Multinuclear uranium-nitrogen cluster formation from exploration of organouranium-ammonolysis

Abstract

Uranium-nitrogen compounds (U_xN_y) pose interesting structural and electronic questions. Understanding the formation and speciation of U_xN_y may provide insights in nitrogen fixation as well as the preparation of potential uranium nitride nuclear fuels, a promising Gen IV nuclear reactor fuel. Small molecular clusters offer the opportunity to study U_xN_y using solution and dynamic spectroscopies. Herein, we have explored a new method for  the synthesis of multinuclear uranium-nitrogen clusters containing uranium-nitride, -imide, and -amide moieties in excellent yield. Ammonolysis of (C₅Me₅)UBn₃ gives a mixture of the tetra- and hexa-nuclear clusters (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(μ-NH₂)₇, 1, (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3. Compound 1 was readily separated from 2 and 3, but compounds 2 and 3 display similar solubilities. Various reaction conditions, including changing the stoichiometry, reaction time, solvent free conditions, or the use of an ammonia/dioxane solution, did not alter the identity of what clusters were produced. Only small variations in the amount of each cluster produced were observed. Once formed, the clusters do not interconvert. The clusters were characterized by single crystal X-ray diffraction as well as ¹H NMR, UV-vis-NIR, and IR spectroscopies.

Uranium–nitrogen compounds offer potential advancements in nitrogen fixation and uranium nitride nuclear fuels. Here, the authors develop a synthetic approach to multinuclear uranium–nitrogen clusters, achieving excellent yields and providing valuable insights into their structural and electronic properties, with implications for Gen-IV nuclear reactor fuels.

Introduction

With rising interest in nuclear power as a reliable and carbon-free form of energy generation, there is a desire for new reactor and fuel designs. Uranium nitride (UN) has been touted as a promising fuel for advanced Generation IV (Gen IV) nuclear reactors^1–7. UN is desirable because it exhibits higher thermal conductivity, higher fissile density, and lower thermal expansion than the traditionally used UO₂ fuel. UN is an insoluble binary ceramic, however, it is easily dissolved in nitric acid, making it highly compatible with mature PUREX reprocessing schemes⁸. Use of UN fuel can lead to enhanced efficiency by increasing fuel burnup, fuel cycle efficiency, and reduced fuel fabrication costs^2,9,10. However, some challenges remain that limit the high quality production of UN¹¹. While other uranium-nitrogen (U_xN_y) solid state and molecular compounds are known and have a more academic interest^2,3,12, their decomposition could offer a novel method of preparing UN^11,13.

In a different vein, Fritz Haber’s foundational 1916 nitrogen fixation work reportedly used uranium, among other metals¹⁴. This work has recently inspired growing interest in the study of molecular U_xN_y complexes with considerable progress toward the isolation and characterization of mono- and polynuclear uranium–nitrogen complexes^12,15–17. Generally, U_xN_y complexes have been pursued by one of two routes, the capture of N₂ by low valent uranium complexes^18–21, or the liberation of N₂ from an azido reagent^22–29. Both of these schemes often make use of an added external reductant^12,17. In many cases, intermediate (N₂)ⁿ⁻ complexes have been isolated^21,30. However, the reactive nature of uranium-nitride moieties has often led to ligand activation^31–34. Also, there is often a focus on uranium-ligand multiple bonding, in particular for monometallic complexes^23,35–37.

The first molecular uranium–nitride complex was described in 2002. The reduction of (DME)K(μ-L)U^III(DME) (L = Et₈-calix[4]tetrapyrrole; DME = 1,2-dimethoxyethane) with K(C₁₀H₈) in DME under an N₂ atmosphere gave the mixed valent [K(DME)₄]{[(DME)K(μ-L)U^IV/V]₂(μ-NK)₂}³⁸. Three years later, a seminal report discussed the preparation of an alternating uranium nitride-azide complex [(C₅Me₅)₂U(μ-N)(C₅Me₅)₂U(μ-N₃)]₄ (Fig. 1a)²². In the years following, various other ligand sets began to be explored. To date, there has been great success from the bulky [OSi(O^tBu)₃]^1– siloxide ligand, where mono- and bis-nitride complexes have been isolated, typically as diuranium complexes^31,39–43. Where other successful ligand sets have included (silyl)amide/TREN^18,44–48 and aryl oxide ligands (Fig. 1)^21,49.

