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

The title compound crystallizes with a single molecule in the asymmetric unit. The phenol ring makes a dihedral angle of 36.56 (3)° with the nitrobenzene ring. In the crystal, molecules are linked by C—H⋯O interactions, forming chains along the b-axis direction.


Chemical context
Over the past 25 years, extensive research has surrounded the synthesis and use of Schiff base compounds in organic and inorganic chemistry, as they have important medicinal and pharmaceutical applications. These compounds show biological activities including antibacterial, antifungal, anticancer and herbicidal activities (Desai et al., 2001;Singh & Dash, 1988;Karia & Parsania, 1999). Schiff bases are also becoming increasingly important in the dye and plastics industries as well as for liquid-crystal technology and the mechanistic investigation of drugs used in pharmacology, biochemistry and physiology (Sheikhshoaie & Sharif, 2006). 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 synthesis, their excited state proton-transfer properties and as fluorescent chemosensors (Faizi et al., 2016Kumar et al., 2018;Mukherjee et al., 2018). We report herein on the synthesis, crystal structure as well as Hirshfeld surface analysis of the title compound (I). The results of calculations by density functional theory ISSN 2056-9890 (DFT) on (I) 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 illustrated in Fig. 1. There is an intramolecular O1-H1Á Á ÁN1 hydrogen bond (Table 1 and Fig. 1); this is a common feature also observed in related imine-phenol Schiff bases. It forms an S(6) ring motif and also induces the phenol ring and the Schiff base to be nearly coplanar, as indicated by the C3-C8-N1-C9 torsion angle of 178.53 (13) . An intramolecular C15-H15BÁ Á ÁO2 interaction is also observed. The phenol ring (C1-C8/O1) is inclined to the tolyl ring (C9-C14) by 37.57 (3) , and the nitro group (N2/O2/O3) is inclined to the tolyl ring (C9-C14) by 35.05 (2) . The configuration of the C8 N1 bond is E. The C4-O1 distance is 1.3455 (18) Å , which 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 1.2782 (19) Å , strongly indicating a C N double bond, while the long C8-C3 bond [1.4486 (18) Å ] implies a single bond. All of these data support the existence of the phenol-imine tautomer for (I) in the crystalline state. These features are similar to those observed in related 4-dimethylamino-N-salicylideneanilines (Pizzala et al., 2000).

Supramolecular features
In the crystal, molecules are linked by two intermolecular interactions, C14-H14Á Á ÁO2 i and C7-H7CÁ Á ÁO1 i , resulting in the formation of an infinite chain along the b-axis direction ( Fig. 2 and Table 1).

Hirshfeld surface analysis and two-dimensional fingerprint plots
Hirshfeld surface analysis, together with 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 fingerprint plots were generated using CrystalExplorer3.1 (Turner et al., 2017). The mappings of d norm and shape-index for the title structure are shown in Fig. 3a and 3c, respectively, with the prominent hydrogen-bonding interactions shown as intense red spots. The red colour indicates regions with shorter intermolecular contacts, while blue colour shows regions with longer contact distance in the Hirshfeld surface. The darkest red spots on the Hirshfeld surface indicate contact points with atoms participating in intermolecular C-HÁ Á ÁO interactions that involve C14-H14 and the O2 of the nitro group (Table 1, Fig. 3b). The two-dimensional fingerprint plots ( Fig. 4a-f) provide information about the percentage contributions of the various interatomic contacts. The most important are HÁ Á ÁH interactions, which contribute 37.2% to the total Hirshfeld surface. Other contributions are from CÁ Á ÁH (30.7%), OÁ Á ÁH (24.9%), NÁ Á ÁH (2.0%) and CÁ Á ÁO (1.8%) contacts. There are also smaller contributions (not shown in Fig. 4)  Symmetry code: (i) Àx þ 3 2 ; y À 1 2 ; z.

Figure 2
A view along the a axis of the chain formed by C-HÁ Á ÁO interactions (dashed lines; see Table 1 for details).

Figure 1
The molecular structure of the title molecule, 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.

DFT calculations
The optimized structure of the title compound in the gas phase was generated theoretically via density functional theory (DFT) using the standard B3LYP functional and 6-311G(d,p) basis-set calculations (Becke, 1993) as implemented in GAUSSIAN09 (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. The electronic, optical and chemical reactivity properties of compounds are predicted by their frontier molecular orbitals (Tanak, 2019 Table 2 Comparison of selected observed (X-ray data) and calculated (DFT) geometric parameters (Å , ).  (14) 124.17

Figure 3
A view of the Hirshfeld surface mapped over (a) d norm (b) C-HÁ Á ÁO interactions and (c) shape-index.

Figure 4
The overall two-dimensional fingerprint plot and those delineated into gap is more polarizable than one with a large gap and is considered a soft molecule because of its high chemical reactivity and low kinetic stability. If the molecule has a large HOMO-LUMO gap, the molecule is more stable and less chemically reactive. The term 'hard molecule' is used to describe such cases. The electron affinity (A = ÀE HOMO ), the ionization potential (I = ÀE 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. As a result of the large ÁE and values (Table 3), the title compound can be classified as a hard molecule. The electron distribution of the HOMOÀ1, HOMO, LUMO and the LUMO+1 energy levels for the title compound is shown in Fig. 5. The DFT study shows that HOMO and LUMO are localized in the plane extending from the whole 2-hydroxy-5-methyl-benzaldehyde ring to the 2-methyl-3-nitrophenylamine ring. The HOMO, HOMOÀ1 and LUMO+1 orbitals are delocalized over the two phenyl rings connected by the Schiff base bridge and HOMO and HOMO-1 can be said to be -bonding orbitals. The LUMO orbital is delocalized on the 2-methyl-3-nitrophenylamine ring and the C atom of the Schiff base. The LUMO and LUMO+1 orbitals exhibit * antibonding character. The energy gap of (I) is 3.7160 eV, similar to that reported for the Schiff bases The energy band gap of the title compound.

Table 3
The energy band gap of the title compound.

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.