Selective Separation and Complexation of Trivalent Actinide and

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

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Selective Separation and Complexation of Trivalent Actinide and Lanthanide by a Tetradentate Soft−Hard Donor Ligand: Solvent Extraction, Spectroscopy, and DFT Calculations Lei Xu,†,§ Ning Pu,†,§ Youzhen Li,† Pingping Wei,† Taoxiang Sun,† Chengliang Xiao,*,‡ Jing Chen,*,† and Chao Xu*,†

Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF TEXAS AT DALLAS on 03/14/19. For personal use only.

†

Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China ‡ College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: Recently, phenanthroline-based ligands have received increasing attention due to their excellent separation capabilities for trivalent actinides over lanthanide. In this work, we designed a soft−hard donor combined tetradentate phenanthroline-based extractant, tetraethyl (1,10-phenanthrolin-2,9-diyl)phosphonate (C2-POPhen), for the selective separation of trivalent Am(III) over Ln(III) from HNO3 media. The solvent extraction and complexation behaviors of Am(III) and Ln(III) by C2-POPhen were investigated both experimentally and theoretically. C2-POPhen could selectively extract Am(III) over Eu(III) with an extremely fast extraction kinetics. NMR titration studies suggest that only 1:1 complexes of Ln(III) with C2-POPhen formed in CH3OH in the presence of a significant amount of nitrate, while both 1:1 and 2:1 complexes species could form between C2-POPhen and Ln(III) perchlorate in CH3OH without nitrate ions. The stability constants for the complexation of Am(III) and Ln(III) with C2-POPhen in CH3OH were determined by spectrophotometric titrations and the Am(III) complexes are approximately 1 order of magnitude stronger than those of Ln(III), which is consistent with the extraction results. Theoretical calculations indicate that the Am−N bonds in Am/C2-POPhen complexes possess more covalent characters than the Eu−N bonds in Eu/C2-POPhen complexes, shedding light on the underlying chemical force responsible for the Am/Eu selectivity by C2-POPhen. This work represents the first report utilizing phenanthroline-based phosphonate ligands for selective separation of actinides from highly acidic solutions.

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INTRODUCTION To establish advanced nuclear fuel cycle systems, chemical separation of long-lived minor trivalent actinides (MAs) over trivalent lanthanides from highly radioactive liquid waste (HLW, PUREX raffinate) is a key step based on the partitioning and transmutation (P&T) strategy.1−4 The aim of P&T strategy is to transmute long-lived MAs into stable elements or short-lived nuclides via neutron fission and is considered an effective method to reduce the long-term radiotoxicity of nuclear waste.5,6 Due to the presence of large amounts of neutron poisonous lanthanides in PUREX raffinate,7 MAs must be first separated from lanthanides to avoid decreasing the transmutation efficiency.8 However, such a separation remains a great challenge as a result of the great physicochemical similarities between trivalent MAs and lanthanides.9 It has been recognized that the less shielding effect of 5f orbitals in actinides than 4f orbitals in lanthanides endows the actinide-ligand bonds with more covalent characters than those of lanthanides.10−12 The soft donor ligands containing N or S © XXXX American Chemical Society

atoms can distinguish this small difference between actinides and lanthanides and thus a large number of S or N donor ligands have been designed for the selective separation of trivalent actinides over lanthanides by liquid−liquid solvent extraction.3,7,13−21 Among these ligands, a new type of tetradentate phenanthroline-derived extractants (Figure 1) such as 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen),22−26 2,9-di(quinazolin-2-yl)-1,10-phenanthroline (BQPhen),10,27 and 2,9-diamide-1,10-phenanthroline (DAPhen)28,29 have attracted increasing attention due to their great potential for actinide separation. The high degree of ligand preorganization in phenanthroline-derived ligands grants them faster extraction kinetics and larger distribution ratios in solvent extraction as compared to their nonpreorganized counterparts.30,31 However, the phenanthrolinederived ligands are still less explored, and further investigation is required to understand their fundamental coordination Received: December 25, 2018

A

DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

POPhen have been thoroughly investigated by batch extraction experiments, NMR and absorption spectrophotometric titrations, and DFT calculations.

