Journal of Nuclear and Radiochemical Sciences, Vol. 6, No. 3, pp. 227-231, 2005

Solvent Extraction of Tc(VII) by Calixarenes Bearing Pyridino Groups
Rainer Ludwig*, a and Nguyen Thi Kim Dzunga, baFreie University Berlin, Inst. of Chem./IAC, Fabeckstr. 34-36, 14195 Berlin, Germany
ITRRE, Vietnam Atomic Energy Commission, 48 Lang Ha Street, Hanoi, Vietnamb

Received: July 31, 2005; In Final Form: September 30, 2005
The distribution of Tc(VII) between aqueous acidic solutions and organic solutions of nitrogen-containing extractants is investigated in order to establish the relationships between extraction power towards the Tc(VII)-anion,
ligand molecular structure, influence of the media and the speciation in this ion-pair extraction. Ligands belonging to the class of calixarenes, derivatized with pyridino groups were newly devloped for these purposes. The
number of substituents per calixarene, the position of the nitrogen and the size of calixarene are varying structuralparameters. The nitrogen atoms are either protonated or quaternized to allow ion-pair extraction. The aqueous
phase contains nitrate or chloride at varying or constant ionic strength. Experiments using Aliquat-336 were conducted under similar conditions.

1. Introduction

2. Experimental

In the course of studies on coupled transport of Tc(VII) andU(VI) by conventional extractants such as TBP1, we realized theimpact of anion competition and acid extraction on the separationof Tc(VII). Furthermore, we were interested whether macrocyclic ligands are able to extract Tc(VII) more selectively comparedwith non-cyclic or monodentate ligands. Among the latter, quaternary amines such as Aliquat-336 extract Tc(VII) well.2, 3 An
example for the macrocyclic effect with cooperative binding to4
the anion was recently observed with respect to HPO2−
4 .
Among macrocyclic ligands, calixarenes5 gained significant
interest in the field of ion recognition. So far, few publications
on the extraction of Tc(VII) by nitrogen-containing crowned
calixarenes were published, in which the relationship between
ligand structure and binding was thoroughly investigated.6, 7Metalated cyclotriveratrylenes extract anionic Tc(VII) andRe(VII) as ion pairs, the selectivity over competing anions
improves when the ligand has a deep cavity.8 Calix[4]arenes
with carbonyl groups9 were reported to complex alkali ions andextract Tc(VII) as ion-pair at high pH. Crowned calixerenes
turned out to be effective in co-extracting Tc(VII) with Cs + ascounterion from both acidic and alkaline media.10
In terms of this work we synthesized a series of calix[n]arenes(n = 4, 6, 8) bearing methylpyridyl (picolyl) groups and investigated their ligating behaviour by means of solvent extraction
from acidic media. The calixarene acts as molecular backbone
to which the pyridino groups are anchored via methoxy groups
in vicinity to each other. The purpose of the pyridino groups
in protonated state is to provide an anion exchange functionality, and the possibility of backextraction by deprotonating them.
The number of pyridino groups per molecule, the position of
the nitrogen atom as well as the size and flexibility of the calixarene vary among the ligands in order to systematically study
the influence of the molecular structure. In addition, ligands
bearing quaternized pyridinium groups were synthesized. All
these substituents were attached to the so-called ‘lower rim’ ofthe calixarene. In the distribution experiments, the concentration and kind of acid as well as the ionic strength were varied.
Extraction by Aliquat-336 ® under the same conditions wasdone for reasons of comparison.

The structures of the extractants developed for this purpose areshown in Scheme 1. In addition to those, di- and tri-substituted
2PyOC4 were investigated in terms of work. The abbreviation*Corresponding author. E-mail: rludwig@mail.chemie.fu-berlin.de.
FAX: +49-30-838-52424.

Calix[4]arene

4

O

O

N

4

O

8

O

2-PicolylDerivatives

N

2PyOC86

4- and 3-PicolylDerivatives

N

N

3PyOC6

OH

2

O

N

2 OH

N

4PyOC4Di

partially substituted
2-Picolyl-Derivatives
2

2PyOC6Tetra

O

2 OH
N

O

OH

3

O

5 OH
N

2

3PyOC6Tetra-

O

6

N

2PyBuC6O

4PyOC4O

6N

2PyOC6

2PyOC4O

Calix[8]arene

Calix[6]arene

Tetra- and Pentasubstituted
3-Picolyl-Derivatives oftOctylcalix[6]arene

3PyOC6Penta-

2 OH

2

O

5 OH
N

N

N

3PyBuC6Tri

3PyBuC6Tetra

3PyBuC6Penta

2

4

O

OH

O

ClN
CH3

N CH
3
Cl-

4PyOC4Di-Q

4PyOC4-Q

Scheme 1. Extractants developed for this work.

© 2005 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/28/2005

Tri-, Tetraand Pentasubstituted
3-PicolylDerivativesoftButylcalix[6]arene

Calix[4]arenes

withquaternizedPicolyl-Groups

228 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005

Ludwigt

Tetra-

substituted

Tetra-

2PyOC41

1

1asubstiHexa- tuted
2PyOC61b0.5logD Tc(VII)logD Tc(VII)

0.5
0
-0.5

0
-0.5

-1

0.1
C

-1

0.9

0.1
C

HNO3 / M

0.9

HNO3 / M

TetraTri-1

substiTetra- tuted
3Py BuC6

1

1c

0.5

Penta- substituted
Hexa3PyOC6

1d

0.5

logD Tc(VII)

Effect of substituent number. Figure 1a shows the distribution coefficients (D = corg /caq) for Tc(VII) in the extraction
by calix[4]arenes bearing 2, 3, or 4 pyridino (py) substituentsat the lower rim. In this series, the nitrogen atom is locatedortho to the anchor group (2PyOC4), thus being located ‘inside’
the cavity. The extractability decreases when the nitric acidconcentration increases, an effect which is discussed later.
There is a significant increase in logD when the number ofpy-groups changes from two to three. Introducing a fourth pygroup contributes little to the extractability. It is concluded thatthree nitrogen atoms are mainly involved in binding to TcO4 −by means of ion-ion, cation-dipole and hydrogen bond interactions. The calix[4]arene backbone may be too rigid to allow
the fourth nitrogen atom to rearrange into a position suitablefor interaction.
Calix[6]arenes are characterized by a higher molecular flexibility and the possibility to introduce up to 6 functional groups.
In Figure 1b, data for the 2PyOC6 are shown. In both Figures
1b and 1a, the nitrogen is located at ortho position. Results fornitrogen in 3-position (3PyBuC6 and 3PyOC6) are given in
Figures 1c and 1d, respectively. However, the anchoring of 3, 4,
or 6 py-groups does not significantly improve the extraction ofTc(VII) as compared with calix[4]arenes as seen in Figures 1b
to 1d. The extraction improves only slightly when more pygroups are contained within the molecule. The reasons may be(i) that the short methoxy spacers do not allow re-orientation of
the py-groups according to an ‘induced fit’ mechanism, and (ii)entropy loss of the ligand during complexation. Data for thehexa-substituted tbutylcalix[6]arene are not included, because
the ligand precipitates upon complete protonation. t t Octylgroups render the ligands sufficiently hydrophobic even in the
protonated state and this was the reason why for this study the
majority are toctylcalixarenes.
Position of the nitrogen atom in the substituents. Figure
2 shows results for calix[4]arenes where the nitrogen atom inthe py-groups is located at the 4-position (4PyOC6), ‘outside’
the cavity. As in Figure 1a, the values for logD are higher forthe tetra-substituted ligand. Changing the acid concentrationin the aqueous phase over a wider range shows two regions forthe distribution coefficient: an increase at the weakly acidic

Tri-logD Tc(VII)

3. Results and Discussion

Di-

0
-0.5

0
-0.5

-1

0.1
C

-1

0.9

0.1
C

HNO3 / M

0.9

HNO3 / M

Figures 1. Effect of substituent number per calixarene on the distribution coefficient of Tc(VII): 2-Py groups on calix[n]arenes (1a: n =4, 1b: n = 6); 3-Py groups on t butyl- (1c) and t octylcalix[6]arene (1d).
Aq. Phase: 99Tc(TcO4 −) 0.05 mM (3 kBq/mL) in HNO3 (0.1 or 0.9 M);
Org. Phase: 2 mM extractant in CHCl3. Tetra- (1a) and Hexa-substituted (1b, 1d) correspond to 2PyOC4, 2PyOC6 and 3PyOC6 respectively in Scheme 1.

