Thermodynamic, Structural, and Computational Investigation on the Complexation between UO22+ and Amine-Functionalized Diacetamide Ligands in Aqueous Solution

Article abstract of DOI:10.1021/acs.inorgchem.7b02971

The stability constants (log β), enthalpies of complexation (βH), and entropies of complexation (βS) for the complexes of uranium(VI) with a series of amine-functionalized diaetamide ligands, 2,2′-benzylazanediylbis(N,N′-dimethylacetamide) (BnABDMA), 2,2′

Full text of DOI:10.1021/acs.inorgchem.7b02971

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Thermodynamic, Structural, and Computational Investigation on the
2+
Complexation between UO2 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, Universita
̀
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 amine-
functionalized diaetamide ligands, 2,2′-benzylazanediylbis-
(N,N′-dimethylacetamide) (BnABDMA), 2,2′-azanediylbis-
(N,N′-dimethylacetamide) (ABDMA), and 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
2+
strengths of the three ligands with UO2 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
1. INTRODUCTION
solvent extraction were determined for actinides and other
In recent decades, alkyl-substituted amide-functionalized
ligands have been investigated extensively because of their
metal ions. These data have demonstrated the effectiveness of
oxydiamide ligands as extractants but do not provide insight
potential to be used in the separation of radioactive elements in
spent nuclear fuel reprocessing.1 These ligands contain only
into the thermodynamic and structural factors that govern the
extraction efficiency. No relationships between the ligand
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
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 complex-
degradation of these types of ligands is much less adverse to the
ation of lanthanide and actinide metal ions (Nd3+, UO22+, and
separation process than that of traditional organophosphorus
+
extractants such as tributylphosphate.2−4 The products of the
NpO2 ) with a series of oxydicarboxylate and oxydiamide
ligands, including 2,2′-oxydiacetic acid (ODA), N,N-dimethyl-
3-oxaglutaramic acid, and N,N,N′,N′-tetramethyldiglycolamide
(TMDGA).12−15 Equilibrium constants, enthalpies and en-
tropies 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
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 oxygen-
linked amides) have been studied as extractants in solvent
extraction for the separation of actinides, including tetraoctyl-
diglycolamide, tetraisobutyldiglycolamide, and N,N′-dimethyl-
Received: November 29, 2017
© XXXX American Chemical Society
A
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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
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′-dimethylaceta-
mide) (MABDMA), as shown in Figure 1. Similar to
TMDGA, which is in tridentate coordination with Nd3+,
2+
samples. The concentrations of UO2 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.
+
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 substitu-
tional 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. Computa-
tional 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 = E0 + 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)
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
+
[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, Calorim-
etry 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
B
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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 )
+
○
2+
2+
+
̀
[UO2L] ; (blue ) [UO2L2] ; (red ) [(UO2)3(OH)5] . Minor hydroxide species of uranium(VI) are not shown. L = BnABDMA, ABDMA, and
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)
a
ligand
reaction
method
log β
ΔH (kJ mol−1
)
ΔS (J K−1 mol−1
)
ref
BnABDMA
H+ + L = HL+
pot, cal
pot, cal
pot, cal
pot, cal
pot, cal
pot, cal
pot, cal
pot, cal
pot, cal
sp, cal
6.36 0.09
4.33 0.03
7.67 0.06
7.12 0.09
5.03 0.03
9.08 0.03
7.64 0.09
5.30 0.03
9.66 0.03
1.71 0.03
2.94 0.01
−(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
17
62
96
11
59
75
34
76
121
58
122
1
3
5
6
3
3
2
2
13
UO2 + L = UO2L2+
this work
2+
2+
2+
2+
UO2 + 2L = UO2L2
ABDMA
MABDMA
TMDGA
H+ + L = HL+
13
UO2 + L = UO2L2+
this work
2+
2+
UO2 + 2L = UO2L2
H+ + L = HL+
13
UO2 + L = UO2L2+
this work
2+
2+
2+
2+
UO2 + 2L = UO2L2
3
UO2 + L = UO2L2+
1
12
2+
2+
UO2 + 2L = UO2L2
sp, cal
19.6 0.5
1
a
pot = potentiometry; cal = calorimetry; sp = spectrophotometry.
