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Trefoil-shaped outer-sphere ion clusters mediate lanthanide(III) ion transport with diglycolamide ligands Derek Brigham, Alexander Ivanov, Bruce A. Moyer, Lætitia H. Delmau, Vyacheslav S. Bryantsev, and Ross J. Ellis J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b07318 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on October 30, 2017

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Journal of the American Chemical Society

Trefoil-shaped outer-sphere ion clusters mediate lanthanide(III) ion transport with diglycolamide ligands Derek M. Brigham*, Alexander S. Ivanov, Bruce A. Moyer, Lætitia H. Delmau, Vyacheslav S. Bryantsev* and Ross J. Ellis*‡ Oak Ridge National Laboratory, 1 Bethel Valley Road, Oak Ridge, TN, 37831 ‡Current institution: Borregaard, P.O Box 162, N-1701 Sarpsborg, Norway *Corresponding authors: [email protected]; [email protected]; [email protected]

ABSTRACT: Outer-sphere ion clusters are inferred in many important natural and technological processes, but their mechanisms of assembly and solution structures are difficult to define. Here, we characterize trefoil-shaped outer-sphere lanthanide chloride and nitrate ion clusters in hydrocarbon solutions formed during liquid-liquid extraction with diglycolamide ligands. These are assembled through steric and electrostatic forces, where the anions reside in equidistant ‘clefts’ between coordinating diglycolamide ligands in positions that satisfy both repulsive and attractive ion-ion interactions. Our study shows how sterically-directed electrostatic interactions may assemble stable outer-sphere ion clusters in organic solutions, elucidating new strategies for controlling ion cluster assembly and extraction.

INTRODUCTION In contrast to the highly directional coordination interactions that fuse anions and cations in the inner-sphere, outer-sphere ion pairing relies instead on electrostatic forces that may or may not have any directional preference. Despite the weak forces involved, outer-sphere ion clustering is responsible for the formation of complex structures that underpin important natural and technological processes.1 For example, outer-sphere ion clustering in solution is now thought to prelude the emergence of minerals and biominerals.2-4 Outer-sphere ion clusters are also important in the extraction and separation of valuable minerals in commercial operations. Indeed, the well-known class of extractants that function by ionpairing, especially the alkylamines, play a central role in separations whose great industrial importance contrasts starkly to this day with the almost total lack of understanding of structure and energetics of the clusters involved.5 In recent years, a great deal of research has been reported on the ability of molecular receptors to control the assembly of outer-sphere ion-pairs and clusters and their development for technological applications.6,7 These often employ a ditopic receptor strategy, with covalently tethered anion and cation receptors in a complex molecule. However, most ion clusters in natural and

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technological processes assemble spontaneously in solution through dynamic processes, where an intricate balance of complex interactions search for the energetic minima. The solution structure of outer-sphere ion clusters in both natural and technological systems remains elusive, difficult to characterize and thus beyond rational control. Nondirectional electrostatic interactions between charges compete with solvation, resulting in loose, dynamic assemblies.8 Early techniques relied on gross size and shape indications from static light scattering and colligative-property measurement such as vapor-pressure lowering.5 In view of the poor state of understanding more than a half century later, characterizing the structure of ion clusters in solution has been identified as one of the great challenges in modern chemistry9 and is essential for developing new strategies for controlling the self-assembly of matter for a range of technological applications. Our study centers on the formation of outer-sphere ion clusters in apolar organic solutions during liquid-liquid extraction of lanthanide chloride and nitrate salts. The controlled assembly of ion clusters in liquid-liquid extraction systems is essential for separating ions, from the treatment of waste water to the multibillion dollar metal refining industry.10 The efficient separation of the rare earth ions (i.e., lanthanides, scandium, and yttrium) has been identified as one of the biggest technological challenges of our time,11 being essential to supporting technologies that include lighting, magnets, catalysts, electronics and renewable-energy. The process generally involves lipophilic ligands (called “extractants”) that leverage the incremental change in ionic radius brought about by the lanthanide contraction.12 These are used in liquid-liquid extraction processes where the extractants are dissolved in a water-immiscible hydrocarbon solution and extract lanthanide ions together with their counterions from water.10 One particularly relevant example is the alkyl diglycolamide family of ligands (e.g., TODGA, Figure 1a), that coordinate to lanthanide(III) cations in tridentate fashion to form 3:1 complexes (Figure 1b).13-19 Strong electrostatic forces in the low-dielectric constant organic solvent necessitates the accommodation of anions in the outer-coordination sphere, which are often either nitrate or chloride that are co-extracted with the lanthanide ion. However, the self-assembly mechanisms of outer-sphere ion clusters in liquidliquid extraction remain largely unexplored, due to difficulties in characterizing the structures in the solution state. This precludes the development of effective strategies for controlling the self-assembly of outer-sphere ion clusters for a range of separation purposes. In a recent study, we characterized organic solutions of lanthanide(III)-DGA complexes formed in organic solutions in a liquid-liquid extraction system.20 The data confirmed the homoleptic [Ln(DGA)3]3+ inner-sphere cationic coordination complex in hydrocarbon solution, with the role of the chloride counterions not defined except that they apparently reside in the outer sphere. Chloride has a high hydration energy and would not be effectively solvated by a hydrocarbon solvent when dissociated from the lanthanide cation, leading us to question how these charges are accommodated in solution. Using


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X-ray absorption spectroscopy (XAS) coupled to molecular dynamics (MD) and density functional theory (DFT) simulations, we present here the first study that pinpoints the position of the anions in the neutral [Ln(DGA)3]3+(Cl−)3 and [Ln(DGA)3]3+(NO3−)3 clusters formed in liquid-liquid extraction. Our results show how sterically-directed electrostatic interactions, akin to a host-guest relationship, assemble stable outer-sphere ion clusters in organic solutions, elucidating new strategies for controlling ion cluster assembly and extraction.

Figure 1. (a) The molecular structure of a DGA extractant, N,N,N,N-tetra(n-octyl)diglycolamide (TODGA) and (b) structure of a generic [Ln(DGA)3]3+ complex. EXPERIMENTAL SECTION Materials and extraction procedure. N,N,N’,N’-Tetraoctyl-diglycolamide (TODGA) and 1-(2,2,3,3,Tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol (Cs-7SB) were procured from Marshallton Research Laboratories Inc. and used as received. ExxalTM 12 Alcohol (isododecanol) and IsoparTM L Fluid (an approximately C12 isoparaffinic hydrocarbon) were obtained from the ExxonMobil Chemical Company and used as received. Concentrated HCl and HNO3 were obtained from Alfa Aesar and J.T. Baker, respectively. Sample solutions of Didymium, a mixture of Nd and Pr, were prepared volumetrically from stock solutions made up in HCl and HNO3 using didymium oxide produced by Molycorp provided to Oak Ridge National Laboratory by Idaho National Laboratory. The ratio of Nd to Pr in the oxide was about 4:1. Didymium solutions made for extraction experiments were at a concentration of 1.24 mM. Extractant solutions were prepared by weighing out appropriate quantities of material followed by volumetric dissolution with Isopar L or Isopar L and Exxal 12 as indicated. Aqueous solutions of 0.5 mL were contacted with 0.5 mL of organic phase containing a given concentration of extractant with or without phase modifier in a 2 mL polypropylene conical vial. Samples were placed on a vertical rotating wheel for 1 hour at 25 °C after which the samples were centrifuged at 3000 rpm and 25 °C for 5 minutes to ensure phase disengagement. The phases were separated from one another, and the aqueous phase was saved for analysis. Metal concentrations in the aqueous phase were



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determined via ICP-MS before and after extraction, and metal content in the organic phase was determined by difference. All experiments were performed in triplicate. Extended X-ray Absorption Fine Structure (EXAFS). Two organic-phase samples were prepared for EXAFS analysis by extracting Nd(III) into a 0.25 M TODGA organic phase from either nitric acid or hydrochloric acid aqueous phases. An aliphatic branched alcohol (Exxal 12, 30% v/v) was added to the hydrocarbon oil (Isopar L) to prevent third-phase formation in the hydrochloric acid extraction experiment. For the HNO3 extraction experiment, no modifier was used, and the hydrocarbon oil was ndodecane. Under these conditions, Nd(III) extraction was near quantitative so that virtually all of the 0.01 M lanthanide salt was transferred into the organic phase. Collection and analysis of EXAFS data was conducted using the procedure reported previously in our publications.21 Briefly, the Nd L3-edge spectra were collected in fluorescence mode at beamline 12-BM-B at the Advanced Photon Source (APS) at Argonne National Laboratory. A 13-element Ge detector (Canberra) was used to detect the fluorescence signal at a sample-to-detector distance of 30 cm. The incident X-ray energy was calibrated using Eu oxide foil. The solutions were injected into 1 mm Kapton capillaries. Three 1-hour scans were averaged for each solution. Analysis of the k3χ(k) EXAFS was performed using EXAFSPAK. Curve fitting used theoretical phase and amplitude functions that were calculated with FEFF8.01. The exact same model and fitting procedure were used as published previously to account for the major and minor oscillations in the FTEXAFS.22 DFT simulations. DFT calculations were carried out with the Gaussian 09,23 Revision D.01 software package using the B3LYP24 density functional. Standard 6-31+G(d) basis set was adopted for main-group elements and hydrogen for geometry optimization. 4f-Elements (Nd/Pr) were modeled using the largecore (LC) relativistic effective core potential (RECP) and the associated (7s6p5d)/[5s4p3d]25 basis set. Since LC RECP calculations include the 4f electrons in the core, they were done on a pseudo singlet-state configuration. DFT calculations were conducted on the 1:3 metal ion:ligand complexes with and without inclusion of the Cl− and NO3− counterions. To reduce the computational demand in our search for the most stable configuration of chloride and nitrate clusters, the hydrophobic n-octyl substituents present in the experimental extractants (TODGA) were replaced by the methyl groups, giving the corresponding N,N,N’N’-tetramethyldiglycolamide (TMDGA) model ligands. Several initial positions for placing counterions in the outer shell were considered. While locating the lowest energy structure for the cluster with chloride counterions was straightforward (Figure S4), the potential energy surface of the nitrate complex was found to be flat with at least two minima lying within ~1 kcal/mol (Figure S5). Based on the results of relative energies, the final chloride and nitrate complexes were constructed by attaching additional methyl groups to TMDGA, resulting in N,N,N’N’-tetraethyldiglycolamide (TEDGA) ligands, and were subsequently optimized at the B3LYP/LC/6-31+G(d) level of theory. Vibrational analyses were


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performed on the corresponding metal ion complexes to confirm that the optimized geometries correspond to true minima on the potential energy surface. The UCSF Chimera package was used for graphical visualization of the considered structures.26 Classical molecular dynamics simulations. All-atom molecular dynamics simulations were performed with the MedeA platform27 using the MedeA LAMMPS28 interface module. We employed the nonpolarizable PCFF+ forcefield, which is based on the PCFF forcefield29 with extension to additional molecular classes and refinements, in particular for nonbonded parameters, to reproduce densities and cohesive energies of molecular liquids in a fashion similar to that used in the development of the COMPASS forcefield.30 The PCFF+ forcefield was validated for a range of hydrocarbons, showing excellent agreement with experimental densities (0.23%) and heats of vaporization (0.28%) (the mean absolute error is shown in parentheses).31 As follows from the DFT calculations, the positions of the O donor atoms change only slightly little with respect to the metal ion center after including counterions. Since we are interested in the coordination behavior of counterions residing in the second coordination sphere, the metal ion and nine O donor atoms of three DGA ligands were kept frozen in the DFT optimized geometry of the complex with counterions during the MD simulations. A formal charge of 3+ was assigned to the trivalent Nd ion. The nonbonded Lennard-Jones (L-J) 9-6 parameters for Nd were selected to be the same as for K+ (ro = 3.292 Å and ε = 0.4691 kcal/mol). The sensitivity analysis showed that the choice of the nonbonded parameters for Nd essentially had no impact on the radial distribution functions reported in this paper. The Waldman and Hagler 6th order combination rules were used for the off-diagonal L-J-9-6 parameters. Three systems were considered in our MD simulations with Cl− counterions: (i) [Nd(TODGA)3]3+(Cl−)3 in pure dodecane, (ii) [Nd(TODGA)3]3+(Cl−)3 in a mixed solvent of dodecane and isododecanol in 2.34:1 molar ratio, and (iii) [Nd(TEDGA)3]3+(Cl−)3 in pure water. A periodic cubic box of 40 Å length containing one metal ion complex and three randomly placed Cl− anions was used to accommodate 160 dodecane molecules or 112 dodecane and 48 isododecanol molecules, corresponding to a density of 0.759 and 0.779 g/cm3, respectively, which closely matches the experimental density of dodecane (0.750 g/cm3 at T = 298.15 K) and the proportional amount of aliphatic alcohol in dodecane (0.778 g/cm3 at T = 298.15 K). For simulation in water, a cubic box of 30 Å length containing one complex, three randomly placed Cl− anions, and 848 water molecules was employed, corresponding to a density of 1.0 g/cm3. Analogous setup with the same number of solvent molecules and simulation box size was used for systems containing NO3− counterions. The initial configurations for MD simulations were generated the MedeA Amorphous Cell Builder,29 which sampled energetically reasonable dihedral angles within the flexible molecules of organic solvents.



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The dispersion interactions were evaluated using a nonbonded cutoff of 9.5 Å with an added long-range van der Waals tail correction. The long-range electrostatic interaction was calculated by means of the Particle–Particle-Particle–Mesh (P3M) Ewald summation method32 with long-range precision smaller than 10-5 for the electrostatic energy. The initial structure of each system was first minimized with 500 steps using the conjugate gradient method, followed by 2 ns of equilibration and 2 ns of production run at 298 K in a canonical NVT ensemble with a time step of 1 fs. The temperature was controlled by the Nosé-Hoover thermostat with a relaxation time (tdamp) of 100 fs and a friction coefficient (drag) of 1.0. Graphical visualization of the second sphere coordination and the analysis of the trajectory were performed with VMD.33

RESULTS AND DISCUSSION The first step to characterizing the outer-sphere ion clusters formed in liquid-liquid extraction was to investigate the ion transport properties under various conditions. This can give insight into the stoichiometry of the self-assembled lanthanide-ligand species in the hydrocarbon solvent, as well as the sensitivity of the system to solvation. We conducted experiments to understand how the speciation and liquid-liquid transport properties of the extracted species is dependent on the solvent make-up and anion type. This involved extracting Nd(III) or Pr(III) from either nitric or hydrochloric acid into a hydrocarbon solution containing the extractant TODGA in a paraffinic diluent with either of two hydrophobic alcohols. In liquid-liquid extraction systems, extraction strength is expressed as the distribution ratio (D), which is the concentration of metal ion in the organic phase divided by its concentration in the aqueous phase. The ability to separate two metal ions (e.g., Nd(III) and Pr(III)) is the selectivity of extraction, expressed as the separation factor (S). This is the ratio of distribution ratios, so the selectivity of separating the two lanthanides Nd(III) and Pr(III) is expressed as SNd(III)/Pr(III) = DNd(III)/DPr(III). The following discussion uses these terms to describe how the anion type and organic solvent environment impact lanthanide ion extraction efficiency and selectivity with DGA extractants. In the simplest terms, liquid-liquid extraction is a competitive solvation experiment, where ions partition between an aqueous solution (usually a mineral acid) and an extractant-oil solution. In rare earth extraction using neutral extractants, the extraction strength depends on the ability of the organic phase to solvate the lanthanide(III) cation along with the attendant anions that maintain charge neutrality. In contrast, the extraction selectivity is mainly dependent on the extractant ability to recognize one lanthanide cation over another. In rare earth solvent extraction systems that require co-extraction of anions with the lanthanide cation, the concentration and type of anion usually has a pronounced effect on both extraction strength and selectivity. Commercial rare earth separation processes exploit this phenomenon to tune extraction, even inverting intra-lanthanide selectivity by switching the anion type.12


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Regarding the present system, our previous results20 showed that three TODGA molecules coordinate the lanthanides in the organic phase, implying that the co-anions are apparently relegated to an outer-sphere role. Assuming that the anions are ion-paired with the 9-coordinate TODGA3-Ln3+ complex cation vs being dissociated, the anion effect occurs as a consequence of anion partitioning to the solvent and ionpairing, effects that are mutually compensating, though we expect domination by the dehydration of the anion. We conducted solvent extraction experiments using TODGA in a paraffinic solvent containing aliphatic alcohols to explore the impact of chloride and nitrate anions on the extraction strength and selectivity for Nd(III) and Pr(III). Experiments were conducted using aqueous lanthanide(III) solutions in either nitric or hydrochloric acid. The organic phase involved various concentrations of TODGA in Isopar L (a commercial paraffinic extraction solvent) with either Exxal 12 (a commercial hydrocarbon alcohol) or Cs-7SB (a commercial fluorinated alcohol) to explore technologically-relevant extraction solvents. The aliphatic alcohols, called “phase modifiers” aid in solubilizing the extractant molecules and extraction complexes, preventing precipitation of third phases or other unwanted behavior such as emulsification. Full solvent extraction results are presented in the Supporting Information, and the key findings that are relevant to this manuscript are summarized in Table 1 and Figure 2. Figure 2 shows the distribution ratios of Nd(III) and Pr(III) as a function of TODGA concentration under the varying conditions of HCl and HNO3 aqueous media, as well as with and without the alcohol modifier. Extraction from HCl without modifier is not presented as a viscous gel-like third phase forms upon mixing. The gradient of the lines in the log-log plots are all between 2.7 and 3.3 regardless of the conditions. This suggests that the stoichiometry of TODGA:Ln(III) species in the organic phase is around 3 in most cases, regardless of whether the anion is nitrate or chloride, or if a phase modifier is present. TODGA is known to aggregate in unmodified hydrocarbon diluents in the presence of nitric acid,34 and so the slopes somewhat less than 3 for the unmodified nitrate system likely reflect the small degree of aggregation of TODGA itself. However, alcohol modifiers have been shown to prevent the aggregation of TODGA,35 and thus, the gradients close to 3 for the alcohol-modified systems are taken to imply by straightforward mass action the association of 3 molecules of TODGA with the metal cation. Interestingly, addition of modifier apparently weakens extraction, implying that the solvation of the TODGA molecules by the modifier outweighs the solvation of the lanthanide complexes by the modifier. Chloride clearly weakens extraction by comparison with nitrate, reflecting its stronger hydration, that is, greater dehydration penalty on transfer to the solvent phase. The separation factor SNd(III)/Pr(III) also remains fairly constant in the range 2.6–3.3, indicating that the conditions have only a slight effect on selectivity. The extraction strength is clearly profoundly impacted by changes in the solvent composition and between the different anions, reflected by the positions of the lines in Figure 2. However, the relatively unchanging separations factors



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and slopes across the large variations in system conditions is remarkable, suggesting that the inner-sphere coordination environment of the TODGA-Ln(III) species remains relatively constant. Our findings are thus consistent with the anions being in the outer-sphere, where they do not affect the discrimination between different lanthanides (this would only occur with coordination), but profoundly influence the extraction strength and introduce acute sensitivity to solvation (i.e., presence of modifier).

10000 Nitrate

Nitrate Exxal12

Chloride Exxal12

1000

D

100

10

1 0.01

0.1 [TODGA] (M)

Figure 2. Pr(III) (solid) and Nd(III) (dashed) extraction from 3 M HNO3 and 5 M HCl by TODGA in Isopar L with and without 30% v/v Exxal 12 as indicated. The initial concentrations of Pr(III) and Nd(III) were 1.00 mM and 0.24 mM. The phase ratio was 1:1, and T = 25.0 ± 0.1 °C.

Table 1. Slope analysis values and separation factors for didymium extraction by TODGA from acidic media. 3 M HNO3 Isopar L Isopar L/Exxal 12 Isopar L/Cs-7SB

Slope Pr(III) Nd(III) 2.71 (0.07) 2.73 (0.06) 2.95 (0.02) 3.06 (0.02) 3.20 (0.01) 3.25 (0.02)

SNd(III)/Pr(III) 2.90 (0.06) 2.89 (0.18) 2.58 (0.09)

2.94 (0.16) 2.98 (0.12)

2.76 (0.21) 3.19 (0.15)

5 M HCl Isopar L/Exxal 12 Isopar L/Cs-7SB

3.10 (0.17) 3.07 (0.17)

To investigate the coordination structure of the extracted species and the effect of anion on this structure, Nd L3-edge extended X-ray absorption fine structure (EXAFS) measurements were employed. Measurements were taken from the 0.25 M TODGA organic phases after extraction of Nd(III) from hydrochloric acid and nitric acid. EXAFS is sensitive to the coordination environment around the probe


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ion (Nd3+), giving structural information on the extracted complex in the two systems. In both cases, the k3.χ(k) EXAFS data (Figure 3a) show sinusoidal-like behavior in the region (2 ≤ k ≤ 9 Å−1). The data are very similar despite the different anions, with only minor differences in the intensities and positions of the oscillations, suggesting that the inner-sphere coordination complex is similar in both cases. Real-space functions were generated from this data using a Fourier transform (FT). The FT-EXAFS provides a physical portrayal of the atomic ordering around the Nd(III) centers, and these are shown in Figure 3 for both the nitrate and chloride extractions. Again, these are very similar, with an intense peak at 1.95 Å (r + ∆) that can be attributed to the inner-sphere donor atoms followed by two minor oscillations above and below 3 Å (r + ∆). The only notable difference between the two sets of data is a more pronounced peak at 3.9 Å (r + ∆) for the solution generated from chloride extraction. Similar EXAFS features were observed previously in a Ln(III)-DGA study by Narita et al., who also noted the similarities in the spectra between the nitrate and chloride Ln(III)-DGA solutions, with a pronounced peak at around 4 Å (r + ∆) apparent in the chloride FT-EXAFS.36 The relative positions and intensities of the first three features of the FTEXAFS in Figure 3b are qualitatively reminiscent of FT-EXAFS data published recently for the homoleptic [Eu(TODGA)3]3+ species by Antonio et al.22 This previous Eu(III) study employed a threeshell model to account for the FT-EXAFS, assuming the major peak is from the coordinating ether and amido O donors and the minor oscillations are from the alpha and beta carbon atoms of the coordinating TODGA ligand (Figure 1b). This model uses fixed coordination numbers, but floats all other variables during the fitting. (a)

(b)

8

F T M ag nitude

2

4 k3 χ(k)

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0

-4

Nitrate

1.5

Chloride

1 0.5 0

-8 2

4

6 k (Å-1)

8

0

2 4 r + Δ (Å)

6

Figure 3. (a) The Nd L3-edge EXAFS data and (b) corresponding Fourier transforms (FTs) for the organic solutions generated via the extraction of Nd(III) into 0.25 M TODGA from nitric and hydrochloric acid. The three-shell O,C,C model24 was used to fit the Nd(III) EXAFS in Figure 3, giving good fits to the EXAFS data (Figure S3) with physically reasonable optimized metrics (Table 2). This suggests that the same homoleptic inner-sphere [Nd(TODGA)3]3+ complex is present in both the nitrate and chloride Nd(III) extraction organic phases. This agrees with the stoichiometry of the extracted species suggested



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by the slopes of the functions in Figure 2, as well as previous EXAFS20 and crystallography.37 For both of the data sets, the positions of the alpha and beta C shells in Table 2 are around 0.9 Å and 1.2 Å longer than inner-sphere O shell, exactly as expected for the homoleptic [Ln(TODGA)3]3+ species characterized previously.24 The difference between solid- and solution-state Nd-C could also be attributed to conformational variability in solution compared to the more rigid conformer in solid as a result of molecule and crystal packing forces. However, there are minor differences in the metrics obtained from the modeling of the chloride and nitrate that may be significant. The inner sphere Nd-O bond length is 0.019 Å longer for the nitrate data than for the chloride, a distance that is just above the error bar. The alpha and beta C atoms distances are 0.04 Å and 0.05 Å longer for the nitrate, again just above the error. These slight differences in inner-sphere coordination might be caused by changes in outer-sphere interactions with the chloride and nitrate anions. Table 2. Metrics from the EXAFS data fitting using the 3-shell O,C,C model. The same model and fitting procedure was used as reported previously.24 This involved fixing coordination numbers during the fitting and floating all other variables. Nitrate Chloride

CN (O,C,C) 9,6,6 9,6,6

r, Å (O,C,C) 2.465(3), 3.39(2), 3.64(2) 2.446(6), 3.35(2), 3.59(3)

σ2, Å2 (O,C,C) 0.0095(3), 0.013(3), 0.012(3) 0.0105(5), 0.007(3), 0.009(4)

∆E(0), eV -0.2(3) 3.4(7)

As mentioned above, aside from the minor differences in the bond distance measurements obtained from modelling, there is also a more pronounced and broad peak in the FT-EXAFS at around 3.9 Å (r + ∆) for the chloride system (Figure 3b). With an atomic number of 17, chloride has significantly higher electron density than the O and N atoms on nitrate, and so chloride back-scatters X-rays with greater intensity. To investigate if chloride might account for the broad peak at 3.9 Å (r + ∆), the model was expanded to include a fourth Cl shell and fitted to the EXAFS for the Nd chloride system. This was performed by first fitting the data with the three shell O,C,C model and then fixing the bond distances of the O,C,C shells to limit the number of free variables before adding the fourth (Cl) shell. All of the parameters of the chloride shell, as well as the σ2 and ∆E values of the O,C,C shells, were floated during the final 4-shell fitting. The model generated physically reasonable metrics for all of the four shells shown in Table 3, accounting for the four peaks in the FT-EXAFS (Figure 4a). Most significantly, the chloride coordination number optimized to a value of 3, which is exactly what is expected for a neutral outersphere [Nd(TODGA)3]3+(Cl−)3 assembly. Notwithstanding the uncertainties associated with all solution-phase experimental techniques used to probe structure, a reasonable structural picture emerges from the EXAFS results. We fit the k3χ(k) EXAFS in a stepwise manner, considering the restrictions imposed by the data length in reciprocal space


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and the range of metrical analysis in real space. The number of independent data points (2 × ∆k × ∆r)/π),38 represents the upper limit for the number of parameters that can be refined (19). Both the threeshell and four-shell models were refined using the same number of variables, namely 7. The goodness-offit (F) value for the four-shell model is significantly better (F = 0.106 × 103) than the three shell model (F = 0.153 × 103), as are the “Additional Statistical Information” (e.g., normalized error, reduced error, weighted F-factor) presented in Table S1. The EXAFS modeling suggests that a similar homoleptic [Nd(TODGA)3]3+ inner-sphere complex prevails in both nitrate and chloride extraction systems and is not affected significantly by the solvent or anion. This is consistent with the extraction stoichiometries and robust separation factors obtained from the liquid-liquid slope analysis, as well as previous crystallographic and EXAFS studies.24,37 The coordination number and Ln-Cl bond length in the fourshell model were unconstrained and optimized to their reported values during the fitting and thus independently represent a statistical minimum for the model. If the long-range feature is assumed to be chloride, then the coordination number and bond length suggests three chlorides that sit equidistant in the near outer-sphere, as suggested in Figure 4b.

Table 3. Metrics from the EXAFS data fitting using the step-wise 4-shell O,C,C,Cl model. CN (O,C,C,Cl)

r, Å (O,C,C,Cl)

σ2, Å2 (O,C,C,Cl)

∆E(0), eV

9,6,6,3(1)

2.446, 3.35, 3.59, 4.45(1)

0.0100(5), 0.009(3), 0.011(5), 0.009(5)

-3.7(3)

Figure 4. (a) The Nd L3-edge FT-EXAFS data for the organic solutions generated via the extraction of Nd(III) into 0.25 M TODGA from hydrochloric acid. The 4-shell O,C,C,Cl model fitting is shown by the dashed grey lines. (b) The proposed structure of DGA ligands and outer-sphere chloride counterions that would account for the 4-shell structure suggested from EXAFS.



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To further support the hypothesis of outer-sphere ion pairing suggested by the EXAFS modelling, and to better understand how the anion might be accommodated in the outer-sphere, we conducted DFT simulations of the [Ln(TEDGA)3]3+(Cl−)3 and [Ln(TEDGA)3]3+(NO3−)3 (Ln = Nd/Pr) complexes. Geometries of the corresponding complexes optimized in the gas phase at the B3LYP/LC/6-31+G(d) level of theory along with the average interatomic distances between the metal cations and the donor oxygen atoms of TEDGA (Ln−O), Cl− anions (Ln−Cl), and the closest to Ln oxygen atoms of NO3− (Ln−ONO2), are shown in Figure 5. The optimized cationic [Ln(TEDGA)3]3+ species without inclusion of the Cl− and NO3− counterions in the second coordination sphere are also shown for comparison.

Figure 5. Structures of the [Ln(TEDGA)3]3+, [Ln(TEDGA)3]3+(Cl−)3, and [Ln(TEDGA)3]3+(NO3−)3 complexes optimized at the B3LYP/LC/6-31+G(d) level of theory. Ln−O (Ln = Nd, Pr) average distances with amide (Oamide) and ether (Oether) oxygen donor atoms of TEDGA, Ln–ONO2, and Ln−Cl distances are given in Å. Color scheme: Nd, purple; Pr, tea green; O, red; N, blue; C, turquoise; H, white; Cl, green.

As seen in Figure 5, the first coordination sphere of the clusters comprises nine oxygen donor atoms of TEDGA, while the counterions are situated in the clefts between the coordinating TEDGA ligands at the distances of around 4.9 Å from the metal center. Application of the M0639 functional to the [Nd(TEDGA)3]3+(Cl−)3 system resulted in the shorter average Nd−Cl distance of 4.70 Å (Figure S5), which is close to the Nd−Cl bond length obtained from the EXAFS data fitting for the 4-shell (Table 3).


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The inner-sphere Ln–Oamide (amide oxygen donor atoms) and Ln–Oether (ether oxygen donor atoms) distances decrease from Pr to Nd, which is consistent with the contraction of the lanthanide ionic radii across the series.40 However, the changes in Ln−Cl/Ln–ONO2 bond lengths follow the opposite trend, with the average Ln−Cl/Ln–ONO2 distance 0.02 Å longer for Nd than for Pr. This behavior can be rationalized by the fact that as the distance between oxygen donor atoms in the first coordination shell gets smaller with decreasing metal ion radius, the Cl−/NO3− anions experience more notable repulsion from electron-rich oxygen atoms of DGA ligands that are due to increased steric crowding pushed farther away from the metal ion center (see the Supporting Information for details). Natural bond orbital (NBO)41 analysis of the complexes was performed to assess the magnitude of the hydrogen-bonding interactions between the aliphatic portion of TEGDA and the Cl−/NO3− anions. Within the NBO framework, the formation of hydrogen bonds is conventionally interpreted as a result of charge transfer from the lone-pair orbitals of hydrogen bond acceptor (݊େ୪ష /݊୒୓ଷష ) to the antibonds of hydrogen bond donor (σ*CH), and thus the strength of hydrogen bonding interactions can be estimated via second-order perturbation theory.42 The results of NBO analysis indicate that each chloride in [Ln(TEDGA)3]3+(Cl−)3 accepts four hydrogen bonds, with CH…Cl− contact distances of 2.69, 2.68, 2.95, and 2.98 Å, and associated second-order stabilization energies (E(2)) of 4.4, 4.4, 1.7, and 1.6 kcal/mol, respectively. In turn, each nitrate in [Ln(TEDGA)3]3+(NO3−)3 forms seven hydrogen bonds (2.05, 2.61, 2.62, 2.70, 2.83, 2.89, 3.03 Å) with E(2) values ranging from 12.6 to 0.3 kcal/mol. The relatively small E(2) values suggest slight ability of the aliphatic chains in the ligands and organic diluent to stabilize the Cl− /NO3− anions, thereby promoting tight ligand-separated ion pairing in nonpolar solvents. Classical MD simulations in explicit solvent can complement static gas-phase DFT calculations by providing more detailed structural information and dynamic properties of counterions in the secondsphere coordination of the [Ln(TODGA)3]3+ cations. To understand the influence of the solvent on the interaction and distribution of three Cl−/NO3− anions in the EXAFS studied [Nd(TODGA)3]3+(Cl−)3 /[Nd(TODGA)3]3+(NO3−)3 complexes, our simulations were performed in pure dodecane, and a 2.34:1 mixture by molar fraction of dodecane and isododecanol. Because the study will be focused on the distribution of the Cl−/NO3− counterions in the second coordination shell, the Nd3+ ion and the oxygen donor atoms of DGA were kept frozen during the MD simulations at their DFT optimized geometries, enabling us to significantly simplify the description of the forcefield atom type for Nd3+, which requires only nonbonded dispersion and electrostatic interaction parameters. This setup is fully justified by the results of DFT optimizations (Figure 5) and EXAFS data (Table 3), showing only subtle changes in the inner-sphere Ln–O bond distances to the addition of counterions. To probe the interaction between the [Nd(TODGA)3]3+ complex and the chloride anions in the liquid phase, we analyzed the structural correlations based on radial distribution functions, g(r). The full



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results, as well as an expanded discussion, are presented in the Supporting Information. As a phase modifier is necessary for successful solvent extraction out of HCl, the mixed solvent (dodecane and isododecanol) system is of particular interest with respect to the experimental conditions for the ion transport and EXAFS studies. A snapshot of the [Nd(TODGA)3]3+(Cl−)3 complex in the mixed solvent system is given in Figure 6a. Again, the same trefoil-shaped ion cluster as observed in the DFT is evident from the three chloride anions placed equidistant from each other and the central lanthanide. This is apparently facilitated by clefts between the three coordinating TODGA ligands, providing a ‘habitat’ for the anions that maximizes the electrostatic attraction with the lanthanide cation whilst minimizing repulsive interactions between the adjacent chlorides. These complementary clefts for the anions exhibit characteristics of recognition, which Lehn defined as binding with a function.43 The function in this case involves the formation of a nonpolar, structurally stable extraction complex facilitating the transport of the rare earth ion into the organic phase. Figure 6b represents a solvent-accessible surface of a coordinatively-saturated lipophilic cationic complex providing a shape that allows complementary and symmetric placements of the Cl− anions into pockets around the lipophilic complex in the close proximity of the metal center. The relatively weak hydrogen bonding between the Cl− anions and the C−H groups within the receptor (see above) suggests that these interactions play only a secondary role in stabilizing the outer-sphere host-guest complex that is stable only in nonpolar organic media. It may be noted that the shape of the unoccupied cleft is nearly the same as the shape of the occupied cleft, implying a degree of preorganization in the anion binding site. Figure 7a shows the radial distribution functions, g(r), between Nd3+ and three Cl− anions in nonpolar solvents. Very sharp peaks are observed in both the pure hydrocarbon solvent and the hydrocarbon solvent with the phase modifier (Figure 7a). The Cl− anions are located somewhat closer to Nd3+ in the pure hydrocarbon diluent at a distance of ~4.43 Å, compared to a distance of ~4.53 Å in the mixed modified hydrocarbon solvent. This originates from H-bonding interactions between the anion and 2 isododecanol molecules on average (Figure S8) and may explain the sensitivity of ion cluster extraction (Figure 2) to solvent environment. The average Nd−Cl distances obtained in our MD simulations are in quantitative agreement with our EXAFS data (~4.45 Å), but slightly shorter than in the static DFT calculations (4.70 Å at M06/LC/6-31+G(d) and 4.89 Å at B3LYP/LC/6-31+G(d)), performed in the absence of solvent molecules. In contrast to the results for the nonpolar medium, MD simulations in water reveal a very different behavior of Cl− counterions in the [Nd(TEDGA)3]3+(Cl−)3 complex. As shown in Figure 7b, there are two poorly defined broad peaks in the radial distribution function of Cl− around Nd3+: a smaller one at ~9 Å and a larger one at ~12 Å, indicating that Cl− anions are completely dissociated from the [Nd(TEDGA)3]3+ complex in water. Qualitatively similar results are observed in the simulation of a


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biphasic water-dodecane system wherein Cl− are unbound and exclusively reside in the water phase. Thus, in a high-dielectric medium the electrostatic attraction between the tripositive charge on the metal center and Cl− counterions is largely suppressed, which is consistent with the aqueous complexation studies showing only weak coordination of Cl− with aqua Ln3+ ions.44 Analogous MD investigation of [Nd(DGA)3]3+(NO3−)3 systems indicates that nitrate ions behave similarly to Cl− in the nonpolar organic (Figure S9) and polar aqueous solvent media.

Figure 6. (a) Snapshot of the [Nd(TODGA)3]3+(Cl−)3 complex with phase-modifier molecules (isodecanol) in organic solvent environment (dodecane) from classical molecular dynamics simulations. Only phase modifiers with a distance

Trefoil-Shaped Outer-Sphere Ion Clusters Mediate ... - ACS Publications

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