Self-assembly of cerium-based metal–organic tetrahedrons for size-selectively luminescent sensing natural saccharidesw Yang Liu, Xiao Wu, Cheng He,* Yang Jiao and Chunying Duan Received (in Cambridge, UK) 29th July 2009, Accepted 20th October 2009 First published as an Advance Article on the web 9th November 2009 DOI: 10.1039/b915358f New Ce-based Werner type tetrahedrons were achieved for size- selectively luminescent detection of natural carbohydrates through incorporating amide groups as both the multiple hydrogen bonding triggers and binding-signalling transductor. The design of artificial carbohydrate sensors operating through non-covalent interactions is a subject of intensive current research, due to their broad utility in wide-ranging applications from the food and cosmetic industries to medicinal and academic arenas. 1 Because of the subtle variation in the sugar structures and the three-dimensional arrangement of their functionalities, the frameworks of carbohydrate receptors must be large enough to be able to fully encapsulate an oligosaccharide nucleus. And the receptors should have various patterns of preorganized, inward-directed H-bond donor and/or acceptor functionality. 2 In this regard, self- assembled metal–organic molecular polyhedrons, appealing as synthetic hosts, are efficient receptors for mimicking biological carbohydrate recognition processes, 3 especially when amide groups, as multiple hydrogen bonding triggers used in nature protein–carbohydrate complexes 4 were incorporated. On the other hand, difficulties in developing saccharide sensors also arise from the fact that saccharides just contain one kind of recognition unit (the hydroxyl functional group) and lack a spectroscopic handle, such as a chromophore or fluorophore, whose modulation could be harnessed in a sensing scheme. Since fluorescent molecular sensing, which translates molecular recognition into tangible fluorescence signals, 5 pro- vides an efficient tool for quantitatively detecting carbo- hydrates with high precision in both solution and complex media. 6 The incorporation of luminescent active lanthanide ions within the metal–organic polyhedrons represents a promising approach in constructing Werner type cage-like molecular capsules for luminescent detection of saccharides in solution and/or in biological media. 7 However, lanthanide ions usually exhibit low stereochemical preferences and high coordination numbers, the rational concepts of related Werner type molecular polyhedrons are quite rare 8 and require the use of highly predisposed and spatially restricted ligands. 9 In order to control the coordination of lanthanide ions and obtain highly ordered architectures, highly predisposed NO 2 tridentate chelators with amide groups were introduced into linear shape molecules H 2 L 1 and H 2 L 2 . Here, two new luminescence-active lanthanide tetrahedrons (TE1 and TE2) were synthesized for the size-selective sensing of saccharides (Scheme 1). The specific electronic structure of Ce 3+ possibly allows the better control of the assembly of highly ordered architecture, through influencing the directional 5d orbitals in the coordination modes. 10 Taking into account the environmentally sensitive character of these parity-allowed electric-dipole 4f–5d transitions to the electronic conformation of the ligands, 11 the formation of hydrogen bonds with the amide groups has the potential to affect the electron transitions associated with the Ce 3+ ions, leading to significant changes in the optical properties. Ligands H 2 L 1 and H 2 L 2 were obtained by reacting salicyl- aldehyde with 2,6-dicarbohydrazide naphthalene and 1,1 0 - dicarbohydrazide 4,4 0 -biphenyl, respectively. Evaporating a CH 3 OH–DMF solution of these ligands with Ce(NO 3 ) 3 6H 2 O in air for several days led to the formation of crystalline solids of compounds TE1 and TE2, in a high yield (65% and 60%), respectively. EA and powder X-ray analysis proved the pure phase of the bulky sample. ESI-MS spectrum of TE1 exhibited two intense peaks at m/z = 1084.84 and 1097.45 with the isotopic distribution patterns separated by 0.33 0.01, demonstrating the presence of