A Systematic approach to the simulation of the voltammetric reduction of [α-SiW12O40]4− in buffered aqueous electrolyte media and acetonitrile
Introduction
The nature of the reduction of polyoxometalate anions is markedly dependent on redox level, solvent, supporting electrolyte and proton concentration. For example, the α isomer of the Keggin anion [SiW12O40]4− undergoes two 1e− [SiW12O40]4−/5− and [SiW12O40]5−/6− reduction steps followed at pH < 5, by an overall (2e−, 2H+) [SiW12O40]6−/[H2SiW12O40]6− reduction process at more negative potentials [1], [2], [3], [4], [5]. The PV analog [PW12O40]3− can be studied only in highly acidic media as, above pH 1.5, it decomposes to the lacunary ion [PW11O39]7− [6]. Its voltammetric behavior in these very acidic conditions is similar to the SiIV form, with two 1e− reduction steps followed by a (2e−, 1H+) third step. However, the 2e− reduced form [PW12O40]5− is stable at pH > 1.5 and voltammetric studies reveal that the third process detected in studies with [PW12O40]3− converts to two resolved [PW12O40]5−/6− and [PW12O40]6−/7− 1e− waves at pH > 4 [1]. In aprotic media, both [SiW12O40]4− and [PW12O40]3−, as their Bu4N+ salts, are reduced in a series of 1e− [SiW12O40]4−/5−/6−/7− and [PW12O40]3−/4−/5−/6−/7− processes [3], [4], [5]. In the case of [PW12O40]3−, Himeno and co-workers have studied the voltammetry in acidified aprotic solvents and simulated the responses [7].
Table 1 summarizes some of the formal reversible potential values reported in the literature for reduction of [α-SiW12O40]4− and [α-PW12O40]3−. Most voltammetric studies have been carried out in very acidic aqueous solutions (e.g. 0.5 M H2SO4, 1 M HClO4) or in aprotic solvents. In the present study, voltammetric studies on the reduction of [α-SiW12O40]4− in less acidic buffered aqueous media over the pH range of 2.1–6.8 have been undertaken. A comparison of the experimental behavior with simulated voltammetric responses is used to establish the details of the reduction mechanism in aqueous media. In addition, results of brief studies on [Bu4N]4[α-SiW12O40] and [Bu4N]3[α-PW12O40] in the aprotic solvent acetonitrile are used to probe the solvent dependence. The work draws upon recent advances in the simulation of complex reaction schemes [8], [9].
Section snippets
Reagents
K4[α-SiW12O40], [Bu4N]4[α-SiW12O40] and [Bu4N]3[α-PW12O40] were synthesized according to the literature procedures [10], [11]. Prior to use, Bu4NPF6 (Aldrich; 98%) was recrystallized twice from ethanol [12], and acetonitrile (Ajax FineChem; UV anhydrous, 99.9%) was purified according to a literature procedure [4]. All other chemical reagents were used as received from the manufacturer. Acetic, chloroacetic, citric and formic acids as well as KOOCCH3 and K2HPO4 were supplied by Ajax. Details of
RDE voltammetry
Voltammograms for [SiW12O40]4− (1 mM) were obtained at a glassy carbon RDE over the pH range 2.1–6.8. However, as the anion decomposes slowly when the pH is greater than 5 [2], only data obtained from pH 2.1 to 4.6 provided limiting current data that were suitable for quantitative evaluation of the diffusion coefficient (Fig. 1a). Nevertheless, data obtained at higher pH values from slowly decomposing solutions were essential for estimation of reversible potentials and other parameters needed to
Conclusions
Detailed simulations of the reduction of [α-SiW12O40]4− in buffered aqueous media allowed investigation of the effect of proton concentration upon the thermodynamics, upon the heterogeneous kinetics of the electron transfer steps and upon the homogeneous kinetics of protonation. Agreement is excellent between simulated and experimental voltammograms over a range of pH values. It can be concluded that the multi-square-scheme mechanism (Scheme 2) allows the essential thermodynamic and kinetic
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Current address: Department of Chemistry, University of Bielefeld, Germany.