Metal-organic polyhedra containing 36 and 24 folds of amide groups for selective luminescent recognition of natural disaccharides

Article abstract of DOI:10.1039/c2cc30497j

An octa-nuclear bicoronal Ce-based triangular prism and tetra-nuclear molecular tetrahedron containing 36 and 24 folds amides within their main backbones were achieved and structurally characterized for the selective luminescent recognition of lactose and sucrose, respectively, over other related natural mono- and disaccharides.

Full text of DOI:10.1039/c2cc30497j

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COMMUNICATION
Metal–organic polyhedra containing 36 and 24 folds of amide groups for
selective luminescent recognition of natural disaccharidesw
Yang Jiao, Jing Zhang, Lejie Zhang, Zhihua Lin, Cheng He* and Chunying Duan
Received 21st January 2012, Accepted 23rd April 2012
DOI: 10.1039/c2cc30497j
An octa-nuclear bicoronal Ce-based triangular prism and tetra-
nuclear molecular tetrahedron containing 36 and 24 folds amides
within their main backbones were achieved and structurally
characterized for the selective luminescent recognition of lactose
and sucrose, respectively, over other related natural mono- and
disaccharides.
bicoronal triangular prism and molecular tetrahedron by
incorporating two kinds of amide groups inside the cavities
of the cage. We envisioned that the functionalization with
amide groups would provide appropriate matches for polar
groups to address the structural diversity of oligosaccharides
and that the difficulty of predicting the favored conformations
of the saccharides could be overcome.
Ligand H4TRBS, N,N0-bis(3,5-bis(2-(2-hydroxybenzylidene)
hydrazinecarbonyl)phenyl) terephthalamde was synthesized
by reacting N,N0-bis(3,5-bis(2-(hydrazinecarbonyl) phenyl)
terephthalamide with salicylaldehyde in a methanol solution.
The ligand contains two ‘‘free’’ amide groups and four amide
moieties, which were the constituent parts of the tridentate
chelating units. Ce-TRBS was obtained as a M8L6 complex by
diffusing methanol into a DMF solution containing H4TRBS
and Ce(NO3)3ꢀ6H2O in the presence of NaOAc. The ESI-MS
spectrum of Ce-TRBS in DMF/CH3CN solution in the presence
of KOH exhibited two intense peaks at m/z = 2293.55 and
2339.92, with the isotopic distribution patterns separated by
0.33 Da. These peaks were assignable to the species of
[Ce6IIICe2IV(TRBS)6-5H]3ꢁ, through the exact comparison of
these peaks with the simulation on the basis of natural isotopic
abundances. It is thus suggested that the M8L6 species is
formed in solution and that it is stable.
Molecular recognition of carbohydrates has been recognized as
a subject of paramount importance in chemistry and biology,1,2
on which a number of biological processes rely, such as cell
adhesion, cell infection and immune response.3 While these
synthetic carbohydrate receptors operating through noncovalent
interactions have provided valuable model systems to study
the underlying principles of carbohydrate-based molecular
recognition process,4 few of them can span or encapsulate oligo-
saccharides, selecting for and among these larger substrates,5
despite the fact that most carbohydrate recognition in biology
involves oligosaccharides.6 The design of highly selective and
effective receptors for these ubiquitous and important bio-
molecules represents a significant challenge,7 due to the fact
that saccharides exhibit high hydrophilicity and non-charged
three-dimensional structures with subtle variations.
Metal–organic polyhedra represent a unique class of func-
tional molecular containers that display interesting molecular
recognition properties mimicking natural enzymes.8,9 Inspired
by the binding motifs within the biotic carbohydrate recognition
and the hydrogen-patterns within the artificial macrocyclic and
acyclic receptors operating through noncovalent interactions,10,11
we have developed metal–organic polyhedra for luminescent
discrimination of natural mono- and disaccharides in solution
through spatial restriction by incorporating amide groups as
both the multiple hydrogen bonding triggers.12 As in natural
complexes, the participation of different types of hydrogen
bonding groups in the recognition process was expected to be
favorable for achieving a high binding affinity and selectivity
towards the target carbohydrate.11 Herein, we modified our
synthetic approach to the new face-driven Ce-based octanuclear
X-ray crystal analysis (ESIw) revealed the formation of an
octa-nuclear bicoronal triangular prism, M8L6 (see Fig. 1).
Each cerium center in a M8L6 fragment is coordinated by three
tridentate chelating groups to complete a nine coordination
geometry. Each ligand connects four cerium centers in two
different types. Six cerium(III) centers are linked by three of the
six ligands, forming a tricycle prism (see Fig. 2). The long and
short edges of each Ce4 tetragon are about 17.9 and 9.8 A,
respectively, on an average. The two phenyl B rings in these
ligands just slightly bend with an angle of 181 and almost do
not twist. The left two Ce centers formed a helical pillar inside
the tricycle though coordinated with the other three ligands,
with the Ce–Ce separations in the helix being about 18.9 A.
Two tridentate chelating moieties in one ligand, but in a
different phenyl ring spanned around the helical axis, the other
two chelating sites coordinate with the two Ce centers on a
tricycle column. From the top view, the structure just looked
like the famous ‘‘Mercedes Benz’’ star. The 24 metal-coordinated
amide groups and 12 ‘‘free’’ amide groups amide groups were
located inside these spaces and the central phenyl ring could
State Key Laboratory of Fine Chemicals, Dalian University of
Technology, Dalian, 116012, China. E-mail: hecheng@dlut.edu.cn
w Electronic supplementary information (ESI) available: Crystal data
in CIF, experimental details and additional spectroscopic data. CCDC
864175 and 864176. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c2cc30497j
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6022 Chem. Commun., 2012, 48, 6022–6024
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Fig. 2 The molecular structure of the octa-nuclear bicoronal triangular
prism of Ce-TRBS showing the trisprand helical fashion of the inside
pillar and the tricycle prism, respectively. The metal, oxygen, nitrogen
and carbon atoms are represented by green, red, blue, and grey, respectively.
Selective bond distance in average (A): Ce–N 2.66, Ce–O(amide) 2.46,
Ce–O(hydroxyl) 2.18, C–N(coordinated amide) 1.31, C–O(coordinated
amide) 1.24, C–N(free amide) 1.36, C–O(free amide), 1.22.
Fig. 1 The molecular structure of the octa-nuclear bicoronal triangular
prism of Ce-TRBS. Hydrogen atoms, and solvent molecules are omitted
for clarity. The metal, oxygen and nitrogen atoms are represented by green,
red, blue. The carbon atoms represented by grey, cyan and organe, respec-
tively. Selective bond distance in average (A): Ce–N 2.66, Ce–O(amide)
2.46, Ce–O(hydroxyl) 2.18, C–N(coordinated amide) 1.31, C–O(coordinated
amide) 1.24, C–N(free amide) 1.36, C–O(free amide), 1.22.
rotate freely to fit with the structural diversity of oligo-
saccharides, Ce-TRBS thus was a suitable candidate in carbo-
hydrate recognition.13
Ce-TRBS exhibited two absorption bands centered at 290
and 375 nm. The addition of several monosaccharides or
disaccharides did not cause any obvious spectral changes on
the absorption spectrum (see Fig. 3). Emission spectrum of
Ce-TRBS (20 mM) exhibited an intense band at about 424 nm,
when the solution was excited at 315 nm. Upon the addition of
lactose (0.6 mM), the luminescence of Ce-TRBS enhanced
gradually with the increasing concentration of the guest. The
Hill-plot profile of the fluorescence titration curves at 424 nm
demonstrated the 1 : 1 host–guest complexation behavior with
the association constants (log Kass) being 3.06 ꢂ 0.24.14 The
addition of other disaccharides and several monosaccharides
did not cause any obvious spectroscopic change. The selectivity
of the fluorescence spectrum should be contributed to by the
spatial selectivity for lactose and the weak interaction of
hydrogen bonds between the Ce-TRBS and lactose. Ce-TRBS
was described as the first artificial receptor that can specifically
discriminate lactose over other disaccharides in a luminescent
enhancement manner. The ESI-MS spectrum of Ce-TRBS upon
the addition of lactose exhibited a new peak at m/z = 2408.58.
The exact comparison of the peak with the simulation on the
basis of natural isotopic abundances suggested the presence of
[(Ce6IIICe2IV(TRBS)6 * (lactose)-5H]3ꢁ. This result supported
the 1 : 1 complexation of host–guest species in the solution.
Fig. 3 Left Picture: Fluorescence responses of Ce-TRBS (red bars)
and Ce-TBAS (blue bars) for saccharides mentioned. Emission intensity
was recorded at 424 nm for Ce-TRBS or 435 nm for Ce-TBAS (20 mM
in DMF/acetonitrile 1 : 9 (v/v), excited at 315 and 340 nm, respectively).
Right: Fluorescence responses of Ce-TRBS upon addition of Lactose
(excitation at 315 nm).
The excellent selectivity of Ce-TRBS toward special oligo-
sacchrides encourage us to further investigate the interaction detail
of the host–guest complexation. A ligand H3TBAS N,N0,N0 0-
tris(4-(2-hydroxybenzylidene)hydrazinecarbonyl)phenyl) benzene
1,3,5-tricarboxamide, that has molecular C3 symmetry was
synthesized. Diffusing of methanol into a DMF solution
containing H3TBAS and Ce(NO3)3ꢀ6H2O in the presence of
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Chem. Commun., 2012, 48, 6022–6024 6023

