5-Diethyl-amino-2-[(E)-(4-methyl-3-nitro-phenyl)-imino-meth-yl]phenol: a redetermination.

The title compound, C(18)H(21)N(3)O(3), is a potential bidentate Schiff base ligand. The whole mol-ecule is disordered with a refined site-occupancy ratio of 0.567 (4):0.433 (4) and not just one ethyl group as reported previously [Sarojini et al. (2007 ▶). Acta Cryst. E63, o4782-o4782]. Using the whole mol-ecule disorder, R values are much smaller than those published. An intra-molecular O-H⋯N hydrogen bond generates a six-membered ring, producing an S(6) ring motif. The dihedral angle between the mean plane of the two benzene rings (major component) is 9.0 (5)°. The crystal structure shows short C⋯C [3.189 (15)-3.298 (12) Å] and C⋯O [2.983 (5)-3.149 (13) Å] contacts. Inter-molecular C-H⋯O inter-actions link neighbouring mol-ecules into dimers with R(2) (2)(18) motifs. In the crystal structure, these dimers are linked together by inter-molecular C-H⋯O inter-actions into one-dimensional extended chains along the b axis. The crystal structure is further stabilized by inter-molecular π-π stacking inter-actions [centroid-centroid distances = 3.458 (8)-3.691 (6) Å].


Comment
Compounds with the structure of -C═N-(azomethine group) are known as Schiff bases, which are usually synthesized by the condensation of primary amines and active carbonyl groups. The chemistry of the carbon-nitrogen double bond plays a vital role in the progresses of chemistry (Patai 1970). They have been used as intermediates in medical substrates and as ligands in complex formation with some metal ions (Tai et al., 2003). Recently multi-dentate complexes of iron and nickel showed high activities of ethylene oligomerization and polymerization (Ittel et al., 2000). They have shown biological activities including antibacterial (Kabeer et al., 2001;Pandeya et al., 1999), antifungal (More et al., 2001;Singh & Dash 1988), anticancer (Isloor et al., 2009;Pathak et al., 2000), anti-inflammatory (Vazzanaa et al., 2004) and herbicidal (Samadhiya & Halve 2001) activities. In addition, Schiff bases have also been used as starting materials in the synthesis of large bioactive and industrial compounds via ring closure, cycloaddition and replacement reactions (Aydoğan et al., 2001).
In the title compound, (I), (Fig. 1), an intramolecular O-H···N hydrogen bond generate a six-membered ring, producing S(6) ring motif (  Fig. 2). The whole molecule is disordered with a site occupancy ratio of 0.567 (4)/0.433 (4) and not just the ethyl group as mentioned in the previously reported article (Sarojini et al., 2007). Using the whole molecule disorder, R-values are much smaller than those published.

Refinement
H atoms of the hydroxy groups were positioned by a freely rotating O-H bond and constrained with a fixed distance of 0.84 Å. The rest of the hydrogen atoms were positioned geometrically and refined using a riding model with C-H = 0.95-0.99 supplementary materials sup-2 Å and U iso (H) = 1.2 or 1.5 U eq (C). A rotating-group model was applied for the methyl hydrogen atoms of the methoxy groups. Rigid bond, similarity and simulation restraints were applied. Fig. 1. The molecular structure of (I), showing 50% probability displacement ellipsoids and the atomic numbering. Intramolecular hydrogen bond is shown as a dashed line. Open bonds show the minor component.

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.
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.