Water-soluble tetrapodal N,O ligands incorporating soft N-heterocycles for the selective complexation of Am(iii) over Ln(iii)

Article abstract of DOI:10.1039/b9nj00319c

A series of four water-soluble N,O-tetrapodal ligands derived from ethylenediamine, bearing hard acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the impact of the softness of N-donors on the complexation properties with trivalent f ions. Two novel ligands of enhanced soft character, bearing three pyridines (L^3py) or three pyrazines (L^3pz), were synthesized and the related lanthanide complexes were studied in solution. The ligand containing three pyridylmethyl moieties L^3py gives complexes with a coordination similar to EDTA, i.e. a hexadentate coordination mode as indicated by NMR and luminescence decays (q = 3) and stability constants in the range log β₁₁₀ = 6.99-9.3 (La-Lu). On the other hand, the softest molecule L^3pz forms much less stable complexes with log β₁₁₀ = 4.0-4.4 (La-Eu). The selective back-extraction of Am(iii) from organic solutions containing 4f and 5f elements was tested with the four water-soluble complexing agents. The ligand L^3pz demonstrates poor stripping ability and selectivity. In contrast, the three ligands L^py, L^pz and L^3py give interesting back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are significantly higher than with HEDTA. This exemplifies the role of the N-heterocycle softness in enhancing the separation between Am(iii) and Eu(iii). Interestingly, the pyrazine-based ligand, L^pz, demonstrates the best stripping properties, with a distribution factor that approaches that of HEDTA in the same conditions (D_Am ≈0.3). This molecule is a good compromise between softness and hardness and forms complexes still stable at pH 3 due to its low basicity.

Full text of DOI:10.1039/b9nj00319c

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Water-soluble tetrapodal N,O ligands incorporating soft N-heterocycles
for the selective complexation of Am(III) over Ln(III)
Marie Heitzmann,a Christelle Gateau,a Laurence Chareyre,b
Manuel Miguirditchian,b Marie-Christine Charbonnelb and Pascale Delangle*a
Received (in Gainesville, FL, USA) 8th July 2009, Accepted 3rd September 2009
First published as an Advance Article on the web 15th October 2009
DOI: 10.1039/b9nj00319c
A series of four water-soluble N,O-tetrapodal ligands derived from ethylenediamine, bearing hard
acetate groups and soft N-heterocycles, either pyridine or pyrazine, was developed to study the
impact of the softness of N-donors on the complexation properties with trivalent f ions. Two
novel ligands of enhanced soft character, bearing three pyridines (L3py) or three pyrazines (L3pz),
were synthesized and the related lanthanide complexes were studied in solution. The ligand
containing three pyridylmethyl moieties L3py gives complexes with a coordination similar to
EDTA, i.e. a hexadentate coordination mode as indicated by NMR and luminescence decays
(q = 3) and stability constants in the range log b110 = 6.99–9.3 (La–Lu). On the other hand,
the softest molecule L3pz forms much less stable complexes with log b110 = 4.0–4.4 (La–Eu).
The selective back-extraction of Am(III) from organic solutions containing 4f and 5f elements was
tested with the four water-soluble complexing agents. The ligand L3pz demonstrates poor
stripping ability and selectivity. In contrast, the three ligands Lpy, Lpz and L3py give interesting
back-extraction results with Eu/Am separation factors ranging from 36 to 46, which are
significantly higher than with HEDTA. This exemplifies the role of the N-heterocycle softness in
enhancing the separation between Am(III) and Eu(III). Interestingly, the pyrazine-based ligand,
Lpz, demonstrates the best stripping properties, with a distribution factor that approaches that
of HEDTA in the same conditions (DAm E 0.3). This molecule is a good compromise between
softness and hardness and forms complexes still stable at pH 3 due to its low basicity.
considered to be hard acids in HSAB theory, the higher
Introduction
spatial expansion of the 5f actinide orbitals with respect to
The separation of trivalent actinides (An(III)) from trivalent
the 4f lanthanide orbitals opens possibilities to discriminate
lanthanides (Ln(III)) is a key step in the partitioning and
them through their relative hardness. Therefore, numerous
extractants containing nitrogen5 or sulfur6 functionalities
transmutation strategy, which is one of the scenario being
seriously considered for the future management of nuclear
which are softer than oxygen donors have been developed to
waste. The aim is to separate long-lived a-emitting minor
actinides from spent nuclear fuel and to transmute them by
favor An(III) over Ln(III) complexes. Several of these soft
molecules extract selectively Am(III) from Am(III)–Ln(III)
mixtures.7
nuclear fission into shorter-lived isotopes. Indeed, the trans-
mutation of the minor actinides will only be possible after
Among heterocyclic N-donors, pyridine and pyrazine
separation from the abundant fission products lanthanides
significantly differ in their soft character and have been
having large neutron capture cross sections.1,2
inserted in tripodal8,9 or tetrapodal10,11 chemical architectures
This separation is one of the most challenging issues owing
to test the effect of the softness of the ligand on its extraction
ability and selectivity. We have recently reported12 two tetra-
to the very similar physicochemical properties of An(III) and
Ln(III). Indeed, lanthanides and transplutonium actinides both
podal ligands derived from ethylenediamine bearing both hard
and soft donors, Lpy and Lpz (see Scheme 1). These ligands
exist predominantly in their trivalent oxidation state in
solution. These cations are hard acids in the Pearson classifi-
cation (HSAB for hard and soft acids and bases)3 with
trivalent f-element complexes and (ii) two N-heterocyclic soft
close ionic radii (e.g. 1.066 for Eu(III) and 1.090 for Am(III),
CN = 8).4 Their interactions with inorganic and organic
complexation with Am(III) and Ln(III) is efficient in water:
ligands are therefore predominantly determined by electro-
bear (i) two hard acetate groups to provide stability to the
groups to provide Am(III) versus Eu(III) selectivity. The
for instance log b110(Eu) = 11.6 (Lpy) and 9.4 (Lpz). More-
static and steric factors. Even if both An(III) and Ln(III) are
over, they show significant selectivities for Am(III) over Eu(III):
K
selectivity(Am/Eu) = b110(Am)/b110(Eu) = 60 (Lpy) and
a CEA, Inac, Service de Chimie Inorganique et Biologique
(UMR_E 3 CEA UJF, FRE CNRS 3200), F-38054 Grenoble,
France. E-mail: pascale.delangle@cea.fr; Fax: +33 4 3878 5090;
500 (Lpz). These ligands only differ in their N-donor moieties
and the related complexes have the same structures in water.
Therefore the difference in the stability constants is correlated
to the softness of the N-heterocycle and the results indicate an
Tel: +33 4 3878 9822
CEA, DEN, DRCP, SCPS, F-30207, Bagnols-sur-Ceze, France
b
`
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Scheme 2 Synthesis of L3pyH. Reagents and conditions: (i) a. HCl (1 M),
b. pyridine-2-carbaldehyde, NaBH4, CH3OH, 55%, (ii) ClCH2COOEt,
K2CO3, CH3CN, 70%, (iii) HCl (2 M), H2O, 50%.
Scheme 1 Design of the ligands.
enhancement of the selectivity when the two pyridine groups
are replaced by two pyrazine groups.
In this paper, the synthesis and lanthanide complexation
properties of two novel ligands with increased soft character in
comparison to Lpy and Lpz are presented. In order to increase
the soft character of the latter ligands, one of their acetate
arms was replaced by one methylpyridyl group in Lpy or one
methylpyrazinyl group in Lpz to obtain the two novel complexing
molecules L3py and L3pz (Scheme 1). The four N,O tetrapodal
ligands presented in Scheme 1 afford a series of hydrophilic
N,O ligands with a range of soft character, which allows us to
evaluate the impact of the softness on the complexation of
f-elements. Furthermore, they are good candidates for the
selective back-extraction of Am(III) from organic solutions
containing 4f and 5f elements, as they efficiently complex
trivalent f cations in aqueous solution. Back-extraction experi-
ments are presented and show that, whereas the three ligands
Lpy, Lpz and L3py are able to selectively strip Am(III) from
the organic phase, L3pz which bears three methylpyrazinyl
moieties is probably too soft to efficiently complex f-elements
in water at pH 3. Moreover it appears that Lpz is a good
compromise between hardness and softness because it
provides enough affinity for Am(III) at pH 3 to significantly
strip this ion from the organic phase and an interesting
selectivity (SFAm/Eu = 40–50).
Scheme 3 Synthesis of L3pzH. Reagents and conditions: (i) a. HCl (1 M),
b. pyrazine-2-carbaldehyde, NaBH4, CH3OH, 27%, (ii) ClCH2COOEt,
K2CO3, CH3CN, 49%, (iii) KOH, EtOH–H2O, 81%.
L3py was obtained as the corresponding hydrochloride salt:
L3pyHꢁ4HClꢁ2H2OꢁEtOH (1 equivalent of ethanol was also
detected in the proton NMR spectra). L3pz was obtained as the
potassium salt: L3pzKꢁ4.5H2O. The ligands L3py and L3pz were
fully characterized by NMR, ESMS and elemental analysis.
The proton and carbon NMR spectra of the two ligands in
D2O indicate the presence of a unique species in which the two
methylpyridine or methylpyrazine arms connected to the same
nitrogen atom are equivalent.
Protonation of the ligands
All potentiometric studies were performed in KNO3 0.1 M at
298 K. The protonation constants of L3py and L3pz could be
obtained from the titrations of the free ligands with KOH and
HNO3. They are defined in eqn (1) and listed in Table 1,
together with the values previously reported with their
analogues Lpy and Lpz.12
Results and discussion
Synthesis of the N,O ligands
Ki = [HiL]/[Hiꢂ1L][H+]
(1)
L3py and L3pz are softer analogues of Lpy and Lpz containing
one acetate function and three pyridyl or pyrazinyl groups.
These functions are introduced on an ethylenediamine bridge.
The synthetic procedures are summarized in Schemes 2
and 3. The ligands L3py and L3pz were obtained from N,N-
bis(2-pyridylmethyl)N0-acetylethylenediamine and N,N-bis(2-
pyrazinylmethyl)N0-acetylethylenediamine, respectively. The
syntheses of these two precursors were described in a previous
report for the synthesis of Lpy and Lpz, from N-acetylethylene-
diamine and 2-(chloromethyl)pyridine or 2-(chloromethyl)-
pyrazine.12 The key step in the synthesis of L3py and L3pz is
the monosubstitution of the second aliphatic nitrogen atom,
which was achieved by a reductive amination using 2-pyridine-
or 2-pyrazinecarbaldehyde and sodium borohydride. Subsequent
reaction with ethylchloroacetate in the presence of potassium
carbonate followed by the deprotection of the ethyl ester
groups gave the expected ligands L3py and L3pz in 19% and
9% yield, respectively. Cleavage of the ethyl ester groups
was performed under acidic conditions for L3py and by
The titration of L3pyHꢁ4HCl is indicative of three acidic
sites. The first protonation (logK1 = 8.18) occurs at the most
basic nitrogen, i.e. the aliphatic nitrogen atom adjacent to the
acetate function. As expected, this logK value is lower than the
Table 1 Protonation constants of the four ligands and stability
constants of their lanthanide complexes LnL from potentiometric
measurements in water KNO3 0.1 M at 298 K. b110 = [ML]/[M][L]
L3py
Lpy a
L3pz
Lpz a
logK1
logK2
logK3
log b110 (La)
log b110 (Nd)
log b110 (Eu)
log b110 (Dy)
log b110 (Lu)
log b110 (Am)
8.18(7)
9.60(6)
5.46(8)
3.4(1)
9.86(6)
11.46(1)
11.62(4)
11.89(2)
13.05(6)
13.4(2)
7.18(9)
2.3(2)
—
4.0(3)
—
4.4(3)
—
—
—
9.35(8)
2.5(1)
—
8.30(7)
8.90(2)
9.37(9)
9.6(1)
9.77(1)
12.1(1)
5.08(6)
3.23(9)
6.99(9)
8.1(1)
8.76(2)
8.74(6)
9.3(1)
—
a
Ref. 12.
saponification for L3pz
.
