Polarity of tetraalkylammonium-based ionic liquids and related low temperature molten salts

Abstract

Solvatochromic measurements and ¹H NMR spectroscopy have been used to investigate the overall polarity of tetraalkylammonium (R₃N⁺–CH₃ with R = n-C₄H₉; n-C₈H₁₇) based ionic liquids (IL) and low temperature molten salts (LTMS), respectively, as function of the counter anion. Solvent acidity (SA), solvent basicity (SB), polarizability (SP) and dipolarity (SdP) parameters have been determined according to the Catalán scale. [Fe^II(1,10-phenanthroline)₂(CN)₂] (Fe), 3-(4-amino-3-methylphenyl)-7-phenylbenzol[1,2-b:4,5-b′]difuran-2,6-dione (ABF), 3-(4-N,N-dimethylaminophenyl)-7-phenylbenzol[1,2-b:4,5-b′]-difuran-2,6-dione (DMe-ABF), 4-tert-butyl-2-(dicyanomethylene)-5-[4-(dimethylamino)benzylidene]-Δ³-thiazoline (Th), and 2-[4-(N,N-dimethylamino)benzylidene]malononitrile (BMN) have been used as solvatochromic polarity indicators. Significant correlations of the SB parameter of anion X⁻ as a function of the chemical shift of the H-atoms of R₃N⁺–CH₃X⁻ are presented and discussed. Thus, IL solvent basicity (SB) of the anion determines the interaction strength towards those CH-bonds in the direct vicinity of the ammonium nitrogen atom.