Colorimetric turn-on sensing of fluoride ions in H₂O/CHCl₃ mixtures by pyridinium boranes

As part of our interest in chemistry of cationic boron compounds as anion receptors, we have synthesized 2-(4′-dimestylborylphenyl)-N-methyl-pyridinium triflate ([3]OTf) and 2-(5′-dimesitylborylthiophen-2′-yl)-N-methyl-pyridinium triflate ([4]OTf) by reaction of the corresponding pyridyl derivatives with MeOTf. Both [3]OTf and [4]OTf extract fluoride ions biphasically (H₂O/CHCl₃) to form the zwitterionic fluoroborates 3-F and 4-F, which have been structurally characterized. Remarkably, fluoride binding induces a red-shift of the low energy absorption band of these boranes (from 319 nm to 368 nm for 3/3-F and from 355 nm to 430 nm for 4/4-F) leading to the appearance of a yellow color. The origin of this turn-on colorimetric response is attributed to an intramolecular charge transfer process involving the triorganofluoroborate moiety as the donor and the pyridinium moiety as the acceptor.