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

Decamethylferrocene image, ferrocene (Cp2Fe), 1,1′-dimethylferrocene. ((MeCp)2Fe), and 1,2-diferrocenylethane (Fc2C2H4) were studied as donors for charge transfer complexation with iodine (I2) 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 I2 or DDQ in CH2Cl2 showed the characteristic bands of the triiodide ion, image and the DDQradical dot- anion radical. With CHL, the same behavior occurred only for image to give image and CHLradical dot- radicals. Elemental analyses of the isolated solid complexes indicated the formation of [ferrocene]I3 for ferrocenes and I2, except for (Fc2C2H4) which gave [(Fc2C2H4)]I5. Ferrocene–DDQ complexes were 1:1 except for (MeCp)2Fe and (Fc2C2H4) which gave 1:2 complexes. By using n-hexane as a non-polar solvent instead of CH2Cl2, the CT band of the image adduct has been observed and the formation constant for this CT complexation was found to be 3900 (±500) dm3 mol−1.