A zinc(ii) polymer constructed with 3,5-pyrazoledicarboxylic acid and 1,4-bis(imidazol-1-ylmethyl)butane: Syntheses, crystal structures, and photoluminescence properties

2.1 Structural analysis

Compound 1 crystallizes in the triclinic space group P 1 ¯ with three crystallographically independent Zn(ii) ions, two pdc³⁻ ligands, two and a half bib ligands, two coordinated water molecules, four crystallized water molecules, and a half free bib molecule in an asymmetric unit. As shown in Figure 1, each Zn(ii) ion is five-coordinated in the distorted {ZnO2N3} trigonal bipyramidal geometry. Zn(1) ion is defined by two nitrogen atoms (N3, N5) from two imidazole rings of two bib ligands, one nitrogen atom (N1) from the pyrazole ring of the pdc³⁻ ligand, occupying the equatorial plane, and one carboxyl oxygen atom (O3) from pdc³⁻ anions and one coordinated water molecule (O1W), occupying apical sites. The Zn(2) ion attaches to three nitrogen atoms (N2, N7, and N9) from two different bib ligands, one pdc³⁻ ligand, and two carboxyl oxygen atoms (O1, O5) from two different pdc³⁻ ligands. The coordination pattern of the Zn(3) ion is the same as the Zn(1) ion. The Zn–O bond distances range from 1.984(2) to 2.206(3) Å, and the Zn–N distances vary from 2.012(3) to 2.220(3) Å. The N(O)–Zn–O(N) angles fall in the 77.71(10)°–170.67(11)° range (Table 1).

Figure 1

The coordination environment of the Zn(ii) ion in 1. Crystallized water molecules and hydrogen atoms are omitted for clarity.

In 1, the bib ligand adopts an anti-conformation bridging mode with a dihedral angle between two imidazole rings of 0°. Each pdc³⁻ ligand adopts the μ ₂ coordination mode, bridging Zn(ii) atoms into tri-nuclear subunits, which are associated with 2D networks with cavities (Figure 2), which is similar to our reported structure [24]. More significantly, free bib molecules are located in voids, as listed in Figure 2, which may be used for adsorption in the material field.

Figure 2

Two-dimensional network in 1.

Further exploration of the crystal packing of compound 1 indicates that there are π–π interactions between the imidazole rings of bib ligands (Figure 3). The centroid-to-centroid distance between the adjacent rings is 3.487(3) Å for N5C20N6C22C21 and N11C36N12C38C37 (symmetry codes: −1 + x, y, z) imidazole rings. The perpendicular distance is −3.3965(17) Å for N5C20N6C22C21 and N11C36N12C38C37 (symmetry codes: −1 + x, y, z) imidazole rings. Additionally, hydrogen-bond interactions (Figure 4) are present in 1 between carboxylate oxygen atoms, crystallized water molecules, and coordinated water molecules (Table 2). Undoubtedly, these interactions stabilize the structure of complex 1 and form a 3D supramolecular architecture. From the view of topology, both ligands can be viewed as linkers, while metal ions can be viewed as three nodes, based on these simplified treatments, and the backbone structure of the complex can be described as a binodal (3,3)-connected net with the topological symbol of {5.8²}{5².8}₂ (Figure 5).

Figure 3

π–π interactions in 1.

Figure 4

H-bond interactions in 1.

Figure 5

Topological structure of 1.

2.2 Powder X-ray diffraction (PXRD) pattern and infrared spectrum analysis

The PXRD pattern of compound 1 is shown in Figure 6. The main peak positions observed are in good agreement with the simulated ones, indicating the phase purity of the original sample 1.

Figure 6

PXRD analysis of complex 1: bottom-simulated, top-experimental.

The IR spectrum of 1 is exhibited in the wave-number range of 4,000 to 400 cm⁻¹ (Figure 7). The peaks observed at 1,600 cm⁻¹ for the complex are assigned to the stretching bands of ν _as(COO−), whereas those at about 1,398 cm⁻¹ are assigned to the stretching bands of ν _s(COO−). The strong bands in the 656–759 cm⁻¹ region can be attributed to the ν _(C−N) stretching of N-heterocyclic rings of the bib ligand [25,26,27].

Figure 7

IR spectrum of complex 1.

2.3 Fluorescence spectrum

CPs with d¹⁰ metal centers possess numerous potential applications, such as in chemical sensors, photochemistry, and electroluminescence displays [28]. Hence, in our work, the photoluminescence properties of H₃pdc, bib, and complex 1 were investigated in the solid state at room temperature. The emission peaks are shown in Figure 8.

