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; 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.


■ INTRODUCTION
Iron terminal imido species are often implicated as key intermediates in iron-catalyzed nitrogen-group-transfer reactions: e.g., alkene aziridation, sulfur imidation, and C−H bond amination. 1−6 A range of iron-mediated nitrogen fixation reactions have also proposed with iron imido species as intermediates en route to the final products, NH 3 and N(SiMe 3 ) 3 . 7−10 The importance of these chemical transformations has attracted great interest in the formation, structural and spectroscopic features, and reactivity of iron imido species. Traditional wisdom had viewed such complexes as highly reactive, nonisolable intermediates, and this knowledge had been largely restricted to theoretical and mass spectrometry studies on the bare iron imide [FeNH] + . 11 Recent studies showed that, by the use of appropriate ancillary ligands, iron terminal imido species can be effectively stabilized and are amenable to isolation. Since Lee's report on the first isolable iron terminal imido complex, [Fe 4 (μ 3 -NBu t ) 4 (NBu t )-Cl 3 ], in 2000, 12 more than 50 structurally well characterized iron terminal imido complexes have been reported. 13 −39 In addition, modern spectroscopic methods have enabled probing iron imido species that merely persist at low temperatures, 40−42 further enriching our knowledge of this type of metal−ligand multiple-bond species.
The aforementioned status quo prompted us to explore the chemistry of iron(II) imido species. Herein, we present a study on a two-coordinate iron(II) imido complex with Nheterocyclic carbene (NHC) ligation, [(IPr)Fe(NAr Trip )] (Chart 1), which was prepared from the reaction of its iron(0) precursor with the bulky organic azide Ar Trip N 3 . Spectroscopic characterizations and DFT calculations collectively established a high-spin (S = 2) ground state for this iron(II) imido complex, which is unprecedented for iron(II) imidos. Reactivity studies revealed that the iron(II) imido complex can react with electron-deficient alkenes, e.g. 3,5bis(trifluoromethyl)phenylethylene, to produce enamines, but it is inert toward electron-rich alkenes. Furthermore, the reactions of [(IPr)Fe(NAr Trip )] with CO and PMe 3 were found to give distinct outcomes; nitrene transfer products, an isocyanide and iron(0) species, were formed in the former reaction, whereas the ligand exchange product, the low-spin iron(II) imido complex [(Me 3 P) 3 Fe(NAr Trip )], was obtained for the latter. [(IPr)Fe(NAr Trip )] exhibits limited hydrogen atom abstraction activity toward 1,4-cyclohexadiene, THF, and toluene, but it is capable of activating a much stronger C(sp)− H bond to afford iron(II) amido complex when it is treated with a terminal alkyne. Interestingly, intramolecular C(sp 3 )−H bond activation was observed by adding an diazo compound to the iron(II) imido complex. These observations signify the unique reactivity of the iron(II) imido complex in comparison to those of mid-and high-valent iron imidos and also earlytransition-metal imidos. 2,4,45,46 The high negative charge population on the imido nitrogen atom and the low coordination number of the iron center are considered the key factors that endow the iron imido complex with such unique reactivity.
The attainment of 2 rather than the desired iron(II) imido species (IPr)Fe(NAr Mes ) hints that the iron(II) imido species might be highly reactive to initiate intramolecular C−H bond activation, which points to the necessity of further increasing steric hindrance in order to protect the FeN core. Toward this goal, the reaction of 1 with the sterically more demanding aryl azide Ar Trip N 3 (Ar Trip = 2,6-bis(2′,4′,6′-triisopropylphenyl)phenyl) 49 was examined, which to our delight led to the preparation of the targeted iron(II) imido complex. Specifically, treatment of 1 with 1 equiv of Ar Trip N 3 in n-hexane results in the precipitation of a brown solid. After workup and recrystallization, an iron(II) imido complex, [(IPr)Fe-(NAr Trip )] (3), was isolated in 75% yield as a brown crystalline solid (Scheme 1). Complex 3 is paramagnetic, and its measured solution magnetic susceptibility (Evans method, μ eff = 5.5(1) μ B in C 6 D 6 at room temperature) is close to those of the reported low-coordinate high-spin iron(II) complexes. 50,51 The 1 H NMR spectrum of 3 (measured in C 6 D 6 ) features 13 broad signals in the range +125 to −180 ppm ( Figure S21), suggesting C 2v molecular symmetry. Complex 3 is air-, moisture-, and heat-sensitive. Under a dinitrogen atmosphere at room temperature, complex 3 in C 6 D 6 solution showed complete decomposition in 2 days, as evidenced by the disappearance of its characteristic 1 H NMR signals. 1 H NMR analysis indicated the formation of new species, but the attempts to isolate them were unsuccessful.
A single-crystal X-ray diffraction study established the molecular structure of 3 as a two-coordinate NHC-iron-  52 and apparently different from the right angle (90°) observed for [(IPr)Co(NAr Mes )]. 47 Complex 3 is the first example of a twocoordinate iron(II) complex with NHC ligation. 53,54 Its Fe− C(carbene) distance (2.042(2) Å) is found to be close to those of the two-coordinate iron(I) complexes [(cyIDep) 2 Fe]-[BAr F 4 ] (1.996(7) Å) 55 and [(IPr)Fe(N(SiMe 3 )(Dipp))] (2.014(2) Å). 56 The Fe−N(imido) bond distance of 1.715(2) Å determined for 3 is longer than those of the lowspin iron(III), iron(IV), and iron(V) imido complexes 2,4,20,33 and comparable to those of the high-spin iron(III) complexes [( Ar DP Mes )Fe III (NR)] (R = Mes, 1.708(2) Å) 37  Magnetic susceptibility measurements ( Figure 2) at variable temperature with a superconducting quantum inference device (SQUID) show that the effective magnetic moment of 3 at room temperature (μ eff ≈ 6.2 μ B ) is higher than the spin-only value (4.9 μ B ) expected for an S = 2 state, suggesting an unquenched orbital momentum contribution. Interestingly, with decreasing temperatures μ eff first increases, and then monotonically plummets; consequently, it reaches a maximum around 6.4 μ B at 80 K. Similar situations were encountered for two-coordinate S = 2 iron(II) alkyl, amido, and aryloxide complexes. 57−59 Our exploring simulations revealed that the overshooting behavior of μ eff found for 3 originates mainly from large g-anisotropy. The variable-temperature and variable-field magnetization studies revealed that all isofield curves are nearly superimposable, which reflects an exceedingly large magnetic anisotropy in 3. The simulation of the combined SQUID data yielded g || = 2.08(6), and g ⊥ = 3.05 (6), but the D and E/D values only can be narrowed down to D < −10 cm −1 and 0 < E/D < 0.11, because the computed relevant error surface is rather flat (Figures S45 and S46).
Zero-field 57 Fe Mossbauer measurements at various temperatures ranging from 80 to 293 K exhibited a broad and asymmetric quadrupole doublet ( Figure S17), probably due to intermediate spin relaxation. 60 As depicted in Figure 3,   Fields were applied perpendicular to the γ-rays. Solid lines represent the best fit using the following parameters: δ = 0.56 mm/s, ΔE Q = − 2.08 mm/s, η = 0 (fixed), A ∥ = +65.8 T, D = −42 cm −1 (fixed), E/D = 0.07 (fixed), g || = 2.08 (fixed), and g ⊥ = 3.05 (fixed). The spectra show the sample contains 25% diamagnetic impurity. However, the much sharper features of 3 can be readily identified and the existence of the impurity does not affect our further analysis.

