Towards understanding the correlation between UO22+ extraction and substitute groups in 2,9-diamide-1,10-phenanthroline

Article abstract of DOI:10.1007/s11426-018-9227-1

2,9-Diamide-1,10-phenanthroline (DAPhen) ligands represent a new family of tetradentate extractants given their strong affinity to actinides and the CHON principle. Among this family, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen), initially reported by us, exhibits excellent selectivity towards actinides (U, Th, Am, Pu) over lanthanides and thus can be potentially applied in the group actinide extraction (GANEX) process for the group separation of actinides. In this article, by tailoring the lengths of alkyl chains, we synthesized other four DAPhen ligands with different substitute groups in the diamide moieties, and characterized the relationship between properties and substitute groups of DAPhen ligand. The extraction results show that three of the ligands exhibit high performance in UO₂²⁺ extraction from an acidic solution and the extracted UO₂²⁺ can be easily stripped by only using ultrapure water. Spectrophotometry titration confirms that UO₂²⁺ combined with all the four ligands in 1:1 mode. The extended X-ray absorption finestructure (EXAFS) study shows that six donor atoms comprise the first equatorial shell of the UO₂²⁺ ions bonded by the DAPhen ligands, among which two nitrogen and two oxygen atoms are from the DAPhen ligand, while other two oxygen atoms are from one nitrate ions. This article promises to provide basic data for assessing the feasibility of this kind of DAPhen ligands applied in actinides separation from nuclear wastes.

Full text of DOI:10.1007/s11426-018-9227-1

SCIENCE CHINA
Chemistry
•ARTICLES• . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . https://doi.org/10.1007/s11426-018-9227-1
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2+
Towards understanding the correlation between UO2 extraction
and substitute groups in 2,9-diamide-1,10-phenanthroline
Xinrui Zhang1, Liyong Yuan1*, Zhifang Chai1,2 & Weiqun Shi1*
1Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China;
2School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation Center of Radiation Medicine of Jiangsu
Higher Education Institutions, Soochow University, Suzhou 215123, China
Received January 19, 2018; accepted February 23, 2018; published online April 28, 2018
2,9-Diamide-1,10-phenanthroline (DAPhen) ligands represent a new family of tetradentate extractants given their strong affinity
to actinides and the CHON principle. Among this family, N,N′-diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline (Et-Tol-
DAPhen), initially reported by us, exhibits excellent selectivity towards actinides (U, Th, Am, Pu) over lanthanides and thus can
be potentially applied in the group actinide extraction (GANEX) process for the group separation of actinides. In this article, by
tailoring the lengths of alkyl chains, we synthesized other four DAPhen ligands with different substitute groups in the diamide
moieties, and characterized the relationship between properties and substitute groups of DAPhen ligand. The extraction results
show that three of the ligands exhibit high performance in UO22+ extraction from an acidic solution and the extracted UO22+ can
2+
be easily stripped by only using ultrapure water. Spectrophotometry titration confirms that UO2 combined with all the four
ligands in 1:1 mode. The extended X-ray absorption finestructure (EXAFS) study shows that six donor atoms comprise the first
equatorial shell of the UO22+ ions bonded by the DAPhen ligands, among which two nitrogen and two oxygen atoms are from the
DAPhen ligand, while other two oxygen atoms are from one nitrate ions. This article promises to provide basic data for assessing
the feasibility of this kind of DAPhen ligands applied in actinides separation from nuclear wastes.
