Electron Transfer Reactivity of the Aqueous Iron(IV)-Oxo Complex. Outer-Sphere vs Proton-Coupled Electron Transfer

The kinetics of oxidation of organic and inorganic reductants by aqueous iron(IV) ions, Fe-IV(H2O)(6)O2+ (hereafter (FeaqO2+)-O-IV), are reported. The substrates examined include several water-soluble ferrocenes, hexachloroiridate(III), polypyridyl complexes M(NN)(3)(2+) (M = Os, Fe and Ru; NN = phenanthroline, bipyridine and derivatives), HABTS(-)/ABTS(2-), phenothiazines, Co-II(dmgBF(2))(2), macrocyclic nickel(II) complexes, and aqueous cerium(III). Most of the reductants were oxidized cleanly to the corresponding one-electron oxidation products, with the exception of phenothiazines which produced the corresponding oxides in a single-step reaction, and polypyridyl complexes of Fe(II) and Ru(II) that generated ligand-modified products. (FeaqO2+)-O-IV oxidizes even Ce(III) (E-0 in 1 M HClO4 = 1.7 V) with a rate constant greater than 10(4) M-1 s(-1.) In 0.10 M aqueous HClO4 at 25 degrees C, the reactions of Os(phen)(3)(2+) (k = 2.5 X 10(5) M-1 s(-1)), IrCl63- (1.6 X 10(6)), ABTS(2-) (4.7 X 10(7)), and Fe(cp)(C5H4CH2OH) (6.4 X 10(7)) appear to take place by outer sphere electron transfer (OSET). The rate constants for the oxidation of Os(phen)(3)(2+) and of ferrocenes remained unchanged in the acidity range 0.05 < [H+] < 0.10 M, ruling out prior protonation of (FeaqO2+)-O-IV and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k(22) + E-Fe(0)/0.059) = 17.2 +/- 0.8, where k(22) and E-Fe(0) are the self-exchange rate constant and reduction potential, respectively, for the (FeaqO2+)-O-IV/(FeaqO+)-O-III couple. Comparison with literature work suggests k(22) < 10(-5) M-1 s(-1) and thus E-0 ((FeaqO2+)-O-IV/(FeaqO+)-O-III) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E-0 ((FeaqO2+)-O-IV; H+/(FeaqOH2+)-O-III) >= 1.95 V.