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.