Application of nano-baskets for extraction of lanthanides

Article abstract of DOI:10.3184/174751912X13527973569178

Eight proton di-ionisable diacid conformers of nano-baskets including 25,26-di(carboxymethoxy)calix[4]arene-crown- 3, -crown-4, -crown-5 and -crown-6 in the cone conformation have been synthesised and shown to extract lanthanide cations effectively. Their selectivities were greatly influenced by the acidity of the solution and the conformations of the calix-crown. The extraction loading was improved by having the p-tert-butyl-moiety in the upper rim. The scaffold bearing the crown-4 ether moiety showed the least loading, while scaffolds with crown-3- and crown-5-ether moieties had the best selectivities.

Full text of DOI:10.3184/174751912X13527973569178

740 RESEARCH PAPER
DECEMBER, 740–743
JOURNAL OF CHEMICAL RESEARCH 2012
Application of nano-baskets for extraction of lanthanides
Bahram Mokhtari and Kobra Pourabdollah*
Razi Chemistry Research Center, Shahreza Branch, Islamic Azad University, Shahreza, Iran
Eight proton di-ionisable diacid conformers of nano-baskets including 25,26-di(carboxymethoxy)calix[4]arene-crown-
3, -crown-4, -crown-5 and -crown-6 in the cone conformation have been synthesised and shown to extract lanthanide
cations effectively. Their selectivities were greatly influenced by the acidity of the solution and the conformations of
the calix-crown. The extraction loading was improved by having the p-tert-butyl-moiety in the upper rim. The scaf-
fold bearing the crown-4 ether moiety showed the least loading, while scaffolds with crown-3- and crown-5-ether
moieties had the best selectivities.
Keywords: calixcrown, calixarenes, lanthanides, extraction, nano-baskets
Nano-baskets of calixarenes and calixcrowns are a versatile
class of macrocycles, which have been subject to extensive
research in the development of many extractants, transporters
and stationary phases.1–8 Functionalisation of calix[4]arenes at
both the upper and lower rims has been extensively studied.
Attaching donor atoms to the lower rim of a calix[4]arene can
improve the binding strength of the parent calixarene dramati-
cally. The two main groups of lower-rim functionalised
calix[4]arenes are calix[4]arene podands and calixarene-crown
ethers.9,10 Distal hydroxyl groups can be connected to give 1,3-
bridged calix[4]crowns, while connection between proximal
hydroxyl groups leads to 1,2-bridged calix[4]crowns.
It was observed that calixarenes with phosphorus-contain-
ing pendant arms are the best extractants for lanthanides
and actinides. Moreover, it was found that the 1,3-bridged
calix[4]crowns exhibit high binding affinity and selectivity
toward alkali and alkaline earth metal cations.11 However,
researches on 1,2-bridged calix[4]crowns lag far behind.
Attachment of proton-ionisable groups to calixcrowns can
further improve their extraction properties because the ionised
group not only participates in metal ion coordination, but also
eliminates the need to transfer aqueous phase anions into the
organic phase.12 Combining crown ethers with calix[4]arenes
increases the cation binding ability of the parent calixarenes.3,14
The selectivity can be affected by the crown ether size, the
identity of donor atoms on the crown ether moiety and the
conformation of the calixarene platform.15–21 To further explore
the influence of these factors on the extraction characteristics
of p-tert-butylcalix[4]-1,2-crown ethers toward metal ions, a
series of di-ionisable p-tert-butylcalix[4]-1,2-crown-3 com-
pounds in the cone conformation and the 1,2-alternate confor-
mation, as well as p-tert-butylcalix[4]arene-1,2-thiacrown-3 in
the cone conformation have now been synthesised.
Two kinds of side chain in the calixcrown skeleton have
been studied, including two ionisable carboxylic acid moieties
and the crown-ether moieties. The ionisable moieties not only
participate in cooperative metal ion coordination, but also
eliminate the need to transfer the anions from the aqueous
phase into the organic phase by operating in a cation-exchange
mode with the metal cation. In this work, two proton-ionisable
