Azacrown-oxabridged macrocycle: A novel hybrid fluorogenic chemosensor for transition and heavy metal ions

Article abstract of DOI:10.1039/b820479a

A novel hybrid of azacrown ether-oxabridged fluorogenic chemosensor (L1) has been synthesized: the metal ion binding ability of the hybrid molecule shows higher affinity towards transition metal ion Zn²⁺ and heavy metal ion Hg²⁺ comp

Full text of DOI:10.1039/b820479a

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Azacrown-oxabridged macrocycle: a novel hybrid fluorogenic
chemosensor for transition and heavy metal ionsw
Faiz Ahmed Khan,* Karuppasamy Parasuraman and Kalyan Kumar Sadhu
Received (in Cambridge, UK) 17th November 2008, Accepted 20th February 2009
First published as an Advance Article on the web 13th March 2009
DOI: 10.1039/b820479a
A novel hybrid of azacrown ether-oxabridged fluorogenic
chemosensor (L1) has been synthesized: the metal ion binding
ability of the hybrid molecule shows higher affinity towards
transition metal ion Zn2+ and heavy metal ion Hg2+ compared
to the parent azacrown ether which shows poor metal ion
selectivity: the binding modes of the Zn2+ and Na+ towards
L1 have been discussed.
This was accomplished in excellent yield by treating 1 with
NaBH4 in THF under reflux condition, a convenient method
reported by us for related systems,6 which furnished 2
Ever since the discovery of crown ethers by Pedersen in 1967,
they have been playing a prominent role as essential materials
in the field of host–guest chemistry.1 The design and synthesis
of crown ether-based fluorescent sensors2 that display high
selectivity and sensitivity towards metal cations, particularly
transition metal ions, has attracted considerable attention. The
selectivity of the crown ether molecules towards a specific
metal ion may be modulated by varying its size or the shape.
While the influence of cavity size on selectivity is easily
comprehensible but subtle changes in the shape might cause
unexpected or sometimes dramatic variation in selectivity.
One way to influence the shape is to embed the crown
ether moiety onto a constrained framework and perform a
comparative study with the parent crown ether to ascertain the
impact of such geometrical constraints. However, very few
examples of crown ethers annulated to constrained scaffolds
are available in the literature.3 Recently, we had demonstrated
an easy access to sterically constrained oxabridged derivatives.4
It occurred to us that building an aza crown around the
oxabridge moiety would lead to a oxabridge-aza crown hybrid
molecule whose shape is constrained due to the rigid tricyclic
molecular scaffold. In this account, we wish to disclose the
unexpected selectivity and sensitivity of the oxabridge-aza
crown hybrid molecule towards Zn2+ and Hg2+
.
The tricyclic oxabridged derivative 1, obtained in two steps
from the Diels–Alder adduct of 5,5-dimethoxy-1,2,3,4-
tetrachlorocyclopentadiene with cyclohexene, was selected
for the proposed elaboration to a hybrid macrocycle for
studying its selectivity for various metal ions based on the
widely employed receptor-spacer-fluorophore concept.2a,5
Reduction of the ester groups in 1 would furnish a diethylene
glycol moiety, a common building block for crown ethers,
such that the central ether oxygen constitutes the oxabridge.
Department of Chemistry, Indian Institute of Technology,
208016 Kanpur, India. E-mail: faiz@iitk.ac.in;
Fax: +91 512-2597436; Tel: +91512-2597864
w Electronic supplementary information (ESI) available: Experimental
details, spectral data of all new compounds, X-ray analysis of
compounds L1, L1 + Na. CCDC 707106 & 707107. For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b820479a
Scheme 1 Synthesis of L1.
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(Scheme 1). The diol 2 was then treated with bis(2-chloroethyl)
ether in the presence of 50% NaOH under PTC condition7 to
obtain the coupled product 3 in 85% yield. Compound 3 was
reacted with p-toluene sulfonamide and K2CO3 in acetonitrile
to furnish the tosyl protected mono-aza-18-crown-6 4 in good
yield. Deprotection of tosyl group using Na/naphthalene in
DME8 followed by coupling the resultant mono-aza-18-
crown-6 5 with 9-bromo methyl anthracene yielded the target
macrocycle L1.z
The photophysical properties of the hybrid macrocycle L1
were studied in dry acetonitrile (Fig. S1).w The UV-Vis
absorption data of the metal free compound was found to
be similar to that of the characteristic 9-substituted anthracene
spectra.9 The band at 385 nm suggests a S0 - S1 transition
and the Frank-Condon vibrational structures are observed at
366 and 347 nm. To investigate the binding properties of L1
towards metal ions, hydrated perchlorate salts of metal ions
were used. In the presence of alkali and alkaline earth metal
ions, no positional shift in the anthracene absorption band
was observed. However small red shift (B4 nm) was observed
when a transition metal ion such as Co(II), Ni(II), Cu(II), Zn(II),
or a heavy metal ion such as Ag(I), Cd(II), Hg(II), Pb(II)
was added.
