Cu(II) complex of an abiotic receptor as highly selective fluorescent sensor for acetate

Article abstract of DOI:10.1016/j.tetlet.2009.08.021

A copper complex having quinoline moiety as fluorophore has been synthesized. The anion recognition behavior of the receptor and its copper complex has been studied in acetonitrile and in acetonitrile: H₂O (95:5 v/v). The copper complex shows high selectivity toward acetate over other anions studied such as F^-, Cl^-, Br^-, I^-, OAc^-, dl-malate, l-mandelate, benzoate, isophthalate, H₂ PO₄^-, NO₃^- and HSO₄^-.

Full text of DOI:10.1016/j.tetlet.2009.08.021

Tetrahedron Letters 50 (2009) 5994–5997
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cu(II) complex of an abiotic receptor as highly selective fluorescent sensor
for acetate
*
Shyamaprosad Goswami , Rinku Chakrabarty
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A copper complex having quinoline moiety as fluorophore has been synthesized. The anion recognition
behavior of the receptor and its copper complex has been studied in acetonitrile and in acetonitrile:
H2O (95:5 v/v). The copper complex shows high selectivity toward acetate over other anions studied such
Received 27 May 2009
Revised 31 July 2009
Accepted 7 August 2009
Available online 19 August 2009
as FÀ, ClÀ, BrÀ, IÀ, OAcÀ, DL-malate,
L
-mandelate, benzoate, isophthalate, H2PO4À, NO3 and HSO4
.
À
À
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Acetate recognition
Cu(II) complex
Fluorescent sensor
The design and synthesis of abiotic receptors for anion recogni-
tion is a rapidly expanding area of interest.1 Since anions play an
important role in many biological and environmental processes,2
selective recognition of a particular anion is an important chal-
lenge in the field of supramolecular chemistry. Anions are mainly
recognized through hydrogen-bonding interactions,3 electrostatic
interactions,4 coordination through metal ions,5 etc. Development
of metal-based receptors6 has recently been the center of interest
as metal ions pre-organize the binding sites structurally for opti-
mal anion-binding through hydrogen bonding and metal ion coor-
dination, resulting in strong affinities than purely organic
receptors.
phore. The formation of copper complex locks the cavity and orga-
nizes the molecule to selectively bind acetate anion. Again the
copper cation provides additional binding site to the acetate anion
resulting in high selectivity measured in fluorescence spectra.
The receptors 1 and 2 were synthesized as shown in Scheme 1.
Isophthalic acid upon esterification yielded the corresponding di-
methyl ester. The diester was then refluxed with ethylenediamine
to afford the diamido diamine (B). We prepared receptor 1 directly
from compound B and compound A under reflux in about 27%
yield. The 1H NMR spectrum of receptor 1 revealed the presence
of two NH protons at 8.66 and 7.82 ppm. Complex 2 was prepared
from compound 1 when it was reacted with Cu(ClO4)2.
Binding of anion through hydrogen bonding is really difficult in
water as anions become highly solvated in water and there exists a
competition of water and anions toward binding with the host
molecules. Additional binding sites like metal coordination may
help in recognizing anions in the presence of water.
The green-colored complex 2 was synthesized by reacting
receptor 1 (yellow) with Cu(ClO4)2Á6H2O (Scheme 1) in dry aceto-
nitrile at refluxing condition. The HRMS mass spectrum reveals
the formation of complex 2 [m/z = 684.6462 (found), the calculated
value is 684.6458]. The attempts to grow single crystals of complex
2 which are suitable for X-ray structure determination were not
successful.8 Energy minimization using PCMODEL Serena software
helped us to understand the interactions between the Cu(II) and
receptor 1. Previously reported copper containing receptors9 which
recognize anions show current interest and progress in the area of
anion recognition research.
The photophysical behavior of receptors 1 and 2 was investi-
gated using acetonitrile as a solvent. The emission maximum for
receptor 1 appears at 399 nm when it is excited at 298 nm, but
for complex 2, the emission maximum appears at 411 nm at an
excitation wavelength of 305 nm using acetonitrile as a solvent.
When acetonitrile:H2O (95:5 v/v) is used (receptor 2), the emission
maximum gets shifted to 390 nm at an excitation wavelength of
302 nm. The appearance and nature of the emission bands of
Selective sensing of acetate is particularly topical7 due to its sig-
nificance in numerous metabolic processes for example, a signifi-
cant number of metal enzymes having cations in their active
sites influence various hydrolysis processes. The rate of acetate
production and sedimentation has frequently been used to indicate
the organic decomposition in marine sediments. Carboxylate an-
ions play an important role in nylon industry. They are also used
in the manufacture of paper, plastics, dyes and paints.
We herein report a new metal (copper)-based receptor which
selectively binds acetate anion. The receptor employs nitrogen
atoms as donor centers. Here, quinoline moiety acts as a fluoro-
* Corresponding author. Tel.: +91 3326684561 3; fax: +91 3326682916.
E-mail address: spgoswamical@yahoo.com (S. Goswami).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.021

