Crystal Engineering of Covalent Organic Frameworks Based on

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Crystal Engineering of Covalent Organic Frameworks based on Hydrazine and Hydroxy-1,3,5-Triformylbenzenes Renata Avena Maia, Felipe Lopes Oliveira, Michael Aleksander Nazarkovsky, and Pierre Mothé Esteves Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01110 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 10, 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

Crystal Engineering of Covalent Organic Frameworks based on HydraHydrazine and Hydroxyydroxy-1,3,51,3,5-Triformylbenzenes Renata A. Maia,† Felipe L. Oliveira,† Michael Nazarkovsky,‡ Pierre M. Esteves*,† †

Instituto de Química, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 149, CT, Bl. A-622, Cid. Universitária, Ilha do Fundão, Rio de Janeiro, RJ, 21941-909 (Brazil) ‡ Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente, 225, Gávea, Rio de Janeiro, RJ, 22435-900 (Brazil) Covalent Organic Frameworks, Crystallinity, Conformational Locks, Modulator.

ABSTRACT: Covalent organic frameworks (COFs) were prepared through imine condensation reaction of hydrazine hydrate with hydroxy-1,3,5-triformylbenzenes, containing a varying number of hydroxyl groups, affording the microporous materials called RIO-11, RIO-12 and RIO-13. The role of intramolecular hydrogen bonding formation (conformational locking effects) in the crystallinity of the resulting COFs was evaluated. The results indicate that the increase of the number of conformational locks increases the symmetry of moieties during nucleation and crystal growth, resulting in less defects in the product structure. The use of aniline as modulator, with in situ formation of an intermediate imine, was also evaluated and proved to be beneficial in the case where the number of conformational locks are insufficient to afford high crystallinity. The use of the modulator for RIO-11 resulted in greater crystallinity and a 5.3-fold increase of its pristine BET surface area. Narrower monomodal pore size distributions, with cylindrical pores, were shown to be responsible for the greater surface area in these cases.

INTRODUCTION Covalent Organic Frameworks (COFs)1 are materials with high surface area, which can be used, for example, in gas storage,2 heterogeneous catalysis3 and thermal insulation.4 This class of material, which is simultaneously fully organic, crystalline and nanoporous, can be synthetized from pre-designed building blocks with well-defined topology, which will bond in an organized extended structure. The final topology of the material depends on the shape of the building blocks. High crystallinity results in COFs with high surface area, usually a characteristic essential to its performance.5. Since this new class of materials is in its infancy, one drawback to overcome in their synthesis is related to the so called “crystallization problem”.6 IUPAC defines7 crystallization as the formation of a crystalline solid from a solution, while a precipitation is defined as the sedimentation of a solid material (a precipitate) from a liquid solution. Attempts of crystallization can yield solids with different fractions of amorphous and crystalline phases, which is the crystallization problem. COF properties, e.g. surface area, are directly related to its crystallinity degree, thus, overcoming this issue is of utmost importance to control its eventual applications. Methods aiming to obtain highly crystalline COFs take into account several strategies, such as, nucleation and/or crystal growth control, either by seeding,8 aging,9 use of a modulator10,11 or by Ostwald ripening.12 Some examples are chemical conversion of protected linkages,6 de novo synthesis13 and more recently, transimination.11,14 Furthermore, it is known that hydrogen bonding can induce molecular aggregation of the building blocks,15 and long range interaction may be a key factor for organized self-assembly in crystal engineering. Indeed, Cheng16 showed that increasing the sites of hydrogen bonding

enhances the crystallinity and porosity of imine-linked porphyrin COFs. Likewise, Li17 synthetized a hydrogen bond assisted azine-linked COF, which exhibited a strong orange-red luminescence and Chandra18 reported COF TpPa-(OH)2, which bears intramolecular hydrogen bonds and exhibited significant specific capacitance.

Figure 1. RIO-11, RIO-12 and RIO-13 synthesized from the condensation of hydrazine (1) and 1,3,5-triformylphenol (2), 1,3,5-triformylresorcinol (3) and 1,3,5-triformylphloroglucinol (4).

