N,N′-Bis(4-bromo-2-fluorobenzylidene)ethane-1,2-diamine

The molecule of the title Schiff base compound, C16H12Br2F2N2, 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. Within the molecule, the planar units are parallel, but extend in opposite directions from the dimethylene bridge. An interesting feature of the crystal structure is the short intermolecular Br⋯F interactions [3.2347 (16) Å, which is shorter than the sum of the van der Waals radii of these atoms]. These interactions link neighbouring molecules along the c axis. The crystal structure is further stabilized by intermolecular C—H⋯N hydrogen bonds.

The molecule of the title Schiff base compound, C 16 H 12 Br 2 F 2 N 2 , 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. Within the molecule, the planar units are parallel, but extend in opposite directions from the dimethylene bridge. An interesting feature of the crystal structure is the short intermolecular BrÁ Á ÁF interactions [3.2347 (16) Å , which is shorter than the sum of the van der Waals radii of these atoms]. These interactions link neighbouring molecules along the c axis. The crystal structure is further stabilized by intermolecular C-HÁ Á ÁN hydrogen bonds.

Comment
Schiff bases are one of most prevalent mixed-donor ligands in the field of coordination chemistry. There has been growing interest in Schiff base ligands, mainly because of their wide application in the field of 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 been characterized (Calligaris & Randaccio, 1987). As an extension of our work (Fun & Kia 2008a,b,c;Fun et al., 2008) on the structural characterization of Schiff base ligands, and the halogen-halogen interactions in the halogen-subtituated Schiff bases, the title compound (I), is reported here.
The molecule of the title compound, (I), (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 related structures (Fun & Kia 2008a,b,c;Fun et al., 2008). 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 Br···F interactions [symmetry code: 1 -x, -1/2 + y, 1/2 -z] with the distance of 3.2347 (16) Å, which is shorter than the sum of the van der Waals radii of these atoms. 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). These interactions link neighbouring molecules along the c-axis (Fig. 2). The crystal structure is further stabilized by intermolecular C-H···N hydrogen bonds (Table 1).

Experimental
The synthetic method has been described earlier (Fun & Kia 2008a). 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 = 0.93 or 0.97 Å and refined in riding model with U iso (H) = 1.2 U eq (C). The highest peak is located 1.73 Å from Br1 and the deepest hole is located 0.7 Å from Br1. Fig. 1. The molecular structure of (I) with atom labels and 50% probability ellipsoids for non-H atoms [symmetry code for A: -x, -y, -z.

Figures
supplementary materials sup-2 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 > σ(F 2 ) is used only for calculating Rfactors(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.