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 degrees ') 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 linearE degrees '(IV/V) againstE degrees '(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 ofE degrees '(III/IV) andE degrees '(IV V-1) correlate 1) with the pK(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 constantsK(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 constantsk(I)andk(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)versusE degrees '(III/IV) plots are lower for fast-to-oxidize S than for slow-to-oxidize S. The log k(I)versusE degrees '(III/IV) plot suggests that the manmade TAML catalyst can never be as reactive toward H(2)O(2)as a horseradish peroxidase enzyme.