Recent developments in synthesis and structural chemistry of nonaqueous actinide complexes

Evidence for ligand- and solvent-induced disproportionation of uranium(IV)

Disproportionation, where a chemical element converts its oxidation state to two different ones, one higher and one lower, underpins the fundamental chemistry of metal ions. The overwhelming majority of uranium disproportionations involve uranium(III) and (V), with a singular example of uranium(IV) to uranium(V/III) disproportionation known, involving a nitride to imido/triflate transformation. Here, we report a conceptually opposite disproportionation of uranium(IV)-imido complexes to uranium(V)-nitride/uranium(III)-amide mixtures. This is facilitated by benzene, but not toluene, since benzene engages in a redox reaction with the uranium(III)-amide product to give uranium(IV)-amide and reduced arene. These disproportionations occur with potassium, rubidium, and cesium counter cations, but not lithium or sodium, reflecting the stability of the corresponding alkali metal-arene by-products. This reveals an exceptional level of ligand- and solvent-control over a key thermodynamic property of uranium, and is complementary to isolobal uranium(V)-oxo disproportionations, suggesting a potentially wider prevalence possibly with broad implications for the chemistry of uranium.

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.

Destruction and reconstruction of UO22+ using gas-phase reactions

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

Synthesis and Characterization of Two Uranyl-Aryl "Ate" Complexes

Reaction of [UO₂ Cl₂ (THF)₃ ] with 3 equivalents of LiC₆ Cl₅ in Et₂ O resulted in the formation of first uranyl aryl complex [Li(Et₂ O)₂ (THF)][UO₂ (C₆ Cl₅ )₃ ] ([Li][1]) in good yields. Subsequent dissolution of [Li][1] in THF resulted in conversion into [Li(THF)₄ ][UO₂ (C₆ Cl₅ )₃ (THF)] ([Li][2]), also in good yields. DFT calculations reveal that the U-C bonds in [Li][1] and [Li][2] exhibit appreciable covalency. Additionally, the ¹³ C NMR chemical shifts for their C_ipso environments are strongly affected by spin-orbit coupling-a consequence of 5f orbital participation in the U-C bonds.

Coordination of trivalent lanthanum and cerium, and tetravalent cerium and actinides (An = Th(IV), U(IV), Np(IV)) by a 4-phosphoryl 1H-pyrazol-5-olate ligand in solution and the solid state

Structural investigations of three actinide(iv) 4-phosphoryl 1H-pyrazol-5-olate complexes (An = Th(iv), U(iv), Np(iv)) and their cerium(iv) analogue display the same metal coordination in the solid state. The mononuclear complexes show the metal centre in a square antiprismatic coordination geometry composed by the two O-donor atoms of four deprotonated ligands. Detailed solid state analysis of the U(iv) complex shows that dependent on the solvent used altered arrangements are observable, resulting in a change in the coordination polyhedron of the U(iv) metal centre to bi-capped trigonal prismatic. Further, single crystal analyses of the La(iii) and Ce(iii) complexes show that the ligand can also act as a neutral ligand by protonation of the pyrazolyl moiety. All complexes were comprehensively characterized by NMR, IR and Raman spectroscopy. A single resonance in each of the 31P NMR spectra for the La(iii), Ce(iii), Ce(iv), Th(iv) and Np(iv) complex indicates the formation of highly symmetric complex species in solution. Extended X-ray absorption fine structure (EXAFS) investigations provide evidence for the same local structure of the U(iv) and Np(iv) complex in toluene solution, confirming the observations made in the solid state.

Intrinsic chemistry of [OUCH]+: reactions with H2O, CH3C[triple bond, length as m-dash]N and O2

We report the first experimental study of the intrinsic chemistry of a U-methylidyne species, focusing on reaction of [OUCH]+ with H2O, O2 and CH3C[triple bond, length as m-dash]N in the gas phase. DFT was also used to determine reaction pathways, and establish the mechanism by which [OUCH]+ is formed through collision-induced dissociation of [UO2(C[triple bond, length as m-dash]CH)]+.

