O2 Activation in a Dinuclear Fe(II)/EDTA Complex: Spin Surface Crossing As a Route to Highly Reactive Fe(IV)oxo Species
O2 Activation in a Dinuclear Fe(II)/EDTA Complex: Spin Surface Crossing As a Route to Highly Reactive Fe(IV)oxo Species
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DOI：
10.1021/jp9033672
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发表时间：
2009-10-29
影响因子：
2.9
通讯作者：
Baerends, Evert Jan
中科院分区：
文献类型：
--
作者：
Belanzoni, Paola;Bernasconi, Leonardo;Baerends, Evert Jan
本文研究了在大气氧和EDTA配体存在下，Fe（Ⅱ）-生成Fe（Ⅲ）自氧化反应的中间体[（EDTAH）Fe（O-2）Fe（EDTAH）]（2-）的O-2裂解。研究了局域高自旋Fe中心之间的交换耦合在O-O解离中的作用。利用破缺对称性（BS）密度泛函理论（DFT）计算结果，我们证明了该体系可以被模拟为两个高自旋（HS）S = 5/2 Fe（III）d（5）中心通过桥连过氧O-2（2-）配体耦合，与文献中提出的假设一致.我们表明，在这种电子配置的O-O裂解反应是禁止的（自旋）对称性。O-2（2-）基团解离成产物基态，只有当O-O键被拉伸时，系统才能经历向较低自旋多重数（S = 4）状态的转变。结果表明，[（EDTAH）Fe（O-2）Fe（EDTAH）]（2-）中两个Fe离子之间的交换耦合作用在确定该体系中O-2活化的化学过程中只起次要作用。过氧/氧相互转换涉及初始S = .5状态的海森堡自旋阶梯之外的状态。在S = 4的状态下，双核配合物演变为两个氧代配合物，[EDTAH中心点Fe（IV）O]（-），总能垒仅为86 kJ mol（-1）。根据最近的理论工作，后者物种是非常强的氧化剂，使它们成为有机氧化（包括C-H键羟基化）的理想候选催化剂。我们强调的（自旋）对称性禁止的性质的S = 5的表面上的反应和它的对称性允许字符的电子配置与S = 4。
We study the cleavage Of O-2 in gas phase [(EDTAH)Fe(O-2)Fe(EDTAH)](2-) a proposed intermediate in the aqueous Fe(II)-to-Fe(III) autoxidation reaction in the presence of atmospheric dioxygen and EDTA ligand. The role of the exchange coupling between the locally high-spin Fe centers in the O-O dissociation is investigated. Using results from broken symmetry (BS) density functional theory (DFT) calculations, we show that the system can be modeled as two high-spin (HS) S = 5/2 Fe(III) d(5) centers coupled through a bridging peroxo O-2(2-) ligand, consistent with hypotheses advanced in the literature. We show that in this electronic configuration the O-O cleavage reaction is forbidden by (spin) symmetry. Dissociation of the O-2(2-) group to the product ground state may only take place if the system is allowed to undergo a transition to a state of lower spin multiplicity (S = 4) as the O-O bond is stretched. We show that the exchange coupling between the two Fe ions in [(EDTAH)Fe(O-2)Fe(EDTAH)](2-) plays only a minor role in defining the chemistry Of O-2 activation in this system. The peroxo/oxo interconversion involves a state outside the Heisenberg spin ladder of the initial S = .5 state. In this S = 4 state, the dinuclear complex evolves to two oxo complexes, [EDTAH center dot Fe(IV)O](-), with an overall energy barrier of only similar to 86 kJ mol(-1). According to recent theoretical work, the latter species are exceptionally strong oxidants, making them ideal candidate catalysts for organic oxidations (including C-H bond hydroxylation). We highlight the (spin) symmetry forbidden nature of the reaction on the S = 5 surface and its symmetry allowed character in the electronic configuration with S = 4.