A chiral quadruple-stranded helicate cage for enantioselective recognition and separation

Article abstract of DOI:10.1021/ja212132r

The self-assembly of enantiopure pyridyl-functionalized metallosalan units affords a homochiral helicate cage, [Zn₈L₄Cl₈], in which the optical rotation of each ligand is increased by a factor of 10 upon coordination. The octanuclear cage featuring a chiral amphiphilic cavity exhibits enantioselective luminescence enhancement by amino acids in solution. The cage exists in two different crystalline polymorphic forms that possess porous structures built of helicate cages interconnected by 1D channels or pentahedral cages and have the ability to separate small racemic molecules by adsorption but with different enantioselectivities.

Full text of DOI:10.1021/ja212132r

Communication
pubs.acs.org/JACS
A Chiral Quadruple-Stranded Helicate Cage for Enantioselective
Recognition and Separation
Weimin Xuan, Mengni Zhang, Yan Liu, Zhijie Chen, and Yong Cui*
School of Chemistry and Chemical Technology and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong
University, Shanghai 200240, China
S
* Supporting Information
Scheme 1. Assembly of Helicate Cage 1 from H2L
ABSTRACT: The self-assembly of enantiopure pyridyl-
functionalized metallosalan units affords a homochiral
helicate cage, [Zn8L4Cl8], in which the optical rotation of
each ligand is increased by a factor of 10 upon
coordination. The octanuclear cage featuring a chiral
amphiphilic cavity exhibits enantioselective luminescence
enhancement by amino acids in solution. The cage exists
in two different crystalline polymorphic forms that possess
porous structures built of helicate cages interconnected by
1D channels or pentahedral cages and have the ability to
separate small racemic molecules by adsorption but with
different enantioselectivities.
ince chiral recognition is one of the fundamental processes
crystalline samples can serve as hosts for adsorption separation
in nature and plays vital roles in diverse fields of science
S
of racemic organic molecules.
and technology, many efforts have been devoted to the
synthesis and utilization of artificial chiral architectures.1 In
particular, metal-directed self-assembly opens up unique
opportunities to make well-defined coordination structures
and has led to great progress in the construction of functional
porous cages.2,3 Incorporating chiral functional groups into
such hollow structures means that they can be used for
enantioselective processes,4,5 but there is still difficulty in
making chiral molecular cages that are optically pure.4−6 In fact,
with a few notable exceptions, chiral molecular coordination
architectures have not been explored for enantioselectivities.6
Although helicates constructed from flexible oligodentate
strands and metal ions have long attracted chemists with their
elegant morphologies and intrinsic chirality,7 helicate cages that
have available inner cavities are very rare, and none to date have
been reported to be optically pure.7,8 Meanwhile, chiral salen
ligands and their metal complexes have diverse applications in
asymmetric catalysis and separation.9 In contrast, salan ligands,
the reduced forms of salen ligands, are less studied, but
potential benefits such as increased ligand flexibility and
stronger nitrogen donors make them attractive ligand targets.10
Realizing that the salan ligand might establish a pair of unique
asymmetric NH recognition sites,1b we examined the
coordination assembly of twisted enantiopure metallosalans
for creating chiral helical cavities and functionalities for
chirotechnology. We report here the assembly of a homochiral
helicate cage from the pyridyl-functionalized salan ligand H2L
and ZnCl2 (Scheme 1). The cage exhibits enantioselective
luminescence enhancement by chiral amino acids, and its
The chiral salan ligand H2L was prepared in an overall 56%
yield by the Schiff base condensation of 5-tert-butyl-3-(4-
vinylpyridyl)salicylaldehyde and enantiopure 1,2-diaminocyclo-
hexane followed by reduction with NaBH4. Reaction of (R)-
H2L with ZnCl2 (1:2 molar ratio) in a mixed solvent of DMF,
THF, and H2O by slow evaporation at room temperature or by
heating at 65 °C afforded complexes [Zn8L4Cl8]·5THF [(R)-
1a] or [Zn8L4Cl8]·THF·H2O [(R)-1b] in good yields. The
formulations were supported by the results of microanalysis, IR
spectroscopy, and thermogravimetric analysis (TGA). The
solution behaviors of 1a and 1b are essentially the same, so only
that of the former will be presented.
1
The H NMR spectrum of (R)-1a shows more complicated
signals than the free H2L ligand (Figure 1). First, there is one
complete set of proton resonances for the L ligand, as could be
deduced from the number of signals and the appearance of two
sets of signals for the tert-butyl groups at 1.12 and 1.25 ppm.
