Efficient separation of perrhenate as analogue to pertechnetate in nitric acid solution with a DOTA-tetraamide ligand: Solvent extraction, complexation and structure study

Article abstract of DOI:10.1016/j.molstruc.2020.128330

The two DOTA-tetraamide ligands, DOTAM-n-butyl (1,4,7,10-tetrakis[(N-n-butyl; carbamoyl)methyl]-1,4,7,10-tetraazacyclododecane) and DOTAM-benzyl (1,4,7,10- tetrakis[(N-benzylcarbamoyl)methyl]-1,4,7,10-tetraazacyclododecane), were synthesized and character

Full text of DOI:10.1016/j.molstruc.2020.128330

Journal of Molecular Structure 1216 (2020) 128330
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Efficient separation of perrhenate as analogue to pertechnetate in
nitric acid solution with a DOTA-tetraamide ligand: Solvent extraction,
complexation and structure study
1
1
Xueyu Wang , Xiaoyang Hu , Lianjun Song , Xiuying Yang , Qian Xiao , Haowei Xu ,
*
Songdong Ding
College of Chemistry, Sichuan University, Chengdu, 610064, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 January 2020
Received in revised form
The two DOTA-tetraamide ligands, DOTAM-n-butyl (1,4,7,10-tetrakis[(N-n-butyl; carbamoyl)methyl]-
1,4,7,10-tetraazacyclododecane) and DOTAM-benzyl (1,4,7,10- tetrakis[(N-benzylcarbamoyl)methyl]-
1
,4,7,10-tetraazacyclododecane), were synthesized and characterized by MS, NMR and FTIR spectra. In
16 April 2020
1
acid solution, DOTAM-n-butyl was liable to be protonated. FTIR, H NMR, ESI-MS spectra and X-ray
Accepted 23 April 2020
Available online 28 April 2020
diffraction analyses revealed that the nitrogen atoms of cyclen ring combined two protons to form a
cation with two positive charges. The protonated DOTAM-n-butyl had strong extraction ability and good
ꢀ
ꢀ
ꢀ
3
in the 1:2
selectivity for ReO
complex of DOTAM-n-butyl with HNO
an anion-exchange extraction model. Meanwhile, X-ray crystal structure analyses of ligand complexes
4
. It was extracted by means of the substitution of one ReO
4
for one NO
Keywords:
Cyclen
DOTA-Tetraamide
Pertechnetate
Perrhenate
3
. The solvent extraction and X-ray crystallography demonstrated
ꢀ
ꢀ
with HReO
4
or HNO
3
also indicated that ReO
4
or NO
3
ions were located in the gap between the upper
and lower protonated ligands through multiple hydrogen bonds and electrostatic interactions.
Solvent extraction
Complexation
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Generally, liquid-liquid solvent extraction is one of the most
commonly used separation methods in experiment and engineer-
Over the past few decades, about 400 metric tonnes of
ing for reprocessing of spent nuclear fuel [10e14]. Up to now, some
99
ꢀ
technetium-99 ( Tc) have been produced during the nuclear
extractants for separating TcO
4
have been developed such as ter-
fission of 2 U or
35
239
Pu [1,2]. Tc as a b-emitter with a long half-life
99
tiary amine TOA and quaternary amine Aliquat-336 [15e18].
5
ꢀ
of 2.13 ꢁ 10 years is one of the most problematic radionuclides in
Through electrostatic interaction between TcO
4
and positively
acidic nuclear waste [3e5], and primarily exists in the þ7 oxidation
charged extractant, technetium as a hydrophobic ion pair is
ꢀ
state as TcO
4
in both the nuclear fuel cycle and environments under
extracted from aqueous phase into organic phase. These extractants
ꢀ
oxic conditions [6,7]. However, due to large water solubility
have good extraction performances for TcO
4
. However, they suf-
ꢂ
(
11.3 mol/L for the sodium salt at 20 C) and extremely high
fered from very low extraction capacity, resulting in the frequent
appearance of the second organic phase or emulsification [18,19].
This greatly hampers their applications in practical process.
ꢀ
mobility in the environment, TcO
4
has a real threat to environ-
mental and public health [6,8]. Additionally, during the nuclear
waste vitrification process, high volatility of technetium would also
lead to the generation of volatile technetium compounds such as
Therefore, it is significant to explore the new and efficient extrac-
ꢀ
tant for TcO
4
separation.
Tc
2
O
7
, which is not good for its ultimate disposal [9]. Thus, in order
In recent years, it was found that macrocyclic polyamine re-
ceptors had an excellent selective recognition and trapping of
pertechnetate in aqueous solution [20e22]. For example, the p-
xylyl aza-cryptand receptor consists of a macro-bicyclic polyamine
which is made of two bis-tren units linked by p-xylyl spacers.
Owing to the stabilization of the inclusion complex by multiple
to mitigate long-term nuclear waste storage issues, it is very
essential to removal of radioactive pertechnetate from acidic nu-
clear waste.
*
Corresponding author.
E-mail address: dsd68@163.com (S. Ding).
These authors contributed equally to this work.
1
https://doi.org/10.1016/j.molstruc.2020.128330
022-2860/© 2020 Elsevier B.V. All rights reserved.
0

2
X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
strong H-bonding interactions with the protonated imino groups
2. Experimental
ꢀ
in the cavity, there is so very high affinity for TcO
4
that the affinity
constant is about two and three orders of magnitude higher than
2.1. Reagents
ꢀ
ꢀ
for ClO
4
and NO
3
, respectively [23]. After the introduction of a
ꢀ
fluorescent unit such as anthracenyl group in the aza-cryptand’s
ReO
4
in HNO
3
solution was prepared from NH
4
ReO
and SO solution were prepared
4
SO (99.9%, Aladdin, China). And all the
4
(99.999%,
ꢀ
ꢀ
2ꢀ
4
in HNO
framework, the receptor was able to selectively recognize and
Alfa Aesar). Cl , H
from NaCl, NaH
other reagents used in the experiments were of analytical grade
without further purification unless otherwise stated.
