Improved recognition of alkylammonium salts by ion pair recognition based on a novel heteroditopic pillar[5]arene receptor

Article abstract of DOI:10.1016/j.tetlet.2012.09.043

A novel pillar[5]arene-based heteroditopic receptor for ion pair recognition of alkylammonium salts with different alkyl chain lengths and different counterions was prepared, which showed the best association constant enhancement (71 times) to n-BuNH₃⁺Cl^- compared with monotopic receptor 1,4-dimethoxypillar[5]arene over corresponding Br ^- and CF₃COO^- salts in chloroform.

Full text of DOI:10.1016/j.tetlet.2012.09.043

Tetrahedron Letters 53 (2012) 6409–6413
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Improved recognition of alkylammonium salts by ion pair recognition based
on a novel heteroditopic pillar[5]arene receptor
⇑
Mengfei Ni, Yangfan Guan, Lin Wu, Chao Deng, Xiaoyu Hu, Juli Jiang, Chen Lin , Leyong Wang
Key Laboratory of Mesoscopic Chemistry of MOE, Center for Multimolecular Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2012
Revised 5 September 2012
Accepted 11 September 2012
Available online 19 September 2012
A novel pillar[5]arene-based heteroditopic receptor for ion pair recognition of alkylammonium salts with
different alkyl chain lengths and different counterions was prepared, which showed the best association
+
À
constant enhancement (71 times) to n-BuNH
3
Cl compared with monotopic receptor 1,4-dimethoxypil-
À
À
lar[5]arene over corresponding Br and CF
3
COO salts in chloroform.
Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
Keywords:
Supramolecular chemistry
Ion pair recognition
Pillar[n]arene
Alkylammonium
Heteroditopic receptor
The research on cation receptors based on host–guest chemistry
has attracted considerable interest in supramolecular chemistry
due to their great importance in biological, analytical, catalytic,
environmental applications, drug delivery, molecular machines,
pairs. Due to the superior recognition ability and selectivity to
ion pairs, heteroditopic receptors have been applied for helping
salt dissolution, extraction, and transportation. Nevertheless, to
6
the best of our knowledge, heteroditopic receptors based on
macrocycles for organic cation recognition especially with the im-
proved binding affinity by ion pair recognition are rarely reported
1
and so on. When the cation recognition processes occur in apolar
solvents, most recognition studies of cation guests were carried out
with their low-coordinating counterions such as tetrafluoroborate
and hexafluorophosphate where the counterion effect could be
neglected, ascribed to their good solubility and weak ion-pairing
binding affinity in apolar solvents. However, those low-coordinating
counterions were hardly observed in our nature environment and
real life system, and as a matter of fact, cation guests usually exist
7
and most of them are based on crown ethers or calixarenes. For
8
instance, Huang and co-workers reported dibenzo-24-crown-8
derivatives for the improved recognition of dibenzylammonium
salts. So we are interested in developing a novel heteroditopic
receptor based on new-type macrocycle host for organic cation
recognition with improved binding affinity by ion pair recognition.
Recently, a new class of macrocycle host named pillar[n]arene9
has attracted much attention of supramolecular chemists, and the
emergence of monofunctionalized pillar[5]arene has expanded the
application of pillar[n]arene in the areas of molecule recognition,
À
3À
À
2
with Cl , PO , HCO3 etc. as counterions, where the coordinating
4
power of the counterions could not be neglected due to their
Coulomb interaction with the cation guests. As a consequence, it
is quite important to study the recognition of cation guest with
competitive coordinating counterions in apolar solvents for well
understanding the role of counterions in recognition.3
self-assembly, supramolecular polymer construction, and so
9j,k,p
on.
Unlike basket-shaped calix[n]arene, this novel host is
In the cation recognition process in apolar solvents, the coun-
terion can inhibit the cation recognition. However, on the other
hand, the binding affinity can be improved by employing heterodi-
topic receptors, which consist of two different binding sites, thus
capable of binding two ionic partners of an ion pair simulta-
neously. In 1991, Reetz and co-workers5 firstly reported a good
example of heteroditopic receptors for potassium associated ion
para-bridged and displays perfectly symmetric architecture. Be-
cause of the column-shaped and -electron rich cavity, pil-
lar[n]arene can form inclusion complexes with organic cations
p
9
a,d
9g,n
like paraquats,
alkylammoniums,
and bis(imidazolium)
9
f
derivatives. Among those organic cation guests, the recognition
of alkylammoniums is rather significant and also full of challenge,
because of their physiological activity and applications in many
4
1
0
1
1
fields such as herbicide detection.
Therefore, in order to prove that the new designed pil-
lar[5]arene based heteroditopic receptor could improve the recog-
nition of the alkylammounium salts compared to monotopic one,
⇑
Corresponding author. Tel.: +86 025 83592529; fax: +86 025 83597090.
E-mail address: linchen@nju.edu.cn (C. Lin).
0
040-4039/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.043

