Molecular and Electronic Structure of Dinuclear Uranium Bis-μ-Oxo Complexes with Diamond Core Structural Motifs

Controlled and sequential single-electron reduction of the uranyl dication

A flexible tripodal pyrrole-imine ligand (H3L) has been used to facilitate the controlled and sequential single-electron reductions of the uranyl dication from the U(VI) oxidation state to U(V) and further to U(IV), processes that are important to understanding the reduction of uranyl and its environmental remediation. The uranyl(VI) complexes UO2(HL)(sol) (sol = THF, py) were straightforwardly accessed by the transamination reaction of H3L with UO2{N(SiMe3)2}2(THF)2 and adopt 'hangman' structures in which one of the pyrrole-imine arms is pendant. While deprotonation of this arm by LiN(SiMe3)2 causes no change in uranyl oxidation state, single-electron reduction of uranyl(VI) to uranyl(V) occurred on addition of two equivalents of KN(SiMe3)2 to UO2(HL)(sol). The potassium cations of this new [UVO2(K2L)]2 dimer were substituted by transmetalation with the appropriate metal chloride salt, forming the new uranyl(V) tetra-heterometallic complexes, [UVO2Zn(L)(py)2]2 and [UVO2Ln(Cl)(L)(py)2]2 (Ln = Y, Sm, Dy). The dimeric uranyl(V)-yttrium complex underwent further reduction and chloride abstraction to form the tetrametallic U(IV) complex [UIVO2YIII(py)]2, so highlighting the adaptability of this ligand to stabilise a variety of different uranium oxidation states.

Synthesis, structure and redox properties of single-atom bridged diuranium complexes supported by aryloxides

Single-atom (group 15 and group 16 anions) bridged dimetallic complexes of low oxidation state uranium provide a convenient route to implement multielectron transfer and promote magnetic communication in uranium chemistry, but remain extremely rare. Here we report the synthesis, redox and magnetic properties of N3-, O2-, and S2- bridged diuranium complexes supported by bulky aryloxide ligands. The U(IV)/U(IV) nitride [Cs(THF)8][(U(OAr)3)2(μ-N)], 1 could be prepared and characterized but could not be reduced. Reduction of the neutral U(IV)/U(IV) complexes [(U(OAr)3)2(μ-X)] A (X = O) and B (X = S) led to the isolation and characterization of the U(IV)/U(III) and U(III)/U(III) analogues. Complexes [(K(THF)4)2(U(OAr)2)2(μ-S)2], 5 and [K(2.2.2-cryptand)]2[(U(OAr)3)2(μ-S)], 6 are the first examples of U(III) sulphide bridged complexes. Computational studies and redox properties allow the reactivity of the dimetallic complexes to be related to their electronic structure.

Metal-Halide Covalency, Exchange Coupling, and Slow Magnetic Relaxation in Triangular (CpiPr5)3U3X6 (X = Cl, Br, I) Clusters

The actinide elements are attractive alternatives to transition metals or lanthanides for the design of exchange-coupled multinuclear single-molecule magnets. However, the synthesis of such compounds is challenging, as is unraveling any contributions from exchange coupling to the overall magnetism. To date, only a few actinide compounds have been shown to exhibit exchange coupling and single-molecule magnetism. Here, we report triangular uranium(III) clusters of the type (CpiPr5)3U3X (1-X; X = Cl, Br, I; CpiPr5 = pentaisopropylcyclopentadienyl), which are synthesized via reaction of the aryloxide-bridged precursor (CpiPr5)2U2(OPhtBu)4 with excess Me3SiX. Spectroscopic analysis suggests the presence of covalency in the uranium-halide interactions arising from 5f orbital participation in bonding. The dc magnetic susceptibility data reveal the presence of antiferromagnetic exchange coupling between the uranium(III) centers in these compounds, with the strength of the exchange decreasing down the halide series. Ac magnetic susceptibility data further reveal all compounds to exhibit slow magnetic relaxation under zero dc field. In 1-I, which exhibits particularly weak exchange, magnetic relaxation occurs via a Raman mechanism associated with the individual uranium(III) centers. In contrast, for 1-Br and 1-Cl, magnetic relaxation occurs via an Orbach mechanism, likely involving relaxation between ground and excited exchange-coupled states. Significantly, in the case of 1-Cl, magnetic relaxation is sufficiently slow such that open magnetic hysteresis is observed up to 2.75 K, and the compound exhibits a 100-s blocking temperature of 2.4 K. This compound provides the first example of magnetic blocking in a compound containing only actinide-based ions, as well as the first example involving the uranium(III) oxidation state.

Formation of N,N,N',N'-tetramethylethylenediamine via coupling of the two charge reversed C-N bonds of Me3NO in the presence of an Eu(II) bis(trimethylsilyl)amide complex

An N,N,N',N'-tetramethylethylenediamine (TMEDA) complex of Eu(II) was synthesized via an unprecedented self-aminomethylation of Me3NO in the presence of Eu[N(SiMe3)2]2(THF)2. The neutral TMEDA ligand is formed via the coupling of two charge reversed carbon centres from the same precursor Me3NO. In contrast, the reactions of LnII[N(SiMe3)2]2(THF)2 (Ln = Yb, Sm) with Me3NO led to the oxygen-bridged dinuclear complexes of Ln(III).

Accessing five oxidation states of uranium in a retained ligand framework

Understanding and exploiting the redox properties of uranium is of great importance because uranium has a wide range of possible oxidation states and holds great potential for small molecule activation and catalysis. However, it remains challenging to stabilise both low and high-valent uranium ions in a preserved ligand environment. Herein we report the synthesis and characterisation of a series of uranium(II-VI) complexes supported by a tripodal tris(amido)arene ligand. In addition, one- or two-electron redox transformations could be achieved with these compounds. Moreover, combined experimental and theoretical studies unveiled that the ambiphilic uranium-arene interactions are the key to balance the stabilisation of low and high-valent uranium, with the anchoring arene acting as a δ acceptor or a π donor. Our results reinforce the design strategy to incorporate metal-arene interactions in stabilising multiple oxidation states, and open up new avenues to explore the redox chemistry of uranium.

