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

The title compound, C15H15NO, shows an intramolecular O—H⋯N hydrogen bond and the aromatic rings are tilted by 45.73 (2)°.

In the title compound, C 15 H 15 NO, the configuration of the C N bond of the Schiff base is E, and an intramolecular O-HÁ Á ÁN hydrogen bond is observed, forming an intramolecular S(6) ring motif. The phenol ring is inclined by 45.73 (2) from the plane of the aniline ring. In the crystal, molecules are linked along the b axis by O-HÁ Á ÁN and C-HÁ Á ÁO hydrogen bonds, forming polymeric chains. The Hirshfeld surface analysis of the crystal structure indicates that the most important contributions for the packing arrangement are from HÁ Á ÁH (56.9%) and HÁ Á ÁC/CÁ Á ÁH (31.2%) interactions. The density functional theory (DFT) optimized structure at the B3LYP/ 6-311 G(d,p) level is compared with the experimentally determined molecular structure, and the HOMO-LUMO energy gap is provided. The crystal studied was refined as an inversion twin.

Chemical context
Azomethines (known as Schiff bases), having imine groups (CH N) and benzyl rings alternately in the main chain and being conjugated, are interesting materials for a wide spectrum of applications, in particular as metal-ion complexing agents and in biological systems (Hö kelek et al., 2004;Moroz et al., 2012;Kansız & Dege, 2018). Schiff bases are important in various areas of chemistry and biochemistry because of their biological activity (El-masry et al., 2000) and photochromic properties. They also have applications in various fields such as the measurement and control of radiation intensities in imaging systems and optical computers (Elmalı et al., 1999), and electronics, optoelectronics and photonics (Iwan et al., 2007). They are used as anion sensors (Dalapati et al., 2011) and as non-linear optics compounds (Sun et al., 2012). The present work is part of an ongoing structural study of Schiff bases and their utilization in the synthesis of new organic, excited-state proton-transfer compounds, and fluorescent chemosensors (Faizi et al., 2016Kumar et al., 2018;Mukherjee et al., 2018). We report herein the crystal structure as well as the Hirshfeld surface analysis of the title Schiff base (E)-4-methyl-2-{[(4-methylphenyl)imino]methyl}phenol, (I). A comparison between the calculated structure [obtained using density functional theory at the B3LYP/ 6-311 G(d,p) level] and the experimental data is also presented.

Structural commentary
The molecular structure of the title compound is illustrated in Fig. 1. An intramolecular O-HÁ Á ÁN hydrogen bond is observed, which forms an S(6) ring motif (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 À169.8 (3) , contributes to the general non-planarity of the molecule. The phenol ring (C9-C14) is inclined by 45.73 (2) to the aniline ring (C2-C7). The configuration of the C8 N1 bond of this Schiff base is E. The C14-O1 bond is 1.335 (5) Å , which is close to reported values of single C-O bonds in phenols and salicylideneamines (Ozeryanskii et al., 2006). The N1-C8 bond is short at 1.273 (4) Å , strongly indicating the existence of a conjugated C N bond, while the longer C8-C9 bond [1.460 (5) 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 (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 (Faizi et al., 2017a,b).

Supramolecular features
In the crystal of (I), molecules are linked by C-HÁ Á ÁO interactions, forming sheets propagating along the b-axis direction ( Fig. 2 and Table 1). There are no other significant intermolecular interactions present.

Figure 2
A view along the b axis of the polymeric chain formed via C-HÁ Á ÁO intermolecular hydrogen bonds (see Table 1).

Figure 3
View of the three-dimensional Hirshfeld surfaces of (I) plotted over (a) d norm and (b) shape-index.

Figure 1
The molecular structure of (I), with the atom-labelling scheme. Displacement ellipsoids are drawn at the 40% probability level. The intramolecular O-HÁ Á ÁN hydrogen bond (see Table 1) is shown as a dashed line.
plotted over d norm (Fig. 3a), white indicates contacts with distances equal to the sum of van der Waals radii, while the red and blue colours indicate distances shorter (in close contact) or longer (distinct contact) than the van der Waals radii, respectively (Venkatesan et al., 2016). The bright-red spots indicate their roles as respective donors and/or acceptors. The shape-index of the HS is a tool to visualizestacking by the presence of adjacent red and blue triangles; if there are no adjacent red and/or blue triangles, then there are nointeractions. Fig. 3b clearly suggests that there are nointeractions in (I).

DFT calculations
The optimized structure in the gas phase of compound (I) was generated theoretically via density functional theory (DFT) using the 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 DFT calculations provide some important information on the reactivity and site selectivity of the molecular framework, E HOMO and E LUMO , which clarify the inevitable charge-exchange collaboration inside the studied material. These data, which also include the electronegativity (), hardness (), electrophilicity (!), softness () and fraction of electrons transferred (ÁN) 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 phenol ring. The energy band gap [ÁE = E LUMO -E HOMO ] of the molecule is 2.742 eV, the frontier molecular orbital energies E HOMO and E LUMO being À1.6411eV and À5.8477 eV, respectively. The dipole moment of (I) is estimated to be 2.61 Debye.   C8-N1-C5 120.6 (3) 120.6 N1-C8-C9 120.6 (3) 120.6 C5-N1-C8-C9 À169.8 (3) À169.8

Synthesis and crystallization
The title compound (I) was obtained following a published method (Hanika et al., 1971;Samant & Mayadeo 1982). Single crystals of compound (I) were obtained by slow evaporation of an ethanol solution after 4 d.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms, with C-H = 0.93-0.96 Å and U iso (H) = 1.2U eq or 1.5U eq (C,O). The crystal studied was refined an a perfect inversion twin.

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.