Article pubs.acs.org/IC
Coordination Chemistry and f‑Element Complexation by Diethylenetriamine‑N,N″‑bis(acetylglycine)‑N,N′,N″‑triacetic Acid Colt R. Heathman,* Travis S. Grimes, and Peter R. Zalupski* Aqueous Separations and Radiochemistry, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States S Supporting Information *
ABSTRACT: Potentiometric and spectroscopic techniques were used to evaluate the coordination behavior and thermodynamic features of trivalent felement complexation by diethylenetriamine-N,N″-bis(acetylglycine)-N,N′,N″triacetic acid (DTTA-DAG) and its di(acetylglycine ethyl ester) analogue [diethylenetriamine-N,N″-bis(acetylglycine ethyl ester)-N,N′,N″-triacetic acid (DTTA-DAGEE)]. Protonation constants and stability constants of trivalent lanthanide complexes (except Pm3+) were determined by potentiometry. Six protonation sites and three metal−ligand complexes [ML2−, MHL−, and MH2L(aq)] were quantified for DTTA-DAG. Four protonation sites and one metal−ligand complex [ML(aq)] were observed for DTTA-DAGEE, consistent with the presence of two ester groups. Absorption spectroscopy was utilized to measure the stability constants for complexation of trivalent neodymium and americium by DTTA-DAG and trivalent neodymium by DTTA-DAGEE. The coordination environment of trivalent europium in the presence of DTTA-DAG was investigated at various acidities by luminescence lifetime measurements. Decay constants indicate one water molecule in the inner coordination sphere across the 1.0 < pH < 5.5 range, presumably due to octadentate coordination by DTTA-DAG. A translanthanide pattern of complex stabilities for DTTA-DAG was found to be analogous to that observed for DTPA, with a ∼106 reduction of the complex stability. The lessened strength of complexation, relative to DTPA, was attributed to significant reduction of the total ligand basicity for DTTA-DAG due to the electronic influence of amide functionalization. When DTTADAG is used as an aqueous holdback complexant in liquid−liquid distribution experiments, the preferential coordination of Am3+ in the aqueous environment offers efficient An/Ln differentiation. The separation extends to pH 2 conditions, where the kinetics of phase transfer in such liquid−liquid systems are aided by the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex.
1. INTRODUCTION One of the most challenging tasks in the field of separation science is the group differentiation of trivalent 4f and 5f elements due to similarities of their physical and chemical properties (i.e., oxidation state, ionic radius, charge density, and hydration). The availability of multiple oxidation states of some actinide elements offers one option to differentiate them from lanthanides. However, access to higher oxidation states of actinide elements like americium is hindered by the high potentials required for oxidation.1−3 An alternative option to differentiate trivalent americium and curium from trivalent lanthanides is built around aqueous reagents containing soft donor atoms (relative to oxygen) to facilitate stronger complexes of actinide ions. The original study by Diamond and co-workers demonstrated this concept when a hydrochloric acid eluant was used to separate trivalent americium from trivalent lanthanides using a strong cation-exchange resin.4 One rendition of the soft-donor concept capitalizes on the ability of nitrogen-containing aminopolycarboxylate reagents to form stronger complexes with trivalent actinides, relative to trivalent lanthanides. Aminopolycarboxylates have been extensively studied,5−8 and their high affinity for metal coordination is This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
utilized in a wide spectrum of industrial products such as agrochemicals, cleaners, detergents, bleaching agents, and magnetic resonance imaging contrast agents.9,10 Perhaps the most well-known reagent in this family, diethylenetriamineN,N,N′,N″,N″-pentaacetic acid (DTPA), was first used by Orr11 in an eluent mixture to efficiently separate trivalent americium from promethium using an ion-exchange method. Later Weaver and Kappelmann used DTPA as an aqueous holdback reagent in a liquid−liquid formulation known as the TALSPEAK (Trivalent Actinide−Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes) process.12,13 The efficient An3+/Ln3+ differentiation afforded by this chemistry originates from the competitive equilibrium between DTPA complexation in the aqueous phase and the coordination of lanthanides by a lipophilic organophosphorus phase-transfer reagent. However, this option of An3+/Ln3+ separation is hindered by slow reaction kinetics, often attributed to the energetic cost of structural rearrangement required to dissociate the M3+/DTPA complex.14 This Received: September 12, 2016
A
DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
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Figure 1. Synthetic scheme for the preparation of DTTA-DAG (bottom) and DTTA-DAGEE (upper right). (DTTA-DAGEE) were prepared by reacting 1 equiv of diethylenetriamine-N,N,N′,N″,N″-pentaacetic dianhydride with 2 equiv of glycine ethyl ester and 4 equiv of triethylamine (TEA; Figure 1). The DTTADAG product was prepared by saponification of DTTA-DAGEE with sodium hydroxide to give the pentasodium salt of DTTA-DAG (Na5L form; Figure 1). See the Supporting Information for a full description of the synthesis and purification of DTTA-DAG and DTTA-DAGEE. Lanthanide stock solutions (except cerium and promethium) were prepared by dissolving 1 equiv of metal oxides (>99.99% Pangea International, China) in 5.9 equiv of HNO3 (TraceSELECT, SigmaAldrich). The undissolved oxide contents were filtered out, and the stock mixtures were adjusted to pH ∼ 3 using HNO3. Sodium carbonate (Sigma-Aldrich) was first added to the metal nitrates to convert lanthanide nitrate stocks to their carbonate salts. The metal carbonate precipitates were filtered, washed with copious amounts of 18 MΩ H2O, and dissolved using 5.9 equiv of HClO4 (TraceSELECT, Sigma-Aldrich). The undissolved carbonate solids were filtered out, and the stock mixtures were adjusted to pH ∼ 3 using HClO4. The preparation of cerium perchlorate followed the same procedure using a commercial source of cerium nitrate (99.99%, Molycorp). Solutions were degassed prior to standardization using a degassing unit (TA Instruments) to remove dissolved CO2. Metal concentrations were determined by complexometric titrations with tetrasodium ethylenediamine-N,N,N′,N′-tetraacetic acid (Na4EDTA) to a xylenol orange end point in a 1 mM DEPP buffer solution at pH 5.00. End-point identification was aided by a DP-5 phototrode (Mettler Toledo) attached to a Mettler Toledo T-90 graphix autotitrator. A stock solution of americium perchlorate [243Am(ClO4)3] was prepared via an initial purification step using extraction chromatography.19 Normal diglycolamide resin (Eichrom, 2 mL cartridge, 50− 100 μm) was used to adsorb americium. The eluted 243Am was converted from chloride to perchlorate by a series of evaporative cycles and a final dissolution of pH ∼ 4 with HClO4. The americium stock solution concentration was determined using a spectrophotometric method reported by Tian and Shuh.24 Tracer solutions of americium-241, cerium-139, and europium-154 were purchased from Eckert and Ziegler. Spiking solutions containing all three isotopes were prepared by dilution in 0.01 M HNO3. All manipulations of radioisotope-containing mixtures were performed in radiological HEPA-filtered ventilation hoods approved for handling radiological materials. Extreme caution should be taken when handling radiological materials. Sodium perchlorate and sodium nitrate (GFS Chemicals) crystals were purified by dissolving the received salts in 18 MΩ H2O and filtering through a medium-porosity glass-fritted filter, with the aid of a vacuum pump. The salt solutions were heated to evaporate excess water and the purified salts recrystallized from solution upon cooling.
