Insight into the Extraction Mechanism of Am(III) over Eu(III) with

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A: Molecular Structure, Quantum Chemistry, and General Theory

Insight into the Extraction Mechanism of Am(III) over Eu(III) with Pyridylpyrazole: A Relativistic Quantum Chemistry Study Xiang-He Kong, Qunyan Wu, Cong-Zhi Wang, Jianhui Lan, Zhi-Fang Chai, Changming Nie, and Wei-Qun Shi J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00177 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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The Journal of Physical Chemistry

Insight into the Extraction Mechanism of Am(III) over Eu(III) with Pyridylpyrazole: A Relativistic Quantum Chemistry Study

Xiang-He Kong,‡a,b Qun-Yan Wu,‡a Cong-ZhiWang,a Jian-HuiLan,a Zhi-Fang Chai,a,c Chang-Ming Nieb and Wei-Qun Shi*a

a

Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese

Academy of Sciences, Beijing, 100049, China b

School of Nuclear Resources Engineering, University of South China, Hengyang

421001, China c

School of Radiological and Interdisciplinary Sciences (RAD-X), and Collaborative

Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is one of the most important steps in the spent nuclear fuel reprocessing. However, it is very difficult and challenging to separate them due to their similar chemical properties. Recently the pyridylpyrazole ligand (PypzH) has been identified to show good separation ability towards Am(III) over Eu(III). In this work, to explore the Am(III)/Eu(III) separation mechanism of PypzH at the molecular level, the geometrical structures, bonding nature, and thermodynamic behaviors of the Am(III) and Eu(III) complexes with PypzH ligands modified by alkyl chains (Cn-PypzH, n=2, 4, 8) have been systematically investigated using scalar relativistic density functional theory (DFT). According to the NBO (natural bonding orbital) and QTAIM (quantum theory of atoms in molecules) analyses, the M–N bonds exhibit a certain degree of covalent character, and more covalency appears in Am–N bonds compared to Eu–N bonds. Thermodynamic analyses suggest that the 1:1 extraction reaction, [M(NO3)(H2O)6]2+ + PypzH + 2NO3- → M(PypzH)(NO3)3(H2O) + 5H2O, is the most suitable for Am(III)/Eu(III) separation. Furthermore, the extraction ability and the Am(III)/Eu(III) selectivity of the ligand PypzH is indeed enhanced by adding alkyl-substituted chains in agreement with experimental observations. Besides, the nitrogen atom of pyrazole ring plays a more significant role in the extraction reactions related to Am(III)/Eu(III) separation compared to that of pyridine ring. This work could identify the mechanism of the Am(III)/Eu(III) selectivity of the ligand PypzH and provide valuable theoretical information for achieving an efficient Am(III)/Eu(III) separation process for spent nuclear fuel reprocessing.

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1. INTRODUCTION With the development of science and technology, the treatment of spent fuel is one of the key problems that restrict the sustainable development of nuclear energy. High level liquid waste (HLLW), which is discharged through the traditional plutonium and uranium extraction (PUREX)1 process, still contains significant amounts of long-lived minor actinides (MAs), which need be vitrified and geologically disposed due to the potential threat to humans and biosphere. Therefore, MAs should be converted into the short-lived or stable nuclides by transmutation.2-3 However, lanthanides (Ln(III)) can absorb neutrons effectively in HLLW and therefore lower the efficiency of transmutation for actinides (An(III)). So separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is the key factor in the treatment of HLLW. But it is still a significantly challenging task for An(III)/Ln(III) separation due to very similar chemical properties in solutions such as ionic radii, oxidation state, parallel electrostatic as well as steric factors.4-6 According to the hard–soft acid base theory, both An(III) and Ln(III) belong to “hard” Lewis acids, while An(III) are slightly softer than to Ln(III).7-8 The softer ligands bearing nitrogen and sulfur are capable of separating An(III) from Ln(III). So it is necessary to design N-donor extractants which can efficiently separate An(III) from Ln(III) in HLLW.

Scheme 1. Chemical structures of the representative ligands for An(III)/Ln(III) separation.

To date, there have been some studies on An(III)/Ln(III) separation focused on families

of

N-donor

ligands,

such

as

2,2′:6′,2′′-terpyridine

3


(Terpy),9-10


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(TPTZ),11

2,4,6-tris(2-pyridyl)-1,3,5-triazine 2-amino-4,6-di-(pyridine-2-yl)-1,3,5-triazine

(ADPTZ),12

2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl)pyridines

(BTPs),13-15

6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridines

(BTBPs),16-17

and

2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhens),18 and so on (Scheme 1). However, less studies paid close attention to the N-heterocyclic ligands bearing five membered

pyrazole.19

Girnt

and

his

co-workers

synthesized

6-(3,5-dimethyl-1H-pyrazol-1-yl)-2,20-bipyridine (dmpbipy, Scheme 2), which displays low extraction ability and selectivity at low pH values.20 Subsequently, Bremer et al. obtained C5-BPP with Am(III)/Eu(III) separation factor of ~100 at 1 M HNO3 in the presence of 2-bromohexanoic acid.21 It was found that the Am-ligand bonding is more covalent compared to the Eu-ligand bonding based on

15

N NMR

spectroscopy, which may be the origin of the Am(III)/Eu(III) selectivity of C5-BPP.22 Recently, Ding et al. synthesized two tetradentate pyridylpyrazole ligands, 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane

(Bippp)

and

1,2-bis[3-(2-pyridyl)pyrazyl-1-methyl]benzene (Dbnpp),23 which can effectively extract Am(III) over Eu(III) in the presence of 2-bromohexanoic acid. Theoretical calculations revealed the greater covalency in Am–N bonds than in Eu–N bonds. Subsequently, they also designed and synthesized two types of alkyl-substituted pyridylpyrazole ligands,24 Cn-Pypz and Cn-PypzH (n = 0, 2, 4, 8, Scheme2). It has been shown that ligands Cn-PypzH behaves better in selective extraction of Am(III) ions compared to Cn-Pypz due to more steric hindrance of the substituted chain at the 2-position of the pyrazole ring. Moreover, the length of the alkyl-substituted chain has a significant impact on the distribution ratios and the separation factor of Am(III)/Eu(III).24 Therefore, to explore the mechanism of Am(III)/Eu(III) separation of PypzH ligands with different alkyl-substituents and to compare the contribution of pyridine and pyrazole rings to the Am(III)/Eu(III) separation, we have systematically studied the extraction behavior of Cn-PypzH using relativistic quantum-chemical calculations. The following issues are specifically discussed: the molecular electrostatic potential (MEP) and proton affinity of PypzH ligand, the geometrical 4


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structures, bonding nature, molecular orbital (MO) of Am(III) and Eu(III) complexes with PypzH ligand as well as their thermodynamic behaviors. This work can expand our understanding of the An(III)/Ln(III) separation mechanism with PypzH and pave the way for designing new derivatives of PypzH for An(III)/Ln(III) separation.

Scheme 2. Chemical structures of the ligands bearing pyridine and pyrazole rings for An(III)/Ln(III) separation.

2. COMPUTATIONAL DETAILS With the rapid improvement of theoretical methods, computational chemistry could provide a convenient approach in comprehending the extraction mechanism of N-donor ligands with the f-block element.25-32 In this work, density functional theory (DFT)33-34 calculations were performed with Gaussian 09 package35 by using the generalized gradient approximation (GGA) functional BP86,36-37 which could provide comparable results of the lanthanide and actinide species with experimental data.38-39 Scalar relativistic effective core pseudo potentials (RECPs) were applied, which replace 60 core electrons for Am and 28 electrons for Eu, respectively.40-41 The corresponding valence basis ECP60MWB-SEG and ECP28MWB-SEG using segmented contraction pattern, were used for Am and Eu, respectively, and the 6-31G* or 6-311G* basis sets were used for other light atoms. Scalar relativistic (SR) effects and spin-orbit coupling (SO) effects were taken into consideration using the all-electron zero-order regular approximation (ZORA) approach in ADF program42 when evaluating the contribution of relativistic effect to the reaction free energies.

5



The single-point energies calculated with ADF were carried out using BP86 method with the quality of the triple zeta plus polarization (TZP). All the structures of the Am(III)- and Eu(III)- complexes with Cn-PypzH were optimized at the BP86//ECP60MWB-SEG/6-31G* level of theory without symmetry restrictions in the gas phase. We took the structures of 1:1 and 1:2 metal to ligand ratios into consideration according to the experimental observations. The natural bond orbital (NBO) analysis based on optimized structures was performed at the BP86//ECP60MWB-SEG/6-311G* level of theory to understand the electronic and bonding properties.26,

