Article pubs.acs.org/IC
Thermodynamic and Spectroscopic Studies of Trivalent f‑element Complexation with Ethylenediamine-N,N′‑di(acetylglycine)N,N′‑diacetic 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: The coordination behavior and thermodynamic features of complexation of trivalent lanthanides and americium by ethylenediamine-N,N′-di(acetylglycine)-N,N′diacetic acid (EDDAG-DA) (bisamide-substituted-EDTA) were investigated by potentiometric and spectroscopic techniques. Acid dissociation constants (Ka) and complexation constants (β) of lanthanides (except Pm) were determined by potentiometric analysis. Absorption spectroscopy was used to determine stability constants for the binding of trivalent americium and neodymium by EDDAG-DA under similar conditions. The potentiometry revealed 5 discernible protonation constants and 3 distinct metal−ligand complexes (identified as ML−, MHL, and MH2L+). Time-resolved fluorescence studies of Eu-(EDDAG-DA) solutions (at varying pH) identified a constant inner-sphere hydration number of 3, suggesting that glycine functionalities contained in the amide pendant arms are not involved in metal complexation and are protonated under more acidic conditions. The thermodynamic studies identified that f-element coordination by EDDAG-DA is similar to that observed for ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA). However, coordination via two amidic oxygens of EDDAG-DA lowers its trivalent f-element complex stability by roughly 3 orders of magnitude relative to EDTA.
1. INTRODUCTION Neutron capture reactions inside the fissioning core of nuclear reactors produce significant quantities of neptunium, plutonium, americium, and curium.1,2 When used nuclear fuel, UNF, is removed from the reactor’s core; its safe management is of utmost importance due to the radiological hazards associated with those transuranic elements.3 While the radiotoxicity poses environmental concerns, thermal heating of nuclear fuel assemblies4 (due to the energy of radioactive decay deposited in matter) imposes limits on geological storage capacity. The management of heat load inside a repository to be licensed for 10 000 years of uneventful operation remains problematic.4 The spatial capacity constraints, coupled with large quantities of UNF in need of management (currently stored in dry storage casks), impose unrealistic demands on the overall size and/or number of geological destinations.3,5 If the transuranics are separated from UNF safe management becomes more predictable.1,3 The argument for the recovery of transuranic elements from used nuclear fuel becomes more enticing when considering further utilization for energy production (U/Pu) and transmutation (Np/Am) as a means of final management. Significant advances in f-element solution chemistry have been made in the past decades, yielding viable partitioning options.1,3,6 Current research efforts are focused on the efficient differentiation of trivalent minor actinides (An) from trivalent lanthanides (Ln). This separation must be accomplished if An © XXXX American Chemical Society
elements are to be transmuted since Ln metals are neutron poisons. Due to the chemical similarities of trivalent actinides and lanthanides (ionic radii, charge density, hydration, oxidation state), mutual separation remains one of the most challenging tasks in the field of separation science.7,8 Perhaps the most noteworthy methodology developed to differentiate trivalent actinides from trivalent lanthanides is a balanced thermodynamic equilibrium established by the TALSPEAK liquid−liquid distribution process.9,10 This system of two immiscible phases relies on the aqueous complexation chemistry of an aminopolycarboxylate reagent, diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA), to selectively keep the trivalent actinides in an aqueous mixture, while the partitioning of lanthanides into the nonaqueous environment is facilitated by a liquid cation exchanger, di(2-ethylhexyl) phosphoric acid (HDEHP).9,10 The main disadvantages of TALSPEAK chemistry are high operational pH and significant pH dependence.11,12 Efficient An/Ln separation may be accomplished when pH is maintained within the 2−4 range, necessitating the use of buffers to prevent pH migration. Furthermore, this liquid−liquid distribution platform is slow to reach phase transfer equilibrium. Such conditional obstacles have motivated research efforts toward an improved version of Received: December 10, 2015
A
DOI: 10.1021/acs.inorgchem.5b02865 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Synthetic scheme for the preparation of ethylenediamine-N,N′-di(acetylglycine)-N,N′-diacetic acid (EDDAG-DA) (Na4L form).
increased physiological toxicity, presumably due to enhanced rate of Gd3+ exchange by calcium ions inside the body. Accordingly, the alternative contrast agent dissociated more readily, relative to Gd3+−DTPA complex. This finding suggests amidated aminopolycarboxylates may offer an option to enhance the kinetic lability for efficient differentiation of trivalent actinides from trivalent lanthanides without sacrificing the critical thermodynamic driving forces necessary for efficient separations. This work summarizes the thermodynamic and spectroscopic studies for one such alternative structure, ethylenediamine-N,N′-di(acetylglycine)-N,N′-diacetic acid, (EDDAG-DA), as an initial amidated analog of conventional aminopolycarboxylate structures.
