Introduction
Metal-organic frameworks (MOFs) ligands are a class of organic molecules used to synthesize MOFs materials (a kind of hybrid porous materials with unique physical and chemical properties), which bridge with metal ions or metal clusters to form MOFs material with one-dimensional, two-dimensional or three-dimensional infinite network. The obtained MOFs show a wide field of applications such as gas storage and separation, catalysis, biomedicine, sensing, supercapacitors, fuel cells and so on. The commonly used MOFs ligands mainly include carboxylic acids, sulfonic acid, phosphonic acid, N-Heterocyclic, porphyrin, etc. Based on these diverse and rich number of ligands, many new structures and unique properties of MOFs have been synthesized, and the types and numbers are also increasing.
The Importance of Ligands In MOFs
Ligands have important influence on the structure, properties and applications of MOFs. First, the robustness of coordination bond between ligands and metal ions or clusters determines the overall skeleton stability of MOFs. And by adjusting the ligand can change the distance between the ligand and nearby metal atoms, thus improving the overall skeleton stability of MOFs. Secondly, the type of functional group on the ligand determines the morphology, size, specific function and specific pore size distribution of MOFs, thus affecting their applications. Most importantly, ligands largely determine the physical properties of MOFs.
Types of MOF Ligands
In the synthesis of MOFs, there are many types of ligands, including carboxylic acid ligands, nitrogen-containing ligands, sulfonic acid ligands, phosphorous acid ligands, hydroxy ligands, carboxylated nitrogen hybrid ligands, halogen ligands, porphyrin ligands, ions ligands and many others. Here, some typical MOF ligands are briefly introduced.
- Carboxylic Acid Ligands: Carboxylic acid ligands containing one or more carboxylic acid functional groups are one of the most popular ligands in synthesis of MOFs due to their diverse types and strong coordination ability with metal centers. They can be used to synthesize many MOFs with large pore sizes and diverse structures, and can be used to synthesize one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) MOFs. Moreover, flexible carboxylic acid ligands that may be distorted or deformed with the metal center during assembly can be used to construct attractive MOF structures [1].
Fig. 1. Some representative carboxylic acid ligands.
- Nitrogen-containing Ligands: In the synthesis of MOFs, most of nitrogen-containing ligands are N-heterocyclic compounds. The nitrogen atoms in N-heterocyclic ligands can form rigid coordination bonds with transition metal atoms, and thus the formed MOFs usually have high thermodynamic stability and relative kinetic inertia, which can be prepared under harsh conditions, such as hydrothermal synthesis or solvothermal synthesis [2]. In addition, N-heterocyclic ligands can also be employed as proton acceptors or proton sources, and thus the formed MOFs with proton conductive property, showing widespread application in solid electrolytes in fuel cells.
Fig. 2. Some representative N-heterocyclic ligands.
- Sulfonic Acid Ligands: Although sulfonate has a less coordinated tendency to metal centers, stricter synthetic conditions, and more hydrophobic characteristics, sulfonic acid ligands are still widely concerned and applied by MOFs researchers. This because sulfonic acid ligands are easy to deprotonate and form hydrogen bonds with hydrogen donors due to their multiple potential oxygen coordination sites. This makes the coordination mode between sulfonic acid ligands and metal ions more flexible, and is conducive to the synthesis of novel and high-dimensional MOFs.
Fig. 3. Structure of a sulfonic acid ligand.
- Hydroxy Ligands: Hydroxy ligands are also an interesting class of MOFs ligands. Because of their polarity and weak acidity, hydroxy MOFs ligands can be used in designing proton conductive MOFs materials with highly hydrophilic channels and MOFs for acid catalysis.
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References
- Sun Q., et al. Constructing functional metal-organic frameworks by ligand design for environmental applications[J]. Journal of Hazardous Materials, 2023, 447: 130848.
- Han B. X., et al. Proton conductive N-heterocyclic metal–organic frameworks[J]. Coordination Chemistry Reviews, 2021, 432: 213754.
