Hg(II) selective complexation by chromoionophoric Calix[4]arene derivative

Article abstract of DOI:10.1007/s10895-010-0802-2

The present article describes an exploration regarding Hg(II) selective complexation behavior of 5,11,17,23- tetrakis[(N,N-dimethylamino)methyl]-25,26, 27,28- tetrahydroxycalix[4]arene (4). The binding affinity of 4 toward selected transition metal ions such as Cd(II), Co(II), Cu(II), Hg(II), Ni(II), Pb(II) and Zn(II) have been investigated by UV-visible and fluorescence spectroscopic techniques. From the results it has been noticed that 4 confers a pronounced preference for Hg(II) in complexation phenomenon even in the presence of other metal ions. The results of Job's plot analysis reveal 1:1 host-guest complex formation between Hg(II) and 4. The FT-IR spectroscopy also supports the complexation affinity of 4 for Hg(II). Springer Science+Business Media, LLC 2011.

Full text of DOI:10.1007/s10895-010-0802-2

J Fluoresc (2011) 21:1231–1238
DOI 10.1007/s10895-010-0802-2
ORIGINAL PAPER
Hg(II) Selective Complexation by Chromoionophoric
Calix[4]arene Derivative
Mansoor Ahmed Qazi & Imdadullah Qureshi &
Shahabuddin Memon
Received: 6 October 2010 /Accepted: 28 December 2010 /Published online: 8 January 2011
#
Springer Science+Business Media, LLC 2011
Abstract The present article describes an exploration regard-
ing Hg(II) selective complexation behavior of 5,11,17,23-
tetrakis[(N,N-dimethylamino)methyl]-25,26,27,28-
tetrahydroxycalix[4]arene (4). The binding affinity of 4
toward selected transition metal ions such as Cd(II), Co(II),
Cu(II), Hg(II), Ni(II), Pb(II) and Zn(II) have been investi-
gated by UV-visible and fluorescence spectroscopic techni-
ques. From the results it has been noticed that 4 confers a
pronounced preference for Hg(II) in complexation phenom-
enon even in the presence of other metal ions. The results of
Job’s plot analysis reveal 1:1 host-guest complex formation
between Hg(II) and 4. The FT-IR spectroscopy also supports
the complexation affinity of 4 for Hg(II).
determination of extremely low concentrations of mercury
in various samples. Thus, the design and development of
efficient ligands that may be used to selectively complex
these metal ions are turning out to be extremely important.
To this end, calixarenes a class of macrocyclic ligands has
been extensively studied [3]. They have received consider-
able attention because of their potential for the synthesis of
highly efficient and selective receptors [4, 5].
Recently, some selective fluorescent chemosensors for
mercury(II) have been reported based on calixarenes [6–9],
quinolines [10–14], fluorescein [15–18], rhodamine [19–
25], pyrene [26, 27], naphthalimide [28–30] and other
structural moieties [31–35]. However, most of them have
disadvantage in practical use, such as interference from
other metal ions, strict reaction condition or complicated
synthetic route. Consequently, it has been shown that the
presence of soft donor atoms in ligands results in a
considerable increase in stability of their complexes with
soft cations such as mercury, while diminishing the stability
of their alkali, alkaline earth metal ions and hard transition
metal ion complexes [36, 37]. Therefore, development of
simple fluorescent chemosensor that can selectively sense
Hg(II) is significant.
In view of the above studies and our previous experience
[38], herein we report an investigation regarding synthesis,
solvatochromic effect and complexation behavior of
5,11,17,23-tetrakis[(N,N-dimethylamino)methyl]-25,26,27,28-
tetrahydroxycalix[4]arene (4) using UV-visible, fluorescence
and FT-IR techniques.
The p-tert-butylcalix[4]arene 2 as well as its derivatives
3 and 4 illustrated in Scheme 1 were prepared by methods
describe previously [39–41]. The characterization of the
compounds for the confirmation of their structure and
.
.
Keywords Calixarene Solvatochromic effect
.
.
Complexation Metal ions Supramolecular chemistry
Introduction
Mercury is one of those elements that accumulate in living
tissues, thereby causing many harmful effects on human
health [1, 2]. It might be brought into the environment by
various processes including human activities; which has
influenced and modified the natural cycles. However, over
the last two decades, the need has increased for the
:
:
M. A. Qazi I. Qureshi S. Memon (*)
National Center of Excellence in Analytical Chemistry,
University of Sindh,
Jamshoro 76080, Pakistan
e-mail: shahabuddinmemon@yahoo.com

