Enantioselective recognition and separation of racemic 1-phenylethanol by a pair of 2D chiral coordination polymers

Article abstract of DOI:10.1021/ic4021367

A pair of 2D chiral coordination polymers were constructed through the self-assembly of a chiral metal-camphor-10-sulfonate salt and a bidentate linker, which show selective inclusion of S and R enantiomers of 1-phenylethanol respectively with an enantioselectivity of 9:1.

Full text of DOI:10.1021/ic4021367

Communication
pubs.acs.org/IC
Enantioselective Recognition and Separation of Racemic
1‑Phenylethanol by a Pair of 2D Chiral Coordination Polymers
Zi-Jian Li, Jia Yao, Qian Tao, Long Jiang,* and Tong-Bu Lu*
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies,
and School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China
S
* Supporting Information
Herein, we present the synthesis and structures of a pair of 2D
ABSTRACT: A pair of 2D chiral coordination polymers
were constructed through the self-assembly of a chiral
metal-camphor-10-sulfonate salt and a bidentate linker,
which show selective inclusion of S and R enantiomers of
1-phenylethanol respectively with an enantioselectivity of
9:1.
CCPs, {[Cd2(L-cps)4(bix)4]·(CH2Cl2)2(EtOH)(H2O)}n (L-1)
and {[Cd2(D-cps)4(bix)4]·(CH2Cl2)2(EtOH)(H2O)}n (D-1),
which were constructed by the self-assembly of a bix ligand
with Cd(L-cps)2·(H2O)6 and Cd(D-cps)2·(H2O)6, respectively
[bix = 1,4-bis(imidazol-1-yl-methyl)benzene; see Figure 1].
hiral coordination polymers (CCPs) have been attracting
Cunprecedented attention because of their potential
applications in enantioselective separation, heterogeneous
asymmetric catalysis, and nonlinear optics.1−3 Recently, CCPs
have been used for chromatography and membrane-based
separation, exhibiting a new generation of chiral separation
materials.4 To date, the most effective strategy to construct CCPs
is to directly utilize the chiral ligand as the starting material;1 for
instance, Rosseinsky and co-workers used chiral nickel aspartate
[Ni(L-asp)] and 4,4′-bipyridine as building units to construct a
neutral chiral 3D framework with 1D open chiral channels, which
exhibits the ability for enantioselective separation of diols.2c The
advantage of using chiral metal salts as starting materials to
construct CCPs is that CCPs are neutral, preventing open-
channel blockage by counteranions.
Figure 1. Construction of L-1 through the self-assembly of Cd(L-cps)2·
(H2O)6 with the bix ligand. The included guest molecules within the
interlayer are omitted for clarity.
Despite much progress in the chemistry of chiral porous
materials,5 the mechanism for enantioselective separation of
chiral guest molecules by CCPs has not been fully understood.
Single-crystal structural analysis can offer a straightforward
explanation to understand the enantioselective discrimination of
CCPs toward chiral guest molecules, which can further help to
design new CCPs effectively. However, up to now, there are only
a few examples that show the intermolecular interactions
between the chiral porous frameworks and chiral guest molecules
through X-ray crystal structural analysis3 and quantum-
mechanical simulation.6 In most cases, the guest molecules are
disordered, and it is hard to see such host−guest interactions.
On the other hand, during the synthesis of porous CCPs using
chiral ligands as starting materials, undesirable achiral crystalline
solids are often obtained owing to racemization of chiral
ligands.2c,7 Chiral camphor derivatives feature two chiral carbon
atoms in their rigid skeleton and thus are unlikely to undergo
racemization, which could become an appropriate chiral building
unit to construct CCPs.8 Although chiral CCPs based on
camphor-10-sulfonic acid (cpsH) have been reported9 and cpsH
has been also widely used as a chiral resolution reagent,10
enantioselective separation by CCPs constructed by cpsH still
remains unexplored.9a
Dissolving L-1 and D-1 in racemic 1-phenylethanol (PEA) and
CHCl3 (1:1) generated {[Cd2(L-cps)4(bix)4]·(PEA)4}n (L-2)
and {[Cd2(D-cps)4(bix)4]·(PEA)4}n (D-2), respectively. The
results of structural analysis reveal that L-2 and L-2 show a
selective inclusion of (R)-PEA and (S)-PEA, respectively, with an
enantioselectivity of 9:1.
