Emerging functional chiral microporous materials

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.

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

Please cite this article in press as: M. Xue, et al., Mater. Today (2016), http://dx.doi.org/10.1

*Corresponding author: Chen, B. ([email protected])

1369-7021/� 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC B

j.mattod.2016.03.003

(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 strategiesWhen 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

016/j.mattod.2016.03.003

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RESEARCH Materials Today � Volume 00, Number 00 �March 2016

MATTOD-729; No of Pages 13

RESEARCH:Review

synthesis, chirality induction synthesis and post-synthesis are three

main well-established strategies for the construction of homochiral

microporous materials [35–40]. Of these mentioned strategies, the

most effective method is to utilize enantiopure organic linkers as

cross-linking ligands or chiral co-ligands to control the stereochem-

istry at the metal centers. The direct participation of chiral compo-

nents into the assembly process warrants the chirality in the

resulting network structures [41–44]. As shown in Fig. 1, there

are many chiral ligands that have been reported to be used for

the construction of chiral microporous structures.

Heterogenization of chiral molecular catalysts within porous

MOFs has been extensively studied by Kim, Lin, Hupp and Cui

et al. over the past 15 years [45–49]. On the basis of readily

available chiral 1,10-bi-2-naphthol (BINOL), Lin and coworkers

have designed a series of chiral pyridyl, carboxylate, and phos-

phonate bridging ligands with orthogonal functional groups [50].

For example, in 2012 they synthesized a highly porous and fluo-

rescent MOFs [Cd2(L)(H2O)2]�6.5DMF 3EtOH, built from a chiral

tetracarboxylate bridging ligand derived from BINOL and a cad-

mium carboxylate infinitechain secondary building unit (SBU).

This material has been demonstrated to be an enantioselective

fluorescence sensor for amino alcohols (Fig. 2) [51].

Recently, Cui and coworkers designed and synthesized two

chiral carboxylic acid functionalized micro and mesoporous


FIGURE 1

Some chiral ligands used for construction of chiral microporous materials.

2

MOFs, both of which are constructed by the stepwise assembly

of triple-stranded heptametallic helicates with six carboxylic acid

groups [52]. The mesoporous MOF with permanent porosity acts as

a host for encapsulation of an enantiopure organic amine catalyst

by combining carboxylic acids and chiral amines in situ through

acid-base interactions. This material crystallizes in a chiral hexag-

onal space group P6322 with a (4,6)-connected 3D framework. The

1D channel can be viewed as being composed of several cylindrical

cages with D6 symmetry, each of which is enclosed by twelve Zn7

helicates and six Zn4O clusters. The cage has a height of about

1.4 nm and a maximum inner width of about 2.36 nm. The

hexagonal apertures that surround by six free carboxylic groups

on the top and bottom faces have a diagonal distance of about

1.6 nm � 1.4 nm (Fig. 3).

In the past few years, the UiO family of MOFs with the Zr6(m3-

O)4(m3-OH)4 SBUs and linear dicarboxylate organic linkers has

particularly provided an ideal platform for the design of highly

efficient MOF catalysts because of their stability in a broad range of

solvents and harsh reaction conditions [53,54]. Recently, Lin et al.

reported a robust and porous BINAP-MOF with UiO topology and

its post-synthetic metalation with Rh- and Ru complexes [55,56].

The resulting materials can catalyze a range of asymmetric reactions

including the 1,4-addition of aryl boronic acids, 1,2-addition of

AlMe3, and hydrogenation of ketones and alkenes. By incorporating

016/j.mattod.2016.03.003




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


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].)

3




FIGURE 5

Comparison between 1-Ln and 2-Ln. (a) Local coordination environment in1-Ln; (b) the inorganic chain in 1-Ln (water molecules are shown as faded);

(c) 3D-framework of 1-Ln through linkage between chains; (d) local

coordination environment in 2-Ln; (e) the inorganic chain in 2-Ln; (f ) 3D-framework of 2-Ln through linkage between inorganic chains (purple: Ln,

red: O, black: C, purple polyhedra was defined using Ln as central atom

and O as ligand atom). (Adapted with permission from Ref. [58].)

