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, <b>1</b>; Me₂-cAAC, <b>2</b>), four new iron­(IV) imido complexes, [(IPr)­Fe­(NDipp)₂] (<b>3</b>) and [(Me₂-cAAC)­Fe­(NR)₂] (R = Mes, <b>4</b>; Ad, <b>5</b>; CMe₂CH₂Ph, <b>6</b>), 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 <b>1</b> as representative revealed an intra­molecular alkane dehydrogenation of <b>1</b> to produce the iron­(II) complex [(IMes)­Fe­(NHDipp)­(NHC₆H₃-2-Pr^<i>i</i>-6-CMeCH₂)] (<b>7</b>), a Si–H bond activation reaction of <b>1</b> with PhSiH₃ to produce the iron­(II) complex [(IMes)­Fe­(NHDipp)­(NDipp­SiPhH₂)] (<b>8</b>), and a [2+2]-addition reaction of <b>1</b> with PhNCNPh and <i>p</i>-Pr^<i>i</i>C₆H₄NCO to form the corresponding open-shell formal iron­(IV) mono­imido complexes [(IMes)­Fe­(NDipp)­(N­(Dipp)­C­(NPh)­(NPh))] (<b>9</b>) and [(IMes)­Fe­(NDipp)­(N­(Dipp)­C­(O)­N­(<i>p</i>-Pr^<i>i</i>C₆H₄))] (<b>10</b>), as well as [NDipp]-group-transfer reactions with CO and Bu^<i>t</i>NC. 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