Synthesis and structural characterization of bisazocalix[4]arene with melamine: Metal ion extraction studies

Article abstract of DOI:10.1016/j.molliq.2014.12.020

In this study, the synthesis and characterization of novel bisazocalix[4]arene [2-amino-4,6-bis(5-azo-25,26,27- tribenzoyloxy-28-hydroxycalix[4]arene)-1,3,5-triazine] (5), symmetrically derived from the diazo coupling of 2,4,6-triamino-1,3,5-triazine (mel

Full text of DOI:10.1016/j.molliq.2014.12.020

Journal of Molecular Liquids 202 (2015) 134–140
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis and structural characterization of bisazocalix[4]arene with
melamine: Metal ion extraction studies
b
c
a
Serkan Elçin a, , Özlem Özen Karakuş , İzzet Kara , Hasalettin Deligöz
⁎
a
Pamukkale University, Faculty of Engineering, Department of Chemical Engineering, 20070 Denizli, Turkey
Pamukkale University, Faculty of Science, Department of Chemistry, 20017 Denizli, Turkey
Pamukkale University, Faculty of Education, Department of CITE, 20017 Denizli, Turkey
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
In this study, the synthesis and characterization of novel bisazocalix[4]arene [2-amino-4,6-bis(5-azo-25,26,27-
tribenzoyloxy-28-hydroxycalix[4]arene)-1,3,5-triazine] (5), symmetrically derived from the diazo coupling of
2,4,6-triamino-1,3,5-triazine (melamine), were carried out. This compound was prepared by reacting 25,26,27-
tribenzoyloxy-28-hydroxycalix[4]arene (3) and melamine (4) in nitrosyl sulfuric acid and DMF. Purified product
was characterized by elemental analyses, FT-IR, 1H-NMR, MS and thermogravimetric analysis (TGA) techniques.
Complexation properties of synthesized compound were studied by the liquid–liquid extraction of selected metal
cations (Na+, K+, Sr2+, Ag+, Hg+, Hg2+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+, Zn2+, Cr3+, Al3+). It has been observed
that bisazocalix[4]arene shows high affinity to Hg+ and Hg2+ ions, whereas almost less affinity to other metals.
This sensor therefore present a very significant advantage in that it allows for determination of mercury ion in
environmental and industrial wastewater.
Received 1 September 2014
Received in revised form 5 November 2014
Accepted 14 December 2014
Available online 16 December 2014
Keywords:
Calixarene
Azocalix[4]arenes
Bisazo
Spectroscopic properties
Liquid–liquid extraction
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
promote their complexation ability toward transition and heavy metal
ions such as the replacement of the amide carbonyl oxygen atoms of
Calix[4]arenes belong to the class of compounds known as
metacyclophanes and are synthetic cyclic tetramers composed of four
phenolic and four methylene moieties [1]. They have long been of inter-
est both as complexation hosts for ions and molecules, and as frame-
works for elaborating more complex structures. Functional groups and
calix core cavity system of calix[4]arenes, constrained to the 1,3-alter-
nate conformation, have received more attention due to possible appli-
cations as receptors for ions and neutral molecules [2]. This kind of
compounds was found to be exceptionally selective for various metal
cations in possible applications such as nuclear-waste remediation [3].
Nowadays, specific consideration is given to the control of toxic
heavy metal-ion content, like lead, cadmium, copper, silver, mercury,
chromium or arsenic in natural waters, which may cause great risk to
human health [4–6]. This study aimed to design bis and/or tris
azocalix[4]arene compounds possessing the ability of binding these
metal cations.
amide groups by sulfur atoms leading to thioamides [11]. The X-ray crys-
tal structure of p-tert-butylcalix[4]arene diethylthioamide's lead complex
showed that Pb2+ was bound to all four ethereal oxygens and four
thiocarbonyl sulfur atoms of the ligand in the cone conformation [12].
Various studies on the syntheses of azocalix[n]arenes for related ap-
plications have been published previously [13–15]. In our laboratory,
azocalix[4]arene derivatives have been synthesized and used as
extractants in liquid–liquid extraction of transition metal ions (Ag+
,
Hg2+ and Hg+) [16–18]. Lu et al. [19] have obtained calix[4]arene
carboxyphenylazo derivative to detect lead cation. Potassium and cesi-
um selective azocalix[4]crown derivatives have been reported by Kim
et al. [20] and Chawla et al. [21] while a nickel selective azocalix[4]
arene derivative has been reported by Ma and coworkers [22].
The first study, using calixarenes to build dendrimers has been re-
ported by Lhotak and Shinkai in 1995 which introduces a series of
oligo-calixarenes linked through the phenolic oxygen with the help of
aliphatic chains (lower rim-lower rim connections) [23].
Previous studies have shown that versatile ionophoric properties
of chemically modified azocalixarenes depend greatly on the nature
of the different substituents attached to the calixarene scaffold
[7–10]. The introduction of soft donor atoms such as phosphorus or
sulfur atoms into calix[4]arene substituents has been shown to
This study reports the synthesis and the characterization of a novel
azocalix[4]arene appended with melamine (Scheme 1). The structural
characterization of dendrimeric azocalix [4]arene and its liquid–liquid ex-
traction results are also presented as follows. The binding properties of
this novel compound toward a selection of metal ions (Na+, K+, Sr2+
,
Ag+, Hg+, Hg2+, Co2+, Ni2+, Cu2+, Cd2+, Pb2+, Zn2+, Cr3+, Al3+) have
⁎
Corresponding author.
E-mail address: serel20@mynet.com (S. Elçin).
been investigated by liquid–liquid extraction of corresponding metal
http://dx.doi.org/10.1016/j.molliq.2014.12.020
0167-7322/© 2014 Elsevier B.V. All rights reserved.

