with Dithioamide-Based Ligands - ACS Publications - American

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

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Theoretical Insights into the Selective Extraction of Americium(III) over Europium(III) with Dithioamide-Based Ligands Cui Wang,†,‡,∥ Qun-Yan Wu,†,∥ Xiang-He Kong,†,‡ Cong-Zhi Wang,† Jian-Hui Lan,† Chang-Ming Nie,‡ Zhi-Fang Chai,†,§ and Wei-Qun Shi*,† †

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China § Engineering Laboratory of Advanced Energy Materials, Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China Downloaded via KEAN UNIV on July 19, 2019 at 07:07:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Separation of trivalent actinides An(III) from lanthanides Ln(III) is a worldwide challenge owing to their very similar chemical behaviors. It is highly desirable to understand the nature of selectivity for the An(III)/Ln(III) separation with various ligands through theoretical calculations because of their radiotoxicity and experimental difficulties. In this work, we have investigated three dithioamide-based ligands and their extraction behaviors with Am(III) and Eu(III) ions using the scalar-relativistic density functional theory. The results show that the dithioamide-based ligands have stronger electron donating ability than do the corresponding diamide-based ones. All analyses including geometry, Mulliken population, QTAIM (quantum theory of atoms in molecules), and NBO (natural bond orbital) suggest that the Am−S/N bonds possess more covalency compared to the Eu−S/N bonds, and the M−S bonds have more covalent character than the M−N bonds. Thermodynamic results reveal that N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline-2,9-bis(carbothioamide) (L1) has a stronger complexing ability with metal ions owing to its rigid structure and that N6,N6′-diethyl-N6,N6′-di-p-tolyl-[2,2′bipyridine]-6,6′-bis(carbothioamide) (L2) shows a higher selectivity for the Am(III)/Eu(III) separation. In addition, these dithioamide-based ligands possess Am(III)/Eu(III) selectivity higher than those of the corresponding diamide-based ones, although the former have weaker complexing ability with metal ions, probably due to the greater covalency of the M−S bonds. This theoretical evaluation provides valuable insights into the nature of the selectivity for the Am(III)/Eu(III) separation and information on designing of efficient An(III)/Ln(III) separation with dithioamide-based ligands.

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INTRODUCTION

According to the hard−soft acids−bases (HSAB) principle, An(III) ions are slightly softer Lewis acids compared to Ln(III) ions; thus, it is feasible to use ligands with soft atoms to separate An(III) from Ln(III).10,11 Various soft ligands used in the An(III)/Ln(III) separation have gained attention over the past several decades, and some of these ligands have indeed achieved excellent separation performance.12−18 N-Donor ligands such as BTPs (derivatives of 2,6-bis(1,2,4-triazin-3-yl)pyridine),19 BTBPs (derivatives of 6,6′-bis(1,2,4-triazin-3-yl)-2,2′-bipyridine),20,21 and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen)22,23 have been extensively studied for the An(III)/ Ln(III) separation. To the best of our knowledge, S-donor ligands such as Cyanex 3018,24,25 are also used for An(III)/ Ln(III) separation. Dolg et al. theoretically reported that the Am(III)/Eu(III) separation for Cyanex301 is mainly attributed to a stronger covalent interaction between ligand and Am(III),

The safe disposal of nuclear waste is one of the most challenging issues in the field of nuclear energy. Most of the uranium and plutonium can be recovered from spent nuclear fuel through the Plutonium Uranium Redox by Extraction (PUREX) process. However, the long-lived minor actinides (MAs) with much of the radiotoxicity, such as 241Am, 245Cm, and 237Np, are still present together with fission products in the high-level liquid waste (HLLW) from the PUREX process, which have long-term potential risks for the health of human beings and the biosphere.1−3 The partitioning and transmutation (P-T)4 strategy can convert long-lived isotopes of MAs into shorterlived or stable ones in fast neutron reactors.5,6 Because lanthanides as typical fission products have large neutron cross sections, it is greatly important to separate trivalent actinides (An(III)) over lanthanides (Ln(III)) for more efficient nuclear fuel cycles.7 However, it is still particularly challenging to separate An(III) from Ln(III) owing to their very similar physicochemical properties.8,9 © XXXX American Chemical Society

Received: April 24, 2019

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DOI: 10.1021/acs.inorgchem.9b01200 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Structures of L1, L2, and L3

Figure 1. ESP of L1, L2, and L3. The values (kcal/mol) of the negative and positive ESP are presented as green and orange spheres, respectively.