6
410
M. Ni et al. / Tetrahedron Letters 53 (2012) 6409–6413
the urea unit that is well known as a good anion receptor was
introduced to the pillar[5]arene host to achieve a novel heterodi-
topic pillar[5]arene receptor 1. With two different binding sites, a
changes of protons H
À2.88, À2.35, À0.83, and À2.30 ppm, respectively. Meanwhile,
more importantly, urea protons H and H of 1 shifted downfield
a
, H
b
, H
c
, H
d
, and NH
3
of G4c were À2.12,
a
b
p-electron rich cavity and a urea group, it was desired that hetero-
significantly from 5.04, 7.19 ppm to 6.58, 8.07 ppm, respectively,
ditopic pillar[5]arene receptor 1 could bind both the ionic partners
of alkylammonium salts (G) cooperatively, leading to dramatically
improved recognition of the alkylammoniums (Fig. 1). On the other
hand, this is also the first example of heteroditopic pillar[5]arene
receptor designed for ion pair recognition of alkylammonium salts.
Herein, we studied the complexation between heteroditopic recep-
tor 1 and several alkylammonium salts with different counterions
and different alkyl chain lengths, demonstrating that heteroditopic
receptor 1 showed much more improved binding affinity to alkyl-
ammoniums by ion pair recognition than monotopic receptor 1,4-
dimethoxypillar[5]arene (DMP5) which was synthesized for
comparison.
indicating the hydrogen bonding between urea and the counterion,
À
CF
3
COO . The cooperative binding between 1 and G4c also gave
rise to lower resonances of the peaks for H
of 1, while no obvious chemical shift changes were observed
for H and H of 1. These results provided strong and direct evi-
dence that linear n-BuNH
and multiple noncovalent interactions such as urea–anion hydro-
1 2 3 4 5
, H , H , H , H , and
H
6
7
8
+
3
fully threaded into the cavity of 1,
n,12
gen bonding, cation–
p, C–HÁ Á Á
p
, and N–HÁ Á Á
p
interactions9
may contribute to the stabilization of the complex of heteroditopic
receptor 1 with G4c. Moreover, similar complexation phenomena
by ion pair recognition between heteroditopic pillar[5]arene 1
À
À
and n-butylammonium salts with Cl and Br (G4a and G4b) as
Heteroditopic receptor 1 was prepared by the simple modifica-
tion of monofunctionalized pillar[5]arene 29k (Scheme 1). With the
attachment of urea, pillar[5]arene 1 was expected to bind both
alkylammonium and its counterion, forming a neutral ternary sys-
tem consequently. Initially, 1H NMR was employed to investigate
the complexation between heteroditopic pillar[5]arene 1 and
counterions in CDCl
3
could also be observed from the 1H NMR
spectra of their mixtures, respectively (Figs. S7–S8, ESI). However,
in these two cases, some peaks for methylene protons and urea
protons could not be clearly observed because of the broadening
9e,9t
effects
that occurred due to complexation dynamics where
those protons were located in the cavity of pillar[5]arene and
À
À
À
À
n-butylammonium salts with Cl , Br , CF
3
COO , and PF as differ-
shielded by
due to the poor solubility of n-BuNH
tigation of the complexation between 1 and n-BuNH
p
-electron rich cyclic pillar[5]arene. What is more,
6
1
+
À
ent counterions, respectively. Figure 2 showed the H NMR spectra
3
PF in chloroform, the inves-
6
+
À
1
of 1, n-butylammonium trifluoroacetate (G4c), and an equimolar
3
PF by
H
6
mixture of them, respectively, in CDCl
3
. In the presence of 1, all
NMR was limited.
peaks of G4c shifted upfield obviously and it only exhibited one
set of peaks, indicating fast-exchanging complex on the NMR time
scale. By 2D COSY spectrum of the mixture (Fig. S11, ESI), the peaks
for G4c were accurately assigned. Therefore, the chemical shift
Because of the relatively less remarkable broadening effect in
the 1H NMR spectra of heteroditopic receptor 1 mixed with G4c,
the complexation between 1 and G4c was selected for further
study by 2D NOESY NMR. From the spectrum (Fig. S12, ESI),
Figure 1. The cartoon representation of the complexation between heteroditopic receptor 1 and G.

