[FeIV鈺怬(TBC)(CH3CN)]2+: Comparative Reactivity of Iron(IV)-Oxo Species with Constrained Equatorial Cyclam Ligation

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• 作者：Samuel A. Wilson ; Junying Chen ; Seungwoo Hong ; Yong-Min Lee ; Martin Cl茅mancey ; Ricardo Garcia-Serres ; Takashi Nomura ; Takashi Ogura ; Jean-Marc Latour ; Britt Hedman ; Keith O. Hodgson ; Wonwoo Nam ; Edward I. Solomon
• 刊名：The Journal of the American Chemical Society
• 出版年：2012
• 出版时间：July 18, 2012
• 年：2012
• 卷：134
• 期：28
• 页码：11791-11806
• 全文大小：968K
• 年卷期：v.134,no.28(July 18, 2012)
• ISSN：1520-5126

文摘

[Fe^IV鈺怬(TBC)(CH₃CN)]²⁺ (TBC = 1,4,8,11-tetrabenzyl-1,4,8,11-tetraazacyclotetradecane) is characterized, and its reactivity differences relative to [Fe^IV鈺怬(TMC)(CH₃CN)]²⁺ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) are evaluated in hydrogen atom (H-atom) abstraction and oxo-transfer reactions. Structural differences are defined using X-ray absorption spectroscopy and correlated to reactivities using density functional theory. The S = 1 ground states are highly similar and result in large activation barriers (25 kcal/mol) due to steric interactions between the cyclam chelate and the substrate (e.g., ethylbenzene) associated with the equatorial 蟺-attack required by this spin state. Conversely, H-atom abstraction reactivity on an S = 2 surface allows for a 蟽-attack with an axial substrate approach. This results in decreased steric interactions with the cyclam and a lower barrier (9 kcal/mol). For [Fe^IV鈺怬(TBC)(CH₃CN)]²⁺, the S = 2 excited state in the reactant is lower in energy and therefore more accessible at the transition state due to a weaker ligand field associated with the steric interactions of the benzyl substituents with the trans-axial ligand. This study is further extended to the oxo-transfer reaction, which is a two-electron process requiring both 蟽- and 蟺-electron transfer and thus a nonlinear transition state. In oxo-transfer, the S = 2 has a lower barrier due to sequential vs concerted (S = 1) two electron transfer which gives a high-spin ferric intermediate at the transition state. The [Fe^IV鈺怬(TBC)(CH₃CN)]²⁺ complex is more distorted at the transition state, with the iron farther out of the equatorial plane due to the steric interaction of the benzyl groups with the trans-axial ligand. This allows for better orbital overlap with the substrate, a lower barrier, and an increased rate of oxo-transfer.