Suvendu Karak, Sushil Kumar, Pradip Pachfule and Rahul Banerjee

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Porosity Prediction through Hydrogen Bonding in Covalent Organic Frameworks Suvendu Karak, Sushil Kumar, Pradip Pachfule, and Rahul Banerjee J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 30 Mar 2018 Downloaded from http://pubs.acs.org on March 30, 2018

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Journal of the American Chemical Society

Suvendu Karak,1,2‡ Sushil Kumar,2‡ Pradip Pachfule2 and Rahul Banerjee2,3,* 1

Academy of Scientific and Innovative Research, New Delhi, India. Physical /Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India. 3 Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, Mohanpur 741246, India. *Email: [email protected]; Tel: +91-20-2590-2535. 2

ABSTRACT: Easy and bulk-scale syntheses of two-dimensional (2D) covalent organic frameworks (COFs) represent an enduring challenge in material science. Concomitantly, the most critical aspect is to precisely control the porosity and crystallinity of these robust structures. Disparate complementary approaches like solvothermal synthesis have emerged recently, and are fuelled in part by the usage of different modulators and acids that have enriched the COF library. Yet, the fundamental understanding of the integral processes of 2D COF assembly, including their growth from nucleating sites and the origin of periodicity, are intriguing chemical questions that need to be answered. To address these cardinal questions, a green and easy-to-perform approach of COF formation has been delineated involving acid-diamine salt precursors. The role of hydrogen bonding [davg (Namine–H…Oacid); davg signifies the average Namine–H…Oacid distances i.e. the average distance from the ‗H‘-atom of the amine to the ‗O‘-atom of the acid] present in the acid-diamine salts in improving the COFs crystallinity and porosity has further been decoded by thorough crystallographic analyses of the salt molecules. What is particularly noteworthy is that we have established the hydrogen bonding distances davg (Namine–H…Oacid) in the acid-diamine salts that are pivotal in maintaining the reversibility of the reaction, which mainly facilitates highly crystalline and porous COFs formation. Moreover, this reactant-structure to the product-quality relationship has further been utilized for the synthesis of highly crystalline and porous COFs that are unattainable by other synthetic means.

INTRODUCTION The challenge in designing a highly crystalline and uniformly porous polymeric network is to find out a way to gain atomic-level control over its structure and composition. 1 Two dimensional covalent organic frameworks (COFs) are one such type of polymeric porous material having predesigned structures with an opportunity for simultaneous tuning of the functionalities of the nano-channels.2 The dynamic covalent connections between the molecular building blocks among these frameworks proceed through iterations of trial and error during synthesis to achieve crystallinity as well as porosity. 3 However, even a decade after their invention, the pursuit of methods for bulk-scale synthesis of highly crystalline porous COFs remains in its infancy. This is mainly because of the lack of understanding of their growth from the nucleating sites and the origin of their periodicity.4 Although the solvothermal synthetic route5 has produced a library of COFs, several parameters such as appropriate solvent combination, longer reaction times, sophisticated techniques and low yields, limit their industrial prospects. Thus, the foremost challenge is to judiciously choose the building blocks and growth conditions, so that the directionality, rigidity and reversibility can interact harmoniously for the simple and easy synthesis of highly crystalline and porous COFs. Tailoring a COF structure by modulating the supramolecular interactions has been reinvigorated recently.6 Even, the modest alteration of a constituent can lead to a significant change in the architecture.7 In recent times, we have shown that the use of p-toluenesulfonic acid (PTSA) could result in highly crystalline and ultraporous COFs.8 The organic acid

