Revisiting complexation thermodynamics of transplutonium elements up to einsteinium

Tuning Selectivity to f-Elements through Bonding and Solvation Effects of a Sulfur Donor Ligand

Bis(2,4,4-trimethylpentyl)dithiophosphinic acid, commonly referred to as HBTMPDTP or Cyanex301, is a sulfur-donating ligand that shows considerable promise in the challenging task of separating trivalent actinides (An3+) from lanthanides (Ln3+). Although its effectiveness has been established, the specific molecular details about the preference of HBTMPDTP for americium over europium have remained a mystery, puzzling researchers for over two decades. This study presents a comprehensive, dual-driven separation mechanism for this complex system combining experimental and theoretical approaches. A critical finding is the increased covalency in An-S bonds compared to Ln-S bonds, which plays a significant role in HBTMPDTP's intrinsic selectivity for An3+ over Ln3+. This leads to the formation of distinct An3+ and Ln3+ species, enhancing the ligand's actinide selectivity. Additionally, it provides crucial insights into the coordination chemistry of f-elements with sulfur-donating ligands, thereby deepening our understanding of this intricate field.

Recent Advances in the Study of Trivalent Lanthanides and Actinides by Phosphinic and Thiophosphinic Ligands in Condensed Phases

The separation of trivalent actinides and lanthanides is a key step in the sustainable development of nuclear energy, and it is currently mainly realized via liquid-liquid extraction techniques. The underlying mechanism is complicated and remains ambiguous, which hinders the further development of extraction. Herein, to better understand the mechanism of the extraction, the contributing factors for the extraction are discussed (specifically, the sulfur-donating ligand, Cyanex301) by combing molecular dynamics simulations and experiments. This work is expected to contribute to improve our systematic understanding on a molecular scale of the extraction of lanthanides and actinides, and to assist in the extensive studies on the design and optimization of novel ligands with improved performance.

Estimating the Gibbs Hydration Energies of Actinium and Trans-Plutonium Actinides

The use of actinides for medical, scientific and technological purposes has gained momentum in the recent years. This creates a need to understand their interactions with biomolecules, both at the interface and as they become complexed. Calculation of the Gibbs binding energies of the ions to biomolecules, i. e., the Gibbs energy change associated with a transfer of an ion from the water phase to its binding site, could help to understand the actinides' toxicities and to design agents that bind them with high affinities. To this end, there is a need to obtain accurate reference values for actinide hydration, that for most actinides are not available from experiment. In this study, a set of ionic radii is developed that enables future calculations of binding energies for Pu³⁺ and five actinides with renewed scientific and technological interest: Ac³⁺ , Am³⁺ , Cm³⁺ , Bk³⁺ and Cf³⁺ . Reference hydration energies were calculated using quantum chemistry and ion solvation theory and agree well for all ions except Ac³⁺ , where ion solvation theory seems to underestimate the magnitude of the Gibbs hydration energy. The set of radii and reference energies that are presented here provide means to calculate binding energies for actinides and biomolecules.

Polyoxometalates as ligands to synthesize, isolate and characterize compounds of rare isotopes on the microgram scale

The synthesis and study of radioactive compounds are both inherently limited by their toxicity, cost and isotope scarcity. Traditional methods using small inorganic or organic complexes typically require milligrams of sample-per attempt-which for some isotopes is equivalent to the world's annual supply. Here we demonstrate that polyoxometalates (POMs) enable the facile formation, crystallization, handling and detailed characterization of metal-ligand complexes from microgram quantities owing to their high molecular weight and controllable solubility properties. Three curium-POM complexes were prepared, using just 1-10 μg per synthesis of the rare isotope 248Cm3+, and characterized by single-crystal X-ray diffraction, showing an eight-coordinated Cm3+ centre. Moreover, spectrophotometric, fluorescence, NMR and Raman analyses of several f-block element-POM complexes, including 243Am3+ and 248Cm3+, showed otherwise unnoticeable differences between their solution versus solid-state chemistry, and actinide versus lanthanide behaviour. This POM-driven strategy represents a viable path to isolate even rarer complexes, notably with actinium or transcalifornium elements.

Engineering lanmodulin's selectivity for actinides over lanthanides by controlling solvent coordination and second-sphere interactions

Developing chelators that combine high affinity and selectivity for lanthanides and/or actinides is paramount for numerous industries, including rare earths mining, nuclear waste management, and cancer medicine. In particular, achieving selectivity between actinides and lanthanides is notoriously difficult. The protein lanmodulin (LanM) is one of Nature's most selective chelators for trivalent actinides and lanthanides. However, mechanistic understanding of LanM's affinity and selectivity for f-elements remains limited. In order to decipher, and possibly improve, the features of LanM's metal-binding sites that contribute to this actinide/lanthanide selectivity, we characterized five LanM variants, substituting the aspartate residue at the 9^th position of each metal-binding site with asparagine, histidine, alanine, methionine, and selenomethionine. Spectroscopic measurements with lanthanides (Nd³⁺ and Eu³⁺) and actinides (²⁴³Am³⁺ and ²⁴⁸Cm³⁺) reveal that, contrary to the behavior of small chelator complexes, metal-coordinated water molecules enhance LanM's affinity for f-elements and pH-stability of its complexes. Furthermore, the results show that the native aspartate does not coordinate the metal directly but rather hydrogen bonds to coordinated solvent. By tuning this first-sphere/second-sphere interaction, the asparagine variant nearly doubles LanM's selectivity for actinides versus lanthanides. This study not only clarifies the essential role of coordinated solvent for LanM's physiological function and separation applications, but it also demonstrates that LanM's preference for actinides over lanthanides can be further improved. More broadly, it demonstrates how biomolecular scaffolds possess an expanded repertoire of tunable interactions compared to most small-molecule ligands – providing an avenue for high-performance LanM-based actinide/lanthanide separation methods and bio-engineered chelators optimized for specific medical isotopes.

Nature’s most potent protein for f-elements, lanmodulin, relies on subtle first-sphere/second-sphere interactions to bind metal ions. Dissecting lanmodulin’s binding mechanism yielded variants with enhanced actinide/lanthanide selectivity.

Capturing an elusive but critical element: Natural protein enables actinium chemistry

Actinium-based therapies could revolutionize cancer medicine but remain tantalizing due to the difficulties in studying and limited knowledge of Ac chemistry. Current efforts focus on small synthetic chelators, limiting radioisotope complexation and purification efficiencies. Here, we demonstrate a straightforward strategy to purify medically relevant radiometals, actinium(III) and yttrium(III), and probe their chemistry, using the recently discovered protein, lanmodulin. The stoichiometry, solution behavior, and formation constant of the ²²⁸Ac³⁺-lanmodulin complex and its ⁹⁰Y³⁺/^natY³⁺/^natLa³⁺ analogs were experimentally determined, representing the first actinium-protein and strongest actinide(III)-protein complex (sub-picomolar K_d) to be characterized. Lanmodulin’s unparalleled properties enable the facile purification recovery of radiometals, even in the presence of >10⁺¹⁰ equivalents of competing ions and at ultratrace levels: down to 2 femtograms ⁹⁰Y³⁺ and 40 attograms ²²⁸Ac³⁺. The lanmodulin-based approach charts a new course to study elusive isotopes and develop versatile chelating platforms for medical radiometals, both for high-value separations and potential in vivo applications.

