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The crystal structures of the  ortho -X-benzyl derivatives, where X=F, Cl, Br, I, and Me, of 2,5-bis( N , N -dibenzylamino)-3,6-dicyanopyrazine dyes (C 34 H 24 N 6 X 4 ) were analysed to evaluate the effect of a systematic series of structures on the occurrence of polymorphism. Detailed crystal structure analysis indicated that the thermally stable forms of the polymorphic derivatives (Cl and Br derivatives) were close-packed, whereas those of the non-polymorphic derivatives (F and I derivatives) were stabilised by an intermolecular interaction involving the  ortho -substituents. In the thermally metastable forms of the polymorphic derivative, halogen-halogen and halogen-nitrogen interactions contributed to the stabilisation of these crystals in the same way as the thermally stable form of the non-polymorphic derivatives. This indicated that the ease of polymorph occurrence would require an appropriate balance between the crystal energy of the close-packed structure and that of the crystal structure generated mainly by the electrostatic interactions involving the halogens in these halogenated pyrazine derivatives. In addition, the similar tendency of the occurrence of polymorphs in these halogenated pyrazine derivatives was found in 19 sets of halogenated compounds having known crystal structures of F, Cl, Br and I derivatives including at least one polymorphic derivative in the crystal structure database.

Role of halogen substituents in a series of polymorphic 2,5-diamino-3,6-dicyanopyrazine derivatives with highly flexible groups