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

The transient intermediate eta(2)-cydopropene/bicydobutane niobium complex [(TPNb)-Nb-Me2(eta(2)-c-C3H4)(MeCCMe)] A, generated by an intramolecular beta-H abstraction of methane from the methyl cyclopropyl complex [(TPNbMe)-Nb-Me2(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 [(TPNbR)-Nb-Me2(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(eta(2)-cydopropene) 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 [T(P)(Me2)NbFc(c-C3H5)(MeCCMe)]. The reactivity of these complexes with benzene follows qualitatively the strength and the pK(a) of the CH bond that is cleaved. The pentafluorophenyl complex [(TPNb)-Nb-Me2(C6F5)(c-C3H5)(MeCCMe)] undergoes ckdopropyl ring opening and alkyne coupling to give two isomeric eta(4)-butadienyl complexes, with [(TPNb)-Nb-Me2(C6F5)(eta(4)-CMeCMeCHCHMe)] as the major isomer.