Partially Sulfated Pillar[5]arenes: Synthesis and Molecular Recognition Properties

We report the synthesis and characterization of sulfated pillar[5]arene hosts (P5S2 – P5S10) that differ in the number of sulfate substituents. All five P5Sn hosts display high solubility in water (73 – 131 mM) and do not undergo significant self‐association according to 1H NMR dilution experiments. The x‐ray crystal structures of P5S6, P5S6•Me6HDA, P5S8•Me6HDA, and P5S10•Me6HDA reveal one intracavity molecule of Me6HDA and several external molecules of Me6HDA which form a network of close methonium•••sulfate interactions. The thermodynamic parameters of complexation between P5Sn and the panel of guests was measured by direct or competitive isothermal titration calorimetry. We find that the binding free energy toward a guest becomes more negative as the number of sulfate substituents increase. Conversely, the binding free energy of a specific sulfated pillar[5]arene toward a homologous series of guests becomes more negative as the number of NMe groups increases. The ability to tune the host•guest affinity by changing the number of sulfate substituents will be valuable in supramolecular polymers, separation materials, and latching applications.