Crystal structure and Hirshfeld surface analysis of ethyl (E)-4-[(4-hydroxy-3-methoxy-5-nitrobenzylidene)amino]benzoate

The title Schiff base compound displays a trans configuration with respect to the C=N bond, with the two benzene rings being inclined to each other by 31.90 (12)°.

The title Schiff base compound, C 17 H 16 N 2 O 6 , has an E configuration with respect to the C N bond, with a dihedral angle between the two benzene rings of 31.90 (12) . There is an intramolecular O-HÁ Á ÁO nitro hydrogen bond present forming an S(6) ring motif. In the crystal, molecules are linked by pairs of O-HÁ Á ÁO hydrogen bonds, forming inversion dimers enclosing an R 2 2 (4) ring motif. The dimers are linked about an inversion centre by pairs of C-HÁ Á ÁO hydrogen bonds, which enclose R 2 2 (22) loops, forming chains propagating along the [103] direction. Hirshfeld surface analysis and fingerprint plots show enrichment ratios for the HÁ Á ÁH, OÁ Á ÁH and CÁ Á ÁH contacts, indicating a high propensity of such interactions in the crystal. Both the nitro group and the CH 3 -CH 2 -Ogroup are positionally disordered.

Chemical context
Schiff bases are an important class of compounds in the medicinal and pharmaceutical fields. They play a role in the development of coordination chemistry as they readily form stable complexes with most transition metals. These complexes show interesting properties, for e.g. their ability to reversibly bind oxygen, catalytic activity in hydrogenation of olefins and transfer of an amino group, photochromic properties, and complexing ability towards toxic metals (Karthikeyan et al., 2006;Khattab, 2005;Kü çü kgü zel et al., 2006). Recently, hydrazone Schiff base compounds (Cao, 2009;Zhou & Yang, 2010;Zhang et al., 2009) derived from the reaction of aldehydes with hydrazines have been shown to possess excellent biological activities, such as anti-bacterial, anticonvulsant, and antitubercular (Bernhardt et al., 2005;Armstrong et al., 2003). Herein, we report on the synthesis and crystal structure of the title Schiff base title compound, (E)-4-[(4-hydroxy-3-methoxy-5-nitrobenzylidene)amino]benzoate. The Hirshfeld surface analysis was performed in order to visualize, explore and quantify the intermolecular interactions in the crystal lattice of the title compound.

Structural commentary
The molecular structure of the title Schiff base compound is illustrated in Fig. 1. The molecule has a trans or E configuration with respect to the C10 N1 double bond. The dihedral ISSN 2056-9890 angle between the two benzene rings is 31.90 (12) . The C10 N1 bond length of 1.267 (3) Å confirms the azomethine bond formation. There is an intramolecular O-HÁ Á ÁO hydrogen bond present involving the adjacent hydroxyl and nitro substituents on the C11-C16 benzene ring, forming an S(6) ring motif ( Fig. 1 and Table 1).

Supramolecular features
In the crystal, molecules are linked by pairs of O-HÁ Á ÁO hydrogen bonds, forming inversion dimers (Table 1 and Fig. 2). The dimers are linked by pairs of C-HÁ Á ÁO hydrogen bonds, so forming chains propagating along [103]. Within the chains there are two ring motifs present, viz. R 2 2 (4) and R 2 2 (22), as illustrated in Fig. 2.

Database survey
A search of the Cambridge Structural Database (CSD, Version 5.39, update May 2018;Groom et al., 2016) for ethyl-4-(benzylideneamino)benzoate yielded five hits, while a search for the 2-methoxy-4-[(phenylimino)methyl]phenol skelton gave 25 hits. The most significant structure among these results is that of ethyl-4-[(4-hydroxy-3-methoxybenzylidene)amino]benzoate (APAMUB; Ling et al., 2016). The only difference between APAMUB and the title compound is the presence of a nitro group in the title compound. The two benzene rings in APAMUB are inclined to each other by 24.58 (8) compared to 31.90 (12) in the title compound. The crystal packing of the two compounds is significantly different. In APAMUB, molecules are linked by O-HÁ Á ÁN hydrogen bonds, forming chains along [010]. The chains are linked by C-HÁ Á Á and offsetinteractions, resulting in the formation of layers parallel to (102). In the title compound there are only O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds present; no C-HÁ Á Á nor offsetinteractions are present.

Hirshfeld surface analysis
Hirshfeld surfaces and their associated two-dimensional (2D) fingerprint plots (Soman et al., 2014) have been used to quantify the various intermolecular interactions in the title compound. The Hirshfeld surface of a molecule is mapped using the descriptor d norm , which encompasses two factors: one is d e , representing the distance of any surface point nearest to the internal atoms; another one is d i , representing the distance of the surface point nearest to the exterior atoms and also with  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
Crystal packing of the title compound, viewed along the a axis. The O-HÁ Á ÁO and C-HÁ Á ÁO hydrogen bonds (see Table 1) are shown as dashed lines. Only the major components of the disordered atoms (O3, O4, C1, C2 and O1) are shown.

Synthesis and crystallization
The title compound was synthesized by the reaction of a 1:1 molar ratio of ethyl-4-aminobenzoate (0.151 mg) and 4-hydroxy-3-methoxy-5-nitrobenzaldehyde (0.134 mg) in an acetic acid solution (10 ml). The reaction mixture was refluxed for 6 h. The solid product formed during refluxing was filtered, washed with methanol and dried over anhydrous calcium

Figure 3
Hirshfeld surfaces mapped over d norm for the title compound.
chloride in a vacuum desiccator (yield 75%, m.p. 505 K). Brown block-like crystals of the title compound were obtained by slow evaporation of a solution in DMSO.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydroxyl H atom was located in a difference-Fourier map and freely refined. The C-bound H atoms were positioned geometrically and refined as riding: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms. Atoms O3 and O4 of the nitro group are disordered with a refined occupancy ratio of O3/O3 0 = O4/O4 0 = 0.64 (12)

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (