Characterization and Comparison of Cm(III) and Eu(III) Complexed with 2,6-Di(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine Using EXAFS, TRFLS, and Quantum-Chemical Methods

Room temperature crystal field splitting of curium resolved by circularly polarized luminescence spectroscopy

Coordination of Cm(iii) with a chiral decadentate ligand N,N,N',N'-tetrakis[(6-carboxypyridin-2-yl)methyl]-1,2-diaminocyclohexane (tpadac) generated complexes with strong luminescence allowing for the unprecedented measurement of well-resolved Cm(iii) circularly polarized luminescence spectra. Quantitative resolution of the electronic structure of the [Cm(tpadac)][K] complexes was achieved at room temperature, highlighting the strength of the combination of luminescence and circularly polarized luminescence spectroscopies to unravel the fundamental electronic structure of Cm(iii). These results are a clear demonstration that these spectroscopies are powerful yet simple tools for the fundamental understanding of electronic structure, which opens the door to future investigations of other Cm(iii) complexes in geometries relevant to nuclear applications, and even other 5f-elements.

Theoretical investigations into the bonding and separation properties of non-rigid, partially rigid, and rigid ligands derived from Et-Tol-PTA with trivalent lanthanides and actinides

The separation of trivalent actinide elements from lanthanide elements represents one of the most formidable challenges within the context of nuclear waste partitioning and transmutation (P&T) processes. Consequently, we embarked on a systematic investigation aimed at elucidating the bonding properties and thermodynamic behavior of a N-ethyl-N-tolyl-2-amide-1,10-phenanthroline (Et-Tol-PTA) ligand in conjunction with trivalent actinide and lanthanide elements. This investigation involved the utilization of various density functional theory (DFT) methods and a comparative analysis between small-core pseudopotential basis sets and all-electron basis sets. It was found that well-performing results were achieved using the PBE0 functional in both bond length and thermodynamic energy calculations, with minimal impact being exerted by the basis set on the results. Furthermore, an exploration was carried out into the bonding and thermodynamic properties of trivalent actinides and lanthanides with ligands derived from Et-Tol-PTA, encompassing non-rigid (La), partially rigid (Lb, Lc), and rigid (Ld) ligands. Thermodynamically, advantages in the separation of Am(III)/Eu(III) were exhibited by Lb and Lc ligands, while excellent performance in the separation of Am(III)/Cm(III) was demonstrated by the La ligand. Analyses conducted using quantum theory of atoms in molecules (QTAIM), reduced density gradient (RDG), and natural bond orbital (NBO) methodologies revealed the presence of partial covalent character in the bonds between oxygen (O) and metal (M), as well as between nitrogen (N) and metal (M), with a higher degree of covalent character being observed in O-Am and N-Am bonds compared to O-Cm/Eu and N-Cm/Eu interactions.

Synthesis, Crystal Structures, and Ion Pairing of κ6 N Complexes with Rare-Earth Elements in the Solid State and in Solution

The rare earth element complexes (Ln=Y, La, Sm, Lu, Ce) of several podant κ6 N-coordinating ligands have been synthetized and thoroughly characterized. The structural properties of the complexes have been investigated by X-ray diffraction in the solid state and by advanced NMR methods in solution. To estimate the donor capabilities of the presented ligands, an experimental comparison study has been conducted by cyclic voltammetry as well as absorption experiments using the cerium complexes and by analyzing 89 Y NMR chemical shifts of the different yttrium complexes. In order to obtain a complete and detailed picture, all experiments were corroborated by state-of-the-art quantum chemical calculations. Finally, coordination competition studies have been carried out by means of 1 H and 31 P NMR spectroscopy to investigate the correlation with donor properties and selectivity.

