Please cite this article in press as: M. Xue, et al., Mater. Today (2016), http://dx.doi.org/10.1016/j.mattod.2016.03.003 Materials Today Volume 00, Number 00 March 2016 RESEARCH Emerging functional chiral microporous materials: synthetic strategies and enantioselective separations Ming Xue 1 , Bin Li 2 , Shilun Qiu 1 and Banglin Chen 2, * 1 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China 2 Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, United States In recent years, chiral microporous materials with open pores have attracted much attention because of their potential applications in enantioselective separation and catalysis. This review summarizes the recent advances on chiral microporous materials, such as metal-organic frameworks (MOFs), hydrogen- bonded organic frameworks (HOFs) and covalent organic frameworks (COFs). We will introduce the synthetic strategies in detail and highlight the current status of chiral microporous materials on solid enantioselective adsorption, chiral chromatography resolution and membrane separation. Introduction Microporous materials, containing pores with diameters less than 2 nm, typically display permanent porosities due to the retention of their structural integrity after removal of all the guest molecules [1]. On the basis of framework compositions, there are three types of crystalline porous solids: inorganic framework materials (e.g. aluminosilicate zeolites) [2], inorganic–organic hybrid framework solids (variously denoted as MOFs or PCPs) [3–7], and organic framework materials (e.g. HOFs and COFs) [8,9]. Given the fact that both the chirality and porosity are very important roles in chemistry and biology, chiral microporous materials with open pores have attracted increasing attention in recent years due to their potential applications in enantioselective separation and catalysis [10–12]. However, it is still a grand challenge to design a crystalline material that combines both chirality and porosity properties into one framework [13,14]. Many inorganic framework materials have chiral crystal structures in both right- and left- handed forms in the same bulk solids. There are some interesting zeolite frameworks that have been identified as intrinsic chirality, including BEA, CZP, GOO, -ITV, JRY, LTJ, OSO, SFS and STW; however, these materials almost invariably crystallize as racemic conglomerates [15–17]. The syntheses of enantioenriched zeolite and zeolite-type materials have been still a very challenging goal. Some approaches have been developed to prepare such inorganic microporous materials using the chiral structure directing agents (SDA) or chiral induction to transfer the chirality into the inor- ganic frameworks but with a limited success [18]. Compared to the syntheses of zeolites, homochiral MOFs/PCPs/HOFs/COFs can be readily self-assembled using chiral molecules as the primary linkers or chiral molecules as a supplementary or auxiliary ligands [19]. Although the synthesis of homochiral MOFs/PCPs/HOFs/COFs is relatively easy, the use of these frameworks in chiral separations in the liquid phase is rather limited so far. In this review, we will focus on the current methodologies for the synthesis of chiral micropo- rous MOF, PCP, HOF and COF materials and their applications on enantioselective adsorption [20], chiral chromatography resolu- tion [21,22] and membrane separation [23,24]. Synthetic strategies When we discuss chirality in solid materials, two different aspects need to be considered: firstly, whether the components of the structures are chiral themselves; secondly, whether the arrange- ment of components into the solid is chiral [25]. Chemists prefer the concept of chirality in molecules, where such chirality comes from an asymmetric carbon or other chiral centers. The same rule can also apply to crystal structures of solid materials [26–30]. To date, several strategies have been reported for the development of chiral materials, which include the introduction of chiral ligands or chiral templates, the influence of the chiral physical environ- ment, spontaneous resolution without any chiral auxiliary, and post-synthesis by synthetic modification of the organic struts or the metal nodes via guest exchange [31–34]. In general, direct RESEARCH: Review *Corresponding author: Chen, B. ([email protected]) 1369-7021/ß 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). http://dx.doi.org/10.1016/ j.mattod.2016.03.003 1
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RESEARCH:Review
Materials Today � Volume 00, Number 00 �March 2016 RESEARCH
Emerging functional chiral microporousmaterials: synthetic strategies andenantioselective separationsMing Xue1, Bin Li2, Shilun Qiu1 and Banglin Chen2,*
1 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China2Department of Chemistry, University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249-0698, United States
In recent years, chiral microporous materials with open pores have attracted much attention because of
their potential applications in enantioselective separation and catalysis. This review summarizes the
recent advances on chiral microporous materials, such as metal-organic frameworks (MOFs), hydrogen-
bonded organic frameworks (HOFs) and covalent organic frameworks (COFs). We will introduce the
synthetic strategies in detail and highlight the current status of chiral microporous materials on solid
enantioselective adsorption, chiral chromatography resolution and membrane separation.
