Influence of a Heterocyclic Nitrogen-Donor Group on the Coordination

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of a Heterocyclic Nitrogen-Donor Group on the Coordination of Trivalent Actinides and Lanthanides by Aminopolycarboxylate Complexants Travis S. Grimes,*,† Colt R. Heathman,† Santa Jansone-Popova,‡ Alexander S. Ivanov,‡ Santanu Roy,‡ Vyacheslav S. Bryantsev,‡ and Peter R. Zalupski*,† †

Aqueous Separations and Radiochemistry, Idaho National Laboratory, Idaho Falls, Idaho 83415, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

‡

S Supporting Information *

ABSTRACT: The novel metal chelator N-2-(pyridylmethyl)diethylenetriamineN,N′,N″,N″-tetraacetic acid (DTTA-PyM) was designed to replace a single oxygen-donor acetate group of the well-known aminopolycarboxylate complexant diethylenetriamine-N,N,N′,N″,N″-pentaacetic acid (DTPA) with a nitrogendonor 2-pyridylmethyl. Potentiometric, spectroscopic, computational, and radioisotope distribution methods show distinct differences for the 4f and 5f coordination environments and enhanced actinide binding due to the nitrogenbearing heterocyclic moiety. The Am3+, Cm3+, and Ln3+ complexation studies for DTTA-PyM reveal an enhanced preference, relative to DTPA, for trivalent actinide binding. Fluorescence studies indicate no changes to the octadentate coordination of trivalent curium, while evidence of heptadentate complexation of trivalent europium is found in mixtures containing EuHL(aq) complexes at the same aqueous acidity. The denticity change observed for Eu3+ suggests that complex protonation occurs on the pyridyl nitrogen. Formation of the CmHL(aq) complex is likely due to the protonation of an available carboxylate group because the carbonyl oxygen can maintain octadentate coordination through a rotation. The observed suppressed protonation of the pyridyl nitrogen in the curium complexes may be attributed to stronger trivalent actinide binding by DTTA-PyM. Density functional theory calculations indicate that added stabilization of the actinide complexes with DTTA-PyM may originate from π-back-bonding interactions between singly occupied 5f orbitals of Am3+ and the pyridyl nitrogen. The differences between the stabilities of trivalent actinide chelates (Am3+, Cm3+) and trivalent lanthanide chelates (La3+−Lu3+) are observed in liquid−liquid extraction systems, yielding unprecedented 4f/5f differentiation when using DTTA-PyM as an aqueous holdback reagent. In addition, the enhanced nitrogen-donor softness of the new DTTA-PyM chelator was perturbed by adding a fluorine onto the pyridine group. The comparative characterization of N-(3-fluoro-2-pyridylmethyl)diethylenetriamine-N,N′,N″,N″-tetraacetic acid (DTTA-3-F-PyM) showed subdued 4f/5f differentiation due to the presence of this electron-withdrawing group.

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citrate was used for intra-lanthanide separations3) with a hydrochloric acid eluant solution.1 The retention of trivalent actinides on the resin was shorter, relative to trivalent lanthanides, because of the formation of stronger actinide chloride complexes. A later study by Diamond et al.4 proposed that the 4f/5f selectivity originates with the enhanced covalent interactions between trivalent actinides and complexing agents containing “soft” donor atoms. Building on the soft donor principle, Thompson et al. found that the anionic actinide chloride complexes adsorbed more strongly to anion-exchange resins, relative to trivalent lanthanides, when using a concentrated hydrogen chloride (HCl) eluant.5 The adsorption characteristics for trivalent americium6,7 and many other divalent and trivalent metal ions8

INTRODUCTION Concomitant with the advent of transuranic element discovery, and the determination of their nuclear and chemical properties, was the isolation of novel f-elements from complex matrixes. The heavy actinide progress along the 5f-element series aided the rapid development of the challenging trivalent actinide/trivalent lanthanide intergroup differentiation methods.1 Initially, 4f/5f group separation was accomplished using a fluorosilicate precipitation method.2 The separation was based on numerous partial precipitations of a lanthanum fluoride carrier, isolating the element of interest in low yields. Because of the time constraints imposed by the shorter half-lives of the heavier actinide elements, a more rapid and efficient separation was necessary. Perhaps the most important discovery pertaining to trivalent lanthanide/trivalent actinide separations was made in 1950 by Street and Seaborg.1 While utilizing a strong cation-exchange resin, the authors replaced the carboxylic acid (ammonium © XXXX American Chemical Society

Received: November 1, 2017

A

DOI: 10.1021/acs.inorgchem.7b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Synthetic scheme for the preparation of DTTA-PyM (structure 3a) and DTTA-3-F-PyM (structure 3b).

chelator, N-(2-pyridylmethyl)diethylenetriamine-N,N′,N″,N″tetraacetic acid (DTTA-PyM). Thorough thermodynamic characterization of the trivalent f-element coordination by DTTA-PyM has advanced the understanding of the pyridyl functionality in the coordination sphere and its implications on the complexation of trivalent actinides and lanthanides. Further, the electronic properties of the pyridyl moiety were perturbed by the addition of a fluorine-atom substitution to study inductive effects on the reagent protonation reactions and the stability of metal ion complexes.

on a Dowex-1 resin were improved when using LiCl eluant mixtures. Hulet et al.9 evolved the An3+/Ln3+ separation concept further when studying elution trends using 10 M LiCl. The soft donor principle was also demonstrated for the eluant mixture containing thiocyanate.10 Further advances in An3+/Ln3+ differentiation were made when small inorganic anions (Cl− and SCN−) were replaced by polydentate organic nitrogen-donor ligands, i.e., aminopolycarboxylates, initially used for intralanthanide separations.11 The study by Orr showed the utility of diethylenetriamineN,N,N′,N″,N″-pentaacetic acid (DTPA) with an effective separation of Am3+ from Pm3+ and Eu3+ on a cation-exchange resin.12 Weaver and Kapplemann transitioned the solid−liquid An3+/Ln3+ separation concept onto a liquid−liquid platform, introducing the TALSPEAK (Trivalent Actinide Lanthanide Separation using Phosphorus-based Extractants and Aqueous Komplexes) process.13,14 The authors, when probing a thermodynamic balance between the strong liquid cation exchangers and aqueous aminopolycarboxylate complexants, studied numerous soft donor reagents [e.g., N,N-bis(carboxymethyl)glycine (NTA), N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA), ethylenediamine-N,N,N′,N′-tetraacetic acid (EDTA), 1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA), and DTPA]. The extent of Ln3+/An3+ differentiation increased with the number of soft donor nitrogen atoms present in the structure of the aqueous holdback complexant. The utility of DTPA proved most proficient, and until now, the traditional DTPA-based TALSPEAK platform delivers unrivaled trivalent actinide selectivity when utilizing aminopolycarboxylate holdback reagents.14 Several modifications to aminopolycarboxylate structures have been reported to facilitate efficient An3+/Ln3+ separations.15−17 Heathman et al. studied substitutions of acetate pendant arms by amide functionalities.15,16 These alterations enhanced the total ligand acidity of the aqueous complexants, improved the metal complexation performance at higher acidities, and capitalized on the H+-catalyzed mechanism of complex dissociation to speed up the phase-transfer kinetics. The structural variation reported by Grimes et al., where a terminal acetate arm of DTPA was replaced with a hydroxyethyl group, showcased the impact of reduced ligand denticity (octa vs hepta).17 These modifications, while overcoming the kinetic limitations imposed by the energy necessary for the dissociation of a compact [M(DTPA)]2+ complex, lowered the overall stability of the M:L chelates and also reduced the An3+/Ln3+ differentiation. This study aims to enhance the An3+/Ln3+ separation by incorporating an additional N−M−N-type chelate into the coordination environments of trivalent f-elements. Replacing the N-acetate group of DTPA with a N-2-pyridylmethyl pendant arm has produced a new