2
PO
PO
and Na
4
3
ꢀ
sense TcO
4
at
m
mol/L concentration [24]. Consequently, there are
2
4
2
ꢀ
good reasons to expect that an efficient TcO
4
separation may be
achieved in the event that this kind of water-soluble macrocyclic
polyamine receptor is modified as a lipid-soluble extractant. One
of the effective modification methods is to reduce the number of
imino groups to suppress the formation of highly protonated
species with pronounced hydrophilicity. In other words, this also
means to reduce the ring size of macrocyclic polyamine. Among
the macrocyclic polyamines with smaller ring, 1,4,7,10-
tetrazacyclododecane, “cyclen”, is the most representative one.
Its tetraamide derivatives (Fig. 1) have eight donor atoms, i.e. four
N atoms of cyclen ring and four carbonyl O atoms of four side arms.
There is the cooperativity between cyclen ring and cyclen-side
arms, producing an expanded cavity. So these ligands can effec-
tively extract and encapsulate metal ion with different ionic
FTIR spectra were recorded on a Nicolet Nexus 670 Model in-
1
strument. H NMR spectra were measured on a Varian Inova
400 MHz NMR spectrometer using methanol-d4, chloroform-d or
dimethyl sulfoxide-d6 as solvent with tetramethylsilane as internal
standard. High resolution ESI-MS spectra were obtained on a LCMS-
IT-TOF spectrometer. The pH of the aqueous phase was measured
by an INESA PHS-3C pH meter equipped with an Eꢀ201-C pH glass
electrode.
2.2. Synthesis
N-n-butyl bromoacetamide and N-benzyl bromoacetamide
were prepared by previously published methods [32,33]. DOTAM-
n-butyl and DOTAM-benzyl were synthesized according to Scheme
1.
Under a nitrogen atmosphere at room temperature, triethyl-
amine (5.06 g, 50 mmol) was added to a solution of 1,4,7,10-
tetraazacyclododecane (1.03 g, 6 mmol) and bromoacetamide
þ
2þ
3þ
3þ
radius, such as Na , Pd , Eu and Gd [25e29]. For example,
þ
þ
Tsukube et al. investigated the extraction behaviors of Li , Na and
ions by amide-armed cyclen L
extraction percentage of Na was recorded up to 86%, whilst that
of Li or K was less than 10% under the same conditions, showing
good selectivity for Na . Xia et al. also reported a similar armed
þ
K
1
(Fig. 1) in CH
2
Cl
2
[25,26]. The
þ
þ
þ
þ
cyclen L
cations in HNO
exhibited excellent selectivity for Th . At pH > 4.2, about 90%
Th was extracted into organic phase. Furthermore, due to four N
atoms on the ring, the cyclen-based ligands have complicated
protonization behavior. Multistep protonations commonly take
place on the amine groups of cyclen ring [27,28]. In theory, these
protonations may be good for the extraction of negative ions via
2
(Fig. 1) and its extraction behaviors toward some actinide
(24 mmol) in 100 mL CH
DOTAM-benzyl). The reaction mixture was stirred at 65 C for 2
days. After cooling, the white solid was filtered and washed with
3
cold CH CN and ice-cold water, then dried under vacuum and
3
CN (for DOTAM-n-butyl) or THF (for
ꢂ
3
solution using CH Cl as diluent [30]. The ligand
2
2
4
þ
L
2
4
þ
recrystallized from CH
colorless solid.
3
CN to afford the title compound as a
ꢂ
DOTAM-n-butyl. (2.30 g, 61%) mp: 161e162 C. FTIR (KBr,
n
/
ꢀ1
cm ): 3464, 3294(NH), 3236 (NH), 3080, 2958, 2933, 2870, 2823,
1
electrostatic attraction. But unfortunately there have so far been
1659(C]O), 1554, 1456, 1306, 1238, 1101, 715. H NMR (400 MHz,
ꢀ
no reports on the extraction of anions such as TcO
tetraamides.
4
by DOTA-
CD
(s, 16H, ring NCH
(m, 8H, CH CH CH
CD OD, ppm): 173.7 (CO), 59.9 (NCH
40.2 (CH CH NH), 33.0 (CH CH CH
3
OD, ppm):
d
3.21 (t, 8H, CH
CH N), 1.57e1.44 (m, 8H, CH
), 0.95 (t, 12H, CH
CO), 54.9 (ring NCH
), 21.4 (CH CH CH
2
2
CH
2
NH), 3.08 (s, 8H, NCH
CH CH ), 1.43e1.29
). C NMR (100 MHz,
CH N),
), 14.3
2
CO), 2.73
2
2
3
2
2
1
3
It is well known that Tc is a radioactive nuclide and difficult to
3
2
d
2
3 2
CH
be obtained, but Re, which lies below Tc in the periodic table, has
3
2
2
2
ꢀ
stable isotopes and its oxometallate ReO
4
is sufficiently similar to
2
2
3
2
2
3
2
ꢀ
ꢀ
þ
TcO
ReO
4
. They have similar ionic radii (TcO
4
¼ 2.52 Å and
(CH
3
CH
2
CH
2
). ESI-MS (m/z): Found: 625.5083 ([MþH] ), 647.4899
ꢀ
þ
þ
þ
4
¼ 2.60 Å) and metal oxygen bond lengths (TceO ¼ 1.702 Å
([MþNa] ). Calcd: 625.5123 ([MþH] ), 647.4943 ([MþNa] ).
