SCIENCE CHINA
Chemistry
•INVITED REVIEWS• . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . August 2018 Vol.61 No.8: 993–1003
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Coronarenes: recent advances and perspectives on macrocyclic and
supramolecular chemistry
Mei-Xiang Wang*
MOE Key Laboratory of Bioorganic Phosphorous Chemistry & Chemical Biology, Department of Chemistry, Tsinghua University,
Beijing 100084, China
Received June 21, 2018; accepted July 9, 2018; published online July 17, 2018
Synthetic macrocyclic host molecules always play an essential role in the establishment and development of supramolecular
chemistry. Along with the continuous interests in the study of classical macrocycles, recent decades have witnessed the
emergence and rapid development of the chemistry and supramolecular chemistry of novel and functional macrocycles. Owing
to their easy availability, a self-tunable V-shaped cavity resulted from 1,3-alternate conformation, and diversified electronic
features steered by the interplay between heteroatom linkages and aromatic rings, heteracalixaromatics act as a type of versatile
and powerful macrocyclic hosts in molecular recognition and fabrication of supramolecular systems. Very recently, by means of
engineering the bond connectivity or the recombination of chemical bonds within heteracalixaromatics, we have devised
coronarenes, a new generation of macrocycles. In this concise review, macrocyclic and supramolecular chemistry of coronarenes
are summarized in the order of their syntheses, structural features, molecular recognition and self-assembly properties. In the last
part of this article, personal perspectives on the study of macrocyclic and supramolecular chemistry will also be discussed.
coronarenes, heteracalixaromatics, macrocycles, molecular recognition, non-covalent bond interactions
Citation:
Wang MX. Coronarenes: recent advances and perspectives on macrocyclic and supramolecular chemistry. Sci China Chem, 2018, 61: 993–1003,
https://doi.org/10.1007/s11426-018-9328-8
1 Introduction
Design and construction of novel and functional macrocyclic
molecules are one of the central focuses of study in supramolecular science [1]. Tailor-made synthetic macrocycles
offer not only the excellent model systems to study the nature
of various non-covalent interactions but also the essential
building units for the fabrication of sophisticated (supra)
molecular structures, advanced materials and machinery
systems. Furthermore, two- and three-dimensional macrocycles with well-defined cavities are unique molecular tools
in highly selective synthesis and in the study of reaction
mechanisms.
For some 15 years, we and others have been establishing
*Corresponding author (email: wangmx@mail.tsinghua.edu.cn)
and developing the chemistry and supramolecular chemistry
of heteracalixaromatics or heteroatom-bridged calix(het)
arenes (Figure 1) [2–9]. The macrocyclic scaffolds of heteracalix[n]aromatics are constructed by a stepwise fragment
coupling approach through efficient nucleophilic aromatic
substitution reaction or transition metal catalyzed crosscoupling reaction using cheap and mostly commercially
available starting materials. Some symmetric heteracalix[4]
aromatics are easily synthesized from the reaction of 1,3phenylene diol and diamine derivatives with highly reactive
1,3-dihaloarenes in a one-pot reaction manner. Post-macrocyclic chemical manipulations enable the generation of
various tailor-made functionalized heteracalixaromatics.
Because of the dipole-diploe interaction, heteracalix[4]
aromatics generally adopt 1,3-alternate conformation,
yielding a V-shaped cavity or cleft [10]. The cavity size
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Figure 2
Figure 1
General structure of heteracalixaromatics (color online).
varies greatly depending on the electronic and steric effects
of the aromatic and heteroatom building blocks. Most remarkably, the electronic feature of the macrocyclic cavity of
a heteracalixaromatic ring is amenable to regulation due to
the interplay between aromatic moieties and heteroatom
linkages. The rational combinations of individual aromatic
rings and heteroatoms lead therefore to the desired heteracalixaromatics with powerful and versatile capability in
complexing selectively with metal ions, anions and electronneutral guest species [4–9]. To our delight, heteracalixaromacs have been shown to date by a number of research
groups to have many applications ranging from molecular
recognition and self-assembly, the fabrication of supramolecular materials and functional systems [9], to molecular
tools in elucidation of the reaction mechanisms of high valent organocopper compounds [11–15].
To construct cylindroid cavities other than the V-shaped
ones while remaining the salient advantages to tune electronic features and properties through the interplay between
aromatic rings and heteroatoms, we have recently designed
coronarenes, a novel class of macrocycles, based on the replacement of all meta arylenes within heteracalixaromatics
with para ones [16,17]. Coronarenes are therefore referred to
macrocycles which are composed of para arylenes and heteroatoms alternatively in a cyclic fashion (Figure 2). In recent years, we have systematically studied the synthesis and
structure of coronarenes. Molecular recognition property of
coronarenes has also been explored. In this article, research
advances of the chemistry of coronarenes are summarized. I
would also like to give my personal perspectives on macrocyclic and supramolecular chemistry by the end of the
article.
