Until recently, the availability of neutral carbon-based κ1C ligands was limited to carbon monoxide, isocyanides, and carbenes. Compared to phosphorus-based ligands, carbenes tend to bind more strongly to metal centers, avoiding the necessity for the use of excess ligand in catalytic reactions. The corresponding complexes are often less sensitive to air and moisture, and are remarkably resistant to oxidation.[1] As the robustness of carbene complexes is largely due to the presence of strong carbon–metal bonds, other types of carbon-based ligands are highly desirable. It is noteworthy that, although complexes between a carbene and a transition metal have been known for a long time,[2] the recent developments in their application in catalysis[3] have been greatly facilitated by the availability of carbenes that are stable enough to be bottled.[4,5] Moreover, carbenes, especially imidazol-2-ylidenes I[4c] and 1,2,4-triazol-5-ylidenes II,[4e] are also excellent organocatalysts (Scheme 1).[6]

The formal cycloaddition between 1,3-diaza-2-azoniaallene salts and alkynes or alkyne equivalents provides an efficient synthesis of 1,3-diaryl-1H-1,2,3-triazolium salts, the direct precursors of 1,2,3-triazol-5-ylidenes. These N,N-diarylated mesoionic carbenes (MICs) exhibit enhanced stability in comparison to their alkylated counterparts. Experimental and computational results confirm that these MICs act as strongly electron-donating ligands. Their increased stability allows for the preparation of ruthenium olefin metathesis catalysts that are efficient in both ring-opening and ring-closing reactions.

Conspectus Classical carbenes are usually described as neutral compounds featuring a divalent carbon with only six electrons in their valence shell. It was only in 1988 that our group prepared the first isolable example, in which the carbene center was stabilized by a push–pull effect, using a phosphino and a silyl substituent. In the last 30 years, a myriad of acyclic and cyclic push–pull and push–push carbenes, bearing different heteroatom substituents, have been isolated. Among them, the so-called N-heterocyclic carbenes (NHCs), which include cyclic (alkyl)­(amino)­carbenes (CAACs), are arguably the most popular. They have found a vast number of applications ranging from catalysis to material science, and even in medicine. In this Account, we focus on the synthesis, structure, electronic properties, coordination, and applications of a different class of stable cyclic carbenes, namely, 1H-1,2,3-triazol-5-ylidenes. In contrast with NHCs and CAACs, these compounds have no reasonable canonical resonance forms that can be drawn showing a carbene without additional charges. According to the IUPAC, they belong to the family of mesoionic compounds and thus they are named mesoionic carbenes (MICs). In 2010, we prepared the first stable 1,2,3-triazol-5-ylidene, via a CuAAC reaction, followed by alkylation of the resulting 1,2,3-triazole, and deprotonation. Later, we synthesized more robust N3-arylated counterparts from 1,3-diarylated-1H-1,2,3-triazolium salts. Both synthetic routes can be carried out in multigram scales, making these MICS readily available. Importantly, MICs do not dimerize which contrasts with NHCs that can give the corresponding Wanzlick-type olefin. This property leads to relaxed steric requirements for their isolation; even C-unsubstituted MICs can be stored for months in the solid state at room temperature. The practicality and easily scalable syntheses of MICs allow for the preparation of polycarbenes, such as bis­(1,2,3-triazol-5-ylidenes) (i-bitz), the analogues of the well-known 2,2′-bipyridines (bpy). MIC-transition metal complexes are excellent precatalysts for variety of chemical transformations, which include hydrohydrazination of alkynes, olefin metathesis, reductive formylation of amines with carbon dioxide and diphenylsilane, hydrogenation and dehydrogenation of N-heteroarenes in water, cycloisomerization of enynes, asymmetric Suzuki–Miyaura cross-coupling, and water oxidation (WO) reactions. Besides their catalytic applications, MIC–transition metal complexes have found applications in material sciences as exemplified by the preparation of the first iron­(III) complex that is luminescent at room temperature. The peculiar properties of mesoionic triazolylidenes, combined with their enhanced stability, position them as excellent candidates to address some current challenges such as access to high-oxidation-state 3d metal complexes, the stabilization of highly reactive main group elements, the stabilization of nanoparticles, the preparation of efficient catalysts and photosensitizers based...

Direct metalation of bis(1,2,3-triazolium) salts affords mononuclear rhodium(I) complexes, which feature a 1,4-bidentate bis(1,2,3-triazol-5-ylidene) (i-bitz) ligand. The topology of the ligand is similar to 2,2′-bipyridines (bpy) and their congeners, as well as bis(1,2,4-triazol-5-ylidenes) (bitz). As the former, but in contrast to the latter, the free i-bitz can be isolated, which paves the way for various applications.

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