2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril: Encapsulation-Promoted Dimerization of Electrostatically Repulsing Pyridiniums

Article abstract of DOI:10.1002/asia.201701816

Rigid linear compounds G1 and G2, which contained two 4-phenylpyridinium (PhPy⁺) units, have been prepared to investigate their binding with cucurbit[8]uril (CB[8]). X-ray crystallographic structures revealed that in the solid state both compounds were included by CB[8], through antiparallel stacking, to form 2:2 quaternary complexes (G1)₂@(CB[8])₂ and (G2)₂@(CB[8])₂. For the former complex, CB[8] entrapped G1 by holding two heterodimers of its Py⁺ and benzyl units, which were at opposite ends of the backbone. In contrast, for the first time, the second complex disclosed parallel stacking of two cationic Py⁺ units of G2 in the cavity of CB[8] in the solid state, despite the generation of important electrostatic repulsion. Isothermal titrations in water afforded high apparent association constants of 4.36×10⁶ and 6.43×10⁶ m^?1 for 1:1 complexes G1@CB[8] and G2@CB[8], respectively, and ¹H NMR spectroscopy experiments in D₂O confirmed a similar stacking pattern to that observed in the solid state. A previous study and crystal structures of the 2:1 complexes formed between three new controls, G3–5, and CB[8] did not display such unusual stacking of the cationic Py⁺ unit; this may be attributed to the multivalency of the two CB[8] encapsulation interactions.

Full text of DOI:10.1002/asia.201701816

CHEMISTRY
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Accepted Article
Title: 2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril:
Encapsulation-Promoted Dimerization of Electrostatically
Repulsing Pyridiniums
Authors: Zhan-Ting Li, Bo Yang, Shang-Bo Yu, Hui Wang, and Dan-
Wei Zhang
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2:2 Complexes from Diphenylpyridiniums and Cucurbit[8]uril:
Encapsulation-Promoted Dimerization of Electrostatically
Repulsing Pyridiniums
Bo Yang,[a] Shang-Bo Yu,[a] Hui Wang,[a] Dan-Wei Zhang*[a] and Zhan-Ting Li*[a]
Abstract: Rigid linear compounds G1 and G2 which contained two 4-
phenylpyridinium (PhPy+) units have been prepared to investigate
their binding with cucurbit[8]uril (CB[8]). X-ray crystallographic
structures revealed that in the solid state both compounds were
included by CB[8], through anti-parallel stacking, to form 2:2
quaternary complexes (G1)2@(CB[8])2 and (G2)2@(CB[8])2. For the
former complex, CB[8] entrapped G1 by holding two heterodimers of
its Py+ and benzyl units, which were at the opposite ends of the
backbone. In contrast, for the first time, the second complex disclosed
parallel stacking of two cationic Py+ units of G2 in the cavity of CB[8]
in the solid state, despite the generation of important electrostatic
repulsion. Isothermal titrations in water afforded high apparent
association constants of 4.36  106 and 6.43  106 M-1 for 1:1
stoichiometry as well as refined structural parameters in the solid
state. In order to get insight into this potentially useful binding
pattern of higher degree and also to expand the scope of the
linear building blocks, we have prepared rigid compounds G1 and
G2 which contained two 4-phenylpyridinium units. X-ray crystal
structures revealed that both compounds formed new quaternary
2:2 complexes with CB[8] in the solid state. In particular, for the
first time, the crystal structure of the complex of G2 and CB[8]
disclosed the parallel-stacked dimerization of two electrostatically
repulsing pyridinium (Py+) units through the encapsulation of
CB[8]. Isothermal titration calorimetric (ITC) experiments showed
that both complexes possessed highly stable binding in water,
while 1H NMR experiments indicated that parallel-stacked
dimerization of pyridinium (Py+) units of G2 could also occur in
1
complexes G1@CB[8] and G2@CB[8], and H NMR experiments in
D2O confirmed a similar stacking pattern as observed in the solid state. solution. This paper describes the details.
Previous study and crystal structures of the 2:1 complexes formed
between three new controls G3-5 and CB[8] did not display such
unusual stacking pattern of the cationic Py+ unit, which may be
Results and Discussion
attributed to the multivalency of the two CB[8] encapsulation
interactions.
Previous studies by Zhao and co-workers established that CB[8]
encapsulated two PhPy+ units to form antiparallel 1:2 complex.[3b]
We and others have utilized this binding motif to generate a
number of two- and three-dimensional supramolecular organic
frameworks.[6] In this study, longer rod-like compounds G1 and
G2 were prepared to exploit higher binding motifs as well as to
investigate discrete factors that affect the binding stability and
selectivity. For comparison, G3-G5 were also prepared.