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Figure 2. Adsorption-desorption N2 isotherms at 77K for RIO-11, RIO-12 and RIO-13: experimental without modulator (a-c, blue), experimental with modulator (a-b, red) and theoretical (a-c, green). NLDFT pores size distributions: experimental without modulator (d-f, blue), theoretical unit cells of model compounds (d-f, structures). PXRD: experimental without modulator (g-i, blue), experimental with modulator (g-i, red) and theoretical (g-i, green). The asterisk mark corresponds to an unassigned peak. Despite the efforts in obtaining such crystalline materials, a systematic study is still missing on how structural factors and reaction conditions affect the resulting COF. Herein we report the synthesis of COFs (RIO-11, RIO-12 and RIO-13) based on hydrazine (1) and hydroxy-1,3,5-triformylbenzenes (2-4), which present a systematic increase of the hydroxyl groups within the aromatic moiety. Factors that affect their crystallinity, such as hydrogen bonding and use of modulator, were investigated. METHODS

COFs were synthesized based on conventional method of aldehyde-hydrazine condensation.19 Hydrazine hydrate (1) was reacted with 1,3,5-triformylphenol (2), 1,3,5triformylresorcinol (3) and 1,3,5-triformylphloroglucinol (4) to afford COFs called RIO-11, RIO-12 and RIO-13, respectively (Figure 1). The hydroxy-1,3,5-triformylbenzenes were obtained from formylation of phenol, resorcinol and phloroglucinol, according to procedure adapted from MacLachlan et al.20 The materials were named as RIO-1X, where the X represents the number of hydroxyl groups in the aromatic moiety. RIO-13 was previously reported as COF-JLU221 and ATFG-COF19 in the literature. RIO-11 and RIO-12, as far as we are aware, have not been

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Crystal Growth & Design reported. The solvothermal synthesis was performed under a mixture of the starting materials, which immediately precipitate when mixed, 1:3 mesitylene/1,4-dioxane and 6M aqueous acetic acid, which reacted for 72 h at 120°C under argon. The resulting solid was filtered off and sequentially washed with dioxane, DMF, methanol and THF, followed by soaking in THF for three days to unclutter the pores. The resulting material was isolated by filtration and the products were obtained as yellow and red powders (Figure S1). A different procedure was employed when the materials were obtained through transimination by the use of aniline as modulator. Several conditions were employed to optimize COF synthesis, resulting in the following procedure. Firstly 1,3,5triformylphenol (1), 1,3,5-triformylresorcinol (2) or 1,3,5triformylphloroglucinol (3) were individually reacted with aniline for 30 min to form the imine derivative in situ in a 1:3 mesitylene/1,4-dioxane mixture. After that, hydrazine and 6M aqueous acetic acid (catalyst) were added, and the mixture reacted for 72 h at 120°C. The reaction only afforded a precipitate after 24 h of reaction. The product was filtered off and treated in the same way as described above. Geometry optimization of RIO-11 RIO-12 and RIO-13 model structures was carried out using DFT calculations under periodic boundary conditions approach with Quantum Espresso code version 6.2.2,22,23 in order to better understand their properties. Exchange and correlation effects are treated with generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional and nuclei and core electrons are described by ultrasoft pseudopotentials.24 The van der Waals interactions were treated with D3 correction method of Grimme et al.25 The Kohn–Sham orbitals are expanded in a plane-wave basis set with a kinetic energy cutoff of 60 Ry and 480 Ry for charge density. The first Brillouin Zone integrations were performed 26 with 3x3x3 Γ-centered Monkhorst – Pack k-point sampling. Atomic positions and cell parameters were simultaneously fully optimized until the forces acting on the atoms were lower than 10-4 Ry/Bohr. Two layers of each COF were considered in these calculations in order to determine stacking. The optimized structures are shown in Figure 2d-f. Force field-based Monte Carlo algorithm, as implemented in RASPA27 package, was carried out in order to obtain the theoretical gas adsorption and textural properties of such materials. Lennard-Jones potential was used to treat the van der Waals interactions. All simulations were carried out with 10000 initiation cycles and 10000 running cycles in a 2x2x4 supercell. The parameters for adsorbed atoms (N and He) were taken from TraPPE force field.28 RESULTS AND DISCUSSION Fourier-transform infrared spectroscopy (FTIR) analysis of compounds RIO-11, RIO-12 and RIO-13 (Figure 3) showed bands associated to the C=N stretching mode at 1625, 1591 and 1589 cm-1, respectively, where band broadening is due to increase of intramolecular hydrogen bonding towards the more hydroxylated material.29 Furthermore, no residual starting material of 1,3,5-triformylphenol, 1,3,5-triformylresorcinol and 1,3,5-triformylphloroglucinol is detected, due to the absence of the corresponding aldehyde carbonyl peaks in the COFs FTIR. The materials were also characterized by solid state CP-MAS 13C NMR and TGA, for details see supporting information (Figure S2, S3, S4 and S7).

Figure 2a-c shows type I gas adsorption isotherms, which are typical of microporous materials. Simulated BET surface areas (SBET)30 from the theoretical GCMC calculations of nitrogen adsorption in the DFT optimized model structures of RIO-11, RIO-12 and RIO-13 (Figure 2a-c, green) are estimated as 1683 m2/g, 1471 m2/g and 1238 m2/g, respectively, this virtually being the upper limit in terms of microporous surface area that these materials can achieve. Experimental surface area determination by nitrogen adsorption (Figure 2a-c, blue) afforded SBET of 242 m2/g (RIO-11, without modulator), 1008 m2/g (RIO12, without modulator) and 1205 m2/g (RIO-13, without modulator).

Figure 3. FTIR of hydroxy-1,3,5-triformylbenzenes (2-4) starting materials (left) and COFs RIO-11, RIO-12 and RIO-13 (right). Despite their structural similarity, SBET of RIO-12 and RIO-13 are respectively 4.2 and 5 times higher than RIO-11. RIO-13 seems to have reached its maximum surface area, since its experimental SBET reached 97% (1205 m2/g) of its theoretical value (1238 m2/g). Conversely, RIO-11 is still far from its full potential in terms of surface area (reached 14% of its theoretical value), indicating that this material could be further improved. This low surface area for RIO-11 is possibly related to fast and disordered precipitation in the early stages of its synthesis, affording a high fraction of amorphous phase in the solid. Experimental pore size distributions (Figure 2d-f), according to Non-local density functional theory (NLDFT), indicate pores of 13.3 Å for RIO-11, RIO-12 and RIO-13. Meanwhile, theoretical pore size of model compounds are 11.98 Å (RIO-11), 12.05 Å (RIO-12) and 12.11 Å (RIO-13), which are in good agreement with NLDFT data. Figure 2g-i shows powder X-ray diffraction (PDXR) of RIO-11, RIO-12 and RIO-13. The main diffraction peaks are located around 6° (hkl 100), 11° (hkl 210), and 27° (hkl 002), the latter attributed to layers stacking.31 Experimental interplanar distances referring to the layers (hkl 002), according to Bragg’s law,32 are 3.30 Å, 3.27 Å and 3.25 Å for RIO-11, RIO-12 and RIO13 respectively (Table S3). Those values are compatible with similar materials, such as graphite, which has an interlayer distance of 3.4 Å.33,34 Interplanar distances obtained from DFT calculations are 3.71 Å, 3.66 Å and 3.63 Å for RIO-11, RIO-12 and RIO-13 respectively, being in good agreement with experimental results, considering the difficulty associated with the DFT method for treating intermolecular interactions.25 RIO-11 synthesized without the use of a modulator presented significant low crystallinity, as shown by its low SBET and broad-

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Crystal Growth & Design er peaks in PXRD, suggesting a lack of organization in the structure. Indeed, the mean crystallite size for RIO-11, RIO-12 and RIO-13 are (Scherrer method,35 table S1) 48 Å, 56 Å, 54 Å, respectively, thus affording the following mean crystallite size hierarchy: RIO-11< RIO-13 ≈ RIO-12. The shape of all three isotherms (Figure 2a-c, blue) indicates a complex textural structure composed of differently sized and shaped pores (Table S5 and Figure S6). Analyses of the isotherms by the modified Nguyen-Do method36 allowed us to better evaluate the contribution of micro- meso- and macropores, altogether with their shapes into BET surface area (Table S4, Figure S5) and total pore volume. Micropores fraction increases as RIO11 < RIO12 < RIO13 and the fraction of mesopores monotonously decreases (Figure 4a-c).

(49.7%) and voids (39.7%). In the same way mesoporosity is also dictated by cylindrical pores (50.9%) and voids (43.7%). It is noteworthy to notice that RIO-13 do not present a relevant contribution of slit-shaped pores over the whole size scope. As for interparticle voids, they do not have any important impact upon the macroporosity in all the materials, giving only a small contribution in RIO-11. However, they play a significant role in micro- and the mesoporosity for RIO-13. Macroporosity is negligible for total pore volume of RIO-11, and RIO-13, representing 3.4 and 2.6%, respectively. The contribution of slits decreases in the series as RIO-11 > RIO-12 > RIO-13. Thus, volume porosity is due mainly to slit type mesopores only in RIO-11, possibly due to the border of small single crystals or defects in the stacking (Figure 4d). Differently, volume porosity is dictated mainly by cylindrical micro- and mesopores in RIO-12 and RIO-13, which is a consequence of more organized structures. All these results are summarized in Figure 5.

100

Voids Cylindrical Slits

84.8

Figure 4. Pore size and shape (slits, cylinders, voids) distributions PSDV for (a) RIO-11, (b) RIO-12 and (c) RIO-13. (d) Pore shapes for 2D materials. Distribution by pore volume (PSDV) changes drastically from RIO-11 to RIO-12, meanwhile RIO-13 PSDV features fewer dispersion contribution within the mesopores region. All profiles are defined by polymodality bringing out a common peak with a maximum at pore sizes of ~ 10 Å, compatible with DFT calculation. In RIO-11 (Figure 4a), micropores have participation values as high as 11.8% of the total pore volume (Vp) and show important contribution of slit typed pores (87.7%). The mesoporous component contributes with 84.8% to Vp, and the textural pattern is also dictated by slits (59.8%). As the whole, slits (60.9%) are the main donors of total porosity in RIO-11. Meanwhile, in RIO-12 (Figure 4b) micropores have participation of 38.8% of Vp, but the pore shape distribution undergoes a drastic change: cylindrical pores become the main factor in microporosity (51.4%). Mesoporosity contribution to Vp is 49.8%, consisting mainly of cylindrical pores (63.1%). Macropores represent 11.4% of Vp, dominated by slit typed pores (94.5%). The sample is an illustrative example of the cylindrical shape occupation on the porosity. It is noteworthy that slit-like pores are presented uniformly throughout the entire size scale – 14.1, 10.5, 10.8 % of Vp for micro-, meso- and macropores, respectively. Finally, RIO-13 (Figure 4d) is mainly composed of micropores and mesopores - 45.3% and 52.1%, respectively, with respect to Vp. Microporosity composition is dominated by cylindrical pores

Percentage of Vp (%)

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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80 60

