Reactions of low valent transition metal complexes with hydrogen peroxide. Are they "Fenton-like" or not? 4. The case of Fe(II)L, L = edta; hedta and tcma.

The question whether hydroxyl free radicals are formed in the reactions of divalent iron complexes Fe(II)L; L = edta; hedta; tcma (tcma = 1-acetato-1,4,7-triazacyclononane) with hydrogen peroxide in neutral and slightly acidic solutions was studied by using the beta elimination reaction as an assay for the formation of hydroxyl free radicals, OH. The results show that at pH < 5.5 the iron(II)peroxide intermediate complex decomposes rapidly to yield free hydroxyl radicals for L = edta and hedta. This is in contrast to the mechanism of the corresponding Fe(II)nta peroxide complex, which probably decomposes to form Fe(IV)nta which then reacts with organic substrates to yield aliphatic free radicals. Thus, the non-participating ligand L has an appreciable effect on the mechanism of reaction of the metal center with hydrogen peroxide. Blank experiments using ionizing radiation as the source of .CH2CR(CH3)OH, R = H or CH3 radicals indicate that when L = tcma intermediates of the type LFeIII-CH2CR(CH3)OHaq are formed, but their major mode of decomposition is not the beta elimination reaction. Thus, the present assay for the formation of hydroxyl free radicals by the Fenton Reaction does not fit the latter system.