Enediyne scaffold-based highly selective chemosensor for ratiometric sensing of H2PO4-ions

Article abstract of DOI:10.1016/j.tetlet.2012.02.024

Enediyne scaffold - based new chemosensor 1 has been designed and synthesized. The sensor 1 fluorometrically recognizes H₂PO4- in CH₃CN containing 1% DMSO by exhibiting a ratiometric change in emission upon complexation. The associat

Full text of DOI:10.1016/j.tetlet.2012.02.024

Tetrahedron Letters 53 (2012) 2054–2058
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Enediyne scaffold-based highly selective chemosensor for ratiometric sensing of
À
H PO ions
2
4
⇑
Kumaresh Ghosh , Sk Sarfaraj Ali, Soumen Joardar
Department of Chemistry, University of Kalyani, Kalyani, Nadia 741235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Enediyne scaffold - based new chemosensor 1 has been designed and synthesized. The sensor 1 fluoromet-
Received 29 November 2011
Revised 2 February 2012
Accepted 6 February 2012
Available online 14 February 2012
À
rically recognizes H
upon complexation. The association constant of 1 with H
2
PO in CH
3
CN containing 1% DMSO by exhibiting a ratiometric change in emission
4
À
4
À1
2
PO was calculated as (3.06 ± 0.6) Â 10 M
4
.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
À
H
2
PO Binding
4
Enediyne-based chemosensor
Ratiometric sensing
Anthracene labeled benzimidazolium motif
PET sensor
Selective sensing of anions of biological and environmental rel-
evance by synthetic receptors has become an important research
area in supramolecular chemistry. In relation to this, the binding
the Bergman cyclization is assisted in the presence of metal ion,
it is also disfavored or occurred at elevated temperatures in the
presence of metal ions due to the modulation of the distance be-
tween two yne motifs. Due to this characteristic feature, exploi-
tation of enediyne motif in molecular recognition studies for the
substrate recognition is untried. To the best of our knowledge,
the use of enediyne motif in developing hosts for anions is un-
known in the literature. In this Letter, we, for the first time, wish
to report the design, synthesis, and the anion sensing properties
of a new enediyne-based chemosensor 1.
1
1
1
unit, which is capable of binding anions strongly or moderately un-
der the mastery of a synthetic spacer, has the crucial role in giving
the selectivity to the designed receptor. Among the various types of
2
3
4
5
binding units, amide, urea/thiourea, guanidinium, amidinium,
imidazolium, benzimidazolium,6 pyridinium,7 polyammonium
cations etc., are widely used in anion binding and they were found
5d
8
to be creditable in functioning. Benzimidazolium like imidazolium
and pyridinium motifs provides an unconventional C–H bond for
Compound 1 shows selective ratiometric fluorescence sensing
9
À
À
bonding. Further stabilization of the complex is attributed to the
2
4
of H PO over the other anions such as AcO , malonate, succinate,
À
À
À
À
À
electrostatic interaction working in between the opposite charges
of host and guest. The use of this motif in the context of anion rec-
ognition is well established.6 It is to be noted that functioning of
this motif is controlled by the organic framework that holds them
4
3
HSO , F , Cl , Br , and I in CH CN containing 1% DMSO. The coop-
erativity of the two benzimidazolium motifs of 1 in anion binding
was established by considering the model compound 2, which per-
turbed the emission weakly in a non ratiometric fashion.