2 Nomenclature of coronarenes
According to nomenclature recommended by International
Union of Pure and Applied Chemistry (IUPAC), coronarene
compounds belong to cyclophanes and have very long
names. Following the tradition of naming macrocycles, and
based on the coronary conformation of the compounds (infra
General structure of coronarenes (color online).
vide) we suggest the name Xn-corona[n](het)arenes to describe the macrocycles that are composed of para-(het)arylenes and heteroatoms in an alternative manner. While the
type and the number of linking heteroatoms are prefixed to
corona, the bracketed number(s) followed by the name of
(het)aromatic rings after corona indicate the aromatic components. O6-Corona[3]arene[3]tetrazine, for example, means
a macrocycle consisting alternately of three phenylene units
and three tetrazinylene units that are bridged by six oxygen
atoms.
3 Synthesis of coronarenes
The convenient and cost-effective accesses to diverse coronarenes are the prerequisite to the development of novel
chemistry and supramolecular chemistry of coronarenes. We
have established and developed several synthetic strategies
ranging from one-pot de novo construction method, stepwise
fragment coupling approach, macrocycle-to-macrocycle
transformations, post-macrocyclization oxidation of sulfide
linkages and functional group transformation. Based on these
methods, a large number of novel and functionalized coronarenes of different macrocyclic ring sizes and electronic
features have been synthesized.
3.1 De novo construction of coronarene macrocyclic
rings
Taking on the advantage of facile nucleophilic aromatic
substitution reaction of 3,6-dichlorotetrazine 3 [18] with
aromatic diols 1 and dithiols 2, we have established a very
efficient one-pot reaction method to construct macrocyclic
structure of oxygen- [16] and sulfur-linked coronarenes [17],
respectively. Shown in Figure 3 are our first synthesis of
oxygen and sulfur linked corona[3]arene[3]tetrazines 4 and
5. The following features of this synthetic method are worth
addressing. First of all, the starting materials such as aromatic diols, dithiols and 3,6-chlorotetrazine are readily
available either from commercial sources or from synthesis
based on well-documented procedures [18]. No expensive
catalysts, reagents and solvents are required. Second, the
reaction conditions are very mild and operationally simple.
The chemical yields are moderate to good. Consideration of
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Figure 3
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The one-pot reaction method for the synthesis of oxygen and sulfur linked corona[3]arene[3]tetrazines (color online).
formation of six chemical bonds in synthesis, a chemical
yield of 53% from one-pot synthesis means 90% efficiency
in each single bond formation reaction. Preparation is also
scalable. For instance, a gram scale synthesis of O6-corona
[3]arene[3]tetrazine has been demonstrated. In addition, the
method is general and applicable to not only simple 1,4dihydroxy and 1,4-dimercaptobenzene substrates but diols
and dithiols derived from fused arenes as well. Thus, the
employment of 1,5- and 2,6-naphthodiols [19], 4,4′-dihydrox-1,1′-biphenyl, 9H-fluorene-3,6-dithiol [17] in the
synthesis, for example, led to the formation of corona[3]
arene[3]tetrazines with varied cavity sizes. Furthermore, the
use of pre-functionalized diols and dithiols enables the
construction of multifunctional corona[6]arenes [17,19].
Moreover, tetrazine is transformable aromatic subunit
amenable to inverse electron demand Diels-Alder reaction,
offering a great opportunity for the synthesis of pyridazinebearing coronarenes (infra vide).
The one-pot reaction has been successfully extended to the
synthesis of corona[4]arene[2]tetrazines with mixed heteroatom bridges [20]. As illustrated in Figure 4, 4,4′-methyl-,
oxy, thio- and sulfonyl-bridged diphenols and dibenzenethiols react with 3,6-dichlorotetrazines affords the corresponding corona[4]arene[2]tetrazines as the major products.
It should be noted that, being different from the synthesis of
corona[3]arene[3]tetrazines in which no larger macrocyclic
ring analogs are isolated, some reactions depicted in Figure 4
gives low yields of corona[6]arene[3]tetrazines and even
corona[8]arene[4]tetrazines.
