On the "Reverse Gear" Mechanism of the Reversible Dehydrogenation/Hydrogenation of a Nitrogen Heterocycle Catalyzed by a Cp*Ir Complex: A Computational Study
On the "Reverse Gear" Mechanism of the Reversible Dehydrogenation/Hydrogenation of a Nitrogen Heterocycle Catalyzed by a Cp*Ir Complex: A Computational Study
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DOI：
10.1021/om200222j
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发表时间：
2011-06-13
期刊：
影响因子：
2.8
通讯作者：
Wang, Zhi-Xiang
中科院分区：
文献类型：
--
作者：
Li, Haixia;Jiang, Jinliang;Wang, Zhi-Xiang
开发高效的脱氢/加氢对于实现有机氢化物储氢至关重要。利用 B3LYP DFT 计算，我们研究了 Ir 配合物 2cat (Cp*IrPYD', Cp* = eta(5)-C5Me5 和 PYD' = CF3 取代) 催化的可逆 3qu4h/4qu 脱氢/氢化（3qu4h = 四氢喹啉和 4qu = 喹啉）的催化机制2-吡啶酮酸盐），由 Yamaguchi 和 Fujita 等人报道。两种活性物质（7bif 和 12hcl）被确定在催化系统中发挥重要作用。只需旋转 PYD' 配体即可从 2cat 轻松获得具有双功能反应性的物质 (7bif)，而脱氢中的氢转移或氢化中的氢活化后，通过 HPYD' 配体 (HPYD' = CF3-取代的 2-羟基吡啶) 解离生成 12hcl (Cp*IrHCl)。 7bif物种通过氢转移机制介导脱氢，这比β-H消除（BETAHE）机制更有利，证实了我们在类似催化剂催化的醇脱氢研究中得出的结论。 12hcl 物质与底物（例如 4qu）结合形成反应对，该反应对同时具有路易斯酸性和碱性反应性，可激活 H-2，其机制类似于无金属 FLP（受阻路易斯对）激活 H-2。该对的氢活化产生离子对，该离子对进行氢化物转移以完成氢化。由于二聚体 (Cp*IrHCl)(2) 本身不表现出氢活化反应性，但很容易分解为反应性单体 (12hcl)，因此我们推断通过使用二聚体 (Cp*IrHCl)(2) 实验观察到的 4qu 氢化是由单体 (12hcl) 介导的。 7bif 和 12hcl 通过微观可逆性催化脱氢和氢化过程；根据 H-2 的存在或不存在，反应分别进行脱氢或氢化。完整的3qu4h/4qu脱氢/氢化需要通过亚胺(sic)烯胺互变异构和歧化进行异构化步骤。歧化所需的质子化可以通过7bif的二氢中间体(9oh_h)或离子对(12hcl催化的H-2活化的产物)介导。预测的机制合理地合理化了 (3qu4h (sic) 4qu)/2cat 系统中的实验观察结果。
Development of efficient dehydrogenation/hydrogenation is critical to the realization of organic hydride hydrogen storage. Using B3LYP DFT calculations, we have investigated the catalytic mechanisms of the reversible 3qu4h/4qu dehydrogenation/hydrogenation (3qu4h = tetrahydroquinoline and 4qu = quinoline) catalyzed by the Ir-complex 2cat (Cp*IrPYD', Cp* = eta(5)-C5Me5 and PYD' = CF3-substituted 2-pyridonate), reported by Yamaguchi and Fujita et al. Two reactive species (7bif and 12hcl) are identified to play important roles in the catalytic system. The species (7bif) with bifunctional reactivity can be facilely obtained from 2cat by just rotating the PYD' ligand, while 12hcl (Cp*IrHCl) is generated via HPYD' ligand (HPYD' = CF3-substituted 2-hydroxypyridine) dissociation after hydrogen transfer in dehydrogenation or hydrogen activation in hydrogenation. The species 7bif mediates dehydrogenation via a hydrogen transfer mechanism, which is more favorable than the beta-H elimination (BETAHE) one, confirming our conclusion drawn in the study of alcohol dehydrogenation catalyzed by a similar catalyst. The 12hcl species combines the substrates (e.g., 4qu) to form a reactive pair that simultaneously possesses Lewis acidic and basic reactivity to activate H-2 with a mechanism similar to the H-2 activation by metal-free FLP (frustrated Lewis pair). The hydrogen activation by the pair gives an ion pair that undergoes hydride transfer to complete the hydrogenation. Because the dimer (Cp*IrHCl)(2) itself does not show reactivity toward hydrogen activation but can be easily decomposed into the reactive monomer (12hcl), we reason the experimentally observed hydrogenation of 4qu by using the dimer (Cp*IrHCl)(2) is mediated by the monomer (12hcl). The species 7bif and 12hcl catalyze both dehydrogenation and hydrogenation processes via microscopic reversibility; depending on the absence or presence of H-2, the reaction moves toward dehydrogenation or hydrogenation, respectively. The complete 3qu4h/4qu dehydrogenation/hydrogenation requires an isomerization step through imine (sic) enamine tautomerization and disproportionation. The protonation required for the disproportionation can be mediated by the dihydro intermediate (9oh_h) of 7bif or the ion pair (the product of H-2 activation catalyzed by 12hcl). The predicted mechanisms reasonably rationalize the experimental observations in the (3qu4h (sic) 4qu)/2cat system.