Lower rim substituted p-tert-butyl-calix[4]arene. Part 17. Synthesis, extractive and ionophoric properties of p-tert-butylcalix[4]arene appended with hydroxamic acid moieties

Article abstract of DOI:10.1016/j.poly.2014.04.009

The synthesis and characterization of four p-tert-butylcalix[4]arene- hydroxamic acids are reported. The dependence of the metal ion binding, assessed by liquid-liquid extraction of the metal nitrates from water into dichloromethane in individual and competitive experiments, on the ligand structure, is presented. The results showed that those ligands could be successfully used in separation process of transition and heavy metals often present together. Two of the ligands were used as active materials in Pb-ion-selective membrane electrodes. The characteristics of these electrodes, in particular their selectivity coefficients for Pb²⁺ over other metal ions, are discussed.

Full text of DOI:10.1016/j.poly.2014.04.009

Polyhedron 77 (2014) 89–95
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Lower rim substituted p-tert-butyl-calix[4]arene. Part 17. Synthesis,
extractive and ionophoric properties of p-tert-butylcalix[4]arene
appended with hydroxamic acid moieties
J. Kulesza a,b, , M. Guzinski a,c, M. Bochenska a, V. Hubscher-Bruder b, F. Arnaud-Neu b
⇑
a Department of Chemistry and Technology of Functional Materials, Faculty of Chemistry, Gdansk University of Technology, Narutowicza Street 11/12, 80-233 Gdansk, Poland
b Laboratoire de Chimie-Physique, IPHC-DSA, UDS, CNRS, ECPM 25, Rue Becquerel, 67087 Strasbourg Cedex 2, France
c Department of Biomedical Engineering, The University of Memphis, Memphis, TN 38152, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of four p-tert-butylcalix[4]arene-hydroxamic acids are reported. The
dependence of the metal ion binding, assessed by liquid–liquid extraction of the metal nitrates from
water into dichloromethane in individual and competitive experiments, on the ligand structure, is
presented. The results showed that those ligands could be successfully used in separation process of
transition and heavy metals often present together. Two of the ligands were used as active materials
in Pb-ion-selective membrane electrodes. The characteristics of these electrodes, in particular their
selectivity coefficients for Pb2+ over other metal ions, are discussed.
Received 30 January 2014
Accepted 8 April 2014
Available online 18 April 2014
Keywords:
Calix[4]arenes
Hydroxamic acids
Extraction
Ó 2014 Elsevier Ltd. All rights reserved.
Ion-selective electrodes
Heavy metals
1. Introduction
accumulated in the human body, causing serious health disorders
such as kidney and liver dysfunction, respiratory problems and
Naturally occurring compounds possessing hydroxamic acid
moieties, known as siderophores, are produced by fungi and
bacteria and act as sequestering agents for Fe3+ incorporation into
microorganisms [1]. Their excellent binding properties found
biomedical application in removal of Fe3+ and Al3+ from the body.
It has been reported that siderophores show affinity also for other
bivalent metal cations such as Pb2+, Cu2+ or Ni2+ as well as UO22+ [1].
Calixarenes appended with hydroxamic acid groups could be
regarded as synthetic siderophores and were mainly tested in
solvent extraction experiments. They occurred to be excellent
uranophiles [2–4], although they may efficiently extract transition
and precious metal cations such as Fe3+, Cu2+, Zn2+, Ni2+ and Pd2+ as
well [5,6]. Solid-phase extraction of heavy metal ions using
immobilized calix[4]arene-hydroxamic acids was also reported
[7,8].
bone disease. Therefore, the level of these toxic metals in natural
and wastewaters needs to be monitored and controlled. However,
the direct determination of their concentration level is frequently
not sufficiently sensitive and the removal and separation of metal
ions from their mixture is necessary.
Removal and separation of toxic heavy metals reduces the envi-
ronmental impact and if they can be recovered in an economical
way, it may generate some revenue for the industries. Therefore,
the separation of metal ions, often present together, like Pb(II)
and Cd(II) or Cu(II), Zn(II) and Ni(II), is a very important and
challenging task and requires more and more selective ligands
[9–11].
In this paper, we present the synthesis, characterization, extrac-
tive and ionophoric properties of four p-tert-butylcalix[4]arene
hydroxamic acid derivatives (Fig. 1), among which, ligand 1 is
reported here for the first time. These ligands differ by the number
of functional groups attached to the calixarene framework (three
or four arms) and by the substituents on the nitrogen atoms
(R = H for 1 and 3, R = CH3 for 2 and 4). Such structural changes
are expected to influence binding properties of these compounds.
