Synthesis of a novel triptycene-based molecular tweezer and its complexation with paraquat derivatives

Article abstract of DOI:10.1021/ol070017n

(Chemical Equation Presented) A novel triptycene-based molecular tweezer has been synthesized, and its complexation with paraquat derivatives in solution and in the solid state has been studied. Due to its electron-rich cavity, the molecular tweezer could

Full text of DOI:10.1021/ol070017n

ORGANIC
LETTERS
2007
Vol. 9, No. 5
895-898
Synthesis of a Novel Triptycene-Based
Molecular Tweezer and Its Complexation
with Paraquat Derivatives
Xiao-Xia Peng,†,‡ Hai-Yan Lu,† Tao Han,† and Chuan-Feng Chen*,†
Beijing National Laboratory for Molecular Sciences, Laboratory of Chemical Biology,
Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and
Graduate School, Chinese Academy of Sciences, Beijing 100049, China
cchen@iccas.ac.cn
Received January 4, 2007
ABSTRACT
A novel triptycene−based molecular tweezer has been synthesized, and its complexation with paraquat derivatives in solution and in the solid
state has been studied. Due to its electron-rich cavity, the molecular tweezer could form stable complexes with paraquat derivatives with
different functional groups. Moreover, it was also found that formation of the complexes was caused by a charge transfer interaction and the
complexes dissociated upon two one-electron reduction of the bipyridinium ring.
The design of novel efficient synthetic receptors with the
capability for strongly and selectively binding a substrate is
a permanent and challenging topic in the development of
new supramolecular systems with specific structures and
properities.1 Recently, molecular tweezers,2 which are com-
posed of a tether and two flat, generally aromatic pincers,
have attracted increasing interest for their specific structures,
convenient synthesis, and wide potential applications in
biological and supramolecular chemistry.3
the synthesis and binding properties of a novel tweezer-like
receptor 1, which is composed of two triptycene units linked
by two crown ether chains (Figure 1). Due to its electron-
Triptycene,4 with its unique three-dimensional rigid struc-
ture and electron-rich property, has been found to be a useful
building block for the construction of novel receptors.5 We
deduced that with triptycene as the building block, novel
molecular tweezers could also be formed. Herein, we report
† Institute of Chemistry.
‡ Graduate School, Chinese Academy of Sciences.
(1) (a) Lehn, J. M. Supramolecular Chemistry Concepts and PerspectiVes;
Wiley-VCH, Weinheim, 1995. (b) Atwood, J. L.; J. W. Steed, Supramo-
lecular Chemistry; Wiley-VCH: Weinheim, 2000.
Figure 1. Structure and proton designations of the receptor 1 and
guests G1-G6.
(2) (a) Chen, C. W.; Whitlock, H. W. J. Am. Chem. Soc. 1978, 100,
4921-4922. (b) Zimmerman, S. C. Top. Curr. Chem. 1993, 165, 71-102.
(c) Klarner, F.; Kahlert, B. Acc. Chem. Res. 2003, 36, 919-932. (d)
Harmata, M. Acc. Chem. Res. 2004, 37, 862-873.
rich cavity, the receptor 1 could form stable complexes with
the paraquat derivatives6 with different functional groups in
10.1021/ol070017n CCC: $37.00
© 2007 American Chemical Society
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solution and in the solid state. Moreover, it was also found
that formation of the complexes could be caused by a charge-
transfer interaction and the complexes could dissociate upon
two one-electron reductions of the bipyridinium salt.
Synthesis of the receptor 1 is outlined in Scheme 1. First,
We first investigated the complex ability of receptor 1
toward paraquat derivatives in solution. Consequently, mix-
ing 1 and guest G1 (2 mM each) in chloroform and
acetonitrile (1:1) showed a swift color change from pale
yellow to a light red-brown, which might be due to the charge
transfer between the electron-rich aromatic ring of 1 and the
bipyridinium ring of G1. Similar phenomena also took place
for other guests. As shown in Figure 2, the 1H NMR spectrum
Scheme 1. Synthesis of the Receptor 1
Figure 2. Partial 1H NMR spectra (300 MHz, 1:1 CDCl3/CD3CN,
298 K) of (a) free 1, (b) 1 and 1.0 equiv of G1, and (c) free guest
G1. [1]0 ) 2.0 mM.