Expanding the Nonaqueous Chemistry of Neptunium: Synthesis and Structural Characterization of [Np(NR2)3Cl], [Np(NR2)3Cl]-, and [Np{N(R)(SiMe2CH2)}2(NR2)]- (R = SiMe3)

Reaction of 3 equiv of NaNR₂ (R = SiMe₃) with NpCl₄(DME)₂ in THF afforded the Np(IV) silylamide complex, [Np(NR₂)₃Cl] (1), in good yield. Reaction of 1 with 1.5 equiv of KC₈ in THF, in the presence of 1 equiv of dibenzo-18-crown-6, resulted in formation of [{K(DB-18-C-6)(THF)}₃(μ₃-Cl)][Np(NR₂)₃Cl]₂ (4), also in good yield. Complex 4 represents the first structurally characterized Np(III) amide. Finally, reaction of NpCl₄(DME)₂ with 5 equiv of NaNR₂ and 1 equiv of dibenzo-18-crown-6 afforded the Np(IV) bis(metallacycle), [{Na(DB-18-C-6)(Et₂O)_0.62(κ¹-DME)_0.38}₂(μ-DME)][Np{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (8), in moderate yield. Complex 8 was characterized by ¹H NMR spectroscopy and X-ray crystallography and represents a rare example of a structurally characterized neptunium-hydrocarbyl complex. To support these studies, we also synthesized the uranium analogues of 4 and 8, namely, [K(2,2,2-cryptand)][U(NR₂)₃Cl] (2), [K(DB-18-C-6)(THF)₂][U(NR₂)₃Cl] (3), [Na(DME)₃][U{N(R)(SiMe₂CH₂)}₂(NR₂)] (6), and [{Na(DB-18-C-6)(Et₂O)_0.5(κ¹-DME)_0.5}₂(μ-DME)][U{N(R)(SiMe₂CH₂)}₂(NR₂)]₂ (7). Complexes 2, 3, 6, and 7 were characterized by a number of techniques, including NMR spectroscopy and X-ray crystallography.

Stabilization of uranyl(v) by dipicolinic acid in aqueous medium

Preparation of a stable U(v) complex in an aqueous medium is a challenging task owing to its disproportionation nature (conversion into more stable U(vi) and U(iv) species) and sensitivity to atmospheric oxygen. The stable uranyl (UO₂²⁺)/dipicolinic acid (DPA) complex ([U^(VI)O₂(DPA)(OH)(H₂O)]^-) was formed at pH 10.5-12.0, which was confirmed by potentiometric and spectrophotometric titrations, and NMR, ESI-MS and EXAFS spectroscopy. The complex [U^(VI)O₂(DPA)(OH)(H₂O)]^- can be electrochemically reduced on the Pt electrode at -0.9 eV (vs. Ag/AgCl) to [U^(V)O₂(DPA)(OH)(H₂O)]^2- in aqueous medium under an anaerobic environment. According to cyclic voltammetric analysis, a pair of oxidation and reduction waves at E'₀ = -0.592 V corresponds to the [U^(VI)O₂(DPA)(OH)(H₂O)]^-/[U^(V)O₂(DPA)(OH)(H₂O)]^2- redox couple and the formation of [U^(V)O₂(DPA)(OH)(H₂O)]^2- was confirmed by the electron stoichiometry (n = 0.97 ± 0.05) of the reduction reaction of [U^(VI)O₂(DPA)(OH)(H₂O)]^-. The pentavalent uranyl complex [U^(V)O₂(DPA)(OH)(H₂O)]^2- was further characterized via UV-vis-NIR absorption spectrophotometry and X-ray absorption (XANES and EXAFS) spectroscopy. The [U^(V)O₂(DPA)(OH)(H₂O)]^2- complex is stable at pH 10.5-12.0 in anaerobic water for a few days. DFT calculation shows the strong complexing ability of DPA stabilizing the unstable oxidation state U(v) in aqueous medium.

Yttrium and lanthanum bis(phosphine-oxide)methanides: structurally diverse, dynamic, and reactive

Homoleptic yttrium and lanthanum complexes of bis(phosphineoxide) methanides, RE(HPhL)3 and RE2(HMeL)6, promote the first rare-earth mediated Horner-Wittig and acid-base chemistry consistent with multifunctional reactivity (Lewis-acid/Brønstedbase).

Coordination-driven assembly of actinide-organic polyrotaxanes involving crown ether macrocycles

Using macrocyclic DB24C8 as a new kind of host molecule for a pseudorotaxane ligand, actinide-linked crown ether-based coordination polyrotaxanes, UCER-1 and UCER-2, that are linked by two different uranyl nodes, have been synthesised for the first time.

