Recent advances in the synthesis,structural diversity,and applications of mesoionic 1,2,3-triazol-5-ylidene metal complexes

Beyond the classic N-heterocyclic carbenes (NHCs) there is a subclass of NHCs called mesoionic carbenes (MICs). This review focuses on recent advances in the area of 1,2,3-triazol-5-ylidenes as the most abundant class of MICs and their metal complexes. The study of mesoionic 1,2,4- and 1,3,4-trisubstituted 1,2,3-triazol-5-ylidene transition metal complexes is a research area with a history of just ～10 years. During this relatively short period, hundreds of these complexes have appeared in the literature, reflecting their high stability and simpler synthesis compared with NHCs. Specifically, this review is focused on advances in the synthesis of 1,2,3-triazol-5-ylidene metal complexes derived from palladium, silver, gold, ruthenium, iridium, rhodium, iron, molybdenum, cobalt, nickel, platinum, and osmium, together with their catalytic, medicinal, and photophysical applications.