Synthesis and cation binding properties of new arylazo- and heteroarylazotetrathiacalix[4]arenes

Article abstract of DOI:10.1016/j.tet.2005.10.073

New macrocyclic tetrathiacalix[4]arenes have been synthesized by incorporating arylazo-, thiazoleazo- and β-naphthylazo- units in the tetrathiacalix[4]arene molecular architecture through diazotization and coupling reactions. The new compounds have been c

Full text of DOI:10.1016/j.tet.2005.10.073

Tetrahedron 62 (2006) 1150–1157
Synthesis and cation binding properties of new arylazo-
and heteroarylazotetrathiacalix[4]arenes
* *
Ananya Chakrabarti, H. M. Chawla, T. Francis, N. Pant and S. Upreti
Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi-110016, India
Received 11 August 2005; revised 11 October 2005; accepted 27 October 2005
Available online 1 December 2005
Abstract—New macrocyclic tetrathiacalix[4]arenes have been synthesized by incorporating arylazo-, thiazoleazo- and b-naphthylazo- units
in the tetrathiacalix[4]arene molecular architecture through diazotization and coupling reactions. The new compounds have been
characterized by 1H NMR, 13C NMR and FAB-MS spectroscopic analysis. X-ray crystallography for one of the new dyes (4a) reveals that the
compound is present in the cone conformation. The synthesized macrocycles have been examined for their binding with alkali (LiC, NaC,
KC, CsC and RbC), alkaline earth (Ca2C, Mg2C and Ba2C) and transition metal cations (Cr3C, Fe2C, Co2C, Ni2C, Cu2C, HgC, Hg2C
,
Pd2C and Pt2C) by UV–visible spectroscopy to reveal selective bathochromic shifts for heavier alkali metal ions (cesium and rubidium) and
palladium in a 1:1 and 2:1 stoichiometry respectively. The study has a significant bearing on the development of useful ionic filters and sensor
materials.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
probably due to the ease with which the bridging sulfur
atoms can be oxidized to sulphoxide5 or sulphone6
derivatives by many of the oxidation catalysts used for
derivatization which complicates the outcome of reactions.
A few interesting reports on upper rim substitution of
tetrathiacalix[n]arenes have appeared in the literature
recently. For instance, sulphonation,7 bromination,8
nitration,9,10 chloromethylation11 and organophosphoryla-
tion11 of tetrathiacalix[4]arenes have been reported. Lhotak
et al. have reported the synthesis of aminotetrathiacalix[4]-
arenes via diazocoupling and reduction12 of the tetrathia-
calix[4]arene analogue. Tetrathiacalix[4]arenes bearing
ethynylic groups on the lower rim have also been reported
by Parola et al.13 To the best of our knowledge, very little
work seems to have been reported on tetrathiacalix[4]arenes
with chromogens at the upper rim, despite the fact that these
derivatives are attractive targets for supramolecular assem-
blages for molecular recognition through visual colour
change.14–18 We describe herein the synthesis and evalu-
ation of new chromogenic tetrathiacalix[4]arenes bearing
phenylazo-, thiazoleazo-, pyridylazo- and b-naphthylazo-
groups (Table 1) for ionic recognition in the hope to obtain
new molecular filters and devices for specific use. While all
compounds have been identified by physical and spectro-
scopic measurements (UV, IR, NMR and FAB MS),
structure of one of these chromogenic compounds has
been elucidated by X-ray crystallography. The ionic
recognition by the synthesized molecular receptors
reported in this paper has been investigated by UV–visible
spectroscopic methods.
Calix[n]arenes are now well recognized molecular receptors
for chemical and biochemical analysis through principles of
molecular recognition. The replacement of the four
methylene bridges of p-tert-butylcalix[4]arene by sulfide
linkages provides tetrathiacalix[4]arenes which are
expected to have different chemical behaviour from that
of the classical calixarenes first described by Gutsche et al.1
and a large number of subsequent workers.2 The difference
in chemical and spectral behaviour of calixarenes and
tetrathiacalixarenes is primarily due to the possibility of
varying oxidation states of the bridging sulfur atoms. The
parent tetrathiacalix[4]arene architecture was first achieved
by Sone et al. in 1997 via stepwise replacement of
methylene linkages of p-tert-butylcalix[4]arene by sulfur
bridges.3 In the same year, Kumagai et al. reported a facile
synthesis of p-tert-butyltetrathiacalix[4]arene in satisfactory
yields by the reaction of p-tert-butylphenol with elemental
sulfur.4 Considerable efforts have been made thereafter to
derivatize p-tert-butyltetrathiacalix[4]arenes to achieve
molecular scaffolds and novel molecular receptors. Though
a small number of tetrathiacalix[4]arenes with substituents
at the lower rim are known, information on upper rim
derivatization of tetrathiacalix[4]arenes is limited. This is
Keywords: Tetrathiacalix[4]arenes; Chromogenic; Diazotization; Ionic
recognition.
