Synthesis and structure of a terminal uranium nitride complex

Installation of Copper(I) and Silver(I) Sites into TREN-Based Porous Organic Cages via Postsynthetic Metalation

Porous organic cages (POCs) and metal-organic polyhedra (MOPs) function as zero-dimensional porous materials, able to mimic many functions of insoluble framework materials while offering processability advantages. A popular approach to access tailored metal-based motifs in extended network materials is postsynthetic metalation, which allows metal installation to be decoupled from framework assembly. Surprisingly, this approach has only sparingly been reported for molecular porous materials. In this report, we demonstrate postsynthetic metalation of tetrahedral [4 + 4] POCs assembled from tris(2-aminoethyl)amine (TREN) and 1,3,5-tris(4-formylphenyl)benzene. The trigonally symmetric TREN motif is a common chelator in coordination chemistry and, in the POCs explored herein, readily binds copper(I) and silver(I) to form cationic cages bearing discrete mononuclear coordination fragments. Metalation retains cage porosity, allowing us to compare the sorption properties of the parent organic and metalated cages. Interestingly, introduction of copper(I) facilitates activated oxygen chemisorption, demonstrating how targeted metalation can be exploited to tune the sorption characteristics of porous molecular materials.

Reactivity of a triamidoamine terminal uranium(VI)-nitride with 3d-transition metal metallocenes

Reactions between [(TrenTIPS)UVIN] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3-) and [MII(η5-C5R5)2] (M/R = Cr/H, Mn/H, Fe/H, Ni/H) were intractable, but M/R = Co/H or Co/Me afforded [(TrenTIPS)UVN-(η1:η4-C5H5)CoI(η5-C5H5)] (2) and [(TrenTIPS)UIV-NH2] (3), respectively. For M/R = V/H [(TrenTIPS)UIV-NVIV(η5-C5H5)2] (4), was isolated. Complexes 2-4 evidence one-/two-electron uranium reductions, nucleophilic nitrides, and partial N-atom transfer.

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.

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

There is long-standing interest in nonaqueous uranium chemistry because of fundamental questions about uranium's variable chemical bonding and the similarities of this pseudo-Group 6 element to its congener d-block elements molybdenum and tungsten. To provide historical context, with reference to a conference presentation slide presented around 1988 that advanced a defining collection of top targets, and the challenge, for synthetic actinide chemistry to realize in isolable complexes under normal experimental conditions, this Viewpoint surveys progress against those targets, including (i) CO and related π-acid ligand complexes, (ii) alkylidenes, carbynes, and carbidos, (iii) imidos and terminal nitrides, (iv) homoleptic polyalkyls, -alkoxides, and -aryloxides, (v) uranium-uranium bonds, and (vi) examples of topics that can be regarded as branching out in parallel from the leading targets. Having summarized advances from the past four decades, opportunities to build on that progress, and hence possible future directions for the field, are highlighted. The wealth and diversity of uranium chemistry that is described emphasizes the importance of ligand-metal complementarity in developing exciting new chemistry that builds our knowledge and understanding of elements in a relativistic regime.

Arene-, Chlorido-, and Imido-Uranium Bis- and Tris(boryloxide) Complexes

We introduce the boryloxide ligand {(HCNDipp)2BO}- (NBODipp, Dipp = 2,6-di-isopropylphenyl) to actinide chemistry. Protonolysis of [U{N(SiMe3)2}3] with 3 equiv of NBODippH produced the uranium(III) tris(boryloxide) complex [U(NBODipp)3] (1). In contrast, treatment of UCl4 with 3 equiv of NBODippK in THF at room temperature or reflux conditions produced only [U(NBODipp)2(Cl)2(THF)2] (2) with 1 equiv of NBODippK remaining unreacted. However, refluxing the mixture of 2 and unreacted NBODippK in toluene instead of THF afforded the target complex [U(NBODipp)3(Cl)(THF)] (3). Two-electron oxidation of 1 with AdN3 (Ad = 1-adamantyl) afforded the uranium(V)-imido complex [U(NBODipp)3(NAd)] (4). The solid-state structure of 1 reveals a uranium-arene bonding motif, and structural, spectroscopic, and DFT calculations all suggest modest uranium-arene δ-back-bonding with approximately equal donation into the arene π4 and π5 δ-symmetry π* molecular orbitals. Complex 4 exhibits a short uranium(V)-imido distance, and computational modeling enabled its electronic structure to be compared to related uranium-imido and uranium-oxo complexes, revealing a substantial 5f-orbital crystal field splitting and extensive mixing of 5f |ml,ms⟩ states and mj projections. Complexes 1-4 have been variously characterized by single-crystal X-ray diffraction, 1H NMR, IR, UV/vis/NIR, and EPR spectroscopies, SQUID magnetometry, elemental analysis, and CONDON, F-shell, DFT, NLMO, and QTAIM crystal field and quantum chemical calculations.

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.