Solution Structures of Europium Terpyridyl Complexes with Nitrate and Triflate Counterions in Acetonitrile

Lanthanide-ligand complexes are key components of technological applications, and their properties depend on their structures in the solution phase, which are challenging to resolve experimentally or computationally. The coordination structure of the Eu³⁺ ion in different coordination environments in acetonitrile is examined using ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. AIMD simulations are conducted for the solvated Eu³⁺ ion in acetonitrile, both with or without a terpyridyl ligand, and in the presence of either triflate or nitrate counterions. EXAFS spectra are calculated directly from AIMD simulations and then compared to experimentally measured EXAFS spectra. In acetonitrile solution, both nitrate and triflate anions are shown to coordinate directly to the Eu³⁺ ion forming either ten- or eight-coordinate solvent complexes where the counterions are binding as bidentate or monodentate structures, respectively. Coordination of a terpyridyl ligand to the Eu³⁺ ion limits the available binding sites for the solvent and anions. In certain cases, the terpyridyl ligand excludes any solvent binding and limits the number of coordinated anions. The solution structure of the Eu-terpyridyl complex with nitrate counterions is shown to have a similar arrangement of Eu³⁺ coordinating molecules as the crystal structure. This study illustrates how a combination of AIMD and EXAFS can be used to determine how ligands, solvent, and counterions coordinate with the lanthanide ions in solution.

Quantifying the Influence of Covalent Metal-Ligand Bonding on Differing Reactivity of Trivalent Uranium and Lanthanide Complexes

Qualitative differences in the reactivity of trivalent lanthanide and actinide complexes have long been attributed to differences in covalent metal-ligand bonding, but there are few examples where thermodynamic aspects of this relationship have been quantified, especially with U3+ and in the absence of competing variables. Here we report a series of dimeric phosphinodiboranate complexes with trivalent f-metals that show how shorter-than-expected U-B distances indicative of increased covalency give rise to measurable differences in solution deoligomerization reactivity when compared to isostructural complexes with similarly sized lanthanides. These results, which are in excellent agreement with supporting DFT and QTAIM calculations, afford rare experimental evidence concerning the measured effect of variations in metal-ligand covalency on the reactivity of trivalent uranium and lanthanide complexes.

Opportunities and challenges of applying advanced X-ray spectroscopy to actinide and lanthanide N-donor ligand systems

N-donor ligands such as n-Pr-BTP [2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine] preferentially bind trivalent actinides (An3+) over trivalent lanthanides (Ln3+) in liquid-liquid separation. However, the chemical and physical processes responsible for this selectivity are not yet well understood. Here, an explorative comparative X-ray spectroscopy and computational (L3-edge) study for the An/Ln L3-edge and the N K-edge of [An/Ln(n-Pr-BTP)3](NO3)3, [Ln(n-Pr-BTP)3](CF3SO3)3 and [Ln(n-Pr-BTP)3](ClO4)3 complexes is presented. High-resolution X-ray absorption near-edge structure (HR-XANES) L3-edge data reveal additional features in the pre- and post-edge range of the spectra that are investigated using the quantum chemical codes FEFF and FDMNES. X-ray Raman spectroscopy studies demonstrate the applicability of this novel technique for investigations of liquid samples of partitioning systems at the N K-edge.

Prediction of An(III)/Ln(III) Separation by 1,2,4-Triazinylpyridine Derivatives

The effect of frustrated Lewis donors on metal selectivity between actinides and lanthanides was studied using a series of novel organic ligands. Structures and thermodynamic energies were predicted in the gas phase, in water, and in butanol using 9-coordinate, explicitly solvated (H₂O) Eu, Gd, Am, and Cm in the +III oxidation state as reactants in the formation of complexes with 2-(6-[1,2,4]-triazin-3-yl-pyridin-2-yl)-1H-indole (Core 1), 3-[6-(2H-pyrazol-3-yl)pyridin-2-yl]-1,2,4-triazine (Core 2), and several derivatives. These complexations were studied using density functional theory (DFT) incorporating scalar relativistic effects on the actinides and lanthanides using a small core pseudopotential and corresponding basis set. A self-consistent reaction field approach was used to model the effect of water and butanol as solvents. Coordination preferences and metal selectivity are predicted for each ligand. Several ligands are predicted to have a high degree of selectivity, particularly when a low ionization potential in the ligand permits charge transfer to Eu(III), reducing it to Eu(II) and creating a half-filled f⁷ shell. Reasonable separation is predicted between Cm(III) and Gd(III) with Core 1 ligands, possibly due to ligand donor frustration. This separation is largely absent from Core 2 ligands, which are predicted to lose their frustration due to proton transfer from the 2N to the 3N position of the pyrazole component of the ligands via tautomerization.