Inorganic Chemistry
Article magnetic Mossbauer spectra of complex 3 in the solid state, recorded at 1.7 K with 4.0 and 7.0 T applied perpendicular to the γ-rays, display well-resolved magnetic splitting. The relative intensity of the six lines indicates that complex 3 features extremely strong easy-axis magnetization at low temperatures. Assuming that the principal axis system of the electric field gradient tensor is largely collinear with that of D, a satisfactory fit gave δ = 0.56(2) mm/s, ΔE Q = − 2.08(6) mm/s, and A ∥ = +65.8(6) T. Precise determination of the D and E/D values by the applied Mossbauer measurements is hampered by fast relaxation of 3, due to the existence of the unquenched orbital angular momentum. Therefore, during the simulations, D, E/ D, g || , and g ⊥ were fixed to those determined by the above magnetometry investigations. The simulations do not allow accurate determination of the A ⊥ and η values either, because nearly all 57 Fe nuclear spins are polarized along the z axis of the ZFS tensor. According to the observed A ∥ value, complex 3 possesses an internal magnetic field of ∼132 T. The measured Mossbauer parameters are comparable to those found for analogous two-coordinate high-pin ferrous complexes. 61−64 Figure 4 illustrates the electronic structure computed by B3LYP calculations. The computations predicted an electron configuration of (d x 2 −y 2 ) 2 (d xy ) 1 (d z 2 ) 1 (d xz/yz ) 2 for the ground state of 3. The linear coordination environment of the iron center in 3 leads to formation of two π bonds between Fe 3d xz/yz and N 2p x/y atomic orbitals and one σ bond between Fe 3d z 2 and N p z . As borne out by the shape of the orbital labeled as Fe 3d z 2 , significant 3d z 2 −4s mixing considerably reduces the antibonding interaction with the two ligand and hence stabilizes the σ* orbital relative to the two π* orbitals, a bonding feature found for the low-spin complex [(PhB(o-C 6 H 4 PPr i 2 ) 3 )Fe(NAd)] + as well. 4 Furthermore, the remaining Fe 3d x 2 −y 2 and 3d xy orbitals are essentially nonbonding in nature. The orbital occupation pattern of the ground state therefore defines a formal bond order of 1.5 for the Fe− N(imido) interaction. In line with this reasoning, 3 features a longer Fe−N bond distance relative to those measured for the low-spin Fe(II)-imido complexes. 15,26 The Fe−N π interaction in 3 is somewhat ionic, as suggested by the uneven contribution of the Fe and N atomic orbitals to the two Fe− N π* orbitals. As a consequence, the imido ligand of 3 donates less electron density to the Fe center and its N atom thus accumulates sizable negative charge. This notion is consistent with the computed charge population of N (−0.57), which is much higher than that of Fe (−0.18). Our further theoretical results showed that the computed negative charge population on the N atom of 3 is much higher than those of the related iron(III) imido/iminyl complexes [(nacnac terphenyl )Fe(NAd)] 23 (Table S3), which hints at stronger basicity/nucleophilicity of the imido moiety in 3. On the basis of the metal−ligand interaction discussed above for 3, one can envision it has a rather low lying excited state with an electron configuration of (d x 2 −y 2 ) 1 (d xy ) 2 (d z 2 ) 1 (d xz/yz ). A CASSCF (12,13) calculation reveals that this excited state is only 190 cm −1 above the ground state. Both states are hence nearly degenerate, which explains the unusual magnetic properties observed for 3. 61−64 Reactivity of [(IPr)Fe(NAr Trip )]. Complex 3 represents the first example of isolable iron(II) imido species featuring a highspin (S = 2) ground state. As mentioned earlier, structurally well characterized iron(II) terminal imido complexes are scarce, and their chemistry has remained largely unexplored. 15,26 The status quo thus prompted us to further investigate the reactivity of 3.
Reactions with Alkenes. In iron-catalyzed alkene aziridation reactions, iron terminal imido complexes were proposed as key intermediates. 44,65−68 However, isolable iron imido species capable of reacting with alkenes are exceedingly rare. The only known example is Betley's iron(III) iminyl complex [( Ar DP Mes )Fe III ( •− NAd)Cl], which can react with alkenes to afford aziridines and allyl amines. 43 As elaborated below, complex 3 is reactive toward electron-deficient alkenes to give distinct net nitrene transfer products, enamines; more importantly, the transformation displays a disparate reactivity pattern in comparison to that observed for the iron(III) iminyl complexes.
