Crystal structure, spectroscopic characterization and Hirshfeld surface analysis of trans-diaqua[2,5-bis(pyridin-4-yl)-1,3,4-oxadiazole]dithiocyanatonickel(II)

The NiII atom in the mononuclear title complex has an almost regular octahedral N4O2 coordination geometry. In the crystal, the complex molecules are linked in a three-dimensional network through strong O—H⋯N hydrogen bonds.


Hirshfeld surface analysis
In order to visualize the role of weak intermolecular contacts, a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) generated using CrystalExplorer17.5 (Turner et al., 2017). The three dimensional d norm surface of the title compound using a standard surface resolution with a fixed colour scale of À0.6661 to 1.4210 a.u. is shown in Fig. 3. The darkest red spots on this surface correspond to the O-HÁ Á ÁN hydrogen bonds resulting from the interaction between the coordinated water molecules and N atoms of the pyridine and oxadiazole rings.

Figure 2
Packing diagram of the title compound viewed approximately along the c axis. Hydrogen bonds (Table 1) are shown as dotted lines.

Spectroscopic characterizations
FTIR spectra were recorded on a SHIMADZU FT-IR 8400S spectrometer with a Smart iTR attachment and diamond-attenuated total reflectance (ATR) crystal in the range 500-4000 cm À1 . UV-visible absorption spectra were recorded in the range 200-800 nm using a SHIMADZU 2450 spectrophotometer. The complex concentration used for UV-visible measurements was 10 À4 M in methanol solvent. The IR spectrum of the title complex ( Fig. 6) is analogous to that of the 4-pox ligand, except for the presence of a wide band of low intensity around 3409 cm À1 in addition to another sharp and strong band at 2083 cm À1 , attributable to the water molecules [(OH); Du et al., 2002a;Du et al., 2004a] and thiocyanate ions [(CN; Du & Zhao, 2004;Fang et al., 2002], respectively. A comparison of the spectrum with that of 4-pox, which is characterized by its main absorption bands, 3055-3084, 1618, 1569 and 1551-1418 cm À1 , resulting from the C- Acta Cryst. Hirshfeld surface of the title complex plotted over shape-index.

Figure 4
The overall two-dimensional fingerprint plot for the title compound

Figure 6
Infrared spectra of the 4-pox ligand and the title complex in the 500-4000 cm À1 range.
H, C N (oxadiazole), C N (pyridine) and C C (pyridine) bonds, respectively (Table 3; Jha et al., 2010;Formagio et al., 2008), indicates the presence of 4-pox in the complexes as well as water molecules and thiocyanate anions in the isolated product, as evidenced by the XRD study. The UV-vis spectrum of the title complex in methanol ( Fig. 7) displays an intense band at 274 nm that is essentially attributable to intraligand -* electronic transitions in a conjugate system (Mahmoudi & Morsali, 2007;Kudelko et al., 2015). The free 4pox ligand also shows the same band at the same position, indicating that the ligand structure has undergone very few changes upon coordination to the metal.  (Du et al., 2002b), Mn II or Co II (Fang et al., 2002) or Fe II (Du & Zhao, 2004). In all cases, an octahedral geometry around the metal site with pseudohalide and organic ligands in mutually trans positions was observed.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The water H atoms were initially located in a difference-Fourier map and refined with O-H distance restraints of 0.78 Å and with U iso (H) set to 1.5 U eq (O). All other H atoms were located in a difference-Fourier map and refined as riding, with C-H = 0.93 Å and with U iso (H) = 1.2U eq (C).