Anion influences on reactivity and NMR spectroscopic features of NHC precursors.

A series of 16 benzimidazolium salts of the type Pr-bimyHX with various anions X were synthesized and characterized by various spectroscopic and spectrometric methods. Significant anion and solvent effects on the chemical shifts of the C2-H protons were found, which allows for a ranking of the anions in terms of their hydrogen-bond acceptor properties. Stronger acceptors could increase the acidity of their respective salts leading to a faster H/D exchange. Similar but less pronounced anion influences were detected for the C NMR resonances, while coupling constants appear to be anion and solvent independent.