The reaction of chiral L-cpsH with CdO in water produced
Cd(L-cps)2·(H2O)6, which was confirmed by single-crystal X-ray
diffraction analysis. Single crystals of L-1 were obtained by
diffusing an ethanol solution of Cd(L-cps)2·(H2O)6 into the
CH2Cl2 solution of bix (see the Supporting Information, SI). The
result of X-ray crystal structural analysis reveals that L-1
crystallizes in chiral space group P1. The asymmetric unit
consists of two crystallographic independent CdII ions (Figure S1
in the SI), in which each CdII is six-coordinated to four nitrogen
atoms from four bix ligands and two sulfonate oxygen atoms
from two individual L-cps− anions, forming a slightly distorted
N4O2 octahedral geometry. The Cd−O and Cd−N distances are
in the normal range.11 In L-1, CdII ions are connected by bix
ligands to generate a layered structure, with a 4,4 network in the
Received: August 20, 2013
© XXXX American Chemical Society
A
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Inorganic Chemistry
Communication
ac plane (Figure S2 in the SI), and the monocoordinated L-cps−
anions locate on and below the layer to induce a homochiral
layer. The adjacent chiral layers are connected via interdigitation
of L-cps− groups along the b axis to generate the 3D chiral
framework of L-1 (Figure S3 in the SI), and the guest molecules
locate within the interlayer.
(Figure 3). After bonding of the PEA molecules within the
interlayer, the distance between the adjacent layers extends from
The results of solid circular dichroism (CD) spectroscopy
measurements demonstrate that the chirality of L-1 comes from
the starting material of L-cps− anions (Figure S4 in the SI). The
result of thermogravimetric analysis (TGA) indicates that the
guest molecules in L-1 can be removed below 160 °C (Figure S5
in the SI), and the framework can be stable up to 300 °C.
Variable-temperature powder X-ray diffraction (PXRD) of L-1
was performed from room temperature to 300 °C, showing that
the position of the peak at 6.7° gradually moved to higher angles
upon heating (Figure S6 in the SI), demonstrating that the
framework of L-1 shrinks after the loss of guest molecules within
the interlayer.
To see if chiral L-1 has the ability for enantioselective
separation of racemic guest molecules, the crystals of L-1 were
immersed in a mixture of racemic PEA and CHCl3. However, the
quality of the generated crystals is not good enough for X-ray
crystal structural analysis. Thus, the crystals of L-1 were dissolved
in a mixture of racemic PEA and CHCl3 under heating and
evaporated slowly at room temperature, and single crystals of L-2
were obtained. L-2 also crystallizes in the chiral space group P1
and shows a chiral layered structure similar to that of L-1. In
contrast to L-1, one of the imidazole rings of the bix ligand
bonded to Cd2 is disordered in L-2, with an occupancy of 0.6:0.4
for two different positions (Figure 2), and this leads to one L-cps−
Figure 3. Structure of L-2 showing hydrogen-bonding interactions
between the L-cps− anions and chiral PEA molecules [3.62(R)-PEA +
0.38(S)-PEA in one asymmetric unit].
13.50 Å in L-1 to 16.55 Å in L-2 (Figure S8 in the SI). Owing to
the chiral environment induced by L-cps− anions, the three
ordered L-cps− anions in L-2 selectively bond three (R)-PEA
rather than (S)-PEA, while the fourth disordered L-cps− bonds
both (R)- and (S)-PEA enantiomers (Figure 2), with a ratio of
(R)-PEA/(S)-PEA equal to 0.6:0.4. Also, this ratio is the same as
that of the positional occupancy of 0.6:0.4 for the disordered L-
cps− anion. Obviously, it is the disorder of the L-cps− anion that
leads the selectivity of (R)-PEA over (S)-PEA to decrease from
1:0 to 0.6:0.4. The above results demonstrate that the 2D CCP of
L-1 has the ability for enantioselective separation of racemic PEA
molecules within its interlayer, with an ee value of 80% and a
selectivity of 9:1 for (R)-PEA over (S)-PEA.
