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).