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Theoretical Studies on the AnO2n+ (An=U, Np; n=1, 2) Complexes with Di-(2-ethylhexyl) Phosphoric Acid Juan Luo,‡a Cong-Zhi Wang,‡a Jian-Hui Lan,a Qun-Yan Wu,a Yu-Liang Zhao,a Zhi-Fang Chai,a,b Chang-Ming Niec and Wei-Qun Shi*a

Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/

Actinide separation in spent nuclear fuel reprocessing is essential for the closed nuclear fuel cycle. Organophosphorus reagents have been found to exhibit strong affinities for actinides in experiment. In this work, the extraction complexes of AnO2n+ (An=U, Np; n=1, 2) with the traditional organophosphorus ligand HDEHP (di-(2-ethylhexyl) phosphoric acid) have been investigated by using density functional theory together with scalar-relativistic effective core potentials (ECPs) for actinide elements. According to our calculations, the HDEHP dimer prefers to act as a bidentate ligand in most of the studied complexes. HDEHP ligands show a higher extraction ability for An(VI) over An(V), and the formation of Np(VI) complexes is slightly more favorable than those of U(VI) analogues, which is mainly attributed to the stronger donor-acceptor interaction in Np(VI) complexes. The intramolecular hydrogen bonds play an significant role in the stability of the 1:1 type complexes AnO2(HL) 2(NO3)2 (L=DEHP). Moreover, AnO2(HL) 2(NO3) 2 are the most stable species in nitrate-rich acid solutions, while at low nitric acid concentrations, the complexing reaction of AnO2(H2 O) 52+ + 2(HL)2 → AnO2(HL2)2 + 2H+ + 5H2O is probably the dominant reaction in the extraction process. Our results can help to understand the speciation of actinyl complexes in real solvent extraction of actinides with HDEHP at the molecular level.

Introduction To realize the sustainable development of nuclear energy, it is necessary to recycle fissile materials and separate long-lived minor actinides such as Np, Am, Cm from HLLW (high-level liquid waste). A number of solvent extraction processes have been proposed based on the extraction capability of different reagents for actinides and lanthanides, e.g. the organophosphorus compound, such as CMPO (n-octyl(phenyl)N,N-diisobutylmethylcarbamoyl phosphine oxide) used in the TRUEX (transuranium extraction) process1, 2 and HDEHP (di(2-ethylhexyl) phosphoric acid) used in the TALSPEAK (the trivalent actinide-lanthanide separation by phosphorus reagent extraction from aqueous komplexes) process3, 4, and malonamides such as DMDOHEMA (N,N'-dimethyl-N,N'dioctylhexylethoxymalon-amide) used in the DIAMEX (diamide extraction) process5, 6. Hereinto, HDEHP (Figure 1) is one of the most widely used acidic organophosphorus extractants in the large-scale separation of f-element cations and has been extensively studied7-10. It has been found that HDEHP possesses high separation factors for adjacent lanthanide pairs,11 and it can be infinitely mixable with organic diluents with the main form of hydrogen-bonded dimers in bidentate coordination mode to the metal cations12-16. In addition, HDEHP is also a liquid cation exchanger used for back extraction in the CTH (Chalmers Tekniska Högskola) process.17, 18

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O

O P

O

OH

Figure 1. Stucture of HDEHP. Up to now, a series of experimental investigations on liquidliquid extraction of actinides with HDEHP have been carried out. Sato19 studied the extraction behavior of U(VI) with HDEHP from nitric acid media in kerosene. They found that at the low acidity, U(VI) extraction is in conformity to the equation of UO22+ + 2(HL)2 → UO2(HL2)2 + 2H+ (L=DEHP-) and the reaction mechanism of the extraction is mainly ionic exchange, while at higher acidities, the extraction process becomes more complicated and UO2(HL)2(NO3)2 is probably the predominant complex. Nakamura20 examined the extraction of Np(IV), (V), (VI) with HDEHP, and suggested that the extraction behavior of Np(VI) is similar to that of U(VI) in acid solution and HDEHP exhibits higher extractability for hexavalent neptunium and uranium but lower extractability for pentavalent neptunium. Recently, Cocalia et al.13 investigated the extraction of trivalent europium as well as hexavalent uranyl ions by HDEHP in ionic liquid and dodecane. The UV-

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vis (Ultraviolet-visible spectroscopy) and EXAFS (Extended XRay Absorption Fine Structure) results indicated that the HDEHP ligands coordinate to U(VI) and Eu(III) through four and six phosphoryl oxygen atoms, respectively. Actually, understanding the chemical species in solvent based extraction can provide meaningful information for extraction mechanism identification and further help to optimize the extraction parameters. Although numerous experimental studies have been carried out to identify actinide extraction complexes with HDEHP, the chemical structures of these extraction complexes have not been clearly elucidated so far. On the other hand, theoretical calculation has been considered to be an effective compensatory approach for exploring actinide complexes at the molecular scale, which can give accurate prediction towards actinide bonding and coordination. Herein in this study, we investigated a series of U(V), U(VI) and Np(V), Np(VI) complexes with HDEHP in nitric acid solution using density functional theory (DFT), and mainly focused on the structure properties and bonding natures as well as thermodynamic stabilities of the extraction complexes. It is expected that this work could lay a new avenue for studying the formation of extraction complexes of HDEHP.

Computational Details All theoretical calculations were carried out with Gaussian 0921 package using density functional theory (DFT).22, 23 The B3LYP hybrid functional (Becke’s three parameter hybrid functional with the Lee, Yang and Parr correlation functional)24, 25 was adopted for all molecular systems. For U and Np, scalar relativistic effects were taken into account through Stuttgart small-core scalar-relativistic effective core potential (SCRECP) and corresponding basis sets26, while the 6-311G(d, p) basis sets were adopted for H, C, O, N and P. The spin polarization has not been taken into account. Our previous investigations27-32 demonstrated that this level of theory can obtain relatively accurate results of geometries and energetics for actinide complexes with organophosphorus ligands and other traditional reagents. During geometry optimization, the singlet, doublet, doublet, and triplet states were considered as the ground states for the U(VI), U(V), Np(VI), and Np(V) species, respectively. At the B3LYP/6-311G(d, p)/RECP level of theory, harmonic vibrational frequencies were calculated to insure that the optimized structures are local minima on the potential energy surface. NBO (natural bond orbital) analysis33, 34 for the optimized geometries was performed at the same level of theory to evaluate the bonding nature. In addition, the QTAIM (Quantum Theory of Atoms in Molecules) analysis35 was carried out with Multiwfn 3.2 software36. Given the reaction conditions, solvent effects have been considered based on the gas-phase optimized structures using the conductor-like polarizable continuum model (CPCM)37-40 with Klamt’s form of conductor-like screening model (COSMO)37, both in water and organic solution (n-dodecane). The solvation Gibbs free energies of each species were calculated including zero-point energy (ZPE) and thermal corrections obtained in the gas phase. Besides, the experimental value -263.982 kcal/mol41 was adopted for H+.

Results and Discussion The HDEHP ligand Because of the presence of P-OH and P=O bonds, the HDEHP ligands usually form hydrogen bonded dimers or polymers in

2 | J. Name., 2012, 00, 1-3

Journal Name DOI: 10.1039/C4DT03321C nonpolar diluents.14, 42, 43 To better understand the property of HDEHP complexes, the geometrical and electronic structures of the HDEHP dimer and trimer were explored firstly. Figure 2 shows the optimized structures of the dimer and trimer. In the HDEHP dimer and trimer, the eight- and twelve-membered rings are formed by two and three intermolecular hydrogen bonds, respectively. In the dimer, the average O···O bond distance of the hydrogen bonds is 2.576 Å, which is slightly longer than those in the trimer.

Figure 2. Optimized structures of the HDEHP dimer (a) and trimer (b). Green and white sticks represent C and H, while white, yellow and red spheres represent H, P, and O, respectively. The frontier molecular orbitals (FMOs) play an important role in governing chemical reactions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) plots of the HDEHP dimer and trimer are illustrated in Figures S1 and S2 (in the ESI), respectively. For the HDEHP dimer, it can be seen that the HOMO (Figure S1a) and LUMO (Figure S1b) are mainly localized at the eightmembered ring, and the HOMO is largely occupied by the lone pair O 2p electrons in the ring, which are the main components taking part in the coordination reaction. To find the chemically active sites of HDEHP dimer, the MEP (molecular electrostatic potential) has been also calculated. As depicted in Figure S1c, the red and green areas in the diagram identify regions of negative electrostatic potentials (nucleophilic sites) and positive or less negative electrostatic potentials, respectively. Clearly, the negative regions mainly cover the oxygen atoms of the eight-membered ring, indicating higher reactivity of those sites. It is noted that the electrostatic potentials on the O atoms of P=O is more negative than those of P-OH, which is indicative of stronger nucleophilicity for the phosphoryl oxygen atom in the HDEHP dimer. Similar results can be obtained for the HDEHP trimer by the MOs and MEP analyses (Figure S2). In addition, the stability of the HDEHP dimer and trimer has been estimated by the reaction of nHL → (HL)n (n=2,3) in the gas phase and aqueous solution as well as n-dodecane. At the B3LYP/6-311G(d, p) level of theory, the calculated changes of Gibbs free energy ∆Gg, ∆Gaq, ∆Gorg were -14.1, -0.8 and -8.7 kcal/mol for the HDEHP dimer, respectively, while for the trimer, these energies were -19.0, 12.1, -10.2 kcal/mol, respectively. Thus, both the formation of these two complexes is more favorable in organic solution compared to that in aqueous solution. Consequently, it is necessary to take the effect of organic solution into consideration for exploring the metal-ligand complexing process. Based on our calculations, the HDEHP dimer and trimer can simultaneously exist in the organic solution by the virtue of the comparable ∆Gorg values, i.e., -8.7 kcal/mol for the dimer and -10.2 kcal/mol for the trimer. Taking into account the interfacial reaction in the extraction process, the dimer may gain a competitive edge over the trimer because of its lower hydrophobicity compared to the

