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

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.

Reduced Molybenum‐Oxide‐Based Core–Shell Hybrids: “Blue” Electrons Are Delocalized on the Shell

“…(Proton-coupled electron transfers are essential in important biological processes [33][34][35][36] and have been documented for reoxidation of reduced Keggin-type anions. [37][38][39] ) Further work is planned to get insight into the mechanism for the formation of these fascinating hybrid systems, and in particular to specify the role of the related (possible proton-coupled) electron transfers but also to get more insight into the understanding of the extremely high formation tendency of {Mo 72 Fe 30 }-type shells. In this context the related reaction highlighted as chemical Darwinism is especially worthwhile to mention (see also Introduction and references [5,6] [4,5] IR spectra were recorded from KBr pellets at room temperature on a Bio-Rad FT 165 spectrometer.…”

A Stable “End‐On” Iron(III)–Hydroperoxo Complex in Water Derived from a Multi‐Iron(II)‐Substituted Polyoxometalate and Molecular Oxygen

“…In summary, the structural, analytical, spectroscopic and reactivity data for 2 indicate that it is an all-iron(III) compound with "end-on" coordination of two hydroperoxo moieties at the two terminal positions of the polyoxometalate "belt", Na 12 It should be noted that the sum of charges dictates a hydroperoxo rather than a peroxo species. This compound is accessible, because polyoxometalate 1 is a potential sixelectron reducing agent, four of which are required for the reduction of the coordinated dioxygen molecules at the two terminal iron positions.…”

“…[11] Mechanistic understanding of O 2 /polyoxometalate reactions has been based mostly on spectroscopy and kinetic studies; for example, a recent report on the outer-sphere reduction of O 2 with reduced polyoxotungstates devoid of low-valent substitution of transition metals. [12] Herein, we report the isolation and characterization of an "end-on" {Fe III À O 2 } polyoxometalate-based compound in water with unusual properties. The compound is stabilized by hydrogen bonding and is derived from a reaction between a multi-iron(II) polyoxometalate and O 2 .…”

“…A space-filling model based on van der Waals radii of 2 shows a crowded structure that perhaps enforces the linear bond angle (see the Supporting Information). Na 12 2 ]·52 H 2 O was stable in water and did not react as an oxidant. It did, however, efficiently reduce benzoquinone.…”

The kinetics and mechanism of oxidation of reduced phosphovanadomolybdates by molecular oxygen: theory and experiment in concert

“…This rate constant is two orders of magnitude lower than that for oxidation of one electron reduced polyoxotungstates in water. 15 The rate constant for the reoxidation of…”