Redox properties and basicity of Keggin-type polyoxometalate complexes

For Keggin-type polyoxometalate complexes (alpha-[XMo12O40](n-) (X = S, P, As, Si, Ge; n = 2-4) and alpha-[XW12O40](n-) (X = S, P, As, Si, Ge, B, Al; n = 2-5)), the conversion processes of the first two one-electron waves into a two-electron wave were investigated in dipolar aprotic solvents containing H+, Li+, or Na+ as a Lewis acid. A simulation of the cyclic voltammogram indicated that the potential difference (DeltaE(mid)) between the first one- and two-electron redox waves served as a useful criterion for the basicity of the Keggin anions. According to the DeltaE(mid) values, the Keggin anions were classified into the following four groups: (1) [SW12O40](2-); (2) [SMo12O40](2-), [XW12O40](3-)(X = P, As); (3) [XMo12O40](3-) (X = P, As), [XW12O40](4-) (X = Si, Ge); (4) [XMo12O40](4-) (X = Si, Ge), [XW12O40](5-) (X = B, Al). The voltammetric properties of the Keggin anions were systematized with reference to their basicities.