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Hybrid Triazine-Boron 2D Covalent Organic Frameworks – Synthesis, Characterization and DFT Approach to Layer Interaction Energies Krzysztof Gontarczyk, Wojciech Bury, Janusz Serwatowski, Piotr Wieci#ski, Krzysztof Wozniak, Krzysztof Durka, and Sergiusz Lulinski ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09061 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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ACS Applied Materials & Interfaces

Hybrid Triazine-Boron 2D Covalent Organic Frameworks – Synthesis, Characterization and DFT Approach to Layer Interaction Energies Krzysztof Gontarczyk,[a] Wojciech Bury,[a,b] Janusz Serwatowski,[a] Piotr Wieciński,[a] Krzysztof Woźniak,[c] Krzysztof Durka,[a]* Sergiusz Luliński[a]* [a]

Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland [b] Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland [c] University of Warsaw, Biological and Chemical Research Centre, Żwirki i Wigury 101 02-089 Warszawa, Poland Corresponding authors E-mail: [email protected]; [email protected] Keywords: Covalent-organic frameworks, two-dimensional materials, adsorption, periodic DFT calculations, interlayer energy, layer assembly, boron-triazine COFs. Abstract The conversion of 2,4,6-tris(4’-bromophenyl)-1,3,5-triazine to the respective triboronic acid was successfully accomplished by a simple triple Br/Li exchange followed by boronation. Further dehydrative condensation reactions with 2,3,6,7,10,11-hexahydroxytriphenylene or 2,3,6,7-tetrahydroxy-9,10-dilalkylanthracenes (R = Me, Et) resulted in materials featuring good porosity and sorption properties with the nitrogen uptake exceeding 500 cm3/g (STP) and SBET up to 1267 m2/g (T = 77.2 K). In addition, simple dehydration of this compound was employed for the preparation of a hybrid 2D COF composed of triazine, boroxine and benzene rings. The formation of materials was confirmed by the IR analysis and NMR studies on water-decomposed samples. All obtained COFs exhibit high thermal stability with decomposition temperatures in the range 400-600 °C. They also show quite different morphology ranging from regular 0.5-4 µm spherical and ellipsoidal clusters to 5-12 µm bent rod-like particles. The PXRD studies supported by periodic DFT modeling in Crystal09 package revealed the formation of crystalline 2D honeycomb-type lattices with eclipsed stacking models. In addition, the differences between boroxine-triazine material and related COF-1 and CTF-1 structures were investigated by comparing layer interaction energies, work function values as well as atomic charges and electrostatic potential maps plotted on the electron density surfaces. It demonstrates that the interactions between layers are enhanced by the stacking of triazine and boroxine rings. Finally we have investigated the upper limit to space accessible volume using procrystal electron density approach.

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Introduction Covalent Organic Frameworks constitute a class of porous organic materials composed of light elements such as carbon, boron, oxygen, nitrogen, silicon, and sulfur connected by strong covalent bonds.1-5 The seminal results regarding design of these materials were reported by Yaghi and co-workers. Their initial strategy was based on a simple dehydrative autocondensation of 1,4-phenyleneboronic acid and resulted in COF-1 which showed 2D layer hexagonal architecture.6 Alternatively, they used the polycondensation reactions between the multiboronic acids and appropriate polyols (e.g., 2,3,6,7,10,11hexahydroxytriphenylene, HHTP), to prepare various 2D organic frameworks showing improved sorption properties with respect to industrially and environmentally relevant gases such as H2, CH4 and CO2.6-8 The most impressive results were achieved for 3D COFs based on tetraboronic acids C[p-C6H4B(OH)2]4 and Si[p-C6H4B(OH)2]4 for the construction of highly porous 3D COF-102 and COF-103 structures, respectively.9 The significant advantage of COF materials is that they often have a very low density, which drops below 0.2 g·cm-3 for COF-102 and COF-103. In addition, COFs usually exhibit exceptional thermal stability (even up to 600 °C). The presented characteristics make COFs an attractive class of new porous materials. The building blocks of COFs do not undergo significant change in their overall geometry when incorporated into the material framework. This allows also for the design of COFs possessing desired composition, pore size, and aperture which is very important from the practical point of view. Further development in the field is obviously coupled with the improvement of experimental and theoretical methods used for the characterization of these materials. Currently, the chemistry of COFs is no longer limited to boron-based monomers. Among various classes of these materials, systems comprising triazine rings are gaining recently a strong interest due to their promising properties. For instance, unlike the majority of boron COFs, the Covalent Triazine Frameworks (CTFs) are hydrolytically stable which was exploited for specific applications including surfactant adsorption,10 desalination11 or even photocatalytic water splitting.12-13 They were also used as a platform in a Pd-based heterogenous catalyst for cross-coupling reactions14 and other catalytic processes.15 It is already well-documented that CTFs show efficient and highly selective CO2 adsorption which can be used for gas separation.16-22 Considering the continued interest in boron COFs as well as the recent rapid growth of CTF chemistry, we present herein results of our work on novel boron-triazine 2D COFs (BTA-COFs) obtained using 2,4,6-tris(4’-dihydroxyborylphenyl)-1,3,5-triazine as a basic building block. They can be regarded as hybrid materials due to the presence of boron-based as well as triazine structural motifs. They exhibit high thermal stability and N2 sorption parameters comparable to those found for systems possessing similar topologies of 2D COF layers. However, it seems that the sheet stacking can be organized by specific boron-nitrogen interactions resulting in a different geometry of the pores with respect to classical boron-based COFs.


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Results and Discussion Synthesis and characterization of COF precursors. The synthesis of boronated triazine started from 4-bromobenzonitrile, which was subjected to trimerization in CHCl3 using triflic acid as a superacidic catalyst, to give 2,4,6-tris(4bromophenyl)triazine 123-26 as a white solid in good yield (85%). In a consecutive reaction, an orange suspension of a respective trilithio reagent was generated using nBuLi in THF at −78 °C. Then an excess of B(Oi-Pr)3 was added and the mixture was allowed to warm to the room temperature. The mixture was quenched with aqueous H2SO4 and final workup afforded 2,4,6-tris(4’-dihydroxyborylphenyl)-1,3,5-triazine 2 (Scheme 1). However, the boronic acid was isolated in a moderate yield (33%), presumably due to low solubility of precursor 1 in THF resulting in its incomplete conversion. Scheme 1. The synthesis of the boronated triazine 2. B(OH)2

Br

CN TfOH/CHCl3, rt

3

N

1. nBuLi/THF, -78 oC

N

2. B(OiPr) 3 3. H3 O+

N Br

Br

1

Br

N

N N

(HO)2B

2

B(OH)2

Triazine-based triboronic acid was isolated as a white solid well soluble in wet acetone and slow evaporation of a solution resulted in the formation of single crystals suitable for Xray diffraction. Crystal structure determination showed that the molecule of 2 is essentially planar with a slight deviation of one phenyl group from the coplanar arrangement (Figure 1a). Molecules form hydrogen-bonded channelized layers parallel to (203) crystal plane (Figure 1b). The channels are filled with water and acetone connected with triboronic acid molecules via hydrogen bonding interactions.

