Organochalcogen substituted dimesityl borane-imidazoles as fluorogenic fluoride and cyanide sensors in aqueous media

Abstract

Natural and anthropogenic activities leading to excessive toxic biological anions in water have warranted the development of anion sensors, especially for cyanide (CN⁻) and fluoride (F⁻) ions in aqueous media. Herein, a contemporary synthetic route to embed organochalcogen compounds (–SPh and –SePh) in a boron-functionalized imidazole scaffold via metal halogen exchange is reported. Upon methylation, fluorogenic compounds 5-dimesitylboryl-2-phenylthio-1-methylimidazole (7) and 5-dimesitylboryl-2-phenylselone-1-methylimidazole (8) form the corresponding imidazolium salts, 5-dimesitylboryl-2-phenylthio-1-methylimidazolium iodide (9) and 5-dimesitylboryl-2-phenylselone-1-methylimidazolium iodide (10), which are non-luminescent. All of the compounds are thoroughly characterized, and their utility in sensing F⁻, CN⁻, and various other biologically relevant halides has been studied through UV-vis and fluorescence spectroscopy. The substrates and products of 1 : 1 binding of anions (F⁻/CN⁻) with the compounds have been established by spectroscopy, spectrometry, single-crystal X-ray crystallography, and first principles quantum mechanical theoretical calculations. The superior binding ability and micromolar sensitivity towards CN⁻ ions over other anions in aqueous media were elucidated. The reversibility of compound 7 was also tested and it was found that fluoride binding was reversible while cyanide binding was not.