Benzothiazole appended lower rim 1,3-di-amido-derivative of calix[4]arene: Synthesis, structure, receptor properties towards Cu2+, iodide recognition and computational modeling

Article abstract of DOI:10.1016/j.ica.2010.04.005

A new molecular fluorescent sensor (L) for Cu²⁺ has been synthesized by derivatizing the lower rim of calix[4]arene with benzothiazole moiety, through amide linkage to result in 1,3-di-derivative. The receptor molecule, L exhibited fluorescence quenching towards Cu²⁺ among eleven divalent ions, viz., Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Hg ²⁺, Ca²⁺, Mg²⁺ and Pb²⁺, studied. The 1:1 stoichiometry of the complex formed between L and Cu²⁺ has been demonstrated by electronic absorption and ESI-MS. The role of calix[4]arene for the selective sensing of Cu²⁺ has been established by comparing the data with that obtained for an appropriate control molecule. The minimum concentration at which L can detect Cu²⁺ has been found to be 403 ppb. The computations carried out at DFT level have provided the coordination and structural features of the Cu²⁺ complex of L as species of recognition. The Cu²⁺ complex thus formed recognizes iodide by bringing change in the color, among the 14 anions studied.

Full text of DOI:10.1016/j.ica.2010.04.005

Inorganica Chimica Acta 363 (2010) 2833–2839
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Benzothiazole appended lower rim 1,3-di-amido-derivative of calix[4]arene:
Synthesis, structure, receptor properties towards Cu2+, iodide recognition and
computational modeling
*
Roymon Joseph, Jugun Prakash Chinta, Chebrolu P. Rao
Bioinorganic Laboratory, Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
a r t i c l e i n f o
a b s t r a c t
A new molecular fluorescent sensor (L) for Cu2+ has been synthesized by derivatizing the lower rim of
Article history:
Received 29 December 2009
Received in revised form 30 March 2010
Accepted 6 April 2010
calix[4]arene with benzothiazole moiety, through amide linkage to result in 1,3-di-derivative. The recep-
tor molecule, L exhibited fluorescence quenching towards Cu2+ among eleven divalent ions, viz., Mn2+
,
Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Ca2+, Mg2+ and Pb2+, studied. The 1:1 stoichiometry of the complex
formed between L and Cu2+ has been demonstrated by electronic absorption and ESI-MS. The role of
calix[4]arene for the selective sensing of Cu2+ has been established by comparing the data with that
obtained for an appropriate control molecule. The minimum concentration at which L can detect Cu2+
has been found to be 403 ppb. The computations carried out at DFT level have provided the coordination
and structural features of the Cu2+ complex of L as species of recognition. The Cu2+ complex thus formed
recognizes iodide by bringing change in the color, among the 14 anions studied.
Available online 18 April 2010
Dedicated to Animesh Chakravorty
Keywords:
1,3-Di-benzothiazole-amido-derivative of
calix[4]arene (L)
Selective recognition of Cu2+
Fluorescence quenching
S2O2 binding core of L
DFT computations
Ó 2010 Elsevier B.V. All rights reserved.
ESI-MS
1. Introduction
the upper rim of calix[4]arene through an imine moiety. Among
the lower rim functionalized derivatives, 3-alkoxy-2-naphthoic
Copper is one of the top three biologically essential elements [1]
involved in exhibiting a variety of oxidative functions including
electron transfer [2–5], while iodide is involved only in the thyroid
function [6,7]. Insufficiency or overload of this element results in
various neurodegenerative disorders [8,9] and the insufficiency of
iodine results in goiter [10,11]. This necessitates the development
of receptors for selective recognition of Cu2+ and Iꢀ. Calix[4]arenes
[12] are important class of macrocyclic compounds possessing both
hydrophilic and hydrophobic regions and hence are suited for
receiving cations and anions selectively. Calix[4]arene derivatives
can be synthesized easily by introducing moieties having different
functional groups that can bind to metal ions through nitrogen, sul-
fur or oxygen ligating centers [13–19]. Calix[4]arene based recep-
tors for the selective recognition of Cu2+ as well as Iꢀ are rather
limited in the literature [20–28]. This includes, the report of a 1,3-
di-derivative of calix[4]arene bearing anthracene–isoxazolymethyl
at the lower rim by Chung et al. [20], which revealed chemosensor
properties towards Cu2+ by fluorescence quenching. Other litera-
ture reports for Cu2+ sensing by calix[4]arene derivatives includes
quinoline [21] or 5-nitro salicylaldehyde [22] group connected to
acid [23] and coumarin [24] appended calix[4]arenes are a few
examples for Cu2+ sensing. Our research group recently demon-
strated the selective recognition of some biologically relevant cat-
ions, viz., Zn2+ [29,30], Zn2+ and Ni2+ [31] and Cu2+ [32], as well as
environmentally relevant heavy metal ion, viz., Hg2+ [33] in addition
to an anion, viz., iodide [34] by 1,3-di-derivatives of calix[4]arene. In
this paper we report the synthesis, characterization, and cation and
anion binding properties of a new 1,3-di-derivative of calix[4]arene
connected to a benzothiazole moiety through amide linkage (L) as a
fluorescent sensor for Cu2+ and the corresponding complex as
absorption sensor for Iꢀ. Fluorescence, absorption and ESI-MS
techniques have been used for studying the binding properties.
