Homochiral Metal-Organic Cage for Gas Chromatographic Separations

Article abstract of DOI:10.1021/acs.analchem.8b01670

Metal-organic cages (MOCs) as a new type of porous material with well-defined cavities were extensively pursued because of their relative ease of synthesis and their potential applications in host-guest chemistry, molecular recognition, separation, catalysis, gas storage, and drug delivery. Here, we first reported that a homochiral MOC [Zn₃L₂] is explored to fabricate [Zn₃L₂] coated capillary column for high-resolution gas chromatographic separation of a wide range of analytes, including n-alkanes, polycyclic aromatic hydrocarbons, and positional isomers, especially for racemates. Various kinds of racemates such as alcohols, diols, epoxides, ethers, halohydrocarbons, and esters were separated with good enantioselectivity and reproducibility on the [Zn₃L₂] coated capillary column. The fabricated [Zn₃L₂] coated capillary column exhibited significant chiral recognition complementary to that of a commercial β-DEX 120 column and our recently reported homochiral porous organic cage CC3-R coated column. The results show that the homochiral MOCs will be very attractive as a new type of chiral selector in separation science.

Full text of DOI:10.1021/acs.analchem.8b01670

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Subscriber access provided by TUFTS UNIV
Article
Homochiral Metal-Organic Cage for Gas Chromatographic Separations
Sheng-Ming Xie, Nan Fu, Li Li, Bao-Yan Yuan, Jun-Hui Zhang, Yan-Xia Li, and Li-Ming Yuan
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01670 • Publication Date (Web): 10 Jul 2018
Downloaded from http://pubs.acs.org on July 10, 2018
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a service to the research community to expedite the dissemination
of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in
full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully
peer reviewed, but should not be considered the official version of record. They are citable by the
Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore,
the “Just Accepted” Web site may not include all articles that will be published in the journal. After
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web
site and published as an ASAP article. Note that technical editing may introduce minor changes
to the manuscript text and/or graphics which could affect content, and all legal disclaimers and
ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or
consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W.,
Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 1 of 8
Analytical Chemistry
1
2
3
4
5
6
7
8
9
Homochiral Metal-Organic Cage for Gas Chromatographic
Separations
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ShengꢀMing Xie*, Nan Fu, Li Li, BaoꢀYan Yuan, JunꢀHui Zhang, YanꢀXia Li, LiꢀMing Yuan*
Department of Chemistry, Yunnan Normal University, Kunming 650500, People’s Republic of China
ABSTRACT: Metalꢀorganic cages (MOCs) as a new type of porous materials with wellꢀdefined cavities have been extensively
pursued because of their relative ease of synthesis and their potential applications in hostꢀguest chemistry, molecular recognition,
separation, catalysis, gas storage and drug delivery. Here we first reported that a homochiral MOC [Zn3L2] is explored to fabricate
[Zn3L2] coated capillary column for highꢀresolution gas chromatographic separation of a wide range of analytes, including nꢀ
alkanes, polycyclic aromatic hydrocarbons (PAHs) and positional isomers, especially for racemates. Various kinds of racemates
such as alcohols, diols, epoxides, ethers, halohydrocarbons and esters colud be separated with good enantioselectivity and
reproducibility on the [Zn3L2] coated capillary column. The fabricated [Zn3L2] coated capillary column exhibited significant chiral
recognition complementarity to a commercial βꢀDEX 120 column and our recently reported homochiral porous organic cage CC3ꢀ
R coated column. The results show that the homochrial MOCs will be very attractive as a new type of chiral selectors in separation
science.
Supramolecular materials (e.g., macrocyclic, crypt and
cageꢀlike molecules) with wellꢀdefined shapes, sizes and
cavities have attracted considerable interest because of their
fascinating structres and potential applications such as
molecular recognition, separation, catalysis, gas storage,
sensing and drug delivery, etc.1ꢀ4 Selfꢀassembly is an essential
feature of supramolecular chemistry and plays a vital role in
the creation of various supramolecules.5 Metalꢀorganic cages
(MOCs) are a class of discrete coordination assemblies built
from the spontaneous coordinationꢀdriven assembly of suitable
metal ions or metal clusters and polydentate ligands.2,6ꢀ8
Chirality at a molecular and supramolecular level is very
important because it is strongly related to chemistry, physics,
biology, materials, and nanoscience.9 Over the past few
decades, chiral MOCs have evolved to be one of the most
attractive topics within supramolecular chemistry and
materials science.7 To date, a great deal of complicated and
novel structures of chiral MOCs have been reported. In
general, the chiral MOCs can be constructed from optically
pure chiral building blocks (a “hard” approach) and achiral
building blocks (a “soft” approach) in a selfꢀassembled
system. Based on the unique chiral functionalities and wellꢀ
defined cavities of chiral MOCs, many chiral MOCs have
been extensively used for chiral molecular recognition10ꢀ19 and
asymmetric catalysis.20ꢀ25
inner wall of the capillary column is of great importance for
the performance of a coated capillary column in GC.