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1:1 (Metal:Ligand) Mole Ratio Extraction Complexes Geometrical structures Given that the coordination number of actinyl in the equatorial plane can range mainly from four to six,44 here we considered a series of 1:1 type hydrate and nitrate-hydrate complexes with four, five and six-membered rings in actinyl equatorial plane. Besides, the species of AnO2(HL)2(NO3)2 (An=U, Np) predicted by experimental studies19 have also been considered. The structures of these extraction species were optimized at the B3LYP/6-311G(d, p)/RECP level of theory. As show in Figures 3, 4

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ARTICLE DOI: 10.1039/C4DT03321C and S3, S4, for each species with HDEHP dimer, all the dimers coordinate to the uranyl and neptunyl ions as bidentate ligands with two phosphoryl groups in the equatorial plane. Besides, the actinyl complexes with U and Np at the same oxidation state seem to possess qualitatively similar geometrical structures according to our calculations. It is particularly worth mentioning that AnO2(HL)2(NO3)2 have two conformers (Figure S5), i.e. the cis structure with two HDEHP ligands coordinated as dimers through hydrogen-bonding, and the trans structure with two monomer HDEHP ligands coordinated to the metal cations as monodentate ligands. The trans structure is predicted to be the global minimum, which lies 6.3 kcal/mol below the cis structure. This is mainly due to the formation of intramolecular hydrogen bonds via the P-OH group and nitrate ions in the trans structure. In the following discussion, only the trans configuration will be taken into account.

Figure 3. Optimized structures of the 1:1 type UO22+ complexes with HDEHP. Green and white sticks represent C and H, while yellow, red, blue, and pink spheres represent P, O, N, and U, respectively.

Figure 4. Optimized structures of the 1:1 type NpO22+ complexes with HDEHP. Green and white sticks represent C and H, while yellow, red, blue, and purple spheres represent P, O, N, and Np, respectively.

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Table 1. U-O Bond Lengths (Å) and the Symmetrical, Antisymmetrical Stretching Frequencies (υs and υas, cm-1) of U=Oaxial for the Complexes of UO2n+ (n=1, 2) and L (L=DEHP-) Calculated by the B3LYP Method. UO2(H2O)5

2+

__

U-OP-OH __

U-O(H2O)a

U-O(NO3−)a __

2.491(2.41)

υsc b

894(870)

υasc b

981(965)b

UO2(HL2)(NO3)

1.771

2.266

2.314

2.458

__

843

931

UO2(HL2)(NO3)(H2O)

1.777

2.288

2.312

2.498

2.577

832

919

UO2(HL2)(NO3)(H2O)2

1.775

2.324

2.463

2.558

2.605

834

922

UO2(HL2)(H2O)3] Published on 24 December 2014. Downloaded by Tulane University on 12/01/2015 12:08:19.

1.751(1.76)

U-OP=O b

+

1.773

2.256

2.308

__

2.574

837

925

UO2(HL)2(NO3)2

1.770

2.414a

__

2.560

__

847

932

UO2(H2O)5+

1.811

__

__

__

2.585

803

861

[UO2(HL2)(NO3)]-

1.830

2.411

2.445

2.573

__

747

814

1.836

2.450

2.522

2.579

2.698

738

801

1.833

2.504

2.595

2.717

2.727

741

818

[UO2(HL2)(NO3)(H2O)]

-

[UO2(HL2)(NO3)(H2O)2]UO2(HL2)(H2O)3

1.831

2.437

2.453

__

2.673

762

815

UO2(HL)2(NO3)

1.818

2.459

2.499

2.566

__

780

826

b

a

45-49 c

The average bond lengths. The values in the parentheses are the corresponding experimental data calibrated with scaling factor 0.968 at the B3LYP/6-311G(d, p) level50.

