Synthesis and catalytic activity of iron complexes with bidentate NHC ligands

A family of well-defined Fe-II complexes of the type {BnN(N-CH2(CH2)(n)-N'-tert-butyl-imidazole-2-ylidene)(2)}FeCl2 (Bn = benzyl; n = 1 (1) or 2 (2)), {BnN(N-CH2(CH2)(n)-N'-methylbenzimidazole-2-ylidene)(2)}FeCl2 (n = 1 (3) or 2 (4)) and {BnN(N-CH2CH2CH2-N'-methylbenzimidazole-2-ylidene)(2)}FeBr2 (5) has been synthesized. These complexes are rare examples of Fe species supported by bidentate NHC ligands. Complexes 2, 3, 4 and 5 were characterized by X-ray crystallography and in all cases a distorted tetrahedral geometry is observed around the Fe center. The magnetic data is consistent with the complexes containing non interacting high spin Fe-II centers (S = 2) and indicates that a large zero-field splitting (D) is present. The new complexes are highly active pre-catalysts for the homo-coupling of Grignard reagents.