Three-Coordinate Iron(IV) Bisimido Complexes with Aminocarbene Ligation: Synthesis, Structure, and Reactivity

High-valent iron imido species are implicated as reactive intermediates in many iron-catalyzed transformations. However, isolable complexes of this type are rare, and their reactivity is poorly understood. Herein, we report the synthesis, characterization, and reactivity studies on novel three-coordinate iron­(IV) bisimido complexes with amino­carbene ligation. Using our recently reported synthetic method for [LFe­(NDipp)2] (L = IMes, 1; Me2-cAAC, 2), four new iron­(IV) imido complexes, [(IPr)­Fe­(NDipp)2] (3) and [(Me2-cAAC)­Fe­(NR)2] (R = Mes, 4; Ad, 5; CMe2CH2Ph, 6), were prepared from the reactions of three-coordinate iron(0) compounds with organic azides. Characterization data acquired from 1H and 13C NMR spectroscopy, 57Fe Mössbauer spectroscopy, and X-ray diffraction studies suggest a low-spin singlet ground state for these iron­(IV) complexes and the multiple-bond character of their Fe–N bonds. A reactivity study taking the reactions of 1 as representative revealed an intra­molecular alkane dehydrogenation of 1 to produce the iron­(II) complex [(IMes)­Fe­(NHDipp)­(NHC6H3-2-Pri-6-CMeCH2)] (7), a Si–H bond activation reaction of 1 with PhSiH3 to produce the iron­(II) complex [(IMes)­Fe­(NHDipp)­(NDipp­SiPhH2)] (8), and a [2+2]-addition reaction of 1 with PhNCNPh and p-PriC6H4NCO to form the corresponding open-shell formal iron­(IV) mono­imido complexes [(IMes)­Fe­(NDipp)­(N­(Dipp)­C­(NPh)­(NPh))] (9) and [(IMes)­Fe­(NDipp)­(N­(Dipp)­C­(O)­N­(p-PriC6H4))] (10), as well as [NDipp]-group-transfer reactions with CO and ButNC. Density functional theory calculations suggested that the alkane chain dehydrogenation reaction starts with a hydrogen atom abstraction mechanism, whereas the Si–H activation reaction proceeds in a [2π+2σ]-addition manner. Both reactions have the pathways at the triplet potential energy surfaces being energetically preferred, and have formal iron­(IV) hydride and iron­(IV) silyl species as intermediates, respectively. The low-coordinate nature and low d-electron count (d4) of iron­(IV) imido complexes are thought to be the key features endowing their unique reactivity.