How to control the acidity of 1,2,3-triazolium ions: A density functional theory study

An exploratory study for the acidity of 1,2,3-triazolium ions has been carried out by using the density functional theory (DFT). The prototype of 1,2,3-triazolium ion 2a is calculated to have a pKa of 24.0 ± 0.1. Substitution at N (1) or C (4) on the heterocycle by a tolyl group increases the acidity by about 0.3 pKa unit (pKa (3a) = 23.7 ± 0.1). Electron-withdrawing groups on the aromatic ring increases the acidity by approximately three pKa units (pKa (4b) = 20.9 ± 0.4). The 1,2,3-triazolium ions are less acidic than the corresponding 1,2,4-triazolium and imidazolium ions. The weaker acidity may be explained by the position of the carbene precursor C-H group, which is located between one nitrogen and one carbon atom in the heterocycle. This exploratory study shows that the acidity of the 1,2,3-triazolium ions can be modified by aromatic ring substitutions at N (1) or C (4) positions on the heterocycle, and further by the substituents on the aromatic rings.