Thiaoxaaza-macrocyclic chromoionophores as mercury(II) sensors: Synthesis and color modulation

Article abstract of DOI:10.1021/ol900241p

Azo-coupled macrocyclic chromoionophores incorporating benzene (L ¹) and pyridine (L²) subunits were synthesized, respectively. In a cation-induced color change experiment, both receptors showed Hg²⁺ selectivity. However,

Full text of DOI:10.1021/ol900241p

ORGANIC
LETTERS
2009
Vol. 11, No. 6
1393-1396
Thiaoxaaza-Macrocyclic
Chromoionophores as Mercury(II)
Sensors: Synthesis and Color
Modulation
Hayan Lee and Shim Sung Lee*
Department of Chemistry (BK21) and Research Institute of Natural Science,
Gyeongsang National UniVersity, Jinju 660-701, South Korea
sslee@gnu.ac.kr
Received February 4, 2009
ABSTRACT
Azo-coupled macrocyclic chromoionophores incorporating benzene (L1) and pyridine (L2) subunits were synthesized, respectively. In a cation-
induced color change experiment, both receptors showed Hg2+ selectivity. However, L1 gave a larger cation-induced hypsochromic shift than
L2, suggesting that the presence of the pyridine unit in L2 may inhibit the Hg···N-azo interaction. The observed Hg2+-selective color changes
for L1 and L2 were found to be controlled by anion-coordination ability. NMR titration of the proposed receptor ligand with Hg(II) salt was
accomplished.
Synthetic receptors for sensing and recognizing environ-
mentally and biologically important ionic species are cur-
rently of widespread interest.1 In this category of sensor
molecules, N-azo-coupled macrocyclic systems2 represent a
promising research area, not only in terms of their chro-
mophoric function3 but also because of their high selectivity
for the metal species of interest, including heavy metal ions.4
Previously, we reported the synthesis of NO2S2 macro-
cycles5 that show an affinity for Hg2+ and bind to this cation
in an exo-5b,6 or endo-coordination5b,7 manner. More re-
cently, we synthesized an N-azo-coupled analogue that shows
Hg2+ selectivity with the color of the Hg2+ complex also
controlled by the nature of the anion present. As an extension,
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J. Inorg. Chem. 2008, 1559. (b) Kim, J. S.; Quang, D. T. Chem. ReV. 2007,
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2008, 1465. (c) Cheng, T.; Xu, Y.; Zhang, S.; Zhu, W.; Qian, X.; Duan, L.
J. Am. Chem. Soc. 2008, 130, 16160. (d) Yantasee, W.; Warner, C. L.;
Sangvanich, T.; Addleman, R. S.; Carter, T. G.; Wiacek, R. J.; Fryxell,
G. E.; Timchalk, C.; Warner, M. G. EnViron. Sci. Technol. 2007, 41, 5114.
(2) (a) Lee, S. J.; Jung, J. H.; Seo, J.; Yoon, I.; Park, K.-M.; Lindoy,
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J. K.; Lee, S. H.; Kim, J. Y.; Park, K.-M.; Lee, S. S. J. Org. Chem. 2002,
67, 1372.
(5) (a) Park, C. S.; Lee, J. Y.; Kang, E.-J.; Lee, J.-E.; Lee, S. S.
Tetrahedron Lett. 2009, 50, 671. (b) Jin, Y.; Yoon, I.; Seo, J.; Lee, J.-E.;
Moon, S.-T.; Kim, J.; Han, S. W.; Park, K.-M.; Lindoy, L. F.; Lee, S. S.
Dalton Trans. 2005, 788.
(6) (a) Lee, S. Y.; Park, S.; Kim, H. J.; Jung, J. H.; Lee, S. S. Inorg.
Chem. 2008, 47, 1913. (b) Sultana, K. F.; Lee, S. Y.; Lee, J. Y.; Park,
K.-M.; Lee, S. S. Bull. Korean Chem. Soc. 2008, 29, 241. (c) Lee, J.-E.;
Jin, Y.; Seo, J.; Yoon, I.; Song, M. R.; Lee, S. Y.; Park, K.-M.; Lee, S. S.
(3) (a) Nishiyabu, R.; Palacios, M. A.; Dehaen, W.; Anzenbacher, P.,
Jr. J. Am. Chem. Soc. 2006, 128, 11496. (b) Lee, H. G.; Lee, J.-E.; Choi,
K. S. Inorg. Chem. Commun. 2006, 9, 582. (c) Beer, P. D.; Gale, P. A.
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2008, 49, 3770. (b) Choi, K. S.; Kang, D.; Lee, J.-E.; Seo, J.; Lee, S. S.
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10.1021/ol900241p CCC: $40.75
Published on Web 02/23/2009
2009 American Chemical Society

