A flexible hydrogen bonded organic framework that reversibly adsorbs

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A flexible hydrogen bonded organic framework that reversibly adsorbs acetic acid:#-trimesic acid. Marta Sanchez-Sala, Oriol Vallcorba, Concepción Domingo, and José A. Ayllón Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00858 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 18, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

A flexible hydrogen bonded organic framework that reversibly adsorbs acetic acid:  trimesic acid. Marta Sanchez-Sala,a Oriol Vallcorba, *,b Concepción Domingoc and José A. Ayllón.*,a

a Departamento

de Química, Universidad Autónoma de Barcelona, 08193-Bellaterra,

Barcelona, Spain b

ALBA Synchrotron Light Source, Cerdanyola del Vallés, Barcelona, Spain.

c

Instituto de Ciencia de los Materiales de Barcelona (CSIC), Campus UAB, 08193 Bellaterra,

Spain.

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Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract The elusive  phase of trimesic acid (TMA, 1,3,5-Benzenetricarboxylic acid) has been prepared by recrystallization of commercial -TMA in acetic acid. This process yields a modified -TMA phase that contains guest acetic acid solvent molecules with an approximate stoichiometry of -TMA·1HAc. According to 1H-NMR and crystal structure determination, guest molecules are located both in the channels and in relatively isolated cavities. Despite the constricted connection between channels and cavities, solvent guest molecules are easily removed, even at room temperature, yielding guest free -TMA. This process is reversible, since pristine -TMA can reabsorb acetic acid vapor at room temperature, yielding again -TMA·1HAc. Conversely, -TMA only adsorbs negligible amounts of N2 at 77 K or CO2 at 273 K, denoting that the guest-adsorbent interaction is a key factor governing adsorption.

Keywords: Trimesic acid, Hydrogen bonded Organic Frameworks (HOFs), porous materials, metastable compounds, phase transition.


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1. Introduction Organic porous materials have been intensively explored for several applications, such as selective gas separation and storage, sensors and catalysis.1 Among them, hydrogen-bonded organic frameworks (HOFs) are promising candidates to develop functional organic materials due to their easy construction and intrinsic flexibility, arising from reversible hydrogen bond formation.2-5 In addition, HOFs can be easily regenerated by recrystallization, which favors material recycling.6 In the synthesis of porous materials, the challenging parameters are to avoid compact packing and to control the structural interpenetration. Moreover, the stability issues must be considered. Molecules with adequate geometry and different hydrogen-bond donoracceptor peripheral groups have been studied for the synthesis of HOFs, as for example phenols,7 amides,8 2,4-diaminotriazinyl,9,10 and carboxylic acids.11-13 Although most HOFs are usually less stable than porous materials based on covalent and coordination bonds, some remarkable examples of robust compounds have already been reported.6 For instance, the recently described H4TCBP (3,3’,5,5’-tetrakis-(4carboxyphenyl)-1,1’-biphenyl), build from molecules with four carboxylic acid groups arranged symmetrically, is a permanent 3-D porous material (surface area > 2000 m2/g) and shows high thermal stability.11 Molecules with peripheral carboxylic acid groups interact among them through the formation of complementary hydrogen bond acceptor and donor pairs. In the quoted example, eight-atom rings are thus formed, including two pairs of complementary acceptors (-C=O) and donors (-C-O-H), named R22(8) hydrogen bonding synthon.14 Usually, molecules with four to six carboxylic acid groups, disposed in adequate symmetry, are desirable to yield porous materials.11-13 3 ACS Paragon Plus Environment


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A well-known example of a small organic molecule that forms supramolecular network structures is the  phase of trimesic acid, which forms infinite 2-D hydrogen bonded networks with hexagonal 63 topology.15 The structure of -TMA presents inclined triple interpenetration (Fig. 1a). The networks of hydrogen bonded molecules are not planar, but slightly folded due to rotations around the C–CO2H bond. Thus, although each 2D supramolecular hexagonal network shows a hole, after extensive interpenetration the free space is reduced and it is only available as isolated cavities (13.4 v%). Several H3TMA derivatives show structures with non-interpenetrated hexagonal networks, which can be accessed through crystallization with a bulky template molecule that yields host-guestsupramolecular structures where the guest occupies the cavities of the hexagonal net.16,17 Sublimation of TMA at high temperature (613 K) yields TMA·0.05TMA·0.04C6H4(COOH)2. The crystal structure of this material was described by Herbstein et al.18 The partial decomposition of TMA by decarboxylation yields benzenedicarboxylic acid, which together with additional TMA remain as guest in the final structure. The encapsulation of these molecules seems a key factor for the formation of the  phase.18 In fact, the synthesis of pure -TMA has not been reported, although the presence of structural pores suggests that this phase can be considered as a HOF. Crystal structures of the  and  phases of TMA (without guest molecules) are very similar at the short range, with the same interpenetration scheme. However, in -TMA the 2D supramolecular network remains basically planar, i.e., non-folded (Fig. 1b). Due to this modification, the packing is less effective than in the -TMA and the A has channels parallel to the c axis in addition to the relatively isolated cavities as in -TMA.


