Homochiral Metal–Organic Framework with Intrinsic Chiral Topology

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Homochiral metal-organic framework with intrinsic chiral topology and helical channels Fei Wang, Hong-Ru Fu, and Jian Zhang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00209 • Publication Date (Web): 09 Mar 2015 Downloaded from http://pubs.acs.org on March 15, 2015

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Crystal Growth & Design

Homochiral metal-organic framework with intrinsic chiral topology and helical channels Fei Wang*, Hong-Ru Fu, and Jian Zhang* State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, the Chinese Academy of Sciences, Fuzhou, Fujian 350002, P. R. China. ABSTRACT: Presented here is an unusual microporous metal-organic framework (FIR-30) integrating helical channels, intrinsical chirality and permanent porosity. The structure of FIR-30 containing three types of cages has two interpenetrating helical channel systems with opposite handedness, which can be considered as a surface with the same topology as gyroid, and the generation of chirality is related with the intrinsically chiral topology of FIR-30. The results provide new insight towards the construction of chiral MOFs with helical channels.

Porosity and chirality are two essential features for materials in the chemical and pharmaceutical industry. Chiral porous materials, such as chiral inorganic zeolites or chiral metalorganic frameworks (MOFs), are particularly desirable because of their potential application in enantioselective separation and catalysis. 1-20 Among the building strategies on chiral porous structures, to adopt an intrinsically chiral framework topology is a very direct approach. 21-22 Some typical and familiar intrinsically chiral topologies are known as 3-connected srs net (maximum symmetry I4132), 4-connected quartz (denoted: qtz; maximum symmetry P6222) and quartz dual (denoted: qzd; maximum symmetry P6222) etc. 23-24 Of particular interest is the srs net that can intergrow with its enantiomorph to form gyroid. The gyroid is a minimal surface 25, and networks lie on its surface would gain maximum porosity per volume.

and permanent microporosity. Three types of cages are presented in the structure of FIR-30, which lead to two interpenetrating helical channel systems with opposite handedness, which can be considered as a surface with the same topology as gyriod, while the generation of chirality is related with the intrinsically chiral topology of FIR-30.

Gyroid has been recognized in some porous materials with different structural topologies, such as UCSB-7K 1 and STU-1 (gie) 26, SU-M (fcy) 4, ITQ-37 (-ITV) 6, ZIF-14 (ana) 27, ZIF72 (lcs) 28, CPM-17 (lcs) 29, and so on. Among them, UCSB7K and ITQ-37 crystalize in chiral space group I213 and P4132, respectively. The origin of the chirality for UCSB-7K with gie net is related to framework chemical compositions and ordering of T atom (Ga and Ge) sites, because the gie net (ideal symmetry: Ia-3d) is not intrinsically chiral. Thus, the gyroidal MOF STU-1 with the same gie topology does not show any chiral feature. Similar reason can be observed in those gyroidal ZIF-14 (ana), ZIF-72 (lcs) and CPM-17 (lcs). Only the inorganic framework ITQ-37 with -ITV net is special to see the coexistance of intrinsic chirality and gyroid.6 In contrast, MOF structures combining gyroid, intrinsic chirality and porosity together have never been known to date. In this work, we report such an outstanding MOF, Zn15(5etz)16(2-eim)11(OH-)3(H2O)3.x(guest) (FIR-30; FIR denotes Fujian Institute of Research, 5-etz = 5-ethyltetrazole, 2-eim = 2-ethylimidazole) with helical channels, intrinsical chirality

Figure 1 (a) the coordination environment of Zinc atoms; four types of cages: A-cage (b), B-cage (c); C-cage (d) and D-cage (e) in FIR-30; (f) the topology of FIR-30.

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coordinates 0, 3/4, 3/8 and their symmetry equivalents), and D tiles from D-cages at positions 24h (with coordinates 0.125, 0.019, 0.231 and their symmetry equivalents), located at the interstices between B tiles (Figure 2c and 2d). Each B tile is connected to another three B tiles through C and D tiles to form another gyroidal channel with srs topology by considering each tile B as a 3-connected node (Figure 2f). Interestingly, two independent sets of helical channels with opposite handedness are interpenetrating each other and just separated by the crystalline wall of FIR-30 (Figure 2g).

Colorless polyhedral crystals of FIR-30 were solvothermally synthesized and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the cubic chiral space group I4132. 30 The Flack parameter of 0.09(3) demonstrates the homochirality of a single crystal. Since 5-etz and 2-eim have similar chemical structures and both of them act as µ2-linkers, single-crystal X-ray diffraction is difficult to identify them. But the element analysis and NMR results demonstrated the coexistence of 5-etz and 2-eim in the structure. Here, we call them the azolate ligands. In the structure of FIR-30, the Zn centers are alternately linked by these azolate ligands into a non-interpenetrating three-dimensional framework with three types of cages. An A-Cage has inner diameter of 14 Å and is defined as [49.92.103] with 10 member ring (MR) faces (in the symbols [...mn...] means that there are n faces that are m-rings) (Figure 1a). The B-cage defined as [92.142] shows 12MR faces (Figure 1b), and two smaller [42.123] C-cage and D-cage just locate at the interstices between B-cages (Figure 1c). Large free voids in FIR-30 (45612 Å3 per unit cell without considering hydrogen atoms) were calculated by PLATON program. 31 The whole framework of FIR-30 can be topologically represented as a (3,4)-connected network with Point symbol of (42.6)2(42.92.10.11)3(43.62.8)2 (denoted: caz = chiral azole zeolite), 32 which presents a new one out of the network database (Figure 1d). Further analysis by the Systre program reveals its intrinsic chirality with maximum symmetry of I4132. 33 Based on this result, the generation of chirality in FIR-30 is due to

