Synthesis, Structure, and Hydrogenolysis of Pyridine Dicarbene Iron Dialkyl Complexes

Abstract

Two methods for the synthesis of bis(imidazol-2-ylidene)pyridine iron dialkyl complexes, (CNC)Fe(CH₂SiMe₃)₂, have been developed. The first route consists of addition of two equivalents of LiCH₂SiMe₃ to the iron dihalide complex, (CNC)FeBr₂, while the second relies on addition of the free CNC ligand to readily-prepared (py)₂Fe(CH₂SiMe₃)₂ (py = pyridine). With aryl-substituted CNC ligands, octahedral complexes of the type (^ArCNC)Fe(CH₂SiMe₃)₂(N₂) (^ArCNC = bis(arylimidazol-2-ylidene)pyridine) were isolated, where the dinitrogen ligand occupies the site trans to the pyridine of the CNC-chelate. In contrast, the alkyl-substituted variant, (^tBuACNC)Fe(CH₂SiMe₃)₂ (^tBuACNC = 2,6-(^tBu-imidazol-2-ylidene)₂pyridine) was isolated as the five-coordinate compound lacking dinitrogen. Exposure of the (^ArCNC)Fe(CH₂SiMe₃)₂(N₂) derivatives to an H₂ atmosphere resulted in formation of the corresponding iron hydride complexes (^ArCNC)FeH₄. These compounds catalyzed hydrogen isotope exchange between the deuterated benzene solvent and H₂, generating isotopologues and isotopomers of (^ArCNC)Fe(H_n)(D_4-n) (n = 0–4). When (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (3,5-Me₂^MesCNC = 2,6-(2,4,6-Me₃-C6H2-imidazol-2-ylidene)₂-3,5-Me₂-pyridine) was treated successively with H₂ and then N₂, the corresponding reduced dinitrogen complex (3,5-Me₂^MesCNC)Fe(N₂)₂ was isolated. The same product was also obtained following addition of pinacolborane to (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂).

Graphical Abstract

INTRODUCTION

The oxidative addition of non-polar bonds such as H–H, B–H, Si–H and C–H is a fundamental substrate activation step in organometallic transformations¹ and poses a fundamental challenge for catalysis with earth-abundant first row transition metals.² One strategy for enabling this reactivity with iron and cobalt is to introduce pincer ligands with strong σ-donating phosphines or N-heterocyclic carbenes to enforce a strong ligand field and promote formation of low-spin complexes prone to two-electron reactivity.^2a,c Examples of concerted, two-electron oxidative addition with both iron and cobalt are now well documented.²

Aryl-substituted bis(arylimidazol-2-ylidene)pyridine iron dinitrogen compounds, [(^ArylCNC)Fe(N₂)₂] originally synthesized by Danopoulos and coworkers,^3,4 are exemplary of the strong-field ligand strategy and promote the oxidative addition of C–H,^3a Si–H,^3b and H–H bonds.^3c These complexes have also been shown to be highly active pre-catalysts for the hydrogenation of hindered, essentially unactivated olefins.⁵ More recently, a variant with saturated arylimidazolines, (H₄-^arylCNC)Fe(N₂)₂ (H₄-^arylCNC = bis(arylimidazolin-2-ylidene)pyridine), (H₄-^arylCNC)Fe(N₂)₂ (H₄-^arylCNC = bis(arylimidazolin-2-ylidene)pyridine) was synthesized and shown to be an effective catalyst for hydrogen isotope exchange (HIE) and the tritiation of active pharmaceutical ingredients.⁶ The catalyst exhibits both broad functional group tolerance and high HIE activity in polar solvents such as N-methyl pyrrolidone (NMP) and exhibits sterically-driven site selectivity that is complementary to state-of-the-art iridium catalysts by ignoring directing groups and being governed by steric effects.

One challenge associated with these compounds is their synthesis and extreme air- and moisture-sensitivity (Scheme 1a). Typical routes to the dinitrogen complexes involve reduction of iron(II) dihalide precursors with strong reducing agents, typically sodium amalgam or sodium naphthalenide. The success and efficiency of this reaction also proved highly dependent on the nature of the aryl substituents as iron complexes bearing smaller groups required stoichiometric sodium and catalytic naphthalene for reduction to the corresponding iron bis(dinitrogen) complex.⁵ No examples of iron bis(dinitrogen) complexes with N-alkyl rather than N-aryl substituents on the carbene are known. Another limitation of the current synthetic routes is the reliance on Fe[N(SiMe₃)₂]₂ as the iron precursor for preparation of the iron dihalides. This material is a low-melting, green solid that is highly air sensitive and requires distillation for purification.⁷ Alternative synthetic routes that rely on more accessible precursors that are compatible with a range of ligand substitution patterns are therefore valuable and of interest.

Scheme 1.

Synthesis of (CNC)Fe Precatalysts from a) (CNC)Fe dihalide complexes and b) (CNC)Fe dialkyl complexes

Iron dialkyl complexes, (CNC)Fe(CH₂SiMe₃)₂ (CNC = bis(imidazol-2-ylidene)pyridine) are attractive targets for isolable catalyst precursors due to their potential synthetic accessibility and activation by straightforward treatment with H₂, silanes or boranes.⁸ The commercial availability of LiCH₂SiMe₃ and the synthetic accessibility of (pyridine)₂Fe(CH₂SiMe₃)₂ reported by Cámpora⁹ also motivated exploration of routes utilizing these reagents. Previous reports from our laboratory have demonstrated the utility of both iron and cobalt dialkyl complexes of this type as precatalysts for alkene hydrogenation, hydroboration, hydrosilylation, HIE and C(sp^2/3)–H borylation reactions.⁸ Here, we describe the synthesis and characterization of a family of (CNC)Fe(CH₂SiMe₃)₂ complexes, their propensity to coordinate dinitrogen as a function of N-heterocyclic carbene substituent, and their reactivity with hydrogen.

RESULTS AND DISCUSSION

Our studies commenced with the preparation of dialkyl derivatives of [(^iPrCNC)Fe], as this compound is the most studied in its class. Dialkylation of the iron dihalide, (^iPrCNC)FeBr₂ was initially explored with the addition of two equivalents of LiCH₂SiMe₃ in diethyl ether at −35 ºC. A color change to dark violet was observed and filtration, followed by recrystallization from pentane at −35 ºC furnished (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂) (1-Ns₂(N₂)) as a diamagnetic violet solid in 61% isolated yield (Scheme 2). An alternative route was inspired by the work of Cámpora, who demonstrated that addition of tridentate, aryl-substituted pyridine(diimine) ligands to (py)₂Fe(CH₂SiMe₃)₂ resulted in pyridine displacement and formation of the five-coordinate iron dialkyl complexes.⁹ To explore analogous chemistry with the ^iPrCNC chelate, the free carbene was prepared by treatment of the bis(imidazolium) salt ^iPrCNC(HBr)₂ with KHMDS (HMDS = N(SiMe₃)₂).¹⁰ Addition of the resulting free chelate to a readily-generated solution of (py)₂Fe(CH₂SiMe₃)₂ in pentane at −35 ºC resulted in a dark violet solution, which upon concentration and filtration, resulted in isolation of 1-Ns₂(N₂) in 67% yield based on ^iPrCNC.

Scheme 2.

Two Methods for the Synthesis of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂).

