4,4′-[Ethylenebis(nitrilomethylidyne)]dibenzonitrile

The molecule of the title Schiff base compound, C18H14N4, lies across a crystallographic inversion centre and adopts an E configuration with respect to the azomethine (C=N) bonds. The imino groups are coplanar with the aromatic rings with a maximum deviation of 0.1574 (12) Å for the N atom. Within the molecule, the planar units are parallel, but extend in opposite directions from the dimethylene bridge. In the crystal structure, pairs of intermolecular C—H⋯N hydrogen bonds link neighbouring molecules into centrosymmetric dimers with R 2 2(10) ring motifs. An interesting feature of the crystal structure is the short intermolecular C⋯C interaction with a distance of 3.3821 (13) Å, which is shorter than the sum of the van der Waals radius of a carbon atom.


Comment
Schiff bases are among the most prevalent mixed-donor ligands in the field of coordination chemistry in which there has been growing interest, mainly because of their wide application in the areas such as biochemistry, synthesis, and catalysis (Pal et al., 2005;Hou et al., 2001;Ren et al., 2002). Many Schiff base complexes have been structurally characterized, but only a relatively small number of free Schiff bases have had their X-ray structures reported (Calligaris & Randaccio, 1987). As an extension of our work  on the structural characterization of Schiff base ligands, the title compound (I), is reported here.
The molecule of the title compound, (Fig. 1), lies across a crystallographic inversion centre and adopts an E configuration with respect to the azomethine (C═N) bond. The bond lengths (Allen et al., 1987) and angles are within normal ranges and are comparable with the values found in related structures . The two planar units are parallel but extend in opposite directions from the dimethylene bridge. The interesting feature of the crystal structure is the short intermolecular C3···C6 interactions [symmetry code: 1 + x, y, z] with a distance of 3.3821 (13) Å, which is shorter than the sum of the van der Waals radius of carbon atom. In the crystal structure, pairs of intermolecular C-H···N hydrogen bonds link neighbouring molecules into dimer with R 2 2 (10) ring motif (Bernstein et al., 1995) (Table 1, Fig. 2).

Experimental
The synthetic method has been described earlier . Single crystals suitable for X-ray diffraction were obtained by evaporation of an ethanol solution at room temperature.

Refinement
All of the hydrogen atoms were positioned geometrically with C-H = 0.95 or 0.97 Å and refined in riding mode with U iso (H) = 1.2 U eq (C). 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].   (Cosier & Glazer, 1986) operating at 100.0 (1) K.

Figures
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.