O-O Bond Cleavage in Dinuclear Peroxo Complexes of Iron Porphyrins: a Quantum Chemical Study

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• 作者：Margareta R. A. Blomberg ; Adam Johannes Johansson ; Per E. M. Siegbahn
• 刊名：Inorganic Chemistry
• 出版年：2007
• 出版时间：September 17, 2007
• 年：2007
• 卷：46
• 期：19
• 页码：7992 - 7997
• 全文大小：98K
• 年卷期：v.46,no.19(September 17, 2007)
• ISSN：1520-510X

文摘

To gain insight into the mechanisms of O₂ activation and cleavage in metalloenzymes, biomimetic metal complexeshave been constructed and experimentally characterized. One such model complex is the dinuclear peroxo complexof iron porphyrins observed at low temperature in a noncoordinating solvent. The present theoretical study examinesthe O-O bond cleavage in these complexes, experimentally observed to occur either at increased temperature orwhen a strongly coordinating base is added. Using hybrid density functional theory, it is shown that the O-O bondcleavage always occurs in a state where two low-spin irons (S = ±¹/₂) are antiferromagnetically coupled to adiamagnetic state. This state is the ground state when the strong base is present and forms an axial ligand to thefree iron positions. In contrast, without the axial ligands, the ground state of the dinuclear peroxo complex has twohigh-spin irons (S = ±⁵/₂) coupled antiferromagnetically. Thus, the activation barrier for O-O bond cleavage ishigher without the base because it includes also the promotion energy from the ground state to the reacting state.It is further found that this excitation energy, going from 10 unpaired electrons in the high-spin case to 2 in thelow-spin case, is unusually difficult to determine accurately from density functional theory because it is extremelysensitive to the amount of exact exchange included in the functional.