Olive-shaped chiral supramolecules: Simultaneous self-assembly of heptameric lanthanum clusters and carbon dioxide fixation

Full text of DOI:10.1002/anie.200900838

Angewandte
Chemie
DOI: 10.1002/anie.200900838
Heptameric Lanthanum Clusters
Olive-Shaped Chiral Supramolecules: Simultaneous Self-Assembly of
Heptameric Lanthanum Clusters and Carbon Dioxide Fixation**
Xiao-Liang Tang, Wen-Hua Wang, Wei Dou, Jie Jiang, Wei-Sheng Liu,* Wen-Wu Qin, Guo-
Lin Zhang, Hong-Rui Zhang, Kai-Bei Yu, and Li-Min Zheng
Lanthanide-based high-nuclearity clusters have received
considerable attention because of their fascinating self-
assembled structures and potential applications in magnetic,
optical, electronic, and catalytic processes.[1,2] More and more
geometrically intriguing supramolecules, for example, molec-
ular polygons,[3] cages,[4] helicates,[5] wheel-shaped clusters,[6]
and polynuclear clusters with different numbers of lanthanide
ions[7] have been successfully obtained, which may ultimately
extend the range of new materials.[2] However, the precise
prediction and control of such lanthanide polynuclear clusters
needs to be explored further because of the variable and high
(8, 9, or more) coordination numbers that can exist, as well as
the weak stereochemical preferences of lantha-
nide ions.[2d,8] Moreover, cell-like heptanuclear
lanthanide clusters that can effectively fix carbon
dioxide under mild conditions have not been
reported to date.
complexes, the CO32À ion adopts a rare bridging mode and is
incorporated into a triangular arrangement of lanthanides.
However, the design and synthesis of these extended lantha-
nide complexes still represent a formidable challenge.[10,12]
We have investigated the reaction of chiral amino acids
with a variety of lanthanide salts. As multidentate ligands, two
enantiomorphous amino acid containing reduced Schiff bases,
(R)-H2L and (S)-H2L in optically pure form (Scheme 1) not
only retain the chirality of the amino acids but also show
improved metal-bridging capability. We report herein a pair
of unique enantiopure heptanuclear lanthanide clusters,
2À
which are self-assembled with the aid of CO3 ions and
There has been a long-standing interest in the
capture, fixation, and activation of CO2 for the
chemical conversion of CO2 into various carbo-
nates, formic acid, and other C1 feedstock chem-
Scheme 1. Preparation of ligands (S)-H2L and (R)-H2L.
icals.[9] Part of such work has dealt with the
insertion of CO2 from the atmosphere, particu-
larly into coordination complexes as the carbonate anion.[10]
Several ZnII, CuII, and NiII complexes are well known to
perform this reaction, which results in the different bridging
modes of the CO32À ion.[11] Despite intensive studies, the only
example of CO2 insertion into lanthanide complexes to date
was reported by Jeong et al., and comprises a pair of
lanthanide-based polynuclear clusters that are associated
possess olive-shaped nanostructures, by a one-step growth
process under mild conditions. In addition, the overall
structure of the chiral ligands in the cluster represents a
new class of homochiral triple helix.
The clusters were synthesized by the addition of a solution
of La(NO3)3·6H2O in methanol to a transparent solution of
(S)-H2L and two equivalents of Li(OH)·H2O in methanol.
Colorless rhombohedral crystals of [La7{(S)-L}6(CO3)(NO3)6-
(OCH3)(CH3OH)7]·2CH3OH·5H2O (1) suitable for X-ray
diffraction were obtained in good yield by slow evaporation of
the solution in air over two weeks. X-ray crystallographic
studies revealed that the crystal belongs to the monoclinic
space group P21.[13] The asymmetric unit, [La7{(S)-L}6(CO3)-
(NO3)6(OCH3)(CH3OH)7], which contains seven LaIII ions
and six (S)-L2À linking ligands, forms a complete supramolec-
ular cluster (Figure 1a).
