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Nov 29, 2017 - Complexation between UO2. 2+ and Amine-Functionalized. Diacetamide Ligands in Aqueous Solution. Phuong V. Dau,. †. Zhicheng Zhang,. â...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Thermodynamic, Structural, and Computational Investigation on the Complexation between UO22+ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution Phuong V. Dau,† Zhicheng Zhang,† Yang Gao,†,‡ Bernard F. Parker,†,§ Phuong D. Dau,† John K. Gibson,† John Arnold,†,§ Marilena Tolazzi,∥ Andrea Melchior,*,∥ and Linfeng Rao*,† †

Chemical Science Division, Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720, United States College of Nuclear Science and Technology, Harbin Engineering University, Harbin 150001, China § Department of Chemistry, University of CaliforniaBerkeley, Berkeley, California 94720, United States ∥ Laboratori di Chimica, Università di Udine, via delle Scienze 99, 33100 Udine, Italy ‡

S Supporting Information *

ABSTRACT: The stability constants (log β), enthalpies of complexation (ΔH), and entropies of complexation (ΔS) for the complexes of uranium(VI) with a series of aminefunctionalized diaetamide ligands, 2,2′-benzylazanediylbis(N,N′-dimethylacetamide) (BnABDMA), 2,2′-azanediylbis( N , N ′- d i m e t h y l a c e t a m id e ) ( A B D M A ) , an d 2 , 2 ′methylazanediylbis(N,N′-dimethylacetamide) (MABDMA), in aqueous solution were determined by potentiometry and calorimetry. Electronspray ionization mass spectrometry was used to verify the presence of uranium(VI) complexes in solution. The thermodynamic data indicate that the binding strengths of the three ligands with UO22+ follow the order BnABDMA < ABDMA < MABDMA, parallel to the order of the protonation constants as well as the order of the stability of the Nd3+ complexes, suggesting that the complexation of UO22+ with the ligands consist predominantly of electrostatic interactions. Denisty functional theory calculations were conducted to reveal the structures, electronic charge distribution, and energetics of the uranium(VI) complexes, providing insight into the thermodynamic trends of the complexation. Extended X-ray absorption fine structure spectroscopy was used to identify the structures of the uranium(VI) complexes in aqueous solution. N,N′-dihexyldiglycolamide.1,5−11 Distribution coefficients in solvent extraction were determined for actinides and other metal ions. These data have demonstrated the effectiveness of oxydiamide ligands as extractants but do not provide insight into the thermodynamic and structural factors that govern the extraction efficiency. No relationships between the ligand structure and property have been developed. As a result, it is difficult to understand the extraction mechanism and design new ligands to improve the efficiency. In recent years, we have reported systematic thermodynamic studies on the complexation of lanthanide and actinide metal ions (Nd3+, UO22+, and NpO2+) with a series of oxydicarboxylate and oxydiamide ligands, including 2,2′-oxydiacetic acid (ODA), N,N-dimethyl3-oxaglutaramic acid, and N,N,N′,N′-tetramethyldiglycolamide (TMDGA).12−15 Equilibrium constants, enthalpies and entropies of complexation, and structures of the complexes in solid crystal and aqueous solution were obtained. The results help to establish a structure−property relationship that

1. INTRODUCTION In recent decades, alkyl-substituted amide-functionalized ligands have been investigated extensively because of their potential to be used in the separation of radioactive elements in spent nuclear fuel reprocessing.1 These ligands contain only carbon, hydrogen, nitrogen, and oxygen, so that they are completely combustible. Therefore, the volume of solid secondary nuclear wastes from spent nuclear fuel reprocessing can be greatly reduced. Besides, the hydrolytic and radiolytic degradation of these types of ligands is much less adverse to the separation process than that of traditional organophosphorus extractants such as tributylphosphate.2−4 The products of the hydrolytic and radiolytic degradation of tributylphosphate, dibutylphosphate, and monobutylphosphate make the stripping of actinides more difficult and facilitate the formation of a “third phase”, which could lead to safety problems such as criticality and explosion. Several alkyl-substituted oxydiamide ligands (ether oxygenlinked amides) have been studied as extractants in solvent extraction for the separation of actinides, including tetraoctyldiglycolamide, tetraisobutyldiglycolamide, and N,N′-dimethyl© XXXX American Chemical Society

