Fe-B bonding in (dibromoboryl)ferrocene: A structural and theoretical investigation

The geometry and electronic structure of borylferrocenes, FcBR₂, have been investigated by X-ray structure analysis (R = Br) and density functional theory (DFT) calculations (R = H). An interaction between filled d-type orbitals at iron and the empty p orbital of boron causes bending of the BR₂ substituent toward the central iron atom. The dip angle α* is 17.7° (18.9°) for FcBBr₂ (X-ray structure). Slightly larger values were calculated for the sterically unhindered model system FcBH₂ (α* = 21.6° [LDA/NL]). According to the DFT calculations, the Fe-B interaction in FcBH₂ is considerably weaker than the Fe-C_exo bond of the isoelectronic cation FcCH₂⁺ (α* = 41.0° [LDA/NL]). In 1,1′-diborylated ferrocene no energetically preferred rotamer was detected by our DFT calculations.