C-F Bond Activation Of Perfluorinated Arenes Using Nhc-Stabilized Cobalt Half-Sandwich Complexes

A study on the reactivity of NHC cobalt half-sandwich complexes [(eta(5)-C5R51)Co(R(2)(2)Im)(eta(2)-C2H4)] (R-1 = H; R-2 = Me 1, iPr 2) and [(eta(5)-C5R51)Co(R(2)(2)Im)(eta(2)-C2H3{SiMe3})] (R-1 = H, R-2 = Me 3, iPr 4; R-1 = Me, R-2 = Me 5, iPr 6) towards selected perfluoroarenes is presented (R(2)Im = 1,3-di(organyl)-imidazolin-2-ylidene). The reaction with hexafluorobenzene, perfluorotoluene and decafluorobiphenyl at 100 degrees C led to the isolation of the cobalt(ii) complexes [CpCo(iPr(2)Im)(C6F5)] 7, [CpCo(iPr(2)Im)(C7F7)] 8, [CpCo(iPr(2)Im)(C12F9)] 9, [Cp*Co(iPr(2)Im)(C6F5)] 10, [Cp*Co(iPr(2)Im)(C7F7)] 11 and [Cp*Co(iPr(2)Im)(C12F9)] 12 (Cp* = eta(5)-C-5{CH3}(5)), Cp = eta(5)-C5H5). The cobalt(ii) fluoride [CpCo(iPr(2)Im)(F)] was detected as a side product of these reactions. The reaction of [CpCo(R(2)(2)Im)(eta(2)-C2H4)] (R-2 = Me 1, iPr 2) with C6F6 and C7F8 at 60 degrees C afforded the dinuclear complexes [{CpCo(R(2)(2)Im)}(2)(mu-eta(2),eta(2)-C6F6)] (R-2 = Me 13, iPr 14) and [{CpCo(R(2)(2)Im)}(2)(mu-eta(2),eta(2)-C7F8)] (R-2 = Me 15, iPr 16). Furthermore, the dinuclear complexes [{CpCo(Me(2)Im)}(2)(mu-eta(2),eta(2)-C10F8)] 17, [{CpCo(iPr(2)Im)}(2)(mu-eta(2),eta(2)-C10F8)] 18 and mononuclear [CpCo(iPr(2)Im)(eta(2)-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 [(CpCo)-Co-(*)(NHC)(olefin)]. Transfer of [(CpCo)-Co-(*)(NHC)] to the perfluoroarene, which is limited by the activation barrier to replace the alkene ligand, affords mononuclear cobalt complexes [(CpCo)-Co-(*)(NHC)(eta(2)-Ar-F)]. Ligand dismutation at higher temperatures leads to dinuclear complexes [{(CpCo)-Co-(*)(NHC)}(2)(mu-eta(2),eta(2)-Ar-F)], which are precursors for the formation of the cobalt(ii) complexes [(CpCo)-Co-(*)(NHC)(Ar-F)] and [(CpCo)-Co-(*)(NHC)(F)]. One electron oxidative addition prevails in the C-F bond activation step using these cobalt half-sandwich complexes.