NHC Complexes of Cobalt(II) Relevant to Catalytic C–C Coupling Reactions

Abstract

Alkyl compounds of cobalt(II) containing aryl-substituted N-heterocyclic carbene ligands have been prepared by reaction of the precursor chloro complexes [CoCl2(IMes)2] and [Co2Cl2(μ-Cl)2(IPr)2] (IMes = 1,3-dimesityl-imidazol-2-ylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) with Grignard reagents. Examples of alkyl complexes possessing both four-coordinate and three-coordinate geometries are reported. The chloro complex [CoCl2(IMes)2] adopts a pseudotetrahedral geometry displaying an S = 3/2 ground state, whereas the alkyl complex [Co(CH3)2(IMes)2] adopts a square-planar geometry consistent with an S = 1/2 ground state. In contrast to [Co(CH3)2(IMes)2], [Co(CH2SiMe3)2(IPr)] exhibits a three-coordinate trigonal-planar geometry displaying an S = 3/2 ground state. The catalytic efficacy of [CoCl2(IMes)2] in Kumada couplings is examined, as is the chemistry of the alkyl complexes toward CO. The structure and reactivity of these compounds is discussed in the context of C–C coupling reactions catalyzed by cobalt NHCs.