RESEARCH:Review

of asymmetric crystallization from achiral precursors to form

enantiopure or enantioenriched crystalline microporous materi-

als. In 2004, we synthesized a 2D layer coordination polymer

Co(PDC)(H2O)2�H2O containing two helical chains by using an

achiral ligand. The synthesis did not involve any chiral reactant,

solvent and other auxiliary agent. However, the resultant crystals

are not racemic as indicated by the strong signals in the vibrational

circular dichroism (VCD) spectrum [59]. In addition, the absolute

chirality of an intrinsically chiral three-connected net, induced by

the coordination of enantiopure solvent molecules into the frame-

work, was reported by Rosseinsky and coworkers [60]. Recently, Bu

and coworkers demonstrated that an unusual asymmetric crystal-

lization in a new 3D porous material can be entirely constructed

from achiral building units by making use of enantiopure organic

acids (camphoric acid) or amino acids (glutamic acid) as the

chirality-inducing agents (Fig. 6) [61].

As an emerging class of porous crystalline polymers, COF mate-

rials have attracted much attention for a wide range of potential

applications in gas storage and separation, energy storage, and

heterogeneous catalysis [62–64]. COFs are mainly composed of

light elements, such as H, B, C, N, and O, which crystallize into

polymeric networks with highly ordered internal structures by

strong covalent bonds. Since the pioneering work of Yaghi in 2005,

a large number of COFs have been developed in recent years [8].

However, few chiral COFs have been reported. Very recently, Jiang

et al. synthesized a crystalline mesoporous TPB-DMTP-COF, which

is stable in water, strong acids and strong bases [65]. Considering

its high crystallinity and large mesoporous channels, they further

decorated TPB-DMTP-COF into a catalytic chiral open framework

by anchoring chiral centers and organocatalytic sites onto the

channel walls via post-synthetic functionalization (Fig. 7). This

strategy may facilitate the design of COFs in which the combina-

tion of stability, crystallinity, chirality and porosity would be

particularly useful in a variety of functions and applications.

Finally, it is of great importance that the bulk sample must be

homochiral. A combination of bulk measurement, such as circular

dichroism (CD) spectra, VCD spectrum, or multiple single-crystal


4

X-ray structure determination, is the most popular way to attain

enough evidences for the homochirality of the bulk material

[66,67]. Furthermore, homochiral measurements can be further

supplemented by some experiments, particularly chiral separa-

tions [20,68].

Enantioselective adsorption and separationBecause of the significant difference in the biological and phar-

macological properties for the isomers of chiral compounds, much

attention has been dedicated to the chiral resolution. Homochiral

microporous materials have been demonstrated to be not only

potential candidates for heterogeneous asymmetric catalysts but

also enantioselective adsorbents or separators for the production

of optically active organic compounds. Therefore, the investiga-

tion of adsorption and diffusion of enantiomeric molecules in

homochiral microporous materials is essential and important to

promote these materials into the use of chiral resolution. Table 1

summarizes most of chiral microporous materials that exhibit the

corresponding properties on chiral resolution.

In 2000, Kim et al. synthesized a homochiral MOF (D-POST-1),

built up by the oxo-bridged trinuclear metal carboxylates cluster

and the enantiopure chiral ligand derived from D-tartaric acid [69].

The large chiral 1D channels exist along the c axis, and the void

volume of the channels filled with water molecules is estimated to

be around 47% of the total volume. Although D-POST-1 loses

crystallinity after removal of the solvate molecules by evacuation,

it is stable in most of normal organic solvents and its framework

can restore upon exposure of the evacuated sample to ethanol or

water vapor. When D-POST-1 was exposed to a racemic methanol

solution of the chiral [Ru(2,20-bipyridine)3]Cl2 complex, an enan-

tiomeric excess (ee) of 66% was achieved as confirmed by NMR,

UV–vis spectroscopy and CD measurement. The enantioselective

adsorption property was successfully established for the first time

by the homochiral MOFs.