Di-

substituted
4PyOC4

Tetra0.8
0.4logD Tc(VII)

2PyOC4 for example means octyl calix[4]arene functionalizedwith four 2-pyridyl methoxy groups. The compounds 2PyOC4,
4PyOC411 and 2PyBuC612 was reported previously. The synthesis of the other ligands involves treatment of the corresponding
parent calix[n]arene (n = 4, 6, 8) with a base, followed by reaction
with picolylchlorides and work-up by crystallization or column
chromatography, similar to References 13 and 14. Quaternization
was done by reaction with methyliodide. The compounds wereidentified by 1H/13C-NMR, elemental analysis, MS and thin-layer
chromatography. Their analytical data and the synthesis details
will be reported elsewhere.
Solvent extraction was carried out batchwise in sealed PEvials at 295 ± 2 K and equal phase volumes of 3 mL. Magnetic
stirring was carried out overnight. It has not been tested whichtime period is sufficient to reach equilibrium. In previous
anion extraction15 it was two minutes, while in Ag(I) extractiona slow kinetics was observed.11 The organic phase contained
0.002 M calixarene or 0.008 M Aliquat-336 in chloroform.
The aqueous phase contained 0.00005 M 99Tc(TcO4 −) (3 kBq)
and either nitrate or chloride: HNO3 (0.003 to 4 M) or a mixture of HNO3 and N(Me) 4NO3 (3 M NO3−); or HCl (0 to 5 M).
For weakly acidic solutions, the pH at equilibrium was measured
with a glass electrode. One mL of the aqueous phase was subjected to liquid scintillation β-counting. The activity in theorganic phase was calculated from the difference to the initial
one.

0
-0.4
-0.8
-1.2
0.01

0.1

1
C

5

HNO3 / M

Figure 2. Effect of nitric acid concentration on Tc(VII) extraction by
di- and tetra-substituted calix[4]arene. Aq. Phase: 99Tc(TcO4 −) 0.05
mM (3 kBq/mL) in HNO3 (varying concentration); Org. Phase: 2 mM
extractant in CHCl3.

J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005 229

Solvent Extraction of Tc(VII)region results from the protonation of the nitrogen atoms.
Although the protonation constants were not yet measured, we
assume that protonation occurs stepwise due to cooperativity,
as does deprotonation in carboxylic acid derivatives.16, 17 The
distribution coefficient decreases at higher acidity nearly linearly on a half-logarithmic scale.
In Figures 3a and 3b the position of the nitrogen atom, either
ortho or para with respect to the anchor group (2PyOC4 and
4PyOC4, respectively) is compared for partially (3a) and fully
substituted (3b) calix[4]arenes. We observe significant differences: In the di-substituted ligands, the protonation of thenitrogen dominates up to 0.3 M HNO3 and causes an increasein logD. In tetra-substituted calixarene, the 2PyOC4 starts
with high logD-values, while 4PyOC4 first passes the protonation stage before it can extract Tc(VII). We interpret this difference in terms of a cooperative effect in 2PyOC4, which
promotes protonation even at low acidity. On the other hand,
4PyOC4 offers the advantageous possibility of Tc-backextraction with diluted acid.
Figure 4 compares ortho- and meta-position of the nitrogen(2PyOC6 and 3PyOC6, respectively). As for the calix[4]areneson the previous picture, in these calix[6]arenes the nitrogen in
2-position (ortho) appears to be protonated all over the investigated range. The 3Py-derivative (meta position) lies in-between
the ortho- and para ones, showing a region where it is not yet
fully protonated (< 0.1 M HNO3). We therefore conclude that
2PyOC4

2PyOC4

4PyOC4

-0.2

1

-0.4

0.5

-0.6
-0.8
-1

3b

0
-0.5
-1

-1.2
0.01

0.1
C

1

-1.5
0.01

5

0.1

1

2PyOC60.1

1

5HNO3 / M

Figure 5. Comparison of calixarenes with different size. Aq. andorg. phase as in Figure 2.

