Anionic chromogenic chemosensors highly selective for cyanide based on the interaction of phenyl boronic acid and solvatochromic dyes

Abstract

Solutions of two solvatochromic dyes, Reichardt's pyridinium N-phenolate betaine (1) and Brooker's merocyanine (2), are colored in acetonitrile but become colorless when phenyl boronic acid (PBA) is added due to the formation of covalently linked dye-PBA species. Of the various anions added, only F− and CN− caused the reappearance of the original color related to the free dye in solution. These anions act as nucleophiles in acetonitrile and displace the dye leaving group. On the addition of very small amounts of water the system becomes highly selective toward CN− because of the ability of water to strongly solvate F−, inhibiting its nucleophilic action, while CN− is sufficiently naked to react with dye-PBA species. The titrations of the dye-PBA complexes with CN− revealed that the shape of the curves is dependent on the experimental conditions. With the use of large amounts of PBA, the dye, after being displaced, interacts with the free PBA forming hydrogen-bonded complexes, which are disrupted with the addition of an excess of the anion. This was not observed in aqueous acetonitrile since water can form hydrogen bonds with the boronic acid and with the dye, preventing the formation of dye-PBA hydrogen-bonded complexes.