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EXPERIMENTAL SECTION

Chemicals. The analytical-grade rare earth nitrates Ln(NO3)3 6H2O (Ln = La, Nd, Eu, and Lu), tetra-ethylammonium nitrate (Et4NNO3), NaClO4, and other organic reagents, CH3OH and 3nitrobenzotrifluoride, were used as received without further purification. All chemicals reagents used for the synthesis of ligand C2-POPhen are of analytical grade or higher. The solid Nd(ClO4)3 was obtained by reacting Nd2O3 with perchloric acid under stirring and then heating to dryness. The stock solution of Nd(ClO4)3 was prepared by directly dissolving Nd(ClO4)3 solid in CH3OH. The Ln(III) concentration in solutions was determined by complexometric titrations with ethylenediaminetetraacetic acid (EDTA) and using xylenol orange as the indicator.32 The stock solution of 241 Am(III) in CH3OH was prepared according to the method reported in the literature.33 CAUTION: Radioactive 241Am could pose serious health threats; all experimental work was conducted in radiological facilities dedicated to studies on transuranium elements. The concentration of Am(III) in CH3OH was determined on a Quantulus 1220 Ultra Low Level Liquid Scintillation Spectrometer. C2-POPhen was synthesized using the procedures as reported in a previous work.34 C2-POPhen: 1H NMR (600 MHz, CD3OD): δ = 1.41 ppm (t, 12H, −CH3), 4.42 ppm (m, 8H, −CH2−), 8.08 ppm (s, 2H, −Ar−H), 8.23 ppm (dd, 2H, −Ar−H), 8.60 ppm (dd, 2H, −Ar−H) Elemental analysis calcd (%) for C20H26N2O6P2 (C2-POPhen): C 53.10, H 5.79, N 6.19. Found: C 52.99, H 5.91, N 5.89. Solvent Extraction. The aqueous solutions with different HNO3 concentrations (0.1∼3.0 M) were spiked with trace amounts of radioactive 241Am(III) and 152,154Eu(III). The organic phases were

Figure 1. Promising extractants for trivalent lanthanide/actinide separation mentioned in this study.

behavior as well as to extent their applications in actinide separation. Moreover, as is well-known, a phosphate derivative, tributyl phosphate (TBP), has been industrialized in the separation of uranium and plutonium from spent nuclear waste through PUREX process worldwide due to its low volatility, high flash point, and high stability against nitric acid and radiation. To combine the advantages of phenanthroline and phosphate groups, herein, a preorganized 1,10-phenanthroline-drived ligand (C2-POPhen) bearing diethyl-phosphonate moieties has been synthesized and used for selective separation of Am(III) over Ln(III). The solvent extraction properties and the complexation behaviors between Am(III)/Ln(III) and C2-

Figure 2. Distribution ratios (D) of Am(III) and Eu(III) by C2-POPhen in 3-nitrobenzotrifluoride as a function of (A) contact time ([HNO3]i = 1.0 M, [L] = 10 mM); (B) ligand concentration([HNO3]i = 1.0 M); (C) HNO3 concentration ([L] = 10 mM); (D) temperature ([HNO3]i = 1.0 M, [L] = 10 mM). B

DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry obtained by dissolving appropriate amounts of C2-POPhen in 3nitrobenzotrifluoride. Each 0.5 mL of aqueous phase was contacted with 0.5 mL of organic phase, and the mixture was vigorously shaken for 60 min (except for the kinetics experiments) at 298 ± 1K. After phase separation by centrifugation, the relative concentrations of 241 Am(III) and 152,154Eu(III) in aqueous phases before and after extraction were measured using Liquid Scintillation Spectrometer. The distribution ratio (D) was calculated by the ratio between the concentration (radioactivity counts per unit volume) in the organic phase and in the aqueous phase. The separation factors (SF) was determined by the ratio of distribution ratios of 241Am(III) to 152,154 Eu(III). NMR Titration. For NMR titration, the stock solutions of C2POPhen, La(NO3)3 and La(ClO4)3 were prepared by dissolving the C2-POPhen solid, La(NO3)3 6H2O, and La(ClO4)3 in CD3OD (1H NMR) or CH3OH (31P NMR), respectively. An initial 0.5 mL of C2POPhen solution was added into an NMR tube, and its NMR spectrum was collected on a Bruker Avance III Model 600 MHz instrument. Then, appropriate amounts of La(III) solution were added into the NMR tube. After each addition, the resulting mixture was mixed for 5 min to ensure that the complexation reaction reached equilibrium and then the 1H or 31P NMR spectra of the samples were collected. The titration was stopped until no further changes were observed in the titration spectra. Spectrophotometry. The stability constants of Am(III) and some typical Ln(III) ions with C2-POPhen in CH3OH at 298 K were determined by UV−vis−NIR spectrophotometric titrations on a Cary 6000i spectrophotometer (Agilent, Inc.). For nitrate media (I = 0.01 M Et4NNO3), an appropriate volume of 1.0 mM Ln(III) in CH3OH was added into a 0.02 mM solution of C2-POPhen in CH3OH and the absorption was monitored in the range of 240−360 nm. For perchlorate media (I = 0.01 M NaClO4), an appropriate volume of C2-POPhen in CH3OH was added into Am(III) or Nd(III) in CH3OH, and the absorption was monitored in the range of 490−530 nm for Am(III) and 560−620 nm for Nd(III). Preliminary kinetics experiments have shown that the complexation reactions could reach the equilibrium state within a few minutes. All the stability constants in this work were calculated using the nonlinear regression program HypSpec.35 Computational Details. The complexation behaviors of Am(III) and Eu(III) with C2-POPhen were studied by density functional theory (DFT) using the Gaussian 09 package.36 The B3LYP hybrid functional (Becke’s three-parameter nonlocal hybrid exchange correlation functional, Becke−Lee−Yang−Parr) was used.37,38 The Pople style double ζ 6-31G** basis set with polarization functions was used for C, H, O, N, and P atoms. The energy-consistent ECP28MWB effective core potentials as well as (14s13p10d8f6g)/ [6s6p5d4f3g] valence basis sets were used for Eu, and energyconsistent ECP60MWB effective core potentials as well as (14s13p10d8f6g)/[6s6p5d4f3g] valence basis sets were used for Am to consider the scalar quasi-relativistic effects.39−41 All the geometries were optimized in the gas phase without symmetry constraint. For each optimized structure, analytical frequencies were calculated to ensure that the minimum point on the potential energy surface was found. Furthermore, the Hirshfeld charges and Wiberg bond indices were calculated by Multiwfn 3.3.9 program.42 In order to take consideration of the solvent effect, the SMD implicit solvation model was used to calculate the rotation energy barrier of C2-POPhen.

