文摘
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鈥?b>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鈥搗is absorption titrations in aqueous solution. The triarylboranes, [2]I鈥揫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 脳 101 M鈥?) < [3]2+ (3.6 脳 105 M鈥?) < [4]3+ (1.0 脳 107 M鈥?). A similar trend is also observed in the cyanide binding, with K values that are greater by 2鈥? 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 脳 107 M鈥?).