Corresponding authors. Tel.: C91 11 2659 1517; fax: C91 11 2659
1502; e-mail: hmchawla@chemistry.iitd.ernet.in
*
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.10.073

A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
1151
Table 1. Different products of reactions under optimized reaction conditions and their yields
Starting
compound
Diazotizing unit
Product no.
Optimized conditions (base/
solvent)
% Yields
2
2
2
2
2
2
3a
3b
3c
3d
3e
3f
DMF:MeOH (8:5)/CH3COONa
DMF/CH3COONa
37
68
25
74
71
77
DMF:MeOH (8:5)/CH3COONa
DMF: MeOH (8:5)/NaOH
DMF/CH3COONa
DMF/CH3COONa
2
4a
DMF:MeOH (8:5)/CH3COONa
41
2
2
2
4b
5
DMF:MeOH (8:5)/CH3COONa
DMF:MeOH (8:5)/CH3COONa
DMF:MeOH (8:5)/CH3COONa
32
46
47
6a
2
6b
DMF:MeOH (8:5)/CH3COONa
79
2. Results and discussion
performed at 0–5 8C in a mixed DMF/MeOH (8:5) solvent
and sodium acetate as the base11 followed by separation of
the products by column chromatography to yield pure
products in reasonable to good yields (Table 1). Detailed 1H
NMR spectral analysis of the synthesized compounds
revealed that the phenylazo tetrathiacalix[4]arenes 3a–c
exhibited a singlet for the aromatic protons of the
substituted tetrathiacalix[4]arene ring in the range d
8.15–8.24 while protons on the unsubstituted aromatic
tetrathiacalix[4]arene rings gave signals between d
6.77–7.80. 3d exhibited a singlet for the aromatic protons
of the tetrathiacalix[4]arene ring at d 8.24. Since sulfur bridges
in the present compounds have replaced the methylene bridge
2.1. Synthesis and characterization of chromogenic
tetrathiacalix[4]arenes
In this work, seven new diazo-coupled tetrathiacalix[4]-
arenes were synthesized by a weak base catalyzed coupling
reaction of 25,26,27,28-tetrahydroxy-2,8,14,20-tetrathia-
calix[4]arene 2 with diazotized solutions of aniline,
2-aminothiazole, 3-aminopyridine and b-naphthylamine
(Scheme 1). The parent p-tert-butyltetrathiacalix[4]arene
and tetrathiacalix[4]arene 2 were obtained by employing
literature procedures.4,19 The coupling reaction was
Scheme 1.
3a
3b
3c
3d
3e
3f
4a
4b
5
R1ZC6H5–N]N–
R1, R3ZC6H5–N]N–
R1, R2, R3ZC6H5–N]N–
R1, R2, R3, R4ZC6H5–N]N–
R1, R2, R3, R4ZO2N–C6H4–N]N–
R1, R2, R3, R4ZHOOC–C6H4–N]N–
R1ZNSH2C3–N]N–
R1, R2ZNSH2C3–N]N–
R1ZC5H4N–N]N–
R2, R3, R4ZH
R2, R4ZH
R4ZH
—
—
—
R2, R3, R4ZH
R3, R4ZH
R2, R3, R4ZH
R2, R3, R4ZH
R2, R4ZH
6a
6b
R1ZC10H7–N]N–
R1, R3ZC10H7–N]N–

1152
A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
present in classical calixarenes, exact information regarding
the conformation of these chromoionophores could not be
obtained from 1H NMR and 13C NMR spectroscopic
techniques. A single crystal X-ray of one of the derivatives
(4a) revealed that the synthesized compound was present in
the cone conformation. By analogy and comparison of the
NMR spectral data, it appears that the other synthesized
tetrathiacalix[4]arenes probably also have the cone confor-
mation but X-ray structure determinations of compounds
other than 4a are yet to be achieved.
the complexity of the spectral data. However, elemental
analysis and FAB MS spectroscopic measurements proved
useful for identification of compounds 6a and 6b.
