N–H Bond Formation at a Diiron Bridging Nitride

Abstract

Despite their connection to ammonia synthesis, little is known about the ability of iron-bound, bridging nitrides to form N–H bonds. Herein we report a linear diiron bridging nitride complex supported by a redox-active macrocycle. The unique ability of the ligand scaffold to adapt to the geometric preference of the bridging species was found to facilitate the formation of N–H bonds via proton-coupled electron transfer to generate a μ-amide product. The structurally analogous μ-silyl- and μ-borylamide complexes were shown to form from the net insertion of the nitride into the E–H bonds (E = B, Si). Protonation of the parent bridging amide produced ammonia in high yield, and treatment of the nitride with PhSH was found to liberate NH₃ in high yield through a reaction that engages the redox-activity of the ligand during PCET.

Ammonia (NH₃) is a promising energy carrier for fuel cell, hydrogen storage, and alternative fuel applications. It is produced industrially by the Haber-Bosch process, which uses a catalyst that contains metallic iron and various main group promoters. Chemisorption of N₂ at the catalyst surface is understood to form bridging nitrogen adatoms on the metallic surface (typically referred to as surface nitrides),^[1] which then combine with surface hydrogen adatoms (surface hydrides) through successive N–H coupling events to generate ammonia.^[2] Given the high covalency that has been proposed in surface and bulk iron nitride systems,^[3] iron-bound, electrophilic μ-N species are of considerable interest.

Most examples of diiron bridging nitrides exhibit both high formal oxidation states (Fe^≥4+) and saturated coordination spheres in the vicinity of the nitride.^[4,5] And even though nitrides typically exhibit electrophilic reactivity when bound to oxidizing metal centers,^[6] no examples have been provided in the literature of these species undergoing μ-N-based chemistry. Instead, the diiron bridging nitrides that undergo N–H bond formation make use of lower-valent iron centers (Fe^≤3+); two such systems are known. Brown and Peters reported the ability of Fe²⁺₂ and Fe²⁺Fe³⁺ bridging nitrides supported by tris(phosphino)borate ligands (PhBP₃) to cleave H₂, thereby yielding products that contain both bridging imide (μ-NH) and bridging hydride (μ-H) moieties (Scheme 1, top).^[7] More recently, Holland and co-workers reported that an N₂-derived μ-N between two Fe³⁺ ions acts as a strong base (Scheme 1, middle);^[8] related μ-imide products, formed via intermolecular protonation, generate NH₃ following net hydrogen atom transfer and protonolysis.^[9]

Scheme 1.

Structures and reactivity profiles for isolated diiron and dicobalt bridging nitride complexes that show reactivity at μ-N.

We recently described the putative formation of a pair of transient bridging nitrides between two cobalt ions.^[10] The isolation of products that result from intramolecular hydrogen atom abstraction, P=N bond formation, and N-atom insertion into an aliphatic C–H bond provided compelling evidence for the formation of highly electrophilic Co₂(μ-N) cores. An example of this moiety was recently isolated by Fortier and co-workers and shown to undergo intermolecular activation of H₂ (Scheme 1, bottom).^[11] The results presented below describe an isolable diiron bridging nitride that is isoelectronic to our previously reported Co₂(μ-N) species. Both the formation and the reactivity of the diiron nitride appear to rely on the unique ability of the supporting ligand to accommodate geometric changes of the bridging nitrogen.^[12] This species undergoes intermolecular reactivity toward formation of N–H bonds via proton-coupled electron transfer (PCET), E–H bond insertion chemistry (E = Si, B), and protonation.