Unusual Actinyl Complexes with a Redox-Active N,S-Donor Ligand

Understanding the fundamental chemistry of soft N,S-donor ligands with actinides across the series is critical for separation science toward sustainable nuclear energy. This task is particularly challenging when the ligands are redox active. We herein report a series of actinyl complexes with a N,S-donor redox-active ligand that stabilizes different oxidation states across the actinide series. These complexes are isolated and characterized in the gas phase, along with high-level electronic structure studies. The redox-active N,S-donor ligand in the products, C₅H₄NS, acts as a monoanion in [U^VIO₂(C₅H₄NS^-)]⁺ but as a neutral radical with unpaired electrons localized on the sulfur atom in [Np^VO₂(C₅H₄NS^•)]⁺ and [Pu^VO₂(C₅H₄NS^•)]⁺, resulting in different oxidation states for uranium and transuranic elements. This is rationalized by considering the relative energy levels of actinyl(VI) 5f orbitals and S 3p lone pair orbitals of the C₅H₄NS^- ligand and the cooperativity between An-N and An-S bonds that provides additional stability for the transuranic elements.

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.

Nonaqueous Electrochemistry of Uranium Complexes: A Guide to Structure-Reactivity Tuning

Uranium complexes can be stabilized in a wide range of oxidation states, ranging from U^II to U^VI and a very recent example of a U^I complex. This review provides a comprehensive summary of electrochemistry data reported on uranium complexes in nonaqueous electrolyte, to serve as a clear point of reference for newly synthesized compounds, and to evaluate how different ligand environments influence experimentally observed electrochemical redox potentials. Data for over 200 uranium compounds are reported, together with a detailed discussion of trends observed across larger series of complexes in response to ligand field variations. In analogy to the traditional Lever parameter, we utilized the data to derive a new uranium-specific set of ligand field parameters ^UE_L(L) that more accurately represent metal-ligand bonding situations than previously existing transition metal derived parameters. Exemplarily, we demonstrate ^UE_L(L) parameters to be useful for the prediction of structure-reactivity correlations in order to activate specific substrate targets.

An Aluminium Imide as a Transfer Agent for the [NR]2- Function via Metathesis Chemistry

The reactions of a terminal aluminium imide with a range of oxygen-containing substrates have been probed with a view to developing its use as a novel main group transfer agent for the [NR]^2- fragment. We demonstrate transfer of the imide moiety to [N₂ ], [CO] and [Ph(H)C] units driven thermodynamically by Al-O bond formation. N₂ O reacts rapidly to generate the organoazide DippN₃ (Dipp=2,6-ⁱ Pr₂ C₆ H₃ ), while CO₂ (under dilute reaction conditions) yields the corresponding isocyanate, DippNCO. Mechanistic studies, using both experimental and quantum chemical techniques, identify a carbamate complex K₂ [(NON)Al-{κ² -(N,O)-N(Dipp)CO₂ }]₂ (formed via [2+2] cycloaddition) as an intermediate in the formation of DippNCO, and also in an alternative reaction leading to the generation of the amino-dicarboxylate complex K₂ [(NON)Al{κ² -(O,O')-(O₂ C)₂ N-(Dipp)}] (via the take-up of a second equivalent of CO₂ ). In the case of benzaldehyde, a similar [2+2] cycloaddition process generates the metallacyclic hemi-aminal complex, K_n [(NON)Al{κ² -(N,O)-(N(Dipp)C(Ph)(H)O}]_n . Extrusion of the imine, PhC(H)NDipp, via cyclo-reversion is disfavoured thermally, due to the high energy of the putative aluminium oxide co-product, K₂ [(NON)Al(O)]₂ . However, addition of CO₂ allows the imine to be released, driven by the formation of the thermodynamically more stable aluminium carbonate co-product, K₂ [(NON)Al(κ² -(O,O')-CO₃ )]₂ .

Unprecedented pairs of uranium (iv/v) hydroxido and (iv/v/vi) oxido complexes supported by a seven-coordinate cyclen-anchored tris-aryloxide ligand

We present the synthesis and reactivity of a newly developed, cyclen-based tris-aryloxide ligand precursor, namely cyclen(Me)( t-Bu,t-BuArOH)3, and its coordination chemistry to uranium. The corresponding uranium(iii) complex [UIII((OAr t-Bu,t-Bu)3(Me)cyclen)] (1) was characterized by 1H NMR analysis, CHN elemental analysis and UV/vis/NIR electronic absorption spectroscopy. Since no single-crystals suitable for X-ray diffraction analysis could be obtained from this precursor, 1 was oxidized with methylene chloride or silver fluoride to yield [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(X)] (X = Cl (2), F (3)), which were unambiguously characterized and successfully crystallized to gain insight into the molecular structure by single-crystal X-ray diffraction analysis (SC-XRD). Further, the activation of H2O and N2O by 1 is presented, resulting in the U(iv) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UIV(OH)] (4) and the U(v) complex [(cyclen(Me)( t-Bu,t-BuArO)3)UV(O)] (6). Complexes 2, 3, 4, and 6 were characterized by 1H NMR analysis, CHN elemental analysis, UV/vis/NIR electronic absorption spectroscopy, IR vibrational spectroscopy, and SQUID magnetization measurements as well as cyclic voltammetry. Furthermore, chemical oxidation of 3, 4, and 6 with AgF or AgSbF6 was achieved leading to complexes [(cyclen(Me)( t-Bu,t-BuArO)3)UV(F)2] (5), [(cyclen(Me)( t-Bu,t-BuArO)3)UV(OH)][SbF6] (7), and [(cyclen(Me)( t-Bu,t-BuArO)3)UVI(O)][SbF6] (8). Finally, reduction of 7 with KC8 yielded a U(iv) complex, spectroscopically and magnetochemically identified as K[(cyclen(Me)( t-Bu,t-BuArO)3)UIV(O)].

