Coligand role in the NHC nickel catalyzed C-F bond activation: investigations on the insertion of bis(NHC) nickel into the C-F bond of hexafluorobenzene.

The reaction of [Ni(MesIm)] () (MesIm = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of and [Ni(PrIm)] () (PrIm = 1,3-diisopropyl-imidazolin-2-ylidene) into the C-F bond of CF is reported. The reaction of with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes -[Ni(MesIm)(F)(Ar)] (Ar = 4-CF-CF , CF , 2,3,5,6-CFN , 2,3,5,6-CFH , 2,3,5-CFH , 3,5-CFH ) in fair to good yields with the exception of the formation of the pentafluorophenyl complex (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of with CF, in line with a radical pathway for the C-F bond activation step using . The difluoride complex -[Ni(MesIm)(F)] (), the bis(aryl) complex -[Ni(MesIm)(CF)] (), the structurally characterized nickel(i) complex -[Ni(MesIm)(CF)] () and the metal radical -[Ni(MesIm)(F)] () were identified. Complex , and related [Ni(MesIm)(2,3,5,6-CFH)] () and [Ni(MesIm)(2,3,5-CFH)] (), were synthesized independently by reaction of -[Ni(MesIm)(F)(Ar)] with PhSiH. Simple electron transfer from to CF was excluded, as the redox potentials of the reaction partners do not match and [Ni(MesIm)], which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(PrIm)] () and [Ni(MesIm)] () into the C-F bond of CF. For , concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for , a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.