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

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.