Improved complexation between dibenzo-24-crown-8 derivatives and dibenzylammonium salts by ion-pair recognition

Article abstract of DOI:10.1039/b813175a

The introduction of a urea-based anion binding site onto dibenzo-24-crown-8 can enhance the complexation between dibenzo-24-crown-8 derivatives and dibenzylammonium cation by ion-pair recognition. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b813175a

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Improved complexation between dibenzo-24-crown-8 derivatives
and dibenzylammonium salts by ion-pair recognitionw
Kelong Zhu, Mingming Zhang, Feng Wang, Ning Li, Shijun Li and Feihe Huang*
Received (in Durham, UK) 31st July 2008, Accepted 18th September 2008
First published as an Advance Article on the web 29th September 2008
DOI: 10.1039/b813175a
The introduction of a urea-based anion binding site onto
dibenzo-24-crown-8 can enhance the complexation between
dibenzo-24-crown-8 derivatives and dibenzylammonium cation
by ion-pair recognition.
their ClÀ or BrÀ salts, known as tight ion-pairs, in many
real-life situations.8 Therefore, the study of the complexation
of tight ion-paired salts is important for understanding the
operation mechanisms of some natural supramolecular sys-
tems. However, up to now only little has been done to
investigate this field.9 It is well known that urea derivatives
can be used as anion receptors. In order to improve the
preparation efficiencies of threaded structures based on the
well-known DB24C8/dibenzylammonium salt recognition
motif,1 we are interested in improving complexation between the
DB24C8 derivatives and dibenzylammonium salts. Herein, we
report the synthesis of some DB24C8 derivative heteroditopic
hosts 2 (Scheme 1) containing urea-based anion binding sites
and their ion-pair recognition for dibenzylammonium salts 3
with different anions (Scheme 2). We will demonstrate not
only that the complexation between DB24C8 derivatives and
dibenzylammonium salts can be improved but also that the
improvement can be tuned by controlling the binding strength
of anions through the introduction of electron-withdrawing
substituents onto the urea-based anion binding site.
Threaded structures, based on host–guest chemistry, have
been attractive to scientists not only because of their topolo-
gical importance, but also due to their many potential applica-
tions.1 The preparation efficiency of these threaded structures
is strongly dependent on the complexation strength, reflected
by the association constant (Ka), between the host and guest.
Scientists have devoted much effort into raising the Ka values
by both developing new recognition systems and derivating
the known ones.2 For the latter, a classic example is the ion-
pair recognition using heteroditopic receptors.3 It is well
known that the binding of inorganic salts by synthetic recep-
tors in organic solvents, especially in low-dielectric-constant
solvents, is weak because the target salts exist as associated
ion-pairs which can hinder the single-ion recognition process.4
However, the binding can be improved by using heteroditopic
receptors that can simultaneously bind both cations and
anions. Since Reetz et al. reported one of the first successful
examples of heteroditopic receptors for associated ion pairs in
1991,5 much work has been done in this area for its many
applications, such as solubilization, extraction, and membrane
transport.3,6
The key starting material, 4, was prepared in excellent yield
according to the literature.10a The intermediates 5 and 6 were
synthesized according to a similar method reported by Gibson
et al.10b The reaction of 4 with 1.2 equiv. of potassium
phthalimidate afforded 5 (Scheme 1). The amine 6 was
obtained by hydrazinolysis of 5 (Scheme 1). The heteroditopic
hosts 2 were obtained in good yield by the reaction of 6 with
corresponding phenyl isocyanates in CHCl3 (Scheme 1).