Temperature induced single-crystal to single-crystal transformation of uranium azide complexes

The monomeric and dimeric uranium azide complexes {[(CH3)2NCH2CH2NPiPr2]2U(N3)2} (2) and {[(CH3)2NCH2CH2NPiPr2]2U(N3)2}2 (3) were synthesized by treating complex 1 with NaN3 at 60 and -20 °C, respectively. A temperature-induced single-crystal to single-crystal transformation of 3 to 2 was observed. The reduction of either 2 or 3 with KC8 yields a uranium nitride complex {[(CH3)2NCH2CH2NPiPr2]4U2(μ-N)2} (4).

Perplexing EPR Signals from 5f36d1 U(II) Complexes

Metal complexes with unpaired electrons in orbitals of different angular momentum quantum numbers (e.g., f and d orbitals) are unusual and opportunities to study the interactions among these electrons are rare. X-band electron paramagnetic resonance (EPR) data were collected at <10 and 77 K on 10 U(II) complexes with 5f36d1 electron configurations and on some analogous Ce(II), Pr(II), and Nd(II) complexes with 4fn5d1 electron configurations. The U(II) compounds unexpectedly display similar two-line axial signals with g|| = 2.04 and g⊥ = 2.00 at 77 K. In contrast, U(II) complexes with 5f4 configurations are EPR-silent. Unlike U(II), the congenic 4f35d1 Nd(II) complex is EPR-silent. The Ce(II) complex with a 4f15d1 configuration is also EPR-silent, but a signal is observed for the Pr(II) complex, which has a 4f25d1 configuration. Whether or not an EPR signal is expected for these complexes depends on the coupling between f and d electrons. Since the coupling in U(II) systems is expected to be sufficiently strong to preclude an EPR signal from compounds with a 5f36d1 configuration, the results are viewed as unexplained phenomena. However, they do show that 5f36d1 U(II) samples can be differentiated from 5f4 U(II) complexes by EPR spectroscopy.

Two-Electron Redox Reactivity of Thorium Supported by Redox-Active Tripodal Frameworks

The high stability of the + IVoxidation state limits thorium redox reactivity. Here we report the synthesis and the redox reactivity of two Th(IV) complexes supported by the arene-tethered tris(siloxide) tripodal ligands [(KOSiR2 Ar)3 -arene)]. The two-electron reduction of these Th(IV) complexes generates the doubly reduced [KTh((OSi(Ot Bu)2 Ar)3 -arene)(THF)2 ] (2OtBu ) and [K(2.2.2-cryptand)][Th((OSiPh2 Ar)3 -arene)(THF)2 ](2Ph -crypt) where the formal oxidation state of Th is +II. Structural and computational studies indicate that the reduction occurred at the arene anchor of the ligand. The robust tripodal frameworks store in the arene anchor two electrons that become available at the metal center for the two-electron reduction of a broad range of substrates (N2 O, COT, CHT, Ph2 N2 , Ph3 PS and O2 ) while retaining the ligand framework. This work shows that arene-tethered tris(siloxide) tripodal ligands allow implementation of two-electron redox chemistry at the thorium center while retaining the ligand framework unchanged.

f-Element heavy pnictogen chemistry

The coordination and organometallic chemistry of the f-elements, that is group 3, lanthanide, and actinide ions, supported by nitrogen ligands, e.g. amides, imides, and nitrides, has become well developed over many decades. In contrast, the corresponding f-element chemisty with the heavier pnictogen analogues phosphorus, arsenic, antimony, and bismuth has remained significantly underdeveloped, due largely to a lack of suitable synthetic methodologies and also the inherent hard(f-element)-soft(heavier pnictogen) acid-base mismatch, but has begun to flourish in recent years. Here, we review complexes containing chemical bonds between the f-elements and heavy pnictogens from phosphorus to bismuth that spans five decades of endeavour. We focus on complexes whose identity has been unambiguously established by structural authentication by single-crystal X-ray diffraction with respect to their synthesis, characterisation, bonding, and reactivity, in order to provide a representative overview of this burgeoning area. By highlighting that much has been achieved but that there is still much to do this review aims to inspire, focus and guide future efforts in this area.

A trinuclear metallasilsesquioxane of uranium(III)

The silsesquioxane ligand (iBu)7Si7O9(OH)3 (iBuPOSSH3) is revealed as an attractive system for the assembly of robust polynuclear complexes of uranium(III) and allowed the isolation of the first example of a trinuclear U(III) complex ([U3(iBuPOSS)3]) that exhibits magnetic communication and promotes dinitrogen reduction in the presence of reducing agent.

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.