Quinine is the off-the-shelf antimalarial alkaloid. Xiong et al.

prepared a new enantiopure chiral ligand HQA (60-methoxyl-

(8S,9R)-cinchonan-9-ol-3-carboxylic acid) from the quinine, and

utilized it to synthesize a homochiral MOF [Cd(QA)2]. Each Cd2+

ion acts as a 4-connected node to construct a non-interpenetrated

diamond net. To investigate the enantioselective separation prop-

erty, the powder sample of Cd(QA)2 was under solvothermal

reaction in the racemic 2-butanol solution for three days at

1008C. The crystalline sample of (S)-2-butanol@Cd(QA)2 was

obtained and the single-crystal X-ray structural determination

clearly revealed that (S)-2-butanol was included into the chiral

cavity. The ee value of the 2-butanol desorbed from (S)-2-buta-

nol@Cd(QA)2 was estimated to be approximately 98.2%. When the

larger racemic 2-methyl-1-butanol was used, the ee value of (S)-2-

methyl-1-butanol was found to be only 8.2%. Such striking differ-

ence in selectivity for chiral molecules of different size indicates

that a sufficient match between pore dimension and the size of

chiral guest is required for enantioselective adsorption [70].

Rosseinsky et al. made the significant contribution to the enan-

tioseparation of homochiral MOFs. In 2000, they reported that the

stereochemistry of the alcohol bound to the metal can control the

helicity of the chiral framework, which is the first example that

chiral molecules can specifically template helix handedness in a

chiral microporous material [60]. Multiple interpenetrated chiral

016/j.mattod.2016.03.003




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

twofold interpenetration. Furthermore, Ni3(BTC2)(py)6(1,2-pd)3

can be grown homochirally from enantiomerically pure diol tem-

plate. Ni3(BTC2)(py)6(1,2-pd)3 is unstable and nonporous with a


very low BET surface area of 12.6 m2 g�1. However, the isostruc-

tural Ni3(BTC2)(3-pic)6(1,2-pd)3 has the permanent chiral porosity

originated from the replacement of pyridine with 3-picoline

(3-methyl pyridine), which has the enhanced stability and BET

surface area of 930 m2 g�1. Then the enantioselectivity was ob-

served for menthone and fenchoneand binaphthol, and an ee of

8.3% can be achieved for the larger binaphthol [71].

In 2006, Kim and Fedin et al. synthesized a 3D porous homo-

chiral MOF [Zn2(bdc)(L-lac)(DMF)] (Zn-BLD, Fig. 8), possessing the

intrinsic chirality due to the chiral L-lactic acid [72]. This material

016/j.mattod.2016.03.003

5




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


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

016/j.mattod.2016.03.003




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

TABLE 1

Summary of chiral microporous materials for chiral resolution.

Chiral microporous materials Ligands Enantiomers ee (%) Ref

D-POST-1 (4S,5S)-2,2-dimethyl-5-[(4-pyridinylamino)

carbonyl]-1,3-dioxolane-4-carboxylic acid

[Ru(2,20-bipyridine)3]Cl2 66 [69]

[Cd(QA)2] 60-Methoxyl-(8S,9R)-cinchonan-

9-ol-3-carboxylic acid (HQA)

2-Butanol 98.2 [70]

2-Methyl-1-butanol 8.2

Ni3(btc)2(3-pic)6(1,2-pd)3�[(1,2-pd)9(H2O)11]

1,3,5-Benzenetricarboxylic acid (btc) Binaphthol 8.3 [71]

1,2-Propanediol (1,2-pd)

3-Picoline (3-pic)

[Zn2(bdc)(L-lac)(DMF)] 1,4-Benzenedicarboxylate (bdc) Methyl phenyl sulfoxide 20 [72]

L-Lactic acid (L-lac) 1-Bromo-4-(methylsulfonyl)benzene 27

[Zn2(bdc)(L-lac)(DMF)] 1,4-Benzenedicarboxylate (bdc)

L-Lactic acid (L-lac)

p-BrPhSOMe 7 [73]

PhSOMe 60

p-MePhSOMe 38

PhSOi-Pr 55

[Zn2(bdc)(S-lac)(DMF)] L-(�)-Lactate (S-lac)

1,4-Benzenedicarboxylate (bdc)

2-Butanol 14 [74]

2-Methyl-1-butanol 71-Phenyl-1-ethanol 21

1-Phenyl-1-propanol 12

[Zn2(bdc)(L-lac)(DMF)]

membraneL-Lactic acid (L-lac) Methyl phenyl sulfoxide 33 [77]

Fe3O4@SiO2-ZnBLD 1,4-Benzenedicarboxylate (bdc)

L-Lactic acid (L-lac)