In comparison to corona[6]arenes, synthesis of corona[5]
arenes appeared challenging [21]. After a serendipitous
discovery of the macrocycle-to-macrocycle transformation
of S6-corona[3]arene[3]arenes, infra vide, we have established a straightforward three-component reaction to prepare
a number of corona[3]arene[2]tetrazines. In the presence of
diisopropylethylamine (DIPEA), a mixture of 1,4-benzenedithiol 2, 3,6-dichlorotetrazine 3 and 4,4′-methylene-, propane-2,2-diyl-, thio- or sulfonyl-bridged dibenzenethiols 7 in
a 1:2:1 ratio undergoes macrocyclization reaction to generate
corona[5]arene products 10 (Figure 5). Except for sulfonecontaining macrocycle which was obtained in a low yield
(11%), the rest three-component reactions give acceptable
yields (41%–46%) [21].
Stepwise fragment coupling reaction provides another
useful route to coronarenes [22–24]. One of the examples,
which is depicted in Figure 6, is the synthesis of corona[4]
arene[2]tetrazines that contain different combinations of nitrogen atom with O, S, SO2 and CH2. N,N-Bis(4-hydroxyphenyl)acetamide 11 undergoes two directional
nucleophilic aromatic substitution reaction with 3,6-dichlorotetrazine 3 to afford linear tetramer intermediate 12 in
74% yield. Treatment of 11 with diols 6 or dithiols 7 and 11
in warm acetonitrile gives rise to the formation of corona[4]
arene[2]tetrazines 13 [22].
3.2 Synthesis of coronarenes from macrocycle-to-macrocycle transformations
In an attempt to prepare corona[5]arenes by means of a
stepwise fragment coupling route, we mistakenly mixed S6corona[3]arene[3]tetrazine with 4,4′-thiodibenzenethiol.
Astonishingly, the reaction at −20 °C was found to yield S5corona[3]arene[2]tetrazine in 72% yield. Since S6-corona[3]
arene[3]tetrazine is easily available, supra vide, the macro-
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Figure 4 Synthesis of corona[4]arene[2]tetrazines from one-pot reaction
(color online).
Figure 5 One-pot three-component reaction for the synthesis corona[3]
arene[2]tetrazines (color online).
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zine, we have established a powerful macrocycle-to-macrocycle transformation method to construct new coronarene
scaffolds [17,25]. It is noteworthy that all tetrazine rings in
corona[3]arene[3]tetrazines and corona[4]arene[2]tetrazines
are capable of reacting effectively with enamines and norbornadiene, which are respectively electron-rich and highly
strained dienophiles. Demonstrated in Figure 8 are the examples of the synthesis of oxygen and sulfur linked corona
[3]arene[3]pyridazines 14 and 15 from [4+2] cycloaddition
reaction of coronarenes 4 and 5 with enamines followed by
deaminative aromatization [17,25]. Notably, the reaction of
sulfur-bridged coronarenes 5 [17] proceeds more efficiently
than that of oxygen atom linked analogs 4 [25], owing
probably to different electronic effect between sulfur and
oxygen on tetrazine ring.
In addition to the ring transformation of aromatic subunits,
selective oxidation of sulfide linkages constitutes another
simple and efficient macrocycle-to-macrocycle method in the
production of novel coronarenes [17,20]. Under controlled
oxidation reaction conditions, sulfide is known to undergo
selective oxidation to give sulfone and sulfoxide functionalities. As exemplified in Figure 9, in the presence of m-CPBA,
(CH2)2,S4-corona[4]arene[2]pyridazine 15a is oxidized into
(CH2)2,(SO2)4-corona[4]arene[2]pyridazine 16 while selective
oxidation of diphenylsulfide moieties with Selectfluo yields
S4,(SO)2-corona[4]arene[2]pyridazine 17 [20].
3.3 Functional group transformations in the synthesis
functionalized coronarenes
In comparison to de novo synthesis and macrocycle-to-
Figure 6 The fragment coupling approach to corona[4]arene[2]tetrazines
(color online).
Figure 7 Synthesis of corona[5]arenes from corona[6]arenes through
macrocycle-to-macrocycle transformation (color online).
cycle-to-macrocycle transformation of corona[6]arenes has
been developed into a useful protocol to synthesize corona
[5]arenes 10 (Figure 7) [21]. The driving force for the conversion of corona[6]arenes 5 into corona[5]arenes 10 is most
probably duo to the higher thermodynamic stability of the
later macrocycles than that of the former ones. The nature of
dynamic covalent bond between sulfur and triazine, on the
other hand, is attributed to account for the rapid reaction
process.
Tetrazine is viewed as an electron deficient heterodiene
able to undergo inverse electron demand hetero-Diels-Alder
reaction. Taking the advantage of unique reactivity of tetra-
Figure 8 Synthesis of corona[3]arene[3]pyridazines from inverse electron demand Diels-Alder reaction of corona[3]arene[3]tetrazines (color
online).
