Towards a better understanding of the reactivity of nonheme iron-oxido systems

铁-氧化-氯基配合物[(L)FeIV=O(Cl)]+，其中L为四齿双匹啶2,4-二（吡啶-2-基）-3,7-重氮二环壬烷-1- 1，强制氧化物和氯基顺式排列，具有中间自旋电子基态，导致环己烷作为底物选择性卤化，是已知最快的非血红素铁基氧化剂。一个有趣的问题是，观察到的反应性与FeIV/III=O氧化还原电位有多大关系，与氧化基团的亲电性有多大关系，与铁基配合物的五三重态能隙有多大关系。这些都是非血红素氧化铁系统的基本问题，该提案旨在通过实验和计算工作来找到这些问题的答案，这些实验和计算工作涉及比斯必定配合物以及研究单元中研究的其他系统。氧化还原电位的问题在于，目前还不清楚已发表的FeIV/III=O氧化还原电位是否准确和正确，并且各种可用的计算方法都存在至少需要一个准确可靠的实验数据集进行验证和校准的问题。当前项目的一部分就是为了解决这个问题。另一个也是最有趣的观察是这些中间自旋系统的巨大反应性，初步的计算和实验数据表明，这是由于一个非常小的三重-五重能隙。这些数据需要通过一系列额外的实验和理论研究来证实。这将主要涉及配体场的微妙变化，这些是合成可能的，即多达十二个具有四齿比斯必定平台的例子和供体强度的变化是可用的。在实验探索和量子化学分析相结合的数据方面，零场分裂（field-Mössbauer和HF-EPR光谱）是主要的光谱目标。结合详细的低温停流动力学，这可能使我们获得有关铁基配合物的五三重态能隙的实验信息，这将是验证计算数据的重要信息。

The iron-oxido-chlorido complex [(L)FeIV=O(Cl)]+, where L is the tetradentate bispidine 2,4-di(pyridine-2-yl)-3,7-diazabicyclo[3.3.1]nonane-1-one that enforces a cis-disposition of the oxido and chlorido groups has an intermediate spin electronic ground state, leads to selective halogenation with cyclohexane as substrate and is the fastest known nonheme ferryl oxidant. An interesting question is, how much the observed reactivity relates to the FeIV/III=O redox potential, how much to the electrophilicity of the oxido-group and how much it depends on the quintet-triplet energy gap of the ferryl complex. These are fundamental questions for nonheme iron-oxido systems in general, and the proposal aims to find answers to these questions on the basis of experiments and computational work involving the bispidine complexes as well as other systems studied in the research unit. The problem with the redox potential is that it so far is not clear whether any of the published FeIV/III=O redox potentials is accurate and correct, and the various available computational methods have the problem that they need to be validated and calibrated with at least one accurate and reliable experimental data set. Part of the current project aims at solving this problem. The other and most interesting observation is the enormous reactivity of these intermediate-spin systems, and preliminary computational as well as experimental data indicate that this is due to a very small triplet-quintet energy gap. These data need to be confirmed with a set of additional experiments and theoretical studies. This will primarily involve subtle variations of the ligand field, and these are synthetically possible, i.e. up to a dozen examples with the tetradentate bispidine platform and a variation of the donor strengths are available. In terms of data that will be explored experimentally and combined with quantum-chemical analyses, the zero-field splitting (field-Mössbauer and HF-EPR spectroscopy) is the main spectroscopic target. Together with elaborate cryo-stopped-flow kinetics, this might allow us to obtain experimental information on the quintet-triplet energy gap of our ferryl complexes, and this would be an important information for a validation of the computational data.