Ammonium Boranes for the Selective Complexation of Cyanide or Fluoride Ions in Water

With the recognition of aqueous fluoride and cyanide ions as an objective, we have investigated
the anion binding properties of two isomeric ammonium boranes, namely [p-(Mes2B)C6H4(NMe3)]+ ([1]+)
and [o-(Mes2B)C6H4(NMe3)]+ ([2]+). These cationic boranes, which could be obtained by reaction of the
known 4- and 2-dimesitylboryl-N,N-dimethylaniline with MeOTf, have been investigated both experimentally
and computationally. They both react with fluoride and cyanide ions in organic solvents to afford the
corresponding fluoroborate/ or cyanoborate/ammonium zwitterions 1F, 1CN, 2F, and 2CN. In aqueous
solution, however, these cationic boranes behave as remarkably selective receptors. Indeed, [1]+ only
complexes cyanide ions while [2]+ only complexes fluoride ions. In H2O/DMSO 60:40 vol (HEPES 6 mM,
pH 7), the cyanide binding constant of [1]+ and the fluoride binding constant of [2]+ are respectively equal
to 3.9 (±0.1) × 108 and 910 (±50) M-1. Structural and computational studies indicate that both steric and
electronic effects contribute to the unusual selectivity displayed by these cationic boranes. Owing to favorable
Coulombic effects, the para-derivative [1]+ has a very high affinity for cyanide; yet these effects are not
sufficiently intense to allow complexation of the more efficiently hydrated and less basic fluoride anion. In
the case of the ortho-derivative [2]+, the proximity of the ammonium moiety leads to an increase in the
Lewis acidity of the boron center thus making fluoride binding possible. However, steric effects prevent
cyanide coordination to the boron center of [2]+. Finally, cation [1]+ and [2]+ bind their dedicated anions
reversibly and show a negligible response in the presence of other common anions including Cl-, Br-, I-,
NO3-, OAc-, H2PO4-, and HSO4-.