Metal-tunable nanocages as artificial chemosensors

Article abstract of DOI:10.1002/anie.200704206

(Figure Presented) Molecular design: Metal-tunable octahedral nanocages that act as molecular chemosensors are generated by a new strategy (the metals can be transition metals or lanthanides). Amide groups in the supramolecular structures act as guest-accessible functional sites. These interact with glucosamine molecules and function as efficient communicators to convert the recognition information into metal-tunable chromogenic and fluorogenic responses (see picture).

Full text of DOI:10.1002/anie.200704206

Angewandte
Chemie
DOI: 10.1002/anie.200704206
Molecular Sensors
Metal-Tunable Nanocages as Artificial Chemosensors**
Cheng He, Zhihua Lin, Zheng He, Chunying Duan,* Chunhu Xu, Zheming Wang, and
Chunhua Yan*
Supramolecular assembly of predesigned organic and inor-
ganic building blocks is an excellent tool for constructing well-
defined nanosized molecular cavities. The nanocages can
encapsulate and stabilize guest substrates, and chemical
transformations can be catalyzed within these “microreactor”
cagelike structures.[1] Recently, synthetic strategies for con-
structing functional coordination cages with various struc-
tures and novel inclusion properties have been well estab-
lished.[2,3] Substantial developments have been made in the
creation of Werner-type capsules that can be used for sensing
special guests with selective signal responses.[4] In this case,
the major challenge goes beyond achieving a size- or shape-
selective dynamic molecular recognition and includes detect-
ing and amplifying guest-bonding events to produce a
measurable output. Thus, a proper communication system
that is able to transduce the recognition information into an
easy-to-measure signal must be included in the overall
molecular design.[5]
By combining three quinoline groups into a disk-shape
molecule—by meta substitution of a central benzene ring—
we developed a new strategy for the preparation of octahedral
nanocages, which can be used as artificial chemosensors
(Scheme 1, H3L1). Each quinoline group acts as both chro-
mophore and fluorophore, thus serving as an N2O tridentate
coordinating site for the amide groups, which are introduced
as trigger sites to achieve efficient guest interactions and a
consequently good signal response.[6] Since the formation of
donor-type hydrogen bonds with the amide groups may
perturb the electronic distribution on the conjugated back-
bone of the ligand, such host–guest interactions can affect the
charge transfer associated with the quinoline units and lead to
significant changes in the optical properties.[7,8]
Scheme 1. Structure of the ligand H3L1 and constitutive/constructional
fragments of the functional octahedral cages. For ligand H3L1 (quino-
line derivative), the metal ions used are CoII and ZnII. For ligand H3L2
(pyridine derivative), the metal ions used are NiII, EuIII, and TbIII.
to a transition-metal center at an angle of exactly 908.[9] Our
preparation strategy, which is based on using a C3-symmetric
facial ligand that comprises three such tridentate coordinating
sites, forces various metal ions to form metal-variable
isostructural octahedral nanocages with considerable stabil-
ities. This method allows us to finely tune the metal ions and
ligand chromophore units to obtain highly selective and
sensitive responses toward particular guest molecules. By
appropriately modulating the quinoline groups to pyridine
groups, the synthetic strategy can be further extended to
lanthanide ions, which exhibit a low stereochemical prefer-
ence. Interesting metal-dependent luminescent properties are
expected for these isostructural coordination architectures.
The ligand H3L1 was easily obtained by Schiff base
reaction of 2-quinoline carboxaldehyde with 1,3,5-tricarbo-
hydrazine benzene in ethanol. Adding a solution of cobalt(II)
or zinc(II) perchlorate in methanol to a chloroform/methanol
solution of the ligand H3L1 led to high-yield formation of the
CoII compound 1 and the ZnII compound 2.[10] Crystal-
structure analyses reveal that compounds 1 and 2 are
isostructural (Figure 1). Both of them crystallize in the
Tridentate coordinated units are efficient building blocks
that can be used to fix the orientation of ligands coordinated
[*] Dr. C. He, Dr. Z. Lin, Prof. C. Duan
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian, 116012 (China)
Fax: (+86)411-8370-2355
E-mail: cyduan@dlut.edu.cn
Dr. Z. He, C. Xu, Prof. Z. Wang, Prof. C. Yan
State Key Lab of Rare Earth Materials
Chemistry and Applications
Peking University, Beijing 100871 (China)
E-mail: yan@pku.edu.cn
[**] We gratefully acknowledge financial support fromNSFC (20221101,
20490213, and 20571041) and the Start-Up Fund of the Dalian
University of Technology.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or fromthe author.