function groups were incorporated into a calix[4]arene scaf-
fold. A special feature of such modification is that the acidity
of the ionisable moiety can be tuned by changing the func-
tional group from hydroxyl to other groups having different
electron-withdrawing abilities of the functional group. A wide
range of pH environments can be examined when these ioni-
sable groups are incorporated into the calixcrown skeleton.
These extractants exhibit excellent extraction selectivity for
alkali and alkaline earth metals.
(10), cone p-tert-butyl-25,26-di(carboxymethoxy)calix[4]arene
1,2-crown-4 (11), cone p-tert-butyl-25,26-di(carboxymethoxy)
calix[4]arene1,2-crown-5 (12), cone p-tert-butyl-25,26-di
(carboxymethoxy)calix[4]arene1,2-crown-6 (13), cone 25,26-di
(carboxymethoxy) calix[4]arene1,2-crown-3 (23), cone 25,26-
di(carboxymethoxy)calix[4]arene1,2-crown-4 (24), cone 25,26-
di(carboxymethoxy)calix[4]arene1,2-crown-5 (25), and cone
25,26-di(carboxymethoxy)calix[4]arene1,2-crown-6(26). Some
of the synthesised scaffolds have also been reported as inter-
mediates for preparation of N-pheny-sulfonyl oxyacetamide
derivatives, and their extraction of alkaline earth metals
described.22–26
In the following, the synthesis and the extraction procedures
of eight conformers are presented, respectively. Figure 1
depicts the chemical structure of eight calixcrown scaffolds
studied as extracting agents.
Experimental
The synthesis scheme for the preparation of cone p-tert-butyl-25,26-
di(carboxymethoxy)calix[4]arene-1,2-crown-3,4,5,6 (10–13) and cone
25,26-di(carboxymethoxy)calix[4]arene-1,2-crown-3,4,5,6 (23–26) is
presented in Fig. 2. The synthetic and characterisation details of all
compounds in Figs 1 and 2 have been deposited in the Electronic
Supplementary Information.
Reagents were obtained from commercial suppliers and used
directly, unless otherwise noted. Acetonitrile (MeCN) was dried over
CaH2 and distilled immediately before use. Tetrahydrofuran (THF)
was dried over sodium with benzophenone as an indicator and
distilled just before use. Cs2CO3 was activated by heating at 150 °C
overnight under reduced pressure and stored in a desiccator. Melting
points were determined with a Mel-Temp melting point apparatus. IR
spectra were recorded with a Perkin-Elmer Model 1600 FT-IR spec-
1
trometer as deposits from CH2Cl2 solution on NaCl plates. The H
and 13C NMR spectra were recorded with a Varian Unity INOVA
500 MHz FT-NMR (1H 500 MHz and 13C 126 MHz) spectrometer in
CDCl3 with Me4Si as internal standard unless mentioned otherwise.
Chemical shifts (δ) are given in ppm downfield from TMS and
coupling constants (J) values are in Hz.
Lanthanide perchlorates (99%), lanthanide hydroxide, perchloric
acid (1.0 N) and chloroform were obtained from Aldrich. Chloroform
was shaken with deionised water to remove the stabilising ethanol and
stored in the dark.
Lanthanide cations were loaded into the aqueous solutions by
adding stock solutions containing six lanthanide cations. The solu-
tions of six cations were made up as Nd3+, Eu3+, Tb3+, Dy3+, Er3+ and
Yb3+ perchlorate solutions (0.1 mM in each). 0.1 mM lanthanide
hydroxide and 0.01–1.0 M perchloric acid solutions were used to
adjust the pH values of the aqueous phases. The extraction ability
of calixcrown scaffolds was determined in eight solutions with pH
range of 1.0–10.0, in 15 mL conical polypropylene centrifuge tubes.
The samples contained 2.0 mL of the aqueous phase of 0.1 mM lan-
thanide cation solution and 2.0 mL of 5.0 mM calixarene solution in
chloroform.
A preconcentration method was used in the solvent extraction pro-
cedure. The combined aqueous and organic phases were shaken for 5
minutes and were centrifuged for 5 minutes. The pH of the aqueous
phase was measured using a pH-meter with a Corning 476157 combi-
nation pH electrode. In the stripping step, 1.5 mL of the organic phase
In the work reported here, eight diacid proton-ionisable
calixcrowns have been synthesised including cone p-tert-
butyl-25,26-di(carboxymethoxy)calix[4]arene1,2-crown-3
* Correspondent. E-mail: pourabdollah@iaush.ac.ir

