Highly Stable Zr(IV)-Based Metal-Organic Frameworks for Chiral Separation in Reversed-Phase Liquid Chromatography

Article abstract of DOI:10.1021/jacs.0c11276

Separation of racemic mixtures is of great importance and interest in chemistry and pharmacology. Porous materials including metal-organic frameworks (MOFs) have been widely explored as chiral stationary phases (CSPs) in chiral resolution. However, it remains a challenge to develop new CSPs for reversed-phase high-performance liquid chromatography (RP-HPLC), which is the most popular chromatographic mode and accounts for over 90% of all separations. Here we demonstrated for the first time that highly stable Zr-based MOFs can be efficient CSPs for RP-HPLC. By elaborately designing and synthesizing three tetracarboxylate ligands of enantiopure 1,1′-biphenyl-20-crown-6, we prepared three chiral porous Zr(IV)-MOFs with the framework formula [Zr6O4(OH)8(H2O)4(L)2]. They share the same flu topological structure but channels of different sizes and display excellent tolerance to water, acid, and base. Chiral crown ether moieties are periodically aligned within the framework channels, allowing for stereoselective recognition of guest molecules via supramolecular interactions. Under acidic aqueous eluent conditions, the Zr-MOF-packed HPLC columns provide high resolution, selectivity, and durability for the separation of a variety of model racemates, including unprotected and protected amino acids and N-containing drugs, which are comparable to or even superior to several commercial chiral columns for HPLC separation. DFT calculations suggest that the Zr-MOF provides a confined microenvironment for chiral crown ethers that dictates the separation selectivity.

Full text of DOI:10.1021/jacs.0c11276

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Highly Stable Zr(IV)-Based Metal−Organic Frameworks for Chiral
Separation in Reversed-Phase Liquid Chromatography
Hong Jiang, Kuiwei Yang, Xiangxiang Zhao, Wenqiang Zhang, Yan Liu,* Jianwen Jiang,* and Yong Cui*
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ABSTRACT: Separation of racemic mixtures is of great importance and
interest in chemistry and pharmacology. Porous materials including
metal−organic frameworks (MOFs) have been widely explored as chiral
stationary phases (CSPs) in chiral resolution. However, it remains a
challenge to develop new CSPs for reversed-phase high-performance
liquid chromatography (RP-HPLC), which is the most popular chromato-
graphic mode and accounts for over 90% of all separations. Here we
demonstrated for the first time that highly stable Zr-based MOFs can be
efficient CSPs for RP-HPLC. By elaborately designing and synthesizing
three tetracarboxylate ligands of enantiopure 1,1′-biphenyl-20-crown-6, we
prepared three chiral porous Zr(IV)-MOFs with the framework formula
[Zr6O4(OH)8(H2O)4(L)2]. They share the same flu topological structure
but channels of different sizes and display excellent tolerance to water,
acid, and base. Chiral crown ether moieties are periodically aligned within
the framework channels, allowing for stereoselective recognition of guest molecules via supramolecular interactions. Under acidic
aqueous eluent conditions, the Zr-MOF-packed HPLC columns provide high resolution, selectivity, and durability for the separation
of a variety of model racemates, including unprotected and protected amino acids and N-containing drugs, which are comparable to
or even superior to several commercial chiral columns for HPLC separation. DFT calculations suggest that the Zr-MOF provides a
confined microenvironment for chiral crown ethers that dictates the separation selectivity.
