Five-fold-symmetric macrocyclic aromatic pentamers: High-affinity cation recognition, ion-pair-induced columnar stacking, and nanofibrillation

Article abstract of DOI:10.1021/ja206457b

Described in this study is a conceptually new class of five-fold-symmetric cavity-containing planar pentameric macrocycles with their interior decorated by five convergently aligned, properly spaced carbonyl oxygen atoms. These cation-binding oxygens enclose a hydrophilic lumen of 2.85 A in radius and thus display high-affinity binding toward alkali metal cations, and possibly many other cations, too. Arising from their high-affinity recognition of metal ions, these planar macrocycles form cation- or ion-pair-induced one-dimensional columnar aggregates, and subsequently fascinating fibrillation results.

Full text of DOI:10.1021/ja206457b

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Five-Fold-Symmetric Macrocyclic Aromatic Pentamers: High-Affinity
Cation Recognition, Ion-Pair-Induced Columnar Stacking, and
Nanofibrillation
Changliang Ren,† Victor Maurizot,‡ Huaiqing Zhao,† Jie Shen,† Feng Zhou,§ Wei Qiang Ong,† Zhiyun Du,||
Kun Zhang,|| Haibin Su,§ and Huaqiang Zeng*,†
†Department of Chemistry and NUS MedChem Program of the Office of Life Sciences, 3 Science Drive 3,
National University of Singapore, Singapore 117543
‡CNAB—UMR5084, Universitꢀe de Bordeaux, UMR CNRS 5248, Institute Europꢀeen de Chimie et Biologie, 2 rue Robert Escarpit,
33607 Pessac Cedex, France
§Division of Materials Science, 50 Nanyang Avenue, Nanyang Technological University, Singapore 639798
Faculty of Light Industry and Chemical Engineering, Guang Dong University of Technology, Guang Dong 510006, China
S Supporting Information
b
ABSTRACT: Described in this study is a conceptually new
class of five-fold-symmetric cavity-containing planar penta-
meric macrocycles with their interior decorated by five
convergently aligned, properly spaced carbonyl oxygen
atoms. These cation-binding oxygens enclose a hydrophilic
lumen of 2.85 Å in radius and thus display high-affinity
binding toward alkali metal cations, and possibly many other
cations, too. Arising from their high-affinity recognition of
metal ions, these planar macrocycles form cation- or ion-
pair-induced one-dimensional columnar aggregates, and
subsequently fascinating fibrillation results.
Figure 1. (a) Chemical structures of pentamers 1 and 2. (b) Top and
side views of computationally optimized structures for 1 and 2, with the
exterior side chains replaced by methyl groups at the B3LYP/6-31G*
level, illustrating five-fold-symmetric planarity in 1 and 2. (c) Chemical
and crystal structures of dimer 1q.
ne-dimensional (1D) fibers or nanotubes assembled from
O
organic molecules are promising materials1 for applications
As illustrated in Figure 1a, pentameric molecules 1 and 2 are
made of five alkylated 4(1H)-pyridone motifs meta-linked
by secondary amide groups. Both carbonyl oxygens and amide
protons point inward to form a continuous intramolecularly
H-bonded network in a way such that the H-bonding rigidified
backbone becomes increasingly curved and eventually cyclizable
to arrive at a pentagon shape.4 Ab initio calculation at the B3LYP/
6-31G* level (Figure 1b) on pentamers 1 and 2 shows that such a
pentagon shape encloses a hydrophilic oxygen-containing cavity
of 2.85 Å, nearly identical to the average coordination bond
distance between K+ ions and covalently bound oxygen atoms.5a
These convergently aligned, properly spaced oxygens should
therefore suggest high-affinity cooperative recognition of the alkali
metal ions by pentameric molecules 1and 2. Additionally, the planar
geometry in 1 and 2 should promote the formation of cation- or ion-
pair-mediated 1D stacked structures under suitable conditions.
Pentamers 1 and 2 were both made by a stepwise construction
strategy using HBTU-mediated amide coupling requiring 15À16
steps with an overall yield of 1À2%, starting from the commer-
ciallyavailablediethyl3-oxopentanedioate (SchemesS1 andS25b).
