Persistently folded circular aromatic amide pentamers containing modularly tunable cation-binding cavities with high ion selectivity

Article abstract of DOI:10.1021/ja1035804

In this work, we illustrated a novel design strategy that allows systematically tunable interior properties (effective cavity size, steric crowdedness, and hydrophobicity) contained within a novel class of shape-persistent aromatic pentamers to take place on a scale below 3 A. Such finely tunable structural features are complimented by experimentally observable functional variations in ion-binding potential. Results of the selective, differential binding affinities of three circular pentamers for Li⁺, Na⁺, K⁺, Rb⁺, and Cs ⁺, substantiated by metal-containing crystal structures and computational modeling, are detailed.

Full text of DOI:10.1021/ja1035804

Published on Web 06/25/2010
Persistently Folded Circular Aromatic Amide Pentamers Containing Modularly
Tunable Cation-Binding Cavities with High Ion Selectivity
Bo Qin,† Changliang Ren,† Ruijuan Ye,† Chang Sun,† Khalid Chiad,‡ Xiuying Chen,§ Zhao Li,†
Feng Xue,† Haibin Su,§ Gregory A. Chass,| and Huaqiang Zeng*,†
Department of Chemistry, 3 Science DriVe 3, National UniVersity of Singapore, Singapore 117543, Max Planck
Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany, DiVision of Materials Science, Nanyang
Technological UniVersity, 50 Nanyang AVenue, Singapore, and School of Chemistry, UniVersity of Wales Bangor,
United Kingdom LL57 2UW
Received April 27, 2010; E-mail: chmzh@nus.edu.sg
Recently, we reported a highly rigid and structurally well-defined
Abstract: In this work, we illustrated a novel design strategy that
circularly folded pentamer 1 containing five methoxy groups in its
allows systematically tunable interior properties (effective cavity size,
steric crowdedness, and hydrophobicity) contained within a novel
class of shape-persistent aromatic pentamers to take place on a
scale below 3 Å. Such finely tunable structural features are com-
plimented by experimentally observable functional variations in ion-
binding potential. Results of the selective, differential binding affinities
of three circular pentamers for Li+, Na+, K+, Rb+, and Cs+,
interior (Figure 1a).1r Its shape, a nearly planar, rigidified pentagonal
aromatic amide backbone incorporating an inward-pointing, con-
tinuous H-bonding network, is quite unusual among unnatural
foldamers.1e-y The single crystal X-ray structure1r reveals a cavity
radius of ∼2.85 Å from the cavity center to the nucleus of the
interior oxygen atom. Two steric caps comprised of three and two
interior methoxy methyl groups, respectively, on either side of the
foldamer backbone plane completely block the cavity and prevent
any metal cation from entering it. Not surprisingly, our numerous
substantiated by metal-containing crystal structures and computa-
tional modeling, are detailed.
attempts to effect the binding by all of the five alkaline metal cations
were unsuccessful.
Persistently folded aromatic macrocycles with rigid, noncollapsible
backbones, such as porphyrin derivatives or analogues,1a,b arylene
ethynylene macrocycles,1c macrocyclic Schiff bases,1d and hydrogen-
bonded (H-bonded) macrocyclic foldamers,1e-y exhibit novel proper-
ties with potential for applications in chemistry, materials science,
medicine, and biology. The shape persistency and rigidity of these
macrocyclic frameworks are induced by covalent forces,1c built-in
hydrogen bonds (H-bonds),1a,e-y and intrinsic conformational bias of
the backbone2a or functional groups such as urea,1p amide,2a,b or
sulfonamide,2c,d acting alone or in a cooperative manner. Owing to
its robustness, predictability, and directionality, multiple-center H-
bonding has become a top strategic choice for designing tailor-made
macrocyclic aromatic foldamers.1m-y Such H-bonded aromatic mac-
rocycles have been demonstrated to bind either neutral1n,t or cationic
species,1m,2e act as ion transporters across cell membranes,2f and
stabilize DNA G-quadruplex structures.2g However, few synthetic
macrocyclic systems allow systematic fine-tuning of interior properties
while maintaining overall topographic persistence;1w,2h-j even fewer
We envisioned that replacement of one or two interior methoxy
groups in 1 with hydroxyl groups, giving rise to six pentamers 2-4
(Figure 1a-c), should increasingly open the cavity (Figure 1d-f),
allowing selective binding to diverse metal cations to take place. This
reasoning was based on the following premises: (1) the coordination
distances between the oxygen atoms and the majority of metal cations
fall below 2.85 Å; (2) pentamers 2-4 possess a cation-binding site
displaying five oxygen donor atoms in a preorganized manner; (3)
the binding site in 2-4 additionally incorporates at least one phenol
moiety capable of ionic interaction with metal ions upon deprotonation
under basic conditions; and (4) the methoxy methyl groups present in
the interior are sterically bulky and hydrophobic in nature and may
have subtly different orientations, possibly allowing the fine-tuning
of cation-binding potential of 2-4.