45.3

38.8

40 20 0

52.1

49.8

11.8

11.4 3.4

2.6

Micro Meso Macro Micro Meso Macro Micro Meso Macro

RIO-11

RIO-12

RIO-13

Figure 5. Percentage of total pore volume per pore type for RIO-11, RIO-12 and RIO-13. The role of hydrogen bonding in crystallinity Data analyses of COFs synthesized without the use of a modulator showed a significant difference in crystallinity among the materials. Remarkably for RIO-11, which has the lower surface area and crystallinity, when compared to RIO-12 and RIO-13 (Figure 2a-c, blue; g-i, blue). Although there is an increase of a single hydroxyl group from RIO-11 to RIO-12, and from RIO-12 to RIO-13, the difference in the properties (BET surface area and crystallinity) when comparing RIO-11 and RIO-12 is more prominent than when comparing RIO-12 and RIO-13. Thus, the materials properties do not vary linearly with the increase of the number of hydroxyl groups in the aromatic moieties. With all the compounds having hydroxyl and imine groups in their vicinity, it is reasonable to consider that hydrogen bonding might play a role in structure conformation. Non-covalent interactions usually play a key role in structure organization, with hydrogen bonding having a locking effect in previously reported COFs. 16 RIO-11, RIO-12 and RIO-13 form an extended structure by 3 imine linkages throughout the triangular aromatic moieties. It has been proposed37 that higher symmetry in the building blocks may result in higher crystallinity. In RIO-11, only one imine group is locked in a conformation due to hydrogen bond-

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Crystal Growth & Design ing, while two other imine linkages are free to assume four additional conformations (Figure 6a). This disturbs the symmetry of the material, which eventually compromises its crystallinity. The four possible isomers for RIO-11 could be randomly formed in the early nucleation stages of the resulting COF, increasing the amorphous fraction of this material. In RIO-12, two imine groups are conformationally locked by hydrogen bonding, leaving only one conformationally free imine group. Thus, there would be two possible isomers in the resulting COF (Figure 6b), which would justify the higher symmetry of RIO-12 when compared to RIO-11, but could also result in some degree of amorphous phase. On the other hand, all three imine groups are conformationally locked due to hydrogen bonding in RIO-13 (Figure 6c). This results in a single conformation of higher symmetry, affording the most crystalline COF.

nucleation and, consequently, more efficient crystallization. Based on that, we decided to use aniline as modulator in order to decrease the nucleation rate and favor crystal growth, which in theory would prevent fast precipitation of amorphous phase.

Figure 6. Conformational locks for (a) RIO-11, (b) RIO-12 and (c) RIO-13, leading to several possible conformers involved in COF formation.

Scheme 1. Equilibria involved in crystallization of COFs when modulator is used.

The most remarkable consequence of these conformational locks is presented in the BET surface area. RIO-11, RIO-12 and RIO-13 reached 14, 68 and 97%, respectively, of its theoretical surface area (SBET). Since the main difference among these materials is the number of hydroxyl groups in the aromatic moiety, it is reasonable to think that the disparity in SBET is mainly attributed to the increase of crystallinity due to hydrogen bonding effects, demonstrated experimentally by broadening of C=N peaks in FTIR. Influence of aniline as modulator It is known that amorphous COFs, especially from imine linkages, arise from fast disordered nucleation and subsequent precipitation of amorphous imine aggregates.38,39 Thus, affording a kinetic product that cannot anneal in a timely manner to afford a crystalline material, mainly due to strong covalent bonding and π-stacking, which prevents its redissolution.40 The amorphous phase may be formed from a constellation of random combinations of possible isomers, with slightly different shapes, leading to defects.40 It is well known that the crystallization outcome can be influenced by additives, as exemplified by the case of biomass-mediated zeolite synthesis.41

The addition of aniline in the reaction would lead to the formation of an imine as intermediate, which would undergo by transimination with hydrazine to afford the corresponding COF (Scheme 1). Since these steps would not involve the sudden formation of an amorphous aggregate, this transimination would facilitate thermodynamically the self-healing and annealing of the material, resulting in a COF with fewer defects than the one from the previous approach (without modulator). Also, possible oligomeric phases formed would suffer Ostwald ripening in favor of larger and more stable COF crystals. This methodology would lead to a material with greater crystallinity and, consequently, higher surface area.