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Figure 1. Molecular structure of the octahedral cage in the isostruc-
tural compounds 1 and 2. Hydrogen atoms, anions, and solvent
molecules are omitted for clarity. The metal, oxygen, nitrogen, and
carbon atoms are green, red, blue, and gray, respectively.
Figure 2. ESI-MS of compound 1 in a) methanol solution and b) meth-
anol–DMSO solution upon addition of glucosamine. The inserts show
the measured and simulated isotopic patterns at m/z 1007.6 (a) and
m/z 1067.1 (b).
1
9+
¯
space group R3. Each octahedral cationic cage [M6(H9L )]
4
has an ideal C3 symmetry, with one of the four ligands located
on the crystallographic threefold axis. The pseudo S4 symme-
try is achieved by alternatively arranging the four planar
ligands onto the eight triangle faces of the octahedron defined
by the six metal ions. The three rigidly separated N2O
tridentate chelators of each ligand coordinate with three
different metal centers, while six metal centers occupy the
vertical positions of the octahedron, each of them coordinat-
ing with two planar tridentate N2O chelators to form a mer
configuration. These [M4L6] cages are closely related to the
hexanuclear palladium octahedron structures reported by
Fujita and co-workers.[11] The separation between two metal
ions bridged by one ligand is of about 9.77 Š, whereas the
distance between two diagonally opposing metal ions is about
13.8 Š (the average inner volume has been estimated to be
about 440 Š3). The presence of nine disordered perchlorate
anions for each cationic cage suggests that only three amide
groups in the four ligands lost their protons during coordina-
tion. These amide groups—located within the positively
charged cages—provide static, geometric, coordinative, and
functional properties to the cagelike capsules, which are
important for the recognition of monosaccharide derivatives,
such as glucosamine.[12,13]
an exact comparison of the most interesting experimental
peak (which is observed at m/z 1067.14) with the simulation
results obtained on the basis of natural isotopic abundances
reveals that this trivalently charged species can be reasonable
assigned to [Co6(H3L1 )ꢂ(NH2-Glu)]3+, thus providing evi-
4
dence of a 1:1 stoichiometric host–guest complexation. The
peak at m/z 1033.46, on the other hand, can be safely assigned
to the species [Co6(H3L1 )(DMSO)]3+ (DMSO = dimethyl
4
sulfoxide). Upon addition of glucose, instead of glucosamine,
no host–guest complex species were detected in the ESI-MS
spectra under the same experimental conditions, which
suggests a selective recognition of compound 1 by glucos-
amine in solution.
The inclusion phenomenon was further characterized by
means of UV/Vis measurements (Figure 3a). Both the
cobalt(II) (1) and the zinc(II) (2) compounds exhibited
clear ligand-based charge-transfer bands[14] (at about 280
and 415 nm) in an acetonitrile solution (5 ” 10ꢃ6 m). The
addition of glucosamine caused a significant increase in the
absorbance intensity at 415 nm and clear decrease at 340 nm.
The presence of sharp isosbestic points at 300, 315, and
380 nm indicates that only two species coexist in equilibrium.
The individual profile (see the Supporting Information) of the
band at 415 nm (increasing) also demonstrates the occurrence
of 1:1 stoichiometric host–guest complexations with associa-
tion constants (logKass) of 5.52 ꢁ 0.02 and 4.97 ꢁ 0.02 for
compounds 1 and 2, respectively. The addition of glucose does
not cause any obvious spectroscopic changes, which suggests
that the affinities of compounds 1 and 2 for glucosamine are
better than for glucose.
