Template-directed synthesis of pyridazine-containing tetracationic cyclophane for construction of [2]rotaxane

Article abstract of DOI:10.1016/j.cclet.2016.12.002

Benefiting from its bent molecular structure, 3,6-pyridazinyl contained tetracationic cyclophane (1) is synthesized by template-directed method with high isolated yield up to 92%. This template-directed strategy is further utilized to efficiently construct [2]rotaxane.

Full text of DOI:10.1016/j.cclet.2016.12.002

G Model
CCLET 3907 1–6
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
Original article
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3
Template-directed synthesis of pyridazine-containing tetracationic
cyclophane for construction of [2]rotaxane
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Q1
*
Qiu-Sheng Fang, Ling Chen , Qing-Yan Liu
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Department College of Chemistry and Chemical Engineering, Key Laboratory of Functional Small Organic Molecules, Ministry of Education, Institution Jiangxi
Normal University, Nanchang 330022, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Benefiting from its bent molecular structure, 3,6-pyridazinyl contained tetracationic cyclophane (1) is
synthesized by template-directed method with high isolated yield up to 92%. This template-directed
strategy is further utilized to efficiently construct [2]rotaxane.
Received 13 October 2016
Received in revised form 9 November 2016
Accepted 25 November 2016
Available online xxx
© 2016 Ling Chen. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of
Medical Sciences. Published by Elsevier B.V. All rights reserved.
Keywords:
Tetracationic cyclophane
Template effect
Inclusion complex
p
-Stacking
Rotaxane
7
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31
1. Introduction
versatile cavity size, but also bring about the variable conformation
due to the low symmetry of most heterocycles [3,5].
This realization prompted the design of new tetracationic
cyclophane that utilizes the heterocycle pyridazine as part of
8
9
Recently, great progress has been achieved in the design
and synthesis of functionalized molecular machines. In this regard,
interlocked molecules as promising candidates have been received
much attention and inventive applications in catalysts, sensors,
photoelectronics and metamaterials [1]. One attractive approach is
to exploit new molecular recognition leading to highly efficient
synthesis of interlocked structures such as rotaxanes and
catenanes, and molecular devices [2]. Of the potential classes of
macrocyclic compounds for use in interlocked molecules and
molecular devices, tetracationic cyclophanes are often employed
because of their favorable electronic, redox property and diverse
binding ability. These host molecules usually possess a cavity
around with two
linkers, in which a wide range of
bound. Well-defined variations [3] on the
linkers can dramatically influence the supramolecular behaviors.
Although lots of classic structures have been reported, the design
and synthesis of new cationic cyclophanes are still important to be
exploited in supramolecular chemistry [4]. The desired variation
may be executed by introducing a variety of heterocycles as
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p
-deficient moiety. Here, we represented a novel tetracationic
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cyclophane (1) consisted of two 3,6-bis(4-pyridinium)pyridazine
and two 1,4-bis(methylene)phenylene. The new host 1 has a
moderate bent geometry arose from the low symmetry of
pyridazine rings. 1 shows ideal template effect which leads to
high macrocyclization yield up to 92% in presence of appropriate
template. Since template methods are often employed to make
macrocyclic molecules and interlocked molecules, the synthesis of
1 was extended to made [2]rotaxane with excellent yield. As
detailed below, a series of experiments were carried out to
investigate the molecular recognition and template effect by
NMR, UV spectra, X-ray crystallography, and synthesis of a [2]
rotaxane.
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Q2 37
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p
-deficient extended planes and two aromatic
-efficient guest molecules can be
-deficient and aromatic
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p
43
p
44
2. Results and discussion
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Synthetic routes for 1 are shown in Scheme 1. The starting
precursor 3,6-pyridazyl extended bipyridine (2) was prepared by
well-described [4+2] cycloaddition reaction, executed by reacting
4-cyanopyridine with hydrazine hydrate to obtain 3,6-di-4-
pyridyl-1,2,4,5-tetrazine, then [4+2] cycloaddition was carried
out by reacting tetrazine with ethyl vinyl ether [6]. And the
addition product bipyridine 2 is easily obtained on one hundred
gram scale. Then, macrocyclization of 1 was proceeded in two steps
p
-moiety. Heterocyles not only endow the cyclophanes with
* Corresponding author.
E-mail address: chenlingjxnu@163.com (L. Chen).
http://dx.doi.org/10.1016/j.cclet.2016.12.002
1001-8417/© 2016 Ling Chen. Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002