Theoretical Investigation of Catalytic Water Splitting by the Arene-Anchored Actinide Complexes

Actinide complexes, which could enable the electrocatalytic H₂O reduction, are not well documented because of the fact that actinide-containing catalysts are precluded by extremely stable actinyl species. Herein, by using relativistic density functional theory calculations, the arene-anchored trivalent actinide complexes (^Me,MeArO)₃ArAn (marked as [AnL]) with desirable electron transport between metal and ligand arene are investigated for H₂ production. The metal center is changed from Ac to Pu. Electron-spin density calculations reveal a two-electron oxidative process (involving high-valent intermediates) for complexes [AnL] (An = P-Pu) along the catalytic pathway. The electrons are provided by both the actinide metal and the arene ring of ligand. This is comparable to the previously reported uranium catalyst (^Ad,MeArO)₃mesU (Ad = adamantine and mes = mesitylene). From the thermodynamic and kinetic perspectives, [PaL] offers appreciably lower reaction energies for the overall catalytic cycle than other actinide complexes. Thus, the protactinium complex tends to be the most reactive for H₂O reduction to produce H₂ and has the advantage of its experimental accessibility.

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{UIV(OSi(O t Bu)3)(N(SiMe3)2)2}2(μ-N)] (1) and [Cs{UIV(OSi(O t Bu)3)2(N(SiMe3)2)}2(μ-N)] (3-Cs), containing different combinations of OSi(O t Bu)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{UIV/III(OSi(O t Bu)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 N2 4-complex [K3{UV(OSi(O t Bu)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{UVI(OSi(O t Bu)3)2(N(SiMe3)2)([triple bond, length as m-dash]N)}(μ-N)2{UV(OSi(O t Bu)3)2(N(SiMe3)2)}]2, 8, and [K4{(OSi(O t Bu)3)2UV)([triple bond, length as m-dash]N)}(μ-NH)(μ-κ2:C,N-CH2SiMe2NSiMe3)-{UV(OSi(O t Bu)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.

Heterometallic Clusters with Uranium-Metal Bonds Supported by Double-Layer Nitrogen-Phosphorus Ligands

ConspectusHeterometallic clusters with M-M bonds have significantly interested chemists because of their attractive structures and synergistic effects in small-molecule activation and catalysis. However, reports of the isolation of heterometallic clusters with uranium-transition metal (U-TM) bonds remain very limited. In this Account, we describe our research in the construction of heterometallic molecular clusters with multiple U-TM single or multiple bonds supported by novel double-layer N-P ligands. Multimetallic synergistic catalysis and small-molecule activation with these species are also summarized.First, according to the hard-soft acid-base theory, we employed a three-armed N-P ligand, which can be used to construct heterometallic clusters with four or six U-Ni bonds. This strategy was also effective in the construction of complexes with direct rare earth metal-TM bonding. The similar two-armed N-P ligands also are effective platforms for the synthesis of heterometallic complexes with U-Ni, U-Pd, and U-Pt bonds.Second, a set of heterometallic clusters featuring U≡Rh, U≡Co, and U≡Fe triple bonds were constructed under routine experimental conditions. X-ray diffraction analysis of these clusters exhibits the shortest U-TM bond distance (1.9693(4) Å for the U≡Fe triple bond) in these complexes. Theoretical studies reveal that the nature of the triple bond is one covalent σ bond and two TM → U dative π bonds. A large Wiberg bond index (WBI) of 2.93 and a significant degree of covalency for the U≡TM triple bonds were also found in these complexes.Third, these uranium complexes supported by the double-layer N-P ligands exhibit great potential in small-molecule activation. For instance, N₂ cleavage without an external reducing agent was achieved by a U(III)-P(III) synergistic six-electron reduction. The synergism between U(III) and P(III) enables the activation of other small molecules, such as O₂, P₄, and As⁰_(nano), and highlights the importance of the P atom in the double-layer N-P ligand for the activation of small molecules. A heterometallic cluster with U-Rh bonds can break the strong N≡N triple bond in N₂ in the presence of potassium graphite, suggesting a synergistic effect between U and Rh. This multimetallic synergistic effect was also observed in catalytic processes. A heterometallic cluster with U≡Co triple bonds shows excellent selectivity and activity in the hydroboration of a series of alkynes under mild conditions. These results lead to effective methods for the construction of heterometallic molecular clusters with U-TM single or multiple bonds and could promote the application of heterometallic clusters with U-TM bonds in catalysis and the activation of small molecules.

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.

Uranium Going the Soft Way: Low-Valent Uranium(III) Coordinated to an Arene-Anchored Tris-Thiophenolate Ligand

The synthesis of a tripodal, S-based ligand, namely the mesitylene-anchored, tris-thiophenolate-functionalized (mes(^Me,AdArS)₃)^3- (1)^3-, and its coordination chemistry with low-valent uranium to form [U^III((SAr^Ad,Me)₃mes)] (1-U) are reported. Single-crystal X-ray diffraction analysis reveals a C₃-symmetric molecular structure. Full characterization of 1-U was performed using nuclear magnetic resonance, UV-vis-NIR electronic absorption, and electron paramagnetic resonance spectroscopies as well as SQUID magnetometry, thus confirming the U(III) oxidation state. Alternating current magnetic studies show that 1-U exhibits single-molecule magnet behavior at low temperatures in a non-zero external field. Comparison of these results to those of the previously reported mesitylene-anchored complexes, [U^III((OAr^Ad,Me)₃mes)] and [U^III((OAr^tBu,tBu)₃mes)], indicates a drastic change in the electronic structure when moving from phenolate-based ligands to thiophenolate-based 1, which is further discussed by means of computational analysis (NBO, DFT, and QTAIM). Despite the U-O bonds being stronger, a much higher covalency was found for the U-S analogue.

Structure of the {U13} polyoxo cluster U13O8Cl x (MeO)38-x (x = 2.3, MeO = methoxide)

The structure of a new type of polyoxo cluster complex that contains thirteen uranium atoms, {U13}, is reported. The complex crystallized from methanol containing tetra-valent uranium (UIV) with a basic organic ligand, and was characterized as di-chloridoocta-cosa-μ2-methano-lato-octa-kis-(methano-lato)octa-μ4-oxido-trideca-uranium, [U13(CH3O)35.7Cl2.3O8] or [U13(μ4-Ooxo)8Cl x (MeO)38-x ] (x = 2.3, MeO = methoxide) (I), by single-crystal X-ray diffraction. The characterized {U13} polyoxo cluster complex (I) possesses a single cubic uranium polyhedron at the centre of the cluster core. To the best of our knowledge, this is the very first example of a polyoxo actinide complex that bears a single cubic polyhedron in its structure. The cubic polyhedron in I is well comparable in shape with those in bulk UO2. The U-O bonds in the cubic polyhedron of I are, however, significantly shorter than those not only in bulk UO2 but also in another analogue in the {U38} cluster. This shortening of U-O bonds, together with BVS calculations and the overall negative charge (2-) of I, suggests that the central uranium atom in I, which forms the single cubic coordination polyhedron, is presumably oxidized to the penta-valent state (UV) from the original tetra-valent state (UIV). Complex I is, hence, the first example of a polyoxo cluster possessing a single cubic coordination polyhedron of UV.

Stepwise Reduction of Dinitrogen by a Uranium-Potassium Complex Yielding a U(VI)/U(IV) Tetranitride Cluster

Multimetallic cooperativity is believed to play a key role in the cleavage of dinitrogen to nitrides (N^3-), but the mechanism remains ambiguous due to the lack of isolated intermediates. Herein, we report the reduction of the complex [K₂{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-η²:η²-N₂)}], B, with KC₈, yielding the tetranuclear tetranitride cluster [K₆{(OSi(O^tBu)₃)₂U^IV}₃{(OSi(O^tBu)₃)₂U^VI}(μ₄-N)₃(μ₃-N)(μ₃-O)₂], 1, a novel example of N₂ cleavage to nitride by a diuranium complex. The structure of complex 1 is remarkable, as it contains a unique uranium center bound by four nitrides and provides the second example of a trans-N═U^VI═N core analogue of UO₂²⁺. Experimental and computational studies indicate that the formation of the U(IV)/U(VI) tetrauranium cluster occurs via successive one-electron transfers from potassium to the bound N₂^4- ligand in complex B, resulting in N₂ cleavage and the formation of the putative diuranium(V) bis-nitride [K₄{[U^V(OSi(O^tBu)₃)₃]₂(μ-O)(μ-N)₂}], X. Additionally, cooperative potassium binding to the U-bound N₂^4- ligand facilitates dinitrogen cleavage during electron transfer. The nucleophilic nitrides in both complexes are easily functionalized by protons to yield ammonia in 93-97% yield and with excess ¹³CO to yield K¹³CN and KN¹³CO. The structures of two tetranuclear U(IV)/U(V) bis- and mononitride clusters isolated from the reaction with CO demonstrate that the nitride moieties are replaced by oxides without disrupting the tetranuclear structure, but ultimately leading to valence redistribution.

Charge frustration in ligand design and functional group transfer

Molecules with different resonance structures of similar importance, such as heterocumulenes and mesoionics, are prominent in many applications of chemistry, including 'click chemistry', photochemistry, switching and sensing. In coordination chemistry, similar chameleonic/schizophrenic entities are referred to as ambidentate/ambiphilic or cooperative ligands. Examples of these had remained, for a long time, limited to a handful of archetypal compounds that were mere curiosities. In this Review, we describe ambiphilicity - or, rather, 'charge frustration' - as a general guiding principle for ligand design and functional group transfer. We first give a historical account of organic zwitterions and discuss their electronic structures and applications. Our discussion then focuses on zwitterionic ligands and their metal complexes, such as those of ylidic and redox-active ligands. Finally, we present new approaches to single-atom transfer using cumulated small molecules and outline emerging areas, such as bond activation and stable donor-acceptor ligand systems for reversible 1e^- chemistry or switching.

Near-infrared C-term MCD spectroscopy of octahedral uranium(V) complexes

C-term magnetic circular dichroism (MCD) spectroscopy is a powerful method for probing d-d and f-f transitions in paramagnetic metal complexes. However, this technique remains underdeveloped both experimentally and theoretically for studies of U(v) complexes of Oh symmetry, which have been of longstanding interest for probing electronic structure, bonding, and covalency in 5f systems. In this study, C-term NIR MCD of the Laporte forbidden f-f transitions of [UCl6]- and [UF6]- are reported, demonstrating the significant fine structure resolution possible with this technique including for the low energy Γ7 → Γ8 transitions in [UF6]-. The experimental NIR MCD studies were further extended to [U(OC6F5)6]-, [U(CH2SiMe3)6]-, and [U(NC(tBu)(Ph))6]- to evaluate the effects of ligand-type on the f-f MCD fine structure features. Theoretical calculations were conducted to determine the Laporte forbidden f-f transitions and their MCD intensity experimentally observed in the NIR spectra of the U(v) hexahalide complexes, via the inclusion of vibronic coupling, to better understand the underlying spectral fine structure features for these complexes. These spectra and simulations provide an important platform for the application of MCD spectroscopy to this widely studied class of U(v) complexes and identify areas for continued theoretical development.