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EXPERIMENTAL SECTION

Reagents. The novel complexant 2,2′-[[2-[(carboxymethyl)[2[(carboxymethyl)(pyridin-2-ylmethyl)amino]ethyl]amino]ethyl]azanediyl]diacetic acid (IUPAC name for DTTA-PyM) was isolated as the tetra HCl salt according to the synthetic scheme in Figure 1. The novel complexant 2,2′-[[2-[(carboxymethyl)[2-[(carboxymethyl)[(3fluoropyridin-2-yl)methyl]amino]ethyl]amino]ethyl]azanediyl]diacetic acid (IUPAC name for DTTA-3-F-PyM) was synthesized using the same methodology and isolated as a tetra HCl salt. A more detailed synthetic and characterization description for the new ligands is provided in the Supporting Information. Potentiometric (La3+−Lu3+, excluding Pm3+) and spectrophotometric (Nd3+, Am3+, and Cm3+) methods were used to determine the stability constants for the coordination of trivalent f-elements by DTTA-PyM. For DTTA-3-FPyM, the stability constants for the complexation of La3+, Pr3+, Eu3+, Ho3+, and Lu3+ were determined potentiometrically. The lanthanide perchlorate stocks were prepared as previously described.15−17 The Am3+ and Cm3+ solutions from Idaho National Laboratory stocks were prepared separately using diglycolic acid (DGA) extraction chromatographic resin (Eichrom). The desired An3+ was first adsorbed onto a DGA column and eluted using 0.02 M HCl. Multiple evaporation cycles were conducted to remove HCl from the solution, and the final stock solution was dissolved in 0.1 mM perchloric acid (HClO4). The concentration of the purified Am3+ perchlorate stock was determined using a spectrophotometric titration method reported by Tian and Shuh.18 The concentration of the Cm3+ stock was estimated via the Beer−Lambert relationship using the extinction coefficient of 52.9 cm−1 M−1 at 369.4 nm.19 The metal ions investigated using solvent extraction techniques included radioisotopes 241Am3+, 139Ce3+, and 154Eu3+, purchased as chloride solutions from Eckert and Ziegler, and the lanthanides La3+−Ho3+ purchased as oxides from Pangea International. The lanthanide perchlorate stocks were prepared after oxide dissolution in HClO4 (trace metals, Aldrich) and standardized using inductively coupled plasma mass spectrometry (ICP-MS). Sodium perchlorate salt (NaClO4; ACS reagent grade, GFS Chemicals) was used to control the ionic strength of aqueous mixtures used throughout the study. The purchased NaClO4 was purified to remove iron particulates, recrystallized, and standardized using ion exchange according to procedures reported previously.15−17 Sodium hydroxide (NaOH) solutions were prepared by diluting weighed amounts of 50% (w/w) NaOH (Sigma-Aldrich) and a NaClO4 stock solution with 18 MΩ deionized water. All NaOH solutions were B

DOI: 10.1021/acs.inorgchem.7b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry standardized by titration of the primary standard potassium hydrogen phthalate. Potentiometry. Potentiometric titrations were conducted with a Mettler Toledo T90 graphix autotitrator to determine the acid dissociation (Ka) and metal complex stability (βmhl) constants. A steady stream of hydrated nitrogen gas (bubbled through 1.0 M NaOH) was directed toward the top of the titrand solution to prevent carbon dioxide absorption. The titrations were held constant at 25.0 ± 0.1 °C with a jacketed beaker and a circulating water bath. A Ross Orion semimicro glass electrode (with a 5.0 M sodium chloride (NaCl) filling solution) was used to measure changes in pCH. The original electrode filling solution of potassium chloride was replaced with NaCl to prevent potassium perchlorate precipitation inside the glass electrode. The high concentration of NaCl was used for rapid electrode response and to minimize variations in the junction potential. Strong acid (HClO4)/ strong base (NaOH) titration (I = 2.0 M; T = 25.0 ± 1.0 °C) was used for Gran analysis20−22 and to establish the operational pCH scale for the glass electrode based on the hydrogen-ion concentration. Potentiometric data were modeled using the Hyperquad2013 fitting software.23 Spectrophotometric Titrations. The ionic strength for mixtures used in spectrophotometric studies was maintained at 2.0 M adjusted using NaClO4. The optical absorption spectra for the complexation of Nd3+ with DTTA-PyM (550−610 nm, 0.08 nm interval) were collected using an Agilent Cary 6000i UV−vis−near-IR (NIR) spectrophotometer in a 1.0 cm quartz cuvette at ambient temperature (20.0 ± 1.0 °C). A titrand solution ([Nd3+] = 7.50 mM, I = 2.0 M, and pCH = 2.02) was blended with the titrant ([Nd3+] = 7.47 mM, [DTTA-PyM] = 25.0 mM, and pCH ≈ 7.0) in appropriate amounts to produce a DTTA-PyM dependence with constant [Nd3+], until a final pCH of 6.7 was reached. Spectrophotometric titrations monitoring the formation of Am3+ and Cm3+ complexes with DTTA-PyM were performed using a Flame-SVIS-NIR-ES Ocean Optics device coupled with an Ocean Optics DH2000-BAL light source through a 2 m (200 μm) fiber-optic cable in a 1 cm quartz cuvette (Starna) at ambient temperature (20.0 ± 1.0 °C). For americium, the 400−600 nm spectral range was studied at 0.37 nm intervals. For curium, the 365−410 nm spectral range was studied at 0.38 nm intervals. Titrand mixture for americium: [Am3+] = 0.806 mM and pCH = 1.9. Titrant: [Am3+] = 0.787 mM, [DTTA-PyM] = 6.83 mM, and pCH ≈ 7.0. Titrand mixture for curium: [Cm3+] = 0.984 mM and pCH = 2.2. Titrant: [Cm3+] = 0.975 mM, [DTTA-PyM] = 5.90 mM, and pCH ≈ 7.0. The titrant additions varied from 5 to 150 μL to produce final pCH values of 3.9 and 4.7, respectively. Accompanying titrations were performed to measure pCH after each ligand addition for all metal ions. The fitting program HypSpec24 was used to analyze the spectral data. Fluorescence Measurements. Luminescence lifetimes were collected for mixtures of Eu3+ or Cm3+ with DTTA-PyM at pCH 2.1, 3.1, 4.0, and 5.0 conditions. The metal-ion concentrations were 2.0 mM for europium and 50 μM for curium. The total DTTA-PyM concentration was 10.0 mM for all mixtures. The ionic strength was adjusted to 2.0 M using NaClO4. The measurements were made using a Horiba Jobin Yvon IBH Fluorolog-3 fluorometer adapted for timeresolved measurements. The light source, a submicrosecond xenon flash lamp (Jobin Yvon 5000XeF), is coupled with a photomultiplier tube, a wide-band-width preamplifier, and a picosecond constant fraction discriminator. Data were collected in 1.0 cm quartz cuvettes (Starna) at 25.0 ± 0.1 °C. The temperature was maintained by a water-jacketed cuvette holder and a circulating water bath. The software package DAS 6 decay analysis provided by Horiba Jobin Yvon IBH was used for data fitting. Luminescence lifetime data were modeled using single- and double-exponential decay curves with X2 values that ranged between 1.01 and 1.08. Density Functional Theory (DFT) Calculations. Molecular DFT calculations were performed with the Gaussian 09, revision D.01, software package25 using the hybrid B3LYP26,27 functional. Standard 631+G* and def2-TZVPP basis sets were used for main-group elements and hydrogen for geometry optimization. The f-elements were modeled using both large-core (LC)28−30 and Stuttgart small-core (SSC)31 relativistic effective core potential (RECP) and the associated basis sets. In particular, the following energy-consistent scalar-relativistic Wood− Boring-adjusted electron core potentials and corresponding basis sets

were selected: LC RECP and the related (7s6p5d)/[5s4p3d] basis sets for Eu3+ and Am3+ were used in conjunction with the 6-31+G* basis set for light elements, respectively. The largest available basis sets, (8s7p5d3f2g)/[6s5p5d3f2g] and (7s6p5d2f1g)/[6s5p4d2f1g], for Eu3+ and Am3+, respectively, were used in conjunction with the def2TZVPP basis set for light elements. Because LC RECP calculations include the outermost f electrons in the core, they were performed on a pseudo-singlet-state configuration. SSC RECP and the related ECP28MWB_SEG and ECP60MWB_SEG basis sets were used for Eu3+ and Am3+, respectively. For the DTTA-MPy structure, beginning from eight different initial configurations generated by the systematic replacement of one of the carboxylic groups with the 2-pyridylmethyl group in two X-ray structures32,33 of nine-coordinate Eu3+-DTPA complexes containing one water molecule in the primary coordination shell, the most stable structures of ML− complexes (M = Eu3+ and Am3+) were identified. The most stable configuration for each complex was selected for computing the relative energy differences. Chemical bonding analysis was performed with the natural bond orbital (NBO) method35,36 at the B3LYP/SSC/def2-TZVPP level. Typical NBO analysis provides a good quantitative description of interatomic and intermolecular interactions in accordance with the basic Pauling−Slater−Coulson representations of bond polarization and hybridization.35,36 The donor−acceptor interaction energy in the NBOs was estimated via second-order perturbation theory analysis of the Fock matrix.35 For each donor orbital (i) and acceptor orbital (j), the stabilization energy E(2) associated with i → j delocalization is given by eq 1:

