Synthesis and dichromate anion extraction ability of p-tert-butylcalix[4]arene diamide derivatives with different binding sites

Article abstract of DOI:10.1016/j.tet.2007.03.138

The article describes the synthesis and evaluation of the dichromate anion (Cr₂O₇^2-/HCr₂O₇^-) extraction properties of p-tert-butylcalix[4]arene diamide derivatives (5-7) containing different binding sites. Among these compounds, 6 and 7 have been synthesized via aminolysis in a toluene-methanol solvent system with 3-aminomethylpyridine and 3,6-dioxa-1,8-diamino octane, respectively. On the other hand, compound 5 has been synthesized via an acid chloride method due to its inefficiency under aminolysis. The extraction properties of these diamides toward dichromate anions are studied by liquid-liquid extraction. The results show that p-tert-butylcalix[4]arene diamide derivative 7 exhibited a much higher affinity toward dichromate anions than that of 6 due to its special structure, while 5 was an ineffective ligand for these anions.

Full text of DOI:10.1016/j.tet.2007.03.138

Tetrahedron 63 (2007) 5000–5005
Synthesis and dichromate anion extraction ability of p-tert-
butylcalix[4]arene diamide derivatives with different binding sites
*
Aydan Yilmaz, Begum Tabakci, Ezgi Akceylan and Mustafa Yilmaz
Selc¸uk University, Department of Chemistry, 42031 Konya, Turkey
Received 9 November 2006; revised 3 March 2007; accepted 22 March 2007
Available online 27 March 2007
Abstract—The article describes the synthesis and evaluation of the dichromate anion (Cr2O27ꢀ/HCr2O7ꢀ) extraction properties of p-tert-
butylcalix[4]arene diamide derivatives (5–7) containing different binding sites. Among these compounds, 6 and 7 have been synthesized
via aminolysis in a toluene–methanol solvent system with 3-aminomethylpyridine and 3,6-dioxa-1,8-diamino octane, respectively. On the
other hand, compound 5 has been synthesized via an acid chloride method due to its inefficiency under aminolysis. The extraction properties
of these diamides toward dichromate anions are studied by liquid–liquid extraction. The results show that p-tert-butylcalix[4]arene diamide
derivative 7 exhibited a much higher affinity toward dichromate anions than that of 6 due to its special structure, while 5 was an ineffective
ligand for these anions.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
complexant especially for ions in aqueous solution. Al-
though there are numerous examples of molecules that act
as hosts and complexants for cations, relatively few mole-
cules have been reported as hosts for anions.6–9 Thus, the
development of an efficient extractant for anions has re-
ceived considerable attention in recent years.10
Toxic heavy metals like copper, mercury, chromium, lead,
nickel, and cadmium can have a serious impact on the aque-
ous environment, as well as on animals and humans. Chro-
mium and its compounds are widely used in the plating,
leather tanning, dye, cement, and photography industries,
producing large quantities of toxic pollutants.1 Chromium
can exist in several oxidation states, however, only the tri-
valent and hexavalent forms are environmentally important.2
Chromium(III) has been reported to be biologically essential
to mammals as it maintains effective glucose, lipid, and pro-
tein metabolism. Chromium(VI) can be toxic as it can dif-
fuse as Cr2O27ꢀ or HCr2O7ꢀ through cell membranes, and
oxidize biological molecules.3 The maximum permissible
levels of Cr(VI) in potable and industrial wastewaters are
0.05 and 0.25 mg/L, respectively.2 Due to its high solubility,
Cr(VI) is very toxic to living organisms compared to
Cr(III).4 When Cr(VI) is ingested beyond the maximum con-
centration, it can cause health disorders, such as vomiting
and haemorrhage.5 Therefore, treatment of wastewater con-
taining Cr(VI) prior to discharge is essential. Conventional
techniques for removing metal ions from wastewater include
chemical precipitation, membrane separation, reverse osmo-
sis, evaporation, electrochemical treatment, and solvent ex-
traction. Among them, solvent extraction is one of the most
commonly used treatment methods, and employs a selective
Calixarenes, cyclic oligomers of phenolic units linked
through the ortho positions, are a fascinating class of macro-
cycle. Chemical modification from the upper or lower rim
has made this class of synthetic ionophores as effective ex-
tractants for transferring anionic and cationic ions or neutral
molecules from aqueous solution into an organic layer. The
complexation properties of these molecules appear to be
highly dependent upon the nature and number of donor
atoms, and also upon the conformation of the calix[4]arene
moiety.11–14 Therefore, a variety of sophisticated anion com-
plexing ligands containing a calix[4]arene backbone have
been designed and synthesized for use as selective anion ex-
tractants.15–18 These molecules are generally calix[4]arene
derivatives bearing amine or amide functions, and are
capable of interacting with anions by hydrogen bonds or
cation–p interactions.19 In recent years, we have reported
calix[4]arene based receptors that effectively bind anions,
which can be useful in multiple applications such as labora-
tory, clinical, environmental, and industrial process analy-
ses.20–23 In the literature, generally, although amine
derivatives of calix[4]arene are known to be more effective
extractants than its amide derivatives for anions, the prepara-
tion of their amide derivatives is also important since their
stability is more than that of amine derivatives. Therefore,
herein, we report the syntheses and dichromate anion
Keywords: p-tert-Butylcalix[4]arene; Diamide; Dichromate anions; Extrac-
tion.
