Matrix isolation of vapors from 1,2,4-triazolium salts: Exploring the generation of N–heterocyclic carbenes

The generation of the carbenic species 1,4-dimethyl-1,2,4-triazol-5-ylidene DMTr was explored via the sublimation of 1,4-dimethyl-1,2,4-triazolium iodide [DMTrH⁺][I⁻] and the vaporization of 1,4-dimethyl-1,2,4-triazolium trifluoroacetate [DMTrH⁺][TFA⁻]. Thermal stability of both salts was first assessed by thermogravimetric analysis. Vapors were then generated by heating the samples under vacuum and subsequently trapped in a low-temperature Ar matrix (15 K). In both cases, the matrix-isolation IR spectra differed from those of the pure salts, indicating thermal transformations. The outcomes of these transformations varied between the two compounds. For [DMTrH⁺][I⁻], no spectral indication of DMTr carbene was observed. Instead, 1-methyl- and 4-methyl-1,2,4-triazoles, along with CH₃I, were identified. For [DMTrH⁺][TFA⁻], trifluoroacetic acid HTFA was detected, suggesting the concurrent formation of DMTr carbene. These contrasting outcomes highlight the pivotal influence of the counterion basicity (iodide vs trifluoroacetate) in dictating the transformations of the triazolium salt. The less basic iodide attacks one of the methyl groups, leading to C–N bond cleavage and the formation of neutral triazole, whereas the more basic trifluoroacetate promotes proton transfer. Complementary DFT calculations of ion pair interactions and ab initio molecular dynamics simulations of the bulk [DMTrH⁺][TFA⁻] ionic liquid provided further structural insights and helped rationalize the distinct behavior of the two triazolium salts, as revealed by trapping their vapors in the cryogenic Ar matrix.