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

Abstract

Bis-(pentafluorophenyl)boryl ferrocene, 1, was prepared via borylation of ferrocene with HB(C 6 F 5 ) 2 or via a transmetallation reaction involving FcHgCl and ClB(C 6 F 5 ) 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 [Formula: see text] B charge transfer band at ~230 nm, ε = 1.33 × 10 4 ) and solid-state crystallographic data (Fe-B = 2.924 Å) This interaction has an impact on the Lewis acidity of the boron center which, unlike the related compound B(C 6 F 5 ) 3 , does not strongly bind Lewis bases such as acetone, THF, or acetonitrile. However, an adduct between the stronger base PMe 3 and 1 forms readily and this complex (2) was fully characterized. The electron withdrawing -B(C 6 F 5 ) 2 group causes 1 to be oxidized at +450 mV relative to ferrocene. Oxidation of 1 with [NO][BF 4 ], AgOSO 2 CF 3 , or AgC 6 F 5 leads to the zwitterionic ferrocenium borates 3-F, 3-OTf, and 3-C 6 F 5 , 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 σ-donating ability of the borate substituents, zwitterions 3 are weaker oxidizing agents than unsubstituted ferrocenium salts.Key words: organoboranes, ferrocene derivatives, Lewis acids.