Cholesterol appended pyridinium ureas: A case of gel making and breaking for selective visual readout of F-

Article abstract of DOI:10.1039/c2ob26631h

Cholesterol appended pyridinium ureas 1 and 2 have been designed and synthesized. The unsymmetrical urea-based chemosensor 1 fluorimetrically distinguishes F^- from the other anions examined in both CH ₃CN and DMSO with appreciable binding constant values. The pyridinium - based symmetrical compound 2 acts as a low molecular weight gelator (LMWG) in CHCl₃ and is capable of detecting F^- visually by breaking the gel. On the contrary, the chemosensor 1 in DMSO in the presence of tetrabutylammonium fluoride undergoes a change from sol to gel state that produces an unambiguous visual readout of F^-. This journal is

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PAPER
Cholesterol appended pyridinium ureas: a case of gel making and breaking
for selective visual readout of F−†
Kumaresh Ghosh* and Debasis Kar
Received 16th August 2012, Accepted 2nd October 2012
DOI: 10.1039/c2ob26631h
Cholesterol appended pyridinium ureas 1 and 2 have been designed and synthesized. The unsymmetrical
urea-based chemosensor 1 fluorimetrically distinguishes F− from the other anions examined in both
CH3CN and DMSO with appreciable binding constant values. The pyridinium – based symmetrical
compound 2 acts as a low molecular weight gelator (LMWG) in CHCl3 and is capable of detecting F−
visually by breaking the gel. On the contrary, the chemosensor 1 in DMSO in the presence of
tetrabutylammonium fluoride undergoes a change from sol to gel state that produces an unambiguous
visual readout of F−.
organogelators for which the effective hydrogen-bonding inter-
actions between the gelators are disturbed. However, because
Introduction
Supramolecular structures, composed of low molecular weight
organic compounds, are of much interest due to their unique fea-
tures and wide range of potential applications.1 In this context,
low molecular weight organic compounds which are capable of
forming supramolecular gels with various organic solvents draw
attention in the visual detection of some ionic species.2 It is well
documented that supramolecular gels are dimensionally con-
trolled assemblies comprising low-molecular-weight molecules
held together by noncovalent interactions, such as hydrogen
bonding, metal coordination, van der Waals interaction, and π–π
stacking.2,3 Stimuli responsive gels that are highly desirable for
the development of sensor devices are of considerable attention
in recent time. Many research groups in this domain have shown
interest in the construction of anion-responsive supramolecular
gels.4,3a Among these, fluoride-sensing gels have received a
great deal of attention4c,5 because fluoride is involved in prevent-
ing dental caries and in medical treatment for osteoporosis.6
Fluoride and proton are reported to elicit gel to sol transition and
fluorescence emission after interaction with bisurea-functiona-
lized naphthalene organogelators.5f In addition, organogelators
based on urea,5k oxalamide,5m or hydrazide5n motifs are found to
induce gel to sol transitions and optical changes5o after the
addition of fluoride anions. This happens due to the interaction
of fluoride with the amide NHs of these amide-containing
amidic NHs in the organogelators could also interact with other
anions, the initiation of gel to sol transitions and optical changes
would not necessarily be selective to fluoride. Under these cir-
cumstances, the design of receptor that is capable of detecting
and sensing fluoride selectively through gel making and break-
ing processes is of immense interest. In regard to this, during our
on-going research on anion recognition,7 we report here two
supramolecular structures 1 and 2 that are cholesterol appended
pyridinium ureas. Both of them recognize F− selectively either
by forming or breaking gels in organic solvents.
Department of Chemistry, University of Kalyani, Kalyani-741235, India.
E-mail: ghosh_k2003@yahoo.co.in; Fax: +913325828282;
Tel: +913325828750
†Electronic supplementary information (ESI) available: Figures showing
the change in fluorescence and UV-vis titrations of receptor 1 with
various anions in different solvents, binding constant curves, Job plot,
Interestingly, while the structure 1 forms gel in DMSO in the
presence of tetrabutylammonium fluoride, chloroform gel of
symmetric structure 2 shows a sharp conversion to the sol state
1
photograph of gel of 1, IR spectra, H, 13C NMR and mass spectra and
other selected spectra. See DOI: 10.1039/c2ob26631h
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in the presence of the same and thus produces an unambiguous
visual readout of F− over a series of other anions studied.
emission intensity at 450 nm decreased with a blue shift of
∼20 nm (Fig. 1). Such spectral change was also seen while the
titration of 1 was performed with H2PO4−, AcO−. In the case of
H2PO4−, fluorescence intensity of 1 decreased upon the addition
of 2 equiv. amounts of guest. Further addition of H2PO4− caused
an increase in emission which was assumed to be due to decom-
plexation (See ESI†).8 Fig. 2 represents the change in fluor-
escence ratio of 1 at 430 nm, upon addition of 10 equiv.
amounts of individual anion. As can be seen from Fig. 2, the
receptor shows less selectivity in the identification of F−. In the
Results and discussion
Compounds 1 and 2 were synthesized according to Scheme 1.
Initially, cholesterol was converted to the chloride 3 which on
treatment with the pyridine-based unsymmetrical urea 4 and
symmetrical urea 5 gave the chloride salts. The unsymmetrical
urea 4 of 3-aminopyridine was obtained from its reaction with
triphosgene followed by the addition of 6-aminocoumarin in
CH2Cl2. The symmetrical urea 5 was accomplished from the
reaction of 3-aminopyridine in the presence of triphosgene in
CH2Cl2. However, the compounds 4 and 5 were refluxed indivi-
dually with the cholesterol-based chloride 3 in dry CH3CN to
afford their respective mono and dichloride salts. Anion
exchange reaction of the chloride salt of 1 using NH4PF6 gave
the desired compound 1 in appreciable yield. A similar anion
exchange in 2 produced the salt 2a. All the compounds were
characterized by 1H NMR, 13C NMR, FTIR and mass analysis.
−
profile, the other anions such as AcO− and H2PO4 also exhibit
a similar perturbation in emission of 1 as noticed with F− ions.
In the interaction, the 1 : 1 stoichiometry of 1 with F− was
confirmed by Job plot9 (Fig. 3) and the binding constant (Ka)10
was determined to be 2.07 × 103 M−1. This moderate interaction
of F− with the binding site was also realized in the ground state.
Fig. 1 Change in emission of 1 (c = 6.32 × 10−5 M) with an increase
in concentration of F− (λexc = 330 nm) in CH3CN.
Scheme 1 (i) Chloroacetyl chloride, pyridine, dry CH2Cl2, rt, 10 h; (ii)
6-aminocoumarin, triphosgene, Et3N, dry CH2Cl2, rt, 24 h; (iii) triphos-
gene, Et3N, dry CH2Cl2, rt, 24 h; (iv) a. 3, dry CH3CN, reflux, 3 days,
b. aq. NH4PF6, DMF–CH3OH, rt, 12 h; (v) 3, dry CH3CN, reflux, 3 days;
(vi) aq. NH4PF6, DMF–CH3OH, rt, 12 h.
Fig. 2 Change in fluorescence ratio (I0 − I/I0) of 1 (c = 6.32 × 10−5
M) upon addition of 10 equiv. amounts of a particular anion in CH3CN.
Complexation studies
The underlying theme for the design of molecules 1 and 2 in the
gelation study is as follows: (1) cholesterol unit can aggregate in
solution through van der Waals and hydrophobic interaction; (2)
the pyridinium urea motif has the propensity to complex anions
involving hydrogen bonds and charge–charge interactions. To
investigate the relevance of the design in anion recognition
through gelation, complexation properties of 1 and 2 towards a
series of anions were evaluated.
Due to the presence of coumarin probe in 1, UV-vis and fluor-
escence titration experiments were performed in addition to its
gel study to determine its sensing behavior in solution. Com-
pound 1 (c = 6.32 × 10−5 M) showed emission at 430 nm when
excited at 340 nm in CH3CN. Upon titration with F−, the
Fig. 3 Fluorescence Job plot for 1 with TBAF in CH3CN ([H] = [G] =
5.56 × 10−5 M).
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In the UV-vis titration of 1 with F− (Fig. 4) while the absor-
bance at 271 nm decreased, the absorbance at 331 nm increased
upon gradual addition of F− to the solution of 1 in CH3CN and a
well defined ratiometric behavior was noticed. This indicated the
formation of a new species in solution which may remain in
equilibrium with the free receptor 1.
However, in order to ascertain the selectivity in the recog-
nition process, the polarity of the solvent was changed. Interest-
ingly, in DMSO, chemosensor 1 showed greater change in
emission upon addition of F− ions (ESI†). In this regard, Fig. 5
corroborates this feature at the emission 430 nm in the presence
of 10 equiv. amounts of each anion in DMSO. Similar to the
case in CH3CN, the receptor 1 binds F− with the binding con-
stant value10 of 3.21 × 103 M−1 involving a 1 : 1 stoichiometry9
(ESI†).
The selectivity in the binding of F− in DMSO was ascertained
by observing the change in fluorescence in the presence and
absence of other anions. Fig. 6 shows the profile where it is
observed that the other anions negligibly interfered in the
binding of the F− anion.
In addition to this fluorometric behavior in solution, the recep-
tor 1 exhibited some new findings in DMSO. While the receptor
1 was taken in ∼10−3 M concentration range in DMSO, the
addition of tetrabutylammonium fluoride (c = 5.0 × 10−3 M)
brought about a marked color change. The almost colorless solu-
tion of 1 turned into intense yellow color. In the presence of
AcO− and H2PO4− a faint yellow color was noticed (Fig. 7).
We believe that F− being more basic initially interacts with the
pyridinium urea site of 1 through hydrogen bonding and then
deprotonates the more acidic urea protons in the presence of its
excess concentration in solution (Fig. 8). In the deprotonated
species the delocalization of charge resulted in color change in
solution. This is evident from the observation noted when tetra-
butylammonium hydroxide was added to the solution of 1
(Fig. 7). The hydrogen bonding and the deprotonation phenom-
Fig. 4 Change in absorption of 1 (c = 6.32 × 10−5 M) in CH3CN with
1
ena were established by H NMR study. Fig. 9, in this context,
increase in concentration of F−.
describes the change in 1H NMR of 1 in the presence and
absence of equiv. amount of tetrabutylammonium fluoride
(TBAF) in d6-DMSO. Due to the deprotonation of urea protons
(a and b), the signals of both the pyridinium and coumarin
motifs underwent little upfield shift.
Fig. 5 Change in fluorescence ratio (I0 − I/I0) of receptor 1 (c = 3.75 ×
10−5 M) upon addition of 10 equiv. amounts of a particular anion in
DMSO.
Fig. 7 Photographs of colour changes of 1 (c = 1.0 × 10−3 M) upon
addition of F−, H2PO4−, AcO− and OH− (c = 5.0 × 10−3 M) as their tet-
rabutylammonium salts in DMSO.
Fig. 6 Change in fluorescence ratio of 1 (c = 6.35 × 10−5 M) upon
addition of 15 equiv. amounts of F− in the presence of other anions in
DMSO.
Fig. 8 Suggested scheme for (a) hydrogen bonding and (b) deprotona-
tion in 1 in the presence of F− ions.
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Fig. 9 Partial 1H NMR (400 MHz, d6-DMSO) of (A) 1 (c = 4.8 ×
10−3 M) and (B) in the presence of equiv. amount of TBAF (see Fig. 8
for labelling of protons).
Fig. 10 The changes in the DMSO solution of 1 (10 mg/0.5 mL) after
addition of TBAF (2 equiv). Brownish solution slowly gelates after (a)
5 min and (b) 10 min.
Table 1 Results of gelation test for 1
Solvent
Resulta
DMSO
DMF
G (3.0)
S
S
S
S
S
I
Methanol
Chloroform
Ethanol
Acetonitrile
n-Hexane
Diethyl ether
Propanol
I
S
a S = solution; G = transparent gel (mgc = minimum gelatination
concentration in mg mL−1); I = insoluble.
Fig. 11 (a) AFM image of xerogel obtained from 1 and TBAF (scale
bar: = 1.45 μm); (b) SEM image of xerogel from DMSO gel of 1 with
fluoride (scale bar: = 2 μm).