limitation has generally been controlled by using high buffer concentrations to facilitate metal-transfer equilibrium and minimize pH migration.12−14 Several new renditions of TALSPEAK-type chemistry have been developed since its original introduction, focusing on process optimization; however, improvements of the kinetic and thermodynamic balance of this separation are still needed.15−18 This work evaluates the thermodynamic and kinetic impacts of utilizing a new aminopolycarboxylate reagent as an aqueous holdback complexant to facilitate An3+/Ln3+ separation. Previous thermodynamic studies of f-element coordination by ethylene-N,N′-bis(acetylglycine)-N,N′-diacetic acid (EDDAGDA)19 demonstrated that amide substitution largely reduced the basicity of the amine groups [pKa values for EDDAG-DA are 7.31 and 4.32,19 relative to 8.73 and 6.19 for ethylenediaminetetraacetic acid (EDTA)].20 Such a reduction of the overall “softness” of the reagent’s coordination pocket reduced the extent of An3+/Ln3+ selectivity, but it also expanded the pH spectrum where the reagent prefers to complex trivalent f elements over protonation. The diamide-functionalized alternative of EDTA presented an opportunity to enhance the kinetic aspects of complexant-supported An3+/Ln3+ separation through the acid-catalyzed dissociation of f-element complexes in aqueous environments.21,22 The increased concentration of hydrogen ions (lower pH) promotes the acid-catalyzed mechanism of complex restructuring (likely involving amine protonation/deprotonation23) to facilitate reaction kinetics involving aminopolycarboxylate reagents. Because reduction of the basicity compromised the efficient An3+/Ln3+ differentiation for EDDAG-DA, a similar diamide modification has been made to the diethylenetriamine backbone to regain this characteristic with an increased number of nitrogen-donor atoms. Here we report a series of thermodynamic and kinetic studies of trivalent f-element complexation by diethylenetriamine-N,N″-bis(acetylglycine)-N,N′,N″-triacetic acid to demonstrate a new aqueous holdback complexant capable of efficient An3+/Ln3+ separation and enhanced H+-catalyzed reaction kinetics.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Diethylenetriamine-N,N″-bis(acetylglycine)-N,N′,N″-triacetic acid (DTTA-DAG) and diethylenetriamine-N,N″-bis(acetylglycine ethyl ester)-N,N′,N″-triacetic acid B
DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. Potentiometric studies of DTTA-DAGEE and DTTA-DAG protonation. T = 25.0 ± 0.1 °C and I = 2.00 ± 0.01 M (Na+/H+)ClO4. (Left) Titrand: Vinit = 24.913 mL, CDTTA‑DAG = 5.05 mM, CH+total = 0.044 M. Titrant: 0.25 M NaOH and 1.75 M NaClO4. (○) Experimental p[H+], (red solid line) calculated p[H+], (black dashed line) L5−, (red dashed line) HL4−, (green dashed line) H2L3−, (blue dashed line) H3L2−, (magenta dashed line) H4L−, (orange dashed line) H5L(aq), and (purple dashed line) H6L+. (Right) Titrand: Vinit = 24.950 mL, CDTTA‑DAGEE = 4.96 mM, and CH+total = 0.033 M. Titrant: 0.25 M NaOH and 1.75 M NaClO4. (○) Experimental p[H+], (red solid line) calculated p[H+], (black dashed line) L3−, (red dashed line) HL2−, (green dashed line) H2L−, (blue dashed line) H3L(aq), and (orange dashed line) H4L+. procedure, but here solutions A and B did not contain Nd3+. The p[H+] was measured for all investigated mixtures. Spectrophotometric titrations of 243Am3+ solutions were performed using a Flame-S-Vis-NIR-ES Ocean Optics device coupled to an Ocean Optics DH-2000-BAL light source via a 2 m (200 μm) fiber optic cable. Absorbance spectra were collected in 400−600 nm range using a 1 cm quartz cuvette at ambient temperature (20 ± 1 °C). Each spectrum was measured under the following default instrumental settings: (1) single beam mode, (2) 8 ms integration time, (3) 1000 scans averaged, and (4) 0.37 nm data interval. Spectrophotometric absorbance features were monitored when a titrand solution [Vinit = 0.8 mL, [Am3+] = 0.774 mM, and I = 2.00 M (Na/H)ClO4] was progressively blended with a titrant mixture {[DTTA-DAG] = 8.63 mM, [Am3+] = 0.773 mM, and I = 2.00 M (Na/H)ClO4}. The p[H+] of titrand at the beginning of the titration was 1.98, and the p[H+] reading at the conclusion of an experiment was 3.64. Additional titration experiments mimicked the spectrophotometric study to measure p[H+] throughout. Spectrophotometric data were analyzed using HypSpec software to resolve the stability constants for the binding of Nd3+ and Am3+ by DTTA-DAG and for Nd3+ by DTTADAGEE.27 2.4. Luminescence Spectroscopy. Luminescence lifetime measurements were collected on a Jobin Yvon IBH Fluorolog-3 fluorometer (Horiba) adapted for time-resolved measurements. Experiments were maintained at 25.0 ± 0.1 °C using a thermostated cuvette assembly. Perchlorate media were used to ensure full hydration of the metal ion in electrolyte mixtures prior to the addition of DTTADAG.28 The Eu3+/DTTA-DAG solutions containing 5 mM DTTADAG, 1 mM Eu(ClO4)3, and I = 2.00 M (Na/H)ClO4 were adjusted to the following p[H+] values: 5.56, 3.48, 1.93, and 1.01. The p[H+] conditions were selected to maximize the concentrations of ML2−, MHL−, MH2L(aq), and M3+ species, respectively. Analysis of lifetime data was resolved with a single-exponential fit for p[H+] values of 5.56, 3.48, and 1.93. At p[H+] = 1.01, a double-exponential fit was required to resolve the decay constants. 2.5. Liquid−Liquid Distribution Measurements. Liquid−liquid extraction studies were performed using bis(2-ethylhexyl)phosphoric acid (HDEHP; 97%, Sigma-Aldrich) or (2-ethylhexyl)phosphonic acid
The supernatant was decanted, and the resulting crystals were dissolved in 18 MΩ H2O. Stock concentrations were determined by potentiometric titration of the ion-exchange eluents [Dowex 50X8 (H+ form)] collected after aliquots of NaNO3 and NaClO4 were passed through the resin. 2.2. Potentiometry. Acid dissociation constants and metal stability constants were determined using a Mettler Toledo T-90 graphix autotitrator equipped with a Ross Orion semimicro glass electrode. The electrode filling solution was replaced with 5 M sodium chloride to prevent potassium perchlorate precipitation inside the electrode. Calibration of the electrode to the operational p[H+] scale was achieved by titrations of a strong acid with a strong base in 2.0 M (Na/H)ClO4 media (Gran method).25 A circulating water-jacketed beaker was used to maintain a constant temperature at 25.0 ± 0.1 °C. Hydrated nitrogen was used to blanket the titrand mixtures to prevent CO2 absorption. Ligand solutions of DTTA-DAG and DTTA-DAGEE (esterified DTTA-DAG) were prepared in 2.0 M ionic strength (Na/ H)ClO4. Perchloric acid was added to ligand mixtures to ensure that potentiometric titrations begin at a p[H+] of ∼1.8. This pH lowering converts the NaxL form of the ligand to the protonated HnL form. Acid dissociation constants (Ka) and lanthanide binding constants (βMHL) were resolved using HYPERQUAD 2013 software.26 2.3. Spectrophotometric Titrations. Neodymium complexation studies were collected on a QEPro High Performance Spectrometer Ocean Optics Device coupled to an Ocean Optics HD-2000-BAL light source. Solutions were prepared by mixing solution A {DTTA-DAG, 0.497 mM Nd(ClO4)3, p[H+] = 2.01; DTTA-DAGEE, 0.497 mM Nd(ClO4)3, p[H+] = 1.98} and solution B {4.97 mM DTTA-DAG, 0.503 mM Nd3+, pH ≈ 9.5, or 4.98 mM DTTA-DAGEE, 0.508 mM Nd3+, pH ≈ 10.5}, respectively, in variant proportions, yielding a total sample volume of 2.5 mL. The ionic strength for all solutions was adjusted to 2.00 M using NaClO4. The absorbance spectra were collected in the 560−600 nm range using a 100 cm waveguide capillary flow cell (World Precision Instruments Inc.) at ambient temperature (20 ± 1 °C). Each spectrum was measured using the following instrumental settings: (1) single-beam mode, (2) 16 ms integration time, (3) 1000 scans averaged, and (4) 0.776 nm data interval. The preparation of blank solutions matched the above-mentioned blending C
DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. Acid Dissociation Constants for DTTA-DAG and DTTA-DAGEE at 25.0 ± 0.1 °Ca DTTA-DAG 4−
HL H2L3− H3L2− H4L− H5L(aq) H6L+ H7L2+ H8L3+ a
n
−log Kan
DTTA-DAGEE
8 7 6 5 4 3 2 1
± ± ± ± ± ±
2−
9.14 4.98 4.15 3.69 2.97 2.18
0.01 0.01 0.01 0.01 0.01 0.01
HL H2L− H3L(aq) H4L+ H5L2+ H6L3+
m
−log Kam
DTPA
n
−log Kan
6 5 4 3 2 1
± ± ± ±
4−
8 7 6 5 4 3 2 1
9.50 8.31 4.38 2.53 2.41
9.27 4.69 3.79 2.18
0.01 0.01 0.01 0.01
HL H2L3− H3L2− H4L− H5L(aq) H6L+ H7L2+ H8L3+
Data for DTPA displayed for comparison.32 I = 2.00 M (H+/Na+)ClO4.
Figure 3. Potentiometric studies of NdIII/DTTA-DAG and NdIII/DTTA-DAGEE complexation. T = 25.0 ± 0.1 °C and I = 2.00 ± 0.01 M (Na+/ H+)ClO4. (Left) Titrand: Vinit = 25.204 mL, CDTTA‑DAG = 4.96 mM, CH+total = 0.044 M, and CNd3+ = 4.96 mM. Titrant: 0.25 M NaOH and 1.75 M NaClO4. (○) Experimental p[H+], (red solid line) calculated p[H+], (red dashed line) NdL2−, (green dashed line) NdHL−, (blue dashed line) NdH2L(aq), and (black dashed line) Nd3+. (Right) Titrand: Vinit = 25.206 mL, CDTTA‑DAGEE = 4.95 mM, CH+total = 0.033 M, and CNd3+ = 5.02 mM. Titrant: 0.25 M NaOH and 1.75 M NaClO4. (○) Experimental p[H+], (red solid line) calculated p[H+], (red dashed line) NdL(aq), and (black dashed line) Nd3+. mono-2-ethylhexyl ester (HEH[EHP]; 95%, Yic-Vic Pharm. Ltd.). HDEHP was used without further purification, and HEH[EHP] was purified by third-phase formation methodology.29 Time-dependent distribution studies were performed at p[H+] of 3.0, 2.6, and 2.2. Organic phases of HDEHP dissolved in n-dodecane (with adjusted [HDEHP] to afford a liquid−liquid distribution of europium of ∼10) were contacted with 20 mM DTTA-DAG or 20 mM DTPA (recrystallized), I = 1.00 M (Na/H)NO3, adjusted to the respective p[H+] under investigation. The absence of buffer in the aqueous phase was designed to facilitate slower metal partitioning rates to better evaluate the role of acidity. Organic phases were preequilibrated with 1.0 M NaNO3 adjusted to p[H+] values of 3.0, 2.6, and 2.2 with HNO3. All equilibrations were performed on a multitube vortexer (Glas-col, motor speed = 50). The liquid−liquid distribution of europium was monitored using a 154Eu3+ radiotracer. Prior to contact with the organic phase, the aqueous environments of DTTA-DAG and DTPA were spiked from a working stock solution of 154Eu3+ to yield feed solutions containing 800 (±40) Bq/mL of activity. At chosen time increments, samples were centrifuged for 30 s (6500 rpm), and 0.3 mL aliquots of each phase were sampled for γ-ray spectroscopy analysis (Packard D5003 Cobra autogamma counter). The p[H+] dependence for the liquid−liquid distribution of trivalent lanthanides and Am3+ was investigated using 0.75 M HEH[EHP] in n-dodecane. Aqueous phases contained 20 mM DTTA-DAG, 0.5 M malonate buffer, and 1 mM total lanthanides
(lanthanum, cerium, promethium, neodymium, samarium, europium, gadolinium, terbium, dysprosium, and holmium) at p[H+] = 2.99, 2.76, 2.50, 2.36, 2.12, 1.93, and 1.86. The ionic strength was adjusted to 1.0 M using NaNO3. Prior to contact, organic phases were preequilibrated with a 0.5 M malonate buffer, I = 1.0 M adjusted with NaNO3, at the p[H+] corresponding with the distribution study. Samples (0.5 mL of each phase) were contacted for 5 h to ensure that distribution equilibrium was attained. After centrifugation and phase separation, the lanthanide concentrations in aqueous mixtures before and after extraction were determined by inductively coupled plasma mass spectrometry (ICP-MS). The lanthanide distribution was calculated using eq 1. Americium distribution studies were performed in duplicate using a 241Am3+ radiotracer. Isotopes 154Eu3+ and 139Ce3+ were also added to those mixtures to compare the distribution values for Eu3+ and Ce3+ obtained radiometrically with those determined by ICP-MS. The radioisotope distribution in liquid−liquid systems was measured by the quotient of activity (A) counted in the heavy and light phases (eq 2). The centrifuged and separated phases were counted on an ORTEC GEM50P4 coaxial HPGe detector equipped with a DSPEC γ-ray spectrometer (59.54 keV peak for 241Am, 123.07 keV peak for 154Eu, and 165.86 keV peak for 139Ce).