6c–g,10
in a particular array. During our work along this direction,
we
The receptor 1 was achieved in a reasonable yield according to
the Scheme 1. The dibromo compound 6, obtained based on the re-
questioned ourselves whether we can use the enediyne motif as
synthetic spacer in building up a new architecture for the recogni-
tion of anionic substrates. Enediyne is a well known moiety that
undergoes Bergman cyclization under thermal condition to give
1
3
ported procedure, was reacted with anthracene labeled benz-
1
4
imidazole 7 in dry CH
the dibromide salt of 1. Anion exchange of the dibromide salt with
NH PF finally gave the desired compound 1 in an appreciable yiel-
d. Reaction of 7 with butyl bromide under refluxing condition in
dry CH CN followed by anion exchange using NH PF afforded the
compound 2
The interaction properties of 1 toward the anions of different
3
CN under refluxing condition to afford
1
1
benzene-1,4-diradical. This reactive diradical species interacts
with DNA and shows antitumor activity. Research aimed at the fac-
ile cyclization of enediyne motif present in the designed com-
pounds in the presence of suitable cation is being pursued in
4
6
1
5a
3
4
6
1
5b
in a good yield.
1
1,12
different laboratories.
In this horizon, while in some cases
À
À
À
À
À
À
shapes such as F , Cl , Br , I , HSO , AcO , malonate, succinate,
PO (taken as tetrabutylammonium salts) were studied by
fluorescence, UV–vis, and H NMR. Importantly, the receptor 1
4
À
⇑
Corresponding author. Fax: +91 33258282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
and H
2
4
1
0
040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.02.024

K. Ghosh et al. / Tetrahedron Letters 53 (2012) 2054–2058
2055
O
O
N
H
N
H
N+
N+
N+
N
N
2
PF -
6
H
N+
PF6-
N
N
2
PF -
H
6
N+
N
1
2
3
MsO
HO
MsCl/Et N
Br
i)N
Br
N
Pd(PPh )
3
LiBr, dry THF
3
4
7
(a)
1
n-Butyl amine,
dry DCM, 0 C
°
CH CN, reflux
Br propargyl alcohol
3
4
5
6
ii) NH4PF6
MsO
Br
HO
(
b)
i) n-Butyl bromide, dry CH CN, reflux
3
2
N
ii) NH PF , aq. CH OH
N
4
6
3
7
Scheme 1. Syntheses of receptors 1 and 2.
0
0
0
0
0
.4
.3
.2
.1
.0
3
3
2
2
1
1
50
00
50
00
50
00
3
00
350
400
450
500
Wavelength (nm)
50
0
400
450
500
550
600
À5
Figure 2. Change in fluorescence ratio of 1 (c = 2.22 Â 10 M) at 418 nm upon
addition of a particular anion in 10 equiv amounts in CH CN containing 1% DMSO
ex = 370 nm).
Wavelength (nm)
3
(k
À5
Figure 1. Fluorescence titration spectra of 1 (c = 2.22 Â 10 M) upon addition of
À
H
2
PO in CH
3
CN containing 1% DMSO (kex = 370 nm); Inset: UV–vis titration spectra
4
À5
À
4
of 1 (c = 2.22 Â 10 M) with H
2
PO in CH
3
CN containing 1% DMSO.
À
Among the other anions F induces a little more change in the
emission. The smaller sized dicarboxylates malonate and succinate
also did not bring any measurable change in emission.
À
exhibited selective sensing of H
2
PO in CH
3
CN containing 1%
4
À
DMSO. In the presence of H
2
PO ions, the monomer emission of
However, in the event, the anion–receptor interaction is due to
hydrogen bonding and electrostatic interaction between the anion
4
1
was greatly decreased along with the appearance of a new peak
at 500 nm, ascribed to the chelation induced excimer formation
1
target and the benzimidazolium functions. Indeed, H NMR spectra
between the closely spaced anthracene moieties. This characteris-
in DMSO-d
6
show that only the benzimidazolium protons are
À
tic emission change allowed H
2
PO ions to be distinguished from
mainly affected by the anion addition. Due to the presence of
DMSO, a competitive solvent, the downfield shift of the benzim-
idazolium protons was found to be 0.04 ppm. A similar study in H
NMR was performed in CDCl
addition of H
of the benzimidazolium protons (
the presence of equivalent amount of H
4
1
6
the other anions studied. Figure 1 demonstrates the change in
À
1
emission of 1 upon successive addition of H
of 1 in CH
2
PO to the solution
4
3
CN containing 1% DMSO. The appearance of an isosbestic
3
containing 0.8% DMSO-d
PO . Interestingly, a greater downfield movement
6
with the
À
point at 486 nm in Figure 1 indicates the formation of a new spe-
cies that remains in equilibrium with the free receptor in the solu-
tion. Figure 2 highlights the change in emission of 1 in the presence
2
4
D
d = 1.19 ppm) was observed in
À
2
PO . Figure 3 demon-
4
of 10 equiv amounts of a particular anion studied. As can be seen
strates this feature. Upon interaction, the anthracenyl ring protons
(H , H , H , H and H ) suffered upfield shift which anticipated the p
À
from Figure 2, only H
2
PO perturbs the emission considerably.
