Catalytic dinitrogen reduction to hydrazine and ammonia using Cr(N₂)₂(diphosphine)₂ complexes

Abstract

The synthesis, characterization of trans-[Cr(N₂)₂(depe)₂] (1) is described. 1 and trans-[Cr(N₂)₂(dmpe)₂] (2) catalyze the reduction of N₂ to N₂H₄ and NH₃ in THF using SmI₂ and H₂O or ethylene glycol as proton sources. 2 produces the highest total fixed N for a molecular Cr catalyst to date.

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Motivated by the desire to understand and control the challenging multi-proton, multi-electron reaction of N₂ reduction to NH₃, researchers have intensely studied the reactivity of molecular transition metal dinitrogen complexes.¹ Well-defined molecular systems offer a high degree of electronic and structural control to regulate chemical reactivity of N₂.² When combined with effective strategies to form N–H bonds, such as proton-coupled electron transfer (PCET) reagents,³i.e. SmI₂ and a proton source, tens-of-thousands of equivalents of NH₃ can be generated.⁴ The valuable information obtained from these studies includes the identification of viable M–N_xH_y reaction intermediates from spectroscopic data that can be used to delineate the mechanistic steps of a putative catalytic cycle. Such studies can aid in the understanding of the mechanistically complex biological N₂ fixation processes carried out by nitrogenase enzymes,⁵ as well as heterogeneous Haber–Bosch catalysts.⁶

Group 6 N₂ complexes bearing monodentate phosphine ligands, especially with Mo and W, were among the first molecular systems to generate stoichiometric quantities of N₂-derived NH₃ from protonolysis reactions with strong acids nearly 50 years ago.⁷ Recently, a renaissance of examining structurally similar [M(N₂)₂(P–P)₂], (M = Mo, W; P–P = diphosphine) systems has begun, elevating these simple complexes as catalysts for N₂ reduction to NH₃, or other remarkable reactions such as cleavage of the N₂ triple bond.⁸ Masuda and co-workers reported spontaneous N≡N bond cleavage upon one-electron oxidation of trans-[Mo(N₂)₂(depe)₂] (depe = Et₂PCH₂CH₂PEt₂) to form [Mo(N)(depe)₂]⁺.⁹ Chirik and co-workers developed a photocatalytic strategy to form NH₃ from [Mo(N)(depe)₂]⁺ and H₂.¹⁰ Electrocatalytic N₂ fixation with Mo and W-phosphine complexes was described by Peters and co-workers using a tandem catalysis approach.¹¹ Nishibayashi and co-workers showed simple Mo-phosphine complexes catalyzed N₂ reduction to NH₃ using SmI₂ and various proton sources.¹²

While these examples highlight new discoveries using [M(N₂)₂(P–P)₂] (M = Mo, W) complexes, catalytic N₂ reduction with analogous Cr compounds are limited. Recent reports highlighted the utility of molecular Cr complexes using a variety of ligand architectures for N₂ activation,^8a,13 functionalization,¹⁴ or catalytic N₂ silylation.¹⁵ However, molecular Cr complexes that catalyze the direct reduction of N₂ to NH₃ are rare. In 2022, Nishibayashi and co-workers reported a Cr complex bearing a PCP pincer ligand that catalyzed direct N₂ reduction to NH₃ and N₂H₄ at −78 °C to rt. KC₈ and phosphonium salts as H⁺ sources were required for turnover, and this system was not catalytic using SmI₂.¹⁶ Herein we prepared and characterized trans-[Cr(N₂)₂(depe)₂] (1), and report catalytic N₂ reduction to NH₃ and N₂H₄ with 1 and trans-[Cr(N₂)₂(dmpe)₂]¹⁷ (2) (dmpe = Me₂PCH₂CH₂PMe₂) at room temperature using SmI₂ and ethylene glycol or H₂O as proton sources.

Vigorous stirring of yellow trans-[CrCl₂(depe)₂]¹⁸ (1-Cl) in THF with excess Mg powder under a N₂ atmosphere for 24 h furnished 1 as a dark red solid in 70% yield. Isolation of 1 allowed for a comparison of the structural and spectroscopic data with 2 that was reported in 1983.^17a The structure of 1, determined by single crystal X-ray diffraction, shows Cr with four phosphorus atoms of the chelates on the equatorial plane and two axial end-on bound N₂ ligands, Fig. 1, panel a. The average Cr–N, Cr–P, and N≡N bond distances are 1.904 ± 0.005 Å, 2.334 ± 0.007 Å, and 1.104 ± 0.004 Å, respectively. The corresponding Cr–N, and Cr–P, bond distances in 2 (see ESI†), are slightly shorter at 1.8862(17) Å, and 2.294 ± 0.005 Å, and the N≡N distance is 1.110(2) Å.¹⁹ The ligand bite angles for 1 and 2, i.e. P1–Cr–P2, are 81.6° and 83.5°, respectively, and the P–Cr–N angles are near 90°.

