(E)-4-Allyl-2-[(2-hydroxyphenyl)iminiomethyl]-6-methoxyphenolate

The title compound, C17H17NO3, crystallizes in a zwitterionic form with cationic iminium and anionic enolate groups. The zwitterion exists in a trans configuration about the C=N bond. The dihedral angle between the two benzene rings is 13.42 (7)°. The methoxy group is almost coplanar [C—O—C—C = 2.1 (2)°] with the attached ring whereas the allyl unit is oriented at a dihedral angle of 67.9 (1)°. An intramolecular N—H⋯O hydrogen bond generates an S(6) ring motif. In the crystal, the molecules are linked into zigzag chains along [010] by O—H⋯O hydrogen bonds. In addition, weak C—H⋯π interactions are observed.


Comment
Schiff bases have received much attention because of their potential applications. Some of these compounds exhibit 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. In addition, some of them may be used as analytical reagents for the determination of trace elements (Eltayeb & Ahmed, 2005a,b). Previously we have reported the crystal structure of 2-((E)-{2-[(E)-2,3-dihydroxybenzylideneamino]-5-methylphenyl}-iminiomethyl)-6-hydroxyphenolate (Eltayeb et al., 2009) which exists in a zwitterionic form. The title compound is another schiff base which also crystallizes in a zwitterionic form. Herein we report its crystal structure.  Table 1) (Bernstein et al., 1995). The bond distances show normal values (Allen et al., 1987) and are comparable with those observed in related structures (Eltayeb et al., 2009;Tan & Liu, 2009).
In the crystal packing ( Fig. 2), molecules are linked into zigzag chains along the [010] by O-H···O hydrogen bonds involving hydroxy groups, phenolate and methoxy O atoms (Table 1). Within a chain, the adjacent molecules are approximately perpendicular to each other (Fig. 2). The crystal structure is further stabilized by weak intermolecular C-H···π interactions (Table 1) involving the C8-C13 ring (centroid Cg1).

Experimental
The title compound was synthesized by adding 5-allyl-2-hydroxy-3-methoxybenzaldehyde (0.768 g, 4 mmol) to a solution of 2-aminophenol (0.436 g, 4 mmol) in ethanol (30 ml). The mixture was refluxed with stirring for 30 min. The resultant red solution was filtered and the filtrate was evaporated to give a red solid product. Red plate-shaped single crystals of the title compound suitable for X-ray structure determination were obtained from diethyl ether by slow evaporation at room temperature after a few days.

Refinement
Atom H1O1 attached to O1 was located in a difference map and then constrained to ride with U iso = 1.5U eq (O1). The remaining H atoms were also located in a difference map and were isotropically refined. Fig. 1. The molecular structure of the title compound, with 50% probability displacement ellipsoids and the atom-numbering scheme. The dashed line indicates a hydrogen bond.

Special details
Experimental. The crystal was placed in the cold stream of an Oxford Cryosystems Cobra open-flow nitrogen cryostat (Cosier & Glazer, 1986) operating at 100.0 (1) K.
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.