stationary phase in chromatography, especially liquid chroma-
tography (LC), because their porosity permits high flow rates
through it with low back-pressure and, ultimately, favors
miniaturization.6 Consequently, many chiral MOFs have been
explored as CSPs for gas and liquid chromatographic
enantioseparation.7,8 However, with few exceptions, these
CSPs exhibit unsatisfactory separate factors and resolutions,
mainly due to lacking specific interactions between guests and
frameworks.7a,b,8g Moreover, there are no reports utilizing
MOFs as CSPs for RP-HPLC because of their typically limited
chemical stability.6d Inspiringly, the recent discovery of MOFs
made from high-valent metals such as Zr(IV) has greatly
increased MOF stability and robustness in water, acid, and
base solutions,9 which are clearly favorable when utilized as
CSPs in RP-HPLC. We have recently shown that tetracarbox-
ylate ligands of 1,1′-biphenol can form highly stable and
porous Zr-MOFs with a flu topology.10a We envisaged that
INTRODUCTION
■
Separation of enantiomers plays a significant role in the
pharmaceutical and agrochemical industries, due to the
different biological interactions, pharmacology, and toxicity
of pure antipodes.1 This has promoted the enormous
development of chiral resolution techniques.2 Chiral high
performance liquid chromatography (HPLC) that relies on the
use of chiral stationary phases (CSPs) has proven to be the
most popular and efficient method to separate enantiomers.3
In particular, reversed-phase (RP) HPLC columns are the
most popular and account for more than 90% of all HPLC
separations.3a In RP-HPLC, the mobile phase is water or
water-miscible organic mixtures, both of which are safe and
easily accessible. Commercial CSPs based on polysaccharide,
cyclodextrin, and amino acids show excellent performance in
the resolution of a range of racemates, but they lack versatility
across all types of chiral analytes and suffer poor stability.4 So,
the exploration of new functional materials as CSPs in chiral
chromatography has been a challenging topic for decades. Here
we demonstrate, for the first time, that highly stable Zr(IV)-
based metal−organic frameworks (MOFs) can serve as
efficient CSPs for RP-HPLC in enantiomeric separation.
MOFs have shown potential applications in diverse areas for
their permanent porosity and tunable and functionalizable pore
structures.5 MOFs are a priori quite interesting materials as the
Received: October 26, 2020
https://dx.doi.org/10.1021/jacs.0c11276
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A

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Figure 1. (a) A flu-a network constructed by 4-connected linkers and 8-connected Zr nodes. (b) A dodecahedral cage constructed by six Zr6
clusters and eight ligands (for clarity, methyl, crown ether groups, and H atoms are omitted). (c) Face sharing of the dodecahedra to form the flu
network. (d−f) View of the three-dimensional structures of CE-1, CE-2, and CE-3 (the cavities are highlighted by yellow spheres). The color code
is as follows: C, gray; O, red; Zr, green.
incorporating chiral crown ethers into Zr-MOFs may create
specific recognition sites with enantioselective control and
provide a way to engineer CSPs for RP-HPLC separation. This
facilitates the synthesis of three robust chiral Zr(IV)-MOFs
with a flu topology from the custom-designed 3,3′,5,5′-
tetra(benzoate), -tetra(2-naphthoate), and -tetra(4-phenyl-
benzoate) ligands of enantiopure 2,2′-pentaethylene glycol-
1,1′-biphenyl. Crown ether moieties are periodically aligned
within the framework channels that are accessible for guest
molecules. It should be noted that, due to the superior ability
to complex with cationic species, chiral macrocyclic com-
pounds including crown ethers can provide an enantioselective
specific interaction with single enantiomer to resolve race-
mates.11 By using acidic aqueous eluents, the Zr-MOF-packed
HPLC columns afforded high resolution and enantioselectivity
for the separation of a variety of racemates such as amino acids
and amine drugs. The resolution abilities of Zr-MOFs can be
tuned by modifying pore sizes through alteration of the linker
lengths. The different interactions between crown ether groups
of the Zr-MOF and racemates were revealed by DFT
calculations and accounted for the chiral separation capacity.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The tetracarboxylate
ligands H4L1−H4L3 were prepared from enantiopure 6,6′-
dimethyl-3,3′-tert-butyl-5,5′-dibro-1,1′-biphenyl-2,2′-diol in
several steps. The solvothermal reactions of ZrOCl2·8H2O
and H4L1, H4L2, or H4L3 in DMF with formic acid or
trifluoroacetic acid (TFA) as modulating agent at 120 °C for
24 h afforded colorless octahedral crystals of
1
[ Z r
O
( O H ) ( H O ) ( L
) ]
2
( C E - 1 ) ,
6
4
8
2
4
[ Z r 6 O 4 ( O H ) 8 ( H 2 O ) 4 ( L 2 ) 2 ] ( C E - 2 ) , o r
[Zr6O4(OH)8(H2O)4(L3)2] (CE-3), respectively. The prod-
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Figure 2. (a) PXRD patterns and (b) N2 adsorption isotherms for CE-1 under different conditions. (c) TGA curves for 1−3. (d) BET surface areas
for 1−3 under different conditions.