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and the solvent effect plays a major role in the Am(III)/Eu(III) separation.26,27 Mariani et al. studied the Am(III)/Eu(III) separation using 1,10-phenanthroline di(thio)amide and its derivatives in different solvents.28 Ghanty and co-workers explored the actinide selectivity with 1,10-phenanthroline-2,9dithio-carboxyamide (THIOAM) ligand and found that the preferential complexing ability with Am(III) is mainly dominated by a higher percentage of orbital interactions between Am(III) and ligand.29 Ding et al. found that dithiophosphinic acids (DPAHs) ligands achieve the excellent selectivity toward Am(III) over Eu(III), because the unoccupied valence orbitals of Am(III) display a stronger affinity to the sulfur lone pair electron.30 Very recently, Konstantinos and coworkers reported that the dithioamide ligands bearing N−CS groups display higher selectivity for Am(III) over Eu(III) compared to the analogous containing N−CO groups.31 Nowadays, experimental researchers on actinide complexes have great challenges due to the complexes’ radiotoxicity and limitation of experimental conditions. With the impressive advancement in theoretical methodologies, more and more theoretical works on the complexing behaviors of soft donor ligands with An(III) and Ln(III) ions have been conducted,26,27,32−35 which gave some reasonable explanation about the origin of the Am(III)/Eu(III) selectivity of the ligands. We have evaluated diamide-based ligands and their complexing structures with Am(III) and Eu(III) ions and provided the understanding on the origin of the Am(III)/ Eu(III) selectivity for the these analogous ligands.13 In this work, we further studied the three analogous dithioamide-based ligands, N2,N9-diethyl-N2,N9-di-p-tolyl-1,10-phenanthroline2,9-bis(carbothioamide) (L1), N6,N6′-diethyl-N6,N6′-di-p-tolyl[2,2′-bipyridine]-6,6′-bis(carbothioamide) (L2), and N2,N6diethyl-N2,N6-di-p-tolylpyridine-2,6-bis(carbothioamide) (L3) (Scheme 1) and their extraction behaviors with Am(III) and Eu(III) ions using the scalar-relativistic theoretical method. This work theoretically explored the relationship between the Sdonor ligands and the Am(III)/Eu(III) selectivity and revealed the nature of the Am(III)/Eu(III) separation of these ligands, which can provide useful insights into the selectivity of Am(III)/ Eu(III) separation for the S-donor ligands.

COMPUTATIONAL DETAILS

Optimizations were carried out in the gas phase with the Gaussian 16 package36 using the density functional theory (DFT) method37,38 with the BP86 functional,39,40 which has been widely used for actinide complexes.41,42 The relativistic effects were taken into account by using the scalar-relativistic effective core potentials (RECPs), which substitute 28 core electrons for the Eu atom43 and 60 core electrons for the Am atom,43,44 without considering spin−orbit coupling effects. The corresponding ECP28MWB-SEG45 and ECP60MWB-SEG46,47 valence basis sets were used to describe Eu and Am atoms, respectively. As for other light atoms including C, H, O, N, and S, the 6-31G(d) basis set was applied, so mixed basis sets (BS-I) were used for the metal complexes. The electrostatic potential and molecular orbitals of the three ligands were displayed based on the wave function at the BP86/ 6-31G(d) level of theory in the gas phase. The septet electronic state was employed for the Am(III) and Eu(III) complexes according to a previous study.13 In order to access the bonding nature between the metal atom and the ligands, the topological analysis of electron density has been explored by employing quantum theory of atoms in molecules (QTAIM) using Multiwfn software.48 In the QTAIM method, a chemical bond is defined by the presence of a line of maximum electron density along a bond path between each atom pair and the BCP (bond critical point).49 Therefore, it can provide insightful information about the properties of the chemical bonds,50,51 which has been widely used to explore actinide complexes. The solvation effect was taken into account by the conductor-like screening model (COSMO)52,53 with the Klamt atomic radii in water (ε= 78.4) and an organic solution, cyclohexanone (ε= 15.6). Previous studies suggested that the structures optimized in aqueous and organic solution have no obvious effect compared to those in the gas phase.54,55 Thus, solvation energies in the aqueous and organic phases were calculated with the larger 6-311G(d,p) basis set based on the structures optimized in the gas phase, including correction of thermal free energy at 298.15 K. In this work, we take the reaction [M(NO3)(H2O)8]2+ + L + 2NO3− = [ML(NO3)3] + 8H2O to evaluate the thermodynamic properties according to previous work.13 B


Article

Inorganic Chemistry

Figure 2. Optimized structures of the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ (M = Am and Eu) complexes at the BP86/BS-I/RECP level of theory in the gas phase. H, C, N, S, O, and metal atoms are represented by white, gray, blue, yellow, red, and pink spheres, respectively.

