Improved and controlled complexation of paraquat derivatives by the formation of a bis(m-phenylene)-26-crown-8-based lariat ether

Article abstract of DOI:10.1002/ejoc.201000926

A novel bis(m-phenylene)-26-crown-8-based lariat ether (i.e., 3b) was synthesized and characterized. It can bind paraquat derivatives more strongly than bis(m-phenylene)-26-crown-8 in solution. It forms pseudorotaxanes with two paraquat derivatives in the

Full text of DOI:10.1002/ejoc.201000926

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000926
Improved and Controlled Complexation of Paraquat Derivatives by the
Formation of a Bis(m-phenylene)-26-Crown-8-Based Lariat Ether
Mingming Zhang,[a] Yan Luo,[a] Bo Zheng,[a] Binyuan Xia,[a] and Feihe Huang*[a]
Keywords: Crown compounds / Rotaxanes / Controlled assembly / Substituent effects
A
novel bis(m-phenylene)-26-crown-8-based lariat ether
lariat ether 3b. Furthermore, due to the introduction of two
benzyloxy groups, its binding to paraquat derivatives can be
switched off (and back on) by adding K+ (and then dibenzo-
18-crown-6), and the disassociation percentage depends on
the concentration of the added K+ ions.
(i.e., 3b) was synthesized and characterized. It can bind para-
quat derivatives more strongly than bis(m-phenylene)-26-
crown-8 in solution. It forms pseudorotaxanes with two para-
quat derivatives in the solid state. N-Methyl substitution was
found to play an important role on the binding strength of
Introduction
chines.[12] However, up to now, the complexation of bis(m-
phenylene)-26-crown-8 (BMP26C8, 3a) and paraquat deriv-
atives has been rarely explored.[3c]
Threaded structures, such as rotaxanes and catenanes,
have been widely applied in the construction of molecular
machines[1] and mechanically bonded macromolecules.[2]
Paraquat derivatives (N,NЈ-dialkyl-4,4Ј-bipyridinium salts)
are commonly used guests in the preparation of these
self-assembled structures.[3] Bis(p-phenylene)-34-crown-10
(BPP34C10, Figure 1) can form pseudorotaxanes with
paraquat derivatives,[4] and this recognition motif has been
widely used in the construction of supramolecular systems
including dendrimers with interesting electrochemical prop-
erties and guest binding properties,[5] metal–organic frame-
works,[6] and supramolecular polymers.[7] Bis(m-phenylene)-
32-crown-10 (BMP32C10) derivatives, such as 1a and 1b,
mainly form taco complexes with paraquat deriva-
tives,[3a,3d,8] and the related recognition motif has also been
widely used in supramolecular chemistry,[9] because the
symmetric nature of BMP32C10 derivatives avoids issues of
isomers in the fabrication of complicated supramolecular
structures. cis- or trans-Dibenzo-30-crown-10 (DB30C10)
diols 2a and 2b can form inclusion complexes with paraquat
derivatives, and the complexation can be switched by add-
ing small molecules (KPF6 and 18-crown-6).[10] Dibenzo-
24-crown-8 (DB24C8) can also form pseudorotaxanes and
rotaxanes with paraquat derivatives,[11] but the complexes
based on the recognition of DB24C8 to paraquat are less
stable than the complexes based on the binding of DB24C8
to secondary ammonium salts, so the DB24C8/paraquat re-
cognition motif has usually been used as the secondary
binding motif to construct pH-driven molecular ma-
Figure 1. Chemical structures of BPP34C10, BMP32C10 deriva-
tives 1, cis- and trans-DB30C10 diol, DB24C8, BMP26C8 deriva-
tives 3, and paraquat derivatives 4.
Lariat ethers, as “cousins” of crown ethers and cryp-
tands, were designed to have the three dimensionality of
cryptands while retaining the faster complexation dynamics
of crown ethers.[13] They have been widely used in con-
trolled transport in membranes, novel membranes, nucleo-
tide-based molecular boxes, and cation-conducting chan-
nels.[14] However, although lariat ethers have been widely
used as hosts for metal cations, the complexation of lariat
ethers with paraquat derivatives has rarely been reported.
[a] Department of Chemistry, Zhejiang University,
Hangzhou 310027, P. R. China
Fax: +86-571-8795 3189
E-mail: fhuang@zju.edu.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000926.