. The calculated vibrational frequencies are

Table 2. Np-O Bond Lengths (Å) and the Symmetrical, Antisymmetrical Stretching Frequencies (υs and υas, cm-1) of Np=Oaxial for the Complexes of NpO2n+ (n=1, 2) and L (L=DEHP-) Calculated by the B3LYP Method. Np=Oaxiala 1.730(1.75)b

Np-OP=O __

Np-OP-OH __

Np-O(NO3−)a __

Np-O(H2O)a

NpO2(HL2)(NO3)

1.752

2.241

2.300

2.444

NpO2(HL2)(NO3)(H2O)

1.755

2.279

2.308

NpO2(HL2)(NO3)(H2O)2

1.751

2.340

NpO2(HL2)(H2O)3]+

1.750

2.239

Species NpO2(H2O)5

2+

a

υsc 891(863)b

υasc 987(969)b

__

832

932

2.487

2.559

832

929

2.435

2.591

2.610

840

938

2.305

__

2.566

834

942

2.474(2.42)

b

NpO2(HL)2(NO3)2

1.749

2.405

__

2.552

__

845

942

NpO2(H2O)5+

1.794(1.83)b

__

__

__

2.589(2.50)b

797(767)b

861(824)b

1.803

2.405

2.444

2.563

__

762

838

[NpO2(HL2)(NO3)][NpO2(HL2)(NO3)(H2O)]

-

1.824

2.419

2.497

2.577

2.767

741

818

[NpO2(HL2)(NO3)(H2O)2]-

1.817

2.576

2.620

2.629

2.703

720

810

NpO2(HL2)(H2O)3

1.811

2.451

2.470

__

2.674

764

832

1.804

2.465

2.545

2.540

__

782

845

NpO2(HL)2(NO3) a

b

45-49 c

The average bond length. The values in the parentheses are the corresponding experimental data calibrated with scaling factor 0.968 at the B3LYP/6-311G(d, p) level50.

Tables 1 and 2 list the An-O bond lengths and harmonic vibrational frequencies of An=Oaxial for the uranyl and neptunyl extraction complexes. As we all know, although the B3LYP method does account for certain electronic correlation effects, it still overestimates the fundamental normal modes of vibration relative to the experimental values because of the neglect of inharmonic effects and the basis set deficiencies.51, 52 To further improve the consistency between the calculated and observed fundamental vibrational frequencies, all the calculated An=Oaxial vibrational frequencies of the studied complexes are calibrated with scaling factor 0.96850 at the B3LYP/6-311G(d, p) level. For the actinyl pentahydrates UO2(H2O)52+, NpO2(H2O)52+ and NpO2(H2O)5+, the calculated An=Oaxial vibrational frequencies seem to be in accordance with the corresponding experimental data and the average deviation is within the range of about 20~30 cm-1. Besides, the calculated

4 | J. Name., 2012, 00, 1-3

. The calculated vibrational frequencies are

structural parameters were found to be in good agreement with the experimental results. Considering the large molecular systems in the present work, the adopted theoretical methods can obtain reasonable results for the actinyl complexes. Based on the calculations, we note that except for AnO2(HL)2(NO3)2, the An-OP=O bond lengths in each species are shorter than the corresponding An-OP-OH bond lengths, which attributes to the relatively more negative electrostatic potentials on the O(P=O) atom. For both the An(VI) and An(V) species, the An-O axial and equatorial bond lengths of the uranyl complexes are slightly longer than those of the corresponding neptunyl complexes, which may be a result of the actinide contraction. Comparing the An(VI) and An(V) species, all the An-O bond lengths for the former complexes are significantly shorter than the latter ones, indicating stronger metal-ligand bonding in the An(VI) complexes. This seems to be understandable because of higher metal charges of An(VI) ions. Besides, it is well

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established that the strong metal-ligand binding in actinyl equatorial plane can weaken the An=Oaxial bond.53, 54 According to our calculations, for all the 1:1 type extraction complexes, the bond lengths of An=Oaxial increase compared to those for the penta-aquo complexes AnO2(H2O)5n+ (n=1, 2). Accordingly, the An=Oaxial stretching frequencies are significantly red-shifted, which reveals that the ligands are more strongly bonded to the actinyl cations in the extraction complexes. For the hexacoordinated complexes AnO2(HL2)(NO3)(H2O)2 and AnO2(HL)2(NO3)2, the latter complexes with two HDEHP monomers exhibit shorter An=Oaxial bond lengths and stronger An=Oaxial stretching frequencies, indicating weaker An=Oaxial bonds and stronger metal-ligand binding in actinyl equatorial plane in AnO2(HL2)(NO3)(H2O)2. Besides, the An-OP=O bond lengths in AnO2(HL)2(NO3)2 are much longer than that in AnO2(HL2)(NO3)(H2O)2. Given that nitrate ions have higher coordination ability to the actinyl ions than water molecules, it can be concluded that the bond strength between the two HDEHP monomers and the metal ions is much weaker compared to the HDEHP dimers.

In addition to the NBO analysis, the QTAIM analysis was also performed to probe the interaction nature between metal ions and organic ligands. As shown in Tables S1 and S2, the electron density (ρ) values of An-Op, An-ONO3- and An-OH2O bonds for all the studied complexes are lower than 0.1 a.u. and the Laplacian of electron density (∇2ρ) values are all positive. It is worth mentioning that in the QTAIM analysis the ρ and ∇2ρ values may incorrectly reflect the bonding nature for very polar bonds.55, 56 At the An-O bond critical points, the An(V) complexes show lower electron densities compared to the An(VI) complexes, indicating that the An(VI) species have stronger An-O bonding than the An(V) species for both uranium and neptunium. This conclusion agrees well with those observed from the ligand-to-metal charge transfer property discussed above. However, the ρ and ∇2ρ values of U(VI) and U(V) complexes seem to be comparable to those of the corresponding Np(VI) and Np(V) complexes, respectively. Therefore, the ρ and ∇2ρ values cannot well reflect the differences of the metal-ligand bond for actinyl complexes with the same oxidation state.

