# Catalytic and stoichiometric stepwise conversion of side-on bound dinitrogen to ammonia mediated by a uranium complex

Mikhail S Batov, Heather T Partlow, Lucile Chatelain, John A Seed, Rosario

Scopelliti, Ivica Zivkovic, Ralph W Adams, Stephen T Liddle, Marinella

Mazzanti

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Mikhail S Batov, Heather T Partlow, Lucile Chatelain, John A Seed, Rosario Scopelliti, et al. | Catalytic and stoichiometric stepwise conversion of side-on bound dinitrogen to ammonia mediated by a uranium complex. Nature Chemistry, 2025, 10.1038/s41557-025-01867-z. hal-05217600

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# Catalytic and stoichiometric stepwise conversion of side-on bound dinitrogen to ammonia mediated by a uranium complex

Received: 17 October 2024

Accepted: 2 June 2025

Published online: 16 July 2025

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Mikhail S. Batov Ⓔ<fnref n="1" />, Heather T. Partlow Ⓔ², Lucile Chatelain Ⓔ<fnref n="1" />,³,
John A. Seed Ⓔ², Rosario Scopelliti Ⓔ<fnref n="1" />, Ivica Zivkovic Ⓔ⁴, Ralph W. Adams Ⓔ²,
Stephen T. Liddle Ⓔ²✉ & Marinella Mazzanti Ⓔ<fnref n="1" />✉

The well-defined catalytic conversion of dinitrogen ($N_2$) to ammonia ($NH_3$) by molecular complexes is of fundamental interest and important for providing an atomic-level understanding of reactivity that can be related to industrial and biological nitrogen-fixation processes. Molecular catalytic $N_2$ to $NH_3$ conversion currently involves the reduction and protonation of terminal or bridging end-on bound metal–$N_2$ complexes. However, catalytic $N_2$ to $NH_3$ conversion by side-on bound metal–$N_2$ molecular complexes is relevant to both the industrial and biological nitrogen-fixation processes. Here, using a uranium triamidoamine complex, we report catalytic $N_2$ to $NH_3$ conversion involving side-on bound $N_2$ binding. Stoichiometric reactions reveal stepwise reduction of $N_2$ from free $N_2$ to bridging side-on bound forms and subsequently to bridging nitrides, uniquely accessing four different states of side-on bound $N_2$ for the same molecular system. This reveals the roles of $N_2$, $N_2^{2-}$, $N_2^{3-}$, $N_2^{4-}$ and $N^{3-}$ in the catalytic conversion of $N_2$ to $NH_3$ when involving side-on bridging $N_2$.

The well-defined catalytic conversion of dinitrogen ($N_2$) to ammonia ($NH_3$)—promoted by a molecular Mo–triamidoamine complex—was reported by Schrock in 2003 (ref. 1). The proposed mechanism involved the reduction of terminal end-on bound $N_2$ by the stepwise, alternating addition of six protons and six electrons. Since that seminal report, a broad range of mononuclear and dinuclear Fe and Mo complexes$^{2–4}$ capable of binding and catalytically converting $N_2$ into $NH_3$ have been identified$^{5,6}$, with some reaching very high conversion efficiencies$^7$. Since the discovery of the Fe–Mo cofactor, numerous mechanistic studies, mostly focused on Mo and Fe due to their biological relevance, have been carried out on metal–$N_2$ complexes as models for the biological reduction of $N_2$ (refs. 6, 8–10). Although synthetic model chemistry—even based on Fe and Mo—does not provide direct mechanistic information on enzymatic species, such studies can provide relevant

models and permit unforeseen reactivity patterns to be recognized. The catalytic conversion of $N_2$ to $NH_3$ has subsequently expanded beyond Mo and Fe, but involves a still exceedingly small number of metals (Ti, Co, V, Ru and Os)$^{11–14}$.

The plethora of reported mechanistic studies on molecular catalytic $N_2$ to $NH_3$ conversion involve the reduction and protonation of end-on terminal or end-on/end-on bridging metal–$N_2$ complexes$^{5,6,8}$. However, the recent identification of intermediates of nitrogenase reactive sites where substrates such as CO or imide bridge—with only one atom—two neighbouring Fe centres highlights the need for more studies on dinuclear $N_2$-bridged complexes$^{15–17}$. Furthermore, the reduction and hydrogenation of side-on bound systems is also potentially relevant to gaining a better understanding of the mechanism of the Haber–Bosch process for converting $N_2$ and hydrogen gas ($H_2$) into $NH_3$.

<footnote n="1">¹Group of Coordination Chemistry, Institut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. ²Department of Chemistry, The University of Manchester, Manchester, UK. ³Chimie, Electrochimie Moléculaires et Chimie Analytique, Université de Bretagne Occidentale, Brest, France. ⁴Laboratory for Quantum Magnetism, Institute of Physics, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland. ✉e-mail: steve.liddle@manchester.ac.uk; marinella.mazzanti@epfl.ch</footnote>
<figure><img src="image_4.png" /><figcaption>**Fig. 1 |** Selected literature examples (1, A–C) of N₂ binding and reduction by uranium complexes, as well as a summary of this work. Previously reported N₂²⁻-bridged complexes: [{U¹⁴V(TrenDMBS)}₂(µ·η²:η²·N₂)] (1, TrenDMBS = {N(CH₂C₆H₅NSiMe₂Bu)₃}³⁻); N₂³⁻- radical-bridged aryloxide complex [K(2,2,2-crypt)] [{(ArO)₃U}₂(µ·η²:η²·N₂)] (A, OAr = 2,4,6-tri-tert-butylphenoxide)²⁶; N₂⁴⁻-bridged</figcaption></figure>

siloxide complex [K₂[{U¹⁴V(OSi(O'Bu)₃)}₂(µ·O)(µ·η²:η²·N₂)]} (B)²⁵; bis-nitride bridged calix[4]tetrapyrrole complex [{K(DME)(Et₈·calix[4]tetrapyrrole)U}₂(µ·NK₂)][K(DME)₄] (C, DME = 1,2-dimethoxyethane)⁷⁸; this work: stepwise reduction of **1** and catalytic conversion of N₂ to ammonia by **1**. xs., excess.

(refs. 18–20). Although the catalytic N₂ to NH₃ conversion for a side-on bound N₂ complex remains elusive, stoichiometric N₂ to NH₃ conversion has been reported for a few side-on bridged N₂ complexes of group 4 metals²¹,²² and uranium²³⁻²⁸. Side-on coordinated N₂ can undergo stepwise reduction to a variety of formal charge states including N₂²⁻, N₂³⁻ and N₂⁴⁻, but in most cases only one of these activated N₂ species is isolated for a given metal complex²⁹. In f-element chemistry where side-on N₂ complexes dominate, interconversion between N₂²⁻ and the N₂³⁻ radical is documented²⁶,³⁰⁻³³, but further reduction could not be observed for lanthanide ions. Nonetheless, cleavage of side-on bound N₂⁴⁻ to 2N₃⁻ following the addition of external reducing agents has also been reported by some of us for U-complexes²³,²⁶,³⁴, which suggested to us that under suitable conditions, side-on coordinated bridging U–N₂ complexes³⁵⁻³⁸ might prove to be competent for catalytic N₂ to NH₃ conversion. It is also germane to recall that before the use of Fe-based catalysts in the industrial Haber–Bosch process, Haber reported that uranium and uranium nitride materials (UNx) were effective heterogeneous catalysts for the production of ammonia from dinitrogen³⁹. Accordingly, studies on molecular analogues may afford insights into heterogeneous scenarios.

Given our earlier collective work in early metal and f-block N₂-activation²³,²⁵⁻²⁷,³³,⁴⁰⁻⁴³ and U-nitride chemistries²⁶,³⁴,⁴⁴⁻⁶⁵, we developed our investigations using the diuranium–N₂ complex [{U¹⁴V(TrenDMBS)}₂(µ·η²:η²·N₂)] (1, TrenDMBS = {N(CH₂CH₂NSiMe₂Bu)₃}³⁻), reported by Scott³⁵ in 1998 (Fig. 1), as the formation of **1** from its tri-valent precursor [U³³(V(TrenDMBS))] is reversible, suggesting that it could be amenable to catalysis rather than being an irreversible thermodynamic sink.

In this Article we report the stepwise reduction of N₂ from free-N₂ to bridging side-on bound forms to bridging nitrides—overall uniquely accessing four different states of side-on bound N₂ for the same dinuclear U-complex. Experimental characterization and quantum-chemical studies suggest that the addition of reducing agents leads to the formal sequential reduction of the bound N₂ rather than the U-centres. We establish molecular U-mediated catalytic N₂ to NH₃ conversion, moreover involving side-on bound coordination of N₂ in metal-mediated N₂ to NH₃ catalysis. The stoichiometric reactions suggest the relevance of N₂, N₂²⁻, N₂³⁻, N₂⁴⁻ and N₃⁻ in the catalytic conversion of N₂ to NH₃ when involving side-on bridging N₂.

