Topological and metal ion effects on the anion binding abilities of new heteroditopic receptors derived from p-tert-butylcalix[4]arene

Article abstract of DOI:10.1016/j.tetlet.2011.04.014

Two derivatives of p-tert-butylcalix[4]arenes 1 and 2 containing diethyl ester and amidoferrocene units as cation and anion binding sites, respectively, have been synthesized. Both compounds were isomeric with different topology for accommodating ions. ¹H NMR spectroscopy, cyclic voltammetry, and square-wave voltammetry were used to study the binding abilities of receptors 1 and 2 toward anions. Both receptors were found to bind Br^- and I ^- selectively in the presence of Na⁺. The electrochemically oxidized ferrocenium form of para-isomer 2 in the free form was found to sense AcO^- selectively, but demonstrated a negative sensing in the presence of Na⁺. In contrast, the electrochemically oxidized ferrocenium form of meta-isomer 1 was found to enhance sensing of AcO^- and Cl^- in the presence of Na⁺.

Full text of DOI:10.1016/j.tetlet.2011.04.014

Tetrahedron Letters 52 (2011) 2914–2917
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Topological and metal ion effects on the anion binding abilities of new
heteroditopic receptors derived from p-tert-butylcalix[4]arene
Nisachol Nerngchamnong a, Benjamas Chailap a, Pannee Leeladee a, Orawon Chailapakul a,
a,
Chomchai Suksai b, , Thawatchai Tuntulani
⇑
⇑
a Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
b Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Burapha University, Chonburi 20131, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Two derivatives of p-tert-butylcalix[4]arenes 1 and 2 containing diethyl ester and amidoferrocene units
as cation and anion binding sites, respectively, have been synthesized. Both compounds were isomeric
with different topology for accommodating ions. 1H NMR spectroscopy, cyclic voltammetry, and
square-wave voltammetry were used to study the binding abilities of receptors 1 and 2 toward anions.
Both receptors were found to bind BrÀ and IÀ selectively in the presence of Na+. The electrochemically
oxidized ferrocenium form of para-isomer 2 in the free form was found to sense AcOÀ selectively, but
demonstrated a negative sensing in the presence of Na+. In contrast, the electrochemically oxidized fer-
rocenium form of meta-isomer 1 was found to enhance sensing of AcOÀ and ClÀ in the presence of Na+.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 31 January 2011
Revised 21 March 2011
Accepted 1 April 2011
Available online 9 April 2011
For anion recognition, attention has been focused on the design
and construction of anion receptors and sensors.1 In general,
simple anion receptors bind anions directly using noncovalent
interactions, for example, hydrogen bonding and electrostatic
interactions. Nevertheless, to gain high selectivity for a specific an-
ion is quite a challenge. Sessler has shown in his recent review that
heteroditopic receptors or ion-pair receptors can be designed to
demonstrate co-operative binding of ion pairs with different bind-
ing modes.2 Compared to simple anion receptors, ion-pair recep-
tors generally display significantly enhanced affinities for anions
as the result of allosteric effects that can strengthen electrostatic
interactions between the bound cation and anion.3
In 2005, we reported a heteroditopic electrochemical anion sen-
sor consisting of an azacrown ether unit for binding alkali metal
cations and an amidoferrocene for hydrogen-bonding with anions.4
Interestingly, this receptor was found to bind BrÀ selectively with
high stability in the presence of Na+. We continued working on
the synthesis of ditopic receptors and sensors using calix[4]arene
as the building block. Our calix[4]arenes containing amidoferro-
cene at the wide rim were reported as electrochemical anion
receptors and sensors.5 To achieve heterditopic electrochemical
anion receptors and sensors, we have modified p-tert-butylca-
lix[4]arene with ethyl ester and amidoferrocene units on the
narrow rim. In this Letter, two isomers of bis-ethyl ester-calix[4]-
arene-capped amidoferrocene 1 (meta-1) and 2 (para-2) have been
synthesized. The ethyl ester and amidoferrocene units were chosen
as the cation and anion binding functionality, respectively. We ex-
pected that the unique topology of compounds 1 and 2 would pro-
vide different anion binding abilities. Furthermore, the anion
binding abilities can be further affected by cations and electro-
chemical oxidation of the ferrocene unit.