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Figure 9 Synthesis of sulfone and sulphide linked corona[4]arene[2]
pyridazines from selective oxidation of sulphide (color online).
Figure 10 Synthesis of functionalized coronarenes via functional group
transformations (color online).
macrocycle transformation methods aforementioned, postmacrocyclization functional group transformation is a conceivably straightforward and practical means to fabricate
coronarenes. Chemical manipulation of various functional
groups introduced in the macrocyclic ring construction step
would permit the generation of tailor-made functional coronarenes. Simple reduction of all ester groups in O6-corona
[6]arene 18 for instance affords O6-corona[6]arene 19 with
six hydroxymethyl groups which would provide a platform
for further functionalizations [23] while saponification of O6[26] and S6-corona[3]arene[3]pyridazine 20 [26] followed by
salification furnishes the water soluble coronarenes 21
(Figure 10).
For example, O6-corona[3]arene[3]tetrazines give a nearly
equilateral hexagon cavity with the average distances between centroids of two proximal aromatic rings and between
centroids of two distal aromatic rings are around 4.61 to
4.63 Å and 9.21 to 9.26 Å, respectively (Figure 11(a)) [16].
Much larger isogonal hexagon cavities are observed on the
other hand from O6-corona[3](1,1′-biphenyl)[3]tetrazines
(Figure 11(b)) and O6-corona[3](9H-fluorene)[3]pyridazine
(Figure 11(c)) [19]. Furthermore, the change of tetrazines by
pyridazines in corona[6]arenes does not affect the size of
macrocyclic cavity. This is important to engineer the macrocyclic hosts which have substantially different electronic
feature but the same cavity size (infra vide). Moreover, the
nature of the heteroatom linkages plays an important effect
on the tuning of macrocyclic conformation. The replacement
of oxygen atoms by sulfur atoms leads to the slight distortion
and enlargement of hexagon cavities due to larger atomic
radius of sulfur (Figure 11(d, e)) [17]. After exhaust oxidation of sulfide bridges, for instance, the resulting (SO2)6corona[3](9H-fluorene)[3]pyridazine produces a conformer
in which almost all aromatic subunits are perpendicular to
the plane defined by six sulfur atoms of bridging sulfones
(Figure 11(f)) [17]. Finally, the bond lengths and bond angles
indicate the formation of conjugation of bridging oxygen
atoms with their neighboring electron-deficient heterocyclic
ring rather than with the arene ring [16–20].
As depicted in Figure 12, all corona[5]arenes 10 form
nearly similar regular pentagons with the mean side lengths
ranging from 6.06 to 6.24 Å. All bridging atoms in corona[5]
arenes locate nearly on the same plane. The angle (α) between two phenylenes varies from 100° to 114° depending
on the nature of X, while the rest four internal angles are
almost identical (100° to 104°) (Figure 12). Two tetrazine
rings are procumbent on the plane, and p-phenylene rings,
4 Structure of coronarenes
Almost all of the coronarenes obtained so far are crystalline
compounds, and most of them give high quality single
crystals from recrystallization, permitting X-ray diffraction
analysis. X-ray molecular structures show that coronarenes
adopt various coronary conformations in the crystalline state.
Some interesting structural features are worth noting. First of
all, as revealed by their X-ray molecular structures, all six
bridging oxygen atoms in O6-corona[6]arenes which contain
either tetrazine (Figure 11(a, b)) or pyridazine rings are located almost on the same plane [16]. In the case of sulfurlinked analogs, however, six sulfur atoms are hardly in the
same plane [17]. Second, while three heterocyclic rings are
procumbent on the plane, (substituted) benzenes, naphthalenes and biphenyl moieties tend be orthogonal to the plane
(Figure 11(a–e)) [16,17]. In addition, corona[6]arenes yield
approximately hexagon cavities, and the cavity sizes are
dependent on the sizes of building (het)arylene components.
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Some representative X-ray molecular structures of corona[6]arenes with top and side views. Substituents are hidden for clarity (color online).
bulky substituents at 2,4-position, complicated proton and
carbon resonance signals were observed at room temperature. With the increase of the probe temperature, the signals
became gradually broadened, coalesced and well-resolved
peaks (Figure 13) [16]. The VT NMR spectra indicate clearly
the presence of a mixture of conformers in solution. They
undergo very rapid interconversion at an elevated temperature in relative to the NMR time scale.
Figure 12 X-ray molecular structures of corona[5]arenes with top and
side views (color online).
especially, the diethyl terephthalate moiety or the phenylene
between two tetrazine rings tend to be orthogonal to the
plane. The cavity sizes, as defined by the distance (d) between the centroid of terephthalate ring and heteroatome or
methylene carbon (X), decreased from 9.29–9.82 to
9.00–9.08 Å and 8.74 Å along the change of X from sulfur to
carbon and then to oxygen, indicating a fine-tuning effect on
the cavity sizes by the bridging elements [21].
