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, KF, of 8.9(1.9) × 104 M−1 in dichloromethane solution and a B–F bond length of 1.481(2) A for the salt [nBu4N]+[PhMes2BF]− 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, KCN, 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.