Crystal structure, DFT and MEP study of (E)-2-[(2-hydroxy-5-methoxybenzylidene)amino]benzonitrile

The asymmetric unit contains two crystallographically independent molecules in which the dihedral angles between the benzene rings are 13.26 (5) and 7.87 (5)°. An intramolecular O—H⋯N hydrogen bonds results in the formation of an S(6) ring motif. In the crystal, molecules are linked by weak C—H⋯O and C—H⋯N hydrogen bonds, forming layers parallel to (011). π–π stacking interactions complete the three-dimensional network.


Chemical context
Most Schiff bases have antibacterial, anticancer, anti inflammatory and antitoxic properties (Williams, 1972). In addition, Schiff bases are important in diverse fields of chemistry and biochemistry owing to their biological activities (Lozier et al., 1975). On the industrial scale, they have a wide range of applications, such as in dyes and pigments, and Schiff bases have also been employed as ligands for the complexation of metal ions (Taggi et al., 2002). Photochromism and thermochromism are also characteristics of these materials and arise via H-atom transfer from the hydroxy O atom to the N atom (Hadjoudis et al., 1987). In NLO studies, Schiff base provide the key functions of frequency shifting, optical modulation, optical switching, optical logic, and optical memory for the emerging technologies in areas such as telecommunications, signal processing, and optical interconnections (Geskin et al., 2003). The present work is a part of an ongoing structural study of Schiff bases and their utilization in the synthesis of quinoxaline derivatives (Faizi et al., 2016a), fluorescence sensors (Faizi et al., 2016b) and coordination compounds (Faizi & Prisyazhnaya, 2015).
We report herein on the synthesis, crystal structure and DFT computational calculation of the new title Schiff base compound, (I). The results of calculations by density functional theory (DFT) on (I) carried out at the B3LYP/6-311G(d,p) level are compared with the experimentally determined molecular structure in the solid state.

Figure 2
Part of the crystal structure with weak C-HÁ Á ÁO and C-HÁ Á ÁN hydrogen bonds shown as dashed lines.

Figure 3
Part of the crystal structure viewed along the b axis to illustrate thestacking interactions in the crystal. label for c axis not visible compounds, as well as in their quantum chemistry and UV-vis spectra. According to molecular orbital theory, an interaction between HOMO and LUMO orbitals of a structure gives rise to a -* type transition. The frontier orbital gap helps to characterize the chemical reactivity and the kinetic stability of the molecule. A molecule with a small frontier orbital gap is generally associated with a high chemical reactivity, low kinetic stability and is also termed a soft molecule. DFT quantum-chemical calculations for the title compound were performed at the B3LYP/6-311G(d,p) level (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). The DFT structure optimization started from the X-ray geometry and the experimental bond lengths and bond angles were found to match with theoretical values indicating that the 6-311G(d,p) basis set is well suited in its approach to the experimental data. The DFT study of (I) shows that the HOMO and LUMO are localized in the plane extending from the whole phenol ring to the cyano benzene ring. The electron distribution of the HOMOÀ1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 4. The HOMO molecular orbital exhibits both and character, whereas HOMOÀ1 is dominated by -orbital density. The LUMO is mainly composed of density while LUMO+1 has both and electronic density. The HOMO-LUMO gap is 0.12935 a.u. and the frontier molecular orbital energies, E HOMO and E LUMO are À0.21428 and À0.08493 a.u., respectively.

Molecular electrostatic potential surface analysis
Molecular electrostatic potential (MEP) surface analysis is a technique of mapping electrostatic potential onto the isoelectron density surface, providing information about the reactive sites. The surface simultaneously displays molecular size and shape and the electrostatic potential value. In the colour scheme adopted, red indicates an electron-rich region with a partially negative charge and blue an electron-deficient region with partially positive charge, light blue indicates a slightly electron-deficient region, yellow a slightly electronrich region and green a neutral region (Politzer et al., 2002). In addition to these, in the majority of the MEPs, the maximum positive region, which is the preferred site for nucleophilic attack, is shown in blue and the maximum negative region, which is preferred site for electrophilic attack, is red. A threedimensional plot of the MEP surface of one of the two independent molecules of the title compound is shown in Fig. 5. According to this, the negative regions of the molecule are located on the donor oxygen atom, the acceptor nitrogen atom and the benzonitrile group of N2 atom (red region). The positive regions over the methoxy hydrogen atoms and all other hydrogen atoms indicate that these sites are most probably involved in nucleophilic processes. Molecular orbital surfaces and energies of HOMOÀ1, HOMO, LUMO and LUMO+1 for (I).

Synthesis and crystallization
The title compound was prepared by refluxing mixed solutions of 2-hydroxy-5-methoxybenzaldehyde (38.0 mg, 0.25 mmol) in ethanol (15 ml) and 2-aminobenzonitrile (29.5 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 60%, m.p. 414-416 K).

(E)-2-[(2-Hydroxy-5-methoxybenzylidene)amino]benzonitrile
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.