we are currently working on the synthesis of new macro-
cyclic chromoionophores for the selective detection of heavy
metalionsandhaveundertakenacomparativestructure-function
analysis of metal sensing by the N-azo-coupled macrocycles
incorporating benzene (L1) and pyridine (L2) subunits as part
of their macrocyclic backbones (Scheme 1). We herein report
the synthesis of L1 and L2 as an investigation of their
structure-function relationship, with emphasis on the color-
modulation in metal ion sensing.
Scheme 1. Synthesis of Azo-Coupled Macrocycles
Figure 2.
UV/vis spectra of (a) L1 and (b) L2 (40 µM) in the
As key precursors, N-phenylated macrocycles 5 and 6 were
synthesized by dithiol-dichloride coupling reactions of 4
with 3a or 3b, employing Cs2CO3 under high dilution. L1
and L2 were synthesized by the reactions of the diazonium
salt of p-nitronaniline with 5 and 6, respectively. Red single
crystals of L2 were obtained by vapor diffusion of n-hexane
into dichloromethane solution, and the crystal structure of
this product was obtained (Figure 1). In L2, the macrocyclic
ring is flattened with the two torsion angles between the S
and N donors indicating a propensity for each linkage to
adopt an anti-anti conformation.
presence of metal perchlorates (5.0 equiv) in acetonitrile.
The titrations of L1 and L2 with Hg(ClO4)2 resulted in the
absorption at 480 nm gradually decreasing whereas the
absorptions at 378 and 339 nm gradually increased to give
isosbestic points at 415 and 399 nm (Figure 3b), respectively.
Figure 1.
Crystal structure of L2.
The metal-binding properties of L1 and L2 were examined
by UV-vis spectrophotometry with respect to the induced
color changes (Figure 2). L1 and L2 both exhibit intense
absorptions at 480 nm (red, εmax: 27300 for L1 and 23300
for L2). Notably, the addition of Hg2+ resulted in the largest
hypsochromic shift: to 339 nm for L1 (∆λ ) 142 nm) and
378 nm for L2 (∆λ ) 102 nm), changing their solution colors
from red to colorless and pale-yellow, respectively (Figure
2). In contrast, no significant color changes were observed
upon addition of other metal ions, except Cu2+ for L1 and
Ag+ for L2 (Figure 2).
Figure 3.
Spectrophotometric titrations of (a) L1 and (b) L2 (40
µM) with Hg(ClO4)2 in acetonitrile. Inset: (left) titration curves and
(right) Job plots.
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Org. Lett., Vol. 11, No. 6, 2009

In both cases, a 1:1 (L/M) stoichiometry for complexation
was evident from each titration curve and also from Job plots
for each system. The titration plots were very sharp, and
hence, the stablity constants are too high to be calculated
using this data in least-squares nonlinear fitting programs.
In Hg2+ selectivity measurements, the absorbance of the
Hg2+-L1 complex was not affected in the presence of 10
equiv of other competing metal ions such as Ag+, Co2+, Ni2+,
Cu2+, Zn2+, Pb2+, Cd2+, Li+, K+, Ca2+, and Mg2+ (Figure
S10, Supporting Information). These results are different
from those obtained with other Hg2+ sensor molecules that
have usually also been affected by Ag+ and other heavy
metal ions.4 However, in contrast, the absorbance of the
Hg2+-L2 complex showed a 10-15% decrease when Ag+,
Co2+, Ni2+, and Cd2+ were added to the solution.
respective complexes generated, and thus, we decided to attempt
the preparation and crystal structure of each of the colored
species.
We were successful in isolating yellow-red (1) and red
(2) complexes as single crystals from solutions of L2 with
Hg(ClO4)2 and Hg(NO3)2, respectively, and their structures
were obtained by X-ray diffraction (Figure 5). The X-ray
analysis revealed that 1 is a 1:1 complex of type
[Hg(L2)](ClO4)2(DMF) (Figure 5a). The Hg atom is located
at the center of L2 which adopts a bent arrangement. Hg1 is
five-coordinate, being bound to two S, two O and one
pyridine N atom. An intramolecular π···π stacking interaction
(3.29 Å and 16.7°, dashed line in Figure 5a) occurs between
two aromatic groups.
Figure 5.
Crystal structures of L2 complexes with Hg2+ salts: (a)
1, [Hg(L2)](ClO4)2(DMF) and (b) 2, [Hg(L2)(NO3)2]. Noncoordi-
nated anions and solvent molecule are omitted.
On reaction with Hg(NO3)2, L2 forms the 1:1 endotype
complex [Hg(L2)(NO3)2] (2). Unlike 1, this dinitrato complex
is a neutral species; the Hg center in the cavity is coordinated
to an NS2O2 donor set from the macrocycle (but not to the
t-N atom) as well as to two nitrato ions bound in a mono-
and a bidentate manner. Considering the narrow bite angle
[O5-Hg1-O7 44.94(3)°] of the bidentate nitrate ion, the
geometry of the Hg coodination sphere is probably best
described as pseudopentagonal bipyramidal. The NS2O2
donors of L2 define the equatorial plane, with the axial
positions occupied by the two nitrato ligands.
Figure 4.
UV/vis spectra of (a) L1 and (b) L2 (40 µM) in the
presence of Hg2+ salts (5.0 equiv) in acetonitrile.
We also found that the color change with Hg2+ may vary
with the anion present. For instance, the addition of ClO4- or
NO3- resulted in hypochromic shifts to 367 and 350 nm (pale
yellow), respectively (Figure 4). However, no color change was
observed upon addition of Cl-, Br-, I-, AcO-, SCN-, or SO42-
.
This is attributed to the significant coordinating ability of these
latter anions to the Hg2+ ion.2a The results indicate that 1 not
only can be useful as an efficient chromogenic reagent for cation
sensing but also shows promise as a reagent for ClO4- and
NO3- sensing in the presence of a range of other anions. The
observed behavior undoubtedly reflects the structures of the
In both complexes of L2, the distances between Hg and the
t-N atom (3.290 Å for 1 and 4.478 Å for 2) are too long to be
considered as a regular bond,8 with the longer distance in 2
than that in 1 being attributed to the presence of anion
coordination. Consequently, it is the presence of the pyridine
unit in L2 that may inhibit bond formation between Hg and the
Org. Lett., Vol. 11, No. 6, 2009
1395

