Crystal structure of 4,4-dibromo-1-(3,4-dimethoxyphenyl)-2-azabuta-1,3-diene-1-carbonitrile

The substitution of the aryl group at trans position with respect to the N atom in the imine part of 4,4-dihalogeno-1,1-aryl-2-azabuta-1,3-dienes [Ar2C=N—C(H)=CX 2] by a CN group allows to get an almost perfectly planar molecule.


Chemical context
In the context of our interest in developing novel -conjugated dithioether compounds as ligands for coordination chemistry and further organic transformations, we have reported on the synthesis and crystal structure of 4,4-dichloro-1,1-diphenyl-2-azabuta-1,3-diene [Ph 2 C N-C(H) CCl 2 ] and its conversion to [Ph 2 C N-C(H) C(SR) 2 ] and [Ph 2 C N-C(H) C(OPh) 2 ] by reaction with thiolates NaSR or NaOPh, respectively (Jacquot et al., 1999(Jacquot et al., , 2000Jacquot-Rousseau et al., 2006;Kinghat et al., 2016). Several crystal structures of these molecules/ligands and their derived transition metal complexes reveal that despite the overall planarity of the -conjugated chain, one aryl group of the -N CPh 2 imine segment is tilted with respect to the azabutadienic array (Jacquot et al., 1999;Knorr et al., 2003;Kinghat et al., 2008). To circumvent this feature and to modulate the stereoelectronic properties, we examined other synthetic strategies for the synthesis of 2-azabutadienes. The reaction scheme for the synthesis of (1).
-aminonitrile H 2 NCHPhC N with chloral or bromal (Sato & Adachi, 1978), we reinvestigated this reaction to explore the scope for the synthesis of other derivatives. For example, we succeeded in preparing the title compound [C 6 H 3 (OMe) 2 -(C N)C N-C(H) CBr 2 ], (1), bearing two electrondonating methoxy groups at the meta-and para-positions of the aryl ring (see Fig. 1).

Supramolecular features
Each planar molecule of (1) is connected through halogen (Cavallo et al., 2016) bifurcated bonds C12-Br2Á Á Á(O1,O2) to two neighbouring molecules to form a one-dimensional ribbon. The ribbon is further connected through another kind of side halogen bond (C12-Br1Á Á ÁBr1-C12) to other neighbouring molecules with the formation of roughly planar one-dimensional double-wide straight chains ( Fig. 3   An displacement ellipsoid plot of (1) at the 50% probability level.
When projecting the structure down the direction perpendicular to the planes of the planar molecules of (1) (e.g. down from the top in Fig. 4), one sees an interesting overlap in a head-to-tail arrangement of zigzagging unsaturated chains that leads to the formation ofstacking interactions around the symmetry centres located at (0, 1 2 , 1 2 ) and ( 1 2 , 1 2 , 1 2 ). They consist of overlaps between the azadiene C C and C N double bonds and parts of the aryl rings. For clarity, these overlaps are shown separately in Figs. 6 and 7. The mean interatomic separation between the chains built around ( 1 2 , 1 2 , 1 2 ) ( Fig. 6 and Table 3) is 3.523 (5) Å , while a slightly shorter separation of 3.464 (5) Å is observed for the second couple built around (0, 1 2 , 1 2 ) ( Fig. 7 and Table 3).
and discussed this feature in the structures of [Ar 2 C N-C(S t Bu) C(H)S t Bu] (Kinghat et al., 2016).

Synthesis and crystallization
The required -aminonitrile used a starting material was obtained according a literature protocol (Mai & Patil, 1984). An equimolar mixture of N-(dibromoethylenyl)-1-imino-1vertracetonitrile (10 mmol) and tribromoacetaldehyde in 10 ml of acetonitrile was stirred under reflux for 2 h. The solution was then filtered and all volatiles removed under reduced pressure.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. All H atoms were placed in calculated positions and treated in a riding model. C-H distances were set at 0.95 (aromatic) and 0.98 Å (methyl), with U iso (H) = xU eq (C), where x = 1.5 for idealized methyl H atoms refined as rotating groups and 1.2 for all other H atoms.

Special details
Experimental. Absorption correction: SADABS-2014/4 (Bruker,2014) was used for absorption correction. wR2(int) was 0.0938 before and 0.0647 after correction. The Ratio of minimum to maximum transmission is 0.7197. The λ/2 correction factor is 0.00150. 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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )