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

Abstract

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.

Synthesis, characterization and C–H amination reactivity of nickel iminyl complexes

“…We and others have pursued the synthesis of late transition metal, low-coordinate MLMBs to investigate their resulting group transfer reactivity. [1][2][3][4][5][6]8,9,[14][15][16][17][18][35][36][37][38][39][40][41][42][43] Our group has focused on the generation of highly electrophilic MLMBs by employing weak-eld dipyrrin ancillary ligands. 37,39,40,42,44 Ferric imido complexes bearing the dipyrrin ligand feature high-spin (S ¼ 5/2) electronic structures, [37][38][39][40] while the Co III analogues thermally access open-shell excited states.…”

High‐Spin Imido Cobalt Complexes with Imidyl Radical Character**

“…Whereas most isolable imido metal complexes are found in a low‐spin state, those with higher spin‐states tend to be more reactive. [ 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 ]…”

Imido Cobalt Complexes with Imidyl Character

“…[33,41] The formation of a metal-carbon bond however was only observed for a mesityl imido cobalt(III) complex by Betley and coworkers [22] and a transient aryl imido iron(II) complex by Cheng et al. [42] 1 H NMR spectroscopic examination of the reaction of [1]with Mes-N3 revealed an immediate formation of [3] -, even at -30°C, which corroborated the labile nature of [2] -. This hints to the high reactivity of the imido cobalt complex [2] -, which may be attributed to its elongated Co-Nimide bond.…”