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New J. Chem., 2010, 34, 108–116 | 109

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corresponding value found for Lpy, because of the withdrawing
effect of the methylpyridyl group attached to this nitrogen
atom. The other two protonations give similar logK values to
Lpy and can therefore be assigned to nitrogen atoms of
pyridines (logK2 = 5.08 and logK3 = 3.23). Moreover, the
logK value of the carboxylate group of the NCH2COOꢂ
fragment is expected to be lower than 2.7 like in EDTA or
other polyaminocarboxylate ligands. As for Lpy, the second
aliphatic nitrogen atom of L3py does not protonate in our
experimental conditions (pH 4 2.5).
The titration of L3pzH is indicative of only two acidic sites,
as observed for LpzH2. The first protonation (logK1 = 7.18)
also occurs at the aliphatic nitrogen atom adjacent to the
acetate function and is significantly lowered in comparison to
L3py, because the electron withdrawing effect of the methyl-
pyrazinyl group is larger than the electron withdrawing effect
of the methylpyridyl group. The second logK value can hardly
be assigned to the second aliphatic nitrogen atom as in Lpz
because the strong electron withdrawing effect of the methyl-
pyrazinyl group directly attached to this nitrogen atom would
significantly affect this value to give a much lower logK value
than the one found for Lpz (logK2 = 2.5). Therefore, this
second logK can be assigned to the protonation of the
carboxylate function.
Fig.
1 M
Alkalimetric titrations of solutions containing 10ꢂ3
L3pyHꢁ4HCl with 0 or 1 equiv. Ln(NO3)3 in water and 0.1 M KNO3
at 298 K. (A) L3py, (B) LaL3py, (C) EuL3py, (D) LuL3py
.
pH 4 6 with La and Eu. The titration curve with 1 Eu(III) equiv.
is presented in Fig. 2 and could be fitted in the pH-range 2.7–6
(no precipitate) with the formation of the complex (EuL3pz 2+
)
with log b110 = 4.4. At pH 6, 45% of the complex is formed
and 55% of the cation is still free. Therefore the stability
constant is calculated with a lower number of points than for
L3py, which explains the higher uncertainty associated with
this particular value. Other lanthanide complexes of L3pz were
not studied because of experimental difficulties due to the
precipitation of the complexes.
As expected, the ligands containing pyridines are much
more basic than the ligands containing pyrazines. Indeed,
the pyridine group is much more basic than the pyrazine
group: the pKa of 2-methylpyridine is 5.96 whereas the pKa
of 2-methylpyrazine is only 1.65.13 Moreover the withdrawing
effect of methylpyridyl and methylpyrazinyl groups signifi-
cantly decreases the basicity of the nitrogen atoms to which
they are connected. Furthermore, as this effect is much larger
with pyrazine than pyridine, the attachment of one methyl-
pyridyl or one methylpyrazinyl group instead of one
methylcarboxylate group decreases the pKa value of the most
basic site, i.e. the tertiary amine nitrogen atom adjacent to
the acetate function, by 1.4 and 2.2 units, respectively.
Consequently the two ligands L3py and L3pz are less basic than
their analogues with two N-donors Lpy and Lpz.
The evolution of the stability constants as an inverse
function of the cation ionic radii, related to the hardness of
the ion, is represented in Fig. 3 for the four N,O ligands. As
expected, the series of complexes show an increase of the
complex stability when the ionic radius of the cation decreases,
according to the well-known electrostatic trend. Indeed the net
complexation reaction of a Ln(III) ion with a ligand L is the
result of two successive steps corresponding to dehydration
followed by the combination of the partially desolved partners.
The compensation effect assumes that the free energy for the
dehydration process is negligible at room temperature because
DHdehyd E TDSdehyd.
14 Therefore the global free energy of the
complexation process is dominated by the enthalpy-driven
combination step, leading to increased formation constants
Potentiometric studies of the lanthanide complexes
The lanthanide complexes of L3py have been studied by
potentiometric titration for five representative cations of the
4f series, namely La(III), Nd(III), Eu(III), Dy(III) and Lu(III).
Typical titration curves are shown in Fig. 1. For every
cation, these data could be fitted according to the formation
of a unique metallic species, namely LnL2+ for pH values
inferior to 7. The corresponding stability constants are given in
Table 1. Above pH 7, the curves are characteristic of the
formation of hydroxo complexes. Nevertheless, a hysteresis
exists between the direct titration with KOH and the back
titration with HNO3 in this high-pH region and prevented us
from determining reliable thermodynamic constants for the
hydrolysis reactions. The metal titrations were fitted in the
pH-range 2.5–7 (no hysteresis) with the formation of LnL2+
.
Titrations conducted with the pyrazine-based ligand L3pz
indicate the formation of complexes with low stability in
comparison to L3py. Furthermore, precipitation occurs for
Fig.
L3pzH + 2HNO3 with 0 or 1 equiv. Eu(NO3)3 in water and 0.1 M
KNO3 at 298 K. (A) L3pz, (B) EuL3pz
2 M
Alkalimetric titrations of solutions containing 10ꢂ3
.
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complex evidences the loss of symmetry upon the ion
complexation. In particular, the two methylpyridyl arms
connected to the same nitrogen atom are no longer equivalent:
for instance H6 gives two different doublets in the NMR
spectrum of the complex. The methylene signals of protons
H2, H8 and H9 are split into well-resolved AB patterns. Each
methylene of the ethylenediamine bridge, H1 and H7, is also
split into a doublet and a triplet (ABXY system with JAB
E
JXY E JAX E 13 Hz and JAY E JBY E JBX E 0). The
formation of only one complex was confirmed by the spectra
obtained either in excess of ligand or in excess of cation.
Indeed, the same set of signals was obtained for the complex,
in slow exchange with the ligand in default of cation.
Exchange correlations between the signals of the protons
(especially H6) of the two methylpyridine arms connected to
the same nitrogen atom are detected in the EXSY spectrum
(mixing time 100 ms). This feature can be explained by the
successive steps: decomplexation of the cation followed by
the interconversion of these two arms and new coordination of
the cation.
