Crystal structure and Hirshfeld surface analysis of 6,6′-((1E,1′E)-{[1,4-phenylenebis(methylene)]bis(azanylylidene)}bis(methaneylylidene))bis(2-methoxyphenol)

The title compound is a Schiff base that exists in the phenol–imine tautomeric form. The molecular structure is stabilized by an O—H⋯N hydrogen bond, forming an S(6) ring motif.


Chemical context
Schiff bases are used as pigments and dyes, catalysts, intermediates in organic synthesis, and as polymer stabilizers (Supuran et al., 1996). In azomethine derivatives, the C N linkage is essential for biological activity and several azomethines have been reported to possess remarkable antibacterial, antifungal, anticancer and diuretic activities (Gaur, 2003). Schiff bases having an azomethine group of general formula C N-contain various substituted groups (Schiff, 1864). Of particular interest are the two different tautomeric structures for o-hydroxy Schiff bases, which are expressed as keto-amine and phenol-imine, with intrinsic N-HÁ Á ÁO or O-HÁ Á ÁN hydrogen bonds, (Filarowski et al., 2004). There are many studies in the literature on the synthesis of Schiff bases and investigation of tautomeric structures. Phenol-imine and keto-amine tautomeric structures exhibit features of photochromism and thermochromism (Hadjoudis et al., 2004). Tetradentate salen-type ligands have been used in almost all areas of coordination chemistry to prepare complexes that have catalytic and biological activity or which feature interesting structural, electrochemical or magnetic properties (Abd El-Hamid et al., 2019). In this study, a symmetrical tetradentate Schiff base ligand bearing ONNO donor atoms, 6,6 0 -((1E,1 0 E)-{[1,4-phenylenebis(methylene)]bis(azanylylidene)}bis(methaneylylidene))bis (2-methoxyphenol) was synthesized by the interaction of 2-hydroxy-3-methoxy benzaldehyde and 1,4-benzene dimethanamine in ethanol and its crystal structure determined by single-crystal X-ray diffraction.

Structural commentary
The molecular structure of the title Schiff base derivative is illustrated in Fig. 1. The asymmetric unit of the title compound contains one-half of the centrosymmetric molecule (Z 0 = 0.5). There is an intramolecular O2-H2Á Á ÁN1 hydrogen bond (Table 1 and Fig. 1); this is a common feature also observed in related phenol-imine 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 C6-C8-N1-C9 torsion angle of 178.54 (13) . The molecule is non-planar, the 1,4-diethylbenzene ring being inclined to the phenol ring by 74.27 (5) . The C7-C6-C8-N1 torsion angle [3.8 (2) ] further supports the co-planarity of the phenol ring and the Schiff base. The C7-O2 distance is 1.3438 (17) Å , which is close to normal values reported for single C-O bonds in phenols and salicylideneamines (Kaştaş & Albayrak Kaştaş, 2019). The N1-C8 bond is short at 1.2717 (17) Å , strongly indicating a C N double bond, while the long C6-C8 bond [1.451 (2) Å ] implies a single bond. All of these data support the existence of the phenol-imine tautomer for the title compound in the crystalline state.

Supramolecular features
In the crystal, molecules are connected into sheets extending in the bc plane by C8-H8Á Á ÁO2 i hydrogen bonds (Table 1; Fig. 2).

Hirshfeld surface analysis
Hirshfeld surface analysis was used to analyse the various intermolecular interactions in the title compound, through mapping of the normalized contact distance (d norm ) using CrystalExplorer17 (Turner et al., 2017;Spackman & Jayatilaka, 2009). Hirshfeld surface analysis is a valuable tool for assessing the strength of intermolecular interactions, predicting the properties of a crystal and its potential applications (Al-Resayes et al., 2020). The Hirshfeld surface was generated using a standard (high) surface resolution with the threedimensional d norm surface mapped over a fixed color scale of  Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
A view of the crystal packing of the title compound in a view parallel to the bc plane. C-HÁ Á ÁO hydrogen bonds are shown as dashed blue lines.
À0.175 (red) to 1.404 a.u. (blue). The packing of molecules is mainly dependent on HÁ Á ÁH (50.5%) and CÁ Á ÁH (24.3%) interactions and the significant C-HÁ Á ÁO interactions (18%). Blue regions in the d norm map indicate intermolecular interactions with distances longer than van der Waals radius sum of the interacting elements (Fig. 3). The C-HÁ Á ÁO interactions, which appear as red spots in the d norm map, have contact distances shorter than the sum of the van der Waals radii of the oxygen and hydrogen atoms 6. Synthesis and crystallization 0.0225 g (0.148 mmol) of 2-hydroxy-3-methoxy benzaldehyde was dissolved in 20 mL of ethanol and mixed with 0.0100 g (0.074 mmol) of 1,4-benzene dimethanamine dissolved in 20 mL of ethanol (Fig. 4). The reaction mixture was refluxed for 6 h and at the end of the reaction, the solution was allowed to cool. The yellow product obtained was washed with ether and crystallized in ethanol at room temperature (m.p. = 431-434 K, yield 85%).

Refinement
Crystal data, data collection and structure refinement details are summarized in The synthesis of the title compound.

Figure 3
The Hirshfeld surface analysis of the title compound mapped with d norm over À0.175 to 1.404 a.u. showing the C-HÁ Á ÁO hydrogen-bonded contacts. were positioned geometrically (C-H = 0.93, 0.96 and 0.97 Å ) and refined using a riding model, with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )