Crystal structure, Hirshfeld surface analysis and DFT studies of 2-[(2-hydroxy-5-methylbenzylidene)amino]benzonitrile

The title compound crystallizes in the orthorhombic space group Pbca. The phenol ring is inclined to the benzonitrile ring by 25.65 (3)°. The configuration about the C=N bond is E, stabilized by a strong intramolecular O—H⋯N hydrogen bond that forms an S(6) ring motif.


Chemical context
Schiff bases containing the azomethine moiety (-RCH N-R 0 ) are prepared by a condensation reaction between amines and reactive carbonyl compounds, such as aldehydes. 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). They also show biological activities such as antibacterial, antifungal, anticancer, antiviral and herbicidal (Desai et al., 2001;Singh & Dash, 1988;Karia & Parsania, 1999;Siddiqui et al., 2006). Schiff bases are also capable of forming stable complexes by coordination to metal ions via their nitrogen donor atoms (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 and in optical computers, electronics, optoelectronics and photonics (Iwan et al., 2007). ortho-Hydroxy Schiff base compounds such as the title compound can display two tautomeric forms, the enol-imine (OH) and keto-amine (NH) forms. Depending on the tautomers, two types of intramolecular hydrogen bonds are generally observed in ortho-hydroxy Schiff bases, namely O-HÁ Á ÁN in enol-imine and N-HÁ Á ÁO in keto-amine tautomers (Tanak et al., 2010). The present work is a part of an ongoing structural study of Schiff bases and their utilization in ISSN 2056-9890 the synthesis of quinoxaline derivatives , fluorescence sensors Mukherjee et al., 2018;Kumar et al., 2017; and non-linear optical properties (Faizi et al., 2020). We report herein the synthesis of the title compound 2-[(2-hydroxy-5-methylbenzylidene)amino]benzonitrile (I) from 2-hydroxy-5-methylbenzaldehyde and 3-chloro-4-methylaniline, as well as its crystal structure, Hirshfeld surface analysis and DFT computational calculations. 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 is shown in Fig.1. The configuration of the C8 N2 bond of this Schiff base is E, stabilized by the intramolecular O1-H1Á Á ÁN1 hydrogen bond that forms an S(6) ring motif ( Fig. 1 and Table 1). This is a relatively common feature in analogous imine-phenol compounds (see Database survey section). The C10-O1 bond length [1.3503 (17) Å for X-ray and 1.337 Å for B3LYP] indicates single-bond character, while the imine C8 N2 bond length [1.2795 (17)Å for X-ray and 1.291 Å for B3LYP] indicates double-bond character. All bond lengths and bond angles are within normal ranges and are comparable with those in related Schiff base compounds (Faizi et al., 2019;Kansiz et al., 2018;Ozeryanskii et al., 2006). The C10-O1 and C8 N2 bond lengths confirm the enol-imine form of the title compound (Wozniak et al., 1995;Pizzala et al., 2000). The molecule is not planar, with the benzonitrile ring tilted by 25.65 (3) to the plane of the 5-methylphenol moiety. The imine and 5-methylphenol groups are, however, essentially coplanar, as indicated by the C9-C8-N2-C7 torsion angle of À178.75 (13) and the C1-C14-C15-N1 torsion angle [0.31 (3) for X-ray and 0.44 for B3LYP].

Supramolecular features and Hirshfeld surface analysis
The crystal structure of the title compound is consolidated by C-HÁ Á ÁO and C-HÁ Á ÁN interactions, forming corrugated layers perpendicular to the a axis ( Fig. 2 Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
View along the a axis of the unit cell showing the molecular sheets, formed via C-HÁ Á ÁO and C-HÁ Á ÁN interactions (see Table 1 for details).

Figure 3
View along the c axis of the unit cell showing the infinite chains, formed via C-HÁ Á Á interactions (see Table 1 for details).

Figure 1
The molecular structure of (I) with the atom-numbering scheme. Displacement ellipsoids are drawn at the 40% probability level. The intramolecular O-HÁ Á ÁN hydrogen bond (Table1) is shown as a dashed line.
In order to better visualize and analyze the role of weak intermolecular contacts in the crystal, a Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) was carried out and the associated two-dimensional fingerprint plots (McKinnon et al., 2007) generated using CrystalExplorer17.5 (Turner et al., 2017) were analysed. The three-dimensional d norm surface is shown in Fig. 4 with a standard surface resolution and a fixed colour scale of À0.1805 to 1.0413 a.u. The darkest red spots on the Hirshfeld surface indicate contact points with atoms participating in the C-HÁ Á Á interactions involving C11-H11 and the phenyl substituent (Table 1). As illustrated in Fig. 5a, the corresponding fingerprint plots for the compound have characteristic pseudo-symmetric wings along the d e and d i diagonal axes. The presence of C-HÁ Á Á interactions in the crystal is indicated by the pair of characteristic wings in the fingerprint plot delineated into CÁ Á ÁH/HÁ Á ÁC contacts (Fig. 5c, 27.1% contribution to the Hirshfeld surface). As shown in Fig. 5b, the most widely scattered points in the fingerprint plot are related to HÁ Á ÁH contacts, which make a contribution of 39.2% to the Hirshfeld surface. There are also NÁ Á ÁH/HÁ Á ÁN (16.0%; Fig. 5d), OÁ Á ÁH/HÁ Á ÁO (8.3%; Fig. 5e) and CÁ Á ÁC (6.2%; Fig. 5f) contacts, with smaller contributions from CÁ Á ÁN/NÁ Á ÁC (2.6%), CÁ Á ÁO/OÁ Á ÁC (0.4%) and NÁ Á ÁN (0.3%) contacts.

