(E)-2-Ethoxy-6-[(4-ethoxyphenyl)iminomethyl]phenol

In the asymmetric unit of the title compound, C17H19NO3, there are three independent molecules, which are align nearly parallel to each other and adopt the phenol-imine tautomeric form. In each molecule, an intramolecular O—H⋯N hydrogen bond results in the formation of an S(6) ring motif. The dihedral angles between the aromatic rings in the three independent molecules are 13.55 (2), 21.24 (2) and 46.26 (1)°. C—H⋯π interactions are also observed in the crystal structure.

In the asymmetric unit of the title compound, C 17 H 19 NO 3 , there are three independent molecules, which are align nearly parallel to each other and adopt the phenol-imine tautomeric form. In each molecule, an intramolecular O-HÁ Á ÁN hydrogen bond results in the formation of an S(6) ring motif. The dihedral angles between the aromatic rings in the three independent molecules are 13.55 (2), 21.24 (2) and 46.26 (1) . C-HÁ Á Á interactions are also observed in the crystal structure. H atoms treated by a mixture of independent and constrained refinement Á max = 0.45 e Å À3 Á min = À0.50 e Å À3 Table 1 Hydrogen-bond geometry (Å , ).
In general, O-hydroxy Schiff bases exhibit two possible tautomeric forms, the phenol-imine (or benzenoid) and ketoamine (or quinoid) forms. Depending on the tautomers, two types of intra-molecular hydrogen bonds are possible: O-H···N in benzenoid and N-H···O in quinoid tautomers. The H atom in title compound (I) is located on atom O1, thus the phenolimine tautomer is favored over the keto-amine form, as indicated by the C9-N1, C9-C10, C11-O1 and C10-C11 bond lengths. The O1-C11 bond lengths in molecule A, B and C [1.347 (3) There are three crystallographic independent molecules A, B and C in the asymmetric unit ( Fig. 1) with their ethoxy groups pointing in same directions. The molecular structure of (I), is not planar and this non-planarity increase gradually with the sequence of molecule A, B and C. The dihedral angles between the C1-C6 and C10-C15 benzene rings are 13.55 (2), 21.24 (2) and 46.26 (1)° with this A, B, C sequence. It is known that Schiff bases may exhibit thermochromism or photochromism, depending on the planarity or non-planarity of the molecule, respectively. Therefore, one can expect photochromic properties in (I) caused by non-planarity of the molecules. Intramolecular O-H···N hydrogen bonds result in the formation of a nearly planar six-membered ring motif S(6) (Bernstein et al., 1995), which is oriented with respect to the fused aromatic rings at dihedral angles of 1.22 (1) and 12.38 (1)° for molecule A, 3.28 (1) and 20.28 (1)° for molecule B and 3.26 (1) and 45.49 (1)° for molecule C.
supplementary materials sup-2 Refinement Atoms H1A, H1B and H1C were located in a difference map and refined isotropically. The remaining H atoms were positioned geometrically [C-H = 0.93 Å (aromatic) and C-H = 0.96 Å (methyl)] and constrained to ride on their parent atom, with U iso (H) = xU eq (C), where x = 1.5 for methyl H, and x = 1.2 for other H atoms. To recover the slightly deformed shape of the ring C1A-C6A, the SHELXL97 similar U ij and rigid bond restraints (SIMU and DELU) were applied. Fig. 1. A view of (I), with the atom-numbering scheme and 30% probability displacement ellipsoids. Dashed line indicates intramolecular hydrogen bond.

Special details
Experimental. 288 frames, detector distance = 100 mm 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 > σ(F 2 ) is used only for calculating Rfactors(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.