The synthetic route to amidoferrocene calix[4]arenes 1 and 2 is
summarized in Scheme 1. Compounds 1a and 2a were synthesized
by the procedure reported previously.6 Nucleophilic substitution of
bisnitrobenzene calix[4]arene with ethyl chloroacetate in THF
afforded bis-esters 1a and 2a, which were then converted into
the corresponding diamines 1b and 2b in almost quantitative
yields with NH2NH2ÁH2O using Pd/C as the catalyst. Finally, recep-
tors 1 and 2 were obtained in 14% and 12% yields through treat-
ment of the bisamines with chlorocarbonyl ferrocene7 in the
presence of triethylamine. These new calix[4]arene derivatives
were characterized by 1H and 13C NMR spectroscopy, MALDI-TOF
mass spectroscopy and elemental analysis.8,9
Receptors 1 and 2 exist in the cone conformation, as supported
by their NMR spectra, possessing a pair of doublets around 3.24
and 4.43 ppm (J ꢀ 13 Hz) corresponding to the equatorial and the
axial protons of the methylene bridging groups (Figs. S1–S4 in Sup-
plementary data). Moreover, the chemical shift of the NH amide
protons also appeared in the downfield region at 8.77 and
7.37 ppm for receptors 1 and 2, respectively. This indicates that
intramolecular hydrogen bonding interactions between the two
amide groups could occur, as in the solid state structure of an
amidoferrocene calix[4]arene reported previously.10 Interestingly,
the amide protons of 1 were shifted to a more downfield region
than 2, implying that intramolecular hydrogen bonding interac-
tions in 1 were stronger than those in 2, probably due to a shorter
⇑
Corresponding authors. Tel.: +66 2 2187643; fax: +66 2 2187598 (T.T.).
E-mail addresses: jomjai@buu.ac.th (C. Suksai), thawatchai.t@chula.ac.th (T.
Tuntulani).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.04.014

N. Nerngchamnong et al. / Tetrahedron Letters 52 (2011) 2914–2917
2915
Fe
HN
O
NO
O
2 O2N
O2N
R
NO2
R
NH2 H2N
NH
O
O
O
O
O
O
O
O
O
O
O
O
(i)
(ii)
(iii)
R
O
O
R
O
O
O
O
O
R
R
O
O
HO
OH
O
O
O
O
O
O
O
R = OEt
1b meta, 2b para (quantitative yields)
R = OEt
1a, meta (24%)
2a, para (72%)
R = OEt
1, meta (14%)
2, para (12%)
Scheme 1. Synthetic route to receptors 1 and 2. Reagents and conditions: (i) ethyl chloroacetate, NaH, NaI (catalyst), THF, reflux; (ii) Ra/Ni, hydrazine, 40% v/v MeOH in
EtOAc, reflux; (iii) chlorocarbonylferrocene, Et3N, CH2Cl2, rt.
hydrogen bonding distance in the meta-isomer 1. The MALDI-TOF
mass spectra of the two receptors demonstrated intense peaks
due to [Fe(C80H92N2O12)] at m/z = 1351.45 [M+Na+].
amide moieties of each isomer played a crucial role in anion selec-
tivity. Receptor meta-1 exhibited binding ability toward benzoate
only. Strong intramolecular hydrogen bonding interactions be-
tween the amide protons in the structure of 1 may inhibit the bind-
ing of the receptor toward anions. In contrast, the para-isomer 2
could bind spherical iodide, Y-shaped acetate and benzoate and
also tetrahedral dihydrogenphosphate because intramolecular
hydrogen bonding interactions were weaker in para-2 and the an-
ion binding site of para-2 was larger than that of meta-1. However,
low values for the anion binding constants of both compounds sug-
gested that they were poor receptors for anions.
The ability of receptors 1 and 2 to bind metal salts in solution
was studied by 1H NMR spectroscopy in 5% CD3CN/CDCl3. Upon
subjecting receptors 1 and 2 to titration with NaClO4 and KPF6
salts, two sets of resonances were observed for all the proton sig-
nals in the 1H NMR spectra of 1 and 2, resulting from strong bind-
ing interactions with slow complexation/decomplexation kinetics.