Trivalent Actinide Ions Showing Tenfold Coordination in Solution

Trivalent actinides generally exhibit ninefold coordination in solution. 2,6-Bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (nPr-BTP), a tridentate nitrogen donor ligand, is known to form ninefold coordinated 1:3 complexes, [An(nPr-BTP)₃]³⁺ (An = U, Pu, Am, Cm) in solution. We report a Cm(III) complex with tenfold coordination in solution, [Cm(nPr-BTP)₃(NO₃)]²⁺. This species was identified using time-resolved laser fluorescence spectroscopy (TRLFS), vibronic side band spectroscopy (VSBS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT). Adding nitrate to a solution of the [Cm(nPr-BTP)₃]³⁺ complex in 2-propanol shifts the Cm(III) emission band from 613.1 to 617.3 nm. This bathochromic shift is due to a higher coordination number of the Cm(III) ion in solution, in agreement with the formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex. The formation of this complex exhibits slow kinetics in the range of 5 to 12 days, depending on the water content of the solvent. Formation of a complex [Cm(nPr-BTP)₃(X)]²⁺ was not observed for anions other than nitrate (X^- = NO₂^-, CN^-, or OTf^-). The formation of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex was studied as a function of NO₃^- and nPr-BTP concentrations, and slope analyses confirmed the addition of one nitrate anion to the [Cm(nPr-BTP)₃]³⁺ complex. Experiments with varied nPr-BTP concentration show that [Cm(nPr-BTP)₃(NO₃)]²⁺ only forms at nPr-BTP concentrations below 10^-4 mol/L whereas for concentrations greater than 10^-4 mol/L the formation of the tenfold species is suppressed and [Cm(nPr-BTP)₃]³⁺ is the only species present. The presence of the tenfold coordinated complex is supported by VSBS, XPS, and DFT calculations. The vibronic side band of the [Cm(nPr-BTP)₃(NO₃)]²⁺ complex exhibits a nitrate stretching mode not observed in the [Cm(nPr-BTP)₃]³⁺ complex. Moreover, XPS on [M(nPr-BTP)₃(NO₃)](NO₃)₂ (M = Eu, Am) yields signals from both non-coordinated and coordinated nitrate. Finally, DFT calculations reveal that the energetically most favored structure is obtained if the nitrate is positioned on the C₂ axis of the D₃ symmetrical [Cm(nPr-BTP)₃]³⁺ complex with a bond distance of 413 pm. Combining results from TRLFS, VSBS, XPS, and DFT provides sound evidence for a unique tenfold coordinated Cm(III) complex in solution-a novelty in An(III) solution chemistry.