Selective oxo ligand functionalisation and substitution reactivity in an oxo/catecholate-bridged UIV/UIV Pacman complex

The oxo- and catecholate-bridged U^IV/U^IV Pacman complex [{(py)U^IVOU^IV(μ-O₂C₆H₄)(py)}(L^A)] A (L^A = a macrocyclic "Pacman" ligand; anthracenylene hinge between N₄-donor pockets, ethyl substituents on meso-carbon atom of each N₄-donor pocket) featuring a bent U^IV-O-U^IV oxo bridge readily reacts with small molecule substrates to undergo either oxo-atom functionalisation or substitution. Complex A reacts with H₂O or MeOH to afford [{(py)U^IV(μ-OH)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (1) and [{(py)U^IV(μ-OH)(μ-OMe)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (2), respectively, in which the bridging oxo ligand in A is substituted for two bridging hydroxo ligands or one bridging hydroxo and one bridging methoxy ligand, respectively. Alternatively, A reacts with either 0.5 equiv. of S₈ or 4 equiv. of Se to provide [{(py)U^IV(μ-η²:η²-E₂)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (E = S (3), Se (4)) respectively, in which the [E₂]^2- ion bridges the two U^IV centres. To the best of our knowledge, complex A is the first example of either a d- or f-block bimetallic μ-oxo complex that activates elemental chalcogens. Complex A also reacts with XeF₂ or 2 equiv. of Me₃SiCl to provide [{(py)U^IV(μ-X)₂U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5), Cl (6)), in which the oxo ligand has been substituted for two bridging halido ligands. Reacting A with either XeF₂ or Me₃SiCl in the presence of O(Bcat)₂ at room temperature forms [{(py)U^IV(μ-X)(μ-OBcat)U^IV(μ-O₂C₆H₄)(py)}(L^A)] (X = F (5A), Cl (6A)), which upon heating to 80 °C is converted to 5 and 6, respectively. In order to probe the importance of the bent U^IV-O-U^IV motif in A on the observed reactivity, the bis(boroxido)-U^IV/U^IV complex, [{(py)(pinBO)U^IVOU^IV(OBpin)(py)}(L^A)] (B), featuring a linear U^IV-O-U^IV bond angle was treated with H₂O and Me₃SiCl. Complex B reacts with two equiv. of either H₂O or Me₃SiCl to provide [{(py)HOU^IVOU^IVOH(py)}(L^A)] (7) and [{(py)ClU^IVOU^IVCl(py)}(L^A)] (8), respectively, in which reactions occur preferentially at the boroxido ligands, with the μ-oxo ligand unchanged. The formal U^IV oxidation state is retained in all of the products 1-8, and selective reactions at the bridging oxo ligand in A is facilitated by: (1) its highly nucleophilic character which is a result of a non-linear U^IV-O-U^IV bond angle causing an increase in U-O bond covalency and localisation of the lone pairs of electrons on the μ-oxo group, and (2) the presence of the bridging catecholate ligand, which destabilises a linear oxo-bridging geometry and stabilises the resulting products.

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

Synthetic studies of bimetallic uranium nitride complexes with the N(SiMe₃)₂ ligand have generated a new nitride complex of U(III), which is highly reactive toward C-H bonds and H₂. Treatment of the previously reported U(IV)/U(IV) nitride complex [Na(DME)₃][((Me₃Si)₂N)₂U(μ-N)(μ-κ²:CN̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (DME = 1,2-dimethoxyethane), 1, with 2 equiv of HNEt₃BPh₃ yielded the cationic U(IV)/U(IV) nitride complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][BPh₄] (THF = tetrahydrofuran), 3, by successive protonolysis of one N(SiMe₃)₂ ligand and the uranium-methylene bond. Reduction of 3 with KC₈ afforded a rare example of a U(III) nitride, namely, the U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)], 4. Complex 4 is highly reactive and undergoes 1,2-addition of the C-H bond of the N(SiMe₃)₂ ligand across the uranium-nitride moiety to give the U(III)/U(IV) imide cyclometalate complex, [((Me₃Si)₂N)₂(THF)U(μ-NH)(μ-κ²:C,N̵-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂))(THF)], 5. Complex 4 also reacts with toluene at -80 °C to yield an inverse sandwich imide complex arising from C-H bond activation of toluene, [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][{((Me₃Si)₂N)₃U(μ-NH)U(N(SiMe₃)₂)}₂(C₇H₈)], 6. Complex 4 effects the heterolytic cleavage of the C-H of phenylacetylene to yield the imide acetylide [{((Me₃Si)₂N)₂U(THF)}₂(μ-N)][((Me₃Si)₂N)₂U(η¹-CCPh)(μ₂-NH)(μ₂-η²:η¹-CCPh)U(N(SiMe₃)₂)₂], 7. Complex 4 also reacts with H₂ to produce an imide hydride U(III)/U(IV) complex, [{((Me₃Si)₂N)₂U(THF)}₂(μ-NH)(μ-H)], 9. These data demonstrate that nitride complexes of U(III) are accessible with amide ligands and show the high reactivity of molecular U(III) nitrides in C-H bond activation.

Intra- and intermolecular interception of a photochemically generated terminal uranium nitride

The photochemically generated synthesis of a terminal uranium nitride species is here reported and an examination of its intra- and intermolecular chemistry is presented. Treatment of the U(iii) complex L^ArUI(DME) ((L^Ar)²⁻ = 2,2′′-bis(Dippanilide)-p-terphenyl; Dipp = 2,6-diisopropylphenyl) with LiNIm^Dipp ((NIm^Dipp)⁻ = 1,3-bis(Dipp)-imidazolin-2-iminato) generates the sterically congested 3N-coordinate compound L^ArU(NIm^Dipp) (1). Complex 1 reacts with 1 equiv. of Ph₃CN₃ to give the U(iv) azide L^ArU(N₃)(NIm^Dipp) (2). Structural analysis of 2 reveals inequivalent N_α–N_β > N_β–N_γ distances indicative of an activated azide moiety predisposed to N₂ loss. Room-temperature photolysis of benzene solutions of 2 affords the U(iv) amide (N-L^Ar)U(NIm^Dipp) (3) via intramolecular N-atom insertion into the benzylic C–H bond of a pendant isopropyl group of the (L^Ar)²⁻ ligand. The formation of 3 occurs as a result of the intramolecular interception of the intermediately generated, terminal uranium nitride (L^Ar)U(N)(NIm^Dipp) (3′). Evidence for the formation of 3′ is further bolstered by its intermolecular capture, accomplished by photolyzing solutions of 2 in the presence of an isocyanide or PMe₃ to give (L^Ar)U[NCN(C₆H₃Me₂)](NIm^Dipp) (5) and (N,C-L^Ar*)U(N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> PMe₃)(NIm^Dipp) (6), respectively. These results expand upon the limited reactivity studies of terminal uranium–nitride moieties and provide new insights into their chemical properties.

