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 Aug 14;141(32):12682-12696. doi: 10.1021/jacs.9b05277. Epub 2019 Jul 30.
A superoxide-bridged dicopper(II) complex, [Cu(XYLO)(O)] () (XYLO = binucleating -xylyl derivative with a bridging phenolate ligand donor and two bis(2-{2-pyridyl}ethyl)amine arms), was generated from chemical oxidation of the peroxide-bridged dicopper(II) complex [Cu(XYLO)(O)] (), using ferrocenium (Fc) derivatives, in 2-methyltetrahydrofuran (MeTHF) at -125 °C. Using MeFc, a ⇆ equilibrium was established, allowing for calculation of the reduction potential of as -0.525 ± 0.01 V vs Fc. Addition of 1 equiv of strong acid to afforded the hydroperoxide-bridged dicopper(II) species [Cu(XYLO)(OOH)] (). An acid-base equilibrium between and was achieved through spectral titrations using a derivatized phosphazene base. The p of was thus determined to be 24 ± 0.6 in MeTHF at -125 °C. Using a thermodynamic square scheme and the Bordwell relationship, the hydroperoxo complex () O-H bond dissociation free energy (BDFE) was calculated as 81.8 ± 1.5 (BDE = 86.8) kcal/mol. The observed oxidizing capability of [Cu(XYLO)(O)] (), as demonstrated in H atom abstraction reactions with certain phenolic ArO-H and hydrocarbon C-H substrates, provides direct support for this experimentally determined O-H BDFE. A kinetic study reveals a very fast reaction of TEMPO-H with in MeTHF, with (-100 °C) = 5.6 M s. Density functional theory (DFT) calculations reveal how the structure of may minimize stabilization of the superoxide moiety, resulting in its enhanced reactivity. The thermodynamic insights obtained herein highlight the importance of the interplay between ligand design and the generation and properties of copper (or other metal ion) bound O-derived reduced species, such as p, reduction potential, and BDFE; these may be relevant to the capabilities (i.e., oxidizing power) of reactive oxygen intermediates in metalloenzyme chemical system mediated oxidative processes.
一种超氧桥联双核铜（II）配合物，Cu(XYLO)(O)（XYLO 是一种双核 - 二甲苯衍生物，具有桥连的酚氧配体供体和两个双（2-{2-吡啶基}乙基）胺臂），是通过使用 ferrocenium（Fc）衍生物在-125°C 下化学氧化过氧桥联双核铜（II）配合物Cu(XYLO)(O)在 2-甲基四氢呋喃（MeTHF）中生成的。使用 MeFc 建立了 ⇆ 平衡，允许计算出作为 -0.525 ± 0.01 V vs Fc 的还原电位。向 中加入 1 当量强酸，得到过氧桥联双核铜（II）物种Cu(XYLO)(OOH)。通过使用衍生化的磷杂环戊二烯碱进行光谱滴定，实现了 与 的酸碱平衡。因此，在-125°C 的 MeTHF 中，确定 的 p 为 24 ± 0.6。使用热力学正方形方案和 Bordwell 关系，计算得到过氧氢复合物() O-H 键离解自由能（BDFE）为 81.8 ± 1.5（BDE = 86.8）kcal/mol。Cu(XYLO)(O)的观察到的氧化能力，如在某些酚类 ArO-H 和烃类 C-H 底物的 H 原子提取反应中所示，直接支持了这种实验确定的 O-H BDFE。动力学研究表明 TEMPO-H 在 MeTHF 中与 快速反应，k (-100°C) = 5.6 M s。密度泛函理论（DFT）计算揭示了 结构如何最小化超氧部分的稳定化，从而导致其增强的反应性。本文获得的热力学见解强调了配体设计与铜（或其他金属离子）结合的 O 衍生还原物种的生成和性质（如 p、还原电位和 BDFE）之间相互作用的重要性；这些可能与金属酶化学系统中介导氧化过程中的活性氧中间体的能力（即氧化能力）相关。