Near complete conversions into what was presumed to be the
bound form were observed upon the addition of ꢁ1.0 equiv of
M+, which were consistent with a 1:1 binding mode. Binding con-
stants were determined by direct integrations of the host and com-
plex resonances in the 1H NMR spectra as described by
Macomber.11 Binding constants (K) for compounds 1 and 2 with
To improve the anion binding abilities, 1H NMR titrations were
carried out using co-bound Na+ complexes to reveal the cation ef-
fect on the binding affinity of the receptors toward anions. A large
downfield shift of the NH amide proton signals was observed upon
progressive addition of aliquots of bromide and iodide to the co-
bound Na+ complex of 1, [1ÁNa+], Figure 1. The association con-
stants of co-bound Na+ with anions were calculated with the
expression of a 1:1 binding isotherm and the results are summa-
rized in Table 1. Interestingly, the complex formation constants
of [1ÁNa+] toward bromide and iodide were higher than [2ÁNa+],
suggesting that the electrostatic attraction of anions with posi-
tively charged [1ÁNa+] was stronger than that in [2ÁNa+]. This ‘posi-
tive co-operative binding’ of halide anions must be due to
electrostatic interactions with Na+ and preorganization of co-
bound receptors when metal cations are encapsulated in the pseu-
do crown ether cavity.
Na+ (K+) were calculated to be 760 (180) and 505 (236) MÀ1
,
respectively. According to these binding constants, co-bound Na+
complexes were chosen to study the co-operative binding of recep-
tors 1 and 2 with anions. Moreover, the NH protons of co-bound
receptors moved to a more downfield region, indicating that dur-
ing metal co-ordination the two amide units probably moved into
close proximity resulting in stronger intramolecular hydrogen
bonding interactions. (Figs. S5 and S6 in Supplementary data). This
preorganization upon adding the metal ion would influence the an-
ion binding ability of the receptor.
In the absence of Na+, upon adding aliquots of selected anions to
solutions (5% CDCN/CDCl3) of receptors 1 and 2, small changes in
the chemical shifts of the NH amide protons were observed indi-
cating that these protons interacted with the added anions. Deter-
mination of the binding constants was performed on the basis of
the data of the titration profiles using the EQNMR12 nonlinear
curve fitting program. Titration data analysis using a 1:1 binding
model gave the stability constants displayed in Table 1. It should
be noted that the unique topological arrangement of the ferrocene
Table 1
Binding constants (K) for receptors 1 and 2 with anionic guests in the presence and
absence of Na+ in 5% CD3CN/CDCl3
Receptor
K (MÀ1
)
ClÀ
BrÀ
IÀ
BzOÀ
AcO-
H2PO4
À
a
a
a
a
a
1
—
—
—
—
—
—
25
—
—
—
—
—
b
b
b
b
[1ÁNa+]
2
2096
1187
25
886
a
a
—
66
64
32
b
b
b
b
[2ÁNa+]
1119
—
—
—
a
Values are very small and errors are more than 10%.
Values cannot be calculated due to ion-pair formation between co-bound cat-
Figure 1. Change in the NH chemical shift (Dd NH) of receptor 1 and its co-bound
b
cation complex [1ÁNa+] as a function of increasing equivalents of tetrabutylammo-
ions and added anions.
nium (TBA) bromide and iodide in 5% CD3CN/CDCl3.

2916
N. Nerngchamnong et al. / Tetrahedron Letters 52 (2011) 2914–2917
Since Na+ can ‘switch on’ bromide and iodide binding in both
onstrated the emergence of a new voltammetric curve which
developed at the expense of the original following progressive
addition of anionic guests. Significant one-wave cathodic shifts of
the respective Fc/Fc+ redox couples of the receptors were observed
with all anionic guest species (Fig. S9 in Supplementary data) ex-
cept acetate anions. Changes in the Fc/Fc+ redox couples of recep-
receptors meta-1 and para-2, we explored whether the positive
charge of the oxidized forms of 1 and 2 could affect anion binding
abilities. Furthermore, it was of interest to study the potential of
these two cation complexes as ion-pair electrochemical sensors.
The electrochemical properties of the free receptors were investi-
gated by cyclic and square-wave voltammetries. The cyclic voltam-
mograms of 1 and 2 underwent reversible one-electron transfer
(Fig. S7 in Supplementary data). It was found that the redox poten-
tial of 1 (Epa = 0.493 V, Epc = 0.386 V) was higher than that of 2
(Epa = 0.428 V, Epc = 0.321 V), implying that meta-1 was harder to
oxidize than para-2. This may result from stronger intramolecular
hydrogen bonding interactions in meta-1 compared to those in
para-2.