Worthy of mention is that the chiral zeolite ITQ-37 has only one single helical channel and its framework shows opposite chirality to the helical channel system (left-handedness for the srs-type framework and right-handedness for the srs-type channel system). In comparison, FIR-30 reported here has two opposite (non-equal) srs-type helical channel system, and the (3,4)-connected network of FIR-30 just lies on the surface with the same topology as gyriod and separates such two channels. Thus, FIR-30 presents a unique framework different to all the reported materials with G-surface. The TG curve of as-synthesized FIR-30 shows a weight loss of 21 % before 310 oC, corresponding to the release of the DMF guest molecules (Figure 3). For gas sorption studies, FIR-30 was activated by supercritical CO2 method (seen supporting information in details). From the powder XRD (PXRD) patterns, we can see a little difference between the simulated and the experimental results (Figure s3), the deviation is likely

Figure 2 Tiling of FIR-30. (a-d) The A, B, C and D tiles in FIR-30; (e) helical channel based on A tiles. (f) helical channel based on B, C and D tiles; (g) the interpenetrating helical channels presented by two interpenetrating srs nets with opposite handedness.

Figure 3 The overlay of TGA traces of as-synthesized, solventexchanged, and evacuated (activated) samples of FIR-30.

such an intrinsically chiral network.

due to the small structure change of the as-prepared sample as a result of moisture or air sensitivity.

The other prominent structural feature of FIR-30 is the presence of the surface with the same topology as gyriod that separates two helical channels formed by A-cages and B-cages. The packing of these cages can be seen from the illustration of them as tilings in Figure 2. The A tiles from A-cages are at positions 16e (with coordinates 5/8, 7/8, 1/8 and their symmetry equivalents) (Figure 2a). Each tile is connected to three others to form giant gyroidal channels with srs topology by considering each tile A as a 3-connected node (Figure 2e). The B tiles from B-cages are at positions 8a (with coordinates 1/8, 1/8, 1/8 and their symmetry equivalents) (Figure 2b). There are also C tiles from C-cages at positions 12c (with

To characterize the porosity of FIR-30, N2 gas sorption experiments at 77 K on the activated samples were measured (Figure 4). FIR-30 exhibits typical type I reversible sorption isotherms and takes up N2 to 386 cm3/g at 77 K, corresponding to Langmuir and BET surface areas were 1595 and 1420 m2/g, respectively. A single data point at relative pressure 0.95 gives a maximun pore volume of 0.595 cm3/g by the HorvathKawazoe equation. The surface areas and pore volume are much higher than that of those porous materials with gyroidal channels, and is comparable to recently reported ones. 34

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Crystal Growth & Design in Figure 4 and 5. The most significant feature is that FIR-30 displays much more uptake of C3H8 than that of C1 and C2 hydrocarbons. The calcated isosteric heat of adsorption for C3H8 is 27.0 kJ·mol-1 (Figure s4), which is much higher than that of other light hydrocarbons (C2H6: 20.9 kJ·mol-1; C2H4: 20.9 kJ·mol-1; C2H2: 13.1 kJ·mol-1; CH4: 13.2 kJ·mol-1), indicating that FIR-30 may be a potential candidate for the separation of C3H8 from C1 and C2 hydrocarbon mixtures. In summary, we first presented here a unique MOF (FIR-30) integrating helical channels, permanent porosity and intrinsic chirality. The structure of FIR-30 can be considered as a 2dimensional tiling of a surface with the same topology of gyroid, separating two helical channels, the intrinsically chiral topology of FIR-30 made them can’t offset each other. The results provide new insight towards the construction of chiral MOFs with helical channels.

Figure 4 The N2 sorption isotherms of FIR-30.

Supporting Information. Experimental details, TGA diagram, powder X-ray diffraction, and CIF file (CCDC--1010569 (FIR30). This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *E*E-mail: [email protected];[email protected] [email protected];[email protected] ACKNOWLEDGMENT We thank the support of this work by 973 program (2011CB932504 and 2012CB821705), NSFC (21221001, 21103189), and Chunmiao Project of Haixi Institute of Chinese Academy of Sciences (CMZX-2015-001).

REFERENCES Figure 4 The gas sorption isotherms of FIR-30 at 273K: (a) CH4; (b) C2H2; (c) C2H4; (d) C2H6; (e) C3H8. Solid represents adsorption, open represents desorption.

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7 8 9 Figure 5 The gas sorption isotherms of FIR-30 at 289K: (a) CH4; (b) C2H2; (c) C2H4; (d) C2H6; (e) C3H8. Solid represents adsorption, open represents desorption.

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The pure component sorption isotherms of FIR-30 for various hydrocarbons under ambient conditions were also shown

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Homochiral metal-organic framework with intrinsic chiral topology and helical channels Fei Wang*, Hong-Ru Fu, and Jian Zhang*

Presented here is a microporous metal-organic framework (FIR-30) integrating helical channels, intrinsical chirality and permanent porosity, which shows two interpenetrating helical channel systems with opposite handedness.

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