Face-driven octanuclear cerium(iv) luminescence polyhedra: Synthesis and luminescent sensing natural saccharides

Article abstract of DOI:10.1039/c1cc12766g

By incorporating NOO tridentate chelators into rationally designed ligand systems, a well-defined molecular cube Ce-TBDS and a bicoronal trigonal prism Ce-TBBS were achieved via self-assembly and the luminescence recognition of natural saccharides with Ce

Full text of DOI:10.1039/c1cc12766g

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Face-driven octanuclear cerium(IV) luminescence polyhedra: synthesis and
luminescent sensing natural saccharidesw
Liang Zhao, Siyi Qu, Cheng He,* Rong Zhang and Chunying Duan
Received 11th May 2011, Accepted 1st July 2011
DOI: 10.1039/c1cc12766g
By incorporating NOO tridentate chelators into rationally
designed ligand systems, a well-defined molecular cube Ce-TBDS
and a bicoronal trigonal prism Ce-TBBS were achieved via self-
assembly and the luminescence recognition of natural saccharides
with Ce-TBBS were also investigated.
to a transition metal center at an angle of exactly 901.12 By
using a C3-symmetric facial ligand that comprises three such
tridentate coordinating sites, various metal ions were used to
form metal-variable isostructural octahedral nanocages with
considerable stabilities.13 This strategy allows us to finely tune
the metal ions and ligand chromophore units to obtain highly
selective and sensitive responses toward particular guest
molecules. Herein, we further extended our synthetic strategy
to create a molecular cube by incorporating NOO tridentate
chelators into the rationally designed ligands with psuedo-D2
symmetry. Since the high coordination number of lanthanide
ions allows the coordination with three of these tridentate
chelators in a C3 symmetrical coordination configuration, the
promising efficiency and stability of the approach could lead to
other more complex systems.14 Through the participation of
possible directional 5d orbitals in the coordination modes, a
cerium ion is used to assemble highly ordered architectures.15
Ligand H8TBDS was obtained by a simple Schiff-base
reaction of salicylaldehyde with 3,30,5,50-tetracarboxyhydrazide-
1,2-diphenoxyethane in a methanol solution. Evaporating
a DMF solution of Ce(NO3)3ꢀ6H2O and ligand H8TBDS
in air for several weeks lead to the formation of compound
Ce-TBDS (Fig. 1). The ESI-MS spectrum of Ce-TBDS in
DMF solution exhibited two intense peaks at m/z = 1566.23
and 1575.96, with the isotopic distribution patterns separated
by 0.25 Daltons. Through a detailed comparison of the experi-
mental peaks with the simulation results obtained on the basis
of natural isotopic abundances, these peaks were respectively
assigned to the species [Ce8(H2TBDS)2(H3TBDS)4]4ꢁ and
[Ce8(H2TBDS)3(H3TBDS)3K]4ꢁ, demonstrating the formation
of a M8L6 species in the solution. EA and powder X-ray analysis
both proved the pure phase of the bulky sample. The Ce 3d core
level XPS spectrum exhibits a distinct band at about 916 eV,
besides the peaks around the 880–890 and 895–910 distinct
regions. The unique peak is assigned to 4f0 orbital transitions,16
suggesting the presence of the CeIV state in compound Ce-TBDS.
Single crystal X-ray structure analysis reveals that compound
Ce-TBDS is comprised of eight cerium ions and six planar
ligands in the regular cube. Eight CeIV centers site at the eight
corners of the cube and each coordinates to three tridentate
chelating groups in the pseudo-C3 symmetry to complete the
coordination geometry (Fig. 2). The Ce–O (phenol) and Ce–O
(amide) distances of 2.24 A and 2.43 A and the Ce–N distances
of 2.67 A are consistent with the related CeIV compounds.17
Metal–organic polyhedra (MOPs), which are discrete molecular
architectures constructed through the coordination of metal
ions and organic linkers, have attracted considerable attention
due to their high symmetry and stability, as well as their rich
chemical and physical properties potential for a variety
of applications.1,2 The structures are synthesized through
modular and high yield coordination chemistry-based self-
assembly methods. The steric, geometric and electronic
characteristics embedded within the individual components
thus collectively allow the controllable construction, to some
extent, of designed supramolecular entities.3,4 Programming of
the architectures of the inner cluster walls to give well-defined
cavities and gated pores has also resulted in the specific
interaction of the MOPs with their environment and the
selective uptake and binding of guests, as well as the catalysis
of their reactions.5 So far, investigations have mainly involved
the construction of transition metal-based cages with predictable
coordination geometry. In contrast, the use of f or mixed d–f
metal ions in such metal-directed synthesis has been much less
explored,6,7 despite the incorporation of lanthanide centers in
molecular edifices, which has various applications, such as in
light-conversion devices, luminescent probes and switches,
contrast agents for medical imaging, etc.8–10 The low stereo-
chemical preferences of these lanthanide metals, in most cases,
require more precise control of the chelator predisposition and
spatial restriction of the ligands, in order to obtain highly
ordered architectures. However, the high coordination number
of the lanthanide, in some case, might be an important
