Mercury(II) and silver(I) receptors based on tetrathiacalix[4]arene hydrazones

Article abstract of DOI:10.1007/s10847-013-0307-0

The extraction ability of phenyl hydrazone derivatives of cone- and 1,3-alternate tetrathiacalix[4]arenes towards to some alkali, alkaline earth, transition and heavy metal ions has been investigated by picrate extraction method. The synthesized cone-tetrathiacalix[4]arene hydrazones show a high efficiency coupled with an excellent selectivity towards for Ag⁺ and Hg²⁺ ions. The stoichiometry of complexes and the extraction constants have been determined. The influence of calix[4]arene platform's structure and the electron donor substituents in phenyl fragments on the extraction efficiency has been discussed.

Full text of DOI:10.1007/s10847-013-0307-0

J Incl Phenom Macrocycl Chem
DOI 10.1007/s10847-013-0307-0
ORIGINAL ARTICLE
Mercury(II) and silver(I) receptors based
on tetrathiacalix[4]arene hydrazones
•
Sergey N. Podyachev Nadezda E. Kashapova Victor V. Syakaev
•
•
•
Svetlana N. Sudakova Railya R. Zainullina Margit Gruner
•
•
•
Wolf D. Habicher Tatiana A. Barsukova Fafu Yang Alexander I. Konovalov
•
•
Received: 26 November 2012 / Accepted: 12 March 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract The extraction ability of phenyl hydrazone
derivatives of cone- and 1,3-alternate tetrathiacalix[4]are-
nes towards to some alkali, alkaline earth, transition and
heavy metal ions has been investigated by picrate extrac-
tion method. The synthesized cone-tetrathiacalix[4]arene
hydrazones show a high efficiency coupled with an
excellent selectivity towards for Ag? and Hg2? ions. The
stoichiometry of complexes and the extraction constants
have been determined. The influence of calix[4]arene
platform’s structure and the electron donor substituents in
phenyl fragments on the extraction efficiency has been
discussed.
Introduction
Some toxic heavy metals are widely used in various fields of
science and technology. Among them the silver and mercury
should be mentioned particularly. Thus, high-tech electron-
ics and microwave technology successfully apply silver for
the covering of contacts and conductors in high frequency
circuits as well as for coating the inner surfaces of wave-
guides. This is inevitably resulted in an increase of metal
content in the environment. Though the silver does not have a
clinically apparent effect on a human, but it is known, that an
excess amount of this metal in organism can lead to some
diseases, such as coma, pleural edema and hemolysis [1].
Moreover, according to new data, silver can interact with
essential nutrients, vitamins E and B12 that can also testify to
the potential health hazard of silver [2–5].
Keywords Receptor ꢀ Calix[4]arene ꢀ Hydrazone ꢀ
Hg2? ꢀ Ag?
Among toxic metals, the mercury and its compounds are
considered as the most harmful [4, 6, 7]. Soluble mercury
salts which together with food and potable water can
penetrate into the human body are especially dangerous
due to their ability to accumulate in the organism. The
application of the mercury compounds scales step by step
down. However, up to date they play still an important role
in modern industry and their harmful influence continues to
be damaging for people. Therefore, the development of
effective methods for the detection, analysis and detoxifi-
cation of mercury and silver compounds is as before an
actual problem of environmental safety.
S. N. Podyachev (&) ꢀ V. V. Syakaev ꢀ
S. N. Sudakova ꢀ T. A. Barsukova ꢀ A. I. Konovalov
A. E. Arbuzov Institute of Organic and Physical Chemistry
of Kazan Scientific Center of Russian Academy of Sciences,
Arbuzov Str., 8, 420088 Kazan, Russia
e-mail: spodyachev@iopc.ru
N. E. Kashapova ꢀ R. R. Zainullina
Kazan National Research State Technological University,
K. Marks Str., 68, 420015 Kazan, Russia
To detect traces of metal ions, a high selective precon-
centration procedure is usually prerequisite. A high content
of other metals, such as alkali and alkaline earth ions as
well as the comparable amounts of other impurity ions can
reduce the accuracy of evaluation of the toxic metals’
content in test samples. The extraction in a liquid–liquid
system appears to be an advantageous method in
M. Gruner ꢀ W. D. Habicher
Institute of Organic Chemistry, Dresden University of
Technology, Mommsenstrasse 13, 01062 Dresden, Germany
F. Yang
College of Chemistry and Materials, Fujian Normal University,
Fuzhou 350007, China
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J Incl Phenom Macrocycl Chem
comparison with another applied for the preconcentration
of toxic metals, due to its efficiency, selectivity and eco-
nomic benefit. The extraction properties of these systems
are mainly related to the choice of extractant.
Moreover, Kumar et al. [27] have recently reported that the
cone isomer of the calix[4]arene, functionalized 2-[4-(N,N-
dimethylaminophenyl)iminoethoxy] moieties has exhibited
a selective fluorescence response towards Hg2?. In whole,
the obtained results highlighted the availability of these
calix[4]arene derivatives and indicated the further direction
for searching more efficient receptors of toxic ions.
