1H-1,2,3-Triazol-5-ylidenes: Readily Available Mesoionic Carbenes

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, 1 H-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-1 H-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 on earth-abundant transition metals, and the functionalization of self-assembled monolayers (SAMs) on gold.