C-H Bond Activation by an Isolated Dinuclear U(III)/U(IV) Nitride

Neutral inverse-sandwich rare-earth metal complexes of the benzene tetraanion

Rare-earth metal complexes of the parent benzene tetraanion and neutral inverse-sandwich rare-earth metal arene complexes have remained elusive. Here, we report the first neutral inverse-sandwich rare-earth metal complexes of the parent benzene tetraanion supported by a monoanionic β-diketiminate (BDI) ligand. Reduction of the trivalent rare-earth metal diiodide precursors (BDI)MI2(THF) (BDI = HC(C(Me)N[C6H3-(3-pentyl)2-2,6])2; M = Y, 1-Y; M = Sm, 1-Sm) in benzene or para-xylene by potassium graphite yielded the neutral inverse-sandwich rare-earth metal arene complexes [(BDI)M(THF) n ]2(μ-η6,η6-arene) (M = Y, Sm; arene = benzene, 2-M; arene = para-xylene, 3-M). Single crystal X-ray diffraction, spectroscopic and magnetic characterization studies, together with density functional theory (DFT) calculations confirm that these neutral rare-earth metal arene complexes possess an [M3+-(arene)4--M3+] electronic structure with strong metal-arene δ interactions. The arene exchange reactivity shows that 2-Sm has higher stability than 3-Sm. Furthermore, 2-Sm can behave as a four-electron reductant to reduce unsaturated organic substrates. Particularly, while the reaction of 2-Sm with 1,3,5,7-cyclooctatetraene (COT) yielded (BDI)Sm(η8-COT) (4-Sm), 2-Sm reacted with 1,4-diphenylbutadiyne to afford (BDI)Sm(η4-C4Ph2) (5-Sm), the first rare-earth metallacyclopentatriene complex.

Multiconfigurational actinide nitrides assisted by double Möbius aromaticity

Understanding the bonding nature between actinides and main-group elements remains a key challenge in actinide chemistry due to the involvement of f orbitals. Herein, we propose a unique "aromaticity-assisted multiconfiguration" (AAM) model to elucidate the bonding nature in actinide nitrides (An2N2, An = Ac, Th, Pa, U). Each planar four-membered An2N2 with equivalent An-N bonds possesses four delocalized π electrons and four delocalized σ electrons, forming a new family of double Möbius aromaticity that contributes to the molecular stability. The unprecedented aromaticity further supports actinide nitrides to exhibit multiconfigurational characters, where the unpaired electrons (2, 4 or 6 in naked Th2N2, Pa2N2 or U2N2, respectively) either are spin-free and localized on metal centres or form metal-ligand bonds. High-level multiconfigurational computations confirm an open-shell singlet ground state for actinide nitrides, with small energy gaps to high spin states. This is consistent with the antiferromagnetic nature observed experimentally in uranium nitrides. The novel AAM bonding model can be authenticated in both experimentally identified compounds containing a U2N2 motif and other theoretically modelled An2N2 clusters and is thus expected to be a general chemical bonding pattern between actinides and main-group elements.

Accessing a Highly Reducing Uranium(III) Complex through Cyclometalation

U(IV) cyclometalated complexes have shown rich reactivity, but their low oxidation state analogues still remain rare. Herein, we report the isolation of [K(2.2.2-cryptand)][UIII{N(SiMe3)2}2(κ2-C,N-CH2SiMe2NSiMe3)], 1, from the reduction of [UIII{N(SiMe)2}3] with KC8 and 2.2.2-cryptand at room temperature. Cyclic voltammetry studies demonstrate that 1 has a reduction potential similar to that of the previously reported [K(2.2.2-cryptand)][UII{N(SiMe)2}3] (Epc = -2.6 V versus Fc+/0 and Epc = -2.8 V versus Fc+/0, respectively). Complex 1, indeed, shows similar reducing abilities upon reactions with 4,4'-bipyridine, 2,2'-bipyridine, and 1-azidoadamantane. Interestingly, 1 was also found to be the first example of a mononuclear U(III) complex that is capable of reducing pyridine. In addition, it is shown that a wide variety of substrates can be inserted into the U-C bond, forming new U(III) metallacycles. These results highlight that cyclometalated U(III) complexes can serve as versatile precursors for a broad range of reactivity and for assembling a variety of novel chemical architectures.

Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene

The structure of and bonding in two base-free terminal actinide imido metallocenes, [η5-1,2,4-(Me3C)3C5H2]2An═N(p-tolyl) (An = U (1), Th (1')) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An═N(p-tolyl) moieties is significantly larger for 1 than for 1', which makes the bonds between the [η5-1,2,4-(Me3C)3C5H2]2U2+ and [(p-tolyl)N]2- fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(L) (L = OPMe3 (6), dmap (9), PhCN (14), and 2,6-Me2PhNC (17)) with Me3PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me2PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS2, isothiocyanates, thio-ketones, ketones, lactides, and acyl nitriles, forming the four- or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p-tolylCHO, the tetranuclear complex [η5-1,2,4-(Me3C)3C5H2]4[OCH(p-tolyl)CH(p-tolyl)O]2U4O4 (10) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N,N'-diisopropylcarbodiimide (DIC), 1, and the four-membered metallaheterocycle [η5-1,2,4-(Me3C)3C5H2]2U[N(p-tolyl)C(═NiPr)N(iPr)] (12). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph3CN3, CuI, Ph2S2, and Ph2Se2, yielding the uranium(V) imido complexes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(X) (X = N3 (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN3) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3C)3C5H2]2U═N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η5-1,2,4-(Me3C)3C5H2]2U=N(p-tolyl)[=NN=(9-C13H8)] (21), respectively.

Planar Tetranuclear Uranium Hydride Cluster Supported by ansa-Bis(cyclopentadienyl) Ligands

Polynuclear metal hydride clusters play important roles in various catalytic processes, with most of the reported polynuclear metal hydride clusters adopting a polyhedral three-dimensional structure. Herein, we report the first example of a planar tetranuclear uranium hydride cluster [(CpCMe2CMe2Cp)U]4(μ2-H)4(μ3-H)4 (U4H8). It was synthesized by reacting an ansa-bis(cyclopentadienyl) ligand-supported uranium chloride precursor [(CpCMe2CMe2Cp)U]3(μ2-Cl)3(μ3-Cl)2 with NaHBEt3. The presence of hydrides in U4H8 was confirmed by NMR spectroscopy and its reactivity with phenol and carbon tetrachloride. DFT calculations also facilitated the determination of the hydrides' positions in U4H8, featuring four bridging μ2-H ligands and four face-capping μ3-H ligands, with the four U centers arranged in a rhombic geometry. The U4H8 represents not only the first example of planar polynuclear actinide metal hydride cluster but also the uranium hydride cluster with the highest nuclearity reported to date.

Multimetallic Uranium Nitride Cubane Clusters from Dinitrogen Cleavage

Dinitrogen cleavage provides an attractive but poorly studied route to the assembly of multimetallic nitride clusters. Here, we show that the monoelectron reduction of the dinitrogen complex [{U(OC6H2-But3-2,4,6)3}2(μ-η2:η2-N2)], 1, allows us to generate, for the first time, a uranium complex presenting a rare triply reduced N2 moiety ((μ-η2:η2-N2)•3-). Importantly, the bound dinitrogen can be further reduced, affording the U4N4 cubane cluster, 3, and the U6N6 edge-shared cubane cluster, 4, thus showing that (N2)•3- can be an intermediate in nitride formation. The tetranitride cluster showed high reactivity with electrophiles, yielding ammonia quantitatively upon acid addition and promoting CO cleavage to yield quantitative conversion of nitride into cyanide. These results show that dinitrogen reduction provides a versatile route for the assembly of large highly reactive nitride clusters, with U6N6 providing the first example of a molecular nitride of any metal formed from a complete cleavage of three N2 molecules.

Reactivity of a Lewis base-supported uranium terminal imido metallocene towards small molecules

The Lewis base-supported uranium terminal imido metallocene [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(dmap) (1) readily reacts with various small molecules such as internal alkynes, isothiocyanates, thioketones, amidates, organic nitriles and imines, chlorosilanes, copper iodide, diphenyl disulfide, organic azides and diazoalkane derivatives. For example, treatment of 1 with PhCCCCPh and PhNCS forms metallaheterocycles originating from a [2 + 2] cycloaddition to yield [η5-1-(p-tolyl)NC(Ph)CHCC(Ph)CH2Si(Me)2-2,4-(Me3Si)2C5H2][η5-1,2,4-(Me3Si)3C5H2]U (2) and [η5-1,2,4-(Me3Si)3C5H2]2U[N(p-tolyl)C(NPh)S](dmap) (3), respectively. The reaction of 1 with the thioketone Ph2CS forms the known uranium sulfido complex [η5-1,2,4-(Me3Si)3C5H2]2US(dmap) (4), which reacts with a second molecule of Ph2CS to give the disulfido compound [η5-1,2,4-(Me3Si)3C5H2]2U(S2CPh2) (5). The imido moiety also promotes deprotonation reactions as illustrated in the reactions with the amide PhCONH(p-tolyl), the nitrile PhCH2CN and the imine (p-tolyl)2CNH to form the bis-amidate [η5-1,2,4-(Me3Si)3C5H2]2U[OC(Ph)N(p-tolyl)]2 (7), and the iminato complexes [η5-1,2,4-(Me3Si)3C5H2]2U[N(p-tolyl)C(CH2Ph)NH](NCCHPh) (8) and [η5-1,2,4-(Me3Si)3C5H2]2U[NH(p-tolyl)][NC(p-tolyl)2] (9), respectively. Addition of PhSiH2Cl to 1 yields [η5-1,2,4-(Me3Si)3C5H2]2U(Cl)[N(p-tolyl)SiH2Ph] (10). In contrast, the uranium(V) imido complexes [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(I) (11) and [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(SPh) (12), may be isolated upon addition of CuI or Ph2S2 to 1, respectively. Uranium(VI) bis-imido metallocenes [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)(NR) (R = p-tolyl (13), mesityl (14)) and [η5-1,2,4-(Me3Si)3C5H2]2UN(p-tolyl)[NN(9-C13H8)] (15) are accessible from 1 on exposure to RN3 (R = p-tolyl, mesityl) and 9-diazofluorene, respectively. Complexes 2, 3, 5, and 7-15 were characterized by various spectroscopic techniques and, in addition, compounds 2, 3, 5, and 7-13 were structurally authenticated by single-crystal X-ray diffraction analyses.

Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Dinitrogen cleavage and hydrogenation to ammonia with a uranium complex

The Haber-Bosch process produces ammonia (NH₃) from dinitrogen (N₂) and dihydrogen (H₂), but requires high temperature and pressure. Before iron-based catalysts were exploited in the current industrial Haber-Bosch process, uranium-based materials served as effective catalysts for production of NH₃ from N₂. Although some molecular uranium complexes are known to be capable of combining with N₂, further hydrogenation with H₂ forming NH₃ has not been reported to date. Here, we describe the first example of N₂ cleavage and hydrogenation with H₂ to NH₃ with a molecular uranium complex. The N₂ cleavage product contains three uranium centers that are bridged by three imido μ ₂-NH ligands and one nitrido μ ₃-N ligand. Labeling experiments with ¹⁵N demonstrate that the nitrido ligand in the product originates from N₂. Reaction of the N₂-cleaved complex with H₂ or H⁺ forms NH₃ under mild conditions. A synthetic cycle has been established by the reaction of the N₂-cleaved complex with trimethylsilyl chloride. The isolation of this trinuclear imido-nitrido product implies that a multi-metallic uranium assembly plays an important role in the activation of N₂.

A versatile strategy for the formation of hydride-bridged actinide-iridium multimetallics

Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H₃] with UCl₄ and ThCl₄(DME)₂ led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H)₃IrCp*}₄ (1) and Th{[(μ-H₂)(H)IrCp*]₂[(μ-H)₃IrCp*]₂} (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI₃(1,4-dioxane)_1.5, resulted in the octanuclear complex {U[(μ²-H₃)IrCp*]₂[(μ³-H₂)IrCp*]}₂ (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects.

Advances in Catalytic C–F Bond Activation and Transformation of Aromatic Fluorides

The activation and transformation of C–F bonds in fluoro-aromatics is a highly desirable process in organic chemistry. It provides synthetic methods/protocols for the generation of organic compounds possessing single or multiple C–F bonds, and effective catalytic systems for further study of the activation mode of inert chemical bonds. Due to the high polarity of the C–F bond and it having the highest bond energy in organics, C–F activation often faces considerable academic challenges. In this mini-review, the important research achievements in the activation and transformation of aromatic C–F bond, catalyzed by transition metal and metal-free systems, are presented.

Photochemical Synthesis of Transition Metal-Stabilized Uranium(VI) Nitride Complexes

Uranium nitrides play important roles in dinitrogen activation and functionalization and in chemistry for nuclear fuels, but the synthesis and isolation of the highly reactive uranium(VI) nitrides remains challenging. Here, we report an example of transition metal (TM) stabilized U(VI) nitride complexes, which are generated by the photolysis of azide-bridged U(IV)-TM (TM = Rh, Ir) precursors. The U(V) nitride intermediates with bridged azide ligands are isolated successfully by careful control of the irradiation time, suggesting that the photolysis of azide-bridged U(IV)-TM precursors is a stepwise process. The presence of two U(VI) nitrides stabilized by three TMs is clearly demonstrated by an X-ray crystallographic study. These TM stabilized U(V) nitride intermediates and U(VI) nitride products exhibit excellent stability both in the solid-state and in THF solution under ambient light. Density functional theory calculations show that the photolysis necessary to break the N-N bond of the azide ligands implies excitation from uranium f-orbital to the lowest unoccupied molecular orbital (LUMO), as suggested by the strong antibonding N-(N₂) character present in the latter.

Uranium-nitride chemistry: uranium-uranium electronic communication mediated by nitride bridges

Treatment of [U^IV(N₃)(Tren^TIPS)] (1, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-) with excess Li resulted in the isolation of [{U^IV(μ-NLi₂)(Tren^TIPS)}₂] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [U^III(Tren^TIPS)] (3), and together with known [{U^V(μ-NLi)(Tren^TIPS)}₂] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{U^IV(Tren^TIPS)}₂(μ-NH)(μ-NLi₂)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin-orbit coupled nature of the ground multiplet of uranium(IV). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of -4.1 and -3.4 cm^-1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium-uranium bonding was raised, but in-depth computational analysis reveals that if any uranium-uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide-actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the 'diamond-core' dinitride of 2 and matrix-isolated binary U₂N₂.

Uranium: The Nuclear Fuel Cycle and Beyond

This review summarizes the recent developments regarding the use of uranium as nuclear fuel, including recycling and health aspects, elucidated from a chemical point of view, i.e., emphasizing the rich uranium coordination chemistry, which has also raised interest in using uranium compounds in synthesis and catalysis. A number of novel uranium coordination features are addressed, such the emerging number of U(II) complexes and uranium nitride complexes as a promising class of materials for more efficient and safer nuclear fuels. The current discussion about uranium triple bonds is addressed by quantum chemical investigations using local vibrational mode force constants as quantitative bond strength descriptors based on vibrational spectroscopy. The local mode analysis of selected uranium nitrides, N≡U≡N, U≡N, N≡U=NH and N≡U=O, could confirm and quantify, for the first time, that these molecules exhibit a UN triple bond as hypothesized in the literature. We hope that this review will inspire the community interested in uranium chemistry and will serve as an incubator for fruitful collaborations between theory and experimentation in exploring the wealth of uranium chemistry.

Reactivity of Multimetallic Thorium Nitrides Generated by Reduction of Thorium Azides

Thorium nitrides are likely intermediates in the reported cleavage and functionalization of dinitrogen by molecular thorium complexes and are attractive compounds for the study of multiple bond formation in f-element chemistry, but only one example of thorium nitride isolable from solution was reported. Here, we show that stable multimetallic azide/nitride thorium complexes can be generated by reduction of thorium azide precursors─a route that has failed so far to produce Th nitrides. Once isolated, the thorium azide/nitride clusters, M₃Th═N═Th (M = K or Cs), are stable in solutions probably due to the presence of alkali ions capping the nitride, but their synthesis requires a careful control of the reaction conditions (solvent, temperature, nature of precursor, and alkali ion). The nature of the cation plays an important role in generating a nitride product and results in large structural differences with a bent Th═N═Th moiety found in the K-bound nitride as a result of a strong K-nitride interaction and a linear arrangement in the Cs-bound nitride. Reactivity studies demonstrated the ability of Th nitrides to cleave CO in ambient conditions yielding CN^-.

Assembling Diuranium Complexes in Different States of Charge with a Bridging Redox-Active Ligand

Radical-bridged diuranium complexes are desirable for their potential high exchange coupling and single molecule magnet (SMM) behavior, but remain rare. Here we report for the first time radical-bridged diuranium(iv) and diuranium(iii) complexes. Reaction of [U{N(SiMe₃)₂}₃] with 2,2′-bipyrimidine (bpym) resulted in the formation of the bpym-bridged diuranium(iv) complex [{((Me₃Si)₂N)₃U^IV}₂(μ-bpym²⁻)], 1. Reduction with 1 equiv. KC₈ reduces the complex, affording [K(2.2.2-cryptand)][{((Me₃Si)₂N)₃U}₂(μ-bpym)], 2, which is best described as a radical-bridged U^III–bpym˙⁻–U^III complex. Further reduction of 1 with 2 equiv. KC₈, affords [K(2.2.2-cryptand)]₂[{((Me₃Si)₂N)₃U^III}₂(μ-bpym²⁻)], 3. Addition of AgBPh₄ to complex 1 resulted in the oxidation of the ligand, yielding the radical-bridged complex [{((Me₃Si)₂N)₃U^IV}₂(μ-bpym˙⁻)][BPh₄], 4. X-ray crystallography, electrochemistry, susceptibility data, EPR and DFT/CASSCF calculations are in line with their assignments. In complexes 2 and 4 the presence of the radical-bridge leads to slow magnetic relaxation.

Convenient routes to dinuclear complexes of uranium where two uranium centers are bridged by the redox-active ligand bpym were identified resulting in unique and stable radical-bridged dimetallic complexes of U(iii) and U(iv) showing SMM behaviour.

Nitrogen activation and cleavage by a multimetallic uranium complex

Multimetallic-multielectron cooperativity plays a key role in the metal-mediated cleavage of N₂ to nitrides (N³⁻). In particular, low-valent uranium complexes coupled with strong alkali metal reducing agents can lead to N₂ cleavage, but often, it is ambiguous how many electrons are transferred from the uranium centers to cleave N₂. Herein, we designed new dinuclear uranium nitride complexes presenting a combination of electronically diverse ancillary ligands to promote the multielectron transformation of N₂. Two heteroleptic diuranium nitride complexes, [K{U^IV(OSi(O^tBu)₃)(N(SiMe₃)₂)₂}₂(μ-N)] (1) and [Cs{U^IV(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (3-Cs), containing different combinations of OSi(O^tBu)₃ and N(SiMe₃)₂ ancillary ligands, were synthesized. We found that both complexes could be reduced to their U(iii)/U(iv) analogues, and the complex, [K₂{U^IV/III(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)] (6-K), could be further reduced to a putative U(iii)/U(iii) species that is capable of promoting the 4e⁻ reduction of N₂, yielding the N₂⁴⁻complex [K₃{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}₂(μ-N)(μ-η²:η²-N₂)], 7. Parallel N₂ reduction pathways were also identified, leading to the isolation of N₂ cleavage products, [K₃{U^VI(OSi(O^tBu)₃)₂(N(SiMe₃)₂)( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N)}(μ-N)₂{U^V(OSi(O^tBu)₃)₂(N(SiMe₃)₂)}]₂, 8, and [K₄{(OSi(O^tBu)₃)₂U^V)(N)}(μ-NH)(μ-κ²:C,N-CH₂SiMe₂NSiMe₃)-{U^V(OSi(O^tBu)₃)₂][K(N(SiMe₃)₂]₂, 9. These complexes provide the first example of N₂ cleavage to nitride by a uranium complex in the absence of reducing alkali metals.