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Q.-S. Fang et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
Scheme 1. Two-step synthesis of 1.
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from 2 using excess pyrene as template, which closely resembled
the other tetracationic cyclophanes reported before. Using pyrene
as template, bisbromomethyl[bis-p-benzyl-4,40-(3,6-pyridazinyl)-
bispyridine] bis(hexafluorophosphate) (3) was reacted with
equimolar amounts of 2 in dry MeCN at room temperature for
20 days. After removal of template and counterion exchange, 1 was
isolated as tetrakis(hexafluorophosphate) salt with yield up to 92%.
In contrary, the isolated yield dropped to only 17% without pyrene
template. These results indicate that appropriate template such as
pyrene is a key factor for the efficient synthesis of 1. It is also found
that such example of nearly quantitative macrocyclization is
uneasy to be achieved synthetically in supramolecular chemistry
[7], and the newly synthesized 1 allows us to explore it as an useful
host for molecular recognition and interlocked molecules.
and H3 on 3 were shifted upfield, indicating that 3 associated with
pyrene to form a -donor–acceptor complex with -electron
p
p
shielding of the face-to-face oriented aromatic rings (Fig. S12 in
Supporting information). Then, 1H NMR spectrum was recorded
after 4 days (Fig. 1b), showing several new signals which was
assigned to the protons in pyreneꢀ1 complex. The relative
intensities of new signals suggested about 53% of the starting
materials had been converted to pyrene complex as pyreneꢀ1.
During a reaction period between 5 min to 20 days, the signals
corresponding to 2 and 3 disappeared gradually, and those for the
pyreneꢀ1 complex appeared accordingly. The reaction was finally
completed after about 20 days, which exhibited only five groups of
proton signals with respect to pyreneꢀ1 complex, suggesting that
1 was almost exclusively formed in solution (Ha– ). In comparison,
e
At the outset, the macrocyclization process of 1 with excess
pyrene as template was investigated by 1H NMR spectroscopy. As
shown in Fig. 1a, upon addition of excess pyrene to an equimolar
mixture of 2 and 3 in CD3CN in a NMR tube, the signals for H1, H2
the 1H NMR spectra of equimolar-mixed of 2 and 3 without pyrene
template was recorded at the same experimental condition
(Fig. S13 in Supporting information). Although the proton signals
corresponding to 2 and 3 disappeared after 20 days, only several
Fig. 1. Partial 1H NMR spectra of a mixture containing 2 (3 mmol/L), 3 (3 mmol/L) and pyrene (15 mmol/L) in CD3CN at 25 ꢁC for (a) 5 min, (b) 4 days, (c) 8 days, (d) 11 days, (e)
15 days and (f) 20 days.
Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002

