Highly Efficient Trivalent Americium/Europium Separation by Phenanthroline-Derived Bis(pyrazole) Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b00074

The synthesis, Eu³⁺ complexation, and solvent extraction of Am³⁺ and Eu³⁺ from nitric acid solutions by tetradentate phenanthroline-derived bis(pyrazole) (BPPhen) ligands were described. By using meta-nitrobenzotrifluoride as diluent, BPPhen ligands in combination with 2-bromohexanoic acid extracted Am³⁺ and Eu³⁺ with remarkably high efficiency, excellent selectivity, and fast extraction kinetics. Stripping posed no issues. The ligands also showed excellent hydrolytic stability and acid tolerance. 2-Bromohexanoic anion neutralized the charge and increased the lipophilicity of the extracted ion pair. The extraction conformed to a cation exchange model. Slope analysis demonstrated the extraction of 1:2 metal/ligand complexes. Analyses by electrospray ionization mass spectrometry, time-resolved laser-induced fluorescence spectroscopy, Raman, and Fourier transform infrared techniques indicated that the composition of the extracted species is [Eu(nOct-BPPhen)₂(H₂O)]³⁺. The formation of 1:2 complexes was also confirmed by UV-vis spectroscopic titration and microcalorimetric titration methods. Meanwhile, the stability constants (K) and the thermodynamic parameters (ΔH, ΔS, ΔG) for the complexation of Eu³⁺ with nOct-BPPhen were presented too.

Full text of DOI:10.1021/acs.inorgchem.8b00074

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Highly Efficient Trivalent Americium/Europium Separation by
Phenanthroline-Derived Bis(pyrazole) Ligands
Ying Liu, Xiuying Yang, Songdong Ding,* Zhipeng Wang, Lirong Zhang, Lianjun Song, Zhili Chen,
and Xueyu Wang
College of Chemistry, Sichuan University, Chengdu 610064, P. R. China
S
* Supporting Information
ABSTRACT: The synthesis, Eu3+ complexation, and solvent extraction of
Am3+ and Eu3+ from nitric acid solutions by tetradentate phenanthroline-
derived bis(pyrazole) (BPPhen) ligands were described. By using meta-
nitrobenzotrifluoride as diluent, BPPhen ligands in combination with 2-
bromohexanoic acid extracted Am3+ and Eu3+ with remarkably high
efficiency, excellent selectivity, and fast extraction kinetics. Stripping posed
no issues. The ligands also showed excellent hydrolytic stability and acid
tolerance. 2-Bromohexanoic anion neutralized the charge and increased the
lipophilicity of the extracted ion pair. The extraction conformed to a cation
exchange model. Slope analysis demonstrated the extraction of 1:2 metal/
ligand complexes. Analyses by electrospray ionization mass spectrometry,
time-resolved laser-induced fluorescence spectroscopy, Raman, and Fourier
transform infrared techniques indicated that the composition of the
extracted species is [Eu(nOct-BPPhen)2(H2O)]3+. The formation of 1:2
complexes was also confirmed by UV−vis spectroscopic titration and microcalorimetric titration methods. Meanwhile, the
stability constants (K) and the thermodynamic parameters (ΔH, ΔS, ΔG) for the complexation of Eu3+ with nOct-BPPhen were
presented too.
contrast to S-donor ligands, N-donor ligands not only have
better stability against acidity and oxidation but also are
composed of the elements C, H, O, and N only (CHON
principle), which is beneficial to the waste management,
because they are completely incinerable. Thus, more and more
attentions have been paid to the research and development of
N-donor ligands.
At present, the triazinyl-based N-donor ligands are identified
as promising candidates for the An/Ln separation, mainly
including 2,6-bis(5,6-dialkyl-1,2,4-triazin-3-yl) pyridines
(BTPs),6 6,6′-bis(5,6-dialkyl-1,2,4-triazin-3-yl)-2,2′-bipyridines
(BTBPs),7 and 2,9-bis(1,2,4-triazin-3-yl)-1,10-phenanthrolines
(BTPhens).8 These ligands were able to extract Am3+ and Eu3+
in a wide range of acidity from pH level to molar
concentrations of nitric acid solutions with good selectivity
(SFAm/Eu > 100). However, they suffered from the following
defects. First, the DAn values were usually so high that it
hindered the stripping (e.g., for the extraction from HNO3
solution into (0.01 mol/L CyMe4-BTPhen in n-octanol), DAm
≈ 1300 for 4.0 mol/L HNO3; DAm ≈ 17 for 0.001 mol/L
HNO3), unless it was performed by adding another solvent into
organic phase or using water-soluble complexing agents.8a,7c
Second, extraction kinetics is relatively slow in the absence of a
phase-transfer reagent such as N,N′-dimethyl-N,N′-dioctyl-2-
INTRODUCTION
■
Partitioning and transmutation strategy is very significant for
the sustainable development of nuclear energy. As for this
strategy, the long-lived radiotoxic minor actinides (An) need to
be partitioned from the bulk of lanthanides (Ln) prior to their
transmutation, because high neutron absorption cross sections
of certain Ln isotopes give rise to great hindrances to the An
transmutation.1 However, owing to the similar chemical
properties of An and Ln, their separation is difficult.2
It has been shown that the ligands containing soft S- or N-
donor atoms are capable of discriminating between An and
Ln.1c,3 The most important characteristics of ligands are the
distribution ratios of metal ions defined as the ratio of the
concentration of metal ions between organic and aqueous
phases (DM = [M]org/[M]aq) and the An/Ln separation factors
defined as the ratio of DAn to DLn (SFAn/Ln = DAn/DLn). The
extractability and the selectivity of ligands toward metal ions are
usually estimated by DM and SFAm/Eu values, respectively. The
most representative S-donor ligand is bis(2,4,4-trimethylpentyl)
dithiophosphinic acid (Cyanex 301), which was able to extract
Am3+ and Eu3+ with an unprecedented SFAm/Eu value of
∼5900.4 Nevertheless, it can only extract from relatively low
HNO3 (pH 3−4) solution owing to nondissociation and
oxidative degradation of the ligand at low pH.4,5 Besides, the
phosphorus-based Cyanex 301 would produce large volumes of
secondary waste upon incineration at the end its useful life. All
of these greatly limit its applications in industrial process. In
Received: January 9, 2018
© XXXX American Chemical Society
A
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Scheme 1. Synthesis Route of BPPhen Ligands
(2-hexyloxy-ethyl)malonamide (DMDOHEMA) and
N,N,N′,N′-tetraoctyl-diglycolamide (TODGA).9 Ultimately,
these defects render these ligands unsuitable for use in an
industrial process. Accordingly, further great efforts are
demanded to find more efficient N-donor ligands.
