N,N′-Bis(4-bromobenzylidene)butane-1,4-diamine

The molecule of the title Schiff base compound, C18H18Br2N2, lies across a crystallographic inversion centre and adopts an E configuration with respect to the C=N bond. In the crystal structure, molecules are linked into chains along [201] through intermolecular Br⋯Br interactions [3.3747 (3) Å], which are significantly shorter than the sum of the van der Waals radii for Br atoms (3.70 Å). The crystal structure is further stabilized by π–π stacking interactions [centroid–centroid distance 3.6811 (11) Å].


Comment
The condensation of primary amines with carbonyl compounds yields Schiff bases (Casellato & Vigato, 1977) that are still one of the most prevalent mixed-donor ligand in coordination chemistry. In the past two decades, the synthesis, structure and properties of Schiff base complexes have stimulated much interest for their noteworthy contributions in single moleculebased magnetism, materials science, catalysis of many reactions like carbonylation, hydroformylation, reduction, oxidation, epoxidation and hydrolysis (Casellato & Vigato 1977). As an extension of our work (Fun et al., 2008;Fun, Kia & Kargar 2008a,b;Fun & Kia 2008a,b) on the structural characterization of Schiff base ligands, the title compound 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 C═N bond. The bond lengths (Allen et al., 1987) and angles are within normal ranges. 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. An interesting feature of the crystal structure is the short Br···Br [3.3747 (3) Å] interaction ( Fig. 2), which is significantly shorter than the sum of the van der Waals radii for two Br atoms (3.70 Å). The directionality of these interactions, C-X···X-C (X = halogens), has been attributed to anisotropic van der Waals radii for terminally bound halogens or ascribed to donor-acceptor interactions involving a lone pair donor orbital on one halogen and a C-X σ * acceptor orbital on the other (Ramasubbu et al., 1986;Brammer et al., 2003). In the crystal structure, molecules are linked into chains along the [201] direction through the short intermolecular Br···Br interactions (Fig. 2). In addition, the crystal structure is further stabilized by π-π interaction ( Fig. 3) with centroid-to-centroid distance of 3.6811 (11) Å, perpendicular interplanar distance of 3.3617 (8) Å, and centroid···centroid offset of 1.4997 (5) Å.

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

Refinement
All hydrogen atoms were located from the difference Fourier map and refined freely. Fig. 1. The molecular structure of the title compound with atom labels and 50% probability ellipsoids for non-H atoms. The suffix A corresponds to symmetry code (-x, -y + 1, -z).

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.