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

详细信息    查看全文

• 作者：Michael K. Coggins ; Xianru Sun ; Yeonju Kwak ; Edward I. Solomon ; Elena Rybak-Akimova ; Julie A. Kovacs
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：April 17, 2013
• 年：2013
• 卷：135
• 期：15
• 页码：5631-5640
• 全文大小：517K
• 年卷期：v.135,no.15(April 17, 2013)
• ISSN：1520-5126

文摘

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鈥損eroxos have been characterized. The dioxygen chemistry of manganese remains largely unexplored despite the proposed involvement of a Mn鈥損eroxo, 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))] ⁺ (1), 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(渭-O)}²⁺ (2), the oxo atom of which is derived from ¹⁸O₂. At low-temperatures, a dioxygen intermediate, [Mn(S^Me2N₄(6-Me-DPEN))(O₂)]⁺ (4), is observed (by stopped-flow) to rapidly and irreversibly form in this reaction (k₁(鈭?0 掳C) = 3780 卤 180 M^鈥? s^鈥?, 螖H₁ = 26.4 卤 1.7 kJ mol^鈥?, 螖S₁ = 鈭?5.6 卤 6.8 J mol^鈥? K^鈥?) and then convert more slowly (k₂(鈭?0 掳C) = 417 卤 3.2 M^鈥? s^鈥?, 螖H₂ = 47.1 卤 1.4 kJ mol^鈥?, 螖S₂ = 鈭?5.0 卤 5.7 J mol^鈥? K^鈥?) to a species 3 with isotopically sensitive stretches at 谓_O鈥揙(螖¹⁸O) = 819(47) cm^鈥?, k_O鈥揙 = 3.02 mdyn/脜, and 谓_Mn鈥揙(螖¹⁸O) = 611(25) cm^鈥? consistent with a peroxo. Intermediate 3 releases approximately 0.5 equiv of H₂O₂ per Mn ion upon protonation, and the rate of conversion of 4 to 3 is dependent on [Mn(II)] concentration, consistent with a binuclear Mn(O₂^{2鈥?/sup>) Mn peroxo. This was verified by X-ray crystallography, where the peroxo of {[MnIII(SMe2N₄(6-Me-DPEN)]₂(trans-渭-1,2-O₂)}2+ (3) is shown to be bridging between two Mn(III) ions in an end-on trans-渭-1,2-fashion. This represents the first characterized example of a binuclear Mn(III)鈥損eroxo, and a rare case in which more than one intermediate is observed en route to a binuclear 渭-oxo-bridged product derived from O₂. Vibrational and metrical parameters for binuclear Mn鈥損eroxo 3 are compared with those of related binuclear Fe鈥?and Cu鈥損eroxo compounds.}