Synthesis and Characterization of U≡C Triple Bonds in Fullerene Compounds

Despite decades of efforts, the actinide-carbon triple bond has remained an elusive target, defying synthesis in any isolable compound. Herein, we report the successful synthesis of uranium-carbon triple bonds in carbide-bridged bimetallic [U≡C-Ce] units encapsulated inside the fullerene cages of C72 and C78. The molecular structures of UCCe@C2n and the nature of the U≡C triple bond were characterized through X-ray crystallography and various spectroscopic analyses, revealing very short uranium-carbon bonds of 1.921(6) and 1.930(6) Å, with the metals existing in their highest oxidation states of +6 and +4 for uranium and cerium, respectively. Quantum-chemical studies further demonstrate that the C2n cages are crucial for stabilizing the [UVI≡C-CeIV] units through covalent and coordinative interactions. This work offers a new fundamental understanding of the elusive uranium-carbon triple bond and informs the design of complexes with similar bonding motifs, opening up new possibilities for creating distinctive molecular compounds and materials.

Stabilization of Pu(IV) in PuBr4(OPCy3)2 and Comparisons with Structurally Similar ThX4(OPR3)2 (R = Cy, Ph) Molecules

The pursuit of a trivalent plutonium halide phosphine oxide compound, e.g., "PuBr3(OPR)3," instead led to the isolation of the tetravalent trans-PuIVBr4(OPCy3)2, PuBr/Cy, compound by spontaneous oxidation of PuIII. The donating nature of phosphine oxides has allowed the isolation and characterization of PuBr/Cy by crystallographic, multinuclear NMR, solid state, and solution phase UV-vis-NIR spectroscopic techniques. The presence of a putative plutonyl(VI) complex formulated as "trans-PuVIO2Br2(OPCy3)2" was also observed spectroscopically and tentatively by single-crystal X-ray diffraction as a cocrystal of PuBr/Cy. A series of trans-ThX4(OPCy3)2 (X = Cl, ThCl/Cy; Br, ThBr/Cy; I, ThI/Cy) complexes were synthesized for comparison to PuBr/Cy. The triphenylphosphine oxide, OPPh3, complexes, trans-AnI4(OPPh3)2 (An = Th, ThI/Ph; U, UI/Ph), were also synthesized for comparison, completing the series trans-UX4(OPPh3)2 (X = Cl, Br, I), UX/Ph. To enable the synthesis of ThI/Cy and ThI/Ph, a new nonaqueous thorium iodide starting material, ThI4(Et2O)2, was synthesized. The syntheses of organic solvent soluble ThI4L2 (L = Et2O, OPCy3, and OPPh3) are the first examples of crystallographically characterized neutral thorium tetraiodide materials beyond binary ThI4. To show the viability of ThI4(Et2O)2 as a starting material for organothorium chemistry, (C5Me4H)3ThI was synthesized and crystallographically characterized.

Two-Electron Oxidations at a Single Cerium Center

Two-electron oxidations are ubiquitous and play a key role in the synthesis and catalysis. For transition metals and actinides, two-electron oxidation often takes place at a single-metal site. However, redox reactions at rare-earth metals have been limited to one-electron processes due to the lack of accessible oxidation states. Despite recent advancements in nontraditional oxidation state chemistry, the low stability of low-valent compounds and large disparity among different oxidation states prevented the implementation of two-electron processes at a single rare-earth metal center. Here we report two-electron oxidations at a cerium(II) center to yield cerium(IV) terminal oxo and imido complexes. A series of cerium(II-IV) complexes supported by a tripodal tris(amido)arene ligand were synthesized and characterized. Experimental and theoretical studies revealed that the cerium(II) complex is best described as a 4f2 ion stabilized by δ-backdonation to the anchoring arene, while the cerium(IV) oxo and imido complexes exhibit multiple bonding characters. The accomplishment of two-electron oxidations at a single cerium center brings a new facet to molecular rare-earth metal chemistry.

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.

Unusual Hydrogenation Reactivities of a Thiolate-Bridged Dicobalt μ-Nitride Featuring a Bent {CoIII-N-CoIII} Core

Transition metal nitrides have received considerable attention owing to their crucial roles in nitrogen fixation and nitrogen atom transfer reactions. Compared to the early and middle transition metals, it is much more challenging to access late transition metal nitrides, especially cobalt in group 9. So far, only a handful of cobalt nitrides have been reported; consequently, their hydrogenation reactivity is largely unexplored. Herein, we present a structurally and spectroscopically well-characterized thiolate-bridged dicobalt μ-nitride [Cp*CoIII(μ-SAd)(μ-N)CoIIICp*] (2) featuring a bent {CoIII(μ-N)CoIII} core. Remarkably, complex 2 can realize not only direct hydrogenation of nitride to amide but also stepwise N-H bond formation from nitride to ammonia. Specifically, 2 can facilely activate dihydrogen (H2) at mild conditions to generate a dicobalt μ-amide [Cp*CoII(μ-SAd)(μ-NH2)CoIICp*] (4) via an unusual mechanism of two-electron oxidation of H2 as proposed by computational studies; in the presence of protons (H+) and electrons, nitride 2 can convert to dicobalt μ-imide [Cp*CoIII(μ-SAd)(μ-NH)CoIIICp*][BPh4] (3[BPh4]) and to CoIICoII μ-amide 4, and finally release ammonia. In contrast to 2, the only other structurally characterized dicobalt μ-nitride Na(THF)4{[(ketguan)CoIII(N3)]2(μ-N)} (ketguan = [(tBu2CN)C(NDipp)2]-, Dipp = 2,6-diisopropylphenyl) (e) that possesses a linear {CoIII(μ-N)CoIII} moiety cannot directly react with H2 or H+. Further in-depth electronic structure analyses shed light on how the varying geometries of the {CoIII(μ-N)CoIII} moieties in 2 and e, bent vs linear, impart their disparate reactivities.

