Electrochemical Reactions in Nonaqueous and Mixed Solvents

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

“…For the Co(bpy) 3 3+/2+ electrode reaction, solvent dynamics are found to be important in nonaqueous solvents. This is somewhat surprising, since Δ G IR * is expected to be substantial for Co(bpy) 3 3+/2+ self-exchange reactions, in which case solvent dynamics should not be dominant and, indeed, eq 2 should not be valid. ,,,,, For an electrode reaction, however, the fifty-percent rule 21,24,25 predicts that the activation free energy barrier height is only one-half that for the corresponding self-exchange reaction, and besides we have presented here evidence that suggests a difference in mechanism for the Co(bpy) 3 3+/2+ electrode reaction between organic solvents and the aqueous systems on which expectations are typically based. Finally, although the Co(bpy) 3 3+/2+ electrode reaction in water behaves much as do its analogues Co(en) 3 3+/2+ and Co(phen) 3 3+/2+ , for which adiabaticity in self-exchange reactions has been inferred, the reduction of Co(bpy) 3 3+ by Co(sep) 2+ and other cross reactions with a high driving potential appears to be nonadiabatic, leading to failure of eqs 7 and 8.…”

“…For the Co(bpy) 3 3+/2+ electrode reaction in any solvent at each pressure and temperature, the mean reactant diffusion coefficient D was obtained from the averaged peak currents of multiple CV measurements, and the electrode rate constant k el was calculated from the maximum in-phase and 90° out-of-phase alternating currents ( I x and I y , respectively, at potential E max ) of an ACV after correcting for the uncompensated resistance R u . Although k el can, in principle, be obtained from the peak separation of CVs, such a procedure will give erroneous results if R u is significant, which is generally the case for nonaqueous solvents. ,, In our ACV experiments, however, R u was first obtained directly from the cell impedance, measured at a high frequency (typically 8 kHz) and a potential ≥300 mV away from E 1/2 for Co(bpy) 3 3+/2+ , and was then specifically allowed for in the calculation of k el . The in-phase and 90° out-of-phase background currents I b x and I b y were measured to obtain the total cell impedance, from which R u was subtracted to give the double-layer charging impedance Z dl .…”

“…24 Although k el can, in principle, be obtained from the peak separation of CVs, such a procedure will give erroneous results if R u is significant, which is generally the case for nonaqueous solvents. 6,26,27 In our ACV experiments, however, R u was first obtained directly from the cell impedance, measured at a high frequency (typically 8 kHz) and a potential g300 mV away from E 1/2 for Co(bpy) 3 3+/2+ , and was then specifically allowed for in the calculation of k el . 24 The in-phase and 90°o ut-of-phase background currents I bx and I by were measured to obtain the total cell impedance, from which R u was subtracted to give the double-layer charging impedance Z dl .…”

“…The question of the possible involvement of solvent dynamics (solvent friction) in the kinetics of electron transfer processes has been perceptively reviewed by Weaver , and continues to attract much theoretical interest. − Experimental evidence for solvent dynamical influences on the rate constants k el of electrode reactions is nevertheless still limited and sometimes ambiguous. ,,− In essence, if solvent dynamics are rate-controlling, one expects k el to correlate inversely with the longitudinal relaxation time τ L (and hence with the viscosity η) of the solvent. Such effects have been observed for electrode reactions of Cr(EDTA) -/2- and Fe(CN) 6 3-/4- in experiments in which η was adjusted by addition of dextrose or sucrose to the solvent (water or dimethyl sulfoxide). − Solvent friction has also been inferred in the reduction in aqueous acetone of (NH 3 ) 5 Co 3+ groups bound to Hg electrodes through thiophenecarboxylate ligands, and in the redox kinetics of metallocene couples at Hg electrodes in various organic solvents. , Of particular interest is the recent observation by Murray and co-workers 14,15 that the rate constant k el of the Co(bpy) 3 3+/2+ 16 electrode reaction in Debye solvents is proportional to τ L -1 and η -1 and to the diffusion coefficient D of the complexes (which may be assumed to be the same for both the oxidized and reduced species); these correlations hold over a spectacular range of 10 11 in k el .…”

“…[3][4][5] Experimental evidence for solvent dynamical influences on the rate constants k el of electrode reactions is nevertheless still limited and sometimes ambiguous. 1,2,[6][7][8][9][10][11][12][13][14][15] In essence, if solvent dynamics are ratecontrolling, one expects k el to correlate inversely with the longitudinal relaxation time τ L (and hence with the viscosity η) of the solvent. Such effects have been observed for electrode reactions of Cr(EDTA) -/2and Fe(CN) 6 3-/4in experiments in which η was adjusted by addition of dextrose or sucrose to the solvent (water or dimethyl sulfoxide).…”