Phenyl methyl sulfoxide 85.2 [78]Phenyl vinyl sulfoxide 76.7

4-Chlorophenyl methyl sulfoxide 29

Fe3O4@SiO2-ZnBDD 1,4-Benzenedicarboxylate (bdc)

D-Lactic acid (D-lac)

Phenyl methyl sulfoxide 86.2 [78]

Phenyl vinyl sulfoxide 77

4-Chlorophenyl methyl sulfoxide 29.9

Ni2(L-asp)2(bipy) L-Aspartic acid (L-asp)

4,40-Bipyridyl (bipy)

1,2-Propanediol 5.35 [79]

1,3-Butanediol 17.93

1,2-Butanediol 5.07

2,3-Butanediol 1.51,2-Pentanediol 13.9

2,4-Pentanediol 24.5

2-Methyl-2,4-pentanediol 53.772,5-Hexanediol 3.4

1,2-Hexanediol 5

Ni2(L-asp)2bipy membrane L-Aspartic acid (L-asp)


2-Methyl-2,4-pentanediol 32.5 [80]

Ni2(L-asp)2bipy membrane L-Aspartic acid (L-asp)


2-Methyl-2,4-pentanediol 35.5 [81]

Zn3(bdc)3[Cu(SalPycy)]

M0MOF-2

1,4-benzenedicarboxylate (bdc)

Cu(SalPycy)

1-Phenylethanol 21.1 [85]

Zn3(cdc)3[Cu(SalPycy)]

M0MOF-3

1,4-Cyclohexanedicarboxylate (cdc)

Cu(SalPycy)

1-Phenylethanol 64 [85]

Cd3(bdc)3[Cu(SalPyMeCam)]

M0MOF-4

1,4-Benzenedicarboxylate (bdc)

Cu(SalPyMeCam)


2-Butanol 45.2

2-Pentanol 27.9

2-Heptanol <4

Zn3(cdc)3[Cu(SalPyMeCam)]

M0MOF-5


Cu(SalPyMeCam)


2-Butanol 72.52-Pentanol 62.2

2-Heptanol <9

Cd3(bdc)3[Cu(SalPytBuCy)]M0MOF-6

1,4-Benzenedicarboxylate (bdc)Cu(SalPytBuCy)

1-Phenylethanol 46.2 [86]2-Butanol 49.6

2-Pentanol 39.7

2-Heptanol <6

Zn3(cdc)3[Cu(SalPytBuCy)]

M0MOF-7


Cu(SalPytBuCy)


2-Butanol 77.1

2-Pentanol 65.92-Heptanol <10

7

RESEARCH:Review




TABLE 1 (Continued )

Chiral microporous materials Ligands Enantiomers ee (%) Ref

(S)-[DyNaL(H2O)4] 3,30-Di-tert-butyl-5,50-di(3,5-carboxyphenyl-1-yl)-6,60-dimethylbiphenyl-2,20-diol ligand (H4L)

L-Methyl mandelate 93.1 [87]

Ethyl mandelate 64.3i-Propyl mandelate 90.7

D-Methyl mandelate 89.9

Benzyl mandelate 73.5

Zn2(cam)2(dabco)thin film

Camphoric acid (cam)1,4-Diazabicyclo(2.2.2)

octane (dabco)

2,5-Hexanediol 21.6 [88]

[Co-(Tt)2][Cu4(D-cam)4] Tris(triazolyl)borate (Tt)

D-Camphorate (D-cam)

1-Phenyl-2-propanol 13.8 [89]

Cu2(D-cam)2(dabco) D-Camphorate (D-cam) Limonene 17 [90]

Cu2(D-cam)2(bipy) Diazabicyclo[2.2.2]octane(dabco)

35

Cu2(D-cam)2(BiPyB)

thin films


1,4-bis(4-pyridyl)benzene (BiPyB)

8

D-his–ZIF-8 D-Histidine Alanine 78.52 [91]