Main-Group Metals Stabilized Polypyrrolic Uranyl(V) Complexes via Cation-Cation Interaction with the Uranyl exo-Oxo Atom: A Relativistic Density Functional Theory Study

To explore the innovative uranyl(V) complexes by deeply understanding their coordination stability, relativistic density functional theory calculations have been performed to investigate the experimentally reported [(py)(R₂AlOU^VO)(py)(H₂L)] [R = Me (1), ⁱBu (2)] and [{(py)₃MOU^VO}(py)(H₂L)] [M = Li (3), Na (4), K (5)] and their uranyl(VI) counterparts. Structural and topological analyses along with transformation-reaction energies and redox potentials were systematically studied. Geometrical and quantum theory of atoms in molecules analyses implied a linear U-O_exo-M feature in 1-3 and a bent one in 4 and 5. The calculated free energies (Δ_rG) of reactions transforming 1/2 into 3/4/5 confirmed a higher stability of the latter ones, which were further corroborated by their reduction potentials (E⁰). The E⁰ value of 5 versus uranyl(VI) is close to its experimental value, particularly in solvation with spin-orbit coupling. The highest occupied and lowest unoccupied molecular orbitals of uranyl(V) and uranyl(VI) have predominant U(5fδ) character. Compared to mononuclear uranyl(VI), the coordination of aluminum and alkali metals to uranyl exo-oxo significantly contributes to the stabilization of uranyl(V) by altering the E⁰ value from -1.59 to -0.85, -0.91, -1.33, -1.50, and -1.46 V, respectively. The calculation results show a more positive E⁰ than that of the precursor 6^VI/6 without exo-oxo coordination. The calculated E⁰ values of 3-5 are certainly more negative than those of 1 and 2. The alkali metals were found to activate U═O bonds more easily/readily than aluminum by coordination to the exo-oxo atom. In brief, the uranyl exo-oxo cation-cation-interaction enhanced the reduction ability from its uranyl(VI) analogue and raised the stability of the U^V center.

Origin of Bond Elongation in a Uranium(IV) cis-Bis(imido) Complex

The activation of U-N multiple bonds in an imido analogue of the uranyl ion is accomplished by using a system that is very electron-rich with sterically encumbering ligands. Treating the uranium(VI) trans-bis(imido) UI₂(NDIPP)₂(THF)₃ (DIPP = 2,6-diisopropylphenyl and THF = tetrahydrofuran) with tert-butyl(dimethylsilyl)amide (NTSA) results in a reduction and rearrangement to form the uranium(IV) cis-bis(imido) [U(NDIPP)₂(NTSA)₂]K₂ (1). Compound 1 features long U-N bonds, pointing toward substantial activation of the N═U═N unit, as determined by X-ray crystallography and ¹H NMR, IR, and electronic absorption spectroscopies. Computational analyses show that uranium(IV)-imido bonds in 1 are significantly weakened multiple bonds due to polarization toward antibonding and nonbonding orbitals. Such geometric control has important effects on the electronic structures of these species, which could be useful in the recycling of spent nuclear fuels.

Exploring the Oxidation States of Neptunium with Schiff Base Coordination Complexes

A pair of neptunium Schiff base coordination complexes, Np^VIO₂L(MeOH) and Np^IVL₂ {H₂L = N,N'-bis[(4,4'-diethylamino)salicylidene]-1,2-phenylenediamine}, have been synthesized and analyzed by several characterization methods including single-crystal X-ray diffraction, electronic absorption, ¹H NMR, cyclic voltammetry, and theoretical interpretation. Structural analysis reveals that Np^VIO₂L(MeOH) and Np^IVL₂ are isomorphous with the previously reported U^VIO₂L(MeOH) and M^IVL₂ (M = Pu, Ce, U, Th) complexes, respectively, allowing for a direct comparison across the series. The reduction of Np^VIO₂L(MeOH) in situ or direct synthesis from a (Np^VO₂)⁺ source shows evidence of a pentavalent neptunyl (Np^VO₂L)_x^n- species as determined by UV/vis/NIR and ¹H NMR spectroscopy. The synthesis of (Np^VO₂L)_x^n- directly from a (Np^VO₂)⁺ starting material gives a similar spectrum. Theoretical analysis offers insight into the electronic structure for a better understanding of the bonding patterns and relative stability of the different oxidation states. Computational results show that the Np-L covalent interactions in Np^IVL₂ are similar to those in the Np^VIO₂L(MeOH) complex, indicating that neither the presence of the axial oxo ligands nor the oxidation state significantly modify the nature of the Np-L bonds.