Fig. 3 Evolution of the stability constants, log b110 of ML complexes
as an inverse function of the cation ionic radii.
with increasing charge density (or hardness) on the cation.15
For the two pyridine-based ligands, the increase of the stability
constants across the 4f series is common and of the same order
of magnitude as EDTA: Lpy (log bLu ꢂ log bLa E 3.2, 32%);
L3py (log bLu ꢂ log bLa E 2.3, 33%). The electrostatic trend is
The hydration numbers (q) of the europium and terbium
cations in the complexes of L3py were obtained from the
measurement of the luminescence lifetimes of the excited-
states of the complexes (t) in H2O and D2O (see Table 2).
The numbers of coordinated water molecules were calculated
using the equation of Parker and co-workers17 and are 3
within the experimental errors (see Table 2). They are similar
to the values obtained previously for Lpy and Lpz.12 These
values are very close to the values found for Ln(EDTA)ꢂ
complexes. Indeed, depending on the empirical equations
used, the hydration number of the lanthanide ion in
Ln(EDTA)ꢂ complexes was found to be between 2.6 and 3.0
for Eu and between 2.8 and 2.9 for Tb.17,18 This indicates that
the novel molecule L3py acts as a hexadentate ligand like the
parent molecule EDTA, and affords two aliphatic nitrogen
atoms, three aromatic nitrogen atoms of pyridines and only
one oxygen donor of a carboxylate. Three water molecules
complete the Ln(III) coordination sphere to obtain an overall
coordination number of 9.
less marked for the pyrazine-based ligands, Lpz (log bLu
ꢂ
log bLa E 1.5, 18%) and L3pz (log bEu ꢂ log bLa E 0.4, 10%).
This tendency can be attributed to their softer character which
produces smaller electrostatic interactions and therefore a
smoother electrostatic evolution of the stability constants
across the 4f series.
As expected the two novel ligands, L3py and L3pz, bearing
three N-donor sites and only one O-donor acetate group show
lower affinities for the hard lanthanide cations than their
analogues with two acetate moieties. The replacement of one
acetate arm in Lpy by one methylpyridyl group induces a
loss of 2.9 log units in the stability constants, whereas the
replacement of one acetate arm in Lpz by one methylpyrazinyl
group has an even more dramatic effect, loss of 5 log units in
the stability constants.
The three ligands Lpy, Lpz and L3py demonstrate significant
affinities for Ln(III) in water (108–1011 for Eu(III)) whereas the
softest ligand L3pz forms lanthanide complexes of quite low
affinity (104). This indicates that L3pz is too soft to efficiently
compete with the water molecules, which have a large affinity
for the hard lanthanide cations. For instance, ligands containing
only pyrazine groups like tpza8 or tpzen11 have very low
affinities for lanthanides preventing the study of their
complexation properties in water, whereas their pyridine
analogues have larger complexation efficiency even in
water.8,16 L3pz has an intermediary behavior between these
ligands bearing exclusively N-donors and the three other
molecules presented in this paper.
Unfortunately, the lanthanide complexes of L3pz could not
be studied in solution because of the occurrence of precipitation
at low concentration. Therefore the coordination behavior of
Characterization of LnL3py complexes
The proton NMR spectrum of the diamagnetic complex
LaL3py shows the presence of only one set of signals indicating
the presence of one isomer without symmetry. The spectra of
free L3py and its lanthanum complex, LaL3py, in D2O at pD 7
are presented in Fig. 4. They were assigned with classical 2D
NMR experiments. The aromatic protons are slightly shifted
with respect to the free ligand. The spectrum of the lanthanum
Fig. 4 400 MHz 1H NMR spectra of L3py and LaL3py in D2O
(pD = 7.0) at 298 K.
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Table 2 Luminescence lifetimes and calculated hydration numbers (q)
Table 3 Back-extraction results: distribution ratios, percentages of
stripped ions and separation factors for the back-extraction of Eu(III)
and Am(III) by ligands Lpz, Lpy, L3py, L3pz and HEDTAa and
conditional stability constants at pH 3 (298 K, KNO3 0.1 M)b
t
H O/ms
t
D O/ms
qa
2
2
EuL3py+
TbL3py+
0.29(1)
0.91(3)
1.34(1)
2.05(2)
2.9(2)
2.8(2)
Lpz
Lpy
L3py
L3pz
HEDTA
a
The hydration numbers, q, were calculated using the equation of
Parker and co-workers:17 q = ALn(1/tH O ꢂ 1/tD O ꢂ aLn), with
DEu
DAm
%
13
0.31
49
1.37
53
1.16
2
46
1.5
—
46
130
8.3
1
11
0.1
1.6
0.13
38
ATb = 5 ms, AEu = 1.2 ms, aTb = 0.06 msꢂ1 and aE2 u = 0.25 msꢂ1
.
2
7
76
2.9
5.6
41
2
42
2.0
3.8
36
stripped Eu
%
88
stripped Am
log b110(Eu) at pH 3
log b110(Am) at pH 3
SFAm/Eu
6.1
6.5
12
this ligand could not be compared with its analogues. As
pyrazine has only a very low contribution to the stability of
the lanthanide complexes,12 other types of coordination in
which more than one aminoacetate moiety is coordinated to
the cation may be favored leading to insoluble polymolecular
systems.
—
15
a
Aqueous phase: ligand 0.5 M, citric acid 0.4 M, pH = 3. Organic
phase: 0.3 M HDEHP, 0.6 M DMDOHEMA, 152Eu and 241Am trace
b
level, TPH. The conditional stability constants at pH 3 were
calculated from the pK and stability constants given in Table 1.
Back-extraction of Am–Ln mixtures with the ligand series
The separation factor between Am(III) and Eu(III), SFAm/Eu, is
given by eqn (4). The higher SFAm/Eu is, the better the
selectivity of the water-soluble ligand for Am(III) with respect
to Eu(III) is.
The selectivity for Am(III) over Eu(III) of the two previously
reported ligands Lpy and Lpz has been demonstrated in water
solution by the values of the stability constants in aqueous
0.1 M KNO3 at 298 K and makes them interesting candidates
for the separation of the two families of f-elements. Indeed,
soft-donor reagents can be used to separate actinides from
lanthanides either as lipophilic extractant molecules or
as water-soluble complexing agents.2 For instance, the
TALSPEAK process (trivalent actinide–lanthanide separation
by phosphorus reagent extraction from aqueous komplexes)19
and the DIAMEX-SANEX process20 are based on the
partitioning of lanthanides and actinides between an acidic
phosphorus extractant organic solution and an aqueous phase
containing a high concentration of a carboxylic acid buffer
and a polyaminopolycarboxylate complexant. The four water-
soluble molecules Lpy, Lpz, L3py and L3pz were subjected to
back-extraction experiments to test their ability to selectively
strip Am(III) from an organic metal solution containing
Am(III) and Eu(III). The ligand HEDTA, used as a selective
complexing agent in the DIAMEX-SANEX partitioning
process,20 was tested in the same conditions for comparison.
