Selective ammonium nitrate recognition by a heteroditopic macrotricyclic ion-pair receptor

The heteroditopic macrotricyclic molecular receptor 1, which bears a tripodal anion binding domain and 4,10,16-triaza-18-crown-6 cation recognition domain, proves to be an effective ion-pair receptor. In the absence of the cobound cation (TBA(+) salts) receptor 1 preferably binds nitrate and nitrite over other anions, including basic anions such as acetate or dihydrogenphosphate. Ammonium cation binding by the 4,10,16-triaza-18-crown-6 subunit significantly enhances the strength of the nitrate and nitrite complexation at the triamide recognition site of the receptor. In the presence of ammonium cations, the association constants of nitrate binding reach an impressive value of 1050 M(-1) in highly polar DMSO-d6. Interestingly, the binding of other anions such as chloride and bromide is not enhanced in the presence of a cobound NH4(+) cation. The increased affinity of [1·NH4(+)]PF6(-) for anionic species is attributed to a strong cooperative effect that arises from the properly positioned binding sites in the receptor 1 cavity, thus allowing for the formation of the ion pair. Under liquid/liquid conditions, receptor 1 is able to extract NH4NO3 from an aqueous to an organic phase, as inferred from (1)H NMR spectroscopic and nitrite/nitrate colorimetric analyses.