Structure and Characterization of an Americium Bis(O,O'-diethyl)dithiophosphate Complex

A facile synthesis of an americium complex with a sulfur-donor ligand has been developed, allowing characterization of americium bonding from multiple perspectives via several techniques. Reaction of ²⁴³Am with S₂P(OEt)₂^- yields the tetrakis complex [Am(S₂P(OEt)₂)₄]^- that can be crystallized as the tetraphenylarsonium salt. Structures obtained from single crystal X-ray diffraction show bond length discrepancies from the neodymium analogue consistent with the soft-donor bond enhancement common to actinides. Solid state optical spectroscopy confirms interaction of the ligand with 5f orbitals. ³¹P nuclear magnetic reflects the minor paramagnetism of Am(III). Computational investigations through CASSCF calculations, ligand-field density functional theory, and quantum chemical topological analysis allow a quantification of covalency or orbital interaction effects via total energy density and nephelauxetic parameters, both of which indicate greater covalency in the americium species than in the neodymium analogue or the americium aquo complex.

Reaction of Cu(II) Chelates with Uranyl Nitrate to Form a Coordination Complex or H-Bonded Adduct: Experimental Observations and Rationalization by Theoretical Calculations

Four new heterometallic Cu(II)-U(VI) species, [{(CuL¹)(CH₃CN)}UO₂(NO₃)₂] (1), [{(CuL²)(CH₃CN)}UO₂(NO₃)₂] (2), [{(CuL³)(H₂O)}UO₂(NO₃)₂] (3), and [UO₂(NO₃)₂(H₂O)₂]·2[CuL⁴]·H₂O (4), were synthesized using four different metalloligands ([CuL¹], [CuL²], [CuL³], and [CuL⁴], respectively) derived from four unsymmetrically dicondensed N,O-donor Schiff bases. Single-crystal structural analyses revealed that complexes 1, 2, and 3 have a discrete dinuclear [Cu-UO₂] core in which one metalloligand, [CuL], is connected to the uranyl moiety via a double phenoxido bridge. Two chelating nitrate ions complete the octa-coordination around uranium. Species 4 is a cocrystal, where a uranyl nitrate dihydrate is sandwiched between two metalloligands [CuL⁴] by the formation of strong hydrogen bonds between the H atoms of the coordinated water molecules to U(VI) and the O atoms of [CuL⁴]. Spectrophotometric titrations of these four metalloligands with uranyl nitrate dihydrate in acetonitrile showed a well-anchored isosbestic point between 300 and 500 nm in all cases, conforming with the coordination of [CuL¹], [CuL²], [CuL³], and the H-bonding interaction of [CuL⁴] with UO₂(NO₃)₂. This behavior of [CuL⁴] was utilized to selectively bind metal ions (e.g., Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, and La³⁺) in the presence of UO₂(NO₃)₂·2H₂O in acetonitrile. The formation of these Cu(II)-U(VI) species in solution was also evaluated by steady-state fluorescence quenching experiments. The difference in the coordination behavior of these metalloligands toward [UO₂(NO₃)₂(H₂O)₂] was studied by density functional theory calculations. The lower flexibility of the ethylenediamine ring and a large negative binding energy obtained from the evaluation of H bonds and supramolecular interactions between [CuL⁴] and [UO₂(NO₃)₂(H₂O)₂] corroborate the formation of cocrystal 4. A very good linear correlation (r² = 0.9949) was observed between the experimental U═O stretching frequencies and the strength of the equatorial bonds that connect the U atom to the metalloligand.

f-Element silicon and heavy tetrel chemistry

The last three decades have seen a significant increase in the number of reports of f-element carbon chemistry, whilst the f-element chemistry of silicon, germanium, tin, and lead remain underdeveloped in comparison. Here, in this perspective we review complexes that contain chemical bonds between f-elements and silicon or the heavier tetrels since the birth of this field in 1985 to present day, with the intention of inspiring researchers to contribute to its development and explore the opportunities that it presents. For the purposes of this perspective, f-elements include lanthanides, actinides and group 3 metals. We focus on complexes that have been structurally authenticated by single-crystal X-ray diffraction, and horizon-scan for future opportunities and targets in the area.

A Single Small-Scale Plutonium Redox Reaction System Yields Three Crystallographically-Characterizable Organoplutonium Complexes

