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(µ-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 MnIV/MnIV species (1) to give, initially, a MnIII/MnIV 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 MnIII/MnIV species is ultimately converted to monomeric MnII. 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.