Valence Fluctuation of Uranium Ions in Uranium Sesquinitride Revealed by Dynamical Mean-field Theory Merged with Density Functional Theory

The electronic properties, in particular, the occupation number of 5f electrons and the valence state of U ions in uranium sesquinitride (U2 N3 ) are studied by using density functional theory (DFT) calculations merged with dynamical mean-field theory (DMFT). The results demonstrate that j=5/2 and j=7/2 manifolds are in the weakly correlated metallic and weakly correlated insulating regimes, respectively. The quasi-particle weights indicate that LS coupling scheme is more feasible for 5f electrons, which are not in the orbital-selective localized state. The weighted summation of the occupation probabilities of 5fn (n=0,1,2,3,4) atomic configurations suggests that 5f electrons have the inter-configuration fluctuation, or the mixed-valence state for U ions, together with an average occupation number of 5f electrons n5f ∼2.234, which is in good agreement with the electron localization function (ELF) and occupation analysis based on other DFT-based calculations. The 5fn -mixing-driven inter-configuration fluctuation might originate from the dual nature of 5f electrons, and the flexible electronic configuration of U ions. Finally, the so-called quasiparticle band structure is also discussed.

Divergent Stabilities of Tetravalent Cerium, Uranium, and Neptunium Imidophosphorane Complexes

The study of the redox chemistry of mid-actinides (U-Pu) has historically relied on cerium as a model, due to the accessibility of trivalent and tetravalent oxidation states for these ions. Recently, dramatic shifts of lanthanide 4+/3+ non-aqueous redox couples have been established within a homoleptic imidophosphorane ligand framework. Herein we extend the chemistry of the imidophosphorane ligand (NPC=[N=Pt Bu(pyrr)2 ]- ; pyrr=pyrrolidinyl) to tetrahomoleptic NPC complexes of neptunium and cerium (1-M, 2-M, M=Np, Ce) and present comparative structural, electrochemical, and theoretical studies of these complexes. Large cathodic shifts in the M4+/3+ (M=Ce, U, Np) couples underpin the stabilization of higher metal oxidation states owing to the strongly donating nature of the NPC ligands, providing access to the U5+/4+ , U6+/5+ , and to an unprecedented, well-behaved Np5+/4+ redox couple. The differences in the chemical redox properties of the U vs. Ce and Np complexes are rationalized based on their redox potentials, degree of structural rearrangement upon reduction/oxidation, relative molecular orbital energies, and orbital composition analyses employing density functional theory.

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.

Actinide inverse trans influence versus cooperative pushing from below and multi-center bonding

Actinide-ligand bonds with high multiplicities remain poorly understood. Decades ago, an effect known as 6p pushing from below (PFB) was proposed to enhance actinide covalency. A related effect-also poorly understood-is inverse trans influence (ITI). The present computational study of actinide-ligand covalent interactions with high bond multiplicities quantifies the energetic contributions from PFB and identifies a hitherto overlooked fourth bonding interaction for 2nd-row ligands in the studied organometallic systems. The latter are best described by a terminal O/N ligand exhibiting quadruple bonding interactions with the actinide. The 4th interaction may be characterized as a multi-center or charge-shift bond involving the trans ligand. It is shown in this work that the 4th bonding interaction is a manifestation of ITI, assisted by PFB, and provides a long-sought missing piece in the understanding of actinide chemistry.

Tetravalent Uranium and Thorium Complexes: Elucidating Disparate Reactivities of AnIVCl2 (An = U, Th) Supported by a Pyridine-Decorated Dianionic Ligand

