Highly selective Na+-templated formation of [2]pseudorotaxanes exhibiting significant optical outputs

Article abstract of DOI:10.1002/anie.200604166

(Figure Presented) The chosen one: A molecular cage has been shown to form pseudorotaxane-like complexes with threaded anthraquinone and squaraine units in the presence of templating Na⁺ ions (see picture). The complexation and decomplexation o

Full text of DOI:10.1002/anie.200604166

Angewandte
Chemie
DOI: 10.1002/anie.200604166
Host–Guest systems
Highly Selective Na+-Templated Formation of [2]Pseudorotaxanes
Exhibiting Significant Optical Outputs**
Sheng-Yao Hsueh, Chien-Chen Lai, Yi-Hung Liu, Shie-Ming Peng, and Sheng-Hsien Chiu*
The diverse range of binding geometries exhibited by metal
ions and their relatively stable interactions with ligands can be
exploited for the efficient template-directed synthesis of
macrocycles,[1] supramolecular architectures,[2] and inter-
locked molecules.[3] Although the formation of rotaxanes or
pseudorotaxanes has been demonstrated using transition-
metal ions coordinated to four (tetrahedral and square
planar),[4] five (trigonal bipyramidal and square pyramidal),[5]
and six (octahedral)[6] donor atoms, alkali-metal ions, which
are useful templates for the synthesis of crown ethers,[7] are
Scheme 1. Molecular cages 1 and 2.
rarely used to direct the construction of such complexes. In
one example, Li+ ions have been used to assist the formation
of a unique donor–acceptor pseudorotaxane,[8] but it appears
that Na+ ions play a similar role in that system.[9] We became
interested in forming pseudorotaxanes in solution with high
efficiency and selectivity toward a single species of alkali-
metal ion—especially if we could differentiate between the
physiologically important,[10] but chemically similar, Na+ and
K+ ions.[11] Herein, we report a new molecular recognition
system based on the molecular cage 1 (Scheme 1) and two
different threadlike molecules (anthraquinone and squar-
aine), each of which forms a pseudorotaxane complex with 1
in the presence of templating Na+ ions and exhibits high
selectively over the other alkali-metal ions tested. This ion-
specific templating effect was easy to monitor because the
complexation and decomplexation of the pseudorotaxane
complexes in solution occurred with color changes that were
detectable to the naked eye. It appears from the solid-state
structures that the dramatic Na+/K+ selectivity in this
supramolecular system results from the different positions
and ligand complexation geometries of the metal ions bound
by the crown ether motifs of the molecular cage.
Previously, we demonstrated that molecular cage 2 and
anthraquinone (3, Scheme 2) can form pseudorotaxane-like
complexes in solution when K+ ions are present.[12] We
suspected that greater ion-selective templating behavior
would arise if we replaced the two openings resembling
dibenzo[24]crown-8 (DB24C8) in 2 with two relatively
smaller and more rigid units resembling dibenzo[18]crown-6
(DB18C6, Scheme 2). Thus, we synthesized the correspond-
ing molecular cage 1 in three steps from 2,3,6,7-tetrahydroxy-
9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene (see the
Supporting Information).
The 1H NMR spectrum (Figure 1B) of a mixture of
macrocycle 1, anthraquinone, and NaClO4 (2:2:4 mm) in
CDCl3/CD3CN (1:1) exhibits three sets of resonances:
1) those of the Na+-complexed molecular cage 1 (Figure 1A),
2) the free anthraquinone (Figure 1C), and 3) the complex
formed between 1, anthraquinone, and Na+ ions. This finding
implies that the rates of complexation and decomplexation
are both slow on the 1H NMR spectroscopic timescale at
400 MHz and 298 K. The upfield shifts of the signals of the
aromatic protons of both components imply that stacking of
their aromatic rings occurs within the supramolecular com-
plex. By using a single-point method,[13] we determined the
association constant (Ka) of this system to be 660mÀ1 in
CDCl3/CD3CN (1:1). In contrast, the 1H NMR spectrum of a
mixture of macrocycle 1, anthraquinone, and KPF6
(2:2:4 mm) in CDCl3/CD3CN (1:1) exhibits negligible shifts
of the anthraquinone signals relative to those of the uncom-
plexed molecule, which suggests that K+ ions are very poor
templates for the formation of the corresponding pseudo-
rotaxane complex (Figure 1D).