Glutamic 79.44

HOF-2 (R)-1,10-Bi-2-naphthol scaffold

into 2,4-diaminotriazinyl


1-(4-Chlorophenyl)ethanol 79

1-(3-Chlorophenyl)ethanol 662-Butanol 77

2-Pentanol 48

2-Hexanol <10

2-Heptanol <4

RESEARCH:Review

in situ incorporation of chiral auxiliaries into UMCM-1 (Zn4O

(BTB)4/3(BDC)) structure to construct two chiral porous MOFs

iPr-Chir-UMCM-1 and Bn-Chir-UMCM-1. Bn-Chir-UMCM-1 was

successfully applied as the stationary phase for high performance

liquid chromatography (HPLC) enantioseparation. Particularly,

1-phenylethanol shows both selective and enantioselective


FIGURE 8

(a) A view of the 1D chiral chains in the structure. (b) Perspective view of

the structure along the a axis. (c) Projection of the structure in the (110)

plane. Hydrogen atoms and guest molecules are omitted for clarity. Zngreen, N blue, O red, C gray; chiral C atoms of the lactic acid ligand are

shown in white. (Adapted with permission from Ref. [72].)

8

interaction with Bn-Chir-UMCM-1. The potential for enantiose-

paration can be clearly seen from the selectivity, which is high

enough to reach enantiomer separation [75]. In 2011, Yuan et al.

successfully utilized chiral MOFs Cu(sala)n (H2sala = N-(2-hydro-

xybenzyl)-L-alanine) as stationary phases in gas chromatographic

(GC) separation of chiral compounds. The column not only has an

excellent selectivity, but also possesses good recognition ability

toward a wide range of organic compounds such as alkanes,

alcohols, and isomers. To demonstrate that this single-handed

016/j.mattod.2016.03.003

FIGURE 9

Separation of alkyl aryl sulfoxides using Zn2(bdc)(L-lac)(DMF) as the chiral

stationary phase. Eluents: (a, b) 12 cm3 of 0.01 M DMF solution in CH2Cl2,then 1% DMF in CH2Cl2; (c, d) 20 cm3 of 0.01 M DMF solution in CH2Cl2,

then 1% DMF in CH2Cl2. Elution rate = 2 cm3/h. (Adapted with permission

from Ref. [73].)




FIGURE 10

(a) The connection of chiral Ni(L-asp) layers by 4,40-bipyridine ligands

produces framework Ni2(L-asp)2(bipy), which contains channels lined with

amino acid residues. The disordered methanol and water guests thatoccupy the channels are represented with space-filling spheres. Hydrogen

atoms and the minor disorder component of the 4,40-bipyridine ligands are

omitted for clarity. (b) Part of the Ni(L-asp) layer, showing the coordination

environments of the nickel centers. Ni cyan, C gray (chiral centers yellow),H white, N blue, O red, chiral C atoms are shown in yellow. (Adapted with

permission from Ref. [79].)

RESEARCH:Review

helical channel is appropriate for the enantioseparation, they used

metal-complex [Cu(sala)]2�2H2O as a comparison. The results

indicate that the helical channels of this MOF make a significant

contribution to the chiral separation in GC [76].

Meanwhile, membrane separation offers great promise nowa-

days owing to incomparable preponderance over traditional meth-

ods, such as low-energy consumption, large processing capacity,

and a continuous mode of operation. Owing to their well-defined

porosity and stability, zeolites and mesoporous membranes have

attracted intense interest in engineering applications such as gas or

liquid separations, membrane reactors and chemical sensors,

among others. However, it is still very challenging to synthesize

these materials with chirality, which is the core for chiral separa-

tion. In 2012, Jin et al. for the first time reported a new generation

of a chiral separation membrane composed of homochiral Zn-BLD,

which was successfully fabricated on a porous zinc oxide substrate

by a reactive seeding technique [77]. This membrane is stable

enough for chiral separation driven by the concentration differ-

ence across the membrane. The resolution process was carried out

by a ‘side-by-side diffusion cell’ and readily separated by the as-

prepared membrane. After 48 hours separation, the enantioselec-

tivity was observed to be the highest and the ee value can reach 33.

The preferential diffusion of R-MPS across the Zn-BLD membrane

suggests that R-MPS has a weaker affinity for the membrane

compared with S-MPS. This was confirmed by the adsorption

separation behavior of racemic MPSs in the Zn-BLD crystals.

The result was further confirmed by the following simulation data.

It can be found that R-MPS transports faster than S-MPS and the

two enantiomers can be separated, which is in accordance with the

experimental data.