used to identify uranium(VI)/L complexes. Water/ethanol (<5%
water) spray solutions were used in the experiments. In high-
resolution mode, the instrument has a detection range of m/z 50−
2200 and a resolution of m/z ∼0.25. The instrument parameters used
to obtain the MS spectra of these complexes were similar to those
employed in previous reports.16
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
2.5. Computational Studies. Density functional theory (DFT)
calculations were carried out on the complexes and ligands using the
three-parameter hybrid functional B3LYP24,25 because previously this
level of theory has been demonstrated to produce reliable structural
and energetic results for actinide and lanthanide complexes.26,27 The
Stuttgart−Dresden small-core potential28 was employed for uranium
because, in combination with the B3LYP functional, it previously
provided good agreement with experimental values for the reaction
energies and vibrational frequencies of uranium(VI) complexes.26
Other elements were treated using a 6-31++G(d,p) Gaussian-type
basis set. Geometry optimizations were run in a vacuum without
symmetry constraints except for the [UO2(L)2]2+ complexes for
computational convenience. The resulting stationary points were all
minima as no imaginary frequencies were found. The reactant and
product free energies in the gas phase (G°gas) were computed by
adding to the electronic energy the zero-point energy and thermal
corrections calculated using the rigid-rotor and harmonic-oscillator
approximations. The solvent effect was taken into account by using the
polarizable continuum model (PCM),29 and geometry optimizations
were repeated.26,30−32 Energies were also recalculated for the obtained
minima with the same effective core potential for the U atom and the
6-311++G(3df,2p) basis set for the other elements.
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
The charges on the atoms were calculated by the fitting of the
electrostatic potential (ESP) according to the Merz−Kollman
scheme.33 For calculation of the reaction free energies in solution, it
was taken into account that a correction for the standard state was
needed because the gas-phase free energies (G°gas) are calculated with
the standard state of an ideal gas at 1 atm, but the free energies in
2
coordination mode. In all of the fits, an amplitude factor (S0 ) 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.
C
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Figure 3. Thermograms of calorimetric titrations (upper) and fitting of the experimental heat and speciation of uranium(VI) (lower). Symbols in the
lower figures: (magenta ) observed heat; (magenta ) calculated heat; (red ) free UO22+; () [UO2(NO3)]+; (green ) [UO2L]2+; (blue )
○
2+
̀
[UO2L2] . L = BnABDMA, ABDMA, and MABDMA.
using the equilibrium constants from potentiometry; (2) fitting
the equilibrium constants and the enthalpy of complexation
simultaneously. In general, higher uncertainties were associated
with the equilibrium constants and enthalpy if fitted
simultaneously, and the equilibrium constants obtained by
the two fitting approaches were in general agreement within the
uncertainties. We elected to report the results obtained by using
method 1 for fitting. Table 1 shows the enthalpy of
complexation calculated by using the equilibrium constants
from potentiometry. In the calculation, the enthalpy of
protonation of the ligands and the enthalpy of complexation
for UO2(NO3)+ from the literature13,17 were included as known
values. [The enthalpy of complexation for UO2(NO3)+ and the
enthalpy of protonation of BnABDMA, ABDMA, and
MADBDMA in 1.0 M NaNO3 media are 3.9, −31.2, −37.2,
and −33.5 kJ mol−1, respectively.]
The enthalpies of complexation (ΔH) of the three N-
functionalized diamide ligands with uranium(VI) are all
exothermic, in contrast to the endothermic enthalpy of
complexation of the oxydiamide ligand TMDGA (Table 1).
Similar contrast was observed between iminodiacetic acid
(IDA) and ODA: the complexation of uranium(VI) with IDA is
exothermic, while that with ODA is endothermic.38 Evidently,
the interactions between uranium(VI) and the N-atom donor
in the N-functionalized diamide ligands are much stronger than
those between uranium(VI) and the O-atom donor in the
oxydiamide ligand. The higher binding strength of N-
functionalized diamide ligands originates from the more
favorable enthalpy of complexation.
