Transfer of highly hydrophilic ions between immiscible liquids at the three-phase junction using a squaramide-based ion pair receptor

In a recently published paper in Electrochimica Acta¹, Julia presents the results of her investigations of two new squaramide-based ion-pair receptors for ion transfer at a three-phase electrode. The compounds, which were synthesised by colleagues from Warsaw University had previously been used to transfer hydrophilic ions across a liquid-liquid interface, so we decided to try to evaluate them as a potential ionophores for very hydrophilic ions such as fluoride and sulphate. The compounds, lovingly called 5FSQC6 and CF3SQC6, consist of three functional parts: a cation-binding domain (benzo-18-crown-6 ether), an anion-binding domain (squaramide) and a unit assuring both the lipophilicity and increased acidity to the anion-binding domain (pentafluorophenyl or 3,5-bis(trifluoromethyl)phenyl groups).

In the paper we show that both 5FSQC6 and CF3SQC6 can carry fluoride across liquid-liquid interface at much lower potential than normally expected. This encouraged us to use 5FSQC6 in potentiometric measurements to determine the concentration of fluoride in a solution. These measurements were done in collaboration with Łukasz Górski from Warsaw University of Technology. Unfortunately, it turned out that the potentiometric signal was completely dominated by the signal from the cation. Tests with part of the ion-receptor without the cation-binding domain revealed that it is an integral part of the molecule’s function, because the addition of a cation to the crown ether further withdraws electron from the anion-binding squaramide.

Far from giving up, we hope that given the promising results from the voltammetric experiments, a different electron-withdrawing unit that does not bind cations might prove to be a more successful ionophore for fluorides.

1. J. Maciejewska-Komorowska, K. Peret, A. Kubas, M. Zaleskaya-Hernik, J. Romański, M. Karbarz, Ł. Górski, M. Jönsson-Niedziółka
   Transfer of highly hydrophilic ions between immiscible liquids at the three-phase junction using a squaramide-based ion pair receptor, Electrochim. Acta 517, 145778 (2025). (link)(Preprint on ChemRxiv).