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J Fluoresc (2011) 21:1231–1238
Scheme 1 Reaction pathway
for synthesis of ligand a
HCHO/NaOH b Phenol-AlCl3/
(dry)Toluene c Dimethylamine-
HCHO Acetic acid: THF
a
b
OH
OH
OH
OH
HO
OH
1
OH
OH
HO
2
3
c
OH
HO
OH
OH
..
N
..
N
..
..
N
N
4
purity was made by various techniques such as, melting
point, TLC, IR, and elemental analysis.
Analytical Procedure
Stoichometric Ratio of the Hg(II) and 4 in the Complex
Experimental Section
Job’s method [43] (continual variation method) was used to
determine the stoichometric ratio between 4 and Hg(II) for
their complexation in DMF solution. The solutions were
prepared by mixing equimolar concentration (4.3×10−5 M)
of both components in different ratios varying from 1:9 to
9:1. Then the absorbance was measured at 265 nm.
General Experimental Information
Melting points were determined on a Gallenkamp (UK)
apparatus in a sealed capillary tube. Thermo Nicollet
AVATAR 5700 FTIR spectrometer was used for recording
IR spectra using KBr pellets in a wide spectral range, i.e.
4000-400 cm−1. Elemental analyses were performed using a
CHNS instrument model Flash EA 1112 elemental analyz-
er. UV-visible spectral studies of ligand 4 (Scheme 1) and
its metal complexes were performed on a Perkin Elmer
Lambda-35 UV-visible double beam spectrophotometer
using standard 1.00 cm quartz cells. Analytical TLC was
performed on pre-coated silica gel plates (SiO2, Merck
PF254). All the reagents and solvents were of analytical
grade and used without further purification.
General Procedure for Fluorescence Study
Stock solutions of (1.00 mM) 4 and metal nitrate salts were
prepared in DMF. Test solutions were prepared by placing
50 μl of 4 into a cuvette, adding appropriate aliquot (10 eq)
of each metal stock, and diluting the solution up to 3.5 mL
with DMF. Same equivalents (10 eq) were taken for the
interference study of co-existing ions into a solution
containing 4-Hg(II) complex. For all measurements, exci-
tation was 310 nm; excitation and emission slits widths
were both 5 nm each.
Synthesis of Ligand 4
The required starting material p-tert-butylcalix[4]arene (2),
calix[4]arene (3) and tetra amine derivative (4) were
prepared by published procedures [39–41].
Results and Discussion
Usually, a red shifting (positive solvatochromism) [44] is
observed with increase in polarity of solvent because a
polar solvent tends to stabilize the excited state for almost
all molecules and vice-versa is true for negative solvato-
chromism. Moreover, it has been found that solvent plays
an important role in complexation phenomena involving
ionic or neutral species and macrocyclic ligands in general
and calixarenes in particular [45, 46].
Synthesis of Metal Complexes with 4
For FT-IR experiments, 4 with a nitrate salt of Hg(II) and
KBr were mixed and then ground to powder form in an
agate mortar. The resulting mixture was kept in oven at
115 °C for 1 h and then pressed to form pellets [42].

