Enantioselective sorption of alcohols in a homochiral metal-organic framework

Article abstract of DOI:10.1039/c1cc16209h

Single-crystal X-ray diffraction study reveals the host-guest interactions between a homochiral metal-organic framework and two enantiomers of a chiral alcohol providing the key driving force for the enantioselective sorption of alcohols in the framework.

Full text of DOI:10.1039/c1cc16209h

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COMMUNICATION
Enantioselective sorption of alcohols in a homochiral metal–organic
frameworkw
ab
ac
a
c
Kyungwon Suh,z Maxim P. Yutkin,z Danil N. Dybtsev,* Vladimir P. Fedin and
ab
Kimoon Kim*
Received 6th October 2011, Accepted 9th November 2011
DOI: 10.1039/c1cc16209h
Single-crystal X-ray diffraction study reveals the host–guest inter-
actions between a homochiral metal–organic framework and two
enantiomers of a chiral alcohol providing the key driving force for
the enantioselective sorption of alcohols in the framework.
an enantiopure porous framework and two different enantiomers
of a chiral molecule. Comprehensive studies of the intermolecular
interactions would allow the prediction of the enantioselective
properties of a homochiral MOF on one hand, as well as a
suitable framework design for the best resolution of a given chiral
substrate on the other.
Metal–organic frameworks (MOFs) or porous coordination
polymers (PCPs) are an emerging class of porous materials
with rich design possibilities, tunable properties and various
Recently, several groups attempted to simulate optimized
geometries of chiral substrates to fit in homochiral frameworks
1
2,21
1–5
and to calculate their interaction energies.
According to
applications. Enantiopure (homochiral) MOFs are synthesized
by polycondensation of inorganic and organic building units,
where at least one of them (typically the organic ligand) has to be
these reports, the calculated energy difference in intermolecular
interactions between a porous framework and different
ꢀ1
ꢀ1
6–8
enantiomers varies from 1–5 kJ mol to 32 kJ mol , depending
on host–guest systems. In the latter case, such a difference
results in up to 60% enantiomeric excess (ee) in enantioselective
sorption, which allows partial or even full separation of a mixture
of chiral substrates. However, such quantum mechanical
calculations are extremely difficult and time-consuming, which
poses a serious drawback for this approach. The interatomic
van der Waals forces are known to be the weakest interactions,
therefore each small correction, otherwise insignificant, needs
to be taken into account as even tiny deviations in energy
calculations will strongly affect the result.
enantiopure.
Having a regular structure with uniform pore
size, chemical functionality and chiral centers, homochiral MOFs
are attractive especially for enantioselective applications, such
9
–12
13
separation and catalysis.
14–19
as sorption,
Although the
synthesis of homochiral MOFs with desired properties and
functions is still challenging, a number of rationally designed
homochiral MOFs with tunable structural features have been
1
2,19,20
reported recently.
Despite remarkable progress in the
chemistry of homochiral porous materials, there are only a
handful of studies of intermolecular interactions between
1
1,12,21
chiral porous frameworks and substrates.
Many useful
A more straightforward approach to study the inter-
molecular interactions is an in situ structural investigation of
host–guest complexes, e.g. by single-crystal X-ray diffraction
techniques. Unfortunately, this approach also has a number of
limitations due to incomplete guest exchange or severe disorder
of chiral substrate molecules inside a porous coordination
framework. To date, there are only a few reports describing
the synthesis and X-ray diffraction study of homochiral MOF
inclusion compounds with only one enantiomer of a chiral
properties of homochiral MOFs, such as chiral sorption and
catalysis, arise from enantioselective recognition based on
intermolecular interactions. Apparently, this recognition is
attributed to the difference in host–guest interaction between
a
Division of Advanced Materials Science, Pohang University of
Science and Technology, San 31 Hyojadong, Pohang 790-784,
Republic of Korea. E-mail: danil@postech.ac.kr,
kkim@postech.ac.kr; Fax: +82-54-279-8739, +82-54-279-8128;
Tel: +82-54-279-8152, +82-54-279-2113
Center for Smart Supramolecules, Department of Chemistry,
1
0,22,23
guest molecule.
To the best of our knowledge, there are
b
no comprehensive structural analyses of such host–guest
complexes with both enantiomers of a chiral substrate, which
is indispensable to understanding the nature of enantioselective
discrimination. Here, we present the enantioselective sorption
Pohang University of Science and Technology, San 31 Hyojadong,
Pohang 790-784, Republic of Korea. E-mail: kkim@postech.ac.kr;
Fax: +82-54-279-8128; Tel: +82-54-279-2113
Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the
Russian Academy of Sciences, Pr. Lavrentieva 3, Novosibirsk
c
2
properties of the homochiral MOF [Zn (bdc)(S-lac)(dmf)] (1)
630090, Russian Federation. E-mail: cluster@niic.nsc.ru;
Fax: +7-383-3309489
w Electronic supplementary information (ESI) available: Experimental
details for sorption experiments, analysis of 1ꢁguest single crystals.
CCDC 847107 (1ꢁR-PhEtOH) and 847108 (1ꢁS-PhEtOH). For ESI and
crystallographic data in CIF or other electronic format see DOI:
towards chiral alcohol substrates and in situ characterization
of its host–guest complexes 1ꢁR-PhEtOH and 1ꢁS-PhEtOH by
single-crystal X-ray diffraction using synchrotron radiation, and
Differential Scanning Calorimetric (DSC) methods (S-lac =
L-(ꢀ)-lactate; bdc
= 1,4-benzenedicarboxylate; dmf =
10.1039/c1cc16209h
0
z K. Suh and M. P. Yutkin contributed equally to this work.
N,N -dimethylformamide; PhEtOH = 1-phenyl-1-ethanol).
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 513–515 513

