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

Abstract

The electrochemistry of the Dawson-like sulfite polyoxometalate anion alpha-[Mo18O54(SO3)2]6-, derived from the TEAH6{alpha-[Mo18O54(SO3)2]} salt (TEAH+ is the triethanolammonium cation; pKa=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>or=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 E0' 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<or=pH<or=8 are well described by the double-square scheme mechanism: [FORMULAS: SEE TEXT] 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=-0.009 V vs. Fc+/Fc; E=-0.125 V vs. Fc+/Fc and KC=1.5x10(-5) M; values for KA, KB, E and E 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{alpha-[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 [Pn4N]4{alpha-[Mo18O54(SO3)2]} adhered to a glassy carbon electrode in contact with an aqueous 0.1 M Et4NCl electrolyte.