DFT calculations
The optimized structure of the title compound in the gas phase was generated theoretically via density functional theory (DFT) calculations using the standard B3LYP functional and a 6-311G(d,p) basis-set (Becke, 1993) as implemented in GAUSSIAN09 (Frisch et al., 2009). The theoretical and experimental results are in good agreement ( Table 2). The C8 N2 bond length is 1.2795 (17) Å for X-ray and 1.291 Å for B3LYP and the C10-O1 bond length is 1.3503 (17) Å for X-ray and 1.367 Å for B3LYP.
The highest-occupied molecular orbital (HOMO) and the lowest-unoccupied molecular orbital (LUMO) are very important parameters for quantum chemistry. Many electronic, optical and chemical reactivity properties of compounds can be predicted from frontier molecular orbitals (Tanak, 2019). A molecule with a small HOMO-LUMO bandgap is more polarizable than one with a large gap and is considered a soft molecule because of its high polarizibility, while molecules with a large bandgap are considered to be The Hirshfeld surface of compound (I) mapped over d norm .

Table 2
Comparison of selected observed (X-ray data) and calculated (DFT) geometric parameters (Å , ). 'hard molecules'. To better understand the nature of the title compound, the electron affinity (A = -E HOMO ), the ionization potential (I = -E LUMO ), HOMO-LUMO energy gap (ÁE), the chemical hardness () and softness (S) of the title compound were predicted based on the E HOMO and E LUMO energies (Tanak, 2019). For the title compound, I = 6.146 eV, A = 2.223 eV, ÁE = 3.923 eV, = 1.961 eV and S = 0.311 eV. Based on the relatively large ÁE and values, the title compound can be classified as a hard molecule. The electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels are shown in Fig. 6. The DFT study shows that the HOMO and LUMO are localized in the plane extending from the whole 2-hydroxy-5-methyl-benzaldehyde ring to the 2 aminobenzonitrile unit. The HOMO, HOMO-1 and LUMO orbitals are delocalized over the systems of the two aromatic rings and connected by the Schiff base bridge. HOMO and HOMO-1 can be said to bebonding with respect to the C N imine bond, while the LUMO orbital has imine * antibonding character. The LUMO+1 orbital on the other hand is localized only on the aminobenzonitrile ring and the C atom of the Schiff base. With respect to the imine -bond it is mostly non-bonding. From the frontier orbital analysis, it can be concluded that a HOMO-to-LUMO excitation of (I) would be a -* transition that would weaken the imine bond and drive the production of an excited-state keto-amine tautomer from the enol-imine ground state observed in the solid state. The calculated bandgap of (I) is 3.923 eV, which is similar to that reported for other Schiff base materials, such as for example (E) Zhou et al., 2009b). All of these compounds are enol-imine tautomers, feature an E imine configuration and have the same common strong intramolecular O-HÁ Á ÁN hydrogen-bonding interaction that stabilizes the molecular conformation and forms an S(6) ring motif. The dihedral angles between the aromatic rings are generally smaller than the value of 25.65 (3) observed for the title compound, with angles between 1.09 (4) (for FOWXOF and GEJGAE) and 13.84 (13) (for PUJDOO). Only YOVBUH features angles similar to those of (I), with dihedral angles of 21.74 (5), 27.59 (5) and 27.87 (5) for the three independent molecules in its structure. Steric crowding within each molecule seem to be no issue for the eight structures analysed, and the varying torsion angles might be the result of subtle effects from crystal packing forces.

Synthesis and crystallization
The title compound was prepared by combining solutions of 2-hydroxy-5-methyl-benzaldehyde ( Electron distribution of the HOMO-1, HOMO, LUMO and the LUMO+1 energy levels for the title compound. in ethanol (15 ml) and stirring the mixture for 5 h under reflux (yield 60%, m.p. 412-414 K). Single crystals of the title compound suitable for X-ray analysis were obtained by slow evaporation of an ethanol solution.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. C-bound H atoms were positioned geometrically and refined using a riding model, with C-H = 0.93-0.97 Å and U iso (H) = 1.2-1.5U eq (C). The position of the H1 atom was obtained from a difference map; it was placed in a calculated position with a fixed C-O-H angle, but the O-H distance and the torsion angle were allowed to freely refine.

Funding information
This study was supported by Ondokuz Mayıs University under project No. PYOFEN.1906.19.001. Funding for this research was provided by a Startup Project, University Grants Commission, India.