Carbon oxygenate transformations by actinide compounds and catalysts

The Tishchenko reaction mediated by organo-f-complexes: the myths and obstacles

For over a century, the Tishchenko reaction has been a valuable technique for synthesizing esters from aldehydes, serving a variety of applications in different domains. Beyond the remarkable advances in organoactinide and organolanthanide chemistry over the past two decades, there has been a significant increase in the research of the electrophilic d0/fn chemistry of organoactinide and organolanthanide compounds due to the captivating interplay between their structure and reactivity, and their exceptional performance in various homogeneous catalytic processes. The remarkable influence of ligand design, both in terms of steric hindrance and electronic properties, on the catalytic activity of organo-f-element complexes in organic transformations is well-established. However, the traditional view was that the significant oxophilicity of actinide and lanthanide complexes makes them unfavorable for reactions involving oxygen because of catalytic poisoning and their applications have been relatively limited, primarily focused on hydroalkoxylation, small-molecule activation, and cyclic ester polymerization. This review dissects the intricate interplay between ligand design and catalytic activity in actinide and lanthanide complexes, specifically in the context of the Tishchenko esterification.

Cholesky Decomposition-Based Implementation of Relativistic Two-Component Coupled-Cluster Methods for Medium-Sized Molecules

A Cholesky decomposition (CD)-based implementation of relativistic two-component coupled-cluster (CC) and equation-of-motion CC (EOM-CC) methods using an exact two-component Hamiltonian augmented with atomic-mean-field spin-orbit integrals (the X2CAMF scheme) is reported. The present CD-based implementation of X2CAMF-CC and EOM-CC methods employs atomic-orbital-based algorithms to avoid the construction of two-electron integrals and intermediates involving three and four virtual indices. Our CD-based implementation extends the applicability of X2CAMF-CC and EOM-CC methods to medium-sized molecules with the possibility to correlate around 1000 spinors. Benchmark calculations for uranium-containing small molecules were performed to assess the dependence of the CC results on the Cholesky threshold. A Cholesky threshold of 10-4 is shown to be sufficient to maintain chemical accuracy. Example calculations to illustrate the capability of the CD-based relativistic CC methods are reported for the bond-dissociation energy of the uranium hexafluoride molecule, UF6, with up to quadruple-ζ basis sets, and the lowest excitation energy in the solvated uranyl ion [UO22+(H2O)12].

A binuclear guanidinate yttrium carbyne complex: unique reactivity toward unsaturated C-N, C-O and C-S bonds

A guanidinato-stabilized binuclear yttrium carbyne complex [(PhCH2)2NC(NC6H3iPr2-2,6)2]2Y2(μ2-Me)(AlMe3)2(μ4-CH) (1) was synthesized via C-H bond activation and its versatile reactivities were investigated. Complex 1 underwent σ-bond metathesis with PhSSPh and nucleophilic addition with PhCN to form the corresponding yttrium thiolate complex 3 and aza-allyl complex 4 respectively. Additionally, the rare yttrium carbide complex 5 was also prepared by treatment of complex 1 with S8. Interestingly, in the reaction with PhNCS, the C[double bond, length as m-dash]S double bond was cleaved, followed by C-H bond activation to give the yttrium sulfide complex 7 with a ketenimine dianion ligand. Unexpectedly, the reaction of complex 1 with CO (1 atm) resulted in deoxygenative coupling of CO, to afford mono- or dioxo-yttrium complexes at different temperatures. The mechanism of the possible formation processes of complexes 3 and 9 was elucidated by DFT calculations.

Isolation of C1 through C4 derivatives from CO using heteroleptic uranium(iii) metallocene aryloxide complexes

