Crystal structure, Hirshfeld surface analysis and frontier molecular orbital analysis of (E)-4-bromo-N′-(2,3-dichlorobenzylidene)benzohydrazide

The title Schiff base compound, has an E configuration with respect to the C=N bond, and the benzene rings are inclined to each other by 15.7 (2)°. In the crystal, molecules are linked by N—H⋯O and C—H⋯O hydrogen bonds, forming chains along [001] which enclose (6) loops.


Chemical context
Schiff bases are nitrogen-containing compounds that were first obtained by the condensation reactions of aromatic amines and aldehydes (Schiff, 1864). A wide range of these compounds, with the general formula RHC NR1 (R and R1 can be alkyl, aryl, cycloalkyl or heterocyclic groups) have been synthesized. They are of great importance in the field of coordination chemistry as they are able to form stable complexes with many metal ions (Souza et al., 1985). The chemical and biological significance of Schiff bases can be attributed to the presence of a lone electron pair in the sp 2hybridized orbital of the nitrogen atom of the azomethine group (Singh et al., 1975). These compounds are used in the fields of organic synthesis, chemical catalysis, medicine and pharmacy as well as other new technologies (Tanaka et al., 2010). Schiff bases are also used as probes in investigating the structure of DNA (Tiwari et al., 2011) and have gained special attention in pharmacophore research and in the development of several bioactive lead molecules (Muralisankar et al., 2016). They also exhibit photochromic and thermochromic properties and have been used in information storage, electronic display systems, optical switching devices, and ophthalmic glasses (Amimoto & Kawato, 2005). Herein, we report on the crystal structure, the Hirshfeld surface analysis and the molecular orbital analysis of the title compound, (E)-4-bromo-N 0 -(2,3-dichlorobenzylidene)benzohydrazide. ISSN 2056-9890

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. The configuration about the C8 N2 bond, which has a bond length of 1.271 (5) Å , is E. The benzene rings (C1-C6 and C9-C14) are inclined to each other by 15.7 (2) . The bond lengths and angles and the overall conformation of the molecule are close to those reported for a very similar compound, (E)-4-bromo-N 0 -(2-chlorobenzylidene)benzohydrazide (Shu et al., 2009).

Supramolecular features
In the crystal, molecules are linked by N-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds, forming chains that propagate along the [001] direction and which enclose R 1 2 (6) ring motifs (Fig. 2  and Table 1). Here the oxygen atom O1 acts as a bifurcated acceptor. There are no other significant intermolecular interactions present (see Table 2 in Hirshfeld surface analysis).

Hirshfeld surface analysis
Crystal Explorer (Wolff et al., 2012) was used to generate the Hirshfeld surface and two-dimensional fingerprint plots (Rohl et al., 2008). The three-dimensional d norm surface is a useful tool for analysing and visualizing the intermolecular interactions, which are given in Table 2. The d norm values are negative or positive depending on whether the intermolecular contact is shorter or longer than the sum of the van der Waals radii (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). The total d norm surface of the title compound is shown in Fig. 3. The red spots correspond to the N-HÁ Á ÁO and C-HÁ Á ÁO interactions, the most significant interactions in the crystal (Tables 1 and 2).

Table 2
Intermolecular contacts (Å ) for the title compound.

Figure 1
A view of the molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
Hirshfeld surface mapped over d norm for the title compound. [add range of dnorm to legend]

Frontier molecular orbital calculations
The HOMO (highest occupied molecular orbital) acts as an electron donor and the LUMO (lowest occupied molecular orbital) as an electron acceptor. If the energy gap is small then the molecule is highly polarizable and has high chemical reactivity. The energy levels of the title compound were computed using the DFT-B3LYP/6-311G++(d,p) method (Sivajeyanthi et al., 2017). The energy gap between HOMO-LUMO orbitals, which determines the chemical stability, chemical hardness, chemical potential, electronegativity and the electrophilicity index are shown in Fig. 5 and details are given in Table 3. The frontier molecular orbital LUMO is located over the whole of the molecule. The energy gap of the molecule clearly shows the charge-transfer interaction involving donor and acceptor groups. The chemical hardness and softness of a molecule is a sign of its chemical stability. From the HOMO-LUMO energy gap, we can see whether or not the molecule is hard or soft. If the energy gap is large, the molecule is hard and if small the molecule is soft. Soft molecules are more polarizable than hard ones because they need less energy for excitation. From the data presented in Table 3, we conclude that the energy gap is large, hence the title molecule is a hard material and will be difficult to polarize. Molecular orbital energy levels of the title compound. Two-dimensional fingerprint plots of the crystal with the relative contributions of the atom pairs to the Hirshfeld surface.

Synthesis and crystallization
The title compound was synthesized by the reaction of 1:1 molar ratio mixture of a hot ethanolic solution (20 ml) of 4-bromobenzohydrazide (0.213 mg, Aldrich) and 2,3-dichlorobenzaldehyde (0.175 mg, Aldrich), which was refluxed for 8 h. The solution was then cooled and kept at room temperature. The powder obtained was recrystallized from dimethyl sulfoxide (DMSO). Colourless block-like crystals suitable for the X-ray diffraction analysis were obtained in a few days.

(E)-4-Bromo-N′-(2,3-dichlorobenzylidene)benzohydrazide
Crystal data 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.