Received: November 29, 2017

A

DOI: 10.1021/acs.inorgchem.7b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Structures of TMDGA, MABDMA, ABDMA, and BnABDMA. UO2(NO3)2·H2O(s) in a dilute nitric acid solution. Caution! Uranyl salts are chemotoxic and moderately radioactive (α-radiation). Therefore, precautions must be taken to avoid exposure when handling uranyl samples. The concentrations of UO22+ and free acid in the stock solution were determined, respectively, by complexometry titrations using ethylenediaminetetraacetic acid17 and Gran’s titration.18 The ionic strengths of all working solutions used in the potentiometry and calorimetry were adjusted to 1.0 M at 25 °C by adding appropriate amounts of NaNO3 as the background electrolyte. NaNO3 (1.0 M), instead of NaClO4, which is commonly used in thermodynamic studies, was used as the ionic medium in this study because the solubilities of uranium(VI) complexes with BnABDMA, ABDMA, and MABDMA in perchlorate media are low. 2.2. Potentiometry. Potentiometric titrations were performed to determine the equilibrium constants of complexation between uranium(VI) and the three ligands. The titrations were carried out using an automatic titration system consisting of a glass titration cell, a MetroOhm pH meter (model 713) equipped with a combination pH electrode, a MetroOhm dosimat (model 765), and a computer. The temperature of the titration cell was kept at 25.0 ± 0.1 °C using an external circulating water bath. In the potentiometric titrations, the concentration of H+ ions is determined from the measured electromotive force (EMF). The EMF in an acidic region can be expressed by eq 1.

provides guidance in designing and developing more efficient ligands for actinide separations. By substituting the ether O atom linkage in oxydiamide ligands such as TMDGA with the amine N atom linkage, we have synthesized a series of N-functionalized diamide ligands,16 including 2,2′-benzylazanediylbis(N,N′-dimethylacetamide) (BnABDMA), 2,2′-azanediylbis(N,N′-dimethylacetamide) (ABDMA), and 2,2′-methylazanediylbis(N,N′-dimethylacetamide) (MABDMA), as shown in Figure 1. Similar to TMDGA, which is in tridentate coordination with Nd3+, UO22+, and NpO2+, the N-functionalized diamide ligands have been shown to be in tridentate coordination with Nd3+ and the binding strengths of BnABDMA, ABDMA, and MABDMA with Nd3+ are significantly higher than that of TMDGA. Also, importantly, the amine linkage allows variations of substitutional groups on the N atom so that the electronegativity as well as the binding strength of the ligands can be fine-tuned. In the present work, we have conducted thermodynamic, structural, and computational studies on the complexation of uranium(VI) with BnABDMA, ABDMA, and MABDMA. The stability constants, enthalpies and entropies of complexation, and stoichiometries and structures of the uranium(VI) complexes in aqueous solution were determined. Computational data, including optimized structures and electronic charge distribution, provide insight into the energetics of complexation and the trends in the binding strengths of the amine-functionalized amide ligands.

E = E 0 + RT /F ln [H+] + γH[H+]

(1)