## Results and discussion
### Synthesis and characterization

Complex **1** (Fig. 2), was prepared from hexane in 71% yield through a modified procedure, circumventing the sublimation step of the previously reported synthesis³⁵.

The redox reactivity of **1** was then explored (Fig. 2). After sequential addition of **1** equiv. of 2,2,2-cryptand and then KC₈ to a brown-red solution of **1** in toluene, the mixture turned dark-brown, and full consumption of the starting material occurred after 3 h of stirring at room temperature (r.t.). Brown-black crystals of the mono-reduced complex [K(2,2,2-cryptand)][{U¹⁴V(TrenDMBS)}₂(µ·η²:η²·N₂)] (**2-crypt**) were isolated in 83% yield from the filtered reaction mixture when stored at -40 °C for 2 days (Fig. 3a and Supplementary Fig. 2). The N1–N2 distance of 1.336(6) Å in **2-crypt** is significantly longer compared to that found in **1** (1.109(7) Å), the latter having an N=N distance close to that of the free N₂ (1.0975 Å) (Supplementary Table 1). The longer N–N distance in **2-crypt** indicates reduction at the central N₂ rather than at the uranium ions, and the U1–N₂–U2 unit is bent (fold angle of 157.43°, Fig. 3g) compared to the essentially planar angle of 177.83° in **1**. Elongation of the N1–N2 distance in **2-crypt** is associated with population of its antibonding π* orbital and formation of the N₂³⁻ radical. Similar values of the N–N distance were found for the only other N₂³⁻-radical-bridged diuranium system [K(L)][{(ArO)₃U}₂(µ·η²:η²·N₂)] (for L = 2,2,2-cryptand, 1.425(5) Å; for L = (THF)₆, 1.39(1) Å, THF = tetrahydrofuran; ArO = 2,4,6'-Bu·C₆H₅O, Fig. 1, A)²⁶, as well as for the N₂³⁻-radical-bridged lanthanide complexes, which show N=N distances in the range of 1.396(7)–1.405(3) Å (refs. 66,67). The U–N*amine* distances for **2-crypt** (average 2.238 Å) are close to those of [U¹⁴V(TrenDMBS)Cl] (average 2.22 Å)⁶⁸, and the U–N*amine* distance (average 2.743 Å) is longer than in [U¹⁴V(TrenDMBS)Cl] (2.656(19) Å). In contrast to **1**, ¹H NMR studies showed that **2-crypt** is stable towards N₂ loss under dynamic vacuum in the solid state and after three freeze-pump-thaw cycles in toluene-d₈ solution, and no N₂ displacement was evidenced by exposure to coordinating solvents such as tetrahydrofuran and diethyl ether.

Reduction of **1** was also performed without chelating agents. Dark-brown microcrystals of [K{U¹⁴V(TrenDMBS)}₂(µ·η²:η²·N₂)] (**2-K**) were obtained in 62% yield from the filtered reaction mixture left standing at -40 °C for 2 days. When the reduction was performed under similar conditions but with adventitious Li cations present, the complex [Li{U¹⁴V(TrenDMBS)}₂(µ·η²:η²·N₂)] (**2-Li**) was isolated after 3 days of storage
<figure><img src="image_5.png" /><figcaption>**Fig. 2 | Synthesis of uranium–dinitrogen complexes 1–5.** Synthesis of the previously reported pernitride-bridged complex [{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] **1**, from [U<sup>IV</sup>Tren<sup>DMBS</sup>]Cl via a modified procedure, followed by reduction of **1** to yield the N<sub>2</sub><sup>3-</sup> radical-bridged complexes [K(2.2.2-cryptand)] [{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (**2-crypt**), [Li{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (**2-Li**) and [K{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (**2-K**). Further reduction yields the hydrazido</figcaption></figure>

(4-)-bridged complex [Li₂{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(μ-η<sup>2</sup>:η<sup>2</sup>-N₂)] (**3**) and ultimately leads to splitting of the N₂ molecule, affording the bis-nitride complex [Li₄(OEt₂){U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(μ-N₂)] (**4**). Cleavage of the hydrazido moiety is observed upon reaction of **3** with dihydrogen, and allows the isolation of the nitrido/ imido-bridged complex [Li₃{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(μ-N)(μ-NH)] (**5**).

at –40 °C. The N1–N2 distance (1.28(1) Å) in complex **2-Li** is shorter than the N–N distances in **2-K** (1.364(18) Å) and in **2-crypt** (1.336(6) Å), suggesting stronger N–N activation in the K- and K-crypt complexes.

Magnetic susceptibility data were collected for **1** and **2-crypt** under an applied field of 0.1 T and in a temperature range of 2–300 K. The susceptibility data for **1** indicate the presence of two noninteracting 5f² U(IV) centres, with a magnetic moment of 0.8 μB at 2.5 K, 4.2 μB at 300 K (0.6 μB at 2.5 K and 3.0 μB at 300 K per U ion), in agreement with the assignment of **1** as a pernitride-bridged U(IV)/U(IV) complex⁶⁹ (Supplementary Figs. 137–140). By contrast, the magnetic data for **2-crypt** show that the magnetic moment of the complex increases from 4.10 μB (2 K) to 4.90 μB (300 K) (Supplementary Fig. 143). Similar data were obtained for **2-K** (per complex, 3.94 μB at 2 K and 6.84 μB at 300 K under 0.1 T applied field; 3.24 μB at 2 K and 5.12 μB at 300 K under 1 T applied field; Supplementary Fig. 149). These values are increased compared to **1** across the entire temperature range, reflecting the presence of an S = ½ radical in addition to two 5f² U(IV) ions (Supplementary Fig. 146).

The χ versus *T* data for **2-crypt** and **2-K** do not exhibit any maxima feature that could be associated with U···U magnetic exchange coupling (Supplementary Fig. 142). However, the variable temperature behaviour is also not as expected for isolated spins. Interestingly, the magnetic data for **2-crypt** and **2-K** are similar to those found for the N<sub>2</sub><sup>3-</sup> radical complex [K(2.2.2-crypt)][{(ArO)<sub>3</sub>U<sup>IV</sup>}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (1.8 K, 3.6 μB; 300 K, 5.0 μB)<sup>26</sup>. Powdered **1** was found to be electron paramagnetic resonance (EPR)-silent (X-band) at 6 and 298 K, which agrees with its U(IV)/U(IV)-N<sub>2</sub><sup>2-</sup> formulation<sup>70</sup>. In contrast, the X-band EPR spectrum of **2-crypt**, although silent at 298 K, shows a single axial feature at *g* = 8.6 (*H*<sub>0</sub> = 780 Gs; *g* = Landé *g*-factor, *H*<sub>0</sub> = magnetic field) both in solution and in frozen 2-methyltetrahydrofuran glass at 6 K, indicating that the entire molecular species possesses a Kramers ground state

(Supplementary Figs. 134 and 135)⁷¹. A better-resolved EPR signal was observed for the inner-sphere N₂³⁻ radical complex **2-K** in the solid state at 6 K, with *g*₁ = 7.9 (*H*<sub>0</sub> = 850 Gs), *g*₂ = 0.73 (*H*<sub>0</sub> = 9,205 Gs) and *g*₃ = 0.5 (*H*<sub>0</sub> = 1,330 Gs) (Supplementary Fig. 136). Although EPR signals were not previously observed for the radical-bound U-complexes [{(SiMe₂NPh)₃-tacn}U<sup>IV</sup>(η²-N₂Ph₂)]⁷² and [{(tBuArO)₃tacn}U<sup>IV</sup>(η²-NNCPh₂)] (tacn = 1,4,7-triazacyclononane)⁷³, nor the N₂³⁻ radical-bridged diuranium complex [K(2.2.2-crypt)][{(ArO)₃U₂(μ-η²:η²-N₂)]<sup>26</sup>, the magnetic and anisotropic EPR data of **2-crypt** and **2-K** are similar to those of the dip phosphorus radical trianion diuranium(IV) complex [K(2.2.2-crypt)] [{U(Tren<sup>TIPS</sup>)}₂(μ-η²:η²-P₂)] (Tren<sup>TIPS</sup> = {N(CH₂CH₂NSi'Pr₃)₃}³⁻). The latter was found to have a strong U–P radical, but very weak U···U magnetic coupling<sup>74</sup>.

Overall, the magnetic and EPR data are in agreement with the presence of two magnetically independent U(IV) ions bridged by an N₂²⁻ in **1**, and the presence of two N₂³⁻-bridged U(IV) ions in **2-crypt** and **2-K** with a strong uranium radical but weak U···U magnetic exchange coupling.