Examining the reactions of 3 with different alkenes revealed the higher activity of 3 toward electron-deficient terminal alkenes over electron-rich alkenes. For instance, treatment of 3 with 3 equiv of 3,5-bis(trifluoromethyl)phenylethylene (Ar CF3 CHCH 2 ) at room temperature in Et 2 O can lead to the full consumption of 3 in hours. In contrast, 3 is inert toward 4-methoxyphenylethylene, 1-octene, and 4-methylcyclohexene. Our further workup and characterization study revealed that the reaction of 3 with 3,5-bis(trifluoromethyl)phenylethylene gives a mixture of the iron(0) complex [(IPr)Fe(CH 2 CHAr CF3 ) 2 ] (4), the enamine trans-Ar CF3 CH CHNHAr Trip (5), and the iron(II) enamino complex [(aIPr)-Fe(NAr Trip CHCHAr CF3 )(CHMeAr CF3 )] (6; aIPr = 1,3-bis-(2′,6′-diisopropylphenyl)imidazol-4-ylidene) (Scheme 2), along with unidentified fluoro-containing species. Complex 4 has been characterized by NMR and elemental analysis. Its 1 H NMR spectrum displays a resonance pattern similar to that of 1. The yield of 4 in this reaction is low (ca. 6% based on 19 F NMR analysis). On the other hand, the ligand replacement reaction of 1 with 2 equiv of Ar CF3 CHCH 2 proved a more reliable synthetic route for 4. The enamine 5 was isolated in 62% yield when chromatographic separation was performed on the quenched reaction mixture. The high isolated yield of 5 partially comes from protonation of the enamindo complex 6. Compound 5 has been characterized by NMR and mass spectroscopy. Its 1 H NMR spectrum exhibits a diagnostic vicinal, trans coupling constant of 13.8 Hz for its olefinic hydrogens, and DEPT 13 C NMR spectra also support its identity. 19 F NMR analysis suggested a moderate yield (ca. 50%) for 6. Because of its high solubility in common organic solvent, only a small amount of red crystals of 6 could be Probably driven by the relief of steric congestion, the reaction also induced the conversion of IPr into its abnormal form aIPr. Similar normal to abnormal rearrangement has been observed in three-coordinate iron(II) amido NHC complexes. 69 In addition to 4−6, 19 F NMR analysis indicated that a species which shows 19 F NMR signals at −62.18 and −62.54 ppm were also formed in the reaction of 3 with Ar CF3 CHCH 2 .
However, attempts to isolate the species were unsuccessful. In addition to the reaction with 3,5-bis(trifluoromethyl)phenylethylene, complex 3 could also react with ethylene, styrene, and vinyltrimethylsilane, which are more electron rich than 3,5-bis(trimethyl)phenylethylene. The latter reactions, however, proceeded more sluggishly. GC-MS analyses on these reaction mixtures indicated the formation of new organic species having molecular masses anticipated for the corresponding enamines. However, even when the alkenes were used in a large excess (more than 100 equiv), the yields of the desired enamines in 24 h were low (<10%), and further isolation attempts were unsuccessful. The production of enamine in the reaction of 3 with 3,5bis(trifluoromethyl)phenylethylene is different from the formation of aziridines in the reactions of Betley's iron(III) iminyl complexes [( Ar DP Mes )Fe III ( •− NAd)Cl] with substituted styrenes. 43 For the latter, irrespective of electron-donating and -withdrawing groups on the para position of styrenes, all reactions showed rates accelerated over that with unsubstituted styrene. According to this finding, the aziridation reaction was proposed to proceed via the sequential steps of the attack of iminyl radical on styrenes followed by radical rebound. The different reaction outcome reflects the influence of the electronic structure of the nitrogen group, [NAr Trip ] 2− in 3 versus [NAd] •− in [( Ar DP Mes )Fe III ( •− NAd)Cl], on its reactivity toward alkene. On the other hand, the distinct coordination numbers, two-coordinate in 3 over four-coordinate in [( Ar DP Mes )Fe III ( •− NAd)Cl], should also be among the contributing factors, as they might induce the reaction to operate in an "inner-sphere" or "outer-sphere" pathway. 70,71 Although the reaction of 3 with 3,5-bis(trifluoromethyl)phenylethylene bears resemblance to the production of imine and enamine, respectively, in the reactions of [(IPr)Co-(NAr Mes )] 47 Table S3), as a consequence of the gradually increased effective nuclear charge of the metal center. Therefore, we hypothesized that the [NAr] group in the iron(II) imido species is more nucleophilic or less electrophilic than those in its cobalt(II) and nickel(II) imido congeners.