the original TALSPEAK recipe. The substitution of HDEHP with a weaker phosphonic acid-based cation exchanger affords flat pH dependencies when DTPA is substituted by a weaker aqueous complexant N-(2-hydroxyethyl)ethylenediamineN,N′,N′-triacetic acid (HEDTA).13 Some recent studies have chosen a reverse approach. An initial complete partitioning of trivalent actinides and trivalent lanthanides into a nonaqueous environment is followed by selective aqueous complexation of minor actinides using DTPA.14−18 Despite improvements to the original formulation the kinetic limitation remains due to slow dissociation of metal complexes formed in aqueous mixtures containing aminopolycarboxylates. Choppin showed that kinetic inertness of the complexes formed between f-elements and aminopolycarboxylates increases with the number of chelate rings and bulk steric effects due to the energetic cost of complex restructuring.19,20 Danesi and Cianetti studied the interfacial mass transfer of Eu3+ in a liquid−liquid system containing a buffered aqueous solution of aminopolycarboxylate and a nonaqueous mixture of HDEHP in n-dodecane.21,22 For such mixtures the authors suggested the rates of transfer may be enhanced when using a weaker (relative to DTPA) aqueous complexant.21,22 A less saturated coordination sphere of trivalent f-element invites interactions with secondary complexants, i.e., the formation of mixed complexes. Danesi and Cianetti demonstrated that ternary complexation increases the lability of the metal complex, which dissociates more readily upon contact with HDEHP at the liquid−liquid interface. Admittedly, the benefit of using a weaker aminopolycarboxylate to aid the kinetic limitations is handicapped by the thermodynamic challenge of An/Ln differentiation. Weaver and Kappelmann, in their original work on TALSPEAK, demonstrated the separation efficiency between Am(III) and lanthanides is reduced when using weaker chelators.10 Such structures form fewer number of 5-membered rings upon metal coordination. Inherently, the binding pockets contain fewer nitrogen atoms, which are the donor atoms responsible for soft interactions with actinide ions. Therefore, to develop improved aqueous holdback reagents for An(III)/Ln(III) differentiation the lessons from Choppin, Danesi, Cianetti, Weaver, and Kappelman must be combined. The search for a more labile aminopolycarboxylate structure must be limited to subtle structural modifications (relative to DTPA) to enhance the complex dissociation kinetics without drastic perturbation of the An/Ln differentiation. Periasamy et al. replaced two carboxylate pendant arms of DTPA with amide functionality.23 The study sought effective structural alternatives for DTPA as MRI contrast agents. The authors demonstrated that mono-substituted amides of DTPA
2. EXPERIMENTAL SECTION 2.1. Chemicals and Reagents. Ethylenediamine-N,N′-di(acetylglycine)-N,N′-diacetic acid was synthesized by reacting 1 equiv of ethylenediamine-N,N,N′,N′-tetraacetic dianhydride with 2 equiv of glycine ethyl ester and 4 equiv of triethylamine followed by deprotection of the ethyl esters with sodium hydroxide (Figure 1). The product was isolated and purified by recrystallization from methanol to give EDDAG-DA as the tetrasodium salt (Na4L). Spectroscopic analysis of the product (1H, 13C NMR) matches that reported by Motekaitis24 (see Supporting Information for synthetic description). Potentiometric analysis of the pKa values supported the spectroscopic analysis and confirmed the lack of impurities. Lanthanide (Ln) stock solutions were prepared by dissolving solid metal oxides (>99.99% purity) in 98% mole equivalence triflic acid (trifluoromethanesulfonic acid, HOTf). Solutions were filtered to remove undissolved particulates and acidity adjusted to ∼pH 3 with triflic acid. The cerium stock solution was prepared by dissolving the oxide (CeO2) in dilute nitric acid followed by precipitation with sodium carbonate. The solid was washed with 18 MΩ water and dissolved in dilute perchloric acid (HClO4). The solution was degassed to remove any dissolved CO2, and the final pH was adjusted to 3. Metal concentrations were determined by complexometric end point titrations with tetrasodium ethylenediamine-N,N,N′,N′-tetraacetic acid (Na4EDTA) and xylenol orange (indicator) with a DP5phototrode (Mettler Toledo) attached to a Mettler Toledo T 90 graphix autotitrator. The americium-243 stock solution was prepared by loading the metal ion onto a DGA extraction chromatographic resin (Eichrom) using ∼6 M HNO3 as the initial eluent. The column was conditioned with 8 M HCl to flush impurities, and 243Am3+ was eluted with dilute 0.1 M HCl. The matrix was exchanged to dilute perchloric acid (pH ≈ 4) via metastasis. Americium concentration was established by complexometric end point titrations with Na4EDTA, analogous to the Ln stocks. Gamma analysis of the 74.66 keV peak for diluted mixtures of Am-243 (on a ORTEC GEM50P4 coaxial HPGe detector equipped with DSPEC gamma spectrometer) was in good agreement with the end point titration ([Am3+]stock = 0.0223 M). Radiotracer B
