A Computational Approach to Predicting Ligand Selectivity for the Size‐Based Separation of Trivalent Lanthanides

Modeling separation of lanthanides via heterogeneous ligand binding

Individual lanthanide elements have physical/electronic/magnetic properties that make each useful for specific applications. Several of the lanthanides cations (Ln3+) naturally occur together in the same ores. They are notoriously difficult to separate from each other due to their chemical similarity. Predicting the Ln3+ differential binding energies (ΔΔE) or free energies (ΔΔG) at different binding sites, which are key figures of merit for separation applications, will help design of materials with lanthanide selectivity. We apply ab initio molecular dynamics (AIMD) simulations and density functional theory (DFT) to calculate ΔΔG for Ln3+ coordinated to ligands in water and embedded in metal-organic frameworks (MOFs), and ΔΔE for Ln3+ bonded to functionalized silica surfaces, thus circumventing the need for the computational costly absolute binding (free) energies ΔG and ΔE. Perturbative AIMD simulations of water-inundated simulation cells are applied to examine the selectivity of ligands towards adjacent Ln3+ in the periodic table. Static DFT calculations with a full Ln3+ first coordination shell, while less rigorous, show that all ligands examined with net negative charges are more selective towards the heavier lanthanides than a charge-neutral coordination shell made up of water molecules. Amine groups are predicted to be poor ligands for lanthanide-binding. We also address cooperative ion binding, i.e., using different ligands in concert to enhance lanthanide selectivity.

Solvent effects on extractant conformational energetics in liquid-liquid extraction: a simulation study of molecular solvents and ionic liquids

Extractant design in liquid-liquid extraction (LLE) is a research frontier of metal ion separations that typically focuses on the direct extractant-metal interactions. However, a more detailed understanding of energetic drivers of separations beyond primary metal coordination is often lacking, including the role of solvent in the extractant phase. In this work, we propose a new mechanism for enhancing metal-complexant energetics with nanostructured solvents. Using molecular dynamics simulations with umbrella sampling, we find that the organic solvent can reshape the energetics of the extractant's intramolecular conformational landscape. We calculate free energy profiles of different conformations of a representative bidentate extractant, n-octyl(phenyl)-N,N-diisobutyl carbamoyl methyl phosphinoxide (CMPO), in four different solvents: dodecane, tributyl phosphate (TBP), and dry and wet ionic liquid (IL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][Tf2N]). By promoting reorganization of the extractant molecule into its binding conformation, our findings reveal how particular solvents can ameliorate this unfavorable step of the metal separation process. In particular, the charge alternating nanodomains formed in ILs substantially reduce the free energy penalty associated with extractant reorganization. Importantly, using alchemical free energy calculations, we find that this stabilization persists even when we explicitly include the extracted cation. These findings provide insight into the energetic drivers of metal ion separations and potentially suggest a new approach to designing effective separations using a molecular-level understanding of solvent effects.

Small Cyclic Diglycolamides: Tautomerism, Solvent Extraction and Coordination with f-Elements: One Strain to Rule Them All

The search for new effective extractants is an important task for the management of high-level liquid waste (HLW) generated during the reprocessing of spent nuclear fuel. Here, we synthesized a series of diglycolamides with cyclic substituents for the first time. We disclosed their coordination with f-element nitrates [La(NO3)3 and UO2(NO3)2] by SC-XRD study and complexation properties toward Am(III), Ln(III), and U(VI) during solvent extraction from nitric acid solutions. Using dynamic nuclear magnetic resonance (NMR) and density functional theory (DFT) calculations, the importance of tautomerism in the extraction properties of diglycolamides was shown.

Reining in Radium for Nuclear Medicine: Extra-Large Chelator Development for an Extra-Large Ion

Targeted α therapy (TAT) of soft-tissue cancers using the α particle-emitting radionuclide 223Ra holds great potential because of its favorable nuclear properties, adequate availability, and established clinical use for treating metastatic prostate cancer of the bone. Despite these advantages, the use of 223Ra has been largely overshadowed by other α emitters due to its challenging chelation chemistry. A key criterion that needs to be met for a radionuclide to be used in TAT is its stable attachment to a targeting vector via a bifunctional chelator. The low charge density of Ra2+ arising from its large ionic radius weakens its electrostatic binding interactions with chelators, leading to insufficient complex stability in vivo. In this study, we synthesized and evaluated macropa-XL as a novel chelator for 223Ra. It bears a large 21-crown-7 macrocyclic core and two picolinate pendent groups, which we hypothesized would effectively saturate the large coordination sphere of the Ra2+ ion. The structural chemistry of macropa-XL was first established with the nonradioactive Ba2+ ion using X-ray diffraction and X-ray absorption spectroscopy, which revealed the formation of an 11-coordinate complex in a rare anti pendent-arm configuration. Subsequently, the stability constant of the [Ra(macropa-XL)] complex was determined via competitive cation exchange with 223Ra and 224Ra radiotracers and compared with that of macropa, the current state-of-the-art chelator for Ra2+. A moderate log KML value of 8.12 was measured for [Ra(macropa-XL)], which is approximately 1.5 log K units lower than the stability constant of [Ra(macropa)]. This relative decrease in Ra2+ complex stability for macropa-XL versus macropa was further probed using density functional theory calculations. Additionally, macropa-XL was radiolabeled with 223Ra, and the kinetic stability of the resulting complex was evaluated in human serum. Although macropa-XL could effectively bind 223Ra under mild conditions, the complex appeared to be unstable to transchelation. Collectively, this study sheds additional light on the chelation chemistry of the exotic Ra2+ ion and contributes to the small, but growing, number of chelator development efforts for 223Ra-based TAT.

Influencing Bonding Interactions of the Neptunyl (V, VI) Cations with Electron-Donating and -Withdrawing Groups

Neptunium makes up the largest percentage of minor actinides found in spent nuclear fuel, yet separations of this element have proven difficult due to its rich redox chemistry. Developing new reprocessing techniques should rely on understanding how to control the Np oxidation state and its interactions with different ligands. Designing new ligands for separations requires understanding how to properly tune a system toward a desired trait through functionalization. Emerging technologies for minor actinide separations focus on ligands containing carboxylate or pyridine functional groups, which are desirable due to their high degree of functionalization. Here, we use DFT calculations to study the interactions of carboxylate and polypyridine ligands with the neptunyl cation [Np(V/VI)O2]+/2+. A systematic study is performed by varying the electronic properties of the carboxylate and polypyridine ligands through the inclusion of different electron-withdrawing and electron-donating R groups. We focus on how these groups can affect geometric properties, electronic structure, and bonding characterization as a function of the metal oxidation state and ligand character and discuss how these factors can play a role in neptunium ligand design principles.

N-oxide ligands for selective separations of lanthanides: insights from computation

Preorganized ligands such as bis-lactam-1,10-phenanthroline (BLPhen) show unique selectivity trends across the lanthanide series, indicating the synergistic effects of both N and O donors in complexing with lanthanides. We hypothesize that by replacing amide functional groups with an N-oxide functionality would open the door to new ligand architectures with improved selectivities. To test this idea, we computationally examined mixed N,O-donor ligands containing pyridinic N and N-oxide groups and evaluated their relative aqueous La(iii)/Ln(iii) selectivity by computing free energy changes for the exchange reaction between the designed ligands and a reference ligand. Three novel ligands show promise as excellent extractant agents in selectively separating trivalent lanthanides. The extent of conjugation (and hyperconjugation), the complex geometry, and the electron accumulations on the two O-donors of the N-oxide groups are found to be important factors in dictating the selectivity trends.

Elucidating the coordination chemistry of the radium ion for targeted alpha therapy

The coordination chemistry of Ra²⁺ is poorly defined, hampering efforts to design effective chelators for ²²³Ra-based targeted alpha therapy. Here, we report the complexation thermodynamics of Ra²⁺ with the biomedically-relevant chelators DOTA and macropa. Our work reveals the highest affinity chelator to date for Ra²⁺ and advances our understanding of key factors underlying complex stability and selectivity for this underexplored ion.

Atomistic Insights into Structure and Dynamics of Neodymium(III) Complexation with a Bis-lactam Phenanthroline Ligand in the Organic Phase

Rare-earth elements (REEs) such as neodymium are critical materials needed in many important technologies, and rigid neutral bis-lactam-1,10-phenanthroline (BLPhen) ligands show one of the highest extraction performance for complexing Nd(III) in REE uptake and separation processes. However, the local structure of the complexes formed between BLPhen and Nd(III) in a typical organic solvent such as dichloroethane (DCE) is unclear. Here, we perform first-principles molecular dynamics (FPMD) simulations to unveil the structure of complexes formed by BLPhen with Nd(NO₃)₃ in the DCE solvent. BLPhen can bind to Nd(III) in either 1:1 or 2:1 fashion. In the 1:1 complex, three nitrates bind to Nd(III) via the bidentate mode in the first solvation shell, leading to the formation of a neutral complex, [Nd(BLPhen)(NO₃)₃]⁰, in the organic phase. In contrast, there are two nitrates in the first solvation shell in the 2:1 complex, creating a charged complex, [Nd(BLPhen)₂(NO₃)₂]⁺. The third nitrate was found to be far away from the metal center, migrating to the outer solvation shell. Our simulations show that the binding pocket formed by the two rigid BLPhen ligands allows ample space for two nitrates to bind to the Nd(III) center from opposite sides. Our findings of two nitrates in the first solvation shell of the 2:1 complex and the corresponding bond distances agree well with the available crystal structure. This study represents the first accurate FPMD modeling of the BLPhen-Nd(III) complexes in an explicit organic solvent and opens the door to more atomistic understanding of REE separations from first principles.

Interplay of physically different properties leading to challenges in separating lanthanide cations - an ab initio molecular dynamics and experimental study

Lanthanide elements have well-documented similarities in their chemical behavior, which make the valuable trivalent lanthanide cations (Ln3+) particularly difficult to separate from each other in water. In this work, we apply ab initio molecular dynamics simulations to compare the free energies (ΔGads) associated with the adsorption of lanthanide cations to silica surfaces at a pH condition where SiO- groups are present. The predicted ΔGads for lutetium (Lu3+) and europium (Eu3+) are similar within statistical uncertainties; this is in qualitative agreement with our batch adsorption measurements on silica. This finding is remarkable because the two cations exhibit hydration free energies (ΔGhyd) that differ by >2 eV, different hydration numbers, and different hydrolysis behavior far from silica surfaces. We observe that the similarity in Lu3+ and Eu3+ ΔGads is the result of a delicate cancellation between the difference in Eu3+ and Lu3+ hydration (ΔGhyd), and their difference in binding energies to silica. We propose that disrupting this cancellation at the two end points, either for adsorbed or completely desorbed lanthanides (e.g., via nanoconfinment or mixed solvents), will lead to effective Ln3+ separation.

Applied machine learning for predicting the lanthanide-ligand binding affinities

Binding affinities of metal-ligand complexes are central to a multitude of applications like drug design, chelation therapy, designing reagents for solvent extraction etc. While state-of-the-art molecular modelling approaches are usually employed to gather structural and chemical insights about the metal complexation with ligands, their computational cost and the limited ability to predict metal-ligand stability constants with reasonable accuracy, renders them impractical to screen large chemical spaces. In this context, leveraging vast amounts of experimental data to learn the metal-binding affinities of ligands becomes a promising alternative. Here, we develop a machine learning framework for predicting binding affinities (logK1) of lanthanide cations with several structurally diverse molecular ligands. Six supervised machine learning algorithms-Random Forest (RF), k-Nearest Neighbours (KNN), Support Vector Machines (SVM), Kernel Ridge Regression (KRR), Multi Layered Perceptrons (MLP) and Adaptive Boosting (AdaBoost)-were trained on a dataset comprising thousands of experimental values of logK1 and validated in an external 10-folds cross-validation procedure. This was followed by a thorough feature engineering and feature importance analysis to identify the molecular, metallic and solvent features most relevant to binding affinity prediction, along with an evaluation of performance metrics against the dimensionality of feature space. Having demonstrated the excellent predictive ability of our framework, we utilized the best performing AdaBoost model to predict the logK1 values of lanthanide cations with nearly 71 million compounds present in the PubChem database. Our methodology opens up an opportunity for significantly accelerating screening and design of ligands for various targeted applications, from vast chemical spaces.

The role of ligand to metal charge-transfer states on the luminescence of Europium complexes with 18-membered macrocyclic ligands

The stabilization of a divalent Europium provides an efficient pathway for the quenching of the luminescence in ten-coordinate macrocyclic complexes.