Beside the colorimetric response, compound 1 in ∼10−3
M
concentration formed gel in the presence of 5 equiv. amounts of
TBAF. Other anions taken in the study were non-responding in
this new findings. The gelation propensity of 1 was examined in
a wide range of solvents and solvent mixtures (Table 1). Only
the ‘instant metastable gel’ from the DMSO in the NMR tube
was observed when 5 equiv. amounts of fluoride was added
(ESI†). The gelation property of the compound 1 was checked in
various solvents as well as solvent mixtures, and the results are
given in Table 1.
In regard to this gelation study, it is mentionable that com-
pound 1 in high concentration in DMSO (10 mg/0.5 mL)
showed impressive gelation behavior upon addition of TBAF.
Addition of 2 equiv. amounts of F− to the brownish solution of 1
in DMSO induced gelation within 5 min. After 10 min, a com-
plete gel was noticed with a change in color (brown to greenish
brown; see Fig. 10). It is important to note that anions usually
disrupt the supramolecular gels although there are few examples
known where the presence of anions stimulates gel formation
and allows their naked eye detection.4b,7h,11 In our opinion, in
the present study, the sol-to-gel transition induced by F− occurs
as a result of the effective aggregation of gelators. The hydrogen
bonding properties of urea as well as pyridinium motifs set a
complex network in DMSO in the presence of F−. Excess con-
centration of F− carries deprotonation of the urea protons and the
resulting dianionic species as shown in Fig. 8 possibly forms
intermolecular contacts with the pyridinium part and entrapped
HF2− ion via hydrogen bond formation and charge–charge inter-
action and leads to the formation of a network in solution. For-
mation of this network is further encouraged by large
hydrophobic surface of the cholesterol part. During interaction
the formation of bifluoride (HF2−) was supported by the
1
appearance of broad signal at 16 ppm12 in H NMR (ESI†). The
morphology of the gel was investigated by scanning electron
microscopy (SEM) and atomic force microscopy (AFM) as
shown in Fig. 11. The SEM image reveals a microstructure of
the xerogel of 1 with F− in DMSO. Three-dimensional network
is found to contain globules with some imprecise cavities. At
68 °C, gel was converted to the sol state with deep green color.
It is important to be mentioned here that the chloride salt of 1
did not form any gel with the addition of TBAF in DMSO.
Instead, a viscous and yellow colored solution containing some
precipitates was obtained. This is thought to be due to the inter-
ference of Cl− ion in hydrogen bonding.
As we move from compound 1 to 2, a different behavior
towards gelation is noted. Interestingly, when compound 2 is dis-
solved in CHCl3 on warming and then kept at room temperature,
a nice thick yellow colored gel is formed. However, this gelation
property was not at all observed while the chloride anions in 2
−
are exchanged with PF6 ions to have the compound 2a. It is
believed that Cl− ions in 2 plays a key role in establishing a
hydrogen bonded network in solution, especially in CHCl3. The
syn and anti orientations of the urea motif may assemble the
molecules through hydrogen bonding (see Fig. 12). The pyridi-
nium ring protons such as Ho, Ho′ and Hp also can take part in
hydrogen bonding with the interstitial Cl− ions, urea and ester
carbonyl oxygens. Thus this arrangement may establish a
complex network in less polar CHCl3 which is substantially
stabilized by the cholesterol units with large hydrophobic sur-
faces. We further believe that replacement of Cl− by the larger
−
PF6 ion disrupts the assemblies as shown in Fig. 12 by
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Fig. 13 SEM image of xerogel from CHCl3 gel of 2 (scale bar =
2 μm).
Fig. 12 Suggested arrangements of molecules of 2 in establishing the
hydrogen bonded network involving (a) the anti and (b) syn-orientations
of urea motif.
Table 2 Results of gelation test for 2
Solvent
Resulta
DMSO
DMF
S
S
Methanol
Chloroform
Ethanol
Acetonitrile
n-Hexane
Diethyl ether
Propanol
S
G (7.5)
S
S
I
I
S
Fig. 14 Photographs of chloroform gels of 2 (c = 6.58 × 10−3 M)
upon addition of 10 equiv. amounts of each anion.
a S = solution; G = transparent gel (mgc = minimum gelatination
concentration in mg mL−1); I = insoluble.
solution on addition of F− (TBAF) in excess (10 equiv)
(Fig. 14). Addition of less than 10 equiv. of F− caused partial
disintegration of the gel. However, completion of gel to sol tran-
sition occurred within 10 min. at room temperature. The disrup-
tion of the gel structure took place only in the presence of
fluoride anions, rapid gel-to-sol transition were not observed in
the presence of other anions examined (Fig. 14). This is attribu-
ted to the more basic character of F− due to which deprotonation
of urea protons led to the disruption of the hydrogen bonded
network in solution. The F− induced broken gel reappeared in
the presence of BF3·OEt2 (Fig. 15). Due to the reaction of BF3
with F−, the influence of F− on the gel is nullified. However,
further addition of F− transformed the gel into the sol state. This
cycle could be repeated at least three times.
destroying a number of intermolecular hydrogen bonds. This
was realized from FTIR studies of compounds 2 and 2a. While
NH stretching in 2 appeared at 3418 cm−1 as a broad signal due
to hydrogen bonding effects, compound 2a showed stretching at
1
3637 cm−1 for free NH (ESI†). In H NMR the signals for urea
and pyridinium ring protons in 2 appeared in the more downfield
region compared to 2a (ESI†). A similar observation was noted
1
in 1 (ESI†). Thus both FTIR and H NMR studies reveal that
compound 2 and chloride salt of 1 are different from their hexa-
fluorophosphate analogues with respect to the hydrogen bonding
characteristics. Such different hydrogen bonding behavior of Cl−
−
and PF6 is well established.13 However, the gelation propensity
of 2 in other dielectric media was also examined. The results are
summarized in Table 2.
The interaction of F− with 2 was understood by investigating
the UV-vis titration in CHCl3 in the concentration range of
∼10−5 M.
The Tgel at which the gel state of 2 was transformed into the
sol state was recorded as 67 °C and the morphology of the gel
was realized from SEM image. As can be seen from Fig. 13,
compound 2 is self-assembled with a three-dimensional network
composed of some aggregates. FTIR study of the amorphous 2
and its gel from CHCl3 reveals that there is a shift of the urea
It is evident from Fig. 16 that the compound 2 (c = 6.07 ×
10−5 M) typically shows two absorption peaks at 273 nm and
313 nm which undergo ratiometric change with the formation of
three isosbestic points at 326 nm, 297 nm and 277 nm upon titra-
tion with F− in CHCl3. Interaction of 2 with other anions con-
sidered in the study was also followed through UV-vis titration.
Change in absorbance was minor and irregular in some cases
except F− and Cl−. The binding constant values for F− and Cl−
carbonyl stretching to a lower wave number (νamorphous − νgel
=
48 cm−1). This shift provides evidence for the existence of inter-
molecular hydrogen bonds of 2 involving the urea part in the
Gel (ESI†). A chloroform gel of 2 formed a homogeneous
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corroborates these features. To our opinion, Cl− ions enter in the
gelation process due to the formation of 2 in solution on
−
exchange of PF6 ions and sets up a hydrogen bonded network
as shown in Fig. 12. To gain more information about the possi-
bility of gel formation of 2a with F− like the case of 1 with F−, a
similar experiment in DMSO was performed. But we failed.
Although it is believed that similar phenomena (Fig. 8) may
happen, the non appearance of gel of 2a with F− in DMSO is
presumably due to the structural aspect of 2a. The presence of
two pyridinium sites in 2a will follow a different hydrogen
bonded assembly from that of 1 in solution.
Fig. 15 Photographs for gel–sol and vice-versa phase transitions of
chloroform gel of 2 in the presence of F− and BF3·OEt2.
Conclusion
In conclusion, we have shown that the cholesterol appended
pyridinium ureas (symmetrical as well as unsymmetrical) exhibit
excellent property towards gelation in organic solvent. The sol or
gel state of the compounds is the reliable media in selective
sensing of F− in organic solvents. While the compound 1 forms
gel in DMSO in the presence of F−, compound 2 undergoes a
change from gel to sol state in CHCl3 in the presence of the
same anion. This produces an unambiguous visual readout
towards the selective recognition of F− in organic solvent
through the making and breaking of gels. Moreover, in such
process the change in colour of the solutions of the compounds
shows a convenient way to the selective detection of F− anion.
