Iron(II) Dihydrocarbyls Supported by a Biphenyl-Linked Bis(benzimidazol-2-ylidene) Ligand: Syntheses and Characterization

The use of a biphenyl-linked bis­(benzimidazol-2-ylidene)
ligand enables the access of dialkyl-, diaryl-, and diallyliron­(II)
species with bis­(NHC) ligation. Deprotonation of the biphenyl-linked
bis­(benzimidazolium) salt (1) with 2 equiv of KH affords
the biphenyl-linked dibenzotetraazafulvalene 2 in high
yield. Treatment of 2 with 0.5 equiv of [(TMEDA)­FeCl2]2 leads to the formation of the biphenyl-linked
bis­(benzimidazol-2-ylidene)­iron­(II) dichloride 3. Further
salt elimination reactions between 3 and 2 equiv of TMSCH2Li, C6H5CH2K, PhMgBr, 3,5-(CF3)2C6H3MgBr, C3H5MgCl, and 2-Me-C3H4MgCl yield
the corresponding biphenyl-linked bis­(benzimidazol-2-ylidene)­iron­(II)
dihydrocarbyls, denoted as [bis­(NHC)­FeR2] (R = CH2TMS (4), CH2Ph (5), Ph (6), C6H3-3,5-(CF3)2 (7), η3-CH2CHCH2 (8), η3-CH2C­(Me)­CH2 (9)). Compounds 2–9 have been fully characterized by NMR spectroscopy, absorption
spectroscopy, solution magnetic susceptibility measurements, elemental
analysis, and X-ray crystallography, revealing that 3–7 are four-coordinate high-spin iron­(II) compounds
with a formal 14-electron count and 8 and 9 are diamagnetic bis­(η3-allyl)­iron­(II) species with
18 valence electrons. Attempts to prepare the bis­(hydrocarbyl)­iron­(II)
complexes of bulky allyls lead to the isolation of mono­(η1-allyl)­iron­(II) chloride, denoted as [bis­(NHC)­FeCl­(η1-TMSCHCHCHTMS)] (10), in low yield, whose structure
has also been established by an single-crystal X-ray diffraction study.
The achievement of the syntheses of these bis­(hydrocarbyl)­iron­(II)
compounds proves the unique electronic and steric features of the
biphenyl-linked bis­(benzimidazol-2-ylidene).