Controlling the Packing of Metal-Organic Layers by Inclusion of

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Controlling the Packing of Metal-Organic Layers by Inclusion of Polymer Guests Benjamin Le Ouay, Hikaru Takaya, and Takashi Uemura J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b07563 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Journal of the American Chemical Society

Controlling the Packing of Metal-Organic Layers by Inclusion of Polymer Guests Benjamin Le Ouay,† Hikaru Takaya,‡ Takashi Uemura*,†,§,# †

Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8561, Japan. ‡ Institute of Chemical Research, Kyoto University, Gokashou, Uji, Kyoto 611-0011, Japan § CREST, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan. # Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. Supporting Information Placeholder ABSTRACT: The preparation of metal-organic structures with a controlled degree of disorder is currently one of the most promising fields of materials science. Here, we describe the effect of guest polymer chains on the transformation of a MOF. Heating a pillared MOF at a controlled temperature resulted in the exclusive removal of the pillar ligands, while the connectivity of the metal-organic square-grid layers was maintained. In absence of polymer, 2D-layers rearranged to form a new crystalline phase. In contrast, the presence of polymer in the MOF inhibited totally the recrystallization, leading to a turbostratic phase with layers threaded and maintained apart by the polymer chains. This work demonstrates a new synthetic approach toward the preparation of anisotropic metal-organic materials with controlled disorder. It also reveals how guests can dramatically modify the conversion of host MOFs, even though no chemical reaction occurs between them.

Metal-organic frameworks (MOFs) constitute a versatile class of materials, owing to their high porosity associated with virtually infinite structural and functional tunability.1-2 While MOFs have long been considered as ideal crystalline structures, an increasing research effort has been recently dedicated to the characterization of structural defects and paracrystalline domains.3-4 Amorphous MOFs, that present a well-defined coordination environment but lack a long-range order, represent a particularly promising class of materials.5-6 Because the overall molecular environment is maintained but has less strict geometric constraints, amorphous MOFs present properties that are quite different from those of their crystallized counter-parts.7-8 However, synthetic approaches allowing to precisely control the structural disorder remain scarce. Furthermore, conventional methods of amorphization, such as meltquenching9-11 and ball-milling12 usually lead to isotropic disordered structures. Producing materials with an anisotropic disorder remain still a challenging task. In this paper, we report the selective pillar removal from a pillaredlayer type MOF by solid-state reaction, associated with a guest-dependent reactivity (Figure 1). Indeed, the presence of polymer chains in MOF pores prevented the spontaneous recrystallization into a dense phase, and led instead to the formation of a new turbostratic phase with a layer packing reminiscent of the parent MOF. This material was thus prepared in two steps: First, the high crystallinity of the MOF was used to assemble precisely the polymer chains through the layers. The ordered MOF was then partially

decomposed,13-16 to reach a soft material with a controlled organization at the molecular level.

Figure 1. Preparation of a polymer-threaded turbostratic metal-organic phase. Inclusion of functional guests,17 and notably of polymers,18-19 constitutes a convenient strategy to introduce new properties in MOFs, while benefitting from the high porosity of the hosts. For instance, this approach have been used to bring electron conductivity,20-22 photoconductivity,23 hydrophobicity,24-25 and ion exchange properties26-27 to MOFs. In addition, our group reported recently that the inclusion of polymers could increase the thermal and mechanical stability of MOFs.28-29 Here, we show how polymer guests can direct the structural transformation of a MOF, characterized at several length scales. While the polymer had little impact on the local coordination environment and on the macroscopic shape of the crystals, it altered significantly the material’s structure at the nanometer scale. This phenomenon is remarkable, as no chemical bonds existed between host and guests. Thanks to the polymer chains hindering the mobility of the MOF’s components, the composite structure was neither densely packed, nor completely disordered, but presented interesting dynamic properties. In this study, we performed the solid-state conversion by moderate heat-treatment of the MOF [Zn2(BDC)2(DABCO)]n (1; BDC: benzene-1,4-dicarboxylate, DABCO: 1,4-diazabicyclo[2.2.2]octane, Figure S1).30 1 is constituted of Zn2(BDC)2 square-grid layers, with DABCO acting as a pillar ligand. The stacked grids define one-dimensional (1D) nanochannels, in which polystyrene (PSt) was prepared by in situ polymerization,31-33 yielding a MOF-polymer composite (1PSt, Figure S2). Here, the molecular weight (Mn) of PSt