triptycene derivative 27 was obtained by treating anthrancene
with 4,5-dimethoxybenzenediazonium-2-carboxylate in 1,2-
dichloroethane in the presence of 1,2-epoxypropane. Dem-
ethylation of 2 with BBr3 in CH2Cl2 gave the compound 3
in 78% yield. Triptycene derivative 5 was then achieved in
85% yield by the reaction of 8-tosyloxy-3,6-dioxaoctanol 4
and compound 3 in CH3CN in the presence of K2CO3. By
the treatment of compound 5 with TsCl in CH2Cl2 in the
presence of Ag2O and KI, the ester product 6 was obtained
in 94% yield. Finally, the target molecule 1 was produced
in 31% yield by the reaction of 6 and 3 in DMF in the
presence of Cs2CO3. All new compounds were confirmed
by 1H NMR, 13C NMR, MS spectra, and elemental analysis.8
of a mixture of 1 and G1 in CDCl3 and CD3CN (1/1) showed
a great difference with those for 1 and G1 (Figure 2). The
H9 and H10 proton signals of G1 shifted significantly upfield,
which might be due to the strong shielding effect of the
aromatic rings of 1. Similarly, the proton H3 of aromatic
ring linked with the crown ether chains and the proton H4
also showed considerable upfield shifts. Although there are
no obvious changes for the protons H1 and H2 of 1, obvious
changes were observed for the crown protons H5-H7. These
observations all suggested that a new complex between 1
1
and G1 was formed. The H NMR spectroscopic titrations
further gave a quantitative estimate for the complex of 1
and G1 by monitoring the changes of the chemical shift of
the proton H3. The results showed that a 1:1 complex 1‚G1
was formed by a mole ratio plot. Accordingly, the association
(3) Some recent examples: (a) Balzani, V.; Ceroni, P.; Giansante, C.;
Vicinelli, V.; Kla¨rner, F.; Verhaelen, C.; Vo¨gtle, F.; Hahn, U. Angew. Chem.,
Int. Ed. 2005, 44, 4574-4578. (b) Balzani, V.; Bandmann, H.; Ceroni, P.;
Giansante, C.; Hahn, U.; Kla¨rner, F.; Mu¨ller, U.; Mu¨ller, W. M.; Verhaelen,
C.; Vicinelli, V.; Vo¨gtle, F. J. Am. Chem. Soc. 2006, 128, 637-648. (c)
Huang, H.; Drueckhammer, D. G. Chem. Commun. 2006, 2995-2997. (d)
Gianneschi, N. C.; Cho, S.-H.; Nguyen, S. T.; Mirkin, C. A. Angew. Chem.,
Int. Ed. 2004, 43, 5503-5507. (e) Wu, Z. Q.; Shao, X. B.; Hou, J. L.;
Wang, K.; Jiang, X. K.; Li, Z. T. J. Am. Chem. Soc. 2006, 128, 17460-
17468. (f) Uno, H.; Masumoto, A.; Ono, N. J. Am. Chem. Soc. 2003, 125,
12082-12083. (g) Muraoka, T.; Kinbara, K.; Aida, T. Nature 2006, 440,
512-515.
(4) (a) Bartlett, P. D.; Ryan, M. J.; Cohen, S. G. J. Am. Chem. Soc.
1942, 64, 2649-2653. (b) Zhu, X. Z.; Chen, C. F. J. Org. Chem. 2005, 70,
917-924. (c) Zhang, C.; Chen, C. F. J. Org. Chem. 2006, 71, 6626-6629.
(5) (a) Zhu, X. Z.; Chen, C. F. J. Am. Chem. Soc. 2005, 127, 13158-
13159. (b) Zong, Q. S.; Chen, C. F. Org. Lett. 2006, 8, 211-214. (c) Han,
T.; Chen, C. F. Org. Lett. 2006, 8, 1069-1072. (d) Zhu, X. Z.; Chen, C. F.
Chem. Eur. J. 2006, 12, 5603-5609. (e) Zong, Q. S.; Zhang, C.; Chen, C.
F. Org. Lett. 2006, 8, 1859-1862.
constant between 1 and G1 was calculated to be Ka1·G1
)
4.08 × 103 M-1 by the Scatchard plot,5c,9 which is bigger
than that of the complex between dibenzo[24]crown-8 and
the paraquat.10
Furthermore, we tested the complexation of receptor 1
toward paraquat derivatives G2-G6 by NMR approach. The
results showed that similar to the complex 1‚G1, the alkyl
substituted paraquat derivatives G2 and G3 could also form
1:1 complex with the receptor 1. For the guests containing
terminal vinyl group, hydroxyl, and anthracyl group, a similar
but slightly weak complexation with receptor 1 was observed.