D=
D
[M3 +]aq,init − [M3 +]aq,eq [M3 +]aq,eq
(1) DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
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Aorg A aq
DTTA-DAGEE are described by eqs 5−7 and eq 8, respectively.
(2)
M3 + + L5 − ⇄ ML2 −
3. RESULTS 3.1. Acid Dissociation Constants of DTTA-DAG and DTTA-DAGEE. The potentiometric curves collected when DTTA-DAG and DTTA-DAGEE were titrated with NaOH are compared in Figure 2. Changes in the distribution of protonated ligand species are shown throughout the duration of the titration experiments. Theoretical titration curves (represented by solid red lines) were calculated by nonlinear least-squares regression analysis, as implemented in the HYPERQUAD 2013 software package. The best-fitting representation of the experimental results was obtained when six dissociation equilibria for DTTA-DAG and four dissociation equilibria for DTTA-DAGEE were present in the model, according to eqs 3 and 4, respectively. 4−n+
H 9 − nL
Ka n =
3−n+
⇄ H8 − nL
+H
[H+][H8 − nL3 − n +] [H 9 − nL4 − n +]
[H+][H6 − mL3 − m +] [H 7 − mL4 − m +]
[ML2 −] [M3 +][L5 −]
(5)
M3 + + H+ + L5 − ⇄ MHL− [MHL−] DTTA‐DAG β111 = [M3 +][L5 −][H+]
(6)
M3 + + 2H+ + L5 − ⇄ MH 2L(aq) DTTA‐DAG β121 =
[MH 2L(aq)] [M3 +][L5 −][H+]2
M3 + + L3 − ⇄ ML(aq)
(7)
DTTA‐DAGEE β101 =
[ML(aq)] [M3 +][L3 −] (8)
+
+
Experimental data collected above a p[H ] value of 5.5 were excluded from HYPERQUAD’s nonlinear least-squares regression treatment to eliminate the possible contribution of metal hydrolysis to experimental measurement. Resolved stability constants for the complexation of lanthanide ions (except promethium) by DTTA-DAG and DTTA-DAGEE are listed in Table 2. Stability Constants Determined by Absorbance Spectroscopy. The metal binding constants of Am3+ and Nd3+ with DTTA-DAG were investigated by absorbance spectroscopy. The collected absorbance spectra and corresponding molar absorptivities of free metal and metal−ligand species are shown
n = 1, ..., 8 (3)
H 7 − mL4 − m + ⇄ H6 − mL3 − m + + H+ Ka m =
DTTA‐DAG β101 =
m = 1, ..., 6 (4)
Table 1 lists the determined acid dissociation constants for DTTA-DAG and DTTA-DAGEE and indicates the protonation steps outside the operational range of the glass electrode. The three most basic pKa values (n = 6−8) describe the dissociation of three amine groups of the diethylenetriamine structural backbone. The following two constants obtained for DTTA-DAG (n = 4 and 5) quantify the dissociation of carboxylic acids located on two glycine substituents. Those equilibria are absent for the esterified analogue, DTTA-DAGEE. The remaining pKa value is a sole quantifiable carboxylic acid of the acetate pendant arm (one of three present in both reagents). 3.2. Trivalent f-Element Complexation by DTTA-DAG and DTTA-DAGEE. Stability Constants Determined by Potentiometry. The potentiometric titration curves presented in Figure 3 correspond to p[H+] changes resulting from the addition of NaOH to equimolar mixtures of Nd3+ and DTTADAG (left) or DTTA-DAGEE (right). Compared to the potentiometric curves afforded during the measurements of the acid dissociation constants for both reagents (Figure 2), the displacement of hydrogen ions from DTTA-DAG and DTTADAGEE is noticeable. This p[H+] buffering effect results from the formation of metal complexes. The most accurate theoretical description of the collected data yielded a set of complexation reactions used to calculate the distribution of metal-containing species throughout the titration. The distribution curves accompanying the experimental titration curves are shown in Figure 3. Several models were evaluated to accurately describe the M(DTTA-DAG) data. The best fit was obtained using three metal complexes: ML2−, MHL−, and MH2L(aq). Similar to the M(DTTA-DAG) analysis, the complexation model most accurately representing M(DTTADAGEE) included only one metal−ligand complex: ML(aq). The metal complexation equilibria for DTTA-DAG and
Table 2. Stability Constants for the Complexation of Ln3+ and Am3+ with DTTA-DAG and DTTA-DAGEE at 25.0 ± 0.1 °Cb log β101
ligand
M3+
DTTA-DAG
La3+ Ce3+ Pr3+ Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ Am3+ a Nd3+ a La3+
13.98 14.87 15.65 15.98 16.54 16.88 16.70 16.93 16.97 16.70 16.82 16.82 16.62 16.46
Nd3+ Gd3+ Ho3+ Lu3+ Nd3+ a
15.65 16.17 16.26 15.99 15.81
DTTADAGEE
± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
log β111 17.65 18.52 19.29 19.64 20.19 20.50 20.32 20.57 20.59 20.29 20.42 20.39 20.21 19.99
± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
log β121 20.68 21.68 22.34 22.68 23.24 23.55 23.39 23.60 23.63 23.37 23.44 23.41 23.18 23.01 23.79 22.95
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
13.97 ± 0.02 ± ± ± ± ±
0.01 0.01 0.01 0.02 0.01
a Am3+ and Nd3+ stability constants obtained using spectrophotometry, determined at ambient temperature (19 ± 1 °C). bI = 2.00 M (H+/ Na+)ClO4.
