Unprecedented Inversion of Selectivity and Extraordinary Difference in the Complexation of Trivalent f Elements by Diastereomers of a Methylated Diglycolamide

Article abstract of DOI:10.1002/chem.201806161

When considering f elements, solvent extraction is primarily used for the removal of lanthanides from ore and their recycling, as well as for the separation of actinides from used nuclear fuel. Understanding the complexation mechanism of metal ions with organic extractants, particularly the influence of their molecular structure on complex formation is of fundamental importance. Herein, we report an extraordinary (up to two orders of magnitude) change in the extraction efficiency of f elements with two diastereomers of dimethyl tetraoctyl diglycolamide (Me₂-TODGA), which only differ in the orientation of a single methyl group. Solvent extraction techniques, extended X-ray absorption fine structure (EXAFS) measurements, and density functional theory (DFT) based ab initio calculations were used to understand their complex structures and to explain their complexation mechanism. We show that the huge differences observed in extraction selectivity results from a small change in the complexation of nitrate counter-ions caused by the different orientation of one methyl group in the backbone of the extractant. The obtained results give a significant new insight into metal–ligand complexation mechanisms, which will promote the development of more efficient separation techniques.

Full text of DOI:10.1002/chem.201806161

A Journal of
Accepted Article
Title: Unprecedented Inversion of Selectivity and Extraordinary
Difference in the Complexation of Trivalent f-Elements by
Diastereomers of a Methylated Diglycolamide
Authors: Andreas Wilden, Piotr M. Kowalski, Larissa Klaß, Benjamin
Kraus, Fabian Kreft, Giuseppe Modolo, Yan Li, Jörg Rothe,
Kathy Dardenne, Andreas Geist, Andrea Leoncini, Jurriaan
Huskens, and Willem Verboom
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To be cited as: Chem. Eur. J. 10.1002/chem.201806161
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10.1002/chem.201806161
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Unprecedented Inversion of Selectivity and Extraordinary
Difference in the Complexation of Trivalent f-Elements by
Diastereomers of a Methylated Diglycolamide
Andreas Wilden,*[a,b] Piotr M. Kowalski,*[a,b] Larissa Klaß,[a,b] Benjamin Kraus,[a,b] Fabian Kreft,[a,b]
Giuseppe Modolo,[a,b] Yan Li,[a,b] Jörg Rothe,[c] Kathy Dardenne,[c] Andreas Geist,[c] Andrea Leoncini,[d]
Jurriaan Huskens,[d] and Willem Verboom[d]
Abstract: Solvent extraction is primarily used for the removal of
lanthanides from ore and their recycling, as well as for the
Introduction
separation of actinides from used nuclear fuel. Understanding the
complexation mechanism of metal ions with organic extractants,
particularly the influence of their molecular structure on complex
formation is of fundamental importance. Herein, we report an
extraordinary, up to two orders of magnitude, change in the
extraction efficiency of f-elements with two diastereomers of dimethyl
tetraoctyl diglycolamide (Me2-TODGA), which only differ in the
orientation of a single methyl group. Solvent extraction techniques,
Extended X-Ray Absorption Fine Structure (EXAFS) measurements,
and Density Functional Theory (DFT) – based ab-initio calculations
were used to understand their complex structures and explain their
complexation mechanism. We show that the huge differences
observed in extraction selectivity results from a small change in the
complexation of nitrate counter-ions caused by the different
orientation of one methyl group in the backbone of the extractant.
The obtained results give a significant new insight into metal-ligand
complexation mechanisms, which will promote the development of
more efficient separation techniques.
New solvent extraction techniques for the separation of
highly radiotoxic nuclides from used nuclear fuel or
contaminated wastewater from sites of nuclear accidents are
intensively studied worldwide.[1] The separation of trivalent