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Most interestingly, the OꢁꢁꢁH–C interactions between substrates
and pendant dmf ligands appear to play a critical role in the
enantioselective properties of 1. This is consistent with previous
quantum calculations for 1ꢁPhSOMe complexes, where similar
samples, we can estimate that the interaction between the
ꢀ1
chiral framework and S-PhEtOH is 19 kJ mol stronger
than that for R-PhEtOH. This result is consistent with the
1
theoretical study reported earlier.
2,21
12
interactions involving dmf ligands were found.
Being a convenient substrate and showing the highest sorption
enantioselectivity among the simple alcohols, PhEtOH was
chosen as a model guest molecule for a detailed study of the
host–guest interactions in the porous channels of zinc(II) lactate
terephthalate framework 1. Also, PhEtOH as well as other
alcohol substrates studied here are useful precursors for a
The enantioselective sorption properties of homochiral
framework 1 toward chiral alcohols were studied by immersing
as-synthesized 1ꢁDMF single-crystals in their racemic mixtures,
followed by chiral HPLC and/or optical rotation measure-
ments. Typically, it takes 18–24 h to reach equilibrium in
enantioselective sorption (see ESIw) even for bulkier substrates.
Elemental analysis, NMR spectroscopy, TG and single-crystal
X-ray diffraction analysis clearly suggested the complete guest
exchange and formation of target host–guest complexes, with
one alcohol molecule per formula unit of 1, with the exception
of 1-phenyl-1-propanol, the larger size of which results in a final
composition of ca. 0.75 guest molecules per formula unit of 1.
We should note that only guest DMF molecules in 1ꢁDMF are
exchanged with alcohol substrates, while the dmf ligands
coordinated to the metal–organic framework remain intact.
Previous results demonstrated that 1 has great enantioselectivity
towards chiral sulfoxides with up to 60% ee for methylphenyl-
24,25
number of chiral drugs
such as salbutamol or epinephrine,
2
6
the optical isomers of which have different bioactivity and,
thus, their purification is highly critical. The single crystals of
host–guest complexes 1ꢁR-PhEtOH and 1ꢁS-PhEtOH were
prepared by soaking single crystals of as-synthesized 1ꢁDMF
in optically pure R- or S-PhEtOH, respectively, for 11 days
to allow full guest substitution and reach diffusion equili-
brium along the channels. The single-crystalline specimens of
1ꢁR-PhEtOH (and 1ꢁS-PhEtOH) were collected and examined
by single crystal X-ray diffraction analysis using the synchro-
tron radiation at Pohang Accelerator Laboratory (PAL).y The
data revealed formation of the desired complexes where guest
alcohol molecules R-PhEtOH or S-PhEtOH occupy the chiral
channels of the zinc(II) lactate terephthalate 1 host framework.
The inclusion of the guest molecules does not cause any
1
2
sulfoxide. As shown in Table 1, the ee values for different
alcohol substrates turned out to be relatively modest with the
highest ee of 21% (in favor of S-isomer) for racemic PhEtOH.
We investigated the thermal decomposition of both
2
7
notable distortion in the original host structure of 1ꢁDMF.
1
ꢁS-PhEtOH and 1ꢁR-PhEtOH by DSC in the temperature
Upon inclusion, both R- and S-PhEtOH occupy similar positions
inside the porous channels of 1 (Fig. 1).
range 100–300 1C. The decomposition is associated with the
release of coordinated dmf and guest phenylethanol molecules.
As expected, both processes are endothermic with a specific
However, the most notable difference is the direction of the
OH group of the chiral guests. In the complex 1ꢁR-PhEtOH,
the OH group is interacting with the carboxylate oxygen
ꢀ1
heat of 173 or 192 kJ mol
for 1ꢁR-PhEtOH and 1ꢁS-
˚
PhEtOH, respectively (see ESIw for details). Assuming that
the desorption of dmf contributed more or less the same
amount of enthalpy change to the specific heat for both
atoms of the framework (d(O1Rꢁ ꢁ ꢁO2B) = 3.255(4) A,
˚
3.342(4) A) whereas in the complex
d(O1Rꢁ ꢁ ꢁO4B
)
=
1ꢁS-PhEtOH, the OH group is interacting with the carbon
˚
atoms of the coordinated dmf (d(O1Sꢁ ꢁ ꢁC3D) = 3.327(9) A),
Table 1 Stereoselective sorption of 1 toward chiral alcohols
which explains the enantioselective sorption of the chiral
alcohol in 1. Although the above-mentioned distances are
the shortest contacts between the substrate and host, they
are significantly longer than typical hydrogen bond interactions,
which indicates that the overall affinity of 1 towards the chiral
alcohol is relatively weak and explains the modest enantioselective
sorption of the chiral framework. We also note that the
coordinated dmf ligands have already been proved to be involved
c
d
ee , %
Substrate
Formula
a
1ꢁBu
14(R)
a
1ꢁMeBu
7(R)
12
in the enantioselective recognition of chiral sulfoxides by 1.
According to the ab initio quantum calculations, the framework
a
21(S) /20(S)
b
1ꢁPhEtOH
a
12(S) /11(S)
b
1ꢁ(PhPrOH)0.75
Fig. 1 Structure of host–guest complexes 1ꢁR-PhEtOH (left) and
a
b
c
Determined by polarimetry. Determined by HPLC. Bu: 2-butanol;
1
ꢁS-PhEtOH (right). C: gray, O: red, N: blue, Zn: green, R-PhEtOH:
MeBu: 2-methyl-1-butanol; PhEtOH: 1-phenyl-1-ethanol; PhPrOH:
1
d
-phenyl-1-propanol. Letters in brackets specify the preferable isomer.
sky-blue, S-PhEtOH: brown. Shortest intermolecular interactions are
indicated by dashed yellow lines.
5
14 Chem. Commun., 2012, 48, 513–515
This journal is c The Royal Society of Chemistry 2012