S. Goswami, R. Chakrabarty / Tetrahedron Letters 50 (2009) 5994–5997
5995
O
OH
O
OEt
N
N
(i)
A
(ii)
O
O
O
O
O
OMe
OMe
OH
OH
O
(iii)
O
O
+
H2N
NH2
NH
NH2
HN
OMe
OMe
H2N
B
O
O
O
O
OEt
(iv)
(v)
N
NH
NH2
HN
+
Receptor1
Receptor2
H2N
Scheme 1. Reagents and conditions: (i) ethyl chloroacetate, dry K2CO3, dry acetone, TBAB (0.5 mol %); (ii) dry MeOH, conc H2SO4 (two drops), reflux; (iii) and (iv) acetonitrile,
reflux; (v) Cu(ClO4)2Á6H2O, acetonitrile, reflux.
receptors 1 and 2 indicate the coordination of copper with the
quinoline moiety because the two receptors were taken in equal
concentrations.
of a metal center (Cu2+) provides additional binding site to the an-
ion either through electrostatic or covalent bond formation result-
ing in high selectivity of 2 toward acetate in acetonitrile as well as
in acetonitrile–water mixture (9:1 v/v). Similarly the cavity of 2
selectively binds OAcÀ over C6H5COOÀ which may be due to the
steric hindrance of the phenyl ring at the binding site of the copper
complex.
The photophysical property of copper complex as a selective
host for acetate was studied in detail by titration of host and ace-
tate in acetonitrile and in acetonitrile–water (95:5 v/v) using tetra-
butylammonium acetate. The fluorescence intensity increased
gradually (SI: Fig. S3) as we added the guest solution. The associa-
tion constant Ka of complex 2 for acetate in acetonitrile–water was
found to be 5.11 Â 105 MÀ1 which was calculated using the re-
ported literature method.11
The anion-binding ability of the receptors 1 and 2 has been
studied by UV–vis and fluorescence methods (Fig. 1) using tetrabu-
tylammonium salt of particular anions in acetonitrile and acetoni-
trile–water (for 2). The change of fluorescence intensity of receptor
1 after addition of anions as their tetrabutylammonium salts is
very small. After addition of a large amount of acetate the intensity
decreases in acetonitrile–water (9:1 v/v) solvent. Other anions
influence no noticeable changes. However due to the addition of
very larger amounts of anion salts the fluorescence intensity of
receptor 1 decreases in very small amounts like acetate. The inten-
sity changes (of receptor 2) in acetonitrile (Supplementary data)
and acetonitrile–water after addition of particular anion solutions
are depicted in Figure 1. Figure 1 shows that there was no signifi-
From the fluorescence spectra, it is clear that the complex
shows sensitivity toward acetate by increasing the emission inten-
sity. The stoichiometry of complexation can be determined by JOB
plot (Fig. 3a). The JOB plot between the copper complex and ace-
tate shows maxima at mol fraction 0.5, suggesting 1:1 complexa-
tion. Another typical experiment for evaluation of the selectivity
of acetate over other anions was carried out for receptor 2 in ace-
tonitrile and in acetonitrile–water mixture. In this experiment,
other anions which may interfere were added in 10 times higher
concentration with respect to acetate, and the fluorescence inten-
sities were measured. The fluorescence intensities almost remain
unchanged as shown in Figure 3b. Slight interference can be seen
cant change after addition of other anions studied (FÀ, ClÀ, BrÀ, IÀ,
À
DL-malate,
L
-mandelate, benzoate, isophthalate, H2PO4À, NO3 and
HSO4À). Thus complex 2 can be used for selective recognition of
acetate.
The fluorescence ratio of receptor 2 displayed in Figure 2 also dis-
played the enhancement of emission spectra in the cases of iodide,
À
acetate, DL-malate, L-mandelate, benzoate, isophthalate, H2PO4
and NO3À. Slight decrease takes place in the cases of FÀ, ClÀ and
BrÀ (decrease with red shift in case of HSO4À) in acetonitrile.
The preference of acetate over other anions may be due to the
size and shape of the host and guest being complementary to each
other. The acetate anion was encapsulated within the cavity
formed after complexation of receptor 1 with copper cation in a
better way as compared to the other anions studied. Marked and
selective enhancement in the fluorescence emission intensity
was observed in presence of acetate ion. This effect is probably
due to a consequence of the strong complexation (of acetate with
the copper complex) with a decrease in non-radiative rate con-
stant.10 Again besides the size and shape selectivity, the presence
À
in the case of HSO4 when concentration of acetate anion is very
less ꢀ10–30
lM in acetonitrile. But in the case of acetonitrile–
water mixture, such kind of interferences was almost negligible.
This experiment was also carried out in the case of receptor 1 to
clarify any interference of other anions. Here, we noticed no appre-
ciable positive results (SI: Fig. S10).
To understand the binding behavior of receptor 2 with anions,
we performed model study12 (see Supplementary data) using