Having established that the mono-reduced N₂ analogues of **1** are synthetically accessible, we probed the further reduction of the N₂ unit (Fig. 2). To accommodate the increased steric and electronic loadings anticipated from further reductions, and thus facilitate the isolation of the reduced species, we carried out reductions in the presence of added Li cations. Addition at –40 °C of a solution of **1** and 2 equiv. of Lil in diethyl ether to a suspension of 2 equiv. of KC₈ resulted in a colour change from brown-red to maroon over 3 days at –40 °C. After workup, dark-red crystals of [Li₂(U<sup>IV</sup>(Tren<sup>DMBS</sup>))₂(μ-η²:η²-N₂)] (**3**) were obtained after 4 days in 63% yield. The solid-state structure of **3** shows the presence of a central hydrazido(4–) moiety bridging two U ions disordered over two orientations (Fig. 3d and Supplementary Figs. 5 and 6). The N–N bond (Fig. 3g) is substantially elongated
<figure><img src="image_6.png" /><figcaption>**Fig. 3** | Single-crystal X-ray diffraction solid-state structures of the dinitrogen-bridged Tren<sup>DMBS</sup> diuranium complexes with displacement ellipsoids at 50% probability level and selective labelling. **a–f**, Molecular structures of the anionic complex [{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)]<sup>−</sup> in **2-crypt** (**a**), as well as in **2-Li** (**b**) and **2-K** (**c**), the hydrazido-bridged complex **3** (**d**) with disorder in the dinitrogen core and with one lithium position removed for clarity, the bis-nitride complex **4** (**e**) and the nitrido/imido complex **5** (**f**). For all complexes, the triamidoamine</figcaption></figure>

ligand framework is depicted as a wireframe, and hydrogen atoms (apart from the imido-bound H in **5**) and disordered *tert*-butyl groups have been removed for clarity. **g**, Molecular U<sub>2</sub>N<sub>2</sub> core structures and XRD-derived dinitrogen distance summaries for complexes **2–5**. **h**, Correlation between the degree of N–N bond activation and the NH<sub>4</sub>Cl yields obtained after direct quenching with 2 M HCl in diethyl ether.

compared to **1** and **2-crypt** and is within the same range for both orientations (N1–N2 = 1.483(19); N3–N4 = 1.47(2) Å), and essentially the same as free hydrazine (N–N = 1.47 Å). This value falls within the range of the N–N bond distance values found in U(IV) and U(V) hydrazido complexes, namely 1.491(5) Å in K<sub>4</sub>[U<sup>IV</sup><sub>2</sub>(μ-N<sub>2</sub>H<sub>2</sub>)(mTP<sup>R−</sup>)<sub>2</sub>] (mTP = [{2-(OC<sub>6</sub>H<sub>2</sub>)′Bu-2,Me-4)₂CH}-C<sub>6</sub>H<sub>4</sub>-1,3]⁴⁻; R = Me⁴, Bu), 1.40(1) Å (ref. 24) in [K<sub>2</sub>{[U<sup>V</sup>(OSi(O'Bu)<sub>3</sub>)<sub>3</sub>]<sub>2</sub>(μ-O)(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)}]<sup>25</sup> (Fig. 1, **B**) and 1.521(18) Å in [K<sub>3</sub>{[U<sup>V</sup>(OSi(O'Bu)<sub>3</sub>)<sub>3</sub>]<sub>2</sub>(μ-N)(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)}]<sup>23</sup>. Similar N–N distances were also found in Zr(IV) hydrazido complexes (1.457(3)–1.548(7) Å)<sup>21,75</sup>. The metrical parameters of bonding between the triamidoamine framework and the U-centres in **3** (U-N<sub>amide</sub> = 2.402 Å and U-N<sub>amine</sub> = 2.628 Å) are similar to those found in **2-Li**, in agreement with the presence of U(IV) ions (Supplementary Table 1).

With the products of mono- and di-reduction of **1** in hand, we pursued further reduction to split the N<sub>2</sub> molecule and access nitrides. When an excess of 6 equiv. of KC<sub>8</sub> was added to the mixture of **1** and LiI

(6 equiv.) and left stirring for 3 days at –40 °C, the colour of the reaction mixture gradually turned dark-brown. Subsequent workup and crystallization at –40 °C afforded the bis-nitride complex [Li<sub>4</sub>(OEt<sub>2</sub>)<sub>2</sub>{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-N)<sub>2</sub>] (4) as brown-black crystals in 56% yield (Fig. 3e). The solid-state structure of **4** shows two U-centres bridged by two nitrides (N6 and N3) at a non-bonding N···N distance of 2.70(1) Å in a diamond core U<sub>2</sub>N<sub>2</sub> motif (Fig. 3g) with U–N bond distances of U2–N3 = 2.248(9) Å and U2–N6 = 2.184(9) Å (Supplementary Fig. 7 and Supplementary Table 1). Each nitride binds two Li cations, with two different binding modes: the Li cations interacting with N6 are coordinatively supported by two U-bound amide arms (Li1–N6 = 1.96(2); Li2–N6 = 1.98(2) Å), and the two cations capping the second nitride are held by one Tren<sup>DMBS</sup> arm that is dissociated from U (Li3–N3 = 2.16(3); Li3–N3 = 1.97(2) Å).

Complex **4** adds to the limited examples of U-nitrides obtained from the reductive cleavage of N<sub>2</sub> (refs. 27,34,76,77) following the first report (Fig. 1, C)<sup>78</sup> with only one example of a U(IV) bis-nitride obtained
from cleavage of N₂ having been reported⁷⁷. Alternatively, brown-black crystals of 4 could be isolated through reduction of **2-K** with 3 equiv. of KC₈ and 4 equiv. of Lil for 1 day at -40 °C in diethyl ether, or from **3** and 2 equiv. of Lil with the addition of 2 equiv. of KC₈ after 1 day under similar experimental conditions (Supplementary Figs. 24 and 25). Furthermore, it was found that **3** could in turn be accessed from **2-K** by reaction with 1 equiv. of KC₈ and 2 equiv. of Lil (diethyl ether at -40 °C, 1 day; Supplementary Figs. 20 and 21). These results highlight the possibility of controlled, stepwise addition of electrons into the side-on bound N₂ by controlling the amount of external reducing agent. Although the stepwise reduction of end-on bound N₂ in molecular complexes has been studied extensively⁶, here four different products of N₂ reduction (N₂²⁻, N₂³⁻, N₂⁴⁻, N³⁻) have all been isolated for the same ligand system and for the same metal, and all have been shown to be accessible in a stepwise controlled manner. Recently, some of us reported the stepwise reduction of the U₂–N₂ complex [{(ArO)₃U₄}₂(μ-η²:η²-N₂)]²⁶, but in that case reduction of the N₂³⁻–analogue resulted in the immediate splitting of N₂, and identification of a potential hydrazido(4⁻) intermediate was not possible under conditions similar to those used here.

Finally, we explored the reactivity of complexes **3** and **4** with H₂ (1.01 bar) at room temperature. X-ray-quality crystals of the nitrido/ imido complex **complex** [Li₃{(TrenDMBS)U₄}₂(μ-N)(μ-NH)] (5) could be isolated in good yield (Supplementary Figs. 27 and 28) from the reaction mixture, indicating that H₂ can cleave the hydrazido(4⁻) moiety at room temperature. The solid-state structure of **5** (Fig. 3f) reveals an asymmetric diamond-shape U₂–N₂ core (Fig. 3g) with disparate U–N*nitride* distances (U2–N5 = 2.35(1); U1–N5 = 2.02(1) Å) (Supplementary Fig. 8). Loss of one Li-cation is observed following the protonation of the N6 nitrido to give an imido-group (U2–N6 = 2.21(1); U1–N6 = 2.14(1) Å).

Cleavage of metal-bound N₂ by H₂, a reaction directly relevant to the Haber process, has only been reported for group 4 ions at substantially higher temperatures²¹. To unambiguously confirm that the imido-proton originates from the H₂ gas, complex **3** was reacted with D₂. An immediate reaction was observed, producing **5** as a major species, with the ¹H NMR spectrum of the crude reaction mixture dissolved in toluene-d₈ showing the full set of resonances assigned to **5** (Supplementary Figs. 31 and 32), apart from the broad NH singlet at -170.25 ppm. The structure of **5** indicates that a second species must be formed. Other species were observed in the ¹H and ⁷Li NMR spectra of the reaction mixture but could not be isolated. The origin of the imido-proton was further confirmed by observing the shift in the N-H/D IR stretch from 3,313 cm⁻¹ (for **5**) to 2,452 cm⁻¹ (for **5-D₂**) upon isotopic substitution (Supplementary Fig. 127). The formation of **5** could be the result of H⁺/Li⁺ scrambling from a putative bis-imido complex formed upon cleavage of the N-N bond followed by rearrangement and formation of **3** and an imido-amido complex ([Li{(TrenDMBS)U₄}₂(μ-NH₂)(μ-NH)]).