* Corresponding author. Tel.: +90 332 2232786; fax: +90 332 2410106;
e-mail: aydan@selcuk.edu.tr
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.138

A. Yilmaz et al. / Tetrahedron 63 (2007) 5000–5005
5001
(Cr2O27ꢀ/HCr2Oꢀ7 ) binding properties of p-tert-butylcalix-
[4]arene ionophores bearing different diamide functions,
which have often been claimed to act as binding sites in
the complexation of dichromate anions.
time, from the reaction of acid chloride 4 with tert-butyl
amine we obtained p-tert-butylcalix[4]arene diamide 5 in
87% yield. 5,11,17,23-tert-Butyl-25,27-ethoxycarbonyl-
methoxy-26,28-hydroxycalix[4]arene (2) was heated at reflux
with 3-aminomethylpyridine and 3,6-dioxa-1,8-diamino
octane to give the corresponding diamide derivatives of
p-tert-butylcalix[4]arene (6 and 7) in 73% and 56% yields,
respectively. A mixture of toluene–methanol (1:1) was em-
ployed as toluene facilitates the dissolution of diester, while
methanol is beneficial in transforming the ethyl ester to the
more reactive methyl ester prior to aminolysis.25
2. Results and discussion
2.1. Synthesis
In this work, we extend our previous studies and explore the
binding properties of p-tert-butylcalix[4]arene diamide
derivatives (5–7) bearing different binding sites toward di-
chromate anions. The synthetic route for the preparation of
p-tert-butylcalix[4]arene diamide derivatives is described
in Scheme 1. For the synthesis of these p-tert-butylcalix[4]-
arene diamides, the parent compounds 1–4 and 7 were
prepared according to published procedures.23,24 Two syn-
thetic methods, aminolysis25 and acid chloride methods,23
were tried for the synthesis of compound 5. In aminolysis,
no any amide derivatives could be obtained, maybe due to
steric hindrance of the amine derivative. Therefore, we tried
an acid chloride method to synthesize compound 5. This
The diamide derivatives 5–7 were characterized by a combi-
nation of FTIR, 1H NMR, FABMS, and elemental analysis.
The formation of the diamide derivatives of p-tert-butyl-
calix[4]arene (5–7) was confirmed by the appearance of the
characteristic amide bands at about 1680 cmꢀ1 and also by
the disappearance of ester carbonyl band at 1755 cmꢀ1 in
the IR spectra. The conformational characteristics of
calix[4]arenes were conveniently estimated by the splitting
pattern of the ArCH2Ar methylene protons in the 1H
NMR spectrum.26,27 1H NMR spectroscopic data showed
that compounds 5–7 were in the cone conformation.
OH
HO
OH
OH
i
1
Cl
HO
EtO
Cl
OH
OEt
O
O
O
O
O
O
OH
OH
OH
O
O
O
O
O
O
OH
OH
OH
ii
iii
4
3
2
v
vi
iv
N
N
CH3
C
O
CH3
O
C
C
H3
CH3
H3C
O
CH3
HN
NH
NH
NH
NH
HN
O
O
O
O
O
OH
OH
O
O
O
OH
OH
O
O
O
OH
OH
6
7
5
Scheme 1. (i) Ethylbromoacetate, K2CO3, acetone; (ii) NaOH, ethanol; (iii) oxalyl dichloride, THF, pyridine; (iv) tert-butyl amine, THF, pyridine; (v) 3-amino-
methylpyridine, toluene–methanol; (vi) 3,6-dioxa-1,8-diamino octane, toluene–methanol.

