An Fe-N₂ Complex That Generates Hydrazine and Ammonia via Fe═NNH₂: Demonstrating a Hybrid Distal-to-Alternating Pathway for N₂ Reduction

Abstract

Biological N₂ fixation to NH₃ may proceed at one or more Fe sites in the active-site cofactors of nitrogenases. Modeling individual e⁻/H⁺ transfer steps of iron-ligated N₂ in well-defined synthetic systems is hence of much interest but remains a significant challenge. While molecular Fe species have been recently demonstrated to catalyze the formation of NH₃ from N₂, mechanistic details of these processes remain elusive. Herein, we report the synthesis and isolation of a diamagnetic, 5-coordinate formally iron(IV) Fe═NNH₂⁺ species supported by a tris(phosphino)silyl ligand via the direct protonation of a terminally bound Fe-N₂⁻ complex. The Fe═NNH₂⁺ complex is redox-active, and low-temperature spectroscopic data and DFT calculations evidence an accumulation of significant radical character on the hydrazido ligand upon one-electron reduction to S = ¹/₂ Fe═ NNH₂. At warmer temperatures, Fe═NNH₂ rapidly converts to an iron hydrazine complex, Fe-NH₂NH₂⁺, via the additional transfer of proton and electron equivalents in solution. Fe-NH₂NH₂⁺ can liberate ammonia, and the sequence of reactions described here demonstrates that an iron site can shuttle from a distal intermediate (Fe═NNH₂⁺) to an alternating intermediate (Fe-NH₂NH₂⁺) en route to NH₃ liberation from N₂. It is interesting to consider the possibility that similar “hybrid” mechanisms for N₂ reduction may be operative in biological N₂ fixation.

Graphical Abstract

INTRODUCTION

The proton-coupled reduction of dinitrogen (N₂) to ammonia (NH₃) by nitrogenase enzymes sustains life and has been under study for decades. Known nitrogenases employ a cofactor comprised of seven Fe-atoms and one additional metal site (Mo, V, or Fe).¹ Despite a suite of crystallographic, theoretical, and spectroscopic studies,² the mechanistic details of N₂ reduction and the metallic site(s) of N₂ coordination are uncertain.

The feasibility of N₂ reduction at an Fe or Mo site has been tested with synthetic model complexes.³ Well-defined Mo systems have been reported to catalyze the direct reduction of N₂ to NH₃ in the presence of proton and electron equivalents,⁴ and our laboratory has recently disclosed molecular Fe complexes that furnish catalytic yields of NH₃ from N₂.⁵ While synthetic studies of the Mo systems have revealed a number of isolable Mo(N_xH_y) species that inform likely mechanistic scenarios of N₂ activation and overall reduction,^6,4c similar studies on the Fe catalyzed systems are challenged by the high reactivity of the putative Fe(N_xH_y) intermediates and their varied spin states.

An iron hydrazido(2−) complex, Fe═NNH₂⁺, has been invoked as a likely intermediate in Fe-catalyzed reaction mixtures with the tris(phosphine)borane (TPB) system (TPB = tris(2-(diisopropylphosphino)phenyl)borane)).^5,7 Its detection in operando when both strong acid and reductant are present is not feasible; the species is far too reactive under such conditions. We therefore generated {[TPB]Fe═NNH₂}⁺ at low temperature by double protonation of {[TPB]Fe(N₂)}⁻ in the absence of exogenous reductant and characterized this species with a suite of spectroscopic techniques including EPR/ ENDOR, XAS, and Mössbauer spectroscopies.⁷ {[TPB]Fe═ NNH₂}⁺ decays rapidly at temperatures above −78 °C, frustrating our attempts to purify and study it by X-ray crystallography and to map its further reactivity patterns.