c
d
e
f
b
4

2
056
K. Ghosh et al. / Tetrahedron Letters 53 (2012) 2054–2058
-6
6
.0x10
N+
Ha
N
Hg
Hc
-6
5.0x10
H
1
N+
N
-6
2
PF6
-
4.0x10
Hb
He
-6
Hd
3.0x10
Hf
-6
2
.0x10
-6
1
.0x10
0.0
0.0
0.2
0.4
0.6
0.8
1.0
XG
À
À5
Figure 5. UV Job plot of the receptor 1 with H
in CH CN containing 1% DMSO.
2
PO , where [G] = [H] = 2.22 Â 10
M
4
3
Figure 3. 1H NMR (400 MHz) of (a) receptor 1 (c = 1.45 Â 10 M) and (b) with
À3
À
equivalent amount of H
2
PO in CDCl
3
containing 0.8% DMSO-d
6
.
4
À
The binding selectivity of 1 toward H
2
PO was determined from
4
the control experiment in fluorescence. The change in emission of
4
3
3
2
2
1
1
00
50
00
50
00
50
00
1 was noticed in CH
CN containing 1% DMSO after the addition of
3
À
5
5
equiv amounts of H
equiv amounts of other particular anion. Figure 7, in this context,
2
PO to the receptor solution containing
4
highlights the observations. It is evident from Figure 7 that inter-
À
ference of the other anions in the binding of H
2
PO is negligible.
4
However, in comparison to our previously reported ortho-phen-
6
d
ylenediamine-based receptor 3, the receptor 1 is much selective
À
toward H
2
PO with greater binding constant value. Also the selec-
4
À
À
2
4
tivity between H PO and F ions is measurably high. The intramo-
lecular hydrogen bonding between the amides in 3 which cannot
be excluded presumably reduced its affinity and selectivity toward
anion sensing. This is in contrast to 1. Furthermore, the enediyne
5
0
0
motif in 1 holds the benzimidazolium groups at a fixed distance
À
4
00
450
500
550
600
and allows a more compact binding of H
2
PO ion for which the
4
À
decomplexation in the presence of excess concentration of H
did not occur like in the case of 3. Importantly, the cooperativity
2
PO
Wavelength (nm)
4
6
d
À5
À
Figure 4. Fluorescence titration spectra of 1 (c = 2.22 Â 10 M) upon addition of F
in CH CN containing 1% DMSO (kex = 370 nm).
of the two benzimidazolium moieties in 1 was further understood
À
3
from the weak change in emission of 2 upon titration with H
and F ions (Supplementary data).