M. Ni et al. / Tetrahedron Letters 53 (2012) 6409–6413
6411
2
3
R = Br
i
OMe
CH2
OMe
CH2
R = NPht
OMe
CH2
ii
OMe
O
4 R = NH2
4
iii
OMe
DMP5
R
1 R = NHCONH-p-Tolyl
5
Reagents and conditions: (i) phthalimide,K CO , DMF (92%);
2
3
(
ii) NH NH , THF (94%); (iii) p-tolyl isocyanate, CH Cl (85%).
2
2
2
2
Scheme 1. Synthesis of pillar[5]arene 1 and structure of DMP5.
Figure 2. 1H NMR spectra (300 MHz, CDCl
, 298 K) of (a) G4c (8 mM); (b) 1 (8 mM) + G4c (8 mM); (c) 1 (8 mM). Asterisk = water.
3
correlation peaks can be clearly observed between H
and H , H , H , H , and NH of G4c, suggesting that n-BuNH
threaded into the cavity of 1. Job’s plot (Fig. S13, ESI) revealed that
and G4c formed 1:1 complex in chloroform. Furthermore, the 1:1
binding complex between 1 and G4a, G4b, and G4c was confirmed
by ESI-MS, respectively (Figs. S14, S16–S17, ESI). As for the com-
plexation of 1 and G4c, in the positive ion mode, the spectrum
1
, H
2
, and H
3
of
ing structure for all alkylammonium trifluoroacetates. As for
+
+
À
1
a
b
c
d
3
3
n-HexNH
methyl H
Fig. S9, ESI), while all other methylene protons shifted upfield.
3
CF
3
COO (G6), only one peak corresponding to terminal
d
of G6 exhibited no obvious shift (red dashed line in
1
+
À
3 3
And in the case of n-OctNH CF COO (G8), the proton chemical
shifts of only terminal three alkyl groups had no significant change
(red dashed line in Fig. S10, ESI), which agreed well with G6, indi-
cating only first five methylenes connecting to the ammonium end
of G could thread into the cavity of 1 with others outside the cavity.
Furthermore, the complexation between heteroditopic receptor 1
and G6 and G8, respectively, was also studied by ESI-MS spectra
(Figs. S18–S19, ESI), which revealed the same 1:1 cooperative bind-
ing between them.
exhibited a peak at m/z = 986.3, which is corresponding to
+
[
1 + G4c–CF
3
COO] . Meanwhile, a peak at m/z = 1025.3 correspond-
À
ing to [1 + G4c–n-BuNH
3
]
could be observed in the negative ion
mode.
In addition, the ion pair recognition between heteroditopic
receptor 1 and alkylammonium trifluoroacetate salts with different
alkyl chain lengths (G6 and G8) in chloroform was further studied
Because heteroditopic receptor 1 could form 1:1 complexes
with all n-butylammonium salts G4a–c by ion pair recognition,
1
by the H NMR spectroscopy (Figs. S9–S10, ESI). The proton signals
1
for NH
G4c, G6, and G8) showed similar upfield shifts (À2.30, À2.18,
and À2.07 ppm, respectively), indicating the formation of thread-
3
of three types of alkylammonium trifluoroacetate salts
H NMR titration was carried out to investigate their recognition
1
(
process and measure their association constants. The H NMR titra-
3
tion spectra of 1 into G4c (Fig. S20, ESI) in CDCl showed that only

