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

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 superset of DBA(+) or 1 superset of DPA(+)) and external complexes (1 center dot DBA(+) or 1 center dot DPA(+)), whereas binding with di(xylyl methylene)ammonium (DXA(+)) resulted in only an external analogue (1 center dot 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 superset of DXA(+) and 1 center dot DXA(+) are close in energy, steric interactions inhibit formation of 1 superset of DXA(+) and only 1 center dot 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 center dot 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.