t-N atom and results in this dye receptor being less sensitive
than L1 as exemplified by the results presented in Figures 2
and 3 (see also Figure S11, Supporting Information).
(FAB) mass spectra, which show peaks corresponding to
[Hg(L1)ClO4]+ (m/z 977.2) and [Hg(L2)ClO4]+ (m/z 978.2),
respectively (Figure 7). The relative abundance of their
isotope peak patterns are in each case in good agreement
with the corresponding simulated peak patterns.
Figure 6.
1H NMR titration curves for L1 with Hg(ClO4)2 in CDCl3/
CD3CN (v/v 1:6).
Figure 7.
Observed isotope distributions for (a) [Hg(L1)ClO4]+ and
(b) [Hg(L2)ClO4]+ in the FAB mass spectra of Hg(ClO4)2 complexes
of L1 and L2, respectively. The bars represent the predicted mass
spectral distributions for these ions.
Unfortunately, single crystals of the corresponding Hg2+
complexes of L1 were not able to be obtained. Instead, the
binding behavior of L1 toward Hg2+ was further investigated
by NMR titration (Figure 6). The signals of the methylene
protons (H1-4) in L1 (Figure S9a) were well resolved. Upon
addition of Hg(ClO4)2 to a solution of L1, all of the protons
display downfield shifts except for H4, in keeping with L1
forming a stable complex with Hg2+, which shows a fast-
exchange rate between ligand and cation on the NMR time
scale. The titration curves [plotting ∆δ (ppm) vs added equiv
of Hg2+] for each proton clearly show an inflection point at a
mole ratio (Hg/L) of 1.0, indicating a 1:1 solution stoichiometry
for the complex. The order of magnitude of the chemical shift
variation for the aliphatic region is H2 (∆δ: 0.74 ppm) > H3
(0.54 ppm) > H4 (0.19 ppm) > H1 (0.11 ppm). The larger
chemical shift changes shown by the H2, H3, and H4 adjacent
to the S and N atoms can be explained by the likelihood that
the Hg2+ is strongly bound to the S and N donors, while the O
donors interact with the Hg2+ relatively weakly.
In summary, the synthesis and comparative chromogenic
behavior of the N-azo-coupled thiaoxaaza-macrocycles in-
corporating benzene (L1) and pyridine (L2) subunits are
presented. L1 exhibits better Hg2+ selectivity and sensitivity
than L2 because of the coordination of the pyridine nitrogen
in L2 inhibiting the bond formation between Hg2+ and the
macrocyclic t-N atom, with the observed induced color being
very sensitive to this effect. This result may contribute not
only to the design and synthesis of new chromoionophores
for the selective detection of heavy metal ions but also in
the development of related nanosensing materials based on
the structure-function analysis.
Acknowledgment. This work was supported by KOSEF
(Grant No. R01-2007-000-20245-0). The authors thank Prof.
L. F. Lindoy for his advice.
The 1:1 (Hg/L) stoichiometry of the Hg2+ complexes with
each ligand is also confirmed by their fast atom bombardment
Supporting Information Available: Syntheses of L2, 1
and 2; X-ray cystallographic files (CIF); NMR data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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OL900241P
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Org. Lett., Vol. 11, No. 6, 2009