IntroductionMicroporous materials, containing pores with diameters less than
2 nm, typically display permanent porosities due to the retention
of their structural integrity after removal of all the guest molecules
[1]. On the basis of framework compositions, there are three types
of crystalline porous solids: inorganic framework materials (e.g.
Materials Today � Volume 00, Number 00 �March 2016 RESEARCH
MATTOD-729; No of Pages 13
FIGURE 2
(a) Ball-and-stick model of an infinite 1D [Cd4(H2O)4(m2-h1,h1-CO2)4(m2-h
2,h1-CO2)4] chain. (b) Ball-and-stick model showing the structure with the simplified Lligand, viewed along the [010] direction. (c) Analyte inside a MOF channel. The (S)-AA4 molecule is represented by a space-filling model, while the
framework is represented by a stick/polyhedron model. (Adapted with permission from Ref. [51].)
RESEARCH:Review
‘privileged ligands’ into robust and porous MOFs, a new generation
of single-site solid catalysts can be envisioned for broad-scope
asymmetric reactions that are needed for synthetic manipulations
of complex molecules (Fig. 4).
In 2008, Bu and coworkers described unusual integrated homo-
chirality features in six 3D MOFs, in which enantiopure building
blocks embedded in intrinsically chiral topological quartz nets
Please cite this article in press as: M. Xue, et al., Mater. Today (2016), http://dx.doi.org/10.1
FIGURE 3
(a) The mesoporous cage and (b) the 3D porous network in mesoporous
MOF viewed along the c axis. (Adapted with permission from Ref. [52].)
[57]. These materials were prepared by D- or L-camphoric acid and
trivalent or divalent metal ions in the presence of achiral template
cations or molecules under solvothermal conditions. Single crystal
analysis showed that all six compounds have three homochiral
features: 3D intrinsically homochiral net (quartz, quartz dual or srs
net), homohelicity, and enantiopure molecular chirality. It is
worth mentioning that the absolute helicity in each case can be
controlled by the chirality of molecular building blocks. In 2014,
they further successfully developed a low-cost homochiral MOF
platform based on the inexpensive D-camphoric and formic acid,
which can be used to selectively crystallize or enrich specific
lanthanide ions in predesigned MOFs [58]. By systematic synthetic
and structural studies of crystallization of a large series of homo-
chiral rare-earth camphorates, they demonstrated that crystalliza-
tion processes by Ln3+ ions are very sensitive to ionic radii and the
ionic radius difference between two Ln3+ ions dictates the unequal
concentrations of Ln3+ in Ln-MOF crystals. For some Ln3+ combi-
nations, the selectivity for a particular Ln3+ is nearly exclusive,
which permits one-step separation of two Ln3+ elements (Fig. 5).
As discussed above, great progress has been achieved on the
direct synthesis of chiral microporous materials in the past dec-
ades. In comparison, there has been very little progress in the area
016/j.mattod.2016.03.003
FIGURE 4
Post-synthetic metalation of BINAP-MOF (1) to form 1�Ru and 1�Rh.(Adapted with permission from Ref. [55].)
