Design and synthesis of an ortho-phenylenediamine-based open cleft: a selective fluorescent chemosensor for dihydrogen phosphate

Article abstract of DOI:10.1016/j.tetlet.2009.02.215

A new ortho-phenylenediamine-based fluorescent cleft 1 has been designed and synthesized. The open cleft of 1 selectively recognizes tetrabutylammonium dihydrogen phosphate in CH₃CN by exhibiting a significant decrease in the emission of anthra

Full text of DOI:10.1016/j.tetlet.2009.02.215

Tetrahedron Letters 50 (2009) 2392–2397
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Tetrahedron Letters
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Design and synthesis of an ortho-phenylenediamine-based open cleft: a
selective fluorescent chemosensor for dihydrogen phosphate
a
b
Kumaresh Ghosh a, , Indrajit Saha , Amarendra Patra
*
a Department of Chemistry, University of Kalyani, Kalyani, Nadia 741 235, India
b Department of Chemistry, University College of Science, 92 A. P. C. Road, Kolkata 700 009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new ortho-phenylenediamine-based fluorescent cleft 1 has been designed and synthesized. The open
cleft of 1 selectively recognizes tetrabutylammonium dihydrogen phosphate in CH3CN by exhibiting a
significant decrease in the emission of anthracene. The interactions of 1 are also investigated in aq CH3OH
where no measurable change in the emission is observed for tetrabutylammonium dihydrogen phos-
phate, although sodium salts of phosphate, hydrogen phosphate, and dihydrogen phosphate exhibit mod-
erate changes. Tetrabutylammonium hydrogen sulfate is sensed effectively in aq CH3OH. The anion
binding properties of 1 were evaluated by 1H NMR, UV–vis, and fluorescence spectroscopic methods.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 23 October 2008
Revised 20 February 2009
Accepted 28 February 2009
Available online 5 March 2009
Keywords:
Dihydrogen phosphate
Fluoride
Carboxylate recognition
Hydrogen sulfate
Anthracene
Benzimidazole
The design and synthesis of artificial receptors for anions have
attracted considerable attention due to their medical and environ-
mental potential.1–3 In this context, amide,4,5 pyrrole,6 urea,7,8
thiourea9 based receptors with conventional H-bond donors (N–
HꢀꢀꢀX; X = O, N) have been widely used for the complexation of an-
ions. Receptors with positively charged moieties such as imidazo-
lium/benzimidazolium,10,11 guanidinium,12 and pyridinium13 are
also noteworthy in anion recognition. The well-known C+–HꢀꢀꢀX
type unconventional hydrogen bonds in imidazolium/benzimi-
dazolium-based receptors play a key role along with the N–HꢀꢀꢀX
type conventional hydrogen bonds in the formation of strong com-
plexes with anions.11,14 Therefore, the rational use of the benzim-
idazolium moiety in the design of artificial receptors for the
selective complexation of anions is an interesting aspect of supra-
molecular chemistry. Recently, several excellent receptors have
been developed to study the anion binding in both solution and so-
lid states.15,16 In this regard, fluorescent receptors appear to be
attractive due to the simplicity and high detection limit of fluores-
cence. During our ongoing work on supramolecular chemistry, we
herein report a new ortho-phenylenediamine-based fluorescent
However, 1 shows an increase in emission only in the presence
ꢁ
of a large excess of HPO42ꢁ, PO43ꢁ, and HSO4 anions in aq metha-
nol (CH3OH:H2O = 4:1 v/v).
Two hydrogen bond donors from the o-phenylenediamine motif
have been considered the basic unit for anion binding. The use of
such a motif for anion binding is well established.17,18 Our recent
report on anthracene-coupled benzimidazolium-based ortho-
phenylenediamine derivatives for anions can also be mentioned.19
The synthesis of receptor 1 was accomplished according to
Scheme 1. The reaction of ortho-phenylenediamine with chloroace-
tylchloride in dry CH2Cl2 gave diamide 3 in 55% yield. Diamide 3,
on refluxing in CH3CN with compound 2, which was obtained
according to Scheme 1a by the reaction of benzimidazole with 9-
chloromethylanthracene in the presence of NaH in dry THF, affor-
ded chloride salt 4. The subsequent anion exchange of chloride salt
4 using NH4PF6 gave the desired receptor 1 in appreciable yields.
