Air stable pincer (CNC) N-heterocyclic carbene–cobalt complexes and their application as catalysts for C–N coupling reactions

Two new pincer (CNC) N-heterocyclic carbene (NHC)–Co complexes: 3,3′-(dimethylimidazolin-2-ylidene)lutidine chlorocobalt(II) hexafluorophosphate, 3a and 3,3′-(dibenzylimidazolin-2-ylidine)lutidine chlorocobalt(II) hexafluorophosphate, 3b were synthesised in good yields via transmetalation of the corresponding Ag–NHC complexes. Spectroscopic analyses (1H, 13C, 31P, FT-IR) and analytical techniques (melting point, MS-ES+) were used to characterize the complexes. 3b showed distinct paramagnetic properties as observed from its 1H NMR data. Magnetic measurements showed that it has a susceptibility of 4.12 μB, consistent with three un-paired electrons for a high spin paramagnetic Co(II) specie. Both complexes showed excellent activity in catalysing the C–N coupling of aryl halides with primary aryl amines to yield desired secondary aryl amines in good to excellent isolated yields. The catalysts were selective for the desired monoarylated 2° amine products with the suppression of 3° amines and biphenyls. A mechanism is proposed for the selective action of the NHC–Co complexes in this reaction.