Sp ~2 C-H activation of dimethyl fumarate by a [(Cp*Co) _2-μ-(η ~4:η ~4-toluene)] complex

摘要

The well-defined oxidative addition of the vinylic sp ~2 C-H bond of dimethyl fumarate is mediated by the cobalt triple decker complex [(Cp*Co) _2-μ-(η ~4:η ~4-toluene)] (1) at ambient temperature, affording the dinuclear, bridging cobalt hydride, fumaryl compound (2). The C-H activation product has been characterized by mass spectrometry, NMR spectroscopy, and X-ray crystallography. Computational studies of 2 support asymmetric bonding interactions between the two metal centres and the bridging hydride/fumaryl fragments. Monitoring the reaction of dimethyl fumarate with 1 by ~1H NMR spectroscopy allows observation of intermediate [Cp*Co(MeO _2CCHCHCO _2Me)] _n (n = 1 or 2) (3). Addition of 4 equivalents of dimethyl fumarate to 1 results in rapid formation of the bis(ligand) adduct Cp*Co(η ~2-MeO _2CCHCHCO _2Me) _2 (5). Reversibility of the C-H activation was probed by reaction of additional dimethyl fumarate with 2, suggesting ligand induced reductive elimination is possible under ambient conditions. Reaction between 2 and strong σ or π ligands, such as PMe _3 or CO, affords the corresponding Cp*Co(η ~2- MeO _2CCHCHCO _2Me)(L) (L = PMe _3 (7); L = CO (8)) complexes when heated, demonstrating the ability of 2 to undergo two electron redox processes. Further evidence for reversible C-H activation is provided by the isomerization of dimethyl maleate to the corresponding fumarate using 2, suggesting the complex can serve as a source of Co(i) under the appropriate catalytic conditions.