2,9-diamide-1,10-phenanthroline, UO22+, extraction, UV-Vis titration, EXAFS
2+
Citation:
Zhang X, Yuan L, Chai Z, Shi W. Towards understanding the correlation between UO2 extraction and substitute groups in 2,9-diamide-1,10-
phenanthroline. Sci China Chem, 2018, 61, https://doi.org/10.1007/s11426-018-9227-1
1 Introduction
the high level liquid waste (HLLW), which contains a slight
of minor actinides (such as americium, curium and neptu-
nium) [4]. These minor actinides exhibit a long lasting
radiotoxicity, being not only harmful to organism health and
environmental protection, but also difficult to be handled in
geological repository [5]. Partitioning and transmutation
(P&T) process is developed intending to bomb these minor
actinides by neutrons and to transform these long-lived ele-
ments into short-lived fission products [6,7]. However, an
unignorable amount of lanthanides is found to weaken the
P&T process severely by absorbing neutrons [8]. Thus, se-
paration of actinides (An) from lanthanides (Ln) before the
P&T process is indispensable [9,10]. An/Ln separation is a
Growing energy demand and continuous environmental is-
sues have led to increasing interest in nuclear energy all over
the world [1]. The rapid developments in nuclear energy,
however, results in a large quantity of nuclear spent fuel that
contains an intense radioactivity and must be managed in
time [2]. Uranium (95.5%) and plutonium (0.9%) comprise
the major radioactive components in spent nuclear fuel
which can be separated by plutonium and uranium extraction
(PUREX) process maturely using tri-n-buty phosphate
(TBP) as extractant [3]. The rest of the wastes are known as
*Corresponding authors (email: yuanly@ihep.ac.cn; shiwq@ihep.ac.cn)
© Science China Press and Springer-Verlag GmbH Germany 2018 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . chem.scichina.com link.springer.com

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long-standing and challenging problem, a result of the si-
milar chemistries of these elements [11–13]. In the last
decades, several ligands have been developed for the An/Ln
separation, including DMDOHEMA (developed by CEA in
1980s and used in the DIAMEX process) [14], TODGA
(developed by JAEA in 2000s) [15], TOPO (developed by
INETand Tsinghua in 1980s and used in TRPO process) [16]
and CMPO (developed by ANL in 1980s and used in the
TRUEX process) [17]. These ligands always aim at one or
several certain actinides extraction but not the group se-
paration of actinides. Recently, we have reported a phenan-
throline based tetradentate ligand with hard-soft donors
combined in the same molecule, N,N′-diethyl-N,N′-ditolyl-
2,9-diamide-1,10-phenanthroline (Et-Tol-DAPhen), for the
group separation of actinides over lanthanides [18]. The
extraction results show that this ligand is stable and very
efficient for the separation of a series of actinides (U, Th,
Am, Pu) from the lanthanides. The coordination behaviors of
Et-Tol-DAPhen with UO22+, Eu3+ and Th4+ were then studied
by UV-Vis titration experiments and single-crystal structure
analysis. The results suggest that the chemical stoichiometry
of all the test metal complexes with Et-Tol-DAPhen is 1:1. In
addition, the coordination mode of Et-Tol-DAPhen with Eu3+
inspired us to characterize the relationship between proper-
ties and substitute groups of DAPhen ligand in the realm of
group separation of actinides. To achieve this aim, we
modified the structure of DAPhen ligand by varying the
substituent group in the diamide moieties and synthesized
four DAPhen ligands with different alkyl chains on the
2+
diamide. Then, UO2 extraction by the four ligands were
performed in detail. Considering that spectrometry offers a
powerful tool for understanding the coordination behavior of
actinides/lanthanides [21], UV-Vis titration as well as the
extended X-ray absorption finestructure (EXAFS) mea-
surement were explored to characterize the UO22+ complex in
acetonitrile. The results clearly reveal influence of sub-
stituent groups on the properties of the DAPhen ligand.
2 Experimental
2.1 Synthesis of the ligands
The four DAPhen ligands used in this study were prepared
by tailoring the substituent group in the diamide moieties of
Et-Tol-DAPhen reported in our previous work [18]. The
synthesis procedure is given in Scheme 1 and the details
were described as follows.