The studies were compared with appropriate control molecules.
The coordination and structural features of the Cu2+ complex of L
has been demonstrated by computational calculations at DFT level.
2. Experimental
2.1. General
All the metal salts used in the titrations were as their perchlo-
rate salts (Caution: perchlorate salts may explode under certain
* Corresponding author. Fax: +91 22 2572 3480.
E-mail address: cprao@iitb.ac.in (C.P. Rao).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.04.005

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R. Joseph et al. / Inorganica Chimica Acta 363 (2010) 2833–2839
conditions) with a formula, M(ClO4)2ꢁxH2O and these were pro-
studies. The reference molecule L1 has been synthesized as shown
in Scheme 2 (SI 01) and was well characterized.
cured from Sigma Aldrich Chemical Cꢀo., U.S.A. Sodium salts of an-
ꢀ
ions, viz.; Fꢀ, Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, SCN , AcO , SO42ꢀ, CO32ꢀ, NO3
,
ꢀ
ꢀ
ꢀ
HSO3ꢀ, HPO42ꢀ, NO2 and N3 have been used for the titrations
and were procured from local sources. All the solvents used were
of analytical grade and were purified and dried by routine proce-
dures immediately before use. 1H and 13C NMR spectra were re-
2.4. Synthesis and characterization of 5,11,17,23–tetra–tert–butyl-
25,27-bis((2-benzothiazole)carbonylmethoxy)-26,28-
dihydroxycalix[4]arene (L)
corded on
a Varian Mercury NMR spectrometer working at
A suspension of 2-aminobenzothiazole (0.89 g, 5.9 mmol) and
Et3N (1.5 mL, 10.78 mmol) was stirred in dry THF (50 mL) under ar-
gon atmosphere. Diacid chloride, 4 (2.16 g, 2.70 mmol) in dry THF
(80 mL) was added drop wise to this reaction mixture. Immedi-
ately, a yellowish precipitate was formed and stirring was contin-
ued for 48 h at room temperature. After filtering, the filtrate was
concentrated to dryness. A yellow solid was obtained which was
extracted with CHCl3, washed with water and then with brine
and the organic layer was dried over anhydrous MgSO4. Filtrate
was concentrated to dryness and re-crystallised from EtOH/CHCl3
400 MHz. The mass spectra were recorded on Varian Inc, U.S.A.
spectrometer using electrospray ionization method. Steady state
fluorescence spectra were measured on Perkin–Elmer LS55. The
absorption spectra were measured on Shimadzu UV2101 PC. The
elemental analysis was performed on ThermoQuest microanalysis.
FTIR spectra were measured on Perkin–Elmer spectrometer using
KBr pellets. Single crystal X-ray diffraction data were collected
on an OXFORD DIFFRACTION XCALIBUR-S CCD system by
x
ꢀ2h
scan mode and the absorption corrections were applied by using
multi-scan method. The structure was solved by direct methods
to get
L as white solid. Yield (50%, 1.42 g) C62H68N4S2O6
and refined by full-matrix least squares against F2 using SHELXL
-
(1029.35). Anal. Calc. for C62H68N4S2O6. C2H5OH: C, 71.48; H,
97. Non-hydrogen atoms were refined with anisotropic thermal
parameters. All the hydrogen atoms were geometrically fixed and
allowed to refine using a riding model.
