Núñez-Zarur Francisco, Solans-Monfort Xavier, Restrepo Albeiro
Instituto de Química, Universidad de Antioquia , Calle 70 No. 52-21, 050010 Medellín, Colombia.
Departament de Química, Universitat Autònoma de Barcelona , 08193 Bellaterra, Spain.
Inorg Chem. 2017 Sep 5;56(17):10458-10473. doi: 10.1021/acs.inorgchem.7b01464. Epub 2017 Aug 15.
Alkane metathesis transforms small alkanes into their higher and lower homologues. The reaction is catalyzed by either supported d metal hydrides (M = Ta, W) or d alkyl alkylidene complexes (M = Ta, Mo, W, Re). For the silica-supported tantalum hydrides, several reaction mechanisms have been proposed. We performed DFT-D3 calculations to analyze the viability of the proposed pathways and compare them with alkane hydrogenolysis, which is a competitive process observed at the early stages of the reaction. The results show that the reaction mechanisms for alkane metathesis and for alkane hydrogenolysis present similar energetics, and this is consistent with the fact that the process taking place depends on the concentrations of the initial reactants. Overall, a modified version of the so-called one-site mechanism that involves alkyl alkylidene intermediates appears to be more likely and consistent with experiments. According to this proposal, tantalum hydrides are precursors of the alkyl alkylidene active species. During precursor activation, H is released and this allows alkane hydrogenolysis to occur. In contrast, the catalytic cycle implies only the reaction with alkane molecules in excess and does not form H. Thus, the activity for alkane hydrogenolysis decreases. The catalytic cycle proposed here implies three stages: (i) β-H elimination from the alkyl ligand, liberating ethene, (ii) alkene cross-metathesis, allowing olefin substituent exchange, and (iii) formation of the final products and alkyl alkylidene regeneration by olefin insertion and three successive 1,2-CH insertions to the alkylidene followed by α abstraction. These results relate the reactivity of silica-supported hydrides with that of the alkyl alkylidene complexes, the other common catalyst for alkane metathesis.
烷烃复分解反应将小分子烷烃转化为其更高和更低的同系物。该反应由负载型d金属氢化物（M = Ta、W）或d烷基亚烷基配合物（M = Ta、Mo、W、Re）催化。对于二氧化硅负载的钽氢化物，已经提出了几种反应机理。我们进行了DFT-D3计算，以分析所提出途径的可行性，并将其与烷烃氢解反应进行比较，烷烃氢解是在反应早期观察到的竞争过程。结果表明，烷烃复分解反应和烷烃氢解反应的机理具有相似的能量学，这与反应过程取决于初始反应物浓度这一事实是一致的。总体而言，涉及烷基亚烷基中间体的所谓单中心机理的改进版本似乎更有可能且与实验结果一致。根据这一建议，钽氢化物是烷基亚烷基活性物种的前体。在前体活化过程中，会释放出H，这使得烷烃氢解反应得以发生。相比之下，催化循环仅意味着与过量的烷烃分子发生反应，且不会形成H。因此，烷烃氢解反应的活性降低。这里提出的催化循环包括三个阶段：（i）从烷基配体上消除β-H，释放出乙烯；（ii）烯烃交叉复分解反应，实现烯烃取代基的交换；（iii）形成最终产物，并通过烯烃插入以及随后依次进行的三次1,2-CH插入到亚烷基中，接着进行α-氢提取来实现烷基亚烷基的再生。这些结果将二氧化硅负载的氢化物的反应活性与烷基亚烷基配合物（烷烃复分解反应的另一种常见催化剂）的反应活性联系了起来。
