Tianqiong Ma,†,‡ Jian Li,‡ Jing Niu,† Lei Zhang,‡ Ahmed Etman

in COFs, the application of this novel technique disclosed the 9- ..... Wojtas, L.; Larsen, R. W.; Eddaoudi, M.; Zaworotko, M. J. J. Am. Chem. Soc. 20...
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Observation of Interpenetration Isomerism in Covalent Organic Frameworks Tianqiong Ma, Jian Li, Jing Niu, Lei Zhang, Ahmed S. Etman, Cong Lin, Dier Shi, Pohua Chen, Lihua Li, Xin Du, Junliang Sun, and Wei Wang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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Tianqiong Ma,†,‡ Jian Li,‡ Jing Niu,† Lei Zhang,‡ Ahmed Etman,§ Cong Lin,‡ Dier Shi,‡ Pohua Chen,‡ Lihua Li,† Xin Du,‡ Junliang Sun,*,‡,§ and Wei Wang*,†,¶ †

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China ‡

College of Chemistry and Molecular Engineering, Beijing National Laboratory for Molecular Sciences, Peking University, Beijing 100871, China §

Department of Materials and Environmental Chemistry, Stockholm University, Stockholm, 10691, Sweden



Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300071, China

Supporting Information Placeholder ABSTRACT: We report herein the first example of interpenetration isomerism in covalent organic frameworks (COFs). As a wellknown three-dimensional (3D) COF, COF-300 have been synthesized and characterized by the Yaghi group in 2009 as a 5-fold interpenetrated diamond structure (dia-c5 topology). We found that adding an ageing process prior to the reported synthetic procedure afforded the formation of an interpenetration isomer, diac7 COF-300. The 7-fold interpenetrated diamond structure of this new isomer was identified by powder X-ray diffraction (PXRD) and rotation electron diffraction (RED) analyses. Furthermore, we proposed a universal formula to accurately determine the interpenetration degrees (N) of dia-based COFs only from the unit cell parameters and the length of the organic linker. This work not only provides a novel example to the category of interpenetration isomerism, but also sheds new lights to the further development of 3D COFs.

Diversity of the nature has inspired mankind to construct a vast range of elegant structures. With the question1 of "how far can we push chemical self-assembly?" in mind, chemists are devoted to the ingenious assembly of functional materials from the atomic/molecular level. For example, chemists have been paying increasing attention to interpenetration2 which is an intriguing phenomenon in supramolecular chemistry and crystal engineering. In an interpenetrated structure, two or more independent networks are inextricably interlocked to each other but no chemical bonds exist in between. Through interpenetration, the void space is reduced and the whole system is further stabilized. In this regard, interpenetration isomerism3 can occur as a vivid example of diversity, because the optimal filling into the void space under different conditions may result in different degrees of interpenetration. These structural isomers have therefore provided a unique platform4 to investigate the structure-performance relationship, to understand the exquisite principles beyond, and to develop the universal acknowledge for precise control. Toward this goal, we

report herein the first observation of interpenetration isomerism in covalent organic frameworks (COFs). COFs5 represent a new class of crystalline porous polymers, the structures of which are diversely constructed via the covalent bonding of pre-designed organic monomers. Since the first report6 in 2005, most research in this fascinating area has been focusing on two-dimensional (2D) COFs. By contrast, 3D COFs are more complicated in structure and therefore, have rarely been synthesized.7 Compared to 2D COFs, 3D COFs are mostly featured by the interpenetration nature, with the maximum of 11-fold8 interpenetration reported so far. However, in difference from hydrogen or coordination bonding,9 the strong and directional covalent connection between the rigid monomers is unlikely to afford a diversity of interpenetration. In this contribution, we discovered an interpenetration isomer of a well-known 3D COF, COF-300.10 By combining powder X-ray diffraction (PXRD) and rotation electron diffraction (RED) analyses, we verified this new isomer as the 7-fold interpenetrated diamond topology (dia-c7) which is different from the reported 5-fold interpenetration (dia-c5) structure10. Based on this observation, we further deduced a general formula to calculate the interpenetration degree of dia-based COF structures. Synthesized by the Yaghi group in 2009, COF-30010 is the first example of imine-linked 3D COFs. As shown in Figure 1 (left), solvothermal condensation of the tetrahedral monomer 1 and the linear monomer 2 produced COF-300 with dia-c5 topology. Interestingly, we found that adding an ageing process before the synthetic procedure described10 in the literature resulted in an interpenetration isomer, dia-c7 COF-300 (Figure 1, right). More in detail, the reaction mixture was allowed to stand at room temperature for 72 h and then warmed at 50 °C for 72 h before heating at 120 oC for 72 h. The covalent connection throughout dia-c7 COF-300 framework has been confirmed by solid-state nuclear magnetic resonance (Figure S1) and Fourier transform infrared (Figure S2) spectroscopy. These data are essentially identical to those of dia-c5 COF-300, which means that two COF-300 isomers are indeed constructed from the identical covalent-bonding at the atomic level. Also, no differences in thermal stability or

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hydrothermal stability can be observed between two COF-300 isomers.

