Crystal structure, Hirshfeld surface analysis and DFT studies of (E)-2-{[(3-chloro-4-methylphenyl)imino]methyl}-4-methylphenol

In the title Schiff base compound, the hydroxy group forms a intramolecular hydrogen bond to the imine N atom generating an S(6) ring motif. The 3-chlorobenzene ring is inclined to the phenol ring by 9.38 (11)°. The configuration about the C=N bond is E.


Chemical context
Schiff bases contain the azomethine moiety (-RCH N-R 0 ) and are prepared by condensation reactions between amines and active carbonyl compounds. Schiff bases are employed as catalyst carriers (Grigoras et al., 2001), thermo-stable materials (Vančo et al., 2004), metal-cation complexing agents and in biological systems (Taggi et al., 2002). Schiff bases show biological activities including antibacterial, antifungal, anticancer, antiviral and herbicidal activities (Desai et al., 2001;Singh & Dash, 1988;Karia & Parsania, 1999;Siddiqui et al., 2006). Moreover, Schiff base ligands are potentially capable of forming stable complexes by coordination of metal ions with their nitrogen atoms as donors (Ebrahimipour et al., 2012). They are important for their photochromic properties and have applications in various fields such as the measurement and control of radiation intensities in imaging systems, optical computers, electronics, optoelectronics and photonics (Iwan et al., 2007). The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of quinoxaline derivatives , fluorescence sensors (Faizi et al., 2016;Mukherjee et al., 2018;Kumar et al., 2017Kumar et al., , 2018 and non-linear optical properties (Faizi et al., 2020). We report herein on the synthesis (from 2-hydroxy-5methylbenzaldehyde and 3-chloro-4-methylaniline), crystal structure, Hirshfeld surface analysis and DFT computational calculations of the title compound, (I). The results of calculations by density functional theory (DFT) carried out at the B3LYP/6-311 G(d,p) level are compared with the experimentally determined molecular structure in the solid state.

Structural commentary
The molecular structure of the title compound (I) is shown in Fig. 1. An intramolecular O-HÁ Á ÁN hydrogen bond is observed (Table 1 and Fig. 1). This is a relatively common feature in analogous imine-phenol compounds (see Database survey section). The imine group, which displays a C9-C8-N1-C5 torsion angle of À177.49 (18) , contributes to the general non-planarity of the molecule. The chlorobenzene ring (C2-C7) is inclined by 9.38 (11) to the phenol ring (C9-C14). The configuration of the C7 N1 bond of this Schiff base is E, and the intramolecular O1-H1Á Á ÁN1 hydrogen bond forms an S(6) ring motif ( Fig. 1a and Table 1). The C14-O1 distance [1.354 (2) Å ] is close to normal values reported for single C-O bonds in phenols and salicylideneamines (Ozeryanskii et al., 2006). The N1-C8 bond is short at 1.281 (3) Å , indicating the existence of an imine bond, while the long C8-C9 bond [1.446 (3) Å ] implies a single bond. All these data support the existence of the phenol-imine tautomer for (I) in its crystalline state. These features are similar to those observed in related 4-dimethylamino-N-salicylideneanilines (Wozniak et al., 1995;Pizzala et al., 2000). The C-N, C=N and C-C bond lengths are normal and close to the values observed in related structures .

Hirshfeld surface analysis
The intermolecular interactions were investigated quantitatively and visualized with Crystal Explorer 17.5 (Turner et al., 2017;Spackman et al., 2009). The shorter and longer contacts are indicated as red and blue spots, respectively, on the Hirshfeld surfaces, and contacts with distances approximately equal to the sum of the van der Waals radii are represented as white spots. The d norm (a-d) and shape index (e) surface mappings are shown in Fig. 3. The most important red spots on the d norm surface represent O1Á Á ÁCl1 interactions (Fig. 3b) and C1-H1CÁ Á ÁCg1 interactions (Fig. 3c). Some additional interactions indicated by light-red spots are corresponding to contacts around phenolic and chlorobenzene rings (Fig. 3d). The red and blue triangles are absent on the shape-index surface, which indicates there are no strongstacking interactions in the crystal structure.
Cg1 is the centroid of the C2-C7 ring.

DFT calculations
The optimized structure in the gas phase of compound (I) was generated theoretically via density functional theory (DFT) using standard B3LYP functional and 6-311 G(d,p) basis-set calculations (Becke, 1993) as implemented in GAUSSIAN 09 (Frisch et al., 2009). The theoretical and experimental results are in good agreement ( Table 2). The highest-occupied molecular orbital (HOMO), acting as an electron donor, and the lowest-unoccupied molecular orbital (LUMO), acting as an electron acceptor, are very important parameters for quantum chemistry. When the energy gap is small, the molecule is highly polarizable and has high chemical reactivity (Fukui, 1982;Khan et al., 2015).   the molecular framework, E HOMO and E LUMO , which clarify the inevitable charge-exchange collaboration inside the studied material, electronegativity (), hardness (), electrophilicity (!), softness () and fraction of electron transferred (ÁN). These data are recorded in Table 3. The significance of and is for the evaluation of both the reactivity and stability. The electron transition from the HOMO to the LUMO energy level is shown in Fig. 5. The HOMO and LUMO are localized in the plane extending from the whole 2-{[(3-chloro-4methylphenyl)imino]methyl}-4-methylphenol ring. The energy band gap [ÁE = E LUMO À E HOMO ] of the molecule is 4.0023 eV, the frontier molecular orbital energies E HOMO and E LUMO being À5.9865 eV and À1.9842 eV, respectively. The dipole moment of (I) is estimated to be 4.30 Debye.  Pekdemir et al., 2012), the dihedral angle between the two benzene rings is 12.4 (2) . For 4-[(2-hydroxy-5-methoxybenzylidene)amino]benzonitrile (XIGNEI; Chiang et al., 2013), a complex with zinc is reported. In N-(5-hydroxysalicylidene)-2,4,6-trimethylaniline (ZIKNOW; Tenon et al., 1995), the angle between the planes of the benzene rings is 74.5 (1) and chlorine is absent.

Synthesis and crystallization
The title compound was prepared by refluxing mixed solutions of 2-hydroxy-5-methylbenzaldehyde (34.0 mg, 0.25 mmol) in ethanol (15 ml) and 3-chloro-4-methylaniline (35.4 mg, 0.25 mmol) in ethanol (15 ml). The reaction mixture was stirred for 5 h under reflux. Single crystals of the title compound suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution (yield 65%, m.p. 383-386 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The hydroxy H atom was located in a difference-Fourier map and its positional parameters were refined freely with U iso (H) = 1.5U eq (O). Other H atoms were fixed geometrically and treated as riding with C-H = 0.96 Å (methyl) or 0.93 Å (aromatic), and U iso (H) = 1.2U eq (C) for aromatic H atoms or U iso (H) = 1.5U eq (C) for methyl H atoms.      (Farrugia, 2012), XP in SHELXTL (Sheldrick, 2008).

(E)-2-{[(3-Chloro-4-methylphenyl)imino]methyl}-4-methylphenol
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.