Redox Properties and Basicity of Keggin-Type Polyoxometalate Complexes

For Keggin-type polyoxometalate complexes (α-[XMo 1 2 O 4 0 ] n - (X = S, P, As, Si, Ge; n = 2-4) and α-[XW 1 2 O 4 0 ] n - (X = S, P, As, Si, Ge, B, Al; n = 2-5)), the conversion processes of the first two one-electron wavesinto 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 (ΔE m i d ) between the first one- and two-electron redox waves served as a useful criterion for the basicity of the Keggin anions. According to the ΔE m i d values, the Keggin anions were classified into the following four groups: (1) [SW 1 2 O 4 0 ] 2 - ; (2) [SMo 1 2 O 4 0 ] 2 - , [XW 1 2 O 4 0 ] 3 - (X = P, As); (3) [XMo 1 2 O 4 0 ] 3 - (X = P, As), [XW 1 2 O 4 0 ] 4 - (X = Si, Ge); (4) [XMo 1 2 O 4 0 ] 4 - (X = Si, Ge), [XW 1 2 O 4 0 ] 5 - (X = B, Al). The voltammetric properties of the Keggin anions were systematized with reference to their basicities.