Cooperative anion recognition by a novel heteroditopic receptor based on dibenzo[18]crown-6 fullero-bis(pyrrolidine)

Article abstract of DOI:10.1080/10610270903254183

A novel C₂ symmetrical fullero-bis(pyrrolidine) dibenzo[18]crown-6 conjugate 3 with trans-1 addition pattern on the fullerene sphere has been prepared and characterised by elemental analysis, ¹H NMR, ¹³C NMR and FAB-MS. NMR spectra are consistent with the isolation of a single regioisomer, assigned to the trans-1 bis-adduct of C60. Preliminary coordination studies by fluorimetry and UV-vis spectroscopy revealed that on inclusion of K⁺ in the crown ether cavity, cyclophane structured receptor 3 showed good selectivity in complexation towards H₂PO₄^- as compared to other anions via favourable electrostatic effects. Cyclic voltammetric studies demonstrate that the presence of K⁺ amplifies the electrochemical response of 3 towards H₂PO₄⁺ as compared to other anions. These results were corroborated by ¹H NMR titration study which confirmed the binding site for both H₂PO₄⁺ and K⁺ in the receptor.

Full text of DOI:10.1080/10610270903254183

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Supramolecular Chemistry
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Cooperative anion recognition by a novel heteroditopic
receptor based on dibenzo[18]crown-6 fullero-
bis(pyrrolidine)
Anish Kumar a & Shobhana K. Menon a
a Department of Chemistry , School of Sciences, Gujarat University , Ahmedabad, 380009,
Gujarat, India
Published online: 16 Sep 2009.
To cite this article: Anish Kumar & Shobhana K. Menon (2010) Cooperative anion recognition by a novel heteroditopic
receptor based on dibenzo[18]crown-6 fullero-bis(pyrrolidine), Supramolecular Chemistry, 22:1, 46-56, DOI:
10.1080/10610270903254183
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Supramolecular Chemistry
Vol. 22, No. 1, January 2010, 46–56
Cooperative anion recognition by a novel heteroditopic receptor based on
dibenzo[18]crown-6 fullero-bis(pyrrolidine)
Anish Kumar and Shobhana K. Menon*
Department of Chemistry, School of Sciences, Gujarat University, Ahmedabad 380009, Gujarat, India
(Received 16 June 2009; final version received 31 July 2009)
A novel C2 symmetrical fullero-bis(pyrrolidine) dibenzo[18]crown-6 conjugate 3 with trans-1 addition pattern on the
fullerene sphere has been prepared and characterised by elemental analysis, 1H NMR, 13C NMR and FAB-MS. NMR spectra
are consistent with the isolation of a single regioisomer, assigned to the trans-1 bis-adduct of C60. Preliminary coordination
studies by fluorimetry and UV–vis spectroscopy revealed that on inclusion of Kþ in the crown ether cavity, cyclophane
structured receptor 3 showed good selectivity in complexation towards H2PO24 as compared to other anions via favourable
electrostatic effects. Cyclic voltammetric studies demonstrate that the presence of Kþ amplifies the electrochemical
1
response of 3 towards H2PO24 as compared to other anions. These results were corroborated by H NMR titration study
which confirmed the binding site for both H2PO24 and Kþ in the receptor.
Keywords: fulleropyrrolidine; dibenzo[18]crown-6; dihydrogen phosphate; sensor; Prato reaction
Introduction
The ubiquity and importance of oxo-anions, in
particular dihydrogen phosphate anion, impart an urgency
to design receptors that can bind them efficiently as well as
selectively. Cooperativity for binding of H2PO24 is greatly
enhanced in the presence of bound cation, especially Kþ in
the receptor (6). Such receptors can be easily designed by
introduction of 18-crown-6 ether which is a well-known
ionophore for recognition of Kþ (7). Moreover, the
auxiliary hydrogen bonding sites can be constructed by
functionalising 18-crown-6 ether with a urea group having
NH as a hydrogen bond donor. The presence of an electron
acceptor unit can play a crucial role in discriminating
dihydrogen phosphate, which has got two OH groups,
from other structurally related anions such as sulphate and
acetate. In parallel, fullerenes, especially C60, are
particularly an attractive scaffold for a wide range of
applications (8). Its multiple reactive sites along with a
range of reported functionalisation techniques make it an
ideal candidate for preparation of new chemical entities
(9). Though generally studied for their biological
application and in preparation of artificial photosynthetic
systems, the development of fullerene derivatives for
recognition of ions is comparatively nascent (10).
Fulleropyrrolidines prepared by well-established Prato’s
1,3-dipolar cycloaddition reaction (11) could be an
interesting prospect in designing receptors for dihydrogen
phosphate anion because (a) C60 with its electron-
withdrawing nature can also efficiently interact with the
One of the emerging fields in coordination chemistry has
been the design of heteroditopic receptors for anionic and
cationic guest species (ion pair recognition) (1). A number
of artificial receptors have been developed for recognition
and sensing of either cations or anions; however, the
recognition of ion pairs by synthetic receptors remains a
less explored area. For recognition of ion pairs,
heteroditopic receptors having both cation and anion
binding sites have been studied. These receptors can
increase the lipophilicity of ionic guests and therefore
enhance ion pair solubility in non-polar media, leading to
their exploitation in both extraction and membrane
transport system (2). In addition, ion pair receptors are
capable of coordinating with biological molecules such as
zwitter ionic amino acids and peptides (3). In these
systems, the cation may be bound by a number of common
motifs, while the anion is coordinated using Lewis acidic,
electrostatic and/or hydrogen bonding interactions. Novel
cooperative and allosteric metal salt complexing behaviour
in which binding of metal cationic guests can
enhance, through electrostatic and conformational
effects, the subsequent coordination of the pairing
anion has been demonstrated by a number of ditopic
crown ether functionalised receptor systems (4). In
addition, such systems have recently been shown to
solubilise and transport alkali metal salts across lipophilic
membranes (5).
*Corresponding author. Email: shobhanamenon07@gmail.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2010 Taylor & Francis
DOI: 10.1080/10610270903254183
http://www.informaworld.com

Supramolecular Chemistry
47
hydroxyl group of dihydrogen phosphate (b). The use of
60 as a spectroscopic as well as electrochemical probe for
C
monitoring the cation as well as anion binding can also be
conceived.
Hence, a combination of 18-crown-6 ether, urea
linkage as well as fulleropyrrolidine would be highly
efficient and selective in recognising dihydrogen phos-
phate anion. With this perspective, we designed and
synthesised a dibenzo-18-crown-6 fullero-bis(pyrrolidine)
conjugate (Figure 1) with urea linkage, so that in addition
to Kþ, anion binding can be effected by a number of
factors such as electrostatic interaction with Kþ, hydrogen
bonding with the urea NH group (hydrogen bond donor)
and interaction of the non-bonding electrons of the OH
group with the fullerene cage. There are a few reports
where fullerene has been attached to crown ethers (12), but
there are no reports where fullerene derivatised crown
ether has been used as a ditopic receptor.
Figure 2. Regioisomers possible for bis-adducts on a fullerene
sphere.
Results and discussion
Synthesis and characterisation
Prato’s 1,3-dipolar cyclo-addition reaction was adopted to
synthesise the desired ditopic receptor 3 (Scheme 1).
The synthesis of dibenzo[18]crown-6 (15), nitration to
4,40-dinitrodibenzo[18]crown-6 (16) and reduction to 4,40-
diaminodibenzo[18]crown-6 (17) were carried out by
reported procedures. p-[1,3-Dioxolane-2-yl] benzoic acid
was prepared by a procedure similar to that reported by
Imahori and co-workers (18) (procedure given in
Supporting Information). The condensation of 4,40-
diaminodibenzo[18]crown-6 with p-[1,3-dioxolane-2-yl]
benzoic acid in the presence of diphenyl phosphoryl azide
(DPPA) and triethylamine (Curtius reaction) gave 1, which
on treatment with trifluoroacetic acid (TFA) in dichlor-
omethane gave crown ether-based dialdehyde 2 in ,70%
yield. Crown ether-based dialdehyde 2 was then refluxed
with C60 and sarcosine in toluene for 6 h to give the
dibenzo-18-crown-6 fullero-bis(pyrrolidine) conjugate 3
in ,33% yield. The desired trans-1 isomer was isolated
from other regioisomers by column chromatography, since
the trans-1 isomer was having a higher Rf value as
compared to others. One possible reason for the higher Rf
value of the trans-1 isomer could be the lower dipole
moment brought about by the symmetrical structure of the
molecule. All the intermediates as well as the final product
Taking note of the structural complications involved in
the design of anion recognition motifs, an easy-to-prepare
and functionalised cis-diamino[18]crown-6 was chosen as
the molecular structure to synthesise the desired ligand.
Moreover, the presence of the amino group made it
possible to introduce urea linkage by means of a modified
Curtius reaction (13). Fullerene C60 with its multiple
reactive sites made it possible to obtain a cyclophane-type
structure, which complements the binding of Kþ as well as
H2PO24 . Of all the regioisomers possible for the bis-adduct
of fullerene (14) (Figure 2), the trans-1 isomer has the best
architecture required for efficient cooperative binding of
both cation and anion, since the distance between the
cation binding site (crown ether cavity) and the anion
binding site (urea, fullerene cage) will be less as compared
to other isomers.
Herein, we report the first regioselective synthesis of a
crown ether–urea–C60 hybrid system with a simple and
modular strategy along with studies that justify its use as a
heteroditopic receptor.
1
were characterised by elemental analysis, H NMR, 13C
NMR and FAB-MS/EI-MS. The trans-1 geometry of the
synthesised compound was supported by the fact that only
1
two signals were obtained in the H NMR spectra for the
urea protons, indicating a C2 symmetry in the molecule.
Moreover, the chemical shift values at d 5.25 and 4.45 as
doublets (J ¼ 9.3 Hz) for CH2 and at d 2.85 corresponding
to ZNCH3 protons of the pyrrolidine ring clearly support
the trans-1 geometry of the synthesised molecule.
13C NMR showed only 28 signals corresponding to sp2
carbons of the fullerene cage, which is also an indication of
the symmetrical trans-1 geometry. The trans-1 geometry of
the synthesised molecule was more or less confirmed by
comparing its UV–vis spectra in the 400–700 nm region
with the previously reported Bingel (19) as well as
Prato bis-adducts (20) (see Supporting Information).
Figure 1. Structure of dibenzo-18-crown-6 fullero-
bis(pyrrolidine) conjugate 3.

48
A. Kumar and S.K. Menon
Scheme 1. Synthesis of receptor 3.
Moreover, its spectral properties were similar to the trans-1
fullerene dibenzo-18-crown-6 conjugate reported by
Echegoyen and co-workers (21).
Complexation studies of the receptor
The ability of receptor 3 to bind both cations and anions
was studied. The addition of tetrabutylammonium
hexafluorphosphate (TBAPF6) to receptor 3 does not
cause any change in either its photophysical or electro-
chemical properties. Hence, tetrabutylammonium cation
1
More insight into the H NMR data for 3 (Figure 3)
showed peaks at d 8.65 and 8.35 as singlets corresponding
to NH1 and NH2, respectively, integrating for two protons
each. The aromatic protons appeared as multiplet between
d 7.0 and 6.6. The ZOCH2Z protons of the crown ether
ring appeared between d 4.1 and 3.7 as multiplet. Other
peaks like singlet at 4.89 corresponding to ZCH and
doublets at 5.25 and 4.45 corresponding to H8CH and
H10CH, respectively, of the pyrrolidine ring were also
observed. The peaks corresponding to aromatic protons
have been resolved to observe doublets for protons H3
(4J ¼ 1.5 Hz), H4 (3J ¼ 8.3 Hz) and H7 (3J ¼ 8.3 Hz),
while doublet of doublets appeared for H5 (3J ¼ 8.4 Hz,
4J ¼ 1.5 Hz) and H6 (3J ¼ 8.2 Hz, 4J ¼ 1.2 Hz). 13C NMR
showed a peak at d 154.7 corresponding to the C of the
urea linkage. The peaks corresponding to sp3 C of C60
were observed at 75.0 and 68.3, respectively. FAB-MS
showed a molecular ion peak at 1458 (Mþ) and a base peak
at 720 for C60. Mass fragmentation pattern of 3, FAB-MS,
1H NMR, 13C NMR and FT-IR spectra of 3 are given in
the Supporting Information.
Figure 3. Numbered protons of compound 3 in the order in
which they appear in the 1H NMR spectra (downfield to upfield).

Supramolecular Chemistry
was chosen as the counterion for anion recognition while
49
hexafluorphosphate anion was chosen as the counterion for
cation recognition studies. The presence of the fullerene
moiety in the receptor having characteristic photophysical
and electrochemical properties made it possible to study
the ion binding by UV–vis, fluorimetric as well as
electrochemical techniques. Moreover, the presence of
crown ether and urea NH protons enabled us to study ion
1
binding by H NMR spectral analysis.
Photophysical studies
Fluorescence studies were carried out by excitation at
450 nm (where only the fullerene cage is excited) of a
solution of receptor 3 (1 £ 1025 M) in benzonitrile with
emission at 708 nm corresponding to decay of the singlet
excited state of the fullerene cage. The effect of cations on
fluorescence was observed by adding a solution of Mþ
(2 £ 1025 M) to receptor 3 (1 £ 1025 M) in benzonitrile.
Figure 4. Fluorescence spectþra of receptor 3 (1 £ 1025 M) in
the absence and presence of K (2 £ 1025 M) in benzonitrile at
298 K.
not cause an appreciable change in the emission spectra.
Only a slight increase in fluorescence intensity was observed
in the case of dihydrogen phosphate, which was not enough
to draw any sort of conclusion. One possible reason for
such observation could be the repulsive interaction of the
anion with the crown ether O atoms, thus preventing the
urea group of the receptor from interacting with the anions.
Hence, anion complexation studies were carried out by
mixing an equimolar solution of cation (Kþ) and receptor 3
with subsequent addition of anions. It was found that the
fluorescence intensity at 708 nm increased remarkably on
complexation with H2PO24 , whereas other anions
ðAcO2; HSO42; ClO24 Þ do not cause too much perturbation
in the fluorescence spectra (Figure 5). These observations
indicate that there is efficient binding of H2PO24 by the 3:Kþ
complex. The O atoms of the crown ether ring, which were
responsible for repulsive interaction with anions, were now
involved in the non-covalent interaction with Kþ, thus
making way for better interaction of dihydrogen phosphate
anion with the urea group of the receptor.
Metal ions Mnþ (Naþ, Kþ, Mg2þ, Ca2þ, Ba2þ, Cu2þ, Ni2þ
)
were added as hexafluorophosphate salts. It was found that
the fluorescence intensity of 3 decreased on addition of Kþ
ion (F ¼ 3.7 £ 1024 to 2.6 £ 1024), while other cations did
not cause too much change in the spectra of 3. This
observation can be attributed to the quenching of the singlet
excited state of the fullerene cage on inclusion of Kþ in the
crown ether cavity (Figure 4). The stoichiometry of
complexation of 3 with Kþ was found to be 1:1 while the
association constantwas found to be3580M21 (seeTable 1).
Apart from benzonitrile, other solvent systems such as
toluene, chloroform, THF and methylcyclohexane were also
used for fluorimetric analysis (see Supporting Information).
But since the shift in emission spectra of 3 on addition of Kþ
was best observed in benzonitrile, further studies were
carried out in the benzonitrile solvent system.
On the contrary, addition of 2 equiv. of anions
A2ðH2PO42; HSO42; AcO2; ClO24 ; Cl2 and Br2Þ as tetra-
butylammonium salt to solution of 3 in benzonitrile does
Table 1. Photophysical properties of uncomplexed and complexed receptor 3 along with the respective association constants.
Compound
Absorptiona (lmax) nm
Emission quantum yieldb F ( £ 1024
)
Association constantc K (M21
)
3 (1 £ 1025 M)
438
433
443
435
434
433
433
433
439
329
331
3.70
2.60
3.65
3.00
2.80
2.69
2.67
2.69
3.60
–
3þKþþ (1 equiv.)
3580
2456
650
515
276
180
276
20
3þK þH2PO24 (4 equiv.)
3þKþþþAcO2 (4 equiv.)
3þK þHSO24 (4 equiv.)
3þKþþCIO24 (4 equiv.)
3þKþþþCl2 (4 equiv.)
3þK þBr2 (4 equiv.)
3þH2PO24 (4 equiv.)
1þKþþ (1 equiv.)
–
–
1þK þH2PO24 (4 equiv.)
–
a Solvent system CHCl3ZCH3CN (4:1), 258C.
b lmax ¼ 708 nm, solvent system benzonitrile, 258C.
c Association constant calculated by the Benesi–Hildebrand plot for a 1:1 stoichiometry.

50
A. Kumar and S.K. Menon
Figure 5. Fluorescence spectra of ligand 3:Kþ complex
(1 £ 1025 M) in the presence of (a) H2PO24 , (b) AcO2, (c)
HSO24 and (d) ClO24 (5 £ 1023 M) in benzonitrile at 298 K.
Figure 7. Absorption spectra of 3:Kþ (1 £ 1025 M) upon
addition of 4 equiv. of: (a) H2PO24 , (b) AcO2, (c) HSO42 and (d)
without anions in CHCl3ZCH3CN (4:1) at 298 K.
Fluorimetric titrations were carried out to evaluate the
binding constant and stoichiometry of anion complexation
by 3:Kþ. Figure 6 shows the change in fluorescence of
3:Kþ on increasing the concentration of H2PO24 .
A plot of 1/(F 2 F0) ! 1/[A2] produced a straight line
for titration with H2PO24 , indicating the formation of a 1:1
complex (see Supporting Information). F0 is the
fluorescence intensity of 3:Kþ and F is the fluorescence
intensity after complexation with H2PO24 . The association
constant Ka can be found by the following modified
Benesi–Hildebrand equation using the fluorescence data:
host–guest complexation in the case of H2PO24 with Ka
evaluated as 2456 M21. The Ka for H2PO42 binding was
much higher as compared to other anions. The binding
constant values Ka calculated by fluorimetric analysis have
been summarised in Table 1 (fluorimetric titration curves
3:Kþ with AcO2 and HSO24 are given in the Supporting
Information).
The UV–vis spectra of receptor 3 (1 £ 1025 M) in
CHCl3ZCH3CN (4:1) showed characteristic fullerene
peaks at 202, 308, 438 and 471 nm, which extend all the
way to the energetically low-lying transition at 706 nm.
The addition of KPF6 (1 equiv.) to a solution of 3 induces a
moderate blue shift of 5 nm (438–433 nm) to the charge
transfer band of the UV–vis spectra. Anion complexation
studies were carried out on potassium cation-complexed
1
1
1
;
¼
þ
F 2 F0 ½A2ꢀKaa
a
where [A2] is the concentration of added anion, F0 and F
are the fluorescence intensity of the 3:Kþ complex in the
absence and presence of anions and a is a constant. The Ka
value was obtained from the slope and intercept of the
double reciprocal plot. The plot was linear confirming 1:1
Figure 6. Fluorescence spectra of ligand 3:Kþ complex
(1 £ 1025 M) in the presence of (a) 0 mM, (b) 0.2 mM, (c)
0.4 mM, (d) 1.5 mM, (e) 2.0 mM and (f) 2.5 mM of H2PO42
(dihydrogen phosphate) anions in benzonitrile at 298 K.
Figure 8. Change in UV–vis spectra of 3:Kþ (1 £ 1025 M)
upon increasing concentration of H2PO24 in CHCl3ZCH3CN
(4:1) at 298 K.

Supramolecular Chemistry
51
On successive addition of a solution of H2PO24 (0, 2, 4,
6 and 10 equiv.) to the solution of 3:Kþ in CHCl3ZCH3CN
(4:1), the absorbance of the peak at 433 nm decreased while
the absorbance of the new peak that appeared at 443 nm was
found to increase (Figure 8). The isobestic point at 436 nm
indicates the absence of any long-lived intermediates.
These results indicate that the 3:Kþ complex can bind and
discriminate between H2PO42 anion and structurally related
HSO24 and AcO2 efficiently. UV–vis titrations of 3:Kþ
with H2PO24 showed the formation of a 1:1 host–guest
complex from the nonlinear least-square analysis of the
titration curve (Figure 9). The role of the fullerene moiety in
complexation was also analysed by observing the changes
in fluorescence as well as UV–vis spectra of intermediate 1
on complexation with Kþ and H2PO42. It was found that
H2PO24 induced only minor perturbation in the spectra of
the 1:Kþ complex. The change in photophysical properties
of compound 3 and 3:Kþ complex on interaction with
anions has been summarised in Table 1.
Figure 9. Change in absorbance at 443 nm for 3:Kþ
(1 £ 1025 M) with an increase in the concentration of anions in
CHCl3ZCH3CN (4:1) at 298 K.
receptor 3, because addition of anions ðH2PO24 ; AcO2Þ
to the solution of uncomplexed 3 does not cause a
noticeable shift in the spectra. Figure 7 shows the
absorption spectra of the 3:Kþ complex in the absence and
presence of 4 equiv. of oxo-anions ðH2PO24 ; AcO2Þ. It was
observed that the addition of HSO24 and AcO2 induces a
small bathochromic shift of the peak at 433 nm, while the
addition of H2PO24 causes a comparatively large bath-
ochromic shift of 10 nm (433–443 nm). The shifts caused
by the addition of chloride as well as bromide anions were
also negligible. This shift can be ascribed to the occurrence
of an H-bond interaction involving the two NH fragments
of the urea subunit and the two oxygen atoms of the
dihydrogen phosphate anion. The electron density is
transferred to the urea moiety, which increases the
intensity of dipole and hence shifts the charge transfer
band to a longer wavelength.
Electrochemical studies
In order to gain more insight into the cooperative
enhancement of anion recognition by the presence of
bound cation, electrochemical studies were carried out.
The electrochemical response of 3 (1 £ 1024 M) as well as
3þKþ (1 £ 1024 M) to anion complexation was studied by
cyclic voltammetry in 4:1 CHCl3ZCH3CN with TBAPF6
as the supporting electrolyte. Receptor 3 exhibited three
quasireversible reduction peaks corresponding to the
reduction of the fullerene core (Figure 10(a)). A slight
anodic shift in the reduction wave was observed in 3þKþ
(Figure 10(b)) as compared to free receptor 3, indicating
the increased tendency of the fullerene core to undergo
reduction in the receptor 3:Kþ complex. This change can
be attributed to the electrostatic effect of the Kþ ion bound
Figure 10. Cyclic voltammogram of (a) 3 (1 £ 1024 M), (b) 3þKþ (1 equiv.) and (c) 3þKþþH2PO24 (1.5 equiv.) in CHCl3ZCH3CN
(4:1) at 298 K.

52
A. Kumar and S.K. Menon
in close proximity to the fullerene sphere. The influence of
dihydrogen phosphate as well as other anions on the
reduction waves of the fullerene cage was studied by
adding a solution of tetrabutylammonium salts of anions to
the solution of 3þKþ. A cathodic shift was observed in the
case of dihydrogen phosphate while the other anion does
not cause too much shift. This anion-induced cathodic
shift (Figure 10(c)) can be attributed to the complexation
of the anion near the fullerene cage, hence retarding its
reduction. The change in the redox potential of compound
3 on inclusion of Kþ as well as on interaction with anions
has been summarised in Table 2. Hence, it has been proved
by both photophysical and electrochemical studies that the
synthesised crown ether fulleropyrrolidine is an efficient
heteroditopic receptor for Kþ and H2PO42 ions. However,
the binding mode of dihydrogen phosphate by the receptor
3:Kþ complex could not be made out from either of these
studies. Hence, NMR titration was carried out to study the
influence of inclusion of dihydrogen phosphate as well as
potassium cation on the chemical shifts of the protons of
the ligand.
measurement and electrochemical methods. These results
indicate that dihydrogen phosphate anion is bound to the
urea linkage in the 3:Kþ complex, with the urea group
acting as a H-bond donor. On increasing the concentration
of the dihydrogen phosphate added, the urea signals were
shifted more downfield and were broadened (Figure 11).
A signal at d 2.38 was observed for the proton of
dihydrogen phosphate anion, which is at the upfield
position as compared to dihydrogen phosphate alone,
clearly indicating some kind of interaction of the OH
1
group of H2PO24 with the fullerene cage. The H NMR
titration data of 3:Kþ with AcO2 and HSO24 are given in
the Supporting Information. It was found that the peaks
corresponding to protons of the urea linkage did undergo a
downfield shift but the shift was much less when compared
with H2PO24 . The change in the chemical shift for protons
of ligand 3, after complexation with potassium cation and
subsequently with dihydrogen phosphate anion, has been
described in Table 3. A 1:1 binding stoichiometry with
H2PO24 and other oxo-anions by 3:Kþ was confirmed by
Job plots (22).
NMR titrations
Conclusion
Receptor 3 was titrated with dihydrogen phosphate anion
(added as a tetrabutylammonium salt) in the absence and
presence of Kþ (added as a hexafluorophosphate salt). The
choice of the solvent considering the possibility of species,
such as receptor, ion-pair salt as well as cation–anion–
receptor complex in the solution, was selected as 1:1
CDCl3–CD3CN. The addition of the KPF6 salt (1.5 equiv.)
to the solution of receptor 3 (1 £ 1025 M) caused a
significant upfield shift (Dd ¼ 0.23) in crown ether ZCH2
signals in 1H NMR spectra, which is in confirmation with
the inclusion of Kþ with the crown ether cavity. On
titration with dihydrogen phosphate anions, it was found
that the presence of Kþ significantly increases the
magnitude of the anion-induced perturbation of urea
protons (which were shifted downfield). Moreover, the
addition of dihydrogen phosphate alone does not cause too
much shift in the urea NH signals, which is in good
agreement with the results obtained from photophysical
Hence, from photophysical, electrochemical and 1H NMR
titration experiments, it has been ascertained that the
synthesised dibenzo[18]crown-6 fullero-bis(pyrrolidine)
conjugate 3 on complexation with potassium cation can
efficiently recognise and discriminate dihydrogen phos-
phate from other structurally similar anions. Moreover, 1H
NMR titration studies have revealed that NH groups of urea
linkage in the ligand act as H-bond donors. Additional
factors such as the electrostatic effect from the bound cation
(Kþ) and electron-accepting nature of C60 made the
receptor 3:Kþ complex highly efficient in recognising
H2PO24 . Based on these results, the structure of the complex
will be Kþ in the crown ether cavity and H2PO42 in the space
between crown ether ring and fullerene with H-bonding
between O and H of the urea group in the receptor and OH
groups of H2PO24 pointed towards the fullerene cage.