tors 1 and 2 turned out to be almost identical,
D
E ꢀ À80 mV, after
adding 4 equiv of ClÀ as shown in Table 2. In the presence of AcOÀ,
two-wave voltammograms were observed for the electrolytic solu-
tions of free receptors 1 and 2, Figure 2 and Figure S10. Interest-
ingly, a significant perturbation was found in receptor 2 in which
an unexpected negative shift of the Fc/Fc+ redox couple was ob-
served up to À180 mV. This phenomenon could be considered as
an increase of the equilibrium constant for acetate binding by elec-
trochemically-oxidized receptor 2+.17 The binding enhancement
As reported previously, the binding of ferrocene-based recep-
tors with cations generally resulted in an anodic shift of the Fc/
Fc+ redox couple.13 Therefore, upon addition of increasing amounts
of Na+ to receptor solutions, an anodic shift of the wave was ob-
served and the electrochemical response still remained reversible
(Fig. S8 in Supplementary data), while no change was observed
when K+ was added, as expected from the increasing charge-to-size
ratio of K+.14 The quantitative values of the reaction coupling effi-
ciency, or RCEs,15 for receptors 1 and 2 with Na+ were calculated to
factor (BEF)15 was calculated to be 1100 with a
D
G of À17.36
kJ molÀ1. Therefore, the para-isomer 2 should be a good electro-
chemical sensor for acetate.5b,18 The selective sensing properties
of 2 toward acetate mainly stemmed from the fact that the oxi-
dized Fc+ species possessed more acidic NH amide protons to par-
ticipate in hydrogen bonding with acetate anions as well as
electrostatic attractions with the Fc+ moiety.
In all anion sensing studies, the cyclic voltammograms became
quasi-reversible and completely irreversible after adding more
than four equivalents of anions. This indicated an electrochemi-
cal–chemical (EC) mechanism causing product adsorption.19 The
oxidized forms of the receptors could form adducts with anions
giving insoluble ion pairs that strongly adsorbed onto the electrode
surface and could not be reduced in the reverse scan. Moreover, the
difference in the diffusion coefficients of the reduced forms of the
free receptors and their anion adducts resulted in the differences in
the current intensity observed in the voltammograms.
be 0.19 and 0.76, respectively. The difference in
D
E of 1 with Na+
(42 mV) was much higher than that of 2 (7 mV). Presumably, Na+
resided in a larger crown ether like cavity of para-2 resulting in a
longer distance between the iron center and the bound cation as
compared with meta-1, giving weaker electrostatic repulsions be-
tween Fc+ and Na+. This also implied that the preorganization of
receptor 1 in the presence of Na+ affected the anion recognizing
ability of the receptor.
In the absence of alkali metal cations, interactions of the recep-
tors with anions were investigated electrochemically. Cyclic vol-
tammograms and square-wave voltammograms were recorded
after progressively adding aliquots of anion guests to the electro-
lytic solutions of the receptors and the results are summarized in
Table 2. When aliquots of selected anions were added to the recep-
tor solutions, the initially observed Fc/Fc+ redox couple underwent
significant negative potential shifts. This cathodic shift could be
attributed to hydrogen bonding interactions between the anions
and NH protons of the amide groups, which facilitated the oxida-
tion of Fc–Fc+. Unfortunately, the redox process corresponding to
iodide oxidation interfered with the Fc/Fc+ redox wave. Therefore,
the electrochemical behavior of the receptors upon adding iodide
could not be monitored. Furthermore, the limited solubility of
the anion adducts of the oxidized forms of 1 and 2 with bromide,
benzoate, and dihydrogenphosphate did not allow complete elec-
trochemical studies for these anions.
There were two types of voltammograms observed upon adding
anions to the electrolytic solution of receptors 1 and 2: one-wave
and two-wave voltammograms.16 The former corresponded to a
gradual potential shift of the voltammetric curve. The latter dem-
Table 2
Cathodic shifts (DE) of the ferrocene redox couples of receptors 1 and 2 on addition of
4 equiv of ClÀ and AcOÀa
Receptor
D
Eb (mV)
ClÀ
AcOÀ
1
À70
À80
À106
À180
À120
[1ÁNa+]
2
À100
À80
À90
[2ÁNa+]
a
Experiments were carried out in 40% CH3CN/CH2Cl2 using a Pt working elec-
trode, Ag/AgNO3 as the reference electrode and TBAPF6 as the supporting electrolyte
Figure 2. Cyclic voltammograms (top) and square-wave voltammograms (bottom)
on titration of para-2 with 0, 0.5, 1.0, 2.0, 3.0 and 4.0 equiv of acetate anions in 40%
CH3CN/CH2Cl2 with 0.1 M TBAPF6 at a scan rate of 100 mV/s.
at a scan rate of 100 mV/s.
b
D
E is defined as Epa (complex) – Epa (receptor).