Fig. 1. Summary of uranium-nitrogen cluster syntheses.

Progress in multinuclear uranium-nitride complexes a an octanuclear uranium-nitride-azide ring b a trinuclear uranium-nitride-imide complex c tetra-/hexa-nuclear uranium nitride cubanes d clusters reported here produced through ammonolysis.

Efforts to use ammonia as a nitrogen source in organouranium chemistry are scarce, with the amido complex [C₈H₆(SiⁱPr₃)₂](C₅Me₅)U(NH₂) among the few examples⁵⁰. Other examples include the use of NaNH₂ and NH₄Cl to synthesize the thorium-nitride complexes [M(18-crown-6)L_x][(R₂N)₃Th]₂(μ-N) (M = Na, K; L = Et₂O, THF; x = 1, 2)^51,52. We have sought to use the pentamethylcyclopentadienide ligand for solubility and crystallinity, combined with a strongly basic internal alkyl ligand to deprotonate NH₃. The pentamethylcyclopentadienyl(trisbenzyl)uranium(IV), (C₅Me₅)UBn₃ (Bn, -CH₂C₆H₅)^53,54, is highly soluble in organic solvents and would be an excellent candidate for protonolysis. Such chemistry has been explored with titanium, where the reaction of (C₅Me₅)TiMe₃ or (C₅Me₅)Ti(NMe₂)₃ with excess NH₃ gives (C₅Me₅)₃Ti₃(μ₃-N)(μ-NH)₃ and (C₅Me₅)₄Ti₄(μ₃-N)₄, respectively^55,56. Herein, we sought to synthesize and characterize the product(s) from the reaction of (C₅Me₅)UBn₃ with ammonia in varying stoichiometries and conditions. The resulting products were characterized by ¹H NMR, IR, and UV-vis-NIR spectroscopies, as well as single crystal X-ray crystallography.

Results and discussion

Synthesis

(C₅Me₅)UBn₃ (Bn, -CH₂C₆H₅) was synthesized in moderate yield (55%) by a slight modification of the literature and confirmed by ¹H NMR spectroscopy^53,54, see the Supplementary Information for details (Supplementary Figs. 1 and 2). With a viable starting material in hand, we began to explore its reactivity with ammonia. When a black solution of (C₅Me₅)UBn₃ in C₆D₆ was reacted with an excess (1 atm) of NH₃ (g) in a J-Young NMR tube, an immediate colour change to red occurred. The resulting spectrum displayed evidence of toluene and no evidence of the (C₅Me₅)UBn₃ starting material. New resonances at δ 3.12 and −1.98 ppm with peak intensity ratios of 1:10 were observed (Supplementary Fig. 9).

When this reaction was scaled up and performed in toluene, a similar colour change from black to red was observed. Removal of the volatiles under reduced pressure yielded a reddish-brown residue. Workup inside the glovebox yielded a bronze coloured precipitate and a reddish-brown filtrate. The ¹H NMR spectrum of the precipitate in THF-d₈ indicated a primary resonance at δ −1.68 ppm (Supplementary Fig. 3). Storage of a concentrated THF solution diffused with pentane vapour at −35 °C for three days yielded orange plate-like crystals, suitable for X-ray diffraction. The molecular structure was revealed to be a tetranuclear uranium nitride-imide-amide cluster, (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, 1, Eq. 1. Compound 1 was typically isolated in ca. 55% crystalline yield or ca. 74% as isolated yield, based on (C₅Me₅)UBn₃. An Evans’ method experiment^57,58 (Supplementary Fig. 4) gave an ambient temperature (296 K) magnetic moment of 5.35 μ_B for the entire cluster. A measured magnetic moment of 1.34 μ_B/U would be rather low for any U³⁺, U⁴⁺, or even U⁵⁺ ion. Further, the oxidation states of uranium are known to be difficult to differentiate at ambient temperature⁵⁹. The low magnetic moment for the cluster suggests that some U-U antiferromagnetic (AF) interactions may be present. U-U antiferromagnetic coupling is not commonly observed, is often weak, and may be difficult to parse from crystal field effects^60,61. U-U AF interactions are most commonly reported for U(V) species, with some reports of U(IV) species, typically as bimetallic systems⁶². Of note, several of the systems that report U(IV)-U(IV) AF coupling feature nitrogen-rich coordination environments, and often U-N multiple bonding^47,63–65.

1

A ¹H NMR spectrum of the toluene-soluble portion in C₆D₆ suggested the presence of two products. The spectrum displayed two major paramagnetic resonances at δ −1.92 and 6.31 ppm (Supplementary Fig. 7). The reddish-brown filtrate was dried, redissolved in a minimal amount of toluene, and stored at ambient glovebox temperature for crystallization. Over a week, X-ray quality crystals of two different morphologies were produced, dichroic green-purple needles and dark orange-brown pyramids, separated a la Pasteur. The structures were revealed to be the hexanuclear uranium clusters, (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3, Eq. 1. Experiments with differing reaction times, e.g., 30 min, 2 h, overnight, etc., gave similar results. Despite our best efforts, compounds 2 and 3 have been inseparable; however, through repeated crystallizations, it is possible to obtain 3 with small amounts of 2 present. The crystalline yield of a mixture of compounds 2 and 3, relative to the starting material, was ca. 15–20%, combined, in an approximate 1:1 ratio, based on relative ¹H NMR integration. Taken as a whole, there is a good yield of uranium atoms despite the mixture of products.

A solvent-free gas-solid reaction was tested to see if solution-phase dynamics played a role in the formation of the clusters 1, 2, and 3⁶⁶. When a flask containing solid (C₅Me₅)UBn₃ was evacuated and refilled with 1 atm of NH₃ (g) a reddish-brown paste formed immediately, presumably due to the small amount of toluene formed in the reaction. Upon evacuation, the paste turned to a powder and was worked up in a manner similar to that given above. Compound 1 was confirmed by X-ray diffraction, and a mixture of compounds 2 and 3 was identified by ¹H NMR spectroscopy in an approximate 2:1 ratio (Supplementary Fig. 10). These efforts suggested that compounds 1, 2, and 3 are consistently formed in the presence of excess ammonia.

Next, we sought to explore the impact of ammonia stoichiometry or the presence of coordinating solvents on product formation. Therefore, (C₅Me₅)UBn₃ was reacted with 1.5, 2.0, or 3.0 equivalents of NH₃ (g) or a 0.4 M NH₃/dioxane solution. When 2.0 equivalents of NH₃ (g) were reacted with a toluene solution of (C₅Me₅)UBn₃, an immediate colour change to reddish-brown was observed, typical of the reaction with excess ammonia. The ¹H NMR spectrum of the crude reaction mixture in C₆D₆ displayed resonances corresponding to complexes 2 and 3 in a 2:1 ratio (Supplementary Fig. 11). Complex 1 was identified by X-ray diffraction. When a toluene solution of (C₅Me₅)UBn₃ was treated with 1.5 equivalents of NH₃ (0.4 M NH₃ in 1,4-dioxane), the crude ¹H NMR spectrum in C₆D₆ displayed the presence of 2 and 3 in a 1:2 ratio (Supplementary Fig. 12). Upon workup, compound 1 was also identified by X-ray diffraction. Similarly, treatment of (C₅Me₅)UBn₃ with 2.0 or 3.0 equivalents of NH₃ in 1,4-dioxane also produced mixtures of 2 and 3 in an ~3:1 ratio, as determined by ¹H NMR spectroscopy (Supplementary Fig. 13). Compound 1 was identified by ¹H NMR spectroscopy and X-ray diffraction.

Finally, it was of interest to test whether the complexes exhibit dynamic behaviour or interconvert. This was studied using several strategies: samples under an ammonia atmosphere, samples that had been isolated and were redissolved, and by exposing a mixture of 2 and 3 to vacuum (ca. 10⁻³ torr) for 4 h. No change in the 2:3 ratio was observed by ¹H NMR spectroscopy over the course of a week for any method. This lack of interconversion may be surprising, as all of the clusters are related by the number of nitrogen or uranium atoms, and suggests the energy separation between them may be rather small. Future work may include a better understanding of the lack of preference for the formation of the different clusters. In summary, the reaction of (C₅Me₅)UBn₃ with ammonia in any stoichiometry consistently produced a mixture of compounds 1, 2, and 3 regardless of reaction time, and the presence or absence of non-polar/coordinating solvent. Although the exact ratio of the compounds varied slightly with the conditions, no condition appeared to exclusively produce one product or prevent the formation of another.