Although synthesis, reactivity, and bonding of U(IV) and Th(IV) complexes have been extensively studied, direct comparison of fully analogous compounds is rare. Herein, we report corresponding complexes 1-U and 1-Th, in which U(IV) and Th(IV) are supported by the tetradentate pyridine-decorated dianionic ligand N₂NN' (1,1,1-trimethyl-N-(2-(((pyridin-2-ylmethyl)(2-((trimethylsilyl)amino)benzyl)amino)methyl)phenyl)silanamine). Although 1-U and 1-Th are structurally very similar, they display disparate reactivities with TMS₃SiK (tris(trimethylsilyl)silylpotassium). The reaction of (N₂NN')UCl₂ (1-U) and 1 equiv of TMS₃SiK in THF unexpectedly formed [Cl(N₂NN')U]₂O (2-U) featuring an unusual bent U-O-U moiety. In contrast, a salt elimination reaction between (N₂NN')ThCl₂ (1-Th) and 1 equiv of TMS₃SiK led to thorium complex 2-Th, in which the pyridyl group has undergone a 1,4-addition nucleophilic attack. Complex 2-Th serves as a synthon for preparing dimetallic bis-azide complex 3-Th by reaction with NaN₃. The complexes were characterized by X-ray crystal diffraction, solution NMR, FT-IR, and elemental analysis. Computations of the formation mechanism of 2-U from 1-U suggest reduced U(III) as a key intermediate for promoting the cleavage of the C-O bonds of THF. The inaccessible nature of Th(III) as an intermediate oxidation state explains the very different reactivity of 1-Th versus 1-U. Given that reactants 1-U and 1-Th and products 2-U and 2-Th all comprise tetravalent actinides, this is an unusual case of very disparate reactivity despite no net change in the oxidation state. Complexes 2-U and 3-Th provide a basis for the synthesis of other dinuclear actinide complexes with novel reactivity and properties.

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.

Evaluating f-Orbital Participation in the UV═E Multiple Bonds of [U(E)(NR2)3] (E = O, NSiMe3, NAd; R = SiMe3)

The reaction of 1 equiv of 1-azidoadamantane with [U^III(NR₂)₃] (R = SiMe₃) in Et₂O results in the formation of [U^V(NR₂)₃(NAd)] (1, Ad = 1-adamantyl) in good yields. The electronic structure of 1, as well as those of the related U(V) complexes, [U^V(NR₂)₃(NSiMe₃)] (2) and [U^V(NR₂)₃(O)] (3), were analyzed with EPR spectroscopy, SQUID magnetometry, NIR-visible spectroscopy, and crystal field modeling. This analysis revealed that, within this series of complexes, the steric bulk of the E^2- (E═O, NR) ligand is the most important factor in determining the electronic structure. In particular, the increasing steric bulk of this ligand, on moving from O^2- to [NAd]^2-, results in increasing U═E distances and E-U-N_amide angles. These changes have two principal effects on the resulting electronic structure: (1) the increasing U═E distances decreases the energy of the f_σ orbital, which is primarily σ* with respect to the U═E bond, and (2) the increasing E-U-N_amide angles increases the energy of f_δ, due to increasing antibonding interactions with the amide ligands. As a result of the latter change, the electronic ground state for complexes 1 and 2 is primarily f_φ in character, whereas the ground state for complex 3 is primarily f_δ.

Homoleptic Acetylacetonate (acac) and β-Ketoiminate (acnac) Complexes of Uranium

Transmetalation of potassium salts of differently substituted acetylacetonate (acac) and β-ketoiminate (acnac) with [U(I)₃(dioxane)_1.5] and [U(I)₄(dioxane)₂] resulted in the formation of homoleptic, octahedral complexes [U(^tBuacnac^Ph)₃] (with ^tBuacnac^Ph = 2,2,6,6-tetramethyl-5-(phenylimino)heptan-3-onate) in the oxidation states +III and +IV and the homoleptic, square prismatic complexes [U^IV(^Meacnac^Ph)₄] (with ^Meacnac^Ph = 4-(phenylimino)pentan-2-onate) and the homoleptic, square antiprismatic complexes [U(^tBuacac)₄] [with acac = 2,2,6,6-tetramethyl-3,5-heptanedionate (^tBuacac), 2,2,6,6-tetramethyl,4-methyl-3,5-heptanedionate (^tBuac^Meac), and 2,2,6,6-tetramethyl-4-phenyl-3,5-heptanedionate (^tBuac^Phac)] in oxidation states +III, +IV, and +V. Oxidation of [U^III(^tBuacnac^Ph)₃] (1) with AgOTf yielded [U^IV(^tBuacnac^Ph)₃][OTf] (2), which was fully characterized by single-crystal X-ray diffraction analysis, a combination of ultraviolet/visible/near-infrared, nuclear magnetic resonance, and infrared spectroscopies, and solid-state superconducting quantum interference device magnetization studies. Complexation of the sterically less encumbering ligand derivative ^Meacnac^Ph provided access to the tetravalent, square antiprismatic complex [U^IV(^Meacnac^Ph)₄] (3). Cyclovoltammetric analysis of the square antiprismatic [U^IV(^tBuacac)₄] (4), [U^IV(^tBuac^Meac)₄] (5), and [U^IV(^tBuac^Phac)₄] (6) revealed reversible anodic and cathodic waves, attributable to the U(III/IV) and U(IV/V) redox couples, both being chemically accessible, as tested in the case of 5. The corresponding U(III) and U(V) compounds, [K(2.2.2-cryptand)][U^III(^tBuac^Meac)₄] (7) and [U^V(^tBuac^Meac)₄][SbF₆] (8), were synthesized accordingly. Unfortunately, reduced 7 proved to be too reactive for isolation and could only be detected by electron paramagnetic resonance spectroscopy. Notably, electrochemical studies on homoleptic uranium(IV) complexes with differently derivatized (R) ac^Rac ligands (R = H, Me, or Ph) feature large electrochemical windows of up to 2.91 V, measured between the uranium(III) and the uranium(V) species, in addition to high stability toward repeated potential scans.

