Anion–π interactions in adducts of anionic guests with octahydroxy-pyridine[4]arene: theoretical and experimental elucidation

Abstract

A theoretical investigation has been carried out at the DFT (B3LYP, RI-BP86, RI-B97-D) and RI-MP2 levels of approximation for octahydroxy-pyridine[4]arene and a series of its adducts with anions (BF₄⁻, PF₆⁻, NO₃⁻, CH₃COO⁻ and CF₃COO⁻) and ammonium cations (NH₄⁺). The encapsulated complexes (two hosts to one guest molecule) are predicted to be more stable in the gas phase than the corresponding 1 ∶ 1 adducts. Calculations using Grimme's B97-D functional have provided relative energies, which agree well with those derived at the RI-MP2 level of theory. By taking solvent effects into account using the COSMO routine, the formation of the anionic adducts is significantly less favourable. Complexes of octahydroxy-pyridine[4]arene with the ammonium cation seem to be unstable in solution. The most promising complexes of octahydroxy-pyridine[4]arene with BF₄⁻ and PF₆⁻ have been studied experimentally. Only formation of 1 ∶ 1 anionic adducts has been detected using electrospray ionization mass spectrometry. Probably, in the 1 ∶ 1 complexes the coordinated anion still interacts with a counterion. The excess of anionic guests used in the experiment and the conditions in the ESI source of the mass spectrometer have been considered another possible reason for shifting the equilibrium towards the 1 ∶ 1 aggregates.