Anthracene-based ortho-phenylenediamine clefts for sensing carboxylates

Article abstract of DOI:10.1016/j.tetlet.2008.05.096

Two ortho-phenylenediamine-based new receptors 1 and 2 with an anthracene-coupled benzimidazolium motif have been designed and synthesized. The directed hydrogen bonds (both conventional and unconventional) and charge-charge interactions allowed the open clefts of both 1 and 2 to bind carboxylate, fluoride and dihydrogenphosphate anions with moderate binding constant values. The selectivity and sensitivity were ascertained by ¹H NMR, UV-vis and fluorescence spectroscopic methods. The binding cleft of 2 is found to be more effective than that of 1.

Full text of DOI:10.1016/j.tetlet.2008.05.096

Tetrahedron Letters 49 (2008) 4591–4595
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Tetrahedron Letters
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Anthracene-based ortho-phenylenediamine clefts for sensing carboxylates
*
Kumaresh Ghosh , Indrajit Saha
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741 235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two ortho-phenylenediamine-based new receptors 1 and 2 with an anthracene-coupled benzimidazoli-
um motif have been designed and synthesized. The directed hydrogen bonds (both conventional and
unconventional) and charge–charge interactions allowed the open clefts of both 1 and 2 to bind carbox-
ylate, fluoride and dihydrogenphosphate anions with moderate binding constant values. The selectivity
and sensitivity were ascertained by 1H NMR, UV–vis and fluorescence spectroscopic methods. The bind-
ing cleft of 2 is found to be more effective than that of 1.
Received 31 March 2008
Revised 17 May 2008
Accepted 20 May 2008
Available online 24 May 2008
This paper is dedicated with profound
regards to Professor S. P. Goswami on his
55th birthday.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Carboxylate recognition
Anthracene
Benzimidazole
Urea–amide conjugate
The development of molecular receptors for recognizing cat-
ions, anions, and neutral molecules has been an emerging area of
supramolecular chemistry.1–6 Receptors capable of sensing anions
are of paramount interest due to the various important roles of
anions in environmental and biological processes.7–9 Carboxylate
anion recognition is important owing to its presence in various
biological molecules.7,10,11 There are various receptors which rec-
ognize both mono and dicarboxylates.12–20 Most of them contain
urea/thiourea,10,19 guanidinium ions,14,15 imidazolium cations,18
etc., as potential binders of carboxylates, and they are usually
placed in close vicinity of different fluorescent probes in order to
report successful recognition events. In this connection, the use
of imidazolium and benzimidazolium salts is worth mentioning
for their effective involvement in the binding of anions through
both charge–charge interactions and unconventional ionic (C–
H)+Á Á ÁX (X = O, N, FÀ, ClÀ, BrÀ, IÀ) hydrogen bonds.21–23 The use of
such hydrogen bonding features of benzimidazolium/imidazolium
ions along with other hydrogen bonding motifs is one strategy in
designing new receptors for a specific purpose. As a consequence,
there has been recent interest in using both imidazolium and benz-
imidazolium motifs in devising new receptors for various ana-
lytes.21–23 As part of our ongoing research in supramolecular
chemistry,24 we report here anthracene labelled benzimidazoli-
um-coupled functionalized ortho-phenylenediamines 1 and 2,
which provide a set of convergent hydrogen bonds for recognition
of carboxylates. The open clefts of 1 and 2 exhibit good recognition
abilities for acetate, propanoate, benzoate and dihydrogenphos-
phate. In contrast to 1, the receptor 2 shows a better recognition
ability with modest selectivity towards acetate over the other
anions, particularly mandelate and pyruvate, through N–HÁ Á ÁO,
C–HÁ Á ÁO hydrogen bonds and charge–charge interactions.
In the designs 1 and 2, o-phenylenediamine motif has been con-
sidered as basic unit with two hydrogen bond donors for anion
binding. The use of such a motif for anion binding is well estab-
lished.25,26 To establish new sensor modules on the o- phenylene-
diamine motif, the inclusion of a fluorescent probe and additional
hydrogen bond donors along with charge–charge interactions for
effective complexation and detection of anionic guests in solution
is one strategy and has been fulfilled easily in the receptors 1 and 2.