The complex {[SiP^iPr₃]Fe(N₂)}⁻ ([SiP^iPr₃] = tris(2-(diisopropylphosphino)phenyl)silyl⁽⁻⁾) is isostructural to the {[TPB]Fe(N₂)}⁻ catalyst. While catalytic amounts of NH₃ (7.0(1) equiv of NH₃ per Fe) are generated when {[TPB]Fe-(N₂)}⁻ is exposed to the originally reported catalytic conditions (−78 °C in Et₂O, 1 atm N₂, 50 equiv of {H(OEt₂)₂}{BAr^F₂₄}, 60 equiv of KC₈), {[SiP^iPr₃]Fe(N₂)}⁻ liberates substoichiometric amounts of NH₃ (0.8(1) equiv of NH₃ per Fe) under the same conditions.⁵ We surmised that the doubly protonated form of this species, {[SiP^iPr₃]Fe═NNH₂}⁺, might be more readily characterized than {[TPB]Fe═NNH₂}⁺ owing to its predicted diamagnetism (18-electron species). Herein, we report its synthesis and high-resolution crystal structure. This isolable Fe═NNH₂⁺ species is derived from protonation of its Fe(N₂)⁻ congener. We additionally explore the redox pairs Fe═NNH₂⁺/Fe═NNH₂ and Fe═NNMe₂⁺/Fe═NNMe₂ and demonstrate the overall conversion of Fe NNH₂⁺ to NH₃ via an Fe-NH₂NH₂⁺ intermediate. These observations in sum establish that a molecular iron system can traverse both distal (Fe═NNH₂) and alternating (Fe-NH₂NH₂) intermediates en route to NH₃ formation from N₂, providing synthetic precedent for a new “hybrid” pathway for Fe-mediated N₂ reduction (Scheme 1).^7,8

Scheme 1.

Distal and Alternating Pathways for N₂ Reduction, and the Hybrid N₂ Reduction Pathway Emphasized Herein

RESULTS AND DISCUSSION

As for {[TPB]Fe(N₂)}⁻,⁷ the successful protonation of Fe(N₂) complexes supported by the [SiP^iPr₃] ligand required very low temperatures.⁹ For example, the addition of 1 or 2 equiv of the acid {H(OEt₂)₂}{BAr^F₂₄} to {K(Et₂O)}{[SiP^iPr₃]Fe(N₂)} (1) at −78 °C resulted in the immediate formation of mixtures containing both one-electron oxidized [SiP^iPr₃]Fe(N₂) (2) and two-electron oxidized {[SiP^iPr₃]Fe(N₂)}{BAr^F₂₄} (3). These proton-induced oxidation reactions likely proceed via an unstable and as yet unobserved iron diazenido species, [SiP^iPr₃]Fe(NNH) (4), structurally and electronically related to the previously reported and stable silyldiazenido complex, [SiP^iPr₃]Fe(NNSiMe₃).⁹ An alternative hydride product, [SiP^iPr₃]Fe(N₂)(H), that would derive from protonation at iron instead of N₂ is not observed; [SiP^iPr₃]Fe(N₂)(H) is a very stable complex that has been characterized,¹⁰ and were it produced as the kinetic product of protonation, we would anticipate observing it, as it should also be the thermodynamically preferred isomer.

Combination of 1 with 5 equiv of {H(OEt₂)₂}{BAr^F₂₄} in thawing 2-MeTHF at −135 °C instead produced a pale lavender solution (Figure 1A) with UV–visible features that are distinct from the oxidation products Fe-N₂ 2 and Fe-N₂⁺ 3. The in situ ⁵⁷Fe Mössbauer spectrum (Figure 1B) collected on similarly prepared solutions derived from ⁵⁷Fe-enriched 1 evidences a new integer-spin Fe complex (δ = 0.126 mm/s and ΔE_Q = 1.484 mm/s) assigned as Fe═NNH₂⁺ 5 (vide infra), that constitutes ~90% of the Fe in solution; Fe-N₂⁺ 3 is present as a minor (~10%) component. Compound 5 is persistent for hours in solution at temperatures of −78 °C once prepared in this manner but is increasingly unstable as the solution is warmed further.

Figure 1.

Spectroscopic data collected in situ on compound 5. (A) UV–visible absorbance spectra of 3, 2, and 5. Spectra were collected in 2-MeTHF at −80 °C. (B) Zero-field ⁵⁷Fe Mössbauer spectra of ⁵⁷Fe-enriched 5 as a 3 mM solution in 2-MeTHF prepared from 1 and collected at 80 K. The minor component (10%) was identified as complex 3 derived from competitive oxidation.

The isolation of 5 as a crystalline solid free of Fe-containing impurities was facilitated by the substitution of the BAr^F₂₄ counteranion with a less-soluble analogue. The reaction of 1 with 3 equiv of trifluoromethanesulfonic acid (HOTf) proceeded similarly at −135 °C, but this compound (5′) could be effectively precipitated out of solution in 49% yield by the addition of pentane at temperatures of −78 °C or lower (Scheme 2). The ⁵⁷Fe Mössbauer spectrum of solid 5′ (Supporting Information) reveals a single Fe-containing species with parameters that are similar to those of 5 in frozen solution (Figure 1B). Solid 5′ displays intense vibrational features at 3207 and 3039 cm⁻¹ that shift to 2380 and 2241 cm⁻¹ in 5′-d₂ (prepared from the reaction of 1 with DOTf) assigned to N–H stretching frequencies with strong hydrogen bonding interactions.^11,12 These vibrational features persist in solid samples of 5′ that have been stored for days at −30 °C in the absence of air and moisture.

