Nonheme Iron(IV)-Oxo Intermediates as Biomimetic Models of Dioxygen Activating Enzymes

Abstract

Heme 철 효소에서 cysteine (cytochrome P450), histidine (peroxidase), tyrosine (catalase) 같은 proximal 리간드들이 소위 Compound I (Cpd I) 상태라 불리는 oxoiron(IV) porphyrin π-cation radicals의 반응성을 나타내는데 중요한 역할을 수행한다고 믿어져 왔다. 실제로, 철 포르피린 모델로서 Cpd I의 iron-oxo 부분의 axial 리간드가 변화됨으로 인해 반응성과 분광학적 성질이 현저하게 영향을 받는다는 것이 증명되어 왔다. 그래서 본 논문에서는 nonheme oxoiron(IV) 화합물의 axial 리간드에 의한 화학적 성질의 변화를 연구해 보고자 하였다. 본 논문에서는 [Fe^(IV)(O)(TMC)(NCCH3)]^(2+)(1)^(4a)의 axial 리간드가 치환된 단핵 nonheme oxoiron(IV) 화합물이 근 자외선 영역에서 상대적으로 매우 강한 전자적 흡수를 지닌 새로운 Fe^(IV)=O 종을 형성하는 것을 보고하였다. 이러한 근 자외선 영역의 특징으로 인해, S = 1인 단핵 nonheme oxoiron(IV) 화합물의 Fe=O 진동을 resonance Raman spectroscopy를 통해 최초로 관찰하게 되었다. 또한 nonheme oxoiron(IV) 종의 산화력이 axial 리간드에 따라 현저하게 영향을 받음을 밝혀냈다.;Axial ligand substitution of a mononuclear nonheme oxoiron(IV) complex, [Fe^(IV)(O)(TMC)(NCCH₃)]^(2+)(1) leads to the formation of new Fe^(IV)=O species with relatively intense electronic absorption features in the near-UV region. The presence of these near-UV features allowed us to make the first observation of Fe=O vibrations of S = 1 mononuclear nonheme oxoiron(IV) complexes by resonance Raman spectroscopy. We have also demonstrated that the reactivity of nonheme oxoiron(IV) intermediates is markedly influenced by the axial ligands. There is an intriguing, current controversy on the involvement of iron(III)-hydroperoxo species as a "second electrophilic oxidant" in oxygenation reactions by heme and non-heme iron enzymes and their model compounds. In the present work, we have performed reactivity studies of the iron-hydroperoxo species in nucleophilic and electrophilic reactions, with in situ-generated mononuclear non-heme iron(III)-hydroperoxo complexes that have been well characterized with various spectroscopic techniques. The intermediates did not show any reactivities in the nucleophilic (e.g., aldehyde deformylation) and electrophilic (e.g., oxidation of sulfide and olefin) reactions. These results demonstrate that non-heme iron(III)-hydroperoxo species are sluggish oxidants and that the oxidizing power of the intermediates cannot compete with that of high-valent iron(IV)-oxo complexes. We have also reported reactivities of mononuclear non-heme iron(III)-peroxo and iron(IV)-oxo complexes in the aldehyde deformylation and the oxidation of sulfides, respectively.