X-ray and computational structural study of (E)-2-(4-chlorophenyliminomethyl)-4-methoxyphenol

In the molecule of the title compound, C14H12ClNO2, the two aromatic rings are oriented at a dihedral angle of 5.92 (7)°. An intramolecular O—H⋯N hydrogen bond results in the formation of a nearly planar six-membered ring, which is oriented at dihedral angles of 1.55 (4) and 5.95 (4)° with respect to the phenol and chlorophenyl rings, respectively. In the crystal structure, weak intermolecular C—H⋯O hydrogen bonds link the molecules into a three-dimensional network.


Comment
The present work is part of a structural study of Schiff bases Özek et al., 2007;Odabaşoğlu, Büyükgüngör et al., 2007;Odabaşoğlu, Arslan et al., 2007). We report herein the crystal structure of the title compound, (I).
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 intramolecular hydrogen bonds are possible: O-H···N in benzenoid and N-H···O in quinoid tautomers. The H atom in (I) is located on atom O1, thus the phenol-imine tautomer is favored over the keto-amine form, as indicated by the C2-O1, C8-N1, C1-C8 and C1-C2 bonds ( Fig. 1 and Table 2).
The O1-C2 bond has single-bond character, whereas the N1-C8 bond has a high degree of double-bond character as in 2- Albayrak et al., 2005]. 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 thermochromic properties in (I) caused by the planarity of the molecule; the dihedral angle between rings A (C1-C6) and B (C9-C14) is 5.92 (7)°.
The intramolecular O-H···N hydrogen bond (Table 1)  In the crystal structure, weak intermolecular C-H···O hydrogen bonds (Table 1) link the molecules into a three-dimensional network (Fig. 2), in which they may be effective in the stabilization of the structure.
Ab-initio Hartree-Fock (HF), density-functional theory (DFT) and semi-empirical (AM1 and PM3) calculations and full-geometry optimizations were performed by means of GAUSSIAN 03 W package (Frisch et al., 2004). The selected bond lengths and angles together with the torsion angles are compared with the obtained ones from semi-empirical, ab-initio HF and DFT/B3LYP methods (Table 2). We observe an acceptable general agreement between them. Although the DFT molecular orbital theory was considered as the most accurate method for geometry optimization for free and complex ligands (Friesner, 2005;Liu et al., 2004), the HF method led to better results in regard to the bond lengths and angles.

Experimental
The title compound was prepared by refluxing a mixture of a solution containing 5-methoxysalicylaldehyde (0.5 g 3.3 mmol) in ethanol (20 ml) and a solution containing 4-chloraniline (0.420 g 3.3 mmol) in ethanol (20 ml). The reaction mixture was stirred for 1 h under reflux. The crystals suitable for X-ray analysis were obtained from ethanol by slow evaporation (yield; 76%, m.p. 378-379 K).
supplementary materials sup-2 Refinement H1 atom (for OH) was located in difference syntheses and refined isotropically [O-H = 0.88 (3) Å and U iso (H) = 0.112 (9) Å 2 ]. The remaining H atoms were positioned geometrically, with C-H = 0.93 and 0.96 Å for aromatic and methyl H, respectively, and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C). Fig. 1. The molecular structure of the title molecule, with the atom-numbering scheme. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen bond is shown as dashed line.  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.