Toward characterizing our designs, we first constructed ab initio
models using density functional theory (DFT) at the B3LYP/6-31G(d,p)
level, on pentamers containing either one or two hydroxyl groups. The
geometry-optimized circular conformer structures of pentamers 2a-4a
(see Figure S3a for 2a and Figure 1e-f for 3a and 4a) in their anionic
forms revealed that with incremental substitution of methyl groups
with hydroxyl ones, going from 1 to 2a-4a, the interior cavity
gradually opens up, concomitant with a decrease in steric hindrance
and hydrophobicity imparted by the interior methyl groups. Encouraged
by these computational trends, we prepared all six hydroxyl-containing
pentamers 2-4 by protecting the phenolic hydroxyl groups as benzyl
ethers, followed by a stepwise elongation and subsequent debenzylation
to generate one or two hydroxyl groups decorating the interior of
2-4.4,5
have the proven propensity for selective metal ion recognition,1m,2a
a
feature that may enable important uses as imaging agents, disease-
rectifying therapeutics, etc.2l,m To the best of our knowledge, among
shape-persistent aromatic macrocycles,1 currently there is no example
that maintains a high degree of precisely tunable interior physical
properties including effective cavity size, steric crowdedness, hydro-
phobicity, and cation-binding capacity on the scale below 3 Å.3 Herein,
we characterize, by an experimental-theoretical synergy, a new class
of modular, H-bonded, shape-persistent, macrocyclic receptors, forming
a cation-binding cavity in their anionic forms whose interior properties
can be uniquely and selectively tailored to recognize specific alkali
metal cations.
Examining the single crystal structure of 2a (slowly grown from
methanol:dichloromethane )1:1, v/v) revealed pentamer 2a folds into
an almost planar disk arrangement of nearly perfect C5 symmetry in
the solid state (Figure 1d), remarkably similar to the B3LYP/6-31G(d,p)
structure (Figure S3a). A continuously formed H-bonding network (5-
and 6-membered NH...OMe ) 2.09-2.33 and 1.70-1.99 Å, respec-
† National University of Singapore.
‡ Max Planck Institute for Polymer Research.
§ Nanyang Technological University.
| University of Wales Bangor.
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10.1021/ja1035804  2010 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. (a) X-ray crystal structure of the complex K+@anionic 3a2-; (b)
B3LYP/6-31G(d,p)(SCRF ) PCM, solvent ) CH3CN) calculated structure of
the complex K+@anionic 4a2-;5 (c) X-ray crystal structure of the complex
Cs+@anionic 4a2-. O ) red sphere, K+ ) purple sphere, Cs+ ) deep blue
sphere, weak coordination bonds ) dotted pink lines, and intermediate and
strong coordination bonds ) solid sticks. For clarity of view, all the coordinated
solvent molecules were removed in both (a) and (c).
to 3b that carries two neighboring phenols. Moreover, tunability in
interior properties indeed imparts experimentally measurable ion-
binding selectivity to 3b and 4b. This can be supported by the following
observations: (1) while the binding constants of anionic 3b2- toward
Na+ and K+ were determined to be 1.77 × 105 M-1 and 1.00 × 105
M-1, respectively, no bindings were detected for either 2b- or 4b2-
for both Na+ and K+, and (2) in the case of Rb+ and Cs+, only anionic
4b2- displays appreciable binding constants of 0.12 × 105 M-1 and
0.55 × 105 M-1, respectively. Such noteworthy differences in binding
affinities between neutral and anionic forms of 2b--4b2- and among
anionic 2b--4b2- fully ascertain the potential of our system, leading
to precisely tunable interior chemical and physical properties. Notably,
2b--4b2- display either comparably good or better selectivity against
metal cations with respect to the two crown ethers.
