Cyanide ion complexation by a cationic borane

While we have previously reported that [1-(Mes₂B)-8-(Me₃NCH₂)-C₁₀H₆]⁺ ([2]⁺) complexes fluoride ions to form [1-(Mes₂FB)-8-(Me₃NCH₂)-C₁₀H₆] (2-F), we now show that this cationic borane also complexes cyanide to form [1-(Mes₂(NC)B)-8-(Me₃NCH₂)-C₁₀H₆] (2-CN). This reaction also occurs under biphasic conditions (H₂O–CHCl₃) and may serve to transport cyanide in organic phases. The zwitterionic cyanoborate 2-CN has been fully characterized and its crystal structure determined. UV-vis titration experiments carried out in THF indicate that [2]⁺ has a higher affinity for fluoride (K > 10⁸ M⁻¹) than cyanide (K = 8.0 (±0.5) × 10⁵ M⁻¹). Steric effects which impede cyanide binding to the sterically congested boron center of [2]⁺ are most likely at the origin of this selectivity. Finally, electrochemical studies indicate that [2]⁺ is significantly more electrophilic than its neutral precursor 1-(Mes₂B)-8-(Me₂NCH₂)-(C₁₀H₆) (1). These studies also show that reduction of [2]⁺ is irreversible, possibly because of elimination of the NMe₃ moiety under reductive conditions. In fact, [2]OTf reacts with NaBH₄ to afford 1-(Mes₂B)-8-(CH₃)-(C₁₀H₆) (4) which has also been fully characterized.