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.

Two isomeric pairs of Schiff bases, N,N'-bis(2-methoxybenzylidene)-p-phenylenediamine, C(22)H(20)N(2)O(2), (I), and 2,2'-dimethoxy-N,N-(p-phenylenedimethylene)dianiline, C(22)H(20)N(2)O(2), (II), and (E,E)-1,4-bis(3-iodophenyl)-2,3-diazabuta-1,3-diene (alternative name: 3-iodobenzaldehyde azine), C(14)H(10)I(2)N(2), (III), and N,N'-bis(3-iodophenyl)ethylenediimine, C(14)H(10)I(2)N(2) [JAYFEV; Cho, Moore & Wilson (2005). Acta Cryst. E61, o3773-o3774], differ pairwise only in the orientation of their imino linkages and in all four individual cases occupy inversion centers in the crystal, yet all four compounds are found to assume unique packing arrangements. Compounds (I) and (II) differ substantially in molecular conformation, possessing angles between their ring planes of 12.10 (15) and 46.29 (9) degrees , respectively. Compound (III) and JAYFEV are similar to each other in conformation, with angles between their imino linkages and benzene rings of 11.57 (15) and 7.4 (3) degrees , respectively. The crystal structures are distinguished from each other by different packing motifs involving the functional groups. Intermolecular contacts between methoxy groups define an R(2)(2)(6) motif in (I) but a C(3) motif in (II). Intermolecular contacts are of the I...I type in (III), but they are of the N...I type in JAYFEV.

The non-isomorphous crystal structures of “bridge-flipped” isomers1–5are described and are compared to those of recently reported fluorinated benzylideneanilines.

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.