I. Analysis of the difficulties in structure analysis of MOFs
Werner established the theory of coordination chemistry, and 50 years after his discovery that solvent molecules were present in the crystals of some complexes, it was found that these solvent molecules could be removed from the crystals, leaving spaces that could be used to store gases. The first examples of crystal coordination lattices were reported in 1959, followed by more similar examples, but except for metal cyanides, they were thermally and chemically unstable and had very fragile structures that could not support porosity. It was not until MOF-2 and MOF-5 were reported that the successful synthesis of these two MOFs meant that one could link organic and inorganic units by strong bonds to form porous structures whose composition could be changed and modified by chemical reactions. Due to their structural and functional tunability, MOFs have become one of the fastest-growing materials in chemistry, as evidenced by the increasing number of structures, publications, citations, and a growing number of researchers. MOFs currently have applications in several areas, including photocatalysis, electrocatalysis, energy storage, gas storage and separation, liquid-gas phase separation, and water adsorption.
Structure determines properties, and currently, the main approach to the structural analysis of MOFs is single-crystal X-ray diffraction, and the main synthetic means are an aqueous (solvent) thermal method and diffusion method, etc. For most of the work on MOFs, obtaining a high-quality single crystal and analyzing its structure is an essential element, but single crystal synthesis is time-consuming and requires constant searching of conditions, and many times the effort is still unable to obtain However, single-crystal synthesis is time-consuming and requires constant efforts to figure out the conditions, and in many cases, it is not possible to obtain crystals above 100 μm to meet the requirements of single-crystal X-ray diffraction testing.
II. Analysis of the difficulties in structural analysis of COFs
Conjugated organic frameworks (COFs) are a class of crystalline organic porous materials composed of organic structural units connected by covalent bonds. In addition, COFs are highly porous skeletal structures composed of light elements (e.g., C, H, N, etc.) with low density (down to 0.13 g/cm3) and high specific surface area (up to 5083 cm2 /g), therefore, since the first COF was reported by Yaghi's group in 2005, COFs have shown promising applications in molecular adsorption and separation, catalysis, optoelectronics, sensing, and energy. Therefore, since the first COF was reported by Yaghi's group in 2005, COFs have shown promising applications in the fields of molecular adsorption and separation, catalysis, photovoltaics, sensing and energy, and have made important research progress.
Although the first single crystals of COFs have been synthesized by Tianqiong Ma and he provided an effective strategy to synthesize COF single crystals, the synthesis of the single crystals took a lot of time, and different specific and effective modifiers are needed to improve the crystallinity for different COFs. Currently, most COFs are not synthesized in this way, but only by solving the structure of the obtained COF powder. Due to the large cell parameters of COFs, it is very difficult to solve the structure by powder X-ray diffraction alone, and the structure of many COFs, especially two-dimensional COFs, can only be solved by obtaining the cell parameters from powder diffractograms, building some artificial structural models, and performing some simulations to obtain the pore diameters and specific surface areas, comparing them with experimental values, and taking the model that matches the experimental values. The structure model that matches the experimental value is considered to be the real COF structure, and their refinement of the powder X-ray diffractogram is only the refinement of the peak shape (Pawley or Lebail refinement), but actually, there is no refinement with the structure.
III. New Technology for Structure Analysis｜MicroED Microcrystalline Electron Diffraction
In the study of MOFs/COFs, the slowest step is often to determine the structure of the product. This may no longer be the case. Today, data collection and structural analysis of 100 nm single crystal samples can be performed using MicroED (microcrystalline electron diffraction) for structural analysis. Nanometer-sized crystal culture is much less difficult for researchers. For many difficult MOFs, the single-crystal culture period has also been reduced from weeks to 1 day or even a few hours. Especially for COFs, which are more difficult to culture in single crystals, several COFs with unknown structures have been obtained with MicroED to obtain accurate structures. Because the technology is expected to revolutionize fields such as organic chemistry, MicroED was selected as one of the top 10 technological breakthroughs of the year by the journal Science in 2018.
MicroED, or microcrystalline electron diffraction, is continuously rotating three-dimensional electron diffraction, a series of diffraction patterns recorded continuously at different rotation angles of the goniometer of the TEM, the axis of rotation being the axis of the goniometer of the TEM. Since the rotation never stops during data collection, its data acquisition process takes very little time, only a few minutes for one sample, making it ideal for data collection of electron beam sensitive samples. Since the interaction of electrons with matter is much stronger than that of X-rays, the sample size required for electron diffraction is 2-3 orders of magnitude smaller than that of X-rays, and a 100 nm crystal can be tested. Moreover, the 3D electron diffraction approach also reduces kinetic effects, allowing structure resolution and refinement to be performed by kinematic methods.
Below we present several examples of structure resolution of MOFs and COFs by MicroED.
Junliang Sun's group at Peking University and his collaborators used the cRED (equivalent to MicroED) technique to collect electron diffraction data on Cu3HHTT2, a two-dimensional conducting MOF of size 5-10 μm, and obtained data with a resolution of 1.5 Å (Fig. 1), and its structure was resolved by this data. The structure of this MOF was found to be a rare perfect AA stacking and the distance between adjacent layers is shorter than any other 2D MOF, and the correctness of this structure was verified by N2 adsorption experiments, high-resolution electron microscopy photographs.
Figure. 1 (a)(b) Schematic structure of Cu3HHTT2 (c)(d) Morphology of Cu3HHTT2 and its 3D inverted easy lattice
Xiaodong Zou's group at Stockholm University and his collaborators have resolved the structures of the photochemically active MOFs, PCN-415 and PCN-416, which have very small crystal particles of only 500 nm (Figure 2), MicroED technique was used to obtain the structures of these two MOFs, and the structures were further confirmed by Rietveld refinement of the powder X-ray diffraction data.
Fig. 2 (a)(b) Three-dimensional inverted easy dot matrix reconstructed from cRED data for PCN-415 and PCN-416; (c)(d) Rietveld refinement of PXRD for PCN-415 and PCN-416; (e) schematic structure of PCN-415; (f) schematic fcu topology of PCN-415
Xiaodong Zou's group also synthesized a new type of porous cobalt MOF, Co-CAU-36, which is around 500 nm in size (Figure 3), and obtained 8 sets of high-resolution 3D electron diffraction data by MicroED at a temperature of less than 100 K. It obtained all the metal ions except hydrogen atoms and the position of the linker, and the position of the solvent was found out after refinement, which was achieved for the first time using electron diffraction to determine and refine the position of the solvent.
Figure 3 Schematic diagram of (a) three-dimensional inverted easy lattice (b) structure of Co-CAU-36
Junliang Sun's group at Peking University and Cheng Wang's group at Wuhan University collaborated to obtain the structure of the 3D-TPB series COF by MicroED (Figs. 4-5), and the resolution of the data is in the range of 0.9-1.0 Å. This work shows that it is possible to directly locate all non-hydrogen atoms by 3D electron diffraction techniques.
Figure 4 Shape of 3D-TPB-COF and its 3D inverted easy lattice
Figure 5 Schematic diagram of the structure of 3D-TPB-COF
Single-crystal X-ray diffraction is the main tool for structural analysis of MOFs and COFs, but it can only be performed on large-sized single-crystal samples. MicroED microcrystal electron diffraction technology only requires nanoscale crystals or powder crystals for testing, which greatly reduces the difficulty of crystal cultivation. At present, many unknown structures of MOFs and COFs that have been difficult to solve in the past have been resolved by MicroED.
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