Exposure of a frozen solution of **4** in toluene-d₈ to 1.01-bar H₂ and subsequent warming to room temperature resulted in an immediate colour change from dark-brown to brown. The ¹H NMR and ⁷Li NMR spectra of the reaction mixture indicate that multiple species are formed, but only complex **5** could be isolated and crystallographically characterized. Similarly to the D₂ reactivity of **3**, exposure of **4** to deuterium gas gives a full set of resonances assigned to **5**, with only the NH resonance absent, as the sole identifiable species (Supplementary Figs. 34 and 35). The formation of **5** is probably the result of heterolytic splitting of H₂ by the bis-nitride complex, accompanied by elimination of LiH, but the latter could not be unambiguously identified. Germane to this point, heterolytic splitting of H₂ by a nitride bridged U(IV) complexes to give a rare imide-hydride complex, with the hydride bridging two U(IV) and a Cs cation, has been reported⁵³. Here, the high affinity of the hydride for the Li-cation would account for elimination of the putative LiH.

## Computational analysis

To provide further insight into the U–N₂ complexes reported here, we conducted density functional theory (DFT) calculations on **2-crypt**,

**2-Li, 2-K** and **3**. Additionally, although **1** has already been extensively examined by DFT⁷⁹, we computed **1** at the same level of theory to provide a meaningful benchmark. The closely related analogue of **4**, [{U⁴V(μ-NL₁₂)(TrenTIPS)]₂}, has been computationally analysed in detail previously⁸⁰, so we did not compute **4** here. Given the Li-cation disorder in **3**, we computed two isomers with respect to the positions of the Li cations: the side-on/end-on isomer **3A** and the end-on/end-on isomer **3B**. We find that **3A**, in the gas phase, is more stable than **3B** by 3.37 kcal mol⁻¹, consistent with the experimental observation of crystallographic disorder.

For spin-quintet **1**, there are four α-spin electrons, where HOMO (highest occupied molecular orbital) to HOMO-2 are of essentially pure 5f character, and HOMO-3 is mixed 5f/N₂-π* (46/45%, the B(πg₁) δ-symmetry orbital). HOMO-4 (and its β-spin counterpart) constitute the A(πg⁻) in-plane bonding combination (Supplementary Fig. 151). The addition of an extra electron to give **2-crypt**, **2-Li** and **2-K** in each case introduces an extra α-spin 5f electron, producing spin-sextet formulations. Hence for **2-crypt**, **2-Li** and **2-K**, in each case HOMO to HOMO-3 are 5f-character, and HOMO-4 is the 5f/N₂-π* B(πg₁) (average 33/57%) (Supplementary Figs. 152–154). HOMO-5 and its β-spin counterpart are the A(πg⁻) combination. On moving to spin-quintet **3A/3B**, again, four α-spin electrons of 5f-character are found in HOMO to HOMO-3, and HOMO-4 and HOMO-5 and their β-spin counterparts are the B(πg₁) and A(πg⁻) orbitals, confirming full occupation of the π*-manifold of the N₂ unit (Supplementary Figs. 155 and 156).

The computed uranium MDCq charges (1.90–2.42; MDCq is the multipole derived charge, where q denotes quadrupolar) and MDCm (MDCm is the multipole derived charge, where m denotes multipole) net spin densities (2.09–2.53) for **1**, **2-crypt**, **2-Li**, **2-K** and **3** reflect the formal presence of U(IV) ions in all complexes, consistent with the characterization data (Supplementary Table 4). The analogous MDCq data for the N₂ units progressively increase from -1.88 to -2.73, reflecting the increasing formal N₂ charges across the series. This is also reflected in the MDCm data, where the net spin density of 0.25 electrons on the N₂²⁻ unit in **1** increases to 0.36 (average for the N₂³⁻ radical group in **2-crypt**, **2-Li** and **2-K**, but becomes a net spin density deficiency in **3** (-0.3), reflecting the strongly donating capacity of N₂⁴⁻. Notably, across the series, the N-N bond order progressively weakens, from 1.66 in **1** to 1.27 in **3A**, and this is reflected by quantum theory of atoms in molecules (QTAIM) topological bond analysis and in analytical frequency calculations, where clear N-N stretches are computed at 1,207 cm⁻¹ (1), 1,180 cm⁻¹ (2-K), 1,139 cm⁻¹ (2-crypt) and 990/915 cm⁻¹ (3B/3A).

## Stoichiometric and catalytic conversion of N₂ to NH₃

To provide a baseline for catalytic studies, we first studied the stoichiometric reactions of **1**, **2-crypt**, **3** and **4** with H⁺, because the yield of the resulting NH₃, isolated as the conjugate acid NH₄Cl, could indicate the degree of N₂ activation, with higher yield associated with greater activation (Table 1, entries 1–4). A previous control experiment where [Ti(TrenTMS)Cl] (TrenTMS = {N(CH₂CH₂NSiMe₃)}³⁻) was treated with 1 M HCl only produced 0.04 NH₃ equivalents, suggesting only minor degradation of TrenR ligands under the action of strong acids. This was confirmed by treating [U⁴V(TrenDMBS)Cl] with 2 M ethereal HCl, which produced 0.03 NH₃ equivalents. Treatment of crystalline **1** with excess 2 M HCl affords 0.18 NH₃ equivalents, indicating weak activation of the pernitrile unit (Supplementary Fig. 36). Analogously, crystalline **2-crypt** produced 0.43 NH₃ equivalents, suggesting stronger activation than **1** (Supplementary Fig. 37). This is more NH₃ equivalents than found for [K(2.2.2-crypt)][{(ArO)₃U₄}₂(μ-η²:η²-N₂)] (0.34 equiv.), noting that no NH₃ was detected from HCl treatment of [{(ArO)₃U₄}₂(μ-η²:η²-N₂)]²⁶. For HCl treatment of hydrazido(4⁻)-bridged **3**, 1.57 NH₃ equivalents were detected, indicating much stronger activation of the N-N bond compared to **1** and **2-crypt** (Supplementary Fig. 38). For comparison, 0.54–1.08 NH₃ equivalents were detected upon direct protonation of the dihydrohydrazido(2⁻) complex K₄[U⁴V₂(μ-N₂H₂)(mTPR⁻)]₂.
<table><caption>Table 1 | Stoichiometric and catalytic acidification experiments for the reaction of crystalline 1, 2-crypt, 3 and 4 with 2M ethereal HCl and solutions of 1 with acids and reductants under N₂ to produce NH₃ and N₂H₄</caption><thead><tr><th>Entry<sup>a</sup></th><th>Compound</th><th>Solvent</th><th>Acid</th><th>Reductant</th><th>Acid (equiv.)</th><th>Reductant (equiv.)</th><th>NH<sub>3</sub> (equiv.)</th><th>N<sub>2</sub>H<sub>4</sub> (equiv.)</th><th>Fixed-N (equiv.)</th></tr></thead><tbody><tr><td>1</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>2M HCl</td><td>-</td><td>&gt;1,500</td><td>-</td><td>0.18</td><td>-</td><td>0.18</td></tr><tr><td>2</td><td><b>2-crypt</b></td><td>Et<sub>2</sub>O</td><td>2M HCl</td><td>-</td><td>&gt;1,500</td><td>-</td><td>0.43</td><td>-</td><td>0.43</td></tr><tr><td>3</td><td><b>3</b></td><td>Et<sub>2</sub>O</td><td>2M HCl</td><td>-</td><td>&gt;1,500</td><td>-</td><td>1.57</td><td>-</td><td>1.57</td></tr><tr><td>4</td><td><b>4</b></td><td>Et<sub>2</sub>O</td><td>2M HCl</td><td>-</td><td>&gt;1,500</td><td>-</td><td>2.00</td><td>-</td><td>2.00</td></tr><tr><td>5</td><td>-</td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.00</td><td>0.03</td><td>0.06</td></tr><tr><td>6</td><td>-</td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.00<sup>b</sup></td><td>0.03<sup>b</sup></td><td>0.06<sup>b</sup></td></tr><tr><td>7</td><td><b>1</b></td><td>Hexane</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>10</td><td>4</td><td>0.70</td><td>0.00</td><td>0.70</td></tr><tr><td>8</td><td><b>1</b></td><td>Toluene</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>10</td><td>4</td><td>0.07</td><td>0.00</td><td>0.07</td></tr><tr><td>9</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>10</td><td>4</td><td>1.18</td><td>0.00</td><td>1.18</td></tr><tr><td>10</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Et<sub>3</sub>NH][BPh<sub>4</sub>]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.00</td><td>0.00</td><td>0.00</td></tr><tr><td>11</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Et<sub>3</sub>NH][Cl]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.00</td><td>0.08</td><td>0.15</td></tr><tr><td>12</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Et<sub>3</sub>NH][I]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.00</td><td>0.01</td><td>0.02</td></tr><tr><td>13</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][BAr<sup>F20</sup>]</td><td>KC<sub>8</sub></td><td>129</td><td>129</td><td>0.40</td><td>-</td><td>0.40</td></tr><tr><td>14</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>Na</td><td>600</td><td>600</td><td>0.00</td><td>0.13</td><td>0.26</td></tr><tr><td>15</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>K</td><td>600</td><td>600</td><td>0.00</td><td>0.12</td><td>0.24</td></tr><tr><td>16</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>173</td><td>173</td><td>1.74</td><td>0.46</td><td>2.66</td></tr><tr><td>17</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>173</td><td>173</td><td>0.34</td><td>0.12</td><td>0.58</td></tr><tr><td>18</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>300</td><td>300</td><td>0.88</td><td>0.13</td><td>1.14</td></tr><tr><td>19</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>KC<sub>8</sub></td><td>600</td><td>600</td><td>0.62</td><td>0.05</td><td>0.72</td></tr><tr><td>20</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>Rb</td><td>600</td><td>600</td><td>1.11</td><td>0.07</td><td>1.25</td></tr><tr><td>21</td><td><b>1/1-<sup>15</sup>N<sub>2</sub></b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>RbC<sub>8</sub></td><td>600</td><td>600</td><td>8.84/8.17<sup>c</sup></td><td>0.00<sup>c</sup></td><td>8.84/8.17<sup>c</sup></td></tr><tr><td>22</td><td><b>1/1-<sup>15</sup>N<sub>2</sub></b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>RbC<sub>8</sub></td><td>600</td><td>600</td><td>1.49/1.12<sup>d</sup></td><td>0.05/0.11<sup>d</sup></td><td>1.59/1.34<sup>d</sup></td></tr><tr><td>23</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>Cs</td><td>600</td><td>600</td><td>4.94</td><td>0.06</td><td>5.06</td></tr><tr><td>24</td><td><b>1</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>CsC<sub>8</sub></td><td>600</td><td>600</td><td>2.61</td><td>0.03</td><td>2.67</td></tr><tr><td>25</td><td><b>4</b></td><td>Et<sub>2</sub>O</td><td>[Cy<sub>3</sub>PH][I]</td><td>RbC<sub>8</sub></td><td>600</td><td>600</td><td>4.43</td><td>0.00</td><td>4.43</td></tr></tbody></table>