Single crystal structures

Compound 1, (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, crystallized as orange plates from THF diffused with pentane at −35 °C. The crystals were characterized with orthorhombic symmetry in the Fddd space group. The asymmetric unit cell contains one [(C₅Me₅)U]³⁺ unit connected with one central nitrogen atom and three peripheral nitrogen atoms. When grown, the molecular unit forms a tetrametallic cluster (Fig. 2). A distorted tetrahedral (τ₄ = 0.95) nitrogen atom, N_int, sits at the centre of the cluster, bound to all four uranium atoms. The U-N_int bond distance is 2.238(2) Å and the U-(N_int)-U angles are 102.621(8)°, 111.652(9)° and 114.364(9)°. Each uranium atom sits in a distorted pseudo-octahedral geometry, with the (C₅Me₅)-centroid (cent) and the central nitrogen atom on the axial positions, and four additional external nitrogen atoms forming an equatorial plane. All of the external nitrogen atoms bridge (μ-N_ext) between two uranium atoms. We note that one nitrogen, N4a, is a mixture of an amide (–NH₂) and an imide (–NH), such that a shorter bond length, 2.397(4) Å, was observed, compared to those of N2a and N3a, 2.487(4) and 2.464(4) Å, respectively. This imide/amide mixture is due to the crystallographic symmetry rather than a chemical difference. Where the shorter U-N4a bond is more in line with a uranium-imido bond, the U-N2a and U-N3a bonds are typical of uranium amidos, see Table 1 for comparisons. This was not observed in the 1-d₁₅ analogue, where the U-imido and U-amido were observed separately, see below. The U-cent distance is 2.501(2) Å, and the uranium atom sits 0.681 Å above the N_ext plane with U-(N_ext)-U angles of 89.23(2)°, 99.5(2)°, and 101.2(2)°. The distance between any two uranium atoms was found to be 3.494(3) and 3.704(3) Å.

Fig. 2. Molecular structure of compound 1.

Thermal ellipsoid plot of (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, 1, drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Only non-carbon atoms have been labelled, with atom colour coding uranium (green), nitrogen (blue), and carbon (black).

Table 1.

Bond length (Å) comparisons of 1, 2, and 3, with related literature compounds

Compound	U-Cp*cent	U-N (nitride)	U-NH (imido)	U-NH2 (amido)
Dinuclear
{[UIV(THF)4]2(μ-N)(μ-NH2)2}{BPh4}375		2.062(3), 2.018(3)		2.449(3), 2.455(3)
Trinuclear
K2{[UIV/V(N3P2) (μ-NH)]3(μ-N)}16		2.211(5)–2.222(5)	2.188(6)–2.231(5)
[Cp*UIV(μ-I)2]3(µ3-N)25	2.455–2.462	2.138(3)–2.157(3)
Tetranuclear
[(Et2O)K]2[(Et2O)UIV4(μ-N)4(OArttt)6]49		2.169(3)–2.258(3)
[K6{(OSi(OtBu)3)2UIV}3 {(OSi(OtBu)3)2UVI}(μ4-N)3(μ3-N)(μ3-O)2]74		2.183(7)–2.319(7)
[(U(BIMA)2)2(μ-N)(μ-NiPr)(K2(μ-η3:η3-CH2CHNiPr)]268		2.101(6), 2.132(5)
Cp*4UIV4(µ4-N)(μ-NH)(µ-NH2)7, 1	2.501(2)	2.238(2)	2.397(4)	2.464(4)–2.487(4)
Cp*4UIV4(µ4-N)(μ-ND)(µ-ND2)7, 1-d15	2.497(3), 2.506(3)	2.231(2), 2.240(2)	2.359(2)	2.400(2)–2.504(2)
Hexanuclear
K2[UIV6(μ-N)6(OArttt)8]49		2.156(4)– 2.420(5)
[(Et2O)K]2[(Et2O)UIV4(μ-N)4(OArttt)6]49		2.169(3)–2.258(3)
Cp*6UIV6(µ6-N)(µ3-NH)7(µ3-NH2), 2	2.502(3)–2.506(5)	2.452(8)–2.483(6)	2.330(2)–2.368(2)	2.372(2)
Cp*6UIV6(µ3-N)2(µ-NH2)12, 3	2.497(3)–2.508(4)	2.191(6)–2.278(2)		2.240(7)–2.357(5)
Octanuclear
[Cp*2UIV(μ-N)U(μ-N3)Cp*2]422	2.488–2.528	2.047(6)–2.090(8)

Cp* = C₅Me₅; OAr^ttt = OC₆H₂-^tBu₃-2,4,6; BIMA = N,N-bis(iso-propyl)methylamidinate.