2+
and UO2 was also studied by electrospray ionization mass
spectrometry (ESI-MS) and density functional theory (DFT)
study [19], which gives a similar conclusion. These studies
highlight vast opportunity of Et-Tol-DAPhen applied in
group separation of actinides over lanthanides, and also offer
basic data in coordination and extraction of actinides and
lanthanides with this new kind of ligands. Up to now, how-
ever, the questions of whether or not the peripheral substitute
groups of the DAPhen ligand correlates with the binding
ability and selectivity of the resultant extractants has not
been addressed. Structure optimization by tailoring sub-
stitute groups of the DAPhen ligand for the construction of
more effective extractant is of great significance [20]. This
2.1.1 2,9-Dicarboxylic acid-1,10-phenanthroline
Selenium oxide (13.5 g, 0.12 mol) and 2,9-dimethyl-1,10-
phenanthroline (5 g, 0.02 mol) were added into a mixture of
1,4-dioxane (320 mL) and ultra-pure water (50 mL) in a
500 mL of round-bottom flask. The reaction remained under
reflux for 3 h and filtered while hot, then the filtrate was left
to cool. Golden product, 2,9-dicarbaldehyde-1,10-phenan-
throline (4.62 g, 80.56%), was obtained after another filtra-
tion. Then, 2,9-dicarbaldehyde-1,10-phenanthroline (3 g,
0.013 mol) was added into concentrated nitric acid (65%,
60 mL). After refluxing for 3 h, the solution was cooled by
Scheme 1 Synthesis route of DAPhen ligands.

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poured onto the solid ice and the crude product was separated
out gradually. The crude product was filtered to get the solid
product and dried before the next step [18].
nitric acid solutions was contacted with a 1 mL organic phase
containing 1.0 mM Et-Et-DAPhen, But-But-DAPhen, Hex-
Hex-DAPhen and Oct-Oct-DAPhen in 1-(trifluoromethyl)-
3-nitrobenzene, respectively. A phase ratio of 2:1 aqueous:
organic phase was used here to reduce the usage of organic
diluents and achieve a higher extraction capacity. 1-(Tri-
fluoromethyl)-3-nitrobenzene was used as a diluent in view
of its good dissolving capacity towards DAPhen ligands,
strong hydrophobicity, and suitable density. Prior to extrac-
tion, the organic phase was pre-equilibrated with the aqueous
phase of the same composition as the extraction experiment
except being void of uranium. The kinetics data indicated
that 20 min is enough to reach extraction equilibrium for all
the four ligands. Thus all the extraction experiments were
carried out for 30 min with the aid of a vortex shaker. After
2 min of centrifugation at 6000 r/min, the concentration of
2.1.2 N,N′-Diethyl-N,N′-diethyl-2,9-diamide-1,10-phenan-
throline (Et-Et-DAPhen)
2,9-Diacyl chloride-1,10-phenanthroline was synthesized by
adding 2.9-dicarboxylic acid-1,10-phenanthroline (2.86 g,
10.68 mmol) into the thionyl chloride (70 mL) and refluxing
under 85 °C for 3 h. After removal of the thionyl chloride by
reduced pressure distillation, dichloromethane (70 mL) was
added as the solvent, and diethylamine (1.72 g, 23.51 mmol)
and triethylamine (4.76 g, 47.00 mmol) were added slowly
into the mixture. After 3 h refluxing under nitrogen atmo-
sphere, the solvent was removed by reduced pressure dis-
tillation. The residue was purified with silica gel column
chromatography (eluent: CH3OH/CHCl3=1/50), yielding
white powders of ligand a (3.16 g, 78.08%). ESI-MS: m/z (M
2+
UO2 in the aqueous phase was determined by inductively
coupled plasma optical emission spectrometer (ICP-OES,
Horiba JY2000-2, Japan). For ICP-OES measurement (the
detection limit is below 0.01 ppm), the supernatant was di-
luted 25–100 times to make sure that the UO22+ concentration
in the dilution is 1–5 μg/mL. The distribution ratio (D) was
calculated as follows:
1
+H)+ 379.2. H NMR (500 MHz, CDCl3): δ=8.64 (d, J=
8.0 Hz, 2H), 8.12 (s, 2H), 7.95 (d, J=8.0 Hz, 2H), 3.55 (q, J=
8.0 Hz, 4H), 3.41 (q, J=8.0 Hz, 4H), 1.22 (t, J=8.0 Hz, 12H).
N,N′-Dibutyl-N,N′-dibutyl-2,9-diamide-1,10-phenanthro-
line (But-But-DAPhen), N,N′-dihexyl-N,N′-dihexyl-2,9-dia-
mide-1,10-phenanthroline (Hex-Hex-DAPhen) and N,N′-
dioctyl-N,N′-dioctyl-2,9-diamide-1,10-phenanthro-line
(Oct-Oct-DAPhen), were synthesized following the same
procedure as that used for N,N′-diethyl-N,N′-diethyl-2,9-
diamide-1,10-phenanthroline except that dibutylamine, di-
hexylamine and diotcylamine instead of diethylamine were
used, respectively. All the ligands were characterized by ESI-
MS and NMR, as shown as follows: N,N′-Dibutyl-N,N′-di-
butyl-2,9-diamide-1,10-phenanthroline (yield 66.1%), ESI-
MS: m/z (M+H)+ 491.4. 1H NMR (500 MHz, CDCl3): δ=8.63
(d, J=8.3 Hz, 2H), 8.11 (s, 2H), 7.92 (d, J=8.3 Hz, 2H), 3.51
(q, J=7.5 Hz, 4H), 3.40 (q, J=7.5 Hz, 4H), 1.70–1.64 (m,
8H), 1.44–1.39 (m, 4H), 0.99–0.96 (m, 10H), 1.55 (t, J=
7.5 Hz, 6H). N,N′-Dihexyl-N,N′-dihexyl-2,9-diamide-1,10-
phenanthroline (yield 37.8%), ESI-MS: m/z (M+H)+ 603.5.
(Ci Cf)
2Cf
D =
,
(1)
where Ci and Cf represent the initial and final concentrations
of U(VI) in the aqueous phase, respectively; 2 designates the
volume ratio of organic phase and aqueous phase during the
extraction. Above equations for calculation of the D value
made a convenience that only the aqueous phase was mea-
sured. All values were measured in duplicate with un-
certainty within 5%.
2+
For UO2 stripping, the organic phase loaded with UO22+
was contacted with an equal volume of ultrapure water
during 30 min at room temperature with the aid of a vortex
shaker. The resulting mixture was centrifuged at 6000 r/min
for 2 min and the organic phase was separated completely
2+
from the aqueous phase. The concentration of UO2 in the
1H NMR (500 MHz, CDCl3): δ=8.63 (d, J=8.0 Hz, 2H), 8.11
(s, 2H), 7.92 (d, J=8.0 Hz, 2H), 3.51–3.44 (m, 8H), 1.68–
1.62 (m, 8H), 1.35 (m, 12H), 0.91 (m, 18H), 0.49 (m, 4H). N,
N′-Dioctyl-N,N′-dioctyl-2,9-diamide-1,10-phenanthroline
(yield 22.2%), ESI-MS: m/z (M+H)+ 735.2. 1H NMR
(500 MHz, CDCl3): δ=8.63 (d, J=8.0 Hz, 2H), 8.12 (s, 2H),
7.93 (d, J=8.0 Hz, 2H), 3.50 (q, J=8.0 Hz, 8H), 1.61 (m, 8H),
1.37–1.30 (m, 20H), 0.93–0.88 (m, 26H), 0.67 (m, 6H).
aqueous phase was determined by the ICP-OES method.
2.3 UV-Vis titration
The absorption spectroscopy was measured by a HITACHI
2+
U-3600 (Japan). Stoke solutions of 0.002 M UO2 in acet-
onitrile were prepared by dissolving the UO2(NO3)3·6H2O
solid into HPLC grade acetonitrile; The concentration of li-
gands in acetonitrile was 2.0×10−5 M. And 0.01 M tetra-
ethylammonium nitrate (Et4NNO3) was added into the ligand
solutions to control the ionic strength. Each 10 μL of the
2.2 Solvent extraction experiments
2+
2+
A stock solution of UO2 was prepared by dissolving the
UO2 solutions were titrated into 5 mL of the ligand solu-
UO2(NO3)3·6H2O solid with ultrapure water in a volumetric
flask. A 2 mL aqueous phase consisting of 0.5 mM UO22+ in
tion. After mixing for 15 min, the absorption spectroscopy of
the solution was measured with a 1.0 cm quartz cell. The

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2+
recorded wavelength range was 250–400 nm with a band-
width of 1 nm. The stability constants were calculated with
the HypSpec program [22,23].