Combinations of ligands were used to tune U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> NU complexes yielding a U(iii)/U(iii) nitride, which activates N₂. Parallel N₂ reduction pathways were identified, leading to the first example of N₂ cleavage by U without external alkali reducing agents.

Cation assisted binding and cleavage of dinitrogen by uranium complexes

N2 binding affinity decreases markedly in a series of isostructural U(iii)–alkali ions complexes with increasing cation size. N2 binding is undetectable in the Cs analogue, but the first example of cesium-assisted N2 cleavage to bis-nitride was observed at ambient condition.

Nitride protonation and NH3 binding versus N-H bond cleavage in uranium nitrides

The conversion of metal nitrides to NH₃ is an essential step in dinitrogen fixation, but there is limited knowledge of the reactivity of nitrides with protons (H⁺). Herein, we report comparative studies for the reactions of H⁺ and NH₃ with uranium nitrides, containing different types of ancillary ligands. We show that the differences in ancillary ligands, leads to dramatically different reactivity. The nitride group, in nitride-bridged cationic and anionic diuranium(iv) complexes supported by –N(SiMe₃)₂ ligands, is resistant toward protonation by weak acids, while stronger acids result in ligand loss by protonolysis. Moreover, the basic –N(SiMe₃)₂ ligands promote the N–H heterolytic bond cleavage of NH₃, yielding a “naked” diuranium complex containing three bridging ligands, a nitride (N³⁻) and two NH₂ ligands. Conversely, in the nitride-bridged diuranium(iv) complex supported by –OSi(O^tBu)₃ ligands, the nitride group is easily protonated to afford NH₃, which binds the U(iv) ion strongly, resulting in a mononuclear U–NH₃ complex, where NH₃ can be displaced by addition of strong acids. Furthermore, the U–OSi(O^tBu)₃ bonds were found to be stable, even in the presence of stronger acids, such as NH₄BPh₄, therefore indicating that –OSi(O^tBu)₃ supporting ligands are well suited to be used when acidic conditions are required, such as in the H⁺/e⁻ mediated catalytic conversion of N₂ to NH₃.

Ancillary ligands alter the reactivity of U-nitrides with H⁺, relevant to N₂ conversion to NH₃. The amides lead to complete ligand loss and NH₃ activation, while for siloxides, the nitride is protonated to NH₃ leaving the ancillary ligands intact.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)₂]₂(μ-N)(μ-κ¹:κ¹-BCMA) (7), [(U(BIMA)₂)₂(μ-N)(μ-NⁱPr)(K₂(μ-η³:η³-CH₂CHNⁱPr)]₂ (8), and [U(BIMA)₂]₂(μ-N)(μ-κ¹:κ¹-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. ¹⁵N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N₃)(BCMA)₃ yielded the cationic U(V) species [U(N₃)(BCMA)₃][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)₃, U(N₃)(BCMA)₃, and [U(N₃)(BCMA)₃][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