ꢀ
ꢀ1
and ReeO ¼ 1.719 Å) [4,22]. Consequently, ReO
4
can be often used
DOTAM-benzyl. (2.40 g, 53%) FTIR (KBr,
n
/cm ): 3305, 3203,
ꢀ
as an analogue for TcO
4
in solvent extraction [31]. In the present
3059, 3030, 2945, 2818, 1651(C]O), 1552, 1456, 1365, 1306, 1257,
1
paper, the ligands DOTAM-n-butyl and DOTAM-benzyl (Scheme 1)
1103, 739, 700, 615. H NMR (400 MHz, CDCl
3
, ppm): d 7.31e7.15 (m,
ꢀ
ꢀ
were synthesized, and by employing ReO
4
as a surrogate for TcO
4
,
20H, Ar), 7.10 (t, J ¼ 6.0 Hz, 4H, CONH), 4.29 (d, J ¼ 5.8 Hz, 8H,
ꢀ
13
the extraction behaviors of DOTAM-n-butyl toward ReO
4
in HNO
3
ArCH
NMR (100 MHz, CDCl
43.0. ESI-MS (m/z): Found: 761.4493 ([MþH] ), 783.4300
2
), 2.90 (s, 8H, NCH
2
CO), 2.45 (s, 16H, ring NCH
2
CH
2
N).
C
solution were investigated. And then, the complexation of
3
, ppm):
d
138.4, 128.8, 127.8, 127.6, 59.0, 53.7,
ꢀ
þ
DOTAM-n-butyl with ReO
4
, as well as the complex structures were
1
þ
þ
þ
also studied by using H NMR, FTIR, ESI-MS and single crystal X-
ray diffraction. Moreover, the extraction model was presented too.
([MþNa] ). Calcd: 761.4502 ([MþH] ), 783.4322 ([MþNa] ).
HNO Complex of DOTAM-n-butyl (DOTAM-n-butyl-HNO ). A so-
lution of DOTAM-n-butyl (0.13 g, 0.20 mmol) in CH Cl (2 mL) was
added to a 0.20 mol/L HNO solution (2 mL), and the mixture was
stirred at room temperature for 2 h. After centrifugation, the CH Cl
3
3
2
2
3
2
2
phase was removed and the water phase was subsequently
concentrated under reduced pressure, and the white solid was
obtained. Crystals suitable for X-ray diffraction were obtained by
slow diffusion of ethyl acetate (EA) into the CH
3
OH solution after 3
ꢀ1
weeks. FTIR (KBr,
682(C]O),1648(C]O),1558,1459,1384(NO
DMSO‑d , ppm): 8.20 (s, 4H, CONH), 7.83 (s, 2H, ring NH), 3.57 (s,
CO), 3.27e3.13 (d br, 16H, ring NCH CH N), 3.10e3.05 (q,
CH NH), 1.44e1.37 (m, 8H, CH CH NH), 1.32e1.23 (m, 8H,
CH ), 0.87 (t, 12H, CH CH ). ESI-MS (m/z): Found: 313.2558
n/cm ): 3354, 3268, 3082, 2960, 2932, 2868,
ꢀ
1
1
3
). H NMR (400 MHz,
6
d
8
8
CH
H, NCH
H, CH
CH
2
2
2
2
2
2
2
Fig. 1. Chemical structures of cyclen and its derivatives.
3
2
2
3
2

X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
3
Scheme 1. Synthetic route of DOTAM-n-butyl and DOTAM-benzyl.
2
þ
2þ
ꢀ
ꢀ
2ꢀ
4
and SO were analyzed using ion chromatog-
(
[Mþ2H] ). Calcd: 313.2603 ([Mþ2H] ).
HReO Complex of DOTAM-n-butyl (DOTAM-n-butyl-HReO
solution of DOTAM-n-butyl (0.13 g, 0.20 mmol) in CH Cl (2 mL)
was added to a 0.20 mol/L HNO solution (2 mL) with NH ReO
0.11 g, 0.40 mmol). The mixture was stirred at room temperature
2
such as Cl , H PO
4
4
4
). A
raphy (PIC-10, Qingdao Puren, China) equipped with a conductivity
detector. Meanwhile, the concentrations of ReO
anions in organic phase were calculated from mass balances by the
difference between initial solution and aqueous phase after
extraction. All extraction experiments were carried out in triplicate.
ꢀ
2
2
4
and the other
3
4
4
(
for 2 h. And the white solid was obtained by filtration. Single
crystals were obtained by slow diffusion of EA into a solution in
The extraction efficiency of HNO
3
was gotten as follows:
ꢀ1
CH
3
OH after 3 weeks. FTIR (KBr,
n
/cm ): 3358, 3282, 3081, 2960,
ꢀ
½HNO ꢃ ꢀ ½HNO ꢃ
2
933, 2868, 1683(C]O), 1647(C]O), 1558, 1460, 1385, 917(ReO
4
).
3 ini:
3 eq:
E ¼
ꢁ 100%
(1)
1
H NMR (400 MHz, DMSO‑d
H, ring NH), 3.57 (s, 8H, NCH
NCH CH N), 3.10e3.05 (q, 8H, CH
CH CH NH), 1.32e1.23 (m, 8H, CH CH
ESI-MS (m/z): Found: 313.2586 ([Mþ2H] ). Calcd: 313.2603
6
, ppm):
CO), 3.27e3.12 (d br, 16H, ring
CH NH), 1.44e1.37 (m, 8H,
CH ), 0.87 (t, 12H, CH CH ).
d
8.18 (s, 4H, CONH), 7.83 (s,
½HNO3ꢃini:
2
2
where E, [HNO ]ini. and [HNO ]eq. represent extraction efficiency,
3 3
2
2
2
2
initial concentration and equilibrium concentration of HNO
aqueous phase, respectively.
