Crystal structure, Hirshfeld surface analysis and antioxidant capacity of 2,2′-{(1E,1′E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(5-benzyloxy)phenol

The title Schiff base compound was synthesized via the condensation reaction of 1,2-diaminebenzene with 4-benzyloxy-2-hydroxybenzaldehyde. The molecule is V-shaped and possesses mirror symmetry; the mirror bisects the central benzene ring. There are two intramolecular O—H⋯N hydrogen bonds present forming S(6) ring motifs.


Chemical context
Schiff base derivatives are a biologically versatile class of compounds possessing diverse activities, such as anti-oxidant (Haribabu et al., 2015(Haribabu et al., , 2016, anti-inflammatory (Alam et al., 2012), antianxiety, antidepressant (Jubie et al., 2011), antitumour, antibacterial, and fungicidal properties (Refat et al., 2008;Kannan & Ramesh, 2006). Bis-bidentate Schiff base ligands have been studied extensively and used as building blocks in metallo-supramolecular chemistry (Birkedal & Pattison, 2006;Shahverdizadeh & Tiekink, 2011;Chu & Huang, 2007;Yoshida & Ichikawa, 1997;Kruger et al., 2001). The common structural feature of these compounds is the presence of an azomethine group, linked by a methylene bridge, which can act as a hydrogen-bond acceptor. In view of this interest we have synthesized the title compound, (I), and report herein on its crystal structure. The 1 H NMR NMR spectrum reveals the presence of an imino group (N CH) in the range = 8.5-8.7 p.p.m. The antioxidant capacity of the compound was determined by the cupric reducing antioxidant capacity (CUPRAC) process. ISSN 2056-9890

Structural commentary
The molecular structure of compound (I) is illustrated in Fig. 1. The asymmetric unit consists of half a molecule, with the whole molecule being generated by mirror symmetry. The mirror bisects the central benzene ring, viz. bonds C1-C1 i and C3-C3 i [symmetry code: (i) Àx, y, z]. In the molecule there are two intramolecular O-HÁ Á ÁN hydrogen bonds present (Table 1), which form S(6) ring motifs as shown in Fig. 1. The configuration of the C4 N1 imine bonds is E and the C4 N1 bond length is 1.278 (6) Å . The C3-N1 C4 bond angles are less than 120 [118.9 (4) ], and the imine group has a C3-N1-C4-C5 torsion angle of À176.8 (4) . The molecule is V-shaped and the two arms are non-planar; the central benzene ring forms dihedral angles of 41.9 (2) and 43.6 (2) with the phenol ring (C5-C10) and the outer benzyloxy ring (C12-C17), respectively. The latter two rings are almost normal to each other, with a dihedral angle of 84.4 (2) .

Supramolecular features and Hirshfeld surface analysis
In the crystal of (I), molecules are linked by C-HÁ Á Á interactions (Table 1), forming layers parallel to the (001) plane, as illustrated in Fig. 2.

Figure 2
Crystal packing of compound (I) viewed along the c axis, with the O-HÁ Á ÁN intramolecular hydrogen bonds and the C-HÁ Á Á interactions (see Table 1) illustrated as dashed lines.

Figure 1
View of the molecular structure of compound (I), with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Unlabelled atoms are related to labelled atoms by the mirror symmetry code: (i) Àx, y, z. The intramolecular O-HÁ Á ÁN hydrogen bonds (see Table 1) are shown as dashed lines.  (12), 2.20 (12) and 57.60 (12) . In compound (IV), that possesses twofold rotational symmetry with the twofold axis bisecting the central benzene ring, the phenol rings are inclined to the central benzene ring by 82.30 (5) and to each other by 63.76 (5) . In the title compound, which possesses mirror symmetry, the corresponding dihedral angles are 41.9 (2) and 68.9 (2) .
A search of the CSD for metal complexes of compounds similar to compound (I) gave over 30 hits. The ligands always coordinate in a tetradentate manner. For example, there were 13 hits for transition metal complexes of compound (II). The majority involve square-planar coordinated metal atoms, such as in complexes (5,

Antioxidant activity
The antioxidant activity profile of the synthesized compound (I) was determined by utilizing the copper(II)-neocuprine [Cu II -Nc] (CUPRAC) method (Apak et al., 2004). The CUPRAC method (Fig. 6) (cupric ion reducing antioxidant capacity) is based on the follow-up of the decrease in the increased absorbance of the neocuproene (Nc), copper (Cu +2 )Nc 2 -Cu +2 complex. Indeed, in the presence of an antioxidant agent, the copper-neocuproene complex is reduced and this reaction is quantified spectrophotometrically at a wavelength of 450 nm. Similar compounds to that of the title compound, (I), in the CSD; see Section 4, Database survey.

Figure 3
View of the Hirshfeld surface of (I) mapped over d norm .

Figure 4
The two-dimensional fingerprint plots of (I): (a) all interactions; According to the cupric ion reducing antioxidant capacity assay, the title compound displayed activity with variable potency in all tested concentrations, because the percentage (%) inhibition in the CUPRAC assay is good [A 0.50 = 15.03 AE 1.50 for a 4 mg dosage, compared to the results for buthylated toluene (BHT) [A 0.50 = 8.97 AE 3.94], used as a positive control (see Table 2). Note: In CUPRAC antioxidant activity, the values expressed are the mean AE s.u.s of three parallel measurements (p < 0.05).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydroxyl H atom was located in a difference-Fourier map and initially freely refined. In the final cycles of refinements it was positioned geometrically (O-H = 0.82 Å ) and refined as riding with U iso (H) = 1.5U eq (O). The C-bound H atoms were positioned geometrically (C-H = 0.93-0.97 Å ) and refined as riding with U iso (H) = 1.2U eq (C).

Funding information
We are grateful to the Department of Higher Scientific Research and CHEMS Research Unit, University of Constantine1, Algeria, for funding this research project.