View this journal online at
wileyonlinelibrary.com
Eur. J. Org. Chem. 2010, 5543–5547
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M. Zhang, Y. Luo, B. Zheng, B. Xia, F. Huang
SHORT COMMUNICATION
As mentioned above, BPP34C10, BMP32C10, DB30C10,
Job plots[16] based on UV/Vis absorbance data for the
and DB24C8 can all bind paraquat derivatives, and these charge-transfer band (λ = 375 nm) demonstrated that the
binding systems have been widely used in supramolecular complexes of 3b with 4a and 4b were both of 1:1 stoichiom-
chemistry. Naturally, we turned to the binding of paraquat etry in solution. The association constants (Ka) of com-
using BMP26C8 derivatives. Due to their symmetric nature, plexes 3bʛ4a and 3bʛ4b in acetone were determined by
BMP26C8 derivatives can be easily used in the construction probing the charge-transfer band of the complexes by a
of more sophisticated systems without consideration of the UV/Vis titration method to be 5.3(Ϯ0.5)ϫ103 –1 and
above-mentioned symmetry problem in the fabrication of 7.0(Ϯ0.3)ϫ102 –1, respectively. For the investigation of
complicated supramolecular structures. However, com- the large difference in association constants of complexes
monly BMP26C8 does not show good affinities with para- 3bʛ4a and 3bʛ4b, the complexation between host 3b and
quat derivatives and the association constant of 3aʛ4a is paraquat derivative guest 4c was also studied as a control
only 390 –1 in acetone,[3c] which is not high enough for the experiment. The association constant for 3bʛ4c in acetone
efficient construction of mechanically interlocked threaded was determined to be 6.0(Ϯ0.5)ϫ102 –1, similar with
structures. Herein, we synthesized a new BMP26C8 deriva- 3bʛ4b, indicating that N-methyl substitution has an impor-
tive 3b, which is a lariat ether with two benzyloxy groups. tant influence on the binding strength between BMP26C8-
It forms pseudorotaxanes with paraquat derivatives 4a and based lariat ether 3b and paraquat derivatives.[17] This may
4b, and the complex 3bʛ4a is much more stable than be due to the fact that the methyl protons of 4a are more
3aʛ4a.
or less acidic and besides the pseudorotaxane geometry, the
three-point binding geometry (similar to the complexation
of primary alkylammonium ions and 18-crown-6)[18] can
also exist for 3bʛ4a, increasing the association constant.
The 1:1 stoichiometry of both 3bʛ4a and 3bʛ4b was
Results and Discussion
Host 3b was synthesized from pyrogallol in four steps in
good yield (Scheme 1). Then the complexations between it confirmed by electrospray ionization mass spectrometry
and two paraquat derivatives, 4a and 4b,[15] were studied. (ESI-MS). Peaks were found at m/z = 991.3 (9.1%) and
When equimolar (8.00 m) acetone solutions of 3b and 423.4 (100%) for 3bʛ4a and at m/z = 1051.1 (3.2%) and
either 4a or 4b were made, a bright yellow color appeared 453.4 (100%) for 3bʛ4b, corresponding to [3bʛ4a – PF6]+,
as a result of charge-transfer interactions between the elec- [3bʛ4a – 2PF6]2+, [3bʛ4b – PF6]+, and [3bʛ4b – 2PF6]2+
,
tron-rich aromatic rings of the lariat ether host and the elec- respectively.
tron-poor pyridinium rings of the paraquat derivative
The 1:1 stoichiometry of lariat ether 3b with paraquat
guests, and it was visually observed that the yellow color of derivative 4a or 4b was further evidenced by X-ray diffrac-
the solution containing 4b and lariat ether 3b was not so tion analysis of yellow single crystals grown by vapor dif-
deep as that of the solution containing 4a and 3b. This dif- fusion of pentane into acetone solutions of 3b with 4a or
ference in yellow color intensity indicated that 4a bound 3b with 4b, respectively. The complex 3bʛ4a is a [2]pseudo-
lariat ether 3b more strongly than 4b.
rotaxane in the solid state (Figure 2). This is the first time
Scheme 1. Syntheses of BMP26C8-based lariat ether 3b and chemical structures of paraquat derivatives 4a, 4b, and 4c.
