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)(7)SO3-, "PFOS") were prepared by coupling fluorinated carboxylic acids onto poly(aminomethyl)benzene scaffolds. Binding to PFOS, CF3SO3-, p-TsO-, and Cl- was monitored by H-1 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.