UO2 extraction occurred for Et-Et-DAPhen, whereas the
other three DAPhen ligands exhibit a strong extraction
ability toward UO22+. Under the test conditions in 3 M HNO3
solution, the distribution ratios by But-But-DAPhen, Hex-
Hex-DAPhen and Oct-Oct-DAPhen were determined to be
118, 92 and 90, respectively. Although these values are lower
than that for Et-Tol-DAPhen [25], these ligands represent
2.4 EXAFS measurement
The samples for EXAFS studies were prepared by liquid-
liquid extraction method at room temperature. Specially,
2 mL of 3 M nitric acid aqueous phase containing 17.5 mM
2+
highly efficient UO2 extractants. With regard to Et-Et-
DAPhen, the poor extraction ability probably results from
the fact that the short alkyl chain in diamide moieties make
the ligand hard to dissolve in organic solvent but prefer to
transfer into the aqueous phase during the extraction. Be-
sides, it is notable that among these DAPhen ligands, But-
But-DAPhen shows the highest extraction ability towards
UO22+ regardless of the concentrations of HNO3 and the Hex-
Hex-DAPhen and Oct-Oct-DAPhen were slightly worse,
indicating that the long alkyl chain has the steric effect thus
weaken the coordination of UO22+ with the donor atoms. The
results here clearly reveal the important roles of alkyl chain
in diamide moieties for determing the extraction ability of
DAPhen ligand.
2+
UO2 and 2 mL of 1-(trifluoromethyl)-3-nitrobenzene with
17.5 mM ligands were mixed in a stoppered test tube and
vigorously shaken by a vortex mixer for 30 min. Before the
extraction process, the organic phase was pre-equilibrated
with 3 M HNO3 for 10 min at room temperature. An aliquot
of the organic phase was transferred to the EXAFS sample
holder and sealed with Kapton tape.
The EXAFS spectra at the U-LIII edge (17166 eV) were
recorded in fluorescence mode in the range of 17.0–17.9 keV
[24], using synchrotron radiation at the beamline 1W1B of
Beijing Synchrotron Radiation Facility (BSRF). A silicon
(111) double-crystal monochromator was used to tune the
incident X-ray beam to the desired energies. An yttrium foil
(K-edge 17038 eV) was simultaneously measured in trans-
mission mode for energy calibration of the monochromator.
The data processing was analyzed with the ATHENA code
[23]. The E0 energy was set at the maximum of the absorp-
tion edge. The maximum energy of the absorption edge
confirms the redox states of +VI for uranium (17173.6 eV).
The linear pre-edge background was subtracted and nor-
malized to extract the EXAFS signal. Pseudoradial dis-
tribution functions (PRDF) were acquired by Fourier
transform in k3x (k) with the ATHENA code between 2.5 and
13.5 Å−1. Using ARTEMIS code, the R factor (%) and the
quality factor (QF, reduced chi-square) of the fits were ob-
tained. For EXAFS data analysis, the theoretical phase shift
and amplitude functions for single and double scattering
paths are calculated by the program FEFF6 and optimized as
implemented in the FEFFIT code using the model structure
of U-Et-Tol-DAPhen complex reported in our previous work
[18]. Experimental data were then fitted using four in-
dividual single scattering paths. Fitting procedure was per-
formed on the k3-weighted FT-EXAFS from 1.0 to ~3.0 Å.
The amplitude reduction factor S02 was fixed at 1.0, and the
shifts in the threshold energy ΔE0 were linked to all the paths.
In addition, stripping experiments were also carried out
given continuous operation of the extraction systems and
cyclic utilization of the extractants, in which the extracted
2+
UO2 in the organic phase containing But-But-DAPhen,
Hex-Hex-DAPhen and Oct-Oct-DAPhen ligands, respec-
tively, were simply back-extracted into ultra-pure water. As
shown in Figure 2, the stripping percentage for the ultrapure
water in one stage is more than 90%, and following four
stages of stripping experiments, an almost complete stripping
(~100%) of UO22+ can be achieved. Thus, from the point of
easy recycling view, But-But-DAPhen, Hex-Hex-DAPhen
and Oct-Oct-DAPhen in 1-(trifluoromethyl)-3-nitrobenzene
are potentially suitable to be applied in real UO22+ separation.