offers a suitable ―scaffolding‖ for the growth of the porous 2D structures. This green and easy-to-perform approach led to highly ordered networks that enabled the fabrication of crystalline COFs into different shapes and sculptures. 1f,8a,8b However, a tour de force experimental effort is necessary to expand the choice of reliable organic acids required for broader diversification of this process. Subsequently, an utmost effort should be provided to understand the rationale between the structures of the ‗reactants‘ i.e. the acid-diamine salts, and the properties of the ‗products‘, i.e. the COFs. 9 The molecularlevel insights of the reactant crystal structures can provide an idea of the product (COF) structures in a manner that is complementary to experimental techniques. Keeping this in perspective, herein, we have made an attempt to bridge this gap between the reactant‘s structure and the product‘s properties (crystallinity and porosity), within the COF family using the advantage of X-ray crystallography. In this regard, directional hydrogen bonding10 appears to be the missing link between these two aforementioned aspects. The crucial role of hydrogen bonding has thus been decoded by scrutinizing forty-nine (49) crystal structures of various acid-diamine salts. The analysis delineates the suitable hydrogen bonding distances [davg (Namine–H…Oacid); davg signifies the average Namine–H…Oacid distances i.e. the average distance from the ‗H‘-atom of the amine to the ‗O‘-atom of the acid] which result in highly crystalline and porous COFs. A set of different Bronsted acids with a range of acid dissociation constants (Ka) were chosen for the synthesis of acid-diamine salts that manifest crystalline and porous COFs. Different acids like p-toluenesulfonic acid (PTSA), benzene sulfonic acid (BSA), phenol sulfonic acid (PSA), 4-nitro benzene sulfonic acid (NBSA), 2-amino

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Scheme 1. Schematic representation of COF synthesis using a series of acid-diamine salts with various combinations of acids and diamines. Different diamines have been reacted with acids which resulted in corresponding salts. The reaction of acid-diamine salts with an aldehyde, Tp, afforded 49 COFs.

benzene sulfonic acid (ABSA), phosphoric acid (H3PO4), trifluoroacetic acid (TFA), hydrochloric acid (HCl), sulphuric acid (H2SO4) and oxalic acid (Oxa) have been firstly reacted with diamines, and subsequently the resulting salts are crystallized. These salts have further been used as primary reactants of COF synthesis. The crystal structures of the acid-diamine salts and subsequently, the properties of the COFs have been further correlated via X-ray diffraction and N2 adsorption analysis. The COFs showed the highest surface area and best crystallinity when they were synthesized from acid-diamine salts having a specific hydrogen bonding distance [davg (Namine– H…Oacid)].11 While strongly hydrogen bonded acid-diamine salts have a propensity to hinder the intermediate imine bond formation, weakly hydrogen bonded (Namine–H…Oacid) diamine-acid salts would lead to less crystalline COFs owing to a faster reaction. Thus, an optimum hydrogen bonding interaction is necessary to ensure that the COFs are both highly crystalline as well as porous. These results lead us to propose, for the first time, a remarkable analogy between the crystal structure of the ‗reactants‘ and the properties, of the ‗products‘. This analogy has been further validated by synthesizing sixteen (16) new COFs (based on four different amine molecules), which are unattainable (crystallinity and porosity-wise) by other synthetic protocols (Scheme 1 and Scheme S1, SI). Identification of the high surface area and best crystallinity among the COFs synthesized from the acid-diamine salts having similar characteristic hydrogen bonding distances, davg (Namine–H…Oacid) confirms our new hypothesis on porosity and crystallinity prediction through hydrogen bonding in COFs. RESULTS AND DISCUSSION The p-toluenesulfonic acid has been explored recently as a catalyst to produce highly crystalline and porous COFs. 8 How-

ever, the choice of the particular acid was not decisive enough itself to understand and generalize the COF crystallization. Thus, we have chosen a series of acids with a range of acid dissociation constants (Ka) (Table S12, SI). Firstly, the acids were converted into acid-diamine salts after mixing with different diamines (Figure S1, Table S1, SI). All the assynthesized acid-diamine salts have been characterized by the single crystal X-ray diffraction technique (Table S2, SI). These salts possess different reactivity towards the aldehyde (Tp) depending upon their Namine–H…Oacid hydrogen bonding strength. Further, the transformation of salts into the respective COFs has been carried out by mixing with Tp with the addition of few drops of water, followed by the thermal treatment at 90 °C for 12 h (Figure S2, SI). The crystallinity of the COFs was assessed by the PXRD technique (Figures 1a and 1b, Section S3, SI). TpPa-1, TpPa-NO2, TpBpy, TpBD-(OMe)2, TpBD-Me2, and TpBD-(OH)2 synthesized from PTSAdiamine salts show sharp diffraction peaks corresponding to the planes. The experimental PXRD patterns are in good agreement with simulated PXRD patterns (Figures S6 and S7, SI).8b Interestingly, when these COFs are synthesized via the similar solid state mixing of Tp with acid-diamine salts of respective diamines with BSA, PSA, NBSA, ABSA, H3PO4, TFA, HCl, H2SO4 and Oxa, they mostly show poor crystallinity depicted from their comparatively low intense PXRD patterns and domain size (Tables S5 and S6, SI). This suggests that the resultant COF matrix has poor long-range periodicity. It is noteworthy that TpPa-2 exhibits highest surface area (969 m2g-1, vide infra), when it has been synthesized from phenol sulfonic acid-Pa-2 salt (PSA-Pa-2) (Figure 1b). In contrary to TpPa-1, TpPa-NO2, TpBpy, TpBD-(OMe)2, TpBD-Me2, and TpBD-(OH)2 COFs, the TpPa-2 framework


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Journal of the American Chemical Society exhibits lower surface area (430 m2g-1, vide infra) and modest intensity PXRD patterns when it was synthesized from ptoluene sulfonic acid-Pa-2 salt (PTSA-Pa-2). These results suggest that the crystallinity and the porosity of the COFs may alter with the choice of the acids considered during the synthesis of the salts. The permanent porosities of the as-synthesized COFs have been analyzed using N2 adsorption isotherms. Observation of moderate to high Brunauer−Emmett−Teller surface area (SBET) is consistent with their crystallinity. Assynthesized COFs exhibit a range of surface areas varying from 14 to 832 (TpPa-1), 192 to 969 (TpPa-2), 32 to 850 (TpPa-NO2), 136 to 2336 (TpBpy), 99 to 2646 (TpBD-Me2), 34 to 1365 [TpBD-(OMe)2] and 167 to 496 [TpBD-(OH)2], depending upon the choice of the acid-diamine salts during the COF synthesis (Figure 2, Figure S3, Section S5, SI). The comparison of the pore size distributions of the resulting COFs also indicates the alteration of the pore size distribution of the COFs with respect to the variation of the acid-diamine salts (Section S6, SI). These results indicate that by altering the selection of acid-diamine salts during a particular COF synthesis, the long-range order could be tailored, which further helps in ordering the nano-pores, and subsequently, the surface area. This observation prompted us to undertake a detailed crystallographic examination of the acid-diamine salts and decode the role of the hydrogen bonding, if any, in salt to porous crystalline COF synthesis. The crystal structures of the salts show that each amine functionality (–NH3+) of the diamine molecule is surrounded by three acid molecules, which provide a chain like onedimensional lamellar structure (Figure 3b).12 Notably, this protonation-deprotonation is a reversible reaction.13 Depending upon the nature of the acids, the hydrogen bonding (Namine– H…Oacid) strength varies,14 which has been reflected in the average hydrogen bonding distances (davg) in the acid-diamine salts (Section S7, SI). Further, the addition of aldehyde (Tp) leads to the formation of imine bonds via the Schiff-base reaction, which subsequently undergoes keto-enol tautomerism and leads to the chemically stable COF structure. The first step of this COF formation reaction, i.e. s-cis-imination is reversible, and as per dynamic covalent chemistry, the higher degree of reversibility provides an opportunity for error-correction and reconstruction of the framework structure that leads to the high crystallinity. Thus, starting substrates can impart longrange order to the framework by dragging the first step of the COF formation reaction (s-cis-imine bond formation) towards the backward direction. Herein, the reversible acid-diamine salt formation reaction competes with the s-cis-imination reaction which helps in sustaining the reversibility of the entire COF formation reaction, by slowing down the overall reaction, and thus enhances the long-range ordering and the crystallinity of the framework (Figure 3a, Scheme S2, SI). However, strong hydrogen bonded acid-diamine salts could hinder this imine bond formation because of their lower reactivity with the aldehyde, whereas, weak hydrogen bonding (Namine–H…Oacid) within the diamine-acid salts would lead to a faster reaction which will result in less ordered structures. These results indicate that intermolecular (Namine–H…Oacid) hydrogen bonding of a particular distance (thereby of a specific strength) has a pivotal role in controlling the reactivity of these acid-diamine salts. Although hydrogen bonding controls the reactivity of the salts, the pKa of the acid or pKb of the base may contribute minutely to the quality of the product by affecting the reversibility of the imination reaction (Section S8). Thus, to

