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Dioxygen Activation and Catalytic Reduction to Hydrogen Peroxide by a Thiolate-Bridged Dimanganese(II) Complex with a Pendant Thiol

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posted on 08.07.2015, 00:00 by Marcello Gennari, Deborah Brazzolotto, Jacques Pécaut, Mickael V. Cherrier, Christopher J. Pollock, Serena DeBeer, Marius Retegan, Dimitrios A. Pantazis, Frank Neese, Mathieu Rouzières, Rodolphe Clérac, Carole Duboc
Herein, we describe an uncommon example of a manganese–thiolate complex, which is capable of activating dioxygen and catalyzing its two-electron reduction to generate H2O2. The structurally characterized dimercapto-bridged MnII dimer [MnII2(LS)­(LSH)]­ClO4 (MnII2SH) is formed by reaction of the LS ligand (2,2′-(2,2′-bipyridine-6,6′-diyl)­bis­(1,1-diphenylethanethiolate)) with MnII. The unusual presence of a pendant thiol group bound to one MnII ion in MnII2SH is evidenced both in the solid state and in solution. The MnII2SH complex reacts with dioxygen in CH3CN, leading to the formation of a rare mono-μ-hydroxo dinuclear MnIII complex, [(MnIII2(LS)2(OH)]­ClO4 (MnIII2OH), which has also been structurally characterized. When MnII2SH reacts with O2 in the presence of a proton source, 2,6-lutidinium tetrafluoroborate (up to 50 equiv), the formation of a new Mn species is observed, assigned to a bis-μ-thiolato dinuclear MnIII complex with two terminal thiolate groups (MnIII2), with the concomitant production of H2O2 up to ∼40% vs MnII2SH. The addition of a catalytic amount of MnII2SH to an air-saturated solution of MenFc (n = 8 or 10) and 2,6-lutidinium tetrafluoroborate results in the quantitative and efficient oxidation of MenFc by O2 to afford the respective ferrocenium derivatives (MenFc+, with n = 8 or 10). Hydrogen peroxide is mainly produced during the catalytic reduction of dioxygen with 80–84% selectivity, making the MnII2SH complex a rare Mn-based active catalyst for two-electron O2 reduction.