Brønsted Basicities of N-Heterocyclic Imines and Their Derivatives in Dipolar Aprotic Solvents: A Computational Investigation

The Brønsted basicities (pK_aHs) of 49 imidazolium-, benzimidazolium-, and imidazolinium-based N-heterocyclic imines (NHIs) were calculated in two dipolar aprotic solvents, i.e., acetonitrile (AN) and dimethyl sulfoxide (DMSO), by a theoretical model for acidity/basicity calculation in solution, which was rigorously tested and verified through calibrations and comparisons. The established basicity scales for NHIs span a wide range of 19.6 pK units in both solvents. The influences of substituents on the basicities of these NHIs were investigated through basicity comparisons and rationalized by geometry analyses. In general, the basicity of NHIs is similar to those of commonly used nitrogen-containing organobases, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), but much weaker than those of their structural analogues N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) of similar backbones and substituents. There exists a good linear correlation between the basicity of NHI and that of NHC in solution. The basicities of exocyclic N-H-deprotonated NHIs (imidazolin-2-iminato compounds) in DMSO and AN were also calculated. The results showcase that the basicity of imidazolin-2-iminato compounds is about 30 pK units (∼10³⁰) stronger than that of their precursors. A comparison with several commonly used organobases indicates that imidazolin-2-iminato compounds are among the most basic organosuperbases in solution.