43-44

The QTAIM (quantum theory of atoms in molecules)

analysis45-50 was also carried out to get insight into the bonding characters between the metal ions and the ligands using the Multiwfn 3.3 software.51 The solvation effect was evaluated using the conductor-like polarizable continuum model52 with the Klamt atomic radii default in water (ε=78.4), n-dodecane (ε=2.0) and tert-butylBenzene (ε=2.3). The solvation Gibbs free energies of each species were calculated based on the gas-phase optimized structures at the BP86//ECP60MWB-SEG/6-31G* level of theory including the thermal energy correction obtained in the gas phase.53 The solvation energy of each water molecule was corrected by −4.3 kcalmol-1 in the aqueous phase.54-55 3. RESULTS AND DISCUSSION 3.1 The PypzH ligand. A previous report14 gives us a hint that increasing the lengths of substituted chains can improve the solubility and selectivity of the ligands. Therefore, we firstly studied the MEP, the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) of Cn-PypzH ligands with different substituted chains as shown in Figure 1 and Figure S1 of Supporting Information. The MEP can reflect the chemically active sites, where red and blue regions represent negative and positive electrostatic potential, respectively. Obviously, the negative regions (nucleophilic sites) of PypzH mainly cover cavity, indicating the higher reactivity of two nitrogen atoms. Moreover, the charge value on the pyridine N atom (Npy) is more negative than that on the pyrazole N atom (Npz), showing that the former denotes a higher active site. It can be seen that the energy levels of HOMO and LUMO are 6


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increased when the PypzH modified by alkyl-chains, while the length of the alkyl-chain has no obvious effect on the energy levels of HOMO and LUMO. This result suggests that the PypzH becomes more active by modifying pyrazole with alkyl-substituted chains.

Figure 1. The map of MEP, HOMO and LUMO of PypzH, the natural charges on two nitrogen atoms and the corresponding MO energies (eV) obtained at the BP86//ECP60MWB-SEG/6-31G* level of theory. (The isosurface value is set as 0.02 au.)

Most extraction processes operate in acidic condition and thus Npy and Npz atoms may be protonated, which may have a significant influence on the complexation of metal ion with the ligands.38 To evaluate the level of protonation at Npy and Npz sites, proton affinity of the ligand (L) was calculated according to the following reactions: L+ H+ = [LH]+ and L+ [H3O]+ = [LH]+ + H2O.56 Here, the proton affinity ability was obtained by using Gibbs free energies at the BP86//ECP60MWB-SEG/6-311G* level of theory in the gas phase and aqueous solution. According to Table 1, the ∆G values at Npy site are more negative than those at Npz site for all the ligands, indicating the more easily protonated site at Npy, which agrees with the result of natural charges on nitrogen atom. Besides, the substituted ligands Cn-PypzH (n = 2, 4, 8) possess more negative ∆G values at both nitrogen sites compared to C0-PypzH, revealing their relatively stronger proton affinity. Table 1. Calculated energies of protonation at Npy and Npz sites (∆Gpy and ∆Gpz, kcal mol−1) of ligands with [H]+and [H3O]+ at the BP86//ECP60MWB-SEG/6-311G* level of theory C0-PypzH

C2-PypzH

C4-PypzH

C8-PypzH

Reactions

L+ H+ = [LH]+

gas

∆Gpy

∆Gpz

∆Gpy

∆Gpz

∆GPy

∆Gpz

∆Gpy

∆Gpz

-236.89

-226.18

-240.22

-231.67

-239.98

-232.22

-240.17

-231.17

L+ [H3O]+ = [LH]+ + H2O

gas

-67.39

-56.67

-70.72

-62.17

-70.48

-62.72

-70.67

-61.67

L+ [H3O]+ = [LH]+ + H2O

aq

-26.31

-19.04

-27.48

-21.47

-26.57

-21.06

-26.45

-19.64

7



3.2 Extraction complexes. Due to the diversity of coordinated structures of An(III) and Ln(III) with multidentate ligands, both 1:1 (metal/ligand) and 1:2 type complexes can be formed depending on experimental conditions such as metal : ligand molar ratio, experimental temperature, pH, etc. Narbutt and his co-workers investigated both the 1:1 type M(C2-BTBP)(NO3)3 and 1:2 type [M(C2-BTBP)2]3+ complexes using quantum mechanical calculations.57 Lewis et al. explored the existence of the 1:2 type [Eu(CyMe4-BTPhen)2(NO3)]2+ and subsequently obtained the crystal structure of the 1:1 type Eu(CyMe4-BTPhen)(NO3)3.58-59 Ding et al. confirmed the 1:1 type complexes of both Am(III) and Eu(III) ions with C8-PypzH by slope analysis and discovered 1:2 type single crystal structures of [Eu(PypzH)2(NO3)3].24 Therefore, we have considered both 1:1 and 1:2 type Am(III) and Eu(III) complexes with PypzH theoretically. 3.2.1 Structures of 1:1 type extraction complexes. To explore the most favorable species of Am(III) and Eu(III) ions with Cn-PypzH, we selected ligand C0-PypzH as a representative to study. Five possible optimized structures of the Am(III) and Eu(III) complexes and their binding energies were shown in Supporting Information, Figure S2 and Table S1, respectively. The calculated results suggest that for the same number of nitrate ions, the higher the coordination number is, the more negative the binding energy becomes. Moreover, for the same coordination number, more nitrate ions result to greater binding energies. Therefore, we regard the complexes M(PypzH)(NO3)3(H2O) as the most favorable species. And the structures of 1:1 type complexes of Am(III) and Eu(III) with four Cn-PypzH (n=0, 2, 4, 8) were displayed in Figure 2.