Complexation of Lanthanides and Heavy Actinides with Aqueous Sulfur-Donating Ligands

The separation of trivalent lanthanides and actinides is challenging because of their similar sizes and charge densities. S-donating extractants have shown significant selectivity for trivalent actinides over lanthanides, with single-stage americium/lanthanide separation efficiencies for some thiol-based extractants reported at >99.999%. While such separations could transform the nuclear waste management landscape, these systems are often limited by the hydrolytic and radiolytic stability of the extractant. Progress away from thiol-based systems is limited by the poorly understood and complex interactions of these extractants in organic phases, where molecular aggregation and micelle formation obfuscates assessment of the metal-extractant coordination environment. Because S-donating thioethers are generally more resistant to hydrolysis and oxidation and the aqueous phase coordination chemistry is anticipated to lack complications brought on by micelle formation, we have considered three thioethers, 2,2'-thiodiacetic acid (TDA), (2R,5S)-tetrahydrothiophene-2,5-dicarboxylic acid, and 2,5-thiophenedicarboxylic acid (TPA), as possible trivalent actinide selective reagents. Formation constants, extended X-ray absorption fine structure spectroscopy, and computational studies were completed for thioether complexes with a variety of trivalent lanthanides and actinides including Nd, Eu, Tb, Am, Cm, Bk, and Cf. TPA was found to have moderately higher selectivity for the actinides because of its ability to bind actinides in a different manner than lanthanides, but the utility of TPA is limited by poor water solubility and high rigidity. While significant competition with water for the metal center limits the efficacy of aqueous-based thioethers for separations, the characterization of these solution-phase, S-containing lanthanide and actinide complexes is the most comprehensively available in the literature to date. This is due to the breadth of lanthanides and actinides considered as well as the techniques deployed and serves as a platform for the further development of S-containing reagents for actinide separations. Additionally, this paper reports on the first bond lengths for Cf and Bk with a neutral S donor.

Efficient discrimination of transplutonium actinides by in vivo models

Transplutonium actinides are among the heaviest elements whose macroscale chemical properties can be experimentally tested. Being scarce and hazardous, their chemistry is rather unexplored, and they have traditionally been considered a rather homogeneous group, with most of their characteristics extrapolated from lanthanide surrogates. Newly emerged applications for these elements, combined with their persistent presence in nuclear waste, however, call for a better understanding of their behavior in complex living systems. In this work, we explored the biodistribution and excretion profiles of four transplutonium actinides (248Cm, 249Bk, 249Cf and 253Es) in a small animal model, and evaluated their in vivo sequestration and decorporation by two therapeutic chelators, diethylenetriamine pentaacetic acid and 3,4,3-LI(1,2-HOPO). Notably, the organ deposition patterns of those transplutonium actinides were element-dependent, particularly in the liver and skeleton, where lower atomic number radionuclides showed up to 7-fold larger liver/skeleton accumulation ratios. Nevertheless, the metal content in multiple organs was significantly decreased for all tested actinides, particularly in the liver, after administering the therapeutic agent 3,4,3-LI(1,2-HOPO) post-contamination. Lastly, the systematic comparison of the radionuclide biodistributions showed discernibly element-dependent organ depositions, which may provide insights into design rules for new bio-inspired chelating systems with high sequestration and separation performance.

Probing electronic structure in berkelium and californium via an electron microscopy nanosampling approach

Due to their rarity and radioactive nature, comparatively little is known about the actinides, particularly those with atomic numbers higher than that of plutonium, and their compounds. In this work, we describe how transmission electron microscopy can provide comprehensive, safe, and cost-effective characterization using only single nanogram amounts of highly-radioactive, solid compounds. Chlorides of the rare elements berkelium and californium are dropcast and then converted in situ to oxides using the electron beam. The f-band occupancies are probed using electron energy loss spectroscopy and an unexpectedly weak spin-orbit-coupling is identified for berkelium. In contrast, californium follows a jj coupling scheme. These results have important implications for the chemistries of these elements and solidify the status of californium as a transitional element in the actinide series.

The obtention and study of actinide elements is challenging due to various factors including their radioactivity and scarcity. Herein, the authors characterize the atomic and electronic structure of Am, Cm, Bk, and Cf compounds using a transmission electron microscopy-based workflow that only requires nanogram amounts of the actinide element.