The conversion of C1 feedstock molecules such as CO into commodity chemicals is a desirable, but challenging, endeavour. When the U(iii) complex, [(C₅Me₅)₂U(O-2,6- ^t Bu₂-4-MeC₆H₂)], is exposed to 1 atm of CO, only coordination is observed by IR spectroscopy as well as X-ray crystallography, unveiling a rare structurally characterized f element carbonyl. However, using [(C₅Me₅)₂(MesO)U (THF)], Mes = 2,4,6-Me₃C₆H₂, reaction with CO forms the bridging ethynediolate species, [{(C₅Me₅)₂(MesO)U}₂(μ₂-OCCO)]. While ethynediolate complexes are known, their reactivity has not been reported in much detail to afford further functionalization. For example, addition of more CO to the ethynediolate complex with heating forms a ketene carboxylate, [{(C₅Me₅)₂(MesO)U}₂(μ ₂:κ ²:η ¹-C₃O₃)], which can be further reacted with CO₂ to yield a ketene dicarboxylate complex, [{(C₅Me₅)₂(MesO)U}₂(μ ₂:κ ²:κ ²-C₄O₅)]. Since the ethynediolate showed reactivity with more CO, we explored its reactivity further. A [2 + 2] cycloaddition is observed with diphenylketene to yield [{(C₅Me₅)₂U}₂(OC(CPh₂)C([double bond, length as m-dash]O)CO)] with concomitant formation of [(C₅Me₅)₂U(OMes)₂]. Surprisingly, reaction with SO₂ shows rare S-O bond cleavage to yield the unusual [(O₂CC(O)(SO)]^2- bridging ligand between two U(iv) centres. All complexes have been characterized using spectroscopic and structural methods, and the reaction of the ethynediolate with CO to form the ketene carboxylate has been investigated computationally as well as the reaction with SO₂.

Route to Chemical Accuracy for Computational Uranium Thermochemistry

Benchmark spinor-based relativistic coupled-cluster calculations for the ionization energies of the uranium atom, the uranium monoxide molecule (UO), and the uranium dioxide molecule (UO₂) and for the bond dissociation energies of UO and UO₂ are reported. The accuracy of these calculations in the treatments of relativistic, electron-correlation, and basis-set effects is analyzed. The intrinsic convergence of the computed results and the favorable comparison with the experimental values demonstrate the unique applicability of the spinor representation of quantum-chemical methods to open-shell uranium-containing atomic and molecular species with uranium oxidation states ranging from U(0) to U(V).

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.

A Sacrificial Iminato Ligand in the Catalytic Cyanosilylation of Ketones Promoted by Organoactinide Complexes

Four new complexes containing the bis(pentamethylcyclopentadienyl)thorium(IV) moiety, Cp*₂Th(L1)(Me) (Th2), Cp*₂Th(L2)(Me) (Th3), Cp*₂Th(L1)Cl (Th5), and Cp*₂Th(L2)Cl (Th6), were synthesized in quantitative yields via the protonolysis reaction of the metallocene precursor complexes Cp*₂Th(Me)₂ (Th1) and Cp*₂Th(Me)Cl (Th4) and the respective six- and seven-membered N-heterocyclic neutral imine ligands L1H and L2H. The molecular structures of all the complexes were established by single-crystal X-ray structure analyses. The synthesized complexes along with the precursor complexes were employed as catalysts for the cyanosilylation reaction of ketones with trimethylsilyl cyanide (Me₃SiCN). The removal of the iminato ligand is necessary to trigger the reaction, allowing the formation of the active catalyst.

Base-free transfer hydrogenation of aldehydes and ketones catalyzed by imidazolin-2-iminato actinide complexes

A series of thorium and uranium complexes decorated with unsymmetrical imidazolin-2-iminato ligands were found to be effective as catalysts in the transfer hydrogenation of aldehydes and ketones with 2-propanol to form the corresponding alcohols.

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.

Reactions of a Dilithiomethane with CO and N2 O: An Avenue to an Anionic Ketene and a Hexafunctionalized Benzene

Synthesis of value-added products from simple C1 feedstocks is an attractive alternative avenue to traditional fossil fuels. Hexa-substituted benzene derivatives are highly useful molecules but are often challenging to prepare. Herein, we report that the lithium complex [(Ph₂ P(S))₂ CLi₂ (THF)]₂ 1 reacts with CO lead to C-C bond formation and migration of a Ph₂ P(S)-fragment affording 2. Subsequent reaction with N₂ O results in oxidative cleavage of a P-C bond affording [Ph₂ P(S)OLi(THF)₂ ]₂ 4 and the anionic ketene-derivative Ph₂ P(S)CCOLi(THF)₂ 5. Heating 5 prompts cyclotrimerization giving the hexa-substituted benzene derivative [Ph₂ P(S)CCOLi(THF)₂ ]₃ 6 regioselectively. This transition metal-free protocol to a hexa-substituted benzene is viable on a gram scale and permits the incorporation of ¹³ C labels. The mechanisms of these reactions are detailed via extensive DFT computations.