S. Elçin et al. / Journal of Molecular Liquids 202 (2015) 134–140
135
O
C
O
AlCl3
+
+
NaOH
Toluen
H
Re ux
H
HO
OH
OH
OH
OH
HO
OH
OH
(1)
OH
(2)
O
C
Cl
Pyridine
ice bath
C O
O
C O
O
C
O
O
OH
H2N
N
N
N
H2N
N
N
N
N
N
HO
O
O C
O
O C
O
O C
NaNO2 / H2SO4
DMF
NH2
+
N
N
H2N
(5)
(4)
(3)
HO
O
O C
O
C
O
O C
O
Scheme 1. Synthesis of bisazocalix[4]arene.
picrates from water into chloroform. The results have been compared to
those previously obtained with the azocalix[4]arene [18], which is re-
synthesized in this study.
(Merck), sulfuric acid (Sigma-Aldrich), acetic acid (Merck), HCl (Sigma-
Aldrich), and NaOH (Merck), were acquired commercially and used
without further purification. Melting points of the compounds were mea-
sured by Electrothermal IA9100 digital melting point apparatus in capil-
laries. 1H-NMR spectra were referenced to tetramethylsilane (TMS) at
0.00 ppm as internal standard and were recorded on a Bruker 400 MHz
spectrometer at room temperature (25 1 °C). Mass spectroscopy was
performed on Bruker Daltonics of flight LC/MS. IR spectra were recorded
on a Mattson 1000 FT-IR spectrometer using KBr pellets. UV–Vis spectra
were obtained on a Shimadzu 160A UV–Visible recording spectropho-
tometer. The elemental analyses were performed in the TUBITAK Labora-
tory (Center of Science and Technology Research of Turkey).
2. Experimental
The solvents, ethyl acetate (Sigma-Aldrich), hexane (Merck), DMF
(Merck), benzene (Sigma-Aldrich), acetone (Sigma-Aldrich), chloroform
(Sigma-Aldrich), ethanol (Sigma-Aldrich), DMSO (Merck), and benzene
(Merck) were acquired commercially and used without further purifica-
tion. The compounds, 2,4,6-triamino-1,3,5-triazine (Merck), sodium ni-
trite (Merck), sodium acetate (Merck), TLC (Merck), glacial acetic acid
Fig. 1. The IR spectrum of bisazocalix[4]arene (5).