criterion in the assembly of special Plato polyhedrons.11
Tridentate coordinated units are efficient building blocks
that can be used to fix the orientation of ligands coordinated
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 files, experimental details and additional spectroscopic data.
CCDC 825073–825074. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1cc12766g
c
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Fig. 3 The assembly process of the two octanuclear polyhedrons
showing the difference between the two bridging modes corresponding
to the four metal centers connected in one ligand, the factors influencing
the assembly configuration of the polyhedrons and positions of these
metal ions.
only one third of the amide groups were deprotonated during
coordination.
Fig. 1 A schematic of the ligands, the constitutive/constructional
fragments and structures, showing the face-driven manner. The metal,
oxygen, nitrogen and carbon atoms are green, red, blue and other
colors, respectively.
To further criticize the stability of the face-driven assembly
strategy and to look for larger congener for molecular recognition,
a full substituted benzyl group was inserted within the C–C
bond of the spacer in H8TBDS to get the ligand H8TBBS.
Compound Ce-TBBS was synthesized by the above mentioned
procedure. The ESI-MS spectrum of Ce-TBBS in DMF solution
exhibited an intense peak at m/z = 1764.32, with the isotopic
distribution patterns separated by 0.25 Daltons. This peak was
assignable to [Ce8(H2TBBS)3(H3TBBS)3]4ꢁ in a comparison
of the experimental peaks with the simulation results obtained
on the basis of natural isotopic abundances, revealing the
successful assembly of the octanuclear Ce-based polyhedron.
Preliminary crystal structure analysis of Ce-TBBS confirmed
the formation of the octanuclear cerium polyhedron with six
four-connected TBBS ligands. Each cerium center is also nine-
coordinated by three tridentate chelating groups and each
ligand also connects to four metal centers. Since the spacer
in ligand TBBS is much larger than that in ligand TBDS, the
Ceꢀ ꢀ ꢀCe separations in the two bridging models are quite
different (9.45 vs. 19.55 A) and the flexibility of the ligand
backbone makes the four ceriums connected not in a plane,
but positioned in a twist configuration. Accordingly, the octa-
nuclear polyhedron could be described as a bicoronal trigonal
prism (Fig. 3). Intramolecular pꢀ ꢀ ꢀp stacking interactions
between the pair of the benzyl spacer rings positioned on each
tetragonal face was found to stabilize the special configuration of
the polyhedron. The volume of the polyhedron is about 750 A3
and the narrow diameter of the openings was thus enlarged to
8.0 A, possibly allowing natural mono- or disaccharides in and
out. The amide groups located within the cages make it a
suitable candidate for a luminescence carbohydrate chemosensor,
inspired by natural protein–carbohydrate interactions.20
The valence of the two cerium centers were calculated as 3.9
and 4.0 for Ce(1) and Ce(2), respectively.18 Six ligands sit on
the six faces of the cube and each connects four metal centers.
As shown in Fig. 3, there were two bridging models for the
Ceꢀ ꢀ ꢀCe edge on each face; one was the two Ce centers joined
by two carboxyhydride salicylazone tridentate chelators on the
meso site within one benzene ring (model I), the other was the
two Ce centers joined by two tridentate chelators on the
different benzene rings in the ligand, but in a cis configuration
(model II). The Ceꢀ ꢀ ꢀCe separations in the Ce-TBDS are quite
similar, ranging 11.56–11.74 A, so that the two meso-substituted
benzene rings acting as the linker in the ligand remain almost
coplanar (the dihedral angle of 9.01 is the average) and the
preferred shape of the cage is a regular cube. The inner volume
is ca. 500 A3 and the narrow edge of the opening is less than
5.0 A – hardly allowing small molecules in and out. The C–O
and C–N distances of 1.26 A and 1.34 A, respectively, are
intermediate between formal single and double bonds, indicating
extensive delocalization over the entire molecular skeleton.19
The absence of any anions in the crystal structure suggests that
the cube Ce-TBDS is neutral and these phenol protons and
Compound Ce-TBBS exhibited two absorption bands at
290 and 375 nm. When excited at 340 nm (20 mM in DMF
solution), it exhibited one broad intensive band centred at
about 450 nm (Fig. 4). The addition of mono- and disaccharides
caused significant fluorescence enhancement. The Hill plot
profile of the fluorescence titration curve at 465 nm demon-
strated a 1 : 2 stoichiometric host–guest behavior (Kass ranging
Fig. 2 The crystal structure of the Ce-TBDS (left) and Ce-TBBS
(right) cages with the atomic numbering scheme showing the pseudo-
Oh symmetry and bicoronal trigonal prism geometry of the octa-
nuclear cerium Ce8L6, respectively. H-atoms and solvent molecules
were omitted for clarity. The metal, oxygen, nitrogen and carbon
atoms are green, red, blue and other colours, respectively.
1.15–5.37 ꢂ 105 ꢁ2) for these monosaccharides, and
M
1 : 1 stoichiometric host–guest behavior for the disaccharides
c
9388 Chem. Commun., 2011, 47, 9387–9389
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This work is supported by NCSF (20801008).
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 9387–9389 9389