POPhen are even faster than that by the state-of-the-art BTPhen type ligands,22 which requires up to 20 min to reach extraction equilibrium. Therefore, the kinetics for actinide separation can be greatly improved by using C2-POPhen as the extractant, which is a significant advantage in real industry applications. Figure 2B displays the extraction results of Am(III) and Eu(III) from 1.0 M HNO3 to 3-nitrobenzotrifluoride solution by varied concentrations of C2-POPhen. As the concentration of C2-POPhen increases from 1 to 20 mM, the D values of both Am(III) and Eu(III) increases obviously. The average separation factor of SFAm(III)/Eu(III) is about 7, which indicates that C2-POPhen does possess the potential to separate trivalent Am(III) over Ln(III) in solvent extraction. To the best of our knowledge, this is the first time that a phenanthroline-derived phosphonate ligand has been reported for partitioning of actinides over lanthanides. The slopes of log DAm(III) and log DEu(III) versus log[C2POPhen] are 1.44 ± 0.07 and 1.43 ± 0.02, respectively. The nearly identical and nonunit slopes for Am(III) and Eu(III) indicate that the stoichiometries of their extracted species should be similar and both 1:1 and 2:1 ligand/metal complexes should coexist in the extraction systems. The formation of both 1:1 and 2:1 complexes with Am(III) and Ln(III) have also been found for some other tetradentate N-donor phenanthroline-derived ligands such as R-BTPhen,23,43,44 BQPhen,10 and DAPhen.28 NMR and absorption spectroscopic titration studies in the following sections on the complexation behaviors of Am(III) and some typical lanthanides ions with C2-POPhen will provide more information to illustrate the species between Am(III)/Ln(III) and C2-POPhen. The influence of HNO3 concentration on the extraction behaviors of Am(III) and Eu(III) by C2-POPhen was also studied. As depicted in Figure 2C, C2-POPhen exhibits stronger extraction ability toward Am(III) than Eu(III) over a wide range of HNO3 concentrations, implying that C2POPhen has good potential to selectively separate Am(III) over Ln(III) from highly acidic radioactive liquid waste (HLW). Moreover, it was found that both the distribution ratios of Am(III) and Eu(III) increase quickly when the concentration of HNO3 increases from 0.01 to 2.0 M, suggesting that an increase of nitrate concentration is beneficial for the extraction. In combination with the results of slope analysis, the extraction mechanism of Am(III) could be deduced as Am 3 +aq + 3NO3−aq + 1.47(L)org ↔ Am(NO3)3 · 1.47(L)org

(1)

It should be noted that eq 1 is an averaged extraction reaction. It was used to facilitate the data analysis and to obtain some quantitative information to help us better understand the extraction system. The apparent extraction equilibrium constant Kex of the extraction reaction can be defined as eq 2.

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RESULTS AND DISCUSSIONS Solvent Extraction. Solvent extraction experiments were performed to test the capability of C2-POPhen to selectively extract Am(III) over Eu(III) in HNO3 media, and the results are shown in Figure 2. First, the effect of contacting time on the extraction was investigated. As shown in Figure 2A, both the distribution ratios of Am(III) and Eu(III) became constant in 2 min, showing extremely fast extraction kinetics without the addition of any phase-transfer agents. The extraction by C2-

Kex =

[Am(NO3)3 · 1.47(L)]org [Am 3 +]aq [NO3−]3aq [L]1.47 org

(2)

