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 of 1 and [Ni((i)Pr(2)Im)(2)] (1ipr) ((i)Pr(2)Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C–F bond of C(6)F(6) is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes(2)Im)(2)(F)(Ar(F))] (Ar(F) = 4-CF(3)-C(6)F(4)2, C(6)F(5)3, 2,3,5,6-C(6)F(4)N 4, 2,3,5,6-C(6)F(4)H 5, 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 complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C(6)F(6), in line with a radical pathway for the C–F bond activation step using 1. The difluoride complex trans-[Ni(Mes(2)Im)(2)(F)(2)] (9), the bis(aryl) complex trans-[Ni(Mes(2)Im)(2)(C(6)F(5))(2)] (15), the structurally characterized nickel(i) complex trans-[Ni(I)(Mes(2)Im)(2)(C(6)F(5))] (11) and the metal radical trans-[Ni(I)(Mes(2)Im)(2)(F)] (12) were identified. Complex 11, and related [Ni(I)(Mes(2)Im)(2)(2,3,5,6-C(6)F(4)H)] (13) and [Ni(I)(Mes(2)Im)(2)(2,3,5-C(6)F(3)H(2))] (14), were synthesized independently by reaction of trans-[Ni(Mes(2)Im)(2)(F)(Ar(F))] with PhSiH(3). Simple electron transfer from 1 to 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)] (1ipr) and [Ni(Mes(2)Im)(2)] (1) into the C–F bond of C(6)F(6). For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.