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.