbance wavelength of 664 and 550 nm, respectively.

2.5 X-ray crystallography

Crystallographic data for complex 1 were collected on a Bruker Smart CCD diffractometer with MoKα radiation (λ = 0.71073 Å) using the ω − 2θ scan mode at T = 173(2) K. An absorption corrections was applied by the multi-scan program Sadabs. The structure was solved by Direct Methods and refined by full-matrix least-squares techniques based on F² using the program package Shelxl-2014 [15]. All nonhydrogen atoms were treated anisotropically. The hydrogen atoms of the organic ligands were geometrically generated using a riding model and isotropically refined. Residual minor interstitial solvent molecules in crystals of 1 were highly disordered and removed from the diffraction data using the routine SQUEEZE of Platon [16]. The final crystal structure data as summarized in Table 1 refers to the complex 1 without additional crystal solvent molecules. Selected bond lengths and angles are listed in Table 2.

CCDC 1982748 for 1 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.

3.1 Description of the structure

Crystal structure analysis revealed that complex 1 crystallizes in the orthorhombic space group Cmca and involves one Cu(II) ion, one OBA^2– ligand and one-half 1,3-BMIB ligand in the asymmetric unit. As described in Figure 1, the Cu(II) center adopts square pyramidal {CuO₄N} geometry encircled by four oxygen atoms (O2, O5A, O4B and O1C) from four different OBA^2– ligands and one nitrogen atom (N1) from one 1,3-BMIB ligand (symmetry code: A = x, y – 1/2, 1/2 – z; B = 1/2 – x, 1 – y, z – 1/2; C = 1/2 – x, 1/2 – y, –z). The bond lengths around Cu(II) ions are in the region of 1.964(4)–2.141(5) Å. The coordination angles around the Cu(II) centers are in the range from 88.78(19) to 167.13(17)° (Table 2).

Figure 1:

A view of the local coordination of the Cu^II cations in complex 1. The molecular entity shown has crystallographic mirror symmetry, the mirror plane passing through the atoms C(19) and C(21). Symmetry codes: A = x, y – 1/2, 1/2 – z; B = 1/2 – x, 1 – y, z – 1/2; C = 1/2 – x, 1/2 – y, –z; D = –x, y, z; E = x – 1/2, –y + 1/2, –z; F = x – 1/2, –y + 1, z – 1/2; G = –x, y – 1/2, –z + 1/2.

In complex 1, each H₂OBA ligand is completely deprotonated and links four Cu(II) ions with (κ¹-κ¹)-(κ¹-κ¹)-μ₄ coordination mode to generate a [Cu(OBA)]_n layer (Figure 2b) based on paddle-wheel [Cu₂(OCO)₄] secondary building units (SBUs) with a Cu···Cu distance of 2.683 Å (Figure 2a). Moreover, these [Cu(OBA)]_n layers are extended into a complicated 3D framework by 1,3-BMIB co-ligands acting as bridging pillars (Figure 3).

Figure 2:

View of the [Cu₂(OCO)₄] SBUs (a) and of a layer of complex 1 (b). The Cu dimer in (a) has crystallographic inversion symmetry, as have the large rings shown in (b). For symmetry codes see caption to Figure 1.

Figure 3:

View of the 3D structure of complex 1.

From the viewpoint of network topology, each [Cu₂(OCO)₄] SBU connects six adjacent [Cu₂(OCO)₄] SBUs and is 6-connected. The whole framework can be simplified as a 6-connected net with {4¹².6³} topology. The three-dimensional coordination polymer of complex 1 leaves a large cavity. In order to stabilize the framework, the potential void space is filled by another identical network, leading to the two-fold mutually interpenetrating three-dimensional architecture (Figure 4).

Figure 4:

A schematic representation of the twofold interpenetrating framework in the structure of complex 1.

3.2 Thermal analysis

As shown in Figure 5, the weight of complex 1 is stable up to T = 120 °C and then the 1,3-BMIB and H₂OBA ligands start to decompose in the temperature range 120–773 °C. Finally, the remaining product corresponds to CuO (found: 18.3%; calcd: 18.1%).

Figure 5:

Thermogravimetric analysis of 1.

3.3 Photocatalytic properties

The photocatalysis activity of complex 1 was investigated by use of two organic model pollutants (MB and RhB). As shown in Figure 6, the characteristic absorption peaks obviously decreased as the reaction time increased. From Figure 7, the degradation rates of MB and RhB were 13 and 14% without complex 1, respectively. After adding complex 1, the rates increased to 96% (MB) and 91% (RhB) within 210 min, which indicated that complex 1 shows remarkable photocatalytic activity and may be a potential candidate for photocatalytic degradation of MB and RhB.

Figure 6:

UV/Vis absorption spectra of the MB (a) and RhB (b) solution during the decomposition reaction in the presence of complex 1.

Figure 7:

Photocatalytic decomposition rate of the MB and RhB solution under UV irradiation.