Introduction
Cucurbit[n]urils (CB[n]) represent a family of rigid macrocycles
that can bind discrete aliphatic and/or aromatic molecules driven
mainly by hydrophobicity in water.[1] Among others, CB[8] has
been revealed to encapsulate homo- or heterodimers of aromatic
units.[2,3] Currently this unique 1:2 binding motif has found wide
applications in the construction of a variety of supramolecular
polymers, gels and frameworks.[4-6] Incorporation of two aromatic
units into one backbone is expected to lead to enhanced binding
through higher 2:2 binding stoichiometry, which is potentially
useful for the construction of more stable advanced
supramolecular architectures. In this context, several elegant
examples have been reported.[7-9] However, all the complexes
have been investigated only in solution, and X-ray crystal
structures have never been available, even though they are
crucially important for providing direct evidences for the binding
[a]
B. Yang, S.-B. Yu, Prof. H. Wang, Prof. D.-W. Zhang, Prof. Z.-T. Li
Department of Chemistry, Collaborative Innovation Centre of
Chemistry for Energy Materials (iChEM), and Shanghai Key
Laboratory of Molecular Catalysis and Innovative Materials
Fudan University
220 Handan Road, Shanghai 200433, China
E-mail: zhangdw@fudan.edu.cn, ztli@fudan.edu.cn
Compound G1 was prepared by the methylation of 1[10] in 70%
yield (Scheme 1). For the preparation of G2 (Scheme 1), acid 2
was first treated with thionyl chloride to afford the corresponding
acyl chloride and then reacted with 3 in dichloromethane to
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produce 4 in 65% yield. The ester was then hydrolysed to acid 5
in 80% yield and the latter was treated with methyl iodide in DMF
to afford 6 in 90% yield. This acid was further coupled with 7 in
DMF to give rise to 8 in 52% yield. Finally, methylation of 8 in DMF,
followed by ion exchange, produced G2 in 70% yield. G3 was
prepared according to the reported method,[10] and G4 and G5
were obtained by the methylation of 4 or 5 in DMF both in 90%
yield.
a)
b)
c)
Figure 1. Crystal structures of the 1:2 complexes formed between CB[8] and a)
G3, b) G4 and c) G5.
Scheme 1. Synthesis of compounds G1 and G2.
Single crystals of the complex formed between G1 and CB[8]
were obtained by slow evaporation of their 1:1 mixture in water
containing cadmium nitrate which was used to increase
crystallinity. The X-ray crystal structure revealed a triclinic system
with space group P1 and a 2:2 binding motif in which two
molecules of G1 stacked in a antiparallel manner in the cavity of
two CB[8] molecules (Figure 2a). G1 had three aromatic units,
that is, AB, CD and E (see the structure for definement), that
potentially underwent CB[8]-encapsulation-enhanced homo- or
hetero-dimerization. To avoid steric hindrince and probably also
as a result of the formation of weak donor-acceptor interaction, it
was the heterodimer formed by the Py+-connected B and E
benzene units of G1 that seized the cavity of CB[8]. The existence
of the CH2 unit forced this stacking occurred in a staggered
manner. The encapsulation also caused the two central C
benzene units to stack closely. The amide unit formed strong
NHO hydrogen bonding (HO distance = 2.01 Å), which
The crystal structures of the complexes formed between
compounds G3-G5 and CB[8] are provided in Figure 1. All the
three complexes displayed the 2:1 binding stoichiometry, with the
antiparallel dimer of the PhPy+ unit of the guest being
encapsulated in the cavity of CB[8], as revealed previously by
Zhao and co-workers.[3b] Probably due to large steric hindrince,
the carboxylic group of G3 and G5 did not form any intermolecular
hydrogen bonding, whereas the amide group of G4 and G5
formed strong NHO hydrogen bonding with the carbonyl
oxygen of CB[8] with the HO distance being 2.17 or 1.95 Å,
respectively. The formation of these strong hydrogen bonds also
caused considerable torsion (35 and 46) of the amide unit from
the benzene ring to which the amide was connected. The
introduction of the methyl group to the carboxylic acid-bearing
pyridine of G5 caused this pyridine ring to twist to be nearly
perpendicular to the PhPy+ unit.
