Coordination Chemistry of Homoleptic Actinide(IV)–Thiocyanate Complexes

The synthesis, X‐ray crystal structure, vibrational and optical spectroscopy for the eight‐coordinate thiocyanate compounds, [Et₄N]₄[Pu^IV(NCS)₈], [Et₄N]₄[Th^IV(NCS)₈], and [Et₄N]₄[Ce^III(NCS)₇(H₂O)] are reported. Thiocyanate was found to rapidly reduce plutonium to Pu^III in acidic solutions (pH<1) in the presence of NCS^?. The optical spectrum of [Et₄N][SCN] containing Pu^III solution was indistinguishable from that of aquated Pu^III suggesting that inner‐sphere complexation with [Et₄N][SCN] does not occur in water. However, upon concentration, the homoleptic thiocyanate complex [Et₄N]₄[Pu^IV(NCS)₈] was crystallized when a large excess of [Et₄N][NCS] was present. This compound, along with its U^IV analogue, maintains inner‐sphere thiocyanate coordination in acetonitrile based on the observation of intense ligand‐to‐metal charge‐transfer bands. Spectroscopic and crystallographic data do not support the interaction of the metal orbitals with the ligand π system, but support an enhanced An^IV–NCS interaction, as the Lewis acidity of the metal ion increases from Th to Pu.     

Electrode processes of plutonium ions in phosphate media

Summary The electrolytic behaviour of plutonium ions in a mixture of phosphate and nitrate solutions was studied by flow-coulometry with column electrodes of glassy carbon(GC)-fibers and voltammetry with a GC disc electrode, and compared with that in phosphate-free media. The redox processes, PuO ₂ ²⁺ /PuO ₂ ⁺ and Pu⁴⁺/Pu³⁺, were demonstrated to be reversible even in the phosphate media and their half wave potentials shifted more negatively due to the formation of PuO₂(H₂PO₄)⁺ and Pu(HPO₄)₂. The rate of the irreversible reduction of PuO ₂ ⁺ to Pu³⁺ increased in the presence of phosphoric acid and the quantitative reduction was attained with the column electrode even at +0.35 V vs. saturated KCl-Ag/AgCl. The reduction process of PuO ₂ ⁺ was elucidated considering an intermediate Pu(IV)-species, PuO²⁺, which decomposed into Pu⁴⁺ by a post-chemical reaction. Analytical advantages of the use of phosphate media are discussed.     

Ligand‐Supported Facile Conversion of Uranyl(VI) into Uranium(IV) in Organic and Aqueous Media

Reduction of uranyl(VI) to U^V and to U^IV is important in uranium environmental migration and remediation processes. The anaerobic reduction of a uranyl U^VI complex supported by a picolinate ligand in both organic and aqueous media is presented. The [U^VIO₂(dpaea)] complex is readily converted into the cis‐boroxide U^IV species via diborane‐mediated reductive functionalization in organic media. Remarkably, in aqueous media the uranyl(VI) complex is rapidly converted, by Na₂S₂O₄, a reductant relevant for chemical remediation processes, into the stable uranyl(V) analogue, which is then slowly reduced to yield a water‐insoluble trinuclear U^IV oxo‐hydroxo cluster. This report provides the first example of direct conversion of a uranyl(VI) compound into a well‐defined molecular U^IV species in aqueous conditions.     

Substituted sulphoxide ligands in piperidinium based ionic liquid: novel solvent systems for the extraction of Pu<Superscript>4+</Superscript> and PuO<Subscript>2</Subscript><Superscript>2+</Superscript>

Sulphoxide ligands in piperidinium based ionic liquid were demonstrated as highly efficient, selective and environmentally benign systems for the extraction of plutonium from acidic aqueous solution. The extraction followed ‘cation-exchange mechanism’ via [Pu(NO₃)·L]³⁺ and [PuO₂(NO₃)·L]⁺ species. The extraction efficiency followed the trend: APSO > BPSO > BMSO. The phenyl substituted sulphoxides showed higher affinity for plutonium due to a combination of steric as well as electronic factors. Extraction process was thermodynamically spontaneous for all three solvent systems. Oxalic acid and sodium carbonate were suitable for quantitative stripping of Pu⁴⁺ and PuO₂ ²⁺, respectively. APSO in ionic liquid showed good radiolytic stability.     