Structural and spectroscopic characterization of an einsteinium complex

The transplutonium elements (atomic numbers 95-103) are a group of metals that lie at the edge of the periodic table. As a result, the patterns and trends used to predict and control the physics and chemistry for transition metals, main-group elements and lanthanides are less applicable to transplutonium elements. Furthermore, understanding the properties of these heavy elements has been restricted by their scarcity and radioactivity. This is especially true for einsteinium (Es), the heaviest element on the periodic table that can currently be generated in quantities sufficient to enable classical macroscale studies¹. Here we characterize a coordination complex of einsteinium, using less than 200 nanograms of ²⁵⁴Es (with half-life of 275.7(5) days), with an organic hydroxypyridinone-based chelating ligand. X-ray absorption spectroscopic and structural studies are used to determine the energy of the L₃-edge and a bond distance of einsteinium. Photophysical measurements show antenna sensitization of Es^III luminescence; they also reveal a hypsochromic shift on metal complexation, which had not previously been observed in lower-atomic-number actinide elements. These findings are indicative of an intermediate spin-orbit coupling scheme in which j-j coupling (whereby single-electron orbital angular momentum and spin are first coupled to form a total angular momentum, j) prevails over Russell-Saunders coupling. Together with previous actinide complexation studies², our results highlight the need to continue studying the unusual behaviour of the actinide elements, especially those that are scarce and short-lived.

Covalency in AnCl3 (An = Th-No)

The geometric and electronic structures of AnCl3 are studied computationally using scalar relativistic, hybrid density functional theory (PBE0). The An-Cl bond lengths generally decrease across the 5f series, although there is a slight lengthening from Fm-Cl to No-Cl as the metal ions display increasing M(ii) character. Covalency in the An-Cl bond is studied using a wide range of metrics drawn from the Natural Bond Orbital, Natural Resonance Theory and Quantum Theory of Atoms-in-Molecules (QTAIM) methods, including bond order, orbital composition, orbital overlap and electron density topology data. Most metrics agree that the later An-Cl bonds are less ionic than might be anticipated on the basis of trends in the first half of the series, due to energy degeneracy-driven covalency in the β spin manifold; for example, the An-Cl QTAIM delocalisation index (bond order) for MdCl3 (0.88) is almost exactly the same as for NpCl3 (0.89). By contrast, the ratio of the kinetic to potential energy densities at the An-Cl bond critical points indicates that ionicity increases across the series, suggesting that the delocalisation index measures both orbital overlap and energy degeneracy-based covalency, while the bond critical point metric gauges only the former. Recalculation of all the data using the generalised gradient approximation PBE functional finds larger energy degeneracy-driven covalency in the later actinides than using hybrid DFT. Hence, we find that conclusions concerning the covalency of the An-Cl bond are dependent not only on the metric used to evaluate it, but also on the underlying electronic structure method.

Ligand rigidity and electronic effect on the complexation of hexavalent plutonyl with three dicarboxylic acids: a combined spectrophotometric and computational study

Both the ligands’ rigidity and electronic structure contribute to the stability and coordination mode of Pu(vi) complexes with three dicarboxylic acids.

Multiple Bonding in Lanthanides and Actinides: Direct Comparison of Covalency in Thorium(IV)- and Cerium(IV)-Imido Complexes

A series of thorium(IV)-imido complexes was synthesized and characterized. Extensive experimental and computational comparisons with the isostructural cerium(IV)-imido complexes revealed a notably more covalent bonding arrangement for the Ce═N bond compared with the more ionic Th═N bond. The thorium-imido moieties were observed to be 3 orders of magnitude more basic than their cerium congeners. More generally, these results provide unique experimental evidence for the larger covalent character of 4f⁰5d⁰ Ce(IV) multiple bonds compared to its 5f⁰6d⁰ Th(IV) actinide congener.