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Published on the web January 16, 2010
A Coumarin-appended Pseudo-crown for the Selective Recognition of Fe3+
Shyamaprosad Goswami* and Rinku Chakrabarty
Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah-711 103, India
(Received October 9, 2009; CL-090909)
A coumarin-appended pseudo-crown (receptor 1) demon-
OH
OH
O
strates remarkable selectivity for Fe3+ in an acetonitrile-water
(9:1 v/v) medium (which can be determined by UV-vis and
fluorescence methods) over others studied e.g., Li+, Na+, K+,
(i)
OMe
(ii)
(iv)
(iii)
Receptor 1
+
OTs
O
O
TsO
O
+
O
CHO
Ca2+, Mg2+, Ba2+, Pb2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
and Mn2+ which are used as their perchlorate salts.
,
Scheme 2. Synthetic scheme of preparation of receptor 1.
Reagents and conditions: (i) K2CO3, TBAB, dry acetone, rt,
24 h. (ii) aq. KMnO4, 3 h. (iii) oxalyl dichloride, dry CH2Cl2, dry
DMF (cat. amount), N2-atmosphere, 3 h. (iv) dry CH2Cl2, NEt3,
rt, 12 h.
During the recent few decades an upsurge of interest is
taking place regarding the development of fluorescent probes for
transition-metal ions due to their important and fundamental
roles in a wide range of biological and environmental
processes.1 Although many works regarding the selective
detection of transition-metal ions e.g., Cu(II), Pb(II), Zn(II),
and Hg(II) have been reported, the development of receptors for
the selective detection of Fe3+ is still rare.2 Fluorescent sensors
are particularly attractive due to their simplicity, high sensitivity,
and instantaneous response. They also allow nondestructive
and quick detection of ionic species by a simple fluorescence
response.
carboxylic acid and acid chloride formation ultimately furnished
receptor 1 upon reaction with 4-hydroxy coumarin as a colorless
semisolid in an overall ca. 10.2% yield.
The preliminary UV-vis and fluorescence studies of
receptor 1 were carried out with different metal cations to
determine the binding ability of receptor 1 in CH3CN-H2O (9:1
v/v) medium. The preliminary fluorescence studies of receptor 1
shows fluorescence quenching i.e., “switching off ” with Fe3+ in
the presence of other interfering metal ions viz. Ni2+, Zn2+
,
Iron is the most abundant transition-metal ion present in the
earth’s crust. It is an important biological element due to its
diverse functions. Iron plays an indispensible role in the growth
and development of living systems.3 Iron provides the oxygen
carrying capacity of heme and acts as a cofactor in many
enzyme-catalyzed reactions which are involved in the mitochon-
drial respiratory chain. Intracellular iron level needs to be
controlled because lack of iron causes severe deficiency
symptoms, whereas excess iron may be equally harmful or even
fatal.4
We report here a designed pseudo-crown based fluorescent
sensor (receptor 1) which selectively binds Fe3+ in CH3CN-H2O
(9:1/v/v). The incorporated coumarin moiety acts as a fluoro-
phore. The benzene rings attached immediately to the crown part
get involved in cation-³ interaction. They create a hydrophobic
cavity so that the metal ions will be better encapsulated
(Scheme 1) resulting in high complex stability.
Pb2+, Hg2+, and alkali and alkaline earth metal ions even in very
large amounts in ¯M level (only a small quenching takes place
in the case of Cu2+). Therefore, for the quantitative detection of
Fe3+, receptor 1 can be efficiently and successfully used.
The binding abilities of receptor 1 toward cations have
been studied by UV-vis as well as fluorescence methods in
acetonitrile-water (9:1/v/v) medium with metal ions e.g., Li+,
Na+, K+, Ca2+, Mg2+, Ba2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+
,
Cd2+, Hg2+, Mn2+, and Pb2+ as their perchlorate salts. The
UV-vis titration spectra of receptor 1 (c = 1.21 © 10¹5 M)
exhibits a broad absorption peak at 280 nm. Upon the addition
of perchlorate salts of metal ions, the titration spectra almost
remain unchanged except in the case of Fe3+. In this case, a
remarkable and continuous increase is observed with the
concomitant increase of two newly generated peaks at 309 and
359 nm (Figure 1a). The concentration of the guest cations was
taken in the order of 10¹4 M. The value of association constant
(Ka) determined by this method has been found to be 5.23 © 105
(see Supporting Information).6 The strong peaks at higher
energy region are well-resolved and the peak at 359 nm can be
assigned due to the pseudo-crown oxygen atoms and the guest
Fe3+ charge-transfer transitions.5
Receptor 1 has been synthesized as delineated in Scheme 2.
The tetraethyleneglycol ditosylate was reacted with vanillin to
produce the desired dialdehyde which then after oxidation to
O
O
O
The 1:1 stoichiometry of complexation is further confirmed
from the value of mole fraction (0.5) in Job plot (Figure 1b).
The fluorescence titration was also performed for receptor 1
using all the perchlorate salts stated earlier to confirm the
selective binding ability of receptor 1 with Fe3+. Except for
Cu2+ and Fe3+, other metal ions show very little influence on the
spectrum of receptor 1 (-max for receptor 1 is 343 nm). After
addition of Cu2+ a small decrease takes place in the spectrum
of 1. Prominent and remarkable decrease takes place in the case
of Fe3+. This probably happens owing to the strong complex
O
O
O
O
O
Fe 3+
O
O
O
O
Fe(ClO4)3
Me Me
O
O
Me Me
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
Scheme 1. Receptor 1 and its complexation with Fe3+ cation.
Chem. Lett. 2010, 39, 100-101
© 2010 The Chemical Society of Japan
Goswami, Shyamaprosad