Their identity was unambiguously confirmed by excellent
as diverse as nanodevices,1b,c,g,h sensors,2aÀc and water trans-
porters.2d These organic columnar aggregates are predominantly
stabilized by hydrogen-bonding or πÀπ stacking forces, or a
combination of both.1aÀh,2eÀ2m Although metal ions can offer
unique properties and geometries for functional diversification,
metalÀligand coordination bonds have received comparatively
much less attention in the construction of fibrillar materials3a
derived from acyclic or macrocyclic organic molecules.1iÀk On
the other hand, macrocyclic molecules suitable for constructing
metal-containing 1D materials are particularly limited and
mostly rely on a G-quartet and its derivatives,1i,3bÀ3d Schiff base
macrocycles,3eÀh and porphyrin and its analogues.3hÀm More-
over, these fibrillar ensembles are largely cation-mediated, where
cations either are sandwiched between macrocycles1i,3b,3dÀ3f or
form additional coordination bonds directly and vertically with
the neighboring macrocyclic ligands,3hÀm while ion-pair-induced
1D columnar aggregation3g is very scarce. We report here a new
class of planar five-fold-symmetric macrocyclic pentamers whose
appropriately sized interiors are decorated by five carbonyl
oxygens that display high-affinity binding toward alkali metal
ions through metalÀoxygen interatomic interactions. Cation- or
scarcely reported ion-pair-induced fibrillation thus results from
these high-affinity bindings and planar geometries.
Received: July 12, 2011
Published: August 08, 2011
r
2011 American Chemical Society
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Table 1. Ka, R, and ΔGo for 2 Complexing Alkali Picrate Salts
from Li+ to Cs+ in H2O/CHCl3 at 25 °C, Obtained by Cram’s
Method5c
the presence of Cs+, predominantly forming a sandwiched
[23 Cs2]2+ species with an almost complete disappearance
3
of the 1:1 complex [2 Cs]+. This finding is consistent with
3
the experimental observation of Cs+-mediated fiber forma-
tion (Table 2 and Figures 2 and 3) and with ab initio
molecular modeling (B3LYP/6-31G*, Figure S9), showing
that Cs+ ions, being largest in size, are more capable of
forming and stabilizing sandwiched assemblies with respect
to other ions.
b
picrate salt, M
Ra
Ka (Â108 MÀ1
)
ÀΔG° (kcal/mol)
Li+
Na+
K+
Rb+
Cs+
0.719
0.715
0.645
0.615
0.602
2.28 ( 0.10
1.78 ( 0.09
0.57 ( 0.04
0.24 ( 0.03
0.18 ( 0.01
11.35
11.20
10.53
10.01
9.84
To enable direct visualization of the solid-state morphologies
resulting from molecular interactions between alkali ions and 1 or 2,
and subsequent intercolumnar associations, transmission electron
microscopy (TEM) was employed. In a typical experiment, MeOH
in the case of 1 or CHCl3 in the case of 2 was slowly diffused over a
few days or weeks into an acetonitrile solution containing a 1:1 ratio
of pentamer (1 or 2) and the respective MBPh4 salt (M = Li+, Na+,
K+, Rb+, and Cs+) at 10 mM. The slowly formed aggregates were
spotted onto TEM grids and examined by TEM. TEM images
(Figures 2 and S10ÀS12) reveal that, except for KBPh4, 1 forms
fibers with all the other four metal ions and that, for 2, appreciable
fibers can be observed only for RbBPh4 and CsBPh4. In other words,
1 carrying benzyl side chains forms good fibers with MBPh4 salts
more often than 2 carrying octyl side chains (Table 2). This is
consistent with the theoretical calculations, revealing the existence of
extensive intermolecular H-bonds among stacked molecules of 1
(Figure S31) that increases the stability of 1D columnar stack.
The potential to use ion-pairs to induce fiber formation of 1 or
2 was also investigated by TEM. A total of 10 salts (MCl and
MBr, where M = Li+, Na+, K+, Rb+, and Cs+) were tested against
both 1 and 2. Akin to the case of MBPh4, examining these 20
combinations (Table 2, Figures 2 and S13ÀS17) highlights a
similar trend: 1 forms fibers with alkali halide salts more often
than 2. Aside from the fibers seen in the majority of cases, defined
nanorods and nanoropes (Figure 2) were also produced when 2was
mixed with LiCl, NaBr, and KBr, and possibly with LiBr (Figure
S11). For instance, KBr salts combine with 2 to produce well-
defined nanorods, typically measuring 50À200 nm in width and
1À3 μm in length, and 2 in the presence of NaBr salts assembles
into very flexible, easily coiled, and virtually endless nanoropes, each
with a uniform diameter from 0.2 to 1 μm (Figures 2 and S14).