2.2. Results obtained from X-ray crystallography of
5-[(1-thiazole)azo]-25,26,27,28-tetrahydroxy-2,8,14,
20-tetrathiacalix[4]arene, 4a
Recrystallization of 4a from chloroform afforded crystals
suitable for X-ray diffraction. It was found that 4a
crystallized with chloroform located outside the tetrathia-
calixarene cavity with torsion angles f and c around
Ar–S–Ar bonds about S1, S2, S3 and S4 as K89.3(3),
94.7(3), K94.5(3), 91.6(3), K86.6(3), 97.9(3), K97.4(3)
and 87.4(3) respectively. The alternate G sequence observed
was consistent with the cone conformation found in parent
p-tert-butyltetrathiacalix[4]arene21and tetrathiacalix[4]-
arene.22 The unit cell of 4a (Fig. 1) consisted of four
molecules, each molecule having one chloroform molecule
near the outer periphery of the tetrathiacalix[4]arene cavity.
AllfouraromaticringsA(C1–C6), B(C7–C12), C(C13–C18)
and D (C19–C24) were found to be almost planar with
angles C2–S1–C24Z101.62(14), C6–S2–C8Z103.28(14),
C12–S3–C14Z101.31(15) and C18–S4–C20Z101.94(15).
The connecting sulfur atoms S1, S2, S3 and S4 formed an
approximate plane where alternate sulfur atoms lay G0.053
It was interesting to note that the solvent used for the
coupling reaction has a profound effect on the outcome of
the reaction, i.e., the extent of substitution as well as the
yield of the final product obtained. For example, when DMF
was employed instead of DMF-methanol in the coupling
reaction, orange crystals of pure 3b in far better yields
(68%) were obtained in contrast to the yields obtained from
the use of DMF-MeOH mixture (43%). In addition, the
latter solvent afforded a mixture of 3a, 3b and 3c.
The thiazoleazo coupled tetrathiacalix[4]arenes 4a and 4b
exhibited a pair of doublets for the protons of the thiazole
moiety in the range of d 7.41 and 8.01 respectively. 4a and
4b were identified as cone conformers by a comparative
study of their NMR patterns with those of known
tetrathiacalix[4]arenes.20 The pyridylazo substituted tetra-
thiacalix[4]arene exhibited a singlet at d 9.06, a pair of
doublets at d 8.00 and 8.60, and a triplet at d 7.34 for the
protons of the 3-pyridylazo moiety and a singlet at d 8.18 for
the protons of the tetrathiacalixaryl aromatic ring bearing
the 3-pyridylazo group and two multiplets for the
tetrathiacalixaryl protons of the unsubstituted rings at d
6.75 and 7.64. The characterization of the tetrathiacalix[4]-
arenes bearing b-naphthylazo substituents on the upper rim
by NMR spectroscopic techniques proved difficult due to
˚
and G0.053 A above and below the plane. The interplanar
angles found between this plane and the rings A (C1–C6), B
(C7–C12), C (C13–C18) and D (C19–C24) were 57.66,
49.22, 61.91, and 50.978 respectively. The interplanar
angles between the rings AC and BD were 60.43 and
79.818 respectively. The dihedral angle between the
substituted azothiazole group plane was found to be 8.268,
which corroborated the alignment of the heterocyclic ring
with the cone conformation of the tetrathiacalix[4]arene
skeleton. The corresponding hydroxyl substituents O1, O2,
Figure 1. (a) ORTEP diagram of 4a showing labeling scheme used and (b) contents of a single unit cell.

A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
1153
O3 and O4 were directed inwards the cavity of the
calixarene architecture. The average distance between two
˚
oxygen atoms was 2.82 A. Although one cannot exclude the
stabilizing role of the solvent, the rather short distance
between adjacent oxygen atoms might indicate the existence
of an intramolecular H-bond array stabilizing the cone
conformation. The average distance between two adjacent
˚
sulfur atoms was 5.51 A which demonstrated that the size of
˚
the cavity in tetrathiacalix[4]arene was approximately 0.5 A
bigger than that in classical calix[4]arene. The exocyclic
chloroform molecule showed three prominent non bonding
interactions between the H of the chloroform molecule and the
thiazole moiety attached to the ring A with average CH–p
˚
distancesof2.399, 2.473and2.944 A respectively. TheCH–p
interaction between two adjacent tetrathiacalix[4]arene
molecules is given in Fig. 2(a). There was significant S–S
interaction between adjacent tetrathiacalix[4]arene molecules
˚
with a distance of 3.550 A (Fig. 2(b)).
Figure 2. (a) CH–p interaction between two adjacent molecules and
(b) H-bonding and S.S interactions in two molecules along b axis.
2.3. UV–visible spectroscopic studies of synthesized
chromoionophores 3a–c and 4a,b.