Besides, the distribution ratio (D) was obtained as the ratio of
the total ion concentration in organic phase ([M]org.) to that in
aqueous phase ([M]aq.).
3
in the
2
2
3
2
2
3
2
2
þ
2
þ
(
[Mþ2H] ).
HReO Complex of DOTAM-benzyl (DOTAM-benzyl-HReO
DOTAM-benzyl (0.038 g, 0.05 mmol) in 30 mL mixed solution of
CH CN/CH OH (v:v, 10:1) was added to 0.20 mol/L HNO solution
1 mL) with NH ReO (0.027 g, 0.10 mmol). The mixture was stirred
4
4
).
3
3
3
½
Mꢃorg:
(
4
4
DM
¼
(2)
at room temperature for 2 h. Then the solvents were removed in
vacuo and the white solid was obtained. Crystals suitable for X-ray
½Mꢃaq:
diffraction were obtained by slow evaporation of CH
3
OH/H
2
O so-
where the subscripts org. and aq. represent the organic phase and
ꢀ
1
ꢀ
lution after 2 weeks. FTIR (KBr,
n
/cm ): 3439, 3340, 3062, 2844,
aqueous phase, separately. The separation factor (SF) of ReO
4
to-
ꢀ
1
684(C]O), 1647(C]O), 1558, 1454, 1387, 1262, 916(ReO
4
), 731,
ward the other ion was calculated by the formula:
1
6
96. H NMR (400 MHz, DMSO‑d
s, 2H, ring NH), 7.21 (s br, 20H, Ar), 4.26 (d, J ¼ 5.8 Hz, 8H, ArCH
.74 (s, 8H, NCH CO), 3.21 (s br, 16H, ring NCH CH N). ESI-MS (m/z):
Found: 381.2290 ([Mþ2H] ). Calcd: 381.2290 ([Mþ2H] ).
6
, ppm):
d
8.73 (s, 4H, CONH), 7.95
DRe
(
3
2
),
SFRe=M ¼ D
(3)
2
2
2
M
2
þ
2þ
2.3. Solvent extraction
2.4. Single crystal X-ray diffraction analyses
Solvent extraction was performed by mixing the organic phase
Single crystal X-ray diffraction measurements were carried out
using an Agilent Technologies Gemini diffractometer. Using Olex2
[34], the structure was solved with the ShelXT [35] structure solu-
tion program using Direct Methods and refined with the ShelXL [36]
refinement package using Least Squares minimization. All of the
non-hydrogen atoms were refined anisotropically and the
hydrogen atoms were refined by using riding coordinates. Hydro-
gens bonded to carbon and nitrogen were positioned in geometric
positions and given thermal parameters equivalent to 1.2 times (or
1.5 times for methyl groups) those of the atoms to which they were
bonded. Crystal data, details of the data collection and the structure
refinement of complexes were given in Table S1. Crystallographic
data for the single crystals reported in this work have been
deposited with the Cambridge Crystallographic Data Centre as a
supplementary publication, CCDC 1449197 (DOTAM-n-butyl),
ꢀ
4
ꢀ
4
and the aqueous phase. The 2.7 ꢁ 10 mol/L ReO
in HNO
or the pure HNO
3
solu-
ꢀ
tion, the mixed solution of anions with ReO
4
3
solution was used as the aqueous phase. DOTAM-n-butyl dissolved
in benzyl alcohol or other diluents were employed as the organic
ꢀ
phase. Before the extraction of ReO
4
, the organic phase was pre-
solu-
, there
equilibrated with the corresponding concentration of HNO
tion. During an investigation for the extraction of alone HNO
3
3
was no need for this pre-equilibration step. Equal volumes (1.0 mL)
of the organic phase and the aqueous phase were stirred in a 10 mL
ꢂ
stoppered glass tube in water bath at 25.0 ± 0.5 C for the desired
time. After phase separation by centrifugation, the aqueous phase
was taken out for analysis. The equilibrium concentrations of HNO
3
in aqueous phase were determined by acid-base titration or using
ꢀ
pH meter (PHSe3C, Shanghai INESA, China). Those of ReO
4
were
measured by an inductively coupled plasma optical emission
spectrometer (ICP-OES, Optima 8000, PerkinElmer). The anions
1485999 (DOTAM-n-butyl-HNO
3
), 1505095 (DOTAM-n-butyl-
). These data can be
HReO ) and 1506110 (DOTAM-benzyl-HReO
4
4

4
X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[DOTAM-n-butyl] also gave a straight line with a slope value of 1.0,
revealing the formation of 1:1 complex of DOTAM-n-butyl with
ꢀ
ReO
ReO
4
. Therefore, according to these results, it could be inferred that
ꢀ
ꢀ
4
was extracted by means of the substitution of one ReO
4
for
3
. Results and discussion
ꢀ
one NO
3
in the 1:2 complex with HNO
3
. The extraction equilibrium
equation could be expressed as follows:
3.1. Solvent extraction
Prior screening tests had shown that both DOTAM-n-butyl and
h
#
ꢀi
2
þ
ꢀ
4
aq:
H L ,2NO3
2
þ ReO
DOTAM-benzyl had poor solubility in the commonly used diluents
such as kerosene and toluene. Especially for DOTAM-benzyl, there
were few hydrophobic solvents to dissolve it. Consequently, its
extraction behaviors were not examined. Nevertheless, for DOTAM-
n-butyl, it is fortunate that it could be dissolved in benzyl alcohol,
org:
h
i
(4)
2þ
ꢀ
ꢀ
þ NO ꢀ
H L ,NO3 ,ReO4
2
3
aq:
org:
where L represents ligand and the subscripts org. and aq. denote
the organic phase and aqueous phase, respectively.