All the compounds were thoroughly characterized by the usual
spectroscopic techniques.20
The molecular modeling21 study shows that the benzimidazoli-
um protons Hc, amide protons Ha and methylene protons Hb of 1
(see the labeled structure 1) are oriented into the cavity for the
complexation of anions. The appended anthracenes are not paral-
lel. One anthracene unit is nearly perpendicular to the g-surface
of another anthracene maintaining the shortest distance of 3.39 Å
in the gas phase (Fig. 1).
ꢁ
receptor 1, which shows selective complexation of the H2PO4
ion in CH3CN. The concomitant decrease in the emission of anthra-
ꢁ
cene in 1 upon the binding of the H2PO4 ion is worthy and signif-
icant to distinguish it from the other anions in the present study.
The anion binding properties of receptor 1 were investigated
in dry CH3CN and aq methanol (CH3OH:H2O = 4:1 v/v) using
* 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 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.215

K. Ghosh et al. / Tetrahedron Letters 50 (2009) 2392–2397
2393
N
N
N
i.
a
N
H
2
O
O
Cl
NH HN
O
O
N
N
NH2
NH2
NH
NH
ii.
H
H
iv.
iii.
b
1
+
N
N
+
2Cl-
Cl
3
4
Scheme 1. Reagents and conditions: (i) NaH, dry THF, 9-chloromethylanthracene, reflux, 10 h, 64% yield; (ii) chloroacetyl chloride, Et3N, dry CH2Cl2, 1 h, 91% yield; (iii) 2 in
dry CH3CN, reflux, 5 days, 55% yield; (iv) NH4PF6, DMF–water, stirring for ½ h, 80% yield.
wavelength. In the presence of H2PO4ꢁ, a weak, broad peak at
525 nm was noticed (Fig. 2). This is presumably attributed either
O
O
to the formation of an excimer between the appended anthracenes
or an exciplex between the anthracene and benzimidazolium ion
NH HN
a
b
ꢁ
N
N
in 1 upon the strong binding of the H2PO4 ion. In the presence
H
c
of carboxylates (acetate, propanoate, benzoate) the emission of 1
N+
N+
-
ꢁ
2PF6
was hardly perturbed. In the case of other anions such as HSO4
,
Clꢁ, Brꢁ, and Iꢁ, the emission of 1 was decreased to a small extent.
The Stern–Volmer plot clearly demonstrates the quenching behav-
ior of 1 in the presence of different anions (Fig. 3). In the presence
of 5.0 equiv amounts of Fꢁ, a sharp decrease in the emission of 1
was observed. In contrast to this, a significant decrease in the emis-
sion of 1 was noticed up to the addition of 2 equiv amounts of
1
Figure 1. Energy optimized geometry of 1 (E = 22.95 kcal/mol).
ꢁ
H2PO4 to the receptor solution of 1 in CH3CN. On further addition
ꢁ
of the H2PO4 ion, a small increase in emission followed. We pro-
fluorescence. Receptor 1 (c = 3.72 ꢂ 10ꢁ5 M) in CH3CN on excita-
ꢁ
pose that in the presence of more than 2 equiv amounts of H2PO4
,
tion at 366 nm displayed an emission at 419 nm. Upon the com-
the flexible receptor 1 may suffer a conformational change for
which the effect of PET quenching is reduced.
plexation of anions such as AcOꢁ, propanoate, benzoate, HSO4
,
ꢁ
H2PO4ꢁ, Fꢁ, Clꢁ, Brꢁ, and Iꢁ, the emission of anthracene in 1 was
quenched to different extents. Figure 2, in this regard, represents
the changes in the emission intensity of 1 in the presence of
2 equiv amounts of those different anions. As can be seen from Fig-
ure 2, the emission of 1 is significantly quenched in the presence of
Figure 4 displays the fluorescence ratio (I0 ꢁ I)/I0 upon the addi-
tion of 5 equiv amounts of a particular anion and exhibits a marked
change in the presence of Fꢁ and H2PO4 anions. The stoichiome-
ꢁ
tries of the complexes were evaluated from the titration curves
in Figure 5. It is evident from Figure 5 that receptor 1 forms a
2:1 (guest:host) complex with H2PO4ꢁ. This was further confirmed
by the Job plot in fluorescence (see Supplementary data). In the
ꢁ
the H2PO4 anion. More importantly, during titration with the an-
ions except H2PO4ꢁ, no additional peak was observed at a higher
-----receptor
25
----- acetate
acetate
600
----- propanoate
----- benzoate
-----dihydrogenphosphate
-----hydrogensulfate
----- Fluoride
----- iodide
----- chloride
----- bromide
propanoate
dihydrogenphosphate
hydrogensulfate
perchlorate
fluoride
chloride
bromide
500
400
300
200
100
0
20
15
10
5
iodide
0
400
450
500
550
600
0
5
10
15
20
25
30
35
Wavelength (nm)
[G] x 10-5(M)
Figure 2. Change in fluorescence intensity of 1 at 419 nm (kex = 366 nm) in the
presence of 2 equiv amounts of anions.
