Carbon–Carbon Bond Formation Reactivity of a Four-Coordinate NHC-Supported Iron(II) Phenyl Compound

The
preparation and characterization of a NHC-coordinated (NHC
= N-heterocyclic carbene) ferrous phenyl complex [(IPr2Me2)2FePh2] (1; IPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene)
as well as its C–C bond formation reactivity have been studied.
The four-coordinate iron­(II) phenyl complex was prepared from the
reaction of ferrous chloride with PhMgBr and IPr2Me2. It reacts with nonactivated primary and secondary alkyl
bromides and chlorides to furnish cross-coupling products and the
iron­(II) monophenyl species (IPr2Me2)2FePhX (X = Br (2), Cl). When it is treated with cyclooctatetraene
(cot) or [Cp2Fe]­[BArF4] in the presence
of PMe3, it undergoes coordination or one-electron oxidation
induced reductive elimination of biphenyl to form the corresponding
iron(0) or iron­(I) species [(IPr2Me2)2Fe­(η4-cot)] (3) or [(IPr2Me2)2Fe­(PMe3)2]­[BArF4] (4). All of these iron-containing
products have been fully characterized by various spectroscopic methods.
Complex 1 and (IPr2Me2)2FeCl2 catalyze the reaction of n-C8H17Br with (p-tolyl)­MgBr to afford
the cross-coupling product in moderate yields (49% and 47%), whereas
the reactions employing 4 and 1/PMe3 as catalysts give the cross-coupling product in very low
yields. The results reflect the complexity of the reaction mechanism
of iron-catalyzed coupling reactions.