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Figure 1. Comparison between the packing in (a) α-TMA, (b) γ-TMA and (c) γ-TMA·HAc (HAc molecules are not included for clarity).



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In this work, a simple and straightforward method to prepare -TMA by recrystallization of -TMA in acetic acid (HAc) is presented. In this synthesis, the volatile nature of the guest molecules allows the preparation of pure -TMA. The reversible adsorption of acetic acid by this material was also demonstrated (Scheme1).

Scheme 1. Synthetic route for γ-TMA·HAc and the interconversion between γ-TMA·HAc and γ-TMA.

2. Experimental 2.1. Materials Trimesic acid and glacial acetic acid were both from commercial sources (Sigma-Aldrich and Panreac, respectively) and used without further purification. All the reactions and manipulations were carried out in air.


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2.2 Characterization Synchrotron X-ray diffraction was carried out at the MSPD-BL04 beamline19 in ALBA Synchrotron. Single-crystal diffraction experiments were performed on the microdiffraction endstation (focused beam spot of 15 x 15 μm full width at half maximum) and the powder diffraction measurements on the high-resolution powder diffraction endstation of the beamline.

2.2.1 Synchrotron single-crystal X-ray diffraction (SCXRD) SCXRD data were collected at 29.2 keV energy (0.4246 Å wavelength, determined from the Sn K-edge) using a Rayonix SX165 CCD detector (165 mm diameter, frame size 2048 x 2048 pixels, 79 µm pixel size, dynamic range 16 bit) placed at 145 mm of the sample. A colorless crystal was mounted on a loop and measured using a step-scan data collection (vertical axis of rotation, ω, from -90 ° to 90 ° with steps of 0.2 °). A second scan with the crystal rotated by 90 ° (orthogonal axis) was performed (ω from -60 ° to 60 °, 0.2 ° steps) and data from both scans was merged afterwards. The diffraction data were indexed, merged and integrated using XDS software.20 The crystal structure was solved by intrinsic phasing (SHELXT) and refined with SHELXL (version 2014/7)21 using Olex2 as graphical interface.22 Non-H atoms were refined using anisotropic displacement parameters. H-atoms were geometrically placed and refined using a riding model with isotropic displacement linked to the bonded atom. The disordered HAc solvent molecules contained inside the voids of the crystal structure could not be satisfactorily modeled and the disordered density was masked using the PLATON/SQUEEZE method23 in the final refinement (1.2 Å probe 7 ACS Paragon Plus Environment


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radius, 0.2 Å grid space). However, an additional refinement with a rough modeling of the HAc molecules was performed to be used as a model for powder diffraction refinements. Crystallographic data and refinement details are summarized in Table S1 and the CIF files have been deposited in the Cambridge Crystallographic Data Centre (ref. numbers CCDC 1841046-1841049).

2.2.2 Synchrotron powder X-ray diffraction (PXRD) PXRD data were collected at 20 keV energy (0.61872 Å wavelength, determined from the Si NIST-640d reference) using the microstrip Mythen-II detector (six modules, 1280 channels/module, 50 µm/channel, sample-to-detector distance 550 mm). Variable temperature measurements were performed using a Cyberstar hot gas blower with an Eurotherm temperature controller. A ramp from 30 to 420 °C at 5 °/minute was performed continuously collecting a diffraction pattern every 20 sec. Cell parameters at all temperatures have been refined with full pattern matchings using DAjust software.24 Fitting of the single-crystal models to powder diffraction data have been performed with Rietveld refinements by RIBOLS software introducing TMA and HAc molecules as rigid bodies.