Fig. 1 (a) Synthesis and molecular structure of 1. Thermal ellipsoids are drawn at 50% probability. Hydrogen atoms are omitted for clarity. Crystals of 1 contain two molecules per asymmetric unit with comparable metric parameters; only one molecule is shown. Selected bond distances (Å) and angles (°): Cr1–N1 = 1.9081(10); N1–N2 = 1.1003(14); Cr–P1 = 2.3343(3); Cr–P2 = 2.3249(3). Cr2–N3 = 1.9008(10); N3–N4 = 1.1069(14); Cr–P3 = 2.3425(3); Cr–P4 = 2.3346(3). P1–Cr1–P2 = 81.650(9); P3–Cr2–P4 = 81.583(10); P1–Cr1–N1 = 89.25(3); P2–Cr1–N1 = 90.21(3); P3–Cr2–N3 = 89.29(3); P4–Cr2–N3 = 90.59(3). (b) ¹⁵N{¹H} NMR spectra of 1^15N (red) and 2^15N (black) recorded at 25 °C in THF-d₈. (c) Cyclic voltammograms of 1 and 2 in THF showing the Cr^I/0 wave.

The ³¹P{¹H} NMR spectrum of 1 in THF-d₈, displays a singlet at 79.9 ppm (68.8 ppm for 2) consistent with four magnetically equivalent P atoms. Complexes 1 and 2 were characterized by ¹⁵N NMR spectroscopy to augment the cumulative library of tabulated ¹⁵N NMR data of phosphine-supported group 6 N₂ complexes.^13h The ¹⁵N₂-labelled complexes 1^15N and 2^15N, were prepared by mixing the respective Cr–N₂ complexes in THF-d₈ under 1 atm ¹⁵N₂. The ¹⁵N NMR spectra were collected after mixing for 24 h. The ¹⁵N{¹H} NMR spectra contain two resonances; a doublet (J_NN = 7.0 Hz) and a multiplet (∼2.5 Hz ³¹P coupling) (1^15N: −31.1 ppm, −24.2 ppm, and 2^15N: −37.6 ppm, −26.4 ppm), assigned as the distal (N_d) and proximal (N_p) nitrogen atoms, respectively, (Fig. 1, panel b).¹³ⁱ

Cyclic voltammetry (CV) experiments established the redox behaviour of the Cr(0)-N₂ complexes. Voltammograms were recorded using a glassy carbon working electrode at 0.1 V s⁻¹ in THF. The voltammogram for each complex displays a reversible, one-electron Cr^I/0 wave with the half-wave potential (E_1/2) of −1.49 V and −1.34 V (vs. Cp₂Fe^+/0) for 1 and 2, respectively (Fig. 1, panel c). The electrochemically reversible Cr^I/0 couples indicate N₂ dissociation does not occur upon oxidation to Cr(I) during the CV experiments. The reversibility of the waves for 1 and 2 contrasts other cis- or trans-[Cr(N₂)₂(P₄)] complexes measured by CV that exhibit quasi-reversible or irreversible Cr^I/0 waves due to rapid N₂ loss upon oxidation.^13b,c,i In the current study, an irreversible anodic wave was assigned to the Cr^II/I redox feature at E_pa = −0.48 V and E_pa = −0.63 V, for 1 and 2, respectively, due to N₂ dissociation at more positive potentials, (Fig. S17 and S18 ESI†). The CV results suggest a one-electron chemical oxidation to form trans-[Cr(N₂)₂(P-P)₂]⁺ should be possible; however, our attempts to isolate such a species have been unsuccessful. Owing to the more electron-rich metal centre of 1, the ν_NN band in the infrared spectrum at 1906 cm⁻¹ (THF) appears at lower energy than the ν_NN band for 2 at 1917 cm⁻¹ (THF).

Complexes 1 and 2 were examined as catalysts for the direct reduction of N₂ to NH₃ and N₂H₄. The catalysis studies were performed in THF at room temperature using the PCET reagent SmI₂ and ethylene glycol and/or water as proton donors. A typical catalytic run used 583 equiv. SmI₂, 1166 equiv. ROH per Cr centre and was stirred for 48 h. Quantification of NH₃, N₂H₄ and H₂ (see ESI for details†) products assessed the total fixed N generated in each reaction. Selected catalytic data are listed in Table 1 (see ESI for all tabulated results†).

