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

Abstract

The preparation and characterization of a NHC-coordinated (NHC = N-heterocyclic carbene) ferrous phenyl complex [(IPr₂Me₂)₂FePh₂] (<b>1</b>; 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 (<b>2</b>), Cl). When it is treated with cyclooctatetraene (cot) or [Cp₂Fe]­[BAr^F₄] 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)] (<b>3</b>) or [(IPr₂Me₂)₂Fe­(PMe₃)₂]­[BAr^F₄] (<b>4</b>). All of these iron-containing products have been fully characterized by various spectroscopic methods. Complex <b>1</b> and (IPr₂Me₂)₂FeCl₂ catalyze the reaction of <i>n</i>-C₈H₁₇Br with (<i>p</i>-tolyl)­MgBr to afford the cross-coupling product in moderate yields (49% and 47%), whereas the reactions employing <b>4</b> and <b>1</b>/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