J Fluoresc (2011) 21:1231–1238
1233
1.07
1.0
0.7
(a)
0.6
DMF
DMSO
1.0
0.8
0.6
0.4
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.5
0.4
A
A
0.2
0
0.3
(b)
0
0.2
0.4
0.6
0.8
1
0.2
0.1
0.0
Mole Fraction
0.1
0.0
255 260
270
280
290
300
310
320
330
340
350
360
nm
260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
nm
Fig. 1 UV-visible spectral behavior and inset naked eye color
changes of compound 4 in a DMF b DMSO (4.3×10−5 M)
Fig. 3 UV-visible titration spectra of 4 (4.3×10−5 M) upon addition
of various equivalents of Hg(II); (inset) Job’s plot of 4 and Hg(II)
In order to determine solvatochromic property of ligand
4, different solutions of ligand 4 have been prepared with a
fixed concentration of 4.3×10−5 M in various solvents
(Fig. 1) such as acetone (ACO), acetonitrile (ACN),
chloroform (CF), dichloromethane (DCM), tetrahydrofuran
(THF), methanol (MeOH), dimethylformamide (DMF) and
dimethylsulfoxide (DMSO) to evaluate the spectral changes
in these solvents.
absorption band of maximum intensity within standard
absorption limits and shows no any aggregation as
compared with DMSO or other solvents.
Complexation Behavior of 4
In DCM, CF and THF, the UV-visible spectra of 4 show
aggregation and too much noise even at low concentration,
whereas in ACO and ACN, 4 has partial solubility
therefore, all of them have been discarded. The color of
ligand 4 and changes in the absorption spectrum (Fig. 1) in
DMF and DMSO suggest its potential as a probe for these
solvents. Ligand 4 shows its absorption maxima at 285 nm
accompanied by its shoulder at 310 nm in DMF/DMSO
may be attributed to the π→π* and n→π* transitions.
The naked eye color changes of 4 in DMF gives clear
evidence for hydrogen bonded interactions (i.e. N···H)
between DMF and ligand 4. Thus, DMF was preferred for
absorption as well as fluorescence studies since it gives
Macrocyclic based Mannich bases (i.e. calixarene) have
been proved to be ideal candidates for the formation of
stable complexes with soft/borderline metal cations such as
Cu(II), Hg(II) and Pb(II) etc [47]. Therefore, in this work,
we have selected calix[4]arene derived Mannich base for its
complexation study owing to ease of its synthesis and
possessing borderline nitrogen donor atom with macrocy-
clic effect.
Thus, different titration experiments were carried out in
order to determine whether ligand 4 can form complexes
with the selected metals and consequently different param-
eters were checked to evaluate its selectivity.
1.1
ligand 4-Hg(II) complex
1.20
1.0
1.1
1.05
0.9
0
0.90
0.75
0.60
0.45
0.30
0.15
0.00
4
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.00
20 min
40 min
80 min
100 min
120 min
140min
160min
180 min
200 min
4+Hg(II)
4+Cu(II)
4+Ni(II)
4+Co(II)
4+Pb(II)
4+Cd(II)
4+Zn(II)
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
A
A
0
20 40 60 80 100120140160180200
Time (min)
260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
nm
260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
Fig. 4 Time-dependent UV-visible spectrum of 4-Hg(II) complex
(4.3×10−5 M) in DMF upon continuous irradiation of UV light;
(inset) graph showed stability of 4-Hg(II) complex with respect to
time
nm
Fig. 2 Comparative UV-visible spectra of 4 (4.3×10−5 M) before and
after titration with selected transition metals (10 eq)

1234
J Fluoresc (2011) 21:1231–1238
950
1.4
1.3
1.2
1.1
1.0
0.9
0.8
4
4-Hg(II)
4+Hg(II)+Cu(II)
4+Hg(II)+Co(II)
4+Hg(II)+Ni(II)
4+Hg(II)+Pb(II)
4+Hg(II)+Cd(II)
4+Hg(II)+Zn(II)
4
A
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
475
260 270 280 290 300 310 320 330 340 350 360 370 380 390 400
0.00
nm
280
565
850
Wavelength (nm)
Fig. 5 Interference study of Hg(II)-complex of 4 with selected
Fig. 7 Fluorescence spectra of 4 (4.3×10−5 M) in DMF solution
transition metals (10 eq) in DMF
before and after adding nitrate salt of transition metals (10 eq)
UV-Visible Study
tions between Hg(II) and the nitrogen atoms of amine
functions on macrocycle [48]. However, other metal ions
even borderline metal ions like Co(II), Cu(II), Ni(II) and Zn
(II) does not take significant part in complexation process with
4. This shows that alone soft/hard nature of ligand or metal
ion is not only the important factor for selectiveness but also
other factors like thermodynamic stability, ionic radii, cavity
size as well as geometry of ligand and metal ion confers the
selectivity of ligand toward a specific metal ion.
In an attempt to investigate the quantitative analytical
behavior of ligand 4 (4.3×10−5 M) for Hg(II) complexation.
The absorption profile as a function of metal ion concentration
was obtained followed by increase in the intensity of
absorbance with respect to increased Hg(II) concentration
(Fig. 3). After 10 equiv, the intensity did not significantly
change, which implies that 4 could quantitatively intimate
the concentration of Hg(II) up to 4.3×10−4 M.
Preliminary measurements were made to checkout the
complexation behavior of 4 in DMF solution (4.3×
10−5 M) for selected metal ions such as Cd(II), Co(II), Cu
(II), Hg(II), Ni(II), Pb(II) and Zn(II). The UV-visible
spectra of 4 (free) show a strong band at 285 nm which is
attributed to π→π* and its shoulder at 310 nm due to
n→π* transition (Fig. 2). Generally, an enhancement in the
intensity of the band, appearance of new bands and/or
shifting of bands to shorter or longer wavelengths than that
of the free ligand after complexation is an informative sign
of the complex formation. However, in this case, addition
of each metal cation (10 equiv) in 4 causes little
enhancement in the absorption intensity of the previous
bands as shown in Fig. 2, whereas addition of Hg(II) ion
causes significant hypsochromic shifting of band to 265 nm
(~20 nm) along with appearance of new band at 345 nm.
This shifting and appearance of new band could be
assigned to metal-ligand charge transfer (MLCT) absorp-
The method of continuous variation was applied by
varying the concentration of both ligand 4 and Hg(II) to
950
475
0.0
18
16
14
12
10
I/I0
8
6
4
2
0
4
Cu(II) Co(II) Cd(II) Hg(II)
Ni(II)
Pb(II) Zn(II)
280
565
850
Wavelength (nm)
Metal Ions
Fig. 6 Fluorescence spectra of selected (free) transition metal ions
Fig. 8 Ratiometric behavior (I/I0) of 4 in the presence of selected
transition metals in DMF solution
(4.3×10−5 M) in DMF solution