DOI: 10.1021/acs.inorgchem.5b02865 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry solutions of 241Am3+, 139Ce3+, and 154Eu3+ were purchased from Eckert and Ziegler. The working tracer solutions 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 radioactive materials. EXTREME CAUTION should be taken when handling radiological materials. Sodium perchlorate (NaClO4) and sodium nitrate (NaNO3) were both purchased from GFS Chemicals and purified by recrystallization from hot water. The NaClO4 and NaNO3 salts were dissolved in HPLC-grade water. The concentrated solutions of NaClO4 and NaNO3 were initially filtered through a glass fine frit filter to remove undissolved particulates and recrystallized. Resulting crystals were dissolved in minimal amounts of 18 MΩ H2O and standardized by ion exchange (Dowex 50 × 8, H+ form). The column eluent was titrated with standardized NaOH (triplicate analysis). All working solutions were prepared in 18 MΩ H2O at an ionic strength of 2.00 ± 0.01 M. 2.2. Potentiometry. Potentiometric titrations were carried out using a Mettler Toledo T-90 graphix autotitrator equipped with a Ross Orion semimicro glass electrode (5 M NaCl filling solution). The electrode was calibrated to an operational p[H+] scale (p[H+] = −log [H+], where [H+] is the molar hydrogen ion concentration in solution) by titration of a strong acid with a strong base at 2.0 M ionic strength. Gran analysis25 was performed to check for carbonate contamination and develop an electrode response equation. All titrations were performed at 25.0 ± 0.1 °C (maintained with a circulating water bath attached to a jacketed beaker). Sodium perchlorate was used to control ionic strength. Hydrated nitrogen gas was blanketed over the titration solutions to prevent CO2 absorption. Prior to potentiometric analyses HClO4 was added to convert EDDAG-DA from its sodium salt to the protonated form. A small excess of HClO4 was added to suppress the dissociation of the H5EDDAG-DA+ ion and to allow the quantification of the five independent protonation steps. Potentiometric titrations of standardized EDDAG-DA solutions with standardized NaOH were performed in triplicate and analyzed using Hyperquad 2013 software26 to resolve the acid dissociation constants (Ka) and lanthanide binding stability constants (βMHL). 2.3. Spectrophotometric Titrations. Absorbance spectra of 243 Am3+ solutions were collected in the wavelength range of 400−600 nm (0.65 nm intervals) using a Flame-S-Vis-NIR-ES Ocean Optics device coupled with an Ocean Optics DH-2000-BAL light source and 2 m (200 um) UV−vis−NIR fiber optic cables. Spectra were collected in a 1 cm quartz cuvette with an extended neck screw cap at ambient temperature (19 ± 1 °C). The titrand solution contained [H+] = 0.01 M, [Am3+] = 0.712 mM, and I = 2.00 M (Na+,H+)ClO4 and was titrated with [EDDAG-DA] = 0.0205 M, [Am3+] = 0.712 mM, [H+]Total = 0.0234 M, and I = 2.00 M (Na+,H+)ClO4. Titrant additions were mixed thoroughly for 2 min prior to spectra collection. A reference cell containing 2.0 M NaClO4 was used to zero the instrument prior to each spectra collection. The p[H+] was measured for each corresponding solution condition. Initial p[H+] of the cell solution was 2.03 and increased throughout the titration ending at a p[H+] of 4.08. The metal−ligand stability constant (β121) was resolved by nonlinear least-squares regression analysis using the HypSpec program.27 Neodymium absorbance spectra were collected in the wavelength range of 560−600 nm (0.768 nm intervals) on a QEPro High Performance Spectrometer Ocean Optics Device coupled to an Ocean Optics DH-2000-BAL light source with 2 m (200 μm) UV−vis−NIR fiber optic cables. Spectra were collected in a 100 cm waveguide capillary flow cell from World Precision Instruments Inc. (WPII) at ambient temperature (20 ± 1 °C). Solutions were prepared to 2.5 mL volumes by mixing varying volumes of solution A (0.798 mM Nd(ClO4)3, 0.011 M HClO4, I = 2.00 M NaClO4) and solution B (11.47 mM EDDAG-DA, 0.082 mM Nd(ClO4)3, pH ≈ 4.5, I = 2.00 M NaClO4). Blank solutions containing no metal and appropriate ligand concentrations were used to baseline the instrument prior to addition of new solutions. The p[H+] of individual solutions were measured throughout.