Ei(2) , j = − oi

⟨i|F(̂ i , j)|j 2 ⟩ εj − εi

(1)

where oi is the donor orbital occupancy, F̂(i,j) is the Fock operator, and εi and εj are the orbital energies. Solvent Extraction. The pCH dependency studies for liquid−liquid partitioning of trivalent f-elements were performed using DTTA-PyM and DTTA-3-F-PyM as aqueous holdback complexants according to a previously described method.16 The metal ion distribution was monitored radiometrically (ORTEC GEM50P4 coaxial HPGe detector and DSPEC γ spectrometer) for 241Am, 139Ce, and 154Eu, measured as the ratio of radioisotope activity in the organic and aqueous phases. Partitioning of La3+−Ho3+ metal ions was also determined using ICPMS, measuring changes in the aqueous metal concentration before and after equilibration with the nonaqueous phase. The organic extractant (2-ethylhexyl)phosphonic acid mono(2-ethyhexyl) ester (HEH[EHP]) was purchased from Marshallton Inc. and used as received. The organic diluent n-dodecane (99%, Sigma-Aldrich) was used without further purification. Organic phases (uniform throughout at 0.1 M HEH[EHP]) were prepared by dissolving weighed amounts of HEH[EHP] in n-dodecane. The nonaqueous mixtures containing HEH[EHP] were initially preequilibrated using the corresponding aqueous solution without metals. The aqueous mixtures were prepared to contain 20 mM holdback complexant, 0.5 M sodium malonate (99%, Alfa Aesar), and 0.1 mM total lanthanide content, adjusted to an ionic strength of 2.0 M using NaClO4. The pCH of each aqueous condition was adjusted using HClO4. The pCH readings were 2.84, 3.10, 3.36, 3.65, 3.83, 4.12, 4.35, and 4.59 for mixtures containing DTTA-PyM and 2.11, 2.37, 2.57, 2.85, 3.09, 3.35, 3.62, and 3.85 for mixtures containing DTTA-3-F-PyM. A liquid−liquid distribution equilibrium was attained by rapid overnight mixing using a Glas-Col multitube vortexer at room temperature (21.0 ± 1.0 °C). The pCH readings did not change after equilibration with nonaqueous solutions.

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RESULTS Determination of the Acid Dissociation Constants and Stability Constants by Potentiometric Titration. The acid dissociation constants and stability constants for complexation of La3+−Lu3+ by DTTA-PyM and La3+, Pr3+, Eu3+, Ho3+, and Lu3+ by DTTA-3-F-PyM were determined using potentiometry. The formation of ML− and MHL(aq) complexes were studied using C

DOI: 10.1021/acs.inorgchem.7b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry spectrophotometry for trivalent neodymium, americium, and curium. Good agreement was found for the two analytical techniques when used to investigate Nd3+ /DTTA-PyM complexation. Acid Dissociation Constants. Figure 2 shows a representative free acid titration curve, a calculated best fit, and distribution of

Figure 3. Potentiometric titration of a 1:1 mixture of Nd3+/DTTA-PyM, calculated best fit, and the distribution of Nd3+ species. Titrant: 0.303 M NaOH and I = 2.0 M (Na+/H+)ClO4. Titrand: CDTTA‑PyM = 4.56 mM DTTA-PyM, CH+ = 0.040 M, CNd3+ = 5.08 mM, Vinit = 25.064 mL, I = 2.0 M (Na+/H+)ClO4, and T = 25.0 ± 0.1 °C. Experimental pCH (○) and calculated pCH (solid green line), Nd3+ (blue dashed line), NdHL(aq) (red dashed line), and NdL− (black dashed line). Figure 2. Potentiometric free acid titration of DTTA-PyM to determine the protonation constants, calculated best fit, and distribution of ligand species. Titrant: 0.303 M NaOH, I = 2.0 M (Na+/H+)ClO4. Titrand: CDTTA‑PyM = 4.56 mM DTTA-PyM, CH+ = 0.040 M, Vinit = 25.031 mL, I = 2.0 M (Na+/H+)ClO4, and T = 25.0 ± 0.1 °C. Experimental pCH (□), calculated pCH (solid green line), H6L2+ (navy-blue dashed line), H5L+ (olive dashed line), H4L (magenta dashed line), H3L− (blue dashed line), H2L2− (red dashed line), HL3− (black dashed line), and L4− (purple dashed line).

polycarboxylate complexant and metal complexation. The sharp pCH increase signals the complete complexation of the neodymium ion. The best-fit representation of the experimental titration curve indicates the formation of [Ln(DTTA-PyM)]− and [LnH(DTTA-PyM)](aq) complexes according to eqs 3 and 4. The determined conditional, concentration-based stability constants for trivalent lanthanide complexes with DTTA-PyM and DTTA-3-F-PyM are summarized in Table 2 and compared to those previously determined with DTPA.37 The metal complex protonation equilibrium may be described by eq 5.

the protonated DTTA-PyM species in the investigated pCH range. The pyridine nitrogen (pKa ≈ 5) was included in the fitting iterations, and a best-fit representation of the experimental data was obtained using a six protonation model. The generalized proton dissociation reaction for DTTA-PyM is defined by eq 2, where n designates each dissociation step.

M3 + + L4 − ⇄ ML−

Ka n =

β111 =

+4 − n

[H ][H8 − nL

[H 9 − nL+5 − n]

]

[ML−] [M3 +][L4 −]

(3)

M3 + + H+ + L4 − ⇄ MHL(aq)

H 9 − nL+5 − n ⇄ H8 − nL+4 − n + H+ +

β101 =

n = 1, ..., 8

[ML(aq)] 3+

[M ][H+][L4 −]

(4)

(2)

ML− + H+ ⇄ MHL(aq)

The conditional acid dissociation constants determined for DTTA-PyM and DTTA-3-F-PyM are listed in Table 1 and compared to those reported in the literature for DTPA.37 Stability Constants. Figure 3 shows the potentiometric pCH curve collected when an equivalent molar content of Nd3+ and DTTA-PyM is titrated with base. The initial resistance to pCH change (buffering) is due to deprotonation of the amino-

K111 =

[ML(aq)] [ML−][H+]

(5)

Spectrophotometric Titrations. Spectrophotometric titrations were conducted to determine the stability constants for complexation of Nd3+, Am3+, and Cm3+ with DTTA-PyM. The optical absorption spectral changes throughout the titration experiments are shown in Figure 4 for neodymium (A),

Table 1. Comparison of the Acid Dissociation Constants Determined for DTTA-PyM and DTTA-3-F-PyM at 25.0 ± 0.1 °C with Those Determined for DTPA37 [I = 2.00 M (H+/Na+)ClO4] n 8 7 6 5 4 3 2 1

DTTA-PyM 3−

HL H2L2− H3L− H4L(aq) H5L+ H6L2+ H7L3+ H8L4+

pKn 9.44 ± 0.01 7.71 ± 0.01 5.11 ± 0.01 3.79 ± 0.01 2.35 ± 0.02 1.95 ± 0.01

DTTA-3-F-PyM 3−

HL H2L2− H3L− H4L(aq) H5L+ H6L2+ H7L3+ H8L4+ D

pKn

DTPA37

pKa

9.31 ± 0.01 7.43 ± 0.01 4.23 ± 0.01 2.95 ± 0.01 2.09 ± 0.01 1.64 ± 0.01

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

9.50 8.31 4.38 2.53 2.41

DOI: 10.1021/acs.inorgchem.7b02792 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Cumulative Stability Constants log β101 and log β111 for Lanthanide (La3+−Lu3+) Complexes with DTTA-PyM (Determined Using Potentiometry at 25.0 ± 0.1 °C) and Nd3+, Am3+, and Cm3+ Complexes with DTTA-PyM (Determined Using Spectroscopy at 20.0 ± 1.0 °C) in 2.0 M (Na+/H+)ClO4a log β101

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ndb Amb Cmb

16.02 ± 0.02 16.99 ± 0.01 17.80 ± 0.01 18.50 ± 0.02 19.07 ± 0.01 19.20 ± 0.02 19.36 ± 0.01 19.70 ± 0.01 19.49 ± 0.01 19.25 ± 0.01 19.51 ± 0.01 19.39 ± 0.01 19.08 ± 0.01 19.16 ± 0.02 18.26 ± 0.02 20.00 ± 0.02 19.77 ± 0.04

log β111

19.64 ± 0.02 3.62 ± 0.01 20.28 ± 0.01 3.29 ± 0.01 20.90 ± 0.01 3.10 ± 0.01 21.58 ± 0.02 3.08 ± 0.01 21.98 ± 0.01 2.91 ± 0.01 22.27 ± 0.02 3.07 ± 0.01 22.18 ± 0.01 2.82 ± 0.01 22.41 ± 0.01 2.71 ± 0.01 22.17 ± 0.01 2.68 ± 0.01 22.22 ± 0.01 2.96 ± 0.01 22.12 ± 0.01 2.61 ± 0.01 22.05 ± 0.01 2.66 ± 0.01 21.84 ± 0.01 2.76 ± 0.01 22.20 ± 0.02 3.04 ± 0.01 21.02 ± 0.01 2.76 ± 0.02 22.48 ± 0.01 2.48 ± 0.02 22.37 ± 0.02 2.60 ± 0.04 DTTA-3-F-PyM

log β101

log K111

18.02 19.06 19.64 20.23 20.79 21.03 21.15 21.15 21.23 21.43 21.41 21.1 20.96 21.14

