Low Temperature N₂ Binding to 2-coordinate L₂Fe⁰ Enables Reductive Trapping of L₂FeN₂⁻ and NH₃ Generation.

Abstract

The 2-coordinate (CAAC)₂Fe complex [CAAC = cyclic (alkyl)(amino)carbene] binds dinitrogen at low temperature (T < −80 °C). The resulting putative 3-coordinate N₂-complex, (CAAC)₂Fe(N₂), was trapped by one electron reduction to its corresponding anion [(CAAC)₂FeN₂]⁻ at low temperature. This complex was structurally characterized and features an activated dinitrogen unit that can be silylated at the β-nitrogen. The redox linked complexes (CAAC)₂Fe^IBAr^F₂₄, (CAAC)₂Fe⁰ and [(CAAC)₂Fe^−IN₂]⁻ were all found to be active for the reduction of dinitrogen to ammonia upon treatment with reductant (KC₈) and acid (HBAr^F₂₄·2Et₂O) at −95 °C (up to 3.4 ±1.0 equivalent of ammonia per Fe center). The N₂ reduction activity is highly temperature dependent, with significant N₂ reduction to NH₃ only occurring below −78 °C. This reactivity profile tracks with the low temperatures needed for N₂ binding and an otherwise unavailable electron transfer step to generate reactive [(CAAC)₂FeN₂]⁻.

While hundreds of transition-metal-N₂ complexes have been prepared and studied,^[1] comparatively few systems afford access to productive N₂ functionalization.^[2] This is particularly true for the case of N₂ functionalization by protons and electrons to produce NH₃ (or N₂H₄). Building on extensive early Mo and W model work,^[3] Schrock^[4] and Nishibayashi^[5] have reported Mo-containing coordination complexes (A and B in Figure 1, respectively) that facilitate catalytic N₂ reduction to NH₃ in the presence of suitable acids and reductants. Because Fe is (i) the only transition metal known to be essential to enzymatic nitrogenase function,^[6] and (ii) the predominant transition metal catalyst used in the Haber-Bosch ammonia synthesis,^[7] studying N₂ reduction chemistry at well-defined Fe model complexes is of interest.^[8,9] Recently, our lab demonstrated that Fe coordination complexes (C in Figure 1) are capable of modest catalytic N₂ reduction to NH₃.^[10]

Figure 1.

Representative Mo and Fe complexes studied for catalytic N₂ reduction to NH₃. R = tBu; R’ = H, OMe; HIPT = hexaisopropylterphenyl; E = B, C; M = K, Na.

The functional Fe-N₂ reduction systems we have studied to date are phosphine supported 5-coordinate XL₃Fe-N₂ adducts. It is of interest to explore whether other donor ligands and geometries might expose similar reactivity patterns for Fe-N₂ species.^[11,12] For example, Holland and co-workers recently reported a solution characterization of a 3-coordinate Fe complex with an N₂ ligand linearly bridged between Fe and a solvated Mg cation (D in Figure 1).^[13,14] The N₂ ligand in this complex is highly activated. However, only trace NH₃ (< 2% per Fe) could be detected upon attempts to mediate its reductive protonation. Related low-coordinate Fe-N₂ species can undergo reductive N₂ cleavage which, when followed by a separate acidic work-up, can achieve higher ammonia yields (ca. 35% overall).^[8c] Understanding subtle factors that lead to productive reactivity at N₂ is essential to improving molecular nitrogen fixation catalysts.

We recently reported the utilization of a π-accepting cyclic (alkyl)(amino)carbene (CAAC)^[15] to synthesize a structurally unusual 2-coordinate, formally L₂Fe⁰ complex by reduction of its corresponding 2-coordinate Fe^I cation.^[16] We reasoned that the polarizability of the CAAC ligand, by comparison to typical NHCs,^[17] might better facilitate N₂ binding and productive functionalization. Note that low-valent Fe-carbene complexes^[18] have to date been surprisingly resistant to N₂ binding, with the tripodal, N-heterocyclic carbene (NHC) containing, systems of Meyer^[19] and Smith^[20] underscoring this point. To our knowledge, a bis(imidazol-2-ylidene)pyridine scaffold is the only Fe-carbene derivative where N₂ binding has been established.^[21] Here we show that the 2-coordinate (CAAC)₂Fe complex binds N₂, but only at low temperature, and that this binding event exposes an otherwise unavailable 1-electron reduction step to form 3-coordinate (CAAC)₂Fe(N₂)⁻ and desirable reactivity at the coordinated N₂.

