| Molecular receptors enhance extraction efficacy when incorporated into a receiving phase. Extraction selectivity is defined as the ratio of the relative concentrations of analyte and interfering species in each phase. Recognition-based extractions are highly selective if target/receptor interactions dominate the free energy change for the extraction process. As the receiving phase becomes a poorer solvent, selectivity increases. Noncovalent interactions between target and receptor are strengthened while the solubility of interfering compounds is decreased. Perfluorinated (fluorous) liquids are exceptionally non-polar, poor solvents that should increase the extraction selectivity by reducing the extraction of interfering species. Synthetic chemists have focused on fluorous biphasic separations of catalysts, reactants, and products based on fluorophilicity using covalently attached perfluoroalkyl tags. Until recently, noncovalent approaches had not been explored. This work is focused on developing a fundamental understanding of noncovalent interactions in fluorous environments. Quantification of the strength of such associations should have application reaching far beyond the synthetic community.;The application of perfluoropolyether carboxylic acid receptors to fluorous biphasic extractions of pyridine-like bases, including porphyrins, is explored. UV, IR, X-ray crystallography, and quantitative continuous variations plots leads to an understanding of complex formation in the fluorous phase. We quantified free energy of formation of the hydrogen bond between the receptor and pyridine (-39 kJ mol-1) in perfluorohexanes at room temperature. This is the first reported hydrogen bond strength in a fluorous environment.;The solvent environment affects complex formation significantly. We wondered if nonpolar fluorous liquids could support proton transfer. We show that the protonated pyridine is ‘solvated’ by carboxylic acids, making the local environment more polarizable than the bulk solvent. Investigations of complexation between N-heterocyclic bases and carboxylic acids in a range of solvents showed that proton transfer occurs in fluorous environments when both the acid/base ratio and the ΔpKα between the acid and conjugate acid of the base are sufficiently high. Remarkably, we have observed solvation of a hexavalent porphyrin cation in the fluorous phase. The fundamental understanding of molecular recognition in fluorous liquids should provide opportunities for development in sensing and separation applications. |