Electronic structure analysis The NBO analysis was performed to provide insight into the bonding nature of the uranyl and neptunyl complexes. The calculated WBIs of the An-O bonds and the quantity of charge transfer from the NO3-, L (L=DEHP-) and H2O ligands to the metal ion are shown in Tables 3 and 4. For all of these complexes, the WBIs of An-OP=O bonds are higher than those of the corresponding An-OP-OH bonds, which is in agreement with the trends in the An-O bond distances. Besides, the An-O WBIs of the An(VI) complexes are higher than the An(V) complexes, suggesting more covalent character of the An-O bonds in the An(VI) complexes. As for most U(VI) and Np(VI) complexes, the WBIs of the An-OP bonds in the Np(VI) complexes are slightly higher than those in their U(VI) analogues, which is in accordance with the results of An-O bond lengths. With regard to U(V) and Np(V) species, higher bond orders are found in U(V) complexes. Being consistent with the bond lengths, AnO2(HL)2(NO3)2 complexes yield lower An-OP=O WBIs values compared to the AnO2(HL2)(NO3)(H2O)2 complexes, implying less covalent character in the metal-ligand bond in the former complex with monomer HDEHP ligands. As depicted in Tables 5 and 6, for most of the An(VI) complexes, the charge transfer from HDEHP ligands to UO22+ cation is less significant compared to that to NpO22+. All of these results suggest the stronger donor-accept interaction of Np(VI) complexes. However, U(V) species show significantly more ligand-to-metal charge transfer than that of Np(V) analogues, indicating stronger donoraccept interaction in U(V) complexes. Compared with the An(V) species, the ligands in the An(VI) species donate obviously more electrons to the metal cations, implying that the HDEHP ligands have higher extractability for An(VI) over An(V), which has been already confirmed by experiments.20 It is interesting to note that compared with the complexes with HDEHP dimers, the monomer HDEHP ligands in AnO2(HL)2(NO3)2 complexes exhibit obviously lower charge transfer to actinyl ions, revealing weaker donor-accept interaction between HDEHP ligand and actinyl ion. In addition, the ligand-to-metal charge transfer of the HDEHP ligands is even smaller than that of the NO3- ions, which seems to be inconsistent with the result of the WBIs. This may be attributed to the reason that the NO3- ions donate a portion of electrons to the P-OH groups forming intramolecular hydrogen bonds, which enhances the stability of AnO2(HL)2(NO3)2. From AnO2(HL2)(NO3) to AnO2(HL2)(NO3)(H2O)2 complexes, more electrons was donated from the ligands with the increasing of the water molecules.

Table 3. Wiberg Bond Indices (WBIs) of U-O Bonds for Complexes of UO2n+ (n=1, 2), and L (L=DEHP-) Obtained by the B3LYP Method.

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Species

U-OP=O

U-OP-OH

U-O(NO3−)a

U-O(H2O)a

UO2(HL2)(NO3)

0.647

0.548

0.449

__

UO2(HL2)(NO3)(H2O)

0.662

0.593

0.444

0.339

UO2(HL2)(NO3)(H2O)2

0.610

0.467

0.430

0.353

+

UO2(HL2)(H2O)3]

0.719

0.611

__

0.355

UO2(HL)2(NO3)2

0.489a

__

0.402

__

[UO2(HL2)(NO3)]-

0.433

0.387

0.321

__

[UO2(HL2)(NO3)(H2O)]-

0.419

0.379

0.342

0.262

[UO2(HL2)(NO3)(H2O)2]

0.385

0.329

0.292

0.263

UO2(HL2)(H2O)3

0.446

0.424

__

0.278

UO2(HL)2(NO3)

0.381

0.330

0.304

__

-

a

The average WBIs.

Table 4. Wiberg Bond Indices (WBIs) of Np-O Bonds for Complexes of NpO2n+ (n=1, 2), and L (L=DEHP-) Obtained by the B3LYP Method. Np-OP=O

Species

Np-OP-OH

Np-O(NO3−)a

Np-O(H2O)a

NpO2(HL2)(NO3)

0.686

0.566

0.460

__

NpO2(HL2)(NO3)(H2O)

0.676

0.591

0.444

0.332

NpO2(HL2)(NO3)(H2O)2

0.584

0.462

0.421

0.335

NpO2(HL2)(H2O)3]+

0.751

0.618

__

0.345

NpO2(HL)2(NO3)2

0.480a

__

0.394

__

[NpO2(HL2)(NO3)]-

0.397

0.349

0.298

__

-

[NpO2(HL2)(NO3)(H2O)]

0.405

0.342

0.312

0.225

[NpO2(HL2)(NO3)(H2O)2]-

0.322

0.309

0.315

0.255

NpO2(HL2)(H2O)3

0.385

0.366

__

0.247

NpO2(HL)2(NO3)

0.353

0.284

0.299

__

a

The average WBIs.

