Chemical Variants of the Dicyanamide Anion, and a Landscape for Basic and Superbasic Ionic Liquids

Abstract

We describe the properties of three new aprotic ionic liquids (ILs) with irreducible cations and anions which are close relatives of the commonly used −N(CN)2 anion, namely −N(CHO)2 (diformylamide) and −N(CO)2(CH2)2 (succinimide), having pK a values which are slightly more basic than the pK a of OH− ions of aqueous solution chemistry. In addition to using the well-known N-butyl-N-methyl pyrrolidinium cation, P14 +, we report the use of the tetrabutylammonium cation, +N[CH3(CH2)3]4, in forming an ionic liquid with the diformylamide anion. The formation and purity of these ILs was assessed by 1H NMR spectroscopy. The broadband dielectric spectra for all three ionic liquids were analyzed and the glass transition temperatures (T g) estimated from the dielectric data matched well with those obtained from differential scanning calorimetry. The results demonstrate that pure aprotic ILs with high-pK a equivalent anions can be obtained by metathesis reactions despite their low pK a difference of 2 to 3. By contrast, protic analogues of these cation-anion pairs would fail to produce ILs, for which larger pK a differences are required. These new aprotic ILs are glass-forming with T g’s near 210 K and show DC-conductivities near 0.1 mS cm−1 at ambient temperatures.