The ¹H NMR spectrum of 1-Ns₂(N₂) in benzene-d₆ is consistent with a C_2v symmetric complex with resolved coupling of the pyridine protons and the methylene groups of the neosilyl ligand detectable at –1.11 ppm. The carbene carbon atoms give rise to a signal in the ¹³C NMR spectrum at 223.1 ppm, consistent with other (CNC)Fe complexes.^3–6,11 The ¹³C resonances of the alkyl carbons directly bound to iron, however, were not observed by ¹³C NMR spectroscopy.

Single crystals of 1-Ns₂(N₂) suitable for X-ray diffraction were obtained from a concentrated pentane solution at –35 °C. The solid-state structure of 1-Ns₂(N₂) is best described as an idealized octahedral complex with the two alkyl ligands oriented trans to each other, although slightly deviated from linearity (C40–Fe1–C33 = 163.26(14)°, Figure 1). The coordination site trans to the pyridine is occupied by a dinitrogen ligand, the coordination of which is maintained in solution as judged by IR spectroscopy (νN₂ (C₆D₆) = 2129 cm⁻¹). The bite of the CNC chelate results in a deviation of the C(11)-Fe(1)-C(10) angle from linearity (C11–Fe1–C10 = 159.68(15)°) despite a yaw angle of 15.7° (obtained from (N1-C11-Fe1)-(N2-C11-Fe1))/2). Having two strong-field, mutually trans X-type ligands and a neutral ligand trans to the pyridine in the CNC-plane around the iron center is a general structural feature of this class of complexes as it has also been observed in related iron dihydride and silyl–hydride complexes.^3b,c The zero-field, solid-state ⁵⁷Fe Mößbauer spectrum of 1-Ns₂(N₂) was measured at 80 K and exhibited a doublet with an isomer shift (δ) of 0.13 mm/s and a quadrupole splitting (|ΔE_Q|) of 1.71 mm/s, consistent with a low-spin iron(II) complex (Figure 2a).¹²

Figure 1.

Solid-state molecular structure of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂) (1-Ns₂(N₂)) at 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C10–Fe1 1.936(3), C11–Fe1 1.933(3), N3–Fe1 1.877(3), N6–Fe1 1.829(3), C33–Fe1 2.137(3), C40–Fe1 2.135(3); N6–Fe1–N3 177.39(13), N6–Fe1–C11 100.79(14); N3–Fe1–C11 79.80(14), N6–Fe1–C10 99.50(14), N3–Fe1–C10 79.88(13), C11–Fe1–C10 159.68(15), C40–Fe1–C33 163.26(14).

Figure 2.

Zero-field ⁵⁷Fe Mößbauer spectra of (a) (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂) (1-Ns₂(N₂)), (b) (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (2-Ns₂(N₂)), (c) (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (3-Ns₂(N₂)), (d) (^tBuACNC)Fe(CH₂SiMe₃)₂ (4-Ns₂) at 80 K in the solid state. Simulated parameters (blue lines) are (a) δ = 0.13 mm/s, |ΔE_Q| = 1.71 mm/s; (b) δ = 0.13 mm/s, |ΔE_Q| = 1.62 mm/s; (c) δ = 0.11 mm/s, |ΔE_Q| = 1.90 mm/s; (d) δ = 0.13 mm/s, |ΔE_Q| = 3.89 mm/s.

For the synthesis of other iron dialkyl complexes of this type, the route involving pyridine displacement from (py)₂Fe(CH₂SiMe₃)₂ was preferred because it obviates the use of Fe[N(SiMe₃)₂]₂. While the yields using the two routes are similar, the iron dialkyl precursor obviates the distillation of an air-sensitive iron precursor. Addition of the free pyridine dicarbene pincer ^MesCNC to a pentane solution of (py)₂Fe(CH₂SiMe₃)₂ followed by filtration furnished (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (2-Ns₂(N₂), ^MesCNC = 2,6-(2,4,6-Me₃-C₆H₂-imidazol-2-ylidene)₂pyridine) as a violet-brown solid in 72% yield (Scheme 3). Using a similar procedure, the related 3,5-dimethyl pyridine substituted iron dialkyl dinitrogen complex (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (3-Ns₂(N₂), 3,5-Me₂^MesCNC = 2,6-(2,4,6-Me₃-C₆H₂-imidazol-2-ylidene)₂ 3,5-Me₂-pyridine) was isolated as a brown solid in 90% yield (Scheme 3). The dimethyl substituents at the 3- and 5-positions of the central pyridine ring were originally reported by Danopoulos and coworkers to prevent deleterious metalation at those sites and generally gave improved yields.^3a

Scheme 3.

Synthesis of (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) and (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂)

Both diamagnetic iron complexes 2-Ns₂(N₂) and 3-Ns₂(N₂) were characterized by a combination of NMR, IR and Mößbauer spectroscopies. ¹H and ¹³C NMR spectra exhibit features similar to 1-Ns₂(N₂), with the exception that the carbene carbons bound to iron were not located by ¹³C NMR spectroscopy. Dinitrogen coordination was confirmed by benzene solution IR spectroscopy with strong bands located at 2126 cm⁻¹ and 2118 cm⁻¹ for 2-Ns₂(N₂) and 3-Ns₂(N₂), respectively. The zero-field ⁵⁷Fe Mößbauer spectra for both complexes exhibit parameters as expected for octahedral iron complexes in a strong ligand field (δ = 0.13 mm/s; |ΔE_Q| = 1.62 mm/s (2-Ns₂(N₂); δ = 0.11 mm/s; |ΔE_Q| = 1.90 mm/s (3-Ns₂(N₂)) (Figure 2b, c).¹² Both the infrared and Mößbauer spectroscopic data support a more electron-donating pincer upon introduction of methyl groups in the 3,5-positions of the central pyridine ring. The solid-state structure of 3-Ns₂(N₂) was determined by X-ray diffraction of single crystals obtained from a pentane/Et₂O solution at –35 °C, confirming the idealized octahedral geometry of the complex with trans alkyl ligands (Figure 3).

Figure 3.

Solid-state molecular structure of (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (3-Ns₂(N₂)) at 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C11–Fe1 1.8969(14), C10–Fe1 1.8983(15), C32–Fe1 2.1587(14), C36–Fe1 2.1256(15), Fe1–N6 1.8241(13), Fe1–N3 1.8822(12); N6–Fe1–C11 99.29(6), N3–Fe1–C11 80.69(6), N6–Fe1–C10 99.56(6), N3–Fe1–C10 80.57(6), C11–Fe1–C10 160.77(7), N6–Fe1–N3 178.53(5), C36–Fe1–C32 169.46(6).

With a reliable synthetic method in hand, variants with alkyl substituents on the imidazol-2-ylidenes of the CNC-ligands were targeted (Scheme 4). Addition of ^tBuACNC (^tBuACNC = 2,6-(^tBu-imidazol-2-ylidene)₂pyridine) to a pentane solution of (py)₂Fe(CH₂SiMe₃)₂ followed by filtration furnished a brown diamagnetic solid in 77% yield identified as (^tBuACNC)Fe(CH₂SiMe₃)₂ (4-Ns₂). In contrast to the aryl-substituted derivatives described above, no infrared band assignable to a dinitrogen stretch was observed in the benzene solution infrared spectrum. The ¹H NMR spectrum of 4-Ns₂ exhibited signals for the methylene groups of the neosilyl ligand at 8.34 ppm, as identified by a cross-peak between the methylene group and the carbon atom of the SiMe₃-group in the ¹H-¹³C HMBC (heteronuclear multiple bond correlation) NMR spectrum. The triplet assigned to the 4-pyridine proton is shifted significantly downfield to 11.01 ppm in comparison to 1-Ns₂(N₂) (7.18 ppm) and 2-Ns₂(N₂) (7.11 ppm). Although the signal for this carbon was not directly observed by ¹³C NMR spectroscopy, the HSQC (HSQC = heteronuclear single quantum coherence) NMR spectrum exhibited a cross peak at 81.7 ppm. A signal for the carbene carbon was again not observable. Although 4-Ns₂ is diamagnetic, the anomalous NMR chemical shifts are likely a result of population of low-lying triplet state.