6.93; N, 5.22; S, 5.96. Found C 71.57, H 6.61, N 5.68, S 5.65%. FTIR:
(KBr, cmꢀ1): 1702 ( OH). 1H NMR: (CDCl3, d ppm): 1.10
mC@O), 3424 (m
(s, 18H, C(CH3)3), 1.19 (s, 18H, C(CH3)3), 3.51 (d, 4H, Ar–CH2–Ar,
J = 13.14 Hz), 4.23 (d, 4H, Ar–CH2–Ar, J = 13.14 Hz), 4.81 (s, 4H, –
CH2CONH–), 7.04 (s, 4H, Ar-H), 7.06 (s, 4H, Ar-H), 7.20 (t, 2H, ben-
zothiazole H, J = 7.33 Hz), 7.37 (t, 2H, benzothiazole, J = 7.03 Hz),
7.53 (d, 2H, benzothiazole H, J = 7.94 Hz), 7.66 (d, 2H, benzothia-
zole, J = 7.94 Hz), 8.72 (s, 2H, –OH), 12.29 (s, 2H, –NH). 13C NMR:
(CDCl3, 100 MHz d ppm): 31.2, 31.7 (C(CH3)3), 32.7 (Ar–CH2–Ar),
34.0, 34.4 (C(CH3)3), 74.3 (OCH2CO), 120.8, 121.6, 123.7, 125.8,
125.9, 126.4, 126.6, 132.3, 132.6, 143.2, 148.7, 149, 149.8, 156.6
(benzothiazole and calix-Ar-C), 166.5 (C@O). m/z (ES-MS)
1029.45 ([M]+ 100%), 1030.45 ([M+H]+ 25%,). Single crystals of L
were obtained by slow evaporation of the solvent mixture (EtOH/
CHCl3) at room temperature.
2.2. Solutions for fluorescence and absorption studies
Fluorescence emission spectra were measured by exciting the
solutions in acetonitrile at 300 nm and the emission spectra were
recorded in 320–500 nm range. The fluorescence studies per-
formed in CH3CN solution uses always a 50
the measurements were made in 1 cm quartz cell and maintained
a final L concentration of 10 M. During the titration, the concen-
ll solution of L. All
l
tration of metal perchlorate was varied accordingly in order to re-
sult in requisite mole ratios of metal ion to L and the total volume
of the solution was maintained constant at 3 mL in each case by
adding CH3CN. All the absorption studies were carried out in Shi-
madzu UV-2101 PC by using 10 lM solution of L. Same procedure
2.5. Synthesis and characterization of 2-(4-tert-butylphenoxy)-N-
has been followed for the titration of the reference molecule, L1
(benzothiazol-2-yl)acetamide (L1)
with metal ions.
To a solution of 1c (0.5 g, 2.40 mmol) in CH2Cl2 (85 mL) was
added Et3N (1.40 mL, 9.59 mmol), 1-ethyl-(3-dimethylaminopro-
pyl)-3-carbodiimide hydrochloride (EDCI. HCl) (0.69 g, 3.60 mmol)
and a catalytic amount of 1-hydroxybenzotriazole (HOBT) and stir-
red the solution at 0 °C for 30 min under N2 atmosphere. 2-Amino-
benzothiazole (0.54 g, 3.60 mmol) was added to this reaction
mixture and stirred at room temperature overnight. The resulting
mixture was washed with water followed by saturated NaHCO3
and brine. The product has been purified by silica gel column
2.3. Synthesis and characterization of receptor and control molecule
The receptor molecule L has been synthesized by four known
steps [35–38] starting from p-tert-butylcalix[4]arene as given in
Scheme 1 (SI 01). All the precursor molecules and the receptor
molecule, L have been well characterized by NMR, ESI MS, IR and
elemental analysis. The receptor molecule, L exists in cone confor-
mation as confirmed by both NMR spectra and single crystal XRD
S
S
N
N
EtO
HO
OEt
Cl
OH
Cl
HN
NH
O
O
O
O
O
O
O
O
OH
R
OH
OH OH
OH
O
OH
O
OH
O
OH
O
OH
O
OH
O
OH
OH
O
O
b
c
d
a
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
1
3
L
2
4
Scheme 1. Synthesis of lower rim calix[4]arene-1,3-di-amido-benzothiazole derivative, L: (a) ethyl bromoacetate/K2CO3/ acetone; (b) NaOH/C2H5OH, reflux; (c) SOCl2/
benzene, reflux; (d) 2-aminobenzothiazole/Et3N/THF. R = tert-butyl.

