Multi-nuclear Iron Clusters as Biomimics of Nitrogenase Enzyme Metallocofactors

项目摘要/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发育。大自然使用固氮酶 酶介导生物不活跃的大气氮气转化为更活跃的氮源 如NH3。在所有固氮酶中普遍存在的是多核过渡金属辅因子，它们充当 氮气结合、还原和转化为其还原底物的活性部位。尽管发挥了基础性作用 关于氮气固定循环对生物学的影响，固氮酶金属辅酶促进的详细机制 对氮气的固定在很大程度上是不确定的。合成化学家们正在积极研究氮气固定的机理。 由固氮酶金属辅因子通过配位化学设计的分子结构 束缚和还原氮气。在这方面取得了重大进展，特别是在减少氮气排放方面 单核过渡金属化合物，但单核体系是否能提供 多核金属因子位点的精确模型。为此，利用多核高自旋跃迁 金属络合物作为固氮酶金属辅因子的功能模型的探索要少得多。这个 这里描述的研究项目涉及新的全单价化合物的合成、表征和反应活性 我们提出的[Fe3]分子结构可以作为功能模型来探索Fe-的形成机理。 固氮酶的钼辅助因子(FeMoCo)。最初，一种新的三阴离子，六齿，配体支架[NPL]3-将是 合成了可负载三种单价第一排过渡金属离子的化合物。[NPL]_3-与的三金属化反应 一价铁(I)将被进行，以形成所提议的一价三铁络合物，(NPL)Fe3。单晶 将使用X射线衍射、SQUID磁测量、EPR光谱和循环伏安法来确定 (NPL)Fe3的结构、自旋态和氧化还原性质。将评估新的[Fe3]团簇的反应性 通过(NPL)Fe3与简单的化学氧化剂和固氮酶底物(即氮气和一氧化碳)反应。化合物 (NPL)Fe3将与氧化性N基团转移试剂或N_2替代品反应，以建立 在氮气完全转化的过程中涉及到N-键的中间体。的氧化基团转移反应性 还将使用S-、O-和C-基团转移试剂探索(NPL)Fe3，其中建议的硫化物络合物， [(NPL)Fe3(µ3-S)]-，将被用来模拟硫化物配体在FeMoCO固氮过程中的作用。我们将综合 单氢化物和多氢化物，(NPL)Fe3H1-3，可以作为FeMoco的E4状态的合成模型， 它是被提议与N_2结合的中间体。氢化物络合物的反应性将用 氮气、一氧化碳和乙炔等固氮酶底物与烷烃还原胺的形成 将对基质进行监测。对于有前景的模型化合物，催化还原氮气或一氧化碳的效果将是 在适当的化学氢/电光源存在下或通过电解进行研究。如果成功，这些研究 以期阐明FeMoco还原氮气过程的关键机理步骤。

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.