where R is the gas constant, F is the Faraday constant, T is the absolute temperature, and γH[H+] is the electrode junction potential for the H+ ion, which is assumed to be proportional to the concentration of H+ ions. Prior to each titration, an acid/base titration with standard HNO3 and NaOH was performed to obtain the electrode parameters of E0 and γH. These parameters allowed calculation of the H+ ion concentrations from the measured EMF in the subsequent titration. In a typical titration, a solution (22 mL or more) containing appropriate amounts of uranium(VI) and the ligand was titrated with a standard solution of HNO3 or NaOH. Multiple titrations were conducted with solutions of different concentrations of uranium(VI) (CM), the ligand (CL), and the total proton (CH). The equilibrium constants of uranium(VI) complexation were calculated by fitting the potentiometric titration data with the Hyperquad2008 program.19 In the calculation, the equilibrium constants for protonation of the ligands, formation of UO2(NO3)+, and hydrolysis of uranium(VI) [UO2(OH) +, (UO2) 2(OH) 22+, and (UO2)3 (OH) 5+] from the literature16,20,21 were included. [The equilibrium constants (logK) for UO2(NO3)+, UO2(OH)+, (UO2)2(OH)22+, and (UO2)3(OH)5+ in 1.0 M NaNO3 are −0.62, −5.26, −5.96, and −16.54, respectively.] 2.3. Calorimetry. The enthalpies of complexation between uranium(VI) and the ligands were determined by calorimetric titrations using an isothermal microcalorimeter (ITC 4200, Calorimetry Science Corp.). Detailed information about this microcalorimeter and the calibration process were reported previously.22 Multiple titrations using different concentrations of uranium(VI), the ligand, and H+ in the initial solution were performed. From the reaction heat, the enthalpy of complexation was calculated by using the HypDeltaH program.23 2.4. Electrospray Ionization Mass Spectrometry (ESI-MS). ESI-MS experiments were performed using an Agilent 6340 quadrupole ion-trap mass spectrophotometer. An Agilent 6340 quadrupole ion-trap mass spectrometer with a micro ESI source was

2. EXPERIMENTAL SECTION 2.1. Chemicals. BnABDMA, ABDMA, and MABDMA were prepared based on Scheme 1. Details of the preparation and characterization are provided in the literature.16 The starting materials and solvents used in the preparation were purchased from commercial suppliers (Sigma-Aldrich, Alfa Aesar, EMD, and TCI) and used without further purification. Milli-Q water was used in the preparation of all aqueous solutions. The stock solution of uranium(VI) was prepared by dissolving

Scheme 1. Synthesis of BnABDMA, ABDMA, and MABDMA.13

B

DOI: 10.1021/acs.inorgchem.7b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Representative potentiometric titrations of uranium(VI) complexation with BnABDMA (left), ABDMA (middle), and MABDMA (right). Symbols: (magenta ○) observed pCH (=−log [H+]); (magenta ) calculated pCH; (black ) free UO22+; (yellow ) [UO2(NO3)]+; (green ) [UO2L]2+; (blue ) [UO2L2]2+; (red ) [(UO2)3(OH)5]+. Minor hydroxide species of uranium(VI) are not shown. L = BnABDMA, ABDMA, and̀ MABDMA. The right y axis is for pCH, and the left y axis is for the percentage of uranium(VI) species.

Table 1. Thermodynamic Data for the Complexation of Uranium(VI) with Amine-Functionalized Diacetamide Ligands (BnABDMA, ABDMA, and MABDMA), in Comparison with TMDGA (I = 1.0 M; t = 25 °C) ligand BnABDMA

ABDMA

MABDMA

TMDGA a

reaction +

+

H + L = HL UO22+ + L = UO2L2+ UO22+ + 2L = UO2L22+ H+ + L = HL+ UO22+ + L = UO2L2+ UO22+ + 2L = UO2L22+ H+ + L = HL+ UO22+ + L = UO2L2+ UO22+ + 2L = UO2L22+ UO22+ + L = UO2L2+ UO22+ + 2L = UO2L22+

methoda pot, cal pot, cal pot, cal pot, cal pot, cal pot, cal pot, cal pot, cal pot, cal sp, cal sp, cal

log β

ΔH (kJ mol−1)

ΔS (J K−1 mol−1)

ref

± ± ± ± ± ± ± ± ± ± ±

−(31.2 ± 0.3) −(6.2 ± 0.3) −(15.3 ± 0.9) −(37.2 ± 2.1) −(11.2 ± 0.2) −(29.5 ± 0.1) −(33.5 ± 0.6) −(7.65 ± 0.20) −(19.0 ± 0.9) 7.47 ± 0.27 19.6 ± 0.5

