Examination of a series of "bridge-flipped" halogen-and nitrile-substituted benzylideneanilines for isomorphism by single-crystal X-ray diffraction has resulted in the identification of three non-isomorphous pairs:Their crystal structures are rendered unique by differences in solid-state molecular conformation and intermolecular interactions. The crystal structures of N-(4-cyanobenzylidene)-2-bromoaniline (2Bran) and N-(4-cyanobenzylidene)-2-iodoaniline (2Ian) were determined, but upon attempts to prepare their bridge-flipped isomers the unexpected dianilines 1-(2-bromophenyl)-N,N′-bis(4-cyanophenyl)methanediamine (2Brdian) and 1-(2-iodophenyl)-N,N′-bis(4-cyanophenyl)methanediamine (2Idian) were obtained. The crystal structures of 2Brdian and 2Idian were found to be isomorphous, as were the crystal structures of the three pairs 2Clan/2Bran, 2Ian/2CH 3 an (N-(4-cyanobenzylidene)-2methylaniline), and 2CNan/XOXLOM (N-(4-methylbenzylidene)-2-cyanoaniline). Isomorphism involving iodine/methyl and nitrile/methyl exchanges is rare in the Cambridge Structural Database (CSD). The unexpected product meso-(E,E)-1,1′-[1,2bis(4-methylphenyl)ethane-1,2-diyl]-bis [(2-trifluoromethyl)phenyldiazene] (2CF 3 azo) was obtained in an attempt to prepare the phenylhydrazone, and its crystal structure is reported here. Only one other example of this type of compound currently appears in the CSD. Examples of isomorphous bridge-flipped isomers among benzylideneanilines and phenylhydrazones remain rare in spite of their conformational flexibility and steric similarity.

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.