Activation of fluorinated arenes using NHC-stabilized nickel(0) complexes: Selectivity and mechanistic investigations

The reaction of [Ni-2((i)Pr(2)Im)(4)(COD)] 1a or [Ni((i)Pr(2)Im)(2) (eta(2)-C2H4)] 1b with different fluorinated arenes is reported. These reactions occur with a high chemo- and regioselectivity. In the case of polyfluorinated aromatics of the type C6F5X such as hexafluorobenzene (X = F) octafluorotoluene (X = CF3) trimethyl(pentafluorophenyl)silane (X = SiMe3) or decafluorobiphenyl (X = C6F5) the C-F activation regioselectively takes place at the C-F bond in the para position to the X group to afford the complexes trans-[Ni((i)Pr(2)Im)(2)(F)(C6F5)] 2, trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F4)] 3, trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F5)] 4, and trans-[Ni((i)Pr(2)Im)(2)(F)(4-(CF3)C6F5)] 5. Complex 5 was structurally characterized by X-ray diffraction. The reaction of 1a with partially fluorinated aromatic substrates C6HxFy leads to the products of a C-F activation trans-[Ni((i)Pr(2)Im)(2)(F)(2-(C6F4)] 7, trans-[Ni((i)Pr(2)Im)(2)(F)(3,5-(C6F2H3)] 8, trans-[Ni((i)Pr(2)Im)(2)(F)(2,3-(C6F2H3)]9a and trans-[Ni((i)Pr(2)Im)(2)(F)(2,6-(C6F2H3)] 9b, trans-[Ni((i)Pr(2)Im)(2)(F)(2,5-(C6F2H3)] 10, and trans-[Ni((i)Pr(2)Im)(2)(F)(2,3,5,6-(C6F4H)] 11. The reaction of 1a with octafluoronaphthalene yields exclusively trans-[Ni((i)Pr(2)Im)(2)(F)(1,3,45,6,7,8-(C10F7)] 6a, the product of an insertion into the C-F bond in the 2-position, whereas for the reaction of 1b with octafluoronaphthalene the two isomers trans-[Ni((i)Pr(2)Im)(2)(F)(1,3,4,5,6,7,8-C10F7)] 6a and trans-[Ni((i)Pr(2)Im)(2)(F)(2,3,4,5,6,7,8-C10F7)] 6b are formed in a ratio of 11:1. The reaction of 1a or of 1b with pentafluoropyridine at low temperatures affords trans[Ni((i)Pr(2)Im)(2)(F)(4-C5NF4)] 12a as the sole product, whereas the reaction of 1b performed at room temperature leads to the generation of trans-[Ni((i)Pr(2)Im)(2)(F)(4-(C5NF4)] 12a and trans-[Ni((i)Pr(2)Im)(2)(F)(2-C5NF4)] 12b in a ratio of approximately 1:2. The detection of intermediates as well as kinetic studies gives some insight into the mechanistic details for the activation of an aromatic carbon-fluorine bond at the {Ni((i)Pr(2)Im)(2)} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni((i)Pr(2)Im)(2)(eta(2)-(C6F6)] 13 and [Ni((i)Pr(2)Im)(2)(eta(2)-C10F8)] 14, have been detected in solution. They convert into the C-F activation products. Complex 14 was structurally characterizd by X-ray diffraction. The rates for the loss of 14 at different temperatures for the C-F activation of the coordinated naphthalene are first order and the estimated activation enthalpy Delta H double dagger for this process was determined to be Delta H double dagger = 116 +/- 8 kJ mol(-1) (Delta S double dagger = 37 +/- 25 J K-1 mol(-1)). Furthermore, density functional theory calculations on the reaction of la with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.