M4L4 cerium cages assembled from ligands with lower symmetry as a molecular flask

Article abstract of DOI:10.1016/j.inoche.2013.09.071

Cerium-based tetranuclear polyhedra Ce-YB₁ and Ce-YB₂ assembled from lower symmetrical ligands without three fold axle, can work as molecular flasks to catalyze cyanosilylation reactions of aldehyde molecules with suitable sizes. The

Full text of DOI:10.1016/j.inoche.2013.09.071

Inorganic Chemistry Communications 39 (2014) 147–150
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
M4L4 cerium cages assembled from ligands with lower symmetry as a
molecular flask
⁎
Yang Jiao, Cheng He, Chun-Ying Duan
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Cerium-based tetranuclear polyhedra Ce-YB1 and Ce-YB2 assembled from lower symmetrical ligands without
three fold axle, can work as molecular flasks to catalyze cyanosilylation reactions of aldehyde molecules with
suitable sizes. The size-selectively catalytic behavior of the cages was investigated by using large aldehydes as
the substrate, based on which the corresponding cyanosilylation reactions could not be carried out on the
same condition.
Received 12 August 2013
Accepted 9 September 2013
Available online 8 October 2013
Keywords:
Cages
© 2013 Published by Elsevier B.V.
Molecular flask
Cyanosilylation
Supramolecular assembly of predesigned organic and inorganic
building blocks has been recognized as an excellent tool in constructing
well-defined molecular hollows [1–3]. Similar to the pockets of enzymes
[4–6], cavities of these synthetic hosts can encapsulate and stabilize
guest substrates and fix them into orientations, favoring specific reaction
paths that exhibit excellent size discrimination properties [7–9]. These
hollow molecular structures could encapsulate guest molecules as mo-
lecular flask which could provide an environment for some reactions
[10–13]. They can enable one to realize the unique properties to promote
entropically unfavorable reactions and catalyze chemical transforma-
tions within the cage-like structures [14–16]. Certainly, the first step to
construct a molecular flask is to obtain cage-like structures with enough
inner space. Recently, substantial developments have been made in the
creation of Werner-type capsules having larger inner volume that can
be used for recognizing special guests [17,18] and worked as a catalyst
to catalyze the relevant reactions of the special guests [19–21].
We have well established the strategy for assembling a series of
well-defined Ce-based molecular polyhedra through incorporating
NOO tridentate chelators into the rationally designed ligands with
predesigned higher symmetries [22–24]. For example, by combining
three phenol–imine groups as metal-bonding sites within one C3 sym-
metrical central benzene ring, a Ce-based M4L4 tetrahedron with ideal
T symmetry was obtained [25]. However, the tetrahedron just has a
very small cavity with its inner volume being estimated as 220 Å3,
which is difficult to encapsulate any substrates. To further apply such
constructing strategy on molecular flask, we reported here the prepara-
tion of new cerium molecular polyhedra by incorporating three such
NOO chelating groups into related bigger ligands without C3 symmetry.
The resulting M4L4 polyhedra have larger cavity for recognizing
aldehyde molecules and prompting the corresponding cyanosilylation
reaction in a homogeneous phase (Scheme 1).
Ligand H6YB1 was easily obtained by a Schiff base reaction of
salicylaldehyde with 5-(4-(hydrazinecarbonyl)phenoxy)-isophthalo-
hydrazide dimethyl in methanol and identified by the relatively broad-
ened and shifted resonance signals in 1H NMR spectra. Compared to
ligand NATB [25], the bigger ligand H6YB1 has the same three tridentate
chelating units but lose the three-fold axial symmetry. H6YB1 and
Ce(NO3)3·6H2O were dissolved in DMF to give a black solution. The
black crystal was isolated with 50% yield. Single crystal X-ray structural
analysis reveals that compound Ce-YB1 is comprised of four cerium ions
and four ligands, keeping the M4L4 structural feature as the reported Ce-
based tetrahedron Ce-NATB constructed from ligand NATB. But the mo-
lecular structure of compound Ce-YB1 is a warping tetrahedron because
of the flexibility of ligand H6YB1. Each cerium ion is chelated by three
tridentate chelating groups from three different ligands to form a ter-
nate coronary trigonal prism coordination geometry having a pseudo-
C3 symmetry. As shown in Fig. 1, four Ce centers site at the four corners
of the warping tetrahedron and the Ce–O (phenol) and Ce–O (amide)
distances of 2.209 Å and 2.444 Å and the Ce–N distance of 2.626 Å are
consistent with the related Ce compounds. Every flexible ligand site
on four corners is a face of warping tetrahedron. The three rigidly
tridentate chelating groups in one deprotonated flexible ligand coordi-
nate to three different metal centers with the Ce⋯Ce separation bridged
ca. 13.276 Å and 9.731 Å. The inner volume of the cube is about 345 Å3
and the opening of the cube is a rectangle having the size of
5.6 × 7.2 Å2, potentially allowing small molecules' ingress and egress
[26]. The absence of any anions in the crystal structure suggests that
the cube Ce-YB1 is neutral and these phenol protons and only one
third of the amide groups were deprotonated during coordination. The
middle benzene groups of each ligand are parallel to each other and
are likely work as π–π accumulation sites.
⁎
Corresponding author. Tel.: +86 411 84986316; fax: +86 411 84986314.
E-mail address: cyduan@dlut.edu.cn (C.-Y. Duan).
1387-7003/$ – see front matter © 2013 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.inoche.2013.09.071