In solution, most of the corona[6]arenes and corona[5]
arenes give one simple set of proton and carbon resonance
signals at room temperature in their 1H and 13C NMR spectra,
respectively [17,19,20,22–24]. However, in the case of
oxygen-linked corona[3]arene[3]tetrazines [16] and corona
[3]arene[3]pyridazines [25] in which phenylene units contain
5 Electronic
properties
spectral
and
electrochemical
All tetrazine-bearing coronarenes are red-colored compounds, and their electronic absorption spectra exhibit two
absorption bands at λmax=320–337 nm and at λmax=518–
535 nm with molar extinction coefficients (ε) being around
104 and 103 mol−1 cm−1, respectively. The strong absorption
bands at a short wavelength region correspond to π→π*
transitions while the low energy absorptions are attributable
to n→π* transitions of a tetrazine component
[16,17,19,20,22]. Under irradiation, O6-corona[3]arene[3]
tetrazines display a fluorescence emission band at 558–
584 nm in dichloromethane (DCM). Due to probably the
heavy atom effect, very weak or virtually no fluorescence
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Figure 13 Partial variable temperature 1H NMR (a) and 13C NMR (b, c (expanded)) spectra of O6-corona[3]arene[3]tetrazine derived from 2,6-dibromo1,4-dihydroxybenzene in d6-DMSO.
emission was detected for sulfur-linked macrocycle analogs
[16,22].
Most of the oxygen- and sulfur-linked corona[3]arene[3]
tetrazines gave a very similar electrochemical response in
their cyclic voltammograms (CV), showing a characteristic
reversible redox couple due to one electron reduction and
oxidation of the tetrazine moiety [16,17]. The half-wave
reduction potentials measured by CV and differential pulse
voltammetry (DPV) are in the range of −792 to −1135 mV
versus Fc/Fc+. Notably, S6-coronacorona[3]arene[3]tetrazine,
which is derived from diethyl 2,5-dimercaptoterephthalates,
proceeded through a sequential one-electron redox process at
−939, −987 and −1194 mV while S6-corona[3]arene[3]tetrazine experienced one-electron redox simultaneously with
two tetrazine rings at −990 mV followed by redox for the last
tetrazine at −1130 mV [17]. All corona[3]arene[2]tetrazines,
on the other hand, proceed reversibly through a sequential
one-electron redox process, giving E1/2(1) from −978 to
−1085 mV and E1/2(2) from −1083 to −1196 mV [21]. The
occurrence of electronic communication between the three or
two equivalent tetrazine redox centers within the corona[6]
arenes and corona[5]arene macrocycles is due to probably
electron sharing through space. It should be pointed out that
all S6-corona[3]arene[3]pyridazine macrocycles are inert
toward electrochemical reduction in the range of −600 to
−2200 mV. However, after the sulfide linkages were oxidized, the resulting sulfone-bridged corona[3]arene[3]pyridazines gave the half-wave reduction potentials ranging
from −1365 to −1848 mV [17]. It has also been observed that
the presence of stronger electron-withdrawing groups on the
phenylene rings lowered the reduction potentials of coronarenes [16,17]. It is worth addressing that the substantial
difference of redox potentials suggests the formation of a
library of coronarenes of varied electronic features. They can
be used therefore as hosts to interact selectively with various
electron-deficient and electron-rich guest species.
6 Molecular recognition
The easy availability, cylindroid cavity of different geometries and sizes, and tunable electronic feature render coronarenes useful synthetic macrocycles in the study molecular
recognition and self-assembly. Although the investigation
into the applications is still in the early stage, examples reported in literature have demonstrated indeed that coronarenes are versatile hosts to form complexes with cations,
anions and electron-neutral organic molecules.
6.1
Anion recognition
In the first publication of coronarene, we reported the observation of complexation of O6-corona[3]arene[3]tetrazine
with chloride. Owing to its electron deficiency, each tetrazine
in O6-corona[3]arene[3]tetrazine is able to form complex
with chloride by mainly the typical anion-π interaction.
Evidenced by X-ray molecular structure, the chloride is located above the centroid of tetrazine ring with the distance
(dCl…tetrazine centroid) being shorter than the van der Waals radius.
In addition to attractive anion-π interaction, the C–H bonds
of phenylene moieties also form non-conventional hydrogen
bonds with chloride [16].
