Recent advances in the use of N-heterocyclic carbene adducts of N, P, C elements as supporting ligands in organometallic chemistry

Abstract

With the rapid development of N-heterocyclic carbene (NHC) coordination chemistry, the potential of NHCs as strong electron-donating ligands has been fully explored. At the same time, an increasing number of p-block element NHC adducts, due to their possession of lone pairs or polarized carbon-element π-bonds, have gradually become important ligands of significant interest in coordination chemistry. Among these, N-heterocyclic imine (NHI), N-heterocyclic phosphinidene (NHCP) and N-heterocyclic olefin (NHO) adducts, formed by linking the 2-position of the N-heterocycle with nitrogen, phosphorus, and carbon elements, respectively, have been widely applied in both main-group and transition metal chemistry. The efficient stabilization of positive charges by the N-heterocycle allows the exocyclic atoms to accumulate negative charges, thereby significantly enhancing the basicity and nucleophilicity of the exocyclic elements. The improvement in nucleophilicity has facilitated the successful synthesis of numerous complexes with unique element–metal bonds. Additionally, by modifying the N-substituents, the kinetic stabilization requirements of highly reactive species can be fulfilled. This review aims to summarize the latest research progress on NHIs, NHCPs, and NHOs in main-group and transition metal compounds. It is important to emphasize that this review focuses on the commonly used five-membered N-heterocyclic carbene adducts, including imidazolin-2-ylidenes, imidazolidin-2-ylidenes, and benzimidazolin-2-ylidenes.