Selective Recognition of Quaternary Ammonium Cations

Selective recognition of ammonium cations fundamentally relies on their degree of substitution. In biological systems, proteins can preferentially bind more substituted ammonium cations over less substituted homologues. To date, a general methodology to mimic this behavior remains elusive as synthetic hosts principally observe the inverse order (i.e., 1°>2°>3°>4°). Here we show that, through combining supramolecular recognition with solid-phase abstraction, we can overturn the canonical order of synthetic receptor selectivity across a diverse range of ammonium cation scaffolds. Quaternary ammonium cations access a lower energy solid-state than tertiary counterparts through multipoint binding to an adaptive array of isostructural BINOL·counterion networks. The preferential abstraction of quaternary ammonium cations from mixtures of homologous cations proceeds under thermodynamic control with excellent selectivity and remains operative even under aqueous conditions.