Compound 2a in the study is also capable of detecting Cl− ions
in CHCl3 by forming yellowish brown gel. It is further mention-
able that unsymmetrical urea-based chemosensor 1 fluorimetri-
cally distinguishes F− from the other anions examined in both
CH3CN and DMSO with appreciable binding constant values.
The extent of selectivity towards F− ion has been noted to be
admirable in DMSO.
Fig. 16 UV-vis titration spectra of 2 (c = 6.07 × 10−5 M) upon
addition of 10 equiv. amounts of F− (c = 1 × 10−3 M) in CHCl3.
Experimental Section
Chloro-acetic acid 17-(1,5-dimethyl-hexyl)-10,13-dimethyl-2,3,4,7,8,9,
10,11,12,13,14,15,16,17-tetradecahydro-1H-cyclopenta[a]phenanthren-
3-yl ester (3)
To a stirred solution of cholesterol (1 g, 2.59 mmol) in 30 mL
dry CH2Cl2 was added chloroacetyl chloride (0.31 mL,
3.88 mmol) and pyridine (0.1 mL, 1.3 mmol) under nitrogenous
atmosphere. The mixture was allowed to stir for 10 h at room
temperature. After completion of reaction, the solvent was evap-
orated and the crude was extracted with CHCl3 (3 × 50 mL). The
organic layer was washed several times with water and separated
and dried over Na2SO4. Evaporation of the solvent gave white
solid compound. Recrystallization from petroleum ether afforded
pure product 3 (1.18 g, yield 98%), mp 148 °C. 1H NMR
(400 MHz, CDCl3) δ 5.37 (m, 1H), 4.72 (m, 1H), 4.03 (s, 2H),
2.36 (m, 2H), 2.02–0.85 (m, 38H), 0.67 (s, 3H); FTIR (KBr,
cm−1): 2939, 2907, 2821, 1753, 1620, 1195.
Fig. 17 Photographs showing the phase changes of 2a (c = 8.76 ×
10−3 M) in CHCl3 upon addition of 2 equiv. amounts of (a) TBAF, (b)
TBACl, (c) TBABr and (d) TBAI (concentration of all halides was 1.6 ×
10−2 M).
were determined to be 3.14 × 103 M−1 and 4.20 × 104 M−1
,
respectively. On the other hand, in 1H NMR either broadening or
deprotonation of urea protons was the main hindrance to conduct
the titration experiment smoothly for determining the binding
constant values for 2 with the anions accurately.
An additional feature was observed while the compound 2
was converted into 2a. Compound 2a did not form any gel
under the condition as maintained for 2. But addition of TBACl
to the solution of 2a in CHCl3 induced gel formation. Under this
condition, addition of TBAF and TBAI did not induce any gel in
the medium. Only TBABr made the solution viscous. Fig. 17
1-(2-Oxo-2H-chromen-6-yl)-3-pyridin-3-yl-urea (4)
To a stirred solution of triphosgene (0.920 g, 3.10 mmol) in
10 mL dry CH2Cl2, 3-aminopyridine (0.292 g, 3.10 mmol)
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dissolved in 10 mL CH2Cl2 was added dropwise along with
Et3N (0.45 mL, 3.10 mmol). The reaction mixture was allowed
to stir for 1 h. Then 6-aminocoumarin (0.5 g, 3.10 mmol), dis-
solved in 10 mL CH2Cl2, was added dropwise from a dropping
funnel. The reaction mixture was allowed to stir for further 24 h.
After completion of reaction, solvent was evaporated off. The
crude mass was extracted with CHCl3 (3 × 30 mL). The organic
layer was washed with water and dried over Na2SO4. Evapor-
ation of the solvent in vacuo gave crude mixture which was chro-
matographed on a silica gel column using 80% ethyl acetate in
petroleum ether as eluent. The desired product 4 was obtained in
68% yield (0.6 g), mp 130 °C. 1H NMR (400 MHz, CDCl3 con-
taining two drops of d6-DMSO) δ 8.47 (m, 3H), 8.24 (d, 1H, J =
4 Hz), 8.13 (m, 1H), 7.96 (d, 1H, J = 2.40 Hz), 7.73 (d, 1H, J =
8 Hz), 7.37 (s, 1H), 7.33 (dd, 1H, J1 = 8 Hz, J2 = 4 Hz), 7.24
(m, 1H), 6.42 (d, 1H, J = 8 Hz); 13C NMR (100 MHz, d6-
DMSO) δ 160.0, 152.6, 148.7, 144.2, 142.8, 139.9, 136.3,
135.8, 125.5, 123.6, 122.7, 118.7, 116.7, 116.5, 116.4; FTIR
(KBr, cm−1): 3466, 1720, 1695, 1567, 1552.
