Metal-organic framework Co(D-cam)1/2(bdc)1/2(tmdpy) for improved enantioseparations on a chiral cyclodextrin stationary phase in gas chromatography

Article abstract of DOI:10.1002/cplu.201402067

Initial efforts to combine a chiral metal-organic framework (MOF), Co(D-Cam)_1/2(bdc)_1/2(tmdpy) (D-Cam=D-camphoric acid, bdc=1,4-benzenedicarboxylic acid, tmdpy=4,4′-trimethylenedipyridine), with peramylated β-cyclodextrins to investigate whether the use of a MOF can enhance enantioseparations on a cyclodextrin stationary phase are reported. Compared with columns of peramylated β-cyclodextrin incorporated in a MOF containing sodium chloride, the column of peramylated β-cyclodextrin+MOF shows excellent selectivity for the recognition of racemates, and higher resolutions are achieved on the peramylated β-cyclodextrin+MOF stationary phase. Experimental results indicate that the use of Co(D-Cam) _1/2(bdc)_1/2(tmdpy) can improve enantioseparations on peramylated β-cyclodextrins. This is the first report that chiral MOFs can improve enantioseparations on a chiral stationary phase for chromatography. Copyright

Full text of DOI:10.1002/cplu.201402067

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DOI: 10.1002/cplu.201402067
Metal–Organic Framework Co(d-Cam)1/2(bdc)1/2(tmdpy) for
Improved Enantioseparations on a Chiral Cyclodextrin
Stationary Phase in Gas Chromatography**
Hong Liu,[a] Sheng-Ming Xie,[a] Ping Ai,[a] Jun-Hui Zhang,[b] Mei Zhang,[b] and Li-
Ming Yuan*[a, b]
Initial efforts to combine a chiral metal–organic framework
(MOF), Co(d-Cam)1/2(bdc)1/2(tmdpy) (d-Cam=d-camphoric acid,
bdc=1,4-benzenedicarboxylic acid, tmdpy=4,4’-trimethylene-
dipyridine), with peramylated b-cyclodextrins to investigate
whether the use of a MOF can enhance enantioseparations on
a cyclodextrin stationary phase are reported. Compared with
columns of peramylated b-cyclodextrin incorporated in a MOF
containing sodium chloride, the column of peramylated b-cy-
clodextrin+MOF shows excellent selectivity for the recognition
of racemates, and higher resolutions are achieved on the pera-
mylated b-cyclodextrin+MOF stationary phase. Experimental
results indicate that the use of Co(d-Cam)1/2(bdc)1/2(tmdpy) can
improve enantioseparations on peramylated b-cyclodextrins.
This is the first report that chiral MOFs can improve enantiose-
parations on a chiral stationary phase for chromatography.
Introduction
Chirality is one of the major phenomena in nature and it is
also an essential feature of biological systems.[1] For many
chiral drugs, different enantiomers are known to have different
physiological and therapeutic effects. Usually, only one of the
two enantiomers is pharmaceutically active, whereas the other
may be inactive or toxic.[2] The separation of chiral compounds
is of great importance in many industries.
different derivatized CDs to separate enantiomers in TLC,
HPLC, and GC.[8]
Metal–organic frameworks (MOFs), which are a subclass of
the broader family of coordination polymers, consist of an ex-
tended network of metal ions coordinated to multidentate or-
ganic molecules.[9] MOFs have been shown to hold great prom-
ise for applications in catalysis, adsorption, microreactors,
anion exchange, and many other areas owing to their fascinat-
ing structures.[10] In recent years, a large number of chiral
MOFs have been synthesized.[11] The selective adsorption and
high thermal and chemical stability of MOFs have also made
these materials useful for GC[12] and HPLC.[13] Chiral MOFs show
great potential for the separation of enantiomers in GC and
HPLC.[14] The marriage of the unique properties of chiral MOFs
and excellent features of peralkylated b-CDs to develop novel
stationary phases should be promising for enhanced GC sepa-
ration of enantiomers.
Cyclodextrins (CDs), which are cyclic oligosaccharides com-
posed of d-(+)-glucopyranose units, can form host–guest com-
plexes with organic compounds. They can be derivatized with
different functional groups at different positions. Much prog-
ress has been made since b-CD was used to separate some op-
tical isomers on capillary columns[3], and Koenig and co-work-
ers introduced hydrophobic groups into CDs in 1988.[4] Owing
to their high melting points, Schurig et al. dissolved peralkylat-
ed CDs in polysiloxanes, such as OV-1701, to obtain a high
column efficiency.[5] Commercial columns of permethylated b-
CDs have been achieved.[6] CD derivatives are widely used and
show highly selective separations, especially for positional iso-
mers and enantiomers.[7] In addition, Armstrong et al. also used
For the first time, we report that MOFs can improve the
enantioseparation of a chiral stationary phase in chromatogra-
phy. Herein, we report our efforts to a combine chiral MOF,
Co(d-Cam)1/2(bdc)1/2(tmdpy) (d-Cam=d-camphoric acid, bdc=
1,4-benzenedicarboxylic acid, tmdpy=4,4’-trimethylenedipyri-
dine), with peramylated b-CDs to investigate whether the use
of a chiral MOF can enhance enantioseparations on a chiral CD
stationary phase. The Co(d-Cam)1/2(bdc)1/2(tmdpy) compound
possesses a 3D framework containing enantiopure building
blocks embedded in intrinsically chiral topological nets and
has excellent thermal stability, a high surface area, and unusual
integrated homochirality features. Moreover, the stationary
phase has three homochiral features: a 3D intrinsically homo-
chiral net, homohelicity, and enantiopure molecular chirality
(Figure S1 in the Supporting Information). The MOF Co(d-
Cam)1/2(bdc)1/2(tmdpy) was synthesized by heating a mixture of
[a] H. Liu, S.-M. Xie, Prof. P. Ai, Prof. L.-M. Yuan
Department of Chemistry
Yunnan Normal University
Kunming 650500 (P. R. China)
Fax: (+86)871-65941088
E-mail: yuan_limingpd@aliyun.com
[b] Dr. J.-H. Zhang, Dr. M. Zhang, Prof. L.-M. Yuan
Department of Chemistry
East China Normal University
Shanghai 200241 (P. R. China)
[**] d-Cam=d-camphoric acid, bdc=1,4-benzenedicarboxylic acid, tmdpy=
4,4’-trimethylenedipyridine.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201402067.
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d-camphoric acid, 1,4-benzenedicarboxylic acid, 4,4’-
trimethylenedipyridine, and CoCO3 in ultrapure water
at 1208C for 5 days, according to the method report-
ed by Zhang et al,[15] and purple crystals were ob-
tained (see the Supporting Information). The success-
ful synthesis of Co(d-Cam)1/2(bdc)1/2(tmdpy) was con-
firmed by powder XRD analysis (Figure S2 in the Sup-
porting Information). Meanwhile, the thermogravi-
Table 2. McReynolds constants of columns A, B, and C at 1208C.
Column Benzene 1-Butanol 2-Pentanone 1-Nitropropane Pyridine Av.
A
B
C
78
25
34
393
113
105
249
62
62
225
101
100
208
92
149
230.6
78.6
90
metric analysis curve of the Co(d-Cam)1/2(bdc)1/2(tmdpy)
sample showed that the MOF was thermally stable below
3008C, and therefore, suitable for GC usage (Figure S3 in the
Supporting Information).
lytes.[17] The McReynolds constants show that the polarity of
the MOF is moderate and the peramylated b-CD is a low-polar
stationary phase.