with CO3 ions at high temperature.[12b] In this class of
2À
[*] X.-L. Tang,[+] Dr. W.-H. Wang,[+] Dr. W. Dou, J. Jiang, Prof. Dr. W.-S. Liu,
Prof. W.-W. Qin, G.-L. Zhang, H.-R. Zhang
College of Chemistry and Chemical Engineering and State Key
Laboratory of Applied Organic Chemistry, Lanzhou University
Lanzhou 730000 (P.R. China)
Fax: (+86)931-891-2582
E-mail: liuws@lzu.edu.cn
Prof. Dr. L.-M. Zheng
State Key Laboratory of Coordination Chemistry
Nanjing University, Nanjing 210093 (P.R. China)
In the cluster, which has five LaIII ions as apexes, La(2)
···La(6) form an elongated trigonal bipyramid with La ···La
distances that range between 5.063 ꢀ and 5.304 ꢀ. La(2) and
La(6) occupy the axial sites of the bipyramid and the rest lie in
the equatorial plane. The other two LaIII ions, La(1) and
La(7), are symmetrically located on the axis at both sides of
the trigonal bipyramid. The distances La(1)· ··La(2) and La(6)
···La(7) are 3.768 ꢀ and 3.763 ꢀ, respectively. Thus, seven
LaIII ions extend over a trigonal bipyramidal geometry and
are linearly arranged in the mode 1:1:3:1:1 to form an olive-
Prof. Dr. K.-B. Yu
Chengdu Institute of Organic Chemistry
Chinese Academy of Sciences, Chengdu 610041 (P.R. China)
[+] These authors contributed equally to this work.
[**] This work was financially supported by the National Natural Science
Foundation of China (grant nos. 20771048 and 20621091)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900838.
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2À
In addition to the bridging of the CO3 ion by three
equatorial LaIII ions, six bridging ligands play important roles
in linking all the LaIII ions to construct the cluster. The
carboxylate anion of the ligand bridges two metal ions in a m2-
k1O; k2O,O’ fashion, and a phenolic oxygen atom also bridges
two LaIII ions (Figure 1c). As a result, each (S)-L2À group
bridges four LaIII ions, and thus acts in different coordination
modes: 1) as a monodentate ligand for polar La(1) or La(7)
through the Ophenoxo atom, and through the Oamide atom for the
equatorial LaIII ions; 2) as a bidentate ligand for equatorial
LaIII ions through the carboxylate ion; and 3) as a tridentate
ligand for La(2) or La(6) through the Ophenoxo , Namino , and
Ocarboxylate atoms. Furthermore, in order to saturate the
coordination sphere of the lanthanide ions, a methanol
molecule and three nitrate groups coordinate to La(1) or
La(7) in a bidentate mode, and two solvent molecules
coordinate to each equatorial LaIII ion. Thus, most LaIII ions
in the cluster unit have a coordination number of ten, except
La(2) and La(6), which have a coordination number of 9.
Interestingly, the whole framework of the cluster exists as
a homochiral helix (Figure 1b). In order to understand the
structure further, the cluster molecule can be disassembled
schematically into three equatorial [{(S)-L}–La–{(S)-L}] units,
which are related by quasi-C3 molecular symmetry (Figure 2).
2À
The units are interconnected by the central CO3 and four
LaIII ions on the major axis of the cluster {La(1), La(2), La(6),
La(7)} to form a completely integrated triple M helix. The
pitch length of the major axis {La(1) ···La(7)} is about 16.15 ꢀ.
The intrinsic structural information that is encoded in the
discrete helical units reflects the direct relationship between
the chiral ligand and supramolecular structure of the cluster.
That is, the chirality of the organic ligand, with the aid of
lanthanide ions, is reproduced in the outer chiral helical
cluster, which is a remarkable chirality transfer process.
Figure 1. a) Molecular structure of lanthanide-based cluster 1 contain-
ing seven LaIII ions and six (S)-L2À ligands. The CO32À ion is
encapsulated by LaIII ions and shown in a space-filling representation.
Hydrogen atoms and solvent molecules are omitted for clarity.
b) Space-filling representation of 1 with a chiral helical arrangement of
six (S)-L2Àligands. Hydrogen atoms, nitrate groups, and solvent
molecules are omitted for clarity. Each {[(S)-L]–La–[(S)-L]} unit is
represented in a different color. c) The simplified structure of 1. All
LaIII ions are bridged by oxygen atoms and form extend trigonal
bipyramidal structures. d) The m3-tridentate bridging mode of the
2À
To further confirm the formation of CO3 ions in the
cluster core and understand the chirality transfer process from
the ligand to the supramolecular structure, the chiral counter-
part of 2, [La7{(R)-L}6(CO3)(NO3)6(OCH3)(CH3OH)5-
(H2O)2]·2CH3OH·4H2O was prepared by employing (R)-
2À
CO3 ion in the center of the cluster core. C gray, O red, N blue, La
violet.
shaped cluster unit (Figure 1c). This arrangement has, to the
best of our knowledge, not been reported to date.
