Synthesis, Structure, and Reactivity of Organo-Iron(II) Complexes with N-Heterocyclic Carbene Ligation

The synthesis, structure, and reactivity of some organo-iron complexes with monodentate N-heterocyclic carbene (NHC) ligation were studied. Mononuclear ferrous complexes [(IEt)2FeR2] (IEt = 2,5-diethyl-3,4-dimethylimidazol-1-ylidene, R = Me (2a), CH2TMS (2b)) and [(IPr)FeMes2] (3, IPr = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) were prepared in good yields via salt elimination reactions of [(NHC)2FeCl2] (1) with alkylation reagents. The interaction of 1 with PhLi gave a mixture of dinuclear complexes [Cl(IEt)Fe(IEt′)2Fe(IEt)Cl] (4a) and [Ph(IEt)Fe(IEt′)2Fe(IEt)Ph] (4b) (IEt′ = 3-Et-4,5-Me2-2-ylideneimidazolyl anion), in which N−C(ethyl) bond cleavage of the NHC ligand was involved. Complexes 2a−4b were characterized by 1H NMR, elemental analyses, and single-crystal X-ray diffraction studies. Solution magnetism measurement by Evan’s method revealed the high-spin electronic configuration for the mononuclear organo-iron(II) complexes 2a, 2b, and 3. Reactivity studies showed the tetrahedral complex 2a was inert toward many unsaturated organic substrates, whereas the trigonal-planar complex 3 could react with CO and carbodiimide PriNCNPri to yield dimesityl ketone and [(IPr)Fe(Mes)(η2-PriNC(Mes)NPri)] (5), respectively. Relevant to iron-catalyzed Kumada couplings, both complexes 2b and 3 were found reactive with PhI to yield the corresponding carbon−carbon bond formation products Ph−CH2TMS and Ph−Mes.