Hill Anthony F, Ward Jas S
Research School of Chemistry, The Australian National University, Canberra, ACT 2601, Australia.
Dalton Trans. 2017 Jun 6;46(22):7291-7308. doi: 10.1039/c7dt01023k.
The reaction of BrHB·SMe with primary and secondary phosphines affords a range of boronium salts of the form [HB(PR)]Br (PR = PHCy1; PHPh, 2; PHCy, 3), which have been fully characterised including solid-state determinations. Reactions of bulky tertiary phosphines, e.g., PCy and PPh, with BrHB·SMe do not proceed beyond the phosphine-stabilised bromoborane adducts, however, the smaller tertiary phosphine PMePh readily proceeds to form [HB(PMePh)]Br (4). The formation of the unsymmetrical boronium salts [HB(PHCy)(PHCy)]Br (5) and [HB(PHCy)(PHPh)]Br (6) was observed by in situ NMR spectroscopy, however, the compounds were found to spontaneously disproportionate to their respective homophosphine boronium cations, even on prolonged storage in solution at -78 °C. Di- and triphosphines were found to form ring-closed boronium salts to afford [HB(κ-P,P'-diphosphine)]Br (diphosphine = dppe, 7; dcpe, 8; dmpe, 9; dppf, 10; dppf with [AsF] counterion, 11; amphos, 12). The analogous methodology with BrHB·SMe proved less generally applicable due to an accessible decomposition pathway being available to boronium salts bearing primary and secondary phosphines, leading to the formation of phosphonium salts, although [BrHB(dcpe)]Br (13), [BrHB(PMePh)]Br (14) and [BrHB(amphos)]Br (15) were synthesised with varying degrees of success. Reaction of BrHB·SMe with diphars afforded the boronium salt [HB(κ-P,P'-diphars)]Br (16), which featured two pendant arsine arms. Similarly, triphos was found to react with BrHB·SMe to give [HB(κ-P,P'-triphos)]Br (17), which featured a pendant phosphine arm. Substitution of the bromide counter anion with either hexafluoroarsenate or hexfluoroantimonate anions revealed weak hydrogen bonds between the P-H bonds of the boronium cations and the anions, that appeared through NMR studies to be retained in solution (where hydrogen bonding order was determined to be Br > [SbF]/[AsF]). This was further demonstrated by comparison of solid-state structures and solution NMR data of 1 with [HB(PHCy)][SbF] (18), 4 with [HB(PMePh)][AsF] (19) and 17 with [HB(triphos)][AsF] (20).
BrHB·SMe与伯膦和仲膦反应可得到一系列形式为[HB(PR)]Br的硼鎓盐（PR = PHCy₁；PHPh，2；PHCy，3），这些盐已得到充分表征，包括固态测定。然而，体积较大的叔膦，如PCy和PPh，与BrHB·SMe的反应不会超过膦稳定的溴硼烷加合物，不过，较小的叔膦PMePh很容易反应生成[HB(PMePh)]Br（4）。通过原位核磁共振光谱观察到了不对称硼鎓盐[HB(PHCy)(PHCy)]Br（5）和[HB(PHCy)(PHPh)]Br（6）的形成，然而，发现这些化合物即使在-78°C的溶液中长时间储存也会自发歧化为各自的同膦硼鎓阳离子。发现二膦和三膦会形成闭环硼鎓盐，得到[HB(κ-P,P'-二膦)]Br（二膦 = dppe，7；dcpe，8；dmpe，9；dppf，10；带有[AsF]抗衡离子的dppf，11；amphos，12）。由于带有伯膦和仲膦的硼鎓盐存在易于发生的分解途径，导致形成鏻盐，因此用BrHB·SMe的类似方法证明适用性较差，不过，[BrHB(dcpe)]Br（13）、[BrHB(PMePh)]Br（14）和[BrHB(amphos)]Br（15）已不同程度地成功合成。BrHB·SMe与二胂反应得到硼鎓盐[HB(κ-P,P'-二胂)]Br（16），其具有两个悬垂的胂臂。同样，发现三膦与BrHB·SMe反应生成[HB(κ-P,P'-三膦)]Br（17），其具有一个悬垂的膦臂。用六氟砷酸根或六氟锑酸根阴离子取代溴离子抗衡阴离子后，发现硼鎓阳离子的P-H键与阴离子之间存在弱氢键，通过核磁共振研究表明在溶液中得以保留（其中氢键顺序确定为Br > [SbF]/[AsF]）。通过比较1与[HB(PHCy)][SbF]（18）、4与[HB(PMePh)][AsF]（19）以及17与[HB(三膦)][AsF]（20）的固态结构和溶液核磁共振数据进一步证明了这一点。
