Crystal structure and Hirshfeld surface analysis of dimethyl 3,3′-{[(1E,2E)-ethane-1,2-diylidene]bis(azanylylidene)}bis(4-methylbenzoate)

The title Schiff base compound, synthesized by the condensation reaction of methyl 3-amino-4-methylbenzoat and glyoxal in ethanol, crystallizes in the the monoclinic space group P21/n. The molecule is Z-shaped with the C—N—C—C torsion angle being 47.58 (18)°. In the crystal, pairs of molecules are linked via C—H⋯N hydrogen bonds, forming centrosymetric dimers with an (8) ring motif; this conectivity leads to the formation of columns running along the a-axis direction.

The title Schiff base compound, C 20 H 20 N 2 O 4 , synthesized by the condensation reaction of methyl 3-amino-4-methylbenzoat and glyoxal in ethanol, crystallizes in the the monoclinic space group P2 1 /n. The molecule is Z-shaped with the C-N-C-C torsion angle being 47.58 (18) . In the crystal, pairs of molecules are linked via C-HÁ Á ÁN hydrogen bonds, forming centrosymetric dimers with an R 2 2 (8) ring motif; this connectivity leads to the formation of columns running along the a-axis direction. Hirshfeld surface analysis and two-dimensional fingerprint plots were used to explore the intermolecular interactions and revealed that the most significant contributions to the crystal packing are from HÁ Á ÁH (49.4%), HÁ Á ÁO/OÁ Á ÁH (19.0%) and HÁ Á ÁC/CÁ Á ÁH (17.5%) contacts. Energy frameworks were constructed through different intermolecular interaction energies to investigate the stability of the compound. The net interaction energies for the title compound were found to be electrostatic (E ele = À48.4 kJ mol À1 ), polarization (E pol = À9.7 kJ mol À1 ), dispersion (E dis = À186.9 kJ mol À1 ) and repulsion (E rep = 94.9 kJ mol À1 ) with a total interaction energy, E tot , of À162.4 kJ mol À1 .

Chemical context
In this study, the title Schiff base compound was synthesized by the condensation reaction of methyl 3-amino-4-methylbenzoat and glyoxal in ethanol. Schiff bases are studied widely because of their synthetic flexibility, selectivity and sensitivity towards the central metal atom, structural similarities with natural biological compounds and because of the presence of an azomethine group (-N CH-), which is important for elucidating the mechanism of the transformation and racemization reaction biologically (Sharghi et al., 2003). Schiff bases having chelation with oxygen and nitrogen donors and their complexes have been used as drugs and are reported to possess a wide variety of biological activities against bacteria, fungi and certain types of tumors; in addition, they have many biochemical, clinical and pharmacological properties (Przybylski et al., 2009;Barbosa et al., 2020). In recent years, these molecules, which belong to a large family of click reactions, have attracted a lot of interest for their role in the development of self-healing hydrogels (Xu et al., 2019) . Over the past few years, some metal complexes of Schiff bases have attracted great interest in many fields. The binding interactions of metal complexes with DNA have been studied (Shahabadi et al., 2010). Schiff bases have different applica-tions in many research areas including organic, inorganic, biological and materials chemistry (Fan et al., 2020) and as dyes for the textile and related industries. These compounds also have unique characteristics that make them promising candidates for photovoltaic and photonic materials applications (Abdel-Shakour et al., 2019;Imer et al.,2018). We report herein XRD data and Hirshfeld surface analysis of a new Schiff base compound, dimethyl 3,3 0 -{[(1E,2E)-ethane-1,2diylidene]bis(azanylylidene)}bis(4-methylbenzoate), for which energy frameworks of the crystal packing were calculated.

Structural commentary
The molecular structure of the title complex is illustrated in Fig. 1. The molecule is located in a special position related to the inversion centre 8i (mm2) at the middle of the C10-C10 i bond [symmetry code: (i) 1 À x, 1 À y, 1 À z]. The molecule is Z-shaped with the C10-N1-C7-C8 torsion angle being 47.58 (18) . The benzene rings are located in planes parallel to each other. The values of the C1-O2, O2-C2 and C2-O1 bond lengths and the O1-C2-O2, C2-O2-C1 bond angles are close to those reported for similar complexes (see Database survey). Some selected geometric parameters of the molecule are given in Table 1. The azomethine C N bond length is 1.2713 (17) Å , which is quite close to the corresponding values reported by Gumus et al. (2021) and  [1.276 (6) and 1.287 (6) Å and 1.287 (5) Å , respectively].

Supramolecular features
Although no classical hydrogen bonds are found in the crystal structure, weak hydrogen bonds are present (Table 2, Fig. 2). The role of hydrogen bonds in the formation of the crystal lattice is shown in Fig. 2a  Hydrogen-bond geometry (Å , ).
Cg1 is the centroid of the C3-C8 ring

Figure 2
A view of the crystal packing of the title compound.

Figure 1
The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 40% probability level.
bonds, leading to the formation of columns running along the a-axis direction. A weak C9-H9AÁ Á ÁCg1 contact is also present ( Table 2), which reinforces the crystal structure and plays a major role in the supramolecular framework stabilization, see Fig. 2b.