The most important advantage of the novel stationary
phase is its enantioselective and resolving abilities in GC. The
following 10 racemates were separated on columns A, B, and
C: (Æ)-2-hexanol, (Æ)-linalool, (Æ)-citronellal, (Æ)-methyl l-b-hy-
droxyisobutyrate, (Æ)-limonene, (Æ)-rose oxide, (Æ)-dihydro-
carvyl acetate, (Æ)-menthol, dl-valine, and dl-leucine; details
are provided in Table 3. The enantiomeric resolutions on three
different stationary phases are exhibited in Figure S4 in the
Supporting Information. Table 3 shows the temperatures, re-
tention factors (a1’) for the first eluted enantiomers, and sepa-
ration factors (a). The retention factor k1’ is (t1Àt0)/t0 and the
To investigate whether the use of Co(d-Cam)1/2(bdc)1/2(tmdpy)
could improve enantioseparations on a peramylated b-CD sta-
tionary phase, three capillary columns were prepared for GC:
column A (20 m longꢁ0.25 mm i.d.) contained the chiral MOF,
column B (20 m longꢁ0.25 mm i.d.) contained sodium chloride
and peramylated b-CD, and column C (20 m longꢁ0.25 mm
i.d.) contained chiral MOF and peramylated b-CD. Columns A
and B were prepared for comparison. Column A was coated
through a dynamic coating method. Column B was coated
with sodium chloride crystals by a dynamic method and was
then coated by a static method employing a solution of
peramylated b-CD and OV-17 (4.5 mgmLÀ1; 3:7 w/v) in di-
chloromethane at 368C. The method for the preparation of
column C was the same as that for column B. The thickness of
the static coating method can be calculated[16] and is about
0.28 mm on the column. The dynamic method was used for
the coating of crystals and the thickness was controlled
through the coating velocity and concentration. It can be seen
from the SEM image that this is approximately 1 mm. Thus, the
thickness of the coatings on columns A, B, and C is approxi-
mately 1.28 mm.
Table 3. Separations of racemates on three capillary columns.
Column
A
Compound
T [8C]
k1
1.93
a
2-hexanol
110
140
140
120
140
120
1.00
1.00
1.11
1.00
1.02
1.00
1.00
1.06
1.13
1.00
1.00
1.01
1.00
1.00
1.03
1.05
1.00
1.00
1.02
1.07
1.00
1.02
1.00
1.02
1.08
1.05
1.07
1.05
1.02
1.03
1.08
1.10
1.04
(Æ)-linalool
2.65
2.45
2.05
2.68
3.72
3.97
4.74
3.09
2.61
3.99
2.07
4.26
2.13
3.72
2.35
5.79
6.45
16.35
0.44
1.38
6.81
1.93
5.77
4.11
4.05
3.06
4.17
5.39
30.7
0.55
1.08
8.22
(Æ)-citronellal
(Æ)-methyl l-b-hydroxyisobutyrate
(Æ)-limonene
(Æ)-cis-rose oxide
(Æ)-trans-rose oxide
(Æ)-dihydrocarvyl acetate
(Æ)-menthol
130
130
150
140
65
100
120
85
dl-valine[a]
dl-leucine[a]
Results and Discussion
B
2-hexanol
(Æ)-linalool
Table 1 summarizes the chromatographic properties of col-
umns A, B, and C. The plate numbers (“n”) for the three col-
umns followed an increasing order of A (2767)<B (3563)<C
(4268); this indicated that the MOF layer enhanced the coating
properties of the b-CDs.
(Æ)-citronellal
(Æ)-methyl l-b-hydroxyisobutyrate
(Æ)-limonene
140
90
(Æ)-cis-rose oxide
(Æ)-trans-rose oxide
(Æ)-dihydrocarvyl acetate
(Æ)-menthol
85
150
130
110
70
70
115
93
Table 2 gives the McReynolds constants of the five reference
analytes on columns A, B, and C. The average of the five
McReynolds constants is sometimes used as an approximate
polarity scale. The respective constants are thought to measure
various interactions between the stationary phase and the ana-
dl-valine[a]
dl-leucine[a]
C
2-hexanol
(Æ)-linalool
(Æ)-citronellal
(Æ)-methyl l-b-hydroxyisobutyrate
(Æ)-limonene
130
100
(Æ)-cis-rose oxide
(Æ)-trans-rose oxide
(Æ)-dihydrocarvyl acetate
(Æ)-menthol
Table 1. Characteristics of the columns (A, B, C) upon using n-dodecane
as the tested compound at 1208C.