Scheme 4.

Synthesis of (^tBuACNC)Fe(CH₂SiMe₃)₂

Single crystals of 4-Ns₂ suitable for X-ray diffraction were obtained from a diethyl ether solution stored at –35 °C and a representation of the solid-state structure is depicted in Figure 4. The coordination environment around the iron is between square pyramidal and trigonal bipyramidal with the two neosilyl ligands in a mutually trans disposition (C20–Fe1–C24 = 148.63(7)°) and in plane with the two carbene ligands. The ^tBu-groups shield the sixth coordination site, likely preventing coordination of dinitrogen and resulting in the isolation of 4-Ns₂ as a five-coordinate, 16-electron complex.

Figure 4.

Solid-state molecular structure of (^tBuACNC)Fe(CH₂SiMe₃)₂ (4-Ns₂) at 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): C10–Fe1 1.9467(17), C11–Fe1 1.9392(17), C20–Fe1 2.0633(16), C24–Fe1 2.0716(17), Fe1–N3 1.8205(14); N3–Fe1–C11 81.53(6), N3–Fe1–C10 81.55(6), C11–Fe1–C10 163.03(7), C20–Fe1–C24 148.63(7).

The diamagnetism of 4-Ns₂ is noteworthy as other five-coordinate iron dialkyl complexes with tridentate pincer ligands usually adopt a trigonal bipyramidal geometry and have either S =1 or 2 ground states.^8j-l,9,13 To our knowledge, an octahedral coordination environment has thus far been a prerequisite for diamagnetism among iron dialkyl complexes.¹⁴ The higher anisotropy of the electron density around the Fe(II) center this coordination environment confers has implications for Mößbauer spectroscopy. The zero-field ⁵⁷Fe Mößbauer spectrum of 4-Ns₂ exhibits a doublet with an isomer shift of 0.13 mm/s and a quadrupolar splitting of 3.89 mm/s, the latter being significantly higher than observed for the octahedral, N₂-containing complexes as well as trigonal bipyramidal, paramagnetic iron bis(neosilyl) complexes (Figure 3d).^8j Similar behavior was also observed with square-planar, ferrous complexes when compared to their tetrahedral counterparts.¹⁵

Synthesis of (ACNC)Fe(CH₂SiMe₃)₂ analogs with smaller alkyl substituents on the imidazol-2-ylidene were unsuccessful as addition of ^MeACNC (^MeACNC = 2,6-(Me-imidazol-2-ylidene)₂pyridine) to (py)₂Fe(CH₂SiMe₃)₂ yielded a black solid, which was insoluble in benzene-d₆ and did not exhibit any assignable resonances in the ¹H NMR spectrum in THF-d₈. Iron dialkyl complexes with the pincer containing saturated N-heterocyclic carbenes were not obtained using this method as deprotonation of (H₄-^iPrCNC)(HBr)₂ and (H₄-^MesCNC)(HBr)₂ produced a complex mixture of unidentified products, likely due to the decreased stability of free imidazolin-2-ylidenes when compared to their unsaturated counterparts.¹⁶

With the various (CNC)Fe dialkyl complexes in hand, reactivity studies were conducted, specifically focused on the generation of reduced or hydride complexes, with potential application as hydrogenation and HIE catalysts.^3c,5,6 First, ligand-induced reductive elimination of the alkyl ligands was targeted.^14a,17 Exposure of a diethyl ether solution of 1-Ns₂(N₂) to a carbon monoxide atmosphere did not yield the reductive elimination product (^iPrCNC)Fe(CO)₂^3b,11 but resulted in clean substitution of the dinitrogen ligand (Scheme 5). The diamagnetic iron product, (^iPrCNC)Fe(CH₂SiMe₃)₂(CO) (1-Ns₂(CO)) was isolated in 81% yield following recrystallization from a Et₂O/pentane mixture and was stable under an N₂ atmosphere. As expected, both the ¹H and ¹³C NMR spectra exhibited similar features to 1-Ns₂(N₂). The zero-field, solid-state ⁵⁷Fe Mößbauer spectrum exhibited a doublet with an isomer shift of –0.03 mm/s and a quadrupole splitting of 0.65 mm/s. Coordination of CO was also corroborated by infrared spectroscopy in toluene solution (ν(CO) = 1919 cm⁻¹).

Scheme 5.

Synthesis of (^iPrCNC)Fe(CH₂SiMe₃)₂(CO)

Single crystals suitable of 1-Ns₂(CO) for X-ray diffraction were obtained from a pentane solution at –35 °C and a representation of the solid-state structure presented in Figure 5 establishes an idealized octahedral geometry nearly identical to 1-Ns₂(N₂) with the CO ligand trans to the pyridine and two alkyl ligands mutually trans. While the Fe-carbene and Fe-alkyl bond lengths are similar to those in 1-Ns₂(N₂), the Fe-pyridine bond length is significantly longer (1.925(2) Å in 1-Ns₂(CO), 1.829(3) Å in 1-Ns₂(N₂)), likely a result of the stronger trans influence of the CO ligand as compared to N₂.

Figure 5.

Solid-state molecular structure of (^iPrCNC)Fe(CH₂SiMe₃)₂(CO) (1-Ns₂(CO)) at 30% probability ellipsoids. Hydrogen atoms and one aryl substituent are omitted for clarity. Selected bond distances (Å) and angles (deg): C44–Fe1 1.730(3), N3–Fe1 1.925(2), C10–Fe1 1.931(3), C11–Fe1 1.915(3), C36–Fe1 2.137(2), C40–Fe1 2.154(3); C44–Fe1–C11 100.45(12), C44–Fe1–N3 178.25(11), C11–Fe1–N3 79.81(10), C44–Fe1–C10 100.14(12), C11–Fe1–C10 159.37(12), N3–Fe1–C10 79.64(10), C36–Fe1–C40 170.16(11).

Addition of the stronger field ligand, carbon monoxide to 4-Ns₂ overcomes the apparent steric penalty associated with formation of a six-coordinate complex as diamagnetic 4-Ns₂(CO) was isolated. CO coordination was confirmed by the observation of a strong band centered at 1919 cm⁻¹ in the benzene-d₆ solution infrared spectrum. The ¹H NMR spectrum of 4-Ns₂(CO) also differs slightly from 4-Ns₂ with the methylene groups of the neosilyl ligand (−1.34 ppm) and 4-pyridine (7.10 ppm) protons significantly shifted upfield, comparable to the spectroscopic features observed with 1-Ns₂(CO).

