Square-Planar Cobalt Complexes with Monodentate N-Heterocyclic Carbene Ligation: Synthesis, Structure, and Catalytic Application

The unique cobalt(I)-NHC complex [Co(IEt)4][BPh4] (1) (IEt = 1,3-diethyl-4,5-dimethylimidazole-2-ylidene) and its analogues employing other carbene ligands were prepared by the reactions of [Co(PPh3)3Cl] with free carbene ligands followed by anion exchange. X-ray diffraction revealed 1 features a homoleptic cation [Co(IEt)4]+ with square-planar geometry. Electrochemical studies showed the square-planar complexes support the redox series [Co(IEt)4]+/2+/3+, of which the divalent cation has been synthesized and structurally characterized as in [Co(IEt)4][BF4]2 (2). Both EPR and DFT studies indicated 2 has a low-spin Co(II) center. 1 can efficiently catalyze the oxidative homocoupling reactions of aryl Grignard reagents. Investigations on the stoichiometric reactions of 1 with organic halides and 2 with aryl Grignard reagents established an interesting radical mechanism involving the [Co(IEt)4]+/2+ redox shuttle for these cobalt-NHC complex catalyzed homocoupling reactions.