Synthesis, structure, and reactivity of uranium(vi) nitrides

Uranium nitride compounds are important molecular analogues of uranium nitride materials such as UN and UN2 which are effective catalysts in the Haber-Bosch synthesis of ammonia, but the synthesis of molecular nitrides remains a challenge and studies of the reactivity and of the nature of the bonding are poorly developed. Here we report the synthesis of the first nitride bridged uranium complexes containing U(vi) and provide a unique comparison of reactivity and bonding in U(vi)/U(vi), U(vi)/U(v) and U(v)/U(v) systems. Oxidation of the U(v)/U(v) bis-nitride [K2{U(OSi(O t Bu)3)3(μ-N)}2], 1, with mild oxidants yields the U(v)/U(vi) complexes [K{U(OSi(O t Bu)3)3(μ-N)}2], 2 and [K2{U(OSi(O t Bu)3)3}2(μ-N)2(μ-I)], 3 while oxidation with a stronger oxidant ("magic blue") yields the U(vi)/U(vi) complex [{U(OSi(O t Bu)3)3}2(μ-N)2(μ-thf)], 4. The three complexes show very different stability and reactivity, with N2 release observed for complex 4. Complex 2 undergoes hydrogenolysis to yield imido bridged [K2{U(OSi(O t Bu)3)3(μ-NH)}2], 6 and rare amido bridged U(iv)/U(iv) complexes [{U(OSi(O t Bu)3)3}2(μ-NH2)2(μ-thf)], 7 while no hydrogenolysis could be observed for 4. Both complexes 2 and 4 react with H+ to yield quantitatively NH4Cl, but only complex 2 reacts with CO and H2. Differences in reactivity can be related to significant differences in the U-N bonding. Computational studies show a delocalised bond across the U-N-U for 1 and 2, but an asymmetric bonding scheme is found for the U(vi)/U(vi) complex 4 which shows a U-N σ orbital well localised to U[triple bond, length as m-dash]N and π orbitals which partially delocalise to form the U-N single bond with the other uranium.

Intrinsic Semiconducting Behavior in a Large Mixed-Valent Uranium(V/VI) Cluster

We disclose the intrinsic semiconducting properties of one of the largest mixed-valent uranium clusters, [H₃ O⁺ ][U^V (U^VI O₂ )₈ (μ₃ -O)₆ (PhCOO)₂ (Py(CH₂ O)₂ )₄ (DMF)₄ ] (Ph=phenyl, Py=pyridyl, DMF=N,N-dimethylformamide) (1). Single-crystal X-ray crystallography demonstrates that U^V center is stabilized within a tetraoxo core surrounded by eight uranyl(VI) pentagonal bipyramidal centers. The oxidation states of uranium are substantiated by spectroscopic data and magnetic susceptibility measurement. Electronic spectroscopy and theory corroborate that U^V species serve as electron donors and thus facilitate 1 being a n-type semiconductor. With the largest effective atomic number among all reported radiation-detection semiconductor materials, charge transport properties and photoconductivity were investigated under X-ray excitation for 1: a large on-off ratio of 500 and considerable charge mobility lifetime product of 2.3×10^-4  cm²  V^-1 , as well as a high detection sensitivity of 23.4 μC Gy_air ^-1  cm^-2 .

U2N@I h(7)-C80: fullerene cage encapsulating an unsymmetrical U(iv)[double bond, length as m-dash]N[double bond, length as m-dash]U(v) cluster

For the first time, an actinide nitride clusterfullerene, U₂N@I_h(7)-C₈₀, is synthesized and fully characterized by X-ray single crystallography and multiple spectroscopic methods. U₂N@I_h(7)-C₈₀ is by far the first endohedral fullerene that violates the well-established tri-metallic nitride template for nitride clusterfullerenes. The novel UNU cluster features two UN bonds with uneven bond distances of 2.058(3) Å and 1.943(3) Å, leading to a rare unsymmetrical structure for the dinuclear nitride motif. The combined experimental and theoretical investigations suggest that the two uranium ions show different oxidation states of +4 and +5. Quantum-chemical investigation further reveals that the f¹/f² population dominantly induces a distortion of the UNU cluster, which leads to the unsymmetrical structure. A comparative study of U₂X@C₈₀ (X = C, N and O) reveals that the U–X interaction in UXU clusters can hardly be seen as being formed by classical multiple bonds, but is more like an anionic central ion X^q− with biased overlaps with the two metal ions, which decrease as the electronegativity of X increases. This study not only demonstrates the unique bonding variety of actinide clusters stabilized by fullerene cages, showing different bonding from that observed for the lanthanide analogs, it also reveals the electronic structure of the UXU clusters (X = C, N and O), which are of fundamental significance to understanding these actinide bonding motifs.

A novel actinide cluster, UNU, is stabilized inside a C₈₀ fullerene cage. The U(iv)NU(v) cluster features two UN bonds with uneven bond distances of 2.058(3) Å and 1.943(3) Å, leading to an unsymmetrical structure.