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Two isomeric pairs of Schiff bases, N,N′-bis­(2-methoxy­benzyl­idene)-p-phenyl­enediamine, C22H20N2O2, (I), and 2,2′-dimeth­oxy-N,N-(p-phenyl­enedimethyl­ene)dianiline, C22H20N2O2, (II), and (E,E)-1,4-bis­(3-iodo­phen­yl)-2,3-diaza­buta-1,3-diene (alternative name: 3-iodo­benzaldehyde azine), C14H10I2N2, (III), and N,N′-bis­(3-iodo­phen­yl)ethylenedi­imine, C14H10I2N2 [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)°, 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)°, respectively. The crystal structures are distinguished from each other by different packing motifs involving the functional groups. Inter­molecular contacts between meth­oxy groups define an R22(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.

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Three polymorphs of 4,4′-diiodobenzalazine (systematic name: 4-iodo­benzaldehyde azine), C14H10I2N2, have crystallographically imposed inversion symmetry. 4-Chloro-4′-iodobenzalazine [systematic name: 1-(4-chloro­benzyl­idene)-2-(4-iodo­benzyl­idene)diazane], C14H10ClIN2, has a partially disordered pseudocentrosymmetric packing and is not isostructural with any of the polymorphs of 4,4′-diiodo­benzalazine. All structures pack utilizing halogen–halogen inter­actions; some also have weak π (benzene ring) inter­actions. A comparison with previously published methyl­phenyl­ketalazines (which differ by substitution of methyl for H at the azine C atoms) shows a fundamentally different geometry for these two classes, namely planar for the alazines and twisted for the ketalazines. Density functional theory calculations confirm that the difference is fundamental and not an artifact of packing forces.
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