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should also make a contribution for the encapsulation to take
place. This strong hydrogen bonding might also play a key role in
the selective encapsulation of CB[8] for the B/E heterodimer, but
not the A/E heterodimer, even though donor-acceptor interaction
between E and cationic A (Py+), if any, should be stronger than
that between E and B. To the best of our knowledge, such
heterodimerization of two different benzene units in the cavity of
CB[8] had not been reported in the literature, which may be
attributed to the multivalency of two encapsulation events in this
2:2 complex.[11]
being stacked in the cavity of the two CB[8] macrocycles. The
introduction of the central N-methyl Py+ (C) enlarged the spatial
separation of the two PhPy+ units. The ionic PhPy+ unit should
have a relatively high dipole moment. Previous study by Zhao and
co-workers and the crystal structures of G3-G5 all revealed that
PhPy+ preferred to stack in an antiparallel manner in the cavity of
CB[8], which could produce two dipolar attractions.[3b] However,
in the solid state, G2 did not form linear supramolecular polymers
with CB[8], which could be stabilized by such a binding pattern.
Instead, a 2:2 complexation motif was formed selectively, in which
two pairs of electrostatically repulsing Py+ units stacked in the
cavity of two CB[8] marcocycles, which also forced the two central
Py+ units to stack. This stacking should cause the third
unfavorable electrostatic repulsion, although their anti-parallel
stacking might weaken this repulsion. Again, it should be the
multivalency of the two encapsulations that led to this binding
pattern, which did not exist for the linear binding pattern. The
strong NHO hydrogen bonding (HO distance: 2.00 Å)
formed between one of the amide units of G2 and the carbonyl
oxygen of CB[8] should also stabilize the complex. However, this
hydrogen bonding could also stabilize the above mentioned linear
polymeric structure and thus was not an important factor for
favoring the 2:2 binding pattern. The crystal obtained from the
solution containing cadmium chloride and hydrochloric acid also
displayed the similar 2:2 stacking pattern (Figure 2c), although a
little larger dislocation was observed. Moreover, both the amide
units of G2 formed NHO hydrogen bonding (HO distances:
2.09 and 2.16 Å) with the neighboring carbonyl oxygen atoms of
CB[8].
The binding behavior between G1 and G2 with CB[8] was also
investigated in water. Isothermal titration calorimetry (ITC)
experiments were first conducted by gradually adding the guest
to the aqueous solution of CB[8], from which the apparent
association constant Ka of their 1:1 complexes was determined to
be 4.36(±0.93)106 and 6.43(±0.44) 106 M-1, respectively.[9] The
associated enthalpy changes (H) were obtained to be
58.86(±0.75) and 57.05(±0.21) kJ, respectively, whereas the
entropy changes (TS) were determined to be 20.96 and 18.19
kJ, respectively. The large Ka values clearly showed the high
binding stability of the complexes in water, although there existed
three unfavorable electrostatic repulsions. For both cases, the
enthalpy and entropy changes were quite comparable, indicative
of similar binding pattern. The results also revealed that the
binding of CB[8] for the two guests were driven enthalpically, as
demonstrated by Scherman and co-workers in the encapsulation
of CB[8] for shorter 4,4’-diphenylviologens.[9,12]
a)
b)
c)
Figure 2. Crystal structures of the 2:2 complexes formed between CB[8] and a)
G1, b) and c) G2. Two different single crystals were grown for the 1:1 mixture
of CB[8] and G2 from the aqueous solution of Cd(NO3)2 or hydrochloric acid
containing CdCl2.
1H NMR experiments were further carried out to get insight into
the sites of the guests that were encapsulated by CB[8]. 2D
NOESY and COSY experiments were first recorded for G1 and
G2 and their 1:1 mixture with CB[8] in D2O to assign the signals
of G1 and G2 in the absence and presence of CB[8] (Figures 4, 5
and S1-S8). The 1H NMR spectra of G1, G1+CB[8] and CB[8] are
provided in Figure 4. In the presence of CB[8], large upfield
shifting was observed for the signals of the B and E rings and the
H-c, H-i and H-j signals (: 0.50~1.38 ppm) as shown in the
dotted rectangles, which indicated that these signals underwent
strong shielding of the CB[8] ring as a result of direct
Single crystals of the complex formed between G2 and CB[8]
were obtained from their 1:1 aqueous solution containing
cadmium nitrate or cadmium chloride. The two crystal structures
are provided in Figures 2b and 2c. The first crystal structure
displayed a triclinic system with space group P1. Again, the two
molecules formed a 2:2 binding motif with two molecules of G2
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Time (s)
4000
encapsulation. In contrast, the H-b, H-f, H-g and H-h signals
underwent no or much smaller shifting (: 0~0.09 ppm), which
suggested that these protons were not encapsulated by CB[8] to
an important extent. The small shifting might also be caused by
the stacking of the two guests, although the influence of CB[8]
could not be excluded. The CH3 signal shifted downfield notably,
reflecting that only this group suffered important de-shielding from
CB[8]. All these observations supported a binding pattern similar
to that in the solid state. The NOESY spectrum did not display
correlated signals between the two G1 molecules or between G1
a)
0
2000
6000
8000
12
8
4
and CB[8] (Figure S3)[9,12]
.
