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

The hydrodefluorination reaction of perfluorinated arenes using [Ni 2 ( i Pr 2 Im) 4 (COD)] ( 1; i Pr 2 Im = 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 Et 3 SiH for 4 days at 80 °C results in the selective formation of 1-(CF 3 )-2,3,5,6-C 6 F 4 H. Stoichiometric transformations of the complexes cis -[Ni( i Pr 2 Im) 2 (H)(SiPh 3 )] and cis -[Ni( i Pr 2 Im) 2 (H)(SiMePh 2 )] with hexafluorobenzene at room temperature lead to the formation of trans -[Ni( i Pr 2 Im) 2 (F)(C 6 F 5 )] ( 2 ) and trans -[Ni( i Pr 2 Im) 2 (H)(C 6 F 5 )] ( 4 ) with elimination of the corresponding silane or fluorosilane. The reactions of the C–F activation products trans -[Ni( i Pr 2 Im) 2 (F)(C 6 F 5 )] ( 2 ) and trans -[Ni( i Pr 2 Im) 2 (F)(4-(CF 3 )C 6 F 4 )] ( 3 ) with PhSiH 3 and Ph 2 SiH 2 afford the hydride complexes trans -[Ni( i Pr 2 Im) 2 (H)(C 6 F 5 )] ( 4 ) and trans -[Ni( i Pr 2 Im) 2 (H)(4-(CF 3 )C 6 F 4 )] ( 5 ), which convert into the compounds trans -[Ni( i Pr 2 Im) 2 (F)(2,3,5,6-C 6 F 4 H)] ( 7 ), trans -[Ni( i Pr 2 Im) 2 (F)(3-(CF 3 )-2,4,5-C 6 F 3 H)] ( 9a ), and trans -[Ni( i Pr 2 Im) 2 (F)(2-(CF 3 )-3,4,6-C 6 F 3 H)] ( 9b ), respectively. In the case of the rearrangement of trans -[Ni( i Pr 2 Im) 2 (H)(4-(CF 3 )C 6 F 4 )] ( 5 ) the intermediate [Ni( i Pr 2 Im) 2 (η 2 - C, C -(CF 3 )C 6 F 4 H)] ( 8 ) was detected. Reaction of 8 with perfluorotoluene gave the C–F activation product trans -[Ni( i Pr 2 Im) 2 (F)(4-(CF 3 )C 6 F 4 )] ( 3 ). All these experimental findings point to a mechanism for the HDF by [Ni( i Pr 2 Im) 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 η 2 -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( i Pr 2 Im) 2 (η 2 - C, C -(CF 3 )C 6 F 4 H)] ( 8 ) rearranges to the isomers trans -[Ni( i Pr 2 Im) 2 (F)(3-(CF 3 )-2,4,5-C 6 F 3 H)] ( 9a; major) and trans -[Ni( i Pr 2 Im) 2 (F)(2-(CF 3 )-3,4,6-C 6 F 3 H)] ( 9b; minor) in a ratio of 80:20. DFT calculations performed on conversion of trans -[Ni( i Pr 2 Im) 2 (H)(4-(CF 3 )C 6 F 4 )] 5 into the two products trans -[Ni( i Pr 2 Im) 2 (F)(3-(CF 3 )-2,4,5-C 6 F 3 H)] 9a and trans -[Ni( i Pr 2 Im) 2 (F)(2-(CF 3 )-3,4,6-C 6 F 3 H)] ( 9b ) using the commonly accepted intramolecular concerted pathway via η 2 -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.