Pyramid shaped dark orange-brown crystals of 2, (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, crystallized from toluene at ambient glovebox temperature. The crystals were characterized with monoclinic symmetry in the I2/m space group. The asymmetric unit contains two [(C₅Me₅)U]³⁺ units bridged by three nitrogen atoms (µ-N) and another nitrogen atom is bound to one uranium centre. When the structure is grown, the molecular unit forms an octahedron with a [(C₅Me₅)U]³⁺ at each vertex, one internal (μ₆-N_int), and eight (μ₃-N_ext) units (Fig. 3). Where two [(C₅Me₅)U]³⁺ units form the axial (U_ax) positions with (C₅Me₅)¹⁻ disorder and the other four [(C₅Me₅)U]³⁺ units form the equatorial (U_eq) positions. The U-cent distances are 2.5118(6) Å (U_ax) and 2.5247(8) Å (U_eq) with U_ax-(N_int) and U_eq-(N_int) distances of 2.4522(8) and 2.4834(6) Å, respectively. The U-(N_int)-U angles range from 89.68(3) to 90.32(3)°. The uranium centres can be described as distorted pseudo-octahedral geometries, with the uranium atom sitting 0.926 Å to 0.965 Å above the N_ext plane. The periphery (μ₃-N_ext) atom distances range from 2.330(2) to 2.372(2) Å with U-(N_ext)-U angles ranging from 95.1(7) to 96.2(5)°. In this case, the U-(N_ext) bonds all fall in a somewhat narrow range, 2.330(2)–2.372(2) and are more typical of U-imido bonds. For charge balance, the longest U-(N_ext) bond is assigned as a U-amido. Due to the high symmetry of the compound and the possibility for a single hydrogen atom to be hydrogen bonded between two nitrogen atoms, the exact identity of a single amido was difficult to assign. The distances between any two uranium atoms range from 3.4898(2) to 3.4904(9) Å.

Fig. 3. Molecular structure of compound 2.

Thermal ellipsoid plot of (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, drawn at the 30% probability level. Hydrogen atoms, disorder, and lattice solvent are omitted for clarity. Only non-carbon atoms are labelled, atom colour coding uranium (green), nitrogen (blue) and carbon (black).

Concurrently with the isolation of 2, dichroic green-purple needles of (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3, crystallized from toluene at ambient glovebox temperature. The crystals were characterized with monoclinic symmetry in the I2/m space group. Although compounds 2 and 3 were characterized in the same space group, the unit cell of 3 is ca. 10% larger than that of 2 (Supplementary Table 2). The asymmetric unit of 3 contains two [(C₅Me₅)U]³⁺ units bridged by two nitrogen atoms, with three more nitrogen atoms bound to a single uranium centre. When the structure is grown, the molecular unit forms a highly distorted octahedron with a [(C₅Me₅)U]³⁺ at each vertex, two internal (μ₃-N_int), and 12 (μ-N_ext) units (Fig. 4). Where there are two axial (U_ax) positions and four equatorial (U_eq) positions. The U-cent distances are 2.5078(4) (U_ax) and 2.4973(3) Å (U_eq). The (μ₃-N_int) atom sits approximately equally between three uranium centres and distorts the geometry away from octahedral. The two (μ₃-N_int) atoms are well separated by 2.50(2) Å. The U-(μ₃-N_int) distances range from 2.191(6) to 2.278(2) Å and the U-(μ-N_ext) distances range from 2.240(7) to 2.357(5) Å. In this case, the two N_int nitrides are balanced by the 12-N_ext amides. The distances between any two uranium atoms range from 3.7108(6) to 3.809(5) Å. Where visually complex 2 appears to be a well-formed octahedron, complex 3 could be viewed as the dimerization of two [(C₅Me₅)U]₃(μ₃-N)(μ-NH₂)₆ units.

Fig. 4. Molecular structure of compound 3.

Thermal ellipsoid plot of (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3, drawn at the 50% probability level. Hydrogen atoms and disordered lattice solvent are omitted for clarity. Only non-carbon atoms are labelled, with atom colour coding uranium (green), nitrogen (blue), and carbon (black).

