Zwitterionic Guanidinium Compounds Serve as Electroneutral Anion Hosts

Abstract

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 × 104 M-1 in DMS...