Selective Extraction of Americium(III) over Europium(III) Ions with Pyridylpyrazole Ligands: Structure–Property Relationships

Article abstract of DOI:10.1002/ejic.201601011

To clarify the structure–property relationships of pyridylpyrazole ligands and provide guidance for the design of new and more efficient ligands for the selective extraction of actinides over lanthanides, a series of alkyl-substituted pyridylpyrazole ligands with different branched chains at different positions of the pyrazole ring were synthesized. Extraction experiments showed that the pyridylpyrazole ligands exhibited good selective extraction abilities for Am^IIIions, and the steric effects of the branched chain had a significant impact on the distribution ratios of Am^IIIand Eu^IIIions as well as the separation factor. Moreover, both slope analyses and UV/Vis spectrometry titrations indicated the formation of a 1:1 complex of 2-(1-octyl-1H-pyrazol-3-yl)pyridine (C8-PypzH) with Eu^IIIions. The stability constant (log K) for this complex obtained from the UV/Vis titration was 4.45 ± 0.04. Single crystals of the complexes of 3-(2-pyridyl)pyrazole (PypzH) with Eu(NO₃)₃and Sm(NO₃)₃were obtained; PypzH acts as a bidentate ligand in the crystal structures, and the N atom with a bound H atom did not participate in the coordination. In general, this study revealed some interesting findings on the effects of the alkyl-chain structure and the special complexation between pyridylpyrazole ligands and Ln^IIIions.

Full text of DOI:10.1002/ejic.201601011

A Journal of
Accepted Article
Title: Selective Extraction of Am(III) over Eu(III) with Pyridylpyrazole Ligands:
Structure-properties Relationship
Authors: Dongping Su; Ying Liu; Shimeng Li; Songdong Ding; Yongdong Jin; Zhi-
peng Wang; Xiaoyang Hu; Lirong Zhang
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European Journal of Inorganic Chemistry
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Selective Extraction of Am(III) over Eu(III) with Pyridylpyrazole
Ligands：Structure-properties Relationship
Dongping Su, Ying Liu, Shimeng Li, Songdong Ding,* Yongdong Jin,* Zhipeng Wang, Xiaoyang Hu,
and Lirong Zhang
Abstract: To clarify the structure-properties relationship of
pyridylpyrazole ligands and provide guidance to design new and
more efficient ligands for the selective extraction of actinides over
lanthanides, a series of alkyl-substituted pyridylpyrazole ligands with
different branched-chain at different position of pyrazole ring were
soft N or S atom than Ln(III). In other words, An(III)/Ln(III)
separation can be carried out by N– or S– donor ligands.[7,8]
Over the years, some N– and S– donor ligands have been
developed for An(III)/Ln(III) separation, such as 2,6-bis(5,6-
dialkyl-1,2,4-triazin-3-yl)pyridine (BTP),[9–11] 6,6′-bis(5,6-dialkyl-
1,2,4-triazin-3-yl)-2,2′-bipyridine (BTBP),[12] 2,9-bis(1,2,4-triazin-
synthesized.
The
extraction
experiments
showed
that
pyridylpyrazole ligands exhibited good selective extraction ability for
Am(III) and the steric effects of branched-chain had a significant
impact on the distribution ratio of Am(III), Eu(III) as well as the
separation factor. Moreover, both slope analyses and UV–vis
spectrometry titration indicated the formation of 1:1 complexes of
C8-PypzH with Eu(III). And the stability constant (log K) for this
complex obtained from UV–vis titration was 4.45 ± 0.04. Meanwhile,
the single crystals of PypzH with Eu(NO3)3 and Sm(NO3)3 were
obtained and in the crystal structures PypzH acted as a bidentate
ligand and the N atom combining with H atom did not participate in
the coordination. In general, this study exhibited some interesting
findings on the effect of alkyl chain structure as well as the special
complexation between pyridylpyrazole ligands and Ln(III).
3-yl)-1,10-phenanthroline
pentyl)dithiophosphinic
(BTPhen),[13]
acid,[14,15]
bis(2,4,4-trimethyl-
and
bis(chlorophenyl)dithiophosphinic acid.[16,17] These ligands all
exhibited good separation efficiency for An(III) over Ln(III), but a
persistent problem about radiolytic and hydrolytic instability
made them not meet the needs of practical application.
Consequently, it is very necessary to seek more efficient ligands
for An(III)/Ln(III) separation.
Compared with S– donor ligand, N– donor ligand has better
chemical and irradiation stability. It consists only of C, H, O and
N elements and could be totally incinerated to gas products
without any sulfur-bearing residual wastes which are difficult to
be managed. Thus, N– donor ligand has become the research
focus for nearly two decades.[18–20]
To learn more about the extraction and complexation
properties of BTP-type ligands on molecular level, some
changes were performed on the strcuture of BTP-type ligands.