Figure 1. Molecular structure of 2 (a) and packing diagrams showing the hydrogen bond interaction patter within (203) plane (b). Hydrogen bonds are marked as red dashed lines. We have decided to use two types of polyols for the condensation with 2: 2,3,6,7,10,11hexahydroxytriphenylene (HHTP), 2,3,6,7-tetrahydroxy-9,10-dialkylanthracenes (9,10dimethyl - THDMA and 9,10-diethyl THDEA), (Scheme 2). The synthesis of the latter ones 3 ACS Paragon Plus Environment


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involved the treatment of veratrole with ethanal or propanal, respectively, in the mixture of acetonitrile and concentrated H2SO4. This tandem electrophilic Friedel-Crafts type substitution and subsequent oxidation of the initial cyclization products gave rise to respective tetramethoxy intermediates TMDMA and TMDEA. Subsequently, cleavage of methoxy groups with BBr3 afforded targeted tetrahydroxy linkers (Scheme 2).27-29 Scheme 2. Structures of the polyols used as linkers for the preparation of triazine boron-based COFs.

Synthesis and characterization of COFs. The syntheses of materials BTA-COF1−3 were performed by dehydrative polycondensation reactions of 2 with 1 equivalent of HHTP, or 1.5 equivalents of THDMA and THDEA, respectively (Scheme 3). A general protocol similar to those developed for the preparation of other boronate ester COFs was employed.1,2,6-9 Thus, a stoichiometric mixture of 2 and a polyol was stirred in a 1:1 mesitylene – 1,4-dioxane mixture at 85 °C followed by repeated washing of a crude product with anhydrous THF and final drying under high vacuum (0.002 Torr) at 200 °C (Scheme 3). It is noticeable that using a higher initial concentration of substrates led to gelification of the obtained reaction mixture. The same approach was used to obtain BTA-COF4 via a dehydrative self-condensation of 2. The prepared materials are greyish (BTA-COF1−3) or white (BTA-COF4) powders. Their compositions were confirmed by recording 1H NMR spectra of respective solutions of samples subjected to hydrolytic degradation in DMSO-d6 with added D2O (Figure 2). At this point we would like to stress that this process is slow and obtained COF materials are in general quite stable under air. The 1H NMR analyses revealed that the obtained COFs contain significant amounts of mesitylene retained in materials dried at 85 °C under medium vacuum (0.1 Torr) for 24 hours. Furthermore BTA-COF2-3 are contaminated by significant amounts (ca. 1.5-2 equiv.) of unreacted or partially reacted triazine triboronic acid 2. On the other hand the hydrolytic decomposition of BTA-COF1 leads to the recovery of starting materials in almost stoichiometric proportions. The further drying of all materials at 250 °C removed most of mesitylene, whereas its complete removal was observed at 300 °C (0.1 Torr). The performed 1 H NMR analyses on samples dried at 300 °C prior hydrolysis showed that starting materials are in almost stoichiometric proportions with no impurities indicating that the boronic acid 2 can be efficiently removed from the mixture. This is due to observed decomposition of 2 to triphenyltriazine above 200 °C, which further sublimes from the mixture under these conditions. Finally we have found that the activation of obtained COFs at 200 °C under high vacuum (0.002 Torr) for 12 hours is sufficient to remove all volatiles prior to sorption


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measurements. This important observation should be taken into account when preparing other boronic COFs since residual molecules may significantly influence sorption properties. It is worth noting that BTA-COFs are synthesized using general procedure applied for boron-based COFs - solvothermal conditions with heating at 85 °C, whereas the synthesis of pure CTF materials proceeds under harsh conditions (molten ZnCl2 at 400 oC). Furthermore, TGA analysis revealed that a significant (ca. 5wt%) content of ZnCl2 resides in the material after purification.30 Thus, in our opinion, the approach based on a use of self-complementary building blocks can open new possibilities for various hybrid COF materials. Scheme 3. Preparation of COFs based on the boronated triazine 2.

B

R N

2 + 3/2 THDMA mesitylene/1,4-dioxane (or THDEA) - H2 O

N

O

O B

N

O

O R

R = Me: BTA-COF2 R = Et: BTA-COF3

B



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B

N N

O B B

N

dehydrative polycondensation 2

O O B

O B B

N O

N

O B

B N

N N

O B B

N

O O B

B

BTA-COF4

Figure 2. The representative 1H NMR spectra of BTA-COF2 samples initially dried at 85 °C, 250 °C, 300 °C and subjected to hydrolytic decomposition in wet DMSO-d6. The 1H NMR spectra of remaining COF materials are provided in Supporting Information. The thermal stability of BTA-COF1− −4 materials (dried only at 85 °C prior to analysis) was investigated under N2 atmosphere by TGA technique (Figure 3). A minor mass loss (up to 5%) is observed at lower temperatures (up to ca. 150 °C). It can be associated with the removal of adsorbed gases, and some volatile impurities (e.g., traces of water resulting from completion of condensation reactions). A more significant mass loss occurs in the temperature range of 150− −300 °C. Based on the 1H NMR analyses of hydrolyzed samples it can be assigned to the removal of remaining solvents (mostly mesitylene). Notably, the mass loss is much lower for BTA-COF1 (ca. 10%), whereas it reaches 20− −25% for BTA-COF2− −3, which 1 is in agreement with the amounts calculated from H NMR analysis. This indicates the higher 6 ACS Paragon Plus Environment

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potential of BTA-COF2− −3 for penetration with mesitylene molecules due to the higher pore diameter of these 2D COFs. Further heating up to 450− −500 °C does not result in any significant mass changes. Then a gradual decomposition starts as the total mass loss at 600 °C reaches 50% for BTA-COF3. To summarize, all obtained boronate ester COFs exhibit comparably high thermal resistance consistent with the exclusive presence of strong covalent bonds. This is also characteristic for many boron-based porous materials as well as for those −3, the boroxine BTA-COF4 shows a featuring triazine rings.31-33 Unlike BTA-COF1− relatively flat TGA curve up to 400 °C (the mass loss reaches 11% at that temperature). This indicates that BTA-COF4 does not incorporate substantial amounts of solvent molecules, which is naturally due to a smaller pore size. On the other hand, it seems to be slightly less stable in comparison with BTA-COF1− −3 as it gradually starts to decompose at ca. 400 °C and 32% of the total mass is lost in the temperature range of 400− −600 °C.