n-octanol and dichloromethane. Therefore, the influence of the
ꢀ
ꢀ
diluent on the extraction of ReO
4
by DOTAM-n-butyl was listed in
The extraction of ReO4 in HNO3 solution by DOTAM-n-butyl
Table S8. It was found that benzyl alcohol was the best fit for the
extraction. Thus, in the following extraction experiments, benzyl
alcohol was chosen as a diluent.
follows an anion-exchange extraction model. At first, the ligand
was protonated in organic phase, resulting in the formation of 1:2
complex of the ligand with HNO3, namely, H DOTAM-n-
2
2
þ
ꢀ
ꢀ
3
DOTAM-n-butyl is a basic amine ligand. In order to achieve
3
butyl ∙2NO . And then one of the two NO ions was exchanged
ꢀ
ꢀ
with ReO4 ion in aqueous phase, producing a more hydrophobic
extraction for ReO
4
, it must initially be converted to an appropriate
2
þ
ꢀ
ꢀ
ꢀ
amine salt in the organic phase by reacting it with an acid in the
medium. This reaction is known as “pre-equilibration” [37,38]. The
results of pre-equilibration experiment were listed in Table 1. It was
clear that the acidity of the aqueous phase did not change anymore
2 3 4
ion-pair of H DOTAM-n-butyl ∙NO ∙ReO . As a result, ReO ions
4
were extracted from aqueous phase to organic phase. The extrac-
tion process can described by Scheme 2.
Due to the electrostatic repulsion, the protonated DOTAM-n-
butyl hardly extracted metal cations under acidic condition
after being equilibrated with 0.10 mol/L HNO
indicating a complete HNO pre-equilibration. This means that the
ligand was totally converted to its amine salt. Meanwhile, it can also
be seen that approximate 0.17 mol/L HNO were extracted into the
organic phase of 0.10 mol/L DOTAM-n-butyl in benzyl alcohol,
3
solution thrice,
ꢀ
3
(Fig. S17). Nevertheless, the negatively-charged ions such Cl ,
ꢀ
2ꢀ
H PO and SO , which commonly present in acidic nuclear waste,
2
4
4
ꢀ
3
might have a big impact on the ReO4 extraction. And for this, the
extraction selectivity of DOTAM-n-butyl for ReO was investigated.
As shown in Table 2, the distribution ratios of ReO4 were signifi-
2
cantly greater than those of the other anions of Cl , H PO and
SO . It could be attributed to the following two main reasons. On
one hand, the hydration ability of H PO and SO
stronger than that of ReO . On other hand, Cl had the similar
hydration energy to ReO , but its ionic radius was obviously less
ꢀ
4
ꢀ
suggesting the possible formation of 1:2 ligand/HNO
3
extracted
ꢀ
ꢀ
ꢀ
4
complexes. Hence, during the subsequent ReO
4
extraction, three
2
ꢀ
times pre-equilibrium with the corresponding concentration of
HNO solution was necessary.
The influence of contact time, acidity, NO
4
ꢀ
2ꢀ
4
3
2
4
was much
ꢀ
ꢀ
ꢀ
3
concentration and
4
ꢀ
ꢀ
ligand concentration on the extraction of ReO
4
by DOTAM-n-butyl
4
ꢀ
in benzyl alcohol were investigated with the results shown in Fig. 2.
Fig. 2a displayed the influence of contact time on the extraction.
Within about 40 min, the extraction equilibrium reached. To ensure
the complete equilibrium, 60 min was chosen as two-phase contact
than that of ReO , meaning that the latter was more hydrophobic
4
and easier extracted than the former. Meanwhile, the separation
ꢀ
factors of ReO4 over the three anions could also reach the level of
ꢀ
several tens, indicating a good selectivity for ReO . Moreover, un-
4
time. The influence of HNO
3
concentration on the extraction was
der the same conditions, DOTAM-n-butyl had an obviously higher
ꢀ
illustrated in Fig. 2b. The sole benzyl alcohol has very weak
loading capacity of ReO than that of TOA or Aliquat-336 (Fig. S18).
4
ꢀ
ꢀ
4
extraction ability toward ReO
4
with DRe value less than 1.0. Once
This is also good for the extraction separation of ReO .
DOTAM-n-butyl ligand was added into benzyl alcohol, DRe value
increased steeply, especially at low acidity, indicating that DOTAM-
3
ReO
.2. Complexation and structure of DOTAM-n-butyl with HNO
3
and
ꢀ
n-butyl can extract effectively ReO
4
. With increasing HNO
3
con-
ꢀ
4
centration, DRe value decreased, which could be attributed to the
ꢀ
competition extraction of NO
3
.
Thus, the influence of
Generally, study on the complexation of extracted ions with the
extractant can not only help in obtaining information of the com-
plex composition and structure, and also contribute to deeply un-
derstanding the extraction model. Thus, the complexation behavior
of DOTAM-n-butyl with ReO
FTIR spectra analyses. The white solid complexes of DOTAM-n-
ꢀ
NO
3
concentration was also examined and showed in Fig. 2c. The
ꢀ
plot of lgDRe vs lg[NO
3
] gave a straight line with a slope value close
ꢀ
to ꢀ1, suggesting that one NO
3
was involved in the extraction re-
action. To determine the number of DOTAM-n-butyl molecules in
the extraction reaction, slope analysis was also used on the basis of
the influence of extractant concentration on the extraction shown
in Fig. 2d. It was clear that DRe values increased with the rise of
DOTAM-n-butyl concentration. Meanwhile, the plot of lgDRe vs lg
ꢀ
1
4
was investigated using H NMR and
ꢀ
ꢀ
3
were obtained by means of solvent
butyl with both ReO
diffusion as described in the experimental section.
