Triazole- and azo-coupled calix[4]arene as a highly
sensitive chromogenic sensor for Ca
2+
and Pb
2+
ions
Kai-Chi Chang,
aIn-Hao Su,
aGene-Hsiang Lee
band Wen-Sheng Chung
a,*a
Department of Applied Chemistry, National Chiao-Tung University, Hsinchu 30050, Taiwan, ROC
b
Instrumentation Center, National Taiwan University, Taipei 106, Taiwan, ROC
Received 8 July 2007; revised 13 August 2007; accepted 14 August 2007 Available online 16 August 2007
Abstract—A novel chromogenic calix[4]arene 3, which has within a molecule both the triazoles and the hydroxyl azophenols as the metal-binding sites and the azophenol moiety as a coloration sites was designed and synthesized. Calix[4]arene 3 is highly sensitive to Ca2+and Pb2+ions, which can be detected by the naked eye. Furthermore, the association constants for the 1:1 complexes of 3ÆCa2+ and 3ÆPb2+were determined to be 7.06· 104
M 1and 8.57· 103
M 1, respectively. 2007 Elsevier Ltd. All rights reserved.
The design and synthesis of new chemosensors for metal ions is an important subject in the field of supramolecu-lar chemistry due to their fundamental role in biological,
environmental, and chemical processes.1 Chromogenic
ionophores have been intensively investigated as a spe-cific metal ion indicator since Vo¨gtle reported the use of 4-(4-nitrophenyl)azo-coupled crowns and azacrowns as chromoionophores, which showed large UV/vis band
shifts when cations were added.1a
Calix[4]arenes have been shown to be useful molecu-lar scaffold in the development of chromoionophores,
especially for metal ion recognition.2 Shinkai and
co-workers reported that calix[4]arene having a 4-(4-nitrophenyl)azophenol unit with three ethyl ester groups showed a perfect lithium ion selectivity with respect to the UV/vis band shift.2a,bChang et al. reported a batho-chromic shift of a p-tert-butylcalix[4]arene bearing a
1,3-diazophenol unit upon calcium ion complexation.2d
Reinhoudt et al. also reported that a calix[4]arene with monoalkylated azophenol unit and triamides on the
lower rim is a highly selective Pb2+ sensor, in which
the direction of the shift was dependent on the confor-mation of the calix[4]arenes.2e
In continuation of our interests in the design and
synthe-sis of chromogenic3,4 and fluorogenic chemosensors,5,6
we report here the synthesis of a novel chromogenic
calix[4]arene using the Click chemistry7of an azide and
an alkyne to form a triazole cationic binding site.
The synthesis of host 3 is illustrated in Scheme 1. Our
synthesis began with
25,27-bis(O-propargyl)calix[4]-arene 18 followed by diazo coupling reaction using
p-anisidine in HCl and NaNO2 in acetone and pyridine
gave the desired product 2 in 62% yield.9Cu(I)-catalyzed 1,3-dipolar cycloaddition reaction of 2 with 1-(azido-methyl)-benzene in the Click condition afforded the
5,17-bis(p-methoxy-phenyl)azo-25,27-bis(1,2,3-triazole)-calix[4]arene 3 in 71% yield.10 Control compound 411
was synthesized in 79% yield using a method similar to that used in the preparation of 2. Besides traditional
organic spectroscopic identification (1H and 13C
NMR, MS, and HRMS spectra) of all these calix[4]ar-enes, single-crystal X-ray analysis of 2 and 3 confirmed the structures to be in cone conformations (Fig. 1).12
The absorption maxima (kmax) and molar extinction
coefficients of the chromogenic calix[4]arenes and con-trol compound synthesized in this work are summarized
in Table 1. Next, we then investigated the affinities of
these azo-compounds 2–4 for a series of groups 1A,
2A, and transition-metal ions in MeCN/CHCl3(v/v =
1000:4).
Excess perchlorate salts (10 equiv) of Li+, Na+, K+, Mg2+, Ca2+, Ba2+, Cr3+, Pb2+, Cd2+, Ag+, Ni2+,
Mn2+, and Zn2+ions were tested to evaluate the metal
0040-4039/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2007.08.045
* Corresponding author. Tel.: +886 3 513 1517; fax: +886 3 572 3764; e-mail:[email protected]
ion binding properties of 2–4. Ligand concentration in
all titration experiments was fixed at 10 5M in
MeCN/CHCl3 (v/v = 1000:4). Free hosts 2, 3, and 4
exhibited absorption bands at 364, 365, and 360 nm in
MeCN/CHCl3 (v/v = 1000:4), respectively. The
tri-azole–azophenol host 3, having triazoles as the metal ligating groups, is found to exhibit remarkable selectiv-ity toward Ca2+and Pb2+ions over all other metal ions.