Subtle Interactions and Electron Transfer between UIII,NpIII, or PuIII and Uranyl Mediated by the Oxo Group

A dramatic difference in the ability of the reducing An^III center in AnCp₃ (An=U, Np, Pu; Cp=C₅H₅) to oxo‐bind and reduce the uranyl(VI) dication in the complex [(UO₂)(THF)(H₂L)] (L=“Pacman” Schiff‐base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At‐first contradictory electronic structural data are explained by combining theory and experiment. Complete one‐electron transfer from Cp₃U forms the U^IV‐uranyl(V) compound that behaves as a U^V‐localized single molecule magnet below 4 K. The extent of reduction by the Cp₃Np group upon oxo‐coordination is much less, with a Np^III‐uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest Np^IVU^V but single‐crystal X‐ray diffraction and SQUID magnetometry suggest a Np^III‐U^VI assignment. DFT‐calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The Pu^III–U^VI interaction is too weak to be observed in THF solvent, in agreement with calculated predictions.     

Subtle Interactions and Electron Transfer between UIII,NpIII, or PuIII and Uranyl Mediated by the Oxo Group

A dramatic difference in the ability of the reducing An^III center in AnCp₃ (An=U, Np, Pu; Cp=C₅H₅) to oxo‐bind and reduce the uranyl(VI) dication in the complex [(UO₂)(THF)(H₂L)] (L=“Pacman” Schiff‐base polypyrrolic macrocycle), is found and explained. These are the first selective functionalizations of the uranyl oxo by another actinide cation. At‐first contradictory electronic structural data are explained by combining theory and experiment. Complete one‐electron transfer from Cp₃U forms the U^IV‐uranyl(V) compound that behaves as a U^V‐localized single molecule magnet below 4 K. The extent of reduction by the Cp₃Np group upon oxo‐coordination is much less, with a Np^III‐uranyl(VI) dative bond assigned. Solution NMR and NIR spectroscopy suggest Np^IVU^V but single‐crystal X‐ray diffraction and SQUID magnetometry suggest a Np^III‐U^VI assignment. DFT‐calculated Hirshfeld charge and spin density analyses suggest half an electron has transferred, and these explain the strongly shifted NMR spectra by spin density contributions at the hydrogen nuclei. The Pu^III–U^VI interaction is too weak to be observed in THF solvent, in agreement with calculated predictions.     

Catalytic Dinitrogen Reduction to Ammonia at a Triamidoamine–Titanium Complex

Catalytic reduction of N₂ to NH₃ by a Ti complex has been achieved, thus now adding an early d‐block metal to the small group of mid‐ and late‐d‐block metals (Mo, Fe, Ru, Os, Co) that catalytically produce NH₃ by N₂ reduction and protonolysis under homogeneous, abiological conditions. Reduction of [Ti^IV(Tren^TMS)X] (X=Cl, 1A ; I, 1B ; Tren^TMS=N(CH₂CH₂NSiMe₃)₃) with KC₈ affords [Ti^III(Tren^TMS)] ( 2 ). Addition of N₂ affords [{(Tren^TMS)Ti^III}₂(μ‐η¹:η¹‐N₂)] ( 3 ); further reduction with KC₈ gives [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹:η²:η²‐N₂K₂)] ( 4 ). Addition of benzo‐15‐crown‐5 ether (B15C5) to 4 affords [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹‐N₂)][K(B15C5)₂]₂ ( 5 ). Complexes 3 – 5 treated under N₂ with KC₈ and [R₃PH][I], (the weakest H⁺ source yet used in N₂ reduction) produce up to 18 equiv of NH₃ with only trace N₂H₄. When only acid is present, N₂H₄ is the dominant product, suggesting successive protonation produces [{(Tren^TMS)Ti^IV}₂(μ‐η¹:η¹‐N₂H₄)][I]₂, and that extruded N₂H₄ reacts further with [R₃PH][I]/KC₈ to form NH₃.     

Extension of the Plutonium Oxide Nanocluster Family to Include {Pu16} and {Pu22}

Plutonium(IV) oxide nanoclusters represent an interesting scientific problem regarding structure-function relationships and are also relevant to global concerns surrounding plutonium reprocessing, the remediation of plutonium-contaminated soils, and the storage or disposal of plutonium-containing wastes. Here we report the synthesis of five unique Pu^IV oxide nanoclusters containing 38, 22, or 16 metal centers and capped only with inorganic ligands, such as chlorine and water, which prevent formation of the extended PuO₂ structure. The products are interrogated through single-crystal X-ray diffraction, bond valence calculations, and comparisons to other known metal-oxide nanoclusters. Of particular interest is the incorporation of hydroxide linkages into the smaller plutonium oxide nanoclusters, which underscores previous observations and hypotheses regarding the formation of plutonium polymers and colloids.     

