The decamethylferrocene(+/0) electrode reaction in organic solvents at variable pressure and temperature

Half-wave potentials E-1/2 relative to a Ag/Ag+ electrode, mean diffusion coefficients D, and standard electrode reaction rate constants k(el) are reported for the decamethylferrocene(+/0) couple (DmFc(+/0)) in nine organic solvents at variable pressure and (for five solvents) temperature. Limited data on the ferrocene(+/0) (Fc+/0) and Fe(phen)(3)(3+/2+) electrode reactions are included for comparison. Although E-1/2 for DmFc(+/0) correlates only loosely with the reciprocal of the solvent dielectric constant epsilon at ambient pressure, its pressure dependence expressed as the volume of reaction DeltaV(cell) is a linear function of Phi = (1/epsilon)(partial derivative In epsilon/partial derivativeP)(T) (the Drude-Nernst relation). Interpretation of the temperature dependence data is made difficult by enthalpy-entropy compensation. Measurements of D for solutions containing 0.5 mol L-1 tetrabutylammonium perchlorate (TBAP) at 25 degreesC and ambient pressure are inversely proportional to the viscosities eta of the pure solvents as expected from the Stokes-Einstein relation, despite the fact that increasing [TBAP] results in increased eta. The activation volume DeltaVdiff(double dagger) for diffusion of DmFc(+/0) ranges from 7 to 17 cm(3) mol(-1) and generally increases with increasing eta and thus with increasing [TBAP]. The activation volumes DeltaV(el)(double dagger) for the electrode reactions of DmFc(+/0) and Fc(+/0) are all positive, equaling the corresponding DeltaV(diff)(double dagger) values within the experimental uncertainty and contrast sharply with the negative DeltaV(ex)(double dagger) values characteristic of the corresponding self-exchange reactions in homogeneous solution. These facts, together with the thermal activation parameters, point to solvent dynamical control of the electrode (but not the homogeneous self-exchange) reactions. The apparent radii of the electroactive species according to the Drude-Nernst and Stokes-Einstein relations cannot be satisfactorily related to their crystallographic radii and are better regarded as adjustable parameters with limited physical significance.