The electrospray ionization mass spectrometry (ESI-MS)
data demonstrate that compounds 1 and 2 are substantially
stable in solution (see the Supporting Information). As shown
in Figure 2a, compound 1 exhibits an intense peak at m/z
ꢀ 1007.56 and two moderate peaks at m/z values of 1040.88
and 1074.56, with the isotopic distribution patterns being
separated by a distance of (0.33 ꢁ 0.01) Dalton. These signals
can be assigned to the positively charged species [Co6-
(H3L1 )]3+, [ Co(H4L1 )(ClO4)]3+, and [Co6(H5L1 )(ClO4)2]3+.
Furthermore, the coordination of zinc(II), blocks photo-
induced electron transfer (PET) between the quinoline and
the amide groups in the ligand, which causes a fluorescence
4
6
4
4
In the case of a solution of compound 1 in the presence of
an equimolar amount of glucosamine (NH2-Glu; Figure 2b),
878
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Chemie
of a new peak at about 16.2 ppm could be an indicator of the
formation of hydrogen bonds with the amide groups, which
act as donors (see the Supporting Information).
To control the assembly of lanthanide ions and obtain
highly ordered architectures, the ligands must be highly
predisposed and the lanthanide ions must be modified using
auxiliary ligands to prevent further connections.[18] Using
pyridine—instead of quinoline—moieties gave the ligand
H3L2, which retained the N2O tridentate units for the
assembly of octahedral structures. This ligand can introduce
lanthanide ions with high coordination numbers into highly
symmetric architectures by decreasing the spatial restriction
around the metal center. Treating the ligand H3L2 with
nickel(II) tetrafluoroborate gave the nickel(II) compound 3,
Figure 3. Spectra of compound 2 in acetonitrile: a) UV/Vis spectra (at
a concentration of 5.0”10ꢃ6 m) and b) fluorescence spectra (at a
concentration of 5.0”10ꢃ6 m, excitation at 380 nm), upon addition of a
standard solution of glucosamine.
¯
which crystallizes in the highly symmetric space group Fd3,
with the octahedral cationic cage [Ni6(H9L2 )]9+ exhibiting an
4
ideally octahedral symmetry.[19]
Using Eu(NO3)3·6H2O in the synthetic procedure gave
compound 4. Single-crystal structural characterization dem-
onstrated the formation of a lanthanide-based octahedral
cage (Figure 5).[19] The [Eu6(H3L2)4(NO3)12]6+ cage crystallizes
enhancement and, at the same time, opens a photoinduced-
charge-transfer (PCT) channel.[7a,15] The metal-tunable emis-
sion band of compound 2 in acetonitrile (1 ” 10ꢃ5 m, excitation
at 380 nm) appears at about 530 nm (see the Supporting
Information), and the addition of glucosamine causes a
significant enhancement of the luminescence (Figure 3b). It
is clear that the amide group is an efficient binding–signaling
transductor, which can convert the recognition information
into both UV/Vis and luminescent responses. From a
mechanistic viewpoint, the formation of a donor-type hydro-
gen bond between the amide groups and the guest molecule
could favor electronic delocalization and lower the highest
occupied molecular orbital (HOMO) energy of the electronic
donors, which would lead to a further blocking of the PET
processes that take place between the amide and the
quinoline groups and to a significant enhancement of the
metal-tunable luminescence signal.[16]
The host–guest complexation of the d10 ZnII compound (2)
1
with glucosamine was also examined by means of H NMR
spectroscopic titration (see Figure 4). The observed downfield
shifting of the glucosamine signals suggest the inclusion of this
molecule into the positively charged cage,[17] and the presence
Figure 5. Structure of the octahedral cage [Eu6(H3L2)4(NO3)12]6+ (of
compound 4). Hydrogen atoms are omitted for clarity. The metal,
oxygen, nitrogen, and carbon atoms are green, red, blue, and gray,
respectively.
¯
in the highly symmetric space group R3 and exhibits crystallo-
graphic threefold symmetry, with one of the four ligands
positioned on a threefold axis. Two EuIII ions bridged by one
ligand are separated by a distance of about 10.5 Š, and each
metal center is coordinated by two N2O tridentate chelating
units and two bidentate coordinating nitrate anions. Replac-
ing Eu(NO3)3·6H2O with Tb(NO3)3·6H2O gave compound 5.
