Selective Recognition of Quaternary Ammonium Ions by Structurally Flexible Cages†

The binding behaviors of three structurally similar and flexible cages to a series of quaternary ammonium ions have been systematically studied. The cage with phenyl sidewalls is new; moreover, its structure and conformational states have been characterized by NMR, 2D NMR, and X-ray crystallography. The results reveal that the three cages show quite high binding affinities and selectivities to different guests. Enthalpy-entropy compensation was invoked to explain the different binding performance. Moreover, a six-component, high-fidelity self-sorting system was successfully constructed by using these cages and three guests. This suggests that conformationally flexible hosts may also achieve high binding affinities and selectivities, which is in contrast to the traditional point of view — only preorganized and rigid hosts are able to do so. This research advocates that conformational flexibility should also be seriously considered in host design.