C–F bond activation of perfluorinated arenes using NHC-stabilized cobalt half-sandwich complexes

Abstract

A study on the reactivity of NHC cobalt half-sandwich complexes [(η⁵-C₅R¹₅)Co(R²₂Im)(η²-C₂H₄)] (R¹ = H; R² = Me 1, iPr 2) and [(η⁵-C₅R¹₅)Co(R²₂Im)(η²-C₂H₃{SiMe₃})] (R¹ = H, R² = Me 3, iPr 4; R¹ = Me, R² = Me 5, iPr 6) towards selected perfluoroarenes is presented (R₂Im = 1,3-di(organyl)-imidazolin-2-ylidene). The reaction with hexafluorobenzene, perfluorotoluene and decafluorobiphenyl at 100 °C led to the isolation of the cobalt(II) complexes [CpCo(iPr₂Im)(C₆F₅)] 7, [CpCo(iPr₂Im)(C₇F₇)] 8, [CpCo(iPr₂Im)(C₁₂F₉)] 9, [Cp*Co(iPr₂Im)(C₆F₅)] 10, [Cp*Co(iPr₂Im)(C₇F₇)] 11 and [Cp*Co(iPr₂Im)(C₁₂F₉)] 12 (Cp* = η⁵-C₅{CH₃}₅), Cp = η⁵-C₅H₅). The cobalt(II) fluoride [CpCo(iPr₂Im)(F)] was detected as a side product of these reactions. The reaction of [CpCo(R²₂Im)(η²-C₂H₄)] (R² = Me 1, iPr 2) with C₆F₆ and C₇F₈ at 60 °C afforded the dinuclear complexes [{CpCo(R²₂Im)}₂(μ-η²,η²-C₆F₆)] (R² = Me 13, iPr 14) and [{CpCo(R²₂Im)}₂(μ-η²,η²-C₇F₈)] (R² = Me 15, iPr 16). Furthermore, the dinuclear complexes [{CpCo(Me₂Im)}₂(μ-η²,η²-C₁₀F₈)] 17, [{CpCo(iPr₂Im)}₂(μ-η²,η²-C₁₀F₈)] 18 and mononuclear [CpCo(iPr₂Im)(η²-C₁₀F₈)] 19 were isolated from the reaction of 1 and 2 with octafluoronaphthalene at room temperature. Based on the experimental data a mechanism is proposed for the C–F bond activation of perfluoroarenes with complexes [Cp^(*)Co(NHC)(olefin)]. Transfer of [Cp^(*)Co(NHC)] to the perfluoroarene, which is limited by the activation barrier to replace the alkene ligand, affords mononuclear cobalt complexes [Cp^(*)Co(NHC)(η²-Ar^F)]. Ligand dismutation at higher temperatures leads to dinuclear complexes [{Cp^(*)Co(NHC)}₂(μ-η²,η²-Ar^F)], which are precursors for the formation of the cobalt(II) complexes [Cp^(*)Co(NHC)(Ar^F)] and [Cp^(*)Co(NHC)(F)]. One electron oxidative addition prevails in the C–F bond activation step using these cobalt half-sandwich complexes.