4
and NO
3
.2.1. 1H NMR spectra analysis
NMR spectroscopy is the most powerful mean for the identifi-
Table 1
Equilibration experiment of 0.10 mol/L DOTAM-n-butyl in benzyl alcohol with equal
volume of HNO solution.
3
cations of structural groups, whose main feature is that the
chemical shift measured by this technique is strongly dependent on
the chemical environment of the investigated nuclei [41]. Fig. 3
showed the H NMR spectra of DOTAM-n-butyl, DOTAM-n-butyl-
3 4 6
HNO and DOTAM-n-butyl-HReO in DMSO‑d . On the basis of the
Organic phase
Initial
equilibrated once
equilibrated twice
equilibrated thrice
[HNO
3
]ini., mol/L
[HNO
3
]
eq., mol/L
4
E, %
1
0.10
0.10
0.10
0.10
< 1 ꢁ 10ꢀ
0.035
0.091
100
65
9
chemical shift and the splitting, the observed chemical shifts were
assigned in Table 3. The H(a) atoms can be assigned to NeH group
0.10
0

X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
5
ꢀ
ꢀ
ꢀ4
Fig. 2. Distribution ratios of ReO
4
by DOTAM-n-butyl in benzyl alcohol as a function of (a)contact time ([L] ¼ 0.10 mol/L, [ReO
4
] ¼ 2.7 ꢁ 10 mol/L); (b) HNO
3
concentration
ꢀ
ꢀ4
ꢀ
þ
ꢀ
ꢀ4
(
[L] ¼ 0.10 mol/L, [ReO
4
] ¼ 2.7 ꢁ 10 mol/L); (c) [NO
3
] concentration ([L] ¼ 0.10 mol/L, [H ] ¼ 0.10 mol/L, [ReO
4
] ¼ 2.7 ꢁ 10 mol/L); (d) ligand concentration ([HNO ] ¼ 0.10 mol/
3
ꢀ
ꢀ4
L, [ReO
4
] ¼ 2.7 ꢁ 10 mol/L).
ꢀ
Scheme 2. The extraction process of ReO
4
in HNO
3
solution by DOTAM-n-butyl.
of pendant arm, while H(c) and H(d) atoms belong to the methylene
groups next to carbonyl bonds and N atom of amide, respectively.
DOTAM-n-butyl2 . Thus, a conclusion can be drawn from these
þ
H
2
2
þ
results that a diprotonated species H
2
DOTAM-n-butyl
was
ꢀ
ꢀ
After extracting NO
3
or ReO
4
, there was a new resonance band of
formed by a reaction of tertiary amine N atoms of cyclen ring with
þ
ꢀ
3
H(b) at 7.83 ppm in the two complexes, which was attributed to the
protonated tertiary amine NeH group of cyclen ring. Meanwhile,
Figs. S10 and S12 yet gave the high resolution ESI-MS of DOTAM-n-
butyl-HNO and DOTAM-n-butyl-HReO . Both of the two highest
3 4
intensity peaks at m/z 313.2558 for the former and 313.2586 for the
H
in the extraction process. Moreover, after extracting NO
or
ꢀ
ReO
4
, the resonance bands of H(a) largely shifted from 7.89 ppm to
lower magnetic field by 0.31 and 0.29 ppm, respectively. Whilst the
chemical shift of H(c) and H(d) of the two complexes were identical
with each other, increasing 0.51 and 0.13 ppm respectively. This is
due to the fact that the protonation of N atoms of cyclen ring in
latter originated from the diprotonated molecular species of

6
X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
Table 2
Table 3
ꢀ
The chemical shifts of the 1H NMR spectra of the samples in Fig. 3.
The distribution ratios and the separation factors of ReO
4
over the other anions at
various acidity. Organic phase: 0.10 mol/L DOTAM-n-butyl in benzyl alcohol;
Aqueous phase: 3.0 ꢁ 10ꢀ4 mol/L of each anions in HNO
Samples
Chemical shifts (ppm)
3
solution.
Anion
DG
h
ꢂa, kJ/mol Volume , Å 0.01 mol/L
a
3
0.05 mol/
0.10 mol/L
HNO
H(a)
H(b)
H(c)
H(d)
HNO
3
LHNO
3
3
DOTAM-n-butyl
DOTAM-n-butyl-HNO
DOTAM-n-butyl-HReO
7.89
8.20
8.18
e
3.06
3.57
3.57
2.95
3.08
3.08
3
7.83
7.83
D
A
SFRe/A
D
A
SFRe/A
D
A
SFRe/A
4
ꢀ
Cl
H
ꢀ347
ꢀ465
ꢀ1090
ꢀ330
24.8
33.5
51.0
73.6
3.0
0.94 86
3.0
73
27
3.1
1.9
0.80 36
29
9.4
15
1.4
2.1
0.68 28
19
14
9.0
ꢀ
2
PO
4
2
ꢀ
SO
4
27
e
ꢀ
ꢀ
4
, respectively. The
ꢀ
ReO
4
e
e
asymmetric stretching vibration of NO
slightly higher wavenumber in DOTAM-n-butyl-HReO
arisen from the intermolecular interactions between ReO
3
and ReO
a
4
may be
These data were obtained from the literature [39,40].