Figure 3. TGA curves for BTA-COF1− −4 recorded with the heating rate of 10 K/min. The FT-IR spectra of the obtained COFs were recorded using the ATR technique. They feature strong bands of B-O stretching vibrations in the range of 1324-1344 cm-1. There are also intense bands in the range of 1505-1512 cm-1 which can be assigned to C=N stretching vibrations in the triazine ring. In a recent study of IR spectral features of boron COFs34 it was pointed out that a strong and sharp band in the range of ca. 1220−1240 cm−1 of C−O stretching vibrations is highly diagnostic of boronate ester formation. In the case of our materials the bands which can be assigned to these vibrations were indeed observed but they were located slightly above of the given region for BTA-COF1 (1242 cm−1), whereas a negative and stronger deviation was found for COFs based on anthracene tetraols (1208− −1209 cm-1). An additional evidence for the boronate ester formation is the presence of a medium strong band at 645 cm−1. It has been noted previously35 that the vibrational mode between 633 and 658 cm−1 (involving out-of-plane displacements of boron atoms that are syn to out-ofplane displacements of aryl hydrogen atoms) is diagnostic for boronate esters. On the other hand, the appearance of an analogous band at 679 cm-1 for BTA-COF1 is in agreement with the observation that the region from 677 to 714 cm−1 is diagnostic for boroxine rings. These out-of-plane vibrations are especially diagnostic in 2D COFs and COF-like assemblies. 7 ACS Paragon Plus Environment


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Nitrogen sorption properties of BTA-COF1−4. The porosity of obtained materials was investigated using N2 gas adsorption at 77 K. Prior to the measurements, samples were activated by heating at 200 °C under high vacuum (0.002 Torr) for 12 hours to remove guest molecules. All recorded isotherms show a sharp increase of N2 uptake at low relative pressures (below 0.02 P/P0) which is a common feature of microporous materials (Figure 4) including classical boron COFs. In general BTA-COF1 and BTA-COF4 exhibit similar type-I sorption isotherms with relatively slow and almost constant increase of sorption in the pressure range of 0.2−0.8 P/P0. The NLDFT pore size distribution (PSD) plot (Figure S9) for BTA-COF1 demonstrates two maxima at 11.7 Å and 16.7 Å, which is in agreement with PXRD data (vide infra). Interestingly, the NLDFT PSD plot for BTA-COF4 shows only one major maximum below 10 Å, which also agrees well with its PXRD analysis. BTA-COF2 and BTA-COF3 show similar sorption behavior and both exhibit type-IV isotherm (Figure 4) typical for mesoporous 2D COFs.36 The BTACOF2 demonstrates slightly higher gas uptake than BTA-COF3 in the range of 0.4−0.8 P/P0, which might be a result of bulkier ethyl substituents present in the pores of BTA-COF3. In addition, desorption measurements revealed that the isotherms are almost reversible for BTACOF1 whereas only a slight hysteresis was observed for remaining materials. It was pointed out previously that this is characteristic for materials containing hexagonally aligned 1D mesopores with widths < 40 Å.37 The Brunauer-Emmett-Teller model was used to calculate the relevant parameters including apparent surface area SBET, with Rouquerol’s consistency criteria,38 and the pore volume Vp at P/P0 = 0.90 (Table 1). As expected, the SBET values for boronate ester COFs were significantly higher (1013-1267 m2 g-1) than for the boroxine BTA-COF4 (611 m2 g-1). This is consistent with a larger size of pores and their increased volume for BTA-COF1-3. Obtained results are comparable with those reported for related boron 2D COFs such as COF-1, COF-56 and BTP-COF.39

Figure 4. Nitrogen sorption isotherms (T = 77 K) for BTA-COF1−4. Table 1. Summary of sorption porosity measurements, pore volumes and surface areas derived from promolecular density and Connolly surface approximations for idealized models of BTA-COF1-4 structures. Sorption

SBET/ m2 g-1 Total pore volume /

BTA-COF1 1 267 0.55

BTA-COF2 1 013 0.67


BTA-COF3 1 054 0.59

BTA-COF4 611 0.37

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cm3 g-1 (P/P0 = 0.9) CrystalExplorer Pore volume per unit 675 (48%) 4013 (69%) 3 cell / Å (% of the unit cell) 1770 Surface area 2091 per gram / m2g-1 0.62 1.62 Pore volume per gram / cm3g-1 3782 (65%) Connolly surfacea Pore volume per unit 585 (42%) cell / Å3 (% of the unit cell) Pore volume 0.54 1.52 per gram / cm3g-1 a Values referred to DFT optimized geometries of most stable models. a

3905 (67%)

171 (28%)

2042

1605

1.41

0.27

3590 (62%)

131 (21%)

1.30

0.20

Morphology, structure characterization and modeling Scanning electron microscopy (SEM) indicates differences in morphology of studied materials (Figure 5). BTA-COF1 exhibits a uniform 2-4 µm size sphere- and ellipsoidshaped particles. In turn, anthracene-based BTA-COF2 crystallites adopt rod-like shapes with length and width dimensions of about 5-12 and 1-4 µm, respectively. They are usually bent reflecting the soft nature of the material. In SEM images of BTA-COF3, the irregular flaketype clusters are observed together with smaller ellipsoid-shaped agglomerates. BTA-COF4 material exhibits similar features, however, the size of corresponding particles are much smaller and oscillate in the range of 0.5-2 µm. The structures of BTA-COFs were investigated by powder X-ray diffraction (PXRD) measurements supported by the computational modeling (Figure 5). The PXRD pattern of BTA-COF1 exhibit six well resolved peaks assigned to the (100), (210), (200), (310), (410) and (001) facets. In the case of BTA-COF2-3 the intense peaks appear at 2θ = 2.1 o, however, other peaks are much broadened and are characterized by lower intensities. PXRD pattern of BTA-COF4 also demonstrated broadening of peaks, likely resulting from a relatively low crystallinity of the material, which is frequently observed in COFs. Nevertheless, in all cases the Pawley refinements enabled reliable peak assignment and revealed no diffraction peaks that could be attributed to the starting materials. The appearance of intense peaks at lower 2θ angle assigned to (100) facets and broad peaks at ca. 25o resulting from the π-stacking between the (001) layers indicates the formation of crystalline networks with 2D honeycombtype lattices. To investigate the crystal lattice packing, two types of usually considered models with respect to the stacking of the (001) layers were evaluated: fully eclipsed models with AA stacking and a staggered model with AB type stacking, where adjacent layers were translated ଵଵ

with respect to each other by [

ଶଶ

1] vector. In all cases the eclipsed stacking models

reproduce the peak position and intensity of the PXRD pattern. A full Pawley refinement produced the unit cell parameters (Table 2) staying in agreement with the observed diffraction patters.



