Weaving of organic threads into a crystalline covalent organic framework

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Science  22 Jan 2016:
Vol. 351, Issue 6271, pp. 365-369
DOI: 10.1126/science.aad4011

Weaving organic threads

Woven fabrics are inherently flexible. Liu et al. created a molecular fabric analog using metal-organic frameworks (see the Perspective by Gutierrez-Puebla). Phenanthroline ligands on a copper metal complex directed the addition of organic linkers via imine bonds to create helical organic threads with woven texture. Removing the copper allowed the strands to slide against each other and increased the elasticity of the material 10-fold.

Science, this issue p. 365; see also p. 336


A three-dimensional covalent organic framework (COF-505) constructed from helical organic threads, designed to be mutually weaving at regular intervals, has been synthesized by imine condensation reactions of aldehyde functionalized copper(I)-bisphenanthroline tetrafluoroborate, Cu(PDB)2(BF4), and benzidine (BZ). The copper centers are topologically independent of the weaving within the COF structure and serve as templates for bringing the threads into a woven pattern rather than the more commonly observed parallel arrangement. The copper(I) ions can be reversibly removed and added without loss of the COF structure, for which a tenfold increase in elasticity accompanies its demetalation. The threads in COF-505 have many degrees of freedom for enormous deviations to take place between them, throughout the material, without undoing the weaving of the overall structure.

Weaving, the mutual interlacing of long threads, is one of the oldest and most enduring methods of making fabric, but this important design concept has yet to be emulated in extended chemical structures. Learning how to link molecular building units by strong bonds through reticular synthesis (1) into weaving forms would be a boon to making materials with exceptional mechanical properties and dynamics. To successfully design weaving of chains into two- and three-dimensional (2D and 3D) chemical structures (Fig. 1, A and B), long threads of covalently linked molecules (i.e., 1D units) must be able to cross at regular intervals. It would also be desirable if such crossings serve as points of registry, so that the threads can have many degrees of freedom to move away from and back to such points without collapsing the overall structure. Structures have been made by weaving metal-organic chains (2), but designing well-defined materials and assembling their structures by weaving is challenging, and weaving in crystalline inorganic or covalent organic extended structures is undeveloped.

Fig. 1 Weaving and entanglement.

Illustrations of weaving of threads in two (A) and three (B) dimensions, compared with entanglements of sheets (C), 3D arrangements (D), and their interlocking of rings (insets).

We report on a general strategy and its implementation for the designed synthesis of a woven material [covalent organic framework-505 (COF-505)]. This COF has helical organic threads interlacing to make a weaving crystal structure with the basic topology of Fig. 1B, and we show that this material has an unusual behavior in elasticity. Although terms such as interweaving (3), polycatenated (2), and interpenetrating (46) have been used to describe interlocking of 2D and 3D extended objects (Fig. 1, C and D), most commonly found in MOFs, we reserve the term “weaving” to describe exclusively the interlacing of 1D units to make 2D and 3D structures (Fig. 1, A and B). Weaving differs from the commonly observed interpenetrating and polycatenated frameworks because the latter are topologically interlocking (i.e., interlocking rings) (Fig. 1, C and D, insets), whereas the weaving constructs that we envision have many more degrees of freedom for enormous spatial deviations, by each of the threads, to take place independently and still preserve the underlying topology. Such freedom may enable reversible control over the mechanical properties of materials.

Our synthetic strategy is shown in Fig. 2, where we start with the aldehyde functionalized derivative of the well-known complex salt Cu(I)-bis[4,4'-(1,10-phenanthroline-2,9-diyl)dibenzaldehyde] tetrafluoroborate, Cu(PDB)2(BF4) (Fig. 2A). The position of the aldehyde groups approximates a tetrahedral geometry and can be used in reticular synthesis as a building block to be linked with benzidine (BZ) and make an imine-bonded PDB-BZ threads weaving arrangement, with the tetrafluoroborate anions occupying the pores (Fig. 2B). The orientation of the PDB units in a mutually interlacing fashion ensures that the threads produced from linking the building units are entirely independent, with the Cu(I) ions serving as templates (points of registry) to bring those threads together in a precise manner at well-defined intervals. Because the PDB-BZ threads are topologically independent of the Cu(I) ions, the resulting woven structure is formally a COF (termed COF-505). The overall tetrahedral geometry of the aldehyde units ensures the assembly of the threads into a 3D framework (Fig. 2B). The topology of this framework is that of diamond, as expected from the principles of reticular chemistry (1). We show that when we remove the Cu(I) ions, the structure and its topology remain intact regardless of how the threads deviate from their points of registry, and upon remetalating, the overall structure is reversibly restored. We find a tenfold increase in elasticity when going from the metalated to the demetalated forms of the material.

