Complexation of Lanthanide(III) and Actinide(III) Cations with Tridentate Nitrogen-Donor Ligands: A Luminescence and Spectrophotometric Study

Next-Generation 3,3'-AlkoxyBTPs as Complexants for Minor Actinide Separation from Lanthanides: A Comprehensive Separations, Spectroscopic, and DFT Study

Progress toward the closure of the nuclear fuel cycle can be achieved if satisfactory separation strategies for the chemoselective speciation of the trivalent actinides from the lanthanides are realized in a nonproliferative manner. Since Kolarik's initial report on the utility of bis-1,2,4-triazinyl-2,6-pyridines (BTPs) in 1999, a perfect complexant-based, liquid-liquid separation system has yet to be realized. In this report, a comprehensive performance assessment for the separation of 241Am3+ from 154Eu3+ as a model system for spent nuclear fuel using hydrocarbon-actuated alkoxy-BTP complexants is described. These newly discovered complexants realize gains that contemporary aryl-substituted BTPs have yet to achieve, specifically: long-term stability in highly concentrated nitric acid solutions relevant to the low pH of unprocessed spent nuclear fuel, high DAm over DEu in the economical, nonpolar diluent Exxal-8, and the demonstrated capacity to complete the separation cycle with high efficiency by depositing the chelated An3+ to the aqueous layer via decomplexation of the metal-ligand complex. These soft-N-donor BTPs are hypothesized to function as bipolar complexants, effectively traversing the organic/aqueous interface for effective chelation and bound metal/ligand complex solubility. Complexant design, separation assays, spectroscopic analysis, single-crystal X-ray crystallographic data, and DFT calculations are reported.

3–(2–Pyridyl)pyrazole Based Luminescent 1D-Coordination Polymers and Polymorphic Complexes of Various Lanthanide Chlorides Including Orange-Emitting Cerium(III)

A series of 18 lanthanide-containing 1D-coordination polymers 1∞[Ln2(2–PyPzH)4Cl6], Ln = La, Nd, Sm, dinuclear polymorphic complexes α–, β–[Ln2(2–PyPzH)4Cl6], Ln = Sm, Eu, Gd, α–[Tb2(2–PyPzH)4Cl6], and [Gd2(2–PyPzH)3(2–PyPz)Cl5], mononuclear complexes [Ce(2–PyPzH)3Cl3], [Ln(2–PyPzH)2Cl3], Ln = Tb, Dy, Ho, and Er, and salt-like complexes [Gd3(2–PyPzH)8Cl8]Cl and [PyH][Tb(2–PyPzH)2Cl4] were obtained from the reaction of the respective lanthanide chloride with the 3–(2–pyridyl)pyrazole (2–PyPzH) ligand at different temperatures. An antenna effect through ligand-to-metal energy transfer was observed for several products, leading to the highest luminescence efficiency displayed by a quantum yield of 92% in [Tb(2–PyPzH)2Cl3]. The Ce3+ ion in the complex [Ce(2–PyPzH)3Cl3] exhibits a bright and orange 5d-based broadband emission with a maximum at around 600 nm, marking an example of a strong reduction of the 5d-excited states of Ce(III). The absorption spectroscopy shows ion-specific 4f–4f transitions, which can be assigned to Nd3+, Sm3+, Eu3+, Dy3+, Ho3+, and Er3+ in a wide spectral range from UV–VIS to the NIR region.

Machine learning-based analysis of overall stability constants of metal-ligand complexes

The stability constants of metal(M)-ligand(L) complexes are industrially important because they affect the quality of the plating film and the efficiency of metal separation. Thus, it is desirable to develop an effective screening method for promising ligands. Although there have been several machine-learning approaches for predicting stability constants, most of them focus only on the first overall stability constant of M-L complexes, and the variety of cations is also limited to less than 20. In this study, two Gaussian process regression models are developed to predict the first overall stability constant and the n-th (n > 1) overall stability constants. Furthermore, the feature relevance is quantitatively evaluated via sensitivity analysis. As a result, the electronegativities of both metal and ligand are found to be the most important factor for predicting the first overall stability constant. Interestingly, the predicted value of the first overall stability constant shows the highest correlation with the n-th overall stability constant of the corresponding M-L pair. Finally, the number of features is optimized using validation data where the ligands are not included in the training data, which indicates high generalizability. This study provides valuable insights and may help accelerate molecular screening and design for various applications.

Advancing the Am Extractant Design through the Interplay among Planarity, Preorganization, and Substitution Effects

Advancing the field of chemical separations is important for nearly every area of science and technology. Some of the most challenging separations are associated with the americium ion Am(III) for its extraction in the nuclear fuel cycle, ²⁴¹Am production for industrial usage, and environmental cleanup efforts. Herein, we study a series of extractants, using first-principle calculations, to identify the electronic properties that preferentially influence Am(III) binding in separations. As the most used extractant family and because it affords a high degree of functionalization, the polypyridyl family of extractants is chosen to study the effects of the planarity of the structure, preorganization of coordinating atoms, and substitution of various functional groups. The actinyl ions are used as a structurally simplified surrogate model to quickly screen the most promising candidates that can separate these metal ions. The down-selected extractants are then tested for the Am(III)/Eu(III) system. Our results show that π interactions, especially those between the central terpyridine ring and Am(III), play a crucial role in separation. Adding an electron-donating group onto the terpyridine backbone increases the binding energies to Am(III) and stabilizes Am-terpyridine coordination. Increasing the planarity of the extractant increases the binding strength as well, although this effect is found to be rather weak. Preorganizing the coordinating atoms of an extractant to their binding configuration as in the bound metal complex speeds up the binding process and significantly improves the kinetics of the separation process. This conclusion is validated by the synthesized 1,2-dihydrodipyrido[4,3-b;5,6-b]acridine (13) extractant, a preorganized derivative of the terpyridine extractant, which we experimentally showed was four times more effective than terpyridine at separating Am³⁺ from Eu³⁺ (SF_Am/Eu ∼ 23 ± 1).

Evidence for Participation of 4f and 5d Orbitals in Lanthanide Metal-Ligand Bonding and That Y(III) Has Less of This Complex-Stabilizing Ability. A Thermodynamic, Spectroscopic, and DFT Study of Their Complexation by the Nitrogen Donor Ligand TPEN

The formation constants (log K₁) of lanthanide(III) (Ln) ions [except for Pm(III)] and the Y(III) cation have been measured with the ligand TPEN (N,N,N',N'-tetra-2-picolylethylenediamine). These log K₁ values show a typical variation with ionic radius, with a local maximum at Sm(III) and a local minimum at Gd(III), with an overall increase in log K₁ from La(III) to Lu(III) as the ionic radius decreases. The log K₁ for the Y(III)/TPEN complex is much lower than expected from its ionic radius, while the literature log K₁ for Am(III) is much higher. The latter effect is thought to be due to greater covalence in the M-L (metal-ligand) bond than for Ln(III) ions: the low log K₁ for Y(III) is interpreted as being due to lower covalence. The f → f transitions in the Nd(III) and Pr(III) complexes were examined for effects that might indicate the participation of f orbitals in M-L bonding. The intensity of the f → f transitions in the Nd(III)/TPEN complex was greatly increased compared to that of the Nd³⁺ aqua ion, which appeared to be due to additional sharp peaks, possibly parity forbidden transitions where parity rules were broken by covalence in the M-L bond. The Pr(III)/TPEN complex showed that all of the f → f transitions shifted to longer wavelengths by some 5 nm, with modest increases in intensity. The effects seen in the f → f transitions of Nd(III) and Pr(III) with TPEN with its six nitrogen donors were present to a much smaller extent in the EDTA and other complexes with fewer nitrogen donors. The changes in the f → f transitions of the TPEN complexes of Er(III) and Ho(III) were small, suggesting a smaller contribution of f orbitals to M-L bonding in these heavier Ln(III) ions. The intense Laporte allowed f → d transitions in Ce(III) complexes show large shifts to longer wavelengths as complexes of, for example, EDTA with increasing numbers of nitrogen donors, suggesting the participation of both f and d orbitals, or either, in M-L bonding. The nature of M-L bonding in M(III)/TPEN complexes was further investigated via density functional theory calculations.

Fluorescence and Metal-Binding Properties of the Highly Preorganized Tetradentate Ligand 2,2'-Bi-1,10-phenanthroline and Its Remarkable Affinity for Cadmium(II)

The metal-ion-complexing properties of the tetradentate ligand 2,2'-bi-1,10-phenanthroline (BIPHEN) in 50% CH₃OH/H₂O are reported for a variety of metal ions. BIPHEN (with two reinforcing benzo groups in the backbone) was compared to other tetrapyridyls, 2,9-di(pyrid-2-yl)-1,10-phenanthroline (DPP; with one benzo group) and 2,2':6',2″:6″,2‴- quaterpyridine (QPY; with no benzo groups), with levels of preorganization BIPHEN > DPP > QPY. Formation constants were determined by following the variation of the intense π → π* transitions in the absorbance spectra of BIPHEN in the presence of metal ion as a function of the pH. The log K₁ values show that the increased level of preorganization produced by the two benzo groups, reinforcing the backbone of the BIPHEN ligand, leads to increased complex stability with large metal ions (an ionic radius greater than 0.9 Å) compared to the less preorganized tetrapyridines DPP and QPY. In particular, the large Cd^II ion [log K₁(BIPHEN) = 12.7] shows unusual selectivity over the small Zn^II ion [log K₁(BIPHEN) = 7.78]. The order of levels of preorganization BIPHEN > DPP > QPY leads to enhanced selectivity for Sm^III over Gd^III with increased preorganization, which is of interest in relation to separating Am^III from Gd^III in the treatment of radioactive waste. Am^III is very close in ionic radius to Sm^III, so that the size-based selectivity produced by the enhanced preorganization of BIPHEN should translate into enhanced Am^III/Gd^III selectivity. The chelation-enhanced fluorescence (CHEF) effect in BIPHEN complexes is discussed. The CHEF effect in the Zn^II complex is somewhat smaller than that for Cd^II, which is discussed in terms of decreased overlap in the Zn-N bonds formed by the too small Zn^II, leading to a partial photoinduced-electron-transfer quenching of fluorescence. The structure of the complex [Cd(BIPHEN)₂](ClO₄)₂ is reported and shows that the Cd-N bonds are largely normal for the unusual 8-coordination observed, except that steric clashes between the terminal pyridyl groups of each of the BIPHEN ligands, and the rest of the orthogonal BIPHEN ligand, lead to some stretching of the outer Cd-N bonds.

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.

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.

Determining Metal Ion Complexation Kinetics with Fluorescent Ligands by Using Fluorescence Correlation Spectroscopy

Fluorescence correlation spectroscopy (FCS) has been extensively used to measure equilibrium binding constants (K) or association and dissociation rates in many reversible chemical reactions across chemistry and biology. For the majority of investigated reactions, the binding constant was on the order of ∼100 M^-1 , with dissociation constants faster or equal to 10³  s^-1 , which ensured that enough association/dissociation events occur during the typical diffusion-determined transition time of molecules through the FCS detection volume. However, complexation reactions involving metal ions and chelating ligands exhibit equilibrium constants exceeding 10⁴  M^-1 . In the present paper, we explore the applicability of FCS for measuring reaction rates of such complexation reactions, and apply it to binding of iron, europium and uranyl ions to a fluorescent chelating ligand, calcein. For this purpose, we exploit the fact that the ligand fluorescence becomes strongly quenched after binding a metal ion, which results in strong intensity fluctuations that lead to a partial correlation decay in FCS. We also present measurements for the strongly radioactive ions of ²⁴¹ Am³⁺ , where the extreme sensitivity of FCS allows us to work with sample concentrations and volumes that exhibit close to negligible radioactivity levels. A general discussion of the applicability of FCS to the investigation of metal-ligand binding reactions concludes our paper.

