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J Am Chem Soc
J Am Chem Soc
ja
jacsat
Journal of the American Chemical Society
0002-7863
1520-5126
American Chemical Society

39136967
10.1021/jacs.4c07332
Article
Tetradentate Ligand’s Chameleon-Like Behavior Offers Recognition of Specific Lanthanides
Pramanik Subhamay †
Li Bo ‡
https://orcid.org/0000-0001-8859-8016
Driscoll Darren M. †
https://orcid.org/0000-0002-1323-0222
Johnson Katherine R. ⊥
https://orcid.org/0000-0002-2574-2567
Evans Barbara R. †
https://orcid.org/0000-0003-3409-0190
Damron Joshua T. †
https://orcid.org/0000-0002-8193-6673
Ivanov Alexander S. †
https://orcid.org/0000-0001-5167-0731
Jiang De-en ‡
https://orcid.org/0000-0002-6403-4301
Einkauf Jeffrey †
https://orcid.org/0000-0002-9767-3353
Popovs Ilja *†
https://orcid.org/0000-0002-0690-5957
Jansone-Popova Santa *†
† Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
‡ Department of Chemical and Biomolecular Engineering, Vanderbilt University, Nashville, Tennessee 37235, United States
⊥ Nuclear Energy and Fuel Cycle Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
* Email: popovsi@ornl.gov.
* Email: jansonepopos@ornl.gov.
13 08 2024
18 09 2024
146 37 2566925679
03 06 2024
30 07 2024
29 07 2024
© 2024 The Authors. Published by American Chemical Society
2024
The Authors
https://creativecommons.org/licenses/by-nc-nd/4.0/ Permits non-commercial access and re-use, provided that author attribution and integrity are maintained; but does not permit creation of adaptations or other derivative works (https://creativecommons.org/licenses/by-nc-nd/4.0/).

The surging demand for high-purity individual lanthanides necessitates the development of novel and exceptionally selective separation strategies. At the heart of these separation systems is an organic compound that, based on its structural features, selectively recognizes the lighter or heavier lanthanides in the trivalent lanthanide (Ln) series. This work emphasizes the significant implications resulting from modifying the donor group configuration within an N,O-based tetradentate ligand and the changes in the solvation environment of Ln ions in the process of separating Lns, with the unique ability to achieve peak selectivity in the light, medium, and heavy Ln regions. The structural rigidity of the bis-lactam-1,10-phenanthroline ligand enforces size-based selectivity, displaying an exceptional affinity for Lns having larger ionic radii such as La. Modifying the ligand by eliminating one preorganization element (phenanthroline → bipyridine) results in the fast formation of complexes with light Lns, but, in the span of hours, the peak selectivity shifts toward middle Ln (Sm), resulting in time-resolved separation. As expected, at low nitric acid concentrations, the neutral tetradentate ligand complexes with Ln3+ ions. However, the change in extraction mechanism is observed at high nitric acid concentrations, leading to the formation and preferential extraction of anionic heavy Ln species, [Ln(NO3)x+3]x−, that self-assemble with two ligands that have undergone protonation, forming intricate supramolecular architectures. The tetradentate ligand that is structurally balanced with restrictive and unrestrictive motifs demonstrates unique, controllable selectivity for light, middle, and heavy Lns, underscoring the pivotal role of solvation and ion interactions within the first and second coordination spheres.

Basic Energy Sciences 10.13039/100006151 DE-SC00ERKCG21 Basic Energy Sciences 10.13039/100006151 ERKCK60 Basic Energy Sciences 10.13039/100006151 ERKCC08 document-id-old-9ja4c07332
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pmcIntroduction

Rare earth elements, including scandium, yttrium, and 14 lanthanides (excluding radioactive promethium), are frequently found together in mineral deposits due to their similar physical and chemical properties.1−3 Separating these elements one from another is a requisite for clean energy applications, economic growth, and national security. Due to their unique properties with limited viable substitutes, modern civilization remains reliant on rare earth element-containing products and services, including but not limited to communication, transport, and medicine.4 The minute differences in ionic radii among trivalent lanthanides5,6 are utilized to achieve their separation using techniques such as crystallization,7−11 chromatography,12,13 and solvent extraction.14−16 Current large-scale production and separation of individual lanthanides heavily rely on solvent extraction processes developed several decades ago. However, meeting the upsurge in rare earth element demand17 necessitates the advancement of separation technologies that are more efficient, selective, and environment-friendly.18−21

The tetradentate heterocycles containing N- and N,O-binding motifs have gained recognition for their efficiency in separating actinides from lanthanides22−39 and rare earth elements40−42 (Figure 1). In the series of triazine-based ligands, bis-triazin-1,10-phenanthroline (BTPhen)43,44 surpasses bis-triazin-bipyridine (BTBP)45 with a remarkable 100-fold increase in selectively separating americium (Am) from europium (Eu). This is attributed to the higher degree of preorganization incorporated at the metal ion binding site in BTPhen. A similar trend is observed when comparing the performance of pyridine-2,6-dicarboxamides (DAPy) with the preorganized counterpart, bis-lactam-pyridine (BLPy).46 The latter shows improved efficiency and selectivity in separating Am. The comparison of bis-lactam-1,10-phenanthroline (BLPhen) to the less preorganized 2,9-diamide-1,10-phenanthroline (DAPhen) reveals not only enhanced affinity for Am over Eu47 but also a superior ability in separating light trivalent Lns.48 However, it is unknown whether the presence or absence of other preorganizing elements in multidentate ligands exerts a profound influence on Ln separation. A recent computational evaluation of phenanthroline- and bipyridine-based tetradentate N,O-ligands in separating Am and Eu49 underscores the need for design and experimental validation of tetradentate ligands with varied degrees of preorganization. The scarcity of such studies can be attributed to the limited availability of robust synthetic protocols for ligands with heightened levels of complexity.

Figure 1 Reported tridentate and tetradentate ligands with increasing levels of preorganization.

Herein, we present a systematic study that assesses the performance of four distinct tetradentate ligands, presented in Figure 2, in separating 14 trivalent Lns. These ligands vary in terms of their conformational rigidity at the two N-donor and two O-donor binding sites. While derivatives of ligands 1, 3, and 4, with differing substitution patterns, are known and have been previously reported in Ln separations,47,50,51 ligand 2—bis-lactam-bipyridine (BLBPy)—is a novel addition to the series showing an unexpected selectivity trend. The experimental separation results are further corroborated using spectroscopic and computational studies, offering an explanation for the shifting selectivity trends observed across the trivalent Ln series.

Figure 2 Chemical structures of tetradentate ligands 1–4 with different levels of preorganization. R substituents correspond to 2-octyldodecyl. Black arrows indicate the C–C bond around which free rotation is possible.

Results and Discussion

Two strategies exist for the synthesis of bis-lactam-1,10-phenanthroline ligands (Figure 3). The first involves the utilization of Rh(II) catalyzed C–H annulation reaction.52 Alternatively, the second method employs a palladium catalyzed biaryl coupling reaction, a technique successfully applied in preparing BLPy ligands.53,54 Each of these approaches enables the incorporation of distinct substituents, offering a means of manipulating the physical properties of the ligand. A C–H bond activation using the Pd catalyst was successfully employed to preorganize the amide groups and to prepare ligands 1 and 2 on a multigram scale. The detailed synthesis description for ligands 1–4 is provided in the Supporting Information (SI) file, sections 2 and 3 (Figures S1–S17). The incorporation of branched substituents (i.e., R = 2-octyldodecyl and R‴ = t-Bu) was necessary to render ligands soluble in common organic solvents, such as 1,2-dichloroethane (DCE). Due to increased rigidity and larger number of annulated aryl-groups, BLPhen 1 shows reduced solubility in nonpolar solvent compared to BLPhen prepared via a Rh(II)-catalyzed route.47

Figure 3 Chemical synthesis approaches for the family of tetradentate bis-lactam ligands.

