Highly efficient separation of actinides from lanthanides by a phenanthroline-derived bis-triazine ligand

Article abstract of DOI:10.1021/ja203378m

The synthesis, lanthanide complexation, and solvent extraction of actinide(III) and lanthanide(III) radiotracers from nitric acid solutions by a phenanthroline-derived quadridentate bis-triazine ligand are described. The ligand separates Am(III) and Cm(II

Full text of DOI:10.1021/ja203378m

ARTICLE
pubs.acs.org/JACS
Highly Efficient Separation of Actinides from Lanthanides by a
Phenanthroline-Derived Bis-triazine Ligand
†
,†
†
†
‡
Frank W. Lewis, Laurence M. Harwood,* Michael J. Hudson, Michael G. B. Drew, Jean F. Desreux,
‡
‡
§
§
§
Geoffrey Vidick, Nouri Bouslimani, Giuseppe Modolo, Andreas Wilden, Michal Sypula,
|
|
||
Trong-Hung Vu, and Jean-Pierre Simonin
†
Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, U.K.
‡
Coordination and Radiochemistry, University of Li ꢀe ge, Sart Tilman B16, B-4000 Li ꢀe ge, Belgium
§
Sicherheitsforschung und Reaktortechnik, Forschungszentrum J €u lich GmbH, D-52425 J €u lich, Germany
Laboratoire PECSA (UMR CNRS 7195), Universit ꢁe Pierre et Marie Curie, Case 51, 4 Place Jussieu, 75252 Paris Cedex 05, France
S Supporting Information
b
ABSTRACT: The synthesis, lanthanide complexation, and solvent ex-
traction of actinide(III) and lanthanide(III) radiotracers from nitric acid
solutions by a phenanthroline-derived quadridentate bis-triazine ligand
are described. The ligand separates Am(III) and Cm(III) from the
lanthanides with remarkably high efficiency, high selectivity, and fast
0
extraction kinetics compared to its 2,2 -bipyridine counterpart. Structures
of the 1:2 bis-complexes of the ligand with Eu(III) and Yb(III) were
elucidated by X-ray crystallography and force field calculations, respec-
tively. The Eu(III) bis-complex is the first 1:2 bis-complex of a quad-
ridentate bis-triazine ligand to be characterized by crystallography. The
faster rates of extraction were verified by kinetics measurements using the
rotating membrane cell technique in several diluents. The improved
kinetics of metal ion extraction are related to the higher surface activity of
the ligand at the phase interface. The improvement in the ligand's properties on replacing the bipyridine unit with a phenanthroline
unit far exceeds what was anticipated based on ligand design alone.
’
INTRODUCTION
other fission and corrosion products that are also present in the
acidic PUREX waste streams because the lanthanides have high
neutron capture cross sections. The separation of the radio-
During the production of electricity from nuclear power, a
4
typical 1000 MW(e) light water reactor produces 20ꢀ30 t of
1
active minor actinides from the lanthanides is therefore one of
the principal current challenges in nuclear waste reprocessing.
This separation is made all the more difficult given the chemical
spent nuclear fuel per annum. This spent fuel consists mainly
(
>98.5%) of uranium and short-lived fission products, which do
not pose a long-term hazard. However, approximately 1 wt % of
the spent fuel is composed of plutonium and the minor actinides
5
similarities between the two groups of elements. However, it is
thought that the greater availability of the valence orbitals in the
actinides means that there is a more covalent contribution to
metalꢀligand bonding than with the lanthanides. The origins of
this covalency are still not fully understood and remain the
(
Am, Cm, Np), which are highly radiotoxic. Plutonium is the
main contributor to the long-term radiotoxicity of spent nuclear
fuel. Its separation through the PUREX (plutonium and uranium
extraction) process and reuse as fuel (MOX fuel) in nuclear
6
2
subject of ongoing debate and study.
reactors is current industrial practice. The remaining waste still
It has been shown that ligands with soft N-donor atoms can
exploit this small but significant difference between the actinides
and lanthanides, and many N-heterocyclic ligands have been
investigated for their ability to carry out this separation by solvent
contains the minor actinides, necessitating the containment and
separation of the waste from the biosphere for many thousands of
years, a situation that represents a serious environmental concern.
If the minor actinides could be removed, the mandatory
storage time of the remaining waste decreases from several
thousand years to a few hundred years. One strategy for reducing
the radiotoxicity of the waste involves partitioning and transmu-
tation of the long-lived minor actinides into shorter-lived or
7
extraction. Two ligand classes have emerged as the most
promising: the terdentate 2,6-bis(1,2,4-triazin-3-yl)pyridine li-
8
0
gands (BTPs) and the quadridentate 6,6 -bis(1,2,4-triazin-3-yl)-
3
stable elements by neutron fission. However, it is first necessary
Received: April 12, 2011
Published: July 19, 2011
to separate the actinides from the bulk of the lanthanides and
r 2011 American Chemical Society
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ARTICLE
Scheme 1. Synthesis of the BTPhen Ligand 9
Figure 1. Conformations of the annulated BTBP ligand 1 and structure
of 2,9-di(2-pyridyl)-1,10-phenanthroline 2.