Selective 1,4‐Reduction of Chalcones with Zn/NH4Cl/C2H5OH/H2O

In this paper, the Zn/NH₄Cl/C₂H₅OH/H₂O system was applied for the selective 1,4‐reduction of chalcones under mild conditions with high selectivity. The merit lies in inexpensive reagent, simple operation and environmental friendliness. The structures of the products were characterized by IR, ¹HNMR, MS spectra and elemental analysis. A possible mechanism is also suggested.     

1,10‐Phenanthroline and Non‐Symmetrical 1,3,5‐Triazine Dipicolinamide‐Based Ligands For Group Actinide Extraction

The synthesis and evaluation of new extractants for spent nuclear fuel reprocessing are described. New bitopic ligands constituted of phenanthroline and 1,3,5‐triazine cores functionalized by picolinamide groups were designed. Synthetic routes were investigated and optimized to obtain twelve new polyaza‐heterocyclic ligands. In particular, an efficient and versatile methodology was developed to access non‐symmetric 2‐substituted‐4,6‐di(6‐picolin‐2‐yl)‐1,3,5‐triazines from the 1,3,5‐triazapentadiene precursor in the presence of anhydride reagents. Extraction studies showed the ability of both ligand series to extract and separate actinides selectively at different oxidation states (U^VI, Np^V,VI, Am^III, Cm^III, and Pu^IV) from an acidic solution (3 M HNO₃). Phenanthroline‐based ligands show the most promising efficiency for use in the group actinide extraction (GANEX) process due to a higher number of donor nitrogen atoms and a suitable pre‐organization of the dipicolinamide‐1,10‐phenanthroline architecture.     

Simultaneous recovery of plutonium and americium from assorted analytical waste solutions using extraction chromatography

A rapid extraction chromatography based methodology was developed for simultaneous recovery of plutonium and americium from various kinds of analytical waste obtained during chemical quality control of plutonium based nuclear materials using sulphonic acid based actinide? resin. Efforts were made to understand the effect of initial feed acidity, gamma radiation and the concentrations of Am³⁺ and Pu⁴⁺ on their k _d. values. Processing of assorted analytical waste solutions through this method revealed that more than 95 % of Am³⁺ and 90 % of Pu⁴⁺ were adsorbed on the resin while iso- propanol can be successfully employed for the quantitative recovery of both the actinides from the loaded resin phase.     

Photolysis and Thermolysis of Platinum(IV) 2,2′‐Bipyridine Complexes Lead to Identical Platinum(II)–DNA Adducts

Two Pt^IV and two Pt^II complexes containing a 2,2′‐bipyridine ligand were treated with a short DNA oligonucleotide under light irradiation at 37 °C or in the dark at 37 and 50 °C. Photolysis and thermolysis of the Pt^IV complexes led to spontaneous reduction of the Pt^IV to the corresponding Pt^II complexes and to binding of Pt^II 2,2′‐bipyridine complexes to N7 of guanine. When the reduction product was [Pt(bpy)Cl₂], formation of bis‐oligonucleotide adducts was observed, whereas [Pt(bpy)(MeNH₂)Cl]⁺ gave monoadducts, with chloride ligands substituted in both cases. Neither in the dark nor under light irradiation was the reductive elimination process of these Pt^IV complexes accompanied by oxidative DNA damage. This work raises the question of the stability of photoactivatable Pt^IV complexes toward moderate heating conditions.     

Determination of plutonium in environmental samples by controlled valence in aniion exchange

The title method was successfully used for collecting^239,249Pu from 200 litres of seawater by coprecipitation with 16 g FeSO₄·7H₂O under redcing conditions witout filtering. The plutonium is leached by concentrate HNO₃+HCl from the coprecipitate and the solid particles. The precipitate is heated at 400°C and digested in aqua regia. Na₂SO₃ and NaNO₂ have been applied to obtain the Pu⁴⁺ valence state in 0.5–1N HNO₃ for different samples. Plutonium and thorium are coadsorbed on anionic resin from 8N HNO₃. The column is eluted with 8N HNO₃ containing fresh NaNO₂ to keep the Pu⁴⁺ state for uranium decontaination. The system of the column is changed from 8N HNO₃ to concentrated HCl with 50 ml concentrated HCl containing a few milligrams of NaNO₂. Furtheer decontaimination of torium was achieved by elution with concentrated HCl instead of 9N HCl. The plutonium is successfully stripped by H₂O, NaOH, 2N HNO₃ and 0.5N HNO₃ containign 0.01M NaNO₃. The chemica yield of plutonium for a 2001 seawate sample is 60–80%. The resolution of the electroplated thin source is very good.     