Furthermore, the linker between the crown ether and the
fullerene moiety could be further modified to incorporate
Table 2. Voltammetric data of 3 (1 £ 1024 M) upon complexation with Kþ and anions, obtained in CHCl3ZCH3CN (4:1) at 298 K.
a
a
Compound
DE11=2a (mV)
DE21=2 (mV)
DE31=2 (mV)
Min. equiv.b
Max. equiv.c
3þKþþd
60
259
56
257
58
255
0.1
0.1
1
1.5
e
3þK þH2PO24
3þKþþþAcO2e
228
224
227
222
227
223
0.25
0.5
1.0
1.0
e
3þK þHSO24
a Difference calculated with reference to E1/2 values of compound 3 (E1 ¼ 21.46 V, E12=2 ¼ 21.81 V, E31=2 ¼ 22.28 V).
1=2
b Minimum equivalents of salt required to give detectable electrochemical response.
c Maximum equivalents required to saturate voltammetric response.
d Added as the hexafluorophosphate salt.
e Added as the tetrabutylammonium salt.

Supramolecular Chemistry
53
Figure 11. 1H NMR spectral changes observed for the protons of 3þKþ (1 £ 1025 M), in CDCl3–CD3CN (1:1), after the addition of
(a) 0, (b) 0.5, (c) 1.0 and (d) 1.6 equiv. of H2PO24 . B represents the protons under consideration. † represents the protons of dihydrogen
phosphate.
other anion recognition elements (e.g. thiourea, amides,
sulfanamides, etc.) as such, additional fine tuning of anion
binding properties may be envisioned. This work opens
interesting perspectives for the design of efficient C60-
based sensors for ion-pair recognition.
60-F254. Melting point was taken on Veego (VMP-DS)
using a Mel-Temp apparatus. Silica gel (Merck, 0.040–
0.063 mm) was used for column chromatography. FAB-
MS was recorded on a Jeol SX-102/DA 600 mass
spectrometer using argon/xenon as the accelerating gas
and nitrobenzylalcohol as the matrix. EI-MS were
recorded on a SHIMADZU QP-5050A. 1H and 13C
NMR spectra were recorded on a DRX 400 spectropho-
tometer operating at 400 and 125 MHz, respectively, with
TMS as the internal standard. The following abbreviations
for stating the multiplicity of the signals in the NMR
spectra were used: s (singlet), d (doublet), dd (doublet of
doublet), t (triplet), m (multiplet), Cq (quaternary carbon).
1H and 13C NMR spectra were referenced to the residual
Experimental
Materials and methods
All the chemicals including C60 were purchased from
Sigma Aldrich and E. Merck and were of analytical grade.
The cycloaddition reaction was performed under argon.
The cycloaddition reactions as well as other preceding
steps were monitored by TLC using Merck silica gel
Table 3. 1H NMR chemical shifts and chemical shift changes of ligand 3 (1 £ 1025 M) protons on complexations with Kþ and on
subsequent complexation with H2PO24 (1.6 equiv.).
b
Protons
3 (d ppm), d1
3þK þa (d ppm), d2
Dd ppm, d22d1
31Kþaþ H2PO42 (d ppm), d3
Dd ppm, d32d2
1
2
3
4
5
6
7
8
8.65
8.35
6.99
6.88
6.75
6.66
6.59
5.25
8.48
8.20
7.01
6.90
6.77
6.67
6.60
5.26
20.17
20.15
0.02
0.02
0.02
0.01
0.01
0.01
0.01
10.17
9.97
7.04
6.92
6.78
6.69
6.61
5.08
4.75
1.69
1.77
0.03
0.02
0.01
0.03
0.02
20.18
20.16
20.18
0.08c
20.26
9
10
11, 12
13
4.90
4.45
4.91
4.47
0.02
0.23c
20.01
4.29
3.97–3.56
2.58
4.12–3.70
2.85
3.89–3.48
2.84
a Added as KPF6 (1.5 equiv.).
b Added as TBAH2PO4 (1.5 equiv.).
c Calculated as the average change for all the crown ether protons.