Fig. 2 Fluorescence response of L1 and L2 in the presence of various
metal ions in acetonitrile. [L1], [L2] = 1.9 ꢁ 10ꢂ7 M, [Mn+] = 1.9 ꢁ
10ꢂ5 M. lex = 350 nm.
parent monoaza-18-crown-6 (L2),2a,10 under similar experimental
conditions, clearly demonstrated that the rigid tricyclic scafold
plays a crucial role, through subtle changes in the shape of the
macrocycle, in modulating its selectivity towards Zn2+ and Hg2+
which is totally absent in the parent monoaza-18-crown-6. The
importance of sensing Zn2+ is an emerging area of contemporary
interest.11 In this context the ligand (L1) appears to offer a
potential analytical tool for the detection of Zn2+. Interestingly,
L1 also shows reasonable selectivity towards the Hg2+ ion, a
toxic heavy metal ion. Designing the macrocyclic chemosensor to
detect trace amount of Hg2+ ion in the environment is an active
area of current research.12
The emission behavior of the L1 was attributable to the
anthracene monomeric emission with the typical characteristic
emission bands between 375–525 nm with a (0, 0) band at
390 nm (lex = 350 nm). The ligand L1 showed very low
fluorescence quantum yield (F = 0.004) in the absence of
metal ions in dry CH3CN due to efficient intramolecular
photoinduced electron transfer (PET) process (Fig. 1). In
presence of an alkali metal ion (Li+, Na+, K+) no considerable
fluorescence enhancement was observed, while addition of
Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Cd2+, and Pb2+ ions afforded
slightly enhanced fluorescence emission (F = 0.07–0.11). In case
of heavy metal ions, only Ag+ (F = 0.228, 57 fold) and Hg2+
(F = 0.277, 69 fold) exhibited moderate values of quantum
yields. In the presence of Zn(II), highest enhancement of the
fluorescence (F = 0.393, 98 fold) was observed (Fig. 2).
Comparison of metal ion binding results of L1 and that of the
To understand the coordination mode of the metal ions with
ligand L1, titration experiment was performed for Zn2+ and
Hg2+ in dry acetonitrile and the emission band due to the
9-substituted anthracene was monitored. The intensity of the
emission gradually increases attaining a maximum (Fig. 3a)
upon addition of one equivalent of Zn2+ ion. Thereafter, no
Fig. 3 (a) Fluorescence spectral changes of L1 (1 ꢁ 10ꢂ7 M) with the
addition of Zn2+ in acetonitrile solution. (b) Plot of quantum yield
versus equivalence of Zn2+ added indicating the formation of 1 : 1
complexes. lex = 350 nm.
Fig. 1 Fluorescence spectra of the L1 alone and in the presence of
different ionic inputs in acetonitrile. [L1] = 1.9 ꢁ 10ꢂ7 M, [Mn+] =
1.9 ꢁ 10ꢂ5 M. lex = 350 nm.
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Notes and references
z Crystal data for L1 (Fig. S11w): Pale yellow crystal. C36 H47 N O7, M
= 605.75, 0.11 ꢁ 0.09 ꢁ 0.06 mm3, Orthorhombic, space group P 21
21 21 with a = 8.2548(11) A, b = 18.988(3) A, c = 20.873(3) A, a =
901, b = 901, g = 901 V = 3271.7(8) A3, Z = 4, rc = 1.230 g cmꢂ3
,
F(000) = 1304, Absorption coefficient = 0.084 mmꢂ1, Mo-Ka radia-
tion, l = 0.71073 A, T = 100 K, 2ymax = 56.61, Number of measured
and independent reflections = 21795/4548, Rint. = 0.0527, R1 =
0.0480, wR2 = 0.1124. Maximum and Minimum residual electron
density is 0.147 and ꢂ0.138 eAꢂ3, respectively. CCDC: 707106;
Crystal data for L1 + Na+: Pale yellow crystal. C36 H47 Cl N Na
3
ꢀ
O11, M = 728.19, 0.09 ꢁ 0.06 ꢁ 0.05 mm , Triclinic, space group P1
with a = 11.4813(19) A, b = 13.416(2) A, c = 13.433(2) A, a =
69.085(3)1, b = 73.360(3)1, g = 66.898(3)1, V = 1751.9(5) A3, Z = 2,
rc = 1.380 g cmꢂ3, F(000) = 772, Absorption coefficient = 0.184
mmꢂ1, Mo-Ka radiation, l = 0.71073 A, T = 100 K, 2ymax = 521,
Number of measured and independent reflections = 9825/6726, Rint.