Recent developments in highly basic N-heterocyclic iminato ligands in actinide chemistry

In the last decade, major conceptual advances in the chemistry of actinide molecules and materials have been made to demonstrate their distinct reactivity profiles as compared to lanthanide and transition metal compounds, but some difficult questions remain concerning the intriguing stability of low-valent actinide complexes, and the importance of the 5f-orbitals in reactivity and bonding. The imidazolin-2-iminato moiety has been extensively used in ligands for the advancement of actinide chemistry owing to its unique capability of stabilizing the reactive and highly electrophilic metal ions by virtue of its strong electron donation and steric tunability. The current review article describes recent developments in the chemistry of light actinide metal ions (thorium and uranium) bearing these N-heterocyclic iminato moieties as supporting ligands. In addition, the effect of ring expansion of the N-heterocycle on the catalytic aptitude of the organoactinides is also described herein. The synthesis and reactivity of actinide complexes bearing N-heterocyclic iminato ligands are presented, and promising apposite applications are also presented. The current review focuses on addressing the catalytic behavior of actinide complexes with oxygen-containing substrates such as in the Tishchenko reaction, hydroelementation processes, and polymerization reactions. Actinide complexes have also found new catalytic applications, as demonstrated by the potent chemoselective carbonyl hydroboration and tandem proton-transfer esterification (TPTE) reaction, featuring coupling between an aldehyde and alcohol.

Instantaneous and Phosphine-Catalyzed Arene Binding and Reduction by U(III) Complexes

Neutral arenes such as benzene have never been considered suitable ligands for electropositive actinide cations, yet we find that even simple U^III UX₃ aryloxide complexes such as U(ODipp)₃ bind and reduce arenes spontaneously at room temperature, forming inverse arene sandwich (IAS) complexes X_nU(μ-C₆D₆)UX_m (X = ODipp, n=2, m=3; X = OBMes₂ n=m=2 or 3) (ODipp = OC₆H₃ⁱPr₂-2,6; Mes = 2,4,6-Me₃-C₆H₂). In some of these cases, further arene reduction has occured as a result of X ligand redistribution. These unexpected spontaneous reactions explain the anomalous spectra and reported lack of further reactivity of strongly reducing U^III centers of U(ODipp)₃. Phosphines that are not considered suitable ligands for actinides can catalyze the formation of the IAS complexes. This enables otherwise inaccessible asymmetric and less congested IAS complexes to be isolated and the bonding in this series compared.

Delivery of a Masked Uranium(II) by an Oxide-Bridged Diuranium(III) Complex

Oxide is an attractive linker for building polymetallic complexes that provide molecular models for metal oxide activity, but studies of these systems are limited to metals in high oxidation states. Herein, we synthesized and characterized the molecular and electronic structure of diuranium bridged U^III /U^IV and U^III /U^III complexes. Reactivity studies of these complexes revealed that the U-O bond is easily broken upon addition of N-heterocycles resulting in the delivery of a formal equivalent of U^III and U^II , respectively, along with the uranium(IV) terminal-oxo coproduct. In particular, the U^III /U^III oxide complex effects the reductive coupling of pyridine and two-electron reduction of 4,4'-bipyridine affording unique examples of diuranium(III) complexes bridged by N-heterocyclic redox-active ligands. These results provide insight into the chemistry of low oxidation state metal oxides and demonstrate the use of oxo-bridged U^III /U^III complexes as a strategy to explore U^II reactivity.

Lithium Dicyclohexylamide in Transition-Metal-Free Fischer-Tropsch Chemistry

Lithium dicyclohexylamide (Cy₂NLi) reacts with syn-gas or CO to generate transient intermediates with carbene character, which are capable of reacting further with CO or H₂, effecting sequential C-C and C-H bond formations from CO or H₂, thus providing a transition-metal-free avenue to the fundamental reactions of the Fischer-Tropsch process. Further experimental and computational data indicate that reactions with CO and H₂ are thermodynamically accessible, with a kinetic bias toward CO homologation.

A relativistic DFT probe for small-molecule activation mediated by low-valent uranium metallocenes

DFT calculations rationalize the capability of uranium metallocenes in activating small molecules, and the experimentally inaccessible CO₂ adduct is addressed.