Photolysis of the U(iv) azide L^ArU(NIm^Dipp) generates a reactive uranium nitride intermediate that can be intercepted by nucleophilic substrates – the first example of intermolecular chemistry of a rare photochemically generated uranium nitride.

The synthesis and versatile reducing power of low-valent uranium complexes

This synthesis and diverse reactivity of uranium(iii) and uranium(ii) complexes is discussed.

Synthesis and reactivity of a terminal aluminium–imide bond

The Al–N_imide bond in a new anionic aluminium imide complex reacts via a [2+2] cycloaddition with CO₂ to afford the dianionic carbamate ligand.

Tuning the structure, reactivity and magnetic communication of nitride-bridged uranium complexes with the ancillary ligands

The reactivity of the nitride ligand is increased in complexes of uranium(iv) when bound by the OSi(O^tBu)₃ ligand as opposed to N(SiMe₃)₂, but magnetic exchange coupling is decreased.

Molecular uranium nitride complexes were prepared to relate their small molecule reactivity to the nature of the U Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> U bonding imposed by the supporting ligand. The U⁴⁺–U⁴⁺ nitride complexes, [NBu₄][{((^tBuO)₃SiO)₃U}₂(μ-N)], [NBu₄]-1, and [NBu₄][((Me₃Si)₂N)₃U}₂(μ-N)], 2, were synthesised by reacting NBu₄N₃ with the U³⁺ complexes, [U(OSi(O^tBu)₃)₂(μ-OSi(O^tBu)₃)]₂ and [U(N(SiMe₃)₂)₃], respectively. Oxidation of 2 with AgBPh₄ gave the U⁴⁺–U⁵⁺ analogue, [((Me₃Si)₂N)₃U}₂(μ-N)], 4. The previously reported methylene-bridged U⁴⁺–U⁴⁺ nitride [Na(dme)₃][((Me₃Si)₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NSiMe₃)U(N(SiMe₃)₂)₂] (dme = 1,2-dimethoxyethane), [Na(dme)₃]-3, provided a versatile precursor for the synthesis of the mixed-ligand U⁴⁺–U⁴⁺ nitride complex, [Na(dme)₃][((Me₃Si)₂N)₃U(μ-N)U(N(SiMe₃)₂)(OSi(O^tBu)₃)], 5. The reactivity of the 1–5 complexes was assessed with CO₂, CO, and H₂. Complex [NBu₄]-1 displays similar reactivity to the previously reported heterobimetallic complex, [Cs{((^tBuO)₃SiO)₃U}₂(μ-N)], [Cs]-1, whereas the amide complexes 2 and 4 are unreactive with these substrates. The mixed-ligand complexes 3 and 5 react with CO and CO₂ but not H₂. The nitride complexes [NBu₄]-1, 2, 4, and 5 along with their small molecule activation products were structurally characterized. Magnetic data measured for the all-siloxide complexes [NBu₄]-1 and [Cs]-1 show uncoupled uranium centers, while strong antiferromagnetic coupling was found in complexes containing amide ligands, namely 2 and 5 (with maxima in the χ versus T plot of 90 K and 55 K). Computational analysis indicates that the U(μ-N) bond order decreases with the introduction of oxygen-based ligands effectively increasing the nucleophilicity of the bridging nitride.

A complete series of uranium(iv) complexes with terminal hydrochalcogenido (EH) and chalcogenido (E) ligands E = O, S, Se, Te

We here report the synthesis and characterization of a complete series of terminal hydrochalcogenido, U-EH, and chalcogenido uranium(iv) complexes, U≡E (with E = O, S, Se, Te), supported by the (^Ad,MeArOH)₃tacn (1,4,7-tris(3-(1-adamantyl)-5-methyl-2-hydroxybenzyl)-1,4,7-triazacyclononane) ligand system. Reaction of H₂E with the trivalent precursor [((^Ad,MeArO)₃tacn)U] (1) yields the corresponding uranium(iv) hydrochalcogenido complexes [((^Ad,MeArO)₃tacn)U(EH)] (2). Subsequent deprotonation of the terminal hydrochalcogenido species with KN(SiMe₃)₂, in the presence of 2.2.2-cryptand, gives access to the uranium(iv) complexes with terminal chalcogenido ligands [K(2.2.2-crypt)][((^Ad,MeArO)₃tacn)U≡E] (3). In order to study the influence of the varying terminal chalogenido ligands on the overall molecular and electronic structure, all complexes were studied by single-crystal X-ray diffractometry, UV/vis/NIR, electronic absorption, and IR vibrational spectroscopy as well as SQUID magnetometry and computational analyses (DFT, MO, NBO).

