Electron-deficient hemicryptophanes for the recognition of anions through anion-π interactions

The selective recognition and encapsulation of anions are crucial in many biological and environmental processes. Anion-π interactions, characterized by attractive non-covalent interactions between anions and electron-deficient aromatic surfaces, have gained significant attention for their potential in anion binding. However, due to the weak nature of these interactions, receptors that rely solely on anion-π interactions are scarce. To promote anion binding via electron-deficient surfaces, we constructed a cavity within the hemicryptophane framework using tetrafluoroxylylene arms and a benzene triimide unit, thereby displaying multiple electron-deficient surfaces. Binding studies revealed that the electron-deficient receptor exhibits significantly higher anion binding constants compared to a less electron-deficient hemicryptophane. For example, the affinity for chloride (K_a = 207) is ten times higher for the most electron-deficient hemicryptophane. More impressive is the unique affinity for triiodide (K_a = 416), representing the first example of triiodide recognition by a BTI-based receptor. The enhanced binding properties of this receptor result from the combination of different π-acidic surfaces in a single cage molecule.