0
2
,2 -bipyridine ligands (BTBPs). The BTBP ligands are cur-
9
rently the most suitable ligands for these separations. The
annulated BTBP 1 has extraction and back-extraction proper-
ties that could be suitable for a continuous separation process
1
0,11
(
Figure 1).
Unlike the BTBPs bearing alkyl side chains on
the triazine rings, 1 is stable to hydrolysis and resistant toward
radiolysis and degradation by free radicals because the labile
1
2
benzylic hydrogens have been replaced by methyl groups.
Scheme 2. Modified and ImprovedSynthesis of the Diketone 8
2
+
Recently, some unusual uranyl (UO ) complexes of 1 have
2
1
3
been isolated and characterized.
However, the rates of extraction of the actinides by 1 was
0
rather slow, and a phase-transfer agent (N,N -dimethyl-N,
0
N -dioctyl-2-(2-hexoxyethyl) malondiamide) was required to im-
10
prove the kinetics of extraction. This represents a disadvantage
for the use of 1 in the proposed generation IV fast reactors. Since
a conformational change by 1 from the trans-conformation to its
less-favored cis-conformation is required prior to metal ligation,
we hypothesized that this need for molecular reorientation was
responsible for the slow rates of extraction by 1.
The cis-locked 1,10-phenanthrolines are versatile ligands in a
wide range of applications. Their rigidity and the juxtaposition of
their donor atoms means that complex formation is both more
rapid and more thermodynamically favored than their 2,
0
14
2
-bipyridine analogues. For example, it has been reported that
15
the preorganized, phenanthroline-based ligand 2 exhibits high-
from 2,9-dimethyl-1,10-phenanthroline 3 following known
procedures. The conversion of dioxime 5 to dinitrile 6 gave
a low yield (37%) of product in our hands. Consequently, we
developed a one-pot method for the synthesis of the dinitrile 6
from the dialdehyde 4. The reaction of the dinitrile 6 with
hydrazine hydrate in EtOH gave the novel diamide dihydrazide
er levels of complex stability and selectivity toward Cd(II) than
19
16
related ligands such as quaterpyridine. In addition, phenan-
0
17
throlines have larger dipole moments than the 2,2 -bipyridines
18
and often coordinate strongly to water via hydrogen bonding.
These properties could lead to improved extraction kinetics in a
separation process since the ligand may interact more favorably
with the organic/water interface. On the basis of this, we
predicted that a cis-locked quadridentate bis-triazine ligand
containing a 1,10-phenanthroline moiety would show improved
7
in near quantitative yield. The condensation of 7 with
diketone 8 in THF at reflux afforded 2,9-bis(1,2,4-triazin-3-yl)-
,10-phenanthroline (BTPhen) 9 in 59% yield (see Supporting
1
Information). The ligand 9 was obtained as a stable hydrate, as
0
extraction properties compared to its 2,2 -bipyridine counterpart
1
indicated by its H NMR spectrum, the water remaining bound
1
. Herein, we report the synthesis, lanthanide complexation, and
to the ligand in vacuo (0.1 mmHg) and after heating at 120 ꢀC
solvent extraction chemistry of the first of a new class of bis-
triazine ligands in which the 2,2 -bipyridine moiety of 1 has been
for 24 h.
0
20
The known diketone 8 was synthesized by a modified
replaced by a 1,10-phenanthroline moiety.
procedure (Scheme 2). The diester intermediate 11 proved very
difficult to synthesize on a large scale using the literature method
(
oxidative coupling of pivalic acid using H O /FeSO , followed
2 2 4
21
’
RESULTS AND DISCUSSION
by esterification of the resulting diacid), but we have since
developed a new synthesis of 11 involving the alkylation of
ethyl isobutyrate 10 with ethane-1,2-disulfonate esters. This
Synthesis and X-ray Crystallography. The new ligand 9 was
synthesized as shown in Scheme 1. The dinitrile 6 was synthesized
2
2
1
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ARTICLE
2
4
procedure gave the diester 11 in 69ꢀ70% yield. This contrasts
with when 1,2-dihaloethanes were used in attempted alkylation
reactions, where none of the desired diester 11 was formed and
The X-ray crystal structure of the ligand 9 (Figure 2) shows
one of the bound water molecules located in the coordination
cavity of the ligand. The two outer triazine rings of BTPhen 9 are
twisted away from the conformation required for metal ligation
such that the N(11)ꢀCꢀCꢀN(21) and N(32)ꢀCꢀCꢀN(41)
torsion angles are ꢀ159.0(2)ꢀ and ꢀ159.0(2)ꢀ, respectively.
23
the major product was the ethyl R-haloisobutyrate. The diester
1
1 was converted to the diketone 8 as previously described (see
20a
Supporting Information).
Using this modified procedure,
o
diketone 8 was readily synthesized on a multigram scale in much
improved overall yield (44% overall compared to <5% using the
previous method).
The central N(21)ꢀCꢀCꢀN(32) torsion angle is ꢀ8.6(3) .
The Eu(III) complex of BTPhen 9 was synthesized (see Sup-
porting Information) and structurally characterized by X-ray
diffraction. We were surprised to find that the bis-complex of
stoichiometry [Eu(9) NO ] had been formed (Figure 3), even
25
2+
2
3
though a single equivalent of 9 was used in the crystal growing
experiment.
This is the first time that a 1:2 lanthanide complex with any
quadridentate bis-triazine ligands has been isolated and structu-
rally characterized. Although it is known that the BTBP ligands
can form both 1:1 and 1:2 complexes in solution, only 1:1
complexes have been previously characterized by X-ray
26
diffraction. The metal is 10-coordinate, being fully enclosed
by two molecules of 9 in addition to a single bidentate nitrate ion.
The geometry can best be considered as a capped square
antiprism with the bidentate nitrate group occupying the capping
site. The bond lengths from the metal to the outer triazine
nitrogen atoms are equivalent, with a range of 2.546(6)ꢀ2.616-
ꢁ
(
5) Å to those to the inner phenanthroline nitrogen atoms with a
ꢁ
range of 2.561(6)ꢀ2.598(5) Å. Indeed the mean bond lengths
ꢁ
are 2.587 and 2.582 Å, respectively. These metalꢀnitrogen bond
lengths are very similar to those found in the 1:1 complexes
26
formed by the BTBP ligands. By contrast to the ligand
conformation, now the outer torsion angles N(11)ꢀCꢀCꢀ
N(21) and N(32)ꢀCꢀCꢀN(41) are 6.0(10)ꢀ and ꢀ6.0(10)ꢀ
Figure 2. Molecular structure in the crystal of the BTPhen ligand 9.
Thermal ellipsoids are at 30% probability. Hydrogen bonds are shown as
dotted lines. Distances are O(1)ꢀN(21) 3.003(2), O(1)ꢀN(45)
o
with the central torsion angle being ꢀ0.6(8) . The correspond-
3.125(2) Å. Other solvent molecules are omitted for clarity.
ing angles in the second ligand are 4.6(11)ꢀ, 10.6(10)ꢀ, and
2
+
2ꢀ
Figure 3. Molecular structure in the crystal of the [Eu(9)
2
NO
3
]
cation. Thermal ellipsoids are at 30% probability. The [Eu(NO
3
)
5
]
counterions
ꢁ
and solvent molecules have been omitted for clarity. Selected bond lengths [Å]: Eu(1)ꢀN(51) 2.546(6), Eu(1)ꢀN(72) 2.561(6), Eu(1)ꢀN(32)
2
2
.571(7), Eu(1)ꢀN(21) 2.598(5), Eu(1)ꢀN(81) 2.572(6), Eu(1)ꢀN(61) 2.598(5), Eu(1)ꢀN(11) 2.615(7), Eu(1)ꢀN(41) 2.616(5), Eu(1)ꢀO(1)
.540(6), Eu(1)ꢀO(3) 2.593(5).
1
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1
the H relaxation time of these molecules increases as the latter
are no longer in the immediate vicinity of the paramagnetic
centers. The relaxation rate thus decreases and a plateau is
reached when the metal complex is fully formed. NMRD
3
+
titrations are presented in Figure 4 for Gd perchlorate and
nitrate salts to which BTPhen 9 is progressively added in
anhydrous acetonitrile. The clear breaks in the titration curves
for a 1:2 ratio indicate that stable bis-complexes are formed in
both cases. However, the curvature in the NMRD titration plot
of the nitrate salt indicates that the nitrate ion is better able to
compete with the BTPhen ligand 9 than the perchlorate ion. In
keeping with these data, NMR peaks due to the free ligand are
observed as soon as 1:2 metal:ligand ratios are exceeded. The
number of resonances and their shifts in all the NMR spectra
discussed below remain identical whether this ratio is 1:1 or 1:2,
and the bis-complex is thus the major solution component in
these conditions.
The NMR spectrum of the diamagnetic bis-complex
Figure 4. Nuclear magnetic relaxation dispersion (NMRD) titration
curve of an anhydrous CD CN solution of Gd(ClO ) (.) and
formed between anhydrous Lu(ClO ) and BTPhen 9 in
4
3
3
4 3
acetonitrile displays two methyl peaks, three aromatic peaks,
and a multiplet between 1.45 and 1.65 ppm that originates
from the rapidly inverting cyclohexenyl groups (see Support-
ing Information). These peaks broaden somewhat at lower
temperatures, but the changes are insufficient for a quanti-
tative analysis that would lead to an estimation of the
activation energy. Attempts in chloroform/acetonitrile mix-
tures at 223 K were also unsuccessful and were not pursued,
Gd(NO
3
)
3
(~) by BTPhen 9.
ꢀ
4.9(11)ꢀ, respectively. The four donor nitrogen atoms in each
ꢁ
ligand are approximately planar (rms deviations 0.01, 0.07 Å)
ꢁ
with the metal 0.729(3) and 0.585(4) Å, respectively, from each
N4 plane. These two N4 planes intersect at 69.8(2)ꢀ. The bond
lengths to the two nitrate oxygen atoms are 2.540(6) and
ꢁ
2
.593(5) Å.
as the rigidification of cyclohexene itself is reached only at
pK Determination and NMR Studies of Lanthanide Com-
a
28
1
09 K. A NOESY spectrum was recorded to verify the
plexes. It is rather paradoxical that metal ions are so well
extracted from concentrated acid aqueous solutions by ligands
such as 1 and 9 despite their easily protonatable central
bipyridine or phenanthroline units. The basicity of these ligands
must be strongly decreased by the electron-withdrawing tria-
assignments (see Supporting Information). Cross-peaks
were observed between the cyclohexenyl protons and be-
tween the aromatic protons but not between these two
groups. No cross-peaks were observed between two different
3
+
protons on separate ligands. The spectrum of Lu(BTPhen)2
zine substituents, but little or no information on the pK of
these ligands is yet available to substantiate this hypothesis. The
a
was used as a reference for deducing the paramagnetic shifts
3
+
1
induced by the Yb ion. The latter is known to induce essen-
pK of the ligand 9 was thus determined by H NMR
a
2
7
tially pure dipolar shifts from which solution structures can be
spectroscopy. The dependence of the NMR shifts of BTPhen
upon the DCl concentration in deuterated methanol
29
inferred1.
9
ꢀ
7
The H NMR spectrum of a 1:2 mixture of Yb(ClO ) and
4 3
(10 ꢀ3 M, > 99% methanol) was recorded (see Supporting
BTPhen 9 in anhydrous acetonitrile is shown in Figure 5.
All resonances are easily assigned from their respective areas and
from COSY spectra. Three aromatic protons resonate at high and
low fields, and the cyclohexenyl substituents give rise to two
methyl peaks and two methylene peaks. The aliphatic substitu-
ents are thus rapidly inverting their conformation on the NMR
time scale (over a 17 ppm shift range). This remains true between
Information). The NMR titration curves display a well-defined
shift jump between ꢀlog[DCl] = 4 and 2 followed by a smooth
shift increase at higher acid concentrations. The first proton-
ation pK of 9 (3.1 ( 0.1) is assigned to the protonation of the
phenanthroline ring; it is close to the value reported for the BTP
ligands in 76% methanol (pK = 3.3) and is much lower than
a
7
c
a
the pK of phenanthroline itself in water (4.92). The electron-
a
2
98 and 230 K.
withdrawing effect of the triazine rings of 9 is also responsible
A molecular modeling calculation was made following the
for the low value of the second pK (0.3 ( 0.1, 0.2 ( 0.1, and
a
procedure used for a related bis-triazine ligand derived from
0
.03 ( 0.03 for phenanthroline protons 3, 4, and 5, re-
0 0 00
,2 :6 ,2 -terpyridine (force-field approach with parameters
11b
2
spectively), which is tentatively assigned to the second proton-
ation of phenanthroline.
published for lanthanide complexes). The optimized struc-
3
+
ture of the 1:2 bis-complex of 9 with Yb is shown in Figure 6.
As expected, the most stable conformation is an arrangement in
which the two ligands are perpendicular to each other. All
cyclohexenyl groups in the modeled structure are in the half-
chair conformation, in agreement with molecular calculations
Using lanthanides as actinide surrogates, the complexation
behavior of BTPhen 9 toward the lanthanides was studied by
NMR spectroscopy to gain an understanding of the species likely
to be involved in the extraction of the trivalent actinides.