Scheme 2.

Functionalization of [SiP^iPr₃]Fe(N₂) Complexes

Characterization of {[SiP^iPr₃]Fe═NNH₂}⁺

The stability of 5 and 5′ in solution at −78 °C permitted growth of single crystals suitable for X-ray diffraction, and their respective structures are depicted in Figure 2. The structures differ in that 5 features an independent Et₂O molecule hydrogen bonded to each of the protons of the NNH₂ ligand, and in 5′ the NNH₂ protons feature tight hydrogen bonding interactions with the triflate anion, and these interactions form the basis of dimeric (5′)₂ units in the crystal lattice (Supporting Information). The structures are nonetheless highly similar with respect to the Fe═NNH₂⁺ subunit; short Fe–N distances (~1.67 Å) are found that reflect substantial Fe–N multiple bond character (Table 1), a characteristic feature of most terminal metal hydrazido(2−) complexes.^6,11 The ^αN-atoms display linear geometries, and the location of the nitrogen-bound protons in the Fourier difference map divulges trigonal-planar ^βN-atoms. The N–N distances (~1.27 Å) are markedly increased from that displayed by {Na(12-crown-4)₂}{[SiP^iPr₃]Fe(N₂)} (1.132(4) Å) and 1 (1.146(4) Å) (Supporting Information).^9a The N–N distance in 5′ correlates with a broad feature centered at 1443 cm⁻¹ in the IR spectrum that shifts to 1401 cm⁻¹ in ¹⁵N-5′ and is assigned to the ν(NN) stretching frequency. IR features of similar energy have been observed in Mo- and W═NNH₂ complexes.¹¹ While a number of X-ray diffraction studies on mononuclear¹³ and dinuclear¹⁴ ═ Fe complexes that support the isomeric diazene ligand (HN NH) have been disclosed, 5 and 5′ are the first crystallographically characterized complexes that contain a terminal Fe═NNH₂ unit. The structural parameters they reveal are consistent with those recently deduced from XAS and ENDOR spectroscopies for the catalytically relevant species, [TPB]Fe═NNH₂^+.7

Figure 2.

X-ray diffraction crystal structure of 5 and core-atom structures of 5′, 6, and 8 with thermal ellipsoids drawn at 50% probability. Hydrogen atoms (excepting the N–H′s), the BAr^F₂₄ counteranion of 5, the triflate counteranion of 6, and cocrystallized solvent molecules have been removed for clarity. Refer to the Supporting Information for complete crystallographic details.

Table 1.

Crystallographic Bond Metrics

	5	5′	6	8
Fe–Na	1.672(2)	1.668(2)	1.691(2)	1.773(1)
N–Na	1.272(3)	1.273(3)	1.270(3)	1.276(2)
Fe–N–Nb	175.3(2)	175.0(2)	174.7(2)	158.63(9)
Si–Fe–Nb	173.99(6)	170.82(7)	164.64(9)	174.19(4)

^aBond distances in Å.

^bBond angle in degrees.

Although Fe═NNH₂⁺ 5′ is a stable solid, solutions of 5′ decompose at temperatures of 0 °C and higher to an intractable mixture of Fe-containing species. Seeking to prepare a more stable analogue of 5′, we reacted 1 with excess MeOTf at −78 °C which, upon warming the reaction mixture to room temperature, precipitated {[SiP^iPr₃]Fe(NNMe₂)}{OTf} (6) as a purple solid. Unlike the isoelectronic Fe═NNH₂⁺ species, Fe═NNMe₂⁺ 6 is quite stable both in the solid state and in solution. The relevant metrical data derived from the solid-state crystal structure of 6 (Figure 2) are similar to those of 5′ (Table 1).