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Table 1
Q7
The binding parameters of PAHs toward with 1 determined by UV–vis spectroscopy
titration.
PAHs
p-electron count
DG (kJ/mol)
Ka (L/mol)
Naphthalene
Phenanthrene
Anthracene
10
14
14
16
16
16
18
9.14
40
18.13
16.65
20.67
15.46
21.24
23.37
1.50 ꢂ102
8.27 ꢂ102
4.18 ꢂ 103
5.11 ꢂ102
5.27 ꢂ103
1.24 ꢂ104
Pyrene
2,7-Di-tert-butyl-pyrene
Fluoranthene
Triphenylene
89
90
weak peaks of 1 could be observed, indicative of the macro-
cyclization process occurred in low efficiency.
Fig. 2. Chemical structures of seven PAHs.
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Subsequently, seven polycyclic aromatic hydrocarbons (Fig. 2,
PAHs) guests varying from two to four rings were selected to study
their capability for the forming inclusion complexes with 1 by UV
spectroscopy titration [8]. As shown in Table 1, the association
constants (Ka) for the seven guests with 1 ranging from 40 L/mol
for naphthalene to 1.24 ꢂ104 L/mol for triphenylene. The two
guests pyrene and fluoranthene which have similar molecular size
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118
face-to-face oriented
p-stacking [9], while the small shift values
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of Ha indicate that the stacking mainly occurs between pyrene/
anthracene and the central of pyridazine-bridged bipyridinium.
Moreover, the significant downfield shift for the para-xylylene
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119
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120
(C6H4) protons on 1 could be related to the CꢃꢃHꢄ ꢄ ꢄ
p interaction
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121
between the hydrogen atoms on guests and para-xylylene plane.
From the 1H NMR spectrum it is clearly deduced that the
noncovalent interaction between aromatic guest and 1 is mainly
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122
and
1. On contrary, although pyrene and 2,7-di-tert-butyl-pyrene have
the same -electron count and aromatic core, the Ka value for 2,7-
p-electron count displayed very close binding strength toward
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103
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112
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124
p
consisted of face-to-face and edge-to-face p-stacking interaction.
125
di-tert-butyl-pyrene is far smaller than that for pyrene. This could
be attributed to steric hindrance of tert-butyl group that seriously
restricts the penetration of pyrene plane from the 1,6- or 2,7-
Moreover, the complexation of pyreneꢀ1 and anthraceneꢀ1 was
also proved by ESI-MS experiments. As expected, the peaks at m/z
219.60, and 213.60 could be clearly assigned to [(14+ + pyrene)/4]+
and [(14+ + anthracene)/4]+, respectively.
Next, in order to probe the structural characteristics of 1 upon
binding with PAHs, two inclusion complexes anthraceneꢀ1 and
pyreneꢀ1 were investigated in the solid state by single crystal
X-ray diffraction. As shown in Fig. 4, both the two guests are bound
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orientation, the restricted
p-stacking therefore drop the binding
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strength of 2,7-di-tert-butyl-pyrene.
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The molecular binding of pyrene and anthracene by 1 was
selected to investigate in CD3CN by 1H NMR spectroscopy. As can
be seen in Fig. 3, the proton signals of two complexes (pyreneꢀ1
and anthraceneꢀ1) exhibited fast-exchange equilibrium on
1H NMR time scale. The two complexes displayed remarkable
upfield shifts for almost all of the aromatic protons on both the
host and guests except the slight upfield shifts for Ha, whereas
there was only a significant downfield shift corresponding to the
resonances associated with the para-xylylene protons (Hd). These
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in the cavity by face-to-face and edge-to-face
p-stacking interac-
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tion. The pyridazine rings locate in the central of bipyridiniums
moieties with almost planar conformation, bringing two geomet-
rical characteristics.
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(1) The pyridazine bridged bipyridinium has bent geometry.
Nitrogen atoms have smaller radius than carbon atoms, and
upfield shifts were originated from mutual p-electron shielding of
1
Fig. 3. Partial H NMR spectra of (a) pyrene, (b) pyreneꢀ1 complex, (c) 1, (d) anthraceneꢀ1 complex, (e) anthracene in CD3CN at 25 ꢁC, respectively (400 MHz,
[pyrene] = [1] = [anthracene] = 4 mmol/L).
Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002

