Crystal structure and photocatalytic degradation properties of a new two-dimensional zinc coordination polymer based on 4,4ʹ-oxy-bis(benzoic acid)

2.1 Chemicals and reagents

All starting materials and solvents used during the experiments were purchased commercially and used without further purification.

2.2 Physical measurements

The Fourier transform infrared (FT-IR) spectra were recorded on a Bruker Vector 22 spectrophotometer with KBr disks in the range 400–4000 cm⁻¹. Elemental analyses for C, H, and N were performed on a Perkin Elmer 240C elemental analyzer. TGA was carried out on a Perkin-Elmer Pyris 1 TGA analyzer with a heating rate of 10 K min⁻¹ under nitrogen. The UV/vis absorption spectra of methylene blue (MB) and methyl orange (MO) aqueous solutions were measured using a Perkin Elmer Lambda 25 spectrophotometer.

2.3 Synthesis of complex 1

A mixture of H₂OBA (25.8 mg, 0.1 mmol), 1,3-BMIB (23.8 mg, 0.1 mmol), Zn(NO₃)₂·6H₂O (29.7 mg, 0.1 mmol), and H₂O (6 mL) was placed in a Teflon-lined stainless steel vessel, heated to T=150°C for 3 days, and then cooled to room temperature over 24 h. Colorless crystals of 1 were obtained. Yield: 42% based on Zn(NO₃)₂·6H₂O. Elemental analysis calcd. for C₂₈H₂₂N₄O₅Zn (%): C 60.06, H 3.96, N 10.01; found C 60.22; H 3.97; N 10.04. IR (cm⁻¹): 3425 w, 3072 w, 2985 m, 2821 m, 2671 m, 2540 m, 1940 w, 1688 s, 1597 s, 1501 s, 1423 s, 1302 m, 1250 s, 1157 m, 1105 w, 1005 w, 933 m, 860 s, 777 s, 692 m, 650 w, 615 w, 546 s, 501 m.

2.4 Photocatalytic measurement

The potential of complex 1 as a photocatalyst was evaluated via the degradation of MB and MO dyes under irradiation by a 300 W medium-pressure mercury vapor lamp. Thirty milligrams of complex 1 and 10 μL of 30% H₂O₂ were added to 50 mL of aqueous solutions of MB and MO (10 mg L⁻¹). Prior to irradiation, the solution was magnetically stirred in the dark for 30 min to ensure the establishment of an adsorption/desorption equilibrium. The MB and MO concentrations were determined by following the maximum absorbances at 664 and 464 nm, respectively. At specific intervals, 1 mL of the reaction solution was taken and analyzed with a UV/vis spectrophotometer.

2.5 X-ray crystallography

Crystallographic data for complex 1 was collected on a Bruker SMART APEX II CCD-based diffractometer with graphite-monochromatized MoK_α radiation (λ=0.71073 Å) at room temperature. Absorption correction was performed by using the program Sadabs [11]. The structure was solved by Direct Methods using the program Shelxs-2014 and refined by full-matrix least-squares on F² with Shelxl-2014 [12]. All non-hydrogen atoms were located in difference Fourier maps and refined anisotropically. All H atoms were refined isotropically, with the isotropic displacement parameters related to the non-H atom to which they are bonded. The crystallographic data is summarized in Table 1. The selected bond lengths and angles are listed in Table 2.

CCDC 1945252 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

3.1 Description of the structure

X-ray crystallography has revealed that complex 1 crystallizes in the triclinic space group P1̅ with an asymmetric unit consisting of one divalent Zn atom, one fully deprotonated H₂OBA ligand, and one 1,3-BMIB ligand. As shown in Fig. 1, Zn1 is four-coordinated by two oxygen atoms from two different OBA²⁻ ligands (Zn1–O1=1.959(3) Å, Zn1–O5B=1.966(3) Å) and two N atoms from the imidazole ring of two 1,3-BMIB ligands (Zn1–N1=2.069(3) Å, Zn1–N4A=2.048(3) Å) to form a slightly distorted tetrahedral geometry (symmetry code: A: x, y+1, z; B: x+1, y, z+1.). The distortion of the tetrahedron can be confirmed by the calculated value of the τ₄ parameter, which is 0.83 for Zn1 (for a perfect square planar geometry, τ₄=0; for perfect tetrahedral geometry τ₄=1) [13]. In complex 1, the dihedral angles between the central benzene ring and the two imidazole rings in the 1,3-BMIB ligand are 115.9° and 69.1°. Each 1,3-BMIB ligand links two Zn1 atoms to form zigzag chains [Zn(1,3-BMIB)]²ⁿ⁺_n along the b-axis with a Zn1···Zn1 distance of 11.585 Å (Fig. 2). Adjacent [Zn(1,3-BMIB)]²ⁿ⁺_n chains are connected into a ruffled neutral [Zn(1,3-BMIB)(OBA)]_n (4, 4) grid CP with its layers parallel to the (1 0 1̅) crystal direction through tethering OBA²⁻ ligands, which span a Zn1···Zn1 distance of 14.811 Å (Fig. 3).

Fig. 1:

Coordination environment around the Zn(II) ion in complex 1 (symmetry code: A: x, y+1, z; B: x+1, y, z+1).

Fig. 2:

View of chain formed by the Zn(II) atoms and the 1,3-BMIB ligands in complex 1 (distance in Å).

Fig. 3:

View of 2D structure of complex 1.

Three weak hydrogen bonds (2.796(6) Å for C12···O4, 3.208(6) Å for C16···O4B, 3.249(5) Å for C18···O1; symmetry codes: B: 1+x, y, z+1) exist in each layer. These layers are linked by four weak contacts (3.407(5) Å for C7···O2C, 3.409(5) Å for C21···O2D, 3.381(5) Å for C26···O4E, 3.025(5) Å for C27···O4F; symmetry codes: C: 1−x, 2−y, 1−z; D: −1+x, −1+y, z; E: −x, 1−y, 1−z; F: 1+x, −1+y, 1+z) to form a three-dimensional supramolecular structure.

3.2 Thermal Analysis

As shown in Fig. 4, complex 1 was stable upon heating to T=360°C. Then there was a weight loss from 360 to 800°C, indicating the decomposition of the organic ligands, leading to the generation of ZnO (found 14.7%, calculated 14.53%).

Fig. 4:

Thermogravimetric curve of complex 1.

3.3 Photocatalytic properties

Here, we have selected two commonly used organic dyes (MB and MO) as model pollutants in aqueous media to evaluate the photocatalytic efficiency of complex 1, considering that these two organic dyes are commonly used as representatives of widespread organic dyes that are very difficult to decompose in waste streams. As shown in Figs. 5 and 6, the absorption peaks of MB and MO decreased with the reaction time and no other new peaks were observed, which indicates that complex 1 does not give rise to new pollutants during the process of degradation. From Fig. 7, it can be seen that 85% MB and 86% MO could be successfully photodegraded in the presence of complex 1 under UV irradiation after 110 and 100 min, respectively. However, when the experiment was conducted in the blank solutions, the concentration of MB reduced by only 13.8% in 110 min and MO by only 17.0% in 100 min, as illustrated. These results indicate that complex 1 is very active in the decomposition of MB and MO.

Fig. 5:

Absorption spectra of aqueous MB solution under UV light irradiation for complex 1.

Fig. 6:

Absorption spectra of aqueous MO solution under UV light irradiation for complex 1.

Fig. 7:

Plot of concentration versus irradiation time for MB and MO under irradiation with a 300 W mercury lamp in the presence of complex 1.