Nonheme iron enzymes often utilize a high-valent iron(IV) oxo species for the biosynthesis of natural products, but their high reactivity often precludes structural and functional studies of these complexes. In this work, a combined experimental and computational study is presented on a biomimetic nonheme iron(IV) oxo complex bearing an aminopyridine macrocyclic ligand and its reactivity toward olefin epoxidation upon changes in the identity and coordination ability of the axial ligand. Herein, we show a dramatic effect of the pH on the oxygen-atom-transfer (OAT) reaction with substrates. In particular, these changes have occurred because of protonation of the axial-bound pendant amine group, where its coordination to iron is replaced by a solvent molecule or anionic ligand. This axial ligand effect influences the catalysis, and we observe enhanced cyclooctene epoxidation yields and turnover numbers in the presence of the unbound protonated pendant amine group. Density functional theory studies were performed to support the experiments and highlight that replacement of the pendant amine with a neutral or anionic ligand dramatically lowers the rate-determining barriers of cyclooctene epoxidation. The computational work further establishes that the change in OAT is due to electrostatic interactions of the pendant amine cation that favorably affect the barrier heights.

中文翻译：

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pH 值变化对具有附加氨基丙基官能团的非血红素铁 (IV) 氧代复合物产生轴向配体效应

非血红素铁酶通常利用高价铁 (IV) 氧代物种进行天然产物的生物合成，但它们的高反应性通常妨碍对这些复合物的结构和功能研究。在这项工作中，对带有氨基吡啶大环配体的仿生非血红素铁 (IV) 氧代配合物及其在轴向配体的身份和配位能力发生变化时对烯烃环氧化的反应性进行了联合实验和计算研究。在此，我们展示了 pH 值对与底物的氧原子转移 (OAT) 反应的显着影响。特别是，这些变化是由于轴向结合的侧胺基团的质子化而发生的，其中它与铁的配位被溶剂分子或阴离子配体取代。这种轴向配体效应影响催化作用，我们观察到在未结合的质子化侧胺基团存在下环辛烯环氧化产率和周转数增加。进行了密度泛函理论研究以支持实验并强调用中性或阴离子配体替换侧胺显着降低了环辛烯环氧化的速率决定障碍。计算工作进一步确定 OAT 的变化是由于侧链胺阳离子的静电相互作用对势垒高度产生有利影响。进行了密度泛函理论研究以支持实验并强调用中性或阴离子配体替换侧胺显着降低了环辛烯环氧化的速率决定障碍。计算工作进一步确定 OAT 的变化是由于侧链胺阳离子的静电相互作用对势垒高度产生有利影响。进行了密度泛函理论研究以支持实验并强调用中性或阴离子配体替换侧胺显着降低了环辛烯环氧化的速率决定障碍。计算工作进一步确定 OAT 的变化是由于侧链胺阳离子的静电相互作用对势垒高度产生有利影响。