PO
2
4
À
stacking interaction between the pendant anthracenes. It is to be
noted that F being smaller sized and highly basic, perturbed the
3 3 2
In aqueous CH CN (CH CN: H O = 1:1 v/v) containing few drops
À
of DMSO (used to make solution homogeneous) the interaction of 1
was further studied with the same anions. Almost equal change in
emission of 1 established its non efficiency in discriminating the
anions fluorometrically (Fig. 8) in aqueous organic solvent. Even,
under identical condition, mere change in emission of 1 upon addi-
tion of the different sodium salts of phosphate derivatives as well
as phosphate group containing biomolecules such as ATP, ADP and
emission of 1 relatively little more than the other halides and also
À
À
AcO ions. This is attributed to the dimensional matching of F into
the narrow cavity of 1. Figure 4 shows the emission titration spec-
À
tra for 1 with F . The complexation induced quenching of emission
of 1 in the present case is explained due to the activation of PET
process occurring in between the binding site and the excited state
of anthracene. The significant PET-based quenching of monomer
3 3 2
AMP, dissolved in aqueous CH CN (CH CN: H O = 1:1 v/v) (Supple-
emission of 1 followed by the growth of an excimer peak in the
mentary data) indicated weak or no interaction.
À
presence of H
sensing in CH
2
PO is worth mentioning for its selective ratiometric
Further to obtain insight into the ground state interaction, we
4
3
CN containing 1% DMSO. The ratiometric chemosen-
3
recorded UV–vis spectra of 1 in CH CN containing 1% DMSO in
À
sors offer advantages over the conventional monitoring of fluores-
cence intensity at a single wavelength. A dual emission system can
minimize the measurement errors because of the factors such as
phototransformation, receptor concentrations, and environmental
the presence of the same anions. Among the anions, only H
2
PO
4
brought a marked change with a red shift of ꢀ6 nm in the UV–
vis spectra of 1 and thereby indicated its significant interaction (in-
set of Fig. 1) in the ground state also. On the contrary, the minimal
change in the absorption of 1 in the presence of other anions cor-
roborated their weak or non involvement in the binding (Supple-
mentary data). Thus the enediyne scaffold in 1 acts as a suitable
spacer that holds the benzimidazolium groups for providing a
channel like molecular cleft for the complexation of H
effectively.
1
7
effects. A literature survey indicated that ratiometric chemosen-
À
6g,18
sors for H
2
PO are rare.
In the interaction process, the 1:1 stoi-
4
À
chiometry of the complex of 1 with H
2
PO was established from
4
Job plot1 (Fig. 5; also see Supplementary data) and the binding
9
4
À1 20
À
affinity was determined to be (3.06 ± 0.6) Â 10 M
.
Based on
2
PO ion
4
these experimental observations we presume a binding structure
as shown in Figure 6a. The compactness of the binding cavity, com-
posed of benzimidazolium units only, was realized from the MM3
As the enediyne motif is sensitive to Bergman cyclization to give
benzene-1,4-diradical under thermal condition, we verified its sus-
2
1
1
optimized geometry (Fig. 6b).
tainability as a spacer by recording the H NMR at variable temper-

K. Ghosh et al. / Tetrahedron Letters 53 (2012) 2054–2058
2057
-
2
PF6
N+
N+
H
H
O-
O
N
N
P
HO OH
(
a)
(b)
À
2
4
Figure 6. (a) Proposed binding structure for 1 with H PO ; (b) MM3 optimized geometry of 1.
atures (Supplementary data). As can be seen from Figure 7S, no
structural change was observed upto temperature 120 °C. More-
over, to be acquainted with the actual temperature range at which
the enediyne scaffold in 1 is cycloaromatized to the benzene dirad-
2
2
ical, the differential scanning calorimetric (DSC) experiment was
carried out. In this regard, in DSC curve an inflection at 168.95 °C
which presumably corresponds to the enediyne cyclization was
observed (Supplementary data). Interestingly, in the presence of
À
equivalent amount of the H
2
PO the inflection at 168.95 °C is
4
shifted to 196.4 °C (Supplementary data). This is ascribed to the
higher stability of the system upon complexation. Thus the exper-
imental findings in the present communication further provide the
information that the enediyne motif in 1 is sound in anion recogni-
tion without exhibiting chelation induced Bergman cyclization
over a wide range of temperature.
In conclusion, we have thus designed and synthesized a new
À
molecular architecture 1 which is found to recognize H
2
PO ratio-
4
metrically. In the design, enediyne motif has been exploited as
synthetic spacer in building up the receptor site for anion. Although
there are reports on enediyne based molecules which on complex-
ation with cations undergo facile Bergman cyclization, there is no
report on anion binding receptor built on enediyne scaffold till date.
