Triazolylidenes have rapidly emerged as a powerful subclass of N-heterocyclic carbene ligands for transition metals. They are readily available through regioselective [2+3] cycloaddition of alkynes and azides and subsequent metallation according to procedures established for related carbenes. Due to their mesoionic character, triazolylidenes are stronger donors than Arduengo-type imidazol-2-ylidenes. Spurred by these attractive attributes and despite their only recent emergence, triazolylidenes have shown 10 major implications in catalysis. This feature article summarises the synthetic accessibility of triazolylidene metal complexes and their electronic and structural characteristics, and it compiles their applications, in particular, as catalyst precursors for various bond forming and redox reactions, as well as first approaches into photophysical and biochemical domains.

Transmetalation of a 1,4-diphenyl-substituted 1,2,3triazolylidene silver complex with an electrophilic metal center, e.g., Ru II , Ir III , or Rh III , induces spontaneous and chemoselective cyclometalation involving C−H bond activation of the N-bound phenyl group exclusively. Less electrophilic metals such as Ir I , Rh I , and Pt II yield a monodentate triazolylidene complex, while cyclometalation with borderline cases (Pd II ) or the activation of the C-bound phenyl ring requires acetate as a promoter.

A ruthenium cymene complex bearing a bidentate ligand composed of the N-mesoionic donor N-[1-methylpyridin-4(1H)-ylidene]-amide and the C-mesoionic donor 1,2,3-triazolylidene was prepared. Spectroscopic analyses including UV−vis, electrochemical, and NMR methods demonstrate that the pyridylideneamide ligand adapts to its environment and switches, depending on the solvent, between a formally anionic and a neutral donor. A mesoionic pyridinium-amidate structure predominates in polar solvents, whereas a neutral pyridylidene imine structure prevails in apolar solvents. The implications of these solventdependent electronic characteristics have been exploited in redox catalysis involving alcohol dehydrogenation and transfer dehydrogenation. The results indicate that the ligand resonance flexibility provides a new approach to enhance catalytic performance.

Iridium complexes containing a triazolylidene ligand with an appended methylpyridinium site undergo either aromatic C(sp2)–H bond activation or exocyclic C(sp3)–H bond activation of the N-bound methyl group. The selectivity of these bond activations is controlled by the remote substituent R of the triazolylidene ligand. Iterative computational and synthetic experiments provide evidence for more facile C(sp2)–H bond activation for a variety of remote substituents with R = Me, CH2C6F5, CH2CH2C6H5. For triazolylidene ligands with a benzylic substituent, C(sp2)–H bond activation of this benzylic group is the lowest energy pathway and is competitive with aromatic pyridinium C–H bond activation. The generated cyclometalated species is metastable and undergoes, via an oxidative addition/reductive elimination sequence, a transcyclometalation with exclusive activation of the methyl C–H bond and thus leads to the C(sp3)–H bond activated product. An experimental determination of activation energies as well as isomer ratios of the intermediates validates the computed pathways. The application of a transcyclometalation procedure to activate more challenging C(sp3)–H bonds is unprecedented and constitutes an attractive concept for devising catalytic processes.

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