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

metal

log β101

log β111

log K111

La Pr Eu Ho Lu

15.60 ± 0.01 17.32 ± 0.01 18.55 ± 0.01 18.66 ± 0.02 18.33 ± 0.01

18.33 ± 0.02 19.66 ± 0.02 20.81 ± 0.02 20.99 ± 0.02 20.68 ± 0.02

2.73 ± 0.02 2.34 ± 0.01 2.26 ± 0.01 2.33 ± 0.01 2.34 ± 0.01

(6)

Cm ηH O(Cm 3+) = 0.65 × 10−3kobs − 0.88

(7)

2

Luminescence lifetimes τ, the calculated hydration numbers ηH2O, pCH, and the corresponding aqueous distribution of M3+/ DTTA-PyM species are summarized in Table 3. The luminescence lifetimes for mixtures of Eu3+/DTTA-PyM increase as pCH is increased, while mixtures of Cm3+/DTTAPyM remain more constant with a small observed increase. For Eu3+-containing mixtures, a double-exponential function best represented the luminescence decay at pCH = 2.1, and singleexponential treatments matched the pCH = 3.1−5.0 data. The calculated hydration numbers for europium show a transition from two to one water molecule as the aqueous acidity decreases. The luminescence data for Cm3+ mixtures with DTTA-PyM show similar decay patterns across the investigated pCH range, requiring a single-exponential representation throughout. The luminescence lifetime decay showed small differences (45 μs) between pCH = 2.1 and 5.0. The ηH2O calculations indicate two water molecules in the inner coordination sphere of trivalent curium at all pCH conditions. DFT. The DFT calculations were performed to describe the structures of [Eu(DTTA-MPy)]− and [Am(DTTA-MPy)]− complexes. The M−O and M−N bond distances for the considered ML− complexes, optimized at the B3LYP/SSC/ def2-TZVPP level of theory, are summarized in Table 4. Further calculations for the optimized metal complex structures found differences for the ligand binding energy for AmIII over EuIII when small-core RECP and the associated bases sets ( f-invalence) were used. The LC RECP calculations did not yield significant differences for [Eu(DTTA-MPy)]− and [Am(DTTAMPy)]− complexes. Using the second-order perturbation theory, the NBO analyses found differences in the forward- and backdonation bonding interactions in [Eu(DTTA-MPy)]− and [Am(DTTA-MPy)]− complexes, as summarized in Table 5. Lanthanide/Actinide Separations. The measured radioisotope distribution trends for the investigated trivalent felements are presented in Figure 6 as a function of the changing aqueous acidity. Figure 6A demonstrates the aqueous complexing performance of DTTA-PyM, while Figure 6B summarizes the pCH-dependent study for DTTA-3-F-PyM. The An3+/Ln3+ differentiation is maintained throughout the entire range of investigated aqueous conditions. A good balance between the strong cation exchanger HEH[EHP] (when present at a 0.1 M concentration in the nonaqueous solution) and DTTA-PyM complexant (20 mM in an aqueous solution) yields low extractability of Am3+ (D < 1) and high partitioning of Ln3+ throughout. For lanthanides, the distribution of lanthanum is lowest at pCH = 2.84 (SFLa Am = 30). Neodymium shows the lowest distribution for all conditions of lower acidity (SFNd Am = 73 ± 8). The efficient 4f/5f group separation is shifted to conditions of higher aqueous acidity when utilizing DTTA-3-F-PyM as the aqueous complexant (Figure 6B). Collectively, liquid−liquid distribution values are lowered presumably because of stronger complexation by DTTA-3-F-PyM with both 4f- and 5f-elements in the aqueous environment. At pCH = 2.11, 2.37, and 2.57, the partitioning of the lanthanum ion is the lowest, replaced by neodymium in the 2.85 < pCH < 3.85 region. Complete 4f/5f group separation is reduced for DTTA-3-F-PyM, relative to

DTPA log K111

Eu ηH O(Eu 3+) = 1.05 × 10−3kobs − 0.44 2

37

DTTA-PyM metal

decay of Eu/DTTA-PyM and Cm/DTTA-PyM solutions, respectively.

a

Stability constants collected for Ln/DTPA complexes in 2.0 M (Na+/ H+)ClO4 using potentiometry are tabulated for comparison.37. b Denotes stability constants determined using spectroscopy.

americium (C), and curium (E). During each titration, the absorption attributed to the presence of free metal ions (λmax = 574, 503, and 375 and 396 nm for Nd3+, Am3+, and Cm3+, respectively) decreased upon the addition of DTTA-PyM. Metal ion complexation by DTTA-PyM was manifested by new redshifted absorbance peaks in each titration. Those included peaks at 580, 582, 585, and 587 nm for Nd3+/DTTA-PyM, the peak at 508 nm for Am3+/DTTA-PyM, and peaks at 380, 384, and 400 nm for Cm3+/DTTA-PyM. Each spectrophotometric titration data point was best represented when the presence of three distinct absorbing species, M3+, ML−, and MHL(aq), was considered. The calculated molar absorptivities of each species are shown in Figure 4 for neodymium (B), americium (D), and curium (F). The stability constants for Nd3+, Am3+, and Cm3+ complexation by DTTA-PyM are summarized in Table 2. Luminescence Lifetime Data. Luminescence lifetimes were measured using fluorescence spectroscopy to determine the number of water molecules in the metal ion primary hydration sphere. The coordination environment of Eu3+ was studied with a 5:1 ligand/metal ratio and a 200:1 ligand/metal ratio for the Cm3+ mixtures. Measurements were taken at pCH = 2.1, 3.1, 4.0, and 5.0, chosen to vary the content of M3+, ML−, and MHL(aq) species. Hydration numbers for Eu3+ and Cm3+ were calculated using the empirical equations (eqs 6 and 7) developed by Kimura.38,39 Parts A and B of Figure 5 shows the luminescence E

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Figure 4. (A) Nd(ClO4)3, (C) Am(ClO4)3, and (E) Cm(ClO4)3 showing spectrophotometric titrations with DTTA-PyM. (B) Nd3+, (D) Am3+, and (F) Cm3+ showing the calculated molar absorptivities for the species observed during titrations. (A) Titrant: CNd3+ = 7.5 mM, CDTTA‑PyM = 25.0 mM, pCH = 7.0, and I = 2.0 M (Na+/H+)ClO4. Titrand: 7.5 mM Nd(ClO4)3, pCH = 2.0, I = 2.0 M (Na+/H+)ClO4, and T = 20.0 ± 1.0 °C. (C) Titrant: CAm3+ = 0.787 mM, CDTTA‑PyM = 6.80 mM, pCH = 7.0, and I = 2.0 M (Na+/H+)ClO4. Titrand: CAm3+ = 0.806 mM, pCH = 1.95, I = 2.0 M (Na+/H+)ClO4, T = 20.0 ± 1.0 °C. (E) Titrant: CCm3+ = 0.975 mM, CDTTA‑PyM = 5.9 mM, pCH = 7.0, and I = 2.0 M (Na+/H+)ClO4. Titrand: CCm3+ = 0.984 mM, pCH = 2.2, I = 2.0 M (Na+/H+)ClO4, and T = 20.0 ± 1.0 °C.

DTTA-PyM, with the minimum SFLn Am growing from 6 (pCH = 2.11), 30 (pCH = 2.37), and ultimately 54 ± 4 at lower acidity.

DTPA. This difference may stem from an asymmetric electron perturbation expected from the replacement of a DTPA acetate group.41 The equilibrium observed for pK6 matches that expected of the pyridine nitrogen atom,43 particularly when comparing these values for DTTA-PyM and DTTA-3-F-PyM. The electron-withdrawing influence of fluorine renders the pK6 more acidic. The fourth amine protonation equilibrium occurs on the terminal site connected to the 2-pyridylmethyl arm. Here, the substantially delayed protonation may be attributed to stabilization of the amine site through hydrogen bonding with the protonated pyridyl nitrogen atom.41 A similar suppression of the amine protonation equilibria has been observed by Cacheris et al. for symmetrically disubstituted N,N″-bis(2-pyridylmethyl)diethylenetriamine-N,N′,N″-triacetic acid.43 Further protona-

■

DISCUSSION Thermodynamic Impact of the N-Methyl-2-pyridyl Group. Ligand Protonation. Similar equilibrium constants for the final hydrogen-ion dissociation reaction for DTTA-PyM, DTTA-3-F-PyM, and DTPA (pK8 in Table 1) indicates that the initial protonation occurs on the central amine group, as reported for DTPA. 4 0−4 2 The central nitrogen atom of the −NCH2NCH2N− group is least affected by a substituent on a terminal amine. The second protonation step occurs on the nitrogen atom at the imidodiacetate end, showing a substantial decrease of pK7 for DTTA-PyM and DTTA-3-F-PyM, relative to F