ꢀ
4
and
protonated DOTAM-n-butyl. Remarkably, during the preparation
ꢀ
DOTAM-n-butyl
adjacent eCH and eNH groups, and gives rise to a shift of the
protons of H(a), H(c) and H(d) to a lower field.
leads
to
a
deshielding
of
the
process of DOTAM-n-butyl-HReO
4
complex, the ReO
4
concentra-
ꢀ
tion was equal to that of NO
3
. However, in the FTIR spectrum of
, no characteristic absorption peak of NO
2
ꢀ
DOTAM-n-butyl-HReO
4
3
ꢀ
appeared obviously, suggesting that the ReO
finity for protonated DOTAM-n-butyl than NO .
4
had a stronger af-
ꢀ
3
3.2.2. FTIR spectra analysis
For better understanding the complexation of DOTAM-n-butyl
ꢀ
with ReO
4
, the FTIR spectra analyses of DOTAM-n-butyl, DOTAM-n-
and DOTAM-n-butyl-HReO were also performed. As
shown in Fig. 4, with respect to the DOTAM-n-butyl FTIR spectrum,
3.2.3. Single crystal X-ray diffraction analysis
butyl-HNO
3
4
Single crystal X-ray diffraction can provide detailed structural
information as an essential prerequisite to gain a fundamental
understanding of structure property relationships. So the single
ꢀ
1
ꢀ1
the vibrational band at 3200ꢀ3400 cm and 1659 cm could be
assigned to the stretching vibrations of NeH and C]O groups,
respectively. Both of them also existed in the FTIR spectra of
crystals of the ligand, HNO
cultivated and analyzed.
3 4
complex and HReO complex were
DOTAM-n-butyl-HNO
the vibrational band at 1659 cm translated into a split carbonyl
3
and DOTAM-n-butyl-HReO
4
. Nevertheless,
The crystal structures of the ligand DOTAM-n-butyl and its nitric
acid complex DOTAM-n-butyl-HNO were shown in Fig. 5. It can be
seen that the ligand and HNO complex crystallized in the mono-
clinic space group of P2 /c with isostructure. Therein, DOTAM-n-
butyl possed a centrosymmetric structure with S symmetry, hav-
ing two pairs of equivalent opposite pendant arms (Fig. 5a). One
pair of arms (i.e. one above and the other below the cyclen ring)
was further connected to the cyclen ring through intramolecular
hydrogen bonds. Whilst the other pair of arms extending and
ꢀ
1
3
ꢀ1
band at 1682 and 1648 cm
for the former and at 1683 and
3
ꢀ
1
1
647 cm for the latter, separately. The main reason may be that
1
these C]O groups were involved in the different hydrogen bonding
interactions and showed the different bond distances in their
crystal structures (Tables S3 and S5) [42]. Besides, the two obvious
2
ꢀ1
new bands appeared at 1384 and 917 cm in the FTIR spectra of
DOTAM-n-butyl-HNO and DOTAM-n-butyl-HReO , assigned to the
3
4
Fig. 3. 1H NMR spectra for DOTAM-n-butyl, DOTAM-n-butyl-HNO
and DOTAM-n-butyl-HReO in DMSO‑d .
3 4 6

X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
7
ꢀ
interaction with two counterions of NO
agreement with the above result of HNO
3
, which was also in
3
extraction. Meanwhile, it
was also observed a bilayer structure with an ABAB stacking mode
ꢀ
in the crystal of DOTAM-n-butyl-HNO
3
(Fig. 5d), which NO
3
ions
were located in the gap between the upper and lower structural
layers. There were also multiple hydrogen bond interactions be-
ꢀ
2þ
tween NO
3
ions and H
2
DOTAM-n-butyl (Table S5).
As far as the HReO
4
complexes be concerned, only the single
4
crystal of DOTAM-benzyl-HReO was acquired. Great efforts were
also made to obtain the single crystal of DOTAM-n-butyl-HReO
4
complex. Unfortunately, although fast and careful handling of the
crystals during the measurement, the crystal structure suffered
from a significant decrease in quality since the carbon chains of side
arms were disordered by interacting on each other and the highest
residual electron density peaks were located near the Re atoms,
reflected in a number of corresponding A and B level alerts in the
checkcif report (CCDC 1505095). Despite all this, it can still be seen
ꢀ
clearly that two ReO
diprotonated H
structure of DOTAM-benzyl-HReO
4
ions have electrostatic interactions with one
2
þ
2
DOTAM-n-butyl
cation (Fig. S16). The crystal
was shown in Fig. 6. It was clear
4
that two trans-disposed nitrogens of cyclen ring were protonated,
and four arms all were extended to one side of cyclen ring plane to
Fig. 4. FT-IR spectra for DOTAM-n-butyl, DOTAM-n-butyl-HNO
HReO
3
and DOTAM-n-butyl-
ꢀ
compatible with the tetrahedral ReO
4
ion (Fig. 6a). Similar to the
4
.