Extraction experiments were carried out in individual and compet-
itive conditions to evaluate the ability of these compounds to
selectively separate heavy metal ions.
Heavy metals such as lead, cadmium, copper, nickel, cobalt, zinc
or others, are present in our environment, mainly in wastewaters
coming from various industries. These metals are easily
⇑
Corresponding author. Current address: Escola de Ciências
e Tecnologia,
Universidade Federal do Rio Grande do Norte (UFRN), Campus Universitário Lagoa
Nova, 59078-970 Natal, RN, Brazil. Tel.: +55 84 33422347; fax: +55 84 33422303.
E-mail address: kulesza.joanna@gmail.com (J. Kulesza).
http://dx.doi.org/10.1016/j.poly.2014.04.009
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

90
J. Kulesza et al. / Polyhedron 77 (2014) 89–95
The reagent grade organic solvents, dichloromethane (Fluka
Analytical), ethyl ether (Fluka Analytical), acetone (Riedel-
de-Haën), ethyl alcohol (Carlo Erba) were used without further
purification. Tetrahydrofuran (THF) p.a. (from POCh) was dried
and freshly distilled before use. Dimethylformamide (DMF) (from
POCh) used in reactions was dried before use.
2.3. Synthesis
The four calix[4]arene-hydroxamic acids were prepared via the
so-called mixed anhydrides method [16]. The intermediate com-
pounds I–IV (Schemes 1 and 2) were synthesized according to
the known procedures [17–19].
The tris-substituted calix[4]arene-hydroxamic acids (1 and 2)
were prepared as follows (Scheme 1): calix[4]arene-triscarboxylic
acid (II) was dissolved in dry CH2Cl2 (10 mL) and the solution
was stirred and cooled to ꢁ10 °C. Subsequently, 3.2 equivalents
of NEt3 and 3.2 equivalents of ethyl chloroformate were added.
The solution was stirred at ꢁ10 °C for 30 min. and then 18 equiv-
alents of an appropriate hydroxylamine hydrochloride (NH2OH.HCl
or CH3NHOH.HCl, respectively) in 5 mL of CH2Cl2 were added.
Before introducing amine hydrochloride to the reaction mixture,
it was released from its hydrochloride form by adding the same
equivalent of NEt3. After 1 h, the mixture was diluted with CH2Cl2
(15 mL), washed with water (15 mL), then with 0.1 mol Lꢁ1 HCl
(15 mL) and again with water (15 mL). The water phase was
extracted twice with CH2Cl2 (30 mL). The combined organic layers
were dried over MgSO4, filtered and the solvent was evaporated
under reduced pressure. The residue was crystallized from CH2Cl2/
Et2O mixture to give the pure products.
Fig. 1. Chemical structures of the p-tert-butylcalix[4]arene hydroxamic acids
synthesized and studied in this work.
Functionalized calixarenes play an important role as ionophores
in the field of chemical sensors [12–14]. In the previous paper,
ligands 2 and 4 were preliminary tested as Pb-ionophores in ion-
selective membrane electrodes (ISEs) showing promising proper-
ties [15]. In this work, the results of the continued studies in this
field are presented.
2. Experimental
2.1. General
TLC was performed on silica gel plates Merck 60 F254. Melting
points were measured and are uncorrected. 1H NMR spectra were
recorded in DMSO on a Varian instrument (200 or 500 MHz). IR
spectra were obtained on a Mattson Genesis II spectrometer. Ele-
mental analyses were performed on a Carlo Erba Instrument CHNS
EA 1108-Elemental analyzer. The absorbances were measured by
atomic absorption spectrometry with an air-acetylene flame (Var-
ian-55).
The calibration curves were obtained by measuring the absor-
bance of several samples of known metal cation concentration
under the same conditions as the unknown. Therefore, the metal
concentration of the unknown sample could be calculated.
The EMF measurements were done on a 16 – channel LAWSON
LAB potentiometer (16 EMF, USA).
The tetrakis-substituted compounds (3 and 4) were prepared
similarly by reacting calix[4]arene-tetra-carboxylic acid (IV) with
4.2 equivalents of ethyl chloroformate and 4.2 equivalents of tri-
ethyl amine. Subsequently, 24 equivalents of an appropriate
hydroxylamine hydrochloride (NH2OH.HCl or CH3NHOH.HCl,
respectively) were added (Scheme 2).
