The Redox Chemistry of [Co₆C(CO)₁₅]^2–: A Synthetic Route to New Co-Carbide Carbonyl Clusters

The oxidation and reduction reactions of [Co₆C­(CO)₁₅]^2– have been studied in detail, leading to the isolation of several new Co-carbide carbonyl clusters. Thus, [Co₆C­(CO)₁₅]^2– reacts in tetrahydrofuran (THF) with oxidants such as HBF₄·Et₂O and [Cp₂Fe]­[PF₆], resulting first in the formation of the previously reported [Co₆C­(CO)₁₄]⁻; then, in CH₂Cl₂, the new dicarbide [Co₁₁C₂(CO)₂₃]^2– is formed. The latter may be further oxidized, yielding the isostructural monoanion [Co₁₁C₂(CO)₂₃]⁻, whereas its reduction with (cyclopentadienyl)₂Co affords the unstable trianion [Co₁₁C₂(CO)₂₃]^3–, which decomposes during workup. Oxidation of [Co₆C­(CO)₁₅]^2– in CH₃CN with [C₇H₇]­[BF₄] affords the same major products, and besides, the new monoacetylide [Co₁₀(C₂)­(CO)₂₁]^2– was obtained as side-product. Conversely, the reduction of [Co₆C­(CO)₁₅]^2– in THF with increasing amounts of Na/naphthalene results in the following species: [Co₆C­(CO)₁₃]^2–, [Co₁₁(C₂)­(CO)₂₂]^3–, [Co₇C­(CO)₁₅]^3–, [Co₈C­(CO)₁₇]^4–, [Co₆C­(CO)₁₂]^3–, and [Co­(CO)₄]⁻. The new [Co₁₁C₂(CO)₂₃]⁻, [Co₁₁C₂(CO)₂₃]^2–, [Co₁₀(C₂)­(CO)₂₁]^2–, [Co₈C­(CO)₁₇]^4–, [Co₆C­(CO)₁₂]^3–, and [Co₇C­(CO)₁₅]^3– clusters were structurally characterized. Moreover, the paramagnetic species [Co₁₁C₂(CO)₂₃]^2– and [Co₆C­(CO)₁₂]^3– were investigated by means of electron paramagnetic resonance spectroscopy. Finally, electrochemical studies were performed on [Co₁₁C₂(CO)₂₃]^<i>n</i>− (<i>n</i> = 1–3)