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Figure 2. Optimized structures of the 1:1 type complexes of Am(III) and Eu(III) ions with Cn-PypzH (n=0, 2, 4, 8) in the gas phase.

As shown in Figure 2, the Am(III)/Eu(III) centers in these 1:1 type complexes form nine-coordination to three bidentate nitrate anions and one unidentate water molecule. The average M–N and M–O bond lengths and the ∠NpyMNpz bond angle are listed in Table 2. Four complexes have similar structures with the ∠NpyMNpz angle values of ~61°. The predicted Am-N/Eu–N bond lengths are in the range of 2.5-2.7Å, which is in coincidence with previously reported values from 2.5 to 2.8 Å.57 Moreover, all of the Am–N bonds are shorter than the corresponding Eu–N bonds, indicating stronger interaction between Am(III) ions and the ligands. It is interesting to point out that there is more negative charge on Npy atom than that on Npz atom, but the M–Npz bonds are shorter than the M–Npz bonds, revealing the stronger binding affinity of Npz atom towards metal ions. Furthermore, the M–Npy bonds keep similar lengths, while the M–Npz bonds obviously decrease when the ligands are modified by the alkyl-substituted chain. This result indicates that the binding affinity of Npz atom towards metal ion is significantly affected by the length of alkyl-chains. In addition, the bond lengths between Am(III) ions and oxygen atoms of nitrate ions (Am–ON) are similar to those of Eu–ON bonds, which indicates that there is no obvious difference in coordination ability of nitrate ions towards Am and Eu cations owing to the hard acid feature of both Am(III) and Eu(III) ions.

9



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Table 2. Average M–N and M–O bond lengths (Å) and∠ ∠NpyMNpz bond angle (deg.) in M(Cn-PypzH)(NO3)3(H2O) complexes (M=Am, Eu) at the BP86//ECP60MWB-SEG/6-31G* level of theorya M-Npy

M-Npz

M-ON

M-OW

∠NpyMNpz

M(C0-PypzH)(NO3)3(H2O)

2.675/2.704

2.563/2.585

2.501/2.502

2.548/2.535

61.39 /61.45


2.673/2.700

2.554/2.577

2.503/2.503

2.551/2.538

61.53/61.57


2.676/2.690

2.554/2.555

2.502/2.506

2.553/2.533

61.50/61.55


2.668/2.690

2.552/2.554

2.503/2.503

2.551/2.534

61.49/61.67

a

…/… represent the results of Am(III) and Eu(III) complexes, respectively.

3.2.2 Bonding nature. NBO analysis which can help to better understand the bonding nature was performed at the BP86//ECP60MWB-SEG/6-311G* level of theory. Natural charges on the central metal ions and nitrogen atoms as well as WBIs of the M–N and M–O bonds are provided in Table 3. Natural charges loaded on Am and Eu atom are about 1.30 and 1.19 for all complexes, respectively, and therefore, there is a significant charge transfer from ligands to central metal ions. It is clearly seen that the natural charges on Npy hardly change when the ligand is substituted, while that on Npz gets more negative, which is probably the reason for the trend of M–N bond as discussed above. The natural charge on Npy and Npz is -0.428 and -0.243 in the free C0-PypzH ligands, respectively. Therefore, there is more charge transfer from Npz to metal ion than that from Npy, which also indicates that the Npz atom has an important role in affinity ability of ligand towards metal ions. Moreover, the natural charge on Npz atom in the Am(III) complexes is more negative than that in Eu(III) complexes, revealing more charge transfer from Npz atom to Am than to Eu. In addition, the charges on metal and nitrogen keep almost constant with the alkyl-chain changing from n= 2 to 8, respectively, which indicated the similar electronic structures of the substituted complexes. The electron occupation of Am(III) and Eu(III) in the complexes M(Cn-PypzH)(NO3)3(H2O) (M=Am, Eu; n=0, 2) at the BP86//ECP60MWB-SEG/6-311G* level are provided in Table S2, which denote that the electron occupations of Am(III) and Eu(III) mainly reside in the 5f and 4f shells, respectively.