Calix[n]arenes are commonly utilized as a convenient
platform for the creation of a great variety of receptors due to
their 3D structure, and as a rule, they possess more effective
binding affinity towards cations, anions and neutral mole-
cules than their acyclic analogues [8–10]. The thermody-
namics of calixarene solutions and their metal-ion
complexes, involving the interaction with Ag? and Hg2?
were extensively investigated by Danil de Namor et al.
[11–16]. According to the hard-soft acid-base principle, the
functionalizing of calix[n]arene molecules by groups con-
taining soft donor nitrogen and, especially, sulphur atoms
would provide a high binding efficiency towards soft metal
ions. Indeed, the calix[4]arenes functionalized by thioethers
[17], thiocrown ethers [18, 19], thioamides [13, 20, 21]
thiophosphate [14, 22] and ethylthioethoxy [15, 16] groups
have revealed a high recovering ability towards Ag? and
Hg2?. However, only some of them have an acceptable
selectivity in a wide range of metal ions. In addition, from the
view of the environmentally sound approach the utilizing
compounds with N,O-donor groups seems to be more pref-
erable. It is also known that some of calix[n]arenes with
nitrogen and harder oxygen donor atoms can effectively bind
Ag? and Hg2? ions [23–26]. The screening of the selective
and harmless receptors for Ag? and Hg2? in a wide range of
metal ions continues to be an actual problem for up to date
scientific investigations [14, 27–29].
Consequently, following our intention to develop the
effective receptors of Ag? and Hg2?, we present herein the
synthesis of new phenyl hydrazone derivatives of tetrathi-
acalix[4]arene and perform a comparative investigation of
their binding properties with the ones obtained for some
calix[4]arene derivatives synthesized earlier. The tuning of
coordination behavior of the calix[4]arenes has been carried
out by varying the isomer forms of calix[4]arene platform
and the substituents in phenyl hydrazone fragments. To
evaluate the receptor properties of these calix[4]arenes in a
wide range of metal ions the solvent extraction method was
applied. The extraction efficiency and selectivity as well as
stoichiometry of the complexes and extraction constants for
Ag? and Hg2? ions have been established.
Experimental
Reagents
All chemicals were applied as commercially received
without further purification. CHCl3 was distilled over P2O5.
DMSO-d6 (99.5 % isotopic purity) for NMR spectroscopy
was used from Aldrich. The metal salts for extraction
experiments were the following chlorides and nitrates:
LiCl, NaCl, KCl, CsCl, CaCl2, CoCl2ꢀ6H2O, NiCl2, CuCl2,
ZnCl2, AgNO3, CdCl2ꢀ2.5H2O, Hg(NO3)2ꢀH2O, Pb(NO3)2,
LaCl3ꢀ7H2O, Gd(NO3)3ꢀ6H2O and LuCl3ꢀ6H2O.
In recent years, we have reported the synthesis of
hydrazide and hydrazone derivatives of calix[4]arenes and
their extraction performance against a wide row of metals
by liquid–liquid extraction. It was established that some of
investigated compounds may be successfully applied for
separation of certain transition and heavy metals [27–34].
The key role of preorganization of binding groups on
calix[4]arene platform in the complex formation process
was unambiguously demonstrated. The influence of the
isomeric form of calix[4]arene and the effect of tert-butyl
groups at the upper rim of the macrocycle on the extraction
properties were also shown.
Synthesis
The synthetic routes and the structural formulae of the
investigated compounds are shown in Fig. 1. The hydra-
zones 1 [31], 2a [35] 5a, 5b [36], 5e, 5f and 6c [37] and
tetrahydrazide calix[4]arene derivatives 3a [32], 3b, 4b
[33] and 4a [38] were obtained as described earlier.
These investigations allowed us to establish that pyrid-
inyl hydrazone derivatives of 1,3-alternate terathiaca-
lix[4]arene can separate selectively Cu2?, Ag? and Hg2?
among other metal ions [30]. On the other hand, the
research of binding properties of hydrazone derivatives
anchored on the calix[4]resorcine platform has shown that
the replacement of pyridinyl groups in hydrazone frag-
ments by the phenyl ones leads to excellent selectivity
towards Hg2?. Other metal ions including Cu2? were
practically not extracted from water solutions by phenyl
hydrazone derivatives of calix[4]resorcines [31].