actinides from lanthanides is one of the most challenging tasks,
due to the chemical similarity of these two groups of elements.
The separation of Am3+ from Cm3+ is in equal measure very
difficult owing to the similar ionic radii. Nevertheless, Am/Cm
separation has gained tremendous interest, as the separation
and recycling of Am (without Cm) in advanced nuclear fuel
cycles was shown to have a major impact on the size (volume
and footprint) and cost of a geological disposal facility for high
level nuclear waste.[2] Furthermore, the recycling of lanthanides
from end-of-life products is very important to reach a sustainable
economy.[3] Consequently, different types of ligands have been
developed for the selective complexation of actinides and
lanthanides.[4] For this purpose we have focused on
diglycolamides both as individual ligands[5] and pre-organized on
a molecular platform.[6]
Diglycolamides are widely used in solvent extraction for the
separation of trivalent lanthanides and actinides from used
nuclear fuel solutions.[4c] One of the most prominent members of
the class of diglycolamides used for this purpose is TODGA
(N,N,N’,N’-tetraoctyl diglycolamide).[7]
A
large number of
derivatives has been studied to date, with main focus on
derivatization of the amidic side chains.[4c] Additionally, we are
interested in understanding the influence of modifications in the
central backbone of the diglycolamides. In particular, methylated
derivatives were found to be suitable ligands for solvent
extraction processes, as they show lower co-extraction of
unwanted metal ions (esp. Sr2+)[5a, 5b] and higher loading capacity
e.g. towards Pu loading. Consequently, the development of new
processes for grouped actinide extraction utilising methylated
diglycolamide ligands has become quite topical.[8]
[a]
Dr. A. Wilden, Dr. P. M. Kowalski, L. Klaß, B. Kraus, F. Kreft, PD Dr.
G. Modolo, Dr. Y. Li
Forschungszentrum Jülich GmbH
Institut für Energie- und Klimaforschung-Nukleare Entsorgung und
Reaktorsicherheit- (IEK-6)
Wilhelm-Johnen-Str. 1
52428 Jülich, Germany
E-mail: a.wilden@fz-juelich.de, p.kowalski@fz-juelich.de
JARA High-Performance Computing
Schinkelstrasse 2
[b]
[c]
52062 Aachen, Germany
Dr. A. Geist, Dr. J. Rothe, Dr. K. Dardenne
Karlsruhe Institute of Technology (KIT)
Institute for Nuclear Waste Disposal (INE)
76021 Karlsruhe, Germany
Dr. A. Leoncini, Prof. J. Huskens, Dr. W. Verboom
Laboratory of Molecular Nanofabrication
Mesa+ Institute for Nanotechnology
University of Twente
To our best knowledge, a paucity of information exists
regarding
the
complexation
behaviour
of
individual
diastereomeric ligands. In the case of two diastereomers of
1,4,7-triazacyclononane-1,4,7-tris-(glutaric acid) no difference
was found in the complexation reaction with 67GaCl3 and the
affinity for αv,β3 integrin.[9] Similar conclusions were obtained by
Sun et al. in the case of diastereomerically pure bifunctional
chelators for copper radiopharmaceuticals.[10] Ishimori et al.
describe the extraction of Am3+ and Eu3+ with different
[d]
P.O. Box 217, 7500 AE Enschede, The Netherlands
Supporting information for this article is given via a link at the end of
the document.
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10.1002/chem.201806161
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diastereomers of
a tripodal pyridine ligand and observed
differences in metal extraction of a factor of 2-3.[11] Yamada et al.
studied the lanthanide complexation and luminescence behavior
of un-, mono-, and disubstituted tripodal pyridine ligands and
found that their complexation behavior was “rarely influenced by
ligand chirality” with an enhancement in Tb luminescence of ca.
three times for one diastereomer over the other.[12] Crown ether
compounds are known to show very high selectivity for metal
ions with ionic radii fitted to the cavity size of the crown
compound. Differences in the extraction with the cis-syn-cis and
cis-anti-cis isomers of dicyclohexano-18-crown-6 (DCH18C6)
were found for the extraction of Ca2+, Sr2+ Ba2+, Ra2+and K+[13]
and Pu(IV)[14] ranging between a factor of 2-3. Bond et al.
