N,N′-Dihexylbenzimidazolium salts. Anion-controlled bilayer structures via π–π dimer or C–H⋯π catemer motifs

Abstract

A series of N,N′-dihexylbenzimidazolium salts with three anions, Br⁻, I⁻ and PF₆⁻, have been synthesized and characterized by single-crystal X-ray diffraction. These salts exhibit some anion-independent structural properties, which include the U-shaped conformation, the interdigitated bilayer packing and the thickness of the ionic layer. These salts also show anion-controlled inter-headgroup interactions; the benzimidazolium headgroups form a dimer motif via π–π interaction when the anions are Br⁻ and I⁻, however, a 1D catemer motif is formed via C–H⋯π interactions when the anion is PF₆⁻. The influence of the anion sizes on the arrangement of the benzimidazolium headgroups and the alkyl chains is discussed, with particular reference to the areas of the four adjacent headgroups and the degree of the alkyl chain tilting angles