An approach to obtaining substantial amounts of data from a hazardous starting material that can only be obtained and handled in small quantities is demonstrated by the investigation of a single small-scale reaction of cyclooctatetraene, C₈H₈, with a solution obtained from the reduction of Cp'₃Pu (Cp' = C₅H₄SiMe₃) with potassium graphite. This one reaction coupled with oxidation of a product has provided single-crystal X-ray structural data on three organoplutonium compounds as well as information on redox chemistry thereby demonstrating an efficient route to new reactivity and structural information on this highly radioactive element. The crystal structures were obtained from the reduction of C₈H₈ by a putative Pu(II) complex, (Cp'₃Pu^II)^1-, generated in situ, to form the Pu(III) cyclooctatetraenide complex, [K(crypt)][(C₈H₈)₂Pu^III], 1-Pu, and the tetra(cyclopentadienyl) Pu(III) complex, [K(crypt)][Cp'₄Pu^III], 2-Pu. Oxidation of the sample of 1-Pu with Ag(I) afforded a third organoplutonium complex that has been structurally characterized for the first time, (C₈H₈)₂Pu^IV, 3-Pu. Complexes 1-Pu and 3-Pu contain Pu sandwiched between parallel (C₈H₈)^2- rings. The (Cp'₄Pu^III)^- anion in 2-Pu features three η⁵-Cp' rings and one η¹-Cp' ring, which is a rare example of a formal Pu-C η¹-bond. In addition, this study addresses the challenge of small-scale synthesis imparted by radiological and material availability of transuranium isotopes, in particular that of pure metal samples. A route to an anhydrous Pu(III) starting material from the more readily available Pu^IVO₂ was developed to facilitate reproducible syntheses and allow complete spectroscopic analysis of 1-Pu and 2-Pu. Pu^IVO₂ was converted to Pu^IIIBr₃(DME)₂ (DME = CH₃OCH₂CH₂OCH₃) and subsequently Pu^IIIBr₃(THF)_x, which was used to independently synthesize 1-Pu, 2-Pu, and 3-Pu.

Enantiomerically Pure Tetravalent Neptunium Amidinates: Synthesis and Characterization

The synthesis of a tetravalent neptunium amidinate [NpCl((S)-PEBA)3 ] (1) ((S)-PEBA=(S,S)-N,N'-bis-(1-phenylethyl)-benzamidinate) is reported. This complex represents the first structurally characterized enantiopure transuranic compound. Reactivity studies with halide/pseudohalides yielding [NpX((S)-PEBA)3 ] (X=F (2), Br (3), N3 (4)) have shown that the chirality-at-metal is preserved for all compounds in the solid state. Furthermore, they represent an unprecedented example of a structurally characterized metal-organic Np complex featuring a Np-Br (3) bond. In addition, 4 is the only reported tetravalent transuranic azide. All compounds were additionally characterized in solution using para-magnetic NMR spectroscopy showing an expected C3 -symmetry at low temperatures.

A Uranium(VI)-Oxo-Imido Dimer Complex Derived from a Sterically Demanding Triamidoamine

The reaction of [UO₂(μ-Cl)₄{K(18-crown-6)}₂] with [{N(CH₂CH₂NSiPrⁱ₃)₃}Li₃] gives [{UO(μ-NCH₂CH₂N[CH₂CH₂NSiPrⁱ₃]₂)}₂] (1), [{(LiCl)(KCl)(18-crown-6)}₂] (2), and [LiOSiPrⁱ₃] (3) in a 1:2:2 ratio. The formation of the oxo-imido 1 involves the cleavage of a N-Si bond and the activation of one of the usually robust U═O bonds of uranyl(VI), resulting in the formation of uranium(VI)-imido and siloxide linkages. Notably, the uranium oxidation state remains unchanged at +6 in the starting material and product. Structural characterization suggests the dominance of a core RN═U═O group, and the dimeric formulation of 1 is supported by bridging imido linkages in a highly asymmetric U₂N₂ ring. Density functional theory analyses find a σ > π orbital energy ordering for the U═N and U═O bonds in 1, which is uranyl-like in nature. Complexes 1-3 were characterized variously by single crystal X-ray diffraction, multinuclear NMR, IR, Raman, and optical spectroscopies; cyclic voltammetry; and density functional theory.

Nature of the Arsonium-Ylide Ph3 As=CH2 and a Uranium(IV) Arsonium-Carbene Complex

Treatment of [Ph₃ EMe][I] with [Na{N(SiMe₃ )₂ }] affords the ylides [Ph₃ E=CH₂ ] (E=As, 1As; P, 1P). For 1As this overcomes prior difficulties in the synthesis of this classical arsonium-ylide that have historically impeded its wider study. The structure of 1As has now been determined, 45 years after it was first convincingly isolated, and compared to 1P, confirming the long-proposed hypothesis of increasing pyramidalisation of the ylide-carbon, highlighting the increasing dominance of E⁺ -C^- dipolar resonance form (sp³ -C) over the E=C ene π-bonded form (sp² -C), as group 15 is descended. The uranium(IV)-cyclometallate complex [U{N(CH₂ CH₂ NSiPrⁱ ₃ )₂ (CH₂ CH₂ SiPrⁱ ₂ CH(Me)CH₂ )}] reacts with 1As and 1P by α-proton abstraction to give [U(Tren^TIPS )(CHEPh₃ )] (Tren^TIPS =N(CH₂ CH₂ NSiPrⁱ ₃ )₃ ; E=As, 2As; P, 2P), where 2As is an unprecedented structurally characterised arsonium-carbene complex. The short U-C distances and obtuse U-C-E angles suggest significant U=C double bond character. A shorter U-C distance is found for 2As than 2P, consistent with increased uranium- and reduced pnictonium-stabilisation of the carbene as group 15 is descended, which is supported by quantum chemical calculations.