Energy-dispersive X-ray analysis (EDX), a microanalytical
technique for elemental mapping, was carried out in an effort
to identify elements present in the above-formed fibers. Selective
EDX analyses on four types of as-produced fibers formed be-
tween 1 and CsBPh4 or KCl, and between 2 and NaBr or KBr,
reveal the elemental occurrence of Cs, K/Cl, Na/Br, and K/
Br in these fibers, respectively, unambiguously confirming
their incorporation into the fibers (Figures S18, S19, S21,
S23, and S24).
a [Guest]/[host] ratio in CHCl3 layer at equilibrium. b Averaged values
over three runs with the assumption of 1:1 guest/host binding
stoichiometry.4e,5d
matching between their experimentally obtained high-resolution
masses and the calculated ones;5b their highly symmetrical nature
was revealed by their highly simplified 1H NMR spectra at 110 °C
in DMSO-d6 for 1 and 33% CDCl3/67% DMSO-d6 for 2 that
showed three sets of proton signals corresponding to protons
on the pyridone motif and amide bonds.5b The crystal structure
of dimer molecule 1q demonstrates the ability of the interior
carbonyl oxygen to form intramolecular H-bonds (2.00À2.27 Å)
with the amide proton, restricting the conformational freedom
of the amide bond and biasing the aromatic backbone into a
crescent shape (Figures 1c and S2).5e
An indication suggesting good binding of alkali metal ions
by 1 and 2 came from the solubility test: 1 is sparingly soluble
in DMSO (<0.01 mM at room temperature), but addition
of 1 equiv of alkali metal ions of their tetraphenylborate
(BPh4) salts from Li+ to Cs+ invariably dissolves >20 mM 1 into
DMSO at room temperature. Following Cram’s method using
picrate extraction experiments at 10 mM,5b,c the association
constants (Ka) for complexation of 2 to five alkali metal ions
were evaluated in a H2O/CHCl3 system. The data compiled in
Table 1 show high-affinity binding of ∼108 MÀ1 with a good
degree of selectivity toward alkali ions by 2. The strong binding
between 2and alkali metal ions is established further by being able to
detect the complex formation using thin-layer chromatography
(TLC). In the presence of 5 equiv of KBPh4 salt, 2 mostly exists
in the complex form in TLC plates; free 2 is very minimal (lanes 7
and 8, Figure S3). This trend persists when 5 equiv of other MBPh4
salts (M = Li+, Na+, Rb+ and Cs+) were used.
The high-affinity binding exhibited by 2 led us to explore the
possibility of 2 stabilizing the columnar stacking in the presence
of alkali ions (Li+, Na+, K+, Rb+, and Cs+). This was first
investigated by high-resolution ESI mass spectroscopy. In addi-
tion to the 1:1 complex [2 M]+, other aggregated species, such
3
as [22 M]+, [23 M2]2+, [24 M3]3+ (M = Li+, Na+, K+, Rb+, and
3
3
3
Cs+) and [22 Li2 H2O]2+, were observed (Figures S4ÀS8)
To gain further insights into the above-formed 1D stacked
structures at the molecular level, ab initio theoretical investigations at
3
3
Most noteworthy is the strongest tendency of 2 to aggregate in
Table 2. Cation- and ion pair-induced fibrillationa of 1 or 2 in the presence of various alkali metal salts.
alkali MBPh4 salts
alkali MX Salts
host
Li+
yes
Na+
Rb+
Cs+
LiCl
LiBr
NaCl
yes
NaBr
KCl
KBr
RbCl
yes
RbBr
CsCl
CsBr
e
f
1
2
yes
yesc
yes
yes
yes
yesb
nanorodd
yes
nanorodc,d
yes
nanoropeb
yesb
yes
yesb
nanorodb
À
yesb
yesb
À
e
e
e
e
À
À
À
À
yes
yesc
a Obtained by slow diffusion of ethyl acetate into 1-containing DMSO solution, or MeOH into 2-containing CHCl3 solution; in some cases, a tiny
amount of H2O was added to dissolve the salts. In all the salts studied, only KBPh4 does not induce any noticeable fiber formation. b High quality with
smooth surfaces. c Moderate or bad quality. d Irregular shapes. e No significant fibers were observed. f A mixture of irregular nanorods and fibers.