As shown earlier, the maximum stoichiometry of lanthanide
complexes in an organic solvent can be deduced from a nuclear
magnetic relaxation dispersion (NMRD) titration in which the
relaxation rate of a Gd solution is measured for different metal:
ligand ratios. The progressive formation of a metal complex is
accompanied by the removal of solvent molecules. Consequently,
11a
2
8
performed for cyclohexene. The white arrows in Figure 6
indicate the smaller interatomic distances found between two
ligands in the bis-complex. All these distances are larger than
the sum of the van der Waals radii. The structure is thus not
sterically crowded, and there is ample room for additional
coordinating anions or solvent molecules. The full dipolar
3+
1
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1
Figure 5. H NMR spectrum of a 1:2 Yb(ClO
4
)
3
/BTPhen 9 mixture in anhydrous CD
3
CN at 263 K. Assignments: (+) methyl protons, (O) methylene
protons, (ꢁ) aromatic protons (broad peak at ꢀ3.7 ppm: traces of water).
computed from a molecular model. The factors θ , ψ , and r are
i
i
i
the polar coordinates of proton i in the set of axes of the magnetic
susceptibility tensor with the metal ion at its origin. This equation
has been used successfully for obtaining the solution structure of
29
lanthanide chelates or of proteins modified by grafting a
30
paramagnetic metal complex. In the latter case, the spectra
display so many shifted resonances that the orientation of the
susceptibility axes can be included in a fit between the experi-
mental and calculated paramagnetic shifts. This is not the case for
the BTPhen complex that exhibits only seven NMR resonances,
but this complex features symmetry elements with which the axes
of the magnetic susceptibility tensor must coincide. The z axis
was thus oriented with the C axis that bisects the two phenan-
2
throline groups. In addition to this axis, there are two symmetry
planes, either through the phenanthroline rings or between these
rings (i.e., at 45ꢀ). The x and y axes can be located in either plane.
For symmetry reasons, the same correlation between experi-
mental and calculated paramagnetic shifts will be obtained, but
the sign of the magnetic susceptibility terms will change. This is
also true if the x (or y) axis is exchanged with the z axis (see
Supporting Information). As shown in Figure 7, there is a very
good correlation between the calculated and experimental para-
magnetic shifts based on the dipolar eq 1. The correlation is
especially good if one takes into account that the cyclohexenyl
rings are rapidly inverting. The geometric factors of each ex-
changing group were obtained by computing the mean of the
factors for two proton sets exchanging their positions during the
ring inversion process. No account was taken of the transition
states nor of the fact that cyclohexene adopts various less stable
Figure 6. Force field simulation of the optimized structure of the 1:2
3+
bis-complex of BTPhen 9 with Yb . The arrows show the shortest
interatomic distances in the metal complexes. These distances are larger
than the sum of the van der Walls radii. See Supporting Information for a
list of structural parameters.
equation 1 was used to unravel the solution structure of the
3
+
Yb BTPhen 9 bis-complex.
"
ꢀ
ꢁ
2
1
1
Æ3 cos θi ꢀ 1æ
δi ¼ 1
χ ꢀ ðχ þ χyyÞ
zz
xx
3
2π
2
ri
ꢂ
2
Æsin θi cos 2ψ æ
i
28
þ ðχ ꢀ χ Þ
ð1Þ
geometries in addition to the half-chair conformation. Not
xx
yy
3
ri
surprisingly, the datum point that is the most removed from the
correlation line in Figure 7 originates from one of the methyl
proton sets undergoing rapid inversion. The geometric factors of
the exchanging protons are very different in this particular case,
and larger errors are thus expected.
29
In this equation, the paramagnetic shifts depend on mag-
netic susceptibility terms that are identical for all nuclei and on
geometric terms that are different for each nucleus and are easily
1
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3
+
Figure 7. Correlation between the calculated and experimental paramagnetic shifts of the bis-complexes of BTPhen 9 with Yb perchlorate (.) or
nitrate (~); y axis on left: perchlorate, y axis on right: nitrate.
There is one feature worthy of note in these calculations that
has not been mentioned in the literature so far. As the two ligands
are planar in an environment of 2-fold symmetry, an agreement
indistinguishable from the one presented in Figure 7 is obtained
if the angle between the two ligands is increased or decreased.
The geometric factors are of course modified, but the magnetic
susceptibility terms are also changing and the correlation be-
tween calculated and experimental paramagnetic shifts remains
unaltered. For instance, if the angle between the two BTPhen
ligands is 90ꢀ, 75ꢀ, 60ꢀ, and 45ꢀ, the magnetic susceptibility
factors in eq 1 become 1392 and ꢀ4038, 1804 and ꢀ3600, 1617
ꢁ
ꢀ3
and ꢀ3788, and 1543 and ꢀ3863 Å , respectively. We are
confronted here with a limit of the dipolar shift analysis method
that does not arise with nonplanar ligands such as one based on 2,
0
0 00
11b
29
2
:6 ,2 -terpyridine or other complexes investigated so far. A
consequence of this is that a good correlation will still be
obtained if the structure of the complex becomes flatter to allow
for the entrance of an anion or a solvent molecule in the first
coordination sphere. A good correlation between experimental
and calculated shifts is thus still expected for the bis-BTPhen
nitrate complex depicted in Figure 3, as it features a pseudo-2-
fold symmetry axis and a larger opening for accommodating
an anion.
