Solvent Dynamics and Pressure Effects in the Kinetics of the Tris(bipyridine)cobalt(III/II) Electrode Reaction in Various Solvents

The volume of activation ΔV el ⧧ for the Co(bpy)3 3+/2+ electrode reaction in aqueous NaCl (0.2 mol L-1) is −8.6 ± 0.4 cm3 mol-1 at 25.0 °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 ΔV el ⧧ correlate with the corresponding mean diffusion coefficients D for the couple and the diffusional activation volume ΔV diff ⧧, respectively. In organic solvents, however, ΔV el ⧧ is strongly positive (9.1 ± 0.3, 10.2 ± 0.7, and 12.2 ± 0.9 cm3 mol-1 for CH3CN, acetone, and propylene carbonate, respectively, with 0.2 mol L-1 [(C4H9)4N]ClO4 at 25 °C) and correlates with ΔV diff ⧧, while k el correlates with D. These results support the proposition of Murray et al. (J. Am. Chem. Soc. 1996, 118, 1743; 1997, 119, 10249) that solvent dynamics control the rate of the Co(bpy)3 3+/2+ electrode reaction in organic solvents. In aqueous solution at near-ambient temperatures, solvent dynamical influences would not be revealed by pressure effects, but in any event the aqueous Co(bpy)3 3+/2+ electrode reaction appears to be mechanistically different from the nonaqueous cases. For the reduction of Co(bpy)3 3+ with Co(sep)2+ in homogeneous aqueous solution, the rate constant is lower, and the volume of activation more negative, than can be accommodated by extended Marcus theory, suggesting nonadiabatic behavior. These observations are consistent with the view that, although the self-exchange and electrode reactions are generally adiabatic, cross reactions involving CoIII/II couples (and presumably others) become increasingly nonadiabatic as the driving potential is increased.