J Fluoresc (2011) 21:1231–1238
1235
in terms of appearance of new band as well as enhancement
in emission intensity indicates that not only the binding
sites of ligand 4 are more compatible to Hg(II) than other
metal ions but there are other factors that are also
responsible for the selective complexation as discussed
earlier.
18
16
14
12
10
I/I0
8
6
4
2
0
The ratiometric behavior of 4 in the presence of selected
transition metal ions also supports the pronounced Hg(II)
selectivity of 4 (Fig. 8); where upon interaction of 4, the
ratio (I/I0) for the 4-Hg(II) system increased 17-fold
compared with free ligand (4) and 3-fold with Cu(II).
To gain more insight into the practical applicability of 4
in Hg(II) signaling, competitive experiments on the
signaling of Hg(II) ions by ligand 4 in the presence of
various co-existing ions were carried out as illustrated in
Fig. 9. Fluorescence spectroscopic study of interfering ions
also supports the results obtained in the UV-visible spectra;
showing no any shifting (bathochromic or hypsochromic
shift), quenching or enhancement in the fluorescence
intensity of 4-Hg(II) complex after the addition of metals
and gave approximately similar results except Cu(II).
Moreover, fluorescence changes (I/I0) of the 4-Hg(II) were
not significantly affected by presence of 10 equiv of other
metal ions except Cu(II).
Regarding another complexation possibility between the
phenolic OH functionalities of host and the metal ions, it
may be presumed that the metal may be able to interact
with the phenolic moieties at lower rim of calix[4]arene
[49, 50], but due to intramolecular hydrogen bonding of
lower rim, metal ion may not be accommodated within this
region of lower cavity of calix[4]arene. Nonetheless, all the
UV-visible and fluorescence spectra suggest that 4 may be
used as a potential Hg(II) selective chromoionophore. Thus,
according to the Job’s plot analysis as discussed above, it
has been revealed that Hg(II) forms 1:1 metal ligand
complex with 4. Therefore, the proposed mechanism for
metal-ligand interaction is shown in Fig. 10 [51–55].
4
I
II
III
IV
V
VI
VII
Interfering Ions
Fig. 9 Ratiometric behavior (I/I0) of 4 in the presence of Hg(II) and
other co-existing ions in DMF solution. I=4+Hg(II), II=4+Hg(II)+
Cd(II), III=4+Hg(II)+Cu(II), IV=4+Hg(II)+Pb(II), V=4+Hg(II)+Ni
(II), VI=4+Hg(II)+Co(II), VII=4+Hg(II)+Zn(II)
determine the stoichiometry of 4-Hg(II) complex. Fig. 3
(inset graph) shows typical Job’s plots of 4-Hg(II) com-
plexation at 265 nm, the maximum point of the mole
fractions was found as 0.5, which refers to ligand-metal
ratio of 1:1 in the complex.
Response time measurement of chromoionophore based
on macrocyclic compounds is an intense area of study.
Therefore, it has been aimed to investigate the response
time and stability of 4-Hg(II) complex. The results reveal
that 4 responds very fast and when titrated with Hg(NO3)2,
a significant hypsochromic shifting accompanied by en-
hancement in absorption of a band at 265 nm and
appearance of new band at 345 nm occurs very rapidly
within 1 min. and remains stable up to a long time (Fig. 4),
i.e. remains same even after three to 4 days.
Selectivity of ligand 4 toward Hg(II) ion in the presence
of interfering ions was examined by the competitive
experiments carried out in the presence of Hg(II) by mixing
Cd(II), Co(II), Cu(II), Ni(II), Pb(II) and Zn(II) (in 1:1 ratio)
separately as shown in Fig. 5. It was observed that except
Cu(II), other metal ions do not interfere on the absorption
spectra of 4-Hg(II) complex.
OH
HO
HO
HO
Fluorescence Study
Fluorescence spectral properties of free metal ions as well
as cationic fluorescence sensing ability of ligand 4 toward
these cations was investigated and illustrated in Figs. 6 and
7. Upon addition of 10 eq. of each cation, despite the
border line nature of Cu(II), Co(II), Ni(II) and Zn(II), all
metal ions give more or less same response as 4. However,
addition of Hg(II) causes remarkable enhancement in the
emission intensity of band from 51 to 873 a.u at 373 nm
followed by appearance of new band at 623 nm. Such a
considerable difference between Hg(II) and other metal ions
N
N
(NO3)2
Hg2+
N
N
4
Fig. 10 Proposed interaction between compound 4 and Hg(II).