JOURNAL OF CHEMICAL RESEARCH 2012 741
Fig. 1 The chemical structure of eight calixcrown scaffolds.
Fig. 2 Synthesis of cone p-tert-butyl-25,26-di(carboxymethoxy)calix[4]arene-1,2-crown-3,4,5,6 (10–13) and cone 25,26-di(carboxy
methoxy)calix[4]arene-1,2-crown-3,4,5,6 (23–26).
was transferred to a capped conical centrifuge tube containing 3.0 mL
of 0.10 M HClO4. The stripping process involved 5 minutes of mixing,
then 5 minutes of centrifuging. After that, 1.0 mL of the aqueous
phase was diluted to 10.0 mL for analysis by ICP-AES.
The average coefficients of variation for lanthanide cations were
calculated from the replicate analyses of the six samples and are listed
in Table 2. The maximum and minimum CV values were determined
as 5.5% for Tb3+ and 1.0% for Yb3+, respectively. The average CV
values of all lanthanide cations were calculated to be 3 1%.
The percent of cation extraction (E%) was defined as:
[M3+
]org ×Vorg
[M3+
]aq ×Vaq
E% =
×100
(1)
Results and discussion
Upon ionisation, the di-ionisable cone calix[4]crowns form
metal ion complexes with two anionic centres on the same
side of the crown unit. The perchlorate anions in the solution
neutralise the last positive charge in the complex and enhance
the extraction of uncharged complexes to the organic media.
Therefore, 1:1:1 ligand:metal:perchlorate complexes are
formed and transferred.27
where [M3+]org and [M3+]aq represent the concentration of cations in the
organic phase and the aqua solution (before extraction), and V and
org
V depict the corresponding volume of phases respectively.
aq
An ICP-AES spectrometer (model Seiko SPS-1500R) was used in
the experiments. The spectrometer was equipped with a computer-
controlled sequential Czerny-Turner scanning monochromator with
the focal length of holographic grating 3600 grooves per mm. From
the emission intensity data for the sample and standard solutions, the
concentrations of solutes were calculated and corrected for spectral
interferences among solutes. Table 1 presents the wavelengths of the
emission lines used in the ICP-AES measurements. It was found that
blank corrections were unnecessary. The recovery tests showed that
the cations were recovered by 96 3%.
Competitive solvent extractions of lanthanide cations (Nd3+,
Eu3+, Tb3+, Dy3+, Er3+ and Yb3+) were performed for 5.0 mM
solutions of di-ionisable calix[4]crown ligands 10–13 and 23–
26 in chloroform, and plots of metal ion loading of the organic
phase vs. the equilibrium pH of the aqueous phase were
obtained, as depicted in Figs 3 and 4, respectively. All of the
Table 1 The wavelengths of emission lines used in the ICP-AES measurements.
Cation
Nd3+
Eu3+
Tb3+
Dy3+
Er3+
Yb3+
Wavelength/nm
430.358
381.967
350.917
353.170
369.265
328.937
Table 2 The average deviations of ICP-AES measurements of lanthanide cations.
Cation
Nd3+
Eu3+
Tb3+
Dy3+
Er3+
2.0
Yb3+
1.0
Deviation/%
3.0
4.1
5.5
3.5

