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Mechanism of Manganese-Catalyzed Oxygen Evolution from Experimental and Theoretical Analyses of 18O Kinetic Isotope Effects

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journal contribution
posted on 04.12.2015 by Sahr Khan, Ke R. Yang, Mehmed Z. Ertem, Victor S. Batista, Gary W. Brudvig
The biomimetic oxomanganese complex [MnIII/IV2(μ-O)2(terpy)2(OH2)2]­(NO3)3 (1; terpy = 2,2′:6′,2″-terpyridine) catalyzes O2 evolution from water when activated by oxidants, such as oxone (2KHSO5·KHSO4·K2SO4). The mechanism of this reaction has never been characterized, due to the fleeting nature of the intermediates. In the present study, we elucidate the underlying reaction mechanism through experimental and theoretical analyses of competitive kinetic oxygen isotope effects (KIEs) during catalytic turnover conditions. The experimental 18O KIE is a sensitive probe of the highest transition state in the O2-evolution mechanism and provides a strict constraint for calculated mechanisms. The 18O kinetic isotope effect of 1.013 ± 0.003 measured using natural abundance reactants is consistent with the calculated isotope effect of peroxymonosulfate binding to the complex, as described by density functional theory (DFT). This provides strong evidence for peroxymonosulfate binding being both the first irreversible and rate-determining step during turnover, in contrast to the previously held assumption that formation of a high-valent Mn-oxo/oxyl species is the highest barrier step that controls the rate of O2 evolution by this complex. The comparison of the measured and calculated KIEs supplements previous kinetic studies, enabling us to describe the complete mechanism of O2 evolution, starting from when the oxidant first binds to the manganese complex to when O2 is released. The reported findings lay the groundwork for understanding O2 evolution catalyzed by other biomimetic oxomanganese complexes, with features common to those of the O2-evolving complex of photosystem II, providing experimental and theoretical diagnostics of oxygen isotope effects that could reveal the nature of elusive reaction intermediates.