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Table 5. The Charge Transfer (∆Q) from Ligands to Metal for UO22+, NpO22+ Complexes with L (L=DEHP-).a Species

Charge Transfer (∆Q)

AnO2(HL2)(NO3) AnO2(HL2)(NO3)(H2O) AnO2(HL2)(NO3)(H2O)2 [AnO2(HL2)(H2O)3]+

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AnO2(HL)2(NO3)2

AnO22+

HL2→An

NO3_b→An

H2Ob→An

1.231/ 1.302 1.448/ 1.482 1.542/ 1.522 1.390/ 1.422 1.522/ 1.526

0.692/ 0.737 0.725/ 0.747 0.644/ 0.651 0.822/ 0.866 0.489/ 0.487c

0.539/ 0.565 0.534/ 0.548 0.520/ 0.518 __

__

0.517/ 0.520

0.189/ 0.187 0.189/ 0.176 0.190/ 0.185 __

a

···/··· represents the results of UO22+ and NpO22+ complexes, respectively. bThe average values of charge differences after chelating to metal cations. c The charge transfer from (HL)2 to metal.

Table 6. The Charge Transfer (∆Q) from Ligands to Metal for UO2+, NpO2+ Complexes with L= DEHP-.a Charge Transfer (∆Q) Species AnO2 [AnO2(HL2)(NO3)]-

[AnO2(HL2)(NO3)(H2O)]

[AnO2(HL2)(NO3)(H2O)2]-

AnO2(HL)2(NO3)

+

0.826/ 0.768 0.991/ 0.906 1.033/ 1.010 0.959/ 0.854 0.742/ 0.697

a

_

HL2→An

NO3 b→An

0.450/ 0.413 0.451/ 0.416 0.420/ 0.374 0.510/ 0.458 0.333/ 0.292c

0.376/ 0.355 0.406/ 0.369 0.349/ 0.372

UO2+

__ 0.409/ 0.407

H2Ob→An __ 0.135/ 0.121 0.132/ 0.132 0.150/ 0.132 __

NpO2+

···/··· represents the results of and complexes, respectively.b The average values of charge differences after chelating to metal cations. c The charge transfer from HL to metal.

Stability At the B3LYP/6-311G(d, p) level of theory, the stabilities of the studied actinide complexes have also been

evaluated by the metal-ligand complexing reactions with AnO2(H2O)52+ or AnO2(H2O)5+ as reactants in aqueous solution and organic solution. As shown in Figures 5 and 6 as well as Table S3, for the An(VI) complexes, we note that the reaction of AnO2(H2O)52+ + (HL)2 + 2NO3- → AnO2(HL)2(NO3)2 + 5H2O are more favorable than other reactions in both the aqueous phase and organic solution according to the changes of Gibbs free energy (∆G). Thus, AnO2(HL)2(NO3)2 are the most favorable complexes in the extraction process, which is in accordance with the experimental findings19. This may be attribute to the strong affinity of two bidentate nitrate ions to AnO22+ cations in AnO2(HL)2(NO3)2. Besides, the two intramolecular hydrogen bonds also play an important role in the stability of AnO2(HL)2(NO3)2. In addition, the AnO2(HL2)(NO3) complexes are less stable and prefer to binding a water molecule forming AnO2(HL2)(NO3)(H2O) complexes. However, when two water molecules are bound to the AnO2(HL2)(NO3) complexes, these reactions become unfavorable. This may be due to the steric effect of these ligands, and indicates that the most appropriate coordination number of the nitrate-hydrate complexes with HDEHP dimers is five for the An(VI) species in the equatorial plane. As for the formation of [AnO2(HL2)(H2O)3]+ complexes, the values of ∆Gorg are endergonic in organic solution. As expected, the ∆Gorg values are more negative than the ∆Gaq values in most An(VI) complexes, indicating that these extraction complexes are more soluble in n-dodecane. In addition, for each reaction, the Np(VI) complexes can be comparable to the U(VI) complexes in energetics, accompanying with very close ∆G values in both aqueous phase and organic solution. In the case of the An(V) species except for UO2(HL)2(NO3), all the reactions show positive ∆G values, which can be deduced that the HDEHP ligand shows poor extractability for the U(V) and Np(V). Given that the absolute values of solvation energy depend on the adopted theoretical methods to a large extent, the calculated trends in the Gibbs free energy of solvation here can provide us more useful information for studying the speciation of actinide extraction complexes with HDEHP.

