Anion-π Interactions: Generality, Binding Strength, and Structure

Anion-pi interactions have been systematically studied using tetraoxacalix[2]arene[2]triazine 1, an electron-deficient and cavity self-tunable macrocyclic host, as an electron-neutral molecular probe. As revealed by electrospray ionization mass spectrometry (ESI-MS), fluorescence titration and X-ray crystallography, tetraoxacalix[2]arene[2]triazine has been found to form 1:1 complexes with four typical polyatomic anions of different geometries and shapes in the gaseous phase, in solution, and in the solid state. The association constants for the formation of anion-pi complexes in acetonitrile are in the range of 239 to 16950 M-1, following the order of 1 center dot NO3- > 1.BF4- > 1.PF6- > 1.SCN-. X-ray molecular structures of the complexes showed that two opposing triazine rings of tetraoxacalix[2]arene[2]triazine act as a pair of tweezers to interact with the included anions through cooperative anion-pi and lone-pair electron-g interactions. The generality of anion-pi interactions and diverse anion-pi interaction motifs can provide a new dimension in the study of molecular recognition and self-assembly. Moreover, this study potentiates the effect of anion-pi interactions in chemical and biological systems, especially those involving anion and electron-deficient aromatic species.