Given the performance of [(CNC)Fe] complexes in catalytic alkene hydrogenation and HIE chemistry,⁶ hydrogenolysis of the iron dialkyl complexes was studied (Scheme 6a). Exposure of a benzene-d₆ solution of 1-Ns₂(N₂) to 4 atm of H₂ resulted in a color change from violet to brown. Monitoring the progress of the reaction by ¹H NMR spectroscopy revealed formation of SiMe₄ along with a new diamagnetic product with upfield resonances between −8 and −10 ppm, consistent with formation of iron hydrides (Scheme 6b). In analogy to spectroscopic features reported for (H₄-^iPrCNC)Fe(N₂)₂ under similar reaction conditions,^3c these resonances were assigned to the isotopologues of (^iPrCNC)FeH₄, generated from hydrogenolysis of the iron alkyl ligands and subsequent HIE with the benzene-d₆ solvent. Performing a similar procedure with 2-Ns₂(N₂) and 3-Ns₂(N₂) also produced ¹H NMR signals consistent with formation of isotopologues and isotopomers of the corresponding iron dihydride-dihydrogen complexes.^3c Exposure of 4-Ns₂ to dihydrogen also liberated SiMe₄, but no signals assignable to an organometallic iron compound were observed.

Scheme 6.

(a) Reaction of (CNC)Fe(CH₂SiMe₃)₂(N₂) complexes with H₂ in the Presence of C₆D₆ and (b) High-Field Excerpts of the Corresponding ¹H NMR Spectra

Attention was also devoted to the synthesis of reduced iron dinitrogen complexes, (CNC)Fe(N₂)₂ using the dialkyl derivatives as precursors. Such routes would obviate harsh and hazardous alkali metal reduction reactions. The aryl-susbtituted compound, 3-Ns₂(N₂) was chosen as a representative example (Scheme 7). Stirring a suspension of 3-Ns₂(N₂) in pentane successively under 4 atm of H₂ followed by 1 atm of N₂ resulted in the isolation of (3,5-Me₂^MesCNC)Fe(N₂)₂ (3-(N₂)₂) as a diamagnetic, brown solid in 96% yield. The same complex was also obtained in 94% isolated yield following addition of 1.2 equiv of HBPin (Pin = pinacolato) to a pentane suspension of 3-Ns₂(N₂). When this procedure was conducted in a sealed NMR tube in benzene-d_6, concomitant formation of a 1:1 mixture of TMS and PinBCH₂SiMe₃ was observed by ¹H NMR spectroscopy, supporting initial generation of an iron alkyl-hydride or alkyl-boryl complex and subsequent reductive elimination. Excess HBPin was detected in the reaction mixture by ¹H NMR spectroscopy, indicating that HBPin does not undergo irreversible oxidative addition to the reduced iron product. This observation is also supported by the near quantitative isolated yield of 3-Ns₂(N₂). Addition of HBPin to a benzene-d₆ solution of 4-Ns₂ produced no reaction over the course of one hour at ambient temperature. Heating the mixture to 70 ºC induced loss of SiMe₄ and formation of PinBCH₂SiMe₃ but no defined organometallic product could be identified by ¹H NMR spectroscopy.

Scheme 7.

Synthesis of (3,5-Me₂^MesCNC)Fe(N₂)₂

The benzene solution infrared spectrum of 3-(N₂)₂ displayed four strong bands assigned as N₂ stretches, the relative intensities of which were concentration dependent. Under dilute conditions (~0.007 M), the bands at 2094 cm⁻¹ and 2022 cm⁻¹ were dominant and assigned to the monomeric, five-coordinate bis(dinitrogen) complex, (3,5-Me₂^MesCNC)Fe(N₂)₂ (Figure 6a). Recording the infrared data in the solid state (KBr pellet) also produced four bands, likely due to the presence of both monomer and dimer ([(3,5-Me₂^MesCNC)Fe(N₂)]₂(µ₂-N₂)). These features are reminiscent of (^MesCNC)Fe(N₂)₂, where both monomer and dimer were observed in the crystal lattice.⁵ The equilibrium process was also detected by ¹H NMR spectroscopy as seven broad singlets were observed at ambient temperature while the ¹³C NMR spectrum needed to be recorded in dilute solution (<0.007 M) to observe the signals with the carbene carbons detected at 205.1 ppm. Mößbauer spectroscopy was used to assess the purity and identity of the obtained 3-(N₂)₂ complex, which exhibited a clean doublet with an isomer shift of 0.29 mm/s and quadrupole splitting of 0.55 mm/s, very similar to the features observed for (^iPrCNC)Fe(N₂)₂ (Figure 6b).¹¹

Figure 6.

Benzene solution IR spectra of (3,5-Me₂^MesCNC)Fe(N₂)₂ 3-(N₂)₂ in C₆D₆ at low (0.007 M upper spectrum) and high (0.03 M, lower spectrum) concentration. (b) Zero-field ⁵⁷Fe Mößbauer spectrum of (3,5-Me₂^MesCNC)Fe(N₂)₂ at 80 K in the solid state. Simulated parameters (blue line) are δ = 0.29 mm/s, |ΔE_Q| = 0.55 mm/s.

CONCLUDING REMARKS

A synthetic method has been developed to access a series of (CNC)Fe dialkyl complexes. With aryl-substituted N-heterocyclic carbenes on the pincer, dinitrogen coordinates to the position trans to the pyridine of the chelate to form diamagnetic, idealized octahedral low-spin iron(II) complexes. The dinitrogen ligand is labile and in one instance was displaced by CO to from the corresponding carbonyl complex. The synthetic route to iron dialkyl complexes was also compatible with iron complexes bearing tert-butyl rather than aryl substituents on the carbene portion of the pincer. The resulting iron dialkyl complex is five-coordinate and diamagnetic but exhibits NMR spectroscopic features to suggest contributions from a paramagnetic excited state. Hydrogenation of the aryl-substituted iron dialkyl complexes generated isotopologues and isotopomers of the resulting iron hydrides, demonstrating activity in catalytic hydrogen isotope exchange with benzene-d₆. The iron bis(dinitrogen) complex, (3,5-Me₂^MesCNC)Fe(N₂)₂ was synthesized from (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) by successive reaction with H₂ and N₂ or reaction with HBPin in the presence of N₂. This straightforward method to access reduced (CNC)Fe complexes and in situ generation of (CNC)Fe hydride complexes circumvents the use of Fe[N(SiMe₃)₂]₂, metallic reductants and isolation of very sensitive dinitrogen complexes and permits a higher variability of the ligand substituents. It will therefore allow a more rapid assessment of catalytic activity in hydrogenation and HIE, and generally facilitate reactivity studies of (CNC)Fe complexes. Studies along these lines are currently underway in our laboratory.

EXPERIMENTAL SECTION

General Considerations.