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was 45,000 (Figure S3), corresponding to chains expanding across ca. 100 adjacent unit-cells along the c-axis of 1. Owing to the difference in coordination strength, apical DABCO amines could be removed selectively at high temperature, while the Zn-carboxylate paddlewheels remained in position. Thermogravimetric analysis (TGA, Figure 2A) of the pristine 1 revealed a decomposition in two distinct steps. The first step started at 300 °C and corresponded to the removal of intact DABCO molecule from 1 (Figure 2B). This result is in accordance with the hydrolytic decomposition of 1, where water displaced DABCO from the Zn centers.34-35 However, in our experiment under inert atmosphere, the pillar removal was spontaneous upon heating, and not caused by a displacement. 1 presented a second step around 430 °C, corresponding to BDC decomposition. 1PSt TGA showed that DABCO and BDC removals occurred at the same temperatures as for 1, with a negligible influence of PSt. In addition, PSt was degraded above 400 °C and depolymerized into monomeric styrene. However, PSt has an almost zero vapor pressure below its decomposition temperature. In order to remove DABCO without degrading the rest of the structure, we performed the heat-treatment of 1 and 1PSt at 320 °C, giving 1HT and 1HTPSt respectively. TGA curves of 1HT and 1HTPSt presented no more DABCO loss at 300 °C, but showed the same decomposition steps for BDC and PSt as their parent materials. BDC integrity and DABCO quantitative removal were further confirmed by NMR (Figure S4). GPC and DSC indicated that long PSt chains were maintained and remained confined in the pores after the heat-treatment (Figure S3 and S5).

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environment around Zn centers (Figure 3 and S6 to S25).36-37 Zn K-edge EXAFS spectra demonstrated the similarities of local geometric and electronic structures at Zn centers within each pair: pillared (1 and 1PSt) and heat-treated (1HT and 1HTPSt) (Figure 3A). Furthermore, only small changes between pillared and depillared samples were observed, indicating that the Zn-O bond lengths from the Zn(BDC) substructure in the initial 1 were overall maintained even after DABCO removal. Indeed, structural analysis using free energy force field (FEFF) fitting confirmed the preservation of the 4-fold paddlewheel layer structures in 1HT and 1HTPSt, with only minimal distortion of geometry. This was further characterized using O K-edge NEXAFS (Figure 3B). Here, 1HT and 1HTPSt spectra exhibited sharp peaks at 533 eV, characteristic of the O-1s→C=O-* transition, differing substantially from that of pillared 1 and 1PSt. The high similarity between the spectra of 1HT and 1HTPSt confirmed that their local coordination environments and electronic states were affected in a similar way after DABCO removal. This peak enhancement induced by the pillar removal could be rationalized by the changes of orbital hybridization along with a distortion of the Zn-O-C angle (from 126.69° in 1 to 121.35°) upon decoordination of the electron-donating axial ligand DABCO; the large electronic perturbation affected the O-1s→* transitions despite negligible changes of Zn-O and O-C bond lengths and O-Zn-O bond angles.

Figure 3. EXAFS spectra of 1, 1PSt, 1HT, and 1HTPSt. A: Zn K-edge EXAFS spectra in R-space. B: O K-edge NEXAFS spectra.

Figure 2. (A) TGA of 1, 1PSt, 1HT and 1HTPSt (Reference: mass at 200 °C). The vertical dotted line corresponds to the heat-treatment temperature (320 °C). (B) Mass spectroscopy traces during the TGAs. The species monitored are DABCO (m/z = 112, blue), styrene (m/z = 104, black) and benzene (from BDC decomposition, m/z = 78, red). Having shown the removal of DABCO during the heat-treatment, we used extended X-ray absorption fine structure analysis (EXAFS) to study its consequences on the coordination

Powder X-ray diffraction (PXRD) was also used, to probe the longrange ordering of the samples (Figure 4). PXRD measurements of 1HT and 1HTPSt revealed the disappearance of diffraction peaks from crystalline 1, revealing a structural transformation upon DABCO removal. Variable temperature PXRD confirmed the formation of new phases as soon as DABCO was removed, and their maintenance upon cooling (figure S26). Furthermore, depending on the presence of PSt, the obtained phases in 1HT and 1HTPSt were quite different, although their local coordination environments were very similar. 1HT presented relatively sharp diffraction peaks, revealing the conversion of 1 into another crystalline phase (Figure S27). By contrast, 1HTPSt did not give any sharp peak

