Crystal structures and antioxidant capacity of (E)-5-benzyloxy-2-{[(4-chlorophenyl)imino]methyl}phenol and (E)-5-benzyloxy-2-({[2-(1H-indol-3-yl)ethyl]iminiumyl}methyl)phenolate

The title Schiff base compounds, (I) and (II), were synthesized via the condensation reaction of 2-amino-4-chlorophenol for (I), and 2-(2,3-dihydro-1H-indol-3-yl)ethan-1-amine for (II), with 4-benzyloxy-2-hydroxybenzaldehyde. In both compounds, there is an intramolecular hydrogen bond forming an S(6) ring motif; an O—H⋯O hydrogen bond in (I), but a charge-assisted N+—H⋯O− hydrogen bond in (II).

The title Schiff base compounds, C 20 H 16 ClNO 2 (I) and C 24 H 22 N 2 O 2 (II), were synthesized via the condensation reaction of 2-amino-4-chlorophenol for (I), and 2-(2,3-dihydro-1H-indol-3-yl)ethan-1-amine for (II), with 4-benzyloxy-2hydroxybenzaldehyde. In both compounds, the configuration about the C N imine bond is E. Neither molecule is planar. In (I), the central benzene ring makes dihedral angles of 49.91 (12) and 53.52 (11) with the outer phenyl and chlorophenyl rings, respectively. In (II), the central benzene ring makes dihedral angles of 89.59 (9) and 72.27 (7) , respectively, with the outer phenyl ring and the mean plane of the indole ring system (r.m.s. deviation = 0.011 Å ). In both compounds there is an intramolecular hydrogen bond forming an S(6) ring motif; an O-HÁ Á ÁO hydrogen bond in (I), but a charge-assisted N + -HÁ Á ÁO À hydrogen bond in (II). In the crystal of (I), molecules are linked by C-HÁ Á Á interactions, forming slabs parallel to plane (001). In the crystal of (II), molecules are linked by pairs of N-HÁ Á ÁO hydrogen bonds, forming inversion dimers. The dimers are linked by C-HÁ Á ÁO hydrogen bonds, C-HÁ Á Á interactions and a weak N-HÁ Á Á interaction, forming columns propagating along the a-axis direction. The antioxidant capacity of the synthesized compounds was determined by cupric reducing antioxidant capacity (CUPRAC) for compound (I) and by 2,2-picrylhydrazyl hydrate (DPPH) for compound (II).

Chemical context
Schiff bases of the general type RR 0 C NR 00 exhibit a wide structural diversity and have found a wide range of applications (Jia & Li, 2015). 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), anti-tumour, antibacterial, and fungicidal properties (Refat et al., 2008;Kannan & Ramesh, 2006). They can be used as potential materials for optical memory and switch devices (Zhao et al., 2007). Besides their biological applications, many Schiff bases also reversibly bind with oxygen, coordinate with and show fluorescent variability with metals, exhibiting photo-chromism and/or thermochromism, and have been used as catalysts, pigments and dyes, corrosion inhibitors, polymer stabilizers, or precursors in the formation of nanoparticles (Gupta & Sutar, 2008;Gupta et al., 2009;Mishra et al., 2012). The common structural feature of these compounds ISSN 2056-9890 is the presence of an azomethine group linked by an -methylene bridge, which can act as hydrogen-bond acceptors. In view of this interest we have synthesized the title compounds, (I) and (II), and report herein on their crystal structures. The 1 H NMR spectra revealed the presence of an imino group (N CH) in the range = 8.5-8.6 p.p.m. Cupric reducing antioxidant capacity (CUPRAC) of (I) was estimated, and the antioxidant capacity of compound (II) was determined by in vitro 2,2-diphenyl-1-picrylhydrazil hydrate (DPPH) radical scavenging.

Supramolecular features
In the crystal structures of both compounds C-HÁ Á Á interactions predominate; see Table 1 for details concerning compound (I), and Table 2 for details concerning compound (II). In the crystal of (I), molecules are linked by C-HÁ Á Á View of the molecular structure of compound (II), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular charge-assisted N + -HÁ Á ÁO À hydrogen bond (see Table 2) is shown as a dashed line.

Figure 1
View of the molecular structure of compound (I), with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular O-HÁ Á ÁN hydrogen bond (see Table 1) is shown as a dashed line.
interactions, forming slabs lying parallel to (001), as illustrated in Fig. 3. In the crystal of (II), molecules are linked by pairs of N-HÁ Á ÁO hydrogen bonds, forming inversion dimers. The dimers are linked by C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á interactions, and a weak N-HÁ Á Á interaction, forming columns propagating along the a-axis direction. The different hydrogen bonds and X-HÁ Á Á (X = C, N) interactions are illustrated in Fig. 4, and the overall crystal packing is illustrated in Fig. 5. There are no other significant intermolecular contacts present in either crystal structure.

Database survey
The structures of Schiff bases derived from hydroxyaryl aldehydes have recently been the subject of a general survey, in which a number of structural errors, often involving misplaced H atoms, were pointed out (Blagus et al., 2010).    Table 1), and only the H atoms (grey balls) involved these interactions have been included.

Figure 4
A view of the hydrogen bonds (dashed lines) and C-HÁ Á Á and weak N-HÁ Á Á interactions (blue arrows) in the crystal structure of compound (II); centroid Cg1 is blue, centroid Cg2 is green and centroid Cg4 is red (see Table 2). Only the H atoms involved in these interactions have been included. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Figure 5
A view along the a axis of the crystal packing of compound (II). The hydrogen bonds and C-HÁ Á Á interactions are shown as dashed lines (see Table 2) and only the H atoms involved in these interactions have been included. yl}phenolate (WOJBEE; Ghichi et al., 2014b). In RUTQOO there is an intramolecular O-HÁ Á ÁO hydrogen bond, as in compound (I). In KOSCUS and WOJBEE there are intramolecular charge-assisted N + -HÁ Á ÁO À hydrogen bonds, as observed for compound (II).

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 (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.
The current results indicate that Schiff base compound (I) has a low cupric ion reducing antioxidant capacity, because the absorbance in the CUPRAC assay is large (A 0.50 > 100) for a 4 mg dosage (see Table 3). The current results indicate that the Schiff base compound (II), has a low free-radical scavenging activity (Blois, 1958), because the percentage inhibition in the DPPH assay is large (IC 50 > 100) for a 1 mg dosage, by comparison with buthylated toluene (BHT) IC 50 = 22.32 AE1.19, used as a positive control (see Table 3).
Note: Compound (I): the activity is cupric ion reducing antioxidant capacity (CUPRAC) with the BHT (positive control). Compound (II): the BHT positive control or standard reference is different for each antioxidant activity test (percentage inhibition).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. In compound (I), the hydroxyl H atom was located in a difference-Fourier map and initially freely refined. In the final cycles of refinement it was positioned geometrically (O-H = 0.82 Å ) and refined with U iso (H)= 1.5U eq (O). In compound (II), an H atom was located in a difference-Fourier map close to atom N1 of the C11 N1 bond, and was freely refined, as was the indole NH H atom. For both compounds, the C-bound H atoms were positioned geometrically (C-H = 0.93-0.97Å ) 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 Constantine 1, Algeria, for funding this research project.