Theoretical Investigation of the Electronic Structure and Magnetic Properties of Oxo-Bridged Uranyl(V) Dinuclear and Trinuclear Complexes

The uranyl(V) complexes [UO₂(dbm)₂K(18C6)]₂ (dbm = dibenzoylmethanate) and [UO₂(L)]₃(L = 2-(4-tolyl)-1,3-bis(quinolyl)malondiiminate), exhibiting diamond-shaped U₂O₂ and triangular-shaped U₃O₃ cores respectively with 5f¹-5f¹ and 5f¹-5f¹-5f¹ configurations, have been investigated using relativistic density functional theory (DFT). The bond order and QTAIM analyses reveal that the covalent contribution to the bonding within the oxo cores is slightly more important for U₃O₃ than for U₂O₂, in line with the shorter U-O distances existing in the trinuclear complex in comparison to those in the binuclear complex. Using the broken symmetry (BS) approach combined with the B3LYP functional for the calculation of the magnetic exchange coupling constants (J) between the magnetic centers, the antiferromagnetic (AF) character of these complexes was confirmed, the estimated J values being respectively equal to -24.1 and -7.2 cm^-1 for the dioxo and trioxo species. It was found that the magnetic exchange is more sensitive to small variations of the core geometry of the dioxo species in comparison to the trioxo species. Although the robust AF exchange coupling within the U_xO_x cores is generally maintained when small variations of the UOU angle are applied, a weak ferromagnetic character appears in the dioxo species when this angle is higher than 114°, its value for the actual structure being equal to 105.9°. The electronic factors driving the magnetic coupling are discussed.

Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes

The boroxide ligand [OBAr2]- (Ar = Mes, Trip) is shown to be able to support both UIII and UIV centres for the first time. The synthesis and structures of homoleptic and heteroleptic UIII and UIV complexes are reported. The UX3 complex with larger substituents, [U(OBTrip2)3]2, exhibits greater thermal stability compared to less encumbered [U(OBMes2)3]2 but reacts with a smaller range of the small molecules tested to date. Initial studies on their capacity to participate in small molecule chemistry show that dark purple [U(OBMes2)3]2 binds and/or reductively activates a variety of small molecules such as pyridine-oxide, triphenylphosphineoxide, sulfur, and dicyclohexylcarbodiimide. While [U(OBMes2)3]2 shows no reaction with CO or CO2, [U(OBTrip2)3]2 is oxidised by both, in the former case forming [U(OBTrip2)4], and in the latter case forming a small quantity of the structurally characterised μ-carbonate product [(μ-CO3){U(OBTrip2)3}2].

Facile N-functionalization and strong magnetic communication in a diuranium(v) bis-nitride complex

Uranium nitride complexes are of high interest because of their ability to effect dinitrogen reduction and functionalization and to promote magnetic communication, but studies of their properties and reactivity remain rare. Here we have prepared in 73% yield the diuranium(v) bis-nitride complex [K2{[U(OSi(O t Bu)3)3]2(μ-N)2}], 4, from the thermal decomposition of the nitride-, azide-bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-N3)}], 3. The bis-nitride 4 reacts in ambient conditions with 1 equiv. of CS2 and 1 equiv. of CO2 resulting in N-C bond formation to afford the diuranium(v) complexes [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-S)(μ-NCS)}], 5 and [K2{[U(OSi(O t Bu)3)3]2(μ-N)(μ-O)(μ-NCO)}], 6, respectively. Both nitrides in 4 react with CO resulting in oxidative addition of CO to one nitride and CO cleavage by the second nitride to afford the diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-CN)(μ-O)(μ-NCO)}], 7. Complex 4 also effects the remarkable oxidative cleavage of H2 in mild conditions to afford the bis-imido bridged diuranium(iv) complex [K2{[U(OSi(O t Bu)3)3]2(μ-NH)2}], 8 that can be further protonated to afford ammonia in 73% yield. Complex 8 provides a good model for hydrogen cleavage by metal nitrides in the Haber-Bosch process. The measured magnetic data show an unusually strong antiferromagnetic coupling between uranium(v) ions in the complexes 4 and 6 with Neel temperatures of 77 K and 60 K respectively, demonstrating that nitrides are attractives linkers for promoting magnetic communication in uranium complexes.

DFT Investigations of the Magnetic Properties of Actinide Complexes

Over the past 25 years, magnetic actinide complexes have been the object of considerable attention, not only at the experimental level, but also at the theoretical one. Such systems are of great interest, owing to the well-known larger spin–orbit coupling for actinide ions, and could exhibit slow relaxation of the magnetization, arising from a large anisotropy barrier, and magnetic hysteresis of purely molecular origin below a given blocking temperature. Furthermore, more diffuse 5f orbitals than lanthanide 4f ones (more covalency) could lead to stronger magnetic super-exchange. On the other hand, the extraordinary experimental challenges of actinide complexes chemistry, because of their rarity and toxicity, afford computational chemistry a particularly valuable role. However, for such a purpose, the use of a multiconfigurational post-Hartree-Fock approach is required, but such an approach is computationally demanding for polymetallic systems—notably for actinide ones—and usually simplified models are considered instead of the actual systems. Thus, Density Functional Theory (DFT) appears as an alternative tool to compute magnetic exchange coupling and to explore the electronic structure and magnetic properties of actinide-containing molecules, especially when the considered systems are very large. In this paper, relevant achievements regarding DFT investigations of the magnetic properties of actinide complexes are surveyed, with particular emphasis on some representative examples that illustrate the subject, including actinides in Single Molecular Magnets (SMMs) and systems featuring metal-metal super-exchange coupling interactions. Examples are drawn from studies that are either entirely computational or are combined experimental/computational investigations in which the latter play a significant role.

Oxidation of uranium(iv) thiocyanate complexes: cation–cation interactions in mixed-valent uranium coordination chains

Oxidation of Cs₄[U(NCS)₈] in different solvents results in two mixed-valent uranium compounds. Spectroscopic, magnetic and computational data support a unique [U^IVU^VU^IV][U^VI] oxidation state assignment in [U(DMF)₈(μ-O)U(NCS)₅(μ-O)U(DMF)₇(NCS)][UO₂(NCS)₅].

