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 Bu^tNC. 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+2]-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.