The metal-containing organic solution was obtained by
equilibrating an aqueous solution of lanthanides (25 mM)
spiked with 152Eu(III) and 241Am(III) in HNO3 3 M, with an
organic solution containing the malonamide derivative
DMDOHEMA (0.6 M) and the dialkyl phosphoric acid
HDEHP (0.3 M) in TPH, which have been demonstrated to
extract these cations in the organic phase.21 Then, this metal-
containing organic solution was contacted with 0.5 M water
solutions of the ligands at pH 3 buffered with 0.4 M citric acid.
The results are reported in Table 3. The distribution ratios,
DM, were calculated as the ratio of the analytical concentra-
tions of the relevant cation in the organic and aqueous phase
(eqn (2)). Therefore, the smaller DM is, the more efficient the
back-extraction is
SFAm/Eu = DEu/DAm
(4)
Most of the distribution ratios given in Table 3 are above 1
indicating that a small percentage of the cations are back-
extracted in the water phase. In particular, it appears that the
softest molecule L3pz has too low an affinity to perform
significant cation back-extraction. This ligand gives the largest
distribution ratios but also a poor selectivity factor, which may
indicate that the pyrazine N-donors play no role in the
coordination of the cations. The four other ligands give DAm
below 1 (Lpz and HEDTA) or very close to 1 (Lpy and L3py
)
and are thus able to strip Am(III) from the organic solution in
significant amounts. The conditional stability constants at
pH 3 are also reported in Table 3. These values have been
calculated from the known stability constants of the Eu(III)
and possibly Am(III) complexes and the logK values of the
ligands. As expected, large conditional stability constants give
efficient back-extraction and small distribution ratios DEu or
D
Am. The high conditional stability constant values calculated
for the two ligands HEDTA and Lpz explain their better
efficiency to strip Am(III) at pH 3. Indeed, Lpz shows lower
affinity constants than Lpy but is less basic, therefore, its
conditional stability constants at pH 3 are higher than those
of Lpy, making the former a more efficient complexant. For the
same reason, the two pyridine-based ligands Lpy and L3py give
nearly the same distribution ratios, even though the stability
constants of their complexes are quite different, because L3py is
less basic than Lpy. As the DAm values of these two ligands are
very similar, this allows us to roughly estimate the conditional
stability constant of L3py at pH 3: log b110pH3(AmL3py) E
log b110pH3(AmLpy) E 4, which gives an estimation of the
stability constant: log b110(AmL3py) E 11. As expected, this
indicates that L3py, which is softer than Lpy, has a greater
selectivity for Am(III) over Eu(III) in water: Kselectivity(Am/Eu) =
DM = [M]org/[M]aq
(2)
The percentage of metal ion stripped in the water phase can also
be calculated from the distribution factor according to eqn (3).
b
110(Am)/b110(Eu) E 102.24 = 170.
The three ligands Lpy, Lpz and L3py give interesting back-
1
extraction results which are illustrated in Fig. 5, with separation
factors ranging from 36 to 46. The SF values do not reflect the
%
¼
ð3Þ
stripped M
1 þ DM
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Table 4 Back-extraction results: distribution ratios, percentages of
stripped ions and separation factors for the back-extraction of Eu(III)
and Am(III) by Lpza
[Lpz] = 0.5 M pH = 2b
[Lpz] = 0.05 M pH = 3c
DEu
DAm
%
320
7.7
0.3
11
650
13
0.2
7
stripped Eu
%
SFAm/Eu
stripped Am
42
50
a
Organic phase: 0.3 M HDEHP, 0.6 M DMDOHEMA, 152Eu and
241Am trace level, TPH. Aqueous phase: [Lpz] 0.5 M, pH = 2.
c
b
Aqueous phase: ligand 0.5 M, citric acid 0.4 M, pH = 3.
Fig. 5 Percentage of stripped ions and separation factors for the
back-extraction of Eu(III) and Am(III) by 0.5 M water solutions of Lpz
Lpy, L3py, L3pz and HEDTA at pH 3.
,
bonds with their ligands. The scale of affinity is the following:
Lpy 4 Lpz Z L3py c L3pz. The ligand containing three
pyridylmethyl moieties L3py gives similar complexes to Lpy
and Lpz with a hexadentate coordination mode as indicated by
NMR and luminescence decays (q = 3). The coordination
behavior of the softest molecule L3pz is less clear as the
complexes precipitate in the mM range, which prevents
detailed solution studies. Nevertheless, potentiometric
titration shows that L3pz complexes are much less stable than
those obtained with the other three ligands.
thermodynamic selectivity, Kselectivity(Am/Eu), determined in
homogeneous water solutions. This may be due to the different
media used to determine the affinity constants (homogeneous
water, KNO3 0.1 M) and to perform the back-extraction
experiments (different ionic strength, presence of citric acid,
extractants DMDOHEMA and HDEHP in the organic
phase). In particular, the formation of ternary complexes with
citric acid cannot be excluded. Nevertheless, the separation
factors obtained with the three ligands containing soft
N-donors, Lpy, Lpz and L3py, give higher SF than the ligand
HEDTA, exemplifying the role of the ligand’s softness in
enhancing the separation between Am(III) and Eu(III).
Selective back-extraction is a promising process for the
separation of minor actinides from fission products.19 The
series of water-soluble complexing agents presented in this
paper was subjected to back-extraction to test their stripping
ability in regard to their complexation properties for trivalent
f-elements. The softest ligand L3pz appears not to be an
efficient stripping agent as its complexing ability is too low.
Moreover, its selectivity factor is similar to that obtained with
HEDTA which suggests that the pyrazine groups may not
coordinate the cations in these low-affinity complexes. In
contrast, the other three molecules give similar selectivity
factors (B40) which are higher than that obtained with
HEDTA and evidence the role of the N-heterocycles in
enhancing the Am(III)–Eu(III) selectivity. The second pyrazine-
based ligand, Lpz, demonstrates the best stripping properties.