[*] S.-Y. Hsueh, Y.-H. Liu, Prof. S.-M. Peng, Prof. S.-H. Chiu
Department of Chemistry
National Taiwan University
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan (ROC)
Fax: (+886)2-3366-1677
E-mail: shchiu@ntu.edu.tw
Homepage: http://www.ch.ntu.edu.tw/english/efaculty/people/
chiu-eng.html
Prof. C.-C. Lai
Institute of Molecular Biology
National Chung Hsing University and
Department of Medical Genetics
China Medical University Hospital
Taichung, Taiwan (ROC)
We grew single crystals suitable for X-ray crystallography
by liquid diffusion of diisopropyl ether into a solution of
molecular cage 1, NaClO4, and anthraquinone (1:2:1) in
CH3CN. The solid-state structure (Figure 2) reveals the
expected [2]pseudorotaxane binding geometry of the com-
plex [3ꢀ1ꢁNa2]2+;[14,15] the rodlike anthraquinone unit pene-
trates through the two 28-membered rings while its two
[**] This study was supported by the National Science Council, Taiwan
(NSC-95-2113M-002-016-MY3 and NSC-95-2752M-002-002-PAE.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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common alkali-metal ions. As the
absorption arising from charge-transfer
interactions between the electron-rich
phenol units of the molecular cage and
the electron-deficient anthraquinone
was not very striking to the naked eye,
we turned our attention toward com-
plexes formed using the rodlike squar-
aine (4, Scheme 2), which is an impor-
tant near-IR dye with potential applica-
tions in many areas, such as nonlinear
optics, photovoltaics, biological labeling,
and photodynamic therapy.[16]
The 1H NMR spectrum of a solution
of macrocycle 1, squaraine, and NaClO4
(2:2:4 mm) in CDCl3/CD3CN (1:1)
exhibits proton shifts that are similar to
those of the [3ꢀ1ꢁNa2][2ClO4] system,
which suggests that the geometry of the
complex [4ꢀ1ꢁNa2][2ClO4] is likely to
Scheme 2. Complexation of molecular cage 1 withthreadlike guests 3 and 4 in the presence of
templating Na+ ions.
Figure 2. Ball-and-stick representation of the solid-state structure of
[3ꢀ1ꢁNa2][2ClO4]. Atom labels: C, gray; O, orange; Na, purple; Cl,
green.
Figure 1. Partial 1H NMR spectra (400 MHz, CDCl3/CD3CN (1:1),
298 K) of A) a mixture of 1 (2 mm) and NaClO4 (4 mm), B) a mixture
of 1 (2 mm), 3 (2 mm), and NaClO4 (4 mm), C) 3, and D) a mixture of
1 (2 mm), 3 (2 mm), and KClO4 (4 mm). The descriptors (C) and (UC)
refer to the signals of the complexed and uncomplexed states of the
components, respectively.
oxygen atoms coordinate to the two Na+ ions, which are
themselves bound within the [18]crown-6 units of the
molecular cage.
The solution of 1, 3, and NaClO4 (4:4:8 mm) in CDCl3/
CD3CN (1:1) displayed a clearly observable orange color,
possibly as a result of charge-transfer interactions between
the molecular cage and the anthraquinone in the pseudo-
rotaxane-like complex (Figure 3). The addition of Li+, K+, or
Cs+ ions at the same concentration did not cause a noticeable
color change of the molecular cage/anthraquinone solution,
thus suggesting that Na+ ions templated the formation of the
pseudorotaxane-like complex quite selectively among these
Figure 3. Photograph of vials containing solutions of 1 and anthraqui-
none (4 mm each) in CDCl3/CD3CN (1:1) in the a) absence and b)–e)
in the presence of b) Li+, c) Na+, d) K+, and e) Cs+ ions (8 mm each).
be that of a pseudorotaxane (see the Supporting Informa-
tion). The Na+ ions templated the formation of the complex
[4ꢀ1ꢁNa2][2ClO4] much more efficiently than they did the
2014
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corresponding anthraquinone complex; the binding constant
(Ka) between the Na-complexed molecular cage 1 and
squaraine in CDCl3/CD3CN (1:1) was 9600mÀ1 [13]
which is
,
more than tenfold higher than that of the anthraquinone
complex. The stronger binding affinity of squaraine to the Na-
complexed molecular cage 1, relative to that of anthraqui-
none, may be the result of greater steric compatibility and/or
the greater electron densities of the oxygen atoms of 4.
