Emerging functional chiral microporous materials: synthetic strategies and enantioselective separations

Emerging functional chiral microporous materials: synthetic strategies and enantioselective separations

Materials Today  Volume 00, Number 00  March 2016 RESEARCH RESEARCH: Review Emerging functional chiral microporous materials: synthetic strategie...

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Materials Today  Volume 00, Number 00  March 2016

RESEARCH

RESEARCH: Review

Emerging functional chiral microporous materials: synthetic strategies and enantioselective separations Ming Xue1, Bin Li2, Shilun Qiu1 and Banglin Chen2,* 1 2

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130012, PR China 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), hydrogenbonded 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 lefthanded 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

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

(SDA) or chiral induction to transfer the chirality into the inorganic 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 microporous MOF, PCP, HOF and COF materials and their applications on enantioselective adsorption [20], chiral chromatography resolution [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 arrangement 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 environment, 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

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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 stereochemistry at the metal centers. The direct participation of chiral components 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 phosphonate bridging ligands with orthogonal functional groups [50]. For example, in 2012 they synthesized a highly porous and fluorescent MOFs [Cd2(L)(H2O)2]6.5DMF 3EtOH, built from a chiral tetracarboxylate bridging ligand derived from BINOL and a cadmium 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

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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 hexagonal 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(m3O)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

FIGURE 1

Some chiral ligands used for construction of chiral microporous materials. 2 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

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FIGURE 2

(a) Ball-and-stick model of an infinite 1D [Cd4(H2O)4(m2-h1,h1-CO2)4(m2-h2,h1-CO2)4] chain. (b) Ball-and-stick model showing the structure with the simplified L ligand, 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].)

‘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 homochirality 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 homochiral rare-earth camphorates, they demonstrated that crystallization 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+ combinations, 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 decades. In comparison, there has been very little progress in the area

FIGURE 4

Post-synthetic metalation of BINAP-MOF (1) to form 1Ru and 1Rh. (Adapted with permission from Ref. [55].) 3

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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 separations [20,68].

Enantioselective adsorption and separation

RESEARCH: Review

FIGURE 5

Comparison between 1-Ln and 2-Ln. (a) Local coordination environment in 1-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 ) 3Dframework 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].)

of asymmetric crystallization from achiral precursors to form enantiopure or enantioenriched crystalline microporous materials. In 2004, we synthesized a 2D layer coordination polymer Co(PDC)(H2O)2H2O 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 framework, was reported by Rosseinsky and coworkers [60]. Recently, Bu and coworkers demonstrated that an unusual asymmetric crystallization 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 materials 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 combination 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

Because of the significant difference in the biological and pharmacological 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 investigation 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 enantiomeric 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 property, 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-butanol@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)-2methyl-1-butanol was found to be only 8.2%. Such striking difference 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 enantioseparation 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

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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 chiral induction. D-Camphoric acid initially controls the absolute chirality of [Mn3(HCOO)4]n2+ 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].)

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

very low BET surface area of 12.6 m2 g1. However, the isostructural 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 g1. Then the enantioselectivity was observed 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 homochiral MOF [Zn2(bdc)(L-lac)(DMF)] (Zn-BLD, Fig. 8), possessing the intrinsic chirality due to the chiral L-lactic acid [72]. This material 5

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RESEARCH: Review 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 threecomponent condensation followed by a click reaction. (Adapted with permission from Ref. [65].)

displays significant enantioselective sorption ability for the aromatic 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

less explored for these applications. In 2007, Fedin and Bryliakov et al. presented a quantitative study of the enantioselective sorption 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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RESEARCH

TABLE 1

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)-cinchonan9-ol-3-carboxylic acid (HQA)

2-Butanol 2-Methyl-1-butanol

98.2 8.2

[70]

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

1,3,5-Benzenetricarboxylic acid (btc) 1,2-Propanediol (1,2-pd) 3-Picoline (3-pic)

Binaphthol

8.3

[71]

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

1,4-Benzenedicarboxylate (bdc) L-Lactic acid (L-lac)

Methyl phenyl sulfoxide 1-Bromo-4-(methylsulfonyl)benzene

20 27

[72]