Compounds 5′ and 6 exhibit diamagnetic ground states, permitting their further characterization by multinuclear NMR spectroscopies (Figure 3).¹⁵ A single broad resonance is found in the ³¹P{¹H} NMR spectrum of 5′, consistent with averaged 3-fold symmetry in solution. Compound ¹⁵N-5′ exhibits two resonances in the ¹⁵N NMR spectrum at δ = 518 and 198 ppm, corresponding to the ^αN- and ^βN-atoms, respectively (Figure 3A).^6b,16 The resonance at δ = 198 ppm appears as a triplet of doublets (¹J_NH = 96 Hz, ¹J_NN = 11 Hz) whereas the feature at 518 ppm is broadened due to unresolved coupling to the phosphine ligands. In the ¹H NMR spectrum, ¹⁵N-5′ displays a broad doublet (¹J_NH = 97 Hz) at δ = 9.5 ppm assigned to the NNH₂ protons. The magnitude of the ¹J_NH coupling constant in 5′ is consistent with sp² hybridization at the ^βN-atom¹⁷ and similar to that found in other terminal metal-hydrazido(2−) complexes.^6b,16 These data confirm that the structure of 5′ found in the solid state is maintained in solution. Related NMR data for 6 reproduces the salient features exhibited by 5′ (Supporting Information).

Figure 3.

NMR spectra of 5′ recorded at −60 °C in 9:1 THF-d₈:CD₃CN. (A) ¹⁵N NMR spectrum of ¹⁵N-5′. (B) ³¹P{¹H} NMR spectrum of 5′. (C) Overlaid ¹H and ¹H{¹⁵N} spectra of ¹⁵N-5′. The central feature in the ¹H spectrum results from contamination of ¹⁵N-5′ with the natural abundance 5′.

Redox Chemistry of [SiP^iPr₃]Fe═NNR₂⁺

The intermediacy of Fe═NNH₂⁺ 5′ in the formation of NH₃ requires additional proton or electron equivalents. Both 5 and 5′ were found to be stable at −78 °C to the presence of additional proton equivalents; we therefore explored the one-electron reduction chemistry of 5′ and Fe═NNMe₂⁺ 6 to generate neutral [SiP^iPr₃]Fe═NNH₂ (7) and its methylated derivative [SiP^iPr₃]Fe═NNMe₂ (8), respectively. Cyclic voltammetry measurements on THF electrolytes of Fe═NNMe₂ 6 reveal a reversible reduction event at −1.73 V (Supporting Information). The chemical reduction of 6 with 1 equiv of Na(Hg) (Scheme 2) and subsequent workup furnished paramagnetic 8, whose crystal structure (Figure 3) reveals a lengthened Fe–N distance (from 1.69 to1.77 Å) concomitant with substantial bending at the ^αN-atom from 175° to 159° (Table 1). The ^βN-atom retains sp² hybridization, and the N–N bond length is essentially unchanged. The X-band EPR spectrum of 8 indicates an S = ¹/₂ ground state (Figure 4A, g_avg = 2.04), consistent with its room temperature magnetic moment in C₆D₆ (μ_eff = 1.7 μ_B). Magnetically perturbed ⁵⁷Fe Mössbauer studies of 8 (Figure 4B) demonstrate strong ⁵⁷Fe hyperfine coupling and much slower relaxation properties compared to Fe(N₂) 2 (Supporting Information); distinctive features that span a range of 5 mm/s at temperatures of 80 K and lower are observed.

Figure 4.

(A) X-band EPR spectra of Fe═NNH₂ 7 and 7-d₂, derived from the in situ reduction of 5′ or 5′-d₂, respectively, with Cp*₂Co; Fe═NNMe₂ 8 and ¹⁵N-8 collected at 77 K in 2-MeTHF glasses. Signals derived from S = ¹/₂ Fe-N₂ 2 have been subtracted from the displayed spectra of 7 and 7-d₂ for clarity. (Inset) Prominent features of 8 that differ in ¹⁵N-8. These features arise from hyperfine coupling to single ³¹P and single ^14/15N nuclei of comparable magnitude. (B) ⁵⁷Fe Mössbauer spectra of in situ-prepared 7 and 8 obtained by subtracting out quadrupole doublet impurities from the raw data. A 50 mT magnetic field was applied (left) perpendicular and (right) parallel to the propagation of γ-beam. The solid lines are theoretical fits to an S = ¹/₂ spin Hamiltonian operating in the slow relaxation regime. Refer to the Supporting Information for a detailed discussion and the derived spin Hamiltonian parameters.