Hydrodeoxygenative Cyclotetramerization of Carbon Monoxide by a Trinuclear Titanium Polyhydride Complex

The reductive coupling of carbon monoxide (CO) by metal hydrides is of fundamental interest and practical importance. Herein we report an unprecedented hydrodeoxygenative cyclotetramerization of CO by a trinuclear titanium polyhydride complex [(C₅Me₄SiMe₃)Ti]₃(μ₃-H)(μ₂-H)₆ (1). The reaction of CO with 1 at -78 °C gave an ethen-1,2-diyl species [CH═CH]^2- through the hydrodeoxygenative dimerization of two molecules of CO, which upon cycloaddition to another two molecules of CO afforded a cyclobuten-3,4-diyl-1,2-diolate unit [C₄H₂O₂]^4-. The hydrogenolysis of the [C₄H₂O₂]^4- species with H₂ yielded a tetrahydrocyclobuten-1,2-diolate species [C₄H₄O₂]^2-, which on heating at 100 °C gave a cyclobuten-2-yl-1-olate product [C₄H₄O]^2-. The acidolysis of the [C₄H₂O₂]^4- and [C₄H₄O]^2- species with HCl afforded γ-butyrolactone and cyclobutanone, respectively.

Single-Atom Catalysts across the Periodic Table

Isolated atoms featuring unique reactivity are at the heart of enzymatic and homogeneous catalysts. In contrast, although the concept has long existed, single-atom heterogeneous catalysts (SACs) have only recently gained prominence. Host materials have similar functions to ligands in homogeneous catalysts, determining the stability, local environment, and electronic properties of isolated atoms and thus providing a platform for tailoring heterogeneous catalysts for targeted applications. Within just a decade, we have witnessed many examples of SACs both disrupting diverse fields of heterogeneous catalysis with their distinctive reactivity and substantially enriching our understanding of molecular processes on surfaces. To date, the term SAC mostly refers to late transition metal-based systems, but numerous examples exist in which isolated atoms of other elements play key catalytic roles. This review provides a compositional encyclopedia of SACs, celebrating the 10th anniversary of the introduction of this term. By defining single-atom catalysis in the broadest sense, we explore the full elemental diversity, joining different areas across the whole periodic table, and discussing historical milestones and recent developments. In particular, we examine the coordination structures and associated properties accessed through distinct single-atom-host combinations and relate them to their main applications in thermo-, electro-, and photocatalysis, revealing trends in element-specific evolution, host design, and uses. Finally, we highlight frontiers in the field, including multimetallic SACs, atom proximity control, and possible applications for multistep and cascade reactions, identifying challenges, and propose directions for future development in this flourishing field.

Mild catalytic deoxygenation of amides promoted by thorium metallocene

The organoactinide-catalyzed (Cp*2ThMe2) hydroborated reduction of a wide range of tertiary, secondary, and primary amides to the corresponding amines/amine-borane adducts via deoxygenation of the amides is reported herein. The catalytic reactions proceed under mild conditions with low catalyst loading and pinacolborane (HBpin) concentration in a selective fashion. Cp*2ThMe2 is capable of efficiently catalysing the gram-scale reaction without a drop in efficiency. The amine-borane adducts are successfully converted into free amine products in high conversions, which increases the usefulness of this catalytic system. A plausible mechanism is proposed based on detailed kinetics, stoichiometric, and deuterium labeling studies.

Heterotrimetallic {LnOVPt} complexes with antiferromagnetic Ln-V coupling and magnetic memory

The new PtVO(SOCR)4 lantern complexes, 1 (R = CH3) and 2 (R = Ph) behave as neutral O-donor ligands to Ln(OR)3 with Ln = Ce, Nd. Four heterotrimetallic complexes with linear {LnOVPt} units were prepared: [Ln(ODtbp)3{PtVO(SOCR)4}] (Ln = Ce, 3Ce (R = CH3), 4Ce (R = Ph); Nd, 3Nd (R = CH3), 4Nd (R = Ph); ODtbp = 2,6-ditertbutylphenolate). Magnetic characterization confirms slow magnetic relaxation behaviour and suggests antiferromagnetic coupling across {Ln-O[double bond, length as m-dash]V} in all four complexes, with variations tunable as a function of Ln and R.

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.