Recently, a type of trident N-ligands of 2,6-bis-triazolyl-pyridines,
exhibited particular selectivity for Am. Kiefer et al. examined the
complexation behaviors of 2,6-bis(1-(ptolyl)-1H-1,2,3-triazol-4-
yl)pyridine (BTTP) with Eu(III), Sm(III) and Cm(III) by means of
single-crystal X-ray diffraction and TRLFS.[21] Afterwards,
Macerata et al. found that hydrophilic clicked 2,6-bis-triazolyl-
Introduction
High level liquid waste (HLLW) issued from the PUREX process
contains a bulk of fission products and minor actinides (MA). It is
very hazardous and poses long-term harm to the environment,
[1,2] which is predominately produced from MA. In order to reduce
the environmental risks, one rational treatment way is to convert
MA into short lived or stable elements through transmutation
technology using high-energy neutrons from fast reactors or
accelerator driven system.[3,4] However, the accompaniment of
abundant lanthanides (Ln) with strong ability to absorb neutrons
can prevent the transmutation of MA.[5] Therefore, the separation
of An over Ln is very essential for HLLW treatment.
Nevertheless, owing to the similar chemical characters, same
oxidation state of +3 and similar ionic radii of Ln and An, their
separation is so difficult as to be considered as one of the most
challenging tasks. Even so, it is still possible to achieve a mutual
separation according to the slight differences between An(III)
and Ln(III). In comparision with Ln(III), the 5f, 6d, 7s and 7p
orbitals of An(III) all show similar spatial extensions, giving rise
to a 5f covalent contribution to the ionic bonding of An(III).[6]
Thus, An(III) are easier to coordinate with the ligands containing
pyridines could selectively strip the actinides from
a
diglycolamide-containing kerosene solution into an acidic
aqueous phase.[22] These findings provided new informations
pertaining to the relationship between molecular structure and
extraction for An(III)/Ln(III) separation.
Furthermore, it was also found that a new kind of N-donor
extractants composed of pyridine and pyrazole had good
separation efficiency for An(III) over Ln(III). Bremer et al.
investigated the extraction behaviors of Am(III) and Eu(III) from
HNO3 solution with 2,6-bis(5-(2,2-dimethyl-propyl)-1H-pyrazol-3-
yl)pyridine (C5-BPP, Fig. 1).[23,24] In the examined acidity range
of 0.1−1.0 mol/L, separation factor SFAm/Eu could reach around
100 in the case of the extraction of Am(III) and Eu(III) by 0.01
mol/L C5-BPP and 0.5 mol/L 2-bromohexanoic acid in kerosene.
Besides, the particular coordination mode of trivalent actinides
and lanthanides with C5-BPP is worth exploring at a deeper
level. In comparison with BTPs, there are a fewer numbers of N
atoms in pyridylpyrazole ligands, suggesting higher chemical
and irradiation stability. However, it is unfortunate that there are
no systematic studies on this new kind of extractant until now.
Further researches on pyridylpyrazole ligands are also
imperative.
D. Su, Y. Liu, S. Li, Prof. S. Ding, Prof. Y. Jin, Z. Wang, X. Hu, L.
Zhang.
Department of chemistry
Sichuan University, Chengdu, 610064, China
Fax: (+86) 028-85410259
E-mail: dsd68@163.com; jinyongdong@scu.edu.cn
Http://chem.scu.edu.cn/Zh/DingSongDong
Not long ago, we reported the extraction behaviors for Am(III)
and Ln(III) by two tetradentate pyridylpyrazole ligands: 1,3-bis[3-
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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(2-pyridyl)pyrazol-1-yl)propane (Bippp, Figure 1) and 1,2-bis[3-
(2-pyridyl)pyrazyl-1-methyl]benzene (Dbnpp, Figure 1).[25] In
terms of the chemical structures, the two pyridylpyrazole groups
are linked by an aliphatic and aromatic bridge for Bippp and
Dbnpp respectively. In the presence of 1.0 mol/L 2-
bromohexanoic acid, 0.2 mol/L Bippp or Dbnpp in tert-butyl
benzene gave the maximum SFAm/Eu value of only more than 20
at pH 1.96, which is much lower than that of C5-BPP. These
results clearly showed that the structure of pyridylpyrazole
ligands has a great influence on their extraction properties.
Solvent extraction
Using 0.5 mol/L 2-bromohexanoic acid as lipophilic anion source
and TBB as diluent, Table 1 listed the distribution ratios and
separation factors of Am(III) and Eu(III) for all pyridylpyrazole
ligands. Compared with Cn-PypzH liangd, Cn-Pypz ligand
exhibited really poor extraction ability towards both Eu(III) and
Am(III), suggesting that the branched-chain on the 2-position of
pyrazole ring would cause serious steric hindrance for the
extraction. Nevertheless, for all Cn-PypzH ligands, DAm and DEu
increased just slightly with decreasing branched-chain length,
indicating that the length of the branched-chain had no obvious
effect on the extraction ability of three Cn-PypzH ligands.
As for Cn-Pypz ligands, the extraction ability varies in the
order of C2-Pypz > C8-Pypz > C4-Pypz and C4-Pypz fared the
worst. That is probably because the steric effect of n-butyl
substituent group just prevented the coordination to a great
extent and the steric hindrance of n-octyl or ethyl were too large
or too small to hinder coordination like n-butyl. Meanwhile, in the
examination of acidity range, PypzH had the highest DM values
and the lowest SFAm/Eu values. This implies that the introduction
of suitable branched-chain is good for the separation of Am(III)
over Eu(III).