Hirshfeld surface analysis
The intermolecular interactions present in the crystal structure were visualized by drawing contact and shape descriptors using Crystal Explorer17.5 (Turner et al., 2017). The Hirshfeld surfaces mapped over d norm , curvedness, shape-index and electrostatic potential are shown in Fig. 3. The molecular Hirshfeld surfaces were calculated using a standard (high) surface resolution and with the three-dimensional d norm surfaces mapped over a fixed colour scale from À0.083 (red) to 1.171 (blue) a.u. Red spots in Fig. 3a correspond to the neartype HÁ Á ÁO contacts resulting from C-HÁ Á ÁO and N-HÁ Á ÁO hydrogen bonds. The shape-index surface (Fig. 3b) shows red concave regions with 'bow-tie' patterns, indicating the presence of aromatic stacking interactions (C-HÁ Á Á). In Fig. 3c, the curvedness plots show flat surface patches characteristic of planar stacking. The molecular properties can be described by mapping the molecular electrostatic potential (À0.067 to 0.025 a.u.), which plays a key role in identifying reactive positions on the molecular surface. The Fig. 3d map is useful for predicting the position of nucleophile and electrophile attacks. The blue and red regions observed on the surface around the different atoms correspond to positive and negative electrostatic potentials, respectively. It shows clearly that the electron-rich sites are mainly localized around the oxygen atoms.
Intermolecular contacts and the location of electron-rich regions provide an indication of the stacking in the crystal. To understand this stacking, the crystal voids [calculated with Crystal Explorer17.5 (Turner et al., 2017)] were visualized (Fig. 4). The void parameters of the title compound give a void volume of 76.77 Å 3 , an area of 340.15 Å 2 , a globularity of 0.257 and asphericity value of 0.807. Fig. 5a shows the two-dimensional fingerprint plot of the sum of all the contacts contributing to the Hirshfeld surface represented in normal mode. The HÁ Á ÁH contacts make the largest contribution to the overall crystal packing at 49.4%. This contribution arises as widely scattered points of high density due to the large hydrogen content of the molecule with the two tips at d e + d i = 2.43 Å (Fig. 5b). Scattered points of the HÁ Á ÁO/OÁ Á ÁH interactions contribution (19.0%) have a tip at d e + d i = 2.68 Å . (Fig. 5c) . The pair of characteristic wings in Fig. 5d   The Hirshfeld surface of the title compound mapped over (a) d norm , (b) shape-index, (c) curvedness and (d) electrostatic potential.

Figure 4
A view of the crystal voids.
surface, are also represented by a pair of sharp spikes at d e + d i = 2.76 Å , Fig. 5e. As seen in Fig. 5f, the CÁ Á ÁC contacts (4.9%) have an arrow-shaped distribution of points with its tip at d e = d i = 3.59 Å . The contribution of the CÁ Á ÁO/OÁ Á ÁC contacts to the Hirshfeld surface (2.9%) is negligible, Fig. 5g.

Interaction energies
Interaction energies for the title compound were calculated using the CE-B3LYP/6-31G(d,p) quantum level of theory, as available in CrystalExplorer (Turner et al., 2017). The total intermolecular interaction energy (E tot ) is the sum of four energy terms: electrostatic (E ele ), polarization (E pol ), dispersion (E disp ) and exchange-repulsion (E rep ) with scale factors of 1.057, 0.740, 0.871 and 0.618, respectively. The relative strengths of the interaction energies in individual directions are represented by cylinder-shaped energy frameworks. The energy-framework calculations were analysed to understand the topologies of the pair-wise intermolecular interaction energies. The energy framework is constructed to compare the different energy components, i.e. repulsion (E rep ), electrostatic (E ele ), dispersion (E dis ), polarization (E pol ) and total (E tot ) energy (Mackenzie et al., 2017). The energies between molecular pairs are indicated as cylinders joining the centroids of pairs of molecules with the thickness of the cylinder radius being directly proportional to the amount of interaction    energy between the pair of molecules . As seen in Fig. 6, the red molecule with symmetry (x, y, z) located at a distance of 4.60 Å from the centroid of the selected molecule has shown the highest total interaction energy of À63.7 kJ mol À1 , whereas the purple molecule at the symmetry position (Àx + 1 2 , y + 1 2 , Àz + 1 2 ) located at a distance of 15.88 Å from the centroid of the selected molecule has the lowest total interaction energy of À13.4 kJ mol À1 . The net interaction energies for the title compound are electrostatic (E ele ) = À48.4 kJ mol À1 , polarization (E pol ) = À9.7 kJ mol À1 , dispersion (E dis ) = À186.9 kJ mol À1 , repulsion (E rep ) = 94.9 kJ mol -1 and total interaction energy (E tot ) = À162.4 kJ mol À1 . The dispersion energy is dominant.

Synthesis and crystallization
27.3 mg (0.165 mmol) of 2-amino-3-methylphenol were dissolved in 20 ml of ethanol. To this was added 11.98 mg (0.083 mmol) of glyoxal (40wt % in H 2 O) dissolved in 20 ml of ethanol and the mixture was refluxed for 12 h. At the end of the reaction, the solution was allowed to cool. The orange product obtained was washed with hexane and crystallized from isopropyl alcohol at room temperature (m.p. = 427-430 K, yield 84%).
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )