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Thermodynamic, Spectroscopic, and Computational Studies of f‑Element Complexation by N‑Hydroxyethyl-diethylenetriamineN,N′,N″,N″‑tetraacetic Acid Travis S. Grimes,*,† Colt R. Heathman,† Santa Jansone-Popova,‡ Vyacheslav S. Bryantsev,‡ Sriram Goverapet Srinivasan,‡ Masahiko Nakase,§ and Peter R. Zalupski*,† †

Aqueous Separations and Radiochemistry, Idaho National Laboratory, Idaho Falls, Idaho, 83415, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States § Actinide Chemistry Group, Japan Atomic Energy Agency, Material Science Research Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo-pref 679-5148 Japan ‡

S Supporting Information *

ABSTRACT: Potentiometric and spectroscopic techniques were combined with DFT calculations to probe the coordination environment and determine thermodynamic features of trivalent f-element complexation by N-hydroxyethyldiethylenetriamine-N,N′,N″,N″-tetraacetic acid, HEDTTA. Ligand protonation constants and lanthanide stability constants were determined using potentiometry. Five protonation constants were accessible in I = 2.0 M (H+/Na+)ClO4. UV−vis spectroscopy was used to determine stability constants for Nd3+ and Am3+ complexation with HEDTTA. Luminescence spectroscopy indicates two water molecules in the inner coordination sphere of the Eu/HEDTTA complex, suggesting HEDTTA is heptadentate. Luminescence data was supported by DFT calculations, which demonstrate that substitution of the acetate pendant arm by a N-hydroxyethyl group weakens the metal−nitrogen bond. This bond elongation is reflected in HEDTTA’s ability to differentiate trivalent actinides from trivalent lanthanides. The trans-lanthanide Ln/HEDTTA complex stability trend is analogous to Ln/DTPA complexation; however, the loss of one chelate ring resulting from structural substitution weakens the complexation by ∼3 orders of magnitude. Successful separation of trivalent americium from trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback complexant in a liquid−liquid system. Time-dependent extraction studies for HEDTTA were compared to diethylenetriamineN,N,N′,N″,N″-pentaacetic acid (DTPA) and N-hydroxyethyl-ethylenediamine-N,N′,N′-triacetic acid (HEDTA). The results indicate substantially enhanced phase-transfer kinetic rates for mixtures containing HEDTTA. trivalent lanthanides from Am3+ and Cm3+. The TALSPEAK process balances the organic soluble organophosphorus extractant against an aqueous medium with metal ions, DTPA, and carboxylic acid buffer, adjusted to pH 3.0−4.0. The An3+/ Ln3+ differentiation is accomplished due to selective coordination of An3+ metal ions by DTPA in the aqueous environment, while a transport of Ln3+ metal ions to the organic phase is facilitated by a liquid cation exchanger. Carboxylic acid buffer is needed to maintain aqueous phase acidity as the organophosphorus extractant exchanges H+ for metal ions during extraction. Although it efficiently separates trivalent actinides from trivalent lanthanides, one major drawback to TALSPEAK chemistry is slow attainment of liquid−liquid distribution equilibrium. The kinetic limitation has been attributed to energetics of structural rearrangement necessary for dissociation of [M(DTPA)]2− complexes.13 This process obstacle has been subdued by the addition of high concentrations of carboxylic acid

1. INTRODUCTION Aminopolycarboxylic acids are well-known Lewis bases that contain both N and O donor atoms. The nitrogen donor groups are found in the amine backbone, while the oxygen donor atoms are available through acetic acid pendant arms functionalized to the amine groups. These chelators have high affinity for metal ions, forming strong complexes through polydentate coordination.1 Since aminopolycarboxylic acid metal complexes are very stable they are present in a variety of household commodities, such as foods, shampoos, and detergents, and find many applications in industrial (photography, water treatment, soil remediation) and medical (imaging, radioimmunotherapy, chelation) sectors.2−10 The aminopolycarboxylate, diethylenetriamineN,N,N′,N″,N″-pentaacetic acid (DTPA) ubiquitous in industrial products, is also a key component for successful 4f−5f group separations. Weaver and Kapplemann11,12 developed the TALSPEAK process (Trivalent Actinide Lanthanide Separations using Phosphorus-reagent Extraction from Aqueous Komplexes), a liquid−liquid extraction system, which separates © 2017 American Chemical Society

Received: November 29, 2016 Published: January 24, 2017 1722

DOI: 10.1021/acs.inorgchem.6b02897 Inorg. Chem. 2017, 56, 1722−1733

Article

Inorganic Chemistry

Figure 1. Synthetic scheme for the preparation of N-hydroxyethyl-diethylenetriamine-N,N′,N″,N″-tetraacetic acid, HEDTTA (structure 3).

buffer (1.0−2.0 M) at the expense of extraction system “ideality” and thermodynamic predictability.14,15 Recently, several new processes16−21 have been developed using the TALSPEAK framework to improve Ln3+/An3+ separations, but the kinetic shortcomings still plague the new systems. This work, a third contribution in a series by this group,22,23 continues to evaluate thermodynamic and kinetic impacts of new aminopolycarboxylic acid holdback reagents designed to facilitate 4f−5f element separations using a TALSPEAK-type chemistry platform. In the first study22 ethylenediamineN,N,N′,N′-tetraacetic acid was modified by the substitution of two acetate pendant arms with two acetylglycine functionalities, forming hexadentate ethylenediamine-N,N′-di(acetylglycine)N,N′-diacetic acid (EDDAG-DA). The induction effects of the amide groups reduced the overall “softness” of the amine backbone, subsequently reducing An3+ selectivity, but promoted metal complex formation over ligand protonation in lower pH regimes. The same amide substitution was made on a diethylenetriamine backbone in the second study,23 forming octadentate diethylenetriamine-N′,N″-bis(acetylglycine)N,N′,N″-triacetic acid (DTTA-DAG) in an attempt to regain ligand softness required for efficient An3+/Ln3+ differentiation. The study demonstrated a pathway to enhanced phase-transfer kinetics via H+-catalyzed dissociation of the aminopolycarboxylate metal complexes. The bisamide substitution enhanced the reagent’s ability to complex metal ions through more efficient competition with the protonation equilibria while maintaining efficient separation of Am3+ from all lanthanides below pH = 2.0. In this study a different approach is used to enhance the phasetransfer kinetics. Danesi and Cianetti studied the interfacial mass transfer of Eu3+ in a liquid−liquid system containing a buffered aqueous solution of an aminopolycarboxylate and a nonaqueous mixture of the liquid cation exchanger (bis-2-ethylhexylphosphoric acid, HDEHP) in dodecane.13 The authors demonstrated that mass transfer across the liquid−liquid boundary is fully controlled by the rates of interfacial chemical reactions, with no diffusional resistance due to migration through the stagnant interfacial zones.13 Accordingly, the authors recommended the use of smaller, easier to unravel (relative to DTPA) complexants as the only option to speed up the liquid−liquid partitioning equilibria. This strategy is employed here through a specific structural modification of DTPA. A single acetate pendant arm of DTPA was replaced by a hydroxyethyl group, thus reducing the denticity of the reagent. A thorough thermodynamic and kinetic characterization of N-hydroxyethyl-diethylenetriamineN,N′,N″,N″-tetraacetic acid, HEDTTA, is presented. The coordination environment of a trivalent f-element/HEDTTA complex was probed by luminescence spectroscopy, and its theoretical geometry was described via ab initio molecular dynamics simulations based on density functional theory (DFT). Effects of reduction of the f-element coordination environment on the stability of metal chelate were studied using potentiometry and spectroscopy. The differentiation of trivalent actinides from

trivalent lanthanides was demonstrated when HEDTTA was utilized as aqueous holdback complexant in a liquid−liquid system. Interestingly, time dependencies for the partitioning of trivalent f-elements indicate the enhancement of kinetic rates beyond those expected based solely on progressive reduction of ligand’s denticity.

2. EXPERIMENTAL SECTION 2.1. Reagents. 2,2′-((2-((Carboxymethyl)(2-((carboxymethyl)(2hydroxyethyl)amino)ethyl)amino)ethyl)azanediyl)diacetic acid (IUPAC name for HEDTTA) was isolated as triflate or HCl salts according to synthetic scheme presented in Figure 1. A detailed description of the synthetic methodology and product characterization is provided in the Supporting Information. Metal ions investigated using potentiometric and spectrophotometric methods to determine stability constants include Y3+, the lanthanide series (excluding Pm3+), and Am3+. The Y3+ and lanthanide perchlorate stocks were prepared as previously described.22 The Am3+ perchlorate stock solution was obtained from Idaho National Laboratory stocks and purified using N,N,N′,N′-tetra-noctyldiglycolamide (DGA) extraction chromatographic resin (Eichrom). The Am3+ was first adsorbed on a DGA column and then eluted using dilute HCl. Several evaporation cycles were performed to drive off the HCl, and the final material was dissolved in 0.1 mM HClO4. The Am3+ concentration was determined using a spectrophotometric titration method reported by Tian and Shuh.24 Metal ion solutions investigated with solvent extraction techniques include Am3+ and La3+− Ho3+ group of lanthanides, prepared from lanthanide nitrate stocks previously standardized using ICP-MS. Sodium perchlorate and sodium nitrate salts (ACS reagent grade) were purchased from GFS Chemicals. Concentrated background electrolyte stock solutions were prepared separately (5.0 mol/kg). Salt crystals were dissolved in 18 MΩ deionized water, filtered through a fine glass frit filter, and recrystallized by heating to reduce the volume until the salts crashed out of solution. After cooling, the remaining solution was decanted thoroughly. The salt crystals were dissolved in a minimal amount of 18 MΩ deionized water, and each solution was standardized using ion-exchange chromatography (Dowex 50WX8 beads, H+ form, 100−200 mesh) and potentiometric titrations. Sodium hydroxide solutions were prepared by adding the desired amount of 50% w/w NaOH (Sigma-Aldrich) and NaClO4 in 18 MΩ deionized water. The ionic strength was adjusted to match the titrand solutions to maintain constant ionic strength throughout the titrations. All NaOH solutions were standardized by titration of primary standard potassium hydrogen phthalate (KHP). Organic extractant 2-ethylhexyl phosphonic acid mono-2-ethyhexyl ester (HEH[EHP]) was purchased from Yick-Vic Chemicals. The HEH[EHP] was purified using the third phase formation method25 and determined to be >99% by potentiometric titration. The organic diluent n-dodecane (Sigma-Aldrich) was used without further purification. The organic phases were prepared by dissolving weighed amounts of HEH[EHP] in n-dodecane. 2.2. Potentiometry. Acid dissociation (Ka) and metal complex stability (βmhl) constants were determined by potentiometric titration using a Mettler Toledo T-50 graphix autotitrator. Titrand solution temperatures were held constant at 25.0 ± 0.1 °C with a jacketed beaker and a circulating water bath. All titrations were blanketed with hydrated nitrogen gas (bubbled through 1.0 M NaOH) to prevent CO2 1723


Article

Inorganic Chemistry absorption. A 1.5 L solution of 5.0 mM HEDTTA (I = 2.0 M adjusted with (H+/Na+)ClO4) was prepared at pCH 1.65. This pCH condition guaranteed access to the lowest obtainable Ka by a pH electrode and control of the starting pCH for each titration, thus eliminating the need for pCH corrections in the fitting exercises. A Ross Orion semimicro glass electrode was used to measure changes in pCH. The electrode filling solution was changed from KCl to 5.0 M NaCl to prevent KClO4 precipitation. The high concentration of NaCl was used for rapid electrode response and to minimize variations in the junction potential. The electrode was calibrated (I = 2.0 M, T = 25.0 ± 0.1 °C) by strong acid−strong base titration with standardized reagents. Gran analysis26−28 of the titration data provided a calibration curve (pCH versus mV) to determine the H+ concentration from hydrogen ion activity. Acid dissociation and metal complex stability titration data were analyzed using Hyperquad2013 fitting software.29 2.3. Spectrophotometric Titrations. Absorption spectra for [Nd(HEDTTA)]− complexation (550−610 nm, 0.1 nm interval) were collected using an Agilent Cary-6000i UV−vis−NIR spectrophotometer equipped with a 100 cm waveguide capillary flow cell (World Precision Instruments) at ambient temperature (20 ± 1 °C). Solution A ([Nd3+] = 0.533 mM, I = 2.0 M, pCH = 2.04) and solution B ([Nd3+] = 0.538 mM, [HEDTTA] = 14.482 mM, I = 2.0 M, pCH = 9.02) were mixed in appropriate amounts to produce an HEDTTA dependence at constant volume (2.5 mL), [Nd3+], and I = 2.0 M. The solutions were mixed to produce 20 spectra for the titration. Absorption spectra for [Am(HEDTTA)]− complexation (400−600 nm, 0.37 nm interval) were collected in a 1 cm quartz cuvette (Starna) using a Flame-S-VIS-NIR-ES Ocean Optics device coupled with to an Ocean Optics DH-2000-BAL light source through a 2 m (200 μm) fiber optic cable at ambient temperature (20 ± 1 °C). Titrand solution A ([Am3+] = 0.796 mM, I = 2.0 M, pCH = 1.92, 0.8 mL) was titrated with 0.8 mL of solution B ([Am3+] = 0.788 mM, [HEDTTA] = 16.721 mM, I = 2.0 M, pCH = 4.58) using titrant additions that varied from 5 to 150 μL. Separate titrations were performed to measure pCH after each ligand addition. Stability constants of the [M(HEDTTA)]− complexes were resolved using the HypSpec30 spectral data fitting program. 2.4. Fluorescence Measurements. Luminescence lifetimes were collected using a HORIBA Jobin Yvon IBH FluoroLog-3 fluorometer adapted for time-resolved measurements. A submicrosecond xenon flash lamp (Jobin Yvon, 5000XeF) coupled to a double-grating excitation monchromator for spectral selection was used as the light source. The detector consists of a single-photon detection module (HORIBA Jobin Yvon IBH, TBX-04-D) coupled with a fast rise time photomultiplier tube, a wide bandwidth preamplifier, and a picosecond constant fraction discriminator. Data were collected using an IBH Data Station Hub and analyzed using the DAS 6 decay analysis software package from HORIBA Jobin Yvon IBH. Lifetime data were modeled using single- and double-exponential decay curves. The χ2 values ranged between 1.03 and 1.05 for all curve-fitting iterations. Samples were analyzed in 1.0 cm quartz cuvettes (Starna); the temperature was controlled at 25.0 ± 0.1 °C using a water-jacketed cuvette holder and a circulating water bath. 2.5. Density Functional Theory Calculations. Theoretical calculations were performed to simulate the participation of −CH2CH2OH in the coordination of europium metal ion. Density functional theory (DFT)-based molecular dynamics (MD) simulations were carried out in the Born−Oppenheimer approximation using the VASP software.31−34 A detailed description of calculations is provided in the Supporting Information. 2.6. Solvent Extraction. Solvent extraction experiments were conducted to test the selectivity of HEDTTA toward actinides. Aqueous phase (1.0 mM Ln3+, 20 mM HEDTTA, 0.5 M malonate buffer, pCH 3.0, I = 2.0 M adjusted (H+/Na+)NaNO3) was pre-equilibrated using a Glass-Col multitube vortexer with neat n-dodecane for 30 min and then centrifuged for 5 min. Organic phase (0.08 M HEH[EHP]/n-dodecane) was pre-equilibrated with metal ion free aqueous phase for 30 min and then centrifuged for 5 min. Equal phase ratios (0.5 mL each) of preequilibrated aqueous and organic phases were contacted for 30 min and then centrifuged for 5 min. Aqueous phases were sampled before and after equilibrium; the samples were diluted with 2% nitric acid and analyzed using ICP-MS to determine lanthanide concentration. The

distribution ratios were calculated using the following equation, where [M]i,aq corresponds to initial concentration of metal in aqueous mixture and [M]f,aq the final concentration of metal in aqueous mixture after equilibration with nonaqueous solution

DM =

[M]i,aq − [M]f,aq [M]f,aq

(1)

When radioisotope tracers were utilized, distribution ratios were determined from the ratio of activity in organic and aqueous phase. Phase-transfer kinetic experiments (aqueous into organic) were performed by monitoring time-dependent partitioning of radioisotopes 241 Am, 139Ce, and 154Eu (Eckert and Ziegler) between two immiscible liquid phases. Nonaqueous phases contained liquid cation exchangers (2-ethylhexyl)-2-ethylhexyl-phosphonic acid, HEH[EHP], or di(2ethylhexyl)phosphoric acid, HDEHP, in n-dodecane. Preliminary equilibration experiments identified the required content of ion exchanger to ensure partitioning of the majority (>80%) of metal ions into the nonaqueous environment at equilibrium. Aqueous phases contained 20 mM holdback complexant (except for 5 mM DTPA) in 1 M (H+/Na+)NaNO3 and were adjusted to pCH = 3.0 using NaOH. To slow down the phase-transfer equilibrium and enhance reagent differences on a kinetic scale measurable by solvent extraction, the buffers were omitted. Phase-transfer kinetic studies for HEDTTA were compared to conventional aminopolycarboxylate reagents HEDTA and DTPA. Organic solutions containing either HEH[EHP] or HDEHP were pre-equilibrated using a 1 M (H+/Na+)NaNO3, pCH = 3.0 solution prior to contact with the aqueous complexant mixtures containing radioisotopes. Two-phase experiments were vortexed on a Glass-Col multitube shaker (motor speed setting 50) and centrifuged, and the liquid phases were sampled for radiometric measurements. Liquid− liquid distribution of Eu3+ only was monitored by measuring its activity (15−300 keV energy window) using the Packard D5003 Cobra Gamma counter. Partitioning of mixed radioisotope samples was measured on an ORTEC GEM50P4 coaxial HPGe detector equipped with a DSPEC gamma spectrometer (59.54 keV peak for Am-241, 123.07 keV peak for Eu-154, and 165.86 keV peak for Ce-139).

3. RESULTS 3.1. Potentiometric Determination of Acid Dissociation Constants and Ln/HEDTTA Stability Constants. Experimental and theoretical titration curves combined with HEDTTA speciation are shown in Figure 2. Best fits were obtained when using a five-dissociable proton model to represent the experimental data. The acid dissociation reactions for HEDTTA are summarized by cumulative equilibria expression as described by eq 2. Table 1 lists the acid dissociation constants for HEDTTA resolved in this study, together with pKa values previously determined at I = 2.0 M for DTPA and DTTA-PA for comparison.35,36 The proton associated with the ethyl alcohol (pKa ≈ 16) is not readily dissociable in the pCH range typical of potentiometric titration; therefore, it was treated as an inert pendant arm for modeling purposes. The H6R2+ and H7R3+ species have weak proton association, and the acidic pKa values were not accessible using potentiometric titration methods due to the limitations of the glass electrode. H8 − nL+4 − n ⇄ H 7 − nL+3 − n + H+K an =

[H+]·[H 7 − nL+3 − n] [H8 − nL+4 − n]

, n = 1...7 (2)

Stability constants for [Ln(HEDTTA)]− complexation were determined by titrating 1:1 metal/HEDTTA mixtures. The results of a typical Ln3+/HEDTTA titration, calculated best fit curve, and Nd3+ complex speciation are shown in Figure 3. The large inflection point at ∼7.75 equiv of NaOH is typical of 1:1 1724


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Inorganic Chemistry

Figure 3. Potentiometric titration of HEDTTA in the presence of Nd3+, superimposed on the distribution of Nd3+ species. Titrant: 0.328 M NaOH, I = 2.0 M (Na+/H+)ClO4. Titrand: CHEDTTA = 4.68 mM HEDTTA, CH+ = 0.022 M, CNd3+ = 4.95 mM, Vint = 25.068 mL, I = 2.0 M (Na+/H+)ClO4, T = 25.0 ± 0.1 °C. Experimental pCH (○), calculated pCH (solid orange line), Nd3+ (green dashed line), NdHL(aq) (red dashed line), NdL− (blue dashed line).

Figure 2. Potentiometric titration of HEDTTA to determine protonation constants overlaid on distribution of ligand species. Titrant: 0.330 M NaOH, I = 2.0 M (Na+/H+)ClO4. Titrand: CHEDTTA = 4.68 mM HEDTTA, CH+ = 0.022 M, Vint = 25.098 mL, I = 2.0 M (Na+/H+)ClO4, T = 25.0 ± 0.1 °C. Experimental pCH (□), calculated pCH (solid orange line), H5L+ (dashed black line), H4L (red dashed line), H3L− (green dashed line), H2L2− (blue dashed line), HL3− (cyan dashed line), and L4− (magenta dashed line).

ML− + H+ ⇄ MHL(aq)

HEDTTA β101 =

[ML−] [M3 +]·[L4 −]

(3)

[MHL(aq)] 3+

[M ]·[H+]·[L4 −]

[ML−]·[H+]

3.2. Spectrophotometric Determination of Stability Constants. Absorbance spectra of free metal and metal ligand complexes are shown in Figure 4A and 4B. Typically, Nd3+ (ionic radius =1.109 Å) is used as a nonradioactive metal ion surrogate of Am3+ (ionic radius = 1.09 Å) due to similar ionic radii. Optical absorbances due to f−f transitions at λmax = 575 nm for trivalent neodymium (4I9/2 → 4G5/2) and λmax = 503 nm for trivalent americium (7F0 → 5L6) were monitored. Both transitions are sensitive to metal/ligand coordination.39,40 In both titrations, the intensities of both peaks corresponding to free metal decreased as HEDTTA concentration increased. The optical absorbance features were red shifted (585 nm for Nd3+, 507 nm for Am3+), resulting from the coordination of trivalent f-elements by HEDTTA. The spectrophotometric titration data was analyzed using HypSpec software.30 The best theoretical representation of spectral information was afforded when three absorbing species (M3+, ML−, and MHL(aq)) were present in the model. The calculated molar absorptivities for Nd3+ and Am3+ species are shown in Figure 4C and 4D. The formation equilibria of two absorbing metal complexes ML− and MHL(aq) are described by

M3 + + H+ + L4 − ⇄ MHL(aq) HEDTTA β111 =

[MHL(aq)] (5)

metal/ligand complex formation. In previous f-element/DTPA complexation studies, both [M(DTPAH)]− and [M(DTPA)]2− complexes have been reported in acidic aqueous media.37,38 On the basis of the analogous structure of HEDTTA the same metal complexation equilibria were used to model the Ln3+/HEDTTA titration data. In this study, the conventional nomenclature βmhl and Kmhl are used where subscripts define the complex stoichiometry with respect to metal ion (m), hydrogen ion (h), and ligand (l) for the overall reactions and stepwise reactions, respectively. The stability constants for M3+/HEDTTA complexation are shown in Table 2. Metal ion complexation equilibria for HEDTTA are expressed by eqs 3, 4 and 5. M3 + + L4 − ⇄ ML−

HEDTTA K111 =

(4)

Table 1. Acid Dissociation Constants for HEDTTA at 25.0 ± 1 °Ca

a

HEDTTA

n

−log10 Kan

DTTA-PAb

m

−log10 Kam

DTPA

−log10 Kam

HL3− H2L2− H3L− H4L(aq) H5L+ H6L2+ H7L3+

7 6 5 4 3 2 1

9.49 ± 0.02 8.19 ± 0.01 4.31 ± 0.02 2.56 ± 0.02 2.15 ± 0.02

HL4− H2L3− H3L2− H4L− H5L(aq) H6L+ H7L2+ H8L3+

8 7 6 5 4 3 2 1

9.64 8.86 4.52 3.54 2.79

HL4− H2L3− H3L2− H4L− H5L(aq) H6L+ H7L2+ H8L3+

9.50 8.31 4.38 2.53 2.41

I = 2.0 M (H+,Na+)ClO4. Data for DTTA-PA and DTPA displayed for comparison.35,36 bI = 0.1 (H+,K+)NO3 1725


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Inorganic Chemistry

Table 2. Cumulative Stability Constants log10 β101 and log10 β111 for M3+/HEDTTA Complexes Determined Using Potentiometry (25.0 ± 0.1 °C) and Spectroscopy (20 ± 1 °C) in 2.0 M (Na+/H+)ClO4a HEDTTA

DTPA

metal

log10 β101

log10 β111

log10 K111

log10 β101

log10 K111

Y3+ La3+ Ce3+ Pr3+ Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ Nd3+ b Am3+ b

17.18 ± 0.01 15.38 ± 0.01 16.35 ± 0.01 17.00 ± 0.01 17.28 ± 0.01 18.15 ± 0.01 18.10 ± 0.01 18.09 ± 0.01 18.18 ± 0.01 18.12 ± 0.01 18.05 ± 0.01 17.92 ± 0.01 17.79 ± 0.01 17.72 ± 0.02 17.68 ± 0.02 17.50 ± 0.06 18.34 ± 0.08

19.83 ± 0.01 18.31 ± 0.03 19.15 ± 0.02 19.35 ± 0.02 19.42 ± 0.02 20.22 ± 0.03 20.42 ± 0.01 20.27 ± 0.01 20.34 ± 0.01 20.43 ± 0.01 20.43 ± 0.01 20.47 ± 0.01 20.63 ± 0.01 20.77 ± 0.02 20.69 ± 0.02 20.41 ± 0.02 20.67 ± 0.04

2.65 ± 0.01 2.93 ± 0.02 2.81 ± 0.01 2.36 ± 0.01 2.14 ± 0.02 2.07 ± 0.01 2.32 ± 0.01 2.18 ± 0.01 2.15 ± 0.01 2.31 ± 0.01 2.38 ± 0.01 2.56 ± 0.01 2.83 ± 0.01 3.05 ± 0.01 3.01 ± 0.01 2.91 ± 0.05 2.33 ± 0.07

20.13 18.02 19.06 19.64 20.23 20.79 21.03 21.15 21.15 21.23 21.43 21.41 21.10 20.96 21.14

1.74 2.36 1.72 1.71 1.34 1.59 1.54 1.23 1.61 1.58 1.33 1.25 1.35 1.57 1.07

a

Stability constants are compared to Ln/DTPA complexes determined in 2.0 M (Na+/H+)ClO4 using potentiometry.35 Stepwise log10 K111 constants for the protonation of Ln3+/HEDTTA and Ln3+/DTPA complexes are tabulated for comparison. bDenotes stability constants determined using spectrophotometry.

Figure 4. Spectrophotometric titrations of (A) Nd(ClO4)3 and (C) Am(ClO4)3 with HEDTTA accompanied by calculated molar absorptivities for (B) Nd3+ and (D) Am3+ absorbing species. (A) Titrant: CNd3+ = 0.3536 mM, CHEDTTA = 14.482 mM, pCH = 9.02, I = 2.0 M (Na+/H+)ClO4. Titrand: 0.533 mM Nd(ClO4)3, pCH = 2.04, I = 2.0 M (Na+/H+)ClO4, T = 20.0 ± 1.0 °C. (C) Titrant: CAm3+ = 0.788 mM, CHEDTTA = 16.721 mM, pCH = 4.58, I = 2.0 M (Na+/H+)ClO4. Titrand: CAm3+ = 0.5796 mM, pCH = 1.92, I = 2.0 M (Na+/H+)ClO4, T = 20.0 ± 1.0 °C.

3.3. Luminescence Lifetimes of Eu3+/HEDTTA Solutions. Luminescence lifetime measurements were collected for trivalent europium in the presence of HEDTTA in aqueous environment adjusted to pCH = 1.0, 2.4, and 5.0. The experimental data and calculated best fit lines are shown in

eqs 3 and 4, respectively. The stability constants for Nd3+/ HEDTTA complexes determined using spectroscopy were comparable to potentiometric results given differences in experimental conditions (Table 2). 1726


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Inorganic Chemistry Figure 5. Trivalent europium luminescence is produced by relaxation of electronic transitions from 5D0 (lowest excited

Table 3. Luminescence Lifetimes and Waters of Hydration for Eu/HEDTTA Complexes at Various pCH Conditionsa % species −1

pCH

τ (μs)

kobs (s )

nH2O (±0.5)

1.0 2.4 2.4 5.0

109 120 458 483

9196 8354 2185 2071

9.2 8.3 1.9 1.7

Eu

3+

100 31.6 0.0 0.0

EuL−

EuHL(aq)

0.0 0.0 37.5 99.8

0.0 0.0 30.9 0.2

a

Aqueous phase: [Eu3+]total = 0.001 M, [HEDTTA]total = 0.010 M, I = 2.0 M (H+/Na+)ClO4.

ion and significantly dehydrated metal due to formation of EuHL(aq) and EuL−complexes. 3.4. Liquid Liquid Distribution Measurements. Figure 6 shows the distribution of trivalent lanthanides and Am3+ between Figure 5. Luminescence lifetime data for aqueous mixtures containing Eu3+ ion and HEDTTA, collected at different pCH conditions. For all solutions CEu = 1.0 mM, CHEDTTA = 10.0 mM, I = 2.0 M (Na+/H+)ClO4, T = 25.0 ± 0.1 °C. Solid red lines represent best fits calculated using exponential functions.

state) to the ground state 7Fj (where j = 0, 1, 2, 3...). Luminescence yields are quenched by nonradiative vibrational decay through OH oscillators (typically coordinated H2O molecules). The degree of quenching is directly proportional to the number of H2O molecules found in the inner coordination sphere. Horrocks and Sudnick41 established the correlation between luminescent lifetime and inner coordination sphere H2O molecules. Waters of hydration were calculated using the empirical equation developed by Kimura and Kato,42 where kobs is the decay constant for the luminescent lifetime and is expressed in s−1. Eu NH2O = 1.05 × 10−3kobs − 0.44

Figure 6. Distribution of (blue squares) La3+−Ho3+ (except Pm3+), (green triangles) 139Ce3+ and 154Eu3+, and (red circles) 241Am3+ between a nonaqueous solution of ion exchanger and an aqueous mixture of HEDTTA complexant. Aqueous phase: 1.0 mM total Ln3+, 0.5 M Malonate, 20.0 mM HEDTTA, 1.0 M (H+/Na+)NO3, pCH = 3.0. Organic phase: 0.1 M HEH[EHP] in n-dodecane.

(6)

The aqueous acidities were chosen to maximize the concentrations of free Eu3+, EuHL(aq), and EuL− species in the Eu3+/HEDTTA mixtures. As the pCH was increased luminescence lifetimes became longer. Single-exponential functions were used to obtain luminescence lifetimes (τ) for pCH 1.0 and 5.0 systems (best fit with χ2 = 1.03 and 1.05, respectively), indicating the presence of a single luminescent species. A single-exponential function was used to model the pCH 2.4 system which produced a best fit of χ2 = 2.1 and suggested two waters of hydration. Therefore, based on the poor fit using a single-exponential function and the known speciation from stability constant determinations, a double-exponential function was used to model the pCH 2.4 system. The fitting iterations indicated two distinguishable luminescent components and produced a best fit with χ2 = 1.05. Luminescence decay rates, hydration numbers calculated using eq 6, and Eu3+/HEDTTA speciation are shown in Table 3. The short 109 μs lifetime (τ), measured at a pCH of 1.0, is consistent with full quenching from nonradiative vibrations through OH oscillators, indicating the presence of a fully hydrated (n = 9) Eu3+ metal ion in solution. At pCH 5.0 the longer quenching (483 μs) yielded 1.7 coordinated waters according to eq 6, suggesting the presence of 2 waters of hydration in the inner coordination sphere of Eu3+ complex. At pCH 2.4 a mixture of free Eu3+, EuHL(aq), and EuL− species is present in solution. The best fit representation of the quenching data confirms the presence of a fully hydrated europium metal

nonaqueous environment containing HEH[EHP] in n-dodecane and aqueous electrolyte mixture containing HEDTTA holdback reagent. The figure also shows the distribution ratios for 139Ce3+ and 154Eu3+ determined using radiometric techniques, illustrating good agreement with the ICP-MS data. The extraction efficiency increases throughout the investigated lanthanide group (La3+− Ho3+). Three tetrads are evident in the extraction data. The increased lanthanide distribution across the series observed in the HEDTTA system closely resembles TALSPEAK chemistry with the exception of La3+. Here the distribution ratio of La3+ is ∼1, compared to ∼10 in the original TALSPEAK formulation. The distribution data shows that effective Ln/An differentiation is facilitated by HEDTTA suppressing the partitioning of Am3+ to the organic phase. The minimum group separation factor (SFLn An = DLn/DAn) was driven by lanthanum (DLa/DAm = 8) for 20 mM HEDTTA, pCH = 3.0 aqueous complexing environment. Figure 7a compares the time-dependent partitioning trends collected when ion exchangers facilitated the forward extraction of Eu3+ from aqueous mixtures of HEDTTA, HEDTA, and DTPA. The distribution of europium increases steadily for each system until the phase-transfer equilibrium is manifested by a 1727


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HEDTTA may be inferred from the luminescence results (nH2O = 1.7) assuming seven inner-sphere waters of hydration are expelled from aqua metal ion due to inner-sphere coordination by HEDTTA. This heptadentate coordination matches other literature accounts of f-element complexation by DTTA-type reagents, where a single acetate group of DTPA has been replaced by coordination-inert moieties.43,44 The comparison of luminescence quenching at pCH 1.0 and 5.0 suggests the hydroxyethyl functionality is inert to metal binding, likely extending out into bulk solvent. This is consistent with a report by Moeller and Horowitz45 that suggested the hydroxyethyl functionality of HEDTA does not participate in the ligand complex. The presence of a protonated complex MHL(aq) at pCH 2.4 cannot be distinguished from the influence of the ML− complex on luminescence lifetime decay signatures. The doubleexponential fit resolves two main components contributing different luminescence quenching properties. The resolved nH2O = 1.9 value for the coordinated Eu3+ species represents the average hydration number for both EuL− and EuHL (aq) complexes, suggesting heptadentate coordination by HEDTTA in both cases. Tian and Rao46 argued that octadentate coordination of metal ion remains the same for the [Eu(DTPA)]2− complex and its protonated form. The authors combined the DFT calculations and luminescence lifetime data to suggest the protonation of metal complex likely occurs on the oxygen of one acetate group, which undergoes a rotation to orient carbonyl oxygen toward the metal to retain the octadentate coordination. The heptadentate coordination of europium ion by HEDTTA in EuL− and EuHL(aq) complexes reiterates the inert nature of the alcohol moiety to metal ion coordination. The density functional theory calculations were performed to gain further understanding of alcohol’s influence of structural organization of the metal complex. Several plausible complex geometries for two different alcohol binding modes (A, coordinating; B, noncoordinating) were optimized for Eu3+− HEDTTA interaction. As shown in Figure 8, Eu3+ is coordinated

Figure 7. (a) Time-dependent liquid−liquid distribution of 154Eu between organic solutions of ion exchanger in n-dodecane and aqueous mixtures containing HEDTTA, HEDTA, or DTPA in 1 M (H+/ Na+)NO3, pCH = 3.0. T = 20.0 ± 1.0 °C: (red open diamonds) 0.065 M HEH[EHP]/20.0 mM HEDTTA; (blue open circles) 0.065 M HEH[EHP]/20.0 mM HEDTA; (black open squares) 0.04 M HDEHP/5.0 mM DTPA. (b) Time-dependent distribution of 241Am (black open squares), 139Ce (blue open circles), and 154Eu (red open diamonds) between 0.065 M HEH[EHP] in n-dodecane and 20 mM HEDTTA, 1 M NaNO3, pCH = 3.0.

plateau. The kinetic trends follow the HEDTTA > HEDTA > DTPA order as translated from time required to reach the constant liquid−liquid distribution of metal ion (i.e., plateau). The equilibrium is reached within ∼30 min for HEDTTA and ∼90 min for HEDTA and is not attained until ∼5 h of phase contact for DTPA mixtures. Time-dependent forward extraction of 241Am, 139Ce, and 154Eu is presented in Figure 7b for a liquid− liquid system containing 0.065 M HEH[EHP] in nonaqueous ndodecane phase and 20 mM HEDTTA adjusted to pCH 3.0 in the aqueous phase. Efficient separation of trivalent lanthanides from trivalent actinides is on display here, yielding good separation Eu factors SFCe Am ≈ 50 and SFAm ≈ 150 at equilibrium.

Figure 8. Optimized structures of Eu3+−HEDTTA complexes in water. (a) Complex A with the −CH2CH2OH group directly coordinated to the metal ion. (b) Complex B with the −CH2CH2OH group not coordinated to the metal ion. Eu atom is pink, O atoms are red, N atoms are blue, C atoms are cyan, and H atoms are white.

4. DISCUSSION 4.1. Coordination Environment for Trivalent f-Element Complexes with HEDTTA. Data in Table 3 shows luminescent lifetimes, decay rates, number of inner-sphere waters, and percent abundance of Eu3+ species as a function of aqueous acidity. At pCH 1.0, the protonation reactions of HEDTTA outcompete the metal ion complexation equilibria leaving the metal ion fully hydrated. At pCH 5.0 the luminescence lifetimes are extended due to displacement of the inner-sphere waters by HEDTTA forming the [Eu(HEDTTA)]− complex (99% abundance). The heptadentate coordination of Eu3+ by

to four carboxylate groups and three amino groups. The remaining two sites are occupied by (i) one water molecule and a neutral hydroxyl group of the ligand in complex A and (ii) two water molecules (the hydroxyl group is unbound) in complex B. Table 4 compares the calculated average Eu−N and Eu−O bond lengths for the coordination scenarios A and B with those optimized for [Eu(DTPA)]−2 complex based on crystallographic data reported by Liu and co-workers.47 Tian and Rao46 1728


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Inorganic Chemistry Table 4. Calculateda Eu−N and Eu−O Bond Distances (Angstrsoms) for Eu3+/HEDTTA Complexes

Eu−O1 Eu−O2 Eu−O3 Eu−O4 Eu−O5 Eu−Ow1 Eu−Ow2 Eu−OOH Eu−N1 Eu−N2 Eu−N3

[Eu(DTPA)]−2

A: [Eu(HEDTTA)]−

B: [Eu(HEDTTA)]−

octadentate

octadentate (Figure 8a)

heptadentate (Figure 8b)

2.37 2.44 2.40 2.51 2.49 2.67

2.39 ± 0.08 2.42 ± 0.08 2.39 ± 0.08 2.44 ± 0.09

2.40 ± 0.08 2.37 ± 0.08 2.39 ± 0.07 2.46 ± 0.09

2.48 ± 0.09

2.51 ± 0.10 2.40 ± 0.07 5.98 ± 0.17 2.64 ± 0.08 2.70 ± 0.09 3.04 ± 0.14

2.87 2.76 2.89

2.53 ± 0.10 2.70 ± 0.09 2.66 ± 0.08 2.84 ± 0.10

Averaged for the last 15 ps of trajectory in the canonical ensemble (T = 300 K). Scenario A assumes metal coordination by the −CH2CH2OH group. Scenario B assumes inertness of the alcohol group in metal binding. Corresponding bond distances for the Eu3+/DTPA complex are shown for comparison.44 a

demonstrated that protonation of [Eu(DTPA)]−2 complex significantly alters the calculated Eu−O average bond distance for the affected acetate group. The other Eu−O average bond lengths were unchanged. Similarly, the calculated Eu−O average bond lengths for complexes A and B of HEDTTA do not differ significantly from those observed for DTPA complex, showing that four acetate groups of HEDTTA are not influenced by alcohol substitution. For Eu−N bonds Tian and Rao46 showed significant weakening (manifested through longer bond distances) for the amine group directly connected to acetate that protonates. The calculated Eu−N3 average bond length for the coordination scenario A, where participation of alcohol in the binding of europium is postulated, does not change relative to equivalent Eu−N3 bond of [Eu(DTPA)]−2 complex. This finding does not adequately translate the thermodynamic information as the Eu3+/HEDTTA stability constant indicates that interaction between metal ion and ligand weakens substantially due to acetate/ethanol pendant arm replacement. This weakening is rather demonstrated by the elongated Eu−N3 bond distance calculated when the noncoordinating alcohol group is modeled. This theoretical outcome points to alcohol’s ability to pull away a weakly coordinated amine atom (N3) from the metal center. The weakened interaction between the metal ion and one of the amine groups was also manifested in the thermodynamic characteristics of HEDTTA, as discussed in the following section. 4.2. Thermodynamic Profile of HEDTTA. Table 1 compares the determined acid dissociation constants for HEDTTA with those reported for DTPA35 and diethylenetriamine-N,N,N″,N″-tetraacetic-N′-propionic acid, DTTA-PA, determined by Sawyer and Powell.36 The HEDTTA pKa values are similar to those reported for DTPA in 2.0 M (H+/Na+)ClO4. Nearly statistical agreement of all amine protonation sites of HEDTTA and DTPA indicates that pendant arm substitution has minimal inductive influence on the diethylenetriamine backbone. Based solely on such an equivalence of the inductive effects between HEDTTA and DTPA, the capacity of those reagents to differentiate trivalent actinides from trivalent lanthanides should be similar. The inspection of the carboxylic acid dissociation constants of HEDTTA reiterates the similarity to DTPA when comparing the least acidic carboxylic protons, pK4. In all, HEDTTA is a weaker acid, relative to DTPA, and its operational capacity to delay protonation in favor of the metal complexation reactions is subdued. The ethanolic substituent of

HEDTTA may be compared with the structural modification of DTTA-PA, where one carboxylate group is moved away from diethylenetriamine central bridge. Although the equilibrium constant for the first protonation of DTTA-PA seems unusually low (relative to DTPA at I = 0.1 M),37 the remaining pKa values describing protonation of amine sites show minimal effect of increased separation between the amine backbone and the carboxylate group. The replacement of one acetate group of the DTPA reagent with an ethanol functionality is strongly manifested in the reduction of stability constants for the complexation of trivalent f-elements by HEDTTA (Table 2). The log10β101 values for [Ln(HEDTTA)]− complexes are ∼3 orders of magnitude weaker relative to equivalent 1:1 complexes of DTPA. Such a decrease in the binding strength is likely due to the somewhat weakened electrostatic attraction between M3+ and the complexing conjugate base, with a major impact coming from the elimination of a five-membered chelate ring from the coordination pocket. The magnitude of weakening follows the trend expected from conventional aminopolycarboxylates capable of formation of five-membered chelate rings.48 A simple transitioning from DTPA (7 rings) to EDTA (5 rings) to nitrilotriacetic acid, NTA (3 rings) reduces the log10β101 values from ∼21 to ∼16 to ∼11, respectively, for an average 102.5 reduction per chelate ring.49 The log10K111 values for the MHL(aq) complex are similar to the protonation of H3HEDTTA− (pK4 = 2.56), suggesting that protonation of the ML− complex occurs on a carboxylic acid pendant group. Grimes and Nash35 showed the electrostatic interaction between the hydrogen ion and H3DTPA2− is ∼10 times stronger, relative to its isoelectronic [Eu(DTPA)]2− complex. The authors attributed this to a “spherical” compactness of [Eu(DTPA)]2− complex, where carboxylate groups are shielded from the electrostatic interactions with hydrogen ions. This relative shift in the protonation equilibrium due to metal ion coordination is not observed for HEDTTA. The protonation of ML− matches the protonation of H3HEDTTA−, perhaps reiterating the argument of a coordination environment more susceptible to interactions with bulk solvent. The trans-lanthanide trend of metal HEDTTA complex stability constants (Table 2) matches the pattern displayed by DTPA. The log10β101 values for DTPA range from 18.0 to 21.1 across the lanthanide series,35 whereas those for HEDTTA range from 15.4 (La3+) to 17.7 (Lu3+). The complex stabilities steadily 1729


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Inorganic Chemistry increase for La3+−Sm3+ group, presumably due to increasing electrostatic binding. The stability constants reach a plateau (Sm3+−Ho3+), and even slightly decrease for Er3+−Lu3+ group. For DTPA this leveling off trend in stability constants has been attributed to decreased entropic stabilization due to lower hydration of heavier lanthanides, and increased steric repulsion due to tight coordination pocket for such contracted felements.48 The similarity of stability constant trends collected for HEDTTA, relative to DTPA, is somewhat surprising, considering the lowered constraints imposed by a heptadentate complexant. Further thermodynamic studies are anticipated to determine the enthalpic and entropic contributions to Gibbs energies of [Ln(HEDTTA)]− complex formation to explain the trans-lanthanide pattern in more detail. The comparison of the stability constants for the formation of [Nd(HEDTTA)]− and [Am(HEDTTA)]− complexes (determined using spectrophotometry) show the enhanced affinity for coordination of trivalent actinides, relative to trivalent lanthanides. This is partly due to slight differences in charge density and partly due to small degree of covalency afforded the An3+ elements. Using ionic radius data produced by Shannon50 for metal ions with coordination number 8, the dimensionless electrostatic parameter can be calculated using the simple equation ξ = z2/r, where z corresponds to effective ionic charge and r is the ionic radius. The higher ξ for americium ion (Nd3+ = 8.12, Am3+ = 8.25) implies a higher charge density, relative to neodymium ion, resulting in slightly more favorable ionic binding of Am3+. The remaining enhancement in the Am3+ complex stability has been typically attributed to small degree of covalent interactions made possible by the greater spatial extension of the 5f and 6d orbitals for actinides.51 The unshielded orbitals are more available for overlap with the relatively soft nitrogen donor groups in the HEDTTA backbone. The small degree of covalency experienced by the actinides is absent for lanthanide metal ions due to shielding of the 4f orbitals by 5 d orbitals. The electrons occupying the 4f orbitals are considered core electrons, and are not available for orbital mixing with incoming ligands. Accordingly, the lanthanide interactions are strictly ionic and defined by electrostatic attraction. 4.3. Ln3+/An3+ Separation Using HEDTTA as Holdback Reagent. The liquid/liquid distribution results shown in Figure 8 for Ln3+ and Am3+ demonstrate HEDTTA’s ability to effectively suppress the distribution ratio of Am3+ and produce 4f−5f trivalent element group separation. The separation factor between Nd3+ (typically the least extracted Ln3+ in TALSPEAKtype chemistries) and Am3+ is slightly lower in the HEDTTA 11,12 Nd system, (SFNd Am = 26) compared to TALSPEAK (SFAm = 34). 3+ The difference in the stability constants for Nd and Am3+ complexation by HEDTTA is smaller (Δlog10β101 of 0.84) than that measured for DTPA (Δlog10β101 = 1.3).49 The diminished capacity to differentiate Am3+ from Nd3+ may be attributed in part to the disruption of the octadentate environment of DTPA’s binding pocket through the introduction of the alcohol. The elongated Eu−N3 bond predicted by the DFT calculations supports this experimental finding, suggesting that alcohol weakens the interaction between the amine and the trivalent metal. Accordingly, the overall softness of the diethylenetriamine binding pocket of HEDTTA is reduced, relative to DTPA, as reflected by differences in the stability constants. The original TALSPEAK recipe balances the use of DTPA with phosphoric acid ion exchanger, producing an initial La3+ > Ce3+ > Pr3+ > Nd3+ trend of metal partitioning, which is reversed for the latter members of the 4f group.11,12 The overall lowering

of trivalent f-element complexation strength for HEDTTA requires the use of phosphonic acid ion exchanger (higher pKa) to efficiently balance the partitioning equilibrium. The distribution trend for the investigated lanthanides (La3+−Ho3+) shown in Figure 6 steadily increases throughout, with the least extracted lanthanum limiting the overall differentiation of trivalent americium from the lanthanides. This partitioning trend varies appreciably from that observed when DTPA is utilized as the aqueous holdback complexant.11,12 Careful comparison of trans-lanthanide patterns of stability constants for HEDTTA and DTPA explains the observed liquid−liquid distribution differences very well. The prediction of the equilibrium partitioning of trivalent f-elements in liquid−liquid mixtures described here follows the combined stoichiometric constant Kex × Kd. The Kex is the extraction constant of the lanthanides to the organic medium, and Kd is the dissociation constant of the Ln3+ ligand complex in the aqueous mixture. Accordingly, the strength of the liquid cation exchanger and the relative ease of Ln3+ ligand complex dissociation balance the partitioning equilibrium. The trans-lanthanide trends of stability constants for HEDTTA and DTPA show that light lanthanides form less stable, more readily dissociable complexes, relative to heavier 4f elements. Figure 9 illustrates a relative measure of this

Figure 9. Relative differences in the Gibbs free energies of dissociation of (black squares) [Ln(HEDTTA)]− and (red circles) [Ln(DTPA)]2− complexes as compared to energy required to dissociate equivalent complexes of Lu3+.

energy balance, showing the net difference in the Gibbs free energy of complex dissociation as compared to the complex of lutetium. The plot demonstrates that dissociation of [La(DTPA)]2− requires 18 kJ/mol less energy than [Lu(DTPA)]2−, while the energetic difference is 13 kJ/mol for the equivalent pair of HEDTTA complexes. This consistent difference of ∼5 kJ/mol is sustained throughout the lanthanide series until holmium. As such the influence by the phase-transfer reagent on the liquid− liquid partitioning of lanthanum relative to lutetium (as indicated by Kex × Kd) will not be as pronounced for HEDTTA as it is for the DTPA-containing mixtures. This is manifested in the liquid− liquid distribution of lanthanides shown in Figure 6 where the initially decreasing partitioning trend for light metal ions observed with DTPA is not reproduced when utilizing HEDTTA. The lower extractability of light lanthanides, when coupled with slightly reduced “softness” of HEDTTA due to 1730


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One plausible explanation for HEDTTA’s unexpectedly enhanced facilitation of kinetics of f-element phase transfer may reside with the presence of the liquid−liquid interface. A substitution of the metal-coordinating acetate with an inert ethanol functionality introduces a less rigid coordination environment and labels the metal chelate with a hydrophobic “antennae” directed outward into bulk solvent. Since the microscopic interfacial layer of water is more compact, resembling glycerol-like environment,61 the diffusion of ions through this “stagnant” region to the interface may be altered by the enhanced hydrophobic character of the chelate.62 The interfacial structuring is mainly determined by the surface active molecule present in the nonaqueous environment; however, the ethanol group of HEDTTA may incorporate some surfactant character to the metal chelate of this aminopolycarboxylate reagent.62 Further surface tension measurements will be performed to understand this kinetic enhancement in more detail. It is important to note that the phase-transfer kinetic trend for the partitioning of Eu3+ supported by HEDTTA outperforms that collected for HEDTA (Figure 7a), which also contains ethanol functionality. The time-dependent partitioning trends for Am3+, Ce3+, and Eu3+ (Figure 7b) demonstrate that HEDTTA operates efficiently as a holdback complexant of trivalent actinides at pCH = 3.0. The phase-transfer partitioning trends of the americium and cerium ions are faster relative to europium, as expected from metals of lower charge density.63 The partitioning studies containing up to 1.0 mM Eu3+ in the aqueous environment afforded an efficient, fast option for differentiation of trivalent actinides from trivalent lanthanides.

alcohol functionalization, yields an overall trivalent 4f−5f separation factor of 8 in pCH 3.0 aqueous environments. 4.4. Kinetics of Liquid−Liquid Distribution Supported by HEDTTA. Sluggish equilibration of liquid−liquid partitioning systems has been related to the extraordinary stability of aminopolycarboxylate metal chelates.13 The main kinetic limitation resides with the energetic costs associated with reorganization of the metal complex during the partitioning process.13 Accordingly, the complexant denticity directly impacts the rates of complex dissociation.52 The partitioning trends presented in Figure 7a demonstrate this polydentate kinetic inhibition for HEDTA and DTPA. The partitioning of Eu3+ is much slower when octadentate DTPA is present, relative to pentadentate HEDTA. The observed distribution differences cannot be attributed to differences in the experimental pH, and the corresponding enhancement of the rate of complex dissociation due to catalytic influence of hydrogen ion.52 The kinetic variance originates from the stability of a complex containing 8 five-membered chelate rings (DTPA), and nearly fully dehydrated metal ion, well-shielded from the aqueous environment. The hydration sphere of a trivalent metal ion is disrupted to a much lesser extent by HEDTA, and other EDTAlike reagents, offering easier interaction with bulk water, or secondary coordination reagents.53 To illustrate, the water exchange rate of Gd3+/DTPA chelate (k298ex = 3.30 × 106 s−1)54 is approximately 10 times slower relative to chelates formed using the EDTA-type complexants, such as bisamide-substituted EDTA (k298ex = 44 × 106 s−1) reported by Platas-Iglesias et al.55 The time-dependent trend for the partitioning of Eu3+ facilitated by HEDTTA is surprisingly enhanced relative to that expected solely from the argument of polydenticity. On the basis of luminescence studies presented here HEDTTA is heptadentate in aqueous solutions adjusted to pCH = 3. Presumably the task of unravelling such a coordination environment must be of intermediate difficulty relative to HEDTA and DTPA. The observed Eu3+ partitioning rate enhancement for HEDTTA must therefore originate with the specific structural modification of this reagent, where one acetate pendant arm of DTPA has been replaced by an ethanol group. Sarka et al. studied kinetic stability and water exchange rates of Gd3+ complexes of DTPA-type amide-substituted complexants.56,57 The authors indicated that either single or multiple replacement of an acetate pendant arm of DTPA by an amide does not appreciably alter the kinetic stability of the chelate. Laus et al. showed that substitution of an acetate group of DTPA by a propionate group increases the water exchange rate for the Gd3+ complex by an order of magnitude (k298ex = 31 × 106 s−1).58 Octadentate coordination of the gadolinium ion is maintained for reagents studied by the Sarka and Laus groups, suggesting that kinetic stability may be influenced by the flexibility of the pendant arm functionality. A reduction of polydenticity to heptadentate coordination was demonstrated by Moriggi et al., who did not see significant changes in the kinetic stability of the metal complex when a single acetate group was replaced by a methyl group relative to bisamide-substituted DTPA reagent.59 In contrast, Silverio et al. found a much enhanced water exchange rate of a Gd3+ complex of DTTA-type compound, where acetate was substituted by a benzyl group (k298ex = 77 × 106 s−1).60 This has been attributed to steric strain on the water binding site imposed by the bulky benzyl group within the metal complex.60 Clearly, no definitive indication for significant kinetic instability for heptadentate DTTA-type chelates may be deduced from these aqueous aminopolycarboxylate systems.