Figure 1. Solvothermal condensation of tetra-(4-anilyl)methane 1 and terephthaldehyde 2 under different conditions resulted in two interpenetration isomers of COF-300 respectively: the dia-c5 COF-300 (left, reported by the Yaghi group10) was obtained without an ageing procedure; while the dia-c7 COF-300 (right, this work) was obtained with an ageing procedure. However, the difference in their PXRD patterns (Figure 2) implied that two COF-300 isomers are structurally different. All the peaks in the PXRD pattern of dia-c7 COF-300 were indexed, affording a body-centered tetragonal unit cell of a = b = 20.4 Å and c = 8.8 Å . Note that these parameters are distinct from those of diac510 (a = b = 28.280 Å and c = 10.079 Å ). According to the geometry of monomers and their connectivity, we inferred that this new isomer should also be crystallized in a diamond network and most likely with different interpenetration degrees. According to the formula of N = 2a/c suggested for determining the interpenetration degree (N) in ideal dia-cN structures10, the new COF-300 isomer we obtained could be of 4- or 5-fold. However, the simulated PXRD pattern in neither case fits the experimental data (Figure S4). Here, we deduced a formula of N  2 4L2  a 2 c to calculate N by analyzing the trigonometric relationship in dia-cN structures (Section I in SI). The key point is that the N value is determined not only by the unit cell parameters of a and c, but also by the length (L) of the organic linker between two tetrahedral nodes. Accordingly, the N value for this new isomer was calculated to be 7. The structure model with dia-c7 topology was therefore built and the simulated PXRD pattern matches well with the experimental data (Figure S4). Furthermore, we applied the 3D RED technique to obtain directly the single-crystal structure of dia-c7 COF-300.

Figure 2. Comparison of the PXRD patterns and indexed results of dia-c5 and dia-c7 COF-300 interpenetration isomers.

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The RED technique has recently been applied as a powerful tool for the structure determination of micro- and nano-sized crystals.11 The key feature of this technique is the complete collection of three-dimensional electron diffraction (ED) data from nano-sized crystals with reduced dynamical effects, which can then be used to determine the crystal structures through the standard single-crystal diffraction techniques. As the only report in COFs, the application of this novel technique disclosed the 9fold interpenetration structure of COF-320 in 2013.12 We obtained the dia-c7 COF-300 crystals with a maximal crystal dimension of ~500 nm (Figure 3a inset and Figure S6) which can be directly used for RED measurements. The data collection was carried out at 93 K to reduce the electron-beam damage. In total, 506 ED frames were collected, from which the 3D reciprocal lattice was successfully reconstructed13 (Figure 3a). Then a bodycentered tetragonal unit cell with a = b = 20.196 Å and c = 8.558 Å was obtained and these data are well consistent with those derived from the PXRD simulation. The reflection conditions obtained from the RED data proposed the possible space group as I4/mmm (No. 139) or I41/a (No. 88). We eventually solved the singlecrystal structure of dia-c7 COF-300 with the space group of I41/a. The Rietveld refinement with rigid-body restraints was further performed on the structure of dia-c7 COF-300, which resulted in the refined unit cell of a = b = 20.4140(36) and c = 8.8216(23) with Rwp = 4.65%, Rp = 3.42%, and χ2 = 2.35 (Figure 3b and Table S2).

Figure 3. Reconstructed 3D RED data and Rietveld refinement of dia-c7 COF-300. (a) Overview of 3D reciprocal lattice of dia-c7 COF-300. Inset: the TEM image of the dia-c7 COF-300 crystal (0.5 × 0.2 × 0.2 μm3), from which the RED data were collected. (b) Rietveld refinement of dia-c7 COF-300. Observed plot (red), calculated plot (black), difference plot (green), and peak positions (blue). The structural difference between these two interpenetration isomers is depicted in Figure 4. The basic adamantane-like cages of these isomers are different in shape although they are constructed from the same monomers with the identical covalent bonding. For example, the dimensions of this cage is of 36 × 28 × 28 Å 3 in dia-c5 COF-300 and of 62 × 20 × 20 Å 3 in dia-c7 COF300, respectively. The length between two tetrahedral nodes (C atoms) is 17.9 Å in dia-c5 COF-300 and 18.5 Å in dia-c7 COF300. The angles of tetrahedron nodes in dia-c5 COF-300 are measured as 103.4° and 112.6°, respectively, which are slightly distorted and somehow close to the ideal tetrahedral angle (109.5°); while in dia-c7 COF-300, these angles are severely distorted to 66.9° and 134.1°, respectively. As mentioned above, the degree of interpenetration (N = 7) for dia-c7 COF-300 determined by structural analysis cannot match with the value calculated from the formula of N = 2a/c10. The reason is that this formula was established only for the ideal10 dia-cN structures which possess the tetrahedral angles of the standard value, i.e., 109.5°, or