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

1,4-Benzenedicarboxylate (bdc) L-Lactic acid (L-lac)

p-BrPhSOMe PhSOMe p-MePhSOMe PhSOi-Pr

7 60 38 55

[73]

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

L-()-Lactate (S-lac) 1,4-Benzenedicarboxylate (bdc)

2-Butanol 2-Methyl-1-butanol 1-Phenyl-1-ethanol 1-Phenyl-1-propanol

14 7 21 12

[74]

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

L-Lactic

Methyl phenyl sulfoxide

33

[77]

Fe3O4@SiO2-ZnBLD

1,4-Benzenedicarboxylate (bdc) L-Lactic acid (L-lac)

Phenyl methyl sulfoxide Phenyl vinyl sulfoxide 4-Chlorophenyl methyl sulfoxide

85.2 76.7 29

[78]

Fe3O4@SiO2-ZnBDD

1,4-Benzenedicarboxylate (bdc) D-Lactic acid (D-lac)

Phenyl methyl sulfoxide Phenyl vinyl sulfoxide 4-Chlorophenyl methyl sulfoxide

86.2 77 29.9

[78]

Ni2(L-asp)2(bipy)

L-Aspartic acid (L-asp) 4,40 -Bipyridyl (bipy)

1,2-Propanediol 1,3-Butanediol 1,2-Butanediol 2,3-Butanediol 1,2-Pentanediol 2,4-Pentanediol 2-Methyl-2,4-pentanediol 2,5-Hexanediol 1,2-Hexanediol

5.35 17.93 5.07 1.5 13.9 24.5 53.77 3.4 5

[79]

Ni2(L-asp)2bipy membrane

L-Aspartic acid (L-asp) 4,40 -Bipyridyl (bipy)

2-Methyl-2,4-pentanediol

32.5

[80]

Ni2(L-asp)2bipy membrane

L-Aspartic acid (L-asp) 4,40 -Bipyridyl (bipy)

2-Methyl-2,4-pentanediol

35.5

[81]

Zn3(bdc)3[Cu(SalPycy)] M0 MOF-2

1,4-benzenedicarboxylate (bdc) Cu(SalPycy)

1-Phenylethanol

21.1

[85]

Zn3(cdc)3[Cu(SalPycy)] M0 MOF-3

1,4-Cyclohexanedicarboxylate (cdc) Cu(SalPycy)

1-Phenylethanol

64

[85]

Cd3(bdc)3[Cu(SalPyMeCam)] M0 MOF-4

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

1-Phenylethanol 2-Butanol 2-Pentanol 2-Heptanol

45 45.2 27.9 <4

[86]

Zn3(cdc)3[Cu(SalPyMeCam)] M0 MOF-5

1,4-Cyclohexanedicarboxylate (cdc) Cu(SalPyMeCam)

1-Phenylethanol 2-Butanol 2-Pentanol 2-Heptanol

75.3 72.5 62.2 <9

[86]

Cd3(bdc)3[Cu(SalPytBuCy)] M0 MOF-6

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

1-Phenylethanol 2-Butanol 2-Pentanol 2-Heptanol

46.2 49.6 39.7 <6

[86]

Zn3(cdc)3[Cu(SalPytBuCy)] M0 MOF-7

1,4-Cyclohexanedicarboxylate (cdc) Cu(SalPytBuCy)

1-Phenylethanol 2-Butanol 2-Pentanol 2-Heptanol

82.4 77.1 65.9 <10

[86]

acid (L-lac)

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Summary of chiral microporous materials for chiral resolution.

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TABLE 1 (Continued ) Chiral microporous materials

Ligands

(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)

Enantiomers

ee (%)

Ref

93.1

[87]

L-Methyl mandelate Ethyl mandelate i-Propyl mandelate D-Methyl mandelate Benzyl mandelate

64.3 90.7 89.9 73.5

RESEARCH: Review

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) Cu2(D-cam)2(BiPyB) thin films

Diazabicyclo[2.2.2]octane (dabco) 4,40 -Bipyridyl (bipy) 1,4-bis(4-pyridyl)benzene (BiPyB)

D-his–ZIF-8

D-Histidine

Alanine Glutamic

HOF-2

(R)-1,10 -Bi-2-naphthol scaffold into 2,4-diaminotriazinyl

1-Phenylethanol 1-(4-Chlorophenyl)ethanol 1-(3-Chlorophenyl)ethanol 2-Butanol 2-Pentanol 2-Hexanol 2-Heptanol

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. Zn green, N blue, O red, C gray; chiral C atoms of the lactic acid ligand are shown in white. (Adapted with permission from Ref. [72].)

