Excellent selectivity for actinides with a tetradentate 2,9-diamide-1,10-phenanthroline ligand in highly acidic solution: A hard-soft donor combined strategy

Article abstract of DOI:10.1021/ic402784c

In this work, we reported a phenanthroline-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. The synthesis and solvent extraction as well as complexation behaviors of the ligand with actinides and lanthanides are studied experimentally and theoretically. The ligand exhibits excellent extraction ability and high selectivity toward hexavalent, tetravalent, and trivalent actinides over lanthanides in highly acidic solution. The chemical stoichiometry of Th(IV) and U(VI) complexes with Et-Tol-DAPhen is determined to be 1:1 using X-ray crystallography. The stability constants of some typical actinide and lanthanide complexes of Et-Tol-DAPhen are also determined in methanol by UV-vis spectrometry. Density functional theory (DFT) calculations reveal that the An-N bonds of the Et-Tol-DAPhen complexes have more covalent characters than the corresponding Eu-N bonds, which may in turn lead to the selectivity of Et-Tol-DAPhen toward actinides. This ligand possesses merits of both alkylamide and 2,9-bis-(5,6-dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthroline (R-BTPhen) extractants for efficient actinide extraction and the selectivity toward minor actinides over lanthanides and hence renders huge potential opportunities in high-level liquid waste (HLLW) partitioning.

Full text of DOI:10.1021/ic402784c

Article
pubs.acs.org/IC
Excellent Selectivity for Actinides with a Tetradentate 2,9-Diamide-
1,10-Phenanthroline Ligand in Highly Acidic Solution: A Hard−Soft
Donor Combined Strategy
Cheng-Liang Xiao,† Cong-Zhi Wang,† Li-Yong Yuan,† Bin Li,‡ Hui He,‡ Shuao Wang,§ Yu-Liang Zhao,†
Zhi-Fang Chai,*,†,§ and Wei-Qun Shi*,†
†Key Laboratory of Nuclear Radiation and Nuclear Energy Technology and Key Laboratory For Biomedical Effects of Nanomaterials
and Nanosafety, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P.R. China
‡Division of Radiochemistry, China Institute of Atomic Energy, Beijing 102413, P.R. China
§School of Radiological and Interdisciplinary Sciences, Soochow University, Suzhou 215123, P.R. China
S
* Supporting Information
ABSTRACT: In this work, we reported a phenanthroline-
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 separa-
tion of actinides over lanthanides. The synthesis and solvent
extraction as well as complexation behaviors of the ligand with
actinides and lanthanides are studied experimentally and
theoretically. The ligand exhibits excellent extraction ability
and high selectivity toward hexavalent, tetravalent, and
trivalent actinides over lanthanides in highly acidic solution.
The chemical stoichiometry of Th(IV) and U(VI) complexes
with Et-Tol-DAPhen is determined to be 1:1 using X-ray
crystallography. The stability constants of some typical actinide
and lanthanide complexes of Et-Tol-DAPhen are also determined in methanol by UV−vis spectrometry. Density functional
theory (DFT) calculations reveal that the An−N bonds of the Et-Tol-DAPhen complexes have more covalent characters than the
corresponding Eu−N bonds, which may in turn lead to the selectivity of Et-Tol-DAPhen toward actinides. This ligand possesses
merits of both alkylamide and 2,9-bis-(5,6-dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthroline (R-BTPhen) extractants for efficient
actinide extraction and the selectivity toward minor actinides over lanthanides and hence renders huge potential opportunities in
high-level liquid waste (HLLW) partitioning.
multiple extractant system in the second cycle. Actually, several
extractant-combined systems have been investigated prelimi-
narily, such as tributyl phosphate (TBP) and 2,6-bis(5,6-dialkyl-
1,2,4-triazin-3-yl)-2,2′-bipyridine (R-BTBP)5,8−10/2,9-bis-(5,6-
dialkyl-1,2,4-triazin-3-yl)-1,10-phenanthroline (R-BTPhen)5
(Figure 1), N,N-di-2-ethylhexyl-butylamide (DEHBA) and R-
BTBP (Figure 1),7 N,N,N′,N′-tetra-n-octyl-diglycolamide
(TODGA), and N,N′-dimethyl-N,N′-dioctyl-2-(2-
hexyloxyethyl)malonamide (DMDOHEMA).6
To simplify the GANEX process and improve the separation
efficiency, a simple alternative extractant seems more promising
in comparison with the multiple extractant combination. This is
a difficult task because the challenge of the GANEX process
comes from the separation of MA(III) from Ln(III). MA(III)
and Ln(III) have similar chemical properties, such as oxidation
state, ionic radius, hydration, and complexation mode.