In order to obtain insight into the metal affinity of the
chromogenic tetrathiacalix[4]arenes, the changes in their
lmax upon interaction with a variety of hard and soft metal
cations was investigated as follows:
2.3.1. Interaction with alkali and alkaline earth metal
ions. The affinity of tetrathiacalix[4]arenes 3a–c and 4a,b
for group I (LiC, NaC, KC, CsC and RbC) and group II
(Ca2C, Mg2C and Ba2C) cations was examined in solution
using chloroform/methanol (1:1) as the solvent. Fig. 3
depicts the wavelengths of absorption of the different
chromoionophores. The changes in lmax of these chromoio-
nophores upon addition of various cations are listed in
Table 2. It has been observed that 3a–c and 4a,b exhibited
Figure 3. UV–visible spectra showing lmax of chromoionophores 3a–c and
4a,b.
Table 2. Dlmax/nm of the synthesized chromoionophores 3a–c and 4a,b on addition of 100 equiv. of metal salts using methanol–chloroform (v/vZ1/1) as the
cosolvent
Metal salts
Blank solutions of ligandsa
3a
3b
3c
4a
4b
298
389
378
392
301
469
320
473
lmax/nm of chromoionophores after adding metal salts
LiC
nc
nc
nc
C19
C21
C91 (np at 320)
C77 (np at 318)
ns
ns
C20 (np at 335)
nc
K39
K41 (np at 293)
K9
s(327)
K15
nc
nc
nc
nc
C9
C10
K22
nc
nc
C12
C12
K22
K20
nc
nc
—
K22
nc
nc
nc
NaC
KC
C21
C6
C21
C10
C5
nc
C33
nc
C51
ns
C19
C24
nc
C7
C24
C26
nc
C14
C10
C60
C73
nc
nc
C12
nc
C66
nc
K17
nc
K26
nc
nc
ns
nc
s(329)
s(332)
C61
C46
K19
nc
C18
C13
C18
C18
nc
nc
C25
nc
C90
nc
C11
C16
nc
C20
C34
C55
C57
nc
K36
C19
nc
K22
nc
K44
K54
K46
nc
nc
K20
nc
C148
nc
C13
C16
C54
C46
K43
nc
nc
nc
K67
nc
K51
K57
K50
K46
nc
C17 s(554)
nc
s(603)
nc
CsC
RbC
Mg2C
Ca2C
Ba2C
Sr2C
Cr2C
Fe2C
Co2C
Ni2C
Cu2C
Cd2C
Hg2C
HgC
AgC
Pd2C
Pt2C
nc
nc
K43
K14
K24
K17
nc
K20
nc
nc
nc
nc
K20
nc
nc
nc
nc
C18
C20
nc
C20
ns
nc
nc
nc
nc
K45
nc
nc
nc
nc
nc
nc
a Concentration for 3a–c and 4a,b was 10K4 M. C and K indicate bathochromic and hypsochromic shifts respectively. lmaxZlcomplexKlmax (free host). nc
denotes no detectable change in lmax upon metal ion complexation. ns indicates negligible shifts in lmax. np indicates new peak position. s indicates shoulder
in addition to the main peak.

1154
A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
Figure 4. (a) UV–visible spectra of (1)10K4 M solution of 4a and shifts in its lmax upon the addition of 10K4 M solutions of (2) Cs2CO3, (3) Rb2CO3 and
(4) K2PdCl4 in a Methanol–chloroform (v/vZ1/1) cosolvent; (b) changes in the UV–visible spectra of 4a upon titration by Cs2CO3 in a methanol–chloroform
(v/vZ1/1) cosolvent where the concentration of Cs2CO3 (1) 1!10K4 (2) 2!10K4 (3) 3!10K4 (4) 4!10K4 (5) 5!10K4 (6) 6!10K4 (7) 7!10K4
(8) 8!10K4 (9) 9!10K4 (10) 1!10K3 (11) 2!10K3 (12) 3!10K3 (13) 4!10K3 (14) 5!10K3
.
little or no shifts for the smaller alkali metal ions (LiC and
NaC) but significant bathochromic shifts were observed in
the case of larger alkali metal ions (CsC and RbC) indicating
that the availability of the required number of donor oxygen
atoms is not the major factor for the interaction of alkali metal
cations and the synthesized tetrathiacalix[4]arenes. The
cavity size of these azo coupled tetrathiacalix[4]arenes
seems to have been affected by somewhat restricted
conformational isomerization which fails to provide appro-
priate binding sites for relatively smaller alkali metal ions.
For example, 3b showed bathochromic shifts of 91 and 77 nm
with concomittant appearance of new peaks at 320 and
318 nm respectively in their UV spectra on addition of
solutions of Cs2CO3 and Rb2CO3 in methanol/chloroform.
