Modulating the binding of polycyclic aromatic hydrocarbons inside a hexacationic cage by anion-π interactions

Abstract

We report the template-directed synthesis of Blue-Cage6+, 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, BlueCage·6PF6 exhibits a lower association constant Ka than its progenitor ExCage·6PF6. A close inspection reveals that the six PF6- counterions of BlueCage6+ occupy the cavity in a fleeting manner as a consequence of anion-π interactions and, as a result, compete with the PAH guests. This conclusion is supported by a one order of magnitude increase in the Ka value for pyrene in BlueCage6+ when the PF6- counterions are replaced by much bulkier anions. The presence of anion-π interactions is supported by X-ray crystallography, and confirms the presence of a PF6- counterion inside its cavity.