3. RESULTS AND DISCUSSION
3.1. Equilibrium Constants of Uranium(VI) Complexes
with BnABDMA, ABDMA, and MABDMA. The potentio-
metric titration conditions were chosen to be in the pH region
of 2.5−5.5 to minimize the hydrolysis of uranium(VI).
Representative titrations for the three ligands are shown in
Figure 2, while detailed titration conditions are provided in
Table S1. The best fit was obtained by using a model
containing two uranium(VI) complexes with the ligand (L):
UO2L2+ and UO2L22+, described by reactions (2) and (3):
2+
2+
UO2 + L = UO2L
(2)
(3)
2+
2+
UO2 + 2L = UO2L2
where L = BnABDMA, ABDMA, and MABDMA. The
calculated equilibrium constants for reactions (2) and (3) are
summarized in Table 1.
The binding affinity between uranium(VI) and these ligands
follows the trend BnABDAM < ABDMA < MABDMA (Table
1), parallel to the trend in protonation of the ligands. This
means that the interactions between uranium(VI) and the
amine-functionalized diamide ligands are dominantly electro-
static interactions and the ligand with higher electronegativity
forms stronger complexes with uranium(VI). Similar trends
have been observed for the complexation of neodymium(III)
with these ligands.16
3.2. Enthalpy and Entropy of Complexation. On the
basis of the results from potentiometric titrations, appropriate
conditions of the calorimetric titrations were chosen so that the
pH of the solutions was in the range from 2 to 4.5 to minimize
the formation of hydrolyzed uranium(VI) species. In the
presence of sufficient ligands, the hydrolyzed uranium(VI)
species were negligible in this pH region. Figure 3 shows the
thermograms and fits of the calorimetric data. The titration
conditions are provided in Table S2. Two methods were tried
to fit the data: (1) fitting only the enthalpy of complexation by
The entropy of complexation (ΔS) was calculated from the
enthalpy (ΔH) and Gibbs free energy (ΔG = RT ln β). The
entropies of complexation for BnABDMA, ABDMA, and
MABDMA are all positive, ranging around 60 J K−1 mol−1
for ML and 100 J K−1 mol−1 for ML2 (Table 1). These values
are similar to those for TMDGA, which has been shown to
form tridentate complexes with uranium(VI), suggesting these
D
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2+
three N-functionalized diamides also coordinate with UO2 as
tridentate ligands. The tridentate coordination mode of the
ligand in the uranium(VI) complexes is verified and further
discussed in the sections Computational Results and EXAFS
Results.
uranium(VI)/diacetamide complexes in PCM water are shown
in Figure 5, and selected bond lengths are reported in Table 2.
In the calculation of the minimum-energy structures for each
[UO2(L)(H2O)3]2+ complex, two geometries characterized by
the different conformations of the ligand were found: one
(isomer a in Figure 5, upper rows) with the distorted
diacetamide (the O−C···C−O dihedral angle ∼30°) and the
other (isomer b in Figure 5, lower rows) with the ligand in a
planar arrangement (the O−C···C−O dihedral angles ∼0°;
similar to the structure with TMDGA15). An equatorial view of
the same structures is shown in Figure S3. During the
minimum-energy search in the gas phase for isomer b, one
water molecule was released from the first coordination shell of
the metal to form an outer-sphere complex (Figure S4). On the
contrary, when optimizations were carried out in water, all
remained in the first coordination shell of uranium for all
isomers. The calculated bond distances are compatible with
those obtained by EXAFS (see sections below), with the U−N
bond slightly underestimated with respect to the experimental
one in solution. By a comparison of the bond distances of the
two conformers (Table 2), it appears that, in complexes b, the
3.3. Identification of Uranium(VI) Complexes by ESI-
MS. The 1:1 and 1:2 uranium(VI) complexes with BnABDMA,
ABDMA, and MABDMA were observed by ESI-MS experi-
ments as singly or doubly charged species with OH− as
counteranions (Figure 4). As the insets in Figure 4 show, the
U−OH O bonds are markedly longer (one at ∼2.54 Å and the
2
other two at ∼2.72 Å) than those in complexes a (2.48−2.55
Å), while the opposite is found for the U−Oamide bonds [∼2.38
Å (a) and ∼2.42 Å (b); Table 2].