3.2 UV-Vis titration of the UO22+ complexes
To further understand the coordination of UO22+ with Et-Et-
DAPhen, But-But-DAPhen, Hex-Hex-DAPhen and Oct-Oct-
DAPhen, UV-Vis titration was explored here to determine
2+
the stoichiometry and stability constant of the UO2 com-
plexes in acetonitrile. Figure 3 shows the UV-Vis absorption
2+
spectra of UO2 with Et-Et-DAPhen, But-But-DAPhen,
Hex-Hex-DAPhen and Oct-Oct-DAPhen at various UO22+
2+
concentrations. As can be seen, with the increasing of UO2
concentration, the UV-Vis spectra for the four ligands exhibit
a similar variation trend. Specifically, the distinct absorption
band for ligands at 273 nm gradually decreases and a new
3 Results and discussion
2+
3.1 Solvent extraction studies
absorption band assigned to UO2 complexes appears at
2+
The effects of HNO3 concentration on UO2 extraction by
301 nm. No obvious band shift and shape variations of the
spectra were observed for all the four ligands. We thus as-
Et-Et-DAPhen, But-But-DAPhen, Hex-Hex-DAPhen and
Oct-Oct-DAPhen in 1-(trifluoromethyl)-3-nitrobenzene sol-
vent were investigated. As shown in Figure 1, almost no
2+
sume that a similar UO2 complex with the same stoichio-
metry and the similar structure formed when changed the

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trend Et-Et-DAPhen≈Oct-Oct-DAPhen<Hex-Hex-DAPhen
<But-But-DAPhen (Table 1), paralleling to the trend in the
extraction results (Figure 1). When the length of alkyl chain
becomes longer, the stability constant becomes smaller,
which probably attributed to the steric effect on the com-
plexation as mentioned above. Although different solvents
were used in this article and that for Et-Tol-DAPhen, the
larger stability constant of Et-Tol-DAPhen denotes a better
2+
binding affinity between UO2 and Et-Tol-DAPhen, which
are in good agreement with the fact that Et-Tol-DAPhen
2+
shows better UO2 extraction under the same condition.
These results indicated that the substituent group in the
diamide of DAPhen may tailor the extraction ability of this
2+
Figure 1 UO22+ extraction by 1.0 mM Et-Et-DAPhen, But-But-DAPhen,
Hex-Hex-DAPhen and Oct-Oct-DAPhen in 1-(trifluoromethyl)-3-ni-
trobenzene as a function of HNO3 concentration at room temperature (color
online).
ligand by affecting its binding affinity with UO2
.
2+
3.3 EXAFS analysis of the UO2 complexes
EXAFS spectra at the U-LIII edge were recorded to further
2+
characterize the UO2 coordination structure in organic
phase. Given the poor signal-to-noise ratio of the spectra for
2+
the UO2 complex with Oct-Oct-DAPhen, we here only
2+
collected the EXAFS spectra of the UO2 complexes with
Et-Et-DAPhen, But-But-DAPhen and Hex-Hex-DAPhen
(see experimental section for the details). Figure 4 shows the
comparison of the k3-weighted EXFAS spectra and the FT
magnitude of the three solution samples (A, B and C). Also
shown in Figure 4 is the spectra of crystalline UO22+ complex
with Et-Tol-DAPhen for reference (D). It can be seen that
both the oscillation mode and intense FT peaks at the range
of 1–4 Å (not phase correction) for spectrum A, B and C are
in good agreement with each other as well as with that of
crystalline UO22+ complex with Et-Tol-DAPhen. Specifically
in the FT magnitude (Figure 4(b)), the EXAFS spectra are
dominated by the two axial Oyl contributions (peak 1).