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1
has higher affinity towards a particular enantiomer, the
6 K. P. Bryliakov and E. P. Talsi, in Transition Metal Chemistry:
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interactions between dmf and sulfoxide molecules. In our
case, similar dmfꢁꢁꢁguest H-bond interactions appear to be
responsible for the enantioselective sorption of 1 in favor of
S-PhEtOH.
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1
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participate in the formation of the scaffold of the homochiral
metal–organic framework, they do play a crucial role in the
stereoselective recognition. Such a phenomenon could hardly
be predicted a priori, but was independently confirmed by
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study for guest molecules of different nature. This clearly
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We are grateful to WCU Programs of MOEST (Project
No. R31-2008-000-10059-0) for support of this work. X-Ray
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2
2
F
with appropriate software implemented in SHELXTL program
25NO Zn , M = 578.17,
(No. 19), a = 10.334(2), b = 11.672(2), c =
package. Crystal data for 1ꢁR-PhEtOH. C22
H
9
2
orthorhombic, P2
1 1 1
2 2
3
ꢀ3
,
˚
˚
2
0.341(4) A, V = 2453.5(9) A , Z = 4, T = 100 K, rcalcd = 1.565 g cm
ꢀ1
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GOF = 1.136, Flack = 0.058(11). Crystal data for 1ꢁS-PhEtOH.
25NO Zn , M = 578.17, orthorhombic, P2 (No. 19), a =
0.337(2), b = 11.649(2), c = 20.464(4) A, V = 2464.2(9) A , Z = 4,
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H
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1 1 1
2 2
3
˚
˚
1
ꢀ3
ꢀ1
,
T = 100 K, rcalcd = 1.558 g cm , m(synchrotron) = 1.997 mm
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47107 (1ꢁR-PhEtOH) and CCDC 847108 (1ꢁS-PhEtOH) contain the
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1
This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 513–515 515