(6) Allwood, B. L.; Spencer, N.; Shahriari-Zavareh, H., Stoddart, J. F.;
Williams, D. J. J. Chem. Soc., Chem. Commun. 1987, 1064-1066.
(7) Gong, K. P.; Zhu, X. Z.; Zhao, R.; Xiong, S. X.; Mao, L. Q.; Chen,
C. F. Anal. Chem. 2005, 77, 8158-8165.
(9) Connors, K. A. Binding Constants; J. Wiley and Sons: New York,
1987.
(10) Huang, F.; Jones, J. W.; Slebodnick, C.; Gibson, H. W. J. Am. Chem.
Soc. 2003, 125, 14458-14464.
(8) See the Supporting Information.
896
Org. Lett., Vol. 9, No. 5, 2007

According to the Scatchard plot, the association constants
between 1 and the guests were further calculated.8 As shown
in Table 1, the substituted groups in the bipyridinium salts
the absence and presence of the receptor 1 was also studied.
As expected, the [G]2+ showed two reversible one-electron
reduction processes. The half-wave potential values are
summarized in Table 1, and the cyclic voltammetric (CV)
curve for [G2]2+ was shown in Figure 3. It was found that
the CV patterns for reduction of the paraquat derivatives were
Table 1. Half-Wave Potentials (E1/2/V vs SCE)a and
Association Constants
bpy2+ f bpy+
bpy+ f bpy
Kab (M-1
)
[G1]2+
-0.697
-0.622
-0.385
-0.338
-0.343
-0.270
-0.324
-0.245
c
-1.179
-1.049
-0.871
-0.813
-0.826
-0.751
-0.783
-0.695
c
1‚[G1]2+
[G2]2+
4.08 × 103
2.98 × 103
3.09 × 103
1.22 × 103
9.86 × 102
3.44 × 103
1‚[G2]2+
[G3]2+
1‚ [G3]2+
[G4]2+
1‚[G4]2+
[G5]2+
1‚[G5]2+
[G6]2+
-
-
-0.314
-0.250
-0.776
-0.693
1‚[G6]2+
a Solvent: CH2Cl2/CH3CN (9:1), [NBu4]PF6 0.1M, [G]/[1] ) 1:4, [G]
) 1.25 × 10-3 M. b From the 1H NMR titration experiments. Solvent:
CDCl3/CD3CN (1:1). c Adsorption on the electrode surface.
had no obvious influence on the stabilities of the complexes,
in which the driving forces might be mainly attributed to
the noncovalent interactions between the molecular tweezer
1 and the bipyridinium ring of the guests.
The electrospray ionization mass spectra provided another
evidence for formation of 1:1 stable complexes between 1
and paraquat derivatives G1-G6. As a result, the strong
peaks at m/z 493.7, 507.7, 591.9, 519.7, 523.7, and 670.0
for [1‚G1-2PF6]2+, [1‚G2-2PF6]2+, [1‚G3-2PF6]2+,
[1‚G4-2PF6]2+, [1‚G5-2PF6]2+, and [1‚G6-2PF6]2+, re-
spectively, were all observed.8
Figure 4. Two views (a) and (b) of the crystal structure of the
complex 1‚G1. Hydrogen bond distances (Å): a ) 2.31; b ) 2.53;
c ) 2.61; d ) 2.62; e ) 2.60; f ) 2.54; g ) 2.71; h ) 2.45; i )
2.49; j ) 2.70. (c) Crystal packing of the complex viewed along
-
the c-axis. Solvent molecules, PF6 counterions, and hydrogen
atoms not involved in the noncovalent interactions are omitted for
clarity.
remarkably affected by the presence of 1, which showed that
the formation of the complexes involved an interaction
between 1 and the bipyridinium salts. In particular, both the
cathodic and anodic peaks corresponding to the first and
second one-electron reduction process of the bipyridinium
core moved to less negative values upon the addition of 1,
which is different from that in the presence of other
hosts.3a,b,11 Such a behavior suggested that formation of the
complexes between 1 and the paraquat derivatives was
caused by a charge transfer interaction and the complexes
Figure 3. CV curves for a solution of [G2]2+ (1.25 × 10-3 M) in
CH2Cl2/CH3CN (9:1)-(NBu4)PF6 (0.1 M) in the absence (black
line) and presence (red line) of the receptor 1 (5.0 × 10-3 M).