2.4. Luminescence Spectroscopy. The emission spectra and lifetime measurements of Eu3+−EDDAG-DA solutions were collected in a Jobin Yvon IBH FluoroLog-3 fluorometer (HORIBA) adapted for time-resolved measurements. Spectra were collected in a 1 cm quartz cuvette at 25.0 ± 0.1 °C. Solutions were prepared to contain 10 mM EDDAG-DA and 1.0 mM Eu(OTf)3 at an ionic strength of 2.0 M adjusted with (Na+,H+)ClO4 at p[H+] = 2.10, 3.25, and 5.00. Lifetime data collected was analyzed using a single-exponential fit. The most acidic condition was analyzed with a double-exponential expression. 2.5. Liquid−Liquid Distribution Measurements. The extractant (2-ethylhexyl)phosphonic acid mono-2-ethylhexyl ester HEH[EHP] was purchased from Yick-Vic Pharmaceuticals Ltd. (Hong Kong) and purified via third phase formation as reported by Hu et al.28 Dodecane and malonic acid were obtained from Sigma-Aldrich (≥99%) and used as received. Liquid−liquid distribution measurements with lanthanide metal ions were performed with 0.1 M HEH[EHP] dissolved in n-dodecane, and EDDAG-DA (Na4L form) dissolved in 0.5 M malonate buffer at pH 3. The ionic strength was adjusted to 2.0 M using NaNO3. Organic phases were pre-equilibrated with pH 3 2.0 M NaNO3, prior to contact with the aqueous phase containing EDDAG-DA, malonate buffer, NaNO3 (electrolyte), and 1 mM total lanthanides. Samples were contacted under vigorous shaking for 30 min, centrifuged for 5 min, and sampled. The distribution experiments were conducted using equal volumes (0.5 mL) of heavy and light phases, with 0.3 mL of each phase sampled for radiometric counting. Radiometric distribution values were calculated as the ratio of activity (A) in the organic and aqueous phases (D = AOrg/AAq). Lanthanide concentrations were determined by ICP-MS before and after contact with organic phases; 10 and 100 μL samples were diluted to 10 mL in 2% HNO3, respectively. The ICP-MS distribution values were calculated by D = ([M]Aq,Init − [M]Aq,eq)/([M]Aq,eq). All experiments were performed in triplicate.
3. RESULTS 3.1. Stability Constants of Ln(III)/EDDAG-DA and Am(III)/EDDAG-DA. The stability constants for the complexation of trivalent lanthanides by EDDAG-DA were determined with potentiometry. Spectrophotometric titrations were performed to study the binding of trivalent americium and trivalent neodymium by EDDAG-DA. Ln(III)/EDDAG-DA Complexation. Figure 2 shows the representative potentiometric titration for H5EDDAG-DA
Figure 2. Potentiometric investigation of EDDAG-DA protonation, T = 25.00 ± 0.01 °C, I = 2.00 ± 0.01 M (Na+/H+)ClO4−: (○) Experimental; (red solid line) calculated p[H+]. Titrand: Vinit = 24.993 mL, CEDDAG‑DA = 4.99 mM, CH+Total = 0.0394 M. Titrant: 0.2494 M NaOH, 1.75 M NaClO4. For speciation, (black dash line) H5L+; (red dash line) H4L; (green dash line) H3L−; (blue dash line) H2L2−; (magenta dash line) HL3−; (purple dash line) L4−. Negative values of CH/CEDDAG‑DA represent the region where hydroxide is in excess (−CH/CEDDAG‑DA = COH/CEDDAG‑DA). C
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Inorganic Chemistry proton dissociation, together with the progressive changes in the speciation of EDDAG-DA throughout the course of titration. The corresponding potentiometric study for Nd(III)/EDDAG-DA complex formation is shown in Figure 3.
Table 2. Thermodynamic Data of the Complexation of Ln(III) and Am(III) with EDDAG-DA at 25.0 ± 0.1 °Ca M3+ 3+
Y La3+ Ce3+ Pr3+ Nd3+ Sm3+ Eu3+ Gd3+ Tb3+ Dy3+ Ho3+ Er3+ Tm3+ Yb3+ Lu3+ Am3+ b Nd3+ b
Figure 3. Potentiometric investigation of Nd(III)/EDDAG-DA complexation, T = 25.00 ± 0.01 °C, I = 2.00 ± 0.01 M (Na+/ H+)ClO4−: (○) Experimental; (red solid line) calculated p[H+]. Titrand: Vinit = 26.990 mL, CEDDAG‑DA = 4.62 mM, CH+Total = 0.0364 M, CNd3+ = 4.45 mM. Titrant: 0.2494 M NaOH, 1.75 M NaClO4. For speciation, (black dash line) Nd3+; (red dash line) NdH2L+; (green dash line) NdHL; (blue dash line) NdL−.
