(E)-4-[(4-Diethylamino-2-hydroxybenzylidene)amino]benzoic acid

In the title compound, C18H20N2O3, a potential bidentate N,O-donor Schiff base ligand, the benzene rings are inclined at an angle of 12.25 (19)°. The molecule has an E conformation about the C=N bond. One of the ethyl groups is disordered over two positions, with a refined site-occupancy ratio of 0.55 (1):0.45 (1). An intramolecular O—H⋯N hydrogen bond makes an S(6) ring motif. In the crystal, pairs of O—H⋯O hydrogen bonds link molecules, forming inversion dimers with R 2 2(8) ring motifs.

In the title compound, C 18 H 20 N 2 O 3 , a potential bidentate N,Odonor Schiff base ligand, the benzene rings are inclined at an angle of 12.25 (19) . The molecule has an E conformation about the C N bond. One of the ethyl groups is disordered over two positions, with a refined site-occupancy ratio of 0.55 (1):0.45 (1). An intramolecular O-HÁ Á ÁN hydrogen bond makes an S(6) ring motif. In the crystal, pairs of O-HÁ Á ÁO hydrogen bonds link molecules, forming inversion dimers with R 2 2 (8) ring motifs.

Related literature
For background to Schiff base ligands and their metal complexes, see: Kargar et al. (2011Kargar et al. ( , 2012; Kia et al. (2010). For standard bond lengths, see: Allen et al. (1987). For hydrogenbond motifs, see: Bernstein et al. (1995 Table 1 Hydrogen-bond geometry (Å , ). In continuation of our work on the crystal structure analysis of Schiff base ligands (Kargar et al., 2011(Kargar et al., , 2012Kia et al., 2010), we determined the crystal structure of the title compound.
The asymmetric unit of the title compound, Fig. 1, comprises a potential bidentate N,O-donor Schiff base ligand. The bond lengths (Allen et al., 1987) and angles are within the normal ranges. The intramolecular O3-H3···N1 hydrogen bond (Table 1) makes an S(6) ring motif (Bernstein et al., 1995). The dihedral angle between the benzene rings is 12.25 (19)°, and the molecule has an E conformation about the C8═N1 bond. One of the ethyl groups was disordered over two position with a refined site occupancy ratio of 0.55 (1)/0.45 (1).

Experimental
The title compound was synthesized by adding 4-diethylaminosalicylaldehyde (2 mmol) to a solution of 4-carboxyaniline (2 mmol) in ethanol (30 ml). The mixture was refluxed with stirring for 30 min. The resultant solution was filtered. Paleyellow single crystals of the title compound, suitable for X-ray structure analysis, were obtained by recrystallization from ethanol, by slow evaporation of the solvents at room temperature over several days.

Refinement
The O-bound hydrogen atoms were located in a difference Fourier map and constrained to ride on the parent atoms with U iso (H) = 1.5 U eq (O). The rest of the hydrogen atoms were included in calculated positions and treated as riding atoms: C -H = 0.93, 0.96 and 0.97 Å for CH, CH 3 and CH 2 H atoms, respectively, with U iso (H) = k × U eq (C), where k = 1.5 for CH 3 H atoms, and = 1.2 for other H atoms. A rotating group model was applied to the methyl group. One of the ethyl groups was disordered over two position with a refined site occupancy ratio of 0.55 (1)/0.45 (1). Since the crystal was very small and not optimal and did not diffract significantly, the ratio of observed to unique reflections is only 32%.   Table 1 for details).

Figure 2
The crystal packing diagram of the title compound viewed along the b-axis, showing the inversion dimers with an R 2 2 (8) ring motif. Only the hydrogen atoms involved the hydrogen bonding are shown -see Table 1  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.14 e Å −3 Δρ min = −0.13 e Å −3 Special details 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.