A series of N-bridgehead tripodal receptors bearing amide functionality is reported which displays reversible binding of nitrate anions via solvent polarity controlled molecular capsule formation through a dynamic self-assembly process.

A series of neutral C(3)-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.

A nitrate-templated stereoselective photochemical [2+2] solid-state synthesis that is promoted by a supramolecular encapsulation/release of the anion template is reported. The complete reaction process with the same nitrate template can be recycled in a repetitive manner.

The halide functionality on N-bridged tripodal receptors has shown a distinct behavior on their self-assembly structures and binding ability toward HgCl(2) and ClO(4)(-) anions. The receptors containing fluoro, chloro, and iodo groups crystallized to form hemicarcerands in the solid state, whereas the receptor with a bromo group forms a molecular capsule via C-H...Br and C-H...pi interactions. The cavity of the molecular capsule is tunable and is capable of reversible encapsulating-releasing the guest molecules by pH modulation.