Fe═NNH₂ 7 is far less stable than 8 and required characterization at cryogenic temperatures. Compound 5′ reacted with Cp*₂Co in 2-MeTHF at −135 °C to produce dark brown solutions that rapidly bleached when warmed to −78 °C or higher temperatures (vide infra). EPR (Figure 4A) and ⁵⁷Fe Mössbauer spectra (Figure 4B) collected on similarly prepared frozen reaction mixtures derived from 5′ confirmed the generation of a new S = ¹/₂ species (g_avg = 2.04) as the major Fe-containing component. Notably, the ⁵⁷Fe Mössbauer spectrum of this complex is nearly identical to that displayed by Fe═NNMe₂ 8, allowing us to assign it as the isoelectronic species 7. Accordingly, the theoretically predicted gas-phase optimized geometry and electronic structure of 7 are very similar to those of 8 (Supporting Information). Compounds 7 and 8 are predicted to have substantial radical character on the NNR₂ and phosphine ligands, as also evident from their respective EPR data. Differences between the X-band EPR spectra of ¹⁵N-8 and 8 establish strong hyperfine coupling (ca. 30 MHz) to a single N-atom (Inset of Figure 4A). In addition, marked differences in the EPR spectra of Fe═NNH₂ 7 and Fe═NND₂ 7-d₂ demonstrate significant hyperfine coupling to one or both nitrogen-bound H-atom(s); this value is estimated to be as high as 25 MHz through analysis of the second derivative EPR spectrum at g₂ and g₃ (Supporting Information. The related S = ¹/₂ {[TPB]Fe(NNH₂)}⁺ exhibited ¹H hyperfine coupling as large as 18 MHz between g₂ and g₃.⁷

Conversion of [SiP^iPr₃]Fe═NNH₂⁺ to [SiP^iPr₃]Fe-NH₂NH₂⁺

Upon warming to temperatures of −78 °C and higher, solutions that contain Fe═ NNH₂ 7 and Fe═NNH₂⁺ 5′ undergo a spontaneous disproportionation to a mixture of Fe species that include the previously reported iron hydrazine complex, {[SiP^iPr₃]Fe(NH₂NH₂)}{OTf} (9),^9a as a major component. Thawing THF solutions of 5′ were combined with stoichiometric Cp*₂Co and allowed to warm slowly to room temperature over 10 min. After minimal workup, NMR analyses of the resulting mixtures (Supporting Information) revealed the formation of roughly equal amounts of Fe-NH₂NH₂⁺ 9 and Fe-N₂ 2 as major products, alongside small amounts of [SiP^iPr₃]Fe(OTf) (10) and {[SiP^iPr₃]Fe(NH₃)}⁻ {OTf} (11).⁹ 9 was also detected in a one-pot reaction from Fe-N₂⁻ 1 via the sequential addition of 2 equiv of HOTf and 0.5 equiv of Cp*₂Co to 1 in 2-MeTHF at −135 °C. Significant quantities of free N₂H₄ (0.53(6) equiv per Fe) and NH₃ (0.16(2) equiv) were detected when these reaction mixtures were quenched with HCl 10 min after warming. This product distribution was found to be time dependent: reaction mixtures quenched after standing at room temperature for 24 h revealed the presence of 0.27(6) equiv of N₂H₄ and 0.39(5) equiv of NH₃, establishing further conversion of N₂H₄ to NH₃ in these mixtures.

The overall formation of both N₂H₄ and NH₃ from an Fe═NNH₂⁺/Fe═NNH₂ redox pair is interesting, given that the complex [SiP^Ph₃]Fe(N₂) was observed to liberate significant amounts of N₂H₄ upon exposure to HBF₄(Et₂O) and CrCl₂ some years ago,¹⁸ whereas the anion {[SiP^iPr₃]Fe(N₂)}⁻ was instead observed to liberate NH₃ in the presence of KC₈ and {H(OEt₂)₂}{BAr^F₂₄};⁵ N₂H₄ is anticipated to gradually degrade to NH₃ under the latter conditions,⁵ and hence, even if formed as an intermediate product, its concentration may not build up.

The formation of the Fe-containing products Fe-NH₂NH₂⁺ 9 and Fe-N₂ 2 can most simply be rationalized by the reaction sequence shown in Scheme 3. As discussed above, ⁵⁷Fe Mössbauer and EPR studies indicate that 5′ is reduced by Cp*₂Co to generate neutral Fe═NNH₂ 7 at temperatures as low as −135 °C. At higher temperatures, we speculate that in situ-generated 7 reacts bimolecularly with remaining 5′ in solution. Exchange of H⁺ and e⁻ equivalents between these two compounds results in the formation of neutral Fe-N₂ 2 and cationic Fe-NH₂NH₂⁺ 9 as the overall reaction products. Whereas 2 is stable to the presence of Cp*₂Co, 9 is slowly reduced by Cp*₂Co to afford detectable quantities of NH₃ and thereby a mixture of 2 and Fe-OTf 10. DFT studies predict that the conversion of 5′ + 7 → 2 + 9 is highly exergonic (−45 kcal/ mol) (Supporting Information).

