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  • Hydroxyl COFs Linkers

    Introduction

    Covalent organic frameworks (COFs) are a kind of crystalline organic porous polymers formed by the covalent bond of organic units, which have a wide range of applications in material adsorption, storage and separation, heterogeneous catalysis, sensing, photoelectric and other aspects. Organic linkers are incorporated into the periodic porous crystalline structures and held together through condensation to form the chemical bonds of boroxines, boronate-ester, imines, borosilicates, and so on. A hydroxy is a functional group with the chemical formula −OH and has high reactivity, which can be condensed with aldehyde group, amine group, boric acid group, etc. Therefore, hydroxy-containing organic molecules such as alcohols and phenols are important organic linkers in COFs. Among them, the most widely used class of hydroxyl COFs linkers is catechol, and the most famous boronate ester linked COFs are constructed from them. In addition, they can be used to build 1, 4-dioxin linked COFs. At present, hydroxyl COFs linkers have been used to construct the synthesis of COFs with various linkages, which greatly increases the structural and functional diversity of COFs and strongly promotes the development of COFs research field.

    Fig. 1. Typical examples of catechol monomers for the synthesis of COFs.Fig. 1. Typical examples of catechol monomers for the synthesis of COFs.

    Applications

    Hydroxyl-based linkers can be used in the synthesis of various COFs, and they mainly include the following types.

    • Synthesis boronate ester linked COFs: The reversible covalent reaction of catechol with boric acid can form boronate-ester linked COFs. COF-5, the earliest synthetic COF, was obtained by this strategy via a condensation reaction between 1,4-benzenediboronic acid (BDBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) at 3:2 stoichiometric ratios. In addition to 2,3,6,7,10,11-hexahydroxytriphenylene, many other catechol compounds have also been reported to successfully react with boric acid monomer for generating boronate ester-linked COFs, such as linear catechols 1,2,4,5-tetrahydroxybenzene (THB) and polyol 2,3,6,7-tetrahydroxy-9,10-dimethyl-anthracene (THDMA), cyclotricatechylene (CTC), 9,10- hydroxyphenanthrene cyclotrimer (HPCT), AEM-1, AEM-2, etc. The obtained COFs include BTP-COF, COF-66, CTC-COF, CTC-COF-2, CTC-COF-3, DBA-COF-1, Py-DBA-COF-1, Star-COF-1, Star-COF-2, Star-COF-3, AEM-COF-2, etc [1].

    Fig. 2. Schematic representation of the synthesis of Star-COF-1, Star-COF-2, Star-COF-3 by 9,10- hydroxyphenanthrene cyclotrimer (HPCT).Fig. 2. Schematic representation of the synthesis of Star-COF-1, Star-COF-2, Star-COF-3 by 9,10- hydroxyphenanthrene cyclotrimer (HPCT).

    • Synthesis of 1,4-dioxin linked COFs: Catechol can react with ortho-difluorobenzene or pyridine to synthesize COFs with a 1,4-dioxin linkage in the present of a base catalyst. The resulting COFs exhibit high chemical stability owing to the irreversible dioxin linkage. Three typical 1,4-dioxin linked COFs have been successfully synthesized through this strategy [2]. COF-316 (= JUC-505) has been prepared by the reaction of 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) and tetrafluorophthalonitrile (TFPN). Meanwhile, HHTP and 2,3,5,6-tetrafluoro-4-pyridinecarbonitrile are condensed to form COF-318 in a mixture of 1,4-dioxane (DOX) and mesitylenee (1/1 v/v). JUC-506 has been synthesized by the condensation of HHTP and 2,3,6,7-tetrafluoroanthraquinone catalyzed by K2CO3.

    Fig. 3. Schematics for the synthesis of COF-318, COF-316 (JUC-505), and JUC-506 from HHTP. Fig. 3. Schematics for the synthesis of COF-318, COF-316 (JUC-505), and JUC-506 from HHTP.

    • Synthesis of ester linked COFs: A series of ester-linked COFs, COF-119, COF-120, COF-121 and COF-122 were constructed by a solvothermal synthetic method by trans-esterification reaction of the hydroxyl-functionalised tri- or tetratopic units and ditopic 2-pyridinyl aromatic carboxylates. Thereinto, the reaction between a hydroxyl-containing monomers, tetrakis(4-hydroxyphenyl)ethylene (THPE) or 1,3,5-tris(4-hydroxyphenyl)benzene (THPB) and di(pyridin-2-yl) terephthalate (DPT) yields COF-119 with the kgm topology and COF-120 with hcb topology, respectively. Furthermore, the reticulated COF-121 was obtained by reacting THPB with di(pyridin-2-yl) [1,10- biphenyl]-4,40 -dicarboxylate (DPBP) [3].

    Fig. 4. Schematics for the synthesis of COF-120 and COF-121 from THPB.Fig. 4. Schematics for the synthesis of COF-120 and COF-121 from THPB.

    • Synthesis of other COFs: In addition to boronate ester linked COFs, 1,4-dioxin linked COFs and ester linked COFs, hydroxyl-based linkers can be also used in the synthesis of silicate linked COFs, spiroborate linked COFs, and mixed imine-boroxine or boronic linked COFs [4].

    Alfa Chemistry offers a series of hydroxyl-based COFs linkers including various catechol compound and many other alcohol and phenol compound. These linkers have been widely used to construct boronate ester linked COFs, 1,4-dioxin linked COFs, ester linked COFs, silicate linked COFs, spiroborate linked COFs, and mixed imine-boroxine or boronic linked COFs, etc. You can click on our product list for a detailed view. At the same time, we also offer product customization according to customer's detailed requirements. If you are interested in our products or have any questions or needs, please feel free to contact us. We will be happy to provide you with support and services.

    References:

    1. Lohse M. S. and Bein T. Covalent organic frameworks: structures, synthesis, and applications[J]. Advanced Functional Materials, 2018, 28(33): 1705553.
    2. Geng K., et al. Covalent organic frameworks: design, synthesis, and functions[J]. Chemical Reviews, 2020, 120(16): 8814-8933.
    3. Zhao C., et al. Ester-linked crystalline covalent organic frameworks[J]. Journal of the American Chemical Society, 2020, 142(34): 14450-14454.
    4. Bhambri H., et al. Nitrogen-rich covalent organic frameworks: a promising class of sensory materials[J]. Materials Advances, 2022, 3(1): 19-124.

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