Treatment of the diamagnetic, bridging chloride complex [(³PDI₂)Fe₂(μ-Cl)(PMe₃)₂][OTf] ([Fe₂Cl]⁺) with NaN₃ in THF resulted in a color change from brown to purple at room temperature, corresponding to the formation of the diiron bridging nitride [(³PDI₂)Fe₂(μ-N)(PMe₃)₂][OTf] ([Fe₂N]⁺, Scheme 2). A crystal structure of the product revealed conversion from the folded ligand architecture^[13] of [Fe₂Cl]⁺ to an unfolded ligand that supports a linear coordination environment about the nitride (∠ Fe–N–Fe= 177.1(2)°, Figure 1). The Fe–N_μ distances of 1.677(3) and 1.673(3) Å are indicative of multiple bond character,^{[7b, 14]} and the Fe–N_PDI distances (avg. Fe–N_im = 1.93 Å; avg. Fe–N_py = 1.86 Å) are consistent with other low- or intermediate-spin iron centers bound by PDI scaffolds.^[15] The constraints of the Fe₂N core induce a modest twist in the ³PDI₂ macrocycle (∠py-py = 23.1°), which causes the two iron centers to be crystallographically distinct from one another (see Supporting Information). In order to evaluate the amount of electron density on the ligand, we make use of the empirically derived Δ parameter. Δ is defined as the difference between the average C_im–C_py distance and the average of the C_im–N_im and C_py–N_py distances on a pyridinediimine ligand,^[16] and this parameter was recently adapted for use with ³PDI₂.^[13] Values near 0.105 Å represent (³PDI₂)²⁻, and Δ decreases as the ligand is reduced. The value of 0.073 Å for [Fe₂N]⁺ indicates the presence of three electron’s worth of electron density on the ligand, as (³PDI₂)⁴⁻ is not reached until Δ ≅ 0.055 Å.

Scheme 2.

Synthesis of a diiron bridging nitride complex and its N–H bond formation chemistry.

Figure 1.

Depictions of cationic portions of the molecular structures of [Fe₂N]⁺ (top left), [Fe₂NH₂]⁺ (top right), [Fe₂NHSi]⁺ (bottom left) and [Fe₂NHB]⁺ (bottom right); all counterions and hydrogen atoms were removed for clarity, except for the hydrogens on N and Si.

The [Fe₂N]⁺ product is diamagnetic in solution. Its ¹H NMR spectrum reveals overall C_2h symmetry, and one singlet was observed in its ³¹P{¹H} NMR spectrum at 4.11 ppm. Together, these data indicate that the phosphine ligands remain bound on the NMR timescale but that the inequivalence of the two iron centers observed crystallographically is not observable in solution. The use of ¹⁵N-labelled NaN₃ (each N₃⁻ contains one terminal ¹⁵N atom) allows for isotopic enrichment at μ-N ([Fe₂N*]⁺; ca. 50% ¹⁵N). A signal at 815 ppm was observed in the ¹⁵N NMR spectrum of this material, comparable to that reported by Peters for {[(PhBP₃)Fe]₂(μ-N)}^– (¹⁵N NMR δ = 801 ppm).^[7b] These values are considerably upfield of those reported for terminal Fe nitrides (950–1150 ppm), all of which formally contain Fe⁴⁺ ions,^[17] but the downfield chemical shifts of the nitrides likely result from temperature-independent paramagnetism, thereby complicating the assignment of trends in these data to changes in the physical oxidation state of the nitride.^[7b] The IR spectrum of [Fe₂N]⁺ reveals an Fe–N–Fe stretching mode at 991 cm⁻¹ (965 cm⁻¹ for [Fe₂N*]⁺). These values are comparable to the handful of values that have been reported in the literature for terminal and bridging nitrides on late transition metals (between 910 to 1000 cm⁻¹).^[5b,18]

Initial investigations into the reactivity of [Fe₂N]⁺ have revealed diverse reaction chemistry toward formation of N–H bonds. Monitoring the treatment of [Fe₂N]⁺ with 4.0 equiv of Ph₂PH in PhF at 60 °C revealed clean conversion (>85 %) to a diamagnetic bridging amide species, [(³PDI₂)Fe₂(μ-NH₂)(PMe₃)₂][OTf] ([Fe₂NH₂]⁺, Scheme 2), along with the formation of 1.0 equiv of Ph₂P–PPh₂ over 4 h. This is consistent with the PCET of two equivalents of hydrogen atoms to μ-N. The resulting μ-NH₂ protons were observed at 6.25 ppm by ¹H NMR spectroscopy in C₆D₆ (5.37 ppm in CD₃CN), a signal that significantly decreased in intensity with the use of Ph₂PD (95 % D) as a reagent. A ²H NMR spectrum of the deuterated product confirmed deuteration at the bridging amide and gave no indication of deuteration at other positions within the molecule. Only a single N–H stretching feature was observed in the IR spectrum of [Fe₂NH₂]⁺.^[19] This signal at 3334 cm⁻¹ shifted to 2448 cm⁻¹ for [Fe₂ND₂]⁺, and the incomplete deuteration of the sample ([Fe₂NHD]⁺) fortuitously allowed for the observation of signals that result from DN–H and HN–D stretches at 3311 and 2447 cm⁻¹, respectively (see Supporting Information). The identity of this product was further supported by independent synthesis via treatment of [Fe₂Cl]⁺ with 1.5 equiv of NaNH₂ to give a product with identical analytical data in 69 % yield.