5002
A. Yilmaz et al. / Tetrahedron 63 (2007) 5000–5005
70
60
50
40
30
20
10
0
A typical AX pattern was observed for the methylene bridge
ArCH2Ar protons at d 3.39 and 4.11 ppm (J¼13.4 and
13.3 Hz, respectively) for 5, d 3.27 and 3.84 ppm (J¼
13.3 Hz) for 6, and d 3.28 and 4.12 ppm (J¼13.5 Hz) for 7
1
in H NMR. The high field doublets at d 3.39 ppm for 5,
d 3.27 ppm for 6, and d 3.28 ppm for 7 were assigned to
the equatorial protons of methylene groups, whereas the
low field signals at d 4.11 ppm for 5, d 3.84 ppm for 6, and
d 4.12 ppm for 7 were assigned to the axial protons in the
1H NMR.
2.2. Extraction studies
Dichromate anions are important because of their high tox-
icity and presence in soils and waters.23 For a molecule to be
effective as a host, it is necessary that its structural features
are compatible with those of the guest anions. The dichro-
mate ions (Cr2O27ꢀ/HCr2O7ꢀ) are anions where the periphery
of the anion has oxide moieties. These oxides are potential
sites for hydrogen bonding to the host molecule. It is known
that calix[4]arenes with a nitrogen functionality such as
amide, amino, and nitrile on their lower rim are efficient
extractants for oxoanions.20,21,28–31 We were interested in
synthesizing p-tert-butylcalix[4]arene diamide derivatives
containing different binding sites in the cone conformation,
and examining their extraction properties for dichromate
ions. The present work determines the strategic require-
ments for two-phase extraction measurements. A prelimi-
nary evaluation of the extraction efficiencies of 5–7 has
been carried out by solvent extraction of Na2Cr2O7 from
water into dichloromethane at the pH range of 1.5–4.5.
The results are summarized in Table 1.
1.5
2.5
3.5
4.5
pH
Figure 1. Plots of extraction (E%) versus pH following the two-phase
solvent extraction of dichromate anion with compounds 5–7.
100
80
60
40
20
7
0
6
The extraction data (Table 1, Figs. 1 and 2) showed that the
diamide derivatives 6 and 7 were effective extractants for
the extraction of dichromate anions at low pH while the
1.5
2.5
5
3.5
4.5
pH
Figure 2. Extraction percentages of dichromate anion with 5–7 at pH
1.5–4.5.
Table 1. Percentage extraction of dichromate by extractants 5–7 at different
pH valuesa,b
Compound
pH
extraction yield by 5 was negligible. The percentage of di-
chromate anions extracted was 24.8 for 6 and 64.4 for 7
when the pH of the aqueous solution was 1.5, and they at-
tained minimum <1.0 for 6 and 35.1 for 7 when pH of the
aqueous solution increased to 4.5. p-tert-Butylcalix[4]arene
diamide derivative 6 provides suitable binding sites for di-
chromate anions at low pH due to the presence of protonable
amine moieties. Therefore, an anion-switchable complex is
formed in the two-phase extraction system (Scheme 2),
1.5
2.5
3.5
4.5
5
6
7
<1.0
24.8ꢁ0.3
<1.0
11.5ꢁ0.2
<1.0
10.3ꢁ0.3
<1.0
<1.0
35.1ꢁ0.2
64.4ꢁ0.2
60.2ꢁ0.1
55.0ꢁ0.3
a
Averages and standard deviations calculated for data obtained from three
independent extraction experiments.
Aqueous phase, [metal dichromate]¼1ꢂ10ꢀ4 M; organic phase, di-
b
chloromethane, [ligand]¼1ꢂ10ꢀ3 M at 25 ꢃC, for 1 h.
Cr2O72-
-
HCr2O7
+
+
+
H
H
N
N
N
N
N
NH
NH
NH
-
NH
NH
NH
NH
+
HO
Cr O2-
/
HCr2O7
-
O
O
O
O
O
O
2
7
+
+
H
OH
OH
OH
O
O
O
OH
O
O
O
OH
OH
Scheme 2. The proposed interactions of compound 6 with dichromate anions.

A. Yilmaz et al. / Tetrahedron 63 (2007) 5000–5005
5003
because of the proton transfer to the nitrogen atom of the
O
O
O
O
amine unit in 6. This also reflects the fact that the complex
with dichromate ions is more stable in low pH medium.
According to our knowledge the increase in extraction
efficiency of 7 can be attributed to a number of reasons.