studied the extraction of Ca2+, Sr2+, and Ba2+ in a synergistic
system of four stereoisomers of DCH18C6 with di-n-
octylphosphoric acid (HDOP). Due to the combination of the
high selectivity of HDOP and the crown compounds, differences
in the extraction of up to ca. a factor 100 were observed.
However, in all cases any explanation of the observed
differences is lacking.
Scheme 1. Synthesis of the Me2-TODGA ligands 6a and 6b.
Both diastereomers 6a and 6b are symmetrical, which is
reflected in their 1H NMR spectra (Figure 1 a,b), although they
are not easily distinguishable because of their free rotation.
However, upon addition of lanthanum(III) triflate (La(III)(OTf)3),
the corresponding complexes were formed, locking the ligands
and preventing rotation. In the case of the La3+-complex of 6a
We report an extraordinary difference in the extraction
efficiency of trivalent f-elements and even a reverse in selectivity
for trivalent actinide (i.e. Am3+, Cm3+) extraction for two
diastereomers
of
dimethyl
tetraoctyl
diglycolamide
(Me2-TODGA), a derivative of the well-known TODGA, differing
only in the orientation of a single methyl group. We investigate
the origin of this phenomenon by a combination of solvent
extraction techniques, EXAFS analysis, and quantum chemical
calculations.
1
the symmetry in the H NMR spectrum is broken, while that of
the La3+-complex of 6b is maintained (Figure 1 c,d).
Subsequently, we conclude that 6a and 6b have Cs and C2
symmetry, with RS- and SS-configuration, respectively.
Complexation of the ligands with La3+ via the amide carbonyl
groups induces a change in their geometry. In the case of 6a
steric hindrance of the two central methyl groups will force the
molecule in a twisted position upon complexation resulting in the
loss of symmetry. Upon complexation of 6b, the methyl groups
will only move further apart maintaining the symmetry.
Results and Discussion
We previously reported the synthesis of Me2-TODGA 6 as a
mixture of diastereomers.[5a] The major diastereomer was
isolated and its behaviour and complex stoichiometry towards
trivalent lanthanide and actinide extraction was studied.[5a, 5b] In
the current study, the individual diastereomers were prepared
via a slightly modified procedure from 2-bromopropionate (1)
and ethyl (S)-lactate (2) (Scheme 1). 2-Bromopropionate (1) was
reacted with ethyl (S)-lactate (2) to give the diesters 3 in a
diastereomeric ratio of 4:1, which were isolated by flash
chromatography. Apparently, lactate 2 reacts faster with one of
the enantiomers of 1 as opposed to the other. The unreacted
lactate 2 undergoes racemization which is known to happen for
α-bromo esters in the presence of Br-.[15] Saponification of the
ester groups in 3a,b afforded the dicarboxylic acids 4a,b. Using
the recently developed approach involving Schotten-Baumann
conditions,[16] 4a,b were converted into the target ligands 6a and
6b via the in situ prepared diacyl dichlorides, in 50% and 68%
yield, respectively.
5.80
5.00
4.20
3.40
2.60
5.80
5.00
4.20
3.40
2.60
Figure 1. 600 MHz 1H NMR spectra in CDCl3 of a, b) the free ligands 6a and
6b and c, d) upon addition of excess of La(III)(OTf)3.
Both diastereomers 6a and 6b were tested as extractants in
solvent extraction experiments dissolved in TPH (hydrogenated
tetrapropene) for the extraction of 241Am, 244Cm and 152Eu, as
well as 10-5 mol/L of each lanthanide (w/o Pm, incl. Y + La) from
HNO3 solution. Me2-TODGA is a relatively weak extractant in
comparison to the parent molecule TODGA[7a, 17] and the mono-
5b]
methylated analogue Me-TODGA.[5a,
Significant distribution
ratios (the ratio of activity or concentration of a metal ion in the
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10.1002/chem.201806161
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organic phase vs. the one in the aqueous phase) for the studied
metal ions are only reached at relatively high HNO3
concentrations of ≥ 2 mol∙L-1.
metal ions bound through the three oxygen donor atoms of each
ligand, resulting in an overall nine-fold coordination.[5b] Nitrate
ions, required for charge compensation, are presumably located