E
DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Spectrophotometric titration of Am(ClO4)3 and Nd(ClO4)3 with DTTA-DAG. I = 2.00 ± 0.01 M (Na+/H+)ClO4 and T = 19 ± 1 °C. (a) Titrand: 0.497 mM Nd(ClO4)3 and p[H+] = 2.01. Titrant: CNd3+ = 0.503 mM, CDTTA‑DAG = 4.97 mM, and p[H+] ≈ 5; p[H+]init = 2.01 and p[H+]final = 2.81. (b) Calculated molar absorptivities for Nd3+ and NdH2L(aq) complex. (c) Titrand: 0.775 mM 243Am(ClO4)3 and p[H+] = 1.98. Titrant: CAm3+ = 0.773 mM, CDTTA‑DAG = 17.28 mM, and p[H+] ≈ 5; p[H+]init = 1.98 and p[H+]final = 3.64. (d) Calculated molar absorptivities for Am3+ and AmH2L(aq) complex.
Figure 5. Spectrophotometric titration of Nd(ClO4)3 with DTTA-DAGEE. (Left) Titrand: 0.497 mM Nd(ClO4)3 and p[H+] = 1.98. Titrant: CNd3+ = 0.508 mM, CDTTA‑DAGEE = 4.98 mM, and p[H+] ≈ 5; p[H+]init = 1.98 and p[H+]final = 9.80. I = 2.00 ± 0.01 M (Na+/H+)ClO4 and T = 19 ± 1 °C. (Right) Calculated molar absorptivities for Nd3+ and NdL(aq) complex.
final CDTTA‑DAG/CNd and CDTTA‑DAG/CAm ratios were 9.5 and 11.1, respectively. The measured p[H+] changes were minimal (2.0−2.5 for Nd3+ and 2.0−2.2 for Am3+) until titrations reached the CDTTA‑DAG/CNd/Am ≈ 2.5 ratio, again demonstrating the p[H+] buffering effect due to metal-ion complexation. Analyses of the M3+/DTTA-DAG spectrophotometric titration data in HypSpec27 identified the best-fit model to contain only two absorbing species: free M3+ and MH2L(aq) complex. Attempts to include the MHL− and ML2− species
in Figure 4. In both experiments, the absorbance due to unbound metal (Nd3+ or Am3+) decreased as the concentration of DTTA-DAG increased. As the metal-ion coordination increased, the absorption peaks corresponding to the 4I9/2 → 4 G5/2, 2G7/2 and 7F0′ → 5L6′ transitions for Nd3+ and Am3+ were red-shifted, reestablishing the new peak maxima at λmax = 584.4 and 507.5 nm, respectively. In both titrations, the metal−ligand absorptivity increased as the CDTTA‑DAG/CM ratio approached 2.5. No spectral changes were observed beyond this point. The F
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exponential model to represent the experiment data. The collected information on the europium hydration, incurred from luminescence data, is tabulated in Table 3. The hydration
(observed in the potentiometric analysis) resulted in nonconvergence, and negative molar absorptivities for those metal−ligand complexes were calculated. In a previous contribution,19 a similar observation for diamide−EDTA lanthanide complexes was attributed to matching coordination environments for all three metal complexes. To quantify this conclusion for DTTA-DAG, complexation of Nd3+ by DTTADAGEE was studied using spectrophotometry. The esterified terminal glycine groups of DTTA-DAGEE exclude the formation of protonated metal complexes, MHL− and MH2L(aq), as demonstrated by the potentiometric studies. The absorbance spectra collected for Nd3+ complexation by DTTADAGEE (Figure 5) show a striking resemblance to the absorption features collected during NdIII/DTTA-DAG titration. Analysis of the spectrophotometric titration data identified two absorbing species: free M3+ (λmax = 574.4 nm) and ML(aq) complex (λmax = 584.4 nm). The resolved stability constants determined from the spectrophotometric analysis are tabulated in Table 2. 3.3. Luminescence Lifetimes of EuIII/DTTA-DAG Solutions. Insights into the coordination environment of Eu3+ in the presence of DTTA-DAG were gathered using luminescence lifetime measurements. The luminescence decay data for solutions containing Eu3+ and DTTA-DAG at p[H+] of 5.56, 3.48, 1.93, and 1.01 are shown in Figure 6. For the fully
Table 3. Fluorescence Decay Constants and Waters of Hydration for the Inner Coordination Sphere of Eu3+ in Solutions Containing 5.0 mM DTTA-DAG at Various p[H+] Valuesb % species p[H+]
τ, μs
Kobs, s−1
nH2O(exp) (±0.5)
Eu3+
EuL
5.56 3.48 1.93 1.01 1.01
593.8 587.4 563.5 485.7 108
1684 1702 1775 2059 9259
1.3 1.3 1.4 1.7a 9.3a
0.0 0.0 1.3 97.2 97.2
98.8 28.7 0.1 0.0 0.0
EuHL EuH2L 1.2 39.9 3.4 0.0 0.0
0.0 31.4 95.2 2.8 2.8
a At p[H+] = 1.01, a double-exponential fit was required to resolve the decay constants. b[Eu3+] = 1.2 mM and I = 2.00 M (H+/Na+)ClO4.
number of Eu3+ was calculated using the relationship described by eq 9, developed by Kimura and Choppin.30 3+
Eu NH2O = 1.03 × 10−3kobs − 0.44
(9)
The identified consistency in the coordination environments for solutions containing Eu3+ and DTTA-DAG at p[H+] values of 5.56, 3.48, and 1.93 suggests that the hydration of europium ion is constant for ML2−, MHL−, and MH2L(aq) complexes. A single water molecule in the inner hydration sphere of Eu3+ was inferred from eq 9 (NH2O = 1.3 ± 0.5), indicating octadentate coordination of Eu3+ by DTTA-DAG. 3.4. Liquid−Liquid Distribution Measurements. The influence of p[H+] on the distribution of lanthanides and americium between the organic solution of 0.75 M HEH[EHP] in n-dodecane and an aqueous electrolyte mixture containing 0.02 M DTTA-DAG and 0.5 M malonate was investigated. Figure 7 summarizes the collected metal distribution trends for La3+−Gd3+ (except Pm3+) and Am3+. Minimal pH dependencies of metal distributions were observed across the investigated p[H+] range. The least extractable lanthanides were La3+ (D values of 1−2) below p[H+] = 2.12 and Nd3+ (D
Figure 6. Luminescence lifetime measurements of EuIII/DTTA-DAG solutions at p[H+] = 5.56, 3.48, 1.93, and 1.01. CEu3+ = 1.2 mM, CDTTA‑DAG = 4.9 mM, and I = 2.00 ± 0.01 M (Na+/H+)ClO4. Solid lines represent the fit results of least-squares analysis used to determine lifetime decay constants.
