Bridge-flipped' isomers may be defined as pairs of molecules related by a reversal of a bridge of atoms connecting two major parts of the individual molecules. This kind of isomerism is commonly found among benzylideneanilines and phenylhydrazones. Isostructural pairs might be suitable for co-crystallization and are thus useful in the preparation of new solid materials. Although most of the examples of bridge-flipped isomeric benzylideneanilines and phenylhydrazones in the crystallographic literature are not isostructural, a small number of isostructural pairs have been reported by previous workers. This paper describes the molecular and crystal structures of four pairs of bridge-flipped isomers: two isostructural phenylhydrazones, (E)-2-bromobenzaldehyde 4-cyanophenylhydrazone (I) and (E)-4-cyanobenzaldehyde 2-bromophenylhydrazone (II); two pairs of isostructural benzylideneanilines, N-(2-trifluoromethylbenzylidene)-2-methylaniline (III) and N-(2-methylbenzylidene)-2-trifluoromethylaniline (IV), and N-(2-bromobenzylidene)-2-methylaniline (V) and N-(2-methylbenzylidene)-2-bromoaniline (VI); and a pair of benzylideneanilines with closely similar unit-cell dimensions but different packing arrangements, N-(4-methylbenzylidene)-4-cyanoaniline (VII) and N-(4-cyanobenzylidene)-4-methylaniline (VIII). The structure of (V) is disordered. The packing arrangement of (VIII) resembles that of the chloro-/methyl-substituted benzylideneanilines MBZCLA/MBZCLB [N-(4-methylbenzylidene)-4-chloroaniline and N-(4-chlorobenzylidene)-4-methylaniline]. Although intermolecular hydrogen bonding plays a part in the isostructuralism of the two phenylhydrazones, the other examples of isostructuralism occur in the absence of similar, relatively strong intermolecular interactions.

Comparison of the crystal structures of compounds we have designated "bridge-flipped isomers," which on the molecular level differ only in the orientation of a bridge of atoms connecting two larger parts of the molecule, offers a useful context for the examination and evaluation of intermolecular interactions and their robustness as supramolecular synthons. Intermolecular interactions in the crystal structures of five pairs of bridge-flipped isomeric benzylideneanilines are examined here, including interactions between nitrile groups and ring hydrogen atoms, between halogen atoms, and between nitrile groups and halogen atoms. Of these, only the halogen-nitrile interaction in which the halogen is iodine is present in both bridge-flipped isomers of the pair, although its influence on the molecular packing arrangement is insufficient to compel the two isomers to assume identical packing arrangements. Pairwise analysis of crystal structures of bridge-flipped isomers thus provides a perspective beyond that provided by the isolated analysis of the structures of the individual compounds.

The graph set assignment for "Motif 3" should be corrected from R 2 2 (10) to R 4 2 (10) in the caption to Scheme 5 and in the text where this motif is described.