4-tert-Butylphenoxyacetylhydrazone
of 4-methoxybenzaldehyde (2b)
To a boiling solution of 5 mmol (1.11 g) 4-tert-butylphe-
noxyacetylhydrazide [39] in 10 ml EtOH under stirring were
added 5 mmol (0.68 g) of 4-methoxybenzaldehyde. The
reaction mixture was refluxed for 2 h. The precipitate was
filtered off, washed several times with a mixture of water/
EtOH (1:1), and recrystallized from EtOH. Yield: 1.54 g
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J Incl Phenom Macrocycl Chem
Fig. 1 Synthetic routes and structural formulae of investigated compounds. Numbering system of atoms used in the experimental section
(91 %) as a white powder. mp 150–151 °C. 1H NMR
(600.13 MHz, CDCl3) d: 1.19, 1.20 (s, 9H, C(1)H), 3.75,
3.76 (s, 3H, (O–CH3)), 4.54, 5.01 (s, 2H, C(7)H); 6.83, 6.84
(br m, 2H, C(12)H), 6.81, 6.83 (br m, 2H, C(5)H); 7.21, 7.24
(br m, 2H, C(4)H); 7.51, 7.62 (br m, 2H, C(11)H); 7.73, 8.05
(br s, 1H, C(9)H); 10.2 (s, 1H, NH). 13C NMR (150.9 MHz,
CDCl3) d: 31.3 C(1); 34.1 C(2), 53.3 C(O–CH3); 65.6, 67.3
C(7); 114.3 C(12), 114.5 C(5); 126.2, 126.6 C(4); 128.8,
129.6 C(11), 144.3, C(3), 145.5, 149.9 C(9), 151.1, 155.0
reaction mixture was heated for 24 h at 80 °C for 5c, 5d
and at 40 °C for 6a, 6b and 6d. The solvent was removed
from the reaction mixture by distillation under vacuum.
Hexane was added to the residue and the mixture was
heated again. After cooling, the precipitate was filtered off,
washed several times with hexane and EtOH, recrystallized
from the mixture of EtOH and DMF.
5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetrakis[N2-(4-
methoxybenzylidene)hydrazinocarbonyl-methylenoxy]-
2,8,14,20-tetrathiacalix[4]arene (1,3-alternate-5c)
C(6); 161.5, 161.9 C(13); 165.3, 170.6 C(8). IR (v/cm-1
,
Nujol): 3438, 3190 (vNH), 1679 (vC=O). Elemental analysis
calc. for C20H24N2O3 (340.42): C, 70.56; H, 7.11; N, 8.23.
Found: C, 70.21; H, 7.30; N, 7.98.
Prepared similarly to the general procedure from 3a
(0.25 g) using 4-methoxybenzaldehyde (0.204 g). Yield:
0.33 g (88 %) as a white powder. Mp 231–235 °C. 1H
NMR (600.13 MHz, DMSO-d6, 30 °C) d: 0.89–1.27 (br m,
36H, C(1)H); 3.75–3.81 (br m, 12H, O–CH3); 4.40, 5.10
(br s, 8H, C(7)H); 6.92 (br m, 8H, C(12)H); 7.27 (br m, 8H,
C(4)H); 7.45, 7.66 (br m, 8H, C(11)H); 7.73, 8.60 (br s, 4H,
C(9)H); 10.50, 11.50 (br s, 4H, NH). 13C NMR
General procedure for the preparation of hydrazone
derivatives of calix[4]arene 5c, 5d, 6a, 6b and 6d
To a suspension of tetrahydrazide 3a, 3b, 4a or 4b
(0.25 mmol) in 6 ml EtOH and 6 ml DMF under stirring
1.5 mmol of the corresponding aldehyde was added. The
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J Incl Phenom Macrocycl Chem
25,26,27,28-Tetrakis[(N2-(4-
methoxybenzylidene)hydrazinocarbonylmethylenoxy]-
2,8,14,20-tetrathiacalix[4]arene (cone-6b)
(150.9 MHz, DMSO-d6, 30 °C) d: 30.6 C(1); 33.6, 34.0
C(2); 55.2 C(O–CH3); 69.6, 70.7 C(7); 113.9 C(12); 126.5
C(5); 128.5 C(11); 129.9 C(4); 142.8 C(3); 142.5, 148.5
C(9); 156.3 C(6); 160.5, 160.7 C(13); 163.7, 168.3 C(8). IR
(m/cm-1, Nujol): 3401, 3215 (mNH), 1694 (mC=O), 1538
(dNH). Elemental analysis Calc. for C80H88N8O12S4
(1481.86): C, 64.84; H, 5.99; N, 7.56; S, 8.66. Found: C,
64.57; H, 5.75; N, 7.18; S, 9.04. Mass spectrum (MALDI-
TOF): m/z: 1482 [M?H]?; 1503 [M?Na]?.