85
140
130
90
Column
Column
dimensions
[mꢁmm.i.d.]
T
[8C]
Capacity
factor
[k]
Linear
velocity
[cmsÀ1
Column
efficiency
[nmÀ1
]
dl-valine[a]
dl-leucine[a]
]
A
B
C
20ꢁ250
20ꢁ250
20ꢁ250
120
120
120
1.33
2.97
2.98
13.67
11.91
12.02
2767
3563
4268
[a] Trifluoroacetyl isopropyl ester derivate. Because there are cis and trans
forms for (Æ)-rose oxide. The values of k1’ for cis-rose oxide and trans-
rose oxide have been displayed in Table 3, respectively.
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separation factor, a, is k2’/k1’, in which t0 is the column void
time, which was tested by using methane. As seen from
Table 3 and Figure S4 in the Supporting Information, the sepa-
ration of all of the tested enantiomers was weak on column A,
except for the separation of citronellal (column A (1.11)>col-
umn C (1.08)>column B (1.00)). Column A was only able to
separate four racemates. Column B, in which contained pera-
mylated b-CDs, was able to separate six chiral compounds. Col-
umn C, in which Co(d-Cam)1/2(bdc)1/2(tmdpy) had been incor-
porated with peramylated b-CDs, was able to separate nine
chiral compounds.
were 0.31 and 2.2% for retention time and peak area, respec-
tively. Furthermore, three similar columns of MOF-incorporated
peramylated b-CD stationary phase (20 mꢁ250 mm i.d.) were
prepared to investigate the column–column reproducibility,
the relative standard deviations for the first-eluted enantiomer
retention factor of leucine was 6.2%. The results showed that
the technique was reliable and reproducible. The above results
demonstrate that incorporated Co(d-Cam)1/2(bdc)1/2(tmdpy)
indeed plays a significant role in the enhanced separation of
racemates.
To study the reason for this resolution enhancement, some
segments were cut from five capillary columns, then analyzed
by means of a scanning electron microscope. SEM results
reveal the presences of differ-
It is clear that (Æ)-linalool (Figure 1), (Æ)-rose oxide
(Figure 2), and dl-valine (Figure 3) could not be separated on
ent stationary phases on these
columns (Figure 5). Figure 5a
shows a SEM image of the cross
section of the pretreated open
tubular column; there is nothing
on it. Figure 5b shows an image
of the cross section of the pera-
mylated b-CD column coating,
which was uniformly coated on
the inner wall. Fabricated colum-
n A had a MOF coating approxi-
mately 1 mm thick on the inner
wall (Figure 5c). SEM results of
columns B and C revealed that
sphere-shaped sodium chloride
particles and MOF were incorpo-
rated with the b-CDs on the
inner surface of the columns, re-
spectively (Figure 5d and e).
Figure 1. a) Linalool at 1408C under a N2 linear velocity of 17.14 cmsÀ1 on column A. b) Linalool at 1008C under
a N2 linear velocity of 11.91 cmsÀ1 on column B. c) Linalool at 708C under a N2 linear velocity of 8.83 cmsÀ1 on col-
umn C.