Traditional cyclodextrin derivatives (CDs) and newly reported
chiral POCs as GC stationary phases exhibited excellent chiral
resolution ability toward enantiomers.40ꢀ43,45ꢀ46 In constrast to
MOFs and COFs, chiral MOCs with similar properties of CDs
and POCs are wellꢀdefined discrete cageꢀlike molecular
entities, which can be dissolved in some common organic
solvents. Therefore, some chiral MOCs are easily processable
and can be coated on the inner wall of capillary to form
membranes by
a
static coating method. All these
characteristics make chiral MOCs attractive candidate as new
chromatographic separation medias to prepare chiral MOCꢀ
coated capillary columns for highꢀresolution gas
chromatography. To the best of our knowledge, no work on
the utilization of chiral MOCs as stationary phases for highꢀ
resolution capillary GC separation of enantiomers has been
reported so far.
Herein we report the first exploration of a homochiral MOC
[Zn3L2] as the stationary phase for highꢀresolution GC
separation of various analytes, including normal alkanes,
polycyclic aromatic hydrocarbons (PAHs), positional isomers
and enantiomers with excellent selectivity and good
reproducibility.
In recent years, a large number of chiral porous materials
such as metalꢀorganic frameworks (MOFs),26ꢀ27 covalent
organic frameworks (COFs),28ꢀ30 hydrogenꢀbonded organic
frameworks (HOFs)31 and porous organic cages (POCs)32 have
been reported. Some of them have been successfully applied
for enantioselective separation of various racemic
compounds.29ꢀ45 Compared with other chromatographic
methods, capillary gas chromatography (GC) is an excellent
chromatographic technique for the separation and analysis of
some volatile organic compounds (VOCs). It is well known
that a thin and uniform coating of stationary phases on the
EXPERIMENTAL SECTION
Chemicals and Reagents. All chemicals and reagents used
were at least of analytical grade. Zn(CH3COO)2ꢀ2H2O, 4ꢀtertꢀ
butylꢀ2,6ꢀdiformylphenol and (1R, 2R)ꢀdiaminocyclohexane
were purchased from SigmaꢀAldrich (USA). All racemates
were obtained from SigmaꢀAldrich. The normal alkanes (nꢀC10
to nꢀC16) were from Beijing Chemical Reagent Company
(China). Xylene, dinitrobenzene and nitrotoluene, α,βꢀionones
and cis,transꢀcitrals were purchased from Aladdin Chemistry
Co. Ltd. (China). Naphthalene, acenaphthene, fluorine,
1
ACS Paragon Plus Environment

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Analytical Chemistry
Page 2 of 8
1
2
3
4
5
6
7
8
phenanthrene,
fluoranthene,
pyrene,
chrysene
and
coated capillary columns were conditioned under nitrogen
flow from 30 °C to 200 °C (2 °C min−1) and held at 200 °C for
3 h before use.
benzo[a]pyrene were purchased from Acros Organics (USA)
and SigmaꢀAldrich (USA). Dichloromethane (DCM), ethanol
and acetonitrile were from Tianjin Guangfu Fine Chemical
Research Institute (China). Untreated fusedꢀsilica capillary
RESULTS AND DISCUSSION
column
was
purchased
from
Yongnian
Ruifeng
Chromatogram Apparatus Co. Ltd. (China).
Characterization of the Synthesized Homochiral MOC
[Zn3L2] and the MOC [Zn3L2] coated Capillary Column.
The preparation of homochiral MOC [Zn3L2] was based on
coordinationꢀdriven selfꢀassembly of Zn2+ ions with
enantiopure [3+3] triphenolic Schiffꢀbase macrocycle derived
from transꢀ1,2ꢀdiaminocyclohexane and aromatic dialdehyde.
Figures S1 and S2 (Supporting Information) showed that the
successful synthesis of enantiopure [3+3] triphenolic Schiffꢀ
base macrocycle (Scheme S1, Supporting Information). As
can be seen from Scheme S2 (Supporting Information), two
enantiopure deprotonated [3+3] macrocyclic units are
connected by three Zn2+ ions to form the cageꢀlike molecule
[Zn3L2]. The successful synthesis of [Zn3L2] was confirmed by
NMR data (Figures S3 and S4, Supporting Information). The
experimental PXRD pattern matches well with the simulated
pattern, further demonstrating the successful synthesis of
[Zn3L2] (Figure 1a). The thermal gravimetric analysis (TGA)
curve shows that the [Zn3L2] is stable up to 400 °C (Figure 1b).
