Physicochemical properties of tri-n-butylalkylphosphonium cation-based room-temperature ionic liquids

The physicochemical properties of novel four tri-n-butylalkylphosphonium-based room-temperature ionic liquids (RTILs), tri-n-butylmethylphosphonium dimethylphosphate ([P(4,4,4,1)][DMP]), tri-n-butyl(2-hydroxymethyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P(4,4,4,2OH)][Tf2N]), tetra-n-butylphosphonium O,O'-diethylphosphorodithioate ([P(4,4,4,4)][DEPDT]), and tri-n-butyldodecylphosphonium 3,5-bis(methoxycarbonyl)benzenesulfonate ([P(4,4,4,12)][MCBS]), were examined in this study. All RTILs showed a favorable thermal decomposition temperature exceeding 560 K. Of these, [P(4,4,4,12)][MCBS] exhibited a fairly high thermal stability compared with common phosphonium cation-based RTILs reported to date. Interestingly [P(4,4,4,1)][DMP] formed an ionic plastic crystal phase within a range of 279-290 K, but that was not the case with [P(4,4,4,4)][DEPDT], which is similar in the cation and anion structures to the [P(4,4,4,1)](+) and [DMP](-). [P(4,4,4,2OH)][Tf2N] showed a relatively high conductivity of 0.48 mS cm(-1) at 303 K among the RTILs consisting of tri-n-butylalkylphosphonium cation and usual fluoroanion.