| Summary: | Heme iron and nonheme dimanganese catalases protect biological systems against oxidative damage caused by hydrogen peroxide. Rubrerythrins are ferritine-like nonheme diiron proteins, which are structurally and mechanistically distinct from the heme-type catalase but similar to a dimanganese KatB enzyme. In order to gain more insight into the mechanism of this curious enzyme reaction, non-heme structural and functional models were carried out by the use of mononuclear [Fe<sup>II</sup>(L<sub>1–4</sub>)(solvent)<sub>3</sub>](ClO<sub>4</sub>)<sub>2</sub> (<b>1</b>–<b>4</b>) (L<sub>1</sub> = 1,3-bis(2-pyridyl-imino)isoindoline, L<sub>2</sub> = 1,3-bis(4′-methyl-2-pyridyl-imino)isoindoline, L<sub>3</sub> = 1,3-bis(4′-Chloro-2-pyridyl-imino)isoindoline, L<sub>4</sub> = 1,3-bis(5′-chloro-2-pyridyl-imino)isoindoline) complexes as catalysts, where the possible reactive intermediates, diiron-perroxo [Fe<sup>III</sup><sub>2</sub>(μ-O)(μ-1,2-O<sub>2</sub>)(L<sub>1</sub>-L<sub>4</sub>)<sub>2</sub>(Solv)<sub>2</sub>]<sup>2+</sup> (<b>5</b>–<b>8</b>) complexes are known and well-characterized. All the complexes displayed catalase-like activity, which provided clear evidence for the formation of diiron-peroxo species during the catalytic cycle. We also found that the fine-tuning of iron redox states is a critical issue, both the formation rate and the reactivity of the diiron-peroxo species showed linear correlation with the Fe<sup>III</sup>/Fe<sup>II</sup> redox potentials. Their stability and reactivity towards H<sub>2</sub>O<sub>2</sub> was also investigated and based on kinetic and mechanistic studies a plausible mechanism, including a rate-determining hydrogen atom transfer between the H<sub>2</sub>O<sub>2</sub> and diiron-peroxo species, was proposed. The present results provide one of the first examples of a nonheme diiron-peroxo complex, which shows a catalase-like reaction.
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