Amino acids, which are cheap and commercially available, are

naturally occurring ligands as chiral building blocks for homochiral

MOFs. In 2006, aspartic acid (NH2CH(COOH)CH2COOH, aspH2)

was utilized to synthesize the microporous chiral MOF [Ni2(L-

asp)2(bipy)] [79]. Neutral chiral Ni(L-asp) layers are connected by

4,40-bipyridine linkers to afford a pillared structure with one-dimen-

sional channels of 3.8 A � 4.7 A. The immobilization of the chiral

carbon atoms of aspartate units into the channels imparts chiral

functionality on the internal surface of this material (Fig. 10). The

enantioselective adsorption of nine chiral diols with closely related

functionalities was investigated at 278 K, which demonstrated that

a good match of size and shape between the small chiral guest and

the chiral pore of the homochiral framework is the decisive factor for

chiral resolution application. 2-Methyl-2,4-pentanediol shows the

highest enantiomer excess 53.7%, attributable that both hydroxyl

groups of (S)-2-methyl-2,4-pentanediol are involved in hydrogen

bonding within the chiral channels.

Recently we facilely synthesized this homochiral MOF mem-

brane using an in situ growth method on the nickel net (Fig. 11).

The MOF membrane possesses chiral channels and has excellent

thermal stability. A diol isomer mixture (2-methyl-2,4-pentane-

diol) was used to test their separation efficiency. The higher

penetration amount of R diols through the membrane is largely

attributed to our assumption that there is a geometry-dependent

interaction between the chiral channel and the optical isomer

guests, so it is easier for R-diols to enter the membrane pores than

S-diols. A temperature–pressure-related membrane performance of

homochiral MOF membranes was observed for the first time,


which could be an important issue in the development of chiral

resolution. As the temperature increases, less S enantiomers are

adsorbed and R enantiomers can diffuse in the resulting free

volume. The selectivity of the membrane can be improved as

the temperature increases, and the resulting ee value reaches

32.5% at 2008C [80].

Kitagawa and Hupp et al. pioneered the research on construc-

tion of porous mixed-metal-organic frameworks (M0MOFs) by

making use of M-Salen metalloligands [82–84]. Such a novel

approach eventually led to several porous M0MOFs for enantiose-

lective separation. Recently, we successfully used this pre-con-

structed building block approach to introduce chiral pockets

within the M0MOFs. To make use of chiral (R,R)-1,2-cyclohexane-

diamine to construct the chiral metalloligand Cu(SalPyCy), enan-

tiopure M0MOF Zn3(BDC)3[Cu(SalPycy)]�(G)x (M0MOF-2) can be

readily assembled by this chiral building block with Zn(NO3)2 and

016/j.mattod.2016.03.003

9




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


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

016/j.mattod.2016.03.003




FIGURE 12

X-ray crystal structures of M0MOF-3 and M0MOF-3@S-PEA. (a) The hexagonal

primitive network topology (Schafli symbol 36418536) and (b) the three-

dimensional (3D) pillared framework with chiral pore cavities for M0MOF-3.

(c) The hexagonal primitive network topology and (d) the 3D pillaredframework exclusively encapsulating S-PEA molecules for M0MOF-3@S-PEA.

(Zn, pink; Cu, cyan; O, red; C, gray; N, blue; H, white). (Adapted with

permission from Ref. [85].)

SCHEME 1

Schematic diagram for the synthesis of four mixed-metal-organic

frameworks (M0MOFs) of tunable chiral pores by making use of different

diamines (red), Terminal alkyl moieties (blue) and organic linkers (green).

(Adapted with permission from Ref. [86].)

RESEARCH:Review

explore novel functional microporous M0MOFs with tunable chiral

pore spaces for the recognition and separation of small molecules.