All air- and moisture-sensitive manipulations were carried out using vacuum line, Schlenk and cannula techniques or in an MBraun inert atmosphere (nitrogen) dry box unless otherwise noted. All glassware was stored in a pre-heated oven prior to use. The solvents used for air- and moisture-sensitive manipulations were dried and deoxygenated using literature procedures.¹⁸ Hydrogen gas was purchased from Airgas National Welders and passed through a column of MnO₂ supported on vermiculite and 3 Å molecular sieves prior to use on a Schlenk manifold. HBpin and KHMDS were purchased from Sigma Aldrich and used as received. LiCH₂SiMe₃ (1.0 m in pentane) was purchased from Sigma Aldrich, concentrated, recrystallized from pentane at –35 °C and dried under vacuum to obtain LiCH₂SiMe₃ as a colorless crystalline solid which was stored at –35 °C. The following compounds were prepared according to literature procedures: 2,6-diiodo-3,5-dimethylpyridine,¹⁹ 1-mesityl-1H-imidazole,^{20 iPr}CNC(HBr)₂,^{21 Mes}CNC(HBr)₂,^{21 tBu}ACNC(HBr)₂,²² (py)₄FeCl₂.²³

¹H NMR spectra were recorded on either Bruker AVANCE 300 or 500 spectrometers operating at 300.13 MHz, and 500.46 MHz, respectively. All ¹H and ¹³C NMR chemical shifts are reported in ppm relative to SiMe₄ using the ¹H (chloroform-d: 7.26 ppm; benzene-d₆: 7.16 ppm) and ¹³C (chloroform-d: 77.16 ppm; benzene-d₆: 128.06 ppm) chemical shifts of the solvent as a standard. ¹H NMR data for diamagnetic compounds are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, p = pentet, sept = septet, br = broad, m = multiplet), coupling constants (Hz), integration, assignment. Elemental analyses were performed at Robinson Microlit Laboratories, Inc., in Ledgewood, NJ. Infrared spectroscopy was conducted on a Thermo-Nicolet iS10 FT-IR spectrometer calibrated with a polystyrene standard. For some iron dinitrogen complexes a satisfactory elemental analysis could not be achieved, which is likely due to the highly sensitive nature of these compounds. The identity and purity of the bulk material can however adequately be assessed by the combination of NMR and Mößbauer spectroscopies.

Zero-field ⁵⁷Fe Mößbauer spectra were recorded on a SEE Co. Mößbauer spectrometer (MS4) at 80 K in constant acceleration mode. ⁵⁷Co/Rh was used as the radiation source. WMOSS software²⁴ was used for the quantitative evaluation of the spectral parameters (least-squares fitting to Lorentzian peaks). The temperature of the sample was controlled by a Janis Research Co. CCS-850 He/N₂ cryostat within an accuracy of 0.3 K. Isomer shifts were determined relative to α-iron at 298 K.

Single crystals suitable for X-ray diffraction were coated with polyisobutylene oil in a drybox, transferred to a nylon loop and then quickly transferred to the goniometer head of a Bruker SMART APEX DUO diffractometer equipped with a molybdenum X-ray tube (λ = 0.71073 Å) and a Cu X-ray tube (λ = 1.54178 Å). Preliminary data revealed the crystal system. The data collection strategy was optimized for completeness and redundancy using the Bruker COSMO software suite. The space group was identified, and the data were processed using the Bruker SAINT+ program and corrected for absorption using SADABS. The structures were solved using direct methods (SHELXS) completed by subsequent Fourier synthesis and refined by full-matrix least-squares procedures.

Preparation of 3,5-Me₂^MesCNC(HI)₂.

A glass ampoule was filled with 2,6-diiodo-3,5-dimethylpyridine (1.44 g, 4.0 mmol, 1.0 equiv) and 1-mesityl-1H-imidazole (2.24 g, 12 mmol, 3.0 equiv). The ampoule was attached to a Schlenk line, evacuated, and then flame sealed. The sealed ampoule was placed in an oven at 200 °C for 7 days, allowed to cool to RT and opened. The solid residue was taken up in CH₂Cl₂ and the resulting mixture was concentrated to a volume of ca. 5 mL. Et₂O (ca. 400 mL) were added and the resulting precipitate was collected through filtration and dried to yield 2.84 g (97% yield) of 3,5-Me₂^MesCNC(HI)₂ as a sandy solid. ¹H NMR (500 MHz, CDCl₃, 25 ºC): δ 10.73 (t, J = 1.6 Hz, 2H, 2-imidazolium H), 8.87 (t, J = 1.8 Hz, 2H, 4/5-imidazolium H), 7.98 (s, 1H, p-pyr H), 7.35 (t, J = 1.8 Hz, 2H, 4/5-imidazolium H), 7.02 (s, 4H, Ar-H), 2.62 (s, 6H, m-pyr-(CH₃)₂), 2.33 (s, 6H, 4-Ar-CH₃), 2.16 (s, 12H, 2,6-Ar-(CH₃)₂). ¹³C{¹H} NMR (126 MHz, CDCl₃, 25 ºC): δ 147.6 (p-pyr C), 142.3 (m-pyr C), 141.8 (4-Ar C), 138.3 (2-imidazolium C), 134.0 (2,6-Ar C), 131.7 (o-pyr-C), 130.3 (1-Ar C), 130.2 (3,5-Ar C), 124.9 (4/5-imidazolium C), 123.2 (4/5-imidazolium C), 21.2 (4-Ar-CH₃), 18.3 (m-pyr-(CH₃)₂), 18.1 (2,6-Ar-(CH₃)₂)

Preparation of ^iPrCNC.

^iPrCNC was prepared using a slightly modified literature procedure.^10,25 In a N₂-filled glovebox a 100 mL round bottom flask was charged with ^iPrCNC(HBr)₂ (347 mg, 0.5 mmol, 1.0 equiv) and THF (ca. 10 mL) and the resulting suspension was cooled in the cold well (–196 °C) for 20 min. A 20 mL scintillation vial was charged with KHMDS (210 mg, 1.05 mmol, 2.1 equiv) and THF (ca. 5 mL) and the resulting suspension was cooled in the freezer (–35 °C) for 20 min. The cold solution of KHMDS in THF was added to the cold suspension of ^MesCNC(HBr)₂ and the resulting mixture was allowed to warm to RT while being stirred for 1 h. All volatiles were removed in vacuo and the residue was triturated in pentane (ca. 20 mL). All volatiles were removed in vacuo and the residue was taken up in toluene (ca. 30 mL). The mixture was filtered through celite (eluent: toluene) and all volatiles were removed from the filtrate in vacuo to yield 264 mg (99% yield) of ^iPrCNC as an off-white solid. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 8.44 (d, J = 8.0 Hz, 2H), 8.06 (d, J = 1.7 Hz, 2H), 7.26 (dd, J = 8.4, 7.1 Hz, 2H), 7.16 – 7.11 (m, 4H), 7.07 (t, J = 8.0 Hz, 1H), 6.62 (d, J = 1.7 Hz, 2H), 2.91 (hept, J = 6.9 Hz, 4H), 1.20 (d, J = 6.8 Hz, 12H), 1.11 (d, J = 6.9 Hz, 12H). The characterization data matched previously reported data.¹⁰

Preparation of ^MesCNC.

^MesCNC was prepared in an analogous fashion to ^iPrCNC using ^MesCNC(HBr)₂ (609 mg, 1.0 mmol, 1.0 equiv) and KHMDS (419 mg, 2.1 mmol, 2.1 equiv). This procedure yielded 430 mg (96% yield) of ^MesCNC as an off-white solid. ¹H NMR (300 MHz, C₆D₆, 25 ºC): δ 8.54 (d, J = 8.0 Hz, 2H), 8.12 (d, J = 1.8 Hz, 2H), 7.11 (t, J = 8.0 Hz, 1H), 6.81 (s, 4H), 6.46 (d, J = 1.7 Hz, 2H), 2.15 (s, 6H), 2.13 (s, 12H). The characterization data matched previously reported data.²⁵

Preparation of 3,5-Me₂^MesCNC.

