Voltammetric Reduction ofα- andγ*-[S₂W₁₈O₆₂]^4-and α-, β-, and γ-[SiW₁₂O₄₀]^4-:  Isomeric Dependence of Reversible Potentials of Polyoxometalate Anions Using Data Obtained by Novel Dissolution and Conventional Solution-Phase Processes

Abstract

Comparative studies on the voltammetric reduction of the alpha and gamma isomers of Dawson [S(2)W(18)O(62)](4)(-) and alpha, beta, and gamma forms of Keggin [SiW(12)O(40)](4)(-) polyoxometalate anions have been undertaken. For the six reversible one-electron [S(2)W(18)O(62)](4)(-)(/5)(-)(/6)(-)(/7)(-)(/8)(-)(/9)(-)(/10)(-) processes in acetonitrile, reversible potentials (E(0)(')) were found to be independent of isomeric form within experimental error (+/-5 mV). However, because both the alpha and gamma* isomers of [Bu(4)N](4)[S(2)W(18)O(62)] are insoluble in water, solid-state voltammetric studies with microcrystals adhered to electrode surfaces in contact with aqueous Et(4)NCl and Bu(4)NCl electrolyte media were also possible. Although no isomeric distinction was again detected in the solid-state studies, it was found that reduction of adhered solid by four or more electron equivalents led to rapid dissolution. When Et(4)NCl was the electrolyte, this dissolution process coupled with potential cycling experiments enabled conventional solution-phase data to be obtained in water for the analogous six one-electron reduction steps previously detected in acetonitrile. A strong medium effect attributed to Lewis acidity effects was apparent upon comparison with E(0)(') data obtained in water and acetonitrile. In contrast, with the [SiW(12)O(40)](4)(-) system, E(0)(') values for the [SiW(12)O(40)](4)(-)(/5)(-)(/6)(-)(/7)(-) processes in acetonitrile exhibited a larger (about 70 mV) dependence on isomeric form, and the isomerization step, [gamma-SiW(12)O(40)](6)(-)--> [alpha-SiW(12)O(40)](6)(-), was detected on the voltammetric time scale. The influence of isomeric form on reversible potential data is considered in terms of structural and charge density differences exhibited in the [S(2)W(18)O(62)](4)(-) and [SiW(12)O(40)](4)(-) systems studied in this paper and published data available on the alpha, beta, gamma, and gamma isomers of [As(2)W(18)O(62)](6)(-) and [P(2)W(18)O(62)](6)(-) Dawson anions and Keggin systems.

Redox Chemistry of Heteropolyacid Microemulsions

“…[21c, 25,27] For the purposes of this work, we sought to maintain the structural integrity of the a-Keggin isomer throughout and three-phase electrode DPV of the third phases (c) formed by their extraction into 20 % TBP/n-octane from 1 m HNO 3 aqueous supporting electrolytes. In the aqueous response for H 3 PT (a), the electrode potentials for the two reduction events (I and II) in the negative scan direction (bottom, c) are at 12 and À248 mV, respectively; the corresponding oxidation events (top, c) in the positive scan direction are at À16 and À276 mV, respectively.…”

“…c) Three-phase electrode CV curve of the third phase, formed from extraction of H 3 PT into 20 % TBP/n-octane with a 1 m HNO 3 supporting aqueous electrolyte solution as a function of seven scan rates n = 4, 9,16,25,36,49,64,81, and 100 mV s…”

β,β-Isomer of Open-Wells–Dawson Polyoxometalate Containing a Tetra-Iron(III) Hydroxide Cluster: [{Fe4(H2O)(OH)5}(β,β-Si2W18O66)]9−

“…Baker and Figgis predicted the existence of six possible structural isomers of the Wells-Dawson POM (α, β, γ, α*, β*, γ*) [31]. So far, only the α-, β-, γ-, and γ*-isomers have been experimentally confirmed [32][33][34][35][36][37]. The α-Wells-Dawson POM has D 3h symmetry ( Figure 1, upper left).…”

Bioinspired Electrocatalytic CO2Reduction by Bovine Serum Albumin‐Capped Silver Nanoclusters Mediated by [α‐SiW12O40]4−

“…[14] Polyoxometalates (POMs), for example, [a-SiW 12 O 40 ] 4À , [15] are molecular oxide anionst hat undergo multiple reversible electroreduction processes covering aw ide potential range. Therefore, as uitable redox process can be chosen to provide the drivingf orce required for CO 2 reduction if aP OM is used as the electron transfer mediator.…”

“…These reduction processes were also observed previously in acetonitrile media and were assigned to the [a-SiW 12 one-electron reduction processes. [15] At as can rate of 100 mV s…”