2868, 1737, 1719, 1543, 1182; HRMS (TOF MS ES+):
C44H58N3O5PF6 , (M − PF6 + 1) requires 708.4371 found
709.3343.
+
Receptor 2
Compounds 3 (0.454 g, 0.98 mmol) and 5 (0.1 g, 0.45 mmol)
were taken in dry CH3CN (30 mL) and the mixture was refluxed
for 3 days. Precipitate appeared was filtered off and washed with
hot CH3CN to afford the pure compound 2 (0.66 g) in 83%
yield.
1H NMR (400 MHz, d6-DMSO) δ 10.9 (brs, 2H), 9.37 (s,
2H), 8.72 (d, 2H, J = 8 Hz), 8.51 (d, 2H, J = 8 Hz), 8.17 (t, 2H,
J = 8 Hz), 5.75–5.69 (m, 4H), 5.63 (m, 2H), 4.63 (m, 2H), 2.35
(m, 4H), 1.98–0.81 (m, 76H), 0.83 (s, 6H); FTIR (KBr, cm−1):
3435, 1745, 1640, 1594; Mass (ES+): 1067.4 (M − 2Cl− − 1)+.
Receptor 2a
The dichloride salt 2 (0.5 g, 0.438 mmol) was dissolved in
DMF–MeOH (1 : 1 v/v; 10 mL) and an aqueous solution of
NH4PF6 (0.187 g, 1.08 mmol) was added under hot condition.
After stirring the mixture for 30 min, the precipitate was filtered.
Repeated crystallization of the precipitate from diethyl ether
afforded pure compound 2a (0.57 g) in 96% yield, mp 232 °C.
1H NMR (400 MHz, d6-DMSO) δ 11.04 (brs, 2H), 9.37 (s, 2H),
8.71 (d, 2H, J = 8 Hz), 8.51 (d, 2H, J = 8 Hz), 8.16 (t, 2H, J =
8 Hz), 5.68 (m, 4H), 5.36 (brs, 2H), 4.61 (m, 2H), 2.35 (m, 4H),
1.95–0.85 (m, 76H), 0.83 (s, 6H); 13C NMR (100 MHz, d6-
DMSO) δ 166.2, 162.7, 141.2, 139.4, 135.8, 128.3, 126.5,
124.1, 122.9, 76.4, 61.3, 56.5, 56.1, 49.8, 42.3, 39.7, 39.3, 37.9,
36.8, 36.5, 36.2, 35.7, 31.7, 31.2, 28.2, 27.8, 27.6, 24.3, 23.8
(two carbons unresolved), 23.0, 22.8, 21.0, 19.3, 18.9; FTIR
(KBr, cm−1): 3395, 1747, 1653, 1595, 1563; Mass (TOF MS
ES+): 1214.32 (M − PF6− + 1)+, 1068.29 (M − 2PF6−)+.
1,3-Di-pyridin-3-yl-urea (5)14
To a stirred solution of triphosgene (0.92 g, 3.10 mmol) in 5 mL
dry CH2Cl2, 3-aminopyridine (0.5 g, 5.37 mmol) in 10 mL
CH2Cl2 along with Et3N (0.38 mL, 2.68 mmol) was added drop-
wise from a dropping funnel. The reaction mixture was stirred
for 24 h. After completion of reaction, solvent was evaporated
off. The crude mass was extracted with CHCl3 (3 × 30 mL). The
organic layer was washed with water and dried over Na2SO4.
Evaporation of the solvent gave crude mass which was purified
by column chromatography using silica gel and 80% ethyl
acetate in petroleum ether. Compound 5 was isolated in 73%
yield (0.42 g), mp 209 °C.