For example, the addition of 10 equiv of Ca2+and Pb2+
ions induced a bathochromic shift of triazole ionophore
3 from kmax 365 nm to 527 and 541 nm, respectively
(Fig. 2). However, the UV/vis spectra of control
com-pounds 2 and 4 showed a weak bathochromic shift to
Cr3+ ion only, and the rest of the metal ions did not
show any change (seeFigs. S7 and S8).
The two triazole moieties of 3 are proven to form an effi-cient metal ion binding site, whereas compounds 2 and 4 are in lack of such an efficient metal ion binding site. Furthermore, the geometry of the binding site of the host, comprising the two nitrogen atoms of triazole units and two hydroxyls of the azophenol units, seems to be ideal in terms of size and arrangement for recogni-tion of doubly charged metal carecogni-tions. Of primary impor-tance is the electrostatic interaction of metal cations with two azophenol moieties as well as the ion–dipole interaction of metal ions with the triazole unit.
Upon interaction with Ca(ClO4)2, the chromogenic
sen-sor 3 in MeCN/CHCl3 (v/v = 1000:4) solution
experi-enced a marked bathochromic shift in its kmax as
shown in Figure 3. The absorption maximum at
365 nm gradually decreased in intensity with the forma-tion of a new absorpforma-tion band at ca. 527 nm
(Dkmax= 162 nm). Three isosbestic points are 270, 305,
OH OH O O O OHOH O 1 N N N OCH3 2 i N OCH3 OH N N OCH3 4 OH i 62% 71% 79% OH OH O O N N N N N N N NN OCH3 N OCH3 3 ii
Scheme 1. Synthesis of chromogenic calix[4]arene 3. Reagents and conditions: (i) p-anisidine/acetone, NaNO2/4 N HCl, pyridine, 0C, 18 h; (ii) 1-(azidomethyl)benzene, CuI, THF/H2O, 50C, 18 h.
C45 O6 C42 C41 C43 C40 C44 C39 C9 C35 O5 N4 N3 C10 C31 C32 C8 C30 C33 C14 C15 C11 C29 C34 C7 C12 N2 C13 C26 C16 C38 C6 C4 N1 C37 C27 C3 C5 C17 O2 C36 C21 C20 O3 C2 C25 C22 C18 C19 C1 O1 C23 C28 C24 O4 C46 C47 C48 C62 O6 C60 C59 C61 C58 C22 C23 C56 C57 C21 N9 C24 C3 N10 C1 C20 C45 C28 C4 C2 C39 C46 C19 N4 C44 O4 C40 C53 N5 C5 O5 C17 C54 C25 C18 C52 C47 C55 C41 C43 N6 N7 C6 O1 C16 C49 C48 C35 C36 C27 C42 C51 N8 O2 C50 C15 C9 C7 C14 C10 C8 C34 C37 C11 C26 C12 O3 C13 C38 C33 N1 N2 C29 C30 N3 C32 C31
Figure 1. X-ray single-crystal structures of 2 and 3.
300 400 500 600 0.0 0.2 0.4 0.6 0.8 541 527 3 + Mn2+ 3 + Cr3+ 3 + Pb2+ 3 + Ca2+ 3 and 3 + other metal ions Abs. Wavelength (nm)
Figure 2. UV/vis spectra of 3 (10 lM) before and after adding 100 lM concentration of various metal perchlorates in MeCN/CHCl3(1000:4, v/v).
Table 1. kmax and corresponding extinction coefficients of azo-compounds 2–4 in MeCN/CHCl3(1000:4, v/v)
Compound kmax(nm) e(M 1cm 1)
2 364 61,000
3 365 55,000
and 422 nm for the titration spectra of 3 by Ca(ClO4)2.
The spectral features in Figure 3are consistent with a
1:1 binding ratio between calix[4]arene 3 with Ca2+
ion. Further support of the 1:1 binding ratio comes from
a Job plot experiment,13 where the absorptions of the
complex at 527 nm were plotted against molar fractions of 3 under the conditions of an invariant total concen-tration. As a result, the concentration of 3ÆCa2+complex approached a maximum when the molar fraction of [3]/ ([3] + [Ca2+]) was about 0.5 (seeFig. 4).