2.2.3 Nuclear Magnetic Resonance (NMR) 1H-NMR

spectra were recorded on an NMR-FT Bruker AC-250MHz spectrometer in

deuterated dimethyl sulphoxide ((CD3)2SO) at 250 MHz. Chemical shifts are referenced to the residual proton signal of the deuterated solvent and are given in ppm. Solid state 13C{1H}-NMR

spectra were obtained on a Bruker Avance II 400 MHz spectrometer at 298

K using rotational frequencies at 10 kHz. Samples (approximately 60-80 mg of material)


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were carefully packed in ZrO2 rotors and the standard CPMAS pulse sequence was applied.25-26

2.2.4. Gas adsorption measurements The gas adsorption behavior was determined by N2 and CO2 adsorption at 77 and 273 K, respectively, using an ASAP 2000 Micromeritics Inc. Samples were first degassed at 333 K for 24 h.

2.3 Synthesis. 1.50 g of TMA were dissolved in 100 mL of glacial acetic acid by heating to boiling and under agitation. The HAc was distillate until a white precipitate appears. At this moment, heating was stopped and the mixture was allowed to cool to room temperature. The precipitate was filtrated and washed with 5 mL of fresh HAc. The solid was dried by pressing between sheets of filter paper until an easily handling powder was obtained. Integration of 1H-NMR peaks of this samples determines the -TMA·1HAc stoichiometry. A sample of this precipitate was spread in a petri dish and maintained under air flow at room temperature overnight. After this treatment, the amount of retained guest was reduced to approximately ¼HAc. Finally, a prolonged aeration at room temperature followed of either a short treatment at 100 °C in the ventilated furnace or overnight vacuum at room temperature yield pure -TMA without residual HAc, according to

1H-NMR

characterization.



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Re-adsorption experiments. A powdered sample (200 mg) of -TMA was spread at the bottom of a beaker (4 cm diameter) forming an even layer. The beaker was placed into a bigger container that contains 1 mL of HAc, thus avoiding the direct contact between the liquid HAc and the powdered material. The container was then closed to create in the interior a saturate atmosphere of HAc vapor. After a day at room temperature, the solid sample was recovered and characterized by powder XRD and 1H-NMR.

3. Results and discussion Recrystallization of commercial α trimesic acid in hot acetic acid yields a crystalline solid. This solid has been characterized by single-crystal diffraction giving the -TMA phase, with an approximate content of 1 molecule of HAc per TMA (referred as -TMA·HAc). The solved crystal structure of -TMA·HAc (Fig. 1c) shows a supramolecular framework very similar to the reported for -TMA,18 including the characteristic 1D channels and relatively isolated cavities, but with wider channels. The PXRD pattern of the bulk sample was consistent with the single-crystal structure (Fig. S1) and the analysis of the data indicate that both channels and cavities contain highly disordered HAc molecules. The presence of HAc was confirmed by 1H-NMR, after sample dissolution in d6-DMSO (Fig. S2). The presence of by-products, coming from the decarboxylation of TMA, was not observed in this case, probably due to the low temperature used in the process compared with the formation of -TMA by sublimation. Further characterization by 13C-SSNMR confirms that the sample contains acetic acid (Fig. 2). The presence of two peaks assigned to the carbon of the methyl groups (at 18.45 and 24.45 ppm) and two assigned to the carbon of the carboxylic acid group of acetic acid (at 179.71 and 178.30 ppm) suggests that the 10 ACS Paragon Plus Environment

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acetic acid molecules are confined in two different environments, which would correspond to the channels and the cavities. Peaks from TMA also differed from the commercial TMA. The analysis of the channels and cavities in the crystal structure of -TMA·HAc with the PLATON/SQUEEZE tool23 resulted in a solvent accessible volume (SAV) of 1380 Å3 in the whole unit cell (23.1 v%) formed by two 273 Å3 channels and two 394 Å3 cavities (Fig. 3c). The electron count inside the voids with SQUEEZE indicates that both the channels and the cavities contain HAc molecules,

Figure 2. 13C-SSNMR spectra of commercial α-TMA (bottom) and of γ-TMA·with different amounts of acetic acid guest. Asterisks denote spinning side bands.

around 3 and 4, respectively (Table S2). Hence, a total of ca. 14 HAc were included in each unit cell (corresponding to -TMA·0.6HAc). This value is slightly lower than the HAc content determined by 1H-NMR (-TMA·1HAc), likely due to differences in each method 11 ACS Paragon Plus Environment


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accuracy. Besides, XRD data correspond to a single crystal, while the NMR reflects the average composition of the bulk sample.


