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 α-[Mo₁₈O₅₄(SO₃)₂]⁶⁻, derived from the TEAH₆{α-[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 α-[Mo₁₈O₅₄(SO₃)₂]⁴⁻. In buffered media at pH ≥ 4, the cyclic voltammetric response for α-[Mo₁₈O₅₄(SO₃)₂]⁶⁻ reveals two partially resolved one-electron oxidation processes corresponding to the sequential generation of α-[Mo₁₈O₅₄(SO₃)₂]⁵⁻ and α-[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:

where A, B and C correspond to species α-[Mo₁₈O₅₄(SO₃)₂]⁴⁻, α-[Mo₁₈O₅₄(SO₃)₂]⁵⁻ and α-[Mo₁₈O₅₄(SO₃)₂]⁶⁻ respectively. The following thermodynamic values could be deduced: E^0/_A/B = −0.009 V vs. Fc⁺/Fc; E^0/_B/C = −0.125 V vs. Fc⁺/Fc and K_C = 1.5 × 10⁻⁵ M; values for K_A, K_B, E^0/_AH/BH and E^0/_BH/CH could not be determined. Protonated α-[HMo₁₈O₅₄(SO₃)₂]⁵⁻, deduced to be the major species present at pH < 4, is highly stable in aqueous media. In contrast, α-[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 α-[HMo₁₈O₅₄(SO₃)₂]⁵⁻ anion imply that the TEAH⁺ cation is important in the isolation of (TEAH)₆{α-[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 α-[Mo₁₈O₅₄(SO₃)₂]^{4−/5−/6−/7−/8−} was confirmed by cyclic voltammetric studies of water insoluble [Pn₄N]₄{α-[Mo₁₈O₅₄(SO₃)₂]} adhered to a glassy carbon electrode in contact with an aqueous 0.1 M Et₄NCl electrolyte.