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.