Figure 8. Extraction of Am(III) and Eu(III) by BTPhen 9 in octanol
(0.01 M) as a function of initial nitric acid concentration (D =
distribution ratio, SF = separation factor, 2 = DAm, b = DEu, 9 =
SFAm/Eu, mixing time: 60 min, temperature: 22 ( 1 ꢀC).
susceptibility terms should indeed be totally different because
of the presence of a charged species directly coordinated to the
metal ion. However, the molecular model remains valid, as
shown by the good correlation between experimental and
calculated shifts presented in Figure 7. Nevertheless, the degree
of opening of the bis-complex to accommodate the nitrate ion
remains unknown, and it appears that NMR spectroscopy is of no
help in solving this particular problem.
A similar analysis was carried out for the 1:2 bis-complex of
1
BTPhen 9 with Eu shown in Figure 3. The H NMR spectrum of
a 1:2 mixture of 9 and Eu(ClO ) was recorded, and the analysis
4
3
of the induced paramagnetic shifts was performed as for the
Yb(III) bis-complex (see Supporting Information). However,
unlike Yb(III), the contact paramagnetic shifts for Eu(III) are
larger than the dipolar paramagnetic shifts, and thus the dipolar
eq 1 cannot be used as reliably for solution structure deter-
Solvent Extraction Studies with Am(III), Cm(III), and Eu(III)
Radionuclides. Preliminary solvent extraction experiments were
then carried out to determine the ability of 9 to selectively extract
An(III) over Ln(III). Solutions of the ligand 9 in octanol (0.01
2
9d,31
minations.
Only a qualitative agreement was obtained, from
2
41
M) were contacted with nitric acid solutions containing Am
which the solution structure of the complex could not be reliably
established.
152
and Eu radiotracers. The distribution ratios (D) for Am(III)
and Eu(III) and the separation factors (SFAm/Eu) at different
nitric acid concentrations are shown in Figure 8. Very high D
values for Am(III) were observed (DAm > 1000), indicating that
the extraction of Am(III) by 9 is very efficient. The D values for
Eu(III) were approximately 2 orders of magnitude lower than
1
In the H NMR spectrum of a 1:2 mixture of Yb(NO ) and
3
3
BTPhen 9 in acetonitrile (see Supporting Information), the
entrance of a nitrate ion into the first coordination sphere is
clearly indicated by paramagnetic shifts that are totally different
from those observed in the perchlorate case. The magnetic
1
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3
Figure 9. Extraction of Am(III) and Eu(III) from 1 M HNO by
Figure 10. Extraction rate constants (k ) in different diluents for 0.01
ext
BTPhen 9 in octanol (0.01 M) as a function of time (9 = DAm, b = DEu
,
M solutions of BTBP 1 and BTPhen 9 (aqueous phase: 2 M HNO
3
).
temperature: 22 ( 1 ꢀC).
6
0 min of contact time to reach its equilibrium D value of
Am
those of Am(III) (DEu < 10), and the resulting separation factors
were in the range 68ꢀ400. These results show that 9 is able to
extract Am(III) in preference to Eu(III) with very high selec-
tivity. The formation of stable 1:2 bis-complexes by 9 certainly
contributes to the higher D values observed for Am(III) com-
10a
approximately 4.5. The faster rates of extraction by BTPhen
9
N -dimethyl-N,N -dioctyl-2-(2-hexoxyethyl) malondiamide,
which is needed to improve the extraction kinetics of BTBP 1,
is not necessary. Furthermore, as DAm is greater than 10 even
after 5 min of contact, an efficient extraction can still take place
using shorter contact times if desired.
mean that the use of a phase-transfer agent such as N,
0
0
34
10
7c
pared to previous N-donor ligands. The distribution ratios for
Am(III) for 9 are about 2 orders of magnitude higher that those
10a
for the related BTBP ligand 1. The considerable improvement
in the extraction of Am(III) by 9 compared to 1 far exceeds what
was anticipated on the basis of molecular design. The high D
values observed for 9 make it especially suitable even for low-level
waste decontamination where only trace levels of radionuclides
are present.