Heteroleptic actinocenes: a thorium(iv)-cyclobutadienyl-cyclooctatetraenyl-di-potassium-cyclooctatetraenyl complex

Despite the vast array of η n -carbocyclic C5-8 complexes reported for actinides, cyclobutadienyl (C4) remain exceedingly rare, being restricted to six uranium examples. Here, overcoming the inherent challenges of installing highly reducing C4-ligands onto actinides when using polar starting materials such as halides, we report that reaction of [Th(η8-C8H8)2] with [K2{C4(SiMe3)4}] gives [{Th(η4-C4[SiMe3]4)(μ-η8-C8H8)(μ-η2-C8H8)(K[C6H5Me]2)}2{K(C6H5Me)}{K}] (1), a new type of heteroleptic actinocene. Quantum chemical calculations suggest that the thorium ion engages in π- and δ-bonding to the η4-cyclobutadienyl and η8-cyclooctatetraenyl ligands, respectively. Furthermore, the coordination sphere of this bent thorocene analogue is supplemented by an η2-cyclooctatetraenyl interaction, which calculations suggest is composed of σ- and π-symmetry donations from in-plane in- and out-of-phase C[double bond, length as m-dash]C 2p-orbital combinations to vacant thorium 6d orbitals. The characterisation data are consistent with this being a metal-alkene-type interaction that is integral to the bent structure and stability of this complex.

Dehydration of UO2Cl2·3H2O and Nd(NO3)3·6H2O with a Soft Donor Ligand and Comparison of Their Interactions through X-ray Diffraction and Theoretical Investigation

We investigated whether the relatively Lewis basic imidazole-2-thiones could be used to substitute water ligands bound to f-element cations and generate f-element soft donor complexes. Reactions of 1,3-diethylimidazole-2-thione (C₂C₂ImT) with Nd(NO₃)₃·6H₂O and UO₂Cl₂·3H₂O led to the isolation of the anhydrous thione complexes Nd(NO₃)₃(C₂C₂ImT)₃ and UO₂Cl₂(C₂C₂ImT)₂, characterized by single crystal X-ray diffraction. Differences in the strength of metal-thione interactions have been examined by means of the crystal structure analysis and density functional theory (DFT) calculations. The C₂C₂ImT ligands were found to be affected by both coordination and noncovalent interactions, making it impossible to deconvolute the effects of one from the other. Calculated partial atomic charges indicated greater ligand-to-metal charge transfer in the [UO₂]²⁺ complex, indicative of a stronger interaction. The reactivity of C₂C₂ImT demonstrates its usefulness in the preparation of f-element soft donor complexes from readily available hydrates that could be useful intermediates for promoting the coordination and studying the effects of soft donor anions.

Probing a variation of the inverse-trans-influence in americium and lanthanide tribromide tris(tricyclohexylphosphine oxide) complexes

The synthesis, characterization, and theoretical analysis of meridional americium tribromide tris(tricyclohexylphosphine oxide), mer-AmBr3(OPcy3)3, has been achieved and is compared with its early lanthanide (La to Nd) analogs. The data show that homo trans ligands display significantly shorter bonds than the cis or hetero trans ligands. This is particularly pronounced in the americium compound. DFT along with multiconfigurational CASSCF calculations show that the contraction of the bonds relates qualitatively with overall covalency, i.e. americium shows the most covalent interactions compared to lanthanides. However, the involvement of the 5p and 6p shells in bonding follows a different order, namely cerium > neodymium ∼ americium. This study provides further insight into the mechanisms by which ITI operates in low-valent f-block complexes.

Polyoxometalate-based complexes as ligands for the study of actinide chemistry

The complexation of actinide cations by polyoxometalates (POMs) has been extensively studied over the past 50 years. In this perspective article, we present the rich structural diversity of actinide-POM complexes and their contribution to the extension of our knowledges of actinide chemistry, especially regarding aspect of their redox chemistry, as well as application for the capture and separation of these cations in the context of nuclear fuel remediation. These heterometallic assemblies have also proven highly valuable as model for heterogeneous systems based on actinides supported by metal oxide surfaces. In particular, activation of the An-O bond of actinyl fragments upon complexation with lacunary POMs has been reported, creating opportunities for future developments regarding the reactivity of these heterometallic assemblies.

