A novel single-side azobenzene-grafted Anderson-type polyoxometalate for recognition-induced chiral migration

Article abstract of DOI:10.1039/c4cc04442h

A three-component supramolecular hybrid system based on host-guest recognition and electrostatic interaction has been developed for a consecutive chiral transfer from an alpha-cyclodextrin to cationic dyes via the bridge of a new azobenzene-grafted Anderson-type polyoxometalate cluster. the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04442h

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A novel single-side azobenzene-grafted
Anderson-type polyoxometalate for
recognition-induced chiral migration†
Cite this: Chem. Commun., 2014,
50, 10823
Received 11th June 2014,
Accepted 18th July 2014
Bin Zhang, Liang Yue, Yang Wang, Yang Yang and Lixin Wu*
DOI: 10.1039/c4cc04442h
www.rsc.org/chemcomm
A three-component supramolecular hybrid system based on host–
To obtain stable chiral POMs in aqueous solution through a
guest recognition and electrostatic interaction has been developed different route from reported ones, we synthesized a new
for a consecutive chiral transfer from an alpha-cyclodextrin to cationic single-side azobenzene-grafted Anderson-type POM. The main
dyes via the bridge of a new azobenzene-grafted Anderson-type reason why we prepared the umbrella-shaped Anderson-type
polyoxometalate cluster.
POM hybrid is that the covalently grafted azobenzene group on
one side of the disc-like POM could be used as an antenna
Polyoxometalates (POMs) are a class of discrete nano-scale group for chiral induction and could be stably sustained in
inorganic polyanion clusters, which possess various advantageous solution. On the other hand, the empty space on the free side of
functions for potential applications in the fields of catalysis, the POM could draw in dye counter ions that can serve as the
medicine, photochromic materials, etc.1 Recently, the covalent chiral acceptor, forming a consecutive chiral transfer chain
and non-covalent organic modifications of POMs draw much more (Scheme 1). By just adjusting the multi-supramolecular interactions
research attention due to their synergistic properties compared within the three-component system, we successfully carried out the
to each individual component.2 Molecular hybridization plays a modulation of the chiral transmission.
significant role in the construction of self-assembled architec-
The single-side-modified Anderson-type hybrid bearing azoben-
tures in crystals, films and solutions. Interestingly, the strategy zene group covalently, is synthesized in tetrabutyl ammonium
has been extended to the creation of chiral POM clusters and their (TBA) salt following a modified procedure previously reported in
self-assemblies. Typically, chiral organic units are introduced into literature.7,8 FT-IR and 1H NMR (Fig. S1 and S2, ESI†) spectra and
POM’s framework via coordination, hydrogen bonding or covalent elemental analysis (Table S1, ESI†) confirm the proposed chemical
linkage, yielding stable enantiomers of chiral hybrids.3 Alternatively, structure. The single crystal of the POM is also obtained from
chiral cations are also used to replace the counter ions, forming the mixed solvent of distilled water and ethanol (1 : 4 in v/v) at
surface-enwrapped POMs (SEPs) through electrostatic interaction. pH 5, and the structure analysis identifies the cluster structure
This approach can be also applied to induce the chirality of of [C6H5NQNC6H4OCH2CONHC(OCH2)3AlMo6O18(OH)3]3À (named
heteropoly blues.4 However, it is still a challenge to transfer the as Azo-POM). Further details of the X-ray structural analysis are
chirality of POM clusters because of their structural rigidity and summarized in Table S2 (ESI†). Compared to the hybrid with
quick racemization in a solution system. The induced circular three TBA counter ions preparing at pH 6–7, only two TBA counter
dichroism (ICD) of achiral molecules, derived from chiral units
through various interactions, has been widely studied.5 Due to the
fast attenuation of chirality against the distance from the chiral
centre, so far, effective methods to deal with the continuous chiral
transfer referring to POMs via multi-supramolecular interactions
have rarely been reported. The first example of recognition between
guest group-modified POM and cyclodextrins (CD) was reported
recently; however, the inclusion chirality was not involved.6
Scheme 1 Schematic drawing of chiral recognition and transfer by
using hybrid POM as a bridge of a-cyclodextrin and dye molecules. Ball-
and-stick and polyhedral representation of the prepared umbrella-shaped
Azo-POM, where colour code: MoO6 and AlO6 units in red and blue
polyhedrons; C, N, and O atoms in grey, blue and red sphere, respectively.
H atoms are omitted for clarity.
