Preparation of a highly fluorophilic phosphonium salt and its use in a fluorous anion-exchanger membrane with high selectivity for perfluorinated acids

Abstract

Fluorous solvents are the most nonpolar, nonpolarizable phases known, whereas ions are inherently polar. This makes it difficult to create salts that are soluble in a fluorous solvent. Here we present the synthesis and characterization of a new fluorophilic phosphonium salt, tris{3,5-bis[(perfluorooctyl)propyl]phenyl}methylphosphonium methyl sulfate. The salt has a solubility of at least 14 mM in perfluoro(perhydrophenanthrene), perfluoro(methylcyclohexane), and perfluorohexanes. It also shows immediate potential for use as a phase-transfer catalyst in fluorous biphasic catalysis, but in this work it is used as an anion exchanger site in the first potentiometric fluorous-membrane anion-selective electrode. The membrane sensor exhibited the exceptional selectivity of 3.9 × 1010 to 1 for perfluorooctanesulfonate over chloride, and of 2.5 × 107 to 1 for perfluorooctanoate over chloride. With improvements to the sensor’s detection limit and lifetime, it has the potential to be an attractive alternative to the expensive, time-consuming methods currently employed for measurement of perfluorinated acids.

Cleaning of pH Selective Electrodes with Ionophore‐doped Fluorous Membranes in NaOH Solution at 90 °C

“…To what extent this unique method of cocktail preparation is specific to membranes with ionophore 1 and perfluoropolyether 3 is unclear at this point. However, it is noteworthy that similar problems of sub‐Nernstian responses caused by ineffective membrane preparation procedures were not observed in our previous work with perfluoro(perhydrophenanthrene) as the fluorous sensing matrix instead of the perfluoropolyether . This may be due to a somewhat lower solubility of the ionophore in the linear perfluoropolyether than in perfluoro(perhydrophenanthrene).…”

Fluorous Polymeric Membranes for Ionophore-Based Ion-Selective Potentiometry: How Inert Is Teflon AF?

“…Unlike in our previous experiments with perfluorocarbons of low molecular weight as fluorous matrixes, no porous support to hold the sensing phase was used. 7,8,18,19 However, the poor solubility of 3 in the linear perfluorooligoether (0.19 mM, as determined by 1 H NMR spectroscopy) resulted in an unacceptably high electrical resistance and a poor reproducibility of the potentiometric responses. Based on the hypothesis that a salt consisting of tetrakis[3,5-bis(perfluorohexyl)phenyl]borate as anion and an ionophore complex as cation would be more soluble in the fluorous phase than the sodium salt 3 , it was decided to perform all subsequent experiments with ionophore-doped membranes.…”

“…If there were no ionizable groups on Teflon AF2400, these membranes would respond to anions, as previously demonstrated for fluorous membranes made of the same cationic site and perfluoroperhydrophenanthrene as matrix. 18 Instead, the membranes with Teflon AF2400 give a Nernstian pH response in the range of pH 1.6 to 4.2 (for a graph, see Figure S1 in Supporting Information). This is the well-known behavior of potentiometric membranes that contain cationic sites and functional groups acting as negatively charged ionophores 37,42,43 for H + , as it is expected for carboxylate groups.…”

“…The first cation exchanger-based ISE with a fluorous membrane was shown to exhibit a selectivity range 8 orders of magnitude wider than for conventional sensors, 7,8 and an anion sensor gave the selectivity of 3.9×10 10 to 1 for perfluorooctanesulfonate over Cl − . 18 Also, a series of fluorophilic ionophores for H + have been used recently for the first ionophore-based ISEs with fluorous sensing membranes. 19 However, the perfluoroperhydrophenanthrene-based matrix of this first generation of fluorous membrane ISEs had only a limited mechanical stability.…”

Syntheses of fluorous quaternary ammonium salts and their application as phase transfer catalysts for halide substitution reactions in extremely nonpolar fluorous solvents

“…13 Yasuda, Sakakura, Stuart, Horvá th, Bü hlmann, and ourselves have prepared a number of fluorous quaternary aliphatic phosphonium halides and triflates (XIII-XVI). [14][15][16][17] Those grouped in XIII have been shown to be highly soluble in perfluoromethylcyclohexane at elevated temperatures. 15 With the triarylphosphonium salt XIV, solubilities exceed 0.014 M in FC-72, perfluoromethylcyclohexane, and perfluoroperhydrophenanthrene at room temperature.…”