Bis(η5:η1‐pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives.

Abstract: Transition‐metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low‐valent bis(η⁵:η¹‐(di‐p‐tolyl)‐pentafulvene)niobium chloride. Owing to the π‐η⁵:σ‐η¹ coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η⁵:η¹‐pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η⁵:η¹ cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α‐hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N−H bond activation of primary amines to niobium imido complexes. [ABSTRACT FROM AUTHOR]