Spontaneous Resolution in the Ionothermal Synthesis of Homochiral

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Spontaneous Resolution in the Ionothermal Synthesis of Homochiral Zn(II) Metal Organic Frameworks with (10,3)-a Topology Constructed from Achiral 5-Sulfoisophthalate Qing-Yan Liu,*,† Yu-Ling Wang,† Na Zhang,† Yun-Liang Jiang,† Jia-Jia Wei,† and Feng Luo*,‡ † ‡

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, P. R. China College of Biology, Chemistry and Material Science, East China Institute of Technology, Fuzhou, Jiangxi, P. R. China

bS Supporting Information ABSTRACT: Two homochiral enantiomers {(EMIM)[Zn(SIP)(IM)]}n (1a and 1b, EMIM = 1-ethyl-3-methylimidazolium) possessing chiral (10,3)-a topology constructed from an achiral rigid 5-sulfoisophthalate (SIP) ligand and imidazole (IM) have been obtained by spontaneous resolution under ionothermal conditions. The homochiral enantiomers show second-order nonlinear optical effects and photoluminescence.

T

he search for chiral materials is of current interest and great importance because these materials have potential applications in enantioselective catalysis and separation, nonlinear optics, and magnetism.1 Recently developed metal organic framework materials (MOFs), which contain both inorganic and organic units within the framework, have shown great promise for the synthetic design of chiral framework materials.2 There are two general approaches to prepare chiral MOFs. The most effective method is using a chiral organic bridging ligand to link the metal centers in the framework.3 The other one, which was first reported by Rosseinsky and co-workers,4 uses a chiral molecule as an auxiliary ligand that does not bridge the metal centers but forces chirality by coordinating to the metal center. Furthermore, chiral MOFs can be obtained from totally achiral components via spontaneous resolution.5 In this approach, the individual crystal will be homochiral, whereas the bulk material will generally contain a 50:50 (racemic) mixture of the two enantiomorphs. We have followed this strategy for the synthesis of one pair of enantiomorphs based on the rigid 5-sulfoisophthalate ligand. However, most of the achiral ligands used for the construction of chiral MOFs are flexible because flexible ligands can have variable conformation and coordination modes to satisfy the needs for the formation of ultimate chiral frameworks.6 Ionothermal synthesis,7 the use of an ionic liquid (IL), as solvent and template in the preparation of crystalline solids, offers many advantages over traditional hydrothermal and solvothermal materials synthesis methods. Compared with the traditional synthesis methods, the change from molecular to r 2011 American Chemical Society

ionic reaction media leads to new types of material being accessible, with structural properties that may be traced directly to the chemistry of the IL. There are some examples in which ILs have been successfully applied to the syntheses of novel MOFs.8 Of particular interest is a study by Morris’ group on the use of an enantiopure anion as one component of the IL to induce homochirality in a nickel(II) structure constructed of entirely achiral building blocks, despite the fact that the anion of the IL is not occluded by the material.9 Nevertheless, there is no other chiral MOF with an achiral ligand prepared under the ionothermal reaction reported. Herein, we report two homochiral enantiomers {(EMIM)[Zn(SIP)(IM)]}n (1a and 1b) obtained from spontaneous resolution under ionothermal reaction. To our knowledge, this is the first report of the spontaneous resolution upon crystallization from IL. Ionothermal reaction of Zn(NO3)2, imidazole (IM), and 5-sulfoisophthalic acid monosodium salt (NaH2SIP) in 1-ethyl3-methylimidazolium tetrafluoroborate (EMIM-BF4) at 160 °C for 6 days afforded colorless crystals of 1. Its IR spectrum exhibits sharp peaks centered at 1641, 1567, and 1433 cm 1, which are the expected absorptions for the stretching vibrations of the carboxylate groups. The sharp absorption at 624 cm 1 can be assigned to the C S stretching vibration of the 5- sulfoisophthalate (SIP) ligand. The peak at 3443 cm 1 is the N H stretching Received: May 12, 2011 Revised: July 20, 2011 Published: July 28, 2011 3717

dx.doi.org/10.1021/cg200606r | Cryst. Growth Des. 2011, 11, 3717–3720

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Figure 1. The enantiomeric nature of 1a and 1b. (The red tetrahedron represents the ZnO3N coordination sphere.)

Figure 2. (a) The small left-handed helical chain in 1a and (b) righthanded helical chain in 1b.

band of the IM ligand. The structures of 1 were characterized by single crystal X-ray diffraction analyses,10 and the results showed that 1 contains single crystals of 1a and 1b with the chiral space group P212121. In the absence of inversion symmetry elements in the space group P212121, the crystal is homochiral with all Zn atoms of the same Λ- or Δ-configuration. The overall ensemble of the crystals in a batch of 1 can be expected to be racemic, which indicates that spontaneous resolution occurred during crystal growth. The powder X-ray diffraction (PXRD) pattern is in good agreement with the ones simulated from single crystal structural data, which confirmed the purity of the bulk sample (Figure S1 in Supporting Information). The enantiomeric nature of 1a and 1b can be simply represented by their mirror structures (Figure 1). Since 1a and 1b are enantiomers, the structure of 1a is detailed here. The asymmetric unit of 1a contains one Zn(II) ion, one SIP3 trianion, one IM

Figure 3. The 3D [Zn(SIP)(IM)]nn framework of 1a consisting of two types of single-stranded helical chains. (The IM ligands are omitted for clarity.)