A crystal structure of [Fe₂NH₂]⁺ revealed that the ligand had refolded to form a geometry similar to that of [Fe₂Cl]⁺ (Figure 1).^[13] The ligand isomerization between [Fe₂Cl]⁺, [Fe₂N]⁺, and [Fe₂NH₂]⁺ supports the view that the ³PDI₂ ligand is able to adapt to the stereoelectronic requirements of the cluster core, from an unfolded geometry suitable for supporting an sp-hybridized μ-nitride to a folded ligand geometry that accommodates sp³-hybridized fragments like a μ-amide. Beyond this geometric change, we were interested to find that the the ³PDI₂ ligand retains three electron’s worth of electron density (Δ = 0.087 Å, see Supporting Information),^[20] and the Fe–N_PDI (avg. Fe–N_im = 1.93 Å; avg. Fe–N_py = 1.82 Å) and Fe–P (avg. 2.26 Å) bond lengths are virtually identical to those for [Fe₂N]⁺ (see above; avg. Fe–P = 2.25 Å).

Other main group hydrides – PhSiH₃ and HBpin (pin = pinacolate) – were also found to react with the μ-nitride of [Fe₂N]⁺. Both formed amide products, but in these cases, the amides resulted from insertion of the bridging nitride into the E-H bond (E = Si, B; Scheme 2).^[21] Treatment of [Fe₂N]⁺ with 1.0 eq. of either reagent in fluorobenzene at 80 °C cleanly generated the diiron silylamide [(³PDI₂)Fe₂(μ-N(H)SiH₂Ph)(PMe₃)₂][OTf] ([Fe₂NHSi]⁺) and borylamide [(³PDI₂)Fe₂(μ-NHBpin)(PMe₃)₂][OTf] ([Fe₂NHB]⁺) complexes, respectively. The latter reaction is of particular interest given the high B–H BDE of ca. 110 kcal/mol.^[22] As with the parent amide [Fe₂NH₂]⁺, the crystal structures of [Fe₂NHSi]⁺ and [Fe₂NHB]⁺ revealed folded ligand geometries (Figure 1), with apparent Fe–Fe bonding interactions (d_Fe–Fe = ca. 2.58 Å, see Supporting Information) and sp³-hybridized μ-NHE residues.^[13] The silyl- and borylamide products were found to be diamagnetic in solution, as observed for both [Fe₂NH₂]⁺ and the isoelectronic, folded-ligand complex [Fe₂Cl]⁺. The μ-NHR complexes were also found to exhibit C_s symmetry in solution, indicating that the folded ligand geometries observed in the solid state persist in solution. For [Fe₂NHSi]⁺, the SiH₂ and μ-NH protons were observed at 6.46 ppm (2H) and 3.23 ppm (1H) by ¹H NMR spectroscopy, and for [Fe₂NHB]⁺, the μ-NH proton was located at 3.93 ppm (1H). A very weak N–H stretch was observed in the IR spectrum of [Fe₂NHSi]⁺ at 3274 cm⁻¹, while [Fe₂NHB]⁺ provided a more intense V_N–H at 3328 cm⁻¹. The Si–H stretching frequencies in [Fe₂NHSi]⁺ were found at 2091 and 2125 cm⁻¹, for the symmetric and asymmetric SiH₂ vibrational modes.