Recently, transimination was reported by Vitaku and Dichtel14 by slow release of benzidine from its imine form with benzophenone. Although this method provided materials with high crystallinity and high surface area, the imine formation involves the use of 1,4-diazobicyclo[2.2.2]octane (DABCO), Titanium(IV) chloride, chlorobenzene and under inert atmosphere. Meanwhile our approach provides the transimination building block 10 in a simpler manner: condensation of 1,3,5-triformylphenol (2) with 7.4 eq. of aniline (9) in methanol under reflux for 24 h (Scheme 2). When performing this methodology in two steps (Scheme 2a), using 10 as a building block for synthesis of RIO11, the latter was obtained as a material with a higher degree of crystallinity and BET surface area of 1290 m2/g (77% of the theoretical SBET). Another approach was the in situ transimination (Scheme 2b). It consists in reacting 1,3,5-triformyphenol and aniline for 30 min in mesitylene/1,4-dioxane 1:3. The formation of 10 in situ is visually detected, since the yellow suspension of 1,3,5-triformylphenol readily becomes a clear red solution right after aniline is added. After that, hydrazine and catalytic acetic acid are added, thus affording COF RIO-11 after 72 h at 120°C.

Thus, one could consider that controlling the equilibrium within the reaction vessel could increase the crystallinity of materials that are not favored by chemical structure per se, such as RIO-11. Competition of one of the building blocks with a small monopodal molecule is known as a modulator approach.10 This means that since nucleation is favored by similar rates of product formation and its reagent regeneration, reaction equilibrium control would lead to a path of slow and controlled

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Crystal Growth & Design

Scheme 2. Two-step (a) and in situ (b) transimination methodologies for RIO-11 formation. The quantity of aniline equivalent for RIO-11 in situ transimination was investigated (Table 1). The best result was achieved with a 1:5 (Entry 4) NH2:CHO ratio (2.0 eq.), affording the higher BET surface area (1050 m2/g) for the in situ synthesis. Additionally, 1:3 (Entry 3) and 1:1 (Entry 5) NH2:CHO ratio provide 940 m2/g and 851 m2/g, respectively. Thus, 2.0 eq. of aniline result in the most favorable condition. NH2:CHO ratios of 1:15 (Entry 2) and 1:0.4 (Entry 6) and 1:30 (Entry 1) afforded BET surface areas of 571 m2/g, 621 m2/g and 965 m2/g, respectively. Thereby, very small or very large NH2:CHO ratios are not favorable for formation of more crystalline RIO-11 by in situ transimination. Although Entry 4 affords 81% of surface area obtained in Entry 7, the in situ methodology is a more straightforward and convenient synthesis, given that the two-step synthesis consumes more reaction time and chemicals. Table 1. Optimization conditions for reverse transimination of RIO-11.

Entry

Aniline eq.

NH2:CHO ratio

SBET (m2/g)

1

0.1

1:30

965

2

0.2

1:15

571

3

1.0

1:3

940

4

2.0

1:1.5

1050

5

3

1:1

851

6

7.5

1:0.4

621

-

-

1290

7b b Result

Differential PSDV (a.u.)

