Introduction
When it comes to covalent organic frameworks (COFs) linkers, COFs linkers containing boric acid and its derivatives have to be mentioned. Boronic acid-containing compounds are the first to be employed as linkers in the synthesis of COFs (In a pioneering work in 2005, Yaghi and co-workers reported the 2D COFs (named COF-1) as well as COFs (named COF-5) were synthesized by condensation of boronic acids linkers, which are the first two COFs). It can be said that the boric acid linkers set the foundations for an exceedingly growing, exciting field of research. Boronic acids can react through a reversible condensation reaction both with diols to give boronate esters as well as self-condense to yield boroxines. Due to the reversibility of these reactions, the use of boronic acid compounds and their derivatives have been demonstrated for the formation of various types of COFs [1]. Nowadays, boronic-acid-derived highly defined two-dimensional (2D) and three-dimensional (3D) COFs have been one of the largest categories of COFs, and these COFs have been studied for a wide range of applications, such as gas storage, sieving/separation of molecules, batteries/energy storage, optoelectronics, etc.
Fig. 1. Synthesis scheme and structures of (a) COF-1 and (b) COF-5.
Applications
Just as mentioned above, linkers of boric acid and its derivatives can be used to form boroxine linkage, boronate ester linkage and other linkages, and thus constructing the corresponding 2D and 3D COFs materials. The details are shown in below.
- Synthesis of boroxine-linked COFs: The self-condensation of boronic acids monomers can yield cyclic six-membered boroxine linked COFs with a planar structure, and water byproduct, which is the typical synthetic strategy to synthesize COFs. The obtained boroxine-linked COFs have good crystallinity, large surface area, and high thermal stability. There are many COFs obtained by this synthetic strategy, and COF-1 is a typical representative, which is synthesized by the dehydration of 1,4-benzenediboronic acid (BDBA) under mesitylene-dioxane solution (as shown in Fig. 1). In addition, 2D PPy-COF have been synthesized from the self-condensation of pyrene-2,7-diboronic acid (PDBA), in a sealed Pyrex tube at 120 °C. Interestingly, a Td-symmetric monomer tetra(4-dihydroxyborylphenyl)methane (TBPM) and its silane analog tetra(4-dihydroxyborylphenyl)silane (TBPS) upon self-condensation can yield a 3D COF-102 and 3D COF-103, respectively.
Fig. 2. (Left ) schematic for the synthesis of PPy-COF and (Right) the structures of TBPM and TBPS.
- Synthesis of boronate ester-linked COFs: The condensation of boronic acids and catechols to produce boronte esters is another synthetic strategy to synthesize boronic-acid-derived COFs. COF-5 is a typical representative one constructed by this synthetic strategy, which is obtained via a condensation reaction between BDBA and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) at 3:2 stoichiometric ratios (as shown in Fig.1). In addition to BDBA, many BDBA derivatives such as 4-formylphenylboronic acid, fluoro-substituted (3-fluoro-4-formylphenyl)boronic acid, (2,3-difluoro-4-formylphenyl) boronic acid, and 5,10,15,20-(tetra-4-dihydroxy-borylphenyl) porphyrin, or other boric acids linkers have also been reported to successfully react with many catechol compounds for generating boronate ester-linked COFs. Up to mow, a variety of 2D COFs including COF-6, COF-8, COF-10, TP-COF, D−A COF, TT-COF, T-COFs, HHTP-DPB COF, TPE-Ph COF, H2P-COF, CuP-COF, ZnP-COF, and COF-66, and many 3D COFs such as COF-105, COF-108, MCOF-1, and DBA-3D-COF 1 have been synthesized by using boric acid linkers and its derivatives [2].
Fig. 3. Schematic for the synthesis of COF-108 by using boric acid linkers.
- Synthesis of borosilicate-linked COFs: In addition to boroxine and boronate ester linked COFs, boronic acids can react with silanols to produce borosilicate bonds and then to form borosilicate-linked COFs such as COF-202 [3].
Fig. 4. Schematic for the synthesis of borosilicate-linked COF-202.
Alfa Chemistry offers a series of boric acid COFs linkers and borate COFs linkers. These linkers have been widely used to construct boroxine-linked COFs, boronate ester-linked COFs, borosilicate-linked COFs, etc. The obtained COFs have a wide range of applications, such as gas storage, sieving/separation of molecules, batteries/energy storage, optoelectronics, 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:
- Frey L.,et al. Boronic-acid-derived covalent organic frameworks: from synthesis to applications[J]. New Journal of Chemistry, 2021, 45(33): 14879-14907.
- Geng K., et al. Covalent organic frameworks: design, synthesis, and functions[J]. Chemical Reviews, 2020, 120(16): 8814-8933.
- Wu M. X. and Yang Y W. Applications of covalent organic frameworks (COFs): From gas storage and separation to drug delivery[J]. Chinese Chemical Letters, 2017, 28(6): 1135-1143.