Dibenzylammonium salts 3 were prepared as described in
the literature.9
Crown ethers, including dibenzo-24-crown-8 (DB24C8) 1,
and their derivatives have been widely used in the preparation
of threaded structures as hosts for organic salts, such as
dialkylammonium salts and paraquat derivatives.1 The
threading of secondary dibenzylammonium salts through the
cavity of crown ethers was the genesis of a diverse range of
mechanically interlocked threaded structures including mole-
cular machines.7 Most of the dibenzylammonium cation bind-
ing studies have been conducted in organic solvents and have
used non-competing counterions. Dibenzylammonium hexa-
fluorophosphate derivatives have been used in many cases for
their good solubility and relatively weak ion-pairing in organic
solvents. However, the luxury of the non-competing counter-
ions is not always available and organic salts usually exist as
1
The H NMR spectrum (Fig. 1b) of an equimolar solution
of 2a and 3b in CDCl3–CD3CN (3 : 2, v : v) shows a slow-
1
exchange complexation on the H NMR time scale which is
similar to the complexation between 1 and 3a.1b The new peak
at 4.61 ppm is attributed to the complexed benzylic methylene
protons H11c. The multiplet at 3.41–3.47 ppm corresponds to
the complexed g-OCH2 protons of 2a. The complexation
between 2a and 3b has 1 : 1 stoichiometry as determined by
integration of these peaks. After complexation, the peak at
5.40 ppm corresponding to protons H9 of 2a was split into two
different signals at 5.56 ppm and 6.44 ppm, respectively
(Fig. 1b and c). The same phenomenon was observed for
protons H10 of 2a (Fig. 1b and c). This should result from the
coexistence of a fast-exchange complexation between the urea
moiety and the trifluoroacetate anion and a slow-exchange
complexation between the dibenzo-24-crown-8 cavity and the
dibenzylammonium cation.
Department of Chemistry, Zhejiang University, Hangzhou, 310027,
PR China. E-mail: fhuang@zju.edu.cn; Fax: +86 571 8795 3189;
Tel: +86 571 8795 3189
w Electronic supplementary information (ESI) available: Preparation
of compounds, determination of association constants, and other
materials. See DOI: 10.1039/b813175a
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Scheme 1 Synthesis of dibenzo-24-crown-8 derivatives 2 and structures of chemicals used in this study.
Scheme 2 Cartoon representation of the ion-pair recognition between host 2 and guest 3.
confirming the 1 : 1 stoichiometry of the complexation
between 2a and the trifluoroacetate anion.
As shown in Fig. 1b, c, 2b and c, chemical shift changes after
the complexation between 2a and 3c in CDCl3–CD3CN
(3 : 2, v : v) are similar to those seen after the complexation
between 2a and 3b. The benzylic methylene protons H11 of 3c
moved downfield to 4.62 ppm (Fig. 2a and b), almost at the same
place as in the case for 2a and 3b (Fig. 1a and b). The g-OCH2
ethyleneoxy protons of 2a also have the same chemical shift at
3.41–3.47 ppm (Fig. 2b), the same as in the case for 2a and 3b
(Fig. 1b) upon complexation. The peak corresponding to protons
H9 of 2a in the equimolar solution were also split into two
different signals at 5.58 ppm and 6.37 ppm, respectively
(Fig. 2b and c). The same phenomenon was observed for the
protons H10 of 2a in the same equimolar solution. This difference
should also result from the coexistence of a fast-exchange
complexation between the urea moiety and the chloride
Fig. 1 Partial 1H NMR spectra (500 MHz, 3 : 2 CDCl3–CD3CN,
22 1C) of 2.00 mM 3b (a), 2.00 mM 2a and 3b (b), and 2.00 mM 2a (c).
Complexed and uncomplexed species are denoted by ‘‘c’’ and ‘‘uc’’,
respectively.
anion and
a
slow-exchange complexation between the
dibenzo-24-crown-8 cavity and the dibenzylammonium cation.
The ESI+-MS mass spectrum of an equimolar acetonitrile solu-
tion of 2a and 3c confirmed the 1 : 1 stoichiometry of the
complexation between the dibenzo-24-crown-8 cavity and
dibenzylammonium cation by a mass fragment at m/z 794.4
(100%) for [2aÁ3c À Cl]+. The ESIÀ-MS mass spectrum of the
same solution gave mass fragments at 631.3 (100%) corresponding
to [2a + Cl]À and at 1227.5 (10%) for [2a + 2a + Cl]À.
Therefore, the complexation between 2a and the chloride anion
The ESI+-MS mass spectrum of an equimolar acetonitrile
solution of 2a and 3b confirmed the 1 : 1 stoichiometry of the
complexation between the dibenzo-24-crown-8 cavity and
dibenzylammonium cation by a mass fragment at m/z 794.3
(57%) for [2aÁ3b À CF3COO]+. The ESIÀ-MS mass spectrum
of the same solution gave a single mass fragment at 709.1
(100%) corresponding to the complex [2a + CF3COO]À,
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Table 1 Association constants (Ka/MÀ1) for the 1 : 1 complexes
between hosts (2.00 mM) 1 and 2 and guests (2.00 mM) 3 in different
solvents at 22 1C
3ba
3ab
3bb
3cb
1
59
78
102
176
1687
1589
1200
1377
193
388
413
487
63
266
385
475
2a
2b
2c
a
b
Determined in CD3CN. Determined in CD3CN–CDCl3 (2 : 3).