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theory,13 revealing that the former have the stronger electrondonating ability. Structures of the Am(III) and Eu(III) Complexes. According to a previous report,60 the coordination number of the Eu(III) and Am(III) complexes in the solution is generally between 6 and 12, and the preferable coordination number is 8 and 9. Optimized structures of the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ (M = Am and Eu, L = L1, L2, and L3) complexes are displayed in Figure 2. All three ligands coordinate with the central metal ions via the phenanthroline/pyridine nitrogen and sulfur atoms. The M−N, M−S, and M−O bond lengths in [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ complexes at the BP86/BS-I/RECP level of theory are presented in Table 1. All the Am−N and Am−S bonds are somewhat shorter than the corresponding Eu−N and Eu−S bonds, respectively, indicating that the ligands have stronger complexing ability to Am(III) ions. Obviously, the M−N bonds are significantly shorter than the M−S owing to the longer effective atomic radius of S.61 The M−S bonds are somewhat

RESULTS AND DISCUSSION Geometric Properties of the Ligands. We optimized the structures of the three dithioamide-based ligands at the BP86/631G(d) level of theory. Ligand L2 has syn−anti and anti−anti conformations (Figure S1) due to freely rotating two pyridine rings around the C−C σ bond.56 The energy of the anti−anti conformation is 6.61 kcal/mol higher than that of the syn−anti one owing to the electrostatic repulsion. The size of the ligand cavity is the distance between two S atoms with the values of 7.90, 7.92, and 6.78 Å for the three ligands L1, L2, and L3, respectively. The ionic radii of Am(III) and Eu(III) are 0.98 and 0.95 Å, respectively; a bigger cavity can facilitate the coordination of the ligand with metal ions.41,57 Electrostatic potential (ESP) has been used to evaluate the relative reactivity;58,59 the red and blue areas of the three ligands denote negative and positive ESP in Figures 1 and S2, respectively. A more negative ESP site has a stronger ability to attract metal ions; thus, it is more possibly a reactive site. Figure 1 shows that the negative surface of ESP appears on the S and N atoms, indicating that these sites have stronger ability to complex with metal ions. Therefore, metal ions are more likely to be packed into the cavity of the ligand. The values of the most negative ESP for the ligands L1, L2, and L3 are −57.69, −38.67, and −37.42 kcal/mol, respectively, which indicate that ligand L1 has the strongest nucleophilic ability. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the three dithioamide-based ligands are presented in Figure S3. It shows that the electron density of LUMO is mainly localized on the phenanthroline/pyridine ring, while that of HOMO is predominately distributed on the substituted phenyl rings, which is very similar to those of the corresponding diamide-based ligands.13 In addition, the HOMO energy of the dithioamide-based ligands is higher than that of the corresponding diamide-counterparts at the B3LYP/6-31G(d) level of

Table 1. Average M−S, M−N, and M−O Bond Lengths (Å) of the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ (M = Am and Eu) Complexes at the BP86/BS-I/RECP Level of Theory in the Gas Phase

C

complexes

Eu−S/Am−S

Eu−N/Am−N

Eu−O/Am−O

[ML1(NO3)2]+ [ML2(NO3)2]+ [ML3(NO3)2]+ [ML1(NO3)3] [ML2(NO3)3] [ML3(NO3)3] [M(L1)2(NO3)]2+ [M(L2)2(NO3)]2+ [M(L3)2(NO3)]2+

2.936/2.912 2.923/2.900 2.912/2.858 3.076/3.020 3.059/3.010 3.115/3.044 3.054/2.976 3.067/2.989 2.934/2.890

2.583/2.568 2.580/2.565 2.613/2.615 2.674/2.636 2.678/2.640 2.729/2.714 2.705/2.649 2.709/2.668 2.783/2.737

2.450/2.437 2.452/2.439 2.423/2.409 2.515/2.503 2.517/2.505 2.481/2.474 2.531/2.524 2.531/2.523 2.422/2.426


Article

Inorganic Chemistry

Table 2. Calculated Average WBIs of the M−S, M−N, and M−O Bonds as Well as the WBIs Difference (Δ) between Am−L and Eu−L Bond in the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ Complexes at the BP86/BS-I/RECP Level of Theory in the Gas Phase complexes 1

+

[ML (NO3)2] [ML2(NO3)2]+ [ML3(NO3)2]+ [ML1(NO3)3] [ML2(NO3)3] [ML3(NO3)3] [M(L1)2(NO3)]2+ [M(L2)2(NO3)]2+ [M(L3)2(NO3)]2+

Eu/Am−S

Δ(M−S)

Eu/Am−N

Δ(M−N)

Eu/Am−O

0.493/0.523 0.502/0.532 0.498/0.555 0.396/0.465 0.406/0.461 0.373/0.431 0.484/0.550 0.480/0.546 0.558/0.620

0.030 0.030 0.058 0.069 0.055 0.059 0.066 0.065 0.062

0.260/0.277 0.261/0.278 0.230/0.259 0.251/0.273 0.251/0.279 0.220/0.233 0.276/0.309 0.272/0.299 0.215/0.246

