The dependence of the rates of reduction of cobalt(III)-μ-peroxo complexes on isomeric structure

Abstract

The reaction of μ-peroxybis(ethylenediamine)bis(diethylenetriamine)cobalt(III) with ferrous ions has been studied by stopped flow spectrophotometry and by 59Co nmr. Two separate initial reduction steps with second order rate constants of 1.07 and 3.04 M−1 s−1, respectively, can be discerned in the stopped flow experiments. These two rate constants are assigned to inner sphere reduction of the aef,cd,b and edf,ac,b isomers of the peroxy cobalt complex, respectively. The different isomers can be distinguished by 59Co nmr. If there is insufficient Fe2+ present to completely reduce the complex to Co(II) and water, the change in the relative intensities of the 59Co resonances after partial reduction shows that one isomer has reacted faster than the other. The 59Co resonances can be assigned to the different isomers on the basis of a point charge model for the chemical shifts and line widths. Based on this model it is concluded that the isomer with the central NH of the diethylenetriamine trans to the peroxy ligand reacts faster. This result is compared to literature reports on the relative rates of substitution of isomers of Co(III) complexes with ethylenediamine and diethylenetriamine ligands.