17 ± 1 62 ± 3 96 ± 5 11 ± 6 59 ± 3 75 ± 3 34 ± 2 76 ± 2 121 ± 3 58 ± 1 122 ± 1

13 this work

6.36 4.33 7.67 7.12 5.03 9.08 7.64 5.30 9.66 1.71 2.94

0.09 0.03 0.06 0.09 0.03 0.03 0.09 0.03 0.03 0.03 0.01

13 this work 13 this work 12

pot = potentiometry; cal = calorimetry; sp = spectrophotometry. solutions are computed for a 1 M standard state.34 At 298.15 K, this correction increases Gwater for each reactant and product by 1.89 kcal mol−1. For water, an additional correction (2.38 kcal mol−1) has to be applied, to go from the 1 M standard state to the “pure liquid” (55.34 M) state.34 All calculations were carried out using the Gaussian16 program.35 2.6. Extended X-ray Absorption Fine Structure (EXAFS) Study. Six uranium(VI)/ligand solution samples, two for each of the three ligands, were prepared for EXAFS study at the Stanford Synchrotron Radiation Laboratory. For each ligand, the conditions of the two samples were selected so that the ML or ML2 complex was the dominant uranium(VI) species in solution. The solution samples (each 2.0 mL) were contained in plastic vials and sealed in plastic bags. The samples were mounted on a sample positioner with Scotch tape and measured on Beamline 11-2. U LIII-edge data were collected up to kmax ∼ 15.0 Å−1 in both transmission and fluorescence modes. Four repetitive scans were taken for each sample. Energy calibration was conducted by assigning the first inflection point of the K edge of yttrium (Y foil as the reference) at 17038 eV. Data reduction, including preedge background subtraction followed by spline fitting and normalization, was performed with the program Athena.36 The EXAFS data were extracted above the threshold energy, E0, defined as 17166 eV. The fit of the EXAFS data utilized the theoretical phases and amplitudes calculated by the program FEFF737 with the model crystal structure of a 1:2 uranium(VI) complex, UO2L2(ClO4)2, where L denotes 2,2′-(trifluoroazanediyl)bis(N,N′dimethylacetamide) (CCDC 1584249). For each sample, selected single-scattering (SS) and multiple-scattering (MS) paths from the FEFF calculation were used in the fit based on the proposed coordination mode. In all of the fits, an amplitude factor (S02) and a threshold energy shift (ΔE0) were considered to be global parameters. Hanning windows with a k range (3.0−14.0 Å−1) and a Fourier transform with an R range (0.95−3.5 Å) were used. The fit factor (r) was provided as an indication of the fit quality.

used to identify uranium(VI)/L complexes. Water/ethanol ( MABDMA > BnABDMA in both isomers, indicating that a

Figure 4. ESI-MS characterization of uranium(VI) complexes with BnABDMA (top), ABDMA (middle), and MABDMA (bottom). The singly charged [UO2L(OH)]+ and doubly charged [UO2L2]2+ are labeled in green and blue, respectively. Inset figures show a peak-topeak separation (Δpp) of m/z 0.5 for [UO2L2]2+.

patterns of intensity of the doubly charged uranium(VI) complexes are consistent with the natural isotopic abundances of UO22+ and corresponding ligands. 3.4. Computational Results. 3.4.1. Geometry Optimizations. The minimum-energy structures of the diacetamide ligands are shown in Figure S2. The U−OH2O bond distance (average 2.418 Å) calculated for [UO2(H2O)5]2+ is in good agreement with previous experimental39,40 and computational41 results. The DFT-calculated structures of both 1:1 and 1:2 E

DOI: 10.1021/acs.inorgchem.7b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. DFT-calculated structures of the 1:1 and 1:2 UO22+ complexes with diacetamide ligands in PCM water (upper row, isomer a; lower row, isomer b). The equatorial view of the same structures is shown in Figure S4.