All reactions were conducted twice to ensure reproducibility. Solutions of 1 were as follows: 1.3 mM for entries 5–7, 11, 14–16; 0.65 mM for entries 8–10, 12, 13, 17–25. Entries 1, 2, 3 and 4 were conducted with no reductant present. Entries 5 and 6 were conducted with no U-based catalyst present. <sup>a</sup>Experiments were performed under 1.31-bar N₂ (except for entry 17 (2.21bar) and entry 22 (-1.0bar)) at -78°C, followed by warming to 25°C and then stirring for 17h (MC₈, M=K, Rb, Cs) or 72h (M⁹, M=Na, K, Rb, Cs). <sup>b</sup>Conducted under <sup>15</sup>N₂. <sup>c</sup>Conducted with 1-<sup>15</sup>N₂ and under <sup>15</sup>N₂ (1.31bar). <sup>d</sup>Conducted with 1-<sup>15</sup>N₂ and under <sup>15</sup>N₂ (-1.01bar).

(mTPR<sup>R</sup> = {[2-(OC₆H₂·'Bu-2,R-4)₂CH}-C₆H₄·1,3]<sup>4⁻</sup>; R = Me, 'Bu)<sup>24,81</sup> with [HPy]Cl, and [K₃{[U⁺(OSi(O'Bu)₃)₂](μ-N)(μ-η²:η²-N₂)]}<sup>23</sup> gave 0.5–0.84 NH₃ equivalents after addition of excess ethereal HCl, and 1.54 NH₃ equivalents when the same compound was first exposed to 1.01-bar H₂ gas before HCl. Finally, crystalline bis-nitride 4 quantitatively produced 2.0 equiv. of NH₃ when treated with HCl, as previously found for other U(IV/V/VI)-nitride complexes (Supplementary Fig. 39)<sup>34,54,55,61,82</sup>. These reactions confirm the anticipated range of N–N activation, consistent with the experimental and computational characterization data of 1, 2-crypt, 3 and 4.

We next undertook catalytic studies, and, based on the recently reported titanium-catalysed conversion of N₂ to NH₃ (ref. 14), we examined the reactivity of 1 with the weak acid source [Cy₃PH][I] (Cy = cyclohexyl) and elemental alkali metals (Na, K, Rb, Cs) or MC₈ alkali-graphite (M = K, Rb, Cs) reagents (Table 1). These combinations benefit from the low solubility of both the H⁺ and e⁻ sources, minimizing their sacrificial reaction with each other rather than sequential reactivity with U₂–N₂ species (Supplementary Figs. 40–73).

To provide a firm basis on which to undertake catalytic studies, we established several experimental baselines. The absence of 1 is decisive, with only 0.03 equiv. of N₂H₄ formed when [Cy₃PH][I] and KC₈ are mixed together then worked up (Table 1, entries 5 and 6). To examine the potential effect of solvent (Table 1, entries 7–9), we tested the reactivity of ~1.3 mM solutions 1 with 4 equiv. of KC₈ and 10 equiv. of

[Cy₃PH][I] and found up to 1.18 NH₃ equivalents, with yields in the order diethyl ether > hexane > toluene. We thus focused on using diethyl ether for further reactions. The weakly acidic nature of [Cy₃PH][I] minimizes protonation of the tren-ligand. This is underscored by the fact that attempts to obtain catalytic conversion of N₂ to NH₃ using 1, [Et₃NH][X] (X = BPh₄⁻, Cl⁻ or I⁻) and KC₈ did not result in any NH₃ being detected (Table 1, entries 10–12), suggesting that the [Et₃NH][BPh₄] is too aggressive a H⁺ source, leading to protonation of the Tren<sup>DMBS</sup> ligand (Supplementary Fig. 74). The importance of a halide source to the catalysis, to produce [U(Tren<sup>DMBS</sup>)I] (after fixed-N is eliminated) that can be reduced to regenerate 1, is demonstrated by the failure to generate NH₃ with [Et₃NH][BPh₄], and the use of [Cy₃PH][BAr<sup>F20</sup>] also did not sustain any catalysis (Table 1, entries 10 and 13), with only 0.4 NH₃ equivalents detected (Supplementary Figs. 45–48). Finally, although the catalytic conditions in this study are quite different to those of electrochemical scenarios, to benchmark potential NOₓ-to-NH₃ conversion false positives, we conducted control experiments<sup>83,84</sup> with 1 equiv. of nitrite or nitrate present (Supplementary Table 3), and found that only up to 0.53 equiv. of ammonia are produced. Given that the catalysis described here is in a sealed rather than continuous-flow scenario, with scrupulously cleaned glassware, and that NOₓ impurities in N₂ gases are at low levels, NOₓ-to-NH₃ conversion false positives can thus be ruled out. Having ascertained the optimal solvent and H⁺ source, we examined varying the reductant.
<figure><img src="image_7.png" /><figcaption>**Fig. 4 | Proposed catalytic cycles for the DMBS Tren uranium–N<sub>2</sub> system.**</figcaption></figure>

Two possible mechanisms are shown for the conversion of N₂ to NH₃ by I⁻:
(1) sequential reduction with protonation only occurring at the nitride step;
(2) proton-coupled sequential reduction (grey H atoms).