1.5

2PyOC6

1

3PyOC6logD Tc(VII)

0.5
0
-0.5
-1
0.01

2PyOC4C

2PyOC6

1

-0.5

-1.5
0.01

HNO3 / M

1.5

0

2PyOC85CHNO3 / M

0.5-1

Figures 3. Influence of the position of nitrogen in the pyridino substituent on the extraction of Tc(VII). Left (3a): Disubstituted ligands;
right (3b): fully substituted ligands. Aq. and org. phase composition
as in Figure 2.

logD Tc(VII)1logD Tc(VII)

3a

1.5

4PyOC41.5

logD Tc(VII)logD Tc(VII)

Disubstituted
0in these fully-substituted macrocycles, the protonation constants follow the order 2Py > 3Py > 4Py, with possible stabilization due to intramolecular hydrogen bonding.
Calixarene size. Next, we compared the fully-substituted
calix[n]arenes with n = 4, 6, or 8 in order to find an optimum
extraction performance. In this series, 4, 6, or 8 pyridinogroups respectively are attached to one molecular backbone
with their nitrogen atoms in ortho position. The results areshown in Figure 5. Surprisingly, the ligands show a very similar behaviour, with the exception of the calix[8]arene at lower
acidity. The latter is ascribed to the high molecular flexibilityof this large macrocycle, which may hinder the intramolecularstabilization of the protonated form. However, above 0.1 M
HNO3 the differences between the three ligands level off andthe values of logD decrease with the acid concentration linearly
on a half-logarithmic scale. The question arises why does acompound with 4 ligating groups performs similar to its homologues containing 6 or 8 of these groups? We conclude that not
all of the available nitrogen atoms in the larger calixarenes
contribute to the anion binding. In other words, these ligand
appear to represent not yet the optimum molecular structure tomatch the anion coordination sphere.
Acid competition. In the previous Figures the distribution
coefficient decreased at higher acidity. In order to answer thequestion if this caused by the competitive extraction of nitrate orby the protonation of pertechnetate, we varied the compositionof the aqueous phase. Figure 6 depicts the results. At constant

0.5
0
-0.5
-1

0.1
C

1

5

HNO3 / M

Figure 4. Effect of nitric acid concentration on the extraction ofTc(VII) at different positions of nitrogen in the substituent. Aq. andorg. phase composition as in Figure 2.

-1.5
0.01

0.1

H+ / M

1

5

C

Figure 6. Effect of nitrate concentration on the extraction of Tc(VII)by 2PyOC6. Aq. Phase: 99Tc(TcO4 −) 0.05 mM (3 kBq/mL) in HNO3
(0.03 to 4 M, ●), or in [N(Me) 4NO3 + HNO3] (3 M nitrate, ∇); Org.
Phase: 2 mM extractant in CHCl3.

230 J. Nucl. Radiochem. Sci., Vol. 6, No. 3, 2005
2

Ludwig2.5

Aliquat 336(0.008 M)

1.5

HNO 3
2PyOC61.5

logD Tc(VII)1logD Tc(VII)HNO 3
2PyOC4

0.5
0HCl
2PyOC4

0.5

HCl
2PyOC6

-0.5

-0.5
-1

-1.5
0.01

0.1

1

5

H+ / M

C

Figure 7. Comparison of the extraction by Aliquat-336 at varying
and constant nitrate concentration. Aq. Phase as in Figure 6, Org.
Phase: 8 mM extractant in CHCl3.