This molecule is indeed a good compromise between softness
and hardness. The presence of two hard O-donors gives
f-element complexes of significant stability; therefore the Am
distribution ratio at pH 3 approaches that of HEDTA under
the same conditions (DAm E 0.3). Moreover, the two soft
pyrazine groups provide a significant selectivity for Am(III)
with respect to Eu(III). To obtain selective and efficient water-
soluble complexing agents, soft ligands with sufficient affinity
and also with a low basicity should be developed in order to
maintain the stripping ability at low pH. Therefore a compro-
mise has to be found between softness and affinity as in the
pyrazine-based ligand Lpz.
The most interesting molecule in the series of N,O tetra-
podal ligands presented here is Lpz which combines several
advantages. Lpz exhibits a good selectivity for Am(III) over
Eu(III). Despite its soft character it has a significant affinity for
f ions due to the presence of two acetate hard donors. Finally,
it is less basic than the pyridine-based ligands due to the
strong withdrawing effect of the methylpyrazinyl moiety that
significantly lowers the highest pKa. Therefore Lpz still has an
efficient conditional stability constant at pH 3 leading to a
distribution ratio for Am(III) approaching the value measured
with HEDTA. In this series, Lpz is the only molecule able to
slightly strip Am(III) from Am–Eu mixtures with a good
selectivity under more drastic conditions, i.e. at lower pH or
lower ligand concentration. These results are reported in
Table 4. Of course, the distribution ratios are higher than
those presented in Table 3 and the back-extraction is less
efficient, but the selectivity factors are in the same range:
40–50. Moreover, it can be added that working at higher
pH, e.g. pH 4, should increase the stripping ability of this
selective ligand.
Conclusion
To conclude, a series of N,O-tetrapodal ligands containing
soft N-heterocycles, either pyridine or pyrazine, was developed
to study the impact of the softness of the N-donor on the
complexation properties with trivalent f ions. As expected, as
the softness of the ligand increases, by replacing either a
pyridine with a pyrazine or an acetate with a N-heterocycle,
the affinity for f-elements decreases. Indeed, trivalent f-element
cations are hard acids which interact mainly via electrostatic
Experimental
General details
Solvents and starting materials were purchased from Aldrich,
Acros, Fluka and Alfa Aesar and used without further
purification. Lanthanide salts were purchased from Aldrich
and deuterium oxide (99.9 atom% D) from Euriso-Top. All
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water solutions were prepared from ultrapure laboratory
grade water that had been filtered and purified by reverse
osmosis using a Millipore MilliQ reverse-osmosis cartridge
system (resistivity 18 MO cm). Thin layer chromatography
(TLC) was performed on aluminium oxide 60 F254 neutral
(Merck). Flash chromatography was performed on aluminium
oxide 90 active neutral (+4.9% water wt, 63–200 mm, Merck).
1H NMR and 13C NMR spectra were recorded at 298 K
on a Mercury Varian 400 spectrometer. Chemical shifts are
reported in ppm with the solvent as the internal reference and
coupling constants are given in Hz. Mass spectra were
acquired with a Finigan LCQ-ion trap equipped with an
electrospray source. Elemental analyses were performed by
the Service Central d0Analyse (Solaize, France).
L3py
H
(N,N,N0-tris(2-pyridylmethyl)ethylenediamine-N0-
acetic acid)ꢁ4HCl. A degassed 2 M HCl solution (75 mL)
was added to N,N,N0-tris(2-pyridylmethyl)-N0-(ethylacetate)-
ethylenediamine (369 mg, 0.88 mmol) and the resulting
mixture was refluxed for 16 h under argon, cooled at room
temperature and evaporated. The residue was washed with
ethanol and diethyl ether and dried under vacuum for one day
to afford L3pyꢁ4HCl (270 mg, 50%) as a white solid (found: C,
46.09; H, 6.12; N, 11.38%. Calc. for C22H25N5O2ꢁ4HClꢁ2H2Oꢁ
1EtOH: C, 46.54; H, 6.35; N, 11.31%); dH (400 MHz, D2O,
DSS) 8.73 (2H, d, J 4.9, H6), 8.62 (1H, d, J 5.9, H60), 8.53
(2H, t, J 7.8, H4), 8.33 (1H, t, J 7.8, H40), 8.07 (2H, d, J 7.8, H3),
7.98 (2H, t, J 6.8, H5), 7.81 (2H, m, HAr0), 4.37 (2H, s, H8), 4.30
(4H, s, H2), 3.71 (2H, s, H9), 3.26 (2H, t, J 6.8, H7), 3.03
(2H, t, J 6.8, H1); dC (100 MHz, D2O, DSS) 172.80 (C11),
152.01 (C10), 151.28 (C100), 147.28 (Cpyr), 144.28 (Cpyr),
143.77 (Cpyr), 141.58 (Cpyr), 127.19 (Cpyr), 126.48 (Cpyr),
126.12 (Cpyr), 125.84 (Cpyr), 56.18 (C9), 55.38 (C2), 55.19
(C8), 51.64 (C7), 50.82 (C1); m/z 392.2 (MH+, 100%).
