A cationic water-soluble pillar[5]arene: Synthesis and host-guest complexation with sodium 1-octanesulfonate

Article abstract of DOI:10.1039/c1cc15660h

By the introduction of trimethylammonium groups at both upper and lower rims, a cationic water-soluble pillar[5]arene was prepared, which forms a stable 1:1 host-guest complex with sodium 1-octanesulfonate in water.

Full text of DOI:10.1039/c1cc15660h

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COMMUNICATION
A cationic water-soluble pillar[5]arene: synthesis and host–guest
complexation with sodium 1-octanesulfonatew
Yingjie Ma,a Xiaofan Ji,a Fei Xiang,a Xiaodong Chi,a Chengyou Han,a Jiuming He,b
Zeper Abliz,b Weixiang Chena and Feihe Huang*a
Received 13th September 2011, Accepted 29th September 2011
DOI: 10.1039/c1cc15660h
By the introduction of trimethylammonium groups at both upper
and lower rims, a cationic water-soluble pillar[5]arene was
prepared, which forms a stable 1 : 1 host–guest complex with
sodium 1-octanesulfonate in water.
Compound 1 was prepared in a facile way (Scheme 1). With
the aim of obtaining 1, precursor 3 with bromoalkane groups
was firstly prepared. The stepwise preparation of 3 has been
reported by Hou et al. recently.6a However, several steps were
involved in their method. To prepare 3 easily, we used another
process, which is through the cyclization of monomer 2 with
paraformaldehyde in the presence of boron trifluoride diethyl
etherate (BF3Á(OC2H5)2) in 1,2-dichloroethane.3a We envisaged
that the introduction of suitable substituents on the benzene
component could result in derivatives of 3 with water-solubility.
For this purpose, on the treatment of 3 with excess trimethyl-
amine (30 equiv) in ethanol, compound 1 containing ten trimethyl-
ammonium groups at upper and lower rims was prepared as a
colourless solid (95% yield). Pillar[5]arene 1 can be easily
dissolved in water to give a colourless solution. Furthermore, 1
is so hygroscopic that it will quickly become soft if exposed to a
moist environment for a short time, though it remains solid in a
dry environment.
The arrival of any new class of macrocycles can accelerate the
development of supramolecular chemistry. Macrocycles, such
as crown ethers, cyclodextrins, cucurbiturils and calixarenes,
play a prominent role in host–guest chemistry.1 In 2006, a new
macrocycle, a cyclic pentamer from the co-oligomerization of
1,4-dimethoxybenzene and 1,4-dimethoxy-2,5-xylylene dibromide,
appeared in the literature.2 Later, Ogoshi et al. prepared this
compound in 2008 by a facile method and named these kinds
of compounds ‘‘pillar[5]arenes’’.3a After that, the chemistry of
pillar[n]arenes has developed rapidly with new synthetic routes
pursued and a wealth of inclusion phenomena described.3–9
The studies about the host–guest chemistry of pillar[5]arenes
have been usually carried out in organic media, which is in
sharp contrast to the case of cyclodextrins and cucurbiturils,
resulting from the poor solubility of pillar[5]arenes in aqueous
media. Therefore, in order to investigate the inclusion properties
of pillar[5]arenes in water, water-soluble pillar[5]arenes were
prepared. The first water-soluble pillar[5]arene was synthesized in
2010 by the introduction of carboxylate anions at both upper
and lower rims,4 which could bind cationic viologen salt in
water; the second one, a pillar[5]arene decaamine, was synthesized
in 2011 and revealed to encapsulate linear diacids in neutral,
alkaline and acidic conditions.6a The former water-soluble
pillar[5]arene contains negative anion groups, carboxylate anions;
the latter contains neutral groups, amino groups. Water-soluble
pillar[5]arenes containing cationic groups have not been reported
so far. Consequently, in this study, we reported the preparation of
a cationic water-soluble pillar[5]arene 1 and demonstrated that 1
could strongly bind sodium 1-octanesulfonate in water, mainly
driven by hydrophobic and electrostatic interactions.
The structure of 1 was characterized by 1H NMR,
13C NMR, LRESIMS and HRESIMS (ESIw). LRESIMS and
HRESIMS of 1 revealed peaks at m/z 1055.12, 677.11, 487.66,
374.31 and 298.46, corresponding to [M – 2Br]2+, [M – 3Br]3+
,
[M – 4Br]4+, [M – 5Br]5+ and [M – 6Br]6+, respectively
(Fig. S8 and S9, ESIw). This shows that 1 can lose different
numbers of bromide anions to form a series of fragments
containing different numbers of positive charges.
Because of the presence of ten positive charges on the
macrocyclic ring, 1 can play the role of an anion receptor.
A detailed study of the inclusion property of 1 with anionic
guests revealed that compound 1 displayed high affinities for
sodium alkyl sulfonates in water. For instance, when host 1
was mixed with sodium 1-octanesulfonate (G) in water, a
a Department of Chemistry, Zhejiang University, Hangzhou 310027,
P. R. China. E-mail: fhuang@zju.edu.cn; Fax: +86-571-8795-1895;
Tel: +86-571-8795-3189
b Institute of Materia Medica, Chinese Academy of Medical Sciences
and Peking Union Medical College, Beijing 100050, P. R. China
w Electronic supplementary information (ESI) available: Compound