136
S. Elçin et al. / Journal of Molecular Liquids 202 (2015) 134–140
Fig. 2. The 1H NMR spectrum of bisazocalix[4]arene (5).
2.1. Preparation of the calix[4]arenes
completion of diazotization, diazo liquor was slowly added to a vigor-
ously stirred solution of 25,26,27-tribenzoyloxy-28-hydroxy calix[4]
arene (1.0 g, 1.36 mmol) in 20 mL DMF. The pH of the reaction mixture
was maintained at 7–8 by simultaneous addition of solid sodium acetate
in portions. The mixture was then stirred for 15 h at 0–5 °C. The progress
of the reaction was followed by TLC using an ethyl acetate–hexane (4:6,
v/v). The resulting solid was filtered, washed with cold water and dried
consecutively. Recrystallization from DMF–H2O mixture gave a pale yel-
low product (yield, 0.81 g (73.5%), m.p. 265–269 °C) [Found: C: 75.22%;
H: 4.27%; N: 6.55%]; C101H72N8O14 requires C: 74.80%; H: 4.48%; N:
6.91%. IR (KBr) υ: 3528 cm−1 (–OH), 2924 cm−1 (–−C–H),
1724 cm−1 (–C = O), 1451 cm−1 (–N = N), 1270 cm−1 (C–O). 1H-
NMR (CDCl3, 25 °C) δH: 3.4–3.9 (16H, m, Ar–CH2–Ar), 5.5 (2H, s, –
NH2), 6.6–7.2 (22H, m, Calix-H), 7.2–8.2 (30H, m, Bnz-H), 8.0 (2H, s, –
OH).
p-tert-Butylcalix[4]arene, calix[4]arene and 25,26,27-tribenzoyloxy-
28-hydroxycalix [4]arene were synthesized as described by previously
reported methods [24–27].
2.2. The synthesis of bisazocalix[4]arene [28]
2,4,6-Triamino-1,3,5-triazine (0.057 g, 0.45 mmol) was dissolved in
hot glacial acetic acid (10 mL) and resulting solution was rapidly cooled
in an ice-salt bath to 0 °C. The liquor was then added in portions during
30 min to a cold solution of nitrosyl sulfuric acid [prepared from sodium
nitrite (0.093 g, 1.35 mmol) and concentrated sulfuric acid (4 mL at 70
°C)]. The mixture was stirred for an additional hour at 0 °C. After the
Fig. 3. The mass spectrum of bisazocalix[4]arene (5).

S. Elçin et al. / Journal of Molecular Liquids 202 (2015) 134–140
137
aromatic protons. The sharp multiplet signals of the phenyl protons ap-
pear in the region δ 7.2 to 8.2 ppm.
NH
N
As it is seen in the mass spectrum (Fig. 3), synthesized bisazocalix[4]
arene (5) compound has very weak molecular ion peaks (m/z = 1621).
This indicates that melamine has been coupled to two triester molecules
and melamine's molecular mass is calculated to be 1621 amu. Examina-
tion of mass spectrum refers to the breakdown and removal of some
structures consecutively. Benzoyl and amine groups have been separat-
ed from the main structure during the analysis which can be attributed
to m/z = 1501. Following, entire molecule has been broken down to
give the structure shown in Fig. 4 with molecular weight of 748. This in-
dicates the fragmentation of bisazocalix[4]arene (5) compound.
Thermal analysis methods are useful tools for the determination of
thermal stability and inclusion behavior of calix[n]arenes with guest mol-
ecules such as toluene, xylene, chloroform, acetone and alkylammonium
[29,30]. The TG and DTA curves of tribenzoyl calix[4]arene (3) and
novel bisazocalix[4]arene (5) compounds are shown in Fig. 5.
O
O
O
O C
O C
O C
Tribenzoyl calix[4]arene (3) is the starting material in synthesis of
bisazocalix[4]arene (5) compound. Thermal behavior of bisazocalix[4]
arene becomes similar to that of tribenzoyl calix[4]arene after removal
of triazin group. Tribenzoyl calix[4]arene (3) decomposes in three
stages. First decomposition stage corresponds to removal of 1.5 mol of
methanol group. Theoretical mass loss is (6.130%) compatible with ex-
perimental mass loss (6.507%). Tribenzoyl groups leave the structure
in the second decomposition stage. Finally the remaining structure de-
composes between 460–620 °C. This reaction occurs exothermically
like the decomposition reactions of other calixarene compounds [31].
Decomposition of novel bisazocalix[4]arene (5) molecule occurs in
four stages. The first stage corresponds to removal of DMF, which has
been bound to the structure during recrystallization. This initial decom-
position reaction occurs endothermically and its peak temperature is
192.92 °C. Theoretical mass loss (4.282%) is compatible with experi-
mental mass loss (4.650%). Triazin groups leave the structure in the
second decomposition stage. After the second decomposition stage,
tribenzoyl groups leave the structure. The third decomposition reaction
occurs endothermically in 365–415 °C temperature range. Experimental
mass loss value (36.379%) is very close to theoretical mass loss value
(36.950) for this stage. Final decomposition stage, which breaks down
the remaining structure, occurs exothermically. All of the thermal anal-
ysis results are summarized in Table 1.
Fig. 4. The most stable fragmentation structure of bisazocalix[4]arene (5).
Bisazocalix[4]arene is soluble in acetic acid, benzene, acetone, CHCl3,
DMF, and DMSO, but insoluble in water, EtOH, 10% HCl and 10% NaOH.
3. Results and discussion
The synthetic route was showed in Scheme 1. A novel bisazocalix[4]
arene derivative, was prepared with melamine by using calix[4]arene
tribenzoyl derivative as the new synthetic platform.
The structure and conformation of novel bisazocalix[4]arene (5) have
been elucidated by spectroscopic methods (FR-IR, 1H-NMR, ESI-MS) and
elemental analyses. The absorption band observed at 1451 cm−1 of FT-
IR spectrum can be assigned to the azo group stretching ν(–N = N–) of com-
pound 5 (Fig. 1). The band appearing at 3528 cm−1 can be ascribed to the
presence of hydroxyl ν(OH) group in compound 5. The absorption peak at
1724 cm−1 can be attributed to ν(C = O) ester carbonyl. The weak absorp-
tion bands appeared at 1584–1600 cm−1, ascribed to –NH2 group, also
confirmed formation of compound 5.
1H-NMR data of bisazocalix[4]arene (5) includes a peak (δ 5.5 ppm),
which can be attributed to the presence of amino protons (–NH2). While
the appearance of peak at δ 3.0 ppm can be ascribed to protons of re-
crystallization solvent DMF the appearance of peak at δ 7.2 ppm can
also be attributed to the proton of dissolving solvent CHCl3 (Fig. 2).
The 1H-NMR spectrum of novel bisazocalix[4]arene (5) in CDCl3 re-
vealed a multiplet at δ 6.6–8.2 ppm corresponding to all aromatic pro-
tons. The formation of this compound is confirmed by the sharp
multiplet at δ 6.6–7.2, which can be ascribed to azocalix[4]arene
Metal transportation experiments for picrate salts were carried out
with a H2O–CHCl3 liquid–liquid phase transfer system using diazo
calixarene compounds as cation carriers. The results of cation transpor-
tation experiments are compatible with those of two-phase extraction
measurements.
The ionophoric properties of all 3, 4 and 5 compounds toward the
alkaline-earth and the transition metal cations were investigated by
TGA
%
DTA
uV
TGA
%
DTA
uV
100.00
50.00
0.00
100.00
60.00
50.00
40.00
30.00
20.00
10.00
0.00
40.00
20.00
0.00
b
a
50.00
-20.00
0.00
0.00
200.00
400.00
Temp [C]
600.00
800.00
0.00
200.00
400.00
Temp [C]
600.00
800.00
Fig. 5. The TG and DTA curves of tribenzoyl calix[4]arene(a) and bisazocalix[4]arene(b).