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reactions to terminal amine sites. The authors reported a similar pK6 for protonation of the pyridine nitrogen atom, and the fourth protonation equilibrium was similarly delayed by hydrogen bonding despite different positioning of the group within the diethylenetriamine backbone.44 Trivalent f-Element Complexation. Thompson et al. showed that the addition of the N-methyl-2-pyridyl substituent to imidodiacetic acid increases the stability of trivalent f-element chelates, supporting participation of the pyridyl nitrogen atom in the metal-ion coordination sphere.45 The stability enhancement was similar to that observed for an amide group but ≈8 kJ/mol lower relative to the acetate binding (estimated through a comparison of the stability constants for lanthanide complexation by IDA and NTA).45,46 Similar free energy differences are observed when coordination of the trivalent lanthanides and actinides by DTTA-PyM (Table 2) is compared to that by DTPA.37 Further, Cacheris et al. showed a reduction to the thermodynamic stability of a gadolinium ion complex when two acetate arms of DTPA were replaced by N-methyl-2-pyridyl groups.43 On average, the stability constants collected for lanthanide complexation by DTTA-PyM were slightly higher, relative to the analogous N-amide-functionalized diethylenetriamine-N,N′,N″,N″-tetraacetic acid.47 The stability constants for the trivalent f-element complexation are ≈10 times higher, relative to N-(hydroxyethyl)diethylenetriamine-N,N′,N″,N″tetraacetic acid (HEDTTA), where the N-hydroxyethyl group did not coordinate the metal ions.17 This large degree of enhancement in the chelate stability further supports the participation of the N-methyl-2-pyridyl pendant arm in metal ion complexation. The influence of the fluorine substituent on the pyridine nitrogen atom is clearly manifested through a collective decrease of the metal complex stability for all investigated lanthanides (Table 2). Because the formation of the trivalent f-element/ aminopolycarboxylate complex is predominantly driven by electrostatic forces, the electron density withdrawn by the halogen weakens the ionic bonding for DTTA-3-F-PyM. If protonation of the ML− complex (eq 5) involves an acetate pendant arm, a comparison of the pK4 (the least acidic dissociation constant for carboxylate moieties) and log K111 values for the MHL(aq) complexes indicates that the carboxylate group is more susceptible to protonation after the formation of a metal complex. The opposite trend (pKn > log K111) typically holds true for aminopolycarboxylic acids capable of forming a compact octadentate coordination sphere.37,47 For DTTA-PyM, the higher K111 value, relative to the corresponding protonation constant, agrees with previously reported HEDTTA, where a noncoordinating N-hydroxyethyl group substituted an acetate arm of DTPA.17 Grimes et al. reported the formation of a lesscompact heptadentate coordination sphere (due to the inert noncomplexing hydroxyethyl group) for the trivalent f-element complex with HEDTTA.17 The reduced denticity weakened the complex, presumably promoting the formation of a MHL(aq) complex. For the DTTA-3-F-PyM ligand, formation of the MHL(aq) species is more pronounced at higher acidity, relative to DTTA-PyM. Such an enhancement in the overall acidity of the complex is consistent with the lowered acid dissociation constant of an organic acid due to halogen substitution.48 Alternatively, protonation of the ML− complex may occur on the pyridine nitrogen atom. If so, the differences between the pK6 and K111 values suggest that protonation of the pyridine nitrogen atom is substantially inhibited by the formation of a metal

Figure 5. Luminescence lifetime data for aqueous mixtures of Eu3+ or Cm3+ with DTTA-PyM at different pCH conditions. (A) Increasing lifetimes for Eu3+/DTTA-PyM mixtures as pCH is increased. (B) Overlapping lifetimes for Cm3+/DTTA-PyM mixtures regardless of pCH. For all solutions, CEu3+ = 2.0 mM, CCm3+ = 50 μM, CDTTA‑PyM = 10.0 mM, I = 2.0 M (Na+/H+)ClO4, and T = 25.0 ± 0.1 °C.

Table 3. Luminescence Lifetimes, Waters of Hydration, and % Speciation for Eu/Cm/DTTA-PyM Complexes at Various pCH Conditionsa % species M

3+

Eu

Cm

pCH

τ (μs)

ηH2O (±0.5)

5.0 4.0 3.1 2.1 2.1 5.0 4.0 3.1 2.1

601.4 541.7 474.4 108.5 394.1 246.7 246.1 237.7 201.3

1.3 1.5 1.8 9.2 2.2 1.8 1.8 1.9 2.3

M

3+

free

ML−

MHL(aq)

98.9 89.2 51.1

1.2 10.8 48.9

5.0 99.6 96.1 75.5 15.4

45.2 0.4 3.9 24.5 47.3

49.8

37.3

Aqueous phase: [Eu3+]total = 0.002 M, [Cm3+]total = 50.0 μM, [DTTA-PyM]total = 0.010 M, and I = 2.0 M (H+/Na+)ClO4. a

Table 4. M−O and M−N Distances (Å) in ML− DTTA-PyM Complexes of Eu3+ and Am3+ Computed at the B3LYP/SSC/ def2TZVPP Level M−O1 M−O2 M−O3/OCOOH M−O4 M−O5water M−N1 M−N2 M−N3 M−N4pyridine

EuL(H2O)−

AmL(H2O)−

2.301 2.350 2.335 2.426 2.647 2.752 2.667 2.863 2.660

2.340 2.391 2.377 2.464 2.692 2.773 2.697 2.902 2.696

tion sites, pK4 and pK3, may be assigned to the initial carboxylic acid protons, both lowered with respect to DTPA by the electronic effects originating from substitution of the acetate arm with the 2-pyridylmethyl functionality. Cheng et al. studied a structural modification similar to that of N′-(2-pyridylmethyl)diethylenetriamine-N,N,N″,N″-tetraacetic acid.44 The central position of this functionality directed the initial two protonation G

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Inorganic Chemistry Table 5. Leading Donor−Acceptor NBO Interactions and Their Second-Order Stabilization Energies E(2) (kJ/mol) donor NBO−acceptor NBOa

complex

LPO,carboxylate → n*M (carboxylate oxygen atoms)

LPO,pyridine → n*M (pyridine nitrogen atom)

LPO,amine → n*M (amine nitrogen atoms)

LPO,water → n*M (water oxygen atoms)

back-donation to σ*O−C, σ*N−C, and Ry* orbitals of oxygen and nitrogen (in total)

EuL(H2O)− AmL(H2O)−

2233 1887

271 234

434 400

246 245

16 88

a

The unstarred and starred labels correspond to Lewis (donor) and non-Lewis (acceptor) NBOs, respectively. Functional groups of the ligand contributing to the particular interaction are shown in parentheses. LPO denotes an occupied lone pair; n*M denotes vacant europium/americium orbitals.

Figure 6. pCH-dependent liquid−liquid distribution of tracer 154Eu, 139Ce, 241Am, and trivalent lanthanides (La3+−Ho3+) between 0.1 M HEH[EHP] in n-dodecane and 0.020 M DTTA-PyM (A) or DTTA-3-F-PyM (B). Initial aqueous mixtures contained 0.1 mM La3+ - Ho3+ (each), buffered by 0.5 M malonate, adjusted to I = 2.0 M using NaClO4.

matching the result expected from aminopolycarboxylate reagents forming octadentate chelates.49,50 As pCH decreases, the [EuDTTA-PyM]− complex is protonated. The luminescence data reflect this speciation change, showing a transition from a monohydrated to a dihydrated metal complex. Tian et al. showed that protonation of an acetate group in the [Eu(DTPA)]2− complex does not alter the coordination environment because the pendant arm rotates a carbonyl oxygen atom to maintain the octadentate binding.50 Table 3 shows that the increasing presence of [EuH(DTTAPyM)](aq) (as predicted by the log β111 constant) matches the observed increase in the calculated ηH2O. The increased innersphere hydration points to reduced denticity of the complex. In light of the observations by Tian et al., heptadentate coordination of the europium ion by DTTA-PyM may only be rationalized if the pyridine nitrogen atom is protonated. This change in the europium coordination sphere is reflected in the steadily decreasing luminescence lifetime. The increased hydration of the metal complex channels the excitation energy loss through nonradiative vibrations of the OH oscillators more efficiently.50 The observed luminescence lifetimes for mixtures of Cm3+ and DTTA-PyM show less variation compared to europium for the same region of acidity (Table 3). Much smaller differences for the luminescence quenching infer a more consistent coordination environment of the curium ion when transitioning from the CmL− complex to CmHL(aq). Beginning with the pCH = 5.0 condition, the calculated τ of 247 μs for [Cm(DTTA-PyM)]− is similar to that reported for the [Cm(DTPA)]2− complex (268 μs), well-known for its octadentate coordination.51 Because the curium aqua ion is nine-coordinate,52 the calculated higher

