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 R²₂(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 R⁴₄(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 R²₂(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.