Receptor 1
General procedure of fluorescence titration. Stock solutions
of the hosts and guests were prepared in CH3CN or DMSO and
2 ml of the host solution was taken in the cuvette. The solution
was irradiated at the excitation wavelength maintaining the exci-
tation and emission slits. Upon addition of guest anions, the
change in fluorescence emission of the host was noticed. The
corresponding emission values during titration were noted and
used for the determination of binding constant values.
Compounds 3 (0.49 g, 1.07 mmol) and 4 (0.3 g, 1.07 mmol)
were taken in dry CH3CN (30 mL) and the mixture was refluxed
for 3 days. The precipitate appeared was filtered off and washed
with hot CH3CN to have pure chloride salt of 1. Next the chlor-
ide salt of 1 (0.6 g, 0.8 mmol) was dissolved in DMF–MeOH
(1 : 1 v/v; 10 mL) and an aqueous solution of NH4PF6 (0.262 g,
1.61 mmol) was added under hot condition. After stirring the
mixture for 30 min, the precipitate was filtered. Repeated crystal-
lization of the precipitate from diethyl ether afforded pure com-
pound 1 (0.66 g) in 78% yield, mp 186 °C. 1H NMR (400 MHz,
d6-DMSO) δ 10.0 (brs, 1H), 9.57 (s, 1H), 9.39 (s, 1H), 8.65 (d,
1H, J = 8 Hz), 8.42 (d, 1H, J = 8 Hz), 8.07 (m, 2H), 7.89 (s,
1H), 7.63 (d, 1H, J = 8 Hz), 7.40 (d, 1H, J = 8 Hz), 6.51 (d, 1H,
J = 8 Hz), 5.66 (s, 2H), 5.37 (s, 1H), 4.62 (m, 1H), 2.37 (m,
2H), 1.95–0.99 (m, 38H), 0.67 (s, 3H); 13C NMR (100 MHz, d6-
DMSO) δ 166.3, 162.5, 152.5, 144.4, 141.2, 140.2, 139.9,
139.5, 135.8 (two carbons unresolved), 134.8, 128.1 (two
carbons unresolved), 126.6, 124.1, 122.9 (two carbons unre-
solved), 76.4, 61.3, 56.5, 56.0, 49.8, 42.3, 39.7, 39.4, 37.9, 36.8,
36.5, 36.1, 35.6, 31.7, 31.1, 28.2, 27.8, 27.6, 24.3, 23.6 (two
carbons unresolved), 23.0, 22.8, 21.0, 19.3, 18.9 (one carbon in
the aromatic region is missed); FTIR (KBr, cm−1): 3400, 2950,
General procedure of UV-vis titration. The receptors were dis-
solved in dry UV grade CH3CN or DMSO and taken in the
cuvette. Then anions dissolved in dry CH3CN or DMSO, were
individually added in different amounts to the receptor solution.
The corresponding absorbance values during titration were
noted.
Method for job plot. The stoichiometry was determined by
the continuous variation method. In this method, solutions of
host and guests of equal concentrations were prepared in dry
CH3CN or DMSO. Then host and guest solutions were mixed in
different proportions maintaining a total volume of 3 mL of the
mixture. The related compositions for host–guest (v/v) were
3 : 0, 2.8 : 0.2, 2.5 : 0.5, 2.2 : 0.8, 2 : 1, 1.8 : 1.2, 1.5 : 1.5, 1 : 2,
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G. O. Lioyd, L. Applegarth and K. M. Anderson, New J. Chem., 2010,
34, 2261.
0.8 : 2.2, 0.5 : 2.5, 0.2 : 2.8. All the prepared solutions were kept
for 1 h with occasional shaking at room temperature. Then emis-
sion and absorbance of the solutions of different compositions
was recorded. The concentration of the complex i.e., [HG] was
calculated using the equation [HG] = ΔI/I0 × [H] where ΔI/I0
indicates the relative emission intensity and [H] corresponds the
concentration of pure host. Mole fraction of the host (XH) was
plotted against concentration of the complex [HG]. In the plot,
the mole fraction of the host at which the concentration of the
host-guest complex concentration [HG] is maximum, gives the
stoichiometry of the complex.
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Acknowledgements
We thank DST and UGC, New Delhi, India for providing facili-
ties in the department. D. K. thanks CSIR, New Delhi, India for
a fellowship.
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