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 [TpMe2Nb­(η2-c-C3H4)­(MeCCMe)] A, generated by an intramolecular β-H abstraction of methane from the methyl cyclopropyl complex [TpMe2NbMe­(c-C3H5)­(MeCCMe)] (1), is able to cleave the CH bond of a variety of unsaturated hydrocarbons RH in a selective manner to give the corresponding hydrocarbyl complexes [TpMe2NbR­(c-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 A. 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 [TpMe2NbFc­(c-C3H5)­(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the pKa of the CH bond that is cleaved. The pentafluorophenyl complex [TpMe2Nb­(C6F5)­(c-C3H5)­(MeCCMe)] undergoes cyclopropyl ring opening and alkyne coupling to give two isomeric η4-butadienyl complexes, with [TpMe2Nb­(C6F5)­(η4-CMeCMeCHCHMe)] as the major isomer.