Spectroscopic and Reactivity Comparisons between Nonheme Oxoiron(IV) and Oxoiron(V) Species Bearing the Same Ancillary Ligand
Spectroscopic and Reactivity Comparisons between Nonheme Oxoiron(IV) and Oxoiron(V) Species Bearing the Same Ancillary Ligand
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DOI：
10.1021/jacs.9b05758
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发表时间：
2019-09-25
影响因子：
15
通讯作者：
Company, Anna
中科院分区：
文献类型：
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作者：
Dantignana, Valeria;Serrano-Plana, Joan;Company, Anna
这项工作直接比较了氧化铁（IV）和氧化铁(V)配合物的光谱和反应性，它们是由相同的中性四齿n基PyNMe3配体支撑的。对氧化铁（IV）的完整光谱表征(2)表明该化合物是两种同分异构体的混合物。热力学上更稳定的氧化铁（IV）异构体（2b）的反应活性与先前报道的1e（-）氧化类似物[Fe-v(O)(OAc)(PyNMe3)](2+)(3)的反应活性直接比较。我们的数据表明，2b在C-H键的氢原子转移（HAT）中比3慢4到5个数量级。这种巨大差异的根源在于氧化铁单元在HAT后形成的O-H键的强度，由3衍生的O-H键比2b衍生的O-H键强约20 kcal.mol（-1）。（FeO）-O-IV-H键的估计键强度为100 kcal.mol(-1) i，与细胞色素P450化合物i的高活性合成模型的报道值非常接近。此外，本对比研究提供了直接的实验证据，证明由高价氧化铁配合物在初始HAT后形成的碳中心自由基的寿命取决于新生铁- oh配合物的氧化态。配合物2b产生长寿命的碳中心自由基，在溶液中自由扩散，而配合物3产生短寿命的笼状自由基，迅速形成产物C-OH键，因此只有配合物3参与立体保持羟基化反应。因此，铁中心的氧化态不仅可以调节HAT的速率，还可以调节配体的回弹速率。
This work directly compares the spectroscopic and reactivity properties of an oxoiron(IV) and an oxoiron(V) complex that are supported by the same neutral tetradentate N-based PyNMe3 ligand. A complete spectroscopic characterization of the oxoiron(IV) species (2) reveals that this compound exists as a mixture of two isomers. The reactivity of the thermodynamically more stable oxoiron(IV) isomer (2b) is directly compared to that exhibited by the previously reported 1e(-)oxidized analogue [Fe-v(O)(OAc)(PyNMe3)](2+)(3). Our data indicates that 2b is 4 to 5 orders of magnitude slower than 3 in hydrogen atom transfer (HAT) from C-H bonds. The origin of this huge difference lies in the strength of the O-H bond formed after HAT by the oxoiron unit, the O-H bond derived from 3 being about 20 kcal.mol(-1) stronger than that from 2b. The estimated bond strength of the (FeO)-O-IV-H bond of 100 kcal.mol(-1) i is very close to the reported values for highly active synthetic models of compound I of cytochrome P450. In addition, this comparative study provides direct experimental evidence that the lifetime of the carbon-centered radical that forms after the initial HAT by the high valent oxoiron complex depends on the oxidation state of the nascent Fe-OH complex. Complex 2b generates long-lived carbon-centered radicals that freely diffuse in solution, while 3 generates short-lived caged radicals that rapidly form product C-OH bonds, so only 3 engages in stereoretentive hydroxylation reactions. Thus, the oxidation state of the iron center modulates not only the rate of HAT but also the rate of ligand rebound.