A Five-Coordinate Phosphino/Acetate Iron(II) Scaffold that Binds N₂, N₂H₂, N₂H₄, and NH₃ in the Sixth Site

Abstract

A family of iron(II) complexes that coordinate dinitrogen, diazene, hydrazine, and ammonia are presented. This series of complexes is unusual in that the complexes within it feature a common auxiliary ligand set and differ only by virtue of the nitrogenous N_xH_y ligand that occupies the sixth binding site. The ability of an iron center to bind N₂, N₂H₂, N₂H₄, and NH₃ is important to establish in the context of evaluating catalytic N₂ reduction schemes that invoke these nitrogenous species. Such a scenario has been proposed as an iron-mediated, alternating reduction scheme within the cofactor of nitrogenase enzymes.

Owing to its biological and industrial relevance, establishing mechanisms for the reduction of N₂ to NH₃ is a longstanding goal of chemists.¹ Several mechanisms have been proposed for a metal-mediated reduction, with the distal (i.e., Mⁿ-N≡N → Mⁿ⁺³≡N + NH₃ → Mⁿ + NH₃) and alternating (i.e., Mⁿ-N≡N → Mⁿ-HN=NH → Mⁿ-H₂N-NH₂ → Mⁿ + 2 NH₃) mechanisms representing two limiting schemes.^1c,d,2 Schrock³ and Nishibayashi⁴ have respectively prepared mono- and di-molybdenum complexes that serve as catalysts for this transformation, the former of which is thought to proceed via a distal reduction scheme. To date, there are no examples of well-defined synthetic catalysts thought to proceed via the alternating reduction scheme, though it has been suggested that the biological reduction of N₂ to NH₃ at the FeMo-cofactor of nitrogenase may proceed via such a mechanism.⁵ Circumstantial support of such a hypothesis derives from the observation that both diazene and hydrazine are substrates for nitrogenase.^1d,5 Also, spectroscopic studies of the FeMo-cofactor under turnover conditions appear to be consistent with an iron-bound NH_y species, which is also observed in the reduction of diazene and hydrazine.⁵ Though the site of N₂ coordination and subsequent reduction remains a matter of uncertainty, recent studies have suggested that N₂ is reduced at one or more iron centers.^1d,6 Hence, there is much interest in preparing synthetic iron complexes that coordinate N_xH_y ligands,⁷ as they serve as structural and spectroscopic models to the postulated trapped intermediates.

Mono- and di-iron complexes that coordinate N₂H_y ligands (y = 2,3) remain relatively rare, and exhibit both acid/base and redox reactivity.^7a–c,7e,f Despite this rich reactivity, there are no reported iron systems that can coordinate and/or interconvert a range of N_xH_y ligands that encompass the full range of sp, sp², and sp³ hybridization at the nitrogen atoms, and the accompanying N-atom oxidations states 0, -1, -2, and -3.^7g,h,8 Such systems are rare for any metal, with only two ruthenium⁹ and one manganese¹⁰ systems published in the literature. Herein we describe a series of iron complexes that coordinate N₂, N₂H₂, N₂H₄, and NH₃. These complexes feature identical auxiliary ligands, are all iron(II), and only differ in the extent that the nitrogenous ligand is reduced.

Access to the N_xH_y chemistry of present interest is realized using a 5-coordinate iron(II) complex, [PhBP^CH2Cy₃]Fe(OAc) (1) ([PhBP^CH2Cy₃] = PhB(CH₂P(CH₂Cy)₂)₃⁻). Complex 1 is quantitatively formed in the reaction between [PhBP^CH2Cy₃]FeMe and one equiv of AcOH, and is isolated as an analytically pure grey powder (Scheme 1). Five-coordinate 1 is paramagnetic with a room temperature solution magnetic moment of 4.5 μ_β.

Scheme 1.

Complex 1 serves as a scaffold for which L-type ligands can bind, generating low-spin 6-coordinate mono- and diiron species. For example, cooling solutions of 1 under an N₂ atmosphere results in the coordination of 0.5 equiv of N₂ to generate the pink and diamagnetic μ-N₂ species 2, {[PhBP^CH2Cy₃]Fe(OAc)}₂(μ-N₂). Crystals of 2 suitable for diffraction can be grown from saturated Et₂O solutions of 2 stored at −35 °C in the glove-box, and its solid-state structure has been obtained (Figure 1a). The respective Fe-N and N-N distances of 1.874(3) and 1.120(5) Å indicate a small degree of N₂ activation.¹¹ Consistent with the small degree of N₂ activation, a ν(NN) stretch is observed at 2083 cm⁻¹ that shifts to 2010 cm⁻¹ upon ¹⁵N-isotopic labeling (calc’d: 2012 cm⁻¹).

Figure 1.