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Y. Jiao et al. / Inorganic Chemistry Communications 39 (2014) 147–150
Scheme 1. Schematic of the constitutive/constructional fragments and the structures of the Ce4L4 cubes Ce-YB1 and Ce-YB2.
To expand ligand H6YB1 to H6YB2 which has one more methylene
to the formation of compound Ce-YB2, yield of 60%. The coordination
of H6YB2 to the cerium metal ions is also identified by electrospray ion-
ization mass spectrometry. Compounds Ce-YB1 and Ce-YB2 are soluble
in acetonitrile and several organic solvents. Electrospray ionization
mass spectrometry (ESI-MS) demonstrates substantial stability of
Ce-YB1 and Ce-YB2 cages in solution. As shown in Fig. 2, Ce-YB1 ex-
hibits intense peaks at m/z = 1055.60 and m/z = 1584.53, assign-
able to the negative charged species [Ce4(YB1)4-16H]3− and
[Ce4(YB1)4-16H]2−, based on the simulation of natural isotopic abun-
dances. And Ce-YB2 exhibits an intense peak at m/z = 1074.56 and
m/z = 1612.52, assignable to the negative charged species
[Ce4(YB2)4-16H]3− and [Ce4(YB2)4-16H]2−, based on the simulation
of natural isotopic abundances. In the case of a solution of Ce-YB2 in
the presence of 4-nitrobenzaldehyde, an exact comparison of the most
interesting experimental peak which is observed at m/z = 1137.31
and m/z = 1706.74 with the simulation results on the basis of natural
isotopic abundances reveals that the −3 and −2 charged species can
be reasonably assigned to [Ce4(YB2)4-16H ⊃ (2)·Na]3− and
[Ce4(YB2)4-16H ⊃ (2)·Na]2−, thus providing evidence of a 1:1 stoi-
chiometric host–guest behavior (Fig. 3).
group, ligand H6YB2 was synthesized by the reaction of 5-
(4(methoxycarbonyl)-benzyloxy)isophthalohydrazide with salicyl-
aldehyde in methanol solution and characterized by 1H NMR spectro-
scopic methods. The size of ligand H6YB2 is larger than ligand H6YB1.
Evaporating a DMF solution of H6YB2 with Ce(NO3)3·6H2O in air led
As the polyhedron could encapsulate the aldehyde molecules, we
focused on cyanosilylation reactions of aldehyde molecules which are
important reactions that could provide a convenient route to cyanohy-
drins and the key derivatives in the synthesis of fine chemicals and
pharmaceuticals. As shown in Fig. 4, the loading of 4 mol‰ Ce-YB1 on
the cyanosilylation reaction of 4-nitrobenzaldehyde led to a more than
99% conversion in 2 hours. When change the 4-nitrobenzaldehyde
to other aldehyde molecules, the conversion of 2-nitrobenzaldehyde,
3-nitrobenzaldehyde and 4-hydroxybenzaldehyde is about 76%, 61%
and 83% at the same condition. But the cyanosilylation reaction of 2-
naphthalaldehyde and 9-anthraldehyde hardly occur in the presence
of Ce-YB1, it is postulated that when the size of large aldehyde mole-
cules is big, it could not be encapsulated in the cage Ce-YB1. We also
carried out the cyanosilylation reactions of aldehyde molecules with
Ce-YB2 worked as molecules flasks. As shown in Fig. 4, while the
cyanosilylation reaction of 4-nitrobenzaldehyde, 2-nitrobenzaldehyde
3-nitrobenzaldehyde and 4-hydroxybenzaldehyde hardly take place in
the absence of Ce-YB2, but the loading of 4 mol‰ Ce-YB2 led to a
more than 99%, 78% ,67% and 98% conversion in 2 hours, respectively.
When change the 4-nitrobenzaldehyde to larger aldehyde molecules
2-naphthalaldehyde and 9-anthraldehyde, the cyanosilylation reaction
hardly occur in the presence of Ce-YB2. Because the reaction of large al-
dehyde molecules hardly occur in the presence of Ce-YB1 and Ce-YB2, it
Fig. 1. Crystal structure of Ce-YB1 and its constitutive/constructional fragments. Hydrogen
atoms were omitted for clarity, the Ce, O and N atoms were drawn in green, red and blue.
Ce(1)–O(1) 2.194(6), Ce(1)–O(4A) 2.203(5), Ce(1)–O(5) 2.211(5), Ce(1)–O(3A)
2.344(5), Ce(1)–O(6) 2.477(5), Ce(1)–O(2) 2.519(6), Ce(1)–N(5A) 2.604(6), Ce(1)–
N(1) 2.626(7), Ce(1)–N(3) 2.645(7), Ce(2)–O(13) 2.203(6), Ce(2)–O(10) 2.217(6),
Ce(2)–O(11A) 2.223(6), Ce(2)–O(9) 2.312(6), Ce(2)–O(14A) 2.444(5), Ce(2)–O(12A)
2.568(6), Ce(2)–N(8) 2.598(7), Ce(2)–N(10A) 2.628(7), and Ce(2)–N(11A) 2.654(6).
Symmetry code A: −x, y, −z + 1/2.

