Phosphonium ionic liquids have great promise because previous studies have shown that they have high electrochemical stability. Phosphonium ionic liquids, with the generic formula [PR4]+, have four substituents on the phosphonium cation. Variations in the substituents and available anions represent a large number of possible salts (Figure.1).
Figure 1. The structure of phosphonium ionic liquids
Phosphonium ionic liquids possess high thermal stability, while their C2 protons tend to make them slightly acidic, which can lead to carbene formation. Alkylphosphine salts are generally less dense than water, which may be beneficial for product post-treatment steps involving aqueous layers containing inorganic salt by-products. For these reasons, phosphonium ionic liquids are often used as phase transfer catalysts, electrochemical media, epoxy resin cured strippers for montmorillonite clay catalysts, and they are also used in high temperature polymer carbonate reactions.
Applications
Electrolyte: Phosphonium ionic liquids can be applied as battery electrolyte, solar cell electrolytes, and super-capacitor electrolytes. For example, a binary ionic liquid system consisting of P222(2O1)-TFSI and lithium salt exhibits favorable transport properties and high thermal stability as an electrolyte of a lithium secondary battery. A lithium battery cell containing a P222(2O1)-TFSI electrolyte was found to exhibit higher charge-discharge performance than a lithium battery containing other ammonium ionic liquid electrolyte due to the low viscosity and high conductivity of the phosphonium ionic liquid electrolyte.
- Solvent: Since the environmental issues become very important, solution phase chemistry is an important research topic in green chemistry. Phosphonium ionic liquid plays an important role in it. Phosphonium ionic liquid can be used as a solvent in many important reactions, such as Heck cross-coupling reaction, Suzuki cross-coupling reaction, Diels–Alder reaction, Grignard reaction and so on. For example, the Suzuki cross-coupling reaction is a method of linking an electrophile containing a sp2 carbon or a non-β-hydride with a boronic acid derivative, which is widely used in organic chemistry. In the Suzuki cross-coupling reaction, the ionic liquid [P6,6,6,14][Cl] is used for Suzuki cross-coupling of an arylamide with a boronic acid derivative. In the reaction of figure 3, a soluble palladium catalyst precursor such as Pd2(dba)3·CHCl3 needs to be dissolved in phosphonium ionic liquid to produce a deep orange solution. The solution is stable in the absence of oxygen for an extended period of time and can be recycled after extraction of the reaction product. The system has numerous advantages, including mild and economical reaction conditions, and readily available aryl chloride reactivity.
Figure 2. The structure of P222(2O1) cation
Figure 3. Suzuki cross-coupling reaction using phosphonium ionic liquids as solvent
References
- Tsunashima, K.; et al. (2008). “A lithium battery electrolyte based on a room-temperature phosphonium ionic liquid.” Chemistry letters 37(3), 314-315.
- McNulty, J.; et al. (2002). “Suzuki cross-coupling reactions of aryl halides in phosphonium salt ionic liquid under mild conditions.” Chemical Communications (17), 1986-1987.