0.017 0.016 0.029 0.022 0.027 0.013 0.033 0.026 0.031

0.372/0.409 0.371/0.409 0.347/0.398 0.345/0.365 0.345/0.373 0.335/0.364 0.330/0.375 0.331/0.370 0.368/0.403

longer in [ML1(NO3)3] and [ML1(NO3)2]+ complexes than in [ML2(NO3)3] and [ML2(NO3)2]+ complexes, respectively, probably due to the more rigid phenanthroline skeleton of ligand L 1 . The M−S bonds for [ML 1 (NO 3 ) 3 ] and [ML2(NO3)3] complexes are shorter compared to those in [ML3(NO3)3], while the M−S bonds in [ML1(NO3)2]+ and [ML 2 (NO 3 ) 2 ] + complexes are longer than those in [ML3(NO3)2]+. These results may be caused by steric hindrance and coordination numbers of the complexes. The similar trend is found for the M−N bonds. The Am−S bond lengths are in the range of 2.858−2.912 Å in the three [AmL(NO3)2]+ complexes, which are in excellent accordance with the Am−S bond (2.921(9) Å) in the crystalline structure of Am[S2 P(tBu2C12H6)]4−.62 In addition, the M−L bonds in the [ML(NO3)3] and [M(L)2(NO3)]2+ complexes are somewhat longer than those of the corresponding [ML(NO 3 ) 2 ] + complexes, probably due to the greater steric effect and coordination number. Bonding Nature of the Am(III) and Eu(III) Complexes. To the best of our knowledge, the An(III)/Ln(III) selectivity of the ligand probably originates from the greater covalent character of An−L bonds than that of Ln−L bonds.63 Wiberg bond indices (WBIs) of all the complexes were calculated using NBO analysis as shown in Table 2. All the values of WBIs for the Am−N and Am−S bonds are larger than those for their Eucounterparts, respectively, denoting the more covalency of Am− L bonds. Although the M−S bonds are significantly longer than the M−N bonds, WBI values of the former are larger compared to those of the latter in all complexes, indicating more covalency of M−S bonds. These results confirm that the S atoms play greater roles in bonding between the ligands and metal ions. The differences of the WBI values for the Am−S and Eu−S bonds are larger than those between the Am−N and Eu−N bonds in all complexes. For instance, in [ML1(NO3)3] the difference of the WBI values is 0.069 and 0.022 for the M−S and M−N bond, respectively. It is probably deduced that the Am(III)/Eu(III) selectivity of the three ligands as discussed below mainly originates from the larger difference between the covalencies of the Am−S and Eu−S bonds. In addition, the WBI values of M−S and M−N bonds are smaller in the [ML(NO3)3] complexes compared to those in the [ML(NO3)2]+ and [M(L)2(NO3)]2+ complexes probably due to the steric effect and the number of nitrate ions. The localized NBO analysis was performed to obtain more bonding details of the M−L bonds in the [ML(NO3)2]+ complexes. The natural localized molecular orbitals of the M− S are distinct σ bonds, as displayed in Figures 3 and S4. The composition of the M−S bonds and the percentage of atomic

Figure 3. M−S natural bonding orbitals in the [ML2(NO3)2]+ complexes at the BP86/BS-I/RECP level of theory in the gas phase. The isosurface value was set as 0.02 a.u.

orbital of the M−S bonds are provided in Table S1. It is worth noting that the Am 6d orbitals and the Eu 5d orbitals have predominant contributions to the M−S σ bonding orbitals. For example, the percentages of the Am 6d orbitals and the Eu 5d orbitals are 45.88 and 51.30% in the [AmL2(NO3)2]+ and [EuL2(NO3)2]+ complexes, respectively. In addition, there is some contribution of M p orbitals and a little M f orbitals to the M−S σ bonding orbital. These results indicate that the covalency of the M−S bonds is dominantly from the contributions of Am 6d and Eu 5d orbitals, which is similar to the result that the contribution of the 6d orbitals to covalency are larger in magnitude than that of 5f orbitals in the previous reports.64,65 Second-order perturbation theory was explored based on the analysis of NBO,66−68 which can provide the nature of interaction between the metal ions and the ligand. The larger second-order stabilization energies (E(2)) of donor−acceptor interaction between the N/S atoms and the metal ions as well as the composition of the corresponding metal empty orbitals for the [ML(NO3)3] complexes are listed in Table S2. It shows that the interaction is predominantly between the occupied lone pairs of N/S and the empty orbitals of the metal atom, with the energy in the range of 12−26 kcal/mol. The contribution of the Am/Eu d orbitals is larger compared to that of Am/Eu f, p, and s orbitals, which is the consistent with the result of NBO. Moreover, there are higher E(2) values in the interactions between the metal ions and dithioamide-based ligands compared to those between the metal ions with the corresponding diamide-based analogues as reported in previous work.13 This result indicates that the dithioamide-based ligands possess stronger electron-donating ability, which is agreement with the results of the energy level of HOMO for the ligands. Mulliken population analysis can reflect the covalency of the metal−ligand bond.26,69−73 Mulliken charges on the metal ions and ligands and the transfer of Mulliken charge for all complexes are provided in Table 3. The charge transfer from the ligand to Am(III) ion is larger than that to Eu(III) suggesting that the D


