Solvent extraction of americium(iii) and europium(iii) with tridentate N,N-dialkyl-1,10-phenanthroline-2-amide-derived ligands: extraction, complexation and theoretical study

Article abstract of DOI:10.1039/c6nj02696f

The extraction and complexation behavior of soft-hard combined N,N-dialkyl-1,10-phenanthroline-2-amide (PTA) ligands with Am(iii) and Eu(iii) in HNO₃ solution was investigated. The effects of acidity of the aqueous solution, shaking time, concentration of the extractant and temperature on the distribution ratios were studied. The extraction ability for both Am(iii) and Eu(iii) was found to decrease with an increase in the length of the substituent alkyl chains, and the highest extractability and selectivity for Am(iii) over Eu(iii) were found for the diethyl-substituted ligand. The separation factor for Am(iii) over Eu(iii) reached around 7.6 at low acidity and high salinity. UV-vis titrations revealed that the dialkyl-substituted tridentate PTAs all predominantly formed 1?:?1 complexes with Eu(iii), which agreed well with the results of slope analyses in the extraction experiments. The stability constants (K_EuL) as well as the protonation constants (K_H) were also determined by UV-vis titration. Computational chemistry gave a good explanation of the relationship between the alkalinity and the protonation energy of the proposed PTA ligands. Density functional theory (DFT) calculations on the optimized structures of Am(iii) and Eu(iii) complexes with C2-PTA showed that the selectivity may originate from differences in the degree of covalency of the bonds between the metal ions and the donor N atoms, which fits well with the experimental results.

Full text of DOI:10.1039/c6nj02696f

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Solvent Extraction of Americium(III) and Europium(III) with
Tridentate N,N-dialkyl-1,10-Phenanthroline-2-Amides-Derived
Ligands: Extraction, Complexation and Theoretical Study
Received 00th January 20xx,
Accepted 00th January 20xx
Shiwei Cao,†ac Jieru Wang,†a Cunmin Tan,†abc Xin Zhang,b, Guo Hangxuac, Lei Wang,d Zhi Qin*a
DOI: 10.1039/x0xx00000x
The extraction and complexation behavior of soft-hard combined N,N-dialkyl-1,10-phenanthroline-2-amide (PTA) ligands
for Am(III) and Eu(III) in HNO3 solution was investigated. The effects of acidity in aqueous, shaking time, concentration of
the extractant and the temperature on distribution ratios were studied. The ability for the extraction of both Am(III) and
Eu(III) were found to decrease with the increase of the length of substitute alkyl chains, and the highest extractability and
selectivity of Am(III) over Eu(III) were found for the diethyl-substituted ligand. The separation factor of Am(III) over Eu(III)
reached around 7.6 at low acidity and high salinity. UV-vis titration revealed that the dialkyl-substituted tridentate PTAs all
dominantly formed 1:1 complexes with Eu(III), which agreed well with the results of slope analyses in the extraction
experiments. The stability constants (KEuL) as well as the protonation constants (KH) were also determined by UV-vis
titration. Computation chemistry gave a good explanation on the relationship between the alkalinity and the protonation
energy of proposed PTA ligands. Density functional theory (DFT) calculations on optimized structures of Am(III) and Eu(III)
complexes with C2-PTA showed that the nature of selectivity may originate from the difference of covalency between the
www.rsc.org/
metal
ion
and
the
donor
N
atoms,
which
fits
well
with
the
experimental
results.
and O-donor) is the mainstream approach. In the past three
decades, some ligands containing soft nitrogen atoms, such as
2,6-bis(5,6-dialkyl-1,2,4-trizain-3-yl)pyridine (R-BTP),3 6,6’-
bis(5,6-dialkyl-1,2,4-trizain-3-yl)-2,2’-bipyridine (R-BTBP)4 and
2,9-bis-(5,6-dialkyl-1,2,4-trizain-3-yl)-1,10-phenanthroline (R-
BTPhen)5,6, were exploited as potential extractants, and were
proved to have high selectivity on An(III) (Fig. 1, a-c). Several
ligands that contain hard oxygen atoms, represent by
diglycolamide (DGA) family (Fig. 1d),7-12 display high affinity on
both An(III) and Ln(III). Unfortunately, these ligands are often
less selective compared to many ligands containing soft
Introduction
In the spent fuel reprocessing, the high-level liquid waste
(HLLW) from PUREX process is the major environmental
concern of its high radiotoxicity level. In recent years,
remarkable efforts were done in order to achieve safe
treatment and disposal of HLLW for the sustainable
development of nuclear energy.1 The partitioning and
transmutation (P&T) is one of the most perspective technology
to solve this problem. The P&T strategy aims at transforming
the minor actinides (MAs) such as Am, Np and Cm into the
short-lived or stable isotopes with neutron bombardment, to
reduce the burden on geological storage. However, the
lanthanides in HLLW have very high neutron capture cross-
sections, and need to be removed before the transmutation.2
The principal problems in the separation of An(III) from Ln(III)
are the same oxidation sate and similar chemical properties.
