Investigation of the Pronounced Medium Effects Observed in the Voltammetry of the Highly Charged Lacunary Anions [α-SiW₁₁O₃₉]^8-and [α-PW₁₁O₃₉]^7-

Abstract

A detailed study is reported of the influence of protons, metal cations, and media on the redox chemistry of lacunary anions [alpha-SiW11O39]8- and [alpha-PW11O39]7- of high formal negative charge. Each anion displayed a single chemically reversible one-electron reduction process in carefully dried aprotic CH3CN solution. This process was detected at very negative potentials just prior to the solvent limit. Addition of 0.3 equiv of acid gave rise to a new reduction process at considerably less negative potentials, which is attributed to formation of the protonated species [SiW11O38(OH)]7- and [PW11O38(OH)]6-. Voltammograms derived from simulations based on a double-square scheme are in excellent agreement with experiment. Previous data reported the presence of several processes in CH3CN and appear to have been influenced by the presence of protons and/or adventitious water. Not surprisingly, protonation reactions coupled to charge transfer contribute significantly to the voltammetry of these lacunary anions in buffered aqueous media over the pH range 2-6. A multi-square-scheme mechanism allowed the essential thermodynamic and kinetic features of this system to be captured and an assessment of the relative significance of possible individual pathways. The high formal anionic charges of [SiW11O39]8- and [PW11O39]7- appear to provide highly basic reduced forms that are able to abstract protons from water to produce protonated species which are reduced at potentials more than a volt less negative than those for the processes [SiW11O39]8-/9- and [PW11O39]7-/8- found in dry aprotic media.

Cs Salt of Undecatungstophospho(aqua) Cuprate(II): Microwave Synthesis, Characterization, Catalytic and Kinetic Study for Epoxidation of cis‐Cyclooctene with TBHP

“…It has been reported in literature that lacunary anion with a formal charge of 7 shows one reversible redox process in aqueous buffer medium. Here too we see a single electron redox process with peat potentials at −0.7 V and −0.4 V, which is in good agreement with literature . The slight shift in the peak potentials in this case may be due to change in the counter‐cation.…”

One-step synthesis of a Keggin-type manganese(II)-substituted phosphotungstate: structural and spectroscopic characterization and non-solvent liquid phase oxidation of styrene

“…4a, b). It has been reported that the lacunary anion (PW 11 ) having a formal charge of 7 features one reversible redox process in aqueous buffered media [27]. The obtained values of Epa and Epc (-0.5 and -0.55 V, respectively,) are in good agreement with the literature, indicating one electron reduction in the lacunary anion.…”

Novel poly(hexylmethacrylate) composite carbon electrodes modified with Keggin-type tungstophosphate-tetrabutylammonium salts

“…This could be due to a very thin film of the polymer remaining on top of parts of the exposed graphite particles, which reduces the ease of reduction or oxidation [31]. The mechanism will involve successive additions of electrons and protons (for reduction), as has been observed in non-aqueous solution and where it was also pointed out that there are pronounced medium effects on the observed voltammetry [42].…”