throline (BPPhen; Scheme 1) consisting of a phenanthroline
moiety and two pyrazol moieties were synthesized and
characterized. Their solvent extraction behavior toward Am3+
and Eu3+ in HNO3 solutions was investigated. Meanwhile, the
complexation behavior of ligand with Eu3+ was also examined
by electrospray ionization mass spectrometry (ESI-MS), time-
resolved laser-induced fluorescence spectroscopy (TRLFS),
Raman, Fourier transform infrared (FTIR), UV−vis spectro-
scopic titration, and microcalorimetric titration methods.
In recent years, a new kind of pyrazolyl-pyridine-based N-
donor ligand, such as 2,6-bis(5-alkyl-1H-pyrazol-3-yl)pyridine
(C5-BPP),10 1,3-bis[3-(2-pyridyl)pyrazol-1-yl]propane
(Bippp),11 and alkyl-substituted pyridylpyrazole (Cn-
PypzH)12 has received increasing attention in An/Ln
separation because of high selectivity, fast extraction kinetics,
and good chemical stability. In particular, C5-BPP exhibited
very good effectiveness. With 2-bromohexanoic acid as
lipophilic anion source and kerosene as diluent, C5-BPP can
give an SFAm/Eu value of ∼100 at 0.5 mol/L HNO3.10
Meanwhile, the extraction kinetics was very fast with
equilibrium for Am3+ and Eu3+ being reached after only 15
min.10b Nevertheless, a regular-SANEX process demonstration
test using C5-BPP as extractant showed that the stripping of
An3+ is incomplete by 0.8 mol/L HNO3.13 Although increasing
the acidity of stripping solution is good for the complete
stripping, this way is not feasible because of the formation of
precipitate at HNO3 concentration in excess of 1.0 mol/L.10,13
Consequently, the water-soluble complexing agents such as
(hydroxyethyl)ethylenediaminetriacetic acid (HEDTA), citric
acid,14 and glycolic acid15 must be used, which greatly limit the
viability of C5-BPP for process applications.
To perform the efficient stripping under highly acidic
conditions, it is necessary to improve the acid stability of
ligand. One rational method is to introduce a new moiety, for
example, phenanthroline into ligand structure. In fact, it has
been clearly shown that phenanthroline-based BTPhens have
much better adaptability to acidity than pyridine-based BTPs.
For instance, BTPhens can extract Am3+ and Eu3+ from 4.0
mol/L HNO3 solution,8a whereas BTPs only works from 1.0
mol/L HNO3 solution.6a From this point of view, it could be
expected that there is also better tolerance toward acidity by
replacing pyridine moiety of alkylated bis-pyrazolylpyridines
(BPPs) with a phenanthroline moiety. And the problem
troubling the stripping may be overcomed. Moreover, the
preorganized structure of phenanthroline moiety in the
molecule may improve extraction properties.8,16
EXPERIMENTAL SECTION
■
General. All reagents supplied by Aladdin were analytical grade or
chromatographic grade without purification unless otherwise stated.
Solutions of Eu(NO3)3 or Eu(ClO4)3 were prepared from Eu2O3
(99.99%, Aldrich) in dilute HNO3 or dilute HClO4, respectively. The
trace stock solutions of 241Am or 152Eu in 0.10 mol/L HNO3 solution
were provided by China Institute of Atomic Energy (CIEA).
UV−Vis spectra were performed with a PERSEE TU-1810
spectrophotometer. Microcalorimetric titration was conducted using
an isothermal titration calorimeter (MicroCal VP-ITC). TRLFS was
performed with an HORIBA TEMPRO-01. Raman spectra were
performed with an HORIBA LabRAM HR800. FTIR spectra were
recorded on a Nicolet Nexus 670 model instrument. NMR spectra
were recorded on a Varian Inova 400 MHz NMR spectrometer using
chloroform-d as solvent with tetramethylsilane as internal standard.
