Article

Reduction of O2 to Superoxide Anion (O2 × -) in Water by Heteropolytungstate Cluster-Anions

Department of Chemistry, Emory University, Atlanta, GA 30322, USA.
Journal of the American Chemical Society (Impact Factor: 11.44). 12/2006; 128(51):17033-42. DOI: 10.1021/ja064244g
Source: PubMed

ABSTRACT Fundamental information concerning the mechanism of electron transfer from reduced heteropolytungstates (POM(red)) to O2, and the effect of donor-ion charge on reduction of O2 to superoxide anion (O2.-), is obtained using an isostructural series of 1e--reduced donors: alpha-X(n+)W12O40(9-n)-, X(n+) = Al3+, Si4+, P5+. For all three, a single rate expression is observed: -d[POM(red)]/dt = 2k12[POM(red)][O2], where k12 is for the rate-limiting electron transfer from POM(red) to O2. At pH 2 (175 mM ionic strength), k12 increases from 1.4 +/- 0.2 to 8.5 +/- 1 to 24 +/- 2 M-1s-1 as Xn+ is varied from P5+ (3red) to Si4+ (2red) to Al3+ (1red). Variable-pH data (for 1red) and solvent-kinetic isotope (KIE = kH/kD) data (all three ions) indicate that protonated superoxide (HO2.) is formed in two steps--electron transfer, followed by proton transfer (ET-PT mechanism--rather than via simultaneous proton-coupled electron transfer (PCET). Support for an outersphere mechanism is provided by agreement between experimental k12 values and those calculated using the Marcus cross relation. Further evidence is provided by the small variation in k12 observed when Xn+ is changed from P5+ to Si4+ to Al3+, and the driving force for formation of O2.- (aq), which increases as cluster-anion charge becomes more negative, increases by nearly +0.4 V (a decrease of >9 kcal mol-1 in DeltaG degrees ). The weak dependence of k12 on POM reduction potentials reflects the outersphere ET-PT mechanism: as the anions become more negatively charged, the "successor-complex" ion pairs are subject to larger anion-anion repulsions, in the order [(3(ox)3-)(O2.-)]4- < [(2(ox)4-)(O2.-)]5- < [(1(ox)5-)(O2.-)]6-. This reveals an inherent limitation to the use of heteropolytungstate charge and reduction potential to control rates of electron transfer to O2 under turnover conditions in catalysis.

0 Followers
 · 
367 Views
  • [Show abstract] [Hide abstract]
    ABSTRACT: A crystal-to-crystal transformation of (DAniF)3Mo2(micro-OH)2Mo2(DAniF)3 (1) to (DAniF)3Mo2(micro-O)2Mo2(DAniF)3 (2), where DAniF is the anion (p-anisyl)NC(H)N(p-anisyl), by dioxygen provides rare insight into the deprotonation process effected by dioxygen. In this dimolybdenum system, the conversion occurs without significant loss of crystallinity. Although no intermediates have been directly observed, a compound containing the [(DAniF)3Mo2(micro-OH)(micro-O)Mo2(DAniF)3]+ cation, a proposed intermediate, has been obtained independently. Possible pathways for the overall conversion of 1 to 2 are discussed.
    Inorganic Chemistry 05/2007; 46(8):3245-50. DOI:10.1021/ic062443v · 4.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Efforts to use the Marcus model to describe oxidations of the superoxide radical anion (O(2-)) by transition-metal complexes have failed dramatically, with discrepancies between theory and experiment spanning 13 orders of magnitude. As a result, the prevailing view is that these reactions involve some complex interactions that are not yet understood. We now show that once the familiar form of the Marcus cross relation (MCR) is modified to allow for the relatively small size of O(2-), excellent agreement is obtained between theory and experiment. This simple finding dispels the decades of uncertainty surrounding these reactions and provides a reliable method for determining whether oxidations of (O2)- occur via inner- or outer-sphere pathways. More generally, the modified MCR is applicable to any homogeneous electron-transfer process characterized by significant differences in size between electron donors and acceptors.
    Inorganic Chemistry 02/2008; 47(2):404-6. DOI:10.1021/ic701885q · 4.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The photocatalytic behaviours of the Dawson salt α-K6[P2W18O62] and two isomers of the tetracobalt Dawson-derived sandwich complexes, αββα-Na17[Co4(H2O)(OH)(P2W15O56)2]·51H2O·2NaCl and ααβα-Na16[Co4(H2O)2(P2W15O56)2]·51H2O (abbreviated ββ-{Co4P4W30} and αβ-{Co4P4W30}, respectively), are described and compared.The direct photochemical excitation of the polyoxometalates (POM) in the presence of propan-2-ol as electron donor leads to their reduction. With polyoxometalates as photocatalyst and propan-2-ol as sacrificial electron donor, the reduction of AgI2SO4 from aqueous solutions is observed leading to metallic Agn0 clusters and colloidal metal nanoparticles stabilized by POM.In the case of both {Co4P4W30}, TEM experiments reveal that most of the Agn particles obtained with a slight excess of Ag+ are spherical with a quite large distribution in size between 10 and 100 nm.
    Applied Catalysis B Environmental 12/2008; 84(s 3–4):835–842. DOI:10.1016/j.apcatb.2008.06.018 · 6.01 Impact Factor
Show more