Prepared similar to the general procedure from 4b
(0.196 g) using 4-methoxybenzaldehyde (0.204 g) Yield:
0.28 g (89 %) as a white powder. Mp 174–176 °C. 1H
NMR (600.13 MHz, DMSO-d6, 30 °C) d: 3.76 (br s, 12H,
O–CH3); 5.05, 5.27 (br s, 8H, C(7)H); 6.60 (br m, 4H,
C(3)H); 6.90 (br m, 8H, C(12)H); 7.1 (br m, 8H, C(4)H);
7.50 (br m, 8H, C(11)H), 7.87, 8.45 (br s, 4H, C(9)H); 11.6
(br s, 4H, NH). 13C NMR (150.9 MHz, DMSO-d6, 30 °C)
d: 55.2 (O–CH3); 72.7 C(7); 114.0 (C(12)); 125.6 C(5);
126.2 C(3); 128.5 (C(11)); 130.0 (C(4)); 144.1, 148.5 C(9);
148.7 C(10); 154.3 (C(6)); 160.7 (C(13)); 163.8, 168.9
C(8). IR (m/cm-1, Nujol): 3464, 3214 (mNH), 1681 (mC=O),
1541 (dNH). Elemental analysis Calc. for C64H56N8O12S4
(1257.44): C, 61.13; H, 4.49; N, 8.91; S, 10.20. Found: C,
61.12; H, 4.12; N, 8.54; S, 10.42. Mass spectrum (MALDI-
TOF): m/z: 1257 [M?H]?, 1279 [M?Na]?, 1295
[M?K]?.
25,26,27,28-Tetrakis[N2-(4-
methoxybenzylidene)hydrazinocarbonylmethylenoxy]-
2,8,14,20-tetrathiacalix[4]arene (1,3-alternate-5d)
Prepared similar to the general procedure from 3b
(0.196 g) using 4-methoxybenzaldehyde (0.204 g). Yield:
0.26 g (82 %) as a white powder. Mp 270–271 °C. 1H
NMR (600.13 MHz, DMSO-d6, 30 °C) d: 3.75, 3.86 (br s,
12H, O–CH3); 4.68, 5.15 (br s, 8H, C(7)H); 6.65, 6.75 (br
m, 4H, C(3)H); 6.97, 7.09 (br m, 8H, C(12)H); 7.60, 7.61
(br m, 8H, C(4)H); 7.90, 7.98 (br m, 8H, C(11)H); 7.87,
8.45 (br s, 4H, C(9)H); 9.8, 11.3 (br s, 4H, NH). 13C NMR
(150.9 MHz, DMSO-d6, 30 °C) d: 55.1, 55.3 (O–CH3);
67.3, 68.2 C(7); 114.2, 114.3 C(12); 123.9, 124.3 C(3);
128.3, 128.7 C(5); 129.2, 129.4 C(11); 135.3, 135.9 C(4);
144.1, 147.4 C(9); 157.6, 158.5 C(10); 160.9, 161.0
(C(13)); 162.0 C(6); 162.6, 168.0 C(8). IR (m/cm-1, Nujol):
3480, 3199 (mNH), 1719, 1697 (mC=O), 1547 (dNH). Ele-
mental analysis Calc. for C64H56N8O12S4 (1257.44): C,
61.13; H, 4.49; N, 8.91; S, 10.20. Found: C, 61.45; H, 4.50;
N, 9.08; S, 10.22. Mass spectrum (MALDI-TOF): m/z:
1279 [M?Na]?; 1295 [M?K]?.
25,26,27,28-Tetrakis[N2-(4-
methoxybenzylidene)hydrazinocarbonylmethylenoxy]-
calix[4]arene (cone-6d)
Prepared similar to the general procedure from 4a (0.18 g)
using 4-methoxybenzaldehyde (0.204 g). Yield: 0.18 g
(60 %) as a white powder. Mp 272–276 °C. 1H NMR
(600.13 MHz, DMSO-d6, 30 °C) d: 3.65, 3.74 (br s, 12H,
O–CH3); 3.32, 4.70 (8H, –CH2–bridge); 4.68, 4.92 (br s,
8H, C(7)H); 6.62, 6.88 (br m, 4H, C(3)H); 6.57, 6.93 (br m,
8H, C(12)H); 6.60, 7.35 (br m, 8H, C(4)H); 7.26, 7.53 (br
m, 8H, C(11)H), 7.93, 8.14 (br s, 4H, C(9)H); 11.60, 11.70
(br s, 4H, NH). 13C NMR (150.9 MHz, DMSO-d6, 30 °C)
d: 29.1, 30.2 (–CH2–bridge); 55.1, 55.2 (O–CH3); 72.6,
74.3 C(7); 113.8, 114.4 C(12); 122.8, 125.5 C(3); 126.4,
126.4 C(10); 128.5, 128.8 (C(11)); 128.5, 129.2 (C(4));
134.4, 135.8 C(5); 144.3, 148.5 C(9); 153.7, 155.6 (C(6));
160.5, 160.6 C(13); 165.5, 170.3 C(8). IR (m/cm-1, Nujol):
3464, 3210 (mNH), 1702, 1668 (mC=O). Elemental analysis
Calc. for C68H64N8O12 (1185.28): C, 68.91; H, 5.44; N,
9.45. Found: C, 68.79; H, 5.07; N, 9.14. Mass spectrum
(MALDI-TOF): m/z: 1185 [M?H]?, 1207 [M?Na]?.
