C−H Bond Activation of Benzene by Unsaturated η²-Cyclopropene and η²-Benzyne Complexes of Niobium

We report the synthesis of a niobium cyclopropyl complex, TpMe2NbMe(c-C3H5)(MeCCMe), and show that thermal loss of methane from this compound generates an intermediate that is capable of activating both aliphatic and aromatic C−H bonds. Isotopic labeling, trapping studies, a detailed kinetic analysis, and density functional theory all suggest that the active intermediate is an η2-cyclopropene complex formed via β-hydrogen abstraction rather than an isomeric cyclopropylidene species. C−H activation chemistry of this type represents a rather unusual reactivity pattern for η2-alkene complexes but is favored in this case by the strain in the C3 ring which prevents the decomposition of the key intermediate via loss of cyclopropene.