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, FeIV(H2O)5O2+ (hereafter FeIV aqO2+), 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–/ABTS2–, phenothiazines, CoII(dmgBF2)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. FeIV aqO2+ oxidizes even Ce­(III) (E 0 in 1 M HClO4 = 1.7 V) with a rate constant greater than 104 M–1 s–1. In 0.10 M aqueous HClO4 at 25 °C, the reactions of Os­(phen)3 2+ (k = 2.5 × 105 M–1 s–1), IrCl6 3– (1.6 × 106), ABTS2– (4.7 × 107), and Fe­(cp)­(C5H4CH2OH) (6.4 × 107) 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 FeIV aqO2+ and further supporting the OSET assignment. A fit to Marcus cross-relation yielded a composite parameter (log k 22 + E 0 Fe/0.059) = 17.2 ± 0.8, where k 22 and E 0 Fe are the self-exchange rate constant and reduction potential, respectively, for the FeIV aqO2+/FeIII aqO+ couple. Comparison with literature work suggests k 22 < 10–5 M–1 s–1 and thus E 0(FeIV aqO2+/FeIII aqO+) > 1.3 V. For proton-coupled electron transfer, the reduction potential is estimated at E 0 (FeIV aqO2+, H+/FeIII aqOH2+) ≥ 1.95 V.