Figure 3. Stern–Volmer plot of 1 at 419 nm (kex = 366 nm).

2394
K. Ghosh et al. / Tetrahedron Letters 50 (2009) 2392–2397
in the emission of 1 in the presence of increasing amounts of
H2PO4 ions.
--
iodide
ꢁ
Simultaneous UV–vis experiments were performed to gain an
insight into the binding interaction of 1 in the ground state. The
intensities of the absorption peaks at 338, 354, 369, and 391 nm
for anthracene were decreased weakly on the complexation of
the anions. In the presence of H2PO4ꢁ, the change in the intensity
of anthracene absorption was relatively more compared to the
other anions considered in the present study. In the presence of
an excess concentration of H2PO4ꢁ, the absorption peaks for
anthracene underwent a red shift (Fig. 7). This was not observed
in the presence of other anions (see Supplementary data). The al-
most linear nature of the titration curves indicated the weak inter-
action of 1 with the anions (see Supplementary data). This small
change in the absorbance of anthracene in 1 upon complexation
demonstrates that the designed receptor 1 is an example of the
ideal PET system (fluorophore–spacer–receptor–spacer–fluorophore)
as described by de Silva in his several examples.22,23 To understand
the quenching behavior, the time-resolved emission for 1 was
studied in CH3CN upon excitation at 370 nm. The emission decay
profile of 1, monitored at 420 nm, was affected in the presence of
anions. Receptor 1 followed a single exponential decay with life
bromide
chloride
fluoride
hydrogensulphate
dihydrogenphosphate
acetate
benzoate
propanoate
0.0
0.2
0.4
(I0-I)/I0
0.6
0.8
Figure 4. Fluorescence ratio (I0 ꢁ I/I0) of receptor 1 (c = 3.72 ꢂ 10ꢁ5 M) at 419 nm
upon addition of 5.0 equiv of a particular anion in CH3CN.
dihydrogenphosphate fluoride
500
400
300
200
100
0
time
s = 4.907 ns. In the presence of an equivalent amount of dif-
ferent anions studied, the emission decay of 1 was alꢁtered to a
small extent (see Table 1). For AcOꢁ, EtCO2ꢁ, C6H5COO , H2PO4
,
ꢁ
Clꢁ, the emission life time (
s) of 1 was increased, thereby suggest-
ing a stable excited state due to hydrogen bonding complexation
and the quenching is static in nature. In case of Brꢁ and Iꢁ, dynamic
quenching takes place. This is evidenced from the decrease in the
life time of 1 in the presence of Brꢁ and Iꢁ ions. Figure 8 shows
the fluorescence decay curves of 1 in both the absence and pres-
acetate
iodide
bromide
benzoate
hydrogensulfate
ꢁ
ence of an equivalent amount of the H2PO4 ion.
The quenching of the emission of 1 upon the complexation of
anions is explained due to the activation of photo-induced electron
transfer (PET) that occurs between the binding site and the excited
state of anthracene. The complexation of anions enhances the elec-
tron density into the binding site of 1 and presumably encourages
the electron transfer to the excited state of anthracene. The greater
propanoate
chloride
-100
0
2
4
6
8
10
12
[G]/[H]
Figure 5. Fluorescence titration curves ([guest]/[host] vs change in emission) of 1
ꢁ
(measured at 419 nm).
