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)₆H^x–4_x and IrCl^2–₆ to demonstrate the catalytic rate enhancement caused by the addition of mono-, di- and tri-valent cations.

Increasing protonation of Fe(CN)₆H^x–4_x 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 IrCl^2–₆, 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.