Lpy, Lpz and 2-pyrazinecarbaldehyde were obtained according
to published procedures.12,22
Synthesis
N,N,N0-Tris(2-pyridylmethyl)ethylenediamine. N,N-Bis(2-pyridyl-
methyl)ethylenediamine was first obtained from N,N-bis(2-
pyridylmethyl)-N0-acetylethylenediamine (648 mg, 2.28 mmol)
according to published procedures.12 The crude product was
used for the reductive amination: 2-pyridinecarbaldehyde
(366 mg, 3.42 mmol) was added dropwise to a solution of
N,N-bis(2-pyridylmethyl)ethylenediamine (552 mg) in dry
methanol (50 mL), and the mixture was refluxed for 2 h under
argon. Sodium borohydride (259 mg, 6.84 mmol) was
added and the mixture was stirred at room temperature over-
night, and evaporated. An aqueous solution of saturated
NaHCO3(100 mL) was added to the residue and the whole
was extracted with dichloromethane (3 ꢃ 20 mL). The
collected organic layers were dried over Na2SO4, filtered,
and concentrated. The resulting brown oil was chromatographed
on aluminium oxide (elution with dichloromethane–ethanol
99 : 1) to afford N,N,N0-tris(2-pyridylmethyl)ethylenediamine
(416 mg, 55%) as an orange oil. dH (400 MHz, CDCl3, Me4Si)
8.50 (3H, d, J 5.0, HAr), 7.62 (3H, t, J 7.8, HAr), 7.52 (2H, d,
N,N,N0-Tris(2-pyrazinylmethyl)ethylenediamine. N,N-Bis(2-
pyrazinylmethyl)ethylenediamine was first obtained from
N,N-bis(2-pyrazinylmethyl)-N0-acetyl ethylenediamine (1.39 g,
4.84 mmol) according to published procedures.12 The crude
product was used for the reductive amination: a solution of
2-pyrazinecarbaldehyde (732 mg, 6.77 mmol) in dry methanol
(50 mL) was added dropwise to a solution of N,N-bis(2-
pyrazinylmethyl)ethylenediamine (1.397 g) in dry methanol
(200 mL), and the mixture was refluxed for 2 h under argon.
Sodium borohydride (649 mg, 17.16 mmol) was added and the
mixture was stirred at room temperature overnight, and
evaporated. The crude product was chromatographed on
aluminium oxide (elution with dichloromethane–ethanol
99 : 1) to afford N,N,N0-tris(2-pyrazinylmethyl)ethylene-
diamine (443 mg, 27%) as an orange oil. dH (400 MHz,
CDCl3, Me4Si) 8.70–8.46 (9H, m, HAr), 3.93 (6H, m,
H2/H8), 2.89 (4H, s, H1H7).
N,N,N0-Tris(2-pyrazinylmethyl)-N0-(ethylacetate)ethylene-
diamine. To a solution of N,N,N0-tris(2-pyrazinylmethyl)ethylene-
diamine (443 mg, 1.32 mmol) in dry acetonitrile (60 mL) under
argon, ethylchloroacetate (267 mL, 1.58 mmol) and potassium
carbonate (218 mg, 1.58 mmol) were added. After stirring the
mixture for 2 h at room temperature, the suspension was
refluxed overnight, cooled at room temperature, filtered and
evaporated. The resulting oil was chromatographed on alumi-
nium oxide (elution with ethylacetate–methanol 98 : 2) to
afford N,N,N0-tris(2-pyrazinylmethyl)-N0-(ethylacetate)ethylene-
diamine (271 mg, 49%) as an orange powder. dH (400 MHz,
CD3CN, Me4Si) 8.65 (2H, s, HAr), 8.62 (1H, s, HAr), 8.41
(6H, m, HAr), 4.07 (2H, q, J 7.0, COOCH2CH3,), 3.88
(2H, s, H8), 3.82 (4H, s, H2), 3.37 (2H, s, H9), 2.84 (2H, t,
J 6.3, H1/H7), 2.69 (2H, t, J 6.3, H1/H7), 1.85 (3H, t, J 7.0,
COOCH2CH3).
J
7.8, HAr), 7.29 (1H, d, 1.6, HAr), 7.13 (3H, t,
J
J 5.0, HAr), 3.85 (2H, s, H8), 3.84 (4H, s, H2), 2.81 (4H, s,
H1H7); m/z 334.1 (MH+, 100%).
N,N,N0-Tris(2-pyridylmethyl)-N0-(ethylacetate)ethylenediamine.
To a solution of N,N,N0-tris(2-pyridylmethyl)ethylenediamine
(416 mg, 1.25 mmol) in 30 mL of dry acetonitrile under
argon, were added ethylchloroacetate (160 mL, 1.5 mmol)
and potassium carbonate (207 mg, 1.5 mmol). After stirring
the mixture for 2 h at room temperature, the suspension was
refluxed overnight, cooled at room temperature, filtered and
evaporated. The resulting oil was chromatographed on
aluminium oxide (elution with dichloromethane–methanol
99 : 1) to afford N,N,N0-tris(2-pyridylmethyl)-N0-(ethylacetate)-
ethylenediamine (369 mg, 70%) as an orange powder. dH
(400 MHz, CDCl3, Me4Si) 8.49 (3H, m, HAr), 7.61 (2H, td,
J 7.8 and 1.6, HAr), 7.56 (1H, td, J 7.8 and 1.6, HAr), 7.49
(2H, d, J 7.8, HAr), 7.42 (1H, d, J 7.8, HAr), 7.13 (3H, m, HAr),
4.12 (2H, q, J 6.9, COOCH2CH3), 3.89 (2H, s, H8), 3.81
(4H, s, H2), 3.37 (2H, s, H9), 2.89 (2H, t, J 7.8, H1/H7), 2.72
(2H, t, J 7.8, H1/H7), 1.22 (3H, t, J 6.9, COOCH2CH3).
L3pz
K
(N,N,N0-tris(2-pyrazinylmethyl)ethylenediamine-N0-
acetic acid). N,N,N0-Tris(2-pyrazinylmethyl)-N0-(ethylacetate)-
ethylenediamine (271 mg, 0.641 mmol) was dissolved in a
solution containing degassed 1 M KOH solution (770 mL,
0.77 mmol), H2O (22.5 mL) and ethanol (7.5 mL). The
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resulting mixture was stirred for 16 h under argon at room
temperature. Then the resulting solution was evaporated,
dissolved in acetonitrile and filtrated. The filtrate was concen-
trated under pressure to afford L3pzK (267 mg, 81%) as an
orange oil (found: C, 44.57; H, 5.58; N, 21.35%. Calc. for
C19H21N8O2K, 4.5 H2O: C, 44.43; H, 5.89; N, 21.82%.),
dH (400 MHz, D2O, Me4Si) 8.56 (1H, s, H60), 8.48 (1H, d,
J 2.9, H30), 8.42 (1H, d, J 2.9, H50), 8.36 (2H, s, H6), 8.33
(2H, s, H3), 8.29 (2H, d, J 2.9, H5), 4.51 (2H, s, H8), 3.87
(4H, s, H2), 3.67 (2H, s, H9), 3.57 (2H, t, J 5.8, H7), 3.17
(2H, t, J 5.8, H1); dC (100 MHz, D2O, DSS) 169.96 (COOH),
153.27 (C10), 146.24 (C100), 144.69 (CAr), 144.62 (CAr),
144.58 (CAr), 143.92 (CAr), 142.95 (CAr), 57.01 (C2), 55.51
(C8), 55.02 (C9), 51.71 (C7), 48.76 (C1); m/z 433.1
(MK+, 100%).