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Figure 3. Solvent accessible volume representation in (a) α -TMA, (b) γ-TMA·0.1HAc and (c) γTMA·HAc crystal structures. The percentage of the total cell volume is 13.4 , 17.9 and 21.3 v%, respectively (1.2 Å probe radius and 0.2 Å grid spacing). 13 ACS Paragon Plus Environment


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When standing in air at room temperature for a day in a well ventilated cabinet, TMA·HAc loses HAc gradually producing samples with ca. ¼ HAc, according to 1H-NMR characterization (Fig. S3). The 13C-SSNMR of this sample (Fig 2) indicates that the intensity of one of the two pairs of signals assignable to acetic acid was much more reduced than the other, probably reflecting that the solvent contained in the channels was more easily desorbed than the molecules included in the cavities. To further investigate the HAc loss, a single-crystal of -TMA·~0.1HAc was selected and measured. A crystal structure with cell parameters very close to the reported -TMA values was obtained, but still containing a small amount of HAc. The same analysis of channels and cavities as in TMA·HAc resulted in a SAV of 1111 Å3 (17.9 v%), containing two channel sections that add 158 Å3 and two cavities of 331 Å3 (Fig. 3b). It is worth nothing that the channels suffered a reduction of 42 v% (from 273 to 158 Å3) and the cavities of 16 v% (from 394 to 331 Å3). The electron count inside the voids resulted in no HAc molecules contained in the channels and only one inside the cavities. As for -TMA·HAc, the total content of HAc of this sample determined by X-ray diffraction is only slightly larger (0.1) than by 1H-NMR (0.06) (Fig. S4). Prolonged aeration or vacuum treatment overnight at room temperature leads to solvent free samples that, according to XRD characterization, retained the crystalline framework (Fig. S5 and S6). 13C-SSNMR also indicates that the sample is different to the commercial αTMA (Fig 2). The accelerated transformation of -TMA·HAc was studied by in situ PXRD of a sample confined in a heated open-glass capillary tube (Fig. 4a). Cell parameters were extracted 14 ACS Paragon Plus Environment

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from each measure in the range of 305 to 660K and two sharp variations of the volume could be observed. First one, at ca. 373K was associated to the loss of HAc contained in the channels, while the second at 543K may be either the additional desorption of HAc or a temperature related readjustment. Rietveld refinements at selected temperatures using the single-crystal model with the modeled HAc molecules were performed (Figure S1). At 428K a content of -TMA·~0.2HAc was obtained, while at 573K the content was reduced to ~0.09HAc. Rietveld refinement after returning to RT (from 573K) showed a reasonable fit with the HAc free model, but the refinement improves when a small amount of HAc content (~0.09) is introduced. The unit cell volume of this heated sample (6136 Å3) is slightly bigger than the vacuum treated -TMA (6123 Å3), which indicates that by heating up to 573K the HAc was not completely removed probably due to slow kinetics together with sample disposition in a thin capillary tube with low evacuation open surface (Fig. 4b, Table S3). In fact, heating a sample dispersed on a petri dish in a ventilated furnace at 383K during 24h produces HAc free samples, although also promotes the partial transformation of the sample to the -TMA phase. Although we have measured repeatedly a large number of different crystals, we have not been able to find a well-diffracting truly empty crystal. This is related to a certain degree of instability of the empty crystal, at least at the surface, which decreased the quality of the crystal during diffraction.



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Figure 4. (a) PXRD patterns of γ-TMA·xHAc upon heating from RT to 660K (0.61872 Å wavelength). (b) Variation of the unit cell parameters and cell volume of γ-TMA·xHAc as a function of the temperature. Two sharp variations of the unit cell volume are observed at 373 K and 543 K, related to the loss of HAc in the structure, from x ≈ 1 to x ≈ 0.2 and from x ≈ 0.2 to x ≈ 0.09 respectively.