Stronger Hydration of Eu(III) Impedes Its Competition against Am(III) in Binding with N-donor Extractants

The significance of understanding the interaction between actinide(III)/lanthanide(III) (An(III)/Ln(III)) and N-donor extractants lies in the importance of efficient An³⁺/Ln³⁺ separation in advanced nuclear fuel cycles and the high expectation of the application of N-donor extractants. This work reports a density functional theory study aiming at a plausible explanation of the origin of the selectivity of the ligands in An³⁺/Ln³⁺ separation and an evaluation of the influence of the bridging groups of typical N-donor extractants. Five bis(triazine) N-donor ligands were considered, differing in their denticity dictated by their bridging groups and in the flexibility of these bridging groups. The results showed much stronger hydration of Eu(III) in comparison to Am(III) in the ligand exchange of aqua ligands by N-donor ligands, while there was a moderate difference in their interaction strengths with the N-donor ligands. This implicated that the distinct difficulty in desolvating Eu(III) and Am(III) may govern their selectivity in liquid-liquid extraction. The analysis of the role of the bridging groups of the ligands confirmed the importance of a ligand to be equipped with preorganized binding sites to minimize the perturbation of entropy. We tentatively propose that this conclusion may hold in the explanation of the low selectivity of oxygenated extractants and the high selectivity of extractants with soft donors in An³⁺/Ln³⁺ separation.

Gas-Phase Complexes of Americium and Lanthanides with a Bis-triazinyl Pyridine: Reactivity and Bonding of Archetypes for F-Element Separations

Bis-triazinyl pyridines (BTPs) exhibit solution selectivity for trivalent americium over lanthanides (Ln), the origins of which remain uncertain. Here, electrospray ionization was used to generate gas-phase complexes [ML₃]³⁺, where M = La, Lu, or Am and L is EtBTP 2,6-bis(5,6-diethyl-1,2,4-triazin-3-yl)-pyridine. Collision-induced dissociation (CID) of [ML₃]³⁺ in the presence of H₂O yielded a protonated ligand [L(H)]⁺ and hydroxide [ML₂(OH)]²⁺ or hydrate [ML(L-H)(H₂O)]²⁺, where (L-H)^- is a deprotonated ligand. Although solution affinities indicate stronger binding of BTPs toward Am³⁺ versus Ln³⁺, the observed CID process is contrastingly more facile for M = Am versus Ln. To understand the disparity, density functional theory was employed to compute potential energy surfaces for two possible CID processes, for M = La and Am. In accordance with the CID results, both the rate determining transition state barrier and the net energy are lower for [AmL₃]³⁺ versus [LaL₃]³⁺ and for both product isomers, [ML₂(OH)]²⁺ and [ML(L-H)(H₂O)]²⁺. More facile removal of a ligand from [AmL₃]³⁺ by CID does not necessarily contradict stronger Am³⁺-L binding, as inferred from solution behavior. In particular, the formation of new bonds in the products can distort kinetics and thermodynamics expected for simple bond cleavage reactions. In addition to correctly predicting the seemingly anomalous CID behavior, the computational results indicate greater participation of Am 5f versus La 4f orbitals in metal-ligand bonding.

Time-resolved and steady-state irradiation of hydrophilic sulfonated bis-triazinyl-(bi)pyridines - modelling radiolytic degradation

Efficient separation of the actinides from the lanthanides is a critical challenge in the development of a more sophisticated spent nuclear fuel recycling process. Based upon the slight differences in f-orbital distribution, a new class of soft nitrogen-donor ligands, the sulfonated bis-triazinyl-(bi)pyridines, has been identified and shown to be successful for this separation under anticipated, large-scale treatment conditions. The radiation robustness of these ligands is key to their implementation; however, current stability studies have yielded conflicting results. Here we report on the radiolytic degradation of the sulfonated 2,6-bis(1,2,4-triazin-3-yl)pyridine (BTP(S)) and 6,6'-bis(1,2,4-triazin-3-yl)-2,2'-bipyridine (BTBP(S)) in aerated, aqueous solutions using a combination of time-resolved pulsed electron techniques to ascertain their reaction kinetics with key aqueous radiolysis products (eaq-, H˙, ˙OH, and ˙NO3), and steady state gamma radiolysis in conjunction with liquid chromatography for identification and quantification of both ligands as a function of absorbed dose. These data were used to construct a predictive deterministic model to provide critical insight into the fundamental radiolysis mechanisms responsible for the ligands' radiolytic stability. The first-order decays of BTP(S) and BTBP(S) are predominantly driven by oxidative processes (˙OH and, to a lesser extent, H2O2), for which calculations demonstrate that the rate of degradation is inhibited by the formation of ligand degradation products that undergo secondary reactions with the primary products of water radiolysis. Overall, BTP(S) is ∼20% more radiolytically stable than BTBP(S), but over 90% of either ligand is consumed within 1 kGy.