Figure 1. Comparison of structures, PXRD patterns and surface areas for (a) TpPa-NO2 and (b) TpPa-2 COFs synthesized from different salts. The hydrogen bonding distances and surface areas mentioned herein are in Å and m2g-1 unit, respectively. The CCDC number of each of the salts has been mentioned in brackets; PTSA-PaNO2 (1498465), PSA-PaNO2 (1583915), BSA-PaNO2 (1583918), ABSA-Pa-NO2 (1583919), NBSA-Pa-NO2 (1583920), PTSA-Pa-2 (1498467), PSA-Pa-2 (1584062), BSAPa-2 (1584063) and NBSA-Pa-2 (1584064). The two COFs have been represented herein as an example to showcase the structureproperty correlation.

synthesize a highly crystalline and porous COF from a particular acid-diamine salt, an appropriate hydrogen bonding (Namine–H…Oacid) is essential. To correlate the structure of the reactant, (acid-diamine salts) and the property (crystallinity and porosity) of the product (COFs), thirty-three (33) low to highly crystalline COFs



Figure 2. Forty-nine (49) COFs synthesized via salt mediated crystallization. For the clarity of the figure forty-five COFs have been represented herein. Firstly, amines and acids have been crystallized and the crystals have been further reacted with the aldehyde (Tp). The highest surface area value of each of the COF has been highlighted in the red colored font (Bold font as well). The cross sign signifies the unavailability of the crystal structures of the acid-diamine salts. The amines are drawn in black color. Common aldehyde Tp and acids are marked in red and green color respectively.

have been synthesized from different acid-diamine salts.15 In this regard, davg of the acid-diamine salts have been calculated. The plots of % of the theoretical SBET vs. davg reveal the appropriate hydrogen bonding distance range for tailoring the COF porosity as well as crystallinity (Figures 3c and 3d). It has been observed that the acid-diamine salts having davg distances of 2.06—2.19 Å produce crystalline COFs that achieve the highest percentage of theoretical value (Figure 3c and Tables S8 and S11, SI). In a distinct example, we have observed that the as-synthesized COF TpPa-2 shows the surface area to be as high as 969 m2g-1, when it has been synthesized from PSAPa-2. On the other hand, the same COF shows the surface area of 407 and 430 m2g-1, when synthesized from BSA-Pa-2 and PTSA-Pa-2 salts, respectively (Figure 1b). The suitable davg of 2.119 Å for PSA-Pa-2 justifies this observation as compared to that of davg of PTSA-Pa-2 (1.875 Å), BSA-Pa-2 (1.869 Å) and NBSA-Pa-2 (1.974 Å) salts. The hydrogen bonding interaction (shorter davg distances) is too strong to allow a reaction between the amine and the aldehyde, which in turn affects the long-range order of the framework and results in COF samples with poor crystallinity. Similarly, when the diamine Pa-NO2 has been crystallized with BSA, PSA, NBSA and PTSA, it shows davg values of 1.984, 2.232, 2.441 and 2.055 Å, respec-