Figure 8

Solid-state emission spectrum of complex 1 and ligands at room temperature.

The main emission peak of the bib is at 396 nm. H₃pdc shows an emission peak at 406 nm (λ _ex = 348 nm). The emission bands of these free ligands are likely caused by the π* → n or π* → π transition [29]. Upon the complexation of these ligands with the Zn(ii) ion, the emission peaks occur at 394 nm (λ _ex = 288 nm). For complex 1, the main emission peaks are blue-shifted by 12 nm compared to the free H₃pdc. Because the Zn(ii) ion is difficult to oxidize or reduce due to its d ¹⁰ configuration, the emission of the complex is neither MLCT nor LMCT in nature [30]. As a result, the emission can be assigned to intraligand transitions [31].

2.4 Luminescence sensing of inorganic anions

Before all luminescence detection, the dried samples were ground into a fine powder in an agate mortar to enhance the disparity in H₂O. The fluorescence sensing experiments of complex 1 were carried out by adding the powdered samples of 1 (5.0 mg) into different anionic aqueous solutions containing sodium salt or potassium salt (MnO₄ ²⁻, Cr₂O₇ ²⁻, H₂PO₄ ⁻, NO₃ ⁻, CO₃ ²⁻, Cl⁻, and Br⁻, 10⁻² mol L⁻¹ (M), 2 mL). After ultrasonication for 10 min to form homogeneous solutions, the luminescence data were collected at an excitation wavelength of 350 nm, respectively, at room temperature.

Considering the high water stability and distinct fluorescence properties of the complex in water, we further explored their potential ability to sense inorganic anions. The finely ground samples of 1 (5 mg) were immersed in 2 mL aqueous solutions containing MnO₄ ²⁻, Cr₂O₇ ²⁻, H₂PO₄ ⁻, NO₃ ⁻, CO₃ ²⁻, Cl⁻ and Br⁻ at the same concentration (10⁻² M). As shown in Figure 9a and b, the luminescence intensity of 1 is slightly enhanced by the addition of Br⁻ and Cl⁻. Interestingly, the Cr₂O₇ ²⁻ and MnO₄ ⁻ anions exhibit significant luminescence quenching effects, suggesting a high selectivity for Cr₂O₇ ²⁻ and MnO₄ ⁻.

Figure 9

(a) and (b) Fluorescence emission spectra of 1 dispersed in aqueous solutions with different anions at the same concentration.

4.1 Preparation of [Zn₃(pdc)₂(bib)_2.5(H₂O)₂] _n ·4nH₂O·0.5nbib (1)

A mixture containing Zn(OAc)₂·2H₂O (43.9 mg, 0.1 mmol), H₃pdc (34.8 mg, 0.2 mmol), bib (38.0 mg, 0.2 mmol), 0.2 mL triethylamine, and 10 mL H₂O was placed under hydrothermal conditions in a Teflon-lined Parr and heated at 140°C for 7 days. After cooling to room temperature, yellow block crystals of compound 1 were obtained with a 21% yield in terms of Zn. Elemental analysis for C₄₀H₄₈N₁₆O₁₄Zn₃: C, 40.95; H, 4.12; N, 19.10 wt%. Found: C, 40.28; H, 3.68; N, 18.35 wt%. IR/cm⁻¹ (KBr): 3,129 (m), 1,600 (s), 1,517 (m), 1,398 (m), 1,339 (s), 1,297 (w), 1,231 (w), 1,110 (m), 1,094 (w), 1,041 (w), 952 (m), 843 (m), 786 (m), 759 (w), 656 (m), 627 (w), 521 (w).

4.2 X-ray crystallography

X-ray diffraction analysis of single crystal 1 was performed on a Rigaku RAXIS-RAPID diffractometer equipped with a graphite monochromator (MoKα radiation, λ = 0.71073 Å) at 296(2) K. The structure was solved by direct methods using SIR2014 [32] and refined by the full-matrix least-squares technique on F2 using the SHELXL2015/3 program [33]. All H atoms were found using the generated calculations and the riding model, and the rest of the atoms were refined with anisotropic temperature parameters. Selected bond lengths and angles are listed in Table 1. Crystallographic characteristics, X-ray data, and structure refinement parameters for 1 are summarized in Table 3. The crystallographic data were deposited with the Cambridge Crystallographic Data Centre (CCDC no. 2359903). Copies of this information can be obtained free of charge at www.ccdc.cam.ac.uk or from the CCDC (12 Union Road, Cambridge CB2 1EZ, UK; fax: 0044 1223 336 033; e-mail: deposit@ccdc.cam.ac.uk).