in the outer shell of the complexes, as no direct nitrate
coordination was found by spectroscopic methods or single
crystal structure analyses in the inner-sphere of comparable
complexes.[20] Consequently, we argue that the specific
interaction of the metal-ligand complex with the nitrate anions is
controlled by the orientation of the additional methyl groups in
Me2-TODGA complexes bound with f elements, which causes
the observed differences in extraction performance. Additional to
this, the Gd break and tetrad effects[21] further influence the
complexation process of the lanthanides with diastereomers 6a
and 6b. The dashed lines in Figure 2 illustrate these effects and
contributions to the distribution ratio. The Gd break is explained
by the half-filled electron shell for Gd and the tetrad effect is
related to observations that certain liquid-liquid extraction
systems show discontinuities grouping the lanthanide behavior
in four tetrads.[21] The observed inversion of Am/Cm selectivity
could therefore be caused by the combined effects of
complexation with 6a or 6b and the Gd break and tetrad effects.
To further understand the extraction mechanism, the
molecular structure of the extracted complexes, and the
observed differences in the extraction efficiency with 6a and 6b,
we conducted EXAFS measurements and DFT-based ab-initio
calculations to gain better insight into structures and
thermodynamics parameters of the metal:ligand complexation.
EXAFS measurements were conducted on organic solution
samples of metal complexes (La3+, Eu3+, Er3+, and 243Am3+) with
6a and 6b, which were prepared by solvent extraction to give
sample compositions as representative for separation process
application as possible. The sample preparation procedure and
EXAFS measurements are described in the Supporting
Information. The structural models required to evaluate the
EXAFS data were built from the complex structures resulting
from the DFT calculations (vide infra). They represent the final
relaxed structures of the 1:3 metal:ligand complexes. In order to
get a good description of the complex structures, we applied
PBEsol exchange-correlation functional,[22] which by design
results in better predicted molecular structures than other
standard DFT functionals, including PBE.[23] These structural
models gave very good fits to the experimental EXAFS data
(Figures S1 and S2 and Table S1). The obtained number of
scattering atoms and bond lengths agree well with the
theoretical results. Figure 3 shows the bond distances between
the central metal ions and the first coordination shell oxygen
atoms for 6a and 6b. For both ligand diastereomers, a decrease
in M-O bond distance is observed as a function of the inverse
ionic radius of the central metal ion. This is a result of the
lanthanide contraction. 6a yields lower bond distances
compared to 6b, reflecting the stronger complexation of 6a. 6b
exhibits a slightly steeper dependency of the M-O bond distance
as a function of the inverse ionic radius of the central metal ion.
The average M-O distances of ligands 6a and 6b are slightly
larger than related distances reported on TODGA complexes in
the literature.[20b] This reflects a significantly larger steric demand
of ligands 6a and 6b, due to the additional methyl groups.
Figure 2 shows the distribution ratios of Am, Cm, Y, La and
other lanthanides (w/o Pm) as a function of the inverse ionic
radius for 9-fold coordination[18] for the extraction from
4.3 mol∙L−1 HNO3 using 0.1 mol∙L−1 6a (blue) and 6b (red) in
TPH. Figure 2 shows that the distribution ratios mostly increase
with decreasing ionic radius of the extracted metal ion. This
behavior is generally observed for diglycolamides used in
solvent extraction and has also been observed before for the
methylated TODGA analogues.[5b] Diastereomers 6a and 6b
show a maximum in the distribution ratios for Ho, similar to
previous observations.[5b] We already reported on a shift of the
maximum of distribution ratios by successive introduction of
additional methyl groups into the TODGA backbone.[5b]
When the extraction efficiencies of the two diastereomers,