Mass spectra were obtained in the ESI mode using a liquid
chromatography−mass spectrometry ion trap time-of-flight (LCMS-
IT-TOF) spectrometer.
Synthesis. BPPhen ligands were synthesized according to Scheme
1. 1,10-Phenanthroline-2,9-dicarboxylic acid was prepared referring to
the literature.17
Synthesis of 1,10-Phenanthroline-2,9-dicarboxylic Acid Diethyl
Ester. A 3.9 g amount (15 mmol) of 1,10-phenanthroline-2,9-
dicarboxylic acid and concentrated H2SO4 (16 mL) were refluxed
for 12 h in absolute ethanol (EtOH; 150 mL). Then the solvent was
completely removed under reduced pressure, and the brown residue
was dissolved in dichloromethane (DCM; 200 mL). Subsequently, the
resulting solution was neutralized with 5% NaHCO3 aqueous solution
and washed three times with deionized water. Finally, the organic
phase was dried with anhydrous Na2SO4 overnight. The product was
obtained as the pale yellow solid with a pungent odor after distilling off
DCM. The pure substance was obtained by recrystallization from ethyl
acetate (EtOAc)/DCM/EtOH (v/v, 1:1:1). Yield: 3.5 g (11 mmol,
73%).
Synthesis of 1,1′-(1,10-Phenanthroline-2,6-diyl)bis(5,5-dialkane-
1,3-dione). A 7.4 mmol amount of alkan-2-one (iBu-BPPhen: 0.93 mL
of methyl isobutyl ketone; nBu-BPPhen: 0.92 mL of hexan-2-one;
nOct-BPPhen: 1.4 mL of decan-2-one) was dissolved in freshly
distilled tetrahydrofuran (THF, 30 mL), and then NaH (0.60 g, 60%
Our aim is to develop a new kind of phenanthroline-derived
bis(pyrazole) ligand. In this work, three novel tetradentate N-
donor ligands 2,9-bis(5-alkyl-1H-pyrazol-3-yl)-1,10-phenan-
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dispersion in paraffin oil, 15 mmol) was added gradually under an
argon atmosphere at room temperature. After 0.50 h, 1,10-
phenanthroline-2,9-dicarboxylic acid diethyl ester (1.0 g, 3.1 mmol)
was added gradually. The reaction was continued and refluxed for 12 h
followed by cooling to room temperature. The solvent was removed
under reduced pressure. And then, the 1:5 mixture of glacial acetic acid
and water (30 mL) was added to the burgundy viscous residue. After it
was filtered, washed to neutral with deionized water, and dried in
vacuo, a reddish-brown solid was obtained and used without further
purification.
Synthesis of 2,9-Bis(5-alkyl-1H-pyrazol-3-yl)-1,10-phenanthro-
line. The crude product obtained from the previous step reaction
and hydrazine hydrate (80% in H2O, 6.0 mL, a large molar excess)
were dissolved in absolute EtOH (25 mL) and refluxed at 70 °C for 5
h. After it cooled, the subsequent mixture was filtered and washed with
cold EtOH and then recrystallized from DCM/n-hexane (v/v, 5:1).
Characterization Data. nOct-BPPhen (0.71 g, 43%) FT-IR (KBr,
ν/cm−1): 3339, 3204, 2926, 2852, 1589, 1502, 1466, 1148, 994, 857,
799, 542. 1H NMR (400 MHz, CDCl3, ppm): δ 8.22 (s, 2H, Phen-H),
7.93 (s, 2H, Phen-H), 7.72 (s, 2H, Phen-H), 6.69 (s, 2H, pyrazolyl-H),
2.70 (s, 4H, NCH2), 1.71 (s, 2H, NCH2CH2), 1.28 (s, 20H,
NCH2CH2CH2CH2CH2CH2CH2CH3), 0.90 (s, 6H, CH3). ESI-MS
(m/z): Found: 559.3577 ([M + Na]+). Calcd: 559.3525 ([M + Na]+).
iBu-BPPhen (0.80 g, 50%) and nBu-BPPhen (0.75 g, 47%). The
detailed characterization data are listed in Section S1.
Solvent Extraction. The aqueous phases consisted of trace
amounts of 152Eu or 241Am plus a stable Eu(NO3)3 carrier in different
concentrations of HNO3 solutions. The organic phases were prepared
by dissolving the synthesized ligand and 2-bromohexanoic acid in
diluent. Equal volumes (1.0 mL) of the organic and aqueous phases
were stirred in a 5.0 mL round-bottom flask at 25 0.5 °C for 0.50 h
unless otherwise stated. Then the mixture was centrifuged to separate
phases. The activity of 241Am or 152Eu in each phase was separately
determined with a NaI(Tl) scintillation counter. All the extraction
experiments were performed in triplicate.
RESULTS AND DISCUSSION
Solvent Extraction. Prior screening tests had shown that
■
alone BPPhen ligands in common diluents such as toluene, n-
octanol, and F-3, hardly extract Am3+ and Eu3+ from HNO3
solution. Nevertheless, fortunately, the extractability can be
greatly enhanced in the presence of 2-bromohexanoic acid.
Besides, the influence of diluent on the extraction was also
evaluated (see Table S1). In F-3, nOct-BPPhen not only has
good solubility but also exhibits remarkably high efficiency and
selectivity. Consequently, the extraction behaviors of BPPhen
ligands toward Am3+ and Eu3+ were investigated by employing
F-3 as diluent and 2-bromohexanoic acid as lipophilic anion
source.
