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.

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.

To examine the roles of competing intermolecular interactions in differentiating the molecular packing arrangements of some isomeric phenylhydrazones from each other, the crystal structures of five nitrile-halogen substituted phenylhydrazones and two nitro-halogen substituted phenylhydrazones have been determined and are described here: (E)-4-cyanobenzaldehyde 4-chlorophenylhydrazone, C(14)H(10)ClN(3), (Ia); (E)-4-cyanobenzaldehyde 4-bromophenylhydrazone, C(14)H(10)BrN(3), (Ib); (E)-4-cyanobenzaldehyde 4-iodophenylhydrazone, C(14)H(10)IN(3), (Ic); (E)-4-bromobenzaldehyde 4-cyanophenylhydrazone, C(14)H(10)BrN(3), (IIb); (E)-4-iodobenzaldehyde 4-cyanophenylhydrazone, C(14)H(10)IN(3), (IIc); (E)-4-chlorobenzaldehyde 4-nitrophenylhydrazone, C(13)H(10)ClN(3)O(2), (III); and (E)-4-nitrobenzaldehyde 4-chlorophenylhydrazone, C(13)H(10)ClN(3)O(2), (IV). Both (Ia) and (Ib) are disordered (less than 7% of the molecules have the minor orientation in each structure). Pairs (Ia)/(Ib) and (IIb)/(IIc), related by a halogen exchange, are isomorphous, but none of the 'bridge-flipped' isomeric pairs, viz. (Ib)/(IIb), (Ic)/(IIc) or (III)/(IV), is isomorphous. In the nitrile-halogen structures (Ia)-(Ic) and (IIb)-(IIc), only the bridge N-H group and not the bridge C-H group acts as a hydrogen-bond donor to the nitrile group, but in the nitro-halogen structures (III) (with Z' = 2) and (IV), both the bridge N-H group and the bridge C-H group interact with the nitro group as hydrogen-bond donors, albeit via different motifs. The occurrence here of the bridge C-H contact with a hydrogen-bond acceptor suggests the possibility that other pairs of `bridge-flipped' isomeric phenylhydrazones may prove to be isomorphous, regardless of the change from isomer to isomer in the position of the N-H group within the bridge.

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