We obtained single crystals of the complex suitable for X-
ray crystallography by liquid diffusion of diisopropyl ether
into a solution of molecular cage 1, NaBF4, and squaraine
(1:2:1) in CH3CN. The solid-state structure (Figure 4) con-
firms the expected [2]pseudorotaxane binding geometry of
the complex [4ꢀ1ꢁNa2]2+,[17] in which the rodlike squaraine
unit penetrates through the two 28-membered rings, while its
two oxygen atoms coordinate to the two Na+ ions, which are
themselves complexed to the two [18]crown-6 units of 1.
Figure 5 illustrates that, among all of the alkali-metal ions
tested, the blue color of the solution of 1 and squaraine turned
red only in the presence of Na+ ions. The quantum yield of
squaraine dye at a wavelength of approximately 650 nm
Figure 5. Photographs of the A) visible and B) fluorescence (excitation
at 354 nm) behavior of mixtures of 1 and 4 (2 mm each) in CHCl3/
CH3CN (1:1) in the a) absence and b)–f) presence of 2 equiv of b) Li+,
c) Na+, d) K+, and e) Cs+ and f) 2 equiv of Na+ and 20 equiv of
K+ ions.
decreases significantly in polar solvents.[18] Thus, the color of a
squaraine solution is related to the polarity of the solvent
system; for example, CHCl3 solutions of squaraine and its
mixture with molecular cage 1 displayed similar red-to-blue
color changes when the more-polar CH3CN was added (see
the Supporting Information). This finding suggests that the
red color of the CHCl3/CH3CN (1:1) solution of molecular
cage 1, squaraine, and Na+ ions results from the formation of
the corresponding pseudorotaxane-like complex in solution;
that is, the relatively nonpolar molecular cage protects the
squaraine dye from solvation by polar CH3CN molecules.
Figure 6 provides a comparison of the emission spectra
observed after adding Na+ and K+ ions (2 equiv), respectively,
to an equimolar (1.3” 10 À4 m) solution of molecular cage 1
and squaraine 4; it is clear that the presence of Na+ ions
enhanced the fluorescence of squaraine substantially. Inter-
estingly, the addition of 20 equivalents of K+ ions to the red
solution of [4ꢀ1ꢁNa2][2ClO4] led to a substantial degree of
Figure 6. Fluorescence spectra (CDCl3/CD3CN (1:1), 298 K; excitation
at 400 nm) of a) an equimolar mixture (1.3”10À4 m) of 1 and 4, b) th e
system in (a) after the addition of 2 equiv of NaClO4, c) the system in
(a) after the addition of 2 equiv of KPF6, and d) the system in (b) after
the addition of 20 equiv of KPF6.
Figure 4. Ball-and-stick representations of the solid-state structure
[4ꢀ1ꢁNa2][2BF4]. Atom labels: C, gray; O, orange; N, blue; Na,
purple; B, yellow; F, green.
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dissociation of the pseudorotaxane complex: the color of the
Our finding that the metal-templating motifs in anthra-
quinone and squaraine (that is, the aromatic rings with two
carbonyl groups) both highly favor Na+ over K+ ions in
templating the formation of their respective [2]pseudorotax-
anes seems puzzling, as they are of different sizes. To
determine how the molecular cage 1 coordinates to these
two metal ions we grew single crystals suitable for X-ray
crystallography by liquid diffusion of diisopropyl ether into
solutions of molecular cage 1 and NaBF4 or KPF6 (1:2 molar
ratio) in CH3CN. The solid-state structures reveal that each
Na+ (Figure 7A,B) and K+ (Figure 7C,D) ion is bound to all
six oxygen atoms of the individual crown ether units of
molecular cage 1.[19,20] The [18]crown-6-complexed Na+ ions
are also coordinated by labile CH3CN ligands located within
the cavity of molecular cage 1; clearly these ligands could be
replaced by bidentate ligands such as anthraquinone and
squaraine. In contrast, the crown ether motifs of the
molecular cage 1 position their K+ ions outward from the
internal cavity, thereby leaving no internal coordination sites
for templating bidentate ligands. Thus, it seems that differ-
ences in the coordination geometries and ligand binding sites
of the complexed metal ions are responsible for molecular
cage 1 forming [2]pseudorotaxane structures in the presence
of Na+ ions, but not K+ ions, even when the bidentate ligands
have different sizes.
solution turned immediately from red to blue (compare vials f
in Figure 5A,B). This result suggests that while K+ ions can
compete with Na+ ions for complexation to 1 in solution, the
K+-complexed molecular cage displays a much weaker bind-
ing affinity to the squaraine thread.