Reaction of 0.65 mM solutions of 1 with 600 equiv. of Na and [Cy₃PH][I] under 1.31-bar N₂ produced no NH₃ equivalents and only a small quantity of N₂H₄ (0.13 equiv.), with a maximum of 0.26 equiv. of fixed-N (Table 1, entry 14). Equivalent reactions with K and Rb under 1.31-bar N₂ were similarly unproductive (Table 1, entries 15 and 20), and we attribute the poor performance of Na, K and Rb to kinetic factors. In contrast, however, the equivalent reaction with Cs (Table 1, entry 23) produced 4.94 equiv. of NH₃ along with a trace amount of N₂H₄. We rationalize this on the basis that during the catalytic runs with Cs, a small amount of Cs would be liquid due to the low melting point of Cs (28.5 °C), and this could thus facilitate the reaction kinetics. Noting that 1 exhibited 2.5 turnovers with Cs, we turned our attention to MC₈ reagents using 600 equiv. of MC₈/[Cy₃PH][I] (Table 1, entries 16–19, 21, 22, 24 and 25). Starting with KC₈, we find that reactions conducted with 173, 300 and 600 equiv. of KC₈/[Cy₃PH][I] produced 1.74, 0.88 and 0.62 equiv. of NH₃, respectively, along with modest amounts of N₂H₄ (0.12–0.46 equiv.). On first inspection these observations are counterintuitive, but they probably reflect the complexity of a number of finely balanced factors critical to successful catalytic turnover. In contrast to Cs, CsC₈ produced 2.61 equiv. of NH₃ with trace N₂H₄. However, importantly, RbC₈ reproducibly produced 8.84 equiv. of NH₃ and no N₃H₄, representing 4.4 catalytic turnovers (Supplementary Fig. 56). To confirm the source of the fixed-N over and above the baseline acidifications, we repeated the RbC₈ reaction with 1-¹⁵N₂ and ¹⁵N₂, and found that 8.17 equiv. of NH₃ are formed, which is consistent with the ¹⁴N₂ data (Supplementary Fig. 57). When the same catalytic reaction (600 equiv. RbC₈/600[Cy₃PH][I]) was conducted with nitride complex 4, 4.4 equiv. of NH₃ were formed, suggesting that a nitride species could be involved in the catalytic pathway, but the presence of strongly bound Li cations in 4 may hinder the catalytic efficiency (Supplementary Fig. 73; Table 1, entry 25). This work thus establishes catalytic N₂ fixation to NH₃, albeit modest, with a side-on bound complex of N₂, and, when taken together with the stoichiometric sequential reductions, we propose two possible catalytic cycles (Fig. 4): (1) sequential reduction, with protonation only occurring at the nitride step, or (2) proton-coupled sequential reduction. Notably, the addition of 600 equiv. of weak acid ([Cy₃PH][I]) to the N₃⁻, N₂⁻ and N³⁻ species in the absence of reducing agent results in ammonia formation only for the nitride complex. However, when the addition of acid is carried out in

the presence of 600 equiv. RbC₈ under Ar we observed the formation of
ammonia for the three complexes, albeit to a different extent (1.3 equiv.
for 2-crypt, 0.9 equiv. for hydrazido 3 and 2.0 equiv. for nitride 4). This
suggests that protonation at earlier stages could also play a role in the
catalytic cycle (Supplementary Figs. 70–72 and Supplementary Table 3,
entries 33–35). Finally, given prior work on Mo–triamidoamine com-
plexes, it may well be that the triamidoamine amide centres shuttle
incoming protons to the reduced dinitrogen moiety⁸⁵,⁸⁶, such mecha-
nistic detail will require further mechanistic study to elaborate.

Conclusion

To conclude, we have reported the stepwise reduction of N₂ from free-N₂ to bridging side-on bound forms and subsequently to bridging nitrides, in the process accessing four different states of side-on bound N₂ for the same dinuclear U-complex. Our combined experimental and quantum-chemical calculation characterization data provide a consistent description of the U-complexes in this study and show that although the coordinated N₂ undergoes sequential reduction steps, the U ions largely remain as U(IV) throughout. Although the catalytic turnover is currently modest, we have established molecular U-mediated catalytic N₂ to NH₃ conversion and moreover demonstrated N₂ to NH₃ catalysis with side-on bound coordination of N₂, adding this mode of reactivity to the paradigm of catalytic conversion of N₂ to NH₃ by terminal or bridging end-on metal–N₂ complexes. Our stoichiometric reactions suggest the relevance of N₂²⁻, N₂³⁻, N₂⁴⁻ and N³⁻ in the catalytic conversion of N₂ to NH₃ when involving side-on bridging N₂. The recognition that side-on bound N₂ can be catalytically activated and converted into NH₃ through polymetallic cooperativity provides conceptual homogeneous–heterogeneous links to, for example, Haber–Bosch processes, and offers new vistas for the elaboration of molecular catalytic transformations involving N₂ and probably other small molecules.

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# Methods

## General considerations

All manipulations were carried out under an inert dinitrogen atmosphere using a standard Schlenk line or glovebox techniques. Catalytic trials were carried out using a J. Young sealable two-bulb apparatus (Supplementary Fig. 1). Water and oxygen levels were kept below 0.1 ppm at all times. Glass-coated stir bars were used instead of Teflon-coated ones to prevent unwanted polytetrafluoroethylene (PTFE) reactivity during reduction reactions. All solvents and reagents were rigorously dried and deoxygenated before use. Prepared compounds were characterized by elemental analyses, NMR, infrared (IR) and EPR spectroscopy, superconducting quantum interference device (SQUID) magnetometry, single-crystal X-ray diffraction studies and DFT, natural bond orbital (NBO) and bond topology theoretical calculations. Further details on the synthetic preparations as well as additional characterization for all compounds are provided in Supplementary Information.

## Synthesis of [{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (1), modified procedure

Under an atmosphere of Ar (ref. 35), a pale-green solution of [U<sup>IV</sup>(Tren<sup>DMBS</sup>)Cl] (374.2 mg, 0.493 mmol, 1 equiv.) in *n*-hexane (10 ml) was added to a Schlenk tube containing a glass-coated stir bar. Solid potassium graphite (199.8 mg, 1.478 mmol, 3 equiv.) was then added in portions, which resulted in the formation of a dark-purple reaction mixture that was allowed to stir for 24 h at room temperature. Full conversion to the [U<sup>III</sup>(Tren<sup>DMBS</sup>)] species was confirmed by ¹H NMR spectroscopy. The resulting reaction mixture was filtered on a porosity '4' glass frit, and the solid residue was rinsed with *n*-hexane (3 × 0.5 ml), yielding a dark-purple solution. The volatiles were removed in vacuo, and the resulting purple solid was brought into the dinitrogen glovebox (1.016-bar absolute N<sub>2</sub> pressure) and dissolved in *n*-pentane (2 ml) to immediately form a dark-red solution. The solution was then stored at -40 °C for 2 days to yield dark-red crystals of **1** in 48% yield (178.1 mg, 0.118 mmol) that were dried under N<sub>2</sub> flow. The mother liquor was concentrated to 1 ml, and then stored at -40 °C, yielding additional **1** (48.2 mg, 0.032 mg) after 2 days. The procedure was repeated one more time (concentration to 0.5 ml and storage at -40 °C), yielding a third and final crop of **1** after 2 days (0.024 mmol, 37.1 mg). The total yield was 71% (263.4 mg). It was found that the use of *n*-hexane instead of pentane for the crystallization step resulted in lower overall yield. Elemental analysis: Calcd. for C<sub>48</sub>H<sub>114</sub>N<sub>10</sub>Si<sub>6</sub>U<sub>2</sub> (1): C, 39.06; H, 7.78; N, 9.49. Found: C, 39.22; H, 7.79; N, 9.30. ¹H NMR (400 MHz, toluene-<i>d</i><sub>8</sub>, 298 K): δ 10.95 (s, 12H, CH<sub>2</sub>), 7.21 (s, 12H, CH<sub>3</sub>), 3.84 (s, 54H, C(CH<sub>3</sub>)<sub>3</sub>), -20.24 ppm (s, 36H, Si(CH<sub>3</sub>)<sub>2</sub>) (Supplementary Fig. 9). The preparation of the <sup>15</sup>N<sub>2</sub>-labelled isotopologue, **1**<sup>-15</sup>N<sub>2</sub>, is described in Supplementary Information.

When the synthesis of **1** was performed under 2.21-bar N<sub>2</sub>, analytically clean **1** could be isolated in a better yield (compared to when performed under 1 atm (1.01 bar)) from a single crystallization. Once isolated, complex **1** is stable in the solid state under N<sub>2</sub>, but quickly releases N<sub>2</sub> under dynamic vacuum. Complex **1** (-8.9 mM concentration) in toluene-<i>d</i><sub>8</sub> and cyclohexane-<i>d</i><sub>12</sub> at 1.01-bar N<sub>2</sub> exist in equilibrium with [U<sup>III</sup>(Tren<sup>DMBS</sup>)] in an approximate 2:1 ratio (<sup>1</sup>H NMR, Supplementary Fig. 9). No increase in N<sub>2</sub> uptake was observed by <sup>1</sup>H NMR spectroscopy when lowering the solution temperatures to -80 °C. However, increasing the dinitrogen pressure to 2.51 bar leads to a further shift in **1**: [U<sup>III</sup>(Tren<sup>DMBS</sup>)] equilibrium, from 2:1 to 5:1 (Supplementary Fig. 12) at room temperature. In contrast, the <sup>1</sup>H NMR spectrum of the isolated crystalline **1** in THF-<i>d</i><sub>8</sub> showed predominantly [U<sup>III</sup>(Tren<sup>DMBS</sup>)] resonances, indicating that N<sub>2</sub> loss occurs in the presence of coordinating solvents (Supplementary Fig. 10). This was further corroborated by the <sup>15</sup>N NMR spectroscopy of **1**<sup>-15</sup>N<sub>2</sub>, where only the resonance for free <sup>15</sup>N<sub>2</sub> could be observed (Supplementary Fig. 14), with the absence of a resonance for **1**<sup>-15</sup>N<sub>2</sub> being attributed to its paramagnetism.