The intrinsic characteristics of
Co(d-Cam)1/2(bdc)1/2(tmdpy),
such as a high surface area, un-
usual integration of molecular
chirality, absolute helicity, and
a 3D intrinsic chiral net, enabled
the marriage of chiral MOF with
b-CDs to form a greater chiral
microenvironment, in which the
steric fit between the chiral
channel framework and confor-
mation of the racemates, which
is one possible reason for the
enhanced racemate resolving
ability of the MOFs+CDs capilla-
ry column. Besides this factor,
Figure 2. a) Rose oxide at 1208C under a N2 linear velocity of 16.14 cmsÀ1 on column A. b) Rose oxide at 908C
under a N2 linear velocity of 11.91 cmsÀ1 on column B. c) Rose oxide at 1008C under a N2 linear velocity of
12.37 cmsÀ1 on column C.
columns A and B, whereas they were well separated on col-
umn C. Furthermore, column C gave a higher resolution for the
enantioseparations of dl-leucine (Figure 4) than column B. The
chromatograms of column C show good peaks and baseline or
at least 60% valley separation. The repeatability of the chiral
MOF-incorporated peramylated b-CD column was determined
from five repeated injections of racemates. The relative stan-
dard deviations for the five replicate separations of linalool
the combination of the interactions of two different stationary
phases with racemates may also play some role. However, the
influence of the chiral microenvironment on the chiral proper-
ties of the chromatographic systems is very complicated and
far from being understood, although some information and
suggestions have already been published. Further studies on
these aspects will be carried out.
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Conclusion
We have fabricated Co(d-Cam)1/
2(bdc)1/2(tmdpy)-incorporated
peramylated b-CD columns for
GC separation of racemates with
enhanced resolution and high
column efficiency. The intrinsic
characteristics of Co(d-Cam)1/
2(bdc)1/2(tmdpy), such as large
surface area, unusual integration
of molecular chirality, absolute
helicity, and 3D intrinsic chiral
network, make the Co(d-Cam)1/
2(bdc)1/2(tmdpy)-incorporated
Figure 3. a) Derivatized valine at 1508C under a N2 linear velocity of 18.03 cmsÀ1 on column A. b) Derivatized
valine at 1308C under a N2 linear velocity of 14.32 cmsÀ1 on column B. c) Derivatized valine at 1308C under a N2
linear velocity of 15.89 cmsÀ1 on column C.
peramylated b-CD column at-
tractive for enhanced GC separa-
tion of racemates. This research
should lead to the development
of a wide range of chromato-
graphic columns with improved
enantioselectivity on GC.
Experimental Section
The MOF crystal was synthesized
according to the method reported
by Zhang et al.[15] Briefly, a solution
was obtained by mixing d-cam-
phoric acid (0.1030 g), 1,4-benze-
nedicarboxylic acid (0.0801 g), 4,4’-
trimethylenedipyridine
(0.0956),
Figure 4. a) Derivatized leucine at 1408C under a N2 linear velocity of 17.14 cmsÀ1 on column A. b) Derivatized leu-
cine at 1008C under a N2 linear velocity of 11.91 cmsÀ1 on column B. c) Derivatized leucine at 908C under a N2
linear velocity of 12.02 cmsÀ1 on column C.
and CoCO3 (0.1307 g) in ultrapure
water (5 mL) in a 50 mL Teflon cup;
this was stirred for 20 min at room
temperature.
The
Teflon-lined
bomb was sealed and placed in an
oven at 1208C for 5 days under static conditions, and then cooled
to room temperature. The product was washed with ultrapure
water and chromatographically pure ethanol and dried in vacuo at
1208C to eliminate the remaining water. Purple crystals were ob-
tained (yield, 65%).
Derivatization of amino acids
Owing to poor volatility, amino acids are generally difficult to ana-
lyze by GC directly. They need to be derivatized. The goal of deri-
vatization is to make an analyte more volatile, less reactive, and
thus, improve its chromatographic behavior. For this study, a solu-
tion containing a mixture of amino acid (<10 mg) in isopropanol/
acetyl chloride (1 mL, 3:1, v/v) was heated at 1108C for 30 min, and
dried with nitrogen. Tetrahydrofuran (1 mL) and a small amount of
trifluoroacetic anhydride were then added. The mixture was
heated at 80–1008C for 30 min. The sample was then dried with ni-
trogen and remained in dichloromethane or diethyl ether.[18,19]
The untreated fused-silica column was filled with 1m NaOH and
left for 3 h. Thereafter, the column was washed successively with
ultrapure water for 1 h, 0.1m HCl for 1 h, and ultrapure water
again until the outflow reached neutrality. The column was then
purged with nitrogen for 6 h at 1208C.