Electrospray mass spectrometry also supports the
for-mation of complex 3ÆCa2+and 3ÆPb2+, where a peak at
m/z = 1173.6 corresponding to the mass of [3 + Ca +
ClO4]+and a double charged peak at m/z = 537.4 which
corresponds to [3 + Ca]2+were observed. Furthermore,
a peak at m/z = 1241.6 corresponding to the mass of
[3 + Pb H]+ and a double charged peak at
m/z = 621.2 which corresponds to [3 + Pb]2+were also
observed (seeFigs. S9 and S10for detail).
The association constant for 3ÆCa2+ in MeCN/CHCl3
(1000:4, v/v) was determined to be 7.06· 104M 1 by
Benesi–Hilderbrand plot14(Fig. 5). Similar UV/vis titra-tion behavior and 1:1 binding stoichiometry was also
observed in the case of 3 with Pb2+(seeFigs. S11–S13
for details); and its association constant was estimated to be 8.57· 103
M 1.
Metal ion-induced chemical shift changes in the 1H
NMR (in CD3CN) spectra support that Ca2+is bound
to the two nitrogen atoms of the triazole units and the
two hydroxyl azophenol groups of 3 (see Fig. 6). In
the presence of 10.0 equiv of Ca2+, chemical shifts of
protons Ha–Heon the azophenol unit of 3 changed
sig-nificantly; the peaks of Ha–Hc were upfield shifted by
0.18, 0.17, and 0.42 ppm, respectively, but the peak of
Hewas downfield shifted by 2.74 nm. In particular, the
peak of Hd was split into two peaks, one was upfield
shifted by 0.23 ppm and the other was downfield shifted
by 0.03 ppm. However, the peaks of Hg and Hh were
each split into two sets of signals and upfield shifted.
300 400 500 600 700 0.0 0.2 0.4 0.6 0.8 1.0 270 305 365 422 527 3 0.2 eq 0.4 0.6 0.8 1.0 1.5 2.0 4.0 6.0 8.0 10.0 20.0 50.0 100.0 Abs. Wavelength (nm)
Figure 3. UV/vis spectra of 3 (10 lM) upon titration by various equivalents of Ca(ClO4)2in MeCN/CHCl3(1000:4, v/v).
0.0 0.2 0.4 0.6 0.8 1.0 0.00 0.04 0.08 0.12 0.16 Abs. of complex (527 nm) [3]/([3]+[Ca2+])
Figure 4. Job plot of a 1:1 complex of 3 and Ca2+ ion, where the absorption at 527 nm was plotted against the mole fraction of 3 at invariant total concentration of 10 lM in MeCN/CHCl3(1000:4, v/v).
0 150000 300000 450000 0 4 8 12 Intercept = 1.32 Slop = 1.87 x 10-5 K a = 7.06 x 10 4 M-1 R2 = 0.996 1/Dalta Abs. 1/[Ca2+], M-1
Figure 5. Benesi–Hilderbrand plot of 3 with Ca(ClO4)2.
4.0 4.5 5.0 5.5 6.0 6.5 7. 7. 8.0 8.5 9.0 9.5 10.0 10.5 11.0 11.5 12.0 ppm e a h i k f g b phenyl d c j e j d ph * k i b h h * c d gf h a H2O
(a)
(b)
7.0 7.5 a h i k f d j pheny * k i b h h * h a OH OH O O N N N N N N N NN OCH3 3 OH OH O O NN N N NN N NN OCH3 3 Ca2+ Ca2+ N OCH3 N OCH3 a b c d f g h e i j k h' h d' d g f f' g'Figure 6. 1H NMR spectra of 3 (2.5 mM) in CD3CN (a) and in the presence of 25 mM (10.0 equiv) of Ca(ClO4)2(b), where * denotes an external standard CHCl3.
The protons Hi, Hj, and Hk were little influenced.
Fur-thermore, 13C NMR spectroscopy proved that
iono-phore 3 forms a complex with Ca2+ in a cone
conformation. The methylene carbon atoms bridging the aromatic rings appear at d 31.2 and 32.1, which are typical resonances for a cone conformation of calix[4]arenes15(seeFig. S14). These results suggest that
Ca2+ion not only is bound by triazole–azophenol host
3, it also breaks the symmetry of the host molecule after complexation.