Influence of Contact Time. A drawback of many N-donor
ligands such as nPr-BTP and CyMe4-BTBP is their slow
extraction kinetics.6c,9a The extraction kinetics of BPPhen
ligands was evaluated with the results shown in Figure S10. The
investigation reveals that the extraction kinetics is very fast with
equilibrium reached within 15 min for both Am3+ and Eu3+. To
ensure the complete equilibrium, the following extraction
experiments were performed for 0.50 h.
Influence of HNO3 Concentration. The influence of HNO3
concentration on the extraction is shown in Figure 1. In the
Hydrolysis Study. A solution of 0.010 mol/L nOct-BBPhen and
1.0 mol/L 2-bromohexanoic acid in meta-nitrobenzotrifluoride (F-3)
was contacted with an equal volume of 5.0 mol/L HNO3 over 100 d.
The two phases were stirred for 1.0 h, followed by settling for 2.0 h.
This cycle was repeated one after another. Samples of the organic
phase were withdrawn at regular intervals for the extraction
experiments. The next steps were same as the solvent extraction
part described above. The resulting D and SF values were employed to
evaluate the hydrolytic stability of nOct-BPPhen ligand. In addition,
the organic phases before and after contacting with 10 mol/L HNO3
solution over a month were also analyzed by ESI-MS measurements.
UV−Vis Titration. The UV−vis spectroscopic titration was
Figure 1. Influence of HNO3 concentration on the extraction of Am3+
and Eu3+. Organic phase: 0.010 mol/L ligand and 1.0 mol/L 2-
bromohexanoic acid in F-3; aqueous phase: 1.0 mmol/L Eu(NO3)3
spiked with trace amount of 241Am or 152Eu in HNO3 solution.
performed at 25
0.2 °C in methanol medium. A 2.0 mL nOct-
BPPhen solution with an initial concentration of 2.0 × 10−5 mol/L was
added to a quartz cell of 1 cm path length, and an initial spectrum of
the ligand was recorded. For each titration, the 4.0 × 10−4 mol/L
Eu(NO3)3·6H2O solution was added into the cell in 10 μL aliquots
and shaken, and spectra from 205 to 400 nm were recorded up to the
variation in absorbance became negligible. Spectra obtained were fit
with HypSpec program.18 Spectroscopic experiments were run in
triplicate.
Microcalorimetric Titration. The calorimeter cell containing a
Eu(NO3)3 solution at a concentration of 5.0 × 10−5 mol/L was titrated
with 1.0 × 10−3 mol/L nOct-BPPhen solution. The heat of titrant
dilution (Qdil,j) was determined in a separate run. The net reaction
heat was obtained from the difference: Qr,j = Qex,j − Qdil,j (Qex,j is
resultant of heat released or absorbed during the formation of different
species involved). Raw data were acquired as a plot of heat rate (μJ/s)
versus time (s) and integrated to give a curve of enthalpy change per
mole of injected nOct-BPPhen (ΔH, kJ/mol) versus molar ratio of
reactants (nOct-BPPhen−Eu(NO3)3). The stability constants (K)
were obtained by fitting the data. The experiment was repeated three
times. The Gibbs free energy change (ΔG) was evaluated by using the
equation ΔG = −RT ln K, and then the entropy change (ΔS) was
calculated from the equation ΔS = (ΔH − ΔG)/T.
range of HNO3 concentrations examined (0.50−3.0 mol/L),
both DAm and DEu decrease with increasing acidity. This can be
mainly ascribed to the following two factors. On the one hand,
the ligands are likely to be protonated in contact with aqueous
phase of increasing acidity, resulting in the decrease of available
ligand concentration. On the other hand, the dissociation of 2-
bromohexanoic acid is suppressed under high acidity
conditions, leading to a weakening of synergistic effect.12 In
addition, the SFAm/Eu values decrease with increasing acidity,
which can be explained as that the slight difference in properties
between Am3+ and Eu3+ results in that DAm declines slightly
faster than DEu. Thus, SFAm/Eu values show a downward trend.
Furthermore, from Figure 1, it also can be observed that the
extractabilities of iBu- and nOct-BPPhen are very close and
slightly stronger than that of nBu-BPPhen under the same
experimental conditions. This phenomenon can be explained as
follows. On the one hand, iBu-BPPhen might have a little lower
steric hindrance than nBu-BPPhen, resulting in a favorable
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extraction. Similar result is also observed for BTP derivatives.6a
On the other hand, nOct-BPPhen, in which the steric hindrance
of n-octyl radical is obviously larger than that of n-butyl radical,
does not show a lower extractability than nBu-BPPhen. It is
likely that the greater lipophilic capacity of nOct-BPPhen
promotes extracted species into organic phase.19
For the An/Ln separation, the acidity of feed liquid issued
from the stripping after the coextraction of An and Ln by
CMPO, TODGA, or TRPO, etc.,20 is often in the range of 0.1−
0.5 mol/L HNO3.21 As a result, the influence of HNO3
concentration within this range on the extraction was
investigated particularly as shown in Figure S11. Very high
DAm values up to 1000 are observed, demonstrating that the
extraction of Am3+ by BPPhen ligands is very efficient. DEu
values are less than 10, resulting in high SFAm/Eu values with the
magnitude of 1 × 102. The extraction performances of BPPhen
ligands exhibited are better than most of other N-donor ligands
such as C5-BPP,10 BTBP,7a and BTPhen,8a suggesting that
BPPhens have a good application prospect in the An/Ln
separation.