Significant bathochromic shifts in the absorption maximum
were accompanied by significant colour changes in the visible
region. For instance when UV–visible spectral measurements
were carried out for interaction of 4a with CsC and RbC by
addition of Cs2CO3 and Rb2CO3 in chloroform/methanol to
the chloroform–methanol solution of 4a (Fig. 4(a)), it
revealed that absorption of 4a shifted from 468 to 524 nm
with an isobestic point at 365 nm. Quantitative analysis
through Job’s continuous variation plots indicated that the
metal:tetrathiacalix[4]arene interaction was in a 1:1 stoichio-
metric ratio of 4a with Cs(I) and Rb(I).
with the synthesized tetrathiacalix[4]arenes, they seemed to
have good potential to be developed into molecular filters
for ionic separations.
2.3.2. Interaction with transition metal ions. An
investigation into the ionic recognition properties
(Table 2) of synthesized chromoionophores with transition
metal ions (Cr3C, Fe2C, Co2C, Ni2C, Cu2C, HgC, Hg2C
,
Pd2C and Pt2C) was also investigated but surprisingly
significant interaction was not observed despite the fact that
sulfur containing molecular receptors should be expected to
exhibit pronounced changes on interaction with transition
metal ions. A marked bathochromic shift was however
observed when a chloroform/methanol (1:4) solution of
K2PdCl4 was added to 4a (Fig. 4(a)), which was
accompanied by a visual change from yellow to bluish
green. This observation revealed a change in the coordi-
nation sphere of palladium in the presence of tetrathiacalix
[4]arene, 4a. The absorption intensity of the free tetra-
thiacalix[4]arene at 468 nm was found to gradually decrease
in intensity with the formation of a new absorption band at
617 nm (DdZ148 nm). This observation is consistent with
the earlier observation that complexes of palladium with
thioethers exhibit a typical blue or green colour due to a d–d
transition around 610 nm.23 Two isobestic points at 363 and
414 nm could be easily discerned from the UV–visible
titration spectra of 4a with K2PdCl4 (Fig. 5(a)). The spectral
features in the figure are consistent with a 1:2 binding of 4a
Since significant shifts were not observed in the case of the
smaller alkali metal ions and alkaline earth metal cations
Figure 5. (a) Changes in the UV–visible spectra of 10K4 M solution of 4a upon titration by K2PdCl4 in a methanol–chloroform (v/vZ1/1) cosolvent where the
concentration of K2PdCl4 varies from 2 to 8!10K4 M and (b) Job plot of 2:1 complex of 4a and Pd2C
.

A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
1155
and Pd2C. Analysis of the molar ratio as well as data from
Job’s plot experiment (Fig. 5(b)) confirmed a 1:2 binding
ratio of 4a with Pd2C. The coordination of palladium may
be visualized through metal–oxygen or through metal–
sulfur interactions besides the cavity of the tetrathiaca-
lix[4]arene core although a possible coordination mode
through the N]N bonds is not ruled out. Knowledge of the
exact coordination mode of palladium would require further
investigations. Since negligible or small hypsochromic
shifts were observed in the case of other transition metal
ions, no quantitative experiments were carried out for these
chromoionophores.
6.72–6.80 (m, 3H, tetrathiacalixarene ArH). MS-FAB:
calcd for C30H20N2O4S4: m/zZ600.75 [MC]; found: m/
zZ601 [MC, 90%].
Compound 3c: Anal. Calcd for C42H28N6O4S4: C, 62.36; H,
3.49; N, 10.39. Found: C, 62.52; H, 3.67; N 10.30. 1H NMR
(300 MHz, CDCl3) dH 9.43 (s, 4H, –OH); 8.24 (s, 4H,
tetrathiacalixarene ArH); 8.16 (s, 2H, tetrathiacalixarene
ArH); 7.77 (m, 6H, phenylazo ArH); 7.65 (d, 2H, JZ7.6 Hz,
tetrathiacalixarene ArH); 7.39 (m, 9H, phenylazo ArH);
6.76 (t, 1H, JZ15.4 Hz, tetrathiacalixarene ArH). MS-FAB:
calcd for C42H28N6O4S4: m/zZ808.97 [MC]; found:
m/zZ809[MC, 42%].
