Discrete cyanide-bridged mixed-valence Co/Fe complexes: Outer-sphere redox behaviour

Abstract

The outer-sphere redox behaviour of a series of [LnCoIII-NCFeII(CN)5]- (Ln = n-membered pentadentate aza-macrocycle) complexes have been studied as a function of pH and oxidising agent. All the dinuclear complexes show a double protonation process at pH ≈ 2 that produces a shift in their UV/Vis spectra. Oxidation of the different non-protonated and diprotonated complexes has been carried out with peroxodisulfate, and of the non-protonated complexes also with trisoxalatocobaltate(III). The results are in agreement with predictions from the Marcus theory. The oxidation of [Fe(phen)3]3+ and [IrCl6]2- is too fast to be measured, although for the latter the transient observation of the process has been achieved at pH = 0. The study of the kinetics of the outer-sphere redox process, with the S2O82- and [Co(ox)3]3- oxidants, has been carried out as a function of pH, temperature, and pressure. As a whole, the values found for the activation volumes, entropies, and enthalpies are in the following margins, for the diprotonated and non-protonated dinuclear complexes, respectively: ΔV≠ from 11 to 13 and 15 to 20 cm3 mol-1; ΔS≠ from 110 to 30 and -60 to -90 J K-1 mol-1; ΔH≠ from 115 to 80 and 50 to 65 kJ·mol-1. The thermal activation parameters are clearly dominated by the electrostriction occurring on outer-sphere precursor formation, while the trends found for the values of the volume of activation indicate an important degree of tuning due to the charge distribution during the electron transfer process. The special arrangement on the amine ligands in the isomer trans-[L14CoIIINCFeII(CN) 5]- accounts for important differences in solvent-assisted hydrogen bonding occurring within the outer-sphere redox process, as has been established in redox reactions of similar compounds. © Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.