N. Nerngchamnong et al. / Tetrahedron Letters 52 (2011) 2914–2917
2917
Supplementary data
Supplementary data (additional 1H and 13C NMR spectra of 1
and 2, and results of electrochemical studies) associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2011.04.014.
References and notes
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Soc. Rev. 2003, 32, 192–202; (c) Martínez-Mánez, R.; Sancenón, F. Chem. Rev.
2003, 103, 4419–4476.
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3. (a) Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486–516; (b) Gale, P. A.
Coord. Chem. Rev. 2003, 240, 191–221; (c) Beer, P. D.; Hayes, E. J. Coord. Chem.
Rev. 2003, 240, 167–189; (d) Mahoney, J. M.; Nawaratna, G. U.; Beatty, A. M.;
Duggan, P. J.; Smith, B. D. Inorg. Chem. 2004, 43, 5902–5907; (e) Gale, P. A.;
García-Garrido, S. E.; Garric, J. Chem. Soc. Rev. 2008, 37, 151–190; (f) Lankshear,
M. D.; Dudley, I. M.; Chan, K. M.; Cowley, A. R.; Santos, S. M.; Fejix, V.; Beer, P. D.
Chem. Eur. J. 2008, 14, 2248–2263; (g) Kim, S. K.; Sessler, J. L.; Gross, D. E.; Lee,
C.-H.; Kim, J. S.; Lynch, V. M.; Delmau, L. H.; Hay, B. P. J. Am. Chem. Soc. 2010,
132, 5827–5836.
4. Suksai, C.; Leeladee, P.; Jainuknan, D.; Tuntulani, T.; Muangsin, N.; Chailapakul,
O.; Kongsaeree, P.; Pakavatchai, C. Tetrahedron Lett. 2005, 46, 2765–
2769.
5. (a) Tomapatanaget, B.; Tuntulani, T. Tetrahedron Lett. 2001, 42, 8105–8109; (b)
Tomapatanaget, B.; Tuntulani, T.; Chailapakul, O. Org. Lett. 2003, 5, 1539–
1542.
Figure 3. Square-wave voltammograms of free receptor 1 and [1ÁNa+] with chloride
anions in 40% CH3CN/CH2Cl2 with 0.1 M TBAPF6 at a scan rate of 100 mV/s.
The electrochemical sensing of Na+ co-bound receptors was also
investigated by voltammetry. Table 2 shows cathodic shifts of Fc/
Fc+ redox couples upon addition of chloride and acetate to the
co-bound Na+ complexes of 1 and 2. It can be clearly seen from Fig-
ure 3 and Table 2 that the complex [1ÁNa+] gave a larger
DE than
the free form 1 upon addition of ClÀ. Therefore, [1ÁNa+] provided
‘positive co-operative electrochemical sensing’. We expected that
the suitable distance between the two positive ions 1+ and Na+
with the preorganization of the doubly positive charge [1+ÁNa+]
and the electrostatic attraction of anions led to a greater shift in
the potential of the Fc/Fc+ redox couples. These results indicated
that the degree of anion electrochemical sensing increases in the
electrochemically-oxidized Na+ co-bound receptor [1+ÁNa+] con-
taining double positive charges as compared to reduced form
[1ÁNa+]. However, the co-bound [2ÁNa+] complex demonstrated
‘negative co-operative electrochemical sensing’ for acetate anions.
It can be seen that the longer distance between the two positive
charges of co-bound Na+ and Fc+, resulting from the topology of
the ligand, may decrease the anion association ability.
6. Sukwattanasinitt, M.; Rojanathanes, R.; Tuntulani, T.; Sritana-Anant, Y.;
Ruangpornvisuti, V. Tetrahedron Lett. 2001, 42, 5291–5293.
