Xue Genqiang, Wang Dong, De Hont Raymond, Fiedler Adam T, Shan Xiaopeng, Münck Eckard, Que Lawrence
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455, USA.
Proc Natl Acad Sci U S A. 2007 Dec 26;104(52):20713-8. doi: 10.1073/pnas.0708516105. Epub 2007 Dec 19.
Intermediate Q, the methane-oxidizing species of soluble methane monooxygenase, is proposed to have an [Fe(IV)(2)(mu-O)(2)] diamond core. In an effort to obtain a synthetic precedent for such a core, bulk electrolysis at 900 mV (versus Fc(+/0)) has been performed in MeCN at -40 degrees C on a valence-delocalized Fe(III)Fe(IV)(mu-O)(2)(L(b))(2) complex (1b) (E(1/2) = 760 mV versus Fc(+/0)). Oxidation of 1b results in the near-quantitative formation of a deep red complex, designated 2b, that exhibits a visible spectrum with lambda(max) at 485 nm (9,800 M(-1).cm(-1)) and 875 nm (2,200 M(-1).cm(-1)). The 4.2 K Mössbauer spectrum of 2b exhibits a quadrupole doublet with delta = -0.04(1) mm.s(-1) and DeltaE(Q) = 2.09(2) mm.s(-1), parameters typical of an iron(IV) center. The Mössbauer patterns observed in strong applied fields show that 2b is an antiferromagnetically coupled diiron(IV) center. Resonance Raman studies reveal the diagnostic vibration mode of the [Fe(2)(mu-O)(2)] core at 674 cm(-1), downshifting 30 cm(-1) upon (18)O labeling. Extended x-ray absorption fine structure (EXAFS) analysis shows two O/N scatterers at 1.78 A and an Fe scatterer at 2.73 A. Based on the accumulated spectroscopic evidence, 2b thus can be formulated as Fe(IV)(2)(mu-O)(2)(L(b))(2), the first synthetic complex with an [Fe(IV)(2)(mu-O)(2)] core. A comparison of 2b and its mononuclear analog Fe(IV)(O)(L(b))(NCMe) (4b) reveals that 4b is 100-fold more reactive than 2b in oxidizing weak C H bonds. This surprising observation may shed further light on how intermediate Q carries out the hydroxylation of methane.
中间体Q是可溶性甲烷单加氧酶的甲烷氧化物种，有人提出它具有一个[Fe(IV)₂(μ - O)₂]菱形核心。为了获得这种核心的合成先例，已在-40℃的乙腈中对一个价态离域的[Fe(III)Fe(IV)(μ - O)₂(L(b))₂]³⁺配合物（1b）（相对于Fc⁺/⁰，E₁/₂ = 760 mV）进行了900 mV（相对于Fc⁺/⁰）的本体电解。1b的氧化导致形成一种深红色配合物，命名为2b，其可见光谱在485 nm（9800 M⁻¹·cm⁻¹）和875 nm（2200 M⁻¹·cm⁻¹）处有λmax。2b在4.2 K时的穆斯堡尔谱显示出一个四极双峰，δ = -0.04(1) mm·s⁻¹，ΔE(Q) = 2.09(2) mm·s⁻¹，这是铁(IV)中心的典型参数。在强外加磁场中观察到的穆斯堡尔图谱表明2b是一个反铁磁耦合的二铁(IV)中心。共振拉曼研究揭示了[Fe₂(μ - O)₂]核心在674 cm⁻¹处的诊断振动模式，在¹⁸O标记后下移30 cm⁻¹。扩展X射线吸收精细结构（EXAFS）分析显示在1.78 Å处有两个O/N散射体，在2.73 Å处有一个Fe散射体。基于积累的光谱证据，2b因此可以被表述为[Fe(IV)₂(μ - O)₂(L(b))₂]⁴⁺，这是第一个具有[Fe(IV)₂(μ - O)₂]核心的合成配合物。2b与其单核类似物[Fe(IV)(O)(L(b))(NCMe)]²⁺（4b）的比较表明，在氧化弱C - H键方面，4b的反应活性比2b高100倍。这一惊人的观察结果可能会进一步揭示中间体Q是如何进行甲烷羟基化反应的。