Displacement ellipsoid (50%) representation of (a) the core atoms of μ-N₂ 2, (b) the core atoms of μ-N₂H₂ 4 (major component only), and (c) 5. Protons that were not located in the difference map have been removed for clarity. Select bond distances (Å) and angles (°) for 2: Fe1-P1 2.285(1); Fe1-P2 2.2319(9); Fe1-P3 2.2361(9); Fe1-O1 2.076(2); Fe1-O2 2.083(2); Fe1-N1 1.874(3); N1-N1’ 1.120(5); Fe1-N1-N1’ 174.9(3). Select bond distances (Å) and angles (°) for 4: Fe1-P1 2.219(3); Fe1-P2 2.311(4); Fe1-P3 2.240(4); Fe1-O1 2.082(8); Fe1-O2 2.110(7); Fe1-N1 1.902(8); Fe2-N2 1.898(8); N1-N2 1.31(1); Fe1-N1-N2 128.1(7); Fe2-N2-N1 129.0(7). (d) Diazene resonance in the ¹H{³¹P} NMR spectrum (C₆D₆, 25 °C) of ¹⁵N-4 indicating the AA’XX’ splitting pattern (experimental, black; fit, red). The data was fit with MestReNova using the following parameters: δ = 17.72, ¹J_NH = −71.0 Hz, ²J_NH = −1.1 Hz, ³J_HH = 21.0 Hz, ¹J_NN = 12.0 Hz, linewidth = 3.5 Hz.

In solution, as in the solid-state, 2 exists as a diiron species; the ¹⁵N NMR spectrum of ¹⁵N-2 (THF-d₈, −75 °C) shows a single resonance at 328.6 ppm, which is split into a doublet by the trans phosphine phosphorous atom (²J_PN ≈ 15 Hz). Despite formation of a diiron species, the coordinated N₂ ligand in 2 is labile, and warming solutions of 2 to room temperature regenerates 1.

Treatment of 2 with 0.5 equiv N₂H₄ at −78 °C generates the purple diiron species {[PhBP^CH2Cy₃]Fe(OAc)}₂(μ-η¹:η¹-N₂H₄) (3) (Scheme 1). The presence of a single ¹⁵N NMR chemical shift for ¹⁵N-3 (δ = 103 ppm), coupled with a single NH₂ resonance in the ¹H NMR spectrum (δ = 2.51 ppm) indicates a bridging hydrazine ligand. As for 2, the hydrazine ligand is labile; at −30 °C resonances ascribed to both 1 and 3 are observed by ¹H NMR spectroscopy.

The hydrazine species 3 is not thermally stable, and at −10 °C undergoes a disproportionation reaction to precipitate the dark blue diazene species {[PhBP^CH2Cy₃]Fe(OAc)}₂(trans-μ-η¹:η¹-N₂H₂) (4) from solution, leaving the ammonia complex [PhBP^CH2Cy₃]Fe(OAc)(NH₃) (5) (Scheme 1) in the supernatant.

The presence of a bridging trans diazene ligand in 4 is readily discerned by NMR spectroscopy. A resonance centered at 434 ppm is observed in the ¹⁵N NMR spectrum of ¹⁵N-4, indicative of a moderately activated diazene ligand.^7e,f,12 In the corresponding ¹H{³¹P} NMR spectrum, an AA’XX’ multiplet centered at 17.72 ppm is observed, consistent with the presence of a bridging diazene ligand (Figure 1d, see caption for fitting parameters). The relatively large ³J_HH of 21.0 Hz suggests a trans ligation, and the magnitude of the ³J_HH coupling in π-conjugated systems can further be used to infer bond distances;¹³ a linear relationship exists between ³J_HH and the N-N bond distance for trans ligated diazene complexes (see SI). The observed ³J_HH of 21.0 Hz suggests an N-N bond distance of ca. 1.31 Å in 4, between that expected for an N-N single and a double bond.

The solid-state structure of 4 was obtained, and the core atoms of the structure are shown in Figure 1b. The quality of the dataset is compromised by a total molecule disorder, in which 18 % of the molecules are translocated along the b-axis of the P2(1)/c unit cell. Though the diazene protons could not be located in the difference map, the structure confirms the trans diazene ligation. The average Fe-N-N angle of 128.5° is consistent with sp²-hybridized nitrogen atoms, and the metrical parameters about the Fe-NH-NH-Fe core are similar (within error) to those of other 6-coordinate diiron(II) bridging diazene complexes.^7a,7eFurther, the N-N distance of 1.31(1) Å is in good agreement with that predicted from the ³J_HH coupling constant.

Comparison of the structures for μ-N₂ 2 and μ-N₂H₂ 4 indicates little reorganization of the auxiliary ligands of the iron centers and a net 2e⁻/2H⁺ difference in the nitrogenous ligand. In both structures, the acetate ligands reside on opposite faces of the dinuclear framework, and the Fe-O and Fe-P distances (that are cis to the N₂H_y ligand) are nearly equivalent. Only the Fe-P distance to the phosphine that is trans to the N₂H_y ligand significantly changes, with a ca. 0.02 Å elongation upon going from 2 to 4, consistent with a stronger trans influence exerted by diazene than dinitrogen.