complex. The possible metal complex protonation sites will be considered more closely in the discussion of fluorescence results. A typical cross-lanthanide stability constant trend expected from complexants containing a diethylenetriamine moiety is observed for DTTA-PyM (Table 2).16,17,37,46 The stability constants increase rapidly from La3+ to Sm3+ and level off throughout the remaining part of the lanthanide series. Investigations by Grimes et al. and Heathman et al. showed that replacement of a single acetate pendant arm of DTPA with either an alcohol group17 or an amide group,47 respectively, did not influence this trend appreciably. The increased strength of felement complexes, when transitioning from the lanthanum ion to the samarium ion, was very similar for DTPA37 (−15.8 kJ/ mol), HEDTTA17 (−15.8 kJ/mol), and DTTA-BuA47 (−15.5 kJ/mol). However, the increase in the Gibbs free energy is higher for DTTA-PyM (−17.4 kJ/mol). The larger ΔΔG for binding of La3+ and Sm3+ by DTTA-PyM may perhaps be driven by entropic variations of the coordination by the bulky 2-pyridylmethyl group. Trivalent f-Element Coordination Environment. Coordination Sphere for Eu3+ and Cm3+. Interesting changes in the coordination spheres of trivalent europium and trivalent curium may be incurred when their luminescence decay patterns are studied in the presence of DTTA-PyM at varying aqueous acidity (Figure 5). For europium, at pCH = 5.0, the luminescence lifetimes were resolved using a single-exponential function. Considering a nearly quantitative presence of ML− for this condition, the increased luminescence lifetime, τ, is consistent with low inner-sphere hydration of Eu3+, as calculated using eq 6.39 The presence of a single water molecule in the coordination sphere of the [Eu(DTTA-PyM)]− complex was determined, H

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Inorganic Chemistry hydration number for the CmL− complex (ηH2O ≈ 2), relative to EuL−, implies a heptadentate coordination environment. Kimura and Choppin studied the hydration states of Cm3+/aminopolycarboxylate complexes.39 The authors developed eq 7 relating the luminescence decay constant, kobs (kobs = 1/τ), and the hydration number for curium, stating that “the residual hydration number of Cm3+ is consistently larger than that of Eu3+ for the same complex”.39 The authors reported a ηH2O = 1.8 for the [Cm(DTPA)]2− complex in 0.1 M NaClO4 throughout the 2 < pH < 12 region. Similarly, Tian et al. reported ηH2O = 1.5 for the [Cm(DTPA)]2− complex in 1 M NaClO4.51 Heathman et al. also reported higher hydration numbers for the Cm3+ complex, relative to the Eu3+ analogue, for an octadentate coordination by N-(butyl-2-acetamide)diethylenetriamine-N,N′,N″,N″-tetraacetic acid (DTTA-BuA).47 The consistently overestimated hydration of curium when using eq 7 for the luminescence decay data analysis implies that DTTA-PyM is likely octadentate.47 Interestingly, while Tian and co-workers found doubleexponential decay patterns for mixtures of the free Cm3+ aqua ion and the [Cm(DTPA)]2− complex, a consistent singleexponential decay was observed for all aqueous conditions containing DTTA-PyM.51 These differing luminescence quenching behaviors could imply that a faster luminescence lifetime of a free curium ion (τ = 65 μs) was not measured (due to an inaccurate estimate of the species distribution) or not detected (due to the greater presence of longer decaying luminescence components). Nonetheless, the matching single-exponential decay patterns for Cm3+/DTTA-PyM mixtures across the 2.1 < pCH < 5.0 region of acidity show a remarkable difference between the coordination environment of the Cm3+ and Eu3+ ions. The consistent ηH2O ≈ 2 suggests that octadentate coordination of curium is maintained after [Cm(DTTA-PyM)]− is protonated. Accordingly, a carboxylate group may be a preferred site of protonation for DTTA-PyM complexes with trivalent actinides, whereas the pyridyl nitrogen atom is protonated for lanthanide complexes. An3+/Ln3+ Differentiation. Complex Stability. Heitzmann et al. replaced two pendant acetate arms of EDTA with two N-2pyridylmethyl groups (Lpy) to enhance differentiation of trivalent actinides and trivalent lanthanides.53 The authors, through a comparison of log β101 for the complexation of Nd3+ and Am3+ by EDTA and the modified ligand Lpy, demonstrated a 3-fold increase for actinide binding by Lpy relative to the lanthanide ion46,53 (EDTA, β101(Am/Nd) = 31.6; Lpy, β101(Am/Nd) = 87.1). Jensen et al. studied N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), where all of the acetate groups of EDTA had been substituted by 2-pyridylmethyl functionalities.54 A comparison of the log β101 values for the complexation of Sm3+ and Am3+ by EDTA and TPEN showed a 10-fold Am3+/Sm3+ enhancement for TPEN, as expected from the introduction of additional nitrogen-donor groups.46,53 For DTTA-PyM, a similar comparison with DTPA yields a 2-fold improvement in Am3+/ Nd3+ differentiation46 (DTPA, β101(Am/Nd) = 25.1; DTTA-PyM, β101(Am/Nd) = 55.0). This modest increase in trivalent 4f/5f group differentiation agrees well with the studies by Heitzmann et al. and Jensen et al., illustrating that the replacement of each Nacetate arm by N-2-pyridylmethyl increases soft-donor characteristics while sacrificing the stability of the metal complex.53,54 The stability constant comparison for the trivalent 4f and 5f elements of similar charge density may be visualized by the linear free energy relationship shown in Figure 7. For pure oxygen-

Figure 7. Correlation between the stability constants of trivalent lanthanide complexes (Nd3+ and Sm3+) and of complexes of trivalent americium. A linear relationship was developed for complexants composed of aminoacetate building blocks [iminodiacetate, nitrilotriacetate,46 ethylenediaminetetraacetate,46 trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetate,46 diethylenetriaminepentaacetate,46 ethylenediamine-N,N′-di(acetylglycine)-N,N′-diacetate,46 diethylenetriamine-N,N′′-bis(acetylglycine)-N,N′,N′′-triacetate, 16 and N(hydroxyethyl)diethylenetriamine-N,N′,N′′,N′′-tetracetate].17 Lpy = N,N-bis(2-pyridylmethyl)ethylenediamine-N′,N′-diacetic acid. 53 TPEN = N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine.54 TPAEN = N,N,N′,N′-tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine.56

donor ligands, the Am3+ and Nd3+ chelate stabilities overlap, resulting in a correlation where the slope equals 1 (not shown). For aqueous complexants composed of aminoacetate building blocks, the stronger interaction with trivalent actinides is reflected by a steeper slope (≈1.07) for the plotted stabilities of 1:1 or 1:2 metal/ligand chelates of Am3+ versus Nd3+ and Sm3+ ions.55 Figure 7 illustrates that the aminopolycarboxylate reagents with 2-pyridylmethyl groups consistently show higher Am3+/Nd3+ stability constant ratios, deviating from the linear trend established for traditional aminoacetate molecules.53,54,56 Although this departure is less pronounced for DTTA-PyM, relative to reagents containing a higher number of N-2pyridylmethyl pendant arms [Lpy, TPEN, and N,N,N′,N′tetrakis[(6-carboxypyridin-2-yl)methyl]ethylenediamine (TPAEN)], it indicates the enhanced preference for the binding of trivalent actinides.53,54,56 According to this relative assessment, an improved An3+/Ln3+ separation should be expected if DTPA is replaced by DTTA-PyM as an aqueous complexant in TALSPEAK-related liquid−liquid separation platforms. DFT Considerations. DFT calculations and chemical bonding analyses were utilized to study the origin of the enhanced complexation of trivalent americium by DTTA-PyM, relative to the trivalent europium ion. The calculations only considered the formation of ML− complexes with Am3+ and Eu3+ for this comparative work. The calculated EuL− bond distances for carboxylate oxygen (2.30−2.43 Å) and amine nitrogen (2.67− 2.86 Å) atoms in Table 4 are consistent with the corresponding X-ray-32,33 and DFT-calculated bond lengths for the related EuDTPA2− complex.50 Replacing one carboxylate functionality with a 2-pyridylmethyl group results in a longer bond distance between Eu3+ and the N3 amine nitrogen atom compared to average Eu−N1,2 bond lengths. The bond distance to the pyridine nitrogen atom N4 (2.66 Å) is the shortest among all of the nitrogen atoms. This is consistent with the crystallographic I