This indicates that the C2 symmetry of the ligand is lost in the
cage complex. Second, with respect to the free ligand, the
proton signals from the cyclohexane groups show an obvious
upfield shift, while those from the pyridyl and phenyl groups
show either downfield or upfield shifts, suggesting the
unsymmetrical coordination behavior of the ligand. Finally, all
of the proton signals are very sharp, suggesting the formation of
discrete metal complexes rather than oligomeric species. The
formation of the cage compound was supported by electrospray
Received: December 28, 2011
Published: April 11, 2012
© 2012 American Chemical Society
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trigonal-bipyramidal Zn centers enclosed in the N2O2 pockets
and linked by two phenolato O atoms. The dimeric unit is
therefore based on two identical coordination bonds forming a
four-membered Zn2O2 ring with a Zn1−Zn2 separation of
3.0725(7) Å. The two twisted salan ligands have an antiparallel
orientation around the dimetallic core, and the four pyridyl
groups are oriented toward the same face of the almost planar
Zn2O2 unit and point away from the adjacent units with a
separation distance of 6.8 Å (average N···N distance) to
minimize steric repulsion. A pair of the bowl-like Zn2L2 dimers
are linked together via their peripheral pyridyl groups by
coordination to four Zn centers in the equatorial plane. Each of
the four Zn ions is tetrahedrally coordinated by two pyridyl
groups and two chloride ions. Overall, the octanuclear cage can
be viewed as a P-configured quadruple-stranded helicate that is
locked by the twisted Zn2L2 dimers at both ends, which take a
screwed U shape and are fixed by coordination to the four
ZnCl2 units.
Figure 1. 1H NMR spectra of (a) (R)-H2L and (b) (R)-1a in acetone-
d6.
1b crystallizes in the chiral orthorhombic P212121 space
group, with one formula unit in the asymmetric unit. It is a
polymorph of (R)-1a and has a similar P-configured helicate
structure assembled from Zn2L2 dimers. Space-filling repre-
sentations of 1a (Figure 2b) and 1b clearly show the formation
of porous helicate cages with wide apertures. The cavities have
inner dimensions of ∼13.7 Å × 6.4 Å occupied by one THF
molecule. Strong intermolecular π−π, CH···π, and CH···Cl
interactions direct the packing of the helicates into porous 3D
structures. In 1a, 1D channels with dimensions of ∼4.0 Å × 5.5
Å are formed by stacking of adjacent tetramers along the [100]
directions (Figure 2c), whereas in 1b, distorted pentahedral
cages with a maximum inner width of ∼8 Å are formed via the
arrangement of four adjacent helicate cages (Figure S5b). In
both cases, the chiral coordinated amines of the L ligands are
oriented toward the outside of the helicate cages and are
exposed to the interstitial pores accessible to guest molecules.
Compounds of this type thus have great potential for studies of
host−guest interactions and supramolecular chemistry. 1a and
1b are rare examples of homochiral coordination cages with
built-in chiral functional groups that have been crystallo-
graphically characterized.4−6
ionization mass spectrometry (ESI-MS) of (R)-1a, which
showed the molecular ion [Zn8L4Cl7]+ at m/z 3325.1 and the
fragment ions [Zn4L4 + Na + 3H]4+, [Zn2L2 + NH4]+, and
[Zn4L4 + K]+ at m/z 713.5, 1431.3, and 2865.0, respectively
(Figure S16 in the Supporting Information).
A single-crystal X-ray diffraction (XRD) study of (R)-1a
unambiguously revealed the formation of a chiral helicate cage.
(R)-1a crystallizes in the chiral monoclinic P21 space group,
with one formula unit in the asymmetric unit. The basic
building unit is a Zn2L2 dimer built of ZnL monomers (Figure
2a). The dimetallic core consists of two five-coordinate
The homochiral nature of the salan ligand H2L is essential for
the formation of the helicate cage. Under otherwise identical
conditions, slow evaporation of rac-H2L and ZnCl2 in the
DMF/THF/H2O mixed solvent afforded only a racemic
dimeric complex [Zn2L2] (2) instead of the expected helicate
complex. In 2, the dinuclear unit is built of two opposite-
handed ligands in which the four pyridyl groups are oriented
toward the different faces of the almost planar Zn2O2 core, thus
disfavoring the generation of a helicate coordination structure
as a result of spatial constraints (Figure S7). Calculations using
the PLATON program indicate that 40.9 and 51.1% of the total
volumes of 1a and 1b, respectively, are occupied by solvent
molecules.11 The phase purity of bulk samples of 1a and 1b was
established by comparison of their observed and simulated
powder XRD (PXRD) patterns. TGA revealed that guest
molecules could be removed at 80−150 °C. PXRD indicated
that the evacuated samples retained their crystallinity, although
structural distortions occurred. The apohost structure took up
guest molecules again upon exposure to THF vapor for 2 days
at room temperature, as evidenced by TGA and PXRD. The
permanent porosity of 1a and 1b was examined by N2
adsorption measurements at 77 K, and the BET surface areas
were found to be 78.0 and 283.7 m2/g, respectively.
