An Unprecedented Homochiral Metal–Organic Framework Based on

Communication pubs.acs.org/crystal

An Unprecedented Homochiral Metal−Organic Framework Based on Achiral Nanosized Pyridine and V‑Shaped Polycarboxylate Acid Ligand Qingxiang Yang, Liangfang Huang, Mingdao Zhang, Yizhi, Li, Hegen Zheng,* and Qingyi Lu* State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: A unique homochiral metal−organic framework has been successfully synthesized by solvothermal reaction of an achiral flexible Vshaped ligand and a nanosized π-electron-deficient pyridine ligand based on cobalt(II) salt, [Co(L)(DPNDI)0.5]n (1) (H2L = 4,4′-dicarboxydiphenylamine, DPNDI = N,N′-di-(4-pyridyl)-1,4,5,8-naphthalenediimide); the helixes assembled by H2L and cobalt(II) paddle-wheel centers are left-handed and transform the framework to chiral. Also, the inserting of the DPNDI transforms the original dia net constructed by H2L and cobalt(II) paddle-wheel centers to a 3-fold jsm net. This is the first example of interpenetrated jsm net. In addition, the chiral property of bulk products is confirmed by circular dichroism spectra (CD), and the thermal stability and the magnetic properties are also investigated.

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INTRODUCTION In the field of supramolecular chemistry and crystal engineering, the design and assembly of metal−organic frameworks (MOFs) with intriguing aesthetic architectures and topological features as well as their promising applications as functional materials have stimulated the interest of chemists in recent years.1−3 Achiral molecules have now been investigated to assembly into homochiral porous networks. This has implications for practical processes such as separations, but also for understanding how homochirality  especially in biological systems  arose from achiral or racemic species.4 Chiral MOFs can be built from chiral ligands or by using achiral ligands under spontaneous resolution without any chiral sources.5 The synthesis of chiral species from achiral ligands is the key issue in studying the genesis of chirality in biological systems. But how to generate chiral units from achiral components and induce the chiral information of the chiral units into higher dimensional chiral MOFs without any chiral auxiliary is the main challenge that should be solved. In previous reports, most homochiral structures are low-dimensional (1-D chains or 2-D layers). Previous research indicates that some supramolecular interactions, such as hydrogen bond or π−π interactions, may be the driving force and contribute to the occurrence of symmetry breaking.6 Without the driving force of supramolecular interactions, symmetry breaking is rarely seen in the crystallization of three-dimensional (3D) framework materials.7 For building chiral frameworks, the helix gives a good opportunity to transmit chiral information, and flexible multidentate organic bridging ligands may be efficient candidates to improve the helix elements for generating chiral MOFs.8 We are interested in constructing porous coordination polymers or MOFs with novel topologies from multidentate ligands and investigating the tremendous properties of them.9 © 2013 American Chemical Society

And we have spent a lot of efforts on the V-shaped ligands for constructing MOFs with various interesting structures, such as 4,4′-bis(imidazol-1-yl)diphenyl ether (BIDPE),10 4,4′- sulfonyldibenzoic acid (sdb), and 4,4′-oxybis(benzoate) (oba).11 Recently, we are focusing on the flexible V-shaped ligand (Scheme 1): Scheme 1. Structure of DPNDI and H2L Ligands

4, 4′-dicarboxydiphenylamine (H2L),12 which possesses the following interesting structural characteristics: (a) there are two carboxyl groups with about a 120° angle separated by an imino group and two benzene rings, which would reduce the steric interference/hindrance; (b) this provides an opportunity to study the relative torsional displacement of the benzene rings with respect to the central nitrogen atom. DPNDI is a π-electrondeficient ligand with the length of about 15 Å (Scheme 1),13 and Received: November 1, 2012 Revised: January 7, 2013 Published: January 10, 2013 440

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Table 1. Crystallographic Data for Compound 1 formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z Dc (g cm−3) μ (mm−1) F(000) R(int) observed data R1, wR2 [I > 2σ(I)] S min and max res dens (e·Å−3)

C26H15CoN3O6 524.34 tetragonal P43212 15.8964(6) 15.8964(6) 17.0316(12) 90 90 90 4303.8(4) 8 1.618 0.85 2136.0 0.0883 3068 0.0399, 0.0824 1.009 −0.531, 0.500

Figure 2. (a) Coordination environment of the paddle-wheel SBU and (b) the smallest metallocyclic ring in 1.

Figure 1. Coordination environment of the Co(II) ion in 1. The hydrogen atoms are omitted for clarity. Symmetry codes: (#1) x, 1.5 − y, 1.25 − z; (#2) 2 − x, 1 − y, z; (#2) 1.5 − y, 1.5 − x, 0.5 − z; (#3) 1.5 − y, 1.5 − x, 0.5 − z; (#4) 0.5 + y, −0.5 + x, 0.5 − z.

we suppose it would be interesting to construct a framework with this kind of ligand. In this work, solvothermal reaction of H2L, and Co(NO3)2·4H2O in the presence of DPNDI resulted in the formation of black octahedron crystals, [Co(L)(DPNDI)0.5] (1).