Figuring
out
the
structure-properties
relationship
of
pyridylpyrazole ligands will be good for developing new
pyridylpyrazole based extractants with better performance for
An(III)/Ln(III) separation in the future.
In the present paper, for the sake of simplicity, we chosen 3-
(2-pyridyl)pyrazole (PypzH; Scheme 1) as the model compound
to investigate extraction and complexation behaviors for Am(III)
and Eu(III). However, it is highly preferable to dissolve the
extractants in some paraffinic diluents that could extract the
Ln(III) and An(III) from acidic solutions,[26] the introduction of
branched-chain onto the heterocycle to promote their solubility is
particularly necessary. But systematic knowledge about the
steric effects of different branched-chain in pyrazolyl ring on their
extraction properties is still missing. Therefore, we designed and
synthesized two types of alkyl-substituted pyridylpyrazole
ligands. One is Cn-PypzH, which the branched-chains are at the
3-position, the other one is Cn-Pypz and the branched-chains
are at the 2-position (n = 2, 4, 8). Using a synergistic system of
these ligands and 2-bromohexanoic acid in tert-butyl benzene, a
comparative study about steric effects of the branched-chain on
the selective extraction of Am(III) and Eu(III) was studied from
HNO3 solution. In the mean time, the complexation between
ligand and metal ions were also investigated by slope analyses,
crystal structure and UV-vis titration.
Besides that, the maximum SFAm/Eu could reach 121 at pH
2.48 for C8-PypzH, showing an excellent extraction selectivity
for Am(III). In order to understand more about the extraction
properties of pyridylpyrazole ligands, an intensive study on the
extraction of Am(III) and Eu(III) was carried out by using C8-
PypzH as extractant and kerosene as diluent.
Table 1 Influence of HNO3 concentration on the extraction of Eu(III) and
Am(III). Org. phase: 0.05 mol/L Ligand, 0.5 mol/L 2-bromohexanoic acid in
TBB; Aq. phase: 0.1 mmol/L Eu(NO3)3 or tracer amount of 241Am in HNO3.
Ligand
PypzH
pH
DAm
DEu
SFAm/Eu
2.91
2.48
1.96
119 ± 7
51.8 ± 1.2
13.5 ± 1.3
17.0 ± 0.9
10.1 ± 1.0
6.87 ± 0.93
7.0
5.1
2.0
2.91
2.48
1.96
17.7 ± 0.8
7.28 ± 0.41
1.67 ± 0.21
0.96 ± 0.04
0.06 ± 0.01
0.05 ± 0.01
18
120
33
C8-PypzH
C4-PypzH
C2-PypzH
C8-Pypz
C4-Pypz
C2-Pypz
2.91
2.48
1.96
18.0 ± 1.0
8.39 ± 0.26
1.68 ± 0.24
1.10 ± 0.13
0.12 ± 0.02
0.06 ± 0.02
16
70
28
2.91
2.48
1.96
23.7 ± 1.6
12.4 ± 0.8
3.50 ± 0.25
2.33 ± 0.16
0.49 ± 0.08
0.20 ± 0.04
10
25
18
2.91
2.48
1.96
0.97 ± 0.10
0.55 ± 0.05
0.23 ± 0.04
0.22 ± 0.02
0.14 ± 0.03
0.05 ± 0.02
4.4
3.9
4.6
Figure
1 Chemical structures of some typical ligands for An(III)/Ln(III)
2.91
2.48
1.96
0.48 ± 0.05
0.23 ± 0.02
0.03 ± 0.01
0.07 ± 0.02
0.03 ± 0.06
0.01 ± 0.02
6.9
7.7
3.0
separation.
2.91
2.48
1.96
2.24 ± 0.14
1.49 ± 0.10
0.45 ± 0.04
0.18 ± 0.02
0.09 ± 0.02
0.05 ± 0.01
12
17
9.0
Results and Discussion
The influence of HNO3 concentration on the extraction of
Am(III) and Eu(III) by C8-PypzH was shown in Figure 2. With the
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European Journal of Inorganic Chemistry
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increase of HNO3 concentration, both DAm and DEu decreased
due to the protonation of the ligand and the inability of 2-
bromohexanoic acid to dissociate. In the case of kerosene as
diluent, C8-PypzH could extracted Am(III) from 0.01 mol/L HNO3
with maximum SFAm/Eu about 45. This maximum value is not as
good as that in TBB at this experimental condition, however, this
result is still satisfying owing to using the paraffinic solvent as
diluent.
The dependence of DAm and DEu on the total concentration of
C8-PypzH was illustrated in Figure 3. The plot of logDM vs
log[ligand]total gave two straight lines with an approximate slope
value of 1, revealed that 1:1 complexes of both Eu(III) and
Am(III) with C8-PypzH were dominantly existed in the extraction.
In addition, the extraction of RE(III) observed with C8-PypzH
was also presented in Figure 4. It can be seen that C8-PypzH
could selectively extract Am(III) from RE(III) efficiently with
SFAm/RE ranged from 40 to 71. That is to say, C8-PypzH exhibits
good selectivity for Am(III) over RE(III).
Figure 2 Influence of HNO3 concentration on the extraction of Am(III) and
Eu(III). Org. phase: 0.05 mol/L C8-PypzH and 0.5 mol/L 2-bromohexanoic acid
in kerosene; Aq. phase: 0.1 mmol/L Eu(NO3)3 or tracer amount of 241Am in
HNO3.
Figure 4 Selective extraction of Am(III) over RE(III) by C8-PypzH. Org. phase:
0.05 mol/L C8-PypzH and 0.5 mol/L 2-bromohexanoic acid in kerosene; Aq.
phase: 0.1 mmol/L of each RE(III) or tracer amount of 241Am in 0.01 mol/L
HNO3 solution.
In order to determine the number of extractant molecules in
the extracted species, slope analysis was performed. By
referring to the method in our previous work,[25,27,28] the
regression plot of logDM vs log[ligand]total can give a line. The
slope value of the straight line is just right the coordination
number between ligand and metal ion.
Stability Constant Determination
The complexation behavior of C8-PypzH with Eu(III) was
investigated
in
CH3CN
by
UV-vis
titration.
The
spectrophotometric titrations of C8-PypzH with Eu(III) was
shown in Figure 5. All the spectra show similar changing trend
with the gradual addition of Eu(III) to the solution of C8-PypzH.
The peak at 252 nm decreases gradually and a new peak at 290
nm appears, suggesting that the metal-ligand complex is formed.
Moreover, there are two isosbestic points presented in the
spectra, which demonstrates that only one metal-ligand complex
is presented in the solution.[25,29]
Furthermore, log K could be evaluated by plotting 1/(A－A0) vs.
1/[M] graphically. As can be seen in Figure S19 of the ESI, the
plots of 1/[A－A0] at 252 nm vs 1/[Eu(III)] gave straight line
(correlation coefficient: R2 = 0.997), indicating that the 1:1 Eu(III)
complex with C8-PypzH is formed in the solution. This result is
in keeping with that obtained by slope analyses in the extraction.
The calculated log K for Eu(III) with C8-pypzH is 4.45 ± 0.03,
implied that C8-PypzH has the similar complexation ability with
some N-donor heterocyclic ligands, such as Bippp, Dbnpp and
ADPTZ. [25,30]
Figure 3 Influence of ligand concentration on the extraction of Am(III) and
Eu(III). Org. phase: 0.01−0.10 mol/L C8-PypzH and 0.5 mol/L 2-
bromohexanoic acid in kerosene; Aq. phase: 0.1 mmol/L Eu(NO3)3 or tracer
amount of 241Am in 0.01 mol/L HNO3.
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European Journal of Inorganic Chemistry
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Figure
5 UV−vis absorption spectroscopic titration of C8-PypzH with
Figure 6 Single crystal structure of the Eu(III) complex with PypzH.
Eu(NO3)3·6H2O in acetonitrile (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).
Colorless crystals of Sm(III) complex suitable for X-ray
diffraction were obtained by slow evaporation in EtOH/DCM (v/v,
2:1). As shown in Figure 7, samarium center is surrounded by
two PypzH ligands, two bidentate nitrates, one water molecule
and one EtOH molecule resulting in a 10-coordinated Sm atom.
The coordination pattern is similar to that of PypzH-Eu complex.
Interestingly, there is also an obvious difference in the Sm−N
bond length of the pyridine ring (2.668 Å, see Table S4 of the
ESI†) and pyrazolyl ring (2.532 Å, see Table S4 of the ESI),
suggesting that the pyrazolyl ring is easier to coordinate with
Sm(III) than pyridyl ring. The Sm−O bond length of two bidentate
nitrates are within the expected ranges of 2.530(3)−2.564(3),
denoting that the nitrate ions may be coordinated to Sm(III) in a
symmetric bidentate way.[34] However, the existence of one free
nitrate ion which is hydrogen bonded to pyrazolyl ring is to keep
the charge balance.
Eu(III) and Sm(III) complexes structures
As a result of long alkyl chain in C8-PypzH, it is difficult to
obtain the single crystal of its metal complex. Thus, PypzH
containing the same coordination sites was employed to
investigate complexation behaviors with Ln(III).[31] We tried our
best to cultivate more complex crystals of Ln(III). However,
unfortunately, only two single crystals for Eu(III) and Sm(III)
were obtained.