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Figure 5. Experimental (red solid line), Pawley refinement (green solid line) and Crystal09derived simulated (blue solid line) PXRD patterns for the eclipsed structures of BTA-COF1 (a), BTA-COF2 (b), BTA-COF3 (c), BTA-COF4 (d) together with corresponding SEM images. Table 2. Comparison of BTA-COFs’ parameters derived from experiment (Pawley refinement) and Crystal09 optimizations.

Exptl

Unit cell pmts / Å

Crystal09

V / Å3 Symmetry Unit cell pmts / Å V/ Å3 d / g cm-3 Pore size / Å

BTA-COF1

BTA-COF2a

BTA-COF3 a

a = 22.7 c = 3.5 1553 P-6m2 a = 22.22 c = 3.27 1398 0.781 15.9

a = 43.7 c = 3.5 5822 P6/mmm a = 44.35 c = 3.41 5835 0.427 36.0

a = 43.7 c = 3.5 5822 P6/mmm a = 44.35 c = 3.41 5835 0.473 32.7

BTA-COF4 AA-I a = 15.7 c = 3.4 715 P3m a = 14.71 c = 3.28 614 1.046 8.5

BTA-COF4 AA-II a = 15.7 c = 6.7 1430 P-3m a = 14.74 c = 6.53 1228 1.045 8.5

a

In BTA-COF2 and BTA-COF3 structures the methyl/ethyl groups were replaced by chlorine atoms due to the problems with disordered hydrogen atoms resulted from the location of methyl/ethyl carbon atoms on the mirror plane elements of symmetry. In the next step we have performed quantum chemical geometry optimization using periodic density-functional approach (B3LYP40/TZVP41 level of theory) implemented in the 10 ACS Paragon Plus Environment

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CRYSTAL0942-43 program package. This procedure allowed us to obtain more reliable positions of atoms. In the optimization procedure, the atom positions and lattice parameters were fully optimized without constrains. The simulated powder diffraction patterns are consistent with the experimental ones obtained from Pawley refinement. On the other hand it is noticeable that DFT calculated structures are slightly more compact as indicated by closer interlayer distances and lower cell volumes. According to our calculations BTA-COF1 structure adopts P-6m2 hexagonal space group with a = 22.22 Å and c = 3.27 Å and diameter of pores of 15.9 Å. It resembles the topology of boroxine-based COF-88 structure constructed from 1,3,5-tris[(4-dihydroxyboryl)phenyl]benzene (TBPA) and HHTP building blocks, where each triazine ring is replaced by central phenyl moiety. Similarly, the BTA-COF2 and BTACOF3 structures with P6/mmm symmetry with unit cell parameters of a = 44.3 Å and c = 3.41 Å and 1D mesopores with diameter of 36.0 Å (BTA-COF2) and 32.7 Å (BTA-COF3) are triazine-based congeners of BTP-COF material, obtained from TBPA and THDMA.39 In turn, the boroxine-based BTA-COF4 structure can be compared with COF-16 and CTF-130 structures, which were formed by dehydration of 1,4-phenylenediboronic acid and polymerization of 1,4-dicyanobenzene, respectively. The molecular layer of COF-1 is composed of B3O3 boroxine nodes linked through phenylene units. The CTF-1 layer resembles the topology of COF-1, with each boroxine ring replaced by the triazine one. The BTA-COF4 layer is composed of both types of rings linked via phenyl groups in alternating fashion. Unlike the COF-1, where AB type stacking was observed (P63/mmc symmetry), the sheets in CTF-1 and BTA-COF4 are assembled in an eclipsed fashion. Consequently, BTACOF4 adopts trigonal space group of symmetry with two possible stacking modes. The eclipsed layers may be related by the mirror plane (P3m space group) or by centre of symmetry laying between neighbored phenyl linkers (P-3m symmetry). In the former case, the boroxine and triazine rings stack over the same corresponding ones leading to the formation of infinite columns (eclipsed AA-I model, Figure 6). In the second structure alternating boroxine-triazine stacking sequence is formed (eclipsed AA-II model). Finally the crystal may exhibit random distribution of layers sequence along the [001] direction. As each structure has the same set of symmetry imposed reflection conditions, they would exhibit the same diffraction patterns, and therefore it is not possible to unambiguously determine the stacking structure from experiment. For the further comparison we have also considered three possible staggered models of BTA-COF4: with boroxine-boroxine (AB-I), triazine-triazine (AB-II) and boroxine-triazine (AB-III) layers intersections.



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Figure 6. Proposed stacking modes in BTA-COF4 structure: AA-I (a), AA-II (b), AB-I (c), AB-II (d), AB-III (e). The experimentally obtained BET surface areas and pore volumes are usually underestimated with respect to idealized (fully periodic and homogenous) materials reflecting the presence of crystal defects and residual solvent molecules that may reside inside the pores. Furthermore, under ambient condition some parts of the volume (especially in mesoporous materials) are not used, while under higher pressure they become more relevant for gas uptake. Therefore to provide the upper limit to space accessible to guest molecules in studied materials we have used procrystal electron density approach implemented in CrystalExplorer. According to studies by Spackman,44-47 the isovalue of procrystal electron density surface was set to 0.0003 au as they found it to be the most appropriate to explore the materials with permanent porosities. The comparison of void surface areas and pore volumes obtained from experiment and CrystalExplorer estimations are given in Table 1, while void surfaces of corresponding substances have been visualized on Figure 7. The comparison with the commonly encountered Connolly surface derived parameters is also provided. According to CrystalExplorer promolecular density estimations, the BTA-COFs constructed from anthracene linkers exhibit the largest procrystal surface area of 2091 m2g-1 (BTA-COF2) and 2042 m2g-1 (BTA-COF3) and the largest pore volumes of 4013 Å3 (BTACOF2) and 3905 Å3 (BTA-COF3) per unit cell corresponding to 69% and 67% of the total material volume. This lead to very low density of these materials of 0.427 gcm-3 (BTACOF2), 0.473 gcm-3 (BTA-COF3) and high upper limits for gas storage capacities of 1.62 cm3g-1 and 1.41 cm3g-1. In the case of BTA-COF1 the calculated surface area is 1770 m2g-1 and the cell consists of 48% (675 Å3) of free volume which corresponds to gas storage 12 ACS Paragon Plus Environment

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capacity of 0.62 cm3g-1. BTA-COF4 obtained in autocondensation reaction of 2 has the procrystal surface area of about 1600 m2g-1 with pore volumes of 344 Å3 (28% of total volume) and gas uptake limit of 0.27 cm3g-1.