138
S. Elçin et al. / Journal of Molecular Liquids 202 (2015) 134–140
Table 1
groups may play a role in complexation with azocalix[n]arenes. The re-
sults have shown that compound 5 displays a strong affinity toward soft
metal cations Hg2+ and Hg+. This phenomenon can be attributed to the
soft nitrogen donor atoms of novel compound 5.
These results suggest that the match between cation and azo group
of calix[4]arene derivative is an evident factor in selectivity. As a part
of research study, a novel azocalix[4]arene based sensor 5 has been syn-
thesized and characterized. It has been revealed from the experimental
evidences that 5 exhibits significant sensing behavior toward Hg+ and
Hg2+ among a series of selected transition metals.
An ORTEP diagram for novel bisazocalix[4]arene (5) is shown in
Fig. 7. Torsion angles and around Ar–CH2–Ar bonds about the presence
of two sets of characteristic AB spin systems at 3.40 and 3.90 ppm are
consistent with cone conformation. The inter planar angles between
rings and its distally positioned ring are about 25.0° theoretically,
which suggest that these are almost parallel to each other. Phenyl
rings of the benzoyl group are perpendicular to the respective calixaryl
aromatic ring plane. The oxygen of the carbonyl group remains exo to
the cavity. The dihedral angle between the phenylazo group plane and
ring melamine is 9.0°, which corroborates that one phenylazo ring is
parallel and the other phenylazo ring is perpendicular to the ring,
which remains outward of the calixarene cavity. This brings the
substituted phenylazo group much closer to the adjacent benzoyl ring.
The molecular packing of bisazocalix[4]arene along the axis a is shown
in Fig. 7.
The thermoanalytical results obtained from TG and DTA curves. exp: Experimental, theo
theoretical.
:
Compounds
Ti–Tf
Tpeak
Mass
Mass
Removed groups
loss%exp loss%theo
Tribenzoyl
calix[4]arene
175–205 188.49
305–460 413.46 40.240
357–535 579.46 52.684
6.141
6.130
40.230
54.151
1.5 mol CH3OH
Tribenzoyl groups
Rest of structure
decomposes
Bisazocalix[4]arene 150–197 192.92
281–365 275.15
4.650
9.959
4.282
9.501
36.950
49.267
1 mol DMF
Triazin groups
Tribenzoyl groups
Rest of structure
decomposes
365–515 427.29 36.379
515–648 561.96 49.506
the picrate extraction method initially [18]. The results, expressed as a
percentage of cation extracted (E%), are demonstrated in Table 2 and
shown graphically in Fig. 6.
The extraction studies of cations (Ag+, Hg+, Hg2+, Co2+, Ni2+
,
Cu2+, Cd2+, Pb2+, Zn2+, Cr3+ and Al3+) with ligand 3 had already
been done [18] with same experimental conditions earlier. The ligand
is selected due to its functional groups (tribenzoyl ester groups)
which increases its solubility. This situation increases the efficiency of
extraction. Extraction properties of compound 5 for Hg2+ (61.0%) and
Hg+ (43.2%) are measured to be much higher than those of compound
3 for Hg2+ (17.0%) and Hg+ (15.5%).
Although both compounds of 3 and 5 form complexes with Hg cat-
ions, extraction property of compound 5 toward Hg cations has been
measured higher with respect to that of compound 3. This difference
can be attributed to conjugated chromophore azo (–N = N–) groups
of compound 5. π-interactions of these chromophore azo (–N = N–)
4. Conclusion
This study demonstrated the synthesis of novel bisazocalix[4]arene
(5) by using 25,26,27-tribenzoyloxy-28-hydroxycalix[4]arene (3) and
melamine (4). The mass spectrum of novel compound (5) was measured
Table 2
Extraction of metal picrates with ligands.a
Ligand
Picrate salt extracted (%)
Na+
K+
Sr2+
Ag+
Hg+
Hg2+
Co2+
Ni2+
Cu2+
Cd2+
Pb2+
Zn2+
Cr3+
Al3+
3
4
5
0.8
1.5
15.8
0.9
1.2
17.9
0.6
1.5
19.6
3.3
2.1
21.2
15.5
13.2
43.2
17.0
21.1
61.0
1.2
1.6
19.8
0.7
2.1
19.6
5.1
5.7
26.2
0.1
0.1
20.2
3.0
1.9
24.6
0.3
0.7
20.0
6.5
6.8
19.8
3.2
2.6
23.1
a
H2O/CHCl3 = 10/10 mL (v/v): [picric acid] = 2 × 10−5 M, [ligand] = 1 × 10−3 M, [metal nitrate] = 1 × 10−2 M; 298 K, 1 h contact time. Experimental error was 2%.
70,0
60,0
50,0
40,0
3
4
5
30,0
20,0
10,0
0,0
Na+
K+
Sr2+
Ag+
5
Hg+
Hg2+
Co2+
4
Ni2+
Cu2+
Cd2+
3
Pb2+
Metals
Zn2+
Cr3+
Al3+
Fig. 6. Extraction percentages of compounds 3, 4 and 5.