Similarly, the reaction of Cd(d-cps)2·(H2O)6 with the bix
ligand led to the formation of D-1, and dissolving D-1 in racemic
PEA and CHCl3 (1:1) generated D-2. The results of X-ray crystal
structural analysis demonstrate that the structures of D-1 and D-2
(Figure S9 in the SI) are similar to those of L-1 and L-2,
respectively. One of the imidazole rings of the bix ligand and one
L-cps− bonded to Cd2 in D-2 are also disordered, with a close
positional occupancy of 0.6:0.4. In contrast to L-2, D-2 shows a
selective inclusion of the (S)-PEA enantiomer over the (R)-PEA
enantiomer, with the same ee value of 80% and a selectivity of 9:1
for (S)-PEA over (R)-PEA.
Once L-2/D-2 was immersed in CHCl3, the included PEA
guest molecules can be readily released, allowing its potential
application for enantioselective separation of racemic PEA. The
experiment for enantioselective separation of racemic PEA was
performed by suspending the crystalline solid sample of L-1/D-1
in a mixture of racemic PEA and CHCl3 (1:1) at −18 °C for 1
week; the resulting solid was filtered with filter paper and dried in
air. The solid was then extracted with CHCl3, and the contents of
the (R)- and (S)-PEA enantiomers were analyzed by chiral high-
performance liquid chromatography (HPLC). The ee value for
(R)-PEA over (S)-PEA for L-2 is 28.2%, and the ee value for (S)-
PEA over (R)-PEA for D-2 is 27.3% (see the SI). The lower ee
value is attributed to the residual of racemic PEA on the surface of
the crystalline solid of L-2/D-2. Similar enantioselective
Figure 2. Structure of the disordered L-cps− anion and PEA molecule
around Cd2 in L-2 (the part of the occupancy of 0.4 is drawn as dotted
lines).
anion bonded to Cd2 being disordered with the same occupancy
of 0.6:0.4 (Figure 2). The other three L-cps− anions bonded to
Cd1 and Cd2 are ordered, and their positions are fixed by
forming weak O···H−C hydrogen bonds between the uncoordi-
nated sulfonate oxygen atoms of L-cps− and the carbon atoms of
ordered imidazole rings of bix ligands (Figures 2 and S7 in the
SI), with O···H−C distances of 2.25−2.53 Å. Interestingly, each
L-cps− anion connects one PEA molecule within the interlayer
through O···H−O hydrogen bonding between the uncoordi-
nated sulfonate oxygen atom of L-cps− and the oxygen atom of
the PEA molecule, with O···H−O distances of 1.95−2.09 Å
B
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Communication
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ASSOCIATED CONTENT
* Supporting Information
X-ray crystallographic data in CIF format (CCDC 952244−
952248), experimental details, single-crystal data, IR, CD, TGA,
PXRD patterns, HPLC graphs, and fragments of structures. This
material is available free of charge via the Internet at http://pubs.
acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail: cesjl@mail.sysu.edu.cn. Fax: +86-20-84112921.
*E-mail: lutongbu@mail.sysu.edu.cn. Fax: +86-20-84112921.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(8) (a) Zhang, J.; Bu, X. Angew. Chem., Int. Ed. 2007, 46, 6115−6118.
(b) Zhang, J.; Chen, S.; Zingiryan, A.; Bu, X. J. Am. Chem. Soc. 2008, 130,
17246−17247.
This work was supported by 973 Program of China (Grant
2012CB821705), the NSFC (Grants 21001119, 21121061, and
91127002), and the NSF of Guangdong Province (Grant
S2012030006240).
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