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(2) The orientation of the two guests bound in the cavity is quite
geometry of both two
distinguishable. Indeed, the CꢃꢃHꢄ ꢄ ꢄ
p
complexes is identically parallel to the bipyridiniums moieties.
However, in the crystal structure of anthraceneꢀ1, the most
distant 2,6-H atom being 9.18 Å apart to each other get close
contact of CꢃꢃHꢄ ꢄ ꢄ
p with para-phenylene, the H–p distance is
only 2.60 Å. In the case of pyreneꢀ1, the comparable CꢃꢃHꢄ ꢄ ꢄ
p
contact do not take place on the most distant 2,7-H atom
(dH2–H7 = 8.83 Å) but the 1,6-H atom (dH1–H6 = 7.65 Å) instead
with H–
phenomenon indicates that the noncovalent interaction for
portion than that for
p distance of 3.32 Å (Fig. 4). This unexpected
anthraceneꢀ1 has more CꢃꢃHꢄ ꢄ ꢄ
p
pyreneꢀ1, which can also be explained by the bent structure
of pyridazine-bridged bipyridiniums. Each of the two pyrida-
zine moieties were pushed away from the central of the cavity
for 1.18 Å due to the bent geometry, which subsequently induce
the short and wide pyrene guest to turn to the pyridazines and
obtain more
cene with long and narrow size has less stacked
and incline to locate in the cavity along 1,6-orientation to get
close contact of CꢃꢃHꢄ ꢄ ꢄ as much as possible.
p-stacking surface area. In comparison, anthra-
p-surface area
p
Fig. 4. Crystal structure and molecular size of the complexes for 1 with anthracene
(a) and pyrene (b). Solvent molecules, counter ions and partial hydrogen atoms are
omitted for clarity.
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Above experiments revealed that the bent geometry of
macrocyclic molecule 1 significantly influences the moleclular
recognition and template effect, it is important to extend this
efficient macrocyclization strategy to synthesize interlocked
molecules like rotaxane [12]. Favorable synthetic yield is expect-
able to be achieved if appropriate axles with pyrene core is
selected, which is important in rotaxane synthesis. A tailor-made
axle 4 possessing one pyrene binding site and two tetraphenyl
stoppers was synthesized by Sonogashira reaction. The axle 4 has
limited solubility in acetonitrile (about 3 mmol/L) and good
solubility in chloroform. By mixing the precursors 2, 3, and 4 in
CH3CN for 20 days at room temperature, [2]rotaxane 5 was
obtained with 84% isolated yield at last (Scheme 2). 1H NMR
spectra demonstrated that the reaction has obviously progressed
after 0.5 day, and after 20 days there are still some unreacted
starting material, which might be the limited solubility of axle 4 in
CD3CN. The 1H NMR spectra of purified 5 showed the binding and
blocking of [2]rotaxane were successfully achieved, as discerned
from the appearance of signals for 1 (Fig. 5), further corroborating
that 1 could act as an useful macrocyclic host for the template-
directed synthesis of interlocked molecules.
thus the sp2 bond angles on the neighboring 3,6-position of
pyridazine rings deviate from the idealized 120ꢁ. The bond
nearby the two nitrogen atoms incline to the nitrogen atoms
for about 5ꢁ, and the plane of 3,6-pyridazine-bridged
bipyridinium is bent totally for about 12ꢁ (Fig. 4). It is well-
known that tetracationic cyclophanes have a certain degree of
inner strain due to the bond angle restriction [10]. Bent
aromatic moieties of cationic cyclophane can usually induce
lower bond angle strain and modified the synthetic yield
[3e,4]. Some examples with imidazole groups even display
flexible conformation and an ability to alter ring shape to
accommodate different guests [11]. Here the moderate bent
geometry in tetracationic cyclophane 1 could relax the inner
bond strain to a certain degree while the main binding ability of
1 toward aromatic guests is withheld. This provides modest
encouragement and motivation to the trend of macrocyclic
formation, and eventually results in a highly efficient template-
directed macrocyclization.
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Scheme 2. Synthesis of [2]rotaxane 5.
Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002