On the basis of the above discussion, we proposed the route shown in Scheme 3 to explain the production of enamine. The iron(II) imido complex 3 interacts with the alkene to give a [2π + 2π]-addition product, the four-membered metallacycle A (Scheme 3), as usually found for early-transition-metal imido complexes. 72 The intermediate A then undergoes β-H elimination to form the iron(II) enamino hydride species B, which eventually initiates reductive elimination to form 4 and 5 (Scheme 3). 52 As such, 6 is generated by inserting a 3,5bis(trifluoromethyl)phenylethylene into the Fe−H bond of enamino iron(II) intermediate B, accompanied by the normal to abnormal NHC rearrangement (Scheme 3). Intriguingly, the CC double bond of the enamino ligand in 6 exhibits a trans configuration like that of 5. This observation hints that the steric repulsion between the 3,5-bis(trifluoromethyl)phenyl group and IPr in the four-membered metallacycle A might govern the selectivity of the β-H elimination step. As the nitrogen atoms of amido ligands in low-coordinate iron(II) complexes usually show trigonal-planar geometry, 33,53,73 steric  Reactions with CO and PMe 3 . Nitrene transfer to CO and phosphine to form isocyanates and iminophosphines has been observed for both iron(III) and iron(IV) imido species. 2,4,22,37 To check whether or not iron(II) imidos can perform nitrene transfer reactions with these substrates, the reactions of 3 with CO and PMe 3 were examined. It turned out that a facile transformation was observed when 3 was treated with CO, but the reaction of 3 with PMe 3 does not yield Me 3 PNAr Trip .
The interaction of 3 with an excess amount of CO (1 atm) proceeded quickly and gave the nitrene transfer reaction products, the isocyanate Ar Trip NCO (7) 26 To our surprise, 9 resists further reductive elimination to form Me 3 PNAr Trip even when a mixture of 9 with 20 equiv of PMe 3 in benzene is heated at 60°C for 36 h.
Nitrene-transfer reactions of metal−ligand multiple bond species with phosphines involves orbital interactions of the frontier molecular orbitals of metal−ligand multiple-bond species with the lone pair and σ*(C−P) orbital of phosphines. This notion is well-demonstrated in the nitrogen atom transfer reaction of iron(V) nitrido with PPh 3 by Smith and Harvey. 76 Alkyl phosphines as σ-donating ligands possess rather limited π-accepting ability (high lying σ*(C−P) orbital), whereas CO has pronounced σ-donating and also good π-accepting ability. On the basis of this consideration, again we attributed the reaction product not being Me 3 PNAr Trip to the essentially nucleophilic nature of the imido moiety in 3, in striking contrast to mid-and high-valent iron imido/iminyl species. Interestingly, the low-spin iron(II) imido complex 9 was also found to be inert toward 3,5-bis(trifluoromethyl)phenylethylene. This finding is consistent with the notion that the nucleophilicity of the imido moiety in 3 plays an important role in functionalizing CC bonds, because the calculated negative charge population on the imido nitrogen of 9 (−0.46) is lower than that of its counterpart in 3 (−0.57) (Table S3).
C−H Bond Activation Reactions. Facile hydrogen atom abstraction (HAA) reactions 77,78 have been observed for iron(V), 42 iron(IV), 33,39,40,79−81 and iron(III) imido 37,50,82 species. In this regard, we have also tested the activity of 3 toward cleaving C−H bonds in a range of hydrocarbons with different C−H bond strengths and found that 3 is able to activate a C(sp)−H bond but is inert toward the substrates with weaker C(sp 3 )−H bonds.