In consideration of the solubility of ligand in organic phase as
well as the ligand loss in aqueous phase, nOct-BPPhen was
employed as an optimal ligand in the following extraction
experiments.
Influence of Ligand Concentration. Slope analysis was
employed to determine the number of nOct-BPPhen molecules
involved in the extracted metallic complexes. Figure 2 shows
Figure 3. Influence of 2-bromohexanoic acid concentration on the
extraction of Am3+ and Eu3+. Organic phase: 3.0 mmol/L nOct-
BPPhen and different concentrations of 2-bromohexanoic acid in F-3;
aqueous phase: 15 ppm Eu(NO3)3 spiked with trace amount of 241Am
or 152Eu in 0.01 mol/L HNO3 solution.
plot of log DM versus log[2-bromohexanoic acid] gives two
straight lines with slope values of ∼3, suggesting that three 2-
bromohexanoic acid molecules are involved in the extraction.
On the basis of the above results of slope analysis, the
extraction equilibrium equation can be expressed as follows:
2Lorg + 3HAorg + M3+ ⇌ [ML2]A3org + 3H+
aq
aq
(1)
where L, HA, and M denote ligand, 2-bromohexanoic acid, and
metal ions, respectively. The subscripts (org) and (aq)
represent organic phase and aqueous phase, severally. The
extraction conforms to a cation exchange model.
Stripping. Besides the excellent extraction properties, the
good stripping performances are also essential for a promising
extraction system. For this reason, stripping was investigated by
oxalic acid and nitric acid solutions, which are common
stripping agents.23 Results are listed in Table 1. It can be seen
Table 1. Stripping of Am3+ and Eu3+ from the Loaded
Organic Phase
stripping percentage, %
HNO3, mol/L
0.01 mol/L
oxalic acid
1.0
2.0
3.0
Figure 2. Influence of ligand concentration on the extraction of Am3+
and Eu3+. Organic phase: different concentrations of nOct-BPPhen
and 1.0 mol/L 2-bromohexanoic acid in F-3; aqueous phase: 1.0
mmol/L Eu(NO3)3 spiked with trace amount of 241Am or 152Eu in 1.0
mol/L HNO3 solution.
stripping
stage
Am3+ Eu3+ Am3+ Eu3+ Am3+ Eu3+ Am3+ Eu3+
I
0.12
0.20
0.31
0.40
4.4
6.7
7.8
15
79
97
68
93
100
100
a
84
95
100
a
II
III
IV
10
13
21
27
100
100
98
99
a
the influence of ligand concentration on the extraction of Am3+
and Eu3+. The plot of log DM versus log[nOct-BPPhen] gives
two straight lines with slope values of ∼2, indicating the
formation of 1:2 metal/ligand extracted complexes.
100
a
100
a
a
No metal ions detected in organic phase.
Influence of 2-Bromohexanoic Acid Concentration. As
lipophilic anion source, 2-bromohexanoic acid plays an
important role in the synergistic extraction process for many
neutral N-donor ligands.10,22 For a deeper understanding of its
role, the influence of 2-bromohexanoic acid concentration on
the extraction was also investigated. Considering that the
dissociation of 2-bromoalkanoic acids (pKa ≈ 2.5) may be
suppressed under high acidity conditions, the extraction
experiment was performed at a low acidity of 0.01 mol/L
HNO3.12,22a,c The results are demonstrated in Figure 3. The
that the stripping efficiency of nitric acid is far superior to that
of oxalic acid. And furthermore, the stripping percentage
increases with increasing HNO3 concentration, which agrees
with the influence of acidity on the extraction as shown in
Figure 1. Almost quantitative stripping of Am3+ and Eu3+ can be
achieved at 3−4 stages and at single stage, respectively, by 3.0
mol/L HNO3. It completely avoids employing water-soluble
complexing agents.
Hydrolysis Stability and Acid Tolerance. A recycling
option in process applications for acidic waste treatment
D
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Figure 4. ESI-MS spectra of the organic phases before (a) and after (b) contacting with 10 mol/L HNO3 solution over a month in the range of m/z
= 100−800.
Figure 5. Positive ESI mass spectrum of the Eu-loaded organic phase, where 5.0 mmol/L nOct-BPPhen and 1.0 mol/L 2-bromohexanoic acid in F-3
was contacted with 1.0 mmol/L Eu(NO3)3 dissolved in 0.10 mol/L HNO3. (inset) Computer simulation of the isotope distribution pattern of
[Eu(nOct-BPPhen)2H−1]2+.
requires that the ligands should have an adequate hydrolytic
stability and good acid tolerance. Thereby, the stability of nOct-
BPPhen was studied. The results are shown in Figure S12. It is
found that the DAm, DEu, and SFAm/Eu values remain at the initial
level with no observable changes, suggesting excellent hydro-
lytic stability of nOct-BPPhen on long-term (100 d) exposure
to 5.0 mol/L HNO3.
