Synthesis and applications of lewis acids based on triarylboranes

Abstract

This thesis deals with Lewis acids based on triarylboranes for recognition of anions such as fluoride and cyanide ion. Various triarylborane compounds including not only electron-withdrawing groups and/or cationic substituents for enhancing Lewis acidity of triarylboranes in aqueous system but chromophore and/or fluorophore for efficient detection of anions from an analytical point of view were investigated. The triarylborane derivatives bearing closo -carborane and nido-carborane cage were prepared and characterized. The o-carborane-trarylborane dyad (closo -1) shows high fluoride ion affinity through the trigonal-planar boron center while degradation to nido-carborane leads to turn-on fluorescence response toward fluoride accompanying a vivid emission color change from blue to green. Photophysical data and TD-DFT studies suggest that the resulting nido-1 fluoresces at the apparently red-shifted region due to the greater elevation of HOMO level by degradation of carborane cage. A series of mono-, di-, tri-phosphonium substituted triarylboranes, [p-Mes2BArP]I ([2]I), [p-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 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 101 M-1) < [3]2+ (3.6 x 105 M-1) < [4]3+ (1.0 x 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 fluo...