6a and 6b, are compared, a large and to the best of our
knowledge unprecedented difference is observed. 6a shows
much higher distribution ratios than 6b, with the largest
difference a factor of 94 greater observed for Ho. Furthermore,
the variation in distribution ratios for the lanthanides examined
with 6a and 6b is far greater for the former diastereomer.
6a
100
6b
10
1
Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm YbY Lu
Am Cm
La
0.1
0.78 0.80 0.82 0.804.805.86 0.88 0.90 0.92 0.904.805.96 0.98 1.00 1.02
1/radius / Å−1
Figure 2. Distribution ratios of Am, Cm (filled symbols), Y, La and other
lanthanides (w/o Pm, open symbols) as a function of the inverse ionic radius
for the extraction from 4.3 mol∙L−1 HNO3 using 0.1 mol∙L−1 6a (blue) and 6b
(red) in TPH.
Diastereomer 6a shows a preference for the extraction of
Cm3+ over Am3+ with a separation factor SFCm/Am of 1.4. To the
best of our knowledge a preference for Cm3+ complexation was
always observed for related diglycolamides (even water-soluble
ones).[4c, 7a, 19] As such, this is the first report on a diglycolamide
(diastereomer 6b) showing a preference for Am3+ over Cm3+.
As the two diastereomers 6a and 6b only differ in the
orientation of a single methyl group, the significant variation in
extraction performance is apparently a consequence of this
subtle structural difference. Based on our previous findings, we
postulate that diastereomers 6a and 6b both form 1:3
metal:ligand complexes in the organic phase with the central
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Based on the good agreement between EXAFS structures
and the DFT-based calculations we conclude that 1:3
metal:ligand complexes are formed in organic solution with
diastereomers 6a and 6b. Furthermore, these results support
our previous argument that the difference in orientation of the
methyl groups in the ligands is the key structural feature behind
the observed differences in affinity and selectivity for the
extraction of trivalent actinide and lanthanide ions with
diastereomers 6a and 6b.
in the reaction enthalpies. The computed ΔΔH (difference of ΔH
values for complexes with diastereomers 6a and 6b) is about
~10 kJ/mol. This should correspond to a factor of ~50 difference
in the distribution ratios with diastereomers 6a and 6b (applying
Boltzman distribution assuming ambient conditions (kT
≈
2.5 kJ mol–1), Eq. 3).
10
DM,6a
 e2.5  50
(3)
DM,6b
This is consistent with the average ratio of the distribution
ratios measured for diastereomers 6a and 6b for Sm - Lu
(values between 44 - 94) and shows that DFT can capture the
considered reaction also on the quantitative level. However, we
note that our DFT studies cannot explain the subtle differences
observed between the extraction results for the lighter
lanthanides La – Nd (values between 4 - 17). This could
potentially be attributed to a different number of co-extracted
water molecules for the heavier lanthanides, as described
recently by Baldwin et al.[24] The effect of co-extracted water is
not accounted for in our DFT calculations, as the water content
was not assessed during the solvent extraction experiments.
2.65
6a EXAFS
6b EXAFS
6a DFT
2.60
2.55
2.50
2.45
2.40
2.35
2.30
6b DFT
La
Am
Eu
Er
0.78
0.82
0.86
1/radius / Å−1
0.90
0.94
0.98
-1085
6a
6b
-1090
-1095
-1100
-1105
-1110
-1115
-1120
-1125
Figure 3. Bond distances between central metal ion and first coordination
shell oxygen atoms (mean values) of ligands 6a (blue/green) and 6b
(red/yellow), as determined by EXAFS measurements and DFT calculations
for Am (filled symbols), La, Eu, and Er (open symbols) as a function of the
inverse ionic radius. A tabular representation is given in Table S2.
We attempted to understand the underlying mechanisms
causing the difference in the extraction performance of the two
diastereomers using systematic DFT calculations (PBE
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
functional[23]
compounds.
First, we computed the enthalpies of the extraction reaction:
) of the metal-ion complexes and the related
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
Atomic number