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Table 3. Calculated natural charge on Am/Eu atom and N atom as well as average WBIs of the M–N and M–O bonds in the M(Cn-PypzH)(NO3)3(H2O) complexesa QM

QNpy

QNpz

M-Npy

M-Npz

M-ON

M-OW


1.298/1.197

-0.476/-0.476

-0.304/-0.295

0.262/0.239

0.291/0.263

0.334/0.324

0.283/0.272


1.278/1.196

-0.478/-0.476

-0.314/-0.300

0.266/0.240

0.301/0.267

0.338/0.323

0.287/0.271


1.277/1.192

-0.478/-0.469

-0.315/-0.307

0.266/0.242

0.302/0.281

0.339/0.322

0.286/0.272


1.277/1.192

-0.479/-0.469

-0.315/-0.307

0.266/0.242

0.303/0.281

0.338/0.322

0.287/0.272

a


The values of M-N WBIs are in the range of 0.26–0.31 (Table 3) which indicates that the M–N bonds contain a certain degree of covalent interaction. All the values of the Am–N WBIs are larger than those for Eu–N bonds, denoting the more covalency of Am–N bonds compared to Eu–N bonds. Furthermore, larger WBIs values of M–Npz bonds appears compared to those of M–Npy bonds in all complexes, which indicates there is significant covalency in M–Npz bonds. These results further confirm that the Npz atoms play greater roles in bonding between the ligands and metal ion, which agrees with the analyses of natural charge and M–N bond lengths. In addition, the WBIs values of M-N bonds become large when the ligands are substituted, while these values are slightly variable with the increasing alkyl-chains. As for the M-O bond WBIs, the values almost keep constant for all the complexes. The topological analyses of electron density for M–N and M–O bonds have also been performed using the QTAIM method with Multiwfn code,51 which can get valuable information on the bonding nature of the target complexes.60-61 To test the effects of methods on the bonding covalency,62 the values of electron density ρ(r) and Laplacian (∇2ρ(r)) at the BCPs using BP86 and HF methods are listed in Table S3 and Table S4, respectively. These two theoretical methods show the similar values. All of the ρ(r) at the M–N BCPs are lower than 0.10 au which accompanies with positive ∇2ρ(r) value.38 This indicates a certain degree of covalent character, which is consistent with the WBIs analysis. The ρ(r) values at the Am–N BCPs are somewhat larger than those at the Eu–N BCPs, suggesting greater covalency of the Am–N bonds. Additionally, the ρ(r) at M–Npz BCPs are larger compared to those of M–Npy, which supports the results of 11



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NBO analysis. 3.2.3 Molecular orbital. Molecular orbital (MO) analysis can be used to evaluate the involvement extent of various orbitals for coordinating toms. Here, we chose two representative complexes M(C0-PypzH)(NO3)3(H2O) and M(C2-PypzH)(NO3)3(H2O) as examples to carry out MO analysis. The α-spin valence MOs diagrams are shown in Figure 3 and the specific atomic contributions to the corresponding MOs are given in Supporting Information, Table S5. It is clearly seen that PypzH ligands and the nitrate ions have obvious orbital interactions with metal ions. For instance, the contribution

of

the

Am

5f

orbital

is

82.64%

for

the

SOMO

in

Am(C0-PypzH)(NO3)3(H2O) complex, which is larger than the corresponding Eu 4f (74.31%) in Eu(C0-PypzH)(NO3)3(H2O). Compared to MOs with higher energy levels, the contribution of the metal f orbitals is much less to orbitals with lower energy levels. The lower orbitals displayed in Figure 3 possess σ character and are dominantly contributed by N 2p atomic orbitals from ligands and the O atoms of nitrate anions. Moreover, Npz atom has an obvious contribution to these lower MOs. Besides, the lowest unoccupied MO (LUMO) and the corresponding energy are provided in Figure S3. The gaps between HOMO and LUMO for the Am(III) complexes are larger than those for the Eu(III) complexes, which indicates that the relative stability of Am(III) complexes.

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Figure 3. The energy levels (eV) and diagrams of α-spin valence MOs in the M(C0-PypzH)(NO3)3(H2O) and M(C2-PypzH)(NO3)3(H2O) complexes. (The isosurface value is set as 0.02au.)

