Hexabromide salt of a tiny octaazacryptand as a receptor for encapsulation of lower homolog halides: structural evidence on halide selectivity inside the tiny cage
- 作者：M. Arunachalam ; Eringathodi Suresh ; Pradyut Ghosh
- 关键词：Octaazacryptand ; Halide selectivity ; Anion encapsulation
- 刊名：Tetrahedron
- 出版年：2007
- 期刊代码：107_00404020
- 类别：ch
- 出版时间：12 November 2007
- 卷：63
- 期：46
- 页码：11371-11376
- 文件大小：458 K
摘要
Tiny azacryptand 1,4,7,10,13,16,21,24-octaazabicyclo[8.8.8]hexacosane (L) upon reaction with 48%hydrobromic acid (containing <0.05%chloride contamination) forms hexabromide salt (1). Single crystal X-ray crystallographic investigation of the hexaprotonated bromide (1) shows no guest encapsulation inside the tiny cage. This bromide salt 1 with an empty proton cage has been utilized as the receptor for encapsulation of chloride (2) and fluoride (3). Crystallographic results of mixed chloride/bromide (2) and fluoride/bromide (3) complexes of L are examined, which show monotopic recognition of chloride in the case of 2 and fluoride in the case of 3 inside the proton cage with five bromide and three water molecules outside the cavity. Single crystals obtained from an experiment on mixed anionic system (chloride and fluoride), 1 shows selective encapsulation of fluoride, which supports the formation of complex 3 and crystals obtained upon treatment of 2 with tetrabutyl ammonium fluoride also yields complex 3. In a separate reaction between L and 49%hydrobromic acid containing higher chloride contamination (<0.2%) forms chloride/bromide salt (2). 1H NMR studies of 1 with sodium chloride and fluoride support the encapsulation of the respective anions inside the proton cage.