Decisive Steps of the Hydrodefluorination of Fluoroaromatics using [Ni(NHC)2]

The hydrodefluorination reaction of perfluorinated arenes using [Ni2(iPr2Im)4(COD)] (1; iPr2Im = 1,3-bis­(isopropyl)­imidazolin-2-ylidene) as a catalyst as well as stoichiometric transformations to elucidate the decisive steps for this reaction are reported. The reaction of hexafluorobenzene with 5 equiv of triphenylsilane in the presence of 5 mol % of 1 affords 1,2,4,5-tetrafluorobenzene after 48 h at 60 °C and 1,4-difluorobenzene after 96 h at 80 °C; the reaction of perfluorotoluene and 5 equiv of Et3SiH for 4 days at 80 °C results in the selective formation of 1-(CF3)-2,3,5,6-C6F4H. Stoichiometric transformations of the complexes cis-[Ni­(iPr2Im)2(H)­(SiPh3)] and cis-[Ni­(iPr2Im)2(H)­(SiMePh2)] with hexafluorobenzene at room temperature lead to the formation of trans-[Ni­(iPr2Im)2(F)­(C6F5)] (2) and trans-[Ni­(iPr2Im)2(H)­(C6F5)] (4) with elimination of the corresponding silane or fluorosilane. The reactions of the C–F activation products trans-[Ni­(iPr2Im)2(F)­(C6F5)] (2) and trans-[Ni­(iPr2Im)2(F)­(4-(CF3)­C6F4)] (3) with PhSiH3 and Ph2SiH2 afford the hydride complexes trans-[Ni­(iPr2Im)2(H)­(C6F5)] (4) and trans-[Ni­(iPr2Im)2(H)­(4-(CF3)­C6F4)] (5), which convert into the compounds trans-[Ni­(iPr2Im)2(F)­(2,3,5,6-C6F4H)] (7), trans-[Ni­(iPr2Im)2(F)­(3-(CF3)-2,4,5-C6F3H)] (9a), and trans-[Ni­(iPr2Im)2(F)­(2-(CF3)-3,4,6-C6F3H)] (9b), respectively. In the case of the rearrangement of trans-[Ni­(iPr2Im)2(H)­(4-(CF3)­C6F4)] (5) the intermediate [Ni­(iPr2Im)2(η2-C,C-(CF3)­C6F4H)] (8) was detected. Reaction of 8 with perfluorotoluene gave the C–F activation product trans-[Ni­(iPr2Im)2(F)­(4-(CF3)­C6F4)] (3). All these experimental findings point to a mechanism for the HDF by [Ni­(iPr2Im)2] via the “fluoride route” involving C–F activation of the polyfluoroarene, H/F exchange of the resulting nickel fluoride, reductive elimination of the polyfluoroaryl nickel hydride to an intermediate with a η2-C,C-coordinated arene ligand, subsequent ligand exchange with the higher fluorinated polyfluoroarene, and renewed C–F activation of the polyfluoroarene. Without additional reagents, [Ni­(iPr2Im)2(η2-C,C-(CF3)­C6F4H)] (8) rearranges to the isomers trans-[Ni­(iPr2Im)2(F)­(3-(CF3)-2,4,5-C6F3H)] (9a; major) and trans-[Ni­(iPr2Im)2(F)­(2-(CF3)-3,4,6-C6F3H)] (9b; minor) in a ratio of 80:20. DFT calculations performed on conversion of trans-[Ni­(iPr2Im)2(H)­(4-(CF3)­C6F4)] 5 into the two products trans-[Ni­(iPr2Im)2(F)­(3-(CF3)-2,4,5-C6F3H)] 9a and trans-[Ni­(iPr2Im)2(F)­(2-(CF3)-3,4,6-C6F3H)] (9b) using the commonly accepted intramolecular concerted pathway via η2-C,F-σ-bound transition states predict 9b to be the major product. We thus propose that this reaction mechanism is not valid for the [Ni(NHC)2]-mediated C–F activation of partially fluorinated arenes with special substitution patterns.