Diverse binding of cationic guests by highly substituted [3 + 3] Schiff-base macrocycles

Abstract

We report the novel host–guest chemistry of highly substituted [3 + 3] Schiff-base macrocycle 1 with secondary ammonium-based guest molecules. Complexation of host 1 with dibenzylammonium (DBA⁺) or dipropylammonium (DPA⁺) led to the formation of both internal (1⊃DBA⁺ or 1⊃DPA⁺) and external complexes (1·DBA⁺ or 1·DPA⁺), whereas binding with di(xylyl methylene)ammonium (DXA⁺) resulted in only an external analogue (1·DXA⁺). For 1 + DPA⁺, the internal complex is more stable than the external complex, whereas in 1+DBA⁺, the internal and external complexes are very close in energy and interchange very slowly, allowing both to be observed at room temperature. Although 1⊃DXA⁺ and 1·DXA⁺ are close in energy, steric interactions inhibit formation of 1⊃DXA⁺ and only 1·DXA⁺ can be detected at room temperature. Based on computational analysis, the complexes all exist in a mixed tautomeric state with two keto-enamine and four enol–imine functionalities. That is, introduction of the ammonium-based guests results in a tautomeric change within the macrocyclic host, where the enol–imine form is converted to the keto-enamine. Finally, it was also shown that when DBA·PF₆ is used in the synthesis of 1, higher yields are obtained than when no additive is used, indicating the potential for ammonium-cation templation of Schiff-base macrocycles.