7. Knobloch, F. W.; Rauscher, W. H. J. Polym. Sci. 1961, 54, 651–656.
8. Meta-isomer 1: 1H NMR (400 MHz, CDCl3, ppm): d, 8.77 (t, 2H, J = 6.4 Hz, ArNH),
7.72 (s, 2H, ArH), 7.28 (s, 2H, ArH), 7.18 (t, 2H, J = 8.0 Hz, ArH), 7.08 (s, 4H, ArH),
6.63 (d, 2H, J = 8.0 Hz, ArH), 6.48 (s, 4H ArH), 4.64 (s, 8H, –OCH2CH2O–), 4.68 (s,
4H, FcH), 4.47 (s, 4H FcH), 4.37 (s, 4H, –OCH2COOR), 3.96 (q, 4H, J = 7.2 Hz,
CH3CH2O–), 4.53 (d, 4H, J = 12.8 Hz, ArCH2Ar), 3.14 (d, 4H, J = 12.8 Hz,
ArCH2Ar), 1.30 (s, 18H, tert-butyl), 1.09 (t, 6H, J = 6.8 Hz, CH3CH2OCO–), 0.81
(s, 18H, tert-butyl); 13C NMR (100 MHz, CDCl3, ppm): d 170.5, 170.0, 169.0,
159.8, 155.5, 152.2, 144.8, 139.7, 135.1, 131.7, 129.5, 125.6, 124.9, 112.3, 111.6,
105.3, 79.0, 73.2, 71.9, 71.0, 68.8, 60.6, 51.2, 33.8, 31.1, 13.9. Elemental
analysis: Anal. Calcd for C80H92FeO12N2: C, 72.26; H, 6.98; N, 2.11. Found: C,
72.25; H, 6.94; N, 2.14; ESI m/z: calcd 1329.46; found 1351.187 [M+Na+].
9. Para-isomer 2: 1H NMR (400 MHz, CDCl3, ppm): d 7.37 (s, 2H, ArNH), 7.21 (d,
4H, J = 8.8 Hz, ArH), 6.95 (s, 4H, ArH), 6.68 (s, 4H, ArH), 6.75 (d, 4H, J = 8.8 Hz,
ArH), 4.89 (s, 4H, FcH), 4.69 (s, 4H, FcH), 4.46 (s, 4H, –OCH2COOR), 4.39 (s, 8H, –
OCH2CH2O–), 4.20 (q, 4H, J = 7.1 Hz, CH3CH2O–), 4.65 (d, 4H, J = 12.8 Hz,
ArCH2Ar), 3.24 (d, 4H, J = 12.8 Hz, ArCH2Ar), 1.28 (t, 6H, J = 7.2 Hz, CH3CH2OCO–
), 1.19 (s, 18H, tert-buyl), 0.99 (s, 18H, tert-buyl); 13C NMR (100 MHz, CDCl3,
ppm): d 170.2, 166.8, 155.6, 154.1, 152.3, 145.1, 134.3, 132.7, 130.7, 125.1,
122.0, 114.0, 79.2, 71.3, 69.7, 60.9, 34.0, 31.3, 14.4. Elemental analysis: Anal.
Calcd for C80H92FeO12N2: C, 72.26; H, 6.98; N, 2.11. Found: C, 72.24; H, 6.97; N,
2.15; ESI m/z: calcd 1329.46; found 1351.452 [M+Na+].
10. Suksai, C.; Leeladee, P.; Jennings, C.; Tuntulani, T.; Kongsaeree, P. J. Chem.
Crystallogr. 2008, 38, 363–368.
11. Macomber, R. S. J. Chem. Educ. 1992, 69, 375–378.
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13. Togni, A.; Hayashi, T. Ferrocene; VCH: New York, 1995.
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In conclusion, we have successfully synthesized heteroditopic
receptors 1 and 2 derived from p-tert-butyl-calix[4]arene. Both
compounds showed a ‘turn on’ binding with BrÀ and IÀ employing
Na+ as a ‘switch’. We also showed that the topology of compounds
1 and 2 played a crucial role in the electrochemical sensing abilities
toward anions. The meta-isomer 1 possessed a suitable cationic
cavity for Na+ co-ordination, whereas the para-isomer 2 provided
a larger cationic cavity in which Na+ probably sat deeply in the cav-
ity. Therefore, the incoming anion would interact with the positive
charge of the co-bound cation complexes [1ÁNa+] and [2ÁNa+] to a
different extent. Complex [1ÁNa+] showed ‘positive co-operative
electrochemical sensing’ for chloride and acetate anions, whereas
complex [2ÁNa+] gave a negative result for acetate. However, the
free-form of para-isomer 2 was well-suited for electrochemical
sensing of acetate anions. Therefore, the topology of the ligands
and the presence of metal ions were able to differentiate the anion
binding abilities of the heteroditopic receptors 1 and 2.
17. Bourgel, C.; Boyd, A. S. F.; Cooks, G.; de Cremires, H. A.; Dulairoir, F. M. A.;
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Acknowledgment
Financial support from The Thailand Research Fund
(RTA5380003) is gratefully acknowledged.