The bond metrics of compounds 1, 2, and 3 compare well with available compounds from the literature. The 2.497 to 2.525 Å U-cent distances are similar to the 2.455 to 2.528 Å U-cent distances of several other (C₅Me₅)_xU-N (x = 1, 2) compounds (Table 1 and Supplementary Table 1), supporting the assignment as all compounds containing U⁴⁺ ions. While the 2.240(7) to 2.487(4) Å U-(μ-NH₂) distances are similar to the 2.407(3) to 2.463(2) Å distances of several different (L₃U-NH₂)_x [L = OSi(O^tBu); THF; x = 2] complexes (Table 1 and Supplementary Table 1). The 2.330(2) to 2.397(4) Å U-(μ₃-NH) distances are little longer than 2.179(5) to 2.276(2) Å reported for other U-(μ-NH) bonds reported with different ligands (Table 1 and Supplementary Table 1). The U-(μ₄-N_int) distance in 1 was found to be 2.238(2) Å whereas the U-(μ₆-N_int) bond distances in 2 were 2.452(8) and 2.483(6) Å (Table 1).

Infrared spectroscopy and studies with ND₃

The ¹H NMR spectra of 1, 2, and 3 displayed resonances readily attributable to the (C₅Me₅)^1– ligand. Despite recording ¹H NMR spectra with large (δ + 500 to −500 ppm) windows, resonances associated with the -NH and -NH₂ moieties were not observed. This is likely due to paramagnetic shifting and broadening. Seeking clarity, reactions with ND₃ were performed. The reaction of (C₅Me₅)UBn₃ with 1 atm of ND₃ in C₆D₆ showed the presence of C₆H₅CDH₂ in ¹H spectra (Supplementary Figs. 14–15). Outside of resonances associated with the solvent, C₆H₅CDH₂, or the known (C₅Me₅)¹⁻ resonances, no other resonances in the range of δ + 700 to −700 ppm were observed in the ¹H or ²H spectra. The d-labelled clusters did not exchange with the (C₅Me₅)^1– protons over the course of weeks. Crystals of d-labelled 1, (C₅Me₅)₄U₄(μ₄-N)(μ-ND)(μ-ND₂)₇, 1-d₁₅ (Supplementary Fig. 22) were obtained and refined with similar metrics to 1. However, 1-d₁₅ was characterized in the Fdd2 space group. In this case, the structure is more ordered, and the amide/imide disorder can be cleanly separated, see the Supplementary Information for further details.

The IR spectra of compounds 1 and 1-d₁₅ were recorded on a diamond ATR spectrometer as a THF solution and as a solid. The spectra of solid and solution phase 1 were almost identical except for some small deviations. The spectra of 1 and 1-d₁₅ are nearly identical, with no major shifting in the stretches (Supplementary Fig. 16). In some cases, ν(U-NH/NH₂) stretches have been identified in the region of 3380 to 3450 cm^–116,67, but this is not always observed, as in the case here of complexes 1-3^31,68. All spectra display a broad band centred at ~2900 cm⁻¹, corresponding to the νC-H (C₅Me₅) stretch. (Supplementary Fig. 6). Similar features were observed in a mixture of 2 and 3 (2/3) as a solid (Supplementary Fig. 8). The 2/3 mixture displays two additional features centred at ~3300 cm⁻¹, which may correspond to ν(NH/NH₂) stretches^16,67.

UV-vis-NIR spectroscopy

The UV-vis-NIR spectra of 1 in THF and a mixture of 2 and 3 (2/3) in toluene were recorded at ambient temperature (ca. 20 °C). The spectrum of 1 indicates three main absorption peaks in the visible region and several weak f-f transitions in the NIR region (Fig. 5, black trace). The UV and visible regions are dominated by a strong charge transfer (CT) band. The spectrum of the 2/3 mixture shows only one intense CT absorption in the visible region and several weak absorptions in the NIR region, consistent with LaPorte forbidden 5f → 5f transitions (Fig. 5, red trace). Although it is difficult to assign specific absorptions to compound 2 vs. 3, there are some notable differences between 1 and the 2/3 mixture. In compound 1, there are several absorptions in the 650 < λ <1000 nm (16,000 > ν >11,000 cm⁻¹) region not present in the 2/3 mixture. In the NIR region, both spectra display three broad absorptions that are well defined in the 2/3 mixture and poorly defined in compound 1. The spectra appear to be in line with their assignment as U^IV ions, as compared to other (C₅Me₅)-based organouranium complexes^69–71.

