EPR evidence for one-electron oxidation of phenols by a dimeric manganese(IV/IV) triazacyclononane complex in the presence and absence of hydrogen peroxide

The reaction of [L2Mn2IV(mu-O)(3)](PF6)(2), L = 1,4,7-trimethyl-1,4,7-triazacyclononane (1) with a range of phenols (2-5) in aqueous solution at pH 10.5 has been investigated, At least for electron-rich substrates, the reaction proceeds via a rapid overall one-electron process from the phenolate ion to the Mn-IV/Mn-IV species (1) to give, initially, a Mn-III/Mn-IV species (detected via its characteristic 16-line EPR spectrum in frozen solution at 77 K) and the corresponding phenoxyl radical, detected directly for Trolox (3) by EPR spectroscopy in aqueous solution, The dimeric Mn-III/Mn-IV species is ultimately converted to monomeric Mn-II. In the presence of H2O2, reoxidation of the manganese species is accompanied by an increase in the rate of formation of the phenoxyl radicals. In similar reactions, 4-methoxyphenol (4) and 2,6 dimethoxyphenol (5) are converted into polyphenols (with the formation of phenoxyl radicals trapped in a polymer matrix), Kinetic EPR and UV-VIS studies provide additional evidence of the reaction mechanism in the presence and absence of H2O2. The mechanism of phenolate oxidation by 1 in the presence and absence of H2O2 is discussed.