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(Mes(2)Im)(2)] (1) (Mes(2)Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of1and [Ni((i)Pr(2)Im)(2)] (1(ipr)) ((i)Pr(2)Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C-F bond of C(6)F(6)is reported. The reaction of1with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexestrans-[Ni(Mes(2)Im)(2)(F)(Ar-F)] (Ar-F= 4-CF3-C(6)F(4)2, C(6)F(5)3, 2,3,5,6-C(6)F(4)N4, 2,3,5,6-C(6)F(4)H5, 2,3,5-C(6)F(3)H(2)6, 3,5-C(6)F(2)H(3)7) in fair to good yields with the exception of the formation of the pentafluorophenyl complex3(less than 20%). Radical species and other diamagnetic side products were detected for the reaction of1with C6F6, in line with a radical pathway for the C-F bond activation step using1. The difluoride complextrans-[Ni(Mes(2)Im)(2)(F)(2)] (9), the bis(aryl) complextrans-[Ni(Mes(2)Im)(2)(C6F5)(2)] (15), the structurally characterized nickel(i) complextrans-[Ni-I(Mes(2)Im)(2)(C6F5)] (11) and the metal radicaltrans-[Ni-I(Mes(2)Im)(2)(F)] (12) were identified. Complex11, and related [Ni-I(Mes(2)Im)(2)(2,3,5,6-C6F4H)] (13) and [Ni-I(Mes(2)Im)(2)(2,3,5-C6F3H2)] (14), were synthesized independently by reaction oftrans-[Ni(Mes(2)Im)(2)(F)(Ar-F)] with PhSiH3. Simple electron transfer from1to C(6)F(6)was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes(2)Im)(2)](+), which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni((i)Pr(2)Im)(2)] (1(ipr)) and [Ni(Mes(2)Im)(2)] (1) into the C-F bond of C6F6. For1(ipr), concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.