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It its remarkable that -TMA·HAc easily desorbs the HAc contained in the cavities, even at room temperature, despite the constricted connection between cavities and channels. This behavior resembles that of molecular materials with 0D porosity, in which, frequently, some gases permeability is observed despite there being no obvious dynamic mechanism to facilitate gas uptake, at least in the static view of their crystal structure.27,28 It must be considered that structures based on hydrogen bond show greater flexibility than those based on coordination or covalent bonds. In the studied system, the HAc adsorption process is completely reversible at room temperature. By contrast, the low-temperature (77 K) adsorption isotherm of N2 in pure -TMA indicated negligible adsorption in spite of the presence of empty channels. The obtained surface adsorption values were in the order of 5 m2/g or less. Similar result was obtained for CO2 adsorbed at 273 K. The negligible gas adsorption is related with the lack of surface stability for the -TMA crystals, in which local defects or structure collapse on the particles surface hinders the adsorption of these gases. Contrarily, the acetic acid, taking profit of its ability to form hydrogen bonds, could surpass the surface barrier and penetrate into the channels and cavities of the material. It is worth mentioning that measurements with HAc were performed over a much longer timescale than for gas adsorption. Moreover, when a sample of -TMA was wetted with a small amount of liquid HAc and stowed in a closed vial at room temperature for a week, it was partially transformed to -TMA·HAc (Fig. S7).

In the route to -TMA preparation via -TMA·HAc presented in this work, the initial occupation of the channels and cavities seems essential to the formation of this phase. Indeed, in the first literature reports, the 1D-channels were empty while the cavities were 17 ACS Paragon Plus Environment


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occupied by additional TMA molecules and by dicarboxylic acid from the partial decarboxylation of TMA. A stoichiometric formula TMA·0.05TMA·0.04C6H4(COOH)2 was reported (it seems that the exact nature of the dicarboxylic acid was not determined, but 1,3 dicarboxylic acid seems the most probable).18 Isostructural polyhalide inclusion compounds of stoichiometric formula TMA·0.7H2O·0.09HI5, TMA·0.7H2O·0.09HBr5 and TMA·0.7H2O·0.167HIBr2 have also been prepared from aqueous solutions.29,30 In them, the polyhalide anion is located in the channels, while water and proton seem included in the roughly spherical voids. The variation of the cell parameters between these isomorphs denotes the flexibility of the supramolecular network of -TMA, related to the guest molecules accommodated into the channels. A similar flexible behavior has also been reported for other HOFs, produced by different methods, like guest inclusion, temperature, etc.31 Interestingly, two different solvates of TMA with acetic acid have been reported, precipitated from complex solvent mixtures such as CS2 and benzene for the 1:3 HAc:TMA solvate32 and ethanol, chloroform and 1-naphtylamine for the 1:2 HAc:TMA solvate.33 In both cases, part of the HAc molecules disrupt some of the extended hexagonal hydrogen bond networks, making a clear difference respect to the structures of both -TMA and TMA. In both cases, some of the HAc molecules are arranged as hydrogen bonded dimers (HAc)2 occupying structural cavities. The existence of these phases evidences the wide range of structural possibilities in crystal growth originated by the combination of two relative simple small molecules and how the growth medium determines the specific structure formed in each case.


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3. Conclusions Recrystallization in acetic acid is a simple method to prepare -TMA via the formation of an intermediate containing acetic acid. The porous and flexible crystal structure of this compound allows to accommodate HAc molecules in two different environments, channels and cavities. The variation of the structural parameters of -TMA as a function of the HAc content denotes the flexibility of this hydrogen bonded organic framework. The reversible adsorption-desorption of HAc is a key property. The compound has some degree of selectivity in the adsorption, as -TMA does not adsorb significant amounts of N2 or CO2.

Acknowledgments This work was partially financed by the Spanish National Plan of Research CTQ201456324, CTQ2017-83632 and MAT2015-67593-P (O.V.) projects. C.D./ICMAB acknowledges financial support from the Spanish MEC, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV- 2015-0496). M. S-S. also acknowledges the Universitat Autònoma de Barcelona for his pre-doctoral grant. ALBA synchrotron is acknowledged for the provision of beam time. Authors thank Dr. Pau Nolis for assistance in NMR measurements.



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Graphical Abstract

Synopsys Recrystallization of commercial -TMA in acetic acid is a simple method to prepare -TMA through an intermediate containing acetic acid as a guest in two different environments, channels and cavities. Pure TMA is obtained by a reversible acetic acid desorption process. -TMA does not adsorb significant amounts of non-polar gasses.


A flexible hydrogen bonded organic framework that reversibly adsorbs

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