Due to the poor solubility of complex 3ÆPb2+in CD3CN,
the following titration was in CDC13/CD3CN (v/v =
3:1) co-solvent system. Upon adding 10.0 equiv of
Pb2+ to the solution of 3 (see Fig. S15 for detail), the
peak of Hdwas downfield shifted by 0.21 ppm, but did
not split. The peaks of Hg and Hh stayed intact. The
peak of Hjon the triazole unit of 3 was downfield shifted
by 0.24 ppm, and the peaks of Hi and Hk were also
downfield shifted by 0.08 and 0.12 ppm. Interestingly,
the peak of Hewas upfield shifted by 3.58 ppm. These
results suggest that Pb2+ion can also be bound by host
3, but was forming a symmetrical metal ion complex.16
In conclusion, we have developed a new calix[4]arene sensor with bistriazoles and azophenols as the metal ion binding sites and azo groups as the signal
transduc-tion unit, which showed selective coloratransduc-tion of Ca2+and
Pb2+ions.16The Ca2+
and Pb2+ ion detection gives rise to a large bathochromic shift in the absorption spectrum (from light yellow to red), which is clearly visible to the naked eye.
Acknowledgements
We thank the National Science Council (NSC) and MOE ATU program of the Ministry of Education of Taiwan, ROC for financial support.
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.08.045.
References and notes
1. (a) Lo¨hr, H. G.; Vo¨gtle, F. Acc. Chem. Res. 1985, 18, 65; (b) Buhlmann, P.; Pretsch, E.; Bakker, E. Chem. Rev. 1998, 98, 1593; (c) Gunnlaugsson, T.; Leonard, J. P.; Murray, N. S. Org. Lett. 2004, 6, 1557; (d) Jiang, P.; Guo, Z. Coord. Chem. Rev. 2004, 248, 205; (e) Lee, D. H.; Lee, K. H.; Hong, J.-I. Org. Lett. 2001, 3, 5; (f) Sanceno´n, F.; Martı´nez-Ma´n˜ez, R.; Soto, J. Angew. Chem., Int. Ed. 2002, 41, 1416.
2. (a) Shimizu, H.; Iwamoto, K.; Fujimoto, K.; Shinkai, S. Chem. Lett. 1991, 2147; (b) Yamamoto, H.; Ueda, K.; Sandanayake, K. R. A. S.; Shinkai, S. Chem. Lett. 1995, 497; (c) Kubo, Y.; Tokita, S.; Kojima, Y.; Osano, Y. T.; Matsuzaki, T. J. Org. Chem. 1996, 61, 3785; (d) Kim, N. Y.; Chang, S.-K. J. Org. Chem. 1998, 63, 2362; (e) van der Veen, N. J.; Egberink, R. J. M.; Engbersen, J. F. J.; van Veggel, F. J. C. M.; Reinhoudt, D. N. Chem. Commun.
1999, 681; (f) Kim, J. S.; Shon, O. J.; Ko, J. W.; Cho, M. H.; Yu, I. Y.; Vicens, J. J. Org. Chem. 2000, 65, 2386; (g) Oueslati, F.; Dumazet-Bonnamour, I.; Lamartine, R. Tetrahedron Lett. 2001, 42, 8177; (h) Kim, J. S.; Shon, O. J.; Lee, J. K.; Lee, S. H.; Kim, J. Y.; Park, K.-M.; Lee, S. S. J. Org. Chem. 2002, 67, 1372; (i) Halouani, H.; Dumazet-Bonnamour, I.; Lamartine, R. Tetrahedron Lett. 2002, 43, 3785; (j) Kim, J. S.; Shon, O. J.; Yang, S. H.; Kim, J. Y.; Kim, M. J. J. Org. Chem. 2002, 67, 6514; (k) Kim, J. Y.; Kim, G.; Kim, C. R.; Lee, S. H.; Lee, J. H.; Kim, J. S. J. Org. Chem. 2003, 68, 1933; (l) Lee, S. H.; Kim, J. Y.; Ko, J.; Lee, J. Y.; Kim, J. S. J. Org. Chem. 2004, 69, 2902; (m) Chawla, H. M.; Singh, S. P.; Upreti, S. Tetrahedron 2006, 62, 9758.
