Multi-nuclear Iron Clusters as Biomimics of Nitrogenase Enzyme Metallocofactors

项目概要/摘要 氮进入生物圈对生命的发展和可持续性至关重要，因为这种元素 用于蛋白质、核酸和其他细胞成分的开发。大自然利用固氮酶 酶介导的生物惰性的大气氮转化为更具活性的氮源 例如NH3。多核过渡金属辅因子普遍存在于所有固氮酶中， N2结合、还原和转化为其还原底物的活性位点。尽管它的基础作用 固氮循环的生物学，固氮酶金属辅因子如何促进的详细机制， N2固定在很大程度上是不确定的。合成化学家们正在积极地寻求N2固定的机理解释 通过使用配位化学来设计分子结构， 结合并还原N2。在这方面取得了重大进展，特别是在减少N2排放方面。 单核过渡金属化合物，但单核系统是否提供 多核金属辅因子位点的精确模型。为此，利用多核高自旋跃迁 金属配合物作为固氮酶金属辅因子的功能模型的研究要少得多。的 本文所述的研究计划涉及新型全单价化合物的合成、表征和反应性 [Fe3]我们提出的分子结构可以作为功能模型来探索Fe- 固氮酶的Mo辅因子（FeMoco）。最初，一种新的三阴离子，六齿，配体支架[NPL] 3-将被合成。 合成了能够负载三个一价第一行过渡金属离子的化合物。[NPL] 3-的三次甲基化， 将进行一价Fe（I）的还原以形成所提出的一价三铁络合物（NPL）Fe 3。单晶 X射线衍射、SQUID磁力测定法、EPR光谱法和循环伏安法将用于确定 （NPL）Fe 3的结构、自旋态和氧化还原性质。新的[Fe 3]簇的反应性将被评估 通过使（NPL）Fe3与简单的化学氧化剂和固氮酶底物（即N2和CO）反应。化合物 （NPL）Fe 3将与氧化性N-基团转移试剂或N2替代物进行反应，以建立 N-结合的中间体参与的方式，以充分的N2转化。的氧化基团转移反应性 （NPL）Fe 3也将使用S-、O-和C-基团转移试剂进行探索，其中所提出的硫化物络合物， [（NPL）Fe3（µ 3-S）]-将用于模拟硫化物配体在FeMoco的N2固定中的作用。我们将合成 单氢化物和多氢化物络合物（NPL）Fe3H1 - 3，其可以充当FeMoco的E4状态的合成模型， 其是提议结合N2的中间体。氢化物络合物的反应性将用 固氮酶底物如N2、CO和乙炔以及烷烃的还原胺的形成 将监测基质。对于有前景的模型络合物，催化N2或CO还原的功效将是 在合适的化学H +/e-源的存在下或通过电解进行研究。如果成功，这些研究 的预测，以阐明FeMoco的N2还原过程的关键机械步骤。

Project Summary/Abstract Entry of nitrogen into the biosphere is crucial for the development and sustainability of life, as this element is utilized in the development of proteins, nucleic acids, and other cell constituents. Nature uses nitrogenase enzymes to mediate the transformation of the bioinactive atmospheric N2 into more reactive nitrogen sources such as NH3. Ubiquitous to all nitrogenase enzymes are multi-nuclear transition metal cofactors which act as the active site for N2 binding, reduction, and transformation into its reduced substrates. Despite the foundational role of the N2 fixation cycle to biology, the detailed mechanism of how nitrogenase enzyme metallocofactors facilitate N2 fixation is largely uncertain. Synthetic chemists are actively pursuing mechanistic elucidation of the N2 fixation by nitrogenase metallocofactors by using coordination chemistry to design molecular architectures which can bind and reduce N2. Significant progress has been made in this regard, especially in terms of the reduction of N2 with mononuclear transition metal compounds, but it is in question whether mononuclear systems provide accurate models for polynuclear metallocofactor sites. To this end, the use of polynuclear high-spin transition metal complexes as functional models for nitrogenase metallocofactors have been much less explored. The research program described herein involves the synthesis, characterization, and reactivity of new all monovalent [Fe3] molecular architectures which we propose could be functional models to explore the mechanism of the Fe- Mo cofactor (FeMoco) of nitrogenase. Initially, a new trianionic, hexadentate, ligand scaffold [NPL]3- will be synthesized which can support three monovalent first-row transition metal ions. The trimetallation of [NPL]3- with monovalent Fe(I) will be carried out to form the proposed monovalent tri-iron complex, (NPL)Fe3. Single-crystal X-ray diffraction, SQUID magnetometry, EPR spectroscopy, and cyclic voltammetry will be used to determine the structure, spin-state, and redox properties of (NPL)Fe3. The reactivity of the new [Fe3] cluster will be evaluated by reacting (NPL)Fe3 with simple chemical oxidants and nitrogenase substrates (i.e. N2 and CO). Compound (NPL)Fe3 will be subjected to reactions with oxidative N-group transfer reagents or N2 surrogates to establish the N-bound intermediates involved on the way to full N2 conversion. The oxidative group transfer reactivity of (NPL)Fe3 will also be explored using S-, O-, and C- group transfer reagents where the proposed sulfide complex, [(NPL)Fe3(µ3-S)]-, will be used to model the role of sulfide ligands in the N2 fixation of FeMoco. We will synthesize mono- and poly-hydride complexes, (NPL)Fe3H1-3, which could act as synthetic models for the E4 state of FeMoco, which is the intermediate proposed to bind N2. The reactivity of the hydride complexes will be explored with nitrogenase substrates such as N2, CO, and acetylene and the formation of the reduced amine of alkane substrates will be monitored. For promising model complexes, the efficacy of catalytic N2 or CO reduction will be investigated in the presence of a suitable chemical H+/e- sources or via electrolysis. If successful, these studies are predicted to elucidate key mechanistic steps of the N2 reduction process of FeMoco.