Optical spectroscopy studies of the complexation of bis(azophenol)calix[4] arene possessing chromogenic donors with Ni2+, Co2+, Cu2+, Pb2+ and Hg2+

Article abstract of DOI:10.1016/j.saa.2012.07.097

Due to their potential applicability as selective receptors in electrochemical or optical sensors, a bis(azophenol)calix[4]arene derivative H₂L has been investigated. The complexation properties of this molecule towards Ni²⁺ and Co²⁺ metal ions has been studied. It is revealed that this ligand exhibits tetradentate with N ₂O₂ core when bound to Ni (II) or Co (II) metal ion. The optical response of azo groups of H₂L towards Ni²⁺, Co²⁺, Cu²⁺, Pb²⁺ and Hg²⁺ metal ions has been investigated in DMSO by UV-vis spectroscopy. The absorption spectra of calix[4]arene with cations show marked changes, especially for Co²⁺ ion. Furthermore, Job's plot indicate 1:1 binding-stiochiometry for calix[4]arene with Co²⁺ ion and Benson-Hilderbrand plot is used for the determination of its association constant. The investigation of UV-vis spectra of chromogenic calix[4]arene in different solvents shows that cis-trans isomerization of azo groups probably depends on kind of solvent. Also the different between the polarity and viscosity of organic solvents used is likely responsible for the changes of the band shape of the spectra.

Full text of DOI:10.1016/j.saa.2012.07.097

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 81–85
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
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Optical spectroscopy studies of the complexation of bis(azophenol)calix[4]arene
possessing chromogenic donors with Ni2+, Co2+, Cu2+, Pb2+ and Hg2+
⇑
Behrouz Shaabani , Zohreh Shaghaghi, Ali Akbar Khandar
Research Laboratory of Synthesis of Inorganic Compounds, Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
O
2
N
"
We explored interaction between
some M2+ metal ions and
bis(azophenol)calix[4]arene H2L in
DMSO.
NO
2
N
=
N
OH
HO
CH
N
CH
N
OH
OH
O
O
"
The absorption spectra of
calix[4]arene with cations show
marked changes, especially for Co2+
ion.
H2L
i=3 eq Co2+
a=0.2 eq Co2+
↑
"
"
Job’s plot indicated 1:1 binding-
stoichiometry for calix[4]arene with
Co2+ ion.
1:1 association constant of H2L with
Co2+ is determined on the base of
Bensi-Hilderbrand plot at
k = 317 nm.
Free Ligand
↑
λ
"
It is revealed that the stability
constant of calix[4]arene H2L with
cobalt is Ka = 2.40 ꢀ 104 Mꢁ1
.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 May 2012
Received in revised form 24 July 2012
Accepted 26 July 2012
Available online 8 August 2012
Due to their potential applicability as selective receptors in electrochemical or optical sensors, a
bis(azophenol)calix[4]arene derivative H2L has been investigated. The complexation properties of this
molecule towards Ni2+ and Co2+ metal ions has been studied. It is revealed that this ligand exhibits tet-
radentate with N2O2 core when bound to Ni (II) or Co (II) metal ion. The optical response of azo groups of
H2L towards Ni2+, Co2+, Cu2+, Pb2+ and Hg2+ metal ions has been investigated in DMSO by UV–vis spectros-
copy. The absorption spectra of calix[4]arene with cations show marked changes, especially for Co2+ ion.
Furthermore, Job’s plot indicate 1:1 binding-stiochiometry for calix[4]arene with Co2+ ion and Benson-
Hilderbrand plot is used for the determination of its association constant. The investigation of UV–vis
spectra of chromogenic calix[4]arene in different solvents shows that cis–trans isomerization of azo
groups probably depends on kind of solvent. Also the different between the polarity and viscosity of
organic solvents used is likely responsible for the changes of the band shape of the spectra.
Ó 2012 Elsevier B.V. All rights reserved.
Keywords:
Chromophore
Calix[4]arene
Spectrophotometry
Solvent effect
Azo compounds
Optical properties
Introduction
either the upper and/or lower rims. The modification of calixarenes
has been the subject of numerous studies, due to their potential
Calixarenes are the third generation of supramolecular recep-
tors after cyclodextrines and crown ethers [1]. It is possible to pre-
pare various calixarene derivatives by functionally modifying
applicability as active ligands in electrochemical and/or optical
sensors [2–8] with different selectivities for various guest ions
and small molecules.
