N,N′-Bis[2-chloro-5-(trifluoromethyl)benzylidene]ethane-1,2-diamine

The molecule of the title Schiff base compound, C18H12Cl2F6N2, adopts an E configuration with respect to the azomethine C=N bond. Intramolecular C—H⋯F (× 2) and C—H⋯Cl (× 2) hydrogen bonds generate S(5) ring motifs. The imino group is coplanar with the aromatic ring. Within the molecule, the planar units are parallel, but extend in opposite directions from the methylene bridge, as indicated by the dihedral angle between the two benzene rings of 3.74 (6)°. The interesting features of the crystal structure are weak intermolecular Cl⋯N and F⋯F interactions, with distances of 2.9192 (11) and 3.2714 (10) Å, respectively, which are shorter than the sum of the van der Waals radii of the relevent atoms. These interactions link neighbouring molecules into dimers which are stacked down the b axis.


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 (Kia et al., 2007a,b;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, Kargar & Kia 2008;Fun, Kia & Kargar 2008;Fun, Mirkhani et al., 2008a,b) on the structural characterization of Schiff base compounds, the title compound (I), is reported here.
The molecule of the title compound, (I), (Fig. 1), adopts an E configuration with respect to the azomethine C═N bond.
The bond lengths and angles are within normal ranges (Allen et al., 1987). Intramolecular C-H···F (x 2) and C-H···Cl (x 2) hydrogen bonds generate S(5) ring motifs (Bernstein et al., 1995). The two planar units are parallel but extend in opposite directions from the methylene bridge. The dihedral angle between two benzene rings is 3.74 (6)°. The interesting feature of the crystal structure is weak intermolecular Cl···N and F···F interactions with the distances of 2.9192 (11) and 3.2714 (10) Å, which are shorter than the sum of the van der Waals radii of the relevant atoms, respectively (Table 1). These interactions link neighbouring molecules into dimers which are stacked down the b-axis (Fig. 2).

Experimental
The synthetic method has been described earlier (Fun, Mirkhani et al., (2008a,b)). 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 and 0.97 Å, and refined in riding model 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. Intramolecular interactions are shown as dashed lines.  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.