3.2.4 Energy analysis. Thermodynamic features are very important for understanding the interactions of Cn-PypzH (n=0, 2, 4, 8) with Am(III) and Eu(III) cations. Six probable reactions starting from six initial reactants are considered for forming products [M(Cn-PypzH)(NO3)3(H2O)] as presented in Figure 4. The corresponding changes of Gibbs free energy (∆G) were calculated at the BP86//ECP60MWB-SEG/6-311G* level of theory. The liquid–liquid extraction system is a relatively complex process.57 To simulate the possible solvent extraction process, solvation effects were carried out in water, n-dodecane and tert-butylBenzene solutions. The ∆G values of these reactions in aqueous (aq) and organic (org) phases are calculated according to Eq. (1) and (2): [M(NO3)a(H2O)b](3-a)+aq+(Cn-PypzH)aq +(3-a)NO3-aq →[M(Cn-PypzH)(NO3)3(H2O)]aq + (b-1)H2Oaq(1) [M(NO3)a(H2O)b](3-a)+aq+(Cn-PypzH)org +(3-a)NO3-aq →[M(Cn-PypzH)(NO3)3(H2O)]org + (b-1)H2Oaq(2)

Table S6 gives the ∆G values of the reactions in both aqueous and organic phases. Figure 4 shows ∆G values for 24 possible extraction reactions related to the formation 13



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of Am(III) complexes mentioned above in the aqueous phase. Obviously, with the increasing number of coordinated nitrate anions in the reactants, the ∆G values for the reactions

get

positive.

For

example,

for

the

reactions

with

product

Am(C0-PypzH)(NO3)3(H2O), ∆G for the reaction with reactants [Am(H2O)9]3+ and Am(NO3)3(H2O)3 is -74.31 to -16.08 kcal mol−1, respectively. The similar trend can be observed for the reactions of Eu(III) complexes as shown in Figure S4. In fact, the trivalent metal ion can easily bind one nitrate ion in high HNO3 solution, and the most probable

reaction

is,

[M(NO3)(H2O)6]2+

+

Cn-PypzH

+

2NO3-

→M(Cn-PypzH)(NO3)3(H2O) + 5H2O, in the extraction process based on the ∆G values. The separation factor SFAm/Eu (−RTlnSFAm/Eu = ∆∆GAm/Eu) has been widely used to estimate ligand selectivity for Am(III) over Eu(III) in a two phases extraction system. The differences of ∆G (∆∆GAm/Eu) for the reaction [M(NO3)(H2O)6]2+ + Cn-PypzH + 2NO3- →M(Cn-PypzH)(NO3)3(H2O) + 5H2O have been calculated between the reactions of Am(III) and Eu(III) complexes according to the equation: ∆∆GAm/Eu=∆GAm−∆GEu. As shown in Table 4, all the ∆∆GAm/Eu are negative in both the aqueous phase and organic phase, which indicates that the Cn-PypzH ligands have good selectivity towards Am(III) over Eu(III). The Am/Eu selectivity of Cn-PypzH is improved when the ligand is substituted in the gas phase. Although the absolute values of the ∆∆G for the reactions are different in aqueous and organic phases, the trend of all the ∆∆G values follows the order of C2-PypzH> C8-PypzH> C4-PypzH. To evaluate the relativistic effect on reaction energies, we have carried out the calculations of SR and SO relativistic effects. As shown in Table S7, the SO contributions to the differences between reaction energies are about 0.5 kcal mol-1, while the trend with two relativistic effects is consistent. Therefore, the SO effect do not qualitatively change the results studied here.

14


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Figure 4. Changes of Gibbs free energy (∆G, kcal mol−1) for the 24 extraction reactions of the Am(III) complexes from six reactants in the aqueous phase. Table 4. Differences between the changes of Gibbs free energy (∆∆G, kcalmol−1) for extraction reactions of Am(III) and Eu(III) complexes with Cn-PypzH ligands (n=0, 2, 4, 6) in the gas, aqueous, tert-butylBenzene and n-dodecane phases at the BP86//ECP60MWB-SEG/6-311G* level of theory Reactions

∆∆Ggas

∆∆Gaq

∆∆Gtbb

∆∆Gn-dod

[M(NO3)(H2O)6]2+ +C0-PypzH +2NO3- →M(C0-PypzH)(NO3)3(H2O) +5H2O

-3.77

-4.18

-5.41

-2.93


-5.19

-5.24

-4.35

-6.37


-4.15

-3.19

-2.74

-2.65


-4.46

-3.61

-3.11

-3.02

3.3.1 The structures of 1:2 type extraction complexes. Ding et al. obtained the 1:2 type single crystal structure of Eu(III) with PypzH.24 Hence, we also explored the structures of 1:2 type complexes of the Am(III) and Eu(III) ions with the ligand PypzH. The optimized structures of the three studied complexes are displayed in Figure 5 and the corresponding M–N and M–O bond lengths are presented in Table S8. Obviously, the Am–N bonds are shorter than the corresponding Eu–N bonds in each 1:2 type complex, and the M–Npz bonds are also shorter than the M–Npy bonds in the same complex, which is consistent with the results derived from 1:1 type complexes. Compared with the 1:1 and 1:2 type complexes, all the bonds in M(PypzH)2(NO3)3 complexes are longer than the corresponding values in 15



Page 16 of 24

M(PypzH)(NO3)3(H2O) complexes (Figure S5), which can probably be attributed to the steric interaction and coordinated number.