Table 1 Selected Cr-catalyzed N₂ reduction experiments

Entry	Cr cat.	ROH	NH3 equiv./Cr^a	N2H4 equiv./Cr^b	Total fixed N	Time (h)
Experiments performed using 0.6 μmol catalyst in 15.0 mL THF at 25 °C under 1 atm N2, with 583 equiv. of SmI2, and with 1166 equiv. ROH unless otherwise specified.a Determined by acidification and NH4^+ quantification using ^1H NMR spectroscopy (see ESI†).b Determined by colormetric p-dimethylaminobenzaldehyde method (see ESI†).c 1000 equiv. H2O/Cr.d 10 000 equiv. H2O/Cr.e 25 ppm of H2O.f 250 ppm of H2O.g 583 equiv. (CH2OH)2, 583 equiv. H2O.h Average of two or more trials. H2 quantification by gas chromatography, values are tabulated in ESI.†
1	None	(CH2OH)2	0	0	0	48
2	1	(CH2OH)2	3.7 ± 0.9	1.4 ± 0.8	4.9^h ± 1.5	48
3	1	(CH2OH)2	4.6 ± 0.6	4.0 ± 1.7	8.6^h ± 2.1	100
4^c	1	H2O	1.4	0.7	2.1	48
5^d	1	H2O	3.2	0.6	3.8	28
6	1-Cl	(CH2OH)2	1.2	0.9	2.1	48
7	2	(CH2OH)2	14.6 ± 1.6	5.9 ± 2.9	20.5^h ± 3.8	48
8^e	2	(CH2OH)2	6.2 ± 0.5	6.4 ± 0.8	12.6^h ± 0.3	48
9^f	2	(CH2OH)2	4.4 ± 0.9	6.6 ± 0.6	11^h ± 0.4	48
10^g	2	(CH2OH)2	1.1	5.7	6.8	48
11^d	2	H2O	5.1	5.9	11	3
12	2-Cl	(CH2OH)2	13.5 ± 2.8	5.9 ± 0.6	19.4^h ± 3.4	48

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Analysis of the catalysis results provides insights about the performance of 1 and 2 under identical reaction conditions. 2 afforded more total fixed N than 1 in all catalytic trials. For example, 1 generated up to 5 equiv. of NH₃ and 5 equiv. N₂H₄ per Cr center using ethylene glycol as the proton donor after >100 h. Under identical conditions, 2 produced up to 16 equiv. NH₃ and 10 equiv. N₂H₄ in 48 h. Furthermore, ethylene glycol worked more effectively as the proton donor affording higher total fixed N than using H₂O. The deliterious effect of H₂O on catalysis was noted in reactions with 2 using ethylene glycol as the primary proton source. As the amount of H₂O added to the reaction increased, NH₃ production declined, while the N₂H₄ formed stayed relatively constant. We postulate the Cr complexes may simply be more prone to degradation in the presence of H₂O. Separately, 2 was treated with 500 equiv. H₂O or ethylene glycol in THF-d₈. Free dmpe from complex degradation appeared more rapidly using H₂O, as assessed by ³¹P NMR spectroscopy. Catalysis performed with 2 under an atmosphere of ¹⁵N₂ afforded ¹⁵NH₄⁺ as a doublet at 7.1 ppm (J_15N-1H = 71 Hz) in the ¹H NMR spectrum, identifying ¹⁵N₂ as the source of ¹⁵NH₃.

Catalytic trials using trans-[CrCl₂(dmpe)₂] (2-Cl) and ethylene glycol generated comparable amounts of NH₃ and N₂H₄ as using 2 as the precatalyst. 1-Cl did not catalyze N₂ reduction, affording only 1 equiv. of NH₃ and N₂H₄ per Cr center. SmI₂ and ethylene glycol may be ineffective at reducing the Cr(II) center of 1-Cl to Cr(0) where N₂ is strongly activated. Treatment of 2-Cl with 2 equiv. SmI₂ and 2 equiv. ethylene glycol rapidly generated 2 (see ESI†). However, the same reaction of 1-Cl and SmI₂ with ethylene glycol additive did not form 1 (E_1/2 = −1.49 V, vide supra). 1 or 2 could not be generated from 1-Cl or 2-Cl using excess SmI₂(THF) alone (E° of SmI₂(THF) = −1.41 ± 0.08 V²⁰vs. Fc/Fc⁺). A Cr(I) species could be accessible, but N₂ activation and subsequent functionalization steps may be moderated at Cr(I), limiting catalysis.