Synthesis and Characterization of a Bridging Cerium(IV) Nitride Complex

Complexes featuring lanthanide-ligand multiple bonds are rare and highly reactive. They are important synthetic targets to understand 4f/5d-bonding in comparison to d-block and actinide congeners. Herein, the isolation and characterization of a bridging cerium(IV)-nitride complex: [(TriNOx)Ce(Li₂μ-N)Ce(TriNOx)][BAr^F₄] is reported, the first example of a molecular cerium-nitride. The compound was isolated by deprotonating a monometallic cerium(IV)-ammonia complex: [Ce^IV(NH₃)(TriNOx)][BAr^F₄]. The average Ce═N bond length of [(TriNOx)Ce(Li₂μ-N)Ce(TriNOx)][BAr^F₄] was 2.117(3) Å. Vibrational studies of the ¹⁵N-isotopomer exhibited a shift of the Ce═N═Ce asymmetric stretch from ν = 644 cm^-1 to 640 cm^-1, and X-ray spectroscopic studies confirm the +4 oxidation state of cerium. Computational analyses showed strong involvement of the cerium 4f shell in bonding with overall 16% and 11% cerium weight in the σ- and π-bonds of the Ce═N═Ce fragment, respectively.

Actinide Pnictinidene Chemistry: A Terminal Thorium Parent-Arsinidene Complex Stabilised by a Super-Bulky Triamidoamine Ligand

We report the direct synthesis of the terminal pnictidenes [An(TrenTCHS )(PnH)][M(2,2,2-cryptand)] (TrenTCHS ={N(CH2 CH2 NSiCy3 )3 }3- ; An/Pn/M=Th/P/Na 5, Th/As/K 6, U/P/Na 7, U/As/K 8) and their conversion to the pnictides [An(TrenTCHS )(PnH2 )] (An/Pn=Th/P 9, Th/As 10, U/P 11, U/As 12). Use of the super-bulky TrenTCHS ligand was essential to accessing complete families, and 6 is an unprecedented example of a terminal thorium-arsinidene complex and only the second structurally authenticated parent terminal arsinidene complex of any metal. Comparison of the terminal Th=AsH unit of 6 to the bridging ThAs(H)K linkage in structurally analogous [Th(TrenTIPS ){μ-As(H)K(15-crown-5)}] (TrenTIPS ={N(CH2 CH2 NSiPri 3 )3 }3- ) reveals a stronger Th-As bond in the former compared to the latter, and a large response overall to the nature of the Th=AsH bonding upon removal of the electrostatically-bound K-ion; the σ-bond changes little but the π-bond is significantly perturbed.

A charged diatomic triple-bonded U≡N species trapped in C82 fullerene cages

Actinide diatomic molecules are ideal models to study elusive actinide multiple bonds, but most of these diatomic molecules have so far only been studied in solid inert gas matrices. Herein, we report a charged U≡N diatomic species captured in fullerene cages and stabilized by the U-fullerene coordination interaction. Two diatomic clusterfullerenes, viz. UN@Cs(6)-C82 and UN@C2(5)-C82, were successfully synthesized and characterized. Crystallographic analysis reveals U-N bond lengths of 1.760(7) and 1.760(20) Å in UN@Cs(6)-C82 and UN@C2(5)-C82. Moreover, U≡N was found to be immobilized and coordinated to the fullerene cages at 100 K but it rotates inside the cage at 273 K. Quantum-chemical calculations show a (UN)2+@(C82)2- electronic structure with formal +5 oxidation state (f1) of U and unambiguously demonstrate the presence of a U≡N bond in the clusterfullerenes. This study constitutes an approach to stabilize fundamentally important actinide multiply bonded species.

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.

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.

An Allyl Uranium(IV) Sandwich Complex: Are ϕ Bonding Interactions Possible?

A method to explore head-to-head ϕ back-bonding from uranium f-orbitals into allyl π* orbitals has been pursued. Anionic allyl groups were coordinated to uranium with tethered anilide ligands, then the products were investigated by using NMR spectroscopy, single-crystal XRD, and theoretical methods. The (allyl)silylanilide ligand, N-((dimethyl)prop-2-enylsilyl)-2,6-diisopropylaniline (LH), was used as either the fully protonated, singly deprotonated, or doubly deprotonated form, thereby highlighting the stability and versatility of the silylanilide motif. A free, neutral allyl group was observed in UI2 (L1)2 (1), which was synthesized by using the mono-deprotonated ligand [K][N-((dimethyl)prop-2-enyl)silyl)-2,6-diisopropylanilide] (L1). The desired homoleptic sandwich complex U[L2]2 (2) was prepared from all three ligand precursors, but the most consistent results came from using the dipotassium salt of the doubly deprotonated ligand [K]2 [N-((dimethyl)propenidesilyl)-2,6-diisopropylanilide] (L2). This allyl-based sandwich complex was studied by using theoretical techniques with supporting experimental spectroscopy to investigate the potential for phi (ϕ) back-bonding. The bonding between UIV and the allyl fragments is best described as ligand-to-metal electron donation from a two carbon fragment-localized electron density into empty f-orbitals.