3
aq
Figure 4. The computed enthalpy of reaction [Eq. (1)] with diastereomers 6a
(blue) and 6b (red).
M H O
 3 Me2TODGA  3 NO3 


2

9
(1)


M Me TODGA NO
 9 H2O

 
3 
2

3
3
org
The DFT data presented in Figure 4 do not show the
experimentally observed slight decrease of the distribution ratios
around Gd and Lu. However, these effects result most probably
from the tetrad effect[21] and probably have their origin in the
subtle differences in the electronic structure, especially of f
electrons, which is not taken into account by the standard DFT
methods. The effect of f electrons is minimal for La3+, Gd3+ and
Lu3+, because these elements in trivalent state have either no f
electrons, half-, or fully-filled f shells, respectively.[25] This also
explains the observed inversion of selectivity in the extraction of
Am3+ and Cm3+. As Cm3+ has a half-filled 5f shell, the f-electron
effect is zero, leading to a lower extraction with 6b compared to
Am3+. With 6a, however, the steeper increase in the extraction of
La3+ – Gd3+, as shown by the dashed lines in Figure 2, leads to a
and the difference between the cases with the two
diastereomers, namely:




E M 6a NO
E M 6b NO
3 E6a E6b
(2)

  3 

  3 







3
3


3
3

The results are given in Figure 4. For both diastereomers,
the computed reaction enthalpies decrease along the lanthanide
series. However, the reaction enthalpies in the case of 6a are
-
lower, which indicates
a
higher stability of M-(6a-NO3 )3
complexes and a higher extraction ability, which is observed
experimentally. Since the difference between the diastereomers
6a and 6b in the reaction [Eq. (1)] is just the orientation of
methyl groups, we assume
a negligible reaction entropy
difference for the two cases and that the difference in the
reaction Gibbs free energies can be estimated by the difference
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higher extraction of Cm3+ compared to Am3+, despite of the
tetrad effect.
significantly more stable. This suggests that the nitrate anions
have a significant influence on the relative stability of the
considered M-ligand complexes. Comparison of the relaxed
structures of the La-(6a)33+ and La-(6b)33+ with the La-(6a-NO3)3
and La-(6b-NO3)3 complexes showed that the orientation of the
backbone methyl groups caused a structural change in the
nitrate coordination. In the case of diastereomer 6b the steric
interaction between the methyl and nitrate groups is increased,
which apparently causes the reduced complexation strength
observed experimentally and in the DFT calculations. This also
explains why diastereomer 6a yields stronger complexes,
although the NMR study of La(III)(OTf)3 complexes (Figure 1)
suggests a higher stability of complexes with diastereomer 6b.
The electrostatic interaction between the metal cation and
the extractant is one of the factors influencing the selectivity. If
this would be the only factor, we should expect to see a linearly
increasing trend in the measured distribution ratios along the
lanthanide series. However, experimental studies of the
extraction of trivalent lanthanides with diglycolamides (especially
TODGA) show an increase of the distribution ratios for light
lanthanides and nearly constant ones for heavier ones.[24] This is
also clearly visible in our data using diastereomers 6a and 6b
(Figure 2). Recently, it was shown by Baldwin et al. and Ellis et
al. that this trend in distribution ratios is determined by
interactions of water and nitrate in the outer-sphere. They called
this effect “aqueous phase selectivity”.[24, 26] Consistently with the
previous findings,[24] these values increase along the lanthanide
series, resulting in an increased selectivity towards the heavier
lanthanides (details are described in SI).
Other factors known to influence the performance of organic
extractants are the HOMO-LUMO levels and their related
parameters.[27] We thus computed the electronic structures and
HOMO-LUMO energies of the two diastereomers 6a and 6b and
the metal-ligand complexes. These are given in Tables S3 and
S4. Interestingly, the HOMO-LUMO parameters of the
diastereomers themselves are nearly identical, showing that
there is no difference in the absolute electronegativity and
hardness that could lead to a possible explanation of the
different performance of the two diastereomers.[27a] The only
significant difference involves the energy of the HOMO levels
and the band gap of the M-(Me2-TODGA-NO3)3 complexes.
While these parameters do not vary significantly between
different metal ions, they are significantly different for the two
diastereomers. For diastereomer 6a the average HOMO and
HOMO-LUMO gap are 4.3 and 2.9 eV, respectively, while for
diastereomer 6b they are 3.9 and 2.2 eV, respectively. These
values already indicate the enhanced stability of M-(Me2-
TODGA-NO3)3 complexes with diastereomer 6a. This may result
from slightly shorter metal-oxygen bond lengths (steric effect).
As shown in Figure S5, DFT calculations showed only a slight
difference in the M-Oin bond lengths for 6a and 6b complexes,
Figure 5. The HOMO state in the La-(6a-NO3)3 complex. The only contributors
are the p states of oxygen atoms from nitrates.
Our results are fully in line with the conclusions of a recent
theoretical investigation on the complexation of metal ions with
TEDGA (N,N,N’,N’-tetraethyl diglycolamide) ligands by Baldwin
-
et al.[24] They state that the M-NO3 distance is an important
indicator of the extraction selectivity.[24] Therefore, we plotted the
-
whereas
a
slightly higher difference in the EXAFS
average M-N(NO3 ) distances in Figure 6. Interestingly, we
measurements (Figure 3) was observed with 6a complexes
yielding slightly shorter M-Oin bond lengths. Thus, the
coordination of metal cation alone cannot be responsible for the
