Cyclometallation reactions of a three-coordinate cobalt(i) complex bearing a nonsymmetric N-heterocyclic carbene ligand

The reactions of a three-coordinate cobalt(i) complex [(IAdMes)(PPh3)CoCl] (1) (IAdMes = 1-mesityl-3-adamantyl-4,5-dimethylimidazol-2-ylidene) with LiCH3, LiCH2SiMe3 and NaCp gave ligand-based C(sp(3))-H bond activation products [(IAd ' Mes)Co(eta(6)-C6H5PPh2)] (2), [(IAdMes ')(PPh3)Co(CH2SiMe3)] (4) and [(IAd ' Mes)Co(eta(5)-Cp)] (5), respectively (IAd ' Mes and IAdMes ' denote adamantyl- and mesityl-cyclometallated N-heterocyclic carbene (NHC) ligands, respectively). All the cobalt-NHC complexes have been characterized by various spectroscopic methods, and their structures have been established by single-crystal X-ray diffraction studies. These cyclometallation reactions are among the rare examples of C(sp(3))-H bond activation reactions observed in structurally well-defined low-valent cobalt complexes. Different cyclometallated NHC ligands ([IAd ' Mes](1-)versus [IAdMes '](1-)) and different oxidation states of the metal center (cobalt(i) versus cobalt(ii)) in the resultant cobalt complexes hint at different mechanisms for these cobalt-mediated C(sp(3))-H bond activation reactions.