n 7.31 4.32 3.75 3.05 1.81
± ± ± ± ±
0.01 0.01 0.01 0.01 0.01
14.10 13.76 14.32 13.97 14.34 14.50 15.32 15.24 15.09 15.09 15.14 15.38 15.29 16.03 15.24
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
log β121
0.02 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02
17.04 16.77 17.29 16.94 17.23 17.47 18.30 18.19 18.10 18.07 18.07 18.28 18.21 18.92 18.19 18.60 17.81
± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02 0.01 0.02
tert-butyl-substituted EDTA compounds, also reporting ML and ML2 complexes for gadolinium.30 The reported stability constants by those two research groups substantially differ (log β101 of 10.3 and 12.79 for the propyl and isopropylsubstituted EDTA amides, respectively). Attempts to resolve the ML and ML2 species were not successful for Gd(III)/ EDDAG-DA potentiometric titration data. [H+]·[Hn − 1L(n − 5)]
HnL(n − 4) → Hn − 1L(n − 5) + H+ K a n =
[HnL(n − 4)] (1)
M3 + + L4 − → ML−
−
β101 =
M3 + + H+ + L4 − → MHL
[ML ] [L4 −]·[M3 +] β111 =
(2)
[MHL] [L ]·[M3 +]·[H+] 4−
(3) +
M3 + + 2H+ + L4 − → MH 2L+
β121 =
[MH 2L ] [L4 −]·[M3 +]·[H+]2
(4)
Am(III) and Nd(III)/EDDAG-DA Complexation. The absorbance spectra collected for the spectrophotometric titration of Nd(III) and Am(III) with EDDAG-DA are shown in Figure 4a and 4c, respectively. The intensity of the absorption band at λmax = 502.8 nm, corresponding to the 7F0′ → 5L6′ transition of fully hydrated Am(III) ion, decreased as the concentration of EDDAG-DA was increased. The absorption at λmax = 574.4 nm (4I9/2 → 4G5/2,2G7/2) for Nd(III) aquo ion also decreases upon EDDAG-DA addition. The absorbance bands for Am(III) and Nd(III) were red shifted due to complexation by EDDAG-DA. New peak maxima for Am(III) and Nd(III) appeared at 506.1 and 579.0 nm, respectively. For both metal ions the intensities of those absorption bands increased until CL/CM ≈ 2.5 was obtained. No further spectral changes were observed as the concentration of EDDAG-DA was increased to a CL/CM of 14.33 for Am3+ and 11.8 for Nd3+ titrations. The p[H+] monitored throughout the titrations increased from 2.0 to 3.7
−log10 Kan 1 2 3 4 5 6
log β111
0.02 0.01 0.01 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.01 0.01 0.01 0.01 0.02
I = 2.00 M (H+,Na+)ClO4. bStability constants derived from spectrophotometric titrations performed at 19 ± 1 °C.
Table 1. Acid Dissociation Constants Determined for EDDAG-DA at 25.0 ± 0.1 °Ca species
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
a
Theoretical representations of the titration curves, calculated by Hyperquad, are shown by solid red lines. The nonlinear leastsquares regression analysis yielded best results when five distinct protonated species (HnL) were considered. Two dissociation equilibria of highest basicity agree well with the values reported by Motekaitis et al.24 The authors also report two carboxylate protonation constants. The reported values, however, are lower relative to those reported here, presumably due to an unresolved H5L+ species. The protonation equilibria are generalized by eq 1. Various models including a variety of M3+/HnL complexes were tested, and the best fit for the metal−ligand titrations was achieved with a model that includes successive formation of the mononuclear complexes, i.e., ML−, MHL, and MH2L+, as represented by eqs 2−4. Data analysis of the metal−ligand titrations was limited to below p[H+] = 5.5 due to onset of metal hydrolysis at higher pH. Acid dissociation constants for EDDAG-DA are listed in Table 1 and stability constants for metal ion complexation by EDDAG-DA in Table 2. PlatasIglesias et al. reported the formation of ML and ML2 complexes when evaluating the complexation of gadolinium by EDTAbispropylamide.29 Wang et al. studied bis-isopropyl- and bis-
HL3− H2L2− H3L− H4L(aq) H5L+ H6L2+
log β101 10.54 10.18 10.73 10.33 10.66 10.89 11.79 11.64 11.52 11.57 11.59 11.87 11.85 12.54 11.85
7.37b 4.38b 3.51b 2.87b
a
I = 2.00 M (H+,Na+)ClO4, L represents EDDAG-DA. bReported by Motekaitis et al.24 in I = 0.1 M KNO3 D
DOI: 10.1021/acs.inorgchem.5b02865 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 4. Spectrophotometric titration of M(ClO4)3 with EDDAG-DA. (a) Titrand: 0.798 mM Nd(ClO4)3, pH = 1.97. Titrant: CNd3+ = 0.082 mM, CL = 11.47 mM; pH ≈ 4.5. Ionic strength adjusted to 2.00 M with NaClO4; ambient temperature (20 ± 1 °C); p[H+] measured at each condition, p[H+]Init = 1.97, p[H+]Final = 3.66. (b) Calculated molar absorptivity of Nd3+ and NdH2L+ complexes. (c) Titrand: 0.712 mM 243Am(ClO4)3 at pH = 2.034. Titrant: CAm3+ = 0.712 mM, CL = 20.44 mM; pH ≈ 4.8. Ionic strength adjusted to 2.00 M with NaClO4; ambient temperature (19 ± 1 °C); p[H+] measured at each condition in a duplicate experiment, p[H+]Init = 2.01, p[H+]Final = 4.08. (d) Calculated molar absorptivity of Am3+ and AmH2L+ complex.
