Three-Coordinate Iron(II) Expanded Ring N-Heterocyclic Car-bene Complexes

A sterically demanding 7-membered expanded ring N-heterocyclic carbene (NHC) ligand allows access to rare examples of 3-coordinate iron(II)-NHC complexes incorporating only halide co-ligands of the general formulae, [Fe(NHC)X2] (NHC = 7-DiPP, X = Br or Cl), 1 and 2, respectively. Reducing the steric influence of the ancillary NHC ligand through modulation of the N-aryl substituents leads to either 4- or 3-coordinate complexes of the general formula, [Fe(NHC)Br2(THF)], 3 or [Fe(NHC)Br2], 4 (NHC = 7-Mes) dependent upon the solvent of recrystallization. The further reduction of NHC steric influence results in 4-coordinate geometries at iron in the form of either the dimeric species, [Fe(NHC)Br(μ-Br)]2, 5 or [Fe(NHC)Br2(THF)], 6 (NHC = SDiPP), again dependent upon the solvent of recrystallization. The series of compounds 1-6 have been analyzed by 1H NMR spectroscopy, X-ray crystallography, elemental microanalysis, Mössbauer spectroscopy (for 1, 3-5) and Evans method magnetic susceptibility. In addition to these measurements the 3-coordinate species 1 and 4 have been further analyzed by SQUID magnetometry and CASSCF calculations which show significant magnetic anisotropy that is extremely sensitive to the coordination geometry.