Under an atmosphere of N₂, a solution of (CAAC)₂Fe in pentane exhibited drastic changes in the absorption spectrum upon cooling at temperatures below −80 °C (Figure 2A). This temperature dependent change in the optical profile of (CAAC)₂Fe under N₂ was reversible over several cooling-warming cycles. To confirm that the change in the optical spectrum was indeed related to the presence of N₂, the solution was thoroughly degassed by five freeze-pump-thaw cycles and variable temperature UV-vis spectra were re-collected (See SI Figure S7, right). In this case, the standard spectrum of (CAAC)₂Fe was recovered. Re-exposing the solution to N₂ resulted in the previously observed temperature dependent profile. Thermodynamic parameters were extracted from a van’t Hoff plot of the experimental data (See SI). As expected, binding of dinitrogen is exothermic (ΔH = −22.1 kcal mol⁻¹) and is associated with negative entropy (ΔS = −8.1 cal K⁻¹ mol⁻¹). These parameters are similar to those recently reported for N₂ coordination at Co.^[21] The N₂ binding constant at room temperature is very low (K_eq = 0.2), consistent with our initial report of a 2-coordinate (CAAC)₂Fe complex.^[16]

Figure 2.

A. Variable temperature UV-vis spectra of (CAAC)₂Fe in a pentane solution. a) under N₂ atmosphere; b) under vacuum. B. X-Band EPR spectra at 10 K: a) [(CAAC)₂Fe(N₂)][K(18-crown-6)] in a frozen Et₂O glass; b) (CAAC)₂Fe(N₂SiEt₃) in a frozen methylcyclohexane glass. C.⁵⁷Fe Mössbauer spectra for microcrystalline samples obtained at 80 K in boron nitride pellets. a) [(CAAC)₂Fe(N₂)][K(18-crown-6)]; b) (CAAC)₂Fe(N₂SiEt₃). Solid red lines correspond to the simulated data. Solid blue line in a) correspond to a minor species (~ 9% of Fe) parameters of δ = 0.74 mm s⁻¹ and ΔE_Q = 3.79 mm s⁻¹. D. Structure of [(CAAC)₂Fe(N₂)][K(18-crown-6)] in the solid state. Ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity.^[23] Selected bond lengths (Å) and angles (^°): C1–Fe1 = 1.924(2), C2–Fe1 = 1.919(2), C1–N1 = 1.386(3), C2–N2 = 1.411(3), Fe1–N3 = 1.778(3), N3–N4 = 1.035(4), N4–K1 = 2.726(4); C1–Fe1–C2 = 140.81(10), C1–Fe1–N3 = 101.91(11), C2–Fe1–N3 = 117.08(11), Fe1–N3–N4 = 175.5(3).

To structurally confirm the binding of dinitrogen at low temperature, we sought to trap the putative complex (CAAC)₂Fe(N₂) (Scheme 1). The presence of an additional π-accepting ligand (N₂) facilitates the further reduction of (CAAC)₂Fe⁰ to a formal Fe^−I. Hence, reduction of (CAAC)₂Fe with KC₈ at −95 °C in diethyl ether in the presence of 18-crown-6 yielded, after work-up, dark brown crystals of [(CAAC)₂Fe(N₂)] [K(18-crown-6)] in moderate yield (42%) (Scheme 1). Temperature control was crucial: reduction at room temperature resulted instead in decomposition of the starting material to intractable products, and at −78 °C, only traces of [(CAAC)₂Fe(N₂)]⁻ could be detected by IR spectroscopy. An XRD study of crystals of [(CAAC)₂Fe(N₂)] [K(18-crown-6)] confirmed the binding of N₂ to a 3-coordinate Fe center in an end-on fashion with an additional ion-pair of the β-N-atom to the K(18-crown-6) cation (Figure 2D).

Scheme 1.

Reduction of (CAAC)₂Fe and subsequent trapping with silylating reagents. Dipp = 2,6-diisopropylphenyl; R = Me, Et.

The structure of [(CAAC)₂Fe(N₂)] [K(18-crown-6)] reveals a distorted trigonal planar Fe center (Σ_angle = 359.8°), with a very wide C-Fe-C angle (140.81(10)°). The C-N distances of the CAAC ligand are substantially elongated: they are 1.386(3) and 1.411(3) Å, compared with 1.315(3) Å for the free ligand.^[15c] This C-N lengthening is indicative of a substantial amount of spin leakage from the highly reduced Fe center to the supporting CAAC ligand.^[16] Complex [(CAAC)₂Fe(N₂)] [K(18-crown-6)] represents a rare example of a structurally characterized pseudo-terminally bonded, 3-coordinate Fe-N₂ complex.^[13,14]