Figure 7. Void surfaces generated for BTA-COF1 (a), BTA-COF2 (b), BTA-COF3 (c) and BTA-COF4 (d) shown in the (001) plane. The procrystal density value was set to 0.0003 au. Finally, the periodic optimization in Crystal09 of nine COF structures including BTACOF4 (5 stacking modes), COF-1 (AA and AB) and CTF-1 (AA and AB) led to slight differences in stacking distances between layers for eclipsed and staggered models. In eclipsed structured the interlayer distance is equal to about 3.3 Å, while for staggered structures it decreases to about 2.9 Å. It seems that eclipsed structures are more stable as indicated by more favorable total crystal energies (∆EBulk, AA relative to AB, Table 3). However, it is noticeable that this relative stability difference is rather small for COF-1 (∆EBulk = 11 kJmol-1). This seems to be in agreement with previous observations showing that the staggered arrangement is more preferred for COF1, however, it may undergo phase transition to the eclipsed structure after removal of residual solvent molecules. The further computations in Crystal09 allowed for direct comparison of interlayer interaction energies EL [kJmol-1] - given per unit cell, and ES [kJmol-1Å-2] - related to the unit area of the molecular layer. At this point it should be stressed that the contribution of conformational energy resulted from the distances and interactions between atoms in layers, to the total crystal structure stabilization is not negligible leading to some differences between ∆EBulk and EL values when comparing the corresponding COF structures. Regarding BTA-COFs, the most advantageous interlayer energies were found for eclipsed AA-II and staggered AB-III structures (EL = –170 kJmol-1; ES = –0.9 kJmol-1Å-2) which probably results from the favorable interaction between boroxine and triazine rings being in alternate sequence in these structures. On the other hand the total crystal energy of the latter structure is by 35 kJmol-1 less advantageous reflecting the important contribution of the layer conformational energy. The layer interaction energies in BTA-COFs with AB-I and AB-II stacking modes are similar to the values obtained for corresponding COF-1 and CTF-1 structures with AB layer sequence, which is in agreement with similar stacking arrangement of boroxine-boroxine or triazine-triazine rings in these materials. Furthermore the staggered COF-1 structure is characterized by lower value of interaction energy as compared to its eclipsed counterpart (AA: EL = –162 kJmol-1 vs. AB: EL = –166 kJmol-1). On the other hand the stronger interlayer interactions seems to be compensated by slightly less advantageous conformation of layer in AB structure. In turn, in 2D CTFs the interaction energy between layers is significantly more advantageous for eclipsed structures (AA: EL = –167 kJmol-1 vs. AB: EL = –152 kJmol-1), which is in agreement with the structure derived from the analysis of PXRD pattern.43 It is 13 ACS Paragon Plus Environment


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also noticeable that the CTF-1 AA structure is characterized by the most advantageous average interaction energy per unit area among the series (ES = –0.92 kJmol-1Å-2), which is related to the most compact character of this structure and probable results from the favorable π-stacking interactions between aromatic ring planes. The comparison between staggered structures of all COF materials clearly demonstrates that the interactions between triazine rings are less favored in comparison with boroxineboroxine stacking. The most advantageous situation occurs for AA-II and AB-III structure featuring both rings in the alternate stacking sequence. This can be also illustrated by comparing corresponding atomic charges and electrostatic potentials (ESP) mapped on electron density surface calculated for the fragments of BTA-COF4, COF-1 and CTF-1 layers (Figure 8). According to expectations, the strongly electropositive regions appear in proximity of boroxine nodes. The electrostatic potential is also slightly positive directly above the triazine rings, while it becomes strongly negative close to the nitrogen atoms. Thus, it can be expected that interaction between triazine and boroxine rings are more favorable than interactions between moieties of the same type. Naturally, the regions of ESPs are not fully complementary as the π-stacking interaction are generally more dispersive in nature. Nonetheless, it is noticeable that the electrostatic potential is slightly negative in the aromatic ring regions indicating that some offset between layers may be expected.48 Recent development in 2D COF materials and single-layered COF chemistry suggest that these materials may serve as platforms for organocatalysis and solar energy conversion.12-15, 49-51 In this context, photocatalytic activity may be related to work function (WF) of the material and surface stability. For most systems, the more stable surfaces are characterized by lower WF values.52-54 Although there are some exception from this general principle,55 this approach can be reasonably used for the comparison of systems with similar layer structure and topology. According to our calculations, the WF values increase in the following order COF1 (ca. 4.1 eV), BTA-COF4 (4.33-4.39 eV) and CTF1 (4.51-4.57 eV), Table 3. This resembles the order calculated by Wang, Jiang and Zhao for some theoretical COF structures based on pure boronic (6.13 eV) or triazine (7.13 eV) nodes joined by diazine (-N=N-) linker.50 Not surprisingly, WF values for BTA-COF4 structures comprising equimolar amounts of boroxine and triazine nodes are placed in-between those obtained for COF1 and CTF1. Furthermore, comparison of staggered and eclipsed structures within each COF group indicates that the higher stability is accompanied by the lowering of WF values. For COF1, WF values are similar for staggered (4.100 eV) and eclipsed (4.112 eV) arrangements, once again pointing to the similar stability of both structures. In the case of CTF1, the AA layer arrangement is characterized by significantly lower WF value than the AB one (4.516 eV vs. 4.573 eV). Similarly, the calculations for BTA-COFs also revealed that eclipsed stacking arrangements are more preferred than staggered ones and AA-II (4.326 eV) and AB-III (4.343 eV) are characterized by the lowest WF values within each group of structures. Table 3. Summary of the periodic Crystal09 energy calculations (B3LYP/TZVP) for COFmaterials. EBULK corresponds to the total crystal energy given relative to the most stable structure in COF series, EL is interlayer interaction energy per unit cell, ES is average interlayer interaction energy given relative to the unit area of the molecular layer, dL donates to optimal interlayer distance (equal to c or c/2), WF stands for work function. 14 ACS Paragon Plus Environment