Figure 1. Chemical structures of 2 (a), 3 (b), and 4 (c) and top view of the
determined crystal structure of 2a in its neutral form (d), ab initio-optimized
structure of 3a2- (e) and 4a2- (f), both in their anionic forms, showing one or
two interior methyl groups form a hydrophobic cap above the pentamer plane;
a second hydrophobic cap comprised of two methyl groups stays below the
plane. CPK models of methoxy and hydroxyl groups were built based on the
van der Waals radius (Gray: H ) 1.20 Å; Green: C ) 1.70 Å; Red: O ) 1.52
Å, O- ) 1.40 Å).
tively) brings the five aryl-bonded oxygens in a circular manner to
enclose a cavity with a radius of ∼2.88 Å, near-identical to that found
in 1 (Figure 1a).1r Two hydrophobic caps comprising four methyl
groups and one hydrogen atom are found below and above the plane
(Figure 1d). Gratifyingly, the structural study demonstrated that
replacing methoxy groups with hydroxyl groups does not interrupt the
continuous H-bonding network nor does it adversely alter the cavity
shape and size necessary for efficient cation interactions.
Table 1. Association Constants (Ka × 10-5 M) for Complexation of
Alkaline Metal Cations (Li+, Na+, K+, Rb+, and Cs+) by Hosts of
a b
,
Anionic 2b2--4b2- as well as 15-Crown-5 and 18-Crown-6
Ethers Determined by Isothermal Titration Calorimetry (ITC) in
Acetonitrile at 25 °C
Li+
Na+
K+
Rb+
Cs+
To facilitate evaluating the solution binding profiles of pentamers
2-4 toward metal cations, octyloxy side chains were introduced around
the periphery of pentamers (2b-4b; Figure 1a-c) to enhance their
solubilities in organic solvents. The circularly folded conformation of
2b-4b in solution was supported by the amide protons resonating at
the very low field (δ > 10.6 ppm relative to TMS)6 and by NOEs
observed between interior methyl peaks and their adjacent amide pro-
tons in 2D NOESY spectra recorded in CDCl3.5 Upon deprotonation
of phenolic hydroxyl groups in 4b with tetrabutylammonium hydrox-
ide, two amide protons significantly downfield shift to δ > 15 ppm
(Figure 3a-b; see Figure S2 for 3b5), affirming the presence of
exceptionally strong intramolecular H-bonds that are mediated by
negatively charged oxygen atoms, rendering amide linkages more rigid
and reinforcing the crescent-shaped aromatic backbone in anionic 3b.
Next, we studied how the cation-binding function is related to subtle
structural changes upon deprotonation by isothermal titration calorim-
etry (ITC) in acetonitrile at 25 °C for anionic 2b-4b and in acetone
at 25 °C for neutral 2b-4b due to their insolubility in acetonitrile.
Analysis of the resulting binding isotherms after subtraction of the
heat of dilution gave the binding affinity, number of binding sites,
and essential thermodynamic parameters (∆G, ∆H, and ∆S).5
From results listed in Table 1, three general trends can be noted.
First, none of neutral 2b-4b binds any alkaline metal cation in
acetone.5 Second, a comparison in binding constants between 2b and
either 3b or 4b shows that the presence of more ionizable hydroxyl
groups enhances the cation-binding potential of the resultant pentamers,
as we designed. Third, separation of the two interior phenolic hydroxyl
groups with one methoxy group (as in pentamer 4b) significantly alters
the binding profile toward all five alkaline metal cations, with respect
c
15-Crown-5 3.43 ( 1.33 0.95 ( 0.09 0.19 ( 0.05
s
0.04 ( 0.01
18-Crown-6
2b-
s
s
0.51 ( 0.15 3.23 ( 0.24 0.76 ( 0.11 0.76 ( 0.19
s
s
s
s
s
s
3b2-
1.77 ( 0.38 1.00 ( 0.18
d
16.8/0.15
0.85 ( 0.23
4b2-
s
s
0.12 ( 0.03 0.55 ( 0.16
a Anionic 2b2--4b2- were generated by deprotonating hydroxyl
groups using tetrabutylammonium hydroxide; samples were fully dried
using vacuum oven. b Either 1:1 binding stoichiometry or no binding
was observed unless specified. c 1:2 Binding stoichiometry. d 2:1
Binding stoichiometry for two binding site mode (2Li+@anionic 3b2-
)
is possible, but the uncertainties are greater than the binding constants.