quenching of the emission of 1 in the presence of H2PO4 is ex-
ꢁ
plained due to the strong complexation of H2PO4 in the cleft
according to the suggested modes A and B in Figure 9. Both the
forms A and B may remain in equilibrium in solution. We further
suggested that fluoride initially forms a weak hydrogen bonded
1:1 complex either in mode C or in mode D in Figure 10. In these
forms, PET is weakly activated and as a consequence, a small
quenching in emission occurs. Upon the addition of more than
plot, a maximum at 0.6 mol fractions was observed along with a
small inflection at 0.5 mol fraction of the guest. This, indeed,
underlines the fact that receptor 1 initially forms a 1:1 complex
ꢁ
with the H2PO4 ion and then equilibrates to a 2:1 stoichiometry
ꢁ
in the presence of excess H2PO4 ions. Figure 6 shows the change
600
1.2
1.0
0.8
0.6
0.4
0.2
0.0
500
500
400
400
300
200
300
100
200
0
-5
0
5
10
15
20
25
30
35
[G] x 10-5
M
100
0
400
450
500
550
600
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Figure 6. Change in emission spectra of 1 (c = 3.72 ꢂ 10ꢁ5 M) upon addition of
Figure 7. Change in absorbance of receptor 1 (c = 3.72 ꢂ 10ꢁ5 M) upon addition of
ꢁ
H2PO4 (as tetrabutylammonium salt) in CH3CN; Inset. Change in emission of 1 at
ꢁ
ꢁ
419 nm with increasing concentration of H2PO4 ion.
H2PO4 (as tetrabutylammonium salt) in CH3CN.

K. Ghosh et al. / Tetrahedron Letters 50 (2009) 2392–2397
2395
1 equiv of Fꢁ deprotonation occurs, and, accordingly, the electron-
ically rich binding site encourages the PET process resulting in a
significant decrease in the emission of 1.
Table 1
Emission life times of 1 in the absence and presence of equivalent amount of guests in
CH3CN
Receptor 1 and with guests
Life time in ns (
s
)
v2
However, the change in the emission of 1 in both the presence
and absence of the same anions was followed in aqueous methanol
(CH3OH:H2O = 4:1 v/v). The emission of 1 was increased only in the
Receptor 1
4.907
6.147
5.102
6.408
5.105
4.962
4.993
5.310
4.522
4.834
1.023
1.205
1.150
1.090
1.290
1.240
1.150
1.160
1.180
1.130
1 with AcOꢁ
ꢁ
ꢁ
1 with EtCO2
1 with C6H5CO2
1 with H2PO4
presence of an excess concentration of HSO4 ions (see Supple-
ꢁ
mentary data) and the binding stoichiometry was found to be
ꢁ
ꢁ
ꢁ
1:1. In the case of H2PO4 and other anions, the emission of 1
1 with HSO4
1 with Fꢁ
1 with Clꢁ
1 with Brꢁ
1 with Iꢁ
was hardly perturbed. Due to the common occurrence of the so-
dium salt of phosphate in nature, we further investigated the inter-
3ꢁ
action of 1 with the sodium salts of H2PO4ꢁ, HPO42ꢁ, and PO4 in
aqueous methanol. In such cases, the emission of 1 was increased
ꢁ
only in the presence of large excess concentrations of H2PO4
,
HPO42ꢁ, and PO4 ions (see Supplementary data). The increase
3ꢁ
2ꢁ
in the emission of 1 in the presence of HSO4ꢁ, H2PO4ꢁ, HPO4
,
3ꢁ
and PO4 is presumably due to the inhibition of PET occurring
in between the binding site and the excited state of anthracene.
To learn about the binding potencies of 1 with the anions, bind-
ing constant values were determined based on emission and
absorption data (Table 2) using the Benesei–Hildebrand equa-
tion.24,25 From the values in Table 2, it is evident that receptor 1
----- prompt
----- receptor 1
----- receptor 1 with H2PO4 (1:1)
1000
100
10
-
moderately binds H2PO4 and Fꢁ anions and shows selectivity.
ꢁ
But in aq CH3OH, receptor 1 exhibits selectivity for the tetrabutyl-
ammonium salt of HSO4ꢁ. Thus the significant diminution of affin-
ity of receptor 1 for tetrabutylammonium dihydrogen phosphate in
aq CH3OH as compared to dry CH3CN is another piece of evidence
indicating a strong hydrogen bonding interaction between the
ꢁ
receptor 1 and H2PO4 ion. The binding constant values for the so-
dium salts were determined from the fluorescence method and
they appeared as 1.81 ꢂ 102, 2.34 ꢂ 102, 1.61 ꢂ 102 Mꢁ1 for NaH2-
PO4, Na2HPO4, and Na3PO4, respectively, with complex
stoichiometries.
