Ionic Liquids and Phase Transfer Catalysis

The traditional use for phosphonium salts is phase transfer catalysis (PTC). While they are typically more expensive than the corresponding ammonium salts, they have the advantage of being more thermally and chemically stable at elevated reaction temperatures. A good example where phosphonium salts out-perform their ammonium counterparts is the biphasic conversion of aromatic chlorides to fluorides using KF at temperatures exceeding 130ºC.

Phosphonium salts also find utility as catalysts in epoxy curing and as catalyst in high temperature polycarbonate systems.

Such "catalytic" phosphonium salts are available under the CYPHOS^® trademark as phosphonium salt. Typical examples are CYPHOS 443 and 442 - tetrabutylphosphonium chloride and bromide and CYPHOS 3453 and 3472 - tributyl(tetradecyl)phosphonium chloride and tributyl(hexadecyl)phosphonium bromide.

Prior to the current interest in using low melting or room temperature, liquid nitrogen based salts as "ionic liquids", tetrabutylphosphonium bromide in the molten state was already reported to be a useful and unique solvent for several large scale petrochemical applications such as hydroformylation and carbonylation. Lately many low melting phosphonium salts have been developed specifically for "ionic liquid" applications either as reaction media or for their unique fluid properties. For a given anion, phosphonium salts are more thermally stable than the corresponding ammonium salts. They have wide electrochemical windows and unlike imidazolium salts, they are compatible with strongly electropositive metals such as sodium, potassium and magnesium.

Phosphonium based "ionic liquids" are available under the CYPHOS^® trademark. They include salts with the traditional halide anions such as CYPHOS IL 101 and 102 - trihexyl(tetradecyl)phosphonium chloride and bromide which are liquid at room temperature and have glass transition temperatures as low as -65ºC. Salts containing many other anions such as tosylate, dicyanamide, methylsulfate diethylphosphate, phosphinate, bistriflamide, tetrafluoroborate and carboxylates are also available.