In a separation process, it is desirable to back-extract (strip)
the actinides from the loaded organic phase after the extraction
has taken place so that the ligand can be reused in further
extraction cycles. Ideally this requires DAm values < 1 for efficient
stripping. Figure 8 shows that for 9 the distribution ratios for
Am(III) are lowest at low acidity but still exceed 1. The effect of
the less polar diluents toluene and hydrogenated tetrapropene
In common with other bis-triazine ligands, BTPhen 9 is able to
extract Am(III) from nitric acid solutions of high acidity (1ꢀ4 M
(
TPH, a dodecane-like diluent) as co-diluents was then studied
HNO ). Thus, although the ligand is likely to be protonated in
3
in order to reduce the D values for Am(III) at low acidities (see
Supporting Information). With toluene as the co-diluent, the
distribution ratios for Am(III) and Eu(III) both decreased as the
volume fraction of toluene increased. This effect was most
contact with HNO solutions (pK = 3.1ꢀ3.2), proton competi-
3
a
tion does not prevent metal ion extraction. This is in contrast
0
0
00
with other N-donor ligands based on 2,2 :6 ,2 -terpyridine,
which can extract Am(III) only from weakly acidic (<0.1 M
pronounced at low acidities. With 0.001 M HNO as the aqueous
3
3
2
HNO ) solutions. The precise reasons for this extraction ability
3
phase, DAm decreased from 1.57 in octanol/toluene (80:20) to
are unclear at this point. It should be noted that, although the
competition between protonation and extraction leads to a
decrease in D values, an increase in acidity is also an increase
in the nitrate ion concentration that favors the extraction. The
formation of highly thermodynamically stable 1:2 bis-complexes
by BTPhen 9, coupled with the high hydrophobicity of the
ligand, could also explain the observed extraction results. In
0
.11 in octanol/toluene (20:80). When toluene was the sole
diluent, a DAm value of 0.02 was observed (aqueous phase: 0.01
M HNO ). With TPH as the co-diluent, D values of 0.48 and
0
3
Am
.45 were observed in the extraction from 0.001 M HNO into
3
octanol/TPH mixtures (60:40 and 50:50), respectively.
2
41
Extraction experiments were then carried out using Am and
244
Cm radiotracers in order to probe the co-extraction of both
addition, a very low pK value of ꢀ1.77 has been estimated for
Am(III) and Cm(III) by BTPhen 9 (see Supporting In-
formation). Both Am(III) and Cm(III) were efficiently co-
extracted into octanol/toluene (40:60) solutions by 9 with
a
1
,2,4-triazine from a correlation between pK ’s and ionization
a
3
3
energies, and the triazine rings of 9 are thus believed to remain
unprotonated even at very high acidities. The extraction of
virtually no selectivity between the two actinides (SFAm/Cm =
lanthanide and actinide salts from concentrated HNO solutions
by 9 is thus less thwarted by protonation than for other N-donor
ligands.
0.7ꢀ4). In the extraction of both Am(III) and Cm(III) from
3
0.001 M HNO into octanol/toluene mixtures, the D values for
3
both metals decreased as the volume fraction of toluene in-
creased, and D values below 1 were observed for both metals in
40:60, 20:80, and 0:100 octanol/toluene mixtures. These results
indicate that the stripping of both Am(III) and Cm(III) from a
loaded organic phase is feasible at lower acidities.
The waste streams produced in the PUREX process contain
high concentrations of Y(III) and all the trivalent lanthanides in
addition to the minor actinides. Information is therefore needed
Figure 9 shows the kinetics of extraction of Am(III) and
Eu(III) by BTPhen 9 from 1 M HNO into octanol. For Am(III),
3
relatively fast extraction kinetics are observed even in the absence
of a phase-transfer agent, and distribution ratios close to the
equilibrium value are reached within 15 min of phase contact
time. Thus the kinetics of extraction by the BTPhen ligand 9 are
significantly faster than the BTBP ligand 1, which requires up to
1
3099
dx.doi.org/10.1021/ja203378m |J. Am. Chem. Soc. 2011, 133, 13093–13102

Journal of the American Chemical Society
ARTICLE
9
at the interface. Hence BTPhen 9, which exhibits surface
activity in all diluents, gives extraction kinetics that are faster
than BTBP 1, which is surface active in only one diluent
(
3-methylcyclohexanone). The greater surface activity observed
for BTPhen 9 compared to 1 is in keeping with what was
predicted based on ligand design. The higher polarity of 9 and
its ability to strongly coordinate to water are probably respon-
sible for its ability to interact favorably with the interface.
’
CONCLUSIONS
In summary, we have reported the first example of a promis-
ing new class of highly selective solvent extraction reagents for
the partitioning of trivalent actinides from trivalent lanthanides
in nuclear waste streams and have demonstrated that preorga-
nization of the donor atoms for metal ligation with a rigid cis-
locked 1,10-phenanthroline motif leads to a rapid and highly
efficient separation of actinides from lanthanides. Interfacial
tension measurements suggest that the improved extraction
Figure 11. Back-extraction rate constants (k ) in different diluents for
str
0.01 M solutions of BTBP 1 and BTPhen 9 (aqueous phase: 2 M
HNO ).