The weight loss at 100°C arose from the escape of solvent
molecules inside the pores of [Zn3L2]. A distinguishing feature
of [Zn3L2] and other porous frameworks (eg., MOFs and COFs)
is that they are soluble in some organic solution such as
dichloromethane and chloroform. The good thermal stability
and solubility of [Zn3L2] make it particularly suitable for GC
usage.
Instrumentation. All GC separations were performed on a
Shimadzu GCꢀ2014C system (Japan) with a flame ionization
detector. The instrument control and data acquisition were
carried out by Nꢀ2000 software. 1H and 13C NMR spectra were
recorded on a Bruker DRX 500 NMR ultrashield spectrometer
(Germany). The thermogravimetric analysis (TGA)
experiment was performed on a NETZSCH STA 449F3
simultaneous thermal analyzer (Germany) from room
temperature to 800 °C at a heating rate of 10 °C min−1.
Scanning electron microscopy (SEM) images were recorded
on a FEI Quanta FEG 650 scanning electron microscope
(USA). Commercial βꢀDEX 120 capillary column (30 m ×
0.25 mm i.d. × 0.25 ꢀm film thickness, Supelco Inc., USA)
was employed for comparison.
Synthesis of Enantiopure [3+3] Macrocycle (L) and
Homochiral MOC [Zn3L2]. The L was synthesized according
to the method of Lisowski et al.47 Typically, a 20 mL
acetonitrile of 4ꢀtertꢀbutylꢀ2,6ꢀdiformylphenol (2.062 g, 10
mmol) was added into a 15 mL acetonitrile of (1R, 2R)ꢀ
diaminocyclohexane (1.142 g, 10 mmol) at room temperature.
Then, the resulted yellow suspensions was stirred at 50 °C for
12 h. A yellow product (2.26g, 76%) was obtained by
filtration and washed with acetonitrile. Scheme S1 (Supporting
Information) is the synthesis diagram of L.
The homochiral MOC [Zn3L2] was synthesized according to
the method of Lisowski et al.47,48 Typically, a solution of
Zn(CH3COO)2·2H2O (0.1098 g, 0.50 mmol) in methanol (10
mL) was added to the stirred suspension of the enantiopure
[3+3] macrocycle L (0.2840 g, 0.334 mmol) in methanol (30
mL). The mixture was refluxed for 2h, cooled down and then
placed in the fridge overnight. A light yellow product (0.194g,
62%) was obtained. Finally, the product was recrystallized
from chloroform and dried under vacuum. Scheme S2
(Supporting Information) is the synthesis diagram of [Zn3L2].
Capillary Pretreatment and Preparation of the
Homochiral Capillary Columns. Fusedꢀsilica capillary
column (15 m long × 0.25 mm i.d.) was pretreated according
to the following method prior to coating: the column was
washed with 1 M NaOH for 3 h, ultrapure water for 1 h, 0.1 M
HCl for 1 h and again using ultrapure water until the washings
were neutral. Finally, the capillary was dried via a nitrogen
purge at 120 °C for 6 h.
The homochiral capillary columns were prepared by using
the static coating method. The static coating process as follows:
one end of the capillary column was sealed when the solution
of stationary phase was introduced into the pretreated capillary
column and the other end was connected to a vacuum system
to gradually remove the solvent under vacuum at 36 °C.
Column A was statically coated with a mixture by mixing 1
mL solution of homochiral MOC [Zn3L2] (3 mg mL−1) in
dichloromethane (DCM) and 1 mL solution of polysiloxane
OVꢀ1701 (4.5 mg mL−1) in DCM. The preparation of column
B was the same as that of column A except for the use
enantiopure [3+3] macrocycle as stationary phase. Finally, the
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Column A coated with homochiral MOC [Zn3L2] diluted
with OVꢀ1701 was fabricated by a static method. The MOC
[Zn3L2] coated capillary columns were characterized by
scanning electron microscopy (SEM). The capillaries were cut
to expose the inner wall for SEM measurement. As can be
seen from Figure 1c, an approximately 300 nm thick [Zn3L2]
coating was deposited on the inner wall of column A.
The column efficiency of column A was measured by using
nꢀdodecane as analyte at 120 °C, and the number of theoretical
plates was 2300 plates m−1.
Figure 1. (a) Comparison of the experimental and simulated XRD
patterns of homochiral MOC [Zn3L2]; (b) TGA of homochiral
MOC [Zn3L2]; (c) SEM image of the cross section view of the
inlet of homochiral MOC [Zn3L2] coated capillary column.