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Journal of the American Chemical Society associated to a crystalline phase, but presented only broad peaks centered around 9.5 ° and 19 °. The positions of these broad peaks suggest the formation of a turbostratic structure with an interplane distance of 9.3 Å, reminiscent of the interlayer distance in the crystalline 1 (d = 9.608 Å). Since PSt chains have a lateral diameter larger than a DABCO pillar, the slight shrinkage is compatible with a maintenance of PSt threaded through the Zn(BDC) layers, and not with an intercalation between layers. In addition, 1 was successfully reconstructed from either 1HT or 1HTPSt in contact with a DABCO solution (Figure S28). However, treating 1HT resulted mostly in the formation of other unfavourable phases, with 1 being only a minor product. When treating 1HTPSt, the proportion of other phases was significantly decreased, giving 1 as the main product of the reaction. Since PSt content in 1PSt was easily tuned by changing the monomer conversion (Figure S29), dependence of the PSt loading amount on the phase transformation was studied. For 1HTPSt with a conversion of 24% and above, only the turbostratic phase was detected. Since traces of crystalline phase were still observed in 1HTPSt with low PSt contents, and traces of turbostratic phase were detected in 1HT, one can infer that a repacking of free Zn(BDC) layers occurred during the depillaring. In 1HT, most of the layers were able to repack to form the crystalline phase. The presence of PSt prevented this repacking and the turbostratic phase was stabilized instead.

guests modified significantly the aspect of the composite in the micrometer range.

Figure 5. A-D: SEM images of single crystals of 1 (A), 1PSt (B), 1HT (C), 1HTPSt (D) (Scale-bar: 100 m). E, F: Close-up views of 1HT (E) and 1HTPSt (F) (Scale-bar: 10 m).

Figure 4. PXRD patterns of 1HTPSt with various PSt contents. In addition to the transformations at the molecular scale, the phase transitions were also characterized at the mesoscopic scale using scanning electron microscopy (SEM) (Figure 5). 1 presented rectangular prismatic crystals of several hundred micrometers (Figure 5A). 1PSt presented the same morphology, and no PSt was detected outside of the crystals, confirming the polymerization in situ (Figure 5B).38 After heat-treatment, crystals’ general shape and dimensions were maintained for both 1HT and 1HTPSt (Figures 5C and 5D). However, the surface of 1HT was rough, with multiple cracks below one micrometer (Figure 5E). These cracks are likely caused by the recrystallization, as the volume per unit cell of 1HT is much smaller than that of 1. On the contrary, 1HTPSt had a smooth surface (Figure 5F), comparable to that of the original 1PSt. This might be caused by the absence of recrystallization and the limited volume change during the transformation into a turbostratic phase. The SEM study revealed thus that: (i) Solid-state heat-treatment is a powerful strategy to convert MOFs and change their micro-structures, without altering their general shape. (ii) By constraining the mobility of MOF at the molecular scale, polymeric

This type of new metal-organic materials would show interesting properties characteristic of the controlled disorder structures. Here we studied gas adsorption behaviors for 1HT and 1HTPSt with a styrene conversion of 52% and 26% (Figure 6). All samples showed a low adsorption capacity at low pressure, revealing a collapse of the microporosity (Figure S31). Moreover, 1HTPSt with 52% conversion exhibited almost no overall adsorption, possibly because too much PSt inhibited the porosity. However, 1HTPSt with a conversion of 26% showed a significant adsorption at P/P0 > 0.7 associated with a hysteretic loop, that could correspond to a gate-opening phenomenon. This indicates that, despite preventing the layers repacking, the PSt chains might offer enough local mobility to allow for a dynamic behavior. PXRD changes in solvents were also considered (Figure S32). 1HT presented the typical behavior of a layered MOF, with some solvents such as acetone and nitromethane being able to intercalate reversibly. By contrast, 1HTPSt presented little variations of PXRD in these solvents, showing that the turbostratic phase was stabilized by the polymer guest. Furthermore, a hysteretic behavior was also observed for 1HTPSt upon adsorption of n-heptane at 293 K, even if this alkane was not susceptible of intercalation in 1HT (Figure S33). The solid-state conversion of the pillared-layers MOF by heattreatment was thus a valuable synthetic route for the preparation of new metal-organic materials and composites, unreachable by wet synthetic route. Notably, it afforded the conversion of a polymerMOF composite while maintaining the microscopic organization of the assembly. This allowed to identify a guest-dependent network transformation, as PSt chains hindered the mobility of the depillared MOF layers and prevented their repacking into a crystalline phase. Instead, the layers were maintained apart and remained approximately in their original position, yielding a turbostratic phase

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Figure 6. N2 adsorption isotherms (77 K) of heat-treated composites. reminiscent of the parent material with an interesting dynamic behavior. This change is remarkable, as it is not due to a direct chemical reaction between host and guest, but originates from a non-reactive polymer hindering the network mobility. This work demonstrates how new materials with interesting properties can be reached, through the rationalized disassembly of MOF composites, in association with polymers as unique guests that extend over numerous unit-cells.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Synthetic procedures and additional characterizations (PDF).

AUTHOR INFORMATION Corresponding Author * [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the JST-CREST program (JPMJCR1321) and a Grant-in Aid for Science Research on Innovative Area “Coordination Asymmetry” (JP16H06517) from the Ministry of Education, Culture, Sports, Science and Technology, Government of Japan.

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