Influence of the Flexibility of the Diimidazolium Cations on Their Organization into Crystalline Materials

Diimidazolium cations self-assemble into molecular networks by electrostatic interactions and hydrogen bonds. To explore the influence of the molecular flexibility on the crystalline networks, three alkylene spacers [-(CH2)(n)-, with n = 1, 2, or 3] have been investigated. Semiempirical PM6 methods were used to predict the most stable geometries of the dications and the formation of the H-bonds network. PM6 calculations provided a qualitative agreement with the obtained crystalline structures: The increase of the flexibility results in a more complex H-bond network. Surprisingly, a transition in the crystal organization occurs when n = 2. This transition represents a different organization from the classical packing of imidazolium salts (where the structure is hold together by ionic bonds and H-bonds), toward the formation of a well-defined supramolecular inclusion of bromide anions in the network only on one direction. For n = 3, novel architectures are formed by complementary pi-stacking interactions that connect the diirnidazolium cations together and by the inclusion of water molecules into the network.