Organoboron compounds as Lewis acid receptors of fluoride ions in polymeric membranes

Newly synthesized organoboron compounds – 4-octyloxyphenylboronic acid (OPBA) and pinacol ester of 2,4,6-trifluorophenylboronic acid (PE-PBA) – were applied as Lewis acid receptors of fluoride anions. Despite enhanced selectivity, the polymer membrane electrodes containing the lipophilic receptor OPBA exhibited non-Nernstian slopes of the responses toward fluoride ions in acidic conditions. Such behavior was explained by the lability of the B–O bond in the boronic acids, and the OH−/F− exchange at higher fluoride content in the sample solution. In consequence, the stoichiometry of the OPBA–fluoride complexes in the membrane could vary during the calibration, changing the equilibrium concentration of the primary anion in membrane and providing super-Nernstian responses. The proposed mechanism was supported by 19F NMR studies, which indicated that the fluoride complexation proceeds more effectively in acidic solution leading mainly to PhBF3− species. Finally, the performances of the membranes based on the phenylboronic acid pinacol ester, with a more stable B–O bond, were tested. As it was expected, Nernstian fluoride responses were recorded for such membranes with worsened fluoride selectivity.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Organoboron Lewis acid receptors for fluoride anions in ion-selective electrodes. ► Super-Nernstian potentiometric response of PVC membrane based on phenylboronic acid. ► Mechanism of the electrode response proposed and supported by 19F NMR studies. ► Variation of the stoichiometry of the boronic acid/fluoride complexes in membrane. ► Nernstian response measured for membranes based on phenylboronic acid ester.