Materials Today � Volume 00, Number 00 �March 2016 RESEARCH
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FIGURE 6
(a) The [Mn(adc)]n chain based on achiral adc ligand with m4-coordination; (b) the porous [Mn3(HCOO)4]n2+ channel based on inorganic Mn–O–Mn
connectivity; (c) two types of enantiopure catalysts used for synthesis and chiral induction; The directions of arrows show the possible mechanism of chiralinduction. D-Camphoric acid initially controls the absolute chirality of [Mn3(HCOO)4]n
2+ frameworks but is later displaced by adc. (d) The 3D hybrid
framework, showing the achiral [Mn(adc)]n chains attached to the wall of the nanosized channels; (e) the solid-state CD spectra. Each curve represents the
signal from the sample of an independent synthesis. (Adapted with permission from Ref. [61].)RESEARCH:Review
(10,3)-a networks are consisting of the octahedral metal centers
coordinated to the tridentate 1,3,5-benzenetricarboxylate (BTC)
ligand. The alcohol and pyridine molecules are coordinated to the
equatorial plane of the metal center. Ethylene glycol (eg) bound in
a unidentate fashion to form Ni3(BTC)2(py)6(eg)6 with fourfold
interpenetration, while chiral 1,2-Propanediol (1,2-pd) is coordi-
nated as a bidentate ligand to form Ni3(BTC2)(py)6(1,2-pd)3 with
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FIGURE 7
Synthesis and structure of stable crystalline porous COFs. (a) Synthesis of TPB-DMTP-COF through the condensation of DMTA (blue) and TAPB (black). Inset:
The structure of the edge units of TPB-DMTP-COF and the resonance effect of the oxygen lone pairs that weaken the polarization of the C N bonds and
soften the interlayer repulsion in the COF. (b) Graphic view of TPB-DMTP-COF (red, O; blue, N; gray, C; hydrogen is omitted for clarity). (c) Synthesis of chiral
COFs ([(S)-Py]x-TPB-DMTP-COFs, x = 0.17, 0.34 and 0.50; blue, DMTA; black, TAPB; red, BPTA; green, (S)-Py sites) via channel-wall engineering using a three-component condensation followed by a click reaction. (Adapted with permission from Ref. [65].)
RESEARCH:Review
displays significant enantioselective sorption ability for the aro-
matic sulfoxides with small substituents (ee values 20% and 27%)
in favor of S isomer. Although various porous materials including
zeolites, activated carbon, silica gel and various polymer resins
have shown to be useful stationary phases in gas chromatography,
liquid chromatography and electrochromatography, MOFs are far
Please cite this article in press as: M. Xue, et al., Mater. Today (2016), http://dx.doi.org/10.1
6
less explored for these applications. In 2007, Fedin and Bryliakov
et al. presented a quantitative study of the enantioselective sorp-
tion properties of Zn-BLD in detail [73]. This work represents the
first example of chiral liquid chromatographic (LC) column for
separation of racemic mixtures of chiral alkyl aryl sulfoxides
(Fig. 9). In 2011, Kaskel and coworkers reported the synthesis of
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FIGURE 11
(a) Leica picture of the surface of the Ni2(L-asp)2(bipy) membrane. SEM pictures of the surface of (b) the Ni2(L-asp)2(bipy) membrane and (c) details of the
densely packed crystallites. (d) A cross-section SEM picture of the Ni2(L-asp)2(bipy) membrane. (Adapted with permission from Ref. [80].)