C(sp3)−H Oxidative Addition and Transfer Hydrogenation Chemistry of a Titanium(II) Synthon: Mimicry of Late‐Metal Type Reactivity

Two‐electron reduction of the Ti^IV compound (^ketguan)(Im^DippN)Ti(OTf)₂ ( 3 ) gives the arene‐masked complex (^ketguan)(η⁶‐Im^DippN)Ti ( 1 ) in excellent yield. Upon standing in solution, 1 converts to a Ti^IV metallacycle ( 4 ) through dehydrogenation of a pendant isopropyl group. Spectroscopic evidence shows this transformation initially proceeds via the oxidative addition of a C(sp³)?H bond and can be reversed upon exposure of 4 to H₂. Interestingly, treatment of 1 with cyclohexene gives cyclohexane and 4 via a titanium‐mediated transfer hydrogenation reaction, a process that can be extended to catalytically hydrogenate other unsaturated hydrocarbons under mild conditions. These results, rare for the early‐metals, suggest 1 possesses chemical characteristics reminiscent of noble, late‐metals.     

A new method for estimating the dispersity of deposited metallic nanoparticles and extent of their interaction with the support matrices

Using a fully relativistic DV cluster method, we study the electronic structure of a large fragment of the crystal lattice of zircon ZrSiO₄ with a plutonium dopant atom replacing a Zr⁴⁺ zirconium atom. Three possible states of the impurity center are considered: Pu⁴⁺ (isovalent substitution), Pu³⁺ (non-isovalent substitution), and Pu³⁺ with an oxygen vacancy in the nearest environment that provides charge compensation. Relaxation of the ZrSiO₄ crystal lattice near a defect is simulated using a semi-empirical method of atomic pair potentials (GULP program). An analysis of overlap populations and effective charges on atoms shows that the chemical bonding of plutonium with a matrix is covalent, while isovalent substitution yields a more stable system than a Pu³⁺ impurity. In the presence of vacancies the structure of chemical bonding is intermediate with respect to substitutions Pu⁴⁺ → Zr⁴⁺ and Pu³⁺ → Zr⁴⁺.     

Rational Design,Synthesis, and Evaluation of Tetrahydroxamic Acid Chelators for Stable Complexation of Zirconium(IV)

Metals of interest for biomedical applications often need to be complexed and associated in a stable manner with a targeting agent before use. Whereas the fundamentals of most transition‐metal complexation processes have been thoroughly studied, the complexation of Zr^IV has been somewhat neglected. This metal has received growing attention in recent years, especially in nuclear medicine, with the use of ⁸⁹Zr, which a β⁺‐emitter with near ideal characteristics for cancer imaging. However, the best chelating agent known for this radionuclide is the trishydroxamate desferrioxamine B (DFB), the Zr^IV complex of which exhibits suboptimal stability, resulting in the progressive release of ⁸⁹Zr in vivo. Based on a recent report demonstrating the higher thermodynamic stability of the tetrahydroxamate complexes of Zr^IV compared with the trishydroxamate complexes analogues to DFB, we designed a series of tetrahydroxamic acids of varying geometries for improved complexation of this metal. Three macrocycles differing in their cavity size (28 to 36‐membered rings) were synthesized by using a ring‐closing metathesis strategy, as well as their acyclic analogues. A solution study with ⁸⁹Zr showed the complexation to be more effective with increasing cavity size. Evaluation of the kinetic inertness of these new complexes in ethylenediaminetetraacetic acid (EDTA) solution showed significantly improved stabilities of the larger chelates compared with ⁸⁹Zr‐DFB, whereas the smaller complexes suffered from insufficient stabilities. These results were rationalized by a quantum chemical study. The lower stability of the smaller chelates was attributed to ring strain, whereas the better stability of the larger cyclic complexes was explained by the macrocyclic effect and by the structural rigidity. Overall, these new chelating agents open new perspectives for the safe and efficient use of ⁸⁹Zr in nuclear imaging, with the best chelators providing dramatically improved stabilities compared with the reference DFB.     

The analysis of plutonium uranium carbide-iron cermets by electrochemical method

Summary Electrochemical methods have been developed for the direct determination of the main metallic constituents of 15 w/o (Pu, U) C — 10 w/o Fe cermets. Samples are dissolved in nitric acid, followed by wet oxidation with a mixture of sulphuric and nitric acids to destroy organic matter. Plutonium is determined by a potentiometric titration procedure based upon the reduction of Pu^VI to Pu^IV with ferrous iron added in slight excess, followed by the titration of this excess with ceric sulphate. The precision of this determination on 4 mg amounts of plutonium is 0.1%. Both uranium and iron are determined by controlled-potential coulometry, but the method for iron includes a contribution from plutonium and allowance has to be made for this. The precision of the uranium determination on 10 mg amounts is 0.25%, and the value for iron on 1 mg amounts is also 0.25%. Results for the complete analysis of typical cermet samples are given.