nitrate concentration (c = 3 M), logD of Tc(VII) remains constant within the error limits over a wide range of acid concentration. It shows that nitrate is extracted as competing anion. At
the nitrate/pertechnetate concentration ratio of 60.000 in thisseries, 16% Tc are extracted. At a concentration ratio of 500,
90% Tc is extracted. The selectivity is reasonable, but should befurther improved in subsequent research works. The results also
show that the protonation of TcO4 − is of minor importance foramine-type extractants, in difference to solvating extactants
such as TBP.
Similar experimental conditions were applied using Aliquat
as extractant, because literature data for this extractant werereported only for varying ionic strength. As seen in Figure 7,
the behaviour is very similar to that of calixarenes and showsthat nitrate ions compete with pertechnetate ions. As the concentration of extractant in these series was four times higher
than for calixarenes, the values of logD cannot be compareddirectly. However, by extrapolating literature data 2 one canestimate that D is about 3 times higher for fully-substituted
calixarenes at given acid and ligand concentration. We therefore conclude macrocyclic effect in the extraction of pertechnetate by pyridino-substituted calixarenes. In comparison, thetertiary amine tri-n-dodecylamine (0.008 M) in its protonatedstate extracts only 10% Tc(VII) from 1 M HNO3, significantly
less than pyridino- and quaternary amino-type ligands.
Does the extraction of TcO 4 − improve when smaller, lesspolarizable anions are present instead of nitrate? In order toanswer this question, we subjected Tc(VII) to extraction fromhydrochloric media by ortho-substituted calixarenes 2PyOC4
and 2PyOC6. Figure 8 shows the results. Indeed, logD
increases by one order of magnitude when the aqueous phase
contains HCl instead of HNO3. The order of anion interferenceagrees with the Hofmeister series. Another difference to thenitrate-containing system represents the protonation of
2PyOC4 which occurs below 0.1 M HCl.
Quaternized nitrogen atoms. Does the extraction improves
when the pyridino groups are quaternized? Figure 9 clearly
demonstrates for nitrogen atoms in para position an efficient
extraction from weakly acidic media by quaternized 4PyOC4Q. The extractants are in a cationic form already in this pHrange and therefore more effective than the protonizable
ligands. At higher acidity, there are negligible differences inlogD between protonated and quaternized calixarene.
However, we noticed from a loss during synthesis work-up thatthe quaternized ligand 4PyOC4-Q is slightly soluble in water.
This is attributed to the presence of four highly polar groups.
In the di-substituted compound 4PyOC4Di-Q, the hydrophobic

0.1
C

5

1
+

H /M

Figure 8. Anion competition in the extraction of Tc(VII). Aq.
Phase: 99Tc(TcO4 −) 0.05 mM (3 kBq/mL) in HNO3 (0.03 to 4 M); or
HCl (0.03 to 5 M); Org. Phase: 2 mM extractant in CHCl3.

1.5

4PyOC4Di
4PyOC4Di-Q

1logD Tc(VII)

0

0.02

-1.5

4PyOC4

0.5

4PyOC4-Q

0
-0.5
-1
-1.5
0.01

0.1
C

1

5

HNO3 / M

Figure 9. Comparison of protonated and quaternized nitrogen atoms
and of the number of substituents per calix[4]arene. Aq. and org.
phase composition as in Figure 2.

character dominates and it is not soluble in water.
4. Conclusions
It turned out that (i) the extraction improves with more pyridino groups per calixarene molecule; (ii) the number of substituents and the position of the nitrogen atom in the pyridine
ring influence the ligand protonation and thus the extraction
behaviour towards Tc(VII) at lower acid concentration which
follows the order 2Py > 3Py > 4Py; and (iii) molecular flexibility, cavity size and the number of pyridino groups in fully substituted calix[n]arenes (n = 4, 6, 8) compensate each otherwhich causes a similar extractability towards Tc(VII) amongthem.
By varying the composition of the aqueous phase, it was
shown that nitrate competes with pertechnetate. The competition by chloride is weaker. Protonation of TcO 4 − contributed
negligibly to the decrease of extraction up to 4 M acid in the
aqueous phase. Quaternized nitrogen is a better ligand thanprotonated one in weakly acidic media. A macrocyclic effectwas observed. The binding is explained by electrostatic andhydrogen bonding interactions.
Acknowledgement. The authors thank Cognis, Mining- &Ion-Transfer Chemicals Technology for a free sample of Aliquat336.
Financial support by the National Science Council/Fundamental Research Projects of Vietnam is appreciated.

Solvent Extraction of Tc(VII)

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