= 0.0371; R1 = 0.0654 (I 4 2s(I)), Rw = 0.1660 (observed data).
Maximum and Minimum residual electron density is 0.423 and
ꢂ0.431 eAꢂ3, respectively. CCDC: 707107.w
1 (a) C. J. Pedersen, J. Am. Chem. Soc., 1967, 89, 7017;
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Fig. 4 X-Ray crystal structure of L1 + Na+ complex. Hydrogen
atoms and the perchlorate ion are omitted for clarity.
change in intensity was observed (Fig. 3b insert) suggesting
1 : 1 binding ratio of L1 : Zn2+. The association constant was
found to be 6.7 ꢁ 105 [Mꢂ1]. Similarly, for the Hg2+, the
titration experiment indicated a 1 : 1 complex formation in
acetonitrile (Fig. S4).w
3 (a) A. P. Marchand, K. A. Kumar, A. S. McKim, K. Mlinari0c-
Majerski and G. Kragol, Tetrahedron, 1997, 53, 3467;
(b) A. P. Marchand, A. McKim and K. A. Kumar, Tetrahedron,
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When we performed the binding study with a mixture of
metal ions, the fluorescence intenstiy of the Zn2+ was not
changed in the presence of metal ions like Co2+, Ni2+, Cu2+
,
Cd2+, and Pb2+. Interestingly, the Zn2+ induced emission
was quenched upon addition of Na+. This must be due to
stronger binding of Na+ with the crown ether. Thus, the
present system acts like an INHIBIT logic gate13
(Fig. S13–S14)w where Zn2+ and Na+ are the two inputs.
Intrigued by this observation we decided to probe the
coordination mode of the sodium ion with ligand L1. A single
crystal X-ray analysis of sodium-ligand complex (L1 + Na+)
4 F. A. Khan, J. Dash, Ch. Sudheer, N. Sahu and K. Parasuraman,
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New York, 1970.
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N. Gunaratne, J.-L. Habib-Jiwan, S. L. A. Peiris, R. A. A.
D. Rupasinghe, T. K. S. D. Samarasinghe, K. R. A.
S. Sandanayake and J.-P. Soumillion, J. Chem. Soc., Perkin Trans.
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13 M. de Sousa, M. Kluciar, S. Abad, M. A. Miranda, B. de Castro
and U. Pischel, Photochem. Photobiol. Sci., 2004, 3, 639.
14 The data was collected on a Bruker SMART APEX diffractometer.
The structure was solved using SIR-92 and refined using
SHELXL-97. CCDC:707106 (L1); CCDC:707107 (L1 + Na+).
14
was performed (Fig. 4).z The X-ray structure demonstrated
that sodium ion is coordinated to the oxygen atoms of the
crown ether (2.43–2.70 A) and is remote from the tertiary
nitrogen (3.24 A) atom (Table S3).wThis is reflected in the
fluorescence emission of L1 in solution state study with Na+
ion as the PET remains operative.
In conclusion, we have reported a novel hybrid of aza
crown-oxabridged fluorogenic chemosensor L1 which shows
high selectivity towards Zn2+ and Hg2+ compared to the
parent monoaza-18-crown-6. Binding nature of Zn2+ and
Na+ towards L1 was explored using solution studies and
a single crystal X-ray analysis of sodium-ligand complex
(L1 + Na+).
We thank the Department of Science and Technology
(DST), New Delhi for financial assistance. We thank Prof.
P. K. Bharadwaj for useful discussions. F.A K. acknowledges
the DST for a Swarnajayanti Fellowship. K.P. and K K S.
thank CSIR, New Delhi for fellowship.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2399–2401 | 2401