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Inorganic Chemistry

Table 3. Mulliken Charges on the Metal Ions and the Charge Transfer from the NO3− [ΔQ(NO3−)] and Ligand [ΔQ(L)] to the Metal Ion in the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ Complexesa complexes 1

+

[ML (NO3)2] [ML2(NO3)2]+ [ML3(NO3)2]+ [ML1(NO3)3] [ML2(NO3)3] [ML3(NO3)3] [M(L1)2(NO3)]2+ [M(L2)2(NO3)]2+ [M(L3)2(NO3)]2+

Eu/Am

ΔQ(M(III))

ΔQ(NO3−)

ΔQ(L)

1.104/0.989 1.095/0.977 1.114/0.994 1.148/1.015 1.138/1.006 1.156/1.048 0.866/0.696 0.859/0.689 0.688/0.544

1.896/2.011 1.905/2.023 1.886/2.006 1.852/1.985 1.862/1.994 1.844/1.952 2.134/2.304 2.141/2.311 2.312/2.456

0.519/0.539 0.519/0.537 0.507/0.569 0.466/0.487 0.466/0.485 0.462/0.486 0.478/0.544 0.474/0.540 0.483/0.551

0.858/0.934 0.868/0.948 0.872/0.868 0.453/0.523 0.464/0.538 0.458/0.494 1.656/1.760 1.667/1.771 1.829/1.905

a

Where .../... denotes the values of the Eu/Am complexes, respectively.

Table 4. Calculated Topological Properties (a.u.) at M−S, M−N, and M−O BCPs in the [ML(NO3)3] and [ML(NO3)2]+ Complexes at the BP86/BS-I/RECP Level of Theory in the Gas Phasea [ML(NO3)2]+

[ML(NO3)3]

L1

L2

L3

bond

ρ

∇ρ

ρ

∇2ρ

M−N M−S M−O M−N M−S M−O M−N M−S M−O

0.032/0.040 0.022/0.030 0.041/0.049 0.031/0.040 0.023/0.030 0.041/0.049 0.029/0.035 0.021/0.028 0.045/0.053

0.110/0.134 0.059/0.071 0.153/0.177 0.109/0.133 0.061/0.072 0.152/0.176 0.099/0.115 0.054/0.066 0.166/0.190

0.039/0.047 0.032/0.039 0.049/0.058 0.039/0.047 0.032/0.040 0.048/0.058 0.037/0.043 0.033/0.043 0.052/0.061

0.136/0.155 0.075/0.083 0.180/0.206 0.137/0.156 0.077/0.085 0.179/0.205 0.127/0.138 0.077/0.089 0.194/0.222

2

a

Where .../... represents the results of Eu(III) and Am(III) complexes, respectively.

Figure 4. Composition and energy level (eV) of the α-spin valence MOs of the [ML2(NO3)3] complexes obtained at the BP86/BS-I/RECP level of theory in the gas phase. The isosurface value was set as 0.02 a.u.

stronger interaction exist in the Am−ligand bonds, which agrees with the shorter Am−S bond in all complexes. This is probably the reason why this dithioamide-based ligand shows high

selectivity for Am(III) over Eu(III) as discussed below. In addition, the charge transfer from ligand to metal ions is much larger than that from NO3− to metal, which is similar result in E


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Inorganic Chemistry

Figure 5. Composition and energy level (eV) of the α-spin valence MOs of the [ML2(NO3)2]+ complexes obtained at the BP86/BS-I/RECP level of theory in the gas phase. The isosurface value was set as 0.02 a.u.

Figure 6. Composition and energy level (eV) of the α-spin valence MOs of the [ML2(NO3)3] and [MLb(NO3)3]complexes obtained at the BP86/BSI/RECP level of theory in the gas phase. The isosurface value was set as 0.02 a.u.

previous work,69 indicating that the three ligands possess stronger coordination ability to metal ions than nitrate ion. Moreover, the charge value on the metal atom is sensitive to the number of the nitrate ions. Topological analysis of the electron density of the metal− ligand bonds has been explored by the QTAIM method with

Multiwfn software,48 which is widely applied to evaluate the ionic/covalent nature of the actinide complexes.74,7576−79The electron density (ρ) and its Laplacian (∇2ρ) at the BCPs may provide valuable information about the strength and characteristics of the bonds.65 Generally, the values of ρ at the BCP > 0.20 a.u. and ∇2ρ < 0 refer to a typical covalent bond. In contrast, ρ < F


