Electron transfer behavioir of multi-iron sandwich-type polyoxometalates and electrocatalytic reduction reactions

The physicochemical and electrocatalytic behaviors of eight multi-iron Wells-Dawson sandwich-type polyoxometalates were studied with specific emphasis on the Fe-III centers. Magnetization measurements were used to identify and quantify the antiferromagnetic interactions between the edge-sharing Fe-III units. Electrochemical studies show the stepwise reduction of the Fe-III centers, in complete agreement with magnetization conclusions. The location of the potential of each wave depends on the pH of the solution, as well as the concentration and composition of the electrolyte. In addition, ion-pairing studies show there is a positive shift of the Fe-III centers with an increase in ion pairing (i.e. K+ > Na+ > Li+). The electrocatalytic reduction of dioxygen and hydrogen peroxide is efficient for all the complexes. However, there is a pronounced increase in efficiency as the number of Fe-III centers in the complex increases (i.e. 4 Fe-III > 3 Fe-III > 2 Fe-III). The mixed-metal complexes [alphabetabetaalpha-Na-14-((MnOH2)-O-II)(2)(Fe-III)(2)(As2W15O56)(2) and alphabetabetaalpha-Na-14((MnOH2)-O-II)(2)(Fe-III)(2)(P2W15O56)(2)] are also efficient catalysts for the reduction of NO and HNO2. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).