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Fig. 5. 1H NMR spectra of a mixture containing 2, 3, and 4 for (a) 0.5 day; (b) 20 days; (c) 20 days followed with purification.
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3. Conclusion
collected and column chromatographed over a shot silica gel using
MeCN to obtain pure 3 as a yellow solid (3.1 g, 82%). 1H NMR
200
In conclusion, this work has demonstrated the introduction of
pyridazine as part of
(400 MHz, CD3CN):
8.60 (s, 2H), 7.59 (d, 4H, J = 8.2 Hz), 7.56 (m, 4H), 5.83 (s, 4H), 4.64 (s,
4H). 13C NMR (100 MHz, CD3CN):
151.32 (s), 145.36 (s), 140.31 (s),
d 8.96 (d, 4H, J = 6.9 Hz), 8.84 (d, 4H, J = 6.9 Hz),
201 Q5
202
p-moiety can dramatically improve the
template effect of the tetracationic cyclophane 1, which showed
excellent yield in macrocyclization and synthesis of [2]rotaxane.
This host–guest system ideal template effect can act as significant
candidate in the construction of more advanced molecular
machines.
d
203
132.93 (s), 130.20 (s), 129.70 (s), 127.55 (s), 126.11 (d, J = 7.8 Hz),
117.42 (s), 63.94 (s), 32.62 (s). HRMS (ESI): m/z calcd. for
(C30H26Br2F6N4P)+ [MꢃPF6]+, 745.0165; found, 745.0158.
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4.3. Synthesis of 1
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4. Experimental
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3 (1.20 g, 1.34 mmol), 2 (0.315 g, 1.36 mmol), and the template
pyrene (1.20 mg, 5.93 mmol) to dry MeCN (500 mL) and stirring
under N2 at room temperature for 15 days. Then tetraethylammo-
nium bromide was added to get the inclusion complexes pyreneꢀ1
(bromide) as red precipitate. The precipitate was collected by
filtration, washed with MeCN and dissolved in water (1 L). Then the
aqueous solution was extracted 30 times with CH2Cl2/toluene
(v/v = 1:1, 100 mL per extraction), while the color of the solution
change from red-orange to white. Then NH4PF6 (1.0 g) in 50 mL H2O
was added and precipitate was formed and collected by filtration to
obtain gray yellow crude product (1.60 g, 95%). The pure 1 was
obtained by further refluxing in CHCl3/acetone (v/v = 2:1, 100 mL)
for 1 h. After cooling down, the precipitate was collected by
filtration and then dried in vacuum to yield the product as gray
yellow solid (1.55 g, 92%) which was pure after examination of
4.1. General
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All chemicals were commercially available unless otherwise
noted. 1H NMR spectra were performed on a BrukerAV400
spectrometer. Mass spectra were performed on Agilent
6520 Q-TOF LC/MS (ESI). Absorption spectra were recorded on a
Hitachi U-3310 UV–vis spectrometer. The X-ray intensity data
for pyreneꢀ1 and anthraceneꢀ1 were collected on a Rigaku
MM-007 rotating anode diffractometer equipped with
Saturn724 CCD Area Detector System, using monochromated Mo
–K = 0.71073 Å) radiation at T = 296(2) K.
a
a (l
218
4.2. Synthesis of 3
0-Dibromo-p-xylene (10.0 g, 37.9 mmol) was added to MeCN
a,a
1H NMR. 1H NMR (400 MHz, CD3CN):
(d, 4H, J = 6.9 Hz), 8.44 (s, 4H), 7.67 (d, 8H, J = 8.2 Hz), 5.80 (s, 8H).
13C NMR (100 MHz, CD3CN):
154.28 (s), 150.98 (s), 144.87 (s),
d 8.95 (d, 4H, J = 6.9 Hz), 8.64
219
220
221
222
223
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(50 mL) and refluxed under N2. After all the solid dissolved, 1 (1.0 g,
4.3 mmol) in hot MeCN (250 mL) was added slowly over 3 h. Then
the mixture was reflux for further 12 h and yellow precipitate
formed. The yellow solid was collected by filtration, washed with
MeCN and CH2Cl2, and then dissolved in cold (ꢅ25 ꢁC) MeOH
(ꢆ2 L). Addition of NH4PF6 (1.0 g) in 50 mL H2O and then H2O (ꢆ2 L)
resulted in 3 as yellow precipitate. The yellow precipitate was
d
135.89 (s), 130.38 (s), 127.27 (s), 126.20 (s), 117.44 (s), 64.34 (s).
HRMS (ESI): m/z calcd. for (C44H36F12N8P2)2+ [(Mꢃ2PF6)/2]+,
483.1172; found, 483.1162.
Other compounds were synthesized using the method
described in the Supporting information. The spectra of all
Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002

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compounds were also deposited in Supporting information. The
crystallographic data of anthraceneꢀ1 and pyreneꢀ1 (CCDC
1508889 and 1508890, respectively) can be obtained in Supporting
information and free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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We gratefully acknowledge the financial support of NNSF of
China (Nos. 21402069 and 21361011) and the Project of Jiangxi
Provincial Education Department (No. GJJ14264).
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in
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Please cite this article in press as: Q.-S. Fang, et al., Template-directed synthesis of pyridazine-containing tetracationic cyclophane for
construction of [2]rotaxane, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.12.002