The distribution ratio of Am3+ (DAm) can be described as eq 3. DAm = C

[Am(NO3)3 · 1.47(L)]org [Am 3 +]tot,aq

(3) DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Since Am3+ in the aqueous phase could form complexes with nitrate ion, the concentration of Am3+(aq) would depend on the stability constants (βn) of Am(III)/nitrate complexes. DAm can be described as eq 4.45 DAm =

[Am(NO3)3 · 1.47(L)]org

(

[Am 3 +]aq 1 + ∑ βn[NO−3 ]naq

)

(4)

where βn is the stability constant for Am(NO3)n complex in the aqueous solution. Combining eqs 2 and 4 gives eq 5. Kex =

(

DAm 1 + ∑ βn[NO3−]naq

)

[NO3−]3aq [L]1.47 org

(5)

Figure 3. Apparent extraction equilibrium constants log Kex as a function of temperature. Initial solutions: organic phase, 10 mM C2POPhen in 3-nitrobenzotrifluoride; aqueous phase, trace amount of 241 Am(III) and 152,154Eu(III) in 1.0 M HNO3 solution.

Generally, Am3+ could form up to two stepwise complexes with NO3−; therefore, eq 5 can be rewritten as eq 6.46 Kex =

DAm(1 + β1[NO3−]aq + β2[NO3−]2 aq ) [NO3−]3aq [L]1.47 org

complexes formed between ligand and diamagnetic metal ions in solutions.23,47−49 In this work, the complexation behaviors of lanthanides with C2-POPhen were qualitatively studied via 1 H and 31P NMR titrations in CD3OD and CH3OH, respectively. Considering the paramagnetic nature of Eu(III), we used the diamagnetic La(III) to replace Eu(III) in all the NMR titration experiments. The changes of 1H NMR spectra of C2-POPhen in the range of 8.0−9.0 ppm upon the titration of La(NO3)3 are presented in Figure 4A. Without the presence of La(III) in solution, three groups of well-defined peaks at 8.60, 8.23, and 8.08 ppm corresponding to the protons of H1, H2, and H3 of C2-POPhen, respectively, could be observed. Upon introduction of La(NO3)3 into the ligand solution, all the three groups of 1H NMR peaks were broadened and shifted downfield significantly until a M/L ratio of 1 was reached (Figure 4A). With further addition of La(NO3)3 and the M/L ratio above 1, the chemical shifts of the three protons became constant, and the fine structure appeared again, suggesting that a sole La(III)/L species exists in the solution. The above change of 1 H NMR spectra is consistent with the gradual formation of a 1:1 La(III)/C2-POPhen complex during the titration. Similar trends were also observed in the 31P NMR spectra of C2POPhen titrated with La(NO3)3 in CH3OH (Figure 4B). The original 31P signal of C2-POPhen at 10.5 ppm shifted downfield and were greatly broadened upon the addition of La(NO3)3. When the M/L ratio was above 1, a narrow 31P signal appeared again at a constant chemical shift of 18.5 ppm. The formation of only a 1:1 Ln(III)/L complex was also found in the case of another tetradentate 2,9-diamide-1,10-phenanthroline ligand with Ln(NO3)3 in CH3OH.28 Therefore, we conclude that the 1:1 complex is the dominant species for these phenanthroline-derived tetradentate ligands with lanthanides nitrate in CH3OH. In contrast to the above results from titrations with La(NO3)3, the 1H and 31P NMR spectra of C2-POPhen titrated with La(ClO4)3 in CD3OD/CH3OH (Figure 4C,D) show that more than one type of complexes have formed between C2-POPhen and La(III) in the solution, emphasizing the great influence of the counteranion on the complexation behavior. As shown in Figure 4C, when the M/L ratio was less than 0.5, a group of 1H signals corresponding to the protons of H1, H2, and H3 appeared at 9.04, 8.45, and 8.37 ppm,

(6)

The values of β1 and β2 can be obtained from the literature as 1.82 and 0.38, respectively.46 Since trace amount of Am(III) was used in the extraction system, the equilibrium concentrations of ligand and NO3− are approximately equal to their initial concentrations. Finally, the apparent extraction equilibrium constants of Am(III) with C2-POPhen can be calculated using eq 6. The same analysis process could also be applied to the extraction of Eu(III). At 293 K, the log Kex of Am(III) and Eu(III) were determined as 4.66 and 3.83, respectively. The thermodynamic properties of the extraction were further evaluated by temperature variation experiments. Figure 2D shows that when the temperature increases from 283 to 323 K, the distribution ratios of Am(III) and Eu(III) decrease significantly from 25.4 to 6.2 and from 3.3 to 1.1, respectively, suggesting that the extraction of Am(III) and Eu(III) by C2POPhen from HNO3 media are exothermic processes and increasing the temperature has adverse effect on the extraction. The apparent thermodynamic parameters including enthalpy change (ΔH), entropy change (ΔS) as well as Gibbs free energy change (ΔG) were calculated using the Van’t Hoff eq 7 and thermodynamic eq 8 from the linear relationship between log Kex and 1/T (Figure 3). log Kex = −ΔH /(2.303RT ) + ΔS /(2.303R )