1
0
20
40
60
time (ns)
80
100
120
ꢁ
The strong hydrogen bonding interactions of 1 with H2PO4 and
Fꢁ were further confirmed by 1H NMR study in DMSO-d6. During
complexation, amide protons Ha along with the benzimidazolium
Figure 8. Fluorescence decay of 1 (c = 3.74 ꢂ 10ꢁ5 M) and in the presence of an
ꢁ
equivalent amount of H2PO4 ions in CH3CN.
OH
P
OH
O
O
O
O
O
N
H
N
H
NH
H
N
H
+
H
+
N
+
H
N
H
O
P
N
H
N
O
+
O
P
N
H
H
N
H
H
O
-
N
N
O
-
O
O
O
H
O
H
H
A
B
ꢁ
Figure 9. Suggested modes of binding of H2PO4 ion into the cleft of 1.
F
O
O
O
NH
N
H
N
H
N
H
+
O
+
+
N
N
N
+
N
F
N
N
N
N
C
D
Figure 10. Suggested modes of binding of the Fꢁ ion into the cleft of 1.

2396
K. Ghosh et al. / Tetrahedron Letters 50 (2009) 2392–2397
Table 2
Binding constant values determined in CH3CN and in aq CH3OH
Anionsb
Ka in Mꢁ1 in CH3CN
Ka in Mꢁ1 in aq CH3OH
Fluorescence method
UV method
Fluorescence method
c
c
a
a
a
a
a
a
a
a
Dihydrogen phosphate
Acetate
Propanoate
Benzoate
Fluoride
Chloride
Bromide
Iodide
Hydrogen sulfate
5.41 ꢂ 103
6.26 ꢂ 103
3.57 ꢂ 102
2.53 ꢂ 102
3.03 ꢂ 103
1.14 ꢂ 103
4.40 ꢂ 103
3.22 ꢂ 103
1.12 ꢂ 103
3.55 ꢂ 103
a
a
a
c
c
1.85 ꢂ 103
a
3.44 ꢂ 103
1.71 ꢂ 103
2.31 ꢂ 103
a
a
Binding constant values were not determined due to minor changes (see Fig. 4).
Tetrabutylammonium salts were used.
b
c
Based on K11
.
protons Hc underwent a downfield shift to different extents in the
presence of an equivalent amount of different anions. The protons
Hb of –CH2– groups also moved to the downfield direction upon
complexation. In the presence of an equivalent amount of
H2PO4ꢁ, the amide protons Ha and benzimidazolium protons Hc
of 1 underwent downfield shifts of 1.31 and 0.20 ppm, respectively
(see Supplementary data). In addition, the Hb protons of the –CH2–
linker exhibited a downfield shift of 0.25 ppm. These observations
Acknowledgements
We thank the CSIR [01(2240)/08/EMR-II], Government of India,
New Delhi, for financial support. I.S. thanks CSIR for a research fel-
lowship. KG and IS thank DST, Government of India, New Delhi, for
providing facilities in the Department under DST-FIST program.
Supplementary data
ꢁ
again substantiated the proposed modes of binding of H2PO4 as
shown in Figure 9.
Emission and absorption spectra of 1 upon the addition of a par-
ticular anion, Job plots of 1 with dihydrogen phosphate, binding
constant curves from fluorescence and UV for 1 with dihydrogen
phosphate and acetate ions, 1H NMR titration and binding constant
curves, plot of fluorescence ratio (I0 ꢁ I/I0) upon the addition of
25 equiv of different guests in aq CH3OH, fluorescence decay of 1
in the presence of different guests in CH3CN, and the energy opti-
mized structure of 1 with AcOꢁ are available. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.02.215.