Scheme 3.

Comparison of the Reaction Products Observed in the Reduction of (A) Fe═NNH₂⁺ 7 and (B) Mo═NNH₂ Supported by the Tri(amido)amine [HIPTN₃N]³⁻ Ligand Framework¹⁹

CONCLUSIONS

The present study has described the thorough characterization, including the first crystallographic evidence, of a terminally bonded Fe═NNH₂ species; this formally iron(IV) complex is derived from the activation and protonation of N₂ coordinated to iron. Numerous examples of iron oxos, nitrides, and imides featuring strong, covalent iron-to-ligand multiples bonds have been characterized in the past 15 years.²⁰ The possibility to use such covalency as a strategy for N₂ reduction to NH₃ is a plausible one, and the stoichiometric chemistry established with the present tris(phosphino)silyl iron system underscores this point. Our ability to isolate diamagnetic [SiP^iPr₃]Fe═NNH₂⁺ and its more stable methylated analogue, [SiP^iPr₃]Fe═ NNMe₂⁺, enables their thorough characterization and also a study of their one-electron reduction chemistry. The stable S = ¹/₂ complex [SiP^iPr₃]Fe═NNMe₂ has been structurally characterized, and its other spectroscopic parameters are very similar to those of the far less stable S = ¹/₂ species [SiP^iPr₃]Fe═NNH₂, which must instead be characterized at very low temperature. Each S = ¹/₂ species evidences significant spin-leakage onto the hydrazido ligand.

A fascinating transformation occurs as solutions containing in situ-generated [SiP^iPr₃]Fe═NNH₂ and [SiP^iPr₃]Fe═NNH₂⁺ are allowed to warm, disproportionating to [SiP^iPr₃]Fe-NH₂NH₂⁺ and [SiP^iPr₃]Fe-N₂. Some [SiP^iPr₃]Fe-NH₃⁺ is also produced in this process; we had previously shown that [SiP^iPr₃]Fe-NH₂NH₂⁺ can liberate [SiP^iPr₃]Fe-NH₃⁺ and free NH₃ in solution. Hence, these collective observations show that iron-bound N₂ can be protonated to generate a distal⁸ intermediate, [SiP^iPr₃]Fe═NNH₂⁺, and further reduced/ disproportionated to an alternating intermediate,⁸ [SiP^iPr₃]Fe-NH₂NH₂⁺, that serves as a source of NH₃ via late-stage N–N cleavage. The conversion of N₂ to NH₃ via an N₂H₄ intermediate therefore does not require an alternating pathway; it can instead be initiated along a distal pathway. Such a scenario is distinct from the early stage N–N cleavage pathway to generate terminal nitrides that is thought to occur in the molybdenum N₂ reduction catalysts of Schrock and Nishibayashi, respectively.⁴

The catalytically relevant [TPB]Fe-N₂⁻ system is thought to proceed via a distal S = ¹/₂ [TPB]Fe═NNH₂⁺ intermediate.⁷ This species cannot be isolated and independently studied owing to its greater instability and the presence of additional iron components, and it remains unclear whether NH₃ production in this case derives from similar late-stage cleavage to first produce N₂H₄, akin to [SiP^iPr₃]Fe═NNH₂, or if an early-stage cleavage pathway instead generates a terminal iron-bound nitride, such as [TPB]Fe═N or (TPB)Fe═N⁺. The greater flexibility of the Fe–B bond relative to the Fe–Si bond may afford access to different intermediates. However, that [SiP^iPr₃]Fe-N₂⁻ generates appreciable amounts of NH₃ under the same conditions as [TPB]Fe-N₂⁻, and that its isostructural carbon analogue [CP^iPr₃]Fe-N₂⁻ is a catalyst for N₂-to-NH₃ conversion but is not nearly as flexible as the [TPB]Fe system,^5b suggest the possibility and perhaps even likelihood of a unifying distal-to-alternating mechanistic sequence en route to NH₃ for these three iron systems.

While we have here demonstrated the viability of a hybrid distal-to-alternating reaction pathway for NH₃ generation via N₂H₄, we still caution that different Fe-mediated N₂ reduction systems, with variable reaction conditions, may sample alternative pathways.