Molecular packing preferences in “bridge-flipped” isomeric aryl-2-pyridylhydrazones and 2-pyridinecarboxaldehyde arylhydrazones

Abstract

The aryl-2-pyridylhydrazones, 4-chlorobenzaldehyde 2-(2-pyridinyl)hydrazone (1NNC), 4-bromobenzaldehyde 2-(2-pyridinyl)hydrazone (2NNC), 4-cyanobenzaldehyde 2-(2-pyridinyl)hydrazone (3NNC), and 4-cyanobenzaldehyde 2-(5-bromo-2-pyridinyl)hydrazone (4NNC), are found to assume solid-state molecular packing arrangements different from those of their "bridge-flipped" isomers, the 2-pyridinecarboxaldehyde arylhydrazones, 2-pyridinecarboxaldehyde 2-(4-chlorophenyl)hydrazone (two polymorphs, 1CNNa and 1CNNb), 2-pyridinecarboxaldehyde 2-(4-bromophenyl)hydrazone (2CNN), 2-pyridinecarboxaldehyde 2-(4-cyanophenyl)hydrazone (3CNN), and 5-bromo-2-pyridinecarboxaldehyde 4-cyanophenylhydrazone monohydrate (4CNN).The isomers are structurally differentiated on the basis of preferred hydrogen bonding motif, with the "2-pyr-NNC" structures 1NNC, 2NNC, 3NNC, and 4NNC incorporating an R 2 2 (8) hydrogen-bonded dimer motif and the "2-pyr-CNN" structures 1CNNa, 1CNNb, 2CNN, and 3CNN incorporating a C(6) hydrogen-bonded chain motif. The hydrate 4CNN incorporates an R 4 4 ( 16) hydrogen bonding motif mediated by bridging water molecules. Examination of corresponding crystal structures retrieved from the Cambridge Structural Database shows that these preferences are general and predictable for these two types of hydrazone and that hydrates among these compounds occur notably less frequently than among their aroylhydrazone analogues. Examination of these structures also shows a strong correlation between the R 2 2 (8) motifs and crystallographic inversion centers as well as a strong correlation between the C(6) chains and screw axes extending parallel to the chain axes. These results may prove useful in structure prediction and crystal engineering efforts directed toward these compounds.

Bridge-Flipped Isomers: Searching for Isomorphous Benzylideneanilines and Phenylhydrazones Reveals Unexpected Structures

“…At the same time, bridge-flipped isomeric pairs that are not isomorphous may serve as seeds for the preparation of new polymorphs of their formerly non-isomorphous counterparts, providing yet another means for obtaining new solid materials. Isomorphism among bridge-flipped isomers is rare, commonly being prevented by differences in structural features such as preferred molecular conformations and favored intermolecular interactions that include hydrogen bonding and Lewis acid–base contacts. − On the other hand, several examples of isomorphous pairs are known. − Our search for isomorphous bridge-flipped isomers has employed the premise that similar intermolecular interactions occurring in the two isomers would promote their isomorphism. This strategy has been pursued successfully by previous workers who used strong hydrogen-bonding interactions between carboxylic acids and amides to obtain isomorphous crystals in a chloro/methyl exchange .…”