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Zusammenfassung Elektrochemische Verfahren wurden ausgearbeitet zur direkten Bestimmung der metallischen Hauptbestandteile in Metall-keramischen Materialien (Cermets) der Zusammensetzung 15% (Pu,U)C-10% Fe. Hierbei wird die Probe in Salpetersäure aufgelöst und anschließend zur Zerstörung organischer Substanz einer nassen Oxydation mit einem Gemisch von Schwefel- und Salpetersäure unterworfen. Plutonium wird mit Hilfe eines potentiometrischen Verfahrens bestimmt, das auf der Reduktion von Pu^VI zu Pu^IV mit Fe^II in geringem Überschluß und nachfolgender Titration des Überschusses mit Cer^IV-sulfatlösung beruht. Die Genauigkeit beträgt hierbei 0,1% bei Mengen von 4 mg Pu. Uran und Eisen werden durch Coulometrie mit Potentialkontrolle bestimmt; jedoch wird bei der Eisenbestimmung Plutonium miterfaßt und muß bei der Berechnung berücksichtigt werden. Die Genauigkeit der Uranbestimmung beträgt 0,25% bei 10 mg U, die der Eisenbestimmung 0,25% bei 1 mg Fe. Ergebnisse von vollständigen Analysen typischer Cermetproben werden angeführt.

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Dedicated to Prof. Dr. M. von Stackelberg on his 70th birthday.     

Mechanism of PtIV Sonochemical Reduction in Formic Acid Media and Pure Water

Sonochemical synthesis of platinum nanoparticles (Pt NPs) in formic acid solutions and pure water was investigated using a 20 kHz ultrasonic irradiation. The obtained results gave new insights on the underneath Pt^IV reduction mechanism in formic acid media under argon and in pure water under Ar/CO atmosphere. It was shown that in pure water sonochemical reduction of platinum ions occurs by hydrogen issued from homolytic water molecule split. Pt^IV ion reduction appears to be a very slow process under argon atmosphere in pure water due to formation of oxidizing species like OH radicals and H₂O₂ leading to reoxidation of intermediate Pt^II ions. Sonochemical reduction is accelerated manifold in the presence of formic acid or Ar/CO gas mixture. Solution and gas‐phase analyses reveal that both CO and HCOOH act as OH^. radical scavenger and reducing agent under ultrasonic irradiation. Their ability to reduce platinum ions at room temperature is enhanced due to the local heating in the liquid shell surround the cavitation bubble. An innovative synthesis route for monodispersed Pt NPs in pure water without any templates or capping agents in the presence of Ar/CO gas mixture is then proposed. Obtained Pt NPs within the range of 2–3 nm exhibited a strong stability towards sedimentation in water. Since Ar/CO atmosphere is the only restriction of the process, this procedure can be applied in various media and is also compatible with a large array of experimental conditions.     

Expansion of the Na3MIII(Ln/An)6F30 Series: Incorporation of Plutonium into a Highly Robust and Stable Framework

Na_nMAn₆F₃₀ is an extremely versatile framework structure for incorporating tetravalent actinides (An) and cerium along with divalent or trivalent d-metals (M); moreover, the structure exhibits a high resistance to harsh chemical conditions. This extreme robustness can potentially be exploited for the sequestration of plutonium in a stable matrix; however, no Na_nMPu₆F₃₀ compounds have been reported so far. Herein, we present four new plutonium fluorides that have been prepared as single crystals by mild hydrothermal synthesis methods. Structural characterizations revealed their compositions to be Na₃AlPu₆F₃₀, Na₃FePu₆F₃₀, Na₃CoPu₆F₃₀, and Na_2.4Mn_1.6Pu₆F₃₀. Surprisingly, in the plutonium series, it was found that Co²⁺ and Mn²⁺ precursors oxidized to form Na₃Co^IIIPu₆F₃₀ and Na_2.4Mn^II/III_1.6Pu₆F₃₀, whereas the analogous reactions for cerium result in reduction of the transition metal, even when beginning with a M³⁺ precursor. While cerium is often used as a surrogate for plutonium, this work serves as an example that deviations between their chemistries do occur.