C−F Activation of Fluorinated Arenes using NHC-Stabilized Nickel(0) Complexes: Selectivity and Mechanistic Investigations

The reaction of [Ni2(iPr2Im)4(COD)] 1a or [Ni(iPr2Im)2(η2-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(iPr2Im)2(F)(C6F5)] 2, trans-[Ni(iPr2Im)2(F)(4-(CF3)C6F4)] 3, trans-[Ni(iPr2Im)2(F)(4-(C6F5)C6F4)] 4, and trans-[Ni(iPr2Im)2(F)(4-(SiMe3)C6F4)] 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(iPr2Im)2(F)(2-C6FH4)] 7, trans-[Ni(iPr2Im)2(F)(3,5-C6F2H3)] 8, trans-[Ni(iPr2Im)2(F)(2,3-C6F2H3)] 9a and trans-[Ni(iPr2Im)2(F)(2,6-C6F2H3)] 9b, trans-[Ni(iPr2Im)2(F)(2,5-C6F2H3)] 10, and trans-[Ni(iPr2Im)2(F)(2,3,5,6- C6F4H)] 11. The reaction of 1a with octafluoronaphthalene yields exclusively trans-[Ni(iPr2Im)2(F)(1,3,4,5,6,7,8-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(iPr2Im)2(F)(1,3,4,5,6,7,8-C10F7)] 6a and trans-[Ni(iPr2Im)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(iPr2Im)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(iPr2Im)2(F)(4-C5NF4)] 12a and trans-[Ni(iPr2Im)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(iPr2Im)2} complex fragment. The intermediates of the reaction of 1b with hexafluorobenzene and octafluoronaphthalene, [Ni(iPr2Im)2(η2-C6F6)] 13 and [Ni(iPr2Im)2(η2-C10F8)] 14, have been detected in solution. They convert into the C−F activation products. Complex 14 was structurally characterized 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 ΔH⧧ for this process was determined to be ΔH⧧ = 116 ± 8 kJ mol−1 (ΔS⧧ = 37 ± 25 J K−1 mol−1). Furthermore, density functional theory calculations on the reaction of 1a with hexafluorobenzene, octafluoronaphthalene, octafluorotoluene, 1,2,4-trifluorobenzene, and 1,2,3-trifluorobenzene are presented.