So far, selective extraction with soft or hard ligands (e.g. N, S
Fig. 1 Structures of extractants discussed in this paper.
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atoms11,13
.
Based on the investigations of these ligands, much effort in
improving the extraction ability and selectivity has been done.
Earlier G. Modolo et al. studied the binary extractants system
composed of both soft-donor 6,6’-bis(5,5,8,8-tetramethyl-
5,6,7,8-tetrahydrobenzo-[1,2,4]trizazin-3-yl)-[2,2’]-bipyridine
(CyMe4BTP) and hard-donor N,N,N’,N’-tetraoctyldiglycolamide
(TODGA). This mixture shows better and more efficient
extraction than using CyMe4BTBP alone.14 However, the binary
mixture of extractants make the system much complicated and
difficult for further investigation of the extraction mechanics.
An ideal solution is to combine both hard and soft donor
atoms in one structure simultaneously. Recently, N,N’-diethyl-
N,N’-ditolyl-2,9-diamide-1,10-phenanthroline (EtTolDAPhen)
(Fig. 1e) was developed by Xiao. et al., and its extraction
behavior for various metal ions such as U(VI), Th(IV), Am(III)
and Eu(III) was studied experimentally and theoretically.15-17
Similar diamide derivations for the separation of An(III) from
Ln(III) were studied by M. Yu. Alyapyshev et al.18 These
tetradentate ligands contain two O atoms on amide groups
and two N atoms on rigidity phenanthroline skeleton. Also, the
extraction behavior of mono-substituted phenanthroline
and is shown in Fig. 2.15,19,21 The final crude product was
purificated with silica gel column chromatography
(methanol/dichloromethane = 1/15), and then recrystallized
from ethanol. NMR spectra were measured on a Bruker
AVANCE III HD 400 MHz spectrometer. FT-IR spectra were
recorded on a Bruker VERTEX 70 instrument. ESI-MS data were
obtained on a Bruker microTOF-Q II spectrometer.
C2-PTA: 1H NMR (CDCl3, 400 MHz, ppm): δ 9.21 (dd, 1H, J7-9
=
derivatives
like
N-Octyl-N-tolyl-1,10-phenanthroline-2-
1.6 Hz, J8-9 = 3.6 Hz, H9), 8.34(d, 1H, J = 8.2 Hz, H4), 8.27(d, 1H, J
= 8.0 Hz, H7), 7.92(d, 1H, J = 8.4 Hz, H3), 7.83(m, 2H, J = 3.6 Hz,
H5 and H6), 7.65(dd, 1H, J7-8 = 8.0 Hz and J8-9 = 4.4 Hz, H8), 3.50-
3.58, 3.62-3.69(m, 4H, J = 7.1 Hz), 1.23-1.29, 1.32-1.38 (t,6H, J
carboxamide (OcTolPTA) (Fig. 1f) and N-methyl-N-phenyl-1,10-
phenanthroline-2-carboxamide (MePhPTA) on Eu(III) was
studied by T. Kobayashi et al.19 Synergistic extraction of Ln(III)
with MePhPTA and pivaloyltrifluoroacetone (HA) was studied
by Y. Hasegawa. et. al.20 Due to the rigid skeleton and hard- = 7.1 Hz). 13C NMR (CDCl3, 100 MHz, ppm): 168.73, 154.89,
soft combined donor atoms, these ligands exhibit fast
dynamics and high distribution ratio in solvent extraction of
some An(III) and Ln(III). Even so, little attention has been
150.42, 146.13, 144.66, 136.92, 136.07, 129.03, 128.57,
127.37, 126.25, 123.10, 122.51, 43.52 and 40.61, 14.47 and
12.99. FT-IR: (KBr, ν/cm-1): 3381, 2976, 2936, 2889, 1624,
1460, 1379, 1094, 1049, 881, 758. ESI-MS: 280.1443 (M+H+),
318.0994 (M+K+), 559.2792 (2M+H+), 581.2604 (2M+Na+),
597.2351 (2M+K+). Yield: 66%; pale-pink powder; mp 120-121
oC.
devoted
to
N,N-dialkyl
substituted
2-amide-1,10-
phenanthroline-derived ligands for An(III)/Ln(III) extraction and
separation by present. As the hard-soft donor combined
strategy exhibits such advantages in the designing of novel
ligands for separation, further and necessary research should
be conducted.