around the metal. The relaxation of the 5D0 → 7F1 magnetic dipole transition state was monitored at 590 nm. Luminescence lifetime measurements of Eu(III) in perchlorate media (τ = 108.2 μs)31 indicate that perchlorate does not disrupt the inner coordination sphere of the metal ion. Figure 5 shows the luminescence lifetime measurements for Eu(III) in the presence of EDDAG-DA in aqueous mixtures of various
and 4.1 for Nd3+ and Am3+, respectively, due to a buffering effect of excess EDDAG-DA (partially neutralized). Analyses of the absorption spectra by the HypSpec27 program indicate the presence of two absorbing species, i.e., the free M(III) and one M(III)/EDDAG-DA complex for Nd(III) and Am(III). Various models, including the model used for analysis of the potentiometric data (containing the ML−, MHL, and MH2L+ complexes), were tested. The best fit indicates the formation of the MH2L+ only, as described by eq 4. The identification of only a single complex (in contrast to three found using the potentiometric method) may indicate that absorption features of the subsequent metal complexes MHL and ML− are nearly identical to that observed for MH2L species. The molar absorptivities of free M(III) and MH2L+ complex were calculated by HypSpec software,27 shown in Figure 4b for Nd(III) and Figure 4d for Am(III). The stability constants calculated for the formation of the NdH2L+ and AmH2L+ complexes are listed in Table 2. 3.2. Luminescence Lifetime of Eu(III)/EDDAG-DA Solutions. Luminescence decay was studied for solutions containing mixtures of Eu3+ and EDDAG-DA. The relaxation of Eu3+ from 5D0 lowest excited state to the ground state manifold 7 FJ (J = 0, 1, 2, 3,...) produces luminescence signal, which, for highly hydrated f elements, is quenched due to partial energy transfer to the O−H vibration of water molecules coordinated to the metal. Thus, the relative magnitude of quenching may be related to the number of coordinated water molecules, yielding valuable information regarding the coordination environment
Figure 5. Luminescence lifetime measurements of Eu(III)/EDDAGDA solutions. Lifetime data collected were at p[H+] = 2.10, 3.25, and 5.00. Aqueous solutions contained: [Eu3+] = 1.0 mM, [EDDAG-DA] = 10 mM, I = 2.00 ± 0.01 M adjusted with NaClO4. Lines in the figure show least -squares fitting results of the data to determine decay lifetimes. E
DOI: 10.1021/acs.inorgchem.5b02865 Inorg. Chem. XXXX, XXX, XXX−XXX
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4. DISCUSSION 4.1. Coordination Modes of EDDAG-DA. The collected potentiometric data for the protonation of EDDAG-DA is best represented by five independent protonation reactions (eq 1). The quantified equilibria describe the dissociation of three carboxylic acid protons of EDDAG-DA, together with two amine protonation equilibria. One carboxylic proton is too acidic and two amide functionalities are too basic to allow accurate determination of their protonation using a glass electrode.33 The potentiometric titration curve collected when EDDAG-DA is present in solution with an equal molar amount of trivalent neodymium (Figure 3) shows features characteristic of metal ion complexation. The dissociation of protons due to complexation reactions effectively buffers the acidity of an aqueous electrolyte mixture around pH ≈ 3. Table 2 lists the observed formation constants for three distinct metal−ligand complexes (ML−, MHL, and MH2L+). The comparison of the protonation constants for the identified metal complexes with free ligand protonation constants reveals that log β111 − log β101 ≈ pKa3 and log β121 − log β111 ≈ pKa4. Such agreement may indicate that two glycine functionalities contained in the amide pendant arm groups of EDDAG-DA are not involved in metal coordination. As such, the successive protonations of the ML− complex would not perturb the metal ion coordination environment. Luminescence lifetime measurements of Eu3+ complexation by EDDAG-DA at various acidities were performed to further substantiate this conclusion. Some structural characteristics of EDDAG-DA match those found for ethylenediamine diacetate (EDDA), with an additional presence of two glycine-type amide pendant arms. A comparison of 1:1 metal−ligand complex formation constants for EDDA and EDDAG-DA points to the participation of the amide groups in the metal ion complexation. Relative to EDDA, the observed strength of binding of Nd3+ by EDDAG-DA is increased by ∼2.5 log units.34,35 Motekaitis et al. suggested an overall lowering of basicity of the ethylenediamine backbone (relative to EDDA) due to the electron-withdrawing effects of the amide functionalities for EDDAG-DA.24 The observed increase in the complex stability of Nd3+/EDDAG-DA must therefore overcome this influence through the formation of two additional five-membered chelate rings in the coordination sphere of the f-element. Motekaitis et al. estimated the thermodynamic gain to the overall complex stability resulting from the participation of the amidic oxygens in metal binding. For Ni2+ the authors calculated a 1.2 log units enhancement and 2.8 log units for Co2+.24 For the purely electrostatic-driven (ionic) binding of a deca-coordinate trivalent neodymium,36 where ligand field stabilization effects are negligible, the observed difference in the EDDAG-DA and EDDA stability constants may predominantly stem from the further dehydration of the metal through chelation. Further evidence of the formation of five chelate rings upon f-element coordination by EDDAG-DA arises from luminescence lifetime measurements. The decay data, summarized in Table 3, show the presence of 3 or 4 water molecules in the inner coordination sphere of Eu3+ in solution with excess EDDAG-DA. Little variance in the hydration sphere of trivalent europium at all investigated aqueous acidities suggests EDDAG-DA is hexadentate for all three (ML−, MHL, and MH2L+) species. The luminescence results matched those reported by Platas-Iglesias et al.29 and Naughton et al.37 for complexants of similar structures to EDDAG-DA (different
acidity. Table 3 lists the collected luminescence data, showing close agreement of the quenching characteristics for all investigated pH conditions. 3+
Eu NH2O = 1.05 × 10−3 × kobs − 0.44
(5)
Table 3. Fluorescence Decay Constants and Waters of Hydration of the Inner Coordination Sphere of Eu3+ in 10 mM EDDAG-DA at Various pH. [Eu3+] = 0.001 M for All Experimentsa % species p[H+]
τ (μs)
Kobs (s−1)
nH2O(exp) (±0.5)
2.10 3.25 5.00
269.9 278.2 279.5
3705 3595 3578
3.5 3.3 3.3
a
Eu3+
EuL
1.4 0.0 0.0
0.4 25.5 96.8
EuHL EuH2L 11.2 41.9 3.2
87.0 26.4 0.0
I = 2.00 M (H+,Na+)ClO4.
