Catalytic influence of the environment on outer-sphere electron-transfer reactions in aqueous solutions

Abstract

The continuous-flow method with integrating observation (CFMIO) has been used to investigate irreversible electron-transfer (ET) reactions between negatively charged, substitution-inert transition metal complexes. Special attention has been paid in order to distinguish between the different contributions to the energy of activation such as size of reactants, long-range charge interactions, influence of the free energy of reactions (difference in redox potential) and the composition (electrolytic content) of the solutions. We selected the ET reaction between Fe(CN)6Hx–4x and IrCl2–6 to demonstrate the catalytic rate enhancement caused by the addition of mono-, di- and tri-valent cations.Increasing protonation of Fe(CN)6Hx–4x decreases the rate of ET; strong association with M2+ and M3+(where M indicates the metal) has no catalytic effect. All alkali-metal ions show an increasing catalytic effect with increasing size; the four tetra-alkylammonium ions show the opposite. The Arrhenius plot of the above-mentioned ET reactions in the presence of cations is strongly curved; the decreasing slope at higher temperatures indicates a complex reaction mechanism.In the ET reaction between silvertetraphenylporphyrin tetrasulphonate and IrCl2–6, ET occurs at the axial position of the complex, far away from the negatively charged sulphonate groups. A catalytic effect similar to that found in the reaction with Fe(CN)4–6 is observed; this result precludes the cations having a bridge-like function during ET. Monovalent cations of varying size show a maximum rate enhancement when their ionic radius is ca. 0.23 nm. If long-range Coulomb interactions are shielded, and a situation in which the free-energy change of reaction is zero is simulated, we extrapolate a maximum for the ET rate constant of 1011 dm3 mol–1 s–1.