Extractants containing soft sulfur or nitrogen atoms are
INTRODUCTION
■
After the emergence of nuclear power plants, the environmental
issues associated with spent fuels accompanying electricity
generation have been of great concern.1 Though U and Pu can
be recovered by the traditional PUREX (Plutonium and
Uranium Recovery by Extraction) process, minor actinides
(MA) still remaining in the high-level liquid waste (HLLW)
become the predominant long-lived radiotoxic elements and
will pose long-term risks toward the environment. The so-
called partitioning and transmutation (P and T) strategy2−4
may provide a reasonable approach to achieve the minimization
of nuclear waste and to reduce the threat of MA.
To avoid nuclear proliferation and meet the requirements of
the nuclear fuel cycle in the Generation IV nuclear energy
system, a conceptual GANEX (Group ActiNide EXtraction)
process5−10 was proposed to recover almost all actinides (U,
Np, Pu, Am, and Cm) together from highly acidic spent fuel
solution. After the first cycle for the selective separation of
uranium with a monoamide extractant, transuranium elements
in varous oxidation states are simultaneously extracted using a
Received: November 18, 2013
© XXXX American Chemical Society
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Figure 1. Promising ligands for lanthanide/actinide separation mentioned in this work.
Scheme 1. Synthesized Route of Ligand 1
preferred to recognize MA(III) over Ln(III).11−18 2,6-Bis(5,6-
dialkyl-1,2,4-triazin-3-yl)pyridine (R-BTP),19−23 R-
BTBP,11,24−30 and R-BTPhen16,24,27,31 ligands have been the
successful representatives for the separation of Ln(III)/MA(III)
in the past 15 years. However, light actinides (U, Np, Pu)
normally favor ligands containing hard oxygen atoms such as
alkylamide and alkylphosphate.32−37 If solely considering the
selectivity for light actinides using hard-donor ligands, the
separation of MA(III) from Ln(III) would be difficult to
achieve. Although some attempts to design single extractants
for group separation of actinides from lanthanides have been
carried out by Miguirditchian and his co-workers,38,39 the
distribution ratios of actinides and separation factors of
actinides toward lanthanides are relatively low in highly acidic
solutions. It is possible that the cavity of 6,6″-(2,2′:6′,2″-
terpyridine)diamide (R-DATP)38,39 (Figure 1) defined by the
terpyridine and two amide groups does not match all actinides.
In addition, the rotation of σ bonds linked with two pyridine
heterocycles also hinders the complexation of the ligand with
actinide ions. Additionally, Charbonnel et al.40 reported a kind
of 2,9-dipyridyl-l,10-phenanthroline ligands grafted with one or
two amide groups, which showed difficult to separate all the
actinides over lanthanides. Therefore, it is still highly desirable
to design and synthesize more highly efficient extractants for
selectively complexing all actinides over lanthanides.
In this work, we first synthesized and characterized a novel
N-heterocyclic tetradentate ligand (ligand 1) consisting of a
phenanthroline moiety with soft nitrogen atoms showing
selectivity for MA(III) over Ln(III) and two amide moieties
with hard oxygen atoms in favor of light actinides, such as Th,
U, Np, and Pu. The solvent extraction experiments of ligand 1
toward some actinides and lanthanides were investigated in
cyclohexanone solvent. The crystal structures of ligand 1 with
2+
UO2 and Th4+ were characterized to elucidate the extraction
mechanism. The stability constants (log β) for some actinides
and lanthanides complexes with ligand 1 were also determined
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evaporate slowly. Flaky yellow crystals suitable for X-ray crystallo-
graphic measurement were obtained over 3 days.
in methanol by UV−vis spectrometry. To further understand
the mechanism of exceptional selectivity for actinides, the
coordination modes and bonding nature of ligand 1 with some
actinide and lanthanide ions were studied by quantum chemical
calculations with denstity functional theory (DFT) method.
Th(NO3)4 Complex Crystals 3. The ligand 1 (50.01 mg, 0.10 mmol)
was dissolved in acetone (4 mL). To this mixture was added a solution
of Th(NO3)4·5H2O (57.02 mg, 0.10 mmol) in deionized water (4
mL). After stirring for half an hour, the solution was left to evaporate
slowly. Light-yellow flaky crystals suitable for X-ray crystallographic
measurement were obtained over 4 days.
EXPERIMENTAL SECTION
■
Solvent Extraction Experiments. All the radioactive experiments
were performed in the glovebox. The aqueous phases were prepared
with stock solution of 241Am3+, UO22+, Th4+, La3+, Eu3+, Yb3+, Co2+,
Ni2+, Zn2+, and Sr2+ in different HNO3 concentration (0.1−4.0 mol/
L). The organic phases were prepared by dissolving ligand 1 (0.01
mol/L) in cyclohexanone. Each aqueous phase (1 mL) was vigorously
shaken with each organic phase (1 mL) for 60 min at 25 °C. This
contact time was enough to reach extraction equilibrium. After phase
separation by centrifugation, the activity of α-ray emitters 241Am was
measured by a low-background α scintillation detector (FJ414, Beijing
Nuclear Instrument Factory). The concentrations of other metal ions
were determined by inductively coupled plasma optical emission
spectrometry (ICP-OES, ULTIMA 2, Horiba). The distribution ratio
(D) was calculated by the ratio between the concentration
(radioactivity counts) in the organic phase and aqueous phase. The
separation factor (SF) was calculated by the ratio of distribution ratios
between two metal ions.