2
ACS Paragon Plus Environment

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Page 3 of 8
Analytical Chemistry
1
2
3
4
5
6
7
8
Separation of Normal Alkanes, PAHs and Positional
coherent boiling points (eg. xylene: pꢀxylene, 138.4 °C; mꢀ
xylene, 139 °C). In this work, column A also offered good
selectivity for the separation of some positional isomers,
including ionone, citral, xylene, dinitrobenzene and
nitrotoluene. Citral and ionone are basic raw material of
perfume and medical industry, while xylene, dinitrobenzene
and nitrotoluene are useful intermediates for the production of
important chemical products such as polymers, insecticides
and dyes etc. The chromatograms of five isomers on column A
are shown in Figure 4, and the chromatographic data is listed
in Table 1. The elution sequence of all these substituted
aromatics isomers followed the order of boiling points on
column A as observed on some commercial capillary columns
in GC.
Isomers. To test the overall chromatographic properties of
column A, many different types of compounds were selected
as targets, including normal alkanes, PAHs and positional
isomers.
Alkanes are important constituents of raw chemicals in the
petroleum and chemical industry.49 The separation of normal
alkanes is a very important process in petroleum refining. As
can be seen from Figure 2, all the seven nꢀalkanes (nꢀC10 to nꢀ
C16) with a wide range of boiling points were baseline
separated with sharp peak shapes on column A and eluted in
the order of their boiling points. The highꢀresolution
separation of nꢀalkanes on column A mainly arises from the
van der Waals interaction between nꢀalkane molecules and
MOC [Zn3L2] stationary phase.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 2. GC chromatograms on the column A (15 m long × 0.25
mm i.d.) for the separation of nꢀAlkanes at a N2 flow rate of 15.2
cm s−1 using a temperature program: 100 °C for 0.5 min, then
35 °C min−1 to 220 °C, and finally 220 °C for the remainder of the
measurement.
Figure 4. GC chromatograms on the column A (15 m long × 0.25
mm i.d.) for the separation of (a) αꢀ, βꢀionone isomers at a N2
flow rate of 12.5 cm s−1 under 137 °C; (b) cisꢀ, transꢀcitral
isomers at a N2 flow rate of 13.5 cm s−1 under 122 °C; (c) oꢀ, mꢀ,
pꢀxylene at a N2 flow rate of 14.2 cm s−1 under 85 °C; (d) oꢀ, mꢀ,
pꢀdinitrobenzene at a N2 flow rate of 13.3 cm s−1 under 180 °C; (e)
oꢀ, mꢀ, pꢀnitrotoluene at a N2 flow rate of 13.8 cm s−1 under
130 °C.
Polycyclic aromatic hydrocarbons (PAHs) are a type of
persistent organic pollutants (POPs) with highly carcinogenic
at relatively low levels. Therefore, it is of vital importance for
the detection and analysis of PAHs in the environment. In this
work, we chose a mixture of eight PAHs as analytes. Figure 3
shows that eight PAH molecules were baseline separated with
good peak shapes on column A using a temperature program.
The result indicates that MOC [Zn3L2] coated capillary column
has potential application for GC separation of some
environmentally important POPs.
Table 1. Separation factor (α) and resolution (Rs) of isomers on
column A
separation
factor (α)
α1 α2
resolution (Rs)
isomers
T (°C)
Rs1
Rs2
ionone
citral
xylene
dinitrobenzene
nitrotoluene
137
122
85
180
130
1.37
1.24
7.60
2.57
1.09
1.23
1.27
1.25
1.62
1.10
0.53
1.55
2.41
1.47
3.21
1.36
Separation of Racemates. The most striking feature of
homochiral MOC [Zn3L2] is its chiral cavity, which makes it
attractive as a new media to prepare chiral stationary phase for
separation of racemates. To evaluate the resolving ability and
enantioselectivity of the homochiral MOC [Zn3L2], various
kinds of racemates were selected as analytes. The following
twelve racemates were separated on column A: 2ꢀbutanol, 1ꢀ
methoxyꢀ2ꢀbutanol, 2ꢀpentanol, 4ꢀmethylꢀ2ꢀpentanol, 1,2ꢀ
butanediol, 1,2ꢀepoxypropane, 1ꢀmethoxyꢀ2ꢀhydroxypropane,
epichlorohydrin, epibromohydrin, methyl 3ꢀhydroxybutyrate,
ethyl 3ꢀhydroxybutyrate and mandelic acid. Those racemates
include alcohols, diols, epoxides, ethers, halohydrocarbons
Figure 3. GC chromatograms on the column A (15 m long × 0.25
mm i.d.) for the separation of PAHs (first peak: solvent toluene; 1:
naphthalene; 2: acenaphthene; 3: fluorene; 4: phenanthrene; 5:
fluoranthene; 6: pyrene; 7: chrysene; 8: benzo[a]pyrene) at a N2
flow rate of 16.6 cm s−1 using a temperature program: 120 °C for
0.5 min, then 40 °C min−1 to 320 °C, and finally 320 °C for the
remainder of the measurement.
Selective separation of some positional isomers is of great
importance in industry and also a challenging task because of
their similar chemical and physical properties, and their
3
ACS Paragon Plus Environment