RESEARCH:Review
H2BDC, leading to the chirality [85]. Furthermore, such chiral
cavities can be straightforwardly tuned by incorporation of differ-
ent bicarboxylate CDC (CDC = 1,4-cyclohexanedicarboxylate) in
Zn3(CDC)3[Cu(SalPycy)]�(G)x (M0MOF-3), which exhibits signifi-
cantly enhanced enatioselective recognition of 1-phenylethyl al-
cohol (PEA). M0MOF-2 and -3 are isostructural three-dimensional
frameworks, exhibiting two chiral pore cavities of about 6.4 A in
diameter (Fig. 12). Particularly, the enantiopure M0MOF-3 could
exclusively take up S-PEA to form M0MOF-3@S-PEA (Zn3(CDC)3
[Cu(SalPyCy)]�S-PEA). The incorporated S-PEA can be easily
extracted from the chiral pores by immersing the as-synthesized
M0MOF-3@S-PEA into methanol, suggesting its potential for enan-
tioselective separation of R/S-PEA. Furthermore, the chiral recog-
nition and enantioselective separation of the R/S-PEA racemic
mixture were also examined for using the bulky as-synthesized
M0MOF-2 and -3 materials. Chiral HPLC analysis of the desorbed
PEA from the PEA-included M0MOF-2 yields an ee value of 21.1%,
and the absolute S configuration for the excess was confirmed by
comparing its optical rotation with that of the standard sample. It
must be noted that the used M0MOF-2 keeps high crystallinity and
can be regenerated simply by the immersion into the excess
amount of methanol, and thus for further resolution of racemic
Please cite this article in press as: M. Xue, et al., Mater. Today (2016), http://dx.doi.org/10.1
10
R/S-PEA. The second and third regenerated M0MOF-2 samples
provide an ee value of 15.7 and 13.2%, respectively. The low
enantioselectivity of M0MOF-2 for the separation of R/S-PEA might
be attributed to its large chiral pore environments, which limits its
high recognition of S-PEA. The smaller chiral pores within M0MOF-
3 have significantly enhanced its enantioselectivity for separation
of R/S-PEA with the much higher ee value of 64% compared with
that of M0MOF-2. The regenerated M0MOF-3 can also be further
used for the separation of R/S-PEA with the slightly lower ee value
of 55.3 and 50.6%, respectively. The chiral pores within M0MOF-2
and M0MOF-3 basically match well with the size of S-PEA, which
are not able to separate larger alcohol enantiomers, such as 1-(p-
tolyl)-ethanol, 2-phenyl-1-propanol and 1-phenyl-2-propanol.
Fine tuning of micropores within porous materials is very
crucial and important to maximize their size-selective effects for
separation. This new M0MOF approach has provided us an ideal
platform to tune and functionalize the micropores within this
series of isoreticular M0MOFs. The main strategies involve the
incorporation of different secondary organic linkers, the immobi-
lization of different metal sites such as Ni2+, Co2+, Zn2+, Pd2+ and
Pt2+, and derivatives of the precursor by the usage of other organic
groups such as t-butyl instead of methyl group. This enables us to
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FIGURE 13
X-ray crystal structure of HOF-2 featuring (a) multiple hydrogen bonding
(light-blue dashed lines) among adjacent units to form three-dimensionalhydrogen-bonded organic framework exhibiting 1D hexagonal pores with
the diameter of about 4.8 A along the c axis and (b) the uninodal 6-
connected {3355667} network topology. X-ray crystal structure of HOF-2�R-1-PEA indicating (c) the enantiopure R-1-PEA molecules residing in thechannels of the framework along the c axis and (d) the chiral cavities of
the framework for the specific recognition of R-1-PEA which is further
enforced by the hydrogen-bonding interactions among the –OH groups ofR-1-PEA (green molecule) and oxygen atoms of the diethoxy groups from
the HOF-2 framework. Comparison of X-ray crystal structures of (e) HOF-
2�S-1-PEA and (f ) HOF-2�R-1-PEA, indicating the different recognition of
the HOF-2 for these two enantiomers (C, gray; H, white; N, pink; O, red).(Adapted with permission from Ref. [92].)
RESEARCH:Review
(DAT) is a very powerful hydrogen-bonding motif for the construc-
tion of porous robust HOFs and the BINOL is the organic backbone
for asymmetric induction, we successfully synthesized the first ex-
ample of porous homochiral HOFs with the highly enantioselective
separation of small molecules (Fig. 13) [92]. HOF-2 systematically
displays higher enantioselective separation for aromatic secondary
alcohols than for aliphatic secondary alcohols (1-PEA > 1-(4Cl-