Kinetics of β-Carotene Oxidation in the Presence of Highly Active Forms of µ-Carbido Diiron(IV) Tetraphenylporphyrinate

Resumo

The oxidative destruction of β-carotene in the presence of highly oxidized forms of µ-carbido-bis[(5,10,15,20-tetraphenyl-21H,23H-porphyrinato)iron(IV)] (1 → 3) or its analog with axially coordinated imidazole (2 → 4) obtained under the action of tert-butyl hydroperoxide ^tBuOOH was studied by spectrophotometry. It was found that compound 3 is the oxo form of compound 1 singly oxidized at the macrocyclic ligand (π radical cation) under the action of which β-carotene is oxidized with a rate constant k = 3.3 L² mol^–2 s^–1. A conclusion is drawn that short-lived compound 4 has unique EAS and is capable of oxidizing ^tBuOOH to form O₂, which makes it possible to consider it the model of peroxidase. The value of k for the reaction with the participation of β-carotene and compound 4 (k = 10.3 L² mol^–2 s^–1) is three times higher than that with the participation of compound 3. If a new portion of β-carotene is added, the process of its oxidative destruction in the presence of compounds 3 or 4 occurs without additives of the dimeric complex and peroxide. A possible nature of compound 4 is discussed, as well as the influence of N-base in the coordination sphere of the complex on the nature of active intermediates and the rate of β-carotene decomposition.