3,5-Me₂^MesCNC was prepared in an analogous fashion to ^iPrCNC using 3,5-Me₂^MesCNC(HI)₂ (731 mg, 1.0 mmol, 1.0 equiv) and KHMDS (419 mg, 2.1 mmol, 2.1 equiv). This procedure yielded 470 mg (99% yield) of 3,5-Me₂^MesCNC as an off-white solid. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 7.70 (d, J = 1.7 Hz, 2H, 4/5-imidazolylidene H), 7.12 (s, 1H, p-pyr H), 6.80 (s, 4H, Ar-H), 6.43 (d, J = 1.7 Hz, 2H, 4/5-imidazolylidene H), 2.73 (s, 6H, m-pyr-(CH₃)₂), 2.15 (s, 6H, 4-Ar-CH₃), 2.12 (s, 12H, 2,6-Ar-(CH₃)₂). ¹³C{¹H} NMR (101 MHz, C₆D₆, 25 ºC): δ 221.3 (2-imidazolylidene C), 148.9 (o-pyr C), 146.6 (p-pyr C), 139.1 (Ar C), 137.4 (Ar C), 135.5 (Ar C), 129.2 (3,5-Ar C), 124.9 (m-pyr C), 119.98 (4/5-imidazolylidene C), 119.96 (4/5-imidazolylidene C), 21.0 (4-Ar-CH₃), 20.5 (m-pyr-(CH₃)₂), 18.2 (2,6-Ar-(CH₃)₂).

Preparation of ^tBuACNC.

^tBuACNC was prepared in an analogous fashion to ^iPrCNC using ^tBuACNC(HBr)₂ (485 mg, 1.0 mmol, 1.0 equiv) and KHMDS (419 mg, 2.1 mmol, 2.1 equiv). This procedure yielded 270 mg (83% yield) of ^tBuACNC as a yellow-orange solid. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 8.51 (d, J = 8.0 Hz, 2H, m-pyr H), 8.07 (d, J = 1.9 Hz, 2H, 4/5-imidazolylidene H), 7.18 (t, J = 8.0 Hz, 1H, p-pyr H), 6.77 (d, J = 1.8 Hz, 2H, 4/5-imidazolylidene H), 1.49 (s, 18H, C(CH₃)₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 25 ºC): δ 216.2 (2-imidazolylidene C), 153.1 (o-pyr C), 140.3 (p-pyr C), 116.6 (4/5-imidazolylidene C), 116.1 (4/5-imidazolylidene C), 111.0 (m-pyr C), 56.5 (C(CH₃)₃), 31.2 (C(CH₃)₃).

Preparation of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂).

In a N₂-filled glovebox a 20 mL scintillation vial was charged with (^iPrCNC)FeBr₂ (500 mg, 0.67 mmol, 1.0 equiv) and Et₂O (ca. 5 mL) and the resulting slurry was cooled in the freezer (–35 °C) for 20 min. Another 20 mL scintillation vial was charged with LiCH₂SiMe₃ (125 mg, 1.33 mmol, 2.0 equiv) and Et₂O (ca. 10 mL) and the resulting solution was likewise cooled in the freezer (–35 °C) for 20 min. The cold solution of LiCH₂SiMe₃ was added dropwise to the cold and rapidly stirred slurry of (^iPrCNC)FeBr₂ and the resulting mixture was allowed to warm to RT while being stirred for 3 h. All volatiles were removed in vacuo, the residue was extracted with pentane, and the resulting mixture was filtered through Celite to give a purple solution. All volatiles were removed in vacuo and the residue recrystallized from pentane at –35 °C to yield 325 mg (61% yield) of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂) as a violet solid. Single crystals suitable for X-ray diffraction were grown from a concentrated solution in pentane at –35 °C. IR (C₆D₆): ν_NN = 2129 cm^-1. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 7.23 (d, J = 2.1 Hz, 2H, 4/5-imidazolylidene H), 7.18 (t, J = 9.2 Hz, 2H, p-Ar H), 7.11 (d, J = 7.6 Hz, 4H, m-Ar H), 7.00 (t, J = 7.8 Hz, 1H, p-pyr H), 6.75 (d, J = 2.1 Hz, 2H, 4/5-imidazolylidene H), 6.61 (d, J = 7.8 Hz, 2H, m-pyr H), 3.28 (sept, J = 6.8 Hz, 4H, CH(CH₃)₂), 1.39 (d, J = 6.7 Hz, 12H, CH(CH₃)₂), 1.08 (d, J = 6.8 Hz, 12H, CH(CH₃)₂), −0.50 (s, 18H, CH₂Si(CH₃)₃), −1.11 (s, 4H, CH₂Si(CH₃)₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 25 ºC): δ 223.1 (2-imidazolylidene C), 153.6 (o-pyr C), 147.3 (Ar C), 136.7 (Ar C), 129.9 (p-Ar C), 126.6 (4/5-imidazolylidene C), 126.4 (p-pyr C), 124.0 (m-Ar C), 112.8 (4/5-imidazolylidene C), 100.00 (m-pyr C), 28.6 (CH(CH₃)₂), 27.1 (CH(CH₃)₂), 23.0 (CH(CH₃)₂), 2.8 (CH₂Si(CH₃)₃), −15.0 (CH₂Si(CH₃)₃, located by HSQC analysis).

Alternative Preparation of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂).

In a N₂-filled glovebox a 20 mL scintillation vial was charged with (py)₄FeCl₂ (260 mg, 0.59 mmol, 1.2 equiv) and pentane (ca. 5 mL) and the resulting suspension was cooled in the freezer (–35 °C) for 20 min. Another 20 mL scintillation vial was charged with LiCH₂SiMe₃ (113 mg, 1.20 mmol, 2.46 equiv) and pentane (ca. 5 mL) and the resulting solution was likewise cooled in the freezer (–35 °C) for 20 min. The cold solution of LiCH₂SiMe₃ was added to the cold suspension of (py)₄FeCl₂ and the resulting mixture was allowed to warm to RT while being stirred for 1–2 h. The resulting violet reaction mixture was filtered through Celite (eluent: pentane) and all volatiles were removed in vacuo. The residue was taken up in pentane (ca. 5 mL) and the resulting solution containing (py)₂Fe(CH₂SiMe₃)₂ was cooled in the freezer (–35 °C) for 20 min. A 100 mL round bottom flask was charged with ^iPrCNC (260 mg, 0.49 mmol, 1.0 equiv) and cold pentane (ca. 30 mL, –35 °C). The cold solution containing (py)₂Fe(CH₂SiMe₃)₂ was added to the cold suspension of ^MesCNC and the resulting mixture was allowed to warm to RT while being stirred for 1 h. All volatiles were removed in vacuo from the resulting violet reaction mixture and the remaining solid was washed with cold pentane (3 × 3 mL) and dried under vacuum to yield 260 mg (67% yield) of (^iPrCNC)Fe(CH₂SiMe₃)₂(N₂) as a violet solid. All characterization data matched the material obtained above.

Preparation of (^MesCNC)Fe(CH₂SiMe₃)₂(N₂).