54
A. Kumar and S.K. Menon
solvent peak (1H d(CHCl3)) at 7.27 ppm and to the solvent
peak (13C d(CDCl3)) at 77.23 ppm. The absorption spectra
were recorded on a Hitachi U-3210 UV–visible
spectrophotometer using a 10-mm quartz cell. Elemental
analysis was done on a Carlo Erba 1108 analyser. FT-IR
spectra were recorded on a BRUKER-TENSOR 27 FT-IR
spectrophotometer as KBr pellets. Cyclic voltammograms
were recorded on a CH Instruments 620A electrochemical
analyser using a three-electrode system. All the solutions
were purged prior to electrochemical and spectral
measurements using nitrogen gas. For all electrochemical
studies, the voltammetric cell was maintained at
25.0 ^ 0.28C. Nitrogen was passed through all the
solutions for 20 min to remove oxygen. A platinum
electrode was used in all experiments as the working
electrode. A silver–silver chloride reference electrode and
a platinum wire counter electrode were used in all
experiments. The scan rate for cyclic voltammetry was
100 mV/s and the supporting electrolyte was TBAPF6
(0.1 M). The square wave pulse was 25 mV, the step height
was 4 mVand the square wave period was varied randomly
over a range of values. All of the potential values reported
are relative to the Fcþ/Fc couple at room temperature. In
1H NMR titrations, aliquots of anions (tetrabutylammo-
nium salts, 0.5 M, 2.5 £ 1024 M in 0.5 ml of the deuterated
solvent) were added to a solution of receptor 3 (0.01 M,
5 £ 1026 in 0.5 ml of the deuterated solvent). Thirteen
aliquots were added (0, 2, 4, 5, 6, 8, 10, 12, 14, 16, 18, 20,
30 ml) corresponding to 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0, 1.2,
1.4, 1.6, 1.8, 2.0, 3.0 equiv. of anion. For titration in the
presence of potassium cation, the above procedure was
repeated for addition of anions to a mixture of receptor 3
(5 £ 1026 in 0.4 ml of the deuterated solvent) and KPF6
(5 £ 1026 in 0.4 ml of the deuterated solvent).
4J(H,H) ¼ 1.7 Hz, 2H, ArH), 6.88 (d, 3J(H,H) ¼ 8.1 Hz, 2H,
3
4
ArH), 6.77 (dd, J(H,H) ¼ 8.2 Hz, 2H, J(H,H) ¼ 1.3 Hz,
3
4
ArH), 6.68 (dd, J(H,H) ¼ 8.3 Hz, J(H,H) ¼ 1.2 Hz, 4H,
3
ArH), 6.58 (d, J(H,H) ¼ 8.5 Hz, 4H, ArH), 5.90 (s, 2H,
OZCHZO), 4.3 (s, 8H, ZOCH2ZCH2OZ), 4.25–4.18 (m,
8H, ZOCH2 crown ether), 4.06–4.00 (m, 8H, ZOCH2
crown ether); 13C NMR (125MHz, CDCl3, Me4Si, 298 K) d
(ppm) 152.2 (2C, CONH), 144.7 (2C, Ar-Cq), 140.1 (2C, Ar-
Cq), 137.3 (2C, Ar-Cq), 132.8 (2C, Ar-Cq), 130.4 (2C, Ar-
Cq), 127.1 (2C, Ar-CH), 120.3 (2C, Ar-CH), 115.0 (2C, Ar-
CH), 114.1 (2C, CH ofdioxolane), 112.6 (2C, Ar-CH), 106.0
(2C, Ar-CH), 73.3 (4C, CH2 of dioxolane), 72.7–70.5 (8C,
crown ether); EI-MS m/z (relative intensity) 772 (Mþ , 100),
320 (60); Anal. calcd for C40O12H44N4: C, 61.38; H, 5.62; N,
6.90. Found: C, 61.50; H, 5.71; N, 6.50.
1,10-(6,7,9,10,17,18,20,21-Octahydrodibenzo[b,-
k][1,4,7,10,13,16]hexaoxacyclooctadecine-2,14-diyl)
bis(3-(4-formylphenyl)urea) 2. Compound 1 (350 mg,
0.01 mol) was mixed with 2.5 ml TFA in 75 ml of
dichloromethane and 10 ml of water. The biphasic mixture
was stirred at room temperature for 12 h. The solution was
diluted with dichloromethane and washed with saturated
sodium bicarbonate until the organic phase became light
brown. The organic layer was separated and dried over
Na2SO4. The solvent was removed in vacuo, and the light
yellow solid residue was purified by column chromatog-
raphy on silica gel with toluene–methanol ¼ 50/1 to get
compound 2 as a pure white product (230 mg; 70%); mp
1588C (dec); IR (KBr; cm21) 1130, 1262, 1638, 1710,
3320; 1H NMR (400 MHz, CDCl3, Me4Si, 298 K) d (ppm)
9.91 (2H, s, ZCHO), 8.60 (s, 2H, ZCONH-Ar), 8.40 (s,
4
2H, ZCONH-Ar), 6.95 (d, J(H,H) ¼ 1.6 Hz, 2H, ArH),
6.89 (d, 3J(H,H) ¼ 8.1 Hz, 2H, ArH), 6.74 (dd,
4
3J(H,H) ¼ 8.1 Hz, J(H,H) ¼ 1.6 Hz, 2H, ArH), 6.64 (dd,
4
3J(H,H) ¼ 8.5 Hz, J(H,H) ¼ 1.3 Hz, 4H, ArH), 6.58 (d,
3J(H,H) ¼ 8.4 Hz, 4H, ArH), 4.30–4.20 (m, 8H, ZOCH2
crown ether), 4.00–3.91 (m, 8H, ZOCH2 crown ether);
13C NMR (400 MHz, CDCl3, Me4Si, 298 K) d 190.0 (2C,
CHO), 152.2 (2C, CONH), 144.7 (2C, Ar-Cq), 140.1 (2C,
Ar-Cq), 137.3 (2C, Ar-Cq), 132.8 (2C, Ar-Cq), 130.4 (2C,
Ar-Cq), 127.8 (2C, Ar-CH), 120.3 (2C, Ar-CH), 115.0
(2C, Ar-CH), 112.6 (2C, Ar-CH), 106.0 (2C, Ar-CH),
73.3–70.7 (8C, crown ether); EI-MS m/z (relative
intensity) 685 (Mþ1, 100), 320 (57); Anal. calcd for
C36O10H36N4: C, 63.15; H, 5.26; N, 8.18. Found: C, 63.50;
H, 5.41; N, 8.10.