2.3.1. 25-hydroxy-26, 27, 28-tris(N-hydroxycarbamoylmethoxy)-p-
tert-butylcalix[4]arene (1)
C50H65O10N3; M.W. = 868.2 g/mol; m.p. = 188–190 °C; yield:
76%; IR mmax (C@O) 1645 cmꢁ1 1H NMR (500 MHz, DMSO): cone
;
conformation, d [ppm]: 0.96 (s, 18H, C–(CH3)3); 1.20 (s, 9H, C–
(CH3)3); 1.22 (s, 9H, C–(CH3)3); 3.30 (d, 2H, Ar–CH2–Ar,
J = 13.67 Hz); 4.26 (d, 2H, Ar–CH2–Ar, J = 13.67 Hz); 4.40 (d, 2H,
Ar–CH2–Ar, J = 13.19 Hz); 4.45 (d, 2H, Ar–CH2–Ar, J = 13.19 Hz);
4.22 (s, 2H, –O–CH2–CO); 4.41 (s, 4H, –O–CH2–CO); 6.78 (s, 2H,
Ar–H); 6.91 (s, 2H, Ar–H); 7.07 (s, 2H, Ar–H); 7.20 (s, 2H, Ar–H);
7.10 (s, 1H, N–H); 7.24 (s, 2H, N–H); 9.09 (s, 1H, O–H); 10.53 (s,
1H, O–H); 10.83 (s, 2H, O–H).
2.2. Chemicals
p-tert-Butylcalix[4]arene, ethylbromoacetate, ethylchlorofor-
mate, triethylamine, DMSO, BaO, Ba(OH)2.8H2O, N-hydroxylamine
hydrochloride (NH2OH.HCl), N-methylhydroxylamine hydrochlo-
ride (CH3NHOHꢀHCl) were purchased from Aldrich; CH3COOH
and CH3COONa were purchased from Prolabo and Merck,
respectively.
Experimental Anal. Calc. for C50H65O10N3ꢀ2H2O: C, 66.36; H,
7.63; N, 4.64. Found: C, 66.81; H, 7.45; N, 3.34%.
For extraction experiments, the metal salts: Ni(NO3)2ꢀ6H2O,
Zn(NO3)2ꢀ6H2O (Fluka, purum), Pb(NO3)2, Cd(NO3)2ꢀ4H2O,
Fe(NO3)3ꢀH2O (Merck, p.a.), Cu(NO3)2ꢀ3H2O (Prolabo) and
Co(NO3)2ꢀ6H2O (Strem Chemicals) (analytical grade) were dried
under vacuum at room temperature before use.
2.3.2. 25-Hydroxy-26,27,28-tris(N-methyl, N-
hydroxycarbamoylmethoxy)-p-tert-butylcalix[4]arene (2)
C53H71O10N3; M.W. = 910.2 g/mol; m.p. = 217–221 °C; yield:
79%; IR mmax (C@O) 1650 cmꢁ1 1H NMR (500 MHz, DMSO): cone
;
conformation, d [ppm]: 0.94 (s, 18H, C–(CH3)3); 1.17 (s, 18H, C–
(CH3)3); 3.14 (s, 9H, –N–CH3); 3.18 (d, 2H, Ar–CH2–Ar,
J = 12.7 Hz); 3.22 (d, 2H, Ar–CH2–Ar, J = 12.7 Hz); 4.37 (d, 2H, Ar–
CH2–Ar, J = 12.7 Hz); 4.54 (d, 2H, Ar–CH2–Ar, J = 12.7 Hz); 4.95 (s,
6H, –O–CH2–CO); 6.70 (s, 2H, Ar–H); 6.81 (s, 2H, Ar–H); 6.94 (s,
2H, Ar–H); 7.02 (s, 2H, Ar–H); 9.70 (s, 1H, O–H); 9.84 (s, 3H, O–H).
Experimental Anal. Calc. for C53H71O10N3ꢀ2H2O: C, 67.22; H,
7.93; N, 4.44. Found: C, 67.72; H, 7.70; N, 4.48%.
Poly(vinylchloride) (PVC, high molecular fraction), o-nitrophe-
nyloctyl ether (NPOE) and the lipophilic salt, potassium tetra-
kis(p-chlorophenyl)borate (KTpClPB), were purchased from Fluka
(Selectophore). All aqueous salt solutions used for ISE studies:
NaCl, KCl, CaCl2, ZnCl2, CdCl2 (POCh all p.a. grade) and Pb(NO3)2
and Cu(NO3)2 (Aldrich) were prepared using ultrapure water from
a Hydro-lab (RO) station (conductivity below 0.1 l
S cmꢁ1).

J. Kulesza et al. / Polyhedron 77 (2014) 89–95
91
Scheme 1. Synthesis of tris-substituted calix[4]arene-hydroxamic acids (1 and 2).
Scheme 2. Synthesis of tetrakis-substituted calix[4]arene-hydroxamic acids (3 and 4).
