sp2 C–H activation of dimethyl fumarate by a [(Cp*Co)2-μ-(η4 : η4-toluene)] complex

Abstract

The well-defined oxidative addition of the vinylic sp² C–H bond of dimethyl fumarate is mediated by the cobalt triple decker complex [(Cp*Co)₂-μ-(η⁴ : η⁴-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(MeO₂CCHCHCO₂Me)]_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(η²-MeO₂CCHCHCO₂Me)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 PMe₃ or CO, affords the corresponding Cp*Co(η²-MeO₂CCHCHCO₂Me)(L) (L = PMe₃ (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.