(7)

ΔG = ΔH − T ΔS

(8)

where R is the gas constant. It was found that the enthalpies of complexation reactions (ΔH) for both Am(III) (−26.2 ± 1.2 kJ mol−1) and Eu(III) (−19.7 ± 1.2 kJ mol−1) are exothermic, which is favorable for the extraction. In contrast, the entropies (ΔS) for Am(III) (−0.2 ± 3.4 J mol−1 K−1) and Eu(III) (6.7 ± 3.6 J mol−1 K−1) are small and contribute less to the extraction. From a thermodynamic point of view, the extraction selectivity of Am(III) over Eu(III) by C2-POPhen mainly comes from the more favorable extraction enthalpy for Am(III) than for Eu(III). The negative ΔG values for Am(III) (−26.2 ± 1.6 kJ mol−1) and for Eu(III) (−21.7 ± 1.2 kJ mol−1) at 293 K indicate that the extraction reactions occur spontaneously. Complexation Behavior. NMR Titrations. NMR titrations have been widely used to interpret the stoichiometries of the D

DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (A) 1H NMR spectra of C2-POPhen (5.0 mM) titrated with La(NO3)3 in CD3OD. (B) 31P NMR spectra of C2-POPhen (10.0 mM) titrated with La(NO3)3 in CH3OH solution. (C) 1H NMR spectra of C2-POPhen (5.0 mM) titrated with La(ClO4)3 in CD3OD. (D) 31P NMR spectra of C2-POPhen (10.0 mM) titrated with La(ClO4)3 in CH3OH solution. M/L denotes the metal/ligand equivalents.

methanol, the complexation between La(III) and nitrate will be even stronger because the solvation ability of methanol is much weaker than water. Therefore, for the complexation of C2-POPhen with La(NO3)3 in methanol, nitrate will compete strongly with C2-POPhen to coordinate with La(III) and the 1:2 La/C2-POPhen complex can hardly form. The different complexation behaviors have been further confirmed and quantified by results from absorption spectroscopic titrations as discussed below. Absorption Spectrophotometric Titrations in Nitrate Media. The complexation of C2-POPhen with Am(III) and Ln(III) was first studied by spectrophotometry in nitrate/ CH3OH solution (I = 0.01 M Et4NNO3). As shown in Figure 5, the absorption spectra of C2-POPhen titrated with Am(III) and Eu(III) exhibit similar changing trends. Upon addition of Am(III) or Eu(III), the intensity of the absorption band at 274 nm corresponding to free C2-POPhen ligand gradually decreased while a new absorption band at longer wavelengths appeared. A typical isosbestic point can be found at 277−278 nm, suggesting the presence of two absorbing species.51,52 Factor analysis and fitting with Hyperquad program53 also confirmed that there were only two absorbing species, i.e., free C2-POPhen ligand and 1:1 metal/C2-POPhen complex, in the solution, which is in accordance with the NMR titration results. The stability constants of C2-POPhen complexes with Am(III) or Eu(III), as well as the molar absorption spectra of each species, were also obtained (Figure 5, middle). The complexation of C2-POPhen with another two lanthanides, La(III) and Nd(III), were also investigated in nitrate/CH3OH

respectively. The intensity of these signals reached the maximum at M/L = 0.5. With further increase of the M/L ratio, the intensity of this group of 1H signals decreased and a new group of signals appeared at 9.08 ppm (H1), 8.54 ppm (H2), and 8.40 ppm (H2) and became constant at M/L > 1.0. The above trends are consistent with the successive formation of two M/L complexes, LaL2 and LaL, during the titration. At relatively small M/L ratios (1), the 1:1 complex would be the dominant species in the solution. The successive formation of 1:1 and 1:2 complexes between La(III) and C2-POPhen is also supported by the variation of 31P NMR spectra of C2POPhen titrated with La(ClO4)3 in CH3OH solution (Figure 4D). With the increase of La(ClO4)3 concentration, two welldefined 31P NMR signals at about 17.2 and 18.1 ppm were observed successively, which correspond to the 1:2 and 1:1 complex, respectively. Apparently, the different complexation behaviors of La(NO3)3 and La(ClO4)3 with C2-POPhen in methanol can be ascribed to the different complexation ability of the two anions. As is well-known, the perchlorate ion can usually be regarded as a noncoordinating anion to metal ions even in weakly solvating solvents;50 thus, C2-POPhen can form both 1:1 and 1:2 complexes with La(III) in methanol without the presence of strong competing ligand. In contrast, the nitrate ion has shown much stronger coordination ability to metal ions. As mentioned earlier, Am(III) or Ln(III) could easily form up to two complexes with nitrate in aqueous solutions.46 In E

DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Spectrophotometric titrations of C2-POPhen with Am(III) and Eu(III) in CH3OH solution (I = 0.01 M Et4NNO3, initial volume V0 = 2.20 mL). (A) Top: normalized absorption spectra of 0.017 mM C2-POPhen titrated with 1.03 mM Am(NO3)3; middle: calculated molar absorptivities of C2-POPhen/Am species; bottom: distribution curves of C2-POPhen/Am species during titration. (B) Top: normalized absorption spectra of 0.020 mM C2-POPhen titrated with 1.01 mM Eu(NO3)3; middle: calculated molar absorptivities of C2-POPhen/Eu species; bottom: distribution curves of C2-POPhen/Eu species during titration.

stronger complexation ability with Am(III) than Ln(III), which is consistent with the extraction selectivity of Am(III) over Eu(III) by C2-POPhen. In addition, the stability constant of Eu(III) with C2-POPhen was larger than those with Et-TolDAPhen28 (log β = 3.81) or PDMA54 (log β = 4.17), suggesting that C2-POPhen has stronger complexation ability with Eu(III) than the ligands Et-Tol-DAPhen and PDMA. Absorption Spectrophotometric Titrations in Perchlorate Media. The complexation of C2-POPhen with Am(III) and Ln(III) was also studied by spectrophotometry in perchlorate/ CH3OH solution (I = 0.01 M NaClO4). We first tried the titration by monitoring the absorption change of the ligand as we did in nitrate media. However, the obtained spectra cannot be well fitted to give reasonable results. This failure could be ascribed to the less sensitivity of the absorption of C2-POPhen to its complexation in different complexes. As indicated by NMR titration results, C2-POPhen could form up to two successive complexes (LnL3+ and LnL23+) with Ln(III). The ligands in the two complexes have similar coordination environment (tetradentate to the metal) and their absorption spectra are expected to be similar and thus hard to discriminate. To obtain more reliable information on the complexation in perchlorate media, we alternatively conducted

solution by spectrophotometric titrations and the results are shown in Figures S2−S5. The stability constants for Am(III) and Ln(III) complexes with C2-POPhen are listed in Table 1. In nitrate/CH3OH solutions, the stability constant (log β) of AmL3+ (L denotes C2-POPhen), 6.10 ± 0.05, is more than 1 order of magnitude larger than those of EuL3+ (4.92 ± 0.03), LaL3+ (4.56 ± 0.02), and NdL3+ (5.15 ± 0.01). Obviously, C2-POPhen has much Table 1. Stability Constants (log β) for the Complexes of Am(III), Eu(III), Nd(III), and La(III) with C2-POPhen (L) by Using Spectrophotometric Titration Method in CH3OH at 298 K metal ion

L + Am ⇌ AmL 2L + Am3+ ⇌ AmL23+ L + Am3+ ⇌ AmL3+ L + Nd3+ ⇌ NdL3+ 2L + Nd3+ ⇌ NdL23+ L + Nd3+ ⇌ NdL3+ L + Eu3+ ⇌ EuL3+ L + La3+ ⇌ LaL3+ 3+

Am3+

Nd3+ Eu3+ La3+

log β

reaction 3+

7.6 12.5 6.10 6.76 11.0 5.15 4.92 4.56

± ± ± ± ± ± ± ±

0.1 0.2 0.02 0.42 0.9 0.01 0.03 0.02

ionic medium 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01

M M M M M M M M

NaClO4 NaClO4 Et4NNO3 NaClO4 NaClO4 Et4NNO3 Et4NNO3 Et4NNO3 F

DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. Spectrophotometric titrations of Am(III) and Nd(III) with C2-POPhen in perchlorate/CH3OH solution (I = 0.01 M NaClO4, initial volume V0 = 2.20 mL). (A) Top: normalized absorption spectra of 0.2 mM Am(III) titrated with 1.0 mM C2-POPhen; middle: calculated molar absorptivities of Am(III) species; bottom: distribution curves of Am(III) species during titration. (B) Top: normalized absorption spectra of 1 mM Nd(III) titrated with 10.0 mM C2-POPhen; middle: calculated molar absorptivities of Nd(III) species; bottom: distribution curves of Nd(III) species during titration.