25,26,27,28-Tetrakis[N2-
(benzylidene)hydrazinocarbonylmethylenoxy]-2,8,14,20-
tetrathiacalix[4]arene (cone-6a)
Prepared similarly to the general procedure from 4b
(0.196 g) using benzaldehyde (0.16 g). Yield: 0.25 g
(88 %) as a white powder. Mp 180–182 °C. 1H NMR
(600.13 MHz, DMSO-d6, 30 °C) d: 4.95, 5.25 (br s, 8H,
C(7)H); 6.60 (br m, 4H, C(3)H); 7.25 (4H, C(13)H); 7.30
(br m, 8H, C(4)H), 7.38 (br m, 8H, C(12)H); 7.58, 7.64 (br
m, 8H, C(11)H); 8.57, 7.91 (br s, 4H, C(9)H); 11.60 (br s,
4H, NH). 13C NMR (150.9 MHz, DMSO-d6, 30 °C) d: 73.3
C(7); 124.0 C(3); 125.6 (C(5)); 127.5, 127.8 C(11); 129.1
C(12); 130.7 C(4); 134.5 (C(13)); 144.9, 149.3 C(9); 150.0
C(10); 154.3 C(6); 164.8, 170.6 C(8). IR (m/cm-1, Nujol):
3467, 3217 (mNH), 1692 (mC=O), 1540 (dNH). Elemental
analysis Calc. for C60H48N8O8S4 (1137.33): C, 63.36; H,
4.25; N, 9.85; S, 11.28. Found: C, 63.19; H, 3.89; N, 9.59;
S, 10.94. Mass spectrum (MALDI-TOF): m/z: 1137
[M?H]?; 1159 [M?Na]?; 1175 [M?K]?.
Apparatus
Microanalyses of C, H and N were carried out with a CHN-
3 analyzer. Melting points of compounds were measured
with a Boetius hotstage apparatus. IR absorption spectrum
of Nujol emulsion of compound 5–6 were recorded on a
Vector-22 Bruker FT-IR spectrophotometer with a resolu-
tion of 4 cm-1. NMR experiments were performed on a
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J Incl Phenom Macrocycl Chem
Bruker AVANCE-600 spectrometer at 303 K equipped
with a 5 mm diameter broadband probe head working at
The maximum at the plot dependence of picric acid content
in organic layer versus its initial mole fraction in water
indicates the ratio of the stoichiometry coefficients of picric
acid and ligand in the complex. Taking the eq. 1 (see bel-
low) as well as the metall cation’s charge into account, the
complex’s stoichiometry was determined.
1
600.13 MHz in H and 150.864 MHz in 13C experiments.
1
Chemical shifts in H and 13C spectra were reported rela-
tive to the solvent as internal standard (DMSO
d(1H) = 2.50 ppm, d(13C) = 39.5 ppm). Assignment was
accomplished by means of 2D 1H–13C HSQC and 2D
1H–13C HMBC methods. The pulse programs for all NMR
experiments were taken from the Bruker software library.
A mass spectrum (MALDI) was detected on a Bruker Ul-
traflex III MALDI-TOF/TOF mass spectrometer.
Results and discussion
Synthesis of receptors
Extraction procedure and measurements
The hydrazone derivatives of calix[4]arene: octahydrazone
1, monohydrazones 2a, 2b as well as tetrahydrazones in
different isomer forms: 5a–f (1,3-alternate) and 6a–6d
(cone) were obtained by condensation of corresponding
hydrazides with aldehydes (Fig. 1). The synthesis of some
of these hydrazones was described earlier in our previous
works [31, 35–37] (See details in ‘‘Experimental’’ part).
The condensation of cone isomers of tetrahydrazide tetra-
thiacalix[4]arene, having tert-butyl groups at the upper rim,
usually results in bisacetylhydrazone derivatives with N,N0-
diacetylhydrazine intramolecular bridge [37, 40, 41].
Therefore, in this work we have synthesized the cone iso-
mers of thia- and calix[4]arene hydrazones 6a–d which do
not have tert-butyl groups at the upper rim of the macro-
cyclic platform.
The CHCl3 was saturated with H2O before use to prevent
volume changes during the extraction. Aqueous metal
picrate solutions (5 ml) buffered at pH 6.0 and the solu-
tions of extractant (5 ml, 2.5 9 10-5 to 1 9 10-3 M) in
CHCl3 were magnetically stirred in a flask. The extraction
equilibrium was reached after vigorous stirring for 1.5 h at
25 °C. Then two phases were allowed to settle for 1 h and
afterwards separated by centrifugation. The absorbances
A1 of aqueous phase after extraction and A0 of aqueous
phase before extraction were measured at kmax = 355 nm
(the wavelength of maximum absorption of the picrate ion).