The receptors 1 and 2 were synthesized according to the
Scheme 1. In each case, anthracene-coupled benzimidazole 3 (ob-
tained by N-alkylation of benzimidazole using 9-chloromethylan-
thracene in the presence of NaH in dry THF; see Scheme 1a) was
attached to o-phenylenediamine-based synthons 5 and 7 to afford
the chloride salts of 1 and 2, respectively. Subsequent anion ex-
change using NH4PF6 gave the desired receptors 1 and 2 in appre-
ciable yields. All the compounds were thoroughly characterized by
spectroscopic techniques.27
To understand the conformational flexibility as well as the
binding features, a conformational search on both the receptors 1
and 2 was performed.28 The lowest energy conformation, in each
* Corresponding author. Tel.: +91 33 25828282/306; fax: +91 33 25828282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.096

4592
K. Ghosh, I. Saha / Tetrahedron Letters 49 (2008) 4591–4595
NO2
N
H
d
N
H
d
N+
H
c
N+
c
H
b
N
H
H
a
H
b
e
N
H
H
N
a
N
N
O
O
O
O
1
2
N
N
N
i. NaH in dry THF
ii. 9-chloromethylanthracene
a)
N
H
64%
3
O
Cl
NH2
NH2
NH2
NH
butyryl chloride / Et3N in dry CH2Cl2
2-chloroacetyl chloride / Et3N
i. 3,
dry CH3CN; reflux
54%
b)
1
2
67%
H
N
H
N
dry CH2Cl2
ii. NH4PF6
85%
89%
4
5
O
O
O
Cl
NH2
NH
2-chloroacetyl chloride / Et3N
i. 3,
dry CH3CN; reflux
ii. NH4PF6
35%
71%
p-nitroaniline / triphosgene
H
H
dry CH2Cl2
Et3N in dry CH2Cl2
78%
53%
H
H
N
N
N
N
7
6
O
O
NO2
NO2
Scheme 1. Syntheses of receptors 1 and 2.
Figure 1. AM1 optimized geometries of: (a) the complex of 1 with acetate (heat of formation = 17.25 kcal); (b) the complex of 2 with acetate (heat of formation = 75.69 kcal).
case, was identified and optimized at the AM1 level. The AM1 opti-
mized hydrogen bonding complexes of 1 and 2 with acetate are
shown in Figure 1.
It is worth noting that all the N–H and C–H hydrogen bond do-
nors of both 1 and 2 are cooperatively involved in bonding with
carboxylate guests. The charge–charge interaction additionally sta-
bilizes the complexes significantly. In each case, the hydrogens of
the linker–CH2–group also form hydrogen bonds in the gas phase
during complexation (Fig. 1).
The interactions of the receptors 1 and 2 in solution with the
anions (acetate, propanoate, benzoate, dihydrogen phosphate,
mandalate, pyruvate and fluoride, added as their tetrabutylammo-
nium salts) were studied using 1H NMR, UV–vis and fluorescence
spectroscopic techniques. The 1H NMR spectra of both receptors

K. Ghosh, I. Saha / Tetrahedron Letters 49 (2008) 4591–4595
4593
Table 1
Change of chemical shift (
D
d) of the interacting protons in 1:1 complexes of the receptors 1 and 2 with anions
Receptor 1
Receptor 2
Anion
NHa
NHb
CH2c
(C–Hd)+
NHa
NHb
NHc
(C–Hd)+
CH2e
Acetate
Propanoate
Benzoate
Dihydrogen phosphate
Pyruvate
Mandelate
Fluoride
+1.82
+1.34
+1.50
+2.22
+0.52
+0.18
+1.02
+1.00
+0.75
+0.60
+1.35
+0.28
+0.10
+0.51
+0.11
+0.07
+0.14
+0.26
+0.02
+0.0
+0.07
+0.04
+0.07
+0.36
+0.03
À0.01
+0.02
+2.01
+2.19
+2.10
+1.71
+1.09
+1.19
+0.90
+1.07
+ 1.28
+0.77
+2.12
+0.49
+0.39
f
+2.22
+ 2.34
+1.77
+1.98
+0.88
+0.88
f
+0.08
+ 0.08
+0.06
+0.20
+0.11
+0.01
f
+0.07
+0.07
+0.08
+0.16
+0.06
À0.13
+0.06
À0.02
+: Indicates downfield shift, À: indicates upfield shift, f: indicates disappearance due to deprotonation.
