Zwitterionic guanidinium compounds serve as electroneutral anion hosts

The complexation of anions in solvating phases by artificial receptors requires the total or at least partial replacement of their solvation shells by the mutual supramolecular interactions. Supplementing well-known approaches that rely on net electrostatic charges or extensive hydrogen bonding for guest complexation the present work focuses on the alternative concept of charge separation in the host to create a Cationic site suitable for strong interaction with negatively charged guests. An anionic moiety balancing the overall host charge to zero is incorporated in a way to prevent collapsing into an ion pair. Using bicyclic guanidinium anchor groups for direct guest binding and a closo-borane cluster as an anionic countercharge the ditopic zwitterionic host 17 was designed and prepared in a convergent synthetic scheme. Despite its electroneutrality 17 binds inorganic and organic oxoanions such as sulfate, oxalate, squarate and p-nitrophenyl phosphate with affinity constants reaching 3.1 x 10(4) M-1 in DMSO (squarate(2-)) or 1.1 x 10(5) M-1 in acetonitrile (p-nitrophenyl phosphate(2-)). Clean 1:1 host-guest stoichiometry is found in dilute solutions, whereas at higher concentrations different complex compositions and host dimerization are observed. Titration calorimetry reveals the major role of entropy in host-guest association. Tn essence, anion binding by 17 and its congeners in strongly solvating; solvents such as DMSO is favored by massive positive association entropies that in the case of sulfate complexation to 17 override an unfavorable positive binding enthalpy constituting an exclusively entropy-driven process.