3. Experimental
3.1.2. Synthesis of 5,17-bis[(1-phenyl)azo]-25,26,27,28-
tetrahydroxy-2,8,14,20-tetrathiacalix[4] arene (3b). A
solution of aniline diazonium chloride, which was prepared
from aniline (1.396 g, 15.0 mmol), sodium nitrite (0.69 g,
10.0 mmol) and conc. HCl (7 mL) in water (25 mL) was
added slowly into a cold (5 8C) solution of tetrathiacalix[4]-
arene (0.5 g, 1.0 mmol) and sodium acetate trihydrate
(2.72 g, 20 mmol) in DMF (15 mL) to give an orange
suspension. After being allowed to stand for 3 h at room
temperature, the suspension was acidified with aqueous HCl
(50 mL, 2 M). The resulting red precipitate was filtered,
washed with water and methanol and dried to yield (0.36 g,
68%) of 3b as a bright yellow solid. Anal. Calcd for
C36H24N4O4S4: C, 61.34; H, 3.43; N, 7.95. Found: C, 61.28;
All the reagents used in the study were purchased from
Sigma-Aldrich or Merck and were chemically pure. The
solvents used were distilled or dried according to the
requirement. Column chromatography was performed on
silica gel (60–120 mesh) obtained from Merck. Melting
points were recorded on an electric melting point apparatus
1
(Toshniwal, India) and are uncorrected. H NMR spectra
were recorded on a 300 MHz Bruker DPX 300 instrument at
room temperature using tetramethylsilane (TMS) as an
internal standard. X-ray data was recorded using a Bruker
SMART CCD single crystal diffractometer. UV–visible
spectra were obtained on a Perkin Elmer (Lambda-3B)
recording spectrophotometer. The FAB mass spectra were
recorded on a JEOL SX 102/DA-6000 Mass spectrometer/
Data System using Argon/Xenon (6 kV, 10 mA) as the FAB
gas.
1
H, 3.54; N, 8.01. H NMR (300 MHz, CDCl3) dH 9.37 (s,
4H, –OH); 8.15 (s, 4H, tetrathiacalixarene ArH); 7.75 (m,
4H, phenylazo ArH); 7.66 (d, 4H, JZ7.4 Hz, tetrathiacalix-
arene ArH); 7.38 (m, 6H, phenylazo ArH); 6.77 (t, 2H, JZ
15.3 Hz, tetrathiacalixarene ArH). MS-FAB calcd for
C36H24N4O4S4: m/zZ704.86 [MC]; found: m/zZ705
[MC, 47%].
3.1. Preparation of the starting materials
p-tert-Butyltetrathiacalix[4]arene4 and tetrathia calix[4]-
arene19 2 were synthesized by methods reported earlier.
3.1.3. Synthesis of 5,11,17,23-tetrakis[(1-phenyl)azo]-
25,26,27,28-tetrahydroxy-2,8,14,20-tetrathiacalix[4]-
arene (3d). A solution of aniline diazonium chloride, which
was prepared from aniline (1.396 g, 15.0 mmol), sodium
nitrite (0.69 g, 10.0 mmol) and conc. HCl (7 mL) in water
(25 mL) was added slowly into a cold (5 8C) solution
of tetrathiacalix[4]arene (0.5 g, 1.0 mmol) and sodium
hydroxide (0.8 g, 20 mmol) in MeOH-DMF (15 mL, 5:8,
v/v) to give a dark red suspension. After being allowed to
stand for 3 h at room temperature, the suspension was
acidified with aqueous HCl (50 mL, 2 M). The resulting
brownish precipitate was filtered and washed with water and
methanol to give (0.68 g, 74%) of 3d as a light brown solid.
Anal. Calcd for C48H32N8O4S4: C, 63.14; H, 3.53; N, 12.27.
3.1.1. Synthesis of 5-[(1-phenyl)azo]-25,26,27,28-tetra-
hydroxy-2,8,14,20-tetrathiacalix[4]arene (3a) and 5,11,
17-tris[(1-phenyl)azo]-25,26,27,28-tetrahydroxy-2,8,14,
20-tetrathia calix[4]arene (3c). General procedure. A
solution of phenyl diazonium chloride prepared from aniline
(1.396 g, 15.0 mmol), sodium nitrite (0.69 g, 10.0 mmol)
and conc. HCl (7 mL) in water (25 mL), was added slowly
into a cold (5 8C) solution of tetrathiacalix[4]arene (0.5 g,
1.0 mmol) and sodium acetate trihydrate (2.72 g, 20 mmol)
in MeOH-DMF (15 mL, 5:8, v/v) to give an orange
suspension. After allowing it to stand for 3 h at room
temperature, the suspension was acidified with aqueous HCl
(50 mL, 2 M). The resulting red precipitate was filtered and
washed with water and methanol to yield a crude product
which was purified by column chromatography on silica gel
using chloroform to give 3a (0.22 g, 37%, RfZ0.60) as a
bright yellow solid and 3c (0.20 g, 25%, RfZ0.34) as a red
powder.
1
Found: C, 63.21; H, 3.67; N, 12.12. H NMR (300 MHz,
CDCl3) dH 9.41 (s, 4H, –OH); 8.24 (s, 8H, tetrathiacalix-
arene ArH); 7.37–7.77 (m, 20H, phenylazo ArH). MS-FAB
calcd for C48H32N8O4S4: (m/z)Z913.08 [MC]; found:
m/zZ913 [MC, 34%].
