Triarylborane Lewis acids with indole or phenol group: B/H hybrid receptors for fluoride

Boron based fluoride anion receptors: Electrochemical and sensory applications

“…The sterically encumbered compound 7, preferentially binds to the relatively less sterically crowded fluoride anion and not to the relatively sterically crowded cyanide anion, whereas the corresponding para-isomer 9 binds to the cyanide anion as it lacks as much steric crowding at the boron center.On the other hand, the relatively lower coulombic effect afforded by the distant cationic site is not sufficient to allow complexation of the relatively more efficiently hydrated fluoride (and thus relatively less basic) anion in aqueous medium.38 Neighboring indole and phenol moieties enhance binding of fluoride to boron in case of the triarylboranes. For example, the fluoride binding constants for the triaryl boranes having a phenol or an indole moiety attached to the boron (10 and 11, respectively) are an order of magnitude higher than those for the triarylboranes lacking such substituents (Figure 10) 58. Proton NMR studies showed that in addition to the significant covalent bonding to the boroncenter, the B-F bond in the fluoride adduct is stabilized by the hydrogen-bonding interactions with the neighboring indolyl N-H (11-F) and and the phenolic O-H (10-F) bonds.…”