Syntheses of Tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl Catalysis, Countercation Effect, and Reaction Intermediates

Potassium tricyanofluoroborate, K­[BF­(CN)3], which is the starting material for tricyanofluoroborate room-temperature ionic liquids [N. Ignat’ev et al. J. Fluorine Chem., submitted] was obtained on a molar scale (140 g) from Na­[BF4] and (CH3)3SiCN with a purity of up to 99.9%. The initial product of the reaction that was catalyzed by (CH3)3SiCl was Na­[BF­(CN)3]·(CH3)3SiCN that was characterized by multinuclear NMR and vibrational spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Na­[BF­(CN)3]·(CH3)3SiCN was converted to K­[BF­(CN)3] via a simple extraction protocol. The catalytic effect of (CH3)3SiCl was evaluated and some intermediates of the reaction, including the isocyanoborate anion [BF­(NC)­(CN)2]−, were identified using multinuclear NMR and vibrational spectroscopy. K­[BF2(CN)2] also reacted with (CH3)3SiCN in the presence of (CH3)3SiCl, to result in K­[BF­(CN)3]. The interpretation of the experimental observations was supported by data derived from density functional theory (DFT) calculations. In addition, the influence of selected countercations of the tetrafluoroborate anion on the progress of the (CH3)3SiCl-catalyzed reaction was studied. The fastest reaction was observed for Na­[BF4], while the conversion of [BF4]− to [BF­(CN)3]− was slower with the countercation K+. Li­[BF4] and [Et4N]­[BF4] were converted under the reaction conditions applied to Li­[BF2(CN)2] and [Et4N]­[BF2(CN)2] only.三氰一氟硼酸钾（Potassium tricyanofluoroborate, K[BF(CN)3]）作为制备三氰一氟硼酸盐室温离子液体的起始原料[N. Ignat’ev等，《氟化学杂志》，已投稿]，以摩尔级规模（140 g）由四氟硼酸钠（Na[BF4]）与三甲基氰硅烷((CH3)3SiCN)合成，纯度最高可达99.9%。该反应以三甲基氯硅烷((CH3)3SiCl)为催化剂，初始产物为Na[BF(CN)3]·(CH3)3SiCN，研究人员通过多核核磁共振波谱、振动光谱、元素分析、差示扫描量热法及单晶X射线衍射对该产物进行了表征。随后通过简便的萃取操作，将Na[BF(CN)3]·(CH3)3SiCN转化为K[BF(CN)3]。本研究评估了三甲基氯硅烷的催化效果，并借助多核核磁共振波谱与振动光谱鉴定出包括异氰基硼酸盐阴离子[BF(NC)(CN)2]−在内的多个反应中间体。二氰二氟硼酸钾（K[BF2(CN)2]）在三甲基氯硅烷存在的条件下，同样可与三甲基氰硅烷反应生成K[BF(CN)3]。实验观测结果的阐释得到了密度泛函理论（density functional theory, DFT）计算数据的支撑。此外，本研究还考察了四氟硼酸盐阴离子的不同抗衡阳离子对三甲基氯硅烷催化反应进程的影响：以四氟硼酸钠（Na[BF4]）为底物时反应速率最快；而以钾离子(K+)作为抗衡阳离子时，[BF4]−向[BF(CN)3]−的转化速率相对较慢。四氟硼酸锂（Li[BF4]）与四氟硼酸四乙基铵([Et4N][BF4])在本次实验采用的反应条件下，仅能分别转化为Li[BF2(CN)2]与[Et4N][BF2(CN)2]。