Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62](4-): A unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media
Pjs. Richardt et al., Synthesis and redox characterization of the polyoxo anion, gamma*-[S2W18O62](4-): A unique fast oxidation pathway determines the characteristic reversible electrochemical behavior of polyoxometalate anions in acidic media, INORG CHEM, 40(4), 2001, pp. 703-709
The synthesis and characterization of (Bu4N)(4)[S2W18O62].1.23MeCN .0.27H(2
)O are reported. It crystallizes in the monoclinic space group C2/c with a
= 22.389(6) Angstrom, b = 22.104(3) Angstrom, c = 25.505(5) Angstrom, beta
= 95.690(15)degrees, V = 12560(5) Angstrom (3), and Z = 4. The anion exists
as the gamma* isomer, the second example of this isomer type to characteri
zed structurally. The equivalent molybdenum salt occurs as the alpha isomer
. gamma*-[S2W18O62](4-) in MeCN solution displayed four electrochemically r
eversible one-electron redox processes at E-1/2 values of -0.24, -0.62, -1.
18, and -1.57 V versus the Fc(+)/Fc couple. Upon addition of acid in MeCN/H
2O (95/5 v/v), the two most cathodic processes converted to an overall two-
electron process at -0.71 V. The total data suggested that this process act
ually comprises two one-electron transfer processes, occurring at different
potentials, with associated proton-transfer reactions. The interpretation
is supported by simulation of the effect of acid titration upon the cyclic
voltammetry. While multiple pathways for correlated reduction and protonati
on are present in both the molybdenum and tungsten systems, only a single f
ast oxidation pathway is available. As the reduced forms of [S2W18O62](4-)
are much weaker bases than those of [S2Mo18O62](4-) the individual oxidatio
n pathways are not the same. However, their existence determines the highly
reversible electrochemical behavior that is characteristic of these anions
, and that of polyoxometalate systems in general.