Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage.
Substrate-triggered activation of a synthetic [Fe2(μ-O)2] diamond core for C-H bond cleavage.
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C-H键裂解的合成[Fe2（μ-O）2]钻石核的底物触发的激活。
DOI：
10.1021/ja207131g
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发表时间：
2011-10-19
影响因子：
15
通讯作者：
Que, Lawrence, Jr.
中科院分区：
文献类型：
--
作者：
Xue, Genqiang;Pokutsa, Alexander;Que, Lawrence, Jr.
[FeIV2(μ-O)2] 金刚石核心结构被假定为可溶性甲烷单加氧酶 (sMMO-Q) 的中间体 Q，该氧化剂负责裂解甲烷的强 C-H 键及其羟基化。推而广之，类似的物种可能参与相关二铁羟化酶和去饱和酶的机制。由于缺乏明确的合成实例，关于高价 [Fe2(μ-O)2] 核复合物氧化碳氢化合物底物的机理研究很少（如果有的话）。我们在此报告，水或醇底物可以将由四齿三（吡啶基-2-甲基）胺配体（1 和 2）支持的合成 [FeIIIFeIV(μ-O)2] 配合物活化几个数量级，以实现 C-H 键氧化。在详细的动力学研究的基础上，推测活化是由路易斯碱对[FeIIIFeIV(μ-O)2]核心的攻击导致的，导致形成具有[X–FeIII–O–FeIV=O]开环结构（1-X、2-X、X = OH−或OR−）的更具反应活性的物质。在 -80 °C 下用甲醇盐处理 2，形成高产率的 2-甲醇盐加合物，其特征是 S = 1/2 EPR 信号，表明反铁磁耦合 [S = 5/2 FeIII/S = 2 FeIV] 对。即使在如此低的温度下，该配合物也会发生轻松的分子内 C-H 键断裂，生成甲醛，这表明末端高自旋 FeIV=O 单元能够氧化强度高达 96 kcal mol−1 的 C-H 键。甲醇配体的这种分子内氧化实际上可以与三苯甲烷的分子间氧化竞争，后者的 C-H 键要弱得多 (DC-H 81 kcal mol−1)。 [FeIIIFeIV(μ-O)2] 核的活化通过 2-甲醇对 9,10-二氢蒽的氧化得到了戏剧性的说明，其二阶速率常数比母体金刚石核复合体 2 大 3.6 x 107 倍。这些观察结果为基于 DFT 的概念提供了强有力的支持，即 S = 2 FeIV=O 单元在夺取 H 原子时比其 S = 1 对应物更具反应性，并且表明核心异构化可能是 sMMO-Q 的 [FeIV2(μ-O)2] 金刚石核心的可行策略，以在存在限定二铁活性位点袋的氨基酸残基的较弱 C-H 键的情况下选择性地攻击甲烷的强 C-H 键。
An [FeIV2(μ-O)2] diamond core structure has been postulated for intermediate Q of soluble methane monooxygenase (sMMO-Q), the oxidant responsible for cleaving the strong C–H bond of methane and its hydroxylation. By extension, analogous species may be involved in the mechanisms of related diiron hydroxylases and desaturases. Due to the paucity of well-defined synthetic examples, there are few, if any, mechanistic studies on the oxidation of hydrocarbon substrates by complexes with high-valent [Fe2(μ-O)2] cores. We report here that water or alcohol substrates can activate synthetic [FeIIIFeIV(μ-O)2] complexes supported by tetradentate tris(pyridyl-2-methyl)amine ligands (1 and 2) by several orders of magnitude for C–H bond oxidation. On the basis of detailed kinetic studies, it is postulated that the activation results from Lewis base attack on the [FeIIIFeIV(μ-O)2] core, resulting in the formation of a more reactive species with a [X–FeIII–O–FeIV=O] ring-opened structure (1-X, 2-X, X = OH− or OR−). Treatment of 2 with methoxide at −80 °C forms the 2-methoxide adduct in high yield, which is characterized by an S = 1/2 EPR signal indicative of an antiferromagnetically coupled [S = 5/2 FeIII/S = 2 FeIV] pair. Even at this low temperature, the complex undergoes facile intramolecular C–H bond cleavage to generate formaldehyde, showing that the terminal high-spin FeIV=O unit is capable of oxidizing a C–H bond as strong as 96 kcal mol−1. This intramolecular oxidation of the methoxide ligand can in fact be competitive with intermolecular oxidation of triphenylmethane, which has a much weaker C–H bond (DC-H 81 kcal mol−1). The activation of the [FeIIIFeIV(μ-O)2] core is dramatically illustrated by the oxidation of 9,10-dihydroanthracene by 2-methoxide, which has a second order rate constant that is 3.6 x 107-fold larger than that for the parent diamond core complex 2. These observations provide strong support for the DFT-based notion that an S = 2 FeIV=O unit is much more reactive at H-atom abstraction than its S = 1 counterpart and suggest that core isomerization could be a viable strategy for the [FeIV2(μ-O)2] diamond core of sMMO-Q to selectively attack the strong C–H bond of methane in the presence of weaker C–H bonds of amino acid residues that define the diiron active site pocket.
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影响因子：
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