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

Abstract

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(sp³)–H bond activation products [(IAd′Mes)Co(η⁶-C₆H₅PPh₂)] (2), [(IAdMes′)(PPh₃)Co(CH₂SiMe₃)] (4) and [(IAd′Mes)Co(η⁵-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(sp³)–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(sp³)–H bond activation reactions.