Phosphonium-Based Ionic Liquids

Ionic liquids refer to salts that are mainly composed of organic cations and inorganic or organic anions that are liquid at or near room temperature. They have many unique properties that other solvents cannot match. Compared with traditional liquid substances, ionic liquids are entirely composed of ions. Compared with conventional pure ionic substances, it is liquid at room temperature, has almost zero vapor pressure, is colorless and odorless, has better chemical stability, a wider electrochemical stable potential window and temperature stability range. There are many types of anions and cations in ionic liquids. Common cations include quaternary ammonium salts, phosphonium-based ionic liquids, pyridinium salts, imidazole salts, and pyridinium salts. Among them, low-melting phosphonium-based ionic liquids are widely known as bacteriostatic agents, phase transfer catalysts and wittig reagents. As a green reaction medium, phosphonium-based ionic liquids has received many applications and attentions.

Figure 1. The general chemical structure formula of Phosphonium-Based Ionic Liquids

Applications:

• Catalyst: Phosphonium-based ionic liquids has the advantages of a synthesis, low volatility, stable physical and chemical properties, and excellent catalytic function, and is widely used in Suzuki, Friedel-Crafts, Grignard, Kornblum, Heck, Diels-Alder , Hydroformylation, carbonylation, esterification and many other types of chemical reactions. Especially for some reaction types involving strong alkaline reaction conditions or electrophilic substitution, phosphonium-based ionic liquids have special advantages.
• Green solvents: Most traditional organic solvents are extremely volatile. Phosphonium-based ionic liquids have the characteristics of extremely low vapor pressure, good thermal stability, and low corrosivity and have been developed into a new type of solvent, which is called one of the three "green" solvents juxtaposed with supercritical CO₂ and aqueous two-phase.
• Antibacterial material: Phosphorus atom is an element of the fifth main group. It uses sp³ hybrid orbital to connect with other 4 alkyl carbons to form a tetrahedron with a relatively stable structure. In addition, the ionic radius of the phosphorus atom is large, which has a strong polarization effect, which leads to an increase in the positive charge of the surroundings. The increase in positive charge makes it easy to adsorb to the surface of negatively charged microorganisms and kill the microorganisms, with strong antibacterial properties.
• Electrochemistry: Smaller phosphonium-based ionic liquids can be used as electrolytes for lithium batteries. The lithium battery electrolyte prepared with tetrabutylphosphorus chloride as the new phosphorus source and reaction medium has high discharge capacity.

Reference

• Sadegh Khazalpour, Meysam Yarie, Effat Kianpour, Ameneh Amani, Simin Asadabadi, Jaber Yousefi Seyf, Majid Rezaeivala, Saeid Azizian, Mohammad Ali Zolfigol. Applications of phosphonium-based ionic liquids in chemical processes[J].JOURNAL OF THE Iranian Chemical Society. 2020, 17, 1775–1917.