Interplay of hydrogen bending and other molecular interactions in determining the crystal packing of a series of anti-β-ketoarylhydrazones

The crystal structures of six anti-beta-ketoarylhydrazones are reported: (a1) (E)-2-(4-cyanophenylhydrazono)-3-oxobutanenitrile; (a2) (E)-2-(4-methylphenylhydrazono)-3-oxobutanenitrile; (a3) (E)-2-(4-acetylphenylhydrazono)-3-oxobutanenitrile; (a4) (E)-2-(2-methoxyphenylhydrazono)-3-oxobutanenitrile; (a5) (E)-2-(2-acetylphenylhydrazono)-3-oxobutanenitrile; (a6) (E)-2-(2-nitrophenylhydrazono)-3-oxobutanenitrile. All compounds contain the pi-conjugated heterodienic group HN-N=C-C=O and could form, at least in principle, chains of intermolecular N-H ... O hydrogen bonds assisted by resonance (RAHB-inter). Compounds (a1) and (a2) form this kind of hydrogen bond though with rather long N ... O distances of 2.948(3) and 2.980 (2) Angstrom, and compound (a6) undergoes the same interaction but even more weakened [N ... O 3.150 (1) Angstrom] by the intramolecular bifurcation of the hydrogen bond donated by the N-H group. The intrinsic weakness of the intermolecular RAHB makes possible the setting up of alternative packing arrangements that are controlled by an antiparallel dipole-dipole (DD) interaction between two C=O groups of the beta-ketohydrazone moiety [compounds (a4) and (a5)]. The critical factors that cause the switching between the different packings turn out to be the presence of hydrogen bonding accepting substituents on the phenyl and, most frequently, the intramolecular N-H ... O bond with the O atom of the phenyl o-substituent. The crystal packing is widely determined by RAHB-inter (three cases) or DD (two cases) interactions. Only compound (a3) displays a different packing arrangement, where the DD interaction is complemented by a non-resonant hydrogen bond between a p-acetyl phenyl substituent and the hydrazone N-H group [N ... O 2.907(2) Angstrom]. Crystal densities range from 1.24 to 1.44 Mg m(-3) and are shown to increase with the number of intermolecular hydrogen bonds and other non-van der Waals interactions.