Predicting Properties of Iron(III) TAML Activators of Peroxides from Their III/IV and IV/V Reduction Potentials or a Lost Battle to Peroxidase

A cyclic voltammetry study of a series of iron(III) TAML activators of peroxides of several generations in acetonitrile as solvent reveals reversible or quasireversible Fe III/IV and Fe IV/V anodic transitions, the formal reduction potentials ( E °′) for which are observed in the ranges 0.4–1.2 and 1.4–1.6 V, respectively, versus Ag/AgCl. The slope of 0.33 for a linear E °′(IV/V) against E °′(III/IV) plot suggests that the TAML ligand system plays a bigger role in the Fe III/IV transition, whereas the second electron transfer is to a larger extent an iron‐centered phenomenon. The reduction potentials appear to be a convenient tool for analysis of various properties of iron TAML activators in terms of linear free energy relationships (LFERs). The values of E °′(III/IV) and E °′(IV V −1 ) correlate 1) with the p K a values of the axial aqua ligand of iron(III) TAMLs with slopes of 0.28 and 0.06 V, respectively; 2) with the Stern–Volmer constants K SV for the quenching of fluorescence of propranolol, a micropollutant of broad concern; 3) with the calculated ionization potentials of Fe III and Fe IV TAMLs; and 4) with rate constants k I and k II for the oxidation of the resting iron(III) TAML state by H 2 O 2 and reactions of the active forms of TAMLs formed with donors of electrons S, respectively. Interestingly, slopes of log  k II versus E °′(III/IV) plots are lower for fast‐to‐oxidize S than for slow‐to‐oxidize S. The log  k I versus E °′(III/IV) plot suggests that the manmade TAML catalyst can never be as reactive toward H 2 O 2 as a horseradish peroxidase enzyme.