5. CONCLUSIONS A single acetate pendant arm of DTPA reagent was replaced by a N-hydroxyethyl group, thus reducing the denticity of the reagent. Potentiometric titration of 1:1 Ln/HEDTTA mixtures identified two species, ML− and MHL(aq) , typical of DTPA-type coordination. Stability constant trends for Ln/HEDTTA complexation across the lanthanide series were analogous to previously observed Ln/DTPA complex formation. The replacement of the acetate pendant arm with the hydroxyethyl did reduce Ln/HEDTTA complex stability ∼3 orders of magnitude relative to Ln/DTPA complexes. DFT calculations help account for this by predicting an elongated Eu−N3 bond with the hydroxyethyl-functionalized amine. The theoretical outcome points to alcohol’s ability to pull away a weakly coordinated amine atom (N3) from the metal center, thus reducing the “softness” of the reagent. UV−vis spectroscopy was used to determine stability constants for HEDTTA complexes with Nd3+ and Am3+. The results showed consistency between the two analytical methods as the stability constants for [Nd(HEDTTA)]− determined by both techniques showed good overlap. The stability constant for [Am(HEDTTA)]− was ∼1 order of magnitude larger than [Nd(HEDTTA)]−, demonstrating that HEDTTA maintains selectivity for trivalent actinides over trivalent lanthanides. Luminescence lifetime measurements showed the presence of 2 waters inside the metal’s inner hydration sphere, resulting from displacement of seven water molecules upon coordination by HEDTTA. Due to the reduced complex strength (relative to DTPA) TALSPEAKtype separations were facilitated using a phosphonic acid extractant HEH[EHP]. As predicted by thermodynamic data collected in this study, HEDTTA was an effective holdback reagent for trivalent lanthanide/trivalent actinide differentiation. 1731


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Mediated Advanced Oxidation of Chlorophenols. Water Res. 2009, 43, 684−694. (5) Repo, E.; Warchol, J. K.; Bhatnagar, A.; Mudhoo, A.; Sillanpäa,̈ M. Aminopolycarboxylic Acid Functionalized Adsorbents for Heavy Metals Removal from Water. Water Res. 2013, 47, 4812−4832. (6) Dermont, G.; Bergeron, M.; Mercier, G.; Richer-Laflèche, M. Soil Washing for Metal Removal: A Review of Physical/Chemical Technologies and Field Applications. J. Hazard. Mater. 2008, 152, 1−31. (7) Arwidsson, Z.; Elgh-Dalgren, K.; von Kronhelm, T.; Sjöberg, R.; Allard, B.; van Hees, P. Remediation of Heavy Metal Contaminated Soil Washing Residues with Amino Polycarboxylic Acids. J. Hazard. Mater. 2010, 173, 697−704. (8) Runge, V. M.; Clanton, J. A.; Lukehart, C. M.; Partain, C. L.; James, E. A., Jr. Paramagnetic Agents for Contrast-Enhanced NMR Imaging: A Review. AJR, Am. J. Roentgenol. 1983, 141, 1209−1215. (9) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B. Gadolinium(III) Chelates as MRI Contrast Agents: Structure, Dynamics, and Applications. Chem. Rev. 1999, 99, 2293−2352. (10) Caravan, P. Strategies for Increasing the Sensitivity of Gadolinium Based MRI Contrast Agents. Chem. Soc. Rev. 2006, 35, 512−523. (11) Weaver, B.; Kappelmann, F. A. TALSPEAK: A New Method of Separating Americium and Curium from the Lanthanides by Extraction from Aqueous Solutions of Aminopolycarboxylic Acid Complex with a Monoacidic Organophosphate or Phosphonate. United States Atomic Energy Commission Unclassified Report ORNL-3559, Oak Ridge, TN, 1964. (12) Weaver, B.; Kappelmann, F. A. Preferential Extraction of Lanthanides over Trivalent Actinides by Monoacidic Organophosphates from Carboxylic Acid and from Mixtures of Carboxylic and Aminopolyacetic Acids. J. Inorg. Nucl. Chem. 1968, 30, 263−272. (13) Danesi, P. R.; Cianetti, C. Kinetics and Mechanism of the Interfacial Mass Transfer of Eu(III) in the System: Bis(2-ethylhexyl)phosphoric Acid, n-Dodecane-NaCl Lactic Acid, Polyaminocarboxylic Acid, Water. Sep. Sci. Technol. 1982, 17 (7), 969−984. (14) Nilsson, M.; Nash, K. L. Trans-Lanthanide Extraction Studies in the TALSPEAK System: Investigating the Effect of Acidity and Temperature. Solvent Extr. Ion Exch. 2009, 27, 354−377. (15) Nash, K. L. The Chemistry of TALSPEAK: A Review of the Science. Solvent Extr. Ion Exch. 2015, 33 (1), 1−55. (16) Braley, J. C.; Grimes, T. S.; Nash, K. L. Alternatives to HDEHP and DTPA for Simplified TALSPEAK Separations. Ind. Eng. Chem. Res. 2012, 51 (2), 629−638. (17) Lumetta, G. J.; Gelis, A. V.; Braley, J. C.; Carter, J. C.; Pittman, J. W.; Warner, M. G.; Vandegrift, G. F. The TRUSPEAK Concept: Combining CMPO and HDEHP for Separating Trivalent Lanthanides from the Transuranic Elements. Solvent Extr. Ion Exch. 2013, 31 (3), 223−236. (18) Braley, J. C.; Lumetta, G. J.; Carter, J. C. Combining CMPO and HEH[EHP] for Separating Trivalent Lanthanides from the Transuranic Elements. Solvent Extr. Ion Exch. 2013, 31 (6), 567−577. (19) Lumetta, G. J.; Casella, A. J.; Rapko, B. M.; Levitskaia, T. G.; Pence, N. K.; Carter, J. C.; Niver, C. N.; Smoot, M. R. An Advanced TALSPEAK Concept Using 2-ethylhexylphosphonic Acid Mono-2ethylhexyl Ester as the Extractant. Solvent Extr. Ion Exch. 2015, 33 (4), 211−223. (20) Lapka, J. L.; Nash, K. L. Advanced TALSPEAK Separations Using a Malonate Buffer System. Solvent Extr. Ion Exch. 2015, 33 (4), 346− 361. (21) Gelis, A. V.; Lumetta, G. J. Actinide Lanthanide Separation Process-ALSEP. Ind. Eng. Chem. Res. 2014, 53 (4), 1624−1634. (22) Heathman, C. R.; Grimes, T. S.; Zalupski, P. R. Thermodynamic and Spectroscopic Studies of Trivalent f-Element Complexation with Ethylenediamine-N,N′-di(acetylglycine-N,N′-diacetic Acid. Inorg. Chem. 2016, 55, 2977−2985. (23) Heathman, C. R.; Grimes, T. S.; Zalupski, P. R. Coordination Chemistry and f-Element Complexation by Diethylenetriamine-N,N″bis(acetylglycine)-N,N′,N″-triacetic Acid. Inorg. Chem. 2016, 55, 11600−11611.

The kinetic trends were collected for the partitioning of trivalent f-elements between aqueous mixtures of aminopolycarboxylate reagents and organic extractant solutions. Forward extraction kinetics of 241Am, 139Ce, and 154Eu in liquid−liquid systems demonstrated that significant enhancement in the equilibration rate occurs when single acetate functionality of the DTPA molecule has been “switched-off” from participating in metal ion coordination.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02897. Synthesis, accompanied by the 1H and 13C NMR spectra of di-tert-butyl 2,2′-((2-((2-(tert-butoxy)-2-oxoethyl)(2((2-(tert-butoxy)-2-oxoethyl)(2-hydroxyethyl)amino)ethyl)amino)ethyl)azanediyl)diacetate and 2,2′-((2((carboxymethyl)(2-((carboxymethyl)(2-hydroxyethyl)amino)ethyl)amino)ethyl) azanediyl)diacetic acid TFA salt and 2,2′-((2-((carboxymethyl)(2-((carboxymethyl)(2-hydroxyethyl)amino)ethyl)amino)ethyl)azanediyl)diacetic acid HCl salt; detailed description of DFT-based molecular dynamics (MD) simulations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Travis S. Grimes: 0000-0003-2751-0492 Vyacheslav S. Bryantsev: 0000-0002-6501-6594 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental work conducted by T.S.G., C.R.H., M.N., and P.R.Z. at the Idaho National Laboratory was supported by the U.S. Department of Energy, Office of Nuclear Energy, DOE Idaho Operations Office, under contract DE-AC07-05ID14517. The synthetic work by S.J.-P. and computational studies by V.S.B. and S.G.S. were supported by the Fuel Cycle Research and Development Program, Office of Nuclear Energy, U.S. Department of Energy. DFT calculations used resources of the National Energy Research Scientific Computing Center and the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, both of which are supported by the Office of Science of the U.S. Department of Energy under contract nos. DE-AC0205CH11231 and DE-AC05-00OR22725, respectively.

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REFERENCES

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