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

Abstract

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.