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

Fully oxidized alpha-(AlW12O405-)-W-III (1(ox)), and one-electron-reduced alpha-(AlW12O406-)-W-III (1(red)), 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, Al-27 NMR spectroscopy is used to measure rates of electron exchange between 10, (27Al NMR: 72.2 ppm relative to Al(H2O)(6)(3+); nu(1/2) = 0.77 Hz) and 1(red) (74.1 ppm; nu(1/2) = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in Al-27 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 k(0) + 2 alpha z(1)z(2)mu(1/2)/(l + beta r mu(1/2)), where z(1) and z(2) are the charges of 1(ox) and 1(red), 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 (R-2 = 0.996), whose slope gives a charge product, z(1)z(2), of 29 +/- 2, statistically identical to the theoretical value of 30. Extrapolation to mu= 0 gives a rate constant k(11) 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 10(5) M-1 s(-1)] determined by P-31 NMR for self-exchange between (PW12O403-)-W-V and its 4one-electron-reduced form, (PW12O404-)-W-V. Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1(ox) and 1(red). These results, including independent verification of k(11), recommend 1 red 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 1(ox) and 1(red) provide a working model.