The reaction of 3 with a terminal alkyne, p-trifluoromethylphenylacetylene, forms the iron(II) amido alkynyl complex [(IPr)Fe(NHAr Trip )(CCC 6 H 4 -p-CF 3 )] (10; Scheme 5). The structure of 10 was established by a single-crystal X-ray diffraction study (Figure 7). The presence of a terminal alkynyl ligand, a long Fe−N(amido) distance of 1.922(2) Å, and a bent Fe−N(amido)−C(aryl) angle of 139.24 (19)°in 10 clearly indicated the occurrence of C(sp)−H bond activation in this reaction. On the other hand, no reaction occurred when 3 was treated with THF, toluene, or 1,4-cyclohexadiene (CHD) at room temperature despite the presence of weaker  We note that the observed reactivity pattern for 3 is exclusively distinct from those for HAA reactions of Nam's iron(V) imido species, wherein the reaction rate of C(sp 3 )−H bond activation decreases as the bond dissociation energy of the target C−H bond increases. 42 As such, it is unlikely that the reaction of 3 with the terminal alkyne involves a direct HAA mechanism. As our calculation study on 3 indicated a high negative charge population on the imido N atom, we reasoned that the polarized N−Fe multiple bond might be amenable to invoke a [2π + 2σ]-addition pathway to interact with the H−C(sp) bond and form 10 at the end. Alternatively, a stepwise mechanism that involves the deprotonation of the alkyne by the nucleophilic imide to form iron(II) amido species and alkynyl anion, followed by subsequent binding of the anion to the iron center, could also explain the formation of 9. Notably, such reactivity is typical of early-transition-metal imido complexes 83,84 and is rarely known for late-transition-metal imidos. 47 In contrast to the reluctance of 3 to perform intermolecular C(sp 3 )−H bond activation reactions, we found that the interaction of 3 with (p-Tol) 2 CN 2 can promote an intramolecular C(sp 3 )−H bond activation reaction. Adding 1 equiv of (p-Tol) 2 CN 2 in an n-hexane solution of 3 at room temperature led to the formation of a brown solution. After workup, the iron(II) amido complex [(IPr)Fe(NHAr Trip-H )] (11) was isolated as orange crystals in 49% yield (Scheme 6). NMR analysis of the reaction mixture indicated the formation of the azine (p-Tol) 2 CNNC(p-Tol) 2 as the byproduct in ca. 45% NMR yield relative to (p-Tol) 2 CN 2 , which might arise from iron-promoted decomposition of the diazo compound. 85 The molecular structure of 11 established by XRD clearly shows that the reaction has led to the conversion of the terminal imido ligand [NAr Trip ] 2− of 3 into a bidentate amidoalkyl chelate denoted as [NHAr Trip-H ] 2− in 11 (Figure 8), indicating the involvement of benzylic C−H bond activation on one of the flanking arenes of the imido ligand. The lack of coordination of the diazo ligand (p-Tol) 2 CN 2 or its derivatives in 11 suggests that in the course of the intramolecular C(sp 3 )− H bond activation reaction the diazo compound seems to play the role of "catalyst" in mediating the C−H activation reaction. Accordingly, it can be proposed that the interaction of 3 with (p-Tol) 2 CN 2 might result in the three-coordinate iron imido complex [(IPr)Fe(NAr Trip )(η 1 -NNC(Tol-p) 2 )] (12), which might perform an intramolecular C−H bond activation reaction to form the cyclometalated complex C. Complex C might be attacked by another molecule of the diazo compound to form cyclometalated complex 11 along with the azine and N 2 (Scheme 6).
