Fluorinated phosphonium salts and ionic liquids prepared via thiol-ene click chemistry: a physical and thermal property study

Abstract

• Synthesis of 18 fluorinated phosphonium salts using thiol-ene click chemistry. • 14 of the salts are ionic liquids (ILs), 5 are liquids at room-temperature (RTILs) • 2 ILs show supercooling behavior, eventually solidifying over time. • Several ILs show interesting thermal behavior, possibly due to domain segregation. • X-ray crystal structure shows segregation between fluorous and non-fluorous domains. Using thiol-ene “click” chemistry, a fluorinated thiol was systematically investigated against a series of allyl-quaternized phosphonium substrates in the pursuit of cation-fluorinated ionic liquids with omniphobic (hydrophobic and oleophobic) properties for aerospace applications. A total of 18 new cation-fluorinated salts were prepared containing either the bromide (Br – ), bis(trifluoromethanesulfonyl)imide (Tf 2 N – ), hexafluorophosphate (PF 6 – ), or tetrafluoroborate (BF 4 – ) anions. Melting point analysis by visual inspection and differential scanning calorimetry (DSC) revealed that 14 of the salts are ionic liquids with melting points below 100 °C. Five are liquids at room temperature, although two exhibit supercooling behavior and eventually solidify over time. In addition to the standard characterization techniques used to verify compound identity, thermal behavior was studied via DSC and thermogravimetric analysis (TGA). Single-crystal X-ray diffraction was performed on the ionic liquid, triphenyl[3-(1 H ,1 H ,2 H ,2 H -perfluorodecylthio)propyl]phosphonium bis(trifluoromethanesulfonyl)imide. The crystal structure shows fluorous and non-fluorous domains, complementing the thermal behavior observed via DSC. Density, viscosity, surface tension, solubility, and water content were measured in an effort to explore the materials’ physical properties for future applications.