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Material BTACOF4

COF-1 CTF-1

Model AA-I AA-II AB-I AB-II AB-III AA AB AA AB

dL / Å 3.28 3.26 2.90 2.94 2.92 3.27 2.90 3.28 2.94

EL / kJmol-1

∆EBulk / kJmol-1 11 0 27 61 35 0 11 0 41

ES / kJmolÅ-2 –0.87 –0.90 –0.88 –0.77 –0.90 –0.83 –0.85 –0.92 –0.84

WF / eV

1

–163 –170 –164 –145 –170 –162 –166 –167 –152

4.329 4.326 4.370 4.392 4.343 4.112 4.100 4.516 4.573

Figure 8. Electrostatic potential mapped onto electron density surface (ρ = 0.0027 eÅ-3) generated for COF1, BTA-COF4 and CTF1 fragments (a). Mulliken charges derived from periodic calculations (b). Conclusions In conclusion, a series of four 2D COFs based on triboronated 2,4,6-triphenyl-1,3,5triazine 2 have been synthesized and characterized by TGA, FT-IR (ATR), SEM and PXRD analyzes. The applied two-step synthetic methodology (synthesis of triazine boronic acids and further condensation) can be extended to other porous materials including imine-, hydrazine-, benzimidazole-based COFs, possessing 2D or 3D topology, as well as multicomponent COF materials. Presented BTA-COFs can be considered as reasonable alternatives for pure boron and triazine COFs, combining advantages of both classes of materials – facile synthesis and exceptional thermal stability comparable to CTFs. Simultaneously, the obtained materials show similar nitrogen sorption behavior to related 2D COFs such as COF1, COF5 or CTF1, resulting from a similar topology and pore size. A significant part of the work was devoted to solving the question of self-assembly of 2D COF sheets. For this purpose, we tested a novel concept based on periodic energy calculations (B3LYP/TZVP) using Crystal09 package. Obtained results revealed that the eclipsed 15 ACS Paragon Plus Environment


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structures are more stable than staggered ones reflecting the effect of stacking interactions between adjacent rings which is in general agreement with experimental findings. A closer inspection of possible stacking modes indicates that there is an energetic preference for alternating boroxine-triazine sequence. This stays also in agreement with the ESP analysis showing that the layer self-assembly is dictated by interactions of electron-deficient boron atoms with the regions of increased electron density located at the nitrogen atoms. Obtained results indicate that relatively weak but specific interactions can be utilized for controlling the interlayer organization of 2D COF materials. Experimental Section General comments. THF, 1,4-dioxane, mesitylene used for reactions were dried by heating to reflux with sodium/benzophenone ketyl and distilled under argon and were stored over 4Å molecular sieves. Starting materials including 4-bromobenzonitrile, veratrole, BBr3, trialkyl borates, triflic acid, HHTP were used as received without further purification. In the 13C NMR spectra the resonances of boron-bound carbon atoms were not observed in most cases as a result of their broadening by a quadrupolar boron nucleus. 1H and 13C chemical shifts are reported in ppm from TMS with the residual solvent resonances as internal standards. Synthesis of COF precursors. 2,3,6,7-Tetramethoxy-9,10-dimethylanthracene (TMDMA).27-29 A mixture of veratrole (27.6 g, 0.2 mol), ethanal (8.8 g, 0.2 mol) and acetonitrile (10 mL) was slowly added dropwise to conc. H2SO4 (100 mL) at 0 °C. The mixture was stirred for 30 min and then it was poured onto ice water. The obtained solid was filtered and washed with water (3 × 100 mL). Then it was mixed with acetone (100 mL) and the resulting slurry was stirred for 30 min and filtered. The obtained white solid was extracted with chloroform in Soxhlet apparatus. The obtained suspension was filtered to afford a white solid which was dried in vacuo. Yield 15.6 g (48%). 2,3,6,7-Tetramethoxy-9,10-diethylanthracene (TMDEA).27-29 A mixture of veratrole (55.2 g, 0.4 mol), propanal (23.2 g, 0.4 mol) and acetonitrile (20 mL) of was slowly added dropwise to conc. H2SO4 (200 mL) at 0 °C. The mixture was stirred for 30 min and then it was poured onto ice water. The obtained solid was filtered and washed with water (3 × 200 mL). Then it was dissolved in chloroform (300 mL). The solution was washed with water and saturated NaHCO3 solution. The organic phase was separated, dried over anhydrous MgSO4 and concentrated in vacuo affording a crude product. It was dissolved in chloroform (150 mL) and cooled to ca. −20 °C. The precipitated crystals were filtered to afford pure product as an off-white solid. Yield 25.4 g (41%). 2,3,6,7-Tetrahydroxy-9,10-dimethylanthracene (THDMA).27-29 A mixture of TMDMA (3.66 g, 0.011 mol) and DCM (200 mL) was cooled down to 0 °C. Then BBr3 (8.1 mL, 7 equiv) was added dropwise and the mixture was refluxed for 2 hours. Excess of boron tribromide was carefully decomposed by slow addition of ethanol followed by hydrolysis. The mixture was neutralized with saturated aqueous NaHCO3 and filtered to afford a solid which was washed with water (2 × 100 mL). Then it was stirred with acetic acid (50 mL) and filtered. Drying in vacuo afforded the title product as a beige powder. Yield 1.35 g (44%). 1H NMR (400 MHz, DMSO) δ 9.42 (broad, 4H), 7.34 (s, 4H), 2.69 (s, 6H) ppm. 13C NMR (400 MHz, DMSO) δ 146.3, 125.7, 121.4, 106.0, 14.6 ppm. 16 ACS Paragon Plus Environment