Compound 7 possesses an amide nitrogen, carbonyl, and
oxa oxygen, facilitating hydrogen bonding with the dichro-
mate anion. The next reason is that 7 has a more stable struc-
ture because of the bridging of the two amide moieties by
a 1,8-dioxa octyl unit. Therefore, one reason that the extrac-
tion ability of 6 is less than that of 7, maybe that 6 has a more
flexible structure. Another reason is that the amide oxygen
can be protonated32 at low pH, although this is more difficult
than in 6. This also implies a decrease in extraction effi-
ciency of 7 with increasing pH of the solution, due to the
fact that protonation decreases, especially at pH 4.5. Conse-
quently, all these causes can cooperatively contribute to the
higher extraction efficiency of 7 over 6. On the other hand,
the ineffectiveness of 5 can be attributed to steric hindrance
by its bulky groups.
Cr2O27-
HCr2O7-
H
H
N
N
H
H
N
N
+
+
+
H
H
O
O
H
O
O
+
OH
O
O
OH
O
O
OH
OH
Scheme 3. The proposed interactions of compound 7 with dichromate
anions.
simultaneous extractions of 1:1 (Eqs. 1 and 2) complexes
according to the following equilibria (Schemes 2 and 3):
log Kex
LHþðorgÞ þ HCr2O7ꢀðaqÞ
PLHþðHCr2OꢀÞR
ð1Þ
ðorgÞ
7
log Ke0x
LH22þðorgÞ þ Cr2O72ðꢀaqÞ
PLH22þðCr2O2ꢀÞR
ð2Þ
All these data have been analyzed using the classical slope
analysis method.23 Figure 3 represents the extractions into
dichloromethane at different concentrations of 6 and 7 for
the dichromate anion. A linear relationship between log D
versus log[L] is observed with a slope for dichromate by 6
and 7, which equals 1.07 and 1.25 at pH 1.5, respectively,
suggesting that 6 and 7 form 1:1 complexes with dichromate
anions.
ðorgÞ
7
According to these assumptions, the extraction constants
(Kex) have been calculated from the experimental data.
Calculations of these constant values lead to log Kex¼
log K0ex¼2.74ꢁ0.2 for 6 and log Kex¼log K0ex¼3.96ꢁ
0.2 for 7.
However, it is well known that at more acidic conditions
Na2Cr2O7 is converted into H2Cr2O7, and after ionization
in an aqueous solution it exists in the HCr2Oꢀ7 /Cr2O72ꢀ
form. At higher acidic conditions HCr2Oꢀ7 and Cr2O72ꢀ di-
mers become the dominant Cr(VI) form, and pKa1 and
pKa2 values of these equations are 0.74 and 6.49, respec-
tively. It is apparent to us that the ligands 6 and 7 form com-
plexes mostly with HCr2Oꢀ7 ion. This as well as the
explanations above have allowed us to consider these
3. Conclusions
In conclusion, the synthesis and dichromate anion extraction
ability of p-tert-butylcalix[4]arene based receptors 5–7 were
studied. The spectroscopic data indicated that the diamide
compounds (5–7) are in the cone conformation. The dichro-
mate anion complexation studies showed that compounds 6
and 7 were effective receptors while compound 5 was not
useful for Cr2O27ꢀ/HCr2O7ꢀ anions. The extraction efficiency
of 7 was much higher than that of 6. It could be concluded
that the complexation of Cr2O27ꢀ/HCr2O7ꢀ anions depends
on the structural properties of the receptor such as hydrogen
binding ability, stability or rigidity, and protonation ability.
The calixarene based receptors could be proved to find
remarkable applications in the design of chemical sensors,
using electrochemical transduction/as conventional ion
selective electrodes (ISE) and solid-state sensors (ISFETs).
7
0,3
6
0
-0,3
-0,6
-0,9
-1,2
-1,5
4. Experimental
4.1. General
Melting points were determined on an Electrothermal 9100
apparatus in a sealed capillary and are uncorrected. 1H NMR
spectra were recorded on a Bruker 400 MHz spectrometer in
CDCl3 with TMS as an internal standard. IR spectra were
obtained on a Perkin–Elmer 1605 FTIR spectrometer using
KBr pellets. UV–visible spectra were obtained on a Shi-
madzu 160A UV–visible spectrophotometer. Elemental
analyses were performed using a Leco CHNS-932 analyzer.
FABMS spectra were taken on a Varian MAT 312
-3,7
-3,5
-3,3
Log [L]
-3,1
-2,9
Figure 3. Log D versus log[L] for the extraction of dichromate anions by the
ligands 6 and 7 from an aqueous phase into dichloromethane at 25 ꢃC.

5004
A. Yilmaz et al. / Tetrahedron 63 (2007) 5000–5005
spectrometer. An Orion 420A+ pH meter was used for the
pH measurements. Analytical TLC was performed using
Merck prepared plates (silica gel 60 F254 on aluminum).
