Halide-Controlled Extending-Shrinking Motion of a Covalent Cage

Herein, we present an example of covalent cages, whose flexible framework undergoes extending-shrinking motion under halide control. In the absence of halide anions, the free cage assumes a flattened conformation: the cavity is compressed along the C₃ axis passing through the tertiary amines, and the two tribenzylamine platforms are eclipsed. Halide encapsulation promotes a large conformational rearrangement of the cage, involving an extension of the cavity along the C₃ axis and shrinkage along the equatorial plane. Interestingly, the rearrangement is accompanied by the pyramidal inversion of the tertiary amines and by the rotation of the tribenzylamine-based platforms, which become staggered. The imidazolium-containing arms wrap around the spherical anion, leading to a racemic mixture of the M and P helical complexes. As expected from the flexible structure of the cage, the switch between the two limit conformations can be repeated for several cycles under alternating chemical stimuli (AgNO₃/TBACl). This result is consistent with the low activation barriers determined by computational investigations. These also allowed us to quantify the energy difference between the shrunk and expanded cage conformations and to hypothesize an energetic pathway along which the conformational rearrangement can occur.