Probing the Reversible Binding of Anionic Reactive Sulfur and Nitrogen Species in Imidazolium Receptors with...

H₂S and NO are physiologically important signaling molecules with complex roles in biology and intermolecular crosstalk. Although these species are often referred to as neutral on paper, they are primarily found in anionic and/or oxidized forms in aerobic solutions as HS^- or NO₂^-/NO₃^-, respectively. Despite the prominence of these anions in biology, particularly HS^- and NO₂^-, few investigations have focused on the molecular recognition and reversible binding of these important species. Using a library of imidazolium receptors with C-H hydrogen bonding interactions, we investigate the influences on binding affinity through modulation of charge, multiplicity, and preorganization, while also investigating how anion volume impacts binding. These factors are probed by solution-state titration experiments and solid-state X-ray crystallographic data showing the specific molecular interactions involved in guest binding. Both solution-state NMR and solid-state X-ray crystallography support the importance and abundance of C-H···X^- interactions in facilitating guest binding as well as conformational changes upon anion recognition.