R. Joseph et al. / Inorganica Chimica Acta 363 (2010) 2833–2839
2835
Table 1
Crystallographic parameters for the structure determination and refinement.
L
Empirical formula
Molecular weight
T (K)
C
62H68N4O6S2 2CHCl3 C2H5OH
N
S
1314.15
293
Crystal system
Space group
Triclinic
P1 (no. 2)
14.575 (3)
15.184 (3)
16.347 (3)
ꢀ
HN
HO
EtO
0
a (AÅ)
O
O
O
0
b (AÅ)
0
O
O
c (AÅ)
O
OH
a
(°)
79.82 (3)
74.823 (3)
75.19 (3)
3352.2 (1)
b (°)
(a)
(b)
(c)
c
(°)
0
V (AÅ3)
Z
2
Absorption coefficient (mmꢀ1
Dcalc. (g cmꢀ3)
Reflections collected
Unique reflections
Rint
)
0.373
1.302
52 819
22 167
0.035
22 167
761
R
1c
R
L1
R
1b
R
1a
Scheme 2. Synthesis of L1: (a) ethyl bromoacetate/K2CO3/acetone; (b) KOH/
C2H5OH/water, reflux; and (c) 2-aminobenzothiazole/Et3N/EDCI. HCl/HOBT/CH2Cl2.
R = tert-butyl.
Reflections used
Parameters
Final R
wR2
0.0812
0.2602
chromatography using chloroform–methanol (9.5:0.5) as eluent to
give white solid. Yield (0.51 g, 62%). C19H20N2SO2 (340.44). Anal.
Calc. for C19H20N2SO2: C,66.82; H, 5.98; N, 8.52; S, 9.36. Found. C
67.03, H 5.92, N 8.23, S 9.42%. FTIR: (KBr, cmꢀ1): 1686 ( C@O). 1H
m
NMR: (CDCl3, d ppm): 1.31 (s, 9H, C(CH3)3), 4.73 (s, 2H, OCH2),
6.90 (d, 2H, Ar-H, J = 8.55 Hz), 7.32–7.38 (m, 3H, Ar-H & Benz-H),
7.46 (t, 1H, Benz-H, J = 8.25 Hz), 7.80–7.85 (m, 2H, Benz-H), 9.98
(s, 1H, CONH). 13C NMR: (CDCl3, 100 MHz d ppm): 31.5 (C(CH3)3),
34.3 (C(CH3)3), 67.0 (OCH2CO), 114.2, 121.3, 121.5, 124.3, 126.4,
126.7, 132.3, 145.5, 148.3, 154.5, 156.9 (Benz and Phenyl Ar-C),
167.3 (C@O). m/z (ES-MS) 341.12 ([M+H]+ 25%,).
and recording the emission spectra in the range 320–500 nm
wherein the emission maximum was observed ꢂ355 nm. The metal
ions used in the recognition studies include, Mn2+, Fe2+, Co2+, Ni2+
,
Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, Mg2+ and Ca2+. The fluorescence inten-
sity gradually diminishes as a function of the addition of Cu2+ and
a saturation in its quenching is observed at [Cu2+]/[L] mole ratio
greater than 10 suggesting that the binding is an equilibrium driven
reaction (Fig. 2a). All the other M2+ ions have not shown any appre-
ciable change in the fluorescence intensity indicating their non-
interactive nature towards L (Fig. 2b). The fluorescence quenching
fold observed in case of L with Cu2+ is 8 0.6 and yields a Ka of
18893 1200 Mꢀ1 based on Benesi–Hildebrand equation. The min-
imum concentration at which L can detect Cu2+ has been found to
be 403 ppb as measured based on dilution experiment carried out
by keeping Cu2+ to L ratio at 1:1 (SI 03).