Fig. 2 A general strategy for the design and synthesis of weaving structures.

COF-505 was constructed from organic threads using copper(I) as a template (A) to make an extended weaving structure (B), which can be subsequently demetalated and remetalated.

The copper(I)-bisphenanthroline core of the Cu(PDB)2 (without the aldehyde functionality) has been studied extensively as a discrete molecule for the formation of supramolecular complexes (79); however, as yet it has not been used to make extended structures, especially of the type discussed here. The tolerance for robust reaction conditions (7, 10) makes this complex suitable for imine COF synthesis, especially in weak acidic conditions. Thus, the tetrahedral building unit, Cu(PDB)2, was designed bearing aldehyde groups in the para positions of the two phenyl substituents (Fig. 2A). The synthesis of Cu(PDB)2(BF4) molecular complex was carried out by air-free Cu(I) complexation of 4,4'-(1,10-phenanthroline-2,9-diyl)dibenzaldehyde, according to a previously reported procedure (11). The single-crystal structure of this complex revealed a distorted tetrahedral geometry around the Cu(I) center, with a dihedral angle of 57° between the two phenanthroline planes. This distortion likely arises from the π-π interaction between the phenanthroline and neighboring phenyl planes (12, 13).

We synthesized COF-505 via imine condensation reactions by combining a mixture of Cu(PDB)2(BF4) (15 mg, 0.016 mmol) and BZ (6.0 mg, 0.032 mmol) in tetrahydrofuran (THF, 1 mL) and aqueous acetic acid (6 mol/L, 100 μL). The reaction mixture was sealed in a Pyrex tube and heated at 120°C for 3 days. The resulting precipitate was collected by centrifugation, washed with anhydrous THF, and then evacuated at 120°C for 12 hours to yield 18.7 mg [94.4%, based on Cu(PDB)2(BF4)] of a dark brown crystalline solid (COF-505), which was insoluble in common polar and nonpolar organic solvents.

Fourier-transform infrared spectroscopy (FT-IR) and solid-state nuclear magnetic resonance (NMR) spectroscopy studies were performed on COF-505 to confirm the formation of imine linkages. A molecular analog of COF-505 fragment, Cu(I)-bis[(1,10-phenanthroline-2,9-diyl)bis(phenylene)bis(biphenyl)methanimine)] tetrafluoroborate, Cu(PBM)2(BF4), was used as a model compound and synthesized by condensation of Cu(PDB)2(BF4) and 4-aminobiphenyl (12). The FT-IR spectrum of COF-505 shows peaks at 1621 and 1196 cm−1 [1622 and 1197 cm−1 for Cu(PBM)2(BF4)], which are characteristic C=N stretching modes for imine bonds (14, 15). Furthermore, the 13C cross-polarization with magic-angle spinning (CPMAS) solid-state NMR spectrum acquired for COF-505 displays a series of peaks from 140 to 160 part per million, similar in shape and occurring at chemical shifts characteristic of those expected for C=N double bonds. To differentiate imine bonds from C=N double bonds of the phenanthroline unit, a cross-polarization and polarization inversion (CPPI) technique was applied, which leaves the signal for quaternary 13C groups unchanged, whereas the residual tertiary 13CH signal should approach zero (16). The decreased intensity of the 13CH signal under these conditions confirmed the existence of imine CH=N double bond. Overall, these observations served as initial confirmation of having covalently linked imine extended threads in COF-505.

Before determining the single-crystal structure of COF-505, we studied the morphology and purity of the as-synthesized material. We found, using scanning electron microscopy (SEM), crystallites of ~200 nm are aggregated into spheres of 2 μm in diameter (Fig. 3A), which possibly arises from weak interactions of the synthesized material with the solvent, THF. No other phase was observed from SEM images taken throughout the material (12).

Fig. 3 Morphology and electron microscopy studies of COF-505.