6
412
M. Ni et al. / Tetrahedron Letters 53 (2012) 6409–6413
Table 1
Association constants (Ka/M ) for the complexation of host (1 and DMP5) and guest
G4a, G4b, and G4c), respectively, in CDCl at 298 K
tion experiments to determine the Ka value of the interaction be-
À1
À
À
À
tween receptor 1 and three anions (Cl , Br , and CF
their tetrabutylammonium salts), which was 56.0 ± 4.0 M
3
COO as
(
3
À1
,
À1
À1
G4a
G4b
G4c
42.3 ± 1.4 M , and 31.3 ± 1.4 M , respectively (Figs. S24–S25,
À
3
3
2
4
2
1
(2.01 ± 0.72) Â 10
(2.81 ± 0.28) Â 10
(4.61 ± 1.68) Â 10
(3.54 ± 1.16) Â 10
(2.62 ± 0.73) Â 10
(6.76 ± 1.16) Â 10
Table S1, ESI). The relatively strongest binding of Cl with urea unit
À
À
DMP5
over Br and CF
ment (71 times) for n-BuNH
receptor 1 showed 13 and 39 times association constant enhance-
3
COO could lead to the most remarkable enhance-
+
14
3
binding to receptor 1. However,
+
À
+
À
ment for n-BuNH
3 3 3
Br and n-BuNH CF COO , respectively, com-
the peaks for H
equiv 1 was added. Nevertheless, the peaks for H
of G4c became sharper and the peaks for H of G4c returned to
d
of G4c could be recognizable when less than
pared to DMP5, which does not correlate with the relative
À
À
1
a
, H , and H
b
c
binding affinity of the urea unit to Br and CF COO . This phenom-
3
À
d
enon could mainly result from the different size of Br and
CF COO , and their different binding modes with urea unit. An-
other possible reason was that compared with CF COO , the stron-
ger urea-anion binding of Br makes the charge density of the
À
15
be triplet peaks from broad peaks with more than 1 equiv addition
of 1. It could also be observed that with the addition of 1, the peak
3
À
3
À
for H
shift change after the addition of 1.6 equiv 1. By employing the
nonlinear curve-fitting of H of G4c (Fig. S21, ESI), the association
constant of the complexation between 1 and G4c was calculated
d
of G4c shifted upfield gradually, and showed no significantly
anion weaker, leading to relatively weaker electrostatic interaction
À
+ 16
d
between Br and the included n-BuNH3 .
In summary, a monofunctionalized pillar[5]arene 1 bearing one
ureido group on the arm has been prepared by the simple modifi-
cation of a previously reported monofunctionalized pillar[5]arene.
Such a novel host is easily synthesized and acts as a heteroditopic
receptor for ion pair recognition of alkylammonium salts in chloro-
form, with the linear guest threading into the cavity of pil-
lar[5]arene and the counteranion binding to urea simultaneously.
The uptake of counteranion by urea moiety of heteroditopic 1
weakened the ion pairing, performing a cooperative role in the rec-
ognition of alkylammoniums, and thus enhanced the recognition of
alkylammoniums by heteroditopic receptor 1 in comparison with
monotopic receptor DMP5, in which the best performance is
4
À1
to be (2.62 ± 0.73) Â 10
M . Similarly, the association constants
of the complexation between 1 and G4a and G4b were also ob-
tained under the same conditions (Table 1).
In order to prove that heteroditopic receptor 1 formed by the
introduction of an urea moiety could enhance the cation recogni-
tion by ion pair recognition, monotopic receptor 1,4-dimethoxypil-
lar[5]arene (DMP5) was prepared and employed for comparison
+
with heteroditopic receptor 1 in the recognition of n-BuNH
3
with
different counterions. The ESI-MS spectrum of DMP5 with G4c
+
was conducted and it only showed [DMP5 + G4c–CF
3
COO]
(
1
m/z = 824.3) in the positive ion mode (Fig. S15, ESI), suggesting
+
+
À
:1 binding stoichiometry between DMP5 and n-BuNH
3
, while
71 times binding enhancement for n-BuNH3 Cl . Future work will
be focused on the design of more powerful pillar[n]arene based
heteroditopic receptors for alkylammoniums with good selectivity
and other organic cation guests and their applications in the areas
of extraction and transportation.
no corresponding peak was observed in the negative mode to
identify the binding between DMP5 and the counteranion of G4c
as expected. Furthermore, the 1H NMR titration between DMP5
and G4a, G4b and G4c was performed under the same condition
as that between 1 and G4a–c, and their association constants were
obtained and listed in Table 1. As for the complexation of DMP5
Acknowledgments
d
and G4c (Fig. S22, ESI), the peak for H of G4c shifted upfield much
less upon addition of DMP5 than addition of 1 (Fig. S21, ESI),
suggesting the less stable complexation between DMP5 and G4c
compared to that between 1 and G4c.
We gratefully thank the financial support of National Natural
Science Foundation of China (No. 20932004, 21072093,
21102073), National Basic Research Program of China
(2011CB808600), the Natural Science Foundation of Jiangsu
(BK2011055, BK2011551).
As shown in Table 1, compared to the Ka value, (2.01 ± 0.72) Â
3
À1
1
0 M , of the complexation between 1 and G4a, the Ka value of
+
À
+
À
3 3 3
the complexation of 1 with n-BuNH Br and with n-BuNH CF COO
increased 2 times and 13 times, respectively. Meanwhile, the Ka
Supplementary data
value of the complexation between DMP5 and G4a was very low
À1
+
(
28.1 ± 2.8 M ), and the Ka values of the complexation of DMP5
Supplementary data (synthesis route, additional NMR spectra,
ESI spectra, Job’s plot and determination of association constants)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2012.09.043.
À
+
À
3 3 3
with n-BuNH Br and with n-BuNH CF -COO increased 12 times
and 24 times, respectively. The results revealed that the different
counterion influenced the stability of complexation between neu-
tral hosts and organic cation guests.13 Most importantly, binding
to the same n-butylammonium guest, heteroditopic receptor 1
remarkably showed much higher Ka values than monotopic
References and notes
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À
receptor DMP5, which is 71 times for n-BuNH
3
Cl , 13 times for
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