Voltammetric and Spectroscopic Studies of α- and β-[PW₁₂O₄₀]^3–Polyoxometalates in Neutral and Acidic Media: Structural Characterization as Their [(n-Bu₄N)₃][PW₁₂O₄₀] Salts

Abstract

The structure of the Keggin-type β-[PWO] (PW) polyoxometalate, with n-BuN as the countercation, has been determined for the first time by single-crystal X-ray analysis and compared to data obtained from a new determination of the structure of the α-PW isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CHCN (0.1 M [n-BuN][PF]) reveals that the reversible potential for the β-PW isomer always remains ca. 100 mV more positive than that of the α-PW isomer on addition of the acid CFSOH. Simulations of the cyclic voltammetry as a function of acid concentration over the range 0-5 mM mimic experimental data exceptionally well. These simulation-experiment comparisons provide access to reversible potentials and acidity constants associated with α and β fully oxidized and one- and two-electron reduced systems and also explain how the two well-resolved one-electron W(VI)/W(V) processes converge into a single two-electron process if sufficient acid is present. W NMR spectra of the oxidized forms of the PW isomers are acid dependent and in the case of β-PW imply that the bridging oxygens between the W and W units are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PWWO] indicate that either the W or the W unit in β-PW is reduced in the β-[PWO]/β-[PWWO] process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW and [SiWO] exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.

Can polyoxometalates enhance the capacitance and energy density of activated carbon in organic electrolyte supercapacitors?

“…Furthermore, we postulate that due to the absence of protons in the organic electrolyte and the formation of ion pairs with the TEA cations, the most negative pH-dependent redox wave has split in two. Indeed, this behavior is similar to a previously reported splitting in organic media [40].…”

Electrochemistry of Polyoxometalates: From Fundamental Aspects to Applications

“…In most cases, only the redox potential changes that depend on the concentration of acid (pH) were examined, although the redox reactions were anticipated to lead to disproportionation reactions . Recently, the detailed voltammetric behavior of the α and β forms of [PW 12 O 40 ] 3− was investigated in CH 3 CN containing n‐ Bu 4 NPF 6 in the presence of designated concentrations of acid (CF 3 SO 3 H) . The diffusion coefficients of the unprotonated POMs were calculated from cyclic voltammograms obtained in the absence of acid over the scan rate range of 20 to 500 mV s −1 and the Randles‐Sevcik equation.…”

Self-Supported Smart Bacterial Nanocellulose–Phosphotungstic Acid Nanocomposites for Photochromic Applications

“…In 0.2 M Na 2 SO 4 aqueous acid solution (pH 2) the hybrid composite presented three successive and quasi-reversible redox processes, located at E 1/2 ((E pc +E pa )/2) +0.023, −0.251 and −0.693 V/(Ag/AgCl/KCl sat ), with the cathodic and anodic peaks separations of 72, 84, and 85 mV, respectively. In general, PWA and other Keggin hetero-polyacids structures exhibit a series of well-resolved one-electron redox processes in neutral or slightly acidic aqueous media, but they merge into two or more electron transfer reactions in strong acid aqueous solution (Sadakane and Steckhan, 1998;Ueda et al, 2017). Also, the two-electron redox activity of PWA are accompanied by protonation and pH has a strong effect on its electrochemical behavior.…”