Insights into the Electronic Structure of a U(IV) Amido and U(V) Imido Complex

Reaction of the N-heterocylic carbene ligand i PrIm (L1 ) and lithium bis(trimethylsilyl)amide (TMSA) as a base with UCl4 resulted in U(IV) and U(V) complexes. Uranium's +V oxidation state in (HL1 )2 [U(V)(TMSI)Cl5 ] (TMSI=trimethylsilylimido) (2) was confirmed by HERFD-XANES measurements. Solid state characterization by SC-XRD and geometry optimisation of [U(IV)(L1 )2 (TMSA)Cl3 ] (1) indicated a silylamido ligand mediated inverse trans influence (ITI). The ITI was examined regarding different metal oxidation states and was compared to transition metal analogues by theoretical calculations.

Heterometallic uranium/molybdenum nitride synthesis via partial N-atom transfer

The reaction of a terminal Mo(II) nitride with a U(III) complex yields a heterodimetallic U-Mo nitride which is the first example of a transition metal-capped uranium nitride. The nitride is triply bonded to U(V) and singly bonded to Mo(0) and supports a U-Mo interaction. This compound shows reactivity toward CO oxidation.

Mono- and Disamarium Azacryptand Complexes: A Platform for Cooperative Rare-Earth Metal Chemistry

Mono- (H₃LSm) and disamarium complexes (LSm₂) were prepared by reaction of the azacryptand N[(CH₂)₂NHCH₂-p-C₆H₄CH₂NH(CH₂)₂]₃N (H₆L) with 1 or 2 equiv of Sm[N(SiMe₃)₂]₃, respectively. The disamarium complex features free coordination sites on both metal centers available for bridging ligands shielded by phenylenes from tetrahydrofuran (THF) coordination. The reaction of LSm₂ with KCN and 18-crown-6 yielded the adduct [LSm₂-μ-η¹:η¹-CN][K(18-crown-6)(THF)₂] featuring a bridging cyanide. The complexes were characterized by crystallography, electrochemical analysis, NMR, and optical spectroscopy, and the effective magnetic moments were determined via the Evans method.

First examples of rare-earth metal azacryptand complexes are presented in the form of mono- and dinuclear samarium compounds, which can be prepared selectively. The bridging phenylenes of the azacryptand ligand prevent THF coordination to the samarium ions and provide available binding sites within the complexes. We investigated cyanide coordination using the disamarium complex to demonstrate the utility of the vacant intermetallic space for binding small molecules.

Versatile Coordination Modes of Multidentate Neutral Amine Ligands with Group 1 Metal Cations

This work comprehensively investigated the coordination chemistry of a hexa-dentate neutral amine ligand, namely, N,N',N"-tris-(2-N-diethylaminoethyl)-1,4,7-triaza-cyclononane (DETAN), with group-1 metal cations (Li+, Na+, K+, Rb+, Cs+). Versatile coordination modes were observed, from four-coordinate trigonal pyramidal to six-coordinate trigonal prismatic, depending on the metal ionic radii and metal's substituent. For comparison, the coordination chemistry of a tetra-dentate tris-[2-(dimethylamino)ethyl]amine (Me6Tren) ligand was also studied. This work defines the available coordination modes of two multidentate amine ligands (DETAN and Me6Tren), guiding future applications of these ligands for pursuing highly reactive and elusive s-block and rare-earth metal complexes.

A terminal neptunium(V)-mono(oxo) complex

Neptunium was the first actinide element to be artificially synthesized, yet, compared with its more famous neighbours uranium and plutonium, is less conspicuously studied. Most neptunium chemistry involves the neptunyl di(oxo)-motif, and transuranic compounds with one metal-ligand multiple bond are rare, being found only in extended-structure oxide, fluoride or oxyhalide materials. These combinations stabilize the required high oxidation states, which are otherwise challenging to realize for transuranic ions. Here we report the synthesis, isolation and characterization of a stable molecular neptunium(V)-mono(oxo) triamidoamine complex. We describe a strong Np≡O triple bond with dominant 5f-orbital contributions and σ_u > π_u energy ordering, akin to terminal uranium-nitrides and di(oxo)-actinyls, but not the uranium-mono(oxo) triple bonds or other actinide multiple bonds reported so far. This work demonstrates that molecular high-oxidation-state transuranic complexes with a single metal-ligand bond can be stabilized and studied in isolation.

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^-.