Procedure for preparation of dibenzo[18]crown-6 fullero-
bis(pyrrolidine) conjugate 3 and intermediates 1 and 2
Synthesis of 1,10-(6,7,9,10,17,18,20,21-octahydrodiben-
zo[b,k][1,4,7,10,13,16]hexaoxacyclooctadecine-2,14-
diyl) bis(3-(4-(1,3-dioxolan-2-yl)phenyl)urea) 1. A solution
of 4,40-diaminodibenzo-18-crown-6 (390 mg, 1 mmol),
p-[1,3-dioxolane-2-yl] benzoic acid (388 mg, 2 mmol),
DPPA (800mg, 3 mmol) and Et3N (700mg, 7 mmol) in
benzene (150ml) was heated under reflux for 1.5 h. After
adding methanol (30 ml), the reaction mixture was further
heated for 2 h and then cooled, concentrated invacuo to get a
white solid product. The product was then dissolved in
chloroform, washed with dilute HCl, dried over Na2SO4,
filtered and concentrated under vacuum. The residue was
chromatographed on silica gel with EtOAc–hexane ¼ 1/25
to afford compound 1 as a white solid product (430mg;
55%); mp 1898C (dec); IR (KBr; cm21) 1130, 1262, 1638,
3257; 1H NMR (400MHz, CDCl3, Me4Si, 298 K) d (ppm)
8.70 (s, 2H, ZCONH-Ar), 8.35 (s, 2H, ZCONH-Ar), 7.0 (d,
Dibenzo[18]crown-6 fullero-bis(pyrrolidine) conju-
gate 3. A mixture of dialdehyde 2 (151 mg, 0.22 mmol),
N-methylglycine (40.2 mg, 0.44 mmol) and C60 (120 mg,
0.17 mmol) was refluxed in dry toluene in inert atmosphere
for 6 h. After cooling to room temperature, the brown
mixture was purified by column chromatography (silica
gel) set-up with a gradient of toluene to toluene–ethyl
acetate (20:1) as the eluant. Adduct 3, a brown solid, was
dried under vacuum and characterised by elemental

Supramolecular Chemistry
55
(HSO42, AcO2, ClO42); 1H NMR titration plots of 3þKþ
stoichiometry of H2PO24 complexation by 3:K .
analysis and spectroscopic methods (114 mg; 33%); mp
2288C (dec); IR (KBr; cm21) 526 (C60), 1429 (C60) 1634
(urea CO stretching), 1134 (crown CZOZC stretching),
1268 (crown Ar-OZC stretching), 3250 (urea NH
stretching); UV–vis (CHCl3ZCH3CN) lmax (log 1) 202
(4.88), 308 (4.52), 405 (sh, 3.74), 438 (3.61), 471(3.64),
540 (sh, 2.89), 706 (2.00) nm; 1H NMR (400 MHz, CDCl3,
Me4Si, 298 K) d (ppm) 8.65 (s, 2H, NH of urea on crown
ether side), 8.35 (s, 2H, NH of urea on fullerene side), 6.97
(d, 4J(H,H) ¼ 1.5 Hz, 2H, Ar-H), 6.88 (d,
with HSO24 , AcO2; Benesi–Hildebrand plþot for
a 1:1
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urea group), 154.1 (2C), 153.5 (2C), 151.4 (2C), 150.8
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(4C), 145.7 (2C), 145.6 (2C), 145.3 (2C), 144.6 (2C),
144.0 (2C, Ar-Cq), 142.9 (2C), 142.1 (2C), 141.8 (2C),
141.7 (2C), 141.3 (2C), 140.6 (2C), 140.1 (2C, Ar-Cq),
139.4 (2C), 138.9 (2C), 138.7 (2C, Ar-Cq), 136.4 (2C),
135.8 (2C), 135.7 (2C), 132.8 (2C, Ar-Cq), 130.5 (2C, Ar-
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Ar-CH), 112 (2C, Ar-CH), 106 (2C, Ar-CH), 83.3 (2C,
NCH of the pyrrolidine ring), 77.0 (solvent), 75.0 (2C, sp3
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Acknowledgements
We are thankful to the UGC for the financial assistance and
CDRI, Lucknow for the spectral analysis. A. Kumar would like to
thank the Council of Scientific and Industrial Research (CSIR),
New Delhi for the Senior Research Fellowship.
Supporting Information
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FAB-MS, 1H NMR, 13C NMR and FT-IR spectra of 3; mass
fragmentation pattern of 3; UV–vis spectra of bingel bis-
adduct trans-1 C62 (CO2Et)4 and synthesised compound 3 in
CHCl3; change in fluorescence spectra of 3 upon addition of
Kþ in different solvents; stoichiometry of the compleþx
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