Syntheses of tricyanofluoroborates M[BF(CN)3] (M = Na, K): (CH3)3SiCl catalysis, countercation effect, and reaction intermediates.

Abstract

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.