In a N₂-filled glovebox a 20 mL scintillation vial was charged with (py)₄FeCl₂ (511 mg, 1.15 mmol, 1.2 equiv) and pentane (ca. 10 mL) and the resulting suspension was cooled in the freezer (–35 °C) for 20 min. Another 20 mL scintillation vial was charged with LiCH₂SiMe₃ (222 mg, 2.36 mmol, 2.46 equiv) and pentane (ca. 5 mL) and the resulting solution was likewise cooled in the freezer (–35 °C) for 20 min. The cold solution of LiCH₂SiMe₃ was added to the cold suspension of (py)₄FeCl₂ and the resulting mixture was allowed to warm to RT while being stirred for 1–2 h. The resulting violet reaction mixture was filtered through Celite (eluent: pentane) and all volatiles were removed in vacuo. The residue was taken up in pentane (ca. 5 mL) and the resulting solution containing (py)₂Fe(CH₂SiMe₃)₂ was cooled in the freezer (–35 °C) for 20 min. A 100 mL round bottom flask was charged with ^MesCNC (430 mg, 0.96 mmol, 1.0 equiv) and cold pentane (ca. 30 mL, –35 °C). The cold solution containing (py)₂Fe(CH₂SiMe₃)₂ was added to the cold suspension of ^MesCNC and the resulting mixture was allowed to warm to RT while being stirred for 1 h during which time a microcrystalline solid precipitated from the reaction mixture. The resulting violet suspension was concentrated to ca. 10 mL and filtered. The solid was washed with cold pentane (3 × 3 mL) and dried under vacuum to yield 487 mg (72% yield) of (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) as a violet-brown solid. Anal Calcd for C₃₇H₅₁FeN₇Si₂: C, 62.96; H, 7.28; N, 13.89. Found: C, 62.58; H, 6.79; N, 13.72. IR (C₆D₆): ν_NN = 2126 cm^-1. ¹H NMR (300 MHz, C₆D₆, 25 ºC): δ 7.21 (s, 2H, 4/5-imidazolylidene H), 7.11 (t, J = 7.8 Hz, 1H, p-pyr H), 6.71 (s, 4H, Ar H), 6.61 (d, J = 7.8 Hz, 2H, m-pyr H), 6.36 (s, 2H, 4/5-imidazolylidene H), 2.27 (s, 12H, 2,6-Ar-(CH₃)₂), 2.03 (s, 6H, 4-Ar-CH₃), −0.49 (s, 18H, CH₂Si(CH₃)₃), −0.73 (s, 4H, CH₂Si(CH₃)₃). ¹³C{¹H} NMR (126 MHz, C₆D₆, 25 ºC): δ 153.3 (o-pyr C), 138.3 (4-Ar C), 137.0 (Ar C), 136.4 (Ar C), 129.5 (3-Ar C), 126.0 (p-pyr C), 124.9 (4/5-imidazolylidene C), 113.3 (4/5-imidazolylidene C), 100.3 (m-pyr C), 21.0 (4-Ar-CH₃), 19.2 (2,6-Ar-(CH₃)₂), 2.9 (CH₂Si(CH₃)₃). (CH₂Si(CH₃)₃ and 2-imidazolylidene C not located).

Preparation of (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂).

(3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) was prepared in an analogous fashion to (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) using 3,5-Me₂^MesCNC (412 mg, 0.89 mmol, 1.0 equiv), (py)₄FeCl₂ (460 mg, 1.04 mmol, 1.2 equiv) and LiCH₂SiMe₃ (201 mg, 2.13 mmol, 2.46 equiv). This procedure yielded 564 mg (90% yield) of (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) as a violet/brown solid. Single crystals suitable for X-ray diffraction were grown from a concentrated solution in Et₂O/pentane at –35 °C. Anal Calcd for C₃₉H₅₅FeN₇Si₂: C, 63.82; H, 7.55; N, 13.36. Found: C, 63.49; H, 7.56; N, 12.89. IR (C₆D₆): ν_NN = 2118 cm^-1. ¹H NMR (500 MHz, C₆D₆, 25 ºC): δ 7.70 (s, 2H, 4/5-imidazolylidene H), 6.71 (s, 4H, Ar H), 6.42 (s, 1H, p-pyr H), 6.35 (s, 2H, 4/5-imidazolylidene H), 2.30 (s, 12H, 2,6-Ar-(CH₃)₂), 2.29 (s, 6H, m-pyr-(CH₃)₂), 2.03 (s, 6H, 4-Ar-CH₃), −0.45 (s, 18H, CH₂Si(CH₃)₃), −1.01 (s, 4H, CH₂Si(CH₃)₃). ¹³C{¹H} NMR (126 MHz, C₆D₆, 25 ºC): δ 150.9 (o-pyr C), 138.1 (4-Ar C), 137.0 (Ar C), 136.5 (Ar C), 134.5 (p-pyr C), 129.5 (3-Ar C), 124.1 (4/5-imidazolylidene C), 116.3 (4/5-imidazolylidene C), 111.0 (m-pyr C), 21.0 (4-Ar-CH₃), 19.3 (2,6-Ar-(CH₃)₂), 18.7 (m-pyr-(CH₃)₂), 2.8 (CH₂Si(CH₃)₃). (CH₂Si(CH₃)₃ and 2-imidazolylidene C not located).

Preparation of (^tBuACNC)Fe(CH₂SiMe₃)₂.

(^tBuACNC)Fe(CH₂SiMe₃)₂ was prepared in an analogous fashion to (^MesCNC)Fe(CH₂SiMe₃)₂(N₂) using ^tBuACNC (187 mg, 0.58 mmol, 1.0 equiv), (py)₄FeCl₂ (307 mg, 0.69 mmol, 1.2 equiv) and LiCH₂SiMe₃ (134 mg, 1.42 mmol, 2.46 equiv). This procedure yielded 247 mg (77% yield) of (^tBuACNC)Fe(CH₂SiMe₃)₂ as a brown solid. Single crystals suitable for X-ray diffraction were grown from a concentrated solution in Et₂O at –35 °C. Anal Calcd for C₂₇H₄₇FeN₅Si₂: C, 58.57; H, 8.56; N, 12.65. Found: C, 58.37; H, 8.12; N, 12.66. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 11.01 (t, J = 7.6 Hz, 1H, p-pyr H), 8.34 (s, 4H, CH₂Si(CH₃)₃), 8.26 (s, 2H, 4/5-imidazolylidene H), 8.23 (d, J = 7.8 Hz, 2H, m-pyr H), 7.89 (s, 2H, 4/5-imidazolylidene H), 1.94 (s, 18H, C(CH₃)₃), −0.87 (s, 18H, CH₂Si(CH₃)₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 25 ºC): δ 151.3 (o- or p-pyr C), 125.1 (4/5-imidazolylidene C), 120.5 (m-pyr C), 105.9 (4/5-imidazolylidene C), 81.7 (CH₂Si(CH₃)₃, located by HSQC analysis), 58.4 (C(CH₃)₃), 35.8 (C(CH₃)₃), 1.6 (CH₂Si(CH₃)₃). (2-imidazolylidene C and o- or p-pyr C not located).

Preparation of (^iPrCNC)Fe(CH₂SiMe₃)₂(CO).

