Cheng, J.; Liu, J.; Leng, X.; Lohmiller, T.; Schnegg, A.; Bill, E.; Ye, S.; Deng, L.: A Two-Coordinate Iron(II) Imido Complex with NHC Ligation: Synthesis, Characterization, and Its Diversified Reactivity of Nitrene Transfer and C-H Bond Activation. Inorganic Chemistry 58 (2019), p. 7634-7644
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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; ArTrip = 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl) prepared from the reaction of an iron(0) complex with the bulky azide ArTripN3. 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) Å 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 π acid, readily afford nitrene transfer products, ArCF3CH═CHNHArTrip and ArTripNCO, respectively, yet no similar reaction occurs when 3 is treated with electron-rich alkenes and PMe3. Moreover, 3 is inert toward the weak C(sp3)–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(sp3)–H bond functionalization was observed by adding (p-Tol)2CN2 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.