ucts are insoluble in water and common solvents. They have
been thoroughly characterized by elemental analysis, FT-IR
spectra, and thermogravimetric analysis (TGA). The phase
purity of them was confirmed by comparison of their
experimental and simulated powder X-ray diffraction
(PXRD) patterns.
to CE-1 but with larger pore size due to the extension of the
peripheral arms of the ligands. Similar to CE-1, the Zr6 clusters
were cross-linked by L2 and L3 units to form a flu network
featuring a larger cavity with sizes of about 12.5 × 27.8 × 36.1
and 17.3 × 35.5 × 39.6 Å3 for CE-2 and CE-3, respectively
(Figure 1e and f). The crown ether parts were also modeled by
1
Single-crystal X-ray diffraction (SCXRD) revealed that 1−3
crystallize in the same space group and share identical
topological structures. CE-1 crystallizes in the chiral space
group P212121, with one of the formula in the asymmetric unit.
Six Zr atoms are assembled into an octahedral Zr6(μ3-
OH)4(μ3-O)4 core by four μ3-O and four μ3-OH groups,
with terminal positions occupied by four − OH and four water
molecules. Each L1 connected four hexanuclear Zr clusters via
four bidentate carboxylate groups to form a (4,8)-connected
flu framework (Figure 1a).10 Because of the high symmetry of
the framework and disordered crown ether units, the precise
positions of crown ether groups cannot be modeled by
SCXRD. Thus, we used Material Studio to model the position
Material Studio, and the H NMR spectrum of the digested
samples clearly indicated the existence of the crown ether
moieties (Figures S2 and S3). As a result of the different
lengths of the tetracarboxylate ligands, the crown ether MOF
series is highly porous with a tunable void space with 73.3%,
75.9%, and 84.3%, as calculated by PLATON for 1−3,
respectively.12 As shown in Figure S5, 1−3 have tunable
opening channels of 8.5 × 14.9, 9.9 × 19.5, and 13.7 × 24.2 Å2
along the a axis and of 8.5 × 9.0, 8.9 × 9.9, and 11.4 × 13.7 Å2
along the c axis, respectively.
Solid-state circular dichroism (CD) of 1-3 spectra built from
(R)- or (S)-enantiomers of H4L are mirror images of each
other, revealing their enantiomeric nature (Figure S6).
Scanning electron microscopy (SEM) images showed 1−3
possess a uniform octahedral morphology (Figure S10). The
permanent porosity of CE-1 was demonstrated by N2
adsorption measurement at 77 K. After desolvation of the
acetone-exchanged sample by heating at 120 °C under
dynamic vacuum overnight, it still remained of high
crystallinity and exhibited a type I sorption behavior, with a
Brunauer−Emmett−Teller (BET) surface area of 1016 m2 g−1
(Figure 2b). In contrast, CE-2 and CE-3 exhibited lower BET
values of 375 and 287 m2 g−1, probably due to the distorted
pore structure after solvent removal (Figure S9). This
1
of the crown ether groups inside the cavities. The H NMR
spectrum of the digested CE-1 also clearly confirmed the
existence of crown ether units inside the MOF (Figure S1). A
dodecahedral cage with a cavity of dimensions about 12.5 ×
24.9 × 26.9 Å3 is constructed from six Zr6 clusters occupying
the vertices and eight L1 linkers riding on the faces (Figure 1b
and d). The cage cavities are periodically decorated with the
crown ether groups of biphenol backbones that are accessible
to guest molecules. The dodecahedral cage is surrounded by
12 adjacent cages via sharing faces, thus generating a 3D
porous structure (Figure 1c). CE-2 and CE-3 are isostructural
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Figure 3. (a) Schematic illustration of RP-HPLC separation by CE-MOF packed column. (b−e) Chromatograms for the separation of 16
racemates with CE-1 packed column (mobile phase pH = 1 HClO4 (aq), 0.3 mL/min flow rate, detection wavelength 220 nm, temperature 25 °C).
