4,4′,5,5′-Tetramethyl-2,2′-[1,1′-(propane-1,3-diyldinitrilo)diethylidyne]diphenol

The title Schiff base compound, C23H30N2O2, has crystallographic twofold rotation symmetry. An intramolecular O—H⋯N hydrogen bond forms a six-membered ring, producing an S(6) ring motif. The imino group is coplanar with the benzene ring. The two benzene rings are almost perpendicular to each other, making a dihedral angle of 87.38 (4)°. In the crystal structure, neighbouring molecules are linked along the c axis by weak intermolecular C—H⋯O hydrogen bonds and are further packed into columns along the b axis, forming sheets which are parallel to the bc plane.


Comment
The condensation of primary amines with carbonyl compounds yields Schiff base (Casellato & Vigato, 1977) that are still now regarded as one of the most potential group of chelators for facile preparations of metallo-organic hybrid materials.
In the past two decades, the synthesis, structure and properties of Schiff base complexes have stimulated much interest for their noteworthy contributions in single molecule-based magnetism, materials science, catalysis of many reactions like carbonylation, hydroformylation, reduction, oxidation, epoxidation and hydrolysis (Casellato & Vigato, 1977). Only a relatively small number of free Schiff base ligands have been characterized (Calligaris & Randaccio, 1987). As an extension of our work Fun & Kia, 2008a,b,c) on the structural characterization of Schiff base ligands and their complexes, the title compound (I), is reported here.
The molecule of the title compound, (I), has a crystallographic twofold rotation symmetry (Fig. 1). The bond lengths and angles are within normal ranges (Allen et al., 1987) and is comparable to its related structure (Fun & Kia 2008c).
The asymmetric unit of the compound is composed of one-half of the molecule. An intramolecular O-H···N hydrogen bond forms a six-membered ring, producing a S(6) ring motif (Bernstein et al., 1995). The imino group is coplanar with the benzene ring. The two benzene rings are almost perpendicular to each other with a dihedral angle of 87.38 (4)°. In the crystal structure, neighbouring molecules are linked together along the c-axis by weak intermolecular C-H···O hydrogen bonds and are further packed into columns along the b axis, forming sheets which are parallel to the bc plane( Fig. 2, Fig.   3 and Table 1).

Experimental
The synthetic method has been described earlier (Fun & Kia et al., 2008c). Single crystals suitable for X-ray diffraction were obtained by evaporation of an ethanol solution at room temperature.

Refinement
H atom bound to O1 was located from the difference Fourier map and refined freely. The H atom bound to C9 was located from the difference Fourier map and refined freely. The rest of the hydrogen atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.97 Å and U iso (H)= 1.2 U eq (C). A rotating-group model was applied for the methyl groups. Fig. 1. The molecular structure of (I) with atom labels and 50% probability ellipsoids for non-H atoms. The suffix A corresponds to symmetry code (-x + 1, y, -z + 3/2). Intramolecular interactions are shown as dashed lines.

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.
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.