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, Tp(Me2)NbMe(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 eta(2)-cyclopropene complex formed via beta-hydrogen abstraction rather than an isomeric cyclopropylidene species. C-H activation chemistry of this type represents a rather unusual reactivity pattern for eta(2)-alkene complexes but is favored in this case by the strain in the C-3 ring which prevents the decomposition of the key intermediate via loss of cyclopropene.