Colorless crystals of Eu(III) complex suitable for X-ray
diffraction were obtained by vapor diffusion in diethyl ether over
a week. As shown in Figure 6, two PypzH are bound to Eu(III)
through two N atoms of pyridyl group together with one N atom
of pyrazolyl group. It's worth noting that the N atom combining
with a H atom didn’t participate in coordination. The average
length of Eu−N bonds with pyrazolyl nitrogen atoms is 2.520 Å,
while 2.570 Å with pyridyl nitrogen atoms (see Table S3 of the
ESI,). The slight difference between these two bond lengths
indicates that the coordination ability of pyrazolyl ring is stronger
than that of pyridyl ring, which is in accord with the coordination
pattern of Eu(III) and C5-BPP.[23] Furthermore, there are three
nitrate ions coordinated with Eu(III) and the Eu−O bond length of
nitrate groups are within the expected ranges of
2.495(3)−2.551(4).[32] Outside the primary coordination, one free
CH3CN molecule is coordinated with pyrazolyl ring by hydrogen
bonding. This phenomenon is widespread in the crystallographic
structure and aggregation of molecules may occur in solvent to
hinder efficient extraction,[33] this may shed light on the influence
of diluents on the aggregation of the molecules in polar solvent,
the extraction ability of C8-PypzH was poor in polar solvent (see
Figure S18 of the ESI).
Figure 7 Single crystal structure of the Sm(III) complex with PypzH.
In the crystal experiments, the results showed that 1:2
complex were formed. It was contradictory with the results
obtained from solvent extraction and UV-vis titration. This
discrimination is easy to be understood: on the one hand, the
coordination circumstance in solid state is different from that in
solution state; on the other hand, the steric effect of the
branched-chain in C8-PypzH would make a contribution to this
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European Journal of Inorganic Chemistry
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case. That is to say, in solvent extraction, 1:1 complex is easy to
be formed for C8-PypzH, while in single crystal, 1:2 complex is
liable to be produced for PypzH. Generally speaking, only in a
few cases, the composition of complexes in both crystal and
solution state are consistent.[25,35,36] However, in most cases,
they are different.[37,38] Moreover, in the organic phase, the
dissociated 2-bromohexanoic acid could substitute for nitric acid
to coordinate with Eu(III), which is good for the enhancement of
lipophilicity of the extracted species. But in crystal state, the only
nitrates as the anions are presented in lattice. In the present
paper, the results obtained from X-ray diffraction are
analytical grade without further purification. NMR spectra were recorded
on a Varian Inova NMR spectrometer with CDCl3 as solvent and
trimethylsilane as internal standard. FT-IR spectra were measured on a
Nicolet Nexus 670 Model instrument. MS data were obtained on a
LCMS-2T-TOF spectrometer. UV-vis spectra were performed on
a
PERSEE TU-1810 spectrophotometer. Single-crystal X-ray diffraction
measurements were carried out using an Agilent Technologies Gemini
diffractometer. The pH of aqueous phase was measured by an INESA
PHS-3C pH meter.
General Preparation of ligands and metal complexes experimental:
disagreement with those from slope analysis and UV-vis titration, All the Cn-PypzH and Cn-Pypz ligands were synthesized according to
but the studies for crystal structure are significant for the
comprehension of interaction between pyridylpyrazole ligands
and metal ions in the extraction.
Scheme 1. PypzH was prepared by the method of literature.[39]
Synthesis of Cn-Pypz: PypzH (1.45 g, 10 mmol) was dissolved in 80
mL of dimethyl formamide (DMF) and stirred at room temperature, then
NaH (0.48 g, 60% dispersion in paraffin oil, 12 mmol) was added
gradually. After 30 min, 10 mmol of 1-bromoalkane (C2-Pypz: 0.75 mL 1-
bromoetnane; C4-Pypz: 1.1 mL 1-bromobutane; C8-Pypz: 1.7 mL 1-
bromoctane) in 10 mL of DMF was added, and the reaction mixture was
stirred at a certain temperature for 6 h. Subsequently the solvent was
removed under reduced pressure. The resulting mixture was dissolved in
50 mL dichloromethane (DCM) and washed three times with water, dried
with anhydrous MgSO4, filtered, and concentrated in vacuo. The yellow
viscous residues were purified with silica gel column chromatography
(eluent: EA / PE = 1/2).
Conclusions
A
series of alkyl-substituted pyridylpyrazole ligands with
branched-chain at 2-position or 3-position of pyrazole ring were
synthesized and tested as the selective extracting agent for
Am(III). It has been shown that the branched-chain at 2-position
of pyrazole ring has significant steric effects on the extraction
ability of pyridylpyrazole ligands. With increasing the branched-
chain length, DAm and DEu values of Cn-PypzH ligands
decreased just slightly, whereas those of Cn-Pypz ligands varied
in the order of C2-Pypz > C8-Pypz > C4-Pypz. The length of the
branched-chain had no obvious effects on the extraction ability
of the three synthesized Cn-PypzH ligands. Besides that, it also
demonstrated that the introduction of proper alkyl group at 3-
position of pyrazole ring enhances the selectivity toward Am(III),
and also promotes the solubility of pyridylpyrazole ligands in
paraffinic diluent. Slope analyses indicated that one C8-PypzH
molecule coordinate with one metal ion in solvent extraction,
which is in good accordance with UV−vis titration analyses.
Meanwhile, X-ray crystallography analyses confirmed that
PypzH acted as a bidentate ligand bonding to metal ion through
both the pyridyl N atom and pyrazolyl N atom without H atom.
The N atom combining with H atom did not take part in the
coordination. Although the extraction performance of all the
pyridylpyrazole ligands is not sufficient to meet the practical
needs for An(III)/Ln(III) separation, e.g. all the experiments were
carried out at a very low acidity of the aqueous phase, the
obtained metal-ligand bonding information and extraction
selectivity would still promote an optimized design of new and
more efficient pyridylpyrazole ligands for the separation of An(III)
and Ln(III) in the future.
C2-Pypz: Reaction temperature: 35 °C. The product was a light yellow
liquid: yield 1.62 g (94%). FT-IR (KBr, ν/cm−1): 3050, 2981, 2940, 1593,
1493, 1458, 1228, 993, 765, 696. 1H NMR (400 MHz, CDCl3, ppm): δ
8.63 (d, 1H, pyridyl H6), 7.94 (d, 1H, pyridyl H3), 7.74 (m, 1H, pyridyl H4),
7.46 (d, 1H, pyrazolyl H3), 7.22 (m, 1H, pyridyl H5), 6.93 (s, 1H,pyrazolyl
H4), 4.26 (q, 2H, NCH2), 1.54 (t, 3H, CH3). 13C NMR (101 MHz, CDCl3,
ppm): δ 152.3 (pyridyl C2), 151.2 (pyridyl C6), 149.3 (pyrazolyl C5),
136.9 (pyridyl C4), 129.8 (pyrazolyl C3), 122.3 (pyridyl C3), 120.0 (pyridyl
C5), 104.2 (pyrazolyl C4), 47.4 (NCH2), 15.7 (CH3). ESI-MS: m/z [M+H]+:
174.1017, [M+Na]+: 196.0843.
C4-Pypz： Reaction temperature: 60 °C. The product was a light yellow
liquid: yield 1.93 g (96%). FT-IR (KBr, ν/cm−1): 3050, 2943, 2870, 1591,
1491, 1459, 1229, 994, 767, 699. 1H NMR (400 MHz, CDCl3, ppm): δ
8.63 (m, 1H, pyridyl H6), 7.96 (d, 1H, pyridyl H3), 7.75 (t, 1H, pyridyl H4),
7.44 (d, 1H, pyrazolyl H3), 7.23 (m, 1H, pyridyl H5) 6.95 (s, 1H, pyrazolyl
H4), 4.19 (t, 2H, NCH2), 1.90 (dt, 2H, NCH2CH2) 1.36 (dq, 2H, CH2CH3),
0.95 (t, 3H, CH3). 13C NMR (101 MHz, CDCl3, ppm): δ 152.9 (pyridyl C2),
151.3 (pyridyl C6), 149.3 (pyrazolyl C5), 136.6 (pyridyl C4), 130.5
(pyrazolyl C3), 122.2 (pyridyl C3), 120.0 (pyridyl C5), 104.1 (pyrazolyl
C4), 52.4 (NCH2), 32.5 (NCH2CH2), 19.9 (CH2CH3), 13.7 (CH3). ESI-MS:
m/z [M+H]+: 202.1330, [M+Na]+: 224.1134.
C8-Pypz： Reaction temperature: 60 °C. The product was a light yellow
liquid: yield 2.39 g (93%). FT-IR (KBr, v/cm-1): 3050, 2927, 2856, 1593,
1493, 1458, 1228, 993, 764, 697. 1H NMR (400 MHz, CDCl3, ppm): δ
8.61 (d, 1H, pyridyl H6), 7.91 (d, 1H, pyridyl H3), 7.69 (td, 1H, pyridyl H4),
7.42 (d, 1H, pyrazolyl H3), 7.15 (m, 1H, pyridyl H5), 6.86 (d, 1H,pyrazolyl
H4), 4.16 (t, 2H, NCH2), 1.90 (p, 2H, NCH2CH2) 1.27 (m, 10H,
CH2CH2CH2CH2CH2CH3), 0.82 (m, 3H, CH3). 13C NMR (101 MHz, CDCl3,
ppm): δ 152.4 (pyridyl C2), 151.2 (pyridyl C6), 149.3 (pyrazolyl C5),
136.6 (pyridyl C4), 130.4 (pyrazolyl C3), 122.2 (pyridyl C3), 120.0 (pyridyl
C5), 104.1 (pyrazolyl C4), 52.6 (NCH2), 31.8 (CH2CH2CH3), 30.5
Experimental Section
General：All the lanthanide nitrates in HNO3 solution were prepared
from their oxides (99.99%, Aldrich). Stock solution of 241Am was provided
by China Institute of Atomic Energy. Chromatographic grade acetonitrile
(H2O ≤ 0.001%) purchased from Aladdin was used in UV-vis titration
experiments. And all the other reagents used in the experiments were of
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European Journal of Inorganic Chemistry
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(NCH2CH2CH2CH2CH2), 29.1 (NCH2CH2), 26.6 (NCH2CH2CH2), 22.6
(CH2CH3), 14.1 (CH3). ESI-MS: m/z [M+H]+ 258.1948, [M+Na]+: 280.1742.
pyrazolyl H4), 2.71 (m, 2H, NCH2), 1.69 (dt, 2H, NCH2CH2), 1.41 (dd, 2H,
CH2CH3), 0.96 (t, 3H, CH3). 13C NMR (101 MHz, CDCl3, ppm): δ 149.3
(pyridyl C2, pyrazolyl C3), 136.9 (pyridyl C6, pyrazolyl C5), 122.6 (pyridyl
C4), 120.0 (pyridyl C3, C5), 102.0 (pyrazolyl C4), 31.5 (NCH2), 27.0
(NCH2CH2), 22.4 (CH2CH3), 13.9 (CH3). ESI-MS: m/z [M+H]+: 202.1334,
[M+Na]+: 224.1136.