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2,3,6,7-Tetrahydroxy-9,10-diethylanthracene (THDEA).27-29 A suspension of TMDEA (3.83 g, 0.082 mol) in DCM (200 mL) was cooled to 0 °C. Then BBr3 (7.8 mL, 7 equiv.) was added dropwise and the resulting mixture was refluxed for 2 hours. Further workup was carried out as described for THDEA to give the title product as a beige powder. Yield 2.54 g (80%). 1H NMR (400 MHz, DMSO) δ 9.40 (broad, 4H), 7.34 (s, 4H), 3.22 (q, J = 7.2 Hz, 4H), 1.26 (t, J = 7.2 Hz, 6H). 13C NMR (100 MHz, DMSO) δ 146.5, 128.2, 124.8, 105.5, 21.6, 15.2 ppm. Anal. Calcd for C18H18O4: C, 72.47; H, 6.08. Found: C, 72.13; H, 6.30. 2,4,6-Tris(4-bromophenyl)-1,3,5-triazine (1):23-26 A mixture of 4-bromobenzonitrile (35.7 g, 0.196 mol) and CHCl3 (80 mL) was cooled to 0 °C and then triflic acid (20 mL) was added dropwise. The mixture was stirred overnight at room temperature. Then, an obtained suspension was diluted with an additional portion of chloroform (100 mL). Finally, water (100 mL) was carefully added and a resulting white solid was filtered and washed with water (3 × 100 mL) and acetone (2 × 50 mL). Drying in vacuo afforded 1 as a white powder, m.p. 320−325 °C; yield 30.5 g (85%). 1H NMR (300 MHz, CDCl3) δ 8.59 (d, J = 8.8 Hz, 1H), 7.69 (d, J = 8.8 Hz, 1H) ppm. 2,4,6-Tris(4-dihydroxyborylphenyl)-1,3,5-triazine (2): Compound 1 (10.92 g, 20 mmol) was suspended in anhydrous THF (200 mL). The stirred suspension was cooled to −78 °C and 2 M n-BuLi (30 mL, 60 mmol) was added dropwise. The temperature was allowed to rise to −60 °C and the mixture was stirred for 3 hours at −78 °C. The obtained orange suspension was treated with B(OiPr)3 (13.8 mL, 60 mmol) at −78 °C. The mixture was stirred overnight, while the temperature was allowed to rise to room temperature. A resulting white suspension was hydrolyzed with aqueous 2 M H2SO4 (35 mL) and diluted with Et2O (200 mL). The organic phase was separated, washed with brine (3 × 50 mL), and filtered from unreacted starting materials. Evaporation of solvents afforded a crude product. It was stirred with a mixture of dichloromethane (20 mL) and water (20 mL), filtered and dried in vacuo to give the product as a of white powder. Yield 2.89 g (33%), m.p. > 300 °C. 1H NMR (300 MHz, Acetone) δ 8.73 (d, J = 8.5 Hz, 1H), 8.10 (d, J = 8.5 Hz, 1H). 13C NMR (75 MHz, Acetone) δ 172.4, 139.6 , 138.0, 135.1, 128.4. Anal. Calcd for C21H18B3N3O6: C, 57.22; H, 4.12; N, 9.53. Found: C, 57.13; H, 4.30; N, 9.45. Synthesis of COFs. BTA-COF1: Compound 2 (0.44 g, 1 mmol) and HHTP (0.32 g, 1 mmol) were placed into a 100 mL Schlenk flask. Mesitylene (25 mL) and 1,4-dioxane (25 mL) were added and the mixture was degassed under reduced pressure. Then the mixture was heated at 85 °C with stirring for 72 hours. The obtained suspension was left to stand and the supernatant solvent was carefully decanted with a syringe. THF (50 mL) was added and the mixture was stirred overnight. The washing with THF was repeated twice in the same manner. The obtained slurry was dried in vacuo at 85 °C for 24 hours to afford the product as a grey powder. Final removal of remaining guest molecules was accomplished by heating in vacuo at 200 °C for 12 hours. Yield 0.56 g (86%). BTA-COF2: It was obtained as a grey powder using the method described for BTA-COF1 from 2 (0.44 g, 1 mmol) and THDMA (0.41 g, 1.5 mmol). Yield (0.44 g, 60%). BTA-COF3: It was obtained as a grey powder using the method described for BTA-COF1 from 2 (0.44 g, 1 mmol) and THDEA (0.45 g, 1.5 mmol). Yield (0.60 g, 77%). 17 ACS Paragon Plus Environment


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BTA-COF4: It was obtained as a grey powder using the method described for BTA-COF1 from 2 (0.86 g, 1.94 mmol). However, the product was finally dried at lower temperature (200 °C) for 24 hours. Yield (0.60 g, 92%). Note: The reaction yields were calculated from samples dried at 200 °C. Single-crystal X-ray diffraction analysis of 2. The single crystal of 2 was measured at 100 K on SuperNova diffractometer equipped with Atlas detector (Cu-Kα radiation, λ = 1.54184 Å). Data reduction and analysis were carried out with the CrysAlisPro program.56 The structure was solved by direct methods using SHELXS-9757 and refined using SHELXL2014.58 All non-hydrogen atoms were refined anisotropically. The structure is of moderate quality, which is due to the fact that sample crystalizes in a form of very tiny plates. The high values of R and wR factors result also from the presence of unrefinement residual electron density, which is located in the central part of channels. It can be attributed to the highly disordered solvent molecule (water and acetone). Crystallographic Information File (CIF) has been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no. 1550182. 2: 2(C21H18B3N3O6)·4C3H6O·2(H2O), Mr (1 molecule of 2) = 441.06 a.u.; T = 100 K; triclinic, P-1, a = 15.255 (4) Å, b = 15.812 (7) Å, c = 16.019 (11)Å, α = 61.29 (6)°, β = 71.61 (4)°, γ = 83.00 (3)°, V = 3214 (3) Å3; dcalc = 1.184 gcm−3; µ = 0.725 mm−1; number of collected / independent reflection (Rint = 15.1%) = 13844 / 6968, R[F] / wR[F] (I ≥ 3σ(I)) = 28.1% / 65.0%, ∆ϱres(min/max) = −0.89 / +1.99 eÅ−3. PXRD measurements. Powder X-ray diffraction (PXRD) analyses of BTA-COFs 1−4 were carried out on a BrukerAXS WAXS D8 powder diffractometer equipped with Cu radiation source (Cu Kα, λ = 1.54184 Å), a no background sample holder and a VÅNTEC detector. Data were collected over a 2θ range of 2.1°−35° in Bragg−Brentano geometry with a generator setting of 40 kV and 40 mA, step size of 0.02°, and exposure time per step of 3 s. TGA. Thermogravimetric analysis were performed on a TGA/DSC1 (Mettler-Toledo) system under continuous flow of argon at the ramp rate of 10 K min−1 from 30 °C to 600 °C. The samples of 2–10 mg were prepared in covered ceramic crucibles. An empty crucible was used as a reference. α-Al2O3 was used for instrument calibration. Samples were measured after preliminary drying at 85 °C under 0.1 Torr for 24 hours. Sorption measurements. Micromeritics ASAP 2020 Surface Area and Porosity Analyzer was used to measure the nitrogen adsorption isotherms. Oven-dried sample tubes were evacuated and tared. The samples were transferred to the sample tubes, which were then capped by TranSeals. The samples were heated to 200 °C under a vacuum of 0.002 Torr for 12 h, at which point the outgas rate was less than 0.002 Torr/min. The evacuated sample tubes were weighed again, and the samples’ mass were determined by subtracting the mass of the previously tared tubes. N2 isotherms were measured using liquid nitrogen baths (77 K). Ultra high purity grade (99.999% purity) N2 and He, oil-free valves, and gas regulators were used for the free space correction and measurement. Relative pressure (P/P0) range for BET analysis was selected based on Rouquerol’s criteria.38 SEM studies. In order to determine COFs morphology and size scanning electron microscopy was applied. The samples were observed using SEM/STEM Hitachi s 5500 with maximal resolution of 0.4 nm at acceleration voltage of 30 keV and below 2 nm at 1 keV. The observation was performed using SE (secondary electron) signal. The microscope was 18 ACS Paragon Plus Environment

