Cation–pi interactions in neutral calix[4]resorcinarenes

The interaction of the neutral form of resorcin[4]arene 1 (R=i-Bu) with tetraalkylammonium cations (Me₄N⁺, Et₄N⁺, Pr₄N⁺and Bu₄N⁺) was analyzed in the solid state and in methanol solutions. In methanol, NMR titration results indicate almost no differentiation between Me₄N⁺ and Et₄N⁺ cations (K_ass(Me₄N⁺)=93 M⁻¹ and K_ass(Et₄N⁺)=81 M⁻¹) and a relatively low association constant for Pr₄N⁺( K_ass(Pr₄N⁺)=25 M⁻¹). In the solid state, for the Me₄NCl complex, the resorcinarene assumes the C_2v boat conformation and is surrounded by as many as four Me₄NCl moieties. The structure of the Et₄NCl complex indicates that ethyl group can be conveniently placed in the resocin[4]arene cavity. The larger Bu₄N⁺ cation was found completely external to the cavity.