Actinyl-Modified g-C3N4 as CO2 Activation Materials for Chemical Conversion and Environmental Remedy via an Artificial Photosynthetic Route

With the reported CO₂ activation for the oxidation of benzene to phenol (-ENE → -OL) by the graphitic carbon nitride g-C₃N₄ (CN) via an artificial photosynthetic route as inspiration, high-valent actinyls (An^mO₂)ⁿ⁺ (An = U, Np, Pu; m = VI, V; n = 2, 1) have been introduced for its further modification. Our calculations indicate thermodynamic spontaneity in the feasibility of g-C₃N₄-(An^mO₂)ⁿ⁺ (CN-An^m) formation. The magnificent structural and electronic properties of CN-An^m are utilized for CO₂ activation in terms of the rarely studied -ENE → -OL conversion. The calculated free energies show that most steps of the catalytic cycle are favored by CN-An^m complexes. The first step (carbamate formation) is slightly endothermic in all cases, where CN-U is 0.51 eV higher than CN and CN-Pu is -0.01 eV lower. All benzene addition reactions release energy, with that for CN-U being the lowest. The phenolate formation is favored by some actinyl complexes over CN, and CN-U is only 0.23 eV higher. The phenol release (resulting in formamide complexes) and CO desorption are exothermic for all CN-An^m. The overall process suggests the improved catalytic performance of actinyl-modified CN materials, and the slightly depleted uranyl-carbon nitride could be one of the promising catalysts.

Divergent uranium- versus phosphorus-based reduction of Me3SiN3 with steric modification of phosphido ligands

We describe an example of a two-electron metal- and ligand-based reduction of Me₃SiN₃ using uranium(iv) complexes with varying steric properties. Reaction of (C₅Me₅)₂U(CH₃)[P(SiMe₃)(Ph)] with Me₃SiN₃ produces the imidophosphorane complex, (C₅Me₅)₂U(CH₃)[N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> P(SiMe₃)₂(Ph)] through oxidation of phosphorus. However, a similar reaction with a more sterically encumbering phosphido ligand, (C₅Me₅)₂U(CH₃)[P(SiMe₃)(Mes)] forms the U(iv) complex, (C₅Me₅)₂U[κ²-(N,N)–N(SiMe₃)P(Mes)N(SiMe₃)]. In probing the mechanism of this reaction, a U(vi) bis(imido) complex, (C₅Me₅)₂U(NSiMe₃){N[P(SiMe₃)(Mes)]} was isolated. DFT calculations show an intramolecular reductive cycloaddition reaction leads to the formation of the U(iv) bis(amido)phosphane from the U(vi) bis(imido) complex. This is a rare example of the isolation of a reaction intermediate in f element chemistry.

We describe an example of a two-electron metal- and ligand-based reduction of Me₃SiN₃ using uranium(iv) complexes with varying steric properties. With uranium-based reduction, a U(vi) intermediate is isolated.

Construction of heterometallic clusters with multiple uranium-metal bonds by using dianionic nitrogen-phosphorus ligands

Compared with the prevalent metal–metal bond in transition metals, examples of the actinide–metal bond in heterometallic clusters are rare. Herein, a series of heterometallic clusters with multiple uranium–metal bonds has been prepared based on two newly synthesized nitrogen–phosphorus ligands L1 {O[(CH₂)₂NHP(ⁱPr)₂]₂} and L2 {[CH₂O(CH₂)₂NHP(ⁱPr)₂]₂}. Different P–P distances, 6.069 and 4.464 Å, are observed in the corresponding uranium complexes 1 {O[(CH₂)₂NP(ⁱPr)₂]₂UCl₂} and 2 {[CH₂O(CH₂)₂NP(ⁱPr)₂]₂UCl₂}, respectively, and lead to the different coordination modes with transition metals. The reactions of zero-valent group 10 metal compounds with complex 1 generate heterometallic clusters (3-U2Ni2 and 4-U2Pd2) featuring four uranium–metal bonds; whereas reactions with 2 afford one-dimensional metal-chain 5-(UNi)n, bimetallic species 6-UPd, and a tri-platinum bridged diuranium molecular cluster 7-U2Pt3. Complex 5-(UNi)n represents the first infinite chain containing the U–M bond and 7-U2Pt3 is the first species with multiple U–Pt bonds. This study further highlights the important role of ligands in the construction of multiple uranium–metal bonds and may allow the synthesis of novel d–f heterometallic clusters and the investigation of their applications in catalysis and small-molecule activation.

Compared with the prevalent metal–metal bond in transition metals, examples of the actinide–metal bond in heterometallic clusters are rare.

