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.