Comparative Study of Complexes of Rare Earths and Actinides with 2,6-Bis(1,2,4-triazin-3-yl)pyridine

Complexes of group III metals (rare earth and actinides) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (BTP) have been investigated by computational (DFT) and, in limited cases, by experimental (FT-IR, X-ray) techniques with the goal of determining the characteristics of metal–ligand interactions. The DFT calculations using the M062X exchange-correlation functional revealed that metal–ligand distances correlate with the ionic radii of the metals, in agreement with available X-ray diffraction results on the Sc, Y, La, U, and Pu complexes. A related blue-shift trend could be observed in seven characteristic bands in the IR spectra associated with metal–ligand vibrations. The computations uncovered considerable charge transfer interactions, particularly in the actinide complexes, as important covalent contributions to the metal–ligand bonding. The covalent character of the metal–ligand bonds decreases in the actinides, from U to Cm.

Do An(iii) and Ln(iii) ions form heteroleptic complexes with diglycolamide and hydrophilic BT(B)P ligands in solvent extraction systems? A spectroscopic and DFT study

Formation of heteroleptic complexes would explain a discrepancy between results from solvent extraction and from spectroscopy reported earlier.

Pentavalent Curium, Berkelium, and Californium in Nitrate Complexes: Extending Actinide Chemistry and Oxidation States

Pentavalent actinyl nitrate complexes An^VO₂(NO₃)₂^- were produced by elimination of two NO₂ from An^III(NO₃)₄^- for An = Pu, Am, Cm, Bk, and Cf. Density functional theory (B3LYP) and relativistic multireference (CASPT2) calculations confirmed the AnO₂(NO₃)₂^- as An^VO₂⁺ actinyl moieties coordinated by nitrates. Computations of alternative An^IIIO₂(NO₃)₂^- and An^IVO₂(NO₃)₂^- revealed significantly higher energies. Previous computations for bare AnO₂⁺ indicated An^VO₂⁺ for An = Pu, Am, Cf, and Bk, but Cm^IIIO₂⁺: electron donation from nitrate ligands has here stabilized the first Cm^V complex, Cm^VO₂(NO₃)₂^-. Structural parameters and bonding analyses indicate increasing An-NO₃ bond covalency from Pu to Cf, in accordance with principles for actinide separations. Atomic ionization energies effectively predict relative stabilities of oxidation states; more reliable energies are needed for the actinides.

Coordination chemistry of lanthanides in a AOT–CMPO solvent extraction system: UV-Vis and XAFS studies

Synergistic extraction systems using AOT and CMPO were investigated at the molecular level for improved understanding of the lanthanide coordination environment.

Extraction performance of Eu3+ by using heterocyclic N-donor ligands with different structures in ionic liquids: an experimental and theoretical study

Extraction performance of Eu³⁺ has been studied for heterocyclic N-donor ligands with different structures in [C₂mim][NTf₂] as a diluent.

Complexation of Actinide(III) and Lanthanide(III) with H4TPAEN for a Separation of Americium from Curium and Lanthanides

Previous studies have identified the TPAEN ligand as a potentially appropriate complexing agent in solvent extraction processes for the separation of americium (Am(III)) from the fission products including lanthanide (Ln(III)) and curium (Cm(III)) ions, a challenging issue for advanced nuclear fuel recycling. To get insight into the selectivity of this ligand, the complexation of selected trivalent Ln(III) and actinide (An(III)) cations with TPAEN was investigated in solution. First, the structure and stoichiometry of the TPAEN complex with Am(III) were characterized by extended X-ray absorption fine structure spectroscopy (EXAFS). Then complexation constants and thermodynamics data were acquired for the complexes using different methods: microcalorimetry for the Ln(III) cations, time-resolved laser fluorescence spectroscopy (TRLFS) for Eu(III) and Cm(III), and UV-visible spectroscopy for Nd(III) and Am(III).