In both isomers of the [UO2(L)(H2O)3]2+ complexes, the
U−N bond distances follow the order ABDMA < MABDMA <
BnABDMA, while the (average) U−Oamide bond distances are
similar for all complexes. In the 1:2 complexes (Figure 5), the
U−Oamide and U−N bond distances are slightly lengthened
with respect to those in the 1:1 complexes (Table 2). The U−
N bond is more elongated in the BnABDMA and MABDMA
complexes with respect to the ABDMA complex for both
isomers. On the contrary, the U−Oamide bond is only slightly
affected upon going from 1:1 to 1:2 species (average variation
∼0.01 Å) in isomer a, while a more evident change is observed
for isomer b (average variation ∼0.08 Å). This result could be
explained by the steric repulsion between the methyl groups of
the amide moiety which is minimized by the distorted
conformation in isomer a, while it induces an elongation of
the U−Oamide bond in isomer b. The energy differences
between the isomers (Table S3) indicate that a slightly higher
stability is expected for conformation b in the 1:1 complexes,
while in the 1:2 species, the opposite is true (with the exception
of MABDMA). However, these stability differences are
relatively small and suggest that a mixture of the two isomers
could be present in solution.
3.4.2. Electronic Charge Distribution. The extent of
electron donation from the ligand atoms to the metal ion is
an important factor for tuning the strength of the metal−ligand
interactions.16 The calculated ESP charges (q) for the U, N, and
Oamide atoms in the complexes and free ligands are shown in
Table 3 (values reported for isomer a and in parentheses for
isomer b). As the data in Table 3 show, the ESP charges of N
atoms in the free ABDMA ligand is more negative than those in
the free MABDMA and BnABDMA ligands. In the 1:1 and 1:2
uranium(VI) complexes, the ESP charges of N atoms become
increasingly less negative because of charge transfer to the U
atom (Table 3). The change in the ESP charges of the amine N
atom [Δq = q(bound ligand) − q(free ligand)] upon formation
of the complexes (both 1:1 and 1:2) follows the order ABDMA
> 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-to-
peak 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
2+
of UO2 and corresponding ligands.
3.4. Computational Results. 3.4.1. Geometry Optimiza-
tions. The minimum-energy structures of the diacetamide
ligands are shown in Figure S2. The U−O
bond distance
H2O
(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
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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
a
[UO2(L)2]2+ Complexes for the Conformations a and b in PCM Water (Figure 5)
b
U−N
U−Oamide
U−OH2O
c
a
b
a
b
a
b
[UO2(ABDMA)(H2O)3]2+
[UO2(MABDMA) (H2O)3]2+
[UO2(BnABDMA) (H2O)3]2+
[UO2(ABDMA)2]2+
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
[UO2(MABDMA)2]2+
[UO2(BnABDMA)2]2+
a
b
N = amine N atom. The U−Oamide and U−OH O bond distance are averaged. For [UO2(H2O)5]2+, U−OH O = 2.470 Å. U−Oamide = 2.418 Å in
2
2
[UO2(TMDGA)2]2+.15 min = 2.54 Å; max = 2.72 Å.
c
larger ligand−metal charge transfer occurs in ABDMA than
MABDMA and BnABDMA.
3.4.3. Energetics of Complexation Reactions. The ener-
getics of complexation are calculated for the two stepwise
reactions (4) and (5) and shown in Table 4.
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(H2O)5]2+ + L → [UO2L(H2O)3]2+ + 2H2O
(4)
[UO2L(H2O)3]2+ + L → [UO2L2]2+ + 3H2O
(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
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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
a
Uranium(VI) Complexes (the Values for Isomer b Are Reported in Parentheses)
qN
−0.997
−0.479
ΔqN
q
Δq
Oamide
q
Owater
qU
Oamide
ABDMA
−0.655
−0.652
−0.665
MABDMA
BnABDMA
−0.207
[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+
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 (0.862)
0.679 (0.608)
0.428 (0.551)
1.214 (1.179)
0.955 (0.887)
0.769 (0.710)
−0.534 (−0.638)
−0.517 (−0.613)
−0.530 (−0.633)
−0.466 (−0.416)
−0.440 (−0.412)
−0.435 (−0.470)
0.121 (0.017)
0.135 (0.039)
0.135 (0.032)
0.068 (0.222)
0.077 (0.201)
0.095 (0.163)
−0.820 (−0.838)
−0.825 (−0.850)
−0.821 (−0.849)
1.874 (2.451)
1.872 (2.484)
1.964 (2.434)
1.183 (0.886)
1.106 (0.969)
1.203 (1.223)
a
Δq = q(bound ligand) − q(free ligand). The charges on the same atom type are averaged.