3. Kao, T.-L.; Wang, C.-C.; Pan, Y.-T.; Shiao, Y.-J.; Yen J.-Y.; Shu, C.-M.; Lee, G.-H.; Peng, S.-M.; Chung, W.-S. J. Org. Chem. 2005, 70, 2912.
4. Ho, I.-T.; Lee, G.-H.; Chung, W.-S. J. Org. Chem. 2007, 72, 2434.
5. Shiao, Y.-J.; Chiang, P.-C.; Senthilvelan, A.; Tsai, M.-T.; Lee, G.-H.; Chung, W.-S. Tetrahedron Lett. 2006, 47, 8383.
6. Senthilvelan, A.; Tsai, M.-T.; Chang, K.-C.; Chung, W.-S. Tetrahedron Lett. 2006, 47, 9077.
7. (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004; (b) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596.
8. Xu, W.; Vittal, J. J.; Puddephatt, R. J. Can. J. Chem. 1996, 74, 766.
9. Compound 2: To an ice cold solution of 0.07 g (1.00 mmol) of NaNO2 in 6 mL of 4 N HCl was added a solution of
0.06 g (0.50 mmol) of p-anisidine in 3 mL of acetone, and the mixture was stirred for 30 s. The combined solution was then added to another ice cold solution of 0.10 g (0.20 mmol) of 25,27-dipropargyloxy-26,28-dihydroxy-calix[4]arene 1 in 12 mL of pyridine to produce a colored solution. The reaction mixture was stirred for 18 h at 0C and then treated with 50 mL of 4 N HCl to give a colored precipitate. The solid residue was recrystallized from CH2Cl2/methanol mixture to give 0.095 g (62%) of 2 as a
red solid; mp 226–228C; Rf= 0.5 (hexane/EtOAc = 3:1); 1 H NMR (CDCl3, 300 MHz) d 7.87 (d, J = 8.9 Hz, 4H), 7.73 (s, 4H), 7.62 (s, 2H), 7.00 (d, J = 8.9 Hz, 4H), 6.96 (d, J = 7.6 Hz, 4H), 6.76 (t, J = 7.6 Hz, 2H), 4.84 (d, J = 2.3 Hz, 4H), 4.46 (d, J = 13.5 Hz, 4H), 3.87 (s, 6H), 3.57 (d, J = 13.5 Hz, 4H), 2.63 (t, J = 2.3 Hz, 2H); 13C NMR (CDCl3, 75.4 MHz) d 161.3 (Cq), 155.8 (Cq), 151.3 (Cq), 147.2 (Cq), 145.7 (Cq), 132.7 (Cq), 129.4 (CH), 128.5 (Cq), 125.9 (CH), 124.1 (CH), 123.4 (CH), 114.1 (CH), 78.0 (Cq), 63.6 (CH2), 55.5 (CH3), 31.8 (CH2); FABMS m/z 770 (M++1, 2), 769 (M+, 2), 460 (6), 307 (100); HR FABMS Calcd for C48H40N4O6 768.2948. Found
768.2936.
10. Compound 3: To a mixture of 2 (0.20 g, 0.26 mmol) and 9-(azidomethyl)benzene (0.084 g, 0.63 mmol) in THF and water (v/v = 2:1, 30.0 mL) was added CuI (about 1 mg, 0.005 mmol). The heterogeneous mixture was stirred vigorously at 50C for 24 h. The mixture was extracted thrice with chloroform. The chloroform solution was dried over MgSO4 and evaporated to give the solid crude
product. Chromatography on silica gel eluting with hexane/ethyl acetate (v/v = 1:1) gave 0.19 g (71%) of 3 as an orange solid; mp 179–181C; Rf= 0.3 (hexane/
EtOAc = 1:1);1H NMR (CDCl3, 300 MHz) d 8.04 (s, 2H),
7.87 (d, J = 9.0 Hz, 4H), 7.73 (s, 2H), 7.67 (s, 4H), 7.35– 7.27 (m, 10H), 6.98 (d, J = 9.0 Hz, 4H), 6.93 (d, J = 7.6 Hz, 4H), 6.70 (t, J = 7.6 Hz, 2H), 5.58 (s, 4H), 5.17 (s, 4H), 4.22 (d, J = 13.2 Hz, 4H), 3.83 (s, 6H), 3.37
(d, J = 13.2 Hz, 4H);13C NMR (CDCl3, 75.4 MHz) d 61.2 (Cq), 155.7 (Cq), 151.3 (Cq), 147.1 (Cq), 145.7 (Cq), 143.6 (Cq), 134.9 (Cq), 132.6 (Cq), 129.4 (CH), 129.0 (CH), 128.7 (CH), 128.2 (Cq), 127.9 (CH), 125.8 (CH), 124.1 (CH), 123.5 (CH), 123.4 (CH), 114.1 (CH), 69.6 (CH2), 55.4 (CH3), 54.1 (CH2), 31.4 (CH2); FABMS m/z 1035