3. Results and discussions
3.1. Crystal structure of L
Slow evaporation of a solution mixture of CHCl3 and ethanol
containing L resulted in good quality single crystals suitable for
X-ray diffraction studies and the corresponding crystallographic
The role of calix[4]arene platform in the selective recognition of
Cu2+ has been examined by comparing the fluorescence results of L
with a single strand derivative (L1, Scheme 2) having p-tert-butyl-
phenoxy moiety instead of calix[4]arene platform, with M2+ ions,
viz., Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Ca2+, Mg2+ and
Pb2+. None of the M2+ including Cu2+ showed appreciable change
in the fluorescence intensity during the titration indicating that
L1 is nonselective and hence supports the necessity of calix[4]arene
platform for selective recognition of Cu2+ (Fig. 3).
ꢀ
data fits well with triclinic system with space group P1 (SI 02)
and the corresponding crystallographic parameters were given in
Table 1. The asymmetric unit cell possesses one molecule of L,
two molecules of CHCl3 and one molecule of C2H5OH. The intramo-
lecular hydrogen bonding present at the lower rim of calix[4]arene
fixes the ligand L in cone conformation (Fig. 1a). The inter rim
hydrogen bonds exhibited donorꢁꢁꢁacceptor distances of 2.674(2)
and 2.672(2) Å respectively between the unsubstituted phenolic
–OH and oxygen of the substituted arm. The amide CO of both
the arms point outside the calix[4]arene cavity. The distance be-
tween the CO and S was observed to be 2.705(3) and 2.731(3) Å
in both the arms suggesting an almost symmetric distribution of
these moieties with respect to the calix[4]arene frame. Further,
the amide NH present in each of the arms make a hydrogen bond
with the phenolic-OH located at lower rim with NꢁꢁꢁO distances
of 2.967(3) and 2.874(4) Å. The ethanol molecule sits inside the
arene cavity without extending any interactions. In the lattice,
the molecules are arranged in columns and the chloroform mole-
cules present between these columns do not extend any interac-
tions with L (Fig. 1b).
3.3. Absorption and ESI-MS titration studies
The binding of L with Cu2+ has been further demonstrated by
absorption spectroscopy wherein the spectrum of L exhibits
mainly three bands at 300, 288 and 275 nm, respectively
(Fig. 4a). The bands at 252 and 300 nm showed increase in absor-
bance upon Cu2+ addition (SI 03). The stoichiometry of the Cu2+ and
L complex has been calculated to be 1:1 based on the Job’s plot
(Fig. 4b). A more direct evidence for the formation of 1:1 complex
has been obtained by ESI mass spectrum when one equivalent of
Cu2+ was added to a solution of L in CH3CN (SI 04). The ESI-MS
spectrum of L with Cu2+ resulted in a molecular ion peak at m/z
of 1091.6 corresponding to a 1:1 species of Cu2+ and L, and the
presence of Cu2+ in this species has been shown based on the iso-
topic peak pattern observed with this peak (Fig. 4c). The 1H NMR
3.2. Fluorescence titration studies
The binding ability of L towards M2+ has been studied by fluores-
cence spectroscopy in acetonitrile at 10 lM by exciting L at 300nm

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R. Joseph et al. / Inorganica Chimica Acta 363 (2010) 2833–2839
Fig. 1. (a) Single crystal XRD structure of L as ORTEP. (b) Lattice structure of L inclusive of the solvent molecules.
200
a
b
8
6
4
2
0
150
100
50
0
320
Mn
Fe
Co
Ni
Cu
Zn
Cd
Hg
Ca
Mg
Pb
360
400
440
480
Wavelength, nm
Fig. 2. (a) Fluorescence spectral traces obtained during the titration of L with different mole ratios of Cu2+. (b) Histogram representing the changes in the fluorescence
response of L in presence of different M2+ ions.
titration revealed the formation of Cu2+ complex of L by exhibiting
These titrations resulted in no significant change in the fluores-
cence intensity suggesting that other M2+ ions do not replace
Cu2+ from the binding site of L even after 50 equivalent addition
of these ions (Fig. 5), viz., a selective binding for Cu2+. Thus it
may be concluded that L is a potential Cu2+ selective receptor even
in the presence of other divalent metal ions.
broadened signals owing to the presence of paramagnetic Cu2+
.
3.4. Competitive metal ion titrations
In order to explore the practical utility of L as an ion-selective
fluorescence chemosensor for Cu2+, competitive experiments were
carried out in presence of other metal ions by titrating 1:10 equiv-
alents of L and Cu2+ with different mole ratio of other M2+ ions.