5544 Eur. J. Org. Chem. 2010, 5543–5547
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Improved and Controlled Complexation of Paraquat Derivatives
that threaded structures are observed between BMP26C8 lid-state structures of 3bʛ4a and 3bʛ4b that the benzyloxy
and paraquat derivatives in the solid state. Interestingly, the groups provide good face-to-face π-stacking interactions
crystal structure of 3bʛ4a is quite different from those of with the pyridinium rings of the paraquat derivative guests,
the complexes of BPP34C10 and BMP32C10 derivatives consistent with our design.
with paraquat 4a.[3,4,8] There are no direct hydrogen bonds
between lariat ether host 3b and paraquat guest 4a; in sharp
contrast, usually pyridinium protons of paraquat 4a are di-
rectly hydrogen-bonded to the ether oxygen atoms of the
crown ether hosts in all previously reported crown ether/4a
complexes.[3,4,8] It is noteworthy that the two pyridinium
rings of paraquat 4a are coplanar, which was not observed
in the previously reported complexes of 4a with various
crown ether hosts.[3,4,8] It seems that the solid-state structure
of 3bʛ4a is mostly stabilized by face-to-face π-stacking and
charge-transfer interactions between the four phenylene
rings of host 3b and the two bipyridinium rings of guest 4a
(Figure 2). In the packing structure, two 3bʛ4a complexes
–
are linked by a bridge formed by two PF6 ions and one
water molecule. Face-to-face π-stacking interactions and in-
direct hydrogen bonds keep the whole structure stable in
the solid state.
Figure 3. A ball–stick view of the X-ray packing crystal structure
of 3bʛ4b. Lariat ether 3b is red, paraquat derivative 4b is blue, and
the PF6– ion is purple. Hydrogen atoms are magenta, oxygen atoms
are green, nitrogen atoms are black, fluorine atoms are purple, and
–
phosphorus atoms are yellow. The other PF6 counterions, solvent
molecules, and hydrogen atoms except the ones involved in hydro-
gen bonding between lariat ether 3b and paraquat derivative 4b
were omitted for clarity. Hydrogen bond parameters: H···O (F) dis-
tance [Å], C(O)–H···O (F) angle [°], C(O)···O (F) distance [Å] d,
2.56, 159, 3.44; e, 2.06, 150, 2.80; f, 2.54, 125, 3.16; g, 2.38, 166,
3.32; h, 2.47, 146, 3.28; i, 2.33, 147, 3.19. Face-to-face π-stacking
parameters: centroid–centroid distances [Å] 3.84, 3.85; ring plane/
ring plane inclinations [°]: 21.0, 15.4.
Figure 2. A ball–stick view of the X-ray packing crystal structure
of 3bʛ4a. Lariat ether 3b is red, paraquat derivative 4a is blue,
To further investigate the complexations between 3b and
4a or 4b, 1H NMR spectra of separate equimolar (8.00 m)
CD3COCD3 solutions of 3b with 4a or 4b were examined
(Figures 4 and 5). Both complexation systems 3bʛ4a and
–
PF6 counterions are purple, and the water molecule is magenta.
Hydrogen atoms are magenta, oxygen atoms are green, nitrogen
atoms are black, fluorine atoms are purple, and the phosphorus
atom is yellow. The other PF6– counterions, solvent molecules, and
hydrogen atoms except the ones involved in hydrogen bonding be-
tween lariat ether 3b and paraquat 4a were omitted for clarity. Hy-
drogen bond parameters: H···O (F) distance [Å], C(O)–H···O (F)
angle [°], C(O)···O (F) distance [Å] a, 2.46, 170, 3.38; b, 2.49, 161,
3.41; c, 2.17, 150, 2.92. Face-to-face π-stacking parameters:
centroid–centroid distances [Å] 3.78, 3.66; ring plane/ring plane in-
clinations [°]: 14.5, 13.4.
1
3bʛ4b undergo fast exchange on the H NMR timescale.