(M+, 9), 91 (100); HR FABMS Calcd for C
62H54N10O6
1034.4228. Found 1034.4221.
11. Compound 4: To an ice cold solution of 1.40 g (20.3 mmol) of NaNO2in 10 mL of 4 N HCl was added a solution of
1.50 g (12.2 mmol) of p-anisidine in 15 mL of acetone, and the mixture was stirred for 30 s. The combined solution was then added to another ice cold solution of 0.50 g (4.1 mmol) of 2,6-dimethylphenol in 20 mL of pyridine to produce a colored solution. The reaction mixture was stirred for 18 h at 0C and then treated with 100 mL of 4 N HCl to give a colored precipitate. The solid residue was purified by column chromatography with hexane/ ethyl acetate (v/v = 3:1) to give 0.83 g (79 %) of 4 as an orange solid; mp 120–122C; Rf= 0.6 (hexane/
EtOAc = 3:1); 1H NMR (CDCl3, 300 MHz) d 7.86 (d, J = 9 Hz, 2H), 7.59 (s, 2H), 7.00 (d, J = 9 Hz, 2H), 4.98 (s, 1H), 3.88 (s, 3H), 2.33 (s, 6H); 13C NMR (CD3CN, 75.4 MHz) d 162.6 (Cq), 156.6 (Cq), 147.7 (Cq), 146.9 (Cq), 125.4 (Cq), 124.9 (CH), 123.9 (CH), 115.3 (CH), 56.3 (CH3), 16.6 (CH3); FABMS m/z 257 (M++1, 100), 256
(M+, 85), 136 (55), 121 (45); HR FABMS Calcd for
C15H16N2O2256.1212. Found 256.1206.
12. Crystal structure data for 2: C50H40Cl6N4O6, M = 1005.56,
Triclinic, a = 13.5391(6) A˚ , a= 74.752(1), b = 14.2081(7) A˚ , b= 65.904(1), c = 14.2656(7) A˚ , c= 72.891(1), V = 2361.89(19) A˚3, T = 150(1) K, space group P-1, Z = 2, l = 0.418 mm 1, 25,311 reflections collected (R1= 0.0918, wR2= 0.2296), 8313 independent reflections (R(int) = 0.0433, R1= 0.1094, wR2= 0.2435).
Crystal structure data for 3: C63H57Cl3N10O6, M =
1156.54, Monoclinic, a = 25.3068(11) A˚ , a= 90, b = 10.5670(4) A˚ , b = 101.372(3), c = 21.5706(13) A˚, c = 90, V = 5655.1(5) A˚3, T = 150(2) K, space group P2(1)/c, Z = 4, l= 0.225 mm 1, 23,719 reflections collected (R1= 0.1076, wR2= 0.2617), 9868 independent reflections (R(int) = 0.0822, R1= 0.2174, wR2= 0.3089).
Crystallo-graphic data for the two structures in this letter have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication. Their CCDC num-bers are 2 (CCDC 652909), 3 (CCDC 652910), respec-tively. Copies of the data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK. Fax: +44 1223 336033, e-mail: data_ [email protected].
13. (a) Job, P. Ann. Chim. Appl. 1928, 9, 113; (b) Connors, K. A. Binding Constants; Wiley: New York, 1987.
14. Benesi, H. A.; Hilderbrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703.
15. Jaime, C.; de Mendoza, J.; Prados, P.; Nieto, P. M.; Sanchez, C. J. Org. Chem. 1991, 56, 3372.
16. Based on the results of different metal ion-induced chemical shift changes in the 1H NMR of ionophore
3, one may infer that the binding modes for 3ÆCa2+ and 3ÆPb2+are different; that is, the Ca2+is bound to one
of the two triazole units and the two hydorxyl azo-phenol groups, whereas Pb2+is bound to both the two triazole units and the two hydroxyl azophenol groups of 3.