Table 2. Selected Bond Distances (Å) in the DFT Minimum-Energy Structures of the Ligands, [UO2(L)(H2O)3]2+ and [UO2(L)2]2+ Complexes for the Conformations a and b in PCM Water (Figure 5)a U−Oamideb

U−N 2+

[UO2(ABDMA)(H2O)3] [UO2(MABDMA) (H2O)3]2+ [UO2(BnABDMA) (H2O)3]2+ [UO2(ABDMA)2]2+ [UO2(MABDMA)2]2+ [UO2(BnABDMA)2]2+

U−OH2O

a

b

a

b

a

bc

2.730 2.846 2.878 2.743 2.873 2.991

2.628 2.684 2.726 2.694 2.787 2.828

2.438 2.424 2.422 2.452 2.424 2.406

2.392 2.386 2.384 2.476 2.465 2.459

2.483 2.554 2.554

2.652 2.649 2.655

a

N = amine N atom. The U−Oamide and U−OH2O bond distance are averaged. For [UO2(H2O)5]2+, U−OH2O = 2.470 Å. bU−Oamide = 2.418 Å in [UO2(TMDGA)2]2+.15 cmin = 2.54 Å; max = 2.72 Å.

3.4.3. Energetics of Complexation Reactions. The energetics of complexation are calculated for the two stepwise reactions (4) and (5) and shown in Table 4.

larger ligand−metal charge transfer occurs in ABDMA than MABDMA and BnABDMA. As to the charges on the Oamide atoms in the complexes, no particular trend of Δq is found for the three ligands (Table 3) when a single isomer is considered. Also, it is interesting to note that, for the 1:1 complexes, much larger Δq is found for isomer a than isomer b. In contrast, much larger Δq is found for isomer b than isomer a for the 1:2 complexes. This result, along with the discussions on the U−Oamide distances in section 3.4.1, shows that the N substituents do not cause notable differences in the coordination of the amide O atoms of a given isomer.

[UO2 (H 2O)5 ]2 + + L → [UO2 L(H 2O)3 ]2 + + 2H 2O

(4)

[UO2 L(H 2O)3 ]2 + + L → [UO2 L 2]2 + + 3H 2O

(5)

For reaction (4), the ΔGgas values follow the order BnABDMA < MABDMA < ABDMA, as expected on the basis of the gas-phase basicity of amines.42 The same order is found also for formation of the 1:2 complexes in the gas phase. On the contrary, the values of ΔGwater for reaction (4) follow a F

DOI: 10.1021/acs.inorgchem.7b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 3. ESP Charges q (e−) Calculated in PCM Water for the Namine, Oamide, and U atoms in the Free Ligands and the Uranium(VI) Complexes (the Values for Isomer b Are Reported in Parentheses)a ΔqN

qN ABDMA MABDMA BnABDMA [UO2(ABDMA)(H2O)3]2+ [UO2(MABDMA) (H2O)3]2+ [UO2(BnABDMA) (H2O)3]2+ [UO2(ABDMA)2]2+ [UO2(MABDMA)2]2+ [UO2(BnABDMA)2]2+ a

−0.997 −0.479 −0.207 0.057 (−0.135) 0.200 (0.129) 0.221 (0.344) 0.217 (0.182) 0.476 (0.408) 0.562 (0.503)

1.054 0.679 0.428 1.214 0.955 0.769

(0.862) (0.608) (0.551) (1.179) (0.887) (0.710)

ΔqOamide

qOamide −0.655 −0.652 −0.665 −0.534 −0.517 −0.530 −0.466 −0.440 −0.435

(−0.638) (−0.613) (−0.633) (−0.416) (−0.412) (−0.470)

0.121 0.135 0.135 0.068 0.077 0.095

qOwater

−0.820 (−0.838) −0.825 (−0.850) −0.821 (−0.849)

(0.017) (0.039) (0.032) (0.222) (0.201) (0.163)

qU

1.874 1.872 1.964 1.183 1.106 1.203

(2.451) (2.484) (2.434) (0.886) (0.969) (1.223)

Δq = q(bound ligand) − q(free ligand). The charges on the same atom type are averaged.