General. Chemical reagents such as rare earth nitrates Ln(NO3)3·
6H2O (Ln = La, Eu, and Yb), UO2(NO3)2·6H2O, Th(NO3)4·5H2O,
and other nitrate salts were of analytical grade. Stock solution of 241Am
in 1.0 M HNO3 was provided by China Institute of Atomic Energy
(CIAE). 2,9-Dimethyl-1,10-phenanthroline, selenium oxide, 1,4-
dioxane, thionyl chloride, N-ethyl-p-toluidine, methanol, dichloro-
methane, and other inorganic/organic reagents were of analytical grade
and used without further purification.
Nuclear magnetic resonance (NMR) spectra were measured on a
Bruker Avance III 500 MHz spectrometer with tetramethylsilane as an
internal solvent resonances reference. Fourier transform infrared
spectroscopy (FT-IR) spectra were recorded on a Nicolet Nexus 670
Model instrument using the KBr self-supported pellet technique.
Electrospray ionization mass spectrometry (ESI-MS) data was
obtained on a Bruker Amazon SL instrument in the form of positive
model.
Syntheses. The novel ligand 1 was synthesized according to
Scheme 1. The syntheses of 2,9-dicarbaldehyde-1,10-phenanthroline
and 2,9-dicarboxylic acid-1,10-phenanthroline were mildly modified on
the basis of known procedures.41,42
2,9-Dicarbaldehyde-1,10-phenanthroline. Selenium oxide (5.62 g,
0.05 mol) was added into a mixture of 1,4-dioxane (150 mL) and
deionized water (10 mL) in a 500 mL of three-necked flask. After
heating to reflux, a solution of 2,9-dimethyl-1,10-phenanthroline (5.02
g, 0.024 mol) in 1,4-dioxane (100 mL) was added dropwise into the
flask over 10 min. The reaction remained under reflux for 2 h. The
mixture was filtered while hot, and the filtrate was left to cool. Yellow
needle product (4.78 g, 82.36%) was obtained after filtering and
washing with warm 1,4-dioxane. The crude product was used in the
next step without further purification.
2,9-Dicarboxylic Acid-1,10-phenanthroline. 2,9-Dicarbaldehyde-
1,10-phenanthroline (5.04 g, 0.021 mol) was added into a
concentrated HNO3 solution (65 wt %, 100 mL). After reaction
under reflux for 3 h, the mixture was poured onto solid ice (∼20 g)
and allowed to cool. The light-yellow solid (4.68 g, 83.15%) was
crystallized and filtered from the mother liquor. The crude product
was used in the next step without further purification.
N,N′-Diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline. 2,9-
Dicarboxylic acid-1,10-phenanthroline (2.02 g, 7.54 mmol) was
refluxed with thionyl chloride (50 mL) in an inert atmosphere. After
3 h, the mixture was cooled, and thionyl chloride was removed by
reduced pressure distillation. When the solvent was completely
evaporated, the gray solid was dissolved in dichloromethane (50
mL) and cooled in ice water. Subsequently, N-ethyl-p-toluidine (4.03
g, 29.80 mmol) and triethylamine (6.04 g, 59.68 mmol) were added
slowly into the mixture. The reaction was continued for 3 h at 40 °C
under N2 protection. The solvent was removed under reduced
pressure. The residue was purified with silica gel column
chromatography (eluent: CH3OH/CHCl3 = 1/50) and recrystallized
from CH3OH/CH2Cl2 to yield white powders of ligand 1 (1.82 g,
48.08%). 1H NMR (500 Hz, CDCl3) δ 1.35 (s, 6H, Ben-CH3), δ 2.28
(t, 6H, CH2CH3), δ 4.11 (d, 4H, CH2CH3), δ 7.15 (s, 4H, Ben-H), δ
6.92 (s, 4H, Ben-H), δ 7.59 (d, 4H, Phen-H), δ 8.03 (s, 2H, Phen-H).
FT-IR (KBr, v/cm−1): 1641 (amide), 1550, 1512, 1452, 1395, 1315,
1254, 1186, 1124, 866, 826, 721, 555. ESI-MS: m/z (M + H)+ 503.2,
(M + Na)+ 525.3, (M + K)+ 541.2. The ligand 1 crystals suitable for X-
ray crystallographic measurement were obtained after slow evaporation
from its methanol solution over several days.