Synthesis and Utility of Neptunium(III) Hydrocarbyl Complex

To extend organoactinide chemistry beyond uranium, reported here is the first structurally characterized transuranic hydrocarbyl complex, Np[η⁴ -Me₂ NC(H)C₆ H₅ ]₃ (1), from reaction of NpCl₄ (DME)₂ with four equivalents of K[Me₂ NC(H)C₆ H₅ ]. Unlike the U^III species, the neptunium analogue can be used to access other Np^III complexes. The reaction of 1 with three equivalents of HE₂ C(2,6-Mes₂ -C₆ H₃ ) (E=O, S) yields [(2,6-Mes₂ -C₆ H₃ )CE₂ ]₃ Np(THF)₂ , maintaining the trivalent oxidation state.

Thorium-nitrogen multiple bonds provide evidence for pushing-from-below for early actinides

Although the chemistry of uranium-ligand multiple bonding is burgeoning, analogous complexes involving other actinides such as thorium remain rare and there are not yet any terminal thorium nitrides outside of cryogenic matrix isolation conditions. Here, we report evidence that reduction of a thorium-azide produces a transient Th≡N triple bond, but this activates C-H bonds to produce isolable parent imido derivatives or it can be trapped in an N-heterocycle amine. Computational studies on these thorium-nitrogen multiple bonds consistently evidences a σ > π energy ordering. This suggests pushing-from-below for thorium, where 6p-orbitals principally interact with filled f-orbitals raising the σ-bond energy. Previously this was dismissed for thorium, being the preserve of uranium-nitrides or the uranyl dication. Recognising that pushing-from-below perhaps occurs with thorium as well as uranium, and with imido ligands as well as nitrides, suggests this phenomenon may be more widespread than previously thought.

Despite the burgeoning nature of uranium–ligand multiple bonding, analogous thorium complexes remain incredibly rare. Here the authors report evidence for a transient thorium–nitride species, which, together with data on parent imido derivatives, suggests that the pushing-from-below phenomenon may be more widespread than previously thought.

[Am(C5 Me4 H)3 ]: An Organometallic Americium Complex

We report the small-scale synthesis, isolated yield, single-crystal X-ray structure, ¹ H NMR solution spectroscopy /solid-state UV/Vis-nIR spectroscopy, and density functional theory (DFT)/ab initio wave function theory calculations on an Am³⁺ organometallic complex, [Am(C₅ Me₄ H)₃ ] (1). This constitutes the first quantitative data on Am-C bonding in a molecular species.

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.

Oxidation of uranium(iv) mixed imido-amido complexes with PhEEPh and to generate uranium(vi) bis(imido) dichalcogenolates, U(NR)2(EPh)2(L)2

This work provides new routes for the conversion of U(iv) into U(vi) bis(imido) complexes and offers new information on the manner in which the U(vi) compounds form. Many compounds from the series described by the general formula U(NR)₂(EPh)₂(L)₂ (R = 2,6-diisopropylphenyl, tert-butyl; E = S, Se, Te; L = py, EPh) were synthesized via oxidation of an in situ generated U(iv) amido-imido species with Ph₂E₂. This synthetic sequence provides a general route into bis(imido) U(vi) chalcogenolate complexes, circumventing the need to perform problematic salt metathesis reactions on U(vi) iodides. Investigation into the speciation of the U(iv) complexes that form prior to oxidation found a significant dependence on the identity of the ancillary ligands, with ^tBu₂bpy forming the isolable imido-(bis)amido complex, U(NDipp)(NHDipp)₂(^tBu₂bpy)₂. Together, these data are consistent with the view that the bis(imido) U(vi) motif - much like the uranyl ion, UO₂²⁺- is a thermodynamic sink into which simple ligand frameworks are unable to prevent uranium from falling when in the presence of a suitable retinue of imido proligands.

