CH Bond Activation of Unsaturated Hydrocarbons by a Niobium Methyl Cyclopropyl Precursor. Cyclopropyl Ring Opening and Alkyne Coupling Reaction

The transient intermediate η²-cyclopropene/bicyclobutane niobium complex [Tp^Me2Nb­(η²-<i>c</i>-C₃H₄)­(MeCCMe)] <b>A</b>, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [Tp^Me2NbMe­(<i>c</i>-C₃H₅)­(MeCCMe)] (<b>1</b>), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [Tp^Me2NbR­(<i>c</i>-C₃H₅)­(MeCCMe)] (R = 2-furyl, 2-thienyl, 1-alkynyl, 1-cyclopentenyl, 1-ferrocenyl (Fc), pentafluorophenyl). The activation of the C–H bond occurs stereospecifically via a 1,3-CH addition across the Nb­(η²-cyclopropene) bond of <b>A</b>. Full characterization of several of these complexes includes multinuclear NMR spectroscopy, X-ray diffraction, UV/vis spectroscopy, and electrochemical data. A charge transfer between the ferrocenyl moiety and the niobium center is responsible for the characteristic purple color of the bimetallic complex [Tp^Me2NbFc­(<i>c</i>-C₃H₅)­(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the p<i>K</i>_a of the CH bond that is cleaved. The pentafluorophenyl complex [Tp^Me2Nb­(C₆F₅)­(<i>c</i>-C₃H₅)­(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling to give two isomeric η⁴-butadienyl complexes, with [Tp^Me2Nb­(C₆F₅)­(η⁴-CMeCMeCHCHMe)] as the major isomer