In situ measurement of the Preyssler polyoxometalate morphology upon electrochemical reduction: A redox system with Born electrostatic ion solvation behavior

SAXS (small-angle X-ray scattering) and controlled-potential bulk electrolysis were combined to probe the radius of gyration (R(g)) of the molecular polyoxometalate (POM) Cluster known as the Preyssler anion, [YP(5)W(30)O(110)](n-) dissolved in an aqueous mineral acid electrolyte, as a function of its charge, n. The experimentally-determined R. for the oxidized anion (n = 12) and its 2-, 4- and 10-electron reduced forms following the course of exhaustive electrolyses with a reticulated vitreous carbon electrode polarized at -0.145, -0.255. and -0,555V vs. Ag/AgCl, respectively, is independent of reduction(and charge) under the solution conditions employed here. Within the limits of resolution and precision of our in situ measurements and analyses, +/- 0.2 angstrom, we have found that the R(g) is 5.8-6.0 angstrom, which is in agreement with R(g)s calculated from the atomic coordinates of previously reported crystallographic structures for the solid-state salts of the fully-oxidized cluster, [Y(3+)P(5)W(30)O(110)](12) (abbreviated [YPA](12)). The equivalence indicates that any modification of the P-W-O structure that may arise upon reduction of the Preyssler anion is too small to affect the R(g), Moreover, the identical, experimentally-determined R(g)s (5.9 +/- 0.1 angstrom) for the oxidized solution anions of [La(3+)PA](12), [Ca(2+)PA](13), [Sr(2+)PA](13-), and [Na(+)PA](14-) further demonstrate that the size of metal-ion-exchanged Preyssler anions, [M(n+)PA](n-15), is independent of the charge, n, on M and, hence, the overall cluster charge, n-15. This provides an ideal scenario with which to test the Born model of electrostatic ion solvation, wherein the electrochemical potential difference, Delta E(1)(0), between the first reduction couples of [M(n+)PA](11-15) anions that differ by a unit charge (for M(n) (-)Na(+), Ca(2+), Sr(2+), Y(3+), La(3+), Th(4+)) was used in a derivation of the original Born equation to calculate their Born radius, r. The result, 6.0(2) angstrom, is equivalent to the effective radius calculated for a charged ellipsoid in a dielectric medium (r(eff)infinity 5.9 angstrom), thereby providing validation of the Born model. (C) 2008 Elsevier B.V. All rights reserved.