characterization, synthetic details, determination of association constants,
and other materials. See DOI: 10.1039/c1cc15660h
Scheme 1 Synthesis of a cationic water-soluble pillar[5]arene (1).
c
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indicating a 1 : 1 stoichiometry for the complexation between
1 and G.
After complexation, no large chemical shift changes were
observed for the protons on 1, but both of the peaks for
H1 and H2 were further split into two separated peaks,
corresponding to the trimethylammonium protons that are
close to the anionic sulfonate head group of G and the
trimethylammonium protons that are close to the alkyl tail
group of G in the 1 : 1 complex (Fig. 1), respectively. This
phenomenon resulted from the asymmetric structure of the
guest (Fig. 1).
The ability of 1 to form a 1 : 1 complex with G was assessed
1
by H NMR titration of 1 into a 16.00 mM solution of G in
water (Fig. 3). With the increase of the concentration of 1,
proton NMR signals corresponding to Ha–Hd shifted upfield
considerably. Addition of 1.0 equiv. of 1 resulted in an upfield
shift of 0.90 ppm for the Ha protons of G while protons Hb, Hc
and Hd on G shifted upfield even more. However, when the
concentration of 1 was higher than 1 equiv, chemical shifts of
protons Ha–Hd hardly changed, indicating the formation of a
1 : 1 complex between pillar[5]arene 1 and sodium 1-octane-
sulfonate G in water.
Fig. 1 The formation of a host–guest complex from pillar[5]arene 1
and sodium 1-octanesulfonate G in water.
host–guest complex was formed (Fig. 1). The proton NMR
spectrum of an equimolar (16.00 mM) aqueous solution of
host 1 and guest G showed that the complex is in fast exchange
on the proton NMR time scale (Fig. 2). Protons Ha, Hb, Hc
and Hd on guest G shifted upfield after complexation while no
obvious chemical shift changes were observed for He after
complexation (spectra a and b in Fig. 2). Furthermore, the
overlapped signal corresponding to Hd was obviously split into
four separate peaks (d: À0.17, À1.10, À1.93 and À2.36 ppm).
These phenomena suggested that linear guest G was threaded
through the cavity of cyclic host 1 to form a [2]pseudorotaxane
with its negative sulfonate head close to the positive trimethyl-
ammonium groups of 1, the middle four methylene groups
(Hd) located in the cavity and the tail (He) out of the cavity
(Fig. 1). The formation of the complex might be mainly driven
by hydrophobic and electrostatic interactions, because the
hydrophobic cavity of 1 could hold the hydrophobic alkyl
chain of G and the cationic trimethylammonium groups of 1
could bind the anionic sulfonate group of G via electrostatic
interaction. Further evidence for the formation of the desired
complex was obtained by LRESIMS, revealing a peak at m/z
276.37, corresponding to [1*G – 7Br]7+ (Fig. S13, ESIw),
A mole ratio plot for the complexation between 1 and G
showed that the stoichiometry of the complex between 1 and G
is 1 : 1 (Fig. S12, ESIw), in accordance with the above mentioned
results of LRESIMS and the proton NMR titration. The
association constant (Ka) of 1*G was calculated to be
1.33(Æ 0.94) Â 104 MÀ1 in water using a nonlinear curve-
fitting analysis (Fig. S11, ESIw). This Ka value is on the same
order of magnitude as the corresponding Ka value of the
previously reported complex between an anionic water-soluble
pillar[5]arene and cationic viologen.4
In summary, we have successfully synthesized a cationic
water-soluble pillar[5]arene, which bears trimethylammonium
groups at both of the two rims. The presence of ten positive
charges makes it possible to act as an anion receptor.
Fig. 3 1H NMR spectra (D2O, 293 K, 400 MHz) of G at a concen-
tration of 16 mM with different concentrations (mM) of 1: (a) 0.00,
(b) 3.07, (c) 5.92, (d) 8.57, (e) 11.03, (f) 13.89, (g) 16.51, (h) 21.18,
(i) 25.21, (j) 31.81, (k) 36.99, (l) 44.60, (m) 49.92.
Fig. 2 1H NMR spectra (D2O, 293 K, 400 MHz) of (a) 16.00 mM G;
(b) 16.00 mM 1 + 16.00 mM G; (c) 16.00 mM 1.
c
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Consequently, in aqueous media, this water-soluble pillar[5]arene
could bind sodium 1-octanesulfonate, forming a [2]pseudo-
rataxane mainly driven by hydrophobic and electrostatic
interactions. The successful preparation of this new cationic
water-soluble pillar[5]arene can provide new opportunities for
chemists working in the field of host–guest chemistry. The new
molecular recognition motif between it and sodium 1-octane-
sulfonate can be further used in the preparation of new functional
supramolecular systems, including molecular machines, mole-
cular switches, chemosensors, delivery systems and responsive
supramolecular polymers, which can be operated in water.
This work was supported by the National Natural Science
Foundation of China (20834004, 91027006 and J0830413), the
Fundamental Research Funds for the Central Universities
(2010QNA3008), National Basic Research Program
(2009CB930104), and Zhejiang Provincial Natural Science
Foundation of China (R4100009).
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