35 8 78.52 79.44 92 79 66 77 48 <10 <4

[91] [92]

interaction with Bn-Chir-UMCM-1. The potential for enantioseparation 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-hydroxybenzyl)-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

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].)

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helical channel is appropriate for the enantioseparation, they used metal-complex [Cu(sala)]22H2O 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 nowadays owing to incomparable preponderance over traditional methods, 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 separation. 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 difference across the membrane. The resolution process was carried out by a ‘side-by-side diffusion cell’ and readily separated by the asprepared membrane. After 48 hours separation, the enantioselectivity 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(Lasp)2(bipy)] [79]. Neutral chiral Ni(L-asp) layers are connected by 4,40 -bipyridine linkers to afford a pillared structure with one-dimen˚  4.7 A ˚ . The immobilization of the chiral sional channels of 3.8 A 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 membrane 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-pentanediol) 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,

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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 that occupy 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].)

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 construction of porous mixed-metal-organic frameworks (M0 MOFs) by making use of M-Salen metalloligands [82–84]. Such a novel approach eventually led to several porous M0 MOFs for enantioselective separation. Recently, we successfully used this pre-constructed building block approach to introduce chiral pockets within the M0 MOFs. To make use of chiral (R,R)-1,2-cyclohexanediamine to construct the chiral metalloligand Cu(SalPyCy), enantiopure M0 MOF Zn3(BDC)3[Cu(SalPycy)](G)x (M0 MOF-2) can be readily assembled by this chiral building block with Zn(NO3)2 and 9

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(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].)

H2BDC, leading to the chirality [85]. Furthermore, such chiral cavities can be straightforwardly tuned by incorporation of different bicarboxylate CDC (CDC = 1,4-cyclohexanedicarboxylate) in Zn3(CDC)3[Cu(SalPycy)](G)x (M0 MOF-3), which exhibits significantly enhanced enatioselective recognition of 1-phenylethyl alcohol (PEA). M0 MOF-2 and -3 are isostructural three-dimensional ˚ in frameworks, exhibiting two chiral pore cavities of about 6.4 A diameter (Fig. 12). Particularly, the enantiopure M0 MOF-3 could exclusively take up S-PEA to form M0 MOF-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 M0 MOF-3@S-PEA into methanol, suggesting its potential for enantioselective separation of R/S-PEA. Furthermore, the chiral recognition and enantioselective separation of the R/S-PEA racemic mixture were also examined for using the bulky as-synthesized M0 MOF-2 and -3 materials. Chiral HPLC analysis of the desorbed PEA from the PEA-included M0 MOF-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 M0 MOF-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

R/S-PEA. The second and third regenerated M0 MOF-2 samples provide an ee value of 15.7 and 13.2%, respectively. The low enantioselectivity of M0 MOF-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 M0 MOF3 have significantly enhanced its enantioselectivity for separation of R/S-PEA with the much higher ee value of 64% compared with that of M0 MOF-2. The regenerated M0 MOF-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 M0 MOF-2 and M0 MOF-3 basically match well with the size of S-PEA, which are not able to separate larger alcohol enantiomers, such as 1-(ptolyl)-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 M0 MOF approach has provided us an ideal platform to tune and functionalize the micropores within this series of isoreticular M0 MOFs. The main strategies involve the incorporation of different secondary organic linkers, the immobilization 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 12

X-ray crystal structures of M0 MOF-3 and M0 MOF-3@S-PEA. (a) The hexagonal primitive network topology (Scha¨fli symbol 36418536) and (b) the threedimensional (3D) pillared framework with chiral pore cavities for M0 MOF-3. (c) The hexagonal primitive network topology and (d) the 3D pillared framework exclusively encapsulating S-PEA molecules for M0 MOF-3@S-PEA. (Zn, pink; Cu, cyan; O, red; C, gray; N, blue; H, white). (Adapted with permission from Ref. [85].)