Contributions at R+ΔR<1.0 Å are low frequencies spline
removal artifacts. Equatorial coordination is split into two
distinct contributions (Figure 4, peaks 2 and 3). In crystalline
UO22+ complex with Et-Tol-DAPhe, the equatorial split from
the DAPhen ligand. The similar EXAFS spectra for ions
coordinate DAPhen ligands in a similar mode regardless of
the substituent group in the diamide. After reasonable fittings
of the spectra, the metric parameters including coordination
number (CN), interatomic distance (R), Debye-Waller factor
(σ2), energy shift (ΔE), and R-factor were extracted, as pre-
sented in Table 2. It is clear parameters, which further con-
Figure 2 UO2+ stripping from UO22+-loaded organic phases containing
DAPhen ligands using the same volume of ultra-pure water. Red, green and
blue color represent But-But-DAPhen, Hex-Hex-DAPhen, and Oct-Oct-
DAPhen, respectively (color online).
ligand from Et-Et-DAPhen to Oct-Oct-DAPhen. Then, the
stoichiometries of the complexes were determined by fitting
the experimental data using the HypSpec program [17]. The
results suggest that 1:1 stoichiometry well matches the ex-
perimental data for all the four ligands. That is, one UO22+ ion
bonds one ligand even at a low UO22+ concentration. This is
consistent with that for previously reported Et-Tol-DAPhen
[25], giving an indication that tailoring the substituent group
in the diamide of DAPhen ligand from ethyl to octyl and
2+
2+
even to tolyl does not change the stoichiometry of UO2
firm a similar type of the UO2 coordination in the UO22+
2+
complex. The stability constants (logβ) of the UO2 com-
complexes with all the test DAPhen ligands including re-
ported Et-Tol-DAPhen. Detailedly, U–Oax distances of
plexes were also calculated as listed in Table 1. The stability
constants of Et-Tol-DAPhen is given for comparison. The
results show that all of the four ligands have comparable
complexation ability towards UO22+ in acetonitrile with logβ
ranged from 4.0 to 4.5. The stability constants, i.e., the
binding affinity between UO22+ and these ligands follows the
2+
~1.77 Å obtained are typical of UO2 compounds [26,27],
and the U–Oax σ2 value (0.001–0.002 Å2) are consistent with
the reported value for UO22+ (0.001–0.003 Å2) [28,29]. There
are two O atoms at a distance of ~2.36 Å and four O/N atoms
at a distance of ~2.53 Å, comprising the first equatorial shell

6 . . . . . . . . . . . . . . . . . . . . . . . . . . . .Zhang et al. Sci China Chem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 3 Spectrophotometric titration of Et-Et-DAPhen, But-But-DAPhen, Hex-Hex-DAPhen and Oct-Oct-DAPhen (CL=2×10−5 M) with UO2(NO3)2 in
acetonitrile (CM=0–3.0×10−5 M), T=25 °C, I=0.01 M Et4NNO3 (color online).
2+
Table 1 Stability constants (logβ) for UO2 complexes with Et-Et-DA-
Phen, But-But-DAPhen, Hex-Hex-DAPhen and Oct-Oct-DAPhen in acet-
onitrile determined by UV-Vis spectrometry
to tolyl does not change the coordination behavior of this
ligand with UO22+
.
Ligand
complexation reaction
UO22++L=UO2L2+
logβ
4.07
Et-Et-DAPhen
But-But-DAPhen
Hex-Hex-DAPhen
Oct-Oct-DAPhen
Et-Tol-DAPhen
4 Conclusions
4.51
We report here a detail investigation on UO22+ extraction and
coordination with four tetradentate ligands, N,N′-diethyl-N,N′-
diethyl-2,9-diamide-1,10-phenanthroline (Et-Et-DAPhen),
N,N′-dibutyl-N,N′-dibutyl-2,9-diamide-1,10-phenanthroline
4.32
4.02
4.68 [18]
of UO22+ ions. One N atom at ~2.9 Å was fitted as the second
equatorial shell, and the U–N distance is well consistent with
(But-But-DAPhen),
N,N′-Dihexyl-N,N′-dihexyl-2,9-dia-
mide-1,10-phenanthroline (Hex-Hex-DAPhen) and N,N′-
dioctyl-N,N′-dioctyl-2,9-diamide-1,10-phenanthroline (Oct-
Oct-DAPhen), aiming to characterize the relationship be-
tween properties and substitute groups of DAPhen ligands.