Among calix[4]arene receptors, various types chromogenic ca-
lix[4]arene derivatives have recently been synthesized, which have
shown selective complexation with metal ions [9,10]. McKervy and
⇑
Corresponding author. Tel./fax: +98 4113393144.
E-mail address: b.shaabani@yahoo.com (B. Shaabani).
1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.07.097

82
B. Shaabani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 81–85
Table 1
UV/vis Spectrometer PG Instruments Ltd. Melting points were ob-
tained with an Electrothermal 9100 and are uncorrected.
IR spectra data (cmꢁ1 in KBr) for H2L, NiL and CoL.
Compound
mC=N
mO–H
mC–O
phenolic
H2L
NiL
CoL
1633
1611
1615
3443
3432
3433
1197
1195
1195
Synthesis of Ni(II)complex (NiL)
Ni(II)acetate.tetrahydrate (0.081 mmol, 0.020 g) in ethanol
(5 ml) was added drop wise, over 15 min, to an ethanol-dichloro-
methane solution (15 ml) of calix[4]arene derivative H2L
(0.0081 mmol, 0.100 g). The mixture was refluxed for 4 h, then it
was filtrated and the obtained solid was purified by crystallization
using the same solvents (C2H5OH–CH2Cl2) and a red-brown pow-
der was obtained. Yield: 0.060 g, 55.0%. IR (KBr, cmꢁ1) 3432(O–H
groups), 2955(C–H aliphatic), 1611 (–C@N– imine), 1524 and
1474 (–N@N– cis and trans), 1383, 1334 (NO2 group), 1197(C-O
phenolic), 1150, 1107, 1035, 856, 749. Elem. Anal. Calcd for Ni
Diamond [11,12] have demonstrated that novel chromogenic calix-
arene-based ligands can be used for the optical recognition of alkali
ions. Bodenant et al. [13] have reported a bipyryl calix[4]arene-
based receptor designed for the optical detection of Cu(II) and
Ni(II) metal ions in solution. Bitter et al. [14] have studied the com-
plex forming behavior and selectivity of a calix[4]arene derivative
with alkali and alkali earth ions. Lang and co-workers [15] have re-
ported on the comparative study of alkali metal receptors based on
the calix[4]arene or thiacalix[4]arene scaffolds. Recently, Bingol
and co-workers [16] have synthesized a novel calix[4]arene deriv-
ative containing benzothiazole azo groups at the upper rim and
investigated sensing properties with some heavy metal ions.
In this paper, we report solvent effect on optical behavior of ca-
lix[4]arene derivative containing two nitrophenylazo groups at the
lower rim and its complexation properties towards some heavy
metal ions such as Ni2+,Co2+, Cu2+, Hg2+ and Pb2+ by UV–vis spec-
trophotometery. We found marked changes in the absorption spec-
tra of calixarene ligand with studied heavy metal ions, especially
for Co2+ ion.
C74H80N8O10: C, 68.48; H, 6.31; N, 8.63. Found: C, 68.18; H, 5.99;
N, 8.30. Decomp: 340 °C.
Synthesis of Co(II) complex (CoL)
Co(II)chloride.hexahydrate (0.081 mmol, 0.019 g) in ethanol
(5 ml) was added drop wise, over 15 min, to an ethanol-dichloro-
methane solution (15 ml) of calix[4]arene derivative H2L
(0.081 mmol, 0.100 g). The solution was refluxed for 24 h, then
the solution was evaporated under reduce pressure. The oil residue
was washed n-hexane and then purified by crystallization by using
the same solvents (C2H5OH–CH2Cl2) and a brown powder was ob-
tained. Yield: 0.070 g, 63.0%. IR (KBr, cmꢁ1) 3433(O–H groups),
2957(C–H aliphatic), 1615 (–C@N– imine), 1521 and 1476 (–
N@N– cis and trans), 1386, 1335 (NO2 group), 1197(C–O phenolic),
Material and methods
General
1154, 1107, 1043, 859, 749. Elem. Anal. Calcd for CoC74H80N8O10
.
