Anti-tert-butylperoxyl radical reactivities of copper(II), manganase(II), and iron(III) 3,5-diisopropylsalicylate chelates

Copper(II)-, Fe(III)-, and Mn(II)-3,5-di-iso-propylsalicylate (3,5-DIPS) chelates and Cu(II)2(acetyl-3,5-DIPS)4, and 3,5-DIPS acid were examined as antioxidants in scavenging tert-butylperoxyl radical (tert-butylOOradical dot). Using kinetic EPR methods it was established that the order of anti-tert-butylOOradical dot reactivity is: Mn(II)(3,5-DIPS)2≫Cu(II)2(3,5-DIPS)4∼Fe(III)(3,5-DIPS)3≫Cu(II)2(acetyl-3,5-DIPS)4 and 3,5-DIPS acid. Mn(II)(3,5-DIPS)2 caused the most rapid rate of removal of tert-butylOOradical dot as a result of the oxidation of Mn(II) to Mn(III) by tert-butylOOradical dot. A slower rate of removal of tert-butylOOradical dot was observed for the abstraction of a hydrogen atom from the phenolic hydroxyl group of 3,5-DIPS ligands and subsequent addition of tert-butylOOradical dot to the aromatic ring of the resultant phenoxyl radical. The reaction of tert-butylOOradical dot with Cu(II)2(3,5-DIPS)4 and Fe(III)(3,5-DIPS)3 is due only to hydrogen atom abstraction from the ligand phenolic OH group by tert-butylOOradical dot. Introduction of ethylacetate, which is capable of forming a hydrogen bond with the phenolic OH of the ligand, diminished the anti-tert-butylOOradical dot reactivity of Cu(II)2(3,5-DIPS)4 and Fe(III)(3,5-DIPS)3, similar to its decrease of antioxidant reactivity of phenols, but increased the second-order rate constant for the removal of tert-butylOOradical dot by Mn(II)(3,5-DIPS)2 due mainly to the oxidation of Mn(II) to Mn(III). It is concluded that removal of alkylperoxyl radicals by oxidizable Cu(I), Fe(II), and Mn(II) compounds may partially account for biological activity of their chelates.