complexes are also shown in Figure 6A (middle and bottom). The calculated stability constants are listed in Table 1. The titration spectra and the fitted results of Nd(III) with C2-POPhen in perchlorate/CH3OH solution are shown in Figure 6B. Significant changes were observed in the absorption spectra of Nd(III) during the titration. Upon addition of the ligand, the main absorption band of free Nd(III) in CH3OH solution red-shifted and concurrently new side absorption bands appeared. Similar to the Am(III) system, the spectra could also be well fitted by assuming the successive formation of two Nd(III) complexes. Due to the absorption of C2POPhen in the range of 560−620 nm, the baseline of the titration spectra rose apparently with the addition of C2POPhen. As shown in Table 1, the stability constants of Am(III) complexes (AmL3+ and AmL23+) are approximately 1 order of magnitude larger than those of the corresponding Nd(III) complexes (NdL3+ and NdL23+), further confirming the selectivity of C2-POPhen to Am(III) over Nd(III). Moreover, the stability constants of 1:1 Am(III) or Nd(III) complexes

titrations by monitoring the absorption spectra of Am(III) and Ln(III). In the visible region, Am(III) has characteristic absorptions that are highly sensitive to the coordination environment and can be used to effectively probe the speciation change of Am(III). Moreover, Nd(III) was selected as the representative Ln(III) due to its well-defined and hypersensitive absorption in the UV−vis region. Figure 6A shows the absorption spectra of Am(III) titrated with C2-POPhen in perchlorate/CH3OH solution. The changes in the spectra during the titration can be described in two stages. In stage one, upon addition of C2-POPhen, the intensity of the absorption band at around 503 nm decreased gradually and a new absorption band appeared at 508 nm. In stage two, as the C2-POPhen solution was further added, the new absorption bands decreased gradually and red-shifted slightly. The spectra could be well-fitted by assuming the successive formation of two Am(III) species, i.e., AmL3+ and AmL23+, during the titration. The molar absorption spectra and speciation diagrams for the 1:1 and 1:2 Am(III)-C2-POPhen G

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Figure 7. (A) Defined dihedrals using to calculate the rotation energy barrier between different conformations of ligand C2-POPhen. (B) Calculated rotation energy barrier curves of ligands C2-POPhen. (C) Calculated molecular electrostatic potential surfaces of inward−inward conformation. (D) Calculated molecular electrostatic potential surfaces of outward−outward conformation.

with C2-POPhen in perchlorate media are much larger than those in nitrate media, emphasizing the strong competition effect of nitrate as compared to perchlorate on the complexation of C2-POPhen with the metals. Geometries and Rotation Energy Barrier Calculations on the Structures of C2-POPhen. To illustrate the preorganization behavior of C2-POPhen, the rotation energy barrier of the ligand in different conformations was analyzed using DFT calculations at B3LYP/6-31G**/ level in CH3OH. As shown in Figure 7A, based on the positions of two phosphonate moeties, the initial dihedral O18−P15−C12−C13, which is in coplane with the phenanthroline ring, is defined as 0°. Then, we rotate the dihedral O17−P16−C3−C2 from 0 to 180° with an interval of 30° to obtain the rotation energy barrier curve (Figure 7B1). Similarly, we set the dihedral O18−P15−C12− C13 as 180° and change the dihedral O17−P16−C3−C2 from 0 to 180° to obtain another rotation energy barrier curve (Figure 7B2). It has been found that the largest rotation energy barrier between all the caculated conformers is only 0.3 eV, indicating that the ligand is highly preorgnized by the rigid 1,10-phenanthroline sketelon and could easily transfer between different conformations.55 Therefore, the extremely fast kinetics for the extraction of Am(III) and Eu(III) by C2POPhen should result from the high preorgnization degree of the ligand itself. Furthermore, the molecular electrostatic potential surfaces show that the conformer with two PO groups in inward−inward direction (Figure 7C) has strong negative potential in the complexation cavity, which is favorable for metal complexation. However, the conformer with two PO groups in outward−outward direction (Figure

7D), the negative potential region is mainly distributed outside the complexation cavity and is unfavorable for metal chelation. Population Analysis of Am(III) and Eu(III) Complexes with C2-POPhen. The theoretical calculation methods we employed in this work have been widely used to reveal the differences in the complexation behaviors of trivalent actinides and lanthanides ions with different ligands.56−59 To illustrate the binding characteristics of C2-POPhen with Am(III) and Eu(III), we optimized the structures of 1:1 species AmL(NO3)3/EuL(NO3)3 and 1:2 species AmL23+/EuL23+ on the basis of results from NMR and absorption spectrophotometric studies. As shown in Figure 8, in the 1:1 species, C2-POPhen binds to the Am(III) or Eu(III) ion via two nitrogen atoms from the phenanthroline groups and two oxygen atoms from two phosphonate moieties, which is similar to Am/Ln complexes with other soft−hard donors combined tetradentate ligands.22,60 In nitrate media, three additional bidentate nitrate ions coordinate with the metal, forming a 10-coordinated complex. Similar structures have also been found in lanthanide complexes with other tetradentate ligands as characterized by single crystal X-ray diffraction.28,61,62 The 1:2 species can only form in perchlorate media without the presence of nitrate anions because nitrate anion interact stronger with Am/Ln than perchlorate anion. As shown in Figure 8, in the optimized 1:2 species, both the two ligands coordinate to the metal in tetradentate mode, forming 8-coordinated 1:2 complexes. The average bond lengths between the metal and the donor atoms of C2-POPhen in the optimized complexes are listed in Table 2. In general, the Am−N bond (2.833 Å) is shorter than the Eu−N bond (2.847 Å) for the 1:1 species, suggesting more covalent characters exist in Am−N bond. In contrast, the Am− H

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indicating that C2-POPhen has a stronger affinity toward Am(III) than Eu(III). These results are in good agreement with the results of solvent extraction experiments.