As originally conceived, the above exhibited ion binding specificity
by 3b2- and 4b2- can be interpreted on the basis of complementarity
and preorganization, with emphasis on (1) the essentially nonmobile
oxygen donor atoms in the interior, (2) the extent of coordination bond
formation between metal ions and negatively charged oxygens, and
(3) the subtly different orientations of hydrophobic methyl groups in
the interior. As illustrated in Figure 2a by the crystal structure of the
complex K+@anionic 3a2-,7 K+ ions are stabilized in an off-center
binding pocket in anionic 3a2- formed by a hydrophobic methyl group
(K+---H-CH2O distance ) 3.21 Å), two strongly coordinating anionic
phenolic oxygens (both K+---O- distances ) 2.68 and 2.73 Å), and
three coordinating methoxy oxygens (K+---O-CH3 distances ) 3.13,
2.23, and 3.24 Å). With respect to the position of K+ in 3a2-, smaller
cations such as Li+ (VdW ) 0.76 Å) and Na+ (1.02 Å) remain in
proximity to anionic oxygens while outlying the three methoxy oxygens
and hydrophobic methyl group. This leads to two strong M+---O-
coordination bonds that stabilize the complexes formed between Li+/
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C O M M U N I C A T I O N S
Na+ and 3b2-. In contrast, larger cations, such as Rb+ (1.52 Å) and
Cs+ (1.67 Å), are kept at bay from the two anionic oxygens while
being forced into the vicinity of the hydrophobic methyl group. This
apparently disallows concurrent binding of the two anionic oxygens,
thus preventing Rb+ and Cs+ from favorably binding and subsequently
entering the interior cavity of 3a2- or 3b2-.
to H.Z.), and GIOCOMMS (Toronto/Budapest/Beijing) for computa-
tional resources. GA.C. thanks CAFMaD (Wales, UK) for personal
support.
Supporting Information Available: Synthetic procedures for six
pentamers 2-4 along with a full set of characterization data including
CIF files and crystal data (2a, K+@3a2-, and Cs+@4a2-), 1H/13C NMR,
HRMS, NOESY, ITC, molecular modeling (anionic 1a2--3a2- and
K+@4a2-) and a full list of authors for ref 6b. This material is available
free of charge via the Internet at http://pubs.acs.org.
Compared to the strong interactions between K+ and 3b2- and no
detectable binding between smaller Na+ and 4b2- (Table 1), the
computationally determined very weak binding of ∼5 M-1 (∆G was
computed to be -0.67 kcal/mol which is consistent with ITC result
in Table 1) between K+ and 4a2- is effected through its larger VdW
radius of 1.38 Å, allowing one strong K+---O-CH3 (2.77 Å), two
relatively weak K+---O- (2.86 and 2.96 Å), and two disfavored K+---
H-CH2O (3.28 and 3.25 Å) interactions (Figure 2b). Conversely, as
supported by two strong Cs+---O- coordination bonds (3.00 and 3.05
Å) found in the complex Cs+@4a2- (Figure 2c),7 larger cations (Rb+
and Cs+) do allow two stronger M+---O- interactions to take place
simultaneously, thereby leading to their experimentally observable
strong bindings toward 4b2- (Table 1).
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(5) See Supporting Information.
Figure 3. 1H NMR spectra (5 mM, 500 MHz, 298 K, CDCl3/DMSO-d6 4:1
v/v) of (a) neutral 4a, (b) freshly prepared anionic 4a2- of its tetrabutylam-
monium (TBA+) salt containing two TBA+, (c) a mixture containing 1:1 molar
ratio of anionic 4a2- of its TBA+ salt and cesium tetraphenylborate (TPB-),
and (d) metal complex Cs+@4a2- containing one 4a2-, one Cs+, and one TBA+
crystallized from the condition outlined in (c).
In conclusion, we have demonstrated a novel strategy for the
modular construction of diverse macrocyclic pentamers. These function
as modularly engineerable, scalable, cavity-forming systems, wherein
precise pinpoint modifications, with variable functionalities in both
the interior and exterior, can be readily attained. Their shape-consistent
aromatic skeletons effectively preorganize and orient electronic (e.g.,
oxygen atoms) and steric/hydrophobic (e.g., methyl groups) features
into a convergent arrangement that may eventually enable the circular
pentamers to tightly, yet selectively, bind metal cations possessing a
radius below ∼1.5 Å. Along this line, further refinement in the binding
profiles of circular pentamers is currently ongoing.
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(7) Single crystals were obtained by slow diffusion of hexane (for K+-3a2-
)
or cyclohexane (for Cs+-4a2-) into a solution in acetone (for K+-3a2-) or
THF (for Cs+-4a2-) containing 1:1 molar ratio of cesium tetraphenylborate
and tetrabutylammonium salt of anionic 3a or 4a.
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.) and A*STAR BMRC research consortia (R-143-000-388-305
JA1035804
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