The octahedral cage [Tb6(H9L2 )(NO3)9]6+ in this compound
4
also exhibits a crystallographic threefold symmetry, but it
[19]
¯
crystallizes in the space group P3c1.
Each cage in the
crystal connects three neighbors through intermolecular
interactions, thereby featuring two-dimensional (6,3)
sheet. Adjacent sheets stack to form 1D nanochannels (see
the Supporting Information).
Figure 4. 1H NMR spectra in [D6]DMSO of a) compound 2, b) com-
pound 2 in the presence of an equimolar amount of glucosamine,
c) free glucosamine.
a
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879

Communications
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Solid-state emission spectroscopy data demonstrate the
presence of a strong intrinsic red luminescence signal for
7
compound 4, which can be attributed to the 5D0! FJ (J = 0–4)
transitions.[20] The analogue compound 5 exhibits green
luminescence (see the Supporting Materials). Clearly, this
synthetic strategy is an efficient way to obtain metal-tunable
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structure. Although the poor solubility of the lanthanide
compounds in most organic solvents prevents us from study-
ing the host–guest interactions present in this case, changing
the anion could improve their solubility and thus their sensing
properties.
In summary, we report a new strategy for preparing metal-
tunable octahedral nanocages with potential applications in
molecular sensing. The introduction of an amide group does
not only provide a guest-accessible functional site but also
offers an efficient communicator that is able to convert the
recognition information into an optical signal. Our strategy
can be extended to other metal-tunable Werner-type capsules
composed of lanthanide ions (which exhibit a low stereo-
chemical preference), and could therefore be used to
fabricate size- or shape-selective dynamic molecular chemo-
sensors with various responses and metal-triggered functions.
[10] Crystal
data:
1:
[
6(CCo H105N36O12)]·9ClO4·5CHCl3·
156
¯
25CH3OH·H2O,
hexagonal,
R3,
a = 26.360(1) Š,
c =
61.768(2) Š, V= 37171(1) Š3, T= 180(2) K, R1 = 0.0797, and
wR2 = 0.2268 for 6509 reflections with I ꢄ 2s(I). 2:
[Zn6(C156H105N36O12)]·9ClO4·2HCCl3·16CH3OH·H2O, hexago-
3
¯
nal, R3, a = 25.907(1) Š, c = 61.528(1) Š, V= 35764(1) Š , T=
Received: September 12, 2007
Published online: December 12, 2007
180(2) K, R1 = 0.0770, and wR2 = 0.2028 for 4708 reflections with
I ꢄ 2s(I). CCDC 656405 (1) and 656404 (2) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Keywords: cage compounds · lanthanides ·
molecular recognition · self-assembly · sensors
.
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Angewandte
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c = 44.685(2) Š, V= 32089(2) Š3, T= 180(2) K, R1 = 0.0763, and
wR2 = 0.2039 for 4547 reflections with I ꢄ 2s(I). 5:
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[Tb6(C108H81N36O12)(NO3)9(H2O)]·6NO3·6CHCl3·5CH3OH·
¯
2H2O, trigonal, P3c1, a = 28.858(4) Š, c = 38.894(8) Š, V=
28052(8) Š3, T= 180(2) K, R1 = 0.0779, and wR2 = 0.2438 for
4586 reflections with I ꢄ 2s(I). CCDC 656406 (3), 656407 (4),
and 656408 (5) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[19] Crystal
14.5CH3OH·H2O,
76029(1) Š3, T= 180(2) K, R1 = 0.0789, and wR2 = 0.2167 for
1471 reflections with I ꢄ 2s(I). 4: E(uC108H84N36O12)-
(NO3)12]·6NO3·8CHCl3·7H2O, hexagonal, R3, a = 28.796(1) Š,
data:
3:
cubic,
[
6(CNi H81N36O12)]·9BF4·0.5CHCl3·
108
¯
Fd3,
a = 42.364(1) Š,
V=
[
[20] G. H. Dieke, Spectra and Energy Levels of Rare Earth Ions in
Crystals, Wiley, New York, 1968, p. 242.
6
¯
Angew. Chem. Int. Ed. 2008, 47, 877 –881
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
881