State Key Laboratory of Supramolecular Structure and Materials, College of
Chemistry, Jilin University, Changchun 130012, China. E-mail: wulx@jlu.edu.cn
† Electronic supplementary information (ESI) available: Synthesis procedures,
additional spectra and experimental details. See DOI: 10.1039/c4cc04442h
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ions are found (Fig. S3, ESI†). Considering the weak acidic up-field in corresponding 1D selective ROESY NMR spectrum
environment, it is reasonable to think that one TBA counter ion (Fig. S8, ESI†), confirming the tight inclusion.
was replaced by a protonated water during crystallization. An
Due to the hydration-induced electrostatic dissociation,
elemental analysis of the crystalline product clearly confirms alkaline metal ion counter ions are not closely associated with
the change of TBA counter ions in crystals (Table S1, ESI†). To the polyanion clusters in aqueous solutions. But when organic
increase the solubility of the synthesized cluster in water, sodium cations bearing hydrophobic terminal are added into the cluster
ions are then used to replace the TBA counter ions following solution, their electrostatic connection with Azo-POM can greatly
the previous procedure,9 and Na3(Azo-AlMo6O18) (abbreviated increase. This happens because the charge neutralization leads
1
as Na–Azo-POM) is obtained. H NMR (Fig. S4, ESI†) spectrum to the formation of a stabilized ionic pair accompanied by the
and an elemental analysis (Table S1, ESI†) are also carried out enhanced hydrophobicity. Methylene blue (MB) is an organic
to identify the replacement of the counter ions.
cationic dye, which has strong long-wavelength absorption and
Because the CD recognition-induced chirality is closely related could be considered as an appropriate candidate for the ionic
to the matching space with the guest component, as well as its combination with Azo-POM. The stability of MB after mixing
binding, we evaluated the interaction of synthesized POM with with POM was checked by UV-Vis spectrum (Fig. S9, ESI†), and
CDs bearing different sizes of cavity. The sample solutions were no obvious degradation was found in the present experimental
obtained by simply dropping CDs in water into an aqueous conditions. By adding a suitable amount of MB into the a-CD
solution containing equal molar amounts of Na–Azo-POM to a included Azo-POM aqueous solution (MB : Azo-POM in 3 : 5 mole
final concentration of 2.1 Â 10À4 mmol mlÀ1 at room tempera- ratio) with stirring at room temperature and curing for 12 h, we
ture. After a while of curing, the recognition and chiral transfer obtained a three-component solution. After an extra standing,
from CDs to azobenzene group of Azo-POM were examined the electrostatic interaction between MB and Azo-POM is char-
through circular dichroism spectrum (CDS). As shown in Fig. 1, acterized by 1H NMR spectra in mixture solution of D2O and
g-CD exhibits insensitivity on chiral induction signal for guest DMSO-d6 (1: 1 in v/v) (Fig. 2). In comparison with MB alone, the
Azo-POM, while b- and a-CDs show obvious induction activity proton peaks corresponding to MB.
due to both positive and negative Cotton effects. The activity
In the a-CD included Na–Azo-POM complex system shift
appears at around 345 and 443 nm, apparently sourcing from to the low field, implying the formation of an electrostatic
azobenzene group in POMs. It is known that CDs bear the connection with Azo-POM cluster. This assignment is supported by
internal chirality in inner cavity, and this result indicates that the emergence of deposition when the amount of MB is further
there exists a chiral transfer from CDs to Azo-POM. Thus, the increased in the triple-component solution. We also use an equiva-
achiral POM can be induced to yield chirality that can be stably lent charge ratio of MB to replace the sodium ions of Azo-POM
maintained in aqueous solutions (Fig. S5 and S6, ESI†). To utilize in water. Through the same procedure reported in literature, we
the stronger recognition capability under the same environment intended to obtain a complex (MB–Azo-POM) bearing three MB
and to keep the specificity, we selected a-CD as the recognition counter ions, as confirmed by elemental analysis (Table S1,
host and chiral inducer of the synthesized hybrid POM in the ESI†). The wholly electrostatic complex MB–Azo-POM became
following study. The 2D ROESY NMR spectrum indicates that almost insoluble in water and its NMR spectrum in organic
H–H correlations take place between protons in the inner cavity solvent displayed a similar moving tendency of chemical shifts as
of a-CD and phenyl groups of trans-state Azo-POM, unequivocally those of MB when mixing with Azo-POM in water (Fig. S10, ESI†),
demonstrating the existence of a strong inclusion interaction further confirming the electrostatic interaction between them.
(Fig. S7, ESI†). In contrast, there is no such interaction occurring Therefore, we could figure out the electrostatic interactions
for the cis-state Azo-POM. In comparison to a-CD alone, the when blending MB with Azo-POM regardless of the addition of
peaks corresponding to protons located in the cavity of a-CD a-CD. Apparently, the electrostatic combination is favourable for
in mixture solutions were found to have larger chemical shifts the chiral transfer between the organic–inorganic ionic pairs.
Fig. 2 1H NMR spectra of (A) MB and (B) MB mixing with Na–Azo-POM
Fig. 1 CDS of Na–Azo-POM mixed with equivalent mole of (a) a-CD,
(b) b-CD and (c) g-CD in aqueous solution.
(MB : Azo-POM at molar ratio of 3 : 5) in D2O/DMSO-d6 (1: 1 in v/v) solution.
