Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 1, pp. 51-54, 2005

Accepted
Study on Stabilization Effect of Neutral Soft Donor on Trivalent Lanthanide and Actinide
Dicarboxylate Complexes by Time-Resolved Laser-Induced Fluorescence Spectroscopy
T. Sasaki,*, a S. Kubo,a T. Kobayashi,a A. Kirishima,b T. Kimura,b T. Kubota,c I. Takagi,a and
H. MoriyamaaaDepartment of Nuclear Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto 606-8501, Japan
Department of Materials Science, Japan Atomic Energy Research Institute, Tokai-mura, Ibaraki 319-1195, Japanc
Research Reactor Institute, Kyoto University, Kumatori, Sennan-gun, Osaka 590-0494, Japanb

Received: November 15, 2004; In Final Form: April 21, 2005
Stability constants for trivalent f-elements of Eu, Am, and Cm with dicarboxylates containing neutral donors suchas secondary amine and ether oxygen were determined by the solvent extraction method in 0.1 M NaClO4 at 25
˚C. Six carboxylic acids of suberic acid, 3,6-dioxaoctanedioic acid, ethylenediamine-N,N’-diacetic acid, glutaricacid, diglycolic acid, and iminodiacetic acid were examined for a systematic comparison. The hydration numberof Eu3+ in the 1:2 complex was also determined by time resolved laser-induced fluorescence spectroscopy. The
role of the softdonor atoms in the complex formation of f-elements and the impact of multiple softdonor atoms on
An/Ln selectivity are discussed.

1. Introduction
In the separation chemistry of trivalent f-elements, it has
been recognized that the most efficient group separation isaccomplished through the agency of donor atoms softer thanoxygen, and extensive efforts are being devoted to design more
powerful and useful extractants having softdonor atoms.
Efficient group separation is needed for the development ofadvanced nuclear fuel cycles that include recovery and transmutation of the minor actinides of Np, Am, and Cm from thefission products. A variety of new reagents and processes havebeen developed and/or are being developed around the world.1−6
In the environment, on the other hand, the naturally-occurring
humic substances contain neutral donor atoms that can coordinate heavy metal ions, and they can make highly stable watersoluble complexes. However, the mechanism of the contribution of neutral atom donation to the stabilization of complex isstill unclear due to the heterogeneous structures involved.
In the present study, the complex stabilization of trivalent An/Lnwith simple dicarboxylates containing nitrogen as a softdonor
was investigated on the basis of their stability constants and of
the number of water molecules in the first coordination sphereof the central metal ion; these data were determined by solvent
extraction and by time resolved laser-induced fluorescence
spectroscopy, respectively.7−8 Comparing these values suggested
the possible structures of these complexes in the aqueous phase.
The role of the donor atoms in the complex formation of f-elements
and the impact of multiple donor atoms on An/Ln selectivity
are also discussed.
2. Experimental
Solvent extraction for formation constants. For the solvent
extraction to obtain the distribution ratio of metal ion D, anaqueous phase, containing 10−2 to 10−8 M carboxylic acid (seeFigure 1) and ca. 0.1 kBq 241 Am and 242 Cm ions each, wasadjusted to the desired pH with sodium hydroxide, perchloric
acid and buffer reagents (a mixture of 5 × 10−3 M MES and
MOPS (Dojindo)). The preparation method of the Am and Cm*Corresponding author. E-mail: sasaki@nucleng.kyoto-u.ac.jp.
FAX: +81-75-753-5831.

O

O

X
O

O

O
O

X–1

O

X

O

X
O

O

O
O

N

X–2
(X = CH2, O, NH)

O

N
O

EDTA

O

O

Figure 1. Scheme of chemical structures of six carboxylic acids as anion
forms: C-2, O-2, N-2, C-1, O-1, and N-1, and EDTA for comparison.

mixture was described previously in detail.9 The six carboxylic
acids used in this study are shown in Figure 1; suberic acid (C-2
as abbreviation), 3,6-dioxaoctanedioic acid (O-2), ethylenediamine-N,N’-diacetic acid (N-2), glutaric acid (C-1), diglycolicacid (O-1), and iminodiacetic acid (N-1). The ionic strength was
adjusted to 0.1 M with NaClO4. Organic solution, containing 1
× 10−3 M TTA (4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione)
as an extractant and TBP (tributyl phosphate) as a synergist in
xylene, was pre-equilibrated with 5 × 10−3 M MES and MOPS
and 0.1 M NaClO4 aqueous solution. The aqueous (3.5 mL)
and organic (2.5 mL) phases were brought into contact in aborosilicate glass vial with screw cap, and were shaken horizontally and gently with the aid of a mechanical shaker at 25 ±1 ˚C. A shaking time of 6 hours was found to be long enoughto attain the extraction equilibrium. For comparison, nonradioactive europium was extracted in the same procedure.
The total recovery of these elements usually was more than97%; no unexpected reactions such as precipitation wereobserved. After phase separation, the pH of the aqueous phase
was measured and taken as the equilibrium pH. A combination glass electrode (D-50, Horiba) was used for pH measurement, and its reference electrode was filled with 3.6 M NaCl +0.4 M NaClO4 to avoid precipitation of KClO4 at the junctionof the electrode and the solution. The correction of theobserved pH value to the pH c (the negative logarithms ofhydrogen ion concentrations) was made following the literature.10 The alpha radioactivity of Am and Cm in both phases
was measured by α-ray spectrometry, using an alpha spectrometer (SOLOIST, EG&G ORTEC) with a silicon surface
detector, after stripping from the organic phase with 0.1 M
HNO3 solution and coprecipitation with SmF3.11 The concentration of Eu was determined by ICP-AES (ICPS-5000,© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 07/31/2005

52

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

Sasaki

Shimadzu). All reagents were of analytical grade and usedwithout further purification.
The extraction equilibrium constant, Kex, of a trivalent metal
ion, M, can be described asKex =[MT3 Bl ]o[H+]3[M][HT] 3o [B]ol(1)where the square brackets denote the concentration, and o, T
and B denote the species in the organic phase, TTA, and TBP,
respectively. When the complex formation of M by TTA isnegligible in the aqueous phase, the distribution ratio of M, D0,
in the absence of a complexation ligand can be expressedsimply by
D0 = [M]o / [M](2)

On the other hand, the distribution ratio, D, in the presence of acomplexation ligand (L) is expressed by
D = [M]o / ([M] + [ML] + [ML2] + … ),(3)

where [ML] and [ML 2 ] are the concentrations of metal
complexes in the aqueous phase. The formation constant of
1:n complex MLn can be expressed by

βn =[MLn ][M][L]n(4)

where n is the number of ligand molecules in the complex.
When it is unlikely that more than three molecules coordinate
to one metal ion, n is considered to be equal to one and two.
Therefore, eq 5 can be described from eqs 2–4. From the plotsof log D vs. log [L] in eq 6, the free parameters of β1, β2, andD0 can be obtained by a least-square fitting analysis.12
D0= 1 + [L]β1 + [L]2β2D(5)log D = log D0 – log (1 + [L]β1 + [L]2β2)(6)Time-resolved laser-induced fluorescence spectroscopy(TRLFS). The Eu3+ in the solution was excited to the 5L6 state
by a pulsed laser beam at 394 nm, and the emission from the5
D0 state to the ground state 7F manifold (7F1), 592 nm, wasmeasured. A 394 nm pulsed laser beam was obtained with apulsed 308 nm output of a Lambda Physik COMpex 201 XeCl
excimer laser pumping PBBO (Lambda Physik) in a dioxane
solution in a Lambda Physik SCANmate 2 dye laser head. The
pulse power was typically 3 – 5 mJ for 394 nm and the pulse
width was about 15 ns. The solution sample was taken in astandard 1 cm fluorimetric cell for luminescence measurement.

