Recognition, Encapsulation, and Selective Fluorescence Sensing of Nitrate Anion by Neutral C3-Symmetric Tripodal Podands Bearing Amide Functionality

A series of neutral C3-symmetric acyclic artificial receptors incorporating amide functionality has been designed, synthesized, and fully characterized. Upon protonation, these conformationally flexible N-bridged tripodal podands 1–5 form in situ cone shape conformation through hydrogen bonding and C–H···π interactions. The protonation-induced interior preorganized cavity is capable of entrapping nitrate anions through the amide N–H bonds to form discrete nitrate complexes (1a–5a), which were fully characterized by NMR, HRESI mass spectra, and single crystal structures. By incorporating suitable fluorophores at each branch of the tripod receptor, the resulting fluorescent receptor 5 selectively recognized nitrate anions by fluorescent quenching in a DMSO solution and displayed one of the highest binding affinities for nitrate anions reported so far in polar media. Receptor 5 represents a unique example of a neutral receptor for the recognition of nitrate anions in polar solvent media by its zwitterionic form. The possible mechanism of proton-induced preorganization of these flexible, acyclic receptors in a convergent cone conformation followed by nitrate complexation has been proposed to rationalize the effective nitrate recognition.