8.27 g (20.5 mmol) of tris(pentafluoroethyl)phosphine oxide, (C2F5)3P═O, are mixed with 20 cm3 of dry benzene and 1.67 g (20.3 mmol) of N-methylimidazole in a flask fitted with a reflux condenser held at minus 25° C. 4.37 g (20.4 mmol) of 3,3,4,4,5,5,5-heptafluoropentan-1-ol are added to this mixture at room temperature while stirring using a magnetic stirrer. The reaction mixture is heated at an oil-bath temperature of 100° C. for 20 minutes. After cooling to room temperature, the phosphinate is separated from the benzene phase and washed twice with 10 cm3 of benzene. Drying at 60° C. (2 hours) under a vacuum of 13.3 Pa gives 11.55 g of 1-(3,3,4,4,5,5,5-heptafluoropentyl)-3-methylimidazolium bis(pentafluoroethyl)phosphinate as liquid. The yield is 97.5%.