Upfield shifts were observed for pyridinium protons H10
and H11 and N-methyl protons H12 of paraquat 4a after
complexation (Figure 4, a and b). Aromatic protons H1, H2,
and H3, benzyl protons H6 and ethyleneoxy protons H7
shifted upfield, whereas aromatic protons H4 and H5 and
ethyleneoxy protons H8 and H9 of 3b moved downfield af-
The complex 3bʛ4b is also a [2]pseudorotaxane in the ter complexation (Figure 4, b and i). From BMP26C8 3a to
solid state (Figure 3). The same as for the complex 3bʛ4a, lariat ether 3b, two benzyloxy groups are introduced, mak-
3bʛ4b is also stabilized by face-to-face π-stacking and ing 3b a good host for K+. When KPF6 was gradually
charge-transfer interactions between the four phenylene added to an equimolar (8.00 m) solution of 3b and 4a in
rings of host 3b and the two bipyridinium rings of guest 4b. CD3COCD3, the chemical shifts of protons on 4a evolved
The two pyridinium rings of paraquat 4b are also coplanar to their uncomplexed values; correspondingly, the yellow
in 3bʛ4b (Figure 3). Different from the complex 3bʛ4a color of the solution became lighter and lighter, indicating
(Figure 2), two β-pyridinium protons of paraquat 4b are di- the dissociation of complex 3bʛ4a. When 32 equiv. of
rectly hydrogen-bonded to the ether oxygen atoms of lariat KPF6 was added, 3bʛ4a was mostly disassociated (Fig-
ether host 3b. In the packing structure, two 3bʛ4b com- ure 4, a, g, and i) and the solution was nearly colorless.
plexes are linked by hydrogen bonds formed between the However, when 32 equiv. of DB18C6 was subsequently
hydroxyethyl groups on two 4b molecules and four 3bʛ4b added, chemical shift changes of protons on 4a were ob-
complexes are linked by a PF6– anion. Therefore, the whole served again (Figure 4, a and h), and correspondingly, the
structure is stabilized by face-to-face π-stacking and hydro- yellow color of the solution returned, indicating the refor-
gen-bonding interactions. It can be concluded from the so- mation of complex 3bʛ4a.[19] Therefore, K+ can be used as
Eur. J. Org. Chem. 2010, 5543–5547
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5545

M. Zhang, Y. Luo, B. Zheng, B. Xia, F. Huang
SHORT COMMUNICATION
a switch to control the association of lariat ether 3b and much smaller than that of 3bʛ4a. These changes were also
paraquat 4a, and the disassociation percentage depends on evidenced by 1H NMR spectroscopy. When 2 equiv. of
the concentration of the added K+.
KPF6 was added to an equimolar solution of 8.00 m 3b
and 4b in CD3COCD3, the chemical shifts of protons on
4b returned to almost their uncomplexed values (Figure 5,
a and c), and correspondingly, the yellow color of the solu-
tion disappeared, indicating the dissociation of complex
3bʛ4b. However, when 2 equiv. of DB18C6 was sub-
sequently added, chemical shift changes of protons on 4b
were observed again (Figure 5, a and d), and correspond-
ingly, the yellow color of the solution returned, indicating
the reformation of complex 3bʛ4b.[19]
Conclusions
In summary, based on BMP26C8, we synthesized a lariat
ether host, 3b, for paraquat derivatives and found that it
could form pseudorotaxanes with them. Due to the intro-
duction of two benzyloxy groups, the association constant
increased about 13 times from 390(Ϯ30) –1 for 3aʛ4a to
5.3(Ϯ0.5)ϫ103 –1 for 3bʛ4a. The N-methyl substitution
had an important influence on the binding strength be-
tween BMP26C8-based lariat ether 3b and paraquat deriva-
tives. Moreover, we demonstrated that a change in the K+
concentration could be used as a switch to control the as-
sembly process of lariat ether 3b and paraquat derivative
guests. Further work will be the application of the recogni-
tion of 3b to paraquat derivatives in the efficient fabrication
of more complicated systems, including molecular ma-
chines.
1
Figure 4. H NMR spectra (400 MHz, CD3COCD3, 295 K) of (a)
4a, (b) 8.00 m 3b and 4a; 8.00 m 3b and 4a and (c)16.0 m
KPF6, (d) 32.0 m KPF6, (e) 64.0 m KPF6, (f) 128 m KPF6, (g)
256 m KPF6; (h) 8.00 m 3b and 4a and 256 m KPF6 and
DB18C6, and (i) 3b.
Supporting Information (see footnote on the first page of this arti-
cle): Synthetic procedures, characterizations, crystal data for 3bʛ4a
and 3bʛ4b, Job plots, and UV/Vis data.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (20774086, 20834004, and J0830413) and the
Fundamental Research Funds for the Central Universities
(2010QNA3008).
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[19] The chemical shift changes of the protons on the host or guest
after complexation could not be fully recovered. Three possible
reasons are: (1) the solution was diluted; (2) the ionic strength
of the solution increased; (3) different counterions are intro-
duced, influencing ion pairing. These factors could change the
chemical shifts and/or decrease the complexation percentage.
Received: June 30, 2010
Published Online: September 10, 2010
Eur. J. Org. Chem. 2010, 5543–5547
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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