Complementary interpretation of ET(30) polarity parameters of ionic liquids

Abstract

Reichardt's empirical E_T(30) polarity parameter has been established as appropriate polarity scale for ionic liquids. In this study, the relationships of E_T(30) of ionic liquids with the empirical Kamlet–Taft polarity parameters α (hydrogen bond donating ability), β (hydrogen bond accepting ability) and π* (dipolarity/polarizability) as well as Catalán's parameter set SA (solvent acidity), SB (solvent basicity), SP (solvent polarizability) and SdP (solvent dipolarity) are examined by means of multiple square correlation analyses. Several subtasks were carried out to address this main concern. First, the influence of anion structure on E_T(30) polarity parameters for various ionic liquids are investigated by use of nine differently substituted pyridinio phenolate betaine dyes of the Reichardt type. It is assumed that halide anions can have an effect on the E_T(30) parameter. In the second part, the Kamlet–Taft π* parameters have been independently determined for several protic ionic liquids using 4-tert-butyl-2-(dicyanomethylene)-5-[4-(diethylamino)-benzylidene]-Δ3-thiazoline (Th) and N,N-diethyl-4-nitroaniline (DENA) to show the impact of the hydrogen bond donating ability of the IL on the actual π* values as function of probe. α and SA values have been measured using the dicyano-bis(1,10-phenanthroline) iron(II) complex (Fe) as HBD probe. Finally, the newly determined Reichardt E_T(30), Kamlet–Taft and Catalán parameters of ionic liquids were used in addition to literature data to prove correlations of E_T(30) with α and π* as well as of E_T(30) with SA and SdP. Linear correlations of SdP with the molar concentration of the ionic liquid are highlighted.