All data were obtained from three independent experi-
ments. The aqueous metal picrate solutions ([metal
salt] = 1 9 10-2 M; [picric acid] = 2.5 9 10-4 M) were
prepared by stepwise addition of a 2.5 9 10-4 M aqueous
picric acid solution to the calculated amounts of metal
salts. The obtained solutions were stirred at pH 6.0 with
acetic-acetate buffer for 1 h. For alkali ions tris(hydroxy-
methyl)aminomethane-HCl (0.05 M) was used as a buffer.
Blank experiments showed that no metal-picrate extraction
occurred under the same conditions in the absence of
calixarenes. The picrate transfer in the presence of calix-
arene and buffer (pH = 6) but in the absence of transition
metal cations was not observed also (E \ 2 %).
The presence of several hydrazone fragments and polar
groups in the molecules 5 and 6 leads to a large number of
conformers, as well as to the numerous hydrogen bonding
and steric interactions. Therefore, the IR spectra of these
compounds contain broadened absorption bands, and many
various signals are observed in their 1H NMR spectra. The
assignment of these signals in experimental was performed
similarly to our previous works [34, 41, 42].
Effect of pH on liquid–liquid phase transfer of picric
acid by hydrazone derivatives
The percent of extraction was calculated as ratio
E % = 100 9 (A0 - A1)/A0 = a 9 100 %. E % uncer-
tainties are generally B 2 %. The log Kex and n values for
the extraction process (see below Eqs. 1, 2) were deter-
mined from the plot of log Q versus log[L]org where
Q = a/z(1 - a)z as described elsewhere [31, 32].
To evaluate the stoichiometry of the complex formation
with Ag? and Hg2? the Job plots method was also applied.
Series of isomolar solutions containing various mole
amounts of picric acid and ligand (their total concentra-
tion 2.5 9 10-4 M) and constant amount of metal ions
(1 9 10-2 M) were prepared. The obtained solutions were
stirred at acetic-acetate buffer with pH 6.0 for 1 h, separated
and spectrophotometrically measured at kmax = 355 nm.
The presence of a lone electron pair on the nitrogen atoms
in hydrazone molecules provides their basic properties. As
a result the HPic transfer from the aqueous to organic phase
at pH 2–4 for the investigated compounds 5–6 becomes
dramatically high (E [ 10–70 %) (Fig. 2). Therefore, to
receive the correct data for extraction yields as well as to
simplify the calculations of the complex stoichiometry and
extraction constants for the synthesized hydrazones, we
have performed the liquid–liquid extraction experiments at
pH = 6.0 with the use of buffer. Under such conditions the
hydrolysis of metal ions and the transfer of picric acid from
aqueous to organic phase (E \ 2 % see Fig. 2) appeared to
be negligible.
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J Incl Phenom Macrocycl Chem
spectrophotometrically. The extraction constant (Kex) was
evaluated from Eq. 2:
n
Kex ¼ ½MzþLnPicꢁz ꢂorg= ½Mzþꢂaq½Picꢁꢂazq½Lꢂorg
ð2Þ
The extraction data obtained for the synthesized
hydrazones are summarized in Fig. 3, 4 and Table 1.
It could be clear observed that monohydrazones 2a, 2b do
not extract metal ions from aqua solutions (E \ 3 %). The
preorganization of hydrazone groups on calix[4]arene plat-
form promotes considerably their affinity towards Ag? and
Hg2?. Really, the 1,3-alternate thiacalix[4]arenes become
already capable to bind effectively these ions. The extraction
efficiency goes up after removing tert-butyl groups from
upper rim of 1,3-alternate tetrathiacalix[4]arene isomers
(E5d (Ag?) * 25 %, E5d (Hg2?) * 24 %). In spite of
decreasing of lipophilicity of the molecules, the weakening
of steric difficulties facilitates the process of metal recovery
by these compounds.
Fig. 2 Effect of pH on a degree of transfer of HPic in the systems
containing extractants 1, 2a, 2b, 5a–f and 6a–d. [HPic] = 2.5 9 10-4
M; [L1] = 1.25 9 10-4 M; [L2a,b] = 1 9 10-3 M, [L5a–f, 6a–d] =
2.5 9 10-4
M
The possibility of a cooperative coordination of four
binding hydrazone groups in cone isomers of tetrathi-
acalix[4]arene results in dramatic increase of the extrac-
tion efficiency for Ag? (E6a/E5b & 8.7, E6b/E5d & 2.9,
E6c/E5f & 6) and Hg2? (E6a/E5b & 3.6, E6b/E5d & 2.6,
E6c/E5f & 4.5) (Fig. 3). In the case of applying other
metal ions the extraction yields, including the possible
transfer of picric acid, do not exceed an experimental
error (E \ 4 %). This fact testifies about remarkable
binding selectivity of cone tetrathiacalix[4]arenes 6a–6c
towards Ag? and Hg2? appearing in a wide range of
metal ions.