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interaction environment within the cages, the Ce-based
coordination polyhedra worked as efficient selective chemo-
sensors for special natural saccharides. Accordingly, changing
the topology and the arrangement of hydrogen binding sites in
the confined space of host, the selectivity toward other target
guests could be expected.
This work was supported by the National Natural Science
foundation of China (21171029 and 20923006).
Notes and references
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Fig. 4 Molecular structure of the Ce-TBAS tetrahedral cage. Hydrogen
atoms and solvent molecules are omitted for clarity. The metal, oxygen,
nitrogen and carbon atoms are represented by green, red, blue, and
grey, respectively. Selective bond distance in average (A): Ce–N 2.66,
Ce–O(amide) 2.41, Ce–O(hydroxyl) 2.20, C–N(coordinated amide)
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The ESI-MS spectrum of Ce-TBAS exhibited an intense peak
at m/z = 1451.50, which was assigned to [(Ce4IV(TBRS)4-
(CH3CN)3-7H]3ꢁ species, revealing the formation of M4L4
specie in the solution. For the experimental spectra of
Ce-TBAS in the presence of sucrose, the exact comparison
of the peak at m/z = 1565.84 with the simulation on the basis
of natural isotopic abundance revealed the presence of com-
plexation species [(Ce-TBRS)-3H * (sucrose)ꢀ(CH3CN)3]3ꢁ
.
When Ce-TBAS (20 mM) was excited at 340 nm, it exhibited
a broad emission band at 435 nm in a DMF/acetonitrile
solution. The addition of sucrose (5 mM) caused a significant
luminescence enhancement (1.3 fold). The addition of other
mono- and disaccharides did not cause any obvious spectro-
scopic changes, indicating the selectivity of Ce-TBAS towards
sucrose over other disaccharides.
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In summary, with the tunability of the topology and size of
these polyhedrons, as well as the position of the weak
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6024 Chem. Commun., 2012, 48, 6022–6024
This journal is The Royal Society of Chemistry 2012