X-ray Crystallographic Measurements. X-ray crystallographic
data were collected with graphite monochromated Mo−Kα radiation
(λ = 0.71073 Å) at 293 K on a Bruker Apex II CCD diffractometer.
Data treatments were carried out with the SAINT program.43 The
structures were solved using direct methods and refined on F2 by full-
matrix least-squares with the SHELXTL-97 program.44 The non-
hydrogen atoms were refined with anisotropic displacement
parameters. The hydrogen atoms were placed by geometrical
considerations and were added to the structure factor calculation. X-
2+
ray crystallographic data of ligand 1, UO2 complex 2, and Th4+
complex 3 are summarized in Table S1, Supporting Information [SI].
Selected bond distances and angles are listed in Tables S2−S4, SI.
UV−Vis Spectrophotometric Titration. The stability constants
(β = [MxLyxn+]/[Mn+]x[L]y) were determined by UV−vis spectropho-
tometric titration at 25 °C. The experiments were carried out on a
Shimuzu in a 1.0 cm quartz cell. The concentration of ligand 1 in
methanol was 2.0 × 10−5 M. 0.01 M tetraethylammonium nitrate
(Et4NNO3) was added to control the ionic strength. Each 20 μL of
0.001 M metal nitrates in methanol were titrated into 5 mL of ligand
solution. After mixing vigorously for 5 min, the solution was measured
in a 1.0 cm quartz cell. Such contact time was sufficient to attain the
complexation equilibrium in our experiments. The obtained spectral
data were treated and fitted with the HypSpec program.45,46
Theoretical Methods. All the theoretical calculations were carried
out by using the density functional theory (DFT) method with the
Gaussian 09 package.47 The functional used is B3LYP, which combines
Becke’s three-parameter functional (B3)48 with the Lee−Yang−Parr
(LYP) correlation functional.49 For Am, Eu, Th, and U, relativistic
effects were taken into consideration with the quasi-relativistic effective
core potentials (RECPs).50,51 The adopted small-core RECPs replace
60 core electrons for Am, Th, U, and 28 electrons for Eu. With regard
to light atoms like C, H, O, and N, the standard Pople-style polarized
valence triple-ξ 6-311G(d,p) basis set was used for geometry
optimization.
All the complexes were optimized at the B3LYP/6-311G(d,p)/
RECP level of theory in the gas phase. Harmonic vibrational
frequencies were calculated to verify the minimum character of the
optimized structures. The natural atomic charges and Wiberg bond
indices (WBIs) were calculated by natural bond orbital (NBO)
analyses at the same method. The frequencies of all the chemical
species were calculated at the same level of theory on the basis of the
optimized geometries. Other detailed methods were the same as our
previous studies.14,28,32,52,53 Theoretical Cartesian coordinates of the
optimized structures are listed in Table S5, SI.
UO2(NO3)2 Complex Crystals 2. The ligand 1 (25.10 mg, 0.05
mmol) was dissolved in acetone (2 mL). To this mixture was added a
solution of UO2(NO3)2·6H2O (25.12 mg, 0.05 mmol) in deionized
water (2 mL). After stirring for half an hour, the solution was left to
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Figure 2. X-ray crystal structure of ligand 1, (a) single molecule; (b) packing pattern; (c) π−π stacking between phenanthroline and phenanthroline;
(d) π−π stacking between phenyl and phenyl. The anions and solvents have been omitted for clarity.
ligand crystals are formed with multiple π−π stackings, which
construct quantities of nanosized channels (Figure 2b). The
π−π stacking distances between two neighboring phenanthro-
line moieties (Figure 2c) are 3.817 Å, while those between two
neighboring tolyl moieties are 4.061 Å (Figure 2d).
RESULTS AND DISCUSSION
■
Syntheses and Ligand Structure. As we know,
extractants containing amide groups have strong affinity for
light actinides in high oxidation states.32,34,36,37 Additionally,
phenanthroline-based ligands were reported to have high
selectivity for MA(III) over Ln(III).16,24,27,31,54 Thus, we
intended to design a novel ligand for separating all actinides
from other fission products by combining amide groups with
the phenanthroline skeleton.
N,N′-Diethyl-N,N′-ditolyl-2,9-diamide-1,10-phenanthroline
(Et-Tol-DAPhen, 1) was synthesized in four steps using 2,9-
dimethyl-1,10-phenanthroline as the starting material (Scheme
1). The first two steps with high yields (>80%) were carried out
according to the reported literatures.41,42 To avoid the reaction
of the diacyl chloride product with water during storage, we
developed a one-pot method to synthesize the ligand 1. After
the diacid product was reacted with thionyl chloride and the
excess solvent was removed, dichloromethane and N-ethyl-p-
toluidine were added in the same flask. Additionally, an organic
base, triethylamine, was introduced to accelerate the reaction.