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Inorganic Chemistry

Table 5. Changes of Gibbs Free Energy (ΔG, kcal/mol) for the Reactions [M(NO3)(H2O)8]2+ + L + 2NO3− ⇄ [ML(NO3)3] + 8H2O (M = Am and Eu) and ΔΔG between the Am and Eu Reactions in Gas, Aqueous and Cyclohexanone Phases at the BP86 and B3LYP Functionals with the 6-311G(d,p) Basis Seta method

reactions 1

BP86

B3LYP

L L2 L3 L1 L2 L3

ΔGgas

ΔΔGgas/SFAm/Eu

ΔGaq

ΔΔGaq/SFAm/Eu

ΔGcyc

ΔΔGcyc/SFAm/Eu

−289.02/−294.27 −279.97/−285.33 −279.49/−282.43 −281.86/−282.79 −272.76/−273.99 −272.07/−272.52

−5.25/7105.36 −5.36/8522.23 −2.95/144.69 −0.92/4.73 −1.23/7.94 −0.45/2.15

−38.49/−41.98 −33.70/−37.27 −31.24/−32.00 −29.82/−31.90 −25.22/−-27.51 −21.49/−23.52

−3.50/366.44 −3.57/416.99 −0.76/3.58 −2.08/33.56 −2.29/47.60 −2.03/30.95

−37.70/−41.20 −32.53/−36.12 −30.32/−31.13 −28.96/−31.03 −23.96/−26.25 −20.55/−22.55

−3.51/371.15 −3.59/427.93 −0.80/3.89 −2.07/33.01 −2.29/47.38 −2.00/29.23

a

Where .../... represents the results of Eu(III) and Am(III) complexes, respectively.

the trend of ΔG for the three ligands remains similar. The values of ΔG for the three ligands are negative, indicating that the all ligands show significant complexing ability toward Am(III) and Eu(III). Obviously, the absolute values of ΔG decrease from ligand L1 to L2 to L3 in the gas, aqueous, and cyclohexanone phases, which agrees with the ESP trend of the ligands. Moreover, L1 shows the strongest complexing ability, and L3 shows the weakest one, probably because L3 is a tridentate ligand and L1 is more rigid compared to L2. This result denotes that the contribution of the central phenanthroline moiety to the complexing ability of the metal ions is most pronounced compared to that of its pyridine counterparts which is consistent with the complexing character of Am- and Eu-complexes reported previously.1,22,23 Additionally, the reactions of [ML(NO3)2]+ and [M(L)2(NO3)]2+ complexes are given in Tables S3 and S4. Obviously, the trend of the ΔG values for the three ligands keeps similar for the three reactions of the [ML(NO3)3], [ML(NO3)2]+ and [M(L)2(NO3)]2+ complexes in the gas phase. However, the ΔG values for the reactions of [ML(NO3)3] complexes are more negative than those of [ML(NO3)2]+ and [M(L)2(NO3)]2+ complexes, also indicating that the [ML(NO3)3] complex is more favorable species in the complexation reactions. Generally, the selectivity of ligands for Am(III) over Eu(III) is evaluated by the separation factor in a two-phase extraction system. The differences of ΔG (ΔΔGAm/Eu = ΔGAm − ΔGEu) for the reactions between the Am(III) and Eu(III) ions can reflect the magnitude of the separation factor (SFAm/Eu) according to the formula of ΔΔGAm/Eu = −RT ln SFAm/Eu (R represents the gas constant, T = 298 K). Table 5 shows that the ΔΔGAm/Eu values are negative, suggesting the excellent Am(III)/Eu(III) separation ability of the ligands. It is worth noting that the value of ΔΔGAm/Eu in the [ML2(NO3)3] complex is the most negative, that is, it has the largest SFAm/Eu, demonstrating that L2 has higher Am(III)/Eu(III) selectivity, probably due to the greater covalency of M−S and M−N bonds in the [ML2(NO3)3] complex based on the results of WBIs and Mulliken charge analyses. The calculated ΔΔGAm/Eu is −0.80 kcal/mol for L3 in the cyclohexanone phases, which is similar to the previously computed value of the analog of L3 (−1.61 kcal/mol).31 In addition, the ΔG values for the reactions with diamide-based ligands are more negative than those with dithioamide-based ligand complexes,13 which indicates that the diamide-based ligands have stronger complexing ability with metal ions. However, the values of ΔΔGAm/Eu for the former are more negative than those for the latter, revealing that dithioamidebased ligands have more excellent selectivity for Am(III)/ Eu(III).