The mixed N₂ reduction selectivity to form NH₃ and N₂H₄ provides preliminary evidence for a catalytic cycle that follows, at least in part, an alternating N₂ reduction mechanism, Fig. 2, bottom. A purely distal N₂ reduction pathway, Fig. 2, top, would be selective for NH₃ formation. In a 1986 report, the reaction of 2 with CF₃SO₃H was postulated to form a Cr-hydrazido product, [Cr(NNH₂)(dmpe)₂][CF₃SO₃]₂.²¹ A recent study by Wei, Yi, Xi, and co-workers examining early stage N₂ functionalization of [Cp*Cr⁰(depe)(N₂)]⁻ (Cp* = η⁵-C₅(CH₃)₅) using a variety of electrophiles (H⁺, Me₃Si⁺, Me⁺) also revealed the selective formation of Cr-hydrazido products, consistent with a distal pathway. Contrary to these reaction patterns, protonation studies of related cis- or trans-[Cr(N₂)₂(P₄)] complexes we examined using strong acids or H⁺/e⁻ reagents, as well as the catalytic Cr[PCP] system¹⁶ generated NH₃ and N₂H₄.^13c,i,15a Considering all these examples, and that N₂ reduction mechanisms are sensitive to reaction conditions, (i.e. identity of the H⁺ and e⁻ reagents, solvent, temperature), a hybrid N₂ reduction pathway²² where the third and fourth N–H bonds are formed at the proximal N atom of a Cr-hydrazido intermediate, Fig. 2, middle, cannot be excluded for the current systems. Further studies are warranted to understand the N₂ reduction pathways with Cr.

Fig. 2 Plausible N₂ reduction mechanisms for Cr mediated formation of hydrazine and ammonia.

The proclivity for N₂ ligand substitution in 1 and 2 was evaluated as a metric that could reflect catalyst stability and influence catalytic performance. We examined reactions of 1 and 2 with CO to assess the rate of ligand exchange, Fig. 3. Ligand substitution in these six-coordinate complexes is expected to be a dissociative process; a result of Cr–N or Cr–P bond dissociation. Wilkinson, Hursthouse, and co-workers noted 2 did not react with 7 atm CO for several hours except under u.v. irradiation (in light petroleum) to form cis-[Cr(CO)₂(dmpe)₂] (cis-2-CO).^17b This account was surprising, and the unreactive nature toward N₂/CO exchange seemed uncharacteristic of a complex with terminally bound N₂ ligands. We reacted 2 with 1 atm CO at 25 °C in pentane or THF without u.v. irradiation and monitored the reaction by in situ IR spectroscopy, or ³¹P NMR spectroscopy (see ESI†). In both solvents the reaction was slow, but 2 was not unreactive. In THF, after 26 h ∼85% of 2 converted to a ∼1 : 1 mixture of cis-2-CO and trans-[Cr(CO)₂(dmpe)₂] (trans-2-CO). trans-2-CO converts to ∼95% cis-2-CO (and ∼5% free dmpe) after additional 46 h by ³¹P NMR spectroscopy. In THF, 1 converts directly to cis-[Cr(CO)₂(depe)₂] cis-1-CO (ν_CO = 1829, 1768 cm⁻¹) in ∼3 h by in situ IR spectroscopy (see ESI†). The vastly different rates of N₂/CO ligand exchange underscore the greater kinetic stability of 2 toward Cr–L dissociative processes that could ultimately curtail catalyst deactivation pathways (i.e. ligand loss) improving catalyst performance for N₂ reduction compared to 1.

Fig. 3 Ligand exchange reactions of 1 and 2 with CO display different reaction profiles.

In conclusion, we present a contemporary advancement in the use of trans-[Cr(N₂)₂(P–P)₂] complexes (1 and 2) for direct catalytic reduction of N₂ to form NH₃ and N₂H₄ using the PCET reagent SmI₂ and H₂O and/or ethylene glycol as proton donors. A new complex, trans-[Cr(N₂)₂(depe)₂], was presented herein. Despite having similar electronic structures, we posit 2 is a better catalyst than 1 (using the presented conditions), due to a less negative Cr^I/0 redox couple and greater kinetic stability from Cr–L dissociative processes.

Author contributions

C. Beasley, investigation, methodology, writing, editing; O. L. Duletski, investigation; K. S. Stankevich, investigation; N. Arulsamy, investigation, writing; M. T. Mock, conceptualization, methodology, supervision, writing, editing, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

The authors thank Dr. Bernhard Linden and Mathias Linden for LIFDI-MS analysis. This material is based upon work supported by the National Science Foundation (NSF) under Grant No. CHE-1956161 and CHE-2247748. Support for MSU's NMR Center has been provided by the NSF (Grant No. NSF-MRI: CHE-2018388 and DBI-1532078), the Murdock Charitable Trust Foundation (2015066:MNL), and MSU's office of the Vice President for Research and Economic Development. The authors gratefully acknowledge financial support for the X-ray diffractometer from the NSF (CHE-0619920) and a Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health (Grant # 2P20GM103432).

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, crystallographic details, and additional spectroscopic and electrochemical data. CCDC 2330754 (1) and 2330755 (2). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00702f

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