One of the most notable structural features of 1 in the core
of the cluster is that three equatorial LaIII ions are trigonally
arranged and bonded by the central CO32À ion, which adopts a
rare m3-h2:h2:h2-tridentate bridging mode.[12b] As shown in
Figure 1d, each of the three oxygen atoms of the carbonate
ion acts as a donor unit with an angle of around 1708, and the
LaIII ion acts as an acceptor unit with an angle of around 528.
À
All La O bonds are almost equal and the average bond
length is 2.544 ꢀ. This bridging mode makes the carbon atom
2À
of CO3 lie on the extended bipyramidal quasi-C3 axis and
Figure 2. Preparation of the two lanthanide-based enantiomeric clus-
ters 1 and 2, which are shown in a space-filling representation.
Hydrogen atoms, nitrate groups, and solvent molecules are omitted
for clarity. Each (S or R)-L2À ligand is represented in a different color,
while the lanthanide ions are shown in violet.
2À
the CO3 ligand symmetrically fixed in the center of the
cluster. From another point of view, the LaIII ions are in close
contact and the supramolecular structure is further stabilized
by the introduction of the CO32À ion.
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H2L in place of (S)-H2L.[13] During the crystal growth, the
mixed solution was placed in a sealed tank full of CO2 and
colorless rhombohedral crystals were successfully obtained
over two days, thus showing how the carbonate core can be
efficiently introduced. As expected, 2 also belongs to chiral
space group P21, and, as in 1, the six ligands form a triple helix
that surrounds the CO32À ion, but with the opposite P helical
arrangement. As confirmed by solid-state CD spectroscopy,
the chirality of the ligands is preserved in the crystals
(Figure 3). Crystals of both 1 and 2 in KBr pellets give
mirror-image CD spectra with opposite signs, that is, each
crystal comprises helices of only one sense.
BrÀ, IÀ, NO3À, SO42À, ClO4À, and CO3 ions. The results
2À
2À
indicate that only the CO3 ion can efficiently change the
optical rotation of the system (Figure 4). Thus, optical
rotation analysis of the chiral ligands can be successfully
applied to anion-selective recognition. This method is simple
and sensitive, which has not been reported to date.
Figure 4. Optical rotation responses of the La(NO3)3–(S)-L2À system to
different anions in water. l=589 nm, [(S)-L2À]=[La-
(NO3)3]=6ꢀ[anion]=0.028m.
In conclusion, we have described the preparation and
structural characterization of a pair of novel supramolecular
clusters 1 and 2, in each of which six ligands bind seven
lanthanum ions to form an olive-shaped nanocluster. Fur-
thermore, the supramolecules not only inherit the chirality of
the enantiomerically pure ligands, which are derived from
amino acids, but also possess triple-helical structures that
Figure 3. CD data obtained from a transparent disk with a radius of
5 mm made from a mixture of 1 or 2 (3 mg) and KBr (100 mg). Dotted
line: complex 1, solid line: complex 2.
The presence of the CO32À ion in the cluster core and the
reaction of the ligands with CO2 were further investigated by
using IR and NMR spectroscopy. The IR spectra of the
ligands and crystals were measured in the solid state. Strong
carbonate-based vibrations, which are not observed for the
ligands, are present at approximately 1450 cmÀ1 in the spectra
2À
form homochiral crystals. In the core of the cluster, a CO3
ion, which is derived from atmospheric CO2 , adopts a rare
tridentate bridging mode to link three lanthanum ions, thus
allowing the clusters to efficiently fix CO2. Optical rotation
analysis demonstrates that the system, which includes the
chiral ligand L2À and LaIII ions, has an excellent ability to
2À
of both 1 and 2.[14,15] The presence of the CO3 ion in 1 was
further confirmed by the solid-state 13C NMR spectrum,[15]
2À
which shows one low-intensity peak at d = 170.77 ppm that
selectively recognize CO3 in water. The properties and
2À
can be assigned to the encapsulated CO3
ion.[16] To
applications of these nanoparticles will be the subjects of
future studies.