ligand, and one [EMIM]+ cation. The Zn(II) center shows a slightly distorted tetrahedral geometry and is coordinated by two carboxylate O atoms and one sulfonate O atom from three SIP3 ligands, and one imidazole N atom (Figures 1 and S2). Imidazole as a terminal ligand coordinates to the zinc center. Each SIP3 ligand bridges three Zn(II) ions through its two unidentate carboxylate groups and one unidentate sulfonate group (Figure 1). As depicted in Figure 2a, the Zn(II) ions are bridged by the sulfonate group and one of carboxylate group (O(1) C(7) O(2)) of SIP 3 ligands to form an infinite lefthanded helical chain running along the a-axis. The helix is generated around the crystallographic 21 screw axis. The remarkable feature of P21 2 1 2 1 space group is the existence of three 2 1 screw axes. In 1a, the homo left-handed helical chain constructed by the SIP3 ligands and Zn(II) ions is circumgyrated along the a, b, c axes, and it is the opposite phenomenon (homo right-handed helical chains) in 1b (Figure 2b). The pitch lengths 3718

dx.doi.org/10.1021/cg200606r |Cryst. Growth Des. 2011, 11, 3717–3720

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of the helical chains of the three directions are identical to a, b, and c-axes lengths, respectively. As described above, the SIP3 ligand uses its sulfonate group and one of the carboxylate groups to form the above single-stranded helical chain. The remaining carboxylate group (O(3) C(8) O(4)) provides an additional binding site to assemble the small single-stranded helical chain into a three-dimensional (3D) [Zn(SIP)(IM)]nn anionic framework. Each single-stranded helical chain serves as a secondary building unit and is further linked to its four adjacent neighbors in two orthogonal directions through Zn(1) O(3A) bonds generating a homochiral 3D [Zn(SIP)(IM)]nn anionic framework, as shown in Figure 3. Remarkably, the 3D [Zn(SIP)(IM)]nn anionic framework has a giant elliptical helical nanotube formed by four adjacent small left-handed helical chains (Figures 3 and S3). It should be noted that the large elliptical helical nanotubes have opposite handedness (right-handed) with the small helical chains (Figure 4). The chirality in the structures of 1a or 1b is the result of the 21 axis in the symmetry. Therefore, the chirality in the present compounds is derived from the presence of helices rather than through a chirally enriched component in the reaction mixture. As is known, IL can play multiple roles in the ionothermal syntheses and crystallizations of compounds.11 The imidazolium cations [EMIM]+ of the IL act as extraframework chargebalancing species for the anionic [Zn(SIP)(IM)]nn framework

and are situated in the spaces between the adjacent pitches of the helical species and π-interacted with the benzene component of the helix with a center-to-center separation of 3.84 Å (Figure S4, Supporting Information). Thus, the [EMIM]+ cation not only adopts a charge-compensating and space-filling role in the material, but also directs the formation of the host framework. We have been unable to remove it without causing the structure to collapse (Figure S5, Supporting Information). In the [Zn(SIP)(IM)]nn framework, the zinc center is linked to three individual SIP ligands, and each SIP ligand links three zinc centers. The network can thus be represented topologically by three-coordinate nodes. Careful examination indicated the two three-connected nodes are topologically equivalent nodes and the network is an non-interpenetrated decorated (10,3)-a net (Figures 5 and S6). It has an extended Schl€afli symbol of 105 3 105 3 105, which is assigned to an srs net. This net is one of the five regular 3-periodic nets and is the only chiral one. There are more than 80 examples which possess the srs topology reported in the literature.12 Since the(10,3)-a net is a chiral net, there is a pair of enantiomorphic (10,3)-a nets. However, all the reported examples present only one kind of (10,3)-a net.13 Their enantiomorphic nets are not provided. Compound 1 crystallized in the chiral space group P212121, so we investigated its second harmonic generation (SHG) property. Powder SHG measurements on polycrystalline samples of 1 were

Figure 4. (a) The large right-handed helical nanotube in 1a and (b) left-handed helical nanotube in 1b.

Figure 6. The solid-state photoluminescent spectra of 1 (λex = 328 and λem = 423 nm) at room temperature.

Figure 5. Schematic representation of the (10,3)-a nets in 1a and 1b along the a axis showing the helical chains. (The cyan and red balls represent the zinc centers and SIP ligands, respectively.) Highlighted are the 10-membered shortest circuits (blue). 3719

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Crystal Growth & Design performed on a modified Kurtz-NLO system using 1064 nm radiation.14 The result revealed that 1 exhibits an SHG response, ∼0.1 times that of KDP, which confirm its acentricity. The SHG active could be due to the presence of intramolecular charge separation, which resulted from the anionic framework and imidazolium cations in 1. However, such a slightly weak SHG response could be due to the absence of a good donor acceptor chromophore in 1. There are a number of literature examples that report the use of chromophores that contain a good electron donor and acceptor connected through a conjugated bridge, which are electronically asymmetric and highly dipolar, as a means to enhance the SHG response.15 In addition, compound 1 exhibits a photoluminescent peak with a maximum at 423 nm upon excitation at 328 nm (Figure 6), which may be assigned to ligand-to-metal charge transfer (LMCT). In summary, we demonstrate here for a pair of Zn(II) enantiomorphs (1a and 1b) with (10,3)-a topology ionothermal synthesis by spontaneous resolution upon crystallization. The spontaneous resolution occurs without any chiral source, which may open up new opportunities in the preparation of chiral materials.

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystal structure information (CIF). Experimental details, crystal data, PXRD, and TGA curve. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*(Q.-Y.L.) E-mail: [email protected]; fax: +86-7918120380. (F.L.) E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by the NNSF of China (Grant 20901033), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (SEM), the NSF of Jiangxi (Grant 2009GZH0056), and the Project of Education Department of Jiangxi Province (Grants GJJ10016 and GJJ11381). The authors thank Professor Jiang-Gao Mao for helpful assistance in measuring the SHG properties.

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