Synthesis, Structures, and Reactivity of Chelating Bis-N-Heterocyclic-Carbene Complexes of Iron(II)

Abstract

Distorted tetrahedral (bis-carbene)FeX2 complexes can be synthesized by direct metalation of chelating bisimidazolium salts, containing CH2 or phenylene linkers, with Fe(N(SiMe3)2)2(THF). For methylenebis(N-R-imidazole-2-ylidene) ((RC)2CH2), the high-spin complexes ((RC)2CH2)FeX2 (R = iPr or 2,6-di-isopropylphenyl (DIPP), X = I or Br) are isolated in good to excellent yield. In contrast the phenylene-linked congener ((iPrC)2Ph)FeI2 cannot be separated from two co-products, one of which was characterized by X-ray crystallography and zero-field 57Fe Mössbauer spectroscopy as the square-planar complex [((iPrC)2Ph)2Fe][I]2. The disparate reactivity toward Fe(N(SiMe3)2)2(THF) is due to the increase in linker length and decrease in linker flexibility of the phenylene-linked bis-carbene ligand relative to the methylene analogue. The short, flexible −CH2– linker projects the azole rings into the xy plane (defined as the carbene–Fe bond directions) and away from the pseudoaxial anionic substituent, whereas the inflexible phenylene linker ((RC)2Ph) orientates both azole rings into the z direction, which in ((RC)2Y)FeX2 species (Y = CH2 or phenylene) has the greatest steric crowding. Metallacycle distortion in tetrahedral ((RC)2Ph)FeX2 is necessary to project the azoles into the xy plane, destabilizing this geometry. Addition of CO to ((DIPPC)2CH2)FeI2 forms cis-((DIPPC)2CH2)FeI2(CO)2, in which the carbene ligands are strong σ donors based on ν(CO). Halide metathesis of ((DIPPC)2CH2)FeI2 with NaSMe cleanly forms ((DIPPC)2CH2)FeI(SMe), while attempts to exchange the second iodide for –SMe led to carbene dissociation.