A sharp band was observed in the IR spectrum of [(CAAC)₂Fe(N₂)][K(18-crown-6)] at 1850 cm⁻¹ (ATR-IR; thin film), attributed to the ν_NN stretch. This value is higher in energy than the transiently observed [β-diketimidatoFe(N₂)]₂Mg(thf)₄ species reported by Holland and co-workers (1818 cm⁻¹),^[13] but lower than other anionic terminal Fe-N₂ complexes where the Fe center resides in higher coordination numbers (1905 – 1927 cm⁻¹).^[9,10a] As expected for a formal Fe^−I species, a room temperature solution magnetic moment for [(CAAC)₂Fe(N₂)][K(18-crown-6)] of 1.9µ_B in C₆D₆ was measured, consistent with its frozen glass (Et₂O) EPR spectrum (Figure 2B, a) and indicative of an S = 1/2 ground state.

Exposure of [(CAAC)₂Fe(N₂)][K(18-crown-6)] to trimethylsilylchloride in diethyl ether resulted in a rapid color change from dark brown-red to dark green. A diagnostic broad band at 1675 cm⁻¹ was observed in the IR spectrum of the crude reaction mixture, reminiscent of previously observed ν_NN stretches for Fe-silyldiazenido complexes.^[9,10a] The presumed diazenido product (CAAC)₂Fe(N₂SiMe₃) rapidly decomposed upon attempted work-up to the (CAAC)₂Fe complex. A bulkier silylating reagent led to a more stable product. Hence, a similar change of color from dark brown-red to dark green was observed when triethylsilylchloride was added to the [(CAAC)₂Fe(N₂)][K(18-crown-6)] complex (Scheme 1). The diazenido product, (CAAC)₂Fe(N₂SiEt₃) was isolated as an analytically pure, dark green solid in moderate yield (52%). Its IR spectrum exhibits a broad band at 1690 cm⁻¹. Complex (CAAC)₂Fe(N₂SiEt₃) is high spin S = 3/2 (room temperature solution magnetic moment of 3.9µ_B), consistent with its frozen glass EPR spectrum at 10 K showing broad signals at low field (Figure 2B, b). Its electronic structure is therefore distinct from previously characterized 5-coordinate XL₃Fe(N₂SiR₃) species that are low spin. For instance, (TP^iPrB)Fe(N₂SiMe₃) is an S = 1/2 species.^[9b]

The⁵⁷Fe Mössbauer spectra of [(CAAC)₂Fe(N₂)][K(18-crown-6)] and (CAAC)₂Fe(N₂SiEt₃) were recorded at 80 K in the solid state (Figure 2C). Fits to the data provided isomer shifts at δ = 0.56 and δ = 0.59 mm s⁻¹, respectively, with quadrupole splitting of ΔE_Q = 1.67 and ΔE_Q = 1.60 mm s⁻¹. The isomer shift value is in line with previously reported 3-coordinate bis-carbene Fe complexes.^[16,24] For example, (CAAC)₂FeCl has an isomer shift of 0.52 mm s⁻¹. The spectrum of [(CAAC)₂Fe(N₂)][K(18-crown-6)] displays an asymmetrical doublet presumably caused by anisotropy in the polycrystalline sample.^[25] A minor component (~9% of the total Fe content), whose parameters fit the oxidation product [(CAAC)₂Fe]⁺,^[16] was also detected (Figure 2C, a).

In previous studies of 5-coordinate Fe-N₂ species, we have empirically determined that the ability to successfully silylate the β-N-atom of an Fe-N₂ derivative can translate to efficacy of the system towards reductive protonation.^[9c,10] Attempts to fix N₂ to NH3 at room temperature using (CAAC)₂Fe in the presence of excess KC₈ and HBAr^F₂₄·2Et₂O in diethyl ether, regardless of the order of addition, proved largely ineffective (Table 1; See SI Figure S12). This observation is consistent with a very limited degree of N₂ binding to (CAAC)₂Fe at room temperature, and the requirement that a (CAAC)₂Fe(N₂) species be present to facilitate N₂ fixation. Indeed, when (CAAC)₂Fe was treated at −95 °C with an excess of KC₈ (50 eq), followed by an excess of HBAr^F₂₄∙2Et₂O (50 eq), the formation of 3.3 ±1.1 equivalents of NH₃ per Fe center was observed. Similar yields (3.4 ±1.0 eq) were obtained employing the 2-coordinate Fe(I) cation [(CAAC)₂Fe][BAr^F₂₄] under the same conditions at −95 °C. The enhancement of the N₂-to-NH₃ reduction yield at very low temperature correlates with the variable temperature optical data for (CAAC)₂Fe, which shows that N₂ binding to (CAAC)₂Fe becomes favorable only at low temperature. Indeed, the yield of NH₃ is negligible at −50 °C (0.3 eq) and is still quite low at −78 °C (0.9 eq). Note that significantly lower temperature (−113 °C) was examined but did not improve the NH₃ yield beyond that obtained at −95 °C.