tively. Among these, the davg distance of 2.055 Å in the PTSAPa-NO2 salt results in the highest surface area (850 m2g-1) and better crystallinity as compared to PSA-Pa-NO2 (2.232 Å; 144 m2g-1), BSA-Pa-NO2 (1.984 Å; 700 m2g-1), NBSA-Pa-NO2 (2.441 Å; 32 m2g-1) salts when they have been transformed to TpPa-NO2 COF (Figure 1a). Similarly, the PTSA-BD-(OMe)2 (davg = 2.118 Å) salt produces highly crystalline TpBD(OMe)2 COF with surface area 1365 m2g-1 whereas the BSABD(OMe)2 (davg = 1.937 Å) salt results in the less crystalline and porous TpBD-(OMe)2 COF (SBET = 210 m2g-1). These results illustrate that the superiority of organic acids depends on their specific hydrogen bonding strength with the diamines, which produces high surface area and crystallinity among the resulting COFs. From the relations between the davg (Namine–H…Oacid) distances in respective acid-diamine salts and the COF porosity, it is clear that the average hydrogen bonding distance (2.06— 2.19 Å) within the acid-diamine salts regulates the subsequent reaction with the aldehyde (Tp) and has a pivotal role in determining the COF crystallinity and porosity. To prove the hypothesis, we have further synthesized 16 new acid-diamine salts, combining Pa-Cl2, Pa-Cl-NO2, Pym and BD-Me4


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Journal of the American Chemical Society

Figure 3. (a) Schematic representation of imparting reversibility in the COF reaction by the introduction of salt-amine competitive reversible reaction. (b) Model representation of the orientation of acids with respect to diamine. Only sulfonic acids have been shown for simplicity. (c) and (d) Scattered plot of hydrogen bonding [davg (Namine–H…Oacid)] distance in the salt structure vs. % of theoretical surface area of the corresponding COF. Later highlights the suitable range of average hydrogen bonding that produces highest % of theoretical surface area of each type of COF. The surface area of each category of COF has been represented with a particular color code as mentioned in Figure 2.

diamines with acids such as BSA, PSA, NBSA, HCl and PTSA. The hydrogen bonding distances in the salt structures have been further measured and their effect on COF properties has been analyzed. The COFs synthesized using these aciddiamine salts showed that the resultant frameworks display a similar high degree of crystallinity and porosity when they have been synthesized from the salts having davg distances of 2.06—2.19 Å (Figure 4, Section S3, S5 and S6, SI). The PaCl-NO2 based salts present different davg of 1.882 (BSA-Pa-ClNO2), 1.914 (PSA-Pa-Cl-NO2) and 2.155 Å (PTSA-Pa-ClNO2). When these salts have further been transformed into TpPa-Cl-NO2 COFs, they exhibit variable surface area of 452, 329 and 592 m2g-1, respectively (Figure 4d). The highest surface area of 592 m2g-1 can be attributed to the specific hydrogen bonding distance of 2.155 Å which is within the aforementioned hydrogen bonding range of 2.06 to 2.19 Å. Similarly, BSA-Pa-Cl2, PSA-Pa-Cl2, and PTSA-Pa-Cl2 follow the same trend of low to the high surface area when they have been converted into corresponding TpPa-Cl2 COFs (Table S4,

