A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation

Iron terminal imido species are typically implicated as reaction intermediates in iron-catalyzed transformations. While a large body of work has been devoted to mid- and high-valent iron imidos, to date the chemistry of iron(II) imidos has remained largely unexplored due to the difficulty in accessing them. Herein, we present a study on the two-coordinate iron(II) imido complex [(IPr)Fe(NArTrip)] (3; IPr = 1,3-bis(2',6'-diisopropylphenyl)imidazol-2-ylidene; Ar-Trip = 2,6-bis(2',4',6'-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide (ArN3)-N-Trip. Spectroscopic investigations in combination with DFT calculations established a high-spin S = 2 ground spin state for 3, consistent with its long Fe-N multiple bond of 1.715(2) angstrom revealed by X-ray diffraction analysis. Complex 3 exhibits unusual activity of nitrene transfer and C-H bond activation in comparison to the reported iron imido complexes. Specifically, the reactions of 3 with CH2=CHArCF3, an electron-deficient alkene, and CO, a strong pi acid, readily afford nitrene transfer products, (ArCH)-C-CF3=CHNHArTrip and (ArNCO)-N-Trip, respectively, yet no similar reaction occurs when 3 is treated with electron rich alkenes and PMe3. Moreover, 3 is inert toward the weak C(sp(3))-H bonds in 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave the stronger C(sp)-H bond in p-trifluoromethylphenylacetylene to form an iron(II) amido alkynyl complex. Interestingly, intramolecular C(sp(3))-H bond functionalization was observed by adding (p-Tol)(2)CN2 to 3. The unique reactivity of 3 is attributed to its low-coordinate nature and the high negative charge population on the imido N atom, which render its iron-imido unit nucleophilic in nature.