State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People's Republic of China.
J Phys Chem A. 2012 Feb 9;116(5):1475-85. doi: 10.1021/jp2120302. Epub 2012 Jan 30.
Alkane C-H bond activation by various catalysts and enzymes has attracted considerable attention recently, but many issues are still unanswered. The conversion of ethane to ethanol and ethene by bare Fe(III)═O has been explored using density functional theory and coupled-cluster method comprehensively. Two possible reaction mechanisms are available for the entire reaction, the direct H-abstraction mechanism and the concerted mechanism. First, in the direct H-abstraction mechanism, a direct H-abstraction is encountered in the initial step, going through a collinear transition state C···H···O-Fe and then leading to the generation of an intermediate Fe-OH bound to the alkyl radical weakly. The final product of the direct H-abstraction mechanism is ethanol, which is produced by the hydroxyl group back transfer to the carbon radical. Second, in the concerted reaction mechanism, the H-abstraction process is characterized via overcoming four/five-centered transition states (6/4)TSH_c5 or (4)TSH_c4. The second step of the concerted mechanism can lead to either product ethanol or ethene. Moreover, the major product ethene can be obtained through two different pathways, the one-step pathway and the stepwise pathway. It is the first report that the former pathway starting from (6/4)IM_c to the product can be better described as a proton-coupled electron transfer (PCET). It plays an important role in the product ethene generation according to the CCSD(T) results. The spin-orbital coupling (SOC) calculations demonstrate that the title reaction should proceed via a two-state reactivity (TSR) pattern and that the spin-forbidden transition could slightly lower the rate-determining energy barrier height. This thorough theoretical study, especially the explicit electronic structure analysis, may provide important clues for understanding and studying the C-H bond activation promoted by iron-based artificial catalysts.
最近，各种催化剂和酶对烷烃 C-H 键的活化引起了相当大的关注，但仍有许多问题尚未得到解答。本研究使用密度泛函理论和耦合簇方法全面探讨了裸露 Fe(III)═O将乙烷转化为乙醇和乙烯的反应。整个反应有两种可能的反应机制，即直接 H 抽提机制和协同机制。首先，在直接 H 抽提机制中，在初始步骤中遇到直接 H 抽提，经历共线过渡态 C···H···O-Fe，然后导致弱结合于烷基自由基的 Fe-OH 中间体的生成。直接 H 抽提机制的最终产物是乙醇，其通过羟基基团回迁至碳自由基生成。其次，在协同反应机制中，H 抽提过程的特征在于通过克服四/五中心过渡态 (6/4)TSH_c5 或 (4)TSH_c4。协同机制的第二步可以生成乙醇或乙烯两种产物。此外，主要产物乙烯可以通过两种不同的途径获得，即一步途径和分步途径。这是首次报道从 (6/4)IM_c 到产物的前一种途径可以更好地描述为质子耦合电子转移 (PCET)。根据 CCSD(T) 结果，它在生成产物乙烯中起着重要作用。自旋轨道耦合 (SOC) 计算表明，标题反应应通过两态反应性 (TSR) 模式进行，自旋禁阻跃迁可能会略微降低速率决定的能垒高度。这项深入的理论研究，特别是明确的电子结构分析，可能为理解和研究铁基人工催化剂促进的 C-H 键活化提供重要线索。
