4,4′-[Butane-1,4-diylbis(nitrilomethylidyne)]dibenzonitrile

The title Schiff base compound, C20H18N4, lies across a crystallographic inversion centre and adopts E configurations with respect to the C=N bonds. The asymmetric unit of the compound is composed of one half-molecule. The imino group is coplanar with the benzene ring. Within the molecule, the planar units are parallel but extend in opposite directions from the methylene bridge. In the crystal structure, neighbouring molecules are linked together by weak intermolecular C—H⋯N hydrogen bonds involving the cyano N atoms. These form ten-membered rings, generating R 2 2(10) ring motifs, and link the molecules along the c axis.


Comment
The condensation of primary amines with carbonyl compounds yields Schiff base compounds (Casellato & Vigato, 1977); these are still one of the most prevalent mixed-donor ligands in coordination chemistry. In the past two decades, the synthesis, structure and properties of Schiff base complexes have stimulated much interest due to their noteworthy contributions in single molecule-based magnetism, materials science and the catalysis of many reactions such as carbonylation, hydroformylation, reduction, oxidation, epoxidation and hydrolysis (Casellato & Vigato 1977). However, 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) on the structural characterization of Schiff base ligands, the structure of the title compound, (I), is reported here.
The molecule of the title compound (I, Fig 1), lies across a crystallographic inversion centre and adopts E configurations with respect to the C═N bonds. 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. The imino group is coplanar with the benzene ring. Within the molecule, the planar units are parallel but extend in opposite directions from the methylene bridge. In the crystal structure, neighbouring molecules are linked together by weak intermolecular C-H···N hydrogen bonds involving the cyano N atoms.
These form ten-membered rings, generate R 2 2 (10) ring motifs (Bernstein et al. 1995) and link the molecules along the c-axis.

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 located from the difference Fourier map and refined freely with fixed isotropic displacement parameters.
Figures 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).

Special details
Experimental. The low-temperature data was collected with the Oxford Cyrosystem Cobra low-temperature attachment.
supplementary materials sup-3 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.