3.5. Computational studies
In order to establish the coordination and structural features of
the species of recognition, computational calculations were carried
out using GAUSSIAN 03 package [39]. The crystal structure of L has
been taken as initial guess and was optimized by replacing each
tert-butyl group in L by hydrogen. Replacement of tert-butyl group
by hydrogen has been done in order to reduce the computational
times without compromising the chemical and conformational fea-
tures of the derivative. The calculations were performed in a cas-
cade fashion by going through different levels of theories, viz.,
AM1 ? HF/STO3G ? HF/3-21G. The optimization of the complex
was carried out by simply placing the Cu2+ far above the binding
core so that there are no interactions present between L and Cu2+
in the beginning. The final optimization of the copper complex
1.0
0.8
0.6
0.4
0.2
0.0
*
has been carried out by DFT method using B3LYP/6-31g . The
resultant structure has been further subjected to the interaction
with acetonitrile. The optimization carried out in the presence of
0
5
10
15
20
acetonitrile in B3LYP/6-31g resulted in a complex where Cu2+ ex-
*
[M2+]/[L1]
hibit a distorted trigonal bipyramidal geometry wherein each arm
of L acts as bidentate in filling a total of four coordinations, and the
fifth coordination comes from acetonitrile, resulting in a NO2S2
binding core (Fig. 6a). Comparison of the crystal structure of L with
Fig. 3. Plot of relative fluorescence intensity L1 vs. mole ratio of M2+ during the
titration. The symbols corresponds to j = Mn2+ = Fe2+ N = Co2+ . = Ni2+
J = cu2+; I = Zn2+; ꢀ = Cd2+; s = Hg2+; h = Ca2+; d = Mg2+ and H = Pb2+
;
D
;
;
;
.

R. Joseph et al. / Inorganica Chimica Acta 363 (2010) 2833–2839
2837
0.4
0.3
0.2
0.1
0.0
a
b
c
0.32
0.24
0.16
0.08
0.00
240 270 300 330 360
Wavelength, nm
0.0 0.2 0.4 0.6 0.8 1.0
n
m
Fig. 4. (a) Absorption spectral traces obtained during the titration of L with different mole ratios of Cu2+; (b) Job’s plot to determine the L to Cu2+ stoichiometry; (c) Expanded
ESI-MS spectrum of [CuL-H+]+ showing the isotopic pattern.
0
O1ꢁꢁꢁO5 distance decreases from 6.290 to 3.412 ÅA on going from
1.0
simple L to its Cu2+ complex.
0.8
3.6. Anion recognition studies
0.6
The Cu2+ complex formed with L has been further titrated in ace-
tonitrile solution for studying the recognition of anions. These stud-
ies were carried out at the same concentration as that of the
0.4
fluorescence titrations carried out using metꢀal ions. Among the 14
ꢀ
anions, viz., F-, Clꢀ, Brꢀ, Iꢀ, ClO4ꢀ, SCN , AcO , SO42ꢀ, CO32ꢀ, NO3
,
ꢀ
0.2
0.0
ꢀ
HSO3ꢀ, HPO42ꢀ, NO2 and N3ꢀ, studied, only iodide ion exhibit
change in the color of the solution from colorless to pale-yellow,
where as all other ions showed no change in the color (Fig. 7). Thus
the presence of iodide can be visually detected.
0
10
20
30
40
50
[M2+]/[L+Cu2+(1:10)]
Absorption titration carried out between Cu2+ and Iꢀ resulted in
some spectral changes, such as, the formation of a new band at
362 nm and increase in the absorbance of 244 and 288 nm bands,
as can be seen from Fig. 8. No such changes were observed in the
absorption spectra in case of other anions (SI 06).