1 and 2 were recorded in DMSO-d6 in the presence of anions. All
the NH signals of both 1 and 2 were shifted downfield in presence
of the guest anions. In addition, the hydrogens of the charged (C–
H)+ bond (marked as d) and the linker–CH2–underwent downfield
shifts during complexation. The different extents of the chemical
shifts of those key protons of receptors 1 and 2 in their 1:1 com-
plexes with anions are summarized in Table 1. It is evident from
Table 1 that the changes in chemical shifts of the interacting pro-
tons are significant in the presence of carboxylate and dihydrogen
phosphate anions. This was ascribed to their participation in
hydrogen bonding and the formation of strong hydrogen bonded
complexes such as 2A–B (Fig. 2). In the presence of fluoride (1:1
stoichiometry with the receptor) the signals for the NH protons
of both 1 and 2 were broad and disappeared when excess fluoride
was present. This was attributed to hydrogen bonding followed by
deprotonation, which we12 and other groups29 have noticed
earlier. For example, the change in the 1H NMR spectrum of 2 in
the presence of acetate is shown in Figure 1S (see the Supple-
mentary data). All these 1H NMR observations during complexa-
tion clearly indicated that all the hydrogen bond donors in
designs 1 and 2 are capable of forming strong complexes with
carboxylate, dihydrogen phosphate and fluoride anions.
To gain an insight on the selectivity and sensitivities of both
receptors 1 and 2 towards the anions mentioned in Table 1, fluo-
rescence and UV titrations were performed in DMSO. When recep-
tor 1 was titrated with varying concentrations of guests in DMSO,
the emission of anthracene at 421 nm was quenched. Figure 3 indi-
cates the change in fluorescence of 1 (c = 1.00 Â 10À5 M) upon
addition of acetate ions in DMSO.
Similar observations with significant quenching of the emission
of anthracene were observed in the case of receptor 2. During titra-
tion, no additional peaks for 1 and 2 at higher wavelengths due to
NO2
OH
NO2
R
O
H
N
OH
O
H
H
N
d
P
H
d
N+
O
N+
O
c
H
H
b
e
c
H
H
e
N
H
a
N
H
b
a
N
N
N
N
O
O
O
O
2A
R = CH3, CH3CH2, C6H5, CH3CO, C6H5CH(OH)
2B
Figure 2. Possible hydrogen-bonding structures of the complexes of receptor 2 with carboxylate and dihydrogen phosphate anions.
1000
800
600
400
200
0
800
700
600
500
400
300
200
100
0
400
450
500
380
400
420
440
460
480
500
520
Wavelength
Wavelength
Figure 3. Change in emission of 1 (c = 1.00 Â 10À5 M) in DMSO upon addition of
Figure 4. Change in emission of 2 (c = 4.59 Â 10À5 M) in DMSO upon addition of
tetrabutylammonium acetate.
tetrabutylammonium acetate.

4594
K. Ghosh, I. Saha / Tetrahedron Letters 49 (2008) 4591–4595
excimer or exciplex formation were observed. The degree of
quenching of the emission varied with the nature of the anions.
As shown in Figure 4, receptor 2 displayed a large fluorescence
quenching effect upon addition of acetate and dihydrogenphos-
phate (Figure 2S, see Supplementary data) in DMSO. The fluores-
cence quenching effect was possibly due to the photo-induced
electron transfer (PET) process between the anthracene moiety
and the binding site.
The Stern–Volmer plots (Figs. 5 and 6) clearly demonstrate the
quenching phenomena. The significant differential quenching of
emission of the anthracene moiety of 2 compared to 1 is due to
strong non-covalent interactions of the binding site with different
complementary guests. The linear nature and the close spacing of
the curves in Figure 7 indicate the weak interaction of 1 with the
anions. The breaks at [G]/[H] = 1 in the titration curves of Figure
8 indicated the 1:1 stoichiometries of the complexes with the
receptor 2.