Activating Water and Hydrogen by Ligand-Modified Uranium and Neptunium Complexes: A Density Functional Theory Study

Organometallic uranium complexes that can activate small molecules are well-known. In contrast, there are no known organometallic trans-uranium species capable of small-molecule transformations. Using density functional theory, we previously showed that changing actinide-ligand bonds from U-O groups to Np-N- (amide/imido) bonds makes redox small-molecule activation more energetically favorable for Np species. Here, we determine how general this ligand-modulation strategy is for affecting small-molecule activation in Np species. We focus on two reactions, one involving redox transformation of the actinide(s) and the other involving no change in the oxidation state of the actinide(s). Specifically, we considered the hydrogen evolution reaction (HER) from H₂O by actinide tris-aryloxide species. We also considered H₂ capture and hydride transfer by actinide siloxide and silylamide complexes. For the HER, the barriers for Np(III) systems are much higher than those of U(III). The overall reaction energies are also much worse. An-O → An-N substitutions marginally improve the barriers by 1-4 kcal/mol and more substantially improve the reaction energies by 9-15 kcal/mol. For H₂ capture and hydride transfer, the reaction energies for the U and Np species are similar. For both actinides, like-for-like An-O → An-N substitutions lead to improved reaction energies. Interestingly, in a recent report, it seemingly appears that U-O (siloxide) → U-N (silylamide) leads to complete shutdown of reactivity for H₂ capture and hydride transfer. This observation is reproduced and explained with calculations. The ligand environments of the siloxide and silylamide that were compared are vastly different. The steric environment of the siloxide is conducive for reactivity while the particular silylamide is not. We conclude that small-molecule activation with organometallic neptunium species is achievable with a guided choice of ligands. Additional emphasis should be placed on ligands that can allow for improved transition state barriers.

CO2 conversion to phenyl isocyanates by uranium(vi) bis(imido) complexes

Uranium(vi) trans-bis(imido) complexes [U(κ4-{(tBu2ArO)2Me2-cyclam})(NPh)(NPhR)] react with CO2 to eliminate phenyl isocyanates and afford uranium(vi) trans-[O[double bond, length as m-dash]U[double bond, length as m-dash]NR]2+ complexes, including [U(κ4-{(tBu2ArO)2Me2-cyclam})(NPh)(O)] that was crystallographically characterized. DFT studies indicate that the reaction proceeds by endergonic formation of a cycloaddition intermediate; the secondary reaction to form a dioxo uranyl complex is both thermodynamically and kinetically hindered.

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 of 3-(2-Hydroxyphenyl)-5-(2-Pyridinyl)-1,2,4-triazoles as a potential chelate ligand for Uranyl ion

Two new uranyl complexes with the molecular formula [(UO2)(H2L1)](СH3OH) and [(UO2)(H3L2)](СH3OH) {H4L1 = 2-[5-[[5-[[5-(2-pyridyl)-1H-1,2,4-triazol-3‑yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]phenol and H5L2 = 2-[5-[[5-[[5-[[5-(2-pyridyl)-1H-1,2,4‑triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]methyl]-1H-1,2,4-triazol-3-yl]phenol)} have been synthesized. All compounds have been characterized by NMR and IR spectroscopy. With H4L1 and H5L2 uranyl ion forms mononuclear complexes. In [(UO2)(H3L2)](СH3OH) pyridyl nitrogen was uncoordinated and bonding of H5L2 was realized only through phenol oxygen and N4-nitrogens of triazole cycles.

Theoretical Study of Divalent Bis(Pentaisopropylcyclopentadienyl) Actinocenes

The existence of divalent bis(pentaisopropylcyclopentadienyl) actinocene compounds, An(Cp^iPr₅)₂ (An = Th, U, Pu, Am, Bk, No, and Lr), is assessed by density functional theory (DFT) calculations with scalar-relativistic small-core pseudopotentials. The calculations predict ground states with significant 6d occupation for Th, U, and Lr, whereas Am, Bk, and No exhibit 5f ground states. A mixed ground state with predominant 5f character is found for Pu. The complexes exhibit a linear coordination geometry and high S₁₀ symmetry except for Pu(Cp^iPr₅)₂ and Am(Cp^iPr₅)₂, which are found to be bent by 11 and 12°, respectively. Absorption spectra are simulated with time-dependent density functional theory (TD-DFT) and compared to experimental spectra of known tris(C₄H₄SiMe₃) and tris(C₅H₃(SiMe₃)₂) compounds [ J. Am. Chem. Soc. 2015 , 137 , 369 - 382 . DOI: 10.1021/ja510831n ] as well as recently synthesized divalent lanthanide analogs Dy(Cp^iPr₅)₂ and Tb(Cp^iPr₅)₂ [ J. Am. Chem. Soc. , 2019 , 141 , 12967 - 12973 . DOI: 10.1021/jacs.9b05816 ]. Thermodynamic stability is assessed by calculation of adiabatic reduction potentials of the trivalent precursors [An(Cp^iPr₅)₂]⁺, and the feasibility of further reduction to obtain as yet unknown monovalent molecular actinide complexes is discussed.