## Synthesis of [K(2.2.2-cryptand)][{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (2-crypt)

Under an atmosphere of dinitrogen, a solution of **1** (130.1 mg, 0.086 mmol, 1 equiv.) and 2.2.2-cryptand (32.5 mg, 0.086 mmol, 1 equiv.) in toluene (4 ml) was added to a Schlenk tube containing a glass-coated stir bar. Solid potassium graphite (11.7 mg, 0.086 mmol, 1 equiv.) was then added at room temperature, and the reaction was left stirring for 3 h to yield a dark-brown mixture. The volatiles were then removed in vacuo, and soluble residues were extracted into THF (4 ml) to facilitate graphite removal by filtration on a porosity '4' glass frit. Volatiles were then removed in vacuo from the filtrate, the residue dried for 1 h, and toluene (-2 ml) added. Storage of the brown solution at -40 °C for 2 days yielded **2-crypt·2C<sub>7</sub>H<sub>8</sub>O** as dark-brown crystals in 83% yield (135.6 mg, 0.072 mmol). **2-crypt** is readily soluble in THF and only modestly in toluene; the complex was found to be stable towards nitrogen loss in THF and toluene (no change in the <sup>1</sup>H NMR spectra after dissolution or a freeze-pump-thaw cycle). Elemental analysis: Calcd. for C<sub>69</sub>H<sub>154</sub>KN<sub>12</sub>O<sub>6</sub>Si<sub>6</sub>U<sub>2</sub> (2-crypt·0.5C<sub>7</sub>H<sub>8</sub>O): C, 43.08; H, 8.01; N, 8.67. Found: C, 43.04; H, 8.05; N, 7.93. <sup>1</sup>H NMR (400 MHz, 298 K, THF-<i>d</i><sub>8</sub>): δ 86.21 (s, 12H, CH<sub>2</sub>), 25.89 (s, 12H, CH<sub>2</sub>), 3.06-3.03 (m, 24H, 2.2.2-crypt), 2.04 (m, 12H, crypt), -13.02 (s, 54H, C(CH<sub>3</sub>)<sub>3</sub>), -33.64 (s, 36H, Si(CH<sub>3</sub>)<sub>2</sub>) (Supplementary Fig. 15a). <sup>29</sup>Si(<sup>1</sup>H) NMR (79.5 MHz, 298 K, THF-<i>d</i><sub>8</sub>): δ -181.81 (Supplementary Fig. 15b).

## Synthesis of [K{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (2-K)

Under an atmosphere of dinitrogen, solid potassium graphite (0.217 mmol, 29.3 mg, 3 equiv.), pre-chilled to -40 °C, was added to a cold solution of **1** (108.7 mg, 0.072 mmol, 1 equiv.) in *n*-hexane. The mixture was left to stir for 3 days at -40 °C, then filtered on a porosity '4' glass frit, also pre-chilled to -40 °C, and concentrated to 0.5 ml while cold. Dark-brown/green crystals of **2-K** were obtained from *n*-hexane after 2 days at -40 °C, in 62% yield (69.1 mg, 0.045 mmol). Elemental analysis: Calcd. for C<sub>51</sub>H<sub>121</sub>KN<sub>10</sub>Si<sub>6</sub>U<sub>2</sub> (2-K·0.5C<sub>6</sub>H<sub>14</sub>): C, 39.31; H, 7.83; N, 8.99. Found: C, 39.17; H, 7.78; N, 8.63. <sup>1</sup>H NMR (400 MHz, 233 K, toluene-<i>d</i><sub>8</sub>): δ 5.88 (s, br.), -21.20 (s, very br.), 83.58 (s, very br.) (Supplementary Fig. 16a). (193 K): -12.46 (s, very br.), -27.79 (s, very br.), -36.57 (s, very br.), -132.49 (s, very br.), -150.65 (s, very br.) (Supplementary Fig. 16b).

## Synthesis of [Li<sub>2</sub>{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-η<sup>2</sup>:η<sup>2</sup>-N<sub>2</sub>)] (3)

Under an atmosphere of dinitrogen, crystalline **1** (130.4 mg, 0.088 mmol, 1 equiv.) was combined with solid lithium iodide (23.7 mg, 0.177 mmol, 2 equiv.). At room temperature, diethyl ether (2 ml) was added, and the mixture was stirred until fully dissolved, before being stored at -40 °C for 1 h. The resulting cold mixture was added to cold (-40 °C) solid potassium graphite (23.9 mg, 0.177 mmol, 2 equiv.) in cold diethyl ether (1 ml) and stirred for 3 days at -40 °C before filtration on a porosity '4' glass frit pre-chilled to -40 °C to obtain a dark-brown/red solution. Concentration in vacuo to -1 ml and storage at -40 °C for 4 days yielded dark-red/brown crystals of **3** in 63% yield, obtained in several crops (82.9 mg, 0.056 mmol). Elemental analysis: Calcd. for C<sub>52</sub>H<sub>124</sub>Li<sub>2</sub>N<sub>10</sub>OSi<sub>6</sub>U<sub>2</sub> (3·1C<sub>4</sub>H<sub>10</sub>O): C, 39.93; H, 7.99; N, 8.96. Found: C, 39.56; H, 7.79; N, 8.65. <sup>1</sup>H NMR (400 MHz, 298 K, toluene-<i>d</i><sub>8</sub>): the spectrum shows paramagnetically shifted resonances spanning a wide range from -130 to 200 ppm, indicating the presence of a highly asymmetric structure in solution (Supplementary Figs. 18 and 19).

## Synthesis of [Li<sub>4</sub>(OEt<sub>2</sub>)<sub>2</sub>][U<sup>IV</sup>(Tren<sup>DMBS</sup>)}<sub>2</sub>(μ-N<sub>2</sub>) (4)

Under an atmosphere of dinitrogen, crystalline **1** (107.9 mg, 0.073 mmol, 1 equiv.) was combined with solid lithium iodide (58.8 mg, 0.439 mmol, 6 equiv.). At room temperature, diethyl ether (3 ml) was added and the mixture stirred until fully dissolved, before being stored at -40 °C for 1 h. The resulting mixture was added to cold solid potassium graphite (59.3 mg, 0.439 mmol, 6 equiv.) in diethyl ether (1 ml) and allowed to stir for 3 days at -40 °C before filtration on a cold porosity
'4' glass frit to obtain a dark-brown solution. Concentration in vacuo to -1 ml and storage at -40 °C for 3 days yielded dark-brown crystals of 4·1C₄H₁₀O in 56% yield (61.6 mg, 0.056 mmol). Elemental analysis: Calcd. for C₅₂H₁₂₄Li₄N₁₀OSi₆U₂ (4): C, 39.58; H, 7.92; N, 8.88. Found: C, 39.48; H, 7.89; N, 8.89. ¹H NMR (400 MHz, 298 K, toluene-d₈): the spectrum shows paramagnetically shifted resonances spanning a wide range from -135 to 150 ppm, indicating the inequivalence of many of the 114 protons present in the complex (Supplementary Figs. 22 and 23).

### Synthesis of [Li₃{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(µ-N)(µ-NH)] (5)

A dark-red/brown solution of 3 (37.1 mg, 0.025 mmol, 1 equiv.) in toluene (0.5 ml) was transferred to a J. Young valve-capped tube and connected to a Schlenk line. The resulting solution was degassed by three cycles of freeze-pump-thawing, and hydrogen gas (1.01 bar) was added to the reaction mixture and the tube warmed to room temperature. A gradual colour change to brown was observed over the course of 2 days as full consumption of the starting material occurred. Brown crystals of 5 were collected from the reaction mixture after leaving it to stand at room temperature (7.9 mg) for 3 days. An additional crop (4.4 mg) of crystalline 5 was isolated after 1 day upon placing the reaction mixture at -40 °C. Elemental analysis: Calcd. for C₄₈H₁₁₅Li₃N₁₀Si₆U₂ (5): C, 38.49; H, 7.74; N, 9.35. Found: C, 38.97; H, 7.75; N, 9.32. ¹H NMR (400 MHz, 298 K, toluene-d₈): the spectrum shows paramagnetically shifted resonances spanning a wide range from -170 to 170 ppm, including the NH resonance identified at -170.25 ppm (s, 1H, NH) (Supplementary Figs. 29 and 30). IR (KBr pellet, v/cm⁻¹): 3,313 (w, N-H), 2,953 (s, 2,929 (s), 2,883 (s), 2,852 (s), 2,735 (w), 2,705 (w), 2,672 (w), 1,605 (m), 1,468 (m), 1,405 (w), 1,388 (w), 1,357 (w), 1,246 (m), 1,133(w), 1,113 (w), 1,092 (m), 1,072 (m), 1,057 (m), 1,023 (m), 1,011 (w), 967 (m), 925 (s), 903 (m), 888 (s), 825 (s), 777 (s), 717 (m), 655 (s), 586 (m), 560 (m), 527 (m), 505 (m) (Supplementary Fig. 127).