hydrated, free Eu3+ ion relaxation from the 5D0 excited state to the ground-state manifold 7FJ (J = 0, 1, 2, 3, ...) yields luminescence decays that are quenched by partial energy transfer to O−H vibrations of water molecules in the inner coordination shell. The degree of this quenching varies depending on the extent of inner-sphere hydration of the metal ion. If the displacement of water molecules from the inner coordination shell of Eu3+ is assumed to directly translate into the ligand’s participation in metal binding, direct insight into the coordination environment can be inferred. In this study, the Eu3+ magnetic dipole transition (5D0 → 7F1) was monitored at 590 nm. The luminescence decay curves were fit using a single exponential, indicating the presence of one predominant species capable of fluorescence. At the most acidic condition (p[H+] = 1.01), significant curvature in the luminescence decay data was observed, requiring a double-
Figure 7. Liquid−liquid distribution of 0.1 mM La3+−Ho3+ (each) and Am3+ (tracer) between an organic mixture of 0.75 M HEH[EHP] in ndodecane and an aqueous electrolyte phase containing 0.02 M DTTADAG, 0.5 M malonate, and I = 1.0 M adjusted with NaNO3 at p[H+] = 2.99, 2.76, 2.50, 2.36, 2.12, 1.93, and 1.86. G
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Figure 8. (a) Time dependencies for the liquid−liquid distribution of 154Eu3+ collected at various p[H+] values: (org) HDEHP in n-dodecane; (aq) 20 mM complexant. I = 1.0 M adjusted with NaNO3 and T = 20.0 ± 1.0 °C. (◇) 0.062 M HDEHP/20.0 mM DTPA and p[H+] = 2.20; (□) 0.081 M HDEHP/20.0 mM DTPA and p[H+] = 2.58; (○) 0.095 M HDEHP/20.0 mM DTPA and p[H+] = 2.96; (◆) 0.095 M HDEHP/20.4 mM DTTA-DAG and p[H+] = 2.27; (■) 0.105 M HDEHP/20.0 mM DTTA-DAG and p[H+] = 2.60; (●) 0.120 M HDEHP/20.4 mM DTTA-DAG and p[H+] = 3.01. (b) Comparison of time-dependent partitioning of 154Eu3+ for liquid−liquid distribution systems containing HEDTA and DTTADAG. (○) 0.095 M HDEHP/20.4 mM DTTA-DAG and p[H+] = 2.27; (□) 0.065 M HEH[EHP]/20.0 mM HEDTA and p[H+] = 3.00.
values of 1−2) above p[H+] = 2.12. The minimal lanthanum/ americium separation factor was 15 at p[H+] = 1.86, where DAm was 0.11. As the acidity decreased, group separation factors increased to 24 at p[H+] = 2.99, where the lowest distribution of americium of 0.05 was obtained. Time-dependent trends for the forward extraction of europium by HDEHP in n-dodecane from aqueous environments of DTTA-DAG or DTPA are presented in Figure 8. Because at the beginning of an experiment 100% of europium resides in the aqueous mixture, the percent extraction [% E = 100(D + 1)/D] represents a percentage of total Eu 3+ partitioned into the nonaqueous phase. As expected, when an aqueous mixture is equilibrated with the immiscible liquid phase containing HDEHP, the extraction of europium increases as a function of time for all investigated p[H+] conditions. The role of H+ on the rate of extraction was investigated at three different acidities (DTTA-DAG, p[H+] = 2.27, 2.60, and 3.01; DTPA p[H+] = 2.20, 2.58, and 2.96). The metal partitioning rates were significantly enhanced as [H+]free increased. For the least acidic condition (p[H+] = 3.0), the equilibrium for the partitioning of the metal ion (% E ∼ 90) is not attained even after 24 h of mixing, whereas at p[H+] = 2.6, equilibrium was reached after 4 h of contact. Overlapping phase-transfer kinetic trends were observed for DTPA- and DTTA-DAG-containing mixtures for those p[H+] conditions. At the most acidic condition (p[H+] = 2.2), equilibrium was reached in 2 h for the DTTA-DAG system, whereas only 40 min were required for DTPA.
diethylenetriamine-based aminopolycarboxylates have been reported.31−35 The similarities of the stability constants collected for DTTA-DAG and those refined for the complexation of La3+, Nd3+, Gd3+, Ho3+, and Lu3+ by DTTA-DAGEE (ester-protected analogue of DTTA-DAG) support the argument for equivalent metal coordination environments for ML2−, MHL−, and MH2L(aq) complexes of DTTA-DAG. The stability constants are lower for the esterified compound, possibly indicating small enthalpic differences in metal complexation; however, the observed difference is not sufficient to argue that metal coordination is not equivalent for DTTA-DAG and DTTADAGEE. A thermodynamic impact on the complex stability resulting from the participation of a glycine carboxylate group (eight-membered chelate) as opposed to the amidic oxygen atom (five-membered chelate) would be more significant. This conclusion is further supported by the luminescence lifetime measurements. Fluorescence studies of hydrated europium in the presence of DTTA-DAG at varying acidities show no significant changes in the fluorescence decay rates at p[H+] = 5.56, 3.48, and 1.93 (Figure 6). The hydration number of Eu3+, as described by eq 9, represents the average number of all metal species in solution. Therefore, a consistent displacement of eight water molecules by DTTA-DAG throughout the range of acidic conditions supporting the presence of ML2−, MHL−, and MH2L(aq) complexes indicates a constant coordination environment of europium. This particular coordination environment, indicative of the participation of three nitrogen and five oxygen (three carboxylic and two amidic) atoms, has been reported previously.31−35 Under more acidic conditions (pH = 1.01) two metal-ion environments were observed, with NH2O = 9.3 ± 0.5 and NH2O = 1.7 ± 0.5. The first species corresponds to the fully hydrated europium ion at 97% abundance, whereas the second species suggests that a small quantity of Eu3+ (3% abundance) remains significantly dehydrated, presumably because of the presence of DTTADAG. The increased hydration number of the second Eu3+ species (NH2O = 1.7 ± 0.5), relative to a monohydrate characteristic of ML2−, MHL−, and MH2L(aq), may signal a change in the coordination environment. Assuming that a
4. DISCUSSION 4.1. Coordination Modes of DTTA-DAG. The presence of three distinct metal complexes [ML2−, MHL−, and MH2L(aq)] for mixtures containing trivalent f-elements and DTTA-DAG agrees well with our previous report on trivalent felement complexation by a bis(acetylglycine)-functionalized EDTA complexant (EDDAG-DA).19 The f-element coordination environment did not vary between the three EDDAG-DA complexes, indicating that acetylglycine groups utilize amidic oxygen atoms to bind metal ions.19 Similar configurations of felement coordination pockets for diamide-functionalized H
