Phosphonium Boranes for the Selective Transport of Fluoride Anions across Artificial Phospholipid Membranes

Abstract

With the view of developing selective transmembrane anion transporters, a series of phosphonium boranes of general formula [p‐RPh2P(C6H4)BMes2]+ have been synthesized and evaluated. The results demonstrate that variation of the R group appended to the phosphorus atom informs the lipophilicity of these compounds, their Lewis acidity, as well as their transport activity. Anion transport experiments in POPC‐based large unilamellar vesicles show that these main‐group cations are highly selective for the fluoride anion, which is transported more than 20 times faster than the chloride anion. Finally, this work shows that the anion transport properties of these compounds are extremely sensitive to the steric crowding about the boron atom, pointing to the crucial involvement of the Group 13 element as the anion binding site.

Selective Fluoride Transport in Subnanometer TiO2 Pores

“…25 Inspired by the ultrahigh selectivity of the biological fluoride channels, researchers attempted to construct artificial nanopores mimicking fluoride channels in order to develop advanced technologies for precise F − separation and detection. 31,32 For instance, a fluoride-selective membrane would be highly desirable for the defluorination of fluoride-contaminated groundwater using electrodialysis or electrosorption. 33,34 Selective F − transport can be achieved by carefully tuning the chemistry of subnanometer pores or channels.…”

“…Specifically, F – ions, with their large hydration energy and hydrated size, permeate more slowly in synthetic membranes due to the higher energy penalty required for fluoride ion dehydration at the pore entrance compared to that for other halide anions. − The rapid, facilitated transport of F – ions in biological systems is attributed to their favorable interaction with phenylalanine moieties within the subnanometer fluoride channels . Inspired by the ultrahigh selectivity of the biological fluoride channels, researchers attempted to construct artificial nanopores mimicking fluoride channels in order to develop advanced technologies for precise F – separation and detection. , For instance, a fluoride-selective membrane would be highly desirable for the defluorination of fluoride-contaminated groundwater using electrodialysis or electrosorption. , …”

“…For example, it was reported that strapped calix[4]­pyrroles molecules can bind F – ions and facilitate their permeation through lipid bilayers via hydrogen bonding, resulting in a selectivity of 1.8:1 over Cl – . In other studies, incorporating Lewis acids (e.g., tetraarylstibonium cations and phosphonium boranes) into lipid bilayers was demonstrated to facilitate the complexation of F – ions (Lewis base) and transport them across the lipid bilayers with a selectivity over Cl – of ∼20:1. Despite these achievements using lipid bilayers, the selective transport of F – ions in solid-state nanopores, a more robust platform for practical separations compared to lipid bilayer-inserted channels, has not been well established.…”