electron-withdrawing nature of the urea group, which can
reduce the electron density of the oxygen atoms of the crown
ether cavity and the strength of hydrogen bonds between the
ether oxygen atoms of the DB24C8 cavity and the acidic
hydrogens of the dibenzylammonium cation.15 Because of
the introduction of electron-withdrawing nitro and trifluoro-
methyl groups, 2bÁ3a and 2cÁ3a have even lower Ka values than
2aÁ3a (column 2 in Table 1).
Fig. 2 Partial 1H NMR spectra (500 MHz, 3 : 2 CDCl3–CD3CN,
22 1C) of 2.00 mM 3c (a), 2.00 mM 2a and 3c (b), and 2.00 mM 2a (c).
Complexed and uncomplexed species are denoted by ‘‘c’’ and ‘‘uc’’,
respectively.
In the contrast, the Ka value of 2aÁ3b is higher, about one
fold, than the Ka value of 1Á3b in the mixture of acetonitrile
and chloroform (2 : 3, v : v) (column 3 in Table 1). This is
different from the case of 3a. It means that by the introduction
of an anion binding site onto DB24C8, the complexation
between the DB24C8 derivatives and dibenzylammonium salts
can be improved by ion-pair recognition. Both 2bÁ3b and 2cÁ3b
exhibited even higher Ka values than 2aÁ3b (column 3 in
Table 1). This can be attributed to the increase in the binding
strength of the urea unit to the trifluoroacetate anion resulting
from the introduction of the electron-withdrawing nitro or
trifluoromethyl group onto the urea unit.
Since 3c is a tighter ion-pair than 3b,9 3c-based complexes
have lower Ka values than 3b-based ones (columns 3 and
4 in Table 1). However, the improvement in the Ka value
caused by ion-pair recognition is more obvious when 3c
instead of 3b is used as the guest. An increase, up to more
than 6-fold, in the Ka value was observed for 2cÁ3c compared
with 1Á3c.
has the 1 : 1 and 2 : 1 stoichiometries simultaneously in the gaseous
state. This is because the binding of the spherical chloride anion by
the urea moiety can adopt both two-coordinate complex (Fig. 3a)
and four-coordinate tetrahedral geometry (Fig. 3b),11 while the
binding of the trifluoroacetate anion takes only the Y-shaped
geometry (Fig. 3c).11,12
It is well known that the introduction of electron-with-
drawing groups to the urea group can enhance the acidity of
the urea protons and promote its binding ability to anions.13
In order to investigate how electron-withdrawing substituents
on the phenyl ring of the host can influence the cation binding
properties, nitro and trifluoromethyl groups were introduced
onto the para site of the phenyl ring (Scheme 1). The Ka values
were determined by a single-point method.14 All of the hosts 1,
2a, 2b and 2c exhibited rather low Ka values (column 1 in
Table 1) for guest 3b in acetonitrile, which is probably because
3b exists as a tight ion-pair and the complexation between the
urea moiety and anions is weak in this polar organic solvent.
However, due to the urea moiety providing a weak binding for
anions in acetonitrile, the heteroditopic hosts 2 have a little
higher Ka values for 3b than the simple cation-binding crown
ether host 1. In order to increase the binding strength between
the urea unit and anions, we changed to a less polar solvent
system, a mixture of acetonitrile and chloroform (2 : 3, v : v).
In this solvent system, 2aÁ3a exhibits a lower Ka value than 1Á
3a (column 2 in Table 1). This can be explaÀined by the very
weak binding of the urea group to the PF6 anion and the
In conclusion, we have demonstrated that the complexation
between DB24C8 derivatives and dibenzylammonium salts
could be improved by ion-pair recognition after a urea unit
was introduced. The improvement can be tuned by controlling
the binding strength of anions through the introduction of
electron-withdrawing substituents onto the anion binding site.
Currently, we are using these heteroditopic hosts in the
efficient preparation of mechanically interlocked threaded
structures and exploring how to improve complexation of
other organic guests by ion-pair recognition.
This work was supported by the National Natural Science
Foundation of China (20604020 and 20774086).
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