An Extremely Electron Poor Cationic Triazoliumylidene N‑Heterocyclic Carbene: Experimental and Computational Studies

The electronic properties of the long-known cationic 1,2,4-triazoliumylidene 3a have been determined. The 77Se NMR chemical shift of its Se adduct 9 (δ = 138 ppm) indicates that 3a is only moderately π-acidic. M­(CO)2Cl complexes of 3a (M = Rh, Ir) allowed the IR spectroscopic determination of a TEP value of 2073 cm–1, the highest value known to date for a N-heterocyclic carbene (NHC). The properties of cationic 3a were compared to those of the related neutral triazolylidene 5, which was prepared for comparison. Density functional theory calculations support the experimental findings. Overall, the cationic carbene 3a has to be considered a very poor σ-donor. Nevertheless, 3a is able to form di- (19) and even tricationic bis-NHC complexes (20 and 21).