Table 4. Theoretical Complex Formation Free Energies (kcal mol−1) for Reactions (4) and (5)a ΔGgas

ΔGwater

Δ log K1,t,calc

reaction

ligand

a

b

a

b

a

b

Δ log K1,t,exp

(4)

ABDMA MABDMA BnABDMA ABDMA MABDMA BnABDMA

−63.1 −68.6 −71.4 −46.5 −51.6 −51.8

−64.6 −83.7 −86.0 −46.1 −52.6 −52.2

−19.0 −21.0 −19.3 −20.1 −20.0 −19.5

−21.2 −23.7 −19.3 −18.2 −21.8 −17.9

−0.2 +1.2 0 +0.4 +0.4 0

+1.4 +3.2 0 +0.2 +2.9 0

+0.7 +1.0 0 +1.4 +2.0 0

(5)

The energies were calculated for the minimum-energy structures in the gas phase (ΔGgas) and PCM water (ΔGwater) for both isomers. The ΔGwater data were used to calculate Δ log K1,t,calc = log K1,t,calc(L) − log K1,t,calc(BnBDMA). a

different order for the two isomers: MABDMA < BnABDMA ∼ ABDMA for isomer a and MABDMA < ABDMA < BnABDMA for isomer b (Table 4). For the stepwise formation of the 1:2 complexes (reaction 5), the ΔGwater values show the trends MABDMA ∼ ABDMA < BnABDMA and MABDMA < ABDMA < BnABDMA for isomers a and b, respectively. These results show that isomer b of the complexes provide a better agreement than isomer a with the trends of the experimentally obtained stabilities. However, because the relative stabilities of the two isomers are similar (Table S3), it can be concluded that the overall agreement with the experimental trends is fairly good. The fact that the obtained stability trend is not exactly the same as that for ΔqN suggests that the different N substitutions have an effect, which is not limited to charge transfer but also related to solvation of the complex and the steric strain induced on the ligand, which globally contributes to the calculated reaction free energy. From the difference in the calculated ΔGwater values, the “relative” stabilities with respect to that of the BnBDMA complex, Δ logK1,t,calc = log K1,t,calc(L) − log K1,t,calc(BnBDMA), were calculated (Table 4). For comparison, also the experimentally observed “relative” stabilities, Δ log K1,t,exp = log K1,t,exp(L) − log K1,t,exp(BnBDMA), are reported (Table 1). It results that the trends of the relative stabilities are qualitatively well reproduced for all complexes with the exception of ABDMA in the formation of isomer a. The mean absolute errors on the difference between the calculated and experimental Δ log K1,i (=|Δ log K1,t,calc − Δ log K1,t,exp|) are 0.9 and 1.2 log units for isomers a and b, respectively. The magnitude of the error is in line with the capability of the DFT/ B3LYP method to predict stability constants because previous data indicate that it was possible to obtain relative stabilities with an absolute error of 1 logarithm unit or less.34 Also, we checked the effect of using different basis sets on the results by repeating the calculations with a larger SDD/6-311++G-

