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

Anion−π 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−π complexes in acetonitrile are in the range of 239 to 16950 M–1, following the order of 1·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−π and lone-pair electron−π interactions. The generality of anion−π interactions and diverse anion−π interaction motifs can provide a new dimension in the study of molecular recognition and self-assembly. Moreover, this study potentiates the effect of anion−π interactions in chemical and biological systems, especially those involving anion and electron-deficient aromatic species.