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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Relay race: By using polyprotic-acid-based pendant proton relays (phosphate, L-glutamic acid, and malonic acid), the oxygen reduction reaction (ORR) on a TiO2 surface is switched from a single-electron process to a concerted 4 e(-) /4 H(+) reduction to H2 O. In contrast, monocarboxylic acids (PA and AA) without pendant proton relays only enhance the ORR reduction rate but barely change the reaction pathway.
    Angewandte Chemie International Edition 07/2013; · 11.34 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. · 6.01 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Reversible redox chemistries are an inherent feature of numerous metal oxide cluster anions (POMs). Moreover, as discrete molecular structures with well-defined and controllable solution chemistries, POMs can be deployed as physicochemical probes for studying inorganic reaction mechanisms. In the past decade, we have used an iso-structural series of α-Keggin heteropolytungstate cluster anions to systematically investigate a number of fundamental topics, including electron transfer to dioxygen. The iso-structural series of cluster anions is obtained by varying the heteroatom, Xn+, in the plenary, Td-symmetry α-Keggin ion, Xn+W12O40(8−n)−, from Al3+ to Si4+ to P5+. This results in a stepwise and linear modulation of ion charge and reduction potential, whose concerted effects on reaction rates can be used to better understand electron-transfer processes. Starting from the acquisition of activation parameters associated with electron self-exchange between the POMs themselves, the studies discussed in this review provide a detailed account of electron transfer from reduced α-Keggin heteropolytungstate anions to dioxygen, culminating in the recent discovery of a fundamentally new mechanism for electron transfer to O2 in water.
    Israel Journal of Chemistry (Online) 02/2011; 51(2). · 2.56 Impact Factor