Three structures containing (E)-1,2-bis(benzimidazol-2-yl)ethene groups.

Two of the title compounds, namely (E)-1,2-bis(1-methylbenzimidazol-2-yl)ethene, C18H16N4, (Ib), and (E)-1,2-bis(1-ethylbenzimidazol-2-yl)ethene, C20H20N4, (Ic), consist of centrosymmetric trans-bis(1-alkylbenzimidazol-2-yl)ethene molecules, while 3-ethyl-2-[(E)-2-(1-ethylbenzimidazol-2-yl)ethenyl]benzimidazol-1-ium perchlorate, C20H21N4+.ClO4-, (II), contains the monoprotonated analogue of compound (Ic). In the three structures, the benzimidazole and benzimidazolium moieties are essentially planar; the geometric parameters for the ethene linkages and their bonds to the aromatic groups are consistent with double and single bonds, respectively, implying little, if any, conjugation of the central C=C bonds with the nitrogen-containing rings. The C-N bond lengths in the N=C-N part of the benzimidazole groups differ and are consistent with localized imine C=N and amine C-N linkages in (Ib) and (Ic); in contrast, the corresponding distances in the benzimidazolium cation are equal in (II), consistent with electron delocalization resulting from protonation of the amine N atom. Crystals of (Ib) and (Ic) contain columns of parallel molecules, which are linked by edge-over-edge C-H...pi overlap. The columns are linked to one another by C-H...pi interactions and, in the case of (Ib), C-H...N hydrogen bonds. Crystals of (II) contain layers of monocations linked by pi-pi interactions and separated by both perchlorate anions and the protruding ethyl groups; the cations and anions are linked by N-H...O hydrogen bonds.