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) 2 FeR 2 ] (IEt = 2,5-diethyl-3,4-dimethylimidazol-1-ylidene, R = Me ( 2a ), CH 2 TMS ( 2b )) and [(IPr)FeMes 2 ] ( 3, IPr = 2,5-diisopropyl-3,4-dimethylimidazol-1-ylidene) were prepared in good yields via salt elimination reactions of [(NHC) 2 FeCl 2 ] ( 1 ) with alkylation reagents. The interaction of 1 with PhLi gave a mixture of dinuclear complexes [Cl(IEt)Fe(IEt′) 2 Fe(IEt)Cl] ( 4a ) and [Ph(IEt)Fe(IEt′) 2 Fe(IEt)Ph] ( 4b ) (IEt′ = 3-Et-4,5-Me 2 -2-ylideneimidazolyl anion), in which N−C(ethyl) bond cleavage of the NHC ligand was involved. Complexes 2a − 4b were characterized by 1 H 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 Pr i N═C═NPr i to yield dimesityl ketone and [(IPr)Fe(Mes)(η 2 -Pr i NC(Mes)NPr i )] ( 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−CH 2 TMS and Ph−Mes.