Ligands 1–4 in Figure 2 are arranged according to increasing degrees of rotational freedom. The depicted conformations do not necessarily represent their most stable states in solution. To determine the most stable conformers of the ligands in their free form and in the complexes, the geometry optimization and conformation search using the density functional theory (DFT) calculations with the B3LYP functional,55,56 for ligands 1–4 and the corresponding 1:1 complexes with La(NO3)3, was performed. As shown in Figure 4a, the trans configurations are preferred along the rotational axis or single bond for 2–4 in their free forms, but upon complexation with La, the change to the cis form takes place (Figure 4b). Such reorganization can be quantitatively measured from the root-mean-square deviation (RMSD) between ligand structures before and after the complexation with metal ion: for ligand 1, it is only 0.39 Å, which means that the ligand barely changes before and after the complexation. In comparison, for ligand 2, the trans-to-cis configuration change results in an RMSD of 3.97 Å. Higher reorganization energy is required to reorient the donor groups in 3 and 4 to complex with La, which negatively affects the complex stability (vide infra). The energy penalty is the highest for ligand 4, at 32.4 kcal/mol, which is consistent with the largest RMSD of 8.70 Å due to the trans–cis configuration change of the C–C bond in the bipyridine core and the reorientation of the donor groups.

Figure 4 DFT-optimized lowest energy configurations for ligands 1–4: (a) in the free form; (b) in the 1:1 complexes with La(NO3)3 and the corresponding reorganization energies (ΔEreorg) and root-mean-square deviation (RMSD) for the ligand. ΔEreorg and RMSD are defined as the energy difference and the average distance between atoms, respectively, of the ligand itself between the configuration in the complex and the free form. Color code: N, blue; O, red; La, cyan; C, gray; H, white. La and donor atoms are highlighted as spheres.

The new ligand 2 shows a distinct selectivity trend across the Ln series, contrasting with the trends observed for 1, 3, and 4. The performance of each ligand in separating 14 lanthanides was studied using a liquid–liquid separation system. Each ligand was dissolved in DCE to prepare 7 mM solution, which after pre-equilibration with 0.9 M nitric acid (HNO3) was equilibrated with an equal volume of aqueous solution containing 14 mM Lns (1 mM each, excluding Pm) in 0.9 M HNO3. The concentration of each Ln in the aqueous media before and after contact with the 7 mM ligand solution was determined using inductively coupled plasma mass spectrometry (ICP-MS). The separation profiles of ligands 1–4 after a specific time interval, ranging from 30 min to 11 days, is presented in Figure 5. In agreement with previously reported results,57 the phenanthroline-based tetradentate ligands 1 and 3 show the highest affinity for the light Ln, La, which gradually diminishes along the series toward Lu (Figure 5a,c). Remarkably, ligand 2 yields a strikingly different performance in comparison to ligands 1 and 3, where the peak selectivity within the lanthanide series shifts over time from Ce to Nd (in ∼6 h), eventually settling at Sm (in ∼24 h). Ligand 4, characterized by the lowest degree of preorganization, shows a modest and monotonic affinity for Lns across the series under the given testing conditions. Due to its limited performance, the focus of the study was shifted to ligands 1–3. Although the total concentration of Lns in the organic solvent stabilizes after approximately 3–6 h, slight variations in selectivity among the Lns persist over the 11-day testing period. This is evidenced by the changes in the SFLa/Nd values for ligands 1–3 depicted in Figure 5 (additional SFs are listed in Table S3). While the selectivity trends for 1 and 2 differ, both ligands at the 7 mM concentration show comparable strength in terms of the total lanthanide extracted in organic solvent. The average concentrations of extracted lanthanides are 3.4 mM for ligand 1 and 3.1 mM for ligand 2. In comparison, ligand 3 shows a 50% reduction in the total Ln extracted ([Ln]org = 1.7 mM). The plot of the molar ratio of total Ln extracted by 1 and 2 versus 7 mM ligand concentration at different extraction times indicates that the saturation point is reached faster in the case of ligand 1, additionally supporting on average 1:2 Ln–ligand complex formation (Figure S18).

Figure 5 Change in percent of Ln extracted after contacting 7 mM 1–4 in 1,2-dichloroethane, pre-equilibrated with 0.9 M HNO3, with 14 mM Lns (1 mM each) in 0.9 M HNO3 at different time intervals at 25.5 ± 0.5 °C. Each experiment was conducted in triplicate; deviations in results are presented in Table S1.

The ligand exchange method confirms that ligand 1 is a stronger complexing agent for La than ligands 2 and 3, and ligand 2 is more effective for Sm than ligand 1 (Figure S19). Specifically, adding a stoichiometric amount of ligand 1 to a solution containing ligand 3 complexed with La nitrate results in the quantitative displacement of 3 by 1 in the ligand 3-La nitrate complex, by analyzing the solution structure with 1H NMR spectroscopy. In a setup similar to that of the ligand 2–La nitrate complex, 1 effectively replaces ligand 2. However, when Sm is used instead of La, the ligand 2–Sm complex remains stable and ligand 1 extracts “free” Sm nitrate from the aqueous solution, subsequently forming a new 1–Sm complex in the organic phase.

The observed selectivity trend for ligands 1 and 2 was additionally corroborated using a mixture of just two lanthanides, La and Sm, in the liquid–liquid separation experiments by analyzing the solution structure with 1H NMR spectroscopic and electrospray ionization mass spectrometric (ESI-MS) methods (Figures S20 and S21). Ligands displaying selectivity for middle lanthanides, as opposed to lanthanides at the beginning or end of the series, are rare. One such example is the R,S-isomer of dimethyl tetra-N,N,N′,N′-octyldiglycolamide (Me2TODGA),58 which shows the highest selectivity for Ho in the trivalent lanthanide series. This switch in selectivity, compared with TODGA, is attributed to the small changes in the complexation of nitrate ions in the second coordination sphere caused by the orientation and steric hindrance of one of the two methyl groups present in the ligand.

The effect of nitric acid concentration on the competitive extraction of the 14 Lns shows the unique performance of the bis-lactam-bipyridine 2. The extraction of light Lns increases with increasing nitric acid concentration from 0.2 to 1 M, as shown in Figure 6 (lighter columns in the graphs). At higher than 1 M HNO3 concentrations, the extraction of light Lns (i.e., La–Pr) by phenanthroline-based ligand 1 decreases; at the same time, the extraction of Nd, Sm, and Eu increases. For example, La extraction using 1 decreases by 15%, Sm extraction increases by 10%, and the heavy Ln concentration in organic solvent remains unaffected when increasing the HNO3 concentration from 1 to 5 M (Figure 6a). On the other hand, the weaker phenanthroline-based ligand 3 shows decreased extraction of light Lns (i.e., La–Sm) at >1 M HNO3. Specifically, the La concentration in the organic phase decreases by more than 40% (Figure 6c) at 3 M HNO3 relative to the 1 M HNO3 concentration, with no significant increase in heavy Ln extraction. The bis-lactam-bipyridine 2 shows notably different performance and extracts heavy Lns (i.e., Tm–Lu) significantly more at the 3–5 M nitric acid concentration (Figure 6b). Compared to the results at 1 M HNO3, the Sm concentration decreases by 8% and the Yb concentration increases by 21%. It is shown that with the increase in nitric acid concentration, the size of extracted Ln-ligand aggregates increases as more water and ions are pulled into the organic layer.59 In general, the increase in nitric acid/nitrate content facilitates Ln extraction, as has been observed for neutral or solvating ligand systems such as diglycolamides and bisphosphine oxides, proceeding by the extraction of trivalent Lns as cationic species (i.e., Ln3+ ions).60−63

Figure 6 Change in percent of Ln extracted (1 mM each, excluding Pm) by 7 mM 1–4, pre-equilibrated with the corresponding concentration of HNO3, in 1,2-dichloroethane as a function of nitric acid concentration from 0.2 to 3 or 5 M after 9 h at 25.5 ± 0.5 °C. Each experiment was conducted in triplicate, with deviations indicated by the gray error bars.

The steep increase in heavy Ln extraction using 2 promoted further investigation using 1H NMR spectroscopy. The behavior of ligands 1 and 2 in the presence of increasing nitric acid concentration is presented in Figure 7. Both ligands undergo protonation by the coextracted nitric acid in the organic phase when in contact with 3 and 5 M HNO3, as indicated by the appearance of a sharp singlet in the ∼10 ppm region of 1H NMR spectra (Figure 7 and Figures S22 and S23). Under the same conditions, 1, being more basic, shows faster protonation than 2. In the presence of Lu3+, both ligands show the extraction of [Lu(L)2(NO3)3] species at 1 M HNO3. However, at higher nitric acid concentrations, the 1H NMR spectra display direct change in speciation using 1 and 2, suggesting the extraction of anionic [Lu(NO3)x+3]x– species by protonated ligand(s) ([L-H]+), forming complexes with general formula [(L-H)xLu(NO3)x+3].64,65 The extraction of trivalent Lns as anionic complexes (e.g., [Ln(NO3)x+3]x–) is known using lipophilic organic cations or anion exchangers, for example, quaternary ammonium nitrate ionic liquids.7,66,67 This mechanism, however, does not hold true when extracting light lanthanide La3+ using ligand 1 in the presence of 5 M HNO3 (Figure 7a). 1 predominantly extracts La as a neutral ligand, even though the ligand undergoes protonation in the absence of Ln ions. Upon progressing to Sm in the series, the 1H NMR spectrum becomes more convoluted, as shown in Figure 7a, suggesting the equilibrium between the extraction of Sm3+ by neutral 1 and [Sm(NO3)x+3]x− species by the corresponding protonated ligands. These results suggest a periodic increase in the extraction of anionic lanthanide species in the series from La to Lu using ligand 1. On the other hand, the results depicted in Figure 7b suggest the formation and extraction of anionic Ln species, indifferent of their identity (i.e., La, Sm, or Lu), by protonated bipyridine-based ligand 2 (Figure S23). The efforts to crystallize the complex of ligand 2 with La(III) nitrate were successful. However, the presence of long, floppy aliphatic 2-octyldodecyl chains introduced structural distortions, yielding a poorly refined structure. Despite these distortions, the measured La–Nligand 2 and La–Oligand 2 bond lengths, along with the ∠N–C–C bond angle, align well with the values predicted by DFT calculations, as shown in Figure S24.

Figure 7 1H NMR spectra of 7 mM ligands 1 (a) and 2 (b) in CDCl3 in the presence of 5 M HNO3 with and without 14 mM Ln(NO3)3. Samples were equilibrated for 0.25 h at 25 °C prior to analysis.

These findings, however, do not explain the observed peak selectivity at Sm using ligand 2 (in the presence of 1 M HNO3), which is a significant deviation from the performance of tetradentate ligand 1 with the highest affinity for La. This prompted a further investigation of the coordination environment of Lns in the Ln–ligand complexes. A recent report suggests that the binding pocket in 2:1 BLPhen–Ln complexes reduces in size when advancing from La to Nd to Eu.68 This, in turn, affects the nitrate binding modes, leading to a shift from bidentate to monodentate coordination and consequently resulting in lower coordination numbers for heavier lanthanides.69 These effects are likely to be more pronounced in the case of ligand 2, due to its lower degree of preorganization.

The extended X-ray absorption fine structure (EXAFS) spectroscopy was used to probe the local coordination environment of Ln in 2:1 complexes formed by ligands 1 and 2 with Ln nitrate. Recent studies have shown that BLPhen ligands preferentially form 2:1 complexes with Lns in an organic solvent where the solvating counteranions (i.e., nitrates) complete the solvation shell.70 However, the number and binding mode of these counteranions vary depending on the size of the Ln.71 Similarities in amplitude of the Fourier transformed EXAFS plots (Figure 8) suggest similar binding environments between Sm and ligands 1 and 2. Nevertheless, the differences in coordination of Sm are revealed in the Fourier transformed EXAFS spectra in the radial distance of the first main coordination shell and indicate variable first shell bond distances between Sm-1 and Sm-2 complexes with the Sm-1 average first shell bond length to be noticeably shorter. Fits to the EXAFS region suggest that the average Sm–1 first shell bond distance is 2.47 ± 0.01 Å whereas the Sm–2 first shell bond distance is 2.55 ± 0.01 Å (Table S6 and Figure S38). It is important to note that the determined average first sphere bond distances correspond to the metal ion binding by two ligands and any additional solvating atoms directly coordinated to Sm, particularly the nitrate counteranions within the complex. The variation in bond distance between the two complexes provides clear evidence of differences in the solvating counteranion coordination. In conjunction with our DFT results, the differences in average first shell bonding between the two complexes can be explained through the orientation of the nitrate anion(s). In the case of the Sm-1 complex, the recent literature suggests that two additional nitrate anions can bind directly to the Ln ion, with the anions exhibiting a preference for mixed binding mode (i.e., one monodentate and one bidentate).71 For the Sm-2 complex, the preferred binding complexation of the nitrate anion changes to favor a single nitrate in bidentate configuration. As suggested by the optimized DFT structures, the average coordination position in relation to the Sm ion of bidentate nitrates is approximately 0.1 Å longer than a monodentate nitrate configuration and is consistent with the magnitude of change we observe in the EXAFS data.

Figure 8 Fourier transform magnitude, |χ(R)|, of Sm L3-edge EXAFS for 1:2 complexes of Sm nitrate with ligands 1 and 2 in CHCl3. Spectra are not phase-shift adjusted, and the given peak positions are based on the EXAFS fitting.

Further analysis of the organic solution composed of ligand 2 and La nitrate species using 1H NMR spectroscopy and ESI-MS consistently showed the presence of only 2:1 [La(L)2(NO3)3] complexes (Figure S25). The same observation was made for ligands 1 and 3 in the two immiscible solvent systems with La (Figure 9a and Figure S26). The interchange between 2:1 and 1:1 ligand–Ln complexes was observed only in the mixed solvent system and in the absence of an aqueous phase. Specifically, ligands 1 and 2 were dissolved in CD3OD containing 30 vol % CDCl3, and the change in 1H NMR spectra was recorded after the addition of varied Ln nitrate concentrations (Figure 9 and Figure S27 and S28). In the presence of 0.5 equiv of La(NO3)3, a set of downfield signals appear in the aromatic region corresponding to 2:1 ligand–Ln complexes as confirmed by ESI-MS analysis (the detected molecular ions: [La(L)2]3+ and [La(L)2(NO3)]2+) (Figure S29). Addition of stoichiometric amounts of La(NO3)3 resulted in the formation of both 1:1 and 2:1 ligand–Ln complexes in solution, as shown in Figure 9a. The presence of excess Ln salt (i.e., 10 equiv) facilitated the dissociation of 2:1 complexes and uniform formation of 1:1 ligand–Ln species that were confirmed using ESI-MS with molecular ion peaks corresponding to [La(L)(NO3)2]+ and [La(L)(NO3)]2+. The variation in size of 1:1 and 2:1 complexes of ligands 1 and 2 with Ln nitrate in solution, as ascertained through 1H diffusion-ordered spectroscopy (DOSY) measurements (Figures S30–S36), emphasizes the effects of subtle structural changes on the selective recognition of specific metal ions. The hydrodynamic diameters of species represented by 1:1 complexes of 1 with La and Sm are consistently smaller than those of the corresponding complexes for ligand 2, whereas the opposite trend is observed in complexes with two ligands and one Ln ion (Table S4). This phenomenon might be attributed to the more compact complex formation with lanthanides in the case of ligand 2 that has one less structural preorganization element.

Figure 9 1H NMR spectra of 4 mM ligands 1 (a) and 2 (b) in CD3OD-CDCl3 (70:30 vol %) in the presence of increasing concentration of La(NO3)3. Samples were equilibrated for 0.25 h at 25 °C prior to analysis.

Guided by the experimental results, DFT calculations were employed to optimize the 2:1 complexes of ligands 1–4 with La nitrate (Figure 10). Interestingly, in the complexes with phenanthroline-based ligands 1 and 3, La is predicted to be 12-coordinated–by eight donor atoms originating from two ligand molecules and two bidentate nitrates,12,65,68 whereas, in the complexes with bipyridine-based ligands 2 and 4, La is predicted to be 10-coordinated with the inclusion of only one bidentate nitrate ion in the first-coordination sphere. This is likely due to the increased flexibility13 of bipyridine-based ligands, allowing for closer contact with the metal center, ultimately limiting the space for additional nitrate ions in the inner coordination sphere.14 The thermodynamics of the system were elucidated by computing the Gibbs free energy (ΔG) for the incorporation of each of the two nitrate ions in the first-coordination shell of [La(L)2]3+. The results summarized in Table S5 highlight that the formation of mononitrate complex [La(L)2(NO3)]2+ is an exothermic process for both ligands 1 and 2 (ΔG = −23 kcal/mol) while the inclusion of the second nitrate ion to form [La(L)2(NO3)2]+ remains an exothermic process only in the case of ligand 1. In stark contrast, for the system involving bipyridine-based ligand 2, the incorporation of the second nitrate ion becomes an endothermic process. The increased flexibility of the bipyridine core72 and stereoelectronic properties of ligand 2 guide the interactions with La in the first-coordination sphere.73

Figure 10 Optimized 2:1 complexes of ligands 1–4 with La nitrate in an aqueous phase.

The elimination of one preorganizing element in 1 affects the stability of 2:1 complexes of 2 with Ln due to the expulsion of one or both coordinating nitrate anions in the inner coordination sphere. DFT calculations were used to predict the equilibrium constants (K) for Lns using ligands 1 and 2 according to Scheme S2. The change in logK across the Ln series is presented in Figure 11. In the case of phenanthroline-based ligand 1, where two bidentate nitrate ions are incorporated in the inner coordination sphere (i.e., [Ln(L)2(NO3)2]+), the logK values monotonously decrease in the series from La to Lu, corresponding to the increase in the Gibbs free energy ΔG by 14.2 kcal/mol. This indicates that the extraction reaction of 1 is more favorable with light lanthanides than with heavy lanthanides, which aligns well with the experimental results (Figure 5a). As for the bipyridine-based ligand 2, the model that incorporates two nitrate ions in the first-coordination shell resulted in unstable metal complex geometries with all lanthanides heavier than Eu whereas the model with only one nitrate ion (i.e., [Ln(L)2(NO3)]2+) provided reasonable energetics for all Lns. Figure 11b shows the slight change in predicted logK values between La and Lu (ΔG = 0.3 kcal/mol), while the peak at Pr–Sm indicates that the extraction reaction for these elements is more favorable by ∼2 kcal/mol. This computed trend resembles the experimentally determined trend (Figure 5b), highlighting the relatively small change in binding strength across the lanthanide series with ligand 2; therefore, the solvation free energy change of the metal complexes plays an important role (Figure S37).

Figure 11 DFT-calculated equilibrium constants for Lns with a) ligand 1 and b) ligand 2 (circles) overlapped with the experimentally measured logD values (diamonds).

Conclusions

Our work demonstrates that the connectivity changes in tetradentate ligands with nearly identical molecular formulas have dramatic implications on the stability of complexes with trivalent lanthanides. The elimination of just one preorganizing element within the bis-lactam-1,10-phenanthroline ligand, responsible for maintaining N-based donor atoms in the necessary configuration to complex metal ion, effectively disables the ligand’s size-based selectivity for Ln ions with larger ionic radii. Instead, a shift in the selectivity from Ce to Sm takes place over time, followed by a shift in the peak selectivity from Sm to Yb in the presence of higher nitric acid concentrations. The selectivity change arises from the inherent flexibility of the ligand, which subsequently leads to a decrease in the coordination number of Ln within the 2:1 complexes with neutral BLBPy. The elimination of two preorganizing elements in the bis-lactam-1,10-phenanthroline ligand, which are pivotal to aligning N- and O-based donor atoms, maintains the ligand’s size-based selectivity trend across the Ln series; however, it substantially diminishes its affinity for lanthanides, by 50%. In addition to the four structures presented in this work, two unsymmetrical ligands in the series of differently preorganized tetradentate ligands remain unexplored; their synthesis and performance in separating trivalent lanthanides will be reported in due course. These findings are expected to expedite the development of novel ligands that could exhibit unparalleled performance in the separation of metal ions with similar chemical properties.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.4c07332.Experimental procedures, characterization of 1–4 (1D and 2D NMR, MS, FTIR), Ln-L complexes (NMR, MS, EXAFS, DOSY, and DFT calculations), and extraction results (PDF)

Supplementary Material

ja4c07332_si_001.pdf

The authors declare no competing financial interest.

Acknowledgments

This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Separation Science program, and Materials Chemistry program under award no. DE-SC00ERKCG21 (J.T.D. was supported under award ERKCK60, and J.E. was supported under award ERKCC08). This research used resources of the Advanced Photon Source at beamline 12-BM, a DOE User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract DE-AC02-06CH11357. Portions of this research were carried out at National Synchrotron Light Source II (NIST beamline 6-BM), a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.
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