Single-electron oxidation of N-heterocyclic carbene-supported nickel amides yielding benzylic C–H activation

Abstract

The dimeric Ni(I)–Ni(I) N-heterocyclic carbene complex {(IPr)Ni(μ-Cl)}₂ (3; IPr = 1,3-(2,6-ⁱPr₂C₆H₃)₂imidazolin-2-ylidene)) reacts with the lithium terphenylamides LiNHdmp and LiNHdippp (dmp = 2,6-di(mesityl)phenyl; dippp = 2,6-bis(2,6-di-iso-propylphenyl)phenyl) to give the monomeric Ni(I) amides (IPr)Ni(NHdmp) (4) and (IPr)Ni(NHdippp) (5), respectively. These nickel amides are 1-electron paramagnets, and crystallographic characterization indicates both are stabilized by Ni–C(ipso) interactions with a flanking aryl group of the terphenyl fragment. This results in significant deviation from the linear C_NHC–Ni–N geometry typical for a two-coordinate transition-metal complex (112.17(9)° in 4, 116.41(9)° in 5). One-electron oxidation of 4 by ferrocenium results in intramolecular deprotonation at a terphenyl benzylic position by the amide, giving the diamagnetic Ni(II) complex [(IPr)Ni(κ²-C,N:NH₂C₆H₃(Mes)C₁₀H₉)][B(Ar^F)₄] (7). DFT calculations on oxidized 4 (i.e., 4⁺) indicate short amide N⋯CH₃ interactions. One-electron oxidation of 5 by ferrocenium gives a new high-spin Ni(II) amide complex salt, [(IPr)Ni(NHdippp)][B(Ar^F)₄] (9). The solid-state structure of 9 indicates it maintains the bent C_NHC–Ni–N core. Unlike three-coordinate cationic Ni(II) amides, 9 has not been observed to undergo smooth deprotonation (at N) to afford a two-coordinate imido complex.