A second species observed in the ¹H NMR spectrum (Supplementary Figs. 26 and 27) of the mother liquor measured in toluene-d₈ could not be isolated despite repeated attempts.

### Synthesis of [Li₃{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(µ-N)(µ-ND)] (5-D₂) through the reaction of 3 with D₂ gas

A dark-red/brown solution of crystalline 3 (37.1 mg, 0.025 mmol, 1 equiv.) in toluene (0.3 ml) was transferred to a J. Young valve-capped NMR tube and connected to a Schlenk line. The resulting solution was degassed by three cycles of freeze-pump-thawing and deuterium gas (0.5 bar absolute pressure of D₂) was added to the reaction mixture and the tube was warmed to room temperature. An immediate colour change from dark-red/brown to brown was observed. After 1 h, all volatiles were removed in vacuo and toluene-d₈ (0.3 ml) was added. Dark-brown crystals of 5-D₂ (6.6 mg) were isolated from the reaction mixture after 2 days at -40 °C. ¹H NMR (400 MHz, 298 K, toluene-d₈): the spectrum of the reaction mixture as well as the one of isolated crystalline 5-D₂ showed the full set of resonances assigned to 5 (Supplementary Figs. 31-33), except for the broadened NH singlet at -170.25 ppm (Supplementary Fig. 32), which is not present in the deuterated mixture. IR (KBr pellet, v/cm⁻¹): 2,953 (s), 2,928 (s), 2,883 (s), 2,851 (s), 2,704 (w), 2,452 (w, N-D), 1,468 (m), 1,444 (w), 1,387 (w), 1,357 (w), 1,288 (m), 1,133 (w), 1,111 (w), 1,091 (m), 1,056 (m), 1,024 (m), 968 (m), 924 (s), 902 (m), 889 (s), 825 (s), 779 (s), 717 (m), 654 (s), 584 (m), 526 (w), 505 (w), 484 (w) (Supplementary Fig. 127).

### Reaction of 1, 2-crypt, 3 and 4 with excess HCl

To J. Young valve-capped NMR tubes containing solid complexes 1 to 4, a solution of HCl (2 M, 2.0 ml in total) in diethyl ether was added at -80 °C in a glovebox-fitted cold well. On warming each tube to -40 °C and then subsequently to room temperature, discolouration of the solids occurred, and formation of light-yellow solutions was observed. After 1 h, all volatiles were removed in vacuo and the resulting solids were dissolved in dimethylsulfoxide (DMSO)-d₆ (0.5 ml, 2,5-Dimethylfuran

was added to the tubes directly or in the form of an NMR insert (sealed quartz capillary) as an internal standard for the quantitative detection of NH₄Cl. Formation of 0.18 equiv. of NH₄Cl for 1, 0.43 equiv. for 2-crypt, 1.57 equiv. for 3 and 2.0 equiv. for 4 were detected by quantitative ¹H NMR experiments (Supplementary Figs. 36-39). Under similar experimental conditions, the parent monomeric complex [U<sup>IV</sup>(Tren<sup>DMBS</sup>)Cl] produced 0.03 equiv. of NH₄Cl.

### Acidification of [{U<sup>IV</sup>(Tren<sup>DMBS</sup>)}₂(µ-η²:η²-N₂)] under a ¹⁴N₂ atmosphere

Under an atmosphere of Ar, bulb A was charged with 1 (0.0052 mmol), the acid and reductant, along with a glass-coated stirrer bar. Bulb A was then sealed and transferred to a Schlenk line. Et₂O (stored under Ar, 4-8 ml, vide infra) was added to bulb B via syringe, and then bulb B was submerged in a LN₂ dewar (-196 °C) and frozen. The entire apparatus was placed under a dynamic vacuum (1 × 10⁻³ mbar) and sealed, leaving the apparatus under static vacuum. The LN₂ dewar (-196 °C) was removed from bulb B and immediately transferred to bulb A such that the frozen solvent in bulb B could thaw and distil onto the frozen solids in bulb A. After the distillation had concluded, the apparatus was placed under a dinitrogen atmosphere, sealed, and allowed to warm to room temperature. For reactions involving MC₈ (M = K, Rb, Cs) as the reductant, the reaction mixture was stirred for 17 h. In the case of the alkali metals, the reaction mixtures were subsequently sonicated until the metal was finely dispersed (1-40 min) and the reactions were stirred for 72 h. After this time, HCl (2 M in Et₂O, 2 ml) was added to bulb B via syringe and both bulbs were subsequently submerged into separate LN₂ (-196 °C) dewars and frozen. The entire distillation apparatus was then placed under a dynamic vacuum and sealed, leaving the vacuum under static vacuum, and the LN₂ dewar removed from bulb A. During this process, the reaction mixture in bulb A thaws, and the volatiles were distilled onto the frozen HCl in bulb B. Once the acid distillation was complete (-30 min), bulb A was resubmerged in a LN₂ (-196 °C) dewar and the contents frozen. Bulb B was sealed, and aqueous KOH (30%, 4 ml) was added via syringe to bulb A under a flow of dinitrogen. Once the contents had frozen, the entire apparatus was once again placed under a dynamic vacuum and sealed, leaving the apparatus under static vacuum, and the LN₂ (-196 °C) dewar removed from bulb A. The base distillation was performed with vigorous stirring for 1 h, then both bulbs A and B were sealed, and bulb B was allowed to warm to room temperature and stirred for 10 min. All volatiles in bulb B were subsequently removed in vacuo and the remaining residue analysed for NH₃/NH₄Cl and N₂H₄/N₂H₄·2HCl via ¹H NMR spectroscopy (Supplementary Figs. 40, 45-56, 58, 61-73) and pdmab methods (vide infra). As previously reported, N₂H₄ is only partially transferred under these conditions, and heating the distillation mixture should be avoided because N₂H₄ undergoes thermal decomposition to NH₃, N₂ and H₂. Thus, after the distillation, the solids remaining in bulb A were also analysed for N₂H₄/N₂H₄·2HCl via the pdmab method (Supplementary Figs. 78, 79, 82-105, 108-119). The acidification of 1-¹⁵N₂ under a ¹⁵N₂ atmosphere is described in Supplementary Information.

### Data availability

The X-ray crystallographic coordinates for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition nos. CCDC 2377328 (2-crypt), 2355682 (2-K), 2377329 (2-Li), 2377330 (3), 2377331 (4) and 2377332 (5). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre (www.ccdc.cam.ac.uk/data_request/cif). All other data are presented in the main text and Supplementary Information, and are also available from the corresponding authors on reasonable request. The data that support the findings of this study are openly available in the Zenodo repository at https://doi.org/10.5281/zenodo.15309919 (ref. 87).
## References

87. Batov, M. S. et al. Catalytic and stoichiometric stepwise conversion of side-on bound dinitrogen to ammonia mediated by a uranium complex. Zenodo https://doi.org/10.5281/ zenodo.15309919 (2025).

## Acknowledgements

We acknowledge support from the Swiss National Science Foundation (grants nos 212723 and 236428), the UK Engineering and Physical Sciences Research Council (EP/M027015/1, EP/T011289/1 and EP/W029057/1), Ecole Polytechnique Fédérale de Lausanne, The University of Manchester including the Computational Shared Facility, and the UK National Nuclear Laboratory. This work is part of a larger project, which is funded in part by UK Research and Innovation, through the EPSRC Centre for Doctoral Training in Integrated Catalysis (EP/S023755/1). The Alexander von Humboldt Foundation is thanked for a Friedrich Wilhelm Bessel Research Award to S.T.L. We thank A. Sienkiewicz (EPFL) for his assistance with EPR data collection for **1**, **2-crypt** and **2-K**, N. Jori (EPFL) for stimulating discussions regarding the acidification methodologies, F. Menoud (EPFL) for carrying out the elemental analyses (EPFL) and F. Fadaei-Tirani (EPFL) for important contributions to the X-ray single-crystal structure analysis of **2-K**.

## Author contributions

M.S.B. prepared and characterized all the compounds, performed preliminary acidification trials, as well as several catalytic trials and

product analyses. L.C. performed preliminary synthetic experiments. H.T.P., J.A.S. and R.W.A. undertook aspects of the syntheses, performed spectroscopic analysis, and performed the catalysis and product analysis. R.S. collected and refined the crystallographic data. I.Z. and M.S.B. performed the magnetic measurements and analysed the magnetic data. S.T.L. performed and analysed the quantum-chemical calculations. M.M. and S.T.L. conceived and supervised the work and analysed the results. M.S.B., S.T.L. and M.M. wrote the paper with contributions from all the authors.

## Competing interests

The authors declare no competing interests.

## Additional information

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41557-025-01867-z.

**Correspondence and requests for materials** should be addressed to Stephen T. Liddle or Marinella Mazzanti.

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