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.本研究系统探究了氮杂环卡宾（N-heterocyclic carbene，NHC）配位的亚铁苯基配合物[(IPr2Me2)₂FePh₂]（化合物1；其中IPr2Me2=1,3-二异丙基-4,5-二甲基咪唑-2-叶立德）的制备、表征及其C-C键形成反应活性。该四配位铁(II)苯基配合物由氯化亚铁与溴化苯基镁（PhMgBr）及IPr2Me2反应制得。它可与未活化的一级、二级烷基溴化物及氯化物反应，生成交叉偶联产物与亚铁(II)单苯基物种(IPr2Me2)₂FePhX（X=Br（化合物2）、Cl）。当该配合物与环辛四烯（cyclooctatetraene, cot）或[Cp₂Fe][BArF4]在三甲基膦（PMe3）存在下反应时，会分别经历配位反应或单电子氧化诱导的联苯还原消除过程，得到对应铁(0)物种[(IPr2Me2)₂Fe(η⁴-cot)]（化合物3）或铁(I)物种[(IPr2Me2)₂Fe(PMe3)₂][BArF4]（化合物4）。所有上述含铁产物均通过多种光谱分析方法得到了完整表征。化合物1与(IPr2Me2)₂FeCl₂可催化正辛基溴（n-C8H17Br）与对甲苯基溴化镁((p-tolyl)MgBr)的交叉偶联反应，产物收率中等（分别为49%和47%）；而以化合物4或1/三甲基膦作为催化剂的反应，仅能获得极低收率的交叉偶联产物。上述结果反映了铁催化偶联反应机理的复杂性。