A stimuli responsive system of self-assembled anion-binding Fe4L68+ cages

A new cationic Fe4L6 cage molecule was synthesised from 4,4′-diaminobiphenyl, 2-formylpyridine and iron(II). The cage exists as a system of interconverting diastereomers in solution. The system adapts to the addition of anionic guest molecules, expressing a new combination of diastereomers that synergistically bind the guest molecules. Not only do the cage diastereomers interconvert, the volume of the individual cages adapts physically through the rotation of bonds, providing a tailored binding pocket for the guest lined with hydrogen-bond donors. A model for the resulting complex network of species was developed that allowed the system to be fully described. The anion binding constants and the kinetics of both diastereomer interconversion and guest exchange were measured.