phenomenon is often observed in highly porous MOFs.13 Dye
uptake experiments were then used to certify the structural
integrity of the CMOFs in solution. 1−3 can adsorb 2.3, 3.8,
and 4.6 methyl orange (MO, ∼1.47 nm × 0.53 nm) per
formula unit, and 1.7, 2.3, and 3.6 rhodamine 6G (R6G, ∼1.43
nm × 1.61 nm) in MeOH, respectively (Table S1). The above
results implied that the structural integrity and open channels
of the three MOFs are maintained in solution.
Chemical Stability. As expected, 1−3 showed excellent
thermal stability. TGA showed that guest molecules within
them could be removed in the temperature ranging from 80 to
200 °C, and the frameworks started to decompose at about
400 °C (Figure 2c).
The chemical stability of the frameworks was studied by
PXRD and N2 adsorption after treatment in boiling water,
concentrated HCl (aq), and pH = 12 NaOH (aq) at rt for 1
week, as well as dye uptake measurements. As shown in
Figures 2a and S8, PXRD patterns of 1−3 after these
treatments remained intact, suggesting that no phase transition
or framework collapse happened. The minor shift of peaks for
CE-2 and CE-3 can be explained by a certain degree of the
framework flexibility. Both of the as-treated CMOFs retained
their permanent porosity, as evidenced by BET measurements
(Figure 2d). Dye uptake measurements showed the samples of
1−3 after these treatments could adsorb 1.6/1.4/1.3, 2.4/2/
1.8, and 3.4/3/2.8 R6G per formula unit, respectively,
D
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Figure 4. (a) HPLC chromatograms of racemic Phg on the CE-1 packed column after being used for 4000 injections after 1 year. (b) Effect of
injected mass of Phg on the CE-1 packed column. The injected mass was increased from 4 to 20 μg. (c) HPLC chromatograms of Phg-OMe on the
CE-1 packed column at 288−308 K. (d) The van’t Hoff plots for D- and L-Phg-OMe.
comparable to the untreated sample, indicative of the structural
integrity (Table S2).
conditioned with HPLC-grade water at a flow rate of 0.2
mL/min for 6 h. The performance of the CSP was first
investigated by resolving phenylglycine (Phg) (Figure 3a),
which is an essential intermediate and shows great potential
value in the areas such as pharmacies, food additives, and fine
chemicals.15 After the separation conditions including the
mobile phase compositions (acid and pH) and flow rate were
optimized, racemic Phg was successfully baseline separated on
the CE-1-based CSP with pH = 1 HClO4 (aq) as the mobile
phase at a flow rate of 0.3 mL/min at 25 °C (Figure S12). An
excellent selectivity factor (α = 5.25) and chromatographic
resolution factor (Rs = 6.31) were achieved, and the elution
sequence was the D-enantiomer followed by the L-enantiomer
(Figure 3b). In this HPLC system, the back-pressure is about 9
MPa. It is also worth noting that the mobile phase and flow
rate can significantly affect the retention time, resolution, and
selectivity (Figure S12).
