Redox reactions via outer sphere charge transfer complexation: the interaction of ferrocenes with σ- and π-type acceptors

Abstract

Decamethylferrocene Cp 2 ∗ Fe , ferrocene (Cp 2Fe), 1,1′-dimethylferrocene. ((MeCp) 2Fe), and 1,2-diferrocenylethane (Fc 2C 2H 4) were studied as donors for charge transfer complexation with iodine (I 2) as a typical σ-type CT acceptor and with 2,3-dichloro-5,6-dicyano- p-benzoquinone (DDQ), and p-chloranil (CHL) as typical π-type CT acceptors. Oxidation of the ferrocenes to the corresponding ferricinium salts is proposed to occur via initial formation of outer sphere charge transfer complexes. UV–Vis spectra for the mixtures of any of the ferrocene donors with I 2 or DDQ in CH 2Cl 2 showed the characteristic bands of the triiodide ion, I 3 - and the DDQ - anion radical. With CHL, the same behavior occurred only for Cp 2 ∗ Fe to give Cp 2 ∗ Fe + and CHL - radicals. Elemental analyses of the isolated solid complexes indicated the formation of [ferrocene]I 3 for ferrocenes and I 2, except for (Fc 2C 2H 4) which gave [(Fc 2C 2H 4)]I 5. Ferrocene–DDQ complexes were 1:1 except for (MeCp) 2Fe and (Fc 2C 2H 4) which gave 1:2 complexes. By using n-hexane as a non-polar solvent instead of CH 2Cl 2, the CT band of the Cp 2 ∗ Fe adduct has been observed and the formation constant for this CT complexation was found to be 3900 (±500) dm 3 mol −1.