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

• Published in: Journal of the American Chemical Society Vol. 128; no. 51; pp. 17033 - 17042
• Main Authors: Geletii, Yurii V, Hill, Craig L, Atalla, Rajai H, Weinstock, Ira A
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 27.12.2006
• Subjects: Anions - chemistry; Chemistry; Electron Transport; Exact sciences and technology; General and physical chemistry; Hydrogen Peroxide - chemistry; Kinetics; Kinetics and mechanisms; Organic chemistry; Oxidation-Reduction; Oxygen - chemistry; Reactivity and mechanisms; Superoxides - chemistry; Tungsten Compounds - chemistry; Water - chemistry; Water; Heteropolysalt; Tungsten compound; Ion pair; Proton transfer; Electron transfer; Transition metal; Reduction potential; Reaction rate; Chemical reduction; Anions; Hyperoxides; Ionic strength; Platinum; Reaction mechanism; Silver compound; Catalysis; Kinetics; Solvent
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja064244g

Fundamental information concerning the mechanism of electron transfer from reduced heteropolytungstates (POMred) 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:  α-X n +W12O40 (9- n )-, X n + = Al3+, Si4+, P5+. For all three, a single rate expression is observed:  −d[POMred]/dt = 2k 12[POMred][O2], where k 12 is for the rate-limiting electron transfer from POMred to O2. At pH 2 (175 mM ionic strength), k 12 increases from 1.4 ± 0.2 to 8.5 ± 1 to 24 ± 2 M-1s-1 as X n + is varied from P5+ (3 red) to Si4+ (2 red) to Al3+ (1 red). Variable-pH data (for 1 red) and solvent-kinetic isotope (KIE = k H/k D) 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 k 12 values and those calculated using the Marcus cross relation. Further evidence is provided by the small variation in k 12 observed when X n + 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 ΔG°). The weak dependence of k 12 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.