To expand the scope of the analytes, we examined seven
other amino acids including phenylalanine (Phe), 4-hydrox-
yphenylglycine (Hpg), tyrosine (Tyr), tryptophan (Trp),
alanine (Ala), methionine (Met), and 2-aminobutyric acid
(Abu). Encouragingly, all of them were completely resolved on
the CE-1-based CSPs with α/Rs = 1.49/1.85, 7.77/6.59, 1.60/
1.99, 1.36/1.85, 1.53/1.23, 2.36/3.51, and 1.66/1.02, respec-
tively (Figure 3b, c, and e). Furthermore, amino acid
derivatives, such as N-protected amino acids (N-Boc-Ala and
N-Boc-Phe), amino esters (Phg-OMe, Tyr-OMe, and Phe-
The high chemical stability of these three Zr-MOFs can be
ascribed to the following reasons: First, the highly charged
Zr(IV) ions can polarize the Zr−O coordination bond that
results in a strong bonding between Zr(IV) and carboxylate
oxygens in the ligands.9a Second, the methyl and crown ether
groups at the ortho positions of the central two aryl rings
prevent the free rotation of the C−C linkages to rigidify the
linker and thus further improve the framework stability.14a
Third, the electron-donating nature of the methyl and crown
ether groups leads to an increase of electron density of
carboxylate oxygens, which can enhance the Zr−O bond
strength.10a,14b The excellent stability of the Zr-CMOFs makes
them attractive CSPs for enantioselective chromatographic
separation.
RP-HPLC Enantioseparation. Inspired by the excellent
stability, large porosity, and abundant chiral crown ether
groups of the Zr-based MOFs, we utilized them as CSPs for
RP-HPLC separations. However, the columns packed with the
pure MOF powders exhibit very high back-pressure (>20
MPa), and so we prepared the CSPs of Zr-MOFs mixed with
reverse phase C18 silica gel. The CE-1 packed column was
prepared by loading the mixture of CE-1 and C18 silica gel in
methanol/water (1:9, v/v) into a 25 cm long × 2.1 mm
internal diameter (i.d.) stainless steel column under a pressure
of 5000 psi. Prior to data collection, the column was
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Table 1. Comparison of the Separation Performance of Six Racemic Amino Acids by RP-HPLC Columns Packed with CE-1,
a
Me4L1, OET-1 (Analogue of CE-1 with −OEt Groups), and Several Commercial CSPs
CROWN-
b
b
b
c
,
d
,
e
,
CE-1
Me4L1
OET-1
PAK CR(+)17a
Chirosil RCA(+)17b
Chirobiotic T17c
analytes
α
Rs
α
Rs
α
Rs
α
Rs
α
Rs
α
Rs
Phg
Phe
Tyr
Trp
Ala
5.25
1.49
1.60
1.36
1.45
2.36
6.31
1.85
1.99
1.85
1.99
3.51
2.01
1.25
1.28
1.19
1.25
1.83
1.82
1.56
1.72
0.91
1.22
3.37
1.00
1.00
1.00
1.00
1.00
1.00
f
f
f
f
f
f
3.97
1.20
1.16
g
1.65
1.85
14.27
2.69
2.22
g
1.66
6.75
2.25
1.48
1.44
1.44
1.28
1.39
6.46
2.31
2.00
2.15
1.33
2.06
3.10
1.50
1.50
1.50
1.80
2.20
2.90
2.00
1.90
2.20
2.90
3.30
Met
a
Separation factor α = (tR2 − tM)/(tR1 − tM), resolution factor Rs = 2(tR2 − tR1)/(w1 + w2). tM, the column void time, is determined by using 1,3,5-
b
tri-tert-butylbenzene as analyte. tR1 and tR2 are the retention times, and w1 and w2 are the peak widths of the peaks. Mobile phase pH = 1 HClO4
c
(aq), 0.3 mL/min flow rate, detection wavelength 220 nm, temperature 25 °C. Mobile phase pH = 2 HClO4 (aq), 0.5 mL/min flow rate, detection
d
wavelength set to 200 or 254 nm, temperature 18 °C. Mobile phase 80% CH3OH in H2O + H2SO4 (10 mM), 0.5 mL/min flow rate, detection
e
wavelength set to 210 nm, temperature 20 °C. Mobile phase CH3OH−H2O (60−40, v/v), 1.0 mL/min flow rate, detection wavelength set to 210
f
g
nm. Could not be separated. Not reported.