(3df,2p) basis set (for isomer a only), and similar results for ΔGwater and Δ log K1,t,calc were obtained (Table S4). We believe that the computational protocol adopted in this study can be employed for a preliminary prediction of the complexing ability of other diacetamide ligands (i.e., those with different N substituents) for selective uranyl complexation. 3.5. EXAFS Results. The k3-weighted EXFAS spectra and the Fourier transform (FT) magnitude of the six solution samples are shown in Figure 6. The fitting results are shown in Table 5. Because of the similarity of the EXAFS signals from the U−O and U−N scattering paths, the assignments of the coordinating O and N atoms were obtained by integrating the experimental data of ESI-MS and EXAFS and verified by theoretical calculations. Table 5 shows the experimentally observed structure parameters for the uranium species in six solution samples. It should be pointed out that the coordination numbers (N) are the average numbers for all of the uranium species. Because the solution samples contain different percentages of ML and ML2 complexes, the actual coordination numbers for ML and ML2 complexes were calculated by taking into consideration the speciation of uranium in solutions using the equilibrium constants obtained by potentiometry. Samples II, IV, and VI contain 90% ML2 complexes and minor amounts of ML. As a result, the experimentally obtained NU−Oeq and NU−Neq numbers (4.1, 4.2, and 4.1 for U−O and 1.9 for U−N in Table 5) can be considered to be representative to the coordination numbers in the ML2 complexes. Therefore, we can state that, based on the data in Table 5, for all three ligands (samples II, IV, and VI), there are four equatorial O atoms at a distance (RU−N) of around 2.4 Å and two N atoms at a distance (RU−N) of around 2.85−2.93 Å in the ML2 complexes (samples II, IV, and VI). On the other hand, samples I, III, and V do not have predominantly ML. Therefore, the experimentally obtained G

DOI: 10.1021/acs.inorgchem.7b02971 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

The assignments of one N atom at a distance (RU−N) of around 2.86−2.89 Å in the ML complex and two N atoms at a distance (RU−N) of around 2.85−2.93 Å in the ML2 complex are consistent with the ESI-MS results and in agreement with the DFT structural data (Table 2), confirming that the amide N atom participates in the complexation with uranium(VI) and corroborates well the thermodynamic equilibrium constants in Table 1. The five equatorial O atoms in the ML complex observed by the EXAFS data indicate that, besides the two O atoms from the ligand, there are three water molecules in the equatorial plane of UO22+, in agreement with the computational data (Figure 5). The four equatorial O atoms in the ML2 complex observed by the EXAFS data could be assigned to the two amide O atoms from each of the two ligands, consistent with the trident chelate complexes shown by the computational studies (the lower row of Figure 5). The experimental U−Oeq bond distances, which lie in the 2.39−2.42 Å range (Table 5), are compatible with the calculated ones (Table 2). The calculated U−Namide bond distances for isomer a are in good agreement with the experimental U−Neq data for MABDMA and BnABDMA, while they are slightly underestimated for the ABDMA complexes. In conclusion, the models reported in Figure 5 can be considered representative of the species in solution because they are globally consistent with both the thermodynamic and EXAFS data. It should be noted that the Debye−Waller factors (σ2 in Table 5) are about 0.0080 for U−O but 0.004−0.005 for U−N. The higher Debye−Waller factors for U−O are probably due to the fact that not all of the O atoms in the equatorial O shell (five O atoms for ML and four O atoms for ML2) are at the same distance to the U atom and small differences exist between the O atom from L and the O atom from water (in ML) and between the four O atoms from the two ligands. In fact, the crystal structure of the reference compound (CCDC 1584249) shows that the four O atoms from the two ligands are at distances of 2.33−2.38 Å. To avoid overinterpreting the

Figure 6. EXAFS spectra (left) and FT magnitude (right).

NU−Oeq numbers (Table 5) need to be corrected for speciation. After correction, the NU−Oeq numbers in the ML complexes are 5.3, 5.4, and 5.4. Taking into consideration that the relative uncertainty on N is ±20%, which is equivalent to the absolute uncertainty of ±1 for N ∼ 5, we can still state that, for all three ligands, there are about five equatorial O atoms at a distance (RU−O) of around 2.41−2.42 Å and one N atom at a distance (RU−N) of around 2.86−2.89 Å in the ML complex (samples I, III, and V).

