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

The crystal structures of six anti‐β‐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‐methoxy‐phenylhydrazono)‐3‐oxobutanenitrile; (a5) (E)‐2‐(2‐acetylphenylhydrazono)‐3‐oxobutanenitrile; (a6) (E)‐2‐(2‐nitrophenylhydrazono)‐3‐oxobutanenitrile. All com­pounds contain the π‐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) Å, and compound (a6) undergoes the same interaction but even more weakened [N⋯O 3.150 (1) Å] 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 β‐ketohydrazone moiety [compounds (a4) and (a5)]. The critical factors that cause the switching between the dif‐ferent 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) dis‐plays a different packing arrangement, where the DD interaction is complemented by a non‐resonant hydro‐gen bond between a p‐acetyl phenyl substituent and the hydrazone N—H group [N⋯O 2.907 (2) Å]. 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.