Low-Coordinate Synthetic Models for Nitrogenase Activity

项目摘要 氮气生物还原为NH3完全是由固氮酶完成的，这种酶含有不寻常的铁- 硫化物-碳化物团簇作为活性中心。我们在这里集中讨论铁-钼中的FeMoco辅因子 固氮酶，通过未知的机制完成这一多电子催化反应。这个 FeMoco是生物化学中碳化物(形式为C4-)的唯一例子。这种碳化物大概是 以Fe-CH3、Fe-CH2和Fe-CH中间体的方式插入，这些中间体在硫化铁中是前所未有的 化合物。FeMoco中不寻常的配位化学可能有助于它的还原能力 但生物合成和催化途径中假定物种的先例的缺乏阻碍了 评估建议的机制是否合理的能力。类似地，对 由于文献中缺乏相关化合物，光谱数据很难获得。移动场地 FORWARD需要新的配位化学来阐明相关簇合物的性质和反应活性。 我们的指导性假设是，合成的FeS和FeC团簇具有体积较大的支持配体 活化FeMoco的结构、光谱和反应性属性。因为这些星系团有 明确已知的环境，他们可以建立特定环境的光谱特征 结构特征，将结构和光谱紧密地联系在一起。此外，它们还可用于测试 Fe-C键断裂、Fe-S键断裂、Fe-N键形成等机械性步骤的可行性。 在拟议的研究中，我们将合成具有铁-硫和铁-碳核的新型簇合物。 一个焦点是可以结合氮气和其他固氮酶底物的铁硫化物簇。我们将准备第一个 结合氮气的铁-硫化物团簇，并将探索它们的光谱性质和反应。第二个焦点 是一系列具有不同氢原子数的Fe-C键的化合物，以及 最终形成碳化物桥联的团簇。我们建议将甲基铁、卡宾、卡宾和 碳化物化合物将阐明高自旋铁团簇中不同Fe-C键的基本原理 类似于FeMoco。氢原子的添加和移除会使这些物种相互转化，模拟步骤 在FeMoco的生物合成中。化合物中Fe-C键的形成和断裂也是 与固氮酶活性的机制有关，将使用反应性和光谱来解决 学习。与研究固氮酶的领先光谱学家的合作增强了相关性和 我们与酶的比较的严谨性。 固氮酶之所以令人惊叹，是因为它执行热力学上要求的多电子还原，它 与碳化物具有独特的辅因子结构，并具有罕见的与氮气相互作用的能力。然而， 联系和理解这些观测结果需要在FeS团簇的基本化学方面取得进展。 该项目将提供理解FeMoco所需的化学先例。

Project Summary Biological reduction of N2 to NH3 is performed solely by nitrogenase enzymes, which have unusual iron- sulfide-carbide clusters as active sites. We focus here on the FeMoco cofactor in iron-molybdenum nitrogenase, which accomplishes this multielectron catalytic reaction through an unknown mechanism. The FeMoco is the only example of a carbide (formally C4–) in biological chemistry. This carbide is presumably inserted by way of Fe-CH3, Fe-CH2, and Fe-CH intermediates that are unprecedented in iron-sulfide compounds. The unusual coordination chemistry in the FeMoco presumably contributes to its ability to reduce N2, but the lack of precedents for the putative species in the biosynthetic and catalytic pathways hinders the ability to evaluate whether the proposed mechanisms are reasonable. Similarly, the interpretation of spectroscopic data is difficult because of the lack of related compounds in the literature. Moving the field forward requires new coordination chemistry that elucidates the properties and reactivity of relevant clusters. Our guiding hypothesis is that synthetic FeS and FeC clusters with bulky supporting ligands will have structural, spectroscopic, and reactivity attributes of activated FeMoco. Because these clusters have environments that are known unambiguously, they can establish the spectroscopic signatures of specific structural features, which links structure and spectroscopy firmly. In addition, they can be used to test the feasibility of mechanistic steps such as Fe-C bond cleavage, Fe-S bond cleavage, and Fe-N2 bond formation. In the proposed research, we will synthesize new cluster compounds with iron-sulfur and iron-carbon cores. One focus is iron-sulfide clusters that can bind N2 and other nitrogenase substrates. We will prepare the first iron-sulfide clusters that bind N2, and will explore their spectroscopic properties and reactions. A second focus is on a systematic series of compounds with Fe-C bonds having different numbers of hydrogen atoms, and culminating in carbide-bridged clusters. We propose that comparison of iron methyl, carbene, carbyne, and carbide compounds will elucidate the fundamentals of different Fe-C bonds in high-spin iron clusters resembling the FeMoco. Addition and removal of hydrogen atoms interconverts these species, mimicking steps in the biosynthesis of the FeMoco. The formation and cleavage of Fe-C bonds in the compounds is also relevant to the mechanism of nitrogenase activity, and will be addressed using reactivity and spectroscopic studies. Collaborations with leading spectroscopists who work on nitrogenase enhances the relevance and rigor of our comparisons to the enzyme. Nitrogenase is amazing because it carries out a thermodynamically demanding multielectron reduction, it possesses a unique cofactor structure with a carbide, and it has the rare ability to interact with N2. However, linking and understanding these observations requires advances in the fundamental chemistry of FeS clusters. This project will provide the chemical precedents that are needed to comprehend the FeMoco.