The ligand 1 could be readily synthesized on a multigram scale
in reasonable yields.
Slow evaporation of the solution of ligand 1 in methanol over
three days afforded colorless crystals suitable for X-ray analysis.
The crystal structure of ligand 1 and its packing patterns are
shown in Figure 2. The ligand structure (Figure 2a) has a near
C2 symmetry, with two tolyl groups on one side and two
oxygen atoms on the opposite side. The torsion angles of C17−
N3−C22−C9 and C16−N4−C13−C7 are −4.188° and
−4.679°, respectively. One methanol molecule is located in
the cavity of ligand 1 with hydrogen bonding. The H---N1 and
H---N2 bond lengths are 2.283 and 2.688 Å, respectively. The
Solvent Extraction Studies. The effect of HNO3
concentration on the extraction properties of Th(IV), U(VI),
Am(III), and Eu(III) by ligand 1 was investigated in
cyclohexanone solvent. As shown in Figure 3a, the Et-Tol-
DAPhen ligand exhibits extremely high extraction ability toward
all actinides even in absence of any synergists. The distribution
ratios of Th(IV), U(VI), and Am(III) are determined to be
205, 25, and 6 in 1.0 M HNO3, respectively. Additionally, it is
obvious that the distribution ratios of all actinides are higher
than that of Eu(III) across the range of HNO3 concentrations.
The separation factors (SF) of Th(IV), U(VI), and Am(III)
toward Eu(III) are 2277, 277, and 67 in 1.0 M HNO3,
respectively. Considering Pu(IV) possesses the same oxidation
states and similar chemical properties with Th(IV), the Et-Tol-
DAPhen ligand should have reasonable extractability toward
Pu(IV). To the best of our knowledge, with respect to the
group separation of actinides using a single extractant, the
distribution ratios and selectivity for all actinides over
lanthanides that we achieved in highly acidic solution in this
case are the most competitive compared to the previous
work.38,39 Generally, extractants containing soft sulfur or
nitrogen atoms are preferred to recognize minor actinides
(Am, Cm) over lanthanides, whereas ligands containing hard
oxygen atoms favor light actinides (U, Th, Np, Pu). If one
wants to separate all actinides from lanthanides in acidic
solution, two extractants containing soft and hard atoms,
respectively, should be combined in one solvent.5−10 In this
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Figure 4. X-ray crystal structure of the [UO2(1)NO3]+ 2. (a) View in
front. (b) View from the top. The anion and solvent molecules have
been omitted for clarity. Selected bond lengths (Å): U1−O1 1.737(9),
U1−O2 1.750(9), U1−O7 2.399(8), U1−O6 2.411(9), U1−O4
2.512(9), U1−O3 2.527(10), U1−N1 2.587(10), U1−N2 2.604(9).
Blue green, blue, red, gray, and dark gray colors denote U, N, O, C,
and H atoms, respectively.
Figure 3. (a) Extraction properties of some actinides and lanthanides
by Et-Tol-DAPhen as a function of HNO3 concentration in
cyclohexanone. (b) Comparison of extraction ability of different
metal ions by Et-Tol-DAPhen in 1.0 M HNO3 solution.
average distance between U(VI) and oxygen atoms of the
amide moieties in 1 is 2.406 Å, whereas the U−O bond
distance of 2.351 Å is found in the unsubstituted phenanthro-
line dicarboxylic acid.55 Such difference might be due to the
decrease of affinity for UO22+ after the introduction of ethyl and
tolyl groups into the amide moieties of 1. Additionally, the
uranium atom is bound to one nitrate anion with U−O bond
distance of 2.519 Å. Another nitrate anion is in the second
coordination layer for keeping the charge balance. The uranyl is
almost located in the plane of phenanthroline with an O6−
U1−O7 angle of 171.593°. Two tolyl groups are still in the
same side similar to ligand 1 (Figure 2). Low distortion of the
ligand structure after complexing with uranyl ion indicates that
the preorganized ligand 1 is suitable for adopting metal ions
work, we combined soft nitrogen atoms and hard oxygen atoms
in one ligand and achieved high extraction ability and excellent
selectivity for all actinides. Besides, the Et-Tol-DAPhen ligand
shows weak extraction ability for the fission and corrosion
products, such as Yb(III), Sr(II), Co(II), Ni(II), and Zn(II)
(Figure 3b). Thus, the Et-Tol-DAPhen ligand is quite
promising in the group separation of actinides from nuclear
wastes.
UO2 and Th4+ Complexes Structures. To provide
2+
structural information for investigating the complexation
behaviors and extraction mechanisms, we synthesized the
complex crystals of ligand 1 with uranium(VI) and thorium-
(IV).