2À
determine the origin of the CO3 ion in the clusters, the
same reactions of (S)-L2À with La(NO3)3·6H2O were carried
out in methanol under an inert atmosphere (Ar or N2). Then,
CO2 was slowly bubbled into the methanolic solution and the
process was monitored by using UV/Vis spectrophotome-
try.[15] Obvious changes in the UV absorption spectrum show
the formation of the carbonate complexes. Thus, CO2
molecules from the atmosphere are efficiently captured and
fixed by these clusters through chemical conversion.
Experimental Section
Procedures for the preparation of ligands (S)-H2L and (R)-H2L are
described in the Supporting Information.
Synthesis of 1: A mixture of LiOH·H2O (8.4 mg, 0.2 mmol) and
(S)-H2L (23.8 mg, 0.1 mmol) in methanol (3 mL) was stirred for 5 min
to obtain a transparent solution. Then a solution of La(NO3)3·6H2O
(43.3 mg, 0.1 mmol) in methanol (2 mL) was added and the mixture
was rapidly stirred for 10 min. Colorless rhombohedral crystals
suitable for X-ray analysis were obtained in 67% yield by slow
evaporation of methanol over two weeks at room temperature in air.
The crystals were soluble in water but insoluble in common organic
solvents. Elemental analysis calcd (%) for 1 C75H103N18O53La7 (non-
coordinated solvent molecules were lost upon drying): C 29.27, H
3.37, N 8.19; found: C 29.81, H 3.28, N 8.29.
These results clearly suggest that the CO32À anion plays an
important role as a template in the formation of the
heptameric lanthanum cluster. Moreover, six chiral ligands
self-assemble and transfer their chirality to produce a new
chiral supramolecular structure. We subsequently investi-
gated the response of the system to different anions. We
examined the optical rotation of a mixture of La(NO3)3·6H2O
and (S)-L2À in deionized water in the presence of the FÀ, ClÀ,
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Synthesis of 2: This complex was synthesized following the same
G. Hopfgartner, C. Piguet, Chem. Eur. J. 2003, 9, 1860; c) J.-C. G.
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instead of (S)-H2L. The crystals were obtained in 59% yield by slow
evaporation of methanol over two days in a sealed tank filled with
CO2. Elemental analysis calcd (%) for 2 C73H99N18O53La7 (non-
coordinated solvent molecules were lost upon drying): C 28.76, H
3.27, N 8.27; found: C 29.18, H 3.21, N 8.16.
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Crystallographic data for complexes 1 and 2 were collected with a
Siemens P4 four-circle diffractometer at (287 Æ 2) K and a Bruker
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Received: February 12, 2009
Published online: April 7, 2009
Keywords: anion recognition · carbon dioxide fixation · chirality ·
.
cluster compounds · helical structures
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[13] Crystal data for 1: C77H112La7N18O60, crystal size: 0.80 ꢆ 0.50 ꢆ
0.30 mm3, monoclinic, space group P21, a = 15.6616(16), b =
25.624(3), c = 16.4596(17) ꢀ, a = g = 908, b = 101.924(9)8, V=
6462.9(12) ꢀ3, Z = 2, 1calcd = 1.656 gcmÀ3, m = 2.354 mmÀ1, T=
287(2) K. R1 = 0.0442, wR2 = 0.1136 (I > 2s(I)), R1 = 0.0576,
wR2 = 0.1176 (all data). Crystal data for 2: C75H108La7N18O59,
crystal size: 0.21 ꢆ 0.18 ꢆ 0.10 mm3, monoclinic, space group P21,
a = 15.6719(9), b = 25.5703(16), c = 16.4720(10) ꢀ, a = g = 908,
b = 101.8380(10)8, V= 6460.5(7) ꢀ3, Z = 2, 1calcd = 1.634 gcmÀ3
,
m = 2.353 mmÀ1, T= 294(2) K. R1 = 0.0549, wR2 = 0.1412 (I >
2s(I)), R1 = 0.0732, wR2 = 0.1560 (all data).
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