Figure 5. Optimized structures of the 1:2 type complexes Am(III) and Eu(III) ions with PypzH in the gas phase.

3.3.2 Bonding nature. Table 5 affords the natural atomic charges onthe central metal and nitrogen atoms as well as the M–N and M–O WBIs. Similar to 1:1 complexes, the values of the natural charge on Am atoms are ~0.1 e larger than those on Eu, and the Npy atoms possess more negative charge compared to Npz atoms. In addition, the values of Am–N WBIs are larger than those of Eu–N bonds for all the complexes, and the values of M–Npz WBIs are larger than those of M–Npz, which is also similar with the cases of 1:1 complexes. Table 5. The natural charges on metal and nitrogen atoms and WBIs of the M–N and M–O bonds in 1:2 type complexes of Am(III) and Eu(III) ions with PypzHa QM

QNpy

QNpz

M(PypzH)2(NO3)3

1.180/1.081

-0.449/-0.437

[M(PypzH)2(NO3)2(H2O)2]+

1.096/1.004

[M(PypzH)2(NO3)(H2O)4]2+

1.204/1.099

a

M-Npy

M-Npz

M-ON

-0.292/-0.282

0.253/0.232

0.298/0.283

0.324/0.312

–

-0.472/-0.461

-0.317/-0.305

0.271/0.263

0.323/0.307

0.363/0.342

0.281/0.267

-0.506/-0.487

-0.327/-0.327

0.301/0.276

0.308/0.303

0.368/0.348

0.265/0.252


3.3.3 Energy analysis. The changes of Gibbs free energies of the three reactions in aqueous and tert-butylBenzene and n-dodecane phases were listed in Table 6. Obviously, the ∆G for the reactions with the products M(PypzH)2(NO3)3 turn out to be the most negative values among the three reactions, which indicates that the extraction complexes are more likely to form M(PypzH)2(NO3)3. Similar to 1:1 type complexes, the ∆G for the reaction of Am(III) complexes is more negative than that of Eu(III) complexes. Thus, these results suggest that the N-donor ligands Cn-PypzH have good selectivity towards Am(III) over Eu(III). Compared with 1:1 and 1:2 type 16


M-Ow

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extraction reactions, the ∆G values for the reactions with 1:2 type extraction complexes M(PypzH)2(NO3)3 are more negative than those with 1:1 type complexes M(PypzH)(NO3)3(H2O). For example, the ∆G for reaction [Am(NO3)(H2O)6]2+ + 2PypzH + 2NO3- →Am(PypzH)2(NO3)3+ 6H2O is −50.70 and −50.20 kcal mol−1 in aqueous and tert-butylBenzene solution, respectively, which is more negative than the corresponding

values of

reaction

[Am(NO3)(H2O)6]2+ +

PypzH

+

2NO3-

→Am(PypzH)(NO3)3(H2O) + 5H2O (-47.47 and -41.65 kcal mol−1 in aqueous and tert-butylBenzene solution, respectively). However, the ∆∆G value for the 1:1 extraction reaction is larger than that for the 1:2 extraction reaction. Thus, the 1:1 extraction reaction [M(NO3)(H2O)6]2+ + PypzH + 2NO3- → M(PypzH)(NO3)3(H2O) + 5H2O is better for the Am(III)/Eu(III) separation. It gives us a hint that, to achieve better Am(III)/Eu(III) selectivity of the ligand PypzH, it is better to form 1:1 type extraction complexes by adjusting experimental conditions, such as ligands and metal ions molar ratio, experimental temperature, pH. Therefore, this work can provide the theoretical basis and guide for experiments. Table 6. Changes of Gibbs free energies (∆G, kcal mol−1) for the reactions of 1:2 complexes of Am(III) and Eu(III) ions with PypzH in aqueous and tert-butylBenzene and n-dodecane phases at the BP86//ECP60MWB-SEG/6-311G* level of theorya,b Reactions

∆Gaq

∆Gtbb

∆Gdod

-50.70/-47.23

-50.20/-47.18

-50.05/-47.11

(3.47)b

(3.02)

(2.92)

[M(NO3)(H2O)6] +2PypzH +NO3 →[M(PypzH)2(NO3)2(H2O)2] + 4H2O

-33.61/-30.05

-21.18/-17.91

-18.91/-15.69

[M(NO3)(H2O)6]2+ +2PypzH →[M(PypzH)2(NO3)(H2O)4]2+ + 2H2O

-10.26/-8.62

42.30/43.49

51.51/52.62

[M(NO3)(H2O)6]2+ +2PypzH +2NO3- →M(PypzH)2(NO3)3+ 6H2O

2+

-

+

a…/… b

represent the results of Am and Eu complexes, respectively. the values in parenthesis are ∆∆G (kcal mol−1).