Using the relationship developed by Kimura and Choppin32 (eq 5) the hydration numbers for Eu(III) complexed by EDDAG-DA were calculated. Literature studies on the fully hydrated Eu(III) identified the presence of 9 water molecules in the inner coordination sphere.31 In the presence of EDDAG3+ DA, NHEu2O = 3.4 ± 0.5 indicates 3 or 4 water molecules in the inner coordination sphere. 3.3. Liquid−Liquid Distribution Measurements. Figure 6 shows the liquid−liquid distribution of lanthanides (La3+−
Figure 6. Liquid−liquid distribution of lanthanides (La3+−Ho3+) and Am3+ between an organic mixture of 0.2 M HEH[EHP] in n-dodecane and an aqueous electrolyte phase containing 0.02 M EDDAG-DA, 0.5 M malonate, 1.0 M NaNO3, adjusted to p[H+] of 3.0.
Ho3+) and Am3+ between an organic mixture of 0.2 M HEH[EHP] in n-dodecane and an aqueous electrolyte phase containing 0.02 M EDDAG-DA, 0.5 M malonate, 2.0 M NaNO3, adjusted to a p[H+] of 3.0. In general, the increasing distribution trend for lanthanides follows the increasing charge density across the series as expected from purely electrostatic coordination by HEH[EHP]. Under these conditions the partitioning of lanthanum into the organic phase was the least efficient (DLa = 0.36). The partitioning of trivalent americium was suppressed by the presence of EDDAG-DA (D = 0.10), yielding a collective separation factor of Ln3+/Am3+ of 3.6. F
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Figure 7. Postulated coordination geometries for the EuH2L+, EuHL, and EuL− complexes of EDDAG-DA.
EDDAG-DA exerts a similar impact on the inner coordination sphere of the trivalent f-elements. With this the spectrophotometric signature of the formation of ML−, MHL, and MH2L+ is constant for Am3+ and Nd3+. 4.2. An(III)/Ln(III) Differentiation Supported by EDDAG-DA. To compare the relative ability of any aminopolycarboxylate reagent to support a liquid−liquid differentiation of trivalent actinides and trivalent lanthanides it is appropriate to seek differences between the complexation strength for Nd3+ and Am3+ as respective 4f and 5f metal ions of very similar charge density.40 Assuming deprotonation of amidic carboxylate protons does not produce a significant electron inductive influence on the metal binding pocket the comparison of Nd3+ and Am3+ complexation will produce similar results for all complexes formed by EDDAG-DA. A direct comparison of the log β121 values (log β121 (Nd3+) = 17.81 ± 0.01 and log β121 (Am3+) = 18.60 ± 0.01) suggests that the presence of nitrogens in the ethylene backbone maintains the soft-bonding characteristics. This is critical to the differentiation of trivalent actinides from trivalent lanthanides in liquid−liquid separation processes. The difference between the stability constants for the complexation of Nd3+ and Am3+ by EDDAG-DA (Δlog β121 = 0.79) shows slightly diminished ability to differentiate those metal ions, relative to EDTA (Δlog β121 = 1.5), due to reduced basicity of its ethylenediamine core.41 Efficient An(III)/Ln(III) separation is assisted when the complexes of lighter lanthanides with aminopolycarboxylates dissociate more readily, relative to complexes of lanthanides with smaller radii. The measured stability constants for the complexes formed between lanthanide ions and EDDAG-DA match this behavior well. Thus, a liquid−liquid distribution equilibrium, which uses EDDAG-DA to complex An(III) in the aqueous mixture and transports Ln(III) ions to the nonaqueous environment, when assisted by a strong cation exchanger, is demonstrated in Figure 6. As expected, the partitioning of light lanthanides is least efficient, but a complete An(III)/Ln(III) differentiation is achieved. The lowest experimental separation factor of 3.6 was observed between La(III) and Am(III) . This is mainly due to the consistently increasing trend in extraction constants of trivalent lanthanides by HEH[EHP]. This liquid
amide functionalization). Both groups report hexadentate coordination, with the participation of amidic oxygens in binding, leaving 3 waters in the inner coordination sphere.29,37 Motekaitis et al. also found the fully saturated coordination spheres of Ni2+ and Co2+ in acidic mixtures via the participation of amidic oxygens of EDDAG-DA.24 Combining the results of the potentiometric work and luminescence studies, it is also safe to assume that coordination of trivalent f-elements (hard Lewis acids) utilizes carbonyl oxygens present in the amide pendant arm groups. Similar lanthanide coordination environments have been reported in numerous studies for DTPAbisamides and amide-based macrocycles.38,39 Figure 7 shows the postulated structures of ML−, MHL, and MH2L+ inferred from the arguments presented above. The use of the amidic oxygens of EDDAG-DA forms two additional chelate rings in the process of f-element coordination. The involvement of the amide carboxylate functionalities would be much more difficult to rationalize as the complex formation stands very little energetic gain from what would be a 7-membered