Crystal structure and Hirshfeld surface analysis of N,N′-bis(3-tert-butyl-2-hydroxy-5-methylbenzylidene)ethane-1,2-diamine

The title molecule adopts the phenol–imine form. Two intramolecular O—H⋯N hydrogen bonds each generate an S(6) ring motif. In the crystal, weak C—H⋯O hydrogen bonds link the molecules into inversion dimers.


Chemical context
The key Schiff base condensation reaction involves simply the reaction of an amine with aldehyde to give an imine and is named after Hugo Schiff who first reported this type of reaction (Schiff, 1864). Schiff bases are considered to be an important class of organic compounds being versatile tools and having wide applications in analytical chemistry, in medicine and in biological processes, displaying antifungal, antibacterial and anticancer activities (Przybylski et al., 2009). Schiff base ligands have also played an important role in the development of coordination and supramolecular chemistry (Moroz et al., 2012), having a chelating structure to coordinate metal ions through the imine nitrogen and another group to form complexes (Cozzi et al., 2004;Moroz et al., 2008Moroz et al., , 2010. The complexes of Schiff bases have a wide range of utilization in various areas of science such as in pharmaceutical, agriculture and industrial chemistry (Anis et al., 2013).
In this study, we designed a new type of Schiff base by the reaction of an aromatic aldehyde derivative and ethylenediamine to give N,N 0 -bis(3-tert-butyl-2-hydroxy-5-methylbenzylidene)ethane-1,2-diamine and have also performed the synthesis, characterization and the crystal structure analysis of the target compound. ISSN 2056-9890

Supramolecular features
In the crystal, pairs of C-HÁ Á ÁO hydrogen bond connect the molecules into inversion dimers (Table 1, Fig. 2).

Hirshfeld surface analysis
Hirshfeld surface analysis was used to investigate the presence of hydrogen bonds and intermolecular interactions in the crystal structure. Plots of Hirshfeld surfaces mapped over d norm , d i and d e using a standard (high) surface resolution with a fixed colour scale of À0.080 (red) to 1.716 (blue) a.u. are shown in Fig. 3. Red spots on these surfaces indicate strong hydrogen bonds and interatomic contacts Gü mü ş et al., 2018;Hö kelek et al., 2018;; in the case of the title compound, these correspond to C-HÁ Á ÁO hydrogen-bonding interactions. The red spots identified in Fig. 4 correspond to the near-type HÁ Á ÁO contacts resulting from the C-HÁ Á ÁO hydrogen bond. The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 20% probability level. Hydrogen bonds (Table 1) are shown as dashed lines. Table 1 Hydrogen-bond geometry (Å , ). Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 2
A partial view of the crystal packing. Dashed lines denote the intramolecular O-HÁ Á ÁN and intermolecular C-HÁ Á ÁO hydrogen bonds (Table 1).

Figure 3
The Hirshfeld surface of the title compound mapped over d norm , d i and d e . A view of the three-dimensional Hirshfeld surface plotted over electrostatic potential energy in the range À0.047 to 0.041 a.u. using the STO-3G basis set at the Hartree-Fock level of theory is shown in Fig. 7; the C-HÁ Á ÁO hydrogenbond donors and acceptors are shown as blue and red areas around the atoms related with positive (hydrogen-bond donors) and negative (hydrogen-bond acceptors) electrostatic potential, respectively. The view of the three-dimensional Hirshfeld surface of the title compound plotted over electrostatic potential energy.     Two-dimensional fingerprint plots with a d norm view of the HÁ Á ÁH (77.5%), HÁ Á ÁC/CÁ Á ÁH (16%), HÁ Á ÁO/OÁ Á ÁH (3.1%) and HÁ Á ÁN/NÁ Á ÁH (1.7%) contacts in the title compound.

Synthesis and crystallization
A solution of ethylenediamine (78 mg, 1.3 mmol) in methanol (30 mL) was slowly added over a solution of 3-tert-butyl-2hydroxy-5-methylbenzaldehyde (500 mg, 2.6 mmol) in methanol (30 mL). The reaction mixture was purged with argon at room temperature and heated up to reflux temperature for 12 h. The reaction was monitored by TLC. After completion of the reaction, the mixture was cooled to room temperature. The precipitated Schiff base was filtered off and washed with diethyl ether. The resulting diimine was recrystallized from methanol and dried under vacuum to give the desired product as a yellow powder (Fig. 8). Crystals suitable for X-ray diffraction analysis were obtained by evaporation in methanol. Yield: 85% (450 mg 43, 158.28, 137.26, 130.79, 129.90, 126.78, 118.48, 68.19, 34.76, 29.31, 20.67

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were positioned geometrically and refined using a riding model: O-H = 0.82 Å and C-H = 0.93-0.97 Å with U iso (H) = 1.2U eq (C) or 1.5U eq (O, C-methyl).

N,N′-Bis(3-tert-butyl-2-hydroxy-5-methylbenzylidene)ethane-1,2-diamine
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.