Progress of Azametallacyclopentadienes in the New Century

As the key intermediates in metal‐promoted/catalyzed C−C bond coupling reactions of nitriles and alkynes, azametallacyclopentadienes, M(N=CR1−CR2=CR3), are an important class of azametallacycles. Although the first authentic azametallacyclopentadienes were documented in 1986, their chemistry towards solid‐state structures, intrinsic reactivity, and synthetic application was rarely investigated for a long time. At the beginning of this century, seminal works about the applications of azametallacyclopentadienes in the synthesis of heterocycles, including multi‐substituted pyridines, isoquinolines, furans, and pyrroles were reported. Subsequently, a series of new complexes with this motif, namely the Group 4, aluminum, actinide, and rare‐earth azametallacyclopentadienes were isolated and structurally characterized. Among them, the rare‐earth azametallacyclopentadiene expresses high reactivity towards unsaturated molecules, such as nitriles, isocyanides, and Mo(CO)6 to provide novel fused metallacycles. In this Concept, we reviewed the advances in the preparation, reactivity, and synthetic application of azametallacyclopentadienes in the past twenty years. This Concept article summarizes the progress of azametallacyclopentadienes towards their generated methods, structural characterization, intrinsic reactivity, and application in the heterocyclic synthesis since the new century.