Supportive of the aforementioned mechanism, an intermediate, which can further convert into 11, has been detected from the NMR-scale reaction of 3 with (p-Tol) 2 CN 2 ( Figure  S41). A preparative-scale reaction at −30°C led to the isolation of [(IPr)Fe(NAr Trip )(η 1 -NNC(Tol-p) 2 )] (12). While the low isolated yield and instability of 12 at room temperature do not allow full spectroscopic characterizations, a singlecrystal X-ray diffraction study has unambiguously established its structure as a three-coordinate NHC iron imido complex bearing η 1 -NNC(Tol-p) 2 ligation. As shown in Figure 9, the coordination of an η 1 -NNC(Tol-p) 2 86 and [(((Dipp)NC(Bu t )) 2 CH)Co(NNCPh 2 )], 87 implying the radical anion nature of the η 1 -NNC(Tol-p) 2 ligand. Thus, in addition to exersion of a steric effect, the coordination of the diazo ligand might also change the electronic nature of the iron imido moiety. These changes could be beneficial for further intramolecular C(sp 3 )−H bond activation reactions. It should be noted that at this stage whether the C(sp 3 )−H bond activation reaction from 12 to 11 undergoes a stepwise mechanism of HAA followed by benzylic radical rebound to iron center or a [2π + 2σ]-addition mechanism remained unclear. However, the long Fe···C(benzyl) separations in 12 should make it difficult to accommodate a four-membered transition state for the [2π + 2σ]-addition mechanism.

■ CONCLUSION
In the present work, we reported the preparation of the first two-coordinate iron(II) imido complex, [(IPr)Fe(NAr Trip )], via the reaction of [(IPr)Fe(vtms) 2 ] with the bulky organic azide Ar Trip N 3 . Single-crystal X-ray diffraction revealed a long distance of 1.715(2) Å for its Fe−N multiple bond. Variabletemperature and variable-field magnetometry and 57 Fe Mossbauer measurements coupled with DFT calculations collectively established an S = 2 ground spin state for the two-coordinate iron(II) imido complex, different from Peters' low-spin (S = 0) iron(II) imidos with tris(phosphine) ligation.
The iron(II) imido complex [(IPr)Fe(NAr Trip )] has been subjected to a detailed reactivity study with the aim of probing the hitherto unknown reactivity of iron(II) imido species. Our study revealed that [(IPr)Fe(NAr Trip )] performs nitrene transfer reactions with electron-deficient terminal alkenes and CO, producing enamines and isocyanide, respectively. In contrast, the interaction of [(IPr)Fe(NAr Trip )] with PMe 3 afforded the low-spin iron(II) imido complex [(Me 3 P) 3 Fe-(NAr Trip )] instead of iminophosphine. [(IPr)Fe(NAr Trip )] can activate the C(sp)−H bond of a terminal alkyne to afford an alkynyl iron(II) amido complex, whereas it exhibits limited HAA activity toward weaker C−H bonds. When it is treated with a diazo compound, [(IPr)Fe(NAr Trip )] can convert into an iron(II) complex bearing a cyclometalated arylamido ligand [NHAr Trip-H ], wherein an intramolecular HAA reaction might occur.
The outcomes of the aforementioned C−H bond activation reactions and nitrene-transfer reactions with alkenes are different from those of the mid-to high-valent iron imido/ iminyl complexes. The difference can be attributed to the less covalent iron−imido interaction and the resulting higher negative charge population accumulating on the nitrogen atom of the iron(II) imido complex versus mid-and high-valent iron imidos/iminyls as revealed by DFT studies. This bonding feature renders the imido moiety pronounced basicity and nucleophilicity. Equally important is the low-coordinate nature of the metal center, as it might allow the iron center to interact with substrates via a four-membered transition state required for the proposed [2π + 2σ]-addition mechanism, which is hard to occur with iron imido complexes featuring higher coordination numbers. These results hint at the potential utility of iron(II) imido intermediates for the development of new iron-catalyzed organic transformations.

* S Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01147.
Detailed experimental procedures, details of the calculation study, characterization data, molecular structures of 1 and 2, 57 Fe Mossbauer spectra, NMR spectra, absorption spectra, and electronic configurations of other iron imido species (PDF)

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
The work was supported by the National Natural Science Foundation of China (Nos. 21725104, 21690062, 21432001,