Yellow crystals 2 suitable for X-ray measurements were
obtained by slow evaporation of the solution of UO2(NO3)2·
6H2O and ligand 1 in acetone/water (vol/vol, 1:1). As
with large ionic radii such as UO22+. That might be the reason
2+
that ligand 1 shows such high extraction ability for UO2
.
Th(1)(NO3)4 complexes 3 suitable for X-ray measurements
were prepared using the same method as for the uranyl
complex. The structure of Th(1)(NO3)4 is shown in Figure 5.
The Et-Tol-DAPhen ligand is bound to Th4+ through two
nitrogen atoms of the phenanthroline moiety along with two
oxygen atoms of the amide moieties. Interestingly, the 1:1
structure of thorium complex with ligand 1 is quite different
from the 1:2 structure of the unsubstituted phenanthroline
dicarboxylic acid.55 The steric effect of the substituent groups
and the strong coordinating ability of nitrate ions is the
probable reason for 1:1 coordination of ligand 1 with Th4+. The
average Th−N bond distance is 2.692 Å. In the CSD
database,56,57 we found three structures of thorium 1,10-
2+
displayed in Figure 4, each ligand is bound to one UO2
through two nitrogen atoms of the phenanthroline moiety
together with two oxygen atoms of the amide moieties. The
average bond distance of U−N is 2.596 Å, which is slightly
longer than that of unsubstituted phenanthroline dicarboxylic
acid complexes (2.557 Å).55 In the Cambridge Structural
Database (CSD),56,57 eight structures of uranyl 1,10-phenan-
throline complexes (Figure S1, SI) can be found. Normally, the
U−N bond distances are in the range of 2.557−2.652 Å. A
comparison of the bond lengths between U(VI) and nitrogen
atoms indicates that the structure feature of 1 is similar to the
uranyl 1,10-phenanthroline complexes in the database. The
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Et-Tol-DAPhen. The cavity of Et-Tol-DAPhen perfectly
matches the size of Th4+.
Stability Constant Determination. The complextion
behaviors of ligand 1 with UO22+, Th4+, and Eu3+ were
investigated in methanol by UV−vis spectrometry. The
spectrophotometric titrations of ligand 1 with UO22+, Th4+,
and Eu3+ are shown in Figure 6. All of the spectra have similar
changing trends. With increasing the amounts of metal ions
added, the peak of ligand 1 (273 nm) gradually decreases, and a
new peak (>300 nm) denoting a metal−ligand complex
appears. For every titration curve, obvious isosbestic points
can be seen, which indicates that only one single complexation
Figure 5. X-ray crystal structure of the Th(1)(NO3)4 complex 3. (a)
View in front. (b) View from top. The solvent molecules have been
omitted for clarity. Selected bond lengths (Å): Th−O1 2.447(4), Th−
O2 2.464(5), Th−O10 2.558(5), Th−O4 2.569(5), Th−O12
2.576(5), Th−O9 2.576(5), Th−O3 2.587(5), Th−O13 2.587(5),
Th−O7 2.622(5), Th−O6 2.634(5), Th−N2 2.686(5), Th−N1
2.698(5). Green, blue, red, gray, and dark gray colors denote Th, N,
O, C, and H atoms, respectively.
phenanthroline complexes and only two crystal structures are
available (Figure S2, SI). The Th−N bond distance of the
Th(1)(NO3)4 complex is longer than its hydroxymethyl
analogue (2.630 Å)58 and quite similar to that of the
corresponding dicarboxylic acid complex (2.693 Å).55 Com-
pared to hydroxymethyl analogue, the electron-withdrawing
effect of CO groups is more obvious in Et-Tol-DAPhen,
decreasing the coordination ability of nitrogen atoms toward
Th4+. The average bond distance of Th−Oamide is 2.456 Å,
which is a little longer than that of phenanthroline dicarboxylic
acid (2.430 Å) and shorter than that of phenanthroline
dihydroxymethyl (2.471 Å). This inicates that the complexation
ability of Th4+ with oxygen atoms of different functional groups
decreases in the order of carboxylic acid > amide >
hydroxymethyl. Besides, thorium ion is coordinated to four
nitrates anions with two locating above the phenanthroline
plane and the others below the plane to achieve a coordination
number of 12 for thorium. The average bond distance between
thorium and oxygen atoms of nitrate anions is 2.589 Å.
Compared to the [UO2(1)NO3]+ structure, the orientation of
tolyl groups is changed. The two tolyl groups are located in the
phenanthroline plane on opposite sides (Figure 4b). The
coordination of four nitrate ions to Th4+ makes the space too
crowded to accommodate two tolyl groups at the same side.
This conformation gives the complex the lowest energy to
stabilize in the solid. Furthermore, the Th4+ is perfectly located
in the phenanthroline plane with a O1−Th1−O2 angle of
179.026°, which is attributed to the preorganized structure of
Figure 6. Spectrophotometric titration of ligand 1 (CL = 2.0 × 10−5
M) with (a) UO2(NO3)2, (b) Th(NO3)4, and (c) Eu(NO3)3 in
methanol (CM = 0−3.0 × 10−5 M), T = 25 °C, I = 0.01 M Et4NNO3.