Synthesis and characterization of uranium(iv) tetrachloro complexes in bis-pyrazolylpyridine ligand environments

The most intriguing feature in structures2a–2dis the out-of-plane shift of the U ion from the chelate's three coordinating N atoms (grey background: molecular plane), as exemplarily shown for2band2d.

2,6-Bis(5,6-diisopropyl-1,2,4-triazin-3-yl)pyridine: a highly selective N-donor ligand studied by TRLFS, liquid–liquid extraction and molecular dynamics

Studying the effect of modifications of BTP-ligands on the complexation and separation of Ln(iii) and An(iii).

Hydrophilic sulfonated bis-1,2,4-triazine ligands are highly effective reagents for separating actinides(iii) from lanthanides(iii) via selective formation of aqueous actinide complexes

Tetrasulfonated bis-1,2,4-triazine ligands can selectively complex and separate actinides from lanthanides in aqueous nitric acid with very high selectivities.

We report the first examples of hydrophilic 6,6′-bis(1,2,4-triazin-3-yl)-2,2′-bipyridine (BTBP) and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen) ligands, and their applications as actinide(iii) selective aqueous complexing agents. The combination of a hydrophobic diamide ligand in the organic phase and a hydrophilic tetrasulfonated bis-triazine ligand in the aqueous phase is able to separate Am(iii) from Eu(iii) by selective Am(iii) complex formation across a range of nitric acid concentrations with very high selectivities, and without the use of buffers. In contrast, disulfonated bis-triazine ligands are unable to separate Am(iii) from Eu(iii) in this system. The greater ability of the tetrasulfonated ligands to retain Am(iii) selectively in the aqueous phase than the corresponding disulfonated ligands appears to be due to the higher aqueous solubilities of the complexes of the tetrasulfonated ligands with Am(iii). The selectivities for Am(iii) complexation observed with hydrophilic tetrasulfonated bis-triazine ligands are in many cases far higher than those found with the polyaminocarboxylate ligands previously used as actinide-selective complexing agents, and are comparable to those found with the parent hydrophobic bis-triazine ligands. Thus we demonstrate a feasible alternative method to separate actinides from lanthanides than the widely studied approach of selective actinide extraction with hydrophobic bis-1,2,4-triazine ligands such as CyMe₄-BTBP and CyMe₄-BTPhen.

The strength of actinide-element bonds from the quantum theory of atoms-in-molecules

[AnX(3)](2)(μ-η(2):η(2)-N(2)) (An = Th-Pu; X = F, Cl, Br, Me, H, OPh) have been studied using relativistic density functional theory. Geometric and vibrational data suggest that metal→N(2) charge transfer maximises at the protactinium systems, which feature the longest N-N bonds and the smallest σ(N-N), as a result of partial population of the N-N π* orbitals. There is very strong correlation of the standard quantum theory of atoms-in-molecules (QTAIM) metrics - bond critical point ρ, ∇(2)ρ and H and delocalisation indices - with An-N and N-N bond lengths and σ(N-N), but the correlation with An-N interaction energies is very poor. A similar situation exists for the other systems studied; neutral and cationic actinide monoxide and dioxides, and AnL(3+) and AnL(3)(3+) (L = pyridine (Py), pyrazine (Pz) and triazine (Tz)) with the exception of some of the ∇(2)ρ data, for which moderate to good correlations with energy data are sometimes seen. By contrast, in almost all cases there is very strong correlation of interaction and bond energies with |ΔQ(QTAIM)(An)|, a simple QTAIM metric which measures the amount of charge transferred to or from the actinide on compound formation.

