Modulating the Binding of Polycyclic Aromatic Hydrocarbons Inside a Hexacationic Cage by Anion-π Interactions

We report the template-directed synthesis of BlueCage(6+), a macrobicyclic cyclophane composed of six pyridinium rings fused with two central triazines and bridged by three paraxylylene units. These moieties endow the cage with a remarkably electron-poor cavity, which makes it a powerful receptor for polycyclic aromatic hydrocarbons (PAHs). Upon forming a 1:1 complex with pyrene in acetonitrile, however, BlueCage6PF(6) exhibits a lower association constant K-a than its progenitor ExCage6PF(6). A close inspection reveals that the six PF6- counterions of BlueCage(6+) occupy the cavity in a fleeting manner as a consequence of anion- interactions and, as a result, compete with the PAH guests. This conclusion is supported by a one order of magnitude increase in the K-a value for pyrene in BlueCage(6+) when the PF6- counterions are replaced by much bulkier anions. The presence of anion- interactions is supported by X-ray crystallography, and confirms the presence of a PF6- counterion inside its cavity.