structure of DOTAM-n-butyl-HNO
structure in that of DOTAM-benzyl-HReO
3
, there was also an ABAB bilayer
ꢀ
4
(Fig. 6d). And the ReO
4
ions also resided in the gap between the upper and lower layers,
having multiple hydrogen bonding interactions with NeH groups
and CeH groups of pendant arms, and CeH groups of cyclen rings
pointing away from the cyclen ring was also involved in intermo-
lecular hydrogen bonds (Fig. 5b and Table S2). However, in the
crystal structure of DOTAM-n-butyl-HNO (Fig. 5c), all four arms
3
(Fig. 6b and d). The related bond distances and angles of hydrogen
were extended to one side of the cyclen ring plane and two trans-
disposed N atoms of cyclen ring were protonated to form a cation of
bonds were listed in Table S6. The space-filling models further
2
þ
revealed that the interlayer gap was compatible with the tetrahe-
H
2
DOTAM-n-butyl . A 1:2 association complex (H
2
DOTAM-n-
ꢀ
3
) was generated by means of an electrostatic
ꢀ
2
þ
dral ReO4 ion (Fig. 6c). As a result, it could be deduced that the
butyl ∙2NO
3
Fig. 5. Crystal structures of DOTAM-n-butyl and DOTAM-n-butyl-HNO : (a) asymmetric unit of DOTAM-n-butyl (the black dotted lines represent hydrogen bonds); (b) the
intermolecular hydrogen bonding interactions of the amide groups in DOTAM-n-butyl; (c) asymmetric unit of DOTAM-n-butyl-HNO
viewed along the b axis. Hydrogen atoms are omitted for clarity. Nonrelevant hydrogen atoms are omitted for clarity.
3 2 3
; (d) layer structure of H DOTAM-n-butyl-HNO

8
X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
Fig. 6. Crystal structures of DOTAM-benzyl-HReO
4
: (a) asymmetric unit of DOTAM-benzyl-HReO
4
(the black dotted lines represent hydrogen bonds); (b) hydrogen bonds formed
ꢀ
2þ
ꢀ
2þ
ꢀ
between ReO
and six H
4
and six H
2
DOTAM-benzyl (the green polyhedrons represent ReO
4
ions and six H
2
DOTAM-benzyl are shown in different colors); (c) a space-filling model of ReO
4
2
þ
2
4
DOTAM-benzyl ; (d) layer structure of DOTAM-benzyl-HReO viewed along the b axis. Hydrogen atoms are omitted for clarity.
ꢀ
extraction of ReO
4
was an ion-pair extraction through electrostatic
DOTAM-n-butyl with HNO
extraction ability and good selectivity for ReO
revealed that 1:1 complex of DOTAM-n-butyl with ReO
generated in solvent extraction processes and ReO
3
. DOTAM-n-butyl exhibited strong
ꢀ
interactions as well as hydrogen bonding interactions.
4
. The slope analyses
ꢀ
Table S6
4
was
ꢀ
ꢀ
However, 1:2 complex of the ligand with ReO
4
in crystal struc-
4
was extracted by
ꢀ
ꢀ
ture was not consistent with the extraction result of 1:1 complex in
solution. It is not strange at all. This difference was a common
phenomenon in the research of solvent extraction and might be
attributed to their fully different coordinated environments
means of the substitution of one ReO
complex with HNO
raphy demonstrated an anion-exchange extraction model. X-ray
crystal structure analyses of the ligand-HReO complex indicated
4
for one NO
3
in the 1:2
3
. The solvent extraction and X-ray crystallog-
4
[
43e46]. Besides, the type of complex formed was also depended
that the ion-pairs were formed through electrostatic interactions
on the concentration of ligand and cation or anion [43e45]. For
example, Drew et al. investigated a series of 1:1 complexes of ter-
pyridine ligand with the lanthanides and found that it was more
likely to form the complexes with 1:2 metal to ligand ratio for
heavier lanthanides with increasing the terpyridine concentration
and multiple hydrogen bonding interactions between the proton-
ꢀ
ated ligand and ReO
cient separation method for ReO
extractant. Although the properties of ReO
4
ions. As a whole, this work provided an effi-
ꢀ
4
based on DOTA-tetraamide
ꢀ
4
are not always the
ꢀ
same as those of TcO
4
, the results of this paper may provide
[
43]. In solvent extraction experiments, the concentration of
important guidance and reference to design of new and more effi-
ꢀ
ꢀ
DOTAM-n-butyl was much more than that of ReO
times), whilst ReO
4
(over 300
cient ligands for TcO
4
extraction from acid medium in the future.
ꢀ
4
concentration was only twice as that of the
ligand during the preparation process of ligand-HReO
4
complex.
Declaration of competing interest
ꢀ
The substantial increase of ReO
4
concentration might be likely to
ꢀ
lead to the formation of 1:2 complex of the ligand with ReO
4
. In the
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
present work, although the results obtained from slope analyses in
solvent extraction were inconsistent with those from X-ray
diffraction, the studies on crystal structure still have important
implications for understanding the interaction between DOTA-
CRediT authorship contribution statement
ꢀ
tetraamide ligand and ReO
. Conclusions
The two DOTAM-n-butyl and DOTAM-benzyl ligands were syn-
4
ion in solvent extraction.
Xueyu Wang: Conceptualization, Methodology, Software,
Investigation, Validation, Data curation, Writing - review & editing.
Xiaoyang Hu: Software, Formal analysis, Validation, Resources,
Data curation, Writing - original draft. Lianjun Song: Resources,
Formal analysis, Visualization. Xiuying Yang: Writing - review &
editing, Supervision, Data curation. Qian Xiao: Writing - review &
editing, Supervision, Data curation. Haowei Xu: Writing - review &
editing. Songdong Ding: Writing - review & editing, Visualization,
4
thesized for investigating the extraction and complexation behav-
ꢀ
iors toward ReO
4
in HNO
3
solution. In acid solution, DOTAM-n-butyl
was liable to be protonated. The analyses of crystal structure for
DOTAM-n-butyl-HNO showed the formation of 1:2 complex of
3

X. Wang et al. / Journal of Molecular Structure 1216 (2020) 128330
9
Supervision, Project administration, Funding acquisition.
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[
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