Schematic diagram for the synthesis of four mixed-metal-organic frameworks (M0 MOFs) 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].)

explore novel functional microporous M0 MOFs 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 tertbutyl groups (blue in Scheme 1) and the second organic linkers 1,4cyclohexanedicarboxylate (CDC) and 1,4-benzenedicarboxylate (BDC) (green in Scheme 1). Therefore, we synthesized four isostructural M0 MOFs Cd3(BDC)3[Cu(SalPyMeCam)](G)x (M0 MOF-4), Zn3 (CDC)3[Cu(SalPyMeCam)](G)x (M0 MOF-5), Cd3(BDC)3[Cu(SalPytBuCy)](G)x (M0 MOF-6) and Zn3(CDC)3[Cu(SalPytBuCy)](G)x (M0 MOF-7), which have the same topology with that of M0 MOF-2 and M0 MOF-3. However, the different chiral pores of M0 MOF 4–7 have enabled us to tune their performance for enatioselective separation of small alcohols such as 1-phenylethanol (1-PEA), 2butanol (2-BUT), 2-pentanol (2-PEN), and 2-heptanol (2-HEP) at room temperature. As expected, these porous M0 MOFs display different recognition behaviors for these four small alcohols. Both M0 MOF-5 and M0 MOF-7 constructed from CDC systematically exhibit higher chiral separation for 1-phenylethanol (1-PEA) with ee of 75.3% and 82.4%, respectively, than those of M0 MOF-4 (ee of 45.0%) and M0 MOF-6 (ee of 46.2%) assembled from BDC. Among these M0 MOFs, M0 MOF-7 is the most efficient material for separation of 1-PEA. Such a high enatioselectivity for M0 MOF-7 was mainly

attributed to its larger terminal tert-butyl group, which can notably decrease the chiral pore space in M0 MOF-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 M0 MOFs 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 wellknown 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 M0 MOF 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 potentially implemented in industrial and pharmaceutical applications. However, there is still grant challenge to establish permanent porosities in HOFs due to the weak hydrogen-bonding interactions. To date, only a few HOFs have been shown to possess permanent porosities. The most valuable properties of the homochiral porous materials arise from the unique combination of porosity and chirality. Given the fact that 2,4-diaminotriazinyl

SCHEME 1

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MOFs, HOFs and COFs with tunable pore sizes has made some a promise feasible. On the one hand, chemists will be still searching 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 characterization and molecular modeling will be necessary to figure out their specific recognition mechanism for small chiral molecules. Further research endeavors will be also focused on their practical applications through membrane separations and column separations. It is envisioned that some useful chiral porous materials will be implemented for their enantioselective separations of chiral molecules for the pharmaceutical industry in the future.

Acknowledgements This work was supported by National Natural Science Foundation of China (21390394, 21261130584 and 21571076), and an Award AX-1730 from Welch Foundation (BC). References

FIGURE 13

X-ray crystal structure of HOF-2 featuring (a) multiple hydrogen bonding (light-blue dashed lines) among adjacent units to form three-dimensional hydrogen-bonded organic framework exhibiting 1D hexagonal pores with ˚ along the c axis and (b) the uninodal 6the diameter of about 4.8 A connected {3355667} network topology. X-ray crystal structure of HOF-2R1-PEA indicating (c) the enantiopure R-1-PEA molecules residing in the channels 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 of R-1-PEA (green molecule) and oxygen atoms of the diethoxy groups from the HOF-2 framework. Comparison of X-ray crystal structures of (e) HOF2S-1-PEA and (f ) HOF-2R-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].)

(DAT) is a very powerful hydrogen-bonding motif for the construction of porous robust HOFs and the BINOL is the organic backbone for asymmetric induction, we successfully synthesized the first example 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-(4ClPEA) > 1-(3Cl-PEA) > 2-BUT > 2-PEN > 2-HEX > 2-HEP). The extremely 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 outlook Chiral 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

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