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p[H+] = 1 condition would promote the protonation of a carboxylate or amide site inaccessible to the glass electrode, NH2O ∼ 2 may suggest a contribution from the MH3L+ complex. Tian et al. argued that the hydration number of 1, and the octadentate coordination of Eu3+ by DTPA, does not change when the distribution of metal complexes changes from ML2− to MHL−.36 Sensitivity analyses of luminescence lifetime data and density functional theory calculations revealed that protonation of the carboxylate orients the carbonyl oxygen atom toward the metal ion, maintaining the octadentate coordination sphere.36 Such a structural rearrangement is not manifested in DTTA-DAG measurements at p[H+] = 1, possibly indicating that the carbonyl oxygen atom no longer coordinates the metal because of protonation of the amide. The elevated hydration number may also be associated with the uncertainty from the low abundance of this species. 4.2. Thermodynamic Considerations. The interpretation of the potentiometric data collected for DTTA-DAG (Figure 2, left) yielded six distinct protonation species, arguably corresponding to three amines of highest basicity (HL4−, H2L3−, and H3L2−) and three carboxylic acids of lowest acidity [H4L−, H5L(aq), and H6L+]. The acid dissociation constants of the two remaining carboxylic acid groups are too low for accurate quantification using a glass electrode. Similarly, two amide groups are too basic to allow potentiometric determination. Table 1 lists the determined acid dissociation constants. For comparison, the acid dissociation constants for DTPA, determined by Grimes and Nash in equivalent electrolyte media,37 are also displayed in Table 1. As expected from the strong electron-withdrawing influence of amide functionalization, a significant reduction of the total ligand basicity for DTTA-DAG, relative to DTPA, is evident. The electronic effects reduce the amine total basicities by ∼4 log units (∑pKa,amines = 18.27 for DTTA-DAG and ∑pKa,amines = 22.19 for DTPA).20 Similar impacts on the acid dissociation constants have been observed for other diamide-functionalized DTPA reagents, although Rizkalla et al., Imura et al., and Wang et al. report even greater inductive influences on pKa,3 values.32,34,38 The stability constants for the complexation of trivalent felements by DTTA-DAG are also strongly impacted by the replacement of two conventional acetate functionalities of the DTPA reagent with acetate glycine amide groups. A consistent reduction of the complex formation strength across the lanthanide group is observed for DTTA-DAG, relative to DTPA. The log β101 values for DTPA range from 20.4 to 22.2 across the lanthanide series, whereas those for DTTA-DAG range from 14.0 (La3+) to 16.5 (Lu3+). This significant decrease may be attributed to the lack of entropic stabilization resulting from f-element coordination by the amidic oxygen atoms of diamide alternatives of DTPA.33 Wang et al. discussed this decrease in the complex stability in terms of the overall weakening of the Lewis base donor capacity of the amide groups, coupled with the lower basicity of the terminal amines.38 The overall 106-fold decrease in the stability of the complexes observed here for DTTA-DAG throughout the lanthanide series has been consistently observed for different diamide-substituted reagents.32,33,38−40 The coordination of trivalent f elements by both DTTADAG and DTTA-DAGEE is predominantly driven by electrostatic attraction as expected from complexing reagents from the aminopolycarboxylate family.8 For both reagents, the total
ligand basicity (∑pKa) directly predicts the relative thermodynamic strength of trivalent f-element binding (log β101), as demonstrated by Figure 9. This linear relationship tracks
Figure 9. Linear Gibbs energy correlation of the stability constants for 1:1 NdIII/aminopolycarboxylate complexes with the ligand basicity, ∑pKa. Data for NTA, HEDTA, EDTA, CDTA, and DTPA are from Martell and Smith.20 Data for EDDAG-DA are from Heathman et al.19
particularly well for the subset of aminopolycarboxylic acids capable of forming five-membered chelate rings upon metal-ion coordination. Figure 9 shows the correlation for the binding of Nd3+ by NTA, HEDTA, EDTA, CDTA, and DTPA, together with reagents studied in our previous19 (EDDAG-DA) and current work (DTTA-DAG and DTTA-DAGEE). It is important to note that calculation of the total ligand basicity for DTTA-DAG excludes the dissociation constants for carboxylic acids located on two glycine substituents, which do not partake in metal-ion coordination. The close proximity of the esterified analogue, DTTA-DAGEE, on the linear free energy plot supports this approach. The positioning of DTTADAG and DTTA-DAGEE on this linear Gibbs energy correlation reiterates a significant impact of amide functionalization on the reagent’s ability to complex metal ions, relative to DTPA. Both reagents match the characteristics of EDTA and HEDTA more closely, suggesting that induction effects have weakened the Lewis base donor groups and reduced the electrostatic nature of metal-ion binding by these compounds. The observed trend of the stability constants for lanthanide complexation by DTTA-DAG resembles that reported for DTPA, as listed in Table 2.20,37 The complex stabilities increase from La3+ to Sm3+ (due to increased electrostatic interactions) and plateau beyond samarium due to enhanced entropic compensation of reduced enthalpic energetics resulting from the contraction of heavier lanthanides and changes in hydration.41 The relative ease of dissociation of light lanthanide complexes by DTPA-like reagents produces a La3+ > Ce3+ > Pr3+ > Nd3+ trend of metal partitioning when equilibrated with a strong liquid cation exchanger in a liquid−liquid distribution system.12,13 This trend is followed by the return to a steadily increasing trend of extraction from Nd3+ to Lu3+. Efficient differentiation of trivalent actinides from trivalent lanthanides occurs when such a lanthanide extraction pattern is coupled with the enhanced actinide complexation by reagents like DTPA and DTTA-DAG. 4.3. AnIII/LnIII Separation Using DTTA-DAG. The liquid− liquid distribution results summarized in Figure 7 for trivalent lanthanides and americium demonstrate efficient actinide/ I
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lanthanide separation is maintained at all investigated p[H+] conditions. The afforded separation between Nd3+ and Am3+ (SF = 24) for the p[H+] = 3.0 environment is lower than that observed using an original TALSPEAK formulation (SF = 34) obtained with 50 mM DTPA at p[H+] = 2.9.42 A diminished actinide/lanthanide differentiation may be attributed to the electron-inducing influence of the amide functionalization on the nitrogen donors of the DTTA-DAG diethylenetriamine backbone. This electronic impact may also be predicted by the log β121 values for the complexation of Nd3+ and Am3+ by DTTA-DAG. The relative difference of two stability constants (Δ log β121 = 0.84) is smaller relative to DTPA (Δ log β101 = 1.3).20 One important advantage of using DTTA-DAG as an aqueous holdback reagent is the influence of amide substituents on the reagent’s acid/base equilibria. As predicted by the Gibbs free linear relationship shown in Figure 9, the decrease in the total ligand basicity for DTTA-DAG produces a decrease in the trivalent f-element complex stability relative to DTPA. This reduction of the stability for DTTA-DAG complexes affords an excellent thermodynamic balance when paired with a phosphonate-type phase-transfer reagent such as HEH[EHP]. A nearly constant actinide/total lanthanide separation shown in Figure 7 gives evidence to an effective partnership between DTTA-DAG and HEH[EHP] throughout the investigated range of acidity. The thermodynamic balance is maintained in conditions of higher acidity, i.e. lower pH, relative to the typical operation range for aminopolycarbxylate-type complexants. At p[H+] = 1.86, the observed distribution ratio for americium (DAm = 