0
0
-100
-200
-300
-400
 (ppm):
H-a: 0.12
H-b: 0.02
H-c: 0.95
H-d: 1.18
H-e: 0.50
H-f: 0.09
H-g: 0.09
H-h: 0.00
H-i: 0.84
H-j: 0.53
H-k: 1.38
H-l: 1.24
H-m: 1.38
x
y
z
a)
b)
a
j
b
h
I,g f e c
d
l k,m
0.0
0
0.5
1.0
Ratio
1.5
2.0
c)
Time (s)
4000
b)
2000
6000
8000
9.0
8.0
7.0
6.0
5.0
4.0 ppm
12
8
Figure 4. 1H NMR spectra (400 MHz) of a) CB[8] (0.1 mM), b) G1 (0.1 mM) +
CB[8] (0.1 mM), and c) G1 (0.1 mM) in D2O. The dotted rectangles highlighted
the moieties that were encapsulated by CB[8] in solution.
4
0
0
 (ppm):
H-a: 0.61
x
z
y
H-b: 1.04
H-c: 1.16
H-d: 1.05
H-e: 0.20
H-f: 0.08
H-g: 0.15
H-h: 0.47
H-i: 0.41
H-j: 0.84
H-k: 0.59
H-l: 0.08
H-m: 0.27
H-n: 0.30
a)
-100
-200
-300
-400
ni
a
b)
c)
l,f,e,g
m
c,d,k,j
b
h
9.0
8.0
7.0
6.0
5.0
4.0 ppm
0.0
0.5
1.0
1.5
2.0
Figure 5. 1H NMR spectra (400 MHz) of a) CB[8] (0.1 mM), b) G2 (0.1 mM) +
CB[8] (0.1 mM), and c) G2 (0.1 mM) in D2O. The dotted rectangles highlighted
the moieties that were encapsulated by CB[8] in solution.
Ratio
Figure 3. Isothermal titration thermogram of a) G1 (1 mM) and b) G2 (1 mM)
titrated into the aqueous solution of CB[8] (0.1 mM).
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The 1H NMR spectra of G2, G2+CB[8] and CB[8] are presented
in Figure 5. It can be found that the presence of CB[8] caused
important upfield shifting (: 0.59~1.16 ppm) of the H-a, H-b,
H-c, H-d, H-j and H-k signals as shown in the dotted rectangles of
the structure of G2, again indicating that these protons were
encapsulated by CB[8]. The encapsulation of the methyl group as
well as the A ring, to which it was attached, was different from the
results observed for G1, implying that the methyl group on the
central C ring might impose considerable steric repulsion for the
CB[8] ring on this side, which forced this macrocycle to move
further to this end of G2. All other signals of G2 shifted downfield
considerably, indicating that all these signals suffered obvious de-
shielding of the two CB[8] macrocycles. This observation may be
attributed to the existence of the methyl group on the C ring, which
confined the sliding of the two CB[8] macrocycle within a smaller
range. The single crystals of the above complex were obtained
from their aqueous solution containing cadmium nitrate or
cadmium chloride. The 1H NMR spectra in D2O containing these
two salts were also recorded, which showed on observable
influence of the salts on the spectra (Figures S25 and S26).
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cationic pyridnium units in the cavity of CB[8] has been revealed,
which demontrated the robustness of the multivalency of two
1
CB[8]-encapsulation-based 1:2 binding motifs. ITC and H NMR
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studies in water further supported the high binding stability of the
new 2:2 complexes. Given the high stability of the new 2:2 self-
assembling pattern, their application in the formation of large
porous supramolecular organic frameworks will be explored in the
future.
Acknowledgements
[4]
We thank National Natural Science Foundation of China (No.
21432004) for financial support.
Keywords: cucurbit[8]uril • self-assembly • quaternary complex •
crystal structure • aromatic stacking
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Entry for the Table of Contents
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X-ray crystallographic and 1H NMR
studies for 2:2 quaternary complexes
formed between rod-like dipyridinium
guests and cucurbit[8]uril (CB[8]) have
been reported, which revealed unusual
dimerization of two electrostatically
repulsing pyridinium units in the cavity
of CB[8] in the solid state and solution.
Bo Yang, Shang-Bo Yu, Hui Wang, Dan-
Wei Zhang* and Zhan-Ting Li*
Page No. – Page No.
2:2 Complexes from
Diphenylpyridiniums and
Cucurbit[8]uril: Encapsulation-
Promoted Dimerization of
Electrostatically Repulsing
Pyridiniums
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