Redox Properties and Basicity of Keggin-Type Polyoxometalate Complexes

Abstract

For Keggin-type polyoxometalate complexes (α-[XMo12O40]n− (X = S, P, As, Si, Ge; n = 2–4) and α-[XW12O40]n− (X = S, P, As, Si, Ge, B, Al; n = 2–5)), the conversion processes of the first two one-electron waves into a two-electron wave were investigated in dipolar aprotic solvents containing H+, Li+, or Na+ as a Lewis acid. A simulation of the cyclic voltammogram indicated that the potential difference (ΔEmid) between the first one- and two-electron redox waves served as a useful criterion for the basicity of the Keggin anions. According to the ΔEmid values, the Keggin anions were classified into the following four groups: (1) [SW12O40]2−; (2) [SMo12O40]2−, [XW12O40]3− (X = P, As); (3) [XMo12O40]3− (X = P, As), [XW12O40]4− (X = Si, Ge); (4) [XMo12O40]4− (X = Si, Ge), [XW12O40]5− (X = B, Al). The voltammetric properties of the Keggin anions were systematized with reference to their basicities.

A Systematic approach to the simulation of the voltammetric reduction of [α-SiW12O40]4− in buffered aqueous electrolyte media and acetonitrile

“…This further documents the expectation that the analogous [SiW 12 O 40 ] 4À/5À/6À/7À/8À sequence could be expected in water if a wider potential range were available. The E 00 values obtained from cyclic voltammograms ( Table 6) are in good agreement with those reported in the literature [7,27,28]. The separation of potentials for processes I and II, and III and IV are similar, further supporting the value of E 00 for the [SiW 12 O 40 ] 7À/8À process in water (assumed above for the simulation of that process).…”

“…In the case of [PW 12 O 40 ] 3À , Himeno et al added perchloric acid, to provide a situation where processes II and III when combined with acid-base chemistry become coincident in CH 3 CN [7]. The voltammetry in acidic CH 3 CN solution was simulated assuming reaction Scheme 7.…”

“…In aprotic media, both [SiW 12 O 40 ] 4À and [PW 12 O 40 ] 3À , as their Bu 4 N + salts, are reduced in a series of 1e À [SiW 12 O 40 ] 4À/5À/6À/7À and [PW 12 O 40 ] 3À/4À/5À/6À/7À processes [3][4][5]. In the case of [PW 12 O 40 ] 3À , 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 [a-SiW 12 O 40 ] 4À and [a-PW 12 O 40 ] 3À .…”

Base Catalysis by Mono- and Polyoxometalates

“…For anhydrous H3PW12O40, the protons have been reported to be attached to the most basic bridging oxygen atoms based on infrared (IR) and 17 O NMR spectroscopy results [21]. Himeno and co-workers reported that the potential difference between the first one-and two-electron redox waves of cyclic voltammograms served as a useful criterion for the basicity of Keggin anions [85,86] …”

Electrochemical and UV–visible spectroscopy studies of K6As2W18−xMoxO62 (x = 0–3) Wells–Dawson heteropolyacid catalysts for oxidative dehydrogenation of benzyl alcohol

“…Several researches on the redox nature of HPAs have been conducted through theoretical and experimental studies such as DFT calculation [4], electrochemical measurement [5][6][7][8], UV-visible spectroscopy [8][9][10], and scanning tunneling microscopy [11]. Due to the diversity of mixed-addenda HPAs, however, much progress has not been made on the redox behavior of Wells-Dawson tungstoarsenates.…”