Application of 1,2,3-triazolylidenes as versatile NHC-type ligands: synthesis, properties, and application in catalysis and beyond

Abstract

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.

Synthesis and structure of trans-bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride and diacetate. Suzuki–Miyaura coupling of polybromoarenes with high catalytic turnover efficiencies

“…In the past five years 1,2,3-triazol-5-ylidenes have emerged as promising ligands for transition-metal chemistry [15–20]. 1,2,3-Triazol-5-ylidenes have been termed as abnormal NHCs and mesoionic carbenes because their structures cannot be represented in neutral canonical form [17,19]. Mesoionic NHCs are stronger sigma donors than the normal NHCs (e.g., imidazolylidenes versus triazolylidenes) [21–23].…”

Preparation and characterisation of a Ni2+/Co2+-cyclam modified mesoporous cellular foam for the specific immobilisation of His6-alanine racemase

“…The use of carbene ligands as homogeneous and heterogeneous catalysts has increased after isolation and characterization of free carbene in 1991 by Arduengo. [144][145][146][147] . To date, a large number of reviews have been published for these valuable carbene ligands 146,148 .…”

“…[144][145][146][147] . To date, a large number of reviews have been published for these valuable carbene ligands 146,148 . Gradually, NHeterocyclic carbenes (NHCs) have become the most successful class of ligand in organometallic chemistry.…”

Iridium, ruthenium, and palladium complexes containing a mesoionic fused imidazolylidene ligand

“…1), since substitution at C2 predisposes the ligand to bind via the imidazole C4 or C5 position and hence to produce abnormal/mesoionic carbenes [13][14][15][16][17][18]. …”

“…A large variety of NHC ligand structures have been discovered, characterized and applied in metal coordination [12][13][14][15][16][17][18][19][20]. Ring-fused NHCs are a particular subclass of NHCs that often display distinct characteristics that are different from normal imidazole-derived NHCs [21,22].…”