Worth noting is that the trans diazene in 4 is prone to dissociation. Heating a toluene solution of 4 to 60 °C for 2 h results in complete transformation to acetate 1 and ammonia complex 5, the ammonia in the latter species presumably generated from the dis-proportionation of free diazene. In contrast, the cis diazene in the related complex, {[PhBP^CH2Cy₃]Fe}₂(μ-η¹:η¹-N₂H₂)(μ-NH₂)₂ is stable in solution for days at 60 °C.^7e As the cis isomer of free diazene is less stable than the trans isomer,¹⁴ the robustness observed in the latter species is likely due to the presence of additional bridging ligands that strengthen the fidelity of the bimetallic unit.

The disproportionation reaction of μ-N₂H₄ 3 also generates the ammonia species 5, whose solid-state structure is shown in Figure 1c. This species can alternatively be prepared by the addition of excess NH₃ to a THF solution of 1 (Scheme 1). Though 5 is stable in solution at room temperature, heating solutions of 5 under vacuum results in NH₃ loss and formation of 1.

Hydrazine disproportionation reactions to generate diazene and ammonia have been observed at diruthenium^9b,15 and diiron^7d,e centers, and can be regarded as the first step in the disproportionation of hydrazine to dinitrogen and ammonia:

To determine whether μ-N₂H₂ 4 further reacts with hydrazine to generate N₂ and NH₃, 0.95 equiv of ¹⁵N₂H₄ was added to a THF solution of 4, and both the reaction volatiles and residual solids were analyzed by NMR spectroscopy. ¹H NMR analysis of the volatiles established the formation of ¹⁵NH₃ (ca. 50 % yield) and no ¹⁴NH₃. ¹H and ³¹P NMR spectroscopy of the residual solids indicates that acetate 1 and ammonia 5 were the major iron containing products, with ¹⁴N-4 present as a minor species. These results are consistent with a disproportionation mechanism in which the bound diazene is oxidized to N₂ by free N₂H₄, which itself is reduced to NH₃.

Though 1 can serve as a hydrazine disproportionation catalyst (in the presence of 10 equiv of N₂H₄, a 16 % yield of NH₃ is obtained), the reaction is hampered by ligand degradation; in addition to 1 and 5, (CH₂Cy)₂PMe and other, unidentifiable products are present in the resulting ¹H and ³¹P NMR spectra.

The ability to isolate iron complexes that only differ in the extent of reduction of the nitrogenous ligand suggests the possibility that these complexes may be interconverted via redox reactions. Though the cyclic voltammogram of μ-N₂ 2 (obtained at −35 °C) shows a quasi-reversible reduction at ca. −2.2 V, the chemical reduction of 2 with [Na][C₁₀H₈] results in a mixture of unidentifiable products. Attempts to chemically reduce μ-N₂ 2 to μ-N₂H₄ 4 at −78 °C with well-defined H-atom transfer agents (i.e. catechol, hydroquinone, Bu₃SnH, cyclohexadiene and PhSH) or with combinations of reductants (i.e. Cp*₂Fe, Cp*₂Co, Cp₂Co, [Na][C₁₀H₈]) and acids (i.e. HOAc, HOTf, [lutH][BPh₄]), did not yield the desired transformation, and in most instances, 1 was the only identifiable iron containing species present (ligand degradation also occurred). In contrast, treatment of 4 with oxidants (e.g. p-benzoquinone, Pb(OAc)₄) results in formation of 1, which may proceed through μ-N₂ 2. These results are perhaps not unexpected when one considers the respective gas-phase BDE of NN-H^● (ca. 0 kcal/mol) and HNN-H^● (60.8 kcal/mol).¹⁶ As the N₂ is weakly activated in 2, addition of a net H-atom to 2 would likely generate a high energy and Fe₂(μ-NNH^●) intermediate that would decay back to 2. To circumvent such an intermediate, the direct reduction of 2 to 4 may call for a concerted 2e^—/2H⁺ transfer.

In summary, a series of mono- and diiron(II) complexes that coordinate nitrogen, diazene, hydrazine, and ammonia in an available sixth coordination site have been prepared and characterized. These complexes are structurally related to one another by the ancillary ligands and differ only by the coordinated N_xH_y ligand, and hence present an attractive synthetic system for studying aspects of an alternating N₂ reduction scheme. The finding that μ-N₂H₄ 3 and μ-N₂H₂ 4 react with free hydrazine, reducing the latter to NH₃ as the former is oxidized, suggests that a similar reactivity pattern merits consideration in the reduction of diazene by nitrogenase.