C4-PTA: 1H NMR (CDCl3, 400 MHz, ppm): δ 9.20 (dd, 1H, J7-9
=
1.2 Hz and J8-9 = 4.2 Hz, H9), 8.33(d, 1H, J = 8.2 Hz, H4), 8.26(d,
1H, J = 8.0 Hz, H7), 7.93(d, 1H, J = 8.2 Hz, H3), 7.83(m, 2H, J =
3.6 Hz, H5 and H6), 7.64(dd, 1H, J = 8.0, 4.3 Hz, H8), 3.53-3.60,
3.45-3.52(t, 4H, J = 8.0 Hz), 1.78(m, 4H, J = 8.0 Hz), 1.42-
1.62(m,4H, J = 7.6 Hz), 0.91(t, 6H, J = 7.6 Hz). 13C NMR (CDCl3,
100 MHz, ppm): 168.74, 154.88, 150.32, 144.55, 136.84,
136.00, 128.99, 128.51, 127.38, 126.23, 123.08, 122.79, 49.29
and 46.47, 31.25 and 29.83, 20.56 and 20.00, 13.97 and 13.66.
FT-IR: (KBr, ν/cm-1): 3384, 2958, 2933, 2871, 1625, 1473, 1382,
1120, 1028, 860, 729. ESI-MS: 336.2067 (M+H+), 358.1893
In view of the current situation above, we proposed a series of
substituted 1,10-phenanthroline-2-carboxamide ligands with
different length of alkyl chains, namely, N,N-diethyl-1,10-
phenanthroline-2-carboxamide (C2-PTA), N,N-dibutyl-1,10-
phenanthroline-2-carboxamide (C4-PTA) and N,N-dihexyl-1,10-
phenanthroline-2-carboxamide (C6-PTA). Their extraction
behavior for Am(III) and Eu(III) in HNO3 solution was
investigated. The extraction models were studied by slope
analysis and the UV-vis titration. DFT calculations were carried
out on the basis of the experimental work to provide a better
understanding of the bond nature and selectivity of such (M+Na+), 374.1624 (M+K+), 671.4015 (2M+H+), 693.3821
(2M+Na+). Yield: 54%; yellow oily liquid at room temperature.
ligands with metal ions.
C6-PTA: 1H NMR (CDCl3,400 MHz, ppm): δ 9.21 (dd, 1H, J = 1.6
Hz and 4.0 Hz, H9), 8.34 (d, 1H, J = 8.4 Hz, H4), 8.27 (dd, 1H, J =
Experimental
1.6, 8.0 Hz, H7), 7.93 (d, 1H, J = 8.0 Hz, H3), 7.83 (m, 2H, J = 3.6
Synthesis and characterization of PTA
Hz, H5 and H6), 7.65 (m, 1H, J = 8.0 Hz, H8), 3.44-3.60 (t, 4H, J =
8.0 Hz), 1.77 (m, 4H, J = 7.6 Hz), 1.25-1.36 (m, 12H), 0.89 (t,
6H). 13C NMR (CDCl3, 100 MHz, ppm): 168.76, 154.92, 150.41,
146.17, 144.65, 136.90, 135.99, 129.02, 128.52, 127.41,
The synthesis route of
N,N-dialkyl-2-amide-1,10-
phenanthroline-derived ligands is similar to that of
EtTolDAPhen, MePhPTA and C2-PTA, as described elsewhere,
2 | J. Name., 2012, 00, 1-3
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126.23, 123.14, 122.69, 49.28 and 47.80, 31.80 and 31.69,
29.01 and 28.60, 27.70 and 27.01, 22.71 and 22.65, 14.05 and
13.90. FT-IR: (KBr, ν/cm-1): 3388, 2952, 2931, 2866, 1625,
1458, 1377, 1166, 860, 727. ESI-MS: 392.2690 (M+H+),
414.2505 (M+Na+). Yield: 57%; light-yellow oily liquid at room
temperature.
Theoretical Methods
The theoretical calculations for protonated PTAs were carried
out by Density Functional Theory (DFT) with B3LYP23
correlation function in Gaussian 09d software package.24 The
Pople-style double-ζ 6-31G(d) basic sets25 were used for light
CHON atoms. For metal atoms, the quasi-relativistic effective
core potentials (RECPs) were adopted: the 60 and 28 electrons
were replaced by the employed small-core RECPs for Am and
Eu, respectively. The residual 35 electrons were treated with
the complementary energy-consistent pseudorelativistic basic
sets.26,27 The calculations for the complexes of Am(III) and
Eu(III) were in septet spin multiplicity in ground state, and no
symmetry was adopted for constraint. The original geometry
structures were optimized under the above-mentioned
theoretical level to reach the stationary point. The frequencies
were calculated right after the geometry optimization to make
sure the obtained structures were the minima on the potential
energy surface. The geometry parameters were examined on
the optimized structures. The natural population analysis
(NPA) was carried out in the NBO 5.0 version.28 For energy
calculations in aqueous phase, the polarizable continuum
model (PCM)29 was used to take into account solvation effect.