SI). The COF shows the highest surface area of 989 m2g-1 when it has been synthesized from the salt BSA-Pa-Cl2 (davg = 2.064 Å) whereas the PSA-Pa-Cl2 (davg = 2.110 Å) and PTSAPa-Cl2 (davg = 1.848 Å) salt based COFs result in 632 m2g-1 and 359 m2g-1 of surface area respectively. Again, among the NBSA-Pym (davg = 2.009 Å), BSA-Pym (davg = 2.363 Å) and PTSA-Pym (davg = 2.139 Å,) based COFs, TpPym shows the highest surface area of 408 m2g-1 when it has been synthesized from PTSA-Pym salt (Figure S19 and Table S9, SI). The proposed hypothesis of structure-property correlation has further been validated in the case of TpBD-Me4 COF. Among BSABD-Me4, PTSA-BD-Me4, PSA-BD-Me4, NBSA-BD-Me4 and HCl-BD-Me4 salts, PTSA-BD-Me4 produces the TpBD-Me4 COF having the highest surface area of 559 m2g-1 (Figure S19 and Table S11, SI). PTSA-BD-Me4 also suits the suitable average hydrogen bonding range of 2.06 to 2.19 Å validating the role of hydrogen bonding in determining COF surface area. The heavy atom distances [Davg (Namine…Oacid) ‗N‘-atom of the amine to the ‗O‘-atom of the acid] of the salts further supports



Figure 4. (a) ORTEP diagram (50 % ellipsoid probability) of different acid-diamine salts synthesized to establish the role of hydrogen bonding. The hydrogen bonding distances mentioned herein are in Å unit. The CCDC number of each of the salts has been mentioned in brackets; PTSA-Pa-Cl2 (1583921), PSA-Pa-Cl2 (1583922), BSA-Pa-Cl2 (1583923), NBSA-Pa-Cl2 (1583924), PTSA-Pa-Cl-NO2 (1583925), PSA-Pa-Cl-NO2 (1583926), BSA-Pa-Cl-NO2 (1583927), NBSA-Pa-Cl-NO2 (1583928), PTSA-BD-Me4 (1584067), PSA-BDMe4 (1584065), BSA-BD-Me4 (1584066), NBSA-BD-Me4 (1584068), PTSA-Pym (1583929), BSA-Pym (1583930) and NBSA-Pym (1583931). (b), (c) and (d) Comparison of structures, PXRD patterns and N2 sorption isotherms for TpPa-Cl2 and TpPa-Cl-NO2 synthesized from different acid-diamine salts.

our principle of porosity prediction (Section S9, SI). Thus, a particular crystallographic identity present in the reactants (salts) can actuate the material‘s (COFs) properties (crystallinity and porosity). CONCLUSION In conclusion, we have synthesized a series of COFs via the salt-to-COF synthesis approach. This solvent-free, and green fabrication avenue is a key to achieve moderate to highly crystalline as well as porous COFs. The role of hydrogen bonding [davg (Namine–H…Oacid)] in the acid-diamine salt has been analyzed for the production of porous and crystalline COFs. Although limited information on acid-diamine salts to framework structure could be established from the crystallographic studies, the gas-adsorption measurements and powder X-ray diffraction patterns implicate the possible role of hydrogen bonding in the acid-diamine salts in establishing this structureproperty relationship. Further advances will provide insight into the various applications dependent on the control of longrange ordering and pore channels.

Supporting Information. Synthesis, crystallography and characterization details are provided in Supporting Information file. This material is available free of charge via the Internet at http://pubs.acs.org. CCDC nos. 1583915-1583939 and 1584062-1584068.

* [email protected] ‡S. Ka and S. K. contributed equally. The authors declare no competing financial interests.

S. Ka acknowledges University Grants Commission, Government of India for Senior Research Fellowship. S. K thanks SERB for fellowship (SB/FT/CS-140/2014). R. B. acknowledges DST IndoSingapore Project (INT/SIN/P-05) and DST Nano-mission Project (SR/NM/NS-1179/2012G) for funding. We acknowledge Mr.


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Journal of the American Chemical Society Tiyash Basu for his valuable suggestions during manuscript writing. We also acknowledge Dr. Matthew Addicoat for the theoretical calculations.

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Suvendu Karak, Sushil Kumar, Pradip Pachfule and Rahul Banerjee

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