Earth abundant metal complexes of donor functionalised N-heterocyclic carbene ligands: synthesis, characterisation and application as amination catalysts

Abstract

Six new imidazolium salts of the form 3-R-1-picolylimidazolium bromide (1a: R = 4-nitrophenyl, 1b: R = 4-acetylphenyl, 1c: R = 4-cyanophenyl, 1d: R = allyl, 1e: R = butenyl, 1f: R = pentenyl) were synthesised and isolated in high yields. The corresponding Ag–NHC intermediate complexes 2a, 2d, and 2e were transmetalated to yield [M(NHC)₂Cl₂ M = Co, Ni] complexes in good to excellent yields. The Co–NHC (3a, 3d, 3e) and Ni–NHC (3a′, 3d′, 3e′) complexes were relatively stable in air, insoluble in chlorinated solvents but very soluble in methanol and DMSO. Poorly resolved NMR spectra and magnetic susceptibility values of 2.53 and 2.73 μ_B for 3d and 3e respectively suggest both to be paramagnetic cobalt complexes. All the imidazolium salts, isolated Ag–NHC complexes and the corresponding Co and Ni–NHC complexes were characterised by spectroscopic and analytical techniques. The complexes were found to be active at low catalyst loading (1 mol%) for the C–N coupling of aniline with phenyl bromide under mild reaction conditions. In addition, the in situ generated catalyst obtained from a mixture of NiCl₂/1a (1 : 2 molar ratio) initiated the C–N coupling of several aryl amines with substituted aryl bromides bearing a wide variety of functional groups. Good to excellent yields of the desired diaryl amine products were obtained. The yields are comparable to data obtained with palladium catalysts or to data obtained under harsher temperature conditions of Cu mediated Ullman reactions.