Functional LnFe-Nx Models of Biological N2 Fixation

描述（由申请人提供）：氮酶（N2ase）是一种金属酶，介导生物固氮，因此对维持生命至关重要。因此，了解n2ase介导的氮还原发生的化学机制是普遍感兴趣的。该计划旨在研究仿生LnFe-Nx模型复合物，这将有助于评估一种机制方案，该机制方案强调N2ase的FeMo辅因子（FeMoco）中的单个Fe-Nx结合位点。实验设计侧重于FeMoco的功能模型而不是结构模型。低分子量的LnFe-Nx配合物将被开发出来，以在四面体或三角双锥体几何结构中分离单个铁位点，同时保留一个结合位点以容纳二氮和各种其他Nx功能。Ln供体支架将包含磷和/或硫供体基团，使LnFe物种具有足够的电子丰富，以结合相对惰性的N2。通过类比生物催化O2还原模式（fei -O2 + 2 e-+ 2 H+ -> FeIV= O + H2O），探索通过末端氮化物中间体（Fel-N2 + 3 e-+ 3 H+ -> FeIV=(N + NH3)）进行的N2还原机制序列。为此目的，将产生和研究为整个循环设想的每一种关键中间体。这些中间体包括feli - n2、Fell-N=NH、fell =N- nh2、FeIV =N、FeIII(NH、fei - nh2和feli - nh3等。进一步的兴趣将是了解LnFe-Nx物种的局部几何和电子结构如何控制它们在Fe-Nx键上的相对稳定性和反应性模式。目的是利用这些知识进一步设计可以促进催化N2ase活性的模型系统。为了实现这些共同目标，需要对每种类型的Fe-Nx物种进行物理和理论表征，对与所提出的循环有关的每种逐步转化的机制理解，以及探索性催化研究，这些研究将揭示成功介导固氮的有前途的模型系统。

DESCRIPTION (provided by applicant): Nitrogenase (N2ase) is a metalloenzyme that mediates biological nitrogen fixation and as such is essential to sustained life. Understanding the chemical mechanism by which N2ase-mediated nitrogen reduction occurs is therefore of general interest. The proposed program is to study biomimetic LnFe-Nx model complexes that will help to evaluate a mechanistic scheme that stresses a single Fe-Nx binding site in the FeMo cofactor (FeMoco) of N2ase. The experimental design focuses on functional rather than structural models of the FeMoco. Low molecular weight LnFe-Nx complexes will be developed to isolate a single iron site in either a tetrahedral or a trigonal bipyramidal geometry while at the same time preserving one binding site to accommodate dinitrogen and various other Nx functionalities. The Ln donor scaffolds will incorporate phosphorous and/or sulfur donor groups to render the LnFe species sufficiently electron-rich to bind relatively inert N2. By analogy to the mode of biocatalytic O2 reduction (FeII-O2 + 2 e-+ 2 H+ -> FeIV= O + H2O), a mechanistic sequence for N2 reduction that proceeds through a terminal nitride intermediate (Fel-N2 + 3 e-+ 3 H+ -> FeIV=(N + NH3) will be explored. To this end each of the critical intermediates envisaged for the complete cycle will be generated and studied. These intermediates will include Fel-N2, Fell-N=NH, Felll=N-NH2, FeIV = N, FeIII(NH, FeII-NH2, and Fel-NH3 species. Of further interest will be to understand how the local geometry and electronic structure of the LnFe-Nx species control their relative stabilities and reactivity patterns at their Fe-Nx linkages. The intent is to then use this knowledge to further design model systems that can facilitate catalytic N2ase activity. To accomplish these collective goals will require the physical and theoretical characterization of each type of Fe-Nx species, a mechanistic understanding of each of the stepwise transformations that pertain to the proposed cycle, and exploratory catalytic studies that will expose promising model systems that successfully mediate nitrogen fixation.