a
Table 4. Theoretical Complex Formation Free Energies (kcal mol−1) for Reactions (4) and (5)
ΔGgas
ΔGwater
Δ log K1,t,calc
reaction
(4)
ligand
a
b
a
b
a
b
Δ log K1,t,exp
ABDMA
−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
+1.4
+3.2
0
+0.7
+1.0
0
MABDMA
BnABDMA
ABDMA
(5)
+0.4
+0.4
0
+0.2
+2.9
0
+1.4
+2.0
0
MABDMA
BnABDMA
a
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).
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
(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−O and NU−N numbers
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-
eq
eq
(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
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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.
Figure 6. EXAFS spectra (left) and FT magnitude (right).
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
NU−O numbers (Table 5) need to be corrected for speciation.
eq
After correction, the NU−O numbers in the ML complexes are
eq
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).
a
Table 5. EXAFS Fitting Results of Uranium(IV) Species in Solution
b
c
Solution
Shell
N
R (Å)
σ2
Note
2
I (MABDMA)
75% UO2L2+
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
1.78 0.01
2.41 0.01
2.86 0.02
1.78 0.01
2.39 0.01
2.85 0.03
1.78 0.01
2.42 0.01
2.87 0.03
1.78 0.01
2.40 0.01
2.87 0.03
1.77 0.01
2.42 0.01
2.89 0.04
1.78 0.01
2.41 0.02
2.93 0.05
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
S0 = 1.0
5.1 0.6
1.0 0.3
2.0
ΔE0 = 7.2 eV
r = 0.0020
2+
25% UO2L2
2
II (MABDMA)
10% UO2L2+
S0 = 1.0
4.1 0.6
1.9 0.4
2.0
ΔE0 = 6.4 eV
r = 0.0060
2+
90% UO2L2
2
III (ABDMA)
75% UO2L2+
S0 = 0.93
5.3 0.7
1.1 0.3
2.0
ΔE0 = 8.3 eV
r = 0.0034
2+
25% UO2L2
2
IV (ABDMA)
10% UO2L2+
S0 = 0.98
4.2 0.5
1.9 0.3
2.0
ΔE0 = 9.3 eV
r = 0.0060
2+
90% UO2L2
2
V (BnABDMA)
75% UO2L2+
S0 = 0.83
5.2 0.5
1.2 0.3
2.0
ΔE0 = 6.3 eV
r = 0.0044
2+
25% UO2L2
2
VI (BnABDMA)
10% UO2L2+
S0 = 0.92
4.1 0.5
1.9 0.3
ΔE0 = 8.5 eV
r = 0.043
2+
90% UO2L2
a
b
Subscripts “ax” and “eq” denote respectively axial and equatorial coordination of the linear UO22+ cation. The uranium(IV) speciation (relevant to
c
the total [U]) was calculated by using the equilibrium constants in Table 1. For 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.
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EXAFS data, we did not make attempts to differentiate these
equatorial O atoms.
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.
4. CONCLUSION
Complexation of UO2 with three structurally related amine-
2+
functionalized diamide ligandsBnABDMA, ABDMA, and
MABDMAin aqueous solution was studied by experimental
measurements and theoretical computation. The experimen-
tally determined binding strength of the ligands follows the
order MABDMA > ABDMA > BnABDMA, which is
corroborated by the computational results. The amine-
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b02971.
Experimental conditions of potentiometric and calori-
metric 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_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: andrea.melchior@uniud.it.
*E-mail: lrao@lbl.gov. Tel: 1-510-486-5427. Fax: 1-510-486-
5596.
■
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.
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
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DOI: 10.1021/acs.inorgchem.7b02971
Inorg. Chem. XXXX, XXX, XXX−XXX