The protonation energy for a ligand (L) was defined as the
energy difference of the following reaction:30,31
Solvent Extraction
The stock solution of radioactive 241Am (3.08 MBq/L) + 152Eu
(1.85 MBq/L) radiotracers containing stable Eu3+ carrier (1800
mg/L) was provided by Institute of Modern Physics (IMP),
Chinese Academy of Sciences (CAS). All the other chemical
reagents were analytical grade, and were used without further
purification. The radioactive operations were performed in the
lid-shielding glovebox. The organic phases were prepared with
stock solution of PTA in the diluent (600 μL). Then the pre-
equilibrated organic phases were mixed with equal volume of
the aqueous phases containing HNO3 solution and 50 μL
radiotracers in 10 mL centrifuge tubes. The mixtures were
vigorously shaken for several minutes under constant
temperature to reach equilibrium. After shaking and phase
separation by centrifugation (3000 rpm), 450 μL of the
aqueous phase and the organic phase were transferred
separately into fresh tubes. The γ ray counts of the isotopes
were measured by an HPGe-γ-detector (EG&G Ortec GEM-
30200-P with a relative efficiency of 30%). A DSPEC-502TM
Digital Gamma-Ray Spectrometer was employed for data
acquisition. The energy resolution (FWHM) was 1.70 keV at
1332.5 keV. The areas of characteristic peaks of 241Am (59.5
keV) and 152Eu (121.8 keV) were calculated respectively by
Gaussian fitting. The extraction processes were repeated three
times and the results were averaged. The errors of the net
areas for all peaks were less than 5%.
L + H3O+
→
[LH (H2O)]+
(2)
It was calculated at the same level of theory on the basis of
optimized geometry structures.
Results and discussion
Solvent Extraction Studies
Effect of acidity and the substituted alkyl chain. The extraction
behavior of C2-, C4- and C6-PTA was tested in same condition.
The effect of acidity in aqueous was examined by varying the
initial concentration of nitric acid from 0.0001 to 1.0 mol/L.
The influence on D(Am) and D(Eu) are shown in Fig. 3.
The distribution ratio of metal ions (DM) is defined as the ratio
of analytical concentration in each phase, which is in
proportional to the ratio of corresponding peak areas in γ
spectra:
DM = [M](org.)/[M](aq.) = A(org.)/A(aq.)
(1)
It can be seen that the distribution ratios of both Am(III) and
Eu(III) as well as SFAm/Eu decrease with the increase of nitric
acid concentration. This is due to the protonation of donor N
The separation factor of Am(III) over Eu(III) (SFAm/Eu) was
determined by the ratio of DAm and DEu.
UV-vis Titration
The UV-vis titration was carried out in methanol. To determine
the composition and the stable constant of EuL complexes
(KEuL), 0.2 mmol/L Eu(NO3)3·6H2O solution was added by
aliquot 10 μL to the solution containing 30 μmol/L ligands (C2-,
C4- and C6-PTA). To determine the protonation constant of the
ligands (KH), 2 mmol/L HNO3 solution was added by aliquot 10
μL to the solution containing 30 μmol/L ligands (C2-, C4-, C6-
PTA and non-substituted 1,10-phenanthroline). The mixture
was stirred in the quartz cuvette (b = 1.0 cm) for 1 min, which
was enough to reach the equilibrium. The spectra at 190-360
nm were measured with the UV-vis spectrometer (Thermo
Scientific, Evolution 300) at 293 ± 0.1 K. The spectra at the
wavelength of from 250 nm to 320 nm representing the
superposition of the absorbance of free ligands and the
complexes were used for analysis. The obtained data were
fitted with the available ThordarsonFittingProgram.22
Fig. 3 Effect of initial acidity in aqueous phase by different C2-PTA, C4-PTA and
C6-PTA. Organic phase: 0.2 mol/L PTA in chloroform. Aqueous: HNO3
241Am/152Eu radiotracers. T = 293 K.
+
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atoms on 1,10-phenanthroline moiety. C2-PTA shows a much
higher extraction selectivity of Am(III) over Eu(III). The
maximum values of D(Am) and D(Eu) reach (25.2 ± 0.2) and
(3.3 ± 0.1), respectively. As lower acid concentration will cause
the hydrolysis of Am(III) and Eu(III) in aqueous, the highest
SFAm/Eu is determined to be (7.6 ± 0.3) at the acidity of around
10–4.0 mol/L. Similar trends were also observed in the
extraction of Eu(III) by some other N-donor ligands like 1,3-
bis[3-(2-pyridyl)pyrazol-1-yl]propane (Bibpp), 1,2-bis[3-(2-
pyridyl)pyrazyl-1-methyl]benzene (Dbnpp)31 and OcTolPTA,19
but D(Eu) values by C2-, C4- and C6-PTA are below those of
OcTolPTA. Meanwhile, the distribution ratios of Am(III) and
Eu(III) by C4-PTA and C6-PTA are all much lower than those by
C2-PTA, respectively, while the trend of decrease along with
the increase of acidity remains the same. The distribution
ratios of the two metal ions by C4-PTA are all below 2.6, and
the separation factors are all below 3.0. For C6-PTA, the
distribution ratios are even lower than 0.5, showing that
hardly can C6-PTA extract trace amount of Am(III) or Eu(III)
from HNO3 medium. It suggests that the distribution ratios
decrease as the amidic alkyl chains extend, and that the longer
alkyl chains suppress the extraction. The phenomena may
probably attribute to the following reasons. First, PTAs with
longer alkyl chains have higher alkalinity and are easier to be
protonated (this will be discussed later in this work), which
could inhibit the conversion of the ligands to the tridentate
conformation during the extraction. Second, longer alkyl
chains may have higher steric hindrance for the coordination,
and the long lipophilic alkyl chains may prevent the ligand
from cutting into the aqueous phase at the contact surface of
the two phases. This finding can also be supported by other
amide derivatives.9,11
Fig. 5 Effect of shaking time. Organic phase: 0.2 mol/L C2-PTA in chloroform.