chelate. Accordingly, further displacement of waters of hydration does not occur. This leaves the glycine groups subject to acid/base equilibria, and the coordination pocket around a trivalent felement remains the same for a deprotonated and mono- and diprotonated metal complexes. The similarities between all three metal complexes observed using potentiometry are further highlighted by the spectrophotometric investigation of Am3+ and Nd3+ complexation by EDDAG-DA (Figure 4). The changes in the spectral characteristics clearly indicate a perturbation of the inner hydration sphere of trivalent americium and neodymium as increasing amounts of EDDAG-DA are added. The absorbances at 502.8 (Am3+) and 574.4 nm (Nd3+) decrease as EDDAG-DA is added due to the complexation of the M3+ aqua ions. The metal−ligand peak grows in at 506.1 nm for Am3+ and 579.0 nm for Nd3+, with an increased absorptivity relative to the aqua ion. As more EDDAG-DA arrives the accompanying changes in the pH indicate the initially formed MH2L+ is deprotonated (based on the approximate speciation model for Nd3+/ EDDAG-DA complexation). The insignificant changes in the optical absorption characteristics at that stage of an experiment may suggest the binding pocket remains the same, i.e., G
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cation exchanger is a hard oxygen donor, and its coordination of f-elements is electrostatic in nature. The lowest distribution for lanthanum (Figure 6) may be attributed to weakest electrostatic attraction (as defined by the electrostatic parameter z2/r) between the metal and the phase transfer reagent in a liquid−liquid distribution system. To project a phase transfer kinetic assessment for a liquid− liquid system containing EDDAG-DA a qualitative observation of metal complex association/dissociation kinetics can be correlated to the water relaxivity rates of lanthanide metal ions. The water relaxivity rates of Gd complexes of inner-sphere water molecules have been shown to be sensitive to the metal ion coordination environment. Water exchange rates for aqua ions decrease significantly depending on the degree of complexation by aqueous complexants. For example, the water exchange rates of Gd3+ aqua ion (k298ex = 804 × 106 s−1) are approximately 240 times faster than when the metal ion 6 −1 42 is complexed by DTPA (k298 Relative to ex = 3.30 × 10 s ). DTPA, Platus-Iglesias et al. observed faster water exchange rates for the Gd3+ complexes of bisamide-substituted EDTA 6 −1 (k298 ex = 44 × 10 s ) and slower rates for bisamide-substituted 298 DTPA (kex = 0.45 × 106 s−1), most likely due to the differences of inner-sphere hydration.29 Although the water relaxivities are not directly related to the metal-complex association/dissociation rates, a qualitative comparison may be used for predictive purposes. It may be inferred that the associative/dissociative metal exchange rates for EDDAG-DA (relative to DTPA) should be faster. Accordingly, it is feasible to anticipate that faster dissociation rates of EDDAG-DA complexes of lanthanides will facilitate the phase transfer kinetics across the liquid−liquid interface. Those studies are currently underway in our laboratory.
Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02865. Synthesis of ethylenediamine-N,N′-di(acetylglycine)N,N′-diacetic acid (EDDAG-DA) (PDF)
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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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5. CONCLUSION A new holdback complexant (EDDAG-DA), containing an EDTA-like coordination environment, was synthesized and thoroughly characterized. The stability constants suggest stronger complexation of f-elements, relative to EDDA, indicating the participation of amide pendant groups in metal ion coordination. Potentiometry studies identified three distinct metal−ligand complexes identified as the ML−, MHL, and MH2L+ species. Time-resolved fluorescence studies revealed a similar binding pocket throughout the investigated range of aqueous acidity (2 < pH < 5). EDDAG-DA was shown to consistently displace 6 waters from the inner coordination sphere of Eu3+. From the fluorescence studies a hexadentate coordination of europium by EDDAG-DA was inferred, consisting of two tertiary amine, two carboxylic oxygens, and two amide oxygens, likely coordination to the metal ions. The glycine −COOH groups were not observed to participate in metal coordination but rather undergo proton exchange with the bulk media. Spectrophotometric studies of complexation of Am3+ and Nd3+ by EDDAG-DA identified an increased thermodynamic stability for Am3+ over Nd3+, indicative of the enhanced coordination of soft-bonding N donors to trivalent actinides over trivalent lanthanides. Liquid−liquid distribution studies showed a maintained An3+/Ln3+ differentiation when EDDAG-DA is present in the aqueous electrolyte mixture, and HEH[EHP] facilitates the extraction into the organic medium. H
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