negatively charged species [Ce 4 L 1 6 –3H] 3and [Ce 4 L 1 6 –3H * (H 2 O) 2 ] 3, respectively. Similarly, the two peaks at m/z = 852.20 and 1137.19 in the ESI-MS spectrum of TE2, could be assignaed to negatively charged species [Ce 4 L 2 6 -4H] 4and [Ce 4 L 2 6 –3H] 3, respectively. These results indicated the successful assembly of Ce-based molecular tetrahedrons. Single crystal X-ray structural analysisz confirmed the formation of the tetrahedron in TE1, [Ce 4 (C 26 H 18 N 4 O 4 ) 6 ]4C 3 H 7 NO6H 2 O2CH 3 OH. The Ce 4 L 1 6 tetrahedron comprised Scheme 1 The constitute/constructional fragments of the functional Ce-based tetrahedron Ce 4 L 1 6 and Ce 4 L 2 6 showing the cavities, the windows (drawn in orange) and the positions of functionality groups. State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, 116012, China. E-mail: [email protected]w Electronic supplementary information (ESI) available: Crystal data in CIF, experimental details, magnetic and additional spectroscopic data. CCDC 705880. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b915358f 7554 | Chem. Commun., 2009, 7554–7556 This journal is c The Royal Society of Chemistry 2009 COMMUNICATION www.rsc.org/chemcomm | ChemComm Published on 09 November 2009. Downloaded by Dalian University of Technology on 16/10/2014 13:14:59. View Article Online / Journal Homepage / Table of Contents for this issue
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Self-assembly of cerium-based metal–organic tetrahedrons
for size-selectively luminescent sensing natural saccharidesw
Yang Liu, Xiao Wu, Cheng He,* Yang Jiao and Chunying Duan
Received (in Cambridge, UK) 29th July 2009, Accepted 20th October 2009
First published as an Advance Article on the web 9th November 2009
DOI: 10.1039/b915358f
New Ce-based Werner type tetrahedrons were achieved for size-
selectively luminescent detection of natural carbohydrates
through incorporating amide groups as both the multiple
hydrogen bonding triggers and binding-signalling transductor.
The design of artificial carbohydrate sensors operating
through non-covalent interactions is a subject of intensive
current research, due to their broad utility in wide-ranging
applications from the food and cosmetic industries to medicinal
and academic arenas.1 Because of the subtle variation in the
sugar structures and the three-dimensional arrangement of
their functionalities, the frameworks of carbohydrate receptors
must be large enough to be able to fully encapsulate an
oligosaccharide nucleus. And the receptors should have
various patterns of preorganized, inward-directed H-bond
donor and/or acceptor functionality.2 In this regard, self-
tridentate chelators with amide groups were introduced into
linear shape molecules H2L1 and H2L
2. Here, two new
luminescence-active lanthanide tetrahedrons (TE1 and TE2)
were synthesized for the size-selective sensing of saccharides
(Scheme 1). The specific electronic structure of Ce3+ possibly
allows the better control of the assembly of highly ordered
architecture, through influencing the directional 5d orbitals
in the coordination modes.10 Taking into account the
environmentally sensitive character of these parity-allowed
electric-dipole 4f–5d transitions to the electronic conformation
of the ligands,11 the formation of hydrogen bonds with the
amide groups has the potential to affect the electron transitions
associated with the Ce3+ ions, leading to significant changes in
the optical properties.
Ligands H2L1 and H2L
2 were obtained by reacting salicyl-
aldehyde with 2,6-dicarbohydrazide naphthalene and 1,10-
dicarbohydrazide 4,40-biphenyl, respectively. Evaporating a
CH3OH–DMF solution of these ligands with Ce(NO3)3�6H2O in air for several days led to the formation of crystalline
solids of compounds TE1 and TE2, in a high yield (65% and
60%), respectively. EA and powder X-ray analysis proved the
pure phase of the bulky sample. ESI-MS spectrum of TE1
exhibited two intense peaks at m/z = 1084.84 and 1097.45
with the isotopic distribution patterns separated by 0.33 � 0.01,
demonstrating the presence of negatively charged species
[Ce4L16–3H]3� and [Ce4L
16–3H * (H2O)2]
3�, respectively.