Very recently, N2,O4-corona[4]arene[2]tetrazine 13 has
been shown to form complexes with anions of varied geo-
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metries and shapes yielding diverse assembled structures in
the solid state [22]. Illustrated in Figure 14 is the [13⋅Br−]
complex structure. Bromide anion (Br1) locates over the
center of the tetrazine (T1) ring, and the distance of Br1 to
the plane of tetrazine (T1) (dBr1-plane=3.464 Å) is short. Noticeably, the tetrazine ring in complexation with bromide
adopts a pinched boat conformation with the dihedral angle
of planeN-N-N-N and planeC-N-N being 16.2°. The deformation of
a planar aromatic ring to a flattened boat conformation accords the typical attractive anion-π interactions predicted by
theoretical calculations [27]. Besides, Br1 also has a short
contact (dBr1-plane=3.357 Å) to tetrazine (T2) ring from the
neighboring coroanarene. The bromide anion (Br1) is actually sandwiched by two tetrazine (T1 and T2) rings due to
dual anion-π interactions. To avoid steric hindrance coming
from the macrocyclic backbone, these two tetrazine rings are
not face-to-face paralleled. Instead, they open up slightly to
give a V-shaped alignment, with the nearest and longest
distances of T1 and T2 being 5.999 (dC2-C18) and 7.080 Å
(dC1-C17), respectively. Evidently, directed by dual anion-π
interactions, bromide anions act as a gluing component to
assemble corona[4]arene[2]tetrazines into one-dimensional
structure (Figure 14, right column). Very similar sandwiched
anion-π complexation and assembled structures were observed for the complexes of 13 with spherical iodide (I−),
linear thiocyanate (NCS−) and tetrahedral perchlorate
(ClO4−) [22].
When naphthalene-1,5-disulfonate, an organic bis-anionic
species, was interacted with corona[4]arene[2]tetrazine 13,
each sulfonate group complexes with one tetrazine ring due
to the typical anion-π interactions. As a result, naphthalene1,5-disulfonate looks like encapsulated by two coronarene
molecules (Figure 15). Each complexed capsule then assembles into a linear structure through dimethyl sulfoxide
(DMSO) solvent molecules which associate with the other
tetrazine ring of coronarene via lone-pair electron-π interaction [28,29] (Figure 15).
In contrast to corona[3]arene[3]tetrazines, corona[3]arene[3]
pyridazines contain electron-rich cavity and they are able to
recognize a large number of cation guests. Oxygen- and
sulfur-bridged corona[3]arene[3]pyridazines form 1:1 complexes selectively with aliphatic and aromatic mono-ammonium and di-ammonium cations in acetonitrile. The
binding constants, which are measured by means of UV-vis
and 1H NMR titration, are in a wide range of 102 to 105 M−1
based on data depending on the structure of cation species
[17,22,25,30,31]. Using S6-corona[3]arene[3]pyridazine as a
model macrocyclic host, non-covalent bond interactions
between coronarenes with ammonium cations has been investigated systematically employing isothermal titration calorimetry [30]. It has been found that the presence of a benzyl
group in alkyl(trimethyl)ammoniums enhanced the hostguest complexation. On the other hand, while moderate
bonding was also observed to acetyl choline, the strength of
binding with 1,4-dibenzyl-1,4-diazabicyclo[2.2.2]octane1,4-diium was as high as up to (1.48±0.11)×105 M−1. The
variation of monocation to dication, viz. the introduction of
one more N-benzyl group to 1,4-dibenzyl-1,4-diazabicyclo
[2.2.2]octane, resulted in a nearly 100-fold increase of
binding. The host also shows stronger affinity to heteroaromatic dications than to simple aliphatic ammonium cations.
It is important to note that N-alkyl ammonium and diammonium cations of relatively large volumes interacted
strongly with the host owing to most probably the large
cavity of the macrocyclic host. The thermodynamics on the
basis of ITC titrations reveal that both favorable enthalpy
(∆H) and entropy (T∆S) effects contribute to the free energy
gain in all host-guest binding processes in solution, although
some processes are driven mainly by the enthalpy effect
whereas in other cases the entropy effect plays a dominant
role [31].
It is significant that water soluble coronarenes form com-
Figure 14 X-ray molecular structure of the complex between coronarene
13 and n-Bu4NBr. Cations and solvent molecules are omitted for clarity
(color online).
Figure 15 X-ray molecular structure of complex between coronarene 13
and tetrabutylammonium naphthalene-1,5-disulfonate. Cation and solvent
molecules are omitted for clarity (color online).
6.2
Cation recognition
9. . . . . . . . . . . . . . . . . . . . . . . . . . .