Fig. 5. (a) Plot of fluorescence intensity versus mole ratio of added M2+ in a solution
contains L and Cu2+ present as 1:10 ratio. The symbols corresponds to j = Mn2+
= Fe2+; N = Co2+; . = Ni2+; I = Zn2+; ꢀ = Cd2+; s = Hg2+; h = Ca2+; d = Mg2+ and
H = Pb2+
;
D
.
that of the optimized structure of the copper complex revealed
only marginal changes in the dihedral angles of the arms (SI 05),
while the arms move farther apart to accommodate the Cu2+ in
the binding core formed by the benzothiazole and amide moieties
(Fig. 6b,c). The distances between S1, S2 and O1, O5 were adjusted
4. Conclusions
In summary, an effective molecular receptor, L for the selective
recognition of Cu2+ followed by iodide, has been synthesized by
appropriate derivatization at the lower rim of calix[4]arene and L
has been characterized. The pre-organized binding core present
in L is good enough to hold Cu2+ even after the addition of excess
in such a way that Cu2+ can accommodate in the O2S2 binding core,
0
where the S1ꢁꢁꢁS2 distance increases from 3.769 to 4.989 ÅA and the
0
*
Fig. 6. (a) B3LYP/6-31G optimized structure of [CuL]2+. The bond distances (AÅ) and bond angles (°) around Cu2+ in the coordination sphere are: Cu–N5 = 1.922, Cu–S1 = 2.493,
0
Cu–S2 = 2.522, Cu–O1 = 2.139 and Cu–O5 = 2.095 ÅA; and S1–Cu–S2 = 168.2, N5–Cu–O1 = 124.1, N5–Cu–O5 = 128.5, O1–Cu–O5 = 107.4, S1–Cu–N5 = 96.2, S1–Cu–O1 = 77.3,
S1–Cu–O5 = 95.2, S2–Cu–N5 = 95.5, S2–Cu–O1 = 95.5, S2–Cu–O5 = 77.9°. (b) Space filling model for L based on the crystal structure. (c) Space filling model for the optimized
structure of the copper complex given under (a).

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R. Joseph et al. / Inorganica Chimica Acta 363 (2010) 2833–2839
(a) (b)
(c)
(e)
(d)
(f) (g)
(h) (i)
(j)
(k)
(n)
(l) (m)
(o)
Fig. 7. Color changes observed during the titration of [CuL]2+ by different anions: (a) no anion, (b) SCNꢀ, (c) HSO3-, (d) AcOꢀ, (e) Fꢀ, (f) Clꢀ, (g) Brꢀ, (h) Iꢀ, (i)ClO4ꢀ, (j) NO2ꢀ, (k)
2ꢀ
N3ꢀ, (l) NO3ꢀ, (m) SO42ꢀ, (n) CO32ꢀ, (o) HPO4
.
2.0
1.6
1.2
0.8
0.4
0.0
4
a
(b)
3
2
1
0
200
250
300
350
400
0
5
10
15
20
2+
-
Wavelength, nm
[I ]/[CuL]
Fig. 8. (a) Spectral traces obtained during the titration of [CuL]2+ (5:1) with increasing mole ratios of Iꢀ, (b) plot of absorbance versus mole ratio of [Iꢀ]/[CuL]2+ added. Where
s = 362, d = 288 and H = 244 nm.
amount of other M2+ ions as demonstrated through competitive
titrations. Comparison of the fluorescence results of L with those
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of L1 clearly explains the necessity of calix[4]arene platform for
selective Cu2+ detection. The 1:1 complex formed between Cu2+
and L was evident from ESI-MS and absorption studies. The ligand,
L was able to detect Cu2+ even at a low concentration of 403 ppb.
The 1H NMR titration revealed the formation of Cu2+ complex of
L by exhibiting broadened signals owing to the presence of para-
magnetic Cu2+. The computational calculations carried out in a cas-
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cade fashion revealed the formation of
a distorted trigonal
bipyramidal Cu2+ complex, wherein the entry of Cu2+ ion seems
to be affected by widening the arms so as to take the ‘S’ centers far-
ther and amide-CO’s closer. In the complex, L acts as tetradentate
ligand providing its ligations through O2S2 core, wherein the 5th
coordination is filled by the solvent acetonitrile, thus providing
structural identity for the species of recognition. The 1:1 complex
thus formed exhibits color change in the presence of iodide only
among the fourteen anions studied and hence the recognition of io-
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CPR acknowledges the financial support from DST, CSIR and
DAE-BRNS. RJ thanks UGC and JPC thanks CSIR for their senior re-
search fellowships respectively. We thank SAIF for ESI-MS and DST’s
national single crystal XRD facility of IIT Bombay for crystal data.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.04.005.
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