Metal-organic frameworks (MOFs) are made of inorganic metal ions/clusters and organic ligands, in which the organic ligands act as a linker connecting the MOF skeleton, therefore, the organic ligands are also known as MOFs linkers. Special connections between metal ions/clusters and MOFs linkers lead to a theoretically unlimited number of MOFs with different structural and properties.
When designing MOFs, linkers design is one of the most important factors due to the unlimited possibility to design functional or multi-functional MOFs linkers as well as distinctive chemical properties of organic functional groups. On the one hand, MOFs linkers can enrich the host-guest chemistry between MOFs as host and other small molecules as guests, further expanding the applications of MOFs. Tuning and optimizing the host-guest chemistry for MOFs by MOFs linkers is an excellent, practical, and rational idea for improving the efficiency of MOFs in different applications. On the other hand, MOFs linkers has also crucial effects on the structural properties such as crystallinity, porosity, flexibility, stability, and topology of MOFs through induced structural changes and different types of secondary interactions. Therefore, judicious choice of MOFs linkers size, geometry, and connectivity can create diverse structures and topologies, which can aid in the quest to design MOFs with high-performance.
Fig.1 the The MOFs with various structures, porosity and topology synthesized depending on the kinds of MOFs linkers
The MOFs linkers can be classified based on their functional group into following six major categories including carboxylic MOFs linkers, nitrogen MOFs linkers, phosphorous MOFs linkers, sulfonic acid MOFs linkers, halogen MOFs linkers, hydroxy MOFs linkers and others.
- Carboxylate MOFs linkers: Carboxylate is widely applied in coordination chemistry to construct three-dimensional structures especially MOFs since it can chelate/coordinate to metal ions through deprotonation and formation of metal-carboxylate complexes. On the other hand, carboxylate group is hydrogen donor and hydrogen bond donor/acceptor site which can act as Brønsted acid and hydrogen bond participating site. Because of these characteristics, carboxylate is applied as both coordinating and guest-interactive site in the construction of MOFs. Generally, carboxylate linkers based MOFs have remarkably high surface area and uniform pore size distribution, highly ordered crystallinity, well-defined reticular chemistry, widely using in catalyst, gas adsorption, metal removal and separation, sensing.
- Nitrogen MOFs linkers: Nitrogen linkers are mostly employed in the structure of MOFs. Considering the chemical properties and function skeleton of this major group of MOFs, nitrogen-based linkers are classified in four categories: (I) heterocyclic azine N-based linkers, (II) heterocyclic-azole N-based linkers, (III) noncyclic N-based linkers, and (IV) ionic N-based linkers. MOFs fabricated from nitrogen linkers offer several distinct advantages, in addition to the features carried by the crystalline porous coordination networks such as control over architectures, tunable pore chemistry. Nitrogen linkers based MOFs have found suitability for applications in a wide span such as molecular sieving, gas/solvent storage, magnetism, catalysis, capture of priority pollutants.
Fig.2 The wide range of applications using Nitrogen-based MOFs
- Phosphorous MOFs linkers: Phosphorous linkers are applied in the synthesis of MOFs more than sulfonate yet less than carboxylate. Phosphorous linkers based MOFs are less soluble with strong PO-M bond, increasing their chemical and thermal stability against heat, air, and humidity. And phosphorous groups can provide polar porosity through their free (PO) groups in the pore walls of MOFs which provide strong interaction with ionic/polar/quadrupolar guests like CO2. Due to the above characteristic, phosphorous linkers based MOFs have been applied in proton conductor, gas adsorbent, and sensor, and can serve as luminescence sensor for detection of different analytes, especially when they are combined with electroactive metal centers.
- Sulfonic acid MOFs linkers: Sulfonic acid (–SO3H) is S-based function which is extensively applied in the structure of MOFs as coordinating and guest interactive site. The advantage of this linkers of functionalized MOFs is that they contain a soft and electron rich sulfur atom which is of benefit for designing electron rich and polar frameworks with soft guest interactive sites. And the sulfonic acid is a strong Brønsted acid, thus, sulfonic acid-based MOFs are applied as catalyst in acid catalyzed reactions.
Fig.3 Light-enhanced acid-catalytic reaction over sulfonic acid-based MOF
- Hydroxy MOFs linkers: Since hydroxy contains acidic hydrogen, hydroxy MOFs linkers can be used in designing proton conductive MOFs materials with highly hydrophilic channels and can act as hydrogen donor, hydrogen bond donor and catalytically active acidic site of MOFs. Also, because of its highly negative nature, it is Lewis base, electron donor with high polarity in a way that it can interact with different guests like hydrogen bond donors, Lewis acidic molecules, and molecules containing partially positive atoms. As a result, hydroxyl linkers based MOFs are designed and applied for different purposes such as improvement of CO2 capture and separation, designing efficient proton conductor frameworks, removal and sensing of hazardous chemicals with different mechanisms.
- Halogen MOFs linkers: Fluorinated organic linkers are usually in construction of MOFs compared to other halogen linkers. Only in some cases Cl and Br atoms are applied as substitutes in side chain of MOF aromatic skeleton to improve gas adsorption performance. Fluorine atom is highly electronegative and lowly polarizable, these two characteristics result in a very polar C-F bond with high density of negative charge on fluorine atom as well as hydrophobicity of this bond. As a result, fluorinated organic linkers can be applied in the structure of MOFs for improving the gas framework interaction through high polarity of C-F bond and increasing the water stability and hydrophobicity by tuning the pores with fluorine. In addition to hydrophobicity, fluorophilicity can be tuned as well to interact with special types of guests.
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