(Z)-6-{2-[(E)-2,4-Dihydroxybenzylideneamino]phenylaminomethylene}-3-hydroxycyclohexa-2,4-dienone toluene solvate

The bis-Schiff base title compound, C20H16N2O4·C7H8, crystallized as a toluene solvate. In the solid state, it is present as its prototropic tautomer formed by transfer of one of the ortho-hydroxyl H atoms. The proton transfer is accompanied by a shift of electron pairs, as is evident from the observed C—O and C—N bond distances of 1.305 (2) and 1.315 (2) Å, which are largely consistent with C=O and C—N distances. The actual molecule present in the solid state is thus the charge-neutral β-keto amine, with a small contribution of its zwitterionic valence tautomer via partial delocalization of electron pairs along the N—C—C—C—O atom chain. The dihedral angles between the central benzene ring and the two outer benzene rings of the Schiff base are 51.99 (8) and 12.95 (9)°. Intramolecular O—H⋯N and N—H⋯O hydrogen bonds generate S(6) ring motifs, whereas intramolecular N—H⋯N hydrogen bonds generate S(5) ring motifs. In the crystal structure, O—H⋯O hydrogen bonds and weak C—H⋯O interactions link the molecules into one-dimensional zigzag chains along the b axis; these chains are further stacked by O—H⋯O and weak C—H⋯O interactions along the c axis, forming two-dimensional extended networks parallel to the bc plane. In addition, the crystal structure is further stabilized by weak C—H⋯π and π–π interactions.


Comment
Schiff bases have received much attention because of their potential applications with some of these compounds exhibiting various pharmacological activities, as noted by their anticancer (Dao et al., 2000), anti-HIV (Sriram et al., 2006), antibacterial and antifungal (Karthikeyan et al., 2006) properties. Although numerous transition-metal complexes of Schiff bases have been structurally characterized (Granovski et al., 1993), relatively few free Schiff bases have been similarly characterized. N-substituted salicylaldimines show photochromism and thermochromism in the solid state. These effects are produced by intramolecular proton transfer associated with a change in the π-electron configuration (Hajioudis et al. 1987).
In addition, some of them may be used as analytical reagents for the determination of trace elements (Eltayeb & Ahmed, 2005a,b) such as nickel in some natural food products (Fakhari et al., 2005) or biologically important species (Shahrokhian et al., 2000). As part of a general study of tetradenate and bidentate Schiff bases Fun, Mirkhani et al., 2008a,b), we determined the structure of the title compound.
The title compound was synthesized from o-phenylenediamine by reaction with two equivalents of 2,4-dihydroxybenzaldehyde, and the expected reaction product would thus have been the bis-Schiff base 4-((E)-(2-((E)-2hydroxybenzylideneamino)phenylimino)methyl) benzene-1,3-diol. The actual molecule obtained in the solid state is however its prototropic tautomer formed by transfer of one of the ortho-hydroxyl protons onto the adjacent imine unit. The proton transfer is accompanied by a shift of electron pairs as is evident from the observed C20-O2 and C14-N2 bond distances of 1.305 (2) and 1.315 (2) Å, which are consistent with C═O and C-N distances (Allen et al., 1987), respectively. The formation of a C═O keto group rather than a C-Ophenolate is also obvious by comparison with the other three phenol C-OH groups in the structure, which are about 0.05 Å longer than C20-O2. The actual molecule present in the solid state is thus the charge neutral β-keto amine (Z)-3-hydroxy-6-((2-((E)-2-hydroxybenzylideneamino)phenylamino)methylene)cyclohexa-2,4-dienone (top isomer in Fig. 4), with a small contribution of its zwitter-ionic valence tautomer via partial delocalization of electron pairs along the atom chain N2-C14-C15-C20-O2 (bottom tautomer in Fig. 4). The other imine group did not undergo proton transfer and is present in its original Shiff base state. Both the imine as well as the amine units are stabilized by strong O-H···N and N-H···O hydrogen bonds ( Table 2) that generate S(6) ring motifs whereas the intramolecular N-H···N hydrogen bond between the amine and imine (Table 2) exhibits an S(5) ring motif (Bernstein et al., 1995).
Bond lengths and angles are in normal ranges (Allen et al., 1987) and comparable to those in related structures (Eltayeb et al., 2007a,b;Cakir et al. 2002;Karabiyik et al., 2007). The C8-C13 phenyl ring makes a dihedral angle of 51.99 (8)°w ith the dihydroxyphenyl ring (C1-C6/O1/O3) and 12.95 (9)° with the keto-hydroxyphenyl ring (C15-C20/O2/O4). In the crystal packing (Fig. 2), additional O-H···O hydrogen bonds and weak C-H···O interactions (Table 2) link the molecules into one dimensional zigzag extended chains along the b axis and these chains are further stacked ( Fig. 2 & 3) along the c axis thus forming two-dimensional extended networks parallel to the bc plane. The crystal is further stabilized by weak C-H···π interactions ( Table 2). The short distance between the centroids of the six-membered rings prove an existence of π···π interactions (Table 1).
The title compound was synthesized by adding 2,4-dihydroxybenzaldehyde (0.552 g, 4 mmol) to a solution of o-phenylenediamine (0.216 g, 2 mmol) in ethanol (20 ml). The mixture was refluxed with stirring for half an hour. The resultant yellow solution was filtered. Yellow single crystals of the title compound suitable for X-ray structure determination were recrystallized from a mixture of THF/toluene (2/1) by slow evaporation of the solvents at room temperature over several days.

Refinement
Hydroxyl and amine/imine H atoms were located from the difference Fourier map and refined as riding on the parent atoms with isotropic refinement of the displacement parameters. The remaining H atoms were geometrically located and refined as riding model. A rotating group model was used for the methyl groups. Fig. 1. The asymmetric unit of (I), showing 50% probability displacement ellipsoids and the atomic numbering. Intramolecular hydrogen bonds are drawn as dashed lines.    as those based on F, and R-factors based on ALL data will be even larger.