Simultaneous Fluoride Binding to Ferrocene-Based Heteronuclear Bidentate Lewis Acids

A series of μ2-fluoro-bridged heteronuclear bidentate Lewis acid complexes [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2Cl)F]-
(1-2F), [K(18-crown-6)THF]+ [Fc(BMeF)(SnMe2F)F]- (1-3F), [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2Cl)F]- (2-F),
and [K(18-crown-6)THF]+ [Fc(BMePh)(SnMe2F)F]- (2-2F) (Fc = 1,2-ferrocenediyl) was prepared. Compounds 2-F
and 2-2F were obtained as a mixture of diastereomers, which arise due to the generation of a stereocenter at
boron in addition to their inherent planar chirality. All compounds have been studied in the solid state by single-crystal X-ray diffraction analysis and by multinuclear NMR spectroscopy in solution. As a result of bridging-fluoride
interactions, tetrahedral boron and distorted trigonal-bipyramidal tin centers are observed. Comparison with the
corresponding monofunctional ferrocenylborates further supports the bridging nature of the fluoride anion. Two-dimensional exchange spectroscopy 19F NMR studies provide evidence for facile intermolecular and intramolecular
fluorine exchange processes. All complexes display reversible one-electron oxidation events at lower potentials
than those of the tricoordinate ferrocenylborane precursors, which is typical of ferrocenylborate complexes.