Reduction of O_2 to Superoxide Anion (O_2~(centre dot-)) in Water by Heteropolytungstate CIuster-Anions

摘要

Fundamental information concerning the mechanism of electron transfer from reduced het-eropolytungstates (POM_(red)) to O_2,and the effect of donor-ion charge on reduction of O_2 to superoxide anion (O_2~),is obtained using an isostructural series of 1e~--reduced donors: alpha-X~(n+)W_(12)O_(40)~((9-n)~-),X~(n+) = Al~(3+),Si~(4+),P~(5+).For all three,a single rate expression is observed: -d[POM_(red)]/df = 2k_(12)[POM_(red)][O_2],where /to is for the rate-limiting electron transfer from POM_(red) to O_2.At pH 2 (175 mM ionic strength),k_(12) increases from 1.4 +- 0.2 to 8.5 +- 1 to 24 +- 2 M~(-1)s~(-1) as X~(n+) is varied from P~(5+) (3_(red)) to Si~(4+) (2_(red)) to Al~(3+) (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 (HO_(2~(centre dot)) 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 P~(5+) to Si~(4+) to Al~(3+),and the driving force for formation of O_(2~(centre dot-)) (aq),which increases as cluster-anion charge becomes more negative,increases by nearly +0.4 V (a decrease of > 9 kcal mol~(-1) in DELTA G~o).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-))(O_(2~(centre dot-))]~(4-) < [(2_(ox)~(4-))(O_(2~(centre dot-))]~(5-) < [(1_(ox)~(5-))(O_(2~(centre dot-))]~(6-).This reveals an inherent limitation to the use of heteropolytungstate charge and reduction potential to control rates of electron transfer to O_2 under turnover conditions in catalysis.