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Predicting Properties of Iron(III) TAML Activators of Peroxides from Their III/IV and IV/V Reduction Potentials or a Lost Battle to Peroxidase
Author(s) -
Somasundar Yogesh,
Shen Longzhu Q.,
Hoane Alexis G.,
Kaaret Evan Z.,
Warner Genoa R.,
Ryabov Alexander D.,
Collins Terrence J.
Publication year - 2020
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.202003535
Subject(s) - chemistry , acetonitrile , electron transfer , ligand (biochemistry) , quenching (fluorescence) , redox , ferric , analytical chemistry (journal) , inorganic chemistry , fluorescence , organic chemistry , biochemistry , physics , receptor , quantum mechanics
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.