Aqueous phase: 10-3.63 mol/L HNO3 241Am/152Eu radiotracers. T = 293 K.
+
increases, D(Am) and D(Eu) clearly increase at identical rates, while
SFAm/Eu remains about the same. On the fact of the appearance in
the experiment, it can be inferred that nitrate ions promote the
formation of the complexes at high concentration, acting as
counter-ions or binding ligands of the metal ions17
.
Effect of shaking time. The equilibrium time in the extraction
with 0.2 mol/L C2-PTA in chloroform was examined. As Fig. 5
presents, the extraction process reached equilibrium after
shaking for 60 min at 57 rpm, 293 K. It can be seen that PTA
ligands exhibit a fast kinetics comparable with BTPs, BTBPs and
BTPhens.3,5 This may probably due to the rigidity skeleton of
the two juxtapose nitrogen atoms on the cis- conformation,
which is favorable for coordination with metal cations. To
ensure the complete equilibrium of extraction in such
condition, all the mixtures were shaken for 120 min in this
work.
As C2-PTA showed the greatest extraction ability, it was
chosen as the representative in subsequent experiments.
Effect of nitrate ions. The effect of nitrate ions on the extraction of
Effect of extractant concentration. Fig. S13 shows the
distribution ratios and separation factors dependence of C2-
PTA concentration on Am(III) and Eu(III) extraction. The
concentration of the ligand in chloroform ranges from 0.01 to
0.2 mol/L. As is shown in Fig. S13, both D(Am) and D(Eu)
increase with the increase of C2-PTA concentration, and D(Am)
grows much faster than D(Eu), and as a result, the SFAm/Eu
values also increase within the range of the measurement.
The model for the extraction of trivalent metal ions (M) with
Am(III) and Eu(III) with 0.2 mol/L C2-PTA was processed, see Fig. 4.
Sodium nitrate was chosen as the salting-out agent in the
concentration of 0.1-4.0 mol/L, while the acid concentration was
fixed at 10-3.63 mol/L. In Fig. 4, as the concentration of nitrate ions
PTA can be expressed as:19
3+
M
(aq.) +3NO3-(aq.) +xPTA(org.) =M(PTA)x(NO3)3(org.)
(3)
The apparent equilibrium constant (Kex) and distribution ratio
(D) are defined as
[M(PTA)x (NO3 )3 ](org.)
Kex
=
(4)
(5)
-
3
[M3+
](aq.)[NO3
]
(aq.)[PTA](xorg.)
[M(PTA)x (NO3)3 ](org.)
D =
[M3+
]
(aq.)
By introducing Eq. (5) into Eq. (4), and taking logarithm of both
sides, the linear equation can be obtained:
-
lgD = lgKex + 3lg[NO3 ](aq.) + xlg[PTA](org.)
(6)
When the composition of aqueous is kept consistent, and Kex is
a constant at a given temperature, all the constants can be
merged into C, and the equation can be simplified into:
Fig. 4 Effect of nitrate ion concentration. Organic phase: 0.2 mol/L C2-PTA in
chloroform. Aqueous phase: 10-3.63 mol/L HNO3
+
241Am/152Eu radiotracers +
NaNO3. T = 293 K.
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Fig. 6 Ligand concentration dependence of distribution ratios on the extraction of
Am(III) and Eu(III) by C2-PTA in chloroform. Aqueous phase: 10-3.63 mol/L HNO3
241Am/152Eu radiotracers. T = 293 K.
Fig. 7 Effect of temperature. Organic phase: 0.2 mol/L C2-PTA in chloroform.
Aqueous phase: 10-3.63 mol/L HNO3 241Am/152Eu radiotracers.