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Fig. 4 (A) CDS of MB and a-CD included Na–Azo-POM (3 : 5 : 5 in molar
ratio) in aqueous solution by gradually increasing the concentration of
NaCl to 0, 0.043, 0.086, 0.128, 0.171, 0.214, 0.257, 0.342, 0.428, 0.599,
0.770, 1.112, and 1.454 mmol mlÀ1. (B) CDS of MB, Na–Azo-POM mixed
with a-CD in aqueous solution by gradually increasing the temperature
(20, 30, 40, 50, 60, and 80 1C).
Fig. 3 UV-Vis spectra of (a) MB (3.0 Â 10À5 mmol mlÀ1), (b) MB mixing
with Na–Azo-POM and a-CD, and (c) Na–Azo-POM mixed with a-CD in
aqueous solution (MB : a-CD : Na–Azo-POM at molar ratio of 3 : 5 : 5).
As shown in Fig. 3, when MB cations are added into the a-CD
included Azo-POM solution, the absorptions at ca. 627 and 713 nm
with a broad band decreased markedly at 560 nm, to the visible
region. The bands moving to the long wavelength in comparison to
isolate MB point out the electrostatic interaction-induced MB aggre-
gation around POMs.10 In contrast, the band at 345 nm that is
ascribed to the absorption of Azo group does not show any change
in its state without MB, indicating a constant inclusion state.
To confirm the chiral induction and migration, CDS of the
three-component system were carried out (Fig. S11, ESI†). For the
MB mixed with Na–Azo-POM, no chiral signal was found. When
MB was mixed with a-CD, the chiral signal of MB molecule could
still not be seen due to the bigger size of MB. The 2D ROESY NMR
spectrum also confirms that no inclusion of a-CD to MB occurring
through b-CD works well (Fig. S12, ESI†). In contrast, when the
three components are mixed together, on one hand, a positive
Cotton signal at around 345 nm and a negative signal at about
440 nm can be well ascribed to the host–guest recognition-induced
chirality of the p–p* transition band of trans-azobenzene groups.
On the other hand, the positive Cotton signals at 558 and 720 nm
and much stronger negative Cotton signal at 629 nm appear and
can be unequivocally attributed to the induced chirality of MB.
These results concerning Cotton activity clearly prove the chiral
migration from a-CD to MB bridged by Azo-POM.
Apparently, both electrostatic interactions and host–guest
recognition play crucial roles in the chiral migration and can be
used for modulating the chirality of Azo-POM and the MB dye
molecule. With the addition of sodium chloride in the three-
component system, the intensity of Cotton signals of MB
obviously decreases until the concentration of the salt reaches
ca. 1.45 mmol mlÀ1 (Fig. 4A). It is well known that the existence of
electrolytes can increase the ionic strength of the solution system,
which weakens the electrostatic interaction of ionic pairs.11 As a
result, the interaction between Azo-POM and MB fades gradually
so that the average distance between the cationic dye and the
anionic cluster becomes greater. Nevertheless, the ionic pair could
not be completely decomposed by simply adding electrolytes.
When the concentration of NaCl was gradually increased, the
chemical shifts of MB in Na–Azo-POM-mixed solution moved
close to those of isolated MB solution, indicating the weakening
of the electrostatic interaction (Fig. S13, ESI†). In addition, based
on the comparison of the absorption spectra (Fig. S14, ESI†), the
fact that the bands of MB that are assigned to the aggregation
state shift to the long wavelength demonstrates the increase in the
amount of dissociated MB with the addition of salt. This change
also implies that the chiral transfer from Azo-POM is blocked by
weakening electrostatic interactions. As seen from the same CDS
measurement in Fig. 4A, the intensity of the chiral signal of Azo
group at 345 nm increases with the addition of salt, indicating
that the host–guest recognition is enhanced by the ionic strength
change. In contrary, when heating the three component system,
accompanying by the decrease of chiral signals of MB, we also see
the decrease of chiral intensity of Azo group at 345 nm (Fig. 4B).
Normally, the host–guest recognition can be broken down under
higher temperature. This change implies that the host–guest
interaction is gradually dissociated, and consequently, the inten-
sity of chiral signals of MB decreases greatly at the same time.
1
Variable-temperature H NMR spectra indicate constant electro-
static interaction between MB and Azo-POM cluster while increas-
ing temperature (Fig. S15, ESI†). Therefore, it is reasonable to
think that the chiral transfer is stopped by the decomposition
between the a-CD and Azo-POM during the heating process.
A new single-side azobenzene-modified Anderson-type POM was
synthesized. The novel umbrella-like hybrid cluster displays inter-
esting property when building a three component supramolecular
system with a-CD and MB dye cations via both host–guest recogni-
tion and electrostatic interactions. The chirality of a-CD is amplified
into Azo-POM and MB cations. Furthermore, the chiral migration is
modulated through simply tuning the concentration of electrolyte
and heating. The present results open a new approach to drive
POMs as inorganic bridging ligands for chiral transfer and
amplifications to cationic molecules.
This work was financially supported by the National 973 Program
(2013CB834503), NSFC (91227110, 21221063), Ministry of Education
of China (20120061110047), and open project of SKL-PPC, CAS.
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