Synthesis and characterization of a novel aminopolycarboxylate complexant for efficient trivalent f-element differentiation: N-butyl-2-acetamide-diethylenetriamine-N,N',N'',N''-tetraacetic acid

The novel metal ion complexant N-butyl-2-acetamide-diethylenetriamine-N,N',N'',N''-tetraacetic acid (DTTA-BuA) uses an amide functionalization to increase the total ligand acidity and attain efficient 4f/5f differentiation in low pH conditions. The amide, when located on the diethylenetriamine platform containing four acetate pendant arms maintains the octadentate coordination sphere for all investigated trivalent f-elements. This compact coordination environment inhibits the protonation of LnL^- complexes, as indicated by lower K₁₁₁ constants relative to the corresponding protonation site of the free ligand. For actinide ions, the enhanced stability of AnL^- lowers the K₁₁₁ for americium and curium beyond the aptitude of potentiometric detection. Density functional theory computations indicate the difference in the back-donation ability of Am³⁺ and Eu³⁺ f-orbitals is mainly responsible for stronger proton affinity of EuL^- compared to AmL^-. The measured stability constants for the formation of AmL^- and CmL^- complexes are consistently higher, relative to ML^- complexes with lanthanides of similar charge density. When compared with the conventional aminopolycarboxylate diethylenetriamine pentaacetic acid (DTPA), the modified DTTA-BuA complexant features higher ligand acidity and the important An³⁺/Ln³⁺ differentiation when deployed on a liquid-liquid distribution platform.

Highly Efficient and Selective Recovery of Rare Earth Elements Using Mesoporous Silica Functionalized by Preorganized Chelating Ligands

Separating the rare earth elements (REEs) in an economically and environmentally sustainable manner is one of the most pressing technological issues of our time. Herein, a series of preorganized bidentate phthaloyl diamide (PA) ligands was synthesized and grafted on large-pore 3-dimensional (3-D) KIT-6 mesoporous silica. The synthesized sorbents were fully characterized by N₂ physisorption, FT-IR, ¹³C cross-polarization (CP) and ²⁹Si magic-angle spinning (MAS) NMR, thermogravimetric analysis-differential thermal analysis (TGA-DTA), and elemental analysis. Overall, the grafting of PA-type ligands was found to have significantly improved the extraction performance of the sorbents toward REEs compared to the homogeneous analogues. Specifically, the sorbent modified with the 1,2-phtaloyl ligand shows high preference over lanthanides with smaller size, whereas the 1,3-phtaloyl ligand exhibits selectivity toward elements with larger ion radius. This selectivity drastically changes from the homogeneous models that do not exhibit any selectivity. The possibility of regenerating the mesoporous sorbents through simple stripping using oxalate salt is demonstrated over up to 10 cycles with no significant loss in REEs extraction capacity, suggesting adequate chemical and structural stability of the new sorbent materials. Despite the complex ion matrix and high ionic composition, the exposure of industrial mining deposits containing REEs to the sorbents results in selective recovery of target REEs.

Radiometric evaluation of diglycolamide resins for the chromatographic separation of actinium from fission product lanthanides

Actinium-225 is a potential Targeted Alpha Therapy (TAT) isotope. It can be generated with high energy (≥ 100MeV) proton irradiation of thorium targets. The main challenge in the chemical recovery of ²²⁵Ac lies in the separation from thorium and many fission by-products most importantly radiolanthanides. We recently developed a separation strategy based on a combination of cation exchange and extraction chromatography to isolate and purify ²²⁵Ac. In this study, actinium and lanthanide equilibrium distribution coefficients and column elution behavior for both TODGA (N,N,N',N'-tetra-n-octyldiglycolamide) and TEHDGA (N,N,N',N'-tetrakis-2-ethylhexyldiglycolamide) were determined. Density functional theory (DFT) calculations were performed and were in agreement with experimental observations providing the foundation for understanding of the selectivity for Ac and lanthanides on different DGA (diglycolamide) based resins. The results of Gibbs energy (ΔG_aq) calculations confirm significantly higher selectivity of DGA based resins for Ln^III over Ac^III in the presence of nitrate. DFT calculations and experimental results reveal that Ac chemistry cannot be predicted from lanthanide behavior under comparable circumstances.