Characterization of Metastable Intermediates Formed in the Reaction between a Mn(II) Complex and Dioxygen, Including a Crystallographic Structure of a Binuclear Mn(III)–Peroxo Species

Transition-metal peroxos have been implicated as key intermediates in a variety of critical biological processes involving O₂. Because of their highly reactive nature, very few metal–peroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn–peroxo, either as a precursor to, or derived from, O₂, in both photosynthetic H₂O oxidation and DNA biosynthesis. These are arguably two of the most fundamental processes of life. Neither of these biological intermediates has been observed. Herein we describe the dioxygen chemistry of coordinatively unsaturated [Mn^II(S^Me2N₄(6-Me-DPEN))] ⁺ (<b>1</b>), and the characterization of intermediates formed en route to a binuclear mono-oxo-bridged Mn­(III) product {[Mn^III(S^Me2N₄(6-Me-DPEN)]₂(μ-O)}²⁺ (<b>2</b>), the oxo atom of which is derived from ¹⁸O₂. At low-temperatures, a dioxygen intermediate, [Mn­(S^Me2N₄(6-Me-DPEN))­(O₂)]⁺ (<b>4</b>), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (<i>k</i>₁(−10 °C) = 3780 ± 180 M^–1 s^–1, Δ<i>H</i>₁^⧧ = 26.4 ± 1.7 kJ mol^–1, Δ<i>S</i>₁^⧧ = −75.6 ± 6.8 J mol^–1 K^–1) and then convert more slowly (<i>k</i>₂(−10 °C) = 417 ± 3.2 M^–1 s^–1, Δ<i>H</i>₂^⧧ = 47.1 ± 1.4 kJ mol^–1, Δ<i>S</i>₂^⧧ = −15.0 ± 5.7 J mol^–1 K^–1) to a species <b>3</b> with isotopically sensitive stretches at ν_O–O(Δ¹⁸O) = 819(47) cm^–1, <i>k</i>_O–O = 3.02 mdyn/Å, and ν_Mn–O(Δ¹⁸O) = 611(25) cm^–1 consistent with a peroxo. Intermediate <b>3</b> releases approximately 0.5 equiv of H₂O₂ per Mn ion upon protonation, and the rate of conversion of <b>4</b> to <b>3</b> is dependent on [Mn­(II)] concentration, consistent with a binuclear Mn­(O₂^2–) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[Mn^III(S^Me2N₄(6-Me-DPEN)]₂(<i>trans</i>-μ-1,2-O₂)}²⁺ (<b>3</b>) is shown to be bridging between two Mn­(III) ions in an <i>end-on trans</i>-μ-1,2-fashion. This represents the <i>first characterized example of a binuclear Mn­(III)–peroxo</i>, and a rare case in which more than one intermediate is observed en route to a binuclear μ-oxo-bridged product derived from O₂. Vibrational and metrical parameters for binuclear Mn–peroxo <b>3</b> are compared with those of related binuclear Fe– and Cu–peroxo compounds

Electron-Transfer Studies of a Peroxide Dianion

A peroxide dianion (O₂^2–) can be isolated within the cavity of hexacarboxamide cryptand, [(O₂)⊂mBDCA-5t-H₆]^2–, stabilized by hydrogen bonding but otherwise free of proton or metal-ion association. This feature has allowed the electron-transfer (ET) kinetics of isolated peroxide to be examined chemically and electrochemically. The ET of [(O₂)⊂mBDCA-5t-H₆]^2– with a series of seven quinones, with reduction potentials spanning 1 V, has been examined by stopped-flow spectroscopy. The kinetics of the homogeneous ET reaction has been correlated to heterogeneous ET kinetics as measured electrochemically to provide a unified description of ET between the Butler–Volmer and Marcus models. The chemical and electrochemical oxidation kinetics together indicate that the oxidative ET of O₂^2– occurs by an outer-sphere mechanism that exhibits significant nonadiabatic character, suggesting that the highest occupied molecular orbital of O₂^2– within the cryptand is sterically shielded from the oxidizing species. An understanding of the ET chemistry of a free peroxide dianion will be useful in studies of metal–air batteries and the use of [(O₂)⊂mBDCA-5t-H₆]^2– as a chemical reagent