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

3.2x10

-2

2.4x10

-2

1.6x10

-2

8.0x10

-3

-2

TOTAL Cylindrical Voids

2.5x10

pores, therefore, indicating that a more organized structure arises from both transimination, two-step and in situ, methodologies. In fact, when analyzing the PXRD pattern of RIO-11 (Table S2), it is possible to observe a narrower diffraction profile that reflects the growth of the main crystallite size from 48 Å (without modulator) to 64 Å (two-step transimination) and 60 Å (in situ transimination). Relying on this promising result, we proposed a one-pot reaction, in which the imine is initially formed, followed by COF formation. Successfully, RIO-11 was prepared in situ by transimination of 1,3,5-triphenyliminephenol with surface area of 1050 m2/g. These conditions were used to prepare RIO-12 and RIO13 by transimination to observe if crystallinity and surface area could be improved. When in situ transimination conditions were used for RIO-12, a BET area of 887 m2/g was obtained. This is a lower area when compared with RIO-12 without use of modulator (1008 m2/g), however this lies within the acceptable range of variation in the synthesis of this type of material. On the other hand, the use of modulator in RIO-13 leads to a material with 338 m2/g of area, which corresponds to a decrease of the surface area in relation to the synthesis without use of modulator (1205 m2/g). One possible explanation is the fact that RIO13 forms a keto-imine adduct, as previously reported.19 Such adducts are resistant to nucleophilic attack, which make the use of modulator deleterious to the process, since it does not allow reaction reversibility, which is essential for obtaining high crystalline materials. In general, the majority of COF building blocks do not present intramolecular hydrogen bonding. Thus, this methodology could benefit new and previously reported COFs in reaching their full potential, making the in situ transimination a promising methodology regarding COF synthesis. CONCLUSIONS In this study we have shown that intramolecular hydrogen bonding formation in COFs leads to conformational locking effects, which influence the crystallinity of the resulting materials. These conformational locks increase the symmetry of one of the building blocks (1,3,5-hydroxy-triformylbenzenes). In the absence of these conformational locks, different conformations available during nucleation lead to disordered crystal growth, resulting in defects and a higher fraction of amorphous phase. The annealing never occurs on a reasonable time scale, leading to the formation of a (kinetic) solid product of low crystallinity and smaller surface area and multimodal pore size distribution. Slow conversion of these defects, through equilibria into a more stable and crystalline phase, increase the surface area of the resulting material.

for two-step (ex situ) transimination.

RIO-11: two step transimination

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RIO-11: in situ transimination

-2

2.0x10

-2

1.5x10

-2

1.0x10

0.0

-3

5.0x10

10

100

Pore Size (Å)

1000

0.0

10

100

1000

Pore Size (Å)

Figure 7. RIO-11 PSDV for (left) two-step modulation and (right) in situ modulation. Comparing the results of PSDv of RIO-11 by transimination with the pristine one, there is a remarkable change in the porosity profile. Figure 4a shows that in the conventional synthesis RIO-11, it presents a prominent contribution of slit type mesopores to the total pore volume, which is a possible consequence of irregular stacking of its layers. In Figure 7, it is possible to notice that the total pore volume contribution is dominated by cylindrical micropores and there is no contribution of slit type

When COFs are not benefited from conformational locks, another strategy can be used. The use of aniline as modulator, with in situ formation of an intermediate imine, leads to a different and slower nucleation mechanism, which happens through transimination with hydrazine. This results in COFs with higher crystallinities and surface areas, achieving close to their theoretical limit. This is also reflected in narrower monomodal pore size distributions, with cylindrical micropores being the major responsible for the surface area. The single-shaped pore structure suggests a more crystalline and uniform packing, since apparently one particular shape and size of crystallite is propagated throughout the structure. This is particularly useful for the synthesis of COFs using less symmetrical building blocks, in which conformational locks are not present, such as in the case

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Crystal Growth & Design of RIO-11. In this case, a 5.3-fold increase of BET surface area and a higher degree of crystallinity were observed.

ASSOCIATED CONTENT The Supporting Information is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *[email protected]

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Funding Sources This work was financially supported by CAPES, CNPq and FAPERJ.

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ACKNOWLEDGMENT We acknowledge Prof. A. Faro (IQ/UFRJ) for some isotherm and TGA determinations and Prof. Simon J. Garden for helpful discussion.

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Crystal Growth & Design

For Table of Contents Use Only Crystal Engineering of Covalent Organic Frameworks based on Hydrazine and Hydroxy-1,3,5Triformylbenzenes Renata A. Maia, Felipe L. Oliveira, Michael Nazarkovsky, Pierre M. Esteves

CONFORMATIONAL LOCKS

LESS ORDERED

Fast Nucleation

MORE ORDERED

ANILINE AS MODULATOR

Slow Nucleation

Synopsis COFs based on hydroxy-1,3,5-triformylbenzenes and hydrazine with varying number of hydroxyl groups were prepared. Their crystallinities and surface areas are strongly dependent on conformational locks due to intramolecular hydrogen bonding. The use of aniline as modulator improved the crystallinity of the system that bears the fewer conformational locks.

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