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. Ignatev et al. J. Fluorine Chem., submitted] was obtained on a molar scale (140 g) from Na[BF4] and (CH3)(3)SiCN with a purity of up to 99.9%. The initial product of the reaction that was catalyzed by (CH3)(3)SiCl was Na[BF(CN)(3)]center dot(CH3)(3)SiCN that was characterized by multinuclear NMR and vibrational spectroscopy, elemental analysis, differential scanning calorimetry, and single-crystal X-ray diffraction. Na[BF(CN)(3)]center dot(CH3)(3)SiCN was converted to K[BF(CN)(3)] via a simple extraction protocol. The catalytic effect of (CH3)(3)SiCl 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)(3)SiCN in the presence of (CH3)(3)SiCl, 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)(3)SiCl-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.