+
+
Effect of temperature. According to the Van’t Hoff equation, the
enthalpy change of the extraction reaction has the relationship
with the extraction constant and distribution ratio:
lgD = xlg[PTA](org.) +C
(7)
According to Eq. (7), plots of lgD(Am), lgD(Eu) as a function of
lg[C2-PTA] give two straight lines, presenting the coordination
number of extractant molecules by the value of x, see Fig. 6. It
elucidates that in such condition, C2-PTA forms 1:1 (x = 1.18 ±
0.09) and 2:1 (x = 2.10 ± 0.18) species with Eu(III) and Am(III),
respectively. Though Eu(III) and Am(III) form different species
in such solution, it is still considerable due to the tiny
difference between the ion radius and the electron structures
of the metal atoms in their complexes, and this difference can
usually be observed in the extraction of Am(III) and Eu(III) with
many other N-donor ligands.32-36
∂lnKex
∂(1 / T)
∂lgD
∂(1 / T )
ΔH = -R
= -2.303R
(8)
where R is the gas constant. By plotting lgD versus 1/T, the
straight lines can be obtained, and the values of ΔH in the
extraction on Am(III) and Eu(III) can simply be estimated from
the slope of the corresponding line ((2.34 ± 0.38) and (1.16 ±
0.19), respectively), see Fig. 7. As calculated by Eq. (8), the –ΔH
values are determined to be (44.8 ± 7.3) kJ/mol for Am(III) and
(22.2 ± 3.6) kJ/mol for Eu(III), respectively. This indicates that
the extraction processes are exothermic, and higher
temperature would restrain the extraction reaction.
Generally, both lanthanides (III) and actinides (III) mostly form
complexes of CN = 8, 9, 10 with such tridentate ligands (such
as BTP and PTA), or tetradentate ligands (such as BTBP,
BTPhen and DAPhen), as is reported elsewhere.3,4,15,16 In those
1:1 complexes, one metal ion coordinate with one ligand,
together with three bidentate nitrate ions in inner-sphere,
constituting neutral ML(NO3)3 species (CN = 9 or 10). In some
2:1 complexes, however, one metal ion combine with two
Solution Spectroscopy and Stability Constant Determination
Eu(III)-PTA complexes. UV-vis titration was carried out to
determine the stable constant of the EuL complexes. The
change of absorption during the UV-vis titration of C2-, C4- and
C6-PTA by Eu(III) are shown in Fig. 8. It can be seen that a free
ligand has a strong absorption peak at 268 nm in methanol
solution. With the addition of Eu(III) the absorption at 268 nm
gradually decreases, while the new peak at 287 nm
corresponding to EuL complexes increases. Only one isosbestic
point is found at 276 nm, denoting the 1:1 complexes formed.
This is also consistent with the result of slope analysis, and is
also similar to the complexation behaviors of EtTolDAPhen.15
By fitting the data at the wavelength of 287 nm, the stable
constants (KEuL) were determined, see Table 1.
tridentate or tetradentate ligands as well as one or two nitrate
(3-x)+
ion(s) in the inner sphere like ML2(NO3)x
(x = 1, 2), and the
nitrate ions in outer sphere can be extracted simultaneously
into organic phase (in form of an ionic polymer) to keep the
electric neutrality4. It is reported by T. Kobayashi et al. that the
PTAs with benzene ring (MePhPTA and OcTolPTA) mainly form
2:1 complexes with Eu(III).19 Compared with the conclusion for
C2-, C4- and C6-PTA, the difference is likely to be attribute to
the steric hindrance. In MePhPTA and OcTolPTA, the large
steric hindrance of aromatic rings may prevent the ligand in
the stable trans- conformation from turning to cis-
conformation, and this could result in the absence of
complexation of carbonyl O atom with metal ion. UV-vis
titration of MePhPTA and non-substituted Phen by Eu(III)
revealed that their 2:1 complexes are found, suggesting that a
similar behavior of both coordinating with the two N atoms on
the phenanthroline rings is likely the case.
From the table we can see that there is a slight increasing
trend of the lgKEuL values in order of C2-, C4- and C6-PTA. Still,
such small discrepancies reveal that the differences between
the stabilities of Eu(III) with the three ligands may seem
dramatic but not enormous, whereas the distribution ratios of
C4-PTA and C6-PTA display much lower than those of C2-PTA.
Thus the reasons are brought to the steric hindrance and the
protonation of ligands in acid solutions, which will be
discussed later in this article.
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Fig. 8 The UV-vis titration of C2-PTA (a), C4-PTA (b) and C6-PTA (c) by Eu(III) in
methanol medium. cL = 3.0×10–5 mol/L, cM = 0–3.7×10–5 mol/L, T = 293 K.
Fig. 9 The UV-vis titration of C2-PTA (a), C4-PTA (b) and C6-PTA (c) by H+ in
methanol medium. cL = 3.0×10–5 mol/L, cH+ = 0–9.3×10–5 mol/L, T = 293 K.