Metal extraction
The extraction process at pH 6 for hydrazones 2a, 2b, 5a–f
and 6a–d can be described by Eq. 1:
Mzaþq þ zPicaꢁq þ nLorgꢀ½MzþLnPiczꢁꢂorg
ð1Þ
where Mz?, Pic-, L, [Mz?LnPic-z ] denote the metal ion,
picrate anion, ligand, ion-pair metal complex and the
subscripts aq and org mean that the species exist in the
aqueous or organic phase. The equilibrium concentration of
picrate anion in the aqueous phase was determined
Fig. 3 Extraction percentages
(E %) of metal picrates from
water into CHCl3 at 25o C by
hydrazones 1, 2a, 2b, 5a–f and
6a–d. [HPic] = 2.5 9 10-4 M,
[Mz?] = 1 9 10-2 M,
[L1] = 1.25 9 10-4 M,
[L2a,b] = 1 9 10-3 M,
[L5a–e,6a–d] = 2.5 9 10-4 M;
pH = 6.0
123

J Incl Phenom Macrocycl Chem
The introduction of 4-methoxy- or 4-hydroxy- groups
into benzylidene fragments of hydrazones in 1,3-alternate
(5c–e) and cone (6b–c) isomers also promotes a consid-
erable increase of Ag? and Hg2? ions recovery by these
compounds (for example, 1,3-alternate: E5b (R2 = H) =
7 % ? E5d (R2 = OCH3) = 25 % for Ag?; cone: E6a
(R2 = H, R3 = H) = 65 % ? E6c (R2 = OH, R3 =
t-Bu) = 81 % for Hg2?). The methoxy and, espe-
cially, the hydroxyl groups posses a positive meso-
meric effect and act as electron donors [43]. Such
groups enhance the imine nitrogen basicity in the
hydrazone fragments that leads to a more effective
binding of the mentioned above compounds with Ag?
and Hg2? ions.
E(Pb2?) = 13 %). The selectivity of metal recovery (S) by
this compound in the row of metals was calculated as the
ratio of percentages of the cations extracted: S = E(Ag?)/
E(Pb2?) = 7.6. Review [44] summarizes the data obtained
for the liquid–liquid extraction of toxic ions (Cd2?, Hg2?
and Pb2?) by 57 calixarenes. Among these compounds the
5,11,17,23-tert-butyl-25,26,27,28-(2-N,N-dimethyldithio-
carbamoylethoxy)calix[4]arene has exhibited the highest
selectivity towards Hg2? (S = 20) and Ag? (S = 18) rel-
atively Cd2? and Pb2? ions [45]. Unfortunately, the com-
parison with other well extracted d-ions has not been carried
out. In our case, such selectivity towards Ag? (S = 16 (6a),
18 (6b), 17 (6c)) and Hg2? (S = 16 (6a), 18 (6b), 20 (6c))
was successfully achieved by the synthesized tetrahydraz-
ones 6a–c in a wide range of metal ions. The selectivity data
have been calculated based on the fact that the extraction
degree for other metal ions did not exceed 4 % (Table 1). In
the case of low yields of extraction the experimental error
and the transfer of picric acid can contribute into determi-
nation of selectivity values and underestimate them. There-
fore, the applicaton in future of more sensitive and accurate
analitical methods may help to obtain more perfect and
remarkable results for the selectivity.
The stoichiometry of extracted complexes with Ag? and
Hg2? and the extraction constants have been determined
from log Q vs log [L]org plots (see Experimental Section)
for the most effective receptors 6a–c. All graphs of
extraction dependences for cone isomers 6a–c with Ag?
and Hg2? have an equal slope (n & 1 and 2 respectively)
regardless of excess or insufficiency of the ligand (Fig. 4).
This indicates the 1:1 for Ag? and 1:2 for Hg2? (Mz?/L)
stoichiometry of the extracted complexes in a wide range
of concentration. These results are in accordance with data
obtained for the complex stoichiometry determined by the
Job plots. The transfer of Pic- anion achieved the maxima
at 0.5 mol fraction in all cases (Fig. 5). This fact clearly
confirms that Ag? and doubly charged Hg2? metal ions
form complexes 1:1 and 1:2 (Mz?/L), respectively.
We have also aimed to compare the efficiency and
selectivity of the metal recovery by hydrazone derivatives
based on tetrathia-, calix[4]arene and calix[4]resorcine
platforms. Calix[4]arene 6d as well as its thia- analogue 6b
was found do not bind alkali ions. Calix[4]arene 6d has
comparable extraction efficiency towards Ag? and Hg2?
with its tetrathia- analogue 6b. Thus, this fact indicates that
the bridge sulfur atoms of the tetrathiacalix[4]arene mac-
rocycle 6d do not participate in the binding of transition
metal ions. Obviously, the coordination is realized due to
the terminal hydrazone groups.
