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

The crystal structures of six anti-beta-ketoarylhydrazones are reported: (a 1) (E)-2-(4-cyanophenylhydrazono)-3-oxobutanenitrile; (a2) (E)-2-(4-methylp henylhydrazono)-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-nit rophenylhydrazono)-3-oxobutanenitrile. All compounds contain the pi-conjuga ted heterodienic group HN-N=C-C=O and could form, at least in principle, ch ains of intermolecular N-H ... O hydrogen bonds assisted by resonance (RAHB -inter). Compounds (a1) and (a2) form this kind of hydrogen bond though wit h rather long N ... O distances of 2.948(3) and 2.980 (2) Angstrom, and com pound (a6) undergoes the same interaction but even more weakened [N ... O 3 .150 (1) Angstrom] by the intramolecular bifurcation of the hydrogen bond d onated 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 cr itical factors that cause the switching between the different packings turn out to be the presence of hydrogen bonding accepting substituents on the p henyl and, most frequently, the intramolecular N-H ... O bond with the O at om 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 com plemented by a non-resonant hydrogen bond between a p-acetyl phenyl substit uent and the hydrazone N-H group [N ... O 2.907(2) Angstrom]. Crystal densi ties range from 1.24 to 1.44 Mg m(-3) and are shown to increase with the nu mber of intermolecular hydrogen bonds and other non-van der Waals interacti ons.