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Figure 4. Comparison of dia-c5 COF-300 (a) and dia-c7 COF-300 (b) structures, including single adamantane-like cage, part of framework viewed from c-axis, and interpenetrated adamantane-like cages. C atoms, gray; N atoms, blue; and H atoms were omitted for clarity. approach to 109.5°. In the real cases such as in dia-c7 COF-300, the tetrahedral angles distorted so as to decrease non-bond energy and van der Waals force from the integrated framework (Section J in SI). Realizing this subtle difference, we also analyzed the reported dia-cN COF structures in the literature (Table. S3). As mentioned above, the formula for calculating interpenetration degrees was established here as N  2 4L2  a 2 c for I lattice structures where N is an odd number, or, N  2 4L2  2a 2 c for P lattice structures where N is an even number (Section I in SI). As shown in Table S3, all the values calculated from our formula matched accurately with the reported structures, which verified the universality of the formula. Meanwhile, in the ideal case, the formula was approximated to be N =√2a/c in I lattice and N = 2a/c in P lattice, which represents a special solution of our formula. Note that, unit cell parameters of a and c can be obtained for example from the index of the PXRD patterns, and, the length (L) of the organic linker between two tetrahedral nodes can be calculated directly from the given monomers by molecular dynamics simulation. Then N can be expeditiously obtained: the closer the calculated L to the experimental value, the more accurate the predicted N is. It is also worth to mention that our formula can be applied directly to all the dia-cN structures with tetragonal symmetries, while in the cases that the symmetries are even lower due to severe distortion, the formula can still be used to roughly estimate the interpenetration degree or be further modified into more complicated forms. Similar to those found for the interpenetration isomers in MOFs, 3b,14 the synthetic conditions are crucial herein for obtaining the certain isomer of COF-300. We conducted the orthogonal experiments to systematically screen the synthetic conditions (Table S1) so as to realize the controllable synthesis of COF-300 isomers. It can be seen that, the ageing process at temperatures lower than 60 °C is very important for the controlled formation of dia-c7 COF-300 (entries 14 and 15 in Table S1). According to the Curtin–Hammett principle15, the occurrence of interpenetration isomerism originates from the competition between the thermodynamic and kinetic routes.4b,14b,16 The energy calculation indicates that dia-c7 COF-300 is more stable than dia-c5 COF-300 (Section J in SI). Therefore, the dia-c5 COF-300 should be formed via a faster kinetic route, while the more stable dia-c7 COF-300 formed via a thermodynamic route. In this regard, adding an ageing process at lower temperatures provides enough time to reach the

thermodynamic equilibrium, through which more void space can be filled with the denser packing and the dia-c7 isomer was obtained. In summary, we report herein the first observation of interpenetration isomerism for COFs. It is an unexpected discovery upon the synthesis of well-known COF-30010 with dia-c5 topology. We found that adding an ageing process prior to the reported synthesis procedure resulted in an interpenetration isomer, dia-c7 COF300. The 7-fold interpenetrated diamond structure of this new isomer has been confirmed by PXRD analysis and RED measurements. Considering that the formation of crystalline porous framework via covalent-bonding is a difficult but target-specific task, we believe that our findings have added new information to the controlled synthesis of complicated COF structures. We also expect that more examples for the interpenetration isomerism of 3D COFs will appear in the future because the subtle competition among many thermodynamic and kinetic routes does exist along with the formation of covalently-linked frameworks. Based on the observation of interpenetration isomerism, we further deduced a general formula to calculate the interpenetration degree of diabased COF structures. The methodology involved should be applicable to calculate the interpenetration degree of other 3D COFs if further integrated with the structural features. Added to the fruitful information on the stacking17 or constitutional18 isomerism of 2D COFs, the phenomenon observed and the methodology developed herein for the interpenetration isomerism of 3D COFs will probably boost the research of diversity-oriented construction of complicated 3D COFs, the future of which may further lead to the unique applications of these fantastic structures.

Experimental details, characterization data (Scheme S1, Table S1S5, Figures S1–S8) and discussion. Crystallographic data for diac7 COF-300 (CIF file). This material is available free of charge via the Internet at http://pubs.acs.org.

*J.S. [email protected] ORCID: 0000-0003-4074-0962

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*W.W. [email protected] ORCID: 0000-0002-9263-7927 The authors declare no competing financial interest.

This work was supported by the National Natural Science Foundation of China (Nos. 21632004, 21471009 and 21527803).

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