We have demonstrated that Na+ ions behave as highly
selective templates, relative to K+ and other alkali-metal ions,
for the formation of pseudorotaxanes from the molecular
cage 1 and two threadlike molecules (anthraquinone and
squaraine) in CD3CN/CDCl3 (1:1). In addition to this ion-
specific binding, the complexation and decomplexation of the
pseudorotaxane-like complexes in solution is observable by
the naked eye. We believe that rotaxanes constructed from 1
and squaraine derivatives will protect this sensitive dye from
decay;[16f,g] furthermore, they have potential for application as
chemosensors and molecular switches that exhibit significant
optical outputs. Such systems are currently under investiga-
tion.
Received: October 11, 2006
Published online: February 7, 2007
Keywords: alkali metals · host–guest systems · molecular cages ·
.
pseudorotaxanes · templating effect
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data
for
[(3ꢀ1ꢁNa2)·4MeCN][2ClO4]:
[C74H80O14N4Na2][ClO4]2, Mr = 1494.30, monoclinic, space
group P21/n, a = 14.8664(2), b = 12.0495(2), c = 20.3540(3) Š,
V= 3624.53(9) Š3, 1calcd = 1.369 gcmÀ3, m(MoKa) = 0.181 mmÀ1
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T= 295(2) K, orange columns; 8119 independent measured
reflections, F2 refinement, R1 = 0.0696, wR2 = 0.1915.
[15] CCDC-617617 [(3ꢀ1ꢁNa2)·4MeCN][2ClO4], CCDC-617618
[(1ꢁK2)·3MeCN][2PF6],
CCDC-617619
[(4ꢀ1ꢁNa2)·
4MeCN][2BF4], and CCDC-617620 [(1ꢁNa2)·2MeCN·CH2Cl2·
H2O][2BF4] contain the supplementary crystallographic data for
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ac.uk/data_request/cif.
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[17] Crystal
data
for
[(4ꢀ1ꢁNa2)·4MeCN][2BF4]:
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[C84H100O14N6Na2][BF4]2, Mr = 1637.30, triclinic, space group
¯
P1, a = 13.5847(2), b = 13.7701(2), c = 14.2357(2) Š, V=
2230.43(6) Š3, 1calcd = 1.219 gcmÀ3, m(MoKa) = 0.101 mmÀ1, T=
295(2) K, orange cubes; 7859 independent measured reflections,
F2 refinement, R1 = 0.0730, wR2 = 0.2305.
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[C57H66O13N2Cl2Na2][BF4]2, Mr = 1277.62, trigonal, space group
P1c, a = 21.3476(4), b = 21.3476(4), c = 26.1494(4) Š, V=
10320.3(3) Š3, 1calcd = 1.233 gcmÀ3, m(MoKa) = 0.184 mmÀ1, T=
295(2) K, colorless plates; 6056 independent measured reflec-
tions, F2 refinement, R1 = 0.1487, wR2 = 0.4145.
[13] Titration of molecular cage 1 with NaClO4 under the same
[20] Crystal data for [(1ꢁK2)·3MeCN][2PF6]: [C58H69O12N3K2][PF6]2,
1
conditions resulted in negligible shifts in the H NMR spectral
Mr = 1368.30, monoclinic, space group P21/n, a = 15.5177(8), b =
signals after two equivalents of Na+ ions had been added. The
binding constant was calculated on the basis of the assumption
that the binding affinity between macrocycle 1 and an Na+ ion is
infinite. For examples of the single-point method, see a) P. R.
Ashton, E. J. T. Chrystal, P. T. Glink, S. Menzer, C. Schiavo, N.
15.0057(9),
1.415 gcmÀ3
c = 15.1762(8) Š,
V= 3211.9(3) Š3,
, T= 295(2) K, colorless
1calcd
=
,
m(MoKa) = 0.292 mmÀ1
plates; 7246 independent measured reflections, F2 refinement,
R1 = 0.0980, wR2 = 0.2431.
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