Electron-transfer kinetics and mechanism of di-imine bond formation in tetracyano(ethylenediamine)ferrate(II)

The oxidative dehydrogenation of the ethylenediamine (en) ligand in the complex [Fe(CN)4(en)]2– has been investigated by cyclic voltammetry, spectroelectrochemistry, and by stopped-flow kinetics with the [Fe(CN)6]3– ion, at 25 °C, and I= 0.100 mol dm–3(KCl). The first step in the oxidation process is assigned to a reversible electron-transfer reaction in the [Fe(CN)4(en)]2– complex, with E½= 0.303 V versus a normal hydrogen electrode. The oxidized ion undergoes successive electron transfer in the presence of OH– ions (pH > 11), forming the red FeII–di-imine complex. The observed rate constant is given by kobs.=(1.9 × 104)[Fe(CN)63–][OH–]. The di-imine complex is further oxidized in the presence of [Fe(CN)6]3– ions, with kobs.=(4.7 × 103)[Fe(CN)63–][OH–]. A mechanism is proposed involving FeIII intermediate species which undergo induced electron transfer assisted by OH– ions.