Crystal structure, Hirshfeld surface analysis and HOMO–LUMO analysis of (E)-4-bromo-N′-(4-methoxybenzylidene)benzohydrazide

The title Schiff base compound displays an E configuration with respect to the C=N double bond. The benzene rings form a dihedral angle of 58.06 (9)°. In the crystal, the molecules are linked by N—H⋯O and C—H⋯O hydrogen bonds into chains, which are further connected into a three-dimensional network by C—H⋯π interactions.


Chemical context
Schiff bases are nitrogen-containing compounds that were first obtained by the condensation reactions of aromatic amines and aldehydes (Schiff et al., 1864). A wide range of these compounds with the general formula RHC NR 1 (R and R 1 can be alkyl, aryl, cycloalkyl or heterocyclic groups) have been synthesized. Schiff bases are of great importance in the field of coordination chemistry because they are able to form stable complexes with 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 2 -hybridized 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 and medicine, 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). Schiff bases showing photochromic and thermochromic properties have been used in information storage, electronic display systems, optical switching devices and ophthalmic glasses (Amimoto et al., 2005). Herein the crystal structure of the title compound, (E)-4-bromo-N 0 -(4-methoxybenzylidene)benzohydrazide is reported.

Structural commentary
The asymmetric unit of the title compound ( Fig. 1)

Hirshfeld surface analysis
The three-dimensional d norm surface is a useful tool for analysing and visualizing the intermolecular interactions. d norm takes negative or positive values depending on whether the intermolecular contact is shorter or longer, respectively, than the van der Waals radii (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). The three-dimensional d norm surface of the title compound is shown in Fig. 4. The red points, which represent closer contacts and negative d norm values on the surface, correspond to the N-HÁ Á ÁO and C-HÁ Á ÁO interactions. Two-dimensional fingerprint plots from Hirshfeld surface analysis (   The molecular structure of the title compound with displacement ellipsoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Frontier molecular orbitals
The HOMO (highest occupied molecular orbital) acts as an electron donor and the LUMO (lowest occupied molecular orbital) acts as an electron acceptor. If the energy gap is small then the molecule is highly polarizable and has high chemical reactivity. The energy levels were computed by the DFT-B3LYP/6-311G++(d,p) method (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). The electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels, which determines the chemical stability, chemical hardness, chemical potential, electronegativity and electrophilicity index (Table 2), are shown in Fig. 6. 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. From the HOMO-LUMO energy gap, information on whether or not the molecule is difficult (hard) or delicate (soft) can be derived. If the molecule has a large energy gap, then the molecule can be defined as a hard molecule whereas the presence of a small energy gap classifies the molecule as soft. The soft molecules are more polarizable than the hard ones because they only need a small energy for excitation. Therefore, from the data reported in Table 2, we conclude that the molecule of the title compound belongs to the really hard materials.  Table 2 Calculated frontier molecular orbital energies (eV). FMO Energy

Figure 4
Hirshfeld surfaces of the title compound mapped over d norm .

Figure 5
Two-dimensional fingerprint plots of the title compound and relative contributions of the atom pairs to the Hirshfeld surface.

Synthesis and crystallization
The title compound was synthesized by the reaction of a 1:1 molar ratio mixture of a hot ethanolic solution (20 mL) of 4-bromobenzohydrazide (0.213 mg) and a hot ethanolic solution of 4-methoxybenzaldehyde (0.136 mg). The mixture was refluxed for 8 h, then it was cooled and kept at room temperature. The powder formed was recrystallized from DMSO. Colourless block-shaped crystals suitable for X-ray analysis were obtained after a few days on slow evaporation of the solvent.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms were positioned geometrically (C-H = 0.93-0.9 Å , N-H = 0.86 Å ) and were refined as riding with U iso (H) = 1.2U eq (C, N) or 1.5U eq (C) for methyl H atoms. A rotating model was used for the methyl H atoms. Three outliers (100, 102, 002) were omitted in the last cycles of refinement.

(E)-4-Bromo-N′-(4-methoxybenzylidene)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.