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Cite this: CrystEngComm, 2017, 19,

2718

Received 17th March 2017,Accepted 7th April 2017

DOI: 10.1039/c7ce00522a

rsc.li/crystengcomm

Homochiral porous coordination polymers with arare utk topology and two types of uniformchannels for enantioseparation†

Changchang Zou,a Qianqian Li,a Fujun Cheng,a Haijun Wang,a

Jingui Duan *abc and Wanqin Jin *a

A pair of enantiopure chiral porous coordination polymers (PCPs)

with a rare utk topology were prepared using derivatives of L-/D-

alanine and ZnIJCH3COO)2·2H2O. The framework integrated two

types of parallel and uniform 1D channels that were constructed

by self-stranding of three equal helical chains and a single helical

chain. Among them, the right-handed PCP D-NTU-18 showed

enhanced enantioseparation towards racemic 1-phenyl-1-

propanol.

Porous coordination polymers (PCPs) with facile tuningability have attracted significant attention due to theirintriguing architectures and important applications such asgas storage, separation, sensing, and proton conductivity.1

Note that chiral PCPs with a unique pore systemdemonstrated unique functions in enantioselective separationand chiral catalysis. In the past few years, chiral PCPs havebeen used as chiral stationary phases in chromatographictechniques for chiral analyses and preparation.2 However,creating integrated properties of chirality, helicity, andporosity in a single domain remains a challenge.3

Usually, the strategy for preparing chiral PCPs with helicaland uniform channels/cages includes designing enantiopureligands, spontaneous resolution, and chiral template inducti-on.3e,4 However, direct transfer of inherent chirality from anasymmetric ligand to the entire structure was believed as themost effective and reliable method.2d Moreover, the pore na-

ture, such as the internal framework surface, pore metrics,and chiral functionalities, of the generated PCPs could befinely designed.5 However, despite these advantages, the syn-theses of enantiopure ligands are often laborious, time-con-suming, and expensive.4a,b Due to ready availability, biologi-cal functional properties, and highly selective substrate-binding abilities, multitopic organic ligands derived fromnatural amino acids are good candidates as chiral linkers forthe preparation of chiral PCPs.6 Although some 3D frame-works have been reported using pure amino acids and auxil-iary ligands, chiral PCPs with a uniform channel obtainedfrom amino acid-derived linkers that possess 3D networks aswell as porosity have rarely been reported.6

As a continuation of our previous studies on the assemblyof supramolecular polymers,7 herein, we report a pair of 3Dchiral PCPs: [ZnIJ(L)-HLN)IJCH3COO)]·nGuest (L-NTU-18) and[ZnIJ(D)-HLN)IJCH3COO)]·nGuest (D-NTU-18); these 3D chiralPCPs possess a rare utk/uniform topology and were obtainedfrom a pair of enantiopure amino acid derivatives: (L)-HLN (N-(4-pyridylmethyl)-L-alanine) and (D)-HLN (N-(4-pyridylmethyl)-D-alanine). The chirality of L-/D-NTU-18, derived from chiralitytransfer and reproduction of the involved ligands, further ledto the production of two different and uniform channels withvaried helical chains. Importantly, D-NTU-18 shows enantio-separation towards racemic 1-phenyl-1-propanol.

Compounds of L-/D-NTU-18 were solvothermally synthe-sized and structurally characterized. Single crystal X-ray dif-fraction analysis revealed that they were enantiomers with achiral space group of P41 and P43 (Fig. 1). Therefore, only thestructure of D-NTU-18 has been discussed in detail. The flackparameters were 0.023(17) and 0.052(12), reflecting the homo-chiral nature of the relevant crystals. The fundamental asym-metric unit of D-NTU-18 contained one Zn2+ ion, one (D)-HLNligand, and one coordinated acetate ion (Fig. 1b). Each Zn2+

ion was six-coordinated by two acetate O atoms, one pyridineN atom, and one amino N atom in the equatorial plane andtwo carboxyl O atoms in the axial positions, forming a

2718 | CrystEngComm, 2017, 19, 2718–2722 This journal is © The Royal Society of Chemistry 2017

a State Key Laboratory of Materials-Oriented Chemical Engineering, College of

Chemistry and Chemical engineering, Nanjing Tech University, Nanjing, 210009,

China. E-mail: duanjingui@njtech.edu.cn, wqjin@njtech.edu.cnb Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing

Tech University, Nanjing, 210009, Chinac State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing,

210023, China

† Electronic supplementary information (ESI) available: IR spectra, TG profiles,and PXRD patterns of crystals. CCDC 1520353 and 1520354. For ESI and crystal-lographic data in CIF or other electronic format see DOI: 10.1039/c7ce00522a

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distorted octahedron. Hence, the Zn atom was bonded tothree (D)-HLN ligands in a fan-type arrangement. Each (D)-HLN ligand connected three Zn2+ ions through its carboxylate,amino, and pyridine motif (Fig. 1b).

