Power of a remote hydrogen bond donor: Anion recognition and structural consequences revealed by IR spectroscopy

Natural and synthetic anion receptors are extensively employed, but the structures of their bound complexes are difficult to determine in the liquid phase. Infrared spectroscopy is used in this work to characterize the solution structures of bound anion receptors for the first time, and surprisingly only two of three hydroxyl groups of the neutral aliphatic triols are found to directly interact with Cl^-. The binding constants of these triols with zero to three CF₃ groups were measured in a polar environment, and K_CD3_CN(Cl^-) = 1.1 × 10⁶ M^-1 for the tris(trifluoromethyl) derivative. This is a remarkably large value, and high selectivity with respect to interfering anions such as, Br^-, NO₃^- and NCS^- is also displayed. The effects of the third "noninteracting" hydroxyl groups on the structures and binding constants were also explored, and surprisingly they are as large or larger than the OH substituents that hydrogen bond to Cl^-. That is, a remote hydroxyl group can play a larger role in binding than two OH substituents that directly interact with an anionic center.