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⊃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 ·PF 6 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.