4CH2Cl2: C, 57.17; H, 5.13; N, 6.84. Found: C, 56.99; H, 5.27; N,
7.45. Decomp: 245 °C.
5,11,17,23-Tetra-tert-butyl-25,27-bis[2-[(2-hydroxy-5-(4-nitro-
azo)benzylidene)amino]ethoxy]-26,28-dihydroxycalix[4]arene H2L
was prepared according to literature method [17]. Starting materi-
als were all purchased commercially and used without further puri-
fications. Elemental analyses were performed on Elementar Vario EL
III. IR spectra were recorded on a FT-IR Spectrometer Bruker Tensor
27 in the region 4000–400 cmꢁ1 using KBr pellets. Electronic
absorption spectra in the UV–vis region were recorded with T 60
Stability constant of cobalt complex
The stability constant Ka of 1:1 complex of H2L with Co2+ tran-
sition metal ion was determined from the following Benesi-Hilder-
brand equation [8]:
O N
2
O N
2
NO
2
NO
2
OH
HO
CH
N
CH
N
O
O
CH
N
CH
N
M
Ni(CH3COO)2.4H2O , CoCl2.6 H2O
C2H5OH-CH2Cl2, reflux
OH
OH
O
O
OH
O
O
OH
H2L
M= Ni, Co
NiL and CoL
Scheme 1. Synthesis of NiL and CoL complexes (azo groups have been shown without consideration to trans–cis izomerization).

B. Shaabani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 81–85
83
1.4
1.2
1
Toluene
Dichloromethane
THF
Ethanol
Acetonitrile
DMF
0.8
0.6
0.4
0.2
0
DMSO
280
330
380
430
480
530
580
630
680
λ(nm)
Fig. 1. UV–vis spectra of H2L (ꢃ10ꢁ4 M) in different solvents.
1=D
A ¼ 1=
D
Asat þ 1=ð
D
AsatKa½guestꢂÞ
Where
D
A is the absorption difference between the apparent metal
complex and the free ligand and
DAsat is the absorption difference
at saturation. The association constant value Ka was evaluated
graphically by plotting 1/DA against 1/[metal ion]. In this study,
the concentration of the ligand was kept constant (10ꢁ4 M), and
the concentration of metal ions was kept in the range ([metal
ion]/[ionophore]) = 0.2–3.0 eq. Plot gave a straight line, and the Ka
value was obtained from the slop and intercept of this line.
Results and discussion
Fig. 2. UV–vis spectra of NiL (a) and CoL (b) in DMSO (ꢃ10ꢁ4 M).
IR Spectra
The major difference in the IR spectra of NiL or CoL compared to
H2L is the shift of the imine peak at 1633 cmꢁ1 in H2L to 1611 and
1615 cmꢁ1 at complexes, respectively. This shows nitrogen elec-
tron pairs coordinate to empty orbital’s of Ni(II) or Co(II) metal
ion. Also, shifts in C–O phenolic and O–H peaks for complexes com-
pared to H2L indicate that H2L acts as a tetradendate ligand with N
and O donors (Table 1 and Scheme 1).
UV–vis experiments
Effect of the solvent
The absorption bands of azo compounds in UV–vis spectra are
usually revealed at 300–500 nm [18]. Azobenzene-containing mol-
ecules have two cis and trans isomers [19]. These compounds show
two broad absorption peaks, depending on
sitions of azo groups (–N@N–). In general, n ? p⁄ (So ? S1) transi-
tion of trans-isomer is symmetrically-forbidden, whereas
p ?
p⁄ and n ? p⁄ tran-
Molar conductivity
p
?
p⁄
(So ? S2) is symmetrically-allowed transition [20]. Trans-azoben-
zene can be transformed into cis-azobenzene by rotating or invert-
ing a phenyl ring upon photo-excitation with ultraviolet light; the
reverse transformation from cis-azobenzene to trans-azobenzene
can be obtained through photo-irradiation with visible light or
thermal isomerization. Also, it is demonstrated that visible light
source can be utilized in studying the photo-conversion of azoben-
zene-containing compounds in an appropriate solution without
using any UV light sources [20]. The effects of solvent viscosity
and polarity on the isomerization have been recently reported by
Serra and Terentjev [21]. Different dipole moments or dielectric
constants of different solvents may result in the dissimilar trans–
cis isomerization depending on the solvent.