Figure 5. Changes of the Gibbs free energy (kcal/mol) for Complexing reactions of AnO22+ (An=U, Np) and L (L=DEHP-) in aqueous solution (∆Gaq) and organic solution (∆Gorg) obtained by the B3LYP method.

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AnO2(HL2)(H2O)3

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ARTICLE DOI: 10.1039/C4DT03321C

Figure 6. Changes of the Gibbs free energy (kcal/mol) for Complexing reactions of AnO2+ (An=U, Np) and L (L=DEHP-) in aqueous solution (∆Gaq) and organic solution (∆Gorg) obtained by the B3LYP method.

1:2 (Metal:Ligand) Type Extraction Complexes Geometrical structures At the same level of theory, the 1:2 type An(VI) complexes predicted in experiments13, 19 and the An(V) corresponding species were also optimized. As depicted in Figure 7, two hydrogen-bonded dimers HL2 behave as n+ bidentate ligands coordinated to AnO2 (n=1, 2) at the equatorial plane, which is in accordance with the experimental observation.13 In addition, the calculated U-O axial and equatorial bond distances of UO2(HL2)2 are very close to the experimental data (Figure 7 and Table S4). Similar to the 1:1 type complexes, the An-OP=O bonds are shorter than the An-OPOH bonds in both 1:2 type uranyl and neptunyl complexes. Besides, the An-O bond lengths in the An(V) complexes are longer than those in the An(VI) complexes, and the Np(VI) complexes show slightly shorter An-O bond lengths compared to U(VI) complexes. Furthermore, the An=Oaxial bond lengths of the 1:2 type complexes are longer than those of AnO2(H2O)5n+ and the stretching frequencies of U=Oaxial are also red-shifted (Table S4).

This implies a lower degree of covalence of the An-OP bonds in the 1:2 type complexes, mainly because of steric effect of the two HDEHP dimers. As shown in Table 9, the An-OP WBIs of An(V) complexes are much smaller than the neutral An(VI) complexes, which is similar to the results of the NBO analysis for 1:1 type complexes. For the 1:2 type complexes, the charge transfer in the An(VI) complexes is more obvious compared to the An(V) complexes. Alternatively, more electrons have been donated from ligands to AnO22+ compared with AnO2+ cations, which is in line with the trend of WBIs values. Table 7. The Wiberg Bond Indices (WBIs) and the Absolute Values of Ligand-to-Metal Charge Transfer in AnO2n+ (An=U, Np; n=1, 2) Complexes with L (L=DEHP-). Wiberg Bond Indices

Charge Differences

An-OP=O

An-OP-OH

∆Q AnO2n+

∆QL→An

0.635, 0.633

0.548, 0.539

1.365

0.683

Species UO2(HL2)2 NpO2(HL2)2

0.649, 0.647

0.555, 0.544

1.416

0.708

[UO2(HL2)2]-

0.446, 0.442

0.400, 0.394

0.926

0.489

[NpO2(HL2)2]-

0.439, 0.429

0.393, 0.386

0.920

0.460

Stability To estimate the stability for 1:2 type complexes, the changes of the Gibbs free energy have been calculated according to the complexing reactions of AnO2(H2O)5n+ + 2(HL)2→ [AnO2(HL2)2]n-2 + 2H+ + 5H2O in aqueous solution and organic solution. As listed in Table 8, because of the strong hydrophobicity of the [AnO2(HL2)2]n-2 complexes, the formations of these species become unfavorable in aqueous solution. By comparison of the ∆G values for all the reactions in aqueous solution and n-dodecane, it has been found that these 1:2 type extraction complexes are more favorable in the organic phase. Besides, NpO2(HL2)2 appears to be slightly more stable than UO2(HL2)2. Moreover, the ∆G values for Figure 7. Optimized structures of the 1:2 type complexes for AnO (HL ) are more negative than those for the 1:1-type 2 2 2 uranyl and neptunyl ions. Green and white sticks represent C complexes without nitrite ion participation. Therefore, and H, while yellow, red, blue, pink, and purple spheres AnO2(HL2)2 and AnO2(HL)2(NO3)2 are probably the main represent P, O, N, U, and Np, respectively. The bond lengths (Å) species in the extraction process at higher and lower nitric acid of An-OP=O and An-OP=OH (An=U, Np) bonds are also given for concentrations, respectively, which is in accordance with the clarity. aThe value in the parenthesis is the experimental data of the experimental results.19 Similar to the 1:1 type An(V) complexes, average U-O equatorial bond distances.13 [AnO2(HL2)2]- are predicted to be unfavorable because of the much positive ∆G values. Nevertheless, [UO2(HL2)2]- shows NBO analysis On the basis of NBO analysis, the An-OP WBIs lower ∆G values than [NpO (HL ) ]- in both aqueous and n2 2 2 range from 0.386 to 0.649 for the 1:2 type complexes (Table 7), which are much smaller than those for most 1:1 type complexes.