Table 5. EXAFS Fitting Results of Uranium(IV) Species in Solutiona Solutionb

Shell

Nc

I (MABDMA) 75% UO2L2+ 25% UO2L22+ II (MABDMA) 10% UO2L2+ 90% UO2L22+ III (ABDMA) 75% UO2L2+ 25% UO2L22+ IV (ABDMA) 10% UO2L2+ 90% UO2L22+ V (BnABDMA) 75% UO2L2+ 25% UO2L22+ VI (BnABDMA) 10% UO2L2+ 90% UO2L22+

U−Oax U−Oeq U−Neq U−Oax U−Oeq U−Neq U−Oax U−Oeq U−Neq U−Oax U−Oeq U−Neq U−Oax U−Oeq U−Neq U−Oax U−Oeq U−Neq

2.0 5.1 ± 0.6 1.0 ± 0.3 2.0 4.1 ± 0.6 1.9 ± 0.4 2.0 5.3 ± 0.7 1.1 ± 0.3 2.0 4.2 ± 0.5 1.9 ± 0.3 2.0 5.2 ± 0.5 1.2 ± 0.3 2.0 4.1 ± 0.5 1.9 ± 0.3

R (Å)

σ2

Note

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.0026 0.0080 0.0027 0.0025 0.0055 0.0047 0.0019 0.0086 0.0026 0.0019 0.0085 0.0041 0.0019 0.0075 0.0020 0.0018 0.0085 0.0051

S02 = 1.0 ΔE0 = 7.2 eV r = 0.0020 S02 = 1.0 ΔE0 = 6.4 eV r = 0.0060 S02 = 0.93 ΔE0 = 8.3 eV r = 0.0034 S02 = 0.98 ΔE0 = 9.3 eV r = 0.0060 S02 = 0.83 ΔE0 = 6.3 eV r = 0.0044 S02 = 0.92 ΔE0 = 8.5 eV r = 0.043

1.78 2.41 2.86 1.78 2.39 2.85 1.78 2.42 2.87 1.78 2.40 2.87 1.77 2.42 2.89 1.78 2.41 2.93

0.01 0.01 0.02 0.01 0.01 0.03 0.01 0.01 0.03 0.01 0.01 0.03 0.01 0.01 0.04 0.01 0.02 0.05

Subscripts “ax” and “eq” denote respectively axial and equatorial coordination of the linear UO22+ cation. bThe uranium(IV) speciation (relevant to the total [U]) was calculated by using the equilibrium constants in Table 1. cFor U−Oax, N = 2.0 is held constant for all six solutions. For U−Oeq and U−Neq, the uncertainty of the fitted values of N is estimated to be ±20−25% at the 95% confidence level. a

H

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Berkeley National Laboratory (LBNL). Y.G. acknowledges financial support from the China Scholarship Council for her visit to LBNL. The EXAFS experiments were carried out at Stanford Synchrotron Radiation Laboratory, a user facility operated for the U.S. DOE by Stanford University.

EXAFS data, we did not make attempts to differentiate these equatorial O atoms.

4. CONCLUSION Complexation of UO22+ with three structurally related aminefunctionalized diamide ligandsBnABDMA, ABDMA, and MABDMAin aqueous solution was studied by experimental measurements and theoretical computation. The experimentally determined binding strength of the ligands follows the order MABDMA > ABDMA > BnABDMA, which is corroborated by the computational results. The aminefunctionalized diamides form stronger complexes with UO22+ than analogous diamides linked by ether O atom (such as TMDGA) because of the much more favorable enthalpy of complexation for the amine-functionalized diamides. Besides, the replacement of the ether O atom linkage with the amine N atom linkage allows one to adjust the binding strength of the ligands by tuning the ligand basicity with different substitution groups. It is expected that the amine-functionalized diamide ligands could be of significant interest in the development of complexants for the separation of actinides and lanthanides.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02971. Experimental conditions of potentiometric and calorimetric titrations, minimum structures of the [UO2(L)(H2O)3]2+ complexes with the planar ligand, equatorial view of the structures in Figure 5, and theoretical formation free energies calculated with the larger basis set (PDF) Accession Codes

CCDC 1584249 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Tel: 1-510-486-5427. Fax: 1-510-4865596. ORCID

John K. Gibson: 0000-0003-2107-5418 John Arnold: 0000-0001-9671-227X Linfeng Rao: 0000-0002-1873-0066 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, under U.S. Department of Energy (DOE) Contract DE-AC02-05CH11231 at Lawrence I

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