To determine the association constants for the complexes
formed between receptors 1 and 2 with the anions, UV titrations
were performed in DMSO. Receptor 1 (c = 3.96 Â 10À5 M) showed
absorption bands at 336, 353, 372 and 392 nm for anthracene in
DMSO. Upon titration with acetate, significant changes were
observed in its UV–vis spectra. On increasing the amount of ace-
tate, the absorbances of the peaks due to the anthracene moiety
of 1 decreased in a regular fashion. The changes in absorbance of
1 at 372 nm in presence of other anions were minor (Fig. 9).
350
300
250
200
150
100
50
fluoride
pyruvate
acetate
benzoate
propanoate
mandelate
dihydrogenphosphate
0
0
1
2
3
4
5
6
7
8
[G]/[H]
Figure 7. Fluorescence titration curves ([Guest]/[Host] vs change in emission) of 1
(measured at 421 nm).
fluoride
propanoate
acetate
dihydrogenphosphate
350
300
250
200
150
100
50
pyruvate
benzoate
mandalate
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1.0
fluoride
0
pyruvate
0
1
2
3
4
5
6
7
8
acetate
[G]/[H]
benzoate
Figure 8. Fluorescence titration curves ([Guest]/[Host] vs change in emission) of 2
(measured at 420 nm).
propanoate
mandelate
dihydrogenphosphate
However, the changes in absorbance of receptor 2 in the
presence of acetate, under similar conditions, were appreciable
compared to 1. The initial change in absorbance upto 1:1
stoichiometry was regular and then a downward trend (Fig. 10)
was noticed in the presence of excess acetate. This is presumably
either due to deprotonation by the basic acetate ions which were
0
1
2
3
4
5
6
7
[G] X 10-5M
Figure 5. Stern–Volmer plot of 1 at 421 nm.
acetate
2.0
dihydrogenphosphoate
acetate
0.05
Propanpoate
Fluroride
benzoate
mandelate
propanoate
0.04
1.8
1.6
1.4
1.2
1.0
dihydrogenphosphate
pyruvate
pyruvate
benzoate
fluoride
0.03
0.02
0.01
0.00
mandelate
0
1
2
3
4
5
6
7
0
5
10
15
20
25
30
[G]/[H]
-5
[G] x 10 M
Figure 9. UV titration curves ([Guest]/[Host] vs change emission) for 1 (measured
Figure 6. Stern–Volmer plot of 2 at 420 nm.
at 372 nm).

K. Ghosh, I. Saha / Tetrahedron Letters 49 (2008) 4591–4595
4595
Supplementary data
0.25
0.20
0.15
0.10
0.05
0.00
acetate
1H NMR spectra of receptor 2 and its 1:1 complex with acetate,
change in emission of receptor 2 in DMSO upon addition of tetra-
butylammonium dihydrogenphosphate, changes in the UV spectra
of 1 and 2 in DMSO in presence of varying amounts of tetrabutyl-
ammonium acetate and the binding constant curves for 2 with
acetate, benzoate, dihydrogenphosphate, fluoride are available.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2008.05.096.
dihydrogenphosphate
mandelate
pyruvate
propanoate
fluoride
References and notes
benzoate
1. Martinez-Manez, R.; Sancenon, F. Chem. Rev. 2003, 13, 4419–4476.
2. Suksai, C.; Tuntulani, T. Chem. Soc. Rev. 2003, 32, 192–202.
3. Beer, P. D.; Gale, P. A. Angew. Chem., Int. Ed. 2001, 40, 486–516.
4. Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609–1646.
5. Steed, J. W. Chem. Commun. 2006, 2637–2649.
0
1
2
3
4
5
6
7
8
[G]/[H]
Figure 10. UV titration curves ([Guest]/[Host] vs change emission) for 2 (measured
6. Gale, P. A. Acc. Chem. Res. 2006, 39, 465–475.
at 371 nm).
7. Voet, D.; Voet, J. G. Biochemistry, 2nd ed.; Wiley: New York, NY, 1995.
8. Chemical Sensors and Biosensors for Medical and Biological Applications;
Spichiger-Keller, U. S., Ed.; Wiley-VCH: Weinheim, Germany, 1998.
9. Mason, C. F. Biology of Freshwater Pollution, 2nd ed.; Longman: New York, 1991.
10. Gunnlaugsson, T.; Davis, A. P.; O’Brien, J. E.; Glynn, M. Org. Lett. 2002, 4, 2449–
2452 and references cited therein.