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Journal Name DOI: 10.1039/C4DT03321C

Table 8. Changes of the Gibbs Free Energy (kcal/mol) for Complexing reactions of AnO2n+ (n=1, 2) and L (L=DEHP-). For H+, the experimental value of -263.982 kcal/mol41 was adopted. Reaction

∆Gaq

∆Gorg

UO2(H2O)5 + 2(HL)2 → UO2(HL2)2 + 2H + 5H2O

13.7

-13.2

NpO2(H2O)52+ + 2(HL)2 → NpO2(HL2)2 + 2H+ + 5H2O

11.7

-16.1

UO2(H2O)5 + 2(HL)2 → [UO2(HL2)2] + 2H + 5H2O

41.9

28.8

NpO2(H2O)5+ + 2(HL)2 → [NpO2(HL2)2]- + 2H+ + 5H2O

68.6

58.3

2+

+

+

-

+

Conclusions In this work, we have investigated the geometries, bonding nature and thermodynamic stabilities of the HDEHP dimer and its extraction complexes with AnO2n+ (n=1, 2; An=U, Np) cations in detail by using the scalar relativistic calculations in combination with the density functional theory. For the HDEHP dimer, the P=O and P-OH form an eight-membered ring, and the oxygen atoms become the active sites for coordination with metal cations. The phosphoryl oxygen atom is the preferred site of electrophilic attack by metal ions compared to the oxygen atom in the P-OH group. The NBO and QTAIM analysis for all the uranyl and neptunyl complexes of HDEHP indicates that the HDEHP ligands in the An(VI) species donate more electrons to metal cations compared to those in the An(V) species, i.e. the An(VI)-O bonds are much stronger than the An(V)-O bonds. According to binding energy calculations, we found that the HDEHP ligand possesses relatively lower extraction ability toward pentavalent uranyl and neptunyl cations in both aqueous and organic phases compared to the hexavalent cations, which is in agreement with the trend of donor-acceptor interaction in the extraction complexes. The probable extraction species for hexavalent uranyl and neptunyl ions obtained here are the neutral 1:1 and 1:2-type complexes, i.e. AnO2(HL)2(NO3)2 and AnO2(HL2)2, which may be the most stable species at high and low acidities, respectively. In addition, the 1:2-type complexes of U(VI) seem to be slightly less stable than the Np(VI) complexes. Overall, the present study may be useful for understanding the structural, bonding and thermodynamic properties of actinide extraction complexes with HDEHP ligand.

a

Group of Nuclear Energy Chemistry, Key Laboratory of Nuclear Radiation and Nuclear Energy Technology and Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. E-mail: [email protected] b School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, Suzhou 215123, China. c School of Chemistry and Chemical Engineering, University of South China, Hengyang 421001, China. ‡ These two authors contributed equally to this work. † Electronic Supplementary Information (ESI) available: Figure S1: HOMO (a), LUMO (b) of HDEHP dimer, and MEP mapped on the isodensity surface (c) for the dimer; Figure S2: HOMO (a), LUMO (b) of HDEHP trimer, and MEP mapped on the isodensity surface (c) for the trimer; Figure S3-S4: Optimized structures of UO2+ and NpO2+ complexes by the B3LYP method; Figure S5: The isomers of the AnO2(HL)2(NO3)2 (An=U, Np) complexes; Table S1-S2: QTAIM analysis of the bonds between ligands and metal for the UO22+, NpO22+, UO2+ and NpO2+ complexes; Table S3: Changes of the Gibbs free energy (kcal/mol) for the complexes of AnO2n+ (n=1, 2) and L (L=DEHP-) in aqoeous solution and organic Aolution obtained by the B3LYP method; Table S4: The bond length and the symmetrical and antisymmetrical stretching frequency (υs and υas, cm-1) of An=Oaxial for the 2:1-type complexes; Complete Gaussian 09 reference (Reference 21). See DOI: 10.1039/b000000x/

Notes and references

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Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21201166, 21101157, 11105162, 21261140335), the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of Natural Science Foundation of China (Grant Nos. 91326202, 91026007, 91126006), the “Strategic Priority Research Program” of the Chinese Academy of Sciences (Grant No. XDA030104). The results described in this work were obtained on the ScGrid of Supercomputing Center, Computer Network Information Center of Chinese Academy of Sciences. We also thank the financial support from the State Key Laboratory of NBC Protection for Civilian (No.SKLNB2014-12).

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dodecane, which is consistent with the result of 1:1-type An(V) complexes.

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The AnO2n+ (An=U, Np; n=1, 2) extraction complexes with HDEHP (di-(2-ethylhexyl) phosphoric acid) in nitric acid solutions. 39x22mm (300 x 300 DPI)

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DOI: 10.1039/C4DT03321C