(B0.005 mol Lꢂ1) and the lanthanide triflate salt in deuterium
oxide. Ligand concentrations were determined by potentio-
metric titration, and the metal concentrations by EDTA
titrations using xylenol orange indicator. The pD of the
samples were adjusted to the desired value by adding stock
solutions of DCl or NaOD in D2O. The pD were measured
according to pD = pHread + 0.41.25
Luminescence
Terbium and europium luminescence lifetimes were measured
on a Perkin-Elmer LS50B spectrofluorimeter by recording the
decay of the emission intensity at 545 nm for Tb and 616 nm
for Eu (excitation at 274 nm). The decays of luminescence
intensities followed systematically monoexponential laws and
were analyzed as single-exponential decays. The instrument
settings were as follows: a gate time of 1 ms, a flash count of 1,
excitation and emission slit widths of 10 nm, and a varied
delay time. The complexes were prepared in situ by mixing 0.9
equivalents of the metal solution with one equivalent of ligand.
The concentration selected was 1 mM for europium complexes
and 0.5 mM for terbium complexes either in H2O or D2O. The
pH (or pD) was then adjusted to 7 with a NaOH (or NaOD)
solution. Reported lifetimes, t, are average of three separate
measurements calculated by monitoring the emission intensity
after at least 20 different delay times covering two or more
lifetimes.
Potentiometry
Carbonate-free 0.1 mol Lꢂ1 KOH and 0.1 mol Lꢂ1 HNO3
were prepared from Fisher Chemicals concentrates. Potentio-
metric titrations were performed in 0.1 mol Lꢂ1 aqueous
KNO3 under an argon atmosphere; the temperature was
controlled to ꢄ0.1 1C with a circulating water bath. The
p[H] (p[H] = ꢂlog [H+], concentration in molarity) was
measured in each titration with a combined pH glass electrode
(Metrohm) filled with 3 mol Lꢂ1 KCl and the titrant addition
was automated by use of a 751 GPD titrino (Metrohm). The
electrode was calibrated in hydrogen ion concentration by
titration of HNO3 with KOH in 0.1 mol Lꢂ1 KNO3.23 A plot
of meter reading versus p[H] allows the determination of the
electrode standard potential (E1) and the slope factor (f).
Ligand concentration was determined by potentiometric
titrations and was in accordance with the elemental analysis of
the molecules. Lanthanide salt solutions were prepared by
dissolving the appropriate amount of lanthanide nitrate in
water. The exact metal ion concentration was determined by
colorimetric titration using standardized EDTA solutions
(Fisher Chemicals) and xylenol orange as indicator. Continuous
potentiometric titrations with KOH 0.1 mol Lꢂ1 were conducted
on 20 mL of aqueous solutions containing 10ꢂ3 mol Lꢂ1 of the
ligand and 0, 0.5, 1 and 2 equiv. of the desired metallic cation.
Back titrations with HNO3 0.1 mol Lꢂ1 were systematically
performed after each experiment to check whether equilibration
had been achieved. In a typical experiment, 100 points were
measured with a 2 min delay between the measurements for the
free ligand, and a 5 min delay for metallic complexes.
Back-extraction experiments
Abbreviations: HDEHP: bis(2-ethylhexyl)phosphoric acid,
DMDOHEMA: N,N0-dimethyl-N,N0-dioctylhexylethoxy malon-
amide, TPH (hydrogenated tetrapropylene) is a mixture of
highly branched alkanes containing 11 to 13 carbon atoms,
HEDTA: N-hydroxyethylethylenediaminetriacetic acid.
All extraction experiments were performed at 25 1C. First,
to prepare the organic phase containing the metallic ions,
4 mL of an organic solution (0.3 M HDEHP, 0.6 M
DMDOHEMA, TPH) and 4 mL of an aqueous solution
(HNO3 3 M, [Ln]tot = 25 mM spiked with 241Am(III) and
152Eu(III)) were equilibrated by vortexing for 10 min.
Preliminary experiments showed that a 10 min vortexing time
is adequate to reach equilibrium. The organic and aqueous
phases were separated by centrifugation. The organic phase
was scrubbed by an aqueous solution (citric acid 0.5 M, pH 3)
and used in the following for the back-extraction experiments.
The aqueous solutions used in the back-extraction experi-
ments were prepared with a ligand concentration of 0.5 M
(or 0.05 M), citric acid 0.4 M, and pH adjusted to 3 with
sodium hydroxide (Lpy and L3py) or nitric acid (Lpz and L3pz).
For the back-extraction experiment performed at pH 2, the pH
of the aqueous phase was adjusted to 2 with HNO3. 0.5 mL of
the aqueous solution of ligand and 0.5 mL of the organic
phase containing the metal ions were equilibrated by vortexing
for 60 min at 25 1C. After separation by centrifugation
aliquots of both phases were analyzed using a gamma counting
spectrometer (Hyper pure Ge detector, CANBERRA). 152Eu
and 241Am tracers were used for the measurement of the
Eu–Am distribution. In the range of 0.1–10 the error in D
values is about 5%, while in the ranges of 0.01–0.1 and 10–100
the error is greater, about 10%.
Experimental data were refined using the computer program
Hyperquad 2000.24 All equilibrium apparent constants are
expressed as concentration ratios and not activities. The ionic
product of water at 25 1C and 0.1 mol Lꢂ1 ionic strength is
pKw = 13.78.13 The initial concentrations of ligand, metal and
proton were fixed, as well as the ligand pK values for the
metallic complex stability constant determination. All values
and errors (one standard deviation) reported represent the
average of at least three independent experiments.
NMR spectroscopy
Samples for NMR spectroscopy were prepared by mixing
appropriate volumes of stock solutions of the ligand
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Acknowledgements
We thank Elodie Bosso for participation in this study as an
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