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

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.

含芳基取代氮杂环卡宾配体（N-heterocyclic carbene ligands）的钴(II)烷基化合物，可通过前体氯配合物[CoCl₂(IMes)₂]与[Co₂Cl₂(μ-Cl)₂(IPr)₂]（其中IMes=1,3-二均三甲苯基咪唑-2-卡宾；IPr=1,3-双(2,6-二异丙基苯基)咪唑-2-卡宾）与格氏试剂（Grignard reagents）的反应制备得到。本文报道了同时具备四配位与三配位几何构型的烷基配合物实例。氯配合物[CoCl₂(IMes)₂]呈假四面体几何构型，基态自旋为S=3/2；而烷基配合物[Co(CH₃)₂(IMes)₂]则采取平面正方形几何构型，对应基态自旋为S=1/2。
与[Co(CH₃)₂(IMes)₂]不同，[Co(CH₂SiMe₃)₂(IPr)]呈现三配位平面三角几何构型，基态自旋为S=3/2。本文考察了[CoCl₂(IMes)₂]在熊田偶联反应（Kumada couplings）中的催化效能，同时也研究了该类烷基配合物与一氧化碳（CO）的反应行为。本文将结合钴基氮杂环卡宾配合物催化的C-C偶联反应（C–C coupling reactions），对上述化合物的结构与反应活性展开讨论。