H-PTA complexes. The UV-vis titration graphs of C2-, C4- and
C6-PTA as well as non-substituted 1,10-phenanthroline (Phen)
by H+ in methanol medium are shown in Fig. 9 and Fig. S14. It
can be found that with the addition of H+ the absorption at
268 nm gradually decreases, while that at 280 nm
corresponding to protonated PTA increases. Only one
isosbestic point is found at 272 nm (Fig. 9). The absorption of
Phen, protonated Phen and the corresponding isosbestic point
appear at 263, 271 and 266 nm (Fig. S14). It can be deduced
from the results that the 1:1 protonated HL is formed for each
ligand. The protonation constants (KH) are also calculated by
fitting the obtained spectra data at the wavelength of 280 nm,
and are listed in Table 1. Obviously, there is an increasing
trend of the lgKH values with the increase of alkyl chain length.
All the ligands are alkaline, which are easily to be protonated
in the presence of HNO3. This can explain why the distribution
Table 1 Fitting result of UV-vis titration at 287 nm (Eu3+ → L) and 280 nm (H+ → L).
L
lgKEuL
4.00 ± 0.01
4.10 ± 0.06
4.19 ± 0.02
-
lgKH
C2-PTA
C4-PTA
C6-PTA
Phen
3.77 ± 0.03
4.13 ± 0.01
4.22 ± 0.04
5.39 ± 0.01
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can the EuL species be found ([EuL] < 10-7 mol/L) according to
the calculation, and almost all the ligand molecules are
protonated when the HNO3 concentration is higher than 10–1.5
mol/L. This is well agreed with the conclusion of solvent
extraction experiment (Fig. 3).
DFT Calculations
Protonation energies of the ligands. To explore the relationship
between the protonation constant and the molecular
structures, the protonation energy values of C2-, C4-, C6-PTA
and Phen were calculated at the B3LYP/6-31G(d) level. The
calculated protonation energy, as well as the experimental
lgKH values of the ligands are listed in Table 2.
As is shown, all ligands in the table exhibit large negative
values of protonation energies, indicating that they are easily
to be protonated in the presence of HNO3 solution. The
alkalinity of the ligands increases in order of C2-PTA < C4-PTA <
C6-PTA < Phen. This should be attributed to the difference of
electron density of the delocalized π-electrons on N1 (N(1)
atom of the heterocycle) and N2 (N(10) atom of the
heterocycle) atoms in PTA and Phen molecules, which is
induced by the conjugated effect of electron-withdrawing (–C)
of amide groups and the inductive effect of electron-donating
(+I) of the alkyl chains. As
a consequence, when the
substituted alkyl chain increases, the protonation energy
become more negative, and the structure turn out to be more
difficult to deprotonate. This is consistent with the trend of the
experimental lgKH values. However, the high deprotonation
energy could significantly inhibit their conversion to the
tridentate conformation during the extraction process, and
could be regarded as one of the major reasons why C4-PTA
and C6-PTA have much lower distribution ratios on metal ions
than C2-PTA.
Additionally, as the protonation energy on N2 showed more
negative values than N1 for each PTA ligand, N2 is considered
to be the preferential protonation site with smaller steric
hindrance compared to N1, according to the calculations.
Geometry, Natural charge and WBI of ML(NO3)3 species. In
order to investigate the origin of selectivity of PTA ligands on
Am(III) and Eu(III), the models for the 1:1 neutral analogues
ML(NO3)3 (M = Am, Eu; L = C2-PTA) (CN = 9) in gas-phase were
studied at the B3LYP/6-31G(d)/RECP level. The 2:1 structure
(ML2(NO3)x(3-x)+) was not taken into consideration due to the
concern of time-consuming calculations.
Fig. 10 The species diagrams of Eu(III) complexes with C2-PTA (a), C4-PTA (b)
and C6-PTA (c) as a function of HNO3 concentration.
ratios remarkably decrease with the increase of HNO3
concentrations.
The optimized structures are shown in Fig. S15 and Fig. S16. As
Fig. 10 shows the species diagrams as a function of HNO3
concentration from 0.0001 to 1.0 mol/L for the Eu(III)-PTA
system. The analytical concentration of the ligands and Eu(III)
are set to be 4.4×10–4 and 8.8×10–5 mol/L, respectively. As the
HNO3 concentration increases, the concentration of EuL
species as well as free L drastically decreases, and that of HL
species increases. The protonation constants are comparable
in order of magnitudes to those of the stable constant of Eu(III)
with each PTA ligand (Table 1), so the protonation may
become the significant side-reaction that compete with the
formation of Eu(III)-PTA, and the coordination reaction would
be masked in high acidic solution. As a matter of fact, hardly
Table 2 Calculated protonation energy (kJ/mol) values of the ligands. The energy
is calculated at the B3LYP/6-31G(d) level, with the ZPE and thermal corrections at
298.15 K, 1.0 atm.
a
b
L
ΔE1
ΔE2
C2-PTA
C4-PTA
C6-PTA
Phen
-175.1
-177.9
-178.2
-194.3
-186.0
-188.4
-188.7
-194.3
a: the protonation energy on N1 atoms.
b: the protonation energy on N2 atoms.