0.10 a.u. and ∇2ρ > 0 a.u. reflect an ionic bond.80 The calculated properties of electron densities at M−S, M−N, and M−O BCPs of the [ML(NO3)3] and [ML(NO3)2]+ complexes are presented in Table 4. The values of ρ and ∇2ρ indicate that the M−S, M− N, and M−O bonds are weak covalent interactions. The values of ρ at the Am−S and Am−N BCPs are somewhat larger than those at the Eu−S and Eu−N BCPs, respectively, suggesting that the Am−S and Am−N bonds show a better degree of covalency compared to the Eu−S and Eu−N bonds. This might lead to shorter Am−L bonds.65 In addition, the value of ρ at the BCPs for the [ML(NO3)3] complexes is smaller than that for the [ML(NO3)2]+ complexes, which is consistent with the results of the WBIs. MOs provide useful information about the bonding nature between metal ions and ligands by evaluating the composition and energy level of the MOs. The α-spin valence MO diagrams of the metal−ligand bonding and the specific atomic contributions to the corresponding MOs for the [ML(NO3)3] and [ML(NO3)2]+ complexes are given in Figures 4 and 5 and S5−S8, respectively. It is clearly seen that the metal ions have obvious orbital interactions with sulfur atoms compared to those with nitrogen atoms, which suggests the stronger M−S bonding interaction. The energy levels of MOs in the Am-complexes are somewhat lower than those in the Eu-complexes, indicating that the ligands have a better complexing ability with Am(III) rather than Eu(III) ions. In addition, the MOs of the [M(L)2(NO3)]2+ complexes are given in Figure S9, which also shows that the interaction between the metal ions and ligands mainly originate from the sulfur atoms of the ligands. In order to evaluate the interaction of metal ions with dithioamide-based ligands and the corresponding diamidebased analogous, MOs bearing significant M−O and M−S interaction in [ML(NO3)3] are displayed in Figure 6. The structure of Lb (Lb denotes the amide-based analogous of L2) is shown in Figure S10.13 It shows that the energy levels of MOs in complexes [AmLb(NO3)3] and [EuLb(NO3)3] are lower than those in complexes [AmL2(NO3)3] and [EuL2(NO3)3], respectively, suggesting that the amide-based ligands have the stronger ability to complex to the metal ions. Moreover, the composition of Am and S in the [AmL2(NO3)3] is larger than the corresponding one in the [AmLb(NO3)3] complex, respectively, which also indicates that the dithioamide-based ligands have the stronger electron donating ability. Thermodynamic Properties. According to previous report,13 the reactions [M(NO3)(H2O)8]2+ + L + 2NO3− ⇄ [ML(NO3)3] + 8H2O are considered in this work. The changes of Gibbs free energy (ΔG) of the reactions in the gas, aqueous (aq), and organic (cyc) phases were calculated using BP86 and B3LYP functionals in Table 5. It shows that the ΔG with BP86 functional is more negative than that with B3LYP method, while G


Article

Inorganic Chemistry

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CONCLUSIONS In this work, the extraction mechanism and the nature of the Am(III)/Eu(III) selectivity for the three dithioamide-based ligands were evaluated using scalar-relativistic DFT. The results of the ligands show that S and N atoms prefer to complex with metal ions and ligand L1 has the strongest ability to attract metal ions. Moreover, the dithioamide-based ligands show electrondonating ability better than to that of the corresponding amidebased ones. The structures of the [ML(NO3)2]+, [ML(NO3)3], and [M(L)2(NO3)]2+ complexes were optimized at the BP86/631G(d)/RECP level of theory. The Am−S and Am−N bonds are shorter than Eu−S and Eu−N bonds, respectively, suggesting that the these ligands show good complexing ability toward the Am(III) ions over Eu(III) ions. The M−S bonds have distinct σ character, and Am 6d orbitals and Eu 5d orbitals have predominant contributions to the σ bonding based on the NBO analysis. The analyses of QTAIM, Mulliken charge, and MO suggest that there is a more covalent contribution to Am−L bonding than to Eu−L bonding in the same type of complexes. Moreover, the covalency of the M−S bond is larger than that of the M−N bond. Thermodynamic results reveal that L1 has a stronger complexing ability with metal ions, and L2 shows a higher Am(III)/Eu(III) selectivity. It is concluded that the nature of the Am(III)/Eu(III) selectivity for the dithioamidebased ligands probably originates from the covalency of the M− S bonds owing to the greater M d orbital participation. In addition, the dithioamide-based ligands have a weaker complexing ability with metal ions compared to the amide-based analouges, while the former have the higher selectivity of Am(III)/Eu(III). This work can extend our understanding of the separation mechanism of Am(III)/Eu(III) with dithioamide-based ligands and provides useful information on achieving an efficient Am(III)/Eu(III) separation process.

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obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences.

