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 (Rg) of the molecular polyoxometalate (POM) cluster known as the Preyssler anion, [YP5W30O110]n− dissolved in an aqueous mineral acid electrolyte, as a function of its charge, n−. The experimentally-determined Rg 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 −0.555 V 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 Å, we have found that the Rg is 5.8–6.0 Å, which is in agreement with Rgs calculated from the atomic coordinates of previously reported crystallographic structures for the solid-state salts of the fully-oxidized cluster, [Y3+P5W30O110]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 Rg. Moreover, the identical, experimentally-determined Rgs (5.9 ± 0.1 Å) for the oxidized solution anions of [La3+PA]12−, [Ca2+PA]13−, [Sr2+PA]13−, and [Na+PA]14− further demonstrate that the size of metal-ion-exchanged Preyssler anions, [Mn+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, ΔE1o, between the first reduction couples of [Mn+PA]n−15 anions that differ by a unit charge (for Mn+Na+, Ca2+, Sr2+, Y3+, La3+, Th4+) was used in a derivation of the original Born equation to calculate their Born radius, r. The result, 6.0(2) Å, is equivalent to the effective radius calculated for a charged ellipsoid in a dielectric medium (reff = 5.9 Å), thereby providing validation of the Born model.