(E)-2,3-Bis[(E)-benzylideneamino]but-2-enedinitrile

The asymmetric unit of the title compound, C18H12N4, consists of a half-molecule, where the two halves of the molecule are related by inversion symmetry. The molecule is effectively planar, with the largest deviation from the 22-atom mean plane, measuring 0.024 (2) Å, exhibited by the ortho-C atom of the phenyl ring. The crystal structure exhibits π-stacking, with an interplanar spacing of 3.431 (3) Å.


Related literature
For applications of the title molecule as a semiconductor, see: Tanaka et al. (2009). For applications of the title compound and its various derivatives as a dye, see: Neumer (1977); Begland (1976). For the crystal structures of three di(azomethine) dyes with various substituents at the para position of the benzene ring of the title compound, see: Matsumoto et al. (2004). For a study of the nonlinear optics applications of both the title compound and the mono-condensation product, see: Das et al. (2001). For a review of the chemistry and reactions of the diaminomaleonitrile, see: Al-Azmi et al. (2003).

Experimental
Crystal data C 18 H 12 N 4 M r = 284.32 Triclinic, P1 a = 6.389 (4) Å b = 7.608 (5) Å c = 8.311 (5)  Data collection: CrysAlis CCD (Oxford Diffraction, 2008); cell refinement: CrysAlis CCD; data reduction: CrysAlis RED (Oxford Diffraction, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: publCIF (Westrip, 2010).  (Tanaka et al., 2009;Neumer, 1977;Begland, 1976;Matsumoto et al., 2004 andDas et al., 2001). The title compound is interesting in that although it is orientated in a trans configuration about the ethylene group of the di(azomethine) linkage the DAMN synthon is exclusively cis about this bond. It is proposed that at elevated temperatures during the chemical synthesis an equilibrium is established between the cis and trans isomers of DAMN in solution. Although it has been shown that the cis isomer of DAMN is lower in energy than the trans isomer (Al-Azmi et al., 2003) it is likely that non-bonded repulsion between the hydrogen atoms in the ortho positions of (1) would result in the trans isomer of (1) having a lower energy than the cis isomer. The double condensation reaction required to form (1) is therefore more likely to take place with the trans isomer of DAMN in solution; the trans isomer of (1) would thus be the major product.
Compound (1) crystallized in the triclinic space group P1 with a half molecule in the asymmetric unit. The two halves of the molecule are related by inversion symmetry with an inversion centre at the midpoint of the C═C double bond of the di(azomethine) linkage unit. The molecule is effectively planar with the largest deviations from the 22-atom mean plane exhibited by the ortho C-atoms C3 and C5, measuring 0.022 (2) and 0.024 (2) Å, respectively. The structure shows that adjacent molecules are parallel, but are in a staggered configuration. There appear to be π-π interactions between adjacent molecules with an interplanar spacing of 3.431 (3) Å. The structure shows no genuine hydrogen bonding as all interactions are longer than the sum of their van der Waals radii. The lack of meaningful hydrogen bonds is likely due to a lack of good H-bond donors, as both the cyanide groups and the imine nitrogen atoms are potentially good H-bond acceptors.

Experimental
Benzaldehyde (0.500 g, 4.7 mmol), cis-diaminomaleonitrile (0.255 g, 2.4 mmol) and a catalytic amount of piperidine were dissolved in dry toluene (100 ml) and the solution brought to reflux for four hours. Water was continuously removed from the reaction via a Dean and Stark apparatus.

Refinement
The positions of all hydrogen atoms were calculated using the standard riding model of SHELXL97 with C-H(aromatic) distances of 0.93 Å and U iso = 1.2 U eq .

Figure 1
Labelled thermal ellipsoid plot of (1) (50% probability surfaces). Hydrogen atoms have been rendered as spheres of arbitrary radius. Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'s 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 > 2σ(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.