Reduction–oxidation properties of organotransition-metal complexes. Part 32. One-electron oxidation of [Co(η4-cot)-(η-C5Me5)](cot = cyclooctatetraene): redox-induced polycyclic ring transformations of the resulting dimer, and crystal structure of [Co2(η4:η′4-C16H16)(η-C5Me5)2]

Abstract

The complex [Co(η⁴-cot)(η-C₅Me₅)]4(cot = cyclooctatetraene) undergoes irreversible one-electron oxidation in CH₂Cl₂ at a platinum electrode; chemical oxidation with [Fe(η-C₅H₅)₂]PF₆ gives the dimeric dication [Co₂(η⁵ : η′⁵-C₁₆H₁₆)(η-C₅Me₅)₂][PF₆]₂5 as a diastereomeric mixture. The reduction of 5 with two equivalents of [Co(η-C₅H₅)₂] gives three asymmetric isomers [Co₂(σ,η³ : η′⁴-C₁₆H₁₆)(η-C₅Me₅)₂]8, [Co₂(σ,η³ : η′⁴-C₁₆H₁₆)(η-C₅Me₅)₂]9 and [Co₂(η⁴ : η′⁴-C₁₆H₁₆)(η-C₅Me₅)₂]10. Complex 8 is thermally isomerised to 10 which has been structurally characterised by a single crystal X-ray diffraction study. The two cobalt atoms, each carrying an η-C₅Me₅ ligand, are bonded to opposite sides of a tetracyclic C₁₆H₁₆ ligand which contains fused cyclooctatriene, cyclopentane, cyclopropane and cycloheptadiene rings; the cobalt atoms are bound viaη⁴-1,3- and η⁴-1,5-diene interactions involving the C₇ and C₈ rings respectively. Complex 10 is oxidised by [Fe(η-C₅H₅)₂]PF₆, to give [Co₂(η²,η³ : η′⁵-C₁₆H₁₆)(η-C₅Me₅)₂][PF₆]₂13, and undergoes partial decomplexation with Me₃NO·2H₂O to yield the monometallic compound [Co(η⁴-C₁₆H₁₆)(η-C₅Me₅)]15. Stepwise rather than concerted mechanisms are proposed for the overall two-electron reduction of 5 and two-electron oxidation of 10.