Wang
Sci China Chem
plexes selectively with ammonium cations in pure water. For
example, both sulfur and oxygen-linked corona[3]arene[3]
pyridazines 21 (X=O, S) are able to complex paraquat in a
1:1 stoichiometric ratio with the binding constant up to
(2.67±0.21)×104 M−1 [17,25,26]. The effective binding of S6corona[3]arene[3]pyridazine 21 (X=S) with bisquaternary
ammonium species has been used successfully in the fabrication of a supra-amphiphile which undergoes self-assembly
to give micellar aggregates in water [30].
Interestingly, different supramolecular motifs have been
observed for the host-guest complexes between electron-rich
coronarenes and ammonium cations depending on the
structure of guests. As revealed by the X-ray molecular
structure, 1-methyl-1,3,5,7-tetraazaadamantan-1-ium [17]
and
6,7-dihydrodipyrido[1,2a:2′,1′-c]pyrazine-5,8-diium
[31] are included by one and two S6-corona[3]arene[3]pyridazine in the solid state. One of the fascinating examples is
the formation of a pseudorotaxane structure between macrocyclic host and 1,4-dibenzyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium in which the bulky bridged bicycle dication
acts as the threading component [17] (Figure 16). Based on
the X-ray molecular structures, there are multiple noncovalent bond interactions such as non-conventional hydrogen bonds, π/π stacking and C–H/π interactions between
coronarenes and ammonium cations in the complexes
[17,25,26,31].
6.3
Recognition of organic molecules
In addition to anion and cation species, some electron-neutral
organic compounds also form inclusion complexes with
coronarenes. Among the examples of macrocycle-organic
molecule interactions [19,23–25], selective recognition of
coronarenes with fullerenes C60 and C70 is especially worth
addressing.
In contrast to various heteracalixaromatics [5,32], functionalized O6-corona[6]renes [23,24] and corona[3]arene[3]
pyridazines [19] which all bind strongly with C60 and C70
with virtually no selectivity, intriguingly, O6-corona[3]arene
[3]tetrazines complex only with C70 [19]. The association
Figure 16 X-ray molecular structure of complex between coronarene and
1,4-dibenzyl-1,4-diazabicyclo[2.2.2]octane-1,4-diium hexafluorophosphate.
Cation and solvent molecules are omitted for clarity (color online).
August (2018) Vol.61 No.8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1001
constant (K1:1), which was measured in toluene by means of
fluorescence titration, is up to (3.96±0.08)×104 M−1. Although the high selectivity of tetrazine-bearing corona[6]
arenes in fullerene recognition has been ascribed to an
electronic rather than steric effect of the hosts, further study
is needed [19].
7 Summary
As a new type of synthetic macrocycles, coronarenes enjoy
first of all the great structural diversity. Combination of para(het)arylenes and hetereoatoms should generate in theory
numerous coronarene macrocycles. Coronarenes are also
subject to functionalization both on the aromatic rings and
heteroatom linkages, further expanding the structural diversity. Besides, the use of different numbers and the structurally-varied (het)arylene units permits the construction of
macrocyclic cavities of tunable shapes and sizes. Furthermore, the incorporation of various (het)aromatic rings and
hetereoatoms gives rise to an array of coronarenes of different electronic features. Therefore, coronarenes are applicable in molecular recognition towards both electronneutral organic molecules, positively and negatively charged
guest species. Moreover, coronarenes are readily accessible
by a number of efficient and straightforward synthetic
methods. It is believed that the easy availability, tunable
macrocyclic cavity structures and electronic features, versatile complexation property would render coronarenes very
useful macrocyclic host molecules.
8 Perspectives on macrocyclic and supramolecular chemistry
Supramolecular chemistry, an appealing and provoking term
introduced by Lehn [1] to describe the chemistry of molecular assemblies and of the intermolecular bond, has become
a popular and vigorous discipline since Pedersen’s landmark
work on crown ethers more than 50 years ago. Evolved from
chemistry and now as the indispensable part of chemical,
biological and material sciences, the hard-core of supramolecular chemistry studies the structure, property and transformation of matters in various systems with the focus on
non-covalent interactions of molecules. Despite tremendous
achievements, the future accelerating and sustainable development of supramolecular chemistry depends mainly on
the exploration, comprehension and manipulation of noncovalent bond interactions at various levels in artificial and
biological systems. In addition, it is highly desirable but still
extremely challenging to fabricate assembled structures,
devices and machinery systems with sophisticated functions
such as solar energy harvest and storage and conversion,
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information processing, target-specific diagnose and therapy,
and catalysis. Furthermore, one of the ultimate and formidable tasks is the construction of artificial cells and mimetic
living systems, the Holy Grail in supramolecular chemistry
[33].