The emission light was collected at 90˚ in an Oriel 77257
monochromator and detected by a Hamamatsu R928 photomultiplier tube. The whole emission wavelength range wasobserved. The signal was fed into a digitizing oscilloscope,
which was connected to a computer through a GP-IB interface.
All of the luminescence decays observed in this work wereanalyzed by single-exponential curves.
The apparent and average hydration number of Eu3+ (NH2O)
can be determined from the decay constants (k’obs) of the luminescent excited state of Eu3+ using eq 7.7
NH2O = 1.05 × 10−3 k’obs – 0.44.

(7)

The NH2O,1:2 (for 1:2 complex) was determined at a given pH cby taking into account the fractional abundances of Eu species
Eu3+, EuL+, and EuL 22− (L = dicarboxylate), which were calculated from the stability constants of the complexes and the aciddissociation constants of the ligands.
The contribution from the ligands for C-1, C-2, O-1 and O2 – namely, that from the oxygen donors of carboxyl groups
and ether – to the de-excitation of the excited luminescence isnegligible.7 In the case of N-1 and N-2 complexes, the ligands
contain amino groups capable of coordination with the Eu3+
ion. The N-H oscillators in the ligand have vibronic frequencies closely similar to those of water O-H oscillators, and they
contribute to nonradiative deactivators of the excited Eu(III).
Lis and Choppin estimated that the deactivating frequency of abound N-H oscillator was about 1.5 times higher than that of abound water O-H oscillator for Eu by using dicarboxylatecontaining imino groups.13 Thus, special consideration of theeffect of Eu-N-H bonding on the deexcitation process is required.
3. Results and Discussion
The log β1 and log β2 values for six ligands containing N
and O donors were obtained, as summarized in Table 1. Here,the distribution coefficient to obtain the β did not change
significantly by the addition of pH buffers containing asulfonic acid group. The stability constants increased in theorder of C-1 < O-1 < N-1 and C-2 < O-2 < N-2 for each metal
ion. This general tendency depends on the additional neutral
donors oxygen and nitrogen, which can donate to the metal ion
when the terminal carboxylates are bound to that ion. In
particular, the lone pair electrons on the secondary aminesshow a highly nucleophilic and basic character due to theirhigh electron density, while those on the ether oxygen show aweaker basicity in the Brønsted sense. The variation in theelectric charge on terminal carboxylate groups due to the electron-withdrawing effect by N and O donors can be ignored,since the alkyl chains are present between them. Thus, it was
concluded that the increase of β was caused by the additional
coordination of neutral donors to the central metal ion.

TABLE 1: Summary of Stability Constants of Dicarboxylate Complex (X-1 and -2), NH2O,1:2, and R at 25 ˚C and I = 0.1 M(NaClO4)Eulog β1log β2pHc
NH2O,1:2RAm

log β1log β2log β1log β2
pHc

Cm

C-1

O-1

N-1

C-2

O-2

N-2

EDTA16

3.24 ± 0.04
4.76 ± 0.11
5.30
2.8
3.1

5.57 ± 0.02
9.42 ± 0.03
5.35
2.0
3.5

7.02 ± 0.09
12.6 ± 0.13
5.55
2.2
3.4

2.96 ± 0.05
4.47 ± 0.16
5.30

5.18 ± 0.13
8.90 ± 0.08
5.30
2.0
3.5

9.78 ± 0.10
17.9 ± 0.05
5.55
0.5
4.2

17.29

6.07 ± 0.11
10.23 ± 0.14
6.08 ± 0.13
10.62 ± 0.12
5.40

7.04 ± 0.04
12.42 ± 0.05
7.12 ± 0.05
12.58 ± 0.06
5.60

2.92 ± 0.12
4.49 ± 0.27
3.00 ± 0.08
4.66 ± 0.14
5.35

5.39 ± 0.12
8.59 ± 0.18
5.55 ± 0.12
8.90 ± 0.57
5.50

10.21 ± 0.58
19.59 ± 0.23
10.76 ± 0.71
20.09 ± 0.38
5.60

17.8
18.1

Tetradentate for N-2 and O-2, tridentate for N-1 and O-1, and bidentate for C-1 (and C-2) at its full coordination

J. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

O
O

N H2O

O

N
H

H

H

N

O

O

O

O

Eu

O
O

O
0.5

Eu(O-2)

O

O

O

O

+

1

O

-

Eu(O-2)2

H

O

Eu

O

2

Eu3+

O

4

N

O

mole ratio

6

O

O

O
O

(ii)

Eu

O

O

O

O

8

CH2

O

5

pHc

CH2

O

4

Eu

O

N H2O

O

O
0

3

O

O

Eu(O-1)+
2

O

O

0

O

O

O

0.5

1

O

2

CH2

1

Eu3+

O
O

Eu(O-1)2-

Eu

O

CH2

CH2

O
O

4

CH2

O

O
mole ratio

6

O

O

(i)

8

53

O

Study on Stabilization Effect of Neutral Soft Donor

N

O

O
Eu
O
N
H

N
H

O

Figure 3. Snapshot of possible structures for 1:2 complexes expectedfrom the values of NH2O,1:2 and log β2. The solid and broken triangles
denote an ion-ion and ion-dipole interaction, respectively. The
display of water molecules was omitted.

0

0
1

2

3

4

5

pHc
Figure 2. Fractional abundances of (i) Eu/O-1 and (ii) Eu/O-2 and the
NH2O measured by TRLFS, as a function of pHc. The pHc for the determination of NH2O,1:2 is the same as that for the solvent extraction.

Figure 2 shows the NH2O and fractional abundance of Eu/O-1
and /O-2 species as a function of pHc. The NH 2O obtained inacidic pH was 9.0 ± 0.5, which agreed with the literature value
for the hydrated Eu 3+ ion in H2O.7 The apparent hydration
number NH2O decreases with an increase of the ratio of higher
order complexes.
The summary of NH2O,1:2 for 1:2 complexes, which are calculated on the basis of the fractional abundance, is shown in
Table 1. From the extremely high log β values for ligands
containing N donors, it is suggested that the secondary aminegroups may coordinate directly to the Eu ion. Then, in thecalculation of NH2O for N-ligands, the contribution of a boundN-H oscillator to the luminescence deactivation is assumed to
be as high as 1.3 times that for the O-H oscillator by taking
into account the literature values.13, 14
The number of dehydrated water molecules R in the coordination of one ligand is defined as follows:
NH2O,1:2 + 2R ≡ NH2O,1:1 + R = 9.0,

(8)where it is assumed that R is independent of the order of thecomplex. R values are listed in Table 1. From the NH2O,1:2 of
0.5 for N-2, the R is calculated to be 4.2, and this is reasonable
for a tetradentate ligand. Also, the values of log β1 and log β2
for this complex are the highest in this work, supporting thehigh stabilization by the two amino groups. By considering
the present results of β and R, a snapshot of the possible structures of the europium carboxylate complexes is postulated andillustrated in Figure 3.
In the case of N-2, the stabilization by nitrogen donors is moresignificant in the trivalent actinides than in the lanthanides asshown in Table 1: namely, Eu(N-2)+ < Am(N-2)+ < Cm(N-2)+

