Synthesis and Anion Binding Properties of Multi-phosphonium Triarylboranes: Selective Sensing of Cyanide Ions in Buffered Water at pH 7

A series of mono-, di-, and triphosphonium-substituted
triarylboranes,
[Mes2BArP]I ([2]­I), [MesBArP2]­I2 ([3]­I2), and [BArP3]­I3 ([4]­I3)
(ArP = 4-(MePh2P)-2,6-Me2-C6H2), were prepared from the corresponding
neutral boranes 2a–4a. The crystal
structure of [4]­I3 determined by X-ray diffraction
study reveals
peripheral decoration of aryl groups with phosphonium moieties. The
anion affinity of the cationic boranes for fluoride and cyanide ions
was investigated by UV–vis absorption titrations in aqueous
solution. The triarylboranes, [2]­I–[4]­I3 bind both fluoride and cyanide ions in a DMSO/H2O (7:3 v/v) mixture with high binding constants (K). Comparison of the K values of triarylboranes
for fluoride reveals that fluorophilicity increases with the increasing
number of phosphonium moieties: [2]+ (K = 2.3 × 101 M–1) <
[3]2+ (3.6 × 105 M–1) < [4]3+ (1.0 × 107 M–1). A similar trend is also observed in the cyanide
binding, with K values that are greater by 2–4
orders of magnitude than those in the fluoride binding. These results
indicate an apparent additive effect of multiple phosphonium substitutions
on the Lewis acidity enhancement of triarylboranes. The triphosphonium
borane [4]­Cl3, a water-soluble form of [4]­I3, was further utilized in evaluating the anion
affinity in water. While [4]3+ is shown to
hardly bind fluoride in buffered water at pH 7, it binds cyanide with
a high binding constant (1.7 × 107 M–1).