2.3.3. 25,26,27,28-Tetrakis(N-hydroxycarbamoylmethoxy)-p-tert-
butylcalix[4]arene (3)
ane) was measured in the same conditions. The percentages of cat-
ions extracted from water into dichloromethane were calculated
from the equation: %E = 100(A0 ꢁ A)/A0.
C52H68O12N4; M.W. = 940.1 g/mol; m.p. = 220–222 °C; yield:
82%; IR mmax (C@O) 1640 cmꢁ1
;
1H NMR (500 MHz, DMSO): cone
For the competitive metal ion extraction, two solutions of metal
salts mixture were prepared; one containing Pb2+ and Cd2+ and the
other with Cu2+, Zn2+ and Ni2+ metal salts. The concentration of
each metal ion was 1.06 ꢂ 10ꢁ4 mol Lꢁ1. The experimental condi-
tions were the same as for the individual extraction experiments.
conformation, d [ppm]: 0.91–1.24 (bs, 36H, C–(CH3)3); 3.23 (d,
4H, Ar–CH2–Ar, J = 13.18 Hz); 4.11–4.87 (m, 16H, –O–CH2–
CO + Ar–CH2–Ar, J = 13.18 Hz); 6.68–7.13 (m, 8H, Ar–H).
Experimental Anal. Calc. for C52H68O12N4ꢀ2H2O: C, 63.93; H,
7.38; N, 5.73. Found: C, 64.18; H, 7.41; N, 3.79%.
2.5. Ion-selective electrode studies
2.3.4. 25,26,27,28-Tetrakis(N-methyl,N-hydroxycarbamoylmethoxy)-
p-tert-butylcalix[4]arene (4)
2.5.1. Membrane preparation and EMF measurements
C56H76O12N4; M.W. = 996.2 g/mol; m.p. = 214–215 °C; yield:
The membrane components (2.25 wt.% of ionophore, 32.5 wt.%
PVC, 65 wt.% o-nitrophenyl-octylether (NPOE) or bis(1-butylpen-
tyl)adipate (BBPA), 0.25 wt.% potassium tetrakis(4-chlorophenyl-
borate) (KTpClPB)), a total cocktail mass of about 185 mg, were
dissolved in 1.5 mL of dried and freshly distilled THF. The solution
was poured into a glass ring (diameter d = 24 mm). After overnight
slow solvent evaporation, the membranes could be used to prepare
the electrodes. Several small membranes of d = 7 mm were cut
from the mother membrane and incorporated into Ag/AgCl elec-
trode bodies of IS 561 type (Moeller S.A., Zurich, Switzerland).
The electrode body was filled up with an internal filling solution
(Pb(NO3)2 10ꢁ3 mol Lꢁ1, Na2EDTA 10ꢁ3 mol Lꢁ1). A double-junction
reference electrode Eurosensor EAgClK – 312 was used with 1 mol
Lꢁ1 KNO3 solution in the bridge cell. The measurements were
carried out at 23 °C, using the following cell:
60%; IR mmax (C@O) 1646 cmꢁ1 1H NMR (500 MHz, DMSO): cone
;
conformation, d [ppm]: 1.04 (bs, 36H, C–(CH3)3); 3.14 (m, 16H, –
N–CH3 + Ar–CH2–Ar, J = 12.7 Hz); 4.76 (d, 4H, Ar–CH2–Ar,
J = 12.7 Hz); 4.88 (s, 8H, –O–CH2–CO); 6.78 (s, 4H, Ar–H); 9.72 (s,
4H, –O–H).
Experimental Anal. Calc. for C56H76O12N4: C, 67.45; H, 7.68; N,
5.62. Found: C, 67.24; H, 7.61; N, 5.62%.
2.4. Extraction studies
The organic solutions were made by dissolving a weighted
amount of the ligand in CH2Cl2. The aqueous solutions containing
the metal cation as nitrate were buffered to pH 5.4 with
0.01 mol Lꢁ1 acetate buffer (8.2 ꢂ 10ꢁ3 mol Lꢁ1 CH3COONa and
1.8 ꢂ 10ꢁ3 mol Lꢁ1 CH3COOH) for Co2+, Ni2+, Zn2+, Cu2+, Pb2+ and
Cd2+ solutions or adjusted to pH 2.2 with HNO3 in the case of
Fe3+. The ionic strength was settled as constant at I = 0.1 mol Lꢁ1
using KCl.
Ag|AgCl|internal electrolyte|membrane|sample|KNO3 1 mol Lꢁ1|KCl
1 mol Lꢁ1|AgCl|Ag and a 16 – channel LAWSON LAB potentiometer
(16 EMF USA).