11. Kral, V.; Andrievsky, A.; Sessler, J. L. J. Am. Chem. Soc. 1995, 117, 2953–2954.
12. Ghosh, K.; Sarkar, A. R. Tetrahedron Lett. 2007, 48, 8725–8729 and references
cited therein.
13. Cudic, P.; Vigneron, J. P.; Lehn, J. M.; Cesario, M.; Prange, T. Eur. J. Org. Chem.
1999, 2479–2484.
14. Echavarren, A.; Galan, A.; de Mendoza, J. J. Am. Chem. Soc. 1989, 111, 4994–
4995.
Table 2
Binding constant values (Ka) determined by UV–vis titration in DMSO
Anion
Receptor 1 (Ka) in MÀ1
Receptor 2 (Ka) in MÀ1
Acetate
Propanoate
Benzoate
Dihydrogenphosphate
Mandelate
Pyruvate
2.33 Â 104
1.98 Â 103
2.25 Â 103
1.73 Â 103
1.00 Â 103
5.02 Â 102
3.79 Â 103
3.91 Â 104
1.04 Â 104
2.36 Â 104
1.12 Â 104
4.37 Â 103
5.95 Â 103
1.87 Â 104
15. Raker, J.; Glass, T. E. J. Org. Chem. 2002, 67, 6113–6116.
16. Boiocchi, M.; Bonizzoni, M.; Fabbrizzi, L.; Piovani, G.; Taglietti, A. Angew. Chem.,
Int. Ed. 2004, 43, 3847–3852.
Fluoride
17. Bonizzoni, M.; Fabbrizzi, L.; Piovani, G.; Taglietti, A. Tetrahedron 2004, 60,
11159–11162.
18. Kim, S. K.; Kang, B.-G.; Koh, H. S.; Yoon, Y. J.; Jung, S. J.; Jeong, B.; Lee, K.-D.;
Yoon, J. Org. Lett 2004, 6, 4655–4685 and references cited therein.
19. Liu, S-Y.; Fang, L.; He, Y.-B.; Chan, W.-H.; Yeung, K.-T.; Cheng, Y.-K.; Yang, R.-H.
Org. Lett. 2005, 7, 5825–5828.
20. Kacprzak, K.; Gawronski, J. Chem. Commun. 2003, 1532–1533.
21. Kim, H.; Kang, J. Tetrahedron Lett. 2005, 46, 5443–5445.
22. Singh, N. J.; Jun, E. J.; Chellappan, K.; Thangadurai, D.; Chandran, R. P.; Hwang,
I.-C.; Yoon, J.; Kim, K. S. Org. Lett. 2007, 9, 485–488 and references cited therein.
23. Bai, Y.; Zhang, B.-G.; Xu, J.; Duan, C.-Y.; Dang, D.-B.; Liu, D.-J.; Meng, Q.-J. New J.
Chem. 2005, 29, 777–779.
held less strongly to the binding site or due to decomplexation in
the presence of excess acetate. The same was true for fluoride
and dihydrogenphosphate (Fig. 10). The binding constants were
determined using the Benesi–Hildebrand equation,30 and the
values are given in Table 2.
The 1:1 stoichiometries of the complexes were further realized
from the break in the UV-titration curves at [G]/[H] = 1 (Fig. 10).
The almost linear nature of the curves in Figure 9 again demon-
strated the weak interactions of 1. As we move from receptor 1
to receptor 2, the binding constant values are improved signifi-
cantly and become higher for acetate ions. This is corroborated
by the presence of more hydrogen bonds, and the directed nature
of the urea linkage of 2 forming a six-membered hydrogen bonding
arrangement with the carboxylate oxygen (see Fig. 1). The role of
the basicity of the anions in the binding process cannot be ignored.
In conclusion, this Letter demonstrates a rational way to design
and synthesize anthracene-coupled benzimidazolium-based ortho-
phenylenediamine derivatives 1 and 2 and describes the binding
properties towards various anions. The cleft of receptor 2 shows
preferred binding with acetate, fluoride and dihydrogenphosphate
anions over receptor 1 where the complexes are stabilized by both
conventional (N–HÁ Á ÁO) and unconventional hydrogen bonds [C–
HÁ Á ÁO, (C–H)+Á Á ÁO] and charge–charge interactions. Further optimi-
zation of the binding site of 2 for other anions is underway in our
laboratory.
