Comparative structural and thermodynamic studies of fluoride and cyanide binding by PhBMes2 and related triarylborane Lewis acidsThis article is part of a themed issue on Main Group chemistry.CCDC 765939 ([nBu4N]+[1·F]−), 765940 ([K(18-crown-6)]+[1·CN]−) and 765941 ([K(18-crown-6)]+2[2·(CN)2]2−). For crystallographic data in CIF or other electronic format see DOI: 10.1039/c0nj00120a

• Main Authors: Bresner, Christopher, Haynes, Cally J. E, Addy, David A, Broomsgrove, Alexander E. J, Fitzpatrick, Philip, Vidovic, Dragoslav, Thompson, Amber L, Fallis, Ian A, Aldridge, Simon
• Format: Article
• Language: English
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/c0nj00120a

Lewis acidic boranes containing the -BMes 2 unit (Mes = 2,4,6-Me 3 C 6 H 2 ) 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 ArBMes 2 . In the current study, the fluoride binding capabilities of the simplest such system, PhBMes 2 ( 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) × 10 4 M −1 in dichloromethane solution and a B-F bond length of 1.481(2) for the salt [ n Bu 4 N] + [PhMes 2 BF] − 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) × 10 5 M −1 ; structurally, similar degrees of pyramidalization of the BC 3 framework are observed on coordination of each anion {Σ(C-B-C) = 339.8, 340.1° for [ n Bu 4 N] + [PhMes 2 BF] − and [K(18-crown-6)] + [PhMes 2 BCN] − , respectively}. Linking of two ArBMes 2 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. Comparative thermodynamic and structural parameters associated with fluoride and cyanide binding by simple triarylboranes have been determined experimentally.