The UV–vis spectra of Schiff-base ligand H2L have been re-
corded in solutions with different solvents: tetrahydrofuran,
dichloromethane, toluene, ethanol, acetonitrile, dimethylformam-
ide (DMF) and DMSO (Fig. 1). Ligand H2L shows two broad and
strong absorption bands at k = 389 and 495 nm in DMF and k
= 399 and 518 nm in DMSO, apparently corresponding to the
The molar conductivity of NiL and CoL complexes in DMSO are,
0.007 and 0.074 Cm2
X
ꢁ1molꢁ1, respectively. These values show
that these complexes are non-electrolyte. Therefore, Schiff-base li-
gand H2L exhibits dianionic, tetradentate with N2O2 core when
bound to Ni (II) or Co (II) metal ion.
Table 2
UV–vis data for H2L (ꢃ10ꢁ4 M) in different solvents.
Solvent
kmax (nm)
p
?
p⁄
p
?
p⁄(azo
n ? p⁄ (azo
(aromatic)
groups)
groups)
Toluene
Dichloromethane 292
THF
Ethanol
Acetonitrile
DMF
294
381
377
383
385
378
389
399
487 (weak)
487 (weak)
489 (weak)
482 (weak)
482 (weak)
495 (strong)
518 (strong)
293
291
292
293
292
DMSO

84
B. Shaabani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 81–85
H2L
1eq Cu
1 eq Ni
1 eq Co
1 eq Hg
1 eq Pb
[H2L]/([H2L]+[Co2+])
λ
Fig. 5. Jop plot for H2L and Co2+, where the absorption at 317 nm was plotted
against the mole fraction of H2L at an invariant total concentration of 10ꢁ4 M in
DMSO.
Fig. 3. UV–vis spectra of H2L (0.1 Mm) before and after adding
concentration of various metal acetates in a DMSO solution.
a 0.1 Mm
p
?
p⁄ and n ? p⁄ transitions of azo groups of calix[4]arene mol-
ecule, respectively [15,16,22]. While, in other studied solvents,
p⁄ transitions are observed as strong bands in the range of
more decreasing of
complexation.
p ?
p⁄ transition in intensity upon
p
?
377–388 nm (Table 2). In these solvents, dissimilar to DMF and
Complexes formed in solution
especially DMSO, n ? p⁄ transitions are appeared as weak bands
The binding ability of compound H2L for some heavy metal cat-
ions is investigated by UV–vis absorbance method. Absorption
spectra of 0.1 mM H2L in the presence 0.1 mM of metal ions
(Ni2+, Cu2+, Co2+, Hg2+ and Pb2+) are shown in Fig. 3. In fact, the
absorbance spectra show remarkable changes after addition of me-
tal ions. In all cases, n ? p⁄transition increases in intensity (Gener-
ally, absorbance intensity for Pb2+ and Hg2+ is more than Ni2+, Co2+
in the range of 482–487 nm (p ?
p⁄ transitions are a lot stronger
than of n ? p⁄ transitions in intensity, so that n ? p⁄ transitions
are negligible). It seems these results probably correspond to the
existence of azo groups of calix[4]arene molecule as trans configu-
ration in studied solvents exception DMSO and DMF (because
n ? p⁄ transition is strong in intensity in DMSO and DMF, while,
this transition is symmetrically forbidden and weak for trans-
isomer).
and Cu2+) and the band at around 399 nm, assigned to the
p ?
p⁄
transition of azo groups of calixarene, decreases in intensity. Com-
plexation of these metal ions for compound H2L probably affects
the equilibrium of tautomeric forms and there is likely a subtle bal-
ance between metal complexation-induced release of protons from
the azophenols to the quinone–hydrazone in DMSO solution
[10,23,24]. When H2L treats with Pb2+, a small red shift from
n ? p⁄ transition at 518 nm to 538 nm (+20 nm) is observed.
Whereas, adding one equivalent of Cu2+ ion results a blue shift
(40 nm). It can be seen as color change of solution from red to
orange-yellow. In addition to aforementioned changes, an especial
behavior is seen after adding Co2+ cation. A new absorption band
appears at wavelength at around 317 nm, indicating significant
selectivity towards Co2+ ion. This band increases in intensity with
the addition of concentration of cation.
Also, the different between the polarity [7] and viscosity [21] of
organic solvents used is likely responsible for the changes of the
band shape of the spectra.
