Introduction
Metal organic frameworks (MOFs) are composed of metal ions or clusters of metal ions and organic ligands (also known as organic linkers). There are many kinds and quantities of organic linkers. Organic linkers containing carboxylic acid groups are one of the most popular linkers because of their strong coordination ability and diverse coordination forms. Carboxylic MOFs linkers usually contain one or more carboxylic acid functional groups, and among them, multicarboxylate linkers containing two or more carboxylic acid functional groups have become a research hotspot in the field of MOFs. Because the incorporating multicarboxylic acid groups into MOFs can enhance stability and rigidity of MOFs through higher connectivity with metal nodes. Today, carboxylic MOFs linkers, especially multicarboxylate linkers, are widely used in MOF synthesis. Their special structure and properties give MOFs more application prospects. A large number of MOFs have been successfully constructed by them and show great application potential in catalysis, gas storage and separation, energy, etc.
Fig. 1. Multicarboxylate linkers lead to more robust metal-organic frameworks as a result of higher connectivity.
Classification
Depending on the number of carboxylic groups, common types of carboxylic MOFs linkers are ditopic carboxylate linkers, tricarboxylate linkers, tetracarboxylate linkers, etc.
- Ditopic carboxylate linkers: Ditopic carboxylate connectors have been used extensively for the synthesis of MOFs because of their ready accessibility and well defined structures in combination with secondary building units (SBUs). Ditopic carboxylate linkers can form a variety of structural units of MOFs such as 4-connected paddle-wheel clusters, octahedral clusters, trigonal prismatic clusters, 12-connected clusters, and infinite chain clusters [1]. Fig. 2. shows some representative ditopic carboxylate linkers.
Fig. 2. Some representative ditopic carboxylate linkers.
- Tricarboxylate linkers: Tricarboxylate linkers are good building units used for the construction of many MOFs. One of the most representative tricarboxylate linkers is 1,3,5-Benzenetricarboxylic acid (BTC). Many representative MOFs materials have been synthesized based on it. For examples, HKUST-1 (also known as MOF- 199) with SBET= 692.2 m2 /g was synthesized with BTC as linker and a dicopper paddle wheel SBU as the metal node. MIL-100 (Cr) was also synthesized with BTC as linker and Cr3O(CO2)6 as the metal node. In addition to BTC, some representative tricarboxylate linkers are shown in Fig. 3.
Fig. 3. Some representative tricarboxylate linkers.
- Tetracarboxylate linkers: This type of linker is also good building units used for the MOFs, particularly the linkers with tetrahedral geometry. Because they possess full Td symmetry, the highest symmetry in a linker that is currently attainable through organic synthesis. And high-symmetry building units are preferred in the construction of MOFs for some reasons such as expediting the packing process of repeating units during the formation of crystallites and constructing framework materials with wide channels and/or large pores. In addition, aromatic tetracarboxylate linkers are also particularly welcomed. Because the greater number of coordinating sites as well as the rich π-electron density, which give MOFs with better interactions with gas molecules, and make MOFs with tunable porosity by inducing guest-responsive structural dynamicity [2].
Fig. 4. Some representative aromatic tetracarboxylate linkers.
- Others: In addition to the most common categories mentioned above, there are pentacarboxylate linkers, hexacarboxylate linkers, octacarboxylate linkers and even dodecarboxylate linkers [3]. Among them, pentacarboxylate linkers and hexacarboxylate linkers are relatively used in synthesis of MOFs. For example, a series of isoreticular MOFs such as (PCN-61, -66, -69 and -610) were constructed using hexacarboxylate linkers [5,5′,5′′- benzene-1,3,5-triyltris(1-ethynyl-2-isophthalate) (BTEI) and 5,5′,5′′-(4,4′,4′′ nitrilotris(benzene-4,1-diyl)tris(ethyne- 2,1-diyl))triisoph-thalate (NTEI)] and other similar linkers with a dicopper paddle-wheel precausors. However, MOFs constructed using octacarboxylate linkers and dodecarboxylate linkers are rare as a result of synthetic challenges.
Fig. 5. Some representative pentacarboxylate linkers.
Alfa Chemistry offers a series of carboxylic MOFs linkers, including monomeric carboxylate linkers, ditopic carboxylate linkers, tricarboxylate linkers, tetracarboxylate linkers, pentacarboxylate linkers, hexacarboxylate linkers, octacarboxylate linkers, and even dodecarboxylate linkers. 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:
- Bull O. S., et al. A Review on Metal-Organic Frameworks (MOFS), Synthesis, Activation, Characterisation, and Application[J]. Oriental Journal of Chemistry, 2022, 38(3): 490.
- Bhattacharyya S., et al. Tetracarboxylate Linker-Based Flexible CuII Frameworks: Efficient Separation of CO2 from CO2/N2 and C2H2 from C2H2/C2H4 Mixtures[J]. ACS omega, 2018, 3(2): 2018-2026.
- Ghasempour H., et al. Metal–organic frameworks based on multicarboxylate linkers[J]. Coordination Chemistry Reviews, 2021, 426: 213542.
