Department of Chemistry , Johns Hopkins University , Baltimore , Maryland 21218 , United States.
Department of Chemistry , Stanford University , Stanford , California 94305 , United States.
J Am Chem Soc. 2019 Apr 10;141(14):5942-5960. doi: 10.1021/jacs.9b00795. Epub 2019 Mar 29.
High-valent ferryl species (e.g., (Por)Fe═O, Cmpd-II) are observed or proposed key oxidizing intermediates in the catalytic cycles of heme-containing enzymes (P-450s, peroxidases, catalases, and cytochrome c oxidase) involved in biological respiration and oxidative metabolism. Herein, various axially ligated iron(IV)-oxo complexes were prepared to examine the influence of the identity of the base. These were generated by addition of various axial ligands (1,5-dicyclohexylimidazole (DCHIm), a tethered-imidazole system, and sodium derivatives of 3,5-dimethoxyphenolate and imidazolate). Characterization was carried out via UV-vis, electron paramagnetic resonance (EPR), Fe Mössbauer, Fe X-ray absorption (XAS), and Fe resonance Raman (rR) spectroscopies to confirm their formation and compare the axial ligand perturbation on the electronic and geometric structures of these heme iron(IV)-oxo species. Mössbauer studies confirmed that the axially ligated derivatives were iron(IV) and six-coordinate complexes. XAS and Fe rR data correlated with slight elongation of the iron-oxo bond with increasing donation from the axial ligands. The first reported synthetic H-bonded iron(IV)-oxo heme systems were made in the presence of the protic Lewis acid, 2,6-lutidinium triflate (LutH), with (or without) DCHIm. Mössbauer, rR, and XAS spectroscopic data indicated the formation of molecular Lewis acid ferryl adducts (rather than full protonation). The reduction potentials of these novel Lewis acid adducts were bracketed through addition of outer-sphere reductants. The oxidizing capabilities of the ferryl species with or without Lewis acid vary drastically; addition of LutH to FCmpd-II (F = tetrakis(2,6-difluorophenyl)porphyrinate) increased its reduction potential by more than 890 mV, experimentally confirming that H-bonding interactions can increase the reactivity of ferryl species.
高价铁氧物种（例如（Por）Fe═O、Cmpd-II）是生物呼吸和氧化代谢中涉及的血红素酶（P-450s、过氧化物酶、过氧化氢酶和细胞色素 c 氧化酶）催化循环中观察到或提出的关键氧化中间体。在此，制备了各种轴向配位的铁（IV）-氧合复合物来检查碱基的影响。这些通过添加各种轴向配体（1,5-二环己基咪唑（DCHIm）、连接的咪唑系统和 3,5-二甲氧基苯酚盐和咪唑的钠衍生物）来生成。通过紫外-可见光谱、电子顺磁共振（EPR）、Fe Mössbauer、Fe X 射线吸收（XAS）和 Fe 共振拉曼（rR）光谱学对其进行了表征，以确认它们的形成，并比较这些血红素铁（IV）-氧合物种的轴向配体对电子和几何结构的影响。Mössbauer 研究证实，配位的衍生物是铁（IV）和六配位配合物。XAS 和 Fe rR 数据与轴向配体捐赠的增加相关联，表明铁-氧键的轻微伸长。在质子路易斯酸 2,6-六氢吡啶三氟甲磺酸（LutH）的存在下，合成了第一个报道的氢键合铁（IV）-氧合血红素系统（存在或不存在 DCHIm）。Mössbauer、rR 和 XAS 光谱数据表明形成了分子路易斯酸 ferryl 加合物（而不是完全质子化）。通过添加外部还原试剂，这些新型路易斯酸加合物的还原电位被限制。有无路易斯酸的 ferryl 物种的氧化能力差异很大；向 FCmpd-II（F = 四（2,6-二氟苯基）卟啉）中添加 LutH 将其还原电位增加了超过 890 mV，实验证实氢键相互作用可以增加 ferryl 物种的反应性。
