A cyclic voltammetric study of mangano(II)undecatungstosilicate: an illustrative example of the reduction, protonation and disproportionation pathways of transition metal substituted heteropolytungstates in aqueous solution

Mangano(II)undecatungstosilicate [(H2O)MnSiW11O39]6− was studied by cyclic voltammetry in aqueous solution at various pH values. The pH-dependent wave patterns are influenced by additional processes and were further investigated by variation of scan rates, substance concentration and buffer concentration, focussing on the first 2-electron reduction. Polarographic data and results obtained from [(H2O)CoSiW11O39]6− and [(H2O)2Mn4(PW9O34)]10− were included for comparison. A suitable pH range of 2.5–9 was determined from monitoring the decay of [(H2O)MnSiW11O39]6− by electronic spectroscopy over a period of several weeks. Around pH 5, the cyclic voltammetric wave patterns appear more distorted than at higher and lower pH values. Conditions were specified under which the reoxidation scans exhibit two species with different degrees of protonation. The results were interpreted in terms of protonation and successive disproportionation of reduced species and suggest that a complex network of reversible reaction pathways takes place during the initial reduction processes of [(H2O)MnSiW11O39]6− and related compounds in aqueous solution.