Scan rate: 0.2 Vs-1
.
Because the guests contain the well-known 4, 4′-bipyri-
dinium electroactive unit, their electrochemical behavior in
Org. Lett., Vol. 9, No. 5, 2007
897

dissociated upon two one-electron reduction of the bipyri-
dinium ring.
Interestingly, it was also found that a C-H···π interaction
with distance of 2.78 Å (D) between the two adjacent
molecules existed. In particular, due to the unique structural
property of triptycene unit, the adjacent receptors could be
connected with each other by the complexation with the
paraquat salts (Figure 4b), in which the driven forces were
multiple π-π stacking (G and H) and C-H···π interactions
(I-M) between the bipyridinium ring and phenyl rings of
triptycene units. Consequently, a waveform strcture was
formed, which could further result in a 2D layer viewed along
the c-axis (Figure 4c) and 3D microporous network.8
Further support for the formation of the complex 1‚G1
came from its X-ray diffraction results.12 It was found that
compared to a simple dibenzo[24]crown-8,10 the host 1
showed completely different binding properties with the
paraquat G1 in the solid state. As shown in Figure 4a, in
one crystal cell, there are two tweezer-like molecules with
different orientation, and each molecule tweezered a bipy-
ridinium guest by different complexation modes. In one case,
there existed not only multiple hydrogen bonds between the
polyether oxygen atoms and protons of methyl group and
aromatic protons in G1, but also a C-H···π interaction with
distance of 2.75 Å (A) and π-π stacking interactions
between phenyl rings of triptycene units and the bipyridinium
ring with distances of 3.28 (B) and 3.36 Å (C). While in
another case, multiple hydrogen bonds between the receptor
1 and the guest G1, and π-π stacking interactions between
phenyl rings of triptycene units and the bipyridinium ring
with distances of 3.34 (E) and 3.34 Å (F) were only observed.
Due to the multiple noncovalent interactions, the complex
1‚G1 showed a high stability. Moreover, the guest in the
complex was very closed to the dibenzocrown-8 moiety, and
no obvious interactions between the guest and the other
aromatic rings of triptycene units were observed. These
results are consistent with those of the complex in solution.
In summary, we have synthesized a novel triptycene-based
molecular tweezer and demonstrated that it could form stable
complexes with paraquat derivatives with different groups
in solution and in the solid state. Moreover, it was also found
that formation of the complexes was caused by a charge
transfer interaction and the complexes dissociated upon two
one-electron reduction of the bipyridinium ring. More
applications of the molecular tweezer in supramolecular
chemistry are underway.
Acknowledgment. We thank the National Natural Sci-
ence Foundation of China, National Basic Research Program
(2007CB808004), and the Chinese Academy of Sciences for
financial support. We also thank Dr. H. B. Song for
determining the crystal structure of the complex.
(11) (a) Ceroni, P.; Vicinelli, V.; Maestri, M.; Balzani, V.; Mu¨ller, W.
M.; Mu¨ller, U.; Hahn, U.; Osswald, F.; Vo¨gtle, F. New J. Chem. 2001, 25,
989-993. (b) Jeon, W. S.; Kim, H. J.; Lee, C.; Kim, K. Chem. Commun.
2002, 1828-1829. (c) Ong, W.; Go´mez-Kaifer, M.; Kaifer, A. E. Org. Lett.
2002, 4, 1791-1794. (d) Wang, W.; Kaifer, A. E. Angew. Chem., Int. Ed.
2006, 45, 7042-7046.
Supporting Information Available: Experimental pro-
cedures and characterization data for the receptor and the
complexes. Cyclic voltammetry curves. The crystal structure
for the complex 1‚G1 and its CIF data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) Crystal data for 2(1‚G1)‚2G1‚6CH3CN‚H2O: chemical formula sum
164H172F48N14O17P8; Mr ) 3770.92; monoclinic; space group P1n1; a )
C
20.390(4) Å, b ) 18.300(4) Å, c ) 22.938(5) Å; â ) 92.504(4); V ) 8551-
(3) Å3; Z ) 2; T ) 113(2) K; 23425 reflections collected, 19402
independent, giving R1 ) 0.0913, wR2 ) 0.2487 for observed unique
reflection (I > 2σ(I)) and R1 ) 0.1033, wR2 ) 0.2654 for all data.
OL070017N
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Org. Lett., Vol. 9, No. 5, 2007