5996
S. Goswami, R. Chakrabarty / Tetrahedron Letters 50 (2009) 5994–5997
8
Acetate
O
O
6
4
2
0
NH
HN
HN
O
NH
O
O
O
N
N
Receptor 1
Hydrogen bonding
array for anion
sensing
O
O
Figure 2. Fluorescence ratio (Io À I/Io) of complex 2 at 411 nm upon addition of
20 M of a particular anion in CH3CN–H2O (95:5 v/v).
NH
HN
l
O
NH
NH
O
O
a
4
3
2
1
Cu2+
N
Vacant site for
anion recognition
O
N
Receptor 2
600
500
400
300
200
100
0
Host [Cu(II)complex]
Fluoride
Chloride
Bromide
Iodide
0.0
0.2
0.4
0.6
0.8
1.0
Acetate
[H]/[H]+[G]
DL-malate
L-mandelate
Benzoate
Isophthalate
b
Dihydrogen phosphate
Nitrate
Fluoride+Acetate
Chloride+Acetate
Bromide+Acetate
Iodide+Acetate
Hydrogen sulphate
210
Acetate
DL-malate+Acetate
L-mandelate+Acetate
Benzoate+Acetate
Isophthalate+acetate
180
150
120
400
500
600
H2PO4-+Acetate
NO3-+Acetate
HSO4-+Acetate
Wavelength(nm)
Figure 1. Fluorescence spectra of receptor 2 after addition of 1 equiv of guest
substrates in acetonitrile–water (95:5 v/v).
PCMODEL Serena Software where we can see that the minimum
energy (Emin) determined in this method is 60.2, 40.5, and À23.7
(in KJ molÀ1) respectively (Fig. S15). The Emin values revealed that
copper complex of receptor 1, that is, complex 2 is more stable
than receptor 1. Again complex 2 forms a more stable complex
with acetate ion as the energy value is further lowered after bind-
ing with acetate ion. Due to the more organized structure of the
copper complex, stabilization energy in the case of 2 decreases
upon complexation of 1 with copper. The inclusion of acetate anion
into the rigid pre-organized cavity of the Cu(II) complex induced a
rigidification with the consequence of the decrease of the Emin
value observed.
90
0.00
0.02
0.04
0.06
0.08
0.10
vol. of acetate added
Figure 3. (a) Job plot between complex 2ÁCu(II) and acetate in acetonitrile–water
(95:5 v/v). The concentration of [HG] was calculated by the equation [HG] = I/IoX
[H]; (b) Estimation of acetate in the presence of other anions (10 times) in
CH3CN:H2O (95:5 v/v) (at 390 nm).
D
In conclusion, we have reported the design and synthesis
of receptor 1 and its copper complex (receptor 2) for selective

S. Goswami, R. Chakrabarty / Tetrahedron Letters 50 (2009) 5994–5997
5997
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the selective recognition of acetate anion has been designed and
studied in CH3CN and in CH3CN–H2O (95:5 v/v). The rigid pre-orga-
nized cavity of the Cu(II) complex leads to the selective inclusion of
the acetate anion into its cavity with the consequence of the ob-
served emission enhancement.
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Acknowledgments
We wish to express our appreciation to CSIR [Project No.
01(1913)/04/EMR-II] and DST [Project No. SR/S1/OC-13/2005],
Govt. of India for financial support. R.C. thanks CSIR, Govt. of India,
for a research fellowship.
Supplementary data
Supplementary data (1H NMR, 13C spectra, of receptor 1 and the
mass spectra of receptors 1 and 2, general procedure of titrations,
detailed experimental procedure and energy minimized structures
of receptors) associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2009.08.021.
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