The most striking feature of D-NTU-18 is the integration ofa single helix in a channel and self-stranding of three equalhelical chains in another channel that were formed by com-plicated linkages of 3-connected ligands and six coordinatedZn2+ ions (Fig. 1c and d). To clearly understand the struc-tures, detailed structural elucidation of the relative channelsand helical chains was necessary (Fig. 2 and S3†).

Channel A was formed by three equal infinite right-handed helical chains. In the single chain (Zn-(D-HLN)-Zn-(D-HLN))n, there was a pyridine N atom and one O atom froma carboxylate group and one acetate ion coordinated to theZn2+ centre along the c axis. As a result, this right-handedhelical chain possessed a crystallographic 43 screw axis with

a pitch of 29.952 Å based on a repeated unit containingfour metal centers, four acetate ions, and four ligands.Thus, this flexible chiral ligand with heterocoordinatedgroups acted as a bidentate ligand in the construction ofhelical chains (Fig. 2a–d). With rational self-stranding, threeequal helical chains formed one homocentric square chan-nel with a ∼7 × 7 Å window aperture, viewed down the caxis.

Interestingly, channels A were further connected by thecoordination of an amino group and shared a carboxylategroup in each ligand to develop a new channel, B. In thischannel, the pyridine N atom coordinated to the Zn center,whereas the O atom from a carboxylate group and amino sitefrom the same ligand also joined the coordination in a che-late fashion, as the rotation of alanine motif. The chain alsohad a crystallographic 43 screw axis; however, the pitch waseven sharply reduced to 9.984 Å based on the same four

Fig. 1 View of the mirror isomers of the enantiopure amino acid derivatives ((L)-HLN) and ((D)-HLN) (a); ball and stick model of an asymmetric unitof L-/D-NTU-18 with mirror isomers, showing a six-coordinated zinc center and a three-connected ligand (b); two enantiomeric channels formedby three equal helical chains (c); two enantiomeric channels formed by single-helical chains. Opposite helicity is shown as a gray curved arrowwith a four-fold axis (d).

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repeated units of (Zn-(D-HLN))4. Since the adjacent repeatedunits in this chain were bridged by a six coordinated Zn cen-ter, as well as by an amino N atom, another uniform channelwith a smaller (∼6.5 × 6.5 Å) window aperture was observed(Fig. 2e–h). Remarkably, these two uniform channels were ex-ceptional for general helical structures with a low symmetryligand, which were usually blocked and intertwisted by thesame helical chains.

To identify the topology of the entire network, the ligandswere simplified as a 3-connected linker, whereas the Zn cen-ter was a 3-connected node.8 As a result, L-/D-NTU-18 pos-sessed a three-dimensional 3-c net with a utk/uniform topol-ogy that has not been encountered in PCP chemistry (Fig. 3).Note that due to different solvent systems, some uuh topolog-ical PCPs have been reported based on the same ligand andZn node.6b,c The formation of these unique frameworks in L-/D-NTU-18 should be assigned as the adaptable combinationof the metal centers and chiral ligands. The involved flexibleligands with amino groups could adopt suitable conforma-tions and coordination modes according to the geometric re-quirements by Zn2+ ions. Moreover, beside the coordination,the amino and carboxylate groups took part in the hydrogenbond interactions, which was also important for the forma-tion of the final stable structures.

The total accessible volume of the channels in L-NTU-18(D-NTU-18) was ca. 31.1% (30.8%), calculated using thePLATON program.9 Their thermal stabilities were evaluated

via thermogravimetric analysis (TGA) in flowing N2 (Fig. S7†).TG curves for these two PCPs showed the release of solventmolecules in the range from 30 to 150 °C. Then, the frame-work started to decompose around 170 °C. Their phase pu-rity were verified by powder X-ray diffraction (PXRD), andwhich agree well with the simulated pattern that obtainedfrom single crystal data. To further demonstrate thehomochirality of L-/D-NTU-18, the solid state circular dichro-ism (CD) spectra were obtained under a N2 flow at roomtemperature (Fig. 4). The CD spectrum of D-NTU-18exhibited an obvious positive Cotton effect with a peak at∼220 nm and a weak peak at ∼256 nm, whereas L-NTU-18

Fig. 2 The crystallographic 43 screw axis helical chain with a longer pitch in channel A. (a and b): The simplification and space-filling of twistedthree equal helix chains along the b-axis in channel A. (c and d): The crystallographic 43 screw axis helix chain with shorter pitch in channel B. (eand f): The connection of adjacent ligands in channel B. (g): The space-filling of the single helical chain along the b-axis in channel B. (h): Thepacking view of D-NTU-18 along the c-axis.

Fig. 3 The rare utk/uniform topology that was assembled from a3-connected chiral linker and a 3-connected metal center.

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exhibited a mirror CD signal in the same location, indicat-ing the formation of a pair of enantiomers.

Due to the uniform and chiral channels, we examined theenantioselective sorption of D-NTU-18 towards 1-phenyl-1-propanol (PhPrOH). As PhPrOH is an important precursor forthe synthesis of chiral drugs such as salbutamol, their opticalisomers have different bioactivity.10 We first soaked crystals(∼40 mg) in a PhPrOH/hexane system at 3 °C with variedconcentrations for separation. From the results (Table 1 andFig. S12–S17†), we found that D-NTU-18 gave 34.78 and42.86% enantiomeric excess (ee) at lower concentrations. Theee value decreased to 38.56% when the concentration ofPhPrOH was further increased. Additionally, we also exploredthe separation performance by directly soaking D-NTU-18crystals in racemic PhPrOH with varied amounts and temper-atures. The results were similar to those obtained before.However, the ee value increased to 44.58% when the adsorp-tion temperature was increased to 25 °C, which is almostfour times higher than that of [Zn2IJbdc)IJS-lac)IJdmf)] (12%).11

Similar to the performance of D-NTU-18, L-NTU-18 showed apreferred adsorption of R-PhPrOH, and the ee value reached36.84% (Fig. S18†). To understand the chiral channel-induced separation, UV spectra and the induced Cotton effectwere studied. As shown in Fig. S19,† the gradually decreasedpeak intensity in the UV spectra at 258 nm showed that theracemic PhPrOH molecules can be captured by D-NTU-18.Furthermore, specific rotation (α) of the chiral molecules re-leased from D-NTU-18@PhPrOH changed to −3.667° dm2 g−1

from 0 for their racemic mixtures, which is consistent withthe experimentally observed separation behaviour.

In summary, we successfully designed and prepared twochiral PCPs with a rare utk topology from amino acid deriva-tives. The three-periodic framework included two types ofparallel and uniform 1D channels that were constructed byself-stranding of three equal helical chains and a single heli-cal chain. In addition, D-NTU-18 showed potential for the sep-aration of 1-phenyl-1-propanol. Therefore, we can expect thatthe use of easily obtained chiral amino acid derivatives forthe synthesis of PCPs with functional channels as well as forpotential enantioseparations is feasible.

The authors gratefully acknowledge the financial supportreceived from the National Science Foundation of China(21301148 and 21671102), National Science Foundation ofJiangsu Province (BK20161538), and Innovative ResearchTeam Program by the Ministry of Education of China(IRT13070), Six talent peaks project in Jiangsu Province (JY-030), State Key Laboratory of Coordination Chemistry(SKLCC1616), and State Key Laboratory of Materials-OrientedChemical Engineering (ZK201406).

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Table 1 Stereoselective sorption of D-NTU-18 towards racemic PhPrOH

Adsorption solvent PhPrOH (μL) T (°C) Desorption solvent ee%

Hexane 1 mL 150 3 Ethanol 34.78Hexane 1 mL 500 3 Ethanol 42.86Hexane 1 mL 1000 3 Ethanol 38.56No solvent 50 3 Ethanol 36.54No solvent 150 3 Ethanol 38.58No solvent 50 25 Ethanol 44.58

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