A Two-Coordinate Iron(II) Imido Complex with NHC Ligation:Synthesis Characterizationand Its Diversified Reactivity of Nitrene Transfer and C–HBond Activation

摘要

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(NAr^Trip)] (>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 Ar^TripN3. 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=CHAr^CF3, an electron-deficient alkene,and CO, a strong π acid, readily afford nitrene transfer products,Ar^CF3CH=CHNHAr^Trip and Ar^TripNCO, 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 bondsin 1,4-cyclohexadiene, THF, and toluene, whereas it can cleave thestronger C(sp)–H bond in p-trifluoromethylphenylacetyleneto form an iron(II) amido alkynyl complex. Interestingly, intramolecularC(sp³)–H bond functionalization was observed byadding (p-Tol)2CN2 to >3. The unique reactivity of >3 is attributed toits low-coordinate nature and the high negative charge populationon the imido N atom, which render its iron–imido unit nucleophilicin nature.