Similarly, the two peaks at m/z = 852.20 and 1137.19 in the
ESI-MS spectrum of TE2, could be assignaed to negatively
charged species [Ce4L26-4H]4� and [Ce4L
26–3H]3�,
respectively. These results indicated the successful assembly
of Ce-based molecular tetrahedrons.
Single crystal X-ray structural analysisz confirmed the
formation of the tetrahedron in TE1, [Ce4(C26H18N4 O4)6]�4C3H7NO�6H2O�2CH3OH. The Ce4L
16 tetrahedron comprised
Scheme 1 The constitute/constructional fragments of the functional
Ce-based tetrahedron Ce4L16 and Ce4L
26 showing the cavities, the
windows (drawn in orange) and the positions of functionality groups.
State Key Laboratory of Fine Chemicals, Dalian University ofTechnology, Dalian, 116012, China. E-mail: [email protected] Electronic supplementary information (ESI) available: Crystal datain CIF, experimental details, magnetic and additional spectroscopicdata. CCDC 705880. For ESI and crystallographic data in CIF orother electronic format see DOI: 10.1039/b915358f
7554 | Chem. Commun., 2009, 7554–7556 This journal is �c The Royal Society of Chemistry 2009
COMMUNICATION www.rsc.org/chemcomm | ChemComm
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demonstrated the occurrence of 1 : 1 stoichiometric complexation
behavior with the association constants (log Kass) being
calculated as 4.05 for sucrose, maltose and trehalose on
average. Although it could not be proven beyond a shadow
of a doubt that the recognition of the saccharides occurred in
the cavity of the cage, the size-dependent affinities of Ce4L26
and Ce4L26 to different saccharides, as well as the stability of
corresponding host–guest species in solution all supported this
hypothesis.
ESI-MS spectra of TE1 in the presence of hexoses exhibited
two intense peaks at m/z = 1145.34 and 1085.63, respectively
(Fig. 4). The comparison of the peak at m/z = 1145.34 with
the simulation on the basis of natural isotopic abundances
revealed the presence of 1 : 1 stoichiometric host–guest species
[Ce4L16–3H * (C6H12O6)]
3�. The addition of smaller
pentoses, xylose or ribose, or larger disaccharides did not
arouse any obvious peaks corresponding to the host–guest
species. In the spectra of TE2 with disaccharides including
sucrose, maltose and trehalose, the presence of peak at 1879.67
assignable to [Ce4L26–2H * (C12H22O11)]
2� demonstrated the
1 : 1 stoichiometric complexation behavior. The addition of all
the above mentioned mono-disaccharides did not cause any
obvious peaks corresponding to the host–guest species.
This work was supported by the National Natural Science
foundation of China (20801008 and 20871025) and the
Start-up Fund of The Dalian University of Technology.
Notes and references
z Crystal data of TE1: C170H156Ce4N28O36,M= 3727.71, monoclinic,space group P21/n, black block, a = 24.009 (1), b = 37.450 (1),c = 24.065(1) A, b = 92.650(2)1, V = 21614(1) A3, Z = 4, Dc =1.146 g cm�3, m(Mo-Ka) = 0.891 mm�1, T= 180(2) K. 31 778 uniquereflections [Rint = 0.1333]. Final R1 [with I > 2s(I)] = 0.0849,wR2 (all data) = 0.1991 for 2y = 471. CCDC number 705880.
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Fig. 3 Fluorescence responses of TE1 (red bars) and TE2 (blue bars)
for saccharides mentioned. Emission intensity was recorded at 525 nm
for TE1 (excited at 360 nm) or at 480 nm for TE2 (20 mM in