Inspite of the absence of the amide groups as extra hydrogen bond
donors to anions as found in receptor 3, the linear nature of the
À
2
Figure 7. Change in fluorescence ratio upon addition of 5 equiv amounts of H PO
4
to the solutions of
1 and 1 containing other anions in 5 equiv amounts
À5
(
[H] = 3.13 Â 10 M).
triple bonds in 1 makes the open cleft more compact for effective
À
complexation of H
2
PO . The involvement of polar C–H bonds of
4
benzimidazolium units in hydrogen bonding and the charge–
charge interaction under the guidance of the enediyne motif
altogether are the essential factors that impart selectivity to the
À
receptor 1 in the recognition of H
2
PO ions. To our opinion, this is
4
6
e–g,23
a new addendum to the existing examples
PO in the literature. We are currently exploring this enediyne
of receptors for
À
H
2
4
framework with a view to establishing more sophisticated systems
that might perform as good hosts for other several anions of specific
interest.
Acknowledgments
We thank CSIR, Government of India for financial support.
K.G. thanks DST, Government of India for providing facilities in
the department under FIST program and also to UGC for SAP
program.
Figure 8. Change in fluorescence ratio of 1 upon addition of 13 equiv amounts of a
À5
particular guest in aqueous CH
3 3
CN (CH CN:H
2
O = 1:1 v/v) ([H] = 3.13 Â 10 M).

2
058
K. Ghosh et al. / Tetrahedron Letters 53 (2012) 2054–2058
Jones, P. G. J. Org. Chem. 1994, 59, 7142; (d) Warner, B. P.; Miller, S. P.; Broee, R.
Supplementary data
D.; Buchwald, S. L. Science 1995, 269, 814; (e) Basak, A.; Shain, J. C.; Khamrai, U.
K.; Rudra, K. R.; Basak, A. J. Chem. Soc., Perkin Trans. 1 2000, 1955; (f) Basak, A.;
Shain, J. C. Tetrahedron Lett. 1998, 39, 3029; (g) Rawat, D. S.; Zaleski, M. J. Am.
Chem. Soc. 2001, 123, 9675; (h) Benites, P. J.; Rawat, D. S.; Zaleski, J. M. J. Am.
Chem. Soc. 2000, 122, 7208.
Supplementary data (figures showing the change in absorption
and fluorescence spectra of 1 with anions, the fluorescence Job
plots for H
of 13 equiv amounts of the anions in aq CH CN, binding constant
3
curve, DSC curve, H NMR at variable temperature) associated with
this article can be found, in the online version, at doi:10.1016/j.
tetlet.2012.02.024.
À
2
PO , change in fluorescence ratio of 1 in the presence
4
1
3. (a) Basak, A.; Shain, J. Tetrahedron Lett. 1998, 39, 1623; (b) Nicolaou, K. C.;
Zuccarello, G.; Rimer, C.; Estevez, V. A.; Dai, W. M. J. Am. Chem. Soc. 1992, 144,
7360; (c) Just, G.; Singh, R. Tetrahedron Lett. 1987, 28, 5981.
1
14. Ghosh, K.; Saha, I.; Frohlich, R.; Patra, A. Mini-Rev. Org. Chem. 2011, 8, 31.
1
5. (a) To a solution of the compound 6 (75 mg, 0.242 mmol) in dry CH
3
CN
(225 mg, 0.726 mmol)
3
CN: DMF (10:1 v/v) was added at a
(
10 mL), anthracene appended benzimidazole 7
dissolved in dry mixture solvent of CH
time. The reaction mixture was refluxed for 98 h. On removal of the solvent,
the dibromide salt of 1 was isolated in a 45% yield (60 mg). In the next step, the
References and notes
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3
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1
.
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À1
À
+
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