For example, simply by making use of different chiral diamines

((1R,3S)-1,2,2-trimethyl-1,3-diaminocyclopentane and (1R,2R)-(�)-

1,2-cyclohexanediamine) (red in Scheme 1), chiral pockets with

slightly different pores can be readily constructed. These pores can

be further tuned by both the substituted terminal methyl and tert-

butyl groups (blue in Scheme 1) and the second organic linkers 1,4-

cyclohexanedicarboxylate (CDC) and 1,4-benzenedicarboxylate

(BDC) (green in Scheme 1). Therefore, we synthesized four isostruc-

tural M0MOFs Cd3(BDC)3[Cu(SalPyMeCam)]�(G)x (M0MOF-4), Zn3

(CDC)3[Cu(SalPyMeCam)]�(G)x (M0MOF-5), Cd3(BDC)3[Cu(SalPyt-

BuCy)]�(G)x (M0MOF-6) and Zn3(CDC)3[Cu(SalPytBuCy)]�(G)x(M0MOF-7), which have the same topology with that of M0MOF-2

and M0MOF-3. However, the different chiral pores of M0MOF 4–7

have enabled us to tune their performance for enatioselective

separation of small alcohols such as 1-phenylethanol (1-PEA), 2-

butanol (2-BUT), 2-pentanol (2-PEN), and 2-heptanol (2-HEP) at

room temperature. As expected, these porous M0MOFs display

different recognition behaviors for these four small alcohols. Both

M0MOF-5 and M0MOF-7 constructed from CDC systematically ex-

hibit higher chiral separation for 1-phenylethanol (1-PEA) with ee of

75.3% and 82.4%, respectively, than those of M0MOF-4 (ee of 45.0%)

and M0MOF-6 (ee of 46.2%) assembled from BDC. Among these

M0MOFs, M0MOF-7 is the most efficient material for separation of

1-PEA. Such a high enatioselectivity for M0MOF-7 was mainly


attributed to its larger terminal tert-butyl group, which can notably

decrease the chiral pore space in M0MOF-7. Such a systematic trend

has been also observed in the chiral separation of 2-BUT and 2-PEN.

It needs to be mentioned that the pores of these M0MOFs are flexible

that can be modified by adsorption of different solvent substrates.

Thus their pores can be slightly adjusted to match and maximize

their chiral separation of the 1-PEA, 2-BUT, and 2-PEN. It is well-

known that chiral secondary alcohols are valuable intermediates in

the synthesis of a variety of pharmaceutical, agricultural, and fine

chemicals. Separation of enantiopure chiral secondary alcohols is

thus very important. Owing to the ability to simply tune the chiral

pores by the interplay of both metalloligands and organic linkers,

this M0MOF approach provides great promise for the realization of

new porous materials for the highly selective separation of chiral

small molecules [86].

HOFs have some intrinsic advantages compared with MOFs and

COFs, such as solution processability and characterization, easy

purification, and straightforward regeneration and reusage by

simple recrystallization; so porous HOF materials might be poten-

tially implemented in industrial and pharmaceutical applications.

However, there is still grant challenge to establish permanent

porosities in HOFs due to the weak hydrogen-bonding interac-

tions. To date, only a few HOFs have been shown to possess

permanent porosities. The most valuable properties of the homo-

chiral porous materials arise from the unique combination of

porosity and chirality. Given the fact that 2,4-diaminotriazinyl

016/j.mattod.2016.03.003

11




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-

PEA) > 1-(3Cl-PEA) > 2-BUT > 2-PEN > 2-HEX > 2-HEP). The ex-

tremely high enantioselective separation of HOF-2 for 1-PEA (ee of

92%) is remarkable. This observation indicates that the size of the

chiral pocket needs to match well with the molecular size of the

adsorbates to realize effective enantioselective separations.

Conclusions and outlookChiral porous materials are certainly very promising materials

for enantioselective separations. The significant progress over

the past several years on the diverse chiral building blocks for

their construction of a variety of chiral porous materials such as


12

MOFs, HOFs and COFs with tunable pore sizes has made some a

promise feasible. On the one hand, chemists will be still search-

ing for new chiral building blocks and thus synthesizing new

chiral porous materials; on the other hand, detailed studies at

the molecular level both by crystallographic structure charac-

terization and molecular modeling will be necessary to figure

out their specific recognition mechanism for small chiral mole-

cules. Further research endeavors will be also focused on their

practical applications through membrane separations and col-

umn separations. It is envisioned that some useful chiral porous

materials will be implemented for their enantioselective separa-

tions of chiral molecules for the pharmaceutical industry in the

future.

AcknowledgementsThis work was supported by National Natural Science Foundation

of China (21390394, 21261130584 and 21571076), and an Award

AX-1730 from Welch Foundation (BC).

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Emerging functional chiral microporous materials