differences in the HOMO-LUMO levels.
observe a larger variation in the M-Nout distance values for 6a
complexes than for 6b complexes, which is consistent with the
larger variation of the distribution ratios for 6a (~100) than for 6b
(~10). For 6a, the average M-Nout bond lengths vary from 6.15 Å
(La) to 6.06 Å (Lu). For 6b, they are roughly constant, but
consistently smaller than for 6a. This also causes large
differences in the solvation energy of the metal ion complexes
with 6a and 6b (Figure S6). Apparently, the additional methyl
groups in the backbone of the extractants partly hinder the
approach of nitrate counter ions to the metal ions. This effect is
more pronounced for diastereomer 6a. Furthermore, the
distance between the closest methyl groups and nitrate anions is
much shorter for diastereomer 6b, resulting in steric repulsion
between methyl group and nitrate anion, and consequently in
the lower complexation strength. This finding is consistent with
the shorter M-NNO3 distances of ca. 5.3 – 5.4 Å reported by
In order to understand the difference in the HOMO levels of
the considered metal:ligand complexes we studied the origin of
the HOMO state. The HOMO state of the La-(6a-NO3)3 complex
is plotted in Figure 5 where it is composed only of p-orbitals of
-
outer oxygen atoms belonging to the NO3 groups. It is clearly
visible in the plots of the DOS (Figure S4) that the only
contributors to the HOMO level are the Oout atoms. It indicates
that the arrangement of nitrates in the complexes and interaction
of the outmost parts of the complexes with the organic solution
is
a main effect driving the stability of the metal-ligand
3+
complexes. Notably, calculations of the La-(6a)33+ and La-(6b)3
3+
complexes without nitrate groups resulted in La-(6b)3 being
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Baldwin et al. for unsubstituted TEDGA complexes[24] and Reilly
et al. for a PuIV(TMDGA)3(NO3)4 complex crystal structure.[20e]
orientation of the methyl groups in the backbone of the ligands.
To our best knowledge the herein reported large difference in
complexation and extraction behavior of two diastereomers is
unprecedented and will have a wide impact on the design of
better organic extractants and will promote the development of
more efficient separation techniques. The impact of a change in
6.3
6a
-
6.2
6.1
6.0
5.9
5.8
6b
the counter-ion from nitrate to e.g. ClO4 would be of strong
interest for further investigation.
Experimental Section
Details of the synthesis of diastereomers 6a and 6b, experimental
procedures and analytical details, as well as details of the DFT
calculations are described in the Supporting Information.
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72
Atomic number
Acknowledgements
Figure 6. The computed average Ln-Nout bond length between lanthanide
cation and three outer nitrate groups.
Financial support was provided by the European Commission
(project SACSESS – Contract No. FP7-Fission-2012-323-282
and GENIORS – grant agreement No. 730227) and the German
Federal Ministry of Education and Research (Contract No.
02NUK020E). The authors thank the JARA-HPC (Jülich Aachen
Research Alliance, Section High Performance Computing) for
computing time at the RWTH Aachen University and
Forschungszentrum Jülich GmbH computing resources. We
acknowledge the KIT synchrotron light source for provision of
the INE-Beamline instrumentation and would like to thank the
KIT Institute for Beam Physics and Technology (IBPT) for
operation of the storage ring, the Karlsruhe Research
Accelerator (KARA).
We also have searched for any substantial differences in the
ion structure of the complexes. For this purpose, we computed
the Löwdin charges on each atom[28] showing no significant
differences between diastereomers 6a and 6b. However, the
small variation of the charge on the metal ions (observed for
both 6a and 6b), and shown in Figure S7, indicates an
enhanced stability of the complex going along the lanthanide
series.
Theoretical investigations demonstrate the importance of the
-
NO3 anions and the interaction of the complex with the organic
medium for the stability of the M-Me2-TODGA complexes. We
note that the complex – organic medium interaction in our
calculations (model) is approximated by interaction of the
complex with the continuum, polarizable medium. Such a simple
approximation may be missing some additional factors that
could lead to the measured variation of the extraction along the
lanthanide series. Baldwin et al. have shown the importance of
the interaction of the complex with H2O molecules, which leads
to different amounts of co-extracted water.[24] This effect,
however, is difficult to model accurately on the DFT level and is
not in the scope of this paper. Its omission does not affect the
results and conclusions of this paper.
Keywords: actinides • complexation • diastereomers •
lanthanides • solvent extraction
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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A. Wilden*, P. Kowalski*, L. Klaß, B.
Kraus, F. Kreft, G. Modolo, Y. Li, J.
Rothe, K. Dardenne, A. Geist, A.
Leoncini, J. Huskens, W. Verboom
6a
100
6b
Page No. – Page No.
10
Title
1
Ce Pr Nd
Sm Eu Gd Tb Dy Ho Er Tm YbY Lu
Am Cm
La
0.1
0.78 0.80 0.82 0.804.805.86 0.88 0.90 0.92 0.904.805.96 0.98 1.00 1.02
1/radius / Å−1
Two diastereomers of Me2-TODGA show an extraordinary, change in the extraction
efficiency of trivalent f-elements and even an unprecedented inversion of the
selectivity for Am/Cm. These differences result from a change in the complexation
of nitrate counter-ions caused by the different orientation of just one methyl group in
the backbone of the extractant as supported by EXAFS analysis and DFT-based
ab-initio calculations.
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