(E)-2-{[(2-Aminophenyl)imino]methyl}-5-(benzyloxy)phenol and (Z)-3-benzyloxy-6-{[(5-chloro-2-hydroxyphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one

The synthesis and structures of (E)-2-{[(2-aminophenyl)imino]methyl}-5-(benzyloxy)phenol (I) and (Z)-3-benzyloxy-6-{[(5-chloro-2-hydroxyphenyl)amino]methylidene}cyclohexa-2,4-dien-1-one (II) are reported. The crystal structures of the molecules are stabilized by N—H⋯O, O—H⋯O and C—H⋯π contacts. DFT calculations on the structure of (II) support the Keto–imine tautomeric form found in the solid state structure. The antioxident properties of both molecules are investigated.


Chemical context
Schiff base compounds have been used as fine chemicals and medicinal substrates (Fun et al., 2011). Studies of the tautomerism of Schiff bases (Alpaslan et al., 2011;Blagus et al., 2010;Ü nver et al., 2002) have demonstrated that the stabilization of the keto-amino tautomer in the crystal depends mostly on the parent o-hydroxyl aldehyde, the type of the Nsubstituent, the electron withdrawing or donating of the Nsubstituent, its position and stereochemistry (Blagus et al., 2010). Schiff base compounds exhibit a broad range of biological activities, including antifungal and antibacterial (da Silva et al., 2011). They are used as anion sensors (Dalapati et al., 2011;Khalil et al., 2009), non-linear optical compounds (Sun et al., 2012), and as versatile ligands in coordination chemistry (Khanmohammadi et al., 2009;Keypour et al., 2010). In view of the interest in such materials we have synthesized the title compounds, (I) and (II), and report their crystal ISSN 2056-9890 structures here. The common structural feature of these compounds is the presence of a benzyloxy substituent on the central ring, although each molecule adopts a different tautomeric form. Density functional theory (DFT) calculations on (II), carried out at the B3LYP/6-311+G(d) level, are compared with the experimentally determined molecular structure and confirm that the keto tautomeric form of this compound, similar to that found in the structure determination, is the lowest energy form. The antioxidant capacity of both compounds was determined by the cupric reducing antioxidant capacity (CUPRAC) process.

Structural commentary
The molecular structures of compounds (I) and (II), illustrated in Figs. 1 and 2, respectively, are influenced by intramolecular hydrogen bonds: the O-HÁ Á ÁN hydrogen bond in (I) and the N-HÁ Á ÁO contact in (II) (Tables 1 and 2) both form S(6) ring motifs. In compound (II), the N atom is protonated and the C9-O1 bond length, 1.277 (2) Å confirms this to be double bond. In compound (I), however, the C9 O1 bond length of 1.3498 (19) Å indicates a single bond. Bond C7 C8 [1.395 (3) Å ] is a double bond in compound (II), whereas the corresponding bond in (I) [1.435 (3) Å ] is a single bond. Compound (I) adopts the enol-imine tautomeric form and the configuration of the C7 N1 imine bond is E with a length of 1.288 (3) Å . In contrast the o-hydroxy Schiff base of (II), has a Z configuration about the C7 C8 double bond and the molecule adopts the keto-imine tautomeric form, with the N1-C7 bond length being 1.309 (2) Å . Neither molecule is planar: in (I), the central ring (C8-C13) is inclined to the two outer rings (C1-C6 and C15-C20) by 46.80 (10) and 78.19 (10) , respectively, while for (II), the dihedral angles between these rings are 5.11 (9) and 58.42 (11) , respectively. In compound (II), the C1-N1-C7 angle is 127.15 (17) . The molecular structure of compound (II), with the atom labeling. Displacement ellipsoids are drawn at the 50% probability level. The intramolecular N-HÁ Á ÁO hydrogen bond is shown as a dashed line.

Figure 1
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 is shown as a dashed line. Table 1 Hydrogen-bond geometry (Å , ) for (I).
For (II), strong O2-H2Á Á ÁO1 i hydrogen bonds Table 2, form inversion dimers that enclose R 2 2 (18) rings. These combine with weaker C7-H7Á Á ÁCl1 hydrogen bonds, which also generate inversion dimers but with R 2 2 (14) motifs. Inversion-related C14-H14AÁ Á ÁCg3 ii contacts lead to the formation of sheets of molecules parallel to (120), Fig. 6, which are stacked approximately along the b-axis direction. The overall packing for this structure is shown in Fig. 7.

Figure 5
Overall packing for (I) viewed along the b-axis direction.

Figure 6
Sheets of molecules of (II) parallel to (120).

Figure 3
Zigzag chains of molecules of (I) along the b-axis direction. Hydrogen bonds are drawn as blue dashed lines.

DFT-optimized calculations
DFT quantum chemical calculations were performed on molecule (II) using the hybrid functional B3LYP (Becke et al., 1993;Lee et al., 1988), and base 6-311+G (d). The DFT structure optimization of (II) was performed starting from the X-ray geometry. The DFT and X-ray stuctures are compared in Fig. 8. The calculated values of bond lengths (Table 3) compare well with experimental values with the largest bondlength deviation being less than 0.031 Å from those found in the crystal structure. The adoption of the keto-imine tautomeric form is also predicted by these calculations. The study also shows that the HOMO and LUMO are localized in the plane extending from the chlorohydroxybenzene ring to the central phenol ring. The electron distribution of the HOMO-1, HOMO, LUMO and LUMO+1 energy levels is shown in Fig. 9. The occupied orbitals are predominantly of -character as is the LUMO, while LUMO+1 is mainly of -character. The HOMO-LUMO gap is 0.12449 a.u, with frontier molecular orbital energies, E HOMO and ELUMO of À5.622 and À2.234 eV, respectively.

Antioxidant activity
The antioxidant activity profiles of (I) and (II) were determined using the copper(II)-neocuprine [Cu II -Nc] (CUPRAC) process (Apak et al., 2004). The CUPRAC method (cupric ion reducing antioxidant capacity) follows the variation in the absorbance of the neocuproine (2,9-dimethyl-1,10-phenanthroline, Nc), copper +2 complex Nc 2 -Cu +2 In the presence of an antioxidant, the copper-neocuproine complex is reduced and this reaction is followed and quantified spectrophotometrically at a wavelength of 450 nm. The results indicate that the percentage (%) inhibition (IC 50 ) in the CUPRAC assay is small for both compounds in comparison to that for 740 Ghichi

Figure 8
Comparison of the structures of (II) obtained from (a) the X-ray determination and (b) the DFT calculations.
butylated hydroxytoluene (BHT) that was used as a positive control. In Table 4 the values shown are the means of three separate measurements.