title: 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-[PW12O40]3-(PW12) polyoxometalate, with n-Bu4N+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-PW12isomer, having the same countercation. Analysis of cyclic voltammograms obtained in CH3CN (0.1 M [n-Bu4N][PF6]) reveals that the reversible potential for the β-PW12isomer always remains ca. 100 mV more positive than that of the α-PW12isomer on addition of the acid CF3SO3H. 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.183W NMR spectra of the oxidized forms of the PW12isomers are acid dependent and in the case of β-PW12imply that the bridging oxygens between the WIand WIIunits are preferentially protonated in acidic media. EPR data on frozen solutions of one-electron reduced β-[PWVWVI11O40]4-indicate that either the WIor the WIIIunit in β-PW12is reduced in the β-[PWVI12O40]3-/β-[PWVWVI11O40]4-process. In the absence of acid, reversible potentials obtained from the α- and β-isomers of PW12and [SiW12O40]4-exhibit a linear relationship with solvent properties such as Lewis acidity, acceptor number, and polarity index.
authors.affiliation: Department of Applied Science, Faculty of Science, Kochi University , Kochi 780-8520, Japan.
authors.forename: Tadaharu
authors.identifier: http://orcid.org/0000-0001-6797-5716
authors.initials: T
authors.lastname: Ueda
authors.affiliation: Department of Applied Science, Faculty of Science, Kochi University , Kochi 780-8520, Japan.
authors.forename: Keisuke
authors.initials: K
authors.lastname: Kodani
authors.affiliation: Division of Instrumental Analysis, Department of Instrumental Analysis and Cryogenics, Advanced Science Research Center, Okayama University , Okayama 700-8530, Japan.
authors.forename: Hiromi
authors.initials: H
authors.lastname: Ota
authors.affiliation: X-ray Research Laboratory, Rigaku Corporation , Akishima, Tokyo 196-8666, Japan.
authors.forename: Motoo
authors.initials: M
authors.lastname: Shiro
authors.forename: Si-Xuan
authors.initials: SX
authors.lastname: Guo
authors.forename: John F
authors.initials: JF
authors.lastname: Boas
authors.forename: Alan M
authors.initials: AM
authors.lastname: Bond