Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification.
Tuning the Relative Stability and Reactivity of Manganese Dioxygen and Peroxo Intermediates via Systematic Ligand Modification.
复制标题
通过系统的配体修饰来调整锰二氧化物和过氧中间体的相对稳定性和反应性。
DOI：
10.1021/acs.accounts.5b00260
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发表时间：
2015-10-20
影响因子：
18.3
通讯作者：
Kovacs JA
中科院分区：
文献类型：
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作者：
Kovacs JA
生命的许多基本过程都依赖于储存在分子氧 (O2) 的 O-O 键中的化学能，其中大部分来自光合 H2O 氧化。这些过程中的关键步骤涉及 Mn、Fe 或 Cu 促进 O-O 和 O-H 键的形成或断裂，其机制尚不完全清楚，尤其是 Mn。金属过氧化合物和高价金属氧化合物被提议作为中间体。有利于 O-O 和 O-H 键形成而不是裂解的金属离子特性尚未得到系统探索。在此，我们检查了一系列结构相关的 Mn(II) 配合物的 O2 反应性，并表明观察到了几种亚稳态中间体，其相对稳定性取决于配体结构的细微差异。我们发现，与 Fe 和 Cu 配合物相比，O2 不可逆地与 Mn(II) 结合。通过结晶整个系列的首次报道的 Mn(III)-OOR 过氧化物以及 O2 衍生的双核反式-μ-1,2-桥联 Mn(III)-过氧化物（具有不同程度的 O-O 键活化），我们证明了光谱、结构和反应性质之间存在明显的相关性。限速 O-O 键断裂可提供能够从 2,4-tBu2-PhOH 或 1,4-环己二烯中夺取 H 原子的活性物质，具体取决于配体取代基。弱配位的 N-杂环 Mn⋯Npy,quino 距离与过氧 O-O 键长相关，并调节填充轨道和 Mn dxz 轨道之间的 π 重叠。我们还表明，过氧 → Mn 电荷转移（CT）带与过氧 O-O 键长之间存在很强的相关性。与具有最短和最长 O-O 键的过氧化物相关的 CT 带之间的能量差异表明，这些距离在光谱上是可区分的。我们表明，只要配体环境大致相同，就可以使用该光谱参数来估计没有晶体结构的中间体中的 O-O 键长度，从而估计 O-O 键活化程度。
Many fundamental processes of life depend on the chemical energy stored in the O–O bond of dioxygen (O2), the majority of which is derived from photosynthetic H2O oxidation. Key steps in these processes involve Mn-, Fe-, or Cu-promoted formation or cleavage of O–O and O–H bonds, the mechanisms of which are not fully understood, especially with Mn. Metal–peroxo and high-valent metal–oxo species are proposed to be involved as intermediates. The metal ion properties that favor O–O and O–H bond formation versus cleavage have yet to be systematically explored. Herein we examine the O2 reactivity of a series of structurally related Mn(II) complexes and show that several metastable intermediates are observed, the relative stabilities of which depend on subtle differences in ligand architecture. We show that in contrast to Fe and Cu complexes, O2 binds irreversibly to Mn(II). By crystallizing an entire series of the first reported examples of Mn(III)–OOR peroxos as well as an O2-derived binuclear trans-μ-1,2-bridged Mn(III)–peroxo with varying degrees of O–O bond activation, we demonstrate that there are distinct correlations between spectroscopic, structural, and reactivity properties. Rate-limiting O–O bond cleavage is shown to afford a reactive species capable of abstracting H atoms from 2,4-tBu2-PhOH or 1,4-cyclohexadiene, depending on the ligand substituents. The weakly coordinated N-heterocycle Mn⋯Npy,quino distance is shown to correlate with the peroxo O–O bond length and modulate the π overlap between the filled and Mn dxz orbitals. We also show that there is a strong correlation between the peroxo → Mn charge transfer (CT) band and the peroxo O–O bond length. The energy difference between the CT bands associated with the peroxos possessing the shortest and longest O–O bonds shows that these distances are spectroscopically distinguishable. We show that we can use this spectroscopic parameter to estimate the O–O bond length, and thus the degree of O–O bond activation, in intermediates for which there is no crystal structure, as long as the ligand environment is approximately the same.