A second monoclinic polymorph of 4,4′-[butane-1,4-diylbis(nitrilomethylidyne)]dibenzonitrile

The asymmetric unit of the title Schiff base compound, C20H18N4, contains one half-molecule, lying across a crystallographic inversion centre and adopting an E configuration with respect to the C=N bonds. The imino group is coplanar with the benzene ring with a maximun deviation of 0.096 (1) Å for the N atom. 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, forming R 2 2(10) ring motifs.

The asymmetric unit of the title Schiff base compound, C 20 H 18 N 4 , contains one half-molecule, lying across a crystallographic inversion centre and adopting an E configuration with respect to the C N bonds. The imino group is coplanar with the benzene ring with a maximun deviation of 0.096 (1) Å for the N atom. 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-HÁ Á ÁN hydrogen bonds involving the cyano N atoms, forming R 2 2 (10) ring motifs.

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 syntheses, structures 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). In comparison to the Schiff base metal complexes, only a relatively small number of free Schiff base ligands have been characterized structurally (Calligaris & Randaccio, 1987). As an extension of our work Fun, Kia & Kargar 2008a,b;Fun & Kia 2008a,b) on the structural characterization of Schiff base ligands, we reported herein the crystal structure of the title compound.
The asymmetric unit of the title compound contains one-half molecule ( Fig. 1), lying across a crystallographic inversion centre and adopting E configurations with respect to the C═N bonds. The bond lengths (Allen et al., 1987) and angles are within normal ranges and are comparable with the related structure . The imino group is coplanar with the benzene ring, and 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 (Table 1) involving the cyano N atoms, forming ten-membered rings with R 2 2 (10) ring motifs (Bernstein et al., 1995).

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

Refinement
H atoms were positioned geometrically, with C-H = 0.93 and 0.97 Å for aromatic and methylene H, respectively, and constrained to ride on their parent atoms with U iso (H) = 1.2U eq (C). The highest peak is located 0.68 Å from C5 atom.

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.