The extractive properties of the ligands studied were investi-
gated following the procedure described in [5,20]: 25 mL of an
1.06 ꢂ 10ꢁ4 mol Lꢁ1 aqueous metal ion solution and 5 mL of the
5.3 ꢂ 10ꢁ4 mol Lꢁ1 ligand solution in CH2Cl2 were mechanically
shaken in a stoppered glass tube immersed in a thermostated
water bath at 30 °C for about 12 h. After that time the two phases
were separated and the aqueous layer was analyzed by atomic
absorption spectrometry and the absorbance A was measured.
The absorbance A0 of a blank experiment performed in the absence
of the ligand (aqueous phase extracted with pure dichlorometh-
2.5.2. Electrode characteristics and selectivity coefficients
The potentiometric measurements of heavy and transition
metal ions were performed at pH 4, adjusted with HNO3. Activity
coefficients were estimated using the semi-empirical Pitzer’s
model which is able to describe the non-ideal behavior of the elec-
trolytes up to high concentrations [21–24]. The single ion activities
were calculated using PHREEQC software version 2.17 [25]. The ion
activity was calculated by taking into account the complexation
with Clꢁ, NOꢁ3 and OHꢁ (hydrolysis). The selectivity coefficients
were determined by the Separate Solution Method (SSM) according

92
J. Kulesza et al. / Polyhedron 77 (2014) 89–95
to the procedure described by Bakker et al. and were calculated
according to the following equation [26,27].
As expected for siderophore-like compounds, they are all good
extractants for Fe3+ (%E P 59.8) at low pH (2.2). In particular,
ligand 4 extracts almost quantitatively this cation (%E = 89.3%).
Some experiments were performed at pH 2.2 with the rest of cat-
ions but no significant extraction was observed which is in agree-
ment with the results obtained by Shinkai and co-workers [5]. On
the other hand, at pH 5.4 most of the cations were fairly extracted.
A remarkable result is the almost quantitative extraction of Cu2+
E0M ꢁ EPb0
pot
Pb;M
log K
¼
SPb
where E0M and EP0b are the potentials corresponding to the interfering
metal cation and Pb2+, respectively; S is the experimental value of
the slope of the electrode for Pb2+
.
(%E P 88) with all ligands. The other divalent cations Co2+, Ni2+
,
In order to obtain unbiased values of the selectivity coefficients,
the calibration of the electrodes was performed starting preferably
from the most discriminating cation.
Zn2+, Pb2+ and Cd2+ are in most cases fairly extracted, however less
than Cu2+. No clear cut effect of extraction percentage is observed
when an additional functional group is present in the calixarene
molecule. Actually, the extractive properties depend on the substi-
tution of the nitrogen atoms in the hydroxamic acid moiety. With
unsubstituted compounds 1 and 3, %E values are similar except for
Pb2+ and Cu2+ which are better extracted by 1 than by 3. With the
methylated compounds 2 and 4 Co2+, Ni2+, Pb2+ and especially Cu2+
are better extracted by 4 than by 2. For both tris and tetrakis com-
pounds the unsubstituted 1 and 3 are always much more efficient
than the methylated compounds 2 and 4, except for Cu2+ which is
better extracted by 4 (%E = 98) than by 3 (%E = 89). This is globally
in agreement with the results of Dasaradhi et al. [31] who observed
that compounds bearing secondary hydroxamate functions were
more efficient extractants than those with tertiary hydroxamate
substituents.
3. Results and discussion
3.1. Synthesis
Ligands 1–4 were obtained with good yields (60–82%) via the
mixed anhydrides method. The cone conformation of the synthe-
sized calixarene-derivatives were confirmed by the caracteristic
splitting pattern of the methylene bridge prótons in the 3.2–
4.9 ppm region in their 1H NMR spectra with the coupling constant
around JH–H = 13 Hz. Those patterns are similar to that observed in
the literature for tetrakis- and tris-substituted derivatives of
calix[4]arene [28–30].
The purity of compounds was proved by 1H NMR spectroscopy
and elemental analysis.
3.2.2. Competitive extraction
Competitive metal ion extraction experiments were performed
in order to determine the potential of the synthesized ligands as
selective extractive agents for Pb2+ in the presence of Cd2+ and
for Cu2+ in the presence of Zn2+ and Ni2+, cations which are usually
present together.
The results of the competitive extraction experiments are given
in Table 2 together with those of the individual extraction experi-
ments, recalled in brackets.