Figure 5.
a) SEM image of the pretreated open tubular column. b) SEM
image of the peramylated b-CD column coating. c) SEM image of the Co(d-
Cam)1/2(bdc)1/2(tmdpy) column coating. d) SEM image of sodium chloride in-
corporated peramylated b-CDs. e) SEM image of Co(d-Cam)1/2(bdc)1/
2(tmdpy)-incorporated peramylated b-CDs.
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solution of sodium chloride (6 mL) in methanol (or suspension of
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quickly under stirring. After stirring for 5 min, chloroform (4–8 mL)
was added. After stirring for another 2 min, the solution was trans-
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Capillary column with Co(d-Cam)1/2(bdc)1/2(tmdpy) (column
A)
A pretreated capillary column was coated by a dynamic coating
method. Briefly, a suspension of Co(d-Cam)1/2(bdc)1/2(tmdpy) was
introduced into the column under gas pressure and then pushed
through the column at a rate of 1–2 cmsÀ1 to leave a wet coating
layer on the inner wall of the column. The coated column was
purged with nitrogen and then the coating process was repeated
by exchanging the inlet and outlet of the capillary column. Finally,
the column was purged thoroughly with nitrogen for 1–2 h at
2508C.
Peramylated b-CD column (column B)
The column was prepared by coating sodium chloride on the inner
surface of the capillary. The method was the same as that used for
coating the MOF crystal. The column was heated at 3008C for 1–
2 h. Then, the column was coated by a static method by employ-
ing a solution of peramylated b-CD and OV-17 (4.5 mgmLÀ1; 3:7 w/
v) in dichloromethane at 368C. After coating, the column was set-
tled for 1 h for conditioning under nitrogen. Further conditioning
of the capillary column was carried out by using a temperature
program: 308C for 5 min, ramp from 30 to 2508C at a ramp rate of
28CminÀ1 and 2508C for 5 h.
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Column with Co(d-Cam)1/2(bdc)1/2(tmdpy) and peramylated
b-CD (column C)
First, the column was coated with Co(d-Cam)1/2(bdc)1/2(tmdpy) by
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employing
a
solution of peramylated b-CD and OV-17
(4.5 mgmLÀ1; 3:7 w/v) in dichloromethane at 368C. The subse-
quent steps used were the same as those for the preparation of
column B.
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Acknowledgements
The study was supported by the National Natural Science Foun-
dation (nos. 21275126, 21127012) of China.
Keywords: hiral
chromatography
phases
resolution
·
cyclodextrins
·
gas
·
metal–organic frameworks
· stationary
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Received: March 15, 2014
Published online on && &&, 0000
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FULL PAPERS
H. Liu, S.-M. Xie, P. Ai, J.-H. Zhang,
M. Zhang, L.-M. Yuan*
&& – &&
Metal–Organic Framework Co(d-Cam)1/
2(bdc)1/2(tmdpy) for Improved
Enantioseparations on a Chiral
Cyclodextrin Stationary Phase in Gas
Chromatography
Increase the gap! The incorporation of
peramylated b-cyclodextrins into the
metal–organic framework Co(d-Cam)1/2
(bdc)1/2(tmdpy) (d-Cam=d-camphoric
acid, bdc=1,4-benzenedicarboxylic
acid, tmdpy=4,4’-trimethylenedipyri-
dine) enhances the gas chromatograph-
ic separation of small molecules with
high column efficiency and good repro-
ducibility (see picture).
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 0000, 00, 1 – 7
&
7
&
ÞÞ
These are not the final page numbers!