Solvent dynamics and pressure effects in the kinetics of the tris(bipyridine)cobalt(III/II) electrode reaction in various solvents

The volume of activation Delta V-el(double dagger) for the Co(bpy)(3)(3+/2) electrode reaction in aqueous NaCl (0.2 mol L-1) is -8.6 +/- 0.4 cm(3) mo l(-1) at 25.0 degrees C, as expected on theoretical grounds and by analogy with Co(en)(3)(3+/2+) and Co(phen)(3)(3+/2+), and neither the rate constant k(el) at various pressures nor Delta V-el(double dagger) correlate with th e corresponding mean diffusion coefficients D for the couple and the diffus ional activation volume Delta V-diff(double dagger), respectively. In organ ic solvents, however, Delta V-el(double dagger) is strongly positive (9.1 /- 0.3, 10.2 +/- 0.7, and 12.2 +/- 0.9 cm(3) mol(-1) for CH3CN, acetone, an d propylene carbonate, respectively, with 0.2 mol L-1 [(C4H9)(4)N]ClO4 at 2 5 degrees C) and correlates with Delta V-diff(double dagger), while k(el) c orrelates with D. These results support the proposition of Murray et al. (J . Am. Chem. Sec. 1996, 118, 1743; 1997, 119, 10249) that solvent dynamics c ontrol the rate of the Co(bpy)(3)(3+/2+) electrode reaction in organic solv ents. In aqueous solution at near-ambient temperatures, solvent dynamical i nfluences would not be revealed by pressure effects, but in any event the a queous CO(bpy)(3)(3+/2+) electrode reaction appears to be mechanistically d ifferent from the nonaqueous cases. For the reduction of Co(bpy)(3)(3+) Wit h Co(sep)(2+) in homogeneous aqueous solution, the rate constant is lower, and the volume of activation more negative, than can be accommodated by ext ended Marcus theory, suggesting nonadiabatic behavior. These observations a re consistent with the view that, although the self-exchange and electrode reactions are generally adiabatic, cross reactions involving Co-III/II coup les (and presumably others) become increasingly nonadiabatic as the driving potential is increased.