In a N₂-filled glovebox a 50 mL Schlenk tube with a Teflon valve was charged with (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (125 mg, 0.16 mmol, 1.0 equiv) and Et₂O (ca. 15 mL). The tube was sealed, brought outside of the glovebox and attached to a Schlenk line where the solution was frozen (–196 °C, liquid nitrogen). Following evacuation of the headspace, CO (1 atm) was admitted. The tube was sealed and allowed to warm to RT upon which a rapid color change from purple to pink occurred and the resulting suspension was vigorously stirred for 30 min. The resulting reaction mixture was filtered through celite and all volatiles were removed in vacuo. The residue was recrystallized from a Et₂O/pentane mixture to yield 101 mg (81% yield) of (^iPrCNC)Fe(CH₂SiMe₃)₂(CO) as a burgundy solid. Anal Calcd for C₄₄H₆₃FeN₅OSi₂: C, 66.89; H, 8.04; N, 8.86. Found; C, 66.74; H, 7.97; N, 8.69. IR (toluene): ν_CO = 1919 cm^-1. ¹H NMR (400 MHz, C₆D₆, 25 ºC): δ 7.22 (t, J = 8 Hz, 2H, p-Ar H), 7.09 – 7.15 (m, 7H, 4/5-imidazolylidene H, m-Ar H, p-pyr H), 6.63 – 6.67 (m, 4H, 4/5-imidazolylidene H, m-pyr H), 3.11 (sept, J = 7 Hz, 4H, CH(CH₃)₂), 1.39 (d, J = 7 Hz, 12H, CH(CH₃)₂), 1.05 (d, J = 6.8 Hz, 12H, CH(CH₃)₂), −0.49 (s, 18H, CH₂Si(CH₃)₃), −1.43 (s, 4H, CH₂Si(CH₃)₃). ¹³C{¹H} NMR (101 MHz, C₆D₆, 25 ºC): δ 227.6 (2-imidazolylidene C), 151.6 (o-pyr C), 147.0 (Ar C), 136.7 (Ar C), 130.0 (p-Ar C), 126.4 (4/5-imidazolylidene C), 124.1 (m-Ar C), 112.7 (4/5-imidazolylidene C), 100.4 (m-pyr C), 28.5 (CH(CH₃)₂), 27.0 (CH(CH₃)₂), 23.0 (CH(CH₃)₂), 2.9 (CH₂Si(CH₃)₃) (CH₂Si(CH₃)₃, CO and p-pyr C not located).

Observation of (^tBuACNC)Fe(CH₂SiMe₃)₂(CO).

In a N₂-filled glovebox a 20 mL scintillation vial was charged with (^tBuACNC)Fe(CH₂SiMe₃)₂ (11 mg, 0.02 mmol, 1.0 equiv) and C₆D₆ (ca. 0.5 mL). The brown solution was transferred to a J-Young Tube which was sealed, brought outside of the glovebox, and attached to a Schlenk line where the solution was frozen (–196 °C, liquid nitrogen). Following evacuation of the headspace, CO (1 atm) was admitted. The tube was sealed, and the solvent was slowly melted immediately resulting in a color change to purple. The contents of the tube were analyzed by ¹H, ¹³C NMR and IR spectroscopies. IR (C₆H₆): ν_CO = 1899 cm^-1. ¹H NMR (400 MHz, C₆D₆, 25 ºC) δ 7.10 (t, J = 8.1 Hz, 1H, p-pyr H), 7.07 (d, J = 2.3 Hz, 2H, 4/5-imidazolylidine H), 6.74 (d, J = 2.3 Hz, 2H, 4/5-imidazolylidine H), 6.57 (d, J = 7.9 Hz, 2H, m-pyr H), 1.83 (s, 18H, C(CH₃)₃), −0.57 (s, 18H, CH₂Si(CH₃)₃), −1.34 (s, 4H, CH₂Si(CH₃)₃). ¹³C NMR (101 MHz, C₆D₆, 25 ºC) δ 223.9 (2-imidazolylidine C), 150.9 (o-pyr C), 132.2 (p-pyr C), 121.4 (4/5-imidazolylidine C), 111.7 (4/5-imidazolylidine C), 101.0 (m-pyr C), 58.6 (C(CH₃)₃), 31.7 (C(CH₃)₃), 2.3 (Si(CH₃)₃) (CO and CH₂Si(CH₃)₃ not located).

Preparation of (3,5-Me₂^MesCNC)Fe(N₂)₂.

In a N₂-filled glovebox a 20 mL scintillation vial was charged with (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (200 mg, 0.26 mmol, 1.0 equiv) and pentane (ca. 5 mL). HBPin (45 μL, 0.3 mmol, 1.2 equiv) was added at RT and the resulting suspension was vigorously stirred for 1 h. The resulting brown suspension was filtered and the solid was washed with pentane (3 × 3 mL) and dried under vacuum to yield 143 mg (94% yield) of (3,5-Me₂^MesCNC)Fe(N₂)₂ as a brown solid. IR (C₆D₆): ν_NN = 2094 (major), 2055 (minor), 2022 (major), 1983 (minor) cm^-1. ¹H NMR (500 MHz, C₆D₆, 25 ºC): δ 7.86 (s, 2H, 4/5-imidazolylidene H), 6.79 (s, 4H, Ar H), 6.64 (s, 1H, p-pyr H), 6.48 (s, 2H, 4/5-imidazolylidene H), 2.43 (s, 6H, m-pyr-(CH₃)₂), 2.27 (s, 12H, 2,6-Ar-(CH₃)₂), 1.99 (s, 6H, 4-Ar-CH₃). ¹³C{¹H} NMR (126 MHz, C₆D₆, 25 ºC): δ 205.1 (2-imidazolylidene C), 140.1 (o-pyr C), 138.3 (4-Ar C), 137.4 (Ar C), 136.8 (Ar C), 129.4 (3-Ar C), 123.1 (4/5-imidazolylidene C), 122.0 (p-pyr C), 115.1 (4/5-imidazolylidene C), 110.7 (m-pyr C), 21.1 (4-Ar-CH₃), 19.1 (m-pyr-(CH₃)₂), 18.1 ((2,6-Ar-(CH₃)₂)).

Alternative Preparation of (3,5-Me₂^MesCNC)Fe(N₂)₂.

In a N₂-filled glovebox a 50 mL Schlenk tube with a Teflon valve was charged with (3,5-Me₂^MesCNC)Fe(CH₂SiMe₃)₂(N₂) (200 mg, 0.26 mmol, 1.0 equiv) and pentane (ca. 5 mL). The tube was sealed, brought outside of the glovebox and attached to a Schlenk line where the solution was frozen (–196 °C, liquid nitrogen). Following evacuation of the headspace, H₂ (1 atm) was admitted at –196 °C. The tube was sealed and allowed to warm to RT and the resulting suspension was vigorously stirred for 1 h. The tube was brought back into the glovebox and opened, and the resulting brown suspension was transferred to a 20 mL scintillation vial and vigorously stirred for 1 h under an N₂ atmosphere. The resulting brown suspension was filtered and the solid was washed with pentane (3 × 3 mL) and dried under vacuum to yield 146 mg (96% yield) of (3,5-Me₂^MesCNC)Fe(N₂)₂ as a brown solid. All characterization data matched the material obtained above.

Observation of (CNC)Fe hydrides and isotopologues.

In a N₂-filled glovebox a J-Young NMR-tube was charged with a solution of (CNC)Fe(CH₂SiMe₃)₂(N₂) (10 mg) in C₆D₆ (0.5 mL). The tube was sealed, brought outside of the glovebox and attached to a Schlenk line and the contents of the tube were frozen in liquid nitrogen. Following evacuation of the headspace H₂ (1 atm) was admitted at –196 °C. The tube was sealed, warmed to RT and inverted three times and the solution immediately analyzed by ¹H NMR spectroscopy.