Fluorous Effects in Amide-Based Receptors for Anions

Hybrid receptors designed to recognize both the sulfonate headgroup and the fluorous tail of perfluorooctanesulfonate (CF3(CF2)7SO3 −, “PFOS”) were prepared by coupling fluorinated carboxylic acids onto poly(aminomethyl)benzene scaffolds. Binding to PFOS, CF3SO3 −, p-TsO−, and Cl− was monitored by 1H NMR and isothermal titration calorimetry (ITC). In chloroform solvent, hydrogen-bonding to anions is accompanied by downfield shifts in the amide NH protons of the fluorinated receptors and by evolution of heat. Association constants for 1:1 complexation (K assoc) are >1000 M−1. An analogous hydrocarbon receptor binds weakly to anionic guests (K assoc < 50 M−1). Ab initio calculations indicate that the differences in 1:1 binding strengths between fluorous and nonfluorous hosts cannot be ascribed to differences in NH donor acidities.