C8-PypzH： The product was a light yellow solid: yield 1.60 g (52%, over
two steps). FT-IR (KBr, v/cm−1): 3258, 2920, 2851, 1601, 1568, 1489,
1152, 999, 781, 648. 1H NMR (400 MHz, CDCl3, ppm): δ 8.61 (d, 1H,
pyridyl H6), 7.67 (m, 2H, pyridyl H3, H4), 7.22(m, 1H, pyridyl H5), 6.62 (s,
1H, pyrazolyl H4), 2.70 (m, 2H, NCH2), 1.54 (s, 2H, NCH2CH2), 1.32 (m,
10H, NCH2CH2CH2CH2CH2CH2CH2CH3), 0.87 (t, 3H, CH3). 13C NMR
(101 MHz, CDCl3, ppm): δ 149.2 (pyridyl C2, pyrazolyl C3), 136.9 (pyridyl
C6, pyrazolyl C5), 122.6 (pyridyl C4), 120.0 (pyridyl C3, C5), 101.9
(pyrazolyl C4), 31.9 (NCH2, CH2CH2CH3), 29.3 (NCH2CH2CH2CH2CH2),
27.4 (NCH2CH2), 22.7 (CH2CH3), 14.1 (CH3). ESI-MS: m/z [M+H]+:
258.1953, [M+Na]+: 280.1741.
Scheme 1 Synthesis route of Cn-PypzH and Cn-Pypz.
Synthesis of pyridine-2-carboxylic acid ethyl ester： A 10 g (81.3
mmol) amount of 2-picolinic acid and 2.0 mL of concentrated H2SO4 were
refluxed for 5 h in 120 mL of absolute EtOH. After the solution cooling
down, the solvent was removed under reduced pressure. And the
viscous residue was dissolved in DCM, the resulting solution was
successively washed with 5% NaHCO3 solution and water three times,
dried with anhydrous MgSO4. The product was obtained as the colorless
and transparent liquid after removing the solvent in vacuo. Yield: 11 g
(90%).
PypzH-Eu： PypzH (20 mg, 0.14 mmol) in 10 mL of anhydrous EtOH
was added dropwise to a solution containing Eu(NO3)3∙6H2O (31 mg,
0.07 mmol) dissolved in 5.0 mL of anhydrous EtOH and the reaction
mixture was stirred at room temperature over night. The white solids
were filtered and washed several times with EtOH to yield the product.
After drying in vacum, 5.0 mg of the product was dissolved in 2.0 mL of
CH3CN and crystals suitable for X-ray diffraction were obtained by vapor
diffusion in diethyl ether over a week. FT-IR (KBr, v/cm−1): 3346, 1608,
1464, 1385, 1310, 1092, 778, 708.
Synthesis of 1-(pyridin-2-yl)alkane-1,3-dione： 14 mmol of n-alkan-2-
one (C2-PypzH: 1.2 mL butan-2-one; C4-PypzH: 1.7 mL hexan-2-one;
C8-PypzH: 2.6 mL decan-2-one) was dissolved in tetrahydrofuran (THF,
50 mL), then NaH (0.80 g, 60% dispersion in paraffin oil, 20 mmol) was
added gradually and stirred for 30 min at room temperature. After that,
pyridine-2-carboxylic acid ethyl ester (1.80 g, 12.0 mmol) was added and
the reaction mixture was refluxed for 10 h. Subsequently the solvent was
removed under reduced pressure and a solution of 1:5 mixture of glacial
acetic acid and water (20 mL) was added to the brown viscous residue.
Then extracting the resulting mixture with DCM and the organic phase
was washed three times with water, dried with anhydrous MgSO4, filtered,
and concentrated in vacuo. The crude products were obtained as brown
viscous liquid and were used without further purification.
PypzH-Sm: PypzH (20 mg, 0.14 mmol) in 10 mL of anhydrous EtOH was
added dropwise to a solution containing Sm(NO3)3∙6H2O (31 mg, 0.07
mmol) dissolved in 5.0 mL of anhydrous EtOH and the reaction mixture
was stirred at room temperature over night. The white solids were filtered,
washed several times with EtOH and drying in the vacuo to yield the
product. Crystals suitable for X-ray diffraction were obtained from
DCM/EtOH (2/1) by slow evaporation in two days. FT-IR (KBr, v/cm−1):
3360, 1608, 1468, 1383, 1297, 1091, 778, 708.