Tetrahydrazone 6a binds Ag? and Hg2? ions approxi-
mately two times more effectively than the octahydrazone of
calix[4]resorcine 1 (for comparison: E6a(Ag?) = 62 %,
E6a(Hg2?) = 65 %; E1(Ag?) = 35 %, E1(Hg2?) = 31 %).
An increase of a number of binding groups under going from
thiacalix[4]arene to calix[4]resorcine does not lead to an
expected enhancement of the extraction efficiency. This
phenomenon is obviously explained by unfavourable dis-
position of endo-oriented acetylhydrazone groups in
calix[4]resorcine, which is less effective for the cooperative
binding than in the case of closely located exo-oriented
groups in tetrathiacalix[4]arene.
As it was previously indicated, the calix[4]arenes
functionalized by thio-groups can reveal a high recovering
ability towards Ag? and Hg2?. Indeed, a quantitative
extraction of Ag? (E = 99 %) was observed for the p-tert-
butyltetrathiacalix[4]arene substituted at the narrow rim by
diethylthiophosphate ester groups (-PS(OEt) [21]. How-
ever, the other metal ions have been also notably extracted
by this compound (for example, E(Cu2?) = 11 % and
In our previous investigations we have shown that
tetrahydrazide 4b (cone) could be utilized for the group
separation of d-ions from alkali- and f- metals [33]. Tetra-
hydrazide 4b forms one order more stable complexes
with Ag? (Kex
= 107.5), than tetrahydrazones 6a–c
4b
(Kex
approximately two orders higher extraction constants than
= 106.3–6.5) (Table 1). In the case of Hg2?
,
6a–c
Table 1 The extraction
Cation
Metal:ligand
stoichiometry
log Kex
constants and stoichiometry of
complexes of hydrazones 1 and
6a–c with Ag? and Hg?
1
6a
6.3 0.2
6b
6.5 0.1
6c
6.4 0.1
Ag?
Hg2?
1:1
1:1
1:2
5.8 0.1
9.2 0.2a
–
–
–
14.1 0.2
14.6 0.2
14.5 0.4
a
Data from Ref. [31]
123

J Incl Phenom Macrocycl Chem
Fig. 4 Log Q versus log [L]org
for Ag? and Hg2? extracted by
1 (a), 6a (b), 6b (c) and 6c (d),
4b,c (b) in CHCl3.
[HPic] = 2.5 9 10-4 M,
[Mz?] = 1 9 10-2 M,
pH = 6.0
Fig. 5 Job plots for Pic- extracted by tetrahydrazones 6a–c in the presence of Ag?(a) and Hg2? (b). [HPic]aq ? [L]org = 2.5 9 10-4 M,
[Mz?] = 1 9 10-2 M, pH = 6.0
for Ag? have been obtained for tetrahydrazone derivatives
of thiacalix[4]arenes (Kex
= 1012.6 and Kex
1014.1–14.6). The 1,3-alternate tetrahydrazide 3b has also
revealed a high selectivity in the series of d-ions. The max-
ima of extraction were observed for Ni?, Ag? and Cd2?, but
the other d-metal ions have been also extracted enough
effectively (E3b = 13–40 % for Co2?, Cu2?, Zn2?).
valuesisratherdifficult. Therefore, theextraction constantsfor
calix[4]arenes 6a–c with the weak extracted metal ions have
been evaluated for the maximal value E & 4 % (Table 1))
assuming the 1:1 (Kex(Me?) = 104.2) or 1:2 (Kex(Me2?) =
1011.4) complex stoichiometry. In these cases the extraction
selectivity for Ag? and Hg2? by tetrahydrazones 6a–c achieves
more than two and three orders of magnitude (Kex(Ag?)/
=
6a–c
4b
To determine the selectivity of metal recovery by using Kex
it is commonly required to estimate these values for the dif-
ferent metal ions forming complexes of equal stoichiometry.
However, in the case of low extraction yields to define these
Kex(Me?) = 106.3–6.5/104.2 = 102.1–2.3, Kex(Hg2?)/Kex(Me2?
)
= 1014.1–14.6/1011.4 = 102.7–3.2), respectively.
Whereas the tetrahydrazides 3b and 4b as well as some
of thio-derivatives of calix[4]arenes demonstrate high
123

J Incl Phenom Macrocycl Chem
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6a–c reveal high selectivity of Ag? and Hg2? ion recovery
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We have synthesized novel phenyl hydrazone derivatives
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¨
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,
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Lu3?) metal ions have shown that the new and early syn-
thesized cone calix[4]arene isomers are excellent receptors
for the recovery of Ag? and Hg2? ions from aqueous
environment. The extraction selectivity for Ag? and Hg2?
ions relative to other metal ions (Ag?/Men? C 16–18 and
Hg2?/Men? C 16–20) achieves two and three orders of
magnitude Kex, respectively.
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It was experimentally demonstrated that in order to
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The introduction of electron-donor substituents in phenyl
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