C5H4BR2Bending in Ferrocenylboranes: A Delocalized Through‐Space Interaction Between Iron and Boron

A comparison of the molecular structures of mono‐, di‐ and tetraborylated ferrocenes [Fc{B(R1)(R2)}] (R1/R2=Br/Br, Br/Fc, Br/Me, Me/Me, Me/OH, OMe/OMe), 1,1′‐[fc{B(R1)(R2)}2] (R1/R2=Br/Br, Br/Me, OMe/OMe), and 1,1′,3,3′‐[Fe{C5H3(BMe2)2}2] revealed the boryl substituent(s) to be bent out of the Cp ring plane towards the iron center. The corresponding dip angle α* decreases with decreasing Lewis acidity of the boron atom and with increasing degree of borylation at the ferrocene core. This trend is well reproduced by DFT calculations (including [FcBH2], not yet accessible experimentally). A Bader analysis of the electron density topology of [FcBH2] (α*=26.5°; BP86/TZVP) clearly showed that there is no direct iron–boron bonding in this compound. Instead, strongly delocalized orbital interactions have been identified that involve the boron p orbital, Cipsoof the adjacent Cp ring, d orbitals at iron, and a through‐space interaction with the second Cp ring. A second important factor is attractive electrostatic interactions, which are enhanced upon ligand bending. Cyclic voltammetric measurements on the series [FcBMe2], 1,1′‐[fc(BMe2)2], and 1,1′,3,3′‐[Fe{C5H3(BMe2)2}2] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe2substituents. Addition of 4‐dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the FeII/FeIIIredox transition far beyond the half‐wave potential of parent ferrocene. In the Mössbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe2], to 1,1′‐[fc(BMe2)2], and to 1,1′,3,3′‐[Fe{C5H3(BMe2)2}2]. In contrast, no significant differences are found between the QS values of ferrocene, [Fc(BMe2DMAP)], and 1,1′‐[fc(BMe2−DMAP)2].