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(1) Hudson, M. J.; Harwood, L. M.; Laventine, D. M.; Lewis, F. W. Use of Soft Heterocyclic N-Donor Ligands To Separate Actinides and Lanthanides. Inorg. Chem. 2013, 52, 3414−3428. (2) Demir, S.; Brune, N. K.; Van Humbeck, J. F.; Mason, J. A.; Plakhova, T. V.; Wang, S.; Tian, G.; Minasian, S. G.; Tyliszczak, T.; Yaita, T.; et al. Extraction of lanthanide and actinide ions from aqueous mixtures using a carboxylic acid-functionalized porous aromatic framework. ACS. Central Sci. 2016, 2, 253−265. (3) Macerata, E.; Mossini, E.; Scaravaggi, S.; Mariani, M.; Mele, A.; Panzeri, W.; Boubals, N.; Berthon, L.; Charbonnel, M.-C.; Sansone, F.; et al. Hydrophilic clicked 2, 6-bis-triazolyl-pyridines endowed with high actinide selectivity and radiochemical stability: toward a closed nuclear fuel cycle. J. Am. Chem. Soc. 2016, 138, 7232−7235. (4) Magill, J.; Berthou, V.; Haas, D.; Galy, J.; Schenkel, R.; Wiese, J.; Heusener, G.; Tommasi, J.; Youinou, G. Impact limits of partitioning and transmutation scenarios on the radiotoxicity of actinides in radioactive waste. Nucl. Energy (Br. Nucl. Energy Soc.) 2003, 42, 263− 278. (5) Herrera-Martinez, A.; Kadi, Y.; Parks, G. Transmutation of nuclear waste in accelerator-driven systems: Thermal spectrum. Ann. Nucl. Energy 2007, 34, 550−563. (6) Yapici, H.; Genç, G.; Demir, N. A comprehensive study on neutronics of a lead−bismuth eutectic cooled accelerator-driven subcritical system for long-lived fission product transmutation. Ann. Nucl. Energy 2008, 35, 1264−1273. (7) Arisaka, M.; Kimura, T. Thermodynamic and spectroscopic studies on Am (III) and Eu (III) in the extraction system of N, N, N’, N’-tetraoctyl-3-oxapentane-1, 5-diamide in n-dodecane/nitric acid. Solvent Extr. Ion Exch. 2011, 29, 72−85. (8) Modolo, G.; Odoj, R. The separation of trivalent actinides from lanthanides by dithiophosphinic acids from HNO3 acid medium. J. Alloys Compd. 1998, 271, 248−251. (9) Bhattacharyya, A.; Mohapatra, P. K.; Manchanda, V. K. Separation of Americium (III) and Europium (III) from nitrate medium using a binary mixture of Cyanex-301 with N-donor ligands. Solvent Extr. Ion Exc. 2006, 24, 1−17. (10) Wang, Z.; Ding, S.; Hu, X.; Li, S.; Su, D.; Zhang, L.; Liu, Y.; Jin, Y. Selective extraction of americium (III) over europium (III) ions in nitric acid solution by NTAamide (C8) using a novel water-soluble bisdiglycolamide as a masking agent. Sep. Purif. Technol. 2017, 181, 148−158. (11) Dam, H. H.; Reinhoudt, D. N.; Verboom, W. Multicoordinate ligands for actinide/lanthanide separations. Chem. Soc. Rev. 2007, 36, 367−377. (12) Panak, P. J.; Geist, A. Complexation and extraction of trivalent actinides and lanthanides by triazinylpyridine N-donor ligands. Chem. Rev. 2013, 113, 1199−1236. (13) Wu, Q. Y.; Song, Y. T.; Ji, L.; Wang, C. Z.; Chai, Z. F.; Shi, W. Q. Theoretically unraveling the separation of Am(iii)/Eu(iii): insights from mixed N,O-donor ligands with variations of central heterocyclic moieties. Phys. Chem. Chem. Phys. 2017, 19, 26969−26979. (14) Whittaker, D. M.; Griffiths, T. L.; Helliwell, M.; Swinburne, A. N.; Natrajan, L. S.; Lewis, F. W.; Harwood, L. M.; Parry, S. A.; Sharrad, C. A. Lanthanide speciation in potential SANEX and GANEX actinide/ lanthanide separations using tetra-N-donor extractants. Inorg. Chem. 2013, 52, 3429−3444. (15) Wang, J.; Su, D.; Wang, D.; Ding, S.; Huang, C.; Huang, H.; Hu, X.; Wang, Z.; Li, S. Selective extraction of americium (III) over europium (III) with the pyridylpyrazole based tetradentate ligands: experimental and theoretical study. Inorg. Chem. 2015, 54, 10648− 10655. (16) Kolarik, Z.; Mullich, U.; Gassner, F. Extraction of Am (lll) and Eu (lll) nitrates by 2−6-di-(5, 6-dipropyl-1, 2, 4-triazin-3-yl) pyridines 1. Solvent Extr. Ion Exch. 1999, 17, 1155−1170.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01200.

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REFERENCES

Structure of L2, maps of ESP, diagrams of HOMO and LUMO, NBO orbitals, MOs, the second-order stabilization energies, and changes of Gibbs free energies (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 86-10-88233968. ORCID

Wei-Qun Shi: 0000-0001-9929-9732 Author Contributions ∥

C.W. and Q.-Y.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 11875058, U1867205, and 11675103), the Major Program of National Natural Science Foundation of China (21790373), and the Science Challenge Project (TZ2016004). The results described in this work were H


Article

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with Dithioamide-Based Ligands - ACS Publications - American

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