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EXPERIMENTAL SECTION

Materials and Methods. The reagents and solvents employed were commercially available and used as received. The DPNDI ligand was prepared according to the literature.13 IR absorption spectra of the compounds were recorded in the range of 400−4000 cm−1 on a Nicolet (Impact 410) spectrometer with KBr pellets (5 mg of sample in 500 mg of KBr). C, H, and N analyses were carried out with a Perkin−Elmer 240C elemental analyzer. Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Advance X-ray diffractometer using Cu−Kα radiation (λ = 1.5418 Å), in which the X-ray tube was operated at 40 kV and 40 mA. The as-synthesized samples were characterized by thermogravimetric analysis (TGA) on a Perkin-Elmer thermogravimetric analyzer Pyris 1 TGA up to 1023 K using a heating rate of 10 K min−1 under N2 atmosphere. The circular dichroism (CD) spectra of 1 were recorded at room temperature with a Jasco J-810(S)

Figure 3. (a) The diamond net of 1; (b) large open channels constructed by L2− ligand of 1 along the b axis; (c) one DPNDI molecule inserted in one diamond net of 1; and (d) the single 3D framework of 1 along the b axis. spectropolarimeter (KBr pellets). Temperature-dependent magnetic susceptibility data were obtained on a MPMS XL-7 SQUID 441

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and 0.5 mL of 3% NaOH solution was dissolved in 6 mL of EtOH/ H2O(1:1, v/v). The final mixture was placed in a Parr Teflon-lined stainless steel vessel (15 mL) under autogenous pressure and heated at 95 °C for 3 days. Black octahedron crystals were obtained, which were washed with mother liquid and dried under ambient conditions (yield: 40% based on H2L). Anal. Calcd for C26H15O6N3Co: C, 59.54, H, 2.86, N, 8.02; found C, 59.50, H, 2.89, N, 7.98. IR (KBr, cm−1): 3386(w), 3116(w), 1721(w), 1594(s), 1535(w), 1514(w),1386(s), 1325(s), 1246(m), 1214(w), 1171(m), 1138(w), 1065(w), 1006(w), 882(w), 847(w), 817(w), 778(m), 695(w), 662(m), 627(w), 550(w). Crystal Structure Determination. X-ray crystallographic data of 1 were collected at room temperature using epoxy-coated crystals mounted on glass fiber. All measurements were made on a Bruker Apex Smart CCD diffractometer with graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å). The structure of compound 1 was solved by direct methods, and the non-hydrogen atoms were located from the trial structure and then refined anisotropically with SHELXTL using fullmatrix least-squares procedures based on F2 values.14 The hydrogen atom positions were fixed geometrically at calculated distances and allowed to ride on the parent atoms. The relevant crystallographic data are presented in Table 1, while the selected bond lengths and angles are given in Supporting Information, Table S1.

Figure 4. The dia topology net constructed by H2L ligand. magnetometer under an applied field of 2000 Oe over the temperature range of 2−300 K. Synthesis of Compound 1. A mixture of Co(NO3)2·4H2O (29.1 mg, 0.1 mmol), DPNDI (21.2 mg, 0.1 mmol), H2L (25.7 mg, 0.1 mmol),

Figure 5. Four left-handed cogenerated helical chains constructed by the L2− ligand.