Fig. 5. Electronic absorption spectra of 1 and 2/3.

UV-vis-NIR spectra of (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, 1, in THF (black trace, left axis) and a toluene solution of a 2/3 mixture, (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3, (red trace, right axis).

Attempted reduction of UNs with KC₈

To test if the cluster complex 1 was readily reduced, a preliminary reaction with KC₈ was examined. Freshly prepared KC₈ (~2.0 equiv.) was added to complex 1 in THF, resulting in an initial colour change from red to brown. However, the UV-vis-NIR spectrum was nearly identical to that of authentic 1, with a few small deviations (Supplementary Fig. 17). After recording the spectrum, the sample appeared visually the same as before the reaction. Only the starting material 1 was recovered in good yield when bulk reductions were performed at ambient and reduced (−30 °C) temperatures. It was also of interest to determine whether compounds 2 and 3 could be separated/differentiated using reduction. When an approximate 1:99 mixture of 2 and 3 (as characterized by relative ¹H NMR integration) was treated with freshly prepared KC₈ (~2.0 molar equiv.) in THF, the colour of the solution immediately turned from brown to a reddish-brown. ¹H NMR spectroscopy of the residue obtained from the filtrate in C₆D₆ showed a minor change in spectra with a few new resonances at δ −5.59, −6.16, and −12.21 ppm (Supplementary Fig. 18). However, no products were identified beyond the starting clusters 2 and 3. At this time, the cluster complexes 1, 2, and 3 do not appear to be readily reducible by reaction with KC₈. However, work is ongoing to find suitable reductants.

Reaction of (C₅Me₅)UBn₃ with BH₃•THF

To test the generality of (C₅Me₅)UBn₃ to function as a strong base, the reaction with isostructural BH₃ was examined. When 2.5 equiv. of BH₃•THF was added to a black solution of (C₅Me₅)UBn₃, the colour immediately turned red. After 15 min, a ¹H NMR spectrum in C₆D₆ showed no presence of (C₅Me₅)UBn₃, along with numerous new resonances. Extraction with toluene followed by layering the toluene solution with pentane at −35 °C, yielded red-orange needle shaped X-ray quality crystals. The structure was revealed to be the THF ring opened product [(C₅Me₅)U(OⁿBu)₂(THF)₃][BBn₄], 4 (Eq. 2 and Fig. 6) in low yield. Where four benzyl groups have been transferred to the boron and two THF molecules have been ring opened, presumably with a hydride transfer from borane to give (OⁿBu)¹⁻. Such ring opening of THF is occasionally observed at U(IV) and U(III) centres^72,73.

2

Fig. 6. Molecular structure of compound 4.

Thermal ellipsoid plot of [(C₅Me₅)U(OⁿBu)₂(THF)₃][BBn₄], 4, drawn at the 50% probability level. Hydrogen atoms, disorder, and lattice solvent are omitted for clarity. Only non-carbon atoms are labelled, atom colour coding uranium (green), oxygen (red), carbon (black), and boron (yellow).

Compound 4 was characterized in the $P\overline{1}$ space group. The uranium atom features a distorted pseudo-octahedral geometry with the two (OⁿBu)¹⁻ ligands in equatorial positions and the three THF molecules occupying the axial and two remaining equatorial positions. The U-cent distance is 2.490(2) Å, the U-O_nBu bond lengths are 2.087(3) and 2.090(3) Å, and the U-O_THF bond lengths are 2.449(3), 2.471(3), and 2.539(3) Å. Where the U-O_ax bond length is considerably longer than the equatorial oxygen atoms. The uranium sits 0.524 Å above the O₄ plane. The equatorial O_eq-U-O_eq angles range from 85.30(2) to 90.11(2)°, while the axial O_ax-U-O_eq angles range from 75.36(2) to 78.48(2)°, see the Supplementary Information for more detailed information (Supplementary Table 7).