24. Ghosh, K.; Masanta, G. Tetrahedron Lett. 2008, 49, 2592–2597 and references
cited therein.
25. Brooks, S.; Gale, P. A.; Light, M. E. Supramol. Chem. 2007, 19, 9–15.
26. Brooks, S.; Gale, P. A.; Light, M. E. CrystEngCommun. 2005, 7, 586–591.
27. Receptor 1: Mp 176–180 °C; 1H NMR (DMSO-d6, 400 MHz): d 9.78 (s, NH, 1H),
9.19 (s, NH, 1H), 8.93 (s, 1H), 8.92 (s, 1H), 8.44 (d, 1H, J = 8 Hz), 8.39 (d, 2H,
J = 8 Hz), 8.27 (d, 2H, J = 8 Hz), 8.06 (d, 1H, J = 8 Hz), 7.85 (t, 1H, J = 8 Hz), 7.78
(t, 1H, J = 8 Hz), 7.67–7.60 (m, 4H), 7.51 (d, 1H, J = 8 Hz), 7.33 (d, 1H, J = 8 Hz),
7.14 (t, 1H, J = 8 Hz), 7.08 (t, 1H, J = 8 Hz), 6.80 (s, 2H), 5.29 (s, 2H), 2.19 (t, 2H,
J = 7.2 Hz), 1.52 (q, 2H, J = 7.2 Hz), 0.86 (t, 3H, J = 7.2 Hz); 13C NMR (DMSO-d6,
125 MHz): d 171.1, 163.5, 142.0, 132.0, 131.3, 131.2, 131.1, 130.9, 130.5, 129.4,
128.9, 127.8, 127.0, 126.7, 125.6, 125.0, 124.6, 124.4, 123.2, 121.6, 114.2, 113.8,
48.9, 43.3, 37.7, 18.3, 13.5 (1 carbon is less); FT-IR:
m
cmÀ1 (Nujol): 3597, 3385,
1680, 1651, 1555, 1462; HRMS (TOF MS ES+) C34H31N4O2PF6 (MÀPF6)+ requires
527.2442, found 527.2441; Receptor 2: Mp 165 °C; 1H NMR (DMSO-d6,
400 MHz): d 10.12 (s, NH, 1H), 9.81 (s, NH, 1H), 8.95 (s, 1H), 8.93 (s, 1H),
8.41–8.36 (m, 3H including NH), 8.26 (d, 2H, J = 8 Hz), 8.20 (d, 2H, J = 8 Hz),
8.06 (d, 1H, J = 8 Hz), 7.81 (t, 1H, J = 8 Hz), 7.67–7.62 (m, 9H), 7.25 (d, 1H,
J = 8 Hz), 7.19 (t, 1H, J = 8 Hz), 7.07 (t, 1H, J = 8 Hz), 6.78 (s, 2H), 5.34 (s, 2H); 13
C
NMR (DMSO-d6, 125 MHz): d 163.9, 152.1, 146.3, 142.0, 140.8, 132.2, 132.0,
131.2, 131.0, 130.9, 130.4, 129.3, 128.3, 127.8, 126.9, 126.6, 126.1, 125.5, 125.3,
125.0, 123.8, 123.6, 123.2, 121.6, 117.2, 114.1, 113.8, 48.8, 43.3; FT-IR:
m
cmÀ1
(Nujol): 3277, 1721, 1681, 1598, 1552, 1537, 1500, 1329; HRMS (TOF MS ES+)
C
37H29N6O4PF6 (MÀPF6)+ requires 621.2245, found 621.2242.
28. Calculations were performed using CS Chem 3D version 6.0.
29. Nishizawa, S.; Kaneda, H.; Uchida, T.; Teramae, N. J. Chem. Soc., Perkin Trans. 2
1998, 2325–2328.
Acknowledgement
We thank the CSIR, Government of India for financial support.
I.S. thanks CSIR for a research fellowship.
30. Connors, K. A. Binding Constants: the measurement of molecular Complex
Stability; J. Wiley & Sons: New York, 1987.