In general, the N-donor ligands such as n-alkylated BTPs and
C5-BPP have an unsatisfactory resistance against nitric acid.24,10
To estimate well nOct-BPPhen’s acid tolerance, the acidity of
aqueous phase was elevated to 10 mol/L, which is high enough
for an An/Ln separation process. The ESI-MS spectra of the
organic phases before and after contacting with HNO3 solution
are displayed in Figure 4. The peak at m/z = 537.3670 observed
in the spectra is attributed to nOct-BBPhen associated with H+.
The intensity of this peak is thought to be unchanged after 10
mol/L HNO3 treatment over a month. It is quite different from
n-alkylated bis(triazine) ligands such as n-Pr-BTP that are
almost completely destroyed in contact with 1.0 mol/L HNO3
over several days.25 In addition, no precipitate formed in the
sample under this condition. These results indicate that nOct-
BPPhen is robust resisting degradation resulting from acidic
conditions.
Complexation of Eu3+ with nOct-BPPhen. The complex-
ation behavior of nOct-BPPhen with Eu3+ was investigated in
biphasic and monophasic systems, respectively. In view of the
similarities in the chemical properties of An and Ln, Eu3+ was
taken as the surrogate of Am3+, and the results of the Eu3+
subsequent complexation studies can be extended for the Am3+
system as well.
E
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ESI-MS Analysis. To further identify the extracted species,
ESI-MS analysis of the Eu-loaded organic phase was performed.
Figure 5 shows the positive ESI spectrum of the Eu-loaded
organic phase. The peak at m/z 612.3180 is assigned to the 1:2
complexes [Eu(nOct-BPPhen)2H−1]2+, as confirmed by the
simulation of the corresponding isotopic clusters. This result is
in good agreement with that of slope analysis.
corresponding to this m/z value. All of this leads to the
−
conclusion that NO3 is absent from the inner sphere of the
central Eu3+ in the complex.
Because of the complexity of these organic phases, it is
difficult to discriminate their FTIR spectra. But fortunately
there is a new absorption peak at ∼3237 cm−1 in the spectra of
the Eu-loaded organic phases. It can be assigned to the
stretching vibration of coordinated H2O (the absorption peak
of free H2O at 1730 cm−1), suggesting that H2O might appear
in the inner coordination sphere of the central Eu3+ in the
complex.28
TRLFS Analysis. Further investigation was performed to
determine the number of H2O molecules in the inner sphere of
Eu3+ by TRLFS analysis method. On the basis of a linear
correlation between fluorescence lifetime and number of H2O
molecules in the inner coordination sphere existing (eq 2), the
hydration number can be calculated from the fluorescence
lifetime with an experimental uncertainty of 0.5.29
Generally speaking, trivalent f-elements in their complexes
with tri- or tetradentate N-donor ligands exhibit coordination
numbers of 8, 9, or 10.6b,7d,8a According to the aforesaid results
of ESI-MS and slope analyses, the formation of 1:2 Eu3+/nOct-
BPPhen complexes means that two tetradentate ligands bind
one Eu3+ through eight nitrogen atoms, leaving the
26
−
coordination site to be occupied with either H2O or NO3 .
−
To confirm whether there is H2O or NO3 taking part in the
coordination, Raman, FTIR, and TRLFS analyses were
performed. Meanwhile, the comparison between nitrate and
perchlorate systems was also made, because perchlorate ions
have lower affinity for Eu3+ than nitrate ions.27
n(H2O) = 1.05/τ − 0.70
(2)
Raman and FTIR Spectra Analyses. The Raman and FTIR
spectra of organic phases (pristine, after contacting with HNO3
or HClO4 solution, and Eu-loaded organic phases) are shown
in Figure S14 and Figure 6, respectively. The Raman spectra
The fluorescence decay curves for the Eu-loaded organic
phases prepared from nitrate and perchlorate systems are
shown in Figure 7. The decay curves were fitted with single-
Figure 7. Fluorescence decay curves of Eu3+ after extraction to an
organic phase of 5.0 mmol/L nOct-BPPhen and 1.0 mol/L 2-
bromohexanoic acid in F-3 from an aqueous phase of 0.10 mol/L
HNO3 (a) or HClO4 (b).
Figure 6. FTIR spectra of the various organic phases containing nOct-
BPPhen and 2-bromohexanoic acid in F-3 (initial conditions, organic
phase: 0.10 mol/L nOct-BBPhen and 1.0 mol/L 2-bromohexanoic
acid in F-3; aqueous phase: 0.05 mol/L Eu3+ in the solution of 0.1
mol/L HNO3 or HClO4). Pristine organic phase (a); organic phase
after contacting with 0.10 mol/L HNO3 (b); organic phase after
contacting with 0.10 mol/L HClO4 (c); Eu-loaded organic phase
prepared from nitrate system (d); Eu-loaded organic phase prepared
from perchlorate system (e).
exponential functions to obtain the fluorescence lifetime (τ, in
ms). τ was found to be 0.565 ms for nitrate system and 0.572
ms for perchlorate system. According to eq 2, the hydration
numbers were calculated to be 1.2 and 1.1, respectively,
indicating the existence of a H2O molecule in the inner sphere
of the central Eu3+ in the complex. However, the aforemen-
tioned result of ESI-MS analysis indicates that there is no H2O
in the extracted species responding to the molecular ion peak at
m/z 612.3180 for Eu3+. This might be explained by a loss of the
coordinated H2O in the process of ionization during MS
measurement due to the very weak coordination bond between
the central Eu3+ and H2O in the extracted complexes.28
On the basis of the above results, it is inferred that the inner-
coordination sphere of the extracted 1:2 complexes of Eu3+ may
comprise two nOct-BPPhen ligands and a H2O molecule.
