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

Distorted tetrahedral (bis-carbene)FeX(2) complexes can be synthesized by direct metalation of chelating bisimidazolium salts, containing CH(2) or phenylene linkers, with Fe(N(SiMe(3))(2))(2)(THF). For methylenebis(N-R-imidazole-2-ylidene) (((R)C)(2)CH(2)), the high-spin complexes (((R)C)(2)CH(2))FeX(2) (R = (i)Pr or 2,6-di-isopropylphenyl (DIPP), X = I or Br) are isolated in good to excellent yield. In contrast the phenylene-linked congener (((iPr)C)(2)Ph)FeI(2) cannot be separated from two co-products, one of which was characterized by X-ray crystallography and zero-field (57)Fe Mossbauer spectroscopy as the square-planar complex [(((iPr)C)(2)Ph)(2)Fe][I](2). The disparate reactivity toward Fe(N(SiMe(3))(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 -CH(2)-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 (((R)C)(2)Ph) orientates both azole rings into the z direction, which in (((R)C)(2)Y)FeX(2) species (Y = CH(2) or phenylene) has the greatest steric crowding. Metallacycle distortion in tetrahedral (((R)C)(2)Ph)FeX(2) is necessary to project the azoles into the xy plane, destabilizing this geometry. Addition of CO to (((DIPP)C)(2)CH(2))FeI(2) forms cis-(((DIPP)C)(2)CH(2))FeI(2)(CO)(2), in which the carbene ligands are strong sigma donors based on nu(CO). Halide metathesis of (((DIPP)C)(2)CH(2))FeI(2) with NaSMe cleanly forms (((DIPP)C)(2)CH(2))FeI(SMe), while attempts to exchange the second iodide for (-)SMe led to carbene dissociation.