Computational mechanistic study of the hydrogenation of carbonate to methanol catalyzed by the Ru(II)PNN complex.
Computational mechanistic study of the hydrogenation of carbonate to methanol catalyzed by the Ru(II)PNN complex.
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DOI：
10.1021/ic300175b
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发表时间：
2012-05
影响因子：
4.6
通讯作者：
Haixia Li;Mingwei Wen;Zhi‐Xiang Wang
中科院分区：
文献类型：
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作者：
Haixia Li;Mingwei Wen;Zhi‐Xiang Wang
采用密度泛函理论方法研究了Ru（II）PNN催化剂催化碳酸二甲酯加氢合成甲醇的反应机理。能量结果表明，催化转化包括三个连续的阶段，始终涉及催化剂：（第一阶段）碳酸二甲酯（3）转化为甲酸甲酯（5）和甲醇;（第二阶段）甲酸甲酯5转化为甲醛和甲醇;（第三阶段）甲醛加氢为甲醇。阶段I和II类似地进行，并遵循三个步骤：氢活化，通过阶段I中逐步氢转移至碳酸二甲酯或阶段II中的甲酸甲酯形成半缩醛中间体，并随后分解半缩醛中间体以得到甲醇。通过羰基插入Ru-H键的氢化比逐步氢转移机制更不利。半缩醛的分解首先通过断开半缩醛的O-H键得到醇盐络合物，然后将苄基臂配体去质子化为相邻的甲氧基。比较三个阶段中的氢化步骤，从动力学和热力学两方面来看，阶段I中的氢化最困难，阶段II中的氢化较不困难，而阶段III中的氢化最容易。这可能是由于甲酸甲酯或甲醛中羰基的亲电性比碳酸二甲酯中的强，以及催化剂与甲酸甲酯或甲醛之间的空间效应比催化剂与碳酸二甲酯之间的空间效应小。从热力学上讲，第一阶段和第二阶段都是上坡，但第三阶段明显是下坡，这是催化转化的驱动力。研究表明，甲醇产物可以促进参与转化的氢活化，这意味着通过初始添加甲醇可以加速转化。
Density functional theory computations have been carried out to study the mechanism of hydrogenation-based transformation of dimethyl carbonate to methanol, catalyzed by Ru(II)PNN catalyst. The energetic results show that the catalytic transformation includes three sequential stages consistently involving the catalyst: (stage I) transformation of dimethyl carbonate (3) to methyl formate (5) and methanol; (stage II) transformation of methyl formate 5 to formaldehyde and methanol; (stage III) hydrogenation of formaldehyde to methanol. Stages I and II proceed similarly and follow three steps: hydrogen activation, formation of a hemiacetal intermediate via stepwise hydrogen transfer to dimethyl carbonate in stage I or methyl fomate in stage II, and subsequent decomposition of the hemiacetal intermediate to afford methanol. Hydrogenation via carbonyl insertion into the Ru-H bond is less favorable than the stepwise hydrogen-transfer mechanism. Decomposition of hemiacetal takes places by first breaking the hemiacetal O-H bond to give an alkoxide complex, followed by deprotonation of the benzylic arm ligand to the adjacent methoxy group. Comparing the hydrogenation steps in the three stages, hydrogenation in stage I is most difficult, that in stage II is less difficult, and that in stage III is easiest in terms of both kinetics and thermodynamics. This can be ascribed to the stronger electrophilicity of the carbonyl group in methyl formate or formaldehyde than that in dimethyl carbonate and fewer steric effects between the catalyst and methyl formate or formaldehyde than that between the catalyst and dimethyl carbonate. Thermodynamically, both stages I and II are uphill, but stage III is downhill significantly, which is the driving force for the catalytic transformation. The study indicates that the methanol product could facilitate the hydrogen activation involved in the transformation, implying that transformation could be accelerated by initially adding methanol.