Crystal structure and Hirshfeld surface analysis of a Schiff base: (Z)-6-[(5-chloro-2-methoxyanilino)methylidene]-2-hydroxycyclohexa-2,4-dien-1-one

The title compound, C14H12ClNO3, adopts the keto–enamine tautomeric form. In the crystal, molecules form stacks along the [001] direction. The crystal packing is further stabilized by O—H⋯O and C—H⋯O hydrogen bonds and C—H⋯Cl and C—H⋯π contacts.

The title compound, C 14 H 12 ClNO 3 , is a Schiff base that exists in the ketoenamine tautomeric form and adopts a Z configuration. In the crystal, the dihedral angle between the planes of the benzene rings is 5.34 (15) . The roughly planar geometry of the molecule is stabilized by a strong intramolecular N-HÁ Á ÁO hydrogen bond. In the crystal, pairs of centrosymmetrically related molecules are linked by O-HÁ Á ÁO hydrogen bonds, forming R 2 2 (10) rings. Besides this, the molecules form stacks along the [001] direction with C-HÁ Á Á and C-HÁ Á ÁCl contacts between the stacks. The intermolecular interactions in the crystal were analysed using Hirshfeld surfaces. The most significant contribution to the crystal packing is from HÁ Á ÁH contacts (30.8%).

Chemical context
Schiff bases are widely used as ligands in coordination chemistry (Calligaris & Randaccio, 1987) and they are also of interest in various fields because of their diverse biological activity (Lozier et al., 1975;Costamagna et al., 1992). Some Schiff bases derived from salicylaldehyde have attracted the interest of chemists and physicists because they show thermochromism and photochromism in the solid state (Cohen et al., 1964;Hadjoudis et al., 1987). The origin of their photo-and thermochromism is related to the reversible intramolecular proton transfer associated with a change in the electronic structure (Hadjoudis et al., 1987). The o-hydroxy Schiff bases obtained by the condensation of o-hydroxyaldehydes with aniline have been extensively examined in this context. Such compounds can exist in two tautomeric forms, viz. keto-enamine (N-HÁ Á ÁO) and phenol-imine (NÁ Á ÁH-O) (Stewart & Lingafelter, 1959;Petek et al., 2010). We report herein the synthesis and the crystal and molecular structures of the title compound, as well as an analysis of its Hirshfeld surfaces.

Structural commentary
As shown in Fig. 1., the asymmetric unit of the title compound contains only one molecule, which adopts the keto-enamine tautomeric form: the H atom is located at N1, and the lengths of the N1-C7 and C8-C9 bonds indicate their single-bond character, whereas the O2-C9 and C7-C8 bonds are double (Table 1). Overall, the bond lengths in the title structure compare well with those of other keto-enamine tautomers known from the literature (see the Database survey section). The whole molecule is almost planar, with a dihedral angle of 5.34 (15) between the benzene ring planes. The methoxy C14 atom deviates from the plane of the C1-C6 benzene ring by 0.038 (4) Å . The torsion angles C1-C6-N1-C7 and N1-C7-C8-C9 are 5.8 (5) and À0.6 (5) , respectively. The planar molecular conformation is stabilized by the intramolecular N1-H2Á Á ÁO2 hydrogen bond (Table 2).

Supramolecular features
In the crystal, the molecules are connected via O-HÁ Á ÁO hydrogen bonds into centrosymmetric pairs with an R 2 2 (10) graph-set motif (Table 2, Fig. 2). Molecules related by a [001] translation form stacks with an interplanar distance of 3.420 (3) Å and a shortest intercentroid separation of 3.6797 (17) Å . The molecular packing is further stabilized by C-HÁ Á ÁO, C-HÁ Á ÁCl and C-HÁ Á Á interactions between the molecules of the neighbouring stacks (Fig. 3). Details of all these contacts are given in Table 2.  Table 1 Selected bond lengths (Å ).

Figure 2
A view of the crystal packing of the title compound. Dashed lines denote the intra-and intermolecular hydrogen bonds.

Figure 1
The molecular structure of the title compound with the displacement ellipsoids drawn at the 50% probability level. The intramolecular N-HÁ Á ÁO hydrogen bond is shown as a dashed line.

Hirshfeld surface analysis
The Hirshfeld surface analysis, together with the two-dimensional fingerprint plots, is a powerful tool for the visualization and interpretation of intermolecular contacts in molecular crystals, since it provides a concise description of all intermolecular interactions present in a crystal structure (Spackman & Jayatilaka, 2009;McKinnon et al., 2007). All surfaces and 2D fingerprint plots were generated using Crys-talExplorer3.1 (Wolff et al., 2012). The mappings of d i , d e , d norm , shape-index and curvedness for the title structure are shown in Fig The Hirshfeld surface of the title compound mapped with (a) d norm , (b) d i , (c) d e , (d) curvedness and (e) shape-index. Table 2 Hydrogen-bond geometry (Å , ).

Figure 5
The d norm -mapped Hirshfeld surface showing the intermolecular interactions in the title compound.
percentage contributions of the various interatomic contacts. As can be seen from these plots (Fig. 6) (Kansız et al., 2018;Ö zek Yıldırım et al., 2018). The donor and acceptor centers of the hydrogen bonding are represented as blue (positive) and red (negative) regions on the Hirshfeld surface mapped over the electrostatic potential (Fig. 7). The electrostatic potential of the Cl01 atom is less negative as compared to those of atoms O2 and O3 of the hydroxy groups, as indicated by the lighter red color.

Synthesis and crystallization
The title compound was prepared by mixing solutions of 2,3dihydroxybenzaldehyde (34.5 mg, 0.25 mmol) and 5-chloro-2methoxyaniline (39.4 mg, 0.25 mmol), both in 15 mL of ethanol, with subsequent stirring for 5 h under reflux. Single crystals were obtained by slow evaporation of an ethanol solution (yield 65%; m.p. 442-444 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-bound H atoms were geometrically positioned with C-H distances of 0.93-0.96 Å and refined as riding, with U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (C) for methyl H atoms. The O-and N-bound H atoms were located in a difference map and freely refined.   SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: WinGX (Farrugia, 2012)and PLATON (Spek, 2009).

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 )