Crystal structure and Hirshfeld surface analysis of (E)-4-({2,2-dichloro-1-[4-(dimethylamino)phenyl]ethenyl}diazenyl)benzonitrile

C—H⋯N interactions, C—Cl⋯π interactions, and π-π stacking interactions link molecules in the crystal, forming molecular layers approximately parallel to the (002) plane. The three-dimensional packing is strengthened by additional weak van der Waals interactions between the layers.


Chemical context
Azo dyes find numerous applications in a diversity of areas, including in molecular recognition, optical data storage, nonlinear optics and as molecular switches, antimicrobial agents, colour-changing materials, liquid crystals, dye-sensitized solar cells, mainly because of the ability for cis-to-trans isomerization and the chromophoric properties of the -N N-synthon Viswanathan et al., 2019). Not only isomerization, but azo-hydrazone tautomerisim is also an important phenomenon in the coordination chemistry of azo dyes (Mahmoudi et al., 2018a,b). Modification of azo dyes with functional groups leads to multifunctional ligands, of which the corresponding metal complexes are effective catalysts in oxidation and in C-C coupling reactions (Ma et al., 2020(Ma et al., , 2021Mahmudov et al., 2013;Mizar et al., 2012). Moreover, the functional properties of azo dyes are dependent on noncovalent bond-donor or -acceptor site(s) attached to the -N N-synthon (Gurbanov et al., 2020a,b;Kopylovich et al., 2011;Mahmudov et al., 2020;Shixaliyev et al., 2014). Thus, we have introduced halogen-bond-donor centres to the -N Nmoiety, leading to a new azo dye, (E)-4-({2,2-dichloro-1-[4-(dimethylamino)phenyl]ethenyl}diazenyl)benzonitrile, which provides multiple intermolecular non-covalent interactions.

Figure 3
The crystal packing of the title compound, viewed along the a axis, showing the C-ClÁ Á Á interactions andstacking interactions as dashed lines.
centroids of the C3-C8 and C11-C16 benzene rings, respectively], forming molecular layers approximately parallel to the (002) plane with the molecules having a bellows-like shape when viewed along the a axis (Figs. 2 and 3). Weak van der Waals interactions between these layers increase the stability of the crystal structure.
To visualize the intermolecular interactions in the title molecule, CrystalExplorer17 (Turner et al., 2017) was used to compute Hirshfeld surfaces (McKinnon et al., 2007) and their corresponding two-dimensional fingerprint plots (Spackman & McKinnon, 2002). The Hirshfeld surface mapped over electrostatic potential (Spackman et al., 2008) is shown in Fig. 4. The positive electrostatic potential (blue region) over the surface indicates hydrogen-bond donors, whereas the hydrogen-bond acceptors are represented by a negative electrostatic potential (red region). In the Hirshfeld surface mapped over d norm (Fig. 5), the bright-red spots near atoms H7, H13, N4 and Cl1 indicate the short C-HÁ Á ÁN and C-HÁ Á ÁCl contacts (Table 2). Other contacts are equal to or longer than the sum of van der Waals radii. The most important interaction is HÁ Á ÁH, contributing 33.6% to the overall crystal packing, which is reflected in Fig. 6b  View of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential energy in the range À0.0500 to 0.0500 a.u. using the STO-3 G basis set at the Hartree-Fock level of theory. Hydrogen-bond donors and acceptors are shown as blue and red regions, respectively, around the atoms, corresponding to positive and negative potentials.

Figure 5
Hirshfeld surface mapped over d norm highlighting the regions of C-HÁ Á ÁCl and C-HÁ Á ÁN intermolecular contacts. Table 2 Summary of short interatomic contacts (Å ) in the title compound.

Contact
Distance Symmetry operation  In the crystal of (I), molecules are linked by pairs of C-ClÁ Á Á interactions, forming inversion dimers. A short intermolecular ClÁ Á ÁCl contact [3.2555 (9) Å ] links the dimers, forming a ribbon along the c-axis direction. The crystal structure of (II) is stabilized by C-ClÁ Á Á and van der Waals interactions. In (III), molecules are stacked in columns along the a axis via weak C-HÁ Á ÁCl hydrogen bonds and face-tofacestacking interactions. The crystal packing is further stabilized by short ClÁ Á ÁCl contacts. In the crystals of (IV) and (V), molecules are linked through weak XÁ Á ÁCl contacts [X = Br for (IV) and Cl for (V)] and C-HÁ Á ÁCl and C-ClÁ Á Á interactions into sheets parallel to the ab plane. In (VI), molecules are linked by C-HÁ Á ÁO hydrogen bonds into zigzag chains running along the c-axis direction. The crystal packing is further stabilized by C-ClÁ Á Á, C-FÁ Á Á and N-OÁ Á Á interactions.

Refinement
Crystal data, data collection and structure refinement details are summarized in

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.