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A two-state reactivity rationale for counterintuitive axial ligand effects on the C-H activation reactivity of nonheme FeIV=O oxidants

Title
A two-state reactivity rationale for counterintuitive axial ligand effects on the C-H activation reactivity of nonheme FeIV=O oxidants
Authors
Hirao H.Que Jr. L.Nam W.Shaik S.
Ewha Authors
남원우
SCOPUS Author ID
남원우scopus
Issue Date
2008
Journal Title
Chemistry - A European Journal
ISSN
0947-6539JCR Link
Citation
vol. 14, no. 6, pp. 1740 - 1756
Indexed
SCI; SCIE; SCOPUS WOS scopus
Abstract
This paper addresses the observation of counterintuitive reactivity patterns of iron-oxo reagents, TMC(L)FeO2+,1+; L = CH3CN, CF3CO2 -, N3 -, and SR -, in O-transfer to phosphines versus H-abstraction from, for example, 1,4-cyclohexadiene. Experiments show that O-transfer reactivity correlates with the electrophilicity of the oxidant, but H-abstraction reactivity follows an opposite trend. DFT/B3LYP calculations reveal that twostate reactivity (TSR) serves as a compelling rationale for these trends, whereby all reactions involve two adjacent spin-states of the iron(IV)-oxo species, triplet and quintet. The ground state triplet surface has high barriers, whereas the excited state quintet surface features lower ones. The barriers, on any single surface, are found to increase as the electrophilicity of TMC(L)FeO2+,1+ decreases. Thus, the counterintuitive behavior of the H-abstraction reactions cannot be explained by considering the reactivity of only a single spin state but can be rationalized by a TSR model in which the reactions proceed on the two surfaces. Two TSR models are outlined: one is traditional involving a variable transmission coefficient for crossover from triplet to quintet, followed by quintet-state reactions; the other considers the net barrier as a blend of the triplet and quintet barriers. The blending coefficient (x), which estimates the triplet participation, increases as the quintet-triplet energy gap of the TMC(L)FeO2+,1+ reagent increases, in the following order of L: CH3CN > CF3CO 2 - > N3 - > SR-. The calculated barriers predict the dichotomic experimental trends and the counterintuitive behavior of the H-abstraction series. The TSR approaches make a variety of testable predictions. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.
DOI
10.1002/chem.200701739
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자연과학대학 > 화학·나노과학전공 > Journal papers
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