3.2. Liquid–liquid extraction studies
3.2.1. Individual extraction
The extractive properties of calix[4]arene-hydroxamic acids (1–
4) were studied using the nitrate extraction method at pH 5.4
except for iron, as precipitation occurred above pH 4. In this case,
the pH was lowered to 2.2 with HNO3. Data of the individual
extraction experiments are presented in Table 1. Literature data
concerning the extraction properties of ligand 3 from water into
chloroform are also shown [5]. It can be seen that for ligand 3,
the percentage extraction of Co2+ and Zn2+ from water into diclo-
romethane and from water to chloroform at the same pH 5.4, are
similar and the values are 89.0 and 90, for Cu2+, whereas for Zn2+
are 60% and 63%, respectively. The bigger difference in percentage
With ligand 1 the percentages of Pb2+ and Cd2+ extraction
decrease when the two cations are present together in the solution.
Nevertheless, Pb2+ is still extracted at almost 70% demonstrating a
quite good selectivity over Cd2+. Interesting results were obtained
with ligand 3 for Pb2+ extraction in the presence of Cd2+. In this
case, Pb2+ is almost quantitatively extracted, showing higher per-
centage extraction than in individual experiments. In contrary,
the percentage of Cd2+ extraction decreases as compared to the
individual extraction. Thus, a very high selectivity for Pb2+ over
Cd2+ can be observed. It is evident that the presence of Cd2+ tends
to increase the percentage extraction of Pb2+. Although a synergic
effect is generally ascribed to the situation where two extractants
are present together, it can be assumed, in a way, that similar effect
occurred in this case. A salting effect present due to an excess of
nitrate could also explain these observations. This result is very
important for the removal of Pb2+ and its separation from other
toxic metal ions like Cd2+ in this particular pH conditions.
In the case of ligands 2 and 4, the competitive extraction con-
extraction might be observed for Co2+
(D%E = 8.2) which is slightly
better extracted into chloroform than into dichloromethane in the
same experimental conditions. On the other hand, Ni2+ is much less
extracted into chloroform (17%) than into dichloromethane (54.2%)
also in the same experimental conditions. The percentage extrac-
tion for Fe3+ was compared with the literature data for the exper-
iments conducted at the same value of pH 2.2. The slightly better
Fe3+ extraction was achieved from H2O into CH2Cl2 with
%E = 59.8 compared to %E = 54 for H2O/CHCl3 system.
Table 1
firmed the low %E values of Cd2+ and Pb2+
.
Percentage extraction (%E) of metal nitrates (CM = 1.06 ꢂ 10ꢁ4 mol Lꢁ1) with ligands
1–4 (CL = 5.3 ꢂ 10ꢁ4 mol Lꢁ1) from water into dichloromethane (Organic to aqueous
phase ratio o/a = 5, T = 30 °C, pH 5.4 or 2.2 in the case of Fe3+).
It can be seen from the Table 2 that in all cases Cu2+ is still
almost quantitatively extracted in the presence of Zn2+ and Ni2+
cations. A significant decrease of extraction percentages of Zn2+
and Ni2+ can be observed in all cases. Ligands 2 and 4 show the best
selectivity for Cu2+ over Zn2+ and Ni2+ and, therefore, they could be
successfully used in separation processes.
Ligand/cation
Fe3+
Co2+
Ni2+
Cu2+
Zn2+
Cd2+
Pb2+
1
2
70.7
63.1
59.8
54
48.1
2.1
45.8
54
52.1
15.4
54.2
17
97.7
88.5
89.0
88
62.4
61
60
63
1.6
32.1
5.5
32.7
nd
85.4
2.6
61.6
nd
3
3a
4
89.3
6.2
41.9
98.0
5.5
8.5
3.3. Potentiometric response and membrane selectivity
‘‘nd’’ – not determined.
Values with uncertainties less than 5%.
Compounds 1 and 3, unsubstituted on the nitrogen atoms, were
poorly soluble in THF what precluded the possibility of their
application as ionophores in ion-selective membrane electrodes.
a
Extraction H2O–CHCl3, data from Ref. [5] (CM = 1.06 ꢂ 10ꢁ4 mol Lꢁ1; CL = 5.3 -
ꢂ 10ꢁ4 mol Lꢁ1; T = 30 °C, pH 5.4 or 2.2 in the case of Fe3+).

J. Kulesza et al. / Polyhedron 77 (2014) 89–95
93
Table 2
Percentage of competitive extraction (%E) from water into dichloromethane of mixtures: Pb2+/Cd2+ and Cu2+/Zn2+/Ni2+ (CM = 1.06 ꢂ 10ꢁ4 mol Lꢁ1 of each metal) with ligands 1–4
(CL = 5.3 ꢂ 10ꢁ4 mol Lꢁ1) (Organic to aqueous phase ratio o/a = 5, T = 30 °C, pH 5.4). In brackets, percentage of individual extraction.
