The redox behaviour of ferrocene derivatives XIII. Fluorenyl ferrocenes

The electrochemical behaviour of a series of 2,7-substituted-9-fluorenyl-ferrocenes has been studied. Irrespective of the inductive effects played by the different substituents of the fluorenyl fragment, all of them undergo a chemically reversible one-electron removal at almost coincident potential values. Upon exhaustive oxidation, the original red complexes turn orange–brown and the oxidised solution is EPR silent at liquid nitrogen temperature. Mass spectrometry confirms that the electrogenerated cations maintain the original molecular frame. In order to account for the fact that the constancy in redox potentials and the EPR inactivity at 77 K of the oxidised species, both of which prelude an iron-centred oxidation, conflict with the orange colour of the oxidation products (instead of the usual blue-to-green colour of ferricinium species), which preludes a ligand-centred oxidation, a theoretical EHMO analysis was performed. In spite of some uncertainty bound to the energy scale of the MO ordering, a metal-centred oxidation seems favoured over a ligand-centred process.