A niobium pentafulvene ethylene complex: synthesis, properties and reaction pathways

The π-η⁵:σ-η¹ coordination mode of early transition metal pentafulvene ligands yields a strongly nucleophilic exocyclic carbon atom (C). The substitution of the chlorido ligand of bis(η⁵:η¹-(di-p-tolyl)pentafulvene)niobium chloride (1) by reaction with ethyl magnesium bromide is subsequently followed by a β-C-H activation employing this C, forming the pentafulvene niobium ethylene complex 2. The intermediately formed ethyl complex can be intercepted with water, protonating both pentafulvene moieties and thereby retaining the ethyl moiety to give the terminal oxo complex 3. Complex 2 shows cooperative reactions of the remaining pentafulvene and the ethylene ligand. While the pentafulvene functions as a proton acceptor, the ethylene can be liberated to provide a Nb^III metal center, available for E-H bond addition. Thereby, the imido hydride complex 4 and niobaaziridine hydride 5 are obtained. These hydride complex formations are investigated by deuterium labeling, concluding the redox mechanism. An alternative β-hydride elimination pathway is further disproven by purposely synthesizing a proposed intermediate and thermally treating it, to show that it does not undergo the β-hydride elimination under reaction conditions.