Uranium(iii) complexes supported by hydrobis(mercaptoimidazolyl)borates: synthesis and oxidation chemistry

The reaction of [UI3(thf)4] with the sodium or lithium salts of hydrobis(2-mercapto-1-methylimidazolyl)borate ligands ([H(R)B(timMe)2]-) in a 1 : 2 ratio, in tetrahydrofuran, gave the U(iii) complexes [UI{κ3-H,S,S'-H(R)B(timMe)2}2(thf)2] (R = H (1), Ph (2)) in good yields. Crystals of [UI{κ3-H,S,S'-H(Ph)B(timMe)2}2(thf)2] (2) were obtained by recrystallization from a tetrahydrofuran/acetonitrile solution, and the ion-separated uranium complex [U{κ3-H,S,S'-H(Ph)B(timMe)2}2(CH3CN)3][I] (3-I) was obtained by dissolution of 2 in acetonitrile followed by recrystallization. One-electron oxidation of 2 with AgBPh4 or I2 resulted in the formation of the cationic U(iv) complexes [U{κ3-H,S,S'-H(Ph)B(timMe)2}3][X] (X = BPh4 (6-BPh4), I (6-I)), due to a ligand redistribution process. These complexes are the first examples of homoleptic poly(azolyl)borate U(iv) complexes. Treatment of complex 2 with azobenzene led to the isolation of crystals of the U(iv) compound [UI{κ3-H(Ph)B(timMe)2}2(κ2-timMe)] (7). Treatment of 2 with pyridine-N oxide (pyNO) led to the formation of the uranyl complex [UO2{κ2-S,S'-H(Ph)B(timMe)2}2] (8) and of complex 6-I, while from the reaction of [U{κ3-H(Ph)B(timMe)2}2(thf)3][BPh4] (5) with pyNO, the oxo-bridged U(iv) complex [{U{κ3-H(Ph)B(timMe)2}2(pyNO)}2(μ-O)][BPh4]2 (9) was also obtained. In the U(iii) and U(iv) complexes, the bis(azolyl)borate ligands bind to the uranium center in a κ3-H,S,S' coordination mode, while in the U(vi) complex the ligands bind to the metal in a κ2-S,S' mode. The presence of UH-B interactions in the solid-state, for the nine-coordinate complexes 1, 2, 3, 6 and 7 and for the eight-coordinate complex 9, was supported by IR spectroscopy and/or X-ray diffraction analysis.

A diuranium carbide cluster stabilized inside a C80 fullerene cage

Unsupported non-bridged uranium-carbon double bonds have long been sought after in actinide chemistry as fundamental synthetic targets in the study of actinide-ligand multiple bonding. Here we report that, utilizing Ih(7)-C80 fullerenes as nanocontainers, a diuranium carbide cluster, U=C=U, has been encapsulated and stabilized in the form of UCU@Ih(7)-C80. This endohedral fullerene was prepared utilizing the Krätschmer-Huffman arc discharge method, and was then co-crystallized with nickel(II) octaethylporphyrin (NiII-OEP) to produce UCU@Ih(7)-C80·[NiII-OEP] as single crystals. X-ray diffraction analysis reveals a cage-stabilized, carbide-bridged, bent UCU cluster with unexpectedly short uranium-carbon distances (2.03 Å) indicative of covalent U=C double-bond character. The quantum-chemical results suggest that both U atoms in the UCU unit have formal oxidation state of +5. The structural features of UCU@Ih(7)-C80 and the covalent nature of the U(f1)=C double bonds were further affirmed through various spectroscopic and theoretical analyses.

Double uranium oxo cations derived from uranyl by borane or silane reduction

A new type of double uranium oxo cation [O-U-O-U-O]4+ is prepared by selective oxygen-atom abstraction from macrocyclic uranyl complexes using either boranes or silanes. A significant degree of multiple U[double bond, length as m-dash]O bonding is evident throughout the U2O3 core, but either trans-,cis- or trans-,trans-OUOUO motifs can be isolated as boron- or silicon-capped oxo complexes. Further controlled deoxygenation of the borylated system is also possible.

The [U2 F12 ]2- Anion of Sr[U2 F12 ]

The D_2h -symmetric dinuclear complex anion [U₂ F₁₂ ]^2- of pastel green Sr[U₂ F₁₂ ] shows a hitherto unknown structural feature: The coordination polyhedra around the U atoms are edge-linked monocapped trigonal prisms, the U^V atoms are therefore seven-coordinated. This leads to a U-U distance of 3.8913(6) Å. A weak U^V -U^V interaction is observed for the dinuclear [U₂ F₁₂ ]^2- complex and described by the antiferromagnetic exchange J_exp of circa -29.9 cm^-1 . The crystalline compound can be easily prepared from SrF₂ and β-UF₅ in anhydrous hydrogen fluoride (aHF) at room temperature. It was studied by means of single crystal X-ray diffraction, IR, Raman and UV/VIS spectroscopy, magnetic measurements, and by molecular as well as by solid-state quantum chemical calculations.

Evidence for single metal two electron oxidative addition and reductive elimination at uranium

Reversible single-metal two-electron oxidative addition and reductive elimination are common fundamental reactions for transition metals that underpin major catalytic transformations. However, these reactions have never been observed together in the f-block because these metals exhibit irreversible one- or multi-electron oxidation or reduction reactions. Here we report that azobenzene oxidises sterically and electronically unsaturated uranium(III) complexes to afford a uranium(V)-imido complex in a reaction that satisfies all criteria of a single-metal two-electron oxidative addition. Thermolysis of this complex promotes extrusion of azobenzene, where H-/D-isotopic labelling finds no isotopomer cross-over and the non-reactivity of a nitrene-trap suggests that nitrenes are not generated and thus a reductive elimination has occurred. Though not optimally balanced in this case, this work presents evidence that classical d-block redox chemistry can be performed reversibly by f-block metals, and that uranium can thus mimic elementary transition metal reactivity, which may lead to the discovery of new f-block catalysis.