Small molecule activation by multimetallic uranium complexes supported by siloxide ligands

The synthesis and reactivity of uranium compounds supported by the tris-tert-butoxysiloxide ligand is surveyed. The multiple binding modes of the tert-butoxysiloxide ligand have proven very well suited to stabilize highly reactive homo- and heteropolymetallic complexes of uranium that have shown an unusual high reactivity towards small molecules such as CO2, CS2, chalcogens and azides. Moreover, these ligands have allowed the isolation of dinuclear nitride and oxide bridged complexes of uranium in various oxidation states. The ability of the tris-tert-butoxysiloxide ligands to trap alkali ions in these nitride or oxide complexes leads to unprecedented ligand based and metal based reduction and functionalization of N2, CO, CO2 and H2.

Identification of a uranium-rhodium triple bond in a heterometallic cluster

The chemistry of d-block metal-metal multiple bonds has been extensively investigated in the past 5 decades. However, the synthesis and characterization of species with f-block metal-metal multiple bonds are significantly more challenging and such species remain extremely rare. Here, we report the identification of a uranium-rhodium triple bond in a heterometallic cluster, which was synthesized under routine conditions. The uranium-rhodium triple-bond length of 2.31 Å in this cluster is only 3% longer than the sum of the covalent triple-bond radii of uranium and rhodium (2.24 Å). Computational studies reveal that the nature of this uranium-rhodium triple bond is 1 covalent bond with 2 rhodium-to-uranium dative bonds. This heterometallic cluster represents a species with f-block metal-metal triple bond structurally authenticated by X-ray diffraction. These studies not only demonstrate the authenticity of the uranium-metal triple bond, but also provide a possibility for the synthesis of other f-block metal-metal multiple bonds. We expect that this work may further our understanding of the bonding between uranium and transition metals, which may help to design new d-f heterometallic catalysts with uranium-metal bonds for small-molecule activation and to promote the utilization of abundant depleted uranium resources.

Nitrogen Reduction by Multimetallic trans-Uranium Actinide Complexes: A Theoretical Comparison of Np and Pu to U

There is recent interest in organometallic complexes of the trans-uranium elements. However, preparation and characterization of such complexes are hampered by radioactivity and chemotoxicity issues as well as the air-sensitive and poorly understood behavior of existing compounds. As such, there are no examples of small-molecule activation via redox reactivity of organometallic trans-uranium complexes. This contrasts with the situation for uranium. Indeed, a multimetallic uranium(III) nitride complex was recently synthesized, characterized, and shown to be able to capture and functionalize molecular nitrogen (N₂) through a four-electron reduction process, N₂ → N₂^4-. The bis-uranium nitride, U-N-U core of this complex is held in a potassium siloxide framework. Importantly, the N₂^4- product could be further functionalized to yield ammonia (NH₃) and other desirable species. Using the U-N-U potassium siloxide complex, K₃U-N-U, and its cesium analogue, Cs₃U-N-U, as starting points, we use scalar-relativistic and spin-orbit coupled density functional theory calculations to shed light on the energetics and mechanism for N₂ capture and functionalization. The N₂ → N₂^4- reactivity depends on the redox potentials of the U(III) centers and crucially on the stability of the starting complex with respect to decomposition into the mixed oxidation U(IV)/U(III) K₂U-N-U or Cs₂U-N-U species. For the trans-uranium, Np and Pu analogues of K₃U-N-U, the N₂ → N₂^4- process is endoergic and would not occur. Interestingly, modification of the Np-O and Pu-O bonds between the actinide cores and the coordinated siloxide framework to Np-NH, Pu-NH, Np-CH₂, and Pu-CH₂ bonds drastically improves the reaction free energies. The Np-NH species are stable and can reductively capture and reduce N₂ to N₂^4-. This is supported by analysis of the spin densities, molecular structure, long-range dispersion effects, as well as spin-orbit coupling effects. These findings chart a path for achieving small-molecule activation with organometallic neptunium analogues of existing uranium complexes.