Exploring the Electronic Properties of Cyclic Five-Membered 1,2,3-Triazolium Ions: A Quantum Chemical Study

Cyclic nitrenium ions (CNIs) are generally stable and have found applications in chemistry, including biochemistry. Cyclic five-membered 1,2,3-triazolium ions (CFTIs) are a special class of CNIs; such moieties are found in a few medicinally important species. Though they are isoelectronic to N-heterocyclic carbenes (NHCs), they exhibit electrophilic properties (unlike nucleophilic NHCs). A few quantum chemical studies were reported, but extensive study on CFTIs is required to understand their electronic features and design species with tailored electronic properties. In this work, a thorough quantum chemical analysis has been carried out on the standard CFTI (1,3-dimethyl-1,2,3-triazolium ion). Hydride ion affinity and fluoride ion affinity parameters were estimated to evaluate the Lewis acidic character of the CFTIs. Also, the electrophilicity of these species has been estimated in terms of global electrophilicity index (ω) values. The complexation energies of CFTIs with PMe₃ (Δ<i>H</i>_PMe₃) and 1,3-dimethylimidazol-2-ylidene (Δ<i>H</i>_NHC) have also been estimated. The modulation of the electronic properties of CFTIs as a function of substitution and ring fusion has been evaluated. The correlation between Δ<i>H</i>_PMe₃ and Δ<i>H</i>_NHC is very high (<i>R</i> = 0.99); this correlation helps in identifying CFTIs that may exhibit high Lewis acidic character.