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J Fluoresc (2011) 21:1231–1238
Fig. 11 The TG curves of com-
pound 4 and its complex with
Hg(II) in N2 atmosphere
104
100
4
90
80
70
60
50
40
30
4-Hg2+
20
15
36 50
100
150
200
250
300
350
400
450
500 533
Temperature (0C)
Thermal Analysis
parent molecule becomes absent. The results indicated a
remarkable interaction between the amino groups of 4 and
the Hg(II). In general, the processing temperature limit of
Hg(II) complex of 4 is up to 150 °C.
The thermo gravimetric curves enable us to establish
information on thermal stability, amount confirmation and
purity of the compounds. Thus, thermal stability of the
ligand 4 and its Hg(II) complex was evaluated by thermal
analysis (TG/DTA). It was found that 4 undergo a two-step
thermal degradation (Figs. 11 and 12). The first step (150–
250 °C) could be attributed to the loss of the relative amino
groups of 4, while the second (325–420 °C) is due to the
cleavage of the calixarene backbone. As seen from TG
curve of Hg(II)-4 complex, it shows less stability than its
parent molecule as it shows 46% (1.258 mg) mass loss in
the first step (150–200 °C) whereas it was reduced to 31%
(0.34 mg) by wt at 300 °C in the second step. The DTA
result of the 4 showed the weight loss endothermic weight
loss endothermic peaks whereas 4-Hg(II) complex showed
surprisingly sharp weight loss exothermic peak temperature
of 180 °C, while others at 270, 400 and 458 °C in the
FT-IR Study
Binding mode of ligand 4 with Hg(II) was further
confirmed through FT-IR spectroscopic analysis since it
provides strong evidence of complexation between ligand
and metal ion. FT-IR spectrum showed characteristic bands
of this ligand (KBr/cm−1) at 3180 [υ( O••••H) intermolec-
ular hydrogen bond], 2935 υ(CH2), 2872 δ (CH2), l377
υ(C-N). Coordination of the Hg(II) ion can be readily
verified in the FT-IR spectrum of the complexes. Figure 13
clearly gives the stronger indication for complexation as
shifting in various frequencies of specific functional groups
occurred as a result of introduction of Hg(II) into
ionophoric cavity. For example, marked changes appeared
Fig. 12 The DTA curves of
compound 4 and its complex
with Hg(II) in N2 atmosphere
-6.267
0
4
4
-Hg2+
10
20
30
40
50
60
65
36 50
100
150
200
250
300
350
400
450
500
533
Temperature (0C)

J Fluoresc (2011) 21:1231–1238
1237
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Wavenumbers (cm-1)
Fig. 13 Comparative FT-IR spectral analysis a Compound 4 (——) b
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In conclusion, the chemosensing/chromoionophoric poten-
tial of ligand 4 was found with high slectivity toward Hg(II)
among a series of selected metal ions that attributed to the
changes in the intensities of spectral lines observed in UV-
visible and fluorescence spectra of 4-Hg(II) complex. It is
also confirmed by TG/DTA and FTIR spectral analysis.
Moreover, the design of 4 that comprises four donor
nitrogen atoms as binding sites seems to be an ideal
geometry in terms of size, arrangement and accommodation
of Hg(II). It infers the importance of pre-organization in
designing the ligand. From the results, it may be concluded
that the study may be treated as a test for the detection of
Hg(II). The study will find its applicability in various fields
of analytical and environmental chemistry.
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