Conformational disorder in 4-(5,5'-dibromo-2'-chloro-4,4'-bipyridyl-2-yl)benzaldehyde: role of pi-pi and halogen interactions.

The crystal packing of the title compound, C(17)H(9)Br(2)ClN(2)O, is governed by strong pi-pi stacking, where molecules are tightly bound within infinite (100) planes; these planes interact mainly through non-optimal pi-pi stacking where arene rings are noticeably displaced from perfect overlap, and also through halogen-halogen interactions. The aldehyde group shows conformational disorder, with a significant population difference between the two conformers; this difference is rationalized by the energetic analysis of the crystal packing using the PIXEL method, which also allows a decomposition of intermolecular interaction energy into Coulombic, polarization, dispersion and repulsion contributions. Using such an analysis, it is found that the main reason for this unequal population of the two conformers in the crystal is two hydrogen bonds that are present only for the major conformer.