0.11) compares favorably to that reported by Braley et al. (DAm = 0.28 at p[H+] = 1.8), who used 20 mM DTPA as an aqueous holdback complexant.15 The original TALSPEAK report by Weaver and Kappelmann also showed higher partitioning of americium (DAm = 0.19 at pH = 2.1 for 25 mM DTPA present in 1 M citrate).12 Grimes et al. reported DAm = 0.39 at p[H+] = 2.0 when using 50 mM DTPA in 1 M Lalanine buffer.43 The DTTA-DAG/HEH[EHP] combination compares favorably with a recent report by Lumetta et al., where HEH[EHP] is equilibrated with aqueous solutions containing a HEDTA complexant (DAm = 0.65 at p[H+] = 2.6 for 0.11 M HEDTA present in 0.2 M citrate).16 Lapka and Nash demonstrated the benefit of using a malonate buffer in aqueous mixtures containing HEDTA.17 Compared to citratebuffered systems, where the distribution of americium increases with the increasing acidity of the aqueous environment, the authors demonstrated that Am3+ holdback is enhanced in the 1 < p[H+] < 2 region.17 At p[H+] = 2.0, when using 20 mM HEDTA in 0.5 M malonate, the authors matched the DAm value reported in this study when using DTTA-DAG. However, the challenge of using HEDTA (a complexant of lower denticity relative to DTPA-like structures) in actinide/lanthanide differentiation is emphasized by low extraction of light lanthanides (DLa = 0.1 reported by Lapka and Nash).17 The DTTA-DAG aqueous holdback complexant facilitates efficient retention of Am3+ in the aqueous environment, coupled with preferential reporting of all Ln3+ to the nonaqueous phase. The distribution results presented in Figure 7 demonstrate that actinide/lanthanide differentiation may be obtained using DTTA-DAG in p[H+] = 2.0 conditions, without the risk of substantial extraction of Am3+ or inefficient extraction of light lanthanides. A transition to higher acidity, where a higher concentration of hydrogen ions is available to facilitate the acid-catalyzed mechanism of metal complex
dissociation, significantly enhances the phase-transfer kinetics, as demonstrated in the next segment. 4.4. Kinetics of EuIII Extraction by DTTA-DAG. The collected Eu3+ distribution patterns presented in Figure 8a demonstrate a tremendous influence of hydrogen ions on the rates of metal extraction from aqueous mixtures containing DTTA-DAG and DTPA. The enhancement in the rate of Eu3+ extraction follows the expected linear dependence between the rate of complex decomposition and the concentration of hydrogen ions in solution.22 While minimal differences between the kinetic trends for DTPA and DTTA-DAG are observed at p[H+] = 3.0 and 2.6, the faster rate of extraction observed for DTPA in p[H+] = 2.2 suggests a small enhancement of the acid-catalyzed dissociation of DTPA complexes, relative to DTTA-DAG (differences may partially be attributed to small differences in the experimental p[H+]). Jászberényi et al. observed a similar pH dependence for the water proton relaxivities of trivalent gadolinium complexes of amidefunctionalized DTPA.44 The authors also showed that the rate constant for the kinetic stability of a diamide-subsituted Gd3+ complex, incurred on the basis of metal-exchange reaction studies, is slower relative to the [Gd(DTPA)]2− complex.44 The water exchange rate reported by Powell et al.46 for the [Gd(DTPA-BMA)] complex [bis(methylamide)-DTPA] is 7 times slower than that reported for the [Gd(DTPA)]2− complex. This has been attributed to the steric strain imposed on the amidic oxygen coordination by bulky amide groups, which, in turn, decreases the steric strain of bound water within the metal complex.45 As discussed above, the DTTA-DAG reagent extends the efficient actinide/lanthanide differentiation into regions of increased acidity (p[H+] = 2.0). Accordingly, the observed H+-catalyzed enhancement in the rate of Eu3+ extraction is of great importance here. The observed thermodynamic enhancements for a liquid−liquid distribution system containing DTTA-DAG, relative to those containing DTPA and HEDTA reagents, renders the consideration of possible reagent substitution. To address this option on a kinetic scale, Figure 8b compares the time-dependent Eu3+ partitioning trends for liquid−liquid systems containing DTTA-DAG and HEDTA. For the DTTA-DAG reagent, the extraction trend collected for the p[H+] = 2.3 condition matches the phase-transfer kinetic results obtained for the liquid−liquid formula containing 20 mM HEDTA adjusted to p[H+] = 3.0. The nearly overlapping trends presented in Figure 8b indicate that improved complete differentiation of trivalent actinides from trivalent lanthanides may be afforded when diamide functionalization of an octadentate metal complexant is performed, at no apparent phase-transfer kinetic inhibition due to its denticity.
5. CONCLUSION Two new holdback complexants (DTTA-DAG and DTTADAGEE) containing DTPA-like coordination environments were synthesized and thoroughly characterized. Potentiometric studies in the presence of lanthanide metals with DTTA-DAG identified three distinct metal−ligand complexes [identified as ML2−, MHL−, and MH2L(aq) species] and one complex [the ML(aq) species] for DTTA-DAGEE. Fluorescence decay studies of DTTA-DAG solutions in the presence of Eu3+ revealed identical decay lifetimes from pH 1.9 to 5, indicative of the protonated complex not influencing the coordination environment of the three complexes. Decay constants identify one water molecule in the inner coordination sphere across the J
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pH range studied, resulting from the displacement of eight water molecules by octadentate coordination of DTTA-DAG. Similarities of the log β101 values of DTTA-DAG and DTTADAGEE reveal little to no differences of the complex stability, suggesting that acetyl−glycine carboxylates (adjacent amides) of DTTA-DAG do not participate in the coordination of metal ion. Stability constants indicate favorable coordination of trivalent f-elements, with reduced complex strength (relative to DTPA), facilitating coupling to the weaker phosphonic acid extractant HEH[EHP] (over HDEHP). Solvent extraction studies revealed flat pH profiles for the liquid−liquid distribution of trivalent metal ions and very efficient retention of americium in an aqueous environment throughout the investigated p[H+] range of 1.86−2.99. The demonstrated operational range of acidity for actinide/lanthanide separation supported by DTTA-DAG compares favorably to those offered by the reported TALSPEAK/advanced TALSPEAK and ALSEP process conditions. Kinetic studies of metal partitioning show further advantages of DTTA-DAG-containing chemistry because efficient actinide/lanthanide differentiation at higher acid concentrations supports the acid-catalyzed dissociation of a metal/aminopolycarboxylate complex.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02158.
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Syntheses of DTTA-DAG and DTTA-DAGEE (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS All experimental work was conducted at the Idaho National Laboratory and supported by the U.S. Department of Energy, Office of Nuclear Energy, DOE Idaho Operations Office, under Contract DE-AC07-05ID14517.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.6b02158 Inorg. Chem. XXXX, XXX, XXX−XXX