The UV–vis spectra of NiL and CoL complexes
The UV–vis spectra for NiL and CoL complexes in DMSO in 200–
750 nm are shown in Fig. 2. Both compounds were determined to
show a strong absorption peak at 293 nm apparently correspond-
ing to
p ?
p⁄ transition of aromatic rings. There are broad and
strong bands in the absorbance spectra of NiL and CoL at
k = 521 nm and 485 nm (a blue shit to shorter wavelength), corre-
sponding to n ? p⁄ transition of azo groups respectively. The major
difference in the UV–vis spectra of NiL and CoL compared to H2L is
↑
λλ
Fig. 4. Changes in the UV–vis spectra of H2L (0.1 Mm) upon titration by Co(CH3COO)2 in a DMSO solution, where the concentration of Co(CH3COO)2 varies from 0.02–0.3 mM.

B. Shaabani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 98 (2012) 81–85
85
in DMF and DMSO, while in the other solvents, n ? p⁄ transitions
were very law in intensity. It showed that cis–trans isomerization
of azo groups of calix[4]arene molecule probably depends on sol-
vent. Also, the different between the polarity and viscosity of or-
ganic solvents used is likely responsible for the changes of the
band shape of the spectra. The study of absorbance spectra of ca-
lix[4]arene ligand with Ni(II), Co(II),Cu(II), Pb(II) and Hg(II) cations
R2= 0.9905
showed remarkable changes in the intensity of
p ?
p⁄ and n ? p⁄
intercept= 3.477
slop=1.44× 10-4
Ka= 2.40×104 M-1
transitions. Dissimilar to the spectra of ligand with studied cations,
especial changes were observed in the absorption spectra of ca-
lix[4]arene with Co2+. A new absorption band at k = 317 nm was
appeared. Upon the appearance of new absorption band and
increasing of this band in intensity with the addition of concentra-
tion of Co2+, 1:1 binding-stiochiometry was determined from Job
plot. Finally, association constant H2L with Co2+ obtained on the
base of Bensi-Hilderbrand plot.
1/[Co2+], M-1
Acknowledgement
Fig. 6. Benesi-Hilderbrand plot of H2L with Co(CH3COO)2.
We are grateful to University of Tabriz Research Council for the
financial support of this research.
UV–vis titrations
UV–vis spectra, obtained upon the addition of cobalt ions to the
solution of calix[4]arene H2L, have been presented in Fig. 4. It can
be seen that the absorbance band at k = 518 nm increases in inten-
sity with the addition of concentration of cation. While the contin-
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uous decreasing of the band at k = 399 nm, assigned to the
p–
p⁄
transition of the azo groups of calixarene and appearance a new
absorption band at k = 317 nm can be attributed to the formation
of complexes with cobalt ions. The band at k = 317 nm gradually
increases in intensity and vice versa the band at k = 399 nm disap-
pears with the extra addition of Co2+to the solution of ligand. The
spectra features in Fig. 4 are consistent with a 1:1 ratio between
calix[4]arene H2L and Co2+ ion. Further support of the 1:1 binding
ratio comes from Job’s plot experiment, where the absorptions of
the complex at 317 nm is plotted against molar fractions of H2L un-
der the conditions of an invariant total concentration. As a result,
the concentration of H2LꢄCo2+ complex approaches a maximum
when the molar fraction of [H2L]/([H2L] + [Co2+]) is about 0.5
(Fig. 5).
Among studied cations, especial remarkable changes are only
observed in the absorbance spectra of ligand with different equiv-
alents of Co2+ ion (disappearing of
p ?
p⁄ transition at k = 399 nm
and appearance a new band at 317 nm (Dk = 82 nm)). Therefore,
1:1 association constant of H2L with Co2+is determined on the base
of Bensi-Hilderbrand plot at k = 317 nm. A typical plot for ligand
and Co2+ is shown in Fig. 6. It is revealed that the stability constant
of calix[4]arene H2L with cobalt is Ka = 2.40 ꢀ 104 Mꢁ1
.
Conclusions
The investigation of UV–vis spectra of azo calix[4]arene deriva-
tive in different solvents indicated two strong and broad bands,
corresponding to the
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1658.
p ?
p⁄ and n ? p⁄ transitions of azo groups