On the Use of a Protic Ionic Liquid with a Novel Cation To Study Anion Basicity

The need for reliable means of ordering and quantifying the Lewis basicity of anions is discussed and the currently available methods are reviewed. Concluding that there is need for a simple impurity-insensitive tool, we have sought, and here describe, a new method using NMR spectroscopy of a weak base, a substituted urea, 1,3-dimethyl-2-imidazolidinone (DMI), as it is protonated by Brønsted acids of different strengths and characters. In all cases studied the product of protonation is a liquid (hence a protic ionic liquid). NMR spectroscopy detects changes in the electronic structure of the base upon interaction with the proton donors. As the proton-donating ability, that is, acidity, increases, there is a smooth but distinct transition from a hydrogen-bonded system (with no net proton transfer) to full ionicity. The liquid state of the samples and high concentration of nitrogen atoms, despite the very low natural abundance of its preferred NMR-active isotope (¹⁵N), make possible the acquisition of¹⁵N spectra in a relatively short time. These¹⁵N, along with¹³C, chemical shifts of the carbonyl atom, and their relative responses to protonation of the carbonyl oxygen, can be used as a means, sensitive to anion basicity and relatively insensitive to impurities, to sort anions in order of increasing hydrogen bond basicity. The order is found to be as follows: SbF₆⁻<BF₄⁻<NTf₂⁻>ClO₄⁻>FSO₃⁻<TfO⁻<HSO₄⁻<Cl⁻<MsO⁻.