Solvent extraction: The aqueous phase contained HNO3 of different
concentrations and 241Am, Eu(III), as well as RE(III) (0.1 mmol/L each).
The organic phases were prepared by dissolving the synthesized ligand
and 2-bromohexanoic acid in tert-butyl benzene (TBB) or kerosene.
Equal volumes (1.0 mL) of organic and aqueous phase were stirred in a
10 mL stoppered glass tube in a water bath at 25.0 ± 0.5 °C for 60 min. It
has been proved that 60 min is enough to attain extraction equilibrium
(Figure S17 of the ESI). After centrifugation and separation, the
concentration of 241Am in each phase was measured by a NaI(Tl)
scintillation counter. While the RE(III) in aqueous phase were measured
by an inductively coupled plasma atomic emission spectrometer (ICP-
AES, IRIS Advantage, Thermo Scientific) and those in organic phase
were calculated from mass balances by difference between the initial
solution and the aqueous phase after extraction. Moreover, all the
extraction experiments were carried out in triplicate. The distribution
ratios of metal ions were calculated by the ratio of the total concentration
of metal ion in each phase, DM = [M]org / [M]aq. The subscripts org and aq
represent the organic phase and aqueous phase, separately. The
Synthesis of Cn-PypzH： The crude product obtained before and 4.0
mL of hydrazine hydrate (80% in H2O) were dissolved in 50 mL of EtOH
and refluxed for 3 h. After removing the solvent, the brown viscous liquid
was dissolved in DCM (50 mL) and washed with brine three times, dried
with anhydrous MgSO4 and filtered. The solvent was successively
removed under reduced pressure, a brown viscous residue was obtained
and the residue purified with silica gel column chromatography (eluent:
EA : PE = 1/1).
C2-PypzH： The product was a light yellow solid: yield 0.99 g (48%, over
two steps). FT-IR (KBr, ν/cm−1): 3192, 2970, 2869, 1595, 1566, 1481,
1152, 999, 786, 653. 1H NMR (400 MHz, CDCl3, ppm): δ 8.61 (dt, 1H,
pyridyl H6), 7.74 (dd, 2H, pyridyl H3, H4), 7.22 (dd, 1H, pyridyl H5), 6.63
(s, 1H, pyrazolyl H4), 2.75 (q, 2H, CH2), 1.32 (t, 3H, CH3). 13C NMR (101
MHz, CDCl3, ppm): δ 149.3 (pyridyl C2, pyrazolyl C3), 137.0 (pyridyl C6,
pyrazolyl C5), 122.7 (pyridyl C4), 120.0(pyridyl C3, C5), 101.5 (pyrazolyl
C4), 20.7 (CH2), 13.6 (CH3). ESI-MS: m/z [M+H]+: 174.1011, [M+Na]+:
196.0825.
selectivity for Am(III) over Eu(III) is expressed by SFAm/Eu = DAm / DEu
.
UV−vis titration: The stability constant (log K) was determined by
UV−vis titration at 25 °C. Experiments were carried out in a 1.0 cm quartz
cell. The concentration of C8-PypzH was 2 × 10−5 mol/L in acetonitrile,
while 4 × 10−4 mol/L for Eu(NO3)3∙6H2O. There were no background
C4-PypzH： The product was a light yellow solid: yield 1.08 g (45%, over
two steps). FT-IR (KBr, v/cm−1): 3199, 2941, 2863, 1595, 1469, 1152,
998, 785, 626. 1H NMR (400 MHz, CDCl3, ppm): δ 8.61 (d, 1H, pyridyl
H6), 7.78 (m, 2H, pyridyl H3, H4), 7.25 (m, 1H, pyridyl H5), 6.68 (s, 1H,
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European Journal of Inorganic Chemistry
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FULL PAPER
electrolytes to control the ionic strength. Each 10 µL of metal solution
was titrated into 2.0 mL of ligand solution and mixing vigorously for 5 min
before measuring. Such contact time was sufficient to attain the
complexation equilibrium in our experiments. The log K for Eu(III)
complex with C8-PypzH was calculated from the change in the UV−vis
absorption spectra by means of Benesi−Hildebrand equation.[40–42]
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Keywords: Pyridylpyrazole ligands • Extraction• Am(III) • Eu(III)
• Selectivity.
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Entry for the Table of Contents (Please choose one layout)
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The steric effects of the branched-
chain on the extraction of Am(III) and
Eu(III) were investigated in HNO3
Dongping Su, Ying Liu, Shimeng Li,
Songdong Ding,* Yongdong Jin,*
Zhipeng Wang, Xiaoyang Hu, and Lirong
Zhang
medium by
a
series of alkyl-
substituted pyridylpyrazole ligands. It
has been shown that these ligands
exhibited good selective extraction
ability for Am(III) and the steric effects
of branched-chain had a significant
Page No. – Page No.
Title:
Americium(III) over Europium(III) with
the Pyridylpyrazole Ligands:
Structure-properties Relationship
Selective
Extraction
of
impact on DAm, DEu as well as SFAm/Eu
.
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