4. CONCLUSIONS In summary, we have investigated the extraction mechanisms of the Am(III) and Eu(III) complexes with the ligands Cn-PypzH (n=0, 2, 4, 8) using relativistic DFT. The MEP and ∆Gpro results reveal that the nitrogen atom of the pyridine ring is more negative and more easily protonated compared to that of pyrazole ring. For both the 1:1 and 1:2 type complexes, the M–Npz bonds are shorter than the M–Npy bonds, and 17



the WBIs values of M–Npz bonds are larger than those of M–Npy bonds, which suggest that the Npz atom has stronger coordination ability towards metal ions compared to Npy atom, though the latter has more negative charges. Furthermore, the shorter bond length and larger WBIs values of Am-N bonds compared to Eu–N bonds suggest a stronger interaction between the ligands and Am(III). The NBO and QTAIM analyses reveal that the M-N bonds have a certain degree of covalent character, and the Am-N bonds possess a higher covalency compared to Eu–N bonds. In addition, the adding alky-chain indeed changes the bond lengths and the electronic properties, while the lengths of the substituted chains have almost no effect on the electronic properties. According to the thermodynamic analysis, the Cn-PypzH ligands show good selectivity for Am(III) over Eu(III). The Am/Eu selectivity of PypzH improves when the ligand was modified by the substituted alkyl-chain. Although the ∆G values for 1:2 extraction reactions are more negative than those for 1:1 extraction reactions, the corresponding ∆∆G values are converse for the two types of extraction reactions. Therefore, to achieve a better Am(III)/Eu(III) selectivity with PypzH, it is necessary to form 1:1 extraction complexes by adjusting the experimental conditions. The trend of the ∆∆G values for the 1:1 extraction reactions follows the order of C2-PypzH> C8-PypzH> C4-PypzH. This work can help understanding the extraction mechanism of the Cn-PypzH ligands with Am(III) and Eu(III) ions and provide valuable information for designing novel homothetic N-donor ligands for Am(III)/Eu(III) separation.

ASSOCIATED CONTENT Supporting Information The diagrams of HOMO and LUMO of the Cn-PypzH, optimized structures, binding energies, frontier molecular orbits of the Am(III) and Eu(III) complexes, electron density and Laplacian values, orbital population, thermodynamic data are provided. This material is available free of charge via the Internet at http://pubs.acs.org.

18


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: 86-10-88233968. ‡These two authors contributed equally to this work. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21477130, 21471152, 91426302), the Major Program of National Natural Science Foundation of China (21790373) and the Science Challenge Project (JCKY2016212A504), the Program of Innovative Research for Postgraduate of Hunan Province (CX2016B439). The results described in this work were obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences. REFERENCES (1) 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. (2) Herrera-Martinez, A.; Kadi, Y.; Parks, G. Transmutation of nuclear waste in accelerator-driven systems: Thermal spectrum. Ann. Nucl. Energy. 2007, 34, 550-563. (3) Yapici, H.; Genç, G.; Demir, N. A comprehensive study on neutronics of a lead–bismuth eutectic cooled accelerator-driven sub-critical system for long-lived fission product transmutation. Ann. Nucl. Energy. 2008, 35, 1264-1273. (4) Panak, P. J.; Geist, A. Complexation and extraction of trivalent actinides and lanthanides by triazinylpyridine N-donor ligands. Chem. Rev. 2013, 113, 1199-1236. (5) Cao, X. Y.; Heidelberg, D.; Ciupka, J.; Dolg, M. First-principles study of the separation of Am-III/Cm-III from Eu-III with Cyanex301. Inorg. Chem. 2010, 49, 10307-10315. (6) Wang, X. X.; Li, J. X.; Dai, S. Y.; Hayat, T.; Alsaedi, A.; Wang, X. K. Interactions of Eu(III) and Am-243(III) with humic acid-bound gamma-Al2O3 studied using batch and kinetic dissociation techniques. Chem. Eng. J. 2015, 273, 588-594. (7) Pearson, R. G. Hard and soft acids and bases. J. Am. Chem. Soc. 1963, 85, 3533-3539. (8) Nash, K. L. A Review of the basic chemistry and recent developments in trivalent f-elements separations. Solvent Extr. Ion Exch. 1993, 11, 729-768. 19



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Insight into the Extraction Mechanism of Am(III) over Eu(III) with

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