Multi-iron Wells-Dawson heteropolytungstates. Electrochemical probing of siderophoric behavior in sandwich-type complexes

The demetalation process of 10 multi-iron Wells-Dawson polyoxometalates is studied by cyclic voltammetry and controlled potential coulometry. Eight sandwich-type complexes (alphaalphaalphaalpha-Na-16[(NaOH2)(2)(Fe-III)(2)(X2W15O56)(2)],alphaalphabetaalpha-Na-14[(NaOH2)((FeOH2)-O-III)(Fe-III)(2)(X2W15O56)(2)], alphabetabetaalpha-Na-12[((FeOH2)-O-III)(2)(Fe-III)(2)(X2W15O56)(2)], and alphabetabetaalpha-Na-14[((MnOH2)-O-II)(2)(Fe-III)(2)(X2W15O56)(2)] (where X = P(V) or As(V))) and two monomeric complexes (alpha-Na-11[(P-2((FeCl)-Cl-III)(2)((FeOH2)-O-III)W15O59)] and alpha-Na-11((AS(2)((FeCl)-Cl-III)(2)(FeOH2)-O-III)W15O59)]) were selected for this study. All 10 complexes show Fe(III) waves which are well-separated from the redox activity of the W(VI) centers. At room temperature and under mild conditions, iron release from the complexes is observed upon reduction of the Fe(III) centers. This release is controlled by the ionic strength of the medium, the nature and concentration of the anions present in the supporting electrolyte, and by the pH of the solution. This behavior parallels those described for most siderophores which depend on the same parameters.