The role of equatorial and axial ligands in promoting the activity of non-heme oxidoiron(IV) catalysts in alkane hydroxylation
The role of equatorial and axial ligands in promoting the activity of non-heme oxidoiron(IV) catalysts in alkane hydroxylation
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DOI：
10.1002/ejic.200601238
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发表时间：
2007-07-01
影响因子：
2.3
通讯作者：
Baerends, Evert Jan
中科院分区：
文献类型：
--
作者：
Bernasconi, Leonardo;Louwerse, Manuel J.;Baerends, Evert Jan
FeO 2+部分的关键电子结构特征（其决定其作为烷烃羟基化催化剂的活性）是存在低位受体轨道，即3 σ * 3d（z）（2）-2p（z）反键轨道。这个轨道的能量位置和系统的自旋状态（反过来也影响3 σ * 能量）都取决于周围的配体。用密度泛函理论（DFT）计算了一系列组成为[FeO（H2O）（n）（L）（5-n）]（2+）（n=4，1，0）的配合物，这些配合物是由最近表征的水溶液[FeO（H2O）5]（2+）通过L=NH3，CH 3CN，H2S和BF 3取代配体水分子而得到的.计算结果表明，高自旋（五重态）的青睐较弱的σ捐赠赤道配体，这是与文献一致。高自旋的配置是更积极的，因为重要的3西格玛 * 向上箭头轨道的显着交换稳定。一旦通过赤道配体的明智选择形成五重态，则可以通过改变轴向配体的性质来调节3 σ * 轨道的能量来微调反应性。观察到轴向配体的σ-供体性质（从σ孤对和3 σ * 轨道之间的轨道相互作用的大小估计）和提取反应的活化势垒之间的线性关系，并且与使3 σ * 轨道不稳定的σ供体的“推动效应”有关。我们提出，相对于[FeO（H2O）（5）]（2+）具有增强的夺氢活化性能的物种可以通过用比H2O弱的σ供体取代轴向配体或通过防止配体在轴向位置与铁配位来获得。((c)Wiley-VCH Verlag GmbH & Co. KGaA，69451魏因海姆，德国，2007）。
The key electronic structural feature of the FeO2+ moiety, which determines its activity as an alkane hydroxylation catalyst, is the presence of low-lying acceptor orbitals, namely the 3 sigma* 3d(z)(2)-2p(z), antibonding orbital. Both the energetic position of this orbital and the spin state of the system (which in turn also affects the 3 sigma* energy) depend on the surrounding ligands. We present results of density functional theory (DFT) calculations performed on a series of gas-phase complexes of composition [FeO(H2O)(n)(L)(5-n)](2+) (n=4, 1, 0) derived from the recently characterised aqueous [FeO(H2O)5](2+) by substitution of ligand water molecules with L=NH3, CH3CN, H2S and BF3. The calculations reveal that the high-spin (quintet) state is favoured by the weaker sigma-donating equatorial ligands, which is consistent with the literature. The high-spin configuration is more reactive because of significant exchange stabilisation of the crucial 3 sigma* up arrow orbital. Once the quintet state is formed by a judicious choice of equatorial ligands, the reactivity can be fine-tuned by modulating the energy of the 3 sigma* orbital by varying the nature of the axial ligand. A linear relation between the sigma-donor properties of the axial ligand (estimated from the magnitude of the orbital interaction between the sigma lone pair and the 3 sigma* orbital) and the activation barrier for the abstraction reaction is observed, and is related to a "push effect" of the sigma donors that destabilises the 3 sigma* orbital. We propose that species with enhanced activation properties for hydrogen abstraction relative to [FeO(H2O)(5)](2+) might be obtainable by either replacing the axial ligand with a sigma donor weaker than H2O or by preventing ligands from coordinating to iron in an axial position.((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).