A two-state reactivity rationale for counterintuitive axial ligand effects on the C-H activation reactivity of nonheme FeIV=O oxidants
A two-state reactivity rationale for counterintuitive axial ligand effects on the C-H activation reactivity of nonheme FeIV=O oxidants
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DOI：
10.1002/chem.200701739
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发表时间：
2008-01-01
影响因子：
4.3
通讯作者：
Shaik, Sason
中科院分区：
文献类型：
--
作者：
Hirao, Hajime;Que, Lawrence, Jr.;Shaik, Sason
本文讨论了铁氧试剂TMC（L）FeO 2 +.1+; L = CH 3CN、CF 3CO-、N-3（-）和SR-在O-转移到膦与H-提取（例如1，4-环己二烯）中的反直觉反应模式的观察。实验表明，O-转移反应性与氧化剂的亲电性相关，但H-提取反应性遵循相反的趋势。DFT/B3 LYP计算表明，两态反应性（TSR）作为一个令人信服的理由，这些趋势，其中所有的反应涉及两个相邻的自旋态的铁（IV）-氧代物种，三重态和五重态。基态三重态表面具有高势垒，而激发态五重态表面具有较低的势垒。发现在任何单个表面上的势垒随着TMC（L）FeO 2 +.1+的亲电性的降低而增加。因此，夺氢反应的违反直觉的行为不能通过只考虑单个自旋态的反应性来解释，但可以通过反应在两个表面上进行的TSR模型来合理化。概述了两种TSR模型：一种是传统的，涉及从三重态到五重态的交叉的可变透射系数，随后是五重态反应;另一种认为净势垒是三重态和五重态势垒的混合。估计三重态参与的共混系数（x）随着TMC（L）FeO 2 +.1+试剂的五重态-三重态能隙的增加而增加，其顺序为L：CH 3CN> CF 3CO 2- > N-3 ㈠> SR-。计算的障碍预测的二分法的实验趋势和H-抽象系列的违反直觉的行为。TSR方法做出了各种可测试的预测。
This paper addresses the observation of counterintuitive reactivity patterns of iron-oxo reagents, TMC(L)FeO2+.1+; L = CH3CN, CF3CO-, N-3(-), 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 two-state 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 > CF3CO2- > N-3(-) > 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.