3e and 3f were synthesized according to the literature
procedures.12
Compound 3a: mp 208 8C. Anal. Calcd for C30H20N2O4S4:
C, 59.98; H, 3.36; N, 4.66. Found: C, 60.02; H, 3.51; N,
1
4.58.]. H NMR (300 MHz, CDCl3) dH 9.42 (s, 4H, –OH);
3.1.4. Synthesis of 5-[(1-thiazole)azo]-25,26,27,28-tetra-
hydroxy-2,8,14,20-tetrathia calix[4]arene (4a) and 5,11-
bis[(1-thiazole)azo]-25,26,27,28-tetrahydroxy-2,8,14,20-
tetrathiacalix[4]arene (4b). Compounds 4a and 4b were
8.22 (s, 2H, tetrathiacalixarene ArH); 7.82 and 7.46 (m, 5H,
phenylazo ArH); 7.70 (d, 2H, JZ6 Hz, tetrathiacalixarene
ArH); 7.63 (d, 4H, JZ6 Hz, tetrathiacalixarene ArH);

1156
A. Chakrabarti et al. / Tetrahedron 62 (2006) 1150–1157
prepared as described above for 3a using 2-aminothiazole.
Column purification using chloroform/ethyl acetate (9:1)
gave 4a (0.25 g, 41%, RfZ0.68) as dark red crystals and 4b
(0.23 g, 32%, RfZ0.31) as a dark red powder. It should be
noted that products formed under these condition do not
contain complexed chloroform.
3.2. Crystallography
The crystals suitable for single crystal X-ray diffraction
were obtained by slow cooling of a warm solution of 4a in
chloroform. Dark red crystals of a 1:1 4a. chloroform
complex were obtained with molecular formula C29H18Cl6-
N3O4S5, MZ845.46, monoclinic, space group P2(1)/n with
aZ10.9745(7), bZ22.1150(14), cZ14.8313(9), aZ90.0,
bZ90.96(10), gZ90.08 and DcZ1.560 g/cm3 for ZZ4.
Intensity diffraction data were calculated up to qZ26.868
by using 2u step scanning mode with Mo Ka radiation
Compound 4a: mpO250 8C. Anal. Calcd for
C27H17N3O4S5: C, 53.36; H, 2.82; N, 6.91. Found: C,
1
53.25; H, 2.67; N, 7.01. H NMR (300 MHz, CDCl3) dH
9.37 (s, 4H, –OH); 8.34 (s, 2H, tetrathiacalixarene ArH);
8.02 (bs, 1H, thiazole ArH); 7.70 and 6.82 (m, 6H and 3H,
tetrathiacalixarene ArH); 7.41 (bs, 1H, thiazole ArH).
˚
lZ0.71073 A) at 273 K. A total of 6693 reflections were
calculated and used in structure analysis and refinement. All
the non-hydrogen atoms were refined anisotropically using
restraints on the bond lengths and thermal parameters. All
hydrogen atoms were placed in their geometrical positions
and were not refined. The labeling scheme followed is in
agreement with that followed in calix[4]arene–solvent host
guest complexes.24,25 The final R index using observed data,
refining 455 parameters with no restraints was RallZ0.0769,
RgtZ0.0569, wRrefZ0.1808 and wRgtZ0.1677. Low
e.s.d.’s for other atoms suggest that the overall geometry
and accuracy of the structure has not been compromised to
any significant extent. All the calculations involving
structure solution, using SHELXTL-PC performed refine-
ment and graphics. Crystallographic data for the structure
have been deposited with Cambridge Crystallographic
Database as supplementary publication number CCDC
280307.
Compound 4b: mp 170 8C. Anal. Calcd for C30H18N6O4S6:
C, 50.12; H, 2.52; N, 11.69. Found: C, 50.28; H, 2.34; N,
11.57. 1H NMR (300 MHz, CDCl3) dH 8.41 (s, 2H,
tetrathiacalixarene ArH); 8.36 (s, 2H, tetrathiacalixarene
ArH); 8.01 (bs, 2H, thiazole ArH); 7.70 (d, 2H, JZ7.8 Hz,
tetrathiacalixarene ArH); 7.64 (d, 2H, JZ7.5 Hz, tetra-
thiacalixarene ArH); 7.41 (bs, 2H, thiazole ArH); 6.89 (t,
2H, JZ14.8 Hz, tetrathiacalixarene ArH).
