Computational study of ion-pair recognition by heteroditopic calix[4]diquinone derivative

The complexation characteristics of isophthalamide diether calix[4]diquinone macrobicycle (1) with ion-pairs (alkali metal or ammonium cation and halide anion) were investigated by quantum mechanical calculation methods. The total electronic and Gibbs free energies of the various (monotopic, heteroditopic, and contact motifs) complexes of alkali metal and ammonium cations and/or halide anions with 1 were analyzed and compared. The structures of the endo- or exo-complexes of the various cations and anions with the host (1) were optimized using the mPW1PW91/6-31G(d,p) method, followed by DFT B3LYP/6-31G(d,p) calculations. Although the cone conformer is less stable than the partial cone and 1,3-alternate conformers of the free host (1), the cone-type complexes are more stable than the partial cone and 1,3-alternate analogues. The contact-complexation efficiencies of the sodium-halide ion-pair inside the cavity of the lower rim of the cone-type hosts were better (∼20 kcal/mol) than those of the potassium ion-pair. These calculated outcomes agree with the experimental results of the anion-binding behavior of 1 · M+. The contact-complexation efficiencies of the ion-pairs inside the lower rim pocket of the host 1(cone) were much better (45–50 kcal/mol) than those of its heteroditopic-complexation with host 1. o-complexation of the cone isomer with the ion-pair could be clearly confirmed by differences in the diagnostic CO bands in the free host and the infrared spectra of its complexes.