Fluoride Ion Capture from Water with a Cationic Borane

The reaction of the [Li(THF)4]+ salt of dimesityl-1,8-naphthalenediylborate with [Me2NCH2]I affords a borane (1-(Mes2B)-8-(Me2NCH2)-C10H6) which can be converted into a cationic borane [3]+ ([1-(Mes2B)-8-(Me3NCH2)-C10H6]+) by methylation with MeOTf. This cationic borane promptly complexes fluoride to afford the corresponding zwitterionic ammonium/fluoroborate 3-F (1-(Mes2FB)-8-(Me3NCH2)-C10H6). Cation [3]+ fails to react with chloride, bromide, and iodide indicating that fluoride complexation is selective. Structural, spectroscopic, and computational studies carried out on 3-F show the existence of an unusual C−H···F−B hydrogen bond. Remarkably, [3]+ captures fluoride from water under biphasic conditions (H2O/CHCl3) to form 3-F. The high fluoride affinity of [3]+ can be correlated to the Coulombic forces which stabilize the B−F bond against heterolysis.

二均三甲苯基-1,8-萘二基硼酸根(dimesityl-1,8-naphthalenediylborate)的四(四氢呋喃)合锂(Ⅰ)阳离子盐([Li(THF)4]+)与(碘甲基)二甲基胺([Me2NCH2]I)反应，制得硼烷化合物1-(Mes2B)-8-(Me2NCH2)-C10H6；该化合物可通过三氟甲磺酸甲酯(MeOTf，methyl trifluoromethanesulfonate)进行甲基化，转化为阳离子型硼烷[3]+ ([1-(Mes2B)-8-(Me3NCH2)-C10H6]+)。该阳离子硼烷可快速与氟离子配位，得到对应的两性离子型铵/氟硼酸盐3-F（1-(Mes2FB)-8-(Me3NCH2)-C10H6）。阳离子[3]+无法与氯离子、溴离子及碘离子发生反应，表明其对氟离子的配位具有选择性。对3-F开展的结构、光谱及计算研究表明，体系中存在一种非典型的C−H···F−B氢键(C−H···F−B hydrogen bond)。值得注意的是，[3]+可在两相（水/三氯甲烷）条件下从水中捕获氟离子，生成3-F。[3]+对氟离子的高亲和性可归因于能够稳定B−F键以抵御异裂的库仑力(Coulombic forces)。