Zero-field Mössbauer spectroscopic data were collected on [Fe₂NH₂]⁺, [Fe₂Cl]⁺, and [Fe₂N]⁺ at room temperature (Figure 2). All three spectra are best modelled by fitting the data with two doublets of equal intensity. This is consistent with the crystallographic data that showed the two iron centers within a molecule to be in comparable, albeit distinct, chemical environments. The data for [Fe₂NH₂]⁺ revealed doublets at δ = +0.10±0.03 and +0.22±0.03 mm/s, and those for [Fe₂Cl]⁺ were found at +0.17±0.03 and +0.32±0.03 mm/s (see Supporting Information). Mononuclear (PDI)Fe species with intermediate-spin Fe²⁺ ions typically show δ > +0.30 mm/s when bound by ligands that undergo negligible π-backbonding,^[23] suggesting that [Fe₂NH₂]⁺ and [Fe₂Cl]⁺ may contain less electron density at the metal centers compared to an Fe²⁺ ion. This would be consistent with the electronic structure of [Fe₂Cl]⁺ that was described previously as containing an (Fe₂)⁵⁺ core with intermediate-spin Fe centers.^[12] The Mössbauer data for [Fe₂N]⁺ yielded doublets at δ = −0.15±0.03, −0.04±0.03 mm/s. The more negative isomer shifts for [Fe₂N]⁺ compared to [Fe₂NH₂]⁺ could result from several factors, including an increase in oxidation state or a decrease in spin configuration. A change from intermediate- to low-spin configurations is known to decrease the value of δ for a single oxidation state by ca. 0.2–0.4 mm/s,^[23] which is worth noting given the similarity in both the Δ values and the Fe–N_PDI and Fe–P bond distances between the two structures.

Figure 2.

Mössbauer spectroscopic data for [Fe₂NH₂]⁺, [Fe₂Cl]⁺, and [Fe₂N]⁺; experimental (●) and theoretical (solid lines).

Finally, it was found that treatment of [Fe₂N]⁺ with 3.0 equiv of PhSH results in complete removal of the bridging nitrogen, forming NH₃ in 71% isolated yield (Scheme 3). Limited examples are known of molecular transition-metal nitrides forming free NH₃ following PCET reactions.^{[17, 24]} The transition metal product in the present case is the previously described tris(thiophenolate) [(³PDI₂)Fe₂(μ-SPh)(SPh)₂][OTf] ([Fe₂(SPh)₃]⁺),^[12] which forms in 56% yield following crystallization. The oxidation of the (³PDI₂)³⁻ ligand in [Fe₂N]⁺ to (³PDI₂)⁰ in [Fe₂(SPh)₃]⁺ during this reaction indicates that intramolecular electron transfer occurs in addition to the formation of N–H bonds. Like Ph₂PH (BDE_P–H = ~61 kcal/mol; see Supporting Information), PhSH (BDE_S–H = 79 kcal/mol) is known to act as a hydrogen atom donor,^[25] but treatment of [Fe₂N]⁺ with non-Lewis basic H-atom donors like 1,4-cyclohexadiene or 9,10-dihydroanthracene (~76–80 kcal/mol) did not form [Fe₂NH₂]⁺.^[26] This suggests either that coordination by the phosphine/thiol may precede PCET or that the PCET process may exhibit significant proton transfer character at the outset of the reaction.^[27] Once the bridging amide is formed, a third equivalent of PhSH (pK_a = 8.9 in MeCN) is able to effect the formation of NH₃ via protonolysis.^[28] This interpretation is supported by protonolysis of the phosphine-bound [Fe₂NH₂]⁺ with one equivalent of PhSH, which generates [(³PDI₂)Fe₂(μ-SPh)(PMe₃)₂][OTf] ([Fe₂(SPh)]⁺; Scheme 3) in 62% isolated yield and NH₃ in 89% isolated yield.

Scheme 3.

Formation of ammonia via N-H bond formation at diiron bridging nitride or amide complexes.

In conclusion, we have shown that the geometric flexibility of the ³PDI₂ ligand supports the formation of a linear diiron bridging nitride. N–H bond formation via proton-coupled electron transfer and E–H bond insertion chemistry were both consistent with considerable electrophilic character of the bridging nitride. The formation of free NH₃ was accomplished via treatment with PhSH, suggesting that both PCET and protonolysis contribute to the removal of the bridging nitrogen from the cluster core. Work is underway to determine the mechanism by which the μ-N in [Fe₂N]⁺ performs PCET and E–H bond insertion chemistry, as we investigate the manner in which the unique electronic properties of the ³PDI₂ ligand impart electrophilic character to [Fe₂N]⁺.