Flash chromatography separations were performed on
a Merck silica gel 60 (230–400 mesh). All reactions, unless
otherwise noted, were conducted under a nitrogen atmo-
sphere. All starting materials and reagents used were of
standard analytical grade from Fluka, Merck, and Aldrich,
and used without further purification. Toluene was distilled
from CaH2 and stored over sodium wire. Other commercial
grade solvents were distilled, and then stored over molecular
sieves. Anions were used as their sodium salts. The drying
agent employed was anhydrous MgSO4. All aqueous solu-
tions were prepared with deionized water that had been
passed through a Millipore milli-Q Plus water purification
system.
(18H, s, tBu), 3.27 (4H, d, J 13.3 Hz, ArCH2Ar), 3.84 (4H,
d, J 13.3 Hz, ArCH2Ar), 4.46–4.48 (8H, m, OCH2, Ar–
CH2–NH), 6.77 (4H, s, ArH), 6.94 (4H, s, ArH), 7.04–7.07
(2H, m, PyH), 7.18 (2H, s, OH), 7.56 (2H, d, J 7.8 Hz,
PyH), 8.40 (2H, d, J 4.5 Hz, PyH), 8.55 (2H, d, PyH), 9.10
(2H, t, NH); FABMS m/z: [M+Na]+, found: 968.2.
5. Analytical procedure
The dichromate anion extraction experiments of p-tert-
butylcalix[4]arene diamide derivatives 5–7 were performed
following Pedersen’s procedure.33 An aqueous solution of
sodium dichromate (10 mL of a 1ꢂ10ꢀ4 M; 0.01 M KOH–
HCl solution was used in order to obtain the desired pH at
equilibrium) and calixarene ligand (10 mL of 1ꢂ10ꢀ3 M)
in CH2Cl2 were shaken vigorously in a stoppered glass
tube with a mechanical shaker for 2 min and then magneti-
cally stirred in a thermostated water bath at 25 ꢃC for 1 h,
and finally left standing for an additional 30 min. The con-
centration of dichromate ion remaining in the aqueous phase
was then determined spectrophotometrically as described
previously.23 Blank experiments showed that no dichromate
extraction occurred in the absence of calix[4]arene. The per-
centage extraction (E%) was calculated from the absorbance
A of the aqueous phase measured at 346 nm (for pH 1.5–4.5)
using the following expression:
4.2. Synthesis
Compounds 1–4 and 7 were synthesized according to previ-
ously described methods.23,24 The synthesis of the other
compounds 5 and 6 was firstly reported in this study.
4.2.1. Synthesis of p-tert-butylcalix[4]arene diamide 5.
5,11,17,23-tert-Butyl-25,27-bis(chlorocarbonylmethoxy)-
26,28-hydroxycalix[4]arene 4 (6.2 g, 5.44 mmol) was dis-
solved in dry THF (100 mL). The addition of pyridine
(1 mL) and the solution of tert-butyl amine (3.3 mL,
31 mmol) in THF (25 mL) was made sequentially and added
dropwise over about 1 h with continuous stirring at room
temperature. The reaction mixture was then stirred and
heated at reflux for 5 h, after which most of the solvent
was distilled off in vacuo. The residue was diluted with water
(200 mL) and neutralized by 0.1 M HCl. The solid material
was then filtered and washed with 2 M HCl, NaHCO3, and
distilled water sequentially. Recrystallization of residue
from ethanol furnished 5 (4.14 g, 87%) as white crystals,
mp 287 ꢃC. [Found: C, 77.01; H, 9.07; N, 3.27.
C56H78N2O6 requires: C, 76.84; H, 8.98; N, 3.20%.] nmax
(KBr pellet): 3470, 3379, 1683 cmꢀ1; dH (400 MHz,
E% ¼ ½ðA0 ꢀ A=A0Þꢄ ꢂ 100
ð3Þ
where A0 and A are the initial and final concentrations of the
dichromate ion before and after the extraction, respectively.
Acknowledgements
We thank the Scientific and Technical Research Council of
Turkey (TUBITAK-Grant Number TBAG-104T199) and
Research Foundation of Selc¸uk University, Konya, Turkey
for financial support of this work.
t
t
CDCl3): 0.94 (18H, s, Bu), 1.30 (18H, s, Bu), 1.51 (18H,
s, Bu), 3.39 (4H, d, J 13.4 Hz, ArCH2Ar), 4.11 (4H, d,
t
References and notes
J 13.3 Hz, ArCH2Ar), 4.43 (4H, s, OCH2), 6.80 (4H, s,
ArH), 7.01 (2H, s, OH), 7.09 (4H, s, ArH), 8.31 (2H, s,
NH); FABMS m/z: [M+Na]+, found: 898.2.
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