Study of the electron transfer reaction between [Co(EDTA)]⁻ and [Ru(NH₃)₅py]²⁺ in methanol–water mixtures

Abstract

The title reaction was studied in different water-cosolvent (methanol) mixtures. The results have been rationalized employing the Marcus-Hush treatment. To apply this treatment, the true, unimolecular, electron-transfer rate constants (k et ) were obtained from the experimentally measured rate constants after calculation of the equilibrium constant for the processes of formation of the encounter complex. This calculation was carried out using EigenFuoss (EF) and exponential mean spherical (EMSA) approaches employing effective values of the solvent dielectric constant. These effective values were obtained from the measured association constants corresponding to other ion pairs. The results reveal that in these media there is an additional component of reorganization energy, absent in neat solvents. An explanation of the origin of this component is given. C 2009 Wiley Periodicals, Inc. Int J Chem Kinet 41: 658

Switching Off Electron Transfer Reactions in Confined Media: Reduction of [Co(dipic)2]− and [Co(edta)]− by Hexacyanoferrate(II)

“…By measuring the changes to the redox potential of the [Co­(edta)] −/2– couple and the equilibrium constant of the outer-sphere complex in the different media, the result was a small decrease in the electron transfer rate. This was rationalized in terms of an extra component (∼7.6 kJ mol –1 ) to the reorganization energy that is not present in neat solvent . Thus, although the current study has focused on location, the solvation of these Co­(III) complexes are very much impacted by their location and could contribute to some of the observed differences.…”

“…This was rationalized in terms of an extra component (∼7.6 kJ mol −1 ) to the reorganization energy that is not present in neat solvent. 2 Thus, although the current study has focused on location, the solvation of these Co(III) complexes are very much impacted by their location and could contribute to some of the observed differences.…”

“…In the concentration range used in the present kinetic study, it is estimated that on average there are 0.2 to 1.0 cobalt­(III) complexes in a w o = 7 RM and 1.5 to 7.6 cobalt­(III) complexes in a w o = 20 RM. Using an average radius of 4.5 Å for the metal complexes and 35 Å for the radius of a w o = 20 RM, about 500 metal complexes can geometrically fit within such a RM whereas 19 metal complexes can fit within a w o = 5 RM with a radius of 12 Å . Since the number of waters range from a few hundred to over 6000 between w o = 5 and 20, the actual metal complex occupancy is significantly lower than geometrically possible, so ion–ion interactions will be much smaller even if the concentrations of Fe and Co complexes are comparable.…”

“…Using an average radius of 4.5 Å for the metal complexes and 35 Å for the radius of a w o = 20 RM, about 500 metal complexes can geometrically fit within such a RM whereas 19 metal complexes can fit within a w o = 5 RM with a radius of 12 Å. 2 Since the number of waters range from a few hundred to over 6000 between w o = 5 and 20, 61 the actual metal complex occupancy is significantly lower than geometrically possible, so ion−ion interactions will be much smaller even if the concentrations of Fe and Co complexes are comparable. While the decreasing statistical collision frequency explains the fall off in the rate of reduction of [Co(dipic) 2 ] − at low w o values, it cannot explain the complete suppression of the reduction of [Co(edta)] − within the RM.…”

“…Reagent confinement and solvation are factors of critical importance in directing electron transfer reactions of metal complexes. , Studying such reactions in nonconventional media such as reverse micelles (RMs) provides an opportunity to obtain fundamental information on how reaction dynamics are impacted by local environment and reactant confinement. RMs are a ternary system composed of an organic solvent containing surfactant droplets that contain a confined water pool.…”