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CONCLUSIONS A tetradentate phenanthroline-derived phosphonate ligand (C2-POPhen) was employed for the separation of actinides from lanthanides in HNO3 media. Solvent extraction experiments demonstrate that C2-POPhen has excellent Am(III) extraction ability, apparent Am(III)/Eu(III) selectivity and extremly fast extraction kinetics. Slope analysis suggests the formation of both 1:1 and 1:2 extractive species during the extraction. The complexation of Am(III) and Ln(III) with C2POPhen in methanol was studied by NMR and spectrophotometric titrations. The formation of only 1:1 complex of Am(III) or Eu(III) with C2-POPhen was confirmed in nitrate media while both 1:1 and 1:2 complexes were identified in perchlorate media. DFT calculation results reveal that the wellpreorganized structure of C2-POPhen is responsible for the fast extraction kinetics. Moreover, relatively more covalent character of Am−N bonds than that of Eu−N bonds contributes to the extraction selectivity of C2-POPhen to Am(III) over Eu(III).

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Figure 8. Optimized structures of the 1:1 and 1:2 Am3+ and Eu3+ complexes with C2-POPhen (L).

ASSOCIATED CONTENT

S Supporting Information *

OP bond (2.489 Å) is longer than the Eu−OP bond (2.434 Å). Therefore, the extraction selectivity of C2-POPhen toward Am(III) over Eu(III) should be originated from the nitrogen atoms in the phenanthroline skeleton rather than the oxygen atoms in the phosphonate moieties. In 1:2 species, both the Am−N bond and Am−Op bond are longer than the corresponding Eu−N bond and Eu−Op bond. Considering the relative larger ionic radius of Am(III) than Eu(III), there are few differences between bond lengths of Am(III) and Eu(III) with N and O atoms of the ligand, which can partly explain the relative small selectivity to extract Am(III) over Eu(III) in nitric acid medium. Moreover, the bond nature of M−N and M−O bonds was further analyzed by Wiberg Bond Indices (WBIs), which are regarded as effective factors to measure the degree of covalency.27,28,63 As listed in Table 2, the WBIs are 0.312 for Eu−N and 0.351 for Am−N in 1:1 complexes and 0.455 for Eu−N and 0.464 for Am−N in 1:2 complexes. Such small WBIs values indicate that the electrostatic interactions between metal ion and ligand are dominant in M−N bonds. Furthermore, the relative larger WBIs of Am−N bonds than those of Eu−N implies that more covalent characteristics are present in Am−N bonds. Besides, the Hirshfeld charges on the metal and the N and , O atoms are also calculated and listed in Table 2. Obviously, the Hirshfeld charges on Am(III) are larger than those of Eu(III) in the same type of complexes, whereas the values on O and N atoms are almost the same,

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03592. 1 H NMR spectrum of ligand C2-POPhen, spectrophotometric titrations of ligand C2-POPhen with typical lanthanides(III), and Cartesian coordinates of optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Taoxiang Sun: 0000-0003-4690-3566 Chao Xu: 0000-0001-5539-4754 Author Contributions §

L.X. and N.P. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from National Natural Science Foundation of China (Nos. 21790372, 21822606, and

Table 2. Calculated Bond Lengths (Å), Wiberg Bond Indices (WBIs), and Hirshfeld Charges (Q) of the AmL(NO3)3, EuL(NO3)3, AmL23+, and AmL23+ Complexesa complexes

M−N

M−OP

M−NWBI

M−OWBI

QM

QO

QN

AmL(NO3)3 EuL(NO3)3 AmL23+ EuL23+

2.833 2.847 2.690 2.654

2.489 2.434 2.407 2.359

0.351 0.312 0.464 0.455

0.618 0.635 0.776 0.786

0.595 0.572 0.675 0.647

−0.322 −0.324 −0.348 −0.351

−0.086 −0.085 −0.104 −0.105

a

Op denotes the phosphonate oxygen. I

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11605118), the Science Challenge Project (TZ2016004), and National Postdoctoral Program for Innovative Talents (BX201700129). The calculations were performed using Tsinghua HPC Platform.

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DOI: 10.1021/acs.inorgchem.8b03592 Inorg. Chem. XXXX, XXX, XXX−XXX