Table 1.

Catalytic reduction of N₂ to NH₃^[a]

Run	Catalyst	T	Eq NH3 per Fe
1	(CAAC)2Fe	−113 °C	3.0 ±0.7
2	(CAAC)2Fe	−95 °C	3.3 ±1.1
3	(CAAC)2Fe	−78 °C	0.9 ±0.3[b]
4	(CAAC)2Fe	−50 °C	0.3 ±0.2[b]
5	(CAAC)2Fe	23 °C	0.4 ±0.2[b]
6	(CAAC)2FeBArF24	−95 °C	3.4 ±1.0
7	(CAAC)2FeN2K(18-c-6)	−95 °C	2.6 ±0.6
8	CAAC[c]	−95 °C	< 0.1
9	None	−95 °C	< 0.1

^[a]Catalytic conditions: catalyst: 0.002 mmol, KC₈: 0.1 mmol, HBAr^F₂₄·2Et₂O: 0.1 mmol, Et₂O, 45 min; yields are an average of 8 independent runs.

^[b]Average of 4 runs.

^[c]0.004 mmol of CAAC used.

Although these NH₃ yields are low compared with our two previously reported Fe catalysts^[10] Schrock’s original Mo system,^[4] and especially Nishibayashi’s recently improved Mo system,^[5a] (CAAC)₂Fe is nevertheless a very modest catalyst for nitrogen fixation to ammonia and the yields of NH₃ per Fe equivalent reported here are far greater than those that have been observed for previously studied 3-coordinate Fe-N₂ complexes (< 0.2 eq per Fe), including [β-diketimidatoFe(N₂)]₂Mg(thf)₄ species (Figure 1).^[13] We speculate that the ability of (CAAC)₂Fe/(CAAC)₂Fe(N₂) to perform nitrogen fixation may arise from the relative flexibility of the system, capable of switching between 2- and 3-coordinate geometries, and allowing for the formation of highly covalent Fe-N_x multiple-bond interactions. The recent isolation of a 3-coordinate Fe-bis(imide) complex supported by CAAC by Deng and co-workers is noted in the latter context.^[26]

For comparison, we also explored (CAAC)₂Fe in the context of catalytic N₂ silylation. Catalytic silylation reactions of N₂ under strongly reducing conditions have been known for many years using metals such as Cr,^[27] Ti^[28] and Mo,^[29] and recently, such silylations were extended to systems using Fe pre-catalysts including Fe(CO)₅ and Cp₂Fe.^[30] For example, Fe(CO)₅ was shown to produce up to 25 equivalents of N(SiMe₃)₃ per Fe in the presence of a vast excess sodium metal and TMSCl at room temperature.^[30] These silylation reactions remain mechanistically ill-defined. Only trace yield of N(SiMe₃)₃ could be detected using Nishibayashi’s conditions (Na as reductant, in THF)^[30] with (CAAC)₂Fe as pre-catalyst. However, when a catalytic amount of (CAAC)₂Fe was instead treated with a large excess of KC₈ (600 eq) and trimethylsilylchloride (600 eq), the formation of 24.4 ± 2.7 equivalents of N(SiMe₃)₃ was observed (See Table S1 in the SI for details). Slightly lower TONs (19.4 ±3.0) were obtained using (CAAC)₂FeBAr^F₂₄ as a pre-catalyst instead. The catalytic activity was significantly reduced (7.0 ±1.0) when the reaction was performed at −78 °C. The attenuation in N(SiMe₃)₃ product is presumably related to slower generation of trimethylsilyl radical at low temperature.

Although it is presently unclear what the mechanistic requirements are for the N₂-to-N(SiMe₃)₃ catalysis mediated by various Fe precursors including (CAAC)₂Fe, the N₂-to-NH₃ chemistry described herein appears to require a well-defined molecular Fe-N₂ species. Indeed, significant N₂-to-NH₃ conversion is observed only at the low temperatures where N₂ binds (CAAC)₂Fe sufficiently favorably. The low-temperature N₂ binding event facilitates subsequent electron-transfer, evidenced by the very low temperature required to synthetically reduce (CAAC)₂Fe(N₂) and isolate highly reactive [(CAAC)₂Fe(N₂)]⁻.