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Table 3 Calculated bond length, natural charges and the Wiberg bond
indices (WBI) of optimized structures of ML(NO3)3 (M = Am, Eu; L = C2-PTA).
The extractability of Am(III) and Eu(III) with tridentate N,N-
dialkyl-1,10-phenanthroline-2-amide (PTA) ligands, which
combine hard-soft donors in single molecule, were
investigated. The distribution ratios decrease with the increase
of the length of substituted alkyl chains, and C2-PTA was
proved to have the best performance in extraction at low
acidity among all the three ligands investigated in this work.
The dominant 2:1 and 1:1 complexes of Am(III) and Eu(III) with
C2-PTA were identified by slope analysis, respectively. The
stable constants of the Eu(III)-PTA complexes and the
protonation constants of the ligands were determined by UV-
vis titration. The species diagrams showed that the
protonation was the major reason to reduce the distribution
ratios of metal ions in highly acidic nitric solution. DFT
calculations also revealed that the protonation energy on N2
side directly influenced the alkalinity of the ligands, which can
be modified by the substitute on phenanthroline ring. From a
theoretical point of view, the differences of geometry and the
electron structures in Am(III) and Eu(III) complexes suggested
that the selectivity of PTA ligands might originate from the
discrepancies of covalency in Am-N and Eu-N bonds, and all
the calculation results are in good accordance with the solvent
extraction and UV-vis titration results. With the view of
improving the extraction performance to meet the practical
needs for actinides/lanthanides separation, the electronic
properties of both N atoms should be finely tuned by
introducing electron donating groups (EDG) or electron
withdrawing groups (EWG) onto the aromatic rings.
Complex
Bond Length
AmL(NO3)3
2.678
EuL(NO3)3
2.649
2.707
2.441
1.391
0.410
-0.435
-0.449
-0.616
0.209
0.196
0.308
M-N1
M-N2
M-O
M
(Å)
2.719
2.486
Natural
Charges
(e)
1.439
L
0.395
N1
-0.443
-0.462
-0.620
0.218
N2
O
WBI
M-N1
M-N2
M-O
0.218
0.292
listed in Table 3, the bond lengths of Am-N1 (2.678 Å) and Am-
N2 (2.719 Å) are longer than those of Eu-N1 (2.649 Å) and Eu-
N2 (2.707 Å) in optimized ML(NO3)3 complexes, respectively,
but the differences between M-N1 and M-N2 bonds are
smaller than 0.03 Å. Taking into account that the ionic radii32
of Am3+ (0.98 Å) is larger than that of Eu3+ (0.95 Å), the bond
distances of Am-N are comparable to or even shorter than
those of Eu-N. This trend is also consistent with the M-N bond
in BTBP, BTPhen, TPEN, NTAamide, as well as the M-S bond in
Cyanex 301.6,36-39 The length of Am-O bond (2.486 Å) is about
0.045 Å longer than that of Eu-O bond (2.441 Å), showing that
hard O atom has comparable or even more affinity towards
Eu(III) than Am(III), which is similar to some O-donor ligands,
such as Syanex-272 and DGAs.7,9,37
The small values of calculated Wiberg bond indices (WBIs) of
M-N and M-O, in the range of 0.196–0.308, indicate that the
ionic and the electrostatic interaction play the dominant role
between M-N and M-O bonds. The WBIs also suggest that Am-
N bonds have a little more covalency than Eu-N bonds, while
Am-O bond is less covalent than Eu-O bond. The difference of
these covalency between the metal and the donor atoms
determines the selectivity in Am(III) and Eu(III) ions by
ligands.37,38 The natural charges of Am (1.439 e) are greater
than those of Eu (1.391 e). This may originated from the
stronger electron affinity of Eu, which may be caused by the
well-known lanthanide contraction and the weaker nucleus-
screening ability of the 4f shell.36 Furthermore, the natural
charge values of N (especially N2) atoms in AmL(NO3)3 is more
negative than those in EuL(NO3)3 complexes, and the absolute
charge difference of Am-L bonds seems to be larger than those
of Eu-L bonds.
The conclusions may shed light on the research of the
designing of improved PTA ligands, and contribute to the
improvement of structural modifications to develop novel
extractants for the separations of actinides from lanthanides in
the treatment and disposal of HLLW.
Acknowledgements
The present work is supported by the National Natural Science
Foundation of China (11079006), and the CAS Strategic Priority
Research Program (XDA03010402). The Key Laboratory for
Natural Medicines of Gansu Province, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, is
acknowledged for NMR and MS analyses.
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Table of Contents Entry
The extraction and complexation of Americium (III) and Europium (III) with three
N,N-dialkyl-1,10-Phenanthroline-2-Amides analogues were investigated with
theoretical calculations herein.