(A) Crystallites aggregated on a crystalline sphere observed by SEM. (B) TEM image of a single sub-μm crystal used for 3D-EDT. (C) 2D projection of the reconstructed reciprocal lattice of COF-505 obtained at 298 K from a set of 3D-EDT data. (D) HRTEM image of COF-505 taken with the [1-10] incidence. (E) 2D projected potential map obtained by imposing pgg plane group symmetry on Fig. 1D. (F) Reconstructed 3D electrostatic potential map (threshold, 0.8). (G) Indexed PXRD pattern of the activated sample of COF-505 (black) and the Pawley fitting (red) from the modeled structure.

A single submicrometer-sized crystal (Fig. 3B) from this sample was studied by 3D electron diffraction tomography (3D-EDT) (1719). One EDT data set was collected from the COF-505 (Fig. 3C) by combining specimen tilt and electron-beam tilt in the range of –41.3° to +69.1° with a beam-tilt step of 0.2°. From the acquired data set, 3D reciprocal lattice of COF-505 was constructed that was identified as a C-centered orthorhombic Bravais lattice. The unit-cell parameters were a = 18.9 Å, b = 21.3 Å, c = 30.8 Å, and V = 12399 Å3, which were used to index reflections observed in both powder x-ray diffraction (PXRD) pattern and Fourier diffractograms of high-resolution transmission electron microscopy (HRTEM) images (Fig. 3D to F). The unit-cell parameters were further refined to be a = 18.6 Å, b = 21.4 Å, c = 30.2 Å, and V = 12021 Å3 by Pawley refinement of PXRD pattern (Fig. 3G). The observed reflection conditions were summarized as hkl: h+k = 2n; hk0: h,k = 2n; h0l: h = 2n; and 0kl: k = 2n, which suggests five possible space groups—Cm2a (no. 39), Cmma (no. 67), Cmca (no. 64), Cc2a (no. 41), and Ccca (no. 68). Three of them—Cm2a, Cmma, and Ccca—were excluded because their projected plane group symmetries along [1-10] did not coincide with that of the HRTEM image, pgg (Fig. 3E). Furthermore, by performing Fourier analysis of the HRTEM images and imposing symmetry to the reflections, Cu(I) positions were determined from the reconstructed 3D potential map (Fig. 3F). The structure of COF-505 was built in Materials Studio by putting Cu(PDB)2 units at copper positions and connecting them through biphenyl (reacted BZ) molecules. The chemical compositions were determined by elemental analysis; therefore, once the number of copper atoms in one unit cell was obtained, the numbers of other elements in one unit cell were also determined, which indicates that the unit-cell framework is constructed by 8 Cu(PDB)2 and 16 biphenyl units. However, symmetry operations of the space group Cmca require two PDB units connected to one copper onto a mirror plane perpendicular to a axis, which is not energetically favorable geometry. The final space group determined, Cc2a, was used to build and optimize a structure model. The PXRD pattern calculated from this model is consistent with the experimental pattern of activated COF-505 (12).

According to the refined model, COF-505 crystallizes in a diamond (dia) network with the distorted tetrahedral building units Cu(PBD)2 and biphenyl linkers BZ linked through trans imine bonds. As a result, covalently linked adamantane-like cages 19 by 21 by 64 Å are obtained and elongated along the c axis (dimensions are calculated based on Cu-to-Cu distances). This size allows two diamond networks of identical frameworks to form the crystal. These frameworks are mutually interpenetrating (when the Cu centers are considered) in COF-505 crystals along the c direction, where the frameworks are related by a C2 rotation along the b axis, leaving sufficient space for BF4- counterions (20). We note that when the structure is demetalated, as demonstrated below, the COF is mutually woven (Fig. 2B).

Fundamentally, each of the threads making up the framework is a helix (Fig. 4A). For clarity, only a fragment of one weaving framework is shown. The helices are entirely made of covalently linked organic threads. As expected, they are weaving and being held by Cu(I) ions at their points of registry (Fig. 4B). These threads are propagating in two different directions along [110] and [–110]. Although the helices are chemically identical, they have opposite chirality, giving rise to an overall racemic weaving framework (Fig. 4, C and D) of the same topology as in Fig. 1B. We note that in the context of reticular chemistry, the points of registry play an important role in crystallizing otherwise difficult-to-crystallize threads and to do so into 2D or 3D frameworks. This arrangement is in stark contrast to the parallel manner in which such 1D objects commonly pack in the solid state.

Fig. 4 Single-crystal structure of COF-505.