3
0
kinetics of the ligand relative to its 2,2 -bipyridine counterpart
are related to higher concentrations of the ligand at the inter-
face. The first X-ray crystal structure of a 1:2 bis-complex of a
quadridentate bis-triazine ligand with Eu(III) has been deter-
mined. Lanthanide NMR complexation studies allowed the
solution structure of the 1:2 bis-complex with Yb(III) to be
deduced from the induced paramagnetic shifts. The aliphatic
diketone precursor to the ligand has been synthesized by an
improved procedure that, for the first time, allows the straight-
forward synthesis of BTPhen 9 and related ligands on at least a
multigram scale. Further applications of the ligand in coordina-
tion chemistry are anticipated.
on the extraction of the lanthanides as well as the actinides. The
extraction of the lanthanides by octanol solutions of BTPhen 9 at
several different acidities showed a profile across the lanthanide
series of first increasing, then decreasing D values, with the
maximum D value being observed for Sm(III) (see Supporting
Information). A similar trend was reported for the BTBP ligand
1
0a
1
.
Thus BTPhen 9 efficiently separates both Am(III) and
Cm(III) from the entire lanthanide series.
Kinetics and Surface Tension Measurements. In order to
uncover the reasons for the faster extraction kinetics observed with
9
compared to its non-rigidified analogue 1, a comparison of the
152
extraction kinetics of Eu by BTBP 1 and BTPhen 9 in different
’
ASSOCIATED CONTENT
diluents was carried out using the rotating membrane cell
3
5
technique. With octanol, 2-methylcyclohexanone, 3-methylcy-
clohexanone, and 4-methylcyclohexanone as the diluents, the
S
Supporting Information. Procedures and characteriza-
b
tion data for all compounds. Tables and graphs of solvent
calculated extraction rate constants (k ) for BTPhen 9 are
ext
extraction data. Figures of optimized structures and NMR
substantially larger than those for BTBP 1 in each of these diluents
spectra of lanthanide complexes. Procedure for pK determina-
a
(Figure 10). Conversely, the back-extraction (stripping) rate
tion. X-ray crystallographic cif files for ligand 9 and its Eu
complex. Tables and graphs of kinetic data and interfacial tension
measurements. This material is available free of charge via the
Internet at http://pubs.acs.org.
constants (k ) for BTBP 1 are larger than those for BTPhen 9 in
str
these diluents (Figure 11). These results suggest that BTPhen 9 will
have a faster rate of extraction, but a slower rate of stripping
compared to BTBP 1. The fastest extraction kinetics for both
ꢀ
ꢀ6
6
ligands were observed in cyclohexanone (k = 121 ꢁ 10 cm
ext
for 0.01 M BTBP 1 from 2 M HNO ; k = 74.8 ꢁ 10 cm
ꢀ1
1
’ AUTHOR INFORMATION
s
s
3
ext
ꢀ
for 0.01 M BTPhen 9 from 1 M HNO ), which is known to
improve significantly the kinetics of extraction by the BTBPs.
Corresponding Author
l.m.harwood@reading.ac.uk
3
36
0
0
34,37
Using N,N,N ,N -tetraoctyl diglycolamide
as additive also
accelerated the extraction kinetics of BTBP 1 in octanol, but was
less effective in accelerating the extraction kinetics of BTPhen 9
in octanol (see Supporting Information).
’
ACKNOWLEDGMENT
We thank the Nuclear Fission Safety Program of the European
The interfacial tensions between aqueous phases of 1 M
Union for support under the ACSEPT (FP7-CP-2007-211 267)
contract. We also thank the EPSRC and the University of Reading
for funds for the X-Calibur system. Use of the Chemical Analysis
Facility at the University of Reading is gratefully acknowledged.
HNO and organic phases of BTBP 1 and BTPhen 9 diluted
3
in octanol, 2-methylcyclohexanone, 3-methylcyclohexanone, and
4
-methylcyclohexanone were then measured using the du No €u y
38
ring method. In the case of BTBP 1, surface activity was
observed only when the ligand was diluted in 3-methylcyclohex-
anone and was not observed in the other diluents. In the case of
BTPhen 9, surface activity was observed in all four diluents (see
Supporting Information). The sharpest decrease in surface
tension was observed in octanol. The faster extraction kinetics
observed with BTPhen 9 compared to BTBP 1 thus appears to be
related to the greater surface concentration of the BTPhen ligand
’
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(
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34.5 43 8 2
M = 601.77, monoclinic, space group P2 /n, Z = 4, a = 15.5530(8) Å, b =
1
ꢁ
o 3
ꢁ
10.2067(6) Å, c = 20.833(2) Å, β = 103.417(7) , U = 3216.9(4) Å ,
calc = 1.243 g cm , 8714 independent reflections, 4145 observed
ꢀ
3
d
reflections, R = 0.0568, wR = 0.1203, CCDC 789753.
1
2
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space group P1, Z = 2, a = 13.5768(8) Å, b = 15.3420(10) Å,
2 3
(NO )]
3
)
5
2
2 23
H81Eu N O
ꢁ
3
o
(
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ꢁ
ꢀ3
U = 3943.4(4) Å , dcalc = 1.560 g cm , 21 133 independent reflections,
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8
1
2
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0
00
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