Thermodynamics of plutonium(iii) and curium(iii) complexation with a N-donor ligand

The complexation of Pu(iii) and Cm(iii) with a soft N-donor ligand was investigated using the van't Hoff method, microcalorimetry and DFT calculations. The studies revealed that the strength of the actinide-ligand bond as given by the enthalpic contribution drastically decreases on going from Pu(iii) to Cm(iii), while the complex stability remains nearly constant.

Computing UV/vis spectra using a combined molecular dynamics and quantum chemistry approach: bis-triazin-pyridine (BTP) ligands studied in solution

We report a combined computational and experimental study to investigate the UV/vis spectra of 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridine (BTP) ligands in solution. In order to study molecules in solution using theoretical methods, force-field parameters for the ligand-water interaction are adjusted to ab initio quantum chemical calculations. Based on these parameters, molecular dynamics (MD) simulations are carried out from which snapshots are extracted as input to quantum chemical excitation-energy calculations to obtain UV/vis spectra of BTP ligands in solution using time-dependent density functional theory (TDDFT) employing the Tamm-Dancoff approximation (TDA). The range-separated CAM-B3LYP functional is used to avoid large errors for charge-transfer states occurring in the electronic spectra. In order to study environment effects with theoretical methods, the frozen-density embedding scheme is applied. This computational procedure allows to obtain electronic spectra calculated at the (range-separated) DFT level of theory in solution, revealing solvatochromic shifts upon solvation of up to about 0.6 eV. Comparison to experimental data shows a significantly improved agreement compared to vacuum calculations and enables the analysis of relevant excitations for the line shape in solution.

Modular Approaches to Diversified Soft Lewis Basic Complexants through Suzuki-Miyaura Cross-Coupling of Bromoheteroarenes with Organotrifluoroborates

Remediation or transmutation of spent nuclear fuel obtained as a function of energy production and legacy waste remains a significant environmental concern. Substantive efforts over the last three decades have focused on the potential of soft-Lewis basic complexants for the chemoselective separation of trivalent actinides from lanthanides in biphasic solvent systems. Recent efforts in this laboratory have focused on the concept of modularity to rapidly prepare complexants and complexant scaffolds not easily accessible via traditional linear methods in a convergent manner to better understand solubility and complexation structure/activity function in process-relevant solvents. The current work describes an efficient method for the construction of diversified complexants through multi-Suzuki-Miyaura cross-coupling of bromoheteroarenes with organotrifluoroborates affording efficient access to 22 novel materials in 43-99% yield over two, three, or four cross-couplings on the same scaffold. Optimization of the catalyst/ligand system, application, and limitations are reported herein.

Pd-Catalyzed Diamination of 1,2,4-Triazinyl Complexant Scaffolds

As part of ongoing efforts in this laboratory to design and synthesize multidentate soft-N-donors as effective complexants for chemoselective minor actinide extraction from used nuclear fuel, a series of aminated mono-1,2,4-triazinylpyridines were required. This study focuses on streamlining convergent access to a diverse array of functionalized N-donors using Pd-catalysis from a common synthon affording access to pyridinyl triazines as the 4,4'-amino derivatives which are commercially limited and unsuccessful in traditional condensation chemistry. A general Pd-catalyzed method for the double amination of functionalized pyridinyl-1,2,4-triazines with low catalyst/ligand loadings enabling the formation of 16 novel complexants is presented.