Ion cross-linking assisted synthesis of ZIF-8/chitosan/melamine sponge with anti-biofouling activity for enhanced uranium recovery

A ZIF-8/chitosan/melamine sponge (CMZ8) uranium adsorbent was prepared using chitosan and zinc ions as adjuvants to achieve the integration of anti-fouling, adsorption and separation properties.

A thiolate-bridged FeIVFeIV μ-nitrido complex and its hydrogenation reactivity toward ammonia formation

Iron nitrides are key intermediates in biological nitrogen fixation and the industrial Haber-Bosch process, used to form ammonia from dinitrogen. However, the proposed successive conversion of nitride to ammonia remains elusive. In this regard, the search for well-described multi-iron nitrido model complexes and investigations on controlling their reactivity towards ammonia formation have long been of great challenge and importance. Here we report a well-defined thiolate-bridged Fe^IVFe^IV μ-nitrido complex featuring an uncommon bent Fe-N-Fe moiety. Remarkably, this complex shows excellent reactivity toward hydrogenation with H₂ at ambient conditions, forming ammonia in high yield. Combined experimental and computational studies demonstrate that a thiolate-bridged Fe^IIIFe^III μ-amido complex is a key intermediate, which is generated through an unusual two-electron oxidation of H₂. Moreover, ammonia production was also realized by treating this diiron μ-nitride with electrons and water as a proton source.

Recent progress in rare-earth metal-catalyzed sp2 and sp3 C–H functionalization to construct C–C and C–heteroelement bonds

The review presents rare-earth metal-catalyzed C(sp2/sp3)–H functionalization accessing C–C/C–heteroatom bonds and olefin (co)polymerization, highlighting substrate scope, mechanistic realization, and origin of site-, enantio-/diastereo-selectivity.

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.

[AnI3(THF)4] (An = Np, Pu) Preparation Bypassing An0 Metal Precursors: Access to Np3+/Pu3+ Nonaqueous and Organometallic Complexes

Direct comparison of homologous molecules provides a foundation from which to elucidate both subtle and patent changes in reactivity patterns, redox processes, and bonding properties across a series of elements. While trivalent molecular U chemistry is richly developed, analogous Np or Pu research has long been hindered by synthetic routes often requiring scarcely available metallic-phase source material, high-temperature solid-state reactions producing poorly soluble binary halides, or the use of pyrophoric reagents. The development of routes to nonaqueous Np³⁺/Pu³⁺ from widely available precursors can potentially transform the scope and pace of research into actinide periodicity. Here, aqueous stocks of An⁴⁺ (An = Np, Pu) are dehydrated to well-defined [AnCl₄(DME)₂] (DME = 1,2-dimethoxyethane), and then a single-step halide exchange/reduction employing Me₃SiI produces [AnI₃(THF)₄] (THF = tetrahydrofuran) in a high to nearly quantitative crystalline yield (with I₂ and Me₃SiCl as easily removed byproducts). We demonstrate the synthetic utility of these An-iodide molecules, prepared by metal⁰-free routes, through characterization of archetypal complexes including the tris-silylamide, [Np{N(SiMe₃)₂}₃], and bent metallocenes, [An(C₅Me₅)₂(I)(THF)] (An = Np, Pu)─chosen because both motifs are ubiquitous in Th, U, and lanthanide research. The synthesis of [Np{N(Se═PPh₂)₂}₃] is also reported, completing an isomorphous series that now extends from U to Am and is the first characterized Np³⁺-Se bond.

Synthesis of a heterobimetallic actinide nitride and an analysis of its bonding

Reaction of [K(DME)][Th{N(R)(SiMe₂ CH₂)}₂(NR₂)] (R = SiMe₃) with 1 equiv. of [U(NR₂)₃(NH₂)] (1) in THF, in the presence of 18-crown-6, results in formation of a bridged uranium-thorium nitride complex, [K(18-crown-6)(THF)₂][(NR₂)₃U^IV(μ-N)Th^IV(NR₂)₃] (2), which can be isolated in 48% yield after work-up. Complex 2 is the first isolable molecular mixed-actinide nitride complex. Also formed in the reaction is the methylene-bridged mixed-actinide nitride, [K(18-crown-6)][K(18-crown-6)(Et₂O)₂][(NR₂)₂U(μ-N)(μ-κ²-C,N-CH₂SiMe₂NR)Th(NR₂)₂]₂ (3), which can be isolated in 34% yield after work-up. Complex 3 is likely generated by deprotonation of a methyl group in 2 by [NR₂]^-, yielding the new μ-CH₂ moiety and HNR₂. Reaction of 2 with 0.5 equiv. of I₂ results in formation of a U^V/Th^IV bridged nitride, [(NR₂)₃U^V(μ-N)Th^IV(NR₂)₃] (4), which can be isolated in 42% yield after work-up. The electronic structure of 4 was analyzed with EPR spectroscopy, SQUID magnetometry, and NIR-visible spectroscopy. This analysis demonstrated that the energies of 5f orbitals of 4 are largely determined by the strong ligand field exerted by the nitride ligand.