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

摘要

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) ] ( 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 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) ] ( 1 ~(ipr) ) and [Ni(Mes _(2) Im) _(2) ] ( 1 ) into the C–F bond of C _(6) F _(6) . For 1 ~(ipr) , 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.