Among various research strategies and approaches to
tackle the problems in question, tailor-made synthetic macrocycles offer great opportunities and powerful solutions as
they can give not only the excellent model systems to study
the nature of various non-covalent interactions but also the
essential building units for sophisticated functional assemblies. This has been manifested extraordinarily by the early
studies on the privileged crown ethers [34], cryptands [35],
spherands [36], cyclodextrin derivatives [37] and calixarenes
[38], which generated many, if not all, fundamental concepts
leading to the foundation of supramolecular chemistry [1]. In
the following decades, the functional macrocycles such as
cucurbiturils [39], calixpyrroles [40], heteracalixaromatics
[3–9], cycloparaphenylenes (CPPs) [41] and pillar[n]arenes
[42], among many others, provides driving forces to promote
the major advances of the field. Evidently, one of the salient
advantages of macrocycle strategy is the designability of
macrocyclic hosts or building blocks. Admittedly, however,
there are no universal synthetic hosts in terms of application,
as the function of a given macrocycle is destined by its intrinsic structure. It is therefore always important and exciting
to design the structure of and to establish facile synthetic
methods for novel and multifunctional macrocycles. Pleasingly, a number of new and interesting macrocyclic scaffolds
have been designed very recently. For example, in addition to
heteracalixaromatics [3–9], CPPs [41,43], pillararenes [42]
and coronarenes, the Texas-sized molecular box [44] and
various ExBoxs [1b], cyanostar [45], triptycene-based macrocycles [46], naphthalene-based tub[n]arenes [47] and biphen[n]arenes [48] have been reported as powerful
macrocyclic host molecules. With the advent of new and
efficient methods in chemical synthesis, emergence of more
synthetic macrocycles can be expected in future.
In parallel to create novel and functional macrocycles,
exploration of new noncovalent bond interactions and supramolecular motifs is always of significance. A very recent
and noteworthy example of burgeoning non-covalent interactions is anion/π interactions, the attractive interactions
between negatively charged species and electron-deficient
aromatic rings [49]. There are two general types of anion/π
interaction motifs; a typical anion/π interaction indicates the
attraction of an anion species to the centroid of an aromatic
ring while the weak σ-interactions describe the contacting
modes in which an anion is located over the periphery of an
aromatic ring. Using synthetic macrocycles, the generality,
binding strength and structures of interactions of the electron-deficient aromatic s-triazine with various anion species
of different geometries and shapes have been established on
the basis of the formation of host-guest complexes in solution, gas phase and solid state [50]. The anion/π interactions
have been recognized currently as a new kind of non-covalent bond interactions and they have been finding applications in molecular recognition and self-assembly, catalysis,
and in the regulation of small molecule/protein interactions
and in the fabrication of functional materials [51]. It is believed that the use of designed synthetic receptors as molecular tools along with advanced analytical methods and
powerful computing technology would fruitfully result in the
discovery of new types of non-covalent bond interactions
which would undoubtedly promote the further advances of
supramolecular chemistry.
One of the aims of supramolecular chemistry is to produce
property-orientated molecular assemblies and materials. Indeed, supramolecular chemistry has been becoming a unique
method to obtain hierarchically assembled structures and
complex molecular systems, chemical sensors and optoelectronic devices, and stimuli-responsive and smart materials. However, in comparison to the sophistication of
chemical synthesis based on chemical bond theory, controllable molecular self-assembly by means of non-covalent
bond interactions is still in its infancy. On the other hand,
supramolecular chemistry becomes integrated with chemical
biology and synthetic biology. Based on molecular recognition, selective or specific interactions of an active organic probe or dye molecule with targeting nucleic acids and
proteins may trigger biological events or show bio-images,
respectively, enabling the elucidation of biological networks
and signal transduction. Strong and highly selective interactions of molecules with cells and biological systems lay the
foundation of drug discovery, facilitating the discovery of
either drugable receptors or lead compounds. Successful
gene delivery and the controlled drug-release systems are
also mainly dependent on the construction of dynamic supramolecular systems, which are sensitive and responsive to
the fine tuning of physiological conditions. The prospective
applications of supramolecular chemistry in materials and
life science would be achieved by using rationally designed
functional macrocycles including coronarenes and following
the comprehension and the elaboration of the control of noncovalent bond interactions in multicomponent systems.
Wang
Acknowledgements This work was supported by the National Natural
Science Foundation of China (21732004, 21421064, 91427301, 21132005)
and Tsinghua University. I am indebted to talented research students and
postdoctoral fellows, whose names can be found in references, for their great
contributions to the project of macrocyclic and supramolecular chemistry.
Conflict of interest The author declares no conflict of interest.
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