S. Elçin et al. / Journal of Molecular Liquids 202 (2015) 134–140
139
Fig. 7. (a) ORTEP diagram of bisazocalix[4]arene (hydrogens, solvent molecules, and disordered part have been omitted for clarity); (b) view of the molecular electron density.
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as a significant peak corresponding to m/z = 1501.6. This product is mea-
sured to be thermally stable by TG and DTA methods. The results of ex-
traction studies performed on ligands 3, 4 and 5 suggest that the match
between cation and ligand is an evident factor in their selectivity proper-
ties. It has been observed that novel bisazocalix[4]arene compound (5)
shows high affinity to Hg+ and Hg2+ ions, whereas almost no affinity to
other metal cations. The results have also shown that compound 5 dis-
plays stronger affinity toward soft metal cations Hg2+ and Hg+. This phe-
nomenon can be explained by the (hard–soft) acid–base (HSAB) principle
whereby –N = N group which is a soft base shows a stronger affinity to-
ward soft acidic cations like Hg2+ and Hg+. Bisazocalix[4]arene (5)
showed a thermal behavior and a decomposition in four stages. These
data are very important for the determination of potential application
areas of azocalix[4]arenes. This study suggests that novel bisazocalix[4]
arene compound (5) may have a potential application as a highly selec-
tive sensor toward Hg cations. Studies on the respect of research are un-
derway in our laboratory.
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