Y. Jiao et al. / Inorganic Chemistry Communications 39 (2014) 147–150
149
100
80
60
40
20
0
4-NBA 4-HBA 2-NBA 3-NBA 2-NPA 9-ATA
Guest
100
80
60
40
20
0
4-NBA 4-HBA 2-NBA 3-NBA 2-NPA 9-ATA
Guest
Fig. 4. Conversion for the cyanosilylation reactions of different aldehyde molecules in the
presence of Ce-YB1 (top), and conversion for the cyanosilylation reactions of different
aldehyde molecules in the presence of Ce-YB2 (bottom).
Fig. 2. ESI-TOF spectra of compound Ce-YB1 (top) and compound Ce-YB2 (bottom) in
CH3CN solution.
formation was pseudo-zeroth-order during the first 30 min and 2 h re-
spectively, and the kinetics behavior suggested that the catalytic process
was enzymatic-like catalysis. Because substrate binding is a first equilib-
rium prior to the rate-limiting step of the reaction, the catalysis behavior
is analogous to the Michaelis–Menten mechanism.
In conclusion, we have synthesized two new molecular cages assem-
bled from related bigger ligands without three fold axle and the
resulting molecular structure of the compound remained the M4L4
structural feature and could be looked as a warping tetrahedron. Their
host–guest complexation behavior was investigated by ESI-MS spec-
trum. The molecular polyhedron could encapsulate aldehyde molecules
and acted as a molecular flask to catalyze the cyanosilylation reactions
of aldehyde molecules, showing obvious size selectivity.
is suggested that the complexes Ce-YB1 and Ce-YB2 were worked as a
molecular flask for cyanosilylation reactions of certain size.
To further investigate the process of cyanosilylation reaction, we
use 1H NMR spectra to track the reaction. As shown in Fig. 5, when
the cyanosilylation reaction of 4-nitrobenzaldehyde took place in the
presence of Ce-YB1, the rate constant is 0.16 M·h−1. And when the
cyanosilylation reaction took place in presence of Ce-YB2, the reaction
was slower and the rate constant is 0.045 M·h−1. The product
100
80
60
40
20
0
Fig. 3. ESI-TOF spectra of compound Ce-YB2 upon addition of 4-nitrobenzaldehyde in
CH3CN solution.
0
20
40
60
80
100
120
Time(min)
Reaction conditions: (CH3)3SiCN (0.8 mmol), aldehyde (0.16 mmol), Ce-YB1/Ce-YB2
(0.64 μmol) room temperature under N2 for 2 h in 2 ml DMF/CDCl3 (v/v = 1/99)
solution.
Fig. 5. The conversion of the reaction with 4-nitrobenzaldehyde took place in the presence
of Ce-YB1 (black line) and Ce-YB2 (red line) respectively.

150
Y. Jiao et al. / Inorganic Chemistry Communications 39 (2014) 147–150
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This work was partly supported by the National Natural Science
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Appendix A. Supplementary material
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2013.09.071.
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[26] Crystal structure analysis: The data were collected at a temperature of 273
2 K on
a Bruker Smart APEX II X-diffractometer equipped with graphite monochromated
Mo-Kα radiation (λ = 0.71073 Å) using the SMART and SAINT programs. Indexing
and unit cell refinement were based on all observed reflections from those
72 frames. The structure was solved in the space group C2/c by direct method
and refined by the full-matrix least-squares fitting on F2 using SHELXTL-97.
All non-hydrogen atoms were treated anisotropically. Hydrogen atoms of organic
ligands were generated geometrically, fixed isotropic thermal parameters,
and included in the structure factor calculations. Crystal data for Ce-YB1:
C
194H233Ce4N39O54, Mr = 5401.15, Monoclinic, C2/c, a = 48.657(9) Å,
b = 15.697(3) Å, c = 33.125(6) Å, β = 115.91(1)° V = 22757(8) Å3, Z = 4,
Dc = 1.324 g·cm−3, μ = 0.866 mm−1. 59460 reflections were collected of which
19866 reflections were unique (Rint = 0.0935). The final refinement gave
R1 = 0.1190 and wR2 = 0.2802 for 11573 reflections with I ≥ 2σ(I). The
structures were solved by direct methods and refined on F2 by full-matrix
least-squares methods with SHELXTL version 5.1. Copies of the data can be ob-
tained free of charge on application to CCDC, 12 Union Road, Cambridge, CB21EZ,
UK (fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).