Ligand/metal
Cd2+
Pb2+
Ni2+
Cu2+
Zn2+
1
2
3
4
12.8 (32.1)
2.6 (5.5)
7.7 (32.7)
2.6 (5.5)
69.6 (85.4)
1.7 (2.6)
95.5 (61.6)
5.2 (8.5)
21.0 (52.1)
2.6 (15.4)
15.8 (54.2)
5.3 (41.9)
98.6 (97.7)
89.9 (88.5)
99.5 (89.0)
99.1 (98.0)
20.0 (62.4)
2.2 (61)
23.3 (60.0)
61 (1.6)
Therefore, only ligands 2 and 4 were tested as ionophores in the
membrane of ISEs. The ligands differ by the number of substitu-
ents, three for ligand 2, four for ligand 4. Two plasticizers BBPA
and NPOE of different dielectric constant (5.6 and 24, respectively)
were used. The exemples of characteristics of the ion selective elec-
trode based on 2/BBPA and 4/NPOE membranes for Pb2+, Na+ and
Ca2+ cations are illustrated in Fig. 2 and 3. The full characteristic
parameters of the electrodes studied are presented in Table 3.
The results showed that the studied calix[4]arene-hydroxamic
acids (2 and 4) act as Pb2+-selective ionophores. The electrodes
showed good response for Pb2+in both membranes, PVC/NPOE
and PVC/BBPA.
The electrode based on ionophore 4 showed close to Nernstian
slope value in Pb2+ solution (ꢃ30 mV/dec) within a linear range
10ꢁ2–10ꢁ5 mol Lꢁ1 in both plasticizers. For Na+ and K+ cations,
the electrodes showed still acceptable but less good parameters
especially with NPOE as plasticizer (sub-nernstian slope and nar-
row linear range).
Fig. 3. Response of the ion selective electrode based on 4/NPOE membrane for Pb2+
,
The electrodes based on ligand 2 present a similar behavior
towards Na+ and Ca2+ cations. However, for Pb2+ the slope is typical
for monovalent cations. Preliminary results on the behavior of
electrodes incorporating the ionophore 4 showed close to Nerns-
tian response in Pb2+ solution, whereas those based on ionophore
2 presented a typical monovalent cations slope (SPb = 58.9 mV/
dec) [15]. In this work we confirmed those results. Such a behavior
has already been reported by Lindner et al. and was interpreted as
a sensitivity towards PbX+ species (where X: OHꢁ, Clꢁ, NO3ꢁ, CH3-
COOꢁ) [32]. These authors suggested that the contribution to the
electrode potential is divided among the monovalent form of the
lead complex as well as the free Pb2+ cations. On the other hand,
it has been later shown that the monovalent, divalent or even
mixed response is depending on the molar ratio between ionic
sites and ionophore [33]. Suzuki et al. suggested that during com-
plexation process one proton is dissociated and released from the
Na+ and Ca2+ cations.
membrane which results in monovalent behavior of the divalent
selective electrodes [34]. Later, theoretical consideration showed
that double nernstian slope is possible if ionophore has acid/base
properties which result in various slope depending on the pH of
the sample and ionophore pK [35].
The response for Cu2+ and Cd2+ cations of the electrodes based
on ligand 2 or 4 were in all cases of sub-nernstian slope within nar-
row linear range which might indicate weak interaction between
ionophores and those cations in the membrane.
The potentiometric selectivity coefficients, determined by the
Separate Solution Method (SSM), are presented in Table 4 and
Fig. 4.
These results proved that the electrodes are selective for the
Pb2+ cations.
It can be seen that generally better selectivity coefficients were
obtained with PVC/NPOE membranes than with PVC/BBPA mem-
branes, which is in agreement with the results reported earlier
showing that NPOE is preferred for Pb-sensors (and other divalent
cations), whereas BBPA seems to be the best plasticizer for Na+ or
Li+ sensors [36]. These results may be explained by the strong Pb2+
cation/ionophore interaction in the membrane containing NPOE
due to the high dielectric constant of this plasticizer [37].
Exceptionally, better Pb2+/Cd2+ selectivity coefficients were
obtained in PVC/BBPA membrane which suggests stronger affinity
of Cd2+ for NPOE than for BBPA plasticizer.
The best selectivity was obtained using the 2/NPOE membrane.
Selectivity coefficients over Na+ and Cu2+ are particularly good
pot
Pb;Na
pot
Pb;Cu
(log K
¼ ꢁ5:67, log K
¼ ꢁ5:53, respectively), whereas Ca2+
pot
Pb;Ca
is the most interfering ions with log K
¼ ꢁ2:76.
