Synthesis, characterization and chemistry of bis-(pentafluorophenyl)boryl ferrocene

Bis-(pentafluorophenyl)boryl ferrocene, 1, was prepared via borylation of ferrocene with HB(C6F5)(2) or via a transmetallation reaction involving FcHgCl and ClB(C6F5)(2) in 87-91% yield. The compound is characterized by a deep maroon colour. A significant intramolecular iron-boron interaction is manifested in the solution spectroscopic (Fe --> B charge transfer band at similar to 230 nm, epsilon = 1.33 x 10(4)) and solid-state crystallographic data (Fe-B = 2.924 Angstrom) This interaction has an impact on the Lewis acidity of the boron center which, unlike the related compound B(C6F5)(3), does not strongly bind Lewis bases such as acetone, THF, or acetonitrile. However, an adduct between the stronger base PMe3 and 1 forms readily and this complex (2) was fully characterized. The electron withdrawing -B(C6F5)(2) group causes 1 to be oxidized at +450 mV relative to ferrocene. Oxidation of 1 with [NO][BF4], AgOSO2CF3, or AgC6F5 leads to the zwitterionic ferrocenium borates 3-F, 3-OTf, and 3-C6F5, respectively. Each of these compounds was characterized spectroscopically and via X-ray crystallography. The properties of these compounds relative to 1 suggest that oxidation of the iron center significantly enhances the Lewis acidity of the boron center. Due to the sigma -donating ability of the borate substituents, zwitterions 3 are weaker oxidizing agents than unsubstituted ferrocenium salts.