Figure 2. (a) View of the molecular structure of (R)-1a (dark-green,
Zn; yellow, Cl; blue, C; red, O; purple, N) and (b) its space-filling
model. (c) The 3D supramolecular structure of (R)-1a showing
helicate cages interconnected by channels. The cavities are highlighted
by colored spheres.
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Chiral amplification occurred during the self-assembly of the
enantiopure helicate. The values of the molar optical rotation
(ϕ) of (R)-1a and H2L are 31208.7 and 674.9 deg cm3 dm−1
mol−1, respectively. The cage bearing four ligands has an optical
rotation per mole that is >45 times that of H2L, that is, a 10-
fold increase in optical rotation between a set of four free
ligands and a set of four coordinated ligands. The generation of
a chiral helical superstructure should be responsible for the 10-
fold increase of the optical rotation. This molar optical rotation
value is large in absolute terms, comparable to those of organic
helicenes12a and the resolved trefoil knot.12b The solution CD
spectra of 1a made from (S)- and (R)-H2L are mirror images of
each other, and each exhibited a bisignate π−π* band at 274
and 371 nm. Their intensities are much higher than that of the
free ligand, consistent with the molar optical result. The solid-
state CD spectra showed their enantiopurity and retention of
chirality in the crystalline state.
The presence of large chiral pores and available chiral
functional NH groups in this helicate cage prompted the
exploration of enantioselective recognition and separation. The
fluorescence of cage (R)-1a in THF showed a strong emission
at λ = 534 nm assigned to a ligand-centered π → π* process,
with a quantum yield of 5.60% and lifetimes of 2.31 and 0.92 ns
(corresponding to a biexponential decay). When (R)-1a was
treated with Ala, the emission at 534 nm was shifted to 515 nm
and enhanced by both the D and L enantiomers (Figure 3), but
to 29.1% and the biexponential fluorescence lifetimes changed
slightly to 1.93 and 0.91 ns, respectively.
The observed change in the fluorescence intensity of 1a is
probably a result of static enhancement by the formation of a
hydrogen-bonded cage−amino acid adduct that may induce
changes in the structure of the emitting species, such as a
change and/or rigidification of the conformation and excimer
formation.13,14 The static complexation is suggested by the
nearly stable fluorescence lifetimes of 1a before and after
titration. Because the noncovalent interactions of 1a with
amino acid enantiomers generate different diastereomeric
complexes, a distinct fluorescence enhancement is detected.
The formation of a stable adduct complex between (R)-1a and
L-Ala is supported by ESI-MS, which showed five prominent
peaks at m/z 903.0, 993.4, 1085.8, 1173.5, and 1264.0,
corresponding to [Zn8L4Cl5 + 4L-Ala + H]4+, [Zn8L4Cl5 +
8L-Ala + H]4+, [Zn8L4Cl5]3+, [Zn8L4Cl5 + 3L-Ala]3+, and
1
[Zn8L4Cl5 + 6L-Ala]3+, respectively. H NMR analysis also
suggested the complexation of 1a and Ala (Figure S28).
Other chiral amino acids such as Phe and Val were also found
to enhance the fluorescence of (R)-1a but gave lower
enantioselectivity than Ala, probably because the steric
hindrance of bulky phenyl groups impaired the chiral
recognition ability. It is interesting to note that very weak
enantioselectivity was observed for the luminescence enhance-
ment of (R)-1a by the N-Boc-protected amino acid N-Boc-Ala,
which supports the involvement of amino groups in the
formation of a ground-state hydrogen-bonded complex and an
excited-state proton-transfer complex. Many synthetic receptors
for chiral recognition of amino acids have been synthesized;
however, no assembled helicates or cages have been reported to
exhibit such enantioselectivity.1c,15,7
The separation capabilities of the crystalline cages toward 1-
(methylsulfinyl)benzene, 1-phenylethanol, 1-phenylpropanol,
and 1-phenylethylamine were evaluated by immersing the
apohost samples in diethyl ether solutions of the racemic
adsorbates. Chiral HPLC analyses of the four guests from (R)-
1a desorbed yielded 20.1, 4.7, 13.0 and 14.6% ee, respectively,
with the S enantiomers being in excess. Compared with (R)-1a,
(R)-1b exhibited enhanced enantioselectivities for (S)-1-
(methylsulfinyl)benzene (37.5% ee), (S)-1-phenylethanol
(13.8% ee), and (S)-1-phenylpropanol (18.6% ee) but a
decreased enantioselectivity for (S)-1-phenylethylamine
(10.3% ee). Their different recognition enantioselectivities
may be due to their different porous structures, in which 1a is
built of helicate cages interconnected by channels whereas 1b is
built of helicate cages interconnected by pentahedral cages.
