Pyrazole Ligands, Imidazole Ligands, Triazole Ligands

A pyrazole ligand refers to a ligand having one or more pyrazole rings in its molecular structure. An imidazole ligand refers to a ligand having one or more imidazole rings in its molecular structure. A triazole ligand refers to a ligand having one or more triazole rings in its molecular structure. Pyrazole ligands, imidazole ligands and triazole ligands all belong to azole ligands. The NH on the ligand causes complex hydrogen bonds in the system, and the positional heterogeneity of the hydrogen bond produces several different conformations. On the other hand, the imino group is weakly acidic in solution, and under certain conditions (such as under alkaline conditions), protons are released, so that the remaining N atoms can also participate in the coordination, that is, there are two coordination sites. Therefore, the pyrazole ligand, the imidazole ligand, and the triazole ligand can form a structurally rich coordination compound with the metal, respectively. The metal complexes of different structures have different properties, so their applications are also very extensive.

Figure 1. Pyrazole ligand

Figure 2. Triazole ligand

Applications:

Due to the specificity of the structure, pyrazole ligands, imidazole ligands and triazole ligands have wide applications in biomedicine, agriculture, material chemistry, organic synthesis and analytical chemistry.

• Material chemistry: Metal complexes of pyrazole ligands with transition metal ions (such as Ni²⁺, Cu²⁺, Fe²⁺, Co²⁺, Zn²⁺) provide the most simple magnetic interactions with unpaired electrons and have a wide range of ferromagnetism or diamagnetism, so can be used to prepare magnets. The complex of pyrazole ligand and zinc has strong fluorescent properties and can be used for preparing fluorescent materials such as fluorescent probes, fluorescent lamps and fluorescent decorations. The complex formed by the triazole ligand and Fe(II) also has ferromagnetism and can also be used for the preparation of magnetic materials. The color of the complex of the triazole ligand with some metals can also change with the change of the spin state. With this property, a new generation of molecular display devices can be developed. In addition, the 1,2,4-triazole complex has a high nitrogen content, and the complex formed contains a heterocyclic ring N, an amino group N and a carbonyl group O, and is an important component of the energy-containing complex, and can be used for the preparation of energetic materials.
• Organic synthesis: Pyrazole ligands have good catalytic effects with metal catalysts such as Rh, Ru, Ir, etc., and can be used in organic synthesis for catalytic reactions of various reactions, such as asymmetric hydrogenation, asymmetric reduction, asymmetric addition, formation reaction, Heck reaction, benzoin condensation reaction, coupling reaction, and the like.
• Biology and medicine: As an active part of the drug, the pyrazole ligand and the zinc complex can increase the activity of treating a new drug, have small side effects, and have other biological activities, such as antibacterial, anti-inflammatory, etc., and thus can be used for antibacterial drugs, anti-inflammatory drugs, and preparation of antineoplastic agents. The diverse biological activities of triazole ligands can be applied to the fields of antibacterial, antifungal, anti-inflammatory, antiviral, anticonvulsant, antidepressant, antitumor, antihypertensive, antiparasitic, hypoglycemic, and inhibitory enzyme activities. Pyrazoles have important biological activities and can also be used in the preparation of antibacterial agents.
• Adsorption separation: Coordination polymers constructed from imidazole ligands exhibit high thermal and chemical stability and have broad applications in gas adsorption and separation. The adsorption and separation of gases such as O₂ and CO₂ can be carried out by temperature-controlled selective adsorption.
• Agriculture: The complex formed by the triazole ligand and the metal affects the osmotic action of the cell membrane by inhibiting the synthesis process of the ergosterol biological compound necessary for the growth and metabolism of the fungus, and finally achieves the effect of causing the pathogenic fungus to lose pathogenic ability. Triazole fungicides are widely used in agriculture because of their wide variety of properties, such as safety, high efficiency, and broad spectrum of sterilization. The triazole ligand has herbicidal activity and can be used as a herbicide for weeding in farmland. The metal complex formed by the triazole ligand with certain metals also has the insecticidal ability, especially the introduction of the triazole group into the organic pesticide structure can not only delay the resistance of the original organophosphorus insecticide, but also can increase the insecticidal spectrum. In addition, triazole ligands have growth regulating effects on a variety of plants and can be used as plant growth regulators.

References

1. Guerrero, Miguel. (2016). " Synthesis and characterization of Pd(II), Pt(II), Cu(I), Ag(I) and Cu(II) complexes with N,O-hybrid pyrazole ligand." Polyhedron 119, 555-562.
2. Ismael, Mohamed. (2018), "Synthesis, Characterization, Modeling, and Antimicrobial Activity of FeIII, CoII, NiII, CuII, and ZnII Complexes Based on Tri-substituted Imidazole Ligand." Zeitschrift fuer Anorganische und Allgemeine Chemie 644(20), 1203-1214.
3. Wang, Ji-Ping. (2018), "Diverse architectures and luminescence properties of three low-dimensional Zn(II)/Cd(II) coordination polymers based on a pyridine-imidazole ligand." Inorganic Chemistry Communications 90, 29-33.
4. Moraes, Leonardo C. (2019), "1D coordination polymer based oncopper(II)-containing tetrameric 1,2,3-triazole ligandfrom click chemistry: Magnetic and catalytic properties." Inorganica Chimica Acta 489, 93-99.