Similarly, in the 1:1 complex of 1 with AcOꢁ, the amide protons
Ha, benzimidazolium proton Hc and –CH2– protons Hb showed
downfield shifts of 1.88, 0.11, and 0.09 ppm, respectively (see Sup-
plementary data). This is evidenced from the molecular modeling
(see Supplementary data). However, in the presence of an equiva-
lent amount of Fꢁ, the amide protons Ha of 1 moved downfield by
0.98 ppm. The other interacting protons Hb and Hc of 1 did not ex-
hibit any change in chemical shift values. This suggests that only
the diamide motif of 1 binds Fꢁ through hydrogen bonding interac-
tion. In the presence of excess Fꢁ ion concentrations, the amide
protons Ha and benzimidazolium protons Hc of 1 vanished due to
deprotonation. Fluoride, being smaller sized and more basic in nat-
ure, initially forms a hydrogen bonded complex into the cleft of 1
upto 1:1 stoichiometry and then induces deprotonation at high
concentrations. We determined the binding constant values24 as
References and notes
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13. Ghosh, K.; Sarkar, A. R.; Masanta, G. Tetrahedron Lett. 2007, 48, 8725–8729.
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ꢁ
well as the stoichiometries of the complexes of 1 with H2PO4
(Ka = 587 Mꢁ1; host:guest = 1:2) and AcOꢁ (Ka = 272 Mꢁ1; host:-
guest = 1:1) ions from NMR titration data in DMSO-d6. The binding
constants are scarified due to the DMSO solvent. In this regard, it
can be mentioned that it was difficult to carry out the NMR titra-
tion in the ꢃ10ꢁ3 Mꢁ1 concentration range in CD3CN, as, during
the progression of titration, precipitation appeared on and from
the 1:1 host to guest ratio.
In conclusion, we have designed and synthesized a simple
anthracene appended o-phenylenediamine-based open cleft 1,
which shows strong interactions with H2PO4 and Fꢁ in CH3CN.
ꢁ
The quenching of anthracene emissions upon complexation sig-
nificantly distinguishes these two anions from other anions.
The preferred binding of H2PO4 into the cleft of 1 due to the
ꢁ
17. Brooks, S.; Gale, P. A.; Light, M. E. Supramol. Chem. 2007, 19, 9–15.
18. Brooks, S.; Gale, P. A.; Light, M. E. Cryst. Eng. Commun. 2005, 7, 586–591.
19. Ghosh, K.; Saha, I. Tetrahedron Lett. 2008, 49, 4591–4595.
formation of conventional (NHꢀꢀꢀO), unconventional hydrogen
bonds (C–HꢀꢀꢀO, C+HꢀꢀꢀO) and charge–charge interactions, dramat-
ically encourages the PET mediated quenching of fluorescence
and reports the recognition event conveniently. On the other
hand, such simple receptors respond to the presence of
20. Receptor 1: MP 150 °C (decomposition and turns black); 1H NMR (400 MHz,
DMSO-d6): d 10.9 (br s, 2H, –NH–), 8.98 (s, 2H), 8.90 (s, 2H), 8.38 – 8.35 (m, 4H),
8.24 (d, 4H, J = 8 Hz), 7.95 – 7.89 (m, 2H), 7.77 – 7.69 (m, 4H), 7.63 – 7.54 (m,
10H), 7.38 (t, 2H, J = 8 Hz), 7.02 (br t, 2H), 6.79 (s, 4H), 5.26 (s, 4H); 13C NMR
(100 MHz, DMSO-d6): d 163.3, 141.9, 131.8, 131.1, 131.0, 130.8, 130.4, 129.3,
127.8, 126.8, 126.6, 125.5, 125.0, 124.7, 124.0, 123.7, 121.6, 114.1, 113.4, 48.7,
HPO42ꢁ, PO43ꢁ, and HSO4 by exhibiting an increase in the emis-
ꢁ
sion of anthracene in aq CH3OH only at a high concentration of
guests and show a preference for tetrabutylammonium hydrogen
sulfate with 1:1 stoichiometry. Further work along this direction
is underway in our laboratory.
43.3; FTIR (KBr,
m
in cmꢁ1) 3379, 1700, 1623, 1565, 1542, 1487, 1458, þ1449.
HRMS (TOF MS ES+): C54H42N6O2ꢀ2PF6 requires 951.3006 for ðM ꢁ PF6ꢁÞ and
806.3358 for ðM ꢁ 2PF6ꢁÞþ
, respectively; found 951.3009 and 806.3309,
respectively.

K. Ghosh et al. / Tetrahedron Letters 50 (2009) 2392–2397
2397
21. Energy optimization was done using CS Chem 3D version 7.0.
22. de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P.. Nature 1993, 364, 42–44.
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1173–1175.
24. Connors, K. A. Binding Constants: The Measurement of Molecular Complex:
Stability; John Wiley & Sons: New York, 1987.
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