Emergent Ion-Gated Binding of Cationic Host–Guest Complexes within Cationic M12L24 Molecular Flasks

“Molecular flasks” are well-defined supra­molecular cages that can encapsulate one or more molecular guests within their cavities and, in so doing, change the physical properties and reactivities of the guests. Although molecular flasks are powerful tools for manipulating matter on the nano­scale, most of them are limited in their scope because of size restrictions. Recently, however, increasingly large and diverse supra­molecular cages have become available with enough space in their cavities for larger chemical systems such as polymers, nano­particles, and bio­molecules. Here we report how a class of metallo­supra­molecular cages known as M12L24 polyhedra have been adapted to serve as nano­meter-scale containers for solutions of a pseudo­rotaxane host–​guest complex based on a tetra­cationic cyclo­phane host, cyclo­bis­(paraquat-p-phenylene) (CBPQT4+), and a 1,5-dioxy­naphthalene (DNP) guest. Remarkably, the hierarchical integration of pseudo­rotaxanes and M12L24 super­hosts causes the system to express stimulus-responsive behavior, a property which can be described as emergent because neither the DNP⊂​CBPQT4+ nor the M12L24 assemblies exhibit this behavior independently. The DNP-containing M12L24 molecular flasks are effectively “sealed off” to CBPQT4+ until ions are added as a stimulus to “open” them. The electrolyte stimulus reduces the electro­static screening distance in solution, allowing favorable DNP⊂​CBPQT4+ host–​guest inter­actions to overcome repulsive Coulombic inter­actions between the cationic M12L24 cages and CBPQT4+ rings. This unusual example of ion-gated transport into chemical nano­containers is reminiscent of trans­membrane ion channels which act as gates to the cell, with the important difference that this system is reversible and operates at equilibrium.