pH-Dependence of the aqueous electrochemistry of the two-electron reduced α-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

The electrochemistry of the Dawson-like sulfite polyoxometalate anion alpha-[Mo18O54(SO3)(2)](6-), derived from the TEAH(6){alpha-[Mo18O54(SO3)(2)]} salt (TEAH(+) is the triethanolammonium cation; pK(a) = 7.8), has been investigated in aqueous media using cyclic and rotated disk voltammetry at glassy carbon electrodes and bulk electrolysis, with a focus on the pH-dependence for oxidation to alpha-[Mo18O54(SO3)(2)](4-). In buffered media at pH >= 4, the cyclic voltammetric response for alpha-[Mo18O54(SO3)(2)](6-) reveals two partially resolved one-electron oxidation processes corresponding to the sequential generation of alpha-[Mo18O54(SO3)(2)](5-) and alpha-[Mo18O54(SO3)(2)](4-). At lower pH, using electrolytes containing sulfuric acid, the two waves coalesce but the individual apparent E-0' reversible formal potential values for the two processes can be extracted down to pH 2 by assuming that reversible protonation accompanies fast electron transfer. The results for 2 <= pH <= 8 are well described by the double-square scheme mechanism: [GRAPHICS] where A, B and C correspond to species alpha-[Mo18O54(SO3)(2)](4-), alpha-[Mo18O54(SO3)(2)](5-) and alpha-[Mo18O54(SO3)(2)](6-) respectively. The following thermodynamic values could be deduced: E-A/B(0/) = - 0.009 V vs. Fc(+)/Fc; E-B/C(0/) =- 0.125 V vs. Fc(+)/ Fc and K-C = 1.5 x 10(-5) M; values for K-A, K-B, E-AH/BH(0/) and E-BH/CH(0/) could not be determined. Protonated alpha-[HMo18O54(SO3) (2)](5-), deduced to be the major species present at pH < 4, is highly stable in aqueous media. In contrast, alpha-[Mo18O54(SO3)(2)](6-), which is dominant at higher pH values, slowly decomposes. Data considered in the context of acid - base properties of both the TEAH(+) cation and alpha-[HMo18O54(SO3)(2)](5-) anion imply that the TEAH(+) cation is important in the isolation of (TEAH) 6{a-[Mo18O54(SO3) 2]}. Cyclic and rotating disk electrode voltammetries demonstrate that at least 8 electrons also can be easily added to the [Mo18O54(SO3)(2)](4-) framework in acidic media. The existence of the electron transfer series alpha-[Mo18O54(SO3)(2)](4-/5-/6-/7-/8) was confirmed by cyclic voltammetric studies of water insoluble [Pn(4)N](4){alpha-[Mo18O54(SO3)(2)]} adhered to a glassy carbon electrode in contact with an aqueous 0.1 M Et4NCl electrolyte.