Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, USA.
J Am Chem Soc. 2010 Jun 30;132(25):8635-44. doi: 10.1021/ja100366c.
Fe(IV)(O)(TMG(3)tren) (1; TMG(3)tren = 1,1,1-tris{2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl}amine) is a unique example of an isolable synthetic S = 2 oxoiron(IV) complex, which serves as a model for the high-valent oxoiron(IV) intermediates observed in nonheme iron enzymes. Congruent with DFT calculations predicting a more reactive S = 2 oxoiron(IV) center, 1 has a lifetime significantly shorter than those of related S = 1 oxoiron(IV) complexes. The self-decay of 1 exhibits strictly first-order kinetic behavior and is unaffected by solvent deuteration, suggesting an intramolecular process. This hypothesis was supported by ESI-MS analysis of the iron products and a significant retardation of self-decay upon use of a perdeuteromethyl TMG(3)tren isotopomer, d(36)-1 (KIE = 24 at 25 degrees C). The greatly enhanced thermal stability of d(36)-1 allowed growth of diffraction quality crystals for which a high-resolution crystal structure was obtained. This structure showed an Fe horizontal lineO unit (r = 1.661(2) A) in the intended trigonal bipyramidal geometry enforced by the sterically bulky tetramethylguanidinyl donors of the tetradentate tripodal TMG(3)tren ligand. The close proximity of the methyl substituents to the oxoiron unit yielded three symmetrically oriented short C-D...O nonbonded contacts (2.38-2.49 A), an arrangement that facilitated self-decay by rate-determining intramolecular hydrogen atom abstraction and subsequent formation of a ligand-hydroxylated iron(III) product. EPR and Mossbauer quantification of the various iron products, referenced against those obtained from reaction of 1 with 1,4-cyclohexadiene, allowed formulation of a detailed mechanism for the self-decay process. The solution of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path toward a full understanding of these pivotal biological intermediates.
Fe(IV)(O)(TMG(3)tren)（1；TMG(3)tren = 1,1,1-tris{2-[N(2)-(1,1,3,3-tetramethylguanidino)]ethyl}amine）是一种独特的可分离合成 S = 2 价态的双氧铁(IV)配合物的例子，它可以作为非血红素铁酶中观察到的高价态双氧铁(IV)中间体的模型。与预测具有更活泼的 S = 2 价态的双氧铁(IV)中心的 DFT 计算结果一致，1 的半衰期明显短于相关的 S = 1 价态的双氧铁(IV)配合物。1 的自降解表现出严格的一级动力学行为，并且不受溶剂氘代的影响，表明这是一个分子内过程。这一假设得到了 ESI-MS 分析铁产物的支持，以及在使用全氘代甲基 TMG(3)tren 同系物 d(36)-1 时自降解显著延迟的支持（25°C 时 KIE = 24）。d(36)-1 的热稳定性大大增强，允许生长出具有高分辨率晶体结构的衍射质量晶体。该结构显示出一个 Fe 水平线 O 单元（r = 1.661(2)Å），处于由四齿三角架状 TMG(3)tren 配体的空间位阻较大的四甲基胍基供体强制形成的拟三角双锥几何形状中。甲基取代基与双氧铁单元的接近导致了三个对称取向的短 C-D...O 非键接触（2.38-2.49Å），这种排列通过速率决定的分子内氢原子提取和随后形成配体羟化铁(III)产物来促进自降解。通过与 1 与 1,4-环己二烯反应获得的各种铁产物的 EPR 和 Mossbauer 定量，允许制定自降解过程的详细机制。这个高自旋（S = 2）双氧铁(IV)中心的第一个晶体结构的解决方案代表了朝着完全理解这些关键生物中间体的方向迈出的重要一步。
