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)(2)C6H3MgBr, 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), eta(3)-CH2CHCH2 (8), eta(3)-CH2C(Me)CH2 (9)). Compounds 29 have been fully characterized by NMR spectroscopy, absorption spectroscopy, solution magnetic susceptibility measurements, elemental analysis, and X-ray crystallography, revealing that 37 are four-coordinate high-spin iron(II) compounds with a formal 14-electron count and 8 and 9 are diamagnetic bis(eta(3)-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(eta(3)-allyl)iron(II) chloride, denoted as [bis(NHC)FeCl(eta(1)-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).