OMe), and amino alcohols (Phg-ol), can also be separated,
affording α/Rs = 1.23/0.63 to 3.23/10.21 (Figure 3c−e).
Furthermore, some unprotected amines such as 1-phenylethyl-
amine (1-PEA) and 1-phenylpropylamine (1-PPA) can be
baseline separated with α/Rs = 1.38/0.96 and 1.54/2.34,
respectively (Figure 3e). Importantly, except for Trp and 1-
PPA, all other substrates were separated on the CE-1 column
within 30 min. The short separation time enables a fast analysis
of the samples, which is highly efficient and time-saving for
enantioselective separation.
Also, we prepared columns packed with CE-2 and CE-3
using the same procedure as for CE-1 to compare their
separation behaviors toward amino acids. Both columns can
separate the racemic amino acids with α/Rs = 1.08/0.33 to
4.16/4.04, which are lower than the values for the CE-1
column (Figure S14a and Table S5). In particular, both the
resolution and the separation factor decreased as the channel
sizes of the frameworks increased. The different separation
performance of 1−3 may be ascribed to the different levels of
asymmetric induction ability caused by the various channel
sizes and aryl substituents.
the first report utilizing MOFs as CSPs for RP-HPLC. As
shown in Table 1, the resolution abilities of CE-1-CSP are
comparable to or even better than those of the three widely
used commercial chiral columns CROWNPAK CR(+),
Chirosil RCA(+), and Chirobiotic T, which are derived from
(S)-(3,3′-diphenyl-1,1′-binaphthyl)-20-crown-6, (+)-(18-
crown-6)-2,3,11,12-tetracarboxylic acid, and teicoplanin, re-
spectively.17
A demonstration of the utility of this MOF-based CSP is
presented in the separation of drug enantiomers that is highly
desired in pharmaceutical, agricultural, and fine chemical
industries. Baclofen, a chemical analogue of γ-aminobutyric
acid, is widely used for the treatment of spinal cord injury-
induced spasm.18 The R-enantiomer is the actual active
species, although the racemic compound is used in clinic
practice.18b We used the CE-1 packed column to separate
racemic baclofen. To our delight, racemic baclofen can be
baseline separated with α/Rs = 1.85/3.66 (Figure 5a). Another
The durability of the CE-1 column was evaluated by
studying its performance after 1 year of shelf life and over 4000
multiple injections (Figure 4a). There was a slight decrease of
separation performance with α/Rs reduced from 5.25/6.31 to
4.23/3.93. PXRD studies indicated that the recovered samples
of CE-1 after HPLC measurement remained crystalline and
structurally intact (Figure S8). Furthermore, the repeatability
of CE-1 column was examined by five replicate separations of
Phg and Phe-OMe (Figure S14f and g). The RSD (relative
standard deviation) values of selectivity factor, peak area, peak
height, and theoretical plate number were calculated to be less
than 2.0%, further confirming the separation stability of the
column (Tables S6 and S7). Also, we prepared another CE-1
packed column by using the same procedure. The two
separated CE-1 packed columns exhibited similar separation
performance (Figure S14h and Table S8) and demonstrated
the good preparation reproducibility of this method. It is worth
mentioning that a variety of chiral porous materials including
CMOFs and chiral covalent organic frameworks (CCOFs)
have been explored as CSPs for chiral HPLC separation, but
they typically suffer low stability and narrow analyte scope.8,16
Moreover, they all used organic solvents as eluents that bring
serious pollution. To the best of our knowledge, this represents
Figure 5. HPLC chromatograms of the four racemic drugs including
baclofen, phenibut, Dopa, and phosphinothricin by the CE-1 packed
column (mobile phase pH = 1 HClO4 (aq), 0.3 mL/min flow rate,
detection wavelength 220 nm, temperature 25 °C).
