A study on fluoride detection and assembly of hydroxyaromatic aldoximes caused by tetrabutylammonium fluoride†
Abstract
Tetrabutylammonium fluoride (TBAF) forms cocrystal H2NAP·TBAF with 2-hydroxynaphthaldoxime (H2NAP) or cocrystal 2(H3OHPA)·TBAF with 2,3-dihydroxyphenylaldoxime (H3OHPA), whereas a similar reaction with 2,4-dihydroxyphenylaldoxime (H3PHPA) forms tetrabutylammonium (TBA) salt TBA(H2.5PHPA)2. Formation of these cocrystals or salts is accompanied by a color change which enables the detection of fluoride ions. Cocrystal H2NAP·TBAF has a layered structure formed by hydrogen bonds between the fluoride ions and parent oxime molecules; the tetrabutylammonium cations are held in between layers of anionic assemblies. On the other hand, cocrystal 2(H3OHPA)·TBAF has a grid-like architecture constructed from hydrogen bonds between the parent oxime molecules and fluoride ions. TBA cations are encapsulated within the grids. It is shown that salt TBA(H2.5PHPA)2 forms anionic assemblies to encapsulate tetrabutylammonium cations which are devoid of fluoride ions. By interactions of the tetrabutylammonium fluoride ions with H3PHPA molecules, an anionic assembly is formed by the sharing of protons, which possesses a grid-like structure. The formation of such an assembly causes color change which enables one to detect fluoride ions by visual means.