The Role of Hydrogen Bonding and Halogen Bonding in the Polymorphic Structures of 3,5-Dibromo-2,6-diaminopyridinium Bromide

An analysis is made of the supramolecular interactions in the two polymorphic structures of 3,5-dibromo-2,6-diaminopyridinium bromide, with particular emphasis on the roles of hydrogen and halogen bonding. Because of the extensive hydrogen bonding capabilities, hydrogen bonding interactions dominate in both phases. This causes a competition between C–Br···Br–C and C–Br···Br − halogen bonding interactions, with the former dominating in the orthorhombic Pbca phase and the latter in the monoclinic P2 1 /c phase. For Pbca phase a = 5.5197(11) Å, b = 15.040(3) Å, c = 23.2555(5) Å, and for P2 1 /c phase a = 8.1143(19) Å, b = 14.226(3) Å, c = 8.9005(18) Å, β = 113.18(2)°. Graphical Abstract The two polymorphic structures of 3,5-dibromo-2,6-diaminopyridinium bromide differ mainly in halogen–halogen bonding. In the more compact and more stable P2 1 /c phase the halogen–halogen interaction, C–Br···Br–C, is 0.36 Å longer than in the less compact Pbca phase. On the other hand the charge assisted halogen–halide interactions, C–Br···Br − , is dominant in the P2 1 /c phase while there are no such in-plane type contacts in the Pbca phase.