The electrode with the membrane 2/BBPA is less selective and
the most interfering cation is Na+ (log Kpot ¼ ꢁ1:02). A similar
Pb;Ca
situation occurs with the membrane containing 4, where better
selectivity was obtained using NPOE. In both membranes with 4,
Fig. 2. Response of the ion selective electrode based on 2/BBPA membrane for Pb2+
,
the most interfering cation is Ca2+ (log Kpot ¼ ꢁ1:13 with BBPA
Na+ and Ca2+ cations.
Pb;Ca

94
J. Kulesza et al. / Polyhedron 77 (2014) 89–95
Table 3
Characteristics of the studied ion-selective electrodes with ionophores 2 and 4.
Ion of preference
2/BBPA
2/NPOE
4/BBPA
4/NPOE
S
r* (mV/dec)
LR ꢁloga
S
r* (mV/dec)
LR ꢁloga
S
r* (mV/dec)
LR ꢁloga
S
r* [mV/dec]
LR ꢁloga
Pb2+
Ca2+
Na+
K+
60.3 1.8
27.3 1.7
57.8 0.2
54.5 0.5
21.2 1.1
20.7 2.8
2–5
1–5
1–5
1–3
2–3
2–3
59.0 2.6
29.5 0.6
43.1 0.9
48.7 0.8
19.5 1.2
18.9 2.6
2–5
1–5
1–3
1–2
2–3
2–3
31.2 1.3
32.1 0.9
49.9 0.5
43.4 0.8
20.8 1.1
19.9 0.9
2–5
2–5
1–3
1–3
2–3
2–3
29.2 0.7
27.5 1.5
47.5 1.5
48.9 1.6
22.3 0.9
20.2 1.7
2–5
2–3
1–2
1–2
2–3
2–3
Cd2+
Cu2+
*
Standard deviation of five independent measurements.
Table 4
cations, the electrode 2/NPOE could be successfully used to deter-
pot
Pb;M
*
mine the concentration of Ca2+ ions in real samples.
Potentiometric selectivity coefficients (log K
ionophores 2 and 4.
ꢄ r ) of the electrodes based on
Ion
2/BBPA
0.00
ꢁ1.02 0.21
ꢁ2.02 0.21
ꢁ4.82 0.29
ꢁ1.10 0.15
ꢁ3.74 0.29
2/NPOE
0.00
ꢁ5.67 0.21
ꢁ3.97 0.08
ꢁ3.46 0.05
ꢁ5.53 0.23
ꢁ2.76 0.06
4/BBPA
0.00
ꢁ2.46 0.26
ꢁ2.89 0.28
ꢁ4.24 0.06
ꢁ3.11 0.32
ꢁ1.13 0.03
4/NPOE
0.00
ꢁ3.85 0.11
ꢁ3.55 0.06
ꢁ3.36 0.17
ꢁ4.72 0.39
ꢁ2.11 0.02
4. Conclusions
Pb2+
Na+
K+
In this paper, p-tert-butylcalix[4]arene appended with hydroxa-
mic acid moieties were prepared, among which, the ligand 1 is
reported here for the first time.
Cd2+
Cu2+
Ca2+
The ligands studied here were shown to be efficient extractants
of Fe3+ and Cu2+. For the rest of the cations tested, better extraction
was observed in the case of ligands 1 and 3, unsubstituted on nitro-
gen atoms, with particularly high Pb2+ extraction level.
Competitive extraction experiments revealed that for ligand 3,
the extraction of Pb2+ in the presence of Cd2+ was almost quantita-
tive and higher than for individual experiments, whereas the per-
centage of Cd2+ extraction was lower compared to the individual
extraction. It is very important in the context of the removal and
separation of Pb2+ from other toxic metal ions such as Cd2+. In addi-
tion, it was shown that ligands 2 and 4 possess the best selectivity
for Cu2+ over Zn2+ and Ni2+ and could be successfully used in
separation processes of Cu2+ from Zn2+ and Ni2+, often present
together.
*
Standard deviation of five independent measurements.
Despite weak Pb2+ extraction, ligands 2 and 4 act as Pb2+-selec-
tive ionophores. The studies in ion-selective electrodes showed
that a particularly good selectivity was found with 2/NPOE mem-
branes for Pb2+ over Na+, Cd2+ and Cu2+ and the electrodes based
on ionophore 2 might be useful in analytical application.
Acknowledgements
pot
Pb;M
Fig. 4. Potentiometric selectivity coefficients (log K
ligands 2 and 4.
) for the electrodes based on
The authors thank Gdansk University of Technology and
University of Strasbourg for financial support.
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the selectivity coeffcients of ligands 2 and 4 is difficult since the
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