NMR and TRLFS studies of Ln(iii) and An(iii) C5-BPP complexes

C5-BPP is a highly efficient N-donor ligand for the separation of trivalent actinides, An(iii), from trivalent lanthanides, Ln(iii). The molecular origin of the selectivity of C5-BPP and many other N-donor ligands of the BTP-type is still not entirely understood. We present here the first NMR studies on C5-BPP Ln(iii) and An(iii) complexes. C5-BPP is synthesized with 10% 15N labeling and characterized by NMR and LIFDI-MS methods. 15N NMR spectroscopy gives a detailed insight into the bonding of C5-BPP with lanthanides and Am(iii) as a representative for trivalent actinide cations, revealing significant differences in 15N chemical shift for coordinating nitrogen atoms compared to Ln(iii) complexes. The temperature dependence of NMR chemical shifts observed for the Am(iii) complex indicates a weak paramagnetism. This as well as the observed large chemical shift for coordinating nitrogen atoms show that metal-ligand bonding in Am(C5-BPP)3 has a larger share of covalence than in lanthanide complexes, confirming earlier studies. The Am(C5-BPP)3 NMR sample is furthermore spiked with Cm(iii) and characterized by time-resolved laser fluorescence spectroscopy (TRLFS), yielding important information on the speciation of trace amounts of minor complex species.

Influence of the solvent on the complexation of Cm(iii) and Eu(iii) with nPr–BTP studied by time-resolved laser fluorescence spectroscopy

The systematic investigation of the influence of the water content in methanol–water mixtures on the complexation of Cm(iii) and Eu(iii) with nPr–BTP revealed an immense influence of the solvent.

A remarkable enhancement in Am3+/Eu3+selectivity by an ionic liquid based solvent containing bis-1,2,4-triazinyl pyridine derivatives: DFT validation of experimental results

Different complexation behaviour in RTIL medium resulted in an enhancement in selectivity up to a few thousands with MeBTP.

Probing the difference in covalence by enthalpy measurements: a new heterocyclic N-donor ligand for actinide/lanthanide separation

BQPhen, a new tetra-dentate nitrogen-donor ligand, forms stronger complexes with Am³⁺ than Eu³⁺ or Nd³⁺, due to a more exothermic enthalpy of complexation for the Am³⁺ complexes, implying a higher degree of covalence in the BQPhen complexes with Am³⁺ than Eu³⁺ or Nd³⁺.

Theoretical insights into the separation of Am(iii) over Eu(iii) with PhenBHPPA

Due to the similar chemical properties of actinides An(iii) and lanthanides Ln(iii), their separation in spent nuclear fuel reprocessing is extremely challenging.

Synchrotron applications to f-element research in the nuclear fuel cycle

A summary of innovative techniques using synchrotron radiation for actinide studies in nuclear research, including X-ray spectroscopy and imaging.

Lower denticity leading to higher stability: structural and solution studies of Ln(III)-OBETA complexes