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lone-pair donor (LP)−vacant orbital acceptor (n*M) NBO interactions for [Eu(DTTA-PyM)]− and [Am(DTTA-PyM)]− and their corresponding second-order stabilization energies E(2) (kJ/mol) are summarized in Table 5. Higher E(2) values were calculated for all ligand-to-metal bonding interactions in the [Eu(DTTA-MPy)]− complex. Because the charge density of the Eu3+ ion is higher, relative to Am3+, the respective LP: → n*M dative bonds are more delocalized toward the europium metal center, showing higher stabilization energies. The calculations of natural orbital occupancies (Table SI-1) at the B3LYP/SSC/ def2-TZVPP level, which allow the explicit treatment of f electrons in the valence space, also indicate greater participation of the Eu3+ 5d orbitals, relative to 6d orbitals of Am3+, in the metal−ligand bonds. However, more significant f-orbital mixing is observed in the [Am(DTTA-PyM)]− complex. Along with the dative flow of charge from the ligand to metal, some degree of back-bonding is expected in the Eu3+ and Am3+ complexes. Accordingly, the ligand is both a strong σ donor (Lewis base) and π acceptor (Lewis acid), resulting in a synergistic interplay between σ donation and π back-bonding. The NBO analyses reveal notable back-donation (Table 5) from the singly occupied 5f orbitals of Am3+ to the unfilled Rydberg (Ry*), σ*O−C, and σ*N−C ligand orbitals. The corresponding back-bonding interactions involving 4f orbitals of Eu3+ are less significant. It is worth noting that the back-bonding interactions become particularly important for ligands with unfilled π* orbitals on nitrogen-donor atoms, which are apparently the most suited for secondary nAm → π*L interactions with partially filled Am 5f orbitals. Figure 8 compares the π-back-bonding interactions between singly occupied 5f and 4f orbitals of Am3+ and Eu3+, respectively, and the Rydberg orbitals on the central amine (Ry*N2) and pyridine (Ry*N4) nitrogen atoms of DTTAPyM. The computed stabilization E(2) energies (listed in

study by McLauchlan et al., who reported shorter bond lengths between the lanthanum ion and the pyridine nitrogen atoms of N,N′-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N′diacetic acid, relative to the nitrogen atoms of the ethylenediamine backbone.57 The M−N4 bond in [EuDTTA-PyM]− is still substantially longer than a typical metal−oxygen bond, however. This result reiterates the expected weakening of a metal-ion complex stability when an acetate group of DTPA is substituted by a 2-pyridylmethyl group. At the B3LYP/SSC/def2-TZVPP level in the gas phase, the optimized structures of EuL− and AmL− complexes are very similar, with bond distances for the larger Am3+ ion being 0.037 Å longer on average. Additionally, we have performed DFT-based molecular dynamics (MD) simulations for EuL− in explicit water, which emphasize the dynamic nature of Eu3+ coordination. The results reveal that, while the shorter and stronger Eu−O bonds remain steady, the more labile Eu−N bonds can dynamically break and form. An expanded discussion of bonding and coordination in the EuL− complex in an aqueous environment is presented in the Supporting Information. To seek computational indication of enhanced An3+/Ln3+ differentiation for DTTA-PyM, the exchange reaction (8) was considered. Am(NO3)3 (H 2O)3 + Eu(L)(H 2O)− ⇌ Eu(NO3)3 (H 2O)3 + Am(L)(H 2O)−

(8)

The advantages of using this approach were recently discussed by Ivanov et al.34 when considering the size-based intralanthanide differentiation. It is important to note the challenges of this approach, however, when drawing comparative clues for actinide and lanthanide ions. The utility of LC RECP (f-subshellin-core) may not be as well justified for actinides as for lanthanides. Also, as illustrated by Dolg et al.,58 the inclusion of f electrons in-valence (small-core RECP) can lead to serious artifacts related to the self-interaction error of the most contemporary DFT functionals. The estimation of the energy difference for the Am3+/Eu3+ exchange (eq 8) indicates that the reaction is favored if the calculated value is negative. When using the small-core RECP (fin-valence), the calculation yielded ΔEAm/Eu of −2.3 kJ/mol, showing a tendency to dissociate the [Eu(DTTA-PyM)]− complex to form the Am3+ equivalent. This value is smaller than the experimentally observed −4.6 kJ/mol difference between the stabilities of AmL− and EuL− complexes of DTTA-PyM. This highlights the above-mentioned uncertainties associated with DFT calculations on actinides. The LC RECP (fin-core) calculation yielded 0.6 kJ/mol and failed to capture the observed selectivity for Am3+ complexation by DTTA-PyM. The different responses between the small-core and LC predictions may indicate that the back-donation from singly occupied f orbitals to the unoccupied ligand orbitals (not allowed when f electrons are placed in-core) may contribute to Am3+/Eu3+ selectivity. To gain more insight into the f-orbital participation in the chemical bonding, NBO analysis35,36 was performed. The Eu3+ and Am3+ Lewis acids, with partially filled f shells, when participating in dative interactions with the ligand, yield formal, polarized metal−ligand σ bonds with negative charges on the donor atoms. If treated as coordinate bonding of a closed-shell Lewis base (L:) with a Lewis acid (M), the strength of a donor− acceptor interaction (L: → M) can be estimated within the framework of the second-order perturbation theory.35 Leading

Figure 8. Comparison of π-back-bonding interactions between the representative singly occupied (a) 5f orbital of Am (nAm) and (b) 4f orbital of Eu (nEu) and the Rydberg orbitals on the amine (Ry*N2) and pyridine (Ry*N4) nitrogen atoms, respectively. J

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Inorganic Chemistry parentheses) are higher for the [Am(DTTA-PyM)]− complex, opposite to the trend demonstrated for the ligand-to-metal bonding interactions. In general, the enhancement observed for the interaction of N2 and americium agrees with stronger binding of actinides by aminopolycarboxylates. The added stabilization coming from the pyridine’s nitrogen atom of DTTA-PyM may hint the enhanced An3+/Ln3+ differentiation as observed in the experimental studies. Aqueous Actinide Holdback in Liquid−Liquid Systems. When an efficient An3+/Ln3+ differentiation using a liquid−liquid solvent extraction platform is targeted, an aminopolycarboxylate complexant efficiently holds trivalent actinides in the aqueous environment while trivalent lanthanides partition into the nonaqueous layer.14 When employing DTPA as an aqueous holdback complexant, Nilsson and Nash reported minimum 4f/ 5f group separation factors (quotient of distribution ratios for the least extractable lanthanide and americium ions) ranging from 29 to 37 for the 2.48 < pCH < 4.8 range of aqueous acidities.59 The original TALSPEAK recipe, as reported by Weaver and Kapplemann, with 0.05 M DTPA, consistently shows neodymium as the least extractable lanthanide ion, yielding a 13 Nd maximum SFNd Am of 30 at pH 3.5. Grimes et al. reported SFAm of 28 for a similar liquid−liquid system, when buffering the aqueous phase with L-alanine at pCH = 3.0.60 Gelis et al. utilized DTPA to back-extract Am3+ from a nonaqueous mixture of HEH[EHP] and the neutral diglycolamide extractant containing trivalent f elements.61 The authors showed that 4f/5f group separation was driven by SFNd Am of 36 when DTPA was present in a citratebuffered aqueous phase at pCH = 3.85. The aqueous complexation of trivalent actinides by DTTAPyM, and its fluorinated analogue, performed favorably on a liquid−liquid distribution platform (Figure 6) compared to all previous studies utilizing DTPA. The 2-fold enhancement in the An3+/Ln3+ differentiation, expected from a comparison of the stability constants for coordination of Am3+ and Nd3+ by DTTAPyM, is preserved throughout the 3.00 < pCH < 4.5 region of acidity, yielding SFNd Am ≈ 70. This result arguably indicates the most efficient separation of trivalent actinides from trivalent lanthanides using an aminopolycarboxylate reagent reported to date.13,14,16,17,59−62 At pCH = 2.75, the low extractability of the lanthanum ion may be attributed to weak electrostatic attraction between a lanthanide of lowest charge density and the oxygendonor HEH[EHP]. Here, the 4f/5f group separation is driven by the partitioning of lanthanum, and the SFLa Am is reduced to 30. For DTTA-3-F-PyM, the introduction of a fluorine substituent on the pyridine ring increased the acidity of the ligand, as demonstrated by the changes in the acid dissociation constants (Table 1). Figure 6B illustrates the pCH dependency for the liquid−liquid distribution trends for Am3+ and lanthanide ions when DTTA-3-F-PyM was used to balance the phase-transfer equilibrium facilitated by HEH[EHP]. As expected, the electronwithdrawing influence of fluorine reduces the overall basicity of the DTTA-3-F-PyM reagent, yielding slightly decreased An3+/ Ln3+ differentiation, relative to DTTA-PyM (SFNd Am ≈ 50). This result still outperforms the trivalent actinide/trivalent lanthanide separation trends established by previously reported DTPAbased chemistries.13,14,16,17,59−62 The added benefit of the enhanced total ligand acidity through the incorporation of fluorine is manifested by the shift of efficient An3+/Ln3+ separation to lower pCH regimes. The intergroup differentiation enhanced by the coordination of trivalent f-elements by DTTA3-F-PyM is obtainable for the 2.5 < pCH < 3.75 region, while the partitioning of La3+ drops off below pCH = 2.5. The electronic

influence of fluorine demonstrates the opportunity to further tune the structural features of aminopolycarboxylate complexants to blend features enhancing the An3+/Ln3+ differentiation with those capable of strengthening the f-element complexation abilities in the aqueous regions of low pCH.

