Electron exchange between r-keggin tungstoaluminates and a well-defined cluster-anion probe for studies in electron transfer

Abstract

Fully oxidized [alpha]-AlIIIW12O405-(1ox), and one-electron-reduced [alpha]-AlIIIW12O406-(1red), are well-behaved (stable and free of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Having established this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H2O)63+; [upsilon]1/2 = 0.77 Hz) and 1red (74.1 ppm; [upsilon]1/2 = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in 27Al NMR signals as ionic strength, [mu], is increased by addition of NaCl at the slow-exchange limit of the NMR time scale. The dependence of k on [mu] is plotted using the extended Debye−Huckel equation: log k = log k0 + 2[alpha]z1z2[mu]1/2/(1 + [beta]r[mu]1/2), where z1 and z2 are the charges of 1ox and 1red, [alpha] and [beta] are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of a Keggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whose slope gives a charge product, z1z2,of29 +- 2, statistically identical to the theoretical value of 30. Extrapolation to [mu] = 0 gives a rate constant k11 of (6.5 +- 1.5) X 10-3 M-1 s-1, more than 7 orders of magnitude smaller than the rate constant [(1.1 +- 0.2) X 105 M-1 s -1] determined by 31P NMR for self-exchange between PVW12O403-and its one-electron-reduced form, PVW12O404-. Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend 1red as a well-behaved electron donor for investigating outer-sphere electron transfer to molecules or nanostructures in water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres, for which 1ox and 1red provide a working model.

Citation

Geletii, Yurii V.; Hill, Craig L.; Bailey, Alan J.; Hardcastle, Kenneth I.; Atalla, Rajai H.; Weinstock, Ira A. 2005. Electron exchange between r-keggin tungstoaluminates and a well-defined cluster-anion probe for studies in electron transfer. Inorganic chemistry. Vol. 44, no. 24 (2005): pages 8955-8966