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γ-aminobutyric acid, phenibut, can also be well separated
affording α/Rs = 1.74/2.46 (Figure 5b). 3,4-Dihydroxypheny-
lalanine (Dopa), a drug for the treatment of Pakinson’s disease,
can be separated with α/Rs = 1.65/1.76 (Figure 5c). Besides,
CE-1-based CSP can also separate the agricultural chemical
glufosinate ammonium (phosphinothricin) affording α/Rs =
1.84/1.40 (Figure 5d). These results demonstrate the potential
versatility of CE-1 for the separation of diverse chiral drugs.
To gain further insight into the separation of chiral
molecules, we prepared a column packed with Me4L1 and
evaluated its chiral recognition ability as CSP. The as-prepared
column can separate amino acids with good to excellent
separation resolution, affording α/Rs = 1.19/0.91 to 1.83/3.37
(Figure S14b and Table 1). This suggested that the crown
ether moieties play a significant role in the separation process.
However, as compared to the CE-1 column, the Me4L1 column
typically showed lower separation factor and resolution. This
may be attributed to the fact that the framework can exert extra
confinement environment and thus lead to an enhancement of
enantiospecific interaction. To examine the importance of
crown ether moieties, we have synthesized an isoreticular
framework OET-1 with OEt groups instead of crown ether
moieties. OET-1 also showed excellent chemical stability
toward different harsh conditions (Figures S8 and S9).
However, the OET-1 packed column cannot separate any of
the analytes under the same conditions (Figure S14c and Table
1). Besides, control experiments showed that the pure C18
column does not have the separation ability toward amino
acids (Figure S14e). This proves the significant role of crown
ether groups in enantioselective recognition. To study the
confined space effect of the framework, we used the bulky 3,5-
dibenzyloxy-1-phenylalanine methyl ester with a size of 14 Å ×
16 Å as the analyte (Figure S11). It can be separated in the
Me4L1 packed column with α/Rs = 1.21/1.19, but cannot be
baseline separated in the column packed by CE-1 with a
maximum channel size of 8.5 Å × 14.9 Å (Figure S14d). Taken
together, we can speculate that the crown ether groups
combined with the confined space of the crystalline framework
exert an important influence in chiral recognition.
To examine the loading capacity of the CSP without
compromising resolution, a loading test was carried out using
different inject masses. When the loading was increased from 4
to 20 μg of each racemate, Phg can still achieve baseline
separation (Figure 5b). Besides, the chromatographic peak area
of each single antipode rises in line with the increase of the
injected mass (Figure S13). It is observed that the retention
time of the enantiomer became closer with increasing injection
mass, although it did achieve baseline resolution. The result
may be due to the strong interaction between amine species
and crown ether groups.
To better understand the separation process, the thermody-
namics of Phg-OMe separation was investigated at temper-
atures ranging from 288 to 308 K (Figure 5c). It was observed
that the retention times of the analytes were decreased as the
column temperature increased. The van’t Hoff plots for the
analytes exhibited excellent linear correlation, and this
suggested no changes of the interactions during the separation
process (Figure 5d). The molar adsorption enthalpy/entropy
changes ΔadsHm/ΔadsHm of (D)- and (L)-protonated Phg-OMe
are −3.64 and −11.30 kJ/mol, respectively. The differences
Δ(ΔadsHm)L−D and Δ(ΔadsSm)L−D are calculated as −7.66 kJ/
mol and −70.66 J/(mol K), and this suggests that the
separation process is enthalpy driven. In addition, the
adsorption enthalpy results are consistent with the order of
retention times and the elution sequence of the D- and L-
enantiomers.
To microscopically elucidate the interactions between the
crown ether group in (S)-CE-1 and L/D-protonated Phg-OMe,
density functional theory (DFT) calculations were performed.
As shown in Figure S15, there are two configurations of the
crown ether groups in (S)-CE-1 with the dihedral angles of
two center phenyl rings at about 64° or 112°, respectively.
However, geometric optimization of the host models (Figures
S17 and S18) suggests that only the configuration with the
dihedral angles of two center phenyl rings at 64° could provide
adequate space for the favorable binding of L/D-enantiomers.