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CONCLUSIONS The influence of the heterocyclic N-donor group (2pyridylmethyl) on the coordination of trivalent actinides and lanthanides was investigated via the structural modification of a well-known aqueous complexant DTPA. Thermodynamic characterization of DTTA-PyM using potentiometric titration and UV−vis spectroscopy showed the formation of ML− and MHL(aq) complexes for trivalent lanthanides (La3+−Lu3+) and trivalent actinides (Am3+ and Cm3+). On average, the Ln/ DTTA-PyM stability constants were lowered by 2 orders of magnitude, relative to DTPA. A direct probe of the inner coordination sphere using fluorescence techniques for Eu3+ and Cm3+ bound to DTTA-PyM showed two different binding modes across a 2.1−5.0 pCH range. The luminescence lifetimes showed Cm3+ complexes of DTTA-PyM to be octadentate throughout the measured pCH range. Conversely, over the same pCH range, the data showed that Eu3+ complexation with DTTAPyM changed gradually from heptadentate (pCH = 2.1) to octadendate (pCH = 5.0). This suggests that the pyridyl nitrogen atom is protonated in the low pCH regime and eventually participates in the binding of Eu3+ as the pKa value of the pyridyl nitrogen atom is approached. The solvent extraction results showed SFNd Am ≈ 70 across the 3.00 < pCH < 4.5 range of acidity where Nd3+ is typically the least extracted lanthanide in these TALSPEAK-like separation platforms. This unprecedented result indicates the most efficient separation of trivalent actinides from trivalent lanthanides using an aminopolycarboxylate reagent reported to date. This result also may help to confirm the participation of an additional soft-donor nitrogen atom in the [Am(DTTA-PyM)]− complex presented by the N-2-pyridylmethyl group. In an effort to study electron-withdrawing effects on the diethylenetriamine backbone and potentially preserve the garnered separation factors in a more acidic media, DTPA was also functionalized as previously described with a 3-fluoropyridylmethyl group, producing DTTA-3-F-PyM. Potentiometric titration showed an overall reduction in the pKa values for DTTA-3-F-PyM compared to DTPA. Selected Ln/DTTA-3-FPyM stability constants also showed a reduction of ≈2.5 compared to that of DTPA, confirming the desired electronwithdrawing effect and the increased acidity of the molecule. In the aqueous acidity range of 2.5 < pCH < 3.75, this unique aqueous chelator produced SFNd Am ≈ 50. The results for the DTTA-3-F-PyM molecule demonstrate an overall increased ligand acidity that promotes a reduction in the proton competition enough to produce the first-time 4f/5f-element separation factor of ≈50 at pCH = 2.5 using an aminopolycarboxylate holdback reagent.

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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.7b02792. Synthesis accompanied by the 1H and 13C NMR spectra (Figures SI-1−SI-4) of 2,2′-[[2-[(carboxymethyl)[2K

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Inorganic Chemistry

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Scale Separation of Americium from Rare Earths. J. Inorg. Nucl. Chem. 1956, 3 (5), 327−328. (11) Mayer, S. F.; Freiling, E. C. Ion Exchange as a Separation Method. VI. Column Studies of the Relative Efficiencies of Various Complexing Agents for the Separation of Lighter Rare Earths. J. Am. Chem. Soc. 1953, 75 (22), 5647−5649. (12) Orr, P. B. Ion-exchange Purification of Promethium-147 and its Separation from Americium-241 with Diethylenetriaminepentaacetic Acid as the Eluant. United States Atomic Energy Commission Unclassified Report No. ORNL-3271; Oak Ridge Nationla Laboratory: Oak Ridge, TN, 1962. (13) Weaver, B.; Kappelmann, F. A. Preferential Extraction of Lanthanides over Trivalent Actinides by Monoacidic Organophosphates from Carboxylic Acid and from Mixtures of Carboxylic and Aminopolyacetic Acids. J. Inorg. Nucl. Chem. 1968, 30, 263−272. (14) Nash, K. L. The Chemistry of TALSPEAK: A Review of the Science. Solvent Extr. Ion Exch. 2015, 33 (1), 1−55. (15) Heathman, C. R.; Grimes, T. S.; Zalupski, P. R. Thermodynamic and Spectroscopic Studies of Trivalent f-Element Complexation with Ethylenediamine-N,N′-di(acetylglycine)-N,N′-diacetic Acid. Inorg. Chem. 2016, 55, 2977−2985. (16) Heathman, C. R.; Grimes, T. S.; Zalupski, P. R. Coordination Chemistry and f-Element Complexation by Diethylenetriamine-N,N″bis(acetylglycine)-N,N′,N″-triacetic Acid. Inorg. Chem. 2016, 55, 11600−11611. (17) Grimes, T. S.; Heathman, C. R.; Jansone-Popova, S.; Bryantsev, V. S.; Goverapet Srinivasan, S.; Nakase, M.; Zalupski, P. R. Thermodynamic, Spectroscopic, and Computational Studies of f-Element Complexation by N-Hydroxyethyl-diethylenetriamine-N,N′,N″,N″-tetraacetic Acid. Inorg. Chem. 2017, 56, 1722−1733. (18) Tian, G.; Shuh, D. K. A Spectroscopic Study of Am(III) Complexation with Nitrate in Aqueous Solution at Elevated Temperatures. Dalton Trans. 2014, 43, 14565−14569. (19) Carnall, W. T.; Fields, P. R.; Stewart, D. C.; Keenan, T. K. The Absorption Spectrum of Aqueous Curium(III). J. Inorg. Nucl. Chem. 1958, 6, 213−216. (20) Gran, G.; Dahlenborg, H.; Laurell, S.; Rottenberg, M. Determination of the Equivalence Point in Potentiometric Titrations. Acta Chem. Scand. 1950, 4, 559−577. (21) Gran, G. Determination of the Equivalence Point in Potentiometric Titrations. Part II. Analyst 1952, 77, 661−671. (22) Rossotti, F. J. C.; Rossotti, H. Potentiometric Titrations Using Gran Plots: A Textbook Omission. J. Chem. J. Chem. Educ. 1965, 42 (7), 375−378. (23) Gans, P.; Sabatini, A.; Vacca, A. Investigation of Equilibrium in Solution. Determination of Equilibrium Constants with the HYPERQUAD Suite of Programs. Talanta 1996, 43, 1739−1753. (24) Gans, P.; Sabatini, A.; Vacca, A. Determination of Equilibrium Constants from Spectrophotometric Data Obtained from Solutions of Known pH: the Program pHab. Ann. Chim. 1999, 89, 45−49. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.; Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J. V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.; Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson, T.; Ranashinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.; Liang, W.; Hada, M.; Ehara, M.; Toyaota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.; Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09, revision D.01; Gaussian, Inc.: Wallingford, CT, 2016. (26) Becke, A. D. Density-functional thermochemistry: The role of exact exchange. J. Chem. Phys. 1993, 98, 5648−5652.

[(carboxymethyl)(pyridin-2-ylmethyl)amino]ethyl]amino]ethyl]azanediyl]diacetic acid and 2,2′-[[2[(carboxymethyl)[2-[(carboxymethyl)[(3-fluoropyridin2-yl)methyl]amino]ethyl]amino]ethyl]azanediyl]diacetic acid HCl salts, natural electron configuration of Eu3+ and Am3+ ions in the studied complexes of DTTA-PyM (Table SI-1), and DFT-based first-principles MD simulations for EuL− in explicit water (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Travis S. Grimes: 0000-0003-2751-0492 Colt R. Heathman: 0000-0001-9436-5972 Santa Jansone-Popova: 0000-0002-0690-5957 Vyacheslav S. Bryantsev: 0000-0002-6501-6594 Peter R. Zalupski: 0000-0001-7359-5568 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The experimental work conducted by T.S.G., C.R.H., and P.R.Z. at the Idaho National Laboratory was supported by the U.S. Department of Energy, Office of Nuclear Energy, DOE Idaho Operations Office, under Contract DE-AC07-05ID14517. The synthetic work by S.J.-P. and computational studies by A.S.I., S.R., and V.S.B. were supported by the Fuel Cycle Research and Development Program, Office of Nuclear Energy, U.S. Department of Energy. DFT calculations used resources of the National Energy Research Scientific Computing Center and the Oak Ridge Leadership Computing Facility at the Oak Ridge National Laboratory, supported by the Office of Science of the U.S. Department of Energy under Contracts DE-AC02-05CH11231 and DE-AC05-00OR22725, respectively.

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REFERENCES

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M

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