supplementary materials

Bis-Schiff bases are a class of important compounds used as pharmaceutical, medicinal and industrial materials. Schiff bases have also been used extensively in coordination and inorganic chemistry. Salem and Amer used H₂O₂ to study the kinetics of the oxidation of a manganese complex with bis-Schiff base of salicyldehyde (Salem & Amer, 1995). Many of these Schiff bases were found to form suitable inner coordination spheres between tin atom with O and N atoms as quadridentate chelates (Teoh et al., 1997). Meanwhile, ruthenium complexes of bis-Schiff bases derived from o-vanillin and salicyldehyde were shown to exhibit dibasic tetradentate chelation (Viswanathamurthi et al., 1998). The intramolecular hydrogen bonds formed between O and N atoms in Schiff bases are responsible for the formation of these metal complexes (Cohen et al., 1964). Kabak et al. (2000) prepared the derivative of another isomer of the title compound and studied the photochromic conformational properties of this derivative, while Parra et al. (2007) examined the intercalation of another derivative of bis-Schiff bases with DNA by UV spectroscopy. Recently, we reported the crystal structure of the meta-isomer of the title compound (Al-Douh et al., 2007), while the single-crystal of the second isomer in the ortho-position was obtained and the data are consistent with the reported structure (Liu et al., 2006). The proton and carbon NMR spectroscopies of the title compound and its isomers were also studied (Al-Douh et al., 2008). Our group has been actively involved in synthesizing bis-Schiff bases and investigating their DNA binding ability using spectroscopic techniques employing calf thymus DNA (Shah et al., 2008). We have synthesized the third symmetric Schiff base by the condensation of o-vanillin with p-phenylenediamine and its X-ray crystal structure is presented here.

The title compound, (Fig. 1), lies across a crystallographic inversion centre [symmetry code of unlabelled atoms -x + 1, -y, -z + 2]. The bond lengths (Allen et al., 1987) and angles are within normal ranges. An intramolecular O—H···N hydrogen bond generates a six-membered ring, producing S(6) ring motif (Bernstein et al., 1995). Intermolecular bifurcated C—H···O hydrogen bonds involving the two oxygen atoms of the Schiff base ligand and the hydrogen atom of the chloroform solvent of crystallization generate a R²₁(5) ring motif. There are short contacts [C1–C9 = 3.267 (3) and C2–C9 = 3.399 (3) Å] which are shorter than the sum of the van der Waals radius of carbon atom. The crystal structure is stabilized by intermolecular C—H···π interaction (Cg1 is the centroid of the C1–C6 benzene ring) (Table 1) and intermolecular π-π interaction [Cg1···Cg2^{i, ii, iii, iv} = 3.6158 (10) Å; symmetry codes: (i) x, 1/2 - y, 1/2 + z (ii) x, y, -1 + z (iii) 1 - x, -y, 1 - z (iv) x, y, 1 + z, (Cg2 is the centroid of the benzene ring in the middle of the main molecule)]. In the crystal structure molecule are stacked down the c axis (Fig. 2).