The results suggest that tailoring the substituent group in the
diamide of DAPhen from ethyl to octyl and even to tolyl does
not change the coordination structure of the UO22+ com-
plexes, but affects the extraction ability of the ligands in the
following two ways: (1) a short alkyl chain in diamide
moieties make the ligand hard to dissolve in organic solvent
but prefer to transfer into the aqueous phase during the ex-
traction, thus leading to a poor UO22+ extraction. (2) A long
alkyl chain or a bulky substituent group exhibits clear steric
2+
that in reported crystalline UO2 complex with Et-Tol-DA-
2+
Phen, in which one nitrate ion binds one UO2 ion in bi-
dentate fashion. All the above results clearly suggest that the
UO22+ complexes with Et-Et-DAPhen, But-But-DAPhen and
Hex-Hex-DAPhen have the similar coordination structure
with that of crystalline UO22+ complex with Et-Tol-DAPhen.
2+
That is, one DAPhen ligand bonds one UO2 ion in tetra-
dentate mode through two amide oxygen atoms and two
pyridine nitrogen atom, while nitrate oxygen atoms bridge
2+
one nitrate ion and one UO2 ions in bidentate mode. The
same structure also indicates that tailoring the substituent
group in the diamide of DAPhen from ethyl to octyl and even

7. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zhang et al. Sci China Chem
Table 2 Metric parameters obtained by fitting of the EXAFS spectra
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Sample
Shell
U–Oax
U–O1
CNa)
2f)
R (Å)b)
1.77
2.37
2.52
2.94
1.77
2.41
2.53
2.94
1.79
2.42
2.52
2.95
1.78
2.41
2.52
2.94
s2 (Å2)c)
0.002
0.007
0.007
0.002
0.002
0.008
0.008
0.001
0.001
0.006
0.006
0.001
0.002
0.006
0.007
0.001
DE (eV)d)
9.0
R-factore)
0.013
2.1
3.8
1.2
2f)
9.0
–
UO22+-EED
U–O2/N1
U–N2
9.0
–
9.0
–
U–Oax
U–O1
9.7
0.008
UO22+-BBD
UO22+-HHD
2.2
4.4
0.8
2f)
9.7
–
U–O2/N1
U–N2
9.7
–
9.7
–
U–Oax
U–O1
8.2
0.006
2.3
3.9
1.0
2f)
8.2
–
U–O2/N1
U–N2
8.2
–
8.2
–
U–Oax
U–O1
7.2
0.012
2.1
3.8
1.2
7.2
–
–
–
UO22+-ETD
U–O2/N1
U–N2
7.2
7.2
a) Coordination number, N±~20%; b) interatomic distance, R±~0.03 Å;c) Debye-Waller factor; d) energy shift linked for all the paths; e) goodness of fit
2+
parameter; f) fixed parameter; UO22+-EED, UO22+-BBD, UO22+-HHD and UO22+-ETD denote the UO2 complexes with Et-Et-DAPhen, But-But-DAPhen,
Hex-Hex-DAPhen, and Et-Tol-DAPhen, respectively.
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21471153, 21777161, 21477130, 21790373),
the Youth Innovation Promotion Association, Chinese Academy of Sciences
and the Science Challenge Project (JCKY2016212A504). We are grateful to
the staff of Beijing Synchrotron Radiation Facility (BSRF) for EXAFS
measurement.
Conflict of interest The authors declare that they have no conflict of
interest.
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Figure 4 (a) Raw U LIII-edge k3 weighted EXAFS spectra of the UO2
2+
complex with Et-Et-DAPhen (A), But-But-DAPhen (B), and Hex-Hex-
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effect thus weaken the extraction ability of DAPhen ligand
by reducing its binding affinity (positively correlated to the
stability constants) with UO22+. The findings of this article
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