Supramolecular umpolung: Converting electron-rich resorcin[4]arenes into potent CH-bonding anion receptors and transporters

Abstract

CH hydrogen bonding receptors constitute an emerging class of anion sensors and transporters, which, owing to their high proteolytic and pH resistance, are promising for biological applications. CH groups, however, are considered as weak hydrogen bond donors, requiring activation by strongly electron-withdrawing substituents. Accordingly, electron-rich aromatic rings have never been used for this purpose thus far. In this work, we demonstrate that resorcin[4]arenes, well known as π-electron-rich cation receptors, can be converted into CH-bonding anion receptors by simple substitution with four nitro groups. Owing to their unique geometric features, tetranitroresorcin[4]arenes acquire large dipole moments (up to 15.8 D) and highly positive electrostatic potentials at the lower rim, leading to remarkable anion affinities (Ka(Cl−) = 1.36 × 105 M−1 in THF). Moreover, we show also that the seemingly innocent alkyl chains surrounding the anion binding site enable high anion transport activities (EC50 down to 0.012 mol%) and resistance to competitive aqueous/organic media.