Electron Exchange between α-Keggin Tungstoaluminates and a Well-Defined Cluster-Anion Probe for Studies in Electron Transfer

• Published in: Inorganic chemistry Vol. 44; no. 24; pp. 8955 - 8966
• Main Authors: Geletii, Yurii V, Hill, Craig L, Bailey, Alan J, Hardcastle, Kenneth I, Atalla, Rajai H, Weinstock, Ira A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.11.2005
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic050860m

Fully oxidized α-AlIIIW12O40 5- (1 ox), and one-electron-reduced α-AlIIIW12O40 6- (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, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1 ox (27Al NMR:  72.2 ppm relative to Al(H2O)6 3+; ν1/2 = 0.77 Hz) and 1 red (74.1 ppm; ν1/2 = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in 27Al NMR signals as ionic strength, μ, is increased by addition of NaCl at the slow-exchange limit of the NMR time scale. The dependence of k on μ is plotted using the extended Debye−Hückel equation:  log k = log k 0 + 2αz 1 z 2μ1/2/(1 + βrμ1/2), where z 1 and z 2 are the charges of 1 ox and 1 red, α and β 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 μ = 0 gives a rate constant k 11 of (6.5 ± 1.5) × 10-3 M-1 s-1, more than 7 orders of magnitude smaller than the rate constant [(1.1 ± 0.2) × 105 M-1 s-1] determined by 31P NMR for self-exchange between PVW12O40 3- and its one-electron-reduced form, PVW12O40 4-. 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.