Figure 6. The single framework in 1 along the a and b axis. 442

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RESULTS AND DISCUSSION

Structure of 1. Single-crystal X-ray diffraction revealed that 1 crystallizes in tetragonal chiral space group P43212. The asymmetric unit of 1 contains two independent Co(II) cations, two L2− and one-half DPNDI. As shown in Figure 1, each cobalt atom is five-coordinated by four oxygen atoms from L2−, and one nitrogen atom from DPNDI ligand. Each pair of Co(II) ions is bridged by four carboxylate groups (Co2(COO)4) to generate a well-known paddle-wheel SBU. Since each Co(II) ion has squarepyramidal coordination geometry, its axial position is further coordinated by a DPNDI molecule (Figure 2a). Because of the flexible V-shaped feature of the L2− linker, it can assemble to the twisted chair-form metallocyclic ring consisting of six Co2 (COO)4 units and six L2− ligands, which shows a large dimension about 39.1 × 15.9 Å (Figure 2b). This chair-form metallocyclic rings connecting to each other form a diamond structure, which contains 10 Co2(CO2)4 SBUs and 12 L2− ligands (ca. 51.1−22.48 Å) (Figure 3a). This diamond structure connects each other forming a 3D framework with large channels (Figure 3b). While the paddle-wheel SBUs are simplified as nodes and the bridging L2− ligands are simplified as the linkers, an uninodal four-connected dia topology can be rationalized (Figure 4). In this diamond framework, the L2− ligands bridge two SBUs through forming infinite helical chains along the b-axis (Figure 5). The helical chains have a pitch of 51.0948(4) Å, and the nearest Co···Co distance through the L2− ligand is 15.0463(8) Å. Interestingly, left-handed helices exist in the structures along the b axis, four cogenerated helices are derived with adjacent helical chains, the diamond network exhibits chiral feature, and the helical chains are all left-handed. These helical chains with the same handness are united alternately through L2− ligands and SBUs to exhibit a chiral 3-D metal−organic framework. The adjacent SBUs are further inserted by the DPNDI ligand; for one diamond net, one DPNDI is introduced, leading to the formation of a complicated 3-D network of 1 (Figure 1c,d). There are large open channels along the a and b axis (Figure 6). While the paddle-wheel SBUs are simplified as nodes and the bridging L2− and DPNDI ligands are simplified as the linkers, a rare uninodal six-connected jsm topology is obtained, with the point symbol of {510.64.7} (Figure 7a). The topology was analyzed by TOPOS4.0. Using TOPOS it is possible to retrieve a list of 17 known coordination networks with jsm topology: MOFFES, VOQBUZ, LESBEQ, OJOQAF, OJOQAF01, MOFFES01, MOWQOF, FEQSOK, FIFNIS, DELTAQ01, BOSHUN, DELTAQ, GIMBIN02, GIMBIN05, GIMBIN01, FIFNOY and GIMBIN. They are all not interpenetrated. Although the DPNDI is a large ligand inserted in the diamond framework, in order to minimize the big void cavities of and stabilize the framework, a 3-fold interpenetrated network is formed (Figure 7b). The interpenetration is the same also when the dia 3-fold is considered; the extra connection from dia to jsm is between the same dia net, and so the interpenetration is kept. According to the CCDC database, this is the first example of interpenetrated jsm topology.15 The Dichroism Spectra (CD) Analysis. The combination of single-crystal X-ray structure determinations and circular dichroism spectra (CD) is the most common way of gaining as much evidence as is possible for the homochirality or enantioenrichment of the bulk material. The bulk crystals are chosen to determine the solid-state CD spectra in a KBr matrix.

Figure 7. (a) A single jsm topological net and (b) the 3-fold interpenetrated framework of 1.

Figure 8. The solid-state CD spectra of bulk samples of 1.

As seen in Figure 8, CD spectrum exhibits a negative CD signal, revealing the formation of enantiomers in crystals. Thermal Analysis and PXRD Patterns. To characterize the compounds more fully in terms of thermal stability, their thermal behaviors were studied by TGA (Figure 9). From 25−480 °C, there is no weight loss. Until 480 °C, a rapid weight loss is observed. This result may indicate that compound 1 is a high thermal stable material. To further determine its thermal stability, we performed in situ variable temperature powder XRD study. As shown in Figure 10, with the heating temperature range from 55 to 375 °C, the PXRD spectra of the bulk products are consistent with the simulated PXRD spectrum of the single crystal, and at 405 °C, the network is already collapsed. Magnetic Properties. The temperature dependence of the magnetic susceptibility of 1 in the forms of χMT versus T is shown 443

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observed, which may be ascribed to the inevitable impurity in the reaction of pH > 7 in this work.

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CONCLUSION In summary, we have prepared and characterized a high stable network based on nanosized naphthalenediimide-dipyridyl ligands and a flexible V-shaped ligand. Our research demonstrates that the employment of the achiral flexible V-shaped ligand is a feasible way to construct a homochiral framework showing a new network with the large DPNDI ligand.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data in CIF format, selected bond lengths and angles, additional pictures, IR and PXRD. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 9. TG plots of 1.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 86-25-83314502. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (Nos. 91022011; 20971065; 21021062) and National Basic Research Program of China (2010CB923303).

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REFERENCES

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Figure 10. VT-PXRD spectra of 1 with heating showing high thermal stability.

in Figure 11. The χMT value is 1.528 cm3 K mol−1 at room temperature, which is much lower than the value expected for

Figure 11. Temperature dependence of χMT for 1.

two independent Co(II) centers (3.75 cm3 K mol−1 for S = 3/2 and assuming g = 2.0), suggesting the very strong antiferromagnetic coupling between Co(II) ions in the dinuclear unit. As the temperature decreases, χMT monotonously decreases until 50 K, also giving evidence of strong antiferromagnetic coupling. According to the trend of χMT with the cooling temperature, χMT will approach zero below 50 K. However, a protuberance is 444

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