Comparative structural and thermodynamic studies of fluoride and cyanide binding by PhBMes₂ and related triarylborane Lewis acids

Lewis acidic boranes containing the -BMes₂ unit (Mes = 2,4,6-Me₃C₆H₂) have been widely exploited in molecular sensors for the fluoride ion reflecting, at least in part, the stability to air and moisture of derivatives of the type ArBMes₂. In the current study, the fluoride binding capabilities of the simplest such system, PhBMes₂ (1), have been investigated by spectroscopic and crystallographic methods, with a view to experimentally determining the fundamental thermodynamic and structural parameters associated with this host/guest interaction. A binding constant, K_F, of 8.9(1.9) × 10⁴ M^-1 in dichloromethane solution and a B-F bond length of 1.481(2) for the salt [ⁿBu₄N]⁺[PhMes ₂BF]^- have thus been elucidated and provide a baseline for the analysis of more complex systems. Competitive binding of the cyanide ion is implied by a similar binding constant, K_CN, of 1.9(0.5) × 10⁵ M^-1; structurally, similar degrees of pyramidalization of the BC₃ framework are observed on coordination of each anion {Σ(C-B-C) = 339.8, 340.1°for [ⁿBu₄N] ⁺[PhMes₂BF]^- and [K(18-crown-6)] ⁺[PhMes₂BCN]^-, respectively}. Linking of two ArBMes₂ units via an alkyne spacer results in a 2,2′- bis(dimesitylboryl)tolan system, which is characterized by independent binding of two equivalents of the CN^- anion, rather than cyanide chelation.