F
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reaction is present. One to one stoichiometries of metal ions
complexes with ligand 1 are well fitted to the experimental data
by the HypSpec program.45,46 The 1:1 complexes of UO22+ and
Th4+ are consistent with those found in the solid state.
The calculated stability constants (log β) are listed in Table
1. The corresponding stability constants of 1,10-phenanthro-
Table 1. Stability Constants (log β) for Some Actinide and
Lanthanide Complexes with Ligand 1, PDAM, and PDA in
Methanol Determined by UV−Vis Spectrometry
metal
ion
ionic radius
(Å)
log
b
log
c
d
complexation reaction
β
β1
log β2
a
2+
2+
UO2
Th4+
Eu3+
0.95
UO2 + L = UO2L2+
4.68
4.39
3.81
4.33
5.01
4.17
>13
0.94
0.89
Th4+ + L = ThL4+
Eu3+ + L = EuL3+
25.7
−
a
This is the apparent size because of no ionic radius for irregular-
59
b
2+
shaped UO2
.
Ligand 1 (2.0 × 10−5 M) in methanol, metal
c
Figure 7. Optimized structures and relative electronic energies of
ligand 1 in different conformers by the B3LYP method in the gas
phase. Sky blue, red, orange, and white colors denote N, O, C, and H
atoms, respectively.
nitrates, T = 25 °C, I = 0.01 M Et4NNO3. 1,10-Phenanthroline-2,9-
dicarboxamide (PDAM, 2.0 × 10−5 M) in deionized water, metal
perchlorates, T = 25 °C, I = 0.1 M NaClO4.60,61 1,10-Phenanthroline-
d
2,9-dicarboxylic acid (PDA, 2.0 × 10−5 M) in deionized water, metal
perchlorates, T = 25 °C, I = 0.01 M NaClO4.55,62
ligand 1 with lanthanides and actinides, we optimized the
structures of Am(1)(NO3)3, Eu(1)(NO3)3, [UO2(1)(NO3)]+,
and Th(1)(NO3)4 (Figure 8) at the B3LYP/6-311G(d,p)/
line-2,9-dicarboxamide (PDAM)60,61 and 1,10-phenanthroline-
2,9-dicarboxylic acid (PDA)55,62 are also given for comparison.
The results show that ligand 1 has reasonable complexation
ability for actinides and lanthanides with log β ≈ 4. The stability
constants of UO22+ and Th4+ are somewhat higher than that of
Eu3+, which agree well with the higher distribution coefficients
for actinides in the solvent extraction experiments. With
different solution and metal salts, the stability constants of
ligand 1 and PDAM for actinides and lanthanides hold almost
the same order of magnitude. This suggested that the
coordination mode and capability of 1,10-phenanthroline-2,9-
dicarboxamide with metal ions did not change much after
further functionalization. However, its dicarboxylic acid
analogue has extremely high stability constants for actinides
and lanthanides. Obviously, the complexation ability of the
carboxylic acid with metal ions is stronger than that of amide.
DFT Calculations. Geometries and Electronic Properties
of Ligand 1. As shown in Figure 7, ligand 1 has four
conformers according to the position of tolyl and ethyl groups.
The relative electronic energies are 0.00, 1.95, 2.52, and 2.53
kcal/mol, respectively. The configuration (conformer 1,
outward−inward) with one outward tolyl and another inward
tolyl group has the lowest energy. If any tolyl group changes its
position, the energy will increase. The stability of the confomers
decreases in the order of outward−inward (4) < outward−
downward (3) < downward−downward (2) < outward−
outward (1). However, there are tiny energy differences
between these four conformers. Thus, ligand 1 may form any
configuration from conformers 1−4, and which one exists in
the solid state depends mainly on the packing effects and
solvent effects.63 Actually, we obtained the single-crystal
structure of conformer 4 (Figure 2) from slow evaporation of
a solution of ligand 1 in CH3OH/CH2Cl2. Additionally, the
configurations of ligand 1 in UO2(1)(NO3) (Figure 4) and in
Th(1)(NO3)4 (Figure 5) belong to conformer 4 and conformer
1, respectively. Due to the steric effect of ethyl groups in the
cavity, conformers 2 and 3 may not be suitable for forming
complexes with other metal ions or coordinating species.
Geometries and NBO Analysis of the Complexes. To
understand the coordination modes and bonding nature of
Figure 8. Optimized structures of (a) Am(1)(NO3)3, (b) Eu(1)-
(NO3)3, (c) [UO2(1)(NO3)]+, (d) Th(1)(NO3)4 by the B3LYP
method in the gas phase. Purple, cyan, sea green, green, sky blue, red,
orange, and white colors denote Am, Eu, U, Th, N, O, C, and H
atoms, respectively.
