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

In the crystal structure, the title Schiff base compound, C21H26N2O4, has twofold rotation symmetry. The imino group is coplanar with the aromatic ring. An intramolecular O—H⋯N hydrogen bond forms a six- membered ring, producing an S(6) ring motif. The two benzene rings are almost perpendicular to each other, making a dihedral angle of 85.00 (2)°. The methoxy group is approximately coplanar with the benzene ring, with a C—O—C—C torsion angle of 2.34 (12)°. Neighbouring molecules are linked together by weak intermolecular C—H⋯O hydrogen bonds and a C—H⋯π interaction, forming a sheet parallel to the ab plane. The molecules also adopt a zigzag arrangement along the c axis.

In the crystal structure, the title Schiff base compound, C 21 H 26 N 2 O 4 , has twofold rotation symmetry. The imino group is coplanar with the aromatic ring. An intramolecular O-HÁ Á ÁN hydrogen bond forms a six-membered ring, producing an S(6) ring motif. The two benzene rings are almost perpendicular to each other, making a dihedral angle of 85.00 (2) . The methoxy group is approximately coplanar with the benzene ring, with a C-O-C-C torsion angle of 2.34 (12) . Neighbouring molecules are linked together by weak intermolecular C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á interaction, forming a sheet parallel to the ab plane. The molecules also adopt a zigzag arrangement along the c axis.

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, etc (Kia, Mirkhani, Kalman & Deak, 2007;Kia, Mirkhani, Harkema & van Hummel, 2007;Pal et al., 2005;Reglinski et al., 2004;Hou et al., 2001;Ren et al., 2002). This is due to the fact that Schiff bases offer opportunities for inducing substrate chirality, tuning the metal-centered electronic factor and enhancing the solubility and stability of either homogeneous or heterogeneous catalysts. Only a relatively small number of free Schiff base ligands have been characterized (Calligaris & Randaccio, 1987). As an extension of our work (Fun, Kargar & Kia, 2008;Fun, Kia & Kargar, 2008;Fun et al., 2008a,b) on the structural characterization of Schiff base compounds, 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). 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 an S(6) ring motif (Bernstein et al. 1995). The two benzene rings are almost perpendicular to each other with a dihedral angle of 85.00 (2)°. The methoxy group is coplanar with the benzene ring, with the C10-O2-C4-C3 torsion angle of 2.34 (12)°. In the crystal structure neighbouring molecules are linked together by weak intermolecular C-H···O hydrogen bonds and a C-H···π interaction to form a sheet parallel to the ab plane (Fig. 2). These molecules also adopt a zigzag arrangement along the c axis (Fig. 3).

Experimental
The synthetic method has been described earlier (Reglinski et al., 2004). 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 a difference Fourier map and refined as riding, with U iso (H) = 1.5U eq (O). Other H atoms were positioned geometrically (C-H = 0.93 -0.96 Å) and refined using a riding model. 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 + 1/2).  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.