Conclusions and outlook

We have prepared several multinuclear uranium nitrogen clusters in good yield by ammonolysis of (C₅Me₅)UBn₃. The tetranuclear cluster (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, 1, and hexanuclear clusters (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3 were obtained. The clusters were characterized structurally by single crystal X-ray diffraction and by ¹H nuclear magnetic resonance, electronic absorption, and infrared spectroscopies. The clusters do not interconvert under an atmosphere of ammonia or after exposure to reduced pressure. Studies with ND₃ showed that no exchange with the (C₅Me₅)¹⁻ ligand occurs and did not appear to aid in identifying the N-H_x resonances or stretches. The magnetic moment of 1 was measured to be 5.35 μ_B at 23 °C by the Evans method, suggesting the possibility of some antiferromagnetic coupling between the uranium centres. Therefore, low temperature magnetometry of complex 1 may present interesting magnetic coupling information. Although complexes 2 and 3 have proven difficult to fully separate, complex 3 can be enriched by repeated crystallization (~99%, by NMR spectroscopy). Preliminary reactivity of 1 or a mixture of 2 and 3 with KC₈ showed little evidence of reactivity. The isolation of compounds 1, 2, and 3 differ structurally from the clusters isolated by direct activation of N₂ which typically form dimers or larger cube-based structures^49,74. Reaction of (C₅M₅)UBn₃ with BH₃•THF gave the THF-ring opened product [(C₅Me₅)U (OⁿBu)₂(THF)₃][BBn₄], 4. Efforts are ongoing to better understand the reactivity of these clusters with unsaturated substrates and alternative reducing reagents based on hydrogen. It is also of interest to explore the role of the supporting ligand.

Methods

General procedure for the preparation of compounds 1, 2, 3, and 4

All reactions and materials were performed in the absence of air and water using rigorous Schlenk and glovebox techniques. (C₅Me₅)UBn₃ was prepared in a manner similar to the literature with some modifications⁵⁴. NH₃ was dried over a potassium mirror before addition. NH₃ (0.4 M in dioxane) and BH₃•THF (1.0 M solution in THF) were used as received. See the Supplementary Information for further details.

When powdered or a toluene solution of (C₅Me₅)UBn₃ was reacted with NH₃ gas or an NH₃ solution (0.4 M in 1,4-dioxane), an immediate colour change from black to red occurred. Filtration of the reaction mixture allowed the isolation of THF soluble (C₅Me₅)₄U₄(µ₄-N)(µ-NH)(µ-NH₂)₇, 1. X-ray quality crystals of 1 were grown from THF diffused with pentane vapour at −35 °C. Concentration of the toluene filtrate and standing at ambient temperature (ca. 23 °C) gave crystals of (C₅Me₅)₆U₆(µ₆-N)(µ₃-NH₂)(µ₃-NH)₇, 2, and (C₅Me₅)₆U₆(µ₃-N)₂(µ-NH₂)₁₂, 3. Repeated crystallization allowed for the enrichment of 3 relative to 2.

When a toluene solution of (C₅Me₅)UBn₃ was treated with BH₃•THF (1.0 M solution in THF), an immediate colour change from black to red occurred. Crystallization from toluene layered with pentane at −35 °C yielded red coloured crystals characterized as the mononuclear uranium complex [(C₅Me₅)U (OⁿBu)₂(THF)₃][BBn₄], 4.

Author contributions

C.J.W. conceived and directed the project. V.P. performed the experiments with some assistance from B.G.B. V.P. collected and refined the data with guidance and input from C.J.W. V.P. and C.J.W. performed crystallographic data collection and data refinement. C.J.W. and V.P. wrote the manuscript with input from B.G.B. All authors discussed the results and commented on the manuscript before submission.

Peer review

Peer review information

Communications Chemistry thanks Congqing Zhu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Data availability

Supplementary Information is available and includes detailed experimental procedures, NMR, IR, and UV-vis-NIR spectra, and crystallographic details. The X-ray crystallographic coordinates and Crystallographic Information Files (CIF) for the structures reported in this manuscript, Supplemental files 1-5, have been deposited in the Cambridge Crystallographic Data Centre (CCDC), under the deposition numbers for compounds 1 (2503508), 1-d₁₅ (2503511), 2 (2503509), 3 (2503510) and 4 (2503512), which may be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. Copies of raw NMR, UV-vis-NIR, and IR spectra are available at 10.5061/dryad.zkh1893rd.

Competing interests

The authors declare no competing interests.

Supplementary information

The online version contains supplementary material available at 10.1038/s42004-026-02000-y.