UV−Vis Titration. For a deep insight into the complexation
behavior of nOct-BPPhen with Eu(NO3)3, UV−vis spectro-
scopic titration was performed in methanol medium (see Figure
8). With the addition of Eu(NO3)3, the absorption at 313 nm
show obvious differences between the Eu-loaded organic phases
and the Eu-unloaded organic phases, indicating that Eu3+ is
extracted into organic phase. Meanwhile, the shape of the
Raman spectrum of the Eu-loaded organic phase prepared from
nitrate system is almost the same as that prepared from
perchlorate system. And the characteristic band of NO3− in the
range of 1480−1650 cm−1 was not observed, suggesting that
−
NO3 might not appear in the extracted species. Moreover, if
−
there were a NO3 in the inner coordination sphere, the peak
assigned to [Eu(nOct-BPPhen)2(NO3)]2+ should appear at m/
z = 643.8165 (calculated from molecular weight calculator
program, http://www.alchemistmatt.com) in the ESI-MS
spectrum (Figure 5) due to a strong coordination ability of
21
−
NO3 with Eu3+. But in fact, there is no the peak
F
DOI: 10.1021/acs.inorgchem.8b00074
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. UV−Vis spectroscopic titration of nOct-BPPhen with
Eu(NO3)3·6H2O in CH3OH (initial conditions, [ligand] = 2.0 × 10−5
mol/L, volume = 2.0 mL; titration conditions, [Eu(NO3)3·6H2O] =
4.0 × 10−4 mol/L).
shifts to ∼330 nm. Isosbestic point is observed at 274 nm,
revealing the presence of at least two species in solution.
However, with further additions of Eu(NO3)3, to give a
stoichiometric excess of Eu(NO3)3, there is no significant
change in the shape of the spectral profile, suggesting an
equilibrium being established between two possible species of
1:1 and 1:2 metal/ligand. HypSpec program was used to model
the spectra. The best fits were given with consideration of the
formation of both 1:1 and 1:2 metal/ligand species, confirming
that [Eu(nOct-BPPhen)]3+ and [Eu(nOct-BPPhen)2]3+ occur
over the conditions used in the titrations. This result is in good
accordance with that of experimental observations regarding
the extracted species. In solvent extraction experiment, nOct-
BPPhen was in excess. Nevertheless, the 1:1 species is likely to
be forced to form with excess Eu3+ present. Similar behavior has
been observed by Whittaker.30
Microcalorimetric Titration. Calorimetry titration is the
technique that can not only be used to directly measure the
enthalpy of complexation but also can determine the stability
constants. The complexation of Eu3+ with nOct-BPPhen was
studied in methanol medium using microcalorimetry. Figure 9
shows the data from microcalorimetric titration of Eu(NO3)3
with nOct-BPPhen. The upper panel exhibits the micro-
calorimetric heat flow trace. With additional nOct-BPPhen
injections into the Eu(NO3)3 solution, the concentration of free
Eu3+ decreases, and the reaction gives a smooth exothermic
decrease. The lower panel shows that the titration plot can be
fitted well.
Thermodynamic parameters and the stability constants are
listed in Table 2. In the case of ML1, both ΔH and ΔS are
positive, while ΔG is negative, indicating that the complexation
process is driven by relatively large entropy changes. This large
positive entropy of complexation is likely to arise from the
release of solvent molecules from the inner coordination sphere
of Eu3+ and the ligand during the complexation. Besides, high
endothermic enthalpy and large positive entropy change
indicate a poor stereochemical conformation of the ML1
complexes, which ultimately gives relatively weak complexation
with Eu3+. However, the reaction for ML2 was characterized by
strongly negative and favorable ΔH overcoming negative
unfavorable entropy term (TΔS < 0). In other words, it is
Figure 9. Microcalorimetric titration of nOct-BPPhen with Eu(NO3)3
in methanol. (initial conditions, [Eu(NO3)3·6H2O] = 5.0 × 10−5 mol/
L; titration conditions, [ligand] = 1.0 × 10−3 mol/L). (upper)
Microcalorimetric trace. (lower) Integrated enthalpy value (round)
and fitted result (line) of the reaction.
Table 2. Stability Constants and Thermodynamic
Parameters for the Complexation of Eu3+ with nOct-
a
BPPhen
metal
ion
ΔG
ΔS
species
log K
(kJ/mol)
ΔH (kJ/mol)
(J/mol/K)
Eu3+
ML1
ML2
4.52 0.02
7.10 0.01
−25.8
−40.1
57.2 5.36
278
−107 5.36
−224
a
In methanol at 25 °C.
favorable to form ML2 species in the view of complexation
energetics. This could be attributed to the fact that the rigidity
and the juxtaposition of donor atoms are benefit for metal
ligation.31,32 Similar phenomena have also been observed on
the microcalorimetric titration of CyMe4-BTPhen with Eu-
(NO3)3.32
CONCLUSIONS
■
BPPhen ligands were designed to combine high selectivity for
complexation of Am3+ over Eu3+, fast extraction kinetics, easy
stripping, good hydrolytic stability, and acid-tolerance proper-
ties. Slope analysis indicated the formation of 1:2 metal/ligand
extracted species, which is consistent with the conclusion drawn
from ESI-MS analysis, and this result was also confirmed by
UV−vis spectroscopic titration and microcalorimetric titration.
