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 and Fe 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 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 ) correlate 1) with the pK 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 for the quenching of fluorescence of propranolol, a micropollutant of broad concern; 3) with the calculated ionization potentials of Fe and Fe TAMLs; and 4) with rate constants k and k for the oxidation of the resting iron(III) TAML state by H O and reactions of the active forms of TAMLs formed with donors of electrons S, respectively. Interestingly, slopes of log k versus E°'(III/IV) plots are lower for fast-to-oxidize S than for slow-to-oxidize S. The log k versus E°'(III/IV) plot suggests that the manmade TAML catalyst can never be as reactive toward H O as a horseradish peroxidase enzyme.