The heptadentate ligand OBETA (2,2'-oxybis(ethylamine)-N,N,N',N'-tetraacetic acid) was reported to form complexes with Ln(3+) ions more stable than those formed by the octadentate and more popular congener EGTA (ethylene glycol O,O'-bis(ethylamine)-N,N,N',N'-tetraacetic acid). The structural features leading to this puzzling coordination paradox were investigated by X-ray diffraction, solution state NMR, molecular modeling, and relaxometric studies. The stability constant of Gd(OBETA) (log KGdL = 19.37, 0.1 M KCl) is 2 orders of magnitude higher than that of the higher denticity analogue Gd(EGTA) (log KGdL = 17.66, 0.1 M KCl). The half-lives (t1/2) for the dissociation reactions of Gd(OBETA) and Gd(EGTA) ([Cu(2+)]tot = 0.2 mM, [Cit(3-)]tot = 0.5 mM, [PO4(3-)]tot = 1.0 mM, and [CO3(2-)]tot = 25 mM at pH = 7.4 and 25 °C in 0.1 M KCl solution) are 6.8 and 0.63 h, respectively, reflecting the much higher inertness of Gd(OBETA) near physiological conditions. NMR studies and DFT calculations using the B3LYP functional and a large-core ECP indicate that the [Gd(OBETA)(H2O)2](-) complex most likely exists in solution as the Δ(λλ)(δδδδ)A/Λ(δδ)(λλλλ)A enantiomeric pair, with an activation free energy for the enantiomerization process of ∼40 kJ·mol(-1). The metal ion is nine-coordinate by seven donor atoms of the ligand and two inner-sphere water molecules. The X-ray crystal structure of [C(NH2)3]3[Lu(OBETA)(CO3)]·2H2O is in agreement with the predictions of DFT calculations, the two coordinated water molecules being replaced by a bidentate carbonate anion. The (1)H NMRD and (17)O NMR study revealed that the two inner-sphere water molecules in Gd(OBETA) are endowed with a relatively fast water exchange rate (kex(298) = 13 × 10(6) s(-1)). The higher thermodynamic stability and inertness of Ln(OBETA) complexes, peaking in the center of the 4f series, combined with the presence of two coordinated water molecules suggests that Gd(OBETA) is a promising paramagnetic probe for MRI applications.

On the use of X-ray absorption spectroscopy to elucidate the structure of lutetium adenosine mono- and triphosphate complexes

Although the physiological impact of the actinide elements as nuclear toxicants has been widely investigated for half a century, a description of their interactions with biological molecules remains limited. It is however of primary importance to better assess the determinants of actinide speciation in cells and more generally in living organisms to unravel the molecular processes underlying actinide transport and deposition in tissues. The biological pathways of this family of elements in case of accidental contamination or chronic natural exposure (in the case of uranium rich soils for instance) are therefore a crucial issue of public health and of societal impact. Because of the high chemical affinity of those actinide elements for phosphate groups and the ubiquity of such chemical functions in biochemistry, phosphate derivatives are considered as probable targets of these cations. Among them, nucleotides and in particular adenosine mono- (AMP) and triphosphate (ATP) nucleotides occur in more chemical reactions than any other compounds on the earth's surface, except water, and are therefore critical target molecules. In the present study, we are interested in trans-plutonium actinide elements, in particular americium and curium that are more rarely considered in environmental and bioaccumulation studies than early actinides like uranium, neptunium and plutonium. A first step in this strategy is to work with chemical analogues like lanthanides that are not radioactive and therefore allow extended physical chemical characterization to be conducted that are difficult to perform with radioactive materials. We describe herein the interaction of lutetium(III) with adenosine AMP and ATP. With AMP and ATP, insoluble amorphous compounds have been obtained with molar ratios of 1:2 and 1:1, respectively. With an excess of ATP, with 1:2 molar ratio, a soluble complex has been obtained. A combination of spectroscopic techniques (IR, NMR, ESI-MS, EXAFS) together with quantum chemical calculations has been implemented in order to assess the lutetium coordination arrangement for the two nucleotides. In all the complexes described in the article, the lutetium cation is coordinated by the phosphate groups of the nucleotide plus additional putative water molecules with various tridimensional arrangements. With AMP 1:2 and ATP 1:1 solid-state compounds, polynuclear complexes are assumed to be obtained. In contrast, with ATP 1:2 soluble compound, the Lu coordination sphere is saturated by two ATP ligands, and this favors the formation of a mononuclear complex. In order to further interpret the EXAFS data obtained at the Lu LIII edge, model structures have been calculated for the 1:1 and 1:2 ATP complexes. They are discussed and compared to the EXAFS best fit metrical parameters.