3.1.5. Synthesis of 5-[(3-pyridyl)azo]-25,26,27,28-tetra-
hydroxy-2,8,14,20-tetrathia calix[4]arene (5). It was
prepared as described above for 3a using 3-aminopyridine.
Column purification using chloroform/ethyl acetate (8:2)
gave 5 (0.36 g, 59%, RfZ0.66) as an orange solid. Anal.
Calcd for C29H19N3O4S4: C, 57.88; H, 3.18; N, 6.98. Found:
C, 57.65; H, 3.20; N, 6.93. 1H NMR (300 MHz, CDCl3) dH
9.40 (s, 4H, –OH); 8.18 (s, 2H, tetrathiacalixarene ArH);
9.06 (s, 1H, pyridylazo ArH); 7.34 (t, 1H, JZ15 Hz,
pyridylazo ArH); 8.60 (d, 1H, JZ7.7 Hz, pyridylazo ArH);
8.00 (d, 1H, JZ7.7 Hz, pyridylazo ArH); 6.75 and 7.64 (m,
9H, tetrathiacalixarene ArH), MS-FAB calcd for
C29H19N3O4S4: m/zZ601.74 [MC]; found: m/zZ602
[MC, 88%].
3.3. General procedure for UV–visible experiments
Because of the poor solubility of metal salts in chloroform,
all of the UV–visible experiments reported in this work
were carried out in chloroform–methanol (1:1) unless
otherwise specified.
3.3.1. Job’s plot experiments. Stock solutions of com-
pound 4a (10K4 M) in chloroform–methanol (1:1) and
metal salt solutions Cs2CO3 (10K4 M), Rb2CO3 (10K4 M)
in chloroform–methanol (1:1) and K2PdCl4 (10K4 M) in
chloroform–methanol (1:4) were prepared. The concen-
trations of each chloroform–methanol solutions were
varied, but their volumes were fixed at 5.0 mL. After the
mixture was shaken for 2 min, the UV–visible absorbances
at 524 nm for Cs2CO3, 526 nm for Rb2CO3 and at 617 nm
for K2PdCl4 were recorded. Assuming that only one
complex (MLn) was formed at equilibrium; the value of
‘n’ could be calculated from the plot of cmax [mole fraction
of the ligand (cL) at maximum absorption] by the following
relationship, nZcmax/1Kcmax. The value of cmax was noted
from the plot of absorbance vs cL.
3.1.6. Synthesis of 5-[(b-naphthyl)azo]-25,26,27,28-tetra-
hydroxy-2,8,14,20-tetrathia calix[4]arene (6a) and 5,17-
bis[(1-phenyl)azo]-25,26,27,28-tetrahydroxy-2,8,14,20-
tetrathia calix[4]arene (6b). 6a and 6b were prepared as
described above for 3a using b-naphthylamine. Column
purification using chloroform/hexane (1:1) gave 6a (0.31 g,
47%, RfZ0.68) as a dark red solid and 6b (0.64 g, 79%,
RfZ0.39) as a maroon coloured powder.
Compound 6a: mp 192 8C. Anal. Calcd for C34H22N2O4S4:
C, 62.75; H, 3.41; N, 4.30. Found: C, 62.66; H, 3.39; N,
4.49. 1H NMR (300 MHz, CDCl3) dH 9.37 (s, 4H, OH); 8.20
(s, 2H, tetrathiacalixarene ArH); 6.74 and 7.83 (m, 9H,
tetrathiacalixarene ArH); 7.45–7.57 and 7.88–8.13 (m, 7H,
b-naphthylazo ArH) MS-FAB calcd for C34H22N2O4S4:
m/zZ650.81 [MC]; found: m/zZ651 [MC, 50%].
Compound 6b: mp 161 8C. Anal. Calcd for C44H28N4O4S4:
C, 65.65; H, 3.51; N, 6.96. Found: C, 65.81; H, 3.66; N,
6.84. 1H NMR (300 MHz, CDCl3) dH 9.37 (s, 4H, OH); 8.65
(s, 4H, tetrathiacalixarene ArH); 7.74 (d, 4H, JZ7.7 Hz,
tetrathiacalixarene ArH); 7.38–7.60 and 7.92–8.06 (m, 14H,
b-naphthylazo ArH); 6.80 (t, 2H, JZ15.1 Hz, tetrathia-
calixarene ArH). MS-FAB calcd for C44H28N4O4S4: m/zZ
804.98 [MC]; found: m/zZ804 [MC, 49%].
Acknowledgements
The authors acknowledge the financial assistance received
from the Department of Science and Technology (Govt. of
India) and UGC for a junior research fellowship to one of
the authors (A.C.) and Sophisticated Analytical Instrument
Facility, Central Drug Research Institute, Lucknow for
recording FAB Mass spectra.

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