The weaving structure of COF-505 consists of chemically identical helices (marked in blue and orange because they are of opposite chirality) with the pitch of 14.2 Å (A). The orange helices propagate in the [1-10] direction, whereas the blue helices propagate in the [110] direction with copper (I) ions as the points of registry (B). Neighboring blue helices are woven with the orange helices to form the overall framework (C). Blue and orange helices and their C2 symmetry-related green and gray copies are mutually woven (D). Additional parallel helices in (C) and (D) are omitted for clarity.

The COF-505 structure is a woven fabric of helices, so we sought to remove the Cu centers and examine the properties of the material before and after demetalation. Heating COF-505 in a KCN methanol-water solution (8) yielded a demetalated material. Using inductively coupled plasma (ICP) analysis, we found that 92 to 97% of the Cu(I) copper ions had been removed (12). The dark brown color of COF-505 [from the copper-phenanthroline metal-to-ligand charge transfer (MLCT) (21)] changed to pale yellow as demetalation proceeded (12). Although the crystallinity of demetalated material decreased compared with COF-505, SEM images show similar morphology before and after demetalation (12). Additionally, the imine linkages were also maintained; the FT-IR peaks at 1621 and 1195 cm−1 (12) are consistent with those of COF-505 (1621 and 1196 cm−1, respectively). Furthermore, the material could be remetalated with Cu(I) ions by stirring in a CH3CN/CHCI3 solution of Cu(CH3CN)4(BF4) to give back crystalline COF-505. This remetalated COF-505 has identical crystallinity to the original as-synthesized COF-505, as evidenced by the full retention of the intensity and positions of the peaks in the PXRD (12). In the FT-IR spectrum, the peak representing imine C=N stretch was retained (12), indicating that the framework is chemically stable and robust under such reaction conditions.

Given the facility with which demetalation can be carried out and the full retention of the structure upon remetalation can be achieved, we examined the elastic behavior of the metalated and demetalated COF-505. A single particle of each of these two samples was indented by a conical tip of an atomic force microscope (AFM), and the load-displacement curves were recorded for both loading and unloading process (22). The effective Young’s moduli (neglecting the anisotropy of the elasticity) of the two COF-505 materials was ~12.5 and 1.3 GPa for the metalated and demetalated particles, respectively (12). Notably, this tenfold ratio in elasticity upon demetalation of COF-505 is similar to the elasticity ratio for porous MOFs to polyethylene (23). The distinct increase of elasticity could be attributed to the loose interaction between the threads upon removal of copper. Moreover, the elasticity of the original COF-505 could be fully recovered after the process of demetalation and remetalation, being facilitated by the structure of weaving helical threads that easily “zip” and “unzip” at their points of registry. The large difference in elasticity modulus is caused by loss of Cu(I) ions, which in total only represent a minute mole percentage (0.67 mol%) of the COF-505 structure.

Supplementary Materials

Materials and Methods

Figs. S1 to S21

Tables S1 to S4

Reference (24)

References and Notes

  1. Materials and methods are available as supplementary materials on Science Online.
Acknowledgments: The structures of COF-505 and Cu(PDB)2(BF4) are available free of charge from the Cambridge Crystallographic Data Centre under the reference numbers CCDC-1434851 and CCDC-1434852, respectively. This research was supported by BASF SE (Ludwigshafen, Germany) for synthesis and basic characterization, and the U.S. Department of Defense, Defense Threat Reduction Agency (HDTRA 1-12-1-0053) for mechanical properties. We thank C. Canlas for his assistance with solid-state NMR and A. Schöedel (Yaghi group), B. Zhang, and Y. Liu (Molecular Foundry, Lawrence Berkeley National Laboratory) for helpful discussions. This work was also supported by the Spanish Ministry of Economy and Competitiveness through the Juan de la Cierva program (F.G.); a Grant-in-Aid for Scientific Research (C) (25390023) and JST Research Acceleration Program (Z.L. and K.S.); grants from Vetenskapsrådet (Y.M. and P.O.) and JEOL Ltd (P.O.), Japan; EXSELENT and 3DEM-Natur, Sweden (O.T.); and BK21Plus, Korea (O.T.). Beamline 7.3.3 of the Advanced Light Source is supported by the Director of the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract DE-AC02-05CH11231. The AFM study was supported by the National Science Foundation (NSF) (grant DMR‐1344290). The data reported in the paper are presented in the supplementary materials.
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