Neither adsorbent could take up larger substrates such as 1-(1-
naphthyl)ethanol and 1-(1-naphthyl)ethylamine because of
their small channels. Control experiments showed that the H2L
ligand itself could not resolve the enantiomers of the examined
substrates under otherwise identical conditions, indicating that
the well-organized structure of the chiral centers of the
metallosalan units controls the enantioselective recognition
process. The adsorbents could be regenerated after adsorption
and reused without performance loss. For example, the second
and third recycled samples of 1b provided 31.0 and 32.5% ee,
respectively, for separating racemic 1-(methylsulfinyl)benzene.
PXRD showed that the recycled sample retained crystallinity
after three runs but that the structure became seriously
distorted. Alternatively, the crystalline samples could easily be
recycled by recrystallization from THF at room temperature or
65 °C. The supramolecular engineering of permanent porous
Figure 3. Fluorescence enhancement of (R)-1a (1.0 × 10−5 M in
THF) upon titration with (a) D-Ala (1× 10−3 M) and (b) L-Ala (1×
10−3 M) and (c) the Benesi−Hildebrand plot.
the increase caused by L-Ala was greater than that by D-Ala,
implying enantioselectivity in the fluorescence recognition. The
fluorescence intensity of 1a was maximally increased to 7.6 and
7.0 times that of the original value by L- and D-Ala, respectively.
Figure 3c shows Benesi−Hildebrand plots for (R)-1a (1.0 ×
10−5 M) in the presence of L- and D-Ala in THF. The
association constants KBH were found to be 12363 M−1 with L-
Ala and 3352 M−1 with D-Ala, giving an enantioselectivity factor
K
BH(R−L)/KBH(R−D) of 3.69. The opposite trend in
enantioselectivity was observed for the enhancement of (S)-
1a by Ala, for which the enantioselectivity factor KBH(S−D)/
K
BH(S−L) was 3.57, further confirming a chirality-based
luminescence-enhancing selectivity. Notably, the free ligand
H2L showed no obvious enantioselectivity, suggesting that the
helical structure confers a better-defined chiral environment.13
After titration, the quantum yield of (R)-1a increased from 5.60
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665. (c) Albrecht, M.; Burk, S.; Weis, P. Synthesis 2008, 2963.
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remains challenging.6b Despite the limited enantioselectivity,
the present crystalline cage materials represent a new
generation of chiral porous solids that are capable of chiral
separation of different types of molecules simultaneously.5f,16
Further studies on separating other racemic compounds and
understanding the enantioselective processes are underway.
In conclusion, we have presented the assembly of homochiral
quadruple-stranded helicate cages from metallosalans and
demonstrated their enantioselective abilities to recognize
enantiomers of amino acids via fluorescence enhancement in
solution and to separate small racemic organic molecules by
adsorption in the crystalline state. The readily tunability of such
a modular approach based on metallosalan units promises to
lead a number of chiral assemblies with unique and practically
useful enantioselective functions.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
yongcui@sjtu.edu.cn.
■
Notes
(9) (a) Baleizao, C.; Garcia, H. Chem. Rev. 2006, 106, 3987.
(b) Clever, G. H.; Polborn, K.; Carell, T. Angew. Chem., Int. Ed. 2005,
44, 7204.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSFC (21025103 and
20971085), the “973” Program (2009CB930403), and the
Shanghai Science and Technology Committee (10DJ1400100).
(11) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.
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Chem. Soc. 2004, 126, 15211. (b) Dietrich-Buchecker, C.; Rapenne, G.;
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(14) Preliminary X-ray analysis of a single crystal obtained by slow
evaporation of a solution of 1a and Ala showed that it is isostructural
to 1a, in which the Cl anions bound to Zn ions are not replaced by
Ala. It is likely that the ligand displacement mechanism13 might not be
responsible for the fluorescence enhancement, although it could not be
excluded absolutely.
(15) (a) Soloshonok, V. A.; Ellis, T. K.; Ueki, H.; Ono, T. J. Am.
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