Figure 6 depicts the simplified structures of the most stable
Figure 6. Side (upper) and top (lower) views of the lowest-energy
structures of L- and D-protonated Phg-OMe with (S)-CE-1. The
Gibbs binding energies at 298 K are indicated in the parentheses. The
lengths (in Å) of hydrogen bonds involved in the host−guest
interactions and the dihedral angles of the two center phenyl rings are
labeled. The MOF frameworks surrounding the host−guest
complexes are omitted for clarity. C, gray; H, white; O, red; and N,
dark blue.
conformers of L/D-protonated Phe-OMe with (S)-CE-1. The
corresponding structures with the surrounding frameworks can
be found in Figure S20. Both the L- and the D-protonated Phg-
OMe bind to the crown ether group in (S)-CE-1 through three
hydrogen bonds. At room temperature, the predicted Gibbs
binding energy for the L-enantiomer is 2.9 kcal/mol higher
than that for the D-enantiomer. This demonstrates a marked
chiral recognition of the L-enantiomer relative to the D-
enantiomer by (S)-CE-1, as attributed to the favorable host−
guest binding through hydrogen bonds at optimum distances
and collinear orientations in the confined framework.
CONCLUSIONS
■
We have designed and prepared three chiral crown ether
embedded Zr-MOFs with a flu network topology based on
three enantiopure 1,1′-biphenol-derived tetracarboxylate link-
ers. The obtained porous Zr-MOFs displayed highly chemical
stability and can serve as robust CSPs for RP-HPLC
separation. Thus, a range of racemic N-containing compounds
including amino acids, amino esters, and some drugs were well
separated under acidic eluent conditions, with separation
G
https://dx.doi.org/10.1021/jacs.0c11276
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
pubs.acs.org/JACS
Article
Notes
performances comparable to or even superior to those of some
commercial CSPs. Controlled experiments and DFT calcu-
lations revealed that chiral crown ether moieties combined
with the confined space account for the efficient separation.
This work thus advances MOFs as a platform for chiral
resolution in reversed-phase liquid chromatography and will
promote the design of more robust Zr-MOFs for the
recognition and separation of racemic fine chemicals and
pharmaceuticals.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Science
Foundation of China (grant nos. 21620102001, 91856204,
91956124, and 21875136), the National Key Basic Research
Program of China (2016YFA0203400), the Key Project of
Basic Research of Shanghai (17JC1403100 and 18JC1413200),
the Shanghai Rising-Star Program (19QA1404300), and the
China Postdoctoral Science Foundation (2020M681280). We
thank the staff from the BL17B beamline of the National
Facility for Protein Science in Shanghai (NFPS) at the
Shanghai Synchrotron Radiation Facility, for assistance during
data collection.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c11276.
Experimental procedures and characterization data
(PDF)
REFERENCES
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X-ray crystallographic data for CE-1 (CIF)
X-ray crystallographic data for CE-2 (CIF)
X-ray crystallographic data for CE-3 (CIF)
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AUTHOR INFORMATION
■
Corresponding Authors
Yan Liu − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China;
orcid.org/0000-0002-7560-519X; Email: liuy@
sjtu.edu.cn
Jianwen Jiang − Department of Chemical and Biomolecular
Engineering, National University of Singapore, Singapore
117576, Singapore; orcid.org/0000-0003-1310-9024;
Email: chejj@nus.edu.sg
Yong Cui − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China;
orcid.org/0000-0003-1977-0470; Email: yongcui@
sjtu.edu.cn
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Hong Jiang − School of Chemistry and Chemical Engineering,
Frontiers Science Center for Transformative Molecules and
State Key Laboratory of Metal Matrix Composites, Shanghai
Jiao Tong University, Shanghai 200240, China;
orcid.org/0000-0002-4118-5297
Kuiwei Yang − Department of Chemical and Biomolecular
Engineering, National University of Singapore, Singapore
117576, Singapore
Xiangxiang Zhao − School of Chemistry and Chemical
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Molecules and State Key Laboratory of Metal Matrix
Composites, Shanghai Jiao Tong University, Shanghai
200240, China
Wenqiang Zhang − School of Chemistry and Chemical
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