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, [Mes(2)BAr(p)]I ([2]I), [MesBAr(2)(P)]I-2 ([3]I2), and [BAr3P]I-3 ([4]I-3) (Ar-P = 4-(mePh(2)P)-2,6-Me-2-C6H2), were prepared from the corresponding neutral boranes 2a-4a. The crystal structure of [4]I-3 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]I-3 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 x 10(-1)M(-1)) < [3](2+) (3.6 x 10(5) M-1) < [4](3+) (1.0 x 10(7) 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]Cl-3, a water-soluble form of [4]I-3, 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 x 10(7) M-1).