Reversible Hydrosulfide (HS–) Binding Using Exclusively C–H Hydrogen-Bonding Interactions in Imidazolium Hosts

H2S is a physiologically important signaling molecule with complex roles in biology and exists primarily as HS– at physiological pH. Despite this anionic character, few investigations have focused on the molecular recognition and reversible binding of this important biological anion. Using a series of imidazole and imidazolium host molecules, we investigate the role of preorganization and charge on HS– binding. Using a macrocyclic bis-imidazolium receptor, we demonstrate the unexpected 2:1 host−guest binding of HS–, which was characterized both in solution and by X-ray crystallography. To the best of our knowledge, this is the first example of this binding stoichiometry for HS– binding. Moreover, the short C–H···S distances of 2.53, 2.54, 2.76, and 2.79 Å are well within the sum of the van der Waals radii of the interacting atoms, which is consistent with strong C–H···S interactions.