RECP level of theory in the gas phase. It is a pity that no
crystals of lanthanides complexes with ligand 1 are obtained
even if many endeavors have been made. Fortunately, quantum
chemical calculations can help us to further understand the
structures of these complexes. Similarly to most tetradentate
ligands,30,31,64 ligand 1 is bound to one metal ion through two
nitrogen atoms of the phenanthroline moiety and two oxygen
atoms of the amide moieties forming 1:1 complexes. For the
Am3+ (Figure 8a) and Eu3+ (Figure 8b) complexes, the
coordination number of metal ions is 10 with the nitrate anions
in a bidentate mode. As listed in Table 2, the average bond
distance of Am−N (2.678 Å) is shorter than that of Eu−N
(2.729 Å). This shows that there may be more covalent
characters in the Am−N bond. The bond distance of Eu−OA
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Table 2. Calculated Bond Distances (Å), Natural Charges, and Wiberg Bond Indices (WBIs) of Am(1)(NO3)3, Eu(1)(NO3)3,
UO2(1)(NO3), and Th(1)(NO3)4 Complexes
complex
M−N
M−OA
M−ON
M−NWBI
QM
QN
QO
Am(1)(NO3)3
Eu(1)(NO3)3
[UO2(1)(NO3)]+
Th(1)(NO3)4
2.678
2.729
2.718
2.803
2.569
2.510
2.435
2.508
2.547
2.503
2.468
2.593
0.214
0.205
0.332
0.289
1.390
1.298
1.436
1.081
−0.446
−0.419
−0.419
−0.413
−0.597
−0.602
−0.563
−0.580
(the oxygen atom in the amides) is shorter than that of Am−
OA, which is indicative of stronger coordination of the oxygen
atoms to Eu3+. Thus, the selectivity of ligand 1 toward Am3+
over Eu3+ in the solvent extraction system may be attributed to
the nitrogen atoms in the phenanthroline rather than the
oxygen atom in the amides. The optimized structures of
[UO2(1)(NO3)]+ (Figure 8c) and Th(1)(NO3)4 (Figure 8d)
in the gas phase are similar to the crystal structures in the solid
state. It is noteworthy that the bond distances of M−N and M−
O in the gas phase are longer than those of solid structures.
This phenomenon is very common in theoretical calcula-
tions.14,25,28,32,52,65 For example, the bond distances of U−N
optimized by DFT method are overestimated by 0.03−0.12 Å
compared to the crystal complex of 2,6-bis(5,6-dimethyl-1,2,4-
triazin-3-yl)pyridine (Me-BTP) with uranyl ion.65 These
differences may originate from the neglect of electron
correlation as well as crystal packing effects and solvent effects
in the optimization.
To explore the bonding nature of the M−N bonds, the
natural bonding orbital (NBO) analyses of all the 1:1 species
were performed at the B3LYP/6-311G(d,p)/RECP level of
theory. The Wiberg bond index (WBI) in NBO analysis is
widely accepted to measure the degree of covalency.32,53 As
shown in Table 2, the WBIs of M−N bonds are in the range of
0.205−0.332. Such small values show that the interactions
between metal ions and the ligand are mainly ionic and the
electrostatic interaction dominates the M−N bonds. The WBIs
of An−N bonds are larger than those of Eu−N, suggesting that
more covalent characteristics exist in the An−N bonds. This
finding agrees well with the results of solvent extraction.
Additionally, the natural charges of An (QAn) are also larger
than those on Eu except Th, which indicates that ligand 1 has a
stronger coordinating ability to An than Eu. The natural
charges on the nitrogen atoms of phenanthroline (QN) and the
oxygen atoms of amides (QO) are in the range of −0.446 to
−0.413 and −0.602 to −0.563, respectively.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data and selected bond distances/angles
of ligand 1, UO2 complex 2, and Th4+ complex 3, Cartesian
2+
coordinates of optimized structures. This material is available
free of charge via the Internet at http://pubs.acs.org. Structures
of uranyl/thorium 1,10-phenanthroline complexes in CSD
database.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: shiwq@ihep.ac.cn
*E-mail: zfchai@suda.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Major Research Plan
“Breeding and Transmutation of Nuclear Fuel in Advanced
Nuclear Fission Energy System” of the Natural Science
Foundation of China (Grant Nos. 91326202 and 91126006)
and the National Natural Science Foundation of China (Grant
Nos. 21201166, 11275219, 11105162, and 21261140335), the
“Strategic Priority Research Program” of the Chinese Academy
of Sciences (Grant No. XDA030104), and the China
Postdoctoral Science Foundation (Grant No. 2013M530734).
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We have first synthesized a novel tetradentate phenanthroline-
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