742 JOURNAL OF CHEMICAL RESEARCH 2012
extraction experiments were repeated three times and the mean
value of the extraction loadings were determined and were
presented in the plots.
Competitive solvent extractions by ligand 23: All cations
were detectably extracted into the chloroform phase and the
selectivity order was Nd3+ > Eu3+ >> Tb3+, Dy3+ > Yb3+ > Er3+.
The pH1/2 values were found to be in the range 5.0–8.0.
Figure 4 depicts the plots of cation loading into the organic
phase versus the equilibrium pH of the aqueous phase using
ligand 23.
Competitive solvent extractions by ligand 24: For competi-
tive extraction of those cations, ligand 24 exhibited no
selectivities and the pH1/2 values were obtained to be around
5.0.
Competitive solvent extractions by ligand 10: For competi-
tive extraction of such metal cations, di-ionisable 5,l1,17,23-
tetrakis(1,l-dimethylethyl)-25,26-bis(carboxymethoxy)-27,28-
crown-3-calix[4]arene in the cone conformation (ligand 10)
showed very different selectivity from its analogues (11–13).
All lanthanide cations were detectably extracted into the chlo-
roform phase and the selectivity order was Nd3+ > Eu3+ >>
Tb3+, Dy3+ >Yb3+ > Er3+ for ligand 10. The pH for half loading,
pH1/2, is a measure of the ligand acidity. For compound 10,
the pH1/2 value was obtained to be in the range of 5.5–6.0 for
binding to the lanthanides.
Competitive solvent extractions by ligand 11: As shown in
Fig. 3, ligand 11 exhibited little selectivity for the ion species.
All six lanthanide cations were extracted into the chloroform
phase without any selectivity. The pH1/2 values were obtained
to be around 5.0 and 6.0 for binding to Nd3+ and Eu3+ as well as
Tb3+, Dy3+, Er3+ and Yb3+, respectively.
Competitive solvent extractions by ligand 25: The pH1/2
values were obtained to be in the range of 6.0–7.0 and all cat-
ions were highly extracted into the chloroform phase without
any selectivity.
Competitive solvent extractions by ligand 26: All lanthanide
cations were detectably extracted into the chloroform phase.
The selectivity order was Nd3+ > Eu3+ >> Tb3+, Dy3+ > Yb3+
>
Er3+ for ligand 24. Moreover, the pH1/2 values were found to be
in the range 6.0–7.0.
Competitive solvent extraction by ligand 12: 5,l1,17,23-
Tetrakis(1,l-dimethylethyl)-25,26-bis(carboxymethoxy)-
27,28-crown-5-calix[4]arene in the cone conformation (ligand
12) also exhibited very different selectivity from its analogues
conformers (10,11,13). All cations were extracted into the
chloroform phase without selectivity. The pH1/2 values were
obtained to be in the range of 6.0–7.0.
Conclusions
Proton di-ionisable diacid conformers of cone 25,26-di(carbox
ymethoxy)calix[4]arene-27,28-crown-3,4,5,6 (23–26), and
their analogues including p-tert-butyl moieties in the upper
rim (10–13), were synthesised as potential extractants for
lanthanide cations. Introducing the upper rim moieties (p-tert-
butyl-) into the scaffolds showed an enhancement in the bind-
ing tendency and extraction ability towards lanthanide cations.
This was attributed to the inductive charges from aromatic
rings to the donor atoms of oxygen, which are in the crown
Competitive solvent extractions by ligand 13: Ligand 13
exhibited good selectivities. The order of loadings was Nd3+
>
Eu3+ >> Tb3+, Dy3+ > Yb3+ > Er3+. Moreover, the pH1/2 values
were around 6.0.
Fig. 3 Percent of metals loading versus equilibrium pH of the aqueous phase for competitive solvent extraction of lanthanides into
chloroform by conformers 10–13.

JOURNAL OF CHEMICAL RESEARCH 2012 743
Fig. 4 Percentage of metals loading versus equilibrium pH of the aqueous phase for competitive solvent extraction of lanthanides
into chloroform by conformers 23–26.
7
8
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K. Salorinne and M. Nissinen, J. Incl. Phenom. Macrocycl. Chem., 2008,
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rings, crown-4 showed the least loading percents. The scaf-
folds with crown-3 and crown-5 moieties displayed the best
selectivities.
9
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11 V. Lamare, J.F. Dozol, F. Ugozzoli, A. Casnati and R. Ungaro, Eur. J. Org.
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This work was supported by the Islamic Azad University
(Shahreza branch) and the Iran Nanotechnology Initiative
Council.
12 R. Ungaro, A. Pochini and G.D. Andreetti, J. Incl. Phenom. Molecul.
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Electronic Supplementary Information
The synthetic and characterisation details of all compounds in
Figs 1 and 2 have been deposited in the ESI which is available
through stl.publisher.ingentaconnect.com/content/stl/jcr/supp-
data.
15 B. Mokhtari and K. Pourabdollah, J. Therm. Anal. Calorim., 2012. doi:
10.1007/s10973-011-2014-7.
16 B. Mokhtari and K. Pourabdollah, J. Incl. Phenom. Macrocycl. Chem.,
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Received 9 October 2012; accepted 23 October 2012
Paper 1100867 doi: 10.3184/174751912X13527973569178
Published online: 5 December 2012
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