C−H Bond Activation and C−C Bond Formation in the Reactions of the Methyl Complex [Ir₂(CH₃)(CO)₂(Ph₂PCH₂PPh₂)₂][CF₃SO₃] with Alkynes

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Abstract

The methyl complex [Ir2(CH3)(CO)(μ-CO)(dppm)2][CF3SO3] (1) reacts readily with a variety of alkyne molecules. With alkynes containing electron-withdrawing substituents (RC⋮CR‘; R = R‘ = CO2Me, CF3; R = CO2Me, R‘ = H), the alkyne-bridged products, [Ir2(CH3)(CO)2(μ-alkyne)(dppm)2][CF3SO3], result. With other 1-alkynes (HC⋮CR, R = Me, Ph) reaction at −80 °C results in oxidative addition to give acetylide hydrides [Ir2H(CH3)(CO)2(μ-C2R)(dppm)2][CF3SO3], which upon warming undergo transfer of the hydride ligand to the β-carbon of the acetylide to give the vinylidene-bridged products. At ambient temperature methane elimination occurs, yielding the acetylide-bridged “A-frames”, [Ir2(CO)2(μ-C2R)(dppm)2][CF3SO3]. Reaction of 1 with 1 equiv of acetylene proceeds through analogous acetylide and vinylidene intermediates; however under carbon monoxide exchange of a vinylidene hydrogen and the methyl ligand occurs to give a methylvinylidene-bridged hydride [Ir2(H)(CO)3(μ-CCHMe)(dppm)2][CF3SO3]. Reaction of 1 with an excess of acetylene results in the incorporation of two acetylene molecules, giving [Ir2(CH3)(C2H)(CO)2(μ-H)(μ-CCH2)(dppm)2][CF3SO3], containing terminal methyl and acetylide ligands and bridging hydride and vinylidene groups. Reaction of 1 with a number of internal, nonactivated alkynes results first in the formation of the alkyne-bridged products, which slowly rearrange to the unusual species [Ir2H(CO)2(μ-CHCRC(H)R)(dppm)2][CF3SO3], in which cleavage of two C−H bonds of the methyl group has occurred accompanied by condensation of the resulting methyne group at one end of alkyne linkage and transfer of a hydrogen to the other end. The resulting hydrocarbyl moiety can be viewed as a vinylcarbene. The X-ray structures of three representative products are reported, including one of these vinylcarbenes (R = C2H5).

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