Anion dependent molecular recognition of cations

In DMF-d₇ tetrabenzimidazole cavitands 2 exist as monomeric species and vase-like conformers. Several possible arrangements of the four benzimidazole NHs are indicated by ¹H NMR spectroscopy. The cavitands form 1:1 inclusion complexes with tetraethyl ammonium and phosphonium cations only when strong hydrogen bonding anions like chloride or acetate are present. These complexes are stable on the NMR time scale at 295 K feature a C_2V-symmetrical arrangement of benzimidazole functions. The stability of the C_2V-symmetrical tetramethylammonium acetate complex is independent of the temperature. In contrast, tetramethylammonium and phosphonium chloride complexes exist at 233 K as several isomers. This complicated behavior is, in part, attributed to the hydrogen bonding interactions between the anions and the NH groups of benzimidazole functions.