Chemical stability and application of a fluorophilic tetraalkylphosphonium salt in fluorous membrane anion-selective electrodes

The highly fluorophilic phosphonium salt (R_f8(CH ₂)₂)(R_f6(CH₂)₂) ₃P⁺I^-, 1, was used to provide cationic sites for anion-selective electrodes based on fluorous sensing membranes. The electrodes exhibited the theoretically expected "Nernstian" response slopes, and their selectivities for different anions were determined. For environmental analysis, the selective detection of perfluorooctanesulfonate and perfluorooctanoate is of particular interest. While previously reported electrodes based on a fluorophilic methyltriarylphosphonium salt suffered from OH^- interference to the extent that OH^- selectivities could not be determined, and general base catalysis caused decomposition of the phosphonium sites in the presence of weakly basic anions, electrode membranes based on 1 are much more robust. A loss of sensor response is only observed when the fluorous membranes doped with 1 are exposed to 0.1 M hydroxide solutions for 24 h. NMR and mass spectrometry show that the fluorophilic tetraalkylphosphonium cation of 1 decomposes under these extreme conditions to give two trialkylphosphine oxides and perfluoroalkylethanes. Interestingly, this decomposition is much slower than the base catalyzed exchange of the hydrogens in α position to the phosphonium center, which in the presence of D ₂O results in the quantitative formation of the octadeuterated tetraalkylphosphonium cation. While formation of a pentacoordinated transition state in the decomposition of 1 is likely, no pentavalent complexes of the phosphonium ion with OH^- could be observed by NMR spectroscopy.