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Journal of the American Chemical Society
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Figure 3. Computationally optimized structures of 1D columnar
aggregates possibly formed by (a,b) [1 KBr]n and (c,d) [2 KBr]n at
Figure 2. Selected TEM images illustrating the fibrillation induced by
alkali metal salts M+BPh4À and ion pairs M+XÀ. Fibers were obtained
by slow diffusion of ethyl acetate into 1-containing DMSO solution, or
MeOH into 2-containing CHCl3 solution; in some cases, a tiny amount
of H2O was added to dissolve the salts.
3
3
the B3LYP/6-31G* level under periodic boundary conditions. The top-
down views illustrate two possible packing modes and their relative
energies. Side views ,with the exterior side chains removed, illustrate the
interplanar distances that dictate the strength of ionic interactions. In the
CPK models, K+ = 1.38 Å and BrÀ = 1.95 Å.
the B3LYP/6-31G* level under periodic boundary conditions
were carried out for 1D columns formed between KBr and 1 or 2
(Figure 3). Two packing modes may develop in forming
1D columns: in the eclipsed mode, pentamers are vertically
aligned and superimposable over each other (Figure 3a,c);
in the twist mode, vertically adjacent pentamers are twisted by
32À33° so that the steric hindrance, if any, among exterior side
chains can be possibly released (Figure 3b,d). For 1, the eclipsed
mode (Figure 3a) is energetically less stable than the twist mode
(Figure 3b) by 46.32 kcal/mol. This enhanced stability results
from (1) the sterically very bulky benzyl side chains that lessen
the ionic interactions among K+ and BrÀ ions, as evidenced by
the larger interplanar separation of 6.96 Å in the eclipsed mode
with respect to 6.27 Å in the twist mode, and (2) intermolecular
H-bonds (2.20 Å) formed between one of the aromatic protons
from every exterior benzyl group and the amide carbonyl oxygen
that is right below it (Figure S31). For 2, the difference in relative
energy between the eclipsed and twist modes is quite small, and
the eclipsed packing mode (Figure 3c) is marginally more stable
than the twist mode (Figure 3d), by 1.32 kcal/mol. This is a result
of an increase in favorable hydrophobic interactions and essen-
tially an absence of steric hindrance among exterior octyl side
chains, which can more than compensate the lessened ionic
interactions (6.27 Å, Figure 3c) in the eclipsed packing mode as
compared to the twist mode (6.18 Å, Figure 3c). Overall, the
computationally obtained more stable structures (Figure 3b,c)
show that K+ cations stay in the center of the cavity and are
stabilized by the closest five carbonyl oxygen atoms. Cross-
linking the planar pentameric molecules to form a 1D stacked
BrÀ-sandwiched supramolecular ensemble was achieved by
stabilizing ionic interactions between K+ and BrÀ, whereby the
interplanar distance of 6.27 Å (Figure 3b,c) is 0.39 Å less than
twice the sum of the van der Waals radii of K+ (vdW = 1.38 Å)
and BrÀ (vdW = 1.95 Å). These 1D aggregates are further
stabilized by intermolecular H-bonds between aromatic benzyl
protons and amide carbonyl oxygen atoms in 1, and by hydro-
phobic side-chain interactions in 2.