Inverse Trans Influence in Low-Valence Actinide-Group 10 Metal Complexes of Phosphinoaryl Oxides: A Theoretical Study via Tuning Metals and Donor Ligands

The recognition and in-depth understanding of inverse trans influence (ITI) have successfully guided the synthesis of novel actinide complexes and enriched actinide chemistry. Those complexes, however, are mainly limited to the involvement of high-valence actinide and/or metal-ligand multiple bonds. Examples containing both low oxidation state actinide and metal-metal single bond remain rare. Herein, more than 20 actinide-transition metal (An-TM) complexes of phosphinoaryl oxide ligands have been designed in accordance with several experimentally known analogs, by changing the metal atoms (An = Th, Pa, U, Np, and Pu; and TM = Ni, Pd, and Pt), actinide oxidation states (IV and III) and metal-metal axial donor ligands (X = Me₃SiO, F, Cl, Br, and I). The relativistic density functional theory study of structural (trans-An-X and cis-An-O toward An-TM), bonding (topological electron/energy density), and electronic properties reveals the order of the ITI stabilizing actinide-metal bond. Computed electron affinity (EA) values, related to the electrochemical reduction, linearly correlate with experimentally measured reduction potentials. Although the same ITI order for the ligand donors was shown as in a previous study, the correlation between electrochemical reduction and the ITI was found to be weak when the actinide atoms were changed. For most complexes, the reduction is primarily of an actinide-based mechanism with minor participation of transition metal and phosphinoaryl oxide, whereas that of thorium-nickel complexes is different.

Uranyl-Bound Tetra-Dentate Non-Innocent Ligands: Prediction of Structure and Redox Behaviour Using Density Functional Theory

Computational methods have been applied to understand the reduction potentials of [UO₂ -salmnt-L] complexes (L=pyridine, DMSO, DMF and TPPO), and their redox behavior is compared with previous experiments in dichloromethane solution. Since the experimental results were inconclusive regarding the influence of the uranyl-bound tetra-dentate 'salmnt' ligand, here we will show that salmnt acts as a redox-active ligand and exhibits non-innocent behavior to interfere with the otherwise expected one-electron metal (U) reduction. We have employed two approaches to determine the uranyl (VI/V) reduction potentials, using a direct study of one-electron reduction processes and an estimation of the overall reduction using isodesmic reactions. Hybrid density functional theory (DFT) methods were combined with the Conductor-like Polarizable Continuum Model (CPCM) to account for solvation effects. The computationally predicted one-electron reduction potentials for the range of [UO₂ -salmnt-L] complexes are in excellent agreement with shoulder peaks (∼1.4 eV) observed in the cyclic voltammetry experiments and clearly correlate with ligand reduction. Highly conjugated pi-bonds stabilize the ligand based delocalized orbital relative to the localized U f-orbitals, and as a consequence, the ligand traps the incoming electron. A second reduction step results in metal U(VI) to U(V) reduction, in good agreement with the experimentally assigned uranyl (VI/V) reduction potentials.

Use of 15N NMR spectroscopy to probe covalency in a thorium nitride

The first isolable molecular thorium nitride, [(NR₂)₃Th(μ-N)Th(NR₂)₃]^–, was synthesized by reaction of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] with NaNH₂ and characterized by X-ray crystallography, ¹⁵N NMR spectroscopy, and DFT calculations.

Reaction of the thorium metallacycle, [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] (R = SiMe₃) with 1 equiv. of NaNH₂ in THF, in the presence of 18-crown-6, results in formation of the bridged thorium nitride complex, [Na(18-crown-6)(Et₂O)][(R₂N)₃Th(μ-N)(Th(NR₂)₃] ([Na][1]), which can be isolated in 66% yield after work-up. Complex [Na][1] is the first isolable molecular thorium nitride complex. Mechanistic studies suggest that the first step of the reaction is deprotonation of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] by NaNH₂, which results in formation of the thorium bis(metallacycle) complex, [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)], and NH₃. NH₃ then reacts with unreacted [Th{N(R)(SiMe₂)CH₂}(NR₂)₂], forming [Th(NR₂)₃(NH₂)] (2), which protonates [Na(THF)_x][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] to give [Na][1]. Consistent with hypothesis, addition of excess NH₃ to a THF solution of [Th{N(R)(SiMe₂)CH₂}(NR₂)₂] results in formation of [Th(NR₂)₃(NH₂)] (2), which can be isolated in 51% yield after work-up. Furthermore, reaction of [K(DME)][Th{N(R)(SiMe₂CH₂)}₂(NR₂)] with 2, in THF-d₈, results in clean formation of [K][1], according to ¹H NMR spectroscopy. The electronic structures of [1]^– and 2 were investigated by ¹⁵N NMR spectroscopy and DFT calculations. This analysis reveals that the Th–N_nitride bond in [1]^– features more covalency and a greater degree of bond multiplicity than the Th–NH₂ bond in 2. Similarly, our analysis indicates a greater degree of covalency in [1]^–vs. comparable thorium imido and oxo complexes.