Cooperative Host/Guest Interactions via Counterion Assisted Chelation:  Pseudorotaxanes from Supramolecular Cryptands

Addition of di- or tritopic hydrogen bond accepting anions to solutions of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 and paraquat di(hexafluorophosphate) serves to enhance host/guest interaction. In particular, addition of Et₄N⁺CF₃COO^- effectively boosts K_a 14-fold, as estimated by ¹H NMR studies. Similar increases in apparent K_a values are observed upon addition of n-Bu₄N⁺OTs^-. Evidenced by crystal structures, the increased association results from chelation of the OH moieties of the crown by the di- or tritopic anions, forming supramolecular bicyclic macrocycles and stabilizing the complex in a cooperative manner.