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Issue 40, 2007
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pH-Dependence of the aqueous electrochemistry of the two-electron reduced α-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

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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; 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 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 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 ≤ 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: E0/A/B = −0.009 V vs. Fc+/Fc; E0/B/C = −0.125 V vs. Fc+/Fc and KC = 1.5 × 10−5 M; values for KA, KB, E0/AH/BH and E0/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.

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

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Publication details

The article was received on 20 Apr 2007, accepted on 14 Jun 2007 and first published on 16 Aug 2007


Article type: Paper
DOI: 10.1039/B705991D
Citation: Dalton Trans., 2007, 4599-4607
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    pH-Dependence of the aqueous electrochemistry of the two-electron reduced α-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

    C. Baffert, S. W. Feldberg, A. M. Bond, D. Long and L. Cronin, Dalton Trans., 2007, 4599
    DOI: 10.1039/B705991D

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