The formation of ion-pair-induced 1D columnar structures
can be supported by the exchange experiments,5b where fibers
formed from 1 and KCl or KBr, i.e., 1 KCl or 1 KBr fibers, were
3
3
suspended in MeOH solution and treated respectively with
a large excess of tetrabutylammonium bromide (TBABr) or
TBACl. If the anions (ClÀ and BrÀ) occupy the intercolumnar
spaces, where these loosely bound anions are separated from
the cations by >12 Å on the basis of the dimensionality of 1,
presumably they should remain exchangeable, largely to equal
extents, respectively, by excessive BrÀ and ClÀ anions. The
exchange experiments, however, show that ∼91À96% ClÀ
anions trapped in the 1 KCl fibers can be exchanged by excessive
3
BrÀ anions (Figure S20), while only ∼26À30% BrÀ anions in
the 1 KBr fibers remains exchangeable by excessive ClÀ anions
3
(Figure S22). These experiments suggest that anions stay inside
the column and that KBr stabilizes the 1D fibrillar structures
formed from 1 better than KCl. Consistent with these data, (1)
TEM images reveal significant morphological changes for 1 KCl
3
fibers after exchange with TBABr (Figures S25 vs S26) and little
change in morphology in 1 KBr fibers after exchange with
3
TBACl (Figures S27 vs S28), and (2) powder X-ray diffraction
(XRD) analyses show that the dominant peak at the lower
angle (2θ < 5°) from the 1 KCl fibers after exchange with KBr
3
(Figure S30) is closer to that of the 1 KBr fibers (Figure S15)
3
than that of the 1 KCl fibers (Figure S29). By allowing anions to sit
3
inside the columns and between cations, the 1D fibrillar structures
should become more stable due to the existence of favorable ionic
interactions and a concurrent reduction in electrostatic repulsions
occurring among cations from the vertically aligned adjacent
macrocycles if anions are to occupy the intercolumnar spaces.
The inference on the intercolumnar association was achieved
by carrying out XRD analyses, revealing a possible hexagonal
arrangement involving 1D columns for the fibers prepared from
1 in the presence of LiBPh4 (Figure S10), NaBPh4 (Figure S10),
LiBr (Figure S13), NaCl (Figure S14), NaBr (Figure S14), and
KCl (Figure S15). As an example, the X-ray diffractogram of
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1 NaCl fibers (Figure S14) shows a strong peak at d100 = 19.98 Å
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3
(2θ = 4.42°), along with two deconvoluted weaker peaks at
d110 = 10.91 Å and d200 = 9.75 Å, possibly indicating a hexagonal
lattice with an intercolumnar distance of dhex = 23.07 Å.
Compared to the overall radius of 1.18 nm, inclusive of
0.68 nm from its pentamer core and 0.50 nm from the benzyl
side chains for the 1D columns formed from 1 (Figure 3b), the
overlap among the 1D columns is ∼0.05 nm, suggesting that the
exterior benzyl side chains in the twist packing mode (Figure 3b)
do not penetrate into each other, which is consistent with the rigid
nature of benzyl groups. Assuming formation of a hexagonal lattice,
the calculated intercolumnar distances of 2.24À2.44 nm indicate an
overlap of <0.12 nm for the 1 LiBr, 1 NaBr, and 1 KCl fibers
3
3
3
(Figures S13ÀS15) and no overlap for the1 LiBPh4 and 1 NaBPh4
3
3
fibers (Figure S10). Similar XRD analyses (Figures S10ÀS17) on
other fibers formed from 1 or 2 (Table 1) give inconclusive
information on the intercolumnar arrangement of 1D columns.
In summary, we have developed an entirely new class of
cation-binding foldamer-based pentameric macrocycles capable
of high-affinity recognition of metal ions and self-assembling into
tunable 1D columnar aggregates that further associate to form
unusual cation-containing or ion-pair-induced fibers of varying
shapes and sizes, controllable by alkali metal ions or their halide
salts. The modular nature of the described macrocycles also enables
easy modification of the outer surfaces, and, in combination with
other monomeric building blocks recently reported by us,4e further
allows the interior properties to be finely tuned with respect to both
ion-binding affinity and selectivity. Accordingly, an enriched family
of closely related, appropriately designed structural variations can be
envisioned for some interesting applications.1À3,4e,6
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S
Supporting Information. Procedures and characteriza-
b
tion data. This material is available free of charge via the Internet
at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
chmzh@nus.edu.sg
’ ACKNOWLEDGMENT
We thank National University of Singapore AcRF Tier 1
Grants (R-143-000-375-112 and R-143-000-398-112 to H.Z.).
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The partially refined structure of pentamer 1 does reveal five-fold
symmetry and a nearly planar backbone in 1 (Figure S1).
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