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equipped with Cold Field Emission gun. This type of electron source in the microscope ensures high resolution at relatively low beam current, which is important in case of observation of sensitive materials like COFs. To minimize the effect of surface charging as well as to confine the structure damage by electrons, low beam current and low acceleration voltage (3 keV) were applied. Such observation conditions ensured enough brightness and resolution but were safe for the COFs samples. Additionally, low accelerating voltage allows to observe surface morphology in details, since the electrons penetrate lower thickness of the sample. Theoretical Calculations. All computations within the Crystal09 program package22 were performed at the DFT(B3LYP) level of theory. TZVP basis set proved to be sufficient for the purpose of the conducted calculations. Grimme dispersion correction59-60 was applied. The evaluation of Coulomb and exchange series was controlled by ﬁve thresholds, set to values of 10−7, 10−7, 10−7, 10−7, 10−25. The condition for the SCF convergence was set to 10−7 on the energy difference between two subsequent cycles. Shrinking factor was equal to 4, which refers to 30−36 k-points (depending on space group symmetry) in the irreducible Brillouin zone in the case of the studied systems and assures the full convergence of the total energy. First molecular geometries were obtained from data provided for related COF structures by replacing the boroxine nodes with triazine rings and modifying the structural symmetry. Then, all the structures including atom positions and unit cell parameters were fully optimized with Crystal09 and they served for the subsequent layer interaction energy calculations. For BTACOF2 and BTA-COF3 structures, the methyl/ethyl groups were replaced by chlorine atoms due to the problems with disordered hydrogen atoms resulted from the location of methyl/ethyl carbon atoms on the mirror plane. Such replacement is justified by the fact that the CH3 group has a similar size to the Cl atom bound to the carbon atom (19 Å3 and 21 Å3, respectively). Consequently, the change of the crystal packing manner should be rather small. Crystal09 optimizations yielded more compact molecular packing which is especially reflected in lower interlayer stacking distances (2.9-3.2 Å vs. 3.4 Å from PXRD). In the case of COF-1 AA eclipsed structure the full optimization led to significant bending of the polymeric layers (global minimum). The flat geometry was found at local minimum and this geometry was taken for further considerations. All obtained coordinates are placed in the Supporting Information. In the next step the same sets of coordinates were subjected to crystal interlayer energy computations. The calculation parameters were set identical as in optimization procedure. The single layer was extracted from the crystal structures and p3m layered group of symmetry was set (SLAB option implemented in Crystal09 was used). The calculations include Grimme dispersion corrections and correction for the basis set superposition error. The latter one was calculated by using two upper and two lower layers as ghost functions. To allow for a direct comparison, the obtained interaction energy values are shown in two ways: they are scaled to the unit cell and are also related to the unit area of a given molecular slab. Electrostatic potentials mapped on the electron density surface (ρ = 0.0027 eÅ-3) was calculated in Gaussian09 (B3LYP/6-31g(d,p) level of theory) on the fragment of the molecule including 174 atoms (13 aromatic and 6 triazine/boroxine rings) cut on the Car-B or Car-Ctriaz bonds and terminated with hydrogen atoms. Mulliken population analysis was derived from Crystal09 calculations (keyword PPAN). Following the procedure described on Crystal Tutorial Project webpage,61 the work function was obtained as the 19 ACS Paragon Plus Environment


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difference of the energy of an electron at a large distance from the surface, minus the Fermi energy. As the potential at the large distance from surface is almost zero the work function was taken as negative Fermi energy. In order to elucidate Fermi energy keyword SMEAR (width = 0.01 hartree) was used. To accelerate convergence modified Broyden scheme was used (keyword BROYDEN). The input file consisted of 5-layered slab representing (001) COF surface. To improve the description of the electrostatic potential, ghost functions were added in the vacuum region close to the surface (keyword GHOSTS). Acknowledgements. This work was supported by National Science Centre (Poland) within the framework of the project DEC-UMO-2016/21/B/ST5/00118. PXRD X-ray measurements were undertaken in the Crystallographic Unit of the Physical Chemistry Laboratory at the Chemistry Department of the University of Warsaw. Authors thank Prof. J. Lewiński for providing an access to a physisorption analyzer, Prof. P. Parzuchowski for his assistance in FTIR (ATR) spectroscopy measurements, Dr. D. Pociecha for providing an access to PXRD difractometer and Dr. T. Płociński from Faculty of Materials Science and Engineering WUT for providing an access to SEM. Computational facilities were provided by the Wrocław Centre for Networking and Supercomputing. We gratefully acknowledge the Aldrich Chemical Co., Milwaukee, WI, U.S.A. for a long-term collaboration. Supporting Information. Details of TGA, PXRD and N2 sorption analyses, additional SEM images, copies of FTIR and NMR spectra of starting materials and hydrolyzed COFs samples, details of theoretical calculations. References. (1) Feng, X.; Ding, X.; Jiang, D. Covalent Organic Frameworks. Chem. Soc. Rev. 2012, 41, 6010–6022. (2) Ding, S.-Y.; Wang, W. Covalent Organic Frameworks (COFs): from Design to Applications, Chem. Soc. Rev. 2013, 42, 548–568. (3) Waller, P. J.; Gándara, F.; Yaghi, O. M. Chemistry of Covalent Organic Frameworks. Acc. Chem. Res. 2015, 48, 3053–3063. (4) Xiang, Z.; Cao, D.; Dai, L. Well-defined Two-dimensional Covalent Organic Polymers: Rational Design, Controlled Syntheses, and Potential Applications, Polym. Chem. 2015, 6, 1896–1911. (5) Huang, N.; Wang, P.; Jiang, D. Covalent Organic Frameworks: a Materials Platform for Structural and Functional Designs. Nat. Rev. Mater. 2016, 1, Article no. 16068, doi:10.1038/natrevmats.2016.68. (6) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O'Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 2005, 310, 1166–1170. (7) Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875–8883.


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