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 (Kass(Me4N+)=93 M−1 and Kass(Et4N+)=81 M−1) and a relatively low association constant for Pr4N+( Kass(Pr4N+)=25 M−1). In the solid state, for the Me4NCl complex, the resorcinarene assumes the C2v 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.