and Eu(N-2)2− < Am(N-6)2− < Cm(N-2)2−. The log β1s for Am
and Cm are 3 and 10 times higher than that for Eu, in spite of
their similar ionic radii. 15 It is interesting to compare thepresent result for N-2 with the result for EDTA (ethylenediaminetetraacetic acid), which is known as a hexadentate
nitrogen-containing ligand consisting of two tertiary amines
and four carboxyl groups. The structural frame of EDTA canbe considered as the frame of N-2 (Figure 1) with the additionof two extra carboxyl groups. The literature values of log β1
for EDTA are 17.29 (Eu), 17.8 (Am), and 18.1 (Cm),16 muchlarger than that for N-2. Thus, the selectivity of Am/Eu for
EDTA, defined as ∆log β1 (= log β1,Am – log β1,Eu), is obtained
to be 0.51. This value agrees well with the present finding of
0.43. In the cases except for N-2, however, the selectivity
between Eu and Am/Cm is not significantly observed.
The values of R for N-1 and O-1 are obtained as 3.4 and 3.5,
respectively. They may be consistent with the chemical structure in which each ligand can coordinate as a tridentate. On
the other hand, the R for C-1 is 3.1, which is rather higher than
expected from its bidentate structure. When the C-1 coordinates to Eu3+ in the most stable chemical formation, an alkyl
chain may needed to exclude the hydrated water molecules. In
other words, this extra dehydration needs some energy, as
reflected in the low value of β.
In Figure 4, the logarithms of β1 and β2 for C-1 (m = 3) andC-2 (m = 6) complexes are plotted in comparison with thereference data of dicarboxylic acids containing a different
alkyl length (m = 0, 1 for Eu, and 2, 4 for Gd).16, 17 Althoughsome reference data are not for Eu but for Gd, it is found thatthe values change continuously over all alkyl chain lengths asshown by the broken curves. The values decrease significantly
at the shorter lengths with the increase of alkyl length. From
the result in Figure 4 at the shorter length and from the R forthe bidentate C-1, the decrease of the βs is caused by the additional energies for the change of the ligand conformation andfor the dehydration by an approach of neutral alkyl chain. At
the longer length, the log β1 and log β2 fall into almost constantvalues 3 and 4.5, respectively. Thus, the additional energies
for the coordination reaction seem constant at the longer

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SasakiJ. Nucl. Radiochem. Sci., Vol. 6, No. 1, 2005

tivity between lanthanides and actinides. Especially, softdonor
atoms such as secondary-amine-group nitrogens, which cancoordinate covalently to 5f-elements, show a higher selectivityof 5f elements over 4f elements than the harder oxygen atom.
Further study should be performed to understand the selectivity and stabilization mechanisms of organic ligands withneutral donors such as nitrogen, oxygen and sulfur, in different
chemical frames such as alkyl chains and aromatic rings.

Eu:
Gd:

10

Ref.[16,17]

8

This workfor Eu

logβ2
6

log β

References

4

logβ1
2

0

0

1

2

3

4

5

6 (=m)

HOOC(CH2)mCOOH
Figure 4. Dependence of alkyl chain length (m) in dicarboxylic acids
on the stability constants of Eu- and Gd-complexes16,17 and the present
data for Eu.

length. The R for other ligands with different alkyl chain
lengths is now under investigation for confirmation.
The R for O-2 (3.5) is as same as that for O-1. This meansthat three water molecules are excluded by one O-2 ligand, as
in the case of O-1. By comparing the log β of O-1 and O-2, it
is found that these values are close to each other, in spite of thehigher number of coordination sites for O-2. As indicated in
Figure 3, the two neutral ether oxygens in O-2 may not coordinate strongly to the Eu3+ ion, since the donating power of ether
oxygen to the Eu3+ ion is relatively low compared with that ofamine. The additional energy stability mentioned above may
also contribute to the destabilization of the O-2 complex.
4. Conclusions
For a systematic comparison, the log β1 and log β2 values oftrivalent f-elements for six dicarboxylates containing N and O
donors were obtained in the present study. By taking into
consideration the study of TRLFS, it was shown that the neutral
donors in these dicarboxylates enhance the stabilization of themetal complexes by the presence of negatively charged oxygen
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