C5H4-BR2 bending in ferrocenylboranes:: A delocalized through-space interaction between iron and boron

A comparison of the molecular structures of mono-, di- and tetra-borylated ferrocenes [Fc{B(R-1)(R-2)}] (R-1/R-2 =Br/Br, Br/Fc, Br/Me, Me/Me, Me/OH, OMe/OMe), 1,1'-[fc{B(R-1)-(R-2)}(2)] (R-1/R-2-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 alpha* 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 [FcBH(2)], not yet accessible experimentally). A Bader analysis of the electron density topology of [FcBH(2)] (alpha* = 26.5degrees; 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, C-ipso of 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 [FcBMe(2)], 1,1'-[fc(BMe2)(2)], and 1,1',3,3'-[Fc{C5H3(BMe2)(2)}(2)] indicate a substantial anodic shift in the oxidation potential of the central iron atom upon introduction of BMe2 substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe-II/Fe-III. redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)] 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)] 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 BMe2 substituents. Addition of 4-dimethylaminopyridine (DMAP) does not just counterbalance this effect, but leads to a cathodic shift of the Fe-II/Fe-III. redox transition far beyond the half-wave potential of parent ferrocene. In the Mossbauer spectra, a continuous decrease in the quadrupole splitting (QS) is observed upon going from parent ferrocene to [FcBMe(2)b 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)].