Besides, TRLFS analysis indicated the presence of a H2O
G
DOI: 10.1021/acs.inorgchem.8b00074
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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molecule in the inner coordination sphere of the central Eu
cation in the complex. Furthermore, the large exothermic
enthalpy and the significant decrease in the degree of disorder
for 1:2 metal/ligand species suggested a stronger complex
formation due to a favorable preorganized structure of
phenanthroline moiety in the molecule. Although 2-bromohex-
anoic acid complicates the extraction system and makes
problems with managing the secondary waste as being not
the CHON compound, this work provides significant
information on the extraction performances and the species
relating to the extraction process. The obtained results
contribute to a better understanding of the model of extraction
at molecular level and are also of major importance for
designing new and more efficient ligands derived from BPPhens
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ASSOCIATED CONTENT
* Supporting Information
■
S
(7) (a) Retegan, T.; Ekberg, C.; Dubois, I.; Fermvik, A.; Skarnemark,
G.; Wass, T. J. Extraction of Actinides with Different 6,6′-Bis (5,6-
Dialkyl-[1,2,4]-Triazin-3-yl)-[2,2′]-Bipyridines (BTBPs). Solvent Extr.
Ion Exch. 2007, 25, 417−431. (b) Foreman, M. R.; Hudson, M. J.;
Drew, M. G.; Hill, C.; Madic, C. Complexes formed between the
quadridentate, heterocyclic molecules 6,6′-bis-(5,6-dialkyl-1,2,4-triazin-
3-yl)-2,2′-bipyridine (BTBP) and lanthanides(III): implications for the
partitioning of actinides(III) and lanthanides(III). Dalton Trans. 2006,
13, 1645−1653. (c) Modolo, G.; Wilden, A.; Daniels, H.; Geist, A.;
Magnusson, D.; Malmbeck, R. Development and demonstration of a
new SANEX Partitioning Process for selective actinide(III)/
lanthanide(III) separation using a mixture of CyMe4-BTBP and
TODGA. Radiochim. Acta 2013, 101, 155−162. (d) Narbutt, J.;
Oziminski, W. P. Selectivity of bis-triazinyl bipyridine ligands for
americium(III) in Am/Eu separation by solvent extraction. Part 1.
Quantum mechanical study on the structures of BTBP complexes and
on the energy of the separation. Dalton Trans. 2012, 41, 14416−
14424.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00074.
Characterization data of all compounds, additional
extraction experiments, as well as ESI-MS analysis of
complexes (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: dsd68@163.com.
■
ORCID
Songdong Ding: 0000-0003-4949-3080
Notes
The authors declare no competing financial interest.
(8) (a) Lewis, F. W.; Harwood, L. M.; Hudson, M. J.; Drew, M. G.
B.; Desreux, J. F.; Vidick, G.; Bouslimani, N.; Modolo, G.; Wilden, A.;
Sypula, M.; Vu, T.-H.; Simonin, J.-P.; et al. Highly efficient separation
of actinides from lanthanides by a phenanthroline-derived bis-triazine
ligand. J. Am. Chem. Soc. 2011, 133, 13093−13102. (b) Williams, N. J.;
Dehaudt, J.; Bryantsev, V. S.; Luo, H.; Abney, C. W.; Dai, S. Selective
separation of americium from europium using 2,9-bis(triazine)-1,10-
phenanthrolines in ionic liquids: a new twist on an old story. Chem.
Commun. 2017, 53, 2744−2747.
ACKNOWLEDGMENTS
■
The authors are sincerely grateful for the financial support by
the National Natural Science Foundation of China (Grant Nos.
11675115, 91126016, and 91426302). The comprehensive
training platform of specialized laboratory (College of
Chemistry, Sichuan Univ.) is highly acknowledged for
providing analytical tests. The authors are grateful to Prof. Y.
Su from the Analytical & Testing Center of Sichuan Univ. for
providing the analysis of Raman spectra and to Prof. L. Yuan
from Institute of High Energy Physics (Chinese Academy of
Sciences) for processing spectroscopic titration data and giving
helpful suggestions.
(9) (a) Geist, A.; Hill, C.; Modolo, G.; Foreman, M. R.; Weigl, M.;
Gompper, K.; Hudson, M. J. 6,6′-Bis(5,5,8,8-tetramethyl-5,6,7,8-
tetrahydro-benzo[1,2,4] triazin-3-yl)[2,2′]bipyridine, an Effective
Extracting Agent for the Separation of Americium(III) and Curium-
(III) from the Lanthanides. Solvent Extr. Ion Exch. 2006, 24, 463−483.
(b) Serrano-Purroy, D.; Baron, P.; Christiansen, B.; Malmbeck, R.;
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de(III) Separation using a Mixture of CyMe4-BTBP and TODGA as a
Selective Extractant, Actinide and Fission Product Partitioning and
Transmutation; 10th Information Exchange Meeting, Mito, Japan, Oct
6−10, 2009. NEA No. 6420, Organisation for Economic Co-
Operation and Development-Nuclear Energy Agency: France, 2010;
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DOI: 10.1021/acs.inorgchem.8b00074
Inorg. Chem. XXXX, XXX, XXX−XXX