Synthesis, crystal structure and Hirshfeld surface analysis of a zinc(II) coordination polymer of 5-phenyl-1,3,4-oxadiazole-2-thiolate

The molecular and crystal structure of a zinc coordination polymer with 5-phenyl-1,3,4-oxadiazole-2-thiolate were studied and Hirshfeld surfaces and fingerprint plots were generated to investigate various intermolecular interactions.

Oxadiazole ligands are ideal objects for creating new coordination compounds with great potential in various fields. Scientists have written extensive literature on the biological properties of oxadiazole-based complex compounds, especially on their anticancer effects. In addition to these, in the field of electrical engineering, metal complexes bearing oxadiazole ligands have been used as emitting particles in light-emitting diodes. The introduction of various functionalized oxadiazole ligands makes it easy to control the emission color, thermal stability, and film-forming properties of such complexes (Salassa & Terenzi, 2019).

Structural commentary
The single crystal X-ray structure of 5-phenyl-1,3,4-oxadiazole-2-thiolate Zn II shows a polymeric structure that crystallizes in the centrosymmetric monoclinic space group C2/c ( Table 2). As seen in Fig. 1, its asymmetric unit contains half a zinc atom and one ligand anion. The central Zn II atom has a distorted tetrahedral environment comprising two sulfur and two nitrogen atoms. It is coordinated by four crystallographically independent (L) ligands, forming zigzag chains along the [001] direction, which are linked by two sulfur atoms and two nitrogen atoms of four ligands. The Zn1-S1 and Zn1-N1 bond lengths are 2.3370 (5) Å , 2.0184 (14) Å , respectively. In this case, the bond angles of the atom forming the tetrahedral polyhedron are slightly different from the angles of the ideal tetrahedron [N1-Zn1-N1 = 111.37 (9) , S1-Zn1-S1 = 100.46 (3) and N1-Zn1-S1 = 108.51 (4) ]. It is known from the literature (Razzoqova et al., 2019) that the sulfur atom in the 1,3,4-oxadiazole-2-thione molecule is attached to the ring by a double bond. In this polymer complex synthesized based on Zn II ion, the oxadiazole derivative transforms into the thiol tautomeric form and binds to the Zn ion. The N1 atom in the ligand molecule, on the other hand, forms a bond with another Zn II ion due to its high electron-donating property, resulting in an eight-membered [Zn-S-C-N-Zn-S-C-N] chair-like ring with two Zn II atoms and two ligand molecules (Fig. 2). The dihedral angle between the mean planes of the phenyl (C3-C8) and oxadiazole (C1/ O1/C2/N2/N1) rings of the ligand molecule is 13.42 (8) . The conformation of the oxadiazole-thiol fragment of the ligand is approximately planar (r.m.s. deviation 0.006 Å ), with a maximum deviation from the least-squares plane of 0.009 (1) Å for atom O1. The dihedral angle between the planes of the two neighboring independent oxadiazole-thiol (C1/O1/C2/N2/N1/S1) fragments is 64.10 (9) .

Supramolecular features
The [(ZnL 2 ) n ] unit is given as a monomer of the polymeric chain that extends parallel to the c-axis. Along the polymeric chain, the hydrophilic groups are concentrated within the core of the chain while the phenyl rings project approximately normal to the chain. Neighboring chains across the ab plane are loosely connected via a rather weak C6-H6Á Á ÁS1 hydrogen bond (Table 1, Fig. 3). Table 1 Hydrogen-bond geometry (Å , ). (3) 3.608 (2) 132 (2) Symmetry code: (i) x þ 1 2 ; Ày þ 3 2 ; z þ 1 2 .

Figure 2
The view of the molecular packing showing the polymeric chain extended along the c-axis.

Figure 1
The molecular structure of [Zn 0.5 L] with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are displayed as small spheres of arbitrary radii.

Hirshfeld surface analysis
To further investigate the intermolecular interactions present in the title compound, a Hirshfeld surface analysis was performed, and the two-dimensional fingerprint plots were generated with CrystalExplorer17 (Turner et al., 2017). The Hirshfeld surface mapped over d norm and corresponding colors representing various interactions are shown in Fig. 4. We chose the ZnL 2 molecular fragment as the monomer unit for calculating the Hirshfeld surface of this polymer complex. The large red areas on the Hirshfeld surface correspond to the ZnÁ Á ÁN interactions. The two-dimensional (2D) fingerprint plots (McKinnon et al., 2007) are shown in Fig. 5. On the Hirshfeld surface, the largest contributions (19.2%, 19.5% and 19%) come from short contacts such as van der Waals forces, HÁ Á ÁH, CÁ Á ÁH and SÁ Á ÁH contacts. NÁ Á ÁH (8.1%), OÁ Á ÁH (8%) and CÁ Á ÁC (4.7%) contacts are also observed. These interactions play a crucial role in the overall stabilization of the crystal packing.

Figure 5
Contributions of the various contacts to the fingerprint plot built using the Hirshfeld surface of the title compound.  Computer programs: CrysAlis PRO (Rigaku OD, 2020), SHELXT2014/5 (Sheldrick, 2015a), SHELXL2016/6 (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009). solution of KOH (0.112 g, 0.002 mol) was added. The obtained solutions were mixed together and stirred at 323 K for 20 min. A white precipitate was obtained. The precipitate was filtered and allowed to dry. The solid residue was dissolved in DMF to crystallize for the single crystal X-ray diffraction studies. X-ray quality single crystals were produced after 10 days by slow evaporation of the solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All the hydrogen atoms were located in difference-Fourier maps and refined isotropically.

Funding information
This work was supported by Uzbekistan Ministry of Innovation Development.